Preparation and characterization of a glassy fluorinated polyimide zeolite-mixed matrix membrane

Mixed matrix membranes of 6FDA-6FpDA-DABA, a glassy polyimide, and modified zeolites (ZSM-2) were successfully fabricated using the procedure outlined in this paper. The membranes were cast from solution and then exposed to different gases for the purpose of determining and comparing the diffusivity coefficients, the solubility coefficients, and the permeation rates of He, O2, N2, CH4 and CO2 of the pure polyimide and the composite membrane. FTIR spectra were collected from the pure polyimide, the polyimide and untethered zeolite solutions, and the mixed matrix membrane (MMM) solution. Comparison of the spectra revealed the presence of hydrogen bonding in the MMM solution not present in the other samples. FESEM images and TEM images did not reveal the presence of voids between the polymer and the zeolite. These images also revealed that when given ample time for the solvent to evaporate, the zeolites sediment to one side of the membrane. This develops a polymer-rich phase and a zeolite-rich phase, and many of the ZSM-2 zeolites appear to adopt an orientation with their largest face orthogonal to the direction of the gas flow.

In this study MMM were fabricated from a 6FDA-6FpDA-DABA polyimide and modified ZSM-2 zeolite. The ZSM-2 zeolites were functionalized with amine groups by reacting them with aminopropyltrimethoxysilane in toluene. MMMs were successfully fabricated at 20% weight zeolite and 50% weight zeolite; however, the latter was too brittle to be used to gather data. The amine-tethered zeolites interacted through secondary forces with the carboxylic groups along the polymer backbone as documented by FTIR studies. Band shifts associated with hydrogen bonding of the carbonyl and amine groups were observed in the spectra. These interactions promoted adhesion between the two components. The morphology of the MMM was documented by SEM and TEM studies and verified the absence of voids around the zeolites. This suggested that the zeolite and polymer had good contact at the interface. Permeation data of He, CO2, O2, N2, and CH4 were collected and analyzed. The solubility coefficient for each gas increased, except for N2, which was largely unchanged. The changes in permeability for each gas correlated well with the change in the diffusion coefficient. The permeabilities of He, CO2 and CH4 all decreased, while O2 and N2 increased.