(740h) Investigating Surface and Gas-Phase Chemistries during the Boron Nitride-Catalyzed Oxidative Dehydrogenation of Propane

Authors: 
Venegas, J. M., UW Madison
Hermans, I., University of Wisconsin-Madison
Zhang, Z., University of California - Los Angeles
Agbi, T. O., University of Wisconsin - Madison
Alexandrova, A., University of California, Los Angeles
The exothermic oxidative dehydrogenation (ODH) of light alkanes to generate olefins has the potential to be a disruptive technology in the chemical industry. This process improves on process inefficiencies of non-oxidative dehydrogenation methods or steam cracking technologies predominantly used today. Despite decades of research on metal oxide catalyst systems, selectivity towards the desired olefin product remains low at industrially attractive conversions. As such, new catalyst systems need to minimize undesired combustion side reactions under ODH conditions.

Over the last years, we have identified boron-based materials, such as hexagonal boron nitride, as a new class of ODH catalysts. These materials offer improved selectivity towards propylene during the ODH of propane and show decreased yields of combustion side products. While spectroscopic characterization has identified BO3-type surface structures as possible active sites, the reaction mechanism that affords the unique selectivity of boron-based catalysts remains unknown. In this contribution, we combine catalytic activity measurements with quantum chemical calculations to propose that the remarkable product distribution during boron-catalyzed ODH can be rationalized by a combination of surface-mediated formation of radicals over metastable sites and their sequential propagation in the gas phase. Based on known radical propagation steps, we quantitatively describe the oxygen pressure-dependent relative formation of the main product propylene and by-product ethylene. The free radical intermediates are most likely what differentiates this catalytic system from less selective vanadium-based catalysts. Furthermore, this work also highlights the role of water, formed during the reaction, in modulating the reactivity of boron nitride catalysts. These investigations provide a new paradigm that considers gas phase chemistry for the design of improved ODH catalysts.