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.