From microkinetic model to process: understanding the role of the boron nitride surface and gas phase chemistry in the oxidative dehydrogenation of propane

Abstract

After naphtha steam cracking, endothermic propylene production via direct dehydrogenation (PDH) is one of the most energy-intensive processes in the chemical industry. The exothermic alternative, oxidative dehydrogenation of propane (ODHP), has been investigated for decades over metal-oxide catalysts but still lacks the propylene selectivity necessary for industrial viability. Recently proposed boron-based catalysts for ODHP show improved selectivity to propylene via a surface-initiated gas-phase free radical mechanism that is remarkably selective. Aiming at process improvements that can further boost propylene selectivity, we investigated the mechanism(s) by which propylene selectivity is lost. We find that surface-mediated propylene marginally affects the initial selectivity to propylene. We hypothesize this is likely due to the initial n-propyl vs. i-propyl radical formation rate over the surface as compared to the gas-phase chemistry. This suggests that shifting the reaction flux more towards the gas phase could improve the selectivity. However, we also observed that propylene predominantly over-oxidizes in the gas-phase but not over the surface. Turning to the gas-phase chemistry, we are unable to boost the selectivity above that of the underlying background reactivity in a tube, despite the use of radical accelerants such as NO and O₃. Our work suggests that future process improvements should focus on tuning the radical distribution in the gas-phase chemistry.