Faster transport in hollow zeolites

Nano-zeolites and hierarchical zeolites have shown enhanced transport properties that are generally attributed to a shorter diffusion path length (L). However, the concomitant increase in the external surface of these types of zeolites may also affect overall transport as the interfaces may act as diffusion barriers. Recently, hollow zeolite crystals have been presented as an alternative zeolite morphology. They possess a large inner cavity and an overall size and shape similar to those of plain microporous zeolite. In contrast to classic hierarchical zeolite materials, the inner cavity of the hollow zeolite induces a shortened diffusion path length with no effect on external surface area. In this work, we have studied the impact of diffusion path length on transport properties by comparing the characteristic time of transport of hydrocarbons in plain zeolite crystals and their hollow counterparts, using zero-length column (ZLC) measurements. Our results show that hollow morphology doubles or quadruples the transport speed for Silicalite-1 and Beta zeolites, respectively. Compared to other reports focusing on nanosized or hierarchical zeolites, this study is free of any bias due to major modifications in external surface area, because hollow and plain zeolites have very similar crystal dimensions.