Computational studies on the diffusion behaviour of alkylaromatics in large pore zeolites

Alkylation of aromatics over solid acid catalysts such as zeolites, has eme rged in the recent past as a viable alternative to conventional Friedel-Cra fts alkylation over environmentally hostile catalysts. We studied the diffu sion behaviour of ethylbenzene (EB), isobutylbenzene (IBB), o-, m- and p-is obutylethylbenzene (IBEB) in various zeolites such as offretite (OFF), canc rinite (CAN), ZSM-12 (MTW) and ZSM-18 (MEI) by computational procedures. Th e periodic variations of interaction energy between the molecules and zeoli te framework in the calculated diffusion energy profiles are used to predic t the energy barrier for diffusion. We analyzed the results to understand t he product selectivity in the formation of IBEB in the transalkylation/disp roportionation reaction between IBB and EB. The results indicated that the zeolites with channel-like pores are more suitable than those with cage-lik e pores to achieve better selectivity. The zeolites with channels whose dia meters are close to the dimensions of the molecules and those which do not have intersecting channels are better selective catalysts. The efficiency o f shape selective production of p-IBEB in these zeolites will be in the ord er MEI < OFF similar to MTW < CAN as predicted from their diffusion energy barriers. The detailed analysis of the configurations of the molecules in t he most favourable and unfavourable adsorption location, indicate that the p-IBEB has favourable interaction energy in all the four zeolites with diff erent pore architecture, compared to o- and m-IBEB except for MEI. It could be concluded that the pore architecture plays a dominant role in controlli ng the adsorption and diffusion characteristics of these molecules. The act ual values of interaction energy themselves are indication of their adsorpt ion behaviour inside the pores of the zeolite.