The oxidation of propane over metal oxides: A first principles study

Alkenes are the predominant feedstocks used to produce a wide range of value-added chemicals such as maleic acid, acrylonitrile and acrylic acid. The oxidative dehydrogenation (ODH) of lower alkanes to form alkenes is an attractive route for the conversion of lower alkanes to form the corresponding alkenes. Though a number of metal oxide and mixed metal oxide catalysts have been reported to be active for the ODH of alkanes, the yield to the corresponding alkenes is limited due to complete combustion to carbon-dioxide. Currently there are significant efforts to develop economic industrial processes that selectively oxidize alkanes to olefins or other functionalized chemical intermediates.;Despite the number of experimental and theoretical studies on the subject, a number of aspects related to the oxydehydrogenation of alkanes are still unresolved. This includes the effect of the composition and structure of the catalyst on its activity, the nature of the participating lattice oxygen atoms, and the specific elementary steps that comprise the mechanism. The aim of this dissertation is to use first principle density functional theory to address the issues listed above on model catalytic systems. More specifically we analyze herein elementary steps in the mechanism for the oxidation of propane over model heteropolyacids of the Keggin form and vanadia species supported over titania.;First principle theoretical calculations were carried out to analyze the elementary reaction pathways and the corresponding reaction energies and activation barriers involved in the oxidative dehydrogenation (ODH) of propane over vanadium substituted phosphomolybdic heteropolyacid (H4PVMo 11O40). A Mars van-Krevelen type mechanism was examined which involves the homolytic chemisorption of an initial propane molecule over neighboring lattice oxygen atoms, followed by the formation and desorption of propene and water leaving behind an oxygen-defect containing substrate. Various mechanisms of regeneration of the substrate using molecular oxygen were explored. The largest activation barrier for the overall reaction path was found to be +127 kJ/mol and it corresponds to the alpha-hydrogen abstraction from the adsorbed alkoxy species to form a propene molecule.;Ab initio DFT calculations were used to explore the effect of substrate composition on its redox properties and consequently on the activation of propane. The initial activation of propane was found to be increasingly more exothermic as the percentage of vanadium in the formula Hn+3PV n12-nO40 (n=0-2) was increased. An in-depth analysis of the electronic structure of the corresponding polyanion revealed a correlation between the HOMO LUMO gap and the reducibility of the Keggin unit. The presence of vanadium increased the reduction potential of the Keggin unit by stabilizing the LUMO. The effect of replacing two protons in H4PVMo11 O40 with bivalent metal counteractions (Co, Ni, Zn and Mg) was also examined. The activation of propane was found to be more exothermic by 35 kJ/mol over NiII exchanged HPA, 15 kJ/mol over Co II exchanged HPA and 7 kJ/mol over ZnII exchanged HPA when compared to the original HPA unit. There was no appreciable change in the reaction energy when Mg was exchanged with two protons. It was observed that when transition metal atoms (Co, Zn or Ni) are exchanged for protons, electronic states are introduced into the band gap, which facilitate electron transfer between the HOMO of the HPA molecule to the LUMO thus making the substrate more reducible.;Finally, we examined the initial activation of propane was studied over vanadia supported over titania. Monomeric, dimeric and polymeric vanadia clusters were considered over the (001) surface of anatase titania to elucidate the effect of polymerization on the activation of propane. The activation of propane over V2Ox/TiO2 was found to be energetically more favorable with the increasing vanadia polymerization over the support. The calculated electronic structure showed that the band gap of these materials decreased with increasing domain size of the vanadia cluster leading to higher reducibility of the catalyst.;In conclusion, the overall kinetics of the reaction pathway for the ODH of alkanes depends on the reducibility and the acid/base properties of the oxide substrate. These in turn depend on the nature of the metal atom and the basicity as well as the mobility of the lattice oxygen atoms.