UiO-66 membranes with confined naphthalene disulfonic acid for selective monovalent ion separation

The selective separation of monovalent ions remains a challenge because of the similar sub-nanometer sizes. Metal-organic frameworks (MOFs) are promising candidate membranes for ion selective separation based on the angstrom sized windows for size sieving. Here, to further enhance the monovalent ion selectivity, 1, 5-naphthalenedisulfonic acid tetrahydrate (NTDS) molecules with sulfonate groups are incorporated to offer a binding affinity, generating a UiO-66@NTDS membrane via the window-cavity structure confinement. In addition to the size sieving effect, the charge and affinity interaction can compensate the energy loss induced by the ion dehydration. Thus, the UiO-66@NTDS membranes exhibit elevated separation performance compared to the original UiO-66 membranes under the concentration gradient driven, accompanied by high monovalent cation permeation rates (i.e., 0.3-0.7 mol m-2 h-1) and mono-/divalent cation selectivities (i.e., ∼73 for K+/Mg2+, ∼57 for Na+/Mg2+, and ∼46 for Li+/Mg2+). This work provides guidelines for the development of efficient ion-selective MOF membranes via the combination of size sieving and interaction attraction.