Fundamental Studies of Butane Oxidation over Model-Supported Vanadium Oxide Catalysts: Molecular Structure-Reactivity Relationships

Abstract

The oxidation of n-butane to maleic anhydride was investigated over a series of model-supported vanadia catalysts where the vanadia phase was present as a two-dimensional metal oxide overlayer on the different oxide supports (TiO 2 , ZrO 2 , CeO 2 , Nb 2 O 5 , Al 2 O 3 , and SiO 2 ). No correlation was found between the properties of the terminal V= =O bond and the butane oxidation turnover frequency (TOF) during in situ Raman spectroscopy study. Furthermore, neither the n-butane oxidation TOF nor maleic anhydride selectivity was related to the extent of reduction of the surface vanadia species. The n-butane oxidation TOF was essentially independent of the surface vanadia coverage, suggesting that the n-butane activation requires only one surface vanadia site. The maleic anhydride TOF, however, increased by a factor of 2-3 as the surface vanadia coverage was increased to monolayer coverage. The higher maleic anhydride TOF at near monolayer coverages suggests that a pair of adjacent vanadia sites may efficiently oxidize n-butane to maleic anhydride, but other factors may also play a contributing role (increase in surface Brønsted acidity and decrease in the number of exposed support cation sites). Varying the specific oxide support changed the n-butane oxidation TOF by ca. 50 (Ti > Ce > Zr ∼ Nb > Al > Si) as well as the maleic anhydride selectivity. The maleic anhydride selectivity closely followed the Lewis acid strength of the oxide support cations, Al > Nb > Ti > Si > Zr > Ce. The addition of acidic surface metal oxides (W, Nb, and P) to the surface vanadia layer was found to have a beneficial effect on the n-butane oxidation TOF and the maleic anhydride selectivity. The creation of bridging V-O-P bonds had an especially positive effect on the maleic anhydride selectivity.

Synthesis and functional verification of the unsupported active phase of VxOy catalysts for partial oxidation of n-butane

“…This estimation results in a selectivity of 10% -40% to oxygenated products. These values compare reasonably well to the selectivities observed for model-supported vanadium oxide catalysts [13]. No conversion of C 4 H 10 and O 2 was detectable by conventional MS, setting the level of reaction to below 1% which is reasonable when taking into account that only a few microgram of catalyst were used.…”

“…Ruitenbeek et al and Wachs et al studied nbutane oxidation to maleic anhydride on titania supported VPO catalysts and titania supported V 2 O 5 catalyst, respectively [33,13]. They found that selective oxidation took place at much lower temperature (493 K) than in the case of conventional VPO catalysts.…”

“…Surface vanadia species can become hydrated in the presence of moisture. Wachs et al report a significant coverage of metal oxide supported vanadia species with moisture at temperatures below 473 K (several monolayers) [12,13,34]. The dehydration of the vanadia species seems to start at temperatures higher than 473 K. Almost no moisture could be detected with Raman on these metal oxide supported systems at 573 K. The strong activity increase and selectivity change observed in our experiments when the catalyst was heated from 473 K to 573 K coincided with the dehydroxylation of the nanoparticles as revealed by TG-MS.…”

“…For catalysts containing approximately a monolayer coverage of surface vanadia, the number of converted n-butane molecules per vanadium atom was a strong function of the specific oxide support [13]. In our case, the V x O y nanoparticles have been synthesized in a controlled way via a simple non-aqueous process involving the reaction of vanadium isopropoxide with benzyl alcohol and subsequent solvothermal treatment [14].…”

Chemistry, Spectroscopy and the Role of Supported Vanadium Oxides in Heterogeneous Catalysis

“…IR can also probe the interaction of vanadium oxide with the surface hydroxyl groups of the oxide support since hydroxyl groups give rise to intense infrared [76][77][78][79][80][81][82][83][84][85][86]. Vanadium-oxygen vibrations are present below 1100 cm −1 and RS has the advantage that the support oxides only show weak absorption bands in the 700-1100 cm −1 region.…”