One-pot synthesis of 5-hydroxymethylfurfural from carbohydrates using an inexpensive FePO4 catalyst

Article abstract of DOI:10.1039/c4ra16145a

Catalytic conversion of carbohydrates to 5-hydroxymethylfurfural (5-HMF) provides a way toward obtaining renewable biomass-based fuels and chemicals. Herein, we use an inexpensive FePO₄ catalyst, which is insoluble at low temperature but can be partially dissolved and act as a homogeneous catalyst at high temperature, in a one-vessel biphasic reactor to generate 5-HMF from carbohydrates such as fructose, glucose, sucrose, cellulose, and Camellia oleifera shell (a lignocellulosic feedstock) without the addition of homogeneous acids. The effects of various reaction conditions including reaction temperature, reaction time, feedstock types and the amount of catalyst on fructose conversion and 5-HMF yield were investigated. The highest 5-HMF yield (71.5 mol%) starting from fructose feedstock was achieved using this "one-pot" biphasic water/tetrahydrofuran (THF) reactor system at 140 °C for 15 min. More interestingly, at high temperature, the FePO₄ catalyst was also highly active in the conversion of cellulose and Camellia oleifera shell, which are very difficult to convert to 5-HMF without the addition of mineral acids. A high 5-HMF yield of 48 mol% starting from microcrystalline cellulose was also obtained using the biphasic reaction system. Moreover, the FePO₄ catalyst could be easily separated and recycled from the aqueous solution via precipitation after cooling to room temperature since it is insoluble at low temperature. Possible dehydration reaction mechanisms of these carbohydrates catalyzed by FePO₄ were also proposed. This journal is

Full text of DOI:10.1039/c4ra16145a

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Organic phase (THF)
OH
O
O
H
(Org.)
Aqueous phase
( NaCl)
OH
O
fructose
glucose
O
PO43- + Fe3+
Heating
H
(Aq.)
Cooling
FePO4 (Solid)
Hydrolysis
Heating
Fe(OH)3 + H3PO4
Cooling
cellulose/
camellia oleifera shell
A “double character” FePO4 catalyst, which acts as a homogeneous
catalyst at high temperature, was used to produce 5-HMF starting from
cellulose and woody biomass without the addition of homogeneous
acids.

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One-pot synthesis of 5-hydroxymethylfurfural from carbohydrates using an inexpensive FePO4
catalyst
Li Yang, Xiaopei Yan, Siquan Xu, Hao Chen, Haian Xia*, Songlin Zuo
Jiangsu Key Lab of BiomassꢀBased Green Fuels and Chemicals,
College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, China
* Corresponding author : Haian Xia
Tel: +86ꢀ25ꢀ85427635; Fax: +86ꢀ411ꢀ85428873;
Eꢀmail: haxia@dicp.ac.cn
1

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Abstract
Catalytic conversion of carbohydrates to 5ꢀhydroxymethylfurfural (5ꢀHMF) provides a way toward
obtaining renewable biomassꢀbased fuels and chemicals. Herein, we use an inexpensive FePO4 catalyst,
which is insoluble at low temperature but can be partially dissolved and act as a homogeneous catalyst at
high temperature, in a oneꢀvessel biphasic reactor to generate 5ꢀHMF from carbohydrates such as
fructose, glucose, sucrose, cellulose, and camellia oleifera shell (a lignocellulosic feedstock) without the
addition of homogeneous acids. The effects of various reaction conditions including reaction temperature,
reaction time, feedstock types and the amount of catalyst on fructose conversion and 5ꢀHMF yield were
investigated. The highest 5ꢀHMF yield (71.5 mol%) starting from fructose feedstock was achieved using
o
this “oneꢀpot” biphasic water/tetrahydrofuran (THF) reactor system at 140 C for 15 min. More
interestingly, at high temperature, the FePO4 catalyst was also highly active in the conversion of cellulose
and camellia oleifera shell, which are very difficult to be converted to 5ꢀHMF without the addition of
mineral acids. A high 5ꢀHMF yield of 48 mol% starting from microcrystalline cellulose was also obtained
using the biphasic reaction system. Moreover, the FePO4 catalyst could be easily separated and recycled
from the aqueous solution via precipitation after cooling to room temperature since it is insoluble at low
temperature. Possible dehydration reaction mechanisms of these carbohydrates catalyzed by FePO4 were
also proposed.
Keywords: 5ꢀhydroxymethylfurfural (5ꢀHMF); dehydration; FePO4; rehydration, isomerization
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Introduction
In recent years, much effort has been focused on finding viable alternatives to the fossil fuelꢀbased
industry of today, and as such, the development of a new, long term, environmentally friendly and
sustainable chemical source has received wide attention.1 5ꢀhydroxymethylfurfural (5ꢀHMF), a versatile
and key renewable platform molecule produced from biomass feedstock,1ꢀ11 can be transformed into fuels
and various valuable chemicals through catalytic conversion, including the gasoline additives 2,5ꢀ
dimethylfuran (DMF),12 an alternative polymer precursor of 2,5ꢀfurandicarboxylic acid (FDCA),13ꢀ16 and
levulinic acid.17ꢀ19
The production of 5ꢀHMF was usually performed by mineral acidꢀcatalyzed hydration of biomass
such as glucose, fructose, inulin, and cellulose.1,20,21 The drawbacks of using mineral acids include severe
environmental pollution and corrosion, and therefore, some novel and green reaction systems have been
developed recently. Ionic liquid systems represent a hot research field due to their excellent catalytic
effectiveness for producing 5ꢀHMF with high yield through catalytic hydrolysis of carbohydrates.2,8ꢀ10,22ꢀ25
Zhao et al. have suggested that by using CrCl2 in 1ꢀethylꢀ3ꢀmethylimidazolium chloride ([Emim]Cl), a 5ꢀ
o
HMF yield of 91% was obtained with fructose as a feedstock at 100 C.9 In addition, a biphasic reactor
system has also attracted increasing attention since it could be suitable for large scale industrial
applications due to the use of lowꢀboiling solvents.26 Ma et al. have reported that a high 5ꢀHMF yield of
53 mol% was obtained by direct degradation of cellulose in a biphasic system with concentrated NaHSO4
and ZnSO4 as coꢀcatalysts.27
Besides homogeneous systems, heterogeneous solid acidic catalysts such as metal oxides,28
phosphate catalysts,29ꢀ33 and acidic zeolites34ꢀ37 have also been applied to carbohydate degradation
reactions due to their ease of separation and reusability. Both high activity and selectivity towards the
dehydration of carbohydrates to 5ꢀHMF were achieved. For instance, Ebitani et al. used the combined
catalysts of MgꢀAl hydrotalcite and Amberlystꢀ15 to achieve HMF selectivity of 58% at glucose
conversion of 73%.38 Nijhuis and coꢀworkers showed that aluminum, titanium, zirconium, and niobiumꢀ
based phosphates afforded 5ꢀHMF selectivities in the range of 30ꢀ60% using glucose as a feedstock.31
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Additionally, high 5ꢀHMF selectivities of over 70% from sugars were achieved using Snꢀbeta zeolite and
HCl in a biphasic (H2O/THF) system, which is comparable to activities obtained using metalloenzymes.39
To date, a 5ꢀHMF yield of up to 90% from fructose has been achieved using tantalum hydroxide as the
catalyst in a waterꢀ2ꢀbutanol biphasic system.40 Furthermore, the development of a cheap, reusable, more
effective solid acid catalyst for the largeꢀscale production of 5ꢀHMF in a biphasic reaction system is of
great relevance, with the exception of the use of lowꢀboiling solvents. However, the main disadvantage of
solid acid systems is the lack of sufficiently high enough catalytic activity to degrade raw biomass
feedstocks such as cellulose and lignocellulosic materials with the exception of ionic liquid systems. One
reason is that the contact and interaction between insoluble raw reactants and solid acid active sites is not
sufficient enough despite most solid acids possessing impressive activity for the conversion of soluble
monosaccharide.
Here, we develop an efficient “oneꢀpot” synthesis of 5ꢀHMF from carbohydrates using an
inexpensive catalyst, i.e. FePO4, without the addition of mineral acids. To the best of our knowledge, the
use of FePO4 in the conversion of carbohydrates to 5ꢀHMF has not been reported until now. The FePO4 is
considered as a “soluble” acidic catalyst, which is insoluble at low temperature (solubility product, Ksp =
41
1.3 × 10ꢀ22 at RT) but is partially dissolved and act as homogeneous catalyst during high temperature
reaction. The reaction system successfully avoids the drawback of difficult separation and recycling of
homogeneous catalysts because partially dissolved or hydrolyzed FePO4 will be precipitated again after
cooling to room temperature.
2. Experimental
Microcrystalline cellulose were purchased from the SigmaꢀAldrich company in China.
Microcrystalline cellulose (50 um) were directly used for the dehydration reaction without any other
pretreatment. Camellia oleifera shell was obtained from local resources in Hunan province, China and
was crushed and sieved to around 75 um. FePO4.2H2O (SigmaꢀAldrich) and H3PO4 were used as the
reaction catalyst. Reactions were conducted in a 100 mL autoclave equipped with a thermostat and an
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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.
After the reaction, the liquid products were analyzed by highꢀperformance liquid chromatography
(HPLC, Agilent 1200) using a column (Zorbax SBꢀC18) with a UV detector to analyze 5ꢀHMF yield. The
reducing sugar was detected by DNS method according to the reference.42 The conversion of monoꢀ and
diꢀsaccharides was calculated as follows: conversion (%) = (moles of substrate reacting)/(moles of
substrate starting)*100, while the conversion of cellulose and camellia oleifera shell was calculated by the
weight change of substrate before and after the reaction. The yield of 5ꢀHMF was calculated as: yield (%)
= (moles of 5ꢀHMF in the products)/(moles of feedstocks put into the reactor)*100.
IR spectra of fresh FePO4.2H2O and the used catalyst were recorded on a Thermo Nicolet Nexus 470
spectrometer. The Fe content of the water solution after the reaction was measured by atomic absorption
spectroscopy (AAS).
3. Results and Discussion
3.1 Conversion of fructose into 5-HMF
H3PO4 and FePO4 were tested for their activity toward conversion of fructose to 5ꢀHMF using
both a single phase and a biphasic reactor system. The singleꢀphase system contained the catalyst,
feedstock, and water, while the biphasic system was composed of a THF phase, an aqueous phase of
fructose, and FePO4 or H3PO4 catalyst. The data listed in Table 1, entries 2 and 3, clearly shows that the
use of a biphasic system results in higher 5ꢀHMF yield at the same temperature and reaction time. This is
due to the presence of a waterꢀimmiscible THF phase that extracts 5ꢀHMF from the aqueous phase,
thereby preventing the quick degradation of 5ꢀHMF into undesired byꢀproducts under high temperature.
To improve 5ꢀHMF selectivity, we examined the effects of adding inorganic salts to the aqueous
phase of the biphasic systems. It has been previously shown that addition of inorganic salts to biphasic
systems remarkably improves the partitioning of 5ꢀHMF into the organic phase by means of the saltingꢀ
out effect, whereby electrolytes alter the intermolecular bonding interactions between liquid components,
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decreasing the mutual solubility of the aqueous and organic phases.1 It has also been shown that NaCl
was the most efficient extracting agent of 5ꢀHMF from the aqueous to organic phase and gave the highest
5ꢀHMF selectivity among a number of different salts such as MgCl2, KBr, and NaCl.39 The results listed
in Table 1, entries 4ꢀ8, clearly indicate that the selectivity to 5ꢀHMF is remarkably increased with the
addition of NaCl to the aqueous phase and suggest that the activity of FePO4 is not affected by the
presence of NaCl. For example, compared to experiments performed without salts, a 20 mg/mL fructose
solution saturated with NaCl using THF as the extraction solvent results in an improvement in 5ꢀHMF
yield from 39.3% to 63.4% (Table, entries 3 and 4). The NaCl mainly contributed to the extraction of 5ꢀ
HMF from aqueous phase to the THF phase, thereby suppressed side reactions. Notably, the presence of
NaCl has the additional benefit of allowing higher volumetric ratio of organic to aqueous phases
(Vorg./Vaq.) to be used, thus resulting in higher 5ꢀHMF selectivity, while maintaining biphasic reaction
conditions. Specifically, when the ratio Vorg./Vaq. is equal to 5, the water/THF biphasic system without
NaCl becomes monophasic, whereas the system saturated with NaCl remains biphasic at the same
Vorg./Vaq. ratio, with an HMF yield of 71.5% (Table 1, entry 8). It should be noted that FePO4 catalyst gave
a similar 5ꢀHMF yield to that of a homogeneous H3PO4 catalyst under identical reaction conditions,
which demonstrates that solid acid catalyst has a competing catalytic activity compared to the
corresponding homogeneous acid catalyst.
3.1.1 Effect of temperature and reaction time on 5-HMF yield
The influence of the reaction temperature on fructose conversion into 5ꢀHMF catalyzed by FePO4
o
in the biphasic reaction system was investigated (Table 2). When the reaction was carried out at 120 C,
high fructose conversion was observed after only 15 min though 5ꢀHMF yield was only 35%. When the
o
reaction temperature was heated to 140 and 160 C, 5ꢀHMF yields of 60.0% and 62.6% were obtained,
respectively, for 15 min reaction time. Higher reaction temperatures not only favored the dehydration of
fructose into 5ꢀHMF, but also accelerated the formation of humins or resulted in the rehydration of 5ꢀ
HMF into levulinic acid and formic acid. The effect of reaction time on fructose conversion and 5ꢀHMF
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conversion was examined as well, and the results are shown in Table 2. No significant increase in the 5ꢀ
o
HMF yield was observed at 140 C with increasing reaction time, indicating that the catalyst has very
high activity and that fructose was almost completely converted after the reaction time of 5 min.
3.1.2 Influence of catalyst amount on 5-HMF yield
Fig. 1 shows the effects of different amounts of catalyst on the fructose conversion and 5ꢀHMF
selectivity. In the presence of 0.02 g FePO4, a 5ꢀHMF yield of 44.5% was obtained after 15 min. After
adding more catalyst (0.10 and 0.20 g), the 5ꢀHMF yield increased to 64.8 and 68.8%, respectively, under
the same reaction conditions. On further increasing the amount of catalyst (0.25 g), the 5ꢀHMF yield
increased slowly to 70.6%.
3.1.3 Effect of fructose concentration on 5-HMF yield
The effectiveness of a catalytic reaction system to high feedstock concentration is an important factor
used to evaluate its efficiency and its potential for use in practical applications. In the acidꢀcatalyzed
dehydration reaction of fructose, 5ꢀHMF molecules produced tend to react with fructose molecules and
crossꢀpolymerize to form humins.1,39 Therefore, the fructose concentration has a significant influence on
the 5ꢀHMF yield and selectivity, especially in aqueous solvents.1 The effect of initial fructose
concentration on 5ꢀHMF yield for the reaction in the biphasic system was investigated (Fig. 2). As can be
seen, the yield of 5ꢀHMF produced from fructose decreases with increasing fructose concentration. For
example, when fructose concentrations of 20, 50, and 100 mg/mL were used, the 5ꢀHMF yields were
69.7, 63.7, and 48.2%, respectively. Similarly, the 5ꢀHMF selectivity also decreases on increasing the
fructose concentration. This result is in good agreement with literature reports that increasing the fructose
concentration results in a decrease in the 5ꢀHMF yield and selectivity.1
3.2 Effect of different feedstocks on 5-HMF yield
Because the FePO4 catalyst exhibited good activity in the conversion of fructose, we also used
other carbohydrates such as glucose and sucrose as renewable raw materials. Glucose is the most
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abundant monosaccharide and the cheapest hexose, making it a promising candidate as renewable raw
material for the production of 5ꢀHMF.6,7 Synthesis of 5ꢀHMF from sucrose involves a hydrolysis into
glucose and fructose in the presence of an enzyme or acidic catalyst, followed by isomerization of glucose
to fructose and subsequent dehydration of fructose to 5ꢀHMF.43,44 The data listed in Table 3, entries 2ꢀ3,
shows the results of conversion of glucose and sucrose to 5ꢀHMF catalyzed by FePO4. 5ꢀHMF yields
of ～23.9% and 39.3% at the complete conversion of 100 mg/mL glucose and 100 mg/mL sucrose,
respectively, were achieved in a biphasic H2O/THF system. It is assumed that the conversion of glucose to
5ꢀHMF can occur via an isomerization of glucose to fructose in the presence base catalysts or Lewis acid
catalysts such as Snꢀbeta,35,39 followed by the dehydration of fructose to 5ꢀHMF using acid catalysts. Here
we propose a similar mechanism for FePO4 catalyzed glucose conversion to 5ꢀHMF.
The promising activity of the FePO4 catalyst in synthesize 5ꢀHMF from glucose and sucrose
opens up the possibility of using more complex carbohydrates such as cellulose and lignocellulosic
feedstocks, i.e., camellia oleifera shell, as renewable raw materials. Synthesis of 5ꢀHMF from cellulose
and camellia oleifera shell involves their hydrolysis into their glucose monomers in the presence of an
acidic catalyst, followed by an isomerization of glucose to fructose and subsequent dehydration of
fructose to 5ꢀHMF.39,45 In addition, catalytic dehydration of camellia oleifera shell may produce furfural
besides 5ꢀHMF since it contains semiꢀcellulose. Table 3, entries 4ꢀ5 display the results obtained using
FePO4 catalyst for the conversion of cellulose and camellia oleifera shell to 5ꢀHMF. 5ꢀHMF yields of
31.8% and 7.8%, respectively, were achieved using the biphasic reactor system at 180 oC for 15 min. A 5ꢀ
o
HMF yield of 48% was achieved at 160 C for 60 min. It should be noted that we did not examine the
most optimal reaction conditions for the dehydration of cellulose and camellia oleifera shell, since we
only investigate whether FePO4 catalyst can catalyze dehydration of cellulose and the lignocellulosic
feedstock to generate 5ꢀHMF or not. Further attempts to explore the optimum reaction conditions of
cellulose and camellia oleifera shell are in progress. Moreover, we found that a higher 5ꢀHMF yield was
obtained using cellulose as the feedstock than glucose (Table 3, entries 2 and 6). It is well known that
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dehydration of cellulose to 5ꢀHMF is more difficult than that of glucose because conversion of cellulose
to 5ꢀHMF needs an extra step of hydrolysis of cellulose to glucose monomers catalyzed by an acid
catalyst.9 The higher yield of 5ꢀHMF from cellulose here can be explained by the fact that at high
temperature trace amounts of partially dissolved FePO4 could be hydrolyzed to Fe(OH)3 and H3PO4, with
the latter acting as a highly effective homogeneous acid to hydrolyze βꢀ1,4ꢀglucosidic bonds of cellulose
and camellia oleifera shell to monosaccharide, which is then eventually converted to 5ꢀHMF.
3.3 Recycling of FePO4 catalyst
To investigate the activity and stability of the FePO4 catalyst in the biphasic reaction system,
recycling of the FePO4 catalyst was studied over five successive cycles (Fig. 2). Experiments were
o
performed at 140 C for 15 min. After the reaction, the mixture was cooled to room temperature and
filtered to obtain solid FePO4 catalyst. Then the catalyst was washed using diethyl ether and water. It can
be seen from Fig. 2 that 5ꢀHMF yield of 50.5% was obtained for the first use of the FePO4 catalyst and
the activity was almost the same after five successive recycles. This indicates that the FePO4 catalyst
retained good activity for the conversion of fructose to 5ꢀHMF after repeated use. IR spectra obtained
showed that the structure of the used FePO4 catalyst did not change after the reaction since the band at
1052 cmꢀ1 assigned to PꢀO bond did not change, as shown in Figs. 4a and 4b. Interestingly, it was found
that some lightꢀyellow solid was precipitated from the aqueous phase after cooling to room temperature,
as shown in Fig. 5. The IR spectra further show that the solid is the FePO4 catalyst (Fig. 4c). This
provides direct evidences that part of the FePO4 catalyst was dissolved at high temperature. This could be
explained by the fact that the FePO4 catalyst is almost insoluble at room temperature (Ksp = 1.3 × 10ꢀ22),41
but less amount of the FePO4 catalyst can be dissolved when the temperature is greater than 140 oC (Table
1S, Supporting information). AAS result shows that the Fe ion concentration of the aqueous solution is ca.
0.022 mg/ml after the reaction (i.e. the precipitation reaction had been finished) when using 0.5 g FePO4
catalyst at the reaction temperature of 180 oC. This suggests that the FePO4 loss is smaller than 0.5% after
it was precipitated upon cooling to room temperature. To investigate whether the reaction proceeds
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through homogeneous or heterogeneous catalysis, we used the filtered aqueous solution containing
dissolved FePO4 to carry out the fructose dehydration reaction without the addition of fresh FePO4
catalyst (Table 1, entry 9). It should be noted that the filtered aqueous solution contained high
concentration Fe3+ ions since the solution was immediately filtered after the reaction and the precipitation
phenomenon did not occur (Table 1S, Supporting information). Under the same reaction conditions, we
found that the filtered aqueous solution exhibit an almost comparable 5ꢀHMF yield as compared to solid
FePO4 catalyst (Table 1, entries 6 and 9). Therefore we propose that the reaction mainly proceeds through
homogeneous catalysis by dissolved FePO4 catalyst for the dehydration of fructose. Although the
dehydration reaction take place by homogeneous catalysis, the FePO4 catalyst can be readily recycled and
separated via cooling to the room temperature since the FePO4 catalyst is almost insoluble at room
temperature (Ksp = 1.3 × 10ꢀ22). Therefore, this novel catalytic system can be reused without loss of
catalytic activity, being beneficial to the potential application for large scale 5ꢀHMF synthesis and from
the industrial point of view.
3.4 Possible production pathways of 5-HMF
In general, the production of 5ꢀHMF from cellulosic or lignocellulosic feedstocks must be via acidꢀ
catalyzed depolymerization of cellulose to produce glucose in the first step.9 However, the FePO4 catalyst
is not a proton acid but has the activity of cracking βꢀ1,4ꢀglucosidic bonds of cellulose, so we propose
that partially dissolved FePO4 could be hydrolyzed to produce Fe(OH)3 and H3PO4 at high temperature. A
mixture of Fe(OH)3 and H3PO4 was employed to carry out the catalytic conversion of cellulose at 160 oC,
the result as shown in Table 3, entry 9. It can be seen that the mixture of Fe(OH)3 and H3PO4 gave 36.6%
yield of 5ꢀHMF. It should be noted that it is very difficult to identify the hydrolyzed product of FePO4 in a
sealed and highꢀpressure autoclave during the high temperature reaction. It is wellꢀknown that H3PO4 is
an effective acid catalyst that can hydrolyze cellulose to produce glucose46 and therefore, it is reasonable
that cellulose would be hydrolyzed to glucose by H3PO4 produced from the hydrolysis of FePO4. After the
formation of glucose, the glucose molecule isomerizes into fructose which was then converted into 5ꢀ
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HMF via the acidꢀcatalyzed dehydration. To further investigate the conversion mechanism of cellulose,
two control experiments were carried out by using FeCl3 or Na3PO4 as the catalyst, the results of which
are shown in Table 3, entries 7ꢀ8. It can be seen that the two catalysts gave very low yields of 5ꢀHMF.
This shows that it is very important for the coꢀexist of iron and phosphate ions to obtain high 5ꢀHMF
yield. It is widely accepted that the catalysts containing Lewis acid centers, such as SnꢀBeta and TiꢀBeta,
can isomerize glucose into fructose at relative low temperatures.39 In this work, a Lewis acid catalyst, i.e.
FeCl3, was also proved to be active for the isomerization of glucose into fructose during the
transformation of cellulose, which is agreement with the results that FeCl3 was reported to be active for
the dehydration of fructose, glucose and sucrose to 5ꢀHMF,11,47 Similarly, we propose that the “oneꢀpot”
conversion of sucrose to 5ꢀHMF proceeds through a cascade reaction sequence involving the
homogeneous acid catalyzed depolymerization of sucrose to fructose and glucose, a Lewis acid site (Fe3+)
ꢀ catalyzed isomerization of glucose to fructose, and a homogeneous acid (H3PO4) ꢀ catalyzed dehydration
of fructose to 5ꢀHMF (Scheme 1). Camellia oleifera shell was converted to furfural besides 5ꢀHMF
because it contains cellulose and semiꢀcellulose. The production pathways of furfural and 5ꢀHMF proceed
through the route in which camellia oleifera shell may be firstly hydrolyzed into monosaccharide
including C5 sugars such as xylose, and glucose, which subsequently undergo an isomerization reaction
(glucose) to fructose followed by the catalytic dehydration to generate 5ꢀHMF or a dehydration reaction
(xylose) to produce furfural via xylose intermediate.48
As mentioned above, the higher temperature will result in an increase of FePO4 solubility (Table 1S,
Supporting information), moreover, the soluble FePO4 will be precipitated into the lightꢀyellow solid after
48 h storage at room temperature (Fig. 4). Moreover, we infer that FePO4 could be hydrolyzed to H3PO4
and Fe(OH)3 at high temperature based on the results that the reaction system can hydrolyze cellulose and
camellia oleifera shell at higher temperatures and the catalytic result of the mixture of H3PO4 and
Fe(OH)3 (Table 3, entry 9). After the reaction, H3PO4 might react with Fe(OH)3 to form insoluble FePO4
again upon cooling to room temperature (Fig. 4 and Table 1S). This temperatureꢀcontrolled separation
process overcomes the concerns of recycling cost and pollution of adding mineral acids, which were
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usually used in the conversion of cellulose or lignocellulosic feedstocks to 5ꢀHMF in ionic liquid solvents
or a biphasic reactor system.9,24,49 Therefore, the FePO4 catalyst not only has the comparable catalytic
performances of homogeneous acids (e.g. hydrolysis of cellulose), but it avoids the drawback of
difficulties to be separated and recycled. Although the FePO4 catalyst is not perfect since about 0.5%
FePO4 was final loss into the reaction solution, the strategy of “double character” catalyst in the
dehydration of lignocellulosic materials will provide some new ideas for the industrial application of 5ꢀ
HMF production.
4. Conclusion
In conclusion, a novel, readily recycled FePO4 catalyst was developed for the direct conversion of
carbohydrates including fructose, glucose, sucrose, cellulose, and camellia oleifera shell to 5ꢀHMF
catalyzed by the inexpensive FePO4 without the addition of homogeneous acids. Using the FePO4
catalyst, a 5ꢀHMF yield of 71.5% from fructose could be achieved in the biphasic reaction system.
Furthermore, the catalyst was shown to act as highly effective homogeneous acid to hydrolyze βꢀ1,4ꢀ
glucosidic bonds of cellulose and camellia oleifera shell to monosaccharides, which were eventually
converted to 5ꢀHMF via an isomerization reaction of glucose to fructose and then fructose dehydration
reaction. The “double character” catalyst has the combined character of both homogeneous and
heterogeneous catalysts, which could act as a dissolved homogenous acid at higher temperature and as an
insoluble solid acid that is easily separated and recycled after cooling to room temperature. The catalyst
was stable over five reaction cycles and the biphasic reaction system could be applied to the conversion of
lignocellulosic feedstocks without the addition of homogeneous acids under mild conditions.
5. Acknowledges
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). This work
was also financially supported by the open foundation of State Key Laboratory of Catalysis from Dalian
Institute of Chemical Physics (Nꢀ12ꢀ09) and the Priority Academic Program Development of Jiangsu
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Higher Education Institutions (PAPD).
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Table 1 Results for the conversion of fructose to 5ꢀHMFa
Reaction Conversio
Catalyst
5-HMF
Entry System
Catalyst
time/min
n
amount/g
yield/mol%
/%
1
2
3
4
5
6
7
8
single phase(H2O)
H3PO4
FePO4
FePO4
H3PO4
FePO4
FePO4
FePO4
FePO4
FePO4(
soluble)
0.25
0.10
0.10
0.10
0.10
0.20
0.05
0.20
30
30
15
15
15
15
15
15
97.4
95.4
99.5
99.9
99.7
99.9
99.5
99.9
44.5
17.0
39.3
63.4
65.0
68.8
62.8
71.5
single phase(H2O)
biphasic(H2O/THF)
biphasic(H2O/THF/NaCl)
biphasic(H2O/THF/NaCl)
biphasic(H2O/THF/NaCl)
biphasic(H2O/THF/NaCl)
biphasic(H2O/THF/NaCl)
9b
biphasic(H2O/THF/NaCl)
/
15
99.9
71.7
a Reaction conditions: Fructose (20 mg/mL H2O), the reaction temperature (140 oC). Entries 3ꢀ6: aqueous
to organic phase volume ratio of 3. Entries 7ꢀ8: aqueous to organic phase volume ratio of 5. Entries 4ꢀ8:
NaCl (0.35 g dissolved in 1 g H2O) was added.
b The aqueous phase solution was used as the catalyst, which was obtained by filtering and separating the
reaction system (entry 6) in order to remove both the solid and the organic phase.
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Table 2 Effect of reaction temperature and time on fructose conversion and 5ꢀHMF yield using FePO4 as
the catalyst a
Temperature
Reaction time Conversion
5-HMF yield
/mol%
34.6
Entry
/oC
/min
15
15
15
15
5
/%
1
2
3
4
5
6
120
140
160
180
140
140
98.8
99.9
99.7
99.9
99.7
99.9
62.6
60.2
54.2
59.5
30
62.8
a Reaction conditions: Fructose (0.2 g), FePO4 (0.05 g), H2O (10 mL), THF (30 mL), NaCl (3.5 g).
b Fructose concentration is defined as fructose mass divided by the aqueous volume.
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Table 3 Results for the conversion of different feedstocks to 5ꢀHMFa
Conversion
5-HMF yield
/mol%
50.5
Entry
Catalyst
Feedstock
Temperature/oC
/%
97.4
97.8
96.1
/
1
2
3
4
FePO4
FePO4
FePO4
FePO4
Fructose
Glucose
140
140
140
180
23.1
Sucrose
39.3
Cellulose
Camellia Oleifera
Shell
31.8
5
FePO4
180
/
7.8b
6
7
8
FePO4
Na3PO4
FeCl3
Cellulose
Cellulose
Cellulose
160
160
160
87.4
96.6
82.3
48.0
6.8
8.6
Fe(OH)3 +
H3PO4
9c
Cellulose
160
/
36.6
a
Entries 1ꢀ5: 1.0 g feedstock, 0.5 g catalyst (FePO4), 10 mL H2O, 30 mL THF, 3.5 g NaCl, the reaction
time of 15 min; entries 6: 0.5 g catalyst, 1 g cellulose, 10 mL water, 30 mL THF, 3.5 g NaCl, 60 min;
Entries 7ꢀ8: 0.1 g catalyst, 1 g cellulose, 10 mL water, 30 mL THF, 3.5 g NaCl, 60 min.
b The yield is defined as the 5ꢀHMF mass divided by the feedstock mass;
cThe reaction time of 60 min, 0.0015 mol H3PO4, 0.001 mol Fe(OH)3, 30 Ml THF, 10 mL H2O,
3.5 g NaCl.
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100
90
80
70
60
50
70
60
50
40
0.00 0.05 0.10 0.15 0.20 0.25
Catalyst amount/g
Fig. 1 Effect of FePO4 amount on fructose conversion and 5ꢀHMF yield. Reaction conditions: Feedstock
(0.2 g), H2O (10 mL), THF (30 mL), NaCl (3.5 g), 15 min, 140 oC.
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Fructose conversion
5-HMF yield
5-HMF selectivity
100
80
60
40
20
0
50 mg/mL
100 mg/mL
20 mg/mL
Fructose concentration
Fig. 2 Effect of fructose concentration on fructose conversion, 5ꢀHMF yield and selectivity using FePO4
catalyst in the H2O/THF biphasic system. Reaction conditions: H2O/THF 1:3 phase volume ratio (10 mL
H2O), NaCl (3.5 g), 140 oC, 15 min.
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Fructose conversion
5-HMF yield
100
80
60
40
20
0
1
2
3
4
5
Reaction times
Figure 3 Catalyst recycling experiments. Reaction conditions: Feedstock (1.0 g), catalyst (FePO4, 0.5 g)
o
H2O (10 mL), THF (30 mL), NaCl (3.5 g), 140 C, 15 min. The filtered solid residue was washed and
used to next recycle experiment.
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A
B
solid FePO4
Figure 4 (A) The aqueous solution obtained via filtrating and separating the reaction mixture, and (B)
after 2 days storage of solution A. Reaction conditions: Feedstock (1.0 g), catalyst (FePO4, 0.5 g), H2O
(10 mL), THF (30 mL), NaCl (3.5 g), 140 oC, 15 min.
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c
b
a
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber/cm-1
Fig. 5 IR spectra obtained for (a) fresh FePO4.2H2O, (b) the used FePO4, and (c) the solid obtained from
the precipitated mixture of the aqueous solution (i.e. Fig. 4B)
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OH
O
H
H+
O
fructose
glucose
Lewis acid
Fe3+ + PO4
Fe3+
3-
Heating
Cooling
FePO4
(soluble)
- H2O
glucose
FePO4
cellulose
Fe(OH3) + H3PO4
+ H2O
(insoluble)
camellia
oleifera
shell
Scheme 1 The proposed reaction pathways of FePO4 catalyzed dehydration of carbohydrates to produce
5ꢀHMF.
24