Summary A novel mesoporous and strong acid catalyst UDCaT-1 is found to be the most active and selective in comparison with several others in the alkylation of p-cresol with MTBE to form 2-tertbutyl- p-cresol. Introduction Butylated hydroxytoluene (BHT) is a well known antioxidant and the basic raw material for the manufacture of oil-soluble phenol-formaldehyde resins.1 It is conventionally prepared by the acid catalysed reaction of isobutylene with p-cresol.The reaction is consecutive with formation of 2-tert-butyl-p-cresol followed by 2,6-di-tert-butyl-p-cresol which is popularly called BHT.1,2 Owing to the problems associated with unavailability, transportation and handling of isobutylene, particularly for use in lowtonnage fine chemical and specialty manufacture, it is advantageous to generate pure isobutylene on site.Two attractive sources of isobutylene are tert-butanol and methyl tert-butyl ether (MTBE). tert-Butanol is available as a byproduct in the ARCO process for propylene oxide. The cracking of MTBE for the generation of isobutylene has been discussed by a number of researchers.2–4 MTBE is a good source for the generation of pure isobutylene and the by-product, methanol, is also a very important raw material in the chemical industry.On the contrary, the dehydration of tert-butanol in situ leads to water as a co-product in the alkylation reaction and thus different yields of the alkylated product are expected vis-à-vis MTBE as the alkylating agent. Synthesis of MTBE from tert-butanol and methanol has been studied in this laboratory using a variety of solid acids.6,7 Heteropoly acids (HPA) supported on clays have shown superior catalytic activity compared to others in the etherification/O-alkylation reactions.5210 Different types of solid acids used for the butylation of pcresol include ion-exchange resins, sulfated zirconia, clays, silica- alumina and g-alumina.6 Ion exchange resins, in spite of their good activity, have the severe limitations of temperature instability and loss of acid sites due to leaching.Our earlier studies in the synthesis of BHT have shown sulfated zirconia as a very efficient and reusable catalyst.5 A number of reports are cited for the alkylation of p-cresol with MTBE. Most of the researchers have employed cation exchange resins as catalysts; and the use of molecular sieves for the selective adsorption of methanol formed in situ by the cracking of MTBE has been reported.11 The cation exchangers used for the study include Lewasorb AC-10,12 Wofatit OK-80,13 Amberlyst-1514 and Lewatit SPC 120.15 The formation of methanol, though it does not deactivate the catalyst, Alkylation of p-cresol with methyl tert-butyl ether (MTBE) over a novel solid acid catalyst UDCaT-1 G.D. Yadav,* A. A. Pujari and A. V. Joshi Chemical Engineering Division, University Department of Chemical Technology (UDCT), University of Mumbai, Matunga, MUMBAI-400 019, India. E-mail: gdy@udct.ernet.in Received 18th August 1999 reduces the number of active sites available for the reaction by occupying them. Haubold et al.13 have suggested the use of gasoline for the removal of methanol from the reaction mixture for the reaction of m-cresol with MTBE.Macho et al.15 has suggested the removal of methanol by trapping it at 243 °C. Yadav and coworkers have reported the behavior of HPA supported on K-10 clay, particularly dodecatungstophosphoric acid (DTPA) on K- 10 in etherification, alkylation and condensation reactions.6–10 It appears that different types of catalysts can be used for the butylation of p-cresol with MTBE.The use of mesoporous zeolitic materials was thus envisaged. A mesoporous material with high acidity and surface area should have an immense potential for reactions involving bulky molecules to improve the selectivity of desired products. All crystalline long range mesoporous materials until now reported including the M41S and/or MCM-41 family members are found to have only mild acidity comparable to silica-alumina catalysts.A catalyst prepared in our laboratory with the synergistic combination of hexagonal mesoporous silica (HMS) and sulfate modified zirconia material ‘UDCaT-1’ was found to have interesting catalytic applications.The present work highlights the superior performance of UDCaT-1 vis-à-vis other solid acids in the synthesis of 2-tert-butyl-p-cresol from p-cresol and MTBE including kinetic modeling. Green Chemistry December 1999 269 This journal is © The Royal Society of Chemistry 1999 C G The alkylation of aromatic substrates is important in many sectors of the chemical industry. Traditional methods of manufacture based on environmentally hazardous catalysts such as aluminium chloride are increasingly being replaced by more benign alternatives but there remains a great need for new catalysts.Solid acids are especially popular in this context and here we can read about a new material that brings together the advantages of a mesoporous inorganic solid (good thermal stability and molecular diffusion rates) with those of a known strong solid acid. The article describes the use of this new catalyst in the important alkylation of p-cresol using MTBE.This offers the additional environmental advantage of using a relatively safe in situ source of isobutylene rather than the gas itself. JHC Green ContextExperimental Catalysts and chemicals UDCaT-1, Filtrol-24, K-10, Indion-130 and DTPA/K-10 were used as catalysts.Filtrol-24 was obtained from Engelhardt. K-10 was obtained from Fluka. Indion-130 used was a product of Ion Exchange (I) Ltd. Dodecatungstophosphoric acid (DTPA) supported clay was prepared by a reported method.6,7 p-Cresol was obtained from s.d. Fine Chem Ltd. MTBE was obtained from Albemarle Corporation, USA.Zirconium oxychloride dissolved in distilled water was carefully added dropwise to calcined HMS with vigorous mixing. After each small addition of the solution, the solid was partially dried over a boiling water bath. Ultimately, after all the solution had been added, the solid was dried in an oven at 120 °C for 1 h. The dried material was loaded in a reactor and ammonia gas was passed through it for 3 h.The ammoniated sample was washed with distilled water to remove the chloride ions and dried in an oven at 120 °C for 2 h. The sulfation was carried out by passing dilute sufuric acid through the filter paper containing the dried ammoniated solid material. It was dried in an oven for 1 h at 120 °C and calcined at 550 °C for 3 h to give the active catalyst UDCaT-1.16 Preparation of sulfated zirconia and 20% DTPA/K- 10 was carried out according to a well-established procedure in our laboratory.5,7 Reaction procedure All experiments were carried out in a Parr Autoclave (100 ml) equipped with a four-blade-pitched turbine impeller.The temperature was maintained at ± 0.5 °C of the desired value. The instrument was also equipped with a speed regulator that could maintain the speed at ± 5 rpm of the desired value.Predetermined amounts of reactants and the catalysts were charged into the autoclave and the temperature was raised to the desired value. Once the temperature was attained, the initial sample was withdrawn. Further samples were drawn at periodic intervals. A typical standard experiment contained 0.22 moles (19.61 g) of MTBE, 0.22 moles (24.31 g) of p-cresol and 3.5% w/w catalyst, based on the reaction mixture.The temperature was maintained at 100 °C and the speed of agitation was 700 rpm. Analysis The samples were analysed on a gas chromatograph (Perkin Elmer Model 8500) equipped with a flame ionisation detector. A 2 m 3 0.003 m column was used. The stationary phase was 10% OV-17 supported on Chromosorb WHP.The quantitative analysis was done through calibration with standard synthetic mixtures. Results and discussion Reaction chemistry and product distribution Fig. 1 shows the reaction scheme for the alkylation of p-cresol with MTBE. This reaction involves two distinct steps, namely, the cracking of MTBE giving methanol and isobutylene and the addition of the isobutylene formed in situ with p-cresol giving 2- tert-butyl-p-cresol and 2,6-di-tert-butyl-p-cresol via consecutive reactions. Further, the diisobutylenes and triisobutylenes could be also formed as side products depending on the type of catalysts and reaction conditions.However, product analysis done showed that not all products depicted in the scheme are formed. The use of MTBE as a butylating agent is advantageous.MTBE cracking starts beyond 75 0C in a significant way. Whatever isobutylene is generated in situ is consumed and no free isobutylene in the gas phase is noticed and no oligomerisation takes place at low temperatures. The co-product methanol is recyclable. In the earlier studies, we had noticed that no O-alkyl- Fig. 1 Reaction scheme ated product is formed beyond 50 0C when p-cersol was alkylated with isobutylene in the presence of sulfated zirconia as catalyst.5 In the present case, all standard experiments were done at 100 0C and the analysis showed no O-alkylated product was formed.The selectivity of p-cresol to 2-tert-butyl-p-cresol was >95% with all catalysts (Table 1). In the case of MTBE as alkylating agent the conversion of p-cresol with 1+1 mole ratio was limited to less than 50% in order that the isobutylene formation was not excessive to produce the oligomers.The diisobutylene and triisobutylene content was less than 1% at 100 0C. At higher temperatures the oligomerisation reactions were significant resulting in up to 10% oligomerisation products. In this work the performance of various solid acid catalysts was evaluated vis-a-vis UDCaT-1 catalyst in the absence of mass transfer resistance.These catalysts includes Indion 130 (an ion exchange resin), Filtrol-24 clay, sulfated zirconia, K-10 clay, 20% dodecatungstophosphoric acid (DTPA) on K-10 clay. All Table 1 Activity of various catalysts for the alkylation of p-cresol with MTBE % Selectivity to Catalyst % Conversion 2-tert-butyl-p-cresol 1.UDCaT-1 45 97 2. Indion 130 39 92 3. Filtrol-24 19 96 4. Sul. Zirconia 15 91 5. K-10 12 96 6. DTPA/K-10 30 96 MTBE (220 mmol), p-cresol (220 mmol); Catalyst loading = 3.5% of reaction mixture; T = 100 °C; Speed = 700 rpm; Reaction time 3 h; By-products formed are isobutylene, diisobutylene, triisobutylene and methanol. 270 Green Chemistry December 1999experiments were done in the absence of any mass transfer limitations. Table 1 gives the conversion of p-cresol and selectivity to 2-tert-butyl-p-cresol. All catalysts give very high selectivity of the mono-alkylated product but the conversion is very high with UDCaT-1.As is evident from Table 1 apart from UDCaT-1, the ion exchange resin catalyst Indion 130 and DTPA/K-10 give a reasonably good conversion of p-cresol and very good selectivity to 2-tert-butyl-pcresol. Isobutylene could also form diisobutylenes and triisobutylenes. However, UDCaT-1 is the best catalyst.In addition to the alkylation with p-cresol, there was in situ generation of isobutylene via cracking of MTBE. A good cracking catalyst and a poor alkylation catalyst would result in loss of the generated isobutylene.A reverse phenomenon would result in poor yield of the required product. It was therefore necessary that the best catalyst should give a reasonable activity for the cracking of MTBE as well as the alkylation of isobutylene with p-cresol. Fig. 2 shows p-cresol conversion with time for the various catalysts employed in the reaction.Cracking of MTBE almost Fig. 2 Effect of different catalysts. p-cresol+MTBE = 1:1; Catalyst loading = 3.33 3 1023; Speed of agitation = 700 rpm; T = 100 °C. occurred immediately for all the catalysts at the set temperature. Gas chromatographic analysis showed the presence of methanol even at zero time. Consequently isobutylene generation was also instantaneous. Thus the isobutylene formed was available to react with the p-cresol to give the desired products.Sulfonated ion exchange resins are well known commercial catalysts that work well for a number of reactions. However, they cannot be used for any reaction above 120 °C. For the alkylation of p-cresol with MTBE, Indion-130 gave a conversion of 39% under otherwise similar conditions which was lower than UDCaT-1.The selectivity towards 2-tert-butyl-p-cresol was 92%, a little lower than the other catalysts employed for the reaction. Filtrol-24 is another commercially available clay catalyst. The amount of residual acidity in Filtrol is relatively high in comparison with K-10. Since both have similar surface areas (320 m2 g21), Filtrol-24 is expected to give higher activity for the reaction in comparison to K-10.Consequently, Filtrol-24 was found to give the higher conversion (19%) compared to 12% with K-10. Both the clay catalysts gave similar selectivities of 96%. Another interesting observation in the use of K-10 was its very high activity towards the cracking of MTBE. Substantial formation of methanol was observed in the first half hour of the reaction.This could possibly be due to the selective adsorption of MTBE onto the catalyst surface that could be responsible for such a phenomenon. Our laboratory has studied in detail the activity of superacidic sulfated zirconia and 20% DTPA/K-10 in the alkylation of pcresol with isobutylene, either generated in situ or as the pure gas.10,11 They have reported that 2-tert-butyl-p-cresol attains a maximum during the course of the reaction after which it gets further alkylated to the dialkylated product.However, for the alkylation of p-cresol with MTBE in the current studies sulfated zirconia gave a very low activity. Even the cracking of MTBE proceeded very slowly as was deduced by the formation of methanol. Effects of different parameters were studied under otherwise similar conditions with UDCaT-1 as the catalyst.Reusabitlity of UDCaT-1 Since UDCaT-1 is a newly developed catalyst, it was essential to conduct studies on its reusability. At the end of the reaction, the catalyst was filtered off and used as such in subsequent reactions. Fig. 3 shows the reusability of the catalyst for three runs. The conversions are marginally lower by 5% from the previous use.Fig. 3 Reusability of catalyst. p-Cresol+MTBE = 1+1; Catalyst loading = 3.33 3 1022 g cm23; Catalyst = UDCaT-1; T = 100 °C. Since the particles were fine some losses due to attrition were responsible for this. On unit weight basis of the actual catalyst present in the reaction under otherwise similar conditions the reusability of catalyst is satisfactory.Effect of speed of agitation The reaction was studied at different speeds of 700, 800 and 1000 rpm, keeping all other parameters constant. There was no observed change in the rate of reaction. It was therefore concluded that there were no external mass transfer resistance controlling the reaction at or above 700 rpm (Fig. 4). All further runs were carried out at 700 rpm. Fig. 4 Effect of speed of agitation. p-Cresol+MTBE = 1:1; Catalyst loading = 3.33 3 1022 g cm23; Catalyst = UDCaT-1. Green Chemistry December 1999 271Effect of catalyst loading Fig. 5 shows the plot of conversion of p-cresol with time at different catalyst loading. The rate of reaction increases with the increase in catalyst loading. With an increase in catalyst loading, Fig. 5 Effect of catalyst loading.p-Cresol+MTBE = 1:1; Catalyst = UDCaT-1; Speed of agitation = 700 rpm; T = 100 °C. the number of accessible active sites for catalysing the reaction increases. With regards to selectivity there was a negligible difference in selectivity profiles of the products at catalyst loadings of 1.5 and 6% of the total mass of the reactants (i.e. 1.75–5.55 3 1022 g cat cm23 liquid phase).The figure shows that with an increase in the catalyst loading from 1.5–3%, there is a marginal increase in the reaction rates but from 3–6% there is a substantial increase. This is because, in addition to the increase in the number of active sites, the turnover number of products also increases, resulting in the quicker consumption of p-cresol and the formation of products.Effect of mole ratio of reactants Fig. 6 shows the plot of conversion vs. time with mole ratios of 1+1, 2+1 and 1+2 of p-cresol to MTBE. It was ensured that the Fig. 6 Effect of mole ratio. p-Cresol+MTBE; Catalyst loading = 3.33 3 1022 cm23; Catalyst = UDCaT-1; Speed of agitation = 700 rpm; T = 100 °C. concentration of the catalyst was kept constant at the different mole ratios.A marked difference in the rate of reaction was observed when the concentration of either of the reactants was changed. The rate of reaction as a function of the mole ratio followed the trend 2+1(highest) > 1+1 > 1+2 of p-cresol to MTBE. If this process was a conventional homogeneous reaction the rate of reaction would have increased with the increase in the isobutylene concentration.In the latter case where the isobutylene generated is much higher the observed conversion of p-cresol is still lower. In a heterogeneous system this may be due to the fact that the isobutylene generated is strongly and preferentially adsorbed on the catalytic sites resulting in a low conversion of p-cresol. Effect of temperature Fig. 7 shows the plots of conversion of p-cresol vs.time at 80, 100 and 115 °C. As expected the rate of reaction increases with Fig. 7 Effect of temperature on conversion. p-Cresol+MTB = 1+1; Catalyst loading = 3.33 31022 g cm23; Catalyst = UDCaT-1; Speed of agitation = 700 rpm. increase in temperature. Another important factor is that the selectivity towards 2-tert-butyl-p-cresol decreases with an increase in temperature, due to the increased rate of reaction towards the formation of 2,6-di-tert-butyl-p-cresol.Selectivity towards 2-tert-butyl-p-cresol at 80, 100 and 115 °C was 100, 97 and 91% respectively, i.e., only at temperatures above 100 °C, is the selectivity towards 2-tert-butyl-p-cresol somewhat decreased. Reaction mechanism and kinetics Model development Several mechanisms were considered and the following was found to describe the results.No dehydration of p-cresol was found to take place to give the corresponding ether or for that matter no O-alkylated product (p-cresyl methyl ether) was formed with methanol. This would suggest that there is adsorption of MTBE on the catalytic site which can react with p-cresol either from the adjacent site, or MTBE first gets cracked to isobutylene which is then readsorbed on the catalytic site to react with p-cresol.Yadav and Thorat 5 have shown that p-cresol reacts with isobutylene according to the Eley-Rideal mechanism with sulfated zirconia as a catalyst. Since UDCaT-1 was based on sulfated zirconia and HMS, the same mechanism was assumed to hold and it was found to be in consonance with the experimental data.The O atom of MTBE coordinates with the acid centre of the catalyst designated as H+ forming a surface complex. Adsorption of MTBE (M) on a general catalytic site (S) leads to the formation of adsorbed species MS as shown in eqn. (1) Chemisorbed MTBE (MS) reacts with p-cresol (C) from the liquid phase near the site according to the Eley-Rideal mechanism to form the surface species PS of the 2-tert-butyl-p-cresol (P) and free methanol (A) [eqn. (2)].The chemisorbed MS can 272 Green Chemistry December 1999undergo simultaneous cracking to give isobutylene (IS) and methanol (A) [eqn. (3)]. Isobutylene is also capable of undergoing dimerisation (D) and trimerisation (T) as shown below by eqns. (6) and (7). If the surface reaction is the rate-controlling step, then the rate of reaction of MTBE is equal to the rate of formation of the alkylated product plus the rate of formation of isobutylene.If reactions (2) and (3) occur simultaneously then, rate of reaction of MTBE, with the surface reaction step controlling, will be rM = k2CMSCC 2 k-2 lCPSCA + k3CMS 2 k-3 ICISCA (10) If the fractional coverage of catalysts is given by that of MTBE alone, then the rate of reaction of MTBE is given by (11) where w is the catalyst loading.When the reaction is far away from equilibrium, then rate of reaction of MTBE is given by (12) In the case of weak adsorption of all species including MTBE K1CM << 1 rM = [k2K1CMCC + k3K1CM]w (13) The selectivity to alkylated product is given by the ratio of the formation of P and that of isobutylene (I) (14) S k C k P I C / = 2 3 r k K C C k K C w K C M M C M M = + + [ ] ( ) 2 1 3 1 1 1 r w k K C C k K C k K C k K C K C j M M C P M I 5 M (1+ = - + - - - [ ] ) 2 1 2 4 3 1 3 1 The selectivity depends only on the concentration of p-cresol and the ratio of rate constants k2/k3 The rate of reaction of p-cresol leading to 2-tert-butyl-p-cresol is given by eqn.(15) rC = 2dCC/dt = [k2K1CMCC]w (15) which is a typical second order reaction. For equimolar concentrations of CM and CC at zero time, eqn. (15) can be written in terms of fractional conversion of p-cresol and integrated to get eqn. (16). = CCOwk2K1t (16) = CCOwkt where k = K1k2, rate constant A plot of XC/(1 2 XC) vs. t should give the slope equal to CCOwk from which k can be calculated.When the initial concentration of MTBE and p-cresol are different, let MR be the mole ratio MR = CCO/CMO, ratio of initial concentration of p-cresol to that of MTBE. On substitution, eqn. (15) can be integrated to give eqn. (17). = CCO(MR 2 1)wkt (17) Where MR � 1 A plot of left hand side of eqn. (17) vs. time could give a slope equal to CCO(MR21)wk from which k could be calculated.Reaction kinetics It was found that the weak adsorption of all species on the catalytic sites was valid wherein the surface reaction between adsorbed MTBE with p-cresol from the liquid phase, according to the Eley-Rideal mechanism controlled the overall rate of reaction [eqn. (2)]. Thus, eqn. (16) for equimolar concentrations of MTBE and p-cresol, and eqn.(17) for non-equimolar concentrations, respectively should be only considered. Plots of XC/(1 2 XC) vs. time were made for equimolar concentration (MR = 1) of CM and CC for three different catalyst loadings (w) as shown in Fig. 8, showing a linear relationship as per Fig. 8 Plot of XC/(1 2 XC) vs. time at different catalyst loading. p-Cresol+MTBE = 1+1; Catalyst = UDCaT-1; Speed of agitation = 700 rpm.the model [see eqn. (16)]. The slopes of these plots are equal to CCOwk. The linear regression (best-data fit) shows the R2 values Green Chemistry December 1999 273 X X C C 1- ln ( ) ( ) M X M X R C R C - - 1as 0.9822, 0.9875 and 0.9959 for catalyst loadings of 1.75 3 1022, 3.33 3 1022 and 5.55 3 1022 g cm23, respectively. This is an excellent match with theory.The k values are calculated as 0.364, 0.233 and 0.399 cm6 g21 mol21 s21 respectively giving an average value of 0.332 cm6 g21 mol21 s21. Plots of ln[MR 2 XC/MR(1 2 XC)] vs. time were made for nonequimolar concentrations of CM and CC (MR = 2 and 0.5) as shown in Fig. 9 and the best fit data show the R2 values of 0.9875 Fig. 9 Plot of ln[MR 2 Xc)/MR(1 2 Xc)] vs.time. MR = p- Cresol+MTBE; Catalyst loading = 3.33 3 1022 g cm23; Catalyst = UDCaT-1; Speed of agitation = 700 rpm. and 0.9798 which again validates the model. The slope of these plots is CCO(MR 2 1)wk, from which the average k was obtained as 0.3333 cm6 g21 mol21 s21 Conclusions UDCaT-1 was found to be a highly active and selective catalyst for the alkylation of p-cresol with MTBE leading to 2-tert-butylp- cresol.Among other catalysts ion exchange resins Indion-130 was the second best catalyst. The high acidity and controlled activity of UDCaT-1 makes it an attractive catalyst. It is recyclable and being an inorganic solid it can be employed at higher temperatures in comparison to ion exchange resin catalysts. The kinetic model was developed according to which MTBE gets adsorbed on the catalyst site to react with p-cresol (Eley-Rideal mechanism). Acknowledgments This research was funded under a grant from the Council of Scientific and Industrial Research (CSIR), New Delhi.A. A. P. received an SRF under this grant and A. V. J. received a fellowship from AICTE. G. D. Y. acknowledges financial assistance for research under the Darbari Seth Professorship Endowment.References 1 R. D. Kirk and D. F. Othmer, Kirk-Othmer Encyclopedia of Chemical Technology, Wiley Interscience, New York, 3rd edn., 1978, vol. 2, p. 72. 2 F. Cunill, J. Tejero and J. F. Izquierdo, Appl. Catal., 1987, 34, 341. 3 J. Tajero, F. Cunill and S. Manzano, Appl. Catal., 1988, 38, 327. 4 B. Schleppinghoff, A. Stuewe, H. Niederberger, H. V. Scheef and J. Grub, Eur. Pat., 407840 (Chem. Abstr., 1991, 114, 228348). 5 G. D. Yadav and T. S. Thorat, Ind. Eng. Chem. Res., 1996, 35, 721. 6 N. 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