Researchers have developed an ultrathin polymer-based ordered membrane that can effectively remove salt from seawater and brine while allowing quick water transport. The KAUST-led team showed their separation membranes could offer a viable alternative to the water desalination systems currently in use.
Water desalination membranes need to exhibit both high water flux and high salt rejection, and there are hopes that nanostructures such as carbon nanotubes and graphene will be suitable candidates due to their frictionless surfaces and tendency to form into channels with diameters smaller than one nanometer.
It is difficult to accurately control the membrane nanostructure and porosity for separating molecules or ions of similar sizes but, as reported in Nature Materials [Shen et al. Nat. Mater. (2022) DOI: 10.1038/s41563-022-01325-y], the use of 2D conjugated polymer frameworks (2D CPF) to make membranes was investigated as they offer high molecular permeability and their sub-nm channels provide a sieving effect for high selectivity.
2D CPF materials are mostly synthesized in solution, and it is not easy for solution-based synthesis to generate membrane morphology. In addition, in solution the CPF material tends to grow randomly, which leads to the formation of a disordered 3D structure with poorly defined micropores rather than the desired 2D structure. This pushed the team to develop a novel strategy to fabricate 2D CPF membranes by chemical vapor deposition (CVD) to realize their controlled growth into ultrathin films with a regular structure containing well-defined sub-nm channels.
The monomer triethynylbenzene was deposited on atomically flat single-crystalline copper substrates with an organic base as a catalyst. Triethynylbenzene has three reactive groups that act as anchor points for further monomers, and these show a 120-degree angle with respect to each other, generating organized arrays of well-defined cyclic structures that stack into sub-nm-sized rhombic hydrophobic channels.
Water molecules formed a 3D network inside the membrane rather than moving through the membrane along vertical triangular channels as 1D chains. The membrane improves on carbon nanostructures and has potential for water purification as it can efficiently produce high-purity water from seawater or brine in both forward and reverse osmosis configurations. It also showed strong rejection for divalent ions, in addition to small charged and neutral molecules.
The team are now looking to enhance the membrane’s anti-fouling property and mechanical strength, and long-term chemical stability, as well as fine-tuning its surface charge properties and channel sizes with the goal of producing a versatile multifunctional platform for different applications. As first author Jie Shen told Materials Today, “These structures may inspire further explorations at fundamental and practical levels for various chemical separation processes such as gas separation, organics separation, and ion sieving.”
“These structures may inspire further explorations at fundamental and practical levels for various chemical separation processes such as gas separation, organics separation, and ion sieving.”Jie Shen
a: Top view (top) and side view (bottom) of ABC-stacked CPF membrane. Layer A: grey; layer B: orange; layer C: green. A rhombic channel formed between adjacent layers (A and B) is highlighted with red dashed lines. b: Structure of the rhombic window between two adjacent layers. The channel dimensions were calculated based on the electron density isosurfaces obtained by density-functional theory calculations, which resulted in an accessible window width of (w) ~3.7 Å, a window length (l) of ~10.3 Å and an interlayer spacing (h) of ~1.9 Å. c: Schematic of water and ion separation through the CPF membrane.