Industrial plants, such as those that make cement or steel, emit copious amounts of carbon dioxide. However, the exhaust is too hot for state-of-the-art carbon removal technology. Cooling the exhaust streams requires a lot of energy and water, which has limited the adoption of carbon dioxide (CO2) capture in some of the most polluting industries.
Chemists at the University of California, Berkeley, have discovered that a porous material can act like a sponge to capture CO2 at temperatures close to those of many industrial exhaust streams. The material—a type of metal-organic framework, or MOF—was described in a paper published in the 15 November print edition of the journal Science.
The dominant method for capturing carbon from power or industrial plant emissions employs liquid amines to absorb CO2, but the reaction only works efficiently at temperatures between 40 and 60°C (100–140°F). Cement manufacturing and steelmaking plants produce exhaust that exceeds 200°C (400°F), and some industrial exhaust approaches 500°C (930°F). New materials are being piloted, including a subclass of MOFs with added amines, which break down at temperatures above 150°C (300°F) or work far less efficiently.
Kurtis Carsch, a UC Berkeley postdoctoral fellow and one of two co-first authors of the paper, said, “A costly infrastructure is necessary to cool these hot gas streams to the appropriate temperatures for existing carbon capture technologies to work. “Our discovery is poised to change how scientists think about carbon capture. We’ve found that an MOF can capture carbon dioxide at unprecedentedly high temperatures, which are relevant to many CO2-emitting processes. This was previously not considered possible for a porous material.
Rachel Rohde, a UC Berkeley graduate student and co-first author, said, “Our work moves away from the prevalent study of amine-based carbon capture systems and demonstrates a new mechanism for carbon capture in a MOF that enables high-temperature operation.”
Like all MOFs, it features a porous, crystalline array of metal ions and organic linkers. Its internal area is equivalent to about six football fields per tablespoon, a huge area for adsorbing gases.
“Due to their unique structures, MOFs have a high density of sites where you can capture and release CO2 under the appropriate conditions,” Mr Carsch said.
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 to 30 per cent CO2, and less concentrated emissions from natural gas power plants, which contain about four per cent CO2.
Removing CO2 from industrial and power plant emissions, after which it is stored underground or used to make fuels or other value-added chemicals, is a key strategy for reducing greenhouse gases that are warming Earth and altering the climate globally. While renewable energy sources are already reducing the need for CO2-emitting, fossil fuel-burning power plants, industrial plants that make intense use of fossil fuels are harder to make sustainable, so flue gas capture is essential.
“We need to start thinking about the CO2 emissions from industries, like making steel and cement, that are hard to decarbonise because it’s likely that they’re still going to emit CO2, even as our energy infrastructure shifts more toward renewables,” Ms Rohde said.
Moving from amines to metal hydrides
Ms Rohde and Mr Carsch conducted research in the lab of Jeffrey Long, a UC Berkeley professor of chemistry, chemical and biomolecular engineering, and materials science and engineering. Professor Long has been conducting research on CO2-adsorbing MOFs for more than a decade. His lab created a promising material in 2015 that was further developed by Professor Long’s startup company, Mosaic Materials, which in 2022 was acquired by the energy technology company Baker Hughes. This material features amines that capture 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.
Mr Carsch said that MOFs, like other porous adsorbents, are ineffective at the elevated temperatures associated with many flue gases.
Amine-based adsorbents, like those developed by Professor Long, have been the focus of carbon capture research for decades. However, the MOF studied by Ms Rohde, Mr Carsch, Professor Long, and their colleagues features pores decorated with zinc hydride sites that bind CO2. Ms Rohde said these sites turned out to be surprisingly stable.
“Molecular metal hydrides can be reactive and have low stability,” Ms Rohde said. “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 it comes into contact with, which is really what you need for point-source capture. 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 removed or desorbed by lowering its partial pressure, either by flushing it 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 C,” Professor Long said. “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°C.”
Ms Rohde, Professor Long, and their colleagues are exploring variants of this metal hydride MOF to see what other gases they can adsorb and what modifications will allow such materials to adsorb even more CO2.
“We’re fortunate to have made this discovery, which has opened up new directions in separation science focused on the design of functional adsorbents that can operate at high temperatures,” Mr Carsch said. “There’s a tremendous number of ways we can tune the metal ion and linker in MOFs, such that it may be possible to rationally design such adsorbents for other high-temperature gas separation processes relevant to industry and sustainability.”
Image: Michael/stock.adobe.com
