Enhanced Photobiocatalytic Cascades at Pickering Droplet Interfaces.

Developing new methods to engineer photobiocatalytic reactions is of utmost significance for artificial photosynthesis, but it remains a grand challenge due to the intrinsic incompatibility of biocatalysts with photocatalysts. In this work, photocatalysts and enzymes were spatially colocalized at Pickering droplet interfaces, where the reaction microenvironment and the spatial distance between two distinct catalysts were exquisitely regulated to achieve unprecedented photobiocatalytic cascade reactions. As proof of the concept, ultrathin graphitic carbon nitride nanosheets loaded with Au nanoparticles were precisely positioned in the outer interfacial layer of Pickering oil droplets to produce H2O2 under light irradiation, while enzymes were exactly placed in the inner interfacial layer to catalyze the subsequent biocatalytic oxidation reactions using in situ formed H2O2 as an oxidant. In the alkene epoxidation and thioether oxidation, our interfacial photobiocatalytic cascades showed a 2.0-5.8-fold higher overall reaction efficiency than the photobiocatalytic cascades in the bulk water phase. It was demonstrated that spatial localization of the photocatalyst and the enzyme at Pickering oil droplet interfaces not only provided their respective preferable reaction environments and intimate proximity for rapid H2O2 transport but also protected the enzyme from oxidative inactivation caused by the photogenerated species. These remarkable interfacial effects contributed to the significantly enhanced photobiocatalytic cascading efficiency. Our work presents an innovative photobiocatalytic reaction system with manifold benefits, providing a cutting-edge platform for solar-driven chemical transformations via photobiocatalysis.

Amphiphilic Covalent Organic Framework Nanoparticles for Pickering Emulsion Catalysis with Size Selectivity.

Exploiting advanced amphiphilic solid catalysts is crucial to the development of Pickering emulsion catalysis. Herein, covalent organic framework (COF) nanoparticles constructed with highly hydrophobic monomers as linkers were found to show superior amphiphilicity and they were then developed as a new class of solid emulsifiers for Pickering emulsion catalysis. Employing amphiphilic COFs as solid emulsifiers, Pickering emulsions with controllable emulsion type and droplet sizes were obtained. COF materials have also been demonstrated to serve as porous surface coatings to replace traditional surface modifications for stabilizing Pickering emulsions. After implanting Pd nanoparticles into amphiphilic COFs, the obtained catalyst displayed a 3.9 times higher catalytic efficiency than traditional amphiphilic solid catalysts with surface modifications in the biphasic oxidation reaction of alcohols. Such an enhanced activity was resulted from the high surface area and regular porous structure of COFs. More importantly, because of their tunable pore diameters, Pickering emulsion catalysis with remarkable size selectivity was achieved. This work is the first example that COFs were applied in Pickering emulsion catalysis, providing a platform for exploring new frontiers of Pickering emulsion catalysis.

Spatial Localization of Two Enzymes at Pickering Emulsion Droplet Interfaces for Cascade Reactions.

Developing biocatalytic cascades in abiological conditions is of utmost significance, but such processes often suffer from low reaction efficiency because of incompatible reaction environments and suppressed intermediate transportation. Herein we report a new type of biocatalytic cascade by localizing two different enzymes separately in the outer and inner interfacial layers of Pickering emulsion droplets. This versatile approach enables the localization of two enzymes in their preferred reaction microenvironments and simultaneously in nanoscale proximity of each other. The thus-designed interfacial biocatalytic cascades show outstanding catalytic efficiency in alkene epoxidation and thioether oxidation with in situ generation of hydrogen peroxide under mild conditions, 6.9-13.6 times higher than the catalytic efficiency of the free enzymes in solution and their multi-enzymatic counterparts. The remarkable interfacial effect of Pickering droplets was found to be responsible for the significantly enhanced cascading efficiency.

Boosting Catalytic Selectivity through a Precise Spatial Control of Catalysts at Pickering Droplet Interfaces.

Exploration of new methodologies to tune catalytic selectivity is a long-sought goal in catalytic community. In this work, oil-water interfaces of Pickering emulsions are developed to effectively regulate catalytic selectivity of hydrogenation reactions, which was achieved via a precise control of the spatial distribution of metal nanoparticles at the droplet interfaces. It was found that Pd nanoparticles located in the inner interfacial layer of Pickering droplets exhibited a significantly enhanced selectivity for p-chloroaniline (up to 99.6%) in the hydrogenation of p-chloronitrobenzene in comparison to those in the outer interfacial layer (63.6%) in pure water (68.5%) or in pure organic solvents (46.8%). Experimental and theoretical investigations indicated that such a remarkable interfacial microregion-dependent catalytic selectivity was attributed to the microenvironments of the coexistence of water and organic solvent at the droplet interfaces, which could provide unique interfacial hydrogen-bonding interactions and solvation effects so as to alter the adsorption patterns of p-chloronitrobenzene and p-chloroaniline on the Pd nanoparticles, thereby avoiding the unwanted contact of C-Cl bonds with the metal surfaces. Our strategy of precise spatial control of catalysts at liquid-liquid interfaces and the unprecedented interfacial effect reported here not only provide new insights into the liquid-liquid interfacial reactions but also open an avenue to boost catalytic selectivity.

Amorphous-to-Crystalline Transformation: General Synthesis of Hollow Structured Covalent Organic Frameworks with High Crystallinity.

Morphological control of covalent organic frameworks (COFs) is particularly interesting to boost their applications; however, it remains a grand challenge to prepare hollow structured COFs (HCOFs) with high crystallinity and uniform morphology. Herein, we report a versatile and efficient strategy of amorphous-to-crystalline transformation for the general and controllable fabrication of highly crystalline HCOFs. These HCOFs exhibited ultrahigh surface areas, radially oriented nanopore channels, quite uniform morphologies, and tunable particle sizes. Mechanistic studies revealed that H2O, acetic acid, and solvent played a crucial role in manipulating the hollowing process and crystallization process by regulating the dynamic imine exchange reaction. Our approach was demonstrated to be applicable to various amines and aldehydes, producing up to 10 kinds of HCOFs. Importantly, based on this methodology, we even constructed a library of unprecedented HCOFs including HCOFs with different pore structures, bowl-like HCOFs, cross-wrinkled COF nanocapsules, grain-assembled HCOFs, and hydrangea-like HCOFs. This strategy was also successfully applied to the fabrication of COF-based yolk-shell nanostructures with various functional interior cores. Furthermore, catalytically active metal nanoparticles were implanted into the hollow cavities of HCOFs with tunable pore diameters, forming attractive size-selective nanoreactors. The obtained metal@HCOFs catalysts showed enhanced catalytic activity and outstanding size-selectivity in hydrogenation of nitroarenes. This work highlights the significance of nucleation-growth kinetics of COFs in tuning their morphologies, structures, and applications.

Metal-Nanoparticles-Loaded Ultrathin g-C3N4 Nanosheets at Liquid-Liquid Interfaces for Enhanced Biphasic Catalysis.

Exploiting new interface-active solid catalysts is crucial to construct efficient Pickering emulsion systems for biphasic catalysis. In this work, ultrathin g-C3N4 nanosheets (g-C3N4-NSs) were developed as a new solid emulsifier to directly position catalytic sites at oil-water interfaces for improving the reaction efficiency of a biphasic reaction. Exemplified by a metal-involved biphasic reaction of nitroarenes reduction, the developed Pd/g-C3N4-NSs catalyst with Pd nanoparticles loaded on the surface of g-C3N4-NSs exhibited excellent activity with a catalytic efficiency of 1220 h-1. Such activity was 4.2 and 17.9 times higher than those of Pd/g-C3N4-bulk and the ordinary Pd/C8-SiO2 catalyst, respectively. Also, in the biphasic oxidation reaction of alcohols, Pd/g-C3N4-NSs achieved a 2.3-fold activity enhancement. It was found by analyzing the solidified emulsion droplets that the Pd/g-C3N4-NSs catalyst was parallelly assembled at the oil-water interfaces. Because of the ultrathin thickness of g-C3N4-NSs, such a unique interfacial assembly behavior allowed precise positioning of Pd nanoparticles at the oil-water interfaces. As a result, the oil-soluble reactant could directly react with the water-soluble reactant at the oil-water interface hosting the Pd nanoparticles. Our elaborately designed reaction interface was believed to substantially avoid the diffusion barrier between oil-soluble and water-soluble reactants and then to significantly enhance the reactivity of biphasic reactions. This work highlights the importance of the interfacial location of catalytic sites in biphasic catalysis.

Dual metal nanoparticles within multicompartmentalized mesoporous organosilicas for efficient sequential hydrogenation.

Controlling localization of multiple metal nanoparticles on a single support is at the cutting edge of designing cascade catalysts, but is still a scientific and technological challenge because of the lack of nanostructured materials that can not only host metal nanoparticles in different sub-compartments but also enable efficient molecular transport between different metals. Herein we report a multicompartmentalized mesoporous organosilica with spatially separated sub-compartments that are connected by short nanochannels. Such a unique structure allows co-localization of Ru and Pd nanoparticles in a nanoscale proximal fashion. The so designed cascade catalyst exhibits an order of magnitude activity enhancement in the sequential hydrogenation of nitroarenes to cyclohexylamines compared with its mono/bi-metallic counterparts. Crucially, an interesting phenomenon of neighboring metal-assisted hydrogenation via hydrogen spillover is observed, contributing to the significant enhancement in catalytic efficiency. The multicompartmentalized architectures along with the revealed mechanism of accelerated hydrogenation provide vast opportunity for designing efficient cascade catalysts.

Salt of Organosilicon Framework as a Novel Emulsifier for Various Water-Oil Biphasic Systems and a Catalyst for Dibromination of Olefins in an Aqueous Medium.

Pickering emulsifiers are significant for organic reactions in an aqueous medium because they have the ability of emulsifying water-oil biphasic systems. For this reason, 2,5-bis[(E)-2-(triethoxysilyl)vinyl]pyridine [BTOSVP] containing a pyridine bridging group was selected as a precursor to prepare a novel salt of organosilicon framework (SOF), an amphiphilic mesoporous pyridine hydrobromide nanosphere. We first synthesized a mesoporous organosilicon framework made up of organic groups containing vinyl groups, pyridine groups, and so forth. Then, hydrobromic acid was added to protonate the pyridine groups in the mesoporous organosilicon framework. Eventually, pyridine hydrobromide salt was formed on the surfaces of channels, and the SOF was successfully prepared for the first time. Pyridine hydrobromide salt can be ionized in water into protonated pyridine cations located on the SOF surfaces and free Br-anions swimming around the protonated pyridine cations because of the electrostatic interaction. In the water-oil biphasic systems, hydrophilicity of SOF originates from the protonated pyridine cations and the lipophilicity of SOF comes from organic groups in the framework; thus, this new kind of SOF can be used as a new generation of solid Pickering emulsifiers. Most importantly, the mesoporous SOF nanosphere can also be used as a catalyst for significantly improved dibromination of olefins in an aqueous medium.

Mesoporous RhRu Nanosponges with Enhanced Water Dissociation toward Efficient Alkaline Hydrogen Evolution.

Lowering the energy barrier of water dissociation is critical to achieving highly efficient hydrogen evolution in alkaline conditions. Herein, we reported mesoporous RhRu nanosponges with enhanced water dissociation behavior as a new class of high-performance electrocatalysts for alkaline hydrogen evolution reaction (HER). The obtained nanosponges have a binary alloy structure (fcc) and a highly porous structure with high surface area. Our RhRu catalyst displayed an outstanding HER activity with an overpotential of 25 mV at 10 mA cm-2 and a Tafel slope of 47.5 mV dec-1 in 1.0 M KOH, which significantly outperformed that of commercial Pt/C catalyst and was even comparable to the classic Pt/metal (hydro)oxide catalysts. Density functional theory (DFT) calculations disclosed that charge redistribution on the RhRu alloy surface enabled tuning of the Ru d-band center and then promoted the adsorption and dissociation of water molecules. Based on the experimental results and theoretical modeling, a bifunctional mechanism contributed to the remarkable alkaline HER activity on the RhRu catalyst surface.

Design of 3D Hollow Porous Heterogeneous Nickel-Cobalt Phosphides for Synergistically Enhancing Catalytic Performance for Electrooxidation of Methanol.

The synergistic effect among different components and the structural and morphological control of catalytic nanomaterials have attracted considerable research interest in the field of electrocatalysis, as using a rational design of the catalytic nanomaterials with the desired structure, morphology, and chemical compositions is an effective strategy for enhancing catalytic performance. Here, by changing the Ni/Co atomic ratio of raw materials, a series of samples with a three-dimensional (3D) hollow porous ternary multicomponent heterostructure has been successfully synthesized via a facile template-free solvothermal approach and subsequently annealing and phosphating treatments, and its formation mechanism is also investigated. By virtue of compositional and structural advantages, the optimized Ni1Co2Px (NiCoP/CoP/CoP2) nanoparticles show very high mass activity (436.9 mA mg-1) and area-specific activity (155 mA cm-2), as well as remarkable durability toward the methanol electrooxidation reaction (MOR) in alkaline solution. This catalytic activity is better than those of most of reported Ni/Co-based nonprecious metal catalysts. Particularly, a multicomponent synergistic effect on the MOR was observed. The present study not only provides a simple method for the fabrication of 3D hollow porous multicomponent composite nanomaterials, but also gives insights into the synergistic effect among the porous structure, chemical compositions, and catalytic activity of nanomaterials in the electrocatalytic oxidation of methanol.

Pore structure dependent activity and durability of mesoporous rhodium nanoparticles towards the methanol oxidation reaction.

This work reports a significant effect of porous structures on the electrocatalytic methanol oxidation performances of Rh-based catalysts. It is demonstrated that a three-dimensional net-like mesoporous structure with ultrathin subunits is beneficial to exposing more active sites and boosting electron transfer inside particles, thus presenting the highest activity.

Encapsulating mesoporous metal nanoparticles: towards a highly active and stable nanoreactor for oxidative coupling reactions in water.

We design and prepare a highly active and stable nanoreactor via encapsulating various mesoporous metal nanoparticles with an amphiphilic hollow shell, which presents excellent performance in oxidative coupling reactions in water for efficient production of α,ß-unsaturated ketones.

Janus N-Doped Carbon@Silica Hollow Spheres as Multifunctional Amphiphilic Nanoreactors for Base-Free Aerobic Oxidation of Alcohols in Water.

The hydrophobicity/hydrophilicity of nanocatalysts has a significant impact on their performances via modulating the adsorption, transfer, and desorption of reactants/products. In this work, we reported a novel multifunctional amphiphilic nanoreactor composed of Janus nitrogen-doped carbon@silica hollow nanostructure and ultrasmall Pt nanoparticles. The core/shell polybenzoxazine@mesosilica spheres were used as the precursor for pyrolysis. It was found that the internal polybenzoxazine was decomposed from interior to exterior and transformed into a nitrogen-doped carbon hollow shell that partly embedded into the mesosilica layer, forming the Janus hollow spheres. The obtained nanoreactor showed remarkable activity and selectivity for base-free aerobic oxidation of alcohols in water using air as the oxidant. A one-pot oxidation-condensation cascade reaction was also successfully demonstrated to synthesize imines from alcohols and amines with good yields. The sorption analyses revealed that the superior hydrophilicity/hydrophobicity strengthened both adsorption of hydrophobic alcohols from water and desorption of byproduct water molecules from the active sites. The doped nitrogen atoms in the carbon matrix were used not only as anchoring sites for stabilizing ultrasmall Pt nanoparticles but also as basic active sites for accelerating the deprotonation process. Moreover, due to the anchoring effect of nitrogen and the extremely stable amphiphilicity, this nanoreactor exhibited excellent catalytic stability.

Janus mesoporous silica nanosheets with perpendicular mesochannels: affording highly accessible reaction interfaces for enhanced biphasic catalysis.

A Janus mesosilica nanosheet with perpendicular mesochannels has been explored to use as a highly efficient interfacial catalyst for biphasic reactions. Owing to the excellent interfacial activity and the highly accessible reaction interface, a significantly enhanced catalytic activity is presented in aqueous hydrogenation reactions.

Flow Pickering Emulsion Interfaces Enhance Catalysis Efficiency and Selectivity for Cyclization of Citronellal.

Cyclization of citronellal is a necessary intermediate step to produce the important flavor chemical (-)-menthol. Here, a continuous-flow Pickering emulsion (FPE) strategy for selective cyclization of citronellal to (-)-isopulegol by using water droplets hosting a heteropolyacid (HPA) catalyst to fill a column reactor is demonstrated. Owing to the large liquid-liquid interface and the excellent confinement ability of droplets toward HPA, the FPE system exhibited a much higher catalysis efficiency than its batch counterpart (2-5-fold) and an excellent durability (two months). Moreover, a remarkably enhanced selectivity was observed from 34.8 % for batch reactions to 64 % for the FPE reactions. It was found that the water droplet size and the flow rate significantly impact the catalysis selectivity and efficiency. This study not only represents an unprecedented and sustainable process for the selective cyclization of citronellal but also demonstrates a new flow-interface catalysis effect that can be useful for designing innovative catalysis systems in the future.

Guidance from an in situ hot stage in TEM to synthesize magnetic metal nanoparticles from a MOF.

A series of in situ hot stage experiments using transmission electron microscopy (TEM) were studied to directly observe the transition of a Ni-MOF to Ni nanoparticles wrapped in carbon (Ni-NPC) over temperatures ranging from ambient temperature to 700 °C. Ni-NPC-600 displays high catalytic activity in 4-nitrophenol reduction and high conversion, even after 10 cycles.

Amphiphilic hollow porous shell encapsulated Au@Pd bimetal nanoparticles for aerobic oxidation of alcohols in water.

This work describes the design, synthesis and analysis of an amphiphilic hollow mesoporous shell encapsulating catalytically active Au@Pd bimetal nanoparticles. The particles exhibited excellent catalytic activity and stability in the aerobic oxidation of primary and secondary alcohols to their corresponding aldehydes or ketones in water when using air as an oxidizing agent under atmospheric pressure.

A one-step carbonization route towards nitrogen-doped porous carbon hollow spheres with ultrahigh nitrogen content for CO2 adsorption.

Nitrogen doped porous carbon hollow spheres (N-PCHSs) with an ultrahigh nitrogen content of 15.9 wt% and a high surface area of 775 m(2) g(-1) were prepared using Melamine-formaldehyde nanospheres as hard templates and nitrogen sources. The N-PCHSs were completely characterized and were found to exhibit considerable CO2 adsorption performance (4.42 mmol g(-1)).