Metal-organic framework-based mixed matrix membranes offer considerable opportunities for advancing polymer membranes due to the precise pore sizes and tunable surface properties of MOFs. However, MMMs often face a trade-off between interfacial compatibility and intrinsic pore selectivity. In a study published in ENG. Chem. Eng. , researchers at Dalian University of Technology and Yingkou Institute of Technology report a multi-step crosslinking strategy that simultaneously addresses both challenges.
The team synthesized UiO-66-AC@PEG by covalently bridging MAH-prefunctionalized UiO-66-NH₂ with PEG. SEM images showed that UiO-66-NH₂ exhibited an octahedral structure with a particle size of approximately 150 nm, while UiO-66-AC@PEG-0.5 displayed less sharp crystalline edges with an increased particle size of approximately 250 nm. FTIR spectra confirmed successful PEG grafting, with the characteristic peak at 878 cm⁻¹ attributed to the polyether chain. XRD patterns revealed that all samples retained the characteristic diffraction peaks of UiO-66-NH₂, confirming that amidation and PEG grafting did not disrupt the crystal structure.
N₂ adsorption-desorption isotherms showed that all materials predominantly featured a micropore structure. The characteristic pore size of UiO-66-NH₂ was approximately 0.501 nm, whereas UiO-66-AC@PEG-0.5 exhibited a reduced pore size of 0.377 nm, representing a 23.6 % decrease. This pore constriction enhances molecular sieving for CO₂/N₂ separation. MD simulations revealed that CO₂ adsorption energy increased from −5.368 kcal·mol⁻¹ for UiO-66-NH₂ to −7.568 kcal·mol⁻¹ for UiO-66-AC@PEG, indicating strengthened dipole-quadrupole interactions with CO₂.
The optimized UiO-66-AC@PEG-0.5/Pebax MMM at 5 wt % filler loading achieved a CO₂ permeability of 125.8 Barrer and a CO₂/N₂ selectivity of 81.1, representing improvements of 56.6 % and 33.5 %, respectively, over the UiO-66-NH₂/Pebax MMM which achieved 80.3 Barrer and 69.13. The enhanced CO₂ permeability is primarily attributed to increased CO₂ solubility. The synthesized UiO-66-AC@PEG-1/Pebax MMM reached a CO₂ solubility coefficient of 24.45 × 10⁻² cm³(STP)/cm³·cmHg, representing a 129.8 % increase over UiO-66-NH₂/Pebax MMM. The CO₂/N₂ solubility selectivity increased by 41.2 %. The diffusion selectivity also improved from 4.94 to 5.87, attributed to PEG-induced pore constriction.
DSC analysis showed that the glass transition temperature of pure Pebax was −52.7 °C. With the introduction of UiO-66-NH₂, the T_g increased to −50.2 °C, while UiO-66-AC@PEG-0.5/Pebax MMM further increased to −48.0 °C, indicating enhanced polymer-filler compatibility. Stress-strain curves demonstrated that PEG-functionalized UiO-66-NH₂ effectively restricted chain mobility and improved mechanical properties.
The membrane also exhibited excellent pressure stability. As pressure increased from 0.1 to 0.7 MPa, CO₂ permeability did not show the typical sharp increase associated with membrane plasticization, while selectivity remained stable. Long-term testing over 144 h confirmed stable performance. The membrane exceeded the 2008 Robeson upper bound for CO₂/N₂ separation.
This work demonstrates that rational multi-level crosslinking of MOFs provides an effective pathway for simultaneously enhancing solubility selectivity, diffusivity selectivity, and interfacial compatibility in high-performance gas separation membranes.
ENGINEERING Chemical Engineering
Experimental study
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Constructing CO2-philic PEG domains on UiO-66-NH2 via amide tethering for highly efficient CO2 separation
5-Aug-2026