Research Trends of Thermogravimetric Pyrolysis of carnauba (Copernicia prunifera) and Thermokinetic Models Based on the Bibliometric Analysis

1. Introduction

Thermogravimetric pyrolysis is a technique that combines thermogravimetric analysis (TG curve) with pyrolysis, allowing the thermal decomposition of complex organic and inorganic materials to be analyzed. Pyrolysis is a conversion technique based on the thermal decomposition process in the absence of oxygen, which involves the conversion of biomass into products such as coal, gas, and liquid [1]. The TG curve allows the study of thermal stability and thermal decomposition mechanism and can provide thermodynamic data and kinetic parameters of the decomposition process [2].

Thermogravimetric analysis of biomass has been extensively employed to ascertain the kinetic parameters associated with the pyrolysis process, including the apparent activation energy, reaction constants, and the pre-exponential factor, as well as thermodynamic parameters such as enthalpy variations, entropy, and Gibbs free energy [3]. These parameters are required to describe the rate at which biomass or other materials undergo thermal decomposition. Such insights facilitate the identification of the optimal temperature and residence time needed to achieve the maximum thermo-conversion of biomass into desired products, including bio-oil, biochar, and gases. Furthermore, they assist in modeling and optimizing pyrolysis processes, thereby enhancing efficiency and producing higher value-added products [4,5,6].

Lignocellulosic biomass, including forestry and agricultural waste, can be subjected to a wide range of energy conversion technologies for utilization in both small and large-scale applications. Among these technologies are gasification, cogeneration (method of producing heat and electricity), recovering energy from solid urban waste and landfill gas, and biofuels for the transport sector (ethanol and biodiesel) [7,8,9,10]. In addition, thermo-conversion processes, such as pyrolysis, are employed to produce bio-oil, fuel gas, and biochar. These products may be employed for the generation of bioenergy or subjected to upgrading procedures for utilization as biofuels or other chemical feedstocks [11,12,13].

As indicated by the National Energy Balance (BEN) 2024, the proportion of renewable resources in the Brazilian energy matrix is 49.1%, which has been influenced by an increase in the domestic supply of biomass (8.0%), wind (13.2%), and solar (7.0%) [14]. In particular, biomass has made the most significant contribution as a renewable source, accounting for 16.9% of the total bioenergy supply. To identify new strategies for utilizing the biomass energy potential, there is a need for research into a range of species, including the carnauba tree (Copernicia prunifera (Miller) H. E. Moore), which has been selected for this study and is also known as the tree of life due to its resilience to both rain and drought [15].

The carnauba tree is a fruit species native to the northeast of Brazil. Its exploitation is mainly based on the extraction of ceriferous powder from the leaves [16], in addition to the process of leaves, stems, fiber, fruit, and roots, to manufacture craft and industrial products [17]. The species belongs to the Arecaceae family, and its geographical distribution is located in the Brazilian states of Ceará, Piauí, and Rio Grande do Norte [18]. Given the considerations, the study sought to conduct a bibliometric review to analyze and summarize the scientific output of the thermogravimetric pyrolysis of carnauba (Copernicia prunifera) and the kinetic methodologies employed to ascertain the thermokinetic parameters of pyrolysis. By employing bibliometric techniques, it was feasible to delineate the trajectory of research and to identify focal areas, pivotal authors, and interinstitutional and international collaborative endeavors.

2. Materials and Methods

2.1. Database, Descriptors, and Time Range

The search for data on the thermogravimetric pyrolysis of carnauba (Copernicia prunifera) was conducted using the Scopus, Science Direct, and Web of Science databases. The following search terms were employed: “Pyrolysis AND Carnauba OR Copernicia prunifera”; “Thermal AND degradation AND Carnauba OR Copernicia prunifera”; “Pyrolysis AND Thermogravimetric AND Carnauba OR Copernicia prunifera”; “Thermal analysis AND Carnauba OR Copernicia prunifera”; “Kinetic AND Study AND Thermogravimetric AND Differential Scanning Calorimetry AND Carnauba OR Copernicia prunifera”; “Friedman AND Ozawa-Flynn-Wall AND Kissinger-Akahira-Sunose AND Biomass”; and “Carnauba And Straw And Stalk”. A total of 174 articles were identified in the search results using a time frame spanning from January 2008 to May 2024.

2.2. Exclusion and Inclusion Criteria

Some inclusion and exclusion criteria were used to narrow down the search filter. The inclusion criteria were based on: [A] works that contained the descriptors used in the title, abstract, or keywords; [B] articles that included thermogravimetric pyrolysis data using carnauba; [C] complete articles of in vivo and in vitro studies in English. The exclusion criteria were [A] studies that did not fall into the category of original research (letters to the editor, prefaces, comments, editorials, reviews); [B] case reports, review articles, books, book chapters, theses, and dissertations; [C] repeated studies.

2.3. Documents Selection Procedure

The software VOSviewer (version 1.6.19) and Bibliometrix (version 4.0.0) were used to analyze the collected data. The flowchart shown in Figure 1 outlines the bibliometric analysis procedure, which encompassed authors, co-authors, academic institutions, journal sources, and keywords.

The research yielded clusters and tables that demonstrated the interconnections between the published works and provided an overview of the quantitative data about the scientific output on the thermogravimetric pyrolysis of carnauba.

3. Results and Discussion

A total of 1,983 articles were identified through analysis of the databases, addressing the thermogravimetric pyrolysis of carnauba biomass across the various parts of the tree species. Following the application of the inclusion and exclusion criteria, the remaining 919 articles were selected. Figure 2 shows 786 publications on pyrolysis, thermogravimetric analysis, and carnauba (Copernicia prunifera) from January 2008 to May 2024, with the highest rate of scientific production occurring from 2021 onwards, followed by 2022 and 2023.

3.1. Quantitative Analysis of Frequent Keywords

A preliminary bibliometric analysis was conducted utilizing the VOSviewer software to evaluate the extent of research activity on the thermogravimetric pyrolysis of carnauba. To gain a more profound understanding of the evolution of research in a specific field, it is essential to examine the keywords employed in published documents, as they provide invaluable insights into the subject matter, including applications, trends, relevance, discussions, and other general characteristics of the research. Applying the exclusion of articles written in Portuguese, conference papers, and early access papers, 786 documents were obtained.

Table 1 presents the ranking and total link strength of the 30 principal keywords in this study. These keywords exemplify the most pertinent issues in this field of research over the past 16 years. The keywords “carnauba wax,” “wax,” and “cellulose” are particularly noteworthy.

Figure 3 shows a word cloud generated by the VOSviewer software, comprising clusters of different colors. In this case, the main keywords present in the 786 articles selected were subjected to analysis, resulting in the identification of 3,695 words. Of this number, those cited at least five times in different articles are presented in Figure 3, amounting to 30 words. The larger circles represent words with more significant connections, including with other clusters. The greater the visibility of the connections between words, the stronger the link between them.

The Mendeley tool was used to search the titles of the 786 articles selected. The filter used was Pyrolysis, Thermogravimetric, Carnauba, and Copernicia prunifera, which resulted in a single article entitled ‘Thermogravimetric pyrolysis of residual biomasses obtained post-extraction of carnauba wax: Determination of kinetic parameters using Friedman’s isoconversional method’. A co-citation analysis of the keywords identified in the article was conducted using the VOSviewer software. The resulting cluster, comprising 26 words and numerous connections, is illustrated in Figure 4. The red color for all connections indicates that all terms are part of the same study area.

To examine the prevalence of keywords in the article on thermogravimetric pyrolysis of carnauba, the Bibliometrix software was employed to generate a keyword cloud. The resulting cloud is presented in Figure 5, wherein the size of the words represents the number of times they have been cited. The keywords with the highest occurrence are pyrolysis, activation energy, and cellulose. Although the words thermogravimetric pyrolysis, kinetics parameter, isoconversional method, carnauba stalk, and carnauba straw have occurred less frequently, they are directly related to this study.

3.2. Botanical Aspects of the Carnauba (Copernicia prunifera)

The carnauba tree is an arboreal plant with a stipe-like stem that can reach a height of 10 to 15 m and a diameter of between 15 and 25 cm. At the top are fan-shaped leaves measuring 0.6 to 1.0 m in diameter and displaying a light green hue. The petioles are also present, measuring 1.0 to 1.5 m in length [19]. The average growth rate is approximately 30 cm per year, with botanical maturity (first flowering) occurring between 12 and 15 years of age. The ovoid or globose fruits are 1.5 to 3.0 cm long. The plant is native to the northeastern region of Brazil, which is part of the Caatinga biome. It has various applications in the manufacturing and chemical industries [20,21]. Figure 6 shows the carnauba palm tree.

The extraction of carnauba has made a significant contribution to the generation of wealth and the occupation of a portion of the rural population in the Northeast region of Brazil, particularly in the valleys of the Jaguaribe and Acaraú rivers (Ceará), Parnaíba (Piauí), and Apodi (Rio Grande do Norte) [22]. Figure 7 displays the geographical distribution of carnauba powder and wax production in Brazil, with Piauí and Ceará being the primary producers of carnauba by-products, which indicates this species as a source of employment and income.

The carnauba tree is entirely usable, from the leaves to the ceriferous powder, which is the raw material for carnauba wax. Their leaves are used in handicraft production, providing a source of income for numerous local families [23,24]. Bagana is another by-product of the carnauba tree used to protect and cool the soil, with results that show it reduces temperature and maintains soil moisture levels, primarily due to its prevalence in regions where the wax is produced [25]. Additionally, the stalk has significant utility, serving as a source for handicraft production, toys, furniture, roofing material for the homes of socially vulnerable families, and other applications [26].

3.3. Publications by Countries and Institutions

The analysis utilized the information provided by the authors in the specified fields for affiliation, country, and institution of origin. The 10 countries with the highest scientific output account for 89.60% of the total number of publications (Table 2). The highest number of publications is concentrated in Brazil (402 publications: 64.32%), followed by China (34 publications: 5.44%) and India (25 publications: 4%).

Figure 8 presents a visual representation of the distribution of publications by country, categorized according to period and research area. The analysis focuses on countries that have produced at least one publication. Brazil has the highest number of publications in the field, probably attributed to the greater production potential of carnauba (Copernicia prunifera).

Figure 9a shows a density map of the countries that have published articles on the research in question. The intensity of the color in Brazil indicates its higher contribution to the publication of articles. Figure 9b shows a map of the networks of collaborative links between the scientific groups analyzed. Brazil, the USA, and China are the countries that collaborate the most in this field.

The results from the Web of Science (WoS) database were subjected to analysis using the VOSviewer software. From a total of 220 institutions, 10 represent 51.72% of the total articles, see Table 3. Federal University of Piauí (19 publications), the Federal University of Ceará (18 publications), and the State University of Ceará (10 publications), all from Brazil’s northeast, concentrate the highest number of articles. This can probably be attributed to the carnauba (Copernicia prunifera), which represents one of the economic bases for the region’s small farmers [27].

To facilitate the visualization of the collaboration between the institutions, a network map was constructed using the restriction of a minimum of one publication per institution, with the return of 220 results, see Figure 10. Only 70 institutions showed some connection. The most relevant include the Federal University of Piauí, the Federal University of Ceará (linked to Embrapa Agroindústria Tropical), and the State University of Ceará.

3.4. Scientific Journals with Publications in the Field

Bibliometrix software was used for bibliometric analysis, with the criterion of at least one publication per scientific journal, generating a result of 303 journals, with an average of 2.6 articles per journal. The result suggests a considerable interest in the research topic in several areas, but the number of relevant publications remains relatively low. Table 4 shows the 15 most relevant scientific journals in thermogravimetric pyrolysis, classified by number of publications and impact factors. These journals represent 33.72% of the total publications in the 303 journals, indicating a reduced number of papers per journal.

To generate the data illustrated in Figure 11, a minimum of four publications per journal was established as the requisite, of which 40 met the specified criterion. As can be observed, the scientific journals that have published the most in this field are those related to the food industry, for example, Food Chemistry (37 publications), Food Hydrocolloids (19 publications), and Food Research International (17 publications).

3.5. The Most Cited Researchers and Articles

The relevance of a publication generally is measured by the number of citations it receives [28]. Thus, the Scopus database and VOSviewer software were used to analyze the most cited documents in the field (see Table 5 and Figure 12). The results reveal valuable insights into the researchers involved, such as Da Silva Lacerda et al. (2015), Zhang Y et al. (2018), and Del Río J. C. et al. (2017), who have papers with over 60 citations (Figure 12).

The Web of Science, Science Direct and Scopus databases, and the VOSviewer software were used to quantitatively analyze the authors who have published the most in the area. To generate the word cloud displayed in Figure 13, the criterion of a minimum of three documents per author was selected. Of the 3,921 authors in question, 125 met the established requirement. Table 6 presents the ten authors with the highest productivity, representing 14% of all publications. The number of authors demonstrates a notable interest in pyrolysis, carnauba, and thermogravimetric analysis.

The data collected resulted in a map of 42 clusters (Figure 13), showing authors who work alone and others who form a network of collaborations. The clusters representing the largest collaborative networks are red, green, blue, and yellow. The most representative cluster is that of Monteiro, S.N. and Nascimento, L.F.C. two researchers with many publications (Table 6).

There are collaborative networks between researchers that generate large numbers of citations, such as Monteiro, S. N., the author with the highest number of publications, and Ribeiro, M. P., in the document entitled “Mechanical, thermal and ballistic performance of epoxy composites reinforced with cannabis sativa hemp fabric”, which occupies the six position as the most cited document (Table 5).

As mentioned in section 3.1, the analysis of the selected articles found only one document with the keywords pyrolysis, thermogravimetric analysis, and carnauba (Copernicia prunifera) in the title: Thermogravimetric pyrolysis of residual biomasses obtained post-extraction of carnauba wax: Determination of kinetic parameters using Friedman’s isoconversional method [39], published by Carvalho P. R. et al. from the Federal University of Ceará, in 2023, in the journal Renewable Energy. The article determines the kinetic parameters of the thermogravimetric pyrolysis of carnauba straw and stalk, and to date, it has six citations.

The fact that the analysis by title found only one article does not necessarily indicate a lack of interest in the subject, as 786 articles related to the topic were found in 303 journals, with the collaboration of 220 institutions demonstrating an interest in the area.

3.6. Residual Carnauba Biomass (Copernicia prunifera)

This species is extracted for the ceriferous powder from the leaves, used to make the most commercialized carnauba product: wax. However, other parts of the palm can be commercialized, such as the straw for handicrafts and the stalk for bioenergy [40]. Research was carried out in the Scopus database, focusing on the carnauba stalk and the straw, in which 43 articles were found. Table 7 shows the classification and total link strength of the 19 main keywords.

Figure 14 shows a word cloud of the main keywords in the 43 selected articles. Scopus database yielded 795 words, including those cited at least five times in different works. The biggest circles represent the words with the most occurrences: in green “cellulose”, “lignin”, and “biomass”; in red, “scanning electron microscopy”, “Fourier transform infrared spectroscopy”, and “thermogravimetric analysis”; in blue, “pyrolysis”, “thermogravimetry”, and in yellow “kinetics” and “thermodynamics”. These words met the proposed objectives, as they are directly related to this study.

A bibliometric analysis of the number of citations was carried out on the 43 articles selected. Table 8 presents the ten most cited documents, showing that the articles published by Qin Y. et al. (2016), Hoslett J. et al. (2019), and Nguyen D. M. et al. (2017) received the highest number of citations.

Figure 15 illustrates a density map with the most cited documents related to carnauba, straw, and stalk, in which the regions colored intense yellow indicate the documents have the highest number of citations. The most prominent cluster is represented by “Qin, Y.; Yu, J.; and Yang, J.” with 137 citations (Table 8). Among the ten most cited documents, two stand out because they deal with the use of carnauba straw (Copernicia prunifera), i.e., (i) “Copper removal using carnauba straw powder: Equilibrium, kinetics, and thermodynamic studies” (17 citations) and “Valorization of carnauba straw and cashew leaf as bioadsorbents to remove copper (ii) ions from aqueous solution” (11 citations).

3.7. Research Areas

From the results of the compiled database (2008 to 2024), the 786 selected documents are from 30 research areas related to pyrolysis, carnauba, and thermogravimetric analysis. Figure 16 shows that Agriculture and Biological Sciences were the most prominent fields, with 27.07% of the occurrences. Materials science was the second with 14.59 %, followed by chemistry with 12.35 %.

3.8. Methods for Determining Kinetic Parameters

The International Confederation for Thermal Analysis and Calorimetry (ICTAC) recommends applying isoconversional methods to determine the kinetic parameters of biomass pyrolysis using thermogravimetry data [51]. Isoconversional models, also known as Model-free kinetics, allow kinetic parameters of solid-state reactions to be determined without knowledge of the reaction mechanism for a wide range of temperatures [52]. Non-isothermal models can be divided into two main categories: differential and integral methods [43]. Friedman (FRI) - differential, Ozawa-Flynn-Wall (OFW) - integral, and Kissinger-Akahira-Sunose (KAS) - integral were selected for the analysis.

The Friedman method (FRI) is based on the hypothesis that the reaction model is independent of the heating program. According to this method, for a series of experiments carried out at different heating rates, it is possible to determine the value of the activation energy (Ea), for each mass conversion fraction (α), by linearly fitting the curve of ln(dα/dt) or ln(βdα/dT) as a function of 1/T, only for experiments with dynamic analyses (non-isothermal), i.e., linear heating. In both analysis conditions, the slope of the curve is equal to -Ea/R, as described in Equation (1) [53].

$$ln\left({\displaystyle \frac{d\alpha }{dt}}\right)=ln\left(\beta {\displaystyle \frac{d\alpha }{dT}}\right)=ln\left[Af\left(\alpha \right)\right]-{\displaystyle \frac{E_a}{RT}}$$

(1)

Where: α = mass conversion fraction (f(α)); β = heating rate (dT/dt); T = absolute temperature (K); t = time (s); A = pre-exponential factor (1/s); E_a = activation energy (kJ/mol), and R = universal ideal gas constant (8.314 J/ K mol).

The Ozawa-Flynn-Wall method (FWO) is a kinetic analysis that calculates the dependence of the activation energy, E_a(α), with the degree of conversion, α, for dynamic experiments with different constant heating rates, β, Equation (2) [54].

$$ln\beta =ln\left({\displaystyle \frac{AE}{Rg\left(\alpha \right)}}\right)-5.331-1.052{\displaystyle \frac{E_a}{RT}}$$

(2)

Where: g(α) = integral form of the reaction; T = absolute temperature (K); A = pre-exponential factor (1/s); E_a = activation energy (kJ/mol), and R = universal ideal gas constant (8.314 J/ K mol).

The Kissinger-Akahira-Sunose method (KAS) assumes that the temperature of the maximum reaction rate point is equal to the temperature of the maximum inflection point of the thermal analysis curve. It is a method for calculating the activation energy of a reaction from thermal analysis curves at different heating rates. In the case of f(α) = 1 - α, the Kissinger equation, derived according to the maximum reaction rate condition, is represented by Equation (3) [55].

$$ln\left({\displaystyle \frac{\beta }{T_P^2}}\right)=-{\displaystyle \frac{E_a}{R}}\left({\displaystyle \frac{1}{T_P}}\right)+ln\left({\displaystyle \frac{RA}{E}}\right)$$

(3)

Where: A = pre-exponential factor (1/s); E_a = activation energy (kJ/mol); β = heating rate (dT/dt); T_p = absolute temperature (K), and R = universal ideal gas constant (8.314 J/ K mol).

Bibliometric analysis for publications on kinetic parameters and their isoconversional models in the Scopus database was conducted using VOSviewer and Bibliometrix software. A filter identified 90 articles about the Friedman, Ozawa-Flynn-Wall, and Kissinger-Akahira-Sunose models. Figure 17 illustrates a word cloud generated by the VOSviewer software, comprising clusters of disparate colors. The co-citation of keywords in the 90 selected articles was subjected to analysis. 976 words were identified, of which 30 were cited at least eight times across the selected articles. The most prominent words were activation energy (74 occurrences), kinetics (59 occurrences), pyrolysis (58 occurrences), thermogravimetric analysis (58 occurrences), and kinetic parameters (23 occurrences).

Figure 18 was generated using the Bibliometrix software, with 90 articles selected from the Scopus database. The bibliometric analysis was conducted using the most frequent words in the abstract of each article. The software scanned 2,000 words of articles to determine the most frequent model for assessing pyrolysis parameters in carnauba biomass. The differential Friedman model is the most frequently indicated, followed in almost similar proportions by the FWO and KAS models [56].

The differential Friedman model is the most prominent thermokinetic method probably due to its simplicity, which allows a large variation in a small conversion range (α), determining activation energy (Ea) and pre-exponential factor (A) in a more accessible way [57,58].

4. Conclusions

A literature review of thermogravimetric pyrolysis of carnauba biomass (Copernicia prunifera) was conducted. The analysis encompassed an examination of the established trends within the field and a detailed investigation into the methodologies employed for evaluating thermokinetic parameters of biomass pyrolysis. The study evaluated 919 articles published between January 2008 and May 2024 in the Scopus, Science Direct, and Web of Science databases using three tools: VOSviewer, Bibliometrix, and Microsoft Excel. The database enabled the creation of network maps (clusters), and tables related to scientific production, thus facilitating the comprehension of research trends in this field, for example, valuable insights into thermogravimetric pyrolysis. According to the findings, the following points are highlighted:

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The countries with the highest number of publications were Brazil, China, and India;

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The kinetic parameters of biomass pyrolysis were highlighted in this article, with Friedman’s isoconversional method being the most recommended by researchers;

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The research topics were identified through an analysis of keywords, obtaining the following emerged themes: carnauba wax, cellulose, biomass, activation energy, kinetics, pyrolysis, thermogravimetric analysis, and kinetic parameters;

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The Federal University of Piauí (Brazil) is the core institution in a network of 220 organizations engaged in research about pyrolysis, carnauba, and thermogravimetric analysis. It has produced the highest number of publications in this field, followed by the Federal University of Ceará (Brazil).

The number of journals on this topic shows significant interest from several academic fields. However, the number of relevant publications remains low. Lignocellulosic waste has many potential applications, including energy generation and the production of biochar, which is used in soil to reduce greenhouse gas emissions. This charcoal is made using thermoconversion technology, namely pyrolysis. Given this, it becomes evident that an in-depth study of thermoconversion technologies is necessary to enhance the potential of renewable energies by effectively harnessing biomass and thus solving the dependence on fossil fuels.