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

Alkylation of aromatics over solid acid catalysts such as zeolites, has emerged in the recent past as a viable alternative to conventional Friedel-Crafts alkylation over environmentally hostile catalysts. We studied the diffusion behaviour of ethylbenzene (EB), isobutylbenzene (IBB), o-, m- and p-isobutylethylbenzene (IBEB) in various zeolites such as offretite (OFF), cancrinite (CAN), ZSM-12 (MTW) and ZSM-18 (MEI) by computational procedures. The periodic variations of interaction energy between the molecules and zeolite framework in the calculated diffusion energy profiles are used to predict the energy barrier for diffusion. We analyzed the results to understand the product selectivity in the formation of IBEB in the transalkylation/disproportionation reaction between IBB and EB. The results indicated that the zeolites with channel-like pores are more suitable than those with cage-like pores to achieve better selectivity. The zeolites with channels whose diameters are close to the dimensions of the molecules and those which do not have intersecting channels are better selective catalysts. The efficiency of shape selective production of p-IBEB in these zeolites will be in the order MEI < OFF similar to MTW < CAN as predicted from their diffusion energy barriers. The detailed analysis of the configurations of the molecules in the most favourable and unfavourable adsorption location, indicate that the p-IBEB has favourable interaction energy in all the four zeolites with different pore architecture, compared to o- and m-IBEB except for MEI. It could be concluded that the pore architecture plays a dominant role in controlling the adsorption and diffusion characteristics of these molecules. The actual values of interaction energy themselves are indication of their adsorption behaviour inside the pores of the zeolite.