Catalysts are of the utmost importance in chemical industry. The use of catalysts allows processes to proceed faster and more efficiently, as well as lowering the power usage. As such, catalysts make chemistry more sustainable: less waste and a lower CO2 footprint. The chemists from Amsterdam have investigated the very selective trimerization of ethene towards 1-hexene, which is a precursor for the production of polyethene, one of the most used plastics worldwide. Since this concerns industrial processes at a very large scale, a small improvement of the catalyst can greatly enhance the sustainability of the reaction.
Combining various spectroscopic techniques
Industrial catalysts are often very complex in nature and therefore their mode of operation is not always known. The studied catalysts for the trimerization of ethene consists of four components: a chromium precursor, a pyrrole ligand and two aluminum-based activators. All four components react together in solution to form a variety of species, including an active catalyst.
The scientific challenge is to unravel the full pathway towards formation of the active catalyst and to determine which species are inactive and thus reduce catalytic activity. This is far from straightforward because different chromium species are formed that display different characteristics. They are paramagnetic, making commonly used NMR analysis impossible, and some of them are also EPR silent. Therefore Bas Venderbosch and his colleagues Jean-Pierre Oudsen, Lukas Wolzak, David Martin and Ties Korstanje applied a combination of various spectroscopic techniques.
They used NMR for detecting reaction products of the pyrrole ligand with the aluminum activators together with EPR spectroscopy to detect paramagnetic compounds not visible in NMR. Next, they applied stopped flow UV-Vis spectroscopy to be able to track all different reaction products in time. Finally, X-ray absorption spectroscopy was performed to structurally identify the major species at different moments during the reaction. Combining all the spectroscopic data with DFT calculations, the researchers obtained a detailed overview of all the reactions occurring in the activation phase of the catalysts.
The active catalyst
With their knowledge of the activation of the catalysts at hand, the researchers designed and performed additional catalytic experiments. This enabled them to exclude some of the major species present in the reaction mixture to be the active catalyst. Their research also indicates that a significant amount of the chromium precursor is not transformed into an active catalyst. They conclude that improvement of the activity of this industrial catalytic system is possible by preventing the formation of these inactive species.
Bas Venderbosch, Jean-Pierre H. Oudsen, Lukas A. Wolzak, David J. Martin, Ties J. Korstanje and Moniek Tromp: Spectroscopic Investigation of the Activation of a Chromium-Pyrrolyl Ethene Trimerization Catalyst ACS Catal. 2019, 9, 1197−1210 DOI: 10.1021/acscatal.8b0341