Episode 113: Metal Organic Frameworks

I would like to describe a field in which little has been done. but in which an enormous amount can be done. This field is not quite the same as the others in that it will tell us little of fundamental physics, but it will tell us much about the strange phenomena that occur just below our perception.

In contrast to the natural philosophers of the past, the scientists of this field delve into the recesses of nature and show how she works in her hiding places. Their quest is to understand and create the imperceptible. After all, there is plenty of room...

Hello and welcome to the Materialism Podcast, an exploration of the past, present, and future of material science. My name is Taylor Sparks, and I'm a professor of material science and engineering here at the University of Utah in Salt Lake City, and I am joined by Andrew. How are you?

Doing pretty well. A little bit tired. I haven't had my coffee this morning, but that will happen soon. It is bright and early. Whenever you're listening to this, just know that we woke up. We spared no expense. No personal suffering is too great to bring you.

the greatest of episodes. And we're joined by somebody who's also taken time out of his busy schedule, Matt Cliff, soon to be from Cambridge. Matt and I met on my sabbatical in England a couple of years ago, and he's the best. And he reached out to us and told us, hey. The Nobel Prize just happened for MOFs. We got to do a Nobel Prize MOF episode and it was a great idea. Matt, how are you doing?

Really good, thanks. Yeah, really good. Just at a conference in Bordeaux, France. This is even more international than you'd otherwise expect. Absolutely. Bordeaux, France, that sounds wonderful in October. Yeah, nice weather, sunny outside. good wine, good food, and some molecular magnets as well. So not a MOF conference, but you are a MOF researcher. Yes, I am. My research focuses on metal-organic frameworks.

So we do a bit on the physical properties of these materials. But, you know, I'm interested in moths of all kinds as well. Yeah. Okay. So the hook that I heard when I first learned about moths or metal organic frameworks. is this notion that you've got a material that is extremely extremely porous right and they'll say like what if a single gram of the material had enough you know

surface area to it that it could cover a sort of football field, right? I don't know if that's even a realistic analogy, but that's the one I heard. In other words, we've seen porous materials. Picture the sponge that you use on your kitchen at home.

But what if you were to greatly, greatly, greatly increase that porosity? How would you make that material? What would they be good for? How would you characterize and measure them? That's all some of the questions that you dive into in the world of these metal organic frameworks.

We're going to talk about that not just because the Nobel Prize, but because they're genuinely very interesting materials. I think they're in some ways at the forefront of material science in that we don't understand everything about them yet. We're still developing and learning new things about them. And they haven't...

quite hit the market and had the translational impact that some other more mature class of materials are so that's what we want to cover in this episode so matt get us started when we say a metal organic framework what are we talking about Yeah, so it's partly in the name. So they're materials which are made from metal atoms or metal clusters connected via organic molecules to make extended frameworks, 3D or sometimes 2D.

material so you've got a metal cluster maybe it's got one atom maybe it's got say half a dozen or 12 or so and that's kind of the corner of this network and these are then connected by extended organic molecules, something maybe like therophilic acid, 1,4-benzene diacopic silic acid, these are, you know, nanometer length organic molecules perhaps, into these open structures. So because you've got...

of long organic molecules as your edges, that means there's a lot of space in the middle, potentially. So this is what, you know, you're talking about a football field worth of surface area in a gram.

uh and that's i mean so much as it makes sense to talk about surface areas that massive you know you start getting to some philosophical questions but you you know the record i think depending on how you define it something like 7 000 meters squared per gram Yeah, so this actually is two or three times more than the most porous other kinds of materials, which are mostly activated carbons.

But these are crystalline. That's probably most of the time moths are crystalline. They don't have to be, but nearly all the ones that people are most excited by are crystalline. So they're very, very well ordered as well, which people make use of too.

So you said something. You said that you've got either a metal or a cluster of metal atoms, and these are connected to organic linkers. So that's a very maybe well-known concept for the chemists in the audience. But for the material scientists, how on earth do you connect a metal to an organic might feel a bit like...

You know, how's that happening? Do you want to describe that at all? Yes. So this is kind of through coordination chemistry, coordination bonds. So if you have a group that's negatively charged or has like a. a local negative uh dipole or something so something like a carboxylate so co2 minus that binds pretty well to metals and you know sometimes these kind of things are used as isolated molecules to collate metals you know take them out of

uh solution or whatever um but if you have two of them two of these sort of carboxyl groups on either end or three or four um those can act as the linker so it's that it's that coordination bond so they're basically You've got negatively charged or sort of locally negatively charged linkers that coordinate to your positively charged metal ions. So these are not in there.

metal zero state typically, but into the metal 2+, 3+, 4+. Okay. And what sort of metals can you use? Are there any restrictions or have been MOFs made with everything?

Pretty much everything, to be honest. So the most popular ones would be, as you might imagine, metals that are cheap and form these... uh frames pretty easily so things like zinc is probably the most zinc two plus copper two plus are some of the most well studied um as maybe we'll get on to later there's a lot of interest in uh things like titanium four plus and zirconium four plus materials

But you can really get kind of anything in it. I know that there are groups who work with transactinides, so very radioactive elements who've made moths for... I think curiosity reasons rather than application reasons, I think there are plutonium moths, uranium moths, thorium moths out there. And you can even get edible moths made out of things like sodium and potassium. Wow.

Yeah. I mean, are they moths? Are they more like a salt? Are they still work for frameworks? But they're porous and they have metal ions and organic linkers. So you can make a moth like a cyclodextrin type thing. calcium so i've got a question like in my really basic chemistry understanding right i'm thinking about the the charge that dictates basically the hybridization of the orbitals and hybridization of the orbitals for organic materials dictates

the number of bonds and the orientation of those bonds like take diamond right whether it's diamond or graphite or whatever depends on whether it's sp2 sp3 and all that is that the same thing in other words does the charge of the metal ion dictate the orientation of the bonds

these ligands that are coming off from it, or is it not the case? It's often a bit more like ceramics in some ways, right? These are pretty ionic interactions. So they're not necessarily, the metal side of things, super directional. But what is directional? So they tend to have the coordination numbers you'd expect to see if it's a single metal atom in other kinds of inorganic materials, so octahedral or tetrahedral, depending on the size of the metal and so on.

but if you have a cluster that actually allows you to get a bit more direction on it so um it's probably a little bit hard to describe it oh i see so like the metal cluster has some orientation to it and then the ligands just stick off of those and so that's providing the

the shape as it were of the connections coming off of it yeah exactly exactly cool like whenever i saw moffs the first time it's this cubic kind of looking structure i think that's often what you see in your first case but it turns out You can have tons of other different types of structures depending on that. And you can even have, is it possible to have multiple different metal centers?

Yeah, you can have a whole full range. So I think there's, you know, people have done solid solutions with a kind of proof of principles of, you know, I guess you maybe even call them high entropy, sort of half a dozen or so metals in it. You can also do... using the different chemistries of different metals to kind of come up with more complex structures where one metal say is bonded to one end of an organic molecule.

say it's a carboxyl end and the other one has a nitrogen end of the molecule and you have two different kinds of metals bond those two different sides so you can get you can kind of get into a real complexity there um yeah if you

Well, Matt, tell us about the history and the discovery. You know, so many of the materials that we know and love were sort of observed in nature as like a mineral or sort of existing natural material or they were accidentally discovered or there's these typically... interesting stories of how we came about finding them do these things exist in nature were they observed or were they genuinely sort of built and had never been seen before what did that look like so so actually i think

So the story of how Moffs, I guess, came to win a Nobel Prize is one of sort of chemists, I think, designing and starting from designing and building. and coming up with concepts and building up new sort of structures. So if we take, so there's three Nobel laureates, Richard Robson from University of Melbourne, Omar Yagi, who's from UC Berkeley. and Tsumika Tagawa from the University of Kyoto. And Robson did the earliest work on this in Australia.

And he noticed that kind of like we were talking about earlier, you know, the metal atoms have preferred geometry. So you notice that copper one has this preferred tetrahedral environment. And so if you coordinated it with another tetrahedral, big organic molecule.

then you could make a kind of expanded diamond-type structure out of it. Yep, makes sense. So basically by choosing a metal, choosing an organic molecule, you're dictating the topology of the structure. Because an organic molecule is massive, it means that the kind of gaps that you see in a... in the cartoon crystal structure of diamond actually become real gaps that you can have solvent and other ions in it. Now these weren't super stable.

um but he came he went on and found designed various other topologies of materials did he actually synthesize those or was it just like a pen and paper theory exercise No, no, they grew them. They grew crystals with them. They solved the structures. They did various bits and bobs with them. But the problem with them is that if you try to take the solvent out, they basically fell apart.

know the bonds were not not super strong it's the solvent actually stabilizes the structure yeah yeah yeah exactly these are when you grow a crystal of moth if you're doing in solution which you normally are so much of the actual structure is the the physical quest you're lifting up it is is actually solvent by weight and these are these are really wet uh materials i hear we talk about them being like crazy porous but like what does that even mean if the

solvent has to fill those pores to make up the structure. It's more of a composite at that point, right? Yeah, exactly. And people actually started exploiting this down the line for sort of ionic conductivity, right? If you can have that kind of solid structure that's full of liquid, or maybe you could use that for the conduction of mobile things inside it.

So the other two laureates started a little bit later in this, sort of mid-90s, and what they realised is that you should be able to make materials that are really porous. that do have empty space inside them. And so this required basically beefing up the strength of the bond. So one of the big things that Sayomai Yagi did, as well as coming up with the term MOF,

was realised that if you use a bigger metal cluster, you can often make the material more rigid. So the most famous material, though not the earliest one, is a material called Moth 5, which is a zinc. zinc oxo benzene diacboxylic acid that's very very porous and it has that nice cubic structure that i think you mentioned andrew

But the first measurements of real porosity in it were done by Susumu Kitagawa, working on a slightly more complicated material. So that was, again, in the mid-90s. So they both and other people, of course, around. around the same time built on this, realized that you could take these materials full of solvents and then turn them into things full of empty space and really massively expanded the range of

different chemistries and different enhanced materials that you could make from it. And they've kind of been rolling ever since, you know, just the number of structures going up kind of almost exponentially. And what is the physical mechanism by which having... one versus a cluster of metal atoms strengthens those bonds? It's kind of that there's just less flexibility around that initial cluster, I think, is part of it.

Also, the higher charge tends to mean that the bonds are stronger. So if you take copper one and start pulling the solvent out of it, what will happen is that there's basically a capillary force as you're trying to... and if that almost evaporate that solvent, and that will pull the framework apart from the inside. So you need to make sure it's relatively rigid, or at least it's chemically rigid as well, so that it stays together.

So Matt, these three people clearly made some important contributions, but as is, I imagine, always the case in these NoMo... prices is you have to like draw the line somewhere and people are always going to feel like dang i must have been on the on the bubble uh when it comes to leds for example right shuji nakamura the blue leds then we had a guy in our department jerry stringfell who won basically every prize he could win but that one he was instrumental in figuring out how to

to tune those i wonder are there a bunch of people in the moth world like did they get it right in picking these three people or their key names that you're like boy i would have thought that person I think probably these are the three that, you know, talking to colleagues and stuff, these are probably the three that people would have been most happy with. Like it's not a super, there's not one person left out for really controversial reasons.

Before he passed away, Gerard Ferret was probably the other person in the conversation. And his big contribution, as well as being one of the key early figures, is that... He came up with some of the earliest examples of flexible moths. So despite being crystals, these can be really, really flexible materials. Okay.

So in diving in and learning about this, something that I think new entrants to the field will see right away and be puzzled by is these goofy names that y'all have come up with in this field. HKUST-1 MIL-53, on and on. These are super bizarre. And we just did an episode on steel where one of the things we talked about was nomenclature and trying to understand what's driving this. Is there a system method to the madness? So let me ask you.

Is there a method to the madness? Like, what does all this mean? Well, there's some information in this. So basically, I mean, I think this is also one of, this is kind of the naming conventions were kind of started by Omayagi as well. All the materials that Omar Yagi discovered, his team, are called MOF with a number after them. And initially that number was sequential, but now I understand that it is not sequential and they choose numbers that...

for reasons that are completely opaque to those not in the group. But basically, there's kind of two broad ways. So most MOFs are called some acronym. So MOF, MIL. um my favorite which is is is uio and then a number right and ordinary the number ticks up but it's as i said not not guaranteed so those acronyms at the beginning

typically actually the university or the institute those materials come from. So I guess this is a little bit like the naming of some mesoporous silicates, so things like MCM, if you're aware of that one. So Miller's Material Institut Lavoisier in Paris, UIO is the University of Oslo. That one is really confusing because... Many people very reasonably assume that it must contain uranium or iodine or oxygen. And in fact, it's a zirconium material.

you get a handful of other materials which are kind of given an acronym that tells you about the chemistry so uh you can get uh zifs are probably the most famous zeolitic imidazolate frameworks so these are materials containing the organic ligand imidazole and they look a bit like zeolites in terms of the structures that they have. So it's basically a memorization exercise, unfortunately, but you can get a little bit of information about it.

There's also some moths that have multiple names, which is usually for academic-politic reasons, and it's completely unhelpful.

it is what it is now and we kind of those in the area just have to kind of live with it and try not to contribute to the to the chaos curious how this will evolve as computational design starts to be integrated into the MOF world more and more where rather than it being somewhat of a rare event to discover a MOF, or maybe it wasn't all that rare, but it was at least limited by experimental capacity.

now with the ability to design and find stable moffs quicker than ever, do you think a naming convention will emerge? I hope so. I hope so. I think people love naming things after themselves or their groups, which is so, you know. That human thing's always going to be there. But yeah, I hope that there will be some more rational...

ways of approaching things. So one of the ways, you know, the computational design, I think is really an exciting aspect of things. And often the way they do this is by starting with the kind of structural topology that they want. So there's a series of relatively sensibly chosen three-letter names for topologies, so the way that your nodes and edges are connected in space. That makes much more sense to me. So you've got things like FCU for face-centre-cubic.

you know, REO, which would be the rhenium trioxide structure, things like that. So you use these three-letter codes, and then you can give some details of the chemistry you've used to realize that topology. and um so there's a few people including me who've used those kind of codes to try and give a little more uh information in uh in those apology labels to give a little more information uh in the names of stuff but and hopefully that'll be

That kind of stuff will be more widely adopted in the future. And material scientists, before you feel too smug about your naming conventions, we also have some goofball names, Spinell and Wurzite. Naming them after the minerals that were found is also pretty oddball, right?

All right. So fascinating history. You think this is worth a Nobel Prize? Absolutely. Yeah. How come? I'm a morph chemist. Well, is that just because there's so many people that work in this field and they find it interesting, therefore it must be valuable or why?

Okay, so I think there's a few things. So one is from a sort of chemistry sort of new compound perspective. These are really remarkable in the kind of design. You can go from... kind of a sketch of what you imagine a 3D structure, you know, atoms in space might be, and come up and using these kind of MOF strategies, there's what's Oma Yagi christened reticular chemistry.

uh reticule being like a net like a fishing net um to to design like where these atoms are going to be in space and that seems really really powerful and if you're looking at say ceramics or something like that you know it's often you're a you're kind of just often just using the fact that there's a known compound nearby to do it you there's no it's much harder to come up those kind of design rules um whereas moths do have that ability and this sort of modularity

is really exciting for not just chemical structures, but being able to tune the properties as well. The other reason I think is that they have really very, as we touched on a little bit, really unusual physical properties. very very high porosities porosities with like different kinds of pore you know not only the voids but the edges of the pores can have all kinds of interesting chemistry on them

They've got very high flexibilities, as I mentioned. So really large, despite being crystals, really large thermal expansions, really high compressibilities, negative thermal expansion, negative compressibilities. things that shrink or expand when you take up solvents, crystals changing in volume by 100% or something. There's a lot of very strange things going on in these materials.

So I think that's... In other words, it's a complexity argument. You think because these are such a massive design space with so much tunability, tailorability, it's worthwhile to make a Nobel Prize out of it because it's such a big new area. Yeah, and I think really all three of the laureates were people who recognized this idea of design, not just that these are...

Because actually, if you go back and look at the databases of crystal structures, you find materials that are pretty plausibly moths. They're compounds that could be now categorized as that. And in fact... You asked Eliana, were these discovered in nature? And they have been, but only about 10 years ago. There are some rare...

Russian biogenic minerals containing oxalate that happen to be moss. But it's not that the compound exists, it's that we know and we have these strategies for designing materials, which I think was a really exciting aspect of it.

All right, Matt. So you've said before, we're thinking of this a little bit like Legos, right? You've got building blocks. You've got the metals, which are the vertices, the corners. You've got the linkers, which is these organic molecules. And then you can also have functionalization, right?

as you were to swap out different metals or different ligands like what's driving that what options do you have um yeah like what choices do you have like why would you choose one linker over another so um so there's this kind of straightforward thing so you choose a longer if you choose if you take a ligand that makes them off and you make a longer version so it has the same same geometry the same binding group to the end but it's just bigger in the middle no maybe you've got one

and add an extra benzene ring you'll make it more porous you'll make it more open um and people have you know explored that in fact that's that's now very very standard kind of thing you can do that's one side but probably more interesting is is you can add you know do organic chemistry on the ligand and put on reactive groups and those reactive groups you could either use as tags for other things or in fact they'll infuse the gas sorption properties so

For example, if you put, so most of the time, the moth ligands are basically hydrocarbons aside from the bits that bind to the metals on the end. But if you put something like an amino group, an NH2 group, that'll... often bind with gases much more strongly, so allow you to have selectivity for one gas over another. And that's something that's really being explored at the moment. There's obviously a lot of desire, and I think you covered this in a previous episode, to get...

absorbents that not only have large uptakes overall, but can specifically separate one gas over another. If you can strip out CO2 specifically from the air, then that'd be really exciting in terms of reducing the effects of climate change.

And adding something like an amine group or a benzene group or whatever else, right, also affects the rigidity, right? The mechanical properties of those linkages. And you'd already mentioned before that some of these things are flexible or not flexible. I imagine that plays a role.

Yeah, for sure. If you make it longer, it tends to make the material floppier. Straightforward. You could do kind of wackier things as well, like... uh people put photosensitive groups in in it so there's various sort of chemical groups that are known to change shape is a benzene is a change shape when you shine light on them and people look to materials that way and okay i've seen these sort of molecular motor sort of things where you try and get functionality be a light

Yeah, exactly. That's actually a current topic for a few research groups at the moment, I know. But there's really the full range of things you can do in organic chemistry. If you put binding groups on either end, you can probably get it into a moth.

explore what this organic molecule does in this crystallized material. In doing some preparation for this episode, I came across a number of people who were really interested in using MOFs to basically devise atomic micro-reactors or nano-reactors or maybe angstrom reactors where you can now do particular reactions within the MOF pore on a singular molecule or perhaps on a protein itself.

So I know a lot of people are interested in that. Are you familiar with any research in that realm? Yeah, there's all kinds of different things. So I guess the material that I've only kind of mentioned in passing, but is... kind of the forerunner in terms of porosity would be would be zeolite so these these sort of very porous silicates so they tend to have lower porosities

a less you know a more restrictive range of chemistry but they're really really widely used and one of the reasons they are is because they're catalysts and they are able to have reactions happen inside the porosose materials so they're very hard to do otherwise and this is i think a goal of a lot of moth chemistry as well um so either using the framework to as you know

functional groups on the framework to catalyze reactions, or even just the confinement effects of squishing two molecules that want to react together in the pores. That's something else that people explored. Equally, people have done stuff like...

If you can trap something bigger inside the pores of a moth, you can often protect it from the outside world and keep it together. So proteins would be the example that people are looking at. Proteins and other kinds of biomolecules that can often be fragile. uh on their own outside of solution sometimes uh design them off or come up with them off that would that would help keep them safe oh that's really cool yeah i think like material scientists like we've got our little tiny acrylic

you know, tiny sample holders. It's cool to think of a molecule, as it were, as the sample holder for some other substructure. That's really cool. Yeah, it's a really, really neat idea. And before we move on, what is your research in MOFS? Yeah, so I don't really look so much at the porosity of things. So as I said, I do physical properties. And what I mean by that is kind of looking at the magnetism and conductivity of these guys.

The metals are far apart, and the metals are often where the magnetism and conductivity is. But the organic molecules can themselves have unpaired electrons of spin and charge as mobile. you know opens the door to a lot of different interesting behavior so room temperature room temperature uh ferromagnetism metallicity you know these have recently been found in in moths and related materials so we're kind of looking at moths as quantum materials basically

using that modularity that the idea that you can tune it how do people characterize these x-ray diffraction comes to mind but could you see these structures in like a tem yeah so so X-ray diffraction is the standard, right? So these are solid materials. So X-ray diffraction allows you to see the crystal structure. We can solve the structures from single crystal or in favorable cases, powder diffraction. and work out where all the atoms are and move forward from there.

As TEMs have improved, actually you can get atomic resolution images of these moths as well. You can see the pores, you can see the metal clusters and organic ligands that make the materials.

up so in fact you can even see you know defects uh in these materials so sometimes you're missing a a series of ligands or organic molecules or you're missing a series of metal clusters and you can actually pick that up in the tem now so yeah you can visualize it really direct really directly as well as using these diffraction based techniques so microscopy extra diffraction but you mentioned a an odd one previously we were talking about this that people rely on

BET, right, so that's Brunauer-Emmett-Teller, it's a porosity measurement. It measures surface area, which I wasn't surprised because many of the applications of this are reliant on the surface area. But you pointed out it's not just that. It's that the porosity, if it's very low...

that's an indicator of impurities, right? Because the MOF structure itself, you expect it to have a high surface area. So they'll do sometimes these BET measurements to tell you essentially something about the purity. Like, did you get the pure MOF or is there a bunch of not porous other stuff in there, which I thought was super cool.

Yeah, exactly. So if you're thinking of, say, a typical moth where they have 2,500 meters squared per gram surface area, you know it's going to be that because you've got the crystal structure. You can have a rough geometric guess on what it should be. If you measure your material, you've...

You think you've cleaned it out pretty well, you've activated it in the jargon, and you measure the surface area and it's only 500 meters squared per gram. Either you've got a lot of stuff that's not your moth at all there, and it's only a small fraction of it, or your moth... Pores are full of all kinds of other molecules, maybe solvent residues of the synthesis. So it's one of those techniques that is harder to...

If you get the high surface area, then it's really telling you that you have the material that you expect to a large extent. I mean, interestingly, actually, I mentioned defects. Sometimes you get to the... you get the opposite you have a higher surface area than you should right you look at the crystal structure the perfect crystal structure and the surface area is bigger yeah some hold on a minute

Yeah, and that's often because you've got these defects. You're missing parts of your material. There are bigger pores than you expect, and that can be a sign of things like that. That's the kind of special case. So one thing that we looked in the literature getting ready for this episode is I think it was Kitagawa's group. They did BET measurements, right, as you do to figure out.

But they found something that during these absorption isotherms, there is this what they're calling a gate opening behavior during these flexible frameworks where essentially it's a signature of these soft MOFs where the structure can open in response as a function of the pressure of the gas around it.

Yeah, so this is one of the kind of weird things that is mainly seen in moths. I'm sure people have found it in other kinds of porous materials afterwards, but it's kind of a real signature of these sort of materials. So basically it's saying that the moth binds strongly enough to that gas that it is able to overcome the lattice stiffness to just change its crystal structure, go through a phase transformation to a different phase.

So you can sort of think of it, I don't think these were the initial Kitagawa ones, but some moths look basically like wine racks. So they have sort of a square grid almost. um or where there's very rigid uh edges with flexible corners all right so wine right you can flatten down and in fact if most of these those sort of wine materials often you make them they're in that kind of closed form

They're the kind of form that you'd store in your garage or whatever. And then if you put CO2, you know, a reasonable pressure CO2 or water or whatever the gas your material responds to in. They'll just open up. And this is a very dramatic change and leads to very clear signature in those porosity measurements. The interesting thing there is that it seems to be... quite specific phenomenon. So very often say CO2 will open it because it binds relatively strongly.

uh compared to something like nitrogen so you could perhaps idea use this idea to selectively take up co2 over nitrogen oh interesting that's one of the real uh One of the reasons that people have really pursued these kind of gate opening transitions.

So we've been skirting around one of the key issues with MOFs that we've got to dive into, which is you've said like these are flexible, they can move, there's this gate opening mechanisms. All this sounds great from a functional standpoint, but with the problem lurking in the distance is that...

Maybe these things aren't as stable as they need to be. Is stability a real issue? Tell us about that a little bit. Yeah, stability is definitely an issue, right? But it's one that has been worked on quite a lot. It's been kind of there from the beginning. People have realized that these materials aren't quite as stable as you'd ideally want. So MOV5, that Omer Yagi material, the zinc BDC material, is really exciting because it's really thermally stable.

or a moth like 400 celsius yeah um um but or you know for an organic containing thing it's pretty it's pretty stable uh but that one is is pretty sensitive to water so it'll start turning to a hydroxide um And that was about 25 years ago that material was first discovered. And people have now come up with strategies to make materials that we can reasonably expect are going to be pretty stable in.

uh not only to sort of similar sort of temperatures you know 400 500 celsius but also in acids and bases um and relatively aggressive conditions um and that often is using like higher charge metals so that's so the just increasing the ionicity right increase the bond strength yeah yeah yeah exactly just just crank up the charge to make the bond stronger so these zirconium ones and also um

It seems that if you have more hydrophobic organic molecules as your connectors, that tends to increase the stability as well, because it just means the water won't go in as much and won't start attacking those metal.

uh organic bonds uh quite as much so that seems to be the other big thing that people do so not every moth is stable and something that people you know you still check it you know when you're making materials still checking it uh but It provided that you don't need really high temperatures where your organic is going to just start burning, basically decomposing, or really, really aggressive conditions.

there will probably be a MOF that will be stable in the conditions you're looking at nowadays. So maybe we can transition a little bit to what these could be used for. We've touched on it a little bit with absorption or maybe gas storage. But what are some other interesting applications for MOFs and what are people trying to design for? Yeah, so still I think the main one that people are looking at is gas sorption separation.

so all kinds of different separations people are trying to optimize you know co2 nitrogen i mentioned things even more exotic ones like hydrogen deuterium um but that kind of more niche application too The other, I think, big application at the moment that people are looking at is catalysis. So using these MOFs as catalysts to selectively speed up particular reactions, whether this is sort of more on the organic scale, you know.

If you're sort of an organic chemist, you want to have a kind of small-scale catalyst to use, or something more on the industrial scale. So there's a lot of people looking at things like photocatalysis. there's like water splitting and so on i mean that i think one of the more uh advanced areas that there is sort of a bit of it seems these mosques are pretty good is trying to combine the two and this would be

things where you have absorbent, which is trying to decompose the molecule that it's absorbing. So the sort of example of this would be sort of chemical weapons, right? So if you want to, you know, You want something that's going to not only filter out the dangerous molecule to stop you from breathing in, but also decompose it. That's so cool. And there's a handful of moths that seem to be able to do this pretty effectively.

Is that an electrochemical process, right? In other words, does the MOF backbone have to be conductive? No, no, no. These ones just work. Basically, they specifically... A lot of the chemical weapon things are relatively reactive in a specific way, but they have to react with something in your body to cause the damage. And they are able to kind of mimic that.

speed up that transformation often with water you know just making them react with water and turn into something much more more benign um overall but that's how they work so you don't actually At least the ones I've seen, you don't need to hook them up to a circuit or anything. They're just a passive device that you could put into a filter.

Yeah, the other one I had seen is for natural gas vehicles. I guess storing natural gas can be pretty challenging, but with a MOF, being able to store it in the porosity, it can actually compress these molecules and allow you to increase the amount that you can store in a given tank. Well, and it lowers the pressure, right? Like the tank cylinders doesn't have to be at a high pressure because there's a chemical absorption reducing the physical pressure of containing that gas, right?

Yeah, exactly. So, I mean, it's a bit counterintuitive if you say it like we're taking this gas tank and then we're going to fill it up with something else. and that will allow you to store more things in it. It's the fact that your methane will just stick to the moth better than it does to itself, and so it reduces the pressure that you need for it. They've gone as far as making, I think BSCF have made methane tanks containing a moth to show that you can produce it.

Methane vehicles have not taken off, particularly with the success of batteries, but that kind of big-scale gas storage is one of the things that people are used for. I was just going to say, I mean, like people have been talking about this with zeolites for a while too, but my understanding is it's about how easily you can, not only how easily you can store the gas, how easily you can get it out as well. I think zeolites, it takes pretty high temperatures to do that.

Yeah, so when you're thinking about these absorption processes, there's always a kind of fine balance. I think hydrogen is sort of the example where it's really tricky because hydrogen... doesn't interact very strongly you know in some ways it's like a molecular helium that doesn't interact very strongly with with most absorbents and that means it's very hard to get a large amount of hydrogen into into a material except very high pressures or very low temperatures right um

But there's lots of chemical ways of storing hydrogen where the hydrogen is really strongly bonded. So basically molecules that contain hydrogen and that you could decompose to release that hydrogen. So ammonia boranes and things like that. And so those will have the high capacities that you'd want. The hydrogen as a percentage mass of material is pretty good, but it's impossible to get out.

So there's this kind of trying to find materials that are in that middle space where they have strong enough interactions where the gas you want sticks in, but weak enough that you can get it out again without, you know.

having to uh go to like 500 celsius or whatever is is the real challenge and i think you know the hydrogen in particular that one still seems to be quite tricky um so we three academics are clearly like great at brainstorming potential uses of this But are companies biting on it? Like, are they actually manufacturing large-scale production of MOFs? Yeah, there are definitely large-scale manufacturing of it now. I mean, it depends what you mean by large scale. So if you...

I think thousands of tons seems to be about as much as people have made of some of these materials, which is plenty for lots of applications. In terms of the sort of applications people are looking at,

One of them is, I think, Numat in the US, where they're using MOFs. They have the same kind of gas tank idea, but rather than storing methane, storing... high purity gases for semiconductors so these are often very toxic things like arsene don't really want like as in your lab at all unless you really have to so reducing the pressure of it means that they can become a little bit safer i think it's the basic idea there

Other kinds of applications that I think have been explored are using to store things like ethene and plant hormones. you know, regulate the raping of fruit so you can store them for longer. I think particularly apples, people look at that. I've heard that, right? Is it, I think it causes it to mature more rapidly? Yeah. And so essentially if you, if you absorb that, then you've extended the shelf life of the fruit.

Yeah, I think that's the idea. It's basically the reason why if you put bananas in the fruit bowl with other fruit, everything starts going riper a lot faster. So it's the same kind of thing, but in reverse. I think the big potential application that a lot of people are excited by was probably two. One would be water harvesting. So using moss that have high affinity for water and using the fact that, so if you're in a desert.

I don't need to talk to people in Utah about that. You can often get very big temperature swings and you often need water. So the idea would be perhaps overnight your moth will take water out the atmosphere. And then just the heat of the day will then release that water back out again. So you could use that to effectively generate water from the ambient humidity, even if it's relatively low.

So that's one application that people are looking at on a larger scale. And the other one is things around carbon capture, right? If you can get direct air capture, so capturing CO2 out of the 400 ppm that's in the air.

uh that would be really exciting but that's like a really that's both a really hard challenge sort of academically you know this is a quite low concentration so you have to be really selective to put it out but also is i think a big industrial challenge because there's a lot of co2

you know he's even 400 ppm there's a lot of air on earth and so to to have a meaningful effect you're going to be making this on an enormous enormous scale yeah i think it obviously makes much more sense to capture it where it's concentrated at the at the polluting site right at the smokestack right put it in in line with that you can do a lot better yeah there's also people looking at that as well of course um yeah those are some of the kind of bigger scale things that people are looking at

There seem to be a lot of promising applications, and in many of the demonstrations I've seen, MOFs have shown success. So what do you think really is the limiter here that's kind of preventing a lot of these interesting academic discoveries from having greater market penetration? Is it... the processing of the MOFs themselves, the scaling of those from the benchtop to an actual production process, or is it something else? Do we still not really understand the chemistry as well as we need?

I think probably the first two that you mentioned are the bigger things. So most of the time you're making moths, you're making them as loose powders. And loose powders are...

There's rarely the application you really want, rarely the sort of form of material you really want. If you want it for gas separations, you probably want it as a membrane. If you want it to storage and you need it to have something that's going to not just leak everywhere when you... things in and out so people pelletize it to make it kind of more stable and you have to think about how stable your pellets are and all that kind of more practical thing there the other side is as you say scaling

So the classic way of making a moth is heating up your metal salts in maybe a metal chloride or nitrate, which are often acidic, in a solvent. like dimethylformamide, which is toxic and really not a nice thing to handle, particularly if you're going to be doing it on a massive scale. People have had to come up with ways, and they are. of getting rid of some of these more challenging features and getting the yields up. I think, yeah. Yeah, do the synthesis?

Do processes affect the quality of the outcomes? I have to imagine they do. So you have a target structure, right? But maybe that structure will still be maintained depending on your synthesis route, but you might have more defects. Yeah. I mean, this is part of it. You can't just assume that you're going to get the material you want.

if you change the synthesis parameters. You'll have to do the quality control to make sure that your structure and physical properties or chemical properties that you really desire are still there when you change the parameters of your synthesis. and i think maybe part of the reason why there's this challenge with scale up is just that we haven't you know this hasn't been done very much at all so we don't have the kind of if you take

If you take other kinds of classes of materials, we've now got a lot of experience in what kind of things are going to be really important, how you can get it on the scale that you need. And also, critically, people have an idea of how much it'll cost. MOFs are expensive at the moment, generally. But part of the reason they're expensive is that... The metal? Or what's driving the cost?

Well, so, yeah, I mean, it is possible. So if you decide to make them off out of gold, then it's going to be expensive or, you know, some of the rarer metals. You can't get around that if it involves something that's, if it has an ingredient like that, that is expensive. But a lot of it is that just the... preps aren't optimized because they're done on such a small scale. If you're making, in my lab, making a gram would be something we'd gear up to. We start with much smaller scale.

So of course the cost of that is going to be, if you've worked it out, would be just unviable. And we don't know how cheap you can... if you make a material in my lab, it's not obvious how cheap it will be if you did it properly on a ton scale. For sure. And as the experience in kind of a scale-up happens, then people will be able to say, oh, this morph.

you can probably make that for, you know, how many dollars a kilo or whatever. So my final question for you, Matt, is I'm going to have you pull out your crystal ball for a minute, right? You've been to the conferences, you've talked to everyone. What's at the frontier of this field? Like what's coming down the line that excites you? I think, I mean, I mean, first off, I guess I would say this, but I think some of the things in terms of the magnetic and electronic properties of MOSR.

are still really surprising i don't think we've reached the kind of limit of where we can where we can get there i think they're going to be as well as materials with higher conductivities higher ordering temperatures there's probably going to be some pretty weird things you know the same way there's some pretty weird ceramics with magnetic properties that's probably going to be most doing that i think also it seems that there

We've now reached a point where MOFs are actually getting used industrially. Companies are spending serious money trying to not only make, but also buy these materials and put them into applications. That's, I think, going to have an effect as we realize where the actual challenges are. It would be less academics saying, this is probably going to be useful and more companies saying, actually, we need this.

So I think that's going to be probably a change over the next few years. Matt, you're amazing. Thanks for joining us. Thanks for suggesting the episode. By the way, listeners, love that. Reach out to us. We love to interact. You can find us all over the place. We're on Instagram. We're on YouTube.

uh we read the comments we love it and it helps us come up with better shows matt i hope our paths cross again um i hope we see each other soon and if not enjoy the rest of your stay in bordeaux and we'll see you next time yes thank you very much for having me If you've been listening to the last couple episodes, you've probably heard us talk about the American Ceramic Society and their great on-demand learning center.

If you're a technician or an early career scientist, or maybe you have a lot of experience and you want to learn even more about ceramics and enhance your technical expertise and your know-how, this is a great way to do that. Yeah. One thing to point out is we have been really excited that they offered a special bonus for our listeners. You get a hundred bucks off of any on-demand course registration, but that is until the end of the year.

So time is short. Sign up while you can get that sweet benefit. And they've actually got some new content on there. For example, there's new additions to the lineup. They've got an intro to electronic ceramics, a technician's guide to ceramics. and a foundations of ceramic processing, all cool courses in addition to the many that are already out there. You can use the code materialism and you'll get a hundred bucks off your registration till the end of the year. So check out.

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