Omar Yaghi: Organometallic Scaffolds May Help Solve Water Scarcity in African Deserts
Омар Яги
Why are porous materials the best storage solution for hydrogen and ammonia? And how can they be used to capture CO2? Omar Yaghi, Founding Director of the Global Science Institute at the University of California at Berkeley, shortlisted for the Global Energy Prize, touched these issues in an interview with the Association.

Nick Boykov: So, hydrogen, methane, carbon dioxide, and water are four of the smallest known molecules and they have the highest impact on transition to clean energy worldwide. You have been addressing this challenge over the last twenty-five years. How will your research help modern energy transfer then?

Omar Yaghi: Thank you. Yeah, in the mid-1990s we discovered a way of linking molecular building blocks together through strong bonds, organic and inorganic, to make porous materials, extended structures. And one can vary both the inorganic component and the organic component so that these materials could have a range of porosities. Controlling the pore size, controlling the internal surface area, and also controlling the chemistry of the pores. And so, what this has allowed us to do, because the building blocks are linked by strong bonds, is to make durable materials that can operate day after day, month after month, year after year in a power plant to trap carbon dioxide or, if they are using natural gas, let’s say, for fuelling automobiles, you can store natural gas in to the pores of these materials at levels that you cannot do without the metal-organic frameworks that you have mentioned in your introduction.

So, the key feature of this is not just the strong bond that leads to durability, but also we showed that in fact, pores can remain open and therefore things can move in and out with great ease. And another important feature is that since we can design them on a molecular level we can make them extremely porous what’s called ultra-high porosity. To give you an idea what that is, for example, in one gram of material, which is no more than about a coin, size of a coin, you have a surface area of 7 thousand metres square per gram. Now for anybody that’s listening to this, if they’re not in porous materials, they may not appreciate what this number means. But an easy way to communicate what an ultra-high porosity material like this is, that in that one gram of material, if I was to unravel in on a molecular level, I would cover at least 2 football fields.

Nick Boykov: That’s impressive.

Omar Yaghi: And so that’s the, let’s call it the footage, onto which one can store gases, and because we can go in and design the adsorptive sites for methane, carbon dioxide and so on, the small gases that you mentioned, we can compact them next to each other without having to use very high pressure or low temperature. So, in a tank filled with metal organic frameworks (MOF), in a fuel tank filled with MOF, we can store three or four times, even four times the amount of natural gas that a tank that doesn’t have MOF would. And so this allows the automobile, let’s say, to travel 3 to 4 times the distance without having to refuel without changing the volume of the fuel tank and without changing practical conditions under that, what that fuel tank operates on. So, and that’s because the MOF acts to those gases within its pores because those pores were designed chemically to attract that particular gas molecule and stack it up in the pores to give ultra-high uptake or storage capacity.

Nick Boykov: You almost answered my next question. So you said it is possible to create such, like, framework within which certain gases can be stored double or even triple the amount of it, but can it be used to increase the storage capacities for, like, any other agents?

Omar Yaghi: Certainly. For example, that same analogy, if you apply it to CO2, you can store 25 times the amount in a MOF that you would in a tank that doesn’t have MOF. But furthermore, let’s say, for carbon capture, not only can you store the carbon dioxide, but also to begin with, you can separate it from the different other gases that might be present in flue gas or in the air. Even in small amounts of carbon dioxide that exist in the air can, although small amounts, but they are harmful, they need to be separated. So you can go in and design chemically modified sites covalently using strong bonds to attract with groups that are specific to attract the carbon dioxide out of the air or other flue gas so that you’re not taking up other gases, such as nitrogen, which we don’t need to take up and to avoid also complications let’s say that water may present in terms of competition with carbon dioxide for those adsorptive sites. So we modify the pores chemically so that the MOF material can seek out just carbon dioxide, block it out of that gas mixture and store it into the pores. Now in future – and we are working on this already, we want to be able to not just store it in the pore but also convert it to a starting material like a fuel, so that not only are we capturing the carbon dioxide but we’re using it as a fuel and so, or as a way to create valuable, valuable chemicals so that’s not just a material that is stored under that ground.

Nick Boykov: So a significant part of your work has also been aimed at harvesting water from low humidity air, for example, in deserts. And in the light of our efforts to explore Mars, can this help us find water there and is there a limit to the amount of water that can be extracted from the air?

Omar Yaghi: Very good question. In that application, because we demonstrate how you can trap water from desert atmosphere where you only have 10 grams of water per cubic meter. So it’s very low amount of water in the atmosphere. We also can design these materials to have adsorptive sites where we can extract water from very low concentration. But the key I think, important advance here is that because of the organic and inorganic component of the MOF one can modulate how tightly that water molecule is bound to the pore, so not only can you take it in, but you also don’t need a lot of energy to take it out. That’s why harvesting water from desert air has been very successful using MOFs because you don’t have to apply a lot of energy to remove the water. And so this is an important feature of what I call reticular chemistry is not only can you assemble the materials using covalent bonds, using strong bonds, but also you can modify them on a molecular and an atomic level, sometimes atom by atom, so that they can be suitable to carry out very difficult separations such as what you mentioned, such as water from air, and as we have done and demonstrated, such as carbon dioxide from air and such as storage of natural gas to give us hopefully clean air, clean water, and clean energy.

Nick Boykov: Yes, and as I understand, this is directly related to the concept of energy-water-food nexus, because this installation that harvests water is powered by a PV, as I know, and it turns out that by putting one in the desert you can provide a remote village, for example, in Africa only with food, but also with energy. Is that correct?

Omar Yaghi: Exactly. So, so the idea is that the MOF, because it doesn’t hold on to the water too tightly, it’s selected to water but it doesn’t hold on to it too tightly, you can use just ambient sunlight to remove the water. So in principle you could have a device in the example that you just showed you could have a device that operates at night to take up the water from air and during the day, because of changing temperature and the heating from the sun, it can be, the water can be released and harvested into a drinkable water.

So, now another aspect of this is that if there is electric available then you could always power the device from a solar panel or by other means so that now you can carry out more than one cycle. And now your productivity that you have would be dramatically improved. So we have published results that show that in fact up to 40 litres of water per kilogram of MOF per day could be produced in this way in some of the driest regions of the world. And again I keep coming to the strong bond because without the strong bond you wouldn’t be able to do this, you wouldn’t be able to make these robust frameworks that operate extensively and over a long period of time. These MOFs have been tested thirty thousand cycles already and they are still operating, and the MOF can stay in the device for the lifetime of the device, for 5-6 years. So it’s very exciting and the energy requirements for this process can be supplied by sunlight, ambient sunlight or by a solar panel depending on what kind of productivity you would like to achieve.

Nick Boykov: Thank you. And in the end of our short talk I would like to once again return to the question of CO2 and its capture. What would be the best way to use CO2 further after it is captured? Because I heard it can be used in catalysis, for example.

Omar Yaghi: Well, you know, right now we use processes that yield carbon monoxide, which is an excellent starting material for many chemicals. Well, we’ve already demonstrated that you could convert the CO2 captured in the MOF, in this case it’s not a MOF, but a covalent organic framework, this is another class of porous materials that we invented in 2005 for two-dimensional structures and 2007 for three-dimensional structures, but they are also porous, but they are all organic and made of covalent bond, entirely of covalent bonds.

Now we’ve shown in that example that you can trap carbon dioxide and convert it to carbon monoxide which is a valuable starting chemical, but potentially you could carbon dioxide to methanol. And we’ve shown preliminary evidence that this is possible. You could take methane and convert it to methanol as well because you can go into the MOF and in this case do, in the last case I mentioned, do what enzymes like methane monooxygen is to take methane and convert to methanol except now you are doing it in the MOF that has been functionalised with cup percenters that are suitable for this transformation.

So I would say not only can we do CO2, but potentially you can make hydrocarbons, hydrocarbon fuels and hopefully down the road perhaps we can make important organic molecules like pharmaceuticals using this method from CO2.

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