Tailor-made MOF readily catches and releases CO₂

A custom-designed metal-organic framework (MOF) compound has met a key US Department of Energy target for industrial carbon capture. The material can soak up 90% of the carbon dioxide in a gas stream that simulates the humid flue gas emitted by modern power plants.

The CO₂ can be released by treating the MOF—a porous crystalline material composed of metal ions bridged by organic linking groups—with low-pressure steam, which provides a low-cost method for regenerating the sorbent (Science 2020, DOI: 10.1126/science.abb3976).

Electricity generation at US power plants leads to the annual production of nearly 2 billion metric tons of CO₂, according to the Energy Information Administration. The standard industrial method for capturing CO₂ uses aqueous solutions of amines. Though well established, the technique suffers drawbacks: the solutions are susceptible to oxidation and thermal degradation and require energy-intensive heating to release the absorbed CO₂.

For more than a decade, researchers have sought to bypass those shortcomings by coming up with solid sorbents that can selectively scrub CO₂ and be regenerated repeatedly and inexpensively. Amine-functionalized MOFs emerged from that quest as promising candidates, but they too have shortcomings.

For example, researchers working with Jeffrey R. Long of the University of California, Berkeley, reported several years ago, that grafting diamine groups to the interior surfaces of the pores of a MOF known as Mg₂(dobpdc) produced a sorbent that could selectively snag CO₂ under demanding power plant conditions and release it with little energy input (J. Am. Chem. Soc. 2012, DOI: 10.1021/ja300034j). It turned out, however, that the regeneration step slowly severed the amine groups from the MOF, limiting the material’s usefulness.

The Long group, which includes graduate student Eugene J. Kim, has now come up with a far more stable version of the MOF. The trick was to customize the compound with tetra-amine chains that are exactly the right length to span the MOF pores. These chains are securely bonded on both ends, unlike the shorter diamines in the earlier MOF, which were tied down only on one end.

The new MOF meets DOE’s CO₂ capture target and remained stable through 1,000 regeneration cycles using low-temperature steam, an inexpensive commodity readily available at power plants.

Abhoyjit Bhown, a carbon-capture specialist at the Electric Power Research Institute, predicts that the new MOF and related ones will enable a practical, low-energy process for CO₂ capture. He explains that not only are the tetra-amine-appended MOFs stable to high-temperature regeneration, but also to lower-temperature regeneration using steam, a process that often degrades MOFs. “That gives several options around which a low-energy CO₂ capture process can be designed,” he says.