Chengcheng Liu

Highly mesoporous metal–organic framework assembled in a switchable solvent

The mesoporous metal–organic frameworks are a family of materials that have pore sizes ranging from 2 to 50 nm, which have shown promising applications in catalysis, adsorption, chemical sensing and so on. The preparation of mesoporous metal–organic frameworks usually needs the supramolecular or cooperative template strategy. Here we report the template-free assembly of mesoporous metal–organic frameworks by using CO2-expanded liquids as switchable solvents. The mesocellular metal–organic frameworks with large mesopores (13–23 nm) are formed, and their porosity properties can be easily adjusted by controlling CO2 pressure. Moreover, the use of CO2 can accelerate the reaction for metal–organic framework formation from metal salt and organic linker due to the viscosity-lowering effect of CO2, and the product can be recovered through CO2 extraction. The as-synthesized mesocellular metal–organic frameworks are highly active in catalysing the aerobic oxidation of benzylic alcohols under mild temperature at atmospheric pressure.

Solvent determines the formation and properties of metal–organic frameworks

The formation of a water-sensitive metal–organic framework (MOF), Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylate), in a water/ethanol solvent system was studied systematically. The X-ray diffraction results prove that the MOF cannot form in pure water or in a water/ethanol mixture with a small amount of ethanol. As the ethanol content exceeds 30 vol%, a crystalline MOF can be obtained. The scanning electron microscope images of the as-synthesized MOFs show the formation of MOF nanoparticles with an average size of 20–300 nm. The MOF particle size decreased with increasing ethanol content in the mixed solvent. The FT-IR spectra further support that the MOF formation occurs in water/ethanol mixtures with ethanol volume ratios higher than 30 vol%. Thermogravimetric analysis showed that the MOF is stable up to 300 °C. Moreover, the FT-IR spectra and thermogravimetric analysis gave consistent information on the solvent amount entrapped in the MOF pores. The porosity of the MOFs was determined using a N2 adsorption–desorption method. When the ethanol volume ratio reached 75%, the largest SBET value of 1067 m2 g−1 and Vt value of 0.52 cm3 g−1 were obtained. The possible mechanism for MOF formation in water/ethanol solvent systems and the dependence of the MOF size on the solvent composition was discussed from the view of hydrogen bond strength between solvent molecules and the ligands in different water/ethanol solvent systems.

The ionic liquid microphase enhances the catalytic activity of Pd nanoparticles supported by a metal–organic framework

Here we demonstrate the utilization of the ionic liquid (IL) micro-phase for enhancing the catalytic activities of the metal nanoparticles supported on a MOF. The IL microphase offers an excellent environment for stabilizing metal nanoparticles. A new heterogeneous catalyst Pd/IL/MOF is developed, which combines the advantages of highly dispersed small Pd nanoparticles, the IL microphase and a porous MOF. The as-synthesized Pd/IL/MOF catalysts have shown high catalytic activity and reusability for selective hydrogenation under mild conditions.

High-internal-phase emulsions stabilized by metal-organic frameworks and derivation of ultralight metal-organic aerogels

To design high-internal-phase emulsion (HIPE) systems is of great interest from the viewpoints of both fundamental researches and practical applications. Here we demonstrate for the first time the utilization of metal-organic framework (MOF) for HIPE formation. By stirring the mixture of water, oil and MOF at room temperature, the HIPE stabilized by the assembly of MOF nanocrystals at oil-water interface could be formed. The MOF-stabilized HIPE provides a novel route to produce highly porous metal-organic aerogel (MOA) monolith. After removing the liquids from the MOF-stabilized HIPE, the ultralight MOA with density as low as 0.01 g·cm−3 was obtained. The HIPE approach for MOA formation has unique advantages and is versatile in producing different kinds of ultralight MOAs with tunable porosities and structures.

Cellular graphene aerogel combines ultralow weight and high mechanical strength: A highly efficient reactor for catalytic hydrogenation.

The construction of three-dimensional graphene aerogels (GAs) is of great importance owing to their outstanding properties for various applications. Up to now, the combination of ultralow weight and super mechanical strength for GA remains a great challenge. Here we demonstrate the fabrication of cellular GAs by a facile, easily controlled and versatile route, i.e. the chemical reduction of graphene oxide assemblies at oil-water interface under a mild condition (70 °C). The GA is ultralight (with density <3 mg cm−3) yet mechanically resilient because the walls of the cell closely pack in a highly ordered manner to maximize mechanical strength. The GA has been utilized as an appealing reactor for catalytic hydrogenation, which exhibited great advantages such as large oil absorption capability, exceptional catalytic activity, ease of product separation and high stability.

Room-temperature synthesis of mesoporous CuO and its catalytic activity for cyclohexene oxidation

CuO nanoleaves with a mesoporous structure were formed in an aqueous solution of triethylamine at room temperature. The growth process of the CuO nanoleaves in a triethylamine solution was investigated by varying the reaction time. It is shown that the CuO nanostructures form by reconstructive transformation from Cu(OH)2, going through a 0D nanoparticle → 1D nanowire → 2D nanoleaf dimensional transition process. The mechanism for the amine-induced formation of CuO at room temperature was studied by using different aliphatic amines. It is revealed that the amines play multiple roles on CuO formation, i.e. acting as an alkali, dominating the Cu(OH)2 to CuO transformation, and directing the oriented crystal growth of CuO. This route is simple, rapid, involves no additional alkalis or directing agents, and can proceed at room temperature. The as synthesized CuO exhibits excellent catalytic activity for cyclohexene oxidation with oxygen under solvent-free conditions.

Ionic liquid accelerates the crystallization of Zr-based metal–organic frameworks

The Zr-based metal–organic frameworks are generally prepared by solvothermal procedure. To overcome the slow kinetics of nucleation and crystallization of Zr-based metal–organic frameworks is of great interest and challenging. Here, we find that an ionic liquid as solvent can significantly accelerate the formation of Zr-based metal–organic frameworks at room temperature. For example, the reaction time is shortened to 0.5 h in 1-hexyl-3-methylimidazolium chloride for Zr-based metal–organic framework formation, while that in the conventional solvent N,N-dimethylformamide needs at least 120 h. The reaction mechanism was investigated in situ by 1H nuclear magnetic resonance, spectroscopy synchrotron small angle X-ray scattering and X-ray absorption fine structure. This rapid, low-energy, and facile route produces Zr-based metal–organic framework nanoparticles with small particle size, missing-linker defects and large surface area, which can be used as heterogeneous catalysts for Meerwein–Ponndorf–Verley reaction.Crystallization kinetics of metal-organic frameworks in conventional organic solvents are usually very slow. Here, the authors show that an ionic liquid medium accelerates considerably the formation of Zr-based metal-organic frameworks that are active catalysts in the Meerwein-Ponndorf-Verley reaction.

High internal ionic liquid phase emulsion stabilized by metal–organic frameworks

The emulsification of metal-organic frameworks (MOFs) for the two immiscible phases of water and ionic liquid (IL) was investigated for the first time. It was found that Ni-BDC (BDC = 1,4-dicarboxybenzene) can emulsify water and ILs and favor the formation of high internal phase emulsions (HIPEs) under certain experimental conditions. The microstructures of the HIPEs were characterized by confocal laser scanning microscopy using a fluorescent dye Rhodamine B, which proves that the HIPEs are the IL-in-water type. Further results reveal that the HIPE forms during the IL-in-water to water-in-IL emulsion inversion. The possibilities of the HIPE formation by other MOFs (Cu-BDC and Zn-BDC) were explored and the mechanism for HIPE formation was discussed. The MOF-stabilized HIPE was applied to the in situ synthesis of a MOF/polymer composite by HIPE polymerization. The macroporous MOF/polyacrylamide network and MOF/polystyrene microspheres were obtained from the HIPEs, respectively.

Assembly of Mesoporous Metal–Organic Framework Templated by an Ionic Liquid/Ethylene Glycol Interface

We propose a facile room-temperature synthesis of a metal-organic framework (MOF) with a bimodal mesoporous structure (3.9 and 17-28 nm) in an ionic liquid (IL)/ethylene glycol (EG) mixture. The X-ray diffraction analysis reveals that MOF formation can be efficiently promoted by the presence of the EG/IL interface at room temperature. The MOFs with mesoporous networks are characterized by SEM and TEM. The formation mechanism of the mesoporous MOF in EG/IL mixture is investigated. It is proposed that the EG nanodroplets in the IL work as templates for the formation of the large mesopores. The as-synthesized mesoporous metal-organic framework is an effective and reusable heterogeneous catalyst to catalyze the aerobic oxidation of benzylic alcohols.

CO2/Water Emulsions Stabilized by Partially Reduced Graphene Oxide

Using functional materials to stabilize emulsions of carbon dioxide (CO2) and water is a promising way to expand the utility of CO2 and functional materials. Here we demonstrate for the first time that the partially reduced graphene oxide (rGO) can well stabilize the emulsion of CO2 and water without the aid of any additional emulsifier or surface modification for rGO. More interestingly, such a novel kind of emulsion provides a facile and versatile route for constructing highly porous three-dimensional rGO materials, including rGO, metal/rGO, and metal oxide/rGO networks. The as-synthesized Au/rGO composite is highly active in catalyzing 4-nitrophenol reduction and styrene epoxidation.