TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Materials: The trivalent element is boron and/or aluminum, used at an atomic ratio to silicon of 1:1-100 (preferably 1:2-20). (II) is an alkoxide or acid, specifically of formula M(OR)3 (II') or M(OH)3 (II'').M = trivalent element, preferably B or Al, especially B;R = 1-8C hydrocarbyl.In particular (II) is a boron alkoxide, formed in situ from boron oxide (B2O3) and an alcohol ROH. The support is of porous alumina. Preferred Procedure: The process involves: (a) forming the microporous layer of trivalent element-doped silica using a sol-gel method, by hydrolyzing a silicon alkoxide in an aqueous alcoholic medium containing a doping amount of (II); (b) depositing the sol on the porous support; and (c) heat treating the deposited film to cause conversion into a microporous layer of ceramic based on trivalent element-doped silica. Preferably in step (a) the silicon alkoxide is 0.3-4 M and B2O3 is added to the medium at pH less than 2. Step (b) is optionally preceded by: (a') pretreating the support to give a charge opposite to that of the sol to be deposited (specifically by impregnating an alumina-based support with an aqueous solution of pH above the isoelectric point of alumina); and/or (a'') pre-impregnating the support with the sol produced in step (a), followed by rinsing and heat treating. Step (b) is carried out by immersing the support in the sol. Preferred Product: The microporous layer of doped silica has a thickness of 50-500 nm. EXAMPLE - A mixture of 4 moles water, 4.5 moles ethanol and 0.4 moles hydrochloric acid was treated at 40degreesC with 1 mole tetraethoxysilane followed by 0.1 mole boron oxide. Reaction was carried out under reflux at 40degreesC for 3 hours, giving a mobile doped silica sol of pH 1. An alumina support, in the form of a hollow cylinder of length 25 cm, external diameter 10 mm and internal diameter 7 mm, having an internal layer of mesoporous gamma-alumina (pore diameter 5 nm) deposited on alpha-alumina), was heat treated at 600degreesC for 2 hours, immersed in aqueous ammonia (pH 10.5) for 30 minutes (to give a negative surface charge), drained, pre-impregnated with the sol for 2 hours, rinsed with ethanol and heat treated at 550degreesC for 2 hours. The pretreated support was then totally immersed in the sol (diluted with alcohol to 1/6 of its initial concentration) for 2 hours, dried at 65degreesC for 15 hours, heated from 20degreesC to 100degreesC at 1degreesC per minute, kept at 100degreesC for 2 hours, heated to 550degreesC at 1degreesC per minute, kept at 550degreesC for 2 hours and cooled top 20degreesC at 1degreesC per minute. When used for the separation of helium from a helium-based mixture containing 1% carbon dioxide and 1% methane at 300degreesC at a transmembrane pressure difference of 1-4 bars, the obtained membrane had a permeation value of 10-6 mol.m-2.s-1.Pa-1, a helium-carbon dioxide selectivity of 18 and a helium-methane selectivity of 21.