Carbon capture COF shows impressive ability to survive hundreds of cycles

Researchers at the University of California, Berkeley, have synthesised a new material, COF-999, that can capture carbon dioxide from air and remains both chemically and thermally stable even after 100 cycles of capture and regeneration.

Removing carbon dioxide directly from the air is increasingly likely to be relied on as a tool to tackle the climate crisis as countries delay action to cut emissions. But it’s expensive and energy intensive because, although carbon dioxide has reached dangerous levels, it makes up just over 0.04% of the atmosphere.

To try to overcome some of the shortfalls of existing carbon capture materials, the researchers turned to the design principles of reticular chemistry. The porous crystalline covalent–organic framework (COF) they made was modified to produce polyamines within the pore that are covalently bonded to the framework, preventing them being lost during capture and regeneration cycles.

‘Even though some materials have a higher capacity than ours, they are not cyclable – they might cycle up to 10 times and then that’s it,’ says Omar Yaghi, a reticular chemistry pioneer at the University of California, Berkeley. ‘We cycled 100 times and we didn’t see any degradation of behaviour which then, by extrapolation – because we’re not losing any capacity – you can tell that this material is going to go a long way … thousands and thousands of cycles.’

Yaghi notes that COFs are more stable than the previous material class he developed, metal–organic frameworks (MOFs), that are being used in pilot projects to capture carbon from cement plants.

COF-999’s desorption energy is also lower than many other materials, taking place at 60°C, compared with more than 100°C for other sorbents. At such a low temperature, says Yaghi, waste heat – for example from a power station – could provide the necessary energy.

‘While the DAC performance parameters of [this] COF material, like cyclic CO₂ capacity, CO₂ uptake rate, and regeneration energy and temperature are relatively good, they are not remarkable compared to other DAC absorbents, such as aqueous amino acids,’ says Radu Custelcean, a distinguished research scientist at Oak Ridge National Laboratory in the US, who works on DAC.

However, Yaghi says that his research student, Zihui Zhou, is already close to doubling the capture capacity of COF-999 under laboratory conditions, by modifying the material.

With the current reported capacity of the COF to take up CO₂, almost 50,000 tonnes would be needed for a plant capturing 1 million tonnes of gas a year. Ambitious mitigation strategies call for building as many as 1500 such plants every year between 2030 and 2050.

Basic chemistry

‘There’s nothing exotic about the chemistry. It’s basic organic chemistry … and many of the constituents are being used in industry and scaled up to multi-tonne quantities. So I’m not worried about that at all,’ says Yaghi. But he is already focusing on a more environmentally friendly means of production.

The synthesis currently involves solvents such as dichlorobenzene and butanol, followed by multiple washing cycles with methanol. ‘Much more efficient, scalable and greener synthetic routes for these COFs need to be developed before they become feasible for large-scale DAC application,’ Custelcean says.

Yaghi has founded a start-up to ensure the chemistry can be done ‘in a cyclable way, so that we don’t generate any waste and the constituents are made from harmless products’. However, ‘initially, we have to work with what we have because of the urgency of the situation’. He stresses, though, that the COF itself is not volatile and is optimistic that its lifespan will be measured in years.

Other researchers point out that COF-999 has yet to be tested in a process. Jennifer Wilcox, a chemical engineer and energy policy expert at the University of Pennsylvania, says that while ‘it’s always exciting when new materials are developed with increased CO₂ capacities … this study is only considering the material properties and missing, for instance, engineering characteristics of performance. Success at the end of the day is not just about capacity but the kinetics of capture and ultimately costs of capital and operating, which you only can estimate after applying the engineering to the chemistry.’