Propane Oxidative Dehydrogenation on Nanosized Boron Carbide: Effect of Boron Content and Its Oxidation Implicated by DFT Calculations

Abstract

Metal-free boron-containing materials are growing to be promising choices in oxidative dehydrogenation (ODH) of light alkanes to alkenes, while it remains unclear how the boron content and the second element that constitutes the scaffold to accommodate B in the materials may influence their catalytic activity. Herein, by means of the density functional theory study of nanosized boron carbide, we studied the mechanisms of ODH of propane (ODHP) catalyzed by three types of boron carbides, B13C2, B4C, and h-BC, which differ from each other in their B contents and their chemical topologies. The influence of hydroxyl coverage on the surface was also evaluated to address the structure–catalytic activity relationship in the ODHP reaction. The calculations show that (1) the B element content and the hydroxyl groups could regulate the electronic structure and significantly increase the highest occupied molecular orbital (HOMO) energy to promote chemical reactivity. (2) Boron carbide with higher coverage of hydroxyl groups on the surface (B 13 C 2 -H and B 4 C-H) exhibits higher activity for catalyzing ODHP reaction owing to the enhanced nucleophilicity, which is further confirmed by the noncovalent interaction (NCI) analysis. (3) The reactivity of the catalyst is positively correlated with the B content, and the ODHP catalyzed by B13C2 is kinetically more favorable than that by B4C and h-BC. This study is expected to enrich our understanding of the structure–activity relationship of boron-based catalysts in the ODH system and benefit the optimization of the catalytic systems.

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.…”