Supramolecular Gold Stripping from Activated Carbon Using α-Cyclodextrin

We report the molecular recognition of the Au(CN)2– anion, a crucial intermediate in today's gold mining industry, by α-cyclodextrin. Three X-ray single-crystal superstructures—KAu(CN)2⊂α-cyclodextrin, KAu(CN)2⊂(α-cyclodextrin)2, and KAg(CN)2⊂(α-cyclodextrin)2—demonstrate that the binding cavity of α-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 α-cyclodextrin tori together as a result of [K+···O] ion−dipole interactions. A 1:1 binding stoichiometry between Au(CN)2– and α-cyclodextrin in aqueous solution, revealed by 1H NMR titrations, has produced binding constants in the order of 104 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⊂α-cyclodextrin in aqueous solution is sustained by multiple [C–H···π] and [C–H···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 α-cyclodextrin and Au(CN)2– is favored in the presence of ethanol. We have demonstrated that this molecular recognition process between α-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 α-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.