Researchers from the United Kingdom and the United States, including from the Cardiff Catalysis Institute at Cardiff University in Wales and Lehigh University in Pennsylvania, have demonstrated with synchrotron-based spectroscopy and advanced electron microscopy that gold ions effectively convert acetylene to vinyl chloride monomers (VCM) that are used in producing polyvinyl chloride (PVC).
An image obtained with scanning transmission electron microscopy and high-angle annular dark-field imaging shows isolated species of gold on a carbon support. The Cardiff-Lehigh team found that atomically dispersed atoms of gold achieve acetylene conversion most effectively.
According to a report on the breakthrough, the researchers were able to determine through advanced techniques that atomically-dispersed gold ions, where atoms are separated on carbon support, were the ideal catalysts for this reaction.
PVC is the third most used plastic in the world and acetylene hydrochlorination has long been the most common technique for converting acetylene, a coal by-product, into VCM. This reaction ca be volatile and allows for the release of toxic mercury into the environment. In the early 1980s it was shown that carbon-supported gold catalyst could be used instead, but the process required large amounts of expensive gold and was largely ignored.
“In 2007, Johnson Matthey, a global specialty chemicals company based in the U.K., became interested in Hutchings' results and began working to make a stable gold-on-carbon catalyst using less gold,” the article explained. “The company developed a catalyst named Pricat MFC, which has now gone into commercial use in a large Chinese PVC plant. China, the world's largest producer and consumer of PVC, still relies on coal to produce the VCM product.”
The 2013 Minamata Convention on Mercury, signed by nearly 140 nations, banned new VCM plants that use mercury chloride and expects all plants to be mercury-free by 2022. This new research, building on older work, is another step forward in fulfilling that goal.
The work was recently published in Science. The abstract from the report stated:
“There remains considerable debate over the active form of gold under operating conditions of a recently validated gold catalyst for acetylene hydrochlorination.
“We have performed an in situ x-ray absorption fine structure study of gold/carbon (Au/C) catalysts under acetylene hydrochlorination reaction conditions and show that highly active catalysts comprise single-site cationic Au entities whose activity correlates with the ratio of Au(I):Au(III) present.
“We demonstrate that these Au/C catalysts are supported analogs of single-site homogeneous Au catalysts and propose a mechanism, supported by computational modeling, based on a redox couple of Au(I)-Au(III) species.”