Synthesis And Catalytic Performance Over KIT-6 Supported Cu-based Catalysts In The Epoxidation Of Styrene

The selective oxidation of side chain C=C in styrene has been one of the hot spots in the field of olefins epoxidation.Over the past several decades,the epoxidation of styrene mainly focuses on the development and application of heterogeneous transition metal catalysts.Copper-based catalysts have attracted considerable attention due to the higher styrene epoxide selectivity.Owing to the structural diversity and unique importance on catalysis Palladium-Copper alloy catalysts have been widely used to various catalytic reactions therein.However,there have no any reports of the addition of Cu to 3D hexagonal mesoporous KIT-6 by the pH adjustment method,and the effects of pH on the associated variations in morphology,structure and catalytic performance remain unknown.On the other hand,the development of Pd-Cu composite catalysts in the field of styrene epoxidation remains a significant need and a challenge.Based on the aforementioned consideration,the heterogeneously copper-catalyzed epoxidation of styrene was studied using copper-doped mesoporous KIT-6(Cu-KIT-6x)prepared via pH adjustment(where x is the pH: 1.43,2.27,3.78,3.97,4.24 or 6.62).Variations in the catalyst structure and morphology with pH values were characterized by X-ray power diffraction,nitrogen adsorption-desorption analysis,transmission electron microscopy and X-ray photoelectron spectroscopy.As the pH value applied during the initial synthesis,the resulting Cu-KIT-6x exhibited different structural,textural and surface characteristics,especially in terms of specific copper species and copper content.At a pH value of 3.78,approximately 4.6 wt% copper(II)was successfully incorporated into the framework of the initial KIT-6,in the form of-Cu-O-Si-groups.The catalytic performance of each catalyst was evaluated by following the epoxidation of styrene,employing tert-butyl hydroperoxide as the oxidant and CH3 CN as the solvent.A significant styrene conversion of 43.5% with 86.6% selectivity for the desired styrene epoxide was obtained over the Cu-KIT-63.78.A higher Cu content,an ordered cubic Ia3 d mesoporous architecture and various specific textural characteristics all combined to endow the Cu-KIT-63.78 with high catalytic activity and good stability.On the basis of the researches on single component catalysts a series of composite catalysts PdxCuy/KIT-6(where x:y refers to the Pd:Cu molar ratio)were synthesized by impregnation method and the effects of Pd:Cu molar ratio on catalyst structure,active species composition and catalytic performance were investigated by various characterizations.The results demonstrate that the immobilization of catalytically active species did not destroy the mesoporous structure of catalysts and the active species were mainly dispersed on the external surface of support.XPS data showed the large quantity of copper was in present in the form of Cu2+,while PdCl2 and Pd-O coexist in the active palladium and the relative contents could be regulated with the variation in Pd:Cu molar ratio.Further study in catalytic test indicated that the active species including Pd and Cu govern the catalytic bahavior of as-prepared materials.And PdCl2 and Pd-O were in favor of the transformation of styrene from the view of Pd,but the higher content of Pd-O was the most decisive factor to achieve better catalytic performance.A higher degree of dispersion,better stability and stronger resistance to leaching all combined to make PdxCuy/KIT-6 more superior.In addition,reaction time was optimized and catalytic performance results verified that the optimum catalytic property was obtained under 6 h.