Abstract
A mathematical description was developed for production of saccharides and fermentation inhibitors during the hot-compressed water pretreatment of cassava residue. Pretreatment was conducted at 150 °C, 160 °C, 170 °C, and 180 °C, and reaction times ranged from 0 to 70 min. The formation of saccharides and four main inhibitors (furfural (F), hydroxymethylfurfural (HMF), acetic acid, and formic acid) were studied. A model for predicting the concentrations of F and HMF (CF and CHMF, respectively) as functions of H+ concentration was established. Furthermore, kinetic models were built after introducing the hydrogen ion concentration index mi. Hydrogen ion concentration had a dramatic effect on the dissolution of pentosan but did not greatly affect the dissolution of hexosan or the degradation of hexose or pentose. Additionally, the activation energies for the formation of pentose or hexose were lower than the degradation energies. The coefficients of determination (R2) of the kinetic models for predicting the yield of the four compounds (pentose, hexose, furfural, and HMF) were higher than 0.923. These kinetic models provided a theoretical foundation and technical support for controlling the production of the main carbohydrates and fermentation inhibitors.
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Kinetics of the Formation of Saccharides and Fermentation Inhibitors during the Hot-compressed Water Pretreatment of Cassava Residue
Peng Qiang Lin,a Jing Hong Zhou,a* Xueping Song,a and Shuangfei Wang a,b
A mathematical description was developed for production of saccharides and fermentation inhibitors during the hot-compressed water pretreatment of cassava residue. Pretreatment was conducted at 150 °C, 160 °C, 170 °C, and 180 °C, and reaction times ranged from 0 to 70 min. The formation of saccharides and four main inhibitors (furfural (F), hydroxymethylfurfural (HMF), acetic acid, and formic acid) were studied. A model for predicting the concentrations of F and HMF (CF and CHMF, respectively) as functions of H+ concentration was established. Furthermore, kinetic models were built after introducing the hydrogen ion concentration index mi. Hydrogen ion concentration had a dramatic effect on the dissolution of pentosan but did not greatly affect the dissolution of hexosan or the degradation of hexose or pentose. Additionally, the activation energies for the formation of pentose or hexose were lower than the degradation energies. The coefficients of determination (R2) of the kinetic models for predicting the yield of the four compounds (pentose, hexose, furfural, and HMF) were higher than 0.923. These kinetic models provided a theoretical foundation and technical support for controlling the production of the main carbohydrates and fermentation inhibitors.
Keywords: Cassava residue; Hot-compressed water pretreatment; Kinetic; Pentose; Hexose; Inhibitors
Contact information: a: College of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China; b: Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Nanning 530004, PR China ;*Corresponding author: jhzhoudou@126.com
INTRODUCTION
With the increasing depletion of oil and other fossil fuels, the transformation of renewable lignocellulose resources into bio-ethanol has become a popular research topic (Hideno et al.2012; Moshi et al. 2014). In general, the digestion of lignocelluloses for producing bio-ethanol is a three-step process consisting of pretreatment, enzymolysis, and fermentation. Of these steps, pretreatment is the rate-limiting step in the bio-ethanol production process (Kim et al. 2003).
Cassava residue, an industrial waste by-product, is mainly generated in starch factories. Every year, more than one million tons of wet-cassava residues are produced in Guangxi province, China, and they are utilized as animal feed (Dos Santos Filho et al. 2015), substrate media for bacteria (Li et al. 2012), and agricultural fertilizers (Duarte et al. 2013). Therefore, the finding of more valuable uses for cassava residues is an important issue. Recent studies have demonstrated that cassava residues can be used to produce polyethylene (Farias et al. 2014), fully biodegradable starch-based composites, (Versino and García 2014), all-plant fiber composites (Zhang et al. 2014), methane (Zhang et al. 2011), and bio-ethanol (Rattanachomsri et al. 2009). Compared with other methods for producing bio-ethanol, transforming cassava residues into bio-ethanol yields high-value chemicals and solves serious environmental pollution problems associated with the disposal of cassava residues.
The major components of cassava residues are starch, cellulose, hemicellulose, and lignin. Various pretreatment methods with acid (Zhang et al. 2011), alkali (Gao et al. 2013), hot-compressed water (Rogalinski et al. 2008), and steam explosion all have their own advantages and disadvantages. For ethanol production, hot-compressed water pretreatment is more attractive than the other methods because of its environmental friendliness, ease of chemical handling, and low operational costs (Imman et al. 2013).
In the hot-compressed water pretreatment of lignocellulosic materials, water ionizes to produce self-catalyzing H+. H+ ions permeate into the lignocellulosic materials under high pressure and subsequently destroy oxygen-acetyl groups in hemicellulose, generating acetic acid or other organic acids (Sreenath et al. 1999). These acids promote the self-hydrolyzation of hemicellulose. The pentose and hexose produced by the self-hydrolyzation of hemicelluloses may degrade into furfural (F) and hydroxymethylfurfural (HMF), which further degrade to form formic acid, acetic acid, and levulinic acid under intensified reaction conditions. These by-products not only reduce the yield of sugars, but also inhibit the subsequent fermentation of sugars into ethanol. Therefore, it is meaningful to investigate the formation of the main inhibitors in biomass materials during hot-compressed water pretreatment.
While most research on the hot-compressed water pretreatment of lignocellulose resources has focused on the formation of sugars, F, and HMF (Cuevas et al. 2014), few studies have dealt with the kinetics of formic acid and acetic acid formation. The H+ concentration is critical to effective hot-compressed water pretreatment. Therefore, organic acids such as formic acid and acetic acid undoubtedly exert influence on the yields of sugar, F, HMF, formic acid, and acetic acid, and understanding this influence was the main goal of this study.
EXPERIMENTAL
Raw Materials
The cassava residue was purchased from Tianyang Huaqiao starch factory (Tianyang, China). The material was passed through a 2-mm sieve after air-drying and was then stored in a polyethylene container at 0 °C for 2 days before use to avoid temperature-induced degradation. The chemical composition of the cassava residue was analyzed using the method prescribed by the National Renewable Energy Laboratory (NREL) (Sluiter et al. 2012) and recorded as follows: 29.08% starch, 16.57% cellulose, 40.12% hemicellulose, 52.17% glucose, 11.58% xylose, 4.03% mannose, 4.83% arabinan, 8.64% galactose, 4.35% acid-soluble lignin, 0.92% acid-insoluble lignin, 1.55% ash, and 7.41% other materials, by dry weight.
Batch Tube Reactors
The cassava residue was loaded into a high-temperature, high-pressure reactor with a volume of 300 mL (Xian Taikang Bio-Technique Co., Ltd., Xian, China). The reactor was constructed with Teflon and equipped with a temperature controller, pressure gauge, electromagnetic stirrer, and valve. An auto-tuning heat-up procedure was adopted to minimize the effect of temperature transients. Therefore, the auto-tuning procedure was reset repeatedly before conducting the pretreatment.
Hot-Compressed Water Pretreatment of Cassava Residue
Approximately 12.5 g (oven-dry mass) of cassava residue was placed in the reactor along with 250 mL of water, with the L/W (water volume/cassava residue oven-dry mass) ratio controlled at 20:1 mL/g. This L/W ratio was selected to minimize the effect of mass transfer and obtain appropriate concentrations of pretreated cassava residue components (Rissanen et al. 2014). Once the desired temperature (150, 160, 170, or 180 °C) was reached, a 70-min isothermal treatment was conducted at that temperature. When the desired reaction time was reached, the reactor was then rapidly moved to a water bath for cooling. During isothermal treatment, hot water extracts were collected from the reactor every 10 min. After completing the reaction, the cassava residue was recovered from the reactor, washed with warm water, and air-dried. The pH of the liquids was determined immediately, after which the hot water extracts were stored at 0 °C.
Analysis of the Liquor Samples
The concentrations of sugars (including monomeric sugars and total sugars) in the hydrolysate were measured by an HPLC system (Agilent 1200, Santa Clara, CA, USA) equipped with an ultraviolet absorption (UV) detector and a DiamonsilTM 5 µm C18(2), 250 mm × 4.6 mm column (Dikma Science and Technology, Beijing, China) with a flow rate of 1.0 mL/min at 35 °C. The mobile phase consisted of 20% acetonitrile and 80% ammonium acetate (0.02 mol/L and 10 mL/L of dissolved acetic acid). The treatment of the samples required derivatization, which involved adding 0.4 mL of NaOH (0.3 mol/L) and 0.4 mL of 1-phenyl-3-methyl-5-pyrazolone (PMP; 0.5 mol/L) to 50 µL of the hydrolysate in a headspace vial. The vial was placed in an ultrasonic cleaner for 90 min at 70 °C. HCl (0.5 mL, 0.3 mol/L) and H2O (0.6 mL) were added after the reaction was completed. Finally, 10 mL of chloroform was used to extract the sample under vortex conditions. The supernatant aqueous phase was used for the HPLC tests.
The degradation products (F and HMF) in the hydrolysate were also analyzed using an HPLC system (Waters 2695, Milford, MA, USA) equipped with a diode array detector (DAD), and a Diamonsil C18(2), 250 mm × 4.6 mm × 5 µm column with a flow rate of 0.6 mL/min at 30 °C. In this case, the mobile phase consisted of 10% methyl alcohol and 90% ultrapure water, with 1% acetic acid dissolved into the water.
The other degradation products, namely formic acid and acetic acid, were also analyzed using the same HPLC system at 30 °C. The flow rate used in the case was 1.0 mL/min, and the mobile phase consisted of 65% methyl alcohol and 35% ultrapure water. Sample treatment prior to this HPLC procedure also required derivatization, which was conducted by mixing 0.2 mL of phosphate buffer solution (pH = 7.6) and 0.2 mL of the hydrolysate in a 10-mL test tube, followed by the addition of 1.0 mL of α-bromo-2,3,4,5,6-pentafluorotoluene (20 g/L dissolved in acetone). The mixture was placed in a water bath kettle for 60 min at 60 °C. Finally, 2 mL of n-hexane was used to extract the sample, and the supernatant organic phase was used for HPLC tests after centrifugation.
Data Analysis
The concentrations of pentose and hexose (CP and CH, respectively) were calculated using Eqs. 1 and 2, “pentose” and “hexose” mean just the monomeric form sugar, and the concentrations of pentosan and hexosan (CA and CB) were calculated using Eqs. 3 and 4, respectively. Here the terms “pentosan” and “hexosan” mean just the oligosaccharide sugar forms, as shown below.
(1)
(2)
(3)
(4)
In the above equations, Cara, Cxyl, Cglu, Cman, and Cgal refer to the concentrations of arabinose, xylose, glucose, mannose, and galactose, respectively. Ct,ara, Ct,xyl, Ct,glu, Ct,man, and Ct,galrefer to the concentrations of the five total sugars, respectively, and 132/150 and 162/180 are the constant factors for transforming pentose into pentosan and hexose into hexosan, respectively.
The yields of pentose and hexose in liquid (YP and YH) were calculated using Eqs. 5 and 6, respectively. The total pentose and hexose in the material were analyzed using HPLC by NREL, and the ratio of pentose and hexose (rP and rH) were calculated using Eqs. 7 and 8.
(5)
(6)
(7)
(8)
Kinetic model
Generally, the practical objective of studying a kinetic model is to optimize the process and to obtain equations useful for economical estimations. Various kinetic studies on the acid hydrolysis of lignocellulosic materials have since been reported (Mittal et al. 2009). This report studied the hydrolysis of sugars such as pentose and hexose, and pseudo-homogeneous first-order series reactions were used to describe the hot-compressed water pretreatment of cassava residue. The following generalized model was used,
(9)
where k1 and k2 are the kinetic constants for the transformation of the polymer into the monomer and monomer decomposition, respectively. The decomposition products were F, HMF, formic acid, or levulinic acid (Karinen et al. 2011). Of the variety of decomposition products, this paper dealt with pentose and hexose as well as their derivatives F and HMF. Based on this model, similar models on the decomposition of sugars in the hydrolysate were developed, as shown below:
(10)
(11)
where CA, CP, CF, CB, CH, and CHMF represent the concentrations of pentosan, pentose, furfural, hexosan, hexose, and HMF, respectively, whereas k1, k2, k3, and k4 are the kinetic constants (min−1).
The formation of monomers and inhibitors are linked to acetyl groups, which are hydrolyzed to acetic acid in acidic conditions. Therefore, acetyl groups originating from hemicelluloses and acetic acid generation (Aguilar et al. 2002) were modeled as follows:
(12)
Formic acid, which is generated from the secondary degradation of F and HMF, also contributed slightly to the formation of acetic acid during the pretreatment process. A few researchers have already investigated the kinetics of formic acid formation, which is shown in the simple model below:
(13)
The predictive kinetic model for pentose is based on consecutive, homogeneous, and first-order reactions, which are shown in Eqs. 14 through 20:
(14)
(15)
(16)
The integration of Eqs. 11 through 17, with the initial conditions of CA = CA0, CP = CF = 0 at t = 0 resulted in the analytical expressions given in Eqs. 14 through 17 to describe the pentosane, pentose, and furfural concentrations over time.
(17)
(18)
(19)
(20)
The proton concentration that catalyzes reactions is integrated into all rate constants. Therefore, the reaction rate constants ki (i = 1 to 4) are related to hydrogen ion concentration and reaction temperature in the above reactions. A model for the reaction rate constants (ki) is shown in Eq. 21:
(21)
where ki is the first order rate constant in units of min−1, including the proton concentrations integrated into each (Kumar and Wyman 2008; Shen and Wyman 2011). Ai and Eai are the Arrhenius pre-exponential factors and the activation energies for each reaction with units of L/(mol min) and J/mol, respectively, and mi is the hydrogen ion concentration index. Finally, Ris the gas constant and T is the reaction temperature in K.
The function Fi was defined as the minimization of the sum of square error between the predictive value (xi*) with the experimental value (xi) (Garrote et al. 2001), which was calculated by Eq. 22. The iteration was performed by the solver complement of Excel 2010 software (Microsoft Corporation, Redmond, WA, USA) in order to obtain the kinetic parameters ki and hydrogen ion concentration index mi at different reaction temperatures.
(22)
Based on the classic Arrhenius equation, the activation energy can be calculated from Eqs. 23 and 24, which show the linearized form of the Arrhenius equation:
(23)
(24)
RESULTS AND DISCUSSION
Influence of Temperature and Reaction Time on the Formation of Saccharides
The four parts of Fig. 1 show the effects of the combination of temperature and time on hemicellulose degradation. Increasing the pretreatment temperature dramatically accelerated the digestion of pentose.
As shown in Fig. 1(a), CP and CA in liquid increased with increasing reaction time. CP and CA increased slowly at low temperatures (from 0.013 to 0.137 g/L and from 0.276 to 0.405 g/L at 150 °C from 0 to 70 min, respectively). Meanwhile, the yield of total pentose (YP) in liquid (including 25.28% CP (rp) and 74.72% CA) reached 19.81% at 150 °C after 70 min.
The total value of pentose first increased and then decreased with increasing reaction time at 170 and 180 °C (Fig. (c), (d)), and the maximum value of YP reached 27.40% at 170 °C after 60 min, which was similar to the 23% to 83% of theoretical sugar yield (Studer et al. 2011). Thus, the optimal condition for YP was 170 °C for 60 min. Additionally, the ratio of monomeric pentose (rp) in liquid increased rapidly with reaction temperature and time, and the maximum value of rp was 61.95% at 180 °C after 70 min.
The primary reason for this result was that high temperatures accelerated the removal of acetyl groups to form acetic acid. Acetic acid catalyzes the formation of oligosaccharides and monosaccharides from hemicellulose (Borrega et al. 2011), leading to high monosaccharide contents at high reaction temperatures.
Fig. 1. Effect of temperature on CA and CP contents in liquors from hot-compressed water pretreatment of cassava residue at (a) 150 °C, (b) 160 °C, (c) 170 °C, and (d) 180 °C
As shown in Fig. 2(a), the yield of total hexose (YH) in liquid (including 1.3% CH (rH) and 98.7% CB) reached 65.92% at 150 °C after 70 min. The total hexose concentration of the liquid also first increased and then decreased with increasing reaction time at 160, 170, and 180 °C (Fig. 2(b), (c), (d)), and the peak values of YH were 89.35%, 88.52%, and 82.38% at 160, 170, and 180 °C, respectively.
Fig. 2. Effect of temperature on CB and CH contents in liquors from hot-compressed water pretreatment of cassava residue at (a) 150 °C, (b) 160 °C, (c) 170 °C, and (d) 180 °C
The maximum peak value of YH was 89.35% at 160 °C for 50 min, which was similar to the 35% to 91% theoretical sugar yield (Studer et al. 2011), indicating that the optimal condition for the highest YH was 170 °C for 50 min. Additionally, the ratio of monomeric hexose (rH) in liquid increased with reaction temperature and time, but the rH content reached a maximum value of 14.26% at 180 °C after 70 min, indicating the total amount of hexose present in liquid with oligomeric hexose.
The hot-compressed water pretreatment of lignocellulosic resources had little effect on cellulose hydrolysis, and a small fraction of glucans was hydrolyzed during the process (Vallejos et al. 2015). However, the major components of cassava residues include starch, cellulose, hemicellulose, and the low lignin content. Larger L/W (1:20), smaller size, and greater specific surface area of cassava residues result in easier hydrolysis than for other lignocellulosic resources (Studer et al. 2011). Additionally, starch in the cassava residues could be hydrolyzed into monomeric hexose and oligomeric hexose under severe reaction conditions.
Influence of Reaction Temperature and Time on the Production of the Four Inhibitors
The F molecule is formed by the degradation of pentose through the dehydration of three H2O molecules. Similarly, HMF is generated by the dehydration of hexose. F and HMF degrade into formic acid under severe reaction conditions. Acetic acid is generated by the removal of acetyl groups (Karinen et al. 2011). As stated previously, the generation of the four inhibitors (F, HMF, formic acid, and acetic acid) not only decreases the yield of sugar, but also greatly influences the subsequent fermentation process (Sakaki et al. 1997).
Influence of reaction temperature and time on the formation of F and HMF
The values of the concentrations of F and HMF (CF and CHMF, respectively) in the water extract increased with increased reaction time and temperature (Fig. 3(a) and (b)). While CF and CHMF increased extremely slowly at low temperatures, they increased rapidly with increasing reaction temperature. Because sugars exist at low temperatures in the form of oligosaccharides, only small amounts of monosaccharides were dissolved. However, the concentration of monosaccharides increased rapidly when the reaction conditions were intensified, resulting in increased CF and CHMF values (Kuster 1990).
The reaction conditions were found to exert a great effect on the formation of F and HMF, with different combinations of reaction temperatures and times producing different amounts of H+ through the ionization of acetic acid. Acidic conditions (pH values in Fig. 1) promoted the hydrolysis of cassava residue components. H+ ions catalyzed this reaction so that the degradation of pentose and hexose generated F and HMF, respectively (Mosier et al. 2005a). Several other reactions occurred, including the decomposition of F, the resinification of F, and condensation of the F and pentose intermediates (Mosier et al. 2005b). The formation of HMF is related to that of F because HMF is also generated by the dehydration of monosaccharides under the influence of H+ as a catalyst. Therefore, secondary reactions similar to those mentioned above for F were also observed for HMF. The decomposition of HMF led to the formation of levulinic acid and formic acid (Karinen et al. 2011). Additionally, hydrolysis was suppressed to prevent subsequent fermentation. Thus, H+ took part in the formation of F and HMF, such that the H+ concentration is likely to be inevitably related to the formation of F and HMF.
Fig. 3. Effect of temperature on (a) F, (b) HMF, (c) formic acid, and (d) acetic acid formation
In Fig. 4(a) and (b), the F and HMF concentrations were obviously related to the concentration of H+. Therefore, a model was proposed to predict the relationship between the F and HMF concentration and the concentration of H+. Fitting the model to the experimental results validated these assumptions.
The fitting curves for F and HMF are presented in Fig. 4(c) and (d), respectively, and the prediction models are shown in Eqs. 25 and 26.
(25)
(26)
It was more convenient to measure the concentration of H+ rather than the concentrations of F or HMF during the hot-compressed water pretreatment of cassava residue. Moreover, the concentrations of F or HMF were easily degraded by high temperature and high pressure condition (Karinen et al. 2011). Thus, the prediction models are an effective and convenient method for monitoring the concentrations of F or HMF.
Fig. 4. Influence of the concentration of H+ on the concentrations of (a) F and (b) HMF; the prediction model for (c) F and (d) HMF against the concentration of H+
Influence of reaction temperature and time on formic acid and acetic acid formation
Figures 3(c) and (d) show that the concentrations of formic acid and acetic acid (Cfa and Caa, respectively) in the water extract increased with increasing reaction time and temperature under low temperature conditions. However, the values of Cfa and Caa were lower at 180 °C than at 170 °C. The reason for the lower concentration of formic acid at 180 °C was that an increase in the severity of the reaction conditions led to changes in the selectivity reactions of F and HMF by the resinification of furfural and the condensation between furfural and pentose intermediates, in order for the decomposition of furfural to produce formic acid (Karinen et al. 2011). The fragmentation products of furfural included formic acid, acetaldehyde, lactic acid, and acetol (Antal et al. 1991). Additionally, the decomposition of HMF led to the formation of levulinic and formic acid. The reason for the decrease in acetic acid concentration was that the formation of sugars and inhibitors required adequate amounts of H+ to take part in catalysis under severe reaction conditions, which led to the notable ionization of acetic acid.
Kinetics of the Formation of Saccharides and Inhibitors
The fitting parameter for predicting hydrogen ion concentration
The equation for predicting hydrogen ion concentration is important for calculating the kinetic constants of the model. Figure 5 shows the curve of hydrogen ion concentration vs. reaction time at different temperatures, the curves are merely fits to the plotted data, and Table 1 lists the fitting parameters. Based on the above equations for hydrogen ion concentration and Eq. 21, ki and mi were obtained using Eqs. 18, 19, 21, and 22.
Fig. 5. Changes in hydrogen ion concentration with reaction time at different temperatures
Table 1. Equation Fitting Parameters for Predicting Hydrogen Ion Concentration
*[H+] = At2 + Bt + C
Kinetic parameters related to the formation of pentose
The kinetic constants (k1 and k2) and hydrogen ion concentration indexes (m1 and m2) are associated with the hydrolysis of pentose (Table 2). Figure 6 shows the Arrhenius plots of ln kivs.1/T, used to obtain the activation energies (Eai) for pentose; the results were obtained by plotting ln k against 1/T and fitting using the least squares method (Table 3).
Table 2. Values of ki and mi for the Formation of Pentose and Hexose at Different Temperatures
As shown in Table 2, the hydrogen ion concentration indexes (m1 and m2) were 0.8 and 0, respectively. The value of m1 indicates that hydrogen ion concentration had an obvious effect on the dissolution of pentosan, and this value is close to that found in the literature in the process of dilute-acid pretreatment. The value of m2 suggests that the hydrogen ion catalyzed the degradation of pentose, but the hydrogen ion concentration had little influence on the degradation of pentose (Baugh and McCarty 1988).
By comparing the values of k1 and k2 (Table 2), the kinetic constants associated with the generation of pentose were higher than those for the degradation reaction. Additionally, the values of the kinetic coefficients increased with increased temperature.
Fig. 6. Arrhenius plots of ln ki vs.1/T, used to obtain the activation energies (Eai)
Table 3. Activation Energies of the Kinetic Reactions Calculated from the Arrhenius Equation
The activation energy Ea1 (Table 3) for the formation of pentose from pretreated cassava residue was 72.39 kJ/mol. This value is lower than the value reported in the literature for wood materials (126.6 kJ/mol) (Rodrı́guez-Chong et al. 2004), but it is similar to the activation energy found for rice straw (68.76 kJ/mol) (Zhuang et al. 2009). The activation energy (Ea2) for the degradation of pentose was 42.57 kJ/mol, which was also similar to that of rice straw (47.08 kJ/mol) (Zhuang et al. 2009).
Kinetic parameters related to the formation of hexose
The hexose released during hydrolysis could have been generated by the decomposition of hemicelluloses. It is important to determine the hexose concentration because this sugar is the main carbon resource for most organisms. The kinetic constants (k3 and k4) associated with the formation of hexose are shown in Table 2. The values of ki0 and Eai for hexose (Table 3) were obtained by plotting lnk versus 1/T and fitting using the least squares method (Fig. 6).
As shown in Table 2, the hydrogen ion concentration indexes (m3 and m4) both approached 0. Thus, hydrogen ion concentration had hardly any effect on the dissolution of hexosan and the degradation of hexose, although hydrogen ions catalyze cellulose and hemicellulose hydrolysis. These results confirmed other reports (Xiang et al. 2004).
Comparing the values of k3 and k4 reveals that the kinetic coefficients for the hexose generation reaction are lower than those for the degradation of hexose, which suggests that HMF could undergo decomposition. The activation energy (Ea3) for the generation of hexose was determined to be 81.13 kJ/mol, which is similar to that reported for the hydrolysis of cellulose paper in 0.1 mol/L HCl (68 to 88 kJ/mol) (Calvini et al. 2007). The activation energy (Ea4) for the formation of HMF from hexose was 45.24 kJ/mol. This value is significantly lower than the values reported in the literature (Carrasco and Rivirres 1992; Girisuta et al. 2007). However, it is difficult to compare values directly with other studies because of the use of different raw materials, kinetic models, and experimental methods.
Fig. 7. Effect of reaction temperature on k1/k2 and k3/k4
Based on the kinetic models, the yields of pentose and hexose were associated with k1/k2 and k3/k4, respectively. Figure 7 shows the effect of temperature on k1/k2 and k3/k4. The values of k1/k2 and k3/k4 first increased and then decreased with reaction temperature, and the maximum peak values were 0.521 at 170 °C and 0.702 at 160 °C, respectively. Thus, the optimal temperature for the high yield of pentose and hexose were 170 °C and 160 °C, respectively, which were similar to the results shown in Figs. 2 and 3.
Based on the aforementioned results, the previous kinetic models were verified (Fig. 8). The value of the correlation coefficient (R2) indicated high agreement between the experimental and model data. Proposed kinetic models may be used to explain the conversion and degradation of pentose and hexose during the hot-compressed water pretreatment of cassava residue or other lignocellulosic resources.
Kinetic parameters related to the formation of acetic acid
Acetic acid is generated by the hydrolysis of acetyl groups in the hemicellulose heteropolymer during hot-compressed water pretreatment (Kim et al. 2012). Based on the experimental data, the values of the kinetic coefficient (k5) at different temperatures are shown in Table 4. Figure 6 shows the curves with Arrhenius plots of ln ki vs.1/T, used to obtain the activation energies (Ea5) for acetic acid. The activation energy (Ea5) (Table 3) for the formation of acetic acid was 62.46 kJ/mol.
Fig. 8. (a) Pentose, (b) furfural, (c) hexose, and (d) HMF released in the liquid by hot-compressed water pretreatment of cassava residues
Table 4. Values of k5 and k6 for the Formation of Acetic Acid and Formic Acid at Different Temperatures
Kinetic parameters related to the formation of formic acid
Formic acid was generated by the degradation of F and HMF. Under severe conditions, the presence of acetic acid promoted the formation of secondary degradation compounds such as formic acid and levulinic acid. Based on the experimental data, the values of the kinetic coefficient (k6) are shown in Table 4, and curves fitted by the Arrhenius equation for the generation of formic acid are shown in Fig. 6. The activation energy (Ea6) (Table 3) for the formation of formic acid was 99.21 kJ/mol. This value is higher than the activation energy for the formation of F (42.57 kJ/mol) and HMF (45.24 kJ/mol). Clearly, a high reaction temperature promoted the formation of formic acid.
CONCLUSIONS
- A model to predict the concentrations of furfural (F) and hydroxymethyl furfural (HMF) as functions of H+ concentration was established as follows:
CF0.57=−3.9+165446.4[H+] and CHMF0.56=−6.1+160374.7[H+]
The model was convenient to monitor the concentrations of F and HMF.
- The kinetic models of the saccharides and degradation products were built after introducing the hydrogen ion concentration index (mi). Hydrogen ion concentration had a great effect on the dissolution of pentosan but hardly affected the dissolution of hexosan and the degradation of hexose or pentose. Additionally, the activation energies for the formation of pentose and hexose were 72.39 and 81.13 kJ/mol, respectively, and the activation energies for the formation of these four compounds (F, HMF, acetic acid, and formic acid) were 42.57, 45.24, 62.46, and 99.21 kJ/mol, respectively.
- The coefficients of determination (R2) of the kinetic models for predicting the yields of the four compounds (pentose, hexose, furfural, and HMF) were higher than 0.923. The above kinetic model can provide the theoretical foundation and technical support for understanding the hot-compressed water pretreatment of cassava residue or other lignocellulosic resources.
ACKNOWLEDGEMENTS
This project was supported by the National Natural Science Foundation of China (2136605) and the Guangxi Natural Fund (2013GXNSFFA019005).
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Article submitted: January 20, 2016; Peer review completed: May 15, 2016; Revised version received and accepted: May 22, 2016; Published: June 2, 2016.
DOI: 10.15376/biores.11.3.6193-6210