Figure 2. Acetophenone conversion
(mol%) under different positive external electric field for 24 h with
stainless-steel electrodes and 1-butanol as solvent.
The increase in conversion with increased electric potential in the
first few hours is consistent with the role of external electric field
in controlling the reaction rates of the phase transfer hydrogenation
reaction by controlling mass transfer of the charged reactive species.
This is further supported by the simulation results in Figure 3. As we
can see, the rates of electromigration of both formate and Ru catalyst
were increased with the increase of voltage at the beginning. The
enhancement of reaction rate observed at higher electric field is
consistent with accelerated rates of transport of both formate anions
and the cationic Ru catalysts to the liquid-liquid interface, resulting
in more rapid reaction compared to that under lower electric potential.
The simulation results also suggested that the equilibrium concentration
of Ru catalyst at the interface is lower at higher voltage, which
explains the corresponding lower conversion at longer reaction times.
Additionally, a negatively charged alkoxide intermediate could be formed
during the reaction as suggested by Pavlova et al..37As the reaction continues, a portion of these alkoxide anions could be
attracted to the anode electrode and constrained in the region close to
it, preventing the reactive species from interacting and retarding the
reaction rates. This inhibition is greater when a greater electrostatic
force is applied, leading to the lower reaction rate as observed at an
elevated electric potential for longer reaction times.