Effects of halide in homogeneous Cr(III)/PNP/MAO catalytic systems for ethylene tetramerization toward 1-octene

In homogeneous Cr(III)/methylaluminoxane/cyclopentyl-bisphosphineamine (PNP)/halide ethylene tetramerization catalyst systems, the effects of halide on the 1-octene formation selectivity and the catalytic activity were investigated. The comparative studies showed that both 1-octene formation selectivity and catalytic activity of the four-member catalytic systems containing dichloromethane were higher than those of containing trichloromethane and tetrachoromethane. 1,1,2-Trichloroethane showed much higher 1-octene formation selectivity improvement than 1,1,1-trichloroethane. The improvement of chloride on 1-octene formation selectivity and catalytic activity was much better than that of a corresponding bromide. So we can draw a conclusion that the steric hindrance, the molecular stability, the halides group configuration, and the types of the halides are important factors for ethylene tetramerization toward 1-octene. Some specific interaction modes of halogen groups with active Cr species in the catalytic cycle are proposed to explain the 1-octene selectivity improvement effects of halide in the ethylene tetramerization reaction.

The possible coordination structures of halide with chromium center are shown here. In the interaction mode, geminal chloro groups act as two-point weak Lewis base sites which fit with two Cr sites of dimeric Cr(III)/PNP complex. The specific interaction mode between dimeric Cr(III)/PNP complex and geminal chloro groups is expected to stabilize the active center.Figure optionsDownload as PowerPoint slide