Counter-intuitive comparable diffusivity of xylene isomers in zeolitic mesopores: Confirmation from adsorption and catalysis studies
The reaction of commercial interest, toluene methylation to selectively obtain para-xylene has been studied. For this purpose, zeolite Beta is modified to impart mesoporosity in the crystals such that formation of kinetically favour para-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 ortho-silicate (TEOS). Albeit, TEOS is found to enter the pores of modified zeolite and no para-selectivity could be attained and proportion of meta-isomer higher than thermodynamically determined is obtained. Catalytic- and xylene adsorption kinetic- results led to conclude (1) diffusivity of para-xylene is only 1.33 times higher than that of the meta-isomers in parent and hierarchical zeolite Beta samples and (2) extent of increase in diffusivity of para- and meta-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 para-xylene selectivity in the reaction of toluene methylation.
Aromatic alkylation; Diffusion coefficient; Mesoporous zeolite; Shape selectivity; Zeolite beta
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