MOF captures hot CO2 from industrial exhaust streams

The porous MOF is detailed in a paper published in Science.

Capturing carbon from power or industrial plant emissions employs liquid amines to absorb CO2, a reaction that works efficiently between 40 and 60^oC (100–140^oF).

Cement manufacturing and steelmaking plants produce exhaust that exceeds 200^oC (400oF), and some industrial exhaust approaches 500oC (930oF). New materials that are now being piloted, including a subclass of MOFs with added amines, break down at temperatures above 150^oC (300^oF) or work less efficiently.

"A costly infrastructure is necessary to take these hot gas streams and cool them to the appropriate temperatures for existing carbon capture technologies to work," UC Berkeley postdoctoral fellow Kurtis Carsch said in a statement. "Our discovery is poised to change how scientists think about carbon capture. We've found that a MOF can capture carbon dioxide at unprecedentedly high temperatures - temperatures that are relevant for many CO2 emitting processes. This was something that was previously not considered as possible for a porous material."

Like all MOFs, the material features a porous, crystalline array of metal ions and organic linkers, with an internal area equivalent to about six American football fields per tablespoon.

"As a result of their unique structures, MOFs have a high density of sites where you can capture and release CO2 under the appropriate conditions," said Carsch, one of two co-first authors of the paper.

Under simulated conditions, the researchers showed that this new type of MOF can capture hot CO2 at concentrations relevant to the exhaust streams of cement and steel manufacturing plants, which average 20 per cent to 30 per cent CO2, as well as less concentrated emissions from natural gas power plants, which contain about four per cent.

Carsch and co-first author Rachel Rohde conduct research in the lab of Jeffrey Long. Long’s lab created a promising material in 2015 that features amines that capture the CO2. Next-generation variants are being tested as alternatives to aqueous amines for CO2 capture in pilot-scale plants, and to capture CO2 directly from ambient air.

Those MOFs, like other porous adsorbents, are ineffective at the elevated temperatures associated with many flue gases, said Carsch.

Amine-based adsorbents have been the focus of carbon capture research for decades. The MOF studied by Rohde, Carsch, Long and their colleagues instead features pores enhanced with zinc hydride sites, which also bind CO2.

"Molecular metal hydrides can be reactive and have low stability," said Rohde. "This material is highly stable and does something called deep carbon capture, which means it can capture 90 per cent or more of the CO2 that it comes into contact with. And it has CO2 capacities comparable to the amine-appended MOFs, though at much higher temperatures."

Once the MOF is filled with CO2, the CO2 can be desorbed by lowering the partial pressure of CO2, either by flushing with a different gas or putting it in a vacuum. The MOF is then ready to be reused for another adsorption cycle.

"Because entropy favours having molecules like CO2 in the gas phase more and more with increasing temperature, it was generally thought to be impossible to capture such molecules with a porous solid at temperatures above 200^oC," said Long. "This work shows that with the right functionality - here, zinc hydride sites - rapid, reversible, high-capacity capture of CO2 can indeed be accomplished at high temperatures such as 300^oC."

Rohde, Long and their colleagues are exploring variants of this metal hydride MOF to see what other gases they can adsorb, and modifications that will allow such materials to adsorb more CO2.