Synthesis of Zeolitic Imidazolate Framework ZIF-8 Thin Films and Membranes on Polymer Substrates for Propylene/Propane Separation

Abstract

Propylene is one of the most important chemical feedstock for the production of a variety of polymers and intermediates. The conventional thermally-driven distillation method to separate propylene, typically from propane, unfortunately are capital and energy intensive. Membrane-based system has emerged as an energy efficient alternative to the traditional distillation process. Although polymer membranes have dominated gas separation industries for over three decades, there are no commercially available polymer membranes for propylene/propane separation due to fundamental limitation of polymer such as poorly defined free volume, plasticization, etc. Zeolitic imidazolate framework ZIF-8 with sodalite topology has been heavily investigated as propylene-selective membranes due to its well-fitted effective pore apertures (~4.0 Å), which are in between the van der Waals diameter of propylene and propane. Despite ZIF-8 potential for propylene/propane separation, their large-scale applications unfortunately are impeded by the lack of simple and cost-effective processing to prepare ultrathin membranes on scalable substrates. Other than inexpensive membrane processing, it is also important to increase productivity of ZIF-8 membranes by reducing membrane thickness (i.e., < 1 µm) and synthesizing the membrane on high surface-to-volume ratio (i.e., HFs) substrates. In this dissertation, we developed two state of the art membrane processing techniques to prepare ultrathin ZIF-8 membranes on polymer HFs. PMMOF seeding and KOH-assisted solvothermal seeding process, which are based on polymer modification strategy, enables high quality ZIF-8 seed layers to be deposited selectively on the bore side of polymer HFs. Relevant properties of ZIF-8 seed layers such as seed crystal density and seed layer thickness can be tailored through simple manipulation of alkali treatment time. Subsequent microfluidic secondary growth led to a formation of continuous and defect-free ZIF-8 membranes. The resulting ZIF-8 membranes on polymer HFs, after posttreatment, displayed propylene/propane separation factor as high as 55, satisfying the commercial requirement for propylene-selective membranes.