Can flexible framework fillers keep breathing in mixed matrix membranes to enhance separation performance?

Abstract

With porous materials acting as fillers, mixed matrix membranes (MMMs) can significantly enhance the separation performance of corresponding pristine polymeric membranes. This improvement mainly ascribes to the high selectivity of porous fillers. However, it is unclear whether those porous fillers, especially flexible metal organic framework (fMOF) fillers, can keep their extraordinary target-responded selectivity in confined space of membranes. To illustrate, a full-atomic-molecular dynamic simulation was carried out to explore the flexibility of the ZIF-7-NH2 fMOF in the confined pores produced by the random distribution of Pebax chains. Significantly, not only the “Induced-Fit Suction” deformation of ZIF-7-NH2 by the n-butanol molecule still kept unchanged, but also the Pebax chains could induce benzene rings of ZIF-7-NH2 to flip over, facilitating the “gate-opening” effect. Inevitably, the experimentally produced ZIF-7-NH2/Pebax MMMs exhibited excellent butanol/water separation performance through the pervaporation, that the n-butanol permeability and the separation factor were as high as 3.59 × 10−6 g−1m−1h−1Pa−1 and 18.8 for a 5 wt% n-butanol feed solution at 40 °C, which was 150% and 160% higher than the pristine Pebax membrane, respectively. To the best of our knowledge, it was the highest separation permeability among all the reported Pebax-based membranes. This work provided a clear evidence why and how fMOFs exhibit the deformation in MMMs for the first time, which helps to design task-specific MMMs by incorporating highly selective fMOFs.