Separating molecules by size in SAPO-34 membranes

•SAPO-34 membranes (0.38- nm pores) can separate gas mixtures by molecular sieving.•Mixture selectivities for CO2 or H2 over i-butane (0.5 nm) are larger than 10,000.•Concentration polarization affects fluxes for separations with high selectivities.•Spatial flux mapping with i-butane shows no defects.

SAPO-34 membranes that have CO2/CH4 separation selectivities of ~100 at 295 K and pressures up to 4.6 MPa also show high molecular sieving separations of mixtures containing i-butane (0.50-nm kinetic diameter) or CF4 (0.47 nm). The separation selectivities were 15,000–20,000 for CO2/i-butane from 295 to 420 K, and 12,000–20,000 for CO2/CF4 for feed pressures up to 0.5 MPa at 295 K. These may be the highest gas-phase separation selectivities reported for zeolite membranes, and they are likely lower limits because of concentration polarization, leakage around O-rings, and i-butane diffusion through the silicone O-rings. The low defect concentration was also confirmed by spatial defect distributions measured with i-butane. Thus, less than 1% of the CH4 permeation during CO2/CH4 separations is through defects larger than 0.47 nm. Isobutane, even though it is too large to adsorb in SAPO-34 pores (0.38-nm diameter), appeared to inhibit CO2 permeation by adsorbing on the external surface. These SAPO-34 membranes also separated H2/i-butane with lower limits on selectivities of 6000 at 295 K and 10,000–14,000 at 423 K, and they were also selective for He/CH4 separation (selectivity=20) and H2/CH4 separation (selectivity=35–45). In these separations, both He and H2 appear to speed up CH4 permeation, as expected from molecular simulations reported previously. Also, CH4 appears to slow diffusion of CO2, He, and H2.