Nanoengineered ZIF fillers for mixed matrix membranes with enhanced CO2/CH4 selectivity

Abstract

Zeolitic-imidazolate frameworks (ZIFs) are considered as promising nanoporous solids for the development of membranes with exceptional gas separation properties. In this work, we show how molecular-scale modifications can lead to ZIF variants, which can greatly enhance the CO2/CH4 separation performance of mixed-matrix membranes (MMMs) when used as additives. First the permeabilities of CO2 and CH4 in ZIF-8 are estimated with the help of molecular simulations. Based on these results and a macroscopic model, the performance of MMMs with ZIF-8 as fillers is deduced. The validity of this approach is confirmed through experiments by developing flat-sheet ZIF-8 based nanocomposite membranes with two typical polymers (rubbery Pebax® MH1657 and glassy 6FDA-DAM) having different permeability properties. In a subsequent step, we design computationally new ZIF crystals, by changing the original metal, organic linker and functional group of ZIF-8, and the CO2/CH4 separation performance of MMMs with the new ZIF variants is predicted. Our results reveal that some of these MMMs exhibit an unprecedented CO2/CH4 separation performance, which surpasses any reported MMM and is directly attributed to the enhanced sieving capacity of the modified ZIF fillers. In conclusion, this work highlights the effectiveness of ZIF nanoengineering as a means to develop highly selective CO2 membranes.