Synthesis and catalytic performance of hierarchical MCM-22 zeolite aggregates with the assistance of carbon particles and fluoride ions

Hierarchical MCM-22 zeolite aggregates (MCM-22-FC) were prepared by one-pot hydrothermal synthesis with the assistance of carbon particles and fluoride ions. The effects of carbon particles and fluoride ions on the morphology and catalytic properties of the MCM-22 zeolite were investigated. The hierarchical MCM-22-FC zeolite aggregates were constructed by intergrown and stacked thin MCM-22 lamellas and possessed macro-/mesopores and inherent micropores. The MCM-22-FC zeolite was modified to form the Mo/MCM-22-FC catalyst, which exhibited an improved benzene yield and aromatic selectivity as well as catalyst life in the methane dehydroaromatization (MDA) reaction. Based on NH3-TPD and pyridine-Fourier transform infrared spectroscopy measurements together with thermogravimetric analysis, we found that the improved MDA catalytic performance of Mo/MCM-22-FC resulted from the formation of more active MoCx or MoOxCy species that evolved from increased amounts of Bronsted acids with the assistance of fluoride ions. This promoted the diffusion of large molecule products because of the thin MCM-22 lamellas. Less excess Bronsted acid sites were retained in the Mo/HMCM-22-FC catalyst because the formation of active centers inhibited the formation of coke, which contributed to its improved aromatic selectivity. (C) 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.