Fundamentals of Heterogeneous Selective Ethylene Oligomerisation

Abstract

Sequential treatment of a partially dehydroxylated oxide (i.e. SiO2, γ-Al2O3, or mixed SiO2-Al2O3) with solutions of Cr3 (0.71 wt% Cr) and a Lewis acidic alkyl aluminium-based co-catalyst (Al/Cr = 15) affords initiator systems active for the oligo- and poly-merisation of ethylene. The influence of the oxide support, calcination temperature, co-catalyst, and reaction diluent on the catalytic performance of such oxide-supported chromium initiators have been investigated. The best performing combination generates a mixture of hexenes (61 wt%; 79% 1-hexene), and polyethylene (PE; 16 wt%) with an overall activity of 2403 g gCr–1 h–1. The observed product distribution is rationalised by two competing processes: trimerisation via a supported metallacycle-based mechanism and polymerisation through a classical Cossee-Arlman-type chain growth pathway. This is supported by the indirect observation of two distinct chromium environments at the surface of silica by a solid-state 29Si direct excitation (DE) magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopic study of the Crx/SiO2-600 pro-initiator.

A series of experimental parameters were tested to evidence their impact on the catalytic performance of the Crx/SiO2-600/MMAO-12 initiator at a constant ethylene pressure. These include varying chromium concentration, Al/Cr loadings, reaction temperature, ethylene working pressure, stirrer speed, reaction time, diluent volume, and the impact of potential promoters, namely 1,2-dimethoxyethane (1,2-DME) and Et2Zn.