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Furfural degradation in a dilute acidic and saline solution in the presence of glucose
Carbohydrate Research 375 (2013) 145–152
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Furfural degradation in a dilute acidic and saline solution in the presence of glucose B. Danon ⇑, L. van der Aa, W. de Jong Department of Process and Energy, Delft University of Technology, Leeghwaterstraat 44, 2628CA Delft, The Netherlands
a r t i c l e
i n f o
Article history: Received 21 February 2013 Received in revised form 3 April 2013 Accepted 21 April 2013 Available online 3 May 2013 Keywords: Furfural degradation Glucose Kinetics Diels–Alder
a b s t r a c t A kinetic study has been performed on the degradation of furfural in a dilute acidic and saline solution with and without the presence of glucose. Experiments have been performed in a stirred batch reactor. The degradation of furfural alone was accurately predicted both using a first- and a second-order kinetic model. It was shown that furfural is degrading significantly faster when glucose is present in the reaction mixture. In the series with glucose present distinct second-order reaction kinetics were observed. From experiments with varying concentrations of glucose it turned out that an additional (second-order) reaction had to be added to the reaction mechanism in order to satisfactorily predict the experimental data. This additional reaction incorporated the initial glucose concentration as a constant in the Arrhenius expression for the reaction rate constant. Furthermore, it has been argued that this second-order reaction could well be a Diels–Alder reaction. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction To achieve an all-embracing alternative for the use of fossil resources in modern economies the production of platform chemicals from renewable resources should be considered. Hydrolytic treatment of (lignocellulosic) biomass is a widely accepted method for the production of such platform chemicals. The main constituents of lignocellulosic biomass are lignin, cellulose, and hemicellulose. Cellulose is a homopolysaccharide, consisting of b-D-glucopyranose monomers, whereas hemicellulose is a heteropolysaccharide of various (both C6- and C5-)sugars, but in hardwoods and many non-woody agricultural residues predominantly xylose. Under acidic and saline conditions these polysaccharides are readily hydrolyzed to yield the individual monosaccharides. The hexoses are subsequently hydrolyzed and dehydrated to form 5-hydroxymethylfurfural (HMF) and levulinic acid. The pentose content on the other hand is dehydrated to form furfural. The focus here is on furfural, a versatile multi-purpose platform chemical. Presently, most of the furfural produced worldwide is used as a raw material for further chemical synthesis. The bulk is hydrogenated to furfuryl alcohol, which is subsequently used in the production of for example resins. However, furfural is also used directly as an extractant, fungicide, or nematocide.1 More recently, furfural has also been identified as a potential precursor for the production of biofuels.2
⇑ Corresponding author. Tel.: +31 152785542. E-mail address: [email protected] (B. Danon). 0008-6215/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carres.2013.04.030
The formation of furfural from biomass resources3,4 and from pure pentoses5–10 has been extensively studied over the second half of the last century. Generally, it is observed that the maximum furfural yield of the dehydration of the pentose content of a biomass is lower than that of pure pentoses at comparable conditions (compare for example the maximum yields of 50%3 and 69%8 for olive stones and xylose, respectively). It is noted that the chemistry of the hydrolytic reactions of the C6- and C5-sugars is similar, that is, they proceed at similar reaction conditions and most catalysts will promote the release and hydrolysis of both sugars. Thus, when platform chemicals are to be produced from either the hexose or the pentose content of a biomass stream, the reaction mixture will inevitably contain both sugars. Therefore, it is possible that the presence of glucose has a significant influence on the final furfural yield of the dehydration of the pentoses in a process with a biomass feed. The objective of this paper is the investigation of the influence of glucose on the degradation reactions of furfural. Furfural is known to degrade under the same conditions under which it is formed. Two different degradation products have been identified, that is, formic acid and resinous tars5,11,12 and the term ‘degradation’ is here used accordingly for the combination of both decomposition and polymerization. The importance of the formation of formic acid has been disputed,13 however, in the majority of the studies (small) quantities of formic acid have been measured.9,11,12 In most previous studies the experimental work has been complemented by (detailed) kinetic modeling. If the kinetics of the reactions of a pentose (or of its precursors) are to be determined, the kinetic parameters of the degradation reactions of furfural
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are to be established beforehand, forming a kind of kinetic cascade. The series of kinetic studies performed by Girisuta for the hydrolysis and dehydration of hexoses can be considered exemplary for the kinetic cascade approach.14–17 The kinetic parameters of the degradation of pure furfural have been previously determined in high-temperature water18 and in dilute sulfuric acid5,11,13 or dilute hydrochloric acid9,19 solutions. The kinetic parameters found in all these studies are in the same order of magnitude. Generally, it can be concluded that the activation energies for the degradation reactions are lower for HCl than for H2SO4. Also, all of these studies observe or assume first-order kinetics. The results of the present kinetic study on furfural degradation with glucose present will allow for much more precise kinetic modeling of biomass hydrolysis toward furfural. In this paper, experimental results are presented for the degradation of furfural in dilute acidic and saline solutions both with and without the addition of glucose to the reaction mixture. Additionally, the kinetics of the degradation reactions are extensively modeled. This paper will conclude with some mechanistic considerations regarding the bimolecular degradation of furfural. 2. Methods and materials The experiments were performed in a 1-L mechanically stirred stainless steel autoclave reactor, see also Figure 1. The temperature was regulated by pumping heated oil through the jacket of the reactor. The operating pressure in the reactor was the saturation pressure of the reaction mixture. Furfural (Sigma-Aldrich, P99%), D-glucose (Sigma-Aldrich, P99.5%), hydrochloric acid (J.T. Baker, 37.5%), and sodium chloride (J.T. Baker, P99%) were all commercially available and used without further purification. First, the reactor was filled with 850 ml demineralized water, including the total amount of required acid and salt for the experiment. This mixture was then heated to the desired operating temperature. Once the mixture had reached the desired temperature, about 100 g of a reactant solution (containing the furfural and/or glucose) was introduced using a highpressure syringe pump with a flow of around 90 ml/min. The injection of the reactant solution generally took about 1 min. The 100 g of reactant solution coincides with approximately 100 ml solution, resulting in a total reaction mixture of around 950 ml. It is noted that during the injection of the (relatively cold) reactant solution the temperature in the reactor dropped with about 5 °C. It took the heating system several minutes to restore the desired reaction temperature. Before the reactant solution was inserted, a first sample was taken via the sampling system. The sampling system consists of a sampling line, which runs through an ice bath in order to quench
T P
the sample, with a needle valve at the end. The flow through the sampling line is established by the use of the internal pressure of the reactor. Samples were taken from the liquid phase only. Prior to the actual sampling, around 10 ml solution was vented to wash the sampling line to ensure reproducible results. Samples were taken at various time intervals during the experiment. Analysis of the reaction products was carried out by means of an HPLC apparatus equipped with a Phenomenex Rezex RHMMonosaccharide column, 8% cross linked H+, 300 7.80 mm. A Marathon XT auto-sampler was used to enhance reproducibility. Glucose was measured by means of a Refractive Index (RI) detector (Varian, Model 350 Pro Star), whereas for furfural both the RI and an Ultraviolet (UV) detector (Varian Model 310 Pro Star) were used. Quantification was achieved by converting the detected areas using calibration curves made with the pure substances at various concentrations. A 0.005 N H2SO4 solution in demineralized water was used as the eluent at a flow rate of 0.6 ml/min with a column temperature of 80 °C. In all experiments the initial furfural and glucose concentrations were 50 mM. Furthermore, both hydrochloric acid (HCl) and sodium chloride (NaCl) were added as the catalysts, with concentrations of 50 and 500 mM, respectively. An HCl concentration of 50 mM has been shown to be a high enough concentration for achieving high furfural yields for the dehydration of xylose.7 The concentration of 500 mM NaCl has been chosen on one hand also based on optimal conditions for furfural production from xylose7 and on the other hand in order to resemble the salinity of sea water. The temperature range chosen for the experiments is between 160 and 200 °C. In Table 1 an overview is presented of all the performed experiments. 3. Kinetic model For the degradation of furfural the proposed reaction mechanism is presented in Scheme 1. The reaction rate for the degradation of furfural (R1) is defined as follows,
R1 ¼ k1 ½Fn1
ð1Þ 1
where k1 is the reaction rate constant (min ), [F] the furfural concentration (mM) and n1 the order of the reaction. The adapted version of the Arrhenius equation has been used for the definition of the reaction rate constant,
k1 ¼ A1 e
E R
ð
1 1 1 T To
ð2Þ 1
where A1 is the pre-exponential factor (min ), E1 the activation energy (kJ/mol), R the universal gas constant (kJ/mol K), T the temperature (K) and To the reference temperature (448.15 K). The kinetic parameters (A1 and E1) were directly estimated using a maximum-likelihood approach, based on the minimization of the sum of squared normalized errors (SSNE) between the ensemble of all the experimental and predicted concentrations at the different temperatures. The SSNE is calculated using the following equation,
Table 1 Overview of the performed experiments
Oil bath
HCl = 50 mM – NaCl = 500 mM
Ice bath Furfural (mM) Glucose (mM) Temperature (°C)
Reactant solution
Þ
HPLC pump
Reactor
Figure 1. Schematic of the experimental setup.
50 0 160 170 180 190 200
50 50 160 170 180 190 200
50 25 160 – 180 – 200
0 50 160 – 180 – 200
B. Danon et al. / Carbohydrate Research 375 (2013) 145–152
Scheme 1. Furfural degradation mechanism.
SSNE ¼
" N X y i¼1
ymod i yo yo i
2 # ð3Þ
where N is the total number of measurements, yo the initial concentration and yi, and ymod represent the experimental and modeled i data, respectively. The minimization of errors was initiated by providing initial guesses for each kinetic parameter. The best estimates were obtained using the MATLAB toolbox fminsearch, which is based on the NelderMead optimization method. This method has previously proven to result in accurate estimates.17 Furthermore, the percentage output variation (FIT) was calculated. The FIT is calculated using the following equation,
normðymod yi Þ i FIT ¼ 100 1 ~i Þ normðyi y
ð4Þ
~i is the mean value of yi and the function norm (M) in MATLAB where y returns the largest singular value of the matrix M. A rule of thumb is that a good FIT has a minimal value of 80%. Finally, the single-parameter 95% confidence interval of the estimated kinetic parameters has been determined using the approximate method described by Smith.20 This method is based on the comparison between two sets of predicted values. The first set is calculated with the estimated parameters from the maximum-likelihood approach. The second set is calculated with one of these estimated parameters varied by a small step (1% in this case). The sensitivity co-efficient is then defined as, mod ymod @ymod p þDp ypi i ¼ i i @pi Dpi
ð5Þ
where pi represents the ith estimated parameter and D pi the variation in pi. The overall sensitivity of the model is then represented by a sensitivity matrix A, with size i by i, where the diagonal values are the squared sensitivity co-efficients of the individual parameters. Next, the single-parameter standard error can be calculated from these diagonal values of the mean square error matrix V, which is related to the inverse of the sensitivity matrix A,
V ¼ r2 A1
ð6Þ
2
where r is the sum of square (absolute) errors. The 95% confidence interval is assumed to be approximately two times this singleparameter standard error. Moreover, the non-diagonal values of matrix V represent the correlation co-efficients between the estimated parameters. These values are between 1 and 1, with a lower value in this range representing less correlation, and vice versa. 4. Results and discussion The results of furfural degradation experiments with and without glucose are presented in Figure 2(a) and (b), respectively. As the reaction order for furfural degradation is assumed to be firstorder in most previous studies,19,9,11,13,18 as a first step, first-order kinetics were fitted on the experimental data. The results of the first-order kinetic model are represented by the solid lines in Figure 2. In Table 2 the estimated pre-exponential factors and activa-
147
tion energies and the fitting parameters are overviewed for the model. It is noted here that the value of E1 for furfural without glucose is of the same order of magnitude, but slightly higher, compared to values reported in the literature for dilute hydrochloric solutions (48.4 kJ/mol19 and 67.6 kJ/mol9). This is probably due to the presence of the second catalyst (NaCl), which seems to inhibit the furfural degradation reactions, translated in a higher activation energy. It is observed that the furfural degradation reaction shows a clear dependency on the temperature, with larger degradation rates at higher temperatures. Secondly, the two graphs also clearly indicate that furfural degrades faster when glucose is present. In the estimated kinetic parameters this is translated in a higher value of the pre-exponential factor and a lower value of the activation energy for the series with glucose. The lower value of the E1 further indicates that the increase in furfural degradation due to the presence of glucose is also dependent on the temperature. In other words, the increase in furfural degradation due to the presence of glucose will be higher at higher temperatures. For both of the experimental series the fit of the kinetic model seems to be quite good, expressed by a high value of FIT. Also, the standard errors in the estimated parameters are considered to be acceptable. However, in the case of glucose being present, the correlation co-efficient of the two estimated parameters is not close to minus unity, indicating that the two estimated parameters are not fully independent. The first-order kinetic model adequately predicts the degradation behavior of furfural, especially for the series without glucose. In the series with glucose, the trend in the measured and predicted furfural concentrations seem to disagree. The plotted logarithm of the furfural concentrations has a clear curved form (predominantly visible at higher temperatures), which indicates a different reaction order than first-order. First-order kinetics for furfural degradation have been challenged before. Root observed for the degradation of furfural in sulfuric acid a distinct relation between the initial furfural concentration and the degradation reaction rate.5 In his discussion a mechanism was proposed in which furfural degrades via two separate pathways. The first is via a first-order unimolecular degradation reaction (as assumed in other studies). However, a second pathway is proposed to proceed via a second-order reaction of furfural with an intermediate of its own degradation. This mechanism, including a second-order reaction, could explain the reported experimental data. Furthermore, Dunlop mentioned resins as a degradation product of furfural.12 In the present experiments there were also indications of resin formation. These include few black particles in the samples at longer reaction times and an increasing darkening of the samples with reaction time. The exact structure of these resins remains unresolved, however, it is clear that they are some kind of polymers. Since a polymerization reaction is by definition a poly-molecular reaction, second-order reaction kinetics cannot be excluded for the resinification reactions. Finally, the plot of the inverse of the furfural concentrations of the experiments with glucose versus the reaction time showed clear linearity. This also indicates second-order reaction kinetics. Therefore, the experimental data have been refitted using a second-order kinetic model (n1 = 2). In Figure 3 and Table 3 the results are presented for this model. For the series without glucose, the SSNE and FIT are improved compared to the first-order model, however, the differences are very small. Due to these small differences, the degradation reaction can be perceived as pseudo-firstorder. It remains undecided whether the reaction is of either order. A possibility for this ambiguity is that in reality the degradation of pure furfural comprises of both a first- and a second-order reaction.
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0.1
160
170
180
190
0.1
200
−0.1 −0.2 −0.3 −0.4
170
180
190
200
0 Furfural ln(C/Co) (−)
Furfural ln(C/Co) (−)
0
160
−0.1 −0.2 −0.3
0
10
20
30 40 Time (min)
50
60
−0.4
0
10
20
30 40 Time (min)
50
60
Figure 2. Experimental data (symbols) and first-order (n1 = 1) model (lines) of furfural concentrations (a) without and (b) with glucose present at five different temperatures.
Table 2 Activation energies (E1 [kJ/mol]) and pre-exponential factors (A1 [min1]) for the first-order kinetic model (n1 = 1) for furfural degradation with and without glucose present Estimated value
95% Confidence interval
Correlation co-efficients
Absolute
Relative (%)
A1
E1
Furfural A1 E1 SSNE FIT
1.55E-03 102.1 0.0068 99.76
±7.51E-05 ±4.1
±4.84 ±3.99
1.00 0.92
1.00
Furfural + Glucose A1 E1 SSNE FIT
5.55E-03 65.7 0.0223 99.68
±2.05E-04 ±3.7
±3.68 ±5.64
1.00 0.73
1.00
0.1
160
170
180
190
0.1
200
−0.1 −0.2 −0.3 −0.4
170
180
190
200
0 Furfural ln(C/Co) (−)
Furfural ln(C/Co) (−)
0
160
−0.1 −0.2 −0.3
0
10
20
30 40 Time (min)
50
60
−0.4
0
10
20
30 40 Time (min)
50
60
Figure 3. Experimental data (symbols) and second-order (n1 = 2) model (lines) of furfural concentrations (a) without and (b) with glucose present at five different temperatures.
In the case of the series with glucose present, it is noted that the predicted furfural concentrations show a similar trend as in the experiments. When the total of normalized errors SSNE and FIT are compared for the two models of this series (compare Tables 2 and 3), it turns out that the predictions are improved significantly. However, the high correlation between the two estimated parameters is still present. It is concluded that furfural degradation shows second-order reaction kinetics in more complex solutions. However, in pure furfural solutions the degradation rates can be perceived as a (pseudo-)first-order reaction. In this case the differences in the
predictions of the first- and second-order kinetic models are too small be conclusive regarding the reaction order. Next, the influence of the glucose on the kinetics of furfural degradation was studied in more detail. First, the apparent disappearance of the glucose in these experiments was compared with results of experiments with pure glucose solutions at similar reaction conditions. In Figure 4 the conversion of glucose at three different temperatures (160, 180, and 200 °C) is shown. The differences in the disappearance of glucose in these two series of experiments (with and without furfural present) seem to be relatively small compared to the differences observed for the
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Table 3 Activation energies (E1 [kJ/mol]) and pre-exponential factors (A1 [min1]) for the second-order kinetic model (n1 = 2) for furfural degradation with and without glucose present Estimated value
Furfural A1 E1 SSNE FIT Furfural + Glucose A1 E1 SSNE FIT
95% Confidence interval Absolute
Relative (%)
A1
E1
2.86E-05 114.7 0.0058 99.78
±1.17E-06 ±3.5
±4.09 ±3.07
1.00 0.92
1.00
1.55E-04 85.7 0.0084 99.80
±3.10E-06 ±2.1
±2.00 ±2.47
1.00 -0.71
1.00
0
Glucose ln(C/Co) (−)
−0.5 −1 −1.5 −2 −2.5 −3 −3.5 −4
160 0
10
20
180
30 Time (min)
200 40
50
60
Figure 4. Glucose conversion with (crosses) and without (circles) furfural present at three different temperatures.
disappearance of furfural. Therefore, it is concluded that the presence of furfural has a relatively small effect on the conversion of glucose. Moreover, a kinetic model incorporating glucose as a reactant in an additional reaction was not able to produce reasonable predictions for the experimental data. Accordingly, it can be concluded that glucose itself does not participate as a reactant in an additional furfural degradation reaction. For a quantitative evaluation of the effect of the glucose on furfural degradation, nine experiments are considered with three different concentrations of glucose and three different temperatures (see also Table 1). An extended reaction mechanism is proposed with an additional furfural degradation reaction (R2), which is influenced by the presence of glucose, see also Scheme 2. The additional reaction R2 is formulated as follows,
R2 ¼ k2 ½Fn2
ð7Þ
with the reaction rate constant k2 defined as,
k2 ¼ A2 ½Go e
E R
ð
2 1 1 T To
Correlation co-efficients
Þ
ð8Þ
where [G]o represents the initial glucose concentration (mM). Since glucose cannot be regarded as a reactant, it is introduced as a con-
stant in the reaction rate constant expression. The reaction order of R2 is assumed to be 2. In Figure 5 the results are presented for the experiments with 50 mM furfural and none, 25, and 50 mM of glucose as initial concentrations. For the degradation of furfural independent of the glucose (R1) both first- and second-order kinetics have been implemented, Figure 5(a) and (b), respectively. In Tables 4 and 5 the estimated kinetic parameters are presented. In both cases the model is capable of predicting the influence of the glucose on furfural degradation. The influence of glucose on furfural degradation seems also to be temperature dependent, but in a different way than that of furfural alone. This can be deduced from the differences between the degradation of furfural in the cases with 25 and 50 mM glucose. At both 160 and 200 °C the effect of the concentration of glucose is smaller than at 180 °C. This different temperature dependence is also reflected in a lower activation energy for R2 compared to R1. In contrast with the mechanism consisting of merely one second-order reaction (see Table 3), the correlation co-efficient of R2 indicates that the two estimated parameters are now fully independent. If the two reaction orders for the independent furfural degradation reaction are compared, the results are again ambiguous. At 160 °C the model with first-order R1 better predicts the experimental data, whereas at 200 °C the second-order R1 seems to be more accurate. It is concluded that the influence of glucose on the degradation of furfural is well represented by introducing an additional reaction to the reaction mechanism. This reaction follows second-order kinetics and incorporates the initial glucose concentration as a constant in the Arrhenius equation. Finally, possible reaction mechanisms of such a second-order furfural degradation reaction are considered. As stated before, Dunlop suggested that the degradation products of furfural in (acidic) aqueous solutions are formic acid and resins.12 For the formation of formic acid the hydrolytic fission of the aldehyde group was proposed. In the present study also very small (unquantifiable) amounts of formic acid have been observed in the chromatograms. For the resinification no reaction mechanism was proposed by Dunlop. It seems evident, both based on the present kinetic results and on the concept of polymerization, that the resinification reaction effectively is a second-order reaction (thus involving two furfural molecules).
Scheme 2. Furfural degradation mechanism with glucose present.
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Temperature = 160°C 0.1
G=00mM
G=25mM
Temperature = 160°C 0.1
G=50mM
−0.1 −0.2 −0.3 −0.4
−0.1 −0.2
0
10
20
30 40 Time (min)
50
−0.4
60
0
10
Temperature = 180°C G=00mM
G=25mM
G=50mM
Furfural ln(C/Co) (−)
Furfural ln(C/Co) (−)
30 40 Time (min)
50
60
G=00mM
G=25mM
G=50mM
0
−0.1 −0.2 −0.3
−0.1 −0.2 −0.3
0
10
20
30 40 Time (min)
50
−0.4
60
0
10
Temperature = 200°C 0.1
G=00mM
G=25mM
20
30 40 Time (min)
50
60
Temperature = 200°C 0.1
G=50mM
G=00mM
G=25mM
G=50mM
0 Furfural ln(C/Co) (−)
0 Furfural ln(C/Co) (−)
20
Temperature = 180°C 0.1
0
−0.1 −0.2 −0.3 −0.4
G=50mM
−0.3
0.1
−0.4
G=25mM
0 Furfural ln(C/Co) (−)
Furfural ln(C/Co) (−)
0
G=00mM
−0.1 −0.2 −0.3
0
10
20
30 40 Time (min)
50
60
−0.4
0
10
20
30 40 Time (min)
50
60
Figure 5. Furfural degradation with various concentrations of glucose and (a) n1 = 1 and n2 = 2 and (b) n1 = n2 = 2.
Therefore, a Diels–Alder reaction is considered, see also Scheme 3. In a Diels–Alder reaction a diene and a dienophile react to form a six-membered ring with one double bond.21 Furfural has been shown to be able to act as a dienophile in reaction with a strong diene (butadiene).12 However, no reaction was observed for furfural with a strong dienophile (maleic anhydride) under the same conditions where less polar furans, for example, methylfuran, did react.12 In methylfuran the higher electron density in the furan ring serves as the electron donor (the diene) in the reaction. It is argued here that furfural does act as a diene, that is, in acidic aqueous solutions. The diene is then defined not as the bonds of the furan ring but as one double bond of the furan ring in combination with the double bond of the aldehyde group. In a polar sol-
vent, these two double bonds of furfural are most stable in cisconformation, as is required for a Diels–Alder reaction.22 Moreover, since the oxygen atom of the aldehyde group forms (on average) one hydrogen bond with the surrounding water, the dipole moment of the furfural molecule is increased significantly.22 Since the concentration of protonated water (H3O+) is relatively high in acidic aqueous solutions, hydrogen bonds with these protonated water molecules even further increase the dipole moment of furfural. This results in an even higher electron density in the aldehyde group (the diene) and a lower electron density in the furan ring (the dienophile), which is favorable for the Diels–Alder reaction. Finally, it is noted that only a very low percentage of furfural will be hydrated. This is mainly due to the substituent on the central
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Table 4 Activation energies (E [kJ/mol]) and pre-exponential factors (A [min1]) for n1 = 1 and n2 = 2 based on experiments with none, 25, and 50 mM glucose and at 160, 180, and 200 °C Estimated value
Reaction R1 A1 E1 SSNE FIT Reaction R2 A2 E2 SSNE FIT
95% Confidence interval
Correlation co-efficients
Absolute
Relative (%)
A#
E#
1.55E-03 102.1 0.0068 99.76
±7.51E-05 ±4.1
±4.84 ±3.99
1.00 0.92
1.00
1.66E-06 76.1 0.0571 99.72
±9.35E-08 ±4.3
±5.62 ±5.62
1.00 1.00
1.00
Table 5 Activation energies (E [kJ/mol]) and pre-exponential factors (A [min1]) for n1 = n2 = 2 based on experiments with none, 25, and 50 mM glucose and at 160, 180, and 200 °C Estimated value
95% Confidence interval
Correlation co-efficients
Absolute
Relative (%)
A#
E#
Reaction R1 A1 E1 SSNE FIT
2.86E-05 114.7 0.0058 99.78
±1.17E-06 ±3.5
±4.09 ±3.07
1.00 0.92
1.00
Reaction R2 A2 E2 SSNE FIT
1.79E-06 94.6 0.0586 99.72
±7.22E-08 ±3.8
±4.04 ±4.04
1.00 1.00
1.00
Scheme 3. Proposed reaction mechanism for a Diels–Alder reaction between two furfural molecules.
carbon atom of the aldehyde group, that is, the furan ring. Both as an electron donating group and as a substituent containing nonbonding electrons, the furan ring decreases the equilibrium constant of hydration of the aldehyde group.23 Therefore, it seems plausible that furfural in an acidic aqueous solution can react with itself, albeit in small quantities, in a Diels–Alder reaction with second-order reaction kinetics. Next, it is considered how the added glucose can enhance this reaction. First, since the glucose itself is also dehydrated to (ultimately) formic and levulinic acid, the acidity of the entire reaction solution is increased.15 Higher concentrations of H3O+ therefore further increase the (average) dipole moment of the furfural molecules and thus favor Diels–Alder reactions. This could also explain the fact that for the experiments with glucose present furfural degradation was accurately predicted by including the initial glucose concentration as a constant in the Arrhenius equation.
Secondly, and more speculatively, it might be that glucose itself, or one of its degradation products (e.g., hydroxymethylfurfural or levulinic acid), participates or initiates Diels–Alder or other polymerization reactions with furfural. Zeitsch identified two pathways for furfural degradation in a reaction solution with xylose.1 Apart from a first-order (unimolecular) degradation reaction of furfural, a condensation reaction was proposed for furfural reacting with an intermediate of the dehydration of xylose toward furfural. Zeitsch propounds that this latter reaction can be both uni- and bimolecular, resulting in a mechanism comprising of both first- and second-order kinetics. This reaction could be an analog for the influence of the glucose. It is of course also possible that both of the previous described influences of glucose contribute to one single reality. In order to obtain more detailed insight in the mechanistic effects, additional experiments have to be performed including furfural degradation at different acid concentrations and with different dehydration products of glucose present. This lay however outside the scope of the present study. It has to be noted here that the present experimental work has been performed in a batch reactor. Nowadays, in more advanced production processes of furfural simultaneous extraction of the formed furfural is included. Obviously, if furfural can be extracted from the reaction mixture as soon as it is formed, the implications of the presence of other components (such as glucose) on furfural degradation in the reaction mixture diminishes. Nevertheless, detailed and correct understanding of the fundamental reaction kinetics of furfural would even in these cases be of major importance. 5. Conclusions Furfural degradation has been investigated in a dilute acidic (HCl) and saline (NaCl) solution. The degradation rates have been determined for a series of experiments with and without glucose
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present in the reaction mixture. The obtained activation energy for a first-order reaction of pure furfural is in agreement with values found in the literature. Model predictions of a second-order kinetic model for pure furfural degradation showed comparable results as the first-order model. It remains therefore undecided whether this reaction is of either order. A possibility for this ambiguity is that in reality the degradation of furfural alone also comprises of both a first- and a second-order reaction. When glucose is present in the reaction mixture furfural degrades significantly faster. A mechanism including an additional second-order reaction incorporating the initial glucose concentration, turned out to satisfactorily predict furfural degradation in the presence of glucose. There were presented several considerations that this additional second-order reaction is a Diels–Alder reaction. In the future, more extensive experimental and kinetic studies should be performed on the influence of the acid concentration on these reactions. Acknowledgements The authors wish to gratefully acknowledge Buana Girisuta for the sharing of his wisdom and Michel van den Brink for his support in the laboratory. References 1. Zeitsch, K. The chemistry and technology of furfural and its many by-products In Sugar series; Elsevier, 2000; Vol. 13,
2. Lange, J.-P.; van der Heide, E.; van Buijtenen, J.; Price, R. ChemSusChem 2012, 5, 150–166. 3. Montané, D.; Salvadó, J.; Torras, C.; Farriol, X. Biomass Bioenergy 2002, 22, 295– 304. 4. Bamufleh, H.; Alhamed, Y.; Daous, M. Ind Crops Prod 2013, 42, 421–428. 5. Root, D.; Seaman, J.; Harris, J.; Neill, W. Forest Prod J 1959, 9, 158–165. 6. Garrett, E.; Dvorchik, B. J Pharm Sci 1969, 58, 813–820. 7. Marcotullio, G.; de Jong, W. Green Chem 2010, 12, 1739–1746. 8. Marcotullio, G.; de Jong, W. Carbohydr Res 2011, 346, 1291–1293. 9. Weingarten, R.; Cho, J.; Conner, W., Jr.; Huber, G. Green Chem 2010, 12(8), 1423–1429. 10. Binder, J.; Blank, J.; Cefali, A.; Raines, R. ChemSusChem 2010, 3, 1268–1272. 11. Williams, D.; Dunlop, A. Ind Eng Chem 1948, 40, 239–241. 12. Dunlop, A. Ind Eng Chem 1948, 40, 204–209. 13. Marcotullio, G.; de Jong, W.; Taveres Cardoso, M.; Verkooijen, A. Int J Chem React Eng 2009, 7, A67. 14. Girisuta, B.; Janssen, L.; Heeres, H. Green Chem 2006, 8, 701–709. 15. Girisuta, B.; Janssen, L.; Heeres, H. Chem Eng Res Des 2006, 84, 339–349. 16. Girisuta, B.; Janssen, L.; Heeres, H. Ind Eng Chem Res 2007, 46, 1696–1708. 17. Girisuta, B.; Danon, B.; Manurung, R.; Janssen, L.; Heeres, H. Bioresour Technol 2008, 99, 8367–8375. 18. Jing, Q.; Lu, X. Chin J Chem Eng 2007, 15, 666–669. 19. Rose, I.; Epstein, N.; Watkinson, A. Ind Eng Chem Res 2000, 39, 843–845. 20. Smith, L.; Kitanidis, P.; McCarty, P. Biotechnol Bioeng 1997, 53, 320–331. 21. Ege, S. Organic chemistry – structure and reactivity, 3rd ed.; D.C. Heath and Company, 1994. 22. Rivelino, R.; Canuto, S.; Coutinho, K. Braz J Phys 2004, 34, 84–89. 23. Hilal, S.; Bornander, L.; Carreira, L. QSAR Comb Sci 2005, 24, 631–637.
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Carbohydrate Research journal homepage: www.elsevier.com/locate/carres
Furfural degradation in a dilute acidic and saline solution in the presence of glucose B. Danon ⇑, L. van der Aa, W. de Jong Department of Process and Energy, Delft University of Technology, Leeghwaterstraat 44, 2628CA Delft, The Netherlands
a r t i c l e
i n f o
Article history: Received 21 February 2013 Received in revised form 3 April 2013 Accepted 21 April 2013 Available online 3 May 2013 Keywords: Furfural degradation Glucose Kinetics Diels–Alder
a b s t r a c t A kinetic study has been performed on the degradation of furfural in a dilute acidic and saline solution with and without the presence of glucose. Experiments have been performed in a stirred batch reactor. The degradation of furfural alone was accurately predicted both using a first- and a second-order kinetic model. It was shown that furfural is degrading significantly faster when glucose is present in the reaction mixture. In the series with glucose present distinct second-order reaction kinetics were observed. From experiments with varying concentrations of glucose it turned out that an additional (second-order) reaction had to be added to the reaction mechanism in order to satisfactorily predict the experimental data. This additional reaction incorporated the initial glucose concentration as a constant in the Arrhenius expression for the reaction rate constant. Furthermore, it has been argued that this second-order reaction could well be a Diels–Alder reaction. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction To achieve an all-embracing alternative for the use of fossil resources in modern economies the production of platform chemicals from renewable resources should be considered. Hydrolytic treatment of (lignocellulosic) biomass is a widely accepted method for the production of such platform chemicals. The main constituents of lignocellulosic biomass are lignin, cellulose, and hemicellulose. Cellulose is a homopolysaccharide, consisting of b-D-glucopyranose monomers, whereas hemicellulose is a heteropolysaccharide of various (both C6- and C5-)sugars, but in hardwoods and many non-woody agricultural residues predominantly xylose. Under acidic and saline conditions these polysaccharides are readily hydrolyzed to yield the individual monosaccharides. The hexoses are subsequently hydrolyzed and dehydrated to form 5-hydroxymethylfurfural (HMF) and levulinic acid. The pentose content on the other hand is dehydrated to form furfural. The focus here is on furfural, a versatile multi-purpose platform chemical. Presently, most of the furfural produced worldwide is used as a raw material for further chemical synthesis. The bulk is hydrogenated to furfuryl alcohol, which is subsequently used in the production of for example resins. However, furfural is also used directly as an extractant, fungicide, or nematocide.1 More recently, furfural has also been identified as a potential precursor for the production of biofuels.2
⇑ Corresponding author. Tel.: +31 152785542. E-mail address: [email protected] (B. Danon). 0008-6215/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carres.2013.04.030
The formation of furfural from biomass resources3,4 and from pure pentoses5–10 has been extensively studied over the second half of the last century. Generally, it is observed that the maximum furfural yield of the dehydration of the pentose content of a biomass is lower than that of pure pentoses at comparable conditions (compare for example the maximum yields of 50%3 and 69%8 for olive stones and xylose, respectively). It is noted that the chemistry of the hydrolytic reactions of the C6- and C5-sugars is similar, that is, they proceed at similar reaction conditions and most catalysts will promote the release and hydrolysis of both sugars. Thus, when platform chemicals are to be produced from either the hexose or the pentose content of a biomass stream, the reaction mixture will inevitably contain both sugars. Therefore, it is possible that the presence of glucose has a significant influence on the final furfural yield of the dehydration of the pentoses in a process with a biomass feed. The objective of this paper is the investigation of the influence of glucose on the degradation reactions of furfural. Furfural is known to degrade under the same conditions under which it is formed. Two different degradation products have been identified, that is, formic acid and resinous tars5,11,12 and the term ‘degradation’ is here used accordingly for the combination of both decomposition and polymerization. The importance of the formation of formic acid has been disputed,13 however, in the majority of the studies (small) quantities of formic acid have been measured.9,11,12 In most previous studies the experimental work has been complemented by (detailed) kinetic modeling. If the kinetics of the reactions of a pentose (or of its precursors) are to be determined, the kinetic parameters of the degradation reactions of furfural
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are to be established beforehand, forming a kind of kinetic cascade. The series of kinetic studies performed by Girisuta for the hydrolysis and dehydration of hexoses can be considered exemplary for the kinetic cascade approach.14–17 The kinetic parameters of the degradation of pure furfural have been previously determined in high-temperature water18 and in dilute sulfuric acid5,11,13 or dilute hydrochloric acid9,19 solutions. The kinetic parameters found in all these studies are in the same order of magnitude. Generally, it can be concluded that the activation energies for the degradation reactions are lower for HCl than for H2SO4. Also, all of these studies observe or assume first-order kinetics. The results of the present kinetic study on furfural degradation with glucose present will allow for much more precise kinetic modeling of biomass hydrolysis toward furfural. In this paper, experimental results are presented for the degradation of furfural in dilute acidic and saline solutions both with and without the addition of glucose to the reaction mixture. Additionally, the kinetics of the degradation reactions are extensively modeled. This paper will conclude with some mechanistic considerations regarding the bimolecular degradation of furfural. 2. Methods and materials The experiments were performed in a 1-L mechanically stirred stainless steel autoclave reactor, see also Figure 1. The temperature was regulated by pumping heated oil through the jacket of the reactor. The operating pressure in the reactor was the saturation pressure of the reaction mixture. Furfural (Sigma-Aldrich, P99%), D-glucose (Sigma-Aldrich, P99.5%), hydrochloric acid (J.T. Baker, 37.5%), and sodium chloride (J.T. Baker, P99%) were all commercially available and used without further purification. First, the reactor was filled with 850 ml demineralized water, including the total amount of required acid and salt for the experiment. This mixture was then heated to the desired operating temperature. Once the mixture had reached the desired temperature, about 100 g of a reactant solution (containing the furfural and/or glucose) was introduced using a highpressure syringe pump with a flow of around 90 ml/min. The injection of the reactant solution generally took about 1 min. The 100 g of reactant solution coincides with approximately 100 ml solution, resulting in a total reaction mixture of around 950 ml. It is noted that during the injection of the (relatively cold) reactant solution the temperature in the reactor dropped with about 5 °C. It took the heating system several minutes to restore the desired reaction temperature. Before the reactant solution was inserted, a first sample was taken via the sampling system. The sampling system consists of a sampling line, which runs through an ice bath in order to quench
T P
the sample, with a needle valve at the end. The flow through the sampling line is established by the use of the internal pressure of the reactor. Samples were taken from the liquid phase only. Prior to the actual sampling, around 10 ml solution was vented to wash the sampling line to ensure reproducible results. Samples were taken at various time intervals during the experiment. Analysis of the reaction products was carried out by means of an HPLC apparatus equipped with a Phenomenex Rezex RHMMonosaccharide column, 8% cross linked H+, 300 7.80 mm. A Marathon XT auto-sampler was used to enhance reproducibility. Glucose was measured by means of a Refractive Index (RI) detector (Varian, Model 350 Pro Star), whereas for furfural both the RI and an Ultraviolet (UV) detector (Varian Model 310 Pro Star) were used. Quantification was achieved by converting the detected areas using calibration curves made with the pure substances at various concentrations. A 0.005 N H2SO4 solution in demineralized water was used as the eluent at a flow rate of 0.6 ml/min with a column temperature of 80 °C. In all experiments the initial furfural and glucose concentrations were 50 mM. Furthermore, both hydrochloric acid (HCl) and sodium chloride (NaCl) were added as the catalysts, with concentrations of 50 and 500 mM, respectively. An HCl concentration of 50 mM has been shown to be a high enough concentration for achieving high furfural yields for the dehydration of xylose.7 The concentration of 500 mM NaCl has been chosen on one hand also based on optimal conditions for furfural production from xylose7 and on the other hand in order to resemble the salinity of sea water. The temperature range chosen for the experiments is between 160 and 200 °C. In Table 1 an overview is presented of all the performed experiments. 3. Kinetic model For the degradation of furfural the proposed reaction mechanism is presented in Scheme 1. The reaction rate for the degradation of furfural (R1) is defined as follows,
R1 ¼ k1 ½Fn1
ð1Þ 1
where k1 is the reaction rate constant (min ), [F] the furfural concentration (mM) and n1 the order of the reaction. The adapted version of the Arrhenius equation has been used for the definition of the reaction rate constant,
k1 ¼ A1 e
E R
ð
1 1 1 T To
ð2Þ 1
where A1 is the pre-exponential factor (min ), E1 the activation energy (kJ/mol), R the universal gas constant (kJ/mol K), T the temperature (K) and To the reference temperature (448.15 K). The kinetic parameters (A1 and E1) were directly estimated using a maximum-likelihood approach, based on the minimization of the sum of squared normalized errors (SSNE) between the ensemble of all the experimental and predicted concentrations at the different temperatures. The SSNE is calculated using the following equation,
Table 1 Overview of the performed experiments
Oil bath
HCl = 50 mM – NaCl = 500 mM
Ice bath Furfural (mM) Glucose (mM) Temperature (°C)
Reactant solution
Þ
HPLC pump
Reactor
Figure 1. Schematic of the experimental setup.
50 0 160 170 180 190 200
50 50 160 170 180 190 200
50 25 160 – 180 – 200
0 50 160 – 180 – 200
B. Danon et al. / Carbohydrate Research 375 (2013) 145–152
Scheme 1. Furfural degradation mechanism.
SSNE ¼
" N X y i¼1
ymod i yo yo i
2 # ð3Þ
where N is the total number of measurements, yo the initial concentration and yi, and ymod represent the experimental and modeled i data, respectively. The minimization of errors was initiated by providing initial guesses for each kinetic parameter. The best estimates were obtained using the MATLAB toolbox fminsearch, which is based on the NelderMead optimization method. This method has previously proven to result in accurate estimates.17 Furthermore, the percentage output variation (FIT) was calculated. The FIT is calculated using the following equation,
normðymod yi Þ i FIT ¼ 100 1 ~i Þ normðyi y
ð4Þ
~i is the mean value of yi and the function norm (M) in MATLAB where y returns the largest singular value of the matrix M. A rule of thumb is that a good FIT has a minimal value of 80%. Finally, the single-parameter 95% confidence interval of the estimated kinetic parameters has been determined using the approximate method described by Smith.20 This method is based on the comparison between two sets of predicted values. The first set is calculated with the estimated parameters from the maximum-likelihood approach. The second set is calculated with one of these estimated parameters varied by a small step (1% in this case). The sensitivity co-efficient is then defined as, mod ymod @ymod p þDp ypi i ¼ i i @pi Dpi
ð5Þ
where pi represents the ith estimated parameter and D pi the variation in pi. The overall sensitivity of the model is then represented by a sensitivity matrix A, with size i by i, where the diagonal values are the squared sensitivity co-efficients of the individual parameters. Next, the single-parameter standard error can be calculated from these diagonal values of the mean square error matrix V, which is related to the inverse of the sensitivity matrix A,
V ¼ r2 A1
ð6Þ
2
where r is the sum of square (absolute) errors. The 95% confidence interval is assumed to be approximately two times this singleparameter standard error. Moreover, the non-diagonal values of matrix V represent the correlation co-efficients between the estimated parameters. These values are between 1 and 1, with a lower value in this range representing less correlation, and vice versa. 4. Results and discussion The results of furfural degradation experiments with and without glucose are presented in Figure 2(a) and (b), respectively. As the reaction order for furfural degradation is assumed to be firstorder in most previous studies,19,9,11,13,18 as a first step, first-order kinetics were fitted on the experimental data. The results of the first-order kinetic model are represented by the solid lines in Figure 2. In Table 2 the estimated pre-exponential factors and activa-
147
tion energies and the fitting parameters are overviewed for the model. It is noted here that the value of E1 for furfural without glucose is of the same order of magnitude, but slightly higher, compared to values reported in the literature for dilute hydrochloric solutions (48.4 kJ/mol19 and 67.6 kJ/mol9). This is probably due to the presence of the second catalyst (NaCl), which seems to inhibit the furfural degradation reactions, translated in a higher activation energy. It is observed that the furfural degradation reaction shows a clear dependency on the temperature, with larger degradation rates at higher temperatures. Secondly, the two graphs also clearly indicate that furfural degrades faster when glucose is present. In the estimated kinetic parameters this is translated in a higher value of the pre-exponential factor and a lower value of the activation energy for the series with glucose. The lower value of the E1 further indicates that the increase in furfural degradation due to the presence of glucose is also dependent on the temperature. In other words, the increase in furfural degradation due to the presence of glucose will be higher at higher temperatures. For both of the experimental series the fit of the kinetic model seems to be quite good, expressed by a high value of FIT. Also, the standard errors in the estimated parameters are considered to be acceptable. However, in the case of glucose being present, the correlation co-efficient of the two estimated parameters is not close to minus unity, indicating that the two estimated parameters are not fully independent. The first-order kinetic model adequately predicts the degradation behavior of furfural, especially for the series without glucose. In the series with glucose, the trend in the measured and predicted furfural concentrations seem to disagree. The plotted logarithm of the furfural concentrations has a clear curved form (predominantly visible at higher temperatures), which indicates a different reaction order than first-order. First-order kinetics for furfural degradation have been challenged before. Root observed for the degradation of furfural in sulfuric acid a distinct relation between the initial furfural concentration and the degradation reaction rate.5 In his discussion a mechanism was proposed in which furfural degrades via two separate pathways. The first is via a first-order unimolecular degradation reaction (as assumed in other studies). However, a second pathway is proposed to proceed via a second-order reaction of furfural with an intermediate of its own degradation. This mechanism, including a second-order reaction, could explain the reported experimental data. Furthermore, Dunlop mentioned resins as a degradation product of furfural.12 In the present experiments there were also indications of resin formation. These include few black particles in the samples at longer reaction times and an increasing darkening of the samples with reaction time. The exact structure of these resins remains unresolved, however, it is clear that they are some kind of polymers. Since a polymerization reaction is by definition a poly-molecular reaction, second-order reaction kinetics cannot be excluded for the resinification reactions. Finally, the plot of the inverse of the furfural concentrations of the experiments with glucose versus the reaction time showed clear linearity. This also indicates second-order reaction kinetics. Therefore, the experimental data have been refitted using a second-order kinetic model (n1 = 2). In Figure 3 and Table 3 the results are presented for this model. For the series without glucose, the SSNE and FIT are improved compared to the first-order model, however, the differences are very small. Due to these small differences, the degradation reaction can be perceived as pseudo-firstorder. It remains undecided whether the reaction is of either order. A possibility for this ambiguity is that in reality the degradation of pure furfural comprises of both a first- and a second-order reaction.
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0.1
160
170
180
190
0.1
200
−0.1 −0.2 −0.3 −0.4
170
180
190
200
0 Furfural ln(C/Co) (−)
Furfural ln(C/Co) (−)
0
160
−0.1 −0.2 −0.3
0
10
20
30 40 Time (min)
50
60
−0.4
0
10
20
30 40 Time (min)
50
60
Figure 2. Experimental data (symbols) and first-order (n1 = 1) model (lines) of furfural concentrations (a) without and (b) with glucose present at five different temperatures.
Table 2 Activation energies (E1 [kJ/mol]) and pre-exponential factors (A1 [min1]) for the first-order kinetic model (n1 = 1) for furfural degradation with and without glucose present Estimated value
95% Confidence interval
Correlation co-efficients
Absolute
Relative (%)
A1
E1
Furfural A1 E1 SSNE FIT
1.55E-03 102.1 0.0068 99.76
±7.51E-05 ±4.1
±4.84 ±3.99
1.00 0.92
1.00
Furfural + Glucose A1 E1 SSNE FIT
5.55E-03 65.7 0.0223 99.68
±2.05E-04 ±3.7
±3.68 ±5.64
1.00 0.73
1.00
0.1
160
170
180
190
0.1
200
−0.1 −0.2 −0.3 −0.4
170
180
190
200
0 Furfural ln(C/Co) (−)
Furfural ln(C/Co) (−)
0
160
−0.1 −0.2 −0.3
0
10
20
30 40 Time (min)
50
60
−0.4
0
10
20
30 40 Time (min)
50
60
Figure 3. Experimental data (symbols) and second-order (n1 = 2) model (lines) of furfural concentrations (a) without and (b) with glucose present at five different temperatures.
In the case of the series with glucose present, it is noted that the predicted furfural concentrations show a similar trend as in the experiments. When the total of normalized errors SSNE and FIT are compared for the two models of this series (compare Tables 2 and 3), it turns out that the predictions are improved significantly. However, the high correlation between the two estimated parameters is still present. It is concluded that furfural degradation shows second-order reaction kinetics in more complex solutions. However, in pure furfural solutions the degradation rates can be perceived as a (pseudo-)first-order reaction. In this case the differences in the
predictions of the first- and second-order kinetic models are too small be conclusive regarding the reaction order. Next, the influence of the glucose on the kinetics of furfural degradation was studied in more detail. First, the apparent disappearance of the glucose in these experiments was compared with results of experiments with pure glucose solutions at similar reaction conditions. In Figure 4 the conversion of glucose at three different temperatures (160, 180, and 200 °C) is shown. The differences in the disappearance of glucose in these two series of experiments (with and without furfural present) seem to be relatively small compared to the differences observed for the
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Table 3 Activation energies (E1 [kJ/mol]) and pre-exponential factors (A1 [min1]) for the second-order kinetic model (n1 = 2) for furfural degradation with and without glucose present Estimated value
Furfural A1 E1 SSNE FIT Furfural + Glucose A1 E1 SSNE FIT
95% Confidence interval Absolute
Relative (%)
A1
E1
2.86E-05 114.7 0.0058 99.78
±1.17E-06 ±3.5
±4.09 ±3.07
1.00 0.92
1.00
1.55E-04 85.7 0.0084 99.80
±3.10E-06 ±2.1
±2.00 ±2.47
1.00 -0.71
1.00
0
Glucose ln(C/Co) (−)
−0.5 −1 −1.5 −2 −2.5 −3 −3.5 −4
160 0
10
20
180
30 Time (min)
200 40
50
60
Figure 4. Glucose conversion with (crosses) and without (circles) furfural present at three different temperatures.
disappearance of furfural. Therefore, it is concluded that the presence of furfural has a relatively small effect on the conversion of glucose. Moreover, a kinetic model incorporating glucose as a reactant in an additional reaction was not able to produce reasonable predictions for the experimental data. Accordingly, it can be concluded that glucose itself does not participate as a reactant in an additional furfural degradation reaction. For a quantitative evaluation of the effect of the glucose on furfural degradation, nine experiments are considered with three different concentrations of glucose and three different temperatures (see also Table 1). An extended reaction mechanism is proposed with an additional furfural degradation reaction (R2), which is influenced by the presence of glucose, see also Scheme 2. The additional reaction R2 is formulated as follows,
R2 ¼ k2 ½Fn2
ð7Þ
with the reaction rate constant k2 defined as,
k2 ¼ A2 ½Go e
E R
ð
2 1 1 T To
Correlation co-efficients
Þ
ð8Þ
where [G]o represents the initial glucose concentration (mM). Since glucose cannot be regarded as a reactant, it is introduced as a con-
stant in the reaction rate constant expression. The reaction order of R2 is assumed to be 2. In Figure 5 the results are presented for the experiments with 50 mM furfural and none, 25, and 50 mM of glucose as initial concentrations. For the degradation of furfural independent of the glucose (R1) both first- and second-order kinetics have been implemented, Figure 5(a) and (b), respectively. In Tables 4 and 5 the estimated kinetic parameters are presented. In both cases the model is capable of predicting the influence of the glucose on furfural degradation. The influence of glucose on furfural degradation seems also to be temperature dependent, but in a different way than that of furfural alone. This can be deduced from the differences between the degradation of furfural in the cases with 25 and 50 mM glucose. At both 160 and 200 °C the effect of the concentration of glucose is smaller than at 180 °C. This different temperature dependence is also reflected in a lower activation energy for R2 compared to R1. In contrast with the mechanism consisting of merely one second-order reaction (see Table 3), the correlation co-efficient of R2 indicates that the two estimated parameters are now fully independent. If the two reaction orders for the independent furfural degradation reaction are compared, the results are again ambiguous. At 160 °C the model with first-order R1 better predicts the experimental data, whereas at 200 °C the second-order R1 seems to be more accurate. It is concluded that the influence of glucose on the degradation of furfural is well represented by introducing an additional reaction to the reaction mechanism. This reaction follows second-order kinetics and incorporates the initial glucose concentration as a constant in the Arrhenius equation. Finally, possible reaction mechanisms of such a second-order furfural degradation reaction are considered. As stated before, Dunlop suggested that the degradation products of furfural in (acidic) aqueous solutions are formic acid and resins.12 For the formation of formic acid the hydrolytic fission of the aldehyde group was proposed. In the present study also very small (unquantifiable) amounts of formic acid have been observed in the chromatograms. For the resinification no reaction mechanism was proposed by Dunlop. It seems evident, both based on the present kinetic results and on the concept of polymerization, that the resinification reaction effectively is a second-order reaction (thus involving two furfural molecules).
Scheme 2. Furfural degradation mechanism with glucose present.
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Temperature = 160°C 0.1
G=00mM
G=25mM
Temperature = 160°C 0.1
G=50mM
−0.1 −0.2 −0.3 −0.4
−0.1 −0.2
0
10
20
30 40 Time (min)
50
−0.4
60
0
10
Temperature = 180°C G=00mM
G=25mM
G=50mM
Furfural ln(C/Co) (−)
Furfural ln(C/Co) (−)
30 40 Time (min)
50
60
G=00mM
G=25mM
G=50mM
0
−0.1 −0.2 −0.3
−0.1 −0.2 −0.3
0
10
20
30 40 Time (min)
50
−0.4
60
0
10
Temperature = 200°C 0.1
G=00mM
G=25mM
20
30 40 Time (min)
50
60
Temperature = 200°C 0.1
G=50mM
G=00mM
G=25mM
G=50mM
0 Furfural ln(C/Co) (−)
0 Furfural ln(C/Co) (−)
20
Temperature = 180°C 0.1
0
−0.1 −0.2 −0.3 −0.4
G=50mM
−0.3
0.1
−0.4
G=25mM
0 Furfural ln(C/Co) (−)
Furfural ln(C/Co) (−)
0
G=00mM
−0.1 −0.2 −0.3
0
10
20
30 40 Time (min)
50
60
−0.4
0
10
20
30 40 Time (min)
50
60
Figure 5. Furfural degradation with various concentrations of glucose and (a) n1 = 1 and n2 = 2 and (b) n1 = n2 = 2.
Therefore, a Diels–Alder reaction is considered, see also Scheme 3. In a Diels–Alder reaction a diene and a dienophile react to form a six-membered ring with one double bond.21 Furfural has been shown to be able to act as a dienophile in reaction with a strong diene (butadiene).12 However, no reaction was observed for furfural with a strong dienophile (maleic anhydride) under the same conditions where less polar furans, for example, methylfuran, did react.12 In methylfuran the higher electron density in the furan ring serves as the electron donor (the diene) in the reaction. It is argued here that furfural does act as a diene, that is, in acidic aqueous solutions. The diene is then defined not as the bonds of the furan ring but as one double bond of the furan ring in combination with the double bond of the aldehyde group. In a polar sol-
vent, these two double bonds of furfural are most stable in cisconformation, as is required for a Diels–Alder reaction.22 Moreover, since the oxygen atom of the aldehyde group forms (on average) one hydrogen bond with the surrounding water, the dipole moment of the furfural molecule is increased significantly.22 Since the concentration of protonated water (H3O+) is relatively high in acidic aqueous solutions, hydrogen bonds with these protonated water molecules even further increase the dipole moment of furfural. This results in an even higher electron density in the aldehyde group (the diene) and a lower electron density in the furan ring (the dienophile), which is favorable for the Diels–Alder reaction. Finally, it is noted that only a very low percentage of furfural will be hydrated. This is mainly due to the substituent on the central
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Table 4 Activation energies (E [kJ/mol]) and pre-exponential factors (A [min1]) for n1 = 1 and n2 = 2 based on experiments with none, 25, and 50 mM glucose and at 160, 180, and 200 °C Estimated value
Reaction R1 A1 E1 SSNE FIT Reaction R2 A2 E2 SSNE FIT
95% Confidence interval
Correlation co-efficients
Absolute
Relative (%)
A#
E#
1.55E-03 102.1 0.0068 99.76
±7.51E-05 ±4.1
±4.84 ±3.99
1.00 0.92
1.00
1.66E-06 76.1 0.0571 99.72
±9.35E-08 ±4.3
±5.62 ±5.62
1.00 1.00
1.00
Table 5 Activation energies (E [kJ/mol]) and pre-exponential factors (A [min1]) for n1 = n2 = 2 based on experiments with none, 25, and 50 mM glucose and at 160, 180, and 200 °C Estimated value
95% Confidence interval
Correlation co-efficients
Absolute
Relative (%)
A#
E#
Reaction R1 A1 E1 SSNE FIT
2.86E-05 114.7 0.0058 99.78
±1.17E-06 ±3.5
±4.09 ±3.07
1.00 0.92
1.00
Reaction R2 A2 E2 SSNE FIT
1.79E-06 94.6 0.0586 99.72
±7.22E-08 ±3.8
±4.04 ±4.04
1.00 1.00
1.00
Scheme 3. Proposed reaction mechanism for a Diels–Alder reaction between two furfural molecules.
carbon atom of the aldehyde group, that is, the furan ring. Both as an electron donating group and as a substituent containing nonbonding electrons, the furan ring decreases the equilibrium constant of hydration of the aldehyde group.23 Therefore, it seems plausible that furfural in an acidic aqueous solution can react with itself, albeit in small quantities, in a Diels–Alder reaction with second-order reaction kinetics. Next, it is considered how the added glucose can enhance this reaction. First, since the glucose itself is also dehydrated to (ultimately) formic and levulinic acid, the acidity of the entire reaction solution is increased.15 Higher concentrations of H3O+ therefore further increase the (average) dipole moment of the furfural molecules and thus favor Diels–Alder reactions. This could also explain the fact that for the experiments with glucose present furfural degradation was accurately predicted by including the initial glucose concentration as a constant in the Arrhenius equation.
Secondly, and more speculatively, it might be that glucose itself, or one of its degradation products (e.g., hydroxymethylfurfural or levulinic acid), participates or initiates Diels–Alder or other polymerization reactions with furfural. Zeitsch identified two pathways for furfural degradation in a reaction solution with xylose.1 Apart from a first-order (unimolecular) degradation reaction of furfural, a condensation reaction was proposed for furfural reacting with an intermediate of the dehydration of xylose toward furfural. Zeitsch propounds that this latter reaction can be both uni- and bimolecular, resulting in a mechanism comprising of both first- and second-order kinetics. This reaction could be an analog for the influence of the glucose. It is of course also possible that both of the previous described influences of glucose contribute to one single reality. In order to obtain more detailed insight in the mechanistic effects, additional experiments have to be performed including furfural degradation at different acid concentrations and with different dehydration products of glucose present. This lay however outside the scope of the present study. It has to be noted here that the present experimental work has been performed in a batch reactor. Nowadays, in more advanced production processes of furfural simultaneous extraction of the formed furfural is included. Obviously, if furfural can be extracted from the reaction mixture as soon as it is formed, the implications of the presence of other components (such as glucose) on furfural degradation in the reaction mixture diminishes. Nevertheless, detailed and correct understanding of the fundamental reaction kinetics of furfural would even in these cases be of major importance. 5. Conclusions Furfural degradation has been investigated in a dilute acidic (HCl) and saline (NaCl) solution. The degradation rates have been determined for a series of experiments with and without glucose
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present in the reaction mixture. The obtained activation energy for a first-order reaction of pure furfural is in agreement with values found in the literature. Model predictions of a second-order kinetic model for pure furfural degradation showed comparable results as the first-order model. It remains therefore undecided whether this reaction is of either order. A possibility for this ambiguity is that in reality the degradation of furfural alone also comprises of both a first- and a second-order reaction. When glucose is present in the reaction mixture furfural degrades significantly faster. A mechanism including an additional second-order reaction incorporating the initial glucose concentration, turned out to satisfactorily predict furfural degradation in the presence of glucose. There were presented several considerations that this additional second-order reaction is a Diels–Alder reaction. In the future, more extensive experimental and kinetic studies should be performed on the influence of the acid concentration on these reactions. Acknowledgements The authors wish to gratefully acknowledge Buana Girisuta for the sharing of his wisdom and Michel van den Brink for his support in the laboratory. References 1. Zeitsch, K. The chemistry and technology of furfural and its many by-products In Sugar series; Elsevier, 2000; Vol. 13,
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