Mechanism of electrochemical capture of CO2 via redox cycle of chlorinated 1,4-naphthoquinone in BMIMBF4: An in-situ FT-IR spectroelectrochemical approach

The mechanisms of electrochemical capture of CO2 via redox cycle of 1,4-naphthoquinone (1,4-NQ), 2,3-dichloro-1,4-naphthoquinone (DCNQ) and 2-chloro-1,4-naphthoquinone (CNQ) in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) have been investigated by cyclic voltammetry (CV) and in-situ FT-IR spectroelectrochemistry techniques. In the absence of CO2, 1,4-NQ and DCNQ undergo a reversible two-step one-electron process while CNQ not. In addition, their two successive electron transfer processes in ionic liquid (IL) are different from that of molecular solvent. There is only a small difference in peak potential between two-step reduction, which possibly due to the ion-pair effects. Meanwhile, the reduction potentials of DCNQ and CNQ shift positively due to the electron-withdrawing effects of chlorine groups. The radical anion (Q−) and dianion (Q2−) formed during the electrochemical reduction can be used as nucleophiles to attack the eletrophilic carbon center of CO2 and form a stable CO2 adduct. For 1,4-NQ, 2 equiv. of CO2 can be captured during the electrochemical reduction of 1,4-NQ. Whereas, 1 equiv. of CO2 is captured by 1 equiv. of dianion during the electrochemical reduction of DCNQ and CNQ. Furthermore, CO2 adducts should be the carbonate type products according to the theoretical calculation results by Gaussian 09 package.