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

Abstract

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.

Kinetic Insights into Boron-Based Materials Catalyzed Oxidative Dehydrogenation of Light Alkanes

“…(Figure a and b) A mechanism involving both surface and gas-phase reactions was proposed by Venegas et al, in which radicals were generated on boron sites and water promoted the ODH by enabling an additional pathway for radical generation . (Figure c) This proposed mechanism was supported by the detection of a number of gas-phase radical species such as methyl radical (·CH 3 ) and allyl radical (·C 3 H 5 ) , and computational studies. − The correlation between oxygen partial pressure and the selectivity of C–C bond cleavage was also well described by the proposed mechanism . Meanwhile, understanding of the gas-phase radical mechanism remains incomplete for the following reasons: (1) Oxygen-containing radicals, which are likely involved in the initial activation of alkanes as H-abstractors, have yet been captured to date; (2) the match between experimental and computational results is qualitative, and many key computational predictions cannot be experimentally verified; and (3) the derivation of the second-order kinetics from the reaction mechanism on bulk boron-based catalysts is still vague.…”

Catalyst development for O2-assisted oxidative dehydrogenation of propane to propylene

“…In addition to BN, some other B-containing catalysts, such as exfoliated boron nanosheets, 18 boron carbonitride, 131 boron phosphate (BPO 4 ), 132 boron carbide, 133 boron phosphide and silicon boride, 134 and boron-containing metal–organic frameworks, 7 have also been developed for catalytic O 2 -ODHP. Here we take the application of exfoliated boron nanosheets in O 2 -ODHP as an example for illustration.…”