The field of organic solvent nanofiltration has recently gained significant attention in materials sciences due to its advantages over conventional techniques. A paper published in the Journal of Membrane Science has explored the rapid fabrication of covalent organic polymer membranes for use in this field.
Study: Rapid fabrication of fluorinated covalent organic polymer membranes for organic solvent nanofiltration. Image Credit: Dmitry Kovalchuk/Shutterstock.com
Organic Solvent Nanofiltration
Organic nanofiltration has several advantages over current filtration techniques such as high separation efficiency, cost-effectiveness, and minimal energy consumption. The most effective strategy to achieve superior permeance is to reduce the thickness of the membrane, but membranes fabricated using this strategy often suffer from issues such as mechanical weakness, defects, and poor selectivity, which hampers their performance.
The development of thin-film composite membranes was a major breakthrough in the field. The thickness of the membrane could be significantly reduced whilst retaining the structural integrity of the material. The invention of the interfacial polymerization method facilitated this breakthrough. The method mainly uses amino-functionalized monomers in the aqueous phase and an acid chloride in the organic phase.
Organic nanofiltration has been successfully applied to water treatment and seawater desalination, achieving high levels of rejection. However, the commercial applications of membranes prepared in this manner have thus far proven to be problematic, having issues with the selectivity of small particles. This is due to randomly occurring polymerization and network formation. Moreover, the hydrophilic nature of the membranes does not allow the easy transport of apolar solvents across them.
To overcome these challenges, novel materials must be designed that possess nanoporosity, structural integrity, interconnected channels, and chemical stability. Alternative materials which have emerged in recent years include metal-organic frameworks, porous graphene oxides, and covalent organic frameworks.
Covalent Organic Frameworks
The use of interfacial polymerization and covalent organic frameworks to produce organic nanofiltration membranes has been proposed in recent years to improve on current materials. A reversible reaction at high temperatures which leads to the fabrication of a stable material at room temperature is typically used to prepare covalent organic frameworks. Aldehydes and amino-functionalized monomers are some of the materials used in their preparation.
Two-dimensional ordered crosslinked segments are obtained by this process which possesses a corresponding high crystallinity. These are formed by taking advantage of the reversibility of the reaction while forming the framework or by choosing building blocks with specific geometry or functionalization. These materials have shown promise for the fabrication of membranes that possess high selectivity for challenging separations.
Covalent organic frameworks can synthesize thin films, but despite this, several key challenges exist with them which hinder their industrial-scale application. Firstly, interfacial polymerization requires aggressive solvents. Secondly, there are issues with cost and scalability due to the use of inorganic substrates. Thirdly, reactions commonly take a long time, typically days, which hampers preparation efficiency and limits the feasibility of continuous, large-scale manufacturing processes.
Moreover, despite the expected high uniformity of the material, it is difficult to achieve crystallinity under conditions that would be advantageous for membrane formation such as mild temperatures and fast reaction times. Using building blocks that favor two-dimensional layers can produce a material that is less ordered but has a higher selectivity than is provided by traditional monomers. This process creates chemically stable, tunable materials which are insoluble in organic solvents termed covalent organic polymers.
The Study
The study, published in the Journal of Membrane Science, has explored the synthesis of covalent organic polymers due to the significant advantages they possess over traditional nanoscale filtration membranes in terms of selectivity for difficult separations.
The simplest method for preparing membranes using covalent organic polymer components is incorporating them into polymer matrix as fillers. However, this method has some well-documented drawbacks including leaching, incompatibility with the polymer matrix, agglomeration, and deterioration or loss of favorable transport properties of nanofillers depending on the matrix’s permeability.
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Another problem associated with membrane preparation using this method is the timescale of fabrication, which can reach 12-72 h and high temperatures of 60-120 oC needed. Therefore, a much simpler, less time-consuming, and effective fabrication route is needed to capture the full benefits of covalent organic polymers for nanofiltration.
The authors have presented a simple methodology that takes less than 10 seconds to fabricate covalent organic polymer-based nanofiltration membranes using interfacial polymerization. The duration of the interfacial polymerization reaction is varied, facilitating control of membrane thickness. A key feature of the material is a fluorine-rich polymer backbone, to which the authors have attributed the chemical stability and hydrophobicity of the prepared membrane in the presence of an organic solvent.
Membranes with a thickness of less than 15 nm were achieved using the novel preparation process. The prepared membranes were evaluated for organic solvent nanofiltration for solvents with various polarities. These include acetone, methanol, n-heptane, and toluene. One result demonstrated in the research was a 97.5% rejection of rhodoline blue in toluene, which is significantly higher than other organic nanofiltration membranes.
The observations demonstrate that covalent organic polymers have significant potential as highly-selective nanofiltration membranes, and due to high thermal stability they can be used for filtration in high-temperature applications.
Further Reading
Alduraiei, F et al. (2022) Rapid fabrication of fluorinated covalent organic polymer membranes for organic solvent nanofiltration [online] Journal of Membrane Science 648 120345 | sciencedirect.com. Available at: https://www.sciencedirect.com/science/article/pii/S0376738822000928
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