Impact of Al Activators on Structure and Catalytic Performance of Cr Catalysts in Homogeneous Ethylene Oligomerization – A Multitechnique in situ/operando Study

Abstract

The effect of different AlR 3 activators (R=methyl, ethyl, isobutyl, n‐octyl) has been studied in comparison to modified methylaluminoxane (MMAO) by operando EPR as well as by in situ UV‐vis, ATR‐IR and XANES/EXAFS spectroscopy during oligomerization of ethylene at 20 bar and 40 °C with a homogeneous Cr complex catalyst formed in situ upon mixing a Cr(acac) 3 precursor, a Ph 2 PN( i Pr)PPh 2 ligand (PNP) and the activator. Coordination of PNP to Cr(acac) 3 is initiated only in the presence of an activator. Highest 1‐octene productivity (detected during operando EPR measurements) was obtained with MMAO which promotes bidentate coordination of the ligand to form an active (PNP)Cr II (CH 3 ) 2 chelate complex. Rising bulkiness of R in AlR 3 leads to only monodentate coordination of PNP to the Cr center by one P atom and increasing reduction to Cr I to a maximum extend of around 30 % for AlOct 3 . This lowers the catalytic performance, which is mainly governed by the mode of PNP coordination rather than by the Cr I content.