Catalysts selectivity:Shape selective catalysis over zeolites

Abstract

The catalysts either homogeneous or heterogeneous are more important for shifting selectivity to a desired, value added product rather than turnover, in the present context of environmental safety and by-products disposal. Among the general class of chemo-, regio and stereo selectivity associated with a catalyst, shape-selective catalysis is of particular importance, as the zeolites offer tremendous advantage over amorphous catalysts. The ability to synthesize zeolites and mesoporous materials with precise structural and porous features has proved an impetus to their application in the manufacture of petrochemicals, chemical intermediates and fine chemicals. This presentation highlights some of the recent work on the use of acidic zeolites for the production of alkyl aromatics. The main theme is the alkylation of naphthalene and biphenyl, as these substrates and their alkyl derivatives, both mono- and di-substituted alkyl derivatives provide the challenge for maximum selectivity to either 2,6-dialkylnapthalene or 4,4’-dialkylbiphenyl, respectively, among many isomers that can possibly form on the acid zeolites both within the pore system and outside through associated reactions of isomerization and trans-alkylation. These isomers are important targets for making advanced heat resistant polymers and liquid crystals. Some of the strategies for improved selectivity through passivation of the external acid sites, and the selectivation of the catalyst through controlled coking and deposition of rare-earth metal oxides are discussed with examples drawn from the recent literature.