The origin of the activity and selectivity of silicalite-1 zeolite for toluene methylation to para-xylene

In this study, a series of MFI-type zeolites with different aluminum contents have been prepared and evaluated for toluene alkylation with methanol. All these zeolite samples are well crystallized and exhibit typical characteristics of MFI structure (as indicated by XRD), and their acid amounts decrease continuously with the decreasing aluminum content of the zeolites (as revealed by NH3-TPD). The weakly-acidic aluminum-free silicalite-1 zeolite is demonstrated to be active in catalyzing toluene methylation for the first time. The content of p-xylene in xylenes, namely the reaction para-selectivity, has been substantially improved by increasing the zeolite diffusion resistance. For the silicalite-1 zeolite, the silanol nests inside the micropores are determined to be the active sites for toluene methylation into p-xylene. Meanwhile, the external surface enriching inert isolated silanols is indicated to be responsible for the enhanced para-selectivity, due to a negligible isomerization of as-formed p-xylene into its isomers. Generally, a greatly improved catalytic performance (with a para-selectivity up to 75% and p-xylene yield ca. 13%) has been obtained for silicalite-1 among the recently reported zeolites. (C) 2017 Elsevier B.V. All rights reserved.