Gold recovery using eco-friendly host-guest chemistry is desirable from an environmental perspective. In this talk, we will report a cation-dependent approach to gold recovery facilitated by second-sphere coordination making use of environmentally benign α-cyclodextrin (α-CD). Upon mixing a series of salts composed of the alkali metal cations Na+, K+, Rb+, Cs+ and square-planar anions [AuX4]− (X = Cl and Br) with α-CD in water, precipitates form selectively from aqueous solutions of α-CD with the salts formed between K+, Rb+, Cs+ and [AuBr4]−, from which the combination of α-CD and KAuBr4 gives the highest precipitation yield. The bulk co-precipitation is unique (selective) for α-CD and KAuBr4, and does not occur if β- or γ-CD is substituted for α-CD, or, if KAuCl4 is employed instead of KAuBr4. Single-crystal X-ray crystallographic analyses reveal that, in all complexes, α-CD with [AuX4]− form 2:1 sandwich-type second-sphere coordination adducts, driven by [C–H●●●X–Au] interactions between the [AuX4]− anion and the primary faces of two α-CDs. In the complex formed between α-CD and KAuBr4, a [K(OH2)6]+ cation is encapsulated between the secondary faces of two α-CDs, leading to a highly efficient precipitation as a result of the formation of an infinite, cation and anion alternating ion-wire residing inside a continuous α-CD nanotube. In contrast, in all the other complexes, cations are coordinated with the hydroxyl groups of α-CDs. The stronger coordination strength of Rb+ and Cs+ to the hydroxyl groups, in conjunction with the geometrically favorable binding of [AuBr4]− with α-CDs, facilitates the precipitation of two complexes between α-CD with the salts RbAuBr4 and CsAuBr4. In order to develop a practical green process for gold recovery, the yield of co-precipitation of α-CD and KAuBr4 was optimized in terms of temperature, concentration, and the molar ratio of α-CD to KAuBr4.

Cation-dependent gold recovery with α-cyclodextrin facilitated by second-sphere coordination

251st National Meeting (2016)

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