Datos en relación a la Variable Pensión 65

5.1 Conclusions and Further Work

The improved rates and selectivities of our ligand {P(C6H4C6F13)3}, compared to PPh3 have been shown to arise from the better complexation of Rh under catalytic conditions. With the bis and tris- phosphine complexes existing under relatively low concentrations of free ligand in the fluorinated case, whereas triphenylphosphine, as is used in industry, is required in large excess to achieve high selectivities. This has been explained to a degree by the strongly electron-withdrawing perfluorinated ponytails on our ligands.

The existence of the tris-phosphine complex in the separator of the CFR also explains the low leaching levels of our ligand and rhodium to the organic phase, however the extremely low values observed in the original batch reactions were never repeated in the CFR. This was possibly in part due to the difference in charging the two reactors, but also because of the low conversion in the CFR, resulting in a higher concentration of leaching to the organic phase.

The CFR has provided proof of concept and is, to the best of our knowledge, the first example of a fluorous biphasic system operated in continuous mode under high pressure. The overall activity of the reactions was lower than expected compared with preliminary work done in this group, but this was explained by the difference in volume, efficiency of stirring / mass-transport limitations and residence time of the substrate in the reactor.

An efficient mass balance of the reactions, especially the semi-continuous experiments may have provided better evidence as to where the fluorous solvent was

being lost. As an extremely expensive solvent, reducing the volume used and especially lost, is imperative to the running of this system. Initial studies of the vent gas, by passing through a cold trap, showed that both the fluorous and organic solvents were stripped off by the gas flow in the separator, further work may include a more quantitative analysis of this observation. The importance of maintaining the solvents in the liquid phase was emphasised by the semi-continuous experiments, which eventually lost almost all of the fluorous phase and resulted in much higher levels of leaching to the organic phase.

The CFR itself may also benefit from a number of modifications; introducing a second liquid inlet to allow for separate loading of the catalyst solution and substrate to the reactor. Even though the kinetic reactions showed that this change would simply increase the rate of the reaction it may be found to improve the overall running of the system: introducing a second CSTR might increase the yields for the reactions, creating a series of reactors, with either the whole reaction mixture being moved into a second reactor or just the organic phase being removed to the next reactor containing “fresh” catalyst.

The mechanistic study of the catalyst showed that it performed quite similarly to triphenyl phosphine and we surmise that hydroformylation with a perfluorinated phosphine catalyst follows a similar mechanism to that of triphenyl phosphine. The most obvious difference is the electron-withdrawing effect of the fluorous ponytail on the Rh atom, decreasing the back bonding to the other ligands, CO.

Throughout this work, it has been apparent that in order for the FBC system to become industrially viable, the activity and selectivity of the catalyst would require to be much higher than that of the existing industrial catalysts, in order to overcome the major hurdle which faces this technology: cost. The volume of literature published

involving FBS is testimony to the perceived advantages that this system may eventually afford. Nevertheless, from the results of the continuous-flow reactor, which have proved the concept, application to industry lies some time in the future, if at all.

This system at the time of writing, is not yet commercially viable and would require further investigation before it could be so. This would probably include the identification of a more efficient ligand, with efficiency including improved performance and lower solubility in the organic phase as well as easier preparation because the synthesis of {P(C6H4C6F13)3} was particularly time consuming with a relatively low yield. A study of alternative fluorous solvents may also provide a cheaper alternative to PFMC.

It would have been of interest to investigate further the rhodium dimer complex formed in the HPIR solution and perhaps understand the apparent loss of an aryl group from the bridging phosphorus atom. Had time allowed an investigation of this complex including an attempt to synthesise it would have been carried out.

Investigation of the unassigned bands and peaks in the HPIR and HPNMR may provide further information in the mechanism of the hydroformylation reaction, alternatively some of these peaks may have arisen from the isolated dimer complex. However, no isolated spectroscopy was carried out on the complex.

From what was observed and identified, it does appear clear that the mechanism of the perfluorinated ligand system is very similar to that of triphenylphosphine, but with the strongly electron withdrawing nature of the perfluorinated ligand, the activity and selectivity of this ligand are better than those for the triphenylphosphine system.

The continuous rig may also require some minor alterations to improve its efficiency and overall performance; the introduction of a second reactor either to

preform the catalyst or to be used as a second reactor to increase the overall conversion and the reintroduction of a heat exchanger ought to be investigated. Improvement of the separator or inclusion of a cold trap, to limit the loss of solvent through the vent gas may also improve the economics of the reactor, if the recovered solvent could be recycled.

Ni and co-workers have developed an oscillatory-baffled reactor (OBR)1 that provides a novel method of mixing a reactor containing a series of baffles, which create eddies when the reaction mixture/fluid passes through them. This alternative method of stirring may improve the activity of the reaction, by providing better mixing within the reactor and is a possible area of further research, as the improvement in conversion upon introduction of the gas entrainment stirrer was notable.

This process is not yet ready to compete with current methods of industrial hydroformylation and the high cost of the perfluorinated ligands and solvents may prevent this ever becoming a large-scale industrial process. Yet, this work has shown that fluorous biphasic catalysis is a feasible method of hydroformylation, which provides high selectivity and conversion, with a facile method of catalyst recovery. Other uses, such as small-scale organic synthesis, may benefit from the fluorous biphasic principal better than larger scale processes.

The work described shows the realisation of the concept and the first reported high-pressure continuous-flow fluorous biphasic reaction in a custom built rig.

5.2 Reference List