Selective Catalytic Oxidation Of Glycerol Secondary Carbon-Oxygen Bond

With the mass production of biodiesel,the production of by-product glycerol also increased.Glycerol is usually used as an additive in cosmetics,food and other industries with low added value.Glycerol is a highly functional organic molecule.Therefore,selective conversion of glycerol's C-O bond could ease the saturated glycerol market and enhance glycerol's added value.At present,the way to produce high value-added products from glycerol could be realized by biological fermentation.However,the substrate concentration is low and the fermentation time is long,which leads to low production efficiency in this process.In recent years,the developed chemical glycerol oxide has high efficiency,easy separation of products,and green environmental protection.Therefore,the heterogeneous catalytic method with green oxidant as an oxygen source is developed to produce high value-added products with good economic prospects.The active catalytic site is the essential issue to accelerate the reaction speed.Herefore,the catalysts' rational design and construction are the keys to heterogeneous catalytic oxidation.A special interface structure could be produced in supported catalysts when the active components and supports contact at the atomic level.The interface structure could enhance the interaction between the active species and the support,and the catalyst activity could be improved while constructing the interface structure.In metal oxides,the d orbitals in the outermost layer are often in an incompletely occupied state of the transition metal oxide.Usually,the d orbitals interact with the reactant molecules to accelerate the reaction,and the transition metal oxides are regarded as catalyst materials with excellent application prospects.Among the transition metal oxides,CuO and ZnO metal oxides with full 3d orbitals of metal cations and acid/base sites often exhibit unusual catalytic behavior.Simultaneously,CuO and ZnO have abundant surface hydroxyl species,multiple crystal structures,oxygen vacancy,metal vacancy,and dislocation structure.Hence,it is easy to realize the control of the defect structure and electronic structure of CuO and ZnO.In this paper,CuO and ZnO are used as supports.The dual catalytic interface construction mechanism and formation law are deeply studied to promote the catalysts' performance.The sol-immobilization method was used to prepare Pt/CuO and Pt/TiO2 catalysts,and the catalysts were further applied in the glycerol oxidation reaction.Pt/CuO and Pt/TiO2 catalysts exhibit an enhanced catalytic activity for 1.4 and 11.1 times compared with Pt-Sol catalyst.Moreover,the Pt/CuO exhibits good selectivity of the secondary hydroxyl and Pt/TiO2 shows an enhanced primary hydroxyl selectivity.The in situ infrared Fourier transform spectroscopy was used to study Glycerol's reaction pathway of the catalysts.Moreover,the reaction site for glycerol primary and secondary group activation was revealed.In Pt/CuO catalyst,the Pt-O site could interact with secondary C-O by hydrogen interaction.In Pt/TiO2,the primary hydroxyl interacts with surface Ti to form alkoxide species.In reaction conditions,the C-O bond's electronic density is lowered,and therefore the C-O bond's directional oxidation is improved.The Au/ZnO catalyst with metal-support interface structure was prepared by air calcination reduction method.Further,the electronic structure of Au/ZnO interface was controlled by changing the supports'oxygen vacancy content.The HRTEM pictures show that ZnO overlay the Au species.The EPR results show that Au transfers electrons to ZnO during the catalyst preparation.ZnO with low oxygen vacancy content has a high work function(the lowest energy of the electrons move from the substance inside to the surface when been excited).Therefore,the ZnO supports with high oxygen vacancies promote electrons' transfer from Au to ZnO.Thus,more ZnO overlay and Au5+ species are formed.According to the isopropanol adsorption infrared spectroscopy results,the secondary glycerol alcohol would be adsorbed on the catalyst surface's unsaturated oxygen sites.At the same time,the hydrogen peroxide was detected in the solution after the reaction,indicating that the OOH*species could exist and attack the ? C-H bond during the reaction.Based on the kinetic research,the elementary reaction rate equation was further proposed.The equation was fitted by the steady-state approximation method,which finally verified the elementary reaction step's rationality.In Au/ZnO catalyst,ZnO-U with low oxygen vacancies is beneficial to the electron transfer from Au to ZnO,and Au?+ species are formed during the catalyst's preparation process.For Au/ZnO-U with high content of Au?+ species,the adsorption of OH*on the surface of Au nanoparticles increases and improve the removal rate of secondary alcohol H.Finally,the catalyst performance is enhanced.In a catalyst with a metal-support interface structure,the active component's surface energy is an essential factor that affects the formation of the overlayer and the support interface structure.In this work,the Au nanoparticle size and the thickness of the ZnO interface layer in the catalyst are controlled by adjusting the Au species' precipitation and deposition process.Furthermore,a small number of impurity atoms with relatively high surface energy is introduced into the active component.A negative correlation between the Au nanoparticles' size and the ZnO overlayer's thickness was obtained.Further,the O 1s XPS spectrum and the Raman spectrum showed that the ZnO overlay increased the catalysts' oxygen vacancies content.The in-situ adsorption infrared spectroscopy results of glycerol showed that the AuPt/ZnO catalyst with a smaller particle size enhances the secondary hydroxyl adsorption.Therefore,the introduction of high surface energy metal could increase the active component's surface energy and strengthen the metal-support interaction to construct more ZnO overlay.In this process,a large number of ZnO overlay increase the content of oxygen vacancies.Hence,the adsorption strength of the secondary hydroxyl on the catalyst is improved to enhance the catalytic performance.