Study on Adsorption Behavior of 12-Phosphotungstic Acid on Silica

Article pubs.acs.org/IECR

Study on Adsorption Behavior of 12-Phosphotungstic Acid on Silica Gel Zuo-xiang Zeng, Li Cui, Wei-Lan Xue,* and Nan-Ke Ma Institute of Chemical Engineering, East China University of Science and Technology, 200237 Shanghai, China ABSTRACT: The adsorption behavior of 12-phosphotungstic acid (12-HPW) on silica gel in aqueous solution was studied by a batch method. It was assumed that the adsorption of 12-HPW on silica gel is due to two species: One is (Si OH2)+·(H2PW12O40)− and/or (SiOH2)+·(H5P2W18O62)−, and the other is the bulk 12-HPW species. The two species adsorb by chemisorption and physisorption, respectively. A model is proposed to correlate the saturated chemisorption capacity limit with the molar percentage of (SiOH2)+·(H2PW12O40)−, that is, a qLc−z1 equation. The adsorption equilibrium data (qe−Ce isotherms) were measured and fitted by several models, namely, the Langmuir, Freundlich, and Dubini−Radushkevich (D−R) models, and the mean feature concentration (C′e) was determined at different temperatures to distinguish chemisorption from physisorption. The values of the correlation coefficient (R2) show that the Freundlich model is more suitable than the Langmuir model. From the D−R model, the experimental saturated chemisorption capacity (qSe) was obtained, which suggests that (SiOH2)+·(H5P2W18O62)− hardly exists during the adsorption process (z1 → 1). The values of the mean free energy (E) calculated from the D−R isotherm equation indicate that the adsorption of 12-HPW on silica gel occurs by sequential chemical and physical mechanisms. The thermodynamic parameters ΔG0, ΔH0, and ΔS0 for the chemisorption and physisorption processes indicate that the overall adsorption is spontaneous and that higher temperatures contribute to chemisorption, whereas physisorption is favored at lower temperatures.

1. INTRODUCTION 12-Phosphotungstic acid (12-HPW, CAS no. 12501-23-4, 2880.05 g/mol) has a Keggin molecular structure.1,2 The acidity of 12-HPW is higher than those of some protonic acids such as sulfuric acid and hydrochloric acid.2,3 Hence, 12-HPW is widely applied in several acid-catalyzed reactions such as esterification, hydration, and rearrangements.4−6 However, bulk 12-HPW has a particularly low surface area ( CeF ′

T (K)

C′eF (g/mL)

n

KF [(mL/g)1/n]

R2

SD

n

KF [(mL/g)1/n]

R2

SD

288 298 308 318

0.01711 0.02223 0.02534 0.03241

1.972 2.135 1.985 1.880

0.3560 0.3830 0.5293 0.6871

0.9998 0.9989 0.9996 0.9986

0.06493 0.02116 0.01513 0.03372

0.7107 0.7252 0.9534 1.040

14.47 11.06 4.035 2.662

0.9969 0.9958 0.9967 0.9919

0.04664 0.04421 0.03594 0.04689

E

dx.doi.org/10.1021/ie400818j | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

0.9970 0.9979 0.9889 0.9814 3.394 3.482 3.828 3.815

0.09046 0.04859 0.06121 0.07222

R2 E (kJ/mol) β (×10 mol ·J )

Table 6. Values of C′e and Relative Errors and Average Deviations for C′eL, C′eF, and C′eDR relative errors T (K) 288 298 308 318 average

4.340 4.124 3.412 3.436 2.286 1.838 1.022 0.8267 0.07823 0.07972 0.06443 0.05384 0.9936 0.9900 0.9935 0.9967

R E (kJ/mol) β (×10 mol ·J )

8.616 8.805 8.789 8.829

SD

qmax (g/g)

8 2

0.6735 0.6449 0.6472 0.6414 0.0957 0.1139 0.1451 0.1915 0.0655 0.0770 0.8106 0.1016 1.038 0.9785 0.9299 0.8708 288 298 308 318

0.01441 0.01883 0.02374 0.03022

qmax (g/g) qec (g/g) CeDR ′ (g/mL) (ε′) (×10 J ·mol ) T (K)

Ce ≤ C′eDR

2 −2 8

−2 2

Ce′ (g/g)

(Ce′ − CeL)/Ce′

(Ce′ − CeF)/Ce′

(Ce′ − CeDR)/Ce′

0.01554 0.02075 0.02413 0.03122 deviations

0.02788 −0.02152 0.03398 0.005872 0.0221

−0.1008 −0.07115 −0.05015 −0.03801 0.0648

0.07291 0.09268 0.01616 0.03213 0.0539

Here, to investigate the adsorption behavior of 12-HPW on silica gel, three equilibrium models, namely, the Langmuir, Freundlich, and Dubini−Radushkevich (D−R) models, were fitted with the experimental data. Langmuir Model. Figure 4 shows comparisons between the theoretical (eq 9) and experimental Ce/qe−Ce isotherms for 12HPW on silica gel at different temperatures. From Figure 4, it can be seen that there is a feature concentration (C′eL) that is correlated with temperature (listed in Table 3). When Ce is lower than CeL ′ , the plots of Ce/qe versus Ce (shown as solid lines) are linear; however, when Ce is higher than CeL ′ , the plots (shown as dashed lines) are irregular. The feature concentration is defined as the corresponding concentration of the intersection point of the solid and dashed lines. The slopes and intercepts of the solid and dashed lines were obtained by the linear least-squares method. The Langmuir model parameters (qmax and KL) were then calculated and are listed in Table 3. The correlation coefficients (R2) and standard deviations (SD) are also included in Table 3. The values of R2 and the Langmuir parameters reported in Table 3 suggest that the Langmuir model fits the experimental data well at lower 12-HPW concentrations, indicating that the adsorption occurs in monolayer fashion for Ce ≤ CeL ′ , whereas at higher 12-HPW concentrations, the model is not applicable and multilayer adsorption might occur for Ce > C′eL. Freundlich Model. From Figure 5, it can be seen that a feature concentration, CeF ′ (listed in Table 4), also exists, and the Freundlich parameters were obtained by regressing the experimental data in the concentration ranges of 0 < Ce < C′eF) and CeF ′ < Ce < 0.1117. The resulting values of n, KF, R2, and SD are reported in Table 4. The higher R2 values (0.9919−0.9998) suggest that the Freundlich model can describe the adsorption isotherms much better than the Langmuir model and that the adsorption process might be heterogeneous. In addition, the values of n (0.7107−2.135) indicate that the adsorption is reversible. From Table 4, the values of n and KF at lower Ce values (Ce ≤ C′eF) are different from those at higher Ce values (Ce > CeF ′ ), which indicates that two kinds of adsorption mechanisms might take place in the two different concentration regions. For Ce ≤ C′eF, the n values (1.880−2.135) are higher, suggesting that qe is more sensitive to Ce in this concentration range. On the other hand, for Ce > CeF ′ , the KF values (2.662− 14.47) are much higher, indicating that adsorption capacity in this concentration range is higher and that multilayer physisorption might occur in this concentration range. Dubini−Radushkevich (D−R) Model. To clarify the detailed adsorption behavior of 12-HPW on silica gel, the D−R model (eq 11) was applied to correlate the isotherms in Figure 2, and the results are shown in Figure 6. As can be seen in Figure 6, the plots of ln qe versus ε2 are linear for ε < ε′ and for ε > ε′, where (ε′)2 is the intersection point of the solid line and the dashed line in Figure 6. The feature concentration for the D−R

2 −2

Ce > C′eDR

Article

2

Table 5. D−R Model Parameters Obtained for the Adsorption of 12-HPW in Aqueous Solution onto Silica Gel at Different Temperatures

SD

Industrial & Engineering Chemistry Research

F



Article

Table 7. Thermodynamic Parameters for the Adsorption of 12-HPW onto Silica Gel at Different Temperatures Ce ≤ Ce′

Ce > Ce′

T (K)

ΔG0 (kJ·mol−1)

ΔH0 (kJ·mol−1)

ΔS0 (J·mol−1·K−1)

ΔG0 (kJ·mol−1)

ΔH0 (kJ·mol−1)

ΔS0 (J·mol−1·K−1)

288 298 308 318

−1.814 −2.391 −3.111 −4.081

21.79

74.71

−4.095 −3.761 −3.389 −2.932

−13.20

−45.96

model (CeDR ′ ) was calculated by eq 12, and the results are reported in Table 5. The D−R model parameters (qmax, β) for the adsorption of 12-HPW on silica gel were obtained from the intercepts and slopes, respectively, of the linear fits to ln qe versus ε2 and are listed in Table 5. The values of mean free energy (E) calculated by eq 13 are also included in Table 5. From Table 5, one can see that, in the temperature range of 288−318 K, the E values are higher (8.616−8.829 kJ·mol−1) and lower (3.394−3.828 kJ·mol−1) than 8 kJ·mol−1 for Ce ≤ CeDR ′ and Ce > CeDR ′ ,31 respectively. These results indicate that the adsorption process of 12-HPW on silica gel in aqueous solution is carried out only by chemical interaction for Ce ≤ CeDR ′ and by chemisorption and followed by physisorption for ′ . Ce > CeDR In accordance with the preceding analysis, the saturated chemisorption capacity (qec) was calculated by eq 11 for Ce = CeDR ′ , and the results are listed in Table 5. As one can see, the qec values range from 0.0655 to 0.1016 g/g in the temperature range of 288−318 K. According to eqs 6 and 7, qLc decreases with decreasing 18-HPW concentration. When z2 = 0, the limiting chemisorption capacity is qLc = 0.1102 g/g, which matches the maximum experimental value (0.1016g/g), confirming that 18-HPW does not exist in the temperature range of 288−318 K. Hence, it can be concluded that the state of 12-HPW on the surface of silica gel is only (Si OH2)+·(H2PW12O40)−. From Tables 3−5, the mean feature concentrations (C′e) were estimated from C′eL, C′eF, and C′eDR by the equation ′ + CeF ′ + CeDR ′ )/3 Ce′ = (CeL

Ce′), indicating that physisorption is an exothermic process and that lower temperatures are favorable to physisorption. On the basis of eq 15, the KC values for chemisorption and physisorption were calculated. The results for the thermodynamic parameters (ΔG0, ΔH0, ΔS0) are listed in Table 7. As seen in Table 7, for Ce ≤ C′e, the negative ΔG0 values indicate the spontaneous nature of the chemisorption process for 12-HPW on silica gel and an increased feasibility of chemisorption at higher temperature. The positive value of ΔH0 suggests that chemisorption is endothermic. The positive ΔS0 value results from the increased randomness at the solid− solution interface due to the chemisorption process. On the other hand, for Ce > Ce′, the negative ΔG0 values confirm the spontaneous nature of adsorption. The absolute values of ΔG0 were found to decrease with increasing temperature from 288 to 318 K, which matches with the nature of physisorption. The negative ΔH0 value further confirms the exothermic nature of the physisorption process. Meanwhile, the negative ΔS0 value suggests a decrease of the 12-HPW concentration at the solid− liquid interface and therefore an increase of that in the solid phase, and it also confirms the decreased randomness at the solid−liquid interface during physisorption. This is the normal consequence of the physisorption phenomenon, which takes place through electrostatic interactions.

6. CONCLUSIONS In the present investigation, the mechanism of 12-HPW adsorption onto silica gel was developed on the basis of the isotherm (qe−Ce). The saturated chemisorption capacity limit (qLc) was estimated, and its value (0.1102−0.1477) was found to depend on the amount of 18-HPW. The Langmuir, Freundlich, and Dubini−Radushkevich (D−R) models were fitted to the experimental data, and the mean feature concentration (C′e) was determined at different temperatures to distinguish chemisorption from physisorption. The values of the correlation coefficient (R2) show that the Freundlich model is more suitable than the Langmuir model. From the D−R model, the saturated chemisorption capacity (qec) was calculated to be 0.1016 g/g, which confirms that 18-HPW does not exist during the adsorption process from 288 to 318 K. From the simulation results of the three models, the proposed adsorption mechanism was found to be reasonable and is described as follows: monolayer chemisorption at lower concentrations (Ce ≤ C′e) and monolayer chemisorption and followed by multimolecular physisorption at higher concentration (Ce > Ce′). The values of thermodynamic parameters (ΔG0, ΔH0, and ΔS0) for chemisorption and physisorption were also obtained. The results indicate that both chemisorption and physisorption are spontaneous and that the chemisorption process is endothermic whereas physisorption is exothermic.

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The values of C′e and average deviations for C′eL, C′eF, and C′eDR are listed in Table 6. Table 6 shows that the values of CeL ′ , CeF ′ , and CeDR ′ from the three models are close to each other, with average deviations of 0.0221, 0.0648, and 0.0539, respectively. Thus, the values of C′e can be applied to determine the nature of the adsorption of 12HPW in aqueous solution onto silica gel as a physical or chemical process. At lower concentrations (Ce ≤ Ce′), chemisorption is the main process, whereas at higher concentrations (Ce > Ce′), physisorption primarily takes place. Moreover, the capacity for physisorption is 3 or 4 times larger than that for chemisorption, as seen from Figures 2 and 3. 5.3. Adsorption Thermodynamics. Experiments were conducted at 288, 298, 308, and 318 K to investigate the influence of temperature on the adsorption capacity and to evaluate the adsorption thermodynamic parameters. Figures 2 and 3 also show the effects of temperature on the adsorption behavior of 12-HPW on silica gel at 288−318 K. From Figure 3, the adsorption capacity of silica gel increases with increasing temperature at lower Ce values (Ce ≤ C′e), implying that chemisorption is an endothermic process and that higher temperatures are helpful to chemisorption. In contrast, as shown in Figure 2, the adsorption capacity of silica gel decreases with increasing temperature at higher Ce value (Ce > G



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Article

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-021-6425-3081. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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NOMENCLATURE A = BET surface area of silica gel (m2/g) Ac = surface area occupied by one HPW crystal cell (m2) a, b, c = cell parameters C0 = initial concentration of 12-HPW (g/mL) CAe = equilibrium 12-HPW concentration on silica gel (g/ mL) Ce = equilibrium concentration in solution (g/mL) Ce′ = mean feature concentration (g/mL) CeDR ′ = D−R feature concentration (g/mL) C′eF = Freundlich feature concentration (g/mL) C′eL = Langmuir feature concentration (g/mL) CSe = equilibrium 12-HPW concentration in solution (g/ mL) E = mean free energy (kJ/mol) ΔG0 = standard Gibbs free energy change (kJ·mol−1) ΔH0 = enthalpy of reaction (kJ·mol−1) KC = thermodynamic equilibrium constant KF = Freundlich constant (mL/g)1/n KL = Langmuir adsorption constant (mL/g) l = side length (m) msi = weight of adsorbent (g) MW = molecular weight of HPW (g/mol) Na = Avogadro’s number n = Freundlich adsorption isotherm constant qc = practical chemisorption capacity (g/g) qe = equilibrium adsorption capacity per unit weight of adsorbent (g/g) qLc = saturated chemisorption capacity limit (g/g) qmax = maximum adsorption capacity of adsorbent (g/g) qSe = saturated adsorption capacity (g/g) R = gas constant (8.314J·mol−1·K−1) ΔS0 = entropy of reaction (J·mol−1·K−1) T = absolute temperature (K) Vsi = volume of silica gel (mL) Vwa = volume of aqueous solution (mL) z1 = molar percentage of 12-HPW loaded on silica gel z2 = molar percentage of 18-HPW loaded on silica gel

Greek Letters

β = D−R isotherm constant (mol2·J−2) ε = Polanyi potential, (J·mol−1)

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Study on Adsorption Behavior of 12-Phosphotungstic Acid on Silica

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