Abstract
Catalytic hydroformylation of olefins mediated by rhodium-phosphine complexes is one the largest and most important industrial applications of homogeneous catalysis. Worldwide aldehyde production by hydroformylation capacities has exceeded 10 million metric tonnes in 2008, and aldehydes markets have continued to grow since then. Nevertheless, there are still important mechanistic questions that remain unanswered due to experimental limitations, despite the extensive amount of research carried out over the past 60 years in both academia and industry. In this project, hydroformylation processes are investigated by FlowNMR spectroscopy and related techniques in the Dynamic Reaction Monitoring Facility at the University of Bath to gain new insights into the mechanism of the reaction.The elevated temperatures and pressures needed for hydroformylation require an adaption of the setup of the facility to carry out reactions under optimal and safe conditions. For this, an autoclave reactor is interfaced into the apparatus while the tubing material, connections and positive displacement pumps are investigated under hydroformylation conditions to ensure the system can withstand the pressures and temperatures required.
The hydroformylation of 1-hexene with 10 bar of 1:1 H2/CO in the presence of the catalytic system [Rh(acac)(CO)2]/PPh3 is then studied by real-time multinuclear high-resolution 1H and 31P FlowNMR spectroscopy at 50 °C. Rates, chemo- and regioselectivities are monitored with varying P/Rh loadings by 1H experiments. 31P{1H} and selective excitation 1H pulses are used to characterise and quantify key hydrido-rhodium and acyl-rhodium intermediates formed during turnover as well as dormant dimeric carbonyl complexes.
Sensitivity in FlowNMR spectroscopy for reaction monitoring is enhanced by the addition of [Cr(tmhd)3], as paramagnetic relaxation agent (PRA) which reduce spin-lattice relaxation times, improving the signal-to-noise ratio. [Cr(tmhd)3] significantly enhances 1H and 31P{1H} FlowNMR data quality in the Rh-catalysed hydroformylation without interfering in the reaction progress. This Cr(III) species is then applied in our subsequent hydroformylation experiments investigating a variety of ligands.
Phosphites are industrially important ligands for the hydroformylation process as they produce more active catalysts than phosphines, mainly due to their strong π-acceptor properties facilitating CO dissociation from the metal centre. We investigate this effect on catalyst speciation during turnover using multi-nuclear operando FlowNMR spectroscopy to observe the analogies and differences with PPh3. The quantitative catalyst distribution maps derived explain activity trends across a range of catalytic reaction conditions that show how phosphites produce more active catalysts, including reduced formation of inactive Rh0 dimers in the absence of substrate during pre-activation and at the end of the reaction.
In the absence of H2, the reaction of hydrido Rh phosphine carbonyl complexes of some mono- and bidentate phosphines and phosphites with hydroformylation substrates 1-hexene, cyclooctene and ethylene and overpressures of CO enables detailed NMR characterisation of rhodium acyl complexes. Five-coordinate acyl mono-, di- and tricarbonyl complexes appear to be thermodynamically preferred over the four-coordinate acyl species at low temperatures (-90 to -40 °C) and 1-5 bar of CO. Under non-catalytic reaction conditions (i.e., no H2 present), NMR spectroscopy reveals the kinetic and thermodynamic selectivity of linear and branched acyl formation. Over the range of ligands investigated, the kinetic regioselectivity observed at low temperatures under our conditions roughly predicts the regioselectivity observed for catalytic transformations monitored by 1H FlowNMR spectroscopy at higher temperatures and pressures.
| Date of Award | 12 Dec 2022 |
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| Original language | English |
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| Supervisor | Ulrich Hintermair (Supervisor), John Lowe (Supervisor) & Catherine Lyall (Supervisor) |