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.