Porous organic polycarbene applied for gold recovery from e-waste

The extraction of precious metals from electronic waste water is a long-term challenge for sustainable economy, as thus, we are motivated to study and construct high-performance extracting materials for the selective stripping of precious metals e.g. gold from a mess of waste solutions.
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Background

Gold, well known for its unique physical and chemical properties, is widely applied in jewellery, chemical catalysis, electronic devices, etc. Hence, it is widely present in a broad range of scrap materials, e.g., in electronic wastes (e-wastes)1,2. The continuously growing consumption and the limited supply of gold, which is an irreplaceable high-value element, create a pressing need for increased production and recycling3. As a consequence, the exploration of improved functional material-enabled selective and effective methods for “gold metallurgy” continues, particularly for urban mining of gold from e-wastes because the metal abundance of these materials is 10-100 times higher than that of natural ores.

Hydrometallurgy employs a more environmentally friendly and energy-efficient treatment by using digestive solutions to precipitate gold ions from e-waste, followed by selective gold ion extraction from the complex digestive solutions4. Chemical/physical adsorption is an environmentally benign option for gold recovery. Hence, a plethora of extraction materials, e.g., porous organic polymers (POPs), have been applied for the sustainable utilization of gold resources. Despite good performance in gold recovery, the tedious synthetic procedures of these materials and their high-cost precursors for metal recovery impede their further applications.

Herein, we introduced a poly(ionic liquid)-derived porous organic polycarbene (POPcarbene) adsorbent as the effective extracting materials for gold stripping. The role of N-heterocyclic carbene was combined with porous structures and leads to a highly effective “nanotrap” or “molecular sieve” for metal ions.

Design Strategy

Typically, the cation-methylene-nitrile functionality sequence in the repeating unit of this poly(ionic liquid), in which the pendent nitrile (-CN) groups could undergo coupling reactions; this reaction was catalysed at room temperature by NH35. By taking advantage of an ammonia-catalysed molecular crosslinking mechanism, poly(1,2,4-triazolium)s as polycarbene precursors was used to construct a covalently locked porous polymer. We first chose ‘vigorous stirring’ method, by adding polymer solution dropwise into an excess of ethanol containing 0.5 wt% NH3 under vigorous stirring. Insoluble POPcarbene aggregates formed immediately. However, the resulting materials exhibited a lower specific surface area of only 93 m2 g-1, which was not good for mass transfer in the adsorption process. Because the porosity was determined as it allows high-density packing of carbene units for metal binding, an essential factor governing the gold-capturing efficiency of the adsorbent. In this regard, we have farther improved the preparation process for this POPcarbene adsorbent. By introducing thermally induced phase separation (TIPS) process and supercritical CO2 drying, the resulted materials presented 332 m2g-1, with an adsorption performance for gold up to 2.09 g/g.

Adsorption-Reduction Mechanism

The remarkable adsorption performance for Au3+ prompted us to study Ptriaz-CN-A more comprehensively. X-ray photoelectron spectroscopy along with nuclear magnetic resonance spectroscopy reveals that the high performance of the POPcarbene adsorbent results from the formation of robust metal-carbene bonds plus the ability to reduce nearby gold ions into nanoparticles. Density functional theory calculations indicate that energetically favourable multinuclear Au binding enhances adsorption as clusters.

Behind the result

The tricky problem we encountered during the experiment was how to characterize the C-Au bond. Although the proton at position 5 of the 1,2, 4-triazolium group in Ptriaz-CN is very active, only in a proton-rich environment can deprotonation occur and C-5 position will then coordinate with metal ions in solution. However, the peak strength of C-Au bond is generally weak in 13C NMR, which requires higher sample concentration and longer detection time. For the selection of deuterium reagents, although Ptriaz-CN is soluble in DMSO, proton exchange cannot occur in aprotic environment; water is a common protic solvent, but Ptriaz-CN is insoluble in water. Considering the solubility of Ptriaz-CN, we firstly tried to dissolve 100 mg of Ptriaz-CN and 0.5 eq Au3+ in a H2O/DMSO-d6 mixture (v/v = 1:5) solvent. However, this solvent environment is not proton-rich and the proton exchange rate is still very slow even after a long-time stirring, and thus C-Au bond cannot be detected in the 13C NMR. Based on this, we modified our experimental pathways. Instead of following the sample preparation conditions of 13C NMR, we supposed that we could directly put Ptriaz-CN into Au3+ aqueous solution under stirring to form C-Au bond in proton-rich environment at first. In particular, 100 mg of Ptriaz-CN was put into 20 mL 0.5 eq Au3+ aqueous solution to stir overnight. After that, the powder was filtered and washed with a large amount of deionized water. At this point, the sample changed from pure white to golden yellow. After drying, the sample was dissolved in 0.6 mL DMSO-d6 to perform the 13C NMR test for 6 h. After these attempts, we finally find signal of C-Au bond.

As an inspiring result, we hope our findings will strengthen efforts to expand carbene chemistry further to materials science and promote the development of porous carbene or porous polycarbene materials.

For more details on this work, please read our paper in Nature Communications at https://doi.org/10.1038/s41467-023-35971-w.

References

  1. Wang, J., Zeng, B., Lv, J., Lu, Y. & Chen, H. Environmentally friendly technology for separating gold from waste printed circuit boards: a Combination of suspension electrolysis and a chlorination process. ACS Sustainable Chem. Eng. 8, 16952-16959 (2020).
  2. Syed, S. Recovery of gold from secondary sources—A review. Hydrometallurgy 115-116, 30-51 (2012).
  3. Dodson, J. R. et al. Bio-derived materials as a green route for precious & critical metal recovery and re-use. Green Chem. 17, 1951-1965 (2015).
  4. Jadhav, U. & Hocheng, H. Hydrometallurgical recovery of metals from large printed circuit board pieces. Sci. Rep. 5, 14574 (2015).
  5. Dong, Z. et al. A cationitrile sequence encodes mild poly(ionic liquid) crosslinking for advanced composite membranes. Mater. Horizons 7, 2683-2689 (2020).

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