Researchers at King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia, have created covalent organic crystal networks that act as high selectivity and high-flux membranes for organic solvent filtration.
They believe such structures could hold the key to future industrial membranes that will help treatment technologies rise to the twin challenges of meeting increasingly stringent environmental controls while being cost-effective to produce and operate.
Organic solvent nanofiltration typically involves polymer-based membranes that feature tiny pores, but that form dense and amorphous networks. Well-ordered microporous materials, such as zeolites and metal-organic frameworks, perform significantly better than these conventional membranes in various separation processes. However, they’re not suitable for extensive use in liquid separation because of their poor structural and chemical stability in liquids.
Now a team led by Zhiping Lai, professor of chemical and biological engineering with KAUST’s physical science and engineering division, has developed a synthetic approach that produces well-ordered microporous materials that are stabilized by covalent keto–enamine linkages. These linkages result from the reaction between the amine and aldehyde functional groups of organic compounds.
“In our innovative process, we have used a β-keto-enamine-based reaction based on the very active precursors 1,3,5-triformylphloroglucinol and 9,9-dihexylfluorene-2,7-diamine for the formation of covalent organic framework (COF) membranes. These precursors are stable in both aqueous and organic solvents,” notes Lai.
The β-keto-enamine route was chosen following a careful literature review into how best to generate good crystalline COF membranes. This helped the team formulate defect-free crystalline COF structures in powder form.
To convert the powder into crystalline COF thin films, the team used the Langmuir-Blodgett (LB) method. This involves transferring Langmuir films — or monolayers — from the liquid/air interface to solid supports as these are passed vertically through the monolayers (Figure 1).
Figure 1. During the Langmuir–Blodgett process (left), the amine and aldehyde molecules gradually formed an extended 2D network at the air–water interface (middle). The network consisted of amine and aldehyde molecules paired into hexagonal structures (right). Source: Digambar Shinde.
The LB method reliably produced large-area thin films of well-defined thickness using the amphiphilic aldehyde and amine precursors. The researchers deposited the precursor mixture solutions on a water surface to form weakly bound two-dimensional hexagonal structures. Once the solvent evaporated, they compressed the films laterally and added an organic acid to the mixture, transforming the reversible bonds into covalent keto-enamine linkages and sealing the hexagonal structures in place.
“The special reaction we chose here produced β-keto-enamine-based COF membranes that are thermally as well as chemically stable in any media, i.e., acid, base and aqueous,” adds Lai.
The new membranes outperformed amorphous analogues fabricated using the same method and the best polymer-based systems.
“They share the same chemistry as polymer analogues, resulting in similar hydrothermal, chemical and mechanical stabilities, but their fluxes are higher,” says postdoctoral fellow Digambar Shinde, lead author of a paper published about the work in the Journal of the American Chemical Society.
The new membranes’ organic solvent permeability is almost an order of magnitude higher than that of the best-reported polymer membranes, he adds. In addition, the membranes were more stable than metal-organic frameworks and more cost-effective than inorganic membranes. They also could separate mixtures of dye molecules differing in molecular weights and sizes.
The team currently is working on extending the use of the membranes to a multitude of applications. “The pore sizes of these membranes are suitable for seawater desalination pretreatment, food processing, purification of pharmaceutics and medical processes, such as hemodialysis,” says Shinde. The membranes also can be useful for eliminating heavy metals, viruses and bacteria.
According to Lai, the current work also includes tuning the porous structure for more energy-intensive separation systems, improving the mechanical strength of the film, developing a better chemical approach to simplify the membrane fabrication process on porous supports, and exploring possible methods for membrane scaling up.
“The current membrane fabrication process can make membranes with a size suitable for niche applications, particularly in miniature medical devices, but not suitable for large-scale applications. A novel approach is yet to be developed to make the membrane on flat-sheet or hollow fiber supports. From the chemistry point of view, the wide application of such membranes will also rely on our ability to scale up the synthesis of all precursors — most of which are currently custom-designed from our lab. We also need to study in detail the membrane formation mechanisms,” he explains.
Nevertheless, Lai points out that the industrial significance of the separation technology is clear: “Once we overcome the scaling up issues, most of our membranes will find industrial applications straightforward.”