ULTRAMICROPOROUS SILICA-BASED SUPPORTED INORGANIC MEMBRANES

Ultramicroporous (Pore radius < 10 angstrom) separation layers with thicknesses in the range of 200-1200 A were deposited from polymeric silicate sols onto commercial alumina supports using a dip-coating/casting procedure. The silicate sols were prepared using a two-step acid-catalyzed hydrolysis of tetraethoxysilane under pH conditions where the condensation rate is low, producing polymers of low fractal dimension which readily interpenetrate during film deposition to provide amorphous layers with extremely small pore sizes. At low aging times (t/t(gel) = 0.05), the acid catalyst concentration had a dramatic effect on the deposited membrane permeability. Using 1N HCl a discrete, featureless membrane layer was deposited on top of the mesoporous support (as revealed by TEM), while lower acid concentration (0.44 N) resulted in polymer penetration and filling of the support pores rather than the deposition of a discrete external layer. This was attributed to the effect of acid concentration on the polymer condensation rate during the final stages of membrane deposition and drying. Deposited membrane layer thickness was found to depend on sol aging time, solvent dilution, and dipping/casting time. Helium permeabilities for deposited layers were approximately 1000-2000 barrer. Measured reductions (65-90%) in the helium and nitrogen permeation rates upon membrane deposition indicated pore radii less than 10 angstrom for the deposited membrane layers.