EarthWeek ChemSplainer: Mircea Dincǎ, MOFs, and water scarcity
At the Princeton Materials Institute Symposium last week, Alexander Stewart 1886 Professor of Chemistry Mircea Dincǎ presented a talk on the innovative science surrounding Metal Organic Frameworks (MOFs), a class of highly porous materials being investigated in part for their immense storage properties. Dincǎ’s comments on how MOFs could provide one answer to a world menaced by freshwater scarcity are especially relevant this Earth Day.
Below, a brief Q&A piece based on Dincǎ’s talk and the utility of the MOFs platform, whose time has come.
Q: Can you define MOFs in context of this research?
A: Think of them as sponges. Your average kitchen sponge has a pore size of about 1 millimeter, but MOFs are on the order of one nanometer. So, the pores are a million times smaller. Now imagine you have a cube that is one square meter on the edge; internally that would give you a surface area of six square meters, which is okay. But if you then start to fill that big cube with the tiny cubes, you still have one square meter, about one teaspoon. But the internal surface area of that teaspoon of material is going to be the same as that of a football field. And this is amazing. It’s by far the highest surface area material that we know of.
Q: Why is that important?
A: Now, you have these tiny holes, and you can put tiny things in them, like molecules. Specifically gaseous molecules – CO2, methane, water. They really love to interact with the surface more than they interact with themselves, which means you can put more gas in the same volume if you first fill that volume with the MOFs. It’s kind of counterintuitive.
The Alexander Stewart 1886 Professor of Chemistry, Mircea Dincǎ.
Q: How is your lab involved in research on MOFs and water scarcity?
A: This came about because I was reading how big a problem water scarcity is. Here’s the fact that should scare you: by 2030, half of the world population—so, four billion people—will live under severe water scarcity. They will not have access to fresh water on a daily basis. Even if we could distribute that water, there is not enough fresh water on this planet to supply eight billion people at the same level that we supply Americans today.
Q: Your PMI Symposium talk mentioned atmospheric water …?
A: It turns out there is 13K cubic kilometers of water just in the atmosphere. It’s not an insignificant proportion: that’s .04% of all fresh water just sitting in the atmosphere. So the question is, can we provide some of this shortfall of fresh water with something from the atmosphere? To do that, you need a sponge that will selectively adsorb water from the atmosphere and deliver it. But that sponge needs to be water stable. So this is where MOFs can come in, if you can make them water stable. Most are not.
Q: How do you address that challenge?
A: MOFs are made of these segments where you have metal ions or metal clusters, things like zinc ions, manganese ions, connected through organic ligands, and the connection points are these functional groups – carboxylic acid, pyridines. The weakest link in the material is the metal ligand bond. That is determining the water stability, the hydrophilic stability, the thermal stability, and often the catalytic properties. And so that is the bond I have to concern myself with if I want to make it water stable.
Q: How is your lab responding?
A: By tuning the pore size and hydrophilicity of MOFs, we can engineer materials that uptake water at low humidity and release it with minimal energy. We focus on the thermodynamics and kinetic control of guest molecules within MOF pores for water capture from the air. We have developed theoretical models to predict the relationship between pore size, hydrophilicity, and the critical humidity for capillary condensation.