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Aqueous Phase Conversion of Hexoses into 5‑Hydroxymethylfurfural and Levulinic Acid in the Presence of Hydrochloric Acid: Mechanism and Kinetics Diego Garcés, Eva Díaz, and Salvador Ordóñez* Department of Chemical and Environmental Engineering, University of Oviedo, Julián Clavería s/n, 33006 Oviedo, Spain S Supporting Information *

ABSTRACT: The kinetics of the acid-catalyzed dehydration of two common monosaccharides (glucose and fructose) to 5hydroxymethyl-furfural (HMF) and levulinic acid is studied in this work. Reaction studies were performed in a stirred batch reactor using aqueous sugar solutions (150 g/L). The influence of reaction temperature (363−403 K) and catalyst concentration (0.05−0.25 mol/L) was also studied. The formation of the main products, as well as other minor products such as humins (in both cases) or glucose dimers and anhydroglucose (in glucose processing), is modeled considering a serial-parallel scheme reaction. The proposed model allows predicting the formation of both main and side products. Obtained results suggest that it is possible to work at total conversions of both monosaccharides, and to tune the selectivity to HMF-LA by changing the operation temperature and/or catalyst concentration. As a general trend, higher selectivities are obtained (to HMF or to LA, depending on reaction conditions) when fructose is used as a reactant. By contrast, glucose dehydration leads to a larger amount of side products such as anhydroglucose and glucose oligomers. 1.12−15 The common conversion of glucose to HMF takes place by isomerization of glucose to fructose before the dehydration step. Higher selectivities of HMF from glucose have been reported using ionic liquid and organic solvents,4,16−18 particularly when a solid catalyst is used for the isomerization of glucose to fructose.19 However, the separation of HMF from the organic solvent or ionic liquid is difficult, making more advantageous the use of water as a solvent.6 In the case of aqueous systems, hydrolysis of HMF to levulinic acid and formic acid also takes place. This process can be avoided in biphasic systems using an immiscible organic phase to extract HMF from the aqueous phase.1,18,21 However, levulinic acid is a valuable chemical which can be used for polymer production such as acrylate polymers, and fuel additives such as γvalerolactone, 2-methyl-tetrahydrofuran, or condensation adducts.22,23 Thus, the manufacture of levulinic acid could be desirable from an economic point of view, since current levulinic acid costs around €5500 per ton, versus a HMF market price of €2500 per ton.23 Hence, there is interest to explore acid catalytic dehydration for HMF and levulinic production from glucose and fructose in aqueous media, which, depending on the operation conditions, allows producing both platform molecules.

1. INTRODUCTION The declining of petroleum reserves and tightening environmental policies about greenhouse gas emissions have boosted the research on the efficient exploitation of natural resources, such as lignocellulosic biomass, as feedstocks for the production of chemicals and liquid fuels.1,2 The drawbacks of the production of first-generation biofuels, such as its contribution to the increase in food prices and the effect on deforestation and biodiversity losses,2 promote the development of techniques for obtaining second-generation biofuels from lignocellulosic materials. Among the different components of lignocellulose biomass, cellulose and hemicellulose can be converted into monosaccharides including, for example, glucose (obtained mainly from cellulose) and xylose (obtained from hemicellulose), glucose being the most abundant sugar. Thus, the transformation of glucose into platform chemicals, such as 5-hydroxymethylfurfural (HMF) and levulinic acid, has recently attracted much attention.3−5 HMF is obtained by acid-catalyzed dehydration of glucose or fructose in aqueous media, where fructose is an intermediate in glucose dehydration. HMF is a platform molecule for obtaining chemicals, biofuels, and polymer precursors from renewable resources.6−8 High yields of HMF from fructose have been obtained in biphasic systems,7 with selectivities up to 89%, and organic systems.9−11 On the other hand, in the case of aqueousphase glucose dehydration, lower yields were obtained (>25%) due to both the low reaction rate and the byproduct formation, such as anhydroglucose and other oligomers as shown in Scheme © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

March 7, 2017 April 10, 2017 April 24, 2017 April 24, 2017 DOI: 10.1021/acs.iecr.7b00952 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Reaction Pathway for Glucose Dehydration to HMF, Levulinic Acid, and Other Side Reactions

2.3. Analysis. The reactants were quantified by HPLC (1200 Series, Agilent) equipped with a G1362A RI detector, G1316A column oven, G1322A degasser, G1311A quaternary pump, G1329A automatic liquid sampler, and a Hi-Plex H column (300 × 7.7 mm, Agilent). The column was operated at 333 K, and the detector at 328 K. Next, 5 mM H2SO4 was used as a mobile phase at a flow rate of 0.6 mL/min. The injection volume was 20 μL of a filtered (45 μm) sample with a 1:33 dilution. All samples were analyzed using the RI detector. Since humins are not detected by HPLC, and with the aim of taking them into account in the calculations, it was supposed that all mass not detected by HPLC corresponds to humins. Once the concentration of each compound was determined, selectivities were calculated as follows:

Although most current studies of dehydration of sugars are focused on heterogeneous catalysis, homogeneous catalysis is the industrial method currently used in the production of levulinic acid because of the low price of mineral acids.24 Therefore, the development of accurate kinetic models is needed for the design of these processes. In this work, dehydration reactions of glucose and fructose in an aqueous medium are studied using hydrochloric acid as a homogeneous catalyst. The choice of this acid is based on the wide availability of hydrochloric acid, even being a low cost waste in many industrial processes, and in addition, most of the previous works related to dehydration kinetics of sugar were developed using H2SO4 as a catalyst, an acid stronger than HCl and, for that matter, with different effects on the dehydration process. The kinetics of dehydration of fructose and glucose are discussed separately to estimate the influence of the isomerization step of glucose to fructose in the dehydration process. A study about the variable influence of acid concentration on each one of the reaction steps is carried out.

n

Si = ni moli/∑ nj mol j j=1

(1)

ni and nj being the number of carbons in the components i and j, respectively, and moli and molj, the moles of the components i and j in the reaction mixture, respectively. The sum ∑n1 Cjmolj includes the component i. The solids obtained for both reactions were analyzed in a TGDSC instrument (Setaram, Sensys) using α-alumina as an inert reference material. Samples (20 mg) were treated in a nitrogen flow (20 mL/min) with a temperature program of 5 K/min from 298 to 973 K trying to identify the different compounds involved in these solids. The value of the pH was followed during the experiments, but very small variations have been detected. The measured pH values were lower than 1 in all the experiments, and without significant variations during the reaction. 2.4. Kinetic Modeling. The kinetic model considers the reactor as an ideally stirred batch reactor (BR). Therefore, mass balance for a given molecule follows a differential eq (eq 2). By combining the reactor model with the appropriate power-law rate expressions (eq 3) and Arrhenius eqs (eq 4), the complete model is obtained (a system of ordinary differential equations), which was implemented and solved in Scientist Software (2006, MicroMath, USA).

2. EXPERIMENTAL SECTION 2.1. Materials. D-(−)-Fructose (≥99%), D-(+)-glucose (≥99.5%), and sodium hydrogen carbonate (≥99.7%) were purchased from Panreac Applichem. 5-(Hydroxymethyl)furfural (≥99%), formic acid (98%), and levulinic acid (98%) for HPLC calibration were acquired from Sigma-Aldrich, and hydrochloric acid (37%) was purchased from Fisher Chemical. Solutions were prepared using distilled water. 2.2. Experimental Procedure. Reactions were carried out in a 0.5 L stirred batch autoclave reactor (Autoclave Engineers EZE seal) with a back pressure regulator and a PID temperature controller. The reaction volume was 0.25 L of an aqueous solution of 45 g of D-(−)-fructose, for the first set of experiments, or D-(+)-glucose, for the second set. Five different reaction temperatures were studied (363−403 K). Once the desired temperature was reached, 50 mL of a solution of hydrochloric acid were added, resulting in an acid concentration of the reaction mixture of 0.25 M. Air was purged with N2, and dehydration was carried out with 10 bar of N2 with a stirring of 600 rpm for 4 h in the case of experiments at 363−383 K and for 3 h in the case of experiments of 393 and 403 K. In order to study the effect of reaction media acidity, additional experiments were carried out with acid concentrations of 0.05 and 0.1 M at 393 K for both reactants, keeping constant the other conditions. Samples were taken from the sampling port, filtered using 0.45 μm Nylon syringe filters, and diluted in a 1:33 ratio.

M

dCi /dt =

∑ −αijrj j=1

(2)

with Ci being the concentration of the i component (mol L−1); αij, the stoichiometric coefficient of component i on the reaction j; and rj, the reaction rate for reaction j (mol L−1 s−1). B

DOI: 10.1021/acs.iecr.7b00952 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Table 1. Summary or the Results Obtained for Conversion of Fructose and Glucose to HMF, Levulinic Acid, Formic Acid, AHG and G2a (CGlucose = 180 g L−1; CHCl = 0.25 M) selectivity (%)

a

entry

feed

T (K)

time (h)

conv. (%)

HMF

levulinic acid

formic acid

AHG

G2

1 2 3 4 5 6 7 8 9 10

fructose fructose fructose fructose fructose glucose glucose glucose glucose glucose

363 373 383 393 403 363 373 383 393 403

4 4 4 3 3 4 4 4 4 4

54 78 93 97 100 20 27 38 49 49

69 49 28 11 3 0 9 16 9 6

26 42 59 76 83 0 15 28 51 43

6 9 13 13 14 0 0 6 7 7

0 0 0 0 0 0 0 13 16 21

0 0 0 0 0 100 76 37 17 23

G2: glucose dimer.

Figure 1. Evolution of the HMF (a) and LA (b) yields with reaction time and temperature for fructose dehydration. Temperatures: 363 K (▲), 373 K (●), 383 K (■), 393 K (⧫), and 403 K (×). Solid lines represent the evolution of the concentration of different products according to the theoretical model proposed. N

rj = k j ∏

n Ck jk

k=1

In the above equation, kj0 is the pre-exponential factor in min−1, A is the acid concentration in mol L−1, mj is the reaction order of acid concentration, Eaj is the activation energy in J mol−1, R is the universal gas constant in J mol−1 K−1, and T is the temperature in K. Different kj values were found for each experiment, and then, an Arrhenius fit between kj and temperature (T) was carried out to find the activation energy of each individual reaction keeping the HCl concentration constant. The kinetic model was fitted to

(3)

with kj being the rate constant of the j reaction; Ck, the concentration of the k component (mol L−1); and nk, the reaction order of the k component. kj = kj0 Amj exp( −Eaj /RT )

(4) C


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Figure 2. Comparison between experimental and theoretical data for the evolution of fructose concentration between 363 and 403 K. For symbol codes, see Figure 1.

fructose mixtures in aqueous media in the presence of HCl and reported that the fructose concentration decreased sharply from the beginning of the reaction, whereas the concentration of glucose was reduced much more slowly.15 Nikolla and coworkers used Sn-Beta zeolite as a catalyst necessary for the isomerization of glucose to fructose, while HCl was used for the dehydration of fructose to HMF.19 In this study, the presence of Sn-Beta zeolite enhanced the conversion of glucose from 26% to 75% under the same reaction conditions. Therefore, the absence of a catalyst with Lewis acid sites slows the isomerization of glucose to fructose, making it the controlling step of the overall reaction. As a consequence, a small amount of fructose is rapidly converted into HMF and, later, levulinic and formic acid due to the presence of HCl. This fact was confirmed by the absence of fructose in the samples of the glucose dehydration reactions. Other works argue that there is a direct method of dehydration of glucose to HMF, although it is much less important than the transformation via isomerization.15 Regarding the reaction products, Figure 1 shows the evolution of the HMF and LA yields with reaction temperature for the fructose dehydration reaction. The formation of HMF is highly dependent on the reaction temperature. The maximum HMF yield (57%) was achieved at 393 K in only 40 min. The use of a higher temperature resulted in lower HMF yields since, at a high temperature, HMF is rehydrated to LA and formic acid so that the LA yield showed a continuous increase with increasing reaction temperature. According to Figure 1, obtained results suggest that it is possible to tune the selectivity in fructose dehydration. Especially, it is possible to maximize the selectivity to HMF by an appropriate selection of the reaction time (or space time, in a continuous process), working in the interval 393−403 K. By contrast, decreasing reaction temperature is not an effective way to increase HMF selectivities. Concerning LA selectivities, they are maximized as both reaction temperature and time increase, with almost total conversions to LA being possible. Figure 2 shows the evolution of fructose concentration with the temperature. As expected, fructose decreases more quickly as reaction temperature increases, this effect being more marked between 383 and 393 K.

experimental data by applying the linear regression model using the coefficient of determination (R2). After that, reactions at different acid concentrations were carried at constant temperatures to evaluate the influence of acid concentration on different kinetic reactions. Consequently, the kj vs A ratio was plotted in order to find the reaction order of acid concentration for each reaction step.

3. RESULTS AND DISCUSSION The dehydration of hexoses to HMF in aqueous media using acid catalysis depends on several parameters, such as acid concentration, temperature, and initial sugar concentration. In this work, initial sugar concentration was initially fixed at 150 g L−1 (13 wt %) since, according to the National Renewable Energy Laboratory of the U.S. Department of Energy, the typical range of sugar concentration of a cellulosic hydrolysate is 10−15 wt %. Thus, the effect of both temperature and acid concentration was studied. Scheme 1 shows the main reaction pathway for glucose conversion to 5-HMF including the formation of condensation products (humins) from polymerization of fructose and glucose molecules.8,15 In this mechanism, several side reactions are involved. Among them, the rehydration of HMF to levulinic and formic acid in aqueous media,6−8,11,12,20,25−29 the dehydration of glucose to anhydroglucose (AHG),3,14 and the polymerization of glucose should be highlighted.1 3.1. Effect of the Temperature on Product Distribution. Table 1 summarizes the results of conversion for fructose and glucose dehydration reactions from 363 to 403 K and the selectivity data for different reaction products. The conversion evolution is the most remarkable difference between fructose and glucose dehydration. For example, at 383 K, the conversion of fructose (93% at 4 h reaction time) was considerably higher than the conversion of glucose (38% at the same reaction time and operation conditions). Although the presence of HCl catalyzes sugar dehydration, the efficient transformation of glucose to fructose by aldose-ketose isomerization requires the presence of a Lewis acid site catalyst such as hydrotalcite.3 Toftgaard Pedersen and co-workers studied the dehydration of glucose− D


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Figure 3. Comparison between experimental and theoretical data for the evolution of glucose (a), 5-HMF (b), levulinic acid (c), formic acid (d), a glucose dimer (e), and anhydroglucose (f) concentration between 363 and 403 K. For symbol codes, see Figure 1.

reached at the lowest time as the acid concentration increases. Furthermore, after the maximum of LA selectivity was reached for the highest acid concentration, it did not decrease, indicating the strong stability of LA in the reaction conditions. Table 2 shows the values of conversion for dehydration reactions of fructose and glucose carried out at different acid concentrations. In the case of dehydration of fructose, all the reactions reached a conversion of over 96%. On the other hand, in the case of glucose dehydration, obtained conversions are largely lower (max. 36%), although conversions increase at increasing acid concentrations Figure 4e shows the evolution of the selectivity of HMF with the reaction time of the glucose dehydration at 393 K for different acid concentrations. The evolution of the selectivity of HMF indicates that it is not a primary product since it is formed after the equilibrium between glucose and side products (AHG and dimer glucose) have been established.15 Furthermore, Figure 4a shows the evolution of HMF concentration and theoretical HMF concentration data for dehydration reactions of glucose with different acid concentrations at 393 K. The shape of the evolution of HMF concentration is similar to those found in previous works. The concentrations reached a maximum during the reaction time since the conversion of HMF to levulinic acid and formic acid is much faster than the conversion of glucose to HMF.26 As the dehydration reaction of sugars and the hydration process of HMF are catalyzed by the presence of acid, the higher the acid concentration, the higher the slope of increase and decrease of HMF concentration, as found in other work,10 and the lower the HMF selectivity as shown in Figure 4e. On the other hand, as shown in Figure 4f, the selectivity for LA with the reaction time does not depend on acid concentration. These results are consistent with those obtained by Rivas and co-

Regarding carbon balance, it decreases to 53% in the case of the fructose dehydration at 393 K in the liquid phase. This behavior could more likely be caused by the formation of humins. The behavior of the glucose at these reaction conditions is completely different, with the formation of several byproducts out of the main isomerization−dehydration route being observed. Thus, Figure 3 shows the evolution of the experimental concentration data and theoretical data predicted by the kinetic model, proposed below, of glucose, HMF, levulinic and formic acid, AHG, and a glucose dimer at different temperatures. This evolution shows a remarkable initial decrease of the concentration of glucose, especially at the highest considered temperatures, in good agreement with the strong initial increase of the concentration of side products, such as AHG and the glucose dimer. It is concluded that glucose is primarily converted into side products by equilibrium controlled reactions. After equilibrium, once AHG and glucose dimer concentrations are stabilized, glucose follows the expected degradation route yielding HMF.15 Since the isomerization from glucose to fructose is the slowest step of the overall mechanism, when small amounts of HMF are formed, the process of rehydration to levulinic and formic acids takes place to a large extent, leading to low HMF selectivities, even at the highest studied temperatures. 3.2. Effect of Acidity (pH). Figure 4 shows the influence of the acid concentration on the selectivity of HMF and levulinic and formic acid during the dehydration reaction of fructose. Regarding HMF, being a primary product, its selectivity decreased with the reaction time in all cases, but in the case of the lowest acid concentration, this decrease was slight because the subsequent rehydration of HMF to levulinic and formic acid was facilitated by high acid concentrations, as shown in Figure 4b and c. On the other hand, the maximum selectivity of HMF is E


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Figure 4. Influence of the concentration of hydrochloric acid on the selectivity of HMF (a), LA (b), and FA (c) during the dehydration of fructose at 393 K, and on the selectivity of HMF (d) and LA (e) during the dehydration of glucose at 393 K. 0.05 M (■), 0.1 M (●), and 0.25 M (▲).

dehydration product, its presence is favored by the presence of acid. 3.3. Kinetic Model. The reaction network seen in Scheme 1 was proposed based on separate fructose and glucose dehydration. We propose a novel kinetic model for the acidcatalyzed dehydration of fructose and glucose to levulinic acid, with a particular focus on possible side reactions which had deserved little attention in previous works. In addition, most dehydration reactions have been studied using sulfuric acid as a catalyst, whose effects on the kinetics of dehydration have been sufficiently described in previous works.26,28,29 However, we have chosen in this work to delve into the kinetics of dehydration using hydrochloric acid as a catalyst. To determine the overall reaction mechanism, the different reaction products were classified according to selectivity−conversion plots. Figure S1 shows the evolution of HMF selectivity with conversion for the reaction of fructose dehydration. In this figure, HMF selectivity begins to fall when the value of fructose conversion reaches 25%. This fact suggests that HMF is a primary product formed in fructose dehydration. However, according to Figure S2, levulinic acid, and therefore formic acid, are secondary products formed from HMF rehydration since LA selectivity increases when the fructose conversion exceeds 25%. Although several authors

Table 2. Conversion Values for Different Acid Concentration Reactions from Fructose and Glucose conversion (%) feed

temperature (K)

reaction time (min)

0.25 M

0.1 M

0.05 M

fructose glucose

393 393

180 240

96.77 36.56

98.58 17.49

98.71 10.91

workers.30 According to this study, the absence of a catalyst which facilitates the isomerization step makes the kinetic constant of HMF formation extremely low in spite of the high acid concentration.30 This is tentatively explained considering that the overall process is controlled by the isomerization glucose−fructose step, independently of the acid concentration. On the other hand, Figure 5a and b show the evolution of concentration of glucose dimer and anhydroglucose for experimental and theoretical model data. As mentioned above, both side products are obtained through polymerization and dehydration equilibria, respectively, making these concentrations remain stable after a given reaction time, as predicted by the proposed model. In the case of anhydroglucose, being a F


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Figure 5. Comparison between experimental and theoretical data for the evolution of AHG (a) and glucose dimer (b) concentration for dehydration of glucose at a different acid concentration at 393 K. For symbol codes, see Figure 4.

propose a direct reaction of glucose to obtain HMF,15,31,32 our results suggest that HMF is only formed as a primary product when fructose is used as a parent reactant. Concerning the glucose dehydration, HMF selectivity shows a pattern of an intermediate product (Figure S3), the selectivity being very low at low conversion (suggesting the formation of other primary products such as anhydroglucose and glucose dimers), and also presenting a slight decrease at the highest conversion. This fact is due to HMF being an intermediate product formed after the equilibrium among glucose and anhydroglucose and glucose dimers, but before the formation of levulinic acid and formic acid. By contrast, LA presents the typical pattern (Figure S4) of a secondary product not undergoing further reactions. LA selectivity increases with conversion, this conversion being higher with temperature. All these reactions are acid-catalyzed, so HCl concentration must be included in the reaction rate equations. Although some previous works suggest a linear relation of the different apparent kinetic constants with HCl concentration,15 reaction orders on acid concentration for each reaction were preliminarily evaluated by power law equations to check this ratio. For that, this study suggests that glucose primarily leads to the formation of glucose (R1), and subsequently, dehydration of fructose to glucose takes place (R2), being the rate of these reactions expressed by the following equations:

R1 = k1(CG)nG1

(5)

k1 = k10 Am1 exp( −Ea1/RT )

(6)

R 2 = k 2(C F)nF2

(7)

k 2 = k 20 Am2 exp( −Ea2 /RT)

(8)

As mentioned previously, whereas the dehydration of fructose to HMF takes place through a single step irreversible reaction (R2), the dehydration of glucose occurs via isomerization from glucose to fructose (R1). In this study, it is considered that the isomerization reaction of glucose to fructose, in the absence of a Lewis acid catalyst, is the slow step of the overall process of the dehydration of glucose. Therefore, from the point of view of the proposed kinetic model, it is assumed that the process of dehydration of glucose is a one-step irreversible reaction (R8) whose activation energy would be similar to that corresponding to the isomerization reaction of glucose to fructose because this is the slowest step of the overall process. For that, the rate expressions of R1 and R8 are the same. In parallel, undesired products (humins) are produced from glucose (R3) and fructose (R4).11,33,34 For these products, the reaction rates are defined as follows:

R3 = k 3(CG)nG3 G

(9) DOI: 10.1021/acs.iecr.7b00952 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Table 3. Summary of the Pre-exponential Factors, Activation Energies, and Reaction Order for the Elemental Steps Mentioned in Scheme 1a

a

reaction (j)

reactant (k)

Ea (kJ mol−1)

log10(kjo) (M(1−njk−mj) min−1)

njk

mj

2 3 4 5 6 7 8

fructose glucose fructose HMF glucose glucose/G2 glucose

88 ± 3.04 135 ± 5.42 136 ± 5.39 91 ± 2.21 115 ± 4.87 77b ± 3.27 114 ± 4.01

10.53 ± 0.53 25.13 ± 0.82 15.72 ± 0.64 22.09 ± 0.74 27.39 ± 0.81 16.11 ± 0.54c 25.37 ± 0.78

1.00 2.00 1.00 1.00 1.00 2.00/1.00 1.00

0.35 ± 0.02 1.47 ± 0.03 1.51 ± 0.04 0.49 ± 0.02 1.45 ± 0.05 1.47 ± 0.02/0.67 ± 0.01 0.33 ± 0.01

Model fitting was done considering both experimental data from glucose and fructose. bEa = Ea7b − Ea7f. clog10(k70) = log10(k7f0) − log10(k7b0).

k 3 = k 30Am3 exp( −Ea3/RT )

(10)

R 4 = k4(C F)nF4

(11)

m4

k4 = k40 A exp( −Ea4 /RT )

may be represented by the following ordinary differential equations.

(12)

To confirm that the obtained solids after the reaction were only humins, thermogravimetric analyses (TGA) of the two solid samples were carried in an inert atmosphere (N2). Figure S7 shows the TGA thermograms of two humins from the dehydration of fructose and glucose, respectively, at a heating rate of 5 K/min. The thermal behavior of both solid samples is very similar. The thermal decomposition takes place between 430 and 950 K, losing about 50% of the sample weight. The highest decomposition rate for both solids was observed at 650 K. These data are consistent with those found in previous works in which the thermal decomposition has the same trend and the same shape.35 On the other hand, glucose can also undertake both intra- and intermolecular dehydration reactions (also known as reversible reactions) yielding anhydroglucose (R6) and glucose dimer (R7), respectively.15,36 nG6

R 6 = k6(CG)

(13)

k6 = k60Am6 exp( −Ea6 /RT )

(14)

R 7 = k 7f (CG)nG7f − k 7b(CAHG)nAHG7b

(15)

k 7f = k 7f 0Am7f exp( −Ea7f /RT )

(16)

k 7b = k 7b0 Am7b exp( − Ea7b/RT)

(17)

(18)

k5 = k50Am5 exp( −Ea5/RT )

(19)

(20)

dC HMF = R 2 − R5 dt

(21)

And the concentration of different compounds involved in the dehydration reaction of glucose as a function of time may be represented by the following differential equations. dC G = −R3 − R 6 − R 7 − R 8 dt

(22)

dC HMF = R 8 − R5 dt

(23)

dCG2 = −R 7 dt

(24)

Using Scientist Software and using the rate expressions (eqs 5−19) in combination with the mass balances (eqs 20−24), different reaction orders were tested for each involved chemical and theoretical model for the dehydration of fructose and glucose has been defined, looking for the combination which involves the lowest value of the sum of squared errors as it was used in previous studies.15 The formation of both humins (R3) and glucose dimers (R7) from glucose are better described by second order dependence on glucose concentration.15 In the case of the formation of glucose dimers, according to previous works, the disaccharide formation from glucose is a relatively simple bimolecular reactions.36 All the other reactions are well-defined by first order dependences on each reactant as occurs in previous works.39 Thus, the activation energies, reaction orders, and preexponential factors for the seven rate expressions proposed are given in Table 3. In the case of glucose dehydration, only experiments performed in the 383−403 K temperature interval were considered because the low concentration or reaction products were obtained at lower temperatures. Regarding Figures 1 and 2, the theoretical model fits very well to the experimental evolution of the concentration of fructose at different temperatures and HMF and LA yields for the dehydration of fructose. However, according to Figures 3 and 4, the fit provided by the proposed model is worse in the glucose dehydration experiments. This fact can be explained by the higher complexity of the reaction network, involving the presence of reaction products at very different concentration intervals. However, the model adequately fits the evolution of concentration and selectivity of anhydroglucose and the glucose

Finally, levulinic acid and formic acid are formed when HMF is treated with hydrochloric acid (R5), and the rate of this reaction is expressed as follows: R 5 = k5(C HMF)nHMF5

dC F = −R 2 − R 4 dt

Although some previous works argue that the formic and levulinic acid are formed directly from glucose and fructose, in these experiments, it has been confirmed that the appearance of these acids does not take place immediately but occurs after the formation of HMF.15 Previous works have shown both levulinic and formic acids are stable under similar reaction conditions and do not decompose to humins or other organic compounds in the wide range of reaction conditions applied.26 For a batch reaction setup, the concentration of the different species involved in the dehydration of fructose as a function of time, using the proposed kinetic mechanism shown in Scheme 1, H


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lower rates due to the lack of a catalyst active for glucose to fructose isomerization. Due to that, acid concentration only slightly affects the levulinic acid selectivities in glucose dehydration. The formation of glucose dimers and anhydroglucose is also significant when using glucose as a reactant. A kinetic model was proposed based on an experimental investigation where the temperature and acid concentration were used as variables. The model describes perfectly the evolution of the concentration of major compounds as such fructose, and HMF and levulinic acid in the case of the dehydration of fructose. In the case of the glucose reaction, the model is able to predict the reactivity trends (including the formation of lateral minor products), in spite of the higher complexity of the reaction network.

dimer, primary products of the dehydration of glucose. Therefore, according to Figures 3 and 4, the theoretical selectivity of HMF and levulinic acid is independent of the acid concentration, whereas the concentration of the glucose dimer and anhydroglucose increases with the acid concentration. This fact is due to the slowest step of the process being the isomerization of glucose to fructose, which controls the subsequent steps such as the dehydration of fructose to HMF and the rehydration of HMF to LA, but the isomerization process does not control the dehydration of glucose to AHG and the polymerization of glucose to the glucose dimer. Some previous works about kinetics of homogeneous acid catalyzed fructose dehydration have been found, and the values of activation energy and pre-exponential factors have been compared with data obtained in this study. However, there are few articles studying homogeneous acid glucose dehydration. Swift and co-workers have reported an activation energy for the dehydration of fructose to HMF of 126 kJ/mol.37,38 This is a result higher than that obtained in this work (88 kJ/mol). In water, the activation energy for acid catalyzed dehydration of fructose is around 126 and 160 kJ/mol for pH between 1.0 and 1.8. So taking into account, in our case, the initial pH value being 0.56, HCl acts as a catalyst reducing the activation energy of the reaction, and by increasing the acid concentration, the value of the activation energy is reduced. In the case of rehydration of HMF to levulinic and formic acid, the activation energy reaches a value of 91 kJ/mol, very similar to values found in other works (94−97 kJ/mol).24,36,39 In regard to the activation energy for the formation of humins from glucose and fructose, the values obtained are 165 and 136 kJ/mol, respectively, very similar to those found in previous works (148−171 kJ/mol).40,41 Referring to the HCl reaction orders with respect to dehydration from fructose to HMF, the formation of humins from glucose, the rehydration of HMF to levulinic and formic acid, and the dehydration of glucose to HMF, the values obtained were 0.35 ± 0.02, 1.47 ± 0.03, 0.49 ± 0.02, and 0.33 ± 0.01, respectively. These reaction orders are below those shown in previous works.25 This fact may be due to, according to most of the previous works, H2SO4 being used as a catalyst and H2SO4 being an acid stronger than HCl. This means the presence of H2SO4 affects to a greater degree the dehydration process, and for that, the reaction order with respect to the acid is higher. On the other hand, the highest reaction order with respect to HCl corresponds to the formation of humins. This fact means that the variation of acid concentration affects, to a greater extent, the process of formation of humins than the dehydration process. The presence of acid also influences in a special way the formation step of AHG and dimer glucose from glucose due to the high values of HCl reaction order. In this case, due to the absence of a Lewis acid catalyst, which facilitates the isomerization step from glucose to fructose, the activation energy of the formation of HMF and AHG from glucose is very similar.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00952. (Figure S1) Evolution of the selectivity of HMF with fructose conversion at different temperatures (363−403 K). (Figure S2) Evolution of the selectivity of LA with fructose conversion at different temperatures (363−403 K). (Figure S3) Evolution of the selectivity of HMF with glucose conversion at different temperatures (383−403 K). (Figure S4) Evolution of the selectivity of LA with glucose conversion at different temperatures (383−403 K). (Figure S5) TGA analyses of humins generated during the dehydration of fructose (a) and glucose (b) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +34 985 103 437. Fax: + 34 985 103 434. E-mail: [email protected]. ORCID

Salvador Ordóñez: 0000-0002-6529-7066 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financed by Research Projects of the Regional Government of Asturias (project reference GRUPIN14-078) and Spanish Ministry for Economy and Competitiveness (CTQ2014-52956-C3-1-R). D.G. thanks the Government of the Principality of Asturias for a Ph.D. fellowship (Severo Ochoa Program).

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REFERENCES

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4. CONCLUSIONS In this study the homogeneous acid catalyzed fructose and glucose dehydration was investigated in a BR reactor. HCl was selected as a homogeneous catalyst allowing an increase of the dehydration rate to HMF and, later, an increase of the rehydration rate from HMF to levulinic and formic acid. The influence of temperature and acid concentration was studied. An increase in temperature and acid concentration enhances the dehydration process of fructose, increasing the selectivity to levulinic acid, although dehydration of glucose takes place at I


Article

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