A metal–organic framework (MOF) is a coordination network in which metal-containing units are connected by organic ligands, producing an extended structure with potential voids. Many MOFs are crystalline porous materials containing channels or cavities accessible to guest molecules. Their modular construction allows researchers to vary pore dimensions, internal chemical environments, and framework flexibility. The formal definition does not require demonstrated permanent porosity or crystallinity: a framework may contain solvent-filled voids or change its structure under external conditions. (goldbook.iupac.org)
Definition and historical development
MOFs belong within the broader category of coordination polymers, but not every coordination polymer qualifies as a MOF. The defining combination is an extended coordination network, organic ligands, and potential voids. This terminology distinguishes framework connectivity from experimentally measured accessibility: identifying cavities in a structure does not by itself establish that gases can enter them after solvent removal. (publications.iupac.org)
The field developed from efforts to construct extended solids using principles of coordination chemistry. Richard Robson’s work established important approaches to designing networks from metal centers and geometrically selected organic components. Susumu Kitagawa demonstrated gas uptake in porous coordination materials and developed concepts involving flexible frameworks. Omar M. Yaghi advanced robust porous frameworks and their systematic design. MOF-5, reported in 1999, became an influential example of a permanently porous framework. The three researchers received the 2025 Nobel Prize in Chemistry for the development of MOFs. (nobelprize.org)
Building blocks and structural design
Metal nodes may be individual ions or clusters containing several metal centers. Such clusters can function as secondary building units, providing defined connection points for organic linkers. Common linker families include carboxylates and nitrogen-containing heterocycles. MOF-5, for example, combines Zn₄O clusters with terephthalate linkers to form a three-dimensional network. Its structure illustrates how relatively simple components can generate extensive internal space. (globalscience.berkeley.edu)
Framework design is closely associated with reticular chemistry, which links selected building blocks into extended networks. Isoreticular chemistry varies linker length or functionality while preserving an underlying network arrangement. Longer linkers can enlarge pores, although framework interpenetration—the interweaving of separate networks—may reduce accessible space. Some MOFs are comparatively rigid; others respond to guest uptake, temperature, or pressure through structural changes commonly described as breathing or gate opening. (globalscience.berkeley.edu)
Synthesis, activation, and modification
Many MOFs are prepared by solvothermal synthesis, in which metal precursors and organic linkers react in a heated solvent. Reaction composition and processing conditions affect which framework forms and the dimensions of its particles. Alternative routes include mechanochemistry, using milling or grinding to promote reaction. In situ studies of ZIF-8 synthesis show that milling can produce intermediate phases, amorphization, and subsequent recrystallization, rather than a simple direct conversion to one product. (nature.com)
As-synthesized pores commonly contain solvent and other guests. Activation removes these species to make internal surfaces accessible, often through washing, solvent exchange, and controlled drying or evacuation. Activation is not equivalent to synthesis: unsuitable drying conditions can damage a material’s larger-scale structure, and insufficient guest removal can obstruct pores. Postsynthetic modification changes an already formed framework, for example by reacting pendant amino groups on its organic linkers. This provides a route to additional functionality without constructing the material entirely from new precursors. (nature.com)
Characterization and adsorption
X-ray crystallography determines framework arrangements, while powder diffraction helps identify phases and compare synthesized samples with structural models. Thermogravimetric analysis and infrared spectroscopy provide complementary information about thermal behavior and chemical composition. Combining methods is important because a diffraction pattern alone does not demonstrate accessible porosity. (nature.com)
Gas uptake is measured through an adsorption isotherm, relating the amount adsorbed to pressure at a specified temperature. BET theory is widely used to estimate specific surface area from suitable adsorption measurements. Values depend on the sample’s activation state and measurement conditions. Adsorption occurs through interactions at internal surfaces; it should not be confused with bulk absorption. Surface area, pore volume, uptake capacity, and separation selectivity describe different aspects of performance and are not interchangeable. (nature.com)
Applications and demonstrated functions
MOFs are investigated for hydrogen and methane storage, gas purification, and carbon capture. Separation can arise from differences in guest affinity, pore accessibility, or framework response. Open metal sites, exposed when coordinating guests are removed, can strengthen interactions with selected gases. High uptake is useful only in relation to operating conditions: practical storage also depends on how much gas can be released between charging and discharge pressures. (globalscience.berkeley.edu)
In atmospheric water harvesting, a MOF adsorbs water vapor and subsequently releases it for condensation. A 2018 study demonstrated a solar-driven MOF-801 device in Tempe, Arizona, under arid conditions. MOFs also support catalysis through framework sites or incorporated catalytic species. For example, copper nanoparticles associated with zirconium-containing UiO-66 nodes have been investigated for converting carbon dioxide to methanol. These functions require evaluation of the complete device or reaction system, not just the isolated powder. (nature.com)
Stability and engineering constraints
MOFs differ substantially in chemical and mechanical stability. MOF-5 is moisture-sensitive because water can disrupt its metal–oxygen coordination, illustrating why permanent porosity does not imply resistance to every environment. Processing powders into usable bodies introduces additional considerations: packing density, retained pore accessibility, and structural integrity influence performance. Research on monolithic MOFs demonstrates that controlled washing and drying can alter macroscopic form and volumetric gas-storage behavior even when the underlying framework composition remains unchanged. (nature.com)