Aqueous Electrochemical and pH Studies of Redox-Active Guanidino Functionalized Aromatics for CO2 Capture

Escalating levels of carbon dioxide (CO₂) in the atmosphere have motivated interest in CO₂ capture and concentration from dilute streams. A guanidino-functionalized aromatic 1,4-bis(tetramethylguanidino)benzene (1,4-btmgb) was evaluated both as a redox-active sorbent and as a pH swing mediator for electrochemical CO₂ capture and concentration. Spectroscopic and crystallographic studies demonstrate that 1,4-btmgb reacts with CO₂ in water to form 1,4-btmgbH₂(HCO₃^–)₂. The product suggests that 1,4-btmgb could be used in an aqueous redox pH swing cycle for the capture and concentration of CO₂. The synthesis and characterization of the mono- and diprotonated forms (1,4-btmgbH⁺ and 1,4-btmgbH₂²⁺) and their pK_a values were measured to be 13.5 and 11.0 in water, respectively. Electrochemical pH swing experiments indicate the formation of an intermediate radical species and other degradation pathways, which ultimately inhibited fully reversible redox-induced pH cycling.