First-principles-informed energetic span and microkinetic analysis of ethanol catalytic conversion to 1,3-butadiene on MgO

Kinetic modeling of single-step catalytic conversion of ethanol to 1,3-butadiene is necessary to inform accurate process design. This paper considers the synthesis of 1,3-butadiene on an MgO (100) step-edge using first-principles-informed energetic span and microkinetic analysis to explore the reaction free energy landscapes and kinetic limitations of competing reaction pathways. Previous studies have observed mechanisms proceeding via both dehydrogenation and dehydration of ethanol, and highlighted sensitivity to conditions and catalyst composition. Here, we use the energetic span concept to characterize the theoretical maximum turnover and degree of rate control for states in each reaction pathway, finding dehydrogenation to be more active than dehydration for producing 1,3-butadiene, and suggesting states in the dehydrogenation, dehydration, and condensation steps to be rate-determining. The influence of temperature on the relative rate contribution of each state is quantified and explained through the varying temperature sensitivity of the free energy landscape. A microkinetic model is developed to explore the impact of competition between pathways, interaction with gas-phase species, and high surface coverage of stable reaction intermediates. This suggests that the turnover obtained may be significantly lower than predicted from the free energy landscape alone. The theoretical rate-determining states were found to contribute to high surface coverage of adsorbed ethanol and longer, oxygenated hydrocarbons. The combined energetic span and microkinetic analysis permits investigation of a complex system from two perspectives, each with inherent advantages, and helps elucidate conflicting observations of rate determining steps and product distribution by considering the interplay between the different pathways and the equilibrium with gas-phase products.