Nanoporous metal oxide materials are ubiquitous in the material sciences because of their numerous potential applications in various areas, including adsorption, catalysis, energy conversion and storage, optoelectronics, and drug delivery. While synthetic strategies for the preparation of siliceous nanoporous materials are well-established, nonsiliceous metal oxide-based nanoporous materials still present challenges. Herein, we report a novel synthetic strategy that exploits a metal-organic framework (MOF)-driven, self-templated route toward nanoporous metal oxides via thermolysis under inert atmosphere. In this approach, an aliphatic ligand-based MOF is thermally converted to nanoporous metal oxides with highly nanocrystalline frameworks, in which aliphatic ligands act as the self-templates that are afterward evaporated to generate nanopores. We demonstrate this concept with hierarchically nanoporous magnesia (MgO) and ceria (CeO2), which have potential applicability for adsorption, catalysis, and energy storage. The pore size of these nanoporous metal oxides can be readily tuned by simple control of experimental parameters. Significantly, nanoporous MgO exhibits exceptional CO2 adsorption capacity (9.2 wt %) under conditions mimicking flue gas. This MOF-driven strategy can be expanded to other nanoporous monometallic and multimetallic oxides with a multitude of potential applications.

Nanostructured materials such as porous metal oxides, metal nanoparticles, porous carbons, and their composites have been intensively studied due to their applications, including energy conversion and storage devices, catalysis, and gas storage. Appropriate precursors and synthetic methods are chosen for synthesizing the target materials. About a decade ago, metal-organic frameworks (MOFs) and coordination polymers (CPs) emerged as new precursors for these nanomaterials because they contain both organic and inorganic species that can play parallel roles as both a template and a precursor under given circumstances. Thermal conversions of MOFs offer a promising toolbox for synthesizing functional nanomaterials that are difficult to obtain using conventional methods. Although understanding the conversion mechanism is important for designing MOF precursors for the synthesis of nanomaterials with desired physicochemical properties, comprehensive discussions revealing the transformation mechanism remain insufficient. This Account reviews the utilization of MOFs/CPs as precursors and their transformation into functional nanomaterials with a special emphasis on understanding the relationship between the intrinsic nature of the parent MOFs and the daughter nanomaterials while discussing various experimental approaches based on mechanistic insights. We discuss nanomaterials categorized by materials such as metal-based nanomaterials and porous carbons. For metal-based nanomaterials transformed from MOFs, the nature of metal ions in the MOF scaffolds affects the physicochemical properties of the resultant materials including the phase, composite, and morphology of nanomaterials. Organic ligands are also involved in the in situ chemical reactions with metal species during thermal conversion. We describe these conversion mechanisms by classifying the phase of metal components in the resultant materials. Along with the metal species, carbon is a major element in MOFs, and thus, the appropriate choice of precursor MOFs and heat treatment can be expected to yield carbon-based nanomaterials. We address the relationship between the nature of the parent MOF and the porosity of the daughter carbon material-a controversial issue in the synthesis of porous carbons. Based on an understanding of the mechanism of MOF conversion, morphologically or compositionally advanced materials are synthesized by adopting appropriate MOF precursors and thermolysis conditions. Despite the progressive understanding of conversion phenomena of MOFs/CPs, this research field still has rooms to be explored and developed, ultimately in order to precisely control the properties of resultant nanomaterials. In this sense, we should pay more attention to the mechanism investigations of MOF conversion. We believe this Account will facilitate a deeper understanding of MOF/CP conversion routes and will accelerate further development in this field.

We developed a general synthetic route for preparing nanoporous transition metal/ceria solid solutions with nanocrystalline frameworks (TM x Ce1–x O2−δ, TM = Mn, Ni, Co, or Fe). Their structural properties were characterized using transmission electron microscopy (TEM), X-ray powder diffraction (XRPD), and N2 sorption. Through thermolysis of bimetallic coordination polymers, hierarchically nanoporous frameworks composed of 3–4 nm TM x Ce1–x O2−δ solid solution nanocrystals in which the transition metal ions are well-dispersed in the ceria lattice as evidenced by the Rietveld refinement of the XRPD patterns were synthesized. The electronic properties of the Mn x Ce1–x O2−δ solid solutions at up to 20 mol % were examined by Raman spectroscopy and X-ray photoelectron spectroscopy analysis, and H2-temperature-programmed reduction results demonstrated the altered physicochemical properties, e.g., hydrogen reduction behaviors, due to the doping. CO oxidation studies of Mn x Ce1–x O2−δ reveal that the Mn species are responsible for increasing the catalytic activity by an order of magnitude compared to that of pure ceria, by creating nanostructures with accessible pores and active sites on the inner surface. This facile synthetic approach can create nanoporous solid solutions with nanocrystalline frameworks and devise structures and compositions. Therefore, our approach opens new avenues for developing multimetallic catalyst systems.

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The pseudomorphic conversion of MOFs resulted in the controlled preparation of Co3O4 nanoparticles with different microstructures, which showed different electrochemical properties.