A Simple One-Pot Synthesis of a Sulfonated Carbon/Silica Composite Catalyst from Low Value Molasses for 5‑HMF Production

Abstract

Sulfonated silica–carbon composite catalysts (SSC) were synthesized from molasses and K60 silica gel in a one-pot method that simultaneously carbonized and sulfonated the composite. SSC prepared with 12 M sulfuric acid (SSC-12M) demonstrated a fructose to 5-hydroxymethylfurfural (5-HMF) conversion of 83.0% and a yield of 49.1% at 180 °C for 3 h in water. Silica was vital for enhancing the performance; sulfonated carbons with no silica led to lower yields and significant proportions of levulinic and formic acid byproducts. SSC-12 M demonstrated a 47.5% yield of 5-HMF after 3 cycles, while catalysts were easily regenerated via reflux in additional sulfuric acid. Solvent selection revealed that DMSO combined with SSC-12 M demonstrated a 98.2% conversion of fructose with a 62.7% 5-HMF yield (63.8% selectivity). However, the conversion of glucose to 5-HMF under identical conditions resulted in a 98.0% conversion and 70.2% yield (71.6% selectivity). SSC-12 M could lead to new opportunities in solid acid-catalyzed reactions.

Introduction

Biomasses including agricultural residues, forest residue, animal waste, and dedicated energy crops are essential feedstock for chemicals, materials, and biofuels. , Converting biomass into value-added chemicals is an essential strategy for building a sustainable and resilient biobased economy. Biobased platform chemicals such as 5-hydroxymethylfurfural (5-HMF), levulinic acid, and formic acid can serve as building blocks to produce biofuels, polymers, and pharmaceuticals. , Recent advances have focused on optimizing reaction conditions, catalyst types, and solvent systems to increase the 5-HMF yield and lower the production cost through higher 5-HMF selectivity, while maintaining the industrial viability of 5-HMF production. , A diverse array of homogeneous and heterogeneous acid catalysts has been designed for the conversion of glucose or fructose into 5-HMF, such as ionic liquids, acidic zeolites, metals salts, functionalized silicas, and solid acids. However, long reaction times led to a decrease in 5-HMF yield due to formation of side-products such as levulinic acid, formic acid, and humins as shown in Scheme .

1. Reaction Pathway for the Conversion of Glucose and Fructose to 5-HMF.

Porous carbons and their composites are a class of materials that have been widely used in industry as adsorbents and heterogeneous catalytic supports. , Sulfonic acid-functionalized catalysts have been demonstrated as being highly effective in the synthesis of 5-HMF from sugar. Among these sulfonated materials, carbon microspheres and carbon composites have demonstrated promise for use in 5-HMF production, offering several distinct advantages over a conventional heterogeneous catalyst. These catalysts exhibit high surface areas, outstanding chemical, physical, and thermal stability, and they are crucially resistant to deactivation by water, a common issue in acid-catalyzed processes, improving selectivity and recyclability. − Sulfonation of silica–carbon composites has led to the efficient conversion of fructose or glucose to 5-HMF through acid-catalyzed dehydration, thus providing an eco-friendly and cost-effective method for biobased chemical synthesis. , Moreover, these can be produced from inexpensive carbon sources, particularly those derived from lignocellulosic biomass. Heterogenous catalysts including solid acids are widely used in the conversion of glucose and fructose, owing to the advantages such as easy separation and catalyst reusability. − For example, the recent work of Liu et al. has demonstrated the effective conversion of glucose-based carbohydrates into 5-HMF using bifunctional Hf/SBA-15 S catalysts. By introducing hafnium species and grafting sulfonic acid groups onto mesoporous SBA-15, the catalyst can efficiently convert glucose and cellulose into HMF with yields of up to 74% and 51%, respectively, owing to their ordered mesoporous structure and combined Lewis and Bronsted acidity.

Tian et al. synthesized a high-efficiency sulfonic acid-functionalized mesoporous carbon/silica catalyst for converting fructose to 5-HMF through a modified soft template method and carbonization process. Tetraethyl orthosilicate (TEOS) and Pluronic copolymer (P123) triblock were used to synthesize mesoporous silica, with glucose as a carbon source; a 70% yield was reported in the first cycle and only a slight decrease of yield after 4 cycles of the reaction. Nevertheless, the development of efficient and recyclable catalysts that integrate the advantages of both carbon and silica support remains an ongoing challenge.

Recently, mesoporous carbon/silica nanocomposites modified with both acidic and basic sites were demonstrated as effective catalysts for 5-HMF production. SO₃H groups were grafted onto the surface of the carbon to provide acidity, while 3-aminopropyl groups on the silica enhanced basicity. It was found that the greater acidity of these MCS-SO₃H catalysts led to a significant increase in 5-HMF yield to 56%, with a fructose conversion of 81%. The bifunctional acid–base catalyst (MCS–SO₃H–NH₂) demonstrated superior performance in converting glucose into 5-HMF (39.4% yield of 5-HMF).

Many mesoporous silica materials have been combined with carbine in composites to leverage the benefits of both components for various applications, especially in catalysis, adsorption, and energy storage. , However, the use of templating agents adds additional resources and process steps to the production of such materials.

Herein is the first reported synthesis of a sulfonated silica–carbon composite catalyst (SSC) produced from molasses, a low value sugar cane byproduct, through a simple method using K60 silica gel and sulfuric acid. The resulting catalysts were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, Brunauer–Emmett–Teller analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, and Raman spectroscopy. The SSC were investigated for the effective conversion of fructose to 5-hydroxymethylfurfural (5-HMF), and the reaction parameters including catalyst, time, temperature, catalyst loading, and solvent selection were optimized. These conditions were also applied to the conversion of glucose to 5-HMF and, importantly, catalyst reuse was also investigated. This work demonstrates a potential method for developing new heterogeneous acid catalysts for industrial biorefinery processes.

Methodology

Materials: D(−)­fructose (C₆H₁₂O₆ > 98.0) were purchased from Seohaean, (Korea), Glucose (C₆H₁₂O₆), furfural (C₅H₄O₂), 5-hydroxymethylfurfural (C₆H₆O₃) were obtained from Sigma-Aldrich. Dimethyl sulfoxide (DMSO) (AR), Sulfuric acid (H₂SO₄) (AR), and tetrahydrofuran (THF) were obtained from RCI Lab scan. Cyrene (C₆H₈O) (LR) was kindly supplied by Circa, Australia. Silica K60 (0.063–0.200 nm) was obtained from EMD Millipore Corporation, Germany.

Synthesis of Sulfonated Silica–Carbon Composites’ Catalyst (SSC)

A typical catalyst preparation method is shown in Figure , where 20 g of molasses and 5 g of silica K60 were added into the prepared 200 mL sulfuric acid solution (2 mol L^–1) until dissolved in a 500 mL round-bottom flask with a refluxed condenser. The solution was stirred at 250 rpm for 10 h in an oil bath at 140 °C. After cooling, the mixture was filtered, and then the catalyst was washed with deionized water until the washing solution was neutral and dried at 160 °C for 6 h. Finally, the catalyst was collected and named SSC-2M. In the above steps 4, 6, 8, 10, 12, 14, 16, and 18 mol L^–1 sulfuric solution was used to prepare the SSC-4M, SSC-6M, SSC-8M, SSC-10M, SSC-12 M SSC-14M, SSC-16M, and SSC-18 M catalysts, respectively.

1.

Preparation route of the SSC catalyst.

Reusability Test

The reusability test was performed under optimal reaction conditions. After each catalytic cycle, the solid catalyst was separated by hot filtration, washed thoroughly with deionized water to remove residual sugars and soluble organic species, and then dried at 160 °C for 6 h before being reused in the next cycle. The liquid product was filtered through a syringe filter and analyzed by HPLC. After three consecutive cycles, the spent catalyst was regenerated by stirring with 200 mL of 12 M H₂SO₄ at 140 °C for 10 h. The regenerated catalyst was washed with deionized water until the filtrate became neutral and was subsequently dried at 160 °C for 6 h prior to reuse.

Synthesis of a Sulfonated Carbon Catalyst (SC)

The mix of 20 g of molasses and sulfuric acid solution (12 mol L^–1) was added to a 500 mL round-bottom flask with a reflux condenser. The mixture of solutions was stirred at a speed of 250 rpm for 10 h in an oil bath at a temperature of 140 °C. After cooling, the mixture was filtered, the catalyst was washed with deionized water until the washing solution was neutral, and dried at 160 °C for 6 h. This sample was named SC-12 M and is a carbon catalyst that contains no silica.

Acid Recycling

The acid recycling experiment is as follows; after the conversion reaction was complete the catalyst was separated by a filter paper and the solution of acid was collected. The acid solution in the first batch was used to synthesize the sulfonated silica carbon and was named SSC­(R-1). After recycling in the first batch, the solution was separated again and collected for synthesis in the second batch and named SSC­(R-2).

Glucose to 5-HMF

The reaction of glucose to 5-HMF was carried out in an autoclave reactor. Specifically, 0.1 g of glucose and 10 mg of catalyst were mixed in 10 mL of water as a solvent. The mix was heated to 180 °C for 1, 2, 3, 4, 5, and 6 h in an oven. After the reaction was completed, the solution was separated and filtered by a syringe filter and analyzed using an Agilent 1260 HPLC-Reverse phase equipped with an Agilent Hi-plex H⁺ column (300 × 7.7 mm, 8 μm of particle size). The condition of the mobile phase was 0.005 M sulfuric acid with a flow rate of 0.6 mL min^–1. An injection volume of 50 μL was used with a column temperature of 60 °C and it was detected using a refractive index detector at 55 °C. The total running time of the analysis was 60 min. The conversion (X), yield of glucose (Y), and selectivity (Z) are defined by eqs –)­

$$X(\%)=\frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{g}\mathrm{l}\mathrm{u}\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{e}-\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}\mathrm{g}\mathrm{l}\mathrm{u}\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{e}}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{g}\mathrm{l}\mathrm{u}\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{e}}\times 100$$ 1

$$Y(\%)=\frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{g}\mathrm{l}\mathrm{u}\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{e}}\times 100$$ 2

$$Z(\%)=\frac{\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}{\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}}\times 100$$ 3

Dehydration of Fructose to 5-HMF

The reaction of fructose to 5-HMF was carried out in an autoclave reactor. Specifically, 0.1 g of fructose and 10 mg of catalyst were mixed with 10 mL of water as a solvent. The mix was heated to 180 °C for 1, 2, 3, 4, 5, and 6 h in an oven. After the reaction was completed, the solution was separated and filtered by a syringe filter and analyzed using an Agilent 1260 HPLC-Reverse phase equipped with an Agilent Hi-plex H⁺ column (300 × 7.7 mm, 8 μm of particle size). The condition of the mobile phase was 0.005 M of sulfuric acid with a flow rate of 0.6 mL min^–1. An injection volume of 50 μm was used with a column temperature of 60 °C and it was detected using a refractive index detector at 55 °C. The total running time of the analysis was 60 min. The conversion (X) and yield of fructose (Y) are defined by eqs and )­

$$X(\%)=\frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{f}\mathrm{r}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{s}\mathrm{e}-\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{o}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}\mathrm{f}\mathrm{r}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{s}\mathrm{e}}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{f}\mathrm{r}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{s}\mathrm{e}}\times 100$$ 4

$$Y(\%)=\frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{f}\mathrm{r}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{s}\mathrm{e}}\times 100$$ 5

Catalyst Reuse

The investigation of reuse was as follows: optimal conditions in water or DMSO were applied for the conversion of fructose or glucose; following each reaction cycle the catalyst was filtered and dried at 160 °C, and the product composition was analyzed by HPLC. The solution was filtered by a syringe filter, washed with DI water, and then analyzed using Agilent 1260 HPLC-Reverse phase equipped with an Agilent Hi-plex H⁺ column (300 × 7.7 mm, 8 μm of particle size). The condition of the mobile phase was 0.005 M of sulfuric acid with a flow rate of 0.6 mL min^–1. An injection volume of 50 μm was used with a column temperature of 60 °C and it was detected using a refractive index detector at 55 °C. The total running time of the analysis was 60 min. The conversion (X) and yield of glucose (Y) are defined by eqs –). After 3 consecutive cycles, the catalyst was regenerated by stirring at 250 rpm with 200 mL of 12 M sulfuric acid at 140 °C for 10 h. Following reactivation, the catalyst was washed with deionized water until the washing solution was neutral and dried at 160 °C for 6 h.

Results and Discussion

Material Characterization

FT-IR results of the sulfonated carbon (SC-12M) and the SSC (SSC-2 M to SSC-18M) confirmed the successful carbonization through the simple application of sulfuric acid, Figure A.

2.

(A) FT-IR spectra of sulfonated silica–carbon composites under different concentrations and (B) XRD pattern of the SSC catalyst.

Silica within the composites exhibited several characteristic FT-IR bands including O–H stretching, asymmetry Si–O–Si bond stretching, and a SiO₄ tetrahedron ring at 3500, 1055, and 800 cm^–1, respectively. − The incorporation of silica to form the SSC catalyst demonstrates a silanol stretching band (3100–3600 cm^–1) consistent with the formation of a carbon layer on the silica framework. Moreover, all of the catalysts showed various functional groups at ∼3364 cm^–1 due to the stretching vibration of the –OH functional groups. The bands at ∼1701 cm^–1 and ∼1615 cm^–1 correspond to CO and CC stretching vibration bands. Successful sulfonation of SC-12 M is confirmed by the presence of characteristic –SO₃H vibrational bands in the FTIR spectrum. The asymmetric OS–O stretching band is observed at ∼1130 cm^–1, while the symmetric OS–O stretching of sulfonic acid groups appears at ∼1030–1050 cm^–1. In addition, the band around ∼800 cm^–1 corresponds to S–O or C–S bending vibrations, further supporting the incorporation of sulfonic groups onto the carbon surface. Oxygenated functional groups are produced due to the acid activity causing oxidation within the carbonaceous layer, parallel to the insertion of the –SO₃H groups.

The XRD pattern of the pure K60 silica gel and SSC are shown in Figure B, demonstrating a broad peak at 2θ at 15°–30° associated with an amorphous silica. As expected, the XRD patterns of SSC-2 M to SSC-6 M are broad and featureless, indicating an amorphous carbon with little structural ordering, as the lower acid concentrations are unable to promote dehydration and condensation steps required to form more organized carbon domains. Moreover, diffraction peaks centered at 2θ of 26.5° for the SSC catalyst found (002) planes of graphite carbon. This peak is typically associated with the (002) reflection of turbostratic or partially ordered carbon, suggesting that small aromatic layers start to form and stack to some extent. This indicates that SSC materials are still largely amorphous but begin to develop some short-range ordering when prepared with higher concentrations of acid. In addition, the presence of graphitic carbon within the composite was also corroborated by Raman spectra of the SSC samples that exhibited a peak at 1560 cm^–1 (G-band) corresponding to some graphitized carbon structure in Figure .

3.

Raman spectroscopy pattern of the SSC catalyst.

Within the SSC catalysts, the D band is not a well-defined peak due to the highly amorphous nature of the materials. Instead, a broad feature spanning roughly 1300–1400 cm^–1 is observed, which is typical for carbons derived from biomass precursors, which contain many structure imperfections including edge site and small disordered sp² domains together with oxygen- and sulfur-containing functional groups formed during carbonization and sulfonation. Such defects are observed in disordered carbons, which broadens the D and G bands, making them difficult to distinguish. The weak broad feature in the 2600–2800 cm^–1 also points to limited graphitic stacking, which is expected for sulfonated biomass-derived carbons, as the incorporation of heteroatoms further disrupts long-range ordering. Similar Raman characteristics have been described for sulfonated carbons prepared under comparable conditions, where the presence of these functional groups leads to strong disorder and prevents the development of a well-defined graphitic structure.

Sulfonation was also confirmed by analysis of C_1s, O_1s, and S_2p as shown in Figure and in the Supporting Information. XPS signals of C 1s were observed at 284.1, 284.8, 286.0, 287.3, and 288.6 eV. The signals at 284.1 and 284.8 correspond to the C–C/CC of the aromatic and aliphatic carbon, whereas the 286.0 eV related to the C–O bond. The binding energy of 287.3 eV is typically associated with carbon atoms in carbonyl groups (CO) or single bonds to oxygen (C–O). The C 1s signal at 288.6 was assigned to the C–S–O bond. The O 1s XPS spectrum confirmed the presence of these oxygenated groups. The peaks at 531.0, 532.3, 533.3, and 534.4 correspond to the CO, SO and CC (figures S2 and S3 in Supporting Information), which were attributed to sulfur in the SO₃H group confirming a successful sulfonation.

4.

C 1s XPS spectra of SSC-2 M to SSC-18M.

The surface morphology of all of the catalysts was analyzed by SEM (Figure ). The SSC-2 M catalyst exhibited irregular particles of carbon in the outer layer due to a small amount of sulfuric acid. The irregular particles can refer to dehydration and polymerization of sugar during carbonization. However, the particle of silica is still in its original shape with some carbon coated on the surface. Increasing the acid concentration led to smaller spherical particles of carbon caused by the sulfonation process, resulting in a reduction in the carbonaceous layer. Sulfonation typically leads to the complete breakdown of the pore structure or disrupts the ordered arrangement of the pore. However, the mesopores remain intact even after sulfonation. This stability is attributed to the silica framework, which serves as structural support for the carbon layer and prevents its collapse.

5.

SEM image of SSC-2 M (a), SSC-4 M (b), SSC-6 M (c), SSC-8 M (d), SSC-10 M (e), SSC-12 M (f), SSC-14 M (g), SSC-16 M (h), SSC-18 M (i), and SC-12 M (j).

Porosimetry of the precursor silica and SSC catalyst (Table ) revealed that the BET surface area of pure silica was 406.0 m² g^–1, while those of SSC-2M, SSC-4M, SSC-6M, SSC-8 M SSC-10M, SSC-12M, SSC-14M, SSC-16 M, and SSC-18 M had decreased, with surface areas of 182.5, 188.3, 190.0, 199.0, 186.2, 200.3, 198.3, 187.4 m²·g^–1, respectively. In addition, the total pore volume of the SSC catalyst also reduced significantly. The lower surface area and total pore volume of the SSC catalyst are due to the coating of the pores with a layer of carbon, leading to some pore filling and reduced volume. The decrease in mesoporous size is attributed to the formation of a carbon layer and is consistent with the other silica–carbon composites. The physisorption nitrogen adsorption–desorption isotherm is shown in Figure A. Both the pure silica and SSC exhibited a type IV isotherm, with a hypothesis loop at P/P ₀ ranging from 0.4 to 1.0, characteristic of mesoporous materials. The pore size distributions of all catalysts were mainly concentrated around 6.8 nm (Figure B).

1. Nitrogen Adsorption Porosimetry for SSC.

catalyst	S BET (m2g–1)	S Micro (m2g–1)	S meso (m2g–1)	V total (cm3·g–1)	V micro (cm3·g–1)<	V meso (cm3·g–1)	Vmicro/Vtotal (cm3·g–1)	Vmeso/Vtotal (cm3·g–1)	D (nm)	acid strength (mmol/g)
silica K60	406.0	20.6	385.4	0.70	0.007	0.693	0.010	0.99	6.9	-
SSC-2M	182.5	7.7	174.8	0.34	0.002	0.338	0.005	99.41	7.5	0.21
SSC-4M	188.3	8.0	180.3	0.35	0.002	0.348	0.005	99.43	7.7	0.52
SSC-6M	190.0	9.3	180.7	0.38	0.003	0.377	0.007	99.21	7.9	0.56
SSC-8M	197.0	11.6	185.4	0.39	0.004	0.386	0.010	98.97	7.9	0.63
SSC-10M	199.0	13.5	185.5	0.39	0.004	0.386	0.010	98.97	7.9	1.10
SSC-12M	186.2	11.7	174.5	0.38	0.004	0.376	0.010	98.94	8.1	1.16
SSC-14M	200.3	12.6	187.7	0.40	0.005	0.395	0.012	98.75	8.0	1.48
SSC-16M	198.3	13.0	185.3	0.37	0.004	0.366	0.010	98.91	7.8	1.70
SSC-18M	187.4	14.6	172.8	0.36	0.004	0.356	0.010	98.88	7.9	2.64
SC-12M	273.6	8.19	265.4	0.49	0.002	0.488	0.004	99.50	7.0	1.48

^aNitrogen adsorption porosity for SSC. Surface area (S _BET) was calculated by the Brunauer–Emmett–Teller (BET) isotherm method.

^bTotal pore volume (V _total) was calculated from the saturation plateau at a high relative pressure.

^cMesoporous volume (V _meso) was determined according to the t-plot.

6.

(A) N₂ adsorption–desorption isotherm of SSC catalysts and (B) pore size distribution of SSC.

The element composition of the catalyst is shown in Table . Increasing the sulfuric acid concentration in the carbonization method resulted in a decrease in the carbon content from 62.3 to 12.6 wt % from SSC-2 M to SSC-14M, respectively. It can be observed that in SSC-10 M to SSC-18M, a sharp decline in the carbon content is observed, reaching 12.6% in SSC-18M. This trend suggests that increasing the acid concentration results in the removal of carbon. The oxygen content ranges from 29.2 to 51.4 wt %, which demonstrated that the sulfonation process promotes the production of oxygenated functional groups on the surface. This rise correlates with sulfonation and greater silica content, both of which exhibit a rich oxygen structure. Sulfur content in the sample was determined by XPS analysis. SC-12 M has a carbon content of 14.1% and sulfur content of 5.72%, demonstrating the efficient incorporation of sulfonic groups.

2. Element Composition of SSC as Determined by XPS.

catalyst	chemical composition (wt %)
	C	O	S	Si	N
silica K60	13.5	31.6	-	54.9	-
SSC-2M	62.3	29.2	-	8.5	-
SSC-4M	65.6	30.4	0.95	2.5	-
SSC-6M	58.4	32.4	1.57	2.7	-
SSC-8M	60.0	28.1	1.61	5.6	-
SSC-10M	18.5	37.5	3.68	41.5	2.0
SSC-12M	14.1	51.4	5.72	33.8	0.4
SSC-14M	15.3	50.7	5.86	30.3	-
SSC-16M	13.5	53.6	6.32	25.9	-
SSC-18M	12.6	43.2	7.32	32.0	-
SC-12M	56.13	29.81	13.71	0.03	-

Transmission Electron Microscopy (TEM) shown in figure S4 and S5 in the Supporting Information reveals the SSC-12 M shows textural properties that are comparable with the parent silica. The carbon coating is observed as a brighter portion of the TEM surrounding the dark area, which are silica particles. TEM images (a,i) reveal well-distributed mesoporous structures in the sulfonated silica–carbon composites, suggesting the successful incorporation of ordered silica frameworks within the carbon matrix. These porous features can enhance the accessibility to active sulfonic acid sites, which are critical in biomass dehydration reactions. In contrast, the sulfonated carbon material in image (j) displays a more amorphous, dense morphology with a significantly lower visible porosity. This is consistent with previously reported sulfonated carbons prepared from sugar-rich precursors, which tend to yield disordered structures with limited surface area and pore accessibility. Therefore, the incorporation of silica not only helps maintain mesoporous architecture but also contributes to greater dispersion of active sites, potentially leading to improved catalytic performance in fructose or glucose conversion to 5-HMF.

Effect of the SSC Catalyst on the Fructose Conversion and 5-HMF Yield

The effect of acid concentration on the production of the SSC catalyst (Figure ) was investigated for the fructose to 5-HMF conversion. The observed yield of 5-HMF for the control reaction with no catalyst was only 1.6 mol %, with a fructose conversion of 15.0 mol %. The sulfonated carbon SC-12 M gave a 13.4 mol % yield of 5-HMF; however, the fructose conversion was 98.0 mol %. This demonstrates that SC-12 M with no silica leads to lower selectivity and significant quantities of byproducts, such as levulinic acid and formic acid. The fructose conversion increased for SSC-2M, SSC-4M, SSC-6M but dropped for SSC-8 M and SSC-12M. Application of the SSC catalysts increased 5-HMF yield from 14.2 to 49.1 mol % for SSC-2 M to SSC-12M, respectively. However, the conversion of SSC-14M, SSC-16 M, and SSC-18 M dropped, demonstrating that higher sulfur content and acid functionalities led to the formation of byproducts. The selectivity toward 5-HMF also increased with higher concentrations of acid from SSC-2 M to SSC-12M. These were linked to a balance between acidity and the pore structure of the catalytic sites, promoting the reaction. , In contrast, the carbon catalyst without silica (SC-12M) generated a significantly lower 5-HMF yield compared to the SSC catalyst, and the silica support helps retain a high density of –SO₃H groups, critical for catalyzing dehydration reactions to 5-HMF. Incorporating silica into a carbon matrix improves the catalytic activity of the acidic sites.

7.

Effect of different SSC catalysts on fructose conversion to 5-HMF. Reaction conditions: fructose, 0.1 g (0.55 mmol), solvent, 10 mL of water, catalysts,10 mg, T, 180 °C.

The influence of the time on fructose to 5-HMF conversion and yield was investigated for the SSC catalysts (Figure A and Figure B). 5-HMF yield increased for all the catalysts to 3 h, for SSC-2M, SSC-4M, SSC-6M, SSC-8M, SSC-10M, SSC-14, SSC-16M, and SSC-18 M catalysts, the yield 5-HMF were 14.2, 35.1, 32.6, 39.3, 48.9, 36.4, 34.0, and 31.69 mol %, respectively. A maximum 5-HMF of 49.1 was obtained for the SSC-12 M catalyst for 3 h, after which the 5-HMF yield started to decrease in SSC-14M, SSC-16 M, and SSC-18 M, indicating more acid leads to generated humins formation and 5-HMF degradation. In contrast, extending the reaction time from 1 to 4 h with SC-12 M led to an increase in the fructose conversion from 90.5 to 97.4 mol %, respectively. The 5-HMF yield increased from 3.88 to 30.27 mol % with the time from 1 to 2 h but dropped to 13.4 and 10.5 mol % after 3 and 4 h, respectively. Thus, this indicated that 5-HMF further converted into unwanted byproducts through additional hydrolysis to generate levulinic and formic acids. This effect is not observed for SSC-12M, further demonstrating the advantages of silica within the composite.

8.

(A) Effect of the reaction time on fructose conversion. (B) Effect of the reaction time on 5-HMF yield. Reaction conditions for A and B: fructose, 0.1 g (0.55 mmol), solvent, 10 mL of water, catalysts, 10 mg, T, 180 °C. (C) The effect of temperature on fructose conversion to 5-HMF. (D) The effect catalyst loading on fructose conversion to 5-HMF. Reaction conditions for C and D: (fructose, 0.55 mmol (0.1 g); 10 mL of solvent; SSC-12 M 10 mg, 180 °C; 3 h).

In addition, loss of sulfonic acid groups at the high temperature diminishes the catalytic performance over time. The optimal catalyst for the reaction of fructose to 5-HMF was SSC-12 and resulted in a 5-HMF yield of 49.1 mol %. This catalyst was selected for further investigation and optimization.

The results of temperature optimization on SSC-12 M show that the yield of 5-HMF increased with increasing reaction temperature (Figure C). 5-HMF yield increased from 31.5 to 49.1 mol % with an increase in reaction temperature from 160 to 180 °C, respectively. After this, the yield of 5-HMF decreased in water. As the temperature of the reaction increases, the endothermic nature of the fructose dehydration and reaction rate increases correspondingly. However, at a reaction temperature of 190 °C the 5-HMF yield decreases due to hydrolysis into formic acid and levulinic acid. The effect of catalyst loading is an important parameter that leads to the 5-HMF yield. 5-HMF and fructose conversion increases with greater catalyst loading due to the higher number of active sites promoting dehydration of glucose into 5-HMF; 10 wt % of SSC-12 M was the optimal loading (Figure D).

Effect of Initial Fructose Concentration on HMF Formation

The influence of the initial fructose concentration on HMF production was examined by varying the fructose concentration from 0.002 to 0.011 M under otherwise identical optimal conditions. As shown in Figure , fructose had a significant impact on both the conversion and HMF yield. Fructose conversion increases steadily with concentrations, from approximately 57.3% mol at 0.002 M to 89.3 at 0.008–0.011 M. In contrast, the HMF yield did not follow the same trend. The maximum yield of 43.1% was obtained at 0.005 M, after which the yield decreased to 36.2% at 0.008 M and further to 19.5% at high concentrations. The inverse relationship between conversion and yield at high substrate loading indicates that side reactions become more pronounced when the fructose concentration is high. Previous studies have reported that higher fructose concentration increase humins formation due to enhanced intermolecular condensation as well as the rehydration of HMF to levulinic and formic acids. The selectivity in system decreased from 59.8 to 22.8%, demonstrating that the higher substrate concentration promotes undesired pathways.

9.

Effect of fructose concentration in fructose conversion to 5-HMF. Reaction conditions: fructose, 0.55 mmol (0.1 g); 10 mL of water; SSC-12 M 10 mg; 180 °C; 3 h.

Effect of the Reaction Solvent on Fructose to 5-HMF Conversion

To maximize the reaction rates and yields in any process, solvent selection must be caref,ully considered. Various solvents including DMSO, Cyrene, THF/NaCl·H₂O and TMO/NaCl·H₂O were evaluated for use with the SSC catalyst at the temperature 180 °C leading to 5-HMF yield of 62.7, 40.2, 56.69, and 52.25 mol %, respectively (Figure ).

10.

Effect of solvent in fructose conversion to 5-HMF. Reaction conditions: fructose, 0.55 mmol (0.1 g); 10 mL of solvent; SSC-12 M 10 mg; 180 °C; 3 h.

The yield of 5-HMF in Cyrene fructose conversion is nearly complete; however, the HMF yield is significantly lower, indicating that secondary reactions dominate under these conditions. This behavior has been reported for other green dipolar aprotic solvents, where rapid dehydration is accompanied by increased humins formation or over-reaction pathways. Biphasic systems (THF/NaCl–H₂O and TMO/NaCl–H₂O) show improved HMF yields relative to water alone because the organic phase can extract HMF from the aqueous phase, minimizing rehydration. Previous work also demonstrated that biphasic systems can limit undesired side reactions compared to single phase systems, such as those conducted in water. Recently, TMO was demonstrated as an effective solvent and a safer alternative to THF for the conversion of glucose to 5-HMF with an AlCl₃ catalyst under microwave heating. However, with the SSC-12 M catalyst and under conventional heating the TMO system exhibited comparable selectivity to the water system, with only a marginally improved yield of 5-HMF. Importantly, THF/NaCl·H₂O and TMO/NaCl·H₂O were the only triphasic systems (TMO or THF, NaCl·H₂O and solid SSC) investigated in this study.

Issues that should be kept in mind when a solvent is selected for 5-HMF synthesis are mobility, acute toxicity for humans, chronic toxicity for humans, and bioaccumulation. In this regard, a preliminary ecological evaluation revealed that water was deemed the safest solvent. According to the CHEM21 solvent selection guide (Table ), DMSO is also graded favorably and received a recommended range and generated the greatest 5-HMF yield of 62.7 mol %. Many reports studied the DMSO solvent in the conversion of glucose and fructose into 5-HMF. Moreover, DMSO effectively inhibited the degradation of 5-HMF into levulinic and formic acids due to the minimal presence of water in the reaction system. All the solvents were deemed to be problematic, with THF receiving the highest (worst) combined score for safety, health, and the environment (Table ). As such, water and DMSO systems were investigated for catalyst reuse.

3. Solvent Ranking according to the CHEM21 Selection Guide of Classical and Less Classical Solvents.

solvent	BP (°C)	FP (°C)	H3xx	H4xx	safety score	health score	envi. Score	ranking by default
water	100[ ]	n.a	none	none	1	1	1	recommended
DMSO	189[ ]	95[ ]	none	none	1	1	5	recommended
cyrene	203[ ]	61[ ]	H319	none	1	2	7	problematic
THF	66[ ]	–14[ ]	H351	none	6	7	5	problematic
TMO	112	-	n.a	n.a	4	5	5	problematic

^aSolvent ranking as determined by refs, , n.a = not available.

Reusability Test

The stability of the catalyst is crucial for the intended use; therefore, the reusability of the SSC-12 M catalyst was investigated by conducting eight consecutive reaction cycles under identical conditions, as shown in Figure A.

11.

Reusability of the SSC-12 M catalyst for glucose to 5-HMF reaction in (A) water and (B) DMSO. Reactivation condition: reflux with 12 M of H₂SO₄ for 10 h.

The catalyst initially maintained approximately 49.1 mol % (first cycle) in water, but the yield fell off in cycles 3 and 4. The deposition of byproducts on the catalyst surface may hinder access to active sites and possible leaching of the sulfonic groups was highlighted as potential reasons for the loss activity. − After the fourth cycle, the SSC-12 M reacted with a 12 M sulfuric acid and was washed and dried. The results show that the HMF yield after reactivation was 44.96%, indicating that reactivation is required for extended reuse of the catalyst in water.

The reusability of SSC-12 M in DMSO is shown in Figure B. The maximum yield of 5-HMF was 62.7% in the first cycle. After the first cycle the yield of 5-HMF dropped to 60.32 and then to 24.6% by the fifth cycle. Reactivation of the SSC-12 M with acid demonstrates good performance with a 5-HMF yield of 54.32%. DMSO can eliminate unwanted side reactions during dehydration of fructose, such as hydrolysis of 5-HMF to form humins and decomposition into other compounds. Therefore, the solid acid catalyst can be reused, but reactivation with additional acid is required to maintain performance in multiple consecutive reaction cycles.

The reused catalyst was further characterized by XPS, BET, and TGA analyses to verify its structural stability and catalytic performance after repeated use (Figures S17 and S18 in the Supporting Information). The XPS results demonstrate that the chemical structure of the SSC catalyst undergoes a change in the C 1s spectrum after reuse (Figure S17 in Supporting Information) and increase in % of surface carbon (Table S4 in Supporting Information). The main peaks corresponding to graphitic carbon (∼284.8 eV), C–O/C–OH (∼286.1 eV), and O–CO (∼287.4 eV) remain present after the second and third cycles, indicating that the carbon framework remains largely stable. However, an increase in the intensity of certain C 1s peaks after reuse can be attributed to the accumulation of carbonaceous species such as humins or polymeric intermediates on the catalyst surface after the reaction. These deposited organics typically contain C–C, C–O, and CO functionalities, which can enhance the corresponding peaks in the C 1s spectra. As the number of reaction cycles increases, more carbonaceous residues are absorbed onto the surface, leading to a greater apparent carbon signal. This behavior is commonly observed in acid-catalyzed dehydration systems where humins formation occurs. This observation is further supported by the TGA profiles (figure S18 in Supporting Information), where the main weight-loss is associated with carbonaceous decomposition (between 350 and 550 °C) becoming pronounced in the recycled catalysts. The BET results shown in Table S4 reveal a gradual decrease in the surface area of the catalyst from 186.2 m²/g (cycle 1) to 163.3 m²/g (cycle 2) and 140.6 m²/g (cycle 3). This decline is attributed to the progressive deposition of humins and coke species inside the pores during repeated reactions, which leads to partial pore blockage and reduced accessible surface area.

For the S 2p region, the fresh catalyst exhibits two characteristic peaks at ∼169.1 and ∼170.5 eV corresponding to the S 2p₃/₂ and S 2p₁/₂ components of the –SO₃H groups. After repeated use, these peaks remain detectable but with minor reduction in intensity, indicating a possible partial desulfonation or leaching of sulfonic groups during reaction (figure S17 in the Supporting Information). The presence of clear S 2p signals after three cycles confirms that a substantial portion of the sulfonic acid sites remains anchored to the carbon–silica matrix. Sulfur content (%) according to XPS analysis decreases from 5.72% to 2.10% over 3 cycles, while the carbon content increases from 14.1% to 58.4% over the same 3 cycles. Similar partial sulfur loss has been widely reported for carbon-based solid acids under aqueous or high-temperature dehydration conditions. However, the preserved reduction in sulfur content in our system may be due to the increased carbon content associated with humins/coke formation blocking the active sites. While partial leaching of the sulfonic group may take place, carbon deposition clearly contributes to pore blocking/coking leading to catalyst deactivation during reuse.

Catalytic Performance for the Conversion of Glucose to 5-HMF

As the catalytic performance of SSC-12 M was highly selective for the conversion of fructose into 5-HMF, the optimized conditions were also applied to investigate the conversion of glucose to 5-HMF as shown in Figure . The glucose conversion in water increased from 20.1 to 98.4 mol %, as time was extended from 1 to 6 h, respectively. 5-HMF yield increased from 12.1 to 47.56 mol % in 3 h, after which 5-HMF decomposition or conversion to other products such as levulinic acid or formic acid occurred. DMSO was investigated for glucose conversion to 5-HMF, reaching 99.4 mol % conversion after 3 h with SSC-12M. It can be observed that the 5-HMF yield increased from 58.2 to 70.2 mol % from 1 to 4 h. After which time, the yield started to drop, indicating that it was converted to other products. DMSO in combination with the SSC-12 M catalyst can eliminate byproducts by stabilization of the carbocation intermediate formed by removing the first water molecule from β-d-fructose furanose, thus forming the [carbocation-DMSO] complex. When the second and third water molecules are removed from this complex, it promptly converts into other intermediates, producing 5-HMF. In a general acid-catalyzed reaction, glucose often produces less fructose than anticipated due to side reactions and the reversible nature of the isomerization process. Although glucose can be converted to fructose, the reaction equilibrium tends to favor glucose, limiting the fructose yield. Furthermore, glucose may undergo competing reactions, such as dehydration, which can result in the formation of byproducts such as HMF and levulinic acid, particularly under more severe reaction conditions. However, under these optimal conditions, in this work, glucose gave a higher HMF yield compared to that in the previous experiments using fructose as the substrate. Interestingly, the HMF yield from glucose was found to be higher than that from fructose, which is contrary to the general trends observed in many literature reports. This difference can be attributed to several possible factors. Isomerization of glucose to fructose is frequently slower than the reaction of fructose to 5-HMF or byproducts, thus aiding the regulation of reaction kinetics and preventing a significant accumulation of fructose that may lead to humins. 5-HMF is efficiently synthesized through the controlled release of fructose, in favor of side-product formation. Weak Bronsted acid site selectivity dehydrates fructose to HMF without further degradation. Direct dehydration from fructose without the slower isomerization step leads to more side reactions and lower 5-HMF yields. Enhanced mesoporosity also promotes better mass transport and active site accessibility, contributing to the overall performance.

12.

Glucose to 5-HMF conversion, using SSC-12 M in water and DMSO. Reaction conditions: glucose, 0.55 mmol (0.1 g), 10 mL of water and DMSO, SSC-12 M 10 mg, 180 °C.

Reusability of the SSC-12 M for the conversion of glucose to 5-HMF was investigated in both water and DMSO (Figure ). The result in water showed a 5-HMF yield of 47.6% obtained in the first cycle and started to decrease by the fourth cycle. Reactivation in the fifth cycle led to an increase in the 5-HMF yield of 43.21%. Furthermore, the catalyst activity of glucose conversion to 5-HMF in DMSO as solvent shows the maximum yield of 5-HMF of 70.2% in the first cycle and started to decrease in the fourth cycle. After reactivation, the catalyst can be effectively used to the ninth cycle. The investigation of catalyst reusability after reactivation revealed that the catalyst still provided a good 5-HMF yield. Once again, both sulfonic group leaching and carbon deposition contribute to catalyst deactivation during reuse.

13.

Reusability of the SSC-12 M catalyst for glucose to 5-HMF reaction in DMSO. Reactivation condition: reflux with 12 M of H₂SO₄ for 10 h.

Comparison to Other Solid Acid Catalysts for 5-HMF Synthesis

Table demonstrates a range of solid acid catalysts for glucose and fructose conversion to 5-HMF. Previous research demonstrates a wide range of solid acid catalysts for the conversion of glucose and fructose into 5-HMF, and a substantial number of studies have investigated this transformation. However, many of these catalytic systems suffer from major drawbacks such as harsh reaction conditions and the requirement for expensive and sometimes toxic solvents (e.g., ionic liquids, biphasic systems). Some systems also rely on rare earth metals, which increase economic and environmental concerns, while others achieve high yields only at remarkably high catalyst loadings, limiting their industrial applicability.

4. Difference between Solid Acid Catalysts.

catalyst	reagent	solvent	T (°C)	time (h)	conversion (%)	yield (%)	selectivity (%)	ref
SiO2/C–SO3H	fructose	DMSO	130	5	-	74.0	-
Si50C50-400-SO3H	fructose	EtOH	160	>24	100	30.0	30.0
NbPW-06	fructose	H2O	80	3	59.9	7.8	13.0
PU-Cat	fructose	H2O	140	2	53.6	34.2	63.8
Glu-TsOH	fructose	H2O	130	1.5	67.0	8.0	11.9
SBA-15-SO3H	fructose	DMSO	120	1	100	96.0	96.0
SC-S	fructose	DMSO	130	1.5	-	70.0	-
LCSA-900	glucose	DMSO	170	1	98.0	75.7	77.2
SSC-12 M (this work)	fructose	H2O	180	3	83.0	49.1	59.1	-
SSC-12 M (this work)	fructose	DMSO	180	3	95.4	62.7	65.7	-
t-SiO2@B@A	glucose	H2O	120	6	89.6	69.7	77.7
SO3H-OAC	glucose	THF/H2O	250	3	100.0	93.0	93.0
SnPCP@MnO2-PDA	glucose	DMSO	150	5	93.3	55.8	60.5
LDMCS-700	glucose	aqNaCl-THF	160	2.5	97.7	57.8	59.1
SSC-12 M (this work)	glucose	H2O	180	3	91.0	47.5	52.1	-
SSC-12 M (this work)	glucose	DMSO	180	3	98.0	70.2	71.6	-

A significant number of studies have investigated the conversion of fructose to 5-HMF.

Al-Amsyar et al. developed environmentally friendly heterogeneous catalysts derived from rice husk, a biomass waste rich in both silica and carbon. Sulfonated silica/carbon composites were synthesized by controlling the carbonization temperature of rice husk, followed by sulfonation to introduce SO₃H groups. However, complex and time-consuming synthetic routes, requiring careful control of carbonization temperature and sulfonation conditions, limit the potential of such catalysts. Zhong et al.’s synthesis of sulfonated mesostructured silica–carbon nanocomposites was performed by an evaporation-induced self-assembly (EISA) method. The catalysts combine the advantages of mesoporous silica and carbon, resulting in a hierarchical porosity and tunable acidity. The materials achieved up to ∼80% combined yield of biofuels (mainly EL and EMF) and the HMF yield was 30% after 24 h. Despite their high efficiency, long reaction times and high-water content may promote humin formation and reduce 5-HMF yield. Qiu et al. carried out fructose conversion by the alcohol-mediated heating method. Unfortunately, NbPW-06 demonstrated poor catalytic activity with 7.8% 5-HMF yield in water due to the large amount of humins formed. In contrast, in the conversion with DMSO as a solvent the 5-HMF shows good activity of 96.7% and 100% fructose conversion. Li et al. produced a sulfonated carbon catalyst for the conversion of fructose in water, resulting in a 5-HMF yield of 34.2%. However, this catalyst promoted side reactions in water and could not shift the equilibrium to favor dehydration. A carbon-based solid acid functionalized with p-toluenesulfonic acid (TsOH) was investigated for fructose conversion to 5-HMF. The yield of 5-HMF was 67.0% and the selectivity was 11.9% with water as the solvent. It indicates that the water in the reaction promotes the side-reaction. SBA-15-SO₃H achieved high 5-HMF yields of 96% with 100% fructose conversion in DMSO. DMSO can exhibit reduced generation of humins as compared to water as the performance of SBA-15-SO₃H arises from its highly ordered mesoporous structure with large uniform pores (6–8 nm) and a very high surface area (>700 m² g^–1), and stable grafted –SO₃H groups, which provide excellent molecular diffusion and accessible Brønsted acid sites. Sulfonic acid-functionalized mesoporous carbon/silica catalysts have been investigated for the dehydration of fructose into 5-HMF. This work achieves a maximum yield of 70% with good recyclability, but the performance is lost when excess glucose blocks the pores, leading to reduced efficiency with a thicker carbon layer and lower surface areas. In contrast, our SSC catalysts derived from molasses contain amorphous carbon/silica domains with less-ordered porosity and a lower surface area, resulting in more mass-transfer limitations during fructose dehydration. A portion of the acid sites in SSC catalysts may be embedded within the carbon matrix or partially blocked by residual carbonaceous species, giving lower effective acidity and lower HMF selectivity. Future work could synthesize SSC catalysts with templated silicas, such as SBA-15, and investigate their performance in HMF production. Li et al. prepared a lignin-based activated carbon solid (LCS) with a high surface area. The results show LCS-900 achieved a maximum 5-HMF yield of 75.7% under the optimum reaction conditions using DMSO as the solvent.

A large number of catalysts including some carbon/silica composites have been used for the conversion of glucose to 5-HMF. The t-SiO₂@B@A catalyst comprised of a mesoporous silica grafted with 3-(aminopropyl) triethoxysilane. The Bronsted basic and acidic sites on the catalyst surface aided in achieving a 69.7% yield of 5-HMF, with a glucose conversion of 89.6% in water. The catalyst shows good recyclability of eight cycles with a little drop in 5-HMF yield. Moreover, the catalyst activity also gradually declined due to the carbon balance being incomplete (∼82.0–83.5%), indicating the formation of side products such as levulinic acid and humins. Nahavandi et al. investigated a sulfonated carbon-based acid catalyst (SO₃H-OAC) prepared by a two-step method by oxidation pretreatment of activated carbon (AC) followed by sulfonation. The results show the SO₃H-OAC complete glucose conversion and approximately 93% 5-HMF yield under the optimal conditions (160 °C for 3 h) with THF/H₂O as the solvent. Catalyst deactivation remained an issue, as performance decreased after multiple reuses due to sulfonic group leaching, humin deposition on active sites, and particle loss of surface area. The preparation process also required harsh conditions, including sulfonation with concentrated acid at 250 °C. The use of tetrahydrofuran (THF) as the organic solvent has some toxicity issues, and peroxide formation is a significant risk. The system is also dependent heavily on sodium chloride to achieve high 5-HMF yields through a salting-out effect, and both yield and selectivity declined sharply when NaCl was absent. Li et al., a porous coordination polymer of tin (SnPCP), was synthesized using 5-sulfonicphthalic acid as the ligand on polydopamine-coated MnO2 (MnO₂-PDA). The results show the catalyst achieved an HMF yield of 55.8% in DMSO and 41.2% in the water/THF biphasic system. The catalyst also showed good stability with minimal activity loss observed after 5 consecutive reaction cycles in terms of both glucose conversion and 5-HMF yield. Although the authors explored water/THF mixtures, the yield and selectivity remained moderate, and the need for THF again introducing safety issues. Wang et al. prepared lignin-derived mesoporous carbon solid acid (LDMCS) for the conversion of glucose to 5-HMF in a biphasic solvent system. The catalyst shows good glucose conversion of 97.7% with a 5-HMF yield of 57.8%. However, the conversion of glucose in water shows poor activity compared with a biphasic solvent system.

Importantly, the SSC-12 M demonstrates high activity in both water and DMSO. In many cases, the 5-HMF yields are comparable or exceed other reported materials, while demonstrating improved selectivity. The SSC-12 M can be easily regenerated through a simple process, enabling multiple reuse cycles. These results indicated that SSC-12 M represents a robust pathway for the sustainable synthesis of 5-HMF from glucose or fructose. In addition, the sustainability of these catalysts was further improved through the demonstration that acid used in the synthesis of the materials could be effectively recycled several times (Figure S16 and Table S2 in the Supporting Information). Green metrics demonstrate that the reactions in DMSO consistently outperform H₂O in terms of yield, RME, PMI, and OE for both glucose and fructose. DMSO achieves the highest conversion and overall efficiency in the conversion of glucose. This comparison emphasizes the importance of solvent selection in maximizing the green metrics and sustainability of HMF production.

Conclusion

SSC catalysts were synthesized from molasses and K60 silica gel with 12 M sulfuric acid in a simple process that simultaneously combined carbonization and sulfonation. SSC-12 M was demonstrated to be effective in the conversion of fructose and glucose into 5-HMF in water at 180 °C for 3 h. A carbon catalyst that contained no silica that was produced under comparable conditions to the SSC demonstrated greater byproduct formation and lower 5-HMF yields. Thus, this indicates that silica within enhances the catalyst accessibility of active sites, increases thermal stability, and provides excellent selectivity toward 5-HMF formation, compared to the carbon catalyst with no silica. The use of DMSO as a solvent in combination with SSC-12 M promoted high 5-HMF yields of 62.7 mol % and fructose conversions of 95.4 mol %, with high selectivity of 65.8% and less byproduct formation The conversion of glucose with SSC-12 M was 98.0%, with a 70.2% yield of 5-HMF and 71.6% selectivity. Furthermore, the catalyst was able to be reused over several cycles, and a simple reactivation with acid further enhanced the activity. This study has demonstrated the potential of sulfonated silica carbon (SSC) catalysts derived from low value sugar cane byproducts for the efficient conversion of fructose and glucose into 5-HMF. Future investigation of carbonization temperature, the silica/carbon ratio, and the use of templated silicas such as SBA-15 may further enhance the stability and selectivity of these catalysts, thus accelerating technology transfer and commercialization.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c11171.

• Reaction pathway for glucose and fructose to HMF, carbon mass of reactant and carbon mass balance, O 1s XPS spectra of SSC-2 M to SSC-18M, S 2p XPS spectra of SSC-4 M to SSC-18M, TEM and EDS image of SSC, acid recycling, reusability and structural stability of the catalyst, XPS S 2p and C 1s spectra of the catalyst before the reaction (fresh) and after Cycle 2 and Cycle 3, TGA spectra of the catalyst before the reaction (fresh) and after Cycle 2 and Cycle 3, BET surface area of the fresh and reused SSC-12 M catalysts, structure parameters of SSC-R catalyst, green metric assessment of glucose and fructose conversion, HPLC chromatograms and quantification methods, and summary of the catalytic performances of several porous solid materials (PDF)

The authors declare no competing financial interest.