(6c) Two-Dimensional Molecular Sieves for Selective Transport

Two-dimensional molecular sieves are highly desired building blocks for the fabrication of molecular selective membranes. Miniaturization of the third dimension opens up opportunities for the synthesis of membranes with a thickness of just a few atoms. Due to remarkably short transport path of the molecules, these membranes will have profound impact in the field of separation technology. Apart from membranes, these two-dimensional porous sheets will find applications in catalysis, sensors, low-dielectric constant materials, barriers, etc.

 Zeolite membranes are attractive due to their high performance (permeance and separation factor) and robust chemical and thermal stability. Attempts to synthesize segregated, two-dimensional zeolite sheets have often led to an intergrown or a layered zeolite. For the first time, a purified, dispersed suspension of highly crystalline 1.5 unit-cell-thick zeolite nanosheets was prepared by the exfoliation of layered zeolites followed by solution processing. Compared to time-consuming and expensive coating techniques involving modification of supports by masking and/or functionalization, a simple scalable filter coating technique was developed to fabricate a sub-100 nm-thick, oriented and compact coating of nanosheets. Wetting properties of the nanosheets were tuned, allowing it to deposit on the air-water interface. This allowed a monolayer coating of nanosheets on non-porous supports, opening opportunities for applications in sensors, low-dielectric constant materials etc. Thin, zeolite films prepared by hydrothermal treatment of the nanosheet film showed exceptional performance in the separation of xylene isomers, sparking industrial interest in deploying these membranes for the large scale purification of p-xylene. A mathematical model based on adsorption and diffusion kinetics was used to gain insights into the parameters affecting membrane performance at industrial process conditions.

In my future research, I aspire to expand the list of two-dimensional materials for molecular separations. Especially, one atom thick graphene sheets, MoS₂ sheets, aluminophosphate sheets, etc. hold exciting potential towards the fabrication of the thinnest membrane. Strategies for the generation of pores with tunable size in the non-porous atomic sheets will be explored. The transport of molecules with varying size and physical properties will be modeled to explore the full potential of two-dimensional membranes.