Structure and catalytic properties of supported vanadium oxides: Support effects on oxidative dehydrogenation reactions

Journal Article · Mon Jan 25 04:00:00 EST 1999 · Journal of Catalysis

DOI:https://doi.org/10.1006/jcat.1998.2295· OSTI ID:316345

Khodakov, A; Olthof, B; Bell, A T; Iglesia, E ^[1]

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1. Lawrence Berkeley National Lab., CA (United States)

The effects of support (Al{sub 2}O{sub 3}, SiO{sub 2}, HfO{sub 2}, TiO{sub 2}, and ZrO{sub 2}) on the structure and catalytic behavior of supported vanadia in the oxidative dehydrogenation of propane were examined over a wide range of vanadium surface densities (0.5--15.0 VO{sub x}/nm{sup 2}). X-ray diffraction and Raman and UV-visible spectra showed that vanadia exists as highly dispersed species at surface densities below 7 VO{sub x}/nm{sup 2} on Al{sub 2}O{sub 3}, HfO{sub 2}, TiO{sub 2}, and ZrO{sub 2}, , but as large V{sub 2}O{sub 3} crystallites on SiO{sub 2}. Surface structures evolve from isolated monovanadates to polyvanadate domains and V{sub 2}O{sub 3} and HfO{sub 2} crystallites as VO{sub x} surface density increases. Polyvanadates appear at lower surface densities on ZrO{sub 2} and TiO{sub 2} than on Al{sub 2}O{sub 3} and HfO{sub 2}. UV-visible edge energies decrease as VO{sub x} domains grow with increasing VO{sub x} surface density on all supports. Initial propene selectivities increase with increasing VO{sub x} surface density, as monovanadate species and exposed support sites, which favor primary combustion pathways, decrease in concentration. Oxidative dehydrogenation rates per V-atom reach a maximum on VO{sub x} domains of intermediate size, which provide a balance between the activity of surface VO{sub x} species and their accessibility to reactants. Interactions with supports determine the type of VO{sub x} structures present at a given surface density, but turnover rates do not depend on the identity of the support when differences in VO{sub x} structure are taken into account. Oxidative dehydrogenation turnover rates are similar on polyvanadate species and on surface VO{sub x} sites on bulk V{sub 2}O{sub 5}. The relative rates of oxidative dehydrogenation to form propene and of secondary propene oxidation to CO{sub x} do not depend on the identify of the support or on VO{sub x} surface density or structure. Thus, it appears that these two reactions require similar VO{sub x} surface sites and that these sites are present at similar surface densities on polyvanadate domains and small V{sub 2}O{sub 5} clusters.

• Sponsoring Organization: USDOE, Washington, DC (United States)
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 316345
• Journal Information: Journal of Catalysis, Journal Name: Journal of Catalysis Journal Issue: 2 Vol. 181; ISSN 0021-9517; ISSN JCTLA5
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
ALUMINIUM OXIDES
CATALYST SUPPORTS
CATALYTIC EFFECTS
CHEMICAL REACTION YIELD
DEHYDROGENATION
HAFNIUM OXIDES
OXIDATION
PROPANE
PROPYLENE
SILICON OXIDES
TITANIUM OXIDES
VANADIUM OXIDES
ZIRCONIUM OXIDES