Hexagonal boron nitride and other boron-containing materials are highly selective catalysts for the oxidative dehydrogenation (ODH) of propane, exhibiting high selectivity to olefins (propylene and ethylene) and low selectivity to undesirable products (CO and CO2). The reaction proceeds via a radical gas-phase mechanism initiated by the catalyst surface. While the exact structure of an active site remains up for debate, there is a consensus that a boron oxyhydroxide phase (B2(OH)2xO3-x )is necessary for catalytic activity. This oxyhydroxide phase is molten at reaction temperatures, resulting in a supported liquid phase catalyst that can generate metastable surface species that are distinct from those observed ex situ at room temperature. Operando characterization of boron catalysts has been limited by difficulties associated with analyzing the light elements that comprise the active phase under the harsh reaction conditions (450-550 °C, O2, C3H8) needed for ODH. Here, we used X-ray Raman Spectroscopy (XRS) to track the oxyfunctionalization of hBN during the catalyst induction period under real reaction conditions. The amount of oxidized boron gradually increases before reaching a steady composition, mirroring the propane conversion trend during the induction period. We then used operando XRS to probe the effect of cofeeding water into the gas feed on the surface while monitoring the reaction products by gas chromatography. We found that the addition of water into the reaction feed reversibly removes the oxidized boron phase and decreases the propylene production rate. The reactivity results measured at the beamline were contrary to those we measured in our home lab reactor, where we observed a reversible enhancement of the reaction rate upon the addition of water that we attributed to an increase in the radical pool concentration. We show that the reactions performed at the beamline have minimal contribution from the gas phase chemistry, resulting in no rate enhancement from the cofed water. These results indicate that the effect of water on the surface oxyfunctionalization is less than its effect on the gas phase chemistry. These results also establish operando XRS as a powerful tool that can help unravel surface and gas phase contributions to this complex and promising catalytic system.

Tracking surface changes on hBN during the oxidative dehydrogenation of propane using operando X-ray Raman spectroscopy

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