Adsorption Study of Glycerol in Biodiesel on the Sulfonated

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Adsorption Study of Glycerol in Biodiesel on the Sulfonated Adsorbent Bin Chen, Wusheng Wang, Xiao Liu, Weiming Xue, Xiaoxun Ma, Guoliang Chen, Qiushuo Yu, and Rong Li* Chemical Engineering Research Center of the Ministry of Education for Advanced Use Technology of Shanbei Energy, Shaanxi Research Center of Engineering Technology for Clean Coal Conversion, School of Chemical Engineering, Northwest University, Xi’an 710069, P.R. China S Supporting Information *

ABSTRACT: The adsorption isotherm data of glycerol from biodiesel (FAME, fatty acid alkyl esters) onto the sulfonated adsorbent were obtained via batch equilibrium tests at different temperatures in the range of 303−323 K. Subsequently, these data were fitted by four isotherm models. Freundlich isotherm model was the best fitted (r2 > 0.98), and the model parameter 1/ n implied that the adsorption process was favorable. For the Dubinin−Radushkevich (D-R) isotherm model, the model parameter QD decreased with increasing temperature, and the mean free energy of sorption (E) was less than 8 kJ mol−1. The results of QD and E reflected the exothermic and physical properties of the adsorption process, respectively. The negative values of Gibbs free energy change (ΔG0) and enthalpy change (ΔH0) indicated that the adsorption occurs spontaneously with an exothermic nature, while the positive values of entropy change (ΔS0) suggested the increase in randomness at the solid−liquid interface during adsorption. The isosteric heat of adsorption (ΔHX) suggested that strong hydrogen bonding between glycerol and the −SO3− groups of the sulfonated adsorbent dominated the adsorption process and that there existed adsorbate−adsorbate mutual attractive interaction. Furthermore, the existence of hydrogen bonding was also confirmed by infrared spectra.

1. INTRODUCTION As a renewable and clean energy source, FAME (fatty acid alkyl esters) develops rapidly and has become the policy choice of many countries in recent years. This fuel is normally produced by transesterification reaction of animal fats, vegetable oils, or waste cooking oil with methanol,1 and the crude FAME always contains several impurities, such as glycerol, soap, methanol, metals, free fatty acids, water, catalyst, and glycerides. Trace amount of these impurities will cause problems during the storage and usage of this fuel.2 Glycerol is especially undesired, because it tends to polymerize with other molecules of glycerol or glycerides at higher temperatures, and this will accelerate the formation of coke and tarnish on cylinders and injectors. As a result, it will promote emissions of aldehydes and corrosion of the injection system.3,4 Thus, both the standards of EN 14214 and ASTM D6751 specified that the maximum limit of glycerol in FAME is 0.2 mg g−1. Currently, water washing and dry washing are the two most prevalent techniques adopted to purify the crude FAME in industrial plants.5,6 The latter is mainly achieved by selecting appropriate adsorbents for the adsorption of impurities in FAME. Additionally, the dry washing is now gradually replacing the water washing due to its advantages of no wastewater, lower costs, better fuel quality, being easy to integrate into the existing plant, smaller footprint, and shorter operation time.7 Some adsorbents for the dry washing technique have been developed and evaluated. Mazzieri et al. investigated the efficiency of silica gel for glycerol removal from FAME. Their batch equilibrium study at room temperature showed the saturation capacity was 0.239 g glycerol per gram of silica gel.3 Magnesol, a synthetic magnesium silicate, was introduced into © 2012 American Chemical Society

the marketplace by the Dallas Group of America Inc. The report8 showed that the quality of biodiesel purified with Magnesol can fully meet the residual glycerol specification. Recently, new types of commercial adsorbents, such as Amberlite BD10 Dry and PD206 have been marketed by Rohm & Haas and Purolite, respectively.2 These ion exchange resins were based on sulfonated poly-(styrene-co-divinylbenzene).9 Both these resins could bring the glycerol levels down to the specification of the EN 14214. Jacob Wall9 pointed out that glycerol uptake could primarily be attributed to the adsorption of these sulfonated resins, but the potential adsorption mechanism was still unclear. Theoretically speaking, fully understanding the adsorption mechanism between the sulfonated resins and the glycerol in FAME is essential, since this will inject more vitality not only into the development of new adsorbents for glycerol removal but also into optimization of the dry washing process. The goal of this work is to elucidate the adsorption mechanism of glycerol from FAME onto the sulfonated adsorbent. For this purpose, a homemade ion-exchange resin was obtained via sulfonation of a commercial poly-(styrene-codivinylbenzene) adsorbent, and the test solution was prepared from purified FAME spiked with glycerol. Glycerol adsorption isotherms at different temperatures were conducted and discussed, followed by thermodynamic interpretation. The values of ΔHX for glycerol on the sulfonated resin were Received: Revised: Accepted: Published: 12933

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temperatures (303, 308, 313, and 323 K) were carried out by repeating the above steps. The equilibrium adsorption capacity (qe) of dry 1180Na (mg g−1) was calculated according to the eq 2:

analyzed in detail, and infrared spectra of both sulfonated resin and the sulfonated resin/glycerol blend were further compared and investigated to explain the adsorbate−adsorbent interaction mechanism.

qe = (ρ0 − ρe )·m /Wdry

2. MATERIALS AND METHODS 2.1. Preparation of the Sulfonated Resin. The modified resin was prepared via sulfonation of Amberlite XAD1180, a poly-(styrene-co-divinylbenzene) adsorbent from Rohm & Haas Company. Dry XAD1180 (∼10 g) was obtained after being heated in the oven at 105 °C for 12 h, then wetted with 30 mL of dichloromethane, and stirred to form slurry. Concentrated H2SO4 (200 mL) was added to the slurry. The bath temperature was increased to 98 °C. After 8 h, the reaction was stopped by slowly adding an excess of ice water and stirring the mixture in an ice bath. The final product was then rinsed with deionized water until the pH of the liquid was neutral.10 The obtained ion-exchange resin was named as 1180H. Approximately 10 mL of wet 1180H was loaded in a glass column, followed by passing through ∼50 mL of 1 M NaCl at 5 BV/h, and then rinsed with deionized water. The transformed ion-exchange resin was defined as 1180Na. Finally, XAD1180, 1180H, and 1180Na were dried in the oven at 105 °C for 12 h and then stored in a desiccator. 2.2. Determination of Exchange Capacity of the Sulfonated Resin. Dry 1180H resin (0.5−1.0 g) was immersed in 20 mL of 0.5 M NaOH solution. After a 10 h neutralization in the sealed system at room temperature, 5 mL of the upper liquid was precisely transferred to a 100 mL conical flask. Three drops of phenolphthalein and 20 mL of deionized water were added to this conical flask, followed by titration with 0.5 M HCl solution.11 The exchange capacity (Q, mmol g−1) of the sulfonated resin was determined by eq 1: Q = 4·(5 − V1) × 0.5/Mdry

(2)

where ρo and ρe stand for the concentrations of glycerol at initial and equilibrium state, respectively (mg g−1), m is the weight of FAME/glycerol solution (g), and Wdry is the weight of dry 1180Na (g). 2.5. Models of Adsorption Equilibrium. Langmuir isotherm equation has the following linear form:12 ρe ρ 1 = + e qe qmKL qm (3) where qm is the monolayer capacity (mg g−1) and KL is the equilibrium constant (g mg−1). While plotting ρe/qe against ρe, a straight line with slope 1/qm is obtained; KL will then be calculated from the intercept. Freundlich isotherm equation is expressed by the following linear form:13 ln qe = ln KF +

1 ln ρ n e

(4)

where KF and n (with n > 0) are the empirical constants representing the adsorption capacity and adsorption intensity, respectively. Temkin isotherm is represented as follows:13 qe = BT ln KT + BT ln ρe

(5)

where KT is the equilibrium binding constant related to binding energy (g mg−1) and BT is the Temkin constant related to binding energy. The Dubinin−Radushkevich (D-R) isotherm equation is given in the following linear form:14

(1)

ln qe = ln Q D − Bε 2

where V1 stands for the titration volume of 0.5 M HCl solution (mL) and Mdry is the weight of dry 1180H (g). 2.3. Preparation of FAME/Glycerol Solution. The FAME/glycerol solution was obtained by dissolving an appropriate amount of glycerol into the purified FAME. The preparation steps were described as follows: Using NaOH as a catalyst, the transesterification reaction was carried out at 60 °C for 90 min on refined soybean oil with methanol in the mole ratio of 8:1. After phase separation, the crude FAME layer was evaporated on the rotary evaporator (0.08 MPa vacuum) at 45 °C for 40 min, then washed by water 4 times, and then once again evaporated on the rotary evaporator (0.08 MPa vacuum) at 98 °C for 120 min to obtain the purified FAME. The analytical reagent glycerol was added to the purified FAME and intensively stirred at 50 °C over 20 min to prepare the FAME/ glycerol solution. The solution with a concentration of 0.6 mg g−1 was obtained. 2.4. Batch Adsorption Equilibrium. For each test, dry 1180Na (Q = 3.7 mmol g−1) ranging from 0.06 to 0.3 g was introduced into 100 mL conical flasks containing 50 g FAME/ glycerol solution with a concentration of 0.6 mg g−1. The tests were carried out in an orbital shaker at 200 rpm over 20 h, since the kinetic results showed that almost 95% of the equilibrium state would be achieved within 10 h. Flasks without resin served as the blank runs. The final glycerol contents were analyzed by referring to a method established previously.4 Tests at different

(6)

where QD (mg g−1) is the model constant implying the monomolecular adsorption capacity of the adsorbent and B (mol2 kJ−2) is related to the mean free energy of sorption E (kJ mol−1) and is correlated by the following equation: E= (2B)−0.5. ε (kJ mol−1) is the Polanyi potential, which is given as follows: ε = RT ln(1 + ρe−1)

(7) −1

−1

where R is the gas constant (kJ mol K ) and T is the absolute temperature (K). 2.6. Infrared Measurement. The dry XAD1180 and 1180Na were ground and stored separately. The 1180Na/ glycerol blend was prepared by mixing the 1180Na flour with glycerol at the weight ratio of 1:1, sealed in plastic bag over 2 h. These samples were characterized by a VERTEX70 FTIR (Bruker co., Germany) using the KBr pellet (pressed-disk) technique. The spectra of samples were recorded in the range of 3800−600 cm−1 with an average of 16 scans at a spectral resolution of 4 cm−1.

3. RESULTS AND DISCUSSION 3.1. Characterization of XAD1180 and 1180Na. According to the procedure mentioned previously, 1180Na was prepared from XAD1180. As shown in Scheme 1, XAD1180 was based on poly-(styrene-co-divinylbenzene). 12934

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conducted to evaluate the adsorption mechanism of glycerol on 1180Na. 3.2. Adsorption Isotherms. Figure 2 showed the experimental isotherm data for adsorption of glycerol from

Scheme 1. Preparation of 1180Na

After the treatments of sulfonation and ion exchange, XAD1180 was transformed to be 1180Na. To examine the results of modification, both dry XAD1180 and 1180Na were characterized by IR. The infrared spectrum of XAD1180 (Figure 1a) displayed: (1) a broad band at 3440 cm−1 and a medium band at 1630

Figure 2. Equilibrium adsorption of glycerol on 1180Na at different temperatures.

FAME onto 1180Na at different temperatures. Basically, the equilibrium adsorption capacity of 1180Na decreased with increasing temperature in the range of 303−323 K; this indicated that the adsorption process was exothermic. Four well-known adsorption isotherm models were adopted to study the adsorption process. Among them, the Langmuir isotherm is an ideal model assuming uniform binding sites, monolayer adsorption, and no adsorbate−adsorbate interaction.12 However, in the real adsorption system, some potential phenomena could not be clearly explained by the Langmuir isotherm model. Thus, three other nonideal models were also chosen to fit the experimental date. Freundlich isotherm is always applied to the process of multilayer sorption on heterogeneous surfaces.13 Temkin isotherm can reflect the existence of adsorbate−adsorbate interaction.13 D-R isotherm is always adopted to distinguish between physical and chemical characteristics of the adsorption process.14 The adsorption process of glycerol from FAME onto 1180Na is complicated; it possibly involves not only the interactions between glycerol molecules and 1180Na but also the interactions among the glycerol molecules. All of these need to be further explored, and parameters from different isotherm models are of great significance for deeper understanding of the adsorption process. The fitting results of the experimental data were presented in Table S1, Supporting Information. It is observed that all regression coefficients (r2) of the four models at different temperatures were over 0.95, which means that all these models fit the isotherm data satisfactorily. For the Langmuir isotherm model, qm was 434.8 mg g−1 at 303 K, while this value reduced almost one-third as the temperature elevated to 323 K. It indicated that higher temperature is not beneficial to this adsorption process. The other three models fitted the experimental isotherm data better, and Freundlich isotherm model was the most adequate (r2 > 0.98), which reflected the adsorption process was nonideal. The values of KF decreased with increasing temperature and changed in the range of 309.8−551.9 mg g−1. The values of 1/n were between 0 and 1, which indicated that glycerol was favorably adsorbed by 1180Na at all the temperatures.22 The high regression coefficients for the Temkin isotherm model (r2 > 0.95) reflected the existence of adsorbate−

Figure 1. Infrared spectra of XAD1180 (a) and 1180Na (b).

cm−1 corresponding to H−O−H stretching and bending vibration of H2O that physically adsorbed on the XAD1180 surface, respectively; (2) three weak bands at 3085−3020 cm−1 corresponding to aromatic C−H stretching vibrations; (3) four bands at 1604, 1510, 1487, and 1447 cm−1 corresponding to aromatic skeletal stretching vibrations; (4) two bands at 2963 cm−1 (2931 cm−1) and 2874 cm−1 (2855 cm−1) corresponding to the asymmetric and symmetric stretching vibration of CH3(CH2), respectively; (5) four bands at 902−708 cm−1 corresponding to aromatic C−H deformation vibrations.15,16 Compared with the infrared spectrum of XAD1180, 1180Na (Figure 1b) displayed: (1) stronger absorbance at 3447 and 1630 cm−1 due to the adsorption of more H2O molecules on the polar 1180Na surface;17 (2) weaker absorbance at 3085− 3020 cm−1 owing to aromatic hydrogen atoms substituted after sulfonation. Specifically, several bands emerged after sulfonation: bands at 1187, 1129, and 1046 cm−1 resulted from asymmetric and symmetric stretching vibrations of the aromatic −SO3− groups,18 the band at 1094 cm−1 was caused by stretching vibrations of the aromatic −SO3− groups,19 the band at 1019 cm−1 was attributed to in-plane aromatic C−H bending vibration,20 and two weak bands at 631 and 690 cm−1 were assigned to in-plane bending vibration of the aromatic −SO3− groups.21 Thus, all above results verified that the −SO3− groups were successfully grafted to the phenyl of XAD1180 via sulfonation reaction, resulting in a polar surface of 1180Na. The trial tests showed that only less than 10 mg of glycerol could be captured per gram of dry XAD1180, while the equilibrium concentration of glycerol in the FAME bulk phase had reached 0.4 mg g−1. Conversely, under the same conditions, 1180Na exhibited great affinity to glycerol. This implied that the induced sulfonic groups played a critical role in adsorption of glycerol from FAME. Thus, several tests were 12935

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The values of ΔHX that related to different equilibrium adsorption capacities (qe = 90, 110, 130, 150, 170, 190 mg g−1) were calculated using the Clausius−Clapeyron equation:24,26

adsorbate interaction.23 Furthermore, BT decreased with increasing temperature, which revealed the exothermic characteristic of the adsorption process.24 The D-R isotherm model parameter QD decreased with increasing temperature, which implied that the capacity of 1180Na for glycerol decreased with rising temperature. E provided information about the adsorption mechanism: physical adsorption (1−8 kJ mol−1), ion exchange adsorption (9−16 kJ mol−1), or chemical adsorption (>16 kJ mol−1).25 In present study, the values of E were less than 5 kJ mol−1. This indicated that the adsorption process of glycerol from FAME onto 1180Na was achieved via physical adsorption. 3.3. Estimation of Thermodynamic Parameters. K0 is defined as the thermodynamic distribution coefficient when qe approaches zero, and it changes with temperature. As the concentration of glycerol in the FAME solution decreases and approaches zero, the activity coefficient γ approaches unity. Thus, the K0 can be written as:13,26 K 0 = lim

qe → 0

q γsqe αs = lim = lim e qe → 0 γρ qe → 0 ρ αl l e e

⎛ ∂ln ρe ⎞ −ΔHX ⎜ ⎟ = ⎝ ∂T ⎠q RT 2

(12)

e

or ⎡ ∂ln ρe ⎤ ΔHX = R ·⎢ ⎥ ⎣ ∂(1/T ) ⎦q

e

(13)

where the values of ρe corresponding to certain qe were obtained from the Freundlich isotherm model at different temperatures. As shown in Figure 3, plots of ln ρe versus 1/T

(8)

where αs and αl are the activity values of the glycerol adsorbed on adsorbent and dissolved in solution at equilibrium, respectively. γs and γl are the activity coefficients, accordingly. K0 can be obtained from the intercept of the plot of ln (qe/ρe) versus qe.26,27 The values of ΔG0 for the adsorption process can be calculated using the classical Van’t Hoff equation:28 ΔG0 = −RT ln K 0

Figure 3. Plots of ln ρe versus 1/T.

(9)

ΔG0 is also related to ΔS0 and ΔH0 at constant temperature according to the eq 10: ΔG0 = ΔH0 − T ΔS0

were linear; the values of ΔHX were obtained from the slopes of these plots. Both the values of ΔHX and the regression coefficients (r2) of these plots were listed in Table S3, Supporting Information. Normally, the attraction between adsorbate and adsorbent arises from some of the forces listed below: van der Waals force (4−10 kJ mol−1), hydrophobic bond force (5 kJ mol−1), hydrogen bond force (2−40 kJ mol−1), coordination bond force (40 kJ mol−1), dipole bond force (2−29 kJ mol−1), and chemical bond force (>60 kJ mol−1).32,33 In this study, the ΔHX ranged from −26.04 to −33.47 kJ mol−1. This suggested that all the above forces may be involved for the adsorption of glycerol except coordination and chemical bond forces. Also, if hydrogen bonding is present, it is the main force as compared with van der Waals, hydrophobic bond, and dipole bond forces.34 Thus, we deduced that strong hydrogen bonding interaction dominated the process of glycerol adsorption from FAME onto 1180Na. Furthermore, it was observed that the absolute values of ΔHX increased steadily with the increase of qe. Some researchers24,35 also found the analogical phenomenon, and they attributed it to adsorbate−adsorbate interaction accompanied by the adsorbate−adsorbent interaction. Besides, Fowler and Tempkin36 assumed that ΔHX changed linearly with surface coverage (θ) due to indirect or direct adsorbate−adsorbate interactions:

(10)

Combining the above two equations, one gets: ln K 0 =

−ΔG0 ΔS0 ΔH0 1 = − · RT R R T

(11)

ΔH0 and ΔS0 can be calculated from the slope and intercept of the linear plot of ln K0 versus 1/T.29 Thermodynamic parameters at four operating temperatures were presented in Table S2, Supporting Information. As seen in Table S2, Supporting Information, K0 decreased with increasing temperature, which suggested that the adsorption capacity of glycerol on 1180Na was lower at higher temperature under the same equilibrium concentration of glycerol in the bulk phase. The negative values of ΔG0 implied that the adsorption occurred spontaneously. The absolute values of ΔG0 were smaller than 20 kJ mol−1, which is typical of a physical adsorption process.27,30 The negative values of ΔH0 further confirmed the exothermic nature of this adsorption process. The positive values of ΔS0 suggested the increase in randomness at the solid−liquid interface during adsorption and reflected the affinity of glycerol in FAME solution to 1180Na.31,32 3.4. Isosteric Heat of Adsorption. Isosteric heat of adsorption (ΔHX) is defined as the heat of adsorption at constant amount of adsorbate. It can be used for characterization and optimization of an adsorption process. The values of ΔHX directly reflect the type of the force between adsorbate and adsorbent.

ΔHX = ΔH0(1 − αθ )

(14)

where ΔH0 is the enthalpy of adsorption at zero coverage (θ = 0) and α reflects the degree of adsorbate−adsorbate interactions. The symbol of α is positive for repulsive interactions, whereas it is negative for attractive interactions. 12936

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In this experiment, α was negative due to the attractive interaction. θ is the ratio of qe to qm. Thus, the variation of ΔHX with qe (Table S3, Supporting Information) could be explained: when the values of qe were small, glycerol molecules were dispersed randomly on the active sites of 1180Na, and the value of ΔHX just represented the average strength of attractive interactions between glycerol molecules and the active sites. As qe increased, the adsorbing glycerol molecules tended to cluster together on the 1180Na surface due to intermolecular hydrogen bond attraction. Namely, the adsorbing glycerol molecule was attracted not only by the active site but also by the glycerol molecules attached on the adjacent sites, which resulted in the increase of ΔHX. 3.5. FTIR Analysis. To further verify the hydrogen bonding between glycerol molecules and −SO3− groups of 1180Na, infrared spectra of 1180Na and the 1180Na/glycerol blend were examined. Also, baselines of the spectra were corrected using a “Concave Rubberband Correction” method of the OPUS spectroscopic software for further analysis. The asymmetric stretching of aromatic −SO3− groups with a shoulder peak is clearly seen in the infrared spectrum of 1180Na at 1187 cm−1 (Figure 1b). To investigate the change in bands corresponding to the asymmetric stretching vibration of the aromatic −SO3− groups quantitatively, the broad profiles were curve-fitted as the sum of subpeaks according to the result of a “Residual after 1st Derivative (search Hidden Peaks)” method. Figure 4 shows the curve-fitted results of 1180Na and 1180Na/glycerol blend. Two peaks near 1217 and 1183 cm−1 were assigned to asymmetric stretching of the aromatic −SO3− groups.

weaker interactions between them.37,38 The results were in accordance with those obtained via ΔHX. On the basis of the results of ΔHX and infrared spectra, the adsorption mechanism of glycerol from FAME onto 1180Na can be elucidated in Figure 5. In this model, glycerol molecules

Figure 5. Model for surface of 1180Na after adsorption of glycerol.

are attracted by the −SO3− groups of 1180Na as well as the glycerol molecules on the adjacent sites via the hydrogen bonds. Moreover, the hydrated sodium ions likely cross-link with −SO3− groups and the adsorbed glycerol via hydrogen bonding, coordination, or charge dipole interaction, while the specific mechanism still needs to be further explored.

4. CONCLUSIONS In this study, the glycerol in FAME exhibited a great affinity to the sodium form of cation exchanger (1180Na). All the four adsorption isotherm models presented good agreement with the experimental data, and the Freundlich model was the best fitted. It could be concluded from the model parameters that the adsorption process of glycerol in FAME on 1180Na was a favorable process with the exothermic and physical characteristics. The thermodynamic parameters further confirmed the exothermic, spontaneous, and physical nature during the glycerol adsorption process accompanied by an increase of randomness at the solid−liquid interface during adsorption. The absolute values of ΔHX were less than 40 kJ mol−1 and increased with an increasing amount of glycerol adsorbed. This suggested that a strong hydrogen bonding interaction dominated the glycerol adsorption process and also indicated that there existed the adsorbate−adsorbate mutual attractive interaction. The results of infrared spectra showed that the magnitude of band splitting resulting from the asymmetric stretching vibration of aromatic −SO3− groups decreased with the addition of glycerol, which further verified the existence of hydrogen bonding between −OH of glycerol molecules and oxygen atoms of the aromatic −SO3− groups.

Figure 4. Curve-fitted results of infrared spectra of 1180Na (a) and 1180Na/glycerol blend (b) in the range of 1300−1150 cm−1: (- - -) fitted results, () the infrared spectrum.

It is evident from Figure 4 that the intensity of band at 1183 cm−1 decreased while the shoulder at 1218 cm−1 increased with the addition of glycerol, and the area ratios for 1180Na and 1180Na/glycerol blend were S1218/S1183 = 1.11 and S1217/S1204 = 1.31, respectively. Meanwhile, the magnitude of the band splitting decreased from 34.4 to 13 cm−1 with the addition of glycerol. This is possible since glycerol molecules are adsorbed on the active sites of −SO3− groups by hydrogen bonding interaction. The emergence of the hydrogen bonding between glycerol and −SO3− groups increased the spatial distance between Na+ and aromatic −SO3− groups, which resulted in

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ASSOCIATED CONTENT

S Supporting Information *

All the tables mentioned in the present study. This material is available free of charge via the Internet at http://pubs.acs.org. 12937

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AUTHOR INFORMATION

Corresponding Author

*Phone: +086-15891721954. Tel: +086-029-88307657. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the grants from the National Natural Science Foundation of China (51174281), the Key Science and Technology Program of Shaanxi Province (2009K10-02), the Natural Science Foundation of Shaanxi Province (2011JM2013), the Natural Science Foundations of Shaanxi Provincial Education Department (09JK735&09JK758), and the Scientific Research Fund of Northwest University (PR10028).

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Industrial & Engineering Chemistry Research

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