ETHYLBENZENE HYDROISOMERIZATION OVER BIFUNCTIONAL ZEOLITE BASED CATALYSTS - THE INFLUENCE OF FRAMEWORK AND EXTRAFRAMEWORK COMPOSITION AND ZEOLITE STRUCTURE

The hydroisomerization of ethylbenzene (EB) has been carried out on a series of bifunctional Pt/Al2O3-zeolite catalysts, in which both the s tructure (mordenite, beta, Y, ZSM-5, MCM-22) and the chemical composit ion (for the mordenite and beta samples) of the zeolitic component wer e varied. Postsynthesis methods such as ion-exchange, steaming and che mical treatments were used to modify the composition of the samples (f ramework and extraframework Al content and nature of the compensation cation). Both parameters, zeolite structure and chemical composition, were seen to affect the activity and selectivity of the resultant cata lysts. Large pore zeolites, particularly mordenite and beta, presented the highest selectivity to the desired isomerization products, i.e., xylenes. In the case of mordenites the textural properties (mainly the mesoporosity), played a key role in determining the activity and sele ctivity of the catalysts. Both, zeolite acidity and mesoporosity deter mined the amount of Bronsted sites accessible to the reactant molecule s. It was seen that a reduced amount of accessible Bronsted acid sites in the zeolite favored the isomerization of EB with respect to second ary reactions like cracking of the naphthenes and dealkylation. Select ivities to xylenes above 40% at 60% EB conversion were obtained with m ost of the mordenite-based catalysts. Also, the calcium-exchanged mord enite samples gave larger xylene yields than the corresponding protoni c samples. In the case of the Pt/Al2O3-beta samples, it was seen that formation of xylenes by isomerization of EB is favored in catalysts pr esenting a reduced density of Bronsted acid sites and a high mesoporos ity. Thus, those catalysts prepared from the beta samples obtained by steaming and steaming + acid treatment of the original acidic zeolite were the most selective to xylenes, giving a selectivity close to 40% at ca 60% EB conversion. (C) 1998 Academic Press.