Scientists have successfully prepared ultra-thin molecular-scale separation membranes with high throughput and high selectivity

At 23:00 Beijing time on August 8, 2022, Professor Han Yu, Professor Ingo Pinnau, Professor Vincent Tung of the University of Tokyo, and Professor Jiang Jianwen of the National University of Singapore co-authored an article entitled “Fast water transport and molecular sieving through ultrathin” in the journal Nature Materials ordered conjugated-polymer-framework membranes” new research.

The research results report the strategy of preparing ultra-thin conjugated polymer frame membranes by chemical vapor deposition method, and realize the precise construction of regular sub-nanometer channels. The separation membrane exhibits high throughput and high selectivity for water desalination, providing new ideas for the development of next-generation carbon material membranes for precise separation at the molecular scale. The correspondence is Jiang Jianwen, Ingo Pinnau, Vincent Tung, Han Yu; The first authors are Shen Jie, Aichen Tsai, Chenhui Zhang, and Wei Wan.

The separation membrane with limited-area mass transfer channel has wide application prospects and scientific research value in the fields of chemical separation and purification, seawater desalination, and nanofluidic technology. Carbon-based nanomaterials such as carbon nanotubes, graphene, etc. are considered potential separation membrane materials because they can form sub-nanometer-sized mass transfer channels and have ultra-smooth channel surfaces. However, how to achieve the accurate arrangement of large-area carbon nanotubes and the perfect accumulation of graphene sheet layers, there are technical bottlenecks, resulting in membrane separation performance is difficult to reach theoretical predictions, which greatly limits the practical application of membranes. Two-dimensional porous carbon materials, such as graphyne, two-dimensional conjugated polymers, etc. have received great attention in recent years, and theoretical studies have shown that they have a one-dimensional mass transfer channel perpendicular to the membrane plane, and the size and chemical properties of the channel can be regulated by monomer molecular design. At present, the controllable preparation of two-dimensional porous carbon material membranes is still difficult to achieve, hindering its application in the molecular scale separation process.

In this work, the team of Professor Han Yu of King Abdullah University of Science and Technology, Professor Ingo Pinnau, and Professor Vincent Tung of the University of Tokyo used chemical vapor deposition to prepare conjugated-polymer frameworks (CPF) ultra-thin films for molecular-scale separation processes. The study found that the use of single crystal Cu (111) with an ultra-smooth surface as the growth substrate and the introduction of organic base molecules during the deposition process are key to the formation of a two-dimensional CPF structure. The separation membrane exhibits rapid water mass transfer and molecular sieving, enabling high-performance water desalination. Professor Jianwen Jiang’s team at the National University of Singapore used molecular dynamics simulations to demonstrate that the regular diamond-shaped pores (3.7 × 10.3 Å) formed between CPF membranes can be used for the rapid passage of water molecules and the interception of hydrated salt ions, revealing the mechanism of water and ion separation.

Figure 1: Schematic diagram of the preparation of conjugated polymer frame membranes by chemical vapor deposition.

Figure 2: Membrane separation performance study.

The separation membrane exhibits high throughput and high selectivity for water and ion separation. In the osmotic pressure-driven separation process, the water permeability rate of CPF membrane reached 112 mol m−2 h−1 bar−1, and the water/sodium chloride ion selectivity was 6854, surpassing the separating properties of nanomaterial membranes that have been reported, including carbon nanotubes, graphene, molybdenum disulfide, transition metal carbides, metal-organic frameworks, etc. Compared with the recently reported graphene oxide membrane, the water/sodium chloride selectivity is increased by nearly 35%, and the permeability rate of water is increased by nearly 10 times. In the pressure-driven separation process, the water permeability rate is ~9.5 L m−2 h−1 bar−1, and the sodium chloride retention rate is ~83%, which is 2 times higher than that of commercial nanofiltration membranes. The thickness of the nanoscale (~8 nm) and the well-organized sub-nanometer channels in the CPF allow the membrane to effectively trap salt ions and allow water molecules to pass through quickly.

Figure 3: Molecular simulation computational study.

3D water molecular network limited area mass transfer and molecular screening mechanism. Molecular dynamics simulation studies have shown that the diamond-shaped pores (3.7 × 10.3 Å) formed in the upper and lower CPF layers allow 2-3 water molecules to pass through at the same time and form a three-dimensional network of water molecules in the membrane. In situ infrared spectroscopy studies showed that the average number of hydrogen bonds per water molecule was ~2.7, demonstrating the existence of a three-dimensional network of water molecules in the membrane. The excellent molecular sieving properties of the diamond-shaped pores allow the larger sodium hydrate ions (7.2 Å diameter) and hydrated chloride ions (diameter 6.6 Å) to be completely trapped. In addition, the research team investigated the effects of pH, cation type, adsorption effect, electric field drive and other factors on water and ion mass transfer behavior, which further confirmed that the membrane has good molecular screening characteristics.

In summary, this work breaks through the problem that traditional synthesis methods are difficult to form long-range ordered CPF, realizes the design and preparation of ultra-thin CPF membranes, and is used in efficient water and ion separation processes. In the future, the continuous preparation of nanoscale CPF membranes can be realized by combining roll-to-roll chemical vapor deposition technology (roll-to-roll CVD). The monomer molecular design allows precise customization of membrane pore sizes and surface charge properties, providing the possibility for applications including ion screening, osmotic capacity, and single molecule sensing. (Source: Science Network)

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