Kinetic Study on the Dilute Acidic Dehydration of Pentoses toward

Article pubs.acs.org/IECR

Kinetic Study on the Dilute Acidic Dehydration of Pentoses toward Furfural in Seawater Wijittra Hongsiri,* Bart Danon, and Wiebren de Jong Delft University of Technology, Process and Energy Department, Leeghwaterstraat 44, 2628 CA, Delft, The Netherlands ABSTRACT: Furfural promises to be a very important product of the lignocellulosic feedstock biorefinery. In this study the kinetics of both xylose and arabinose dehydration toward furfural is investigated in a dilute acidic medium under three different salt conditions. These comprise no salts, a 500 mM NaCl solution, and seawater. The results demonstrate that the salts catalyze all disappearance reactions of the pentoses, both toward furfural and toward loss products. Especially at higher temperatures, the increase in the reaction rate toward furfural is larger than toward loss products. The values of the reaction rate constants at different salt conditions and temperatures indicate that different ions catalyze specific (temperature dependent) reactions in the dehydration mechanism of a pentose. Furthermore, the increase in the reaction rates is more pronounced with the combined salts naturally present in seawater compared to only NaCl, even at the same salinity and ionic strength. It is shown in additional experiments with no acid added that the observed salt effects are independent of the acidic environment. Furthermore, the effects of the salts are larger for the dehydration of xylose compared to arabinose. Moreover, for all temperatures the molar furfural yield was improved by the addition of the salts. Finally, it is shown that the salts inhibit furfural loss reactions toward formic acid. The presented results further contribute to the understanding of the effects of saline catalysis on the mechanism of pentose dehydration.

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INTRODUCTION Furfural, a hemicellulose derived platform chemical,1 promises to be a very important product of the lignocellulosic biomassbased biorefinery.2,3 It can easily be produced in a biorefinery system, and is a precursor for many different chemicals, plastics, and biofuels. Currently, the furfural market is quite small: annual production is ca. 300 kton.4,5 Some authors share the opinion that this small scale is due to inefficient production processes and the relatively low oil price.2 The demand for furfural is expected to grow when furfural production processes become more efficient. It is known that pentose dehydration toward furfural is catalyzed by acids.6−9 However, more recently, also the potential of salts as a secondary catalyst for these reactions has been observed. Marcotullio et al. have investigated the kinetics of xylose dehydration with many different salts at various combinations and concentrations.10,11 Their results show that, at fixed chloride concentrations, the differences in the rate constants of the reactions are similar, thus less dependent on the type of cation employed. Moreover, it was found that different halides have different effects on the reaction rate constants, and a combination of, for example, KCl and KI showed a substantial improvement in the selectivity and yield. However, also the cations do exert influence on the dehydration reactions. Gravitis et al. showed that cations catalyze the dehydration of biomass-derived carbohydrates to furfural with a catalytic effect proportional to their ionization potential, which increase for K+, Na+, Ca2+, Mg2+, and Fe3+, respectively.12 Finally, the rate of disappearance of xylose and xylotriose was investigated by Liu et al. for different chloridecontaining saline nonacidic solutions.13 In this study, the different salts (e.g., NaCl, CaCl2, FeCl3) were added at the same weight percentage, resulting in different ion concentrations which made a clear comparison more difficult. © 2014 American Chemical Society

The use of seawater as a reaction medium for biomass processing offers an opportunity for using nonpotable water sources at large scale, especially in coastal localities, and for existing resources such as algae.14 As a solvent for a furfural production process seawater represents a natural, cheap, and sustainable source of NaCl and other salts. These other salts, such as, KCl, CaCl2, MgCl2, MgSO4, NaHCO3, or NaBr, also have a high potential to further improve the furfural yield. Several studies on biorefinery processes, that show the potential of using seawater as a solvent, are available.15,16 Recently, a study of a seawater-based furfural production process integrated with wastewater recycling was published.17 In this paper, experimental results are presented for corncob hydrolysis at various temperatures using acetic acid, seawater, and/or FeCl3. No kinetic analysis has been performed, however. Vom Stein et al. studied cellulose depolymerization using organic acids and inorganic salts in aqueous media at mild conditions.18 Their results suggest that seawater may also be a viable alternative reaction medium for cellulose depolymerization. Moreover, the use of seawater appears to be a promising alternative reaction medium for large scale biorefineries in general,19 and for the production of furfural in particular.17,20 Due to the fact that salts provide aqueous solutions with acidic or basic properties, the Lewis acids and their catagorization as “hard” or “soft” appear to influence catalysis for the dehydration reactions as follows. For the first C5 sugar conversion step, the formation of the 1,2-enediol is preferably catalyzed by the reducing bases strength in the order of Cl−, Br−, I−. On the other hand, the Received: Revised: Accepted: Published: 5455

December 25, 2013 March 8, 2014 March 10, 2014 March 10, 2014 dx.doi.org/10.1021/ie404374y | Ind. Eng. Chem. Res. 2014, 53, 5455−5463

Industrial & Engineering Chemistry Research

Article

the total amount of required acid and/or salt. This mixture was then heated to the desired temperature (160°−200 °C). Next, around 50 g of reactant solution is pumped into the reactor which generally took 30−45 s. The 50 g of concentrated reactant solution coincides with approximately 50 mL, resulting in a total reaction mixture volume of around 900 mL. Then, the total initial reaction solution is 900 mL with a reactant concentration of 50 mM. It is important to note that during the introduction of the concentrated reactant solution the temperature in the reactor dropped about 3 °C. It took the heating system around 5 min to restore the reaction temperature. The pressure in the reactor remained approximately constant throughout the experiment. Before a first sample (of around 5 mL) was taken, the sampling system was cleaned by flushing around 10−12 mL reaction mixture. The reaction time was set at 1 min (including the pumping and flushing time). Then, samples were taken at various time intervals during the experiment. Before each sample the sampling line was flushed. The samples were analyzed using an HPLC apparatus with a Rezex RHM-Monosaccharide column, 8% cross-linked H+, 300 × 7.80 mm (Phenomenex Inc., Torrance, CA, U.S.A.). A 0.005 N H2SO4 solution in demineralized water was used as the mobile phase at a flow rate of 0.6 mL/min with a column temperature of 80 °C. A Marathon XT autosampler (Separations, Ambacht, NL) was used to improve the reproducibility. The sugars were quantified by means of a Refractive Index detector (Varian Model 350), while the other products, mainly furfural, were analyzed using both the Refractive Index detector and a UV detector (Varian Model 310 Pro Star). Finally, elementary compositions of the employed NaCl solution and seawater have been analyzed using a SPECTRO ICP-OES spectrometer instrument (www. sysmex.nl).

second step the selective dehydration to furfural is assisted by the halides preferably in the order of an increase of base strength as in the order of I−, Br−,Cl− as reported by Marcotullio and De Jong.11 Thus, the catalyzing effect of salts, both of separate mineral salts and of the combined salts naturally present in seawater, seems to be evident from the literature. However, more understanding regarding the actual catalysis of these salts on the different reactions during pentose dehydration is required. Also, possible synergistic action of the many different salts present in seawater needs to be further elucidated. This kinetic study aims at providing these insights. First, the kinetics of furfural degradation in an acidic medium (50 mM HCl) for three different salt conditions, that is, no salts, a 500 mM NaCl solution, and seawater, are considered. Next, the dehydration toward furfural of both xylose and arabinose is kinetically investigated for the same three salt conditions and acid concentration. For all these reactions, first-order kinetic reaction rate constants have been determined for three temperatures (160, 180, and 200 °C). In order to verify that the observed trends are for the salt effects specifically, few experiments are repeated without any acid added. Finally, the formation of formic acid from furfural is studied and the effect of the salts thereon.

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METHODS AND MATERIALS A one-liter mechanically stirred stainless steel autoclave reactor is used for the experiments. The reactor is heated to the desired temperature by pumping heated oil through a jacket around the reactor. The operating pressure in the reactor is the saturation pressure of the mixture. A high-pressure syringe pump is used to introduce the reactant solution into the reactor with a flow rate of 90 mL/min. Samples were taken from the liquid phase in the reactor by a sampling system by making use of the pressure in the reactor. The sampling system consists of a sampling line which is circling through an ice bath to cool down the hot samples and stop the reactions at the desired sampling time. A schematic of the experimental setup is presented in Figure 1.

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KINETIC MODEL The kinetics of furfural formation from xylose have been extensively studied by performing experiments at different temperatures, reactant, and catalyst concentrations. The results show that the kinetics are strongly dependent on these conditions2,11,21 and generally a simple reaction scheme is assumed as presented in Figure 2.6,11,22,23 Even though many

Figure 1. Schematic of the experimental setup.

Figure 2. Simplified reaction pathway for the dehydration of xylose to furfural.

The reagents used in the experiments are D-xylose, Larabinose, and furfural, all with a 99% purity (Sigma-Aldrich). In all experiments, the reactants are dissolved in demineralized water or seawater. The seawater was collected from Scheveningen (The Netherlands) in June 2012 for all experiments. The experimental procedure is as follows. First, the reactor is filled with 850 mL demineralized water or seawater, including

different conditions have been studied, intermediates of the reactions have not been identified. This is probably because the intermediates automerize much faster than they react. For this reason, it is assumed that the xylose intermediate concentration did not significantly influence the reaction rates.7,21,24 The reaction scheme for arabinose dehydration is assumed to be analogous to that of xylose as presented in Figure 2. 5456

dx.doi.org/10.1021/ie404374y | Ind. Eng. Chem. Res. 2014, 53, 5455−5463


Article

Following the reaction scheme for xylose (and arabinose) dehydration the first-order reaction rates can be described by eqs 1 and 2: R xylose

dCx = = −(kx1 + kx 2)Cx dt

dCf

R furfural =

dt

= kx1Cx − k f Cf

Table 1. Concentrations of the Major Ions (g/kg) in the NaCl Solution and Seawatera

(1)

(2)

where Cx is xylose concentration (mM) at reaction time t (min), kx1 is the furfural formation rate constant (min−1) and kx2 is the formation rate constant of other (loss) products (min−1). Analogue equations for the reaction rates and concentrations of arabinose can be written. For the rate of degradation of furfural alone, the differential equation is written as follows:

dCf dt

= −k f Cf

a

Y=

S=

Cx ,0 − Cx Cx ,0

Cf Cx,0

(3)

Cx ,0 − Cx

0.0042 0.3341 0.4008 0.6198 6.8296 0.0056 0.0574 15.7548 2.4525 26.4588

NaCl solution (g/kg)

0.0038 8.8602

17.4439 0.1125 26.4204

Iodide was below the detection limited.

I=

1 2

n

∑ mizi2

(7)

i=0

where n is the number of ions, m is the molality (mol/kg) of the ion i and z is the charge (−) of ion i. The calculated values are presented in Table 2. It is observed that the ionic strengths

100

Table 2. Ionic Strength and Hydronium Ion Activity Coefficients of the 50 mM HCl Solution at the Three Different Salt Conditions

(4)

100

Cf

seawater (g/kg)

salinity in seawater. However, in seawater also other ions contribute to the salinity. These other ions are, concerning the cations (in order of concentration) Mg2+ > K+ > Ca2+ > Sr2+ and concerning the anions (in order of concentration) SO42− > Br−. It is concluded that, although both seawater and the NaCl solution have comparable salinities, the main difference between these two solutions is the presence of other ions than Na+ and Cl−. Next, the ionic strength (I) is calculated for the three salt conditions, using the following equation:

where Cf is the furfural concentration (mM) at time t (min), and kf is the furfural degradation rate constant (min−1). The reaction order of the furfural degradation reaction is assumed to be first-order, as in most previous studies.2,3,12,13,18,25 All reaction rate constants are estimated by least-squares fitting of the concentration expressions on the experimental data in MATLAB. The differential equations were solved using the ode45 function and the lsqcurvef it function was used for the fitting of the experimental data. Finally, the conversion C of the pentoses (%), the molar furfural yield Y (%) and selectivity S (%) have been calculated using the following equations: C=

ion B(OH)3 Ca2+ K+ Mg2+ Na+ Sr2+ Br− Cl− SO2− 4 total

(5)

γH3O+ (−)

100 (6)

where the initial xylose concentration Cx,0 (mM) is calculated from the measured weight of inserted reactant solution. All experiments ran for 60 min with around 15 data points per experiment. The values of the conversion, yield and selectivity presented in the next section are the maximum values for each within these 60 min.

salt condition

I (mol/kg)

160 °C

180 °C

200 °C

no salts NaCl solution seawater

0.050 0.517 0.562

0.751 0.559 0.527

0.738 0.539 0.502

0.723 0.513 0.472

of the NaCl solution and seawater are very comparable, as was expected from their comparable salinities. However, they are significantly higher than the ionic strength of the solution without any added salts. These differences in ionic strength have an effect on the activity coefficients (γi) of the individual ions, thus also on the hydronium ions. The activity coefficients of the hydronium ions at the employed reaction temperatures have been calculated using the eNRTL model in the Aspen Plus software and are also presented in Table 2. Again, the values for seawater and the NaCl solution are comparable and significantly lower than the value for the no salts condition. This means that by introducing the salts in the reaction mixture the activity of the acid, that is, of the hydronium ions, is significantly decreased. Regarding the hydronium ion concentration, [H3O+] all the solutions with acid show the same value (50 mM). HBr or H2SO4 can also be formed in the seawater case, which might change the pH value. However, all the results show very low concentrations of these acids which can thus be neglected.

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RESULTS AND DISCUSSION Salinity and Ionic Strength. For the seawater and the NaCl solution the major ions are defined as those ions that have a concentration higher than 1 mg/kg. The concentrations of these major ions are presented in Table 1 for these two solutions, measured by ICP-OES. The salinity then is defined as the total amount (by weight) of dissolved ions per kilogram of solution. The results show that the total amount of major ions is comparable in both solutions. Thus, the employed salinity in all the saline experiments is approximately 26.4 g/kg. In the NaCl solution, as expected, ions other than Na+ and Cl− do not significantly contribute to the salinity. Furthermore, the Na+ and Cl− ions also account for approximately 85% of the total 5457



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Thus, the hydronium ion activity, (aH3O+) is mainly influenced by the presence of the salts due to changes in the γH3O+. Furfural Degradation. First, the kinetics of the degradation of pure furfural was investigated at temperatures between 160 and 200 °C. All experiments were performed with an initial furfural concentration of 50 mM in an acidic solution (HCl 50 mM). Three different salt conditions were investigated: HCl with no salts (FH), HCl with 500 mM NaCl (FHN), and HCl in seawater (FHS). The results of the furfural degradation experiments are presented in Figure 3. The symbols represent the experimental data and the lines represent the first-order kinetic model. The kinetic model appears to result in a quite good fit for all the experiments. It is observed that the degradation rate constant kf (this is the slope of the lines in Figure 3) is dependent both on temperature and salt condition. At higher temperatures the furfural degradation rate is higher, as expected. Conversely, the presence of the salts results in a decrease of the degradation rate. This effect is more pronounced at higher temperatures. Moreover, the combined salts of seawater result in an even larger decrease in the degradation rate than is observed for the NaCl solution. It is known that the destruction of furfural is enhanced by acid, see for example the values of kf at 200 °C with and without acid in Table 3 and Table 4. Thus, the inhibiting effect of the salts is (at least partly) due to the fact that they lower the activity coefficients of the hydronium ions. It can be concluded that the presence of the salts lowers the degradation rate of furfural. Moreover, the combined salts in seawater result in an even lower furfural degradation rate compared to a solution with mainly Na+ and Cl−. Pentose Dehydration. In Figure 4, the concentrations of xylose (a1−a3) and arabinose (b1−b3) versus reaction time at three different temperatures and different salt conditions are presented. The symbols are the experimental data and the lines represent the first-order kinetic model. First, the disappearance rates of the pentoses are considered. The rate of disappearance of xylose, represented by (kx1 + kx2), which is the slope of the lines in Figure 4 (a1−a3), is dependent on both temperature and salinity. At higher temperatures higher rates of xylose disappearance are observed. Moreover, also the presence of the salts contributes to an increase of the rate of disappearance. However, the effect of the temperature seems to be larger. Analogously, these observations for the dehydration of xylose also apply to that of arabinose. However, in the case of arabinose the differences are smaller. Hughes and Acree already observed that the effect of salts on the conversion rate of arabinose is smaller than that of xylose.26 Also, more generally, it is observed that arabinose reacts slower than xylose, as has been reported before for similar reaction conditions.8 The kinetic rate constants derived from Figure 4 are presented in Table 3 and Table 4. It is observed that the reaction rates of xylose (kx1) and arabinose (ka1) toward furfural are higher for the cases when salts are present (even though the hydronium activities are lower in these cases, see Table 2). Moreover, the rate of the reactions of xylose (kx2) and arabinose (ka2) to loss products are also higher for these cases. These observations imply that the presence of salts accelerates all the reactions of the pentoses to their products; both to furfural and to loss products.

Figure 3. Furfural destruction behavior at three different temperatures and salts conditions (a, b, c). F = furfural, H = no salts, N = NaCl (500 mM), S = seawater. All with HCl = 50 mM.

In order to compare these two increases in reaction rates, the ratios of kx1/kx2 (and similarly ka1/ka2) are also printed in Table 3 and Table 4. The values of kx1/kx2 and ka1/ka2 for the experiments at 160 °C indicate that the salts catalyze the formation of loss products more strongly than the reaction toward furfural. This was not expected a priori. At 180 °C the 5458



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Table 3. Kinetic Parameters for the Experiments with Xylosea temp. (°C) 160 160 160 180 180 180 200 200 200 200 200 200 a

HCl (mM) 50 50 50 50 50 50 50 50 50 0 0 0

salt condition no salts NaCl seawater no salts NaCl seawater no salts NaCl seawater no salts NaCl seawater

kx1 (min−1)

kx2 (min−1)

−3

−4

8.5 2.1 3.2 4.3 9.2 1.1 1.2 2.3 2.7 1.7 2.5 4.8

× × × × × × × × × × × ×

10 10−2 10−2 10−2 10−2 10−1 10−1 10−1 10−1 10−2 10−2 10−2

9.1 1.2 1.9 1.3 4.2 2.3 7.4 1.1 9.7 1.3 2.1 6.2

× × × × × × × × × × × ×

10 10−2 10−2 10−2 10−2 10−2 10−2 10−1 10−2 10−2 10−2 10−2

kx1/kx2 (−) 9.3 1.7 1.7 3.3 2.3 4.6 1.7 2.0 2.8 1.3 1.2 0.8

kf (min−1)

C (%)

Y (%)

S (%)

10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3

45.4 90.0 97.3 97.1 99.0 100.0 98.5 99.4 100.0 91.1 95.5 98.9

36.7 55.5 55.2 59.6 66.6 70.6 52.8 64.3 71.7 51.5 46.1 44.8

67.4 60.6 57.6 71.3 72.7 78.0 60.1 72.6 71.0 53.2 58.1 55.0

kf (min−1)

C (%)

Y (%)

S (%)

10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3

36.1 26.2 53.5 90.7 96.6 96.0 99.4 100.0 99.3 84.2 92.8 97.1

21.5 50.5 28.3 48.6 48.4 51.4 34.5 49.1 53.9 24.1 27.0 26.2

55.7 56.0 57.8 56.2 55.0 57.6 55.7 56.0 57.8 41.0 42.3 43.1

1.9 1.3 1.2 3.1 2.1 2.0 8.4 5.6 4.9 2.2 1.8 1.4

× × × × × × × × × × × ×

C = conversion (%), Y = molar furfural yield (%), and S = selectivity (%).

Table 4. Kinetic Parameters for the Experiments with Arabinosea temp. (°C) 160 160 160 180 180 180 200 200 200 200 200 200 a

HCl (mM) 50 50 50 50 50 50 50 50 50 0 0 0

salt condition no salts NaCl seawater no salts NaCl seawater no salts NaCl seawater no salts NaCl seawater

ka1 (min−1)

ka2 (min−1)

−3

−3

4.6 7.0 7.2 2.7 3.1 2.7 5.0 1.1 1.2 1.1 2.3 4.2

× × × × × × × × × × × ×

10 10−3 10−3 10−2 10−2 10−2 10−2 10−1 10−1 10−2 10−2 10−2

2.1 1.0 5.0 1.4 1.9 1.3 5.1 9.0 8.9 2.7 2.2 6.1

× × × × × × × × × × × ×

10 10−2 10−3 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2

ka1/ka2 (−) 2.2 0.7 1.4 1.9 1.7 2.0 1.0 1.2 1.3 0.4 1.1 0.7

1.9 1.3 1.2 3.1 2.1 2.0 8.4 5.6 4.9 2.2 1.8 1.4

× × × × × × × × × × × ×

C = conversion (%), Y = molar furfural yield (%), and S = selectivity (%).

results are different. At this temperature, the addition of only NaCl results in a decrease of the ratio, while in seawater the ratio is increased as compared to the case without any salt. Finally, at 200 °C there is a clear increase in the ratio of kx1/kx2 and ka1/ka2 considering the cases without salts, with the addition of NaCl and in seawater. All these observed trends for the three different temperatures are the same for the xylose and arabinose experiments. Since the (combined) salts catalyze both the dehydration and loss reactions of the pentoses and these reactions follow different mechanisms, and thus also have different temperature dependencies, no temperature independent trend could be identified. Only at 200 °C, the addition of different salts clearly results in an improvement of the furfural selectivity. Next, the furfural yields are examined. In Table 4 the maximum values are presented. First, it is observed that in most cases the furfural yields are higher when salts are present. Next, the yields of furfural are generally higher for xylose than for arabinose, as has been reported earlier.8 Also, the differences between the salt conditions are more pronounced for xylose dehydration than for arabinose. If the experiments with NaCl and seawater are compared among themselves, it is observed that in almost all cases, both for xylose and arabinose, higher furfural yields are obtained with the combined salts of seawater. This indicates that also the additional salts present in seawater (other than Na+ and Cl−) do enhance (some specific) reaction(s) during the dehydration of pentoses. This was also previously observed with different model salts.10,11

It can thus be concluded that salts (and especially the combined salts of seawater), at specific reaction temperatures, significantly enhance the furfural yield and selectivity. Therefore, the addition of (specific) salts as the second catalyst (next to an acidic catalyst) to the reaction solution or the use of seawater as the reaction medium allow for further improvements of the efficiency of existing furfural production processes. Salt Effects without Acid. In this section, results are presented for experiments with the same three salt conditions (no salt, NaCl solution, and seawater) but without the addition of any acid. Again both pure furfural degradation and xylose and arabinose dehydration was investigated. All the experiments without acid are performed at 200 °C. The initial furfural and pentose concentrations were 50 mM. The results of the furfural degradation experiments are presented in Figure 5. Again, as in the case with acid, the rate of furfural degradation decreases with the addition of NaCl and seawater. However, the differences are very small, which is due to the fact that these degradation rates in a medium without any acid are very slow anyway. In Figure 6 the concentrations of xylose (a) and arabinose (b) during nonacidic dehydration are shown. Analogously to the experiments with acid, the rate of dehydration of the pentoses increases with NaCl and seawater. The small difference of the rate of dehydration of arabinose in the NaCl solution and in seawater is due to its slow reaction behavior. As can be seen in Table 3 and Table 4, with no acid and the salts present, both kx1 and kx2 (and ka1 and ka2) increase. However, where a clear increase in the ratio kx1/kx2 and ka1/ka2 could be 5459



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Figure 4. Xylose (a) and arabinose (b) dehydration behavior at three different temperatures and salts conditions (1, 2, 3). X = xylose (⧫), A = arabinose (▲), H = no salts, N = NaCl, and S = seawater. All with HCl = 50 mM.

observed for the experiments at 200 °C with acid, this is not observed for the case without acid. Related to the results in Figure 6, at least two unidentified compounds were detected as coproducts during furfural formation when no acid was added as the catalyst. In a previous study, it was reported that the hydrolysis of an aldose in water can be decomposed by three parallel pathways: via the furanose and pyranose ring isomers, as well as the acyclic isomer.27 For the case of xylose, the xylopyranose isomers form

2,5-anhydroxylose and further dehydrate to furfural. The xylofuranose seem to be relatively stable species. The acyclic xylose isomer rapidly decomposes to glycoaldehyde and pyruvaldehyde, unidentified acids, and other products. These organic acids formed from xylose degradation consist of formic, lactic, acetic, pyruvic, and glycolic acids and were reported in previous studies.27,28 These xylose degradation products lead to changes in the pH of the solution. When salts are used as the sole catalyst, the formation of both furfural and unknown 5460



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In Figure 7, it is first observed that for all experiments the formic acid concentration increases with the reaction time. Then, which is most important, the concentrations of formic acid are significantly lower, while the amount of the other losses are just slightly decreased, when salts are present. Moreover, with the combined salts of seawater the formic acid concentrations are even lower than in the case with NaCl. It seems that the formation of formic acid is inhibited by the presence of the salts. These results are a strong indication of one of the benefits of adding salts as the secondary catalysts, which is, reducing the furfural loss reactions toward formic acid. It was observed that the addition of NaCl or seawater resulted in a significant decrease of the hydronium ion activity, which, based in Figure 7, seems to mainly result in an inhibition of the hydrolytic fission of furfural to form formic acid. This seems logical since most hydrolytic reactions are catalyzed by acids.29 Furthermore, in previous studies, it was demonstrated that formic acid (due to its acidity) not only acts as a catalyst for xylose dehydration to furfural but also accelerates furfural loss reactions.27,30 Therefore, while the salts inhibit the furfural to degrade toward formic acid, they simultaneously avoid further acidification of the reaction mixture, thus also avoiding a further increase in the furfural degradation rates. It can be concluded that the addition of salts, in a concentration as naturally occurring in seawater, inhibits the hydrolytic fission of furfural toward formic acid and thus also avoids further acidification of the reaction mixture.

Figure 5. Furfural degradation without acid catalyst at different salt conditions at 200 °C. F = furfural, S = seawater, N = NaCl solution, and W = water.

(organic acid) products is increased.13 Thus, this autocatalytic effect is increased. However, since the organic acids are not fully dissociated, the effect must be relatively small. Moreover, since part of the autocatalytic effect is actually catalyzed by the presence of the salts, it can be debated how “auto” the autocatalytic effect is. It can accordingly be regarded as an indirect catalytic effect of the salts. It can be concluded that the effect of the NaCl and the combined salts of seawater exert similar influence on the dehydration of pentoses with and without acids present in the reaction mixture. Formation of Formic Acid. In the reaction mixtures from the experiments with pure furfural, small amounts of formic acid were detected. In Figure 7, these concentrations are presented for the experiments at 200 °C. In the experiment at 180 °C, the concentrations of formic acid were very low and at 160 °C they were so low that they became immeasurable. It is known that formic acid is a degradation product of furfural, as a product of the hydrolytic fission of the aldehyde group of furfural.6

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CONCLUSION In this paper, the effect of salts on the kinetics of dilute acidic dehydration of pentoses toward furfural has been investigated. The reaction rate constants for furfural degradation and pentose dehydration reactions have been derived based on first-order kinetic models for solutions without any added salt, with 500 mM NaCl and in seawater. The experimental and kinetic data show that the addition of salts results in lower furfural degradation rates and in higher pentose dehydration rates. In the latter case, the salts enhance both the reactions toward furfural and toward losses. The ratio of these two reaction rate constants indicate that the salts catalyze different

Figure 6. Concentrations of xylose (a) and arabinose (b) during dehydration without acid at difference salt conditions at 200 °C. X = xylose, A = arabinose, S = seawater, N = NaCl, and W = water. 5461



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ACKNOWLEDGMENTS

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REFERENCES

Article

Kasetsart University in Thailand is kindly acknowledged for cofunding this research. Michel van den Brink is thanked for performing the ICP-OES analyses. Finally, Liza van der Aa is kindly acknowledged for her contribution to the experimental part.

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Figure 7. Product formation from furfural degradation at different salt conditions at 200 °C. All with HCl = 50 mM. * First sampling was set at t = 0 including the pumping and flushing time for 1 min.

reactions with different temperature dependencies. The effects of the salts were more pronounced for the case of xylose, compared to arabinose. In the former case, that of decreased furfural degradation with salts, it was shown that the salts inhibited the formation of formic acid from furfural. Furthermore, it was observed that the molar yield of furfural was improved for all temperatures by the addition of salts. Generally, all these effects are more pronounced with the combined salts of seawater compared to those in a NaCl solution, although the salinities and ionic strengths of the two media were comparable. Finally, comparable trends were observed for the experiments without any acid added.

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*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5462



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Kinetic Study on the Dilute Acidic Dehydration of Pentoses toward

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