Thin carbon/SAPO-34 microporous composite membranes for gas separation

Thin carbon/SAPO-34 microporous composite membranes, in which SAPO-34 particles were dispersed into a carbon matrix, were successfully fabricated on porous a-Al2O3 tubes. The SAPO-34 molecular sieve content in the membrane precursor solution and coating cycles had a significant influence on the separation performance of the composite membranes. The membrane (M1) with a SAPO-34 content of 2 wt.% that was dip-coated twice demonstrated the best performance for gas separation. This membrane exhibited improved ideal selectivities for H2/CH4, CO2/CH4 and H2/SF6 of 97, 87 and 250, respectively, with a reduced H2 permeance of 10 × 10−8 mol m2 Pa−1 s−1. However, the H2/CH4 and CO2/CH4 of the pure carbon membrane were 64 and 40, respectively, at 298 K with a pressure drop of 0.1 MPa. The CO2/CH4 and H2/CH4 selectivities of membrane M1 were as high as 258 and 112, respectively, for the equimolar mixture of CO2/CH4 and H2/CH4, which was larger than the corresponding ideal selectivities of 87 and 97 at 298 K, under a 0.1 MPa pressure drop. This effect was due to the blocked permeation of nonadsorbable gas by an adsorbed component or the competitive diffusion effect. It was found that the CO2/CH4 and H2/CH4 separation selectivities decreased with increasing temperature and pressure difference. These results suggest that the SAPO-34 offers multiple roles in the improved selectivity by reducing pore size of carbon/SAPO-34 membrane and by its preferential adsorption and diffusion characteristics.