KMC simulations are used here to simulate intra-zeolite transport of medium-large molecules whose diffusions may limit catalytic reaction rates, govern observed product selectivities, and lead to catalyst deactivation. These KMC simulations are informed by DFT calculations of activation and reaction energies for Brønsted-catalyzed reactions and for diffusion processes. Here, two reactions of interest are discussed: methanol-to-olefins and toluene methylation to xylenes in the MFI framework. Zeolites convert methanol to alkenes during methanol-to-olefin (MTO) processes. During MTO, a hydrocarbon pool of species consisting of alkenes and methylbenzenes co-catalyze the formation of products. Aromatics can grow via methylation reactions and isomerize to eliminate alkenes. Different aromatic species can co-catalyze the formation of products; activation and reaction free energies for the methylation of C6-C12 arenes indicates that formations of C9+ methylbenzenes are not kinetically limited and are thermodynamically favored during MTO—suggesting large, diffusion limited species are responsible for forming C2 products and eventual deactivation. Straight and sinusoidal diffusion barriers are calculated for all methylbenzenes using DFT and incorporated into KMC simulations. At long times on stream, arenes become extensively methylated and ~70% of active sites are blocked by C10+ arenes, causing catalyst deactivation. Toluene methylation to xylenes occurs at lower temperatures than disproportionation or transalkylation reactions but are likely governed by similar selectivity-controlling elementary steps. The conventional wisdom of xylene production is that xylenes, once formed, can rapidly isomerize and that the greater diffusivity of p-xylene (over o- and m-xylene isomers) leads to an increase in selectivity with increasing diffusion path lengths (i.e., larger crystals) or lower diffusivities (pore-constricting zeolite modifications). However, recent work by our group and others have shown that xylene diffusion has no impact on selectivities . MFI and other zeolite frameworks can impose shape-selectivity, with smaller environments (TON) leading to higher p-xylene selectivity than larger (BEA) environments. MFI, possessing both larger (within pore intersections) and smaller (within the short pores) environments, can display different p-xylene selectivities depending on the choice of SDA, thus alluding to an impact of Al site locations.

Combining DFT and KMC to simulate reactions and transport in MFI zeolites over large time and length scales

Where thousands of chemistry professionals meet to share ideas and advance scientific and technical knowledge. Spring 2023 abstracts, posters, and presentations will launch at the end of March.

ACS Spring 2023

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.