Preparation Of Hollow SiO₂-based Confined Catalysts And Their Selective Hydrogenation Of Nitroarenes

Functionalized anilines are important high-value-added intermediates for the production of various industrial products such as chemical,medical,dyes and pigments.For this,scientists have been trying to design various catalytic systems in order to realize the high efficiency and high selective conversion of nitroarenes into the corresponding anilines.However,the supported-metal catalysts still have some disadvantages such as long reaction time,high temperature,and high-pressure hydrogen,seriously hindering their further development.Therefore,it will be of considerable practical significance to develop a catalyst with highly efficient and selective hydrogenation of nitroarenes to corresponding aniline.In this dissertation,double-shelled hollow/porous silica(SiO₂)-based catalysts were constructed by introducing confinement effect into the controlled synthesis of the catalysts,realizing the application of highly efficient and selective hydrogenation of nitroarenes.The structure-activity relationship between the structure of the catalyst and hydrogenation performance of nitroarenes was further investigated.The structure,morphology and transformation paths of intermediates for hydrogenation of nitroarenes were further studied and analyzed.The mechanism of highly selective hydrogenation of nitroarenes was figured out through DFT calculation and molecular dynamics simulation.Specific research contents are as follows:(1)A confined space was innovatively used as the main reaction site for the hydrogenation of nitroarenes instead of the traditional surface hydrogenated reaction of nitroarenes.A new type of confined catalyst(SiO₂-metal@void/SiO₂)was prepared by using metal nanoparticles,double-layer hollow SiO₂,and polystyrene as catalytic centers,confining carrier,and confining space,respectively.The as-synthesized catalyst showed good catalytic activity and stability due to confinement effect.For example,SiO₂-Ag@void/SiO₂ catalyst can reduce 4-nitrobenzene(99.9%)and o-chloronitrobenzene(99.3%)with high selectivity,beyond the hydrogenation efficiency(54?58%)of the traditional silver catalyst loaded on the surface.Furthermore,its selectivity of nitroarenes still reach 99%after fourth runs.In particular,the morphology structure of catalyst remains unchanged obviously,indicating the good chemical stability due to the protective action resulting from outer SiO₂.(2)To further work out the issue of selective catalysis for the substrates of nitroaromatic with similar molecular size,a kind of gold or silver catalyst confined in the inorganic/organic frameworks was prepared by grafting functional poly N-isopropylacrylamide on the surface of the outer SiO₂ according to above confined catalyst.The as-synthesized catalyst can accurately screen and identify the catalytic substrate due to the existence of functional polymer,achieving a perfect substrate-selective(100%)catalysis of nitroarenes.Based on comparative experiments and theoretical calculation(DFT),the double-shelled confined structure of catalyst and the formation of hydrogen bond network between poly N-isopropylacrylamide and nitroaromatic molecules are key factors to realize excellent substrate-selectivity of nitroarenes.(3)To reduce the cost of catalyst,the catalysts were further prepared through transition metals confined in the inorganic-organic frameworks according to the preparation of precious metal catalyst confined in the inorganic-organic frameworks.Although the catalytic activity of the as-synthesized catalyst is lower than that of the precious metal confined catalyst,an excellent substrate-selectivity(100%)still was obtained for the hydrogenation of nitroarenes,again confirming that the double-shelled confined structure of catalyst and the formation of hydrogen bond network between poly N-isopropylacrylamide and nitroaromatic molecules are key factors to realize excellent substrate-selectivity of nitroarenes.(4)To achieve efficient photocatalytic hydrogenation of nitroarenes and get rid of the dependence on organic system,the confined photocatalyst was constructed by using hollow silica(SiO₂)and Ag/Ag₂O composite semiconductor as the supporter and the photocatalyst,respectively.The as-synthesized photocatalyst(SiO₂-Ag/Ag₂O/void@SiO₂)exhibits unique its unique structural characteristics:the outer silica can hinder the escape of hydrogen during the process of photocatalytic hydrogen production because Ag/Ag₂O photosensitizer was confined between two layers of SiO₂.Therefore,photogenerated hydrogen can be quickly transferred to nitroaromatic molecules,greatly promoting the hydrogenation efficiency of catalyst during the process of photocatalytic nitroarenes.Compared with surface-supported Ag/Ag₂O catalyst,the confined photocatalyst shows excellent photocatalytic reduction of nitrobenzene,in which the reduction rate was 99%in water system(yield:99.1%,selectivity 99.3%),and the reduction rate reached100%in the hybrid system(water/isopropyl alcohol),far beyond the hydrogenation efficiency(yield:7.9%,selectivity:28.5%)of surface-supported catalyst under the same conditions.(5)To further enhance the photocatalytic efficiency of nitroarenes in an aqueous system,Cu₂O with narrow band-gap and strong visible-light absorption and hierarchically hollow SiO₂ were selectively used as photocatalytic active centers and confined supporter,respectively,preparing Cu₂O confined photocatalyst(SiO₂-Cu₂O@SiO₂).Compared with Ag/Ag₂O confined photocatalyst,the as-synthesized Cu₂O confined photocatalyst not only exhibited better hydrogenation performance of nitroarenes,but also effectively inhibited the aggregation of cuprous oxide and improved its cycle stability.