New synthesis strategy could speed up PFAS decontamination

Rice chemical engineer Rafael Verduzco and his team have described a new way to synthesize high-quality COFs at low cost and with high throughput in a study published in ACS Applied Materials and Interfaces that will be featured on the front cover of a future issue of the journal. The work includes a careful analysis of the benefits and drawbacks of different synthesis methods and details a versatile, cost-effective way to make COFs. This involves a multiflow microreactor and careful calibration of the input-output process.

"We built a small, continuous production system -- like a minifactory on a lab bench -- where the ingredients for COFs are mixed and reacted in a steady stream instead of all at once in a big container," said Safiya Khalil, a Rice doctoral alumna who is the first author on the study.

The researchers also found that one of the COFs produced via flow synthesis was better than those made using other methods at breaking down perfluorooctanoic acid (PFOA), a PFAS compound associated with a number of health risks, including cancer and reproductive harm.

"This is an encouraging finding that adds to the growing evidence that COFs could emerge as a key player in the development of cleaner, more efficient technologies for contaminant removal," said Verduzco, professor and associate chair of chemical and biomolecular engineering at Rice who is the corresponding author on the study.

COFs are crystalline polymers made of small, repeating units linked together into microscopic spongelike structures. These materials stand out for their porosity, large surface area and tunable molecular structure -- features that could be harnessed for use in a wide range of applications, including semiconductors, sensors, drug delivery and filtration. However, the slow and expensive process of producing COFs has limited their broader deployment.

"We hope this method will make it easier to produce COFs in large quantities and help accelerate the discovery of new formulations," said Khalil, who earned a Ph.D. in chemical and biomolecular engineering from Rice, where she was a part of Verduzco's Polymer Engineering Laboratory.

Khalil likened the new method to making cookies to order in small batches rather than baking them all at once in one large batch. Although it was not the first time flow reactor synthesis was used to make COFs, the Rice researchers' method stands out from previous approaches because it integrates the continuous synthesis and processing of two different COF chemistries, resulting in a more varied range of macroscopic formats.

"This method allows you to continuously have fresh-made cookies while controlling the temperature and mixing at each step to get the best quality every time," Khalil said. "This process is faster, uses less energy and allows for better control over the final product."

Traditional COF synthesis involves the use of high temperatures, high pressure and toxic organic solvents, limiting widespread production and use. The researchers' flow synthesis strategy not only allows for faster COF production but also enables the creation of COFs with superior crystallinity.

The added proof that one of the newly synthesized COFs was very efficient at breaking down a "forever chemical" showcases the practical benefits of the new method. The breakdown process, known as photocatalytic degradation, is activated by light and occurs at room temperature.

"Imagine these COFs as powerful sponges with built-in 'sunlight engines' that can break down harmful chemicals much faster than current methods," Khalil said. "One of the COFs we synthesized was more effective at breaking down PFOA than traditional materials such as titanium dioxide -- a common photocatalyst used in pollution control."

The research was supported by the Ministry of Education of the United Arab Emirates and the Welch Foundation (C-2124).