Green alkylation of anisole with cyclohexene over 20% cesium modified heteropoly acid on K-10 acidic montmorillonite clay

Applied Clay Science 53 (2011) 254–262

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Research Paper

Green alkylation of anisole with cyclohexene over 20% cesium modified heteropolyacid on K-10 acidic montmorillonite clay

Ganapati D. Yadav ⁎, Santosh R. MoreDepartment of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai-400019, India 1

⁎ Corresponding author. Tel.: +91 22 3361 1001; faxE-mail addresses: [email protected], gd.yadav@ic

1 Formerly University of Mumbai Institute of Chemicuniversity.

0169-1317/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.clay.2011.03.005

a b s t r a c t

Article history:Received 3 April 2010Received in revised form 2 March 2011Accepted 9 March 2011Available online 15 March 2011

Keywords:Clay catalysisAlkylationCyclohexeneSolid acidsAnisoleKinetics

Alkylation of anisole with cyclohexene is an industrially important reaction. It was carried out in the liquidphase using in a number of solid acids as catalysts. Acid treated clay (K-10) as support with 20% m/m 20%Cs2.5H0.5PW12O40 (cesium modified dodecatungstophosphoric acid; designated as Cs-DTP/K-10), with nano-sized particles, was found to be the best catalyst. This reaction leads to formation of 2-cyclohexylanisole and4-cyclohexylanisole, which have been used as perfumery compounds and also as precursors to synthe-sise several commercial products. Conversion of cyclohexene was found to be 96% after 3 h at 105 °C. Nooligomerisation of cyclohexene was observed under the optimized reaction conditions. Effect of differentparameters on rate of reaction was studied systematically and the kinetics of reaction deduced.

: +91 22 3361 1002/1020.tmumbai.edu.in (G.D. Yadav).al Technology; now a separate

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The Friedel–Crafts alkylation of aromatic compounds with alkenesis of great significance in the production of drugs, pesticides and dyes(Olah, 1964; Olah et al., 1991; Roberts and Khalaf, 1984). For example,a number of important industrial processes for the production ofethylbenzene, cumene and linear alkylbenzenes are based on thisreaction. In general, Friedel–Crafts alkylation and acylation arecatalysed by AlCl3, H2SO4, HF and other acid catalysts which areused in more than stoichiometric quantities. These homogeneouscatalysts are efficient but their corrosive and toxic nature posespotential environmental hazards and presents operational problems,including difficulty in separation, recovery and reutilization of thecatalyst resulting in higher costs. In order to avoid the disadvantagesand the environmental limitations of the conventional Friedel–Craftshomogeneous catalysts, it is necessary to find suitable, recyclable andenvironmental-friendly heterogeneous solid acid catalysts (Clark,1999; Corma and Martinez, 1993; Thomas, 1992). Common alkylatingagents used for Friedel–Crafts alkylation are alkyl halides, alcohols,ethers and alkenes. Application of alkenes as an alkylating agentseems to be more attractive, as it can provide high atom economy tothe process.

Song et al. (2000) have disclosed Friedel–Crafts alkylation ofaromatics with alkenes using a novel and recyclable Sc(OTf)3 catalystin ionic liquids. Cyclohexylanisole has been produced by coupling ofaryl Grignard reagents (cyclohexyl bromide) with alkyl halides (4-MeC6H4MgBr) by using Fe–PEG catalyst to give 72–78% conversion(Bedford et al., 2006). Neumann and Khenkin (2008) prepared newelectrophilic catalysts, which are based on the NO+ cation. In polarreaction media consisting only of alkenes, significant amounts of C–Ccoupling products were obtained. This is an interesting alternative toFriedel–Crafts type alkylations using alkenes with aromatic substratesundermild reaction conditions. Kurz and Rodgers (1985) showed thatphotolysis of cyclohexyl iodide with aromatic compounds led to aro-matic cyclohexylation. Polyphosphoric acid is also used as a catalyst inalkylation of anisole with 2-propanol and cyclohexanol, and of phenolwith cyclohexanol (Gardner, 1954). Alkylation of anisole withcyclohexanol leads to the formation of 2- and 4-cyclohexylanisoles.Recently, alkylation of p-cresol, phenol, and guaiacol with cyclohex-ene was reported by our laboratory using sulfated zirconia (S-ZrO2),20%DTP/K-10, and ion exchange resins as catalysts (Yadav and co-workers (see relevant papers cited herein)). However, a detailedkinetic investigation of alkylation of anisole with cyclohexene usingheteropoly acid salts such as partially exchanged Cs+, NH4

+ ions,which are regarded as highly active, stable and reusable solid acidcatalysts, is still missing from the reported literature.

Our laboratory has been recognized for the development of eco-friendly novel solid acid catalysts, green chemical processes andselectivity engineering for several industrially relevant reactionsincluding process kinetics which includes catalysis by sulphatedzirconia (Yadav and Nair, 1999), UDCaT series (Yadav and Murkute,

255G.D. Yadav, S.R. More / Applied Clay Science 53 (2011) 254–262

2004a,b,c; Yadav et al., 1999), clay supported heteropoly acids (Yadavet al., 2005), Cs-DTP/K-10 (Yadav and Asthana, 2002; Yadav andSalgaonkar, 2005), DTP/HMS (Yadav and Manyar, 2003) and ionexchange resins (Yadav and Kulkarni, 2000) for a variety of bulk andfine chemical processes involving alkylation, acylation, esterification,nitration, isomerization, etherification, cracking, oligomerisation,cyclization, and dehydration, etc. Supported heteropoly acids arepromising heterogeneous catalyst for the organic synthesis (Okuharaand Nakato, 1998). Recently, the novelty of clay supported partiallysubstituted dodecatungstophosphoric (DTP) acid with cesium by anin situ grafting-reaction deposition method (designated as Cs-DTP/K-10) was reported by our group. It has been explored for a number ofacid catalyzed reactions by our research group (Yadav, 2005; Yadavet al., 2003).

We report an application of a novel solid acid catalyst Cs-DTP/K-10for a solventless Friedel–Crafts alkylation of anisole with cyclohexeneto obtain cyclohexyl anisole including kinetic modeling. The reactionis 100% atom economical and green.

2. Experimental

2.1. Chemicals and catalysts

Anisole, cyclohexene, n-dodecane, cesium chloride, dodecatung-stophosphoric acid hexahydrate, zirconium oxychloride, ammoniasolution and ethanol were obtained from s. d. Fine Chemicals Pvt. Ltd.,Mumbai. Chlorosulphonic acid was procured from Spectrochem Pvt.Ltd., Mumbai. K-10 montmorillonite clay and tetra ethyl ortho silicate(TEOS) were obtained from Fluka chemicals, Germany. All thechemicals were used without any further purification. Ion exchangeresins like Amberlyst-36 and Indion-130 were obtained from s. d. Finechemicals Pvt. Ltd. All these ion exchange resins were obtained in theform of wet granules, which were pretreated before use.

The supported heteropoly acid catalyst, 20% m/m Cs2.5H0.5PW12-

O40 over K-10 montmorillonite (Cs-DTP/K-10) was synthesized by aprocess developed by Yadav et al. (2003) in our laboratory. UDCaT-4,UDCaT-5 and UDCaT-6were synthesized using a process developed byYadav and Murkute (2004a,b). These catalysts have been wellcharacterized and reported. All the catalysts were dried in oven at110 °C for 2 h before use.

2.2. Reaction scheme

OCH3

+

OCH3

OCH3

+

AnisoleCyclohexene 2-Cyclohexylanisole 4-Cyclohexylanisole

Cs-DTP/K-10

A B C D

2.3. Reaction procedure

All experiments were carried out in a 100 cm3 stainless steel Parrautoclave reactor. A four bladed-pitched turbine impeller was used foragitation. The temperature was maintained at ±1 °C of the desiredvalue. Known quantities of reactants and catalyst were charged intothe autoclave, the temperature raised to the desired value andagitation started. Then, an initial sample was withdrawn. Furthersamples were withdrawn at periodic intervals up to 3 h. A standardexperiment consisted of 0.42 mol anisole, 0.06 mol cyclohexene and acatalyst loading of 0.03 g/cm3 with respect to total volume of reactionmixture. The temperature and speed of agitation was maintained at100 °C and 1000 rpm, respectively. The reaction was carried out

without using any external inert solvent. The total volume of reactionliquid phase was 52.5 cm3.

2.4. Method of analysis

Samples were withdrawn periodically and filtered to removecatalyst particles, if any. GC analysis was performed (Chemito Model-8610) by using a stainless steel column (3.25 mm×4 m) packed witha stationary phase of 10% OV-17 supported on chromosorb WHP inconjunction with flame ionization detector. The injector and detectortemperature were kept at 300 °C. Nitrogen gas was used as the carriergas. The conversionwas based on the disappearance of cyclohexene inthe reaction mixture with respect to internal standard. The productswere confirmed by Gas Chromatography-Mass Spectrometry (GC-MS) analysis.

3. Results and discussion

3.1. Efficacies of various solid acid catalysts

Various solid acid catalysts were used to assess their efficacy foralkylation reaction. A 0.03 g cm−3 loading of the particular catalystbased on the volume of the reaction mixture was employed at 100 °Cat a speed of agitation of 1000 rpm. The catalysts used were UDCaT-5(superacidic sulphate-group modified zirconia), UDCaT-6 (mesopor-ous form of UDCaT-5), UDCaT-4, Cs-DTP/K-10, Amberlyst-36 andIndion-130. The order of activity for solid acid catalyst was found to beas follows :

Cs�DTP=K�10ðmost activeÞNUDCaT�5NAmberlyst�36N Indion�130NUDCaT�6NUDCaT�4

Cs2.5H0.5PW12O40/K-10 (Cs-DTP/K-10) showed the highest activity(Fig. 1). It was found to be more active than commercially availableAmberlyst-36 and Indion-130 cation exchange resins. Hence, furtherexperimentswere conductedwith Cs-DTP/K10 under otherwise similarconditions. Catalyst preparation and characterization was reported byYadav et al. (2003) recently; only the key findings are discussed here.

The FT-IR scan of 20% m/m Cs-DTP/K-10 (Fig. 2(e)) is similar tothat of K-10 (Fig. 2(d)) except that there is a prominent peak at821.1 cm−1 for DTP/K-10. However, the shift observed for this peakwhen compared to that of pure DTP (Fig. 2(a)) could be attributed tothe interaction between DTP and K-10. There are sharp peaks in theregion of 1600–1700 cm−1, indicating the presence of H3O+

(Bronsted acidity). 31P NMR analysis of 20% Cs-DTP/K-10, showing apeak at a chemical shift of ca.−15 ppm, with reference to 0 ppmresponse from 85% H3PO4, confirms that [PW12O40] −3 was the onlyspecies present on the support and no additional 31P NMR peaksca.−13 ppm attributed to a defect (P2W21O71)−6 Keggin wereevident. The nitrogen adsorption desorption isotherms for 20% m/mCs-DTP/K-10 shows that it has type IV isotherm with the hysteresisloop of type H3, which is a characteristic of mesoporous solids. BETsurface area of 20%m/m Cs-DTP/K-10 was found to be 207 m2/g, andthe average pore size fall in the range of 50–75 A0, suggesting thatpore sizes of the catalyst lie in the mesoporous region (Table 1). BulkXRD analysis of DTP is characterized by its cubic structure andcrystalline nature. The crystallographic structure is preserved whilepreparing Cs2.5H0.5PW12O40 when protons of bulk DTP are with Cs+.While impregnating Cs2.5H0.5PW12O40 on K-10, some crystallinity waslost.

3.2. Effect of speed of agitation

To assess the role of external mass transfer on the reaction rate, theeffect of the speed of agitation was studied from 800 to 1200 rpm

0

10

20

30

40

50

60

70

80

90

100

Cs- DTP/ K10 UDCaT-5 Amberlyst-36 Indion-130 UDCaT-6 UDCaT-4

Co

nve

rsio

n (

%)

Fig. 1. Activity of various solid acid catalysts. Cyclohexene: 0.06 mol, anisole: 0.42 mol, catalyst loading: 0.03 g/cm3, temperature: 100 °C, speed of agitation: 1000 rpm.

256 G.D. Yadav, S.R. More / Applied Clay Science 53 (2011) 254–262

with an average particle size of 50 μm. The conversion of cyclohexene,the limiting reactant, at different intervals of time is shown in Fig. 3.It was observed that the conversion of cyclohexene was practicallythe same beyond 1000 rpm. Thus, it was ensured that externalmass-transfer effects did not influence the reaction rate. To be on safeside, all further experiments were conducted at 1000 rpm.

3.3. Proof of absence of external mass transfer resistance

The reaction of A (cyclohexene) and B (anisole) lead to the followingreaction product-

A + B→Catalyst

Product ð1Þ

Cyclohexene was taken as limiting reactant in control experimentsand there was no oligomerisation of cyclohexene. At steady state, therate of mass transfer of cyclohexene (A) per unit volume of the liquidphase is given by

RA = kSL−Aap CA0−CAS½ � ð2Þ

(rate of transfer of cyclohexene (A) from bulk liquid to externalsurface of the catalyst particle)

RB = kSL−Bap CB0−CBS½ � ð3Þ

(rate of transfer of anisole (B) from bulk liquid to external surface ofthe catalyst particle)

RA = robs ð4Þ

(observed rate of the reaction within the catalyst particle).Eq. (4) could be represented by a Langmuir–Hinselwood–Hougen–

Watson (LHHW) type or power law model with or without the effec-tiveness factor η, to account for the intraparticle diffusion resistance.

Depending on the relative magnitudes of external resistance tomass transfer and reaction rates, different controlling mechanismshave been put forward. When the external mass transfer resistance issmall, then the following inequality holds:

1robs

≫ 1kSL−AapCA0

and1

kSL−BapCB0ð5Þ

Thus, the observed rate could be given by three types of modelswherein the contribution of intra-particle diffusion resistance couldbe accounted for by incorporating the effectiveness factor η. Thesemodels are:

(a) The power law model if there is very weak adsorption of reac-tant species;

(b) Langmuir–Hinselwood–Hougen–Watson model;(c) Eley–Rideal model.

Hence, effect of speed of agitation, catalyst loading and particlesize were studied to ascertain the absence of external mass transferand intra-particle diffusion resistances so that the true intrinsickinetics could be developed. According to Eq. (5), it is necessary tocalculate the rates of external mass transfer of both anisole andcyclohexene and compare them with the rate of reaction. For aspherical particle, the particle surface area per unit volume is givenby

ap =6wρpdp

= 42:45cm2= cm3 ð6Þ

wherew, catalyst loading used in g/cm3 of the liquid phase, ρp, densityof particle in g/cm3, and dp, particle diameter (cm).

The diffusivity values (D) were calculated by using the Wilke–Chang equation (Reid et al., 1977) at 100 °C and these values are asfollows:

DAB = 9:84 × 10−6cm2=s and DBA = 1:03 × 10−5cm2

=s:

Thus, the corresponding values of the solid liquid mass transfercoefficients for both of the reactants A (cyclohexene) and B (anisole)were calculated from the limiting value of the Sherwood number (e.g.Sh−A=kSL−Adp/DAB) of 2. The actual Sherwood numbers are typicallyhigher by order of magnitude in well-agitated systems but forconservative estimations a value of 2 is taken. The solid-liquid masstransfer coefficient kSL-A and kSL-B were obtained as 3.93×10−2 and4.15×10−2 cm/s respectively. The initial rate of the reaction wascalculated from the conversion profile. A typical calculation showsthat for a typical experiment, the initial rate of reaction was calculated

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140 160 180 200

Time (min)

Co

nve

rsio

n (

%)

800 rpm 1000 rpm 1200 rpm

Fig. 3. Effect of speed of agitation. Cyclohexene: 0.06 mol, anisole: 0.42 mol, catalyst:20% Cs-DTP/K-10, catalyst loading: 0.03 g/cm3, temperature: 100 °C.

30

40

50

60

70

80

90

100

Co

nve

rsio

n (

%)

Fig. 2. FT-IR spectra — a) DTP, b) Cs2.5H0.5PW12O40, c) K-10 clay, d) 20% DTP/K-10 ande) 20% Cs2.5H0.5PW12O40/K-10.

257G.D. Yadav, S.R. More / Applied Clay Science 53 (2011) 254–262

as 5.0×10−7 mol/cm3 s. Therefore, putting the appropriate value inEq. (5).

1robs

≫ 1kSL−AapCA0

and1

kSL−BapCB0ð7Þ

i.e. 2.0×106≫5.33×103 and 0.72×103

This demonstrates that there was no resistance to external masstransfer for both reactants.

Table 1Textural properties of screened solid acid catalysts.

Catalyst Surface area(m2/g)

Pore volume(cm3/g)

Pore diameter(A°)

20% Cs2.5H0.5PW12O40/K-10 207 0.29 58UDCaT-5 83 0.21 40UDCaT-6 877 0.70 32Amberlyst-36 35 0.30 200Indion-130 15-20 NA NA

3.4. Effect of catalyst loading

In the absence of external mass-transfer resistance, the rate ofreaction is directly proportional to the catalyst loading based on theentire liquid-phase volume. The catalyst loading was varied over arange of 0.01 to 0.04 g cm-3 on the basis of the total volume of thereaction mixture. Fig. 4 shows the effect of the catalyst loading on theconversion of cyclohexene. The conversion increased with an increasein the catalyst loading, which is due to the proportional increase in thenumber of active sites. However, beyond a catalyst loading of0.03 g/cm3, there was no significant increase in the conversion.Hence all further experiments were carried out at 0.03 g/cm3 catalystloading. Linearity of the plot − ln (1−XA) vs. time (t) as shown inFig. 5, further confirms that reaction rate is directly proportional tonumber of active catalyst sites.

0

10

20

0 20 40 60 80 100 120 140 160 180 200

Time (min)

0.01 gm/cc 0.02 gm/cc 0.03 gm/cc 0.04 gm/cc

Fig. 4. Effect of catalyst concentration. Cyclohexene: 0.06 mol, anisole: 0.42 mol, catalyst:20% Cs-DTP/K-10, temperature: 100 °C, speed of agitation: 1000 rpm.

y = 0.0081x

y = 0.0137x

y = 0.0292x

y = 0.0334xR2 = 0.9932

R2 = 0.9931

R2 = 0.9888

R2 = 0.9791

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 10 20 30 40 50 60

Time (min)

-ln

(1-

XA)

0.01 gm/cc 0.02 gm/cc 0.03 gm/cc 0.04 gm/cc

Fig. 5. Plot -ln (1-XA) vs. time (t) for catalyst loading. Cyclohexene: 0.06 mol, anisole:0.42 mol, catalyst: 20% Cs-DTP/K-10, temperature: 100 °C, speed of agitation: 1000 rpm.

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140 160 180 200

Time (min)

Co

nve

rsio

n (

%)

1:01 1:03 1:05 1:07

Fig. 6. Effect of mole ratio of cyclohexene to anisole. Catalyst: 20% Cs-DTP/K-10, catalystloading: 0.03 g/cm3, temperature: 100 °C, speed of agitation: 1000 rpm.

258 G.D. Yadav, S.R. More / Applied Clay Science 53 (2011) 254–262

At steady state, the rate of externalmass transfer (i.e. from the bulkphase in which A and B are located with concentration CA0 and CB0,respectively) to the exterior surface of the catalyst is proportional toap, the exterior surface area of the catalyst where the concentrationsof A and B are CAS and CBS, respectively. For a spherical particle, ap isalso proportional to w, the catalyst loading per unit liquid volume. It ispossible to calculate the values of CAS and CBS. For instance,

kSL−Aap CA0−CAS½ � = robs At steady state= 1:89 × 10−7mol=cm3

:sð8Þ

Thus putting in the appropriate values, it is seen that CA0≅CAS,similarly CB0≅CBS. Thus, any further addition of catalyst is not going tobe of any consequence for rate of external mass transfer.

3.5. Proof of absence of intra-particle diffusion resistance

The average particle diameter of the catalyst used in the reactionwas 50 μm and thus a theoretical calculation was done by using theWiesz–Prater criterion (Foglar, 1995) to assess the influence of intra-particle diffusion resistance. According to the Wiesz–Prater criterion,the dimensionless parameter Cwp which represents the ratio of theintrinsic reaction rate to intra-particle diffusion rate can be evaluatedfrom the observed rate of reaction, the particle radius (Rp), effectivediffusivity of the limiting reactant (De) and concentration of thereactant at the external surface of the particle.

(i) If Cwp=−robsρpR2p/De [CAS]≫1, then the reaction is limited

by severe internal diffusion resistance.(ii) If Cwp≪1, then the reaction is intrinsically kinetically

controlled.

The effective diffusivity of cyclohexene (De-A) inside the pores ofthe catalyst was obtained from the bulk diffusivity (DAB), porosity (ε)and tortuosity (τ) as 8.89×10−6 cm2/s, where De-A=DAB (ω/τ). Inthe present case, the value of Cwp was calculated as 1.9×10−2 for theinitial rate which is much less than 1 and therefore, the reaction isintrinsically kinetically controlled. A further proof of the absence ofthe intra-particle diffusion resistance was obtained by the study of theeffect of temperature and it will be discussed later.

3.6. Effect of molar ratio of cyclohexene to anisole

The molar ratio of cyclohexene to anisole was varied from 1:1 to1:7 range under otherwise same operating conditions (Fig. 6). Thisincrease in overall rate of alkylation with an increase in mole ratio ofcyclohexene to anisole can be explained by concept of activity (Fig. 7).It appears that with an increase in amount of anisole, the activity ofcyclohexene increases results in increase in the rate of alkylationreaction. It is also assumed that lower the concentration ofcyclohexene, more the adsorption of cyclohexene on the catalystsites and therefore, more availability of cyclohexenium cation to reactwith anisole. Thus, all the subsequent reactions were carried out witha mole ratio of 1:7.

3.7. Effect of temperature

The reaction was studied from 90 to 105 °C to investigate influenceof temperature on the rate of reaction. No oligomerisation ofcyclohexene was observed in the used temperature range, whichwas also reported earlier (Yadav and Goel, 2000). It was found thatwith an increase in temperature the rate of reaction is increasedsubstantially, which suggested that the reaction was intrinsicallykinetically controlled and activation energy could be determined(Fig. 8).

3.8. Catalyst reusability study

The reusability of the catalyst was studied by filtering the catalystat the end of the reaction. Reusability of 20% Cs-DTP/K-10 was testedby conducting three runs (Fig. 9). After each reaction the catalyst wasfiltered and then refluxed with (3×50 ml) of toluene for 2 h in orderto remove any adsorbed material from catalyst surface and pores anddried at 110 °C for 2 h after every use. There were losses duringhandling since the particle size was very small and typically about 10–15% catalyst would be lost. On the basis of the initial rates based oncatalyst mass (mol/g-cat/s) the catalyst was found to be robustwithout loss of activity. Thus, the results are good. Catalyst was foundto be fairly reusable for three cycles after fresh use.

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140 160 180 200

Time (min)

Co

nve

rsio

n (

%)

Fresh 1st Reuse 2nd Reuse 3rd Reuse

Fig. 9. Reusability of catalyst. Cyclohexene: 0.06 mol, anisole: 0.42 mol, catalyst: 20%Cs-DTP/K-10, catalyst loading: 0.0 3 g/cm3, temperature: 100 °C, speed of agitation:1000 rpm.

y = 0.0076x

y = 0.0112x

y = 0.0174x

y = 0.0284x

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40

Time (min)

1:01 1:03 1:05 1:07

-ln

(1-

XA)

R2 = 0.9866

R2 = 0.9717

R2 = 0.9594

R2 = 0.9937

Fig. 7. Plot − ln (1−XA) vs. time (t) for different mole ratios. Catalyst: 20% Cs-DTP/K-10, catalyst loading: 0.03 g/cm3, temperature: 100 °C, speed of agitation: 1000 rpm.

259G.D. Yadav, S.R. More / Applied Clay Science 53 (2011) 254–262

3.9. Reaction kinetics and modeling

3.9.1. Development of mechanistic modelIt is essential to understand the reaction mechanism to develop a

suitable kinetic model in the present case. From the calculated valuesof mass transfer rates of A and B, and initial observed rates, it isevident that the rate was independent of the external mass transfereffects. It was also evident from the values of activation energy, thatthe intraparticle diffusion resistance was absent. Thus, the reactioncould be controlled by one of the following steps, (a) adsorption, (b)surface reaction, (c) desorption. Therefore, for the further develop-ment of the model, the actual mechanism wasinvestigated. The Eley–Rideal mechanism was assumed to prevail (Yadav and Kumar, 2005).Cyclohexyl carbocation (AS) formed by the interaction of adsorbedcyclohexene (A) on the acid catalyst sites reacts with the anisole (B)from the liquid phase to produce C-alkylated anisole through wheal

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140 160 180 200

Time (min)

Co

nve

rsio

n (

%)

90° C 95° C 100° C 105° C

Fig. 8. Effect of temperature. Cyclohexene: 0.06 mol, anisole: 0.42 mol, catalyst: 20% Cs-DTP/K-10, catalyst loading: 0.03 g/cm3, speed of agitation: 1000 rpm.

and intermediates. To establish this mechanism, suitable kineticmodels have to be developed.

The initial rate data can be analyzed on the basis of Langmuir–Hinshelwood–Hougen–Watson (LHHW) or Eley–Rideal mechanisms.For initial rate data, the following analysis is the most appropriate:

1. Adsorption of cyclohexene (A) on a vacant site S represented by:

A + S⇌KA AS Equilibrium constant KAð Þ ð9Þ

Similarly, adsorption of anisole (B) on a vacant site S is given by:

B + S⇌KB BS Equilibrium constant KBð Þ ð10Þ

It is assumed that the adsorbed anisole (BS) does not react but onlyhas weak adsorption on catalyst site. The anisole (B) from theliquid phase reacts with adsorbed cyclohexene (AS), according tothe Eley–Rideal mechanism.

2. The chemisorbed AS reacts with B from the liquid phase, in thevicinity of the site, leading to the formation of C-alkylated productC (cyclohexyl anisole).The formation of C-alkylated product C and D is given by:

AS + B⇌k1

k01

CS + DS ð11Þ

The desorption of alkylated product is represented as:

CS⇌1=kC C + S ð12Þ

DS⇌1=kD D + S ð13Þ

where, the desorption constants are reverse of adsorptionconstants.The total concentration of the sites, Ct expressed inmol g-cat−1 is given by:

Ct = CS + CAS + CBS + CCS + CDS ð14Þ

where, CS=concentration of the vacant sites, mol g-cat−1.

260 G.D. Yadav, S.R. More / Applied Clay Science 53 (2011) 254–262

The various models can now be developed to determine theoverall rate of reaction.

3.9.1.1. Surface reaction controlled mechanism. The surface reactioncontrolling mechanism is most commonly found to control severalreactions and is considered first. If the surface reaction betweenchemisorbed AS with B from the liquid phase leading C-alkylationreaction, is rate controlling, then overall rate of reaction (roi) of B withAS, sum of all net rates of various surface complexes given by equation

roi = k1CBCAS−k′1CCSCDS ð15Þ

where, various equilibrium constants are given by the appropriateequations.

It is essential to substitute the concentrations of surface species: e.g.

CAS = KACACS ð16Þ

CBS = KBCBCS ð17Þ

CCS = KCCCCS ð18Þ

CDS = KDCDCD ð19Þ

where KA, KB, KC, KD are adsorption equilibrium constants.Putting the appropriate values of concentrations in Eq. (14), the

following is obtained:

Ct = CS + KACACS + KBCBCS + KCCCCS + KDCDCS ð20Þ

CS = Ct = 1 + KACA + KBCB + KCCC + KDCDð Þ ð21Þ

Replacing the total concentration of sites byw, the solid loading in gcm−3 of liquid phase: Ct=ktw, (Where kt is proportionality constant).

If adsorption of anisole on catalyst surface is very weak, KBCB canbe neglected and hence:

CS = ktw= 1 + KACA + KCCC + KDCDð Þ ð22Þ

Thus, the overall rate of the reaction is given by substitutingEqs. (16) and (22) in Eq. (15):

roi = k1KACACBCS−k0

1KCCCCDCS

= k1KACACB−k0

1KCCCCD

� �CS ð23Þ

Substituting the value of Cs from Eq. (22) to Eq. (23) leads to:

roi =ktw k1KACACB−k′1KCCCCDð Þ1 + KACA + KCCC + KDCDð Þ ð24Þ

Eq. (24) suggests that the relative values of the rate constants,equilibrium constants and the concentration of the various specieswill govern the overall rate of reaction of chemisorbed cyclohexene(AS) with anisole (B) from the liquid phase.

3.9.1.2. Adsorption of cyclohexene alone is significant. If adsorption ofcyclohexene (A) is much stronger than all other species then, theentire catalysts surface would be covered by A and hence

1 + KACAð Þ N N KCCC + KDCDð Þ ð25Þ

Since no poisoning of the catalyst was observed by C-alkylatedproduct during the preliminary experiments and the alkylated pro-ducts were assumed to be weakly adsorbed on the catalyst surface, thereversible reaction could be neglected. Thus, term k1

' KCCCCD could beneglected from the Eq. (24). These assumptions appeared to be valid.

Thus, Eq. (24) can be rearranged for initial rates as follows:

roi =wkRKACACB

1 + KACAð Þ ð26Þ

where kR=k1kt.This is the Eley–Rideal equation.If the initial rates of reaction (roi) are measured, then Eq. (23) can

be transformed into a very convenient form as given below:

CBow

roi=

1kRKACAo

+1kR

ð27Þ

A plot of (CB0/roi) against (1/CAo) gives a slope equal to (1/kRKA)and an intercept of (1/kR) from which both the reaction rate constantkR and adsorption equilibrium constant KA could be established.

3.9.1.3. When CBo≫CAo. Then the change in concentration of B withrespect to time is assumed to be negligible.Now CB ≅ CBo and CA=CAo(1−XA)Eq. (26) can be rewritten as:

roi = − dCA

dt= wkRKACA0 1−XAð ÞCB0 ð28Þ

Thus,

dXA

dt= kRKAð ÞCBow 1−XAð Þ ð29Þ

Eq. (above) can be integrated to get the following:

− ln 1−XAð Þ = kSRwCBot ð30Þ

(where kRKA=kSR=reaction rate constant)Hence a plot of − ln(1−XA)vs. time (t) would be a straight line

with slope equal to (kSRwCBo), from which the rate constant kSR couldbe evaluated. Moreover, the linearity of this plot would confirm thepseudo-first order behavior of the reaction.

3.9.2. Establishment of reaction kineticsThe above theory could now be utilized to discern the controlling

mechanism and kinetics of the reaction. A plot of− ln(1−XA)vs. time(t) was obtained at different temperature. It can be seen that all theseplots show straight lines passing through the origin and the data fitwell for the equation (Fig. 10). This confirms the pseudo-first orderbehavior of the reaction. The rate constants can be evaluated from theslopes of the plot − ln(1−XA)vs. time (t) at different temperatures.The slopes obtained at 90, 95, 100 and 105 °C to get the rate constants(kSR) as given in Table 2. The Arrhenius plot of ln kSR vs.1/T (Fig. 11)gives apparent activation energy for C-alkylation reaction. Apparentactivation energy was found to be 21.96 kcal/mol. The value ofactivation energy obtained confirmed that reaction is intrinsicallykinetically controlled. In addition to these, the value of reaction rateconstant kR and equilibrium constant KA was determined using a plotof (CBow/roi) against (1/CAo) (Fig. 12). Initial rates fit well for this plot.Validation of the proposed model was also supported by linearity of− ln(1−XA) vs. time (t) plot for two cases (i) catalyst loading (Fig. 5)and (ii) effect of mole ratio (Fig. 7) parameters. There is a very goodagreement between experimental results and theory. The reactionfollows a pseudo first order kinetics.

4. Conclusion

Alkylation of anisole with cyclohexene using solid acid catalystsleads to the formation of 2- and 4-cyclohexyl anisole and all theseproducts are useful in a variety of chemical industries. For selective

y = -11042x + 30.167

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0.0026 0.00265 0.0027 0.00275 0.0028

1/T (K-1)

lnk S

R

R2 = 0.9473

Fig. 11. Arrhenius plot cyclohexene: 0.06 mol, anisole: 0.42 mol, catalyst: 20% Cs-DTP/K-10, catalyst loading: 0.03 g/cm3, speed of agitation: 1000 rpm. Ea=21.96 kcal/mol.

y = 0.4043x + 140.71

200

300

400

500

600

CB

0w/r

oi

R2 = 0.9411

y = 0.01x

y = 0.0179x

y = 0.0283x

y = 0.0327x

0

0.5

1

1.5

2

2.5

0 20 40 60 80

Time (min)

90° C 95° C 100° C 105° C

R2 = 0.9972

R2 = 0.9933

R2 = 0.9802

R2 = 0.9946

-ln

(1-

XA)

Fig. 10. Pseudo first order plot [− ln (1−XA) vs time]. Cyclohexene: 0.06 mol, anisole:0.42 mol, catalyst: 20% Cs-DTP/K-10, catalyst loading: 0.03 g/cm3, speed of agitation:1000 rpm.

261G.D. Yadav, S.R. More / Applied Clay Science 53 (2011) 254–262

C-monoalkylation of anisole, the reaction should be carried out at100 °C in presence of 20% Cs-DTP/K-10 with mole ratio 1:7. Here 92%conversion is realized for C-alkylated products. The reaction wascarried out without using solvent. A kinetic model was developedfor this reaction which follows a pseudo first order mechanism. Theapparent activation energy was found to be 21.96 kcal/mol. Thecatalyst was found to possess significant reusability.

NomenclatureA reactant species A, cyclohexeneB reactant species B, anisoleAS chemisorbed cyclohexeneBS chemisorbed anisoleC 2-cyclohexylanisoleD 4-cyclohexylanisoleCA concentration of A in g mol/cm3

CAo initial concentration of A at catalyst (solid) surface (mol/cm3)CB concentration of B in g mol/cm3

CBo initial concentration of B in bulk liquid phase, mol/cm3

CBS concentration of B at solid (catalyst) surface, mol/cm3

CC concentration of C in g mol/cm3

CCS concentration of C at solid (catalyst) surface, mol/cm3

CS concentration of vacant sites, mol/cm3

Ct total concentration of the sites, mol/cm3

dP diameter of catalyst particle, cmDAB diffusion coefficient of A in B, cm2/sDBA diffusion coefficient of B in A, cm2/sk1 surface reaction rate constant for forward reactionk’1 surface reaction rate constant for backward reactionkSR reaction rate constant cm6 g mol−1 g−1 s−1

kt dimensionless constant

Table 2Values of rate constants at various temperatures.

Temperature (K) Slope cm6 gmol−1 g−1 s−1 (kSR)

363 0.01 0.7077368 0.0179 1.2668373 0.0283 2.0028378 0.0327 2.3142

K1 surface reaction equilibrium constant, k1/k’1KA, KB… Adsorption equilibrium constant for A, B, C cm3/molro overall rate of reaction based on liquid phase volume, g mol

cm−3 s−1

roi initial rate of reaction based on liquid phase volume, g molcm−3 s−1

S vacant siteSh Sherwood numberw catalyst loading, g cm3 of the liquid phaseXA fractional conversion of A

Greek lettersρP density of catalyst particle, g/cm3

μ viscosity of solventη effectiveness factorε porosityτ tortuosity

0

100

0 200 400 600 800 1000

1/CA0

Fig. 12. Plot (CBow/roi) vs. (1/CAo) for initial rate. Cyclohexene: 0.06 mol, anisole:0.42 mol, catalyst: 20% Cs-DTP/K-10, catalyst loading: 0.03 g/cm3, speed of agitation:1000 rpm.

262 G.D. Yadav, S.R. More / Applied Clay Science 53 (2011) 254–262

Acknowledgement

GDY acknowledges support from the Darbari Seth Endowment andR.T.Mody Distinguished Professor Endowment; also the J.C. BoseNational Fellowship from DST-Govt. of India. SRM acknowledges theWorld-Bank's Technical Education Quality Improvement Program(TEQIP) and CSIR-NMTLI for awarding the Senior Research Fellowship.

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