Deciphering the Diffusion-Improved Selectivity of Ethylene Mediated by the Mesoscale Spatial Pattern of Aromatics in Zeolite-Catalyzed Methanol-to-Olefin Processes.

Modeling the diffusion behavior of nonuniformly distributed systems at the mesoscopic scale presents significant challenges. In this study, we investigate how the nonuniform mesoscale spatial distribution of aromatic compounds, i.e., the hydrocarbon pool, affects olefin selectivity during the methanol-to-olefins (MTO) process. Ab initio molecular dynamics with enhanced sampling methods and kinetic Monte Carlo techniques were employed to analyze olefin diffusion in a "fully filled from the outside to the inside" distribution model. Our results reveal that while the coexistence of olefins with aromatic compounds hinders olefin diffusion, it simultaneously enhances ethylene selectivity. Further analysis of diffusion rate control and olefin residence time distributions within the zeolite model identifies key elementary diffusion processes and elucidates why aromatic compounds preferentially form at the rim of the SAPO-34 zeolite during the MTO process. This integrated approach enables the simulation of catalytic systems over larger spatial and temporal scales, providing a comprehensive understanding of the underlying mechanisms and facilitating the design of more efficient and ethylene-selective catalysts.