(728d) Surface Engineering of Metal–Organic Frameworks for Mixed-Matrix Membranes: From Polymer Grafting to Intrinsic Compatibility

Membrane technology is a dynamically developing field of separation science that is poised to result in new and efficient processes, energy and cost savings, and sustainability benefits. A key challenge in this field is the development of highly selective membranes, which can be addressed by the development of mixed-matrix membranes (MMMs) containing fillers such as metal–organic frameworks (MOFs). However, the lack of interfacial adhesion causes nanosized gaps between the filler and the polymer matrix.

In this study, we aim to elucidate the intrinsic properties of MMMs and bridge the gap between their material constituents. A series of novel membranes comprising MOF nanoparticles with similar chemical and morphological properties but increasing pore size (UiO-66–68-NH₂) were prepared [1]. The nanoparticles' surface was covalently grafted with poly(N-isopropylacrylamide) (PNIPAM) chains, which could then become entangled with the membranes' polymer matrix. Morphological characterization and organic solvent nanofiltration tests revealed that membranes with PNIPAM-grafted fillers do not suffer from the formation of pinholes at the filler–matrix interface that are detrimental to the filtration performance. For the first time, the experimental results showed an excellent match with a predictive model of nanofiltration built around the premise of liquid transport through the highly ordered pores of the MOF filler.

We also propose a multizoning technique for the formation of MMMs with an asymmetric-filler density at the macroscale, in which the MOF fillers are distributed only on the surface of the membrane, and a seamless interface at the nanoscale [2]. Our design strategy demonstrates five times higher MOF surface coverage, which results in a solvent permeance five times higher than that of conventional MMMs while maintaining high selectivity. Practically, MOFs are paired with polymers of similar chemical nature in order to enhance their adhesion without the need for additional surface modification. Our approach offers permanently accessible MOF porosity, which translates to effective molecular sieving, as exemplified by the polybenzimidazole and Zr–BI–fcu-MOF system. Our findings pave the way for the development of composite materials with a seamless interface.

[1] L. Cseri, R. Hardian, S. Anan, H. Vovusha, U. Schwingenschlogl, P.M. Budd, K. Sada, K. Kokado, G. Szekely, Bridging the interfacial gap in mixed-matrix membranes by nature-inspired design: Precise molecular sieving with polymer-grafted metal–organic frameworks, Journal of Materials Chemistry A, 2021, 9, 23793

[2] R. Hardian, J. Jia, A. Diaz-Marquez, S. Naskar, D. Fan, O. Shekhah, G. Maurin, M. Eddaoudi, G. Szekely, Design of Mixed-Matrix MOF Membranes with Asymmetric Filler Density and Intrinsic MOF/Polymer Compatibility for Enhanced Molecular Sieving, Advanced Materials, 2024, 2314206