HAVE you ever stopped to consider the technological marvel that is the humble kitchen sponge? Liquids tend to be unruly, sploshy things, but, with a quick swipe, a sponge can soak up and transport them to wherever they are needed. It would all be rather miraculous were it not so familiar.
But here’s a question that is a little more out of the ordinary: could we make a liquid sponge? It would be kind of like the household variety, only it would suck up gases instead of liquids and it could be pumped over vast distances. That would make it incredibly useful. After all, we are in the middle of a climate emergency caused by the emission of greenhouse gases such as carbon dioxide and methane. A liquid sponge could provide a better way of sucking up those gases and preventing them from causing harm.
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You might expect the concept of a spongy liquid to be a non-starter. As most of us learned in school, a liquid is something that fills up the bottom of any container it is poured into – no holes or spaces allowed. Yet in labs around the world, chemists are creating a rich assortment of cleverly designed liquid sponges and putting them to the test. We are about to find out just how useful this quirky technology really is.
What are porous liquids?
The story of liquid sponges – sometimes called porous liquids – begins in 2007 when chemist began working at Queen’s University Belfast in the UK. He was researching solids known as metal-organic frameworks (MOFs), cage-like compounds made of metal ions and carbon-based molecules. The special thing about them is their extraordinary porosity: a single gram of a MOF can have pores in it with an internal surface area as large as a football pitch. In other words, MOFs are super sponges, the chemical structure of which can be tweaked so that they absorb very specific things. They can be used to mop up environmental toxins, for example, or to suck water vapour from the air.
There is a problem with MOFs, however, as James discovered one day when he got talking to his colleague , a chemical engineer. Rooney pointed out that engineers like him are loathe to work with solids because they are so awkward to handle when it comes to large-scale industrial chemistry. It is much easier to deal with large volumes of liquids than solids because the former can be easily pumped and stirred.
It got James pondering if it would be possible to turn a MOF into a liquid. From the start, he knew it was an outlandish thought. żěè¶ĚĘÓƵs design the structure of solid materials all the time by controlling where the constituent molecules sit in relation to each other. But in a liquid, those molecules are tumbling all over the place, so designing the structure of a liquid to be sponge-like sounds preposterous. James says that, at the time, he wondered: “Is it bonkers to think that you can actually design a liquid from the molecules?”
Within a few months, he had published a . All liquids have tiny and constantly shifting gaps between their molecules. This is why fish can breathe the oxygen dissolved in water: it is carried in these spaces. But James’s plan was to create liquids made of molecules that were, like MOFs, empty cages. This way, the liquid would be far more absorbent, with bigger holes, and could potentially hold much more significant amounts of gas.
James thought the simplest way to make a porous liquid would be to take a powdered MOF and melt it. He dubbed this a type 1 porous liquid and started trying to make one, working with at the University of Liverpool, UK. The trouble was, many MOFs are only liquid at high temperatures, often above 200°C, making them impractical. The researchers also found that the cage-like molecules would often break down when heated that much. “It was a brilliant idea,” says James. “Unfortunately, it didn’t work.”
He quickly came up with a possible workaround, however. One way to turn a solid into a liquid is to melt it – but another is to dissolve it, like the way we stir sugar into a cup of tea. There could be another kind of porous liquid, a type 2, reasoned James, comprising a cage-like molecule dissolved in a solvent.
To see why achieving this was anything but straightforward, picture the cage molecule and the solvent molecules as a mixture of doughnuts and wiggly strands of spaghetti. The trouble is that the holes of the doughnuts – the pores of the liquid sponge – can easily get clogged up by strands of spaghetti, preventing the liquid from soaking up anything else.
To get around this, James and his colleagues moved away from the metal-based MOFs and instead used large, cage-like molecules made predominantly of carbon atoms, which could be chemically tweaked more easily. They tried a huge range of different designs, but still had problems with the solvent getting inside the cage. Then, in 2015, they hit upon the idea of using an unusual solvent made of molecules called crown ethers. These aren’t shaped like spaghetti strands, but more like large dinner plates. They hit the sweet spot of both dissolving the cages and being so big that they couldn’t block the pores within the cages. This strange concoction of cage-like molecules dissolved in crown ether was .
Working with Margarida Costa-Gomes at the University of Lyon in France, James , comparing it with the neat crown ether. The results, published in 2015, were impressive. “Having the empty spaces increased the solubility of methane [in the liquid] by about a factor of eight,” says James.
Removing greenhouse gases
Since that proof of concept, James and Cooper have been creating other porous liquids. One thing they have been working on is getting the viscosity of the liquids to be as low as possible – think cooking oil rather than treacle – to make them easier to pump. They have also patented their concept and started a company called .
Last year, the company began running its first pilot-scale rig to make and test porous liquids, down the road from James’s lab in Belfast. The first goal is to test the ability of the liquid sponges to selectively suck up CO2 being given off by an anaerobic digester, essentially a tank of bacteria used to break down organic material such as food waste. This process gives off a range of gases – methane, hydrogen and CO2 among them. Ideally, we would separate and capture these so they can be reused. This does happen to some extent at the moment, through a process that involves solvents based on chemicals called amines. But it guzzles large amounts of energy, mainly because getting the greenhouse gases back out of the amine solvent is such hard work.
In the pilot rig, mixed gases from a digester get funnelled into a column containing the porous liquid, which selectively absorbs the CO2 . The liquid sponge then gets pumped into another container, where it is heated and put under vacuum so that the CO2 is “wrung out” of it. James says that initial tests suggest the liquid sponge can extract the CO2 using 17 per cent less energy compared with the existing method. “It’s a very exciting time,” he says.
There is plenty of scope for liquid sponges to save us energy in other ways too. It is estimated that separating chemicals accounts for between 10 and 15 per cent of the world’s energy use. “There is an urgent need for advances in chemical separation processes and materials,” says chemical engineer at the Georgia Institute of Technology in Atlanta. “And porous liquids are an important new direction.” In time, James is hoping to develop a range of these liquids, each with different properties to suit various applications (see  “Three more things we could do with a liquid sponge”, below).
at Harvard University was also intrigued by porous liquids, but noticed that they were nearly all based on organic solvents – that is, solvents made of carbon-based molecules. To Mason’s mind this was a serious limitation because it ruled out using them in any living system. Organic solvents are at best incompatible with cells and tissues, and they can be highly toxic. “Anything you want to do that involves a biomedical application is going to have to take place in an aqueous environment,” he says.
Mason and his colleagues set out to make a water-based liquid sponge. But there was an immediate problem. Water is a tiny molecule, far smaller than most organic solvents, and this meant any cage-like molecules dissolved in water could easily have their pores plugged by the water, stopping them absorbing any gas. To circumvent this, Mason and his colleagues took tiny particles of MOFs and chemically modified them by adding groups of atoms on the outside that were water-loving and other groups of atoms on the inside of the cavity that were water-hating. This created cage-like molecules that were happy to dissolve in water, but each had a pore that water molecules couldn’t easily occupy.
To test the performance of this liquid sponge, Mason and his team sucked the air out of their MOF-water solution and measured how much of it was oxygen. They then did the same with a sample of pure water. The liquid sponge held 100 times more oxygen. They also showed that the by bubbling their oxygen-loaded liquid sponge through deoxygenated blood. “You can see the solutions go from like really dark red to bright red as the red blood cells become oxygenated,” says Mason.
The future of porous liquids
He is currently exploring how these biocompatible liquid sponges could be used. One idea is that, in a medical emergency, they could be injected into people who aren’t breathing. It would be a quicker way of getting oxygen into their bloodstream than relying on mechanical ventilation. But their scope could be far wider than this. Using water-based porous liquids in industrial processes – for example carbon capture and storage – would be good because water is such an environmentally friendly, low-impact solvent.
Recently there has even been progress towards making a type 1 porous liquid, the ideal, undiluted sort that James dreamed up in the early days. James decorated his MOF cages with long tails to make them liquid at a lower temperature – but those tails tended to get tangled up in the cages’ cavities, thwarting their capacity to absorb gas.
at Cambridge University instead constructed a cage molecule from one positively charged component and one negatively charged component. This so-called ionic liquid proved to be a fluid below room temperature. Furthermore, the charges repelled the cages from each other, ensuring that the gaps inside them remained accessible to gas molecules. It was the and Nitschke showed that it could suck up CFCs, the chemicals responsible for creating a hole in the ozone layer.
Nitschke admits that his creation has the consistency of honey – not great for pumping through pipes. This underlines the fact that liquid sponges still have a way to go. Nonetheless, porous liquids are beginning to slosh into action. Before long, they could be helping to soak up the mess we have made of our planet.
Three more things we could do with a liquid sponge
CAPTURING CARBON
Carbon-capture-and-storage technology is used to try to sequester greenhouse gases produced at fossil fuel power plants and other high-emitting industries, such as steelworks. Porous liquids could be a cheaper, more efficient way of soaking up the carbon dioxide than the current technology.
PURIFYING CRUDE OIL
The process of separating crude oil into all its components – natural gas, petrol, bitumen and more – currently relies heavily on distillation, which requires a huge amount of energy to heat up the mixture. Porous liquids could be an alternative, lower-energy separation technology.
HARVESTING XENON
Xenon is a rare gas used in physics experiments, as an anaesthetic and in lights. We currently obtain it by liquefying air and then distilling it, an energy-intensive process. Porous liquids could be used to separate xenon from nuclear waste instead.
