Kinetic Requirements for Selectivity Enhancement During Forced Dynamic Operation of the Oxidative Dehydrogenation of Ethane

The viability of alkane oxidative dehydrogenation (ODH) processes specifically, and catalytic partial oxidation reactions more generally, are oftentimes limited by the formation of undesired deep oxidation products such as CO and CO2. The forced dynamic operation (FDO) of catalytic reactors has been proposed as a means for enhancing desired olefin or oxygenate selectivity and yield over those of CO and CO2, but an elucidation of the precise mechanistic bases for the dynamic enhancement observed continues to remain evasive. In this work, we provide an explanation of the extent of dynamic enhancement noted during ethane ODH over supported MoOx catalysts but not VOx ones─an explanation grounded in a quantitative analysis of the density and reactivity of chemisorbed and lattice oxygen species on these two classes of catalysts. Supported vanadia catalysts, unlike molybdena ones, carry oxygen species with similar reducibilities, resulting in highly contrasting trends in dynamic and steady state ODH properties for the two catalysts. Whereas in the case of VOx/Al2O3, oxygen speciation affects the nature of the hydrocarbon activated (ethane or ethylene), in the case of MoOx/Al2O3, it affects the type of product formed (ethylene or COx). Metal oxide loading is shown to be a key parameter impacting dynamic enhancement, with the FDO enhancement of higher loading molybdena samples converging toward that of the vanadia catalyst. The preferential depletion of chemisorbed oxygens is revealed to be a key determinant of the extent of dynamic enhancement, with an asymmetry in modeled O*/OL ratios under dynamic conditions relative to SS ones helping rationalize the effect that modulation frequency has on FDO enhancement. Collectively, the results presented here establish a quantitative, molecular-level basis for dynamic enhancement noted during the ODH of ethane, and point to considerations relating to the reactivity of chemisorbed and lattice oxygens as well as their dynamic and steady state ratios as levers for mitigating side-product formation through FDO.