Production of polyethylene

A process for producing an ethylene polymer comprising:

• contacting ethylene under polymerization conditions with a catalyst system, said catalyst system comprising:
• silylchromate,
• a silica-containing support, dehydrated at about 400 to about 860° C., comprising silica selected from the group consisting of silica having:
• (a) a pore volume of about 1.1 to about 1.8 cm3/g and a surface area of about 245 to about 375 m2/g,
• (b) a pore volume of about 2.4 to about 3.7 cm3/g and a surface area of about 410 to about 620 m2/g, and
• (c) a pore volume of about 0.9 to about 1.4 cm3/g and a surface area of about 390 to about 590 m2/g;
• and
• an alkyl aluminum alkoxide compound, wherein said alkyl aluminum alkoxide is added directly to the catalyst during catalyst preparation;
• and
• adding an alkyl aluminum compound to the polymerization reactor in an amount of from 0.5:1 to 3:1 of equivalents of aluminum from the alkyl aluminum compound to equivalents of chromium,
• to effect a final ratio of equivalents of aluminum from both the alkyl aluminum alkoxide and the alkyl aluminum to equivalents of chromium of from about 5:1 to about 10:1;
• wherein the polymerization is conducted at a space-time-yield value of greater than 8 kg/h/m3;
• and the polymerization is operated at a catalyst productivity of greater than 3000 kg polymer/kg catalyst system and at a reaction temperature at least 2.5° C. higher than the reaction temperature when polymerizing with the same catalyst system in the absence of the alkyl aluminum compound and producing the ethylene polymer at the same polymer molecular weight and density using the same space-time-yield value, ethylene partial pressure, H2C2 gas mole ratio and comonomer to C2 gas mole ratio.