Co-Production of Furfural, Xylo-Oligosaccharides, and Reducing Sugars from Waste Yellow Bamboo Through the Solid Acid-Assisted Hydrothermal Pretreatment

Abstract

Lignocellulosic waste biomass, a versatile natural resource derived from plants, has gained significant attention for its potential in the sustainable production of biobased chemicals. Furfural (FAL), xylo-oligosaccharides (XOSs), and reducing sugars are important platform chemicals, which can be obtained through the valorization of lignocellulosic solid biomass in a green and sustainable way. Waste yellow bamboo (YB) is one kind of abundant, inexpensive, and renewable lignocellulosic biomass resource. In order to improve the high-value utilization rate of raw YB, biochar-based solid acid catalyst (AT-Sn-YB) was utilized to assist the hydrothermal pretreatment for the valorization of YB in water. Under the optimal reaction conditions (200 °C, 60 min, and AT-Sn-YB dosage of 5.4 wt%), the FAL yield reached 60.8%, and 2.5 g/L of XOSs was obtained in the pretreatment system. It was observed that the surface structure of YB became rough and loose, exposing a significant number of pores. The accessibility increased from 101.8 mg/g to 352.6 mg/g after combined treatment. The surface area and hydrophobicity of lignin were 70.7 m²/g and 2.5 L/g, respectively, which were significantly lower than those of untreated YB (195.4 m²/g and 4.1 L/g, respectively). The YB solid residues obtained after treatment were subjected to enzymatic saccharification, achieving an enzymatic hydrolysis efficiency of 47.9%. Therefore, the hydrothermal pretreatment assisted by the AT-Sn-YB catalyst shows potential application value in FAL production and bamboo utilization, providing important references for other biomass materials.

1. Introduction

With the rapid growth of population and economy in modern society, the world is facing severe energy crises and environmental pollution issues. There is an urgent need for the research and development of green and renewable energy sources [1,2]. Lignocellulosic biomass (LB) is one of the widely available and low-cost biomass resources [3]. Currently, LB can be used to gradually replace fossil resources for the production of value-added chemicals, biofuels, and biodegradable materials, thereby addressing the problems of energy consumption and environmental pollution [4,5]. High-value products obtained from the conversion of LB include furfural (FAL), xylo-oligosaccharides (XOSs), ethanol, 5-hydroxymethylfurfural (HMF), acetic acid, etc., which are important bio-based additives or intermediates in the food and pharmaceutical industries, and play significant roles in the fields of resins and energy [6,7]. FAL was identified as a top chemical that has opportunities from biorefinery carbohydrates due to the high level of attention in the literature in recent years [8]. Furthermore, FAL is an important starting material for the synthesis of cyclopentenone [9,10,11,12]. Yellow bamboo (YB) is a lignocellulosic source that is abundant, inexpensive, and easily accessible [13]. The monosaccharides contained in the cellulose and hemicellulose of YB are substrates for biofuel production, but lignin forms a recalcitrant barrier structure on the surfaces of cellulose and hemicellulose, posing a significant obstacle to the valorization of LB [14]. Therefore, the development of an efficient pretreatment strategy to disrupt the natural barriers and enhance enzymolysis efficiency is crucial for the high-value utilization of YB.

Currently, researchers are employing various pretreatment methods to break down the dense structure of biomass and enhance the accessibility of enzymes to cellulose, such as alkaline, acid, ionic liquid, and microwave pretreatments [15]. Hydrothermal pretreatment has been extensively explored due to its simplicity, low environmental impact, and minimal equipment corrosion [16,17]. During hydrothermal treatment, water not only acts as a solvent but also decomposes to produce hydrated hydrogen ions at high temperatures, which can selectively cleave acetyl groups and break down ether bonds, thereby facilitating the disruption of the lignin barrier [18]. This method can reduce pretreatment costs, minimize environmental impacts, and simplify the biorefinery process [19]. However, compared with other pretreatment methods, hydrothermal pretreatment is relatively mild in removing hemicellulose and a small portion of lignin and disrupting the integrity of the biomass structure [20]. Further research is warranted on additional measures to complement hydrothermal pretreatment.

In recent years, the use of heterogeneous catalysts to address the issues of equipment corrosion, environmental pollution, and high energy consumption associated with traditional homogeneous acid catalysts has garnered extensive attention [21]. In previous studies, heterogeneous catalysts employed for FAL production have primarily included metal oxides, zeolites, functional resins, and sulfonated montmorillonite [22,23,24,25]. Beyond these heterogeneous catalysts, the preparation of sulfonated carbon-based heterogeneous catalysts using inexpensive and renewable biomass as carriers has attracted considerable interest from researchers [26,27]. Biochar-based solid acid catalysts not only exhibit good thermal stability and eco-friendliness but also benefit from the large surface area of carbon materials, which enhances substrate adsorption through synergistic effects, thereby achieving highly efficient catalysis [28]. Kim et al. employed the solid acid catalyst Amberlyst-15 to catalyze FAL production in γ-butyrolactone/H₂O at 210 °C for 20 min, with a FAL yield of 32.2% [29]. Our previous work has successfully used YB as a carrier to prepare a biochar-based solid acid catalyst (AT-Sn-YB) and achieved a high yield of FAL (80.3%) by adding a solvent γ-valerolactone (GVL) (GVL/water = 3:1, v/v) and supplementing chloride ions (CuCl₂, 100 mM) [30]. However, the addition of solvents and extra auxiliary catalysts results in a reaction system that lacks economic and environmental advantages. To achieve the high-value utilization of YB in a green and sustainable way, the role of inexpensive biochar solid acid catalyst AT-Sn-YB-assisted clean hydrothermal pretreatment can be attempted for the conversion of biomass into value-added products, avoiding the additional supplementary of auxiliary catalysts and organic solvents.

The objective of the current work was to use a YB-based solid acid catalyst AT-Sn-YB to assist the hydrothermal pretreatment and catalyze the transformation of waste YB into value-added products. To develop an ecologically and economically viable reaction system to fully realize the high-value utilization of waste YB. Key parameters, including the amount of solid acid, reaction time, and reaction temperature, have been optimized. Additionally, the YB samples before and after pretreatment were characterized by the adsorption of Congo Red, Azure B, and Rose Bengal dyes to elucidate the reasons for the enhanced enzymolysis efficiency. The results indicated that solid acid-assisted hydrothermal pretreatment significantly increases FAL yield, providing important insights for the co-production of FAL, XOSs, and reducing sugars from waste biomass.

2. Results and Discussion

2.1. Solid Acid-Assisted Hydrothermal Pretreatment of Waste YB for Production of FAL

Hydrothermal treatment is one of the cleanest and most economical pretreatment methods, as it uses only water as the reagent, avoiding the need for expensive additives or corrosion-resistant designs [31]. It is known that hydrothermal treatment can hydrolyze hemicellulose and alter the structure of lignin, releasing oligosaccharides, monosaccharides, or other products [32]. These changes facilitate the binding of cellulase to cellulose, thereby enhancing enzymolysis efficiency. In this study, the solid acid catalyst AT-Sn-YB was added to the hydrothermal pretreatment process to assist in the pretreatment of YB and produce FAL. As shown in Figure 1, the yield of FAL from hydrothermal pretreatment without the addition of solid acid was only 5.3%. Adding 1.8–7.2 wt% of solid acid catalyst to the reaction system could effectively increase the yield of FAL. When 5.4 wt% of the solid acid was added, the yield of FAL could reach 39.9%. However, further increasing the amount of solid acid did not significantly enhance the yield of FAL. When the dosage of solid acid was 7.2 wt%, the yield of FAL was only 40.5%. This might be ascribed to the fact that an appropriate amount of catalytic active sites can facilitate the dehydration of xylose to form FAL. However, an excess of solid acid catalyst can lead to side reactions such as condensation and resinification, which hinder the increase in FAL yield [33]. A similar observation has been reported by Xu et al. [34]. The highest yield of FAL from corncob was achieved when the catalyst Sn-SLC content was 150 mg. As the catalyst content increased (200–300 mg), a significant downward trend in FAL yield was observed.

The effects of reaction temperature and duration on the formation of FAL were further investigated. As showcased in Figure 2a, increasing the reaction temperature from 140 °C to 220 °C led to an increase in FAL yield from 5.8% to 41.2%. The highest FAL yield of 51.2% was achieved at 200 °C. However, further elevating the temperature resulted in a decrease in FAL yield. Although high temperatures can promote the formation of FAL, unwanted side reactions may occur. Similar situations have been reported previously [35]. The by-products generated from these side reactions can deposit on the acidic active sites of the chemocatalyst, reducing the binding capacity between substrate molecules and solid acids, and thereby lowering the efficiency of FAL synthesis. Additionally, high temperatures can lead to partial decomposition of FAL [33]. At 200 °C, extending the reaction time from 10 min to 60 min increased the furfural yield to 60.8% (Figure 2b). Further extending the reaction time to 120 min only resulted in a slight increase in FAL yield. Therefore, in this work, 200 °C and 60 min were selected as the optimal reaction conditions, with a solid acid loading of 5.4 wt%.

2.2. Formation of XOSs in Pretreatment Solution

Along with the formation of FAL in the pretreatment process, weakly acidic conditions can hydrolyze xylan to produce XOSs and xylose. XOSs, as an excellent material, are currently widely used in the field of functional foods [36,37]. Moreover, the presence of XOSs in filtrate can also indirectly confirm the dissolution of hemicellulose. As shown in Figure 3a, when the solid acid loading was 5.4 wt%, a yield of 2.3 g/L of XOSs could be obtained. As the reaction temperature elevated from 140 °C to 200 °C, the yield of XOSs increased from 1.5 g/L to 2.3 g/L (Figure 3b). During the investigation of reaction time, extending the reaction time to 60 min resulted in a maximum yield of 2.5 g/L of XOSs (Figure 3c). However, further elevating reaction temperature, increasing solid acid loading, or prolonging reaction duration did not effectively enhance the yield of XOSs. Because harsher treatment conditions can lead to partial hydrolysis or degradation of XOSs, resulting in a reduction in molecular weight and the possible formation of monosaccharides or other by-products [38]. Similar observations have been reported in other studies, and higher yields of XOSs could be obtained under moderate conditions [39]. Clearly, the highest yield of 2.5 g/L of XOSs was obtained when the solid acid AT-Sn-YB was added at 5.4 wt% and the reaction was carried out at 200 °C for 60 min.

The pH value is the most intuitive variable in the hydrothermal treatment process. During hydrothermal treatment, water is ionized to produce hydronium ions, which leads to changes in the pH during autohydrolysis [40]. As shown in Figure 4, the pH decreased with increasing LogR₀. This result might be due to that the higher LogR₀ will generate more hydrogen ions, thereby lowering the pH [41]. These above results showcased that the solid acid catalyst-assisted hydrothermal treatment could enhance FAL productivity. Meanwhile, the reaction system might depolymerize xylan in biomass to generate XOSs, showing great potential in the transformation of lignocellulose to high-value platform chemicals.

2.3. Enzymatic Hydrolysis of YB into Reducing Sugars

Catalyst-assisted hydrothermal pretreatment can effectively disrupt the natural barriers of YB, which promote the cleavage of glycosidic bonds in carbohydrates and assist the biological transformation of cellulase [42]. The raw material, YB, consisted of 33.6% glucan, 19.2% xylan, and 22.2% lignin. In this work, the solid acid-assisted hydrothermal pretreatment of YB at 200 °C for 60 min achieved a lignin removal of 42.9%. As shown in

Table 1. Chemical composition of raw material and YB after pretreatment.

Pretreatment Condition			Glucan, %	Xylan, %	Lignin, %	Lignin Removal, %
Catalyst Loading, wt%	Temperature, °C	Time, min
Raw			33.6 ± 0.1	19.2 ± 0.2	22.2 ± 0.3	/
0	180	30	34.8 ± 0.2	13.1 ± 0.2	24.7 ± 0.2	18.5 ± 0.1
1.8			34.6 ± 0.3	12.6 ± 0.1	26.2 ± 0.1	19.4 ± 0.1
3.6			33.6 ± 0.2	12.1 ± 0.2	27.1 ± 0.1	23.5 ± 0.2
5.4			32.8 ± 0.3	10.9 ± 0.1	29.0 ± 0.2	36.2 ± 0.2
7.2			31.1 ± 0.2	10.6 ± 0.1	29.2 ± 0.2	38.9 ± 0.2
5.4	140		34.1 ± 0.2	15.3 ± 0.3	23.5 ± 0.3	15.5 ± 0.1
	160		33.6 ± 0.1	12.7 ± 0.3	26.0 ± 0.2	33.5 ± 0.1
	180		32.8 ± 0.1	10.4 ± 0.1	27.0 ± 0.2	37.5 ± 0.3
	200		31.6 ± 0.2	9.5 ± 0.2	29.6 ± 0.3	41.0 ± 0.2
	220		21.5 ± 0.1	8.1 ± 0.1	31.5 ± 0.1	43.5 ± 0.2
	200	10	33.9 ± 0.3	10.2 ± 0.3	26.6 ± 0.2	36.1 ± 0.3
		20	33.7 ± 0.2	9.9 ± 0.2	28.4 ± 0.2	37.8 ± 0.1
		30	31.6 ± 0.1	9.5 ± 0.1	29.5 ± 0.3	40.1 ± 0.1
		60	33.8 ± 0.2	8.8 ± 0.1	28.2 ± 0.2	42.9 ± 0.2
		120	34.1 ± 0.2	8.3 ± 0.2	28.1 ± 0.2	44.3 ± 0.4

, with the increasing loading of solid acid, the glucan content in the treated YB first increased and then decreased. Meanwhile, the xylan content in YB continuously decreased. When the solid acid dosage was 5.4 wt%, the glucan content was 32.8%, and the xylan content was 10.9%. The increase in glucan content was due to the dissolution of extractives and hemicellulose during the pretreatment process [43]. Upon investigating the effect of reaction temperature, both the glucan and xylan contents in YB showed a downward trend with increasing reaction temperature. At 220 °C, the glucan and xylan contents were only 21.5% and 8.1%, respectively. This indicated that the hydronium ions generated at high temperatures effectively disrupted the cellulose and hemicellulose structures in YB [44]. Extending the pretreatment time to 120 min also caused a decrease in the xylan content of YB, from 19.2% to 8.3%, while the glucan content changed to 34.1%.

During the screening of pretreatment conditions, increasing the solid acid loading, raising the reaction temperature, and extending the reaction time all led to an increase in the lignin content in YB. This is mainly because of the removal of xylan and partial glucan during the pretreatment process, while part of the lignin will be redeposited on the biomass surface [45]. Overall, solid acid-assisted hydrothermal pretreatment can significantly remove xylan and lignin from biomass, overcoming the recalcitrance of biomass. Compared with the raw sample, the treated samples were enriched with higher cellulose content and greater potential for enzymatic hydrolysis.

The efficiency of enzymatic hydrolysis is a crucial parameter for evaluating the performance of pretreatment [46]. Due to the presence of physical barriers, the enzymatic hydrolysis efficiency of YB before pretreatment was only 10.0%. As shown in Figure 5a, with the increase in the amount of solid acid, the enzymatic hydrolysis efficiency increased from 26.5% to 34.5%. Moreover, as the reaction temperature was raised and the reaction time extended (Figure 5b,c), the enzymatic hydrolysis efficiency also showed an upward trend. When the reaction temperature reached 200 °C, the enzymatic hydrolysis efficiency reached 46.2%. Further increasing the reaction temperature to 220 °C only brought about an additional increase of approximately 4%, which might be due to the condensation and deposition of lignin at high temperatures, thereby affecting the enzymatic hydrolysis of cellulose [47]. After 60 min of pretreatment, the enzymatic hydrolysis efficiency increased to 47.9%. As displayed in Figure 5d, a good correlation between lignin removal and enzymatic hydrolysis efficiency was found in this study (R² = 0.85). After pretreatment, lignin in YB was removed, and the original dense structure among cellulose, hemicellulose, and lignin was disrupted, exposing more cellulose on the biomass surface. Therefore, cellulase can effectively bind to the cellulose in YB, significantly improving the enzymatic hydrolysis efficiency. The above results indicate that solid acid-assisted hydrothermal pretreatment may destroy the inherent structure of YB and enhance saccharification efficiency, and this combined pretreatment is conducive to the subsequent efficient production of reducing sugars from bamboo.

2.4. Analysis of Physicochemical Properties of Raw and Treated YB

2.4.1. Accessibility

One of the significant challenges in the utilization of biomass is the physical barrier formed by the combination of lignin with cellulose and hemicellulose, which hinders the binding of cellulase to cellulose molecules in biomass. Therefore, enhancing the accessibility of cellulase is an effective way to improve biomass utilization [48]. In this work, the accessibility of the raw material and the treated YB was measured. The untreated YB had an accessibility of only 101.8 mg/g. However, after treatment with the AT-Sn-YB catalyst-assisted hydrothermal pretreatment at 220 °C for 30 min, the accessibility reached a maximum of 493.6 mg/g (Figure 6a). Under optimal conditions, the accessibility could reach 352.6 mg/g. Figure 6b revealed a good linear relationship between accessibility and enzymatic hydrolysis efficiency (R² = 0.90). The pretreatment removed a large amount of lignin from the surface of YB, and the cellulose structure was released, increasing the binding sites for cellulase and significantly enhancing the accessibility of cellulase. As a result, the enzymatic hydrolysis efficiency of YB was improved. The above results showcased that the high accessibility could facilitate the enzymolysis of biomass and result in high enzymatic hydrolysis efficiency with cellulases [49].

2.4.2. Lignin Surface Area

Lignin hinders the conversion of biomass and inhibits the enzymatic hydrolysis of biomass, while the lignin surface area has become a key indicator for measuring lignin performance [50]. The lignin surface area of YB was determined by analyzing the adsorption of the Azure B dye. Lignin on the surface of YB competes with cellulose and forms irreversible binding with cellulases, thereby exerting a negative impact on the enzymatic hydrolysis of cellulose. The greater the pretreatment intensity in the reaction system, the greater its impact on the depolymerization of lignin structure [51]. As illustrated in Figure 7, the lignin surface area of the raw was 195.4 m²/g. After pretreatment at 220 °C for 30 min, the lignin surface area was reduced to only 44.7 m²/g. Through the treatment at 200 °C for 60 min, the lignin surface area decreased to 70.7 m²/g. These results suggested that the catalyst-assisted hydrothermal pretreatment effectively removed lignin from YB, reduced its surface area, and thereby increased enzymolysis efficiency.

2.4.3. Hydrophobicity

The adsorption of enzymes onto cellulose is the first step of hydrolysis. However, lignin can also non-specifically and irreversibly adsorb enzymes through hydrophobic interactions [49], thereby reducing the amount of enzyme available for cellulose hydrolysis and consequently decreasing the efficiency of enzymatic saccharification [52,53]. In this study, the hydrophobicity of lignin was determined using the Rose Bengal dye adsorption method, and the results are shown in Figure 8. The hydrophobicity of lignin in the raw YB was 4.1 L/g, which decreased to 2.5 L/g after pretreatment at 200 °C for 60 min. After pretreatment, a large amount of lignin might be eliminated from YB, resulting in a reduction in lignin surface area and hydrophobicity. Therefore, the inhibitory effect of lignin on enzymatic hydrolysis was weakened. These research findings further confirm the important role of lignin hydrophobicity in the non-specific adsorption of enzymes.

2.4.4. SEM

SEM can be utilized to characterize the changes in biomass surface [48]. Figure 9 showcased that the original material YB has a dense structure with almost no pores on its surface. After solid acid-assisted hydrothermal pretreatment, the surface structure of YB became rough and loose, exposing a significant number of pores. This demonstrated that the reaction system might effectively disrupt the dense physical barrier on the surface of YB. The rough surface provided abundant binding sites for cellulases, thereby promoting full contact between cellulose molecules and cellulases, which would be conducive to improving the efficiency of enzymatic hydrolysis [54].

2.5. Comprehensive Evaluation of Coproducing FAL, XOSs, and Reducing Sugars Through the Solid Acid-Assisted Hydrothermal Pretreatment

To alleviate energy issues, it is of long-term significance to study environmentally friendly methods for converting biomass into value-added products [49]. The value-added process of YB was illustrated using a mass balance (Figure 10a). Meanwhile, the potential mechanisms of the solid acid-assisted hydrothermal pretreatment of YB for the co-production of FAL, XOSs, and reducing sugars were comprehensively evaluated (Figure 10b). The raw YB (3000 g) contains 1008.0 g of cellulose, 576.0 g of hemicellulose, and 666.0 g of lignin. The catalytic reaction was carried out in a reactor containing 3000 g of YB powder, 2160.0 g of AT-Sn-YB, and 40.0 L of water. Under optimal conditions (200 °C and 60 min), 3000 g of YB powder was successfully converted into 177.8 g of FAL and 100.0 g of XOSs. The remaining residue could be enzymatically hydrolyzed into 228.0 g of glucose by cellulase, with an enzymatic hydrolysis efficiency of 47.9%. Compared to previous work [reaction media: GVL/water (3:1, v/v) containing 100 mM CuCl₂; formed XOSs: 2.8 g/L] [30], this solid acid-assisted hydrothermal pretreatment process was ecologically sustainable because it did not involve the use of organic solvents and auxiliary catalysts, achieving 2.5 g/L of XOSs from YB in water. In this green and sustainable system, YB could be catalyzed, and the effective co-production of FAL (46.3 mM), XOSs (2.5 g/L), and reducing sugars (8.5 g/L).

In this work, the selected hydrothermal pretreatment and biochar-based solid acid catalysts were considered green and clean, without the need for additional chemicals, thereby minimizing environmental impact [55]. The preparation of YB into a solid acid catalyst was also a promising option for valorizing YB resources. As is well known, solid acid catalysts have attracted widespread attention due to their significant advantages in recyclability and reusability during continuous cycles [56]. Biochar-based materials, with their large specific surface area and porous structure, can effectively immobilize a large number of sulfonic acid groups, thereby providing abundant active sites for catalytic reactions [57]. In the research by Teng et al. [58], a novel carbon-based solid acid catalyst derived from rapeseed pollen was prepared, which still maintained its catalytic performance after five repeated uses. Therefore, it is possible to recycle and reuse the biochar-based solid acid catalyst AT-Sn-YB prepared from waste YB, which can provide an effective strategy for the future development of greener and more environmentally friendly solid acid catalysts. The degradation of hemicellulose usually begins with the cleavage of glycosidic bonds, leading to the formation of monosaccharides such as xylose [59]. Under the combined effects of solid acid and hydrothermal pretreatment, the natural recalcitrance of YB was disrupted, with lignin being removed by 42.9%. FAL is generated through the depolymerization of xylan into xylose, followed by the dehydration of xylose [60,61]. XOSs can be primarily obtained through the hydrolysis of xylan, while the hydrolysis of glucan generates glucose [37]. Additionally, the acidic environment spontaneously formed under pretreatment conditions further promotes the disruption of biomass’ physical barriers [14], resulting in a significant reduction in lignin surface area (70.7 m²/g) and hydrophobicity (2.5 L/g). The cellulase accessibility of the treated YB (352.6 mg/g) was effectively enhanced, with a reducing sugar yield of 47.9%. These results indicated that solid acid-assisted hydrothermal pretreatment could efficiently disrupt the dense structure of YB, thereby facilitating the co-production of FAL, XOSs, and reducing sugars. This constructed pretreatment process might reduce equipment usage while maximizing the utilization of YB resources in a green and sustainable way. However, bamboo is more recalcitrant to pretreatment compared to other lignocellulosic biomass materials (such as agricultural residues and wood). Future work will continue to explore the applicability of catalyst-assisted hydrothermal pretreatment systems in the value-added processing of other biomass materials.

3. Materials and Methods

3.1. Chemicals and Materials

The waste yellow bamboo (YB), which is composed of 33.6% glucan, 19.2% xylan, and 22.2% lignin, was obtained from Fuzhou (Sheng Long limited company, Fuzhou, Fujian Province, China). It was ground into particles (60–80 mesh) and then dried in an oven at 40 °C for storage and later use. The dyes Azure B, Rose Bengal, and Congo Red were purchased from National Pharmaceutical Group Chemical Reagent Co., Ltd. (Nanjing, Jiangsu, China). The cellulase (Cellic^® CTec.2) was obtained from Sigma-Aldrich (Shanghai, China).

3.2. Solid Acid Catalyst Assisted Hydrothermal Pretreatment of YB

The detailed preparation and characterization of the solid acid AT-Sn-YB were described in our previous report [30]. In this research, the hydrothermal pretreatment of YB assisted by a solid acid catalyst was carried out in a 100 mL reactor. A certain amount of AT-Sn-YB (0–7.2 wt%) was added to the reactor, along with 3.0 g of YB and 40 mL of deionized water. The mixture was heated to the specified reaction temperature (140, 160, 180, 200, and 220 °C), and the reaction was conducted for 10–120 min. After the reaction was completed, the reactor was rapidly cooled to room temperature by placing it into ice water. The concentrations of FAL and XOSs were measured using high-performance liquid chromatography (HPLC) (LC-2030C 3D SHIMADZU, Kyoto, Japan). The washed YB residue was dried in an oven at 60 °C for constant weight and further collected for storage. The pretreatment experiments conducted under different conditions can be compared using the severity factor (LogR₀), which is defined by Equation (1) as a parameter for evaluating the intensity of pretreatment [49].

$${\mathrm{LogR}}_0=\mathrm{Log}\left[\mathrm{t}\times \exp \left(\mathrm{T}-100/14.75\right)\right]$$

(1)

In this equation, T indicates pretreatment temperature, and t represents pretreatment time.

3.3. Chemical Composition Analysis

The chemical composition of the YB was analyzed as previously mentioned [62]. A total of 0.3 g of treated YB and 3 mL of 72% H₂SO₄ were placed into a vial, which was then placed in a water bath shaker for reaction (30 °C, 150 rpm, and 1 h). After the reaction was over, 84 mL of deionized water was added. The mixture was shaken well and sealed, and then sterilized at 121 °C for 1 h. After cooling to room temperature, the filtrate was obtained by filtration. The components of the acid hydrolysate were determined using HPLC (Aminex HPX-87H column; Bio-Rad, Hercules, CA, USA). The relevant calculation formulas are shown as follows:

$$\mathrm{FAL}\mathrm{yield}={\displaystyle \frac{\mathrm{FAL}\mathrm{produced}\left(\mathrm{g}\right)}{\mathrm{Consumed}\mathrm{x}\mathrm{ylan}\left(\mathrm{g}\right)}}\times {\displaystyle \frac{150}{96}}\times 100\%$$

(2)

$$\mathrm{Solid}\mathrm{recovery}={\displaystyle \frac{\mathrm{Solid}\mathrm{in}\mathrm{treated}\mathrm{YB}\left(\mathrm{g}\right)}{\mathrm{Solid}\mathrm{in}\mathrm{Untreated}\mathrm{YB}\left(\mathrm{g}\right)}}\times 100\%$$

(3)

$$\mathrm{Glucan}\mathrm{recovery}={\displaystyle \frac{\mathrm{Glucan}\mathrm{in}\mathrm{treated}\mathrm{YB}\left(\mathrm{g}\right)}{\mathrm{Glucan}\mathrm{in}\mathrm{Untreated}\mathrm{YB}\left(\mathrm{g}\right)}}\times 100\%$$

(4)

$$\mathrm{Xylan}\mathrm{removal}=[1-{\displaystyle \frac{\mathrm{Xylan}\mathrm{in}\mathrm{treated}\mathrm{YB}\left(\mathrm{g}\right)}{\mathrm{Xylan}\mathrm{in}\mathrm{untreated}\mathrm{YB}\left(\mathrm{g}\right)}}]\times 100\%$$

(5)

$$\mathrm{Lignin}\mathrm{removal}=[1-{\displaystyle \frac{\mathrm{Lignin}\mathrm{in}\mathrm{treated}\mathrm{YB}\left(\mathrm{g}\right)}{\mathrm{Lignin}\mathrm{in}\mathrm{untreated}\mathrm{YB}\left(\mathrm{g}\right)}}]\times 100\%$$

(6)

3.4. Enzymatic Hydrolysis

The enzymatic saccharification of YB before and after pretreatment was carried out using cellulase (15.0 FPU/g of glucan). The reaction system was incubated in a water bath shaker (150 rpm and 50 °C) for 72 h. The enzymatic hydrolysate was centrifuged, and the supernatant was collected for analysis using HPLC. The enzymatic hydrolysis efficiency was calculated according to the equation below:

$$\mathrm{Enzymolysis}\mathrm{efficiency}={\displaystyle \frac{\mathrm{Glucose}\left(\mathrm{g}\right)\times 0.9+\mathrm{Xylose}\left(\mathrm{g}\right)\times 0.88}{\mathrm{Glucan}\mathrm{in}\mathrm{treated}\mathrm{YB}\left(\mathrm{g}\right)+\mathrm{Xylan}\mathrm{in}\mathrm{treated}\mathrm{YB}\left(\mathrm{g}\right)}}\times 100\%$$

(7)

where 0.9 corresponds to the factor that converts glucose to equivalent glucan and 0.88 corresponds to the factor that converts xylose to equivalent xylan.

3.5. Physicochemical Properties of Raw and Treated YB

3.5.1. Accessibility

The Congo Red dye adsorption experiment can be used to assess the cellulose accessibility of YB [63]. In a 50 mL conical flask, the pretreated YB was immersed in Congo Red solutions of different concentrations (0–4.0 g/L). After the reaction, the absorbance of the supernatant was measured with a spectrophotometer at 498 nm. The accessibility of cellulose was calculated by fitting the Langmuir adsorption isotherm.

3.5.2. Lignin Surface Area

The lignin surface area of YB before and after pretreatment can be determined by the Azure B dye adsorption experiment [64]. YB was reacted with sodium phosphate buffer (50 mM and pH 7.0) and different concentrations of Azure B solution (0–0.80 g/L). Subsequently, the absorbance of the supernatant was measured using a spectrophotometer at 647 nm. Based on the Langmuir adsorption isotherm, the lignin surface area of YB before and after pretreatment was calculated by fitting the data.

3.5.3. Hydrophobicity

The hydrophobicity of lignin in the raw and treated YB can be assessed through the Rose Bengal dye adsorption experiment [54]. Different weights of YB (0.04–0.4 g) were immersed in 10 mL of Rose Bengal solution (40 mg/L, pH 4.8). The residual dye in the supernatant was detected over a spectrophotometer at 543 nm. The distribution coefficient of Rose Bengal dye between the solution and YB was linearly related to the concentration of YB, and the hydrophobicity (L/g) could be calculated from the slope of the fitted line.

3.5.4. Scanning Electron Microscopy (SEM)

The surface morphology of YB was observed using a scanning electron microscope (SEM, Hitachi, Japan). Prior to the SEM imaging, a thin layer of gold (Au) was plated on the surface of the YB powder in a vacuum to make it conductive.

4. Conclusions

Biochar-based solid acid catalyst (AT-Sn-YB) was used to assist hydrothermal pretreatment for catalyzing YB in water. At a catalyst AT-Sn-YB loading of 5.4 wt%, YB could be efficiently converted into FAL (46.3 mM) and XOSs (2.5 g/L) at 200 °C for 60 min. The solid acid catalyst AT-Sn-YB-assisted hydrothermal pretreatment endowed the treated YB with higher accessibility (352.6 mg/g), lower lignin surface area (70.7 m²/g), and hydrophobicity (2.5 L/g). It effectively disrupted the recalcitrant structure of YB, achieving an enzymolysis efficiency of 47.9%. The feasibility of incorporating a green biochar-based solid acid catalyst (AT-Sn-YB) into the clean hydrothermal pretreatment system was explored, providing a green and sustainable strategy for the high-value valorization of bamboo resources.