High-Efficiency Gold Recovery Using Cucurbit[6]uril
Huang Wu, Leighton O. Jones, Yu Wang, Dengke Shen, Zhichang Liu, Long Zhang, Kang Cai, Yang Jiao, Charlotte L. Stern, George C. Schatz, J. Fraser Stoddart
Abstract
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.