Turning a Cr-based heterogeneous ethylene polymerisation catalyst into a selective ethylene trimerisation catalyst

A Phillips Cr/SiO2 polymerisation catalyst was converted into an ethylene trimerisation catalyst after assembling new active sites on the silica surface in the presence of TAC (1,3,5-triphenylhexahydro 1,3,5-triazacyclohexane) as ligand and CH2Cl2 as solvent. The reaction conditions play a role in tuning the catalytic activity of this system. It is a true trimerisation catalyst at low ethylene pressures, while, at high pressures, the polymerisation activity becomes dominant. Moreover, at high ethylene pressures, polyethylene characterised by a high crystallinity degree is obtained. Instead, a highly branched heavy oligomer of ethylene characterised by a very narrow molecular weight distribution is obtained at 1 bar. 1-Hexene, formed as the main product in the ethylene trimerisation reaction at low pressures, is partially incorporated into this growing oligomer making redundant the use of a second comonomer feedstock and an expensive catalyst.

A single-site heterogeneous Cr-based catalyst can interchange trimerisation activity with polymerisation activity as a function of ethylene pressure. At low ethylene pressures, branched oligomers of ethylene are obtained and HDPE-type polymers are obtained at high ethylene pressures. At moderate pressures, this system can be considered a one-pot heterogeneous catalyst for production of LLDPE-type polymers.Figure optionsDownload as PowerPoint slide