Led by Thomson Reuters Citation Laureates, Chemists are Going for Gold
For centuries gold was the way that humankind either hoarded its wealth or adorned itself. Its unchanging nature ensured that it remained attractive, but few practical uses were found for the metal. Nor was it of much interest to chemists. Today gold is a hot topic, both academically and industrially. Indeed, as a catalyst it promises to have a major role in greening the world’s chemical industry, by making it possible to produce the things we need by processes that are low in energy input and high in product output.
Already the manufacture of vinyl acetate monomer (VAM), the precursor to the poly-vinyls used as adhesives, fibers, and paints, is starting to rely on a gold-based catalyst developed by Johnson Matthey. The chemical company INEOS has such a plant at Hull, England, which produces 300,000 tons of VAM a year. Another process that might soon benefit from gold catalysts is the making vinyl chloride for PVC.
THE “GRAND OLD MAN”
Did Masatake Haruta, then at Japan’s Government Industrial Research Institute, realize that he was opening a Pandora’s box of delights, not troubles, when he revealed his research on the oxidation of CO to CO2 by means of a gold catalyst? That was in 1985, and the paper appeared in Abstracts of Papers of the American Chemical Society; this reference to his work has never been cited (see the accompanying table, paper #1). Although his discovery appeared to go unnoticed at the time, today Haruta is fêted as the grand old man of gold catalysis, with thousands of citations to his work.
Most-cited Reviews, and Key Reports 2011 – 2013
|1||M. Haruta, H. Sano, “Composite oxide catalysis containing silver or gold for the low-temperature oxidation of hydrogen and carbon-monoxide,” Abstracts of Papers of the American Chemical Society, 189: 171, 1985. [Government Industrial Research Institute, Osaka, Japan]||—|
|2||A. Hashmi, G.J. Hutchings, “Gold catalysis,” Angewandte Chemie Int. Ed., 45 (47): 7896-936, 2006. [University of Cardiff, Wales, UK]||1,162|
|3||M. Haruta, et al., “Novel gold catalysts for the oxidation of carbon-monoxide at a temperature far below 0 degrees C,” Chemistry Letters, 2: 405-8, 1987. [Government Industrial Research Institute, Osaka, Japan]||1,148|
|4||M. Haruta, “Size- and support-dependency in the catalysis of gold,” Catalysis Today, 36 (1): 153-66, 1997. [Osaka University, Japan]||2,209|
Reports published since 2011
|5||H.-L. Jiang, et al.,“Synergistic catalysis of Au@Ag core-shell nanoparticles stabilized on metal-organic framework,” J. Amer. Chem. Soc., 133(5): 1304-6, 2011. [Japan Science and Technology Agency, Saitama, Japan]||86|
|6||C.G. Silva, et al., “Influence of excitation wavelength (UV or visible light) on the photocatalytic activity of titania containing gold nanoparticles for the generation of hydrogen or oxygen from water,” J. Amer. Chem. Soc., 133 (3): 595-602, 2011. [Polytechnic University of Valencia, Spain]||71|
|7||M. Murdoch, et al., “The effect of gold loading and particle size on photocatalytic hydrogen production from ethanol over Au/TiO2 nanoparticles,” Nature Chemistry, 3: 489-92, 2011. [University of Aberdeen, Scotland, UK]||57|
|8||H. Weimin, et al., “An efficient [2+2+1] synthesis of 2,5-disubstituted oxazoles via gold-catalzyed intermolecular alkyne oxidation,” J. Amer. Chem. Soc., 133(22): 8482-5, 2011. [University of California, Santa Barbara]||49|
|9||J.A. Lopez-Sanchez, et al., “Facile removal of stabilizer-ligands from supported gold nanoparticles,” Nature Chemistry, 3(7): 551-6, 2011. [Cardiff University, Wales, UK]||46|
|10||A. Corma, et al., “Gold catalyzes the Sonotgashira coupling reaction without the requirement of palladium,” Chem. Commun., 47(5): 1446-8, 2011.
[University of Valencia, Spain]
|11||M.N. Hopkinson, et al., “AuI/AuIII catalysis: an alternative approach for C-C oxidative coupling,” Chemistry – A European Journal, 17(30): 8248-62, 2011. [University of Oxford, UK.]||42|
|12||K.J. Kilpin, et al., “Gold(1) ‘click’ 1.2.3-triazolylidenes: synthesis, self-assembly and catalysis,” Chem. Commun., 47: 328-30, 2011. [University of Waikato, New Zealand]||42|
|13||J.-F. Soulé, et al., “Powerful amide synthesis from alcohols and amines under aerobic conditions catalysed by gold or gold/iron, -nickel or –cobalt nanoparticles,” J. Amer. Chem. Soc., 133(46): 18550-3, 2011.
[University of Tokyo, Japan]
|14||X. Gu, et al., “Synergistic catalysis of metal-organic framework-immobilized Au-Pd nanoparticles in dehydrogenation of formic acid for chemical hydrogen storage,” J. Amer. Chem. Soc., 133(31): 11822-5, 2011. [National Institute of Advanced Science and Technology, Osaka, Japan]||41|
|SOURCE: Thomson Reuters Web of Science|
Haruta, in fact, was honored last year by being selected among the 2012 class of Thomson Reuters Citation Laureates—researchers whose scientific advances (and substantial citation records) clearly identify them as being “of Nobel class,” deserving of science’s highest prize.
When I turned to Thomson Reuters Research Fronts PDF in preparing this article and submitted the term “gold catalysis,” the resulting core papers included a list headed by reviews, of which the first (#2) was by one of the world’s leading gold chemists, Graham Hutchings of Cardiff University, Wales. Hutchings, as it happens, also earned Citation Laureate status last year, and was actually paired with Haruta as a conceivable winner of the Nobel Prize in Chemistry someday.
Hutching’s review has collected nearly 1,200 citations. In fact, all the first seven core articles were reviews and each one had more than 600 citations. Gold is clearly being researched by chemists around the world, and interest in this element could not be greater.
THE NANOGOLD DIFFERENCE
Gold had never been thought of as a potential catalyst, unlike the other noble metals such as platinum. The fact that it remained unaffected after thousands of years in prehistoric graves seemed to suggest it had a chemical stability that was not conducive to catalysis. What Haruta did was show that the other form of gold, colloidal gold, had very different properties. Colloidal gold had been discovered in the Middle Ages and was used as a pigment to color glass; it was called Purple of Cassius. Today we refer to it as nanogold.
Haruta had stumbled upon a use of nanogold that had been overlooked, and in 1987 he wrote a full paper on his ground-breaking work, and that has now been cited more than a thousand times (#3). Ten years later was to come his paper which has attracted more than twice as many citations (#4), reflecting the recognition that is owed to Haruta, now a professor at Tokyo Metropolitan University.
Haruta demonstrated that gold in the form of clusters of atoms of diameter 5 nm or less can act as a remarkable and selective catalyst, especially for reactions involving oxygen O2. He has shown that the catalytic activity depends on cluster size, method of preparation, and support material. His methods of making nanogold, using wet or dry processes, are now widely applied. In some cases they can produce clusters with fewer than 20 atoms although 200-atom clusters are the norm. Gold can catalyze all kinds of reactions including the hydrogenation of unsaturated carbonyl compounds, previously regarded as the domain of platinum and palladium catalysts.
Hutchings, head of the Catalysis Center of Cardiff University, has been able to extend the range of reactions that can advantageously be catalyzed by gold nanoparticles, such as the epoxidation of propylene and the formation of secondary amines.
In addition to treating highly cited reviews on gold catalysis, I wanted to look at recent research papers. Therefore, in using the Web of Science to prepare the balance of the table, I limited my selection to papers published in the past two years and, of those, to ones that have attracted more than 40 citations. These contain some remarkable findings.
The most-cited paper does not relate to gold alone but to gold/silver nanoparticles and comes from Qiang Xu of the Japan Science and Technology Agency, reporting work done at the National Institute of Advanced Industrial Science and Technology (AIST) at Osaka, Japan, and Tokyo Metropolitan University. He refers to the catalyst as Au@Ag core-shell and these are immobilized on a metal-organic framework produced by a sequence of deposition and reduction methods. The resulting material can be fine-tuned to achieve catalytic abilities in excess of other types of catalyst (#5).
The second-most-cited paper in the list comes from a group headed by Hermenegildo Garcia of the Polytechnic University of Valencia, Spain, which reports that nanogold particles on a titanium dioxide support can decompose water to its elements when exposed not only to UV but to visible light (#6). The most active catalyst contained 0.2% w/w gold and gave yields of 7.5% H2 and 5.0% O2 when exposed to light of wavelength 560 nm (green). Garcia also features in another oft-cited paper in table, #10, discussed further below.
The next paper (#7) also reports on the generation of hydrogen, in this case from ethanol. The object of this research—from the University of Aberdeen, Scotland, and led by Russell Howe—was to determine the best kind of gold catalyst. What size of nanogold particles and which form of titanium dioxide would perform best? The answer was that gold particles, in the range of 3-30 nm, were most active, and that the much rarer mineral form of titanium dioxide known as anatase was a hundred times better than the rutile form.
Gold catalysts now come in various guises. The one used in #8 is called by the somewhat odd title of BrettPhosAuNTf2. The BrettPhos ligand is actually dicyclohexyl-[3,6-dimethoxy-2-(2,4,6-triisopropylphenyl)phenyl]phosphane; its molecular formula is C35H53O2P and it is named after Brett Fors, who devised it. The other component of the catalyst is NFt2 which is short for nuclear transport factor 2. This is a complex protein designed to transport things to the nucleus of a living cell. Liming Zhang of the University of California, Santa Barbara, California, has shown that BrettPhosAuNTf2 acts as a superb catalyst for the synthesis of oxazoles with substituents attached to the 2,5- positions, and in yields of up to 94%. More recently Zhang has used the same gold catalyst for the selective dimerization of aliphatic terminal alkynes—see Synlett, 23(1); 54-56, 2012.
The Cardiff group has addressed the issue of the removal of the ligands attached to nanogold that are necessary to stabilize it during synthesis. However, once the nanoparticles have been deposited on a substrate, these ligands will reduce the catalytic activity and so need to be removed. Previously this has been achieved by rather crude methods such as heating or oxidation, which can affect the efficacy of the catalyst itself. Now Hutchings shows that the ligands can be extracted by aqueous means and without affecting the morphology of the nanoparticles (#9).
Gold has been shown to be an effective catalyst for what it known as Sonogashira coupling, named after its discoverer in 1975. This reaction results in carbon-carbon bond formation between a terminal alkyne and an aryl halide, and it occurs under mild conditions. It has proved particularly useful in the synthesis of new pharmaceuticals. Paper #10, as mentioned above, represents further work by Hermenegildo Garcia’s group at the University of Valencia. In this paper, he reported that gold would perform this reaction in place of the combination of palladium and copper catalyst normally used.
The formation of carbon-carbon bonds is also the subject of a “concept” paper, in this case from Matthew Hopkinson’s group at the University of Oxford, England. Gold was shown to be capable of catalyzing oxidative homo- and cross-coupling reactions that produced such bonds by bringing together reactive partners that are not easily activated using other catalysts (#11).
The self-assembly of macrocycles incorporating gold(1) as “pre-catalysts” has been the object of research headed by Kelly J. Kilpin at the University of Waikato in New Zealand, and this was achieved via the so-called “click” method (#12) – see ScienceWatch March/April 2007. This allows the ready formation of 1,2,3-triazolylidenes that make an ideal ligand for this metal.
GREENING WITH GOLD
Gold catalysts for greening the chemical industry are extolled in the paper by Shu Kobayashi of the University of Tokyo, whose work is attracting international recognition. It reports the synthesis of commercially important amides from alcohols and amines and using oxygen as an oxidant (#13). The paper not only deals with gold as the catalyst, but of its combinations with iron, nickel and cobalt.
Recently, Kobayashi has published a similar paper reporting the formation of imines from alcohols and amines, with oxygen as an oxidant (see Chemical Communications, 49; 355-357, 2013). In this case the catalyst was gold and palladium and it was embedded in a polymer with carbon black added to act as a stabilizer—and it was reusable, a feature that might one day be of commercial significance.
Another paper (#14), which comes from the AIST in Japan, also reports gold that is immobilized. The gold-palladium clusters were trapped in a metal-organic framework known as MIL-101 (MIL refers to Materials from Institut Lavoisier) and were found to be highly active for the conversion of formic acid to hydrogen of a quality convenient for hydrogen storage. Moreover, the gold-palladium nanoparticles were able to tolerate CO without their catalytic activity being compromised, as tends to happen with other noble metals.
The future of gold as a catalyst could hardly have been foreseen when Haruta began his research more than 25 years ago. Nevertheless the chemical literature is now full of reports on its role. What chemists once regarded as a relatively uninteresting element has shown a side of its nature that promises a better world. Few elements could promise more.
Dr. John Emsley is based at the Department of Chemistry, Cambridge University, UK.
The data and citation records included in this report are from Thomson Reuters Web of KnowledgeSM. Web of KnowledgeSM is a registered trademark of Thomson Reuters. All rights reserved.