Iodine-Catalysed Dissolution of Elemental Gold in Ethanol

Abstract

Gold is a scarce element in the Earth's crust but indispensable in modern electronic devices. New, sustainable methods of gold recycling are essential to meet the growing eco-social demand of gold. Here, we describe a simple, inexpensive, and environmentally benign dissolution of gold under mild conditions. Gold dissolves quantitatively in ethanol using 2-mercaptobenzimidazole as a ligand in the presence of a catalytic amount of iodine. Mechanistically, the dissolution of gold begins when I₂ oxidizes Au⁰ and forms a [Au^I I₂ ]^- species, which undergoes subsequent ligand-exchange reactions and forms a stable bis-ligand Au^I complex. H₂ O₂ oxidizes free iodide and regenerated I₂ returns back to the catalytic cycle. Addition of a reductant to the reaction mixture precipitates gold quantitatively and partially regenerates the ligand. We anticipate our work will open a new pathway to more sustainable metal recycling with the utilization of just catalytic amounts of reagents and green solvents.

Keywords: Catalysis; Gold; Iodine; Recycling; Sustainable Chemistry.

Figure 1

Different approaches to dissolving gold in water or in organic solvents. Gold can be dissolved with various methods that are accompanied with different problems related to sustainability of the process. Widely used cyanide leaching (a) is environmentally burdensome and toxic. More than stoichiometric amounts of moderately toxic iodine are needed for quantitative dissolution of gold in water (b, iodine–iodide leaching) or in organic solvents (c), which makes these methods less affordable. Expensive ligands in excess amounts (d) are often needed to dissolve gold in not necessarily sustainable solvents (DMF). Our new approach (e) combines catalytic amount of iodine and inexpensive compounds to dissolve gold in green solvent (EtOH), which makes this method safe and sustainable.

Figure 2

Mechanism of I₂‐catalysed Au dissolution. Elemental Au is oxidized by I₂ and dissolves in EtOH as 1 (identified form ESI‐HRMS negative mode as species with m/z 450.7750). After, 1 undergoes a substitution reaction on Au^I centre where one iodide ligand is exchanged with one 2‐MBI molecule and formed species 2 is detected as [M−H]⁻ with m/z 472.8870. Free iodide(s) is re‐oxidized by H₂O₂ to generate I₂. The second iodide in 2 is again replaced with yet another 2‐MBI molecule and stable species 3 is produced and identified from ESI‐HRMS spectra in positive and negative mode as [M]⁺ (m/z 497.0162) and [M−2 H]⁻ (m/z 494.9998), respectively. The second iodide is again regenerated by H₂O₂ and formed I₂ can return to the start of the catalytic cycle. The change of Gibbs free energy (ΔG) for the substitution reaction of first and second iodide was calculated for high (first value) and low concentration (value in brackets) of I₂ at TPSS‐D3/def2‐TZVP level. OH⁻ or SO₄ ²⁻ can serve as a counterion for 3.

Figure 3

Au dissolution curve in 24 h. Dissolution process was monitored by FAAS for 24 h and dissolution curve was plotted against time. k ₁, k ₂, and k ₃ are predominant dissolution rates at different times of the reaction progress (0–2 h, 2–13 h and 13–24 h, respectively). They were calculated as tangential rates with slopes of 41.7 % h⁻¹, 3.5 % h⁻¹, and −1.1 % h⁻¹, respectively. I₂ concentration drop is manifested as a discoloration of the reaction after 2 h.

Figure 4

Au dissolution and relative intensities for species 1, 2, and 3 in the first 3 h. Intensities for species 1 ([M]⁻ with m/z 451), 2 ([M−H]⁻ with m/z 473), and 3 ([M−2 H]⁻ with m/z 495) were monitored by ESI‐HRMS in negative mode for the first three hours of the dissolution reaction. The highest intensity was set to 100 % and other data was adjusted accordingly. Intensities were combined with FAAS dissolution curve and plotted against time. Intensities of 1 and 2 rise at the beginning and are then constantly present to some extent, while the intensity for 3 closely follows the dissolution curve (which is in agreement with the proposed mechanism). At 3 h, the ratio between 1, 2, and 3 reaches 1.1/7.7/100.0.

Figure 5

2‐MBI derivatives and another Au species identified in the reaction mixture. Different species were identified from ESI‐HRMS in positive mode and ¹H NMR spectrum of the reaction mixture, including disulphide 5, sulphide 6, and trisulphide 7. Benzimidazole (8) and another Au species, (4) were also detected by ESI‐HRMS but with low intensity.

Figure 6

Precipitation of Au and recycling of 2‐MBI with NaBH₄. NaBH₄ was added directly to the scaled‐up reaction mixture with 20 mg of dissolved Au. Black quantitative precipitate was collected and identified with SEM and EDS as elemental Au with 92 % isolated yield. After evaporation of reaction solvent and quenching of the residue with diluted aqueous HCl, pure 2‐MBI was regenerated with 41 % yield.