Construction Of The Catalysts For The Aerobic Oxidation Of Glycerol Based On Layered Rare-Earth Hydroxides

Nowadays,the global and extensive production of biodiesel has resulted in the surplus of glycerol.Accordingly,the valorisation of glycerol to high value-added products is of great importance for sustainable development.Among varieties of conversion pathway,aerobic glycerol oxidation is a clean and effective approach.Since heterogeneous catalysts play a key role in this reaction,it is essential to provide stable catalysts with considerable activity and specific selectivity.Herein,based on the tunability of hydrotalcite-like compounds,a series of layered rare earth hydroxides,cerium（III）doped Mg Al-hydrotalcites,ceria NPs decorated Mg Al-hydrotalcites and carbon nanotube composited Mg Al-hydrotalcites were synthesized.And they were employed as precursors to construct highly efficient noble-metal based catalysts in catalytic glycerol oxidation reaction.Meanwhile,the relationship between properties of catalysts and composition of supports or noble-metals was thoroughly explored.The studies are summarized as follows:（1）Layered rare earth hydroxides（LREH）as a special kind of lamella material,they have promising application prospects in catalysis field.However,they have received little attention compared with the traditional hydrotalcites and rare earth oxides.Herein,a class of LREH nanosheets were synthesized as precursors for preparing supported gold catalysts using in catalytic glycerol oxidation to lactic acid reaction.It was found that the reverse microemulsion method was more practical for synthesizing the LREH（RE=Y,La,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er and Tm）nanosheets,compared with the methods previously reported in literature.By a comparison between the effects of RE³⁺ions on performance of Au/LREO catalysts,it was discovered that the catalysts activity was related to the lanthanide contraction.Among the twelve catalysts,Au/LPr O exhibited the best catalytic performance on converting glycerol into lactic acid,which was attributed to the stronger metal-support interaction.Meanwhile,the active carrier,LPr O,is an important factor for stabilizing gold nanoparticles with small size and high activity.This research indicated that,the layered rare earth hydroxides have great potentials in efficient catalyzing biomass conversion fields.（2）As it is hard to obtain layered cerium hydroxide nanosheets（LCe H）,its catalytic potential cannot be well investigated.In this work,we provide a feasible and effective method to mass synthesize ultrathin LCe H nanosheets.The LCe H that treated by freeze-drying process had a thickness of about 5 nm.After calcined at high temperature,the LCe H derived LCe O nanosheets exhibited a higher specific surface area（～112 m²/g）than that（～29-84 m²/g）of previously reported Ce O₂ nanosheets.The LCe H-derived nanosheet supported Au Pt catalyst（Au Pt/LCe O）was composed of numerous tightly packed Ce O₂nanocrystals and metal nanoparticles.The steric inhibition effect caused by mesopores and oxygen vacancies promoted the dispersion a stability of the supported metals in Au₃Pt₁/LCe O catalyst.Therefore,the Au₃Pt₁/LCe O showed higher activity and selectivity to lactic acid than Au₃Pt₁/Ce O₂-NC catalyst.In addition,the effects of the molar ratio of Au to Pt and the alkalinity of the reaction solution were also investigated.This study not only provided a practicable way to synthesize ultrathin layered cerium hydroxide nanosheets,but also developed an efficient catalyst for glycerol oxidation reaction.（3）Based on cerium element and layered magnesium aluminum hydrotalcite,a series of cerium doped hydroxides(Mg₅Al_1-xCe_x-LDH)and ceria modified hydroxides（x Ce O₂/LDH）were synthesized.They were taken as supports to build highly dispersed gold catalysts by sol-immobilization method.Compared with Au/Mg Al-LDH,Au/Ce O₂ and other supported noble-metal catalysts,Au/LDH-25%Ce O₂ are both active and selective for the aerobic conversion of glycerol to lactic acid at 90℃and 1atm.It was found that the performance of these catalysts were closely related to the concentration cerium in supports.And the synergistic effect between LDH and Ce O₂was a key factor to improve the activity of Au/LDH-25%Ce O₂catalyst.This study provided a new reference for designing highly efficient catalyst of aerobic glycerol reaction.（4）Pt/CNT catalyst was explored to be less active in catalyzing glycerol oxidation reaction under base free.Although Pt/LDH catalyst that rich in basicity was found to be highly efficient in this reaction,it was more easily to cause the over-oxidation of C₃products.To overcome these drawbacks,Mg Al-LDH nanosheets,CNT nanotubes and Pt nanoparticles were employed to construct Pt/CNT-LDH ternary catalyst.In this study,the LDH ingredients not only improved the dispersion and activity of Pt nanoparticles in ternary catalyst,but also promoted the activation of glycerol substrates.The CNT backbones inhibited the excessive adsorption of reactants over surface of the catalyst,which avoided the excessive oxidation of C₃ products.Owing to the synergistic effect between Pt,CNT and LDH ingredients,Pt/CNT-LDH catalyst facilitate the highest yield of glyceraldehyde product.