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Article

From Cellulose to Highly Aromatic Hydrochar: Catalytic Carbonization and Catalytic Aromatization Mechanism of Lanthanide (III) Ions

by
Shuaijie Han
1,
Mingshu Chi
1,*,
Xiuling Xu
2,
Li Bai
1,
Junquan Wu
1 and
Yizhuo Guo
1
1
School of Municipal and Environmental Engineering, Jilin Jianzhu University, Changchun 130118, China
2
Library of Jilin Jianzhu University, Jilin Jianzhu University, Changchun 130118, China
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(3), 245; https://doi.org/10.3390/catal15030245
Submission received: 15 December 2024 / Revised: 5 January 2025 / Accepted: 13 January 2025 / Published: 5 March 2025
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Abstract

:
Hydrothermal carbonization (HTC) is an efficient method for converting lignocellulosic biomass into biofuels. However, traditional Brønsted acid-catalyzed HTC processes face challenges such as high costs and limited catalytic efficiency. In this study, the catalytic carbonization mechanism was investigated within the temperature range of 180–220 °C by analyzing the evolution of functional groups in hydrochar under lanthanide (III)-catalyzed and non-catalyzed conditions. The results indicate that compared to acid catalysis, lanthanide (III) exhibits superior catalytic performance during the low-temperature HTC of cellulose. At 200 °C, lanthanide (III) accelerates the conversion of cellulose into char microparticles, while at 220 °C, it promotes the complete hydrolysis of cellulose into char microparticles enriched with furan structures. Characterization analyses revealed that lanthanide (III) enhances the formation of HMF (5-hydroxymethylfurfural), suppresses its conversion to LA (levulinic acid), promotes the polymerization of HMF into char microparticles, and indirectly accelerates the hydrolysis of cellulose into oligosaccharides.

1. Introduction

The depletion of fossil fuels such as coal and oil has led to a serious crisis in energy and chemical raw materials. Thermochemical processes originally developed for feedstock conversion using fossil energy have been employed to generate chemicals and biofuels from lignocellulosic biomass [1]. Extensive research has been conducted on the conversion of lignocellulosic biomass into high-value biofuel. However, traditional conversion technologies for lignocellulosic biomass treatment, such as gasification and pyrolysis, are not only costly in processing but also have a low conversion efficiency [2]. In recent years, hydrothermal carbonization, as a negative emission technology through carbon capture and storage, has attracted growing attention due to its potential to mitigate climate change [3]. Numerous studies have demonstrated the promising potential of the HTC process in converting lignocellulosic biomass with various moisture contents into high-value fuels [4,5].
The HTC process is a thermochemical method that utilizes water under mild subcritical conditions (150–300 °C, 2–6 MPa) as the reaction medium and reactant to convert biomass into solid-phase carbon material known as hydrochar [6,7]. Hydrochar mainly consists of matrix char and coke microparticles. The matrix char is formed from the unhydrolyzed part in a high-pressure heterogeneous environment, while the hydrolyzed biomass is converted to coke microparticles; this process mainly consists of the hydrolysis of raw products and polymerization of liquid-phase products [8]. It is reported that the coke microparticles have a higher HHV (higher heating value) than the matrix char [9]. Many studies have shown that the fuel composition and properties of biomass have been effectively improved through the HTC process [10]. Generally speaking, the HHV and aromatization degree of the hydrochar are being continuously improved with the increase in the HTC temperature, becoming closer to the high-quality coal in terms of HHV, volatile matter (VM) content, and fixed carbon (FC) [11].
Although a higher HTC temperature can be used to prepare coal with high aromatization, an increase in temperature will inevitably lead to a significant energy cost in the HTC process. Therefore, it is often necessary to introduce an appropriate catalyst to reduce the temperature dependence, lower the treatment cost of the HTC process, and optimize the distribution of hydrochar. Previous studies have found that Brønsted acid and HTC process wastewater containing acid have catalytic effects on the HTC process of lignocellulosic biomass. Wang et al. [10] discovered that organic acids in HTC process wastewater can effectively promote the hydrolysis of lignocellulosic biomass and then the subsequent dehydration of monosaccharides, improving the fuel quality of hydrochar. Furthermore, Leng et al. [12] reported that the acidic intermediate in the process water could still promote acidolysis and condensation reactions of the HTC hydrolysate to form hydrochar. Additionally, Sztancs et al. [13] indicated that acetic acid, as a catalyst, can be beneficial to the improvement of fuel performance and energy recovery efficiency in the HTC process of Chlorella vulgaris microalgae biomass to produce hydrochar in coal-fired power plants. Meanwhile, Kambo et al. [14] mentioned that organic acids could accelerate dehydration and decarboxylation reactions in the HTC process, which will increase the degree of carbonization of hydrochar.
However, these traditional Brønsted acids also have disadvantages such as higher costs and weaker catalytic effects [15]. Moreover, according to the HTC reaction path, although Brønsted acid can promote the hydrolysis of cellulose to hexose and the further dehydration of hexose to HMF under certain concentrations and conditions, the initial product of HMF is easily further rehydrated to LA [16], which will inhibit polymerization and transformation from HMF to hydrochar. In the search for new catalysts to convert hexose into HMF, Kei-ichi Seri et al. discovered that LaCl3 is much more efficient in catalyzing the dehydration from hexose to HMF than conventional Brønsted acid and also inhibits the conversion reaction of HMF to levulinic acid. Presumably, this will increase the initial concentration of aromatic compounds and effectively reduce the conversion of raw materials to gaseous substances [15]. On the other hand, LaCl3 as a heterogeneous catalyst demonstrates high chemical stability, superior flexibility, and adaptability during the hydrothermal carbonization process [17]. These characteristics make it an ideal substitute for traditional homogeneous catalysts, particularly in the field of efficient biomass resource utilization [18]. However, in reality, the catalytic characteristics of rare earth ions are mostly utilized in the process of preparing HMF chemicals. Their application in the field of HTC is relatively scarce, and the catalytic effect and catalytic mechanism in the HTC process are still not clear.
Based on the above, in this study, cellulose was selected as the raw material, and LaCl3 was used as a catalyst for catalytic activity and the catalytic carbonization mechanism in the HTC process. The specific objectives were to (1) elucidate the catalytic HTC mechanism of cellulose to form hydrochar with a highly aromatic structure due to lanthanide (III) ion catalysis; (2) investigate the relationship between catalyst reactivity of lanthanide (III) ions and HTC temperature in the low-temperature HTC process (180–220 °C) based on the conversion of cellulose to coke microparticles; (3) explore the catalyzing effectiveness of lanthanide (III) ions and further comprehend its catalytic mechanism by comparing with hydrochar prepared at a higher temperature and using Brønsted acid-catalyzed processes. The results of this work may provide a theoretical foundation for optimizing the HTC process of lignocellulose.

2. Results and Discussion

2.1. Analysis of the Basic Properties of Samples

2.1.1. The Fuel Quality and Chemical Composition of the Hydrochar

The proximate analyses of hydrochars are shown in Figure 1. Although LaCl3 and acetic acid did not exhibit any catalytic effect at 180 °C in the HTC process, the FC content of LC-200 was significantly increased and the VM was obviously decreased, and the FC/VM of LC-200 even exceeded that of C-220. When the HTC temperature was further raised to 220 °C, compared with the uncatalyzed hydrochar, the FC content of hydrochar catalyzed by LaCl3 and acetic acid increased. Additionally, the VM content decreased. These results indicated that LaCl3 and acetic acid could promote an increase in the degree of coal mineralization and the formation of more stable polymers, which would optimize the fuel quality of the hydrochar (such as increasing the combustion stability). However, among the two catalyzed hydrochars, LC-220 had the lower VM content but the higher FC content. Furthermore, the FC/VM of LC-200 also almost exceeded that of C-250. These phenomena suggested that LaCl3 has a strong catalytic effect on the improvement of the coalification degree of hydrochar in the HTC process of cellulose.
The HHV analysis of hydrochars is also shown in Figure 1. When treated at 180 °C, the addition of acetic acid and LaCl3 had no significant effect on the enhancement of HHV, which might imply that even with the addition of the catalysts, the hydrolysis degree of the cellulose feedstock could not be significantly altered at this temperature, and the reaction was still dominated by the matrix char formation reaction. At 200 °C, although the addition of acetic acid had no significant effect on the increase in heating value, the HHV of the hydrochar was increased from 16.83 MJ/Kg to 21.85 MJ/Kg by the addition of LaCl3. This phenomenon might be attributed to the fact that the hydrolysis of cellulose has already begun at this temperature [19]. Firstly, the conversion of glucose to HMF is effectively promoted by LaCl3, which increases the absolute content of aromatic compounds in the hydrothermal system. Moreover, glucose, as a product of the hydrolysis of cellulose, indirectly promotes the hydrolysis degree of cellulose based on the effective conversion of glucose to HMF, which exacerbates the coke microparticle formation reaction. When the HTC temperature was further increased to 220 °C, both catalysts played a catalytic role. The addition of an acetic acid catalyst increased the HHV of the hydrochar from 17.94 MJ/Kg to 22.04 MJ/Kg, which is basically equal to the heating value of HMF (22 MJ/Kg), and close to the heating value of lignin [20]. With the addition of the LaCl3 catalyst, the HHV of the hydrochar was increased from 17.94 MJ/Kg to 27.04 MJ/Kg, which basically achieved the effect of increasing the hydrothermal temperature from 220 to 250 °C.

2.1.2. The Evolution of the Microscopic Morphology of Hydrochar

The addition of catalysts within the range of 180 to 220 °C also led to the morphology of cellulose hydrochar presenting diverse patterns of change; SEM images of hydrochar produced through catalytic and non-catalytic reactions between 180 and 220 °C are shown in Figure S1. It was found that the HTC temperature at 180 °C did not have a significant change in the morphology of the hydrochar, regardless of the addition of LaCl3 or acetic acid. At 200 °C, although the addition of acetic acid did not significantly impact the micro-morphology of the hydrochar, the addition of LaCl3 facilitated the formation of coke microparticles. Previous studies had shown that the coke microparticles were formed through the polymerization of cellulose pyrolysis and hydrolysis intermediates, and the change in the microspheres also signified the occurrence of hydrolyzed organic molecule polymerization in the aqueous organic molecules [21]. When the HTC temperature was raised to 220 °C, the formation of coke microparticles had already initiated based on the analysis of the previous experiment, and the addition of acetic acid catalyzed the increase in the coke, while the addition of LaCl3 caused the complete destruction of the cellulose, resulting in the formation of hydrochar enriched by the melting of numerous coke microparticles, which was basically similar to the hydrochar formed from glucose as the raw material. This also suggested that the LaCl3 promotes the dehydration of hexose to HMF and the solubilization of cellulose [22].
However, as shown in Figure 2, unlike coke microparticles produced by the LaCl3-catalyzed HTC process at 220 °C, the coke microparticles formed at high temperatures had a smaller particle size. This phenomenon is mainly attributed to the enhanced hydrolysis and polymerization among the microspheres at elevated temperatures. The reduction in microsphere size at this temperature might be attributed to the removal of a large number of oxygen-containing functional groups from the liquid-phase organic matter and surface of coke due to the increase in temperature since the growth of microspheres was accomplished by the connection and bond of furans, phenols, and ketones in the liquid-phase products with the reactive oxygen-containing groups (hydroxyl, carbonyl, and carboxyl) on the outer surface of the micronuclei [23].

2.2. The Variations in Chemical Structure

2.2.1. Evolution of the Functional Groups from Cellulose to Hydrochar

To discuss and explain the alteration of characteristic functional groups of hydrochar, FTIR was used to analyze the microscopic reaction mechanism of the HTC reaction. The FTIR spectra of hydrochar produced through catalytic and non-catalytic reactions within the temperature range of 180 to 220 °C are shown in Figure 3, and the identified functional groups are listed in Table 1.
First, neither LaCl3 nor acetic acid led to the emergence of the peak of C=O and aromatic C-H at 180 °C, whether acetic acid or LaCl3 was added. This was mainly because cellulose was basically not hydrolyzed at this HTC temperature [24]. The remarkable thing is that the catalysis of acetic acid at this temperature resulted in the appearance of the peak of C=O, likely due to the dehydration and esterification of acetic acid with the solid-phase material. At the HTC temperature of 200 °C, although the addition of acetic acid did not lead to the appearance of the peak of aromatic C-H, the addition of LaCl3 led to the appearance of peaks of C=O and the aromatic C-H, which further indicated that cellulose had been hydrolyzed below 200 °C. Peaks of C=C, C=O, and aromatic C-H were increased based on the catalytic effects of the acetic acid or LaCl3, in which the catalytic effect of LaCl3 was more violent. In fact, the peak of C-O was also drastically weakened at 220 °C after catalysis by LaCl3, which could indicate that a large amount of cellulose was hydrolyzed and may indirectly increase the hydrolysis rate of cellulose by the conversion of hexose to HMF. However, the peak of O-H was not significantly weakened, which might imply that the polymerization at 220 °C was not mainly achieved by the removal of O-H.
Furthermore, as shown in Figure S2, although the hydrochar formed catalyzed by LaCl3 at 220 °C also has more distinct C=O, C=C, and aromatic C-H peaks compared with the hydrochar formed at 250 °C and 280 °C, the hydrochar formed at higher temperatures had weaker O-H peaks. These trends also further explained the SEM analysis from the functional group structure.

2.2.2. Evolution of the C-Containing Functional Groups and Chemical Structure of Hydrochar

Although FTIR analysis can also be conducted to determine and identify some information about carbon-containing functional groups, it was difficult to obtain a more detailed carbon skeleton and chemical structure and semi-quantitatively analyze the aromatization degree of hydrochar. Based on the above reasons, 13C NMR spectra were conducted to analyze the deeper chemical structure of the solid-phase products obtained in this experiment. The 13C NMR spectra of hydrochars were shown in Figure 4, which can be roughly divided into four main regions, including the alkyl carbons in the range of δ = 0–50 ppm, C-OH and C-O-C signals between δ = 50 ppm to δ = 110 ppm, aromatic carbons in the range of δ = 110–160 ppm, and C=O signals between δ = 160 ppm and δ = 220 ppm. More precise peak assignments in the 13C NMR spectra of hydrochar of cellulose can be found in Table 2.
Firstly, the spectra of hydrochars with and without catalysis are highly consistent at 180 °C, which also indicated that acetic acid and LaCl3 could not promote further decomposition of cellulose at this HTC temperature. In fact, the β-1,4-glycosidic linkages were formed by C1 and C4 carbons [25]. However, peaks within the ranges of δ = 110–160 ppm and δ = 160–220 ppm were absent for hydrochars catalyzed and uncatalyzed at 180 °C, suggesting that ketones, esters, carboxylic acids, and furanic and arene species had not been converted to the solid-phase products, consistent with the SEM and FTIR analysis results. Therefore, these phenomena may indicate that the cellulose had not been adequately decomposed.
However, at 200 °C, the addition of acetic acid had no significant effect on characteristic peaks of cellulose structure, while the addition of LaCl3 made the weaker peaks at 0–50 ppm, 110–160 ppm, and 160–220 ppm visible, as shown in Figure 4b. This finding further proved, in terms of the carbon backbone structure, that LaCl3 effectively promoted the formation of aromatic ring structure and ketone structure, if the cellulose was slightly hydrolyzed and the hydrolyzed part did not polymerize into secondary coke, which was consistent with the FTIR analysis findings and further explained the increase in HHV and the formation of coke under this condition. As shown in Figure 4c, compared to the uncatalyzed HTC process, the 13C NMR spectra were changed in various degrees by adding acetic acid or LaCl3 at the onset of the HTC reaction at 220 °C, especially for LaCl3. In fact, based on LaCl3 catalysis, the characteristic peak of pure cellulose completely vanished, which further confirmed at the chemical structure level that the cellulose was thoroughly hydrolyzed. Previous research has found that the region of aromatic carbon was composed of three types of functional groups, including carbon at the alpha position (δ = 140–151 ppm), sp2-hybridized aromatic carbon (δ = 125–129 ppm), and carbon at the beta position (δ = 110–118 ppm). In fact, the carbon at the alpha position and the carbon at the beta position were assigned to oxygen-containing aromatics, including furanics and phenolics [22], whose labeling scheme is shown in Figure 5b. It is also worth noting that the peak intensity and area of the Cα atom and Cβ atom were the highest at the 13C NMR spectra, accompanied by the signal intensity of the Cα atom being greater than that of the Cβ atom. However, the peak attributed to the sp2-hybridized aromatic carbon was not found at the 13C NMR spectra. Furthermore, the intensity and area of the characteristic peak corresponding to the region of C-OH and C-O-C signals were much weaker, which may indicate that the content of ether bonds and ester bonds in the hydrochar structure at 220 °C by LaCl3 catalysis was low combined with the FTIR analysis of the peak of O-H. Based on the above analyses and findings, it can be concluded that the hydrochar formed at 220 °C by LaCl3 catalysis was mainly furanic units, which may be mainly connected by a Cβ-Cβ bond.
According to the peak areas of Figure 4c,d, the relative proportion of peak areas in five representative regions was also calculated for HTC hydrochars formed without catalysis at 220 °C, 250 °C, and 280 °C, as well as hydrochars formed catalyzed by acetic acid and LaCl3 at 220 °C, as shown in Figure 6a. With the increase in HTC temperature, the proportion of aromatic carbon in the solid-phase product was significantly increased from 5% at 220 °C to 69% at 250 °C, while it decreased from 69% at 250 °C to 44% at 280 °C. In addition, the HHV of the hydrochar formed at 280 °C was higher than that formed at 250 °C.
These findings were mainly due to the higher proportion of arene species in the hydrochar and arene groups concerned with furan groups with higher HHV, which indicated that hydrochar for high-quality solid fuel was not only concerned with the aromatization rate but also with the furan-to-arene ratio [26]. Therefore, although the aromatization rate of hydrochar formed at 220 °C by LaCl3 catalysis reached 89%, its HHV is equivalent to that of hydrochar formed at 250 °C as shown in Figure 1, based on a very high furan-to-arene ratio. In addition, according to the strength of the O-H peak on the hydrochar in the FTIR spectra (as shown in Figure S2), we can conclude that C-250 has a higher O-H content than LC-220. However, in comparison with the relative proportion of hydrochar in the five regions shown in Figure 6a, the aliphatic C-OH and C-O-C contents of C-250 were higher than those of LC-220, which indicated that the furan ring in C-250 may be connected by ether bonds and the furan ring in LC-220 may be connected mainly by C-C bonds.
In addition, as shown in Figure 4d, it was worth noting that the major aromatic structure of both LC-220 and C-250 was a furan ring, which was similar to the structure of humin. However, the peak attributed to the sp2-hybridized aromatic carbon became the main peak in the region of aromatic carbon for the hydrochar formed at 280 °C, which indicated that the aromatic structure of C-280 was benzofuran. And the proposed structures of LC-220, C-250, and C-280 are shown in Figure 6a. Therefore, we can conclude that although lanthanide (III) ions promote the polymerization of compounds containing furan rings in the aqueous phase at 220 °C, they could not accelerate the polymerization of aqueous organics containing benzene rings.
To further confirm the high catalytic performance of LaCl3 in hydrothermal reactions, Figure 7 illustrates the potential polymerization pathways of HMF. The reduction in HMF is primarily attributed to three factors. First, the acidic environment of subcritical water facilitates the self-condensation of HMF and potentially promotes the aldol condensation reaction between HMF and DHH [27], primarily involving the hydroxymethyl group on HMF and the α-hydrogens on the carbonyl groups of both HMF and DHH, as shown in the pathways in Figure 7A,B. Second, the addition of LaCl3 further enhances the polymerization reactions between furan molecules and between furan and phenolic compounds by removing substituents or even breaking C–H bonds on furan rings and aromatic groups, thereby generating a large number of aromatic radical intermediates [28]. The mechanism of this process is depicted in Figure 7C. This also significantly increases the conversion of HMF into hydrochar, which is consistent with the increased degree of aromatization of hydrochar observed in the 13C NMR analysis.

2.3. Distribution of pH and TOC of Liquid-Phase Products

To further understand the evolution of the catalytic HTC mechanism on account of lanthanide (III) ion catalysis, the liquid-phase by-products formed in the HTC process were determined and analyzed, which indirectly reflected the intensity of some chemical reactions involved in the HTC process. Characterization and analysis of liquid-phase products included TOC and pH. The variation in TOC and pH in the liquid-phase product is shown in Figure 8. As expected, compared with the uncatalyzed HTC process, the pH of the liquid-phase products produced by the catalyzed HTC process obviously decreased at 180–220 °C. This change could be attributed to the enhanced dehydration reaction of organic acids and sugars based on the catalysis of lanthanide (III) ions for HTC processes.
However, the TOC of the liquid products showed a different tendency. When the reaction temperatures were 180 and 200 °C, compared with the uncatalyzed HTC process, the TOC of liquid-phase products produced by the lanthanide (III) ion-catalyzed HTC process dramatically increased, presumably implying that lanthanide (III) ions accelerated the hydrolysis reaction from solid to liquid organic carbon between 180 and 220 °C. Nevertheless, the reaction temperature increased at 220 °C, and the liquid TOC formed by the catalytic HTC process is smaller than that formed by the uncatalyzed HTC process. In detail, the TOC decreased from 1.707 g/L to 1.639 g/L, which might be based on the fact that lanthanide (III) ions enhanced the polymerization reaction from aqueous organics to coke microparticles at this temperature. Similarly, Uddin found that aqueous organics in the liquid phase repolymerized into solid particles, and this repolymerization rate increased with the concentration of dissolved compounds [29].

2.4. Catalytic Carbonization Mechanism

Based on the above measurement results, the catalytic HTC reaction mechanism was deduced as shown in Figure 9. In our experiment, at the lower temperature (T ≤ 200 °C), the pure decomposition of cellulose into matrix char by intramolecular condensation and dehydration without the coke microparticle formation was revealed [30]. As the HTC temperature was at 220 °C in the present work, a small amount of coke microparticle formation was observed.
In the pathway from cellulose to coke microparticles, cellulose is firstly hydrolyzed to glucose, followed by isomerization to form fructose [31,32]. The formation path of HMF by direct conversion of glucose involved ring opening and cleavage of the C-C bond, recombination, intramolecular dehydration, and cyclization [33]. In addition, another reaction path to form HMF was the isomerization of glucose to fructose via cyclic and open-chain mechanisms followed by intramolecular dehydration [34]. As HMF was continuous in production, some of the HMF was often rehydrated and pyrolyzed into DDH, LA, formic acid, phenols, and even 2,5-hexanedione in a subcritical water environment [35]. In fact, HMF, DDH, phenols, and 2,5-hexane dione were also converted into aromatic clusters through polymerization reactions, which included aldol condensation, intramolecular dehydration, removal of single substituents and adjacent position substituents, nucleophilic addition, and even cracking of furan C-H to form the free form [36,37]. As the aromatic clusters reached a certain concentration, it was polymerized into initial coke microparticles [23]. In addition, LA and formic acid in liquid-phase products can be further decomposed into a certain amount of gaseous substances, which mainly include H2, CO, CH4, and CO2 [38,39].
The analysis of 13C NMR and SEM illustrated that the hydrochar catalyzed by LaCl3 at 220 °C was completely transformed into coke microparticles, whose chemical structure is mainly furan rings. It could have presumably meant that polymerization (K5) from furans to coke microparticles was accelerated by an increase in the initial HMF concentration, suggesting a higher order of reaction. Furthermore, the 13C NMR analysis revealed that the rate of cellulose hydrolysis (K1) was also accelerated based on the complete disappearance of the β-1,4-glycosidic linkages in the 13C NMR spectra of LC-220, which confirmed that lanthanide (III) ions also indirectly catalyze the hydrolysis reaction of cellulose. These phenomena can be explained by the following reasons. First, lanthanide (III) (La3+-Lu3+) effectively accelerated the dehydration (K2 and K4) of hexose to HMF, leading to a decrease in the content of hexose and an increase in the content of HMF in the hydrothermal system. The decrease in the concentration of hexose, as a product of the hydrolysis reaction of cellulose, also leads to the intensification of its degree of hydrolysis. Second, the conversion of hexose to HMF and the inhibition of rehydration (K7) of HMF to LA based on rare earth ion catalysis can dramatically increase the concentration of reactants in the polymerization of HMF to coke particles. However, it is difficult to find the benzene ring structure for the hydrochar catalyzed by LaCl3 at 220 °C, which may indicate that lanthanide (III) ions did not catalyze the polymerization from phenols into the coke microparticles.

3. Materials and Methods

3.1. Materials

In this study, pure cellulose, in the form of alpha-cellulose fibers with a particle size of approximately 0.1 mm, was used for hydrothermal carbonization (HTC) experiments conducted in a 100 mL reaction vessel. For catalytic experiments, acetic acid (99+ wt%) was diluted with deionized water to a pH of 2. The cellulose and acetic acid were supplied by Shanghai McLean Biochemical Co., Ltd., Shanghai, China. The reactor (model ZX-100FJ) was purchased from Zhengxin Co., Ltd., Nanjing, China. Lanthanum chloride (LaCl3, 99.99% analytical grade) was obtained from Runyou Chemical (Shenzhen) Co., Ltd., Shenzhen, China.

3.2. Hydrothermal Carbonization Experiment

The reaction temperatures (and pressures) for the cellulose catalytic HTC experiments were set at 180 °C (1.8 MPa), 200 °C (2.0 MPa), and 220 °C (2.4 MPa), respectively. Among them, 180 °C represents the temperature at which cellulose was not hydrolyzed, while 200 °C was, respectively, the temperature at which cellulose was hydrolyzed but no secondary coke formation occurred. Based on the characteristic reactivity at different HTC temperatures in the previous study and the basis of the above measurement results, secondary coke was formed at 220 °C [40]. These solid-phase products produced by this catalytic experiment were labeled A-C-X and L-C-X (among them, AC stands for acetic acid, L for LaCl3, C for cellulose, and X for the reaction temperature). For instance, AC-220 represents the hydrochar of cellulose obtained by acetic acid catalysis at 220 °C, and LC-220 represents the hydrochar of cellulose obtained by LaCl3 catalysis at 220 °C. All experiments were carried out in triplicate.
The cellulose uncatalyzed HTC experiment reaction temperatures (and pressures) were 180 °C (1.8 MPa), 200 °C (2.0 MPa), 220 °C (2.4 MPa), 250 °C (3.8 MPa), and 280 °C (6.5 MPa), respectively. The reaction time was set at 1 h with an agitation speed of 180 rpm. In each HTC experiment reaction, 5 g of cellulose was loaded into the reactor at a mass ratio of 10:1 (50 g of deionized water and 5 g of cellulose). The hydrochar was labeled C-X (C for cellulose and X for the reaction temperature of the uncatalyzed HTC experiment).

3.3. Characterization of the Hydrochars and Aqueous Products

Industrial analysis was performed using an automatic industrial analyzer (YX-GYFX7705, Changsha Youxin Instrument Manufacturing Co., Ltd., Changsha, China) in accordance with the GB/T 30732-2014 [41] standard. The higher heating value (HHV) of hydrochars was determined using an automatic calorimeter (YX-ZR/Q 9704, Changsha Youxin Instrument Manufacturing Co., Ltd., Changsha, China) following the GB/T 213-2008 [42] standard. Functional groups of the solid hydrochars were characterized by Fourier transform infrared spectroscopy (FT-IR, Spectrum Two™, PerkinElmer Co., Ltd., Springfield, IL, USA), with spectra recorded in the range of 400–4000 cm−1. The carbon skeleton structure of the hydrochars was analyzed using solid-state nuclear magnetic resonance (13C NMR, AVANCE III HD 600, Bruker, Ettlingen, Germany). The surface morphology of the hydrochars was examined by scanning electron microscopy (SEM, FEI Quanta 450FEG, Thermo Fisher Scientific, Waltham, MA, USA). The pH of the aqueous phase products was measured using a pH meter (BJ-260, Shanghai Instrument and Electronics Scientific Instrument Co., Ltd., Shanghai, China), while the total organic carbon (TOC) content in the aqueous phase was determined with a LiquiTOC II analyzer (Elementar, Langenselbold, Germany).

4. Conclusions

In this paper, the effect of lanthanide (III) ions on the HTC process of cellulose within the temperature range of 180–220 °C was investigated. The main research results and findings can be listed as follows:
  • Compared with the uncatalyzed HTC process and the traditional acetic acid catalytic process, lanthanide (III) ions showed superior catalytic effects. At 220 °C, it could facilitate the formation of coke microparticles, and as a result, the fuel properties of hydrochar were similar to those of hydrochar catalyzed by acetic acid at 220 °C. The HHV of hydrochar catalyzed by lanthanide (III) ions at 220 °C was comparable to that of the HHV of hydrochar formed at 250 °C.
The catalytic reactivity of lanthanide (III) ions was associated with the degree of hydrolysis of cellulose in the uncatalyzed HTC process.
  • The effective acceleration of the dehydration reaction of glucose to HMF and the inhibition of the rehydration reaction of HMF to LA also indirectly accelerated the hydrolysis of cellulose and the formation of coke microparticles, accelerating the process from cellulose to highly aromatized hydrochar.
Based on the analysis of experimental data and the conclusions, two brief viewpoints for future research are proposed and listed below:
  • Lanthanide (III) ions could effectively promote the conversion of carbohydrates into furanic units in the HTC process of cellulose. Therefore, it is promising to investigate the potential of using LaCl3 as a catalyst for the aromatization of natural lignocellulosic biomass.
  • Although at 220 °C, lanthanide (III) ions could convert cellulose into furan-rich hydrochar, it was unable to accelerate the further conversion to arene-rich hydrochar structures. Therefore, it is necessary to further develop a catalyst to promote the transformation of the furanyl structure into the phenyl structure in the future, increasing the aromatization and graphitization degree of hydrochar.
This study systematically investigates the generation pathways and polymerization mechanisms of key intermediates during LaCl3-catalyzed hydrothermal carbonization, elucidating the essence of its catalytic effects. It provides an environmentally friendly heterogeneous catalyst system that combines high efficiency and sustainability. This work offers a novel approach to the high-value utilization of biomass resources, particularly demonstrating significant potential in the efficient production of high-quality hydrochar.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal15030245/s1: Figure S1. SEM images of hydrochar produced through catalytic and non-catalytic reactions between 180 and 220 °C; Figure S2. The FTIR spectra of C-220, LC-220, C-250, and C-280.

Author Contributions

M.C. and X.X. conceived and supervised the project. S.H. performed the experiments and wrote the manuscript. L.B. and J.W. analyzed the experimental data. Y.G. and J.W. took part in the experiments. All the authors contributed to the discussion during the whole project. All authors have read and agreed to the published version of the manuscript.

Funding

We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China, grant number 52070088, the National Natural Science Foundation of China, grant number 52100146, and the Development Plan Project of Science and Technology of Jilin Province, grant number No. 20230203047SF. We are also grateful for the research collaboration among the groups and universities of the authors.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The proximate analysis and HHV of hydrochars.
Figure 1. The proximate analysis and HHV of hydrochars.
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Figure 2. SEM images of (a) C-220, (b) LC-220, (c) C-250, and (d) C-280.
Figure 2. SEM images of (a) C-220, (b) LC-220, (c) C-250, and (d) C-280.
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Figure 3. FTIR spectra of hydrothermal carbon catalyzed by Lacl3 (a) and acetic acid (b) between 180 and 220 °C.
Figure 3. FTIR spectra of hydrothermal carbon catalyzed by Lacl3 (a) and acetic acid (b) between 180 and 220 °C.
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Figure 4. The 13C NMR spectra of the hydrochars: (a) hydrochars at 180 °C; (b) hydrochars at 200 °C; (c) hydrochars at 220 °C; (d) hydrochars at different temperatures.
Figure 4. The 13C NMR spectra of the hydrochars: (a) hydrochars at 180 °C; (b) hydrochars at 200 °C; (c) hydrochars at 220 °C; (d) hydrochars at different temperatures.
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Figure 5. 13C NMR labeling schemes for cellulose (a) and aromatic rings (furan and phenol) (b).
Figure 5. 13C NMR labeling schemes for cellulose (a) and aromatic rings (furan and phenol) (b).
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Figure 6. Distribution of the C-containing functional groups (a) and the proposed structures (b) of hydrochars formed from raw materials of a high degree of intermediate dissolution and the subsequent hydrolyzed part through polymerization.
Figure 6. Distribution of the C-containing functional groups (a) and the proposed structures (b) of hydrochars formed from raw materials of a high degree of intermediate dissolution and the subsequent hydrolyzed part through polymerization.
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Figure 7. Potential polymerization pathways of HMF: (A) aldol condensation reaction of HMF with HMF; (B) aldol condensation reaction of HMF with DHH; (C) polymerization reactions between furan molecules and between furan and phenolic compounds.
Figure 7. Potential polymerization pathways of HMF: (A) aldol condensation reaction of HMF with HMF; (B) aldol condensation reaction of HMF with DHH; (C) polymerization reactions between furan molecules and between furan and phenolic compounds.
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Figure 8. The evolution of pH and TOC of liquid-phase products.
Figure 8. The evolution of pH and TOC of liquid-phase products.
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Figure 9. The catalytic HTC mechanism of cellulose to form hydrochar with highly aromatic structures on account of lanthanide (III) ion catalysis.
Figure 9. The catalytic HTC mechanism of cellulose to form hydrochar with highly aromatic structures on account of lanthanide (III) ion catalysis.
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Table 1. Functional groups of cellulose and its hydrochars as determined by FTIR analysis.
Table 1. Functional groups of cellulose and its hydrochars as determined by FTIR analysis.
Wavenumber (cm−1)Functional GroupsDescription
3350O-H stretchingAlcohol, phenol, or carboxylic acid
2923, 2848C-H stretchingAliphatic
1732, 1700C=O stretchingCarbonyl, ester, or carboxyl
1602, 1513C=C stretchingAromatic structure or olefins
1050–1249C-O stretchingAlcohol, aliphatic ether, or carboxylic acid
815C-H stretchingAromatic
Table 2. Peak assignments in the 13C NMR spectra of hydrochar of cellulose.
Table 2. Peak assignments in the 13C NMR spectra of hydrochar of cellulose.
δ (ppm)Functional GroupsChemical Formula
185–220KetoneC=O
160–185Acid or esterCOOH/COOR
140–151Cα phenol or linked furanC=C-OH or C=C-O
142Cα free furanC=CH-O
125–129sp2-hybridized aromatic CC-C=C-C
110–118Cβ phenol or linked furanC=C-OH or C-C=C-O
90–110Anomeric CC-O-C
50–90O-alkyl C (alcohol, ether, or aliphatic)C-OH, C-O-C
40Methine or quaternary carbon-C-H<, >C<
30Methylene-CH2-
15Methyl-CH3
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Han, S.; Chi, M.; Xu, X.; Bai, L.; Wu, J.; Guo, Y. From Cellulose to Highly Aromatic Hydrochar: Catalytic Carbonization and Catalytic Aromatization Mechanism of Lanthanide (III) Ions. Catalysts 2025, 15, 245. https://doi.org/10.3390/catal15030245

AMA Style

Han S, Chi M, Xu X, Bai L, Wu J, Guo Y. From Cellulose to Highly Aromatic Hydrochar: Catalytic Carbonization and Catalytic Aromatization Mechanism of Lanthanide (III) Ions. Catalysts. 2025; 15(3):245. https://doi.org/10.3390/catal15030245

Chicago/Turabian Style

Han, Shuaijie, Mingshu Chi, Xiuling Xu, Li Bai, Junquan Wu, and Yizhuo Guo. 2025. "From Cellulose to Highly Aromatic Hydrochar: Catalytic Carbonization and Catalytic Aromatization Mechanism of Lanthanide (III) Ions" Catalysts 15, no. 3: 245. https://doi.org/10.3390/catal15030245

APA Style

Han, S., Chi, M., Xu, X., Bai, L., Wu, J., & Guo, Y. (2025). From Cellulose to Highly Aromatic Hydrochar: Catalytic Carbonization and Catalytic Aromatization Mechanism of Lanthanide (III) Ions. Catalysts, 15(3), 245. https://doi.org/10.3390/catal15030245

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