Concentration of Carbon Dioxide by Electrochemically Modulated Complexation with a Binuclear Copper Complex

Abstract

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 = α,α‘-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)(μ-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%.