Designing Two-Dimensional Materials For Pickering Emulsion Interfacial Catalysis

Oil-water biphasic reactions are widely used in the production of chemicals.However,due to the mass transfer resistance between oil-soluble and water-soluble reactants,their reaction efficiencies are very low.Recently,the Pickering emulsion interface catalysis system has received great attention.The system can not only provide a large oil-water interface area,to improve the reaction rate,but also solve the problems of catalyst separation and recycling,This makes it to become an efficient catalytic system in the scientific community.The controllable interfacial activity of two-dimensional nanosheets is conducive to the construction of a stable Pickering emulsion catalytic system,and their unique anisotropy is conducive to controlling the reaction microenvironment of the catalytic center at the oil-water interface.Therefore,Pickering emulsion catalysis system based on two-dimensional materials has important research significance in regulating the reactivity and selectivity.In this paper,a series of Pickering emulsion interfacial catalysis systems with high-efficiencies were successfully constructed by using Janus m Si O₂nanosheets and ultrathin graphitic carbon nitride nanosheets,which greatly improved the reaction efficiency of the biphasic reactions.A new method to control the reaction selectivity was put forward,and the interfacial catalytic effect in Pickering emulsion catalysis was deeply studied.Specific research contents are as follows:In the second chapter of this paper,the metal nanoparticles were accurately assembled on the oil-water interface using ultrathin nanosheets,which fundamentally solved the problem of mass transfer at the oil-water interface,thus greatly improving the reaction efficiency of the biphasic reactions.Ultrathin graphitic carbon nitide nanosheets(g-C₃N₄-NSs)with a thickness of 4-5 nm were prepared by ultrasonic-assisted liquid exfoliation of bulk g-C₃N₄.The built unit of C₃N₄ material has an appropriate hydrophilic/hydrophobic balance,which endows it good interfacial activity,This makes the g-C₃N₄-NSs materials become a kind of superior solid emulsifier and can obtain stable oil-in-water Pickering emulsion could be obtained.The Pd nanoparticles were deposited on the surface of g-C₃N₄-NSs by a simple impregnation-reduction method.Owing to the ultrathin structure of g-C₃N₄-NSs,the metal nanoparticles loaded on were accurately positioned on the oil-water interface after the formation of the Pickering emulsion,leading to a highly efficient Pickering emulsion interfacial catalysis system.In the biphasic reduction reactions of nitrobenzene,the obtained Pd/g-C₃N₄-NSs catalyst showed an excellent catalytic activity,which was much higher than that of Pd/g-C₃N₄-bulk and other interfacial active catalysts,Its activity was aslo17.5 times higher than that of traditional interfacial catalyst Pd/C₈-Si O₂.Moreover,we found that Pd/g-C₃N₄-NSs possessed a good catalytic stability.after being reused for 6 times,the conversion of nitrobenzene was still as high as 90%.In the third chapter of this paper,Janus nanosheets were used to control the spatial location of metal nanoparticles at the oil-water interface,thus achieving the efficient regulation of reaction selectivity.The influencing factors for regulating the reaction selectivity were aslo studied in detail.Janus mesoporous silica nanosheets(JMSN)were firstly prepared by a coated-selective modification-crushing methods,and then the metal Pd nanoparticles were selectively loaded on the hydrophilic side of JMSN.After the formation of Pickering emulsion,the catalytic active Pd nanoparticles could be accurately assembled at the interfacial water layer of Pickering emulsions.At the same time,we can also load the metal nanoparticles at the hydrophobic side of JMSN to control the same catalytic center at the interfacial oil layer.In the selective hydrogenation of 4-chloronitrobenzene,Pd nanoparticles at the interfacial water layer showed a high catalytic selectivity of 89.3%for 4-chloroaniline,which was much higher than that of the Pd nanoparticles at the interfacial oil layer and the traditional interfacial active catalyst.In different hydrogenation reactions of halogenated nitrobenzene,Pd nanoparticles at the interfacial water layer also showed an excellent catalytic selectivity.Meanwhile,it was found that Pd nanoparticles at the interfacial water layer showed good p-chloraniline selectivity in different oil-water interfacial catalytic systems.In order to further understand the intrinsic factors of high selectivity of Pd nanoparticles at interfacial water layer,we explored the effects of JMSN thickness,reaction temperature,and different additives on the reaction selectivity.The results proved that the reaction microenvironment of interfacial water layer(hydrogen bonding interaction with reactant and product molecules)was the intrinsic factor for obtaining the high selectivity.