New modification strategy gives MOFs bigger pores

Metal-organic frameworks (MOFs)—hybrid materials that stitch organic linkers and metal bits into a meshlike structure—are popular for catalysis and chemical sensing, and even as carriers for drug delivery. Those applications could work better if chemists could make the pores bigger. Now researchers have shown a way to clip off organic ligands in MOFs to turn micropores into larger mesopores (J. Am. Chem. Soc. 2018, DOI: 10.1021/jacs.8b09682).

MOFs with mesopores—pores wider than 2 nm—could accommodate larger drug molecules and prevent the gas diffusion that occurs in catalytic applications. Existing methods to increase pore size rely on complex, custom-made ligands, which are expensive. Simply increasing the length of the ligand leads to networks of interpenetrating pores that don’t increase the size of the gaps. A few methods exist to tailor pore size using different ligands as modulators, but it’s difficult to both create a hierarchy of pore sizes and control the generation of defects necessary for some reactions. In addition, some of these methods use harsh chemical or thermal treatments.

To get around these problems, the trick is to choose ligands that can be selectively snipped by reacting with ozone, opening up smaller pores into larger ones, says Vincent Guillerm, a postdoctoral researcher at the Catalan Institute of Nanoscience and Nanotechnology. He and his colleagues built MOFs from zirconium clusters and two ligands, azobenzene-4,4’-dicarboxylic acid (H₂azo) and 4,4’-stilbenedicarboxylic acid (H₂sti), both of which are approximately 1.33 nm long. Then they introduced ozone into the system, which reacted with the H₂sti. Those ligands converted into terephthalic acid and formylbenzoic acid, effectively severing their links with metallic centers. The H₂azo ligands, on the other hand, were unaffected because, unlike H₂sti, they do not have carbon-carbon double bonds that are sensitive to ozone, Gullierm says.

This process required a washing step to remove the by-products from the ozone clipping step. The researchers also developed a second mesoporous MOF using 4,4’-biphenyldicarboxylic acid and 1,4-phenylenediacrylic acid for the ligands. In this MOF, the products from the cleaved ligands sublimated from the material, eliminating the washing step.

Before clipping the ligands, all the pores were approximately 1.5 nm in diameter. Afterwards, they covered a range of diameters between 2 and 5 nm. The variation arises from the random distribution of the two ligands throughout the material, so some areas had a higher concentration of one than the other. Researchers would like to get better control of the distribution, which would also help them narrow the range of pore sizes, says study coauthor Daniel Maspoch, who leads the Supramolecular Nanochemistry and Materials group at the institute.

In addition to widening the pores, clipping the ligands also frees up some bonding sites that could react with other chemicals. That could prove beneficial for engineering materials other than MOFs. “If you are able to selectively break some bonds inside an object, you can generate some new function in this object,” Maspoch says.

Omar M. Yaghi of the University of California, Berkeley, who specializes in MOFs, says this work adds a new, creative twist to the efforts to improve MOF performance. “It’s elegant, clever, and its precision is a testament to the increasing utility of reticular chemistry”—the process by which MOFs are stitched together—“in controlling matter on the atomic and molecular level,” he says.