Mark Vender
New research conducted at UNSW has confirmed that HFOs – the next generation of synthetic refrigerants billed as a solution for the future – break down into fluoroform, an HFC with ultra-high global warming potential.
Hydrofluoroolefins (HFOs) were developed to replace hydrofluorocarbons (HFCs), which were found to have a high global warming potential (GWP). Through the Kigali Amendment to the Montreal Protocol, most countries now have plans to reduce their consumption of HFCs, and HFOs have been put forward by refrigerant manufacturers as a suitable alternative, due to their ultra-low GWP of less than 1.
But a paper led by Dr Christopher Hansen from UNSW Chemistry, and published in the Journal of the American Chemical Society, has demonstrated that HFOs break down into a small amount of fluoroform, also known as R23, which has a GWP of 14,800.
Chemistry lessons
HFOs break down quickly in the environment, meaning they don’t rise to the upper atmosphere and become long-lived greenhouse gases.
“But as chemists, we look at the structures of these molecules and we start to try and imagine what they are turned into,” says Dr Hansen. “So rather than just go, ‘Oh, this thing only has a lifetime of two weeks, it can’t be a greenhouse gas’, we must see what it’s turned into.
“And most chemists will look at these structures, and they can draw reactions that actually lead to HFCs.”
Dr Hansen and his team used multiple techniques, including two invented for this study, to measure and evaluate the chemical reaction across the full range of pressures expected in the atmosphere. They started with trifluoroacetaldehyde, the chemical produced when some HFOs break down.
“We used a variety of spectroscopic techniques to observe the reaction,” says Dr Hansen. “And we made up a gas mixture at various pressures to simulate an atmosphere polluted with a trace amount of the immediate HFO decomposition product. Then we used a laser to simulate the photons that would otherwise come from the sun to drive the reaction.”
HFOs have been shown to decompose into fluorinated carbonyls such as trifluoroacetaldehyde at a yield up to, or greater than, 100 per cent.
“This means all the molecules of HFO turn into the first product and, for some HFOs, you might get two molecules of that product for each molecule of HFO that breaks down,” says Dr Hansen. “This study reveals that the next step of the reaction, driven by light, produces a small amount of fluoroform from the decomposition of trifluoroacetaldehyde.”
Dr Hansen says the team has demonstrated comprehensively that some of the most important HFOs do break down into HFCs, and has provided the first hard scientific data needed to model and predict the consequences of large-scale emission.
“Although the reaction only produces a small amount of fluoroform, the chemical can exist in the atmosphere for up to 200 years, and with a global warming potential more than 14,000 times greater than CO2, a small yield can still have a significant impact.”
Trifluoroacetaldehyde – also known as CF3CHO or TFE – is a decomposition product for many HFOs.
“HFO-1234ze, for example, decomposes to produce CF3CHO in 100 per cent molar yield,” says Dr Hansen. “So the fate of CF3CHO is the fate of HFO-1234ze. We have shown that the yield depends strongly on pressure (altitude) and it would need to be modelled. However, from our own modelling, we infer an indirect GWP (from this channel alone) of <=10.”
Other experts also looking at this topic have put the number much higher than that. A paper published by Dr Gabriel Salerno in Chemistry Europe, for example, suggests that a 1.1 per cent conversion rate into R23 would lead to a GWP of >150 for several HFOs. This calls into question their status as ultra-low GWP refrigerants.
Precautionary principles
Dr Hansen notes that many atmospheric crises have caught the world by surprise.
“Think leaded petrol, lethal smog events of the 20th century, the ozone hole crisis,” he says. “This wasn’t because our models were not good enough, but rather because the important chemistry was missing from the models.
“Although questions remain, this paper offers crucial evidence that should inform the next steps in addressing the environmental impact of the chemicals we release into the atmosphere.
“We don’t fully understand the environmental impacts of HFOs at this point, but, unlike previous examples such as the CFCs and leaded petrol, we are trying to figure out the consequences of large-scale emissions before we’ve potentially harmed the environment and human health in an irreversible way. We’re trying to try to change the way that science introduces new products.”
Dr Hansen and his team are planning further novel experimental work using other wavelengths of light, where the yield could be higher or lower.
The findings around these downstream impacts add to concerns about HFOs breaking down into PFAS chemicals, and evidence that feedstocks and refrigerants such as R23 used in the manufacturing process for HFOs are escaping into the atmosphere.
Image, courtesy of UNSW, shows Joshua Thompson, a PhD student involved in the research operating the technology.
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