Preparation and Characterization of a Hydrophobic Metal-Organic Framework Membrane

A hydrophobic metal-organic framework (MOF) UiO-66-CH₃ is prepared and its solvothermal stability is investigated in comparison to UiO-66. It is confirmed that the MOF stability is enhanced by introduction of the two methyl groups, while the water adsorption is reduced. Given its hydrophobicity and stability, UiO-66-CH₃ is proposed as an attractive membrane material for gas separation under moisture conditions. The UiO-66-CH₃ membrane is prepared on a 50µm-thick porous Ni support sheet for the first time by use of a secondary growth method. It is found that uniform seed coating on the support is necessary to form a continuous membrane. In addition to growth time and temperature, presence of a modulator in the growth solution is found to be useful controlling parameters for hydrothermal membrane growth on the seeded support.  A dense, inter-grown membrane layer is formed under 140 ^oC and 160 ^oC for 24 h with modulator addition. The membrane surface comprises 500 nm octahedral crystals, which are supposed to grow out of the original 100 nm spherical seeding crystals. The separation characteristics of resulting membranes are characterized with pure CO₂, air, CO₂/air mixture, and humid CO₂/air mixture. CO₂ permeance as high as 1.9E-06 mol/m²/s/Pa at 31^oC is obtained. Unlike the hydrophilic zeolite membranes, CO₂ permeation through this membrane is not blocked by the presence of water vapor in the feed gas. The results suggest that this MOF framework is a promising membrane material worth to be further investigated for separation of CO₂ and other small molecules from humid gas mixtures.