The applications of nanoporous crystalline materials are closely related to the mass transfer of guest molecules. However, the fundamental knowledge of mass transfer, and in particular the surface barriers controlled by the permeation of guest molecules through the external surfaces of materials, is still incomplete. The diversity of surface permeability at the single‐crystal level, caused by the varying origins of surface transport resistance, hinders the rational materials design and needs better understanding. Herein, we probe the molecular transport in single zeolite crystals with fluorescent 4‐(4‐diethylaminostyryl‐1‐methylpyridinium iodide) (DAMPI) using super‐resolution structured illumination microscopy (SIM). It showed that both the inter‐ and intra‐crystal diversity of surface barriers could be monitored by detecting the diffusion behaviors on the center and surface planes in single crystals. This adds a new perspective for studying the origins of the surface barriers as well as the molecular transport mechanisms in nanoporous materials.
The zero‐length column (ZLC) method has been frequently used to measure the effective diffusivity of guest molecules in nanoporous crystalline materials. Recent studies unveiled that the mass transport of guest molecules could originate from either intracrystalline diffusion and/or surface barriers. Directly quantifying the intracrystalline diffusivity and surface permeability of guest molecules. therefore, is essential for understanding the mass transfer and thus optimizing the design of nanoporous crystalline materials. In this work, we extended the ZLC method, based on a derived theoretical expression of the desorption rate, to decouple the surface barriers and intracrystalline diffusion from effective diffusion of guest molecules in nanoporous materials. The diffusivities of ethane, propane and n‐pentane in SAPO‐34 and Beta zeolites have been experimentally measured to verify the effectiveness of this method.
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