Spectroscopic Investigation of the Activation of aChromium-Pyrrolyl Ethene Trimerization Catalyst

摘要

1-Hexene is an important α-olefin for polyethylene production and is produced from ethene. Recent developments in the α-olefin industry have led to the successful commercialization of ethene trimerization catalysts. An important industrially applied ethene trimerization system uses a mixture of chromium 2-ethylhexanoate, AlEt3, AlEt2Cl, and 2,5-dimethylpyrrole (DMP). Here, we have studied the activation of this system using catalytic and spectroscopic experiments (XAS, EPR, and UV–vis) under conditions employed in industry. First, chromium 2-ethylhexanoate was prepared and characterized to be [Cr3O(RCO2)6(H2O)3]Cl. Next, the activation of chromium 2-ethylhexanoate with AlEt3, AlEt2Cl, and DMP was studied, showing immediate reduction (<5 ms) of the trinuclear Cr(III) carboxylate and formation of a neutral polynuclear Cr(II) carboxylate complex. Over time, this Cr(II) carboxylate complex is partially converted into a chloro-bridged dinuclear Cr(II) pyrrolyl complex. In cyclohexane, small quantities of an unknown Cr(I) complex (∼1% after 1 h) are observed, while in toluene, the quantity of Cr(I) is much higher(∼23% after 1 h). This is due to the formation of cationicbis(tolyl)Cr(I) complexes, which likely leads to the observed inferiorperformance using toluene as the reaction solvent. Catalytic studiesallow us to exclude some of the observed Cr(I) and Cr(II) complexesas the active species in this catalytic system. Using this combinationof techniques, we have been able to structurally characterize complexesof this selective Cr-catalyzed trimerization system under conditionswhich are employed in industry.