Triangular building blocks fit together to make sturdy membranes

Wafer-thin membranes tailored for separating specific molecules from liquids could improve the efficiency of oil refining and pharmaceutical manufacturing. Filtering organic solvents – carbon-based liquids such as oils and alcohols that dissolve other substances – is crucial for petroleum, chemical and pharmaceutical companies that must consistently create the purest product.

Traditional extraction techniques, such as distillation, use vast amounts of energy, but emerging green alternatives, such as membranes, face other challenges. For instance, porous materials must withstand often highly reactive solvents while filtering out target molecules of a particular size and shape. Some very efficient membranes are available for separating salt from water in seawater desalination, but they are not as effective at separating smaller molecules in organic solvents.

Now, using carefully crafted molecular building blocks known as trianglamines, a team led by researchers at King Abdullah University of Science & Technology (KAUST) in Saudi Arabia has created an ultrathin porous membrane for filtering organic solvents. The researchers report their work in a paper in Nature Communications.

"You can imagine it like LEGO," explains Suzana Nunes, professor of chemical and environmental science and engineering at KAUST, "where you take preformed hollow triangles and assemble them together in a flat film." By first defining the pore size and electric charge of these triangular molecules, they went on to create a membrane that could separate molecules of different sizes and shapes.

Membrane thickness is also critical to filtering efficiency. "For faster filtration, the film needs to be as thin as possible to avoid unnecessary resistance to the solvent passing through," says Nunes.

To achieve this, the researchers added the two main membrane ingredients (terephthaloyl chloride and the preformed trianglamines) to two different fluids that do not mix (oil and water, respectively), forcing the reaction between the ingredients to occur only at the interface where the fluids meet. "We found this formed an extremely thin layer of a few nanometres, much thinner than common commercial membranes prepared in this way," says Tiefan Huang from the Advanced Membranes and Porous Materials Center at KAUST, who is lead author of the paper. Each film was 3.5–10nm thick, depending on how long the reaction continued.

The researchers tested their membranes on colored dyes with similar yet distinct molecular sizes. All of their membranes filtered out at least 90% of the color molecules that weighed more than 450g per mole, far outperforming some of the other membranes they tested. "The membranes' performance didn't deteriorate after 48 hours of continuous filtration," adds Huang. And they even withstood exposure to harsher substances, including acetone and methanol.

"Molecular purification for pharmaceuticals can involve many steps," explains Nunes. "More selective and sturdy membranes like ours can simplify the process, making it more cost effective. The machinery we used is already widely used in the membrane industry, so it can be easily scaled up for manufacturing."

The membranes in this study were tailored specifically for molecules of around 400g per mole. "We will next work on a portfolio of building blocks so that we can make membranes for selecting molecules of many different shapes and sizes," says Nunes, "and ultimately help make organic solvent separation more sustainable."

This story is adapted from material from KAUST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.