Hydrophobic modification of SAPO-34 membranes for improvement of stability under wet condition

SAPO-34 zeolite membranes show high efficiency for CO2/CH4 separation but suffer from the reduction of separation performance when exposed to humid atmosphere. In this work, n-dodecyltrimethoxysilane (DTMS) was used to modify the hollow fibers supported SAPO-34 membranes to increase the external surface hydrophobicity and thus sustain their performance under moisture environment. The modified membranes were fully characterized. Their separation performance was extensively investigated in both dry and wet gaseous systems and compared with the un-modified ones. The un-modified SAPO-34 membrane exhibited a high separation selectivity of 160 and CO2 permeance of 1.18 × 10−6 mol·m−2·s−1·Pa−1 for separation of dry CO2/CH4 at 298 K. However, its separation selectivity declined to 0.9 and the CO2 permeance was only about 1.7 × 10−8 mol·m−2·s−1·Pa−1 for wet CO2/CH4 at same temperature. High temperature (e.g. 353 K) could reduce the effect of moisture to improve SAPO-34 separation selectivity, but further increasing temperature (e.g. 373 K) led to decrease in CO2/CH4 separation selectivity. A significant decrease of selectivity was observed at higher pressure drop. The modified SAPO-34 membrane showed decreased CO2 permeance but increased separation selectivity for dry CO2/CH4 gas mixture, and super performance for wet CO2/CH4 gas mixture due to the improved hydrophobicity of membrane surface. A separation selectivity of 65 and CO2 permeance of 4.73 × 10−8 mol·m−2·s−1·Pa−1 for wet CO2/CH4 mixture can be observed at 353 K with a pressure drop of 0.4 MPa. Furthermore, the modified membrane exhibited stable separation performance during the 120-hour test for wet CO2/CH4 mixture at 353 K. The hydrophobic modification paves a way for SAPO-34 membranes in real applications.