Hengquan Yang

A Strategy for Separating and Recycling Solid Catalysts Based on the pH‐Triggered Pickering‐Emulsion Inversion

Turn you inside out: A novel method for performing in situ separation and recycling of submicrometer-sized solid catalysts is developed based on the pH-triggered inversion of Pickering emulsions (see scheme; o = oil, w = water). Solid catalysts can be recycled 36 times without significant loss of activity. The method differs from conventional methods in terms of speed, energy consumption, catalyst separation, and recycling effectiveness.

Compartmentalization of Incompatible Reagents within Pickering Emulsion Droplets for One-Pot Cascade Reactions

It is a dream that future synthetic chemistry can mimic living systems to process multistep cascade reactions in a one-pot fashion. One of the key challenges is the mutual destruction of incompatible or opposing reagents, for example, acid and base, oxidants and reductants. A conceptually novel strategy is developed here to address this challenge. This strategy is based on a layered Pickering emulsion system, which is obtained through lamination of Pickering emulsions. In this working Pickering emulsion, the dispersed phase can separately compartmentalize the incompatible reagents to avoid their mutual destruction, while the continuous phase allows other reagent molecules to diffuse freely to access the compartmentalized reagents for chemical reactions. The compartmentalization effects and molecular transport ability of the Pickering emulsion were investigated. The deacetalization-reduction, deacetalization-Knoevenagel, deacetalization-Henry and diazotization-iodization cascade reactions demonstrate well the versatility and flexibility of our strategy in processing the one-pot cascade reactions involving mutually destructive reagents.

The enantioselective cyanosilylation of aldehydes on a chiral VO(Salen) complex encapsulated in SBA-16

A solid catalyst for enantioselective cyanosilylation of aldehydes was prepared by encapsulating a chiral vanadyl Salen complex [VO(Salen)] in the nanocage of SBA-16. After encapsulation, the pore entrance size of SBA-16 was finely tuned through a silylation method to confine the metal complex in the nanocage and allow the free diffusion of the reactants and products during the catalytic process. For the enantioselective cyanosilylation of benzyldehyde, the enantioselectivity of the solid catalyst can achieve as high as 90%. When alkanes such as pentane, hexane and heptane were used as solvents, VO(Salen) confined in the nanocage of SBA-16 exhibits higher enantioselectivity than its homogeneous counterpart. In halogenated alkanes, the enantioselectivity of VO(Salen) confined in the nanocage of SBA-16 is lower than that of the homogeneous catalyst. The different solvent effect for the solid catalyst from the homogeneous counterpart is probably due to the altered microenvironment of VO(Salen) encapsulated in the nanocage.

Asymmetric Catalysis with Metal Complexes in Nanoreactors

Recently, the field of heterogeneous asymmetric catalysis, generally using chiral solid catalysts, has attracted much attention in the production of single enantiomers. Among versatile chiral solid catalysts, chiral metal complexes confined in nanoreactors often exhibit very unique enantioselectivity and catalytic activity compared to homogeneous catalysts. In this Focus Review, we summarize the recent advances in asymmetric reactions on chiral metal complexes confined in nanoreactors with an emphasis on the confinement effect and cooperative activation effect of the nanoreactor and new strategies for the preparation of chiral solid catalysts, such as the encapsulation of chiral metal complexes in the nanocages of mesoporous silica and incorporation of chiral ligands in the network of mesoporous organosilicas.

Magnetic core–shell-structured nanoporous organosilica microspheres for the Suzuki–Miyaura coupling of aryl chlorides: improved catalytic activity and facile catalyst recovery

New magnetic core–shell-structured nanoporous organosilica microspheres consisting of a Fe3O4 core and an organosilica shell with a built-in N-heterocyclic carbene (NHC) moiety were synthesized through surfactant-directed self-assembly on surface. The structure and composition of the material thus-designed were confirmed by N2 sorption, XRD, TEM, FT-IR, etc. Such composite microspheres possess highly open mesopores (1.6 nm) in the shell, moderate surface area, tunable shell thickness, uniform morphology, high magnetization as well as good coordination ability toward Pd. The catalytic activity test with the Suzuki–Miyaura couplings of challenging aryl chlorides reveals that this Pd-coordinated material is highly active for a wide range of substrates, suggesting highly promising application potentials of the magnetic core–shell-structured nanoporous organosilica. In particular, the catalyst derived from this material is superior to NHC-functionalized MCM-41 microspheres and a commercial Pd/C catalyst in terms of activity and recovery. The catalytic activity enhancement effects may be attributable to the purposely designed core–shell structure with significantly decreased diffusion depth, which is crucial for designing high-performance solid catalysts.

Micrometer-Scale Mixing with Pickering Emulsions: Biphasic Reactions without Stirring

A general strategy that avoids stirring for organic/aqueous reactions involving solid catalysts is reported. The strategy involves converting a conventional biphasic system into a Pickering emulsion phase with micrometer-scale droplets ensuring good mixing. In test reactions, nitrotoluene reduction and epoxidation of allylic alcohols, the reaction efficiency is comparable to conventional stirrer-driven biphasic catalysis reaction systems. Short diffusion distances, arising from the compartmentalization of densely packed droplets, play an important role in boosting the reaction efficiency.

pH-responsive gas–water–solid interface for multiphase catalysis

Despite their wide utility in laboratory synthesis and industrial fabrication, gas-water-solid multiphase catalysis reactions often suffer from low reaction efficiency because of the low solubility of gases in water. Using a surface-modification protocol, interface-active silica nanoparticles were synthesized. Such nanoparticles can assemble at the gas-water interface, stabilizing micrometer-sized gas bubbles in water, and disassemble by tuning of the aqueous phase pH. The ability to stabilize gas microbubbles can be finely tuned through variation of the surface-modification protocol. As proof of this concept, Pd and Au were deposited on these silica nanoparticles, leading to interface-active catalysts for aqueous hydrogenation and oxidation, respectively. With such catalysts, conventional gas-water-solid multiphase reactions can be transformed to H2 or O2 microbubble reaction systems. The resultant microbubble reaction systems exhibit significant catalysis efficiency enhancement effects compared with conventional multiphase reactions. The significant improvement is attributed to the pronounced increase in reaction interface area that allows for the direct contact of gas, water, and solid phases. At the end of reaction, the microbubbles can be removed from the reaction systems through changing the pH, allowing product separation and catalyst recycling. Interestingly, the alcohol oxidation activation energy for the microbubble systems is much lower than that for the conventional multiphase reaction, also indicating that the developed microbubble system may be a valuable platform to design innovative multiphase catalysis reactions.

Super-microporous organosilicas synthesized from well-defined nanobuilding units

Super-microporous organosilica with bridging ethylene and pendant vinyl groups has been synthesized by assembling predefined nanobuilding block polyhedral oligomeric silsesquioxanes (POSS) with nonionic surfactant Brij-76 as the template. The material shows wormhole-like super-micropores with uniform size of 1.9 nm, high BET surface area of 872 m2 g–1 and pore volume of 0.52 cm3 g–1. IR and NMR results show that the bridging ethylene, the pendant vinyl groups and the double-4-membered ring structure were successfully transferred from the building blocks to the super-microporous organosilica material. The material shows high hydrothermal stability and can further react with Br2. The advantage of the present approach lies in that the relative contents and proximity of the different organic functionalities in the final material can be well controlled through the starting nanobuilding blocks.

Integral and differential cross sections for the S(1D)+HD reaction employing the ground adiabatic electronic state

We present converged quantum mechanical calculations for the title reaction employing a time-dependent wavepacket method. We obtained integral and differential cross sections over an energy range from 0.23 to 0.35 eV total energy as well as product state distributions for both product channels. The excitation functions decrease with energy and point to statistical dynamics as do the cold vibrational distributions and highly inverted rotational distributions. The differential cross sections oscillate strongly with energy for both product channels. Our differential cross sections for both product channels at 2.5 kcal/mol, one of the experimental energies, compare well to the experimental results. The quantum results obtained in this study are similar to what has been found employing QCT methods, implying that the differences between the experimental and theoretical results are due to the potential energy surface or non-adiabatic effects rather than due to quantum effects or the methods employed.

Dumbbell-Shaped Bi-component Mesoporous Janus Solid Nanoparticles for Biphasic Interface Catalysis

There is a strong desire to design and synthesize catalysts that assemble at the oil-water interface to improve the efficiency of biphasic reactions. Anisotropic dumbbell-shaped bi-component mesoporous carbon-organosilica Janus particles with asymmetric wettability are synthesized through a one-step compartmentalized growth of a mesoporous organosilica sphere attached to a mesoporous resorcinol-formaldehyde (RF) sphere. A library was prepared of tunable Janus particles possessing diverse hollow structures with various functionalities. As a proof of concept, the Janus particle-derived catalyst can assemble at the oil-water interface to stabilize Pickering emulsions. Owing to the increased reaction interface area, the Janus catalyst exhibits a more than three-fold increase in catalytic efficiency compared to the Pt loaded carbon sphere catalyst in aqueous hydrogenation reactions.