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Membrane gas separations (page we plan to edit): We plan to expand the scope of the Membrane Gas Separations wikipedia page to include an in-depth, up-to-date discussion of membranes for CO2 separation. The current page only discusses polymeric and nanoporous membranes and lacks discussion of zeolitic, silica, perovskite oxide membranes, and more. In addition, there is a lack of meaningful discussion which summarizes the current state and future directions of CO2 capture by membrane processes. We will add sections on inorganic and hybrid membranes. Our additions will include information on material development and recent findings from mechanistic investigations.

Lead Section Gas mixtures can be effectively separated by synthetic membranes made from polymers such as polyamide or cellulose acetate, or from ceramic materials.[1] While polymeric membranes are economical and technologically useful, they have are bounded by their performance (permeability must be sacrificed for selectivity and vice versa).[2] Membrane materials have expanded into the realm of silica, zeolites, and perovskites due to their strong thermal and chemical resistance as well as high tunability. Membranes can be used for separating gas mixtures where they act as a permeable barrier through which different compounds move across at different rates or not move at all. The membranes can be nanoporous, polymer, etc. and the gas molecules penetrate according to their size, diffusivity, or solubility.

[2] http://pubs.acs.org/doi/pdf/10.1021/cm200939d

Silica Membranes Silica membranes are mesoporous and can be made with high uniformity. Synthesized membranes are continuous (and display high permeability) and can be modified on the surface to drastically improve selectivity. Amine containing molecules on silica membranes can be used as agents to separate CO2 from flue gas streams.[2] While previously, silica membranes were impractical due to their technical scalability and cost, Zhou et. al have demonstrated a simple method of producing silica-based MFI membranes on an alumina support that can effectively separate CO2 and H2 using economical materials.[3] Ordered mesoporous silica membranes have shown considerable potential for surface modification that allows for ease of CO2 separation. Surface silanol groups can be modified with amino groups that allow for CO2 to be easily separated from other gaseous streams.[4] Silica nanoparticles have been deposited in polymeric membranes to determine the effect of silica-polymer interactions on gas separation / permeability / selectivity.[5]

[3] http://onlinelibrary.wiley.com/doi/10.1002/anie.201311324/full [4] http://www.sciencedirect.com/science/article/pii/S0001868609001092 [5] http://pubs.acs.org/doi/pdf/10.1021/cm504463c

Zeolite Membranes

Zeolites are crystalline aluminosilicates with a regular repeating structure of molecular-sized pores. Zeolite membranes selectively separate molecules based on pore size and polarity and are thus highly tunable to specific gas separation processes. In general, smaller molecules and those with stronger zeolite-adsorption permeate through zeolitic membranes with larger selectivity. The capacity to discriminate based on both molecular size and adsorption affinity makes zeolite membranes an attractive candidate for CO2 separation from N2, CH4, and H2.

Poshusta et al found that the gas-phase enthalpy (heat) of adsorption on zeolites increases as follows: H2 < CH4 < N2 < CO2. It is generally accepted that CO2 has the largest adsorption energy because it has the largest quadrupolar moment, thereby increasing its affinity for charged or polar zeolite pores. At low temperatures, zeolite adsorption-capacity is large and the high concentration of adsorbed CO2 molecules blocks the flow of other gases. Therefore, at lower temperatures, CO2 selectively permeates through zeolite pores. Several recent research efforts have focused on developing new zeolite membranes that maximize the CO2 selectivity by taking advantage of the low-temperature blocking phenomena.

Kusakabe et al have synthesized Y-type (Si:Al>3) zeolite membranes which achieve room-temperature separation factors of 100 and 21 for CO2/N2 and CO2/CH4 mixtures. At similar CO2/CH4 selectivity, Venna et al have demonstrated ZIF-8 zeolite membranes achieve unprecedented CO2 permeance flux, two orders of magnitude above the previous standard. DDR-type (Himeno et al) and SAPO-34 (Li et al) membranes have also shown promise in separating CO2 and CH4 at a variety of pressures and feed compositions.

Perovskite Membranes:

Perovskite are mixed metal oxide with a well-defined cubic structure and a general formula of ABO3, where A is an alkaline earth or lanthanide element and B is a transition metal. These materials are attractive for CO2 separation because of the high tunability of the metal sites as well as their stability at elevated temperatures.

Kusakabe and co-workers investigated the separation of CO2 with an alpha-alumina membrane impregnated with BaTiO3, where the permeability of CO2, N2 and He in Ar were investigated. It was found that adsorption of CO2 was favorable at high temperatures due to an endothermic interaction between CO2 and the material, promoting mobile CO2 that enhanced CO2 adsorption-desorption rate and surface diffusion. This is exhibited in the experimental separation factor of CO2 to N2 obtain at 100-500C, which was found to be 1.1-1.2, much higher than the separation factor of 0.8 predicted by Knudsen diffusion, the predominant permeation mechanism in the alpha-alumina membrane, where the separation factor is the inverse square root ratio of the molecular weight of the two species. Though the separation factor was low due to pinholes observed in the membrane, this demonstrates the potential of perovskite materials in their selective surface chemistry for CO2 separation.

Reference: Kusakabe, K., K. Ichiki, and S. Morooka. “Separation of CO2 with BaTiO3 Membrane Prepared by the Sol-Gel Method.” J. Membrane Sci., 95 (1994). pp. 171-177.