Preparation And Application In Epoxidations Of Olefins Of Hydrotalcite-like Catalysts

Due to their active cyclic ether structure,epoxides could be transformed to various high value-added organic intermediates vie ring-opening reactions with various functional groups.Amongst the epoxides,styrene oxide is a widely used fine chemical,which generally obtained by styrene epoxidation.Howerver,the traditional epoxidation methods of styrene suffer from the disadvantages of complicated processes and generation of large amounts of by-products leading to environmental pollution.Therefore,it is of great importance to explore novel protocols for the green and efficient epoxidation of olefins especially strene.Firstly,a series of layered double hydrotalcites（LDHs）with different magnesium-aluminum ratios and doped with various metals,which were used as catalyst precursors,were prepared by traditional coprecipitation method.Then the LDHs were calcinated to afford the corresponding hydrotalcite-like catalysts catalytic performance of hydrotalcite-like catalysts.The combined characterization results of XRD,FT-IR,N₂ adsorption-desorption,SEM,CO₂-TPD and TGA revealed that the LDHs were transformed into composite oxide hydrotalcite-like catalysts with stroger bacisities and bigger surface areas.The catalysts were evaluated in the epoxidation of styrene as a model substrate with H₂O₂/CH₃CN oxidation system.The results indicated that the catalyst Mg₅Al-9-18-450-ex LDHs with a magnesium-aluminum ratio of 5:1,which was prepared from the calcination at 450°C for 18 h of LDH with an aging time of 9 h in its preparation,showed the best performances in the epoxidation of styrene.A series of LDHs with same composition as that of the precursor of Mg₅Al-9-18-450-ex LDHs were prepared.The LDHs were calcinated to afford the corresponding hydrotalcite-like catalysts.The affecting parameters in the preparation of LDHs were investigated for preparing LDHs with uniform particle distribution and few structural defects to improve the catalytic performances of the corresponding catalysts.The characterization results of XRD,N₂adsorption-desorption,SEM indicated that inner diameter of the Y-type microreactor,residence time and the concentrations of mixed salts and bases had effects on the surface areas,pore volumes and pore diameters of the catalysts.It was revealed that the catalyst Y₃¹..₇⁶ ₅-Mg₅Al-9-18-450-ex LDHs from the calcination of LDHs which was prepared in a Y-type microreactor with 1.6 mm diameter under reaction materal residence time of 3.75 s and concentrations of mixed salts/bases of 0.6/1.67 mol/L exhibited the biggest surface area,uniform pore size distribution and crystalline phase with fewer defects.This catalyst showed excellent catalysis in the epoxidation of styrene with H₂O₂/CH₃CN as oxidation system.Under the optimal reactions which are styrene of 10 mmol,acetonitrile of 20mmol,catalyst of 0.10g,methanol of 9 m L,30wt%H₂O₂of 90 mmol,reaction temperature of 60℃,and reaction time of 4 h,the conversion of styrene reached 85%with a styrene oxide selectivity of.The catalyst Y₃¹..₇⁶ ₅-Mg₅Al-9-18-450-ex LDHs showed much better catalysis than the catalyst Mg₅Al-9-18-450-ex LDH.Bisides,the catalyst Y₃¹..₇⁶ ₅-Mg₅Al-9-18-450-ex LDHs could be recycled 3 times in the epoxidation of styrene.This catalyst exhibited good performances in the epoxidation of cyclohexene compared to that in styrene under the same reactions.