Counter-intuitive comparable diffusivity of xylene isomers in zeolitic mesopores: Confirmation from adsorption and catalysis studies

摘要

The reaction of commercial interest, toluene methylation to selectively obtain empara/em-xylene has been studied. For this purpose, zeolite Beta is modified to impart mesoporosity in the crystals such that formation of kinetically favour empara/em-isomer (as advocated by some schools earlier) facilitates and diffuse out easily. However, when this modification did not meet the expectation, pore openings on crystal surface have been narrowed using tetraethyl emortho/em-silicate (TEOS). Albeit, TEOS is found to enter the pores of modified zeolite and no empara/em-selectivity could be attained and proportion of emmeta/em-isomer higher than thermodynamically determined is obtained. Catalytic- and xylene adsorption kinetic- results led to conclude (1) diffusivity of empara/em-xylene is only 1.33 times higher than that of the emmeta/em-isomers in parent and hierarchical zeolite Beta samples and (2) extent of increase in diffusivity of empara/em- and emmeta/em-xylene due to mesoporosity generation is comparable (about 1.14 times) unlike increase by 2 order of magnitude reported in case of ZSM-5. These observations dispel the early reckoning: large pore zeolites affording empara/em-xylene selectivity in the reaction of toluene methylation." xml:lang="en_US.