High-Efficiency Gold Recovery Using Cucurbit[6]uril

Developing an extremely efficient and highly selective process for gold recovery is urgently desired for maintaining a sustainable ecological environment. Herein, we report a highly efficient gold-recovery protocol on the basis of the instantaneous assembly between cucurbit[6]uril (CB[6]) and [AuX4]- (X = Cl/Br) anions. Upon mixing CB[6] with the four gold-bearing salts MAuX4 (M = H/K, X = Cl/Br) in aqueous solutions, yellow or brown coprecipitates form immediately, as a result of multiple weak [Au-X···H-C] (X = Cl/Br) hydrogen-bonding and [Au-X···C=O] (X = Cl/Br) ion-dipole interactions. The gold-recovery efficiency, based on CB[6]·HAuCl4 coprecipitation, reaches 99.2% under optimized conditions. In the X-ray crystal superstructures, [AuCl4]- anions and CB[6] molecules adopt an alternating arrangement to form doubly connected supramolecular polymers, while [AuBr4]- anions are accommodated in the lattice between two-dimensional layered nanostructures composed of CB[6] molecules. DFT calculations have revealed that the binding energy (34.8 kcal mol-1) between CB[6] molecules and [AuCl4]- anions is higher than that (11.3-31.3 kcal mol-1) between CB[6] molecules and [AuBr4]- anions, leading to improved crystallinity and higher yields of CB[6]·MAuCl4 (M = H/K) coprecipitates. Additionally, a laboratory-scale gold-recovery protocol, aligned with an attractive strategy for the practical recovery of gold, was established based on the highly efficient coprecipitation of CB[6]·HAuCl4. The use of CB[6] as a gold extractant provides us with a new opportunity to develop more efficient processes for gold recovery.