Sulfur, the yellow stuff with a whiff of the bad egg smell, is mentioned 13 times in the Bible and at present lacks the glamorous image of newer ‘meta’ materials. But bolt on ultra-modern materials chemistry and sulfur has been rebooted into new compounds with incredible new properties that could be used to clean our air and water, and provide components for everything from batteries to optics, all while being far more recyclable than plastics.
Dr Tom Hasell at the University of Liverpool is at the forefront of industrially useful sulfur chemistry. He’s interested in porous materials with high surface areas that could have large-scale applications in gas storage, catalysis and filtration. But many trendy new materials are limited by the high cost of production. “We are developing new porous materials from inorganic waste and other low cost or renewable resources,” says Hasell. “A good example is sulfur-polymers, and we recently showed that polymers made from elemental sulfur can be used to filter mercury from water.”
Improving on existing processes
More than 70 million tonnes of sulfur is made each year as an industrial by-product of oil refining. That means its available for just $100 per ton and it’s a win-win to use a waste material from an industrial process that can foul soils by changing its pH. However, polymers made just from neat sulfur are not stable, and decompose back to a powder, even at room temperature.
The game-changer was rediscovering and improving on a process called ‘inverse vulcanisation’ by which sulfur polymers are made – a combination of sulfur and other organic ‘crosslinker’ compounds that tether sulfur polymers together, making them more stable and preventing them from decomposing. Hasell and colleagues found that adding a small amount of a catalyst (metal diethyl dithiocarbamates) would increase reaction rates as well as chemical yield, and lower the temperature needed, thus avoiding the production of highly undesirable hydrogen disulphide gas.
“This makes inverse vulcanization more widely applicable, efficient, eco-friendly and productive than previous techniques,” says Hasell. He adds that this not only broadens knowledge of the fundamental chemistry itself, but opens the door to wider industrialisation because sulfur is a very different element to the carbon found in all plastics, so sulfur polymers have some really interesting properties.
For example, unlike in carbon polymers, infra-red light goes through sulfur polymer lenses providing optical applications like thermal imaging lenses. Then there are novel antimicrobials, more stable lithium batteries, and porous sulfur compounds that can be used to capture harmful (and valuable) heavy metals like gold and mercury, benefitting human health and the environment. Some combinations contain up to a whopping 90% sulfur by weight.
Hasell has utilised collaborations across the world to work towards realising these applications. Closer to home, the Materials Innovation Factory at the University of Liverpool is a world-leading centre were Hasell can characterise the compounds he makes in the chemistry department. “We have great access to the expertise of the people there and the most advanced shared equipment,” he says. “We use techniques like gel permeation chromatography, infrared spectroscopy and powder diffraction to analyse our polymers."
"We’ve also collaborated with Dr John Griffin, University of Lancaster, who used solid-state nuclear magnetic resonance (NMR) to help us understand the structures of the polymers that formed.”
Hasell adds that he thinks Liverpool is at the forefront in the UK in terms of materials chemistry and discovery, a forward-looking place with drive and momentum that includes a collegiate feel and support among young academics.