This MOF is hot to go

Industrial flue gases that come from making cement and steel are, as pop star Chappell Roan might put it, hot to go: factories belch them out at temperatures above 200 °C. Scientists have discovered that a metal-organic framework (MOF) studded with zinc hydride can reversibly capture carbon dioxide between 200–300 °C. The finding offers a way to sequester the greenhouse gas without having to cool it, which can be an expensive and energy-intensive process.

Using MOFs to capture CO₂ from industrial gas streams has been a longstanding goal and has caught the attention of firms like Baker Hughes and BASF. But MOFs don’t work well at high temperatures. That’s because of entropy—the inescapable thermodynamic law of disorder.

“Entropy means gas molecules don’t want to stick to a surface at high temperatures,” says Jeffrey R. Long of the University of California, Berkeley, who led the research. Because CO₂ is acidic, MOFs need a strong base to capture it. Most MOFs use amines, which react with CO₂ to make an ammonium carbamate. But amines typically have many degrees of freedom, and the ammonium carbamate does not, so the transformation requires a large entropy change. “At high temperatures, it’s difficult to do that reaction,” Long says.

Long’s team discovered that a MOF known as ZnH-MFU-4l, which uses ZnH as a base instead of an amine, can snatch CO₂ at high temperatures. ZnH has few degrees of freedom, so there’s a relatively low change in entropy when it reacts with CO₂ to make a zinc formate. Putting the MOF under vacuum at high temperatures removes CO₂ (Science 2024, DOI: 10.1126/science.adk5697).

ZnH-MFU-4l was developed a decade ago in Dirk Volkmer’s lab at Augsburg University. Those researchers showed that they could drive CO₂ off the MOF to form the ZnH, but they didn’t report CO₂ capture.

Long credits graduate student Rachel C. Rohde and postdoctoral scholar Kurtis M. Carsch with making the discovery that the MOF could grab CO₂ at high temperatures.

“It was basic curiosity about reactivity of the ZnH,” Long says. Rohde and Carsch knew that ZnH should readily react with CO₂ to make a formate, but it wasn’t happening in ZnH-MFU-4l at room temperature. The two wondered if they could get the ZnH to react if they ramped up the heat, and it worked.

“Normally, if you’re working with a metal hydride in the lab, it’s extremely reactive, and you wouldn’t just expose it to the atmosphere and air. But this is an inert metal hydride,” Long says. His group has samples of ZnH-MFU-4l sitting out on the bench for 2 years with no reaction.

“That’s because it’s kinetically protected,” Long says. “There’s a pocket that the ZnH sits in that creates a large activation energy for the reaction.”

Gerard Parkin, who studies organometallic chemistry at Columbia University and was not involved in the work, says that interconversion between metal formate and metal hydride usually strongly favors one or the other form. “The observation that the interconversion can be reversible is particularly exciting with respect to the issue of CO₂ capture,” Parkin says in an email. “I imagine that this is the tip of the iceberg and extending this chemistry to other metals could provide enhanced systems.”

Volkmer, who was also not involved in the work, says in an email that it is a “beautiful example of the huge, yet largely unexploited technological potential of MOF compounds.” MOFs were once considered to be useful only for fundamental research, not real-world applications, he says. “The last few years, however, clearly demonstrate that the race is on to find the niches at which MOF compounds will outperform or complement existing porous materials.”