Conversion of glucose into 5-hydroxymethylfurfural by carbonaceous solid acid catalysts loaded with Brønsted acid and Lewis acid in biphasic system

Biomass-derived 5-Hydroxymethylfurfural (HMF) is a vital platform compound for synthesizing biofuel and various high-value chemicals. This work prepared carbonaceous solid acid catalysts with Brønsted acid and Lewis acid using pine biomass as raw materials through chloride salts impregnation, carbonization and sulfonation. The obtained catalysts were characterized by XRD, FTIR, SEM–EDS, BET, and Py-IR. The catalysts were applied to convert glucose into HMF in a biphasic system involving NaCl solution and γ-valerolactone. The results showed that the catalyst of PC-Al-SO3H exhibited larger microspheres and pore sizes compared with the sulfonated catalyst of PC-SO3H without AlCl3 impregnation. Effects of key variables such as reaction temperature, reaction time on conversion of glucose into HMF were examined. By using PC-2Al-SO3H with stronger Lewis acid, glucose conversion and HMF yield achieved 86.53 mol% and 59.62 mol% at 160 °C and 6 h. The cyclic experiments revealed that PC-2Al-SO3H exhibited relatively stable activity.


Introduction
Sustainable biofuel and chemical production using renewable and carbon-neutral biomass resources can cope with fossil resource depletion and global warming [1][2][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][5][6][7][8].Fructose and glucose are both used to produce HMF [9].In mild conditions, fructose can be easily convert to HMF because it contains a five-ring structure similar to HMF [4].However, the cost of fructose Extended author information available on the last page of the article is high [10].Therefore, using cheaper raw materials such as glucose becomes an attractive option for synthesizing HMF [5,11].
The conversion of glucose to HMF can occur in two methods [12,13].The first one is the catalytic isomerization of glucose to fructose with Lewis acid followed by the catalytic dehydration of fructose to HMF with Brønsted acid; The second is the direct dehydration of glucose to produce HMF with Brønsted acid, which is more difficult to occur at low temperature due to the high activation energy [14], For example, Garces et al. [15] used HCl to catalyze glucose degradation at 383 K for 4 h, obtaining a glucose conversion of only 38%.In contrast, the activation energy of glucose conversion to HMF is lower with joint utilization of Brønsted acid and Lewis acid [16].Therefore, the catalytic system consisting 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 high HMF yield because the catalyst is able to thoroughly mix with glucose [11,17,18].However, homogeneous catalysts introduce severe environmental pollution risks, difficult separation and recovery of catalysts 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 catalysts [21][22][23][24][25][26][27].Using an impregnation-calcination method, Shen et al. [25] prepared a sulfated natural attapulgite (ATP)-based catalyst (SO 4 2− /In 2 O 3 -ATP) to catalyze glucose conversion, resulting in 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 produce HMF from glucose, resulting in 65.72% yield of HMF.However, the preparation cost of the above solid catalysts is high.
Biochar can act as a favorable carrier of catalytic sites because it is low-cost, thermally stable, and abundant in oxygen-containing functional groups such as hydroxyl and carboxyl [28][29][30].Mazzotta et al. [31] used TiO 2 and p-toluenesulfonic acid to modify glucose-derived solid carbon and obtained carbon-based catalysts (GluT-sOH-Ti) containing Brønsted acid and Lewis acid catalytic sites.Under the effect of this catalyst, the yield of HMF from glucose achieved 48 mol%, which was increased 38 mol% compared to the value of the case without the catalyst.The above research used TiO 2 which would increase the cost of catalyst.Moreover, previous studies revealed that chloride salts were promising options for introducing Lewis acid to solid catalyst [17,18].However, it is rarely reported in literature that chloride salts are adopted for loading Lewis acid to biochar in order to prepare carbonaceous solid acid catalysts.
The solvent plays a crucial role in the preparation of HMF from glucose.Water is the mostly used solvent since it is eco-friendly and cheap [32].However, in the acidic aqueous phase, HMF yield is easily decreased by side reactions between glucose, HMF, and intermediates that form by-products [33][34][35][36].Xiong et al. [37] used a carbonaceous solid acid catalyst containing Brønsted acid and Lewis acid to catalyze the conversion of glucose in aqueous phase.The generated HMF was largely degraded to LA and the HMF yield was only 4.2 mol%.In addition, Hou et al. [4] 1 3 Conversion of glucose into 5-hydroxymethylfurfural by… suggested that the process of glucose isomerization and fructose dehydration was accompanied by a series of side reactions such as the degradation of HMF to generate LA and formic acid as well as the side reactions of glucose, fructose, and HMF conversion to generate humin and other by-products.
HMF can be produced in higher yield by adopting biphasic solvents containing organic solvents such as tetrahydrofuran (THF) and γ-valerolactone (GVL) [38][39][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 presented better performance 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 revealed that the yield of HMF catalyzed by carbon-based solid acid catalyst (SC-CCA) in biphasic solvent (with GVL: H 2 O volume ratio of 10:1) was 78.1%, in comparison to the yield of HMF only 3.5% in pure water.These results indicate that water/GVL solvent can produce high yield of HMF.Furthermore, biphasic solvents are beneficial to reduce reaction time cased by the poor mass transfer of carbonaceous solid acid catalysts.
This study aims to synthesize carbonaceous solid acid catalysts loaded with Brønsted acid and Lewis acid catalytic sites using pine biomass as raw material via impregnation, carbonization, and sulfonation.Various properties of the catalysts were examined by using XRD, FTIR, SEM-EDS, BET, and Py-IR.A biphasic solvent of NaCl-H 2 O/GVL was used when evaluating the catalytic activity of the prepared catalysts for converting glucose to HMF.Furthermore, the stability of catalyst was investigated without catalyst regeneration during cyclic experiments.

Catalysts preparation
In a typical process, 5 g AlCl 3 •6H 2 O and 5 g pine powder were added into 50 mL 0.5 mol/L H 3 PO 4 solutions.The mixture was impregnated at 60 °C for 48 h and dried at 105 °C for 12 h.Chloride salts such as ZnCl 2 , MgCl 2 , and FeCl 3 were also used for impregnation.After grinding, the solid powder obtained was carbonized for 1 h under N 2 atmosphere at 550 °C.Afterward, the carbonized powder was mixed with 98% H 2 SO 4 (with the mixing ratio of 1 g biochar: 5 mL H 2 SO 4 ), followed by continuous sulfonation at 150 °C for 10 h.After the sulfonation reaction, the mixture was cooled to room temperature and diluted with deionized water.The sulfonated carbon was separated from the mixture and then washed with hot deionized water exceeding 80 °C until the filtrate was neutral and free of sulfate ions.Sulfonated carbon that had been washed were dried at 105 °C to produce carbonaceous solid catalysts with the name PC-M-SO 3 H (M = Al, Zn, Mg, Fe).A similar procedure was also performed without the addition of chloride salts to prepare PC-SO 3 H catalyst for comparison.Furthermore, PC was prepared without chloride salts impregnating and sulfonation, and PC-Al was prepared with AlCl 3 impregnated carbon without sulfonation.To alter the Lewis acid strength of the carbonaceous solid acid catalyst, 10 g AlCl 3 •6H 2 O and 5 g pine powder were mixed to prepare the catalyst of PC-2Al-SO 3 H by the same impregnation and sulfonation method.

Catalysts characterization
An X-ray diffractometer (XRD, PANalytical X'Pert PRO) was used to examine the structural characteristics of prepared carbonaceous solid acid catalysts (λ = 0.1541 nm).The working voltage and current of the X-ray diffractometer were 40 kV and 40 mA, respectively.The patterns were collected with 2θ range from 10-80° at a step of 0.02°.Infrared Fourier transforms spectroscopy (FT-IR, Nicolet iS20) was used to detect the functional groups of catalysts in the range of 400-4000 cm −1 wavenumber.Each sample was mixed with KBr and pressed into tablet.SEM (Zeiss Gemini 500) was used to characterize the surface morphology of catalysts.The element distribution on the surface of catalyst was examined using energy-dispersive X-ray spectroscopy (EDS, Esprit 2.0).Specific surface area and pore structure were determined using nitrogen adsorption-desorption (BET, ASAP 2010).In addition, the strength of Brønsted acid and Lewis acid on the catalyst was determined by using pyridine infrared spectroscopy (Py-IR, Tensor 27).The detailed method for calculating acid strength was provided in the supporting information.

Catalytic activity test
HMF was converted from glucose using a heating reactor with temperature control and magnetic stirring (DF-101 s).In a typical catalytic experiment, the mass ratio of glucose to catalyst was 2:1.0.2 g glucose, 0.1 g catalyst, 0.8 g NaCl, 5 mL deionized water, and 10 mL GVL were added into a test tube.Reactions were carried out at temperature ranging from 145 to 170 °C and at reaction time ranging from 3 to 9 h.The test tube was cooled rapidly with a fan to room temperature as soon as the reaction was completed.After that, a organic filter membrane of 0.22 μm pore size was used to separate the solid catalyst.To prepare the sample for analysis, the organic and aqueous phases were separated and diluted ten-fold with water.

Product analysis
High-performance liquid chromatography (HPLC, Agilent 1200) was used to quantitatively analyze glucose, fructose, HMF, and LA.A differential refractive 1 3 Conversion of glucose into 5-hydroxymethylfurfural by… detector and an ultraviolet detector were both used.A column of BioRad HPX-87H (7.8 mm × 300 mm) was adopted.The mobile phase was 5 mmol/L H 2 SO 4 with a flow rate of 0.5 mL•min −1 .The temperature of the column and detectors was 60 °C.Each sample with 25 μL was injected and completely eluted within 60 min.Furthermore, glucose and fructose peak areas were detected by differential refractive signals.To avoid the interference of organic solvent GVL on the HMF yield, the contents of HMF and LA were calculated using the intensity of the ultraviolet signal.Typical HPLC chromatograms collected in this work were also provided in supporting information.The following formulae were used to calculate glucose conversion, product yield and carbon balance [43]:

Catalyst characterization
The XRD patterns of PC-SO 3 H and PC-Al-SO 3 H are shown in Fig. 1.The (002) plane diffraction peak at 23° correspond to non-oriented structural properties of carbon in the catalyst [19].As the degree of non-orientation increases, the diffraction peak of the (002) plane is broadened [44].Therefore, AlCl 3 -impregnated catalyst had lower degree of non-orientation.The (101) plane weak diffraction peak at 43° correspond to the graphitization of carbon material [45].Graphitization of carbon in carbonaceous catalysts was beneficial to improve the thermal stability.Furthermore, the characteristic peaks of AlPO 4 peaks appeared at 20°, 26°, and 36° for the PC-Al-SO 3 H catalyst, indicating that AlCl 3 was dissolved in H 3 PO 4 solution resulted in the yield of AlPO 4 [37].
The FT-IR spectra of carbon materials and carbonaceous solid acid catalysts are shown in Fig. 2. All samples presented diffraction peaks at 3434 cm −1 [46] which indicated the presence of -OH groups on those catalysts.The diffraction peaks at 2930 cm −1 , 850 cm −1 , and 760 cm −1 with lower intensity were corresponded to stretching vibrations of the C-H bond [47].The diffraction peaks at 1710 cm −1 and 1612 cm −1 corresponded to C=O and C=C functional groups, respectively [48].Similarly, a characteristic peak for O=S=O functional group was observed in both PC-SO 3 H and PC-Al-SO 3 H [19], the 1108 cm −1 and 620 cm −1 was the characteristic peak of Al-O-P and C-S, respectively [37], which indicating that SO 3 H groups were successfully introduced by sulfonation with H 2 SO 4 .
The SEM images of PC-SO 3 H and PC-Al-SO 3 H catalysts at different magnitudes are shown in Fig. 3.Both catalysts exhibited microspheres and irregular pore structures.Compared with PC-SO 3 H catalyst, the PC-Al-SO 3 H catalyst exhibited larger microspheres and pore sizes.In addition, Fig. 4 shows the EDS spectra of the Fig. 1 The XRD patterns of prepared carbonaceous solid acid catalysts Fig. 2 The FT-IR of prepared carbonaceous solid acid catalysts and intermediates 1 3 Conversion of glucose into 5-hydroxymethylfurfural by… catalysts.The S element was detected in PC-SO 3 H catalyst, while S and Al were found in PC-Al-SO 3 H catalyst.The results were consistent with those obtained from XRD and FTIR characterization.
The N 2 adsorption-desorption characterization of PC-SO 3 H and PC-Al-SO 3 H catalyst is shown in Fig. 5a.It was found that the curves of PC-SO 3 H catalyst were the type of I isotherm [49], while those of PC-Al-SO 3 H catalyst was the type of IV isotherm [50].Figure 5b shows the pore size distribution of the catalysts.The majority of the PC-SO 3 H catalyst pores were less than 5 nm, indicating that the catalyst was rich in micropores.However, PC-Al-SO 3 H catalyst had micropores, mesopores, and macropores, which could be evidenced by its pore size distribution.Table 1 shows the BET surface area, pore volume, and average pore size for different catalysts.The specific surface area, total pore volume, and average pore size of PC-SO 3 H catalyst were 990.1 m 2 g −1 , 0.5 cm 3 g −1 , and 2.6 nm, respectively.The activation effect of H 3 PO 4 effectively increases the specific surface area of carbon materials [51].PC-Al-SO 3 H catalyst which was prepared by impregnation in presence of H 3 PO 4 presented smaller specific surface area (12.4 m 2 g −1 ) and pore volume (0.02 cm 3 g −1 ) than PC-SO 3 H catalyst.The reason was when H 3 PO 4 reacted with metal salts without activation, the metal compound would block the pore structure, which decreased specific surface area and pore volume [52].However, the average pore size of PC-Al-SO 3 H catalyst was 7.3 nm, which also proved that the catalyst mainly had mesoporous being beneficial to improve the catalyst's permeability and conversion efficiency [53].
The Py-IR spectra of PC-SO 3 H, PC-Al-SO 3 H, and PC-2Al-SO 3 H catalysts are shown in Fig. 6.The diffraction peaks at 1540 cm −1 and 1643 cm −1 were corresponded to Brønsted acid, while those at 1445 cm −1 , 1575 cm −1 and 1606 cm −1 were corresponded to Lewis acid [54][55][56].In addition, Brønsted acid and Lewis acid had the common diffraction peak at 1490 cm −1 [31,57].PC-SO 3 H catalyst only contained Brønsted acid, while PC-Al-SO 3 H and PC-2Al-SO 3 H catalysts contained both Brønsted acid and Lewis acid.The strength of Lewis acid and

Conversion of glucose into HMF
Figure 7 shows the effects of reaction temperature (145-175 °C) on glucose conversion and HMF yield using PC-Al-SO 3 H catalyst.The glucose conversion increased from 37.09 to 97.17 mol% with increasing reaction temperature from 145 to 175 °C.However, the yield of HMF first increased and then decreased.The maximum HMF yield was 45.40 mol% with the LA yield of 31.31mol% at 160 °C.At 175 °C, the HMF yield decreased to 25.57 mol% and the LA yield increased to 45.79 mol%.High temperature could accelerate the degradation of HMF and produce various byproducts [16,58].The results of this work showed that PC-Al-SO 3 H catalyst was able to achieve high yield of HMF at 160 °C. Figure 8 shows the effects of reaction time (3-9 h) on glucose conversion and HMF yield using PC-Al-SO 3 H catalyst.With increasing reaction time from 3 to 9 h, glucose conversion increased from 59.42 mol% to 94.95 mol%.These results indicated that it took a relatively long time to achieve complete conversion of glucose.However, the extended reaction time would cause low HMF yield.When the reaction time was 6 h, the largest HMF yield of 45.40 mol% was achieved.When the reaction time was extended to 9 h, the yield of HMF decreased to 29.49 mol% while the yield of LA approached 54.23 mol%, indicating that most HMF would degrade into LA when reaction time was extended.The optimal reaction time should be determined by considering both glucose conversion and HMF yield.On the one hand, Extended reaction time was required when using PC-Al-SO 3 H catalyst because glucose was relatively stable at 160 °C and the mass transfer of solid catalyst was relatively poor [59].In the other hand, the reaction time should also be limited to obtain high yield of HMF and avoid HMF degradation to LA and Fig. 7 The effect of reaction temperature on the conversion of glucose to HMF.Reaction conditions: 0.2 g glucose, 0.1 g catalyst, 0.8 g NaCl, 5 mL water, 10 mL GVL, and 6 h Fig. 8 Effect of reaction time on glucose conversion into HMF.Reaction conditions: 0.2 g glucose, 0.1 g catalyst, 0.8 g NaCl, 5 mL water, 10 mL GVL, and 160 °C 1 3 Conversion of glucose into 5-hydroxymethylfurfural by… the yield of other by-products [14,15].The recommended conditions for converting glucose to HMF with PC-Al-SO 3 H were 160 °C 6 h in this work.
Table 2 shows the main products and carbon balance in the conversion of glucose to HMF catalyzed by different types of carbonaceous solid acid catalysts.In the absence of catalyst, the conversion of glucose was only 22.22 mol% and the yield of HMF was only 11.78 mol%.When using PC-SO 3 H catalyst containing Brønsted acid, the glucose conversion and HMF yield were significantly increased to 56.54 mol% and 32.82 mol%, respectively.The main reason was that Brønsted acid could catalyze the dehydration of glucose to generate HMF directly [60].It should be noted that in the tests using PC and PC-SO 3 H catalysts, no fructose was detected in the products.However, when PC-Al-SO 3 H and PC-2Al-SO 3 H catalysts that were produced by AlCl 3 impregnation were used, fructose was detected in the products of glucose hydrothermal conversion, and the corresponding glucose conversion and HMF yield were further improved.Under the effect of PC-2Al-SO 3 H catalyst, the glucose conversion and HMF yield were as high as 86.53 mol% and 59.62 mol%, respectively.The above results were closely related to the introduction of Lewis acid by AlCl 3 impregnation.The introduction of Lewis acid is beneficial for the catalytic isomerization of glucose to fructose [61], allowing glucose to be converted first to fructose and then to produce HMF from fructose.The catalytic performance of PC-2Al-SO 3 H catalyst was better than that of PC-Al-SO 3 H catalyst, which was also the result of introducing stronger Lewis acid. Figure 9 provides a mechanism diagram for glucose conversion to HMF catalyzed by catalysts coupled with Brønsted acid and Lewis acid.Compared to the traditional pathway where glucose was directly dehydrated to produce HMF under the effect of Brønsted acid (Route a), glucose could react via the other pathway (Route b) where glucose was first isomerized to fructose in the presence of Lewis acid and then the produced fructose was converted to HMF under the effect of Brønsted acid by taking advantage of PC-Al-SO 3 H/PC-2Al-SO 3 H catalysts in this work.The reaction conditions of Route b were relatively mild which was beneficial to reduce the activation energy of the reaction for glucose conversion to HMF [14,62].Table 2 also indicated that the main products generated by glucose conversion were fructose, HMF, and LA.When PC-SO 3 H catalyst was used, the carbon balance is 84.08% and some glucose was converted to other by-products.When PC-2Al-SO 3 H catalyst was used to catalyze glucose conversion, the carbon balance approached 99.27% and the amount of other by-products was very small.Overall, the proportion of carbon converted from glucose to fructose, HMF, and LA platform molecules was relatively high in this work.
Table 2 further compares the catalytic effect of PC-Al-SO 3 H catalyst with those of PC-Zn-SO 3 H, PC-Mg-SO 3 H, and PC-Fe-SO 3 H catalysts which had been prepared by impregnating different chloride salts.Among these four types of catalysts, PC-Al-SO 3 H catalyst achieved the highest glucose conversion and HMF yield under the same reaction conditions.Therefore, it could be concluded that AlCl 3 was more suitable for preparing the carbonaceous solid acid catalyst within the scope of this study.
Table 3 compares the typical results of this work with those in literature for glucose conversion and HMF yield.All experiments corresponding to the data shown in Table 3 had been conducted in biphasic system composed of water and organic solvent.Compared with previous studies using homogeneous catalysts [17], the PC-2Al-SO 3 H catalyst used in this study could achieve competitive glucose conversion or HMF yield and could be easily separated, recovered and recycled from the liquid phase.Compared with previous studies using other solid acid catalysts [25,31,63], the PC-2Al-SO 3 H solid acid catalyst used in this study adopted pine wood char which not only helped to reduce the cost of catalyst but also achieved higher HMF yield even under mild condition with lower reaction temperature.

Multicycle performance of the carbonaceous solid acid catalyst
The stability of catalysts during multicycle tests is significant to reduce the cost for HMF production.The cyclic glucose hydrothermal conversion tests were conducted by using PC-2Al-SO 3 H catalyst.After each reaction of the multicycle experiments, the catalyst was firstly separated by filtration, washed with massive deionized water and then dried at 105 °C for 12 h for cyclic utilization.The results are shown in Fig. 10.The PC-2Al-SO 3 H catalyst presented good stability.Conversion of glucose into 5-hydroxymethylfurfural by… After 5 cycles, the yield of HMF decreased by less than 5 mol% from 59.62 to 54.82 mol%.In addition, glucose adsorption and HMF by-product detachment from catalyst surfaces could lead to catalyst deactivation [19,64], resulting in the decrease of glucose conversion and HMF yield during multicycle tests.

Fig. 3
Fig. 3 SEM images of prepared carbonaceous solid acid catalysts.a, b, and c were for PC-SO 3 H catalyst; d, e, and f were for PC-Al-SO 3 H catalyst

Fig. 5 a
Fig.5a N 2 adsorption-desorption isotherms of prepared catalysts, each material's adsorption isotherm is referred as "ads" while desorption isotherm is referred as "des".b BJH pore size distribution of prepared catalysts

Fig. 6
Fig. 6 FTIR spectra following the adsorption of pyridine on various catalysts: a PC-SO 3 H, b PC-Al-SO 3 H, c PC-2Al-SO 3 H

Fig. 9
Fig.9 Cooperation effect of heterogenous catalyst for the conversion of glucose to HMF

Table 1
Physical properties and acid density of synthesized catalysts Conversion of glucose into 5-hydroxymethylfurfural by…Brønsted acid for different catalysts was shown in Table1.PC-SO 3 H catalyst only presented Brønsted acid site with the strength of 5.99 μmol g −1 .PC-Al-SO 3 H catalyst showed Lewis acid and Brønsted acid sites with strength of 19.35 μmol g −1 and 3.02 μmol g −1 , respectively.Compared with PC-Al-SO 3 H catalyst, PC-2Al-SO 3 H catalyst presented stronger Lewis acid and weaker Brønsted acid with strength of 27.68 μmol g −1 and 1.19 μmol g −1 , respectively.

Table 3
Glucose conversion and HMF yield: a comparison of this work with other related studies a With the addition of 20wt% NaCl in the aqueous phase