Exploring the nature of active chromium catalysts during tetramerization of ethylene by (coupled) operando spectroscopy
Analytical Chemistry
Final Report Abstract
The worldwide first experimental setup of simultaneous operando EPR/UV-vis/ATR-IR/XAS spectroscopy for monitoring catalytic gas-liquid phase reactions has been developed and patented. New pressure cells for in situ/operando studies of those reactions by XAS, EPR and ATR-IR spectroscopy have been designed that are suitable for reactions other than ethylene tetramerization as well. Using these techniques, the influence of different Al-containing activators such as AlR3 (R = methyl, ethyl, isobutyl, n-octyl) and modified methylaluminoxane (MMAO), different PNP ligands and Cr precursors on the catalytic performance and the nature of the active species in Cr-catalyzed were studied during ethylene tetramerization at 20 bar and 40 °C. The active Cr complex catalyst is 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 was obtained with MMAO which promotes bidentate coordination of the ligand to form an active (PNP)CrII(CH3)2 chelate complex. Increasing bulkiness of R in AlR3 leads to only monodentate coordination of PNP to the Cr center by one P atom and reduction to CrI to a maximum extend of around 30 % for AlOct3 is found. This reduces the catalytic performance, which is mainly governed by the mode of PNP coordination rather than by the CrI fraction in the reaction mixture. Extensive in situ EPR and XAS experiments have proved an active CrII/CrIV redox cycle for the selective tetramerization of ethylene for all activators. The catalytic performance and the structure of the active Cr complex are influenced by substituents at the phenyl rings of the PNP ligand. Electron-rich para-substituents (p-F or p-Cl) decrease the activity due to possible intermolecular coordination and/or electron withdrawing properties. Ortho-methoxy substituents inhibit the formation of a chromacyclononane intermediate by imposing steric hindrance and/or blocking of vacant coordination sited at the Cr center, which leads to polymer formation only. Finally, it was found that even when a well-defined CrII precursor complex is used with a structure similar to that of the active CrII species formed in situ in the Cr(acac)3/PNP/MMAO system, activation of this CrII complex with an Al activator is necessary to create vacant sites for ethylene coordination and, thus, formation of a chromacycle intermediate. No catalytic activity could be detected with a neutral Cr0(CO)6 complex, due to its high stability.
Publications
- “Chromium Complexes with Oxido and Corrolato Ligands: Metal-Based Redox Processes versus Ligand Non-Innocence”, Chem. Eur. J. 2018, 24, 12613-12622
A. Garai, S. Sobottka, R. Schepper, W. Sinha, M. Bauer, B. Sarkar, S. Kar
(See online at https://doi.org/10.1002/chem.201801452) - “Multivariate analysis of coupled operando EPR/XANES/ EXAFS/UV–vis/ATR–IR spectroscopy: A new dimension for mechanistic studies of catalytic gas-liquid phase reactions”, Chem. Eur. J.
J. Rabeah, V. Briois, S. Adomeit, C. La Fontaine, U. Bentrup, and A. Brückner
(See online at https://doi.org/10.1002/chem.202000436) - „Probe Setup for Combined EPR and XAS Measurements”, WO 2019/154488 A1, 2019
J. Rabeah, A. Brückner, S. Adomeit
- “Impact of Al Activators on Structure and Catalytic Performance of Cr Catalysts in Homogeneous Ethylene Oligomerization – A Multitechnique in situ/operando Study”, ChemCatChem 2020, 12, 1025-1035
R. Grauke, R. Schepper, J. Rabeah, R. Schoch, U. Bentrup, M. Bauer, A. Brückner
(See online at https://doi.org/10.1002/cctc.201901441)
