Equilibrium, Kinetic, and Thermodynamic Studies on Adsorptive

Article pubs.acs.org/IECR

Equilibrium, Kinetic, and Thermodynamic Studies on Adsorptive Desulfurization onto CuICeIVY Zeolite Hua Song,*,†,‡ Youxin Chang,† Xia Wan,†,§ Min Dai,†,§ Hualin Song,*,# and Zaishun Jin*,# †

College of Chemistry and Chemical Engineering and ‡Provincial Key Laboratory of Oil and Gas Chemical Technology, Northeast Petroleum University, Daqing 163318, China § CPE Survey and Design Institute of Xinjiang Oil, Karamay 834000, Xinjiang, China # Key Laboratory of Cancer Prevention and Treatment of Heilongjiang Province, Mudanjiang Medical University, Mudanjiang 157011, Heilongjiang, China ABSTRACT: The CuIY, CeIVY, and CuICeIVY zeolites were successfully prepared and the competive adsorptions of toluene, cyclohexene, and pyridine were investigated. The results indicate that the effects on the metal ion-exchanged Y zeolites for sulfur removal decrease in the order pyridine > cyclohexene > toluene. The CuICeIVY not only has high sulfur adsorption capacity, similar to CuIY, but also has high selectivity for sulfur compounds, similar to CeIVY. The isotherms and kinetics of benzothiophene (BT) adsorption from 1-octane onto CuICeIVY were studied, and the thermodynamic parameters (ΔG, ΔH, ΔS) for the adsorption of BT were calculated. The results show that the isothermal equilibrium can be represented by the Langmuir model and that maximum adsorption capacities (qm) increase with an increase of temperature. The kinetics for the adsorption process can be described by either the Langmuir model or a pseudo-first-order model. The adsorption is spontaneous and exothermic.

1. INTRODUCTION To meet the stringent emission standard stipulated by regulatory organics, ultradeep removal of sulfur from transportation fuels has become imperative for the petroleumrefining industry. Ultralow-sulfur fuels are becoming one of the general trends in fuel energy development.1−4 Moreover, for the rapid development of fuel cells, which also require “no sulfur” fuel, the desulfurization technology needs higher request to meet the stringent specifications.5,6 Conventional hydrodesulfurization (HDS) is highly effective for the removal of thiols, sulfides, and disulfides, but less effective for the removal of thiophene and its derivatives. To meet the new specifications in sulfur content in transportation fuels, the increase of the operating temperature and pressure are required, which would cause a significant reduction of the octane number and consume more olefinic and aromatic molecules.7 Therefore, to produce ultralow-sulfur gasoline to meet the new regulations, other desulfurization processes including adsorption, extraction, oxidation, and bioprocesses have been developed.8−11 Among these methods, adsorptive desulfurization aroused more attention, because adsorption can be accomplished at ambient temperature and pressure, simple to operate.12 An efficient adsorbent is the key to adsorption desulfurization. Hernández-Maldonado and Yang13−17 reported that Cu+-, Ag+-, Ni2+-, and Zn2+-exchanged NaY zeolites are effective for the adsorption of sulfur and the highly selective adsorption of thiophene and its derivatives. Among these metal-exchanged Y zeolites, CuIY shows the highest adsorption capacity. However, the sulfur adsorption capacity decreased rapidly when aromatic compounds were present in the fuel, because the adsorption heat of thiophene (87.864−92.048 kJ mol−1) over CuIY zeolite is very close to that of benzene (83.680−92.048 kJ mol−1), © 2014 American Chemical Society

which leads to aromatics strongly competing with thiophenic sulfur compounds by π-complexation adsorption.18,19 Velu et al.20,21 found that CeIVY shows higher selectivity adsorption for sulfur compounds than for aromatics, but the sulfur capacity is very low. They found that the sulfur compounds are adsorbed over CeIVY zeolites via direct sulfur adsorbent (S−M) interaction rather than via π-complexation. Wang and coworkers22 investigated the selective adsorption of dibenzothiophene (DBT) over Ce/Ni-loaded Y zeolites and found that Ce as a cocation in NiIICeIVY plays a promoting role in sulfur adsorption when coexisting with toluene in the model fuel. Besides the experimental studies, a mathematical modeling technique is an efficient tool for performance analysis of chemical processes and equipment. There are various theoretical and empirical models used to test the experimental data. Developing multiparameter models based on experimental observations is a very useful approach to describe the performance of operating systems. It seems that using this tool in understanding and simulating the adsorptive removal of sulfur compounds from transportation fuels can reduce the cost of experiments and also increases the feasibility and precision of the scaleup of the process. Therefore, the study of adsorption processes from both experimental and theoretical aspects is important. The theoretical models include the Langmuir and statistical rate theories. There are several empirical models for modeling of adsorption kinetics at the solid/solution interface, including the pseudo-first-order, pseudo-second-order, modified pseudo-first-order, two site pseudo-second-order, and Received: Revised: Accepted: Published: 5701

October 8, 2013 January 2, 2014 March 20, 2014 March 20, 2014 dx.doi.org/10.1021/ie403177t | Ind. Eng. Chem. Res. 2014, 53, 5701−5708

Industrial & Engineering Chemistry Research

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photometric detector−gas chromatograph (Shimadzu FPDGC-14C) equipped with a capillary column (PH-1, 60 m × 0.25 mm). To avoid contact with air, the experiments were carried out under N2.

pseudo-n-order ones. The advantages of empirical models are their simplicity and ease of use. To our knowledge, just a few papers have focused on the study of adsorption of sulfur compounds onto CuICeIVY from the viewpoint of the adsorption equilibrium and modeling of the adsorption kinetic. In our former paper,23 the authors have investigated the selective adsorption of TP and BT over bimetal ion-exchanged zeolite CuICeIVY in a fixed-bed unit. In this paper, the effects of toluene, cyclohexene, and pyridine present in model gasoline on BT adsorption onto CuIY, CeIVY, and CuICeIVY zeolites were investigated by a batch method. The effects of the adsorption time, initial BT content, and temperature on desulfurization, including equilibrium isothermal adsorption, kinetics, and thermodynamics of CuICeIVY, were systematically studied.

3. RESULTS AND DISCUSSION 3.1. Effect of Contact Time on Adsorption. The effect of contact time on the sulfur adsorption of CuICeIVY zeolite is shown in Figure 1, using model gasoline FM-1 with an initial

2. EXPERIMENTAL SECTION 2.1. Materials. Benzothiophene (BT, 98%) was purchased from J&K Chemical Ltd., China. NaY zeolite (Si/Al = 5) in powder form was purchased from Nankai University catalyst plant. Cu(NO3)2·3H2O, Ce(NO3)3·6H2O, toluene, pyridine, cyclohexene, and 1-octane were obtained as commercial analytical grade reagents without further purification. 2.2. Model Gasoline Sample. A model gasoline was prepared by adding BT into 1-octane solvent with a total sulfur concentration of 6.3 mmol L−1, represented as MF-1. To investigate the effects of aromatics, nitrides, and olefins on selective adsorptive desulfurization of gasoline over CuICeIVY, CuIY, and CeIVY, several kinds of model gasolines were made by adding certain amounts of toluene, pyridine, and cyclohexene into MF-1, respectively. The obtained model gasolines with a total sulfur concentration of 6.3 mmol L−1 and aromatics, nitrides, and olefins concentration of 15.6 mmol L−1 are denoted MF-2, MF-3, and MF-4, respectively. Model gasolines with different sulfur concentrations of 3.1, 4.7, 7.8, 9.4, 10.9, 12.5, 14.1, and 15.6 mmol L−1 were also prepared. 2.3. Adsorbent Preparation. The ion-exchanged Y zeolites used in this work were prepared using the liquidphase ion-exchange method at room temperature, and the preparation methods are the same as in our previous paper.23 NaY zeolite was used as a starting material. CeIVY and CuIIY zeolites were prepared by ion exchange of NaY with 0.1 mol L−1 Ce(NO3)3 and 0.1 mol L−1 Cu(NO3)2 aqueous solution for 48 h at room temperature, respectively. After ion exchange, the zeolites were filtered, washed thoroughly with deionized water, and subsequently dried at 110 °C for 12 h. Then, the samples were calcined at 550 °C for 4 h in an air atmosphere, and CeIVY zeolite adsorbent was obtained. To obtain CuIY zeolite, the calcined CuIIY needs to be reduced at 190 °C for 3 h in H2. Ce3+ had a higher ion-exchange selectivity than that of Cu2+, and the cation with a higher ion-exchange selectivity should be first exchanged to zeolite.24 Hence, the CuICeIVY was obtained by ion exchange of the CeIVY with 0.1 mol L−1 Cu(NO3)2 aqueous solution as described above. After ion exchange, the zeolites were filtered, washed thoroughly with deionized water, dried at 110 °C for 12 h, and subsequently reduced at 190 °C for 3 h in H2. 2.4. Adsorption Experiment. For each adsorption run, 20 mL of the solution was taken in a conical flask containing 0.2 g of adsorbents. The solution in the flask was agitated at a certain temperature and adsorbed for a certain time. After adsorption, the solution was separated from supernatant liquid and analyzed for residual sulfur concentration with a flame

Figure 1. Effect of contact time on the adsorption of BT onto CuICeIVY (model oil MF-1, 20 mL; TB, 6.3 mmol L−1; adsorbents, 0.2 g; temperature, 30 °C).

BT concentration (c0) of 6.3 mmol L−1 at 30 °C. As can be seen, the adsorption amount of BT (qt) increases rapidly at the beginning of the process and then increases slightly for contact time longer than 60 min. The adsorption equilibrium for CuICeIVY zeolite was achieved after 100 min, and the removal of BT reaches 92.6%. 3.2. Effect of Initial Concentration on Adsorption. The effect of the initial BT concentration on the rate of adsorption at 30 °C is shown in Figure 2. It is observed that the rate of adsorptive removal of BT varied with its initial concentration. At the initial times, adsorption of BT occurs rapidly and then increases gradually with increasing contact time and tends to reach a balance. This indicates that an increase in the initial concentration of BT leads to an increase in the adsorption capacity of BT. The initial rate of adsorption is faster because of the higher BT concentration at the beginning. 3.3. Effect Temperature on Adsorption. The effect of temperature was studied, and the results are shown in Figure 3. It is observed that the adsorption amount of BT (qt) apparently increased and then decreased with the increase in temperature and reached a maximum at 50 °C. The reason may be that increasing the system temperature will greatly increase the diffusion rate of the adsorbate, owing to the decrease in the viscosity of the solution. However, when the temperature is >50 °C, the desorption rate will increase sharply, which causes the qt to decrease. As can be seen, the BT desulfurization rate of the adsorbents was found to be 99.2% at 50 °C. 3.4. Competitive Adsorption. To investigate the effect of toluene, cyclohexene, and pyridine present in model gasoline on BT adsorption, CuIY, CuICeIVY, and CeIVY zeolites were investigated. The results are shown in Figure 4. As we can see, for desulfurization with model gasoline FM-1, CuICeIVY has a high desulfurization ratio similar to that of CuIY and much 5702

dx.doi.org/10.1021/ie403177t | Ind. Eng. Chem. Res. 2014, 53, 5701−5708


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Figure 4. Effect of toluene, cyclohexene, and pyridine on the sulfur adsorption performance onto CuIY, CeIVY, and CuICeIVY (model oil, 20 mL; TB, 6.3 mmol L−1; adsorbent, 0.2 g; temperature, 30 °C; adsorption time, 60 min).

decrease in the order CuIY > CuICeIVY > CeIVY. This indicates that Ce-exchanged Y zeolites have higher selectivity for sulfur compounds than for olefins. Previous research has shown that the CuIY zeolites are based on their π-complexation type interactions with the sulfur species,18,19 and Ce-exchanged zeolites form S−M bonds between the sulfur atoms of sulfur compounds and the Ce4+.25 BT can be adsorbed by CuIY via π-complexation, but toluene and cyclohexene also can be adsorbed via π-complexation, which leads to competitive adsorption onto CuIY. Ceexchanged zeolites can form via direct sulfur adsorbent (S− M) interaction, so the effect of toluene and cyclohexene on CuICeIVY and CeIVY is less than that on CuIY. From what has been discussed above, the CuICeIVY zeolite not only has a high capacity for sulfur removal similar to CuIY but also shows high selectivity for sulfur compound adsorption similar to CeIVY . For all of the adsorbents, the desulfurization results with model gasoline FM-4, which contains pyridine, show a sharp drop in the capacity for sulfur removal, implying that the presence of nitrogen compounds can greatly affect selective adsorptive desulfurization and that ion-exchanged Y zeolites show a preferable adsorption for nitrogen compounds rather than sulfur compounds. This may be due to the pyridine interacting strongly with the acid sites of zeolite Y. In conclusion, the effect on the metal ion-exchanged Y zeolites for sulfur removal decreases in the order pyridine > cyclohexene > toluene. 3.5. Equilibrium Isothermal Adsorption. In this study, the equilibrium isothermal adsorption was conducted by using model gasoline (3.1, 4.7, 6.3, 7.8, 9.4, 10.9, 12.5, 14.1, and 15.6 mmol L−1 BT, respectively) with a contact time of 3 h at 20, 30, and 40 °C. Figure 5 shows the relationship of the equilibrium adsorptive amount of BT per gram of CuICeIVY zeolite (qe, (mmol g−1)) against the equilibrium concentration of BT in the solution (ce, (mmol L−1)). As seen from Figure 5, initially, qe increases sharply with an increase of ce, then changes slightly for ce is higher than 1.5 (mmol L−1). This indicates that most of the BT could be removed by CuICeIVY zeolite for lower initial concentration of BT. However, with a high enough concentration of BT, the adsorption capacity of CuICeIVY zeolite reaches its saturation and qe tends to the maximum adsorption (qm). From Figure 5, one can estimate qm to be

Figure 2. Effect of initial concentration of BT on the adsorption of BT onto CuICeIVY (model oil with different initial concentrations of BT, 20 mL; adsorbent, 0.2 g; temperature, 30 °C).

Figure 3. Effect of temperatrue on the adsorption of BT onto CuICeIVY (model oil MF-1, 20 mL; TB, 6.3 mmol L−1; adsorbent, 0.2 g; adsorption time, 60 min).

higher than that of CeIVY, indicating the introduction of Cu ion to Ce-exchanged Y zeolite improves its performance of sulfur removal. Desulfurization with model gasoline FM-2, which contains toluene, was investigated. It can be seen that there is a noticeable decrease of CuIY for sulfur removal as compared to absorbing model gasoline without toluene. However, the influence on CuICeIVY and CeIVY zeolites is not obvious. The loss of desulfurization rate shows decrease in the order CuIY > CuICeIVY > CeIVY. This indicates that Ce4+ can improve the selectivity of ion-exchanged zeolites for removing sulfur in the model gasoline with toluene. The existence of olefins in fuels would strongly decrease the adsorption of sulfur compounds. Competitive adsorption between cyclohexene and BT was investigated with model gasoline FM-3, which contains cyclohexene. A similar result was obtained with model gasoline FM-3. It can be seen that the loss of desulfurization rate due to the existence of olefins shows a 5703



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Table 1. Langmuir Adsorption Isotherm Parameters parameter

20 °C

30 °C

40 °C

qm (mmol g−1) KL (L mmol−1) R2

1.018 3.653 0.999

1.189 2.456 0.998

1.238 1.838 0.999

Figure 5. Equilibrium isothermal adsorption of benzothiophene onto CuICeIVY zeolite at different temperatures (20, 30, and 40 °C).

around 0.98, 1.12, and 1.21 mmol g−1 at 20, 30, and 40 °C, respectively. The Langmuir model is probably the best known and most widely applied sorption isotherm. It has produced good agreement with a wide variety of experimental data. In general, the interaction between the adsorbent and the adsorbates in the solution contains two processes, namely, adsorption and desorption. The rates of adsorption (vads) and desorption (vd) can be described as26 νads = kadsct(1 − θ )

(1)

νd = kdθ

(2)

Figure 6. Langmuir model linear equations obtained by using the linear fitting method for the adsorption of benzothiophene over CuICeIVY zeolite at different temperatures (20, 30, and 40 °C).

(DBT) in hexadecane onto NaY zeolite is increased with increasing temperature. The similar tendency of increasing temperature benefited the adsorption process observed by Bai.27 They found that as the temperature increased, qm increased, showing that the affinity between adsorbent and adsorbates enhanced with increasing temperature. The values of qm obtained from Figure 6 are in agreement with the results in Figure 5. The Langmuir constant KL decreases with the increase in temperature as observed from Table 1, indicating the adsorptive process is exothermic. 3.6. Adsorption Kinetics. 3.6.1. Pseudo-First-Order, Pseudo-Second-Order, and Langmuir Models. The transport of adsorbate from the solution phase into the pores of the adsorbent is a complicated process, which contains film or external diffusion, pore diffusion, surface diffusion, and adsorption on the pore surface. CuICeIVY binds the organic sulfur compounds through two types of adsorption modes: πcomplexation between Cu+ ions and sulfur rings13 and direct coordination via S atoms with Ce4+ (S−M) interaction.25 The analysis of adsorption kinetics and mass transfer process is important in designing adsorption system. Thus, to gain a better understanding of the adsorption process, various kinetic models are used to test the experimental date. The frequently used kinetic models, namely, pseudo-first-order and pseudosecond-order models, were used to investigate the adsorption of sulfur compounds over CuICeIVY. The pseudo-first-order and pseudo-second-order rate Lagergren models are given by eqs 5 and 6

where kads and kd are the adsorptive and desorptive constants, ct is the concentration of adsorbate at time t, and θ is the dimensionless ratio of coverage of the surface of adsorbent, which can be expressed by the division of adsorptive amount of adsorbate at time t (qt) to its maximum (qm). At the adsorption equilibrium, the values of vads and vd are equal and the concentration of adsorbate (ct) and the adsorption amount (qt) can be written as ce and qe. Therefore, one can obtain the Langmuir isothermal model as KLqmce qe = 1 + KLce (3) where KL is the Langmuir constant and KL = kads/kd. The Langmuir equation could represented in the linear form: ce 1 1 = ce + qe qm KLqm (4) The plots of ce/qe against ce in eq 4 at different temperatures show a good linear relationship (Figure 5). According to eq 4, one can deduce the parameters KL and qm from the slope and intercept of the straight lines. The results and the corresponding regression coefficient (R2) at different temperature are shown in Table 1. The regression coefficients at different temperatures are 0.999, demonstrating that the experimental data meet the Langmuir adsorptive isothermal equation very well. From Figure 6, one can find that the slope of the line decreases slightly with the increase in temperature; this means the qm increases slightly with the increase in temperature. This is accordance with the result obtained by Jiang et al.31 The qm for the adsorption of dibenzothiophene

dqt dt dqt dt

= k1(qe − qt )

(5)

= k 2(qe − qt )2

(6)

−1

where qt (mmol g ) is the amount of adsorbed sulfur on the adsorbent at time t (min), qe (mmol g−1) is the equilibrium 5704



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adsorption of the pseudo-first-order or pseudo-second-order adsorption, k1 (min−1) is the pseudo-first-order rate constant, and k2 (g mmol−1 min−1) is the pseudo-second-order rate constant. Integrating and applying boundary conditions, t = 0 and qt = 0 to t = t and qt = qt, eqs 5 and 6 can be take the form

qt = qe(1 − e−k1t ) qt =

(7)

k 2qe2t 1 + k 2qet

(8)

Although the pseudo-first-order and pseudo-second-order models are very simple to describe and analyze for adsorption kinetics, they are purely empirical and cannot offer any detailed information about the adsorption mechanism. In this study, the Langmuir model is also used to study the adsorption kinetics. By combining eqs 1 and 2, the adsorptive rate (vt) can be described as follows: νt = νads − νd (9)

Figure 7. Nonlinear fitting according to pseudo-first-order, pseudosecond-order, and Langmuir models for CuICeIVY adsorption kinetics (TB, 6.3 mmol L−1; temperature, 40 °C).

Meanwhile, changing the temperature of the system will lead to another equilibrium capacity of the adsorbent as shown in Table 1. The kinetic data have been analyzed by using the pseudo-first-order rate equation and a nonlinear curve fitting analysis method (Figure 8). The model parameters are listed in Table 3. As seen from Table 3, the theoretical adsorptive amount of BT on CuICeIVY zeolite (qe) and the pseudo-firstorder rate constant (k1) increase with an increase in temperature. The results indicate the process of adsorption of BT on CuICeIVY zeolite is more effective at higher temperatures because both the amount of adsorption and the rate of adsorption increase with an increase in the study temperature. The influence of temperature on the adsorptive rate is calculated using the Arrhenius equation

The adsorptive rate can be also written in the differential form: νt =

dqt (10)

dt

Thus, on the basis of eqs 1, 2, 9, and 10, one can obtain dqt dt

= kadsct(1 − θ ) − kdθ

(11)

Using θ = qt/qmand qt = (c0 − ct)/W is represented by 12: ⎛ q q W ⎞⎛ q ⎞ = kads⎜c0 − t ⎟⎜⎜1 − t ⎟⎟ − kd t dt qm V ⎠⎝ qm ⎠ ⎝

dqt

(12)

After rearrangement and integration from eq 12, qt can be expressed as26 qt = qe

⎛ E ⎞ k1 = k 0 exp⎜⎜ − a ⎟⎟ ⎝ R gTk ⎠

qL(1 − e αt ) qe − qL e

αt

(14)

where k0 is the temperature-independent factor (g mmol−1 min−1), Ea is the activation energy of adsorption (kJ mol−1), and Rg and Tk are the universal gas constant (8.314 (J mol−1 K−1)) and the temperature in Kelvin (K), respectively. Equation 14 can be rearranged in the following form:

(13)

where α = (kdKLW(qL − qe))/(Vqm) and qe and qL are the two solutions of the second-order polynomial expression P(pt) = KLWq2t − (KLqmW + KLc0V + V)qt + KLc0Vqm during the rearrangement. In the following, three adsorption kinetic models including a pseudo-first-order model, a pseudo-second-order model, and the Langmuir model were evaluated to fit the kinetics of the adsorption process onto CuICeIVY. As shown in Figure 7, all three adsorption kinetic models give very good fits to the experimental data. The correlation coefficient R2 (Table 2) of the pseudo-first-order model and the Langmuir model are both 0.999 and higher than the correlation coefficient R2 of the pseudo-second-order model (0.963), showing that the pseudofirst-order and Langmuir models are slightly more suitable for describing the adsorption kinetics of adsorptive desulfurization. The value of qe calculated from the Langmuir model is 0.632 mmol g−1 and very close to the qe value (0.625 mmol g−1) of the pseudo-first-order model. This indicates the Langmuir model is fit to describe the adsorption kinetics. 3.6.2. Effect of Temperature on Adsorption Kinetics. Typically, the temperature has two major effects on the adsorption process.31 Increasing the temperature of the system is known to greatly increase the rate of diffusion of the adsorbate, owing to the decrease in the viscosity of the solution.

ln k1 = ln k 0 −

Ea R gTk

(15)

Typically, the slope of the plot of lnk1 versus 1/Tk is used to evaluate the activation energy. According to the data shown in Table 3, the evaluated activation energy is shown in Figure 9. The low activation energy (11.689 kJ mol−1) determined for the adsorption of BT on CuICeIVY zeolite in octane suggests the adsorption is mass transfer controlled. The result is in accordance with the DBT adsorption on NaY zeolite in hexadecane.26 3.6.3. Diffusion Study. In the adsorption process, adsorption behavior may sometimes be represented by the intraparticle diffusion model. The intraparticle diffusion model28 is written as

qt = k it 1/2 + C

(16)

where ki is the intraparticle diffusion rate constant (mmol g−1 min−0.5) and C (mmol g−1) is the constant related to the energy of adsorption. The plot of qt against t1/2 in eq 16 is shown in 5705



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Table 2. Pseudo-First-Order, Pseudo-Second-Order, and Langmuir Model Parameters (TB, 6.3 mmol L−1; Temperature, 40 °C) pseudo-first-order

pseudo-second-order

Langmuir

k1(min−1)

qe (mmol· g−1)

R2

k2 (g mmol−1 min−1)

qe (mmol g−1)

R2

kads (L g−1 min−1)

qe (mmol g−1)

R2

0.0634

0.625

0.999

0.0779

0.797

0.963

0.00178

0.632

0.999

Figure 10. Plot of qt versus t1/2.

Figure 8. Nonlinear fitting of the adsorption kinetics of BT over CuICeIVY at 20, 30, and 40 °C (TB, 6.3 mmol L−1).

ΔG = −R gTk ln KL

Table 3. Pseudo-First-Order Kinetic Parameters Obtained at Different Temperatures by Nonlinear Fitting parameter

20 °C

30 °C

40 °C

k1 (min−1) qe (mmol g−1) R2

0.0496 0.558 0.998

0.0578 0.593 0.997

0.0674 0.623 0.999

(17)

According to the KL shown in Table 1, the value ΔG can be deduced to be −19.983, −19.665, and −19.560 kJ mol−1 for 20, 30, and 40 °C, respectively. These results indicate that the adsorption of BT on CuICeIVY zeolite is a spontaneous process. ΔG = ΔH − TkΔS; hence, eq 17 can be rearranged in the form ln KL = −

ΔH ΔS + R gTk Rg

(18)

where ΔH is the enthalpy of adsorption (kJ mol−1) and ΔS is the enthalpy change of adsorption (J mol−1 K−1). The relationship between ln KL and 1/T has been plotted in Figure 11, which is based on the data shown in Table 1, and then, the values of ΔH and ΔS can be deduced by the slope and intercept of the line in Figure 11. The results of ΔH and ΔS are −26.221 kJ mol−1 and −21.403 J mol−1 K−1, respectively. The negative values of ΔH and ΔS indicate the adsorption of BT on

Figure 9. Plot of ln k1 versus 1/Tk.

Figure 10. The plots are not linear over the whole time range, implying that more than one process is controlling the adsorption process. The initial portion indicates the boundary layer diffusion effect (surface adsorption),29 and the final linear portion is the result of intraparticle diffusion effect.30 The final portion of the plots is nearly parallel (with a slope ranging from 0.002 to 0.009 mmol g−1 min−0.5), suggesting that the rates of adsorption of BT into the pores of zeolite are comparable at all temperatures. 3.7. Thermodynamics. The Gibbs free energy change (ΔG) can be expressed by the Langmuir constant KL in the form32

Figure 11. Plot of ln KL versus 1/Tk. 5706



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CuICeIVY zeolite is an exothermic process and a decrease in the degree of freedom of the adsorbed species.

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4. CONCLUSIONS The CuICeIVY zeolite was successfully prepared by liquid ion exchange. The experimental results indicated that CuICeIVY zeolite could effectively remove refractory sulfur compounds from model gasoline, and the BT desulfurization rate of the adsorbents can reach 99.2% at 50 °C. The adsorption equilibrium for CuICeIVY zeolite was achieved after 100 min. The adsorption capacity of BT is increased with increasing initial concentration of BT and adsorption temperature. The effect on CuICeIVY zeolite for sulfur removal is in the following order: pyridine > cyclohexene > toluene. The equilibrium isotherm shows that the adsorption can be represented by the Langmuir model and that the maximum adsorption capacity (qm) increases with an increase of temperature, whereas KL decreases with an increase of temperature. Kinetic studies indicate that the adsorption process can be well described by either the Langmuir model or the pseudo-first-order model. The adsorptive capacity (qe) and the pseudo-first-order kinetic constantat (k1) at different temperatures increase with an increase in the temperature, and the adsorption is mass transfer controlled. Analysis of the thermodymanics indicates that the adsorption of BT on CuICeIVY zeolite is a spontaneous process and exothermic.

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AUTHOR INFORMATION

Corresponding Authors

*(Hualin Song) Phone/fax: +86 0453 6892892. E-mail: [email protected]. *(Zaishun Jin) E-mail: [email protected]. *(Hua Song) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (81172204).

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