CO₂ capture from ambient air via crystallization with tetraalkylammonium hydroxides

Aqueous solutions of a series of short carbon chain tetra(n-alkyl)ammonium hydroxides, [N][OH] with n = 2: n-ethyl, 3: n-propyl, 4: n-butyl, have been serendipitously found to be potential candidates for direct air carbon capture (DAC) when being used as reagents in more complicated reactions. Aqueous solutions of [N₃₃₃₃][OH], [N₂₂₂₂][OH], or [N₃₃₃₃][OH] with UO₂SO₄·3H₂O and 1,4-diamidoximylbenzene, and [N₄₄₄₄][OH] with cytosine (HCyt) directly absorb CO₂ from the atmosphere upon mild heating in the open atmosphere crystallizing in complexes reaching up to 2 : 1 CO₂/[N]OH ratio. [N₂₂₂₂][HCO₃]·3H₂O (1), [N₂₂₂₂]₂[H(HCO₃)₃]·5H₂O (2), [N₃₃₃₃][HCO₃]·0.5H₂O (3), [N₃₃₃₃][H(HCO₃)₂] (4), [N₃₃₃₃]₂[(tpa)(H₂CO₃)₂] (5; tpa = terephthalate), [N₄₄₄₄][H(Cyt)(HCO₃)]·H₂O (6) and [N₄₄₄₄][H₂(Cyt)₂(HCO₃)]·H₂O (7) have been isolated in crystalline form and structurally characterized by single crystal X-ray diffraction. The compounds are characterized by complex polyanionic formations from bicarbonate dimers ([(HCO₃)₂·(H₂O)]₂^4-) or chains ([H(HCO₃)₂] or [H₂(tpa)(HCO₃)₂]²) to water-bicarbonate associates ([(HCO₃)₂·6H₂O]^2- and [(H₂CO₃·(HCO₃)₂)₂·6H₂O·2H₂O]^2-) and three-component anionic layers ([H(Cyt)(HCO₃)·H₂O] and [H₂(Cyt)₂(HCO₃)·H₂O]) frequently showing proton sharing. While some hydroxides themselves can maintain a high CO₂/[N][OH] ratio, particularly 2 and 4, the presence of secondary hydrogen bond donors/acceptors may increase the sorption efficiency through decreased solubility and enhanced crystallization.