Bibliometric and Co-Occurrence Study of the Production of Bioethanol and Hydrogen from African Palm Rachis (2003–2023)

Abstract

Today, the world is increasingly concerned about energy and environmental challenges, and the search for renewable energy sources has become an unavoidable priority. In this context, Elaeis guineensis (better known as the African oil palm) has been placed in the spotlight due to its great potential and specific characteristics for the production of alternative fuels in the search for sustainable energy solutions. In the present study, bibliometric and co-occurrence analyses are proposed to identify trends, gaps, future directions, and challenges related to the production of bioethanol and hydrogen from oil palm rachis, using VOSviewer v.1.6.20 as a tool to analyze data obtained from SCOPUS. A mapping of several topics related to bioethanol and hydrogen production from oil palm bagasse or rachis is provided, resulting in contributions to the topic under review. It is shown that research is trending towards the use of oil palm rachis as a raw material for hydrogen production, consolidating its position as a promising renewable energy source. The field of hydrogen production from renewable sources has undergone constant evolution, and it is expected to continue growing and playing a significant role in the transition towards cleaner and more sustainable energy sources, potentially involving the adoption of innovative technologies such as solar-powered steam generation. From an economic point of view, developing a circular economy approach to bioethanol and hydrogen production from oil palm rachis and waste management will require innovations in material design, recycling technologies, and the development of effective life cycle strategies that can be evaluated through computer-assisted process simulation. Additionally, the extraction and purification of other gases during the dark fermentation method contribute to reducing greenhouse gas emissions and minimizing energy consumption. Ultimately, the sustainability assessment of bioethanol production processes is crucial, employing various methodologies such as life cycle assessment (LCA), techno-economic analysis, techno-economic resilience, and environmental risk assessment (ERA). This research is original in that it evaluates not only the behavior of the scientific community on these topics over the past 20 years but also examines a less-studied biofuel, namely bioethanol.

1. Introduction

In a world increasingly aware of the importance of renewable energy sources and environmental sustainability, the production of bioethanol and hydrogen has emerged as a priority in the search for alternatives to fossil fuels [1]. These two biofuels represent crucial pillars in the transition to a cleaner and more sustainable energy future [2]. In this context, African palm oil emerges as a potentially valuable source of raw material for the production of bioethanol and hydrogen, which adds a new level of relevance to this plant in the renewable energy agenda [3]. African palm oil, scientifically known as Elaeis guineensis, is a plant native to tropical Africa that has acquired a fundamental role in the agroindustry industry and the production of palm oil [4]. However, its potential goes beyond being just a source of vegetable oil. African palm oil has exceptional characteristics that make it ideal for the production of bioethanol and hydrogen, making it a promising resource in the search for sustainable energy solutions [5].

Currently, no other document has been published that makes such a deep analysis of the last twenty years on this topic. This makes the information provided in this study more valuable, as it provides valuable information on the current state of research in this field and can serve as a basis for future research and technology development. Therefore, the development of this research represents a significant contribution to favorable research trends with respect to the research topic.

The life cycle assessment (LCA) method provides a more comprehensive view for optimizing sustainability, enabling the comparison of different methods within the same production chain. It also helps pinpoint precise areas for the improvement or study of various strategies to reduce environmental impacts. In parallel, a techno-economic analysis offers a focused approach to optimize the productive and financial performance of the process. In this case, assessment tools are employed through multiple parameters, supporting effective decision-making. Environmental risk assessment (ERA) offers pre- and post-production insights by implementing monitoring plans, guidance, and scientifically based evaluations of the environmental risks associated with a product. This approach ensures a proactive and informed understanding of potential environmental impacts across the entire lifecycle of production [6,7,8]. Therefore, this study aims to consider all these methodologies to assess how extensively they have been studied and developed. On the other hand, economic and techno-economic resilience refers to the ability of a process to remain unaffected by changes in the technical or economic environment or, failing that, to recover easily.

We will explore in detail the African palm tree and its unique characteristics, as well as its chemical composition, focusing especially on the compounds relevant to the production of bioethanol and hydrogen [9]. As this exploration progresses, it will be discovered how the African palm tree could play a crucial role in the transition to a more sustainable energy future, leveraging its chemical components to drive the production of key biofuels in the fight against the depletion of fossil resources and the reduction in greenhouse gas emissions [10]. The production of bioethanol and hydrogen from palm oil fronds was analyzed through bibliometric and co-occurrence studies. Bibliometrics is a research technique that uses bibliographic data to analyze scientific production in a given field [11]. In this case, it was used to identify the number of research papers related to the production of bioethanol and hydrogen from palm oil fronds, determining the trends and patterns in the research. In addition, a co-occurrence analysis was performed to identify the most frequent keywords used in the research papers and understand the relationships between the different concepts related to the production of bioethanol and hydrogen from palm oil fronds. The study provides valuable information on the current state of research in this field and can serve as a basis for future research and development of related technologies. This research aims to shed light on the potential of palm oil as a renewable energy source, highlighting its contribution to the diversification of energy sources and its importance in mitigating climate change [12].

2. Materials and Methods

In this research, a bibliographic review of publications and document types by year was carried out from 2003 to 2023 that contains the topic searched through the Scopus database. We analyze its global geographical distribution (top 15 countries) as well as distribution in South America (top 5 countries) in the last 20 years; the behavior of citations of key documents in the research topic; and the behavior of the citations of research papers with respect to the number of documents published from 2003 to 2023. For the co-occurrence graph, VOSviewer software v.1.6.20 was used, in which the most important keywords were taken into account, analyzing their frequency and the connection between them and groups. A graph and co-occurrence analysis were realized according to the trends of the last 15 years.

Later, a detailed procedure is presented for carrying out a bibliometric and co-occurrence study applied to the production of bioethanol and hydrogen from palm oil bunches This study was carried out using the VOSviewer v.1.6.20 software and is based on data collected from the Scopus platform. The main objective is to analyze the scientific literature related to this area of engineering and identify patterns of co-occurrence between relevant concepts, authors, or publications. This bibliometric and co-occurrence approach provides a deep insight into the trends and relationships in the field of study.

2.1. Bibliometric Analysis

In this case study, an initial collection of documents reported in the Scopus database was carried out, focusing on the production of bioethanol and hydrogen from palm oil fronds. The purpose of this search was to contextualize the importance of bioethanol and hydrogen as renewable energy sources while highlighting the potential of palm oil as a raw material for their production. This involved an exploratory search of existing research that would address the relationship between bioethanol and hydrogen with the characteristics of palm oil as a raw material for their production. A thorough analysis was carried out in the Scopus database, paying special attention to the terms present in the keywords, abstracts, and documents. The search began using the first route “Bioethanol and Palm” and then the second route “Hydrogen AND Palm AND empty bunches” was incorporated. This study aimed to identify and analyze the existing scientific literature on this topic and its relevance in the context of renewable energy and the use of palm oil as a key resource for the production of bioethanol and hydrogen. A general search with the words “Bioethanol” and “Hydrogen” was avoided due to the large number of publications. On the other hand, search routes as detailed as “bioethanol and palm and African” and “hydrogen and palm and African” were not considered due to the small number of reported documents, insufficient for the development of the described methodologies.

With the information obtained from SCOPUS, an analysis was carried out of the number of research documents, including research papers, conference proceedings, books, book chapters, and review papers, distributed by year (2003–2023). In addition, an analysis of the distribution of published documents by type was carried out in order to identify the importance of original research and to understand trends in publications. This approach allowed us to explore the role of conferences and professional meetings in the dissemination of knowledge related to the production of bioethanol and hydrogen from palm oil fronds. In summary, we sought to perceive the evolution of research and the influence of different types of documents in the dissemination of knowledge in this specific field. Following this, we observed the top 15 countries with the highest number of publications from 2003 to 2023, closely monitoring the analysis of trends in countries where palm oil production is greatest. For the case of South America, an analysis of the top 5 countries was carried out to get a sense of the impact and trends in the production of documents at the national level. To conclude this section, the number of documents and reviews was compared with the number of citations per year, looking for correlations in the variation of both, in the last 15 years.

2.2. Co-Occurrence Study

By exporting the same detailed bibliometric analysis information in a CSV file, VOSviewer software was used to evaluate the co-occurrence of words. The minimum number of keywords was set at five in order to diminish the noise in the co-occurrences map, to ensure the most relevant keywords to the topics were selected, and to avoid confusion or digressions that could affect the results. This was followed by filtering out frequent words unrelated to the main topics to be developed, such as “documents” and “humans”, which do not help determine trends in the research addressed in this study. After this filtering, the software generated a map of connections and co-occurrence lines. This map shows the frequency of keyword occurrences, highlighting connections between these keywords and emphasizing the number of links and the strength of those connections. Additionally, the keywords were grouped into clusters identifiable by different colors, simplifying the identification of interconnected research trends.

Subsequently, the most relevant keyword clusters were selected for further analysis. Another map was visualized, showing the evolution of trends over the years, which were set at ten due to VOSviewer’s limitations. The analysis of this map involved identifying connections between keywords over specific time intervals, allowing for observation of the evolution in the research field. Changes in the scientific community’s priorities regarding the production of bioethanol and hydrogen from African palm rachis were examined, and advancements in these fields were tracked.

2.3. Relevant Publications Analysis

In order to fulfill the objective set out in the present document, a detailed search was carried out for the most relevant documents already selected, emphasizing the abstract, methods, new advances, and conclusions, creating a new, more exhaustive filter for the documents of interest. In this section, the filtered documents from 2024 are taken into account, as there are still topics under investigation. Despite the exclusion of documents, it was possible to select relevant data for compilation. In this stage, qualitative and quantitative analysis was carried out to initiate the discussion of results, indicate future directions, and draw conclusions. Figure 1 shows a schematic of the methodology used in the elaboration of the bibliometric and co-occurrence study used in the research.

3. Results and Discussion

The following are the data from the results and analysis of the bibliometric and co-occurrence study for the production of bioethanol and hydrogen from palm oil fronds.

3.1. Bibliometric Analysis for “Bioethanol and Palm”

After conducting the search in SCOPUS with the selected route, 491 documents were found. It was observed that, over the years, there has been a general increase in the number of publications (Figure 2), even though the first publication with reference to the search route only came out in 2005, which indicates a growing interest in research related to the relationship that may exist between bioethanol and African palm. It was also observed that, as the current year is about to end, there has been a small decrease in the number of publications. This is due to the new trends and areas of study promoted for the optimization of palm fronds, in addition to the production of biofuels. The most significant jump in the number of publications occurred between 2008 and 2012, from 5 to 19 publications, which represents a fourfold increase compared to the starting year. It was also observed that the year 2020 had a significant increase in the search despite the health measures experienced that year, going from 34 to 50 documents, with the latter being the highest number of publications per year registered. This can be explained by the extra time that researchers had to write, correct, and publish results already obtained, given the closing of research. These peaks between the years 2008–2012 and 2020, which demonstrated increased interest in the topic, could be the result of new technologies or information discovered regarding bioethanol or African palm, as well as greater scientific or research support for those interested in the field.

Figure 3 illustrates the type of documents indexed in Scopus after using the search equation “bioethanol and palm”. A comprehensive analysis shows that research papers predominate with 264 documents, which reinforces the importance of research in the future development of this topic. In second place, conference proceedings appear with 116 documents, highlighting the role of these types of professional spaces in promoting collaboration and facilitating the exchange of knowledge between researchers interested in this topic. Just above the book chapters, we find review papers with 33 documents responsible for summarizing existing research, which details a moderate interest in examining research in this field.

Figure 4 illustrates the 15 countries with the highest publication of research papers and the number of publications from 2003 to 2023. From the data presented in Figure 4, it can be observed that Malaysia leads research production with 125 documents, followed by Indonesia (124), Thailand (41), Japan (28), and Brazil (21). A correlation between the countries that produce the most documents can be observed since these same countries top the list of palm oil-producing countries according to Sundalian [13]. This phenomenon can be attributed to various factors, including the size of their economic capital, and the items allocated to research and development in each country. Brazil and Colombia, being developing countries, appear in the top 10, which represents their research interest in this field, driven in turn by being the largest palm oil producers in South America [14]. Analyzing the results of the publications by continent, it is observed that approximately 68% of them are from Asia, predominated by the contributions of Malaysia (125), Indonesia (124), Thailand (41), and China (10), which confirms the interest of Southeast Asia in recording the scientific advances in the use of this material in the production of bioethanol. South America appears second thanks to the research contributions of Brazil (21) and Colombia (16), which surpass North America, where only the publications of the United States (12) are visible. This can be attributed to the differences between them in the production of palm oil and corn as raw materials for the production of bioethanol. Indonesia and Malaysia, as leading producers, stand out in the food industry, for example in oils or as a secondary ingredient, in cosmetics such as soaps and shampoos, and in the energy industry with biodiesel. Although these countries have shown greater interest in biofuels derived from other raw materials, it is important to emphasize that, as the main players in palm oil production, it would be easier for them to experiment more with bioethanol as one of the ways to utilize waste in creating biodiesel.

Figure 5 presents the five main countries in South America and their contributions to the research path. The scarcity of publications in countries such as Chile and Argentina is evident, opening a window of interest for future interventions by these countries. On the other hand, the role played by Brazil (21) and Colombia (16) stands out, as they contributed via their publications on this type of topic. They are thus a close example of developing countries for countries such as Ecuador (2) and Peru (1), where the number of publications is not as significant in the contribution to the search. While Colombia may not be a significant player in bioethanol production, Brazil certainly is. Brazil has established itself as one of the largest producers of bioethanol globally, driven by its extensive sugarcane plantations and advanced technology in fermentation and distillation processes. The country utilizes bioethanol not only as a fuel alternative but also as a key component of its energy matrix, promoting sustainability and reducing greenhouse gas emissions.

On the other hand, Brazil does not stand out as much in palm oil production compared to Colombia and Ecuador. Both Colombia and Ecuador have increasingly expanded their palm oil industries, capitalizing on the favorable tropical climate for cultivating oil palm. Indonesia and Malaysia, in addition to being two of the world’s leading palm oil producers, stand out for implementing some of the most stringent sustainability policies related to the production and management of this crop. Both countries have established specific regulatory frameworks, such as the Indonesian Sustainable Palm Oil (ISPO) program and the Malaysian Sustainable Palm Oil (MSPO) certification, aimed at reducing deforestation, protecting ecosystems, and improving social and environmental practices in the palm oil industry. Meanwhile, Brazil has made significant progress in the sustainability of bioethanol. As one of the world’s largest bioethanol producers, Brazil has implemented the RenovaBio program, which encourages more sustainable biofuel production with the aim of reducing greenhouse gas emissions [15,16].

In Figure 6, an analysis of the number of research and review papers per year, along with the number of citations of these documents, is represented for the period from 2009 to 2023, limited by the information available in Scopus in the last 15 years. The combined graph shows a trend in which the increase in the publication of documents on the topic is correlated with an increase in the number of citations, indicating a growing relevance and recognition in the field over time. However, a variation in the number of documents published in 2012 compared to 2011 is noticeable, a situation that is replicated in the years 2017–2019 compared to previous years, with the difference that this time the citation rate maintained its trend. This could indicate that the publication of review papers increased. These types of papers often cite a large number of previous works, which would contribute to the increase in the growth rate of citations in relation to the publication of new research papers. This is evidenced in the years 2020 and 2021, when, despite the health emergency, the highest number of publications on the topic were reached in the time period determined for the search. In conclusion, it is observed that in the last years, 2022 and 2023, the publication of documents has been decreasing, which directly affects the citation rate and can explain its decrease in the present year.

Co-Occurrence Study and Research Trend for the Path “Bioethanol AND Palm”

Through keywords obtained from SCOPUS and the VOSviewer tool to create a map of connections and co-occurrences (Figure 7), an analysis of the frequency of occurrence of keywords, connections between them, and groupings of words was carried out. The study revealed that some keywords are repeated more frequently than others in the research, thus highlighting the areas of greatest interest in the field. Apart from the keywords “bioethanol and palm” and their synonyms, words such as “biodiesel” and “biofuel” are observed, which presents the panorama of studies and research on the use of oil palm bunches or rachis in the production of sustainable biofuels. Other frequent words include “cellulose”, “lignin”, and “lignocellulose”, which highlight in the results the correlation that exists between the lignocellulosic biomass present in the vegetable fiber of oil palm and the production of second-generation bioethanol, which uses these non-edible Feedstocks in the search for renewable energy sources.

In terms of connections between keywords, the analysis focuses on terms linked to the production of bioethanol from oil palm rachis. The relevance of the word “fermentation” stands out, which is presented as one of the most interesting and recurring terms in this study, with the highest number of links (269) and a notable connection strength (1750). This suggests that fermentation is a topic of great importance in relation to the bioethanol production process. It is also related to words such as saccharification, enzymatic hydrolysis, and pretreatment, which highlights the experimental procedures and methods currently used for the production of bioethanol. In relation to groups of words or clusters, five groups of words were identified in a map generated by the VOSviewer software, each highlighted with different colors. This identification of clusters provided ideas for focusing and developing specific areas in later sections of the work. Cluster 1, highlighted in red, identified the relationship with certain key themes or trends in the study such as the variety of biofuels, bioenergies, gas emissions, sustainability, and environmental impact. Cluster 2, in green, is related to the experimental field with words such as saccharification, fermentation, hydrolysis, distillation, pH, enzymatic activity, and temperature, among others. In cluster 3, in blue, we find a window on the microorganisms present in the production of bioethanol with words such as fungi, Trichoderma, biosynthesis, and enzymes. In cluster 4, in yellow, we observe words such as Elaeis, agricultural residues, fruits, and biotechnology, which shows the panorama of biomasses related to the search for the production of biofuels. In cluster 5, in purple, we find the main elements responsible for the production of bioethanol with words such as lignocellulose, cellulose, lignin, chemistry, and palm oil.

The analysis of research trends over time (Figure 8) reveals an evolution in the production of bioethanol, which remained focused on first-generation feedstocks such as corn and sugarcane, with particular attention to process efficiency. A growth in the production of second-generation bioethanol from lignocellulosic biomass, such as agricultural and forest residues, will be observed. Among the studies, the biomass of oil palm or rachis stands out. Following this, the production of bioethanol from microalgae and other third-generation sources began to be explored. Although still in the research stages, this transition corresponds to cluster 3 in the analysis of the word group discussed above. Sustainability in bioethanol production has been emphasized with particular attention to the reduction in greenhouse gas emissions [12] and more efficient resource management, so efforts have been devoted to investigating alternative production routes and process improvements with the aim of increasing efficiency and reducing costs [17]. The research landscape has undergone a significant evolution that reflects the fluctuating priorities of the scientific community and the continuous progress of the discipline. This constant change in research manifests the dynamism and adaptation necessary in the scientific field.

3.2. Bibliometric Analysis “Hydrogen and Palm and Empty Bunches”

Using the second search route, 247 documents were found. After applying the year filter, the number was reduced to 245. It can be observed that there were few publications from 2003 to 2009. This could be interpreted as a lack of interest in research and experimentation on the production of hydrogen from oil palm rachis. It can also be observed that the first significant leap occurred from 2009 to 2010, with a jump from one to seven publications. This trend continued exponentially until 2012 when 16 publications were published. This could be explained by the development of research and the establishment of a firm foundation for publications on the topic of using oil palm rachis as a raw material for hydrogen production. Finally, it can be observed that the maximum number of publications, 27, was reached in 2021. However, there has been a marked decrease in publications so far this year. This could be interpreted as an increase in research interest in new raw materials for hydrogen production, but it also suggests a lack of study of possible experimental methodologies and the use of emerging technologies to make better use of oil palm rachis (Figure 9).

Figure 10 presents the types of documents indexed in Scopus after entering the search route “Hydrogen and palm and empty bunches”. A thorough analysis shows that research papers predominate with 187 documents, which consolidates the importance of ongoing research on this topic. In second place, conference proceedings appear with 50 publications, highlighting the role of this type of professional space in fostering collaboration and facilitating the exchange of knowledge between researchers interested in this topic. With only six documents, book chapters appear, indicating timely in-depth research by researchers and publishers to show future improvements on this topic. Finally, and most importantly, review papers with only two documents suggest that the intersection of these keywords may be a relatively underexplored area of research. This could be due to the fact that the topic has not received significant attention in the scientific literature.

Figure 11 illustrates the 15 countries with the most scientific papers published from 2003 to 2023. Based on the data presented in Figure 11, it was observed that Malaysia leads research production with 131 documents, followed by Indonesia (42), Thailand (27), Japan (28), and South Korea (12). The existing correlation between the countries that produce the most documents and their significant contribution to palm oil production could be reaffirmed [18]. This could be evidence of the availability of natural resources, the energy and environmental policies of these countries, as well as government and private investments in research and development. These investments may vary between countries, but they are sufficiently stable to maintain the development of this research topic. Colombia, a developing country, appears in the top 10, which symbolizes its interest in research in this field, also driven by being one of the largest palm growers in South America along with Brazil [14].

Analyzing the results of the publications by continent, it is observed that approximately 90% of them are from Asia, dominated by the contributions of Malaysia (125), Indonesia (124), Thailand (41), Japan (19), South Korea (12) China (10), Taiwan (5), India (4), Pakistan (4) and United Arab Emirates (4). Confirming the interest of the continent in recording scientific advances in the use of this material in the production of bioethanol. Africas represented by Nigeria with three documents, and America is represented by Colombia with five documents, located in the ninth position, which indicates the latent interest of the country in supporting the research and development area of the country’s new technologies related to the topic analyzed. In 2023, it was estimated that oil palm frond (OPF) generation reached around 59.3 million tons in Malaysia and 156.5 million tons in Indonesia. These figures underscore these countries’ strong interest in leading global palm oil production. Products derived from oil palm, including edible oils and various processed goods, are exported on a global scale, making this crop crucial to their economies. However, the strategies used in oil palm plantation expansion continue to draw criticism for their environmental impacts [19].

Regarding the production of documents on hydrogen, nations such as Malaysia, Indonesia, Japan, and South Korea are leaders in research and development in this area. While Malaysia and Indonesia are just beginning to take steps in hydrogen production, China and South Korea are considered cornerstone countries in the development of hydrogen energy. These countries have significantly invested in clean and sustainable energy technologies, including the production and use of hydrogen as an alternative energy source. However, an opposite dynamic is observed compared to what occurs in Malaysia and Indonesia regarding palm oil production. Although Japan and South Korea stand out as two of the leading countries in hydrogen production and usage, sufficient awareness has not yet been generated to establish appropriate sustainability standards. This situation is primarily due to the lack of social acceptance of this new technological energy source [20]. On the other hand, in the production of documents on bioethanol, while some of the countries mentioned above also have a significant presence, other countries such as Brazil, the United States, Colombia, the United Kingdom, and some African countries such as the United Arab Emirates stand out for their focus on the production of biofuels from biomass, especially sugarcane, corn, and other crops.

While there are countries that remain relevant in both topics, their approaches and efforts may differ depending on their resources, policies, and national priorities in energy and sustainability. The differences in document production can be attributed to a variety of reasons. For example, the availability of natural resources, energy, and environmental policies, as well as government and private investments in research and development, can vary significantly between countries. Some countries may focus more on hydrogen production due to their efforts towards a more sustainable economy that is less dependent on fossil fuels, while others may prioritize bioethanol production due to its ability to use the agricultural biomass available in their nation.

Figure 12 presents the top five countries in South America and their contributions to research. It is clear that there is a lack of publications from countries such as Chile, Argentina, Peru, Ecuador, and Brazil. This opens up an opportunity for future contributions from these countries. Due to the lack of comparison, the search margin was expanded to the entire American continent. In this way, the work of countries such as the United States (two) and Canada (one) was observed. North America is one of the largest producers of corn-based bioethanol [21]. This suggests that there is interest in the alternation of raw materials for the production of possible high-quality bioethanol. Colombia, on the other hand, takes on a leading role and continues to contribute with its publications on these types of topics. It is also worth remembering that Colombia is one of the leading producers of palm oil on the continent. This makes research on the production of hydrogen from this resource relevant from an economic and natural resource perspective. The availability of palm oil in the country could boost interest in this area of research, thus contributing to the development of future publications.

Figure 13 displays an analysis of the number of research and review papers published each year, along with the number of citations for these documents in the period from 2009 to 2023, limited by the information available in Scopus in the past 15 years. The combined graph shows a trend in which the increase in the publication of documents on the topic is correlated with an increase in the number of citations, indicating a growing relevance and recognition in the field over time. However, a variation in the number of cited papers compared to the previous year is noticeable in 2014. This trend stabilizes again, taking on strength in the coming years, even disregarding the decrease in publications observed in 2020. This could be explained as an effect related to the health emergency of that year, where the focus of research centered its efforts on other types of topics. The highest number of publications on the topic in the search period is observed precisely in 2021, possibly awakening the interest of the research sector in developing sustainable and renewable energy sources. To conclude, it can be observed that in the last two years, 2022 and 2023, the publication of documents has been decreasing, which directly affects the citation rate and can explain its decrease in the current year.

Co-Occurrence and Trends Study for the Research Path “Hydrogen and Palm and Empty Bunches”

Using keywords obtained from Scopus and the VOSviewer v.1.6.20 tool to create a map of connections and co-occurrences (Figure 14), an analysis was carried out of the frequency of occurrence of keywords, connections between them, and word groupings. The study revealed that some keywords are repeated more frequently than others in research, thus highlighting the areas of greatest interest in the field. In addition to the keywords “Hydrogen, palm oil, empty fruit bunches” and synonyms, words such as “oil palm” and “gasification” are observed, which opens up the panorama of studies and research on the use of palm oil bunches or rachises in hydrogen production. Other frequent words include “exergy”, “gas synthesis”, and “biohydrogen”, which highlight in the results the correlation that exists between the lignocellulosic biomass present in the vegetable fiber of palm oil and the production of hydrogen using new technologies and methodologies.

Regarding the connections between keywords, the analysis focuses on terms linked to the production of hydrogen from palm oil rachises. The relevance of the word “biomass” stands out, which is presented as one of the most interesting and recurring terms in this study, with the largest number of links (174) and a notable connection strength (789). This suggests that research on lignocellulosic biomass is a topic of great importance in relation to the hydrogen production process, and is also related to words such as glucose, pyrolysis, and pretreatment, which provides an overview of the experimental procedures and methods used today for the production of hydrogen.

In relation to word groups or clusters, five-word groups were identified in a map generated by the VOSviewer software, each highlighted in different colors. This identification of clusters provided ideas for focusing and developing specific areas in subsequent sections of the work. Cluster 1, highlighted in red, identified the relationship with certain key themes or trends in the study such as the types of hydrolysis used, mentioning some of the compounds present in the production of hydrogen synthesis. Cluster 2, in green with 42 items, is related to the main field of study, such as hydrogen production, with words such as raw material, gas synthesis, hydrolysis, methane, and exergy, among others. In cluster 3, in blue with 38 items, we find words such as pyrolysis, temperature, degradation, and thermogravimetry, which shows the experimental part and its variables, as well as a window on the methodologies present in hydrogen production. In cluster 4, in yellow with 35 items, we observe words such as kinetic absorption, spectroscopy, X-rays, and Elaeis diffraction, which shows a little of the technical part of the characterizations that involve the production of hydrogen under appropriate conditions. In cluster 5, in purple with 31 items, repeated words were found in other groups or synonyms. For this reason, this group was discarded for a deep analysis of the main elements responsible for the production of hydrogen based on oil palm bunches, with words such as lignocellulose, cellulose, lignin, chemistry, and palm oil.

The analysis of research trends over time (Figure 15) reveals an evolution in hydrogen production. In the early years, attention was focused on exploring various raw material sources, including palm oil biomass. During this period, laboratory experiments and pilot studies were conducted to assess the feasibility of hydrogen production from this biomass. As time progressed, collaborations were established between academic institutions and the industry to expedite research and explore more efficient and sustainable approaches. It was during this time that hydrogen production from palm oil biomass began to gain recognition as a potential source of renewable energy, leading to intensified research efforts and investments in larger-scale pilot projects. In subsequent years, the importance of sustainability and environmental management in this process was emphasized, prompting the exploration of more eco-friendly approaches.

From 2020 onward, significant advancements were achieved in efficiency and cost reduction in hydrogen production, solidifying its position as a promising renewable energy source. In recent years, investments in commercial projects increased, and regulations have been established to promote sustainable hydrogen production. The field of hydrogen production has undergone a continuous evolution, and it is expected to continue growing and playing a significant role in the transition towards cleaner and more sustainable energy sources.

4. Deep Insights in Bioethanol Production from Palm Oil Rachis

The production of bioethanol from African oil palm rachis is a research topic in the field of bioenergy. African oil palm rachis is a lignocellulosic residue that has the potential for second-generation ethanol production [22]. Pilot tests have been conducted on a scale of 9000 L using Colombian African oil palm rachis, achieving an ethanol yield of 70–75 gallons per ton [23]. These tests have demonstrated that it is possible to obtain viable concentrations of total sugars and ethanol yields for industrial use [4]. Furthermore, it has been found that the fermentation time can be reduced from 72 h to 36 h, allowing for continuous ethanol production [24]. These results are promising and pave the way for future research and developments in bioethanol production from African oil palm rachis [25].

The conversion of African oil palm rachis into bioethanol involves various key methods and technologies. Firstly, biomass pretreatment is conducted to eliminate or reduce lignin and hemicellulose, facilitating sugar release [26]. The choice of a pretreatment method highlights its impact on the economy, as it can improve conversion efficiency but may also add costs to the conversion process [27]. Lignocellulosic biomass pretreatment is classified into physical, chemical, physicochemical, and biological processes [28]. Physical processes involve biomass fragmentation, crushing, shearing, or grinding [29], aiming to reduce polymerization and decrease particle size and crystallinity while increasing material surface area, bulk density, and porosity [30].

On the other hand, physicochemical treatments are processes carried out exclusively through chemical reactions to modify the structure of lignocellulosic biomass. These methods include explosive vaporization (autohydrolysis), fiber breakdown with ammonia (AFEX), exposure to carbon dioxide, and hot water pretreatment [31]. Among chemical pretreatments, acid pretreatment uses sulfuric or hydrochloric acid to decompose cellulose and hemicellulose into simple sugars. Lignin is partially decomposed, allowing the separation of biomass components. The resulting sugars can be fermented to produce ethanol or other chemicals [32]. Another method is wet oxidation, where hydrogen peroxide and other oxidants are used to break down lignin and reduce its content in biomass. This method can enhance cellulose and hemicellulose accessibility for further processing [33]. Among other treatments, alkaline pretreatment stands out, where strong bases like sodium hydroxide are used instead of acids to decompose lignin. This process is particularly useful for lignin removal and hemicellulose depolymerization, facilitating sugar extraction [34].

In biological pretreatment, microorganisms such as bacteria and fungi which produce specific enzymes are used to break down lignocellulose [30]. Since microorganisms are involved, this method is environmentally friendly and cost-effective compared to physical and chemical methods [29]. An effective pretreatment allows the separation of lignocellulosic components without the need for additional removal steps. The choice of pretreatment is influenced by factors such as lignocellulose crystallinity, the presence of acetyl groups in the substrate, the available surface area for degradation, and the degree of polymerization [35]. These considerations are crucial for obtaining lignocellulosic materials that are more susceptible to enzyme action, thereby improving cellulose-hemicellulose hydrolysis [36].

The partial removal of lignin may lead to a reduction in the hydrolysis rate and a decrease in digestibility, highlighting the importance of complete lignin removal before hydrolysis to ensure higher sugar production [37]. Subsequently, enzymatic hydrolysis is carried out with the assistance of specific enzymes such as cellulase and hemicellulase. These enzymes act as catalysts, breaking down cellulose and hemicellulose chains into simpler sugar units, such as glucose, xylose, and arabinose [38]. The temperature and pH of the system are controlled to optimize enzyme activity during this hydrolysis process [38]. These sugars are then fermented with yeast or bacteria, producing gases, alcohol, or fatty acids under anaerobic conditions [39]. The effectiveness of the fermentation process relies on efficient hydrolysis and the appropriate selection of microorganisms to minimize the generation of toxic pollutants and achieve high bioethanol production [40].

Table 1. Comparative analysis of fermentation processes.

Advantages and Disadvantages	SSF (Saccharification and Fermentation)	SHF (Separate Hydrolysis and Fermentation)	SSCF (Simultaneal Saccharification and Co-Fermentation)	SHCF (Separate Hydrolysis and Cofermentation)	CBP (Consolidated Bioprocessing)
Advantages	Time efficiency. High ethanol yield. Low cost. Reduced risk of contamination.	Optimization of separate hydrolysis and fermentation conditions. Flexible selection of microorganisms and enzymes.	Enzymatic efficiency. High bioethanol yield. Lower risk of contamination.	High bioethanol yield. Low cost. Lower risk of contamination.	Energy-efficient. Lower operating cost. Simplified process control.
Disadvantages	Inflexible selection of microorganisms and enzymes. Difficulty in process control	Higher cost. Contaminating effects on microorganisms. Lower ethanol yield	Difficulty in process control. Limitations in the selection of microorganisms.	Difficulty in process control. Limitations in the selection of microorganisms.	Lack of suitable organisms. Lower versatility.

provides a parallel comparison of different fermentation processes.

This bioethanol is then subjected to distillation to increase its concentration [41], and dehydration is used to remove water. In some cases, molecular distillation is employed to obtain high-purity ethanol [42]. Additionally, the recovery of solid and liquid waste generated during the process is important [43], as these residues may have additional value in applications such as energy generation or biomaterial production. Optimization and process control are essential to ensure efficiency in conversion. Furthermore, proper management of water and waste is crucial to minimize environmental impact. Collectively, these methods and technologies enable the conversion of oil palm rachis into bioethanol, contributing to the production of sustainable biofuels.

As a result of the search, documents detailing and addressing technical aspects of biomass pretreatment for bioethanol production from oil palm rachis were identified. Sitinjak et al. [44] conducted a comparative study of the amount of glucose obtained. The study revealed that cellulose fiber hydrolysis generated the highest percentage (63.92%), in contrast to cellulose extracted from oil palm leaves (18.46%) and empty fruit bunches (54.81%). Additionally, it was observed that oil palm fibers showed higher ethanol production compared to leaves and empty fruit bunches. These findings highlight significant differences in glucose composition and ethanol production among different cellulose sources from oil palm, which can influence the efficiency and viability of biofuel production. Meanwhile, Maryana et al. [45] investigated a novel second-generation bioethanol production process using oil palm rachis and pretreatment with sodium hydroxide (NaOH). Utilizing continuous and batch methods at low temperatures and atmospheric pressure, response surface methodology (RSM) was applied to optimize the amount of NaOH. The results showed that continuous pretreatment produced a sugar content of 5.9% and a bioethanol concentration of 2.5%, suggesting the feasibility of this method in the efficient production of bioethanol from rachis. Quek et al. [46] investigated the pretreatment of oil palm empty fruit bunches (OPEFBs) using deep eutectic solvents (DESs) with the assistance of ultrasound for lignin removal in the biomass. It was found that OPEFBs pretreated with choline chloride-lactic acid (ChCl-LA) showed the lowest total lignin content, reaching 18.8%, followed by ChCl-U with 19.4%, and ChCl-G with 21.2%. These results highlight the differences in the effectiveness of deep eutectic solvents in reducing lignin in oil palm rachis biomass, which could significantly influence its potential for biofuel production.

Regarding the different fermentation processes, the recent study by Triwahyuni and Muryanto [47] is highlighted, which focused on the valorization of empty fruit bunches of oil palm for bioethanol production through separate hydrolysis and fermentation (SHF), using immobilized cellulolytic enzymes. Hydrolysis was conducted at 50 °C and pH 4.8, followed by fermentation of the hydrolysate with Saccharomyces cerevisiae yeast for bioethanol production. The results showed that a substrate concentration of 150 g/L achieved the highest glucose yield (75.48%) and an ethanol yield of 78.95% during the fermentation process. These findings highlight the potential efficiency of this method for bioethanol production from oil palm empty fruit bunches. Eriyati et al. employed a microbial consortium of S. cerevisiae and T. harzianum in their study for the optimization of simultaneous saccharification and fermentation process of oil palm empty fruit bunches for bioethanol production. The results revealed that the optimized fermentation conditions achieved a production of 9.65 g/L of bioethanol. This approach has proven to be effective in obtaining a significant amount of bioethanol from oil palm empty fruit bunches [48].

Sahlan et al. [49] carried out a laboratory-scale physical cutting pretreatment for OPEFB pretreated with 10% NaOH under the saccharification and fermentation process, resulting in a glucose reduction yield from 20 g/L to 1 g/L and a maximum bioethanol yield of 38.92 g/L at a pH of 5. As a perspective and a window to the future, Sanjuan-Acosta et al. [50] employed an optimization method that uses superstructures to analyze and design the bioethanol production process using these specific residues. It provides a detailed technical perspective on optimizing the bioethanol production process from palm kernel shells, using a structured approach to maximize efficiency and viability in this process.

According to the methodologies used, the life cycle analysis study of bioethanol from oil palm frond sugar juice was examined in the following document [51]. It was found that the fermentation process generated the majority of environmental issues, contributing between 52% and 97% across all evaluated impact categories. The use of sugar juice from oil palm residues is proposed. Lee and collaborators [52], on the other hand, conducted a life cycle analysis study of bioethanol using SimaPro version 9.0, in which they evaluated several impact categories from the palm cultivation stage to its bioethanol production. Their results highlighted a significant environmental impact in the agricultural and pretreatment stages. A similar assessment is presented in the current study, where the analysis was performed using version 8.0 of the same software (SimaPro v8.0), and it was concluded that the stage requiring the most attention regarding its environmental impact is the sugar recovery process, which has a contribution of over 90%. Additionally, it shows high energy consumption in the production of chemicals and enzymes [53]. Other studies compare the life cycle analysis of bioethanol with the life cycle analysis of biodiesel and conclude that the way bioethanol production affects environmental impacts is related to land use and different modeling compared to conventional biodiesel [54]. On the other hand, Vaskan and colleagues not only evaluated the life cycle analysis but also presented a techno-economic assessment based on empty fruit bunches (EFB), which produce bioethanol, heat, electricity, and a syrup for cattle. Their studies show a reduction in environmental impacts, but the economic trends are limited regarding factors such as toxicity and eutrophication [55]. For a techno-economic assessment, evaluation indicators or metrics are also used. Many of the analyzed documents showed economically viable production, and many of them were subsequently subjected to a sensitivity analysis, The use of software is also influenced by the techno-economic analysis in the study of ethanol production from empty trunks and empty fruit bunches using Aspen Plus v.14.5, where profitability greatly depends on the target production capacity [56].

5. Deep Insights in Hydrogen Production from African Palm Rachis

5.1. Gasification

Gasification, an essential process in hydrogen production from oil palm fronds, a byproduct of the palm oil industry, involves the thermochemical transformation of biomass. This conversion takes place in a high-temperature gasification reactor (700–900 °C) with very high pressures, where prepared oil palm fronds decompose into a combustible gas primarily composed of hydrogen (H₂), carbon monoxide (CO), carbon dioxide (CO₂), and other gases [57]. The absence of oxygen during gasification prevents complete combustion, thus promoting hydrogen production. Various chemical reactions, such as pyrolysis, reduction, and steam reforming, occur within the reactor, breaking down biomass into its basic components and forming syngas. Although this gas is rich in hydrogen, purification is necessary to remove impurities such as carbon dioxide and tar. The resulting purified hydrogen can be stored and used for multiple applications, from electricity generation to the production of synthetic fuels. Palm frond gasification stands as a promising source of renewable hydrogen, contributing to sustainability and greenhouse gas emission reduction by harnessing a byproduct of the palm oil industry for clean energy production.

One of the studies detailing gasification was conducted by Chuayboon and Abanades [58], where solar energy facilitates continuous gasification of empty oil palm fruit bunches. In this study, the influence of biomass feeding (0.5–1.8 g/min) and temperature (1100–1300 °C) was evaluated, with the optimal feed rate being 1.4 g/min at a temperature of 1300 °C, thus achieving a maximum yield of 81.1 mmol/g of dry biomass at 1300 °C. On the other hand, Barco-Burgos et al. [59] conducted an analysis for palm kernel shells using steam in a down-draft allothermal gasifier, where the highest H₂ generation of 80% was obtained at a temperature of 850 °C and a biomass feed rate of (57.1 kg/h). In the study by Aprianti et al. [60], hydrogen-rich synthesis gas was generated from empty oil palm fruit bunches, using CaO and bentonite as absorbents and catalysts in the gasification process at temperatures between 550 and 750 °C, at atmospheric pressure in a fixed-bed gasifier. The findings indicate that CaO acted solely as a CO₂ absorbent during the process, resulting in an H₂ concentration of around 78.16%. This approach demonstrates the ability to generate synthesis gas with a high hydrogen content from empty oil palm fruit bunches (EFB).

In the study conducted by Lim et al. [61], the impact of the pretreatment of empty fruit bunches (EFB) through leaching is highlighted, focusing on the reduction of alkali metals, such as potassium (K) and sodium (Na). Under optimal conditions of 800 °C and a flow rate of 2.5 L/min, a significant effect on the composition of the synthesis gas derived from gasification is evident. This pretreatment process leads to a decrease in ash content and an increase in carbon content, potentially increasing the energy value of the resulting gas.

Another study by Chuayboon and Abanades [62] initially focused on a thermodynamic analysis of solar gasification of empty fruit bunches (EFB) from oil palm using gasifying agents H₂O and CO₂. Solar gasification of EFB demonstrated high efficiency in continuous operation, operating at a temperature of 1300 °C and a molar ratio of 2.6 between the gasifying agent and EFB. As a result, maximum values were obtained in the total synthesis gas yield (76 mmol/g of dry EFB) and the lower heating value of the synthesis gas (22 kJ/g of dry EFB), demonstrating the effective conversion of empty fruit bunches into synthetic fuel through a solar gasification process.

The methodologies mentioned for the evaluation of hydrogen production have not been sufficiently studied, as very few documents were found that cover life cycle analysis, technical or economic evaluation, or other previously mentioned methods. This document studies not only hydrogen production but also other gases such as methane and carbon monoxide through life cycle assessment (LCA), using palm kernel shell gasification as feedstock. It was identified that biomass was the main contributor to global warming and eutrophication [63]. Later documents studied the techno-economic feasibility of hydrogen production from palm oil effluent through steam methane reforming, which was diagnosed as viable with a payback period of 8.5 years and a return on investment of 18.48% in year 20 based on its established objectives [64]. Similarly, the POME process is used for the generation of biogas. They evaluate the performance of each piece of equipment in the process and conduct a comparison between different production methods [65].

Type of Gasification Reactors

For biomass gasification, various types of reactors can be used. Several published documents emphasize the importance of selecting the reactor type carefully due to the variables associated with each, and the effective production of biohydrogen partly depends on the reactions taking place within it. One of the main reactors mentioned is the batch reactor, which studies indicate is often chosen for gasification [66]. In a study conducted a few years ago, a batch reactor was used for phosphorus recovery and hydrogen production [67].

Table 2. Type of reactor used in the gasification process.

Type of Reactor	Substrate Used	Variables	Reference
Stainless steel tubular micro-reactor	Walnut shell, wheat straw.	Time of reaction, yield of the gas.	[68]
Ceramic reactor	Dry biomass	Comparison of bioreactor materials	[69]
Bubbling fluidized bed reactor	Biomass	Heat integration alternative	[70]
Downflow reactor	Wood biomass	Reactor modeling, gas evolution, reduction, and combustion reactions.	[71]
Coupling combustion reactor	Biomass	Upper secondary air height, upper secondary air ratio, and upper secondary air injection.	[70]
Biomass gasification tubular reactor	Biomass	Robust stability, steady state, transient behavior, bifurcation behavior, and multiplicity behavior.	[72]
Peak-bed solar reactor	Biomass	Modelling of the reactor	[73]
Supercritical reactor	Empty fruit bunches	Pressure, temperature	[74]

provides a summary of the different types of reactors used in biomass gasification.

5.2. Dark Fermentation

Dark fermentation, described as a promising technique, stands out for its ability to produce biohydrogen from various organic wastes [75], operating under conditions of absence of light and under the action of anaerobic bacteria [76]. It emerges as an attractive technological option for hydrogen production, utilizing a variety of renewable organic materials [77], such as lignocellulosic biomass, organic waste, or algae [78]. These studies highlight the versatility and potential of this technology in the quest for sustainable energy sources.

Noblecourt et al. [79] detail hydrogen production through dark fermentation from pre-fermented food waste (lactates), maintaining a pH of 6 and a temperature of 35 °C, where the hydrogen production was 0.4 mol H2–mol lactate. On the other hand, Ranjan et al. [80] conducted a study using beet juice, integrating dark fermentation with microorganism electrolysis, maintaining a pH of 5.5 and a temperature of 37 °C, resulting in hydrogen production of 3.2 mol/mol hexose. Mishra et al. [81] added a study on biohydrogen production from palm oil mill effluent (POME) using the anaerobic bacterium Clostridium butyricum, maintaining a pH of 6.5 and a temperature of 35 °C, achieving a hydrogen production of 2.42 mol H2/mol mannose. Other studies considering operational conditions for biohydrogen production, such as those conducted by Álvarez-Guzmán and Cisneros [82], evaluated pHs between 3.5 and 10, temperatures between 15 and 55 °C, and a volume or concentration between 3.5 and 41.4 (g dm³), obtaining yields between 0.71 and 0.80 mol H2/L mol lactose. Khongkliang et al. [83] analyzed high-quality hydrogen production from palm oil through two-stage dark fermentation and microbial electrolysis. This was carried out with a pH of 6.0 and a temperature of 55 °C using a UASB reactor, and achieved a production yield of 236 mL H₂/gCOD. On the other hand, Dreschke et al. [84] varied conditions by using a CSTR reactor (Applikon Biotechnology, Delft, the Netherlands) with a pH of 7 and a stable temperature of 80 °C, with an initial hydraulic retention time of 24 h that was reduced to 6 h, resulting in a production rate of 3.2 mol H₂/mol glucose from Thermotoga neapolitana.

5.3. Hydrogen Purification Technologies

In the recorded search, it was found that three different technologies based on various physical principles have been developed more intensively for the purification and recovery of hydrogen present in gas streams.

5.3.1. Cryogenic Separation

Cryogenic separation technology employs distillation at very low temperatures, usually between −150 °C and −200 °C, to separate gas mixtures. This method focuses on separating compounds based on their different boiling points, taking advantage of the fact that hydrogen, with a lower boiling point than other components, remains in a gaseous state while the rest condenses [85]. Although this technique can achieve high hydrogen purity, its industrial implementation is limited due to the associated investment and operating costs related to the extremely low temperatures required.

5.3.2. Membranes Separation

The membrane separation technique involves the use of a selective membrane that allows the passage of certain molecules based on their size or chemical affinity. This methodology separates hydrogen from a gas stream, leveraging the membrane’s ability to let certain compounds pass while blocking others. A significant advantage lies in its economic efficiency, as it operates at more moderate temperatures and pressures, implying lower energy consumption during the separation process [86].

5.3.3. PSA System Separation

The Pressure Swing Adsorption (PSA) system is a highly efficient technology that achieves exceptional purities of up to 99.999%. In the context of hydrogen purification, it is the most widely used method. This system is based on the adsorbent’s loading and regeneration cycle, which occurs through variations in pressure within the container where mass transfer takes place. In contrast, if the separation is carried out through temperature changes, it would be called Temperature Swing Adsorption (TSA) [87]. A study conducted by Laguna et al. [88] which performed an exergoeconomic assessment of the sizing of a plant for hydrogen production from empty fruit bunches of oil palm, implementing PSA technology instead of conventional Selexol purification for synthesis gas, resulted in the production of 1138 t/h of hydrogen with an overall exergoeconomic efficiency of 32.53% for 24.66 t/h of EFB. Muñoz and Posada [89] simulated a process for producing 8.9 kg/h using a PSA-type hydrogen adsorption unit at 97.5% molar from the gasification of 100 kg/h of rice husks (dry basis).

5.4. Concept of Biorefinery Applied to the Palm Rachis

The principles of biorefineries represent a comprehensive solution to overcome inherent economic constraints in independent technological processes. By merging multiple process streams into a unified technological pathway, optimization is achieved, preventing environmental pollution, minimizing resource waste, and reducing significant costs. This integrated strategy offers a more efficient and sustainable alternative compared to individual approaches [90].

In the literature search, studies related to bioethanol were found, such as those conducted by Sanjuan-Acosta [91]. This study employs a computational approach to evaluate the intrinsic safety of the bioethanol production process from palm clusters. It provides a technical perspective on how the computer-assisted Inherent Safety Index method can be applied to ensure safety during the bioethanol production process from this specific biomass. Another noteworthy work is that of Díaz et al. [92], which used computational tools to analyze in detail the environmental impact of bioethanol production using specific pretreatment methods. It offers a technical perspective on how the use of oxalic acid and molecular sieves can influence the sustainability and environmental implications of bioethanol production from empty palm clusters.

For hydrogen, studies like the one performed by Bamidele and Chen [93] focused on evaluating the technological feasibility of a comprehensive biorefinery for the production of biodiesel, hydrogen, and Fischer–Tropsch liquids (FTLs), along with combined heat and power generation (CHP). Based on a palm oil factory processing 250 tons per year of fresh palm fruit (PFC), a total energy consumption of 36.0 GJ/h was estimated. This demand was covered by cogeneration, generating 792 GJ/h, leaving a surplus of 756 GJ/h available for sale. The results of the biorefinery process modeling and simulation indicate the technological feasibility of obtaining valuable products from palm oil, demonstrating its potential as a diverse source of energy and derived products. On the other hand, Ninõ-Villalobos et al. [94] explored the biorefinery approach to produce biodiesel from jatropha and palm oils. The main objective was to enhance the economic viability of this process by using biomass residues as a fundamental resource for obtaining hydrogen. This hydrogen was obtained through two methods: steam reforming of glycerol, and gasification. The study aimed to demonstrate the possibility of converting biomass residues into a profitable and sustainable source for hydrogen production, which could make biodiesel production from vegetable oils more competitive.

Various methodologies have been developed to assess the performance of chemical processes during the initial stages of conceptual design. These approaches are framed within a context that considers sustainability as a fundamental aspect of the analysis. Notably, one study conducted by Vargas-Mira et al. [95] provides a technical and environmental evaluation of six pathways for industrially producing hydrogen from empty palm fruit waste. Using Aspen Plus software v.10.0, the research thoroughly examines different methods for hydrogen generation from these residues, offering a comprehensive perspective on the feasibility and sustainability of each approach.

In a similar vein, another noteworthy study developed by Tu and Hallet [28] meticulously analyzes the biorefinery process, delving into its technical and environmental implications. This research provides a comprehensive view of the feasibility and sustainability of utilizing residual palm fruit biomass for biofuel production, including biobutanol and biohydrogen.

A different approach is taken by Pacheco-Perez et al. [96], focusing on evaluating the efficiency and exergy losses in the hydrogen generation process. The study closely scrutinizes the use of indirect gasification to harness solid waste from palm oil as a raw material, providing a technical perspective on the feasibility of this methodology and its potential for sustainable hydrogen production from renewable resources.

Additionally, Arteaga-Díaz et al. [97] employ computational tools to delve into the efficiency and energy utilization of the biorefinery process. This study offers detailed technical insights into the feasibility of the initiative, which aims to simultaneously produce oils and hydrogen from various components of palm. An exergetic approach is utilized to assess energy efficiency and sustainability.

González-Delgado et al. [98] conducted an in-depth examination of the environmental impact of biohydrogen production from palm residues. The study provides a detailed understanding of how this process affects the environment, facilitating an evaluation of sustainability and environmental implications related to biohydrogen generation using residues from palm cultivation. Continuing with the aid of computational tools, Zúñiga et al. [99] scrutinize the feasibility and potential for large-scale biohydrogen production using palm wastes. The study offers a technical perspective on the possibility of generating significant amounts of biohydrogen from this specific biomass, utilizing a computational approach to evaluate efficiency and viability on an industrial scale.

Moreover, Peralta-Ruiz et al. [100] adopt an exergy-based approach to thoroughly analyze the efficiency and viability of large-scale hydrogen production using biomass resources from African palm bunches. The study provides a technical perspective on large-scale hydrogen generation, utilizing exergy as a metric to assess efficiency and viability on an industrial scale. At the national level, Perez-Zúñiga et al. [101] explore the economic viability of biohydrogen production using these residues under specific Colombian conditions. The study delves into how technical and economic factors impact the viability of biohydrogen production from empty palm fruit bunches, offering insights into the sensitivity of this process to local and economic conditions in Colombia.

6. Technological Advances and Challenges

6.1. Oil Palm Technological Trends

All byproducts of oil palm after milling, splitting, refining, and drying in mills are part of the oil palm fiber. This oil palm fiber originates from each of the oil palm biomass and offers various uses according to the product’s needs. The extraction of oil palm empty fruit bunches (OPEBFs) is usually carried out using different methods, such as microbial, mechanical, chemical, or steam retting. Among these options, mechanical retting by hammering stands out as an environmentally friendlier technique, without generating significant water pollution [102]. Understanding the chemical elements present in oil palm empty fruit bunches (OPBs) is crucial for researchers when selecting the most suitable oil palm fiber for a specific use. Additionally, this information allows for pretreatments to modify the fiber’s properties, adapting it to the desired characteristics of the final product [103]. There is a lack of literature regarding the relationship between different oil palm species, their chemical compositions in fiber, and the influence of the fiber’s age on its physical and chemical properties. This lack of information demands further research to better understand the utilization of oil palm empty fruit bunches (OPBs). Addressing these gaps will effectively contribute to biomass waste management and mitigate associated environmental issues.

Traditionally, residues from oil palm empty fruit bunches (OPBs) have multiple uses in oil palm agriculture, being employed as soil amendments, such as SPEFB, OPT, and OPF, to release nutrients and organic carbon. Farmers use them to prevent erosion and conserve soil moisture. Moreover, certain parts of OPBs are utilized for heat, electricity, and biogas generation, such as oil palm kernel shell (o) and palm oil mill effluent (POME). However, improper management of these residues can lead to negative environmental impacts, emphasizing the importance of proper handling [104].

In the last ten years, studies on the use of oil palm fiber have diversified, exploring multiple applications in sectors such as pulp and paper, automotive, agriculture, animal feed, renewable energy, water treatment, polymers, architecture, and composites. Despite this diversification, it has been observed that some sectors employing oil palm fiber have not experienced significant growth. In 2022, there has been a prominent focus on three specific areas related to oil palm fiber: compounds, architecture, and renewable energies. Compounds, in particular, present themselves as the most promising field. An example is a study conducted by Nair and Dasari [105], detailing that oil palm fiber has the capacity to improve acoustic absorption properties, noise reduction coefficient, and damping characteristics, although this improvement may result in a reduction in the bending strength of construction materials. Furthermore, in the context of composite materials, the incorporation of natural resources like oil palm fiber has significantly improved the mechanical performance of the product while reducing its weight for more diverse applications [106].

An analysis of other sectors shows increasing research in the field of architecture, where OPEFB, Oil Palm Trunk (OPT), oil palm frond (OPF), Oil Palm Mesocarp Fiber (OPMF), and OPKS have been studied for their acoustic behavior, thermal resistance, and properties that make them lighter. A study conducted by Mawardi et al. [107] evidenced a reduction in thermal conductivity in the field of architecture using oil palm stem biomass. On the other hand, under the architecture trend, Mydin et al. [108] conducted a study resulting in flexural and compressive strength using OPT biomass, which also showed resistance to water absorption.

For renewable energy, the work of Husna et al. [109] was found, where the phenol area percentage is reduced thanks to OPF, indicating its potential in future research to continue introducing it as a biofuel. In the work of Ameen et al. [110], high carbon content and calorific value were obtained due to the types of oil palm fiber components such as OPEFB, OPKS, and OMPF providing greater thermal stability and a more porous structure. Starting from OPEBF biomass, Saritpongteeraka et al. [111] showed that both organic and inorganic leaching improved the biofuel quality. While anaerobic digestion generated slightly more energy, hydrothermal treatment showed a greater impact on oil recovery, which could have significant economic implications.

6.2. Barriers and Challenges

At the industrial level, lignocellulosic ethanol production is constantly improving. Although all stages of the process are susceptible to optimization, pretreatment, pentose fermentation, simultaneous saccharification and fermentation (SSF), subsequent processing, and byproduct reduction continue to be areas of focus in research. Despite advances in research and development, such as pretreatment, carbohydrate hydrolysis, fermentation, distillation, and purification, pretreatment persists as one of the costliest components of the thermal process [112].

Around 11% to 27% of the total costs to produce lignocellulosic ethanol are associated with pretreatment stages [113], and this can vary depending on the type of pretreatment method used in the process and the specific preprocessing method employed. To achieve optimal efficiency in the pretreatment stage, it is necessary to ensure high sugar yields that are vital for the subsequent fermentation stage, as well as to maintain low levels of inhibitors during fermentation stages. The separation of lignin and, in certain cases, hemicellulose, which will later be transformed into molecules of interest, is also crucial. The implementation of these steps will contribute to reducing the costs associated with lignocellulosic bioethanol production, leading to a reduction in the impact of extraction, washing, and neutralization stages, as well as a decrease in energy consumption, reagents, catalysts, and water sources.

So far, no pretreatment method for lignocellulosic biomass has been established as dominant over others. The choice of pretreatment technology is influenced by multiple factors, including the type of biomass in question. However, steam explosion is recognized as one of the most effective and economically viable technologies for the pretreatment of agricultural residues, especially in large-scale processes aimed at commercial production [114].

During the hydrolysis phase, the presence of hemicellulose leads to the production of pentoses, which are less prone to fermentation compared to glucose obtained from cellulose [115]. Effective pentose fermentation results in a reduction in ethanol production costs [17], especially considering that pentose derivatives can represent a significant weight percentage, reaching 26% in the case of sugarcane bagasse and 75% in the case of sugarcane [17]. Therefore, research into microorganisms capable of producing ethanol with high yields at elevated substrate concentrations, inhibitors, and products emerges as a crucial avenue for future development.

The subsequent processing of lignocellulosic ethanol is the subject of prominent research. A good separation of bioethanol and water, the latter being present in the process in large quantities, constitutes one of the fundamental challenges in industrial process optimization. Research is focused on technological advancements in materials, such as membranes and adsorbents, to achieve high-performance membranes. Currently, viscosity requires considerable use of water to reduce the environment’s viscosity during fermentation and to counteract bacterial inhibition. However, the need to limit wastewater to prevent environmental pollution drives the exploration of fermentation in more concentrated media as a new research area.

The use of highly flammable, volatile, and potentially toxic organic solvents also requires additional considerations in terms of safety and storage, increasing the overall process costs. Additionally, efficient methods are being researched for separating biomass components after pretreatment and valorizing lignin and hemicellulose. Currently, research is being conducted to better break down lignin and add value to hemicellulose. Similarly, the leachate from the fermentation process and solid residues, including clusters of bacterial and yeast cells, which are nutritionally rich, can be reused in the process, used as animal feed, or introduced into biogas production facilities.

7. Conclusions

Bibliometric studies conducted in the Scopus database on the selected search paths and co-occurrence analysis executed using the VOSviewer software have identified trends in research interests, research gaps, and directions for future challenges regarding bioethanol and hydrogen production from oil palm rachis. A total of 491 documents were identified for the first route and 245 for the second. Sustained growth in the number of publications over time is observed, indicating a consistently increasing interest in research related to “bioethanol and palm.” Although the first reference to this research area dates back to 2005, a surge in publications occurred between 2008 and 2012, with a fourfold increase compared to the starting year. Despite the health measures of 2020, a notable increase in the number of publications was observed, reaching the highest recorded to date. However, as the current year concludes, a slight decrease in interest is detected, suggesting some dispersion in attention to these research topics.

Research papers are predominant with 264 documents, emphasizing the importance of research in the future development of this theme. Conference proceedings, with 116 documents, stand out as significant professional spaces for knowledge exchange. Review papers, though fewer in number, reflect an interest in summarizing existing research and examining the landscape of this field.

Regarding interest in the search query “Hydrogen and palm and empty bunches”, research development has laid a solid foundation for future publications based on the use of oil palm rachis as a raw material for hydrogen production, consolidating its position as a promising renewable energy source. In recent years, investments in commercial projects have increased, and regulations have been established to promote sustainable hydrogen production. The field of hydrogen production has undergone constant evolution, and it is expected to continue growing and playing a significant role in the transition towards cleaner and more sustainable energy sources, potentially involving the adoption of innovative technologies such as solar-powered steam generation.

From an economic perspective, developing a circular economy approach to bioethanol and hydrogen production from oil palm rachis and waste management will require innovations in material design, recycling technologies, and the development of effective life cycle strategies that can be evaluated through computer-assisted process simulation. Research on by-products from the distillation process and the recovery of water from hydrolysis can help reduce the environmental impact of the waste generated in bioethanol production. Additionally, the extraction and purification of other gases produced in the dark fermentation method for biohydrogen production contribute to reducing greenhouse gas emissions and minimizing energy consumption, thus reducing waste generation in hydrogen production processes from oil palm rachis. Ultimately, the sustainability assessment of bioethanol production processes is crucial, employing various methodologies such as life cycle assessment (LCA), techno-economic analysis, techno-economic resilience (which reveals the slack of the process to changes in the techno-economic environment of the process [116]), and environmental risk assessment (ERA).

From an applied standpoint, the analysis facilitated tracking the evolution of sustainable bioethanol production, demonstrating that, overall, studies revealed more positive aspects than negatives. This research approach is perceived as a positive trend, with active participation from various researchers and publication in high-impact journals, which is of great importance for the expansion of the field of study. The following recommendations for future research are strongly suggested to enhance bibliometric analyses: (1) conduct more experimental work on the integration of first and second-generation biofuel production; (2) promote the availability of public materials that facilitate the development of the research area; (3) carry out experimental studies with third-generation biofuels, especially considering the potential environmental benefits of algae as biomass for bioethanol production; and (4) address the need for regulations in the production and use of biofuels. These recommendations aim to enrich and strengthen research in this field.