Spectroscopic investigation of the activation of a chromium-pyrrolyl ethene trimerization catalyst

DOI: 10.1021/acscatal.8b03414

Authors: Bas Venderbosch (University of Amsterdam) , Jean-pierre H. Oudsen (University of Amsterdam) , Lukas A. Wolzak (University of Amsterdam) , David J. Martin (University of Amsterdam) , Ties Johannis Korstanje (University of Amsterdam) , Moniek Tromp (University of Amsterdam)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Acs Catalysis

State: Published (Approved)
Published: December 2018
Diamond Proposal Number(s): 17599

Abstract: 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. Firstly, 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 hour) are observed, while in toluene the quantity of Cr(I) is much higher (~23% after an hour). This is due to the formation of cationic bis(tolyl)Cr(I) complexes, which likely leads to the observed inferior performance using toluene as the reaction solvent. Catalytic studies allow us to exclude some of the observed Cr(I) and Cr(II) complexes as the active species in this catalytic system. Using this combination of techniques, we have been able to structurally characterize complexes of this selective Cr-catalyzed trimerization system under conditions which are employed in industry.

Journal Keywords: Ethene trimerization; chromium; spectroscopy; XAS; EPR; time-resolved

Subject Areas: Chemistry


Instruments: B18-Core EXAFS

Other Facilities: Swiss Light Source