Concentration of carbon dioxide by electrochemically modulated complexation with a binuclear copper complex

The reactions of bicarbonate ion with a series of binuclear Cu(II) complexes in buffered aqueous solution have been studied, and effective binding constants for bicarbonate have been determined at pH 7.4 for the complexes Cu2(taec)]4+ (taec = N,N',N',N'-tetrakis(2-aminoethyl)-1,4,8,11-tetraazacyclotetradecane) and Cu2(tpmc)(OH)]3+ (tpmc = N, N',N',N'-tetrakis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane). Cu2(o-xyl-DMC2)]4+ (o-xyl-DMC2 = alpha,alpha'-bis(5,7-dimethyl-1,4,8,11-tetraazacyclotetradecan-6-yl)-o-xylene) did not react with bicarbonate ion in an aqueous solution buffered at this pH. The complexes were reduced by controlled-potential electrolysis, and the stability of the Cu(I) derivatives in aqueous solution and their affinity for bicarbonate/carbonate ion were investigated. On the basis of these fundamental studies, Cu2(tpmc)(mu-OH)]3+ has been identified as an air-stable, water-soluble carrier for the capture and concentration of CO2 by electrochemically modulated complexation. The carrier binds to the carbonate ion strongly in its oxidized, Cu(II) form and releases the ion rapidly when reduced to the Cu(I) complex. In small-scale electrochemical pumping experiments designed to demonstrate the feasibility of this approach, CO2 has been pumped from an initial 10% CO2/N2 mixture up to a final concentration of 75%.