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-nnnn][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 [N-3333][OH], [N-2222][OH], or [N-3333][OH] with UO2SO4.3H(2)O and 1,4-diamidoximylbenzene, and [N-4444][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-nnnn]OH ratio. [N-2222][HCO3].3H(2)O (1), [N-2222](2)[H(HCO3)(3)].5H(2)O (2), [N-3333][HCO3].0.5H(2)O (3), [N-3333][H(HCO3)(2)] (4), [N-3333](2)[(tpa)(H2CO3)(2)] (5; tpa = terephthalate), [N-4444][H(Cyt)(HCO3)].H2O (6) and [N-4444][H-2(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)](n)(n-) or [H-2(tpa)(HCO3)(2)](n)(2n-)) to water-bicarbonate associates ([(HCO3)(2).6H(2)O](2-) and [(H2CO3.(HCO3)(2))(2).6H(2)O.2H(2)O](2-)) and three-component anionic layers ([H(Cyt)(HCO3).H2O](n)(n-) and [H-2(Cyt)(2)(HCO3).H2O](n)(n-)) frequently showing proton sharing. While some hydroxides themselves can maintain a high CO2/[N-nnnn][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.