Reaction kinetics and mechanism for hydration of cyclohexene over ion-exchange resin and H-ZSM-5

Chemical Engineering Journal 175 (2011) 423–432

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Reaction kinetics and mechanism for hydration of cyclohexene over ion-exchange resin and H-ZSM-5 Xianglei Shan, Zhenmin Cheng ∗ , Peiqing Yuan State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China

a r t i c l e

i n f o

Article history: Received 13 May 2011 Received in revised form 3 September 2011 Accepted 10 September 2011 Keywords: Cyclohexene hydration Ion-exchange resin Zeolite H-ZSM-5 Reaction mechanism

a b s t r a c t The kinetics of liquid-phase cyclohexene hydration over ion-exchange resin and H-ZSM-5 were, respectively, investigated in a batch autoclave reactor. In order to identify the reaction mechanism involved in the heterogeneous cyclohexene hydration, a pseudo-homogeneous (PH) model and three heterogeneous models based on the Langmuir–Hinshelwood–Hougen–Watson (LHHW) mechanism and Eley–Rideal (ER) mechanism were established to correlate the kinetic data. The results indicated that the heterogeneous models exhibited a more satisfactory agreement with the experimental data than the PH model. According to the kinetic studies and spectroscopic analysis, it is proposed that cyclohexene hydration catalyzed by H-ZSM-5 belongs to the ER mechanism, which accounts that the reaction is between adsorbed water and bulk cyclohexene, while it belongs to the LHHW mechanism over the ion-exchange resin. Nevertheless, the activation energies are similar for the two catalysts, which implies similar controlling steps are encountered in the reaction. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The liquid-phase hydration of cyclohexene is an industrially important reaction in the production of cyclohexanol, which has emerged as an important intermediate in the production of adipic acid and caprolactame [1]. It is well accepted that hydration of alkenes belongs to the electrophilic addition reaction, and acid catalysts are required to provide enough protons for the proceeding of hydration reaction. For the hydration of cyclohexene, zeolite HZSM-5 and ion-exchange resin are the widely applied catalysts. A number of studies on cyclohexene hydration using various catalysts have been reported. Okuhara et al. [2] and Nakato et al. [3] investigated the catalytic performance of various catalysts and concluded that H-ZSM-5 zeolite exhibited a better catalytic performance than ion-exchange resins and heteropoly acids. Ogawa et al. [4] studied the hydration of cyclohexene with alkylchlorosilanetreated H-ZSM-5 and claimed that the modified sample could obviously accelerate the reaction with suppression of the formation of by-products. In addition, a patent by Yoshikazu and Tokitaka [5] studied the solvent effect on the hydration of cyclohexene and found that co-solvents such as ethylene glycol monophenyl ether and isophorones could improve the solubility of cyclohexene evidently.

∗ Corresponding author. Tel.: +86 21 64253529; fax: +86 21 64253528. E-mail address: [email protected] (Z. Cheng). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.09.049

However, the study on the kinetics for cyclohexene hydration is not well reported in the literature. By defining the reaction rate equation as: −rCHE = k1 cCHE − k−1 cCHOL , Panneman and Beenackers [6–8] studied the intrinsic kinetics of cyclohexene hydration using macroporous Amberlite XE 307 in a packed bed reactor at temperatures between 353 and 413 K and a pressure of 20 bar. In their studies, the reaction was carried out in a single liquid phase by pre-saturating water with cyclohexene. In addition, the solvent effects of sulfolane were investigated and the kinetic rate constants were measured as a function of solvent composition (0–90 mol% sulfolane in water). Zhang et al. [9] also studied the kinetics of cyclohexene hydration with H-ZSM-5 catalyst (SiO2 /Al2 O3 = 30–50) in a stainless steel reactor at temperatures between 363 and 393 K, and a simple first order kinetic model based on aqueous phase concentrations of cyclohexene and cyclohexanol were proposed to explain the observed kinetic results. It is conceivable that hydration of cyclohexene catalyzed by ion-exchange resin and H-ZSM-5, which are two different catalytic materials, should follow different reaction mechanisms. However, the kinetic studies on these two catalysts in the literature are insufficient since the rate equation is exclusively expressed by the power-law formula, which is hard to reveal the reaction mechanism which is pertinent to the surface adsorption and reaction. It is well known that the Langmuir–Hinshelwood– Hougen–Watson (LHHW) mechanism and the Eley–Rideal (ER) mechanism have been widely used in the kinetic study of heterogeneous catalytic system. Based on the surface reaction between two adsorbed species, the LHHW mechanism predicted

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X. Shan et al. / Chemical Engineering Journal 175 (2011) 423–432

the kinetic data very well in a number of reactions catalyzed by heterogeneous acid catalysts, for instance, the hydrolysis of ethyl benzoate [10], the liquid-phase dimerization of isoamylenes [11], the liquid-phase hydrogenation of cinnamaldehyde [12], the esterification of lactic acid with ethanol [13] and esterification of propanol with ethanoic acid [14], the synthesis of tert-amyl methyl ether [15], and the catalytic hydrogenation of d-lactose to lactitol [16]. The ER mechanism, as a modified version of the LHHW mechanism, takes into account the possibility that the reaction could proceed between an adsorbed reactant molecule and another reactant molecule in the bulk phase. Kalló and Mihályi [17] found that the 1-butene hydration over acidic zeolite and ionexchange resin was a rate-determining surface reaction between the adsorbed water and gas-phase 1-butene, which belongs to the ER mechanism. Kirumakki et al. [18] studied the esterification of alcohols with acetic acid and found that the reaction also followed the Eley–Rideal pathway. Sert and Atalay [19] also found that the kinetics of the esterification of acetic acid with butanol followed the Eley–Rideal mechanism. Yip and Hu [20] also proposed LHHW and ER mechanism for the cyclohexanone ammoximation over a clay-based titanium silicalite-1 (TS-1) composite. However, Umar et al. [21] studied the synthesis of ethyl tert-butyl ether (ETBE) and found that both LHHW model and ER model were unable to predict the behavior of the etherification reaction. It is noted that the mechanism of the reaction catalyzed by heterogeneous catalysts is rather ambiguous from case to case, which should be dependent on the catalyst and the reactants involved. At present, no information is available in the open literature describing the kinetics of cyclohexene hydration with a developed heterogeneous kinetic model. In this work, two different catalytic materials, ion-exchange resin and H-ZSM-5, will be used as the catalyst for the cyclohexene hydration in solid–liquid–liquid mode. The reaction kinetics of the hydration reaction will be investigated based on the LHHW mechanism and the ER one, and the reaction mechanism of the hydration reaction on the two catalysts will be respectively proposed, which is helpful for future directions related to the improvement and optimization of cyclohexene hydration catalyzed by ion-exchange resin and H-ZSM-5. 2. Experimental 2.1. Chemical and materials Cyclohexene was commercially supplied by Shanghai Gonghe Chemical Co., Ltd. with purity greater than 0.950 in mass fraction. Sulfonated cation exchange resin of 0.4–1.0 mm in diameter was purchased from Jiangsu Success Resin Co., Ltd. with a BET surface area of 32 m2 g−1 , a pore volume of 0.254 cm3 g−1 and a maximum service temperature of 453.15 K. Macroporous ion-exchange resin was washed thoroughly with de-ionized water to remove the impurities, and then dried in a vacuum oven at 343 K for 6 h to remove the residual moisture. Compared with the amount of water existed in the reaction system, the amount of residual water after resin drying is so small that it is ignored. The H-ZSM-5 catalyst (SiO2 /Al2 O3 = 25) of 50–700 nm in diameter was supplied by Nankai University catalyst Co., Ltd. with a BET surface area of 333 m2 g−1 and a pore volume of 0.058 cm3 g−1 . Before the H-ZSM-5 was used, the catalyst was calcined at 773 K for 4 h. Washed and dried resin and calcined H-ZSM-5 were stored in desiccators for future use. 2.2. Procedure The hydration reaction was performed in a stainless steel reactor of 500 ml equipped with agitation and temperature control. The experimental conditions used for the present work were listed in

Table 1 Experimental conditions used in this work for hydration of cyclohexene. Catalyst

Ion-exchange resin

H-ZSM-5

Stirring speed/rpm Catalyst loading in the aqueous phase Temperature/K Initial volume ratio of cyclohexene to water

300–900 10–40% (w/w)

500–1500 10–40% (w/w)

383–413 1:2–4:1

373–403 1:2–4:1

Table 1. In each run, a specified amount of catalyst and reactants were first charged into the reactor, and then the reactor was filled with nitrogen at the pressure of 1.0 MPa to ensure the reactants kept liquid under the reaction temperature, and then the reaction mixture was heated up to the desired temperature by heating jacket within 1 h. In order to avoid the occurrence of reaction and the deposition of catalysts, a slow stirring speed of 100 rpm was used during the heating process, which was confirmed by the preliminary experiments. As the reaction temperature was reached, the speed of agitation was increased up to the desired level and the corresponding time was regarded as the zero reaction time. Since there are always two liquid phases existed in the reactor, it is difficult to sample from the reactor during the reaction process. After a fixed reaction time, the autoclave was cooled to ambient temperature by cooling water within 1 h, and then all the mixture in the reactor was withdrawn and centrifuged at high rotation speed for 5 min to ensure that the solid catalyst was separated from the liquid. The obtained liquid consisted of the aqueous phase and the organic phase. The aqueous phase was then extracted by certain amount of ethyl acetate in order to obtain the cyclohexanol and cyclohexene dissolved in the aqueous samples, and then the extract liquor was mixed with the organic phase for composition analysis. In order to minimize the experimental error, each experiment was at least repeated for three times, and the average value of reliable experimental results was adopted. 2.3. Analysis A gas chromatograph (GC) equipped with a flame ionization detector (FID) was used to analyze the obtained samples. The SUPELCOWAXTM 10 capillary column of 60 m length and 0.25 mm diameter was used for chromatographic analysis. Nitrogen gas was used as the carrier gas at 0.07 MPa. The column temperature was first kept at 393 K for 2 min, and was then heated at 15 K min−1 to 473 K and kept for 14 min. Meanwhile, the injector and detector temperatures were set at 523 K and 553 K, respectively. Each GC analysis was replicated three times to ascertain the accuracy of the obtained data. Based on the analysis results, it was found that both H-ZSM-5 and resin catalysts resulted in high cyclohexanol selectivity (>98%), thus the side reactions in the system were ignored. A Fourier transform infrared (FT-IR) spectrometer, Nicolet 5700, was used to give the infrared spectra of the two catalysts. In order to investigate the adsorption of the reactants on the active sites of the catalysts, the samples in contact with different reactants were freeze-dried before FT-IR analysis. The measurement was carried out by the KBr wafer technique in the range of 400–4000 cm−1 . 3. Theoretical basis In order to study the kinetic behavior and reaction mechanism of the heterogeneous hydration of cyclohexene catalyzed by ion-exchange resin and H-ZSM-5, the kinetics of the hydration reaction is expressed using a simple pseudo-homogeneous (PH) model and three more complex models based on the Langmuir–Hinshelwood–Hougen–Watson (LHHW) mechanism

X. Shan et al. / Chemical Engineering Journal 175 (2011) 423–432 Table 2 Reaction mechanisms and rate equations for hydration of cyclohexene. Model

Rate equation

1

−rCHE = k0 exp

2

−rCHE =

3 4

 −E   RT

aCHE aH2 O −

aCHOL Keq



k0 exp(−E/RT )(aCHE aH O −aCHOL /Keq ) 2

Mechanism Pseudo-homogeneous (PH)

(1+KCHE aCHE +KH O aH O +KCHOL aCHOL )2 2 2

Langmuir–Hinshelwood– Hougen–Watson (LHHW)

−rCHE =

k0 exp(−E/RT )(aCHE aH O −aCHOL /Keq ) 2 1+KH O aH O +KCHOL aCHOL

Eley–Rideal (ER)

−rCHE =

k0 exp(−E/RT )(aCHE aH O −aCHOL /Keq ) 2 1+KCHE aCHE +KCHOL aCHOL

Eley–Rideal (ER)

2

2

and Eley–Rideal (ER) mechanism. The detailed illustrations are as follows. Model 1. The hydration system is assumed to be homogeneous and the adsorption of reactants on the catalyst is ignored. Using a PH model expressed in Eq. (1), the reaction rate equation is simply expressed as the concentrations of reactants and product in the bulk aqueous phase. C6 H10 + H2 O  C6 H12 O

425

the adsorption constant of ith component, and the subscripts ‘CHE’ and ‘CHOL’ denote the cyclohexene and cyclohexanol, respectively. Due to the narrow range of temperature studied in this work, it is assumed that the adsorption constants of reactants and product are constant over the range of our interest. In these models, the adsorption sites of the catalyst are assumed to be equivalent, and all the adsorption sites on the catalyst are assumed to be occupied. In addition, the reaction equilibrium constant Keq determined experimentally is used in the models, and the temperature dependence of the rate constant is expressed by the Arrhenius law.It is well known that the solubility of cyclohexene in the water is extremely small and the cyclohexene is almost immiscible with the water, therefore it forms a triphasic reaction system by the two liquid phases and the solid catalyst. By convention, the hydration reaction is considered to occur in the aqueous phase and the compositions of reactants and product in the aqueous phase can be calculated by the liquid–liquid equilibrium (LLE) [22]. In this work, the Dortmund modified UNIFAC method is used to calculate the concentration and corresponding activity coefficients of reactants in the aqueous phase at equilibrium [23].

(1)

Model 2. This model follows the LHHW mechanism (see Eqs. (2)–(5)), where [A] is the adsorption site on the surface of the catalyst. Cyclohexene and water are adsorbed on the catalyst independently, followed by surface reaction to form cyclohexanol. Finally, the cyclohexanol is desorbed and diffuses to the bulk liquid phase. C6 H10 + [A]  [A·C6 H10 ]

(2)

H2 O + [A]  [A·H2 O]

(3)

[A·C6 H10 ] + [A·H2 O]  [A·C6 H12 O] + [A]

(4)

[A·C6 H12 O]  C6 H12 O + [A]

(5)

4. Results and discussion 4.1. Chemical reaction equilibrium The equilibrium constant Keq for the hydration of cyclohexene is determined through the following expression:



Keq =

aCHOL aCHE aH2 O





= eq

xCHOL xCHE xH2 O

  eq

CHOL CHE H2 O



(13) eq

Model 4. This model is also based on the ER mechanism (see Eqs. (9)–(11)). The difference is that in this model cyclohexene is assumed to be the adsorbed specie on the surface of the catalyst, which reacts directly with the water molecular in the aqueous phase to form cyclohexanol.

where xi and  i are the molar fraction and corresponding activity coefficient of the ith component at equilibrium in the aqueous phase, which are calculated by the Dortmund modified UNIFAC method. For cyclohexene hydration with ion-exchange resin and HZSM-5 as catalysts, the activity coefficient of water in the aqueous phase is approximately equal to 1, the molar fractions and activity coefficients of cyclohexene and cyclohexanol in the aqueous phase and the equilibrium constants corresponding to different reaction temperatures are listed in Table 3. For both ion-exchange resin and H-ZSM-5 catalysts, the equilibrium constants varied between 0.1 and 0.3 in the temperature range from 393.15 K to 433.15 K, which are in agreement with the values reported by Panneman and Beenackers [8]. In addition, it is concluded that the reaction equilibrium constants with different catalysts are similar at the same temperature. Equilibrium constants can also be expressed in terms of the ) standard Gibbs free energy of reaction (r Gm

C6 H10 + [A]  [A·C6 H10 ]

Ka = exp

Model 3. The hydration reaction is proposed to follow the ER mechanism (see Eqs. (6)–(8)) by assuming that cyclohexene from the organic phase could directly contact and react with water adsorbed on the catalyst to form cyclohexanol. H2 O + [A]  [A·H2 O]

(6)

C6 H10 + [A·H2 O]  [A·C6 H12 O]

(7)

[A·C6 H12 O]  C6 H12 O + [A]

(8)

(9)

[A·C6 H10 ] + H2 O  [A·C6 H12 O]

(10)

[A·C6 H12 O]  C6 H12 O + [A]

(11)

The reaction rates for the heterogeneously catalyzed cyclohexene hydration can be calculated by the following equation: (−rCHE )V = NCHE0

 dx

CHE

dt



(12)

where −rCHE is the reaction rate of cyclohexene, V is the volume of the reacting mixture, NCHE0 is the initial moles of cyclohexene, xCHE is the conversion of cyclohexene, and t is the reaction time. Based on the above reaction mechanism, the rate equations obtained are listed in Table 2, where k0 is the preexponential factor, E is the activation energy of cyclohexene hydration, T is the reaction temperature, ai is the activity of the ith component in the aqueous phase, Keq is the equilibrium constant in terms of activity, Ki is



−

 r Gm RT





= exp

−

  r Sm r Hm + RT R



(14)

 and  S  are the standard enthalpy of reaction and where r Hm r m standard entropy of reaction for the cyclohexene hydration. In order to obtain the equilibrium constants and the reaction enthalpy, the experiments were lasted long enough so that chemical equilibrium was reached. In Fig. 1, the logarithm of equilibrium constants obtained with these two catalysts are plotted against the reciprocal of the absolute temperature. It can be seen that the equilibrium constant decreases with increasing temperature, which indicates that the hydration of cyclohexene is an exothermic reaction. Assuming that the standard enthalpy of the reaction is independent of  and  S  of the hydration reaction can be temperature, the r Hm r m obtained through the following equations:



Keq1 = exp 3715.23



1 − 10.775 T

(15)

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X. Shan et al. / Chemical Engineering Journal 175 (2011) 423–432

Table 3 Calculated molar fraction, xi , and activity coefficient,  i , of cyclohexene and cyclohexanol by Dortmund modified UNIFAC corresponding to different equilibrium conversions, XCHE , and reaction temperature, T, with resin and H-ZSM-5 as catalysts.a xCHE × 103

xCHOL × 103

 CHE

 CHOL

Keq

0.207 0.229 0.187 0.135

4.515 4.455 4.407 4.372

6.976 6.302 5.535 4.724

200.8 204.1 208.4 211.5

27.10 24.09 22.42 21.59

0.2158 0.1727 0.1364 0.1113

9.489 ± 0.160 8.318 ± 0.221 7.231 ± 0.196

4.603 4.506 4.438

7.897 6.630 5.774

190.3 195.8 200.7

31.12 29.61 27.58

0.2838 0.2199 0.1775

Catalyst

T/K

XCHE /%

Resin (a)

403.15 413.15 423.15 433.15

8.611 7.908 7.039 6.053

H-ZSM-5 (b)

393.15 403.15 413.15

± ± ± ±

a Reaction conditions: catalyst loading: 30% (w/w) in the aqueous phase; initial volume ratio of cyclohexene to water: 1:1; reaction time: 5 h; stirring speed: (a) 700 rpm and (b) 1000 rpm.



Keq2 = exp 3674.86



1 − 10.607 T

(16)

The reaction enthalpy for the ion-exchange resin and HZSM-5 catalyst is estimated to be −30.9 ± 14.1 kJ mol−1 and −30.6 ± 12.7 kJ mol−1 , respectively, which are in good agreement with the theoretically calculated value of −30.2 kJ mol−1 from the standard free energy of formation. In the previous studies, Panneman and Beenackers [8] reported a reaction enthalpy of −37.3 kJ mol−1 for cyclohexene hydration catalyzed by ionexchange resin, and Zhang et al. [9] obtained a reaction enthalpy of −47.0 kJ mol−1 for cyclohexene hydration catalyzed by H-ZSM5. It is noted that the reaction enthalpies obtained here are slightly lower than the values reported in the literature, which is probably due to the different methods used for calculation of the concentration of components at equilibrium. Panneman and Beenackers [8] used the original UNIFAC method to calculate the equilibrium with the presence of cosolvent sulfolane, while Zhang et al. [9] ignored the change of cyclohexene concentration

-1.6

in the aqueous phase. Compared the reaction enthalpy obtained here with that reported in the literature, it is evident that the Dortmund modified UNIFAC is much more suitable to calculate the liquid–liquid equilibrium of cyclohexene hydration. 4.2. Influence of operating variables For the hydration of cyclohexene, the catalytic performances of ion-exchange resin and H-ZSM-5 catalyst were studied in detail. The effects of stirring speed, catalyst loading, reaction temperature, and initial volume ratio of reactants on the hydration reaction were determined and detailed information for the kinetics study was given below. 4.2.1. Elimination of mass-transfer resistance For the determination of the intrinsic kinetics of cyclohexene hydration, both the internal and the external mass transfer resistances should be eliminated. As the results shown in Fig. 2a, the conversion of cyclohexene catalyzed by ion-exchange resin was found to be evidently increased with increasing stirring speed from

a a 0.08 0.06

-2.0

y = 3.71523 x - 10.77559 2 R = 0.9991

xCHE

ln(Keq1)

-1.8

-2.2

0.04 0.02

2.32

2.36

2.40

2.44

2.48

300 rpm 700 rpm

0.00

500 rpm 900 rpm

-1

1000/T /K -1.2

0

50

100

150

200

250

t /min

b

b

0.10 0.08 0.06

xCHE

ln(Keq2)

-1.4

y = 3.67486 x - 10.60689 2 R = 0.9983

-1.6

-1.8 2.40

0.04

500 rpm 1000 rpm 1500 rpm

0.02 0.00

2.44

2.48

2.52

2.56

-1

1000/T /K

Fig. 1. Equilibrium constants (a) Keq1 with ion-exchange resin catalyst and (b) Keq2 with H-ZSM-5 catalyst as a function of temperature. Catalyst loading: 30% (w/w) in the aqueous phase; initial volume ratio of cyclohexene to water: 1:1; reaction time: 5 h; stirring speed: (a) 700 rpm and (b) 1000 rpm.

0

50

100

150

200

t /min Fig. 2. Effect of stirring speed on the conversion of cyclohexene, xCHE , catalyzed by (a) ion-exchange resin catalyst and (b) H-ZSM-5 catalyst. Catalyst loading: 30% (w/w) in the aqueous phase; initial volume ratio of cyclohexene to water: 1:1; reaction temperature: (a) 413 K and (b) 403 K.

X. Shan et al. / Chemical Engineering Journal 175 (2011) 423–432

a

a

427

0.10

0.08 0.08 0.06

xCHE

xCHE

0.06 0.04

0.04

0.02

dp < 0.55 mm

0.02

0.00

0.55mm< dp< 0.83 mm dp > 0.83 mm

0.00

0

50

100

150

200

250

10% 20% 30% 40% 0

50

t /min

b

100

150

200

250

t /min

0.10

b

0.10

0.08 0.08

xCHE

xCHE

0.06 0.04 100 nm 600 nm

0.02

0.06 0.04

10% 20% 30% 40%

0.02

0.00

0.00 0

50

100

150

200

t /min Fig. 3. Effect of particle size of (a) ion-exchange resin and (b) H-ZSM-5 on the conversion of cyclohexene, xCHE . Stirring speed: (a) 700 rpm and (b) 1000 rpm; catalyst loading: 30% (w/w) in the aqueous phase; initial volume ratio of cyclohexene to water: 1:1; reaction temperature: (a) 413 K and (b) 403 K.

300 to 700 rpm. However, there is no appreciable increase in conversion when the stirring speed is increased to 900 rpm. In addition, the breakage phenomenon of the resin particles was observed when the stirring speed is as high as 700–900 rpm. Thus, it is concluded that external diffusion is probably a control step when the stirring speed varied from 300 to 700 rpm. In other words, the 700 rpm is sufficient to avoid external mass transfer resistances. The similar result was observed in Fig. 2b for H-ZSM-5 catalyst, where the stirring speed of 1000 rpm was chosen to eliminate the external diffusion resistance. Since the resin catalyst is composed of small gel-type microspheres consisting of macropores and the particle size of the H-ZSM-5 is quite small (0.05–0.7 ␮m), the internal diffusion resistance could be neglected [9,21]. In order to ascertain the absence of internal diffusion resistance, the effect of particle size of ion-exchange resin and H-ZSM-5 on the cyclohexene conversion was investigated. The H-ZSM-5 zeolite with two different particle sizes was commercial supplied, and the resin catalyst was sieved into three size groups. As shown in Fig. 3, no obvious variation of the cyclohexene conversion was observed, which suggests that the internal diffusion resistance could be ignored. 4.2.2. Effect of catalyst loading In this work, catalyst loading is defined as the weight ratio of the catalyst to water. The hydration of cyclohexene was studied under different catalyst loadings (from 10% (w/w) to 40% (w/w)) using ion-exchange resin and H-ZSM-5 zeolite, which is shown in Fig. 4. It was observed for both catalysts that an increase in the catalyst loading leads to an increase of the reaction rates. The reason for this is that the higher the catalyst loading, the higher the total number of available actives sites for the cyclohexene hydration. However, only a slight increase in the cyclohexene conversion was observed when the catalyst loading was further increased from 30% to 40%,

0

50

100

150

200

t /min Fig. 4. Effect of catalyst loading on the conversion of cyclohexene, xCHE , catalyzed by (a) ion-exchange resin catalyst and (b) H-ZSM-5 catalyst. Initial volume ratio of cyclohexene to water: 1:1; reaction temperature: (a) 403 K and (b) 393 K.

which is probably caused by the existence of mass transfer resistance when excess catalysts were used under the same reaction conditions. Based on the experimental results, the catalyst loading of 30% (w/w) for both ion-exchange resin and H-ZSM-5 was selected for the further experiments. 4.2.3. Effect of reaction temperature In order to obtain the activation energy of the cyclohexene hydration, the experimental temperature was varied from 373.15 K to 413.15 K. Fig. 5a and b, respectively, shows the effect of the reaction temperature on the hydration reaction catalyzed by the resin catalyst and H-ZSM-5. It can be seen that the increase of temperature is apparently favorable to accelerate the hydration reaction. Meanwhile, it is evident from Fig. 5a that the cyclohexene conversion decreases from 8.6% to 7.7% when the reaction temperature increases from 403.15 K to 413.15 K, and the similar phenomenon is also observed in Fig. 5b. This indicates that the hydration of cyclohexene is a reversible exothermic reaction. For cyclohexene hydration catalyzed by ion-exchange resin and H-ZSM-5, 403.15 K and 393.15 K were considered to be the optimum reaction temperatures, respectively. 4.2.4. Effect of initial volume ratio of reactants The initial volume ratio of cyclohexene to water was varied between 0.5 and 4 under otherwise same values of catalyst loading, total volume and reaction temperature. The effect of the initial volume ratio of reactants on the hydration reaction catalyzed by the resin catalyst and H-ZSM-5 is presented in Fig. 6. For both ionexchange resin and H-ZSM-5, the similar tendency of cyclohexene conversion with initial volume ratio of reactants is observed. As can be seen in Fig. 6, the cyclohexene conversion increases with the decrease of initial volume ratio of cyclohexene to water, which is probably due to the increase of catalyst amount as the initial volume of water increased.

428

X. Shan et al. / Chemical Engineering Journal 175 (2011) 423–432

0.10

d

0.08

c

% Transmittance

xCHE

a

0.06 0.04 383.15 K 393.15 K 403.15 K 413.15 K

0.02 0.00 0

50

100

150

200

3600

0.10

xCHE

0.06

373.15 K 383.15 K 393.15 K 403.15 K

0.02 0.00 0

50

100

150

200

t /min Fig. 5. Effect of reaction temperature on the conversion of cyclohexene, xCHE , catalyzed by (a) ion-exchange resin catalyst and (b) H-ZSM-5 catalyst. Catalyst loading: 30% (w/w) in the aqueous phase; initial volume ratio of cyclohexene to water: 1:1.

1800

1200

600

0.10 0.08 0.06 4:1 2:1 1:1 1:2

0.04 0.02 0.00 0

50

100

150

200

250

t /min 0.10 0.08 0.06 0.04

4:1 2:1 1:1 1:2

0.02 0.00 0

In summary, both the ion-exchange resin and the H-ZSM-5 showed good catalytic performance for the hydration of cyclohexene with an approximated maximum conversion of cyclohexene. It is also found that the reaction catalyzed by the two catalysts exhibited much similar characteristics in the study of above operating variables. However, it is impossible to discriminate the reaction mechanism of cyclohexene hydration only judging by the results of the conventional experiment. With the purpose of understanding the adsorption behavior of the reactants and product on the catalyst, infrared analysis was used in the following section. 4.3. FT-IR spectra Infrared spectroscopic analysis has been widely used in the study of the adsorption behavior of adsorbates on the solid materials [20,24–27]. Based on the structural information of the adsorbed species under the reaction conditions, the reaction mechanism on the heterogeneous catalysts can be proposed. In order to get some information about the reaction mechanism of cyclohexene hydration on the active sites of the ion-exchange resin and H-ZSM-5, the IR spectra of the catalysts pretreated by different components were analyzed, respectively.

a

xCHE

2400

Fig. 7. FT-IR spectra of the (a) ion-exchange resin and that in contact with (b) cyclohexene, (c) water, and (d) cyclohexene + water.

0.04

xCHE

3000

Wavenumber /cm-1

0.08

b

a

250

t /min

b

b

50

100

150

200

t /min Fig. 6. Effect of initial volume ratio of cyclohexene to water on the conversion of cyclohexene, xCHE , catalyzed by (a) ion-exchange resin catalyst and (b) H-ZSM-5 catalyst. Catalyst loading: 30% (w/w) in the aqueous phase; reaction temperature: (a) 403 K and (b) 393 K.

4.3.1. FT-IR spectra of ion-exchange resin The FT-IR spectra of the ion-exchange resin in contact with different reactants are shown in Fig. 7. It can be seen that the ion-exchange resin shows a broad characteristic band at 2900–3700 cm−1 , which is attributed to the framework of the resin material polystyrene sulfonate. Fig. 7b is the IR spectrum of the ion-exchange resin treated with cyclohexene. Compared with the untreated ion-exchange resin, a characteristic band at 2900 cm−1 is found, which indicates that the cyclohexene has been adsorbed on the active sites of the ion-exchange resin. It can be seen that there is no evident difference between Fig. 7a and c, which exhibits the IR spectrum of the ion-exchange resin treated with water. However, because the broad characteristic band at 2900–3700 cm−1 overlaps the characteristic band of water (3400 cm−1 ), it cannot be confirmed that there is no water adsorbed on the surface of ionexchange resin. In addition, the IR spectrum of the ion-exchange resin obtained after interacting with cyclohexene and water simultaneously (Fig. 7d) shows similar characteristic bands with those in Fig. 7b, which further proved that cyclohexene must be adsorbed on the active sites of the ion-exchange resin in the hydration reaction. 4.3.2. FT-IR spectra of H-ZSM-5 Fig. 8 shows the FT-IR spectra of the H-ZSM-5 in contact with different reactants. There is no sharp characteristic band of cyclohexene (2900 cm−1 ) existed for all the four catalyst samples. It

X. Shan et al. / Chemical Engineering Journal 175 (2011) 423–432

% Transmittance

d c b a

3600

3000

2400

1800

1200

600

Wavenumber /cm-1 Fig. 8. FT-IR spectra of the (a) H-ZSM-5 and that in contact with (b) cyclohexene, (c) water, and (d) cyclohexene + water.

should be noted that the small characteristic band at 2945 cm−1 is not attributed to cyclohexene, because the band at 2945 cm−1 also appears in Fig. 8a and c, in which the H-ZSM-5 catalyst were not treated by cyclohexene. In addition, no obvious change in the intensity of the characteristic band at 2945 cm−1 is observed in the IR spectra of the four catalyst samples pretreated by different reactants, which further confirms that cyclohexene is not adsorbed on the active sites of H-ZSM-5. Thus, it can be concluded that the mechanism of cyclohexene hydration catalyzed by H-ZSM-5 catalyst should be different from that catalyzed by ion-exchange resin. The IR spectrum of the untreated H-ZSM-5 catalyst (Fig. 8a) shows a characteristic band at 3645 cm−1 , which is attributed to the structural Si(OH)Al groups in the H-ZSM-5 [24]. However, in Fig. 8c and d, the characteristic band at 3645 cm−1 decreases largely in intensity. It should be caused by the interaction between the water molecule and the Si(OH)Al group. 4.4. Kinetic modeling For modeling the hydration of cyclohexene catalyzed by ionexchange resin and H-ZSM-5, experimental data obtained under different reaction conditions were used in the kinetic modeling. In the four models, several parameters were determined: (1) the preexponential factor, (2) the activation energy for the hydration reaction, (3) the adsorption constants of cyclohexene, water and cyclohexanol. The parameters were obtained by minimizing the residual sum of squares (RSS) between the experimental and calculated values of reaction rate, which could be expressed as the following equation: RSS =



(ri,exp − ri,calc )

2

(17)

A Levenberg–Marquardt (LM) method was applied for solving all the differential equations. In addition, the model adequacy and the discrimination between models were determined using the correlation coefficient (2 ) and the model selection criterion (MSC), according to the following equations.



2

 =1−

(ri,exp − ri,calc )



2

2 ri,exp

   2 (ri,exp − ri,exp ) 2p − MSC = ln 2 n (ri,exp − ri,calc )

(18)

(19)

where n is the number of experimental data, p is the amount of parameters fitted, ri,exp and ri,calc are the experimental and calculated values of reaction rate, respectively, and ri,exp is average of the experimental values.

429

4.4.1. Cyclohexene hydration catalyzed by ion-exchange resin For the cyclohexene hydration catalyzed by ion-exchange resin catalyst, the corresponding values of the kinetic parameters and the results of the statistical analysis for the four models are presented in Table 4. According to the values of the residual sum of squares (RSS), correlation coefficient (2 ) and model selection criterion (MSC), it can be found that the model 1 based on the pseudo-homogeneous mechanism gives the largest value of RSS (6.38 × 10−8 ), smallest value of 2 (0.986) and MSC (3.597) among the four models. It implies that the PH model, which neglects the adsorption of reactants and product on the catalyst, is inadequate to represent the cyclohexene hydration at such experimental conditions. For the three heterogeneous models, the model 2 with the LHHW mechanism, which involves more adjustable parameters, instead describes the experimental data with a smaller value of RSS (1.19 × 10−8 ) and a higher value of 2 (0.998) and MSC (5.087) than the other two models with ER mechanism (models 3 and 4). Thus, the model 2 is proved to be more appropriate for the hydration of cyclohexene catalyzed by ion-exchange resin. Based on the model 2, an activation energy of 79.9 kJ mol−1 is obtained, which is lower than the value of 103.2 kJ mol−1 reported by Panneman and Beenackers [7]. This difference can be attributed to the catalytic activity of the ion-exchange resin. The adsorption constants of cyclohexene, water and cyclohexanol are found to be 6.75, 24.3 and 81.9, respectively. It is noticed that the adsorption constants of cyclohexanol and water are larger than that of cyclohexene. It seems reasonable because both cyclohexanol and water, as polar components, should adsorb on the active sites of ion-exchange resin more readily than cyclohexene does. 4.4.2. Cyclohexene hydration catalyzed by H-ZSM-5 For the hydration of cyclohexene using H-ZSM-5 as the catalyst, the same kinetic models are applied to fit the rate data obtained by the kinetic experiment. Interestingly, some different results are observed. In Table 5 the values of kinetic parameters and results of the statistical analysis for the models are listed, the model 1 with the pseudo-homogeneous mechanism shows the worst fits to the experimental data with a largest value of RSS (9.79 × 10−8 ) and a smallest value of 2 (0.982) and MSC (3.398). It is in agreement with the result obtained by the kinetic modeling of the cyclohexene hydration catalyzed by ion-exchange resin, which further confirms that the kinetic study of the heterogeneous system of cyclohexene hydration should take into account the adsorption of reactants and product on the catalyst. Judging from the values of correlation coefficient (2 ) and model selection criterion (MSC), models 3 and 4 with ER mechanism give larger 2 values and larger MSC values than the model 2 with LHHW mechanism, which is proved appropriate for the hydration of cyclohexene catalyzed by ion-exchange resin. However, between the two models with ER mechanism, it is difficult to discriminate by the RSS, 2 and MSC values. Take into account the physically meaningful of the adsorption constant of cyclohexanol, the value of 146 obtained by the model 3 is considered more reliable than the value of 1240 obtained by the model 4. Thus the model 3 is chosen to describe the hydration of cyclohexene catalyzed by H-ZSM-5 catalyst. As shown in Table 5, the adsorption constant of water is estimated to be 19.9, and the activation energy obtained is 77.7 kJ mol−1 , which is close to the value of 70.8 kJ mol−1 reported by Zhang et al. [9]. 4.5. Mechanism discrimination For the hydration reaction, it is conceivable that the Brönsted acid sites are the active sites, and the main feature of the reaction is related to the adsorption of the species on the catalyst surface. According to classical mechanism of the hydration reaction, it is believed that the reaction proceeds via carbenium ion

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Table 4 Estimated parameter values for cyclohexene hydration over ion-exchange resin. Model

Parameter

Value

−1

−1

−4

−3

RSS

2

MSC

1

k0 /mol s kg cat E/kJ mol−1

5.21 × 10 ± 1.09 × 10 83.9 ± 21.8

6.38 × 10−8

0.986

3.597

2

k0 /mol s−1 kg cat−1 E/kJ mol−1 KCHE KH2 O KCHOL

0.593 ± 0.217 79.9 ± 11.7 6.75 ± 4.34 24.3 ± 8.8 81.9 ± 13.1

1.19 × 10−8

0.998

5.087

3

k0 /mol s−1 kg cat−1 E/kJ mol−1 KH2 O KCHOL

1.53 × 10−2 ± 1.00 × 10−3 90.5 ± 40.3 29.9 ± 24.5 38.8 ± 19.7

5.46 × 10−8

0.988

3.628

4

k0 /mol s−1 kg cat−1 E/kJ mol−1 KCHE KCHOL

1.41 × 10−2 ± 7.39 × 10−2 91.3 ± 77.1 27.9 ± 22.4 66.1 ± 27.8

5.62 × 10−8

0.988

3.598

route through olefin adsorbed on the active sites. However, different viewpoints have also been proposed, e.g., Kalló and Mihályi [17] proposed that water had an inhibition effect on the formation of the carbenium ions by the following reaction: T− –Cn + H2n+1 + H2 O  T− –H3 O+ + Cn H2n

(20)

where T− represents the deprotonated site. Thus it is reasonable for the cyclohexene hydration that water adsorbs on the active sites of the catalyst to form hydroxonium ion, which further reacts with the cyclohexene molecule to form cyclohexanol. It is to be noted that a hydration reaction between positively charged carbenium ion and hydroxonium ion is almost impossible. Many reactions catalyzed by ion-exchange resin have been considered to be pseudo-homogeneous, which requires complete swelling of the resin and total dissociation of the sulfonic groups (–SO3 H) [28]. Nevertheless, based on the results of the kinetic modeling for cyclohexene hydration over ion-exchange resin, it is confirmed that the catalysis by ion-exchange resin cannot be treated as homogenous. Actually, only if the adsorption behaviors of cyclohexene and water are all taken into account, could the corresponding model give the best correlated with the data obtained. Thus, it is clear that the cyclohexene hydration catalyzed by ionexchange resin obeys the LHHW mechanism, which is proved to be the most suitable mechanism for many reactions catalyzed by resin catalyst [10,11,13,15]. Based on the classical LHHW mechanism, the process of cyclohexene hydration over ion-exchange resin could be proposed in Fig. 9. Both cyclohexene and water are adsorbed on the surface

of the ion-exchange resin, and the carbonium ion is formed by protonation of the adsorbed cyclohexene and further reacts with the adsorbed neutral water molecule. The reaction completes till a cyclohexanol molecule is desorbed from the resin and the acid site recovers. The addition of a second cyclohexene molecule to the carbonium ion is a possible secondary pathway. However, the selectivity of cyclohexene hydration catalyzed by ion-exchange resin is quite high and few by-products were observed in the experiment. It is mainly due to the existence of water molecule, which has strong polarity and tends to be adsorbed on the sulfonic groups of resin catalyst. The competitive adsorption of cyclohexene and water should effectively inhibit the occurring of the secondary reaction. For the cyclohexene hydration catalyzed by the H-ZSM-5, the infrared analysis shows that the cyclohexene molecule is not adsorbed on the active sites of H-ZSM-5 catalyst. Moreover, the model without regard to the adsorption of cyclohexene is proved to be the best model in the kinetic modeling. Thus, it is suggested that cyclohexene hydration catalyzed by H-ZSM-5 belongs to the ER mechanism accounting for the reaction between the adsorbed water and bulk cyclohexene. In fact, the H-ZSM-5 type catalysts having a high Si/Al ratio and small particle size normally exhibit hydrophilic outer surface and hydrophobic pore wall [29]. Due to the hydrophilicity of outer surface, the H-ZSM-5 catalyst is stable in the aqueous phase, and the hydrophobicity of pore wall is favor of the diffusion of cyclohexnene from the bulk aqueous phase to the pores of H-ZSM-5. According to Jentys et al. [30], water molecules would like to be adsorbed on the acid sites of H-ZSM-5 and tend to form hydroxonium ion H3 O+ . It

Table 5 Estimated parameter values for cyclohexene hydration over H-ZSM-5. Model

Parameter −1

Value −1

−4

−3

RSS

2

MSC

1

k0 /mol s kg cat E/kJ mol−1

6.54 × 10 ± 4.66 × 10 86.4 ± 15.8

9.79 × 10−8

0.982

3.398

2

k0 /mol s−1 kg cat−1 E/kJ mol−1 KCHE KH2 O KCHOL

12.6 ± 10.2 78.0 ± 18.5 4.81 ± 5.91 127 ± 54.2 220 ± 139

2.39 × 10−8

0.996

4.559

3

k0 /mol s−1 kg cat−1 E/kJ mol−1 KH2 O KCHOL

1.56 × 10−2 ± 0.27 × 10−2 77.7 ± 10.7 19.9 ± 10.4 146 ± 24.1

3.63 × 10−9

0.999

6.530

4

k0 /mol s−1 kg cat−1 E/kJ mol−1 KCHE KCHOL

0.126 ± 0.879 77.6 ± 19.4 168 ± 237 1240 ± 450

3.62 × 10−9

0.999

6.524

X. Shan et al. / Chemical Engineering Journal 175 (2011) 423–432

431

Fig. 9. Proposed reaction mechanism for the hydration of cyclohexene with ion-exchange resin catalyst.

Fig. 10. Proposed reaction mechanism for the hydration of cyclohexene with H-ZSM-5 catalyst.

is deduced accordingly that the surface of H-ZSM-5 catalyst in this work must be saturated with adsorbed water and hydroxonium ion in the process of hydration reaction. Thus, it is postulated that all the acid sites are in the form of H3 O+ , which play the role of the Brönsted acid sites in the H-ZSM-5 catalyst. To make sure the proceeding of the hydration reaction, the cyclohexene molecule in the pores of H-ZSM-5 could react directly with the adjacent hydroxonium ions to obtain the cyclohexanol molecule with recovering of the Brönsted acid sites. Hence, the proposed mechanism for cyclohexene hydration over H-ZSM-5 catalyst can be represented as shown in Fig. 10. 5. Conclusions In this work, ion-exchange resin and H-ZSM-5 zeolite were used as the catalyst for the synthesis of cyclohexanol from hydration of cyclohexene. Both of them showed good catalytic performance and behaved much similar in some aspects, nevertheless, the reaction mechanism was found to be different:

(1) In the thermodynamic aspect, with the ion-exchange resin and H-ZSM-5 catalysts, we respectively obtained a reaction enthalpy of −30.9 kJ mol−1 and −30.6 kJ mol−1 , which were found close to the theoretically calculated value of −30.2 kJ mol−1 based on the standard free energy of formation. (2) The IR spectra of the two catalysts in contact with different reactants gave different informations about the adsorption behavior of the reactants. A characteristic band of cyclohexene at 2900 cm−1 was shown in the IR spectrum of the ion-exchange resin in contact with cyclohexene, but was not found in that of the H-ZSM-5 treated with cyclohexene. In addition, it was deduced that the decrease in intensity of the characteristic band of structural Si(OH)Al groups at 3645 cm−1 was due to the interaction between the water molecule and the Si(OH)Al group. (3) In the kinetics aspect, cyclohexene hydration catalyzed by the ion-exchange resin was found to follow the LHHW mechanism based on the adsorbed water and adsorbed cyclohexene, in which the reaction proceeds via the formation of carbenium ion by the adsorbed cyclohexene. Differently, for the

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cyclohexene hydration catalyzed by the H-ZSM-5, an Eley–Rideal (ER) mechanism, in which the water molecule is adsorbed on the catalyst to form a hydroxonium ion H3 O+ , was proved to be suitable. (4) The obtained activation energies for cyclohexene hydration with the two catalysts were almost identical with the value of 79.9 kJ mol−1 and 77.7 kJ mol−1 , which indicates that the two different catalytic materials cannot influence the transition state of cyclohexene hydration obviously. Acknowledgments The present work was conducted under supports from the State Key Laboratory of Chemical Engineering in China via SKL-ChE08C03 and partially from Natural Science Foundation of China via No. 20876043. The authors are deeply indebted to Mr. Ligen Chen who has contributed much on the IR analysis of the catalyst samples. References [1] H. Ishida, Liquid-phase hydration process of cyclohexene with zeolites, Catal. Surv. Jpn. 1 (1997) 241–246. [2] T. Okuhara, M. Kimura, T. Nakato, Hydration of olefins in excess water catalyzed by an insoluble cesium hydrogen salt of dodecatungstophosphoric acid, Chem. Lett. 8 (1997) 839–840. [3] T. Nakato, Y. Toyoshi, M. Kimura, T. Okuhara, Unique catalysis of an acidic salt of heteropoly acid, Cs2.5 H0.5 PW12 O40 , consisting of microcrystallites, Catal. Today 52 (1999) 23–28. [4] H. Ogawa, H. Xiuhua, T. Chihara, Hydration of cyclohexene by alkylimmobilized H-ZSM-5 catalyst in decalin–water system, Chem. Lett. 55 (1998) 121–123. [5] T. Yoshikazu, K. Tokitaka, Process for the preparation of cyclohexanol, U.S. Patent 6,552,235 (2003). [6] H.J. Panneman, A.A.C.M. Beenackers, Effect on the hydration of cyclohexene catalyzed by strong ion-exchange resins. 1. Solubility of cyclohexene in aqueous sulfolane mixtures, Ind. Eng. Chem. Res. 31 (1992) 1227–1234. [7] H.J. Panneman, A.A.C.M. Beenackers, Effect on the hydration of cyclohexene catalyzed by strong ion-exchange resins. 2. Effect of sulfolane on the reaction kinetics, Ind. Eng. Chem. Res. 31 (1992) 1425–1433. [8] H.J. Panneman, A.A.C.M. Beenackers, Effect on the hydration of cyclohexene catalyzed by strong ion-exchange resins. 3. Effect of sulfolane on the equilibrium conversion, Ind. Eng. Chem. Res. 31 (1992) 1433–1440. [9] H. Zhang, S.M. Mahajani, M.M. Sharma, T. Sridhar, Hydration of cyclohexene with solid acid catalysts, Chem. Eng. Sci. 57 (2002) 315–322. [10] M. Lee, P. Chou, H. Lin, Kinetic of synthesis and hydrolysis of ethyl benzoate over Amberlyst 39, Ind. Eng. Chem. Res. 44 (2005) 725–732.

[11] V.J. Cruz, J.F. Izquierdo, F. Cunill, J. Tejero, M. Iborra, C. Fité, R. Bringué, Kinetic modeling of the liquid-phase dimerization of isoamylenes on Amberlyst 35, React. Funct. Polym. 67 (2007) 210–224. [12] A.J. Marchi, J.F. Paris, N.M. Bertero, C.R. Apesteguía, Kinetic modeling of the liquid-phase hydrogenation of cinnamaldehyde on copper-based catalysts, Ind. Eng. Chem. Res. 46 (2007) 7657–7666. [13] P. Delgado, M.T. Sanz, S. Beltr‘an, Kinetic study for esterification of lactic acid with ethanol and hydrolysis of ethyl lactate using an ion-exchange resin catalyst, Chem. Eng. J. 126 (2007) 111–118. [14] P.K. Igbokwe, V.I. Ugonabo, N.A. Iwegbu, P.C. Akachukwu, C.J. Olisa, Kinetics of the catalytic esterification of propanol with ethanoic acid using catalysts obtained from Nigerian clays, J. Univ. Chem. Technol. Metall. 43 (2008) 345–348. [15] W. Mao, X. Wang, H. Wang, H. Chang, X. Zhang, J. Han, Thermodynamic and kinetic study of tert-amyl methyl ether (TAME) synthesis, Chem. Eng. Process. 47 (2008) 761–769. [16] J. Kuusisto, J.P. Mikkola, M. Sparv, J. Wärnå, H. Karhu, T. Salmi, Kinetics of the catalytic hydrogenation of d-lactose on a carbon supported ruthenium catalyst, Chem. Eng. J. 139 (2008) 69–77. [17] D. Kalló, R.M. Mihályi, Mechanism of 1-butene hydration over acidic zeolite and ion-exchange resin catalysts, Appl. Catal. A: Gen. 121 (1995) 45–56. [18] S.R. Kirumakki, N. Nagaraju, K.V.R. Chary, Esterification of alcohols with acetic acid over zeolites H␤, HY and HZSM5, Appl. Catal. A: Gen. 299 (2006) 185–192. [19] E. Sert, F.S. Atalay, Kinetic study of the esterification of acetic acid with butanol catalyzed by sulfated zirconia, Reac. Kinet. Mech. Cat. 99 (2010) 125–134. [20] A.C.K. Yip, X. Hu, Catalytic activity of clay-based titanium silicalite-1 composite in cyclohexanone ammoximation, Ind. Eng. Chem. Res. 48 (2009) 8441–8450. [21] M. Umar, D. Patel, B. Saha, Kinetic studies of liquid phase ethyl tert-butyl ether (ETBE) synthesis using macroporous and gelular ion-exchange resin catalysts, Chem. Eng. Sci. 64 (2009) 4424–4432. [22] Z. Qi, A. Kolah, K. Sundmacher, Residue curve maps for reactive distillation systems with liquid-phase splitting, Chem. Eng. Sci. 57 (2002) 163–178. [23] J. Gmehling, J. Li, M. Schiller, A modified UNIFAC model. 2. Present parameter matrix and results for different thermodynamic properties, Ind. Eng. Chem. Res. 32 (1993) 178–193. [24] P. Wu, T. Komatsu, I.R. Yashima, MAS NMR studies on the incorporation of aluminum atoms into defect sites of dealuminated mordenites, J. Phys. Chem. 99 (1995) 10923–10931. [25] B. Schumacher, Y. Denkwitz, V. Plzak, M. Kinne, R.J. Behm, Kinetics, mechanism, and the influence of H2 on the CO oxidation reaction on a Au/TiO2 catalyst, J. Catal. 224 (2004) 449–462. [26] G.D. Pirngruber, J.A.Z. Pieterse, The positive effect of NO on the N2 O decomposition activity of Fe-ZSM-5: a combined kinetic and in situ IR spectroscopic study, J. Catal. 237 (2006) 237–247. [27] S.M. Aschmann, E.C. Tuazon, W.D. Long, R. Atkinson, Kinetics and products of the gas-phase reactions of divinyl sulfoxide with OH and NO3 radicals and O3 , J. Phys. Chem. A 112 (2008) 8723–8730. [28] A. Chakrabarti, M.M. Sharma, Cationic ion exchange resins as catalyst, React. Polym. 20 (1993) 1–45. [29] M. Misono, T. Inui, New catalytic technologies in Japan, Catal. Today 51 (1999) 369–375. [30] A. Jentys, G. Warecka, M. Derewinski, J.A. Lercher, Adsorption of water on ZSM5 zeolites, J. Phys. Chem. 93 (1989) 4837–4843.