Modeling the intricacies of heterogeneous catalysis can be a stringent test upon the ability of modern theory methods to simulate chemical reactivity which occurs at elevated temperatures on complex dynamics surfaces. This is most true when one studies complex reaction environments such as liquid-gas or liquid-solid interface of confined spaces. In this presentation, I will discuss recent work in simulating the catalysis of highly anharmonic systems such as a liquid oxide catalyst. Boron oxide-based catalysts have been shown to be both active and selective for driving the oxidative dehydrogenation of propane (ODHP) without the use of metal promoters. However, this reaction occurs at temperatures where boron oxide melts, challenging the experimental identification of the molecular structures within the boron oxide phase under reaction conditions, and thus hindering the understanding of its active sites and reaction mechanism(s).
By combining density functional theory computations, ab initio molecular dynamics simulations, in situ Raman characterization, and microkinetic modeling, we propose that dimerized di-coordinated boron sites (>B–B<), which are dynamically formed in the liquid boron oxide phase, are the active species for O₂ activation under reaction conditions. The resulting peroxy-like species (>B–O–O–B<) is responsible for propane activation, but it is a moderate oxidant for ODHP and thus inert to propene. These peroxy-like structures rapidly activate propane, homolytically, cleaving the >B–O–O–B< bond, producing a propyl radical and a >B–O dangling bond. These > B–O, originating from the >B–O–O–B< sites as well as the liquid B₂O₃ structure itself, play a critical role in the abstraction of H atoms from propane and propyl. In fact, microkinetic modeling reveals that the formation of adsorbed C₃H₇* radicals is the main rate-controlling step due to the highly endergonic adsorption of propane into the system. Otherwise, the only activated processes were found to be the dehydration steps that lead to water formation, which exhibit an intriguing dependence on the concentration of surface hydroxyl species. These findings provide significant insights into the ODHP mechanisms on boron-based catalysts and emphasize the importance of understanding the liquid nature of the oxide to account for its catalytic activity.

Jinshu Tian

Appearances