CO 2 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 UO 2 SO 4 ·3H 2 O and 1,4-diamidoximylbenzene, and [N 4444 ][OH] with cytosine (HCyt) directly absorb CO 2 from the atmosphere upon mild heating in the open atmosphere crystallizing in complexes reaching up to 2 : 1 CO 2 /[N nnnn ]OH ratio. [N 2222 ][HCO 3 ]·3H 2 O (1), [N 2222 ] 2 [H(HCO 3 ) 3 ]·5H 2 O (2), [N 3333 ][HCO 3 ]·0.5H 2 O (3), [N 3333 ][H(HCO 3 ) 2 ] (4), [N 3333 ] 2 [(tpa)(H 2 CO 3 ) 2 ] (5; tpa = terephthalate), [N 4444 ][H(Cyt)(HCO 3 )]·H 2 O (6) and [N 4444 ][H 2 (Cyt) 2 (HCO 3 )]·H 2 O (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 ([(HCO 3 ) 2 ·(H 2 O)] 2 4- ) or chains ([H(HCO 3 ) 2 ] n n - or [H 2 (tpa)(HCO 3 ) 2 ] n 2 n - ) to water-bicarbonate associates ([(HCO 3 ) 2 ·6H 2 O] 2- and [(H 2 CO 3 ·(HCO 3 ) 2 ) 2 ·6H 2 O·2H 2 O] 2- ) and three-component anionic layers ([H(Cyt)(HCO 3 )·H 2 O] n n - and [H 2 (Cyt) 2 (HCO 3 )·H 2 O] n n - ) frequently showing proton sharing. While some hydroxides themselves can maintain a high CO 2 /[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.