Modulation of zeolite acidity by post-synthesis treatments in Mo/HZSM-5 catalysts for methane dehydroaromatization

Abstract

The acid properties of HZSM-5 were modulated by treatment with acids (oxalic, citric, nitric), (NH(4))(2)SiF(6), steaming, steaming+acid, and partial exchange with Cs(+). Then, catalysts containing 3wt% Mo were prepared by impregnation and evaluated for MDA at 700 degrees C. The materials were characterized by ICP, XRD, N(2) adsorption, FTIR-pyridine, NH(3)-TPD, and (27)Al MAS NMR. Depending on the particular treatment, the density of Bronsted acid sites (BAS) was modified to different extents and thus the Mo/H(+) balance in the final catalysts. Partial exchange with Cs(+) cations (Cs/Al= 0.07 at/at) drastically reduced coke formation and improved benzene selectivity by about 35% due to preferential neutralization of the strongest BAS. A notable improvement in the benzene/naphthalene ratio was also observed after treatment with oxalic and citric acids, steaming followed by HCl washing, and partial Cs(+) exchange.