Ind. Eng. Chem. Res. 1989,28, 1757-1763
1757
Some Novel Aspects of the Dimerization of a-Methylstyrene with Acidic Ion-Exchange Resins, Clays, and Other Acidic Materials as Catalysts Basab Chaudhuri and Man Mohan Sharma* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India The dimerization of a-methylstyrene [ (1-methy1ethenyl)benzenelwith acidic ion-exchange resins such as Amberlyst 15, “monodisperse” K 2661, Lewasorb AC 10 FT, Nafion NR 50, various acidtreated clays, etc., as catalysts, in a variety of solvents was studied in 0.05- and 0.10-m4.d. mechanically agitated reactors in the temperature range 60-160 “C. In cumene and anisole media, the dimerization was found to be kinetically controlled and followed first-order kinetics with respect to a-methylstyrene. In phenol and substituted phenol media, the dimerization was also found to be kinetically controlled, but the rates for specified catalyst loadings were higher by a factor of 2-3 compared to that in cumene medium. Some experiments were also carried out with aqueous 85% formic acid, aqueous 80% chloroacetic acid, aqueous 50% p-toluenesulfonic acid, etc., to explore their utility for obtaining dimers. T h e selectivity with respect to the individual unsaturated dimers (2,4-diphenyl-4and the saturated dimers (1,1,3-trimethyl-1-pentene and 2,4-diphenyl-4-methyl-2-pentene) was greatly influmethyl-3-phenylindan and cis- and trans-1,3-dimethyl-l,3-diphenylcyclobutane) enced by the catalyst, solvent, and the operating conditions. Optimum conditions for realizing high selectivity (greater than 92 % ) with respect to the industrially important isomer, 2,4-diphenyl-4methyl-1-pentene, have been suggested. The utility of acidic ion-exchangeresins as catalysts for a variety of reactions, such as alkylation, hydration, esterification, etherification, oligomerization, etc., has been well-documented (Klein and Widdecke, 1979; Martinola, 1980). The dimerization of a-methylstyrene (AMS) [(lmethylethenyl)benzene] is academically and industrially relevant in several contexts. The unsaturated dimers of AMS, particularly 2,4-diphenyl-4-methyl-l-pentene, are industrially important and are useful as chain-transfer agents or molecular weight regulators in the production of polymers such as polystyrene, AS resin, ABS resin, SBR, and the like. Further, the effect of solvents, polar or nonpolar, on oligomerization reactions has not received adequate attention. This work was undertaken to make a systematic study to cover the above aspects and also to examine conditions that would avoid the unwanted saturated dimers. The products in the acid-catalyzed dimerization of AMS are a mixture of 2,4-diphenyl-4-methyl-l-pentene, 2,4-diphenyl-4-methyl-2-pentene, and 1,1,3-trimethyl-3phenylindan (the first two are called unsaturated dimers, while the third one is referred to as the saturated dimer). One more saturated dimer, cis- and/or trans-1,3-dimethyl-1,3-diphenylcyclobutane, may be formed under certain conditions. An efficient method that exclusively produces the unsaturated dimers, preferably, 2,4-diphenyl-4-methyl-l-pentene, at concentrations higher than 92% by using acidic ion-exchange resin as the catalyst has not been reported in the open literature. In order to obtain the unsaturated dimer, it is necessary to suppress its cyclization to 1,1,3-trimethyl-3-phenylindan. In this paper, we have investigated in detail the influence of reaction conditions and the solvent on the dimerization of AMS on a cation-exchange-resin catalyst (Amberlyst 15 of Rohm and Haas). The typical characteristics of Amberlyst 15, as reported by the manufacturer, are as follows: skeletal structure, styrene-divinylbenzene; ionic functionality, RS03H; porosity, 32%; surface area, 45 m2/g; ion-exchange capacity, 4.6 mequiv/g. Bayer monodisperse K 2661, having the following characteristics, was also used as a catalyst for the dimerization of AMs: matrix, polystyrene; ionic functionality, RS03H; ion-exchange capacity, 1.35 mol/L (minimum) (in 0888-5885/89/2628-1757$01.50/0
accordance with DIN standards). Some experiments were also carried out with Amberlyst XN 1010, Amberlite XE 383, Nafion NR 50, Lewasorb AClOFT, and clay catalysts to assess their utility for obtaining unsaturated dimers.
Previous Studies Most of the information regarding the dimerization of AMS is in the patent literature. No systematic kinetic study has apparently been published in the literature. Various catalysts have been tried, and reaction temperatures as high as 140 OC have been used. Takahatake and Hasui (1978) have prepared unsaturated dimers of AMS by contacting AMS with montmorillonite at 145 OC for 6 h. They have reported 41 % AMS conversion and 93 % selectivity for 2,4-diphenyl-4-methylpentenes.Wygant (1978) has reported the preparation of unsaturated dimers by treating AMS in the presence of clay catalysts. KOsovtrev et al. (1979) have reported the preparation of AMS dimer by heating AMS in the presence of a catalyst consisting of 0.5-30 wt % formic acid. Bateman (1978) has used sulfuric acid as the catalyst for dimerization of AMs; the reaction was carried out at a temperature of 60 OC for 45 min. A product mixture containing 40% unsaturated was obdimer and 60% 1,1,3-trimethyl-3-phenylindan tained. Kiessling and co-workers (1978) have claimed that the dimerization of AMS at 80 “C over an acidic cationexchange catalyst gives 77.1 % of a mixture of 4-methyl2,4-diphenyl-1- and -2-pentene and only 0.1% of the saturated dimer 1,1,3-trimethyl-3-phenylindan. From the foregoing, it is clear that very limited information is available in the literature on the dimerization of AMS over cation-exchange resins as catalysts, and in particular, a kinetic study has not been made, and conditions conducive for making 2,4-diphenyl-4-methyl-lpentene predominantly or exclusively have not been delineated. Experimental Section Experiments were carried out in a 0.05-m-i.d. fully baffled mechanically agitated reactor. A six-bladed glass-disk turbine impeller was used for agitation. Some crucial experiments were also carried out in a 0.10-m-i.d. 0 1989 American Chemical Society
1758 Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989
fully baffled mechanically agitated reactor. AMS was obtained from Herdillia Chemicals Limited and was typically 95% pure; contaminants were close boiling tert-butylbenzene and cumene. Some experiments were carried out with 99% pure AMS also. All other chemicals were obtained from firms of repute. The cation-exchange-resin catalyst Amberlyst 15 was obtained from Rohm and Haas. The catalyst was initially washed with acetone and deionized water to remove any impurity present on the external surface of the catalyst and then dehydrated under vacuum (1-2 mmHg pressure) at a temperature of 100 OC ( f 2 "C) for 2-4 h to remove traces of moisture adsorbed on the catalyst surface. All the experiments were carried out at the desired temperature by placing the reactor in the constant-temperature bath. The dimerization of AMS was studied in solvents such as cumene, anisole, methanol, tert-butyl alcohol, p-chlorophenol, 2,4-dichlorophenol, o-chlorophenol, guaiacol, and p-cresol. The purity of each solvent was tested by analyzing it on a gas chromatograph. The analysis of the reaction mixtur5 consisting of unconverted AMS and AMS dimers was carried out on a Netel gas chromatograph. A stainless steel column, SE-30 (5%) on Chromosorb L, was used for analysis, the length of which was 2 m. The initial oven temperature was kept at 90 "C; the temperature was increased to 300 "C by programming. Nitrogen flow rate in the column was maintained at 0.5 mL/min. Some important results were also checked by analyzing the sample on a 3.5-m-long 10% OV-17 on Chromosorb L column in a Perkin-Elmer 8500 gas chromatograph. Some samples were analyzed by GCMS for product identification. In some cases, an internal standard was used for GC analysis, and the results were checked by matching the material balance. In all the experiments, the solvent was introduced into the reactor first. The same catalyst was used repeatedly, and the activity of the catalyst was checked from time to time by carrying out the reaction at a specified temperature and matching the conversion level. The solventcatalyst slurry was stirred a t the reaction temperature for some time. AMS was then added to the solvent-catalyst slurry a t the same temperature as the reaction temperature. Samples were withdrawn a t regular intervals and analyzed. It is necessary to exercise caution when using fresh catalysts, as leaching of "free acid" is frequently encountered, particularly in polar solvents in initial experiments. This can give disguised results, and the rates may be even 5-10 times of that realized on a steady-state basis.
Results and Discussion Dimerization of AMS in Cumene Medium. The dimerization of AMS in a cumene medium was studied in the temperature range 60-100 "C. The catalyst loading was varied from 2.5% (w/w) to 10% (w/w). The fractional conversion of AMS to unsaturated dimer was found to be a linear function of the catalyst loading. The rate of dimerization of AMS with fresh catalyst was found to be slightly higher than the steady rate obtained with the repeatedly used catalyst. All the results presented in this paper were thus obtained with used catalyst, and excellent reproducibility of the rates was obtained with repeatedly used catalyst. No deactivation of the catalyst was observed with repeated use for at least 15 batches in cumene and anisole media. There was no effect of the speed of agitation in the range 700-1600 rpm in the 0.05-m-i.d. contactor on the dimerization of AMS in cumene medium. Further, the value of the apparent rate constant at the highest temperature was well below the calculated value of the external mass-
I1
1
Ph-C(CH3),CHZC =CH2
Ph-C(CH31z-CH
II
1
Ph
C
n,c'
Y
'Ph
VI Figure 1. Dimerization of AMs: reaction network/mechanism.
transfer coefficient based on one of the well-known correlations (Levins and Glastonbury, 1972). This proves that there is no external mass-transfer resistance associated with the dimerization of AMS in cumene medium. The initial concentration of AMS (in cumene medium) was varied from 0.45 to 1.80 kmol/m3. There was no effect of the variation in the initial concentration on the fractional conversion of AMs. It is thus likely that the reaction follows first-order kinetics with respect to AMS. This finding has been confirmed by plotting -(ln (1 - XA)) against time, where XAis the fractional conversion on the basis of AMS. This suggests that the dimerization of AMS follows first-order kinetics with respect to AMS, and the formation of the carbocation (11)is the rate-determining step of the reaction. The reaction scheme is presented in Figure 1. In this study, attempts were made to suppress the formation of 1,1,3-trimethyl-3-phenylindan(IV) (step 31, which is undesirable for the important uses of the unsaturated dimers. The other saturated dimer, 1,3-dimethyl-l,3-diphenylcyclobutane, was formed in negligible quantities when the dimerization of AMS was carried out with ion-exchange resin as the catalyst. To assess whether any intraparticle gradient exists, the interruption test of Helfferich (1962) was performed. The catalyst particle size was also varied from 5 X to 2.5 X m (diameter) to find out the presence of an intraparticle gradient. The aforementioned tests proved that there was no intraparticle gradient. The effect of temperature in the range 60-100 "C on the dimerization of AMS indicates that the activation energy of the reaction is 4.4 X lo4 kJ/kmol, which further lends support to the reaction being kinetically controlled. Unsaturated dimers were the main products of the dimerization of AMs. At 60 "C, the selectivity with respect to 2,4-diphenyl-4-methyl-l-pentene was 83%, and this was found to remain almost constant even a t an AMS conversion of 75%. An increase in temperature adversely affected the 2,4-diphenyl-4-methyl-l-pentene selectivity. A t 80 "C, the selectivity with respect to 2,4-diphenyl-4methyl-1-pentene up to an AMS conversion of 75% was found to be 68%. At 100 O C , the selectivity with respect dropped to 58%. The to 2,4-dipheny1-4-methyl-l-pentene rate of isomerization of 2,4-diphenyl-4-methyl-1-pentene to -2-pentene was found to be very low up to a temperature of 80 "C. At 100 "C, however, considerable isomerization of 2,4-diphenyl-4-methyl-l-pentene to -2-pentene took place beyond an AMS conversion level of 60%. Almost
Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989 1759 no saturated/cyclic dimer was detected at 60 "C. At 80 "C, the saturated dimer concentration in the product was 5% at an AMS conversion level of 90%. At 100 "C, however, the tendency of the carbocation product (111)to cyclize to the saturated dimer increases markedly. Thus, at 100 "C, the concentration of saturated dimer in the product was 10.8% at an AMS conversion level of 39.5%, and this increased to 13.5% at an AMS conversion level of 75%. It can, thus, be concluded that the formation of 2,4-diphenyl-4-methyl-l-pentene, -2-pentene, and the from AMS saturated dimer 1,1,3-trimethyl-3-phenylindan in cumene medium with acidic ion-exchange-resin catalyst proceeds simultaneously by parallel step reactions. No trimer was formed in the reaction. This shows that the adsorption constant for the AMs monomer is much higher compared to the dimer. This observation is consistent with the observation of O'Connor et al. (1985) in the oligomerization of isobutene, where the presence of the monomer strongly inhibited the formation of the tetramer. From the foregoing discussion, it is clear that the dimerization of AMS in cumene medium on Amberlyst 15 catalyst is surface reaction (interaction of AMS with surface H+) controlled and fiit order with respect to AMs. The rate constants at 60,80, and 100 "C are 0.127, 0.375, and 0.80 kg-' s-l, respectively. The rate of dimerization of AMS in cumene medium in the presence of dry monodisperse K 2661 was found to be higher than that obtained in the presence of Amberlyst 15 under otherwise comparable conditions. The values of the rate constant at 60,80, and 100 "C were 0.16,0.60, and 2.00 kg-' s-l, respectively, and the activation energy of dimerization in the presence of this catalyst was found to be 5.67 X lo4 kJ/kmol, which is significantly higher than the value of 4.4 X lo4 kJ/kmol for Amberlyst 15 catalyst. The selectivity of 2,4-diphenyl-4-methyl-l-pentene at 60 "C was found to be 86.570,and this was found to remain almost constant up to an AMS conversion of 80%; this selectivity of 2,4-diphenyl-4-methyl-l-pentene may be compared with the value of 83% for Amberlyst 15. At 80 "C, the selectivity of 2,4-diphenyl-4-methyI-l-pentene was found to drop 83.2%, but this was substantially better than the corresponding selectivity of 68% with respect to 2,4diphenyl-4-methyl-1-pentene obtained with Amberlyst 15. At 10 "C, however, the selectivity with respect to 2,4-diphenyl-Cmethyl-1-pentene was found to drop from 71.5% at an AMS conversion level of 45.8% to 65.1% at an AMS conversion level of 70%. This suggests that the rate of isomerization of 2,4-diphenyl-4-methyl-l-pentene to -2pentene becomes appreciably fast beyond a temperature of 80 "C. Hence, for obtaining an increased yield of 2,4diphenyl-4-methyl-I-pentene, the dimerization of AMS should be carried out in the temperature range 60-80 "C. Higher temperature promotes the isomerization of 2,4diphenyl-4-methyl-1-pentene to -2-pentene and the cyclization of the carbocation product (111)and the unsaturated dimers to give the indanic dimer. Monodisperse K 2661 seems to be a more effective catalyst than Amberlyst 15 in the selective dimerization of AMS to unsaturated dimer, 2,4-diphenyl-4-methyl-lpentene. Dehydration of the catalyst is of great importance, and proper drying of the catalyst is necessary for obtaining reproducible results. Variations in drying conditions were found to have a drastic effect on the rate of dimerization as well as on the selectivity with respect to 2,4-diphenyl4-methyl-1-pentene. The selectivity of 2,4-diphenyl-4methyl-1-pentene was found to vary between 86% and 67% at a temperature of 60 "C with Amberlyst 15 as the
catalyst. This variation in selectivity with respect to 2,4diphenyl-4-methyl-1-pentene was probably due to variations in dehydration conditions and also the activity or residual acidity of the catalyst. The importance of dehydration conditions of the catalyst has been brought out by Buttersack et al. (1987a,b), and our observations support their findings.
Isomerization of 2,4-Dipheny1-4-methyl-l-pentene to 2,4-Diphenyl-4-methyl-2-pentene It was thought that the presence of AMS might inhibit the adsorption of the unsaturated dimers, 2,4-diphenyl4-methyl-1-pentene or -2-pentene, on the catalyst surface. It was, therefore, considered useful to separately study the kinetics of the isomerization of 2,4-diphenyl-4-methyl-lpentene to -2-pentene in the presence of Amberlyst 15 as the catalyst. The isomerization reaction was carried out in the temperature range 60-100 "C. An unsaturated dimer mixture containing 93% 2,4-diphenyl-4-methyl-l-pentene and 7% 2,4-diphenyl-4-methyl-2-pentene was isomerized in the presence of Amberlyst 15, and cumene was used as the diluent/solvent for the reaction. The isomerization of 2,4-dipheny1-4-methyl-l-pentene to -2-pentene was found to be first order with respect to 2,4-diphenyl-4-methyl-l-pentene. The kinetics of isomerization was followed by restricting the conversion of 2,4-diphenyl-4-methyl-l-pentene to within 30%. Prolonged contact of the reaction mixture with the catalyst at a temperature of 60 "C or above led to cyclization of the dimer to 1,1,3-trimethyl-3-phenylindan.The rate constants for the isomerization of 2,4-diphenyl-4methyl-1-pentene to -2-pentene at 60,80, and 100 "C were respectively 0.0605, 0.1735, and 0.375 kg-I s-l, and the activation energy was found to be 4.7 X lo4 kJ/kmol. The isomerization reaction is reversible, and a thermodynamic value of the ratio of 2,4-diphenyl-4-methyl-lpentene to -2-pentene equal to 0.5 at 80 "C has been reported by Lorenzoni (Beltrame et al., 1988). Prolonged contact of the dimer mixture with the ion-exchange resin at 50 "C gave a ratio of 2,4-dipheny1-4-methyl-l-pentene to -2-pentene equal to 0.4. The reverse reaction rate constants for 2,4-diphenyl-4-methy1-2-pentene to -1pentene in the temperature range 60-80 "C could thus be calculated from the knowledge of the forward reaction rate constants (2,4-diphenyl-4-methyl-1-pentene to -2-pentene) and the thermodynamic value of the ratio of 2,4-diphenyl-4-methyl-1-penteneto -2-pentene. The calculated values of reverse reaction rate constants were required for testing the dimerization network; this is discussed elsewhere in the text. Cyclization of Unsaturated Dimers of AMS The kinetics of cyclization of the unsaturated dimers to the saturated dimer was studied in the temperature range 60-100 "C in the presence of Amberlyst 15. The unsaturated dimer mixture containing 93 % 2,4-diphenyl-4methyl-1-pentene and 7 % 2,4-diphenyl-4-methyl-2-pentene was diluted with cumene. The reaction was found to follow first-order kinetics. Some deoligomerization of the unsaturated dimers to A M s was observed. AMS, formed on deoligomerization, dimerized back to the dimers, and hence, its concentration passed through a maximum. The rate constant values for cyclization at 80 and 100 "C were found to be 0.04 and 0.148 kg-I s-l, respectively, and the activation energy of the reaction was estimated to be 7.1 X lo4 kJ/kmol. The high activation energy of cyclization clearly suggests that the tendency of carboca-
1760 Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989
Figure 2. Reaction network.
Figure 3. Simplified network.
tion product I11 to cyclize will be promoted at higher temperatures. It is possible to convert AMS completely to 1,1,3-trimethyl-3-phenylindan by carrying out the dimerization of AMS in cumene medium in the presence of Amberlyst 15 at a temperature of 100 "C, and complete conversion was achieved in 4 h. Network Analysis. A detailed network of the dimerization of AMS represented in Figure 2 was analyzed mathematically, but the details are not reported here. A t low-to-moderate temperatures, the network reduces to that represented in Figure 3. The product distribution, however, could not be predicted from the theoretical equations at elevated temperatures. This is probably due to the inhibition of adsorption of the dimers, 2,4-diphenyl-4methyl-l-pentene and -2-pentene, in the presence of the monomer AMs. Such a case of inhibition of adsorption has been reported by O'Connors et al. (1985). At low-to-moderate temperatures, the network of the dimerization could be represented by parallel steps of the and -2formation of 2,4-diphenyl-4-methyl-l-pentene pentene, and their ratio was equal to K,/K, a t all conversions of AMs. Thus, by knowing the ratio of K1 to K2 from the initial rates of formation of 2,4-diphenyl-4methyl-l-pentene and -2-pentene, the product distribution could be predicted. Dimerization of AMS in Anisole Medium. The dimerization of AMS in anisole medium is kinetically controlled and proceeds a t almost the same rates as those in cumene medium. The rate constants of the dimerization in anisole medium a t 80 and 100 "C were 0.370 and 0.77 kg-l s-l, respectively, which compare well with the rate constants in cumene medium. The product dimers in anisole medium at 80 "C consisted of 95% unsaturated dimers (80% 2,4-diphenyl-4methyl-l-pentene and 20% 2,4-diphenyl-4-methy1-2pentene) and 5% saturated indanic dimer at 95% conversion level of AMs. Inhibition of Dimerization in Methanol, Acetone, tert-Butyl Alcohol, and Dimethylformamide (DMF). The dimerization of AMS did not proceed at all in solvents like methanol, acetone, tert-butyl alcohol, and DMF, even with large catalyst loading (10% w/w). This is probably due to preferential interaction of the polar solvent molecules with the surface sulfonic acid groups and blocking of acid sites. AMS thus cannot interact with sulfonic acids to form the carbocation, Ph+C(CHJ2, and hence, no dimerization occurs in the polar solvents. tert-Butyl alcohol is also reported to be an inhibitor in the dimerization of
isobutylene to diisobutylene (Mutsumoto et al., 1988). Dimerization of AMS in Phenol, Substituted Phenol, and Substituted Chlorophenol Media. A variety of polar solvents, such as,phenol, p-cresol, o-chlorophenol, p-chlorophenol, and 2,4-dichlorophenol,were employed for the dimerization of AMs, and only the important results will be highlighted here. The effect of acid leaching from fresh ion-exchange-resin catalysts on the dimerization of AMS in the first two experiments was found to be severe. The dimerization of AMS in chlorophenol media gave some interesting results. The solvents, o- and p-chlorophenols (OCLP and PCLP, respectively) and 2,4-dichlorophenol (DCLP),were stripped of any HC1 present in them by passing nitrogen through the molten materials and then placing them under reduced pressure at 50 "C before use. The dimerization of AMS was carried out in molten OCLP (pK, at 25 "C = 8-48),PCLP (pKa at 25 " C = 9.18), and DCLP (pK, at 25 "C = 7.75) solvents without any catalyst. It was observed that the dimerization proceeded smoothly in PCLP solvent, and an AMS conversion of 90% was achieved in 20 min, but no dimerization took place in OCLP or DCLP medium, although the pKa of PCLP is more than that of either OCLP or DCLP. The results show that the structural aspects play a very important role in catalysis. Thus, in OCLP and DCLP, the phenolic hydrogen is sterically hindered and hydrogen bonded with the adjacent chlorine at the orthoposition and is not available for interaction with the AMS molecule; hence, the carbocation [Ph+C(CH,),] formation, which is the necessary step for the dimerization, does not occur. In PCLP, however, the phenolic H is free for carbocation [Ph+C(CHJ2]formation, and the dimerization proceeds smoothly in this medium. The dimerization of AMs in p-cresol medium was found to be kinetically controlled, and first-order rate constants at 50,60, and 70 "C were, respectively, 0.21,0.29, and 0.389 kg-' s-l; the activation energy was estimated to be 2.94 X lo4 kJ/kmol. The rate constant for the dimerization of AMS in p-cresol medium at 60 "C was thus 2.28 times the rate constant in cumene medium. The higher rate of dimerization in p-cresol medium may be due to the solvent effect arising out of thermodynamic interaction between AMS and the polar solvent. The isomerization of 2,4-diphenyl-4-methyl-l-pentene to -2-pentene was considerably fast in p-cresol medium, compared to cumene medium; the selectivity with respect to 2,4-diphenyl-4-methyl-l-pentene at 60 "C was 70%, and this was still poorer at elevated temperatures. There may be several reasons for the low activation energy of the kinetically controlled dimerization of AMS in p-cresol medium compared to that in cumene medium. It is possible that the rate-determining step of the formation of carbocation Ph+C(CHJ2 (11)is preceded by the interaction of the AMS molecules with the sulfonic acid groups of the ion-exchange resin, which involves loose bonding and which has a negative dependence on temperature. This may decrease the overall activation energy of the process. The faster rate of dimerization of AMS in p-cresol medium may also invite diffusional resistance within the microgel particles in the macroreticular resin, and the overall activation energy may drop significantly. Dimerization of AMS with Nafion NR 50 as the Catalyst. The superacid Nafion NR 50 was used as the catalyst for the dimerization of AMS, and some very interesting results were obtained. In cumene medium, the reaction was found to be very slow, and a conversion of 44.8% with respect to AMS was obtained in 6.5 h at 60 "C. The reaction followed first-
Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989 1761 order kinetics, and the rate constant was found to be 7.2 X kg-' s-l. The selectivity with respect to 2,4-diphenyl-4-methyl-1-pentene was around 85% A remarkable effect of the solvent was observed with Nafion as the catalyst. Thus, in a polar medium like pcresol, the rate of dimerization was found to be extremely fast, and at 60 "C, the rate constant for dimerization was 0.457 kg-' s-l. The rate of dimerization in p-cresol medium was thus 63 times higher than that in cumeme medium. The effect of temperature on the rate of dimerization in p-cresol medium was studied in the temperature range 60-160 "C. The activation energy was found to be 3.78 X lo4 kJ/kmol. There was no effect of the speed of agitation on the rate of dimerization at the highest temperature of 160 "C. This proved that the dimerization was kinetically controlled and no solid-liquid mass-transport resistance was present. The values of the apparent rate constants for the dimerization of AMS in p-cresol medium with Nafion NR 50 catalyst at 60, 80, and 120 "C were respectively 0.457, 1.0, and 3.4 kg-I s-l, which were much higher than the rate constants obtained with macroporous Amberlyst 15 under otherwise comparable conditions. The selectivity with respect to 2,4-diphenyl-4-methyl1-pentene in p-cresol medium was found' to be poorer, being only 76%. The isomerization of 2,4-diphenyl-4methyl-1-pentene to -2-pentene was also faster in this medium. At higher temperatures, the concentration of saturated dimer in the product increased to 30%. Some alkylation of p-cresol with AMS to give o-cumyl-p-cresol also took place in the presence of Nafion NR 50. This was, however, slow at temperatures below 80 "C. No acid leaching was observed from the superacid catalyst. There was no drop in catalytic activity on repeated use of the catalyst, and excellent reproducibility in the rates was obtained. The strong effect of solvent on the dimerization of AMS in the presence of nonporous Nafion NR 50 is difficult to explain. It is likely that polar solvents like p-cresol improve the adsorption characteristics of A M s on the catalyst surface, thus increasing the rate of dimerization.
.
Preparation of 92+ % P u r e 2,4-Diphenyl-l-methyl-1-pentene For the unsaturated dimers of AMS to function as good molecular weight regulators, the 2,4-diphenyl-4-methyl1-pentene content in the product should preferably be greater than 95%. The unsaturated dimer, 2,4-diphenyl-4-methyl-2-pentene, is undesirable, as it hinders the initiating reaction and accordingly requires an undesirably longer induction period. The acid-catalyzed dimerization of AMS gives a mixture of both 2,4-diphenyl4-methyl-1-pentene and -2-pentene. The separation of 2,4-diphenyl-4-methyl-l-pentene from a mixture of 2,4diphenyl-4-methyl-1-pentene and -2-pentene is difficult, as the difference in boiling points at 13.3 Pa is only 7 "C. It was, thus, thought worthwhile to carry out the dimerization of AMS in the presence of some other catalyst and explore conditions that maximize the selectivity of the unsaturated dimer, 2,4-diphenyl-4-methyl-l-pentene. Various catalysts have been tried for the purpose of producing the unsaturated dimer, 2,4-diphenyl-4methyl-1-pentene, predominantly. These include acidic ion-exchange resins Amberlyst XN 1010, Amberlite XE 383, powder Lewasorb AC 10 FT, clay catalyst (montmorillonite type), acid-treated zirconia, etc. The use of a small amount of alcohol in the reaction mixture was also tried. Aqueous formic acid, sulfuric acid, p-toluenesulfonic acid, and chloroacetic acid were also employed as the catalyst.
The use of a tiny quantity of trichloroacetic acid and oxalic acid as the catalysts in phenol medium was found to be highly efficient in producing the unsaturated dimer, 2,4diphenyl-4-methyl-l-pentene, almost exclusively. Pure chloroacetic acid, when used as the medium, gave, however, predominantly the saturated dimer. Use of Ion-Exchange Resins. Both dehydrated and undehydrated forms of the ion-exchange resin were found to work satisfactorily in the dimerization of AMS. The catalyst Amberlyst 15 was washed with deionized water several times and then dried at 90 "C to remove surface moisture (not adsorbed moisture). The catalyst thus treated was used for the dimerization of AMs. In a typical experiment, 80 g of AMS was stirred with 2 g of catalyst at 80 "C. The conversion of AMS to its unsaturated dimers was found to be 60% after 1 h, and the selectivity was more with respect to 2,4-diphenyl-4-methyl-l-pentene than 85%. Under similar conditions, Bayer catalyst K 2661 gave a better selectivity (89.6%) with respect to the desired dimer. The presence of alcohol, such as isopropyl alcohol, in the reaction mixture (pure AMS + catalyst) was found to improve the selectivity with respect to 2,4-diphenyl-4methyl-1-pentene markedly. A selectivity of 93 % with respect to 2,4-diphenyl-4-methyl-l-pentene was realized when the dimerization of AMS was carried out at a temperature of 60 "C and in the presence of 2.5% (w/w) Amberlyst 15 and 4% (w/w) isopropyl alcohol. Various catalysts, such as Amberlyst XN 1010, Amberlite XE 383, and Lewasorb AC 10 FT, gave a very high selectivity (-93%) with respect to 2,4-diphenyl-4-methyl-l-pentene when a small quantity of isopropyl alcohol (2-4% w/w) was used as an additive. Without addition of the alcohol, the selectivity was around 85% for Amberlyst 15. This was found to be a very good method for producing 92+% 2,4-diphenyl-4-methyl-l-pentene directly by dimerizing AMS in the presence of a cation-exchange resin as the catalyst. The presence of isopropyl alcohol in the reaction mixture, however, was found to retard the rate of dimerization drastically, and only 50% conversion with respect to AMS was obtained in 6 h at 60 "C when a catalyst (Amberlyst 15) loading of 2.5% was used. In the presence of alcohol, the dimerization was found to follow first-order kinetics and an AMS conversion of 75% was achieved in 12 h. The selectivity with respect to 2,4-diphenyl-4methyl-1-pentenewas found to remain constant at around 93.2%, even at an AMS conversion level of 85%. The above results show that an additive like alcohol suppressed and the formation of 2,4-diphenyl-4-methyl-2-pentene saturated dimers in the cation-exchange-catalyzed dimerization of AMS and increased the selectivity with respect to 2,4-diphenyl-4-methyl-l-pentene. Use of Acid-Treated Clays. Acid-treated and cationexchanged montmorillonites have been shown to possess an ability to promote organic reactions as acid catalysts. It was thought that the dimerization of AMS with acidtreated clays might exhibit the desired selectivity with respect to the unsaturated dimer, 2,4-diphenyl-4methyl-1-pentene, and hence a variety of clays such as Filtrol24, Tonsil A/C, Tonsil COG, Tonsil Supreme FF, Tonsil Optimum FF, acid-treated Korean earth, and Activol B/C were tried as catalysts; these clays are relatively cheap. The total surface acidity of Filtrol 24, Tonsil A/C, Tonsil COG, and Activol B/C was measured by ammonia adsorption and temperature-programmeddesorption. The acidities of the clays are listed in Table I. The catalyst Filtrol24, having 12% moisture, was found to be a very useful catalyst for dimerizing AMS to 2,4-
1762 Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989
With the fast filtration clay catalysts, Tonsil Supreme FF and Tonsil Optimum FF, a remarkably high selectivity was (96%) with respect to 2,4-dipheny1-4-methyl-l-pentene obtained and the extent of high boiling substances was also low (-6%). 0.4 Clay catalysts are much cheaper than ion-exchange re0.3 sins, and their use as a catalyst may be attractive indusdiphenyl-4-methyl-1-pentene selectively. An AMS contrially. The exact reason for getting 1,3-dimethyl-1,3-diversion of 77% was obtained in 3 h a t a temperature of phenylcyclobutane in such high proportions as observed 60 "C when 20 g of AMS was dimerized in 60 g of cumene with Tonsil A/C is not clear. with 2.5% (w/w) catalyst loading (particle size = 480 pm); Use of Aqueous Solutions of Sulfuric Acid, Alithe selectivity with respect to 2,4-diphenyl-4-methyl-lphatic Acids, and Aromatic Sulfonic Acids. It was considered desirable to probe the behavior of different pentene was 88%. Even at a conversion level of 85%, no acids having varying ionization constants and concentrated saturated dimer was detected in the product. Repeated solutions of aromatic sulfonic acids which exhibit hydrouse of the catalyst gave considerable improvement in the selectivity with respect to 2,4-diphenyl-4-methyl-l-pentene tropic behavior, resulting in a considerable increase in the solubility of AMS in aqueous solutions. Table I1 gives the (9270, in the absence of alcohol). The use of isopropyl alcohol as an additive in the reaction mixture was found salient details of different acids used in this two-phase to suppress the formation of 2,4-diphenyl-4-methy1-2system. pentene and high boiling substances considerably, and a The effect of the speed of agitation on the rate of divery high selectivity with respect to 2,4-diphenyl-4merization was studied, in the fastest reactive system, by methyl-1-pentene (95%) was realized. varying the speed of agitation from lo00 to 2500 rpm. The The dimerization of AMS with Tonsil A/C catalyst gave rate of dimerization was found to be independent of the some intriguing results. AMS of 95% purity was dimerized speed of agitation, which indicates that the reaction is at 60 "C with 5% (w/w) Tonsil A/C (particle size = 5 pm) kinetically controlled. All the experiments with aqueous as the catalyst. A conversion of 80% with respect to AMS acid solutions were thus carried out at an rpm of 2000 and was achieved in 1 h; the selectivity with respect to 2,4a temperature of 70 "C. diphenyl-4-methyl-1-pentene was 89.6%. One of the reThe separation of the aqueous phase took place readily markable features of this catalyst was that it initially gave after agitation was stopped. the saturated dimer, 1,3-dimethyl-l,3-diphenylcyclobutane A t the same weight percent concentration level, p-TSA (DDCB), to a considerable extent, which gradually isomgave a higher rate of dimerization compared to H,S04 erized to give the unsaturated dimers. The concentration under otherwise uniform conditions. The higher rate of of DDCB in the product went to as high as 30% in 20 min. dimerization with aqueous p-TSA solution might be due The formation of this type of saturated dimer seems to be to its hydrotropic property. The use of hydrotropes to unique and warrants further study, and this will be the increase the rates of reaction in the two-phase system is subject of a future publication. A considerable amount of well-known (Pandit and Sharma, 1987). high boiling products was also formed with this catalyst. It is seen from Table I1 that the highest percentage of The concentration of high boiling products in the product 2,4-diphenyl-4-methyl-l-pentene could be as high as 95. dimers was around 11% with Tonsil A/C as compared to The use of aqueous acidic solutions, although useful in 670 obtained with Filtrol 24 catalyst. giving the desired isomer ratio, may not be industrially useful in view of corrosive conditions, possible problems With 2.570 (w/w) catalyst loading of Tonsil A/C, the concentration of the saturated dimer, DDCB, showed an associated with phase separations, superiority of ion-exoscillatory behavior. An additive like isopropyl alcohol change or clay-catalyzed reactions, etc. (2% w/w) was found to suppress the formation of DDCB Use of Trichloroacetic Acid and Oxalic Acid as almost completely. Catalysts. The use of trichloro- or trifluoroacetic acid as Granular Tonsil COG, Korean earth, and Activol B/C the catalyst for the dimerization of AMS in phenol medium has catalysts also produced 2,4-diphenyl-4-methyl-l-pentene to give selectively 2,4-diphenyl-4-methyl-l-pentene been recently published in the literature (Beltrame et al., selectively (2,4-diphenyl-4-methyl-l-pentene content = 1988). The results reported by the authors have been 92-97% in the product dimer), but these catalysts gave confirmed by us. The use of trichloro- or trifluoroacetic a considerable amount of high boiling substances in the product, and their use as catalysts for the dimerization of acid as the catalyst, however, poses several problems. AMS is, thus, not recommended. These acids are extremely hygroscopic and corrosive. The Table I. Total Acidity of Clay Catalysts
mmol of NH,/g of catalyst 0.4 0.3
catalyst Filtrol 24 Tonsil A/C Tonsil COG Activol B j C
Table 11. Use of Aaueous Acid Solutions as Catalssts" catalyst H*SO, p-TSA
ClCHzCOOH HCOOH
% concn
45 45
phase ratio (viv) (aq to
(w/w)
erg)
1.58:l 1.58:l
60 80
1.58:l 1.58:l
85 99
1.2:l 1.2:l
time, s 1800 3600 5400 1800 3600 5400 1200 900 1800 2700 1200 300
ratio of 70conversion of AMS UD, to UDz 13 92:8 26 91.5:8.5 40 91:9 18 93:7 35 92:8 47 91.8:8.2 86.8 87:13 47 95:5 69 94.6:5.4 83 92.9:7.1 84 92.8:7.2 90 93:7
% saturated dimer in the product
0.3 0.5 0.4 0.5 0.9 0.4 0.4
"pK, of acids at 25 "C: p-TSA = -6 to -7; ClCH,COOH = 2.87; HCOOH = 3.75. In all experiments, AMS of 95% purity was used.
Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989 1763 separation of these acids from the product dimer might also create several problems. It was, thus, thought worthwhile to try oxalic acid as the catalyst for dimerizing AMS in phenol medium. It was found that both trichloroaceticacid and oxalic acid, when used as the catalyst, gave p-cumylphenol (PCP) as an additional product other and -2-pentene; than 2,4-diphenyl-4-methyl-l-pentene however, the extent of formation of PCP was less in the presence of oxalic acid In a typical experiment, 27 g of AMS was taken in 43 g of phenol and to it was added 0.70 g of oxalic acid (1% w/w). The reaction was carried out in a mechanically agitated reactor a t a temperature of 60 "C. A conversion of 68% was respect to AMS was achieved in 3 h, the selectivity with respect to 2,4-diphenyl-4-methyl-l-pentene was found to be 97.5%. No saturated dimer was detected in the product, and the PCP concentration in the product was found to be 8%. The above results show that oxalic acid is a very useful catalyst for dimerizing AMS in phenol medium. The use of phenol as the medium is important, as it solvates oxalic acid effectively. The use of a nonpolar medium like cumene did not give any dimerization. The product dimers can be easily separated from a mixture containing AMS, dimers, and PCP by distillation a t reduced pressure.
Conclusions The dimerization of AMS over the cation-exchange resins Amberlyst 15 and macroporous K 2661 was found to be kinetically controlled and first order with respect to AMS in cumene and anisole media. In phenol and substituted phenol media, the dimerization was also found to be kinetically controlled, but the rates, for specified catalyst loading, were found to be 2-3 times higher than those obtained in cumeme and/or anisole medium. The higher rates of dimerization obtained in polar medium are probably due to the solvent effect. A remarkable effect of solvent was observed with superacid catalyst, Nafion NR 50; the rate of dimerization in p-cresol, a polar medium, was found to be 63 times higher than that in cumeme medium. Solvents like tert-butyl alcohol and dimethylformamide were found to inhibit the dimerization of AMS completely. A method for the preparation of the industrially useful unsaturated dimer, 2,4-diphenyl-4-methyl-l-pentene of purity greater than 93%, free of any saturated dimer, has also been suggested. Acid-treated clays have been found to be very useful catalysts for dimerizing AMS to 2,4-diphenyl-4-methyl-l-pentene highly selectively. Acknowledgment B.C. is thankful to the University Grants Commission, New Delhi, for the award of Research Fellowship.
Nomenclature AMS = a-methylstyrene UD1 = unsaturated dimer, 2,4-diphenyl-4-methyl-l-pentene UD2 = unsaturated dimer, 2,4-diphenyl-4-methyl-P-pentene SD = saturated dimer, 1,1,3-trimethyl-3-phenylindan Ki = rate constants of the different steps as shown in Figures 2 and 3 Registry No. K2661,123205-61-8; Amberlyst 15,9037-24-5; Lewasorb AC 10 FT, 123205-62-9;Nafion NR 50,118473-68-0; Filtrol 24, 123205-60-7;Tonsil A/C, 123205-63-0; Tonsil COG, 123205-64-1;Activol B/C, 123205-59-4;Tonsil Supreme FF, 123205-66-3; Tonsil Optimum FF, 123205-65-2;a-methylstyrene, 98-83-9;formic acid, 64-18-6;chloroacetic acid, 79-11-8;ptoluenesulfonic acid, 104-15-4; 2,4-dipheny1-4-methyl-l-pentene, 6362-80-7;2,4-diphenyl-4-methyl-2-pentene, 6258-73-7;1,1,3trimethyl-3-phenylindan, 3910-35-8;cis-1,3-dimethyl-1,3-diphenylcyclobutane, 123099-36-5;trans-1,3-dimethyl-l,3-diphenylcyclobutane, 123099-37-6;sulfuric acid, 7664-93-9;trichloroacetic acid, 76-03-9;oxalic acid, 144-62-7.
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Received for review January 17,1989 Revised manuscript received July 17,1989 Accepted August 2, 1989
