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Single atom catalysts boost efficiency and cut waste in fine chemical production

Related publication: Chen Z., Vorobyeva E., Mitchell S., Fako E., Ortuño M. A., López N., Collins S. M., Midgley P. A., Richard S., Vilé G. & Pérez-Ramírez J. A heterogeneous single-atom palladium catalyst surpassing homogeneous systems for Suzuki coupling. Nat. Nanotechnol. 13, 702–707 (2018). DOI: 10.1038/s41565-018-0167-2

Publication keywords: Heterogeneous catalysis; Atomic resolution imaging

The chemical industry produces a tremendous amount of waste. Often this arises from the need to separate the product from a solution containing by-products and a dissolved catalyst. 85% of all industrial chemical processes use catalysts, chemicals that speed up a reaction but do not get used up in the process, and they are often expensive metals such as palladium.

An international team of researchers sought an efficient solid catalyst that would avoid waste. They investigated a material designed to contain individual atoms of palladium supported by carbon nitride, which showed improved performance in a reaction to produce new carbon-carbon bonds.

The E02 microscope at the electron Physical Science Imaging Centre (ePSIC) is capable of seeing individual atoms, without causing excessive damage to the sample. Using E02, the team were able to obtain high-quality images of the individual palladium atoms, before and after use in chemical reactions. Their results confirmed that the atoms occurred as isolated single atoms and were retained throughout the use of the catalyst. With additional computational work, they were able to show that the single palladium atoms on the carbon nitride hit the sweet spot: they are available to interact with molecules in the desired chemical reaction, but they are stable enough to be used over and over again.

: Figure 1: (a) Structural models for four Pd catalysts studied for C-C coupling reactions. (b) Conversion efficiency and selectivity for the four Pd catalysts in a selected test reaction producing biphenyl.
Adapted by permission from Springer Nature: Nature Nanotechnology, see Related Publication, copyright 2018.

Industrial production of fine chemicals depends critically on catalysis. In fact, 85% of all industrial processes in the chemical industries are catalytic processes¹. Catalysts operate by reducing activation energies in key mechanistic steps during a chemical reaction, leading to increases in the rate of reaction at accessible temperatures. The catalyst, often relying on a transition metal species for carrying out key steps, is not used up during the reaction, and a relatively small quantity of the catalyst can participate in reactions, in principle, many times over to convert starting materials to a specific product.

Broadly, catalysts can be further subdivided into homogeneous and heterogeneous catalysts. Homogeneous catalysts occur as molecules in solution, such as in the form of organometallic complexes. Heterogeneous catalysts are solids with which gaseous or liquid-phase reagents interact. In many heterogeneous catalysts, transition metal nanoparticles, clusters of metal atoms, or molecular complexes constitute the active catalyst, and are anchored to an inert support material. Common support materials are oxides such as silica and alumina.

Homogeneous catalysts are associated with higher levels of waste production during catalysis due to the requirement to separate the catalyst from the final product, as well as from any by-products. While heterogeneous catalysts can be used in processes that intrinsically segregate the flowing products from the stationary solid catalyst, there are many reactions where the homogeneous catalysts significantly outperform heterogeneous systems. Both heterogeneous and homogeneous catalysts may also suffer from catalyst degradation, reducing their catalytic activity over time.

A particular class of reactions requiring the formation of carbon-carbon bonds, known as cross-coupling reactions, have been carried out most successfully with homogeneous catalysts. The Suzuki coupling reaction, recognised by the 2010 Nobel Prize in Chemistry, is traditionally carried out with palladium (Pd) organometallic complexes in solution², such as Pd(PPh₃)₄ (tetrakis(triphenylphosphine)palladium), and Pd(dtbpf)Cl₂ ([1,1′ -bis(di-tert-butylphosphino)ferrocene] dichloropalladium (Fig. 1). Alternatives using solid supports for organometallic complexes have been proposed, including silica functionalised by 3-mercaptopropyl sulphide to anchor a Pd acetate complex³. However, the most successful heterogeneous catalysts exhibit significantly lower conversion rates in many cross-coupling reactions in comparison with their homogeneous counterparts.

: Figure 2: Atomic resolution aberration-corrected electron micrographs (a) before and (b)
after reaction. White arrows highlight a selection of single Pd atoms, appearing as isolated
bright spots in the micrographs. The scale bars are 1 nm. Adapted by permission from Springer
Nature: Nature Nanotechnology, see Related Publication, copyright 2018.

An alternative to homogeneous catalysts, and those heterogeneous designs anchoring Pd complexes to an inorganic support, has emerged with the development of single atom catalysts on carbon and nitrogen derived materials, such as graphitic carbon nitride⁴. Graphitic carbon nitride is a polymeric material consisting of sheets of carbon and nitrogen atoms analogously to graphene. Conceptually, the ability of graphitic carbon nitride to form bonds with bare, single Pd atoms suggests a design chemically very similar to the coordination environment of the most active homogeneous Pd cross-coupling catalysts.

To examine the performance of this proposed catalyst, Pd atoms were deposited onto exfoliated graphitic carbon nitride (ECN) using a microwave-irradiation assisted method. For a particular Suzuki coupling reaction to form biphenyl, the conversion rate and the selectivity were recorded for a number of candidate catalysts (Fig. 1). The Pd-ECN catalyst exhibited the highest figures of merit among the top performing heterogeneous and homogeneous catalysts. Additionally, the Pd-ECN catalyst retained its conversion efficiency for more than 12 hours of reaction, whereas the conversion efficiency of the next best catalyst (the homogeneous Pd(PPh₃)₄ catalyst) decreased by more than a factor of two, after four hours. The catalyst was also studied for many other cross-coupling reactions and showed comparable or higher conversion efficiency and selectivity relative to the best performing Pd(PPh₃)₄ homogeneous catalyst.

In order to understand these performance characteristics, a microscopic study of the catalyst, as originally prepared and after reaction, was carried out (Fig. 2). Electron microscopy is one of the few, if not only, methods capable of resolving individual atoms in these materials. Light microscopes are unable to resolve atoms due to the diffraction limit associated with the wavelength of light. High-energy electrons accelerated to a significant fraction of the speed of light, however, have associated wavelengths much smaller than the size of an atom. The lenses used in electron microscopes, though, are limited by aberrations arising from the physics of focusing electrons with magnetic fields. Sophisticated aberration correctors comprising several small magnetic elements are required to offset the lens aberrations sufficiently to focus electron beams to the size required to resolve single Pd atoms supported on carbon nitride.

Moreover, carbon- and nitrogen-based materials are easily damaged by high-energy electrons. ePSIC houses a state-of-the-art aberration corrected electron microscope which is capable of high spatial resolution at electron energies as low as 60 keV. This capability for operating at relatively low electron beam energy enabled high-quality images to be formed from Pd single atoms on graphitic carbon nitride (Fig. 2).

In tandem with this experimental work, Density Functional Theory calculations were carried out to model the mechanisms associated with Pd- ECN and Pd(PPh₃)₄. These computational results point to how the Pd atoms on ECN are located within an adaptive coordination environment necessary for catalysing several key mechanistic steps. At the same time, the Pd atoms on ECN are bound with sufficient strength to preclude detachment and loss of Pd during the reaction or aggregation of Pd atoms which might otherwise result in fewer catalytically active Pd sites over time.

The development of catalysts with reduced waste and improved atom economy are two objectives at the core of green chemistry, the effort to minimise harmful environmental effects of chemical processing. Moreover, the identification of catalysts that require lower loadings of precious metals with improved lifetime offers key commercial benefits as well. The newly developed single atom Pd catalyst supported on graphitic carbon nitride capitalises on each of these aspects by improving yield, selectivity, and durability. The conclusive demonstration of the single atom character of this catalyst hinged on advanced imaging capabilities for resolving individual atoms made possible through cutting-edge electron microscopy.

References:

Thomas J. M. et al. Some Turning Points in the Chemical Electron Microscopic Study of Heterogeneous Catalysts. ChemCatChem 5, 2560–2579 (2013). DOI: 10.1002/cctc.201200883
Miyaura N. et al. Palladium-Catalyzed Cross-Coupling Reactions of Organoboron Compounds. Chem. Rev. 95, 2457–2483 (1995). DOI: 10.1021/cr00039a007
Molnár Á. Efficient, Selective, and Recyclable Palladium Catalysts in Carbon−Carbon Coupling Reactions. Chem. Rev. 111, 2251–2320 (2011). DOI: 10.1021/cr100355b
Chen Z. et al. Stabilization of Single Metal Atoms on Graphitic Carbon Nitride. Adv. Funct. Mater. 27, 1605785 (2017). DOI: 10.1002/ adfm.201605785

Funding acknowledgement:

ETH Zurich; Swiss National Science Foundation (200021-169679); MINECO (CTQ2015-68770-R); Severo Ochoa Excellence Accreditation 2014-2018 (SEV-2013-0319); Juan de la Cierva-Incorporación postdoctoral program (IJCI-2016-29762); Girton College (Cambridge, UK); EPSRC (EP/R008779/1).

Corresponding author:

Dr Sean M Collins, Department of Materials Science and Metallurgy, University of Cambridge, smc204@cam.ac.uk

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Single atom catalysts boost efficiency and cut waste in fine chemical production