Selective isolation of gold facilitated by second-sphere coordination with α-cyclodextrin

Abstract

Gold recovery using environmentally benign chemistry is imperative from an environmental perspective. Here we report the spontaneous assembly of a one-dimensional supramolecular complex with an extended {[K(OH₂)₆][AuBr₄](α-cyclodextrin)₂}n chain superstructure formed during the rapid co-precipitation of α-cyclodextrin and KAuBr₄ in water. This phase change is selective for this gold salt, even in the presence of other square-planar palladium and platinum complexes. From single-crystal X-ray analyses of six inclusion complexes between α-, β- and γ-cyclodextrins with KAuBr₄ and KAuCl₄, we hypothesize that a perfect match in molecular recognition between α-cyclodextrin and [AuBr₄](-) leads to a near-axial orientation of the ion with respect to the α-cyclodextrin channel, which facilitates a highly specific second-sphere coordination involving [AuBr₄](-) and [K(OH₂)₆](+) and drives the co-precipitation of the 1:2 adduct. This discovery heralds a green host-guest procedure for gold recovery from gold-bearing raw materials making use of α-cyclodextrin-an inexpensive and environmentally benign carbohydrate.

Figure 1. Schematic representation of the spontaneous self-assembly of α·Br.

Upon mixing KAuBr₄ and α-CD in water, a hydrogen-bonded linear superstructure forms spontaneously in <1 min. The cavities of the α-CDs oriented head-to-head, tail-to-tail form a continuous channel, which is filled by an alternating [K(OH₂)₆]⁺ and [AuBr₄]⁻ polyionic chain to generate a cable-like superstructure that then tightly packs one with another (Supplementary Fig. S1) leading to crystals observable to the naked eye.

Figure 2. Formation and co-precipitation of α·Br from KAuBr₄ and α-CD.

When an aqueous solution (20 mM, 1 ml) of KAuX₄ (X=Cl or Br) is added to an aqueous solution (26.7 mM, 1.5 ml) of α-, β-, or γ-CD, a shiny pale brown suspension forms exclusively from the combination of KAuBr₄ and α-CD within 1–2 min (See Supplementary Movie 1).

Figure 3. Morphology of the nanostructures of α·Br.

(a) SEM images of a crystalline sample prepared by spin-coating an aqueous suspension of α·Br onto a silicon substrate, and then air-drying the suspension. (b) TEM images of α·Br prepared by drop-casting an aqueous suspension of α·Br onto a specimen grid covered with a thin carbon support film and air-dried. (c) Cryo-TEM image (left) and SAED pattern (right) of the nanostructures of α·Br. As the selected area includes several crystals with different orientations and the crystals are so small that the diffraction intensities are relatively weak, we can assign the diffraction rings composed of diffraction dots but not the specific angles between different diffraction dots from the same crystal. The scale bars in a and b are 25 (left), 5 (right), 10 (left), 5 μm (right) and in c are 1 μm (left) and 1 nm⁻¹ (right), respectively.

Figure 4. Single-crystal X-ray structure of α·Br.

The structure has the formula {[K(OH₂)₆][AuBr₄](α-CD)₂}_n. (a) Side-on view showing the orientation of the primary and secondary faces of the α-CD rings in the extended structure. (b) Side-on view illustrating the second-sphere coordination of the [K(OH₂)₆]⁺ ion with the [AuBr₄]⁻ ion. (c) Top view of the arrangement of the [AuBr₄]⁻ ion inside the cavity of α-CD. (d) Schematic illustration of the one-dimensional nanostructure extending along the c-axis in which the α-CD tori form a continuous channel occupied by alternating [K(OH₂)₆]⁺ and [AuBr₄]⁻ ions. Hydrogen atoms areomitted for clarity. C, black; O, red; Br, brown; Au, yellow; K, purple. Hydrogen bonds are depicted as purple dash lines.

Figure 5. AFM analysis of α·Br on a mica surface.

(a) AFM image of a spin-coated sample of α·Br on a freshly cleaved mica surface. (b) The cross-sectional analysis of (a). (c) Dimensions of the cross-section of the one-dimensional α-CD channel in α·Br. Scale bar, 100 nm.

Figure 6. Single-crystal superstructures of α·Br, α·Cl, β·Br, β·Cl, γ·Br and γ·Cl.

The rotation angle of the [AuX₄]⁻ anion viewed from the front is defined as φ, and the inclination angle of the [AuX₄]⁻ anion viewed from the side with respect to the central axis of the CD tori is defined as θ. C, black; O, red; Br, brown; Cl, green; Au, yellow; K, purple.

Figure 7. Selective precipitation and separation of gold.

The separation percentage, from mixtures 1 (red) and 2 (blue), is defined as (C_b−C_a)/C_b, where C_b and C_a are the concentrations of each metal before and after addition of α-CD, respectively.

Figure 8. Gold recovery process flow diagram.

Red arrows indicate the flow direction of the gold recovery. Na₂S₂O₅: Sodium metabisulfite.