Abstract: Process for the manufacture of a support containing silica and at least one constituent chosen from alumina and aluminum phosphate, according to which an alcohol, water, a silicon alkoxide and an acid are mixed under conditions such that gelling or precipitation of silica is prevented, an acidic solution of an aluminum compound and/or a solution of a source of phosphate ions are added thereto, a gelling agent is added thereto, a gel is recovered which is washed with water and then by means of an organic liquid, the gel is then dried by atomization until a powder is obtained, and the powder is calcined. Polymerization of olefins in the presence of a catalyst containing chromium on a support as described above.

Abstract: Process for the manufacture of a support for olefin polymerization catalysts comprising silica, aluminium phosphate and optionally alumina, according to which a source of silica, chosen from alkaline aqueous silica sols and alkaline aqueous solutions of inorganic silicate, is added, in a first step, to an aqueous solution of a source of phosphate ions which has a pH of less than 5, the pH of the medium being maintained below 5 throughout the first step, an aluminium compound is added, in a second step, to the medium obtained from the first step and a precipitate is formed, in a third step, by adding a precipitation agent to the medium obtained from the second step, the pH of the precipitation medium being maintained above or equal to 5.

Abstract: Support for catalysts, containing at least two constituents chosen from silica, alumina and aluminium phosphate, having a specific surface of 100 to 800 m.sup.2 /g, a crystallization temperature greater than or equal to 700.degree. C. and a pore volume of 1.5 to 4 cm.sup.3 /g, the specific surface (SS) and the pore volume (PV) corresponding to the relationship:SS<(PV.times.564-358).Process for the manufacture of such a support, according to which an alcohol, water, a silicon alkoxide and an acid are mixed under conditions such that gelling or precipitation of silica is prevented, an acidic solution of an aluminium compound and/or a solution of a source of phosphate ions are added thereto, a gelling agent is added thereto, a gel is recovered which is washed with water and then by means of an organic liquid, the gel is then dried until a powder is obtained, and the powder is calcined. Polymerization of olefins in the presence of a catalyst containing chromium on a support as described above.

Abstract: Process for the manufacture of a support containing silica and at least one constituent chosen from alumina and aluminium phosphate, according to which an alcohol, water, a silicon alkoxide and an acid are mixed under conditions such that gelling or precipitation of silica is prevented, an acidic solution of an aluminium compound and/or a solution of a source of phosphate ions are added thereto, a gelling agent is added thereto, a gel is recovered which is washed with water and then by means of an organic liquid, the gel is then dried by atomization until a powder is obtained, and the powder is calcined. Polymerization of olefins in the presence of a catalyst containing chromium on a support as described above.