(733b) Zeolitic Imidazolate Framework-8 Membranes for CO2 Separation

From the environmental and energy perspective, purification and recovery of carbon dioxide from flue gas and natural gas are of great interest. CO₂ must be separated from CH₄ because it reduces the energy content of the natural gas, and it is acidic and corrosive in the presence of water. Zeolite imidazolate frameworks (ZIFs), a subclass of metal organic frameworks (MOFs), have emerged as a novel type of crystalline porous material which combine highly desirable properties from both zeolites and MOFs, such as microporosity, high surface areas, and exceptional thermal and chemical stability, making them ideal candidates for gas separation applications. Due to its highly porous open framework structure, large accessible pore volume with fully exposed edges and faces of the organic links, pore apertures in the range of the kinetic diameter of several gas molecules, and high CO₂ adsorption capacity, zeolitic imidazolate framework-8 is highly attractive for gas separation applications.

Herein, we present the synthesis, characterization, and CO₂/CH₄ gas separation performance of tubular alumina supported ZIF-8 membranes, synthesized by secondary seeded growth (S.R. Venna and M.A. Carreon JACS 2010,132,76). ZIF-8 membranes displayed unprecedented high CO₂ permeances up to ~ 2.4 x 10^-5 mol/ m²?s?Pa and CO₂/CH₄ selectivities from ~ 4 to 7 at 295 K and a feed pressure of 139.5 KPa, the maximum pressure that the membranes could hold. The addition of multiple layers increased the CO₂/CH₄ selectivity and decreased the CO₂ permeance, most likely due to the reduction of nonzeolite pores and increase in membrane thickness respectively. The high observed CO₂ permeances were associated not only due to the presence of small crystals with narrow size distribution, which led to thin membranes, but also due to the textural properties of the alumina porous support which is composed of an outer layer of 0.2 µm average pore size and a porous area of 0.8 µm average pore size. The outer layer provided a smoother surface for uniform intergrowth of the ZIF-8 crystals with the support surface, while the porous area of 0.8 µm average pore size translated into higher fluxes. The CO₂/CH₄ selectivity of the membranes was low, most likely due to a high concentration of non-zeolite pores. Despite the low CO₂/CH₄ selectivity, the high CO2 permeances contributed to the relatively high separation indexes. Although ZIF-8 is composed of large 11.6 Å pores and small pore apertures of 3.4 Å, density functional theory simulation data suggests that the smaller pores are the preferential adsorption sites for CO₂ molecules. Therefore, the small pore aperture of ZIF-8, may favor the diffusion of CO₂ (kinetic diameter ~ 3.3 Å) over CH₄ (kinetic diameter ~ 3.8 Å).