Surface permeability on zeolite NaCaA enhanced by layer deposition

By PFG NMR diffusion measurements with short-chain length alkanes, the deposition of nano-sized LTA zeolite seed crystals on LTA single crystals and their secondary growth to surface layers is shown to reduce rather than to enhance transport resistances at the crystal surface. This, at first glance contra-intuitive, finding may be rationalised if the transport resistance on the external surface of the nanoporous crystals of the as-synthesized material results from an essentially impenetrable layer with highly dispersed structural defects rather than from a quasi-continuous layer with extremely low guest diffusivity and/or solubility: if accompanied with an – even very small – enhancement of the defect density, irrespective of the increasing layer thickness a subsequent surface treatment will enhance the surface permeability.

Graphical abstractLTA-type zeolites (left) have been covered by a thin LTA shell (right). PFG NMR diffusion studies show that this surface coverage enhances rather than reduces intercrystalline molecular exchange.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Large LTA crystals were surface-modified by deposition of a 1 μm thick LTA layer. ► PFG NMR tracer-desorption technique was applied to study the surface permeability. ► Surprisingly, the additional surface layer reduced the transport resistances. ► An increasing defect density at previously blocked surface areas might serve as an explanation.