Probing Stability of Hybrid Aminopolymer/Metal-Organic Frameworks (MOFs) Adsorbents for CO2 Capture

As the atmospheric concentration of carbon dioxide (CO₂) continues to rise, several methodologies are being explored to mitigate emissions. Of these technologies, metal-organic frameworks (MOFs) have drawn considerable attention. MOFs are a class of microporous solids with metal nodes connected by organic linkers to create two or three-dimensional crystalline structures. They possess several attractive features, ranging from high surface area and porosity to tunable functionality and morphology. In fact, the functionalization of MOFs with amines (molecular constituents or aminopolymers) has demonstrated CO₂ uptake both from flue gas streams and direct air capture (DAC). This research builds upon the knowledge of two branched aminopolymers (polyethylenimine (PEI) and polypropyleneimine (PPI)) and exploits the tunability of MOF structures to probe polymer-MOF interactions and to understand the ramifications of these molecular-scale features on CO₂ uptake, amine efficiency, and material stability. These aminopolymers were supported in two MOFs, MIL-101(Cr) and UiO-67(Zr), which are frameworks with varying structures and density of open metal sites for polymer binding. MIL-101(Cr) loaded with PPI showed promising CO₂ uptake capacities from both simulated flue gas streams and DAC. Additionally, the impact of storage conditions on surface functionality and CO₂ uptake was examined for PEI and PPI loaded onto UiO-67(Zr). Of the tested solvents, PEI-UiO-67(Zr) demonstrated the highest uptake stored in water and lowest uptake in acetone, while PPI demonstrated no ranking. These results demonstrate different polymer interactions with the desired support during use and in storage, which will ultimately lead to design criteria needed for next generation carbon capture adsorbents.