Catalysts 2015 , 5
2299 Table 3.
Cont.
Run Temperature
T, °C Inulin Intake
C
inulin
, gmL CuCl
2
Concentration, C
cat
, M Reaction Time
t, min y
HMF
wt.
a
y
LA
wt.
b
20 160 0.10
0.010 65
23.5 5.2
21 160 0.05
0.005 120
13.6 4.7
22 170 0.10
0.015 65
18.4 14.8
23 180 0.15
0.005 120
1.0 21.6
24 170 0.05
0.010 65
25.4 13.3
25 170 0.10
0.010 65
20.7 10.7
26 180 0.05
0.005 10
30.3 6.5
27 180 0.05
0.015 120
2.0 28.9
28 180 0.15
0.015 120
3.4 27.1
29 170 0.10
0.010 65
19.9 10.6
30 180 0.05
0.005 120
4.4 27.8
a
The yield of HMF is defined in Equation 3;
b
The yield of LA is defined in Equation 4.
An extended version with more significant numbers for the coefficients to be used for among other reactor engineering studies is given in the supplementary information. The analysis of variance of the
model is given in Table 4. The R-squared value for the model is 0.9764, indicating that the model fits the experimental data well. This is supported by a parity plot in Figure 10 showing the data and
model points.
Table 4. Analysis of variance of the preferred model for the yield of HMF.
Source Sum of Squares
df Mean Square
F-value p-Value
Prob F
Model 1831.98 10 183.20 78.50
0.0001 significant
A-T 11.81 1 11.81
5.06 0.0365 B-C
load
247.48 1 247.48 106.04
0.0001 C-C
cat.
0.92 1 0.92 0.39 0.5373
D-t 647.07 1
647.07 277.25
0.0001 AB 91.95
1 91.95
39.40 0.0001 AC 95.09
1 95.09
40.74 0.0001 AD 330.29 1
330.29 141.52
0.0001 CD 63.23
1 63.23
27.09 0.0001 C
2
10.46 1
10.46 4.48 0.0477
D
2
106.06 1
106.06 45.45 0.0001
Residual 44.34 19 - -
-
The yield of HMF as a function of the process conditions as predicted by the model is given in Figure 11. Clearly, the reaction time has a profound effect on the HMF yield, rationalised by assuming
that HMF is an intermediate in the reaction sequence and ultimately will be converted to LA. Due to the interactions between the independent variables, it is difficult to draw general conclusions regarding the
effect of individual independent variables on HMF yield.
Catalysts 2015 , 5
2300
Figure 10. Parity plot for the HMF yield.
a b Figure 11.
Modelled HMF yield versus temperature and inulin intake at a 30 min and b 90 min batch time.
The yield of LA as a function of the process conditions was also modelled, and the results are given in Table 5 and Equation 2. An extended version of Equation 2 with more significant numbers for the
coefficients to be used among other reactor engineering studies is given in the supplementary information. The model describes the data points well, as is expressed by an R-squared value of 0.9651
and a parity plot Figure 12.
y
LA
= 0.17T + 204.63C
load
+ 303.82C
cat
− 1.2t − 1.38T·C
load
+ 7.73 × 10
−3
T·t −25.22
2
Catalysts 2015 , 5
2301
Figure 12.
Parity plot for LA yield.
Table 5.
Analysis of variance of the preferred model of LA.
Source Sum of Squares
df Mean Square
F-Value p-Value
Prob F
Model 1538.79 6 256.47 106.20
0.0001 significant
A-T 515.98 1
515.98 213.29 0.0001
B-C
load
39.77 1 39.77
16.44 0.0005 C-C
cat.
41.54 1 41.54
17.17 0.0004 D-t 644.84
1 644.84
266.56 0.0001 AB 7.60
1 7.60
3.14 0.0895 AD 289.06
1 289.06
119.49 0.0001 Residual 55.64 23 -
- -
The effect of two process conditions temperature and reaction time is presented in Figure 13. The LA yield increases at higher temperatures and longer reaction times, in line with the proposed reaction
network Figure 6 where LA is the final product and HMF an intermediate.
Figure 13. Modelled LA yield versus temperature and reaction time at a fixed inulin intake
0.1 gmL and CuCl
2
concentration 0.01 M.
Catalysts 2015 , 5
2302 3. Experimental Section
3.1. Chemicals Inulin isolated from Dahlia tubers was purchased from Acros Organic Geel, Belgium.
D
-fructose 99 and levulinic Acid
≥97 were obtained from Acros Organic Geel, Belgium. Formic Acid and
D
-glucose were purchased from Merck KGaA Darmstadt, Germany. 5-Hydroxymethylfurfural HMF ≥99 was obtained from Aldrich Steinheim, Germany. Copper II chloride dihydrate ≥99 was
bought from Acros Organics Geel, Belgium. All other salts were obtained from Acros Organics Geel, Belgium. All chemicals were used without purification. Deionized water was used to prepare
the solutions. 3.2. Experimental Procedures
3.2.1. Catalyst Screening Experiments For the catalyst screening experiments, 2 mL of a solution of inulin 0.2 g inulin and the metal salt
5 mM in water was placed in a microwave tube. The tube was sealed with a plastic cap and placed in the microwave device CEM Synthesis Explorer 48, CEM, Matthews, NC, USA, heated to 170 °C, and
stirred for 1 h at this temperature. After reaction, the tube was rapidly cooled; the contents were filtered, and the liquid phase was analysed by HPLC.
3.2.2. Experimental Design Experiments A similar experimental procedure as for the screening experiments was applied; the only difference
being that solely copper II chloride was used as the catalyst. In addition, a broad window of process variables was used, viz temperatures in the range of 160–180 °C, initial inulin loadings in the range of
0.05–0.15 gmL, copper II chloride concentrations between 0.005–0.015 M, and reaction times between 10 and 120 min.
3.3. Analyses HPLC was used to identify and quantify the components in the liquid product after reaction. An
Agilent technologies Santa Clara, CA, USA, 1200 series HPLC equipped with an isocratic pump, a Bio-Rad Hercules, CA, USA Aminex HPX-87H organic acid column 300 mm × 7.8 mm, and a
Waters Milford, MA, USA 410 differential refractive index detector was used. Sulfuric acid in water 5 mM was used as the eluent with a constant flow rate of 0.55 mLmin. The column was operated at
60 °C. The analysis time for each sample was 60 min. The HPLC was calibrated with solutions of known compounds. Using the chromatogram peak area and the external calibration curve, the unknown
concentrations of components in the liquid phase were determined. MALDI-TOFMS on a Voyager-DE PRO Applied Biosystems, Foster City, CA, USA was used to
determine the molecular weight of the inulin sample. 2,5-Dihydroxybenzoic acid DHB was used as the matrix.
Catalysts 2015 , 5
2303
The composition of the inulin sample, particularly the type and amount of C6-sugars, was determined by an acid-catalysed hydrolysis reaction. For this purpose, inulin 2.5 g was dissolved at
70 °C in 150 mL of water under stirring. The pH was adjusted to 1.4–1.6 by adding an aqueous HCl solution. Then, the solution was placed in a water bath for 30 min at 90 °C. A liquid sample was taken
and analysed by HPLC. 3.4. Definitions
The HMF yield y
HMF
is reported on a wt. basis and defined as follows:
HMF HMF
HMF ln
100 C
M V
y W
g HMFg inulin intake 3
The yield of LA is defined as:
LA LA
LA ln
100 C
M V
y W
g LAg inulin intake 4
In Equations 3 and 4, C
HMF
and C
LA
represent the concentration of HMF and LA, respectively molL as measured with HPLC, M
HMF
and ML
A
the molecular weight of HMF and LA g·mol, V the water intake L, and W
in
the intake of inulin on a weight basis g. The yield of HMF was converted from wt. to mol by assuming that inulin consists of linked
glucosefructose units C
6
H
10
O
5
which react to HMF according to the following stoichiometry: C
6
H
10
O
5
→ C
6
H
6
O
3
+ 2 H
2
O 5
As such, the maximum yield of HMF is 78 wt. . Thus, the yield of HMF in mol may be calculated from the yield in wt. by dividing the latter by 0.78.
3.5. Statistical Modelling The yield of HMF as a function of process variables was modelled using the Design-Expert 7 software
package Stat-Ease, Minneapolis, MN, USA using a standard expression given in Equation 6:
4 4
4 HMF
1 1
1 i i
ij i j i
i j
y b
b x b x x
6 The process variables x
i
: inulin intake, temperature, copper II chloride concentration, and reaction time are represented by the indices 1–4. The regression coefficients were obtained by statistical analyses
of the data. Significance of factors was determined by their p-value in the ANOVA analyses. A factor was considered significant if the p-value is below 0.05, meaning that the probability of noise causing the
correlation between a factor and the response is lower than 0.05. Insignificant factors were eliminated using backward elimination, and the significant factors were used to model the data.
4. Conclusions
An exploratory screening study revealed that metal salts may be used as catalysts for the reaction of inulin to HMF in water. Best results were obtained using CuCl
2
, giving an HMF yield of 30.3 wt. 39 mol at 180 °C, 0.05 g inulinmL, a 0.005 M CuCl
2
solution, and a reaction time of 10 min. The
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