A new silica-supported chromium complex catalyst for ethylene trimerization having high activity and high selectivity is presented. The catalyst was prepared by supporting tris[bis(,trimethylsilyl)amido]Cr(III) Cr[N(SiMe3)(2)](3) and isobutylalumoxane on silica calcined at 600 C. The catalyst containing 1.0 wt.% Cr with an isobutylalumoxane/Cr molar ratio of 24 gave a 74.2% 1-hexene selectivity at an activity of 3170 g/(mmol Cr h) in heptane solvent at 120 degreesC and 0.7 MPa ethylene partial pressure. The main by-product was decenes while small amounts of 1-butene, 1-octene and polyethylene were also obtained. To improve the 1-hexene selectivity, 1,2-dimethoxyethane was added as an electron donor, which gave an 86.4% 1-hexene selectivity at a 1,2-dimethoxyethane/Cr molar ratio of 0.25 under the same reaction conditions, but the activity decreased to 1580 g/(mmol Cr h). With the addition of 1,2-dimethoxyethane, the 1-hexene selectivity increased at the expense of decenes. The presence of the silica as a support and the silica calcination temperature were the key factors for the ethylene trimerization. The trimerization activity greatly decreased when the silica support was not used or calcined at low temperature. The trimerization activity obeyed first-order kinetics on the ethylene partial pressure. The ethylene trimerization mechanism, which includes the metallacycle intermediates supported on silica by one Si-O-Cr bond, is proposed. (C) 2002 Elsevier Science B.V. All rights reserved.