(668g) New Insights into the Mechanisms of Selectivity Loss in High-Performing Catalysts for the Oxidative Dehydrogenation of Propane

Propylene is a principal platform chemical used to produce commodity polymers and chemicals such as polypropylene, acrylonitrile, and propylene oxide. Oxidative dehydrogenation of propane (ODHP) is an attractive on-purpose propylene production process that can offer long-term process stability and reduced temperatures compared to commercially practiced non-oxidative dehydrogenation. Recent discoveries demonstrate that boron-containing materials can efficiently catalyze ODHP with high selectivity to propylene via a radical-propagated gas-phase propane oxidation that contributes to the reported high selectivity and activity. Despite this advancement, boron-mediated propylene production from ODHP still has not reached industrial viability. In this work we investigate propylene selectivity loss over hexagonal boron nitride catalyst (hBN). Through a mix of differential reactor studies and microkinetic modeling of the gas-phase we find, surprisingly, that propylene selectivity loss likely occurs in the gas-phase rather than on the surface. We attempt to tune the selectivity distribution in the gas-phase through the use of ozone as a radical initiator but are unable to improve propylene selectivity.

Given the challenges inherent in improving propylene selectivity in the gas-phase, we sought to re-explore metal oxide catalysts for surface mediated ODHP. Previous work found that Ta-promotion improves both the activity and selectivity towards propylene for V/SiO2 catalysts. We find that tantalum doping of supported monolayer V/SiO2 increases the activation energy for propylene overoxidation on the catalytic surface, explaining the higher selectivity towards propylene. Using a combination of density functional theory modeling, spectroscopy, and reactor studies we develop a mechanistic understanding of these observations. Taken together, our work provides insight on state-of-the-art boron-based and vanadium-based ODHP catalysts. We highlight the difficulties inherent in improving product selectivity for radical-mediated mechanisms that occur in the gas-phase, and we elucidate a path forward for the improvement of catalytic performance on the surface of metal oxide catalysts.