Amine-Functionalized MIL-101 Monoliths for CO₂ Removal from Enclosed Environments

Amine-functionalized metal-organic frameworks (MOFs) are facile adsorbents for CO2 removal from enclosed environments. In this study, we prepared polyethylenimine (PEI) and tetraethylenepentamine (TEPA) impregnated MOF-monoliths using a 3D printing technique, through pre- and postfunctionalization approaches, and evaluated their CO2 capture performances. For preimpregnation, the MIL-101 powder was impregnated with PEI or TEPA and printed to form the monoliths. Meanwhile, the postimpregnation technique directly printed the MOF powder and secondarily impregnated the monoliths with TEPA or PEI. The adsorption analysis results indicated that all impregnated monoliths showed improved CO2 capacities from the pristine monolith at dilute concentrations, and preimpregnation yielded higher CO2 uptakes than postimpregnation. Specifically, the preimpregnated TEPA and preimpregnated PEI monoliths, with 3.5 and 5.5 mmol N/g amine content, respectively, displayed a capture capacity of 1.6 and 1.4 mmol/g, respectively, at 3000 ppm and 25 °C. From CHN analysis, postimpregnation yielded ~50 wt % less N content than preimpregnation which was attributed to reduced diffusion of aminopolymers into the MOF pores. This was further evidenced by the textural properties which showed nearly a 3-fold increase in pore volume and a 2-fold increase in surface area for postimpregnation over preimpregnation. In turn, preimpregnated TEPA-MIL-101 exhibited the highest amine efficiency of 0.46 mmol CO2 /mmol N. Furthermore, the TEPA grafting occurred during paste densification at 50 °C and resulted in the enhanced stability of TEPA-MIL-101 monoliths. Despite high adsorption capacity, the adsorptions kinetics were found to be relatively slow over 3D-printed amine-loaded MIL-101 monoliths, especially the preimpregnated monoliths, because the adsorption rate was limited by the CO2 molecular diffusion into the monolith walls. Overall, this study establishes a route to formulate amine-MOF monoliths by a 3D printing technique; however, the monolith dimensions should be tuned to optimize adsorption kinetics