Aluminum monohydroxide, boehmite, sol was prepared by sol-gel method from aluminum iso-propoxide under an acidic condition. This stable boehmite sol was dried at 70℃ to the boehmite concentration of 9.0wt%. This transparent sol was coated on an alumina support. After it was dried for 2 days at room temperature and calcined at 500℃ for one hour. The properties of the membrane were investigated by means of BET experiments and scanning electron microscope(SEM). It had homogeneous ultra fine pores about 3.0nm with a narrow pore size distribution. The thickness of the membrane was in the range of 0.5~8.2μm, depending upon the number of coatings.
Gas permeation characteristics of the membrane with 7.5μm thick were investigated for four gases He, $N_2$, $H_2$ and $CO_2$. Among four diffusion mechanisms, Knudsen diffusion was dominant for these gases. But $CO_2$ and $N_2$ showed much higher permeabilities than the expected values by Knudsen diffusion due to the effect of surface diffusion. The percent porosity of the membrane was 51% tortuosity factor was 9.52. The adsorption energy of $CO_2$ obtained from adsorption isotherms was 22.40 KJ/mol and the calculated surface diffusion coefficients of $CO_2$ and $N_2$ turned out to be $10.66×10^{-9}㎡/sec$ and $10.84×10^{-9}㎡/sec$ at 290K, respectively. The additional flows by surface diffusion of $CO_2$ and $N_2$ was about 43% and 22% of the total flows, respectively. The separtion factors of $CO_2$-$H_2$ and $CO_2$-$H_2$ approached the theoretical values predicted by Knudsen diffusion with increasing temperature.
