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Literature Syntheis 101

How To Synthesise The Existing Research (With Examples)

By: Derek Jansen (MBA) | Expert Reviewer: Eunice Rautenbach (DTech) | August 2023

One of the most common mistakes that students make when writing a literature review is that they err on the side of describing the existing literature rather than providing a critical synthesis of it. In this post, we’ll unpack what exactly synthesis means and show you how to craft a strong literature synthesis using practical examples.

This post is based on our popular online course, Literature Review Bootcamp . In the course, we walk you through the full process of developing a literature review, step by step. If it’s your first time writing a literature review, you definitely want to use this link to get 50% off the course (limited-time offer).

Overview: Literature Synthesis

• What exactly does “synthesis” mean?
• Aspect 1: Agreement
• Aspect 2: Disagreement
• Aspect 3: Key theories
• Aspect 4: Contexts
• Aspect 5: Methodologies
• Bringing it all together

What does “synthesis” actually mean?

As a starting point, let’s quickly define what exactly we mean when we use the term “synthesis” within the context of a literature review.

Simply put, literature synthesis means going beyond just describing what everyone has said and found. Instead, synthesis is about bringing together all the information from various sources to present a cohesive assessment of the current state of knowledge in relation to your study’s research aims and questions .

Put another way, a good synthesis tells the reader exactly where the current research is “at” in terms of the topic you’re interested in – specifically, what’s known , what’s not , and where there’s a need for more research .

So, how do you go about doing this?

Well, there’s no “one right way” when it comes to literature synthesis, but we’ve found that it’s particularly useful to ask yourself five key questions when you’re working on your literature review. Having done so,  you can then address them more articulately within your actual write up. So, let’s take a look at each of these questions.

1. Points Of Agreement

The first question that you need to ask yourself is: “Overall, what things seem to be agreed upon by the vast majority of the literature?”

For example, if your research aim is to identify which factors contribute toward job satisfaction, you’ll need to identify which factors are broadly agreed upon and “settled” within the literature. Naturally, there may at times be some lone contrarian that has a radical viewpoint , but, provided that the vast majority of researchers are in agreement, you can put these random outliers to the side. That is, of course, unless your research aims to explore a contrarian viewpoint and there’s a clear justification for doing so. 

Identifying what’s broadly agreed upon is an essential starting point for synthesising the literature, because you generally don’t want (or need) to reinvent the wheel or run down a road investigating something that is already well established . So, addressing this question first lays a foundation of “settled” knowledge.

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2. Points Of Disagreement

Related to the previous point, but on the other end of the spectrum, is the equally important question: “Where do the disagreements lie?” .

In other words, which things are not well agreed upon by current researchers? It’s important to clarify here that by disagreement, we don’t mean that researchers are (necessarily) fighting over it – just that there are relatively mixed findings within the empirical research , with no firm consensus amongst researchers.

This is a really important question to address as these “disagreements” will often set the stage for the research gap(s). In other words, they provide clues regarding potential opportunities for further research, which your study can then (hopefully) contribute toward filling. If you’re not familiar with the concept of a research gap, be sure to check out our explainer video covering exactly that .

3. Key Theories

The next question you need to ask yourself is: “Which key theories seem to be coming up repeatedly?” .

Within most research spaces, you’ll find that you keep running into a handful of key theories that are referred to over and over again. Apart from identifying these theories, you’ll also need to think about how they’re connected to each other. Specifically, you need to ask yourself:

• Are they all covering the same ground or do they have different focal points  or underlying assumptions ?
• Do some of them feed into each other and if so, is there an opportunity to integrate them into a more cohesive theory?
• Do some of them pull in different directions ? If so, why might this be?
• Do all of the theories define the key concepts and variables in the same way, or is there some disconnect? If so, what’s the impact of this ?

Simply put, you’ll need to pay careful attention to the key theories in your research area, as they will need to feature within your theoretical framework , which will form a critical component within your final literature review. This will set the foundation for your entire study, so it’s essential that you be critical in this area of your literature synthesis.

If this sounds a bit fluffy, don’t worry. We deep dive into the theoretical framework (as well as the conceptual framework) and look at practical examples in Literature Review Bootcamp . If you’d like to learn more, take advantage of our limited-time offer to get 60% off the standard price.

4. Contexts

The next question that you need to address in your literature synthesis is an important one, and that is: “Which contexts have (and have not) been covered by the existing research?” .

For example, sticking with our earlier hypothetical topic (factors that impact job satisfaction), you may find that most of the research has focused on white-collar , management-level staff within a primarily Western context, but little has been done on blue-collar workers in an Eastern context. Given the significant socio-cultural differences between these two groups, this is an important observation, as it could present a contextual research gap .

In practical terms, this means that you’ll need to carefully assess the context of each piece of literature that you’re engaging with, especially the empirical research (i.e., studies that have collected and analysed real-world data). Ideally, you should keep notes regarding the context of each study in some sort of catalogue or sheet, so that you can easily make sense of this before you start the writing phase. If you’d like, our free literature catalogue worksheet is a great tool for this task.

5. Methodological Approaches

Last but certainly not least, you need to ask yourself the question: “What types of research methodologies have (and haven’t) been used?”

For example, you might find that most studies have approached the topic using qualitative methods such as interviews and thematic analysis. Alternatively, you might find that most studies have used quantitative methods such as online surveys and statistical analysis.

But why does this matter?

Well, it can run in one of two potential directions . If you find that the vast majority of studies use a specific methodological approach, this could provide you with a firm foundation on which to base your own study’s methodology . In other words, you can use the methodologies of similar studies to inform (and justify) your own study’s research design .

On the other hand, you might argue that the lack of diverse methodological approaches presents a research gap , and therefore your study could contribute toward filling that gap by taking a different approach. For example, taking a qualitative approach to a research area that is typically approached quantitatively. Of course, if you’re going to go against the methodological grain, you’ll need to provide a strong justification for why your proposed approach makes sense. Nevertheless, it is something worth at least considering.

Regardless of which route you opt for, you need to pay careful attention to the methodologies used in the relevant studies and provide at least some discussion about this in your write-up. Again, it’s useful to keep track of this on some sort of spreadsheet or catalogue as you digest each article, so consider grabbing a copy of our free literature catalogue if you don’t have anything in place.

Bringing It All Together

Alright, so we’ve looked at five important questions that you need to ask (and answer) to help you develop a strong synthesis within your literature review.  To recap, these are:

• Which things are broadly agreed upon within the current research?
• Which things are the subject of disagreement (or at least, present mixed findings)?
• Which theories seem to be central to your research topic and how do they relate or compare to each other?
• Which contexts have (and haven’t) been covered?
• Which methodological approaches are most common?

Importantly, you’re not just asking yourself these questions for the sake of asking them – they’re not just a reflection exercise. You need to weave your answers to them into your actual literature review when you write it up. How exactly you do this will vary from project to project depending on the structure you opt for, but you’ll still need to address them within your literature review, whichever route you go.

The best approach is to spend some time actually writing out your answers to these questions, as opposed to just thinking about them in your head. Putting your thoughts onto paper really helps you flesh out your thinking . As you do this, don’t just write down the answers – instead, think about what they mean in terms of the research gap you’ll present , as well as the methodological approach you’ll take . Your literature synthesis needs to lay the groundwork for these two things, so it’s essential that you link all of it together in your mind, and of course, on paper.

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• Synthesizing Sources | Examples & Synthesis Matrix

Synthesizing Sources | Examples & Synthesis Matrix

Published on July 4, 2022 by Eoghan Ryan . Revised on May 31, 2023.

Synthesizing sources involves combining the work of other scholars to provide new insights. It’s a way of integrating sources that helps situate your work in relation to existing research.

Synthesizing sources involves more than just summarizing . You must emphasize how each source contributes to current debates, highlighting points of (dis)agreement and putting the sources in conversation with each other.

You might synthesize sources in your literature review to give an overview of the field or throughout your research paper when you want to position your work in relation to existing research.

Table of contents

Example of synthesizing sources, how to synthesize sources, synthesis matrix, other interesting articles, frequently asked questions about synthesizing sources.

Let’s take a look at an example where sources are not properly synthesized, and then see what can be done to improve it.

This paragraph provides no context for the information and does not explain the relationships between the sources described. It also doesn’t analyze the sources or consider gaps in existing research.

Research on the barriers to second language acquisition has primarily focused on age-related difficulties. Building on Lenneberg’s (1967) theory of a critical period of language acquisition, Johnson and Newport (1988) tested Lenneberg’s idea in the context of second language acquisition. Their research seemed to confirm that young learners acquire a second language more easily than older learners. Recent research has considered other potential barriers to language acquisition. Schepens, van Hout, and van der Slik (2022) have revealed that the difficulties of learning a second language at an older age are compounded by dissimilarity between a learner’s first language and the language they aim to acquire. Further research needs to be carried out to determine whether the difficulty faced by adult monoglot speakers is also faced by adults who acquired a second language during the “critical period.”

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To synthesize sources, group them around a specific theme or point of contention.

As you read sources, ask:

• What questions or ideas recur? Do the sources focus on the same points, or do they look at the issue from different angles?
• How does each source relate to others? Does it confirm or challenge the findings of past research?
• Where do the sources agree or disagree?

Once you have a clear idea of how each source positions itself, put them in conversation with each other. Analyze and interpret their points of agreement and disagreement. This displays the relationships among sources and creates a sense of coherence.

Consider both implicit and explicit (dis)agreements. Whether one source specifically refutes another or just happens to come to different conclusions without specifically engaging with it, you can mention it in your synthesis either way.

Synthesize your sources using:

• Topic sentences to introduce the relationship between the sources
• Signal phrases to attribute ideas to their authors
• Transition words and phrases to link together different ideas

To more easily determine the similarities and dissimilarities among your sources, you can create a visual representation of their main ideas with a synthesis matrix . This is a tool that you can use when researching and writing your paper, not a part of the final text.

In a synthesis matrix, each column represents one source, and each row represents a common theme or idea among the sources. In the relevant rows, fill in a short summary of how the source treats each theme or topic.

This helps you to clearly see the commonalities or points of divergence among your sources. You can then synthesize these sources in your work by explaining their relationship.

If you want to know more about ChatGPT, AI tools , citation , and plagiarism , make sure to check out some of our other articles with explanations and examples.

• ChatGPT vs human editor
• ChatGPT citations
• Is ChatGPT trustworthy?
• Using ChatGPT for your studies
• What is ChatGPT?
• Chicago style
• Paraphrasing

 Plagiarism

• Types of plagiarism
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• Consequences of plagiarism
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Synthesizing sources means comparing and contrasting the work of other scholars to provide new insights.

It involves analyzing and interpreting the points of agreement and disagreement among sources.

You might synthesize sources in your literature review to give an overview of the field of research or throughout your paper when you want to contribute something new to existing research.

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a thesis, dissertation , or research paper , in order to situate your work in relation to existing knowledge.

Topic sentences help keep your writing focused and guide the reader through your argument.

In an essay or paper , each paragraph should focus on a single idea. By stating the main idea in the topic sentence, you clarify what the paragraph is about for both yourself and your reader.

At college level, you must properly cite your sources in all essays , research papers , and other academic texts (except exams and in-class exercises).

Add a citation whenever you quote , paraphrase , or summarize information or ideas from a source. You should also give full source details in a bibliography or reference list at the end of your text.

The exact format of your citations depends on which citation style you are instructed to use. The most common styles are APA , MLA , and Chicago .

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Get Organized

• Lit Review Prep Use this template to help you evaluate your sources, create article summaries for an annotated bibliography, and a synthesis matrix for your lit review outline.

Synthesize your Information

Synthesize: combine separate elements to form a whole.

Synthesis Matrix

A synthesis matrix helps you record the main points of each source and document how sources relate to each other.

After summarizing and evaluating your sources, arrange them in a matrix or use a citation manager to help you see how they relate to each other and apply to each of your themes or variables.  

By arranging your sources by theme or variable, you can see how your sources relate to each other, and can start thinking about how you weave them together to create a narrative.

• Step-by-Step Approach
• Example Matrix from NSCU
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How to Write a Literature Review

• 6. Synthesize
• Literature Reviews: A Recap
• Reading Journal Articles
• Does it Describe a Literature Review?
• 1. Identify the Question
• 2. Review Discipline Styles
• Searching Article Databases
• Finding Full-Text of an Article
• Citation Chaining
• When to Stop Searching
• 4. Manage Your References
• 5. Critically Analyze and Evaluate

Synthesis Visualization

Synthesis matrix example.

• 7. Write a Literature Review

• Synthesis Worksheet

About Synthesis

Approaches to synthesis.

You can sort the literature in various ways, for example:

How to Begin?

Read your sources carefully and find the main idea(s) of each source

Look for similarities in your sources – which sources are talking about the same main ideas? (for example, sources that discuss the historical background on your topic)

Use the worksheet (above) or synthesis matrix (below) to get organized

This work can be messy. Don't worry if you have to go through a few iterations of the worksheet or matrix as you work on your lit review!

Four Examples of Student Writing

In the four examples below, only ONE shows a good example of synthesis: the fourth column, or  Student D . For a web accessible version, click the link below the image.

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• Download a copy of the "Four Examples of Student Writing" chart

Click on the example to view the pdf.

From Jennifer Lim

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Chapter 7: Synthesizing Sources

Learning objectives.

At the conclusion of this chapter, you will be able to:

• synthesize key sources connecting them with the research question and topic area.

7.1 Overview of synthesizing

7.1.1 putting the pieces together.

Combining separate elements into a whole is the dictionary definition of synthesis.  It is a way to make connections among and between numerous and varied source materials.  A literature review is not an annotated bibliography, organized by title, author, or date of publication.  Rather, it is grouped by topic to create a whole view of the literature relevant to your research question.

Your synthesis must demonstrate a critical analysis of the papers you collected as well as your ability to integrate the results of your analysis into your own literature review.  Each paper collected should be critically evaluated and weighed for “adequacy, appropriateness, and thoroughness” ( Garrard, 2017 ) before inclusion in your own review.  Papers that do not meet this criteria likely should not be included in your literature review.

Begin the synthesis process by creating a grid, table, or an outline where you will summarize, using common themes you have identified and the sources you have found. The summary grid or outline will help you compare and contrast the themes so you can see the relationships among them as well as areas where you may need to do more searching. Whichever method you choose, this type of organization will help you to both understand the information you find and structure the writing of your review.  Remember, although “the means of summarizing can vary, the key at this point is to make sure you understand what you’ve found and how it relates to your topic and research question” ( Bennard et al., 2014 ).

As you read through the material you gather, look for common themes as they may provide the structure for your literature review.  And, remember, research is an iterative process: it is not unusual to go back and search information sources for more material.

At one extreme, if you are claiming, ‘There are no prior publications on this topic,’ it is more likely that you have not found them yet and may need to broaden your search.  At another extreme, writing a complete literature review can be difficult with a well-trod topic.  Do not cite it all; instead cite what is most relevant.  If that still leaves too much to include, be sure to reference influential sources…as well as high-quality work that clearly connects to the points you make. ( Klingner, Scanlon, & Pressley, 2005 ).

7.2 Creating a summary table

Literature reviews can be organized sequentially or by topic, theme, method, results, theory, or argument.  It’s important to develop categories that are meaningful and relevant to your research question.  Take detailed notes on each article and use a consistent format for capturing all the information each article provides.  These notes and the summary table can be done manually, using note cards.  However, given the amount of information you will be recording, an electronic file created in a word processing or spreadsheet is more manageable. Examples of fields you may want to capture in your notes include:

• Authors’ names
• Article title
• Publication year
• Main purpose of the article
• Methodology or research design
• Participants
• Measurement
• Conclusions

  Other fields that will be useful when you begin to synthesize the sum total of your research:

• Specific details of the article or research that are especially relevant to your study
• Key terms and definitions
• Strengths or weaknesses in research design
• Relationships to other studies
• Possible gaps in the research or literature (for example, many research articles conclude with the statement “more research is needed in this area”)
• Finally, note how closely each article relates to your topic.  You may want to rank these as high, medium, or low relevance.  For papers that you decide not to include, you may want to note your reasoning for exclusion, such as ‘small sample size’, ‘local case study,’ or ‘lacks evidence to support assertion.’

This short video demonstrates how a nursing researcher might create a summary table.

7.2.1 Creating a Summary Table

  Summary tables can be organized by author or by theme, for example:

For a summary table template, see http://blogs.monm.edu/writingatmc/files/2013/04/Synthesis-Matrix-Template.pdf

7.3 Creating a summary outline

An alternate way to organize your articles for synthesis it to create an outline. After you have collected the articles you intend to use (and have put aside the ones you won’t be using), it’s time to identify the conclusions that can be drawn from the articles as a group.

  Based on your review of the collected articles, group them by categories.  You may wish to further organize them by topic and then chronologically or alphabetically by author.  For each topic or subtopic you identified during your critical analysis of the paper, determine what those papers have in common.  Likewise, determine which ones in the group differ.  If there are contradictory findings, you may be able to identify methodological or theoretical differences that could account for the contradiction (for example, differences in population demographics).  Determine what general conclusions you can report about the topic or subtopic as the entire group of studies relate to it.  For example, you may have several studies that agree on outcome, such as ‘hands on learning is best for science in elementary school’ or that ‘continuing education is the best method for updating nursing certification.’ In that case, you may want to organize by methodology used in the studies rather than by outcome.

Organize your outline in a logical order and prepare to write the first draft of your literature review.  That order might be from broad to more specific, or it may be sequential or chronological, going from foundational literature to more current.  Remember, “an effective literature review need not denote the entire historical record, but rather establish the raison d’etre for the current study and in doing so cite that literature distinctly pertinent for theoretical, methodological, or empirical reasons.” ( Milardo, 2015, p. 22 ).

As you organize the summarized documents into a logical structure, you are also appraising and synthesizing complex information from multiple sources.  Your literature review is the result of your research that synthesizes new and old information and creates new knowledge.

7.4 Additional resources:

Literature Reviews: Using a Matrix to Organize Research / Saint Mary’s University of Minnesota

Literature Review: Synthesizing Multiple Sources / Indiana University

Writing a Literature Review and Using a Synthesis Matrix / Florida International University

 Sample Literature Reviews Grid / Complied by Lindsay Roberts

Select three or four articles on a single topic of interest to you. Then enter them into an outline or table in the categories you feel are important to a research question. Try both the grid and the outline if you can to see which suits you better. The attached grid contains the fields suggested in the video .

Literature Review Table  

Test yourself.

• Select two articles from your own summary table or outline and write a paragraph explaining how and why the sources relate to each other and your review of the literature.
• In your literature review, under what topic or subtopic will you place the paragraph you just wrote?

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Literature Reviews for Education and Nursing Graduate Students Copyright © by Linda Frederiksen is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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Literature Review How To

• Things To Consider
• Synthesizing Sources
• Video Tutorials
• Books On Literature Reviews

What is Synthesis

What is Synthesis? Synthesis writing is a form of analysis related to comparison and contrast, classification and division. On a basic level, synthesis requires the writer to pull together two or more summaries, looking for themes in each text. In synthesis, you search for the links between various materials in order to make your point. Most advanced academic writing, including literature reviews, relies heavily on synthesis. (Temple University Writing Center)  

How To Synthesize Sources in a Literature Review

Literature reviews synthesize large amounts of information and present it in a coherent, organized fashion. In a literature review you will be combining material from several texts to create a new text – your literature review.

You will use common points among the sources you have gathered to help you synthesize the material. This will help ensure that your literature review is organized by subtopic, not by source. This means various authors' names can appear and reappear throughout the literature review, and each paragraph will mention several different authors. 

When you shift from writing summaries of the content of a source to synthesizing content from sources, there is a number things you must keep in mind: 

• Look for specific connections and or links between your sources and how those relate to your thesis or question.
• When writing and organizing your literature review be aware that your readers need to understand how and why the information from the different sources overlap.
• Organize your literature review by the themes you find within your sources or themes you have identified. 
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Synthesising the literature as part of a literature review

Affiliation.

• 1 University of Manchester, England.
• PMID: 25783281
• DOI: 10.7748/ns.29.29.44.e8957

This article examines how to synthesise and critique research literature. To place the process of synthesising the research literature into context, the article explores the critiquing process by breaking it down into seven sequential steps. The article explains how and why these steps need to be kept in mind if a robust comprehensive literature search and analysis are to be achieved. The article outlines how to engage in the critiquing process and explains how the literature review needs to be assembled to generate a logical and reasoned debate to examine a topic of interest or research in more detail.

Keywords: Critical analysis; critique; evaluation; integrative review; literature review; literature search; research; research question; search strategy; synthesis.

• Research / standards*
• Research Design*
• Review Literature as Topic*

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3.2 Synthesizing literature

Learning objectives.

• Connect the sources you read with key concepts in your research question and proposal
• Systematize the information and facts from each source you read

Putting the pieces together

Combining separate elements into a whole is the dictionary definition of synthesis. It is a way to make connections among and between numerous and varied source materials. A literature review is not an annotated bibliography, organized by title, author, or date of publication. Rather, it is grouped by topic and argument to create a whole view of the literature relevant to your research question.

Your synthesis must demonstrate a critical analysis of the papers you collected, as well as your ability to integrate the results of your analysis into your own literature review. Each source you collect should be critically evaluated and weighed based on the criteria from Chapter 2 before you include it in your review.

Begin the synthesis process by creating a grid, table, or an outline where you will summarize your literature review findings, using common themes you have identified and the sources you have found. The summary, grid, or outline will help you compare and contrast the themes, so you can see the relationships among them as well as areas where you may need to do more searching. A basic summary table is provided in Figure 3.2. Whichever method you choose, this type of organization will help you to both understand the information you find and structure the writing of your review. Remember, although “the means of summarizing can vary, the key at this point is to make sure you understand what you’ve found and how it relates to your topic and research question” (Bennard et al., 2014, para. 10).

As you read through the material you gather, look for common themes as they may provide the structure for your literature review. And, remember, writing a literature review is an iterative process. It is not unusual to go back and search academic databases for more sources of information as you read the articles you’ve collected.

Literature reviews can be organized sequentially or by topic, theme, method, results, theory, or argument. It’s important to develop categories that are meaningful and relevant to your research question. Take detailed notes on each article and use a consistent format for capturing all the information each article provides. These notes and the summary table can be done manually using note cards. However, given the amount of information you will be recording, an electronic file created in a word processing or spreadsheet (like this example Literature Search Template ) is more manageable. Examples of fields you may want to capture in your notes include:

• Authors’ names
• Article title
• Publication year
• Main purpose of the article
• Methodology or research design
• Participants
• Measurement
• Conclusions

Other fields that will be useful when you begin to synthesize the sum total of your research:

• Specific details of the article or research that are especially relevant to your study
• Key terms and definitions
• Strengths or weaknesses in research design
• Relationships to other studies
• Possible gaps in the research or literature (for example, many research articles conclude with the statement “more research is needed in this area”)
• Finally, note how closely each article relates to your topic. You may want to rank these as high, medium, or low relevance. For papers that you decide not to include, you may want to note your reasoning for exclusion, such as small sample size, local case study, or lacks evidence to support conclusions.

An example of how to organize summary tables by author or theme is shown in Table 3.1.

Here is an example summary table template .

Creating a topical outline

An alternative way to organize your articles for synthesis it to create an outline. After you have collected the articles you intend to use (and have put aside the ones you won’t be using), it’s time to extract as much as possible from the facts provided in those articles. You are starting your research project without a lot of hard facts on the topics you want to study, and by using the literature reviews provided in academic journal articles, you can gain a lot of knowledge about a topic in a short period of time.

As you read an article in detail, try copying the information you find relevant to your research topic in a separate word processing document. Copying and pasting from PDF to Word can be a pain because PDFs are image files not documents. To make that easier, use the HTML version of the article, convert the PDF to Word in Adobe Acrobat or another PDF reader, or use “paste special” command to paste the content into Word without formatting. If it’s an old PDF, you may have to simply type out the information you need. It can be a messy job, but having all of your facts in one place is very helpful for drafting your literature review.  Of course, you will not be using other authors’ words in your own literature review, but this is a good way to start compiling your notes.

You should copy and paste any fact or argument you consider important. Some good examples include definitions of concepts, statistics about the size of the social problem, and empirical evidence about the key variables in the research question, among countless others. It’s a good idea to consult with your professor and the syllabus for the course about what they are looking for when they read your literature review. Facts for your literature review are principally found in the introduction, results, and discussion section of an empirical article or at any point in a non-empirical article. Copy and paste into your notes anything you may want to use in your literature review.

Importantly, you must make sure you note the original source of that information. Nothing is worse than searching your articles for hours only to realize you forgot to note where your facts came from. If you found a statistic that the author used in the introduction, it almost certainly came from another source that the author cited in a footnote or internal citation. You will want to check the original source to make sure the author represented the information correctly. Moreover, you may want to read the original study to learn more about your topic and discover other sources relevant to your inquiry.

Assuming you have pulled all of the facts out of multiple articles, it’s time to start thinking about how these pieces of information relate to each other. Start grouping each fact into categories and subcategories as shown in Figure 3.3. For example, a statistic stating that homeless single adults are more likely to be male may fit into a category of gender and homelessness. For each topic or subtopic you identified during your critical analysis of each paper, determine what those papers have in common. Likewise, determine which ones in the group differ. If there are contradictory findings, you may be able to identify methodological or theoretical differences that could account for the contradiction. For example, one study may sample only high-income earners or those in a rural area. Determine what general conclusions you can report about the topic or subtopic, based on all of the information you’ve found.

Create a separate document containing a topical outline that combines your facts from each source and organizes them by topic or category. As you include more facts and more sources into your topical outline, you will begin to see how each fact fits into a category and how categories are related to each other. Your category names may change over time, as may their definitions. This is a natural reflection of the learning you are doing.

A complete topical outline is a long list of facts, arranged by category about your topic. As you step back from the outline, you should understand the topic areas where you have enough information to make strong conclusions about what the literature says. You should also assess in what areas you need to do more research before you can write a robust literature review. The topical outline should serve as a transitional document between the notes you write on each source and the literature review you submit to your professor. It is important to note that they contain plagiarized information that is copied and pasted directly from the primary sources. That’s okay because these are just notes and are not meant to be turned in as your own ideas. For your final literature review, you must paraphrase these sources to avoid plagiarism. More importantly, you should keep your voice and ideas front-and-center in what you write as this is your analysis of the literature. Make strong claims and support them thoroughly using facts you found in the literature. We will pick up the task of writing your literature review in section 3.3.

Additional resources for synthesizing literature

There are many ways to approach synthesizing literature. We’ve reviewed two examples here: summary tables and topical outlines. Other examples you may encounter include annotated bibliographies and synthesis matrices. As you are learning research, find a method that works for you. Reviewing the literature is a core component of evidence-based practice in social work at any level. See the resources below if you need some additional help:

Literature Reviews: Using a Matrix to Organize Research  / Saint Mary’s University of Minnesota

Literature Review: Synthesizing Multiple Sources  / Indiana University

Writing a Literature Review and Using a Synthesis Matrix  / Florida International University

Sample Literature Reviews Grid  / Complied by Lindsay Roberts

Killam, Laura (2013) . Literature review preparation: Creating a summary table . Includes transcript.

Key Takeaways

• It is necessary to take notes on research articles as you read. Try to develop a system that works for you to keep your notes organized, such as a summary table.
• Summary tables and topical outlines help researchers synthesize sources for the purpose of writing a literature review.

Image attributions

Pieces of the puzzle by congerdesign cc-0, adult diary by pexels cc-0.

• Figure 3.2 copied from Frederiksen, L. & Phelps, S. F. (2018). Literature reviews for education and nursing graduate students. Shared under a CC-BY 4.0 license. ↵
• This table was adapted from the work of Amanda Parsons ↵

Guidebook for Social Work Literature Reviews and Research Questions Copyright © 2020 by Rebecca Mauldin and Matthew DeCarlo is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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Synthesizing Sources

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When you look for areas where your sources agree or disagree and try to draw broader conclusions about your topic based on what your sources say, you are engaging in synthesis. Writing a research paper usually requires synthesizing the available sources in order to provide new insight or a different perspective into your particular topic (as opposed to simply restating what each individual source says about your research topic).

Note that synthesizing is not the same as summarizing.  

• A summary restates the information in one or more sources without providing new insight or reaching new conclusions.
• A synthesis draws on multiple sources to reach a broader conclusion.

There are two types of syntheses: explanatory syntheses and argumentative syntheses . Explanatory syntheses seek to bring sources together to explain a perspective and the reasoning behind it. Argumentative syntheses seek to bring sources together to make an argument. Both types of synthesis involve looking for relationships between sources and drawing conclusions.

In order to successfully synthesize your sources, you might begin by grouping your sources by topic and looking for connections. For example, if you were researching the pros and cons of encouraging healthy eating in children, you would want to separate your sources to find which ones agree with each other and which ones disagree.

After you have a good idea of what your sources are saying, you want to construct your body paragraphs in a way that acknowledges different sources and highlights where you can draw new conclusions.

As you continue synthesizing, here are a few points to remember:

• Don’t force a relationship between sources if there isn’t one. Not all of your sources have to complement one another.
• Do your best to highlight the relationships between sources in very clear ways.
• Don’t ignore any outliers in your research. It’s important to take note of every perspective (even those that disagree with your broader conclusions).

Example Syntheses

Below are two examples of synthesis: one where synthesis is NOT utilized well, and one where it is.

Parents are always trying to find ways to encourage healthy eating in their children. Elena Pearl Ben-Joseph, a doctor and writer for KidsHealth , encourages parents to be role models for their children by not dieting or vocalizing concerns about their body image. The first popular diet began in 1863. William Banting named it the “Banting” diet after himself, and it consisted of eating fruits, vegetables, meat, and dry wine. Despite the fact that dieting has been around for over a hundred and fifty years, parents should not diet because it hinders children’s understanding of healthy eating.

In this sample paragraph, the paragraph begins with one idea then drastically shifts to another. Rather than comparing the sources, the author simply describes their content. This leads the paragraph to veer in an different direction at the end, and it prevents the paragraph from expressing any strong arguments or conclusions.

An example of a stronger synthesis can be found below.

Parents are always trying to find ways to encourage healthy eating in their children. Different scientists and educators have different strategies for promoting a well-rounded diet while still encouraging body positivity in children. David R. Just and Joseph Price suggest in their article “Using Incentives to Encourage Healthy Eating in Children” that children are more likely to eat fruits and vegetables if they are given a reward (855-856). Similarly, Elena Pearl Ben-Joseph, a doctor and writer for Kids Health , encourages parents to be role models for their children. She states that “parents who are always dieting or complaining about their bodies may foster these same negative feelings in their kids. Try to keep a positive approach about food” (Ben-Joseph). Martha J. Nepper and Weiwen Chai support Ben-Joseph’s suggestions in their article “Parents’ Barriers and Strategies to Promote Healthy Eating among School-age Children.” Nepper and Chai note, “Parents felt that patience, consistency, educating themselves on proper nutrition, and having more healthy foods available in the home were important strategies when developing healthy eating habits for their children.” By following some of these ideas, parents can help their children develop healthy eating habits while still maintaining body positivity.

In this example, the author puts different sources in conversation with one another. Rather than simply describing the content of the sources in order, the author uses transitions (like "similarly") and makes the relationship between the sources evident.

Writing in the Health and Social Sciences: Literature Reviews and Synthesis Tools

• Journal Publishing
• Style and Writing Guides
• Readings about Writing
• Citing in APA Style This link opens in a new window
• Resources for Dissertation Authors
• Citation Management and Formatting Tools
• What are Literature Reviews?
• Conducting & Reporting Systematic Reviews
• Finding Systematic Reviews
• Tutorials & Tools for Literature Reviews

Systematic Literature Reviews: Steps & Resources

These steps for conducting a systematic literature review are listed below . 

Also see subpages for more information about:

• The different types of literature reviews, including systematic reviews and other evidence synthesis methods
• Tools & Tutorials

Literature Review & Systematic Review Steps

• Develop a Focused Question
• Scope the Literature  (Initial Search)
• Refine & Expand the Search
• Limit the Results
• Download Citations
• Abstract & Analyze
• Create Flow Diagram
• Synthesize & Report Results

1. Develop a Focused   Question 

Consider the PICO Format: Population/Problem, Intervention, Comparison, Outcome

Focus on defining the Population or Problem and Intervention (don't narrow by Comparison or Outcome just yet!)

"What are the effects of the Pilates method for patients with low back pain?"

Tools & Additional Resources:

• PICO Question Help
• Stillwell, Susan B., DNP, RN, CNE; Fineout-Overholt, Ellen, PhD, RN, FNAP, FAAN; Melnyk, Bernadette Mazurek, PhD, RN, CPNP/PMHNP, FNAP, FAAN; Williamson, Kathleen M., PhD, RN Evidence-Based Practice, Step by Step: Asking the Clinical Question, AJN The American Journal of Nursing : March 2010 - Volume 110 - Issue 3 - p 58-61 doi: 10.1097/01.NAJ.0000368959.11129.79

2. Scope the Literature

A "scoping search" investigates the breadth and/or depth of the initial question or may identify a gap in the literature. 

Eligible studies may be located by searching in:

• Background sources (books, point-of-care tools)
• Article databases
• Trial registries
• Grey literature
• Cited references
• Reference lists

When searching, if possible, translate terms to controlled vocabulary of the database. Use text word searching when necessary.

Use Boolean operators to connect search terms:

• Combine separate concepts with AND  (resulting in a narrower search)
• Connecting synonyms with OR  (resulting in an expanded search)

Search:  pilates AND ("low back pain"  OR  backache )

Video Tutorials - Translating PICO Questions into Search Queries

• Translate Your PICO Into a Search in PubMed (YouTube, Carrie Price, 5:11) 
• Translate Your PICO Into a Search in CINAHL (YouTube, Carrie Price, 4:56)

3. Refine & Expand Your Search

Expand your search strategy with synonymous search terms harvested from:

• database thesauri
• reference lists
• relevant studies

Example: 

(pilates OR exercise movement techniques) AND ("low back pain" OR backache* OR sciatica OR lumbago OR spondylosis)

As you develop a final, reproducible strategy for each database, save your strategies in a:

• a personal database account (e.g., MyNCBI for PubMed)
• Log in with your NYU credentials
• Open and "Make a Copy" to create your own tracker for your literature search strategies

4. Limit Your Results

Use database filters to limit your results based on your defined inclusion/exclusion criteria.  In addition to relying on the databases' categorical filters, you may also need to manually screen results.  

• Limit to Article type, e.g.,:  "randomized controlled trial" OR multicenter study
• Limit by publication years, age groups, language, etc.

NOTE: Many databases allow you to filter to "Full Text Only".  This filter is  not recommended . It excludes articles if their full text is not available in that particular database (CINAHL, PubMed, etc), but if the article is relevant, it is important that you are able to read its title and abstract, regardless of 'full text' status. The full text is likely to be accessible through another source (a different database, or Interlibrary Loan).  

• Filters in PubMed
• CINAHL Advanced Searching Tutorial

5. Download Citations

Selected citations and/or entire sets of search results can be downloaded from the database into a citation management tool. If you are conducting a systematic review that will require reporting according to PRISMA standards, a citation manager can help you keep track of the number of articles that came from each database, as well as the number of duplicate records.

In Zotero, you can create a Collection for the combined results set, and sub-collections for the results from each database you search.  You can then use Zotero's 'Duplicate Items" function to find and merge duplicate records.

• Citation Managers - General Guide

6. Abstract and Analyze

• Migrate citations to data collection/extraction tool
• Screen Title/Abstracts for inclusion/exclusion
• Screen and appraise full text for relevance, methods, 
• Resolve disagreements by consensus

Covidence is a web-based tool that enables you to work with a team to screen titles/abstracts and full text for inclusion in your review, as well as extract data from the included studies.

• Covidence Support
• Critical Appraisal Tools
• Data Extraction Tools

7. Create Flow Diagram

The PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) flow diagram is a visual representation of the flow of records through different phases of a systematic review.  It depicts the number of records identified, included and excluded.  It is best used in conjunction with the PRISMA checklist .

Example from: Stotz, S. A., McNealy, K., Begay, R. L., DeSanto, K., Manson, S. M., & Moore, K. R. (2021). Multi-level diabetes prevention and treatment interventions for Native people in the USA and Canada: A scoping review. Current Diabetes Reports, 2 (11), 46. https://doi.org/10.1007/s11892-021-01414-3

• PRISMA Flow Diagram Generator (ShinyApp.io, Haddaway et al. )
• PRISMA Diagram Templates  (Word and PDF)
• Make a copy of the file to fill out the template
• Image can be downloaded as PDF, PNG, JPG, or SVG
• Covidence generates a PRISMA diagram that is automatically updated as records move through the review phases

8. Synthesize & Report Results

There are a number of reporting guideline available to guide the synthesis and reporting of results in systematic literature reviews.

It is common to organize findings in a matrix, also known as a Table of Evidence (ToE).

• Reporting Guidelines for Systematic Reviews
• Download a sample template of a health sciences review matrix  (GoogleSheets)

Steps modified from: 

Cook, D. A., & West, C. P. (2012). Conducting systematic reviews in medical education: a stepwise approach.   Medical Education , 46 (10), 943–952.

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Academic Research: Literature Synthesis

• Research Information Guide
• Research Ethics
• Planning Research
• Study Design
• Data Analysis
• Self-Guided Learning
• Planning Search
• Literature Analysis
• Literature Synthesis
• Information Resources
• Citation Management
• Data Management Plan
• Writing Guidelines
• Academic Writing
• Medical Writing
• Literature Review
• Academic Publication
• Self-Guided learning

Literature synthesis is a process of analyzing various material (textual, statistical, graphical, programming, and other) to understand the studied phenomenon by defining the differences, similarities, relationships, and new ideas, which emerge after interpreting published information, statements, arguments.  Analysis of published literature is a process of asking questions about the information, meaning, goal, and implication of published research. Questions based on -  what, why, when, where, who - provide an insight for analysis of published literature, evaluation, and interpretation.   

Four synthesis types are defined from integrative literature reviews

• Research agenda - identified from the critical analysis through the provocative question or proposition that is used for the next studies
• Conceptual constructs - provides an analysis of the classification of constructs to classify previous research, which provided the direction for new theories
• Alternative models or conceptual frameworks - new thinking about the published topics. Alternative models and conceptions are proposed deriving from critical analysis and synthesis
• Metatheory - integration, and synthesis of literature review provides the foundation for further development of the metatheory across the theoretical domains

Adapted from  Torraco, R. J. (2005). Writing integrative literature reviews: Guidelines and examples.  Human resource development review ,  4 (3), 356-367.

Literature Annotation   

• Passive - light process (e.g., underlining and highlighting)
• Active - thought-process (e.g., questions, observation, summary, and other
• SIFT annotation - symbols, imagery, figurative language, tone, theme)

Questions 

• What are the question and answer?
• What are the main ideas and findings in the article?
• What are the unique methods and techniques?
• What are the patterns in the published literature?
• Is there a collaborative pattern in published research? 
• How this article relevant to the study?
• What are the important concepts?
• Individual interpretation 

Image Annotation 

• Ramisa A, Yan F, Moreno-Noguer F, Mikolajczyk K. BreakingNews: Article Annotation by Image and Text Processing. IEEE Trans Pattern Anal Mach Intell. 2018 May;40(5):1072-1085. Epub 2017 Jun 30. PMID: 28682246.
• Pelka O, Nensa F, Friedrich CM. Annotation of enhanced radiographs for medical image retrieval with deep convolutional neural networks. PLoS One. 2018 Nov 12;13(11):e0206229. PMID: 30419028; PMCID: PMC6231616.

Critically Appraised Topic

Critically Appraised Topic (CAT) - is a “summary of a search and critical appraisal of the literature related to a focused clinical question, which should be kept in an easily accessible place so that it can be used to help make clinical decisions” Dawes, M. (2005). Critically appraised topics and evidence-based medicine journals. Singapore Medical Journal, 46(9), 442.

"5S" Pyramid of Evidence Resources

Graph by Dr. Tmanova. 

Based on the "5S" Pyramid of Evidence Resources from the Indiana University School of Medicine Libraries, the Dartmouth /Yale EBM Page Generator and Haynes RB, ACP J Club 2006;145(3):A8.

What is a Scientific Knowledge?

• Description - observational analysis of the phenomenon by describing its characteristics 
• Analysis - evaluating the studies phenomenon to conclude by testing assumptions through the analytical processes by evaluating studies characteristics 
• Correlation - analysis of relationships among studied phenomena
• Comparison - analysis of the differences and similarities 
• Prediction - predictive forecast on emerging phenomenon based on information and knowledge
• Control/validation - correlation, cause-effect
• Categorization - typology, specification 
• Explanation - study hypothesis, research questions - what, when, why, how, where, who

Selected key concept on scientific information cycle and management at Information Management guide  !  Note: additional resources on the information management and knowledge management are at the designing phase. 

EBM Appraisal Tools

• Critical Appraisal Checklist, CASP
• Critical Appraisal of Qualitative Studies Sheet
• Diagnostics 
• Randomized Controlled Trials (RCT)
• Systematic review
• IDP Review Sheet 

Citation management, annotation, and synthesis 

Tools useful for information and citation management, annotation, and synthesis: 

• EndNote 
• Torraco, R. J. (2016). Writing integrative reviews of the literature: Methods and purposes.  International Journal of Adult Vocational Education and Technology (IJAVET) ,  7 (3), 62-70.
• Klopper, R., Lubbe, S., & Rugbeer, H. (2007). The matrix method of literature review.  Alternation ,  14 (1), 262-276. For more books and journals, browse  DML Book Catalog  and  DML A-Z Journal Catalog , and  DML Database Catalog . Schedule  Consultation  on finding associated sources. 

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Literature Review Basics

• What is a Literature Review?
• Synthesizing Research
• Using Research & Synthesis Tables
• Additional Resources

Synthesis: What is it?

First, let's be perfectly clear about what synthesizing your research isn't :

• - It isn't  just summarizing the material you read
• - It isn't  generating a collection of annotations or comments (like an annotated bibliography)
• - It isn't  compiling a report on every single thing ever written in relation to your topic

When you  synthesize  your research, your job is to help your reader understand the current state of the conversation on your topic, relative to your research question.  That may include doing the following:

• - Selecting and using representative work on the topic
• - Identifying and discussing trends in published data or results
• - Identifying and explaining the impact of common features (study populations, interventions, etc.) that appear frequently in the literature
• - Explaining controversies, disputes, or central issues in the literature that are relevant to your research question
• - Identifying gaps in the literature, where more research is needed
• - Establishing the discussion to which your own research contributes and demonstrating the value of your contribution

Essentially, you're telling your reader where they are (and where you are) in the scholarly conversation about your project.

Synthesis: How do I do it?

Synthesis, step by step.

This is what you need to do  before  you write your review.

• Identify and clearly describe your research question (you may find the Formulating PICOT Questions table at  the Additional Resources tab helpful).
• Collect sources relevant to your research question.
• Organize and describe the sources you've found -- your job is to identify what  types  of sources you've collected (reviews, clinical trials, etc.), identify their  purpose  (what are they measuring, testing, or trying to discover?), determine the  level of evidence  they represent (see the Levels of Evidence table at the Additional Resources tab ), and briefly explain their  major findings . Use a Research Table to document this step.
• Study the information you've put in your Research Table and examine your collected sources, looking for  similarities  and  differences . Pay particular attention to  populations ,   methods  (especially relative to levels of evidence), and  findings .
• Analyze what you learn in (4) using a tool like a Synthesis Table. Your goal is to identify relevant themes, trends, gaps, and issues in the research.  Your literature review will collect the results of this analysis and explain them in relation to your research question.

Analysis tips

• - Sometimes, what you  don't  find in the literature is as important as what you do find -- look for questions that the existing research hasn't answered yet.
• - If any of the sources you've collected refer to or respond to each other, keep an eye on how they're related -- it may provide a clue as to whether or not study results have been successfully replicated.
• - Sorting your collected sources by level of evidence can provide valuable insight into how a particular topic has been covered, and it may help you to identify gaps worth addressing in your own work.
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How to Synthesize Written Information from Multiple Sources

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Content Manager

B.A., English Literature, University of Glasgow

Shona McCombes is the content manager at Scribbr, Netherlands.

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Saul Mcleod, PhD

Editor-in-Chief for Simply Psychology

BSc (Hons) Psychology, MRes, PhD, University of Manchester

Saul Mcleod, Ph.D., is a qualified psychology teacher with over 18 years experience of working in further and higher education. He has been published in peer-reviewed journals, including the Journal of Clinical Psychology.

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When you write a literature review or essay, you have to go beyond just summarizing the articles you’ve read – you need to synthesize the literature to show how it all fits together (and how your own research fits in).

Synthesizing simply means combining. Instead of summarizing the main points of each source in turn, you put together the ideas and findings of multiple sources in order to make an overall point.

At the most basic level, this involves looking for similarities and differences between your sources. Your synthesis should show the reader where the sources overlap and where they diverge.

Unsynthesized Example

Franz (2008) studied undergraduate online students. He looked at 17 females and 18 males and found that none of them liked APA. According to Franz, the evidence suggested that all students are reluctant to learn citations style. Perez (2010) also studies undergraduate students. She looked at 42 females and 50 males and found that males were significantly more inclined to use citation software ( p < .05). Findings suggest that females might graduate sooner. Goldstein (2012) looked at British undergraduates. Among a sample of 50, all females, all confident in their abilities to cite and were eager to write their dissertations.

Synthesized Example

Studies of undergraduate students reveal conflicting conclusions regarding relationships between advanced scholarly study and citation efficacy. Although Franz (2008) found that no participants enjoyed learning citation style, Goldstein (2012) determined in a larger study that all participants watched felt comfortable citing sources, suggesting that variables among participant and control group populations must be examined more closely. Although Perez (2010) expanded on Franz’s original study with a larger, more diverse sample…

Step 1: Organize your sources

After collecting the relevant literature, you’ve got a lot of information to work through, and no clear idea of how it all fits together.

Before you can start writing, you need to organize your notes in a way that allows you to see the relationships between sources.

One way to begin synthesizing the literature is to put your notes into a table. Depending on your topic and the type of literature you’re dealing with, there are a couple of different ways you can organize this.

Summary table

A summary table collates the key points of each source under consistent headings. This is a good approach if your sources tend to have a similar structure – for instance, if they’re all empirical papers.

Each row in the table lists one source, and each column identifies a specific part of the source. You can decide which headings to include based on what’s most relevant to the literature you’re dealing with.

For example, you might include columns for things like aims, methods, variables, population, sample size, and conclusion.

For each study, you briefly summarize each of these aspects. You can also include columns for your own evaluation and analysis.

The summary table gives you a quick overview of the key points of each source. This allows you to group sources by relevant similarities, as well as noticing important differences or contradictions in their findings.

Synthesis matrix

A synthesis matrix is useful when your sources are more varied in their purpose and structure – for example, when you’re dealing with books and essays making various different arguments about a topic.

Each column in the table lists one source. Each row is labeled with a specific concept, topic or theme that recurs across all or most of the sources.

Then, for each source, you summarize the main points or arguments related to the theme.

The purposes of the table is to identify the common points that connect the sources, as well as identifying points where they diverge or disagree.

Step 2: Outline your structure

Now you should have a clear overview of the main connections and differences between the sources you’ve read. Next, you need to decide how you’ll group them together and the order in which you’ll discuss them.

For shorter papers, your outline can just identify the focus of each paragraph; for longer papers, you might want to divide it into sections with headings.

There are a few different approaches you can take to help you structure your synthesis.

If your sources cover a broad time period, and you found patterns in how researchers approached the topic over time, you can organize your discussion chronologically .

That doesn’t mean you just summarize each paper in chronological order; instead, you should group articles into time periods and identify what they have in common, as well as signalling important turning points or developments in the literature.

If the literature covers various different topics, you can organize it thematically .

That means that each paragraph or section focuses on a specific theme and explains how that theme is approached in the literature.

Source Used with Permission: The Chicago School

If you’re drawing on literature from various different fields or they use a wide variety of research methods, you can organize your sources methodologically .

That means grouping together studies based on the type of research they did and discussing the findings that emerged from each method.

If your topic involves a debate between different schools of thought, you can organize it theoretically .

That means comparing the different theories that have been developed and grouping together papers based on the position or perspective they take on the topic, as well as evaluating which arguments are most convincing.

Step 3: Write paragraphs with topic sentences

What sets a synthesis apart from a summary is that it combines various sources. The easiest way to think about this is that each paragraph should discuss a few different sources, and you should be able to condense the overall point of the paragraph into one sentence.

This is called a topic sentence , and it usually appears at the start of the paragraph. The topic sentence signals what the whole paragraph is about; every sentence in the paragraph should be clearly related to it.

A topic sentence can be a simple summary of the paragraph’s content:

“Early research on [x] focused heavily on [y].”

For an effective synthesis, you can use topic sentences to link back to the previous paragraph, highlighting a point of debate or critique:

“Several scholars have pointed out the flaws in this approach.” “While recent research has attempted to address the problem, many of these studies have methodological flaws that limit their validity.”

By using topic sentences, you can ensure that your paragraphs are coherent and clearly show the connections between the articles you are discussing.

As you write your paragraphs, avoid quoting directly from sources: use your own words to explain the commonalities and differences that you found in the literature.

Don’t try to cover every single point from every single source – the key to synthesizing is to extract the most important and relevant information and combine it to give your reader an overall picture of the state of knowledge on your topic.

Step 4: Revise, edit and proofread

Like any other piece of academic writing, synthesizing literature doesn’t happen all in one go – it involves redrafting, revising, editing and proofreading your work.

Checklist for Synthesis

•   Do I introduce the paragraph with a clear, focused topic sentence?
•   Do I discuss more than one source in the paragraph?
•   Do I mention only the most relevant findings, rather than describing every part of the studies?
•   Do I discuss the similarities or differences between the sources, rather than summarizing each source in turn?
•   Do I put the findings or arguments of the sources in my own words?
•   Is the paragraph organized around a single idea?
•   Is the paragraph directly relevant to my research question or topic?
•   Is there a logical transition from this paragraph to the next one?

Further Information

How to Synthesise: a Step-by-Step Approach

Help…I”ve Been Asked to Synthesize!

Learn how to Synthesise (combine information from sources)

How to write a Psychology Essay

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• v.9(7); 2013 Jul

Ten Simple Rules for Writing a Literature Review

Marco pautasso.

1 Centre for Functional and Evolutionary Ecology (CEFE), CNRS, Montpellier, France

2 Centre for Biodiversity Synthesis and Analysis (CESAB), FRB, Aix-en-Provence, France

Literature reviews are in great demand in most scientific fields. Their need stems from the ever-increasing output of scientific publications [1] . For example, compared to 1991, in 2008 three, eight, and forty times more papers were indexed in Web of Science on malaria, obesity, and biodiversity, respectively [2] . Given such mountains of papers, scientists cannot be expected to examine in detail every single new paper relevant to their interests [3] . Thus, it is both advantageous and necessary to rely on regular summaries of the recent literature. Although recognition for scientists mainly comes from primary research, timely literature reviews can lead to new synthetic insights and are often widely read [4] . For such summaries to be useful, however, they need to be compiled in a professional way [5] .

When starting from scratch, reviewing the literature can require a titanic amount of work. That is why researchers who have spent their career working on a certain research issue are in a perfect position to review that literature. Some graduate schools are now offering courses in reviewing the literature, given that most research students start their project by producing an overview of what has already been done on their research issue [6] . However, it is likely that most scientists have not thought in detail about how to approach and carry out a literature review.

Reviewing the literature requires the ability to juggle multiple tasks, from finding and evaluating relevant material to synthesising information from various sources, from critical thinking to paraphrasing, evaluating, and citation skills [7] . In this contribution, I share ten simple rules I learned working on about 25 literature reviews as a PhD and postdoctoral student. Ideas and insights also come from discussions with coauthors and colleagues, as well as feedback from reviewers and editors.

Rule 1: Define a Topic and Audience

How to choose which topic to review? There are so many issues in contemporary science that you could spend a lifetime of attending conferences and reading the literature just pondering what to review. On the one hand, if you take several years to choose, several other people may have had the same idea in the meantime. On the other hand, only a well-considered topic is likely to lead to a brilliant literature review [8] . The topic must at least be:

• interesting to you (ideally, you should have come across a series of recent papers related to your line of work that call for a critical summary),
• an important aspect of the field (so that many readers will be interested in the review and there will be enough material to write it), and
• a well-defined issue (otherwise you could potentially include thousands of publications, which would make the review unhelpful).

Ideas for potential reviews may come from papers providing lists of key research questions to be answered [9] , but also from serendipitous moments during desultory reading and discussions. In addition to choosing your topic, you should also select a target audience. In many cases, the topic (e.g., web services in computational biology) will automatically define an audience (e.g., computational biologists), but that same topic may also be of interest to neighbouring fields (e.g., computer science, biology, etc.).

Rule 2: Search and Re-search the Literature

After having chosen your topic and audience, start by checking the literature and downloading relevant papers. Five pieces of advice here:

• keep track of the search items you use (so that your search can be replicated [10] ),
• keep a list of papers whose pdfs you cannot access immediately (so as to retrieve them later with alternative strategies),
• use a paper management system (e.g., Mendeley, Papers, Qiqqa, Sente),
• define early in the process some criteria for exclusion of irrelevant papers (these criteria can then be described in the review to help define its scope), and
• do not just look for research papers in the area you wish to review, but also seek previous reviews.

The chances are high that someone will already have published a literature review ( Figure 1 ), if not exactly on the issue you are planning to tackle, at least on a related topic. If there are already a few or several reviews of the literature on your issue, my advice is not to give up, but to carry on with your own literature review,

The bottom-right situation (many literature reviews but few research papers) is not just a theoretical situation; it applies, for example, to the study of the impacts of climate change on plant diseases, where there appear to be more literature reviews than research studies [33] .

• discussing in your review the approaches, limitations, and conclusions of past reviews,
• trying to find a new angle that has not been covered adequately in the previous reviews, and
• incorporating new material that has inevitably accumulated since their appearance.

When searching the literature for pertinent papers and reviews, the usual rules apply:

• be thorough,
• use different keywords and database sources (e.g., DBLP, Google Scholar, ISI Proceedings, JSTOR Search, Medline, Scopus, Web of Science), and
• look at who has cited past relevant papers and book chapters.

Rule 3: Take Notes While Reading

If you read the papers first, and only afterwards start writing the review, you will need a very good memory to remember who wrote what, and what your impressions and associations were while reading each single paper. My advice is, while reading, to start writing down interesting pieces of information, insights about how to organize the review, and thoughts on what to write. This way, by the time you have read the literature you selected, you will already have a rough draft of the review.

Of course, this draft will still need much rewriting, restructuring, and rethinking to obtain a text with a coherent argument [11] , but you will have avoided the danger posed by staring at a blank document. Be careful when taking notes to use quotation marks if you are provisionally copying verbatim from the literature. It is advisable then to reformulate such quotes with your own words in the final draft. It is important to be careful in noting the references already at this stage, so as to avoid misattributions. Using referencing software from the very beginning of your endeavour will save you time.

Rule 4: Choose the Type of Review You Wish to Write

After having taken notes while reading the literature, you will have a rough idea of the amount of material available for the review. This is probably a good time to decide whether to go for a mini- or a full review. Some journals are now favouring the publication of rather short reviews focusing on the last few years, with a limit on the number of words and citations. A mini-review is not necessarily a minor review: it may well attract more attention from busy readers, although it will inevitably simplify some issues and leave out some relevant material due to space limitations. A full review will have the advantage of more freedom to cover in detail the complexities of a particular scientific development, but may then be left in the pile of the very important papers “to be read” by readers with little time to spare for major monographs.

There is probably a continuum between mini- and full reviews. The same point applies to the dichotomy of descriptive vs. integrative reviews. While descriptive reviews focus on the methodology, findings, and interpretation of each reviewed study, integrative reviews attempt to find common ideas and concepts from the reviewed material [12] . A similar distinction exists between narrative and systematic reviews: while narrative reviews are qualitative, systematic reviews attempt to test a hypothesis based on the published evidence, which is gathered using a predefined protocol to reduce bias [13] , [14] . When systematic reviews analyse quantitative results in a quantitative way, they become meta-analyses. The choice between different review types will have to be made on a case-by-case basis, depending not just on the nature of the material found and the preferences of the target journal(s), but also on the time available to write the review and the number of coauthors [15] .

Rule 5: Keep the Review Focused, but Make It of Broad Interest

Whether your plan is to write a mini- or a full review, it is good advice to keep it focused 16 , 17 . Including material just for the sake of it can easily lead to reviews that are trying to do too many things at once. The need to keep a review focused can be problematic for interdisciplinary reviews, where the aim is to bridge the gap between fields [18] . If you are writing a review on, for example, how epidemiological approaches are used in modelling the spread of ideas, you may be inclined to include material from both parent fields, epidemiology and the study of cultural diffusion. This may be necessary to some extent, but in this case a focused review would only deal in detail with those studies at the interface between epidemiology and the spread of ideas.

While focus is an important feature of a successful review, this requirement has to be balanced with the need to make the review relevant to a broad audience. This square may be circled by discussing the wider implications of the reviewed topic for other disciplines.

Rule 6: Be Critical and Consistent

Reviewing the literature is not stamp collecting. A good review does not just summarize the literature, but discusses it critically, identifies methodological problems, and points out research gaps [19] . After having read a review of the literature, a reader should have a rough idea of:

• the major achievements in the reviewed field,
• the main areas of debate, and
• the outstanding research questions.

It is challenging to achieve a successful review on all these fronts. A solution can be to involve a set of complementary coauthors: some people are excellent at mapping what has been achieved, some others are very good at identifying dark clouds on the horizon, and some have instead a knack at predicting where solutions are going to come from. If your journal club has exactly this sort of team, then you should definitely write a review of the literature! In addition to critical thinking, a literature review needs consistency, for example in the choice of passive vs. active voice and present vs. past tense.

Rule 7: Find a Logical Structure

Like a well-baked cake, a good review has a number of telling features: it is worth the reader's time, timely, systematic, well written, focused, and critical. It also needs a good structure. With reviews, the usual subdivision of research papers into introduction, methods, results, and discussion does not work or is rarely used. However, a general introduction of the context and, toward the end, a recapitulation of the main points covered and take-home messages make sense also in the case of reviews. For systematic reviews, there is a trend towards including information about how the literature was searched (database, keywords, time limits) [20] .

How can you organize the flow of the main body of the review so that the reader will be drawn into and guided through it? It is generally helpful to draw a conceptual scheme of the review, e.g., with mind-mapping techniques. Such diagrams can help recognize a logical way to order and link the various sections of a review [21] . This is the case not just at the writing stage, but also for readers if the diagram is included in the review as a figure. A careful selection of diagrams and figures relevant to the reviewed topic can be very helpful to structure the text too [22] .

Rule 8: Make Use of Feedback

Reviews of the literature are normally peer-reviewed in the same way as research papers, and rightly so [23] . As a rule, incorporating feedback from reviewers greatly helps improve a review draft. Having read the review with a fresh mind, reviewers may spot inaccuracies, inconsistencies, and ambiguities that had not been noticed by the writers due to rereading the typescript too many times. It is however advisable to reread the draft one more time before submission, as a last-minute correction of typos, leaps, and muddled sentences may enable the reviewers to focus on providing advice on the content rather than the form.

Feedback is vital to writing a good review, and should be sought from a variety of colleagues, so as to obtain a diversity of views on the draft. This may lead in some cases to conflicting views on the merits of the paper, and on how to improve it, but such a situation is better than the absence of feedback. A diversity of feedback perspectives on a literature review can help identify where the consensus view stands in the landscape of the current scientific understanding of an issue [24] .

Rule 9: Include Your Own Relevant Research, but Be Objective

In many cases, reviewers of the literature will have published studies relevant to the review they are writing. This could create a conflict of interest: how can reviewers report objectively on their own work [25] ? Some scientists may be overly enthusiastic about what they have published, and thus risk giving too much importance to their own findings in the review. However, bias could also occur in the other direction: some scientists may be unduly dismissive of their own achievements, so that they will tend to downplay their contribution (if any) to a field when reviewing it.

In general, a review of the literature should neither be a public relations brochure nor an exercise in competitive self-denial. If a reviewer is up to the job of producing a well-organized and methodical review, which flows well and provides a service to the readership, then it should be possible to be objective in reviewing one's own relevant findings. In reviews written by multiple authors, this may be achieved by assigning the review of the results of a coauthor to different coauthors.

Rule 10: Be Up-to-Date, but Do Not Forget Older Studies

Given the progressive acceleration in the publication of scientific papers, today's reviews of the literature need awareness not just of the overall direction and achievements of a field of inquiry, but also of the latest studies, so as not to become out-of-date before they have been published. Ideally, a literature review should not identify as a major research gap an issue that has just been addressed in a series of papers in press (the same applies, of course, to older, overlooked studies (“sleeping beauties” [26] )). This implies that literature reviewers would do well to keep an eye on electronic lists of papers in press, given that it can take months before these appear in scientific databases. Some reviews declare that they have scanned the literature up to a certain point in time, but given that peer review can be a rather lengthy process, a full search for newly appeared literature at the revision stage may be worthwhile. Assessing the contribution of papers that have just appeared is particularly challenging, because there is little perspective with which to gauge their significance and impact on further research and society.

Inevitably, new papers on the reviewed topic (including independently written literature reviews) will appear from all quarters after the review has been published, so that there may soon be the need for an updated review. But this is the nature of science [27] – [32] . I wish everybody good luck with writing a review of the literature.

Acknowledgments

Many thanks to M. Barbosa, K. Dehnen-Schmutz, T. Döring, D. Fontaneto, M. Garbelotto, O. Holdenrieder, M. Jeger, D. Lonsdale, A. MacLeod, P. Mills, M. Moslonka-Lefebvre, G. Stancanelli, P. Weisberg, and X. Xu for insights and discussions, and to P. Bourne, T. Matoni, and D. Smith for helpful comments on a previous draft.

Funding Statement

This work was funded by the French Foundation for Research on Biodiversity (FRB) through its Centre for Synthesis and Analysis of Biodiversity data (CESAB), as part of the NETSEED research project. The funders had no role in the preparation of the manuscript.

Sustainable development and enhancement of cracking processes using metallic composites

• Open access
• Published: 23 January 2021
• Volume 11 , pages 1–18, ( 2021 )

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• Abimbola G. Olaremu   ORCID: orcid.org/0000-0002-8463-7564 1 ,
• Williams R. Adedoyin 1 ,
• Odunayo T. Ore   ORCID: orcid.org/0000-0002-5529-1509 2 &
• Adedapo O. Adeola   ORCID: orcid.org/0000-0002-7011-2396 1  

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Metallic composites represent a vital class of materials that has gained increased attention in crude oil processing as well as the production of biofuel from other sources in recent times. Several catalytic materials have been reported in the literature for catalytic cracking, particularly, of crude oil. This review seeks to provide a comprehensive overview of existing and emerging methods/technologies such as metal–organic frameworks (MOFs), metal–matrix composites (MMCs), and catalytic support materials, to bridge information gaps toward sustainable advancement in catalysis for petrochemical processes. There is an increase in industrial and environmental concern emanating from the sulphur levels of oils, hence the need to develop more efficient catalysts in the hydrotreatment (HDS and HDN) processes, and combating the challenge of catalyst poisoning and deactivation; in a bid to improving the overall quality of oils and sustainable use of catalyst. Structural improvement, high thermal stability, enhanced cracking potential, and environmental sustainability represent the various benefits accrued to the use of metallic composites as opposed to conventional catalysts employed in catalytic cracking processes.

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Introduction

In a bid to refining light and heavy crude in the petroleum industry, catalytic cracking processes have evolved over the years [ 1 , 2 , 3 ]. The refining of heavy and extra-heavy crude particularly requires highly sophisticated processes. Cracking of petroleum generally refers to the breakdown of highly complex organic molecules such as kerogen into light hydrocarbon molecules under high temperature and pressure [ 4 , 5 ]. Cracking processes could be thermal or catalytic. The former involves the decomposition of highly polymeric molecules into smaller ones under high temperature, while the latter incorporates the use of a catalyst to facilitate high thermal efficiency at a shorter time under low temperature and pressure [ 6 ]. Other advantages of catalytic cracking over thermal cracking include high product selectivity, high yields of gasoline, high octane number, and high aromatic yield, among others [ 7 ].

Catalyst is an essential entity in conversion and upgrading processes as it facilitates the production of valuable products without being consumed in the process. The activation energy required to reach the conversion state is lowered by the catalyst via the provision of an alternative reaction pathway [ 8 ]. The choice of catalysts used in catalytic cracking is dependent on the regenerative potential, unique microporous structure, economic cost, and thermal stability [ 9 ]. The modern-day petroleum industry commonly uses zeolite as a catalyst during catalytic cracking. Alongside every other single catalyst used in the petroleum industry, it is difficult or almost impossible for a catalyst to perform every desired reaction during cracking. Consequent upon this limitation, the impregnation of existing materials used as catalysts with other materials to form composite materials for added functionality or the synthesis of relatively new materials is an evolving technology in the petroleum industry [ 10 , 11 , 12 , 13 ].

At present, composites being a relatively new terminology do not have a widely accepted definition. Composites are obtained through the accumulation of two or more distinct materials. The fundamental constituents of composites have different distinguishing attributes and features, while the compound offers a new material [ 14 ]. Composites are made up of two or more physically and/or chemically noticeably different materials [ 15 ]. Composite materials are understood to possess their own unmatched, entirely new, and distinct properties relative to constituent components [ 14 ]. Metallic composites are useful in many areas of human endeavours, and have found application in both household materials and industrial appliances for quite some time [ 16 , 17 , 18 , 19 ].

The notion of mixing two dissimilar materials has received large attention in the present-day applied sciences. These combinations make possible the attainment of unique attributes [ 20 ]. The composite industry has begun to acknowledge the commercial application of composites which promise to offer enormous business opportunities in the aerospace and automotive sectors [ 21 ]. The need for structural improvement, tribological, thermal, and chemical or some other material properties is the aim of compounding this material [ 14 ]. Several components constituted the various catalysts used in cracking processes in the refining industry. An acid zeolite is crucial in achieving high activity and selectivity to light olefins; therefore, huge research efforts have been dedicated to knowing its working mechanism, since it was introduced, which is needed to enable rational improvements [ 22 ]. In particular, composite materials are composed of a matrix, constituent with the largest quantity share with other components and reinforce materials that are used to obtain projected properties. The constituents of composite materials do not mix or completely homogeneous, thereby giving rise to the presence of two or more phases within the composite material [ 14 ]. The mitigation of greenhouse gases, direct conversion of methane to methanol, and linear paraffin isomerization are specific reactions in oil refining strongly associated with the use of composite catalysts [ 9 , 23 , 24 ].

Many exciting research studies and reviews have focused on the synthesis, use, and catalytic activity of conventional catalysts [ 25 , 26 , 27 , 28 , 29 , 30 ]. Comprehensive reviews on the use of metallic composites in catalytic cracking processes are of great significance, particularly for the future development of sustainable and efficient catalysts employed in the cracking of oils. Up to date, the use of metal-impregnated catalysts in the conversion and upgrading of oils to valuable products has not been critically and comprehensively reviewed despite many studies reporting their usability and efficiency. This review thus seeks to provide an adequate understanding of the existing and emerging choice of composites for catalytic petrochemical processes; the composition, importance, limitations, areas of improvement, and sustainable approach to the development of metallic composites used in catalytic cracking in the petroleum industry.

Ecological restrictions on quality characteristics of oils

Technological advancement is steered as much by economics as well as government as it is by modernization. There is no superior example of this other than in the petroleum refining and process development. In the last decade, advancement in the petrochemical industry has been steered by increased global demand for petroleum products, need for process efficiency, enhanced product selectivity, segmentation of product via quality improvement, environmental concerns for sulphur content, improved aromatics, and olefins to assist an increasing but cyclical petrochemical sphere as well as improved manufacture of oxygenates to achieve the clean fuel directives [ 31 , 32 ]. These are presently being re-explored because of the increasing concern from the environmental circle of benefits and liabilities suffered by the compulsory incorporation of oxygenates in gasoline [ 2 , 33 ].

The ever-increasing demand for energy as well as environmental problems is part of the major implications of the rapid expansion of human society [ 34 ]. The major environmental issue in recent times has been air pollution, not undermining the damning impact of oil spillage on aquatic and terrestrial species [ 35 ]. It is a fact that the burning of petroleum products belches harmful gases and compounds into the atmosphere, resulting in the greenhouse effect, ozone depletion, and acute and/or chronic toxicities in living things. Crucial emissions of NO x , SO x , CO x , and particulate matter are the sources of the majority concern for environmental pollution [ 36 ]. To reduce their effect, stringent environmental restrictions have been launched in many developed countries across the globe to decrease the sulphur and aromatic contents in automobile fuels aimed at reducing harmful exhaust emissions and improving the quality of air [ 37 ].

Element with significant amounts in petroleum after carbon and hydrogen is sulphur. In crude oil, the average sulphur content varies from 0.03 to 7.89 g/g % [ 31 , 38 ]. The sulphur compounds are usually found in two distinct forms of inorganic and organic. Present in the dissolved or suspended form is the inorganic sulphur like elemental sulphur, H 2 S, and pyrite. In the same vein, organic sulphur compounds like thiols, sulphides, and thiophene compounds constitute the main origin of sulphur found in crude oil [ 31 ]. Substantial attention has been directed towards identifying the sulphur compounds in heavy oils, oil sands, and derived bitumen [ 31 ]. New guidelines as proposed by the Environmental Protection Agency (EPA) in the year 2010 were to limit the sulphur in diesel fuels from low sulphur diesel (LSD) of 500 ppm wt to ultra-low sulphur diesel (ULSD) of less than 15 ppm. In Euro V standard, sulphur content of diesel fuels was restricted to be less than 10 ppm, in a bid to reducing the adverse effects of the consumption of fossil fuels [ 39 ].

Furthermore, worldwide fuel character introduced restrictions on the content of aromatics in diesel fuels to less than 20% in the advancing future. Undoubtedly, the decrease of emissions of exhaust gas [majorly hydrocarbons (HC) and particulate matter (PM)], as well as the improved Cetane Index (CI), is attributed to the reduction of the aromatics in diesel fuels [ 40 , 41 ]. The increasing alertness of the effects on the pollution of the environment by automobiles has the responsibility of pollution control diverted to the refineries. The sulphur specifications for both gasoline and diesel are made more stringent as a consequence of their impact on the environment [ 42 ]. The demand for cleaner fuel has been on the increase, since the concerns for the environment have been emphasized. The strict restriction of heavy oils commonly utilized as combustion oils came as a consequence of the high percentages of sulphur it contains [ 43 ]. For that reason, it was envisaged that ultra-deep hydroaromatization and hydrodesulphurization of gas oils should be considered essential to meet both the market and environmental requirements [ 41 ].

The stricter environmental regulations linked with the rising dependence on heavy oils obtained from bitumen are the huge obstacles confronting the petroleum industries [ 44 ]. In the past years, there has been a reasonable increase in awareness about environmental protection, and consequently, a law limiting the sulphur contents in fuels has been enforced across the globe. At present, the reduction of environmental issues induced by the emissions of SO x into the environment requires high-quality ultra-low sulphur transportation fuels. To this end, the hydrotreatment processes making use of more active and selective catalysts have been proposed [ 45 ].

The environmental laws regarding the standard of automotive fuels as well as the emissions from the refinery are presently the most vital and most costly issues. Pollutants, such as SO x , CO, nitrogen oxides (NO x , x  = 1, 2; 90–95% in the form of NO) [ 35 ], which are the main air pollutants produced by the burning of fuel in stationary and automotive engines, particulates, olefins, and aromatic hydrocarbons, are of major concerns [ 46 ]. The improvement in the environmental status of motor fuel is ascribable to the universal agreement in the legislation by many countries across the globe. In this regard, the reduction of the sulphur contents of diesel and gasoline in the advancing years is expected to be nearing 10 and 50 ppm sequentially. A small degree of sulphur in transportation fuels will not only at-once better the standard of air by decreasing SO x emissions, but will also indirectly reduce the NO x and hydrocarbon emissions by permitting the better work of automotive catalytic converters [ 47 , 48 ]. As a result of the enacting demand for a radical decrease in the sulphur content of automotive fuels (say 30 ppm in gasoline and 15 ppm in diesel) that was fully enforced in 2009, much attention has been given to the deep desulphurization of transportation fuels in the research community globally. The emissions of SO x in combustion gases were boosted by the sulphur in gasoline, thereby decreasing the activities of the catalytic converters of vehicles as well as promoting the corrosion of parts of the engine [ 49 ].

There has been a considerable rise in the demand for heavy oil feedstock for refinery aimed at manufacturing gasoline with low sulphur which is attributed to the stringent environmental promulgations in respect to fossil fuels [ 50 ]. The reinforcement of efficient hydrodesulphurization (HDS) catalysts has been steered by the requisite of manufacturing ultra-clean fuels for protecting the environment [ 51 ]. The production of clean gasoline with sulphur less than 10 ppm is a burning topic with regards to eco-friendly catalysis [ 52 ]. There has been immense attention in recent years directed towards the deep hydrodesulphurization (HDS) of fuels obtained from petroleum due to the environmental requirements suggesting the need to eliminate sulphur. In recent times, low sulphur-containing transportation fuels (less than 10–15 ppm) are in use in many countries globally [ 53 , 54 ]. The enactment and enforcement of laws and an increase in people’s environmental protection awareness are accelerating advanced research into ultra-clean gasoline production.

The use of hydrotreating in improving the quality of oils

Hydrotreating or hydroprocessing encompasses different hydrogenation procedures that saturate unsaturated hydrocarbons accompanied by the removal of sulphur, nitrogen, oxygen, and metals through processes, such as hydrodesulphurization (HDS), hydrodenitrogenation (HDN), hydrodeoxygenation (HDO), and hydrodemetallization (HDM) from various streams of petroleum during refining. Hydrotreating of crude oil is mainly aimed at reducing the emissions of air pollutants, as well as avoiding poisoning of noble metals and acid catalysts utilized in catalytic reforming and cracking and the improvement of the quality of the fuel [ 46 , 55 ]. The application of hydrotreatment in a hypothetical refinery is shown in Fig.  1 . Hydrotreating is a non-destructive process useful for the improvement of product quality without appreciable change(s) in boiling range [ 56 ]. Hydroprocessing of the feed is a productive way not only to lessen the final sulphur content of liquid distillates; nevertheless, it equally promotes an increase in the yields of liquids, while enhancing the standard (cetane number) of the diesel, but reducing the octane number of the gasoline produced [ 47 ]. The main technologies employed for the desulphurization of heavy oil industrially are hydrodesulphurization (HDS) in combination with carbon rejection methods like coking and fluid catalytic cracking (FCC) [ 31 ].

Application of hydrotreatment (HT) in a hypothetical refinery [ 46 ]

The ever-increasing global attention received by the deep desulphurization of fuel oils was due to the more stringent sulphur regulations. Sulphur compounds present in fuels, as it is known caused severe environmental pollution as well as severe corrosion of reactors and equipment used in oil processing. The poisoning activities of sulphur in catalytic processes are undeniable. Thus, considering both the industrial and environmental effects of sulphur, the desulphurization of petroleum products is essential [ 57 ]. In the refining industry, the term hydrotreating, hydroprocessing, hydrocracking, as well as hydrodesulphurization are rather loosely used, since cracking and desulphurization operations occur concurrently in the hydrodesulphurization and hydrocracking processes and it is relative as to which stand out. Here, hydrotreating refers to a moderately mild operation whose principal goal is to decrease the sulphur content (and not to alter the boiling range) of the feed [ 58 ].

The most widely employed commercial technology is the catalytic hydrodesulphurization (HDS), apart from being efficient towards the removal of thiols, sulphides, and disulphides, it is not ideal for the removal of thiophenes specifically the alkyldibenzothiophenes [ 57 ]. A typical HDS unit is shown in Fig.  2 . HDS is a catalytic procedure widely employed in the removal of sulphur from processed petroleum products like diesel fuel, gasoline, jet fuel, as well as fuel oils. The reason for sulphur removal is to decrease the emissions of SO 2 .

A typical hydrodesulphurization unit [ 58 ]

The reaction to the main HDS procedure is:

Sulphur-containing compounds react with hydrogen in the presence of a catalyst and get converted into hydrocarbons and hydrogen sulphide. Hydrogen sulphide produced and obtained is in the gas phase and it is being removed easily [ 59 ]. The HDS procedure entails the catalytic treatment with hydrogen to change the different sulphur compounds present into hydrogen sulphide. Here, some of the hydrogen sulphides are oxidized by air to form sulphur dioxide and the sulphur is thus removed by the following overall reaction:

The HDS is a representative of an essential component of the present petroleum refining operations in terms of environmental as well as economic considerations. Presently, crude oils are becoming heavier with rising amounts of sulphur. Since the size of the organic molecules in the heavy oil fractions is large, the HDS catalyst supports have primarily mesopores as well as micropores. Typical HDS catalysts contain CoMo or NiMo sulphides supported on alumina [ 60 ].

To achieve the sulphur specification as required by the strict regulations, it is imperative to embrace new technologies for the reduction of sulphur [ 61 ], and possible procedures like adsorption, extraction, oxidation, as well as bioprocess are used for ultra-deep desulphurization [ 57 ]. In the last few years, much attention has been given to ultra-deep desulphurization from diesel due to the stringent regulations of sulphur levels in many countries across the globe. At the moment, the development of varieties of desulphurization methods meant for the reduction of sulphur-containing compounds to an ultra-low level from diesel fuel has taken place. One of the alternative methods meant for ultra-deep desulphurization is oxidative desulphurization (ODS). Here, the compounds containing sulphur like benzothiophenes (BT), dibenzothiophene (DBT), and 4, 6-dimethyl dibenzothiophene (4, 6-DMDBT) are oxidized to their corresponding sulphones which can be separated from the diesel using extractant and this achieved the ultra-deep desulphurization of diesel fuel [ 62 ]. The goal of the elimination of sulphur from petroleum products is to decrease the sulphur dioxide (SO 2 ) emissions that come from numerous combustion practices. Moreover, it is vital to remove sulphur from the naphtha streams within petroleum refinery, because, at extremely low concentrations, sulphur poisons the noble-metal catalysts (platinum and rhenium) in the catalytic reforming units that are subsequently utilized to improve the octane rating of the naphtha streams [ 58 ].

However, the design of improved catalysts for hydrotreatment is of predominant importance not only in the reduction of environmental pollution but also in process optimization for feedstocks of low grade [ 37 ]. The research and development circle has greeted these challenges with the introduction of varieties of new catalytic processes and looking for ways of modifying many of the substantial refinery processes of manufacturing the fuels, lubricants, petrochemicals, as well as special products needed by a more value-conscious and environmental alert society. Many of these process innovations have come in place as a result of recent applications of zeolite catalysts [ 33 ]. This can be realized by developing more active or selective catalysts for the hydrotreatment (HDT) processes [ 63 ]. The core of the reforming process is the catalyst and most of the notable improvement in the process was attributed to the development of improved catalyst [ 64 ]. A specific catalyst used for the conventional hydrotreatment of crude oil includes shaped alumina-supported sulphided Mo promoted by either Co or Ni [ 65 ]. The regular process employed in removing heteroatoms as well as quality upgrading of petroleum in the refining industry is the catalytic hydrotreatment using supported Ni–Mo or Co–Mo catalysts, and for many decades, it has been commercialized [ 43 ].

The evolution of the newer generation of catalysts is essential to meet up with the challenges coupled with the HDS of numerous petroleum fractions. The improvement of the activity of the catalyst had experienced various attempts such as changes in the active metal composition, use of disparate types of active metals, supports, and additives, etc. among which the vital one is the variation of supports. Molybdenum promoted with cobalt and nickel supported on alumina makes up most of the HDS catalysts utilized for industrial applications [ 42 ]. Due to their introduction, 40 years ago, into the refinery, zeolite catalysts have been the origin of notable improvements in the yield of gasoline and octane along with the production of cleaner fuels and lubricants with enhanced performance properties [ 33 ].

Gamma alumina, γ-Al 2 O 3 , supported NiMoS and CoMoS are widely utilized for the hydrotreating of petroleum in oil refineries. The amelioration of the aforementioned catalysts is required given the greater environmental concern as a result of the pollution caused by automobiles as well as the subsequent limitations of the sulphur contents in gasoline and diesel fractions due to the stringent environmental regulations [ 66 ]. Platinum supported on gamma-alumina (Pt/γ-Al 2 O 3 ) is a well-known reforming catalyst up to date; its interaction with sulphur is recognized. More importantly, sulphur can cause the modification of the reforming Pt/γ-Al 2 O 3 catalysts through the adsorption over the hydrogenolysis sites of the catalyst, consequently, altering the selectivity of the catalyst in reactions of olefins that are competitive, to which isomerization is preferred to reactions of hydrogenation and hydrocracking. The partial or complete removal of sulphur by hydrogen treatment made this effect reversible, although the poisoning of the catalyst is due to the remaining sulphur that is irreversibly held. Apart from their general use in naphtha reforming, platinum (Pt) catalysts were identified a long time ago as desulphurizing catalysts, which is advantageous. Specifically, the especially good hydrogenating ability of platinum (Pt) makes it suitable as a potential catalyst meant for deep hydrodesulphurization (HDS) and hydroaromatization (HAD) [ 67 ]. Sulphided transition metals have been widely employed as hydrotreating catalysts. Hydrodesulphurization catalysts are predominantly little crystallites of sulphided Mo promoted with either Co or Ni and alumina as the support [ 68 ].

Pt and Pd, either as monometallic or alloyed catalysts, have demonstrated excellent catalytic activity in the hydrodesulphurization (HDS) of dibenzothiophene (DBT) molecules. 4,6-DimethylDBT is the common representative member belonging to the family of the highly refractory 4,6-alkyl-substituted DBTs, often referred to as the molecule of interest (target), to be hydropurified in heavy oil cuts. However, the HDS of this molecule can occur through the hydrogenation (HYD) or the direct desulphurization (DDS) routes. The steric hindrance imposed by the alkyl substituents of 4,6-dimethylDBT has been investigated both experimentally and theoretically, and they have been found to inhibit the scission of the sulphur heteroatom through DDS, consequently making the conversion through HYD salient to the achievement of the required HDS rate [ 69 ].

The developed catalysts for hydrotreating include tungsten and molybdenum sulphided on alumina. These metals are often referred to as hydrogenating catalysts; however, the addition of either sulphided cobalt or nickel caused the modification of their properties. Hydrogenation catalysts include nickel sulphide, nickel thio-molybdate, tungsten, and nickel sulphides, as well as vanadium oxide. Sulphided cobalt and molybdenum catalysts supported on alumina are to a great degree in use today; since they have proven to be tremendously selective, regeneration is undemanding, as well as been resistant to poisons [ 58 ]. Tungsten phosphide (WP) was studied, and it was found that it possessed high hydrogenation/dehydrogenation activity, but was highly sensitive to piperidine inhibition. The methyl groups suppressed the direct desulphurization of 4,6-DMDBT, but significantly promoted the hydrogenation of 4,6-DMDBT and the dehydrogenation of 1,2,3,4-tetrahydro-4,6-dimethyldibenzothiophene (TH-4,6-DMDBT) and 1,2,3,4,4a,9b-hexahydro-4,6-dimethyldibenzothiophene, but decreased the rate of hydrogenation of TH-4,6-DMDBT (Fig.  3 ) [ 70 ].

Adapted from Yang et al. [ 70 ] Copyright 2015 Elsevier BV)

Desulphurization mechanism and pathways (

Typically, there is a need to activate these metals by converting the hydrogenation metals from the oxide to the sulphide form, because they are usually in the oxide state when purchased. The catalysts meant for economical sulphur removal contains cobalt and molybdenum sulphides (CoMoS) supported on alumina. Nonetheless, if nitrogen removal is of remarkable consideration, then the catalysts composed of nickel–cobalt–molybdenum or nickel–molybdenum (NiMo) compounds on alumina supports are more effective.

Composition of metallic composites

Metallic composites are materials with higher specific properties, e.g., stiffness, strength, etc., and less density. They exhibit upgraded properties emanating from combinations of two or more materials. The combination usually comprises of a matrix and a reinforcement; the former is responsible for distributing the applied stress, while the latter ensures the provision of the required mechanical strength [ 71 ]. Metallic composites are made through the dispersion of reinforcing material in a metal matrix [ 72 ]. Table 1 presents a summary of metal composites and supports to promote and expedite chemical processes involving the treatment of heavy/crude oils and conversion to useful, low-molecular-weight petrochemicals. There are two main components of new fluid catalytic cracking (FCC) catalysts which are zeolites and a matrix. Additionally, other catalysts also contained additives which boost octane number of gasoline, enhancing the resistance of catalyst to metal, sulphur dioxide (SO 2 ) emissions reduction, as well as the oxidation of carbon monoxide (CO) [ 73 ].

The matrix is the monolithic material that the reinforcement is embedded, and is completely continuous. The reinforcement surfaces are often coated to forestall a chemical reaction with the matrix. This means that there is a path through the matrix to any point in the material, unlike two distinct materials sandwiched together. The active catalytic component of a catalyst is carried by the catalyst matrix material [ 74 ]. Outstandingly, the active matrix ensures an extensive performance of the FCC catalyst by pre-cracking sizeable oil molecules to enhance penetrability to the zeolite’s micropores. The inert portion of the matrix, referred to as filler, generally functions as a heat sink and transfer medium as well as providing little or no catalytic activity to the catalyst. For optimum fluidization properties, the filler also offers mechanical strength and improves the apparent bulk density (ABD) of the particle. The binder mainly acts as a glue, jointly binding zeolite, matrix, and the filler as well as providing huge attrition resistance. The binder occasionally modifies the coking attributes and acts as an inhibitor for poison species [ 75 ]. Additionally, the matrix in many commercial catalysts is the amorphous silica, alumina, or silica-alumina. Concerning the matrix’s catalytic roles, it is principally responsible for the catalyst’s physical properties [ 73 ].

The multifaceted structure and composition of metallic composites are reasons why they exhibit complex behaviour. The stability, activity, and selectivity of the metallic composite catalyst are predicated upon the intrinsic activity and localization of the acid sites and the metal [ 76 ]. The proximity of the metal and acid sites, a phenomenon described as “site intimacy”, affects the selectivity of the reaction. Mechanical mixing and impregnation are the two commonly employed methods of synthesizing metallic composites [ 77 ].

Support for metallic composites

Presently, the use of new materials as supports for catalysts is one of the numerous efforts aimed at improving the activity of catalysts [ 78 , 79 , 80 ]. The most relevant advances in the history of catalytic cracking were the introduction of porous materials as fillers and supports, specifically zeolites and clay, in commercial FCC catalysts in the early 1960s [ 81 , 82 ] for various industrial processes [ 83 ]. With little capital investment, zeolite catalysts provided a greater profit [ 81 ], cost-effective, and environmental benefits such as the replacement of low-selective harmful mineral acids and chloro-containing catalysts, improved selectivity and yields of reactions, products’ quality, and the catalytic systems’ overall life, as well as the reduction of the energy consumption at the same time [ 2 , 83 ]. The zeolite catalysts have high activity as well as selectivity when compared to amorphous silica–alumina catalysts. However, this higher activity and selectivity implied a more profitable yield of liquid products and additional cracking capacity [ 82 ]. Improvement in catalytic efficiency and stability led to growing scientific interest and research into the development of novel materials and advanced framework architecture [ 84 , 85 , 86 ].

For a long time, the value of the support for catalysts used in HDS has been known, as well as the activities of supports like SiO 2 , MgO, ZrO 2 , TiO 2 , and others has been extensively studied [ 87 , 88 ]. The use of mixed supports has been proposed to repress these catalytic setbacks such as a tendency for poisoning, instability, etc. [ 89 ]. Catalysts made with amorphous silica–alumina supports have demonstrated better results in the HDS of 4,6-DMDBT connected to increased acidity and fewer surface hydroxyls attached to tetrahedral aluminium [ 68 ]. Thus far, the problem that has attracted extensive research efforts has been the efficiency and deactivation of the catalyst. The support’s pore structure and the interaction between the active phases and the support are the two most important factors affecting the efficiency and stability of most catalysts. Jointly, these two factors influenced the active components' dispersion, their reducibility, as well as sulphidability, and the substrate molecule’s accessibility to the catalytic sites. Thus, the vital problem made more crucial by the strict regulations concerning the permissible sulphur level in fuels is the discovery of good support for the Ni–Mo catalysts [ 5 , 44 , 90 ]. The only known support utilized in industrial HDS catalysts is γ-Al 2 O 3 . Various studies have been intensified on many other supports such as SiO 2 , carbon oxides, and mixed oxides, while the promising materials being mesoporous silica oxides which are used as support possessed an ordered mesoporous framework [ 50 ].

Alumina-based supports have proven to be the common supports for the hydrotreatment (i.e., hydrodesulphurization) of petroleum [ 91 ], as heterogeneous catalysts as well as sorbents for huge molecules [ 78 ]. The introduction of metal oxides as supports which investigated the catalytic activities of the sulphided CoMo catalysts and their consequent dependence on the nature of the support and additives incorporation is not new [ 91 ]. Many materials have been employed as supports for HDS catalysts [ 43 , 92 ], having acidic or basic sites such as carbon, alumina, mixed oxides, and porous materials. The most applicable ones reported so far include Al 2 O 3 , SiO 2 , ZrO 2 , TiO 2 , MgO, as well as mixed oxides among others [ 45 ]. Molybdenum-based catalysts have been known for ages to interact strongly with alumina supports. The interaction occurring between the active-phase and the support potentially affected the spread of the catalytic material and its reducibility and sulphidability, and thus, attention has been directed towards the investigation of alternative supports.

The formation of nanocomposite materials via the introduction of catalytic active species into a matrix is an aspect of catalysis with growing interest [ 54 ]. The nanocomposites’ surface properties have been linked to their functionality, including surface-to-bulk atomic ratio, polyhedral surface framework, surface defects’ concentrations (coordinatively ions that are unsaturated due to planes, edges, corners, anion/cation vacancies, and electron excess centers), surface acid–base strength, as well as shape selectivity [ 93 ]. In recent times, much attention has been given to zeolites such as ultrastable Y zeolite (USY) and mesoporous materials like Mobil Composition of Matter number 41 (MCM-41), hexagonal mesoporous silica (HMS), Santa Barbara Amorphous (SBA-15), and mesoporous Al 2 O 3 systems [ 42 , 94 , 95 ]. For instance, it was experimented that CoMo or NiMo catalysts supported on MCM-41 are better catalysts when compared to the commercial alumina-supported Co–Mo, Ni–Mo catalysts employed for desulphurization [ 43 , 78 ]. The higher catalytic activities of CoMo supported on MCM-41 were reported for the conversion of benzothiophene and petroleum residues as well as being more substantially active than CoMo/γ-Al 2 O 3 catalysts at high molybdenum loadings. The relatively higher catalytic activity reported in the study was attributed to the uniform and regular pore structure of the tubular-shaped MCM-41 as opposed to the unimodal pore structure of γ-Al 2 O 3 . This finding is expected to help direct future research into the application of MCM-41 as catalyst support for smaller molecules [ 91 , 96 ]. The findings of the study showed that the use of chelating agents during the preparation of catalyst increases its pore diameter, providing better stability for the catalyst in the process.

The discovery of many other types of ordered mesoporous materials such as HMS, KIT, FSM-16, as well as SBA types have been reported [ 97 , 98 ]. With a variety of templates, these materials synthesized in either acidic or basic media offered the opportunity to synthesize materials with a tunable pore size [ 99 ]. However, materials like MCM-41 have been found to possess poor hydrothermal stability which presents a pressing restriction to their applications [ 66 ]. A proposed configuration for hydrogen production from the fuel cell is shown in Fig.  4 [ 100 ]. To improve the thermal stability of these materials, the wall thickness must be increased [ 43 , 99 ]. In addition to an increased wall thickness, the introduction of heteroatoms such as aluminium and titanium into the material framework could also bring about an improvement in the material’s thermal stability.

Proposed configuration for hydrogen production and electricity from diesel fuel [ 100 ]

Benefits, limitations, and effects of catalytic activities of metallic composites in cracking reactions

Herrera et al. [ 101 ] reported the hydrodesulphurization of dibenzothiophene (DBT) and 4,6-dimethyl dibenzothiophene (4,6-DMDBT), using NiMo catalyst supported on MCM-41 having 2 wt% P 2 O 5 . They concluded that the catalyst was moderately more active than other NiMo catalysts containing phosphorus. The NiMo/MCM-41 catalysts demonstrated a higher catalytic activity in 4,6-DMDBT HDS than the NiMo/Al 2 O 3 used as reference material. Consequently, it can be deduced that the modification of MCM-41 with phosphorous produced high-performance NiMo catalysts chiefly for dibenzothiophene compounds. The analysis of the yield of the product revealed that the inclusion of phosphorus boosts the hydrogenation pathway of HDS [ 102 ]. The modified phosphorus mesoporous molecular sieves demonstrated reassuring attributes as supports for Mo-based hydrotreating catalysts ideal for the removal of refractory sulphur compounds like 4,6-DMDBT [ 101 ]. The production of clean energy fuels would be greatly facilitated by the use of phosphorus-modified catalysts, particularly as it aids the hydrogenation pathway of hydrodesulphurization during upgrading.

The conversions of N and S of NiMo/SBA-15 catalysts at regular state conditions are shown in Fig.  5 . The HDS, as well as HDN activities gotten with NiMo, supported on siliceous SBA-15, are not as effective as their boron or alumina-supported equivalents. This perhaps was due to a very feeble interaction of Mo and the siliceous support that does not afford large dispersion of the sulphided molybdenum kinds. Both conversions of N and S are significantly increased with a rise in Mo loading from 7 to 12 wt%. When Mo loading was further increased to 17 wt%, there was an extensive increase of N conversion, whereas the S conversion was improved slightly. On increasing Mo loading to 22 wt%, there was no significant improvement in both HDS and HDN activities of NiMo/Al-SBA-15. From their studies, it was evident that NiMo/Al-SBA-15 catalyst having 17 wt% Mo is the most potent catalyst for both HDN and HDS of light gas oil (LGO) [ 66 ]. It can be concluded that the catalytic investigation of LGO using Al-SBA-15 supported NiMo catalyst with varying loadings of Mo demonstrated that 17 wt% of Mo loaded catalyst is preferable and more efficient.

Copyright 2008, Elsevier BV)

Comparison of HDN and HDS functionalities of Si-SBA-15, B-SBA-15, and Al-SBA-15 supported NiMo catalysts. Feed = LGO, T  = 370 °C, P  = 8.8 MPa, WHSV = 4.5 h1, and H 2 /oil ratio = 600 (v/v) (adapted with modification from Sundaramurthy et al. [ 66 ]

Dai et al. [ 49 ] reported the sulphur reduction in FCC making use of kaolin in situ crystallization catalyst modified with vanadium. The modification by vanadium(V) of kaolin in situ crystallization catalyst increased the total acidity of the kaolin, and it enhanced the weak Lewis acidity. The crystal structure of the catalyst was not damaged by vanadium. Thus, the reduction in the activity of the catalyst was not significant. There was an increase in the weak Lewis acidity which was as a result of the increase in its vanadium content, thus leading to an enhancement in the extent of reduction of sulphur content. The kaolin modified with 0.6 wt% vanadium gave a sulphur reduction of 34.5% in the sulphur content of the liquid product, while the modified kaolin with 0.45 wt% vanadium gave the best performance [ 49 ]. While it is important to carefully consider the choice of metals used in catalyst modification due to potential toxicity, a relative upsurge in the % wt of metals in catalyst composites would significantly aid the desulphurization processes of conventional and unconventional oils.

Na 2 SiO 3 /SiO 2 , a solid alkaline heterogeneous catalyst that is readily affordable and caused a reduction of the disadvantages of conventional homogeneous catalysts, having enhanced activity for transesterification reaction of rubber seed oil, was used for the production of biodiesel from rubber seed oil. Because of its solid phase nature, it demonstrated its obvious advantages as well as high activity in the regeneration of the catalyst [ 103 , 104 ]. The study revealed that the amount (in percentage) of the catalyst used affected the conversion efficiency of the rubber seed oil (Fig.  6 ). By and large, the increase in the amount of the catalyst gave rise to the conversion efficiency, because more active sites are available. The highest conversion was observed using 3% weight of the catalyst, while further catalyst addition did not affect the conversion efficiency but rather led to saponification as well as energy wastage [ 19 , 103 ]. The biodiesel produced from rubber seed oil with Na 2 SiO 3 /SiO 2 was of better quality as seen from its properties (Table 2 ), in comparison with the ASTM standard for biodiesel. The use of SiO 2 as support created a well-pored structure for the catalyst and aided the efficiency of its regeneration.

Copyright 2010, Springer)

The effect of the amount of catalyst on conversion efficiency (adapted with modification from Nguyen et al. [ 103 ]

The catalytic cracking of used palm oil (UPO) as reported by Chang and Tye [ 17 ] was experimented using four catalysts; H-ZSM5, Cu-HZSM5, Zn-HZSM5, and Mg-HZSM5 at a reaction temperature of 350 °C for 1 h. Mg-HZSM5, among the three metal-loaded zeolite catalysts, gave the highest yield of liquid product (84.7 wt%) and gasoline fraction of 8 vol%, as shown in Table 3 . With the conversion of 26.74 wt%, Zn-HZSM5 catalyst gave the highest conversion. The ratio of mesophase to microphase in the composite catalyst performed a vital role in raising the gasoline fraction and conversion in the catalytic cracking of UPO and soybean [ 17 , 18 ]. A reduction in this ratio caused a corresponding increase in the gasoline and conversion of UPO. It can be inferred that a composite catalyst having 5 wt% of magnesium was found to be the most active in the catalytic cracking of the UPO.

The desulphurization of heavy oil with iron oxide-based catalysts using steam was studied in the catalytic cracking of atmospheric residual oil (AR) by Fumoto et al. [ 105 ]. The reaction was performed at 748 K in a fixed-bed reactor at atmospheric pressure. There was an increase in the yield of hydrogen sulphide with a higher ratio of steam to feedstock, whereas in the catalytic cracking of AR without steam, the hydrogen sulphide produced was minimal [ 105 , 106 ]. There was a reduction in the concentration of sulphur of oil to half of the corresponding feedstock, while some sulphur compounds were deposited on the catalyst. Oxidative cracking of heavy oil making use of oxygen species obtained from steam produced hydrogen species from steam, part of the hydrogen reacted with the heavy sulphur compounds present on the catalyst to produce hydrogen sulphide, light sulphur compounds, and hydrocarbons. Carbon dioxide as well as hydrogen sulphide were obtained when dibenzothiophene was decomposed using the catalyst. Therefore, both cyclic as well as acyclic sulphur compounds were decomposed using the catalyst and steam [ 105 ].

Hajimirzaee et al. [ 107 ] studied the effect of the reaction conditions, catalyst support, as well as zeolite modification in the dehydration of methanol to light olefins upon zeolite/alumina catalysts in a fixed-bed reactor. The production of light olefins more selectively is dependent on the high temperature of 400 °C; however, elevated temperatures led to the deactivation, selectivity towards alkanes as well as lower selectivity to light olefins. The study revealed that huge concentrations of water in the feedstock resulted in higher production of light olefins, and catalytic selectivity to propene and other light olefins was enhanced with the use of γ-alumina as support. Zeolite catalyst having 25 wt% of ZSM-5 present in the sample of the catalyst resulted in maximum selectivity to propene and light olefins, but faster deactivation of these catalysts was noticed [ 107 , 108 ]. The modification of ZSM-5 zeolite with P, Cs, Ca, and Fe led to an increase in shape selectivity to propene in all cases. The exchange of ZSM-5 zeolite ion by Cs resulted in optimal selectivity to propene through the changing of the acid site distribution. The least selectivity to C 5+ compounds as well as the smallest quantity of coking was noticed on this catalyst.

The application of novel zeolite Y nanoparticles in fluid catalytic cracking (FCC) reactions was reported [ 73 ]. Crystalline nanoporous materials such as zeolites are used for a variety of applications in the industries. They possess distinguished properties like huge surface area, uniform porosity, accessible pore volume, large adsorption capacity, ion exchange capacity, and improved catalytic activity, based on shape and size; they are selective, etc. The parent zeolite nanoparticles were synthesized and the catalytic cracking experiments of 1,3,5-triisopropylbenzene (TIPB) were carried out in a laboratory-scale novel riser simulator at 400 °C using nitrogen as the carrier gas. Selectivity towards the production of light hydrocarbons was observed with the use of zeolite Y nanoparticles. Significant conversion was recorded when catalysts containing average sizes of 150 and 450 nm nanoparticles were used. The study indicated the feasibility of developing nanocatalysts with stable structures.

The effect of the method employed in the preparation of Ag-modified Ti-HMS catalyst, especially as it affects its structure and catalytic oxidative desulphurization performance was reported by Wang et al. [ 57 ]. Ag-modified mesoporous molecular sieves Ti-HMS were synthesized in situ (Ag/Ti-HMS-I), deposition–precipitation method (Ag/Ti-HMS-D), as well as impregnation assisted by ultrasound process (Ag/Ti-HMS-U), respectively. The catalysts’ performances in the oxidative desulphurization (ODS) of benzothiophene with hydrogen peroxide (H 2 O 2 ) were studied. Experimentally, the results revealed that the Ag/Ti-HMS-U catalyst demonstrated the best activity and this was attributed to the mesoporous structure which was relatively good as well as the huge Ag dispersion [ 57 , 109 ]. The efficiency of oxidative desulphurization for the removal of aromatic thiophenes, as opposed to catalytic hydrodesulphurization, was reflected in the method’s high selectivity and mild reaction conditions. Ag/Ti-HMS-U catalysts prepared via ultrasound-assisted impregnation have enormous potential in the removal of sulphur from crude and heavy oils, in a bid to meeting the stern environmental regulations. However, oxidative desulphurization processes are relatively more expensive due to the use of oxidative reagents and/or high-energy intensity.

Andrienko et al. [ 110 ] reported the study on the removal of sulphur-containing compounds from fuel oils using a naturally occurring iron oxyhydroxide. Studies have shown that water deferrization waste like iron oxyhydroxide could be used as catalysts for the removal of sulphur-containing compounds (SCC) present in fuel oil through the oxidative desulphurization process. The iron oxyhydroxide was activated via heat treatment. However, the maximum parameters of the oxidation process were obtained; each time, SCC was removed from the fuel oil in the oxidative desulphurization process [ 31 , 111 ]. During this process which consisted of successive stages, the degree of desulphurization obtained for the fuel oil was 92.3% as a result of the successive SCC removal from the fuel oil during the various stages of oxidation of SCC as well as the extraction of the oxidized SCC present in the fuel oil [ 110 ]. The removal of sulphur-containing compounds from the oil feedstock making use of iron oxyhydroxide as a catalyst through oxidative desulphurization could be in huge demand for industrial uses due to the high catalytic activity of this readily available low-cost catalyst in the oxidation processes.

Feedstock recycling is receiving growing awareness due to environmental concerns. Contingent upon this, the conversion of plastic wastes to gasoline range fuels is gaining attention. If not properly guided, the financial costs of these conversion processes may not be economical. Hence, the need arises for the development of efficient, yet cost-effective catalysts to facilitate the conversion processes. Sequel upon this, Qureshi et al. [ 112 ] used a composite catalyst comprising modified Asian clay and pure metallic nanoparticles for the pyrolysis of plastics into liquid fuels in an autoclave reactor under varying reaction conditions. As opposed to the sole use of clay as a catalyst, the nickel-incorporated composite catalyst gave a maximum liquid yield of 79%. This was believed to be facilitated by the increased surface area of the composite catalyst. The properties of the resulting liquid yield satisfied fuel specifications.

The conversion of plastic wastes to liquid fuels was carried out in a stirred autoclave reactor via catalytic hydrocracking using a composite zeolite beta-catalyst. The catalytic activity of fresh catalysts and the regenerated composite zeolite beta-catalysts (BC27 and BC48) were compared under reaction temperature conditions between 360 and 400 °C. Catalytic experiments indicated that 93 wt% conversions were achieved at 400 °C using BC27 and BC48, as opposed to the fresh catalysts [ 113 ]. The regenerated composite zeolite beta-catalysts are a blend of microporosity and mesoporosity. The character of the former demonstrates excellent production of gases, while that of the latter favours the high liquid production. The synthesized composite zeolite beta-catalysts embodied high catalytic stability in the catalytic pyrolysis of plastic wastes to oils.

Wang et al. [ 114 ] investigated the catalytic activity of a series of M-Ni/SiO 2 aerogel catalysts in the cracking of n -decane under temperature conditions of 600–750 °C. Upon comparison with conventional wall-coated catalysts, the M-Ni/SiO 2 aerogel catalysts showed better anti-coking and catalytic activity. Catalyst dosage of only half of the wall-coated catalysts produced these excellent results. The excellent textural properties of aerogel catalysts such as large specific area, low density, and high-temperature resistance are fast widening their popularity as catalyst supports. The use of aerogel catalysts could be further extended to the catalytic cracking of heavier hydrocarbons due to their performance.

The potential of generating naphtha and diesel oil from coal tar oil informed the investigation of the catalytic cracking of coal tar in a low-temperature distillation by aluminium retort using metal-loaded UiO-66 and Y-type zeolite. Both catalysts were synthesized using microwave-assisted and solution thermal methods. The combination of the catalysts resulted in UiO-66-Y metal-loaded catalyst. The use of the composite catalyst ensured the active decomposition of aromatic compounds in coal tar as well as the synthesis of a wide range of light component esters. The study concluded that the composite catalyst has advantages of both molecular sieve materials (high-temperature resistance) and metal–organic frameworks (low density) [ 115 ]. The current global energy quagmire may soon be a forgotten episode upon further investigation into improvement technologies of composite catalysts, with particular emphasis on stability.

Sustainability of metallic composites

In the quest for improved catalytic efficiency, cost reduction of chemical processes, and environmental concerns, significant advancement in material development has taken place in the last 2 decades [ 3 ]. The developments of recyclable/regenerable, sustainable, and eco-friendly metal composite materials have attracted attention and explored globally [ 2 , 116 , 117 , 118 ]. Composite development gravitated towards the use of efficient materials, which include metal-based alloys, polymeric composites, and metal–organic frameworks (MOF) [ 84 ]. Green metal-based composites with significant catalytic properties have been synthesized with plant-assisted processes, as biochemical, environmental-friendly, and cost-effective alternative synthetic routes [ 119 ].

Metal-based catalysts have been embedded/loaded on solid supports to improve interfacial activity and reduce the loss of catalyst, which can be classified as a sustainable approach [ 84 , 116 ]. In the past, carbon-based materials have been harnessed as supports for heterogeneous catalyst, due to high specific surface areas, adjustable porosity, and stability (Fig.  7 ). However, the emergence of N-doped carbon-supported metal catalysts in the last decade has attracted significant attention in catalysis, and the fact that nitrogen doping can be tailored to induce other characteristics to the bulk material, other than catalysis, encourages an integrated and sustainable approach to material development [ 1 , 2 , 3 ]. Integrated approaches include catalysis–adsorption, oxidation–catalysis, photocatalysis–degradation, photodegradation–adsorption, etc., achieved with the same functionalized material/composite [ 117 , 120 , 121 , 122 ]. N-doped carbon-supported metal supports have shown higher catalytic efficiency in several heterogeneous catalytic reactions, than the conventional, pristine carbon supports [ 3 ].

copyright 2019 Nature Research)

Scanning electron microscopic image of carbon-based materials revealing pores suitable for loading of catalyst (adapted with slight modification from Khan et al. [ 127 ]

Metal–matrix composite (MMC) and metal–organic frameworks (MOF) have reported improvements in micro-and macro-structure and mechanical properties such as hardness, tensile strength, and density [ 84 ]. For catalysis and other applications, carefully controlled integration of MOFs and functional materials (e.g., metallic nanoparticles, quantum dots, enzymes, silica, and polymers), enhances the properties, activity, and stability of MOFs and MMCs. Metal–organic frameworks can cooperatively and sustainably minimize catalytic poisoning, and enhance the catalytic activity, selectivity, and stability in a variety of chemical processes [ 84 , 85 , 86 ]. Recent studies recommend the use of sustainable materials such as aluminium–magnesium-based matrix composites and agricultural waste-derived biochars as reinforcements in MMCs for industrial applications. Furthermore, stir casting is regarded as the most cost-effective, simple, and efficient technique for producing MMCs [ 2 , 123 ].

There is a need to reduce the sulphur present in petroleum products to ≤ 1 ppm if it is to be used as feedstock for producing hydrogen meant for fuel cell applications, because of the use of noble-metal catalysts in downstream processes, especially in the fuel cell. Additionally, the conversion of hydrocarbon fuel feeds to the fuel cell set up entails 0.1 ppm sulphur reduction. The implication of this is that a daunting task is required to provide catalysts whose catalytic activities are seven times above the existing catalyst for achieving 500 ppm of sulphur removal. Apart from the high activity, the catalysts should possess distinct activity profiles with respect to different functionalities. The variation of the support is a vital approach that has been attempted for the modification of the catalysts to achieve the above objectives [ 94 , 124 ].

Diverse environmental regulations have focused on the reduction of the extent of environmental pollution through lowering the level of sulphur, nitrogen, and metals, particularly nickel and vanadium contents in commercial fuels. Owing to this, there is a need for the reduction of concentrations of contaminants like sulphur in fractions of petroleum, especially in gasoline and middle distillates [ 91 ]. It is presumed that the hydrotreating processes can occur via the isomerization of a methyl group or the hydrogenation of phenyl rings in sulphur-containing compounds with steric hindrance [ 39 , 125 , 126 ]. The proposed isomerization and hydrogenation processes can be enhanced by the introduction of mesoporous catalysts into support matrices. The hydrodesulphurization processes can also be enhanced by the use of chelating agents during the preparation of the catalyst.

The current industry practice is the predominant use of zeolite as a catalyst during catalytic cracking of crude oil. The selectivity of conventional catalysts for specific reactions during cracking has led to the impregnation of existing catalytic materials with metals to form composite materials. The metallic composites have become prominent catalysts not only for the catalytic cracking of crude oil but also in the catalytic cracking of palm oil and rubber seed oil, leading to the production of usable and reusable high-value products. The metallic composites rely basically on their various properties such as high thermal stability, attrition resistance, and high catalytic activity. Thus, they are the first-choice materials for numerous cracking processes occurring in the petrochemical industries and refineries. However, many challenges still exist in the development of composite catalysts with both mesoporous and microporous advantages. Simultaneously, not every combination of a matrix and reinforcement can be processed into useful composite catalysts except by interfacial modifications, and catalyst poisoning and deactivation are challenges requiring future research endeavour. Therefore, future research should be carried out on surface modification, stability improvement, ease of regeneration, and efficiency optimization of these composite catalysts.

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Department of Chemical Sciences, Adekunle Ajasin University, Akungba-Akoko, Ondo State, Nigeria

Abimbola G. Olaremu, Williams R. Adedoyin & Adedapo O. Adeola

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Olaremu, A.G., Adedoyin, W.R., Ore, O.T. et al. Sustainable development and enhancement of cracking processes using metallic composites. Appl Petrochem Res 11 , 1–18 (2021). https://doi.org/10.1007/s13203-021-00263-1

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Received : 18 November 2020

Accepted : 02 January 2021

Published : 23 January 2021

Issue Date : May 2021

DOI : https://doi.org/10.1007/s13203-021-00263-1

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