Kinetics and Mechanism of Alkali Lignin Catalytic Pyrolysis Based on Thermogravimetric Analysis
ACS Omega 2025, 10, 51, 62906–62915: Figure 3. (a) TG and (b) DTG curves of lignin pyrolysis at a temperature increase rate of 10 °C/min.
Catalytic pyrolysis offers an efficient approach for lignin degradation under anaerobic conditions. This study investigates the influence of Co-based catalysts supported on acid-modified attapulgite on alkali lignin pyrolysis using thermogravimetric analysis.
Both non-catalytic and catalytic lignin pyrolysis proceeded through three stages: early, mid, and final pyrolysis. Increasing heating rates shifted maximum weight-loss peaks to higher temperatures due to heat transfer limitations. The presence of catalysts significantly reduced the activation energy to as low as 44.7 kJ/mol, indicating strong catalytic interactions with lignin functional groups and an enhanced pyrolysis reaction rate.
The original article
Kinetics and Mechanism of Alkali Lignin Catalytic Pyrolysis Based on Thermogravimetric Analysis
Haowen Tang, Shujun Zhou, and Xiaoli Gu*
ACS Omega 2025, 10, 51, 62906–62915
https://doi.org/10.1021/acsomega.5c08587
licensed under CC-BY 4.0
Selected sections from the article follow. Formats and hyperlinks were adapted from the original.
With the rapid development of world economy and the continuous increase in population, the depletion of fossil energy, and increasingly severe environmental problems, human beings are faced with unprecedented demand for energy, and there is an urgent need to develop renewable resources. (1,2) Lignocellulosic biomass reserves are abundant, with an annual growth of about 146 billion tons. (3) Lignin is a natural polymer material second to cellulose among biomass components, with a content of about 20–40%. It is also the only renewable resource that can provide bulk renewable aromatic compounds in nature. (4,5) At present, nearly 150–180 million tons of lignin are produced from the paper industry and ethanol industry as byproducts every year, but only 2–5% of lignin is effectively used, and the vast majority of lignin is burned as low-calorific-value fuel or used as feed, which is not effectively used, resulting in a serious waste of lignin resources. (6,7) Therefore, applying lignin as raw materials to selectively utilize aromatic and oxygen-containing functional groups in the lignin structure to realize high-value-added utilization of lignin is of great significance.
The depolymerization methods of lignin mainly include biological and chemical methods (pyrolysis, (8) hydrolysis, (9) catalytic oxidation, (10) photocatalysis, (11) etc.). The essence of these methods is to break certain chemical bonds between the structural units of lignin to generate small molecular compounds. Pyrolysis is an important method to convert lignin into value-added chemicals and high-grade fuels. This method could break chemical bonds between macromolecules and produce the desired products. In the absence of oxygen, biomass pyrolysis usually generates gas, biological oil, and coke. (12) The gaseous products generated by lignin pyrolysis mainly include carbon monoxide, carbon dioxide, water, gaseous hydrocarbons, etc. The compounds in bio-oil mainly include phenols, acids, esters, alcohols, ethers, aldehydes, ketones, aliphatic hydrocarbons, aromatic hydrocarbons, polycyclic aromatic hydrocarbons, furans, and nitrogen compounds. (13) Among them, alcohols, aliphatic hydrocarbons, aromatic hydrocarbons, and phenolic and furan products have higher added value. The selectivity and yield of target products can be improved by adding appropriate catalysts during the pyrolysis process of lignin. In the process of catalytic pyrolysis of lignin, the activity of catalysts, pH, structure, and other factors will affect the pyrolysis rate and composition distribution of products.
Therefore, increasing attention was directed to the partial substitution of monomeric aromatic compounds from renewable sources by thermochemical conversion technology. However, most researchers focused on transformation of lignin model compounds, while little research investigated biobased phenol production from catalytic pyrolysis of lignin and kinetics of this process. (14) The study of lignin thermal decomposition kinetics is of great significance for a further understanding of the lignin thermal decomposition mechanism and also provides valuable information for rational design and large-scale production of a lignin pyrolysis reactor. (15)
In recent years, many groups have focused on kinetic models of catalytic pyrolysis of biomass. Lu et al. (16) calculated the kinetic characteristics of catalytic pyrolysis of biomass with the Coats–Redfern method, showing that the pyrolysis process could be described by a multistep reaction rather than a simple first-order reaction. Li et al. (17) analyzed thermal decomposition behavior and reaction kinetics of sodium lignosulfonate (SL) catalyzed with an HZSM-5 molecular sieve with the Kissinger method, which inhibited char formation of SL pyrolysis and promoted the degradation of high-molecular-weight compounds to low-molecular-weight compounds with cracking reactions of oxygenated products. Due to the structural complexity of lignin, Leng et al.’s (18) research starts by investigating its general pyrolysis characteristics. It then employs model compounds combined with molecular simulations to reveal the microscopic reaction mechanisms. On this basis, kinetic models of varying complexity have been developed, with particular focus on how inherent metals in biomass alter the entire pyrolysis process and product distribution. The research by Kawamoto, (19) through a combination of model compound experiments and theoretical calculations, revealed that its pyrolysis follows a microscopic mechanism centered on free radical chain reactions, clarifying the competitive pathways from initial cleavage to the formation of char and gases. Furthermore, the study explored the interactions during copyrolysis of lignin with polysaccharide components in wood cell walls, emphasizing the regulatory role of key factors such as hydrogen transfer in determining the distribution of final products. This provides a theoretical foundation for optimizing biomass conversion technologies.
In the experiment of this article, catalytic pyrolysis of lignin was investigated by a thermogravimetric analyzer (TGA). Kinetic parameters, models, and compensatory effects were determined by combining an equal conversion method and a double equal-step method. The lignin and catalyst were pressed and fragmented several times by a solid-tablet method to make them mixed evenly. The ratio of lignin to catalyst was set at 5:1 (w/w). The samples were programmed to start from room temperature, and the heating rates were 5, 10, 15, and 20 °C/min. The catalytic pyrolysis reaction was carried out in an inert atmosphere with N2 as the carrier, whose flow rate was 30 mL/min. The experimental conditions of lignin pyrolysis with a catalyst were the same as those without a catalyst, which made the control test more comparable.
2. Experimental Section
2.3. Thermogravimetric Analysis
The thermogravimetric analysis (TGA) was performed on a DTG-60AH instrument (SHIMADZU, Japan). The lignin and catalyst were subjected to multiple pressing and fragmentation processes using the solid press method before the experiment to make a homogeneous mixture of lignin and catalyst, and the ratio of lignin to catalyst was set at 5:1 (w/w). The samples were programmed to start from room temperature at a rate of 5, 10, 15, and 20 °C/min, and the carrier was N2 to ensure that the catalytic pyrolysis reaction was carried out under an inert atmosphere with a carrier flow rate of 30 mL/min. The experimental conditions for lignin pyrolysis without a catalyst were the same as those for lignin with a catalyst, thus making the control tests more comparable.
3. Results and Discussion
3.1. Thermogravimetric Analysis
Figure 1 and Figure 2 are the mass loss (TG) and DTG curves of lignin (a), lignin + cat.1 (b), lignin + cat.2 (c), and lignin + cat.3 (d) at different heating rates, respectively (cat.1: Co/ATP, cat.2: Co/ATP-CZO, and cat.3: Co–Mo/ATP-CZO). According to the TG curve, the conversion rate of the four samples at any time during the whole pyrolysis process at different heating rates is shown in Figure 3.
ACS Omega 2025, 10, 51, 62906–62915: Figure 1. TG curves of the four samples at different heating rates: lignin (a), lignin + cat.1 (b), lignin + cat.2 (c), and lignin + cat.3 (d).
ACS Omega 2025, 10, 51, 62906–62915: Figure 2. DTG curves of the four samples at different heating rates: lignin (a), lignin + cat.1 (b), lignin + cat.2 (c), and lignin + cat.3 (d).
ACS Omega 2025, 10, 51, 62906–62915: Figure 3. (a) TG and (b) DTG curves of lignin pyrolysis at a temperature increase rate of 10 °C/min.
It can be seen from Figure 1 and Figure 2 that the shapes of the weight loss (TG) and weight loss rate (DTG) curves of lignin pyrolysis under different catalysts are roughly the same. As can be seen in Figure 1a, the pyrolysis temperature range of lignin is wide, which is mainly due to the complex structure of the basic constituent units of lignin such as the guaiac-based type, lilac type, and p-hydroxyphenyl type. The pyrolysis process can be roughly divided into three stages, including the drying and dehydration stage (from room temperature to 100 °C). In the first stage, the lignin underwent slight pyrolysis, which mainly resulted in dry water loss and the removal of small-molecule gases adsorbed on the lignin surface. The second stage (100–700 °C), which belongs to the strong cracking stage of lignin, mainly consists of the breaking of side chains and ether bonds in the structural units of lignin, namely, β–O–4 (aryl ether), α–O–4 (aryl ether), 4–O–5 (diaryl ether), β–5 (phenyl coumarin), 5–5 (biphenyl), β–1 (1,2-diarpropane), and β–β (resin alcohol) bonds, (27) and undergoes depolymerization, decomposition, and benzene condensation reactions to produce a large number of small molecular compounds. (28) The third stage (700–800 °C) is the carbonization stage, and the stage is mainly part of the bond condensation and reforming to produce a small amount of gas.
As can be seen from Figure 2, when the heating rate increases, the pyrolysis temperature tends to shift toward high temperatures. This indicates that the faster the heating rate is, the higher is the pyrolysis temperature required if the pyrolysis achieves the same weight loss result. The main reason is that with the accelerated heating rate, the water vapor generated in the pyrolysis process interferes with the heat transfer, and the heat transfer hysteretic effect occurs, leading to the deviation of the pyrolysis temperature to high temperatures.
4. Conclusions
Compared with relevant literature in this field, this work has three advantages: first, the catalyst used achieves the largest reduction in the energy barrier, addressing the “difficult degradation of lignin” issue with an activation energy reduction of 70.8%; second, the reaction model has the optimal stability, ensuring “product selectivity”; finally, the catalyst is suitable for “pyrolysis over a wide temperature range”, with higher industrialization potential. Li et al. (32) used acidic HZSM-5 catalysts regulated by embedded Mo/Mn to achieve energy-efficient amine regeneration for CO2 desorption. However, the CO2 desorption reaction itself has a low energy barrier (80.3 kJ/mol for the blank group), leaving limited room for catalyst improvement with the activation energy reduced by only 53.2%. They did not mention the stability of the reaction model after cycling and only verified the adaptability of the initial model via Arrhenius fitting. Additionally, the reaction temperature was only 101.7–110 °C, with a narrow temperature range, so the catalyst did not need to adapt to a wide temperature domain. Liu et al. (33) improved the catalytic performance of the oxygen evolution reaction (OER) by modifying α-Fe2O3 with Al doping and Al2O3 heterostructures. However, the OER is limited by the inherent chemical properties of the superoxide radical step, making it difficult to significantly reduce the Gibbs free energy change (ΔG), with the activation energy reduced by 31.4–40.5%. After 1000 cycles, ΔG increased by 0.08 eV, and the reaction model shifted slightly with cycling, leading to a decrease in O2 generation selectivity. This reaction is an electrochemical reaction with a basically constant temperature (room temperature), thus requiring no adaptation to a wide temperature domain.
In this article, two kinetic methods of equal conversion method and double equal-step method were applied to analyze the kinetics of lignin under the action of various catalysts. The activation energy and pre-exponential factor during pyrolysis were calculated by an equal conversion method, and the most probable mechanism function G(α) was deduced by a double equal-step method. The results of thermogravimetric analysis show that the pyrolysis process of the four samples is divided into three stages (early pyrolysis, middle pyrolysis, and late pyrolysis). Moreover, due to the restriction of heat transfer, with the increase in the heating rate, the maximum weight loss peak tends to shift to the high temperature. The compensation effect also showed that there was no change in the reaction models during the pyrolysis process. Each TGA experiment was repeated 3 times under the same conditions, and the relative standard deviation (RSD) of weight loss rate and activation energy was <5%, confirming good reproducibility.
