Ultrafast synthesis of thin all-silica DDR zeolite membranes by microwave heating

All-silica DDR zeolite membrane has great potential for CO2-CH4 separation because of its suitable pore size, great stability and insensitivity to moisture. However, the long synthesis time and tough template removal created great challenges for the synthesis of high quality DDR zeolite membrane. All-silica DDR zeolite membrane was prepared on porous ceramic tubes with microwave-aided hydrothermal synthesis. Template removal of DDR membranes was studied with rapid thermal processing and low temperature calcination under ozone environment. Compared with traditional hydrothermal synthesis, microwave heating significantly accelerated the crystallization kinetics of DDR membranes by reducing the synthesis time from 2 to 6 d to 1 h. Membrane thickness was also reduced from ~5 µm to less than 1.5 µm. These improvements can be attributed to the fast and homogeneous heating from microwave heating, which resulted into fast nucleation and crystallization as well as smaller crystals and thinner membrane. Template removal by rapid thermal processing was not successful for DDR membranes which could be the result of high thermal stress at high calcination temperature. Low temperature calcination under ozone environment can remove template molecules effectively without generating defects, which can be ascribed to small thermal stress at low calcination temperature. Outstanding CO2-CH4 separation performance was obtained, with CO2 permeance of 4.7 × 10-7 mol/(m2 s Pa) and CO2-CH4 selectivity of 190 at 0.14 MPa pressure drop. The high separation permeance is the synergy of optimized seeding, microwave-aided synthesis and mild template removal under ozone environment.