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 N2 permeance and mechanical strength were investigated. The Si–HF calcined at 1250 °C showed relatively high mechanical strength and N2 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 × 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 × 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 α-alumina substrate. The Si-CHA-HF membrane calcined at 900 °C showed no change in CO2 or CH4 permeance.