Abstract
Pickering emulsion systems serve as important platforms for organic transformations in biphasic systems. A series of interfacial active silica-based catalysts were prepared and used to address mass transfer limitations in biphasic catalytic systems. The first part of this work involves the fabrication of interfacial active mesoporous silica nanoparticles with different amino groups and inert octyl groups as base catalysts for Knoevenagel condensation in an aqueous medium. The prepared materials were fully characterized, using PXRD, SEM-EDX, FTIR, TEM, Elemental Analysis, and TGA. Successful grafting of amino and octyl groups over the surface of mesoporous nanoparticles was confirmed by FTIR, TGA, and Elemental Analysis. Higher catalytic activity of Knoevenagel reaction was obtained in Pickering emulsion stabilized with amphiphilic mesoporous silica nanoparticles compared to conventional biphasic system catalyzed with monofunctionalized mesoporous silica nanoparticles. Among the various interfacial active mesoporous silica particles used for Knoevenagel reaction, Pickering emulsion stabilized with MS-OA demonstrated the highest catalytic activity compared to MS-OP and MS-OT. The enhanced catalytic activity is due to less steric hindrance offered by the amino group’s presence in the nanomaterial. In addition, MS-OA retained its catalytic activity even after six catalytic runs.
The second aspect of this work involves using facile and reliable protocols to functionalize mesoporous silica particles with suitable wettability and subsequently introducing Pd nanoparticles on the anchoring sites. A co-condensation method was utilized to tune the surface wettability of mesoporous silica particles, using a mixture of (3-aminopropyl) triethoxysilane, tetraethyl orthosilicate, and methyltrimethoxysilane. Pd nanoparticles were immobilized on interfacially active mesoporous silica via wet impregnation. Under static conditions, the as-synthesized catalysts were used as stabilizers and efficient catalysts for the selective reduction of cinnamaldehyde in the Pickering emulsion system. The catalytic activity of cinnamaldehyde was greatly influenced by surface wettability and concentration of the mesoporous structured catalysts. Moreover, the Pd/MS-MAP15 is highly recyclable after eight consecutive reaction runs.
For the next aspect of my work, we successfully synthesized an interfacially active monometallic Pd/SM-CN and bimetallic PdM/SM-CN (M = Co, Ni) catalysts [SM-CN = silica microsphere (SM) modified with methyltrimethoxysilane = C and (3-aminopropyl) triethoxysilane = N] via wet impregnation method. The synthesized catalysts were used to stabilize a water/toluene emulsion system. The resulting stable droplets formed can serve as a
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microreactor to reduce nitroaromatic compounds at room temperature using NaBH4 as the hydrogen source. Moreover, it was discovered that through tuning the volume ratio of toluene/water, the mass concentration of the stabilizer and enhanced emulsion interface area could be well controlled, thereby facilitating the mass transfer significantly and resulting in higher catalytic efficiency in Pickering emulsion systems. Also, the first efficient application of bimetallic Pd-based catalysts for the hydrogenation of aromatic compounds in the Pickering emulsion system was reported in this study. The improved activity is ascribed to the electronic metal-metal interaction between the Pd and M nanoparticles. Furthermore, the catalysts retained their activity and selectivity after eight consecutive cycles.
Finally, the research focuses on the successful application of Pickering interfacial catalysis, which is dependent on the design of catalyst particles with high catalytic activity and appropriate surface wettability. In this study, the oxidation of styrene with aqueous hydrogen peroxide over interfacial active TiO2 was used as a probe reaction to determine the effect of the catalyst surface wettability on catalytic activity. Interface-active titanium oxide nanospheres (TiO2) are prepared in this study by introducing hydrophobic methyl moiety onto the surface of TiO2 nanosphere through post grafting method. Powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), and nitrogen sorption were used to study the textural compositions of these materials. Fourier transform infrared spectra (FT-IR) revealed the successful incorporation of the methyl group onto the surface of TiO2 nanosphere. In the oxidation of styrene by hydrogen peroxide without cosolvent and under static conditions, the amphiphilic TiO2-C1.0 catalyst demonstrated better catalytic activity in a Pickering emulsion system compared to phase-boundary system and conventional reaction system. In all the studied reactions, enhanced catalytic activity was obtained in the Pickering emulsion system compared to the phase boundary system or conventional biphasic system. The enhanced catalytic activity in Pickering emulsion systems is due to the formation of numerous droplets which function as microreactors.