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.