(641c) Unraveling Complex Reaction Networks Combining DFT and Zeolite-Specific Kinetic Monte Carlo Simulations

Zeolite-catalyzed methanol-to-olefins (MTO) reactions involve complex reaction networks, including diverse sets of chemistry such as aromatic methylation, isomerization, cyclization, and hydride transfers. During MTO, methanol and dimethyl ether can repeatedly methylate alkenes and arenes which can subsequently isomerize and crack to form product olefins in the olefins- or aromatics-based cycles. The transient and convoluted nature of MTO reaction networks complicates kinetic studies, prompting density functional theory (DFT) to provide mechanistic insights. However, any assessment of competing cycles during MTO must account for significant differences in diffusivities of alkenes and arenes (and arenes of different sizes)—which are challenging to incorporate into DFT analyses. Furthermore, DFT studies are limited because reactions cannot be modeled at experimentally relevant time and length scales. We overcome these limitations by employing a novel zeolite-specific kinetic Monte Carlo simulation technique to model H-ZSM-5 crystals across experimentally relevant time and length scales to understand kinetics and mass-transport involved in key MTO reactions. Key reactions in the MTO network, including hydride transfers, arene methylation, and olefin formation, are modeled using DFT. Reaction and activation energies are combined with DFT-calculated diffusion barriers and incorporated into KMC, which can model crystals up to a micron in thickness, to provide fundamental insights of rates, selectivity, and mass-transport limitations during MTO. A preliminary test case examining benzene methylation to form toluene, a key pathway during MTO, is presented in Figure 1. It showed that large aromatics formed by “overmethylation” became trapped because of high diffusion barriers (> 200 kJ mol^−1), and eventually cause catalyst deactivation through site-blocking mechanisms (Figure 1). The combination of KMC simulations and DFT-derived reaction and diffusion barriers allows for rapid analysis of rates, mass-transport, and products of large and complex reaction pathways at multiple sites in zeolites—such as those of MTO.