Preparation Of Supported Pt On Hydrotalcite And Its Catalytic Performance Toward Glycerol Oxidation To Glyceric Acid

Glycerol is a main by-product in the field of biodiesel production,whose output will be excessive with the constant development of biodiesel industry.Therefore,catalytic conversion of glycerol to high value-added chemicals has attracted considerable attention.Among these transformation processes,catalytic oxidation of glycerol in liquid phase is an effective method for the conversion of biomass-based glycerol,through which a series of fine chemicals with high added value can be acquired including glyceraldehyde(GLYD),glyceric acid(GLYA),dihydroxyacetone(DHA),tartronic acid(TTA),glycolic acid(GLYCOA),oxalic acid(OXA)and so on.At present,noble metals(Au,Pt,Pd)have shown promising applications in this field,and Pt-based catalysts are the most extensively studied ones in virtue of their excellent oxidation ability.Although much effort has been made over Pt-based catalysts toward glycerol oxidation,some issues remain unsolved in the preparation of highly selective catalysts and in the recognition of active sites:(1)How to regulate the metal-support interaction so as to optimize the performance of glycerol oxidation;(2)How to understand the active sites and catalytic mechanism,and to reveal the structure-activity relationship are key points for rational catalyst design.To solve the problems above,in this thesis,Pt NPs supported on rehydrated hydrotalcite were prepared via a facile wet impregnation method followed by an in situ rehydration procedure.By changing the Mg/Al molar ratio of hydrotalcite support and the amount of catalysts used,an excellent performance of glycerol catalytic oxidation to GLYA was achieved.Moreover,the rate-determining step was determined through kinetic isotope experiment and the structure-activity relationship was studied,which revealed the mechanism of catalytic oxidation of glycerol.This work provides a new method for the preparation and performance control of supported noble metal catalysts,which can be potentially used in glycerol oxidation reaction.The main contents and results of this thesis are as follows:1.Design of Pt NPs supported on hydrotalcite and their catalytic performance for glycerol oxidationPt NPs supported on rehydrated hydrotalcite(denoted as re-MgxAl1-LDH-Pt)were prepared via a facile wet impregnation method followed by an in situ rehydration procedure.As shown from XRD patterns and SEM images,mixed metal oxides(MMO)recovered to MgxAl1-LDH successfully during the process of in situ rehydration.By changing the Mg/Al molar ratio of MgxAl1-LDH precursors(from 2 to 8)combined with catalytic evaluation,the optimal selectivity of GLYA(77.0%)was attained on re-Mg6Al1-LDH-Pt.By exploring suitable amount of catalyst used in batch glycerol oxidation reaction,the glycerol conversion increased gradually with the increment of catalyst dosage while selectivity increased first and then decreased.The optimal yield of GLYA(58.6%)was attained with a GLY/Pt molar ratio of 500.2.Studies on structure-activity relationship of Pt NPs supported on re-MgxAl1-LDH-Pt toward glycerol oxidationOn the basis of the results above,the structure of re-MgxAl1-LDH-Pt and the reaction kinetic were studied by a series of characterizations and kinetic isotope experiment,by which the structure-activity correlation was established.HRTEM,XAFS and CO-FTIR results showed that electron-deficient Pt?+species were produced at the interface due to the metal-support interaction,whose relative content increased with the increment of Mg/Al molar ratio.Kinetic isotope experiment was carried out using ethanol as probe,which confirmed that the breakage of a-C-H bond is the rate-determining step in glycerol oxidation.The TOF value of glycerol oxidation has a positive correlation with the relative content of Pt?+ species,indicating that electron-deficient Pt?+species promotes the removal of electron-rich a-H?-.Moreover,the selectivity of GLYA increases first and then decreases with the increase of support basicity.The reason can be explained as follows.The catalytic oxidation tends to occur on the intermediate hydroxyl group when the basicity of support is weak;while the desorption of GLYA becomes rather difficult when the basicity is over-strong,resulting in an excessive oxidation.In contrast,a mediate basicity of support is suitable for obtaining a higher GLYA selectivity.