Novel cross-linked metal organic framework/polyimide mixed matrix membranes for gas separations

In membranes for gas separations, the performance is evaluated based on gas permeability and selectivity of the membranes to the gas of interest. Permeability is a measure of membrane productivity, and selectivity is a measure of separation efficiency. High gas permeability and high selectivity are of significant importance for the commercial viability of membranes. However, there is a tradeoff between the permeability and selectivity in polymeric membranes, which requires new approaches to overcome this limitation. Combination of the high gas permeability of metal organic frameworks (MOFs), and the high selectivity obtained by different cross-linking methods holds potential for simultaneously increasing gas permeability and selectivity. Work described in this dissertation addresses this issue via cross-linking of MOF/mixed matrix membranes (MMMs) using three approaches. The first chapter describes the fabrication of mixed matrix membranes using zeolitic imidazolate framework-8 (ZIF-8), porous nano crystals in highly permeable polymer 6FDA-durene, and cross-linking the surface of the MMMs by reacting with ethylene diamine vapor. Addition of 33.3 wt% ZIF-8 yields a greatly improved permeability, while cross-linking increases gas selectivity for H2/CO2, H2/N2, H2/CH4 and O2/N2 separations. This method gives improved permeability and selectivity, however, full potential enhancement in selectivity is not obtained. In the second chapter, asymmetric membranes are fabricated by spin coating a 6FDA-durene polymer layer on highly permeable ZIF-8/6FDA-durene MMM substrates followed by subsequent cross-linking of the polymer layer by reacting with ethylene diamine vapor. This method utilizes the high permeability of the ZIF-8/6FDA-durene MMMs and the high selectivity of the cross-linked polymer layer. The fabricated asymmetric membranes show a desirable selectivity enhancement comparable to the cross-linked polymer and a high permeability similar to that of the pure polymer, transcending the upper bound of the Robeson plot (permeability selectivity trade-off curves) for H2/CO2, H2/N2, and H 2/CH4 separations. In chapter 3, total (bulk) cross-linking of a MMM is reported. ZIF-8 (33.3 wt%) is incorporated to thermally cross-linkable polymer propanediol monoester cross-linkable (PDMC) which results in improved permeability and selectivity. The MMM is cross-linked at 280 °C to obtain cross-linked PDMC (CPDM) MMM which shows improved permeability compared to the uncross-linked MMM.