Construction Of Nickel-Cobalt Oxide Catalysts And Study On Their Catalytic Performance Of Selective Oxidation Of Styrene

Catalytic oxidation of hydrocarbons plays an important role in the chemical industry.Among the various oxidation reactions of organic synthesis,the catalytic epoxidation of alkenes to epoxides,which is of great significance in the development of national economy because of its wide application,high added value and large demand.In recent years,the catalytic epoxidation of olefins has aroused much more attention.As an important epoxide,styrene oxide(SO)is also an versatile imtermediate in fine chemistry and pharmaceutical industry.It is widely utilized in the manufacture of surface coatings,fine chemicals,epoxy resins and the like.The traditional process of SO is halogen method which usually uses organic peracids as the oxidant.Nevertheless,this method suffers from several drawbacks such as producing a large a(?)unt of harmful wastes and exhibiting poor selectivity to styrene oxide.Therefore,from the view of green,economy and sustainable chemistry,these traditional methods have been replaced by the means of utilizing O2,H2O2 or organic peroxides as the oxidant.In this work,several cobalt/nickel-based materials were developed and employed as the efficient and robust heterogeneous catalysts in the selective epoxidation of styrene.The structure-activity relationship between the morphology and structure of the catalyst and the catalytic performance were investigated.1.Preparation,characterization and catalytic performance of the hierarchical NiCo2O4NiCo2O4 microflowers assembled from nanoneedles have been synthesized via a facile hydrothermal method.Various characterization results,such as XRD,SEM,TEM,HRTEM,EDS and N2 adsorption-desorption,show that the NiCo2O4 microflowers assembled from nanoneedles have a well-defined three-dimensional(3D)hierarchical mesoporous structure.When used as catalysts for the selective oxidation of styrene,the 3D NiCo2O4 microflowers show a markedly high styrene conversion with TBHP as oxidant and acetonitrile as solvent.The influence of various reaction conditions including reaction temperature,reaction time and the molar ratio of styrene to TBHP on the catalytic performance were also studided.The optimized reaction conditions are as follows:reaction time of 6 h,reaction temperature of 80?,and ratio of styrene to TBHP is 1:5.Under the optimal reaction conditions,the yield of SO is 79.3%with a total styrene conversion of 99.2%and a selectivity of 79.9%to SO can be achieved.Additionally,both styrene conversion and SO selectivity show very small decrease after recycling the catalyst 5 times under similar reaction conditions,which also demonstrates that the NiCo2O4 catalyst shows excellent catalyst stability.2.Preparation,characterization and catalytic performance of the NiO/NiCO2O4Three-dimensional(3D)hierarchical hollow urchin-like NiO/NiCO2O4 heterostructures have been designed and prepared via a facile one-pot hydrothermal method.Various characterization results,such as XRD,SEM,TEM,HRTEM,EDS,XRF,XPS and N2 adsorption-desorption,show that the NiO/NiCO2O4 displays novel 3D hierarchical hollow urchin-like morphology,which are composed of nanocrystals.Mesoporous structure of the catalyst is proved and the specific surface area of the NiO/NiCO2O4 is 40.2 m2g-1 according to Brunauer-Emmett-Teller method.Moreover,the NiO/NiCo2O4 heterostructures consist of 59 wt%of NiO and 41 wt%of NiCo2O4 according to XRF results.Compared with Co3O4 and NiO catalysts,the high catalytic yerformance of the NiO/N(?)Co2O4 heterostructures can be attributed to the novel 3D hierarchical hollow urchin-like morphology,high specific surface area and the abundant NiO/NiCO2O4 phase boundaries,all of which contribute to the numerous active sites.When used as catalysts for the selective oxidation of styrene,the hierarchical NiO/NiCO2O4 heterostructures show a markedly high styrene conversion with TBHP as oxidant and acetonitrile as solvent.The influence of various reaction conditions including reaction temperature,reaction time and the molar ratio of styrene to TBHP on the catalytic performance were also studided.The optimized reaction conditions are as follows:reaction time of 5 h,reaction temperature of 80? and the ratio of styrene to TBHP is 1:5.The yield of SO is 81.4%with a total styrene conversion of 89.6%and a selectivity of 90.8%to SO can be achieved under the optimized reaction conditions.Moreover,the styrene conversion and SO selectivity show very small decrease after recycling the catalyst 5 times under similar reaction conditions,which demonstrates that the catalyst has good catalyst stability.3.Preparation,characterization and catalytic performance of the hierarchical 0D/2D Co3O4 hybrids prepared by a simple liquid phase reduction strategyThe hierarchical 0D/2D Co3O4 catalyst rich in oxygen vacancies was prepared via a simple liquid-phase NaBH4 reduction strategy.The physicochemical properties of the catalyst were systematically investigated by methods of XRD,Raman,SEM,EDS,HRTEM,XPS and N2 adsorption-desorption techniques and so on.The results show that the initial two-dimensional porous Co3O4 sheets evolved into 0D/2D Co3O4 catalyst with the modification of the valence states with more Co2+ species on the surface,the simultaneous generation of surface oxygen vacancies and a larger specific surface area.After this surface modification,the structural defects and oxygen vacancies were produced on the surface of the catalyst,and the samples showed better epoxidation activity and higher catalytic activity.Moreover,the formation of a large amount of oxygen vacancies is more conducive to the adsorption of styrene and oxidants,so that the oxidation rate can be accelerated.The 0D/2D Co3O4 catalyst was then utilized as an efficient catalyst for the selective oxidation of styrene to SO.The yield of SO is up to 70.9%with a styrene conversion of 99.3%and a SO selectivity of 71.4%under the optimized reaction conditions,which shows a much better catalytic performance than the pristine Co3O4 sheets,the yield of SO(70.9%)was more than double that of Co3O4 catalyst(33.2%).The results show that the excellent performance of 0D/2D Co3O4 catalyst in epoxidation of styrene is uue to its hierarchical porous structure,the modification of surface metal valence states and the synergistic effects of oxygen vacancies.In addition,a possible reaction pathway for the selective epoxidation of styrene to SO with the hierarchical 0D/2D Co3O4 hybrids as the catalyst was reasonably proposed.The nickel and cobalt-based heterogeneous catalysts synthesized in this paper has the characteristics of high activity and excellent selectivity of epoxide,simple preparation,environmental-friendly and easy separation and recovery,which provides a reference for the construction and exploration of new types of catalysts with high efficiency.