Abstract: A catalyst, which is active in the oligomerization of olefins is an X-ray-amorphous silica-alumina-nickel oxide gel having an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of from 30/1 to 500/1, and NiO/SiO.sub.2 molar ratio of from 0.001/1 to 0.01/1, a superficial area of from 500 m.sup.2 /g to 1.000 m.sup.2 /g, and a porosity of from 0.3 ml/g to 0.6 ml/g, the means pore diameter being 1 nm (10 Angstrom), and devoid of pores having a diameter over 3 nm (30 Angstrom). The catalyst selectively dimerizes isobutene into alpha- and beta-isobutene and oligomerizes propylene into its relative dimers and trimers. A process is described for preparing the silica-alumina-nickel oxide gel.

Abstract: Disclosed is a process for carrying out the selective dimerization of isobutene in order to produce alpha- and beta-di-isobutene dimer.sub.3, according to which process an isobutene stream is brought into contact, under dimerization conditions, with a solid silica gel/alumina catalyst which is amorphous when analyzed by X-ray diffraction, displays a molar ratio of SiO.sub.2 /Al.sub.2 O.sub.3 comprised within the range of from 30:1 to 500:1, has a surface-area comprised within the range of from 500 to 1000 m.sup.2 /g, a porosity of from 0.3 to 0.6 ml/g and an average pore diameter of approximately 10 .ANG. (.ANG.ngstrom), and is free from pores with a larger diameter than 30 .ANG..