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Reports: ND950928-ND9: Synthesis and High Pressure Gas Separation Properties of Thin Metal-Organic Framework Membranes

Jerry Y.S. Lin, PhD, Arizona State University

Research conducted in the report period (1/1/2011-8/31/2012) has been focused on synthesis of thin metal-organic framework membranes (MOF-5) and study of gas separation properties of the MOF membranes. Continuous and crake-free MOF-5 membranes of about 15 μm in thickness were prepared by seeding size-reduced MOF-5 crystals followed by the secondary growth synthesis. The membrane quality was examined by a molecule probing method using a serious of organic compounds with sizes ranging from 0.56 nm to 1.4 nm. Negligible pervaporation flux (<1.4×10-6 mol/m2∙s-1) of the large molecule, 2-Dicyclohexylphosphino -2'- (N, N-dimethylamino) biphenyl, was found through the MOF-5 membrane prepared in this work. This shows absence of the defects larger than the MOF-5 crystalline pores in the MOF-5 membranes. Pure gas permeance data were obtained for the high quality MOF-5 membranes in 298-374 K. The permeance decreases with increasing molecular weight of the molecules in the order of H2 > He > CO ~ N2 > CO2 > SF6, showing a diffusion-dominating permeation mechanism for the MOF-5 membranes, with a smaller effect of adsorption. CO2 permeation data for the MOF-5 membranes were analyzed by a permeation model with the input of the adsorption isotherm data to obtain more reliable CO2 diffusivity data in MOF-5, which is in the order of 1×10-9 m2/s with activation energy for diffusion of about 14.8 kJ/mol.

Separation properties of CO2/H2 and CO2/N2 mixtures for the thin MOF-5 membranes were studied at different temperatures, feed pressures and feed composition. The MOF-5 membranes offer selective permeation for CO2 over H2 or N2 with CO2/H2 or CO2/N2 mixture feed under the experimental conditions studied. Compared to pure gas permeance data, the presence of the strongly adsorbing CO2 in the binary mixture separation suppresses less adsorbing H2 or N2, similar to what was observed for zeolite membranes. The MOF-5 membranes exhibit a separation factor for CO2/H2 of close to 5 with a feed CO2 composition of 82% and that for CO2/N2 of greater than 60 with a feed CO2 composition of 88% at 445 kPa and 298K. With the mixture feed, CO2 permeance increases and N2 (or H2) permeance decreases, and hence the CO2/N2 (or H2) separation factor increases, with increasing CO2 partial pressure (through the change of composition or feed pressure).