title: Cation-Dependent Gold Recovery with α-Cyclodextrin Facilitated by Second-Sphere Coordination.
abstract: Herein, we report an alkali metal cation-dependent approach to gold recovery, facilitated by second-sphere coordination with eco-friendly α-cyclodextrin (α-CD). Upon mixing eight salts composed of Na(+), K(+), Rb(+), or Cs(+) cations and [AuX4](-) (X = Cl/Br) anions with α-, β-, or γ-CD in water, co-precipitates form selectively from the three (out of 24) aqueous solutions containing α-CD with KAuBr4, RbAuBr4, and CsAuBr4, from which the combination of α-CD and KAuBr4 affords the highest yield. Single-crystal X-ray analyses reveal that in 20 of the 24 adducts CD and [AuX4](-) anions form 2:1 sandwich-type second-sphere adducts driven partially by [C-H···X-Au] interactions between [AuX4](-) anions and the primary faces of two neighboring CDs. In the adduct formed between α-CD and KAuBr4, a [K(OH2)6](+) cation is encapsulated inside the cavity between the secondary faces of two α-CDs, leading to highly efficient precipitation owing to the formation of a cation/anion alternating ion wire residing inside a continuous α-CD nanotube. By contrast, in the other 19 adducts, the cations are coordinated by OH groups and glucopyranosyl ring O atoms in CDs. The strong coordination of Rb(+) and Cs(+) cations by these ligands, in conjunction with the stereoelectronically favorable binding of [AuBr4](-) anions with two α-CDs, facilitates the co-precipitation of the two adducts formed between α-CD with RbAuBr4 and CsAuBr4. In order to develop an efficient process for green gold recovery, the co-precipitation yield of α-CD and KAuBr4 has been optimized regarding both the temperature and the molar ratio of α-CD to KAuBr4.
authors: Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States.
authors: Zhichang
authors: Z
authors: Liu
authors: Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States.
authors: Avik
authors: A
authors: Samanta
authors: School of Resources and Environmental Engineering, East China University of Science and Technology , 130 Meilong Road, Shanghai 200237, China.
authors: Juying
authors: J
authors: Lei
authors: Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States.
authors: Junling
authors: J
authors: Sun
authors: Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States.
authors: Yuping
authors: Y
authors: Wang
authors: Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States.
authors: J Fraser
authors: JF
authors: Stoddart
mesh_terms: D011232:Chemical Precipitation; D006046:Gold / Q000737:chemistry*; D054879:Halogenation; D008958:Models, Molecular; D008968:Molecular Conformation; D014867:Water / Q000737:chemistry; D047391:alpha-Cyclodextrins / Q000737:chemistry*
chemical_list: D047391:alpha-Cyclodextrins; D014867:Water; D006046:Gold; C032613:alpha-cyclodextrin