Rationally Turning the Interface Activity of Mesoporous Silicas for Preparing Pickering Foam and “Dry Water”

We develop a novel protocol to prepare smart, gas/water interface-active, mesoporous silica particles. This protocol involves modification of highly mesoporous silicas with a mixture of hydrophobic octyl organosilane and hydrophilic triamine organosilane. Their structure and compositions are characterized by transmission electron microscopy (TEM), N₂ sorption, solid state NMR, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectra (FT-IR), thermogravimetric analysis (TGA), and elemental analysis. It is demonstrated that our protocol enables the interface activity of mesoporous silica particles to be facilely tuned, so that the stable gas–water interfaces ranging from air bubbles dispersed in water (Pickering foam) and water droplets dispersed in air (“dry water”) can be achieved, depending on the molar ratio of these two organosilanes. The “dry water” is not otherwise attainable for the analogous nonporous silica particles, indicting the uniqueness of the chosen mesoporous structures. Moreover, these particle-stabilized Pickering foams and “dry waters” can be disassembled in response to pH. Interestingly, it was found that aqueous potassium carbonate droplets stabilized by these interface-active mesoporous silica particles (“dry K₂CO₃-containing water”) could automatically capture CO₂ from a simulated flue gas with enhanced adsorption rate and adsorption capacity when compared to the aqueous potassium carbonate bulk solution. This study not only supplies a novel type of efficient, smart, gas/water interface-active mesoporous silica particles but also demonstrates an innovative application of mesoporous materials in gas adsorption

Hydrophobic Core/Hydrophilic Shell Structured Mesoporous Silica Nanospheres: Enhanced Adsorption of Organic Compounds from Water

Inspired by the structure features of micelle, we attempt to synthesize a novel functionalized mesoporous silica nanosphere consisting of a hydrophobic core and a hydrophilic shell. The obtained solid materials were structurally confirmed by N₂ sorption, X-ray diffraction (XRD), and transmission electron microscopy (TEM). Their compositions were characterized by Fourier transfer infrared spectroscopy (FT-IR), solid state NMR, X-ray photoelectron spectroscopy (XPS), and elemental analysis. Its fundamental properties such as dispersibility in water or organic phase, wettability, and adsorption ability toward hydrophobic organics in water were investigated. It was revealed that these important properties could be facilely adjusted through varying structure and composition. In particular, these materials showed much better adsorption ability toward hydrophobic organic molecules in water than conventional monofunctionalized mesoporous materials, owing to possessing the hydrophobic/hydrophilic domain-segregated and hierarchically functionalized mesoporous structures. The intriguing properties would make mesoporous materials more accessible to many important applications, especially in aqueous systems

Synthesis of pH-Responsive Inorganic Janus Nanoparticles and Experimental Investigation of the Stability of Their Pickering Emulsions

Pickering emulsions exhibit outstanding stability, especially those prepared with Janus particles, whose desorption energy is expected to be up to 3-fold greater than emulsions of homogeneous particles from theoretical calculations. To the best of our knowledge, however, there remains no experimental proof of this behavior in practice. In this study, inorganic Janus nanoparticles were fabricated by regioselective modification of the separate side of SiO₂ nanoparticles with a judiciously selected mixture of trimethoxysilylpropyldiethylenetriamine and <i>n</i>-octyltrimethoxysilane. Janus nanoparticles demonstrated excellent interfacial activity, forming Pickering emulsions with oil phases at oil–water interfacial tensions ranging from 6.6–52.8 mN m^–1. Furthermore, as the interface of the Janus nanoparticles was regionally functionalized with −NH₂ groups, phase inversion could be realized by tuning pH. This is the first example for the Pickering emulsions stabilized with inorganic Janus particles. Importantly, based on the results of centrifugation experiment, the desorption energy of Janus nanoparticles at the interface was 3.2 times larger than that of homogeneous nanoparticles, which is in accordance with the result from theoretical calculations. These experimental results will substantially enrich our understanding of Janus nanoparticle Pickering emulsions and their interfacial assembly behavior

Encapsulation of Hoveyda–Grubbs^2nd Catalyst within Yolk–Shell Structured Silica for Olefin Metathesis

Through postreducing the pore size of a mesoporous shell, Hoveyda–Grubbs^2nd catalyst was successfully encapsulated within yolk–shell structured silica, leading to a heterogeneous catalyst for olefin metathesis. Such a catalyst exhibits much higher activity than the reported encapsulated catalysts in olefin ring-closing metathesis and cross metathesis. This excellent activity can be attributed to the combination of a hollow structure in the interior and permeable mesopores in the shells. This catalyst shows good recyclability, highlighted by eight cycles of reaction. This work not only supplies an excellent heterogeneous olefin metathesis catalyst but also demonstrates that yolk–shell structured silica materials can be used as an innovative scaffold to encapsulate homogeneous catalysts

Controlled Synthesis of Au Nanoparticles in the Nanocages of SBA-16: Improved Activity and Enhanced Recyclability for the Oxidative Esterification of Alcohols

Au nanoparticles with different sizes were introduced into the nanocages of a mesoporous material SBA-16 with the aid of chemical modification, leading to new Au-supported catalysts Au/SBA-16. These catalysts were characterized with Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microspectroscopy (TEM), N₂ sorption, and X-ray photoelectron microspectroscopy (XPS). These results revealed that uniform Au nanoparticles with sizes of a few nanometers were successfully positioned inside the nanocages of SBA-16. Such catalysts were catalytically active in the oxidative esterification of various alcohols even including less reactive straight-chain alcohols. It was found that the activity of this catalyst strongly depended on the Au loading, and the Au loading of 5 wt % (corresponding to Au particles of 2–3 nm in sizes) led to the highest activity. Its activity was much higher than those of the analogous catalysts prepared from commercial silica gel as well as SBA-15. Furthermore, Au/SBA-16 could be reused at least eight reaction cycles without significant decrease in activity and selectivity. Its recyclability was much superior to that of the catalyst derived from commercial silica gel. The underlying reason may be that the unique nanostructure of SBA-16 can effectively prevent the growth of Au nanoparticles into less active, larger particles, as evidenced from TEM investigations. This study not only supplies a new, active, recoverable catalyst for the green transformations of alcohols to esters but also demonstrates that the three-dimensional mesoporous cage-like material SBA-16 has a superior ability in reducing the diffusion resistance and stabilizing metal nanoparticles against growth

Pickering Emulsion as an Efficient Platform for Enzymatic Reactions without Stirring

To address the current limitations of enzymatic reactions, we develop a novel strategy to conduct stirring-free biphasic enzymatic reactions. This strategy involves translation of a conventional biphasic enzymatic reaction to a water-in-oil (W/O) Pickering emulsion system by adding a small amount of solid particle emulsifier. In such a system, enzymes, for example, a <i>Candida Antarctica</i> lipase B (CALB), are compartmentalized within millions of micron-sized water droplets, while organic substrates are dissolved in the oil phase (outside the droplets). It was demonstrated that CALB-catalyzed hydrolysis kinetic resolution of racemic esters in the stirring-free Pickering emulsion system gave favorable reaction efficiency and enantioselectivity as compared to those for the conventional biphasic system under stirring conditions, which was due to the large reaction interfacial area and the short molecule distances created by the Pickering emulsion droplets. The specific activity was found to depend on the water droplet size, highlighting the importance of the presence of droplets in the reaction system. Moreover, the convenient and effective recycling of CALB could be achieved through simple demulsification by centrifugation. After 27 reaction cycles, the ee values of ester and alcohol were still as high as 87.5% and 99%, respectively, which significantly exceed those of the conventional biphasic reaction. The high recyclability may be attributed to avoiding stirring that often causes damage to the three-dimensional structure of enzymes. This study compellingly demonstrates that a Pickering emulsion is an innovative platform to efficiently process enzymatic reactions without need for stirring and immobilization

Facile Preparation of Ag-Coated Superhydrophobic/Superoleophilic Mesh for Efficient Oil/Water Separation with Excellent Corrosion Resistance

We present the facile preparation of a superhydrophobic–oleophilic stainless steel mesh with excellent oil/water separation efficiency and resistance to corrosion through hydrofluoric (HF) acid etching, Ag nanoparticle coating, and stearic acid modification, to construct a superhydrophobic micro/nanohierarchical structure. The surface of the treated mesh exhibits superhydrophobicity, with a water contact angle of 152°, and superoleophilicity, with an oil contact angle of 0°. The effects of variation in the HF etching time and Ag nanoparticle coating on surface wettability were explored. The treated mesh demonstrated a very high separation efficiency, as high as 98% for the optimal preparation, on a series of oil/water mixtures. The durability of the treated mesh was tested by repeated separation of kerosene/water mixtures, with the separation efficiency remaining higher than 97% after 40 cycles. In addition, the mesh exhibited an excellent chemical resistance to both acidic and alkaline conditions, with good wearing in hot water. The improved superhydrophobic–oleophilic mesh represents a feasible and realistic oil/water separation methodology even under harsh conditions, and it could have wide application in industrial processes

Tuning the Interfacial Activity of Mesoporous Silicas for Biphasic Interface Catalysis Reactions

Interface-active particle materials that are able to assemble at the oil/water interface so as to stabilize droplets, are gaining unprecedented interest due to the intriguing applications in catalysis and materials synthesis, etc. In contrast to these potential applications, this kind of materials are still limited and cannot meet some particular demands of practical utilizations such as rationally designed interfacial activity and high stability against concentrated salts. In this contribution, interface-active mesoporous silica nanospheres (MSS@C_<i>x</i>Z_<i>y</i>) are synthesized through simultaneous incorporation of extremely hydrophilic zwitterionic moiety and hydrophobic octyl moiety in the shell. The textural properties of these materials are characterized by transmission electron microscopy (TEM), powder X-ray diffraction (XRD), and nitrogen sorption. The successful decoration of these functionalities in the shell is confirmed by Fourier transform infrared spectra (FT–IR), ¹³C nuclear cross-polar magnetic resonance (¹³C CP/MAS NMR), and ²⁹Si nuclear cross-polar magnetic resonance (²⁹Si CP/MAS NMR). The prepared mesoporous silicas exhibit tunable interfacial activity, so that oil-in-water (O/W) and water-in-oil (W/O) Pickering emulsions can be easily obtained by varying the molar fraction of these two functionalities. The MSS@C_<i>x</i>Z_<i>y</i>-stabilized Pickering emulsions exhibit high stability to coalescence even at 6.0 M NaCl and have relatively low surface coverage of droplets due to electrostatic repulsion, which is normally difficult to obtain for conventional particles. Interestingly, such interface-active mesoporous silicas can also carry polyoxometalate that is hosted in the nanopore to assemble at the oil/water interface and thus efficiently promotes biphasic epoxidation reactions without any external stirring, exemplifying an innovative application of theses developed mesoporous silicas

In Situ Surface Engineering of Mesoporous Silica Generates Interfacial Activity and Catalytic Acceleration Effect

Mesoporous structured catalysts featuring interfacial activity are the most promising candidates for biphasic interface catalysis because their nanopores can concurrently accommodate catalytic active components and provide countless permeable channels for mass transfer between the interior and the exterior of Pickering droplets. However, to date, a convenient and effective strategy for the preparation of an anchor site-containing interfacial active mesoporous catalyst is still lacking. In the present work, we report a novel and efficient interfacial active mesoporous silica (MS) catalyst, which is prepared by a facile cocondensation of two types of organosilanes and subsequent anchoring of Pd NPs onto its surface through the confinement and coordination interactions. The as-prepared catalyst is then applied as emulsifier to stabilize the water-in-oil (W/O) Pickering emulsion and investigated as an interfacial catalyst for the hydrogenation of nitroarenes. An obviously enhanced rate toward the nitrobenzene hydrogenation is observed for the 0.8 mol% Pd/PAP-functionalized mesoporous silica-20 catalyst in the emulsion system (both conversion and selectivity are >99% within 30 min) in comparison to a single aqueous solution. Moreover, the emulsion catalytic system can be easily recycled six times without the separation of the catalyst from the water phase during the recycling process. This finding demonstrates that the incorporation of phenylaminopropyl trimethoxysilane amphiphilic groups during the hydrolysis of tetramethyl orthosilicate not only endows MS with interfacial activity but also improves the catalytic activity and stability

Facile Preparation of Ag-Coated Superhydrophobic/Superoleophilic Mesh for Efficient Oil/Water Separation with Excellent Corrosion Resistance

We present the facile preparation of a superhydrophobic–oleophilic stainless steel mesh with excellent oil/water separation efficiency and resistance to corrosion through hydrofluoric (HF) acid etching, Ag nanoparticle coating, and stearic acid modification, to construct a superhydrophobic micro/nanohierarchical structure. The surface of the treated mesh exhibits superhydrophobicity, with a water contact angle of 152°, and superoleophilicity, with an oil contact angle of 0°. The effects of variation in the HF etching time and Ag nanoparticle coating on surface wettability were explored. The treated mesh demonstrated a very high separation efficiency, as high as 98% for the optimal preparation, on a series of oil/water mixtures. The durability of the treated mesh was tested by repeated separation of kerosene/water mixtures, with the separation efficiency remaining higher than 97% after 40 cycles. In addition, the mesh exhibited an excellent chemical resistance to both acidic and alkaline conditions, with good wearing in hot water. The improved superhydrophobic–oleophilic mesh represents a feasible and realistic oil/water separation methodology even under harsh conditions, and it could have wide application in industrial processes