Metal-Organic Frameworks

I’ve been talking about new technologies for fighting climate change, with an emphasis on negative carbon emissions. Now let’s begin looking at one technology in more detail. This will take a few articles. I want to start with the basics.

A metal-organic framework or MOF is a molecular structure built from metal atoms and organic compounds. There are many kinds. They can be 3-dimensional, like this one made by scientists at CSIRO in Australia:



And they can be full of microscopic holes, giving them an enormous surface area! For example, here’s a diagram of a MOF with yellow and orange balls showing the holes:



In fact, one gram of the stuff can have a surface area of more than 12,000 square meters!

Gas molecules like to sit inside these holes. So, perhaps surprisingly at first, you can pack a lot more gas in a cylinder containing a MOF than you can in an empty cylinder at the same pressure!

This lets us store gases using MOFs—like carbon dioxide, but also hydrogen, methane and others. And importantly, you can also get the gas molecules out of the MOF without enormous amounts of energy. Also, you can craft MOFs with different hole sizes and different chemical properties, so they attract some gases much more than others.

So, we can imagine various applications suited to fighting climate change! One is carbon capture and storage, where you want a substance that eagerly latches onto CO2 molecules, but can also easily be persuaded to let them go. But another is hydrogen or methane storage for the purpose of fuel. Methane releases less CO2 than gasoline does when it burns, per unit amount of energy—and hydrogen releases none at all. That’s why some advocate a hydrogen economy.

Could hydrogen-powered cars be better than battery-powered cars, someday? I don’t know. But never mind—such issues, though important, aren’t what I want to talk about now. I just want to quote something about methane storage in MOFs, to give you a sense of the state of the art.

• Mark Peplow, Metal-organic framework compound sets methane storage record, C&EN, 11 December 2017.

Cars powered by methane emit less CO2 than gasoline guzzlers, but they need expensive tanks and compressors to carry the gas at about 250 atm. Certain metal-organic framework (MOF) compounds—made from a lattice of metal-based nodes linked by organic struts—can store methane at lower pressures because the gas molecules pack tightly inside their pores.

So MOFs, in principle, could enable methane-powered cars to use cheaper, lighter, and safer tanks. But in practical tests, no material has met a U.S. Department of Energy (DOE) gas storage target of 263 cm3 of methane per cm3 of adsorbent at room temperature and 64 atm, enough to match the capacity of high-pressure tanks.

A team led by David Fairen-Jimenez at the University of Cambridge has now developed a synthesis method that endows a well-known MOF with a capacity of 259 cm3 of methane per cm3 under those conditions, at least 50% higher than its nearest rival. “It’s definitely a significant result,” says Jarad A. Mason at Harvard University, who works with MOFs and other materials for energy applications and was not involved in the research. “Capacity has been one of the biggest stumbling blocks.”

Only about two-thirds of the MOF’s methane was released when the pressure dropped to 6 atm, a minimum pressure needed to sustain a decent flow of gas from a tank. But this still provides the highest methane delivery capacity of any bulk adsorbent.

A couple things are worth noting here. First, the process of a molecule sticking to a surface is called adsorption, not to be confused with absorption. Second, notice that using MOFs they managed to compress methane by a factor of 259 at a pressure of just 64 atmospheres. If we tried the same trick without MOFs we would need a pressure of 259 atmospheres!

But MOFs are not only good at holding gases, they’re good at sucking them up, which is really the flip side of the same coin: gas molecules avidly seek to sit inside the little holes of your MOF. So people are also using MOFs to build highly sensitive detectors for specific kinds of gases:

Tunable porous MOF materials interface with electrodes to sound the alarm at the first sniff of hydrogen sulfide, Phys.Org, 7 March 2017.

And some MOFs work in water, too—so people are trying to use them as water filters, sort of a high-tech version of zeolites, the minerals that inspired people to invent MOFs in the first place. Zeolites have an impressive variety of crystal structures:





and so on… but MOFs seem to be more adjustable in their structure and chemical properties.

Looking more broadly at future applications, we can imagine MOFs will be important in a host of technologies where we want a substance with lots of microscopic holes that are eager to hold specific molecules. I have a feeling that the most powerful applications of MOFs will come when other technologies mature. For example: projecting forward to a time when we get really good nanotechnology, we can imagine MOFs as useful “storage lockers” for molecular robots.

But next time I’ll talk about what we can do now, or soon, to capture carbon dioxide with MOFs.

In the meantime: can you imagine some cool things we could do with MOFs? This may feed your imagination:

• Wikipedia, Metal-organic frameworks.



11 Responses to Metal-Organic Frameworks

  1. Richard Gaylord says:

    john:

    nice column. how does it feel to think about chemistry rather than maths? when it comes to the science that really affects humankind, chemistry is more relevant than physics. the survival of the ‘real’ world depends on the behavior of electrons and molecules more than it is on quarks and spacetime. it would be nice if the pop science publications recognized this (eg. ‘Quanta’ doesn’t even list chemistry among the subjects it covers) and if discussions of the philosophy of science considered chemistry as well as physics.

    • John Baez says:

      Richard wrote:

      how does it feel to think about chemistry rather than maths?

      I feel extremely ignorant about chemistry compared to mathematics, and I don’t think I’ll ever become an expert: I’ve spent approximately 45 years studying mathematics quite intensively, I think that’s about what it takes to become an expert, and I’ll never be able to do that with chemistry. But, I enjoy thinking about chemistry, as long as I keep doing enough math to feel like I’m competent in something.

      when it comes to the science that really affects humankind, chemistry is more relevant than physics.

      I basically agree, though I’d say: the sciences that are important are sociology, economics, psychology, biology, ecology, chemistry, etc… physics too, but not particle physics or cosmology.

      the survival of the ‘real’ world depends on the behavior of electrons and molecules more than it is on quarks and spacetime.

      Well, it’s good to know that space is \mathbb{R}^3 and time is \mathbb{R}.

      But I agree with your main point: it’s annoying that pop science romanticizes “fundamental physics” much more than the sciences that may help save us.

  2. Grant Roy says:

    Very interesting. I have a car that runs on methane and it would be nice to not have to gas up so much! But I would say hydrogen is the ideal.

    I guess one way I can envision using MOF’s is as better energy storage for data centers. The DOE has an interesting project investigating the extraction of methane hydrate. One thing I thought about was why not just locate data centers in the arctic sitting on top of this stuff (it’s already cool). Maybe MOF storage coupled with gas turbines with heat recapture would drive the carbon footprint down significantly. I have no idea if it could aid the extraction process itself.

    Of course, you still need high-speed data transfer, but things like starlink will be coming online soon. This may make at least scientific computing viable, and the DOE already runs the nations top supercomputers so perhaps there are some synergies that could be exploited. In turn, the supercomputers could be doing MD to improve MOF design!

  3. domenico says:

    It is very, very interesting.
    I am thinking that there is a natural structure that has a high affinity with the carbon dioxide, the hemoglobin, and I am thinking that it would be possible to incorporate the hemoglobin properties in a metal organic frameworks, building a MOF with three-dimensional repetition of heme coordination complex (biology has optimized the carbon dioxide capture).
    I am thinking that a pure crystal structure using organic chemistry, and iron atoms, could be enough to increase adsorption of carbon dioxide, or other gases.

    • John Baez says:

      I thought hemoglobin had a high affinity with oxygen, not carbon dioxide. Am I confused?

      • Proteomics Postdoc says:

        I wouldn’t be too suprised if Hb binds to CO2 with some affinity… as it binds more strongly both to CO and CN than to O2 (thus their toxicity), but CO2 is transported predominantly in the form of bicarbonate ion, serving to buffer the blood at about pH 7.4.. not bound to hemoglobin. Not sure where the poster is getting their information. Also on a density and stoichiometry perspective, I’d imagine that these MOF frameworks could do a better job than protein. At the atomic scale, proteins are really big!

        If we’re going for protein engineering as a strategy for CO2 drawdown, my money is on RuBisCO (https://en.wikipedia.org/wiki/RuBisCO). RuBisCO fixates CO2 in plants, is likely the most abundant enzyme on Earth, and is involved in the first stage of carbon fixation, reacting Ribulose 1,5-Bisphosphate with CO2. Interestingly O2 competes rather efficiently with CO2 for binding to RuBisCO which, given the amount of time photosynthesis has been around–you’d think that Nature would have figured out a way around that by now. My understanding is that it was long viewed as “good enough” that tunneling through series of maladaptive mutations to reach a more optimized enzyme was disfavored[1].. but making a better RuBisCO is turning out to be harder than synthetic biologists initially thought:

        https://www.sciencedirect.com/science/article/pii/S095816691730099X

        1: Possibly misguided interpretation of undergrad cell biology lectures.

      • domenico says:

        I am not an expert, I thought that the hemoglobin could be a good absorber for carbon dioxide (like in the blood flow); so that I searched on wikipedia the chemical formula and structure, and I see that the affinity of carbon dioxide in carbaminohemoglobin (that I read is reversible): I was wrong to think that the activation site was heme, that work for the oxygen and carbon monoxide.

  4. Arch1 says:

    Me too, and we were half right:-) In addition to transporting oxygen in, hemoglobin transports (10-25% of the body’s respiratory) CO2 out.
    And that’s not all: Hemoglobin also transports other gases including nitrous oxide.
    But wait, there’s more! Hemoglobin also performs other functions besides gas transport, but for the details you’ll have to consult wikipedia ( or perhaps Ron Popeil, who is still with us), as I’ve forgotten them already.

  5. Brian Space says:

    John,
    An earlier MOF we modleled for CO2 capture has a nice video (also on my website ) showing selective CO2 capture from a bath of N2. 14:1 N2/CO2 it’s on YouTube here :

    It might illustrate your focus.

    I very much enjoy your work and how you share it.

    Sincerely
    Brian Space

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