Cavity-controlled diffusion in 8-membered ring molecular sieve catalysts for shape selective strategy

Shape selectivity is the most important feature and advantage of molecular sieve catalysis. Diffusion of reactant and product molecules inside the confined environments with channels and cavities is the essential aspect of shape selective catalysis of molecular sieve catalysts. Elucidating the diffusion mechanism of molecules in the confined space of catalysts is of great importance for the efficient utilization of the porous materials in the catalytic reaction as well as the strategy proposal of reaction control in shape-selective catalysis. However, it is still a great challenge to understand and quantify the diffusion behavior at the molecular level for the establishment of the diffusion mechanism within the crystal of microporous materials as so far. In this work, with advanced pulsed field gradient (PFG) NMR technique, self-diffusion coefficients of alkanes (methane, ethane and propane) in three cavity-type molecular sieves (LEV, CHA and RHO) with very close eight-membered ring (8-MR) windows were determined. Furthermore, molecular dynamics (MD) simulations predicted the energy barrier for crossing 8-MR to be overcome. Based on MD simulations and a continuous-time random-walk (CTRW) method, not only the diffusion trajectory of the molecule can be visually displayed, but also the diffusion behavior is quantitatively described as the inter-cavity hopping process with the success extraction of the jump frequency (f) and jump length (L). The substantial role played by cavity structure and dimension in diffusion within cavity-type molecular sieves with the close 8-MR windows were revealed in depth, which would help to reveal the mechanism of cavity-controlled diffusion and propose the shape-selective strategy in the methanol-to-olefins process. (C) 2019 Elsevier Inc. All rights reserved.