Sustainable biofuel and chemical production using renewable and carbon-neutral biomass resources could cope with fossil resource depletion and global climate change [1–3]. 5-hydroxymethylfurfural (HMF) derived from biomass is an essential intermediate platform compound, which can be converted into bio-liquid fuels and various high-value chemicals, such as 2,5-dimethylfuran (DFF), 2,5-furan dicarboxylic acid (FDCA), and levulinic acid (LA) [4–8]. Fructose and glucose can be degraded into HMF [9]. In mild conditions, fructose can easily convert HMF because it contains a five-ring structure similar to HMF [4]. However, due to its high cost [10], there are better raw materials than fructose for synthesizing HMF. Natural monosaccharides, such as glucose, can be used to prepare HMFs [5, 11].
The conversion of glucose to HMF can occur in two ways [12, 13]. The first method is the isomerization of glucose to fructose, followed by the dehydration of fructose to HMF, which were catalyzed by Lewis acid and Brønsted acid, respectively; The second is glucose directly produces HMF with the Brønsted acid. With Brønsted acid only, the conversion of glucose to HMF requires high activation energy [14], resulting in the difficult conversion at low temperature. For example, Garces et al. [15] used HCl to catalyze glucose degradation at 383 K for 4 h, obtaining a glucose conversion rate of only 38%. In contrast, the activation energy of glucose conversion to HMF is lower as Brønsted acid and Lewis acid are synergistic[16]. Therefore, the catalytic system composed of Brønsted acid and Lewis acid can effectively catalyze the conversion of glucose to HMF under mild conditions.
Homogeneous catalytic systems consisting of homogeneous acid and soluble metal salts can produce a high HMF yield for glucose thoroughly mixed with the catalyst [11, 17, 18]. However, homogeneous catalysts pose severe environmental pollution risks, difficult separation and recovery after reaction, and severe equipment erosion [19, 20].
Heterogeneous solid catalysts such as ion exchange resins, molecular sieves, and metal oxides have been developed in recent years to overcome the shortcomings of homogeneous catalytic systems [21–27]. In an impregnation-calcination method, Shen et al. [25] used a sulfated natural attapulgite (ATP)-based catalyst (SO42−/In2O3-ATP) to catalyze glucose conversion, resulting in a 40.2% yield of HMF. In a study by Rezayan et al. [27], bifunctional phosphate Gallia (GaP) solid catalysts were prepared by solid-state grinding and applied to glucose-to-HMF transformation, resulting in a 65.72% yield of HMF. However, the cost of preparation of the above solid catalysts is high. The carbonaceous solid acid catalysts prepared from biomass are simple to prepare, low-cost, and thermally stable [28].
Furthermore, biochar comprises oxygen-containing functional groups, such as hydroxyl and carboxyl, which can act as carriers of catalytic sites [29, 30]. Mazzotta et al. [31] used TiO2 and p-toluenesulfonic acid to modify glucose-based solid carbon and obtained carbon catalysts (GluTsOH-Ti) containing Brønsted acid and Lewis acid catalytic sites. When glucose was converted to HMF using the catalyst, 48 mol% of HMF was produced, and HMF yield increased by 38 mol% over the control experiment without the catalyst. Furthermore, metal chlorides also exhibited excellent Lewis acidity [17, 18]. However, there are few studies on loading metal chlorides as Lewis acids to biochar to prepare solid acid catalysts.
The solvent plays a crucial role in the preparation of HMF from glucose. Water is the most commonly used solvent since it is eco-friendly and cheap [32]. However, in the acidic aqueous phase, HMF is easily degraded into LA [33]. HMF yield is decreased by side reactions that form by-products between glucose, HMF, and intermediates [34–36].
According to Xiong et al. [37], most of the generated HMF was degraded by solid acids to catalyze glucose in an aqueous phase, and the yield of HMF was only 4.2 mol%. HMF can be produced in higher yields by forming biphasic solvents with organic solvents such as tetrahydrofuran (THF) and γ-valerolactone (GVL) [38–40]. To avoid HMF degradation and side reactions, organic solvents are used to extract and separate the generated HMF from the aqueous phase. Shen et al. [41] found that GVL was better than THF, n-butanol, toluene, n-hexanol, and methyl isobutyl ketone in sodium chloride solution for the extraction of HMF. Huang et al. [42] also found that for a GVL: H2O volume ratio of 10:1, the yield of HMF catalyzed by carbon-based solid acid catalyst (SC-CCA) was 78.1%, while the yield of HMF was only 3.5% in pure water. These results indicate that water/GVL solvent can produce a high yield of HMF. Furthermore, two-phase solvents can cope with carbonaceous solid acid catalysts' poor mass transfer efficiency, leading to longer reaction time.
This paper aims to synthesize carbonaceous solid acid catalysts loaded with Brønsted/Lewis acid catalytic sites using pine biomass as a raw material via impregnation, carbonization, and sulfonation. Various properties were studied using XRD, FTIR, SEM, BET, and Py-IR to characterize the above-prepared catalysts. A biphasic solvent of NaCl-H2O/GVL was also used to evaluate the catalytic activity of the catalysts for converting glucose to HMF. Furthermore, the cycle stability of the catalyst was investigated without regeneration.