Journal of the American Chemical Society, Vol.143, No.4, 1984-1992, 2021
Supramolecular Gold Stripping from Activated Carbon Using alpha-Cyclodextrin
We report the molecular recognition of the Au(CN)(2)(-) anion, a crucial intermediate in today's gold mining industry, by alpha-cyclodextrin. Three X-ray single-crystal superstructures-KAu(CN)(2)subset of alpha-cyclodextrin, KAu(CN)(2)subset of(alpha-cyclodextrin)(2), and KAg(CN)(2)subset of(alpha-cyclodextrin)(2)-demonstrate that the binding cavity of alpha-cyclodextrin is a good fit for metal-coordination complexes, such as Au(CN)(2)(-) and Ag(CN)(2)(-) with linear geometries, while the K+ ions fulfill the role of linking alpha-cyclodextrin tori together as a result of [K+center dot center dot center dot O] ion-dipole interactions. A 1:1 binding stoichiometry between Au(CN)(2)(-) and alpha-cyclodextrin in aqueous solution, revealed by H-1 NMR titrations, has produced binding constants in the order of 10(4) M-1. Isothermal calorimetry titrations indicate that this molecular recognition is driven by a favorable enthalpy change overcoming a small entropic penalty. The adduct formation of KAu(CN)(2)subset of alpha-cyclodextrin in aqueous solution is sustained by multiple [C-H center dot center dot center dot pi] and [C-H center dot center dot center dot anion] interactions in addition to hydrophobic effects. The molecular recognition has also been investigated by DFT calculations, which suggest that the 2:1 binding stoichiometry between alpha-cyclodextrin and Au(CN)(2-) is favored in the presence of ethanol. We have demonstrated that this molecular recognition process between alpha-cyclodextrin and KAu(CN)(2) can be applied to the stripping of gold from the surface of activated carbon at room temperature. Moreover, this stripping process is selective for Au(CN)(2)(-) in the presence of Ag(CN)(2)(-), which has a lower binding affinity toward alpha-cyclodextrin. This molecular recognition process could, in principle, be integrated into commercial gold-mining protocols and lead to significantly reduced costs, energy consumption, and environmental impact.