Selective continuous flow catalysis using transition metal complexes in supported ionic liquid phase with supercritical carbon dioxide (2011)

The present thesis deals with the development of a novel, efficient reaction concept which allows selective catalysis mediated by molecularly defined transition metal complexes to be conducted in a continuous manner with fully integrated product separation. Such systems may open up new opportunities in chemical synthesis and catalysis research, and also pave the way for a broader application of organometallic catalysis in the chemical industry. In this context, the general merits and conceptual challenges of performing continuous flow organometallic catalysis are discussed in Chapter I. Traditional methods of catalyst recovery as employed in contemporary industrial processes are contrasted in order to derive general strategies. Then some novel approaches based on near- and supercritical fluids are discussed in order to highlight both the limitations and the opportunities of these promising systems, that may offer elegant solutions to integrated product separation based on the unique physico-chemical properties of compressible gases. This perspective together with the state-of-the-art of catalyst immobilisation leads to the project formulation of investigating molecular catalysts in supported ionic liquid phase (SILP) with supercritical CO2 (scCO2) as the mobile phase for continuous enantioselective C=C hydrogenation of non-volatile substrates. The design and functioning of a customised continuous flow setup capable of processing supercritical fluids are described in Chapter II. A compact, versatile, and highly accurate flow system with production capacities ranging from lab to pilot scale and fully computerised control was successfully realised. Besides precise control of temperature, pressure, and flow, a self-regulating dosage of liquid substrates based solely on mass flow was integrated. A high pressure view cell for phase behaviour observation and a supercritical fluid chromatograph for automated on-line analysis were also installed. The setup ensured defined process conditions and safe long-term operation, providing a solid basis for subsequent catalyst testing studies. In Chapter III the development and characteristics of supported liquid phase catalysts are briefly reviewed to summarise the current understanding of these materials. The phase behaviour of the model reaction system for enantioselective C=C hydrogenation was successfully investigated to yield important physical data for the continuous flow process. A chiral rhodium catalyst complex was evaluated in the reaction and successfully transferred into a biphasic IL / scCO2 system. This was tested in continuous application as starting point for the subsequent evaluation of the SILP catalyst materials. Various supports and SILP catalysts were investigated with diffuse reflectance IR spectroscopy under inert conditions, yielding valuable insight into the molecular interactions on the surface of the composite materials. Under optimised conditions the catalyst complex used was found to be fully compatible with its immobilisation: solution-like performance in asymmetric catalysis was achieved with the SILP materials as evidenced by unperturbed enantioselectivities (ee > 99%). When applied in continuous flow catalysis with scCO2 fast and irreversible deactivation was initially seen. An unprecedented ageing phenomenon was found to be responsible for this behaviour, and greatly superior performance could be achieved with freshly synthesised SILP catalyst materials. The optimised SILP / scCO2 system afforded very high productivities (STYs up to 1.5 kg L-1 h-1) at excellent catalyst retention (< 1 ppm metal leaching), however, with still some deactivation at longer reaction times. Different ligands, ionic liquids, and supports were screened in order to test the influence of the respective components on the performance and stability of the reaction system. It was found that with suitably surface-modified support materials catalyst stability could be enhanced even further by providing a hydrophobic environment for the water-sensitive active species of the transition metal catalyst. Eventually, valuable insight into SILP catalysis was gained and a viable continuous flow process was demonstrated that outperformed the non-supported IL / scCO2 system, notably in a highly enantioselective hydrogenation reaction mediated by a chiral transition metal complex. In Chapter IV the application of continuous SILP / scCO2 catalysis is proposed as reaction concept for thermodynamically difficult transformations. Based on integrated product separation by scCO2, product would be continuously removed from the equilibrium in the catalytically active phase through extraction. The direct hydrogenation of CO2 with molecular hydrogen to yield formic acid was chosen as model reaction, because it is a well-studied system of suitably challenging thermodynamics and of technological relevance in the context of energy storage. Different transition metal catalysts and organic bases, both modified for application in IL / scCO2 biphasic systems, were evaluated in the reaction. Basic SILP catalyst materials were synthesised and tested in batch as well as continuous catalysis with scCO2. Although continuous production of pure formic acid could not yet be demonstrated, promising progress was made towards successful implementation of this concept. In Chapter V synthetic strategies to tailored ligand structures for asymmetric continuous SILP / scCO2 catalysis are described. Chiral cationic phosphines were targeted in order to permit intensified catalyst loadings, enhanced ligand retention and the potential use of chiral ionic liquids for enantiodiscrimination. Besides the synthesis of some new VariPHOS ligand precursors a new synthetic route to chiral imidazolium-phosphines was found. Further refinement of this route may afford access to a new class of intriguing ligands with different elements of chirality based on the combination of N-heterocyclic carbene chemistry and phosphorus ligands.