Along-overlooked metal-organic framework (MOF) acts as a sponge for ammonia, selectively soaking up and releasing the corrosive gas at moderate pressures and temperatures up to 175 °C. Using the material to separate ammonia from a mix of nitrogen and hydrogen could make ammonia synthesis less energy-intensive, which could change how and where ammonia-based fertilizers and shipping fuels are made.
Researchers in Jeffrey R. Long’s lab at the University of California, Berkeley discovered the ammonia-adsorbing properties of the MOF, which is made of copper and trans-1,4-cyclohexanedicarboxylate (Nature 2023, DOI: 10.1038/s41586-022-05409-2). MOF chemists had largely overlooked this material and others like it because at first glance the materials’ structures aren’t particularly interesting, says Benjamin E. R. Snyder, the report’s first author who, after a postdoctoral fellowship with Long, recently started his own lab at the University of Illinois at Urbana-Champaign. “Once you approach them with the point of view that their bonds are dynamic and can be broken and reformed, and you can have a shape-shifting material, they become much more interesting,” he says.
Snyder says he thought an adsorbate like ammonia, which is highly reactive, might make the MOF change its structure. But the effect was far more dramatic than he originally imagined: the MOF takes up 30% of its own weight in ammonia and transforms from a material that is full of channels to a dense material that’s full of ammonia. “I actually thought it was an instrument malfunction when I first collected the data,” he says. “Based on the uptake, it was about four times higher than I was expecting.”
In addition to the MOF’s massive capacity for ammonia uptake, Snyder says its unusual adsorption mechanism has practical benefits: when ammonia binds to the MOF, it distorts the material’s structure, which results in stored energy. Snyder likens this to the energy in a stretched rubber band. Removing the ammonia releases that energy, so less heat is required to return the material to its original, ammonia-free state.
Edward Cussler, an expert in ammonia purification at the University of Minnesota, calls the MOF-based ammonia separation during synthesis a good idea. “Its commercial value is unproven, but it is a very attractive way to improve ammonia production.”
The Berkeley team hasn’t yet shown that the MOF will have an impact on the ammonia industry. But the material’s mechanism could have broad implications. “We are now exploring whether this new mechanism for cooperative adsorption can be used to separate other strongly coordinating molecules,” Long says in an email.