Direct degradation of cellulose to 5-hydroxymethylfurfural in hot compressed steam with inorganic acidic salts

Abstract

A novel method of direct degradation of cellulose into 5-hydroymethylfurfural (HMF) in hot compressed steam was introduced, with the inorganic acidic salts (NaHSO₄, KHSO₄, NaH₂PO₄ and KH₂PO₄) as catalysts. The water molecules in the steam were absorbed by the catalysts to form an acidic aqueous layer on the surface of the cellulose, where the cellulose was converted into HMF and spread into the gas phase. The relative humidity of steam could influence the reaction route by controlling the acidity of the aqueous layer. Low relative humidity of steam was favoured for the carbonization of cellulose, while high relative humidity was preferred for hydrolysis-dehydration of cellulose to form HMF. A moderate HMF yield of 30.4 mol% was obtained with NaH₂PO₄ as the catalyst. This novel methodology demonstrated an efficient and green HMF production from cellulose, without organic solvents and toxic transition metal cations.

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1 Introduction

Conversion of renewable biomass into liquid fuels and value-added chemicals has attracted world widely attention.¹ 5-hydroxymethylfurfural (HMF), which could be obtained from the abundant lignocellulose, is considered as a key platform for production of liquid fuels, polyester monomers, organic solvents and multiple fine chemicals.²

Though high HMF yields could be obtained from monosaccharides such as glucose³ and fructose, the recalcitrance of cellulose and the instability of HMF⁴ make efficient production of HMF from lignocellulosic biomass a significant challenge. In recent years, great efforts have been focused on developing of novel reaction media and catalyst for HMF production from cellulose. Systems employing ionic liquids or polar aprotic solvents as media and Cr-contained material as catalysts, show excellent performance for this process.⁵ For example, Binder et al. reported CrCl₂–HCl could efficiently catalyze cellulose into HMF with 54% yield in DMA-LiCl solvent.^5a Ding et al. reported a high HMF yield of 69.7% from cellulose in the ionic liquid solvent with the aid of CuCl₂.⁶ Water-containing biphasic systems are another widely studied media for HMF production.⁷ By extracting HMF in the aqueous phase into the organic phase to suppress its further degradation, we obtained a HMF yield of 53% from cellulose in a biphasic system, employing NaHSO₄–ZnSO₄ as co-catalyst.⁸ Recently, Dumesic and his co-workers focused on developing the biomass-derived solvent (gamma-lactones⁹ and alkylphenol¹⁰) as organics solvent in the biphasic system to eliminate the need of petroleum derived solvent. However, these systems (ionic liquid, polar aprotic solvents and biphasic systems) employ organic solvents as the reaction medium and/or toxic metal chlorides as catalysts, posing the negative impacts on economics and the environment.¹¹ HMF production without use of organic solvents and toxic catalysts is more feasible for industrialization. Asghari reported a HMF yield of 32% from cellulose in hot compressed water with the aid of a phosphate buffer.¹² Deng et al. reported a moderated HMF yield of 30% from cellulose in concentrated aqueous ZnCl₂ solution, with HCl as a catalyst.¹³ Zhao et al. developed a novel catalyst (HPA)Cr[(DS)H₂PW₁₂O₄₀]₃ for the production of HMF from cellulose with a high yield of 53% in water as the solvent.¹⁴

Considering the volatility of HMF, the degradation of HMF could be suppressed by transferring the produced HMF into the gas phase. Previously Wei et al. reported a novel method of carrying the gaseous HMF away from the ionic liquid by entrainers (nitrogen, hexane or MIBK).¹⁵ On the other hand, the H⁺ offered by acidic inorganic salts (NaHSO₄, KHSO₄, NaH₂PO₄ and KH₂PO₄) show excellent activity for hydrolysis of cellulose,¹⁶ which is the rate-determining step of HMF production from cellulose.¹⁷ Motivated by these inspirations, we conceived of a process of one-pot transformation of cellulose into HMF in hot compressed steam, with the cheap and green inorganic acidic salts as catalysts. The hot compressed steam and carrier gas (N₂) are conducted into the reactor containing the cellulose–acidic salt mixture. The steam and the acidic salts could convert the cellulose into HMF, and the produced HMF is carried away by the steam to prevent its further degradation. The non-volatile catalysts are left in the solid residue. By this means, organic solvents and toxic catalysts are avoided in our process.

2 Experimental section

2.1 Chemicals

α-cellulose (180 μm), HMF (98%), furfural (FF) (≥99%) and levulinic acid (≥99%) were obtained from Shanghai crystal pure reagent Co., LTD and used without further purification. Analytical grade NaHSO₄, KHSO₄, NaH₂PO₄, KH₂PO₄ were purchased from Tianjin Fu Chen chemical reagent factory. Ultrapure water was used in all the experiments.

2.2 Apparatus and method

The degradation of cellulose was conducted in a homemade experimental device consisting of a high pressure metering pump (Dalian Elite Analytical Instruments Co., Ltd, China), a steam generator (Tianjin Xianquan industry &trade development Co., Ltd, China), a stainless steel tube reactor (20 mm inner diameter), a homemade condenser and liquid–gas separator. (Fig. 1).

Fig. 1 Experimental setup for degradation of cellulose in hot compressed steam.

In a typical experiment, 0.5 g of cellulose–catalyst mixture (mechanically mixed in a mortar with the catalyst loading of 10 wt%) was loaded into the reactor. High purity N₂ (99.99%) was used both to pressurize the reaction system and to carry the volatile products away. The flow rate of the carrier gas was controlled by a mass flow controller (Beijing Huibolong instrument Co., Ltd, China). The reactor was heated to the target temperature and kept at that temperature till the end of reaction. The moment the target temperature was achieved, the steam generated in the steam generator was conducted into the reactor to initiate the reaction. The flow rate of the steam was controlled though the feed rate of water by the pump. Volatile products were vaporized and carried away from the reactor and condensed in a condenser by an ice-water mixture, then separated with the carrier gas in a gas–liquid separator. After the reaction, solid residues were collected, washed with deionized water, dried at 105 °C for 5 h and weighted by an electronic scale. The catalysts were recycled by washing the solid residue and evaporating the filtrate. The qualitative and quantitative analyses of liquid products were conducted on GC-MS and HPLC instruments, respectively.

2.3 Analytical methods

The qualitative analysis of HMF, FF and other volatile products were conducted on a GC-MS instrument (HP5890; MSD, HP5972A) equipped with an INNOWAX capillary column (30 m × 0.32 mm × 0.25 μm). The oven temperature was retained at 60 °C for 2 min, then ramped to 220 °C at 20 °C min⁻¹ and held at this final temperature for 10 min.

The mass concentrations of HMF and FF in the final products were quantified by HPLC equipped with a Hewlett Packard 1050 pump, a Waters XbridgeTM C18 (3.5 μm, 4.6 × 150 mm) column and a UV detector (284 nm). The mobile phase was set at 3 : 7 v/v methanol : water at a total flow rate of 0.50 ml min⁻¹. Quantification of products was determined by the external standard method based on the peak area of each product. All the samples were analyzed without further dilution.

The yield of product i was determined by the following formula, assuming the molecular weight of the constructed D-glucoside unit of cellulose being 162:

2.4 Calculation of relative humidity of the steam

The flow rate of the carrier gas and steam was calculated under the standard conditions of 0.1 MPa, 25 °C. The flow rate of steam (v_steam) was calculated by the following equation:
v_water is the feed rate of water to the steam generator by the high pressure metering pump. The steam partial pressures of the reaction system (P_steam) were calculated by the following equation:
P_reaction is the pressure of the reaction system, v_water and v_{carrer gas} are the flow rate of steam and carrier gas, respectively.

The relative humidity (RH) of the steam was calculated though the following equation:

P_{saturated vapor} is the saturated vapor pressure of water at the corresponding temperature.

2.5 TG analysis of cellulose and salts under steam

The TG analysis of samples under water steam were conducted on a STA449F3 thermal analyzer at atmospheric pressure (0.1 MPa). Steam was conducted into a thermogravimetric chamber after the chamber reached 150 °C for 5 min, and the chamber was kept at 150 °C for 60 min.

2.6 Characterization of solid residue

Elemental analyses of cellulose and the solid residues were conducted on an elemental analyzer (vario-ELI) with a thermal conductivity detector (TCD), and helium was used as the carrier gas.

Structures of the cellulose and the solid residues were analyzed by Fourier transform infrared (FT-IR) spectroscopy (TENSOR27) with a resolution of 4 cm⁻¹. Samples were prepared by mixing the sample powders with KBr and compacting into slices.

3 Results and discussion

3.1 Degradation of cellulose in hot compressed steam with NaHSO₄

3.1.1 Influence of relative humidity of steam on cellulose degradation. Table 1 lists the results of degradation of cellulose in hot compressed steam at the fixed temperature of 210 °C with NaHSO₄ as the catalyst. 95.3 wt% of unreacted cellulose was collected after 180 min with the absence of the catalyst, indicating that the steam has little activity on hydrolysis of the cellulose in such conditions (Table 1, entry 1). Addition of NaHSO₄ greatly increased the reaction rate, meanwhile, the weight of solid residues decreased to less than 50 wt%. However, the HMF yields were not efficiently increased. We found that the relative humidity (RH) of steam also played a key role on degradation of cellulose. Under low relative humidity of 0.016 and 0.037 (generated by low reaction pressure or low steam feed), only negligible yields of HMF were formed (Table 1, entries 2–3), accompanied with multiple by-products detected by GC-MS (Fig. 2a–b) and over 40 wt% of black solid residues. Compared to the cellulose, the residues had a high carbon content of 60 wt% and low oxygen content of 35 wt% (Table 2). These results indicated that degradation of cellulose underwent carbonization/pyrolysis route under these conditions. The furfural was proposed to be formed by pyrolysis of cellulose. With an increase in the relative humidity of the steam, HMF became the main product (Fig. 2c) while the residue weight decreased (Table 1, entries 3–8), suggesting that degradation of cellulose changed from a carbonization route to a hydrolysis route. Modest HMF yields of 21 mol% were approached under the relative humidity of 0.485 (Table 1, entry 8). However, further increasing the relative humidity of steam to 0.855, the HMF yield decreased, accompanied with the increase of the weight of the unreacted cellulose (Table 1, entries 9–10). No levulinic acid was detected in all the reactions, suggesting that the rehydration of HMF was efficiently suppressed.

Table 1 The influence of relative humidity of steam on the degradation of cellulose in hot compressed steam^a

Entry	Preaction (MPa)	Water feed (ml min^−1)	Psteam^b (MPa)	RH	Yield (mol%)		Solid residue (wt%)
					HMF	FF
a Reaction conditions: temperature 210 °C; cellulose, 0.5 g; carrier gas flow rate, 500 ml min^−1; reaction time, 180 min; NaHSO4 as the catalyst with a loading of 10 wt%.b Steam partial pressure.c No catalyst was employed.
1^c	1.3	1.0	0.93	0.485	<1.0	<1.0	95.3
2	1.5	0.01	0.03	0.016	<1.0	1.1	43.7
3	0.1	1.0	0.07	0.037	<1.0	2.3	42.6
4	1.5	0.1	0.30	0.156	5.3	1.4	39.8
5	0.5	1.0	0.36	0.188	6.9	<1.0	33.8
6	1.0	1.0	0.71	0.370	18.6	<1.0	29.7
7	1.5	0.5	0.83	0.433	20.2	<1.0	28.4
8	1.3	1.0	0.93	0.485	21.4	<1.0	26.9
9	1.5	1.0	1.07	0.557	19.1	<1.0	28.1
10	2.3	1.0	1.64	0.855	10.4	<1.0	51.3

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Fig. 2 GC-MS profile of the liquid products collected from catalytic degradation of cellulose. Reaction conditions: (a) 210 °C, 60 min, 10 wt% NaHSO₄ as the catalyst, steam relative humidity of 0.037; (b) 210 °C, 60 min, 10 wt% NaHSO₄ as the catalyst, steam relative humidity of 0.016; (c) 210 °C, 60 min, 10 wt% NaHSO₄ as the catalyst, steam relative humidity of 0.483; (d) 280 °C, steam relative humidity of 0.196, 60 min, NaH₂PO₄ as the catalyst with a dosage of 10 wt%.

Table 2 Elemental analysis of cellulose and solid residues under different relative humidities of steam^a

Material	RH	Elemental percentage (wt%)
		C	H	O
a The solid residues are collected at 210 °C after 180 min under different relative humidities of steam catalyzed by NaHSO4.
Pure cellulose	—	44.1	5.9	50.0
Residue (a)	0.016	60.3	4.5	35.3
Residue (b)	0.037	58.4	4.6	37.1
Residue (c)	0.485	47.1	5.8	44.1
Residue (d)	0.855	44.7	6.1	49.2

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The relative humidity of steam also had great impact on the structure of solid residues. Elemental analysis indicated that the solid residues collected under the conditions of low relative humidity had a higher carbon content and a lower oxygen content compared to those obtained under high relative humidity (Table 2). The carbon content in all residues obtained here (44–60 wt%) were lower than the chars obtained from cellulose in the hydrothermal conditions.¹⁸ In the FT-IR spectrum of the solid residues, the absence of peaks at 1604, 1510 and 1395 cm⁻¹ indicated that no furan rings existed in all residues (Fig. 3), meaning the formation of humins from HMF was suppressed.¹⁹ The absorption peak at 1706 cm⁻¹ associating to the C=O bond suggested that char existed in the residues formed under low relative humidity of 0.037 and 0.483 (Fig. 3).^18,20 The formation of the black char was due to the carbonization/pyrolysis of cellulose under those conditions.²¹ Contrary to the residues obtained under low relative humidity, the presence of peaks from 1200 cm⁻¹ to 1050 cm⁻¹ indicated that the residue obtained under a relative humidity of 0.855 mainly contained unreacted cellulose.

Fig. 3 FT-IR spectra of cellulose and solid residues collected under different relative humidities of steam. Reaction conditions: 210 °C, 180 min, 10 wt% NaHSO₄ as the catalyst.

The influence of the relative humidity of steam on the degradation of cellulose was explained by assuming an acidic aqueous layer formed on the cellulose. The Cellulose–Water Adsorption Isotherm had been studied decades ago.²² Generally, the adsorption isotherm was divided into three regions. For the low relative humidity of less than 10%, adsorption of moisture is caused by the strong hydrogen bonding of water molecules with primary alcoholic groups of cellulose. With a relative humidity of between 10% and 50%, the adsorption of moisture is caused by the weak hydrogen bonding of water molecules and secondary alcoholic groups of cellulose. With a relative humidity of over 50%, the adsorption of water molecules were ascribed to the swelling of the cellulose and the condensation of “free” or “unbound” water in the coarser capillaries. When the cellulose–salt mixture was exposed to the steam, both the cellulose and the salts could adsorb the water molecules. So the salts could be dissolved by the adsorbed water molecules to form an acidic aqueous layer on the surface of the cellulose. Hydrolysis of cellulose and dehydration of monosaccharides could take place in the acidic aqueous layer to form HMF, while the produced HMF could spread into the gas phase and be carried away by the steam to prevent its further degradation (Fig. 4). The acidity of the aqueous layer was controlled by the concentration of the catalyst in the aqueous layer, which was greatly influenced by the relative humidity of the steam. The quality of water molecule adsorption increases with increasing relative humidity of steam. For the case of low relative humidity of the steam, fewer water molecules were absorbed, leading to strong acidity of the aqueous layer, resulting in carbonization of cellulose to form black char and multiple by-products. On the contrary, more water molecules were absorbed by the cellulose and NaHSO₄ under the high relative humidity conditions, leading to weak acidity of the aqueous layer, which could favour the hydrolysis of cellulose.

Fig. 4 Schematic diagram of the acidic liquid layer formation on the surface of the cellulose.

The existence of an acidic aqueous layer was proved by TG analysis of salts and cellulose under steam at 150 °C (Fig. 5). The moment the steam was conducted into the thermogravimetric chamber, a mass increase was observed when only cellulose or NaHSO₄ was loaded, indicating that water molecules in the steam were trapped by the salt and cellulose, respectively. Because the melting point of NaHSO₄ is only 58 °C, the NaHSO₄–water mixture must be present in the liquid phase at such a high temperature of 150 °C. Then the mass is kept at a high level, suggesting that the water molecules were in a dynamic balance between the gas phase and the liquid phase. For the case of the cellulose–NaHSO₄ mixture loading in the chamber, a mass decrease was observed, which was due to the strong acidity of the NaHSO₄ being able to directly catalyze the dehydration/carbonization of cellulose. Black carbon was formed after the TG analysis of the cellulose–NaHSO₄ mixture.

Fig. 5 TG profile of different materials in the atmosphere of steam at 150 °C. Water vapor was conducted into the thermogravimetric chamber at the 5^th minute.

The amount of SO₄²⁻ in the liquid products was detected by ion chromatography analysis (IC). Under the relative humidity of 0.485 (corresponding to Table 1, entry 8), only 1.4 wt% of the initially introduced SO₄²⁻ was detected in the liquid products, indicating that most of the catalyst remained in the solid residue. By washing the solid residues with water and evaporating the aqueous solvent, we separated the acidic salts. This is a significant advantage compared to the reaction systems conducted in solvents with mineral acids or chromium-containing salts as the catalyst, for the products need to be neither separated from the catalyst nor neutralized after the reaction. Comparatively, 83.6% of whole SO₄²⁻ presented in the liquid products under the high relative humidity of 0.855 (corresponding to the reaction conditions in Table 1, entry 10), meaning that a great deal of NaHSO₄ was washed away under that condition.

3.1.2 Influence of reaction time, reaction temperature, flow rate of carrier gas and catalyst dosage. Above results indicated that relative humidity of the steam played a critical role in the degradation of cellulose in the hot compressed steam. We further studied the impact of other parameters such as reaction time, reaction temperature and carrier gas flow rate under the steam relative humidity of near 0.5.

The weight of the solid residue and the yield of HMF via reaction time are shown in Fig. S1.† HMF yield increased quickly in the first 90 min. After 90 min, the yield only increased a little, indicating that most of the cellulose was converted in 90 min. Correspondingly, over 25 wt% of the black char was collected after 180 min, which is much higher than that of the 2 wt% obtained in the hot compressed water,¹² suggesting that the carbonization process would be more favoured in the steam.

The influence of reaction temperature on cellulose degradation in hot compressed steam is shown in Fig. S2.† Increasing the temperature from 190 °C to 210 °C, the HMF yield increased from 15.9 to 20.5 mol%, but going on increasing the temperature to 230 °C, the HMF yield only slightly increased to 21.3 mol%, suggesting that the NaHSO₄ was active enough at the temperature of 210 °C.

Attempts of improving the HMF yield by adjusting the carrier gas flow rate was conducted (Fig. S3†). The HMF yields increased with the increase of the gas flow rate from 100 ml min⁻¹ to 500 ml min⁻¹, but going on increasing the gas flow rate from 500 ml min⁻¹ to 1500 ml, the HMF yield changed slightly, meaning that 500 ml min⁻¹ of carrier gas flow rate had been efficient enough for carrying the HMF molecules away.

The impact of catalyst dosage on HMF production is shown in Fig. S4.† The HMF yield was almost independent of catalyst dosage, increasing the catalyst loading from 5 wt% to 20 wt%, the HMF yield only slightly increased by 3 mol%. However, going on increase the catalyst dosage to 30 wt%, the HMF yield decreased, which was ascribed to that high thickness of aqueous layer blocking the diffusion of HMF molecules to the gas phase.

3.2 Degradation of cellulose with other inorganic acidic salts

Other inorganic acidic salts such as KHSO₄, NaH₂PO₄ and KH₂PO₄ also showed excellent activity on HMF formation from cellulose in a similar process. The performance of KHSO₄ was similar to that of NaHSO₄ (Table 3, entry 1), while the dihydric phosphate salts needed to be operated at relatively higher temperature. Because the intrinsic acidities of the dihydric phosphate salt species are much weaker than those of the sulfate acidic salt species, low reaction temperature of 210 °C and 230 °C led to large amounts of unreacted cellulose with the presence of NaH₂PO₄ (Table 3, entries 2–3). By increasing the reaction temperature, the cellulose degradation rate increased significantly. A 30.4 mol% of HMF yield accompanied with a low solid residue of 17.9 wt% was obtained with the condition of 280 °C and 1.7 MPa (Table 3, entry 5). The HMF yield with NaH₂PO₄ at a high temperature was higher than that obtained with NaHSO₄ as the catalyst at a low temperature, which was ascribed to the fact that a high temperature could be more favourable for the crystalline-to-amorphous transformation of cellulose.²³ A small amount of 5-methylfurfural were also detected (Fig. 2d), which was proposed to be formed by dehydroxylation of HMF. Compared with other works on HMF production from cellulose in a single water solvent, the yields obtained in our cases are comparable with the previously reported 30 mol% obtained in the concentrated ZnCl₂ solvent with HCl as the catalyst²⁴ and the 32 mol% obtained in the subcritical water,¹² but much lower than the 53 mol% obtained with a surfactant-combined heteropolyacid catalyst (HPA)Cr[(DS)H₂PW₁₂O₄₀]₃.¹⁴ However, in our case, an efficient and green HMF production process could be approached without the use of an organic solvent, or erosive and toxic metal chlorides and mineral acids.

Table 3 The performance of acidic salts as catalysts^a

Entry	Catalyst	T (°C)	PReaction (MPa)	RH	HMF (mol%)	FF (mol%)	Residue (wt%)
a Except for special state, the reaction conditions were as follows: cellulose, 0.5 g; water feed, 1.0 ml min^−1; carrier gas flow rate, 500 ml min^−1; reaction time, 180 min; catalyst loading, 10 wt%.
1	KHSO4	210	1.3	0.485	19.7	<1.0	30.7
2	NaH2PO4	210	1.3	0.485	9.1	<1.0	62.4
3	NaH2PO4	230	1.3	0.329	13.7	3.2	41.6
4	NaH2PO4	260	1.3	0.196	25.2	3.7	19.2
5	NaH2PO4	280	1.8	0.200	30.4	5.3	17.9
6	KH2PO4	260	1.3	0.196	22.4	4.9	20.8

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As has been shown above, only around 30% of carbon in the reactant was converted to HMF. The carbonization of cellulose to form a solid residue and gaseous products was responsible for the low HMF yield. The carbon in the solid and liquid products accounted for only about 60 wt% of the total carbon in the substrate, indicating that about 40% of carbon was converted into gaseous products. Due to the low partial vapor pressure of other byproducts in the steam, condensation and identification of these products failed.

3.3 Proposed mechanism for isomerization of glucose into fructose with a Brønsted acid

Generally, isomerization of glucose to fructose is regarded as the main obstacle for HMF production from the glucose-based substrates,²⁵ while transition metal chlorides, such as CrCl₃, SnCl₄, ZnCl₂, GeCl₄, AlCl₃ are all reported to be efficient for this process.^13,25,26 However, only a small amount of literature has reported HMF production from glucose-based substrates with a single Brønsted acid.^12,27 Here, direct conversion of cellulose into HMF by a Brønsted acid was obtained, indicating that the proton H⁺ could also efficiently catalyze such an isomerization reaction. The possible mechanism for isomerization of glucose into fructose by proton catalysis is proposed (Scheme 1). Firstly, the oxygen atom in the pyranoid ring of glucose is attacked by a proton, leading to the opening of the pyranoid ring and the formation of a carbocation at C-1. Secondly, the carbocation transfers from C-1 to C-2, resulting in the hydrogen atom on C-2 shifting to C-1. Thirdly, a furan ring is formed by combing the hydroxyl group on C-5 and the carbocation on C-2, releasing a proton. By this means, glucose was isomerized to fructose.

Scheme 1 Putative mechanisms for the proton-catalyzed isomerization of glucose to fructose.

4 Conclusions

The novel concept of acidic aqueous layer catalysis was developed for direct conversion of cellulose into HMF in hot compressed steam, with cheap and environment friendly inorganic acidic salts (NaHSO₄, KHSO₄, NaH₂PO₄ and KH₂PO₄) as catalysts. The acidity of the aqueous layer could be controlled by the relative humidity of steam. The reaction temperature, the intrinsic acidity of the catalyst and the relative humidity of the steam were identified as the key factors for the production of HMF from cellulose. A moderate HMF yield of 30.4 mol% was obtained under the optimized operating conditions. Though the HMF yield obtained here is less than those obtained in ionic liquid or biphasic systems accompanied with Cr-containing catalysts, this novel technology doesn't involve any transition metals or organic solvents, which is highly desirable for its practical application in transformation of biomass into value added chemicals.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Basic Research Program of China (2012CB215304), the Natural Science Foundation of China (51376185 and 51161140331).

Notes and references

1. (a) A. Takagaki, S. Nishimura and K. Ebitani, Catal. Surv. Asia, 2012, 16, 164; (b) J. C. Serrano-Ruiz and J. A. Dumesic, Energy Environ. Sci., 2011, 4, 83.
2. (a) A. A. Rosatella, S. P. Simeonov, R. F. M. Frade and C. A. M. Afonso, Green Chem., 2011, 13, 754; (b) S. Dutta, S. De and B. Saha, ChemPlusChem, 2012, 77, 259.
3. M. Chidambaram and A. T. Bell, Green Chem., 2010, 12, 1253.
4. (a) S. J. Dee and A. T. Bell, ChemSusChem, 2011, 4, 1166; (b) H. J. Heeres, B. Girisuta and L. P. B. M. Janssen, Chem. Eng. Res. Des., 2006, 84, 339.
5. (a) J. B. Binder and R. T. Raines, J. Am. Chem. Soc., 2009, 131, 1979; (b) C. Z. Li, Z. H. Zhang and Z. B. K. Zhao, Tetrahedron Lett., 2009, 50, 5403; (c) Z. C. Zhang, S. Yu, H. M. Brown, X. W. Huang, X. D. Zhou and J. E. Amonette, Appl. Catal., A, 2009, 361, 117; (d) B. Kim, J. Jeong, D. Lee, S. Kim, H. J. Yoon, Y. S. Lee and J. K. Cho, Green Chem., 2011, 13, 1503; (e) S. Dutta, S. De, M. I. Alam, M. M. Abu-Omar and B. Saha, J. Catal., 2012, 288, 8.
6. Z. D. Ding, J. C. Shi, J. J. Xiao, W. X. Gu, C. G. Zheng and H. J. Wang, Carbohydr. Polym., 2012, 90, 792.
7. (a) L. Rigal, A. Gaset and J. P. Gorrichon, Ind. Eng. Chem. Prod. Res. Dev., 1981, 20, 719; (b) Y. Roman-Leshkov, J. N. Chheda and J. A. Dumesic, Science, 2006, 312, 1933; (c) J. A. Dumesic, Y. Roman-Leshkov, C. J. Barrett and Z. Y. Liu, Nature, 2007, 447, 982.
8. N. Shi, Q. Y. Liu, Q. Zhang, T. J. Wana and L. L. Ma, Green Chem., 2013, 15, 1967.
9. J. M. R. Gallo, D. M. Alonso, M. A. Mellmer and J. A. Dumesic, Green Chem., 2013, 15, 85.
10. (a) Y. J. Pagan-Torres, T. F. Wang, J. M. R. Gallo, B. H. Shanks and J. A. Dumesic, ACS Catal., 2012, 2, 930; (b) P. Azadi, R. Carrasquillo-Flores, Y. J. Pagan-Torres, E. I. Gurbuz, R. Farnood and J. A. Dumesic, Green Chem., 2012, 14, 1573.
11. A. I. Torres, P. Daoutidis and M. Tsapatsis, Energy Environ. Sci., 2010, 3, 1560.
12. F. S. Asghari and H. Yoshida, Carbohydr. Res., 2010, 345, 124.
13. T. S. Deng, X. J. Cui, Y. Q. Qi, Y. X. Wang, X. L. Hou and Y. L. Zhu, Chem. Commun., 2012, 48, 5494.
14. S. Zhao, M. X. Cheng, J. Z. Li, J. A. Tian and X. H. Wang, Chem. Commun., 2011, 47, 2176.
15. Z. J. Wei, Y. X. Liu, D. Thushara and Q. L. Ren, Green Chem., 2012, 14(4), 1220–1226.
16. P. Daorattanachai, S. Namuangruk, N. Viriya-empikul, N. Laosiripojana and K. Faungnawakij, J. Ind. Eng. Chem., 2012, 18, 1893.
17. (a) H. M. Pilath, M. R. Nimlos, A. Mittal, M. E. Himmel and D. K. Johnson, J. Agric. Food Chem., 2010, 58, 6131; (b) J. F. Saeman, Ind. Eng. Chem., 1945, 37, 43; (c) C. B. Rasrendra, J. N. M. Soetedjo, I. G. B. N. Makertihartha, S. Adisasmito and H. J. Heeres, Top. Catal., 2012, 55, 543.
18. S. M. Kang, X. H. Li and J. Chang, Ind. Eng. Chem. Res., 2012, 51, 9023.
19. M. Zhang, H. Yang, Y. N. Liu, X. D. Sun, D. K. Zhang and D. F. Xue, Nanoscale Res. Lett., 2012, 7.
20. A. Chuntanapum and Y. Matsumura, Ind. Eng. Chem. Res., 2010, 49, 4055.
21. R. Weingarten, W. C. Conner and G. W. Huber, Energy Environ. Sci., 2012, 5, 7559.
22. A. G. Assaf, R. H. Haas and C. B. Purves, J. Am. Chem. Soc., 1944, 66, 66.
23. (a) S. Deguchi, K. Tsujii and K. Horikoshi, Chem. Commun., 2006, 3293; (b) S. Deguchi, K. Tsujii and K. Horikoshi, Green Chem., 2008, 10, 623; (c) S. Deguchi, K. Tsujii and K. Horikoshi, Green Chem., 2008, 10, 191.
24. T. S. Deng, X. J.Cui, Y. Q. Qi, Y. X. Wang, X. L. Hou and Y. L. Zhu, Chem. Commun., 2012, 48, 5494.
25. H. B. Zhao, J. E. Holladay, H. Brown and Z. C. Zhang, Science, 2007, 316, 1597.
26. (a) H. J. Heeres, C. B. Rasrendra and I. G. B. N. Makertihartha, Top. Catal., 2010, 53, 1241; (b) S. Q. Hu, Z. F. Zhang, J. L. Song, Y. X. Zhou and B. X. Han, Green Chem., 2009, 11, 1746; (c) Y. Yang, C. W. Hu and M. M. Abu-Omar, Green Chem., 2012, 14, 509.
27. (a) J. N. Chheda, Y. Roman-Leshkov and J. A. Dumesic, Green Chem., 2007, 9, 342; (b) M. Bicker, J. Hirth and H. Vogel, Green Chem., 2003, 5, 280.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45813j

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