High-flux CHA zeolite membranes for H₂ separations

Two types of CHA zeolite membranes (SAPO-34 and SSZ-13) were used for the separations of CO₂/H₂ and H₂/CH₄ binary mixtures at pressures up to 2.8 MPa. Separation performances of both membranes were improved after synthesis modifications. The SAPO-34 membranes had higher permeances than the SSZ-13 membranes because less SAPO-34 crystals were grown into the support pores. The best SAPO-34 membrane showed CO₂/H₂ separation selectivity of 17.6 and CO₂ permeance of 40.3 × 10⁻⁷ mol/(m² s Pa) (CO₂ permeability of 72220 barrers); this membrane also had H₂/CH₄ separation selectivity of 42.2 and H₂ permeance of 14.5 × 10⁻⁷ mol/(m² s Pa) (H₂ permeability of 25980 barrers) for equimolar ratio feed at pressure drop of 0.2 MPa and 298 K. These data were far beyond the upper bounds of the current polymeric and zeolite membranes in CO₂/H₂ and H₂/CH₄ plots. The effects of pressure, temperature, feed flow rate, feed concentration and water vapor on membrane performances through SAPO-34 and SSZ-13 membranes were comparably studied in CO₂/H₂ or H₂/CH₄ mixtures. Both membranes displayed similar trends in permeances and selectivities with the changes of the parameters, except for temperature and pressure dependences of selectivities and CH₄ permeance in H₂/CH₄ mixtures, respectively. The different behaviors were attributed to the differences in the compositions and defect distributions of the two membranes. For both membranes, CO₂/H₂ selectivities increased but H₂/CH₄ selectivities decreased with the increase of pressure. Preferential adsorption dominated the CO₂/H₂ separations and molecular sieving did the H₂/CH₄ separations through both membranes. Single-gas permeances dependence of pressure drop was predicted by the Maxwell–Stefan diffusion model.