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

Boron-based oxidative dehydrogenation of propane (ODHP)is emergingas a promising protocol because of its efficient conversion to propene,while the correlation between the structures of boron-containing materialsand their catalytic activity remains unclear. In this work, by meansof density functional theory calculations, the mechanism of ODHP onthe surface of boron phosphate (BPO4) was studied. Threetypes of surface sites, the tri- (>B-OH, 3coord-B) and tetra- (VBOH,4coord-B) coordinated oxygenated boron sites and the phosphate site(VPOH, 4coord-P), were considered; the calculations indicatedthat under ODH conditions, all of them can display reactivity, andit is marginally harder to dehydrogenate the >BOH site (62.0 kcal/mol)compared to the VBOH (4coord-B, 57.8 kcal/mol) and VPOH(61.1 kcal/mol) sites, while the nascent >B-O & BULL; ismoreactive than the VB-O & BULL; and VP-O & BULL;species in the subsequent dehydrogenation of propane (& UDelta;G (& NOTEQUAL;) = 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 ina stepwise manner, but with a higher free energy barrier to the rate-determiningstep on BPO4, implicating a lower reactivity of BPO4 than that of B2O3. By imposing varioussurface geometric constraints, which introduce a mechanical forceon the active site and modulate its ability to construct a hydrogenbond network with the neighboring polar groups, the free energy barrierto the dehydrogenation of the active site decreases, indicating thatthe mechanical forces in the local environment favor the triggeringof the reaction. This work enriches our understanding of boron-basedODHP systems and benefits the rational design and optimization ofboron-based catalysts.