Oxidative Dehydrogenation of Propane on Boron Phosphate: A Computational Mechanistic Study

Boron-based oxidative dehydrogenation of propane (ODHP) is emerging as a promising protocol because of its efficient conversion to propene, while the correlation between the structures of boron-containing materials and their catalytic activity remains unclear. In this work, by means of density functional theory calculations, the mechanism of ODHP on the surface of boron phosphate (BPO4) was studied. Three types of surface sites, the tri- (>B-OH, 3coord-B) and tetra- (≡BOH, 4coord-B) coordinated oxygenated boron sites and the phosphate site (≡POH, 4coord-P), were considered; the calculations indicated that under ODH conditions, all of them can display reactivity, and it is marginally harder to dehydrogenate the >BOH site (62.0 kcal/mol) compared to the ≡BOH (4coord-B, 57.8 kcal/mol) and ≡POH (61.1 kcal/mol) sites, while the nascent >B–O• is more active than the ≡B–O• and ≡P–O• species in the subsequent dehydrogenation of propane (ΔG≠ = 17.5 (17.3), 19.2 (16.3), 28.7 (17.6) kcal/mol, respectively). Similar to that on boron oxide (B2O3), the reaction on the catalyst surface proceeds in a stepwise manner, but with a higher free energy barrier to the rate-determining step on BPO4, implicating a lower reactivity of BPO4 than that of B2O3. By imposing various surface geometric constraints, which introduce a mechanical force on the active site and modulate its ability to construct a hydrogen bond network with the neighboring polar groups, the free energy barrier to the dehydrogenation of the active site decreases, indicating that the mechanical forces in the local environment favor the triggering of the reaction. This work enriches our understanding of boron-based ODHP systems and benefits the rational design and optimization of boron-based catalysts.