Direct Air Capture of CO2 Using Amine Functionalized MIL-101(Cr)

Direct adsorption of CO2 from ambient air, also known as direct air capture (DAC), is gaining attention as a complementary approach to processes that capture CO2 from more concentrated sources such as flue gas. Oxide-supported amine materials are effective materials for CO2 capture from dilute gases, but less work has been done on metal organic framework (MOF) supported amine materials. Use of MOFs as supports for amines could be a versatile approach to the creation of effective amine adsorbents because of the tunability of MOF structures. In the present work, MIL-101(Cr) materials functionalized with amine species are evaluated for CO2 capture from simulated air. Two amines are loaded into the MOF pores, tris (2-amino ethyl) (TREN) and low molecular weight, branched poly(ethylene imine) (PEI-800), at different amine loadings. The MIL-101(Cr)-TREN composites showed high CO2 capacities for high loadings of TREN, but a significant loss of amines is observed over multicycle temperature swing adsorption experiments. In contrast, MIL-101(Cr)-PEI-800 shows better cyclic stability. The amine efficiency (mol CO2/mol amine) as a function of amine loading is used as a metric to characterize the adsorbents. The amine efficiency in MIL-101(Cr)-PEI-800 showed a strong dependence on the amine loading, with a step change to high amine efficiencies occurring at ∼0.8 mmol PEI/g MOF. The kinetics of CO2 capture, which have important implications for the working capacity of the adsorbent, are also examined, demonstrating that a MIL-101(Cr)-PEI-800 sample with a 1–1.1 mmol PEI/g MOF loading has an excellent balance of CO2 capacity and CO2 adsorption kinetics.