HYDROGEN-PERMSELECTIVE SIO2 MEMBRANE FORMED IN PORES OF ALUMINA SUPPORT TUBE BY CHEMICAL-VAPOR-DEPOSITION WITH TETRAETHYL ORTHOSILICATE

The outer surface of an alpha-alumina porous tube of 1.9-mm i.d. and 2.5-mm o.d. was coated with boehmite sol, which was calcined into a gamma-alumina layer of 1.5-2.5-mu m thickness. Then pores of the gamma-alumina layer (average pore size approximate to 7 nm) were filled with SiO2 formed by thermally activated chemical vapor deposition (CVD) of tetraethyl orthosilicate at 400-600 OC. The reactant was continuously evacuated through the porous support tube by keeping the pressure inside the support tube at 1.7-1.8 kPa initially. At the end of the modification, the pressure was reduced to 10 Pa. This procedure was effective in occluding pinholes that might be formed by other methods. A SiO2 film of 50-100-nm thick was also placed on the top of the porous gamma-alumina layer. The hydrogen permeability of the composite membrane was 10(-7)-10(-8) mol.m(-2).s(-1).Pa-1 at 600 degrees C, and the selectivity of H-2 to N-2 was higher than 1000. The membranes were stable in a steam atmosphere at 500 degrees C.