Kinetic Modelling of Ethanol Conversion to 1,3-Butadiene on MgO and Implications for Catalyst Design

Ethanol conversion to 1,3-butadiene, an important organic intermediate, is a mature process and a sustainable alternative to conventional synthesis. However, it is still poorly understood due to complex chemistry and the influence of different types of catalyst sites on selectivity to various hydrocarbons. Improved understanding of the kinetics is essential for designing better catalysts. Here, we study ethanol conversion on an MgO (100) step-edge, drawing on, and extending, an existing, first-principles mechanism to inform the kinetic modelling. We study multiple pathways, including ethanol dehydrogenation to acetaldehyde and dehydration to ethylene, and competing condensation mechanisms. While the MgO surface is often doped to improve performance, studying the pure surface allows identification of process steps that are rate or selectivity limiting, as well as stable intermediate stages that hinder the turnover.

We apply two levels of resolution to the kinetics, first using energy span theory to estimate the maximum turnover along each of the main proposed pathways and identify important states; and then turning to microkinetic modelling based on the same energy landscapes to better understand coverage effects and gas-surface dynamics. We quantify uncertainty in the kinetic predictions using a correlated error model to account for uncertainty in the underlying first-principles calculations. Turnover determining states are found to have high surface coverage, in agreement with experimental observations. We suggest that different reaction steps contribute more significantly at lower/higher temperatures, for example the microkinetic analysis indicates increasing dominance of the dehydration pathway as temperature increases, which could help to explain differing observations of rate determining steps in previous studies. Steps with the greatest impact on performance should be considered in the context of dopant choice to aid production rates. Together, the kinetic studies help to explore the implications of the energy landscape on the catalytic performance, and the selectivity towards the main (by-)products.