Hydration of cyclohexene with solid acid catalysts

Chemical Engineering Science 57 (2002) 315 – 322

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Hydration of cyclohexene with solid acid catalysts H. Zhang, S. M. Mahajani, M. M. Sharma, T. Sridhar ∗ Department of Chemical Engineering, Monash University, Clayton, VIC-3168, Australia Received 9 February 2001; received in revised form 1 July 2001; accepted 6 October 2001

Abstract The hydration of cyclohexene has been studied in the presence of solid acid catalysts such as ion exchange resins and various zeolites. The zeolite ZSM-5 with SiO2 =Al2 O3 in the range 30 –50 was found to o5er the best performance with more than 99% selectivity towards cyclohexanol. The reaction takes place in a solid–liquid–liquid mode and most of the cyclohexanol is extracted out by the organic phase during the course of the reaction. The data on liquid–liquid distribution of cyclohexanol is generated and a simple 9rst order kinetic model based on aqueous phase concentrations has been proposed to explain the observed results on the reaction kinetics. ? 2002 Elsevier Science Ltd. All rights reserved. Keywords: Catalysts; Zeolites; Ion exchange; Kinetics

1. Introduction Cyclohexene has emerged as a reasonably cheap chemical, especially due to the advent of a new technology for its manufacture through selective hydrogenation of benzene (Struijk, d’Angremond, Lucas-de-Regt, & Scholten, 1992; Nagahara, Ono, & Fukuoka, 1995). A number of processes are being investigated to identify cyclohexene as a raw material to produce various important bulk as well as 9ne chemicals such as cyclohexanol, cyclohexene hydroperoxide, etc. (e.g. Mahajani, Sharma, & Sridhar, 1999; Ishada, 1997). The liquid-phase hydration of cyclohexene to produce cyclohexanol is an industrially important reaction. Cyclohexanol can be neatly converted to cyclohexanone via catalytic dehydrogenation and hydrogen can be recycled for benzene hydrogenation. Cyclohexanone has a large market for making adipic acid and caprolactam. There is also a potential for cyclohexanol to be converted to phenol without co-products unlike the conventional cumene based route that is associated with acetone as a co-product. Cyclohexanol is traditionally manufactured through oxidation of cyclohexane (e.g. see Suresh, Sridhar, & Potter, 1988a, b, c). The process su5ers from several drawbacks such as poor selectivity, extremely large recycles and explosion hazards. ∗ Corresponding author. Tel.: +61-3-9905-3427; fax: +61-3-99059649. E-mail address: [email protected] (T. Sridhar).

The hydration, in comparison, appears to be promising as it is a low-pressure reaction with very high selectivity towards cyclohexanol, and has minimal disposal problems due to the use of solid acid catalyst. An alternate route to produce cyclohexanol from cyclohexene through esteri9cation and trans-esteri9cation has also been suggested (Chakrabarti & Sharma, 1992). The direct hydration with proper catalyst may 9nd edge over this route as it involves a less number of processing steps. The main limitation of liquid phase hydration reaction is the extremely poor miscibility of the two reactants, viz. water and cyclohexene. The solubility of cyclohexene in water at 298 K is about 0.02% w=w. Due to this fact, the reaction rates are substantially low and an extremely active catalyst is required to obtain signi9cant rates. The present study is directed towards the evaluation of di5erent acid catalysts for this reaction. Some of the homogeneous catalysts such as p-toluene sulphonic acid exhibit poor performance as they increase the solubility of cyclohexanol in the reacting aqueous phase. The concentration build-up of cyclohexanol within the reacting phase causes reaction to slow down and terminate at a lower conversion level (Ishada, 1997). Apart from this, complete separation of homogeneous acid from the product is very diGcult and dehydration of cyclohexanol occurs in the downstream processing. Hence, the use of heterogeneous catalyst is recommended for this reaction, as it not only o5ers several engineering bene9ts but also provides a favorable distribution of alcohol in the two liquid phases.

0009-2509/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 0 1 ) 0 0 3 7 5 - X

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H. Zhang et al. / Chemical Engineering Science 57 (2002) 315–322

A number of studies on hydration of various ole9ns using homogeneous and heterogeneous solid acids have been reported in the literature. The various solid acid catalysts used for this reaction are ion-exchange resins, heteropoly acids and zeolite-based catalysts. Panneman and Beenackers (1992) used ion exchange resins and determined the intrinsic kinetics by carrying out the reaction in a single liquid phase by pre-saturating water with cyclohexene. They have recommended the use of co-solvent sulfolane to overcome the solubility constraints. However, apart from the safety considerations, the use of sulfolane will result in an additional cost associated with its separation from the reaction mixture. Okuhara and co-workers have performed a comparative study of various catalysts in two-phase reaction and concluded that zeolite ZSM-5 is the best catalyst that provided more than three fold increase in the reaction rate than that obtained with ion-exchange resins and heteropoly acids (Okuhara, Kimura, & Nakato, 1997 and Nakato, Toyoshi, Kimura, & Okuhara, 1999). The work on this reaction has also been reported in some Chinese papers (Zhang & Wei, 1998). Interestingly, the authors have reported the formation of cyclohexenone as one of the side-products of this reaction. Based on the above 9ndings, zeolite based catalysts seem to be highly promising candidates for this reaction. The detailed study on the intrinsic kinetics for the zeolite-catalysed hydration is not evident in the literature and would be one of the important aspects of the present work. A patent by Asahi Chemicals claims that zeolites, when tailor-made for the reaction of interest, are superior than those obtained from the suppliers (Mitsui & Fukuoka, 1986). The particle size, acid sites on the external surface and hydrophobicity of the catalyst are identi9ed as critical parameters in determining reaction kinetics. Among the various zeolites, ZSM-5 type zeolites with SiO2 =Al2 O3 ratio more than 20, are claimed to exhibit the desired performance. The SiO2 =Al2 O3 ratio should be high enough to confer the desired level of hydrophobicity to the catalyst, allowing suGcient adsorption of organic reactants like cyclohexene. Catalysis with zeolites in a liquid reaction medium is an area, which has not been dealt with in depth as compared to gas phase catalysis with zeolites. Apart from catalysis, the study of this reaction provides a rare opportunity to investigate the engineering aspects of the extractive reactions, which has been the subject of growing interest in the recent past (e.g., Mahajani, Kolah, & Sharma, 1995; Vasudevan & Sharma, 1984; Pahari & Sharma, 1991; Minotti, Doherty, & Malone, 1998). As cyclohexene is almost immiscible in water at the reaction conditions, it forms a separate organic phase in the reactor. The reaction takes place over the catalyst surrounded by the aqueous phase. The reaction product, i.e. cyclohexanol, being a non-polar compound is extracted out into the organic phase simultaneously during the course of the reaction thereby giving signi9cant conversion.

2. Experimental 2.1. Materials Cyclohexanol and cyclohexene were obtained from Sigma Aldrich Chemical Company, Australia. The Zeolyst International Pty. Ltd., USA, supplied the di5erent types of zeolites used in this work, and Amberlyst-15, a cation exchange resin, was obtained from Sigma Aldrich Chemical Company, Australia. The zeolites were supplied in the ammonium form and were activated by heating them at 823 K for 5 h in a furnace. Amberlyst-15 was used in the wet form without any prior treatment. 2.2. Apparatus and procedure The batch reactions were conducted in a liquid– liquid–solid (catalyst) mode. A stainless steel reactor of 1 × 10−3 m3 capacity, equipped with speed and temperature monitoring facility was used for this purpose. Typically, the desired quantities of catalyst and reactants were charged to the reactor and the reaction mixture was heated up to the desired temperature with slow stirring. 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. The samples of both organic and aqueous phases were withdrawn at di5erent time intervals to study the concentration variation of reactants and product with respect to time. The samples were centrifuged at high speed of rotation for about 10 min at ambient temperature (283–290 K) to ensure that it is free of solid catalyst. 2.3. Analysis The reactants and product(s) from both aqueous and organic phases were analysed using a gas chromatograph (GC 6890) equipped with an atomic emission detector (AED). The detector uses the energy emitted by the atom in the plasma state for the quanti9cation of the corresponding compound. A 20-m long capillary column HP-1 was used to separate the di5erent components in the reaction sample. The GC oven was operated under isothermal conditions at 348 K throughout the analysis. The various components were characterized by either authentic samples or by GC-MS instrument. 3. Results and discussion 3.1. General course of the reaction Fig. 1 shows the general course of the reaction performed with ZSM-5 zeolite catalyst at the conditions mentioned therein. As mentioned earlier, we de9ne the zero reaction time as the time at which the desired temperature is

H. Zhang et al. / Chemical Engineering Science 57 (2002) 315–322 0.3

0.24 Amberlyst-15 ZSM5-fresh (Si/Al=30) ZSM-5 fresh Si/Al=50 ZSM-5 (Ishada, 1997) Mordenite (Si/Al = 20)

aqueous phase

0.25

0.2 Fractional Conversion

organic phase Total conversion

% Conversion

317

0.2 0.15 0.1

0.16 0.12 0.08 0.04

0.05

0

0 0

50

100

150

0

200

50

100

Time (min)

obtained. Hence in Fig. 1, we see small extent of reaction that occurred till the desired temperature was attained. The extent of reaction during this heat-up period was found to be relatively higher at high catalyst loading (see Fig. 4). Typically, 90 –92% of the total cyclohexanol formed exists in the organic phase for the organic to aqueous phase ratio of 0.89 (v=v). As the reaction progresses, the concentration of cyclohexanol in both aqueous and organic phases builds up. A decrease in the reaction rate is observed at large reaction time as the reaction in the aqueous phase approaches towards the equilibrium. The formation of the other side-products is negligible. The traces of isomers of methyl cyclopentene, di-cyclohexyl ether and brownish tarry material (due to formation of oligomers) were identi9ed. The extent of oligomerization increases with temperature. However, at lower temperatures ( ¡ 398 K) and in the range of conversion studied, the oligomerization is insigni9cant. This may be attributed to the large amount of water present in the reaction mixture. This is con9rmed by heating pure cyclohexene in the absence of water with catalyst (e.g. ZSM-5, SiO2 =Al2 O3 = 50) at the reaction temperature. The intense dark color of the reaction mixture observed in this case suggests that the degree of oligomerization in the absence of water is much higher than that observed in its presence. The side products obtained with ZSM-5 and Amberlyst-15 were insigni9cant (¡ 1:0% w=w) under the conditions studied. 3.2. Comparison of di2erent catalysts The di5erent catalysts such as ZSM-5, mordenite, Amberlyst-15 were studied for this reaction and the results are compared in Fig. 2. It is evident from the 9gure that ZSM-5 type zeolites with SiO2 =Al2 O3 ratio in the range 30 –50, are the best among all the catalysts. They o5er 4 –5 fold increase in the initial rate compared to that obtained using cation exchange resin, Amberlyst-15. The internal surface of the zeolites being highly hydrophobic possesses aGnity

200

250

300

Fig. 2. Cyclohexene hydration with di5erent catalysts. Catalyst loading: 38% w=w of aqueous phase; Temperature: 393 K. Initial aqueous:organic phase ratio: 0.89 v=v.

0.16

Fractional Conversion

Fig. 1. Typical conversion pro9les for cyclohexene hydration. Temperature: 393 K; Initial aqueous:organic phase ratio: 0.89 v=v. Catalyst ZSM-5 (SiO2 =Al2 O3 = 50) loading: 38% w=w of aqueous phase.

150 Time (min)

0.12

0.08

1500 rpm 800 rpm 400 rpm

0.04

0 0

50

100

150

200

250

300

Time (min)

Fig. 3. E5ect of speed of agitation on cyclohexene hydration. Catalyst: ZSM-5-SiO2 =Al2 O3 = 50 (38% w=w of aq phase); Temperature: 393 K. Initial aqueous:organic phase ratio: 0.89 v=v.

towards non-polar molecules like that of cyclohexene. It is believed that the high concentration of cyclohexene inside the pore volume is responsible for this higher apparent activity of zeolites with SiO2 =Al2 O3 ratio in the range 30 –50. Hence, all the kinetic aspects were studied using either of these two catalysts. However, at very large SiO2 =Al2 O3 (i.e. 400) the reaction kinetics is poor because of the considerable drop in the number of catalytic Bronsted acid sites for the identical catalyst loading. 3.3. Elimination of mass transfer resistance The reaction was performed over a wide range of speed of agitation and it was found that the rate of reaction changes negligibly above 800 rpm (Fig. 3). Hence, all the reactions were performed at 1500 rpm to ensure that there is no resistance to mass transfer at both solid–liquid and liquid–liquid interfaces. This is also supported by independent experiments on phase equilibrium of the liquid–liquid system.

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0.15 Fractional Conversion

Fractional Conversion

0.15

0.1

0.05 38% w/w 16% w/w 10% w/w 50

100

150

Fresh ZSM-5 Reuse-1 Reuse-2 Reuse-3 Reuse-4

0.05

0

0 0

0.1

200

0

50

100

Time (min)

Fig. 4. E5ect of catalyst loading on cyclohexene hydration. Catalyst: ZSM-5-SiO2 =Al2 O3 = 50; Temperature: 393 K; Initial aqueous:organic phase ratio: 0.89 v=v.

3.4. Catalyst loading The reactions were performed in the range of catalyst loading and it was observed that the rate of reaction increases with the catalyst loading. Fig. 4 shows the plot of conversion vs. time at di5erent catalyst loadings. The initial rates calculated on the basis of these plots vary linearly with catalyst loading. 3.5. Catalyst reusability It is a well-known fact that the reactions associated with ole9ns encounter the problems of catalyst deactivation due to formation of dimers=oligomers and carbonaceous material (Ishada, 1997). The catalyst after the 9rst run was 9ltered and washed with water once. The depletion in the rate was insigni9cant; recognizing the fact that small amount of the catalyst is lost during 9ltration and washing. This result gives us con9dence to assume that the deactivation is not severe enough to a5ect the reaction kinetics in the time frame of a single reaction run, typically of 2 h, performed in the present studies. Fig. 5 shows the results obtained from 9ve consecutive runs. The surface area measurement suggests that the total surface area of the catalyst decreases by a factor of about 2 (from 450 to 200 m2 =gm). It remains almost constant (in a range 200 –170 m2 =gm) and drastically reduces to 34:2 m2 =gm at the end of 9fth batch. Hence, from a process viewpoint continuous reactivation of catalyst may be required. 3.6. Aqueous phase holdup The volume ratio of aqueous phase to organic phase was varied over a range of 0.5 –1.5 under otherwise similar values of catalyst weight, total volume and reaction temperature. The cyclohexanol concentration was calculated on the basis of the two-phase volume and is plotted against time in Fig. 6 that shows the inPuence of aqueous phase holdup on the reaction. It was observed that in the initial period, i.e., at

150

200

250

300

Time (min) Fig. 5. Catalyst resusability test of ZSM-5 (38% w=w slurry) for cyclohexene hydration. Temperature: 393 K; Initial aqueous:organic phase ratio: 0.89 v=v.

low conversion, the rate was only a weak function of aqueous phase holdup. However, at higher conversions (¿ 10%) the e5ect of aqueous phase holdup becomes evident. The rate of formation of cyclohexanol decreased with an increase in the aqueous phase holdup. In this region of reaction kinetics, the aqueous phase concentration of cyclohexanol inPuences the rate of reaction and it is the extraction ability of the organic layer that plays the crucial role. For lower aqueous to organic phase ratios, extraction of cyclohexanol is more eGcient because of the relatively large volume of organic phase that exists in the reactor. In other words, at lower aqueous to organic phase ratios more amount of cyclohexanol would form at the large reaction time for the given catalyst weight and total two-phase volume. It may also be concluded that for the same catalyst weight, the aqueous phase holdup does not inPuence the intrinsic kinetics of the reaction. This also suggests that the reaction takes place either at the surface of the zeolite or in the intra-crystalline space of the zeolite. Hence, the rate of the reaction has to be expressed on the basis of catalyst weight. The hydration reaction takes place only in the aqueous phase. Hence, one would anticipate that for a given reaction volume, the rate of reaction would depend on the volume of the aqueous phase. Fig. 6 shows the results of an experiment where the volume of the aqueous phase is changed, keeping the total (aqueous+organic) liquid volume and catalyst weight constant. The smallest aqueous phase holdup gives the largest amount of alcohol at equilibrium. This effect is primarily due to the extraction of alcohol into the organic phase and the consequent increase in the reactor productivity. 3.7. E2ect of temperature The extractive reactions are highly inPuenced by temperature as both intrinsic rate constants and distribution coef9cients are strong functions of temperature. Cyclohexene hydration has been studied over the range of temperature

H. Zhang et al. / Chemical Engineering Science 57 (2002) 315–322

all the reactions were conducted at 393 K. The surface area measurement also suggests that the catalyst gets deactivated rapidly if the reaction is conducted at high temperatures.

0.75

Cyclohexanol formed per total volume of the reactor (mols/lit)

319

0.6

3.8. E2ect of particle size

0.45

0.3 aq:org=0.89 aq:org=1.25 aq:org=1.5 aq:org=0.48

0.15

0 0

100

200

300

400

Time (min)

Fig. 6. E5ect of aqueous phase holdup. Catalyst ZSM-5 (SiO2 =Al2 O3 =30) loading: 38% w=w of aqueous phase. Temperature: 393 K.

363–393 K. Fig. 7a shows the e5ect of temperature on the conversion vs. time plot under otherwise similar conditions. The rate increases signi9cantly with an increase in temperature and as will be shown further in the modeling section, this large increase in the rate suggests that reaction is negligibly a5ected by any pore di5usion limitations. Cyclohexene hydration is an exothermic reaction and the equilibrium shifts in the reverse direction with a rise in temperature. This was con9rmed by changing the reaction after equilibrium is reached and is shown in Fig. 7b. As the reaction temperature was increased from 393 to 403 K, the reverse reaction facilitated. A further deliberate decrease in reaction temperature from 403 to 378 K increases the forward reaction and shifts the equilibrium conversion. From this observation it can be concluded that the decrease in the rate of reaction is because of the approach towards the reaction equilibrium. This is the stage where the capacity of the solvent (organic phase) in the reactor to extract the cyclohexanol diminishes. At high temperatures (¿ 398 K), it was observed that the reaction mixture acquires dark brownish color due to the substantial formation of oligomers of cyclohexene. Hence,

An attempt was made to investigate the e5ect of particle size on the reaction kinetics. The catalyst material was ground from 0:7  to 90 nm with the help of a special grinding machine from Micronirsers Pty. Ltd., Australia. It was con9rmed with the X-ray Di5raction analysis that the crystal structure remains intact even after grinding to such a low particle size. The reaction rate was surprisingly lower than that observed with the coarse catalyst particles. Fig. 8 compares the activity of both coarse and 9ne particles with the same catalyst loading. The state of agglomeration of these primary particles may also contribute to the surprisingly low activity. A detailed study of crystallographic changes due to grinding and resultant changes in acidity is necessary to identify the cause for this observation. 3.9. E2ect of presence of cyclohexane and benzene As mentioned earlier, a commercial process by Asahi Chemical manufactures cyclohexene through hydrogenation of benzene (Yamashita, Hideaki, & Katsuta, 1995). The reaction is associated with a substantial formation of cyclohexane. The separation of cyclohexene and cyclohexane is diGcult, as both of them have similar boiling points. This separation may be partially achieved through hydration of cyclohexene in extractive mode. Hence, we conducted the hydration reaction in the presence of cyclohexane (25% w=w) and studied the reaction kinetics. Similarly the e5ect of the presence of benzene (50% w=w) on the reaction rate was also studied under otherwise similar conditions. Substantial dilution of the organic phase takes place in the presence of benzene and it is anticipated that this will adversely 0.2

Fractional Conversion

Fractional Conversion

0.16 0.14 0.12 0.1 0.08

model results 363 K 373 K 383 K 393 K

0.06 0.04 0.02 0

0.12

0.08

393 K 403 K 378 K

0.04

0

0

(a)

0.16

100

200

300

Time (min)

400

0

500

(b)

100

200

300

400

500

600

Time (min)

Fig. 7. (a) E5ect of temperature and agreement between model and experimental results. Catalyst ZSM-5 (SiO2 =Al2 O3 =50) loading: 38% w=w of aqueous phase. Initial aqueous:organic phase ratio: 0.89 v=v. (b) Change in equilibrium position with respect to temperature. Catalyst ZSM-5 (SiO2 =Al2 O3 = 50) loading: 38% w=w of aqueous phase. Initial aqueous:organic phase ratio: 0.89 v=v.

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30

700 nm 90 nm

0.2

25

Corgol/Caqol

Fractional Conversion

0.25

0.15 0.1

20 15 10

0.05

5 0

0

50

100

150 Time (min)

200

250

300

0

Cyclohexanol /Reactor volume mols/lit

Fig. 8. E5ect of particle size on ZSM-5-catalysed cyclohexene hydration. Temperature: 393 K; Initial aqueous:organic phase ratio: 0.89 v=v.

0.6

0.4

0.2

with benzene (50% w/w) without benzene with cyclohexane (25% w/w)

0 0

50

100

150

200

250

300

Time (min)

Fig. 9. E5ect of the presence of cyclohexane and benzene. Catalyst ZSM-5 (Si=Al = 50) loading: 38% w=w of aqueous phase. Temperature: 393 K.

a5ect the reaction kinetics. However, the initial rate of cyclohexanol formation does not change signi9cantly even in the presence of benzene that suggests a favorable change in the solubility of cyclohexene in the aqueous phase. On the other hand, the presence of cyclohexane adversely a5ects the initial rate of formation cyclohexanol because of the possible decrease in the corresponding equilibrium concentration of cyclohexene in the aqueous phase (Fig. 9). 4. Kinetic modeling The rate equation for cyclohexene hydration is given as ne ol rol = k1 Caq − k2 Caq ;

0

(1)

where, rol is the rate of formation of cyclohexanol per unit mass of the catalyst. As the catalyst external surface is hydrophilic, it always stays in the aqueous phase during the course of the reaction. Hence, the rate is assumed to be a function of aqueous phase concentrations of cyclohexene and cyclohexanol. As water exists in large proportion, the forward reaction is assumed to be pseudo 9rst order with respect to cyclohexene. The concentration of dissolved cyne clohexene (Caq ) in the aqueous phase is assumed to be constant throughout the reaction and it corresponds to solubility of cyclohexene in water at the reaction temperature.

0.02

0.04 Caq

0.06

0.08

ol

Fig. 10. Distribution of cyclohexanol in aqueous and organic phases at 393 K.

This was con9rmed by the use of UNIFAC-LLE model of Aspen Plus software. With a considerable change in total concentration of cyclohexanol in a L–L system containing water-cyclohexene-cyclohexanol, a negligible change in aqueous phase concentration of cyclohexene was observed. The solubility data of cyclohexene in water is available in the literature (Panneman & Beenackers, 1992) and is used in the present exercise. The aqueous phase concentration of ol cyclohexanol (Caq ) was obtained from the distribution data generated by measuring the concentrations of organic and aqueous phases at the reaction temperature (Fig. 10). Eq. (2) gives a nonlinear relationship that explains the data with reasonable accuracy. All the concentrations are expressed in kmol=m3 ol ol ol ol Corg =Caq = KD Caq = 401:0Caq :

(2)

It is con9rmed that the e5ect of temperature, over the range of our interest, on distribution data is not signi9cant to affect the regression analysis. The values of k1 and k2 obtained by regression analysis were found to be less sensitive to a change in KD . Hence, Eq. (2) was used for regression ol analysis for all temperatures. Substituting for Caq and analytically solving the resulting equation, gives the relation between total conversion and time (Eq. (3)). 
 2 2(AC=B + 1) P ; (3) t = 2 [P − AC] − ln B B AC where, A = (k1 Cneo W=NAo );

B = (2k2 (1 + )W )=VT ;

C = 16042 Cneo ;

(4)

√ P = AC − B( 1 + Cx − 1):

(5)

The least square method was used to determine the values of k1 and k2 at di5erent temperatures. Fig. 7a shows the agreement between the experimental and predicted results using these k1 and k2 values. The activation energy plot is given

H. Zhang et al. / Chemical Engineering Science 57 (2002) 315–322

321

Notation

-8 -9

ln(k)

-10 -11 -12 -13 -14

0.0025

0.00255

0.0026

0.00265

0.0027

0.00275

0.0028

1/T

A; B; C C k1 k2 KD N r

Fig. 11. Activation energy plot for forward and reverse reactions.

W x 

3 2.5 ln (K)

2

de9ned by Eq. (4) concentration (kmol=m3 ) forward rate constant (m3 =kg s) backward rate constant (m3 =kg s) empirical constant de9ned by Eq. (2) number of moles (kmol) rate of formation per unit mass of the catalyst (kmol=sec kg) mass of the catalyst (kg) conversion of cyclohexanol organic phase holdup=aqueous phase holdup (vol=vol)

Subscripts

1.5

aq org neo

1 0.5 0 0.0025

0.0026

0.0027

0.0028

1/T Fig. 12. Determination of enthalpy of reaction.

in Fig. 11. The activation energies for forward and backward reactions are 16:85 kcal=gmol and 28:06 kcal=gmol respectively. The reasonably large values of activation energy indicate that the reaction is kinetically controlled and that the assumption of negligible mass transfer limitations is valid. The enthalpy of reaction was determined by plotting equilibrium constant as a function of 1=T (Fig. 12). The value obtained (11:2 kcal=gmol) is slightly higher than that reported by Beenackers and co-workers (Panneman & Beenackers, 1992). The experimental value of enthalpy of reaction reported by them is 8:9 kcal=mol and theoretically calculated value based on heats of formation data is 8:45 kcal=gmol.

5. Conclusions The hydration of cyclohexene has been studied in solid– liquid–liquid mode. Various solid catalysts were evaluated and ZSM-5 (SiO2 =Al2 O = 30–50) o5ered best performance. The initial rate of the reaction is a weak function of the aqueous to organic phase ratio. However, the equilibrium conversion is strongly governed by the fractional organic phase holdup in the reactor. The rate constants and the activation energies of forward and backward reactions were determined. The large values of activation energy indicate the absence of intra-particle=intra-crystalline di5usion limitations. To sum up, cyclohexene hydration is an uncommon reaction that uses very 9ne particles of zeolite and slurry reactor has to be adopted with substantial catalyst loading.

ol

aqueous phase organic phase initial concentration of cyclohexene in organic phase cyclohexanol

Superscripts ol ne o

cyclohexanol cyclohexene initial

References Chakrabarti, A., & Sharma, M. M. (1992). Cyclohexanol from cyclohexene via cyclohexyl acetate. Reactive Polymers, 18, 107–116. Ishada, H. (1997). Liquid phase hydration of cyclohexene with zeolites. Catalysis Surveys of Japan, 1, 241–246. Mahajani, S. M., Kolah, A. K., & Sharma, M. M. (1995). Extractive reactions with cationic exchange resins catalysts (acetalization of aldehydes with alcohols). Reactive and Functional Polymers, 28, 29–38. Mahajani, S. M., Sharma, M. M., & Sridhar, T. (1999). Uncatalysed oxidation of cyclohexene. Chemical Engineering Science, 54, 3967– 3976. Minotti, M., Doherty, M. F., & Malone, M. F. (1998). Design for simultaneous reaction and liquid–liquid extraction. Industrial and Engineering Chemistry Research, 37, 4748–4755. Mitsui, O., & Fukuoka, Y. (1986). Process for producing cyclic alcohols. US Patent No. 4558846. Nagahara, H., Ono, M., & Fukuoka, Y. (1995). Partial hydrogenation of benzene to cyclohexene. In Y. Izumi, H. Arai, & M. Iwamoto (Eds.), Studies in Surface Science (pp. 375 –378). Nakato, T., Toyoshi, Y., Kimura, M., & Okuhara, T. (1999). Unique catalysis of an acidic salt of heteropoly acid, Cs2:5 H0:5 PW12 O40 consisting of microcrystallites. Catalysis Today, 52, 23–28. Okuhara, T., Kimura, M., & Nakato, T. (1997). Hydration of Ole9ns in excess water catalysed by an insoluble cesium hydrogen salt of dodecatungstophosphoric acid. Chemistry Letters, 8, 839–840. Pahari, P. K., & Sharma, M. M. (1991). Recovery of morpholine via reactive extraction. Industrial and Engineering Chemistry Research, 30, 2015–2017.

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Panneman, H. J., & Beenackers, A. A. C. M. (1992). E5ect on the hydration of cyclohexene catalyzed by strong ion exchange resins I: Solubility of cyclohexene in aqueous sulfolane mixture; II: E5ect of sulfolane on reaction kinetics; III: E5ect of sulfolane on equilibrium conversion. Industrial and Engineering Chemistry Research, 31, 1227–1234; 1425 –1433; 1433–1440. Struijk, J., d’Angremond, M., Lucas-de-Regt, W. J. M., & Scholten, J. J. F. (1992). Partial liquid phase hydrogenation of benzene to cyclohexene over ruthenium catalysts in the presence of aqueous salt solution. Applied. Catalysis A, General, 83, 263–295. Suresh, A. K., Sridhar, T., & Potter, O. E. (1988a). Mass transfer and solubility in autocatalytic oxidation of cyclohexane. A.I.Ch.E. Journal, 34, 55–69.

Suresh, A. K., Sridhar, T., & Potter, O. E. (1988b). Autoxidation of cyclohexane-modelling reaction kinetics. A.I.Ch.E. Journal, 34, 69 –81. Suresh, A. K., Sridhar, T., & Potter, O. E. (1988c). Autoxidation of cyclohexane-mass transfer and chemical kinetics. A.I.Ch.E. Journal, 34, 81–86. Vasudevan, T. V., & Sharma, M. M. (1984). Some aspects of process design of liquid–liquid reactors. Industrial and Engineering Chemistry Process Design and Development, 23, 400–406. Yamashita, K., Hideaki, O., & Katsuta, I. (1995). Method for partially hydrogenating a monocyclic aromatic hydrocarbon. US Patent No. 5457251. Zhang, H., & Wei, T. (1998). Hydration of cyclohexene. Journal of Fuel Chemistry and Technology, 23(4), 334–344.