Synthesis Of High-stable Core-shell Catalyst And Applition In Hydrogenation

Recent advances in noble metal controlled synthesis enabled the precise control of the size,shape and composition of noble metal nanoparticles,enabling their use as model catalysts for systematic investigations of the atomic-scale properties affecting catalytic activity and selectivity.However,the high specific surface area and large specific surface energy of nanometer size make noble metal nanoparticles very unstable,easily agglomerated and deformed,which limits their application promotion.Many industrially important hydrogenation processes are carried out under relatively severe reactions.Therefore,this thesis is based on the design of high-performance noble metal catalysts from the aspects of activity,selectivity and anti-deactivation to reveal the influence of catalyst structural characteristics on catalyst performance.In this article,Pd@mSiO₂core-shell nanospheres,Pd@mHSiO₂ Yolk-Shell nanospheres and Pd@Al-MSiO₂Yolk-Shell nanospheres were successfully synthesized by a one-pot method.The main research contents and specific research results are as follows:?1?In chapter 2,the Pd@mSiO₂ core-shell nanospheres were synthesized by a one-pot method.Their morphology was systematically studied by changing the concentration of CTAB,pH,and concentration of silicon source.Furthermore,the thickness and pore size of the shells could be easily tuned by changing the amounts of TEOS and the alkyl length of the surfactant during synthesis,respectively.Pd@mSiO₂CSNSs was applied to the hydrogenation of nitrobenzene.The conversion rate of nitrobenzene reached over 97%and the selectivity of aniline reached over 96%after the reaction of 110°C for 30 min.In addition,the effect of pore size on the transport rate of the reactants and the product selectivity in metal-catalyzed hydrogenation were investigated.The size and distribution of Pd particles could be maintained in high temperature environments thanks to the protection of the stable mesoporous silica shell.?2?In chapter 3,based on the synthesis of Pd@mSiO₂ CSNSs.We have devised a strategy for a true one-pot synthesis of Pd@mHSiO₂ YSNSs without the need for extensive purification and separation steps,which comprises three stages:controlled synthesis of Pd nanocubes,coating of mesoporous silica,and transformation from CSNSs to YSNSs.The formation mechanism of the yolk–shell structure induced by an organosilane has been studied in detail.The incorporation of1,2-bis?trimethoxysilyl?ethane?BTME?modifies the degree of condensation between an outer hybrid silica layer and an inner pure silica section,the addition of BTME will have two stages,namely,preferentially forming a thin layer of hybrid silicon,and secondly providing a combination of organic silicon species and oligomeric silicate species that produced by dissolution of pure silicon to grow on the original hybrid silicon layer.During this period,high temperature water is responsible for dissolving the inner pure silica layer leading to a transition from CSNSs to YSNSs.The mesoporous hybrid silica shell effectively prevents Pd NPs aggregation,while the short and open channels in mesoporous hybrid silica shells provide excellent accessibility to the Pd NPs by the reactants.Using water as the solvent and reacting at 200°C for 4 h,the conversion rate ofLA reached more than 97%,and the selectivity of?-GVL reached more than 98%.Pd@mHSiO₂ YSNSs,as a model yolk–shell catalyst,shows excellent catalytic activity and recycling stability in the hydrogenation of levulinic acid?LA?reactions compared to Pd/mSiO₂ NSs and Pd@mSiO₂ CSNSs.?3?In chapter 4,a series of bifunctional Pd@Al-MSiO₂ YSNSs with Pd core and AlO₄ sites shell were synthesis for selective hydrogenolysis of glucose to 1,2-PG.The structure-performance relationship of Pd@Al-MSiO₂ YSNSs in glucose hydrogenolysis to 1,2-PG has been discussed in detail.The mesoporous silica shell with abundant AlO₄sites can promote the isomerization of glucose to fructose,while the Pd core promotes the hydrogenation of the intermediate C=O bond.The unique hollow space inside the Pd@Al-MSiO₂ YSNSs provides a location for the retro-aldol condensation of fructose and avoids direct hydrogenation of fructose.The synthesized Pd@Al₃-MSiO₂ YSNSs shows superior catalytic activity?1,2-PG selectivity of 47.4%,glucose conversion of95.4%?in the hydrogenolysis of glucose,and far outperforms the prototype Al₃-MSiO₂-supported Pd NPs catalysts.In addition,owing to the influence of the framework Al?AlO₄?on the geometry and chemical properties of the mesoporous shell,the nanospheres show good stability and reusability over multiple catalytic cycles without significant loss of activity.In summary,three catalysts,Pd@mSiO₂ CSNSs,Pd@mHSiO₂ YSNSs and Pd@Al-MSiO₂ YSNSs,were prepared in this paper,which were applied to nitrobenzene hydrogenation,levulinic acid hydrogenation,and glucose hydrogenolysis,respectively.The preparation mechanism of the catalyst and the corresponding relationship between the catalyst structural characteristics and its catalytic activity were mainly investigated.Therefore,this study has a certain role in promoting the design of new catalysts with high activity,high stability,and environmental friendliness.