Two-dimensional material membranes for critical separations
Separation technology is of great importance to the chemical industries which involve many steps of purification. Compared to the traditional thermal-based purification processes such as evaporation and distillation, membrane technology has attracted increasing attention due to its low energy consumption and continuous operation. Two-dimensional (2D) material membranes formed by stacking various kinds of 2D nanosheets, including graphene, graphene oxide (GO), MXenes, layered transition metal dichalcogenides (TMDCs), layered zeolites, 2D metal organic frameworks (MOFs), 2D covalent organic frameworks (COFs), etc., represent a burgeoning class of separation membranes. Owing to their easy fabrication, controllable pore size, that is the stacking space between neighbouring layers, and diverse functionalization, these 2D material membranes show great potential for precise and high-efficiency separation through the synergy of the pore size and surface properties of nanopores or nanochannels. To date, 2D material membranes have been widely used in a broad range of critical separations, including ion separation, mixed organic solvent separation, gas separation, nanofiltration, and chiral molecular separation. In this review, we first summarize the separation mechanism adopted in 2D material membranes, followed by the introduction of the most widely used 2D materials and the synthetic strategies for their single or few layer structures. Finally, we summarize the applications of 2D material membranes in various critical separations, including ion sieving, nanofiltration, gas separation and chiral separation.