Alkylation of phenol with cyclohexene over solid acids: Insight in selectivity of O- versus C-alkylation

Applied Catalysis A: General 286 (2005) 61–70 www.elsevier.com/locate/apcata

Alkylation of phenol with cyclohexene over solid acids: Insight in selectivity of O- versus C-alkylation Ganapati D. Yadav *, Parveen Kumar Department of Chemical Engineering, University Institute of Chemical Technology (UICT), University of Mumbai, Matunga, Mumbai 400019, India Received 7 September 2004; received in revised form 24 February 2005; accepted 1 March 2005 Available online 20 April 2005

Abstract Alkylation of phenol with cyclohexene with acid catalysts leads to the formation of both O- and C-alkylated products, which are all useful in a variety of industries. The O-alkylated product cyclohexyl phenyl ether is a valuable perfume and can also serve as a precursor to diphenyl ether, a very important bulk chemical. The efficacy of various acid catalysts such as sulphated zirconia, sulphonic acid treated hexagonal mesoporous silica (SO3-HMS), 20% (w/w) dodecatungstophospheric acid (DTP) supported on K-10 clay, 20% (w/w) cesium salt of DTP (Cs2.5H0.5PW12O40) supported on K-10 clay (Cs-DTP/K-10) and 20% (w/w) DTP/HMS was studied to improve the selectivity to cyclohexyl phenyl ether. A mixture of 2-cyclohexylphenol, 4-cyclohexylphenol and cyclohexyl phenyl ether was obtained with different selectivities. However, 20% (w/w) DTP/K-10 clay was the most active and selective catalyst for O-alkylation in the range of 45–70 8C at atmospheric pressure. The selectivity to O- versus C-alkylation is strongly dependent on temperature, and at lower temperatures, the selectivity to cyclohexyl phenyl ether increases. The best operating temperature is 60 8C. A mathematical model is built to interpret the kinetic data and develop a mechanism. # 2005 Elsevier B.V. All rights reserved. Keywords: Alkylation; Selectivity; Phenol; Cyclohexene; Cyclohexyl phenyl ether; 2-Cyclohexyl phenol; 4-Cyclohexyl phenol; Heteropoly acid; Clay; Dodectatungstophosphoric acid; Sulphated zirconia; Hexagonal mesoporous silica

1. Introduction Solid acid catalysts are widely used for the development of green technology to replace the traditional polluting catalysts in organic process industries employing alkylation, acylation, isomerization, nitration, condensation, cracking, and esterification, etc. [1]. Zeolites, acid treated clays, ion exchange resins and supported heteropoly acids are investigated by several groups for their applications in pharmaceutical, perfumery, agro-chemicals, dye-stuffs, intermediates and specialty chemical industries. Our laboratory has been engaged in novel applications and modelling of catalysis by sulphated zirconia [2–7], UDCaT * Corresponding author. Tel.: +91 22 24102121; fax: +91 22 24145614. E-mail addresses: [email protected], [email protected] (G.D. Yadav). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.03.001

series[8–20], clay supported heteropoly acids [21–27], CsHPA/K-10 [28–31], HMS supported HPA [32], ion exchange resins [33–42] and zeolites [43,44] for a variety of bulk and fine chemical processes involving alkylation, acylation, esterification, nitration, isomerization, etherification, cracking, oligomerisation, cyclization, hydration, and dehydration, etc. The acronym UDCaT stands for the series of catalysts developed in our laboratory (named after University Institute of Chemical Technology). Alkylation of phenol with cyclohexene has attracted attention of many researchers because of its industrial and academic relevance. Alkylation of phenol with cyclohexene leads to a variety of products such as 4-cyclohexylphenol, 2cyclohexylphenol, and cyclohexyl phenyl ether depending on catalyst and reaction conditions. Cyclohexyl phenyl ether serves as a perfumery compound and its dehydrogenation leads to diphenyl oxide (DPO), which is very relevant. DPO

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Nomenclature solid–liquid interfacial area, cm2/cm3 of liquid phase A reactant species A, cyclohexene B reactant species B, phenol BS chemisorbed B C cyclohexyl phenyl ether CA concentration of A (g mol/cm3) CAO initial concentration of A at solid (catalyst) surface (mol/cm3) CAS concentration of A at solid (catalyst) surface, (mol/cm3) CB concentration of B (mol/cm3) CBO initial concentration of B in bulk liquid phase, mol/cm3 CBS concentration of B at solid (catalyst) surface, mol/cm3 CC, CD concentration of C and D, mol/cm3 CCS, CDS concentration of C and D 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, cm D 4-cyclohexyl phenol DAB diffusion coefficient of A in B, cm2/s DBA diffusion coefficient of B in A, cm2/s E 4-cyclohexyl phenol k1 surface reaction rate constant for forward reaction k10 surface reaction rate constant for reverse reaction kSR second order rate constant, cm6 g mol1 g1 s1 kt dimensionless constant K1 surface reaction equilibrium constant, k1 =k10 KA. . . KE adsorption equilibrium constant for A, B, C, D, and E cm3/mol ro overall rate of reaction based on liquid phase volume, g mol cm3 s1 roi initial rate of reaction, g mol cm3 s1 RB rate of reaction of B, g mol cm3 s1 S vacant site Sh sherwood number w catalyst loading, g/cm3 of liquid phase XA fractional conversion of A rP density of catalyst particle, g/cm3 aP

is made from a polluting route or energy intensive radioactive thoria catalysed dehydration of phenol or from chlorobenzene leading to effluent problems. 4-Cyclohexylphenol and 2-cyclohexylphenol can be used precursors of o- and p-phenylphenol [45], which are again precursors to a number of industrially valuable products.

Alkylation of phenol with cyclohexene and cyclohexanol is reported over HY and modified HY zeolites at 140–225 8C [46]. However, a limited information is available on the Oalkylation. The catalysts used for the synthesis of cyclohexylphenyl ether are ion exchange resins, Amberlyst 15, AmberliteIR-120, Nafion NR-50, and Filtrol-24 [45]. Heteropoly acids are very important class of acid and redox catalysts. The novelty of clay supported heteropolyacids as reusable benign catalysts was reported by us [21] for the first time and a series of commercially important reactions were studied [21–27]. We have also developed a novel nano-catalyst using partially substituted dodecatungstophosphoric (DTP) acid with cesium and supporting it on acid activated clay. Thus, nanoparticles of 20% (w/w) Cs2.5H0.5PW12O40b were created in the pore network of acid treated K-10 clay (designated as Cs-DTP/K-10 in subsequent discussions) and used for a number of acid catalyzed reactions [28–31]. The current work was undertaken to asses the suitability of various solid acid catalysts such as sulphated zirconia, sulphonic acid treated silica, 20% (w/w) dodecatungstophospheric acid supported on K-10 clay, 20% (w/w) Cs-DTP/K10 clay and 20% (w/w) DTP/HMS for predominantly producing cyclohexyl phenyl ether, including the development of kinetics and mechanism.

2. Experimental 2.1. Chemicals Cyclohexene and dodecatungstophosphoric (DTP) acid were procured commercially from M/s. s.d. fine Chemicals Pvt. Ltd,. Mumbai, India. Phenol was obtained from Merck Fine Chemicals, Mumbai, India and K-10 clay from Fluka, Germany. Sulphated zirconia [1] and 20% (w/w) DTP/K-10 clay [2] were prepared by established procedures in our laboratory. K-10 clay was first impregnated with aqueous solution of Cs+ precursor (Cesium chloride), dried at 110 8C and calcined at 300 8C. DTP was then impregnated on the support, from a methanolic solution, dried at 110 8C for 12 h and calcined at 300 8C for 3 h. Thus, 20% (w/w) Cs2.5H0.5PW12O40/K-10 was dried at 110 8C for 12 h and the calcined at 300 8C for 3 h. Bulk Cs2.5H0.5PW12O40 was prepared by adding the Cs precursor solution dropwise to the DTP solution while stirring. The resulting precipitate was dried at 110 8C for 12 h and calcined at 300 8C for 3 h. All catalysts were powdered and dried at 120 8C for 3 h prior to their use. For the synthesis of sulphated zirconia, a known amount of zirconium oxychloride was dissolved in deionized water and precipitated using aqueous ammonia. The precipitate obtained was washed and dried. The dry cake was crushed to desired particle size and sulphated with 1 N H2SO4. It was calcined in air at 650 8C for 3 h to give sulphated zirconia [2–5].

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2.2. Experimental setup A 100 ml flat bottom glass reactor of 5 cm internal diameter and 10 cm height equipped with four baffles and a six-bladed pitched turbine impeller located at 5 mm from the bottom of reactor was used for carrying out reactions. The reactor was placed into an electrically heated oil bath equipped with temperature controller. The desired speed of agitation was achieved by controlling the speed of the stirrer motor regulator. 2.3. Experimental methodology Experiments were carried out by using 0.169 mol (15.9 g) phenol and 0.0988 mol (8.1 g) cyclohexene. The total liquid volume was 25 ml. Reactions were carried out at 60 8C. A typical speed of agitation of 1000 rpm was employed. Samples were collected periodically and analysed by gas chromatography. The quantity of catalyst used was typically 2.5 g in the control experiments and the reaction was carried out for 4 h.

Fig. 1. Reaction scheme for the reaction between phenol with cyclohexene in presence of 20% (w/w) DTP/K-10.

2.4. Analysis Samples were analyzed by gas chromatography (Chemito model 8610) equipped with a flame ionization detector and 2.5-m long stainless steel column of 3.2 mm diameter packed with 10% OV-17 supported on Chromosorb WHP. Nitrogen was used as the carrier gas. Product identification was done by using GC–MS. The functional groups were identified by using IR and UV spectrometer. Quantitative analysis was done through calibration by using synthetic mixture. Fig. 2. Reaction mechanism.

2.5. Reaction chemistry The Friedel–Crafts alkylation of phenol with cyclohexene in the presence of solid acid catalyst leads to the formation of mixture of O-alkylated phenol, namely, cyclohexyl phenyl ether and C-alkylated phenols, namely, 2-cyclohexyl phenol and 4-cyclohexyl phenol which were confirmed by GC–MS. The reaction scheme is shown in Figs. 1 and 2. The solid acid catalyst protonates cyclohexene and the carbocation then combines with phenol to give the above alkylated products.

3. Results and discussion 3.1. Comparison of activities of various catalysts Different catalysts were chosen for this study, based on their likely activity and selectivity for O-alkylated phenol and these included sulphated zirconia, sulphonic acid treated silica, DTP/K-10, Cs-DTP/K-10 clay, DTP/HMS. These catalysts have different acidities and pore size distributions. HMS is a uniform sized mesoporous material. Activities of

all these catalyst were tested under otherwise similar conditions, namely, mole ratio cyclohexene to phenol 1:1.7, 1000 rpm, 60 8C, and 4 h. The conversion and yield values are shown in Table 1. The conversions were in the following order: 20% (w/w) DTP/HMS (max) >20% (w/w) DTP/K10 > sulphonic acid treated silica > sulphated zirconia >20% (w/w) Cs-DTP/ HMS >20% (w/w) Cs-DTP/K-10 (min). These results show that 20% (w/w) DTP/K-10 and 20% (w/w) DTP/HMS give more conversion and selectivity than other catalysts. The characterization of 20% DTP/K-10 has been studied by Yadav and Doshi [17] and that of 20% (w/w) DTP/HMS by Yadav and Manyar [29]. Table 2 lists the pore size and surface areas of various catalysts. The surface area of the 20% (w/w) DTP/K-10 is 107 m2/g whereas that of 20% (w/w) DTP/HMS is 909 m2/g. The initial reaction rates were higher with DTP/HMS than DTP/K-10. The final conversions were little higher with DTP/HMS. The reason for this is the total acidity and accessibility of the catalytic sites for reaction. The narrower pore sizes of K-10 clay based catalyst have given better selectivity to the ether.

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Table 1 Comparison of activities of various catalysts No.

Catalyst

Conversion (%)

Yield (%) Cyclohexyl phenyl ether

2-Cyclohexyl phenol

4-Cyclohexyl phenol

1 2 3 4 5

20% (w/w) DTP-HMS 20% (w/w) Cs-DTP/K-10 20% (w/w) Cs-DTP-HMS Sulphated zirconia SO3-HMS (sulphonic acid treated hexagonal mesoporous silica) 20% (w/w) DTP/K-10

70.00 18.56 25.67 31.59 53.68

63.27 63.12 58.53 66.38 73.44

23.38 25.98 22.16 20.56 19.68

13.35 10.90 19.31 2.86 6.88

69.60

67.56

22.45

9.98

6

3

Reaction conditions: mole ratio of cyclohexene to phenol 1:1.7; catalyst loading: 0.14 g/cm ; temperature: 60 8C; speed of agitation: 1000 rpm.

Table 2 Surface area, pore volume and pore diameter of catalysts No.

Catalyst

Surface area (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

1 2 3 4 5 6 7

K-10 20% (w/w) DTP/K-10 20% (w/w) Cs2.5H0.5PW12O40/K-10 20% (w/w) DTP/HMS Sulfated zirconia 20% (w/w) Cs-DTP-HMS SO3-HMS

230 107 207 909 100 789 870

0.36 0.32 0.29 0.538 0.115 0.48 0.45

6.4 7.1 5.8 2.6 2.8 2.7 2.6

Table 2 gives the pore size distribution and surface areas of these catalysts. The heteropolyacids having more Bronsted acidic sites are better than sulfated zirconia and also when there is a narrower pore size distribution. O-Alkylation is promoted by Bronsted acids. Thus, the results are in order. Since 20% (w/w) DTP/K-10 was a cheaper catalyst to prepare and also because the selectivity to cyclohexylphenylether was more than others, it was used in all further experiments. A typical conversion profile of the limiting reactant and products is given in Fig. 3. It is seen that the Oand C-alkylation reactions are parallel reactions in the current case and there is no isomerization of the O-alkylated product (cyclohexyl phenyl ether) into the C-alkylated isomers (o- and p-cyclohexyl phenol) without any oligo-

merization of cyclohexene under the set of experimental conditions chosen. Yadav and Goel [37] have shown earlier that at low temperatures, there is no oligomerization of cyclohexene. The isomerization of O- into C-alkyl product occurs at much higher temperatures of 140–220 8C, as has been reported for vapour phase reactions [46]. It is also worthwhile to comment on the use of ion exchange resins on this reaction. The conversion of cyclohexene with a mole ratio of phenol to cyclohexene of 3:1 at 60 8C was 50% in 4 h with a selectivity of 60% to cyclohexyl phenyl ether with Amberlite IR-120, the most active catalyst. Further, there was a dialkylation up to 5% [45]. In comparison, DTP/K-10 gave a conversion of 70% with selectivity of 70% to cyclohexyl phenyl ether. Therefore, our catalyst is better than cation exchange resins. The effect of various parameters was studied under otherwise similar conditions and a standard temperature of 60 8C. This is a typical solid–liquid slurry reaction involving the transfer of cyclohexene and phenol to the outer surface of the catalyst, followed by adsorption, surface reactions and desorption. The influence of external solid–liquid mass transfer resistance must be ascertained before a true kinetic model could be developed. Thus, experimental and theoretical analyses were done. 3.2. Effect of external mass transfer

Fig. 3. Concentration profile of alkylation for phenol with cyclohexene. Temperature: 60 8C; speed of agitation: 1000 rpm; catalyst loading: 0.14 g/ cm3 (^) cyclohexene; (&) cyclohexylphenylether; (~) 2-cyclohexylphenol; (&) 4-cyclohexylphenol.

The reactions of A (cyclohexene) and B (phenol) lead to the following three products by parallel paths A þ B ! C ðO-alkylationÞ (1a)

G.D. Yadav, P. Kumar / Applied Catalysis A: General 286 (2005) 61–70

A þ B!D

ðC-alkylation at ortho positionÞ

(1b)

A þ B!E

ðC-alkylation at para positionÞ

(1c)

65

Which can be represented by an overall stoichiometry as follows: A þ B!C þ D þ E (1) Cyclohexene was taken as the limiting reactant in control experiments. And there was no oligomerization of cyclohexene. At steady state, the rate of mass transfer of cyclohexene (A) per unit volume of the liquid phase is given by (2) RA ¼ kSL-A aP ½CA0  CAS (rate of transfer of A from bulk liquid to external surface of the catalyst particle) (3) RA ¼ zkS aP ½CB0  CBS (rate of transfer of phenol (B) from the bulk liquid phase to the external surface of the catalyst particle) (4) RA ¼ robs (observed rate of reaction with in the catalyst particle). Eq. (4) could be represented by a Langmuir–Hinselwood–Hougen–watson type or power law model with or without the effectiveness factor h, to account for the intraparticle diffusion resistance. Depending on the relative magnitude of external resistance to mass transfer and reaction rates, different controlling mechanism were put forward [18]. When the external mass transfer resistance is small, then the following inequality holds 1 1 1

and (5) robs kSL-A aP CA0 kSL-B aP CB0 The observed rate robs could be given by three types of models where in the contribution of intra-particle diffusional resistance could be accounted for by incorporating the effectiveness factor h. These models are: (a) The power law model if there is very weak adsorption of reactant species; (b) Langmuir–Hinselwood–Hougel–Watson model; (c) Eley–Rideal model. It is therefore, necessary to study the effect of speed of agitation, catalyst loading and particle size to ascertain the absence of external mass transfer and intra-particle diffusion resistances so that the true intrinsic kinetics could be developed. In all reactions the cyclohexene to phenol molar ratio of 1:1.7 was used. To ascertain the influence of external resistance to mass transfer of the reactants to the catalyst surface, the speed of agitation was varied over the range of 500–1200 rpm, for average particle size of 2–6 mm. The conversion of cyclohexene, the limiting reactant, at different intervals of time is shown in Fig. 4. It was observed that speed had no effect on conversion beyond 1000 rpm and thus there was no limitation on external mass transfer of

Fig. 4. Effect of speed of agitation. Catalyst loading: 0.14 g/cm3; temperature: 60 8C; cyclohexene:phenol::1:1.7; 20% (w/w) DTP-K/10, particle size, 100–300 mm: (*) 1200 rpm; (^) 1000 rpm; (&) 800 rpm; (~) rpm.

cyclohexene from bulk liquid phase to the outer surface of the catalyst beyond this speed. Therefore, all further experiments were conducted at 1000 rpm. According to Eq. (5), it is necessary to calculate the rates of external mass transfer of both phenol and cyclohexene and compare them with the rate of reaction. For a spherical particle, the particle surface area per unit volume is given by 6w ap ¼ (6) rp dp where w, catalyst loading used in the current studies g/cm3 of liquid phase, rp, density of particle g/cm3, and dp, particle diameter (cm). The diffusivity values (D) were calculated by using the Wilke–Chang equation [47] at 60 8C and these values are as follows: DAB = 0.392  105 cm2/s and DBA = 3.38  105 cm2/s. Thus, the corresponding values of the solid– liquid mass transfer coefficients for both of the reactants A (cyclohexene) and B (phenol) 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 order of magnitude in well-agitated systems but for conservative estimations a value of 2 is taken. The solid– liquid mass transfer coefficient kSL-A and kSL-B were obtained as 1.56  103 and 1.35  102 cm/s, respectively. The initial rate of reaction was calculated from the conversion profile. A typical calculation shows that for a typical experiment, the initial rate of reaction was calculated as 9.436  107 mol/cm3/s Therefore, putting the appropriate value in Eq. (5). 1 1 1 and (10)

robs kSL-A ap ½CA0 kSL-B ap ½CB0 i.e. 1.06  106 0.771  103 and 0.521  102 This demonstrates that there was no resistance to external mass transfer of both reactants.

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observed rate of reaction, the particle radius (Rp), effective diffusivity of the limiting reactant (De) and concentration of the reactant at the external surface of the particle. (i) If Cwp ¼ robs rp R2p =De ½CAS 1, then the reaction is limited by severe internal diffusion resistance. (ii) If Cwp  1, then the reaction is intrinsically kinetically controlled.

Fig. 5. Effect of catalyst loading. Cyclohexene:phenol::1:1.7; speed: 1000 rpm; temperature: 60 8C; DTP/K-10: (^) 20%; (&) 14%; (&) 10%; (~) 6%.

3.3. Effect of catalyst loading per unit volume of liquid phase

The effective diffusivity of cyclohexene (De-A) inside the pores of the catalyst was obtained from the bulk diffusivity (DAB), porosity (e) and tortuosity (t) as 0.418  106 ccm2 s1, where De-A = DAB(e/t). In the present case, the value of Cwp was calculated as 8.07  103 for the initial observed rate which is less than 1 and therefore, the reaction is intrinsically kinetically controlled. A further proof of the absence of the intra-particle diffusion resistance was obtained through the study of the effect of temperature and it will be discussed later. 3.5. Effect of mole ratio of cyclohexene to phenol

In the absence of external mass transfer resistance, the rate of reaction is directly proportional to catalyst loading based on the entire liquid phase volume. The catalyst loading was varied over a range of 0.214–0.06 g/cm3. Fig. 5 shows that the conversion increases with increasing catalyst loading, which is obviously due to the proportional increase in the number of active sites. All further experiments were carried out at 0.14 g/cm3 of catalyst loading. As shown by Eqs. (2) and (3), at steady state, the rate of external mass transfer (i.e. from the bulk liquid phase in which A and B are located with concentration CA0 and CB0, respectively) to the exterior surface of the catalyst is proportional to ap, the exterior surface area of the catalyst where the concentration of A and B are CAS and CBS, respectively. For a spherical particle, ap is also proportional to w, the catalyst loading per unit liquid volume as shown by Eq. (6). It is possible to calculate the values of CAS and CBS for instance kSL-A aP fCA0  CAS g ¼ robs at steady state ¼ 9:436  107 mol=cm3 =s

(11)

The effect of mole ratio was studied at cyclohexene to phenol ratio of 1:0.8 and 1:5 (Fig. 6) under otherwise similar conditions. The conversion was found to increase with an increase in phenol concentration with respect to cyclohexene. It is due to higher occupancy of the sites by phenol. Since there was no substantial difference in the initial rate as well as the conversion of cyclohexene when reaction was carried out at the mole ratio of 1:1.7 and 1:5 (cyclohexene:phenol), subsequent reactions were carried with a mole ratio of 1:1.7 without any solvent. 3.6. Effect of temperature The effect of temperature was studied from 45 to 70 8C (Fig. 7). The conversion found to increase substantially with increasing temperature, which suggested that the reaction was intrinsically kinetically controlled and activation energy values should be determined. When the Arrhenius plot of ln(initial rate) versus inverse of temperature was made it

Thus putting the appropriate values, it is seen that CAS  CA0 Similarly, CBS  CB0. Thus, any further addition of catalyst is not going to be of any consequence for external mass transfer. 3.4. Proof of absence of intra-particle resistance The average particle diameter of the catalyst used in the reaction was 100 mm and thus a theoretical calculation was done by using the Wiesz–Prater criterion [48] to asses the influence of intra-particle diffusion resistance. According to the Wiesz–Prater criterion, the dimensionless parameter Cwp which represents the ratio of the intrinsic reaction rate to intra-particle diffusion rate can be evaluated from the

Fig. 6. Effect of mole ratio of cyclohexene to phenol. Catalyst loading: 0.14 g/cm3; temperature: 60 8C; catalyst: 20% (w/w) DTP/K-10; speed of agitation: 1000 rpm: (&) 1:1.7; (~) EDC 5 ml C6H10 8 ml; C6H5OH 11.6 ml; (&) 1:2.4; (^) 1:5; (*) 1:0.8.

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Fig. 7. Effect of temperature. Cyclohexene:phenol 1:1.7; catalyst loading: 0.14 g cm3; catalyst: 20% (w/w) DTP/K-10; speed of agitation: 1000 rpm: (^) 70 8C; (&) 65 8C; (~) 60 8C; (&) 55 8C; (*) 50 8C; (^) 45 8C.

showed that the activation energy values were high and hence the reaction was kinetically controlled. This aspect will be discussed later. 3.7. Reusability of catalyst

Fig. 8. Reusability of catalyst. Cyclohexene:phenol: 1:1.7; catalyst loading: 0.14 g cm3; catalyst: DTP/K-10; speed of agitation: 1000 rpm; temperature: 60 8C; (^) first; (&) second; (~) third; (&) forth.

3.8.1. Adsorption Adsorption of cyclohexene kA

The reusability of the catalyst was studied by filtering the catalyst at the end of the reaction. Therefore, experiments were done in two ways: giving methanol wash and drying the catalyst without any make-up catalyst and with make-up by fresh catalyst. There were losses during handling since the particle size was very small and typically about 10–15% catalyst would be lost. Thus, there was a drop in the final conversion. The initial rates of reaction based on dry weight of catalyst were practically the same (within 5%) when the losses were accounted for (Fig. 8). In another set of experiments, catalyst loss was made up by adding fresh catalyst to observe that the initial rates and final conversion were practically the same. Thus, it is concluded that there is no deactivation of the catalyst and it is recyclable.

A þ S Ð AS Adsorption of phenol kB

B þ S Ð BS

(13)

3.8.2. Surface reaction Formation of O- and C-alkylated products (C, D, and E) k1

B þ AS Ð CS k2

B þ AS Ð DS k3

B þ AS Ð ES k4

CS Ð DS 3.8. Reaction mechanism and kinetics From the calculated values of mass transfer rates of A and B, and initial observed rates, it was evident that the rate was independent of the external mass transfer effects. It was also evident from the values of activation energy, that the intraparticle diffusion resistance was absent. Thus, the reaction could be controlled by one of the following steps, namely (a) adsorption (b) surface reaction or (c) desorption. Therefore, for further development of model, the actual mechanism was undertaken. It is assumed that phenol and cyclohexene adsorb on catalytic surface weakly. It is the phenol from the liquid phase that reacts with AS, accordingly to Eley–Ridel mechanism. I, II, III, IV, and V in Fig. 1 represent A, B, C, D and E, respectively. Parallel reactions of the chemisorbed AS with B from the liquid phase, in the vicinity of the site, lead to the formation of O- and C-alkylated products.

(12)

k5

CS þ Ð ES

(14) (15) (16) (17) (18)

In this case, the experimental results showed that the isomerization of cyclohexyl phenyl ether (C) to either 2-cyclohexyl phenol (D) or 4-cyclohexyl phenol (E), as given by Eqs. (17) and (18) did not occur at 60 8C. Hence these two reactions could be neglected in further treatment. 3.8.3. Desorption The desorption of three alkylated products from the sites can be represented as: 1=KC

CS Ð C þ S

(19)

1=KD

DS Ð D þ S

(20)

1=KE

ES Ð E þ S

(21)

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where the desorption constants are inverse of adsorption constants. The total concentration of the sites, Ct expressed in mol/ g-cat is given by (22) Ct ¼ kt w ¼ CS þ CAS þ CBS þ CCS þ CDS þ CES where CS, concentration of vacant sites, mol per g-catalyst. The various models can now be developed to determine the overall rate of reaction.

where kR ¼ kt ðk1 þ k2 þ k3 Þ dCA ¼ KA kR CA CB w dt dCA ¼ KA kR wCA0 ð1  XA ÞCB0 ð1  XB Þ  dt



2 ð1  XA ÞðM  XA Þ ¼ kS RwCA0

M¼ 3.8.4. Surface reaction controlled mechanism Surface reaction between chemisorbed AS with B from the liquid phase leading to three parallel reactions, is the rate controlling, then overall rate of reaction (ro) of B with AS, sum of all net rates of various surface complexes given by equation ro ¼ k1 CB CAS  k10 CCS þ k2 CB CAS  k20 CDS þ k3 CB CAS 

k30 CES

(23)

where various equilibrium constants are given by the appropriate equations. It is essential to substitute the concentrations of surface species: e.g. (24) CAS ¼ KA CA CS CCS ¼ KC CC CS ; :::

(25)

where KA, KC, etc. are adsorption equilibrium constants. Putting the appropriate values of concentrations in equation, the following is obtained: Ct CS ¼ ð1 þ KA CA þ KB CB þ KC CC þ KD CD þ KE CE Þ (26) Replacing the total concentration of sites by w, the solid loading in g/cm3 of liquid phase,where Ct a w (catalyst loading in g/cm3) or, (27) Ct ¼ kt w and kt is a proportionality constant kt w CS ¼ ð1 þ KA CA þ KB CB þ KC CC þ KD CD þ KE CE Þ

CB0 CA0

and

ðratio of initial molar concentrationÞ

(31) (32)

(33) (34)

kSR ¼ KA kR

Eq. (33) is integrated to get the following: M  XA CB CAO ¼ ln ¼ ðCBO  CAO ÞkSR wt ln Mð1  XA Þ CBO CA

(35)

Hence a plot of lnðM  XA =Mð1  XA ÞÞ versus t would be a straight line with slope equal to (CBO  CAO)kSR w from which the rate constant kSR could be evaluated. Moreover, the linearity of this plot would confirm the second-order behavior of the reaction. 3.9. Establishment of reaction kinetics Above theory could now be utilized to discern the controlling mechanism and kinetics of the reaction. The rate constants were evaluated from the slopes of the plot of lnðM  XA =Mð1  XA ÞÞ versus t at different temperatures (Fig. 9). The slopes were obtained at 45, 50, 55, 60, and 70 8C to get the rate constants (kSR) at the respective temperatures as 0.9835, 2.796, 3.05, 7.17, and 9.65 cm6 g mol1 g1 s1. These are overall rate constant values. The individual rate constants were obtained from the rates of formation of the respective species using the same Eq. (35) wherein only one reaction is considered and the overall pseudo constant is replaced by the individual constant. The initial rates of formation of all the three products at different temperature can also be used to calculate the activation energy values. Thus, from the slope of the Arrhenius plot (Fig. 10), the activation energy (E) of the cyclohexylphenyl ether and 2-cyclohexylphenol and 4cyclohexylphenol are calculated as 19.8, 9.8, and

(28) Putting Eq. (13) in (12) and neglecting all reverse reactions.   KA kt wðk1 þ k3 þ k2 ÞCB CA r0 ¼ ð1 þ KA CA þ KB CB þ KC CC þ KD CD þ KE CE Þ (29) Eq. (29) suggests that the relative values of the rate constants, equilibrium constants and the concentrations of the various species will govern the overall rate of reaction of chemisorbed cyclohexene (AS) with phenol (B) from the liquid phase. Assuming that the reverse reactions are not negligible, Eq. (29) converted into the following equations: (30) ro ¼ kR KA CA CB w

Fig. 9. Kinetic plots for overall reaction at different temperatures: (^) 45 8C; (&) 50 8C; (~) 55 8C; (&) 60 8C; (*) 70 8C.

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Fig. 10. Arrhenius plots for various products for cyclohexylphenyl ether.

8.23 kcal/mol, respectively. Since three different activation energy values are obtained from the same set of data, there is an error which is acceptable. Thus, these are stated as apparent activation energy values. For cyclohexyl phenyl ether the correlation coefficient is 0.9. The idea was to show that it is possible to extract activation energy values for C-alkylated products also. The other two are not shown in the plot. For O-alkylated product, the use of low temperature would favour the selectivity as was seen from the data at 60 8C. At higher temperatures O-alkyl isomerizes to C-alkylated products and thus, the yield of 2-cyclohexyl phenol increases at the cost of cyclohexyl phenyl ether. Thus, a temperature of 60 8C was the best operating temperature.

4. Conclusion Alkylation of phenol with cyclohexene with acid catalysts leads to the formation of both O- and C-alkylated products, depending on temperature and nature of catalyst and all these products are all useful in a variety of industries. Thus, selective formation of one of the products is quite a challenging. The O-alkylated product, cyclohexyl phenyl ether is a valuable perfume and precursor to a number of fine chemicals. The efficacy of various acid catalysts such as

sulphated zirconia, sulphonic acid treated silica, 20% (w/w) dodecatungstophospheric acid (DTP) supported on K-10 clay, 20% (w/w) Cs-DTP/K-10 clay and 20% (w/w) DTP/ HMS was studied to improve the selectivity to cyclohexyl phenyl ether. 20% (w/w) DTP/K-10 clay was the most active and selective catalyst for O-alkylation in the range of 45– 70 8C at atmospheric pressure. O-alkylated phenol is favored at lower temperatures. The best operating temperature is 60 8C. The kinetics was studied with 20 % (w/w) DTP/K-10 as catalyst where the rate determining step is the surface reaction between chemisorbed cyclohexene and phenol from the liquid phase within pores according to the Eley–Rideal mechanism. It is observed that the reaction is second order under the experimental condition employed. The kinetic model was found to fit the data satisfactorily. The activation energies for formation of cyclohexyl phenyl ether, 2-cyclohexylphenol and 4-cyclohexylphenol are 15.79, 9.89, and 8.23 kcal/mol, respectively.

Acknowledgements Parveen Kumar thanks the UGC for the award of a JRF and G.D.Y. greatly acknowledges Darbari Seth Professor Endowment.

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