FORMATION OF HYDROGEN-PERMSELECTIVE SIO2 MEMBRANE IN MACROPORES OF ALPHA-ALUMINA SUPPORT TUBE BY THERMAL-DECOMPOSITION OF TEOS

A porous alpha-alumina tube of 2.5 mm O.D. and 1.9 mm I.D. was used as the support of an inorganic membrane and was modified for the purpose of separating small-molecule gases. Macropores of the tube, about 150 nm in size, were plugged with silica formed by thermal decomposition of tetraethylorthosilicate at 600-650 degrees C with no special pretre atment. To improve the step coverage of the deposition, the reactant w as evacuated through the porous wall of the support. Silica was decomp osed inside the macropores, forming a layer of 500-1000 nm. At a perme ation temperature of 600 degrees C, the H-2 permeance of the modified membrane was of the order of 10(-8) mol.m(-2).s(-1).Pa-1, while the N, permeance was below 10(-11) mol.m(-2).s(-1).Pa-1. When the membrane w as exposed in mixture of nitrogen and steam at 500 degrees C for 24 h, the H-2 permeance was decreased to half the initial value but was lit tle reduced by a further exposure of 48 h. The membrane was resistant to cyclic temperature changes.