In-situ FTIR spectroscopy in homogeneous catalysis
IR spectroscopy is a fast and sensitive analytical tool. It allows for the time-resolved observation of the working homogeneous catalyst. However, the application of this method is affording if reliable qualitative and quantitative information is desired. Complications may arise from the use of compressed gases as substrates, submilimolar concentrations of the catalysts applied as well as the usually occurring highly overlapping spectra. Respective know how has been developed in cooperation with colleagues from the University of Rostock, Prof. Neymeyr, Institute of Mathematics, and Prof. Ludwig, Institute of Chemistry. With the help of the PCD method (Pure Component Decomposition) the spectra of individual compounds become accessible which are compared with that of supposed structures obtained from DFT calculations. Concentration profiles can be obtained for the entire substrate conversion range. We have shown in detail, that the dynamics of a reaction occurring in the presence of a monophosphit-modified rhodium catalyst can be described with enzyme-type kinetics. An interesting finding was that the rhodium acyl intermediate has the bulky phosphite ligand coordinated in an axial position of the trigonal bipyramidal complex structure, thus being trans to the acyl group.1,2
Hydroformylation and FTIR spectroscopy. Left: autoclave with gas entrainment impeller (A), micro gear pump (P), transmission IR cell (C), Bruker Tensor 27 FTIR spectrometer (S, lab version from Bastian GmbH, Wuppertal). Right: concentration profiles of organic components (IR + GC, left concentration axis) and of rhodium complexes (IR, submillimolar, right axis) as obtained for a hydroformylation reaction of 3,3-dimethyl-1-butene with a CO/H2 = 5:1 gas mixture. Given are experimental data, compared to regression which is based on Michaelis Menten type kinetics.
We could furthermore disclose that the often discussed influence of additional phosphorus ligand coordination on product selectivity is not significant in our reaction. Instead, the observed increase in selectivity is a result of competitive inhibition excerted by the cosubstrate carbon monoxide. The more phosphorus-rich hydrido rhodium complex does not contribute to product formation. The respective kinetic analysis has been possible only by applying appropriate chemometric tools. 3-6 Our high pressure spectroscopic methodology has also been used to study new metals for hydroformylation with the project ‚PROFORMING‘, a grant of the Federal Ministry of Education and Research. 7 Now we are planning to focus also on the investigation of heterogeneous catalysts.
- C. Kubis, R. Ludwig, M. Sawall, K. Neymeyr, A. Börner, K.-D. Wiese, D. Hess, R. Franke, D. Selent, ChemCatChem 2010, 2, 287-295.
- C. Kubis, D. Selent, M. Sawall, R. Ludwig, K. Neymeyr, W. Baumann, R. Franke, A. Börner, Chemistry-A European Journal 2012, 18, 8780-8794.
- C. Kubis, M. Sawall, A. Block, K. Neymeyr, R. Ludwig, A. Börner, D. Selent, Chemistry - A European Journal 2014, 20, 11921-11931.
- M. Sawall, C. Kubis, R. Franke, D. Hess, D. Selent, A. Börner, K. Neymeyr, ACS Catalysis 2014, 4, 2836-2843.
- M. Sawall, C. Kubis, E. Barsch, D. Selent, A. Börner, K. Neymeyr, Journal of the Iranian Chemical Society, 2015, DOI 10.1007/s13738-015-0727-4.
- M. Sawall, C. Kubis, A. Börner, D. Selent, K. Neymeyr, Analytica Chimica Acta 2015, 891, 101-112.
- C. Kubis, W. Baumann, E. Barsch, D. Selent, M. Sawall, R. Ludwig, K. Neymeyr, D. Hess, R. Franke, A. Börner, ACS Catal., 2014, 4, 2097–2108.