Volume 96 Issue 5 | p. 8 | News of The Week
Issue Date: January 29, 2018 | Web Date: January 23, 2018

Shaking up gold and palladium

Mechanochemical method makes noble metal compounds without solvents or harsh reagents
By Mark Peplow, special to C&EN
Department: Science & Technology
Keywords: Inorganic chemistry, gold, palldium, mechanochemistry, catalyst, noble metal
Mechanochemistry created colorful metal salts, including (NH4)2PdCl4, (NH4)2PdBr4, NH4AuCl4 and NH4AuBr4 (top, right to left), which are soluble in water (bottom).
Credit: Tomislav Friščić
Four transparent jars contain metal salts that are orange, brown, yellow and red; four vials contain the same salts dissolved in water.
Mechanochemistry created colorful metal salts, including (NH4)2PdCl4, (NH4)2PdBr4, NH4AuCl4 and NH4AuBr4 (top, right to left), which are soluble in water (bottom).
Credit: Tomislav Friščić

Sometimes, chemistry can be a real grind. But for Tomislav Friščić at McGill University, that’s the whole point. His team has developed a mechanochemical method that turns noble metals like gold and palladium into useful salts and complexes without needing solvents or harsh reagents (Angew. Chem. Int. Ed. 2018, DOI: 10.1002/anie.201712602).

Noble metals are widely used in applications including as catalysts and in electronics, but generating soluble noble metal compounds requires some aggressive conditions. A common way to make their chloride salts, for example, involves reacting the metals with aqua regia, a mixture of concentrated nitric and hydrochloric acids.

Friščić’s team came up with a convenient and less hazardous alternative. The chemists mixed metal powder, pellets, or wire with potassium peroxymonosulfate, an oxidizing agent commonly known as Oxone, and simple halide salts such as potassium chloride or ammonium chloride. Then they put the mixture in a zirconia jar with a 1-cm-wide zirconia ball and gave it a ferocious mechanical shake at room temperature for up to 90 minutes.

This ball milling process created a range of noble metal halides in almost quantitative yields, which the researchers then dissolved in water and recrystallized to isolate salts such as potassium tetrachloropalladate (K2PdCl4). Bromide salts worked just as well, although the yield of iodides was lower. “It’s a remarkable result that illustrates the value of mechanochemistry,” says Carsten Bolm, a mechanochemistry researcher at RWTH Aachen University. “It’s a very benign protocol.”

By adding ligands to the dry mixture and milling it for another 30 minutes, the researchers also created various gold and palladium complexes, including grams of bis(triphenylphosphine)palladium chloride, which they successfully used as a catalyst in Suzuki coupling reactions to create biphenyl compounds. This one-pot approach to producing metal complexes means that “you’re doubling up the solvent savings,” by skipping a separate step of mixing the milled metal salts and ligands in solution, says Duncan L. Browne, who works on mechanochemistry at Cardiff University.

Friščić’s team could even use palladium catalyst waste as the starting material in this process to make fresh Suzuki catalyst, effectively recovering 89% of the palladium from the original spent catalyst. Friščić thinks this could help researchers to recycle valuable catalysts that are often discarded: “In a lab, there are a lot of noble metals that go to waste,” he says.

Friščić is now discussing the process with companies in the metal refining industry and hopes to explore whether it could be used for other kinds of noble metal recycling. “I’m looking forward to getting my hands on some electronic waste and seeing how good we are at extracting gold from it,” he says.

CORRECTION: This story was updated on Jan. 29, 2018, to correct the formulas in the image caption for the two gold compounds.

Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Nasrin hooshmand (Wed Jan 24 15:29:50 EST 2018)
That is a great news for first time to get ride of solvents or harsh reagents!
S. LAL (Thu Jan 25 06:29:51 EST 2018)
I read the technical brief in C & E News about Mechano-chemical formation of soluble noble salts. I would request the authors if they could send me a complete copy this article viae-mail.

I shall have comments and questions after reading the full text.

Please share this technical article with. Thanks.

Dr. S. Lal ( ACS Member, USA- PA )
Dr. Harmon B. Abrahamson (Mon Jan 29 09:59:49 EST 2018)
The formulas of the gold compounds in the figure caption are incorrect. There should be only one ammonium ion per gold, giving gold its common oxidation state of +3. (The formulas are correct in the original Angew. Chem. article.)
Michael Torrice (Mon Jan 29 14:03:00 EST 2018)
Thank you for pointing out that error, Dr. Abrahamson. We've corrected the formulas.
Mohammad Boshir Ahmed (Wed Jan 31 17:51:17 EST 2018)
Great news for chemists, chemical engineers and mechanical engineers. I will definitely read this article. Thanks to the authors for publishing such an important work.
Bob Bucsh (Thu Feb 01 13:02:52 EST 2018)
This reminds a bit of my synthesis of dirhenium octocarbonyl. I had a chlororhenate for starting material and I ground it with an excess of silver sponge. I passed carbon monoxide over the heated mixture and collected the carbonyl on a cold finger. In light of this work I wonder if the reaction started before the carbon monoxide was added.

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