Zeolite Membranes: Separating Molecules by Size

Zeolite membranes have molecular-sized pores and are stable at high temperatures and pressures. Because all the pores are the same size in zeolites, membranes that are composed of intergrown zeolite crystals have the potential for high separation selectivities; they can separate molecules based on size. Two examples of zeolite membranes will be presented. These membranes have been prepared as thin layers on porous tubes. The SAPO-34 membranes, which are composed of Si, Al, P, and O, have pores with 0.4-nm diameters, and these membranes can separate CO2/CH4 mixtures with high selectivities at 70 bar pressure. This separation is of interest for purifying natural gas, since many wells are contaminated with high CO2 concentrations. Carbon dioxide preferentially permeates through these membranes because it is smaller than methane and because it preferentially adsorbs. The MFI membranes, which are composed of Si, B, and O, have pores with 0.6-nm diameters and can separate linear and branched hydrocarbons. The smaller linear hydrocarbons preferentially permeate. Although these membranes are inorganic, our recent studies show that the MFI crystals swell or shrink when certain molecules adsorb in the MFI pores. Although the percentage change in crystal size is small, adsorbate-induced crystal swelling can dramatically decrease the flux through membrane defects and increase the separation selectivity. A combination of methods was used to study how the MFI crystal and the defect sizes change with adsorption.

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