LAUSR

Membrane-based separation technologies, compared with other traditional separation operations such as evaporation, extraction, precipitation and distillation, have the merits of low energy consumption, small land footprint and high efficiency and have, therefore, attracted wide attention in past decades [1,2]. Laminar membranes formed by twodimensional (2D) nanosheet stacking showed high flexibility in adjusting the subnanoscale interlayer spacing to allow the unhindered transport of small ions or molecules, demonstrating high separation performance and economic applicability [3]. Graphene oxide (GO) membranes have been applied in seawater desalination, gas separation, nanofiltration, and ultrafiltration [4,5]. However, the intrinsic flexibility of GO nanosheets may lead to the formation of random laminar structures if stacked into membranes, which limits their selectivity [6,7]. Therefore, the fabrication of nanoconfined highly ordered separation membranes with regular 2D nanochannels is the key to efficient and stable separation capability. Recently, Wang et al. reported that, compared to flexible GO nanosheets, rigid Ti3C2Tx MXene nanosheets could be employed as promising building blocks to construct regular and straight channels without curling [6]. MXene, as a vibrant family of 2D materials, was first reported by Naguib et al. at Drexel University in 2011 [8]. It has a formula of Mn+ 1XnTx, where M is an early transition metal, X is carbon and/or nitrogen, and n ranges from 1 to 3. Tx generally refers to ‒H, ‒O, or ‒F, because the synthesis of MXene usually involves etching the interlayer A element (from groups IIIA and IVA) from the precursor Mn+1AXn phase with HF solution. Ti3C2Tx is the most extensively investigated MXene among more than 30 candidates. It can be assembled into 2D laminar membranes via several methods, including solvent-evaporationinduced self-assembly, and vacuum filtrating the Ti3C2Tx aqueous or organic dispersions (Fig. 1). The ordered and straight nanochannels in the prepared MXene membrane endowed it with precise molecular rejection for molecules larger than approximately 2 nm and unparalleled permeations of 2300 and 5000 L$m$h$bar [6] for water and organics, such as acetone and acetonitrile. This ultrahigh permeation of MXene membranes could be attributed to the regular and straight interlayer nanochannels, which produced uninterrupted and steady water-bonded flow. For other laminar membranes with irregular nanochannels, the large disturbance in water-bonded flow caused relatively low water permeance. This research facilitates the industrial application of regularly channeled MXene laminar separation membranes, especially for water treatment and purification. Ding et al. employed a pristine Ti3C2Tx MXene membrane with regular subnanometer channels for highly efficient H2/CO2 separation and achieved an outstanding separation performance of H2 permeability over 2200 Bar and H2/CO2 selectivity larger than 160 [7]. The free spacing between two adjacent MXene nanosheets was estimated to be approximately 0.35 nm, which is applicable for gas separation (Fig. 2(a)). The data displayed in Fig. 2(b) revealed that the gas separation mechanism of the MXene membrane was mainly attributed to size exclusion. He and H2 with small kinetic diameters demonstrated considerably higher permeabilities than those with larger sizes. The authors indicated that the adsorption forces between gas and MXene also had a role in the gas separation process. The interaction between MXene and CO2 is stronger than that between MXene and N2, which causes low CO2 permeability, which is 50% of N2. However, the kinetic diameter of CO2 is only 9% Received April 24, 2020; accepted June 3, 2020