Functional Analysis of Catalysts for Lower Alkane Oxidation

Abstract

The catalytic performance of 1) crystalline MoVTeNb oxide that exhibits the electronic properties of a n-type semiconductor, 2) submonolayer vanadium oxide supported on meso-structured silica (SBA-15) as an insulating support, and 3) surface-functionalized carbon nanotubes that contain neither a redox active metal nor bulk oxygen, but only surface oxygen species have been compared in the oxidative dehydrogenation of ethane and propane under equal reaction conditions. The catalytic results indicate similarities in the reaction network over all three catalysts within the range of the studied reaction conditions implying that differences in selectivity are a consequence of differences in the rate constants. Higher activity and selectivity to acrylic acid over MoVTeNb oxide as compared to the other two catalysts are attributed to the higher density of potential alkane adsorption sites on M1 and the specific electronic structure of the semiconducting bulk catalyst. Microcalorimetry has been used to determine and quantify different adsorption sites revealing a low V_surface/C₃H_{8 ads} ratio of 4 on M1 and a much higher ratio of 150 on silica-supported vanadium oxide. On the latter catalyst less than one per cent of the vanadium atoms adsorb propane. Barriers of propane activation increase in the order P/oCNT (139 kJ mol⁻¹)≤M1 (143 kJ mol⁻¹)<6V/SBA-15 (162 kJ mol⁻¹), which is in agreement with trends predicted by theory

Isotope Studies in Oxidation of Propane over Vanadium Oxide

“…Slight deviation occurs at low temperatures over 6 V/SBA‐15 for the reasons discussed above (Scheme ). The apparent activation energies (Figures c–d) agree well with published values for M1 and SBA‐15‐supported vanadium oxide, respectively.…”

“…The reaction conditions are limited to fat, dry feed (C 3 H 8 /O 2 /He=10/5/85) and low propane conversion. Under these reaction conditions the selectivity to undesired carbon oxides is higher over silica‐supported vanadium oxide compared to M1 . Factors that determine selectivity in the oxidation of short‐chain alkanes over vanadium oxide catalysts are discussed based on the comparative analysis of the reaction network over the two catalysts.…”

“…Our experimental numbers agree well with calculated KIEs from a 2+4 pathway in which both methyl and methylene C−H bonds are simultaneously involved. However, the calculated barrier for the corresponding transition state (48.3 kcal mol −1 ) is higher compared to the experimentally determined value of 34 kcal mol −1 . The 2+4 mechanism corresponds to a concerted heterolytic C−H activation (two‐electron process) in which all in all 6 electrons are involved simultaneously, including both metal atom and oxygen atom of a M=O species as active site.…”

“…In kinetic studies of oxidative dehydrogenation (ODH) of propane over supported vanadium oxide catalysts the reaction network is, however, usually condensed and presented as a triangle (Scheme , black symbols and lines) . Albeit in traces, oxygenates have been detected in the gas phase, revealing that a more complex underlying network (Scheme , blue symbols) has implications on the selectivity towards the main products propene, CO, and CO 2 . The selectivity to propene is highly dependent on catalyst composition (nature of support, promoters and modifiers, vanadium oxide loading), catalyst structure (molecular structure of vanadium oxide surface species, degree of oligomerization, degree of hydroxylation, texture of catalyst or support) and reaction conditions (partial pressures, temperature, steam content) .…”

“… Simplified reaction network in oxidative dehydrogenation of propane over vanadium oxide catalysts (black symbols) and minimum extension based on detected gas phase intermediates (blue symbols) and the supposed C 3 H 7 . surface/ gas phase intermediate (red).…”