Ethylene Selective Tri-/Tetramerisation with PNP-Cr Mixed Ligands

A catalytic system consisting of diphosphinoamine, chromium(Ⅲ) acetylacetonate and methylaluminoxane is a typical system for ethylene tri-/tetramerisation. Adding molecules with heteroatoms as a second ligand, such as amine, benzoate and triphenylphosphine into this catalytic system can form a mixed ligands system, which can lead to the change of performance of the catalytic system. Our studies demonstrate that, upon adding a second ligand into the system, the selectivity of 1-hexene increases while that of 1-octene decreases. Besides, the mass fraction of polymer and activity will also change. A catalytic system added with triphenylphosphine as the second ligand can achieve 47.2% selectivity of 1-hexene and 44.6% selectivity of 1-octene, which might be 43.4% and 45.1% without the second ligand. Among all the mixed ligands systems, the highest total selectivity of 1-hexene and 1-octene could be up to 91.8%, which was 88.5% without the second ligand. This process is companied with a activity decrease from 20 231 g/(g·h) to 12 975 g/(g·h) and a reduced polymer production. The dosage of PPh3 also shows remarkable effect on selectivity and activity of the catalyst when the mole ratio of PPh3 and Cr(acac)₃ varies from 0 to 1. Adding more PPh3 beyond equivalent of Cr(acac)₃ may not show stronger influence on the properties of the catalytic system. All the changes might be assigned to the coordination of heteroatoms to the metal center, which affects the coordination and insertion of ethylene.