Highly Efficient Production of Furfural from Corncob by Barley Hull Biochar-Based Solid Acid in Cyclopentyl Methyl Ether–Water System

Abstract

Furfural, an important biobased compound, can be synthesized through the chemocatalytic conversion of D-xylose and hemicelluloses from lignocellulose. It has widespread applications in the production of valuable furans, additives, resins, rubbers, synthetic fibers, polymers, plastics, biofuels, and pharmaceuticals. By using barley hulls (BHs) as biobased support, a heterogeneous biochar Sn-NUS-BH catalyst was created to transform corncob into furfural in cyclopentyl methyl ether–H₂O. Sn-NUS-BH had a fibrous structure with voids, a large comparative area, and a large pore volume, which resulted in more catalytic active sites. Through the characterization of the physical and chemical properties of Sn-NUS-BH, it was observed that the Sn-NUS-BH had tin dioxide (Lewis acid sites) and a sulfonic acid group (Brønsted acid sites). This chemocatalyst had good thermostability. At 170 °C for 20 min, Sn-NUS-BH (3.6 wt%) was applied to transform 75 g/L of corncob with ZnCl₂ (50 mM) to generate furfural (80.5% yield) in cyclopentyl methyl ether–H₂O (2:1, v/v). This sustainable catalytic process shows great promise in the transformation of lignocellulose to furfural using biochar-based chemical catalysts.

1. Introduction

The swift depletion of global fossil fuel reserves, coupled with escalating environmental pollution and an intensifying energy crisis, has heightened the focus on developing and utilizing renewable energy sources [1]. Lignocellulosic biomass is a promising feedstock for manufacturing biofuels and other value-added chemicals due to its abundance, low cost, and potential to reduce greenhouse gas emissions compared to fossil fuels [2,3,4,5,6]. It primarily consists of lignin, hemicellulose, and cellulose [7]. Hemicellulose is one kind of non-cellulosic polysaccharide in the cell walls of plants [8], which can frequently be transformed into biobased chemicals such as furfural [9,10]. Furfural is a pivotal furan-based compound with extensive applications across various industries. It is widely utilized in the manufacturing of furans, disinfectants, plastics, flavors, resins, dyes, fine chemicals, and biofuels, and it also serves as an industrial solvent [11].

It is well known that chemocatalysts can effectively facilitate the transformation of lignocellulosic biomass into furfural within suitable solvent systems [12,13,14]. Although water is often suggested as the most eco-friendly reagent, furfural production can be hindered in the single water system due to the poor solubility of furfural [15]. However, furfural is easy to dissolve in some organic solvents. The constructed organic solvent–water system facilitates the furfural generation, and the formed furfural can be extracted into the organic phase, avoiding its degradation during the transformation of biomass into furfural [16,17]. Accordingly, an attempt is needed to establish an organic solvent–H₂O reaction medium for converting biomass into furfural. Currently, many organic solvents (e.g., methyl isobutyl ketone (MIBK), γ-valerolactone (GVL), and cyclopentyl methyl ether (CPME)) have been utilized to acquire furfural from biomass or biomass-derived D-xylose [18,19,20,21]. It is known that CPME is a green and effectual organic solvent. In a CPME-H₂O mixture (1:1, v/v), birch–hydrolysate was transformed into furfural in 68% yield at 190 °C [16].

Industrially, conventional homogeneous acids (H₂SO₄ and HCl) are commonly utilized as chemocatalysts for preparing furfural [22], which are prone to causing equipment corrosion, environmental pollution, and product recovery difficulties. On the contrary, heterogeneous biochar-based chemocatalysts have properties of low corrosion, good chemocatalytic selectivity, excellent thermostability, and easy separation [23]. As green chemocatalysts, biochar-based solid acid chemocatalysts have attracted more and more researchers’ attention [24,25]. Gong et al. prepared a biochar-based chemocatalyst SO₄²⁻/SnO₂-SSXR for transforming xylose-rich hydrolysates into furfural in a yield of 83% in a choline chloride–maleic acid–toluene mixture [12]. Dai et al. prepared biochar-based solid acid S-IRCC to transform D-xylose into furfural in a 74% yield in a MIBK-H₂O (2:1, v:v) mixture containing NaCl (60 g/L) at 190 °C within 1 h [18].

As is well known, hulls are the outermost layers of wheat grains, and barley hulls (BHs) account for 15–20% of the dry weight of grains. The global annual output of barley is around 146 million tons. Accordingly, the effectual utilization of BH resources has gained much attention. However, to date, there are few works on the synthesis of BH-based heterogeneous chemocatalysts for effectual transformation of biomass into furfural in eco-friendly reaction systems. As one of the biological wastes, BH has a unique chemical composition and structural properties [26]. In this research, the merits of a heterogeneous chemocatalyst and a green solvent system were combined. A new tin-based biochar heterogeneous chemocatalyst Sn-NUS-BH was acquired using BH as support, and furfural was acquired through the catalysis of corncob in a CPME-H₂O reaction system. The influences of reaction duration, performance temperature, Sn-NUS-BH dose, solvent type and dosage, metal chloride salt type, and ZnCl₂ loading on the corncob conversion to furfural were tested. An effectual CPME-H₂O reaction system containing Sn-NUS-BH was built for the conversion of corncob into furfural, which afforded a promising process for the valorization of biomass.

2. Results and Discussion

2.1. Characteristics of Sn-NUS-BH

The tin-based sulfonated biochar Sn-NUS-BH chemocatalyst was created using a series of operation steps. The pore structure and surface characteristics of NaOH-ultrasonic-treated BH (NUS-BH) and chemocatalyst Sn-NUS-BH were measured using the BET method. In

Table 1. Structural analysis of NUS-BH and solid acid Sn-NUS-BH.

Sample	Specific Surface Area, m2/g	Pore Volume, cm3/g	Pore Size, nm
NUS-BH	0.6	0.04	11.8
Sn-NUS-BH	63.2	0.15	2.8

, Sn-NUS-BH had an increased specific surface area (63.2 m²/g), an augmented pore volume (0.15 cm³/g), and a declined pore size (2.8 nm). In the preparation of Sn-NUS-BH, protein and other components in BH were removed by an alkali solution (0.5 M NaOH) combined with an ultrasonic treatment (100 W, 50 °C). Solvents (e.g., H₂SO₄, ammonia water, and ethanol) and calcining (550 °C) made the NUS-BH structure slack and tight, thus augmenting specific surface area, enlarging pore volume, and decreasing pore size. The alterations might have had a crucial effect on the chemocatalytic activity of heterogeneous chemocatalysts [27], which facilitated additional loading of Sn⁴⁺ ions and sulfonic acid groups on Sn-NUS-BH. The smaller pore size might block more Sn⁴ ions, which would enhance the chemocatalytic activity of solid acid [28].

SEM was applied to observe the surface morphology changes in NUS-BH and Sn-NUS-BH (Figure 1). The NUS-BH surface was smooth, while the Sn-NUS-BH surface was rough and porous, which was consistent with the results of the augmented specific surface area, enlarged pore volume, and decreased pore size (Table 1). These alterations facilitated the loading of sulfonic acid groups (Brønsted acid sites) on Sn-NUS-BH. Some tiny particles might attach to the surface of Sn-NUS-BH, which helped SnO₂ (Lewis acid site) generate covalent bonds with carbon groups [29]. Brønsted and Lewis acids with strong acidity might attach to the Sn-NUS-BH surface. When the Sn-NUS-BH was entirely in contact with corncob powders, the chemocatalytic efficiency improved.

XRD was applied to visualize the crystal structure of NUS-BH and Sn-NUS-BH (Figure 2). They had apparent characteristic peaks at 26.5°, which corresponded to the crystal structure of carbon. Through the sulfonation, the peak width of Sn-NUS-BH arose at the crystal plane of carbon, verifying that the sulfonic acid group might connect to the abnormal carbon structure. The peaks near 33.8° and 51.9° were ascribed to the tin ion, which might be adequately loaded on the Sn-NUS-BH, leading to a solid crystal structure occurring [13,28]. By comparing NUS-BH and Sn-NUS-BH, the position of abscissa was fundamentally unchanged, testifying that the basic structure of the BH shell was not substantially damaged before and after treatment.

FT-IR was used to characterize NUS-BH and Sn-NUS-BH (Figure 3). The peak around 3435 cm⁻¹ originated from the stretching vibration of O-H groups [30]. The peak near 2826 cm⁻¹ was related to the stretching vibration of C-H. The existence of Brønsted acid sites on Sn-NUS-BH was attributed to a wavelength of 1640 cm⁻¹ [31]. The peaks near 1116 cm⁻¹ and 778 cm⁻¹ were associated with Sn⁴⁺ and SO₄²⁻ on the surface of Sn-NUS-BH, and O=S=O stretching was observed, implying the loading of sulfonic groups as Brønsted acid sites on Sn-NUS-BH [28]. The peak around 622 cm⁻¹ was associated with SnO₂ as Lewis acid sites [30,32], confirming that Sn⁴⁺ ions were loaded on NUS-BH.

The valence state of the Sn element on Sn-NUS-BH could be measured through an XPS analysis (Figure 4). Three valence states of tin ions (0, +2, and +4) were observed. It was found that the contents of Sn⁰3d_5/2, Sn²⁺3d_5/2, and Sn⁴⁺3d_5/2 were 14.38%, 48.95%, and 36.67%, respectively. In the preparation of Sn-NUS-BH, tin ions with +4 valence formed. Additionally, SnO₂ might enhance the hydrolysis of xylan to D-xylose and accelerate the dehydration of D-xylose into furfural [3].

According to the analysis with the temperature-programmed desorption (TPD) with NH₃, the acid sites on Sn-NUS-BH were measured. Generally, strong (400–800 °C), medium (200–400 °C), and weak (100–200 °C) acid sites may be determined [13,33]. As showcased in Figure 5, Sn-NUS-BH had two strong acid sites at 750 and 800 °C. The strong acid sites had a crucial role in the hydrolysis and dehydration reaction, which favored the hydrolysis of xylan in biomass into D-xylose and further dehydration of D-xylose to furfural [34].

2.2. Effect of Organic Solvents on Furfural Yield

It is known that the solubility of furfural is poor in a single water system, so the transformation of biomass into furfural would be restricted during the preparation of furfural. Different from the single aqueous phase, the merit of an organic solvent–water reaction system is that the generated furfural can be rapidly extracted into the organic phase [35]. Utilizing an organic solvent–water reaction system not only avoids the tedious steps of collecting furfural but also alleviates the generation of by-products [12]. To examine the chemocatalytic performance of Sn-NUS-BH in different organic solvent–water media, six organic solvents, including CPME, MIBK, GVL, THF, DMF, and DMSO, were separately supplemented into the aqueous system to promote furfural generation (Figure 6a). The results showcased that the yield of furfural in CPME-H₂O reached the highest amount (58.1%), which was 1.73 times higher than that in the single water system. For other organic solvents, the furfural yields were relatively low, which were ascribed to the fact that the most suitable organic solvents might be different in the different reaction media. In previous studies, furfural was synthesized from carbohydrates in biomass in butanone water, reaching a yield of 56% [36]. As the organic solvent was mixed with water, the hydrogen bond and polarity of the solvent were altered, thus influencing the thermodynamics of the D-xylose dehydration reaction. Figure 6b showcases the relationship of the oil–water partition coefficient (Log P) of organic solvents in the reaction system and the corresponding furfural yield. It could be interpreted that Log P was linearly related to the yield of furfural. A high Log P of solvent could favor the furfural generation, and CPME with the highest Log P could give the maximum furfural yield.

As showcased in Figure 7, upon increasing the ratio of CPME-to-water from 1:3 to 2:1 (v;v), a sharp increase in furfural yield was observed. With a decreasing water ratio in the reaction system, the D-xylose dehydration ability was increased. The generated furfural could be extracted into the CPME phase, and the potential furfural degradation was weakened. Upon raising the volumetric ratio of CPME-to-H₂O from 2:1 to 3:1, a slight increase in furfural yield was observed. Thus, the optimum CPME-to-H₂O ratio was 2:1 (v/v). In the previous work, furfural was acquired from D-xylose by sulfonation-functionalized metal–organic frameworks MIL-101(Cr)-SO₃H, and the yield of furfural reached 70.8% in CPME–water (mass ratio, 2:1), while furfural was acquired in the yield of 50.5% in an aqueous solution [37]. The supplementation of CPME into water to form the CPME–water reaction system substantially promoted the formation of furfural. Accordingly, the appropriate volumetric ratio of CPME-to-water was 2:1 in this research. The proper supplementation of CPME might favor the extraction of furfural and increase the chemocatalytic activity of Sn-NUS-BH, enabling the formed CPME–water system to reduce the decomposition of furfural and improve furfural yield.

2.3. Synthesis of Furfural from Corncob by Sn-NUS-BH in CPME–Water Two-Phase System

Distinct from homogeneous catalysts, heterogeneous catalysts have been extensively utilized and studied due to their advantages of low-performance cost, high chemocatalytic activity, and good reusability [38]. Different from the single water system, the merit of a CPME-H₂O two-phase system was that it might rapidly extract furfural into the organic phase CPME and decline the furfural decomposition, avoiding the generation of by-products. To examine the influence of other factors on the synthesis of furfural from corncob through the catalysis with Sn-NUS-BH in CPME-H₂O (2:1, v/v), the Sn-NUS-BH dose (0–6.0 wt%), reaction duration (5–60 min), and catalytic temperature (150–190 °C) were optimized (Figure 8 and Figure 9).

As the Sn-NUS-BH dose rose from 0.6 to 3.6 wt%, the furfural yield gradually elevated (Figure 8). When Sn-NUS-BH was 3.6 wt%, the yield of furfural reached the maximum. With increasing the dosage of Sn-NUS-BH, the number of acid-catalyzed sites and the acidity increased, which would promote furfural generation. However, an excessive increase in Sn-NUS-BH resulted in unwanted side reactions, which might reduce the furfural yield. Thus, 3.6% was selected as the suitable Sn-NUS-BH loading. Under these conditions, the effects of reaction duration (5–60 min) and performance temperature (150–190 °C) on furfural generation were studied (Figure 9a,b). In 20 min, the yield of furfural reached its highest level (71.4%). As showcased in Figure 9b, the furfural yield was 71.4% at 170 °C in 20 min, and the yield of furfural gradually declined with increasing reaction temperature. Elevated temperatures might not only promote furfural formation but also aggravate some unwanted high-temperature side reactions. The by-products might attach to the Sn-NUS-BH active sites, which would weaken the chemocatalytic activity and decline the yield of furfural [39].

Studies have shown that adding chloride salts in the process of furfural synthesis might stabilize the structure of the transition and states and weaken undesirable side reactions, which leads to increased furfural yield [40]. To explore the influences of chlorides on the catalytic activity of furfural catalyzed by Sn-NUS-BH, twelve chloride salts were individually added into reaction media in order to test the changes in furfural yield. As showcased in Figure 10a, NaCl, BaCl₂, MnCl₂, NiCl₂, CuCl₂, SnCl₄, and CrCl₃ might inhibit the generation of furfural in CPME–water. AlCl₃ had no substantial inhibitory effect on the production of furfural, while ZnCl₂, FeCl₃, MgCl₂, and CaCl₂ could promote the formation of furfural in CPME–water. Notably, ZnCl₂ (50 mM) had a pronounced increase in the yield of furfural, reaching 80.5%. Considering the positive role of ZnCl₂ in the reaction system, diverse doses of ZnCl₂ (0–500 mM) were individually added into the CPME-H₂O system to examine its influence on the chemocatalytic activity of Sn-NUS-BH (Figure 10b). At a concentration of 50 mM ZnCl₂, the yield of furfural reached the highest level (80.5% yield). The supplementation of ZnCl₂ could effectually promote the production of furfural from corncob. ZnCl₂ might participate in the pyrolysis reaction by coordinating with the O atom of D-xylose. The D-xylose-ZnCl₂ complexes that coordinated with two O atoms are usually more stable than those combined with a single O atom. Through the chemocatalysis with ZnCl₂, the acyclic D-xylose degradation channel is more conducive to the formation of furfural and lowers the activation-free energy [41]. Additionally, ZnCl₂ might have a positive role in the depolymerization of xylan chains to selectively form furfural by promoting the hydrolysis of glycosidic bonds. In this work, untreated corncob was composed of 36.5% glucan, 32.6% xylan, and 17.8% lignin. When added to CPME–water, Sn-NUS-BH can dehydrate D-xylose to form furfural, while glucan is generally transformed into 5-HMF and levulinic acid. It was found that Sn-NUS-BH and ZnCl₂ could hydrolyze glucan and xylan, resulting in the dehydration of furan compounds (HMF and furfural) and levulinic acid in CPME-H₂O.

In industrial applications, heterogeneous chemocatalysts are often recycled many times due to their advantages of easy recycling and good reusability [42]. To assess the performance stability of Sn-NUS-BH in CPME-H₂O, six recycling experiments were conducted under the acquired optimum reaction conditions. After each reuse, Sn-NUS-BH was recovered by filtration, water washed, oven-dried (80 °C) for 12 h, and calcined (550 °C) for 4 h, and then the chemocatalytic reaction was conducted after calcination. As showcased in Figure 11, from the first batch to the sixth batch, the yield of furfural declined from 80.5% to 65.7%, which might be ascribed to the gradual loss of tin ions from Sn-NUS-BH with repeated recycling. The active sites and void structures on Sn-NUS-BH were blocked and covered by some impurities in the catalysis process, which led to reduced chemocatalytic activity. Sn-NUS-BH was reused six times, and the yield of furfural weakened by 14.4%. The solid acid SC-FAR-800 was reused six times, and the yield of furfural declined from 60.6% (the first batch) to 30.3% (the sixth batch) [43]. These results displayed that Sn-NUS-BH had good reusability and excellent thermostability, and the good recyclability and reusability of Sn-NUS-BH would effectually decrease the production cost.

It is well-known that biomass extensively serves as an abundant, low-cost, easily obtained, and renewable bioresource for the production of value-added biobased compounds and functional materials [44,45,46,47,48]. Utilizing barley hull (BH) as biobased support, the sulfonated tin-based heterogeneous catalyst Sn-NUS-BH was synthesized. CPME-H₂O (2:1, v/v) was utilized as an environmentally friendly reaction medium for the transformation of corncob into furfural, which would avoid the undesired side-reactions, enhance the chemocatalytic effect of Sn-NUS-BH, and promote the furfural productivity, realizing the green and sustainable production of furfural. Distinct from the used carrier NUS-BH, the prepared chemocatalyst Sn-NUS-BH had a larger relative surface area and pore volume, which could support more SnO₂ (Lewis acid site) and sulfonic acid groups (Brønsted acid site). These catalytic sites on solid acid had a high selectivity for transforming hemicellulose in lignocellulose into furfural. The surface of NUS-BH was rough and porous with a special circular pore structure. More tin ions and –SO₃H groups were loaded on Sn-NUS-BH to generate highly uniform chemocatalytic active sites (Lewis/Brønsted acid sites). The transformation of lignocellulose into furfural was carried out via synergistic catalysis of Brønsted/Lewis acid sites on Sn-NUS-BH. Brønsted acids transformed the depolymerization of xylan into xylose, and the formed D-xylose was subsequently isomerized into xylulose with Lewis acids, and eventually, Brønsted acids catalyzed the dehydration of xylulose to furfural [39,42]. CPME might alter the dense structure of lignocellulose and disrupt the physical and chemical connections among biomacromolecules (e.g., hemicellulose, lignin, and cellulose), thereby depolymerizing and hydrolyzing hemicellulose. At high temperatures, the metal cation ZnCl₂ and CPME broke the intramolecular hydrogen bonds that existed between lignin and hemicellulose. Cl⁻ could form hydrogen bonds with the O-H groups of hemicellulose and lignin [12]. The β-1,4-glycosidic bonds in carbohydrates (hemicellulose plus cellulose) were disrupted by protons on Sn-NUS-BH. Xylan in the corncob could be hydrolyzed into D-xylose in CPME–water. ZnCl₂ could be used as a Lewis acid catalyst. Zn²⁺ might favor the isomerization of D-xylose into xylulose, which then eliminated three molecules of H₂O to generate furfural. The supplementation of ZnCl₂ enhanced the interaction between CPME and Sn-NUS-BH. These synergies could promote furfural production through the enhancement of D-xylose isomerization and xylulose dehydration. This constructed sustainable process manifested high potential application for furfural production from biomass with a biobased chemocatalyst in a benign reaction medium. In a concise summary, a substantial improvement in furfural production from lignocellulose with biobased chemocatalyst Sn-NUS-BH was successfully realized in CPME–water containing ZnCl₂.

3. Materials and Methods

3.1. Chemicals and Materials

Corncob, which was composed of 36.5% glucan, 32.6% xylan, and 17.8% lignin, was harvested from a farm in Zhoukou (Henan Province, China). Barley hulls (BHs) were collected from a brewery in Hangzhou (Zhejiang Province, China) and were composed of 18.5% glucan, 23.7% xylan, and 18.1% lignin. Cyclopentyl methyl ether (CPME), methyl isobutyl ketone (MIBK), tetrahydrofuran (THF), acetonitrile (ACN), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and other chemicals used in this research were from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China).

3.2. Preparation of Solid Acid Sn-NUS-BH

BHs were ground with a grinder (SUS 304, Hefei Rongzhida Sanyo Electric Co., Ltd., Hefei, China) to acquire 40–60 mesh of BH powder (0.2 kg). The powders were soaked in 800 mL ethanol solution containing NaOH (0.50 M) in an ultrasonic instrument (KH2200B, Kunshan Hechuang Ultrasonic Instrument Co., Ltd., Kunshan, China) (100 W, 50 °C) for 240 min. The NaOH-ultrasonic-treated BH (NUS-BH) was then filtered, separated, and rinsed with distilled water until neutral. After drying over an oven (60 °C), the acquired NUS-BH was mixed with 0.10 kg glucose in 500 mL distilled water and then treated ultrasonically for 60 min. Afterward, the mixture was filtered and oven-dried. The dried powders were calcined in a Muffle furnace (550 °C, 240 min). The resulting slurry was mixed with 600 mL ethanol and 40.0 g SnCl₄⋅5H₂O, and the pH of this mixture was regulated to 6.0 with ammonia (25.0 wt%). The resulting colloidal solution was oven-dried (80 °C, 48 h). The resulting dried powder was soaked in 4.0 M H₂SO₄ at 60 °C. After 4 h of sulfonation, the sulfonated solid powders were filtered and further oven-dried (80 °C). Afterward, the resulting black powders were calcined (550 °C). After 240 min, the formed Sn-NUS-BH catalyst was further applied to transform biomass into furfural.

3.3. Chemical Transformation of Corncob into Furfural by Sn-NUS-BH in Organic Solvent–Water System

Next, 40 mL organic solvation–water mixture was mixed with 3.0 g dried and milled corncob powders (~40 mesh) and a certain amount of Sn-NUS-BH in a 100 mL Jqf0002 stainless-steel autoclave (Suzhou Shenghua Instrument Technology Co., Ltd., Suzhou, China) by magnetic stirring (550 rpm) to produce furfural at the designed reaction temperature for the designed reaction time. Several chemical reaction factors influencing the formation of furfural were tested, including different organic solvent types (CPME, THF, ACN, MIBK, DMSO or DMF), the volumetric ratio of organic solvent to water (1:3, 1:2, 1:1, 2:1 or 3:1, v/v; pH 1.5), Sn-NUS-BH load (0.6, 1.2, 2.4, 3.6, 4.8 or 6.0 wt%), performance temperature (150, 160, 170 or 180 °C), and reaction duration (10, 15, 20, 30, 40 or 50 min). The effects of metal chloride salts and zinc ion dose (0, 20, 50, 100, 200, 300, or 500 mM) were explored on the furfural formation in CPME–water (2:1, v/v, 170 °C). After 20 min, furfural was quantified with HPLC. LogR₀ was used to assess the influence of different reaction temperatures and reaction durations during the chemical reaction process. The below equation was calculated from reaction time (t, min) and reaction temperature (T, °C).

$${LogR}_0=\mathrm{L}\mathrm{o}\mathrm{g}\left(\mathrm{t}\times \mathrm{e}\mathrm{x}\mathrm{p}{\displaystyle \frac{\mathrm{T}-100}{14.75}}\right)$$

(1)

3.4. Reuse of Sn-NUS-BH

To assess the reusability and thermostability of Sn-NUS-BH, the recovered Sn-NUS-BH was reused six times to catalyze the transformation of corncob into furfural. After each use, Sn-NUS-BH was recovered by simple filtration, entirely washed with DI water, oven-dried (80 °C), and then Muffle furnace-calcined (550 °C) to eliminate residue from the Sn-NUS-BH surface. The recovered Sn-NUS-BH was used in the next batch of reactions. Each batch of samples was reacted in a 40 mL CPME-H₂O (2:1, v/v) mixture composed of 75 g/L corncob and 50 mM ZnCl₂ by autoclaving at 170 °C for 20 min.

3.5. Structural Features of Corncob and Sn-NUS-BH

The structures of the corncob and Sn-NUS-BH surface were characterized by SEM (Regulus 8100, Hitachi, Tokyo, Japan), FT-IR (Nicolet iS50, Thermo, Madison, WI, USA), XRD (D/MAX-2500; Rigaku, Tokyo, Japan). The accessibility of corncob was assessed through the dye adsorption of Congo red dye [49]. BET (Autosorb-IQ2-MP, Quanta chrome, Boynton Beach, FL, USA) surface area obtained via N₂ adsorption was adopted to characterize surface area/pore size/pore volume for NUS-BH and Sn-NUS-BH. The specific surface area of the sample was analyzed based on the N₂ adsorption isotherm; the pore structure of the sample, including pore volume and pore diameter, was analyzed based on the single-point adsorption of N₂. NH₃-TPD was implemented by using a BELCAT II chemisorption system (BELCAT II, MicrotracBEL, Osaka, Japan) to measure the acid strength of Sn-NUS-BH. Sn-NUS-BH (100 mg) was treated for 60 min with a helium atmosphere (300 °C). The sample was then cooled down to 50 °C. After adsorption of NH₃ at 50 °C for 60 min, Sn-NUS-BH was pretreated with helium to remove excessive NH₃ and then heated to 850 °C for NH₃ desorption at a flow rate of 30–50 mL per minute and a heating rate of 10 °C per minute. The desorption gas was detected by TCD.

4. Conclusions

A biochar-based chemocatalyst, Sn-NUS-BH, was synthesized using BH as a carrier and applied for the transformation of corncob into furfural in a CPME-H₂O (2:1, v/v) system containing ZnCl₂ (50 mM) at 170 °C for 20 min, achieving a high furfural yield of 80.5%. Sn-NUS-BH demonstrated excellent thermal stability and reusability, maintaining its catalytic efficiency over six cycles. This study presents an eco-friendly and efficient process for the conversion of biomass into furfural using Sn-NUS-BH in a CPME-H₂O system, offering significant potential for the sustainable production of valuable biobased chemicals from lignocellulosic biomass.