The issues surrounding widely used plastics, such as PVC, are at the forefront of the nation's minds as our war on plastics intensifies. However, our problem with plastic isn't limited to waste alone; the processes involved in plastic production also cause separate issues in their own right.
It's likely you've never heard of vinyl chloride monomer, or VCM, a large-scale chemical intermediate used as a precursor for PVC. But we annually produce a staggering 13 million tons of this chemical using an extremely volatile and toxic catalyst. This catalyst has been used by industry for decades, but there is growing interest in an environmentally friendly alternative – with a gold-on-carbon catalyst being recently validated as a less volatile replacement.
Researchers from the UK, China and the USA have used the ISIS Neutron and Muon Source to study the activity and reaction mechanisms of this environmentally-friendly catalyst. They hope their findings will help design catalysts with extended lifetimes in future.
An important industrial reaction
VCM is produced by acetylene hydrochlorination - a process traditionally catalysed by the volatile and toxic mercuric chloride. Environmentally friendly, single-site gold species supported on carbon have also been shown to be active sites for acetylene hydrochlorination.
Single-site gold species supported on carbon have been shown to be the active sites for acetylene hydrochlorination. Image credit: G. Malta et al. ACS Catal. 2018, 8, 8493−8505 DOI: 10.1021/acscatal.8b02232.
However, there is still much we don't know about this process, most notably the nature of the acetylene species interacting with the catalytically active gold species and the effect of the acid media on carbon surface species. Prof. Graham Hutching of Cardiff Catalysis Institute (CCI) adds, “studying the deactivation of the catalyst will provide a more profound understanding of the catalytic behaviour and the reaction mechanism. This will allow researchers to generate the information needed to design catalysts that exhibit extended lifetimes under industrial operating conditions."
Inelastic neutron diffraction at the limit of sensitivity
Researchers used vibrational spectroscopy to fill in the gaps in our knowledge of some of the processes occurring on the active gold on carbon catalysts. Due to the use of carbon supports, neutrons were the only viable option to probe these surface species “as the carbon support in the catalyst would be highly absorbent of infrared radiation, we used inelastic neutron scattering spectroscopy" explains Dr. Simon Freakley.
ISIS Neutron and Muon Source's spectrometer Merlin was used to take inelastic neutron scattering measurements of the sample. As the data required was at the limit of sensitivity for inelastic neutron scattering the high flux of Merlin offered researchers the best chance of success.
Grazia Malta summarises the group's findings, “we were able to measure the inelastic neutron spectra of carbons with different acid treatments and correlate these observed surface species with the known activity of these catalysts. Our measurements of the spectrum of acetylene bound to the gold on carbon catalysts has also helped improve our understanding of the reaction mechanism. These findings will help us rationally design better catalysts for the future."
As a nation unable to kick our plastic habit this research into alternative environmentally friendly catalysts goes some way in reducing the harmful consequences associated with plastic production. With plastic use, and thus our need for VCM, on the rise there is an active interest from industry and academia alike to reduce the environmental impact of this process .
Read the full research publication here: G. Malta et al. “Deactivation of a Single-Site Gold-on-Carbon Acetylene Hydrochlorination Catalyst: An X‑ray Absorption and Inelastic Neutron Scattering Study" ACS Catal. 2018, 8, 8493−8505 DOI: 10.1021/acscatal.8b02232.
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