Modeling Competitions Between Transport and Kinetics in Zeolite-Catalyzed Reactions by Combining DFT and KMC Simulations

David Hibbitts

University of Florida

Proton-form zeolites, such as MFI, are attractive catalysts in industry because their pores can catalyze many reactions relevant to the production of chemicals and fuel. Zeolites confine catalytic reactions and accelerate them through solvation; however, these rigid containers create large mass transport restrictions that can alter reaction rates, selectivities, and catalyst stabilities. Indeed, many zeolite-catalyzed reactions rapidly deactivate through the formation of large pore- and site-blocking species, such as polyaromatics, formed by undesired acid-catalyzed C–C coupling reactions.

Here, we combine density functional theory (DFT) calculations with kinetic Monte Carlo (KMC) simulations to model the formation, diffusion, and isomerization of aromatic species in the MFI framework zeolite in calculations relevant to toluene disproportionation, methylation, and methanol-to-olefins (MTO) processes. DFT is used to calculate free energies of activation and reaction for Brønsted acid catalyzed reactions within MFI intersections and activation energies for site-hopping diffusions of aromatics between neighboring intersections via the straight and sinusoidal channels. The large reaction network consisting of all intracrystalline elementary steps are incorporated into a zeolite-specific kinetic Monte Carlo (KMC) simulations that can model reactions over long time (days) and length (micron) scales.