Development of Novel Porous Materials and Thin Membranes For Gas Separations

Abstract

The development of high-flux, high-selectivity, and low-cost membranes has the potential to improve the energy efficiency in the chemical industry by reducing the reliance on energy-intensive separation processes, such as distillation. To achieve this goal, novel porous materials and membrane fabrication methods are being increasingly sought after. Metal-organic frameworks (MOFs) are a new type of microporous materials with tunable pore structures suitable for gas separations. However, the high manufacturing cost and industrially-unattractive throughput hinder the industrial applications of MOF membranes. Fabrication of thin membranes with high throughput has the potential to overcome this barrier. This dissertation focuses on developing synthesis methods for thin MOF membranes by using two-dimensional (2D) MOF nanosheets and an all-vapor-phase zeolitic imidazolate frameworks (ZIFs) membrane synthesis process named ligand-induced permselectivation (LIPS). Crystal growth strategies for 2D MOFs were developed that yield Zn(Bim)OAc MOF nanosheets with desirable aspect ratio and uniformity for membrane formation. Using the Zn(Bim)OAc nanosheets, uniform coatings were successfully prepared on porous supports by vacuum filtration. A novel vapor growth method combining the support surface modification and ligand vapor treatment was developed to transform the nanosheet deposits into thin propylene-selective membranes. In addition, in an effort to reduce the membrane cost by using low-cost polymers, porous Cu(BDC) MOF nanosheets were incorporated into polymer matrices to form mixed matrix membranes that exhibited significantly improved performance for CO2/N2 separation. Besides solution processing of MOF membranes, a novel, well-controlled and cost-effective all-vapor-synthesis LIPS method with a combination of atomic layer deposition (ALD) and ligand vapor treatment was investigated. It was demonstrated that an ALD processing condition allowing a thin non-permeable ZnO deposit formation, as well as efficient ZnO-to-ZIFs conversion during ligand vapor treatment are very critical to realize consistent high membrane performance. With optimized ALD parameters, support and ligand properties, the membranes exhibit superior separation performance, with propylene permeance above 1.3 ×10-7 mol m-2 s-1 Pa-1 and propylene/propane selectivity above 60, which is highly promising for industrial applications.