Decarbonisation Technology May 2025 Issue

excessive water can lead to amine leaching and performance loss. Moreover, their durability is limited, as many amine-based adsorbents suffer from gradual efficiency loss over multiple capture-release cycles. To overcome these issues, researchers are developing stabilised amine formulations using additives and structural modifications to prevent oxidation and extend the material’s lifetime. New formulations based on pore-expanded mesoporous silica (PME) have demonstrated significantly higher CO₂ uptake and stability. Metal-organic frameworks (MOFs): the high-tech CO₂ sponges Metal-organic frameworks (MOFs) have emerged as promising next-generation DAC materials due to their tunable structures and massive surface areas. Some MOFs, such as MIL-101(Cr) and Mg₂-based frameworks, can store large amounts of CO₂ per gram of material. They also exhibit cooperative chemisorption, allowing for faster and more efficient adsorption cycles. MOFs stand out because of their unmatched CO₂ uptake, with some exhibiting adsorption capacities exceeding 6.8 mmol/g, significantly higher than conventional amine sorbents. Their highly tunable properties enable modifications in pore size and surface chemistry to optimise CO₂ selectivity. Additionally, certain MOFs require only mild temperature swings to release CO₂, making them energy efficient. However, MOFs also have limitations. Many are highly sensitive to moisture and degrade in humid environments, making them impractical for real-world DAC applications. Their production costs remain high due to complex synthesis processes, limiting their large-scale deployment. To address these issues, researchers are developing polymer- hybrid MOFs, which encapsulate MOFs in protective polymer coatings. This approach improves stability and moisture resistance while maintaining high CO₂ capture efficiency.

suffer from oxidative degradation and can withstand extreme environmental conditions. Their advantages include high durability, allowing them to function in harsh conditions without performance loss. They are also more moisture-tolerant than MOFs and some amines, making them suitable for humid environments. Furthermore, they are cost-effective to produce, making them attractive for large-scale deployment. Despite these benefits, MMOs have drawbacks. Most MMO-based sorbents require regeneration temperatures above 300°C to release CO₂, making them more energy- intensive than amine-based adsorbents. Additionally, their CO₂ selectivity is lower compared to functionalised amines despite their excellent stability. Current research is focused on hybrid MMO materials that integrate amine functionalities, combining the durability of MMOs with the selectivity of amines. Zeolites and carbon-based adsorbents: the low-cost workhorses Zeolites and carbon-based adsorbents, such as activated carbons, are among the most cost-effective and stable DAC materials. These materials rely on physisorption, meaning they trap CO₂ through weak van der Waals interactions rather than forming chemical bonds. Their primary advantages include low cost and widespread availability, as they are already produced on a scale for various industrial applications. They also exhibit high thermal stability, allowing them to withstand elevated temperatures without degradation. Additionally, they require extremely low energy for regeneration, making them highly energy- efficient compared to chemisorption-based materials. However, zeolites and carbon-based adsorbents have limitations. Their CO₂ selectivity is relatively low at atmospheric concentrations, making them more effective in environments with concentrated CO₂ rather than in DAC applications. Many zeolites also suffer from high moisture sensitivity, which can significantly reduce their performance in humid conditions. To improve their viability for DAC, researchers are exploring hydrophobic

Mixed metal oxides (MMOs): a durable alternative

Mixed metal oxides (MMOs), such as MgO- based adsorbents, are gaining attention for their durability and scalability. Unlike amine- based materials and MOFs, MMOs do not