Porous liquids: An interview with Professor Stuart James
ECG Bulletin January 2024
In July 2023, Dr Rowena Fletcher-Wood and David Owen of the ECG interviewed Professor Stuart James, Queen's University, Belfast about his work on porous liquid technologies. This new technology has recently emerged at the forefront of separation materials which have applications in green chemistry.
Concept
Can you start by outlining how you first developed the concept of porous liquids?
I came up with the concept of porous liquids in 2007. I co- founded Porous Liquid Technologies to spin out this technology in 2017 and implement greener, more sustainable processes.
This followed an initial spin-out in mechanical chemistry. I have a strong interest in solvent-free chemical syntheses. People use solvents, but instead, you can simply grind solids together and get reactions to go. It seems very primitive, it seems like it shouldn't work, but actually, it can work very well.
What started as curiosity became a spin-out company (then called Morph Technologies). Now, the area of mechanical chemistry has boomed over the last 20 years.
Is that where the goal of all your work lies, in green and sustainable chemistry?
It has become that way, I think largely because of global drivers. Maybe it takes a bit of a crisis, for example, the need for sustainability, to make people rethink the ways we are doing things. Then we transition to new technologies. My personal driver was inventing something new; a new idea, a new technology, something that had never been done before. But inevitably, you start to think, how could we apply this to something usefully?
Can you start by outlining how you first developed the concept of porous liquids?
I came up with the concept of porous liquids in 2007. I co- founded Porous Liquid Technologies to spin out this technology in 2017 and implement greener, more sustainable processes.
This followed an initial spin-out in mechanical chemistry. I have a strong interest in solvent-free chemical syntheses. People use solvents, but instead, you can simply grind solids together and get reactions to go. It seems very primitive, it seems like it shouldn't work, but actually, it can work very well.
What started as curiosity became a spin-out company (then called Morph Technologies). Now, the area of mechanical chemistry has boomed over the last 20 years.
Is that where the goal of all your work lies, in green and sustainable chemistry?
It has become that way, I think largely because of global drivers. Maybe it takes a bit of a crisis, for example, the need for sustainability, to make people rethink the ways we are doing things. Then we transition to new technologies. My personal driver was inventing something new; a new idea, a new technology, something that had never been done before. But inevitably, you start to think, how could we apply this to something usefully?
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Technology
Can you tell us more about how porous liquid technology works? Imagine a gas dissolving in a liquid, that could be, for example, carbon dioxide from the air dissolving in water, there is a limit to how much gas can dissolve in that liquid. But what causes that limit? One factor is that if a gas molecule dissolves in a liquid, the liquid molecules have to get out of the way. You have to separate the liquid molecules, and ones such as water stick together very strongly through hydrogen bonds. |
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So, what happens if you make holes in the liquid? The gas molecules don’t have much work to do. They don't have to separate the liquid molecules anymore; the holes are already there. The gas can dissolve in the liquid much more easily and you can get much more gas into the liquid.
That's what we call a porous liquid, a liquid with permanent holes. Each hole is microscopic: about the size of a gas molecule. They're not bubbles; they are far too small to see. Dissolution is then enthalpically driven.
We have increased the solubility of the gas, and so changed the thermodynamics. Empty porous liquids contain naked surfaces, which are thermodynamically unfavourable in terms of ΔH. Binding a gas molecule to the walls of the hole through dipole dipole interactions is exothermic, giving a more negative Gibbs free energy.
How do you introduce holes to liquids?
We have discovered various ways to create these holes. When we first proposed the idea of porous liquids, we built organic cages called metal organic frameworks (MOFs) that could be suspended in a solvent. It was a proof of principle experiment, and it worked.
This was, however, a very time-consuming process, it would take about six months to make 1 mL of porous liquids. But if we were going to ever apply this, we realised this approach would never work. It was far too expensive.
Instead, we discovered that a very simple approach was to disperse a zeolite in a liquid with molecules too big to fit in the zeolite pores, creating something that looks like milk. If the dispersion was stable enough, it could last for a few hours, days, weeks, or months. This is very economical, and it's surprising no one's done it before!
That's what we call a porous liquid, a liquid with permanent holes. Each hole is microscopic: about the size of a gas molecule. They're not bubbles; they are far too small to see. Dissolution is then enthalpically driven.
We have increased the solubility of the gas, and so changed the thermodynamics. Empty porous liquids contain naked surfaces, which are thermodynamically unfavourable in terms of ΔH. Binding a gas molecule to the walls of the hole through dipole dipole interactions is exothermic, giving a more negative Gibbs free energy.
How do you introduce holes to liquids?
We have discovered various ways to create these holes. When we first proposed the idea of porous liquids, we built organic cages called metal organic frameworks (MOFs) that could be suspended in a solvent. It was a proof of principle experiment, and it worked.
This was, however, a very time-consuming process, it would take about six months to make 1 mL of porous liquids. But if we were going to ever apply this, we realised this approach would never work. It was far too expensive.
Instead, we discovered that a very simple approach was to disperse a zeolite in a liquid with molecules too big to fit in the zeolite pores, creating something that looks like milk. If the dispersion was stable enough, it could last for a few hours, days, weeks, or months. This is very economical, and it's surprising no one's done it before!
Choice of materials
What makes zeolites attractive for porous liquid syntheses?
A number of elements. On the one hand, they are already produced on huge scales, and there is literature and knowledge to draw upon. Also, zeolites are chemically and thermally stable, which means we don't have to worry about degradation.
What makes zeolites attractive for porous liquid syntheses?
A number of elements. On the one hand, they are already produced on huge scales, and there is literature and knowledge to draw upon. Also, zeolites are chemically and thermally stable, which means we don't have to worry about degradation.
What liquids do you suspend the zeolites in?
We suspend them in something, massively available, such as polyethylene glycol, which is extremely cheap. In addition, its derivatives are already used to separate CO2 from hydrocarbons. This solvent dissolves a substantial amount of CO2 and is soluble in hydrocarbons. Both the zeolite and solvent are now CO2 selective, boosting the CO2 carrying capacity by a factor of 2 or 3, and, fortunately, the pores of the zeolite are too small to let the solvent into.
We suspend them in something, massively available, such as polyethylene glycol, which is extremely cheap. In addition, its derivatives are already used to separate CO2 from hydrocarbons. This solvent dissolves a substantial amount of CO2 and is soluble in hydrocarbons. Both the zeolite and solvent are now CO2 selective, boosting the CO2 carrying capacity by a factor of 2 or 3, and, fortunately, the pores of the zeolite are too small to let the solvent into.
However, in our porous liquids, most of the gas dissolution capacity comes from the solid, which means you don't necessarily have to stick with common, ordinary solvents. We have found we can use silicones, like polydimethyl silicone (PDMS), which is a terrible solvent for most gases. However, it is cheap, inert, and hydrophobic. We have been working on water-based liquids as well.
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Applications
Can you give us some examples of key separation possibilities? Let's say the holes are about the size of a carbon dioxide molecule, but just too small for a larger molecule like methane. This means that you selectively dissolve carbon dioxide, and exclude methane. This is useful when you have a mixture of gases. A good example is biomethane – making methane from farm waste, which presents as a mixture of methane and carbon dioxide. Although there are existing separation methods, problems such as energy costs and efficiency prevail. |
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We have now developed a continuous process, where a liquid selectively dissolves CO2, pump it around, and release the methane, potentially into the natural gas grid. The porous liquid, which is now full of carbon dioxide, is heated or exposed to a vacuum, and that releases the CO2. Now we need to think about what to do, as we don’t want to release CO2 into the atmosphere. Most likely, what's going to happen is that it’s going to be pumped underground. The porous liquid then goes back to the start, where the process is repeated.
We can also separate carbon dioxide from hydrogen, for example, even though hydrogen is smaller than CO2, so can fit into the pores of a porous liquid as well. This is needed for the purification of ‘blue’ hydrogen gas (H2). However, hydrogen gas binds more weakly than a larger molecule such as CO2, because the covalent two-electron hydrogen bond (H2) is not easily polarised and consequentially does not form strong interactions with the walls of the holes. There is therefore an enthalpic advantage for CO2 binding over H2 binding.
Current indications are that porous liquids are very effective for separating mixtures of gases at relatively low energy costs. For example, you don't have to heat the porous liquid to as high a temperature as you do with other solvents. Or you can use a vacuum.
Opportunities and challenges
How is your technology different or better from other options?
The main competing technology is aqueous, which is already established. Even if we are convinced porous liquids are better – lower energy, less toxic, less corrosive – we have to convince people to transition.
How is your technology different or better from other options?
The main competing technology is aqueous, which is already established. Even if we are convinced porous liquids are better – lower energy, less toxic, less corrosive – we have to convince people to transition.
The market for CO2 solvents is still very dominated by aqueous amines. However, these bind the CO2 very strongly, chemically reacting to make carbonates.
Our technology operates via physisorption into the zeolite and, as such, it takes much less energy to recycle and regenerate the CO2.
Our technology operates via physisorption into the zeolite and, as such, it takes much less energy to recycle and regenerate the CO2.
Could you tell me about the challenges or limitations you're facing?
Our main competition at the moment is incumbent technology. Changeover is challenging. For example, if you propose an organic synthesis, one of the first questions chemists ask is “Which solvent are we going to use?” They never almost never stop to consider whether we actually need a solvent. We do think it's going to become clearer and clearer over the coming years that porous liquids have clear advantages over existing technologies – and it's hard to see the limitations. But the devil with these applications is always in the detail. At every single stage, you've got to go through the TRL process, technology readiness level, where you evaluate your progress towards application.
This is certainly our challenge for biogas applications. We're around about four out of nine on the scale with biogas in the sense that we've demonstrated it in principle in the lab. What we're now doing is going on site to demonstrate it there. That's what we need to do to get to five. The challenges we expect include a dirtier mix of gases and contaminants like water and hydrogen sulfide in small amounts. In these biogas plants, the gas is already passed through a column to take out virtually all of the hydrogen sulfide and ammonia, but there's still going to be traces left afterwards. But we're fairly confident the zeolites can tolerate these and won't chemically react. The performance of our porous liquid may decrease a little as it absorbs more contaminants, but we can mitigate that by heating it up to a higher temperature than we would do for a normal regeneration every few cycles.
Our main competition at the moment is incumbent technology. Changeover is challenging. For example, if you propose an organic synthesis, one of the first questions chemists ask is “Which solvent are we going to use?” They never almost never stop to consider whether we actually need a solvent. We do think it's going to become clearer and clearer over the coming years that porous liquids have clear advantages over existing technologies – and it's hard to see the limitations. But the devil with these applications is always in the detail. At every single stage, you've got to go through the TRL process, technology readiness level, where you evaluate your progress towards application.
This is certainly our challenge for biogas applications. We're around about four out of nine on the scale with biogas in the sense that we've demonstrated it in principle in the lab. What we're now doing is going on site to demonstrate it there. That's what we need to do to get to five. The challenges we expect include a dirtier mix of gases and contaminants like water and hydrogen sulfide in small amounts. In these biogas plants, the gas is already passed through a column to take out virtually all of the hydrogen sulfide and ammonia, but there's still going to be traces left afterwards. But we're fairly confident the zeolites can tolerate these and won't chemically react. The performance of our porous liquid may decrease a little as it absorbs more contaminants, but we can mitigate that by heating it up to a higher temperature than we would do for a normal regeneration every few cycles.
We are also looking at post-combustion carbon capture. That presents challenges include much larger volumes of water vapour. Zeolites are highly hydrophilic and absorb water quite strongly, creating competition with CO2 absorption. We have managed to formulate some porous liquids which do selectively take CO2 instead of water.
What are the future possibilities? Have you looked at syn gas (H2 and CO)?
We haven’t looked at syn gas yet, but there are many possibilities.
What are the future possibilities? Have you looked at syn gas (H2 and CO)?
We haven’t looked at syn gas yet, but there are many possibilities.
We also know that the gases are available for reaction and incorporation into the liquid. Normally, this is done under high pressure, but with a porous liquid, you don't need such high pressures. We think that there is a potential use in reducing the pressure needed to do reactions with dissolved gases.
There are also potential applications in the hydrocarbon industry, such as separating ethane and ethene (ethylene). This is done on a massive scale worldwide. Currently, this is performed by cryogenic distillation, which is very energy-hungry, because you've got to cool and to distil. You might selectively dissolve one in a solvent to separate them, but the problem is there aren't any solvents that really discriminate between ethane and ethene as they’re too chemically similar. There are one or two that people have made, but they have never been applied as far as I know. This is much easier with solids.
There are also potential applications in the hydrocarbon industry, such as separating ethane and ethene (ethylene). This is done on a massive scale worldwide. Currently, this is performed by cryogenic distillation, which is very energy-hungry, because you've got to cool and to distil. You might selectively dissolve one in a solvent to separate them, but the problem is there aren't any solvents that really discriminate between ethane and ethene as they’re too chemically similar. There are one or two that people have made, but they have never been applied as far as I know. This is much easier with solids.
About the interviewers
Rowena Fletcher-Wood, ECG Chair and Bulletin Executive editor, has a background in zeolite chemistry and works as a science communicator.
David Owen, Bulletin Briefs commissioning editor, was a former industrial chemist and founder of TreatChem Ltd.
Rowena Fletcher-Wood, ECG Chair and Bulletin Executive editor, has a background in zeolite chemistry and works as a science communicator.
David Owen, Bulletin Briefs commissioning editor, was a former industrial chemist and founder of TreatChem Ltd.
A selected bibliography on porous liquids and porous materials
N. O'Reilly, N. Giri, S. L. James, Porous liquids. Chemistry: A European Journal, 2007, 13(11), 3020-3025. https://doi.org/10.1002/ chem.200700090.
Y. Liu et al. Porous framework materials for energy & environment relevant applications: A systematic review. Green Energy & Environment, 2024, 9, 217-310. https://doi.org/10.1016/ j.gee.2022.12.010.
D. C. Wang et al. Shining Light on Porous Liquids: From Fundamentals to Syntheses, Applications and Future
Challenges. Adv. Funct. Mater., 2022, 32, 2104162. https://doi.org/ 10.1002/adfm.202104162.
Z. Chen et al. Review: Porous materials for hydrogen storage. Chem., 2022, 8, 693–716. https://doi.org/10.1016/ j.chempr.2022.01.012.
T. D. Bennett et al. The changing state of porous materials. Nat. Mater., 2021, 20, 1179–1187. https://doi.org/10.1038/ s41563-021-00957-w.
R. L. Siegelman. et al. Porous materials for carbon dioxide separations. Nat. Mater., 2021, 20, 1060–1072. https://doi.org/ 10.1038/s41563-021-01054-8.
P. F. Fulvio, et al. Review: Porous Liquids: The Next Frontier. Chem., 2020, 6, 3263–3287. https://doi.org/10.1016/j.chempr.2020.11.005.
N. O'Reilly, N. Giri, S. L. James, Porous liquids. Chemistry: A European Journal, 2007, 13(11), 3020-3025. https://doi.org/10.1002/ chem.200700090.
Y. Liu et al. Porous framework materials for energy & environment relevant applications: A systematic review. Green Energy & Environment, 2024, 9, 217-310. https://doi.org/10.1016/ j.gee.2022.12.010.
D. C. Wang et al. Shining Light on Porous Liquids: From Fundamentals to Syntheses, Applications and Future
Challenges. Adv. Funct. Mater., 2022, 32, 2104162. https://doi.org/ 10.1002/adfm.202104162.
Z. Chen et al. Review: Porous materials for hydrogen storage. Chem., 2022, 8, 693–716. https://doi.org/10.1016/ j.chempr.2022.01.012.
T. D. Bennett et al. The changing state of porous materials. Nat. Mater., 2021, 20, 1179–1187. https://doi.org/10.1038/ s41563-021-00957-w.
R. L. Siegelman. et al. Porous materials for carbon dioxide separations. Nat. Mater., 2021, 20, 1060–1072. https://doi.org/ 10.1038/s41563-021-01054-8.
P. F. Fulvio, et al. Review: Porous Liquids: The Next Frontier. Chem., 2020, 6, 3263–3287. https://doi.org/10.1016/j.chempr.2020.11.005.
G. Singh et al. Emerging trends in porous materials for CO2 capture and conversion. Chem Soc Rev., 2020, 49(13), 4360-4404. https://doi.org/10.1039/d0cs00075b.
G. Cai. et al. Metal-Organic Framework-Based Hierarchically Porous Materials: Synthesis and Applications. Chem Rev., 2021, 121(20), 12278-12326. https://doi.org/10.1021/acs.chemrev.1c00243.
M. Sai Bhargava Reddy et al. Carbon dioxide adsorption based on porous materials. RSC Adv., 2021, 11(21),12658-12681 (2021). https://doi.org/10.1039/D0RA10902A.
Basic research needs for carbon capture: Beyond 2020. Report of the Basic Energy Sciences Workshop for Carbon Capture: Beyond 2020. https://www.osti.gov/servlets/purl/1291240.
M. Sai Bhargava Reddy et al. Carbon dioxide adsorption based on porous materials. RSC Adv., 2021, 11(21),12658-12681 (2021). https://doi.org/10.1039/D0RA10902A.
Basic research needs for carbon capture: Beyond 2020. Report of the Basic Energy Sciences Workshop for Carbon Capture: Beyond 2020. https://www.osti.gov/servlets/purl/1291240.
