Influence of defects on the external crystal surface on molecular uptake into MFI-type zeolites

The interference microscopy technique, which was recently introduced in our laboratory, is applied to study transient intracrystalline concentration profiles in ZSM-5 crystals during adsorption and desorption of isobutane. Two different zeolite samples were used, viz., samples of etched and of nonetched ZSM-5 crystals. Etching has been carried out to remove the outer layer of the crystal surface, which may contain large amounts of defects and impurities. Studying the transient concentration profiles in both samples provides unique information on the influence of surface defects on molecular uptake. It is shown that, depending on their type, the defects of the crystal surface can either increase or decrease the rate of adsorption/desorption. The former effect is associated with adsorption/desorption through cracks in the crystal surface. The latter has its origin in the blockage or structural changes of the external crystal surface, leading to the appearance of surface transport barriers. Owing to the ability of interference microscopy to gain direct insight into the influence of surface defects on molecular uptake, this technique gives more accurate information on the transport diffusivities in zeolite crystals than the classical uptake methods.