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How to Write a Research Paper Introduction (with Examples)

The research paper introduction section, along with the Title and Abstract, can be considered the face of any research paper. The following article is intended to guide you in organizing and writing the research paper introduction for a quality academic article or dissertation.

The research paper introduction aims to present the topic to the reader. A study will only be accepted for publishing if you can ascertain that the available literature cannot answer your research question. So it is important to ensure that you have read important studies on that particular topic, especially those within the last five to ten years, and that they are properly referenced in this section. 1 What should be included in the research paper introduction is decided by what you want to tell readers about the reason behind the research and how you plan to fill the knowledge gap. The best research paper introduction provides a systemic review of existing work and demonstrates additional work that needs to be done. It needs to be brief, captivating, and well-referenced; a well-drafted research paper introduction will help the researcher win half the battle.

The introduction for a research paper is where you set up your topic and approach for the reader. It has several key goals:

• Present your research topic
• Capture reader interest
• Summarize existing research
• Position your own approach
• Define your specific research problem and problem statement
• Highlight the novelty and contributions of the study
• Give an overview of the paper’s structure

The research paper introduction can vary in size and structure depending on whether your paper presents the results of original empirical research or is a review paper. Some research paper introduction examples are only half a page while others are a few pages long. In many cases, the introduction will be shorter than all of the other sections of your paper; its length depends on the size of your paper as a whole.

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Table of Contents

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The introduction in a research paper is placed at the beginning to guide the reader from a broad subject area to the specific topic that your research addresses. They present the following information to the reader

• Scope: The topic covered in the research paper
• Context: Background of your topic
• Importance: Why your research matters in that particular area of research and the industry problem that can be targeted

The research paper introduction conveys a lot of information and can be considered an essential roadmap for the rest of your paper. A good introduction for a research paper is important for the following reasons:

• It stimulates your reader’s interest: A good introduction section can make your readers want to read your paper by capturing their interest. It informs the reader what they are going to learn and helps determine if the topic is of interest to them.
• It helps the reader understand the research background: Without a clear introduction, your readers may feel confused and even struggle when reading your paper. A good research paper introduction will prepare them for the in-depth research to come. It provides you the opportunity to engage with the readers and demonstrate your knowledge and authority on the specific topic.
• It explains why your research paper is worth reading: Your introduction can convey a lot of information to your readers. It introduces the topic, why the topic is important, and how you plan to proceed with your research.
• It helps guide the reader through the rest of the paper: The research paper introduction gives the reader a sense of the nature of the information that will support your arguments and the general organization of the paragraphs that will follow. It offers an overview of what to expect when reading the main body of your paper.

What are the parts of introduction in the research?

A good research paper introduction section should comprise three main elements: 2

• What is known: This sets the stage for your research. It informs the readers of what is known on the subject.
• What is lacking: This is aimed at justifying the reason for carrying out your research. This could involve investigating a new concept or method or building upon previous research.
• What you aim to do: This part briefly states the objectives of your research and its major contributions. Your detailed hypothesis will also form a part of this section.

How to write a research paper introduction?

The first step in writing the research paper introduction is to inform the reader what your topic is and why it’s interesting or important. This is generally accomplished with a strong opening statement. The second step involves establishing the kinds of research that have been done and ending with limitations or gaps in the research that you intend to address. Finally, the research paper introduction clarifies how your own research fits in and what problem it addresses. If your research involved testing hypotheses, these should be stated along with your research question. The hypothesis should be presented in the past tense since it will have been tested by the time you are writing the research paper introduction.

The following key points, with examples, can guide you when writing the research paper introduction section:

• Highlight the importance of the research field or topic
• Describe the background of the topic
• Present an overview of current research on the topic

Example: The inclusion of experiential and competency-based learning has benefitted electronics engineering education. Industry partnerships provide an excellent alternative for students wanting to engage in solving real-world challenges. Industry-academia participation has grown in recent years due to the need for skilled engineers with practical training and specialized expertise. However, from the educational perspective, many activities are needed to incorporate sustainable development goals into the university curricula and consolidate learning innovation in universities.

• Reveal a gap in existing research or oppose an existing assumption
• Formulate the research question

Example: There have been plausible efforts to integrate educational activities in higher education electronics engineering programs. However, very few studies have considered using educational research methods for performance evaluation of competency-based higher engineering education, with a focus on technical and or transversal skills. To remedy the current need for evaluating competencies in STEM fields and providing sustainable development goals in engineering education, in this study, a comparison was drawn between study groups without and with industry partners.

• State the purpose of your study
• Highlight the key characteristics of your study
• Describe important results
• Highlight the novelty of the study.
• Offer a brief overview of the structure of the paper.

Example: The study evaluates the main competency needed in the applied electronics course, which is a fundamental core subject for many electronics engineering undergraduate programs. We compared two groups, without and with an industrial partner, that offered real-world projects to solve during the semester. This comparison can help determine significant differences in both groups in terms of developing subject competency and achieving sustainable development goals.

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With Paperpal Copilot, create a research paper introduction effortlessly. In this step-by-step guide, we’ll walk you through how Paperpal transforms your initial ideas into a polished and publication-ready introduction.

How to use Paperpal to write the Introduction section

Step 1: Sign up on Paperpal and click on the Copilot feature, under this choose Outlines > Research Article > Introduction

Step 2: Add your unstructured notes or initial draft, whether in English or another language, to Paperpal, which is to be used as the base for your content.

Step 3: Fill in the specifics, such as your field of study, brief description or details you want to include, which will help the AI generate the outline for your Introduction.

Step 4: Use this outline and sentence suggestions to develop your content, adding citations where needed and modifying it to align with your specific research focus.

Step 5: Turn to Paperpal’s granular language checks to refine your content, tailor it to reflect your personal writing style, and ensure it effectively conveys your message.

You can use the same process to develop each section of your article, and finally your research paper in half the time and without any of the stress.

The purpose of the research paper introduction is to introduce the reader to the problem definition, justify the need for the study, and describe the main theme of the study. The aim is to gain the reader’s attention by providing them with necessary background information and establishing the main purpose and direction of the research.

The length of the research paper introduction can vary across journals and disciplines. While there are no strict word limits for writing the research paper introduction, an ideal length would be one page, with a maximum of 400 words over 1-4 paragraphs. Generally, it is one of the shorter sections of the paper as the reader is assumed to have at least a reasonable knowledge about the topic. 2 For example, for a study evaluating the role of building design in ensuring fire safety, there is no need to discuss definitions and nature of fire in the introduction; you could start by commenting upon the existing practices for fire safety and how your study will add to the existing knowledge and practice.

When deciding what to include in the research paper introduction, the rest of the paper should also be considered. The aim is to introduce the reader smoothly to the topic and facilitate an easy read without much dependency on external sources. 3 Below is a list of elements you can include to prepare a research paper introduction outline and follow it when you are writing the research paper introduction. Topic introduction: This can include key definitions and a brief history of the topic. Research context and background: Offer the readers some general information and then narrow it down to specific aspects. Details of the research you conducted: A brief literature review can be included to support your arguments or line of thought. Rationale for the study: This establishes the relevance of your study and establishes its importance. Importance of your research: The main contributions are highlighted to help establish the novelty of your study Research hypothesis: Introduce your research question and propose an expected outcome. Organization of the paper: Include a short paragraph of 3-4 sentences that highlights your plan for the entire paper

Cite only works that are most relevant to your topic; as a general rule, you can include one to three. Note that readers want to see evidence of original thinking. So it is better to avoid using too many references as it does not leave much room for your personal standpoint to shine through. Citations in your research paper introduction support the key points, and the number of citations depend on the subject matter and the point discussed. If the research paper introduction is too long or overflowing with citations, it is better to cite a few review articles rather than the individual articles summarized in the review. A good point to remember when citing research papers in the introduction section is to include at least one-third of the references in the introduction.

The literature review plays a significant role in the research paper introduction section. A good literature review accomplishes the following: Introduces the topic – Establishes the study’s significance – Provides an overview of the relevant literature – Provides context for the study using literature – Identifies knowledge gaps However, remember to avoid making the following mistakes when writing a research paper introduction: Do not use studies from the literature review to aggressively support your research Avoid direct quoting Do not allow literature review to be the focus of this section. Instead, the literature review should only aid in setting a foundation for the manuscript.

Remember the following key points for writing a good research paper introduction: 4

• Avoid stuffing too much general information: Avoid including what an average reader would know and include only that information related to the problem being addressed in the research paper introduction. For example, when describing a comparative study of non-traditional methods for mechanical design optimization, information related to the traditional methods and differences between traditional and non-traditional methods would not be relevant. In this case, the introduction for the research paper should begin with the state-of-the-art non-traditional methods and methods to evaluate the efficiency of newly developed algorithms.
• Avoid packing too many references: Cite only the required works in your research paper introduction. The other works can be included in the discussion section to strengthen your findings.
• Avoid extensive criticism of previous studies: Avoid being overly critical of earlier studies while setting the rationale for your study. A better place for this would be the Discussion section, where you can highlight the advantages of your method.
• Avoid describing conclusions of the study: When writing a research paper introduction remember not to include the findings of your study. The aim is to let the readers know what question is being answered. The actual answer should only be given in the Results and Discussion section.

To summarize, the research paper introduction section should be brief yet informative. It should convince the reader the need to conduct the study and motivate him to read further. If you’re feeling stuck or unsure, choose trusted AI academic writing assistants like Paperpal to effortlessly craft your research paper introduction and other sections of your research article.

1. Jawaid, S. A., & Jawaid, M. (2019). How to write introduction and discussion. Saudi Journal of Anaesthesia, 13(Suppl 1), S18.

2. Dewan, P., & Gupta, P. (2016). Writing the title, abstract and introduction: Looks matter!. Indian pediatrics, 53, 235-241.

3. Cetin, S., & Hackam, D. J. (2005). An approach to the writing of a scientific Manuscript1. Journal of Surgical Research, 128(2), 165-167.

4. Bavdekar, S. B. (2015). Writing introduction: Laying the foundations of a research paper. Journal of the Association of Physicians of India, 63(7), 44-6.

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Home » Research Paper Introduction – Writing Guide and Examples

Research Paper Introduction – Writing Guide and Examples

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Research Paper Introduction

Research paper introduction is the first section of a research paper that provides an overview of the study, its purpose, and the research question (s) or hypothesis (es) being investigated. It typically includes background information about the topic, a review of previous research in the field, and a statement of the research objectives. The introduction is intended to provide the reader with a clear understanding of the research problem, why it is important, and how the study will contribute to existing knowledge in the field. It also sets the tone for the rest of the paper and helps to establish the author’s credibility and expertise on the subject.

How to Write Research Paper Introduction

Writing an introduction for a research paper can be challenging because it sets the tone for the entire paper. Here are some steps to follow to help you write an effective research paper introduction:

• Start with a hook : Begin your introduction with an attention-grabbing statement, a question, or a surprising fact that will make the reader interested in reading further.
• Provide background information: After the hook, provide background information on the topic. This information should give the reader a general idea of what the topic is about and why it is important.
• State the research problem: Clearly state the research problem or question that the paper addresses. This should be done in a concise and straightforward manner.
• State the research objectives: After stating the research problem, clearly state the research objectives. This will give the reader an idea of what the paper aims to achieve.
• Provide a brief overview of the paper: At the end of the introduction, provide a brief overview of the paper. This should include a summary of the main points that will be discussed in the paper.
• Revise and refine: Finally, revise and refine your introduction to ensure that it is clear, concise, and engaging.

Structure of Research Paper Introduction

The following is a typical structure for a research paper introduction:

• Background Information: This section provides an overview of the topic of the research paper, including relevant background information and any previous research that has been done on the topic. It helps to give the reader a sense of the context for the study.
• Problem Statement: This section identifies the specific problem or issue that the research paper is addressing. It should be clear and concise, and it should articulate the gap in knowledge that the study aims to fill.
• Research Question/Hypothesis : This section states the research question or hypothesis that the study aims to answer. It should be specific and focused, and it should clearly connect to the problem statement.
• Significance of the Study: This section explains why the research is important and what the potential implications of the study are. It should highlight the contribution that the research makes to the field.
• Methodology: This section describes the research methods that were used to conduct the study. It should be detailed enough to allow the reader to understand how the study was conducted and to evaluate the validity of the results.
• Organization of the Paper : This section provides a brief overview of the structure of the research paper. It should give the reader a sense of what to expect in each section of the paper.

Research Paper Introduction Examples

Research Paper Introduction Examples could be:

Example 1: In recent years, the use of artificial intelligence (AI) has become increasingly prevalent in various industries, including healthcare. AI algorithms are being developed to assist with medical diagnoses, treatment recommendations, and patient monitoring. However, as the use of AI in healthcare grows, ethical concerns regarding privacy, bias, and accountability have emerged. This paper aims to explore the ethical implications of AI in healthcare and propose recommendations for addressing these concerns.

Example 2: Climate change is one of the most pressing issues facing our planet today. The increasing concentration of greenhouse gases in the atmosphere has resulted in rising temperatures, changing weather patterns, and other environmental impacts. In this paper, we will review the scientific evidence on climate change, discuss the potential consequences of inaction, and propose solutions for mitigating its effects.

Example 3: The rise of social media has transformed the way we communicate and interact with each other. While social media platforms offer many benefits, including increased connectivity and access to information, they also present numerous challenges. In this paper, we will examine the impact of social media on mental health, privacy, and democracy, and propose solutions for addressing these issues.

Example 4: The use of renewable energy sources has become increasingly important in the face of climate change and environmental degradation. While renewable energy technologies offer many benefits, including reduced greenhouse gas emissions and energy independence, they also present numerous challenges. In this paper, we will assess the current state of renewable energy technology, discuss the economic and political barriers to its adoption, and propose solutions for promoting the widespread use of renewable energy.

Purpose of Research Paper Introduction

The introduction section of a research paper serves several important purposes, including:

• Providing context: The introduction should give readers a general understanding of the topic, including its background, significance, and relevance to the field.
• Presenting the research question or problem: The introduction should clearly state the research question or problem that the paper aims to address. This helps readers understand the purpose of the study and what the author hopes to accomplish.
• Reviewing the literature: The introduction should summarize the current state of knowledge on the topic, highlighting the gaps and limitations in existing research. This shows readers why the study is important and necessary.
• Outlining the scope and objectives of the study: The introduction should describe the scope and objectives of the study, including what aspects of the topic will be covered, what data will be collected, and what methods will be used.
• Previewing the main findings and conclusions : The introduction should provide a brief overview of the main findings and conclusions that the study will present. This helps readers anticipate what they can expect to learn from the paper.

When to Write Research Paper Introduction

The introduction of a research paper is typically written after the research has been conducted and the data has been analyzed. This is because the introduction should provide an overview of the research problem, the purpose of the study, and the research questions or hypotheses that will be investigated.

Once you have a clear understanding of the research problem and the questions that you want to explore, you can begin to write the introduction. It’s important to keep in mind that the introduction should be written in a way that engages the reader and provides a clear rationale for the study. It should also provide context for the research by reviewing relevant literature and explaining how the study fits into the larger field of research.

Advantages of Research Paper Introduction

The introduction of a research paper has several advantages, including:

• Establishing the purpose of the research: The introduction provides an overview of the research problem, question, or hypothesis, and the objectives of the study. This helps to clarify the purpose of the research and provide a roadmap for the reader to follow.
• Providing background information: The introduction also provides background information on the topic, including a review of relevant literature and research. This helps the reader understand the context of the study and how it fits into the broader field of research.
• Demonstrating the significance of the research: The introduction also explains why the research is important and relevant. This helps the reader understand the value of the study and why it is worth reading.
• Setting expectations: The introduction sets the tone for the rest of the paper and prepares the reader for what is to come. This helps the reader understand what to expect and how to approach the paper.
• Grabbing the reader’s attention: A well-written introduction can grab the reader’s attention and make them interested in reading further. This is important because it can help to keep the reader engaged and motivated to read the rest of the paper.
• Creating a strong first impression: The introduction is the first part of the research paper that the reader will see, and it can create a strong first impression. A well-written introduction can make the reader more likely to take the research seriously and view it as credible.
• Establishing the author’s credibility: The introduction can also establish the author’s credibility as a researcher. By providing a clear and thorough overview of the research problem and relevant literature, the author can demonstrate their expertise and knowledge in the field.
• Providing a structure for the paper: The introduction can also provide a structure for the rest of the paper. By outlining the main sections and sub-sections of the paper, the introduction can help the reader navigate the paper and find the information they are looking for.

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Muhammad Hassan

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How to write an effective introduction for your research paper

Last updated

20 January 2024

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However, the introduction is a vital element of your research paper . It helps the reader decide whether your paper is worth their time. As such, it's worth taking your time to get it right.

In this article, we'll tell you everything you need to know about writing an effective introduction for your research paper.

• The importance of an introduction in research papers

The primary purpose of an introduction is to provide an overview of your paper. This lets readers gauge whether they want to continue reading or not. The introduction should provide a meaningful roadmap of your research to help them make this decision. It should let readers know whether the information they're interested in is likely to be found in the pages that follow.

Aside from providing readers with information about the content of your paper, the introduction also sets the tone. It shows readers the style of language they can expect, which can further help them to decide how far to read.

When you take into account both of these roles that an introduction plays, it becomes clear that crafting an engaging introduction is the best way to get your paper read more widely. First impressions count, and the introduction provides that impression to readers.

• The optimum length for a research paper introduction

While there's no magic formula to determine exactly how long a research paper introduction should be, there are a few guidelines. Some variables that impact the ideal introduction length include:

Field of study

Complexity of the topic

Specific requirements of the course or publication

A commonly recommended length of a research paper introduction is around 10% of the total paper’s length. So, a ten-page paper has a one-page introduction. If the topic is complex, it may require more background to craft a compelling intro. Humanities papers tend to have longer introductions than those of the hard sciences.

The best way to craft an introduction of the right length is to focus on clarity and conciseness. Tell the reader only what is necessary to set up your research. An introduction edited down with this goal in mind should end up at an acceptable length.

• Evaluating successful research paper introductions

A good way to gauge how to create a great introduction is by looking at examples from across your field. The most influential and well-regarded papers should provide some insights into what makes a good introduction.

Dissecting examples: what works and why

We can make some general assumptions by looking at common elements of a good introduction, regardless of the field of research.

A common structure is to start with a broad context, and then narrow that down to specific research questions or hypotheses. This creates a funnel that establishes the scope and relevance.

The most effective introductions are careful about the assumptions they make regarding reader knowledge. By clearly defining key terms and concepts instead of assuming the reader is familiar with them, these introductions set a more solid foundation for understanding.

To pull in the reader and make that all-important good first impression, excellent research paper introductions will often incorporate a compelling narrative or some striking fact that grabs the reader's attention.

Finally, good introductions provide clear citations from past research to back up the claims they're making. In the case of argumentative papers or essays (those that take a stance on a topic or issue), a strong thesis statement compels the reader to continue reading.

Common pitfalls to avoid in research paper introductions

You can also learn what not to do by looking at other research papers. Many authors have made mistakes you can learn from.

We've talked about the need to be clear and concise. Many introductions fail at this; they're verbose, vague, or otherwise fail to convey the research problem or hypothesis efficiently. This often comes in the form of an overemphasis on background information, which obscures the main research focus.

Ensure your introduction provides the proper emphasis and excitement around your research and its significance. Otherwise, fewer people will want to read more about it.

• Crafting a compelling introduction for a research paper

Let’s take a look at the steps required to craft an introduction that pulls readers in and compels them to learn more about your research.

Step 1: Capturing interest and setting the scene

To capture the reader's interest immediately, begin your introduction with a compelling question, a surprising fact, a provocative quote, or some other mechanism that will hook readers and pull them further into the paper.

As they continue reading, the introduction should contextualize your research within the current field, showing readers its relevance and importance. Clarify any essential terms that will help them better understand what you're saying. This keeps the fundamentals of your research accessible to all readers from all backgrounds.

Step 2: Building a solid foundation with background information

Including background information in your introduction serves two major purposes:

It helps to clarify the topic for the reader

It establishes the depth of your research

The approach you take when conveying this information depends on the type of paper.

For argumentative papers, you'll want to develop engaging background narratives. These should provide context for the argument you'll be presenting.

For empirical papers, highlighting past research is the key. Often, there will be some questions that weren't answered in those past papers. If your paper is focused on those areas, those papers make ideal candidates for you to discuss and critique in your introduction.

Step 3: Pinpointing the research challenge

To capture the attention of the reader, you need to explain what research challenges you'll be discussing.

For argumentative papers, this involves articulating why the argument you'll be making is important. What is its relevance to current discussions or problems? What is the potential impact of people accepting or rejecting your argument?

For empirical papers, explain how your research is addressing a gap in existing knowledge. What new insights or contributions will your research bring to your field?

Step 4: Clarifying your research aims and objectives

We mentioned earlier that the introduction to a research paper can serve as a roadmap for what's within. We've also frequently discussed the need for clarity. This step addresses both of these.

When writing an argumentative paper, craft a thesis statement with impact. Clearly articulate what your position is and the main points you intend to present. This will map out for the reader exactly what they'll get from reading the rest.

For empirical papers, focus on formulating precise research questions and hypotheses. Directly link them to the gaps or issues you've identified in existing research to show the reader the precise direction your research paper will take.

Step 5: Sketching the blueprint of your study

Continue building a roadmap for your readers by designing a structured outline for the paper. Guide the reader through your research journey, explaining what the different sections will contain and their relationship to one another.

This outline should flow seamlessly as you move from section to section. Creating this outline early can also help guide the creation of the paper itself, resulting in a final product that's better organized. In doing so, you'll craft a paper where each section flows intuitively from the next.

Step 6: Integrating your research question

To avoid letting your research question get lost in background information or clarifications, craft your introduction in such a way that the research question resonates throughout. The research question should clearly address a gap in existing knowledge or offer a new perspective on an existing problem.

Tell users your research question explicitly but also remember to frequently come back to it. When providing context or clarification, point out how it relates to the research question. This keeps your focus where it needs to be and prevents the topic of the paper from becoming under-emphasized.

Step 7: Establishing the scope and limitations

So far, we've talked mostly about what's in the paper and how to convey that information to readers. The opposite is also important. Information that's outside the scope of your paper should be made clear to the reader in the introduction so their expectations for what is to follow are set appropriately.

Similarly, be honest and upfront about the limitations of the study. Any constraints in methodology, data, or how far your findings can be generalized should be fully communicated in the introduction.

Step 8: Concluding the introduction with a promise

The final few lines of the introduction are your last chance to convince people to continue reading the rest of the paper. Here is where you should make it very clear what benefit they'll get from doing so. What topics will be covered? What questions will be answered? Make it clear what they will get for continuing.

By providing a quick recap of the key points contained in the introduction in its final lines and properly setting the stage for what follows in the rest of the paper, you refocus the reader's attention on the topic of your research and guide them to read more.

• Research paper introduction best practices

Following the steps above will give you a compelling introduction that hits on all the key points an introduction should have. Some more tips and tricks can make an introduction even more polished.

As you follow the steps above, keep the following tips in mind.

Set the right tone and style

Like every piece of writing, a research paper should be written for the audience. That is to say, it should match the tone and style that your academic discipline and target audience expect. This is typically a formal and academic tone, though the degree of formality varies by field.

Kno w the audience

The perfect introduction balances clarity with conciseness. The amount of clarification required for a given topic depends greatly on the target audience. Knowing who will be reading your paper will guide you in determining how much background information is required.

Adopt the CARS (create a research space) model

The CARS model is a helpful tool for structuring introductions. This structure has three parts. The beginning of the introduction establishes the general research area. Next, relevant literature is reviewed and critiqued. The final section outlines the purpose of your study as it relates to the previous parts.

Master the art of funneling

The CARS method is one example of a well-funneled introduction. These start broadly and then slowly narrow down to your specific research problem. It provides a nice narrative flow that provides the right information at the right time. If you stray from the CARS model, try to retain this same type of funneling.

Incorporate narrative element

People read research papers largely to be informed. But to inform the reader, you have to hold their attention. A narrative style, particularly in the introduction, is a great way to do that. This can be a compelling story, an intriguing question, or a description of a real-world problem.

Write the introduction last

By writing the introduction after the rest of the paper, you'll have a better idea of what your research entails and how the paper is structured. This prevents the common problem of writing something in the introduction and then forgetting to include it in the paper. It also means anything particularly exciting in the paper isn’t neglected in the intro.

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How to Write a Research Introduction

Last Updated: December 6, 2023 Fact Checked

This article was co-authored by Megan Morgan, PhD . Megan Morgan is a Graduate Program Academic Advisor in the School of Public & International Affairs at the University of Georgia. She earned her PhD in English from the University of Georgia in 2015. There are 7 references cited in this article, which can be found at the bottom of the page. This article has been fact-checked, ensuring the accuracy of any cited facts and confirming the authority of its sources. This article has been viewed 2,653,534 times.

The introduction to a research paper can be the most challenging part of the paper to write. The length of the introduction will vary depending on the type of research paper you are writing. An introduction should announce your topic, provide context and a rationale for your work, before stating your research questions and hypothesis. Well-written introductions set the tone for the paper, catch the reader's interest, and communicate the hypothesis or thesis statement.

Introducing the Topic of the Paper

• In scientific papers this is sometimes known as an "inverted triangle", where you start with the broadest material at the start, before zooming in on the specifics. [2] X Research source
• The sentence "Throughout the 20th century, our views of life on other planets have drastically changed" introduces a topic, but does so in broad terms.
• It provides the reader with an indication of the content of the essay and encourages them to read on.

• For example, if you were writing a paper about the behaviour of mice when exposed to a particular substance, you would include the word "mice", and the scientific name of the relevant compound in the first sentences.
• If you were writing a history paper about the impact of the First World War on gender relations in Britain, you should mention those key words in your first few lines.

• This is especially important if you are attempting to develop a new conceptualization that uses language and terminology your readers may be unfamiliar with.

• If you use an anecdote ensure that is short and highly relevant for your research. It has to function in the same way as an alternative opening, namely to announce the topic of your research paper to your reader.
• For example, if you were writing a sociology paper about re-offending rates among young offenders, you could include a brief story of one person whose story reflects and introduces your topic.
• This kind of approach is generally not appropriate for the introduction to a natural or physical sciences research paper where the writing conventions are different.

Establishing the Context for Your Paper

• It is important to be concise in the introduction, so provide an overview on recent developments in the primary research rather than a lengthy discussion.
• You can follow the "inverted triangle" principle to focus in from the broader themes to those to which you are making a direct contribution with your paper.
• A strong literature review presents important background information to your own research and indicates the importance of the field.

• By making clear reference to existing work you can demonstrate explicitly the specific contribution you are making to move the field forward.
• You can identify a gap in the existing scholarship and explain how you are addressing it and moving understanding forward.

• For example, if you are writing a scientific paper you could stress the merits of the experimental approach or models you have used.
• Stress what is novel in your research and the significance of your new approach, but don't give too much detail in the introduction.
• A stated rationale could be something like: "the study evaluates the previously unknown anti-inflammatory effects of a topical compound in order to evaluate its potential clinical uses".

Specifying Your Research Questions and Hypothesis

• The research question or questions generally come towards the end of the introduction, and should be concise and closely focused.
• The research question might recall some of the key words established in the first few sentences and the title of your paper.
• An example of a research question could be "what were the consequences of the North American Free Trade Agreement on the Mexican export economy?"
• This could be honed further to be specific by referring to a particular element of the Free Trade Agreement and the impact on a particular industry in Mexico, such as clothing manufacture.
• A good research question should shape a problem into a testable hypothesis.

• If possible try to avoid using the word "hypothesis" and rather make this implicit in your writing. This can make your writing appear less formulaic.
• In a scientific paper, giving a clear one-sentence overview of your results and their relation to your hypothesis makes the information clear and accessible. [10] X Trustworthy Source PubMed Central Journal archive from the U.S. National Institutes of Health Go to source
• An example of a hypothesis could be "mice deprived of food for the duration of the study were expected to become more lethargic than those fed normally".

• This is not always necessary and you should pay attention to the writing conventions in your discipline.
• In a natural sciences paper, for example, there is a fairly rigid structure which you will be following.
• A humanities or social science paper will most likely present more opportunities to deviate in how you structure your paper.

Research Introduction Help

Community Q&A

• Use your research papers' outline to help you decide what information to include when writing an introduction. Thanks Helpful 0 Not Helpful 1
• Consider drafting your introduction after you have already completed the rest of your research paper. Writing introductions last can help ensure that you don't leave out any major points. Thanks Helpful 0 Not Helpful 0

• Avoid emotional or sensational introductions; these can create distrust in the reader. Thanks Helpful 50 Not Helpful 12
• Generally avoid using personal pronouns in your introduction, such as "I," "me," "we," "us," "my," "mine," or "our." Thanks Helpful 31 Not Helpful 7
• Don't overwhelm the reader with an over-abundance of information. Keep the introduction as concise as possible by saving specific details for the body of your paper. Thanks Helpful 24 Not Helpful 14

You Might Also Like

• ↑ https://library.sacredheart.edu/c.php?g=29803&p=185916
• ↑ https://www.aresearchguide.com/inverted-pyramid-structure-in-writing.html
• ↑ https://libguides.usc.edu/writingguide/introduction
• ↑ https://writing.wisc.edu/Handbook/PlanResearchPaper.html
• ↑ https://dept.writing.wisc.edu/wac/writing-an-introduction-for-a-scientific-paper/
• ↑ https://writing.wisc.edu/handbook/assignments/planresearchpaper/
• ↑ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3178846/

About This Article

To introduce your research paper, use the first 1-2 sentences to describe your general topic, such as “women in World War I.” Include and define keywords, such as “gender relations,” to show your reader where you’re going. Mention previous research into the topic with a phrase like, “Others have studied…”, then transition into what your contribution will be and why it’s necessary. Finally, state the questions that your paper will address and propose your “answer” to them as your thesis statement. For more information from our English Ph.D. co-author about how to craft a strong hypothesis and thesis, keep reading! Did this summary help you? Yes No

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Organizing Your Social Sciences Research Paper

• 4. The Introduction
• Purpose of Guide
• Design Flaws to Avoid
• Independent and Dependent Variables
• Glossary of Research Terms
• Reading Research Effectively
• Narrowing a Topic Idea
• Broadening a Topic Idea
• Extending the Timeliness of a Topic Idea
• Academic Writing Style
• Applying Critical Thinking
• Choosing a Title
• Making an Outline
• Paragraph Development
• Research Process Video Series
• Executive Summary
• The C.A.R.S. Model
• Background Information
• The Research Problem/Question
• Theoretical Framework
• Citation Tracking
• Content Alert Services
• Evaluating Sources
• Primary Sources
• Secondary Sources
• Tiertiary Sources
• Scholarly vs. Popular Publications
• Qualitative Methods
• Quantitative Methods
• Insiderness
• Using Non-Textual Elements
• Limitations of the Study
• Common Grammar Mistakes
• Writing Concisely
• Avoiding Plagiarism
• Footnotes or Endnotes?
• Further Readings
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The introduction leads the reader from a general subject area to a particular topic of inquiry. It establishes the scope, context, and significance of the research being conducted by summarizing current understanding and background information about the topic, stating the purpose of the work in the form of the research problem supported by a hypothesis or a set of questions, explaining briefly the methodological approach used to examine the research problem, highlighting the potential outcomes your study can reveal, and outlining the remaining structure and organization of the paper.

Key Elements of the Research Proposal. Prepared under the direction of the Superintendent and by the 2010 Curriculum Design and Writing Team. Baltimore County Public Schools.

Importance of a Good Introduction

Think of the introduction as a mental road map that must answer for the reader these four questions:

• What was I studying?
• Why was this topic important to investigate?
• What did we know about this topic before I did this study?
• How will this study advance new knowledge or new ways of understanding?

According to Reyes, there are three overarching goals of a good introduction: 1) ensure that you summarize prior studies about the topic in a manner that lays a foundation for understanding the research problem; 2) explain how your study specifically addresses gaps in the literature, insufficient consideration of the topic, or other deficiency in the literature; and, 3) note the broader theoretical, empirical, and/or policy contributions and implications of your research.

A well-written introduction is important because, quite simply, you never get a second chance to make a good first impression. The opening paragraphs of your paper will provide your readers with their initial impressions about the logic of your argument, your writing style, the overall quality of your research, and, ultimately, the validity of your findings and conclusions. A vague, disorganized, or error-filled introduction will create a negative impression, whereas, a concise, engaging, and well-written introduction will lead your readers to think highly of your analytical skills, your writing style, and your research approach. All introductions should conclude with a brief paragraph that describes the organization of the rest of the paper.

Hirano, Eliana. “Research Article Introductions in English for Specific Purposes: A Comparison between Brazilian, Portuguese, and English.” English for Specific Purposes 28 (October 2009): 240-250; Samraj, B. “Introductions in Research Articles: Variations Across Disciplines.” English for Specific Purposes 21 (2002): 1–17; Introductions. The Writing Center. University of North Carolina; “Writing Introductions.” In Good Essay Writing: A Social Sciences Guide. Peter Redman. 4th edition. (London: Sage, 2011), pp. 63-70; Reyes, Victoria. Demystifying the Journal Article. Inside Higher Education.

Structure and Writing Style

I.  Structure and Approach

The introduction is the broad beginning of the paper that answers three important questions for the reader:

• What is this?
• Why should I read it?
• What do you want me to think about / consider doing / react to?

Think of the structure of the introduction as an inverted triangle of information that lays a foundation for understanding the research problem. Organize the information so as to present the more general aspects of the topic early in the introduction, then narrow your analysis to more specific topical information that provides context, finally arriving at your research problem and the rationale for studying it [often written as a series of key questions to be addressed or framed as a hypothesis or set of assumptions to be tested] and, whenever possible, a description of the potential outcomes your study can reveal.

These are general phases associated with writing an introduction: 1.  Establish an area to research by:

• Highlighting the importance of the topic, and/or
• Making general statements about the topic, and/or
• Presenting an overview on current research on the subject.

2.  Identify a research niche by:

• Opposing an existing assumption, and/or
• Revealing a gap in existing research, and/or
• Formulating a research question or problem, and/or
• Continuing a disciplinary tradition.

3.  Place your research within the research niche by:

• Stating the intent of your study,
• Outlining the key characteristics of your study,
• Describing important results, and
• Giving a brief overview of the structure of the paper.

NOTE:   It is often useful to review the introduction late in the writing process. This is appropriate because outcomes are unknown until you've completed the study. After you complete writing the body of the paper, go back and review introductory descriptions of the structure of the paper, the method of data gathering, the reporting and analysis of results, and the conclusion. Reviewing and, if necessary, rewriting the introduction ensures that it correctly matches the overall structure of your final paper.

II.  Delimitations of the Study

Delimitations refer to those characteristics that limit the scope and define the conceptual boundaries of your research . This is determined by the conscious exclusionary and inclusionary decisions you make about how to investigate the research problem. In other words, not only should you tell the reader what it is you are studying and why, but you must also acknowledge why you rejected alternative approaches that could have been used to examine the topic.

Obviously, the first limiting step was the choice of research problem itself. However, implicit are other, related problems that could have been chosen but were rejected. These should be noted in the conclusion of your introduction. For example, a delimitating statement could read, "Although many factors can be understood to impact the likelihood young people will vote, this study will focus on socioeconomic factors related to the need to work full-time while in school." The point is not to document every possible delimiting factor, but to highlight why previously researched issues related to the topic were not addressed.

Examples of delimitating choices would be:

• The key aims and objectives of your study,
• The research questions that you address,
• The variables of interest [i.e., the various factors and features of the phenomenon being studied],
• The method(s) of investigation,
• The time period your study covers, and
• Any relevant alternative theoretical frameworks that could have been adopted.

Review each of these decisions. Not only do you clearly establish what you intend to accomplish in your research, but you should also include a declaration of what the study does not intend to cover. In the latter case, your exclusionary decisions should be based upon criteria understood as, "not interesting"; "not directly relevant"; “too problematic because..."; "not feasible," and the like. Make this reasoning explicit!

NOTE:   Delimitations refer to the initial choices made about the broader, overall design of your study and should not be confused with documenting the limitations of your study discovered after the research has been completed.

ANOTHER NOTE : Do not view delimitating statements as admitting to an inherent failing or shortcoming in your research. They are an accepted element of academic writing intended to keep the reader focused on the research problem by explicitly defining the conceptual boundaries and scope of your study. It addresses any critical questions in the reader's mind of, "Why the hell didn't the author examine this?"

III.  The Narrative Flow

Issues to keep in mind that will help the narrative flow in your introduction :

• Your introduction should clearly identify the subject area of interest . A simple strategy to follow is to use key words from your title in the first few sentences of the introduction. This will help focus the introduction on the topic at the appropriate level and ensures that you get to the subject matter quickly without losing focus, or discussing information that is too general.
• Establish context by providing a brief and balanced review of the pertinent published literature that is available on the subject. The key is to summarize for the reader what is known about the specific research problem before you did your analysis. This part of your introduction should not represent a comprehensive literature review--that comes next. It consists of a general review of the important, foundational research literature [with citations] that establishes a foundation for understanding key elements of the research problem. See the drop-down menu under this tab for " Background Information " regarding types of contexts.
• Clearly state the hypothesis that you investigated . When you are first learning to write in this format it is okay, and actually preferable, to use a past statement like, "The purpose of this study was to...." or "We investigated three possible mechanisms to explain the...."
• Why did you choose this kind of research study or design? Provide a clear statement of the rationale for your approach to the problem studied. This will usually follow your statement of purpose in the last paragraph of the introduction.

IV.  Engaging the Reader

A research problem in the social sciences can come across as dry and uninteresting to anyone unfamiliar with the topic . Therefore, one of the goals of your introduction is to make readers want to read your paper. Here are several strategies you can use to grab the reader's attention:

• Open with a compelling story . Almost all research problems in the social sciences, no matter how obscure or esoteric , are really about the lives of people. Telling a story that humanizes an issue can help illuminate the significance of the problem and help the reader empathize with those affected by the condition being studied.
• Include a strong quotation or a vivid, perhaps unexpected, anecdote . During your review of the literature, make note of any quotes or anecdotes that grab your attention because they can used in your introduction to highlight the research problem in a captivating way.
• Pose a provocative or thought-provoking question . Your research problem should be framed by a set of questions to be addressed or hypotheses to be tested. However, a provocative question can be presented in the beginning of your introduction that challenges an existing assumption or compels the reader to consider an alternative viewpoint that helps establish the significance of your study. 
• Describe a puzzling scenario or incongruity . This involves highlighting an interesting quandary concerning the research problem or describing contradictory findings from prior studies about a topic. Posing what is essentially an unresolved intellectual riddle about the problem can engage the reader's interest in the study.
• Cite a stirring example or case study that illustrates why the research problem is important . Draw upon the findings of others to demonstrate the significance of the problem and to describe how your study builds upon or offers alternatives ways of investigating this prior research.

NOTE:   It is important that you choose only one of the suggested strategies for engaging your readers. This avoids giving an impression that your paper is more flash than substance and does not distract from the substance of your study.

Freedman, Leora  and Jerry Plotnick. Introductions and Conclusions. University College Writing Centre. University of Toronto; Introduction. The Structure, Format, Content, and Style of a Journal-Style Scientific Paper. Department of Biology. Bates College; Introductions. The Writing Center. University of North Carolina; Introductions. The Writer’s Handbook. Writing Center. University of Wisconsin, Madison; Introductions, Body Paragraphs, and Conclusions for an Argument Paper. The Writing Lab and The OWL. Purdue University; “Writing Introductions.” In Good Essay Writing: A Social Sciences Guide . Peter Redman. 4th edition. (London: Sage, 2011), pp. 63-70; Resources for Writers: Introduction Strategies. Program in Writing and Humanistic Studies. Massachusetts Institute of Technology; Sharpling, Gerald. Writing an Introduction. Centre for Applied Linguistics, University of Warwick; Samraj, B. “Introductions in Research Articles: Variations Across Disciplines.” English for Specific Purposes 21 (2002): 1–17; Swales, John and Christine B. Feak. Academic Writing for Graduate Students: Essential Skills and Tasks . 2nd edition. Ann Arbor, MI: University of Michigan Press, 2004 ; Writing Your Introduction. Department of English Writing Guide. George Mason University.

Writing Tip

Avoid the "Dictionary" Introduction

Giving the dictionary definition of words related to the research problem may appear appropriate because it is important to define specific terminology that readers may be unfamiliar with. However, anyone can look a word up in the dictionary and a general dictionary is not a particularly authoritative source because it doesn't take into account the context of your topic and doesn't offer particularly detailed information. Also, placed in the context of a particular discipline, a term or concept may have a different meaning than what is found in a general dictionary. If you feel that you must seek out an authoritative definition, use a subject specific dictionary or encyclopedia [e.g., if you are a sociology student, search for dictionaries of sociology]. A good database for obtaining definitive definitions of concepts or terms is Credo Reference .

Saba, Robert. The College Research Paper. Florida International University; Introductions. The Writing Center. University of North Carolina.

Another Writing Tip

When Do I Begin?

A common question asked at the start of any paper is, "Where should I begin?" An equally important question to ask yourself is, "When do I begin?" Research problems in the social sciences rarely rest in isolation from history. Therefore, it is important to lay a foundation for understanding the historical context underpinning the research problem. However, this information should be brief and succinct and begin at a point in time that illustrates the study's overall importance. For example, a study that investigates coffee cultivation and export in West Africa as a key stimulus for local economic growth needs to describe the beginning of exporting coffee in the region and establishing why economic growth is important. You do not need to give a long historical explanation about coffee exports in Africa. If a research problem requires a substantial exploration of the historical context, do this in the literature review section. In your introduction, make note of this as part of the "roadmap" [see below] that you use to describe the organization of your paper.

Introductions. The Writing Center. University of North Carolina; “Writing Introductions.” In Good Essay Writing: A Social Sciences Guide . Peter Redman. 4th edition. (London: Sage, 2011), pp. 63-70.

Yet Another Writing Tip

Always End with a Roadmap

The final paragraph or sentences of your introduction should forecast your main arguments and conclusions and provide a brief description of the rest of the paper [the "roadmap"] that let's the reader know where you are going and what to expect. A roadmap is important because it helps the reader place the research problem within the context of their own perspectives about the topic. In addition, concluding your introduction with an explicit roadmap tells the reader that you have a clear understanding of the structural purpose of your paper. In this way, the roadmap acts as a type of promise to yourself and to your readers that you will follow a consistent and coherent approach to addressing the topic of inquiry. Refer to it often to help keep your writing focused and organized.

Cassuto, Leonard. “On the Dissertation: How to Write the Introduction.” The Chronicle of Higher Education , May 28, 2018; Radich, Michael. A Student's Guide to Writing in East Asian Studies . (Cambridge, MA: Harvard University Writing n. d.), pp. 35-37.

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• Dissertation

How to Write a Thesis or Dissertation Introduction

Published on 9 September 2022 by Tegan George and Shona McCombes.

The introduction is the first section of your thesis or dissertation , appearing right after the table of contents . Your introduction draws your reader in, setting the stage for your research with a clear focus, purpose, and direction.

Your introduction should include:

• Your topic, in context: what does your reader need to know to understand your thesis dissertation?
• Your focus and scope: what specific aspect of the topic will you address?
• The relevance of your research: how does your work fit into existing studies on your topic?
• Your questions and objectives: what does your research aim to find out, and how?
• An overview of your structure: what does each section contribute to the overall aim?

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Table of contents

How to start your introduction, topic and context, focus and scope, relevance and importance, questions and objectives, overview of the structure, thesis introduction example, introduction checklist, frequently asked questions about introductions.

Although your introduction kicks off your dissertation, it doesn’t have to be the first thing you write – in fact, it’s often one of the very last parts to be completed (just before your abstract ).

It’s a good idea to write a rough draft of your introduction as you begin your research, to help guide you. If you wrote a research proposal , consider using this as a template, as it contains many of the same elements. However, be sure to revise your introduction throughout the writing process, making sure it matches the content of your ensuing sections.

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Begin by introducing your research topic and giving any necessary background information. It’s important to contextualise your research and generate interest. Aim to show why your topic is timely or important. You may want to mention a relevant news item, academic debate, or practical problem.

After a brief introduction to your general area of interest, narrow your focus and define the scope of your research.

You can narrow this down in many ways, such as by:

• Geographical area
• Time period
• Demographics or communities
• Themes or aspects of the topic

It’s essential to share your motivation for doing this research, as well as how it relates to existing work on your topic. Further, you should also mention what new insights you expect it will contribute.

Start by giving a brief overview of the current state of research. You should definitely cite the most relevant literature, but remember that you will conduct a more in-depth survey of relevant sources in the literature review section, so there’s no need to go too in-depth in the introduction.

Depending on your field, the importance of your research might focus on its practical application (e.g., in policy or management) or on advancing scholarly understanding of the topic (e.g., by developing theories or adding new empirical data). In many cases, it will do both.

Ultimately, your introduction should explain how your thesis or dissertation:

• Helps solve a practical or theoretical problem
• Addresses a gap in the literature
• Builds on existing research
• Proposes a new understanding of your topic

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Perhaps the most important part of your introduction is your questions and objectives, as it sets up the expectations for the rest of your thesis or dissertation. How you formulate your research questions and research objectives will depend on your discipline, topic, and focus, but you should always clearly state the central aim of your research.

If your research aims to test hypotheses , you can formulate them here. Your introduction is also a good place for a conceptual framework that suggests relationships between variables .

• Conduct surveys to collect data on students’ levels of knowledge, understanding, and positive/negative perceptions of government policy.
• Determine whether attitudes to climate policy are associated with variables such as age, gender, region, and social class.
• Conduct interviews to gain qualitative insights into students’ perspectives and actions in relation to climate policy.

To help guide your reader, end your introduction with an outline  of the structure of the thesis or dissertation to follow. Share a brief summary of each chapter, clearly showing how each contributes to your central aims. However, be careful to keep this overview concise: 1-2 sentences should be enough.

I. Introduction

Human language consists of a set of vowels and consonants which are combined to form words. During the speech production process, thoughts are converted into spoken utterances to convey a message. The appropriate words and their meanings are selected in the mental lexicon (Dell & Burger, 1997). This pre-verbal message is then grammatically coded, during which a syntactic representation of the utterance is built.

Speech, language, and voice disorders affect the vocal cords, nerves, muscles, and brain structures, which result in a distorted language reception or speech production (Sataloff & Hawkshaw, 2014). The symptoms vary from adding superfluous words and taking pauses to hoarseness of the voice, depending on the type of disorder (Dodd, 2005). However, distortions of the speech may also occur as a result of a disease that seems unrelated to speech, such as multiple sclerosis or chronic obstructive pulmonary disease.

This study aims to determine which acoustic parameters are suitable for the automatic detection of exacerbations in patients suffering from chronic obstructive pulmonary disease (COPD) by investigating which aspects of speech differ between COPD patients and healthy speakers and which aspects differ between COPD patients in exacerbation and stable COPD patients.

Checklist: Introduction

I have introduced my research topic in an engaging way.

I have provided necessary context to help the reader understand my topic.

I have clearly specified the focus of my research.

I have shown the relevance and importance of the dissertation topic .

I have clearly stated the problem or question that my research addresses.

I have outlined the specific objectives of the research .

I have provided an overview of the dissertation’s structure .

You've written a strong introduction for your thesis or dissertation. Use the other checklists to continue improving your dissertation.

The introduction of a research paper includes several key elements:

• A hook to catch the reader’s interest
• Relevant background on the topic
• Details of your research problem
• A thesis statement or research question
• Sometimes an outline of the paper

Don’t feel that you have to write the introduction first. The introduction is often one of the last parts of the research paper you’ll write, along with the conclusion.

This is because it can be easier to introduce your paper once you’ve already written the body ; you may not have the clearest idea of your arguments until you’ve written them, and things can change during the writing process .

Research objectives describe what you intend your research project to accomplish.

They summarise the approach and purpose of the project and help to focus your research.

Your objectives should appear in the introduction of your research paper , at the end of your problem statement .

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How to Write an Introduction For a Research Paper

Learn how to write a strong and efficient research paper introduction by following the suitable structure and avoiding typical errors.

An introduction to any type of paper is sometimes misunderstood as the beginning; yet, an introduction is actually intended to present your chosen subject to the audience in a way that makes it more appealing and leaves your readers thirsty for more information. After the title and abstract, your audience will read the introduction, thus it’s critical to get off to a solid start.  

This article includes instructions on how to write an introduction for a research paper that engages the reader in your research. You can produce a strong opening for your research paper if you stick to the format and a few basic principles.

What is An Introduction To a Research Paper?

An introduction is the opening section of a research paper and the section that a reader is likely to read first, in which the objective and goals of the subsequent writing are stated. 

The introduction serves numerous purposes. It provides context for your research, explains your topic and objectives, and provides an outline of the work. A solid introduction will establish the tone for the remainder of your paper, enticing readers to continue reading through the methodology, findings, and discussion. 

Even though introductions are generally presented at the beginning of a document, we must distinguish an introduction from the beginning of your research. An introduction, as the name implies, is supposed to introduce your subject without extending it. All relevant information and facts should be placed in the body and conclusion, not the introduction.

Structure Of An Introduction

Before explaining how to write an introduction for a research paper , it’s necessary to comprehend a structure that will make your introduction stronger and more straightforward.

A Good Hook

A hook is one of the most effective research introduction openers. A hook’s objective is to stimulate the reader’s interest to read the research paper.  There are various approaches you may take to generate a strong hook:  startling facts, a question, a brief overview, or even a quotation. 

Broad Overview

Following an excellent hook, you should present a wide overview of your major issue and some background information on your research. If you’re unsure about how to begin an essay introduction, the best approach is to offer a basic explanation of your topic before delving into specific issues. Simply said, you should begin with general information and then narrow it down to your relevant topics.

After offering some background information regarding your research’s main topic, go on to give readers a better understanding of what you’ll be covering throughout your research. In this section of your introduction, you should swiftly clarify your important topics in the sequence in which they will be addressed later, gradually introducing your thesis statement. You can use some  The following are some critical questions to address in this section of your introduction: Who? What? Where? When? How? And why is that?

Thesis Statement

The thesis statement, which must be stated in the beginning clause of your research since your entire research revolves around it, is the most important component of your research.

A thesis statement presents your audience with a quick overview of the research’s main assertion. In the body section of your work, your key argument is what you will expose or debate about it. An excellent thesis statement is usually very succinct, accurate, explicit, clear, and focused. Typically, your thesis should be at the conclusion of your introductory paragraph/section.

Tips for Writing a Strong Introduction

Aside from the good structure, here are a few tips to make your introduction strong and accurate:

• Keep in mind the aim of your research and make sure your introduction supports it.
• Use an appealing and relevant hook that catches the reader’s attention right away.
• Make it obvious to your readers what your stance is.
• Demonstrate your knowledge of your subject.
• Provide your readers with a road map to help them understand what you will address throughout the research.
• Be succinct – it is advised that your opening introduction consists of around 8-9 percent of the overall amount of words in your article (for example, 160 words for a 2000 words essay). 
• Make a strong and unambiguous thesis statement.
• Explain why the article is significant in 1-2 sentences.
• Remember to keep it interesting.

Mistakes to Avoid in Your Introduction

Check out what not to do and what to avoid now that you know the structure and how to write an introduction for a research paper .

• Lacking a feeling of direction or purpose.
• Giving out too much.
• Creating lengthy paragraphs.
• Excessive or insufficient background, literature, and theory.
• Including material that should be placed in the body and conclusion.
• Not writing enough or writing excessively.
• Using too many quotes.

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Research Paper Introduction Examples

Looking for research paper introduction examples? Quotes, anecdotes, questions, examples, and broad statements—all of them can be used successfully to write an introduction for a research paper. It’s instructive to see them in action, in the hands of skilled academic writers.

Let’s begin with David M. Kennedy’s superb history, Freedom from Fear: The American People in Depression and War, 1929–1945 . Kennedy begins each chapter with a quote, followed by his text. The quote above chapter 1 shows President Hoover speaking in 1928 about America’s golden future. The text below it begins with the stock market collapse of 1929. It is a riveting account of just how wrong Hoover was. The text about the Depression is stronger because it contrasts so starkly with the optimistic quotation.

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“We in America today are nearer the final triumph over poverty than ever before in the history of any land.”—Herbert Hoover, August 11, 1928 Like an earthquake, the stock market crash of October 1929 cracked startlingly across the United States, the herald of a crisis that was to shake the American way of life to its foundations. The events of the ensuing decade opened a fissure across the landscape of American history no less gaping than that opened by the volley on Lexington Common in April 1775 or by the bombardment of Sumter on another April four score and six years later. (adsbygoogle = window.adsbygoogle || []).push({}); The ratcheting ticker machines in the autumn of 1929 did not merely record avalanching stock prices. In time they came also to symbolize the end of an era. (David M. Kennedy, Freedom from Fear: The American People in Depression and War, 1929–1945 . New York: Oxford University Press, 1999, p. 10)

Kennedy has exciting, wrenching material to work with. John Mueller faces the exact opposite problem. In Retreat from Doomsday: The Obsolescence of Major War , he is trying to explain why Great Powers have suddenly stopped fighting each other. For centuries they made war on each other with devastating regularity, killing millions in the process. But now, Mueller thinks, they have not just paused; they have stopped permanently. He is literally trying to explain why “nothing is happening now.” That may be an exciting topic intellectually, it may have great practical significance, but “nothing happened” is not a very promising subject for an exciting opening paragraph. Mueller manages to make it exciting and, at the same time, shows why it matters so much. Here’s his opening, aptly entitled “History’s Greatest Nonevent”:

On May 15, 1984, the major countries of the developed world had managed to remain at peace with each other for the longest continuous stretch of time since the days of the Roman Empire. If a significant battle in a war had been fought on that day, the press would have bristled with it. As usual, however, a landmark crossing in the history of peace caused no stir: the most prominent story in the New York Times that day concerned the saga of a manicurist, a machinist, and a cleaning woman who had just won a big Lotto contest. This book seeks to develop an explanation for what is probably the greatest nonevent in human history. (John Mueller, Retreat from Doomsday: The Obsolescence of Major War . New York: Basic Books, 1989, p. 3)

In the space of a few sentences, Mueller sets up his puzzle and reveals its profound human significance. At the same time, he shows just how easy it is to miss this milestone in the buzz of daily events. Notice how concretely he does that. He doesn’t just say that the New York Times ignored this record setting peace. He offers telling details about what they covered instead: “a manicurist, a machinist, and a cleaning woman who had just won a big Lotto contest.” Likewise, David Kennedy immediately entangles us in concrete events: the stunning stock market crash of 1929. These are powerful openings that capture readers’ interests, establish puzzles, and launch narratives.

Sociologist James Coleman begins in a completely different way, by posing the basic questions he will study. His ambitious book, Foundations of Social Theory , develops a comprehensive theory of social life, so it is entirely appropriate for him to begin with some major questions. But he could just as easily have begun with a compelling story or anecdote. He includes many of them elsewhere in his book. His choice for the opening, though, is to state his major themes plainly and frame them as a paradox. Sociologists, he says, are interested in aggregate behavior—how people act in groups, organizations, or large numbers—yet they mostly examine individuals:

A central problem in social science is that of accounting for the function of some kind of social system. Yet in most social research, observations are not made on the system as a whole, but on some part of it. In fact, the natural unit of observation is the individual person…  This has led to a widening gap between theory and research… (James S. Coleman, Foundations of Social Theory . Cambridge, MA: Harvard University Press, 1990, pp. 1–2)

After expanding on this point, Coleman explains that he will not try to remedy the problem by looking solely at groups or aggregate-level data. That’s a false solution, he says, because aggregates don’t act; individuals do. So the real problem is to show the links between individual actions and aggregate outcomes, between the micro and the macro.

The major problem for explanations of system behavior based on actions and orientations at a level below that of the system [in this case, on individual-level actions] is that of moving from the lower level to the system level. This has been called the micro-to-macro problem, and it is pervasive throughout the social sciences. (Coleman, Foundations of Social Theory , p. 6)

Explaining how to deal with this “micro-to-macro problem” is the central issue of Coleman’s book, and he announces it at the beginning.

Coleman’s theory-driven opening stands at the opposite end of the spectrum from engaging stories or anecdotes, which are designed to lure the reader into the narrative and ease the path to a more analytic treatment later in the text. Take, for example, the opening sentences of Robert L. Herbert’s sweeping study Impressionism: Art, Leisure, and Parisian Society : “When Henry Tuckerman came to Paris in 1867, one of the thousands of Americans attracted there by the huge international exposition, he was bowled over by the extraordinary changes since his previous visit twenty years before.” (Robert L. Herbert, Impressionism: Art, Leisure, and Parisian Society . New Haven, CT: Yale University Press, 1988, p. 1.) Herbert fills in the evocative details to set the stage for his analysis of the emerging Impressionist art movement and its connection to Parisian society and leisure in this period.

David Bromwich writes about Wordsworth, a poet so familiar to students of English literature that it is hard to see him afresh, before his great achievements, when he was just a young outsider starting to write. To draw us into Wordsworth’s early work, Bromwich wants us to set aside our entrenched images of the famous mature poet and see him as he was in the 1790s, as a beginning writer on the margins of society. He accomplishes this ambitious task in the opening sentences of Disowned by Memory: Wordsworth’s Poetry of the 1790s :

Wordsworth turned to poetry after the revolution to remind himself that he was still a human being. It was a curious solution, to a difficulty many would not have felt. The whole interest of his predicament is that he did feel it. Yet Wordsworth is now so established an eminence—his name so firmly fixed with readers as a moralist of self-trust emanating from complete self-security—that it may seem perverse to imagine him as a criminal seeking expiation. Still, that is a picture we get from The Borderers and, at a longer distance, from “Tintern Abbey.” (David Bromwich, Disowned by Memory: Wordsworth’s Poetry of the 1790s . Chicago: University of Chicago Press, 1998, p. 1)

That’s a wonderful opening! Look at how much Bromwich accomplishes in just a few words. He not only prepares the way for analyzing Wordsworth’s early poetry; he juxtaposes the anguished young man who wrote it to the self-confident, distinguished figure he became—the eminent man we can’t help remembering as we read his early poetry.

Let us highlight a couple of other points in this passage because they illustrate some intelligent writing choices. First, look at the odd comma in this sentence: “It was a curious solution, to a difficulty many would not have felt.” Any standard grammar book would say that comma is wrong and should be omitted. Why did Bromwich insert it? Because he’s a fine writer, thinking of his sentence rhythm and the point he wants to make. The comma does exactly what it should. It makes us pause, breaking the sentence into two parts, each with an interesting point. One is that Wordsworth felt a difficulty others would not have; the other is that he solved it in a distinctive way. It would be easy for readers to glide over this double message, so Bromwich has inserted a speed bump to slow us down. Most of the time, you should follow grammatical rules, like those about commas, but you should bend them when it serves a good purpose. That’s what the writer does here.

The second small point is the phrase “after the revolution” in the first sentence: “Wordsworth turned to poetry after the revolution to remind himself that he was still a human being.” Why doesn’t Bromwich say “after the French Revolution”? Because he has judged his book’s audience. He is writing for specialists who already know which revolution is reverberating through English life in the 1790s. It is the French Revolution, not the earlier loss of the American colonies. If Bromwich were writing for a much broader audience—say, the New York Times Book Review—he would probably insert the extra word to avoid confusion.

The message “Know your audience” applies to all writers. Don’t talk down to them by assuming they can’t get dressed in the morning. Don’t strut around showing off your book learnin’ by tossing in arcane facts and esoteric language for its own sake. Neither will win over readers.

Bromwich, Herbert, and Coleman open their works in different ways, but their choices work well for their different texts. Your task is to decide what kind of opening will work best for yours. Don’t let that happen by default, by grabbing the first idea you happen upon. Consider a couple of different ways of opening your thesis and then choose the one you prefer. Give yourself some options, think them over, then make an informed choice.

Using the Introduction to Map out Your Writing

Whether you begin with a story, puzzle, or broad statement, the next part of the research paper introduction should pose your main questions and establish your argument. This is your thesis statement—your viewpoint along with the supporting reasons and evidence. It should be articulated plainly so readers understand full well what your paper is about and what it will argue.

After that, give your readers a road map of what’s to come. That’s normally done at the end of the introductory section (or, in a book, at the end of the introductory chapter). Here’s John J. Mearsheimer presenting such a road map in The Tragedy of Great Power Politics . He not only tells us the order of upcoming chapters, he explains why he’s chosen that order and which chapters are most important:

The Plan of the Book The rest of the chapters in this book are concerned mainly with answering the six big questions about power which I identified earlier. Chapter 2, which is probably the most important chapter in the book, lays out my theory of why states compete for power and why they pursue hegemony. In Chapters 3 and 4, I define power and explain how to measure it. I do this in order to lay the groundwork for testing my theory… (John J. Mearsheimer, The Tragedy of Great Power Politics . New York: W. W. Norton, 2001, p. 27)

As this excerpt makes clear, Mearsheimer has already laid out his “six big questions” in the research paper introduction. Now he’s showing us the path ahead, the path to answering those questions.

At the end of the research paper introduction, give your readers a road map of what’s to come. Tell them what the upcoming sections will be and why they are arranged in this particular order.

Learn how to write an introduction for a research paper .

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What’s Included: Research Paper Template

If you’re preparing to write an academic research paper, our free research paper template is the perfect starting point. In the template, we cover every section step by step, with clear, straightforward explanations and examples .

The template’s structure is based on the tried and trusted best-practice format for formal academic research papers. The template structure reflects the overall research process, ensuring your paper will have a smooth, logical flow from chapter to chapter.

The research paper template covers the following core sections:

• The title page/cover page
• Abstract (sometimes also called the executive summary)
• Section 1: Introduction 
• Section 2: Literature review 
• Section 3: Methodology
• Section 4: Findings /results
• Section 5: Discussion
• Section 6: Conclusion
• Reference list

Each section is explained in plain, straightforward language , followed by an overview of the key elements that you need to cover within each section. We’ve also included links to free resources to help you understand how to write each section.

The cleanly formatted Google Doc can be downloaded as a fully editable MS Word Document (DOCX format), so you can use it as-is or convert it to LaTeX.

FAQs: Research Paper Template

What format is the template (doc, pdf, ppt, etc.).

The research paper template is provided as a Google Doc. You can download it in MS Word format or make a copy to your Google Drive. You’re also welcome to convert it to whatever format works best for you, such as LaTeX or PDF.

What types of research papers can this template be used for?

The template follows the standard best-practice structure for formal academic research papers, so it is suitable for the vast majority of degrees, particularly those within the sciences.

Some universities may have some additional requirements, but these are typically minor, with the core structure remaining the same. Therefore, it’s always a good idea to double-check your university’s requirements before you finalise your structure.

Is this template for an undergrad, Masters or PhD-level research paper?

This template can be used for a research paper at any level of study. It may be slight overkill for an undergraduate-level study, but it certainly won’t be missing anything.

How long should my research paper be?

This depends entirely on your university’s specific requirements, so it’s best to check with them. We include generic word count ranges for each section within the template, but these are purely indicative. 

What about the research proposal?

If you’re still working on your research proposal, we’ve got a template for that here .

We’ve also got loads of proposal-related guides and videos over on the Grad Coach blog .

How do I write a literature review?

We have a wealth of free resources on the Grad Coach Blog that unpack how to write a literature review from scratch. You can check out the literature review section of the blog here.

How do I create a research methodology?

We have a wealth of free resources on the Grad Coach Blog that unpack research methodology, both qualitative and quantitative. You can check out the methodology section of the blog here.

Can I share this research paper template with my friends/colleagues?

Yes, you’re welcome to share this template. If you want to post about it on your blog or social media, all we ask is that you reference this page as your source.

Can Grad Coach help me with my research paper?

Within the template, you’ll find plain-language explanations of each section, which should give you a fair amount of guidance. However, you’re also welcome to consider our private coaching services .

Research involvement of medical students in a medical school of India: exploring knowledge, attitude, practices, and perceived barriers

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Introduction Research in the medical discipline significantly impacts society by improving the general well-being of the population, through improvements in diagnostic and treatment modalities. However, of 579 Indian medical colleges, 332 (57.3%) did not publish a single paper from the year 2005 to 2014," indicating a limited contribution from medical fraternity In order to probe in to the cause of this a study was conducted to assess the knowledge, attitude, practices (KAP) and perceived barriers to research among students of a medical school in Delhi, India.

Methods A cross-sectional study was conducted among medical students and the data on academic-cum-demographic information, assessment of knowledge, attitude, practices and barriers to research was collected using a pre-tested, semi-structured questionnaire. Chi-square test was used to check the association of various factors with the KAP of research. A p-value less than 0.05 was considered significant.

Results A total of 402 (N) subjects were enrolled in the study. Majority were male (79.6%) and from clinical professional years (57%). Majority (266, 66.2%) of the subjects had adequate knowledge. Of the study subjects (61,15%) having inadequate knowledge of research, sixty percent were from pre- and para-clinical years, while around 70 % of those having good knowledge were from clinical professional years. However, only 16.9% of the participants had participated in a research project, and only 4.72% had authored a publication. Sixty one percent of study subjects having a positive attitude towards research, were from pre- and para-clinical years. Among the study subjects having a positive attitude towards research, over 60% were from pre- and para-clinical years. The barriers for conducting research were mostly; lack of funds/laboratory equipment/infrastructure (85.1%), lack of exposure to opportunities for research in the medical (MBBS) curriculum (83.8%), and lack of time (83.3%). There was a statistically significant association between knowledge and attitude towards research with a professional year of study.

Conclusions The study revealed that while most of the students had a positive attitude towards research as well as an adequate knowledge of research, there was a poor level of participation in research. These challenges can be overcome by incorporating research as a part of the medical school curriculum from early years on, setting aside separate time for research, and establishing student research societies that can actively promote research.

Competing Interest Statement

The authors have declared no competing interest.

Funding Statement

This study did not receive any funding.

Author Declarations

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The ethics committee of Dr Baba Saheb Ambedkar Medical College and Hospital, New Delhi gave ethical approval for this work.

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Email id: jhaabhinav677{at}gmail.com , manas.shah1999{at}gmail.com , Ritikgoyal152{at}gmail.com , drdeepakdhamnetiya{at}gmail.com , apoorv1729{at}gmail.com , raviprakashjha{at}gmail.com , dr.prachi.obg{at}bsamch.in

Data Availability

All data produced in the present study are available upon reasonable request to the authors.

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1. introduction, 2. literature review, 3. objectives, 6. discussion, 7. conclusions, author contributions, competing interests, funding information, data availability, completeness degree of publication metadata in eight free-access scholarly databases.

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Lorena Delgado-Quirós , José Luis Ortega; Completeness degree of publication metadata in eight free-access scholarly databases. Quantitative Science Studies 2024; 5 (1): 31–49. doi: https://doi.org/10.1162/qss_a_00286

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The main objective of this study is to compare the amount of metadata and the completeness degree of research publications in new academic databases. Using a quantitative approach, we selected a random Crossref sample of more than 115,000 records, which was then searched in seven databases (Dimensions, Google Scholar, Microsoft Academic, OpenAlex, Scilit, Semantic Scholar, and The Lens). Seven characteristics were analyzed (abstract, access, bibliographic info, document type, publication date, language, and identifiers), to observe fields that describe this information, the completeness rate of these fields, and the agreement among databases. The results show that academic search engines (Google Scholar, Microsoft Academic, and Semantic Scholar) gather less information and have a low degree of completeness. Conversely, third-party databases (Dimensions, OpenAlex, Scilit, and The Lens) have more metadata quality and a higher completeness rate. We conclude that academic search engines lack the ability to retrieve reliable descriptive data by crawling the web, and the main problem of third-party databases is the loss of information derived from integrating different sources.

https://www.webofscience.com/api/gateway/wos/peer-review/10.1162/qss_a_00286

The recent proliferation of bibliographic scholarly databases has stimulated interest in these new platforms, given their possibilities of finding scientific literature and providing different bibliometric indicators. This attention has focused on testing the performance of these new systems in relation to traditional products, such as citation indexes (e.g., Web of Science (WoS), Scopus) and academic search engines (e.g., Google Scholar, Microsoft Academic). These new products could be defined as hybrid databases because they share characteristics with the former. On the one hand, these platforms also extract and process citations for computing ad hoc bibliometric indicators as classical citation indexes. On the other hand, they are similar to search engines because they opt for a free-access model in which users require no subscription fee to search and retrieve documents. Moreover, in some cases, they provide open data through REST APIs or dump files.

However, these hybrid products have some particularities that make them different. Most importantly, they are fed by third-party sources. The appearance of Crossref as a repository of publishers’ metadata, the availability of APIs and dump files from academic search engines (e.g., Microsoft Academic, Semantic Scholar), and the possibility of reusing other bibliographic databases (e.g., PubMed, Directory of Open Access Journals (DOAJ), repositories) have facilitated the emergence of these bibliographic products that have quickly and inexpensively covered much of the scientific literature.

However, this multiple and varied availability of bibliographic data also presents a challenge for these new platforms because the integration of data from different sources requires intense data processing to prevent duplicate records, filter nonscholarly materials, and manage different versions of the same document. All of these actions are addressed to achieve optimal metadata quality, which could be defined as the quantity of information that describes an entity in an accurate and reliable form. The integration of internal and external descriptions determines this quality to a large extent.

For this reason, examining the quality of publication metadata in the new scholarly databases allows us to appreciate the extent to which these processing efforts are accomplished and to value the suitability and reliability of these search tools to provide rich information about scientific literature. This study aims to explore the metadata publication quality of these new databases to obtain a global picture about the richness of the information provided by each platform.

Many studies have focused on the evaluation of the performance of these new academic databases, comparing the coverage and overlap of records ( Gusenbauer, 2019 ; Martín-Martín, Thelwall et al., 2021 ; van Eck, Waltman et al., 2018 ; Visser, van Eck, & Waltman, 2021 ). This quantitative procedure is excessively centered on the size of each platform and overlooks the amount and quality of the content included in each database. In this sense, some articles have described the metadata quality of specific sources as a way of informing about the richness and limitations of those sources. Hendricks, Tkaczyk et al. (2020) described how the Crossref database functions and analyzed the completeness of its metadata. Similar papers were published describing Semantic Scholar ( Wade, 2022 ), The Lens ( Jefferson, Koellhofer et al., 2019 ), Dimensions ( Herzog, Hook, & Konkiel, 2020 ), and Microsoft Academic ( Wang, Shen et al., 2020 ). Many of these studies were descriptive reviews written by their employees with no critical discussion of the quality of the data.

In other cases, the coverage of certain elements or entities in different scholarly databases was studied to test their performance in processing specific information. Hug and Brändle (2017) analyzed in detail the coverage of Microsoft Academic, finding important problems in the assigning of author and publication data in comparison to WoS and Scopus. Ranjbar-Sahraei and van Eck (2018) also tested the problems of Microsoft linking papers with organizations. Guerrero-Bote, Chinchilla-Rodríguez et al. (2021) compared affiliation information between Scopus and Dimensions and found that close to half of all documents in Dimensions were not associated with any country. Purnell (2022) evaluated affiliation discrepancies in four scholarly databases, finding that the larger the database, the greater the disambiguation problems. Kramer and de Jonge (2022) analyzed the information about funders included by Crossref, The Lens, WoS, Scopus, and Dimensions, uncovering important differences when they came to extract and process that information. Lutai and Lyubushko (2022) also analyzed the coverage in six databases, detecting discrepancies and similarities in the identification and indexation of Russian authors.

As for publications, some studies have explored the amount of information and quality of this key entity in scholarly databases. Herrmannova and Knoth (2016) tested the reliability of the publication date in Microsoft Academic Graph, finding that 88% of cases showed a correct date. Liu, Hu, and Tang (2018) detected that approximately 20% of WoS publications have no information in the address field. Basson, Simard et al. (2022) showed that the proportion of open access documents in Dimensions is higher than in WoS because the former indexes more publications from Asian and Latin American countries. Other studies have examined errors and inconsistencies in different academic databases to test their suitability for bibliometric studies or for bibliographic searches only. Thus, some articles have analyzed duplicate records management in Scopus ( Valderrama-Zurián, Aguilar-Moya et al., 2015 ) and WoS ( Franceschini, Maisano, & Mastrogiacomo, 2016 ).

Metadata quality is a concept that has emerged recently as a result of the abundance of available data sets and sources. Bruce and Hillman (2004) defined seven characteristics that should be considered when assessing the quality of metadata: completeness, accuracy, provenance, conformance to expectations, logical consistency and coherence, timeliness, and accessibility. Subsequently, Ochoa and Duval (2009) proposed varied metrics to automatically assess these features. However, many of these studies have been applied to digital libraries or repositories ( Kubler, Robert et al., 2018 ; Tani, Candela, & Castelli, 2013 ), with an important gap in the analysis of scholarly databases.

Is it possible to quantitatively compare the metadata content of different databases using a third source (i.e., Crossref)? What advantages and limitations could this procedure report?

Do similarities or discrepancies among databases allow us to delimit different models of databases, with their advantages and limitations?

Which databases provide the most metadata and have a higher completeness rate?

4.1. Source Selection Criteria

They had to be freely accessible through the web: a free-subscription search interface.

They had to provide metrics for research evaluation.

4.2. Sample Selection and Extraction

Crossref was selected as a control sample for several reasons. The first was an operational question. Crossref is a publishers’ consortium that assigns the Digital Object Identifier (DOI), the most extended persistent identifier of research publications in the publishing system. Although coverage is limited to publisher members ( Visser et al., 2021 ), its use is justified because all these platforms allow publications to be queried by DOIs, thereby facilitating rapid and exact matching. The second reason is related to methodological issues: Crossref enables the extraction of random samples of documents ( https://api.crossref.org/works?sample=100 ). The representativeness of the sample is thus reinforced because it avoids the influence of ranking algorithms, filters, or matching procedures that could distort the quality of the sample. A third motive is that publishers can request a DOI for any published material, regardless of typology, discipline, or language. The Crossref database therefore has no inclusion criteria that could limit the coverage of certain types of documents (e.g., indexes, acknowledgements, or front covers). This nonselective criterion would lead us to clearly appreciate the inclusion policies of the different bibliographic platforms. Finally, Crossref is fed by publishers that deposit metadata about their publications and could be considered the most authoritative source of the reliability and accuracy of their own publications.

4.3. Sources Description

Dimensions : Created in 2018 by Digital Science, this database is supported by external products, Crossref being the main source ( Hook, Porter, & Herzog, 2018 ). It gathers more than 138 million publications, in addition to patents (154 million), data sets (12 million), and grants (7 million).

Google Scholar : One of the most important academic search engines due to its estimated size (389 million) and age (2004) ( Gusenbauer, 2019 ), it obtains data directly from the web. Special agreements with publishers allow it to extract citations and content information from paywalled journals. Its search interface can also access books (Google Books) and patents (Google Patents).

OpenAlex : This is the newest service (2022). It was created by OurResearch, a nonprofit organization that recovered the defunct Microsoft Academic Graph (203 million) to implement a new open product. The core of OpenAlex was then set up by Microsoft Academic Graph, with the addition of data from other open sources such as Crossref, PubMed, and ORCID. OpenAlex now indexes 240 million publications.

Microsoft Academic : The last version of this search engine functioned between 2016 and 2021. Like Google Scholar, it also crawled the web extracting metadata from scholarly publications, reaching 260 million publications. Part of its database was openly released (Microsoft Academic Graph), which has enabled reuse by other bibliographic products.

Scilit : Active since 2016, this database was created by the publisher MDPI as a way to compete in the scholarly database market. It indexes 159 million scholarly publications, taken mainly from Crossref and PubMed.

Semantic Scholar : This search engine was launched in 2015 by the Allen Institute for Artificial Intelligence. Although it uses crawlers to extract information from the web, in 2018, it agreed to include Microsoft Academic Graph as a primary source ( Boyle, 2018 ). It now encompasses almost 214 million research papers.

The Lens : This database was developed by Cambia, a nonprofit organization, in 2000. It initially started as a patent database, but in 2018 incorporated scholarly publications from Crossref, PubMed, and Microsoft Academic. It now provides more than 225 million scholarly works.

4.4. Data Retrieving

A sample of 116,648 DOIs was randomly extracted from Crossref in August 2020 and July 2021, the only limitation being that documents were to have been published between 2014 and 2018. This time window was selected so that publications could accrue a significant number of citations and other metrics. The sample was generated by sending 1,200 automatic requests to https://api.crossref.org/works?sample=100 . Duplicate records produced by this random process were removed to obtain the final list. The resulting distribution by document type largely coincides with the entire database ( Hendricks et al., 2020 ), thereby reinforcing the reliability of the sample. Hendricks et al. (2020) adapt some categories, in which Book presumably includes book and book-chapter, and Preprints might be posted-content ( Table 1 ).

Document type distribution in the Crossref random sample and the total coverage in June 2019 ( Hendricks et al., 2020 ). Only the first 10 most frequent categories are shown

Dimensions : This database was accessed through the API ( https://app.dimensions.ai/dsl/v2 ). An R package (dimensionsR 1 ) was used to extract the data. The results were downloaded in the JSON format because dimensionsR caused problems in the transformation of JSON outputs to CSV format.

Google Scholar : Web scraping was used to automatically query each DOI in the search box because Google Scholar does not facilitate access to its data. The RSelenium 2 R package was used to emulate a browser session and avoid antirobot actions (i.e., CAPTCHAs). Because some DOIs could not be indexed ( Martín-Martín, Orduna-Malea et al., 2018 ), a title search with the query “allintitle:title” was used to complete the results.

OpenAlex : This bibliographic repository was accessed through its public API ( https://api.openalex.org/ ). A Python routine was written to extract and process the data.

Microsoft Academic : Coverage of this service was obtained by several methods. Firstly, SPARQL ( https://makg.org/sparql ) and REST API ( https://api.labs.cognitive.microsoft.com/academic/v1.0/evaluate ) endpoints were used to extract publications using DOIs. Microdemic 3 , an R package, was used to query the API. However, the low indexation of DOIs (37.1%) and their case sensitivity forced us to download the entire table of publications available in Zenodo ( https://zenodo.org/record/2628216 ) and locally match them with the sample, using DOIs and titles.

Scilit : This platform was accessed via a public API ( https://app.scilit.net/api/v1/ ). Because access must be via a POST method, a Python script was designed to extract the data.

Semantic Scholar : This database provides a public API ( https://api.semanticscholar.org/v1 ). The semscholar 4 R package was used to extract the data. However, the API was directly queried subsequently to detect any problems in the retrieval process. A Python script was written.

The Lens : After a formal request, this service provided temporary access to its API ( https://api.lens.org/scholarly/search ). In this case, an R script was written to directly extract the data. However, some relevant fields (i.e., abstract, source_urls, funders) for this study were not properly retrieved, due to technical reasons, in July 2021. We then decided to extract a small sample of 5,000 records directly from the main search page ( https://www.lens.org/lens/ ) to make up for this limitation in January 2023. From that request, 4,996 records were successfully retrieved.

Web source with information about publication metadata in each database

Completeness: the quantity of fields and degree of filling for an entity.

Accuracy: proportion of data integrity with respect to the original data or sources.

Provenance: description of primary data sources.

Logical consistency and coherence: the degree of resemblance in the description and classification of objects (e.g., language, document type).

This study describes the amount and quality of metadata associated with the description of research publications indexed in these databases. Publications are central to the publishing ecosystem and are therefore the main asset of a bibliographic database. A clear and complete description of their elements and characteristics improves the identification and retrieval of these items, and their connection with other entities. As a result, publications are the entity with most fields, ranging from 38 fields in Crossref to 18 in Semantic Scholar. Next, we analyze the fields used by each database to describe the main characteristics of a publication.

5.1. Abstract

This is an important access point to publication content because it provides a summary of the research. All the databases analyzed index this element. For Microsoft Academic, the table with this information ( PaperAbstractsInvertedIndex ) is not yet available. However, early studies detected a coverage of 58% ( Färber & Ao, 2022 ). Google Scholar does not index the publication abstract, but it does extract parts of the document text ( Google Scholar, 2023 ).

Table 3 shows the number and proportion of publications with an abstract. Google Scholar indexes the most articles with a summary (91.66%), although some are merely an extract of the text. Aside from this, Dimensions indexes most articles with their abstract (69.6%), followed by OpenAlex (63.8%). Conversely, Crossref has fewer publications with an abstract (13.7%). This last percentage is lower than that reported by Waltman, Kramer et al. (2020) (21%), perhaps because our study also gathers other materials, such as book chapters and conference papers, which do not always include a formal abstract. This low percentage of abstracts in Crossref shows that this information is not usually provided by publishers. Indexation services therefore need to process documents to obtain this data, which would explain the overall low availability of abstracts in free-access databases, in particular in Scilit (50.5%) and Semantic Scholar (54.45%). Only OpenAlex and The Lens ( χ 2 = 2.2881, p -value = 0.1304) show no statistical differences, perhaps due to the small sample size of The Lens or because both databases use Microsoft Academic as a source.

Proportion of publications with abstract in each database. Margin of error at 99%

5.2. Access

Today, a positive feature of scholarly databases is that they provide some type of access to original publications. Widespread electronic publishing allows for the provision of links to different venues where the document is partially or fully hosted. All the databases include external links to the original publication. Microsoft Academic and Crossref have no specific field for open access publications. In March 2023, OpenAlex changed some fields about access: Landing _ page _ url is designated as an external link, but only includes DOI links (99.8%), and oa_url indexes external links to open access versions only. We therefore concluded that OpenAlex includes external links for open access publications only. This is also the case for Dimensions, which only indexes external links ( linkout ) for open_access ( open _ access ) articles.

Figure 1 and Table 4 depict the percentage of publications with external links to the original source and information about whether or not they are open access per database. Google Scholar (97.1%) includes the most external links (97.1%), followed by The Lens (82.9%), Microsoft Academic (80.8%), and Crossref (79%). It is evident that the academic search engines Google Scholar and Microsoft Academic stand out in this aspect because they only index documents that are accessible on the web. The remaining 19.2% of documents without links in Microsoft Academic is explained by the removal of handles. Färber and Ao (2022) detected more documents with links (94%), which would explain this difference, as well as the coverage of The Lens, because it also uses Microsoft Academic Graph as a source. In the case of Crossref, it could be because publishers deposit their landing pages to generate incoming traffic to their publications. Conversely, Semantic Scholar (39.1%) and Scilit (45.4%) provide fewer URLs, despite the fact that the former uses Crossref as a source. The reason is that Semantic Scholar only includes URLs of the venues, but not of the papers, and Scilit only indexes URLs with a pdf ( pdf _ url ). This also occurs in Dimensions, where the proportion of publications with external links is the same as for open access articles (44.5%).

Proportion of bibliographic records with information about open access and external links.

Fields, publications, and percentage of publications with external links and information about open access by database. Margin of error at 99%

According to open access information, Google Scholar again identifies more open versions (52.2%), followed by Dimensions (44.5%), The Lens (43.6%), and OpenAlex (42.6%). These differences between Google Scholar and the other databases could be because Google Scholar indexes any open copy accessible on the web, regardless of whether the publications were released as open access or not (green open access). Conversely, Semantic Scholar (35.4%) and Scilit (25.4%) capture the fewest open documents. Figure 2 depicts two Venn diagrams showing the overlap between databases according to open access records. Overall, the picture shows that, although the databases index a similar proportion of open access documents, the overlap is not significant. Figure 2(a) shows that OpenAlex and Dimensions share the largest proportion of records (81.1%), whereas OpenAlex and Scilit only have in common 46.1% of the records. This low overlap is surprising because both services use Unpaywall to detect open access publications. A possible explanation would be that Scilit only covers open access publications in pdf format ( unpaywall _ pdf _ url ). Semantic Scholar ( Figure 2(b) ) also shows a disparity with Scilit (49.8%) and OpenAlex (50%). In this case, Semantic Scholar uses its own criterion to detect open access publications.

Overlap among databases identifying open access publications.

5.3. Bibliographic Information

A critical element in a bibliographic database is the correct identification of indexed publications. Journal articles are identified using information that enables the document to be placed into the journal. Volume, issue, and pages are three fields that facilitate correct identification. All the databases include these fields, with the exception of Semantic Scholar, which has no field for issue.

Figure 3 and Table 5 depict the proportion of bibliographic data for journal articles in each database. Google Scholar is not included because it does not provide bibliographic information. In general, all the databases show high rates of completeness, including more information about volume than pages and issues. In this sense, Dimensions is again the platform with the highest completeness rates, with 100% of volume and 94.3% of pages, followed by The Lens with the highest number of pages (94.2%). The most noteworthy result is the low completeness degree of OpenAlex, with 50.2% of issue, 57.6% of pages, and 62.4% of volume. These figures are much lower than those reported by Microsoft Academic, its primary source. A manual inspection confirmed this lack of data, in which almost all the records ingested in December 2022 failed to include this information. This high completeness is also observable in the considerable overlap among confidence intervals, which shows that this information is essential in any scholarly database.

Proportion of bibliographic records with information about volume, pages, and issue.

Percentage and number of articles with bibliographic info (volume, issue, and pages) in each database. Margin of error at 99%

5.4. Document Type

Although journal articles make up more than 70% of the scientific literature, there is a large variety of scholarly documents (book, book chapters, conference papers, etc.) that also provide relevant scientific information and that are incorporated by many scholarly databases in their indexes. Scholarly databases categorize these typologies to inform about the academic nature of each item. However, the range of categories in each database varies significantly. For instance, although Crossref includes 33 document types, Dimensions summarizes its classification in only six classes ( Table 6 ).

Number of document typologies and completeness degree in each database. Margin of error at 99%

Table 6 displays the number of different document types and the number of records categorized in each database. Again, Google Scholar is excluded because the database has no document types. All the publications in Crossref (100%) and Dimensions (100%) are assigned to a typology, and OpenAlex (100%), The Lens (99.2%), and Scilit (99.8%) only find assignment problems in exceptional cases. This high completeness could be due to the fact that all these sources take document typologies from Crossref. Therefore, this lack of statistical differences could be more related to completeness than to data origin.

However, Microsoft Academic (80.3%) and Semantic Scholar (41.3%) present serious problems when classifying their records by typology. A possible explanation is that both search engines extract metadata from the web, and this information is not always available. Worth mentioning is the case of Semantic Scholar, which appears to use an automatic procedure to assign more than one typology based more on content criteria ( Review , Study , CaseReport , etc.) than on formal criteria.

Figure 4 shows different alluvial graphs illustrating document type transfers between the Crossref classification and the systems of each database. The aim is to elucidate how each database assigns document types to their records. To improve the clarity of the graph, only the 10 most common categories in Crossref are displayed. For instance, Dimensions significantly reduces the document categories, integrating book-chapter (99.9%), component (78.7%), reference-entry (100%), and other (100%) into the chapter category, and dataset (100%) and journal-article (99.1%) into article . Microsoft Academic shows important problems when classifying book chapters (46.8%) and proceeding-articles (65.3%). Up to July 2023, OpenAlex directly used the Crossref scheme without any variation. Today, monograph (100%) is entered under book , and some marginal typologies (i.e., others , component , journal , proceeding , journal-issue ) are grouped under the paratext category. Scilit also presents slight variations to the Crossref framework, creating Conference-Paper to group any document related to a conference, regardless of whether it is published as a journal-article , book-chapter , or proceeding-article . Semantic Scholar has serious problems when classifying most of the document typologies because only 46.2% of proceeding-articles are classified as Conference and 35.3% of journal-articles as JournalArticle . Finally, The Lens also shows similarities with the Crossref classification: proceeding-articles are split into conference proceedings (56.2%) and conference proceeding articles (35.7%), and posted-content is entered under other (94.4%).

Alluvial graph with the transfer of document types between Crossref and the other databases. The stratum on the left shows the original Crossref classification and on the right the classification system of each database. Some labels were omitted to avoid overlaps.

5.5. Publication Dates

Electronic publishing has led to the appearance of multiple dates associated with the same document, describing different lifespan stages. This variety of dates also causes problems in the management of these publications ( Ortega, 2022 ). Crossref includes more dates (up to eight), followed by Dimensions with five, and Microsoft Academic and The Lens with four. Crossref ( created ), Dimensions ( date _ inserted ), Microsoft Academic ( CreatedDate ), and The Lens ( created ) display the date when the record was created; and Crossref ( published-print , published-online ), Dimensions ( date _ print , date _ online ), and Scilit ( date _ print , publication _ year ) distinguish between the print and online date.

Publication date is common to all the databases and allows us to analyze the reliability of this information in each. A way to test the accuracy of these data is to compare the matching percentage with Crossref dates. Crossref is directly fed by publishers, which could make it the most authoritative source for exact and correct publication dates. Crossref fields that most match the publication date are published (88%), published-online (6%), and created (5%).

Figure 5 and Table 7 depict the proportion of publication dates that match some Crossref dates (i.e., published-print , published-online , created , deposited , indexed , and issued ) and the percentage of publications with no date. Google Scholar only includes publication year. The comparison is thus done by year, not by date, resulting in a much higher match. That said, Google Scholar matches only 79.6%. The bar graph shows that Scilit (94.9%) and Dimensions (91.9%) have the best match with Crossref, and Microsoft Academic (70.1%) and OpenAlex (72.6%) have lower matching rates. These results could be explained because Dimensions and Scilit take their data from Crossref, and OpenAlex is an adaptation of the Microsoft Academic database. The most interesting result is perhaps the high proportion of publications with no date in Semantic Scholar (57.9%) and The Lens (35.8%). In the case of Semantic Scholar, it could be due to parsing problems when information is extracted from websites. For The Lens, however, this absence of information could be due to technical problems. It has the lowest proportion of no matching publication dates (4.3%), which could indicate that The Lens is also extracting the publication date from Crossref.

Percentage of publication dates matching with Crossref dates.

Percentage and number of articles with and without publication date matching and not matching with Crossref. Margin of error at 99%

5.6. Language

A relevant factor to consider in a scholarly database is the language of the full text. The release of research documents in a language other than English is growing, and there is increasing demand for publications by local research communities. However, this information is only supplied by Crossref ( language ), Microsoft Academic ( LanguageCode ), The Lens ( languages ), and Scilit ( language ). In our study, we extracted this information from Crossref, Scilit, and Microsoft Academic. Technical problems made it impossible to retrieve this information from The Lens. The results show that Scilit is the platform that identifies the language of most publications (99.9%), followed by Microsoft Academic (77.6%) and Crossref (57.1%). A manual inspection of the language assignment seems to indicate that Crossref assigns language according to venue, Scilit according to titles and abstracts, and Microsoft takes the language from webpage metadata.

Figure 6 displays a Venn diagram plotting the overlap between Crossref, Microsoft Academic, and Scilit when identifying the same publication language. The results show that Scilit largely matches Crossref (93.3%) and Microsoft Academic (89.8%), and the coincidence between Crossref (38%) and Microsoft Academic (54.9%) is low. These differences show how the methodological differences between Crossref (venues) and Microsoft Academic (webpages) influence language assignation, and how the use of content elements (title and abstract) improves language detection in Scilit.

Overlap between databases assigning the same language.

5.7. Identifiers

Much of the current proliferation of scholarly databases is due to the consolidation of external identifiers that make it possible to individualize publications (duplicate management) and connect with other sources, thereby enriching the information about publications. Apart from DOIs, many databases index different external identifiers. Semantic Scholar ( externalIds ), The Lens ( external _ ids ), OpenAlex ( ids ), and Microsoft Academic ( AttributeType ) have a specific field for external identifiers. Crossref, Scilit, and Dimensions have different fields for each identifier. Google Scholar provides no identifier.

Figure 7 shows the proportion of the three most frequent identifiers (ISSN, MAG, and PMID) in each database. ISSN is the code that identifies journals and series, MAG is the Microsoft Academic identifier, and PMID is the PubMed ID. The aim is to discover the sources of these databases and to explore the identification of publication venues. The results show that all databases index or identify publications from PubMed to a similar extent, ranging from 21.5% for Crossref to 26.5% for Semantic Scholar ( Table 8 ). Only three databases—OpenAlex, Semantic Scholar, and The Lens—take data from Microsoft Academic. Semantic Scholar (97.8%) and OpenAlex (87.4%) index the most publications. Their differences suggest, on the one hand, that Semantic Scholar is highly dependent on Microsoft Academic and, on the other, that OpenAlex is already using other sources (Crossref) to expand its database. As for ISSN, all the services have a similar coverage of ISSNs (≈80%), although Microsoft Academic (22.9%) and Semantic Scholar (37.4%) have rather low proportions, suggesting deficient journal identification possibly deriving from the web extraction process.

Percentage of different identifiers in each database.

Percentage and number of articles with different identifiers. Margin of error at 99%

This comparative analysis between free-access bibliographic databases has mainly focused on the completeness degree and quality of their metadata. This quantitative approach has made it possible to detect coverage limitations, allowing us to speculate about the technical reasons for these results. The study reports important outcomes about data sources and how information is processed. Overall, the results allow us to distinguish between academic search engines (Google Scholar, Microsoft Academic, Semantic Scholar) and third-party databases (Dimensions, The Lens, Scilit, OpenAlex). The former reveal clear problems with the number of fields that describe publications and their completeness. One explanation could be that these databases mainly obtain their data by crawling the web, and the information that webpages make available could be insufficient to correctly describe a publication. Thus, Semantic Scholar only includes abstracts for 54.5% of its publications and is the service with the lowest proportion of open access documents (35.4%) and external links (39.1%). Almost 60% of its publications have no document type, and it is the database that has the most publications with no publication date (57.9%). These figures reveal that Semantic Scholar has serious problems processing bibliographic information, which could be the root of the low quality of its metadata. In addition, the high proportion of records with the Microsoft Academic id (97.8%) and the similar coverage of PMIDs (Microsoft Academic = 26.2% (±0.4); Semantic Scholar = 26.5% (±0.4)) and no matching dates with Crossref (Microsoft Academic = 29.8% (±0.4); Semantic Scholar = 29.1% (±0.4)) suggests that the core of Semantic Scholar relies on Microsoft Academic and less on crawling the web.

To a lesser extent, Microsoft Academic also shows a low document type classification (79.5%), lack of information about open access publications, and the lowest proportion of ISSNs per publication (22.9%). According to document type, some recent studies observed that more than half of publications include this information ( Färber & Ao, 2022 ; Visser et al., 2021 ). This disparity with our results could be because patents are not included in our study (approximately 20%). It does, however, highlight the coverage of external links (80%). These results are important for understanding how other products, based on their data (Semantic Scholar, The Lens, and OpenAlex), have inherited or solved these problems.

The main issue with Google Scholar is that it provides very little information about publications. Basic information, such as document type, bibliographic information, publication date, or identifiers, is missing from this database. With the exception of citations and versions, Google Scholar barely adds value to their records. However, as a search engine, it is the service that provides the most external links (97.1%) and detects the most open versions (52.2%), thereby confirming that it is the best gateway to scientific literature on the web ( Martín-Martín et al., 2021 ).

Conversely, third-party databases supported on Crossref (Dimensions, The Lens, and Scilit) have more metadata details, with a high completeness degree. Dimensions could be considered the database that most enriches and improves information provided by Crossref. It indexes the most publication abstracts (69.6%), identifies the most open access articles (44.5%) ( Basson et al., 2022 ), has the best coverage of bibliographic info (volume = 100%, issue = 81%, pages = 92%), and 100% of publications are listed according to their typology. These results illustrate that Dimensions endeavors to improve Crossref metadata by adding abstracts, document typology, open access status, and so forth, to their records, with a high completeness rate. However, other sources, such as Scilit and The Lens, show signs of low data processing efficiency. For instance, Scilit is the commercial product that indexes the smallest number of abstracts (50.5%) and the lowest proportion of open access documents (25.4%). The Lens has reported serious problems with publication dates (35.8%). These findings reveal that the main risk of third-party databases is that they require a considerable processing effort to improve the quality of their metadata. A similar case is OpenAlex, which is based on both Microsoft Academic and Crossref. This integration of different sources would cause a loss of information, with a high proportion of missing bibliographic data (volume = 62%, issue = 50%, pages = 51%). The results for OpenAlex indicate that this new database is similar to Microsoft Academic (same proportion of abstracts and dates, the second database with the highest number of Microsoft Academic identifiers), but with the addition of DOIs and document types from Crossref ( Scheidsteger & Haunschild, 2023 ). This active processing of different sources illustrates the importance of these tasks for offering a reliable scholarly bibliographic product ( Priem, Piwowar, & Orr, 2022 ).

Any comparative analysis of bibliographic databases is always limited by the reference database used in the study. In our case, Crossref presents some particularities that should be considered in the interpretation of the results. Crossref is merely a repository of unprocessed publishers’ data. Its coverage is determined by its partners, who decide on the data and how they are deposited, which could influence the representativeness of the sample and the quality of the metadata. A way of lessening this limitation is to make a random selection of the sample. This problem would also influence reliability because publishers may deposit incomplete records, lacking some fields and containing errors, including incorrect or null data about their own publications.

The results lead us to conclude that a random Crossref sample enables the comparison of a wide range of scholarly bibliographic databases, and the benchmarking of the amount of information and completeness degree of these databases, with regard to different facets. Accordingly, the numbers of publications with abstract, external links, open access status, document type, or publication date have been measured across databases. Extraction of the same data from each database has shed light on overlaps, which has led us to identify possible connections between databases.

The results show that databases based on external sources can generate more and improved metadata than academic search engines extracting information from the web. Search engines have the power to reach distant publications and detect more open copies, but they lack the ability to retrieve reliable descriptive data about publications from webpages. However, this integration of different sources also produces problems, such as the loss of information (The Lens and publication date, or OpenAlex and bibliographic info), or limitations inherited from the primary sources (OpenAlex and the publication dates of Microsoft Academic).

Finally, Dimensions provides the greatest number of fields about publications and the highest completeness degree. OpenAlex, The Lens, and Scilit also include a varied range of fields but display certain integration problems, with a lack of information and low completeness rates in specific fields. Conversely, search engines such as Semantic Scholar and Google Scholar lack important fields for identifying and searching publications (document types, certain bibliographic info). Microsoft Academic is the search engine that provides the most publication fields, and its completeness rate is high, though it lacks information on open access status and document type for some publications.

Lorena Delgado-Quirós: Data curation, Formal analysis, Resources, Software. José Luis Ortega: Conceptualization, Funding acquisition, Investigation, Methodology, Writing—original draft, Writing—review & editing.

José Luis Ortega is member of the Scilit advisory board, which facilitated access to API documentation.

This work was supported by the research project (NewSIS) “New scientific information sources: Analysis and evaluation for a national scientific information system” (Ref. PID2019-106510GB-I00) funded by the Spanish State Research Agency (AEI) PN2019.

For legal reasons, data from many of the databases (Dimensions, Google Scholar, Scilit, and The Lens) cannot be made openly available. Data from Crossref, OpenAlex, Microsoft Academic, and Semantic Scholar are openly available because they have been released under a CC-BY license. We have uploaded the instructions on how to retrieve the data in each database ( https://osf.io/yw6j4 ). In this form, readers with credentials can download the data and reproduce the study.

https://github.com/massimoaria/dimensionsR .

https://docs.ropensci.org/RSelenium/ .

https://docs.ropensci.org/microdemic/ .

https://github.com/njahn82/semscholar .

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• Salima Essih 1 ,
• Enrique Vilarrasa-García 2 ,
• Diana Cristina Silva Azevedo 2 ,
• Daniel Ballesteros-Plata 1 ,
• Isabel Barroso-Martín 1 ,
• Antonia Infantes-Molina 1 ,
• Enrique Rodríguez-Castellón 1 ,
• Francisco Franco 1 &
• Juan Antonio Cecilia   ORCID: orcid.org/0000-0001-5742-4822 1  

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Five phyllosilicates (kaolinite, montmorillonite, saponite, sepiolite and palygorskite) have been selected as starting materials for the synthesis of zeolites. Among them, kaolinite and montmorillonite display the lowest Si/Al molar ratio leading to aluminosilicates with high crystallinity. Thus, the hydrothermal treatment under basic conditions forms 4A zeolite when kaolinite is used as starting material while 13X zeolite is obtained when montmorillonite is used as starting material. The microporosity and CO 2 -adsorption capacity of the prepared zeolites are directly related to its crystallinity. Thus, in order to improve it, raw phyllosilicates were subjected to a microwave-assisted treatment to remove undesired Mg or Fe-species, which have a negative effect in the assembling of the zeolites by hydrothermal basic conditions in a second step. The highest adsorption value was 3.85 mmol/g at 25 °C and 760 mm of Hg for Mont-A-B sample after the consecutive treatments.

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Introduction

The increase in anthropogenic CO 2 emissions due to the ever-increasing population growth and energy demands, together with the massive consumption of fossil fuels, has aggravated global warming, causing severe effects in the world such as the melting of the polar ice shield and the rise in sea levels. Moreover, in some regions, extreme weather events and rainfall are becoming more common, while others are experiencing more extreme heat waves and droughts. Predictions are not very positive, since these impacts will intensify in the next decades if the CO 2 content in the atmosphere continues to increase (Kessel 2000 ).

Taking into account the serious effects of greenhouse gases, governments are meeting to establish more restrictive legislations in order to diminish the CO 2 -anthropogenic emissions. The main challenge is the quest of alternative energy sources, away from traditional fossil fuels, to achieve net zero emissions. However, renewable energy sources are still developing technologies that cannot supply the world’s great energy demands. On the other hand, the design of more efficient processes in which CO 2 emissions can be minimized is another challenge the scientific community is facing, yet these processes have only been studied on a small scale and have not been applied on a large scale due to the high energy demands. A proposed alternative until more efficient processes or more sustainable energy sources are found is the capture, storage and subsequent valorization of CO 2 (Abdulla et al. 2021 ). In the CO 2 capture and storage (CCS) processes, the most expensive step is CO 2 capture (50–90% of the total cost), depending on the source of CO 2 emission (Pera-Titus 2014 ). The most consolidated technology to capture CO 2 from gaseous effluents streams is chemical absorption using alkylamines (Rochelle 2009 ), obtaining brilliant results in some processes as flue gas. Despite the high potential for CO 2 capture, the intrinsic toxicity and corrosivity of the amine-derived compounds, the high energy demands in the regeneration step and the discontinuity of the process are limitations to their use (Aresta and Dibenedetto 2003 ). In recent years, less corrosive and toxic technologies have been developed, among which the use of membranes, cryogenic distillation or the use of adsorbents can be highlighted thanks to the remarkable results in CO 2 capture. However, these technologies also display limitations related to the costs of large-scale implementation. In addition, their efficiency is diminished for streams with low CO 2 concentration (Cerón et al. 2018 ; Font-Palma et al. 2021 ). In recent years, inorganic or organic–inorganic solids have emerged as an alternative to capture CO 2 through physical or chemical interactions (Pera-Titus 2014 ). In this sense, it has been reported that some inorganic compounds with medium and strong basicity such as alkaline-earth oxides, can chemically capture CO 2 molecules, since these oxides are prone to suffer carbonation processes. Even though these inorganic compounds are inexpensive and reach high CO 2 uptakes, these materials have a serious drawback related to the high temperature required for their reuse due to the strong chemical interaction of CO 2 molecules with these oxides (Grasa and Abanades 2006 ; Wang et al. 2012 ).

On the other hand, porous materials have also been used in CO 2 adsorption processes since CO 2 molecules can be trapped in narrow pores due to their high quadrupole moment. In this sense, metal–organic frameworks (MOFs) (Yu et al. 2017 ), covalent-organic frameworks (COFs) (Zeng et al. 2016 ) and graphene-organic frameworks (GOFs) (Haque et al. 2017 ) have shown excellent CO 2 -uptake at laboratory scale, thanks to their narrow and modulable channels. However, the cost for pilot-scale is too high to be sustainable and competitive.

Porous silicas with different morphologies have been proposed for CO 2 capture. However, CO 2 uptakes with these materials are relatively low, therefore some methodologies like grafting and impregnation have been proposed to incorporate amine species and improve the adsorption capacity by increasing the amount of chemical adsorption sites (Cecilia et al. 2020 ; Chen et al. 2017 ; Vilarrasa-García et al. 2020 ; Yan et al. 2011 ). Porous activated carbons are other thoroughly studied adsorbents that exhibit high adsorption values thanks to their narrow pore size and high surface area. Nonetheless, the use of low cost starting materials is required to achieve sustainable and competitive processes (Abuelnoor et al. 2021 ; Chouikhi et al. 2021 ; Pevida et al. 2008 ; Serafin et al. 2017 ). Zeolites are hierarchical aluminosilicates with a narrow pore size that, like activated carbons, can also retain a large amount of CO 2 (Boycheva et al. 2021 ; Murge et al. 2019 ; Zagho et al. 2021 ). For the synthesis of zeolites, a source of silicon and aluminum is required in a basic medium under hydrothermal conditions (Derbe et al. 2021 ; Koohsaryan and Anbia 2016 ). Considering that CO 2 capture is the most expensive step in the CCS process, the use of low-cost silicon and aluminum sources is necessary to obtain inexpensive zeolites with high CO 2 -adsorption capacity (Khaleque et al. 2020 ). In this sense, several raw materials have been proposed such as blast furnace slag (Sugano et al. 2005 ), rice husk ash (Mohamed et al. 2015 ; Saceda et al. 2011 ), paper sludge ash (Mun and Ahn 2001 ), coal fly ash (Amoni et al. 2019 ), waste of iron mine tailings (Zhang and Li 2018 ), waste glass materials (Tsujiguchi et al. 2014 ) or minerals and rocks such as obsidian (Belviso 2016 ; Mamedova 2016 ), perlite (Filho et al. 2018 ; Wajima and Onishi 2019 ) or clay minerals (Abdullahi et al. 2017 ; Belviso et al. 2017 ; Johnson and Arshad 2014 ; Mackinnon et al. 2010 ; Youssef et al. 2008 ). In the present research, several phyllosilicates (kaolinite, montmorillonite, saponite, sepiolite and palygorskite) have been used as starting materials for the synthesis of zeolites through the alkaline fusion method (Belviso et al. 2017 ; Chen et al. 2014 ; Khalifa et al. 2000 ). To remove the potential impurities of the starting phyllosilicates, the samples were subjected to a microwave-assisted acid treatment, since this treatment causes a partial leaching of the phyllosilicates in short times (Cecilia et al. 2018a ; Franco et al. 2016 , 2020 ). Once the materials were synthesized by hydrothermal treatment under basic conditions, the obtained samples were tested in CO 2 adsorption processes.

Materials and methods

Starting materials.

Five clay minerals (kaolinite, montmorillonite, saponite, sepiolite and palygorskite) were selected as starting materials to the synthesis of zeolites. These materials were treated with a microwave-assisted acid treatment to modify their chemical composition due to a partial leaching of the octahedral sheet (Cecilia et al. 2018a ; Franco et al. 2016 , 2020 ).

The synthesis of potential zeolites from raw clay minerals and those modified by microwave-assisted acid treatment was carried out through hydrothermal treatment under basic conditions, following the methodology described in previous work (Cecilia et al. 2022 ). Briefly, 1 g of raw clay or a microwave-assisted acid-modified clay mineral was treated with a solution of 40 mL of NaOH (2 M) in a Teflon-lined stainless-steel autoclave for 48 h at 100 °C. The obtained material was centrifuged and washed several times to remove Na + excess. Finally, the samples were dried at 80 °C overnight.

The samples were labeled with the initials Kao for kaolinite, Mont for montmorillonite, Sap for saponite, Sep for sepiolite and Pal for palygorskite. The nomenclature clay-A was used for the clay treated by microwave-assisted acid treatment, clay-B for the raw clay subjected to hydrothermal treatment under basic conditions and clay-A-B for those clays modified by microwave-assisted acid treatment, and then by hydrothermal treatment under basic conditions.

Characterization of the materials

The crystalline structure of the clays and potential zeolites was determined by X-ray diffraction (XRD) using a PANalytical X’Pert PRO equipment and recorded in Bragg–Brentano reflection geometry (θ/2θ). This diffractometer is equipped with a Ge (111) monochromator to use monochromatic Cu Kα 1 radiation (λ = 1.54059 Å) strictly.

The quantification of the main elements of the adsorbents was carried out in a MagiX X-ray fluorescence (XRF) spectrometer supplied by PANanalytical. A Varian 220-FS QU-106, atomic absorption (AA) spectrometer was used for the determination of Na-species. Loss of ignition (LOI) was determined at 950 °C.

The morphology of the particles was examined by scanning electron microscopy (SEM) using a JEOL SM-6490 LV combined with X-ray energy dispersive spectroscopy (EDX). The samples for SEM observation were previously gold sputtered to avoid charging of the surface.

The obtained materials were also characterized by attenuated total reflection (ATR) in a FT-IR Vertex70 spectrophometer (Bruker), showing a single reflection gold-gate diamond ATR system. For the acquisition of each spectrum, a standard resolution of 4 cm −1 , 64 accumulations and a spectral range between 4000 and 500 cm −1 were used.

The textural properties of the microporous materials were determined from CO 2 adsorption–desorption at 0 °C. CO 2 was selected to analyze the textural properties instead of N 2 since this gas is more appropriate to analyze the microporosity due to the hindered access to narrow micropores of N 2 -molecules. The micropore volume was calculated through the Dubinin-Radushkevich equation (Dubinin and Radushkevich, 1947). In all cases, the samples were previously outgassed at 200 °C and 10 −4  mbar for 12 h.

The prepared materials were also characterized by solid-state NMR. For these samples, 29 Si and 27 Al MAS NMR spectra were analyzed in a Bruker AVIII HD 600 NMR equipment (field strength of 14.1 T) at 156.4 MHz with a 2.5 mm triple-resonance DVT probe that used zirconia rotors at the spinning rates of 15 kHz ( 29 Si) and 20 kHz ( 27 Al). 29 Si MAS NMR analysis was carried out with proton decoupling (continuous wave sequence), applying a single pulse of π/2, an excitation pulse of 5 µs and a 60 s relaxation delay to obtain 10,800 scans. 27 Al MAS NMR analysis were also carried out with proton decoupling (continuous wave sequence), applying a single pulse of π/12, an excitation pulse of 1 µs, and a 5-s relaxation delay to obtain 200 scans. The chemical shifts were referenced to as an external solution of tetramethylsilane for 29 Si and to an external solution of 1 M of Al(NO 3 ) 3 27 Al, respectively.

CO 2 adsorption studies

To evaluate the CO 2 adsorption capacity of the raw clays, the clays treated by microwave-assisted acid treatment and the clays treated under hydrothermal conditions in a basic medium, the samples were degassed at 150 °C for 12 h. Then, the analyses were carried out at 25 °C, between 0 and 760 mm of Hg using a Micromeritics 2420 apparatus.

In a next step, the isotherms were fitted to a Toth model.

where q m is the maximum adsorbed concentration, b is the affinity parameter between adsorbent and adsorbate and t represents the surface heterogeneity of the adsorbent.

The accuracy of each fit was carried out for the Average Relative Error:

where N T is the total number of the data points, and q i,est and q i,exp are the estimated and experimental amounts of CO 2 adsorbed, respectively.

Characterization of the samples

The analysis of crystallinity and the identification of crystalline phases in the raw clays were determined by XRD (Fig.  1 ). In the case of the Kao sample, the diffractogram shows higher crystallinity compared to the other samples, with two well-defined reflections located around 2θ of 12 and 25°, which are assigned to [001] and [002] reflections of kaolinite (Franco et al. 2014 ). Bragg equation applied to [001] reflection reveals that this sample displays a basal spacing of 7.2 nm. The presence of another set of diffraction peaks located at 34–36, 38–42, 45–50 and 54–63° also stands out, whose intensity depends on the origin of the kaolinite (Franco et al. 2014 ). On the other hand, the signal located at 2θ of 62.3° (1.49 nm) is ascribed to the [060] reflection and confirms the presence of a dioctahedral clay of the kaolin group. Finally, the presence of a small signal located at 2θ of 26.2° is also noted, which is assigned to small impurities of quartz.

X-ray diffractograms of the starting raw clays: Kaolinite (Kao), Montmorillonite (Mont), Saponite (Sap), Sepiolite (Sep) and Palygorskite (Pal)

The diffractogram of the Mont sample (Fig.  1 ) shows a material with less crystallinity than the Kao sample. The [001] reflection located at the lowest 2θ value shows two bands, which indicates a variability in the hydration of the interlayer space (12.6–15.0 nm). In addition, as observed in the Kao sample, a diffraction peak located at 62.3° (1.49 nm) stands out, which confirms the presence of a dioctahedral smectite such as montmorillonite (Cecilia et al. 2018a ). Finally, the presence of the main diffraction peak of quartz (Qtz) and tremolite (Tr) as impurities is also observed.

For the Sap sample (Fig.  1 ), the intensity of the signals is lower, suggesting a sample with poorer crystallinity. A broad diffraction peak is also observed at lower 2θ values, so the presence of variable H 2 O-content is expected. The analysis of the [060] reflection located at 2θ of 60.9° (1.52 nm), which is typical of a trioctahedral smectite as saponite (Cecilia et al. 2018a ). Regarding the impurities, small proportions of quartz and calcite (Cal) can be observed.

In the case of the Sep sample, the typical diffraction peaks of sepiolite are detected (Franco et al. 2014 ), although an impurity of quartz is also observed. Regarding the Pal sample, the diffractogram confirms the presence of palygorskite (Pardo-Canales et al. 2020 ) with the presence of small impurities of smectite, sepiolite and quartz.

In the synthesis of zeolites, a drawback is the presence of Mg- and Fe-species, since these species are not soluble in a basic medium, so they can negatively affect the assembly of silicate and aluminate to form zeolites. It is well known that trioctahedral clays have a high Mg-content. These Mg-species can be removed under acid treatment by leaching. In the same way, microwave-assisted acid treatment accelerates and increases the efficiency of partial leaching in trioctahedral clays (Cecilia et al. 2018a ).

The possible modification of the crystallinity in the clays after microwave-assisted acid treatment for 16 min using 0.2 M HNO 3 solutions was evaluated by XRD and registered diffractograms are displayed in Fig.  2 . The study of the kaolinite samples after the acid treatment (Kao-A) shows that this sample is barely affected by microwave-assisted acid treatment, confirming that the trioctahedral TO structures are highly resistant to acid treatment. Similarly, the acid treatment of the Mont sample (Mont-A) hardly modifies the crystallinity of the sample since its TOT structure is maintained, although under the acid treatment some impurities such as tremolite are removed. Regarding the Sap sample treated with microwave-assisted acid treatment (Sap-A), the intensity of the diffraction peaks decreases notably. In this sense, the absence of the [001] reflection, which should appear at 2θ of 7°, suggests that the lamellar structure of saponite disappears. Similarly, acid treatment also removes the impurity of CaCO 3 . However, other impurities such as quartz seem to be more resistant to acid treatment. A similar trend was observed for Sep-A sample, where the [110] reflection also disappears after acid treatment, suggesting that the fibrous structure of the sepiolite is seriously damaged. In the case of palygorskite treated with microwave-assisted acid treatment (Pal-A), the [110] reflection is maintained, evidencing that the structure of palygorskite seems to be more resistant than that of sepiolite. In this sample, the impurities ascribed to quartz and smectite remain, so the smectite can be a dioctrahedral clay as montmorillonite due to its resistance to acid treatments. When the raw clays and the acid treatment-modified clays are subjected to a hydrothermal treatment in a basic medium, the diffractograms evidence that crystalline phases are only obtained from kaolinite or montmorillonite as starting materials (Fig.  3 A and B). The use of saponite, sepiolite or palygorskite as starting materials to synthesize zeolites leads to amorphous materials even when the samples are subjected to microwave-assisted acid treatment. Regarding crystalline materials, two diffraction profiles can be observed. When the starting clay is kaolinite, the diffraction peaks are assigned to the formation of zeolite 4A (PDF N. 01–075-1151) while the use of montmorillonite as starting clay leads to the typical diffraction peaks of zeolite 13X (PDF N. 00–038-0237). In all cases, although a high crystallinity of the synthesized zeolites is observed, the presence of amorphous phases should not be ruled out.

X-ray diffractograms of the phyllosilicates subjected to microwave-assisted acid treatment

X-ray diffractograms of the aluminosilicates obtained after the hydrothermal treatment in basic medium of the raw phyllosilicates ( A ) and phyllosilicates subjected to microwave-assisted acid treatment and then a hydrothermal treatment in basic medium ( B )

To evaluate the modifications in the chemical composition of clays and potential zeolites to be used in CO 2 capture processes, XRF studies were carried out (Table  1 ). The study of raw clay minerals by XRF is directly related to the results obtained by XRD. Thus, it can be observed how Kao is an aluminosilicate where the Mg-content is very low, which confirms a dioctahedral clay, as already observed in the [060] reflection. The absence of alkaline cations suggests the presence of a TO structure, while the low proportion of other elements confirms the absence of impurities. In the case of the Mont sample, it can be observed that the clay is Al-rich smectite, as suggested by XRD, although a small proportion of Mg-species is also noted. On the other hand, the presence of alkaline cations suggests the presence of a TOT structure where the cations are located in the interlayer space. Regarding the Sap sample, the Mg content is much higher than the Al content, leading to a trioctahedral clay. The presence of Ca-, K- and Na-species, which must be in the interlayer space, suggests that this material is a Mg-smectite. In the case of the fibrous clays (Sep and Pal), it can be observed how the Sep sample displays a higher Mg-content than Pal sample. In both samples, the percentage of Na-, K- or Ca-species is very low, discarding the presence of a high proportion of smectites or the inclusion of these cations in the microchannels of the fibrous structure.

Regarding possible modifications due to microwave-assisted acid treatment, it can be observed how the Al-rich phyllosilicates are highly resistant to acid treatment with microwave, as in the XRD (Figs.  1 and 2 ) where the Kao and Mont samples hardly suffer modifications after the acid treatment. However, trioctahedral clays, i.e. those phyllosilicates with higher Mg-content, suffer a clear loss of Mg-content when the samples are undergone to microwave-assisted acid treatment (Cecilia et al. 2018a ; Franco et al. 2016 ). This leaching may cause a collapse in the lamellar structure in Sap sample and the fibrous structure in Sep sample, leading to amorphous materials for Sap-A and Sep-A, as observed by XRD (Figs.  1 and 2 ). In the case of those clays with Ca species, leaching of Ca species is also observed after acid treatment. The loss of Mg and Ca also affects the purification of clays since some crystalline phases such as calcite or tremolite are removed after acid treatment. On the other hand, it is also striking that the Fe-species are resistant to acid treatment.

After hydrothermal treatment in a basic medium to try the synthesis of the zeolites, the atomic concentration, determined by XRF, varies notably, increasing the Na-content significantly (Table  1 ). It is necessary to remember that the zeolitization process takes place through the following steps: (I) dissolution of the aluminosilicate into aluminate and silicate species; (II) condensation of the aluminate and silicate species forming oligomers; and (III) assembly and growth of oligomers to give rise to highly ordered three-dimensional structures.

In the case of the Kao and Kao-A samples, where the Si/Al molar ratio is the highest, the absence of impurities, mainly Mg-species, forms an aluminosilicate with a well-defined structure (zeolite 4A) (Fig.  3 A). When Mont and Mont-A are used as starting materials to synthesize the zeolite, the assembly of the silicate and aluminate oligomers leads to another three-dimensional distribution forming a 13X zeolite (Fig.  3 A), given the different Si/Al molar ratio for Kao and Mont.

Regarding the other clay minerals (Sap, Sep and Pal), the Si/Al molar ratio is higher than that observed for Kao and Mont, so the oligomerization and assembly must be different for these phyllosilicates. Moreover, the presence of a high Mg content, which is insoluble under basic conditions, also has an adverse effect on the assembly of the oligomers to synthesize the zeolites, obtaining in these cases an amorphous aluminosilicate (Fig.  3 A). The microwave-assisted acid treatment aimed to minimize the Mg content but has an adverse effect on the synthesis of zeolites (Fig.  3 B). Moreover, the Si/Al molar ratio is quite far from that of the zeolites synthesized from Kao and Mont, which are highly crystalline. Thus, the low Al content of Sap, Sep and Pal seems to have an adverse effect on the assembly of crystalline zeolites.

To analyze the morphology of raw clays, clays modified under microwave-assisted acid treatment, and amorphous aluminosilicates or zeolites synthesized by hydrothermal treatment under basic conditions, SEM images were performed (Fig.  4 ). The SEM image of the natural kaolinite confirms the existence of laminar habits, although it can be observed that these laminar structures present a house of cards structure as a consequence of the hydrothermal alterations to which the clay is subjected as well as the grinding treatment. After the microwave-assisted acid treatment, SEM image shows that the sample partially maintains a lamellar structure, evidencing a mixture of large stacks of kaolinite with lamellar structures smaller than those of the starting materials. The most striking change occurs when the zeolite is treated under hydrothermal and strongly basic conditions, since the lamellar structure disappears, giving rise to particles with cubic morphology. The structural changes agree with those observed by XRD where a highly crystalline zeolite (4A) is formed after hydrothermal treatment under basic conditions (Fig.  3 B).

SEM images of raw phyllosilicates, phyllosilicates subjected to microwave-assisted acid treatment and, phyllosilicates subjected to microwave-assisted acid treatment and then hydrothermal treatment in basic medium. Scale: 5 µm

In the case of the Mont sample (Fig.  4 ), the image shows particles of variable size formed by stacked sheets, confirming the laminar structure suggested by XRD (Fig.  1 ). The acid treatment only causes slight modifications in the particles since it seems to affect the edges of the sheets in such a way that the acid treatment under these conditions hardly affects the montmorillonite structure, which agrees with the XRD data (Fig.  2 ). However, the basic treatment causes a strong modification in the morphology and size of the particles, obtaining small particles with a diameter of less than 1 µm, which must be highly ordered according to XRD (Fig.  3 B), where zeolite 13X is observed.

For the Sap sample (Fig.  4 ), a low ordering material is observed, which seems to agree with the XRD data where the order is relatively low in comparison to other TOT smectites such as montmorillonite. The microwave-assisted acid treatment leads to a material with a different morphology from that of the starting material, confirming the efficiency of the acid treatment for the saponite and obtaining a material with poor crystallinity as detected by XRD (Fig.  2 ). Hydrothermal treatment under basic conditions also modifies the structure of the sample, leading to smaller particles with variable morphology.

In the case of the fibrous clays (Fig.  4 ), a different behavior can be observed after microwave assisted acid treatment since sepiolite is more vulnerable to acid treatment than palygorskite, as previously observed in literature (Cecilia et al 2018b ). After hydrothermal treatment under basic conditions both materials also suffer drastic changes, obtaining small particles, which in the case of the sample from Pal are piled up, while in that from Sep large cavities are noticeable.

The samples were also characterized by ATR. The hydroxyl region of the Kao sample shows four bands (Fig.  5 A): one at 3688 cm −1 , assigned to the in-phase symmetric stretching vibration; two weaker bands located at 3671 and 3652 cm −1 which are attributed to out-of-plane stretching vibration modes; and a last band located at 3620 cm −1 , ascribed to the inner hydroxyl groups located between the tetrahedral and octahedral sheets (Madejova 2003 ). In the region between 1300 and 400 cm −1 (Fig.  5 B), several well-defined and strong bands can be observed between 1150 and 1000 cm −1 , which are assigned to Si–O stretching modes. The bands located between 970 and 850 cm −1 are assigned to Al 2 -OH bending modes of the dioctahedral clays (Madejova 2003 ) while the bands located at 795 and 745 cm −1 are assigned to Si–OH-Al bending modes (Madejova 2003 ). The weak band located between 700 and 600 cm −1 is assigned to trioctahedral minerals. The strong band located at 530 cm −1 is assigned to Si–O-Al (octahedral Al) bending vibration modes (Madejova 2003 ). The microwave-assisted acid treatment causes modification in the bands of Kao-A spectrum, confirming that this acid treatment barely causes modifications in its structure as was observed in the XRD (Figs.  1 and 2 ) and SEM images (Fig.  4 ). The hydrothermal treatment under basic conditions causes a drastic modification of the ATR spectra since the typical bands of hydroxyl groups located between 3800 and 3500 cm −1 disappear. Regarding the 1300–500 cm −1 region, two bands are observed. The main one is shifted to a lower wavenumber value in comparison to the Kao and Kao-A samples, with a maximum at 970 cm −1 assigned to the Si–O stretching vibration mode; while the band with a maximum at ca. 550 cm −1 assigned to the Si–O-Al bending vibration mode (Wang et al. 2019 ).

ATR spectra of kaolinite (Kao), kaolinite subjected to microwave-assisted acid treatment (Kao-A), kaolinite subjected to hydrothermal treatment in basic medium (Kao-b) and kaolinite subjected to microwave-assisted acid treatment and then hydrothermal treatment in basic medium (Kao-A-B)

The study of the Mont sample by ATR in the hydroxyl region (Fig.  6 A) shows a band located around 3625 cm −1 , which is assigned to Al(OH)Al-stretching vibrations (Cecilia et al. 2018a ). Other authors have reported the presence of a band at higher wavenumber value attributed to the existence of pyrophyllite-like local structural fragments (Zviagina et al. 2004 ). The analysis of the 1300–400 cm −1 region (Fig.  6 B) shows a main band located at ca. 1000 cm −1 , which is assigned to the Si–O stretching mode. The bands located at 950–800 cm −1 are assigned to -OH bending bands of the dioctahedral clays (Al 2 OH or Fe 2 OH) (Madejova 2003 ) while the band located at 520 cm −1 is attributed to Si–O-Al bending vibrations (Wang et al. 2019 ). The analysis of the Mont-A spectrum confirms that the acid treatment does not affect the morphology of the clay since the ATR profile is maintained unaltered. After zeolitic treatment, the band related to hydroxyl stretching reactions disappears, while the number of bands located between 1300 and 500 cm −1 diminishes to the Si–O stretching band (965 cm −1 ). In addition, some impurities between 700 and 600 cm −1 appear, which may be ascribed to the Mg 2 OH bending vibration modes (Madejova 2003 ).

ATR spectra of montmorillonite (Mont), montmorillonite subjected to microwave-assisted acid treatment (Mont-A), montmorillonite subjected to hydrothermal treatment in basic conditions (Mont-B) and montmorillonite subjected to microwave-assisted acid treatment and then hydrothermal treatment in basic medium (Mont-A-B). ATR spectra of saponite (Sap), saponite subjected to microwave-assisted acid treatment (Sap-A), saponite subjected to hydrothermal treatment in basic conditions (Sap-B) and saponite subjected to microwave-assisted acid treatment and then hydrothermal treatment in basic medium (Sap-A-B)

The ATR study of the saponite sample (Sap) (Fig.  6 C) shows a band located at about 3675 cm −1 , which can be ascribed to the -OH stretching vibration modes of the Mg(OH) 2 groups located in the octahedral sheets of the smectite (Bisio et al. 2008 ), while the band located at about 3625 cm −1 can be assigned to -OH stretching modes of the Si(OH)Al groups due to the isomorphic substitutions of Si(IV) by Al(III) in its tetrahedral sheet (Cecilia et al. 2018a ; Zviagina et al. 2004 ). Microwave-assisted acid treatment causes a loss of most of the bands attributed to the -OH stretching modes. In fact, only the presence of the band located at about 3625 cm −1 , which is assigned to Si(OH)Al, is striking. Thus, from these data, it can be inferred that Mg-species of the octahedral sheet are leached while the small proportion of Al species located in the octahedral sheet seems to be more resistant to acid treatment. The analysis of the region between 1300 and 500 cm −1 (Fig.  6 D) shows a main band whose maximum is located at 970 cm −1 , which is attributed to the Si–O stretching mode, while the band located at 655 cm −1 is assigned to the Mg 3 OH bending vibration modes (Madejova 2003 ). Acid treatment provokes a shift in the Si–O stretching bands to higher wavenumber values due to the Si–O stretching mode of amorphous silica (Madejova 2003 ). This band is accompanied by another weak band located at about 800 cm −1 , to which the presence of amorphous silica is also attributed (Madejova 2003 ). Regarding the samples treated under hydrothermal conditions in a basic medium, the typical band of the Si–O stretching mode assigned to the presence of an amorphous silica can be observed.

In the case of the raw sepiolite sample, the -OH stretching vibration (Fig.  7 A) shows two peaks located at 3685 and 3625 cm −1 , which are attributed to the stretching vibration modes of the -OH groups coordinated to Mg-species located in the octahedral sheet, while the band located at 3560 cm −1 is related to H 2 O coordinated with Mg-species (Franco et al. 2014 ). The region between 1600 and 500 cm −1 can be divided in two sections according to literature (Fig.  7 B) (Frost et al. 2001 ): the bands between 1230 and 900 cm −1 are assigned to Si–O stretching vibration modes, where the maximum at 976 cm −1 corroborates the presence of Si–O-Mg in the octahedral sheet, while the bands between 700 and 600 cm −1 are attributed to M-OH translation (Frost et al. 2001 ). The study of the Sep samples after the microwave assisted acid treatment, Sep-A, reveals a modification in the profile between 1600 and 500 cm −1 . It can be observed how the band of the Mg-OH band is deformed and the typical band of amorphous silica appears, as already observed in the case of the Sap-A sample. Similarly, the bands ascribed to Mg-OH deformation also disappear. When these samples are hydrothermally treated under basic conditions, the band is ascribed to the formation of amorphous silica. On the other hand, the absence of Al in this clay discards the presence of other bands ascribed Si–O-Al.

ATR spectra of sepiolite (Sep), sepiolite subjected to microwave-assisted acid treatment (Sep-A), sepiolite subjected to hydrothermal treatment in basic conditions (Sep-B) and sepiolite subjected to microwave-assisted acid treatment and then hydrothermal treatment in basic medium (Sep-A-B). ATR spectra of palygorskite (Pal), palygorskite subjected to microwave-assisted acid treatment (Pal-A), palygorskite subjected to hydrothermal treatment in basic conditions and palygorskite subjected to microwave-assisted acid treatment and then hydrothermal treatment in basic medium (Pal-A-B)

Finally, the study of raw Pal by ATR in the -OH stretching region (Fig.  7 C) shows a band at about 3615 cm −1 , which is typical of Al-species located in the octahedral sheet (Suárez and García-Romero 2006 ). Regarding the band located at about 3540 cm −1 , previous authors have pointed out that this band is ascribed to Al–Fe-OH or Al–Mg-OH stretching vibration modes (Frost et al. 2001 ; Suárez and García-Romero 2006 ). In the region between 1600 and 500 cm −1 (Fig.  7 D), three bands, with maxima located at 1187, 1118 and 1015 cm −1 , are detected, which are attributed to periodically inverted Si–O-Si bonds in the tetrahedral sheet (Pardo-Canales et al. 2020 ; Wilson 2014 ). On the other hand, the band with a maximum about 980 cm −1 is assigned to the Si–O-Mg stretching vibration modes, as observed in the Sep sample. The band located about 910 and 880 cm −1 are assigned to Al–OH-Al and Al–OH-Fe (Suárez and García-Romero 2006 ). After the microwave-assisted acid treatment, Pal-A, it is observed how the bands are maintained, although those located in the -OH stretching region, between 3700 and 3500 cm −1 , decrease in intensity. From these data, it can be inferred that the acid treatment only slightly affects the Pal structure, as suggested by XRD (Figs.  1 and 2 ) and SEM (Fig.  4 ). After basic treatment under hydrothermal conditions, most of the bands disappear, confirming that the structure is destroyed after acid treatment. Thus, a main band ascribed to the presence of amorphous silica can be observed. Likewise, the band with a maximum located at 660 cm −1 is attributed to Mg 2 OH bending vibration modes (Madejova 2003 ).

The analysis of the textural properties was carried out from CO 2 adsorption–desorption isotherms (Table  2 ). As already mentioned, since the aim of this study is the use of clays and aluminosilicates obtained by hydrothermal treatment using basic conditions for the CO 2 capture, CO 2 has been used as probe molecule given its eased access to small pores, overcoming N 2 thermodynamic limitations to assess microporosity. The study of the textural properties of raw clays shows how Kao, which displayed a higher order, exhibits the poorest pore volume and surface area. In the case of smectites (Mont and Sap), the surface area and pore volume are greater than those of Kao. Between them, the Sap sample displays a higher surface area and pore volume than the Mont sample due to its lower crystallinity or delamination, which promotes microporosity in its disordered structure. Regarding fibrous clays, the porosity of these materials must be located in their zeolitic channels, although the higher porosity of the Sep sample in comparison to that of Pal must be ascribed to the higher disorder of the Sep sample.

Samples obtained after the microwave-assisted acid treatment generally show a slight increase in microporosity, except for the Kao sample, which showed increased resistance to acid treatment as observed in the XRD data (Fig.  2 ). The improvement in microporosity is more pronounced in the case of the Pal sample. However, those samples subjected to a strong modification in their chemical composition (Sap-A and Sep-A) that eventually lead to a material rich in amorphous silica, hardly improves their microporosity. After the hydrothermal treatment in basic conditions of all samples, it can be observed how the textural properties only improve when Kao and Mont samples are used as starting materials, either in their raw or in their acid treatment-modified form, obtaining the best textural properties for the Mont sample treated through microwave-assisted acid treatment and then by hydrothermal treatment under basic conditions (Mont-A-B), which attained a micropore volume of 0.244 cm 3 /g and an equivalent surface area of 610 m 2 /g.

Considering that the zeolites were only formed from the Kao and Mont samples, these samples were selected to be studied by 27 Al and 29 Si MAS ssNMR (Fig.  8 ). The analysis of the 29 Si MAS ssNMR spectrum of the raw Kao (Fig.  8 A) shows two signals located about − 90.8 and − 91.7 ppm. These signals are attributed to the silicate species located in the tetrahedral sheet. In the case of the raw Mont sample (Fig.  8 C), the most intense signal located at − 93.9 ppm is also attributed to similar tetrahedral species interconnected with each other (Cecilia et al. 2022 ). Another weak band located at about − 107 ppm is also noteworthy, which could be ascribed to silicon bound in amorphous silica with a three-dimensional shape because of a small impurity of quartz (Breen et al. 1995 ). The analysis of the 27 Al MAS ssNMR of the raw Kao sample (Fig.  8 B) shows only one peak with a maximum at about 4 ppm typical of Al-species with octahedral coordination (Cecilia et al. 2018a ; Hatakeyama et al. 2011 ). Interestingly, in the case of the Mont sample (Fig.  8 D), this signal can be observed together with another one with a maximum at about 60 ppm, characteristic of Al-species with tetrahedral coordination (Hatakeyama et al. 2011 ). From these spectra, the presence of Al-pentacoordinated can be ruled out (Hatakeyama et al. 2011 ).

29 Si NMR ( A ) and 27 Al NMR ( B ) for the raw kaolinite and the kaolinite subjected to microwave-assisted acid treatment and then hydrothermal treatment in basic conditions. 29 Si NMR ( C ) and 27 Al NMR ( D ) for the raw montmorillonite and the montmorillonite subjected to microwave-assisted acid treatment and then hydrothermal treatment in basic conditions

Hydrothermal treatment under acid and basic conditions causes a slight shift in the 29 Si MAS NMR spectrum for the Kao-A-B sample to 89.7 ppm, attributed to Q 4 sites (Si(4Al)) (Fig.  8 A), in Zeolite A type materials (Shi et al. 1996 ). In this sample, contributions at higher and lower fields can also be elucidated, which indicates a change in the environment of the Si-species due to the incorporation of aluminum into the structure with a tetrahedral coordination (Kirkpatrick 1988 ). This fact is also corroborated by the 27 Al signal, where only tetrahedral species are present (Fig.  8 B).

In the case of the Mont-A-B sample (Fig.  8 C), the 29 Si MAS NMR spectrum is more complex since three main bands are observed at − 99.0, − 94.0, − 89.1 and − 84.8 ppm, which are assigned to Q 1 (Si(1Al)), Q 2 (Si(2Al)), Q 3 (Si(3Al)) and Q 4 (Si(4Al)) environments, respectively (Kirkpatrick 1988 ). In the case of the 27 Al MAS NMR spectra, a band located at about 61 ppm is observed (Fig.  8 D), confirming the presence of Al-species with tetrahedral coordination (Cecilia et al. 2018a ).

Adsorption studies

Once the clays and their respective materials synthesized from hydrothermal conditions in a basic medium had been characterized, the next step was the study of their CO 2 adsorption capacity.

In the case of raw clay, the CO 2 -adsorption isotherms compiled in Fig.  9 show how the adsorption is quite linear, which suggests a weaker interaction than other adsorbents whose pore diameter is narrower. Also, it can be observed that samples with higher crystallinity and poorer textural properties, i.e. the Kao sample, display the lowest CO 2 adsorption capacity, achieving a maximum value of 0.05 mmol/g at 25 °C and 760 mm of Hg. Between the smectites (Mont and Sap), the highest adsorption capacity is observed for the Sap sample. Considering that only the surface is responsible for the adsorption capacity of the materials since previous studies have discarded adsorption in the interlayer spacing (Chouikhi et al. 2021 ), the higher CO 2 -adsorption capacity takes place for the Sap sample, since this material shows less crystallinity in such a way that the CO 2 molecules can be retained in the smaller pores of its disordered structure, which could resemble a house of cards structure. In fact, this low crystallinity of the Sap sample favors CO 2 adsorption, a with an adsorption value of 0.49 mmol/g, while the Mont sample only reaches 0.16 mmol/g at 25 °C and 760 mm of Hg. As for the fibrous clays (Sep and Pal samples), a similar trend to that observed with smectites is observed, since the Sep sample, which displays less crystallinity, reaches a higher adsorption capacity. According to the morphology of fibrous clays where the tetrahedral sheet is periodically inverted, nanocavities are formed, thus it is expected that these microcavities could host a higher amount of CO 2 molecules (Cecilia et al. 2018b ; Suárez and García-Romero 2006 ). However, these fibrous materials show a low CO 2 adsorption capacity to be competitive, since the Sep sample attains an adsorption capacity of 0.53 mmol/g while Pal, which showed a higher ordering by XRD, only achieves a CO 2 adsorption capacity of 0.33 mmol/g at 25 °C and 760 mm of Hg. The isotherms were fitted to the Toth model as shown in Table  3 . One of the most striking data obtained from the Toth equation is related to the b parameter, which defines the strength of the interaction between adsorbate and adsorbent, that is, the interaction between CO 2 molecules and the clay. These data reveal that those materials with higher adsorption capacity and higher disorder in the clay structure also promote a stronger interaction CO 2 -clay, as shown by the b parameter. The t parameter defines the heterogeneity of the adsorbent. The data reported in Table  3 indicates that the adsorbents with more homogeneous adsorption sites are the fibrous clays probably due to the existence of nanocavities where CO 2 adsorption must take place, as previously reported in literature (Cecilia et al. 2018b ).

CO 2 adsorption isotherms at 25 °C of the raw phyllosilicates

The CO 2 -adsorption capacity of clays after microwave-assisted acid treatment hardly improves their data (Fig.  10 ). Thus, both the Kao-A and the Mont-A samples maintain similar values to those observed before treatment (Fig.  9 ). These data agree with the characterization results, since these clays hardly suffer any modification after the acid treatment. In the case of the clays most prone to undergo modifications by acid treatment (Sap and Sep), it can be observed how the Sap-A sample significantly improves the CO 2 adsorption capacity due to the formation of an amorphous material with higher porosity (Cecilia et al. 2018a ), reaching a value of 0.82 mmol/g at 25 °C and 760 mm of Hg. In contrast, the Sep sample only slightly improves adsorption. In this sense, the collapse of the microchannels by the leaching of Mg-species must be the cause of this small improvement in the CO 2 adsorption capacity. However, the Pal-A sample improves the CO 2 adsorption capacity in comparison to the Pal sample (0.82 mmol/g and 0.49 mmol/g, respectively) probably because Pal is more resistant to acid treatment, therefore its fibrous structure does not collapse under this treatment. Regarding the fitting with the Toth model (Table  4 ), the b parameter is in the same range as that of raw clays, showing a poor affinity with CO 2 molecules. The analysis of the t parameter is far from the unity, indicating that the adsorption process is not homogeneous but that there are preferential CO 2 adsorption sites. In fact, phyllosilicates modified by microwave-assisted acid treatment display slightly higher values than those observed for raw clays probably due to partial leaching of the octahedral sheet after the acid treatment. This treatment is more effective in the case of saponite (Sap-A), which could be related to an increase in microporosity and the formation of a greater proportion of adsorption centers to capture CO 2 .

CO 2 adsorption isotherms at 25 °C of the raw phyllosilicates subjected to microwave-assisted acid treatment

The study of the CO 2 adsorption capacity of clay minerals after hydrothermal treatment under basic conditions (Fig.  11 ) causes a notable increase in the adsorption capacity of those materials synthesized from kaolinite and montmorillonite (samples Kao-B and Mont-B). These materials showed structures with high crystallinity, forming well-described zeolites such as 4A in the case of Kao-B and 13X for Mont-B. It is well known that both zeolites display narrow pores, which allows them to host a higher proportion of CO 2 molecules, as observed by previous authors when synthesizing zeolites from other Si- and Al- sources (Barrer 1981 ). Thus, these samples reach a remarkably higher adsorption than that observed for their respective clays, achieving an adsorption capacity of 3.00 mmol/g for Kao-B while Mont-B shows an CO 2 -adsorption of 2.55 mmol/g at 25 °C and 760 mm of Hg.

CO 2 adsorption isotherms at 25 °C of the raw phyllosilicates subjected to hydrothermal in basic conditions

In the case of Sap-B, Sep-B and Pal-B, the adsorption capacity is negligible in comparison to those of the Kao-B and Mont-B samples, so the formation of aluminosilicate with high crystallinity and narrow pore size is necessary to attain high CO 2 -adsorption capacity. For amorphous aluminosilicates, the polymerization of the aluminate and silicate species takes place in an uncontrolled manner, giving rise to aluminosilicate structures with undefined porosity and crystallinity, therefore with low microporosity and poor CO 2 -adsorption capacity. Moreover, the fitting of these isotherms to the Toth model (Table  5 ) shows that those aluminosilicates with defined crystallinity (Kao-B and Mont-B) have a higher affinity for the adsorbate, i.e. CO 2 molecules, as indicated by their higher values of the parameter b. This trend is also observed for the t parameter since those adsorbents with higher microporosity and greater interaction with the adsorbate are also the one with the most homogeneous sites in which CO 2 is preferentially adsorbent in the nanocages formed after the ordered assembly of the zeolites.

The analysis of the CO 2 adsorption capacity for those samples subjected to an acid treatment and then a hydrothermal treatment under basic conditions (Fig.  12 ) reveals that it improves the CO 2 capacity in the case of montmorillonite (Mont-A-B), obtaining a value of 3.87 mmol of CO 2 /g at 25 °C and 760 mm of Hg, while Kao-A-B only slightly improves the CO 2 -adsorption of the Kao-B sample. In this sense, the Mont sample showed a small proportion of impurities, which are removed after acid treatment. These impurities are insoluble in the basic media used for the synthesis of zeolites, so their presence has an adverse effect on their formation. After the acid treatment, these species that interfere with the synthesis of the aluminosilicates are removed, significantly improving the crystallinity of the obtained zeolite.

CO 2 adsorption isotherms at 25 °C of the raw phyllosilicates subjected to microwave-assisted acid treatment and then hydrothermal in basic conditions

In the case of the kaolinite sample, it seems that the presence of impurities is lower, so the inclusion of an additional stage, such as the acid treatment, has a less decisive role in the zeolite synthesis than in the case of montmorillonite. For Sap-A-B, Sep-A-B and Pal-A-B, the CO 2 adsorption values are much lower than in the other synthesized materials. In this sense, the removal of Mg-species by leaching through microwave-assisted acid treatment should avoid the presence of unwanted species in the synthesis of crystalline aluminosilicates. However, the absence or low contents of Al species prevent the formation of crystalline zeolites, at least under these synthetic conditions. This implies the formation of poorly ordered structures with low microporosity and therefore low CO 2 capture capacity. The analysis of the adsorption isotherms according to the Toth model (Table  6 ) shows how those materials with higher adsorption capacity (Kao-A-B and Mont-A-B) also display a greater affinity for CO 2 molecules. The analysis of the t parameter also indicates that the highly crystalline zeolites also display more homogeneous adsorption sites due to the formation of more ordered microporous adsorbents.

Conclusions

Several phyllosilicates (Kao, Mont, Sap, Sep and Pal) have been selected as starting materials to try the synthesis of zeolites and their application in CO 2 capture.

The analysis of the raw materials displays a poor adsorption capacity even in the case of the fibrous phyllosilicates where the presence of microcavities in their structure should promote higher adsorption capacity than Kao, Mont or Sap samples. Thus, the highest adsorption capacity for the starting materials was only of 0.53 mmol/g at 25 °C and a pressure of 760 mm of Hg for the Sep sample.

To improve the adsorption capacity, the phyllosilicates were subjected to a microwave-assisted acid treatment, where the trioctahedral clays, i.e. Sap and Sep, are more prone to suffer modification in its structure by the leaching of the Mg-species located in the octahedral sheet. Despite the improvement of the microporosity of the clays, the CO 2 adsorption capacity is relatively low, reaching a maximum value of 0.82 mmol/g at 25 °C and 760 mm of Hg for Sap-A and Pal-A samples.

All these materials were subjected to a hydrothermal treatment in a basic medium to try the synthesis of zeolites with high microporosity. Among them, Kao, Mont and their respective materials subjected to acid treatment (Kao-A and Mont-A) were the only starting materials that gave rise to zeolites with high crystallinity. Specifically, the kaolinite-based materials gave rise to zeolite 4A while montmorillonite-based materials formed zeolite 13X. In both cases, the obtained zeolites are highly microporous, achieving a maximum CO 2 adsorption capacity of 3.85 mmol/g at 25 °C and 760 mm of Hg for Mont-A-B sample. On the other hand, the aluminosilicates synthesized from Sap, Sep or Pal display poor CO 2 adsorption capacity after the hydrothermal treatment under basic conditions. The low adsorption capacity is attributed to the high Si/Al molar ratio, which is not appropriate for a good assembly of the aluminate and silicate species to form crystalline zeolites with high microporosity.

Data availability

The authors confirm that all data generated or analyzed during this study are available from the corresponding author. These materials can be requested directly from the corresponding author if needed.

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Acknowledgements

The authors thank the Servicios Centrales de Apoyo a la Investication (SCAI) for its availability.

Funding for open access publishing: Universidad Málaga/CBUA This research was funded by the Spanish Ministry of Science and Innovation, project PID2021- 126235OB-C32, funded by MCIN/AEI/ https://doi.org/10.13039/501100011033 / and FEDER funds.

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Salima Essih, Daniel Ballesteros-Plata, Isabel Barroso-Martín, Antonia Infantes-Molina, Enrique Rodríguez-Castellón, Francisco Franco & Juan Antonio Cecilia

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S.E.: Formal analysis and experimentation, E.V.G.: Validation and data curation; D.C.S. A.: Supervision and Project administration, D.B.P.: Formal analysis and data curation: I.B.M.: Formal analysis and data curation, A.I.M.: Data curation, writing—review & editing, project administration and funding acquisition; E.R.C.: writing—review & editing, project administration and funding acquisition; F.F.: Data curation, writing—review & editing and visualization; J.A.C.: Conceptualization, methodology, validation, data curation, writing—original draft, writing—review & editing.

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Essih, S., Vilarrasa-García, E., Azevedo, D.C.S. et al. Zeolites synthesis from phyllosilicates and their performance for CO 2 adsorption. Environ Sci Pollut Res (2024). https://doi.org/10.1007/s11356-024-33685-0

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DOI : https://doi.org/10.1007/s11356-024-33685-0

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