Shape selective methylation of biphenyl within zeolites: An example of transition state selectivity

Shape selective alkylation of binuclear aromatic compounds was studied using the methylation of biphenyl as a test reaction. The slurry phase reaction was carried out in a stainless steel autoclave using n-heptane as a solvent. By simultaneously varying the zeolite catalyst type and the alkylating agent shape selective effects to the least bulky para-isomers 4-methyl and 4,4'-dimethyl-isomers were observed. Alkylation using methanol turned out to be unselective, transalkylation of biphenyl using polymethylbenzenes and an ultrastable faujasite type zeolite produced mainly 4- and 4,4'-isomers. This effect was due to a supposed transition state selectivity. By reducing the amount of the acid sites on the outer as well as the inner surface of the zeolite catalyst through a treatment with phosphorous containing compounds and with water, undesired side reactions were suppressed and the yield of para-products was enhanced. Although the catalysts suffered deactivation during the reaction because of formation of carbonaceous residues, which mainly consisted of irreversibly adsorbed reaction partners, they could nearly be completely reactivated by burning these residues in a stream of synthetic air. To illustrate the potential industrial applicability of the reaction, the synthesis of the required alkylating agents polymethylbenzenes starting from xylenes and methanol was investigated and based on these results a possible 4,4'-dimethylbiphenyl production process was outlined. (C) 1997 Elsevier Science B.V.