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

Article abstract of DOI:10.1039/c5ra23716e

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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High efficient conversion of carbohydrates into 5-hydroxymethylfurfural using the
bi-functional CrPO4 catalyst
Siquan Xu†, Xiaopei Yan†, Quan Bu*‡, Haian Xia*†
†Jiangsu Key Lab of BiomassꢀBased Green Fuels and Chemicals,
College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, China
‡School of Agricultural Equipment Engineering,Jiangsu University,Zhenjiang212013，China
*Corresponding author : Quan Bu, Haian Xia
Tel: +86ꢀ25ꢀ85427635; Fax: +86ꢀ25ꢀ85428873;
Eꢀmail: haxia@dicp.ac.cn; quan.bu@email.wsu.edu
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Abstract
High efficient synthesis of 5ꢀhydroxymethyfurfural (HMF) from carbohydrates was achieved
using inexpensive and biꢀfunctional CrPO4 catalyst in biphasic system. The effect of various
reaction conditions including reaction temperature, time, solvent, etc. on HMF yields were
explored. A HMF yield up to 83% was obtained using fructose as the reactant at 140 ℃ for 15 min.
A maximum HMF yield of 63% was also achieved from glucose when the reaction was carried out
at 140 for 30 min. Among the reported catalysts, CrPO4 showed one of the most effective in the
conversion of glucose into HMF, which is comparable to ionic liquid reaction system. Moreover,
the CrPO4 catalyst exhibited high activity to convert microꢀcrystalline and lignocellulosic
feedstock to HMF without the addition of homogeneous mineral acids. The possible conversion
mechanism of carbohydrates into HMF catalyzed by the biꢀfunctional CrPO4 catalyst was
discussed.
Keywords: 5ꢀhydroxymethylfurfural; isomerization; dehydration; CrPO4.
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1. Introduction
With the depletion of fossil resources and increasing concerns for global warming, the search
for renewable alternative resources is extremely urgent. The abundant renewable biomass is
considered as a promising alternative for nonꢀrenewable resources and possesses a great potential
to become raw materials for the production of valuable chemicals 1ꢀ3. Among those products,
5ꢀhydroxymethylfurfural (HMF), which can be produced by the dehydration of hexose 1ꢀ3, has
attracted much attention. HMF can be further transformed into useful chemicals such as 2,
5ꢀfurandicarboxylic acid, 2,5ꢀdihydroxymethylfuran, and dimethylfuran through oxidation,
hydrogenation and hydrogenolysis 3ꢀ13. So far, a great deal of scientific effort has been put into the
HMF production derived from lignocellulosic biomass and carbohydrate compounds.
The production of HMF was usually performed by mineral acidꢀcatalyzed hydration of
biomass such as glucose, fructose, inulin, and cellulose 11, 14ꢀ22. The use of mineral acid has some
drawbacks, including some environmental pollutions and corrosion. Therefore, a new type of
catalytic system has been developed recently. Since ionic liquids (ILs) have shown to be excellent
reaction media for many chemical reactions in terms of their transcendent advantages 9, 23ꢀ25. Some
ILs can dissolve various carbohydrates such as monosaccharides, polysaccharides, even
lignocellulosic materials. Zhao et al. reported that a 5ꢀHMF yield of 91% was obtained with
fructose as a feedstock at 100 ℃ by using CrCl2 in 1ꢀethylꢀ3ꢀmethylimidazolium chloride
([Emim]Cl) 26. In addition, acidic ionic liquid [HSO3BMIM]HSO4 and [BMIM]HSO4 in
[BMIM]Cl also efficiently catalyzed the hydrolysis of corn stalk 27. However, the ILs system is
high cost and not easy to separate and recovery, making it difficult to be industrialized 2. Therefore,
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some novel, inexpensive, and costꢀefficient routes which are suitable for largeꢀscale 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 largeꢀscale industrial applications 28ꢀ31. Ma et al. reported that a
high HMF yield of 53 mol % was obtained by direct degradation of cellulose in a biphasic system
with concentrated NaHSO4 and ZnSO4 as coꢀcatalysts 28. The HMF yield of 57% was achieved
from glucose using Snꢀbeta 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, when the tantalum hydroxide treated by H3PO4 was used as a catalyst in the
water–2ꢀbutanol biphasic system 32. Using Titanium hydrogenphosphate (TiHP) as catalyst, the 55%
and 35% HMF yield was obtained starting from fructose and glucose, respectively 33. Nijhuis and
coꢀworkers showed that aluminum, titanium, zirconium, and niobiumꢀbased phosphates afforded
5ꢀHMF selectivity in the range of 30–60% using glucose as a feedstock 34. Maximum yields of
HMF of 77 and 50 mol% were achieved from fructose and glucose, respectively, using largeꢀpore
mesoporous tin phosphate in a water/methyl isobutyl ketone biphasic solvent 35. Use phosphated
36
TiO2 as catalyst, the HMF yield of 81% from glucose was obtained at 175
.
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 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
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to improve the HMF yield and 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 were also proposed.
2. Experimental (Materials and Method)
2.1 Materials
Microcrystalline cellulose and DꢀFructose were purchased from the SigmaꢀAldrich company
in China. Microcrystalline cellulose (50 mm) was directly used for the dehydration reaction
without any other pretreatment. CrPO4 were used as the reaction catalyst. All 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 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 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 SBꢀC18) with a UV detector to
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analyze 5ꢀHMF yield. The reducing sugar was detected by DNS method according to the literature
report 39. The conversion of monoꢀ and diꢀ saccharides was 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 substrate before and after the
reaction. The yields of 5ꢀHMF using fructose and glucose as starting materials were calculated
using the following equation: yield (%) = (moles of 5ꢀHMF in the products)/(moles of feedstocks
put into the reactor) × 100. For cellulose and wheat straw, the yields of 5ꢀHMF were defined as
follows: yield (%) = (moles of 5ꢀHMF in the products)/(moles of glucose unit put into the reactor)
× 100.
Results and discussion
2.3 Effect of catalyst dosage on the yield of HMF produced from fructose
The influence of catalyst dosage on the fructose conversion into HMF in biphasic H2O/THF
system was investigated and the results are shown in Table 1. On the basis of our previous work 37
,
we firstly used the temperature of 140 ℃ and the ratio of H2O/THF equal to 3 to perform the
dehydration reaction. In H2O/THF biphasic system, the conversion of fructose into HMF was
obviously dependent on the catalyst used. Without any catalyst addition, the 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 was decreased. The reason might be that
excessive catalyst accelerated the formation of HMF from fructose while it also favored the
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rehydration of HMF into levulinic acid, which offsets increase in HMF yield 40. Thus, the
optimum amount of catalyst was 0.125 g for the maximum HMF yield.
3.2 Effect of reaction temperature and time on 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 ℃, the
reaction was very slow. A HMF yield of 61% was obtained after 15 min. As the reaction was
further carried out at 130 ℃ for 15 min, the yield of HMF increased to 71 %. When the reaction
proceeded at 140 ℃ for 15 min, the highest HMF yield of 83% was obtained. When it was 150 ℃,
the HMF yield was lower than that at 140 ℃, and only 74%. This is because that higher
temperature gave rise to side reactions that formed undesired byproducts. Therefore, 140 ℃ was
determined the optimal reaction temperature. Impact of the reaction time on the conversion of
fructose and HMF yield was also evaluated at 140 ℃. The yield of HMF improved from 69 % to
74% upon 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, HMF yield decreased to 64%. After a long period of
reaction, HMF was rehydration and formation of humins. The results demonstrated that both
reaction temperature and time had significant influences on the production of HMF. Thus, a
reaction temperature of 140 ℃ and a reaction time of 15 min were selected as the optimum
conditions for conversion of fructose into HMF.
3.3 The effects of solvents on yield of HMF from conversion of fructose
Catalytic conversion of fructose into HMF was then carried out in different polar solvent
systems, and the result is listed in Table 2. A lower HMF yield of 33% was obtained when the
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protic solvent MIBK was used (Table 2, entry 1). The HMF yield of 63% and 81% were obtained
using nꢀbutanol and 2ꢀbutanol as organic solvent, respectively (Table 2, entries 2 and 3). When
H2O/THF biphasic system was used, the yields of HMF was also improved (Table 2, entry 4).
Subsequently, the influence of different ratio of H2O/THF on the degradation of fructose was
investigated (Table 2, entries 4ꢀ7). It was found that the HMF yield was up to 83 % when the ratio
of H2O/THF was 1:3. In order to reveal that 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 exhibit an almost comparable
HMF yield as compared to solid CrPO4 catalyst (Table 2, entries 5 and 8). It is obvious that the
reaction mainly proceeded through homogeneous catalysis by dissolved CrPO4 catalyst.
3.4 The effect of different biomass materials on the HMF production
It is well reported that a variety of feedstocks have been used to produce HMF, including
monosaccharides, disaccharides, or more complex, highꢀmolecularꢀweight 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 renewable raw material for the production of 5ꢀHMF 3,
40. In order to test catalytic activity of CrPO4 toward different biomass materials, glucose, cellulose,
and wheat straw were also used for the HMF production, and the result is 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. And a HMF yield of 51.7% was obtained from glucose with a high conversion at 140 ℃
for 15 min. For cellulose, a good HMF yield of 37% at the conversion of 65% was also achieved.
The results provide a promising alternative for the conversion of lessꢀexpensive and renewable
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carbohydrates into HMF.
Subsequently, the reaction conditions, including reaction time, reaction temperature, catalyst
dosage, etc. on the conversion of glucose were investigated. The results are presented in the Table
3. It can be found that the reaction time, reaction temperature and catalyst amount significantly
affected the yield of HMF. A yield of HMF up to 63% was achieved at 140 ℃ 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 ionic liquid reaction system 38. So, CrPO4 is one of the effective catalysts
in the conversion of glucose to HMF, which is comparable to the ionic liquid reaction system 38
.
In order to further improve the HMF yield, we tried to use other catalyst 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 ionic liquor reaction system, suggesting that solvents play an
important role in the reaction 38. In addition, we used NaH2PO4 to modify the pH value of 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 a single CrPO4 or NaH2PO4 as a catalyst.
2ꢀ
The possible explanation is that the presence of H2PO4 changes the pH value and hydrolysed
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 the yield of HMF
was 22%.
3.5 The reaction mechanism of carbohydrates into HMF
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In general, the production of 5ꢀHMF from cellulosic or lignocellulosic feedstocks must
include acid catalyzed depolymerization of cellulose to produce glucose in the first step 41, 42. A
glucose isomerization to fructose 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 acidꢀ Brønsted base bifunctional site
20. However, the CrPO4 catalyst was not a proton acid but has the activity of cracking βꢀ1,
4ꢀglucosidic bonds of cellulose. It is well know that H+ can hydrolyze cellulose to produce glucose
15, 43ꢀ45. Hence, we tested the pH of the aqueous phase which was obtained by filtering and
separating the reaction system (Table 1, entry 5), the result of pH measurement was 2.48. In
contrast, pH of aqueous phase containing dissolved FePO4 is 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 were 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 was then converted into 5ꢀHMF via the acidꢀcatalyzed dehydration. So, we prospect a
possible reaction route about conversion of carbohydrates to 5ꢀHMF proceeded through a cascade
reaction sequence which 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 5ꢀHMF, as shown in
Scheme 1.
Conclusions
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CrPO4 shows to be an efficient, inexpensive biꢀfunctional catalyst for renewable HMF
production, and even the conversion of cellulose and lignocellulose without the addition of
mineral acid. A HMF yield up to 83% was obtained using fructose as the reactant at 140℃ for 15
min. A maximum HMF yield of 63% was also achieved from glucose when the reaction was
carried out at 140 for 30 min. The high activity of the catalyst is attributed to the hydrolysis of
the Cr (Ⅲ) ion releases H+ and Cr hydroxylated species. This Lewis acidꢀderived (intrinsic)
Brønsted acidity was primarily responsible for the fructose dehydration to HMF when no external
Brønsted acids were added. Besides, the catalyst was shown to act as a highly effective
homogeneous acid to hydrolyze βꢀ1, 4ꢀglucosidic bonds of cellulose and wheat straw to
monsaccharides, which were eventually converted to HMF via an isomerization reaction of
glucose to fructose and then fructose dehydration reaction. The CrPO4 catalyst has a great
potential to facilitate the costꢀefficient conversion of carbohydrates into HMF in 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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Figure captions
Figure 1 Effect of reaction temperature on fructose conversion and HMF yield.
Reaction condition: fructose (1.0 g), NaCl (3.5 g), CrPO4 (0.125 g), H2O (10 mL), THF (30 mL),
15 min.
Figure 2 Result for the conversion on different feedstocks to HMF.
Reaction condition: feedstock (1.0 g), NaCl (3.5 g), CrPO4 (0.125 g), H2O (10 mL), THF (30 mL),
140 ℃, 15 min.
Scheme 1 The proposed reaction pathways of CrPO4 catalyzed conversion of cellulose to 5ꢀHMF.
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Table 1 The effects of catalyst dosage and reaction time on conversion of fructose to 5ꢀHMF a
Entry Amount of catalyst/g
Reaction
Time/min
Conversion%
HMF Yield/mol%
1
2
3
4
5
6
0
15
15
15
5
97
48
81
83
69
74
64
0.065
0.125
0.250
0.250
0.250
100
100
98
15
30
99
100
a Reaction conditions: Fructose (1.0 g), NaCl (3.5 g), H2O (10 mL) and THF (30 mL), 140 ℃.
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Table 2 Dehydration of fructose into HMF in different solvent systems a
Entry
System (mL/mL)
H2O/MIBK (10/30)
H2O/nꢀButanol (10/30)
H2O/ 2ꢀButanol (10/30)
H2O/THF (10/20)
H2O/THF (10/30)
H2O/THF (10/40)
H2O/THF (10/60)
H2O/THF ( — )
Conversion/%
HMF Yield/mol%
1
2
3
4
5
6
7
8b
98
33
63
81
56
83
78
70
79
99
100
100
99
100
98
100
a Reaction conditions: Fructose (1.0 g), CrPO4 (0.125 g), NaCl (3.5 g), H2O (10 mL), 140 ℃, 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 ℃, 15 min.
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Table 3 The effects of reaction conditions on the conversion of glucose to 5ꢀHMF a.
Catalyst
Reaction
Reaction
Time/min
HMF
Entry
Conversion/%
amount/g
Temperature/℃
Yield/mol%
1
2
3
4
5
6
7
8
0.125
0.125
0.125
0.125
0.125
0.125
0.065
0.250
130
140
150
140
140
140
140
140
30
30
30
5
98
46
63
50
50
52
49
33
36
99
98
98
15
45
30
30
100
98
99
100
a Reaction conditions: Glucose (1.0 g), NaCl (3.5 g), H2O (10 mL) and THF ( 30 mL).
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Table 4 The effects of different catalysts on the conversion of glucose to 5ꢀHMF a
Entry
Catalyst
CrPO4
Conversion/%
HMF Yield/mol%
1
99
97
96
95
98
63
20
26
15
22
2b
3
CrPO4 + NaH2PO4
NaH2PO4
CrCl3
4
5
H3PO4
a Glucose (1.0 g), Catalyst (0.125 g), NaCl (3.5 g), H2O (10 mL) and THF (30 mL), 140 ℃, 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 ℃, 30 min.
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100
80
60
120
130
140
150
Temperature/ oC
Figure 1 Effects of reaction temperature on (●) fructose conversion and (■) HMF yield. Reaction
condition: fructose (1.0 g), NaCl (3.5 g), CrPO4 (0.125g), H2O (10 mL), THF (30 mL), 15 min.
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Conversion/%
HMF yield/%
100
80
60
40
20
0
Fructose
Glucose
Cellulose
Wheat straw
Figure 2 Result for the conversion of different feedstocks to HMF. Reaction condition: feedstock
(1.0 g), NaCl (3.5 g), CrPO4 (0.125g), H2O (10 mL), and THF (30 mL), 140 ℃, 15min.
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Fructose
[Cr(H2O)5OH]2+
isomerization
H+
-3 H2O
H2O
CrPO4
Glucose
hydrolysis
H+
depolymerization
HO
O
O
H
or
Cellulose
Wheat straw
HMF
Scheme 1 A proposed reaction pathways of CrPO4 catalyzed conversion of cellulose to HMF.
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Organic phase
(THF)
HMF
→
Aqueous phase
(NaCl)
[Cr(H2O]x(OH)y](3-y)+
Transfer
Isomerization
Glucose
Fructose
+H2O
H+
Dehydration
H+
Hydrolysis
+H2O
+H2O
CrPO4
HMF
Cellulose