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 AlR3 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 Ph2PN(iPr)PPh2 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)CrII(CH3)2 chelate complex. Rising bulkiness of R in AlR3 leads to only monodentate coordination of PNP to the Cr center by one P atom and increasing reduction to CrI to a maximum extend of around 30 % for AlOct3. This lowers the catalytic performance, which is mainly governed by the mode of PNP coordination rather than by the CrI content.

Stereoselective Chromium‐Catalyzed Semi‐Hydrogenation of Alkynes

“…The catalyst activation by addition of DIBAL−H was studied by in‐operando X‐ray absorption spectroscopy (XAS) at the chromium K‐edge [13c,19] . Figure 2 shows the XANES (X‐ray absorption near edge structure) spectra for the successive additions of up to 6 equiv.…”

Chromium Complexes Supported by Phenyl Ether‐Pyrazolyl [N,O] Ligands as Catalysts for the Oligo‐ and Polymerization of Ethylene

“…[ 27,108,109 ] The role of Al activators on catalytic performance of chromium catalysts in homogeneous ethylene oligomerization has been recently studied by Tromp, Brückner and co‐workers. [ 110,111 ] With regards to product distribution within the liquid fraction, Cr3 /PMAO‐IP catalytic system leads to a more balanced distribution of oligomers, with a drastic decreases in α‐C 4 (30.20 mol %) and increase in α‐C 8 (16.80 mol %) and α‐C 10 (10.80 mol %) fractions (Figures S9 and S10).…”

Alkene oligomerization via metallacycles: Recent advances and mechanistic insights

“…For example, Brückner & Bauer's teams supports a Cr II complex as the active species in the Cr(acac) 3 /PNP/MMAO thanks to EPR, XANES and EXAFS studies. [70] When using AlEt 3 , Al i Bu 3 and AlOct 3 as co-catalyst, Cr I is detected at a 15 to 30% ratio (compared to other generated Cr oxidation states) but the different mixtures were twice less active than system with MMAO (2% of Cr I detected). The EXAFS and XANES studies suggested the presence of a Cr II species for this system.…”