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

直链烯烃，如1-己烯和1-辛烯，是聚合物生产的重要组成部分。它们通常是在铝基活化剂存在的情况下，使用复合铬催化剂(由铬前体和合适的配体原位组装而成)进行均相催化乙烯齐聚生产的。虽然带有双齿二膦胺配体(PNP)的1-己烯的产率达到90%，但1-辛烯的最大产率仍然低得多，并且根据铬前体、铝活化剂和PNP配体的类型而显著不同。对这些组分在活性铬络合物形成中的作用和催化性能的详细了解是非常必要的，以支持合理的催化剂设计。然而，到目前为止，关于原位形成的活性铬物种的价态和结构的结论几乎完全是通过与乙烯处理定义明确的单独制备的铬络合物或从反应混合物中分离出来的铬物种的非原位表征而得出的。因此，这些结果可能不能反映在反应条件下的真实情况。本项目的目的是通过结合光谱操作技术(EPR、XAS、XES、UV-VIS、FTIR-ATR和拉曼光谱)监测相关条件下(流动温度为60℃、压力为15-20bar)下的乙烯四聚反应中催化活性物种以及中间反应步骤和失活途径的性质。特别强调在同一实验中结合几种方法，因为这扩大了在完全可比较的条件下可获得的信息的范围。除了LIKAT已经提供的同步拉曼/ATR/UV-Vis光谱外，EPR和同步辐射X射线光谱将在同一实验中首次同时耦合。这将有助于识别不同共存的铬物种，因为EPR可以选择性地检测单独的铬(I)和铬(III)部分，而XAS对所有的铬价态都是活跃的，但只反映了所有存在的铬物种的平均值。利用这一先进的表征平台，以Ph2P-N(IPR)-PPh2/Cr(Acac)3/MAO环己烷催化体系为参照，研究了PNP配体(Ph和N位不同取代基)、铬前驱体和/或铝活化剂对原位生成的活性铬络合物结构和价态的影响，从而对催化性能的影响。

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.