Structure and properties of zirconia-supported molybdenum oxide catalysts for oxidative dehydrogenation of propane

Journal Article · Tue Jan 25 04:00:00 EST 2000 · Journal of Catalysis

DOI:https://doi.org/10.1006/jcat.1999.2720· OSTI ID:20014539

Chen, K; Xie, S; Iglesia, E; Bell, A T

Oxidative dehydrogenation (ODH) of propane was studied on zirconia-supported molybdenum oxide catalysts. The structure of the ZrO{sub 2} support and of the dispersed MoO{sub x} species was characterized by X-ray diffraction and by Raman and UV-visible spectroscopies. The structure of dispersed molybdena depends on the Mo surface density and on the temperature at which catalyst precursors are treated in air. Polymolybdate domains were detected by Raman at Mo surface densities below 5 Mo/nm{sup 2}. At higher surface densities, MoO{sub 3} and ZrMo{sub 2}O{sub 8} are present; their relative concentrations depend on the pretreatment temperature. Below 773 K, MoO{sub 3} is the predominant structure at high surface densities, but ZrMo{sub 2}O{sub 8} forms above 773 K. UV-visible edge energies decrease with increasing surface density for samples containing polymolybdate species, suggesting that MoO{sub x} domains become larger as the Mo surface density increases. ODH turnover rates decrease with increasing Mo surface density on samples containing polymolybdate species and MoO{sub 3}. This trend is accompanied by an increase in the initial propene selectivity and in the vibrational frequency of Mo=O bonds. Higher Mo{double{underscore}bond}O vibrational frequencies reflect stronger Mo=O bonds, which show lower ODH reactivity; therefore, the lower ODH reaction rates (per Mo atom) at higher Mo surface densities arise from the lower reactivity of Mo=O bonds, while higher initial propene selectivities arise either from the decrease of exposed Mo-O-Zr bonds or the lower reactivity of Mo=O bonds as the size of MoO{sub x} domains increases with increasing Mo surface density. At similar Mo surface densities, samples containing predominantly ZrMo{sub 2}O{sub 8}/ZrO{sub 2} show higher turnover rates and lower initial propene selectivities than those containing MoO{sub 3} species because the vibrational frequency of the Mo=O bond for ZrMo{sub 2}O{sub 8}/ZrO{sub 2} also decreased with increasing Mo surface density, ultimately due to the increase of the particle size which leads to lower propane accessibility.

• Research Organization: Lawrence Berkeley National Lab., CA (US); Univ. of California, Berkeley, CA (US)
• Sponsoring Organization: US Department of Energy
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 20014539
• Journal Information: Journal of Catalysis, Journal Name: Journal of Catalysis Journal Issue: 2 Vol. 189; ISSN 0021-9517; ISSN JCTLA5
• Country of Publication: United States
• Language: English

Related Subjects

03 NATURAL GAS
10 SYNTHETIC FUELS
CATALYST SUPPORTS
CATALYTIC EFFECTS
DEHYDROGENATION
MICROSTRUCTURE
MOLYBDENUM OXIDES
OXIDATION
PROPANE
PROPYLENE
ZIRCONIUM OXIDES