Adsorption Behaviors of Glycerol from Biodiesel on Sulfonated

Article pubs.acs.org/EF

Adsorption Behaviors of Glycerol from Biodiesel on Sulfonated Polystyrene−Divinylbenzene Resins in Different Forms Bin Chen, Wusheng Wang, Xiaoxun Ma, Chen Wang, 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, People’s Republic of China S Supporting Information *

ABSTRACT: The mechanisms of glycerol adsorption from biodiesel onto the sulfonated resins in both the hydrogen form (1180H) and sodium form (1180Na) were investigated. 1180H displayed higher adsorption capacity of glycerol in comparison to 1180Na. Parameters from the four isotherm models indicated that the adsorption process for 1180H was non-ideal, physical, and endothermic but exothermic for 1180Na. The values of Gibbs free energy change (ΔG0) and entropy change (ΔS0) suggested that the adsorption processes for both 1180H and 1180Na occurred spontaneously with an increase in randomness of the system. The isosteric heats of adsorption (ΔHX) for glycerol on both 1180H and 1180Na implied that interactions between the glycerol molecules and these modified resins were dominated by strong hydrogen bonding and that there existed an adsorbate−adsorbate mutual attractive interaction. The existence of hydrogen bonding was also confirmed by infrared spectra.

1. INTRODUCTION As a renewable and environmentally friendly energy, biodiesel has attracted growing attention in recent years. This fuel is normally produced by the transesterification reaction of vegetable oils, animal fats, or waste cooking oil with methanol.1 Typically, the crude biodiesel contains several impurities, such as glycerol, soap, metals, methanol, free fatty acids, catalyst, glycerides, and water. Among these impurities, glycerol is especially undesired because of the formation of coke and tarnish on injectors and cylinders.2 As a result, both the standards of EN 14214 and American Society for Testing and Materials (ASTM) D6751 specify that the maximum limit of free glycerol in biodiesel is 0.2 mg g−1. Currently, dry washing is gradually becoming the most prevalent technique for purifying the crude biodiesel in industrial plants.3 The dry washing is mainly achieved by selecting appropriate adsorbents to remove the impurities in biodiesel. Some adsorbents used in this technique, such as silica gel4 and synthetic magnesium silicate,5 have been developed and evaluated. Recently, new types of adsorbents for the dry washing, such as Amberlite BD10 Dry and PD206, have been marketed by Rohm and Haas and Purolite, respectively. These resins were in the hydrogen form of sulfonated polystyrene−divinylbenzene (PS−DVB) co-polymer,6 and either of them could bring the glycerol level down to the specification of the EN 14214. Soon afterward, Lewatit GF202, a sulfonated PS−DVB co-polymer in the sodium form, has been specifically developed for glycerol removal from biodiesel by Lanxess.6 Wall6 pointed out that the glycerol uptake by these sulfonated resins could be primarily attributed to the adsorption mechanism. Accompanying the glycerol adsorption, resins in the hydrogen form would be gradually converted to the sodium form because of the ion exchange with residual sodium ions in the crude biodiesel. © 2012 American Chemical Society

Nevertheless, such a hydrogen−sodium transformation of the resins may cause variation of their glycerol uptake capabilities, which results in process parameters of plants having to be adjusted accordingly. Therefore, investigating the adsorption behaviors of sulfonated resins in different forms will be of great significance for not only the development of new adsorbents for glycerol removal but also the optimization of the dry washing process in plants. However, the potential mechanism for glycerol adsorption on the sulfonated resin in either hydrogen or sodium form is still unclear. The present study mainly focuses on elucidating the mechanisms of glycerol adsorption from biodiesel onto the sulfonated resins in both hydrogen and sodium forms. For this purpose, the homemade sulfonated resins in different forms were prepared from a commercial polymeric adsorbent, and the test solution was prepared from purified biodiesel spiked with glycerol. The glycerol adsorption capacities of both polymeric adsorbent and its sulfonated variants with a series of exchange capacities were compared. The adsorption isotherms of glycerol on sulfonated resins were conducted at four constant temperatures, followed by thermodynamic interpretation. The infrared spectra of both sulfonated resins and the sulfonated resin/glycerol blends were further investigated to explain the adsorption mechanisms.

2. EXPERIMENTAL SECTION 2.1. Preparation of the Sulfonated Resins. The sulfonated resins were prepared by modifying the XAD1180, a PS−DVB adsorbent by Rohm and Haas. First, the sulfonation reaction of XAD1180 was conducted by following the procedure described in ref 7, and the product in the hydrogen form was named as 1180H. Then, the resin in the sodium form was prepared as follows. Received: August 27, 2012 Revised: October 9, 2012 Published: October 11, 2012 7060

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where KT is the equilibrium binding constant (g mg−1) and BT is the Temkin constant related to the binding energy. The Dubinin−Radushkevitch (D−R) isotherm equation is given in the following linear form:13

A total of 15 mL of wet 1180H was packed in a column, followed by passing through 150 mL of 1 M (mol L−1) NaCl at 5 bed volumes per hour (BV/h), and then rinsed with deionized water. The obtained sulfonated resin in the sodium form 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. Exchange capacities (Q, mmol g−1) of the sulfonated resins were determined by titration, following the method of GB/T 8144-2008.8 2.2. Preparation of Biodiesel/Glycerol Test Solution. The biodiesel/glycerol test solution was obtained by dissolving an appropriate amount of glycerol into the purified biodiesel. The preparation steps were described as follows. The crude biodiesel was produced from refined soybean oil and methanol using NaOH as the catalyst.9 After phase separation, it was washed with water 4 times and then evaporated under vacuum to obtain the purified biodiesel. Analytical-reagent glycerol was added to the purified biodiesel and intensively stirred at 50 °C over 20 min to prepare the biodiesel/glycerol solution. 2.3. Batch Adsorption Tests of the Resins with Different Exchange Capacities. Approximately 0.15 g of dry resins with different exchange capacities was introduced into 100 mL conical flasks containing 50 g of biodiesel/glycerol test solution with a glycerol concentration of 0.44 mg g−1, and the water content of the test solution was 861 parts per million (ppm) determined by the Karl Fischer coulometer (831 KF, Metrohm Co.). The flasks were shaken at 303 K in an orbital shaker at 200 revolutions per minute (rpm) over 8 h. The glycerol contents were determined by referring to a method established previously.10 2.4. Batch Adsorption Equilibrium Tests. Tests at four temperatures (303, 308, 313, and 323 K) were carried out. For each test, the dry resin (Q = 3.7 mmol g−1) ranging from 0.06 to 0.3 g was transferred to 100 mL conical flasks containing 50 g of test solution with a glycerol concentration of 0.60 mg g−1, and the water content of the solution was 853 ppm. The flasks were shaken in an orbital shaker at 200 rpm over 20 h. The equilibrium adsorption capacity (qe) of dry resin (mg g−1) was calculated according to eq 1

qe = (ρ0 − ρe )m/Wdry

ln qe = ln Q D − Bε 2

where Q D (mg g ) 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. Further, ε (kJ mol−1) is the Polanyi potential, which is given as follows:

ε = RT ln(1 + ρe−1) −1

where R is the gas constant (kJ mol K ) and T is the absolute temperature (K). 2.7. Infrared Measurement. The dry XAD1180, 1180H, and 1180Na were ground. The 1180H/glycerol blend or 1180Na/glycerol blend was prepared by mixing the resin flour with glycerol at the weight ratio of about 1:1 and then sealed in a plastic bag for over 2 h. These samples were characterized by a VERTEX70 FTIR (Bruker Co., Germany) using a 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 Its Modified Resins. As shown in Scheme 1, XAD1180 was the matrix Scheme 1. Preparations of 1180H and 1180Na

(1)

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

Table 1. Glycerol Adsorption Capacities of Resins with Different Exchange Capacities

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 a slope of 1/qm is obtained. KL will then be calculated from the intercept. The Freundlich isotherm equation is expressed by the following linear form:12

q (mg g−1)

(3)

where KF and n are the empirical constants representing the adsorption capacity and adsorption intensity, respectively. The Temkin isotherm is represented as follows:12 qe = BT ln KT + BT ln ρe

(6) −1

where ρo and ρe stand for the concentrations of glycerol at initial and equilibrium states, respectively (mg g−1), m is the weight of the test solution (g), and Wdry is the weight of the dry resin (g). 2.5. Reusability Test for the Sulfonated Resins. The reusability test included adsorption and regeneration processes. The adsorption process was carried out as the procedure mentioned in section 2.3. As for the regeneration procedure, the used resins were separated from the test solution by filtration and then transferred to flasks containing 50 mL of methanol. These flasks were shaken at 303 K in an orbital shaker at 200 rpm for 30 min to wash down the glycerol, and the regenerated resins were then dried in an oven at 105 °C for 4 h. 2.6. Models of Adsorption Equilibrium. The Langmuir isotherm equation has the following linear form:11 ρe ρ 1 = + e qe qmKL qm (2)

1 ln qe = ln KF + ln ρe n

(5)

−1

Q (mmol g−1)

XAD1180

0.00 0.15 0.46 0.92 2.80 3.13 3.70

7.02

1180H

1180Na

16.2 29.6 106.0 132.0 137.5 140.0

15.0 27.2 96.8 116.7 119.3 120.5

(4) 7061

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Table 2. Comparison of the Regenerated Resins for Glycerol Adsorption in Biodiesel

Table 4. Thermodynamic Parameters of Glycerol Adsorptions on 1180H and 1180Na

q (mg g−1) a

resin

service 1

service 2

service 3

1180H 1180Na

191.5 154.6

143.4 147.2

133.3 143.4

resin

temperature (K)

K0

ΔG0 (kJ mol−1)

303 308 313 323 303 308 313 323

8.52 8.51 8.77 8.76 7.81 7.56 7.46 7.40

−5.40 −5.48 −5.65 −5.83 −5.18 −5.26 −5.23 −5.37

1180H

a

Services 1, 2, and 3 stand for the resin in fresh and the resin regenerated once and twice, respectively. 1180Na

ΔH0 (kJ mol−1)

ΔS0 (J mol−1 K−1)

1.29

22.08

−2.07

10.14

Figure 2. Equilibrium adsorptions of glycerol on 1180H and 1180Na at different temperatures.

without sulfonic groups, 1180H was prepared via sulfonation of XAD1180, and 1180Na would be obtained from 1180H in virtue of ion exchange. To examine the results of modification, the dry resins, namely, XAD1180, 1180H, and 1180Na, were characterized by means of infrared spectra. The infrared spectra of XAD1180 (Figure 1a) displayed (1) a broad band at 3440 cm−1 and a medium band at 1630 cm−1 corresponding to the H−O−H stretching and bending vibration of H2O physically adsorbed on the XAD1180 surface, respectively, (2) three weak bands at 3085−3020 cm−1 corresponding to aromatic C−H stretching vibrations, (3)

Figure 3. Plots of ln ρe versus 1/T.

four bands at 1604, 1510, 1487, and 1447 cm−1 corresponding to aromatic skeletal stretching vibrations, (4) two bands at 2963 (2931) cm−1 and 2874 (2855) cm−1 corresponding to the asymmetric and symmetric stretching vibration of CH3(CH2), respectively, and (5) four bands at 902−708 cm−1 corresponding to aromatic C−H deformation vibrations.14,15

Table 3. Isotherm Parameters for Glycerol Adsorptions on 1180H and 1180Na at Different Temperatures 1180H isotherm model

Langmuir isotherm model

Freundlich isotherm model

Temkin isotherm model

D−R

1180Na 2

qm

KL

r2

434.8 416.7 370.4 294.1 KF

4.600 4.000 3.857 4.250 1/n

0.9863 0.9604 0.9574 0.9506 r2

551.9 474.2 403.3 309.8 KT

0.6047 0.6052 0.5815 0.5248 BT

0.9994 0.9967 0.9921 0.9882 r2

temperature (K)

qm

KL

r

303 308 313 323 temperature (K)

1111 1667 250000 833.3 KF

4.500 3.000 0.020 6.000 1/n

0.8461 0.4227 0.000020 0.3882 r2

303 308 313 323 temperature (K)

2241.5 3468.6 7264.1 28085.3 KT

0.8069 0.8794 1.0262 1.3434 BT

0.9731 0.9455 0.9443 0.9477 r2

303 308 313 323 temperature (K)

65.27 75.76 77.39 74.94 QD

188.4 207.3 264.3 354.0 E

0.9957 0.9943 0.9942 0.9983 r2

44.36 37.19 36.18 37.90 QD

97.85 92.39 83.04 68.10 E

0.9822 0.9675 0.9682 0.9563 r2

303 308 313 323

840.6 1060 1592 3180

4.603 4.437 4.545 4.248

0.9914 0.9709 0.9654 0.9656

345.1 310.8 279.1 229.4

4.880 4.704 4.673 4.789

0.9876 0.9743 0.9734 0.9510

7062

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above results verified that −SO3− groups were successfully grafted to the phenyl of XAD1180 via the sulfonation reaction. In addition, the infrared spectra of 1180Na (Figure 1c) presented similar bands mentioned above. However, differing from the infrared spectra of 1180Na (Figure 1c), 1180H (Figure 1b) displayed a new band at 1349 cm−1 and a strengthen band at 897 cm−1 caused by asymmetric stretching vibration of SO and stretching vibration of S−O, respectively. This indicated that some aromatic −SO2OH groups of 1180H were dissociated into −SO3− groups and H5O2+ and others were not dissociated but cross-linked via intermolecular hydrogen bonds.20,21 3.2. Comparison of XAD1180 and Its Modified Resins for Glycerol Uptake. XAD1180, with a large surface area and abundant pores, is typically used for adsorption of molecules via van der Waals force. Besides the similar matrix structure, 1180H and 1180Na are still grafted with sulfonic groups. However, whether sulfonic groups or the matrix of these modified resins will play the critical role on glycerol adsorption is unclear. Thus, XAD1180 and its sulfonated variants with a series of Q were prepared and tested for their adsorption capacities (q, mg g−1) of glycerol. The results were listed in Table 1. As seen in Table 1, when Q = 0, XAD1180 with no sulfonic groups displayed limited uptake of glycerol. When Q > 0, q for both 1180H and 1180Na increased sharply with rising Q and then approached a plane. Their maximum adsorption capacities were almost 17−20 times larger than that of XAD1180. In addition, 1180H exhibited higher values of q than that of 1180Na, and this phenomenon was more significant as Q increased. All of these suggested that the sulfonic groups and not the matrix of the modified resins played the critical role on glycerol adsorption. It seemed that there existed an interaction between the glycerol molecule and the sulfonic group. Such an interaction could be influenced by the hydrogen or sodium bound to the sulfonic groups, and for resins in different forms, the differential of their q was enhanced at higher Q. Thus, 1180H and 1180Na with the highest exchange capacity (Q = 3.7 mmol g−1) were adopted to study their behaviors of glycerol adsorption in biodiesel. 3.3. Evaluation on the Reusability of the Sulfonated Resins. Both 1180H and 1180Na (Q = 3.7 mmol g−1) were cyclically regenerated and reused to test their reusability for glycerol uptake. As shown in Table 2, 1180Na regained almost 93% of its adsorption capacity after 2 cycles of regeneration,

Table 5. Isosteric Heats of Adsorption for Glycerol on 1180H and 1180Na 1180H

1180Na

qe (mg g−1)

ΔHX (kJ mol−1)

r2

ΔHX (kJ mol−1)

r2

90 115 140 165 190

14.25 16.88 19.50 22.12 24.75

0.8558 0.8894 0.9116 0.9269 0.9380

−26.04 −28.48 −30.43 −32.06 −33.47

0.9649 0.9798 0.9878 0.9925 0.9954

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

In comparison to the infrared spectra of XAD1180, 1180H (Figure 1b) displayed (1) stronger absorbance at 3440 and 1643 cm−1 because of the adsorption of more H2O molecules on the 1180H surface,16 as well as the hydroxyl of introduced sulfonic groups, and (2) weaker absorbance at 3085−3020 cm−1, owing to aromatic hydrogen atoms substituted after modification. Particularly, several bands emerged after modification: bands at 1218, 1173, and 1044 cm−1 resulted from asymmetric and symmetric stretching vibrations of the aromatic −SO3− groups,17 a band at 1093 cm−1 caused by stretching vibrations of the aromatic −SO3− groups,18 a band at 1016 cm−1 attributed to in-plane aromatic C−H bending vibration,19 and two weak bands at 626 and 676 cm−1 assigned to in-plane bending vibration of the aromatic −SO3− groups.17 Thus, all

Figure 5. Adsorption models of glycerol on the surfaces of (a) 1180H and (b) 1180Na. 7063

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displayed for 1180Na. 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 the present study, the values of E were less than 5 kJ mol−1, which indicated that the adsorption process of glycerol from biodiesel onto either 1180H or 1180Na was achieved via physical adsorption. 3.5. Estimation of Thermodynamic Parameters. K0 is defined as the thermodynamic distribution coefficient when qe approaches zero; it changes with the temperature and can be written as12,26 q γsqe α = lim e K 0 = lim s = lim qe → 0 α l qe → 0 γρ qe → 0 ρ (7) l e e

while 1180H regained less. This implied that glycerol molecules were more tightly grasped by 1180H than 1180Na. 3.4. Adsorption Isotherms. Figure 2 presented the experimental isotherm data for 1180H and 1180Na. In the range of 303−323 K, 1180H displayed higher qe than that of 1180Na. The qe of 1180H increased with the rising temperature, but 1180Na exhibited the opposite tendency. Namely, the difference of their qe would be enlarged as the temperature increased. This indicated that the adsorption processes for 1180H and 1180Na were endothermic and exothermic, respectively. Four well-known adsorption isotherm models were adopted to fit these experimental data. Among them, the Langmuir isotherm is an ideal model assuming uniform binding sites, monolayer adsorption, and no adsorbate−adsorbate interaction.11 However, in the real adsorption system, some phenomena cannot be clearly explained by the Langmuir isotherm model. Thus, three other non-ideal models were also chosen to fit the experimental date. The Freundlich isotherm is applied to the process of multilayer sorption on heterogeneous surfaces.12 The Temkin isotherm can reflect the existence of an adsorbate−adsorbate interaction.12 The D−R isotherm is adopted to distinguish between physical and chemical characteristics of the adsorption process.13 The adsorption process of glycerol from biodiesel onto the modified resin is complicated, because it possibly involves not only the interactions between the resin and glycerol molecules 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 a deeper understanding of the adsorption process. As shown in Table 3, regression coefficients (r2) of the Langmuir isotherm model indicated that this model was invalid for 1180H while valid for 1180Na. Other models at different temperatures fitted the isotherm data satisfactorily (r2 > 0.94). For the Langmuir isotherm model, the parameter qm for 1180Na was 434.8 mg g−1 at 303 K, while it reduced almost one-third as the temperature elevated to 323 K. This indicated that a higher temperature is not beneficial to glycerol adsorption on 1180Na. The Freundlich isotherm model was the most adequate for 1180Na (r2 > 0.98), and it was also suitable for 1180H (r2 > 0.94), which reflected that the surfaces of both 1180H and 1180Na were heterogeneous and there existed multilayer sorption. For 1180Na, the parameter 1/n ranged from 0 to 1, which implied that the glycerol was favorably adsorbed.22 For 1180H, the shape of fitted lines converted from favorable (1/n < 1) to unfavorable (1/n > 1) as the temperature increased from 303 to 323 K, but another model parameter, KF, increased steeply with the rising temperature. Thus, the total effect was that a higher temperature is beneficial for glycerol uptake. The Temkin isotherm was the most suitable model for 1180H (r2 > 0.99), and it was also satisfactory for 1180Na (r2 > 0.95). This suggested the existence of an adsorbate−adsorbate interaction for both 1180H and 1180Na.23 In addition, 1180H revealed greater glycerol binding energy than 1180Na because of the larger values of BT for 1180H. Also, BT for 1180H increased with the rising temperature, while that for 1180Na presented the adverse trend. This revealed the endothermic and exothermic natures of the adsorption process for 1180H and 1180Na, respectively.24 The D−R isotherm model parameter QD increased with the rising temperature for 1180H, but an inverse trend was

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

(8)

ΔG0 is also related to ΔS0 and ΔH0 (enthalpy change) according to eq 9. ΔG0 = ΔH0 − T ΔS0

(9)

Combining the above two equations, one obtains ln K 0 =

−ΔG0 ΔS0 ΔH0 1 = − RT R R T

(10)

ΔH0 and ΔS0 can be calculated from the slope and intercept of the linear plot of ln K0 versus 1/T. The thermodynamic parameters at different temperatures were presented in Table 4. As seen in Table 4, K0 for 1180Na decreased with the rising temperature, suggesting that less glycerol would be adsorbed on 1180Na at higher temperatures with the same equilibrium concentration of glycerol in the bulk phase. Also, K0 for 1180H displayed higher values than those for 1180Na, which implied that 1180H had greater affinity to glycerol than 1180Na. The negative values of ΔG0 implied that glycerol adsorption on both 1180H and 1180Na occurred spontaneously. The absolute values of ΔG0 were smaller than 20 kJ mol−1, which are typical of the physical adsorption processes for 1180H and 1180Na.27,29 The positive and negative values of ΔH0 further confirmed endothermic and exothermic natures of the adsorption processes for 1180H and 1180Na, respectively. The positive values of ΔS0 suggested an increase in randomness of the systems during adsorption.30 Combining the positive values of ΔH0 and ΔS0 for 1180H, it suggested that the spontaneous adsorption of glycerol from biodiesel on 1180H was an entropy-driven process. 3.6. Isosteric Heat of Adsorption. ΔHX is defined as the amount of heat released at a constant amount of adsorbate. It can be used for characterization of the adsorption process. The values of ΔHX at different equilibrium adsorption capacities (qe = 90, 115, 140, 165, and 190 mg g−1) were calculated using the Clausius−Clapeyron equation24,26 ⎛ ∂ ln ρe ⎞ −ΔHX ⎜ ⎟ = ⎝ ∂T ⎠q RT 2 e

7064

(11)

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together on the adsorbent surface because of intermolecular hydrogen bond attraction. Namely, the adsorbing glycerol molecule was attracted by not only the active site but also the glycerol molecules attached on the adjacent sites, which resulted in rising absolute values of ΔHX. Interestingly, the values of ΔHX for 1180H were positive (Table 5). Three main factors affecting the value of ΔHX for 1180H were the breaking of intermolecular hydrogen bonds among the acid groups of 1180H, glycerol−released site interaction, and glycerol−adsorbed glycerol interaction. Although the latter two were similar to those for 1180Na and were exothermic, the former group-breaking step was endothermic. Obviously, the former factor dominated the overall glycerol adsorption, resulting in positive values of ΔHX. In addition, the combined effect of these factors lead to the steadily increase of ΔHX with rising qe. 3.7. FTIR Analysis. To further verify the existence of hydrogen bonding between glycerol molecules and −SO3− groups of the modified resins, infrared spectra of 1180H, 1180Na, 1180H/glycerol blend, and 1180Na/glycerol blend were examined. The asymmetric stretching of aromatic −SO3− groups was displayed in not only the infrared spectra of 1180H (Figure 1b) at 1218 and 1173 cm−1 but also those of 1180Na (Figure 1c) at 1187 cm−1, with a shoulder peak. To quantitatively investigate the changes in bands corresponding to the asymmetric stretching vibration of the aromatic −SO3− groups, the broad profiles were curve-fitted as the sum of subpeaks according to the results of a “Residual after 1st Derivative (Search Hidden Peaks)” method from the Origin 8.0 software. Figure 4 showed the curve-fitted results. Two subpeaks near 1218 and 1170 cm−1 were assigned to the asymmetric stretching vibration of the aromatic −SO3− groups. It was evident from Figure 4 that the intensity of the band at 1170 cm−1 decreased, while the band at 1218 cm−1 increased with the addition of glycerol, and the area ratios for 1180H and 1180H/glycerol blend were S1218/S1170 = 1.08 and S1209/S1166 = 2.84, respectively. Meanwhile, the magnitude of the band splitting decreased from 48.2 to 43.5 cm−1 with the addition of glycerol. 1180Na showed similar fitting results. That was possible because glycerol molecules were adsorbed on the aromatic −SO3− groups via hydrogen bonds and the emergence of these bonds increased the spatial distance between hydrated H+ (or Na+) and aromatic −SO3− groups, which resulted in weaker interactions between them.34 The existence of hydrogen bonds verified was in accordance with that summarized via ΔHX. On the basis of the results of ΔHX and infrared spectra, the adsorption mechanism of glycerol from biodiesel onto either 1180H or 1180Na can be elucidated in Figure 5. In this model, glycerol molecules are attracted by not only the −SO3− or −SO2OH groups but also the glycerol molecules on the adjacent sites via hydrogen bonding. Intermolecular hydrogen bonds among the acid groups of 1180H tend to be broken as the temperature increased, and both the SO and hydroxyl groups released are capable of grasping the glycerol molecules tightly via hydrogen bonding. Moreover, the hydrated H+ (or Na+) likely cross-link with −SO3− groups and the adsorbed glycerol through hydrogen bonding, coordination, or charge dipole interaction, while the specific mechanism still needs to be further explored.

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

e

(12)

where the values of ρe corresponding to certain qe were obtained from the best fit isotherm model. As shown in Figure 3, plots of ln ρe versus 1/T were linear and 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 5. Normally, the attraction between the adsorbate and adsorbent arises from some of the forces listed: 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).30 If hydrogen bonding is present, it is the main force, as compared to van der Waals, hydrophobic bond, and dipole bond forces.31 In this study, ΔHX for 1180H ranged from 14.25 to 24.75 kJ mol−1, while that for 1180Na ranged from −26.04 to −33.47 kJ mol−1. Thus, it could be deduced that the processes of glycerol adsorption from biodiesel onto either 1180H or 1180Na were dominated by strong hydrogen bonding. Additionally, the negative values of ΔHX for 1180Na suggested that the adsorption process was exothermic, meaning that fewer glycerol molecules would be adsorbed at higher temperatures. This could be ascribed by the fact that some hydrogen bonds between glycerol and 1180Na would be broken as the temperature increased. However, the positive values of ΔHX for 1180H implied that more glycerol molecules would be adsorbed with the rising temperature, probably because more intermolecular hydrogen bonds among the acid groups of 1180H would be broken at higher temperatures, resulting in more hydroxyl and SO groups released to grasp glycerol molecules via hydrogen bonding. Also, the glycerol molecules were more tightly grasped by these released groups of 1180H than the −SO3− groups of 1180Na and could not be easily desorbed at higher temperatures. This was in accordance with the higher values of K0 for 1180H. Furthermore, the absolute values of ΔHX increased steadily with the increase of qe. The analogical phenomenon had once been found by some researchers, and they attributed it to the adsorbate−adsorbate interaction that accompanied the adsorbate−adsorbent interaction.24,32 Fowler and Tempkin assumed that ΔHX changed linearly with surface coverage (θ) because of the adsorbate−adsorbate interactions33

ΔHX = ΔH0(1 − αθ )

(13)

where ΔH0 is the enthalpy of adsorption at zero coverage (θ = 0). α reflects the degree of adsorbate−adsorbate interaction, and the symbol of α is positive for repulsive interactions while negative for attractive interactions. θ is the ratio of qe/qm. Notably, the glycerol molecules tend to cluster because of the intermolecular hydrogen bonging. Thus, the symbol of α in the present study was negative because of the attractive interaction among the glycerol molecules. For 1180Na, the variation of ΔHX with qe (Table 5) could be explained as follows: when the values of qe were low, glycerol molecules were dispersed randomly on the active sites of the adsorbent and the value of ΔHX just represented the average strength of the attractive interaction between glycerol molecules and the active sites. As qe increased, the adsorbing glycerol molecules tended to cluster 7065

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4. CONCLUSION The sulfonic groups and not the matrix of modified resins played the critical role on glycerol uptake from biodiesel. In comparison to 1180Na, 1180H exhibited greater affinity to glycerol in the temperature range of 303−323 K. The adsorption process of glycerol from biodiesel onto 1180H was non-ideal, physical, and endothermic but exothermic for 1180Na. For both 1180H and 1180Na, the adsorption processes occurred spontaneously with an increase of randomness in the system. Strong hydrogen bonding dominated the adsorption processes, and there also existed an adsorbate− adsorbate mutual attractive interaction.

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ASSOCIATED CONTENT

S Supporting Information *

Regression equations for glycerol adsorption on 1180H and 1180Na at different temperatures (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +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 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 and 09JK758), and the Scientific Research Fund of Northwest University (PR10028).

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