In response to increasing CO2 levels, scientists are actively seeking sustainable methods for extracting carbon dioxide from the atmosphere, known as direct air capture (DAC). A promising new material, a covalent organic framework (COF) with amines attached, distinguishes itself due to its resilience and effective intake and release of CO2 at relatively low temperatures. This innovative material is compatible with existing carbon capture systems designed for point source emissions.
Reducing the carbon dioxide generated by human activities is crucial for decreasing atmospheric greenhouse gases and mitigating global warming. However, current carbon capture technologies are primarily effective only for concentrated emissions like those from power plants. These methods struggle to capture carbon dioxide from the surrounding air, which contains much lower concentrations of CO2.
Nonetheless, direct air capture (DAC) is seen as vital in reversing the increase in CO2 levels, which have surged to 426 parts per million (ppm), 50% higher than pre-Industrial Revolution levels. According to the Intergovernmental Panel on Climate Change, without DAC, we may not achieve the global target of limiting warming to 1.5 °C (2.7 °F) above historical averages.
Researchers from the University of California, Berkeley, have developed a new absorbing material that could facilitate achieving negative emissions. This porous covalent organic framework (COF) effectively captures CO2 from the air without being degraded by water or other contaminants, a challenge faced by existing DAC technologies.
“We tested this material by running Berkeley’s outdoor air through it, and it completely removed CO2,” said Omar Yaghi, the James and Neeltje Tretter Professor of Chemistry at UC Berkeley, who is the senior author of a study set to be published on October 23 in the journal Nature.
“I’m thrilled about this material because its performance is unmatched, paving new paths in our climate change efforts,” he added.
Yaghi explained that this new material can seamlessly integrate into existing carbon capture systems targeting emissions from refineries and atmospheric CO2 capture for underground storage.
According to UC Berkeley graduate student Zihui Zhou, the first author of the paper, just 200 grams of the material—less than half a pound—can absorb as much as 20 kilograms (44 pounds) of CO2 over a year, which is comparable to what a tree accomplishes.
“While capturing flue gas can help mitigate climate change by preventing CO2 from being released, direct air capture aims to restore atmospheric conditions to those of more than a century ago,” Zhou remarked. “The current atmospheric CO2 concentration exceeds 420 ppm and is projected to rise to around 500 or 550 before flue gas capture is fully operational. To reduce the concentration back to approximately 400 or 300 ppm, the use of direct air capture is essential.”
COF vs MOF
Yaghi is the creator of both COFs and metal-organic frameworks (MOFs), both of which possess rigid crystalline structures featuring evenly spaced internal pores that afford a significant surface area for gas adhesion or adsorption. Some MOFs developed by Yaghi’s lab can absorb water even in dry climates and release it upon heating, making it suitable for drinking. He began researching MOFs for carbon capture in the 1990s, long before DAC became a significant focus, as he explained.
Two years ago, Yaghi’s lab produced a highly effective material, MOF-808, which was able to adsorb CO2. However, they observed that after numerous cycles of CO2 uptake and release, the MOFs deteriorated. These MOFs were filled with amines (NH2 groups), known for their strong bond with CO2, and are commonly utilized in carbon capture methods. Typically, carbon capture involves passing exhaust gases through liquid amines to separate CO2. Yaghi pointed out that the high energy costs of regenerating liquid amines limit their practicality for widespread industrial use.
Collaborating with his colleagues, Yaghi identified why certain MOFs fail in DAC applications: they are unstable in basic environments (where amines exist), unlike acidic conditions. Together with Zhou and partners in Germany and Chicago, they devised a more robust material called COF-999. Unlike MOFs, which rely on metal atoms for structure, COFs are built with covalent carbon-carbon and carbon-nitrogen bonds, recognized as some of the strongest chemical bonds.
Similar to MOF-808, COF-999’s pores are also decorated with amines that enhance CO2 absorption capabilities.
“Capturing CO2 from the air is a complex challenge,” remarked Yaghi. “It demands a material with high CO2 capacity, selectivity, stability against water, resistance to oxidation, recyclability, low regeneration temperature, and scalability. Meeting all these requirements is quite a challenge.” He noted that current methods mainly rely on amine solutions or solid materials that gradually degrade over time.
Yaghi and his team have invested the past two decades in creating COFs that are resilient against diverse contaminants, including acids, bases, water, sulfur, and nitrogen, which typically harm other porous materials. They designed COF-999 with a backbone of olefin polymers and attached amine groups. After creating the porous structure, the material is treated with additional amines that bond with NH2 groups to form short amine polymers inside the pores, each capable of binding to one CO2 molecule.
When air containing 400 ppm CO2 is pushed through COF-999 at room temperature (25 °C) and 50% humidity, it reaches half its capacity in around 18 minutes and is completely saturated in about two hours. However, these times could improve drastically with optimization. Heating the material to a mild temperature of 60 °C (140 °F) allows the release of CO2, enabling COF-999 to absorb CO2 again. The material can hold up to 2 millimoles of CO2 per gram, setting it apart from other solid adsorbents.
Yaghi added that not all amines within the internal polyamine chains are currently capturing CO2, suggesting there is potential to expand the pores to accommodate even more binding sites.
“This COF is chemically and thermally stable with a durable backbone, requires less energy, and maintains capacity even after 100 cycles with no degradation. Such performance is unparalleled among existing materials,” Yaghi stated. “Truly, it stands as the most effective material available for direct air capture.”
Yaghi is hopeful that the integration of artificial intelligence can facilitate the design of even more effective COFs and MOFs for carbon capture and additional applications, specifically by identifying the chemical parameters necessary for synthesizing their crystalline forms. He serves as the scientific director at UC Berkeley’s Bakar Institute of Digital Materials for the Planet (BIDMaP), which utilizes AI to develop cost-efficient and easily implementable MOF and COF solutions aimed at addressing climate change challenges.
“We are extremely enthusiastic about the possibilities of merging AI with our ongoing chemistry research,” he shared.
This research received funding from the King Abdulaziz City for Science and Technology in Saudi Arabia, Yaghi’s carbon capture venture Atoco Inc., Fifth Generation’s Love, Tito’s, and BIDMaP. Collaborators on this project include Joachim Sauer, a visiting scholar from Humboldt University in Berlin and computational scientist Laura Gagliardi from the University of Chicago.
