catalysts
Article
Efficient Dehydration of Fructose to 5-Hydroxymethylfurfural Catalyzed by Heteropolyacid Salts
Yanlei Song 1 , Xincheng Wang 1 , Yongshui Qu 2 , Chongpin Huang 1, *, Yingxia Li 1
and Biaohua Chen 1
1
2
*
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology,
Beijing 100029, China; [email protected] (Y.S.); [email protected] (X.W.); [email protected] (Y.L.);
[email protected] (B.C.)
National Key Laboratory of Biochemical Engineering, Institute of process Engineering,
Chinese Academy of Sciences, Beijing 100190, China; [email protected]
Correspondence: [email protected]; Tel.: +86-10-6441-2054
Academic Editor: Rafael Luque
Received: 14 November 2015; Accepted: 16 February 2016; Published: 23 March 2016
Abstract: 5-Hydroxymethylfurfural (5-HMF), which is derived from numerous industrial biomass
resources, has attracted attention in recent years due to its potential as a building block. In this
paper, a range of heteropolyacid salts had been investigated for the dehydration of fructose to 5-HMF.
CePW12 O40 demonstrated the best catalytic activity. Effects of fructose concentration, reaction
temperature and reaction time on 5-HMF yield were investigated and optimised through a central
composite design and response surface methodology. The optimal 5-HMF yield was 99.40% under
the optimized reaction conditions of 5.48 mg/mL fructose loading, 158 ˝ C temperature and 164 min
reaction time. A kinetic analysis of the fructose conversion was also performed, and the activation
energy and pre-exponential factor were obtained.
Keywords: 5-hydroxymethylfurfural; dehydration; fructose; heteropolyacid salt; response-surfacemethodology optimisation
1. Introduction
The massive consumption of unsustainable fossil resources has stimulated the search for
renewable resources [1,2]. Biomass has the potential to serve as sustainable resources for the production
of fuel and chemicals [3,4]. 5-Hydroxymethylfurfural (5-HMF) is one of the top biomass-derived
building blocks from which a wide range of value-added compounds can be synthesised, such as
5-dimethylfuran, 2,5-furandicarboxylic acid, levulinic acid, and so on [5–9]. The dehydration of
fructose to 5-HMF has attracted considerable attention in the past few years [10]. Water, organic
solvents, organic/water mixtures and a few other biphasic systems have been used as reaction solvents
during this process [11–13].
Preparation of 5-HMF by chemical catalysis involves mainly dehydration of sugars, which can
be efficiently catalyzed by acid catalysts [14]. Mineral acids (H2 SO4 , HCl and HNO3 ) and transition
metal salts have been widely used in the production of 5-HMF [15–19]. However, the homogeneous
methods suffers from complicated separation processes especially considering the regeneration of
the system. Therefore, heterogeneous catalysts have been proposed. It’s worthwhile to mention
that the heterogeneous acid catalysts have shown superior activity to homogeneous catalysts in
terms of 5-HMF selectivity [20]. H-form zeolites and metal phosphates showed 5-HMF selectivities of
60%–90% with fructose conversions in the 30%–60% range [21,22]. Dumesic et al. reported that acidic
ion-exchange resin could effectively catalyze the dehydration of 30 wt. % fructose to 5-HMF, resulting
into 60% selectivity with 89% fructose conversion [13]. Sulfated zirconia was also demonstrated
Catalysts 2016, 6, 49; doi:10.3390/catal6040049
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Catalysts 2016, 6, 49
Catalysts 2016, 6, 49 2 of 11
2 of 11 to be active for the production of 5-HMF with yield as high as 72.8% [23]. Heteropolyacid salts are
active for the production of 5‐HMF with yield as high as 72.8% [23]. Heteropolyacid salts are well‐
well-known solid acid catalysts that have been utilised in various important commercial reactions, such
known solid acid catalysts that have been utilised in various important commercial reactions, such as esterifications, isomerisations and etherifications. Shimizu evaluated the activity of heteropolyacid
as esterifications, isomerisations and etherifications. Shimizu evaluated the activity of heteropolyacid by simple
could catalyze catalyze the the
by simple water
water removal
removal methods
methods and
and found
found that
that Amerlyst-15
Amerlyst‐15 and
and FePW
FePW1212O40
40 could conversion
of
high
concentration
fructose
(50
wt.
%)
to
5-HMF
with
yields
reached
approximately
conversion of high concentration fructose (50 wt. %) to 5‐HMF with yields reached approximately 100% and 50%, respectively [24]. In addition, H33PW
O4040 and the corresponding cesium salts were and the corresponding cesium salts were
100% and 50%, respectively [24]. In addition, H
PW12
12O
used for dehydration of 50 wt % fructose in a DMSO-polyvinylpyrrolidone system with 52% and 58%
used for dehydration of 50 wt % fructose in a DMSO‐polyvinylpyrrolidone system with 52% and 58% 5-HMF yields [25]. Wang et al. demonstrated that Cs
HPW
O40 exhibited 5-HMF selectivity of
5‐HMF yields [25]. Wang et al. demonstrated that Cs
2.52.5
H0.5
12O12
40 exhibited 5‐HMF selectivity of 94.7% 0.5 PW
94.7% with a final yield of 74% in a biphsic system of water-methylisobutylketone [26].
with a final yield of 74% in a biphsic system of water‐methylisobutylketone [26]. In the present study, several types of heteropolyacid salts were synthesised and utilised for
In the present study, several types of heteropolyacid salts were synthesised and utilised for the the catalytic
dehydration
of fructose
5-HMFwith withoptimum optimumyields yields over over 99%. 99%. sec‐Butanol sec-Butanol was was
catalytic dehydration of fructose to to
5‐HMF demonstrated to be the best reaction solvent. A central composite design (CCD) and response surface
demonstrated to be the best reaction solvent. A central composite design (CCD) and response surface methodology (RSM) (RSM) were were employed employed to to design design experiments experiments and and optimise optimise the the conversion.
Finally,
methodology conversion. Finally, several kinetic parameters for the reaction were obtained.
several kinetic parameters for the reaction were obtained. 2. Results and Discussion
2. Results and Discussion 2.1. Effect of Reaction Medium on the Fructose to 5‐HMF Conversion Yield 2.1. Effect of Reaction Medium on the Fructose to 5-HMF Conversion Yield
The comparative XRD and IR spectra of the heteropolyacid salts are presented in Figures 1–2. In The comparative XRD and IR spectra of the heteropolyacid salts are presented in Figures 1 and 2.
−1, the fingerprint vibrational bands of Keggin anions are ´1 , the fingerprint vibrational bands of Keggin anions
the wavenumber region from 700 to 1100 cm
In the wavenumber region from 700 to 1100 cm
observed. The XRD measurement indicated that the crystalline structures of the HPAs still remained are observed. The XRD measurement indicated that the crystalline structures of the HPAs still remained
in metal‐substituted form. in the the metal-substituted
form. Different Different heteropolyacid heteropolyacid salts salts and and solvents solvents were were employed employed for for the the
dehydration dehydration of of fructose. fructose. The The blank blank references references of of DMSO, DMSO, n‐butanol n-butanol and and sec‐butanol sec-butanol showed showed that that
DMSO was the most effective solvent without catalyst. Different catalysts significantly affected the DMSO was the most effective solvent without catalyst. Different catalysts significantly affected the
conversion of fructose into 5‐HMF (Table 1, entries 5–9). Among the catalysts tested, CePW
O40
40 and conversion of fructose into 5-HMF (Table 1, entries 5–9). Among the catalysts tested, CePW1212O
and
Zn1.5
OO
40 showed the strongest and weakest catalytic activities, affording 88.78% and 26.22% 5‐
PW1212
1.5PW
40 showed the strongest and weakest catalytic activities, affording 88.78% and 26.22%
HMF, respectively (Table 1, entries 5 and 9). In addition, the yield of 5‐HMF increased as the reaction 5-HMF, respectively (Table 1, entries 5 and 9). In addition, the yield of 5-HMF increased as the reaction
time was prolonged with only slightly higher conversion of fructose (Table 1, entries 9 and 11). time was prolonged with only slightly higher conversion of fructose (Table 1, entries 9 and 11).
CePW12O40
Cs3PW12O40
Intensity
Cu1.5PW12O40
Ni1.5PW12O40
Zn1.5PW12O40
H3PW12O40
10
20
30
40
A
Figure 1. XRD patterns of heteropolyacid salts. Figure 1. XRD patterns of heteropolyacid salts.
50
Catalysts 2016, 6, 49
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Catalysts 2016, 6, 49 3 of 11 CePW12O4
Transmittance (a.u)
Cu1.5Pw12O4
Cs3PW12O4
Zn1.5PW12O4
Ni1.5PW12O4
H3PW12O4
1100
1000
900
800
700
Wave number (cm-1)
600
500
Figure 2. FTIR spectra of heteropolyacid salts.
Figure 2. FTIR spectra of heteropolyacid salts. Table 1. Conversion of fructose to 5-HMF with different solvents and catalysts.
Table 1. Conversion of fructose to 5‐HMF with different solvents and catalysts. Entry Entry
1 1
2 2
3 3
4 4
5
5 6
6 7
7 8
8 9
9 10
11
10 12
11 13
14
12 15
13 14 Different
15 Solvent Catalyst
Reaction Time/min
Conv. % Yield % Solvent
Catalyst
Reaction Time/min
Conv. %
Yield %
DMSO Blank 60 18.11 17.53 DMSO
Blank
60
18.11
17.53
n‐Butanol Blank 60 1.56 0.58 n-Butanol
Blank
60
1.56
0.58
sec-Butanol
Blank
60
2.15
1.04
sec‐Butanol Blank 60 2.15 1.04 sec-Butanol
K
PW
O
60
75.85
71.43
3
12
40
sec‐Butanol K3PW12O40 60 75.85 71.43 DMSO
Zn1.5 PW12 O40
60
27.52
26.22
DMSO Zn
1.5PW12O40
60 27.52 26.22 DMSO
Ni1.5 PW12 O40
60
62.42
60.67
DMSO Ni1.5
12O40
60 62.42 60.67 DMSO
CsPW
60
75.73
73.59
3 PW12 O40
DMSO CsCu
3PW
12O
60 75.73 73.59 DMSO
O 40
60
85.68
82.71
1.5 PW
1240
DMSO
CePW
O40
60
89.61
88.78
40
DMSO Cu1.5
PW1212O
60 85.68 82.71 DMSO
Cu1.5 PW12 O40
120
93.79
86.22
DMSO CePW
12O40 60 89.61 88.78 DMSO
CePW12 O40
120
96.23
93.86
DMSO Cu1.5
PW12O
40
120 93.79 86.22 DMAc
CePW
120
91.35
90.13
12 O40
DMSO CePW
12
O
40
120 96.23 93.86 MIBK
CePW12 O40
120
88.92
86.74
n-Butanol
CePW
120
98.56
97.48
DMAc CePW
1212
OO
4040
120 91.35 90.13 sec-Butanol
CePW
O
120
99.10
98.15
12
40
MIBK CePW12O40 120 88.92 86.74 n‐Butanol CePW12O40 120 98.56 97.48 protic
(butanols)CePW
and aprotic
were employed
sec‐Butanol 12O40 (MIBK, DMSO,
120 etc.) solvents
99.10 98.15 in the
dehydration of fructose with CePW12 O40 as the catalyst (Table 1, entries 11–15). Protic solvents
performed
than
the aproticand onesaprotic and sec-butanol
determined
to be the most
solvent
Different better
protic (butanols) (MIBK, was
DMSO, etc.) solvents were efficient
employed in the (Table
1,
entry
15),
showing
that
CePW
O
exhibited
the
highest
catalytic
activity
in
the
fructose
12 as 40 the catalyst (Table 1, entries 11–15). Protic solvents dehydration of fructose with CePW12O40
conversion among the protic solvents. Moreover, the CePW O40 -sec-butanol combination behaved as
performed better than the aprotic ones and sec‐butanol 12was determined to be the most efficient a thermoregulated system and there was a phase change from heterogeneous towards homogeneous
solvent (Table 1, entry 15), showing that CePW12O40 exhibited the highest catalytic activity in the catalysis during the reaction (Figure 3). After cooling down, the reaction solution turned heterogeneous
fructose among the beprotic solvents. Moreover, the CePW
12O40‐sec‐butanol combination againconversion and the catalysts
could
removed
from the
reaction mixture
by simple
filtration.
behaved as a thermoregulated system and there was a phase change from heterogeneous towards homogeneous catalysis during the reaction (Figure 3). After cooling down, the reaction solution turned heterogeneous again and the catalysts could be removed from the reaction mixture by simple filtration. Catalysts 2016, 6, 49
4 of 11
Catalysts 2016, 6, 49 4 of 11 Figure 3. The conversion of fructose to 5‐HMF over CePW12O40 in sec‐butanol. Figure 3. The conversion of fructose to 5-HMF over CePW12 O40 in sec-butanol.
2.2. Optimization of Process Conditions 2.2. Optimization of Process Conditions
The conversion of fructose into 5‐HMF catalyzed by CePW12O40 in sec‐butanol was optimised by The
conversion of fructose into 5-HMF catalyzed by CePW
was optimised by
12 O40 in sec-butanol
1), temperature (X
2) and reaction time CCD in the RSM. The correlation among fructose loading (X
CCD(X
in3) was estimated by ANOVA. The 5‐HMF yield was used as the response to evaluate the conversion the RSM. The correlation among fructose loading (X1 ), temperature (X2 ) and reaction time
(X3 ) was
estimated by ANOVA. The 5-HMF yield was used as the response to evaluate the conversion
efficiency and the targets of optimisation. efficiencyThe results of ANOVA with the 5‐HMF yield as the response are shown in Table 2. The Model and the targets of optimisation.
F‐value of 1275.81 implies that the model is significant. The “Model F‐Value” could obtain this value The results of ANOVA with the 5-HMF yield as the response are shown in Table 2. The Model
with only a 0.01% chance because of noise. Values of “Prob > F” less than 0.05 indicate that the model F-value
of 1275.81 implies that the model is significant. The “Model F-Value” could obtain this value
terms are significant. In this case, B, C, AB, AC, BC, A2, B2 and C2 were the significant model terms. with only a 0.01% chance because of noise. Values of “Prob > F” less than 0.05 indicate that the model
The “Lack of Fit F‐value” of 4.55 and “p‐value” of 0.0862 (>0.05) imply that the Lack of Fit is not terms are significant. In this case, B, C, AB, AC, BC, A2 , B2 and C2 were the significant model terms.
significant. “Adeq Precision” measures the signal‐to‐noise ratio. A ratio greater than 4 is desirable. The “Lack
of Fit F-value” of 4.55 and “p-value” of 0.0862 (>0.05) imply that the Lack of Fit is not
The ratio of 107.66 indicates an adequate signal. The results showed that this model was statistically significant.
“Adeq
measures
ratio.
ratio greater
than
4 iscoded desirable.
sound and can Precision”
be utilised to navigate the
the signal-to-noise
design space. The final A
equation in terms of the The ratio
of 107.66 indicates an adequate signal. The results showed that this model was statistically
factors can be expressed as: sound and can be utilised to navigate the design space. The final equation in terms of the coded factors
98.22 0.68
6.66
12.78
13.65
21.51
12.94
6.21
(1)
can be expressed as:
8.96
9.94
3.19
2.66
2.38
3.99
yThe similarity between the predicted and actual values is described in Figure 4. The coefficient “ 98.22 ´ 0.68x1 ´ 6.66x2 ` 12.78x3 ´ 13.65x12 ´ 21.51x22 ´ 12.94x32 ´ 6.21x1 x2
of determination (R2) value was 0.9001, indicating a close agreement between the experimental and (1)
´8.96x1 x3 ´ 9.94x2 x3 ` 3.19x12 x2 ` 2.66x12 x3 ` 2.38x1 x22 ` 3.99x12 x22
theoretical values. The similarity between theTable 2. ANOVA table for the quadratic model. predicted and actual values is described in Figure 4. The coefficient
of determination (R2 ) value was 0.9001, indicating a close agreement between the experimental and
Source Sum of Squares DF *
Mean Square
F‐Value
Probability > F theoretical values.
Model 14047.75 A 2.62205 Table 2.
B 250.6561 C 923.6402 Source
Sum308.7613 of Squares
AB 14047.75
Model
AC 641.8945 A BC 2.62205
790.8265 B A2 250.6561
2237.45 C 2
923.6402
5554.388 B
AB
308.7613
2010.072 ACC2 641.8945
33.64851 BCA2B 790.8265
2237.45
23.48121 A2A2C 5554.388
18.70431 B2AB2 2010.072
50.90518 C2A2B2 2B
33.64851
5.081933 AResidual 2C
23.48121
A
Lack of Fit 2.42 18.70431
AB2
Pure Error 2.661933 2 B2
50.90518
ACor Total 14052.83 Residual
Lack of Fit
Pure Error
Cor Total
5.081933
2.42
2.661933
14052.83
13 1080.60 1275.81
<0.0001 Significant 1 2.62 3.10 0.1290 ‐ ANOVA table for the quadratic model.
1 250.66 295.94 <0.0001 ‐ 1 923.64 1090.50
<0.0001 ‐ DF
Mean
Square
F-Value <0.0001 Probability
1 *
308.76 364.54 ‐ > F
<0.0001
13
1080.60
1275.81
Significant
1 641.89 757.85 <0.0001 ‐ 2.62
3.10
0.1290
1 1
790.83 933.69 <0.0001 ‐ 250.66
295.94 <0.0001 <0.0001
1 1
2237.45 2641.65
‐ 1
923.64
1090.50
<0.0001
1 1
5554.39 6557.81
‐ 308.76
364.54 <0.0001 <0.0001
1 1
2010.07 2373.20
‐ 641.89
757.85 <0.0001 <0.0001
1 1
33.65 39.73 ‐ <0.0001
790.83
933.69 0.0007 2237.45
2641.65 0.0019 <0.0001
1 1
23.48 27.72 ‐ 5554.39
6557.81 0.0033 <0.0001
1 1
18.70 22.08 2010.07
2373.20 0.0002 <0.0001
1 1
50.91 60.10 ‐ 33.65
39.73
6 1
0.85 ‐ ‐ 0.0007
‐ 23.48
27.72
0.0019 Not significant
1 1
2.42 4.55 0.0862 18.70
22.08
5 1
0.53 ‐ ‐ 0.0033
‐ 1
50.91
60.10
19 ‐ ‐ ‐ 0.0002
‐ -
6 * DF = degree of freedom. 0.85
1
2.42
5
0.53
19
* DF = degree of freedom.
4.55
-
0.0862
-
Not significant
-
Catalysts 2016, 6, 49
Catalysts 2016, 6, 49 Catalysts 2016, 6, 49 5 of 11
5 of 11 5 of 11 100
100
Predicted
PredictedValue
Value
80
80
2
=0.9001
RR2=0.9001
60
60
40
40
20
20
00
00
20
20
40
40
60
60
80
80
100
100
ActualValue
Value
Actual
Figure 4. Predicted vs. actual 5‐HMF yield. Figure 4. Predicted vs. actual 5‐HMF yield. Figure 4. Predicted vs. actual 5-HMF yield.
The 3‐D response surface and the 2‐D contour plots that show the effects of the variables on the The 3‐D response surface and the 2‐D contour plots that show the effects of the variables on the The 3-D response surface and the 2-D contour plots that show the effects of the variables on the
5‐HMF yield are are displayed displayed in in Figure Figure 5. 5. The The interactive interactive effects effects of of independent independent variables variables on on the the 5‐HMF yield 5-HMF yield
are displayed
in Figure
5. The interactive
effects of independent
variables on the responses
responses were obtained by remaining one variable constant and changing the other two variables. responses were obtained by remaining one variable constant and changing the other two variables. were obtained by remaining one variable constant and changing the other two variables. The interaction
The interaction between factors A and B on the 5‐HMF yield is shown in Figure 5A. As the fructose The interaction between factors A and B on the 5‐HMF yield is shown in Figure 5A. As the fructose between factors A and B on the 5-HMF yield is shown in Figure 5A. As the fructose loading and
loading and temperature increased, the 5‐HMF yield rapidly increased, peaked and then decreased loading and temperature increased, the 5‐HMF yield rapidly increased, peaked and then decreased temperature increased, the 5-HMF yield rapidly increased, peaked and then decreased when the
when the the reaction reaction time time was was prolonged. prolonged. This This result result indicated indicated that that the the degradation degradation of of 5‐HMF was was when reaction
time was prolonged.
This result indicated
that the degradation
of 5-HMF was5‐HMF significant.
significant. Tarry materials were also formed; these materials were likely generated by the significant. Tarry materials were these
also materials
formed; were
these likely
materials were by
likely generated by and
the Tarry materials
were
also formed;
generated
the polymerisation
polymerisation and and cross‐polymerisation cross‐polymerisation of of 5‐HMF 5‐HMF and and intermediates intermediates that that readily readily occur occur at at high high polymerisation cross-polymerisation
of 5-HMF and intermediates
that readily
occur at high temperatures
and fructose
temperatures and fructose amount. This finding could account for the decreased yield of 5‐HMF. Figs. temperatures and fructose amount. This finding could account for the decreased yield of 5‐HMF. Figs. amount. This finding could account for the decreased yield of 5-HMF. Figs. 5B and 5C also show
5B and 5C 5C also show show the the presence presence of two‐way two‐way interactions interactions between between variables, variables, namely, namely, fructose fructose 5B the and presencealso of two-way
interactionsof between variables,
namely, fructose
loading and temperature,
loading and temperature, fructose loading and reaction time as well as temperature and reaction time. loading and temperature, fructose loading and reaction time as well as temperature and reaction time. fructose loading and reaction time as well as temperature and reaction time. Moreover, the sequence
Moreover, the sequence of the three factors that affected the 5‐HMF yield from strong to weak was Moreover, the sequence of the three factors that affected the 5‐HMF yield from strong to weak was of the three factors that affected the 5-HMF yield from strong to weak was C > B > A. This finding is
C > B > A. This finding is consistent with the results listed in Table 2. C > B > A. This finding is consistent with the results listed in Table 2. consistent with the results listed in Table 2.
Design-Expert?Software
Design-Expert?Software
Factor Coding: Actual
Factor
Coding:
5-HMF
yield Actual
5-HMF
yield Points
Design
Design
99.06Points
99.06
15.87
15.87
180.00
5-HMF yield
5-HMF yield
70
180.00
70
70
80
70
80
60
60
172.00
172.00
X1 = A: Fructtose amount
X1X2
= A:
Fructtose
amount
= B:
Temperature
X2 = B: Temperature
Actual Factor
Actual
Factor Time = 120.00
C: Reaction
C: Reaction Time = 120.00
80
70
70
60
60
B: Temperature
B: Temperature
5-HMF yield
5-HMF yield
Design-Expert?Software
Design-Expert?Software
Factor Coding: Actual
Factor
Coding:
5-HMF
yield Actual
5-HMF
yield points above predicted value
Design
Design
points
above
predicted
value
Design
points
below
predicted
value
Design
99.06points below predicted value
99.06
15.87
15.87
110
110
X1 = A: Fructtose amount
X1X2
= A:
Fructtose
amount
= B:
Temperature
100
X2 = B: Temperature
100
Actual Factor
90
Actual
Factor Time = 120.00
C: Reaction
90
C: Reaction Time = 120.00
80
164.00
164.00
6
6
156.00
156.00
50
50
40
405.00
5.00
148.00
148.00
90
80
180.00
180.00
172.00
7.00
172.00
7.00
164.00
8.00
164.00
156.00
8.00
A: Fructose amount
156.00
9.00
A: Fructose amount
148.00
9.00
148.00
B: Temperature
10.00 140.00
B: Temperature
10.00 140.00
6.00
6.00
90
80
70
80
70
80
140.00
140.00
5.00
5.00
6.00
6.00
7.00
8.00
7.00
8.00
A: Fructose amount
A: Fructose amount
(A)
(A)
Figure 5. Cont.
Figure
5. Cont.
Figure 5. Cont.
9.00
9.00
10.00
10.00
Catalysts 2016, 6, 49
Catalysts 2016, 6, 49 6 of 11
6 of 11 Design-Expert?Software
Factor Coding: Actual
5-HMF yield
Design points above predicted value
Design points below predicted value
99.06
Design-Expert?Software
Factor Coding: Actual
5-HMF yield
Design Points
99.06
5-HMF yield
180.00
70
15.87
15.87
110
Actual Factor
A: Fructtose amount = 7.50
Actual Factor
A: Fructtose amount = 7.50
90
5-HMF yield
150.00
X1 = B: Temperature
X2 = C: Reaction Time
100
80
70
C: Reaction Time
X1 = B: Temperature
X2 = C: Reaction Time
100
6
120.00
60
90
90.00
50
80
70
70
40
140.00
180.00
148.00
60
150.00
60
50
156.00
60.00
120.00
164.00
B: Temperature
90.00
172.00
180.00
140.00
148.00
156.00
164.00
172.00
180.00
C: Reaction Time
60.00
B: Temperature
(B)
Design-Expert?Software
Factor Coding: Actual
5-HMF yield
Design points above predicted value
Design points below predicted value
99.06
Design-Expert?Software
Factor Coding: Actual
5-HMF yield
Design Points
99.06
5-HMF yield
180.00
15.87
15.87
110
X1 = A: Fructtose amount
X2 = C: Reaction Time
Actual Factor
B: Temperature = 160.00
90
Actual Factor
B: Temperature = 160.00
80
70
150.00
C: Reaction Time
100
5-HMF yield
X1 = A: Fructtose amount
X2 = C: Reaction Time
100
6
120.00
90
60
90.00
50
80
70
180.00
40
5.00
60
150.00
6.00
7.00
A: Fructose amount
90.00
9.00
10.00
70
60.00
120.00
8.00
5.00
C: Reaction Time
6.00
7.00
8.00
9.00
10.00
60.00
A: Fructose amount
(C)
Figure 5. Response fitted surface area and contour plot for 5‐HMF yield based on (A) temperature Figure 5. Response fitted surface area and contour plot for 5-HMF yield based on (A) temperature and
and fructose loading; (B) reaction time and temperature and (C) fructose loading and reaction time. fructose loading; (B) reaction time and temperature and (C) fructose loading and reaction time.
2.3. Optimisation and Confirmation Experiments 2.3. Optimisation and Confirmation Experiments
The optimal 5‐HMF yield was 99.40% under the following reaction conditions: 5.48 mg/mL The optimal 5-HMF yield was 99.40% under the following reaction conditions: 5.48 mg/mL
fructose loading; 158 °C temperature; and 164 min reaction time. Under the same conditions, the fructose loading; 158 ˝ C temperature; and 164 min reaction time. Under the same conditions, the actual
actual value (99.60%) obtained from the experiments was within the 95% prediction interval, value (99.60%) obtained from the experiments was within the 95% prediction interval, implying that
implying that the prediction models were reasonable and credible. the prediction models were reasonable and credible.
2.4. Catalyst Recycling 2.4.
Catalyst Recycling
After the reaction, the mixture was distilled and collected to pure sec‐butanol. The remaining After
the reaction, the mixture was distilled and collected to pure sec-butanol. The remaining
mixture was extracted with ethyl acetate after the addition of 0.5 mL water according to the procedure mixture was extracted with ethyl acetate after the addition of 0.5 mL water according to the procedure
reported previously. CePW
reported
previously. CePW1212OO4040 and sec‐butanol were recycled at least six cycles without significant and sec-butanol were recycled at least six cycles without significant
loss of activity and the yield of 5‐HMF was still 93.3% after six runs (Figure 6). The slight decrease in loss of activity and the yield of 5-HMF was still 93.3% after six runs (Figure 6). The slight decrease in
5‐HMF yield could be ascribed to the intensive formation of humins during the reaction; this could 5-HMF
yield could be ascribed to the intensive formation of humins during the reaction; this could be
be observed from the viscous solutions produced after long reaction times. observed from the viscous solutions produced after long reaction times.
Catalysts
2016, 6, 49
Catalysts 2016, 6, 49 7 of 11
7 of 11 Catalysts 2016, 6, 49 7 of 11 HPLC yield
Isolated yield
100
HPLC yield
Isolated yield
5‐HMF yield (%)
5‐HMF yield (%)
100
80
80
60
60
40
40
20
200
1
2
3
4
5
6
5
6
Recycle time
0
1
2
3
4
˝
Recycle time
Figure
6. Recycle of CePW1212OO
Figure 6. Recycle of CePW
40 and sec‐butanol (5.48 mg/mL of fructose, 1.0 g of catalyst, 158 °C, 164 min). 40 and sec-butanol (5.48 mg/mL of fructose, 1.0 g of catalyst, 158 C,
164 min).
Figure 6. Recycle of CePW12O40 and sec‐butanol (5.48 mg/mL of fructose, 1.0 g of catalyst, 158 °C, 164 min). 2.5. Kinetic Model 2.5. Kinetic Model
Various kinetic analysis of acid‐catalysed hydrolysis of carbohydrates have been reported in the 2.5. Kinetic Model Various kinetic In analysis
of acid-catalysed
hydrolysis
have been reported
the
literature [27,28]. the present study, a kinetic analysis ofof carbohydrates
the fructose dehydration catalyzed inby Various kinetic analysis of acid‐catalysed hydrolysis of carbohydrates have been reported in the CePW12[27,28].
O40 was performed in DMSO so as to make direct comparison with other catalysts used in the literature
In the present study, a kinetic analysis of the fructose dehydration catalyzed by
literature [27,28]. In the present study, a kinetic analysis of the fructose dehydration catalyzed by same reaction solvent. The kinetic curves shown in Figure 7 correspond to temperatures from 80 to CePW
O
was
performed
in DMSO so as to make direct comparison with other catalysts used in the
12 40
CePW
12O40 was performed in DMSO so as to make direct comparison with other catalysts used in the 160 °C. The values of ln(1 − X) (where X is the conversion of fructose) were plotted against the reaction same
reaction solvent. The kinetic curves shown in Figure 7 correspond to temperatures from 80 to
same reaction solvent. The kinetic curves shown in Figure 7 correspond to temperatures from 80 to ˝ C. The values of ln(1 ´ X) (where X is the conversion of fructose) were plotted against the reaction
160time (t) to obtain the pseudo‐first‐order rate constant (k). This model has been employed in most of 160 °C. The values of ln(1 − X) (where X is the conversion of fructose) were plotted against the reaction the kinetic studies because it provides reasonable levels of agreement with experimental data [29]. time (t) to obtain the pseudo-first-order rate constant (k). This model has been employed in most of the
time (t) to obtain the pseudo‐first‐order rate constant (k). This model has been employed in most of Table 3 shows that the value of k increased when increasing the reaction temperature, indicating that kinetic
studies because it provides reasonable levels of agreement with experimental data [29]. Table 3
the kinetic studies because it provides reasonable levels of agreement with experimental data [29]. a higher reaction temperature is beneficial for the dehydration process. shows
that
the value of k increased when increasing the reaction temperature, indicating that a higher
Table 3 shows that the value of k increased when increasing the reaction temperature, indicating that reaction
temperature
is beneficial
for the dehydration process.
100
a higher reaction temperature is beneficial for the dehydration process. Fructose conversion (%)
Fructose conversion (%)
100
80
80
60
60
40
40
20
20
0
0
50
100
0
150
200
250
300
250
300
Reaction time (min)
0
50
100
150
200
80 C
100 C
120 80 CC
140 CC
100 160 CC
120 350
140 C
160 C
350
Figure 7. Fructose conversion when increasing the temperature from 80 to 160 °C. Reaction time (min)
Figure 7. Fructose conversion when increasing the temperature from 80 to 160 °C. Table 3. Rate constants (k) of fructose conversion at different reaction temperatures with CePW
Figure
7. Fructose conversion when increasing the temperature from 80 to 160 ˝ C. 12O40 as the catalyst. Table 3. Rate constants (k) of fructose conversion at different reaction temperatures with CePW12O40 Table 3. Rate
constants
(k) of fructose conversion
at different
reaction
temperatures with CePW12 O40
−1)
Entry Temperature (°C) k (min
Correlation Coefficient as the catalyst. as the catalyst.
−5
1 80 Entry Temperature (°C) 2 100 ˝
Entry
Temperature
1 80 ( C)
3 120 2 100 14 80
140 120 23 100
5 160 34 120
140 45 140
160 5
160
0.001131 ± 9.1531 × 10
0.9963 k (min−1)
Correlation Coefficient 0.001761 ± 4.0587 × 10−5
0.9934 −5
Correlation
Coefficient
k (min´1 )
0.001131 ± 9.1531 × 10
0.9963 0.002755 ± 3.3841 × 10−4
0.9959 −5
´5
0.001761 ± 4.0587 × 10
0.9934 0.9963
0.001131 ˘ 9.1531 ˆ 10 −4
0.004421 ± 2.4758 × 10
0.9872 −4
´5
0.002755 ± 3.3841 × 10
0.9959 0.9934
0.001761
˘
4.0587
ˆ
10
−4
0.006069 ± 1.1199 × 10
0.9973 ´4
0.9959
0.004421 ± 2.4758 × 10
0.9872 0.002755 ˘ 3.3841 ˆ 10−4
´4
0.9872
0.004421 ˘ 2.4758 ˆ 10−4
0.006069 ± 1.1199 × 10
0.9973 0.006069 ˘ 1.1199 ˆ 10´4
0.9973
Catalysts 2016, 6, 49
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An Arrhenius plot was generated by using the values of k. The activation energy and
pre-exponential factor for the CePW12 O40 -catalysed dehydration of fructose to 5-HMF are listed
in Table 4. The activation energy was 27.21 kJ/mol, which is much lower than the value (55.77 kJ/mo)
obtained while using 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate [(C2 OHMIM)BF4 ] as the
catalyst and DMSO as the solvent [30]. This result indicated that the system involving CePW12 O40
efficiently reduced the activation energy and increased the reaction rate, indicating the high catalytic
activity of CePW12 O40 .
Table 4. Kinetic parameters of the degradation of fructose with the CePW12 O40 catalyst.
Kinetic Parameters
Value
Reaction order, n
Activation energy, Ea (kJ/mol)
Pre-exponential factor, A (min´1)
Correlation coefficient
1.0
27.21
11.72
0.9869
3. Experimental Section
3.1. Materials and Catalyst Preparation
5-HMF, cesium carbonate (Cs2 CO3 ) and cerous carbonate (Ce2 (CO3 )3 ) were purchased from
Shanghai Jingchun Chemical Reagent Company (Shanghai, China); sec-butanol, dimethyl sulfoxide
(DMSO), dimethylacetamide (DMAc), N-methylpyrrolidone, fructose, zinc nitrate [Zn(NO3 )2 ], cupric
nitrate [Cu(NO3 )2 ] and nickel nitrate [Ni(NO3 )2 ] were purchased from Beijing Chemical Reagent
Company (Beijing, China). All reagents were utilised as received without further purification.
The heteropolyacid salts were synthesised through the following method: phosphotungstic acid
(30.0 g, 9.6 mmol) was dissolved in deionised water (100 mL) followed by the dropwise addition
of cesium carbonate (4.7 g, 14.5 mmol). The obtained solution was then stirred for another 2 h at
40 ˝ C under nitrogen atmosphere. Cs3 PW12 O40 was obtained after evaporating under vacuum at
60 ˝ C for 6 h. The yield of Cs3 PW12 O40 was 90.1%. Other heteropolyacid salts were prepared using
similar procedures.
3.2. General Procedure for the Conversion of Fructose to 5-HMF
In a typical reaction, 1.0 g of fructose, 0.1 g of catalyst and 60 mL of solvent were placed in a 100 mL
flask fitted with a condenser under a nitrogen atmosphere and heated to the appropriate temperature
for the selected amount of time. After the reaction, the mixture was analysed by high-performance
liquid chromatography (ProStar 310 with a UV detector, Varian, Palo Alto, CA, USA) equipped with a
C18 column using methanol/water (40/60, v/v) as the eluent at a flow rate of 1.0 mL /min. The yield
of 5-HMF (%) was evaluated on a carbon basis as:
Y“
Moles of 5 ´ HMF
ˆ 100%
Staeting amount of fructose
(2)
3.3. Design of Experiments
The effect of three independent variables (temperature, reaction time and fructose loading) on the
response (5-HMF yield) was studied by employing a factorial CCD of RSM, which was a collection of
mathematical and statistical techniques for designing experiments, analysing the effects of variables,
developing models and optimising the process variables to obtain the optimum response [31–34].
The experimental data were fitted with a low-order polynomial equation to evaluate the effect of each
independent variable to the response, which was later analysed to determine the optimum process
conditions. In this study, the following polynomial quadratic equation was employed:
Catalysts 2016, 6, 49
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y “ β0 `
3
ÿ
βi xi `
3
ÿ
βij xi x j
(3)
i “1 j “ i `1
i “1
i “1
3
3
ÿ
ÿ
βii xi2 `
where y is the response function, xi and x j are the independent variables, β0 is the constant coefficient,
βi is the linear coefficient, βii is the quadratic coefficient and βij is the interaction coefficient among
the variables. For fitting second-order models, CCD is one of the most commonly utilised response
surface designs.
Each variable was investigated at three coded levels (´1,0,1), as listed in Table 5. The complete
experimental design matrix and 5-HMF yields are shown in Table 6. The data obtained from the
CCD were analysed by employing Design-Expert software version 8.0.6 (Stat-Ease Inc., Minneapolis,
MN, USA).
Table 5. Range and levels of independent variables.
Variable
Unit
Symbol
Fructose loading
Temperature
Reaction time
mg/mL
˝C
min
x1
x2
x3
Range and Level
´1
0
1
5.0
140
60
7.5
160
120
10
180
180
Table 6. Experimental design matrix and results.
Run
1
2
3
4
5
6
7
8
9
10
11
12
Experimental Variable
A (mg/mL)
B (˝ C)
C (min)
5.00
5.00
7.50
7.50
10.00
5.00
10.00
10.00
7.50
5.00
10.00
7.50
140.00
180.00
160.00
193.64
140.00
140.00
180.00
180.00
160.00
180.00
140.00
140.00
180.00
60.00
120.00
120.00
60.00
60.00
180.00
60.00
180.00
180.00
180.00
120.00
Conv. %
5-HMF Yield (% Mole)
85.61
78.23
99.63
30.31
26.87
37.25
93.92
53.45
88.32
95.64
83.59
50.71
83.45
75.81
99.06
26.17
24.31
35.69
42.10
50.12
79.10
70.15
82.45
47.32
4. Conclusions
The conversion of fructose into 5-HMF catalyzed by heteropolyacid salts was investigated. Among
the catalysts tested, CePW12 O40 showed the highest catalytic activity for the dehydration of fructose to
5-HMF. The optimal 5-HMF yield was 99.40% under the following reaction conditions: 5.48 mg/mL
fructose loading; 158 ˝ C temperature and 164 min reaction time. A kinetic analysis was also conducted,
and the activation energy and pre-exponential factor for the CePW12 O40 -catalysed conversion of
fructose into 5-HMF were obtained.
Acknowledgments: This project was sponsored by the National Natural Science Foundation of China under
grant number 21476021.
Author Contributions: The experimental work was designed by Y.S. and C.H.; Y.S. and X.W. performed the
experiments; Y.Q., Y.L. and B.C. analyzed the data; X.W. drafted the paper. The manuscript was amended using
the comments of all authors. All authors have given approval for the final version of the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
Catalysts 2016, 6, 49
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