5. Conclusion
The reported work demonstrated experimentally controllable reaction rates by low voltage external electric fields for the catalytic transfer hydrogenation of acetophenone. The influence of applied electric potential on reaction performance was evaluated experimentally and the controllable mass transfer of reactive species by electric fields was supported by simulation results. Significant improvements were shown at low values of voltage relative to a control of zero volts, with an optimum voltage of positive15 V. Increases beyond this value to positive 30 V and 50 V showed significant reduction in performance due to lower equilibrium concentration of Ru catalyst at the interface and possible formation and migration of the alkoxide intermediate during reaction. The degradation products from the electrolytic corrosion of the stainless-steel electrodes did not significantly impact the hydrogenation reaction. The influence of different external electric fields on the concentrations of reactive species at the interface was suggested as the main reason for the differences in reaction performance. Enantiomeric excess values were measured, and no significant changes were observed with variation in the positive voltage values. A mechanism and reaction rate expression were proposed based on the observations, suggesting the importance of external electric fields in reactions involving reactive ions.
The direction of the applied electric field was proved to be important as both reaction conversion and enantioselectivity were significantly reduced when the electric field was in the negative orientation. A monotonic increase in conversion was observed as voltage increased from negative 5 V to negative 50 V due to possible temperature increase resulted from the increase of current. Catalyst decomposition by the influence of negative voltages was concluded as the main reason for reduced enantioselectivity, as implied from the NMR analysis.
Overall, this work extends the scope for controllable synthesis of organic reactions conducted in two phase liquid systems and raises an opportunity for green engineering with minimal energy consumption in the proposed reaction system. It is also important for understanding the influence of external electric fields on mass transfer and kinetics in biphasic liquid systems used for organic synthesis and interfacial catalysis on macroscale.