Two-dimensional montmorillonite membranes with controllable sub-nanochannel for precise separation of mono/divalent cations

Two-dimensional (2D) membranes with highly ordered nanoscale interlayer channels have received considerable attention to emerge as a desirable candidate for ions separation application. However, the unstable channel height of 2D membranes in aqueous environment results in labile ions separation selectivity. Herein, tetraethylenepentamine (TEPA) is introduced to the 2D montmorillonite (MMT) membrane, which plays a significant role in the precise control of channel height. This is due to that protonated TEPA can exchange into the interlayer of MMT membranes and the hydrophobic chain of TEPA can prevent water molecules from entering the channel. Based on the precise control of channel height by using different amounts of TEPA, the optimum channel heights with undetectable Mg2+ ions were found. Furthermore, the suitable channel heights for K+/Mg2+, Na+/Mg2+ and Li+/Mg2+ ions separation are proved to be 0.59 nm, 0.57 nm and 0.5 nm, respectively. Also, the permeation selectivity of K+/Mg2+, Na+/Mg2+ and Li+/Mg2+ are 676, 399 and 96, respectively. This strategy realizes the precise control of the channel height in the sub-nanometer scale and demonstrates the potential of MMT membranes for mono/divalent cations separation, providing a feasible and scalable pathway for developing highperformance 2D channel membranes.