One-pot synthesis of sulfonated graphene oxide for efficient conversion of fructose into HMF

Abstract

Sulfonated graphene oxide (SGO) is a promising solid acid catalyst for many acid-catalyzed reactions, but its synthesis typically requires complicated post-functionalization. Herein, a one-pot method was developed to directly synthesize SGO. Characterizations showed that the SGO has abundant acid sites, including SO₃H (1.62 mmol g⁻¹), OH and COOH, and a two-dimensional (2D) structure. An efficient reaction system using SGO as a solid acid catalyst was developed for the dehydration of fructose into 5-hydroxymethylfurfural (HMF). A high yield of HMF up to 94% was obtained with a low loading of SGO. Moreover, the SGO is also capable of converting high-concentration fructose (20 wt%) with an acceptable yield of 67%. The high catalytic activity was attributed to the abundant active sites and the 2D structure which enhances the accessibility of the active sites.

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1. Introduction

The exhaustion of fossil fuel reserves and environmental issues have been aggravated in recent years, which seriously threatens sustainable development.^1,2 Therefore, the development of sustainable industry based on renewable feedstock is extremely important.³ Lignocellulosic biomass is a promising alternative feedstock because of its abundance, environmental benignity and renewability. Therefore, biorefinery technologies have been greatly developed to upgrade biomass into value-added chemicals, materials and fuels.⁴ In biorefinery, the transformation of biomass into platform chemical 5-hydroxymethylfurfural (HMF), a triple dehydration product of hexoses, is a key reaction.¹ As a versatile intermediate, HMF can be converted into various value-added chemicals, including levulinic acid (LA), γ-valerolactone (GVL) and furan-2,5-dicarboxylic acid (FDCA).⁵ The conversion of cellulosic biomass into HMF involves a series of complicated reactions, including the hydrolysis of cellulose into glucose, isomerization of glucose to fructose, and the dehydration of fructose to HMF.^6,7 The deconstruction of lignocellulose into sugars is catalyzed by acids or enzymes in the presence of water.⁸ However, the presence of large amounts of water is harmful to the dehydration process. The direct conversion of lignocellulose and cellulose usually leads to good relative yield and selectivity of HMF.^9,10 Therefore, the rational design of reaction systems for each step is very important for the establishment of a commercially viable HMF production process.⁸

Many homogenous catalysts, such as H₂SO₄, HCl, H₃PO₄ and transition-metal ions, could convert fructose into HMF.¹¹ However, these homogeneous catalysts suffer from several drawbacks, including environmental pollution and separation problems. To overcome the disadvantages of homogenous catalysts, some solid acids, including Amberlyst®-15, poly-benzylic ammonium chloride resin, sulfonic acid-functionalized metal–organic frameworks (MOF–SO₃H), phosphotungstic acid encapsulated in a metal organic framework and titanium hydrogen phosphate were used for the dehydration of fructose to HMF.^6,12–15 Moreover, carbon-based solid acids bearing SO₃H, COOH, and phenolic OH groups prepared by sulfonation of carbonized biomass were reported to be powerful solid acid catalysts for biomass conversion.^16–20 However, high loading of the solid catalyst is typically required due to the limited number of accessible active sites in these solid catalysts, and this will be an obstacle to the practical application of these catalysts on a large scale.²¹ Therefore, it is necessary to increase the accessibility of active sites in solid acid catalysts to improve their catalytic performance.

Compared with conventional solid acid catalysts, graphene oxide (GO), which is prepared by the chemical oxidation of natural graphite followed by exfoliation, seems to be more interesting due to its abundant functional groups, including COOH and OH, as well as its unique two-dimensional (2D) structure.²¹ Several reports have shown that GO and its derivative, sulfonated graphene oxide (SGO), are highly active catalysts for many acid-catalyzed reactions, such as hydrolysis of cellulose, hydrolysis of glycitein and hydration of alkynes.^21,22 However, the preparation of SGO requires complicated and harsh processes: chemical oxidation of natural graphite, reduction of GO by a reductant and sulfonation of reduced GO by concentrated H₂SO₄, fuming sulfuric acid or chlorosulfonic acid.^21,23

In this work, we develop a one-pot method to synthesize SGO for the first time. The prepared SGO was characterized by elemental analysis, FT-IR, Raman, XRD, XPS, TGA and TEM. To evaluate its catalytic performance, the prepared SGO was used to catalyze the dehydration of fructose and glucose into HMF. The influence of reaction temperature, time, catalyst loading and reaction medium was investigated to optimize the reaction conditions.

2. Experimental

2.1. Materials and reagents

Fructose (99%), glucose (99%), 5-hydroxymethylfurfural (99%) and graphite powder (99%) were purchased from Tianjin Heowns Biochem LLC (Tianjin, China). The graphite powder was ball-milled and then sifted with a 100 mesh sieve. The sifted graphite powder (<150 μm) was used for the preparation of graphene oxide. Ionic liquid 1-allyl-3-methylimidazolium chloride (AMIMCl) was purchased from Shanghai Chengjie Chemical Reagent Co. Ltd., China. All the other chemical reagents were purchased from commercial sources in China and used as received.

2.2. Catalyst preparation

GO and SGOs were prepared using different approaches, as described in Scheme 1. GO was prepared by the modified Hummers method according to the literature.²⁴ Briefly, graphite powder (3.0 g) and NaNO₃ (1.5 g) were added to concentrated H₂SO₄ (69 mL) in an ice bath. Keeping the reaction temperature below 20 °C, KMnO₄ (9.0 g) was added slowly in portions to the solution. Then the system was warmed to 35 °C with stirring and kept for 30 min. After water (138 mL) was added slowly, the system was self-heated to 98 °C and the reaction temperature was maintained at 98 °C for 40 min. Afterward, the reaction mixture was cooled, diluted and further treated with additional 30% H₂O₂ (3 mL). After air cooling, the mixture was centrifuged (10 000 rpm for 30 min), and the supernatant was decanted away. The remaining solid material was then washed in succession with water, 30% HCl and ethanol to remove remnant salt. The obtained solid was dewatered by freeze drying to obtain the graphite oxide powder.

Scheme 1 Preparation of GO and SGOs by different approaches.

SGO-1 and SGO-2 were prepared by the Tours method according to a previous report.²⁵ For SGO-1, graphite powder (3.0 g) was added to a 9 : 1 mixture of concentrated H₂SO₄/H₃PO₄ (360 : 40 mL) in an ice bath. Keeping the reaction temperature below 20 °C, KMnO₄ (9.0 g) was added slowly in portions to the solution. Afterward, the reaction mixture was then heated to 50 °C and stirred for 12 h. After the reaction, the mixture was purified as described for GO. SGO-2 was prepared according to the same process by changing the amount of KMnO₄ from 9.0 g to 18.0 g.

SGO-3 was prepared according to a modified Tours method. Briefly, graphite powder (3.0 g) and NaNO₃ (1.5 g) were added to a 9 : 1 mixture of concentrated H₂SO₄/H₃PO₄ (360 : 40 mL) in an ice bath. Keeping the reaction temperature below 20 °C, KMnO₄ (18.0 g) was added slowly in portions to the solution. Then the reaction mixture was heated to 50 °C and stirred for 12 h. After the reaction, the mixture was purified as described for GO.

2.3. Catalyst characterization

The elemental composition of the samples was determined by elemental analysis using an elemental analyzer (Euro EA3000, America). The acidity distributions were measured by the Boehm titration method.¹⁷ X-ray photoelectron spectra were performed using an ESCALAB 250Xi X-ray Photoelectron Spectrometer (Thermo Fisher Scientific, America). A PHI ACCESS ESCA-V6.0 F software package was used for data analysis. A Shirley-type background was subtracted from the signals. All recorded spectra were fitted using Gaussian–Lorentzian curves to more accurately determine the binding energies of the different element core levels. The powder X-ray diffraction (XRD) patterns were recorded on an X'pert MFD diffractometer using Cu-Kα radiation (λ = 1.5406 Å) generated at a voltage of 40 kV and a current of 40 mA. Scans were obtained from 2θ = 5 to 70° at a scanning rate of 5° min⁻¹. The Fourier transform infrared (FTIR) spectra were recorded using an FTS 6000 FTIR spectrometer (Bio-rad, USA). The test samples were prepared using the KBr-disk method. The spectra were measured from 4000 to 400 cm⁻¹ at a resolution of 2 cm⁻¹ in the transmission mode. The Raman spectra were recorded with a DXR Raman system (Thermo Fisher Scientific, America) with a laser frequency of 514 nm as an excitation source. The surface morphology of all the samples was analyzed by a Tecnai G2 F20 S-TWIN high resolution transmission electron microscope (FEI, America).

2.4. Catalytic tests

In a typical procedure, 10 mg of catalyst was added into 2 g of DMSO in a 10 mL quartz tube reactor. The catalyst was sufficiently dispersed into DMSO by sonication at 400 W for 30 min, and then 100 mg of fructose was added to the reactor. The reactor was sealed, and then heated to 120 °C in an oil bath and maintained for 6 h. After the desired reaction time had elapsed, the reactor was cooled to room temperature with a cold water bath within 1 min, and water (1.0 g) was added to the reactor. The sample was diluted with water and then filtered with a 0.45 μm polytetrafluoroethylene filter membrane to remove the insoluble solid. The concentration of HMF was measured with a UV3100PC spectrophotometer (Shanghai Meipuda instrument Co., LTD, China) at 284 nm by using the standard curve method. The concentration of glucose and fructose was measured with a high-performance liquid chromatography (HPLC) system equipped with an evaporative light scattering detector (SofTA, ELSD Model 300s) and an Xtimate® Sugar-Ca analytical column (7.8 × 300 mm, 5 μm). The column oven temperature was set at 80 °C, and the mobile phase was ultrapure water at a flow rate of 0.50 mL min⁻¹. The sugar conversion, HMF yield and selectivity were calculated according to the following equations:

All results were replicated at least three times, and the reproducibility of sugar conversion, HMF yield and HMF selectivity were within 3% standard deviation.

3. Results and discussion

3.1. Catalyst structure and properties

3.1.1. Composition and acidity distributions. After drying, all the samples were subjected to elemental analysis. As shown in Table 1, the C contents in GO, SGO-1, SGO-2 and SGO-3 are 53.51, 50.31, 45.45, and 44.52 wt%, respectively, revealing that the overall order of oxidation is SGO-3 > SGO-2 > SGO-1 > GO. The S contents in GO, SGO-1, SGO-2, and SGO-3 are 1.43, 4.21, 2.54 and 5.19 wt%, respectively. Since all the sulfur atoms are present in the form of SO₃H groups,^21,26 as is demonstrated by X-ray photoelectron spectroscopy in Section 3.1.3., the corresponding concentrations of SO₃H were calculated to be 0.45, 1.31, 0.80 and 1.62 mmol g⁻¹. The acidity distributions were analyzed by the Boehm method and are listed in Table 2, indicating that those materials contain abundant acid sites, including carboxylic groups, phenolic groups, lactonic groups and sulfonic groups.

Table 1 Elemental analysis of the GO and SGO samples

Sample	C [wt%]	H [wt%]	O [wt%]	N [wt%]	S [wt%]	Concentration of –SO3H [mmol g^−1]
GO	53.51	2.01	42.97	0.10	1.43	0.45
SGO-1	50.31	2.01	43.23	0.27	4.21	1.31
SGO-2	45.45	2.29	48.73	0.03	2.54	0.80
SGO-3	44.52	2.20	47.98	0.14	5.19	1.62

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Table 2 Acidity distributions of the GO and SGO samples

Sample	Sulfonic groups^a (mmol g^−1)	Carboxylic groups^b (mmol g^−1)	Phenolic groups^c (mmol g^−1)	Lactonic groups^d (mmol g^−1)	Total acid sites^e [mmol g^−1]
a Calculated from S content by elemental analysis.b Obtained by subtracting concentration of sulfonic groups from titration results with NaHCO3.c Obtained by subtracting titration results using NaHCO3 from calculated values of Na2CO3.d Obtained by subtracting titration results using Na2CO3 from calculated values of NaOH.e Obtained by titration with NaOH.
GO	0.45	0.55	0.68	0.23	1.91
SGO-1	1.31	0.48	0.54	0.17	2.50
SGO-2	0.80	0.63	0.75	0.24	2.42
SGO-3	1.62	0.50	0.59	0.14	2.85

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3.1.2. FTIR spectroscopy. As shown in Fig. 1, all of the samples show a band between 1732 and 1739 cm⁻¹ attributed to the C=O stretch vibration, which is associated with carboxylic acids and carbonyl moieties.²³ The bands at 1574 and 1443 cm⁻¹ are assigned to the skeleton vibration of the C=C bonds of unoxidized graphitic domains.²⁷ The FIIR spectra of SGO-1, SGO-2 and SGO-3 show significant bands at 1393, 1057 and 857 cm⁻¹, which were identified as the O=S=O stretching vibration,²³ –SO₃-symmetrical stretching vibration, and S–OH stretching vibration, respectively. These bands are the characteristic absorptions of SO₃H, confirming that SO₃H is grafted onto the graphene surface during the oxidation process.

Fig. 1 FTIR spectra of GO and SGOs prepared by different methods.

3.1.3. X-ray photoelectron spectroscopy. The wide-range scanning XPS spectra (Fig. 2a) show the absorption peaks of C with a binding energy of 286 and 978 eV, the absorption peaks of O with a binding energy of 533 eV, and the absorption peak of S with a binding energy of 168 eV, confirming the existence of C, O, and S elements in all the samples.²¹ The high-resolution C 1s XPS spectra (Fig. 2b) show peaks corresponding to C–C, C–O, C=O and COOH, confirming the presence of these oxygen-containing groups.^21,24

Fig. 2 Wide-range scanning XPS spectra (a), high-resolution C 1s XPS spectra (b), and high-resolution S 2p XPS spectra (c) of GO and SGOs prepared by different methods.

To investigate the existing state of sulfur, the high-resolution S 2p spectra were recorded. As shown in Fig. 2c, there is a single Gaussian distribution peak at 169.0 eV corresponding to SO₃H in all the samples,²⁶ while the peak at 163.9 eV corresponding to sulfide is absent.²⁴ However, whether the peak around 168 eV should be assigned to a C–S bond energy site or to an O–S bond energy site is contradictory in different literatures. Zhao et al. attributed the peak at 168.2 eV in GO to sulfate, that is to an O–S bond energy site between SO₃H and O.²⁴ Wei et al. prepared GO using a similar method, but they assigned the peak at 167.8 to a C–S bond energy site between SO₃H and C.²¹ Although it is difficult to distinguish C–S and O–S bonds, the high-resolution S 2p spectra demonstrated that all the S was grafted onto the graphene nanosheets in the form of SO₃H. It is notable that the concentration of SO₃H in SGO-1 and SGO-3 is comparable with that of previously reported SGO prepared by a post-modification method.^21,23 All the acidity sites determined by acid–base titration are listed in Table 2, also supporting the relatively high acid strength of SGO-1 and SGO-3. The post-modification method usually involves complicated and harsh processes, including partial reduction of GO with hydrazine and sulfonation of reduced GO with sulfanilic acid, chlorosulfonic acid, concentrated sulfuric acid or fuming sulfuric acid.^21,23,28,29 In the present work, SGO bearing abundant SO₃H functional groups can be obtained directly though a one-pot reaction. Compared with previous methods, our method is very convenient and effective for the preparation of SGO.

3.1.4. X-ray powder diffraction. The X-ray powder diffraction (XRD) spectrum was used to characterize the crystalline nature of the GO and SGOs (Fig. 3). For all samples, the diffraction peak at 2θ = 26.4° that characterizes the 0.335 nm interlayer distance of pristine graphite²⁷ has disappeared completely, indicating that almost no starting material (graphite flakes) is present in these samples. The interlayer spacings are 0.82, 0.87, 0.88 and 0.93 nm for GO, SGO-1, SGO-2 and SGO-3, respectively. Usually the degree of oxidation of GO is considered to be proportional to the interlayer spacing, since the interlayer spacing increases as the number of polar oxygen-containing groups increases during the oxidation process.²⁵ The degree of oxidation revealed by the XRD spectra is consistent with elemental analysis, demonstrating that SGO with a higher degree of oxidation could be obtained through Tour's method and a modified Tour's method.

Fig. 3 XRD spectra of GO and SGOs prepared by different methods.

3.1.5. Raman spectroscopy. All these solid materials were characterized with Raman spectroscopy, a non-destructive technique which is widely used to analyze the structure of carbon material.

As shown in Fig. 4, both D peaks at around 1590 cm⁻¹ and G peaks at around 1350 cm⁻¹ were identified in all samples,²⁵ suggesting that all four materials were grossly similar. The ratio between the area of the D and G bands (I_D/I_G) is generally used as a measure for the oxidation level and the size of the sp² ring clusters in an sp³/sp² hybrid network of carbon atoms.²⁷ The I_D/I_G ratios are 1.35, 1.46, 1.56 and 1.58 for the GO, SGO-1, SGO-2 and SGO-3, respectively. The increased I_D/I_G ratio is consistent with the increased structural disorder originating from the modification of the carbon surface by the introduction of sulfur groups and oxygen-containing groups.

Fig. 4 Raman spectra of GO and SGOs prepared by different methods.

3.1.6. Transmission electron microscopy. The morphology of the as-synthesized GO and SGOs was investigated using a high-resolution transmission electron microscope (HR-TEM). The TEM images (Fig. 5) show that all samples have irregular edges and transparent lamella, indicating that they are composed of large flakes of GO a few layers thick.

Fig. 5 High-resolution transmission electron microscope images of GO and SGOs prepared by different methods.

3.2. Conversion of fructose into HMF

3.2.1. Conversion of fructose into HMF with different catalysts. To evaluate the performance of the prepared GO and SGOs as a heterogeneous solid acid catalyst, the conversion of fructose into HMF was conducted in the medium of DMSO. DMSO could not only dissolve fructose, but also promote the dispersion of the catalyst after ultrasonic exfoliation. Fig. 6 shows the catalytic performance of different catalysts for the dehydration of fructose to HMF in DMSO at 120 °C for 60 min. Among the investigated catalysts, SGO-3 shows the best catalytic performance during the reaction process and reaches a maximum value of fructose conversion of 89% with a high HMF yield (85%). SGO-1 can also afford a good HMF yield (81%), while the fructose conversion and HMF yield for GO and SGO-2 are relatively low. The catalytic activity is influenced not only by the density of active sites, including strongly acidic SO₃H groups and the weakly acidic OH, COOH groups, but also by the accessibility of active sites.²⁴ Compared with other samples, SGO-3 has the highest degree of oxidation with the highest density of SO₃H groups and largest interlayer spacing, thus leading to its superior catalytic performance. Apart from fructose conversion and HMF yield, HMF selectivity for different catalysts was also evaluated. All the catalysts could convert fructose into HMF with an excellent selectivity (around 95%), suggesting that by-products were rarely formed under the investigated conditions.

Fig. 6 The comparison of catalytic performance of GO and SGOs prepared by different approaches. Reaction conditions: fructose (100 mg), catalyst (10 mg), DMSO (2 mL), 120 °C, 1 h.

3.2.2. Influence of temperature and catalyst loading on HMF yield. In the subsequent experiment, the influence of reaction temperature and time on fructose dehydration to HMF was studied. Generally, a high temperature could not only promote the dehydration of fructose, but also accelerate the side-reactions, including polymerization of HMF with sugars to form humins, and rehydration of HMF to form levulinic acid and formic acid.¹¹ As shown in Fig. 7a, at 80 °C the yield of HMF increased slowly to 59% in 240 min. When the temperature increased to 120 °C, the yield of HMF increased to 85% in only 60 min. When the temperature was further increased to 140 °C, the yield of HMF increased quickly to 84% in the initial 60 min, but a prolonged reaction time will lead to an obvious decrease in HMF yield, possibly as a consequence of the further reaction of the produced HMF.

Fig. 7 Effect of temperature (a) on the yield of HMF using 10 mg of SGO-3 and effect of catalyst loading (b) on the yield of HMF at 120 °C. Reaction conditions: fructose (100 mg), DMSO (2 mL).

Afterwards, we tested the effect of catalyst loading on the conversion of fructose at 120 °C. As shown in Fig. 7b, in the absence of a catalyst the HMF yield was less than 25% during a reaction time of 120 min. When the catalyst loading was increased from 10 mg to 20 mg, there was a slight decrease in the HMF yield, which may result from the excess acid-active sites that catalyze not only the dehydration of fructose to HMF but also the degradation of the formed HMF into other products, such as humins.¹⁷

3.2.3. Influence of solvent on HMF yield. It has been shown that DMSO is a very effective reaction medium for fructose dehydration, as is consistent with previous reports.^11,30 In order to search for an alternative reaction medium, several different solvents including 1,3-dimethyl-2-imidazolidinone (DMI), γ-valerolactone (GVL) and ionic liquid 1-allyl-3-methyl-methylimidazolium chloride (AMIMCl) were tested as reaction mediums for the production of HMF. DMI and GVL cannot give HMF in a high yield in these reaction conditions. When ionic liquid AMIMCl was used as reaction medium, the catalyst cannot be exfoliated by ionic liquid AMIMCl after 2 h of ultrasonic treatment (Fig. 8a), in accord with the previous report.³¹ Although SGO cannot form a stable dispersion in AMIMCl, we observed a relatively high HMF yield of 81% from AMIMCl using SGO as a catalyst (Fig. 8b). It has been reported that an HMF yield up to 72% with a fructose conversion approaching 100% was obtained from ionic liquid [EMIM]Cl in the absence of a catalyst.³² In order to confirm that the excellent HMF yield was not just the consequence of the ionic liquid, we did a control experiment without a catalyst. As shown in Fig. 8c, the HMF yield was less than 53% during the whole reaction process in the absence of a catalyst. When a mixture of AMIMCl and DMSO (w/w 1 : 1), which could not only dissolve a high concentration of fructose but also disperse SGO, was used as the reaction medium, a superior HMF yield up to 94% with a fructose conversion approaching 100% was obtained.

Fig. 8 Photographic images of SGO-3 dispersed in different solvents (a). Effect of solvents on the yield of HMF using SGO-3 (10 mg) as catalyst (b) and without catalyst (c). Reaction conditions: fructose (100 mg), solvent (2 mL), 120 °C.

3.2.4. Influence of initial fructose concentration on HMF yield. One major disadvantage of solid acid catalysts is that they cannot process high-concentration feeds, which seriously restricts their practical applications. As mentioned above, the unique 2D structure of SGO may enhance the accessibility of the active sites. In the subsequent experiment, we increased the initial fructose concentrations to investigate their influence on HMF yield. As shown in Table 3, along with an increase in the fructose amount from 5 to 10%, the HMF yield decreased from 95 to 88%. Even if the initial concentration of fructose increases to 20 wt%, the HMF yield remains acceptable (67%). It should be noted that the loading of catalyst in the present work is much lower than that of other solid acids, while the HMF yield here is comparable to or better than the reported values. Although the reaction conditions, including temperature, time and solvent are not exactly the same, this comparison demonstrates that the SGO-3 prepared in this work has a superior catalytic performance. Since both the type and density of acid sites in SGO-3 are similar to those of other carbonaceous solid acids, the superior catalytic performance of SGO-3 is mainly due to the synergy between the abundant acid sites and its easily accessible 2D structure.

Table 3 Comparison of the catalytic performance of the SGO-3 with other reported solid acid catalysts

Entry	Catalyst	Catalyst loading (wt%)	Fructose loading (wt%)	HMF yield (%)	Ref.
a C–SO3H is a carbonaceous solid material prepared by incomplete hydrothermal carbonization of cellulose followed by both chemical activation with KOH and sulfonation with H2SO4.^13b β-Cyclodextrin–SO3H is a carbonaceous solid acid catalyst prepared by a one-step hydrothermal carbonization of β-cyclodextrin and p-TSA.^24
1	SGO-3	0.5	5	94	This work
2	SGO-3	0.5	10	88	This work
3	SGO-3	0.5	15	75	This work
4	SGO-3	0.5	20	67	This work
5	MIL-101(Cr)–SO3H	6	10	76	5
6^a	C–SO3H	5	20	71	13
7^a	C–SO3H	1	5	<50	13
8^b	β-Cyclodextrin–SO3H	0.5	1	<50	24

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3.2.5. Recycling of catalyst. To study the recyclability and stability of the catalyst, a recycling experiment was performed for five cycles. In this experiment, fructose dehydration was catalyzed by SGO-3 in DMSO at 120 °C for 60 min. After five recycles, the conversion of fructose and the HMF yield are 85% and 81%, respectively (Fig. 9). This result indicates that the catalyst can be reused with a slight loss of catalytic activity.

Fig. 9 Catalyst recycling. Reaction conditions: fructose (100 mg), SGO-3 (10 mg), DMSO (2 mL), 120 °C, 60 min.

3.3. Conversion of glucose into HMF

Besides fructose, the reaction system was further extended to produce HMF using glucose as feedstock. Under the optimized reaction conditions for fructose, both the conversion of glucose and HMF yield were remarkably inferior. We optimized the reaction conditions by increasing the reaction temperature and reaction time. However, even at 140 °C for 6 h, the yield of HMF is around 30% with a glucose conversion of less than 60%. As commonly considered, isomerization of glucose to fructose is a necessary step in the transformation of glucose to HMF. The low yield of HMF suggests that the catalyst is not capable of catalyzing the isomerization of glucose to fructose (Fig. 10).

Fig. 10 Influence of reaction temperature and time on the dehydration reaction of glucose. Reaction conditions: SGO-3 (10 mg), glucose (100 mg), AMIMCl-DMSO (w/w 1 : 1, 2 mL), 120 °C.

4. Conclusion

GO and SGOs were synthesized using different approaches, and the characterization indicated that SGO bearing abundant SO₃H can be obtained through a one-pot reaction according to a modified Tours method. Using the SGO as a solid acid catalyst, we developed an efficient reaction system for the dehydration of fructose into HMF. A high yield of HMF up to 94% was obtained with a low loading of catalyst. The excellent catalytic activity of SGO is due to the abundant active sites and its 2D structure. In addition, the catalyst exhibits good reusability.

Acknowledgements

This work was supported by the International Joint Research Projects in the Science & Technology Pillar Program of Tianjin, China (13RCGFSF14300), Research Projects in the Science & Technology Pillar Program of Tianjin, China (14TXGCCX00012), Research Projects in the Science & Technology Program of Jinnan District Tianjin, China (2015JNKW0005) and Research Projects in the Science & Technology Pillar Program of Tianjin, China (15JCTPJC63300).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23420h

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