Structure and density of Mo and acid sites in Mo-exchanged H-ZSM5 catalysts for nonoxidative methane conversion
- Lawrence Berkeley National Lab., CA (United States). Materials Sciences Div.
Mo/H-ZSM5 (1.0--6.3 wt % Mo; Mo/Al = 0.11--0.68) catalysts for CH{sub 4} aromatization were prepared from physical mixtures of MoO{sub 3} and H-ZSM5 (Si/Al = 14.3). X-ray diffraction and elemental analysis of physical mixtures treated in air indicate that MoO{sub x} species migrate onto the external ZSM5 surface at about 623 K. Between 773 and 973 K, MoO{sub x} species migrate inside zeolite channels via surface and gas phase transport, exchange at acid sites, and react to form H{sub 2}O. The amount of H{sub 2}O evolved during exchange and the amount of residual OH groups detected by isotopic equilibration with D{sub 2} showed that each Mo atom replaces one H{sup +} during exchange. This stoichiometry and the requirement for charge compensation suggest that exchanged species consist of (Mo{sub 2}O{sub 5}){sup 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{sub x} monolayer on the external zeolite surface ({approximately}4 wt % Mo for the H-ZSM5 used), Mo species sublime as (MoO{sub 3}){sub n} oligomers or extract Al from the zeolite framework to form inactive Al{sub 2}(MoO{sub 4}){sub 3} domains detectable by {sup 27}Al NMR. These (Mo{sub 2}O{sub 5}){sup 2+} species reduce to form the active MoC{sub x} species during the initial stages of CH{sub 4} conversion reactions. Optimum CH{sub 4} aromatization rates were obtained on catalysts with intermediate MO contents ({approximately}0.4 Mo/Al), because both MoC{sub x} and acid sites are required to active CH{sub 4} and to convert the initial C{sub 2}H{sub 4} products into C{sub 6+} aromatics favored by thermodynamics.
- Sponsoring Organization:
- USDOE, Washington, DC (United States); National Science Foundation, Washington, DC (United States); Korea Science and Engineering Foundation (Korea, Republic of)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 682114
- Journal Information:
- Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical, Journal Name: Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical Journal Issue: 28 Vol. 103; ISSN 1089-5647; ISSN JPCBFK
- Country of Publication:
- United States
- Language:
- English
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