Hierarchical zeolites with micropore lengths on the order of nanometers have been synthesized with the aim of reducing mass transfer limitation. However, due to large external surface to volume ratios, the mass transport in these materials can be hindered by a secondary rate limitation step imposed on the external surface of the zeolites. This has led to the general phenomenon referred to as “surface barriers”, which cause the enhancement in mass transport being far lower than expected. In order to fully unlock the potential of hierarchical zeolites, it is imperative to fundamentally understand the molecular transport in these new types of materials.
This dissertation presents studies of molecular transport in hierarchical zeolites and the investigations on the mechanisms of surface barrier. By studying the adsorptions and diffusions of four different probing molecules on silicalite-1 zeolites with different crystal sizes, it was observed that the onset of surface barrier in small zeolites is related to the strong sorbate-sorbent interaction at the external surface. Micropore re-entry caused by surface diffusion extend the characteristic micropore diffusion length, leading to the slower than expected overall mass transport in these materials. In order to minimize the occurrence of micropore re-entry, a new composite material consisting 80 nm silicalite-1 and 35 nm non-porous silica nanoparticle was developed. The absence of micropore opening on the external surface of silica nanoparticle significantly reduce the chance for diffusion molecule to re-enter the micropore, leading to a much faster mass transport as compared to pure 80 nm silicalite-1.