A Facile Synthesis of Precious Metal Single Site Catalysts using Organic Solvents
Gold based single site catalysts (SSCS) for the direct acetylene hydrochlorination reaction are often prepared using harsh solvent such as aqua regia. We have developed a new and facile way of making SSCS that can be applied to make several different types of precious metal catalysts.
By Tanja Parmentier and Grazia Malta
High metal-loading HgCl2/C catalysts have been used to produce vinyl chloride monomer (VCM), precursor to polyvinylchloride (PVC), via the acetylene hydrochlorination process since the 1950s. During the past decade, the manufacture of VCM via this process has increased markedly in China (ca. 13 million p.a.), partly because of the availability of coal as a feedstock for acetylene production. However, after the “Minamata convention on Mercury” in 2013,  the use of mercury-free catalyst in this large-scale industrial process became mandatory. The recent commercialization of Au/C catalyst for this reaction by Johnson Matthey at an industrial scale  has resulted from nearly 35 years of research, when Hutchings, in 1985, predicted that gold should be the best catalyst for this reaction . The validation of an Au/C catalysts for this important industrial process has prompted a lot of interest in this reaction and, especially, in the investigation of the nature of active site for the design of mercury-free catalysts.
Using operando X-ray absorption spectroscopy (XAS), we identified the atomically dispersed cationic gold chloride species as the active site for this reaction . The identification of Au single-site catalysts (SSCS) as active materials for the acetylene hydrochlorination reaction, prompted us to find alternative and more facile methods to prepare Au/C SSCs, moving away from the conventional use of aqua regia.
By screening of different solvents, we found that their polarity played a key role in the synthesis of SSCs. Using the acetylene hydrochlorination setup, we tested several Au SSCs and we were very excited to find excellent activity for the catalyst prepared using acetone as impregnation solvent.
Could this facile method of making Au/C SSCs be applied for the preparation of other precious metal SSCs? The answer is yes! We prepared Pt/C, Ru/C and Pd/C SSCs catalysts following the same preparation method. These catalysts were all tested for acetylene hydrochlorination reaction and, although they were not as active as the Au/C catalyst, the same order of activity was found as in their aqua regia prepared counterpart , as you can see in the figure below (Figure 1).
Figure 1 – Correlation between the steady-state productivity of acetylene hydrochlorination at 100 min of reaction and the standard electrode potential of the constituent catalyst metal chlorides.
The combination of two characterisation methods,XAS and aberration-corrected scanning transmission electron Microscopy (AC-STEM), , provided reliable evidence of the stability of these SSCs, even after the harsh acetylene hydrochlorination reaction conditions. These experiments were performed at the Diamond Light Source,the UK’s synchrotron facility.
Thus far, we have successfully made 4 different active and stable carbon supported precious metal SSCs using this new and facile preparation method. In the next future, we are looking at the synthesis of other type of metal catalysts and exploring new reactions and applications.
Interested in reading more? Our Nature Chemistry paper can be found at: https://eur03.safelinks.protec...
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