Influence of surface chemistry and channel shapes on the lithium-ion separation in metal-organic-framework-nanochannel membranes

Metal-organic-framework-based channel (MOFC) membranes have great potential in ion separation due to their high flexibility than polycrystalline MOF membranes. Although a few single-MOFC membranes with high ion selectivity and permeability have been reported, the scale-up of MOFC membranes from single-channel membranes to multichannel membranes remains a challenge as defects with the system are escalated, limiting their practical application. Minimizing defects in multichannel MOFC membranes is the key to solving this scale-up challenge. Herein, the surface chemistry of polyethylene terephthalate (PET) nanochannel substrate was firstly tuned by a chemical modification to enhance the binding between the PET nanochannel walls and UiO-66-(COOH)2 MOF crystals. Further investigation of the influences of PET nanochannels' size, shape, and channel density on the selectivity revealed that the cylindrical and ethylenediamine (EDA)-functionalized single PET-UiO-66-(COOH)2 MOFC membrane exhibited an ultrahigh Li+/Mg2+ selectivity of 3077-fold over those of previous single-MOFC works. Furthermore, cylindrical, EDA-functionalized, multichannel UiO-66-(COOH)2 MOFC membranes with a channel density of 106/cm2 have been fabricated to achieve 50-fold Li+/Mg2+ selectivity and a vast enhancement of Li + conductance for 6 orders of magnitude. As the first example of scaling up the MOFC membrane, this work provides a fundamental basis and experimental techniques for MOF membrane synthesis and lithium-ion extractions.