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Ind. Eng. Chem. Res. 1998, 37, 3540-3544
Benzylation of Biphenyl with Benzyl Chloride over Amorphous Silica-Alumina: A Kinetic Study Paolo Beltrame* and Giovanni Zuretti Dipartimento di Chimica Fisica ed Elettrochimica, Universita` degli Studi di Milano, Via C. Golgi 19, I-20133 Milano, Italy
The Friedel-Crafts reaction of biphenyl with benzyl chloride in cyclohexane was carried out over amorphous silica-alumina, in a slurry batch reactor, under magnetic stirring. Mainly benzylbiphenyl isomers (M) but also some dibenzyl derivatives (D) were obtained and determined. Within the monobenzylated fraction, a para-selectivity of 60%, almost independent of conversion, was reached. The chemically controlled reaction kinetics was interpreted by a pseudohomogeneous model of two consecutive reactions; no catalyst deactivation had to be accounted for. The best measurements were effected when the catalyst had been activated at 150 °C: the kinetic model, applied on the basis of an optimization procedure, was able to interpret runs carried out at different doses of catalyst and different initial concentrations of the benzylating agent. The behavior of the amorphous silica-alumina was compared with that of the HY zeolites previously used as catalysts. Introduction
The reactions were carried out in solvent cyclohexane. Hydrochloric acid was mostly released as a gas.
Although polychlorinated biphenyls (PCB) had to be discarded for environmental reasons, there is interest for biphenyl and its derivatives because of their high boiling point, thermal stability, and physicochemical properties. Some of them are useful in heat-transfer and other applications, for instance, as dye solvents.1 Among these derivatives, also benzylbiphenyls have been considered.2,3 The Friedel-Crafts reactions, typically employed for the synthesis of such compounds, may be catalyzed by solid acid catalysts. In the particular case of benzylbiphenyls, catalysts such as montmorillonite and zeolites have been mentioned in the patent literature for their synthesis from biphenyl and benzyl chloride.4,5 The kinetics of the reaction of biphenyl with benzyl chloride over dealuminated HY zeolites has been the subject of a previous paper.6 For an analogous reaction, that is, the alkylation of biphenyl with propene, a comparison as to conversion, yields, and products distribution between crystalline (zeolitic) catalysts and an amorphous silica-alumina has been published.7 To realize a similar comparison for the reaction of biphenyl with benzyl chloride, an amorphous silica-alumina catalyst was considered in the present work. The reactions involved were, as before, the following:
C6H5-C6H5 + C6H5CH2Cl f BzCl BIP C6H5-C6H4-CH2C6H5 + HCl M (three isomers) C6H5-C6H4-CH2C6H5 + C6H5CH2Cl f M BzCl dibenzyl derivatives + HCl D (several isomers) * To whom correspondence should be addressed. Telephone: +39-2-26603250. Fax: +39-2-70638129.
Experimental Section Materials. Reagents and solvents were the same as those for the previous work.6 Catalyst. The catalyst was an amorphous silicaalumina: Akzo LA-30-5P, pellets, SiO2/Al2O3 bulk molar ratio ) 11.4, surface area ) 140 m2/g. Pellets were crushed and sieved, employing only particles of
