Scientists have discovered a faster, more sustainable method for making metal-encapsulated covalent organic frameworks (COFs), materials that have the potential to play a crucial role in catalysis, energy storage, and chemical sensing. Their new one-step, room-temperature process, developed through a collaborative effort between Clark Atlanta University and the Molecular Foundry at Lawrence Berkeley National Lab (Berkeley Lab), eliminates the need for toxic solvents and significantly reduces the production time from several days to just one hour.
Metal-encapsulated COFs are widely studied for their use as catalysts, materials that speed up chemical reactions without being consumed in the process. COFs are like highly organized, tiny Lego structures arranged in a lattice with an abundance of ordered pores or openings. When metals are anchored within these pores to form metal-encapsulated COFs, the material can accelerate reactions and improve overall efficiency.
Metal-encapsulated COFs are traditionally made using a multi-step process over several days in which COFs are made first, and the metal species are incorporated afterwards. This method also requires hazardous solvents, high temperatures, and air-free conditions. In contrast, the new method developed by researchers from Clark Atlanta University and the Molecular Foundry is a one-step process that occurs in a single pot - like a weeknight one-pot pasta recipe - in one hour at room temperature without the need for toxic solvents or air-free conditions. Their results were recently published in ACS Sustainable Chemistry and Engineering.
"We were able to expedite the COF synthesis while also making it as sustainable as possible," explained Normanda Brown, graduate student at Clark Atlanta University and first author of the study. Improving the efficiency and sustainability of this process may enable the broader use of these unique materials.
This new, rapid way of making metal-encapsulated COFs is made possible by mechanochemistry, the process by which mechanical force is used to induce chemical reactions to occur. "Mechanochemistry simply initiates a chemical reaction through mechanical force," explained Brown. "Whether that be grinding, shearing, or milling." In this particular method, a ball mill - an instrument in which materials are crushed using stainless steel balls - is used to combine the ingredients needed to create metal-encapsulated COFs.
To confirm that the correct metal-encapsulated COFs were made and to evaluate their properties, the Clark Atlanta University team worked with Molecular Foundry scientists Jeff Urban and Yi Liu. "The Foundry has developed robust tools and expertise towards structured porous materials to support the characterization needs of our users," said Liu. Through both on-site visits and remote communication, Molecular Foundry staff worked with the Clark Atlanta researchers to understand the required characterization methods for their metal-encapsulated COFs. The COFs were evaluated to see how porous and crystalline they are and how much metal was added to the structure. Additionally, powerful transmission electron microscopes were used to visualize the COF structure and the distribution of metal throughout.
"The Molecular Foundry is like family," said Xinle Li, assistant professor at Clark Atlanta University and principal investigator of the study. Li applied to the User Program at the Molecular Foundry to overcome the challenges of starting his independent career during the pandemic, gaining access to expert collaborators, and subsequently has been able to take advantage of tools and resources that are not available at a smaller university. "Clark Atlanta University was established by the consolidation of Atlanta University, the nation's first graduate school for African Americans, and Clark College, the nation's first four-year liberal arts college to serve a primarily African-American student population," explained Li. "This past fall, there was an enrollment of more than 4000 students, making the university the largest private institution among the Historically Black Colleges and Universities (HBCUs) in Georgia."
As a proof of concept, the metal-encapsulated COF material developed through the team's new, one-step approach was successfully used to catalyze the Suzuki-Miyaura coupling reaction, a widely used method in organic chemistry for forming carbon-carbon bonds. Looking to the future, this one-pot mechanochemical synthesis method can be applied to different metals and different COFs for a range of applications across catalysis, energy storage, gas storage, chemical sensing, and adsorption and separation.
The Molecular Foundry is a DOE Office of Science user facility at Berkeley Lab.
The work was supported in part by the DOE Office of Science.
