Kui Shen

Ordered macro-microporous metal-organic framework single crystals

Mesoporous metal-organic frameworks The diffusion limitations on gas storage and catalytic reaction of microporous materials can often be overcome if they are incorporated into a mesoporous structure with much larger pores. Shen et al. grew ordered arrays of microcrystals of the ZIF-8 metal-organic framework, in which zinc ions are bridged by 2-methylimidazole linkers, inside a porous polystyrene template. These materials showed higher reaction rates for the Knoevenagel reaction between benzaldehydes and malononitriles and better catalyst recyclability. Science, this issue p. 206 A double-solvent method and templating are used to grow ordered arrays of metal-organic framework microcrystals. We constructed highly oriented and ordered macropores within metal-organic framework (MOF) single crystals, opening up the area of three-dimensional–ordered macro-microporous materials (that is, materials containing both macro- and micropores) in single-crystalline form. Our methodology relies on the strong shaping effects of a polystyrene nanosphere monolith template and a double-solvent–induced heterogeneous nucleation approach. This process synergistically enabled the in situ growth of MOFs within ordered voids, rendering a single crystal with oriented and ordered macro-microporous structure. The improved mass diffusion properties of such hierarchical frameworks, together with their robust single-crystalline nature, endow them with superior catalytic activity and recyclability for bulky-molecule reactions, as compared with conventional, polycrystalline hollow, and disordered macroporous ZIF-8.

Multimetal-MOF-derived transition metal alloy NPs embedded in an N-doped carbon matrix: highly active catalysts for hydrogenation reactions

Transition metal NPs and their alloy NPs have attracted significant attention due to their low cost and potential to replace noble-metal-based catalysts in many reaction systems; however, major challenges are still encountered when exploring cost-effective and scaleable strategies to prepare and attach them with optimal supports for maximizing their catalytic efficiency. Here, we report a facile and repeatable route to synthesize transition metal based nano-catalysts by first developing a series of new and novel N-donor multimetallic M–M′-MOFs [(M–M′(1,4-bdc)2(dabco)]·4DMF·1/2H2O, M/M′ = Co, Ni, Cu) by a facile mixed-metal approach and then directly pyrolyzing these hetero-nuclear MOFs under inert gas. In the pyrolysis process, the transition metal ions (two of Co, Ni, and Co) of M–M′-MOFs could be transformed into transition alloy nanoparticles while the surrounding N-containing ligands were polymerized to N-doped graphitic carbon, resulting in highly dispersed M/M′ alloy NPs embedded in the N-doped carbon matrix. The detailed characterization results revealed that the metal elements including Co, Ni, and Cu were uniformly distributed in every individual alloy NP and there existed an obvious synergetic activation of different transition metals and strong coordination interactions between metals of alloy NPs and N atoms. When being used in the transfer hydrogenation of nitriles in the absence of basic additives, the optimal Co–Ni(3 : 1)@C–N showed the best catalytic performance with 100% conversion of benzonitrile and over 98% yield for the desired product, which was almost 5 times more active than its monometallic counterparts. We believe that this MOF-templating strategy provides a facile and controllable route for the preparation of nanocatalysts based on transition metal NPs with high transfer hydrogenation performance.

Greening the Processes of Metal–Organic Framework Synthesis and their Use in Sustainable Catalysis

Given the shortage of sustainable resources and the increasingly serious environmental issues in recent decades, the demand for clean technologies and sustainable feedstocks is of great interest to researchers worldwide. With regard to the fields of energy saving and environmental remediation, the key point is the development of efficient catalysts, not only in terms of facile synthesis methods, but also the benign utilization of such catalysts. This work reviews the use of metal-organic frameworks (MOFs) and MOF-based materials in these fields. The definition of MOFs and MOF-based materials will be primarily introduced followed by a brief description of the characterization and stability of MOF-related materials under the applied conditions. The greening of MOF synthesis processes will then be discussed and catalogued by benign solvents and conditions and green precursors of MOFs. Furthermore, their suitable application in sustainable catalysis will be summarized, focusing on several typical atom-economic reactions, such as the direct introduction of H2 or O2 and C-C bond formation. Approaches towards reducing CO2 emission by MOF-based catalysts will be described with special emphasis on CO2 fixation and CO2 reduction. In addition, driven by the explosive growth of energy consumption in the last century, much research has gone into biomass, which represents a renewable alternative to fossil fuels and a sustainable carbon feedstock for chemical production. The advanced progress of biomass-related transformations is also illustrated herein. Fundamental insights into the nature of MOF-based materials as constitutionally easily recoverable heterogeneous catalysts and as supports for various active sites is thoroughly discussed. Finally, challenges facing the development of this field and the outlook for future research are presented.

Hollow-ZIF-templated formation of a ZnO@C–N–Co core–shell nanostructure for highly efficient pollutant photodegradation

Semiconducting metal oxides have been considered as effective photocatalysts for the degradation of organic pollutants to decolorize contaminated water. However, the poor performance and difficulty in recycling greatly hinder their practical application. Herein, we designed and fabricated, for the first time, a novel ZnO@C–N–Co core–shell nanocomposite towards efficient and recyclable photocatalysis by directly pyrolyzing a hollow Zn/Co–ZIF matrix consisting of a ZIF-8 shell with some Co–ZIF nanoplates inside the hollow cavity. It was demonstrated that the ZIF-8-shell-derived ZnO nanoparticles could spontaneously agglomerate and move to the hollow cavity while the internal Co NPs transferred inversely to the ligand-derived N–C shell at suitable pyrolysis temperature, resulting in the unique ZnO@C–N–Co core–shell structure. This unique nanostructure possessed the following superior properties: (1) a core–shell structure may make the intermediate ZnO much more stable during the reaction; (2) a porous carbon shell can not only provide a high BET specific area and thus high adsorption capability for reactants, but can also inhibit the recombination of photogenerated electrons and holes; (3) the embedded Co NPs were able to provide richer electron traps that further suppress the recombination of electrons and holes. As exemplified for the degradation of methyl orange, the ZnO@C–N–Co showed a significantly improved performance and excellent recyclability due to the highly synergistic effects between the C–N–Co shell and the robust ZnO. This strategy for the synthesis of MOF-derived core–shell nanomaterials could offer prospects for developing highly efficient photocatalysts.

Phase-controllable synthesis of MOF-templated maghemite–carbonaceous composites for efficient photocatalytic hydrogen production

Solar water splitting to produce H2 represents a solution with high potential for the current severe energy and environmental issues. Iron oxides are earth-abundant and nontoxic, with narrow bandgaps and suitable valence band positions for the visible-light-driven water oxidation reaction; however, an energy limitation for hydrogen generation is encountered due to the improper conduction band level. Herein, we demonstrated that this limitation could be overcome by the incorporation of small amounts of GO in the metal–organic framework (MOF)-templated synthesis of iron oxide, affording a uniform and highly ordered ferrite octahedral nanostructure embedded on graphene nanosheets. Such structural superiorities would result in a highly synergistic effect between Fe2O3 and reduced graphene oxide (rGO), affording an elevated flat band potential and a promoted photogenerated charge carrier separation and transportation. As a consequence, the resulting maghemite–carbonaceous composite exhibited a high photocatalytic H2 evolution rate of 318.0 μmol h−1 g−1 in the absence of noble metal cocatalysts and external bias. This work provides for the first time an ideal pathway for the utilization of Fe2O3 as the dominant component of a nanocomposite in efficient photocatalytic H2 production, as well as the prospect of developing highly active photocatalysts for overall water splitting. In addition, different phases of iron oxide, including maghemite (γ-Fe2O3), hematite (α-Fe2O3) and magnetite (Fe3O4), and their carbonaceous composites can be obtained through the cautiously selected thermolysis of Fe-MOF and MOF/GO composites. The maghemite phase could be maintained with high saturation magnetization values under a large temperature gradient, implying a great potential for applications in magnetism and biomedicine-related research fields.

Fabricating Sandwich-Shelled ZnCdS/ZnO/ZnCdS Dodecahedral Cages with “One Stone” as Z-scheme Photocatalysts for Highly Efficient Hydrogen Production

Affording two semiconductors with one template in one step to construct a composite with delicate structures may endow low-cost photocatalysts with desired characteristics, i.e., high activity, stability and recyclability. Herein, novel sandwich-shelled ZnCdS/ZnO/ZnCdS cages are fabricated with “one stone”—zeolitic-imidazolate-framework-8. ZnS and ZnO are formed simultaneously in the sulfidation stage, where ZnS serves as a barrier to localize the ZnO particles filling up the voids between the ZnS layers. It could therefore be very advantageous in photocatalysis that the ZnCdS, derived from cation-exchanged ZnS, and ZnO with well-defined interfaces also have staggered band structure configurations by virtue of the fine adjustment of the composition. The Zn0.5Cd0.5S/ZnO/Zn0.5Cd0.5S cages exhibit a H2 production rate of 28.6 mmol g−1 h−1 and long-term durability, achieving the highest activities among the ZnCdS and ZnO families under noble-metal-free conditions. This remarkable performance could be ascribed to the unique morphology of the sandwich-shell and hollow interior integrating multiple vital merits for photocatalysis, including the enhanced light-harvesting ability, abundant active sites, shortened charge diffusion distances, and Z-scheme mechanism featuring preserved strong redox ability and improved charge separation and migration. This facile strategy may offer great opportunities in developing highly active metal sulfide/oxide-based photocatalysts for practical applications.

Seed-induced and additive-free synthesis of oriented nanorod-assembled meso/macroporous zeolites: toward efficient and cost-effective catalysts for the MTA reaction

We reported a general seed-induced strategy for the synthesis of both meso/macroporous ZSM-5 and ZSM-11 (denoted as N-ZSM-5 and N-ZSM-11, respectively) under additive-free, low template/SiO2 ratio (from 0.011 to 0.0014) and seed-assisted hydrothermal conditions. It was found that both N-ZSM-5 and N-ZSM-11 were primarily composed of 20–50 nm oriented nanorods, which showed excellent physicochemical properties, such as high crystallinity, large surface area, auxiliary meso/macroporous structures, and uniform size. When being used in the methanol-to-aromatics reaction (MTA), both Zn/N-ZSM-5 and Zn/N-ZSM-11 exhibited high catalytic efficiencies, as reflected by their far longer lifetimes and higher selectivities for total aromatics than conventional Zn/ZSM-5 and Zn/ZSM-11, due to their unique meso/macroporous structure and ultra-small crystal size that resulted in substantial improvements in the mass-transport properties. This seed-induced strategy inspires new ideas for the design and fabrication of oriented nanorod-assembled hierarchical zeolites with lower cost and good catalytic performance.