Molybdenum/Vanadium Incorporated Mesoporous Silica:Enhanced Support Effects And Catalytic Hydrogenation

Mesoporous silica has a well-defined pore structure,a high specific surface area,and good thermal stability,and thereby holds the promise in the fields of catalysis,adsorption,and drug carriers.As a noble metal support,the defined pores provide space-confined growth of ultrafine metals,which is beneficial to increase the dispersion of the catalytic center.However,the ultrafine metal nanoparticles are highly susceptible to surface oxidation and formation of a passivation layer due to their high surface energy,restricting the catalytic activity.For example,the d-band center will be lifted up due to oxidation,leading to the difficult desorption of chemisorbed hydrogen and consequently the unfavored catalytic hydrogenation.Therefore,it is necessary to rationally modify the inner surface of the mesoporous silica to strengthen the interactions with loading metals.In this dissertation,KIT-6with three-dimensional interconnected mesopores is used to synthesize molybdenum and vanadium-doped KIT-6 carriers by self-assembly method,thereby activating the inner surface of the otherwise inert silicon oxide and strengthening the interaction with noble metals（such as Ir,Pt,Pd,etc.）.We further study the support effects on modified KIT-6 in the hydrogenation of amide carboxylic acid derivatives andα,β-unsaturated aldehydes,which identifies the improvement by molybdenum and vanadium incorporation.The main research and results of this thesis are as follows:1.A one-step co-assembly method was used to synthesize molybdenum-incorporated mesoporous silica to obtain a highly dispersed Ir/xMo-KIT-6 catalyst for metal Ir,and used for hydrogenation of 4-acetylmorpholine and cinnamaldehyde.It was proved that molybdenum was successfully doped into the pore wall of KIT-6 and partially reduced by hydrogen overflow in the reduction step（H-MoO_x）,which activated the surface of silica and avoided the disadvantage of easy surface oxidation of ultrafine metal.The evidenced electron-transfer from Mo-KIT-6to loading Ir resulted in more Ir⁰ metallic species.The effects by molybdenum content and reduction temperature on the structure and properties of the catalyst were studied.It was found that the increase of the ratio of Ir⁰ species can enhance hydrogen activation and dissociation.The acidic surface and defects enriched on H-MoO_x were capable of activating substrate molecules,thereby optimizing the conversion and selectivity of 4-acetylmorpholine and cinnamaldehyde.The conversion of4-acetylmorpholine at 130℃ for 4 h was 85%,the selectivity was close to 100%,and the stability of the catalyst was good.2.In this part,the V-KIT-6 was synthesized according to the procedure of Mo-KIT-6.It was found that the pH value（3.0 to 7.5）was varied during the synthesis,and the V-KIT-6 structure was significantly affected.The optimum metal species and reaction conditions（pH,reaction temperature,pressure,reaction time）were explored for the hydrogenation of 1-acetylpiperidine.Material characterization and catalytic tests showed that pH made influenceds on the mesoporous structure and the hydrogenation performance.At pH 5.0,the Pt/V-KIT-6 catalyst with a salt solution of Pt[（NO₃）]₂ exhibited higher activity and selectivity in the hydrodeoxygenation of1-acetylpiperidine.The dissertation proves that the inert surface of mesoporous silica can be activated via Mo and V-incorporation,thereby avoiding the oxidation of the loading metals and enhancing the electronic interactions between the loading metals and the supports.The improved hydrogenation was well indentified.This work will provide new ideas for the development of efficient catalysts in hydrogenation.