Direct Air Capture of CO 2 Using Amine Functionalized MIL-101(Cr)

Direct adsorption of CO 2 from ambient air, also known as direct air capture (DAC), is gaining attention as a complementary approach to processes that capture CO 2 from more concentrated sources such as flue gas. Oxide-supported amine materials are effective materials for CO 2 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 CO 2 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 CO 2 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 CO 2 /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 CO 2 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 CO 2 capacity and CO 2 adsorption kinetics.