Exploring Catalyst Deactivation Routes on H-MFI and CHA Zeolites during Methanol-to-Hydrocarbons Chemistry

This study explored cyclization pathways toward forming polyaromatic species in MFI and CHA framework zeolites, which are relevant during the conversion of methanol-to-hydrocarbons (MTH) during zeolite-catalyzed chemistries. Polyaromatic species have been linked to catalyst deactivation because their size prevents them from diffusing out of zeolite pores. We specifically explored cyclization pathways to form naphthalene and diphenylmethane products, which are model compounds for “sheeted” and “branched” polycyclic species. The formation of these products was modeled with density functional theory (DFT) calculations as implemented in the Computational Catalysis Interface. For the case of the MFI framework, we developed calculations at both the straight channel (~0.55 nm) and channel intersections (~0.7 nm), and the preliminary data suggest that cyclization barriers for both naphthalene and diphenylmethane occur with higher activations at tighter environments. This hypothesis was further examined at CHA framework pores (~0.42 nm). While barriers in CHA are indeed higher than those occurring at MFI intersections, they are not higher than those occurring in the straight channels of MFI. This suggests that pore size alone is not a suitable descriptor for polyaromatic formation barriers. This, in turn, indicates that intrinsic differences also influence barriers in MFI and CHA frameworks, which stabilize the relevant cyclization transition states to different extents that cannot be decoupled into differences in pore size only. Future work will resolve these discrepancies by expanding the zeolite models subset to TON, BEA, and MEL.