PNNL

Abstract

The capture of carbon dioxide (CO2) is critical for reducing greenhouse emissions and meeting net-zero emission targets. While current solutions, such as liquid amines, efficiently remove CO2 from coal-burning power plants, their corrosive properties and high regeneration costs necessitate additional research towards more cost-effective alternatives. Metal-organic frameworks (MOFs) are a promising solution for carbon capture due to their structural versatility, porosity, and pore modularity. However, the synthesis of MOFs often relies on costly and environmentaly not friendly reagents, including heavy metal salts and toxic solvents such as N,N-dimethylformamide (DMF). Beyond the synthetic limitations, many MOFs with promising CO2 capture capabilities are susceptible to hydrolysis or show reduced CO2 uptake in humid conditions, which represents a significant obstacle to their implementation in industrially relevant conditions. To address this challenge, herein, we report the use of MIL-120 as a cost-efficient and water-stable MOF for the selective capture of CO2 from wet flue gas. Synthesized using inexpensive and environmentally benign reagents in water, MIL-120 possesses one-dimensional pores decorated with hydroxylbridged Al(III) ions and benzene rings with an interstitial spacing of 4.78 Å. MIL-120 is an ultramicroporous material with pore volume 0.081 cm3 g-1 . Carbon dioxide isotherms show steep uptake at low pressure, and the affinity of MIL-120 for CO2 is 44 kJ mol-1. CO2-loading 13C solidstate Nuclear Magnetic Resonance (NMR) spectra acquired after the loading of MIL-120 with CO2 and in-situ Fourier transform infrared (FTIR) spectra tracking the sorption of CO2 into MIL-120 revealed that the interplay of pore size, functionality, and dimensionality is vital for CO2 restriction within the pores of MIL-120. Once CO2 molecules have interacted with the pores of MIL-120, they cannot be displaced by water, nor can they be removed under vacuum at room temperature. The selectivity of MIL-120 towards CO2 over N2 in postcombustion flue gas concentrations is 42.5 Energy & Environmental Science Page 20 of 84 3 at 150 mbar. Breakthrough experiments reveal that MIL-120 can capture CO2 from dry and wet flue gas with uptake capacities of 1.215 and 1.118 mmol g-1 , respectively. MIL-120 can be easily regenerated and reused with consistent CO2 uptake capacities. Our work highlights the synthetic benefits of MIL-120 and elucidates its selective capture of CO2 from wet flue gas. Comparisons with other solid-state materials reveal MIL-120 to be one of the best MOFs reported to-date for CO2 capture.