Hydrolysis kinetics of inulin by imidazole-based acidic ionic liquid in aqueous media and bioethanol fermentation

Chemical Engineering Science 151 (2016) 16–24

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Hydrolysis kinetics of inulin by imidazole-based acidic ionic liquid in aqueous media and bioethanol fermentation Zhi-Ping Zhao n, Xiao-Lan Wang, Gui-Yin Zhou, Yong Cao, Peng Lu, Wen-Fang Liu School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing 100081, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T


A potentially environment-friendly technology was developed to produce ethanol.
Hydrolysis rate by ILs in water media was higher than by dilute sulfuric acid.
A concise kinetic model of inulin hydrolysis by ILs was firstly proposed.
The kinetic model predicted well the inulin hydrolysis by ILs.
Conversion efficiency of inulin-type sugars to ethanol reached 94.21%.

art ic l e i nf o

a b s t r a c t

Article history: Received 26 December 2015 Received in revised form 12 April 2016 Accepted 7 May 2016 Available online 9 May 2016

This article focused on the inulin-containing energy biomass, Jerusalem artichoke, to explore environment-friendly processes for bioethanol production. An imidazole-based acidic ionic liquid (VImaILs) was prepared as the catalyst of inulin hydrolysis in aqueous media. The hydrolysis kinetics was studied under different conditions. The kinetic parameters of hydrolysis by VImaILs and dilute sulfuric acid were estimated and compared. This work demonstrated that the hydrolysis rate of inulin into reducing sugars by VImaILs was obviously faster than that by the latter. The proposed kinetic model successfully predicted the inulin hydrolysis in wider ranges of experimental conditions. The hydrolysate was fermented into ethanol by Saccharomyces cerevisiae which activity was not inhibited by the VImaILs. The conversion efficiency of inulin-type sugars to ethanol was greater than 92.5% of the theoretical yield. And the ethanol production capacity reached 123.76 g/(L). This system integrated the chemical and biological processes to prepare ethanol in an environment-friendly way. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Inulin-containing biomass Acidic ionic liquid Hydrolysis kinetics Fermentation Bioethanol

1. Introduction With the development of industry and agriculture, a great deal of energy has been consumed. One of the major problems facing on the world today is that the shortage of fossil energy source is becoming more and more serious. Sustainability of biofuels is increasingly taken into account (Klein-Marcuschamer and Blanch,

n

Corresponding author. E-mail address: [email protected] (Z.-P. Zhao).

http://dx.doi.org/10.1016/j.ces.2016.05.017 0009-2509/& 2016 Elsevier Ltd. All rights reserved.

2015; Menegaki and Tsagarakis, 2015). Raw plant biomass is an abundant bioresource for energy sustainability. At present, the main raw materials for bioethanol production include cassava starch, corn starch, sugarcane, wheat starch, sweet potato starch and sweep sorghum starch, most of which are starchy grains and food (Balat et al., 2008; Jensen et al., 2008; Lai et al., 2011; Mojovic et al., 2006). However, the starchy grains should be gelatinized before hydrolysis, and this process consumed much energy (Alex Marvin et al., 2012). On the other hand, it has been recognized globally that our population is increasing faster than the supply of food, moreover, local natural disasters are frequent and the supply

Z.-P. Zhao et al. / Chemical Engineering Science 151 (2016) 16–24

of grain falls short of demand (Chi et al., 2011). Under this background, the production of ethanol as fuel from grain, particularly corm, has been politically knocked down in most countries (Bai et al., 2008; Kunz, 2008). Simultaneously, the non-grain biomass resources have attracted increasing attention. Although lignocellulose is the most abundant renewable non-grain biopolymers on earth, the hydrolysis processes with a complex pretreatment are progressing slowly. And the economics of the complete life-cycle for lignocellulosic derived biofuels production remains unclear (Langan et al., 2011). Among the different raw materials, Jerusalem artichoke is an ideal non-food energy crop, due to it is a salt-tolerance species that is easily grown in saline and alkaline soils. The growing traits of Jerusalem artichoke such as cold and drought tolerance, wind and sand resistance, strong fecundity and high pest and disease resistance make it be widely cultivated (Long et al., 2014; Zhao et al., 2010). Tubers yield of Jerusalem artichoke is between 10 and 15 t of dry weight per hectare (Denoroy, 1996). Long et al. (2013) also reported Jerusalem artichoke commonly yields around 7 and potentially up to 14 t of carbohydrate per hectare. For the reason of the preferential advantages of the Jerusalem artichoke, it has recently received increasing attention as a renewable and abundant raw material for fructose syrup production and ethanol fermentation. In this way, sustainable production technologies are needed (Matías et al., 2011). Thus, we focus on this particular raw material in this article. Jerusalem artichoke contains 10–20 (w/w)% carbohydrates of which approximate 78% is inulin (Pan et al., 2009). Inulin is a mixture of polysaccharides, the number of fructose units can range from a few units to more than 60, depending on the source. Inulin consists of linear chains of D-fructose units in the β (2–1) position, the chain is terminated by a glucose residue through a sucrose-type linkage at the reducing end (Zhang et al., 2010). The main steps of inulin bioethanol production include inulin hydrolysis, hydrolyzates fermentation and ethanol separation. And the hydrolysis of inulin may be considered as a key step in the processing for bioethanol production. This reaction can be carried out by employing acidic catalysts or biocatalysts (Blecker et al., 2002; Hu et al., 2015; Ricca et al., 2009). To obtain bioethanol, traditional processes which employ several acids such as dilute sulfuric acid, maleic acid, and fumaric acid to hydrolysis cellulosic materials can be applied. Especially dilute sulfuric acid which is used to cellulosic hydrolyzation is the most popular catalyst (Lenihan et al., 2010; Rafiqul and Mimi Sakinah, 2012). However, it has drawbacks such as equipment corrosion and issues in the recovery and recycle of the acids and it does not fit in with the green chemical concept. Enzymatic process is specific and it provides an efficient raw material usage (Cateto et al., 2011). Sometimes, this conventional process tends to be expensive for the high cost and a long time cultivation of microorganism (Hu et al., 2015). Therefore, more efficient and economical processes of hydrolyzing inulin should be applied. There has been much interest in the application of ionic liquids (ILs) in biomass processing recently. As ILs have some properties favorable to chemical reactions, they are currently used as “green” solvents and catalysts, and especially the acidic functionalized ILs (Amanda et al., 2002; Ding and Armstrong, 2005; Wilkes, 2004). Compared with traditional organic solvents, ILs are non-volatile, and possess good dissolving capacity (Hsu et al., 2011). As novel catalysts, they show many advantages of both homogenous and heterogeneous catalysts. When the biomass is treated by acidic functionalized ILs, anions which form strong hydrogen bonds are capable to interfere with the inter and intramolecular hydrogen bond network of cellulose, leading in effect to dissolution (Amarasekara and Owereh, 2009; Gupta and Jiang, 2015). Furthermore, The ILs which has acidic centers on both the cation and anion, for

17

example, SO3H-functionalized acidic ionic liquids, shows strong acid strength and good hydrolysis performance of cellulose (Liu et al., 2013). Besides, the ILs possess the characteristics of easy separation and recyclability. Several separation methods to recover the ILs had been proposed (Alvarez et al., 2014). So the inulin hydrolyzation by the acidic functionalized ILs could be regarded as a green catalytic process. This study focused on exploring a novel and efficient catalytic hydrolysis system of inulin, using an imidazole-based acidic ILs, 1(3-sulfonic group)propyl-3-vinylimidazolium hydrosulfate ([(CH2)3SO3HVIm]HSO4, VImaILs) that contain terminal olefinic bond, as the catalyst. The hydrolysis kinetics was systematically studied. Meanwhile, the kinetic parameters of inulin hydrolysis processes by VImaILs and dilute sulfuric acid were estimated and compared, respectively. The other purpose was to establish the hydrolysis kinetic model for predicting the conversion of inulin into reducing sugars by the VImaILs. The general objective was to obtain ethanol from inulin via environment-friendly chemical and biological technologies.

2. Materials and methods 2.1. Materials Inulin (polysaccharide content 90.86%, free sugars content 3.90%, water and ash content 5.24%), main storage carbohydrate of Jerusalem artichoke, was purchased from Likang Company (Gansu, China). Saccharomyces cerevisiae was purchased from Angel Company (Wuhan, China). 1-vinylimidazole (Z98%) and 1, 3-propane sultone (Z99%) were purchased from Baishun Chemical Technology Company (Beijing, China). Other chemicals (analytical grade) were commercially available and used without further purification. 2.2. Preparation of VImaILs The 1-vinylimidazole (9.4 g, 0.1 mol) was dissolved in acetone (40 mL) in a flask, and 1,3-propane sultone (12.2 g, 0.1 mol) was added dropwise. Then the reaction mixture was stirred at 0 °C for 3 h before filtrated. The retentate was purified by washing with ether and dried under vacuum to obtain (CH2)3SO3HVIm (18.1 g, 83.8% yield) as a white solid. The (CH2)3SO3HVIm (18.1 g, 0.838 mol) was dissolved in 50 mL of distilled water, where an anion exchange reaction occurred after adding equal molar sulfuric acid and the mixture was stirred at 50 °C for 12 h. The solvent was removed by vacuum distillation, the residual crude ionic liquid was purified by ether and dried under vacuum to get acidic ionic liquid VImaILs (25.6 g, 81.5% yield) as a yellowish liquid. The VImaILs was analyzed by 13C nuclear magnetic resonance spectroscopy (NMR, Varian mercuryplus-400) and Fourier transform infrared spectroscopy (FTIR, Nicolet IS10). The reaction route is shown as supplemental Fig. 1. 2.3. Hydrolysis of inulin In order to investigate the effect of hydrolysis conditions on reducing sugars yield, a series of experiments were carried out. The inulin aqueous solutions with needed concentration (w/w%, the weight ratio of the inulin to water, the same as the following) were prepared and heated to the required temperature in a 50 mL glass flask which was equipped with a magnetic agitator. Then the VImaILs was added according to the needed concentration (w/w%, the weight ratio of VImaILs to water and VImaILs). During the hydrolysis experiments, the solution was sampled at regular intervals to analyze the concentration of reducing sugars.

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sugars concentration (w/w%) of inulin hydrolysate, the theoretical conversion is 0.511 (g Ethanol/g Glucose).

2.4. Microbial fermentation of inulin hydrolysate Before fermentation, the pH value of inulin hydrolysate was adjusted using 1 M NaOH aqueous solution, and the hydrolysate was treated for sterilization in an autoclave at 115 °C for 20 min. At the same time, an appropriate amount of dry Saccharomyces cerevisiae was added in glucose solution (2 w/w%) at 38 °C for 20 min, subsequently the activation was acted at 34 °C for 60 min. Then, the activated yeast liquid was added in 50 mL hydrolysate in a 100 mL three-mouth flask with magnetic agitating of 100 r/min in thermostated water bath. All fermentation processes were carried out at 40 °C until the experiments terminated.(Mojovic et al., 2006) It is should be pointed out that the total fermentable sugars in inulin hydrolysates include the hydrolyzed reducing sugars and free sugars from the inulin.

2.5. Analytical methods As mentioned above, inulin consists of linear chains of D-fructose units in the β(2–1) position, the chain is terminated by a glucose residue through a sucrose-type linkage at the reducing end (Zhang et al., 2010). Hydrolysis reaction removes the terminal fructose residues from the non-reducing end of the inulin molecule in one step, producing fructose as main products and glucose as minor products. The reducing sugars content in inulin or hydrolysis system was determined by the DNS method, according to the absorbency spectrum curve and Lambert-Beer Law (Miller, 1959). The inulin hydrolysis ratio (%) was defined as follows,

RI =

mR mP0

× 100%

(1)

where mR is the reducing sugars concentration (kg/L) produced by the polysaccharides hydrolysis, and mP0 is the total reducing sugars concentration (kg/L) produced by the polysaccharides complete hydrolysis. In particular, the free sugars content (3.90%) in raw materials should be deducted from the examined sugars content to obtain the reducing sugars concentration mR. At the same time, the reducing sugars molar concentration (mol/L) was defined as follows,

CR =

1000mR MR

(2)

where MR is the molar mass of the reducing sugars (180.16 g/mol). Ethanol concentration of fermentation liquid of inulin hydrolysates was measured by a gas chromatograph (GC7890 II, Tian Mei Scientific Instrument Company, Shanghai, China) equipped with a thermal conductivity detector. Two parallel determinations of each sample were done. Ethanol production was calculated after the fermentation processes terminated, and the ethanol yield was defined as follows,

YE =

CE × 100% 0.511CS

(3)

where CE is the ethanol concentration (w/w%), CS is the total

2.6. Hydrolysis kinetics analysis In order to evaluate the impact of experimental factors on the hydrolysis of polysaccharides in inulin, the kinetics of hydrolysis by VImaILs was studied and the corresponding kinetic model was established. As the inulin consists of a mixture of polysaccharides with different degree of polymerization (DP), it is difficult to detect the concentrations of polysaccharide directly (Muñoz-Gutiérrez et al., 2009). So the reaction process was carried out by measuring the total amount of reducing sugars produced, the concentrations of reactants and products were related to each other by a mass balance based on the stoichiometry of the reaction scheme. The reaction formula is:

where “n” refers the polymerization degree of polysaccharides. In fact, this hydrolysis process is complex, for the polysaccharides are mixture of polymers rather than the single substrate, and the conversion of inulin into reducing sugars is a series-parallel mechanism rather than a single-stage reaction (Ricca et al., 2009). The hydrolysis mechanism can be interpreted as the ILs attack the glucosidic bond in polysaccharide molecule, leading to the breaking of the bonds between the fructose at random, and fructose is main production and glucose is minor production (Zhang et al., 2010). For deriving the kinetic equation, two hypotheses were proposed to the reaction system. Water was assumed as a reactant in excess in an aqueous solution. And considering the concept of average molecular weight was widely used in polymer science, inulin was not considered as a mixture of polymers but as a single molecule characterized by a degree of polymerization equal to average degree of polymerization. This hydrolysis reaction follows a pseudo first order kinetic dependence on polysaccharide concentration (Barclay et al., 2012). According to the law of mass action, the kinetic equation is

r = kCp =

1 dCR n dt

(4)

where r is the reaction rate, k is the first order rate constant, t is the reaction time, n is the polymerization degree of inulin and the range of n is from 2 to 60 (Saengthongpinit and Sajjaanantakul, 2005), Cp and CR are the polysaccharides concentration and reducing sugars concentration in mol/L, respectively. Based on the relationship between polysaccharides and reducing sugars, this equation can be written as

r=

⎛ C ⎞ 1 dCR = k⎜ CP0 − R ⎟ ⎝ n dt n⎠

(5)

where CP0 is the initial concentration of polysaccharides in mol/L, and CP0 = φCI0 , where CI0 is the initial inulin concentration, and φ (90.86%) is the percent content of polysaccharides in the inulin used in this work. Integrating Eq. (5), a model expressing the hydrolysis ratio (RI) can be obtained in following,

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RI =

CR = 1 − exp( − kt ) Cmax

19

(6)

where Cmax equals to the final reducing sugars concentration when the inulin is hydrolyzed completely. And the dependence of the hydrolysis rate constant k on temperature T was expressed in Arrhenius equation.

⎛ E ⎞ k = Aexp⎜ − a ⎟ ⎝ RT ⎠

(7)

where Ea is the activated energy (kJ/mol), R is the molar gas constant (8.3145  10  3 kJ/mol K), T is the temperature (K), A is the pre-exponential factor (min  1). Considering that the activation energy and the pre-exponential factors are dependent on VImaILs concentration CIL, Ea may be expressed by:

⎛ C ⎞ Ea = aexp⎜ − IL ⎟ + c ⎝ b ⎠

(8)

where CIL is the concentration of ILs (w/w%, the weight ratio of VImaILs to water and VImaILs), a, b and c are constants. And A was expressed by (Jensen et al., 2008; Romero et al., 2010; Yat et al., 2008):

A = A 0 CILm

(9)

where A0 is the pre-exponential parameter for inulin hydrolysis (min  1), m is the ILs concentration exponent for the rate constant k (dimensionless).

3. Results and discussion 3.1. Inulin hydrolysis by VImaILs One major objective of our experimental program was to find an efficient catalyst to hydrolyze inulin. The hydrolysis effect of inulin depends on the factors such as the concentration of the ILs, temperature, substrate concentration and reaction time. In order to evaluate the hydrolysis performance of VImaILs, these hydrolysis conditions were investigated. 3.1.1. Effect of VImaILs concentration In the first step, the effect of VImaILs concentration on the hydrolysis of inulin was considered. The substrate concentration represented by the ratio of substrate to water was fixed at 1:5, which corresponded to an initial inulin concentration of 20%. The VImaILs in various concentrations were used to hydrolyze inulin at 75 °C, and the hydrolysis results were shown in Fig. 1. Not surprisingly, the hydrolysis rate increased with the concentration of VImaILs. However, when VImaILs concentration was lower than 1.0%, the hydrolysis rate was very low and inulin was not completely hydrolyzed in 60 min. When VImaILs concentration was above 2.0%, the hydrolysis rate had no marked further change after 15 min and inulin was hydrolyzed completely in 20 min. For lower VImaILs concentrations, the number of active sites that ILs supplied to catalyze the hydrolysis reaction was too less, resulting in the same conversion rate needed a longer time. After VImaILs concentration was greater than an appropriate value (about 2.0%), the active sites were excess and could not been fully utilized, so that the hydrolysis rate had no significant increase. Furthermore, when the concentration of VImaILs increased to 5.0%, the reducing sugars concentration declined in the later stage of experiment. This phenomenon suggests that a high ILs concentration could lead to further reducing sugar degradation to a certain extent. Similar results were reported when Jiang et al. (2011) carried out cellulose hydrolysis. By the in situ 13C NMR of cellulose hydrolysis i

Fig. 1. Effect of VImaILs concentration on the hydrolysis rate of inulin. Hydrolysis conditions: inulin concentration 20%, temperature 75 °C.

upon the acidic catalyst of [C4SO3Hmim]Cl, they found that the majority of the cellulose hydrolysis products in the system was glucose at the early stage of reaction and the degradation of glucose to 5-hydroxymethylfurfural took place at the later period. Considering the selectivity of reaction products and the economical evaluation of the overall process, it is desired to cut down properly the VImaILs consumption. Thus, a VImaILs concentration of 1.0–2.0% was recommended. 3.1.2. Effect of temperature Reaction temperature has influences on many factors such as kinetics, selectivity, and the activity of catalysts in the IL-based inulin conversion. In this experiment, the effects of temperature were studied by varying reaction temperature between 55 and 75 °C at VImaILs concentration of 2.0% and inulin concentration 20%. Fig. 2. demonstrates the effect of reaction temperature on the production of reducing sugars. It was found that the temperature had a noticeable effect on the hydrolysis of inulin, and a rise in temperature resulted in a rapid increase in the hydrolysis rate. The research results showed that the hydrolysis rate of inulin was enough high at 65–75 °C. From the view of energy conservation and economy, it is discouraged for hydrolysis of inulin at a higher temperature. And reducing sugars are easy to degrade at high temperature, which subsequent fermentation does not hope. The hydrolysis time decreased from 90 to 20 min when the temperature rose from 55 to 75 °C. It could be explained by the fact that the temperature influences not only the hydrolysis of the

Fig. 2. Effect of temperature on the hydrolysis rate of inulin. Hydrolysis conditions: concentration of VImaILs 2.0%, concentration inulin 20%.

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glycosidic linkages but also the nonreactive external factors, such as molecule transfer between the polysaccharide surface and the bulk medium. In the experimental range, the viscosity of inulin solutions is from 1.01 to 1.67 cP, tested using a Brookfield viscometer (DV-Ⅱ þ). As a result of the temperature increasing, solution viscosity declined and molecule diffusion was improved. This not only helped the hydrolyzed fructose (reducing sugars) release to medium and expose more glycosidic linkages to be hydrolyzed, but also helped more VImaILs easily reach at the glycosidic linkages. 3.1.3. Effect of inulin concentration The added amount of inulin would affect the solution properties, such as viscosity, molecule transfer and mixing rate. The concentrations of inulin were in the range of 10–50%, the concentration of VImaILs was fixed at 2.0% and the temperature was 75 °C. The experimental results are presented in Fig. 3. It shows that with the increasing concentration of inulin, the growth of hydrolysis ratio was slowed down and it took a longer time for inulin to be hydrolyzed completely. When the concentration of inulin was lower than 30%, the inulin could be completely hydrolyzed in 25 min. But as a contrast, the hydrolysis rate significantly decreased and the time for completely hydrolysis of inulin was about 75 min and 90 min with inulin concentrations of 40% and 50%, respectively. Dissolution mechanism and diffusion mechanism could help explain this phenomenon. The solubility of inulin is about 6–35% under different temperature (Ricca et al., 2007), so the inulin with concentrations of 40% and 50% was not completely dissolved in aqueous solution, resulting in the hydrolysis rate decreased. In addition, compared to a solution with low concentration of inulin, the ILs molecule diffusion to the polysaccharide surface was slowed and the released fructose (reducing sugar) at the polysaccharide surface was not easy to escape into the bulk medium and could reattach to the polysaccharide, in a solution with higher concentration. Considering economy, a higher initial inulin concentration is more favorable, since it is desired to get higher ethanol concentration and decrease the costs of subsequent ethanol-water distillation and decrease the volume of reactors. In conclusion, a VImaILs concentration of 1.0–2.0%, a inulin concentration of 30%, a reaction temperature of 65–75 °C and a reaction time of  50 min are the selected conditions for conversion of inulin into reducing sugars.

Fig. 4. Comparison of inulin hydrolysis with dilute sulfuric acid and VimaILs. Hydrolysis condition: inulin concentration 20 (w/w)%, catalyst dosage 0.634 mmol.

3.2. Comparison of inulin hydrolysis by VImaILs and dilute sulfuric acid As discussed above, VImaILs presented a good performance for inulin hydrolysis. To illustrate the hydrolysis mechanisms by the ILs and inorganic acid, the comparative experiments were carried out using VImaILs and dilute sulfuric acid on the equimolar condition, respectively. In this study, the molar concentration of dilute sulfuric acid was 0.0634 mol/L that corresponds to the 2.0% VImaILs in regard to proton concentration. The results of comparative experiments are shown in Fig. 4. It is clear that the hydrolysis property of VImaILs was better than that of dilute sulfuric acid. At the temperature of 75 °C, the compete hydrolysis time using dilute sulfuric acid was more than 40 min, which took twice as much time as the VImaILs. When dilute sulfuric acid was used, the hydrolysis rate of inulin only depended on the proton H þ concentration. The hydrolysis mechanism is that proton H þ attacks the glycosidic bond between fructose units (Blecker et al., 2002; Lenihan et al., 2010). For the VImaILs, there are different mechanisms from general acid-catalyzed hydrolysis. Two catalytic hydrolysis machenism paths were reported for hydrolysis of cellulose by acid ILs ([C4SO3Hmim]Cl) (Li et al., 2015). And the hydrolysis machenism paths are similar in this system. One path is that the –SO3H group in the side chain of the cation ((CH2)3SO3HVIm) functions as a proton donor, protonating the glycosidic oxygen, and the anion ( HSO−4 ) acts as a nucleophile, attacking the anomeric carbon, leading to breaking of the glycosidic bond. Another path is that the -SO3H group in the side chain of a cation ((CH2)3SO3HVIm) acts as a proton donor/ acid, and the –SO−3 anion group in another cation ((CH2)3SO3HVIm) functions as a proton acceptor/base. Then it formed protonated glycosides in polysaccharide carbon and the oxygen atom of a water molecule attacks at the anomeric carbon concomitantly with deprotonation by the – SO−3 group, completing the catalytic cycle of glycosidic bond hydrolysis. The cation of VImaILs should affects the reaction rate in the VImaILs hydrolysis system. There is a synergistic effect between the imidazole groups and hydrogen ions. This synergistic effect made VImaILs get better hydrolysis activity. 3.3. Kinetic parameters estimation

Fig. 3. Effect of the inulin concentration on hydrolysis rate. Hydrolysis conditions: concentration of VImaILs 2.0%, temperature 75 °C.

3.3.1. Hydrolysis by VImaILs Another major objective of our experiment was to establish a kinetic equation that relates reaction rate with reaction time, reaction temperature and ILs concentration or activation energy. In this section, the kinetic model was established based on the above

Z.-P. Zhao et al. / Chemical Engineering Science 151 (2016) 16–24

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Table 1 The kinetics parameters of inulin hydrolysis by VImaILs. VImaILs concentration (w/w%)

0.2 0.5 1.0 2.0 3.0 5.0

k (min  1) 55 °C

60 °C

65 °C

70 °C

75 °C

0.0008 0.0022 0.0082 0.0327 0.0569 0.0753

0.0015 0.0036 0.0129 0.0528 0.0904 0.1264

0.0024 0.0055 0.0197 0.0781 0.1471 0.1996

0.0035 0.0092 0.0311 0.1273 0.2428 0.2885

0.0058 0.0146 0.0519 0.1905 0.3037 0.4320

Ea (kJ/mol)

lnA

R2

91.45 89.71 86.77 83.68 82.52 82.12

26.45 26.75 26.97 27.25 27.40 27.55

0.9946 0.9989 0.9976 0.9989 0.9894 0.9980

Table 2 Comparison of hydrolysis kinetics parameters by VImaILs and dilute sulfuric acid.a Catalyst

VImaILs Dilute sulfuric acid a

k (min  1) 55 °C

65 °C

75 °C

0.0327 0.0173

0.0781 0.0621

0.1905 0.1278

Ea (kJ/mol)

lnA

R2

83.68 95.12

27.25 30.90

0.9989 0.9802

The corresponding hydrolysis data were presented in Figs. 2 and 4., respectively.

Fig. 5. Reducing sugars yield curve as a function of process time.

theory deduction and experimental data. The kinetic parameters at the considered concentrations of VImaILs are shown in Table 1. With the VImaILs concentration variation from 0.2% to 5.0%, the activation energy decreased from 91.45 kJ/mol to 82.12 kJ/mol, and the pre-exponential factor in lnA increased from 26.45 to 27.55. The reduction of activation energy means the change of the catalytic reaction pathway, that is, as the molecular number of ionic liquid increases, more catalysts (VImaILs) and reactants (inulin) could form an intermediate with lower energy barrier. This is related to the hydrolysis mechanism

of VImaILs. The two kinds of hydrolysis pathes mentioned above are in competition with each other, and the second path in which two ionic liquid molecules involved appears the intermediate with lower energy barrier (Li et al., 2015). As the concentration of ILs increased, the synergistic action of catalyst molecules dominated gradually in the hydrolysis reaction. As a result, the activation energy reduced with the increasing of ILs concentration. In addition, the pre-exponential factor is related to the number of effective active sites (Yat et al., 2008). When the concentration of VImaILs was 0.5% or 0.2%, the number of catalytic active sites was

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few and the pre-exponential factor was small. As a result, the hydrolysis rate was too slow to completely hydrolyze the inulin within the experimental time. From the experimental results shown in Table 1, the potential relationship Eq. (8) between activation energy Ea and VImaILs concentration CIL may be further stated as the following equation:

⎛ 100CIL ⎞ Ea = 11.74exp⎜ − ⎟ + 81.79 ⎝ 1.15 ⎠

(10)

And the Eq. (9) between the pre-exponential factor A and VImaILs concentration CIL may be specifically stated as the following equation:

A = 2.56 × 1012CIL0.350

(11)

As above-mentioned, there are two catalytic hydrolysis machenism paths for inulin hydrolysis by ionic liquid. Two kinds of hydrolysis pathes are in competition with each other, and the second path in which two ionic liquid molecules involved in coordination gradually dominates with the increase of ionic liquid concentration. So Arrhenius Factor A is a non-linear function of the ionic liquid concentration and the ILs concentration exponent m is lower than 1. 3.3.2. Comparison between VImaILs and dilute sulfuric acid methods The kinetic parameters of VImaILs and dilute sulfuric acid hydrolysis that were calculated from the hydrolysis experimental results in Figs. 2 and 4., respectively, were summarized in Table 2. In this contrast experiments, hydrolysis reactions were carried out on the same mole dosage catalysts. It could be seen that the preexponential factor of dilute sulfuric acid system was higher than that of VImaILs system, which was due to the concentration of H þ in dilute sulfuric acid system was higher than in VImaILs system. However, the hydrolysis rate constants by VImaILs were greater than by dilute sulfuric acid because the activation energy of former was apparently smaller than that of latter. This result was consistent with the hydrolysis mechanism mentioned above. As far as inorganic acid catalysis was concerned, Blecker et al. (2002) reported that the speed of inulin hydrolysis was dependent on the proton H þ concentration in the inorganic acid. For VImaILs as the catalyst, the cation released from VImaILs could interact with the glycosidic oxygens by its attack on the oxygen atoms and protonated glycosides in inulin, which further illustrates there existed an above-mentioned synergistic effect between the imidazole groups and the hydrogen ions. So the hydrolysis performance of ionic liquid was better than that of acid hydrolysis.

because at high concentration such as 30% dissolution was not complete. The solution viscosity increases with the increase of the inulin concentration, leading to molecular diffusion reduced. This not only hindered the hydrolyzed fructose (reducing sugars) release to medium and expose more glycosidic linkages to be hydrolyzed, but also hindered more VImaILs easily reach at the glycosidic linkages. In addition, under higher inulin concentration, the relative concentration of ionic liquid decreases and the hydrolysis mechanism is more toward the first path, which requires the synergistic action of anions and cations in a single ionic liquid molecule and forms an intermediate with higher energy barrier. Fortunately, along with the hydrolysis the inulin concentration decreased, and it was gradually dissolved. So, according to the results of the verification, even at higher concentrations of inulin, hydrolysis kinetic model established in this paper can well predict the time of hydrolysis completely. Meanwhile, the response surface methodology (RSM) experimental design was carried out in which the effects of two independent variables on the yield of reducing sugars. Fig. 6a shows the surface representation of Eq. (6) at 2.0% VImaILs concentration over the temperature range (298.15–348.15 K) and process time (0–80 min). And Fig. 6b provides the surface representation of Eq. (6) at 65 °C over the 0.2–5.0% VImaILs concentration range and process time. This kind of response surface provides useful

3.4. Model validation According to the obtained kinetic model and parameters, the specific models, to describe the inulin hydrolysis ratio or reducing sugars concentration as a function of experimental conditions (temperature T, process time t, VImaILs concentration CIL), include Eqs. (6), (7), (10) and (11). By comparing model predictions to experimental data of the reducing sugars yield, the validation of obtained models can be performed as a function of single operation variable or double ones. Fig. 5(a and b) demonstrate that the theoretical calculation results are in agreement with the experimental data very well with the inulin concentration  20%, as a function of process time. The relative error was less than 5.0%. And the average variance was less 1.9%. However, Fig. 5(c and d) show that the theoretical values were slightly higher than the experimental data at the early stage, but in the later stage, the theoretical values were also in very good accordance with the experimental results. The corresponding average variances were 6.8% and 14.6%, respectively. This is

Fig. 6. Reducing sugars yield surfaces as (a) a function of temperature and process time at 2.0% VImaILs concentration, (b) a function of VImaILs concentration and process time at 65 °C. Inulin concentration: 20 (w/w)%.

Z.-P. Zhao et al. / Chemical Engineering Science 151 (2016) 16–24

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Table 3 Fermentation characteristics of inulin hydrolysates.a Entry

a b c d

CI (w/w%)

20 20 20 30 a b

pH

4.5 5.5 5.5 5.5

Yeast(g)

0.15 0.15 0.08 0.15

Ethanol production CE(%)

YE (%)

YE/Ib

PC (g/L  h)

8.64 70.31 9.34 70.34 9.92 70.36 13.74 70.49

92.487 3.32 93.277 3.39 94.217 3.42 92.477 3.30

0.441 70.016 0.454 70.016 0.466 70.017 0.421 70.015

1.417 0.05 1.687 0.06 1.777 0.06 2.38 7 0.08

pH regulator: 1 M NaOH aqueous solution; fermentation time: 52 h. The total sugars content of inulin is 94.7%, the theoretical ethanol yield is 0.511 g (Ethanol)/g(Glucose).

increasing the pH value of hydrolysate properly can improve the ethanol fermentation efficiency. Under the same pH value, the fermentation rate of 0.08 g yeast (Fig. 7c) was lower than that of 0.15 g yeast (Fig. 7b) in the first half fermentation process, but gradually increased over the latter after 32 h. In fermentation process, the yeast utilized the reducing sugars and other nutrients in the inulin hydrolysates for growth, and a higher mass of yeast added would consume more reducing sugars, which lead to a reduction in ethanol production. Fig. 7(b and d) compared the fermentation processes with different inulin concentrations. It could be seen that the higher the inulin concentration was, the higher the fermentation efficiency of the yeast and the production capacity. In consideration of that high concentration of ethanol is conducive to the subsequent separation of ethanol, 30% inulin was recommended for this hydrolysis and fermentation system.

Fig. 7. Production of ethanol from inulin hydrolysate by saccharomyces cerevisiae: (a) inulin 20 (w/w)%, pH¼ 4.5, yeast 0.15 g; (b) inulin 20 (w/w)%, pH ¼5.5, yeast 0.15 g; (c) inulin 20 (w/w)%, pH ¼5.5, yeast 0.08 g; (d) inulin 30 (w/w)%, pH ¼ 5.5, yeast 0.15 g.

information about operational conditions and results. For example, for obtaining the inulin complete hydrolysis with 2.0% VImaILs in 20 min, 75 °C (348.15 K) at least is required. In contrast, the hydrolysis time will be extended to 70 min in a VImaILs concentration of 1.5% and a temperature moderate condition of 65 °C. 3.5. Ethanol fermentation To evaluate the fermentability of hydrolyzates obtained from the inulin, the ethanol fermentation was performed with 50 mL hydrolysates from different initial inulin concentrations at a fixed VImaILs concentration of 1.0%. The fermentation characteristics of inulin hydrolysates, including the ethanol concentration (CE), ethanol yield (YE), production per unit mass of inulin (YE/I) and production capacity (PC), were presented in Table 3. It could be seen that the final fermentation efficiency of hydrolysate to ethanol was greater than 92.47% of the theoretical ethanol yield, demonstrating that the hydrolyzates containing reducing sugars from VImaILs hydrolysis of inulin did not bring any evidently negative influence on the fermentation for ethanol production. He et al. (2015) found that ethanol production of the recovered hydrolyzates containing glucose from enzymatic hydrolysis of AE-ILCS (alkali (NaOH) extracted-[BMIM]Cl-HCl-water pretreated corn stover) represents 88.0% of the theoretical yield, also indicating ionic liquids had no evidently negative influence on the fermentation. Comparing the fermentation results of yeast under different pH values (Fig. 7(a and b)), the production capacity at pH 4.5 was smaller than that at pH 5.5, due to the effect of a relatively acidic environment on yeast activity. So, after hydrolysis by VImaILs,

4. Conclusions In this study, an imidazole-based acidic ILs ([(CH2)3SO3HVIm] HSO4) was synthesized to hydrolyze inulin for subsequent bioethanol fermentation. We have revealed that the hydrolysis rate by VImaILs is obviously faster than dilute sulfuric acid. From the perspective of hydrolysis efficiency and industrial economy, VImaILs concentration of 1.0–2.0%, inulin concentration of 30%, reaction temperature of 65–75 °C and reaction time of  50 min were chosen as the optimum conditions for hydrolysis of inulin. Furthermore, the proposed concise kinetic model of inulin hydrolysis successfully predicted the inulin hydrolysis with high credibility in wider ranges of experimental conditions. Finally, the inulin hydrolysates were fermented into ethanol by Saccharomyces cerevisiae. The conversion efficiency of inulin-type sugars to ethanol was greater than 92.47% of the theoretical ethanol yield. And the ethanol production capacity reached 2.38 g/(L h). This hydrolysis and fermentation system is a potentially environmentfriendly technology to prepare bioethanol from Jerusalem artichoke.

Acknowledgments The authors would like to thank the support of the Doctoral Fund of the Ministry of Education (No. 20131101130005) and the National Natural Science Foundation of China (No. 21576024).

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ces.2016.05.017.

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