Chromium oxide catalyst for ethylene polymerization

A solid catalyst system comprising a first chromium compound, a second chromium compound, a reaction product of an alkyl aluminium compound and a nitrogen containing compound and a silicon oxide support, wherein

• the first chromium compound is chromium trioxide or a compound convertible to chromium trioxide,
• the second chromium compound is a silylchromate compound,
• the nitrogen containing compound is a cycloalkylamine compound having the general formula:
• R3—NH2, wherein R3 is selected from C3-C8 cycloalkyl groups,
• the molar ratio of the amount of Al in the solid catalyst system with respect to the amount of Cr in the solid catalyst system is 1.4 to 1.7.