Structure and Density of Mo and Acid Sites in Mo-Exchanged H-ZSM5 Catalysts for Nonoxidative Methane Conversion

Abstract

Mo/H-ZSM5 (1.0-6.3 wt % Mo; Mo/Al ) 0.11-0.68) catalysts for CH 4 aromatization were prepared from physical mixtures of MoO 3 and H-ZSM5 (Si/Al ) 14.3). X-ray diffraction and elemental analysis of physical mixtures treated in air indicate that MoO x species migrate onto the external ZSM5 surface at about 623 K. Between 773 and 973 K, MoO x species migrate inside zeolite channels via surface and gas phase transport, exchange at acid sites, and react to form H 2 O. The amount of H 2 O evolved during exchange and the amount of residual OH groups detected by isotopic equilibration with D 2 showed that each Mo atom replaces one H + during exchange. This stoichiometry and the requirement for charge compensation suggest that exchanged species consist of (Mo 2 O 5 ) 2+ ditetrahedral structures interacting with two cation exchange sites. The proposed mechanism may provide a general framework to describe the exchange of multivalent cations onto Al sites in zeolites. As the Mo concentration exceeds that required to form a MoO x monolayer on the external zeolite surface (∼4 wt % Mo for the H-ZSM5 used), Mo species sublime as (MoO 3 ) n oligomers or extract Al from the zeolite framework to form inactive Al 2 (MoO 4 ) 3 domains detectable by 27 Al NMR. These (Mo 2 O 5 ) 2+ species reduce to form the active MoC x species during the initial stages of CH 4 conversion reactions. Optimum CH 4 aromatization rates were obtained on catalysts with intermediate Mo contents (∼0.4 Mo/Al), because both MoC x and acid sites are required to activate CH 4 and to convert the initial C 2 H 4 products into C 6+ aromatics favored by thermodynamics.

Direct conversion of methane under nonoxidative conditions

“…Iglesia and co-workers prepared the Mo/HZSM-5 catalysts using solid-state reaction method and studied the location and change of the Mo species during the preparation process in greater detail [47]. They found that the MoO x species migrated onto the external surface first at ca.…”

Study of the carbonaceous deposits formed on a Mo/HZSM-5 catalyst in methane dehydro-aromatization by using TG and temperature-programmed techniques

“…In their recent publications, Iglesia and co-workers studied Mo/HZSM-5 catalysts prepared by using mechanical mixtures of MoO 3 and HZSM-5 via solid exchange reaction [13][14][15][16]. The authors presented evidence that most of the Mo species migrated into the channels and were properly reduced by CH 4 into Mo 2 O x C y .…”

The synergic effect between Mo species and acid sites in Mo/HMCM-22 catalysts for methane aromatization

“…It was suggested that Mo species have a molybdate dimer structure (Scheme 1). 12 We question such a model due to the fact that a dimer, once introduced into the channels and exchanged with the Brønsted acid sites, requires the presence of Si(2Al) sites, which has been proved to be of little possibility to exist in HZSM-5 zeolites. Later, the model was revised and the authors claimed that two Mo atoms will bond to two Si(1Al) sites, 13 which seems to be more reasonable theoretically.…”