Enhancing CO2 capture utilizing aminopolymers supported on metal-organic frameworks (MOFs)

Abstract

Due to rapidly increasing anthropogenic CO2 emissions and consequential climate change, effective carbon capture technologies are imperative. Amine-functionalized supports are considered less energy-intense alternatives to liquid amine solutions, which are conventionally used to capture CO2 from flue gas streams. Metal-organic frameworks (MOFs) are attractive supports for these amines, with unique structural properties such as high pore volumes and potential thermochemical stability, that allow them to host amines and facilitate CO2 uptake. UiO-67(Zr) and MIL-101(Cr) represent supports without and with undercoordinated metal sites, respectively, which may affect mechanisms of aminopolymer stabilization and CO2 adsorption. Specifically, interactions with active metal sites can be probed in two distinct phases of MIL-101(Cr) (MIL-101(Cr)-ρlow and MIL-101(Cr)-ρhigh), containing different defect densities resulting from missing linker or Cr sites. Here, we demonstrate the potential of nanoporous UiO-67(Zr) and MIL-101(Cr) materials functionalized with poly(ethylenimine) (PEI) and poly(propylenimine) (PPI) for CO2 capture from simulated flue gas (10% CO2) and direct air capture (400 ppm CO2). PEI and PPI are incorporated into MOFs at 10-50 wt. % loadings and placed in varying liquid storage environments to probe the effect of solvent environments on CO2 uptake capacities, aminopolymer-MOF interactions, and overall material stability. N2 physisorption and Fourier-transform infrared spectroscopy (FTIR) indicate that PEI and PPI are physiosorbed within MOF voids but not tethered to metal nodes of carboxylate backbones. Exposure of 20 wt. % PEI/UiO-67(Zr) and 20 wt. % PPI/UiO-67(Zr) to solvents of different polarities, heteroatom presence, hydrogen bonding natures, and steric sizes for 7 days indicate 20 wt. % PEI/UiO-67(Zr) is more sensitive to solvent-induced aminopolymer conformational changes, exhibiting the highest amine efficiency in methanol and the lowest in acetone. Amine efficiencies of PEI and PPI incorporated into both phases of MIL-101(Cr) indicate a strong dependence of CO2 efficacy on amine group accessibility, which can be tuned through polymer properties and MOF defect interactions. Finally, PEI/MIL-101(Cr)-ρlow and PPI/MIL-101(Cr)-ρlow displays successful CO2 uptake at elevated flue gas temperatures and in dilute DAC CO2 concentrations. In all, this work provides insight into the effect of molecular-scale features on material stability and CO2 uptake and can be utilized to form design criteria in the development of next-generation carbon capture adsorbents.