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.