Methane dehydroaromatization over Mo/HZSM-5 catalysts in the absence of oxygen: effects of silanation in HZSM-5 zeolite

TEOS as the exotic silicon source was used to modify HZSM-5 zeolite by chemical vapor deposition (CVD) in a vapor phase flow system. Mo/HZSM-5 catalysts prepared by using silicon-modified zeolite showed higher activity and stability than those prepared by using non-modified HZSM-5 zeolite in methane dehydro-aromatization (MDA) under non-oxidative conditions. The effect of silanation on the acid sites and the interaction between the Mo species and the acid sites have been studied by using FT-IR, NH3-TPD and H-1 MAS NMR. The exotic silicon species not only selectively covered and eliminated almost all of the Bronsted acid sites on the external surface of HZSM-5 zeolite, but also affected the internal Bronsted acid sites, which are close to the channel openings. This results in an obvious decrease in the number of Bronsted acid sites per unit cell. In the MDA reaction, the Bronsted acid sites provided active sites not only for the polymerization and aromatization of the active intermedia, but also for the formation of carbonaceous deposits, which is a crucial factor leading to the deactivation of Mo/HZSM-5 catalyst. Only a small fraction of Bronsted acid sites, ca. 0.6 per unit cell, was required to accomplish the aromatization, and the superfluous free Bronsted acid sites would cause severe carbonaceous deposits under non-oxidative condition at a temperature as high as 973 K. (C) 2004 Elsevier B.V. All rights reserved.