Exploring the nature of active chromium catalysts during tetramerization of ethylene by (coupled) operando spectroscopy

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Linear olefins such as 1-hexene and 1-octene are important building blocks for polymer production. They are generally produced by homogeneous catalytic ethylene oligomerization using complex Cr catalysts (assembled in situ from a Cr precursor and suitable ligands) in the presence of Al-based activators. While yields of >90 % are achieved for 1-hexene with bidentate diphosphinoamine ligands (PNP), maximum 1-octene yields are still much lower and differ significantly depending on the type of Cr precursor, Al activator and PNP ligands. Detailed knowledge of the role of these components for the formation of the (still widely unknown) active Cr complex and the catalytic performance is highly desired to support rational catalyst design. However, so far conclusions on valence state and structure of the active Cr species formed in situ have been derived almost exclusively from treatment of well-defined separately prepared Cr complexes with ethylene or from ex situ characterization of Cr species isolated from the reaction mixture. Consequently, these results may not reflect the true situation under reaction conditions.It is the aim of this project to explore the nature of the catalytically active species as well as of intermediate reaction steps and deactivation pathways in tetramerization of ethylene under relevant conditions (flowing ethylene at 60°C and a pressure of 15-20 bar) by monitoring the reaction with a tailored combination of spectroscopic operando techniques (EPR, XAS, XES, UV-vis, FTIR-ATR and Raman spectroscopy). Special emphasis is put on coupling several methods in the same experiment since this widens the range of accessible information under exactly comparable conditions. Apart from simultaneous Raman/ATR/UV-vis spectroscopy already available at LIKAT, EPR and synchrotron X-ray spectroscopy will be simultaneously coupled for the first time in the same experiment. This will facilitate identification of different coexisting Cr species, since EPR can detect selectively individual Cr(I) and Cr(III) moieties while XAS is active for all Cr valence states but reflects an average of all present Cr species only. Using this advanced characterization platform and the catalytic system Ph2P-N(iPr)-PPh2/Cr(acac)3/MAO cyclohexane as a reference, the influence of dedicated modifications of the PNP ligand (different substituents at Ph and N), the Cr precursor and/or the aluminium activator on structure and valence state of the in situ formed active Cr complex and, consequently, on the catalytic performance will be studied.