Preparation and gas permeation properties of all-silica CHA zeolite hollow fiber membranes prepared on amorphous-silica hollow fibers

A porous, amorphous silica hollow fiber (Si-HF) was prepared by the phase inversion method. The effects of the calcination temperature on the N-2 permeance and mechanical strength were investigated. The Si-HF calcined at 1250 degrees C showed relatively high mechanical strength and N-2 permeance. An all-silica CHA zeolite membrane was synthesized on the Si-HF support (Si-CHA-HF membrane) using various H2O/Si ratios and pH values for secondary growth. The Si-CHA-HF prepared with H2O/Si = 6.7 and pH = 7 showed relatively high CO2 permeance and CO2/CH4 selectivity. Si-CHA-HF membranes were also prepared by the seeding method using a Si-based seed crystal (0.01% mass fraction). Seeding is a common method for shortening synthesis time and reducing crystal size due to parts dissolved from seed crystals. A dense zeolite layer was not obtained without addition of the seed crystal. A membrane prepared for 20 h showed high CO2 permeance (4.1 x 10(-6) mol m(-2) s(-1) Pa-1) with CO2/CH4 selectivity of 59. By comparison, a membrane prepared for 72 h showed CO2/CH4 selectivity of 126 and permeance of about 2.9 x 10(-7) mol m(-2) s(-1) Pa-1. This performance was better than that of the selectivity/permeance trade-off line for CHA zeolite membranes. In addition, this membrane showed high thermal stability compared with a conventional CHA zeolite membrane containing aluminum on an alpha-alumina substrate. The Si-CHA-HF membrane calcined at 900 degrees C showed no change in CO2 or CH4 permeance.