Molecular Self-Gating Inside a Zeolite Catalyst.

Diffusion is a ubiquitous process that is strongly correlated with concentration. Based on developed three-dimensional free energy and a continuous-time random-walk coarse-graining method, we found the optimal diffusion pathway under confinement, determined all diffusional energy barriers, and identified the major units of zeolite where molecular diffusion is limited. Interestingly, a novel diffusion mechanism was determined in the nanopore of a zeolite catalyst by molecular dynamics simulation, pulsed field gradient, and 2D exchange spectroscopy (EXSY) NMR experiments. We describe a "molecular self-gating effect" that effectively predominates the diffusion process in cage-type (e.g., RHO and MER) zeolites through a "traffic jam" and a "smooth traffic" process. Initially, transport is hindered by molecules forming a gate (traffic jam); then, as the number of molecules reaches a certain threshold, diffusion increases rapidly due to the synergistic collisions of aggregated molecules upon the gate (smooth traffic). This unique diffusion behavior is observed here for the first time and illustrates a microscopic mechanism dictated by the molecular self-gating effect in a confined space. The exploitable diffusion disclosed herein should shed new light on the fundamental understanding of transport, as well as enrich diffusion behavior under confinement.