Influence of Elevated Temperature and Pressure on the Chromium‐Catalysed Tetramerisation of Ethylene

Abstract A catalyst system comprising a diphenylphosphineamine (PNP) ligand, chromium(III) acetylacetonate and a methylaluminoxane‐based activator was studied for the selective tetramerisation of ethylene. The reaction was investigated over a broad temperature and pressure range and the resulting product mixture was interpreted in the light of the recently published, enlarged metallacycle mechanism. Vapour‐liquid equilibrium (VLE) data were calculated for the binary ethylene‐cyclohexane mixture over the relevant temperature and pressure ranges to deconvolute the influence of ethylene concentration and temperature. Good agreement of the experimental data with the proposed mechanism was found. Enlargement of the metallacycloheptane ring by insertion of ethylene was found to be dependent on the ethylene concentration, albeit to a lesser extent than assumed. The 1‐octene selectivity, which reaches a maximum of 72–74 mass %, thus seems to be primarily dependent on the temperature. The formation of the cyclic side products methyl‐ and methylenecyclopentane was in effect independent of the ethylene concentration. This is in good accordance with the proposed mechanism, since it indicates that the formation of the products occurs via rearrangement of a metallacycle intermediate.