Polyethylene Glycol-Mediated High-Performance Mixed Matrix Membranes via a “Nesting Effect” for CO₂ Separation

Various metal-organic framework (MOF)-based porous nanomaterials have been adopted to fabricate mixed matrix membranes (MMMs) aiming at breaking through the Robeson upper bound of individual polymer membranes for gas separation. However, the insufficient compatibility between the polymeric matrix and MOF nanofillers limits the further improvement of their gas separation performance. To overcome this challenge, we constructed relatively ideal Pebax/PEG/NH₂-UiO-66 mixed matrix membranes inspired by θ solvent theory of polymer solutions. Polyethylene glycol (PEG) molecules were introduced to selectively swell the polar poly(tetramethylene oxide) (PTMO) domains of the Pebax matrix via hydrophobic-hydrophilic interactions and in turn furnished dispersed “nests” for NH₂-UiO-66 nanofillers with polar nature to reduce the interparticle agglomeration (PEG nesting effect). MOF nanofillers tended to be relatively undisturbed, in which either the rigidity of polymer chains or the pore blocking of MOFs seen in unideal MMMs was restricted. The PEG “nesting effect” proposed herein could bring benefits to further improve the allowed maximum loading capacity of MOF nanofillers as well as adjust the interactions therein and therefore maximize the CO₂ separation performance of Pebax/PEG/NH₂-UiO-66 MMMs. Compared with individual Pebax membranes, Pebax/PEG/NH₂-UiO-66 with a 15 wt % loading of NH₂-UiO-66 showed a 73% higher CO₂ permeability (210 barrer) and a 220% higher CO₂/N₂ selectivity (130), which demonstrated the most dramatical increases among Pebax-based MMMs until now and as well significantly exceeded the Robeson upper bound. Therefore, the proposed PEG “nesting effect” for the preparation of θ-type MMMs holds promising potential for membrane-based carbon capture and is possibly extended to the preparation of other MMMs.