Synthesis and gas permeation properties of phosphorous-modified chabazite membranes

Abstract

Among the numerous zeolites, chabazite (CHA) zeolites have attracted attention as a membrane material to separate CO2 with high separation performance, permeability, and stability. The phosphorus modifications improve the physicochemical properties, such as thermal and hydrothermal stability because the modified phosphorus species interacting with the framework aluminum prevents dealumination. In addition, the pore character control provided by phosphorus modifications is expected to improve the gas selectivity compared with conventional CHA membranes. Therefore, phosphorus-modified CHA zeolite (P-CHA) membranes were prepared by the secondary growth method using tetraethyl phosphonium hydroxide (TEPOH) and N,N,N-trimethyl-1-adamantammonium hydroxide as organic structure directing agents (OSDAs). The phosphonium species in P-CHA membrane affected the affinity with carbon dioxide and narrowed the pore size distribution. The effect of phosphorous-modification on the gas permeation properties was confirmed. The P-CHA membrane exhibited high permeances for H2, CO2, and N2, which are 1.5 × 10−6, 6.7 × 10−7, and 2.5 × 10−7 mol m−2s−1Pa−1, respectively. The separation performance of the P-CHA membrane for CO2/CH4 was close to the upper bound in the relationship between permeance and selectivity. For H2/CH4 and N2/CH4, separation performance of the P-CHA membrane exceeded the upper bound.