Macro- and Mesoporous Polymers Synthesized by a CO2-in-Ionic Liquid Emulsion-Templating Route

Green solvent systems for material syntheses and chemical reactions

It is of great significance to develop environmentally benign, non-volatile and recyclable green solvents for different applications. This feature article overviews the properties of green solvent systems (e.g., ionic liquids, supercritical carbon dioxide, deep eutectic solvents and mixed green solvent systems) and their applications in (1) framework material syntheses, including metal-organic frameworks, covalent organic frameworks and hydrogen-bonded organic frameworks, and (2) CO2 conversion reactions, including photocatalytic and electrocatalytic reduction reactions. Finally, the future perspective for research on green solvent systems is proposed from different aspects.

Engineering linker defects in functionalized UiO-66 MOF nanoparticles for oil-in-water Pickering emulsion stabilization

Designing metal-organic framework (MOF)-based solid nanoparticles to stabilize Pickering emulsions by fine-tuning their hydrophobicity and lipophobicity is vital for essential applications and fundamental understanding. We demonstrate in situ grafting of palmitic acid in UiO-66 MOF through its linker defects. Our designed and activated nanoparticles (denoted as UP') stabilized the Pickering emulsions of n-heptane-in-water. Furthermore, we showed how UP' stabilized emulsion droplets disperse in media by covering each tiny droplet with a nanoscale layer made of UP'. To support our claim, we carried out the freeze-drying process to remove the liquid part from the emulsion, leaving behind the solid shell-like microstructures that we further characterized through several microscopic techniques. The stable n-heptane-in-water emulsion was confirmed by dilution (drop test), conductivity, zeta potential, and theoretical surface electrostatic potential measurements. Rheological studies indicate that the Pickering emulsions of n-heptane-in-water stabilized by UP' are much more resistant to deformation and flow imparting higher (mechanical) stability and shelf-life. Pickering emulsions stabilized by UP' emerged as a versatile way to design smart functional materials of UiO-66 through engineering linker defects that may have potential applications in interfacial catalysis, dye or contaminant separation, etc.

Pickering emulsions stabilized by metal-organic frameworks, graphitic carbon nitride and graphene oxide

Pickering emulsion is a heterogeneous system consisting of at least two immiscible liquids, which are stabilized by solid particles, in which organic solvent or water is dispersed into other phase in form of micrometre-sized droplets. Compared to traditional emulsions stabilized by surfactant, solids are cheap and can be easily separated and recycled by centrifugation or filtration after use. Moreover, the properties of Pickering emulsions can be adjusted by using different types of solid particles. Up to now, Pickering emulsions have been applied in a wide range of areas such as material science and catalysis. Here we review recent studies on Pickering emulsions stabilized by metal-organic framework, graphitic carbon nitride and graphene oxide.

Self-Stabilized Giant Aggregates in Water from Room-Temperature Ionic Liquids with an Asymmetric Polar-Apolar-Polar Architecture

We report the assembly of four imidazolium bromides, each of which bears a naphthyl on one side of the imidazolium cation and a branched alkyl chain on the other. This design creates a new type of amphiphilic ionic liquid with an apolar-polar-apolar structure and a low melting point (m_p, <-20 °C), which has not been achieved by reported counterparts bearing linear alkyl chains. In solvent-free states, microphase segregation occurs where polar and apolar domains arrange bicontinuously as proved by molecular dynamics (MD) simulations. When dispersed in water, self-stabilized giant aggregates formed with ultrahigh colloidal stability (up to years). MD simulations provide clues of discrete bicontinuous phases within the giant aggregates. These newly discovered self-assemblies provide a heterogeneous reservoir that can accommodate guest molecules including the highly apolar fullerene C₆₀, paving the way for a wide range of potential applications.

Enhanced selectivity for photodegrading ciprofloxacin by a magnetic photocatalyst modified with a POPD–CdS heterojunction embedded imprinted layer

A POPD–CdS heterojunction embedded magnetic imprinted photocatalyst achieves the purpose of selective photodegradation of ciprofloxacin and effectively suppresses the secondary pollution caused by CdS photocorrosion.

Controllable Synthesis of Multiheteroatoms Co-Doped Hierarchical Porous Carbon Spheres as an Ideal Catalysis Platform

The synthesis of porous carbon spheres with hierarchical porous structures coupled with the doping of heteroatoms is particularly important for advanced applications. In this research, a new route for efficient and controllable synthesis of hierarchical porous carbon spheres co-doped with nitrogen, phosphorus, and sulfur (denoted as NPS-HPCs) was reported. This new approach combines in situ polymerization of hexachlorocyclophosphazene and 4,4'-sulfonyldiphenol with the self-assembly of colloidal silica nanoparticles (SiO₂ NPs). After pyrolysis and subsequent removal of the SiO₂ NPs, the resulting NPS-HPCs possess a high surface area (960 m²/g) as well as homogeneously distributed N, P, and S heteroatoms. The NPS-HPCs are shown to be an ideal support for anchoring highly dispersed and uniformly sized noble metal NPs for heterogeneous catalysis. As a proof of concept, Pd NPs are loaded onto the NPS-HPCs using only methanol as a reductant at room temperature. The prepared Pd/NPS-HPCs are shown to exhibit high activity, excellent stability, and recyclability for hydrogenation of nitroarenes.

Macroporous poly(ionic liquid)/ionic liquid gels via CO2-based emulsion-templating polymerization

A high internal phase emulsion composed of ionic liquids and CO₂ serves as a template for producing unprecedented macroporous poly(ionic liquid) gels.

Pickering emulsions stabilized by a metal-organic framework (MOF) and graphene oxide (GO) for producing MOF/GO composites

Herein we demonstrate the formation of a novel kind of Pickering emulsion that is stabilized by a Zr-based metal-organic framework (Zr-MOF) and graphene oxide (GO). It was found that the Zr-BDC-NO₂ and GO solids assembling at the oil/water interface can effectively stabilize the oil droplets that are dispersed in the water phase. Such a Pickering emulsion offers a facile route for fabricating Zr-MOF/GO composite materials. After removing water and oil by freeze drying from Pickering emulsions, the Zr-MOF/GO composites were obtained and their morphologies, structures and interaction properties were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectrometry, respectively. The influences of the concentration of GO and Zr-MOF on the emulsion microstructures and the properties of the MOF/GO composites were studied. Based on experimental results, the mechanisms for the emulsion formation by Zr-MOF and GO and the as-synthesized superstructures of the Zr-MOF/GO composite were proposed. It is expected that this facile and tunable route can be applied to the synthesis of different kinds of MOF-based or GO-based composite materials.

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.

Synthesis of Functional Nanomaterials in Ionic Liquids

Utilization of ionic liquids (ILs) in material synthesis is a promising field. The unusual properties of ILs provide new opportunities for the design of functional materials, and much excellent work has been reported. Here, the progress in material design and synthesis using ILs, especially nanomaterials, is discussed, including the unitization of ILs as synthetic media, templates, precursors, or components in the synthesis of various categories of nanomaterials. The challenges and opportunities in this interesting and rapid developing area are also discussed.

Spindle-shaped nanoscale yolk/shell magnetic stirring bars for heterogeneous catalysis in macro- and microscopic systems

A new type of spindle-shaped nanoscale yolk/shell magnetic stirring bar containing noble metal nanoparticles was prepared. The as-synthesized Pd-Fe@meso-SiO2 not only showed impressive activity and stability as a heterogeneous catalyst in a macroscopic flask system, but also acted as an efficient nanoscale magnetic stir bar in a microscopic droplet system.

A Pd–Cu2O nanocomposite as an effective synergistic catalyst for selective semi-hydrogenation of the terminal alkynes only

A new type lead-free Pd–Cu₂O nanocomposite catalyst shows “double” selectivities for hydrogenation of alkynes: only terminal alkynes hydrogenated and only alkenes produced, i.e. no internal alkyne is hydrogenated.

CO2-in-PEG emulsion-templating synthesis of poly(acrylamide) with controllable porosity and their use as efficient catalyst supports

Porous poly(acrylamide)s nanoparticles prepared from CO₂-in-PEG emulsions have high catalytic activity for benzene hydrogenation reaction.

Boronic Acid Linear Homopolymers as Effective Emulsifiers and Gelators

We report emulsion studies using poly(vinylphenyl boronic acid) (PVPBA) linear homopolymer as an effective emulsifier and gelator. Two stabilizing regimes were identified depending on the pH of PVPBA aqueous solutions, i.e., emulsions stabilized by the hompolymer nanoparticles (Pickering emulsions) at pH < pKa and emulsions stabilized by the homopolymer unimers at pH > pKa. In both cases, gelled emulsions were obtained from medium to high internal phase volume fractions with the unimers exhibiting more effective emulsification and gelling properties. Hydrogen bonding between the boronic acid units is proposed to account for the high strength of the emulsions. The emulsions were shown to be pH- and sugar-responsive. Finally, the stable emulsions were used as templates to directly prepare PVPBA macroporous materials and to fabricate multilayered capsules. This remarkable observation that a simple homopolymer can serve as an effective emulsifier and gelator may dramatically extend the scope of potential emulsifiers and inspire further research in the design of new types of efficient emulsifying agents.

Dual template effect of supercritical CO2 in ionic liquid to fabricate a highly mesoporous cobalt metal–organic framework

Dual template effect of supercritical CO₂ in ionic liquid to fabricate a highly mesoporous cobalt metal–organic framework.

Facile and scalable synthesis of nanoporous materials based on poly(ionic liquid)s

A simple, fast, sustainable, and scalable strategy to prepare nanoporous materials based on poly(ionic liquid)s (PILs) is presented. The synthetic strategy relies on the radical polymerization of crosslinker-type ionic liquid (IL) monomers in the presence of an analogous IL, which acts as a porogenic solvent. This IL can be extracted easily after polymerization and recycled for further use. The great advantages of this synthetic approach are the atom-efficiency and lack of waste. The effects of different monomer/porogen ratios on the specific surface area, porosity, and pore size have been investigated. Finally, the potential of the materials as CO2 sorbents has been evaluated.

Preparation of catalytic materials using ionic liquids as the media and functional components

Ionic liquids (ILs) have attracted much attention due to their unique properties and wide application potential in a variety of fields. The unusual properties of ILs provide numerous opportunities to design and prepare arious advanced materials, including highly efficient catalysts. In recent years, synthesis of different kinds of catalytic materials and their applications in chemical reactions have been studied extensively and have become a very interesting area. Herein, we present a review on the synthesis of catalytic materials using ILs as the media and/or functional components; the important and widely investigated topics are discussed, including mainly metal nanocatalysts/IL, functional IL/support, metals or metal oxides/IL/support, polymeric ILs (PILs) catalysts, and the performances of catalytic systems are highlighted. An outlook for this interesting area is also given at the end of the article.

Amphiphile self-assemblies in supercritical CO2 and ionic liquids

Supercritical (sc) CO2 and ionic liquids (ILs) are very attractive green solvents with tunable properties. Using scCO2 and ILs as alternatives of conventional solvents (water and oil) for forming amphiphile self-assemblies has many advantages. For example, the properties and structures of the amphiphile self-assemblies in these solvents can be easily modulated by tuning the properties of solvents; scCO2 has excellent solvation power and mass-transfer characteristics; ILs can dissolve both organic and inorganic substances and their properties are designable to satisfy the requirements of various applications. Therefore, the amphiphile self-assemblies in scCO2 and ILs have attracted considerable attention in recent years. This review describes the advances of using scCO2 or/and ILs as amphiphile self-assembly media in the last decade. The amphiphile self-assemblies in scCO2 and ILs are first reviewed, followed by the discussion on combination of scCO2 and ILs in creating microemulsions or emulsions. Some future directions on the amphiphile self-assemblies in scCO2 and ILs are highlighted.

Porosity control in mesoporous polymers using CO2-swollen block copolymer micelles as templates and their use as catalyst supports

Mesoporous polymers with tunable large mesopores (14–22 nm) were synthesized through a CO₂-swollen micelle templating route.