Selectivity Engineering of Cation-Exchange Resins over Inorganic

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Ind. Eng. Chem. Res. 2007, 46, 3119-3127

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Selectivity Engineering of Cation-Exchange Resins over Inorganic Solid Acids in C-Alkylation of Guaiacol with Cyclohexene Ganapati D. Yadav* and Ganesh S. Pathre Department of Chemical Engineering, UniVersity Institute of Chemical Technology (UICT), UniVersity of Mumbai, Matunga, Mumbai 400 019, India

Friedel-Crafts alkylation of guaiacol with cyclohexene gives both C- and O-alkylated products, which are commercially valuable. The C-alkylated products are important intermediates for the production of resins, antioxidants, drugs, dyes, polymer additives, agrochemicals, and antiseptic substances. The objective of the current work was to achieve maximum selectivity for the C-alkylated products, using an active catalyst with minimum separation cost. A variety of eco-friendly solid acid catalysts such as sulfated zirconia, Amberlyst15, Filtrol-24, cesium-modified dodecatungstophosphoric acid (Cs-DTP) supported over K-10 clay (20% w/w Cs2.5H0.5PW12O40/K-10), and 20% w/w dodecatungstophosphoric acid (DTP)/K-10 clay were screened to study the effect of their nature of acidity on product distribution. Amberlyst-15 was determined to be the best catalyst to achieve maximum selectivity for C-alkylated product at 80 °C. The O-alkylation versus C-alkylation not only was observed to be highly temperature sensitive, but the nature of the acidic sites also had an important role. Effects of various parameters on rates and product distribution are discussed to deduce the kinetics of the reaction. 1. Introduction Alkylation reactions are commercially very predominant in a variety of industries, ranging from bulk organics to fine chemicals, particularly those involving aromatic substrates with olefins, alkyl halides, alcohols, and ethers as alkylating agents, which are catalyzed by Lewis and Bronsted acids. Alkylation of aromatics that contain different functional groups, such as OH, SH, NH2, and CN, present a formidable task of getting the desired selectivity at the O, S, N, or C centers. In addition to the nature of alkylating species, temperature and mole ratios of the substrates, relative to the alkylating agent, have vital roles. These are the so-called Friedel-Crafts reactions, which have been traditionally performed with highly polluting liquid acids. Pressure from legislative and environmental bodies, together with a growing awareness within the chemical industry, has led to a massive search for new eco-friendly processes to replace unacceptable outdated reactions. Therefore, a process that could be environmentally friendly and also inexpensive is the most desirable. The use of a solid acid catalyst offers many advantages over homogeneous acid catalysts, such as ease of separation, mild reaction conditions, better selectivity, waste minimization, less-expensive construction material, etc., which results in cleaner and less-expensive chemical process. In our laboratory, we have been seeking new methods to replace the use of strongly acidic, polluting, homogeneous, and corrosive catalysts, to make the processes cleaner and greener in a variety of industries, such as petrochemicals, pharmaceutical and drugs, perfumery and flavors, and polymers.1-4 The alkylation of guaiacol with cyclohexene yields O- and C-alkylated products, both of which have commercial value. The O-alkylated product is a promising perfumery chemical, whereas C-alkylated products are important intermediates for the production of resins, antioxidants, drugs, dyes, polymer additives, agrochemicals, and antiseptic substances.5-7 It has * To whom correspondence should be addressed. Tel.: 91-22-2410 2121, 2414 5616. Fax: 91-22-2410 2121, 2414 5614. E-mail: [email protected], [email protected].

been a challenging task to produce one of the alkylating products selectively to reduce waste and subsequently separation cost to achieve the dual goals, i.e., make the process economically feasible and environmentally friendly. Furthermore, as the selective hydrogenation of benzene to cyclohexene has been commercialized, cyclohexene-based processes are gaining much importance. Hence, cyclohexene can be used as a starting material for many industrially important chemicals, such as cyclohexanol, cyclohexanone, cyclohexylphenols, and cyclohexyl esters and ethers.8 A survey of literature showed that no detailed studies have been made on the alkylation of guaiacol with cyclohexene. Most of the work reported is in the form of patents. Allev et al.9 have reported a yield of 53% of 4-cyclohexyl guaiacol on reacting 0.5 mol of guaiacol and 0.31 mol of cyclohexene with 0.19 mol of 98% H2SO4 catalyst. Starkov et al.10 studied the reaction using KU catex cation-exchange resin. The use of ZnCl2-HCl solution for alkylation of cyclohexene has also been reported.11 The same reaction has also been reported using cyclohexanol as an alkylating agent.12,13 The choice of catalyst system for alkylation is generally based on the general acid-base characteristics (Bronsted, Lewis, strong or weak) of the materials. As per the literature reports, catalysts that possess strong Bronsted acidic sites favor ring alkylation,14 whereas Lewis acidic sites promote O-alkylation.15,16 Ion-exchange resins are ideal catalysts to convert polluting processes into greener processes, because of the aforementioned characteristics that they possess. In a large number of organic reactions, Amberlyst-15, which is a cation-exchange resin with Bronsted sites, is used as a catalyst in processes such as MTBETAME synthesis, the manufacture of alkyl phenols and bisphenol A, the esterification of a variety of carboxylic acids and alcohols, the hydration of alkenes, the dimerization of isobutylene, etc.17,18 It is obvious from the foregoing discussion that no systematic study has been conducted on the alkylation of guaiacol with cyclohexene to obtain mono C-alkylated product exclusively. Therefore, we undertook the current investigation to study the reaction systematically over a wide temperature range to gain a better insight in the operating conditions for

10.1021/ie060645t CCC: $37.00 © 2007 American Chemical Society Published on Web 09/22/2006

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Table 1. Activity of Various Catalysts in Alkylation of Guaiacol with Cyclohexenea Selectivity (%) catalyst

conversion of cyclohexene (%)

O-alkylated product

C-alkylated productsb

dialkylated product

Amberlyst-15 Indion-130 Filtrol-24 20% w/w DTP/K-10 clay 20% w/w Cs-DTP/K-10 clay 20% w/w ZnCl2/K-10 clay 20% w/w FeCl3/K10 clay sulfated zirconia

96 77 43 61 81 0 0 78

01 20 61 57 40 0 0 68

94 71 35 37 51 0 0 27

05 09 04 06 09 0 0 05

a Conditions: guaiacol concentration, 0.226 mol; cyclohexene concentration, 0.045 mol; catalyst loading, 0.05 g/cm3; temperature, 80 °C; speed of agitation, 1000 rpm; time, 120 min. b Mixture of isomers.

producing the C-alkylated products selectively. Several solid acids with different pore structure and acidities, such as sulfated zirconia, Amberlyst-15, Filtrol-24, cesium-modified dodecatungstophosphoric acid supported over K-10 clay (20% w/w Cs2.5H0.5PW12O40/K-10), and 20% w/w dodecatungstophosphoric acid (DTP)/K-10 clay, were placed on a short list. Here, the purpose of using these catalysts was to study the effect of the nature of acidity, its strength, and the pore size distribution on product selectivity. The activity and selectivity patterns are discussed, and a kinetic model is also presented. 2. Experimental Section 2.1. Chemicals and Catalysts. Guaiacol and cyclohexene were obtained from M/s s.d. Fine Chemicals, Pvt., Ltd., Mumbai, India. Amberlyst-15 was obtained from Rohm and Haas (Philadelphia, PA). The ion-exchange resin Indion-130 was obtained from Ion Exchange, Ltd., India, and Filtol-24, which is a commercially available clay, was obtained from Engelhard, USA. All other chemicals were procured from M/s s.d. Fine Chemicals, Pvt., Ltd. The following catalysts were prepared by well-developed procedures and characterized in our laboratory: 20% w/w dodecatungstophosphoric acid (DTP)/K-10,19,20 20% w/w ZnCl2/ K-10 montmorillonite clay,21 20% w/w FeCl3/K-10 montmorillonite clay,22 20% w/w Cs2.5H0.5PW12O40/K-10,23-25 and sulfated zirconia.26 All the catalysts were dried in an oven at 110 °C for 1 h before use. 2.2. Experimental Setup. All experimental studies were conducted in a standard glass reactor with an inner diameter (id) of 5 cm and a height of 10 cm with four glass baffles and a four-bladed disk turbine impeller located at a height of 0.5 cm from the bottom of the vessel and mechanically agitated with an electric motor. The reactor was kept in an isothermal temperature oil bath, whose temperature was maintained at the desired level through proper control. In all the experiments, the guaiacol-catalyst slurry was heated first to the desired reaction temperature with stirring; cyclohexene was then added to the reactor. The moment that cyclohexene was added into the reactor was considered to be the starting time of the reaction. In a typical reaction, 0.226 mol of guaiacol were reacted with 0.045 mol of cyclohexene with a catalyst loading of 0.05 g/cm3 of the total volume of the reaction mixture. The total volume of the reaction mixture was 30 cm3. The reaction was performed at 80 °C at a speed agitation of 1000 rpm. 2.3. Method of Analysis. Clear liquid samples were withdrawn periodically and gas chromatography (GC) analyses were performed (Chemito Model 8510) using a stainless steel column (3.25 mm × 4 m) that had been packed with a liquid stationary phase of 10% OV-17. The injector and detector

temperatures were maintained at 300 °C. The oven temperature was programmed from 100 °C to 300 °C, with a ramp rate of 10 °C/min. Nitrogen was used as the carrier gas at a flow rate of 0.5 cm3/s. The conversions were based on the disappearance of cyclohexene, which was the limiting reactant, in the reaction mixture. The products were confirmed by gas chromatography-mass spectroscopy (GC-MS) and their physical constants. 3. Results and Discussions 3.1. Effect of Different Catalysts. Table 1 compares the performance of Amberlyst-15, Filtrol-24, 20% dodecatungstophosporic acid (DTP/K-10) clay, 20% w/w Cs2.5H0.5PW12O40/ K-10 clay, 20% w/w ZnCl2/K-10 clay, 20% w/w FeCl3/K-10 clay, Indion-130, and sulfated zirconia (S-ZrO2), in regard to the conversion of cyclohexene and the selectivity of the products at 80 °C. Figure 1 presents the reaction network, and Figure 2 shows the mechanism. The effects of nature of acidity, acid strength, and pore size distribution of the various catalysts can be discussed to obtain preferential C-alkylation. The acid strength of inorganic catalysts such as 20% w/w dodecatungstophosphoric acid (DTP)/K-10 clay,19 20% w/w ZnCl2/K-10 montmorillonite clay,21 20% w/w FeCl3/K-10 montmorillonite clay,22 cesium-modified dodecatungstophosphoric acid (CsDTP) supported over K-10 clay (20% w/w Cs2.5H0.5PW12O40/ K-10),25 and sulfated zirconia27 was determined using the temperature-programmed desorption of ammonia. The phrase “mainly B” means that Bronsted acidity is the major contributor and is greater than the Lewis acidity (L). This has been already reported by us earlier, and, hence, the details are avoided. The acidic nature of Amberlyst-15 and Indion-130 was reported elsewhere.28,29 These catalysts contain only B-type acidity. Filtrol-24 contains both L and B acidity. Table 2 shows physical properties of various catalysts. It is evident that Amberlyst-15 and Indion-130 contain only Bronsted acidity, and, hence, these catalysts gave good selectivity for C-alkylated products, whereas sulfated zirconia is a superacid with mainly Lewis acidity, as compared to Bronsted acidic sites, and many pores in the mesoporous region, which could be responsible for the selective O-alkylation product at low temperature. The catalysts 20% w/w Cs2.5H0.5PW12O40/K-10 clay, 20% w/w dodecatungstophosphoric acid (DTP)/K-10 clay, and Filtrol-24 contain both types of acidity and, hence, shows selectivity for both (i.e., C- and O-alkylation). Surprisingly, 20% w/w ZnCl2/K-10 and 20% w/w FeCl3/K-10 did not show any activity under these reaction conditions, even though these are mainly Lewis acids with a mesoporous structure. These catalysts can also lead to polymerization and can block pores to render them inactive. Because Amberlyst-15 gave the maximum selectivity and conversion for the C-alkylated product, all further experiments were conducted

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Figure 1. Reaction scheme for alkylation of guaiacol with cyclohexene over sulfated zirconia.

using Amberlyst-15. The concentration profiles of different products for this reaction at 80 °C with a mole ratio of 5:1 and a catalyst loading of 0.05 g/cm3 are shown in Figure 3. It is observed that the O-alkylated product is formed first, which isomerizes to C-alkylated products. Thus, its concentration goes through a maximum. 3.2. Effect of External Mass Transfer. The effect of speed of agitation on the conversion and rates of reaction was evaluated at different speeds in the range of 800-1200 rpm. Figure 1 suggests that there are two parallel reactions that lead to C- and O-alkylated products. Thus, cyclohexene (A) and guiaiacol (B) are transferred from the organic phase to the external surface of the catalyst and then into the pore space to allow chemisorption, surface reaction, and the desorption of products to occur. Depending on the relative magnitude of external resistance to mass transfer and reaction rates, different controlling mechanisms were put forward. When the external mass-transfer resistance is small, then the following inequality holds:27

1 1 1 ) and robs kSL-AapCA0 kSL-BapCB0

(1)

Therefore, it is necessary to study the effect of speed of agitation, catalyst loading, and particle size to ascertain the absence of external mass transfer and intraparticle diffusion resistances, so that the true intrinsic kinetics could be developed. Figure 4 shows that the conversion remains practically unchanged in the range of 800-1200 rpm, which indicates the absence of external solid-liquid mass-transfer resistance. Theoretical analysis was also conducted, to ensure that the external mass-transfer resistance was, indeed, absent, as delineated below.

According to eq 1, it is necessary to calculate the rates of external mass transfer of both guaiacol and cyclohexene and compare them with the rate of reaction. For a spherical particle, the particle surface area per unit volume is given by

ap )

6w Fpdp

(2)

where w is the catalyst loading of liquid phase used in the current studies (expressed in units of kg/m3), Fp the particle density (also expressed in units of kg/m3), and dp the particle diameter (given in meters). The diffusivity (D) values were calculated using the Wilke-Chang equation30 at 80 °C, and these values are as follows: DAB ) 6.278 × 10-9 m2/s and DBA ) 5.48 × 10-8 m2/s. Thus, the corresponding values of the solid-liquid mass-transfer coefficients for both of the reactants A (cyclohexene) and B (guaiacol) were calculated from the limiting value of the Sherwood number (e.g., ShA ) kSL-AdP/DAB) of 2. The actual Sherwood numbers are typically higher by an order of magnitude in well-agitated systems; however, for conservative estimations, a value of 2 is used.31 The solid-liquid masstransfer coefficient kSL-A and kSL-B were obtained as 1 × 10-4 m/s and 8.7 × 10-5 m/s, respectively. The initial rate of reaction was calculated from the conversion profile. A typical calculation shows that, for a typical reaction, the initial rate of reaction was calculated to be 2.5 × 10-4 kmol m-3 s-1 Therefore, putting the appropriate value in eq 1, one obtains 4 × 103 . 3.69 and 0.85 m3 s/kmol. This demonstrates that there was no resistance to external mass transfer of either of the reactants. 3.3. Effect of Catalyst Loading. In the absence of external mass-transfer resistance, the rate of reaction is directly proportional to catalyst loading, based on the entire liquid-phase

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Figure 2. Reaction mechanism of alkylation of guaiacol with cyclohexene. (H+ is a general solid acid site.) Table 2. Physical Properties of Catalysts

catalyst Filtrol-24 20% DTP/K-10 clay sulfated zirconia Amberlyst-15 20% cesium-substituted DTP/K-10 clay 20% ZnCl2/K-10 clay 20% FeCl3/K-10 clay Indion-130 a

porosity (vol %)

maximum operating temperature (°C)

type of acidity (B or L)a

cation exchange capacity (CEC; mequiv/g)

7.5 7.1 2.8 macroporous 5.8

32 25 22 30-35 NDb

300 285 650 150 300

B and L mainly B B and L only B mainly B

0.3 0.39 NDb 4.9 0.29

6.8 6.8 macroporous

28 28 NDb

285 285 150

only L only L only B

N.B. N.B. 4.8

size (µm)

surface area (m2/g)

average pore diameter (nm)

Engelhard prepared prepared Rohm and Haas prepared

30-400 50-200 50-300 500 50-200

350 107 100 55 207

prepared prepared Indion exchange resin

50-200 50-200 420-1200

145 145 NDb

source

B denotes Bronsted acidity, L denotes Lewis acidity. b No data available.

volume. The catalyst loading was varied over a range of 0.010.07 g/cm3, based on the total volume of reaction mixture. Figure 5 shows the effect of catalyst loading on the conversion of cyclohexene. The conversion of cyclohexene increases as the catalyst loading increases, which is obviously due to the proportional number of active sites. However, beyond a catalyst loading of 0.05 g/cm3, there was no significant increase in the conversion, and, hence, all further experiments were performed

at this catalyst loading. At this loading, the intraparticle diffusion resistance sets in. 3.4. Proof of an Absence of Intraparticle Resistance. The effect of particle size of the catalyst on the reaction rate was studied by taking three different particle size ranges, from 200 µm to 710 µm, to assess the influence of intraparticle resistance (Figure 6). For an average particle size of 250 µm, there was no effect of particle size on the conversion of cyclohexene,

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Figure 5. Effect of catalyst loading on the conversion process: (b) 0.01 g/cm3, ([) 0.02 g/cm3, (2) 0.03 g/cm3, (9) 0.05 g/cm3, and (×) 0.07 g/cm3. Conditions: guaiacol concentration, 0.226 mol; cyclohexene concentration, 0.045 mol; speed of agitation, 1000 rpm; particle size, 200-300 µm; and temperature, 80 °C.

Figure 3. Concentration profile of various products in alkylation of guaiacol with cyclohexene: ([) cyclohexene, (9) O-alkylated product, (2) Calkylated products, and (b) dialkylated product. Conditions: guaiacol concentration, 0.226 mol; cyclohexene concentration, 0.045 mol; Amberlyst15 loading, 0.05 g/cm3; particle size, 200-300 µm; temperature, 80 °C; and speed of agitation, 1000 rpm.

Figure 6. Effect of particle size on the conversion process: ([) 200-300 µm, (9) 300-500 µm, and (2) 500-710 µm. Conditions: guaiacol concentration, 0.226 mol; cyclohexene concentration, 0.045 mol; Amberlyst15 loading, 0.05 g/cm3; speed of agitation, 1000 rpm; and temperature, 80 °C. Figure 4. Effect of the speed of agitation on the conversion process: (2) 800 rpm, (9) 1000 rpm, and ([) 1200 rpm. Conditions: guaiacol concentration, 0.226 mol; cyclohexene concentration, 0.045 mol; Amberlyst15 loading, 0.05 g/cm3; particle size, 200-300 µm; and temperature, 80 °C.

which, in turn, supported the fact that the intraparticle diffusion resistance was absent. This was further confirmed using the Wiesz-Prater modulus or criterion.32 According to the WieszPrater criterion, the dimensionless parameter CWP, which is defined as

CWP )

robsFpRp2 De[As]

and represents the ratio of the intrinsic reaction rate to the intraparticle diffusion rate, can be evaluated from the observed rate of reaction, the particle radius (RP), the effective diffusivity of the limiting reactant (De), and the concentration of the reactant at the external surface of the particle: (i) If CWP . 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 of the catalyst was obtained from the bulk diffusivity (DAB), porosity (), and tortuosity (τ) as 7.324 × 10-10 m2/s, where De-A ) DAB(/τ). In the present case, the value of Cwp was calculated to be 2.7 × 10-2 for the initial observed rate, which is ,1, and, therefore, the reaction is intrinsically kinetically controlled. Further experiments were performed by crushing the particles and sieving them to study the effect of particle size in the range of 50-100 µm. These values were practically the same as those at 200-300 µm. Thus, there was no effect of particle size at or below 300 µm. The WieszPrater criterion was also used earlier to demonstrate this fact. Furthermore, the value of the apparent activation energy also showed that there was no effect of particle size. 3.5. Effect of Mole Ratio. The mole ratio of guaiacol to cyclohexene was varied from 1:1 to 5:1, to assess its effect on the rate and selectivity. The overall reaction rate of cyclohexene increased as the mole ratio of guaiacol to cyclohexene increased from 1:1 to 3:1. Further increases in the mole ratio did not have

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Figure 7. Effect of the guaiacol:cyclohexene molar ratio on the conversion process: ([) 1:1, (9) 3:1, and (2) 5:1. Conditions: Amberlyst-15 loading, 0.05 g/cm3; speed of agitation, 1000 rpm; particle size, 200-300 µm; and temperature, 80 °C.

any effect on the conversion. Thus, all the subsequent reactions were performed with a mole ratio of 5:1 (see Figure 7). It is also assumed that the lower the concentration of cyclohexene, more the adsorption of cyclohexene on the catalyst sites and, therefore, the greater the availability of cyclohexenium cation to react with guaiacol. Furthermore, with a change in the mole ratio from 1:1 to 5:1, the rate of reaction increased remarkably and the selectivity for the C-alkylated product increased from 57% to 76% for the same conversion level (78% of cyclohexene under identical conditions). The increase in selectivity for the C-alkylated products with an increase in mole ratio of guaiacol to cyclohexene may be due to the fact that, with an increase in the amount of guaiacol, the rate of intramolecular rearrangement of the ether to mono-alkylated product increases. The observation made by Kolzlov et al.33 provides a sound base for the aforementioned explanation, because they have observed that the rate of intramolecular rearrangement of ether to C-alkylated products increases in the presence of excess phenolic compound. 3.6. Effect of Temperature. The effect of temperature on the conversion and selectivity of products was studied in the range of 60-80 °C. With an increase in temperature from 60 °C to 80 °C, the rate of reaction increased significantly, with an increase in the selectivity for the C-alkylated product, from 50% to 76% for the same conversion level (78%) of cyclohexene, under identical conditions. Therefore, the C-alkylation reaction seems to have a higher activation energy than the etherification. Figure 1 clearly indicates that the reaction network consists of a combination of series-parallel reactions. Cyclohexyl guaiacol ether is one of the intermediate products of one of the series. Thus, initially, the concentration of cyclohexyl guaiacol ether is sufficiently high, and the rate of its rearrangement to the mono C-alkylated guaiacol increases, in comparison to the rate of its formation (from guaiacol and cyclohexene), leading to the decrease in the selectivity. No oligomerization of cyclohexene occurred in this temperature range. Yadav and Goel2 have shown earlier that the oligomerization of cyclohexene did not occur at -0.97 at different temperatures for all data points was considered to be satisfactory. Indeed, the selectivity is a function of temperature. The initial rate data gives a much better fit, with only C-alkylation being predominant. From Figure 9, the slopes that have been obtained at 60, 70, and 80 °C were determined to be 1.08 × 10-4, 1.95 × 10-4, and 4.8 × 10-4 s-1, respectively. Hence, the rate constants (kSR) at different temperatures can be calculated as

kSR(60 °C) ) 3.61 × 10-7 m6 kmol kg-1 s-1 kSR(70 °C) ) 6.5 × 10-7 m6 kmol kg-1 s-1 kSR(80 °C) ) 1.5 × 10-6 m6 kmol kg-1 s-1 The Arrhenius plot was constructed (Figure 10) to obtain the apparent activation energy (E) of the reaction, which was 16.78 kcal/mol. This value is for both C- and O-alkylation, which are the two parallel reactions. This high value also suggests a kinetically controlled reaction. 3.8. Reusability of Amberlyst-15. The reusability of Amberlyst-15 was tested by conducting two runs (Table 3). After the reaction, the catalyst was filtered and then refluxed with 50 cm3 of toluene for 30 min, to remove any adsorbed material from the catalyst surface and pores, and then was dried at 110 °C. Only a marginal decrease in conversion was observed. There was no effect on the selectivity of the products. 3.9. Inorganic Solid Acids as Catalysts. Among the various inorganic solid acids, sulfated zirconia was determined to be the best catalyst to achieve maximum selectivity for the O-alkylated product, cyclohexylmethoxyphenyl ether. It is a solid superacid with a wide pore size distribution and it has a presence of both Lewis (more) and Bronsted sites, with the maximum number of pores being in the mesoporous region. The overall reaction rate increased as the catalyst loading, reaction temperature, and molar ratio of guaiacol to cyclohexene increased. For selective O-alkylation, the reaction should be performed at 80 °C in the presence of sulfated zirconia with a molar ratio of 5:1.34 Thus, it is possible to produce selectively either C- or O-alkylated products of guaiacol. 4. Conclusion The alkylation of guaiacol with cyclohexene was successfully performed in the presence of several catalysts with different activities and pore size distributions, to study the effect of the nature of acidity and its accessibility in getting more C-alkylated product than O-alkylated product. Amberlyst-15 was determined to be the best catalyst for C-alkylation, whereas sulfated zirconia favored O-alkylation. Amberlyst-15 possesses only Bronsted acidity and is a mesoporous material with an average size of

30 nm. The overall reaction rate increased as the catalyst loading, reaction temperature, and molar ratio of guaiacol to cyclohexene each increased. For selective C-alkylation, the reaction should be performed at 80 °C in the presence of Amberlyst-15 with a molar ratio of 5:1. Here, 91% selectivity is realized for C-alkylated products. The apparent activation energy was determined to be 16.78 kcal/mol. The reaction was performed without the use of a solvent, to make the process cleaner and greener. The catalyst has excellent reusability. Acknowledgment This paper is dedicated to Professor M. M. Sharma, F.R.S., on the occasion of his 70th birthday. Professor Sharma has made many innovative contributions to catalysis by ionexchange resins and clays, and G.D.Y. has enjoyed immense support and encouragement from him over the past 32 years. G.D.Y. acknowledges Darbari Seth Endowment for supporting the Chair and Contingency Grant. G.S.P. is thankful to CSIR, New Delhi, for awarding him a Junior Research Fellowship during this work. Nomenclature A ) reactant species A, cyclohexene A.S ) chemisorbed A B ) reactant species B, guaiacol aP ) solid-liquid interfacial area (m2/m3 of liquid phase) B.S ) chemisorbed B CA ) concentration of A (kmol/m3) CA0 ) bulk concentration of A at the solid (catalyst) surface (kmol/m3) CAS ) concentration of A at the solid (catalyst) surface (kmol/m3) CB ) concentration of B (kmol/m3) CB0 ) bulk concentration of B in the bulk liquid phase (kmol/m3) CBS ) concentration of B at the solid (catalyst) surface (kmol/m3) CC ) concentration of C (kmol/m3) CCS - CDS ) concentration of C and D at the solid (catalyst) surface (kmol/m3) CD ) concentration of D (kmol/m3) CS ) concentration of vacant sites (kmol/m3) Ct ) total concentration of the sites (kmol/m3) DAB ) diffusion coefficient of A into B (m2/s) DBA ) diffusion coefficient of B into A (m2/s) dP ) diameter of the catalyst particle (m) K1 ) surface reaction equilibrium constant; K1 ) k1/k′1 k1 ) surface reaction rate constant for the forward reaction k′1 ) surface reaction rate constant for the reverse reaction KA ) adsorption equilibrium constant for A (m3/kmol) KB ) adsorption equilibrium constant for C and D (m3/kmol) KC) adsorption equilibrium constants for C (m3/kmol) KD ) adsorption equilibrium constants for D (m3/kmol) kSR ) second-order rate constant (m6 kmol-1 kg-1 s-1) kt ) dimensionless constant kSL-A ) solid-liquid mass-transfer coefficient for the transfer of A kSL-B ) solid-liquid mass-transfer coefficient for the transfer of B robs ) observed rate of reaction based on the liquid-phase volume (kmol m-3 s-1) ro ) initial rate of reaction (kmol m-3 s-1) S ) vacant site Sh ) Sherwood number

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ReceiVed for reView May 23, 2006 ReVised manuscript receiVed August 11, 2006 Accepted August 14, 2006 IE060645T