Highly efficient conversion of carbohydrates into 5-hydroxymethylfurfural using the bi-functional CrPO₄ catalyst

Abstract

The highly efficient synthesis of 5-hydroxymethylfurfural (HMF) from carbohydrates was achieved using the inexpensive and bi-functional CrPO₄ catalyst in a biphasic system. The effect of various reaction conditions, including reaction temperature, time, and solvent, on HMF yields was explored. A HMF yield of up to 83% was obtained using fructose as the reactant at 140 °C for 15 min. A maximum HMF yield of 63% was also achieved from glucose when the reaction was carried out at 140 °C for 30 min. Among the reported catalysts, CrPO₄ was shown to be one of the most effective in the conversion of glucose into HMF, which is comparable to an ionic liquid reaction system. Moreover, the CrPO₄ catalyst exhibited high activity to convert microcrystalline and lignocellulosic feedstock to HMF without the need for the addition of homogeneous mineral acids. The possible conversion mechanism of carbohydrates into HMF catalyzed by the bi-functional CrPO₄ catalyst is discussed.

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1. Introduction

With the depletion of fossil resources and increasing concerns about global warming, the search for renewable alternative resources is extremely urgent. Abundant renewable biomass is considered as a promising alternative for non-renewable resources and possesses great potential to become raw materials for the production of valuable chemicals.^1–3 Among those products, 5-hydroxymethylfurfural (HMF), which can be produced by the dehydration of hexose,^1–3 has attracted much attention. HMF can be further transformed into useful chemicals, such as 2,5-furandicarboxylic acid, 2,5-dihydroxymethylfuran, and dimethylfuran, through oxidation, hydrogenation, and hydrogenolysis.^3–13 So far, a great deal of scientific effort has been put into HMF production from lignocellulosic biomass and carbohydrate compounds.

The production of HMF is usually performed by the mineral acid-catalyzed hydration of biomass such as glucose, fructose, inulin, and cellulose.^11,14–22 The use of mineral acids has some drawbacks, including environmental pollutions and corrosion. Therefore, a new type of catalytic system has been developed recently. Ionic liquids (ILs) have been shown to be excellent reaction media for many chemical reactions in terms of their transcendent advantages.^9,23–25 Some ILs can dissolve various carbohydrates such as monosaccharides, polysaccharides, even lignocellulosic materials. Zhao et al. reported that a 5-HMF yield of 91% was obtained with fructose as a feedstock at 100 °C by using CrCl₂ in 1-ethyl-3-methylimidazolium chloride ([Emim]Cl).²⁶ In addition, acidic ionic liquid [HSO₃BMIM]HSO₄ and [BMIM]HSO₄ in [BMIM]Cl have also efficiently catalyzed the hydrolysis of corn stalk.²⁷ However, the ILs system is high cost and separation and recovery are not easy to accomplish, making the system difficult to industrialize.² Therefore, novel, inexpensive, and cost-efficient routes suitable for large-scale production of HMF must be developed.

A biphasic reactor system has attracted increasing attention due to the use of low boiling solvents, so it could be suitable for large-scale industrial applications.^28–31 Ma et al. reported that a high HMF yield of 53 mol% was obtained by the direct degradation of cellulose in a biphasic system with concentrated NaHSO₄ and ZnSO₄ as the co-catalysts.²⁸ An HMF yield of 57% was achieved from glucose using Sn-beta zeolite and HCl in a biphasic (H₂O/THF) system.³⁰ The HMF yield from fructose conversion was up to 90% and the HMF yield of 58% was obtained by using glucose as feedstock, with tantalum hydroxide treated by H₃PO₄ used as the catalyst in a water–2-butanol biphasic system.³² Using titanium hydrogenphosphate (TiHP) as a catalyst, 55% and 35% HMF yields were obtained starting from fructose and glucose, respectively.³³ Nijhuis and co-workers showed that aluminum, titanium, zirconium, and niobium-based phosphates afforded 5-HMF selectivity in the range of 30–60% when using glucose as the feedstock.³⁴ Maximum yields of HMF of 77 and 50 mol% were achieved from fructose and glucose, respectively, using large-pore mesoporous tin phosphate in a water/methyl isobutyl ketone biphasic solvent.³⁵ Using phosphated TiO₂ as the catalyst, an HMF yield of 81% from glucose was obtained at 175 °C.³⁶

In our previous study, the FePO₄ catalyst exhibited high catalytic activity toward the conversion of lignocellulosic materials.³⁷ In addition, CrCl₃ was also reported to have higher catalytic activity.³⁸ Therefore, in this work, the inexpensive CrPO₄ catalyst was applied to produce HMF with a high yield from carbohydrate conversion. To the best of our knowledge, the use of CrPO₄ in the conversion of carbohydrate has not been reported yet. The aim of this study is to improve the HMF yield and then to disclose the reaction mechanism. To this end, the effects of various reaction conditions on HMF yields were explored. Possible dehydration reaction mechanisms of these carbohydrates catalyzed by CrPO₄ are also proposed.

2. Experimental (materials and methods)

2.1. Materials

Microcrystalline cellulose and D-fructose were purchased from the Sigma-Aldrich company in China. Microcrystalline cellulose (50 mm) was directly used for the dehydration reaction without any other pretreatment. CrPO₄ was used as the reaction catalyst. All the other chemicals were purchased from Jianghua Chemical Reagent (Nanjing, China). Wheat straw was obtained from local resources in Hebei province, China, and was crushed and sieved to around 75 mm. Structural carbohydrates and the acid insoluble lignin mass fraction for the wheat straw were determined using the standard NREL laboratory analytical procedures.³⁷ The glucan content of wheat straw is about 35.5%.

2.2. Methods

Bath catalytic experiments were conducted in a 100 mL autoclave equipped with a thermostat and an electronically controlled magnetic stirrer. The reactor was pressurized at 5 bar of N₂ and heated to the desired reaction temperature. Once the reaction temperature was reached, the monitoring of the reaction started.

Alter the reaction, the liquid products were analyzed by high performance liquid chromatography (HPLC, Agilent 1200) using a column (Zorbax SB-C18) with a UV detector to analyze the 5-HMF yield. The reducing sugar was detected by the DNS method according to the literature report.³⁹ The conversions of the mono- and disaccharides were calculated as follows: conversion (%) = (moles of substrate reacting)/(moles of substrate staring) × 100, while the conversion of cellulose and wheat straw was calculated by the weight change of the substrate before and after the reaction. The yields of 5-HMF using fructose and glucose as the starting materials were calculated using the following equation: yield (%) = (moles of 5-HMF in the products)/(moles of feedstocks put into the reactor) × 100. For cellulose and wheat straw, the yields of 5-HMF were defined as follows: yield (%) = (moles of 5-HMF in the products)/(moles of glucose unit put into the reactor) × 100.

3. Results and discussion

3.1. Effect of catalyst dosage on the yield of HMF produced from fructose

The influence of the catalyst dosage on the fructose conversion into HMF in the biphasic H₂O/THF system was investigated and the results are shown in Table 1. On the basis of our previous work,³⁷ we first used a temperature of 140 °C and a ratio of H₂O/THF equal to 3 to perform the dehydration reaction. In the H₂O/THF biphasic system, the conversion of fructose into HMF was obviously dependent on the catalyst used. Without any catalyst addition, a low yield of HMF of 48% was obtained. It can be seen from Table 1 that the addition of CrPO₄ remarkably promoted the production of HMF, indicating that CrPO₄ can efficiently catalyze the conversion of fructose into HMF. The yield of HMF initially increased with increasing the CrPO₄ dosage. However, when 0.25 g catalyst was used, the yield of HMF decreased. The reason for this might be that excessive catalyst accelerated the formation of HMF from fructose while it also favored the rehydration of HMF into levulinic acid, which offsets the increase in HMF yield.⁴⁰ Thus, the optimum amount of catalyst was 0.125 g for the maximum HMF yield.

Table 1 The effects of catalyst dosage and reaction time on the conversion of fructose to 5-HMF^a

Entry	Amount of catalyst/g	Reaction time/min	Conversion %	HMF yield/mol%
a Reaction conditions: fructose (1.0 g), NaCl (3.5 g), H2O (10 mL) and THF (30 mL), 140 °C.
1	0	15	97	48
2	0.065	15	100	81
3	0.125	15	100	83
4	0.250	5	98	69
5	0.250	15	99	74
6	0.250	30	100	64

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3.2. Effect of reaction temperature and time on the HMF yield

The influence of reaction temperature on the yield of HMF was also investigated, and the results are shown in Fig. 1. When the dehydration of fructose was carried out at 120 °C, the reaction was very slow. A HMF yield of 61% was obtained after 15 min. When the reaction was further carried out at 130 °C for 15 min, the yield of HMF increased to 71%. When the reaction proceeded at 140 °C for 15 min, the highest HMF yield of 83% was obtained. When it was 150 °C, the HMF yield was lower than that at 140 °C, and was only 74%. This is because that higher temperature gave rise to side reactions that formed undesired byproducts. Therefore, 140 °C was determined as the optimal reaction temperature. The impact of the reaction time on the conversion of fructose and HMF yield was also evaluated at 140 °C. The yield of HMF improved from 69% to 74% upon the increase of the reaction time from 5 min to 15 min (Table 1, entries 4 and 5). However, when the reaction time reached 30 min, the HMF yield decreased to 64%. After a long reaction period, HMF was rehydrated and formed humins. The results demonstrated that both the reaction temperature and time had significant influences on the production of HMF. Thus, a reaction temperature of 140 °C and a reaction time of 15 min were selected as the optimum conditions for the conversion of fructose into HMF.

Fig. 1 Effect of reaction temperature on (●) fructose conversion and (■) HMF yield. Reaction conditions: fructose (1.0 g), NaCl (3.5 g), CrPO₄ (0.125 g), H₂O (10 mL), THF (30 mL), 15 min.

3.3. The effects of solvents on the yield of HMF from the conversion of fructose

The catalytic conversion of fructose into HMF was then carried out in different polar solvent systems, and the results are listed in Table 2. A lower HMF yield of 33% was obtained when the protic solvent MIBK was used (Table 2, entry 1). HMF yields of 63% and 81% were obtained using n-butanol and 2-butanol as the organic solvent, respectively (Table 2, entries 2 and 3). When the H₂O/THF biphasic system was used, the yields of HMF also improved (Table 2, entry 4). Subsequently, the influence of different ratios of H₂O/THF on the degradation of fructose was investigated (Table 2, entries 4–7). It was found that the HMF yield was up to 83% when the ratio of H₂O/THF was 1 : 3. In order to reveal whether the reaction occurred through homogeneous or heterogeneous catalysis, the filtered aqueous solution containing dissolved CrPO₄ was used to carry out the fructose dehydration reaction without the addition of fresh CrPO₄ catalyst (Table 2, entry 8). The results suggested that the filtered aqueous solution exhibited an almost comparable HMF yield compared to solid CrPO₄ catalyst (Table 2, entries 5 and 8). It is obvious that the reaction mainly proceeded through homogeneous catalysis by the dissolved CrPO₄ catalyst.

Table 2 Dehydration of fructose into HMF in different solvent systems^a

Entry	System (mL/mL)	Conversion/%	HMF yield/mol%
a Reaction conditions: fructose (1.0 g), CrPO4 (0.125 g), NaCl (3.5 g), H2O (10 mL), 140 °C, 15 min.b The aqueous phase solution (soluble CrPO4) was used as the catalyst, which was obtained by filtering and separating the reaction system (entry 5) in order to remove both the solid and the organic phase. Reaction conditions: fructose (1.0 g), NaCl (3.5 g), THF (30 mL), 140 °C, 15 min.
1	H2O/MIBK (10/30)	98	33
2	H2O/n-butanol (10/30)	99	63
3	H2O/2-butanol (10/30)	100	81
4	H2O/THF (10/20)	100	56
5	H2O/THF (10/30)	99	83
6	H2O/THF (10/40)	100	78
7	H2O/THF (10/60)	98	70
8^b	H2O/THF (—)	100	79

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3.4. The effect of different biomass materials on HMF production

It is well reported that a variety of feedstocks have been used to produce HMF, including monosaccharides, disaccharides, and more complex, high-molecular-weight polysaccharides or raw lignocellulosic biomass.^2,3,38,39 Glucose is the most abundant monosaccharide and the cheapest hexose, making it a promising candidate as a renewable raw material for the production of 5-HMF.^3,40 In order to test the catalytic activity of CrPO₄ toward different biomass materials, glucose, cellulose, and wheat straw were also used for HMF production, and the results are shown in Fig. 2. It can be seen from Fig. 2 that a high HMF yield (83%) was obtained using fructose as a starting material. Also a HMF yield of 51.7% was obtained from glucose with a high conversion at 140 °C for 15 min. For cellulose, a good HMF yield of 37% at a conversion rate of 65% was also achieved. The results provide a promising alternative for the conversion of less-expensive and renewable carbohydrates into HMF.

Fig. 2 Results for the conversion of different feedstocks to HMF. Reaction conditions: feedstock (1.0 g), NaCl (3.5 g), CrPO₄ (0.125 g), H₂O (10 mL), THF (30 mL), 140 °C, 15 min.

Subsequently, the reaction conditions, including reaction time, reaction temperature, catalyst dosage, etc., on the conversion of glucose were investigated. The results are presented in Table 3. It can be seen that the reaction time, reaction temperature, and catalyst amount significantly affected the yield of HMF. A yield of HMF of up to 63% was achieved at 140 °C for 30 min (Table 3, entry 2). It was reported that a high HMF yield of 67% was achieved starting from glucose using CrCl₃ as the catalyst in an ionic liquid reaction system.³⁸ So, CrPO₄ is one of the most effective catalysts in the conversion of glucose to HMF, comparable to the ionic liquid reaction system.³⁸

Table 3 The effects of reaction conditions on the conversion of glucose to 5-HMF^a

Entry	Catalyst amount/g	Reaction temperature/°C	Reaction time/min	Conversion/%	HMF yield/mol%
a Reaction conditions: glucose (1.0 g), NaCl (3.5 g), H2O (10 mL) and THF (30 mL).
1	0.125	130	30	98	−46
2	0.125	140	30	99	−63
3	0.125	150	30	98	−50
4	0.125	140	5	98	−50
5	0.125	140	15	100	−52
6	0.125	140	45	98	−49
7	0.065	140	30	99	−33
8	0.250	140	30	100	−36

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In order to further improve the HMF yield, we tried to use other catalysts to perform the dehydration reaction of glucose, and the results are shown in Table 4. Interestingly, when using CrCl₃ as the catalyst, a low HMF yield of 15% was obtained in the H₂O/THF system (Table 4, entry 4). This finding is completely different from the result that CrCl₃ gave a high HMF yield of 67% starting from glucose in the ionic liquor reaction system, suggesting that the solvent plays an important role in the reaction.³⁸ In addition, we used NaH₂PO₄ to modify the pH value of the aqueous phase in order to improve the HMF yield. However, the results showed that the yield of HMF decreased with the addition of NaH₂PO₄ compared to the use of CrPO₄ or NaH₂PO₄ alone as the catalyst. The possible explanation for this is that the presence of H₂PO₄²⁻ changes the pH value and hydrolyzed species thus suppressing the glucose isomerization to fructose. The catalytic activity of H₃PO₄ under identical reaction conditions was also tested as a control experiment, and a yield of HMF of 22% was obtained.

Table 4 The effects of different catalysts on the conversion of glucose to 5-HMF^a

Entry	Catalyst	Conversion/%	HMF yield/mol%
a Glucose (1.0 g), catalyst (0.125 g), NaCl (3.5 g), H2O (10 mL) and THF (30 mL), 140 °C, 30 min.b Glucose (1.0 g), CrPO4 (0.125 g), NaH2PO4 (0.05 g), NaCl (3.5 g), H2O (10 mL) and THF (30 mL), 140 °C, 30 min.
1	CrPO4	99	63
2^b	CrPO4 + NaH2PO4	97	20
3	NaH2PO4	96	26
4	CrCl3	95	15
5	H3PO4	98	22

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3.5. The reaction mechanism of carbohydrates into HMF

In general, the production of 5-HMF from cellulosic or lignocellulosic feedstocks must include the acid-catalyzed depolymerization of cellulose to produce glucose in the first step.^41,42 A glucose isomerization to fructose is followed by its subsequent dehydration to HMF. Vlachos et al. have already reported that the hydrolyzed Cr(III) complex [Cr(H₂O)₅OH]²⁺ was the most active Cr species in glucose isomerization and probably acts as a Lewis acid–Brønsted base bi-functional site.²⁰ However, the CrPO₄ catalyst was not a proton acid but has the activity of cracking the β-1,4-glucosidic bonds of cellulose. It is well know that H⁺ can hydrolyze cellulose to produce glucose.^15,43–45 Hence, we tested the pH of the aqueous phase, which was obtained by filtering and separating the reaction system (Table 1, entry 5), and the result of this pH measurement was 2.48. In contrast, the pH of the aqueous phase containing dissolved FePO₄ was 3.75 at room temperature. Therefore, we propose that partially dissolved CrPO₄ could be hydrolyzed to produce [Cr(H₂O)₅OH]²⁺ and H⁺, both of which are responsible for the catalytic conversion of cellulosic or lignocellulosic feedstocks. It is reasonable that cellulose can be hydrolyzed to glucose by H⁺ produced from the hydrolysis of CrPO₄. After the formation of glucose, the glucose molecule isomerizes into fructose, which is then converted into 5-HMF via the acid-catalyzed dehydration. So, we propose a possible reaction route for the conversion of carbohydrates to 5-HMF through a cascade reaction sequence that involves the homogeneous acid-catalyzed depolymerization of carbohydrates to glucose, a Lewis acid site [Cr(H₂O)₅OH]²⁺-catalyzed isomerization of glucose to fructose, and a homogeneous acid (H⁺)-catalyzed dehydration of fructose to 5-HMF, as shown in Scheme 1.

Scheme 1 The proposed reaction pathways of CrPO₄ catalyzed conversion of cellulose to 5-HMF.

4. Conclusions

CrPO₄ has been shown to be an efficient, inexpensive bi-functional catalyst for renewable HMF production, and even for the conversion of cellulose and lignocellulose without the addition of mineral acid. A HMF yield of up to 83% was obtained using fructose as the reactant at 140 °C for 15 min. A maximum HMF yield of 63% was also achieved from glucose when the reaction was carried out at 140 °C for 30 min. The high activity of the catalyst is attributed to the hydrolysis of the Cr(III) ion, which releases H⁺ and Cr hydroxylated species. This Lewis acid-derived (intrinsic) Brønsted acidity was primarily responsible for the fructose dehydration to HMF when no external Brønsted acids were added. Moreover, the catalyst was shown to act as a highly effective homogeneous acid to hydrolyze the β-1,4-glucosidic bonds of cellulose and wheat straw to monosaccharides, which were eventually converted to HMF via an isomerization reaction of glucose to fructose, followed by the fructose dehydration reaction. The CrPO₄ catalyst has a great potential to facilitate the cost-efficient conversion of carbohydrates into HMF in the future.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (grant no. 31200445), and the Natural Science Foundation of Jiangsu province (grant no. BK2012416 and BK20140972). This work was also financially supported by the open foundation of Jiangsu Key Laboratory of Biomass Energy and Materials (JSBEM201503) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23716e

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