Evaluating the viability of ethylenediamine-functionalized Mg-MOF-74 in direct air capture: The challenges of stability and slow adsorption rate

Carbon removal technologies, such as direct air capture (DAC), hold great potential in mitigating anthropogenic CO2 emissions. Amine-tethered metal-organic frameworks (MOFs) that capture CO2 selectively via chemisorption have been highlighted as frontrunners for CO2 removal technologies. To this end, ethylenediamine (ED) was employed to decorate the metal sites of Mg-MOF-74, and both bare and amine-modified frameworks were thoroughly characterized and studied for DAC using an automated fixed-bed sorption device. The material exhibited a promising CO2 capacity of up to 1.8 mmol/g from 400 ppmv CO2 in humid conditions, although the amount of adsorbed H2O was several times higher. The highest adsorption capacities were measured at 25–35 °C, while decreased capacity was observed at 12 °C due to slower adsorption rate. In dry cyclic adsorption-desorption tests, the cyclic CO2 capacity reduced slightly in 18 cycles. However, at 2 vol% humidity, the capacity dropped rapidly over successive cycles, revealing poor hydrolytic stability. Preliminary coating experiments were conducted on stainless steel plates and cordierite monoliths, suggesting that reasonably even coating layers could be achieved on these substrates with relatively simple coating techniques. High water adsorption, slow adsorption rate at low temperatures, and the rapid cyclic capacity decrease in humid conditions may limit the application of the studied adsorbent for DAC. The vital aspects of the real application of MOFs in DAC, such as adsorption kinetics and stability in humid conditions, are rarely explored in detail in the literature, and these results indicate that these aspects warrant extensive study for the development of practically applicable DAC adsorbents.