metal-organic framework

metal-organic framework. A metal-organic framework is a class of porous, crystalline materials constructed from metal ions or clusters coordinated to organic linkers. This modular architecture creates extended networks with exceptionally high surface areas and tunable pore geometries. Their design flexibility has led to significant research interest across chemistry, materials science, and engineering for advanced applications.

Definition and structure

The defining structural motif of these materials is the coordination bond between inorganic nodes and multidentate organic struts. The inorganic secondary building units often consist of ions from the transition metals, such as zinc or copper, or clusters like paddle-wheel units. Common organic linkers include carboxylates like terephthalic acid or nitrogen-donor molecules such as imidazolates, which form frameworks like ZIF-8. This coordination yields two- or three-dimensional networks with permanent porosity, as exemplified by the structures of MOF-5 and the University of Oslo series. The geometry is frequently characterized using X-ray crystallography at facilities like the Advanced Photon Source.

Synthesis and modulation

These materials are typically synthesized under solvothermal or hydrothermal conditions, often employing solvents like N,N-Dimethylformamide. Pioneering work by Omar Yaghi and his team at the University of California, Berkeley established many foundational methods. Post-synthetic modification is a critical strategy for tailoring functionality, involving chemical reactions on the installed linker or metal site after framework formation. Techniques such as atomic layer deposition can be used to composite frameworks with other materials. The BASF corporation has been instrumental in scaling production for commercial evaluation.

Properties and characterization

A hallmark property is an extraordinarily high Brunauer–Emmett–Teller surface area, with materials like NU-110 and DUT-60 setting successive records. Their porosity enables high gas uptake capacities, relevant for storing hydrogen or methane. Framework flexibility can lead to phenomena like breathing, observed in the MIL-53 series. Characterization relies heavily on powder diffraction, gas sorption analysis, and electron microscopy. Thermal stability is assessed via thermogravimetric analysis, while functional properties are probed using infrared spectroscopy and nuclear magnetic resonance spectroscopy.

Applications

Research is heavily directed toward energy and environmental technologies. They are prominent candidates for carbon capture from flue gases, with projects supported by the United States Department of Energy. In fuel systems, they are studied for onboard storage of compressed natural gas. Their catalytic sites are exploited for reactions like carbon dioxide reduction, with work advanced by institutions like the Max Planck Society. Other uses include water harvesting from arid air, sensing of volatile compounds, and drug delivery systems investigated by Johnson & Johnson. Frameworks like ZIF-67 are templates for deriving functional nanoparticles.

History and development

Early concepts of coordination polymers date to the 1960s, but the modern field coalesced in the 1990s with reports from researchers like Omar Yaghi and Susumu Kitagawa. The term itself was popularized following Yaghi's 1995 report on structures with robust porosity. The 1999 disclosure of MOF-5 was a landmark, demonstrating unprecedented surface area. Subsequent decades saw explosive growth, with major contributions from groups led by Férey Gérard at the University of Versailles Saint-Quentin-en-Yvelines, who developed the MIL series, and Michael O'Keeffe who established systematic topology design. The field continues to evolve through international collaborations and conferences like the International Zeolite Association meetings.

Category:Coordination polymers Category:Porous materials Category:Nanomaterials