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The global chemical industry has been grappling with a big problem: manufacturing releases carbon dioxide. The industry emits nearly a billion tonnes of CO2 each year, more than double Australia’s national output.
Industrial chemicals, which end up everywhere from fertilisers on farms to polyester in T-shirts, require energy to make, sometimes in very high amounts.
But these materials also often have emissions built-in to their design, with fossil fuel feedstocks that release CO2 as they’re turned into other things.
One way to improve the industry’s footprint, according to three Australian chemists, is to use liquid metals with “atomic intelligence”.
“It’s very futuristic in my opinion,” says Professor Kourosh Kalantar-Zadeh, head of the school of chemical and biomolecular engineering at the University of Sydney.
“It’s actually entering science fiction.”
This does not, perhaps fortunately, mean self-assembling robots. The substances Kalantar-Zadeh is talking about are simple metals like tin, bismuth, mercury, and gallium – or alloys thereof.
These elements all have low melting points compared to most other metals, and so don’t need super-high temperatures to work with them as liquids.
But at the atomic scale, they have smart properties that can improve the way we make (or remove) other stuff, like fuel, pharmaceuticals, fertiliser, and plastics.
“There is a massive landscape they can offer to chemical engineering, and other fields as well,” says Kalantar-Zadeh, who has co-authored a perspective on liquid metals in Science.
The perspective argues that liquid metals can be very powerful catalysts: substances that provide pathways for chemical reactions to happen. Chemists and chemical engineers use catalysts to speed reactions up, make them work at much lower temperatures, and control the products of these reactions.
At the moment, many industrial reactions use solid catalysts to spur their reactions, where atoms sit in fixed arrangements.
“Here, we are adding one extra dimension of freedom to the system. Atoms can move around,” says Kalantar-Zadeh.
This means that the metals can process reactions much more dynamically, creating more complex products and giving us more influence over the reaction.
“I think the low hanging fruit is looking at basic production of hydrogen,” says Kalantar-Zadeh.
“Everybody’s talking about hydrogen. It still is very polluting.”
While hydrogen can be made in a zero-emissions way, it’s currently much cheaper to make it by taking methane (CH4) and turning it into hydrogen (H2) and carbon dioxide (CO2). The methane used is typically a mined fossil fuel, but it can come from other places like biogas and biomass.
Kalantar-Zadeh says that liquid metals can prevent CO2 from forming in this process.
“We could create a kind of a catalyst that doesn’t produce the C=O bond that becomes CO2,” he says. Instead, it could turn the leftover carbon atoms into long, ungainly molecules that become solid at room temperature.
“The byproduct doesn’t go to the air, we’d collect it for other purposes,” says Kalantar-Zadeh. Building materials might be a possible use. If waste biomass was used in the first place, this would be a carbon-negative way to make hydrogen.
Kalantar-Zadeh says that, in the short term, liquid metals could be also used to make ammonia for fertilisers, currently a very carbon-intensive process, and help to deal with plastic pollution.
“Liquid metals can really work well with removing plastic and reacting with long [polymer] chains, breaking them down into useful molecules that we want.”
Molten metal is not a new concept – indeed, it’s thousands of years old. Yet Kalantar-Zadeh says that liquid metals are a “forgotten field” in chemistry.
“Perhaps one of the biggest problems has been, since the 80s, that mercury has considered a really toxic, hazardous material. And liquid metal, until the 80s, was exemplified by mercury,” he says.
“In my childhood, we hold mercury in our hands. Nowadays, you cannot even buy mercury for a university, unfortunately.”
Gallium, with a melting point of 30°C, can melt in the palm of your hand, but mercury is the only metal that’s liquid at most room temperatures. The absence of mercury from labs led to low visibility for liquid metals.
“So many things we have forgotten to do, to touch, because they’re not in front of our eyes,” says Kalantar-Zadeh.
But now, both the pressure of the climate crisis, and the increased availability of other substances – like gallium, which is being mined more because it can be used in semiconductors and LEDs – has led to a renewed interest in the field.
“Suddenly, this field that was put aside for about three decades came back, and it came back at the right time,” says Kalantar-Zadeh.
And unlike mercury, gallium is very safe to work with. Tin and bismuth, both of which have melting points below 300°C, are also non-toxic metals with a lot of potential.
“So now we have a new vision. It’s safe and it’s cheap,” says Kalantar-Zadeh.
“The intelligence that we didn’t have access to now is in our hands.”
