Understanding the Selectivity of Selective Oxidation of Propane to Acrylic Acid on a Mo-Te-Nb-O M1 Catalyst Using...

In this work, we have performed a quantum chemical investigation for the selective oxidation of propane toward acrylic acid on the M1 phase of a mixed metal oxide (MMO) catalyst, consisting of Mo-Te-Nb-O. The M1 phase of the catalyst has a complex surface structure, which involves different arrangements of metal sites with variable oxidation states. This complexity makes it inherently difficult to understand its activity and selectivity in catalytic reactions. We have used a multilayer cluster model of the main catalytically active site of M1 and a hybrid DFT methodology to establish the minimum energy pathways for the propane oxidation to acrylic acid via propylene, allyl alcohol, and acrolein as the key intermediates. In addition, the reactivity of propyl radicals toward the formation of isopropanol, which leads the reaction toward an unselective path of CO/CO₂ generation instead of acrylic acid production, has also been depicted. We show that the formation of isopropanol has rather a low activation barrier and is therefore competing with the formation of propylene from the propyl radical after C-H activation of propane. Once propylene has formed, the allyl position can easily be activated to form acrolein, which can be further oxidized to acrylic acid. In addition, we have developed a more general linear scaling relation for C-H activation chemistry to estimate activation barriers on M1 catalysts only based on four key energetic descriptors, which are the hydrogen binding energy (E _H) on the surface site, the C-H bond dissociation energy (E _BDE) of the reactant molecule in the gas phase, the interaction energy at transition state structure (E _int ^TS), and the interaction energy between metal site and the oxygen atom of oxygenated gas molecules (E _MO).