According to statistics, more than 2 billion people around the world do not have access to clean and safe drinking water, so the shortage of fresh water resources is a serious challenge for mankind in the 21st century. Desalination can help to alleviate the fresh-water crisis facing the world. At present, desalination technology mainly includes: reverse osmosis (RO), forward osmosis (FO), multi-effect distillation (MED), multi-stage flash distillation (MSF), and membrane distillation (MD). The number of desalination plants worldwide has grown rapidly in the past few decades, with desalination production reaching about 38 billion cubic meters per year and more than 50% of desalination plants using RO technology. However, RO is an energy-intensive technology, and minimizing energy consumption during desalination is particularly important given the interdependence between energy and water output. In addition, the ability to use renewable energy for water treatment is indispensable due to the poor energy infrastructure in less developed regions.

Fig 1. Schematic illustration of membrane distillation process

What is membrane distillation? Membrane distillation (MD) is a process of desalination where the difference in vapor pressure between the two sides of the membrane is used as the driving force and water vapor is driven thermally to pass through the porous hydrophobic membrane material. MD is a combination of membrane technology and low temperature volatilization technology. It combines the advantages of reverse osmosis and distillation technologies and can achieve nearly 100% removal of salt ions from solution, which has great advantages in treating highly concentrated and polluted brine and using low-grade heat such as industrial waste heat as well as solar and geothermal energy. In addition, MD desalination systems are simple in equipment and operation compared to traditional thermal desalination processes (MED and MSF), and have the flexibility to constitute a large-scale production system. Therefore, MD, a sustainable and low-energy desalination technology, has certain competitiveness and application prospects in many desalination projects. However, membrane distillation membranes suffer from inevitable gradual wetting and low permeate flux, greatly restricting its further development.

Where is the way out for MD? The core of the MD is a porous hydrophobic membrane. The ideal membrane for MD should have a high hydrophobicity, a large porosity, small pore size and small bending factor at the same time, and the traditional empirical formula suggests that a large pore hydrophobic membrane is more conducive to increase the flux. However, traditional MD membrane materials and membrane preparation methods are difficult to simultaneously achieve the above requirements and precise structural control. Therefore, it is important to develop new membrane materials and separation membrane preparation methods to promote the development of MD.

Covalent organic frameworks (COFs) are a class of crystalline, organic porous structures constructed by linking together organic building blocks by covalent bonds, featuring atomically ordered structures. The high porosity, periodic open pore channels and functionalizability of COFs materials make them ideal MD membrane materials and bring new opportunities for the development of new generation MD membranes.

In view of this, Wang Bo and Feng Xiao, professors from the School of Chemistry and Chemical Engineering of Beijing Institute of Technology (BIT), leading the team, realize the preparation of COFs membrane with pore size and intra-pore affinity and hydrophobic environment varying with depth gradient by introducing a competitive reversible covalent bonds based on 2D COFs membrane with regular penetrating nanopores. The enhanced effect of water evaporation in the restricted nano-pore channel of COFs membrane is used to realize ultra-high flux membrane distillation for seawater desalination. The results were published in Nature Materials under the title Hydrophilicity gradient in covalent organic frameworks for membrane distillation. Corresponding authors are Feng Xiao, Wang Fengchao, and Wang Bo; while the first authors are Zhao Shuang, Jiang Chenghao, and Fan Jingcun. The experimental part of the paper was completed by BIT, and the theoretical calculation was completed by Wang Fengchao's team at University of Science and Technology of China.

Fig 2. Schematic illustration of water transport channels in different membrane materials

The lower flux of the traditional polymeric MD membranes has been criticized, and this problem is mainly attributed to the lack of precise modulation of the membrane structure. COFs composite membranes based on competitive reversible covalent bonding strategy have the following advantages: 1. ultra-thin hydrophobic separation layer (short mass transfer channel) and vertically penetrating nanopore channels (low curvature) with a dramatic reduction in the transmembrane resistance to water vapor (as shown in Figure 2); 2. a fast evaporation rate in the confined nanopores. The two together enable the COFs composite membrane to exhibit the highest MD desalination performance reported in the literature so far. It produces a water flux of 220 l m–2 h–1 with a NaCl rejection higher than 99.99% (3.5wt% NaCl solution as the feed, 65℃), three times higher than that of the commercial MD membrane. The pure COF separation layer (COFDT-E18) has a theoretical flux of 1800 L m-2 h-1.

Fig 3. Anti-pollution and anti-infiltration performance as well as long-term operational stability of composite membranes of COFs based on competitive reversible covalent bonding strategy

In addition, two other serious challenges facing MD in practical applications are fouling behavior and wetting behavior, with severe fouling behavior leading to extremely rapid flux decay and wetting behavior leading to direct loss of separation performance of the membrane material. Achieving fine design and modulation of gradient nanopores (Figure 3c), the pore size as well as the hydrophobic gradient constructed in the COFs film confer excellent anti-fouling performance to the membrane material. The enhancement of surface hydrophilicity reduces the adhesion of oily contaminants (Figure 3a). The enhanced electrostatic repulsion within the nanopores retarded salt crystallization at the pore wall and reduced the risk of wetting behavior, conferring the ability to handle high concentrations of brine (17.5% NaCl solution) with long-term operational stability (Figure 3b, d).

Fig 4. Molecular dynamics simulation of water evaporation in the restricted nano-pore channel

Most of the traditional commercial MD separation membranes use polymeric hydrophobic macroporous membranes, such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE) and polypropylene (PP) microfiltration membranes. Molecular dynamics simulations of the evaporation behavior of water molecules within the pore channel show that the evaporation energy barrier of the liquid layer at the liquid-hydrophobic wall interface within the restricted nanopore is lower than that at the center of the liquid-vapor interface (Figure 4). The evaporation of water molecules increases in the restricted nano-pore channel compared to the macropore, and the evaporation rate exhibits a size-dependent property, which means, the smaller the pore diameter, the faster the evaporation rate. To verify this idea, COFPT (2.2 nm) and COFTT (1.8 nm) films with different pore sizes were synthesized, and both exhibited high fluxes of 235 and 250 L m-2 h-1, respectively (3.5 wt.% NaCl solution as the feed and 65℃). This result overturns the conventional knowledge that MD membrane materials require large pore size pores and provides theoretical support for the development of next-generation high-performance MD membranes.