ZSM-5 type zeolites were synthesized by the hydrothermal treatment of the reaction mixture of silica sol, aluminum nitrate, sodium hydroxide, and TPABr at the temperature range of 150-180 degrees C in the autoclave. The shape of the ZSM-5 zeolite calcined at 450 degrees C was spherical or polyhedral and its crystalline size was 0.5-3 mu m. The synthetic ZSM-5 was found to be highly hydrophobic and active for CO2 adsorption. ZSM-5 zeolite composite membranes supported with porous a-alumina tubes have been synthesized by dip coating or pressurized coating of the reaction sol-mixture followed by hydrothermal treatment. The permeation mechanism of CO2 through ZSM-5 membranes was a surface diffusion and the membranes prepared by the pressurized-coating hydrothermal treatment showed a fairly high CO2/N-2 separation factor of 9.0 and a permeability of 10(-8)-10(-7) mol/m(2).s.Pa at room temperature.