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Controlled Modified Mesoporous Silica Supported Tungsten-contained Catalysts: Synthesis, Characterization and Application in Heterogeneous Catalysis

Author: LeYingYi
Tutor: DaiWeiLin
School: Fudan University
Course: Physical and chemical
Keywords: controlled modification mesoporous silica phosphotungstic acid immobilization heterogeneous catalyst
CLC: O643.36
Type: Master's thesis
Year: 2009
Downloads: 314
Quote: 2
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Abstract


Mesoporous silicas have always been regarded as suitable supports for the transformation of homogeneous catalysts into heterogeneous catalysts.To modificate the silica surface with organosilane is a commonly used method to functionalize silicas for the production of efficient heterogeneous catalysts.There is considerable interest in the immobilization of catalysts on modified silicas by chemical bonding with grafted organic groups.Recently,many efforts have been focused on controlled modification with different organic groups in different distribution pattern.It is of significance to investigate the influence of characteristics and distribution pattern of grafted groups on the catalytic performance of immobilized catalysts.The results of these investigations could lead to the birth of new method to produce novel and highly efficient heterogeneous catalysts.In this dissertation,controlled modified mesoporous silica supported tungsten-contained catalysts have been successfully produced for selective oxidations with aqueous hydrogen peroxide as oxidant.Different tungsten-contained heterogeneous catalysts were obtained by adjusting the characteristics and distribution pattem of grafted groups.The physical and chemical properties of modified silica materials have been studied through various characterization methods so as to investigate the relationship between catalytic performance and intrinsic structure of catalysts.1.Preparation of tungsten complexes immobilized by phosphonate ligands modified mesoporous silica and its application in the selective oxidation of cyclopentene to glutaraldehydePhosphonate ligands has been successfully grafted onto the surface of SBA-15.With the functional SBA-15 as the solid support,tungsten complexes were immobilized on the mesoporous materials by the interaction between phosphonate ligands and tungsten species.The structure of the complexes synthesized by such interaction was proposed on the basis of characterization results(XRD,ICP,XPS, BET,TG-DTA,TEM,FT-IR,31p MAS NMR and Raman).It is proposed that one single tungsten atom has formed W-O-P bonds with two phosphorous atoms and most tungsten species in the catalyst are in form of isolated tetrahedral WO4.It is found that HPWX-SBA shows high conversion and selectivity in the selective oxidation of cyclopentene（CPE） to produce glutaraldehyde（GA） using H2O2 as oxidant and can preserve high activity after several cycles of reaction.2.Preparation of phosphotungstic acid immobilized by modified mesoporous silicas with site-direction and its application in the selective oxidation of cycloocta-1,5-dieneWith the assistance of template,organoamino-groups were site-directedly grafted onto the external and internal surface of MCM-41.Owing to the existence ofγ-aminopropyl groups,phosphotungstic acid（HPA） was controllably immobilized on the external or internal surface of selectively modified MCM-41.In the oxidation of cycloocta-1,5-diene,externally immobilized catalyst HPA（out）-MCM enjoys good selectivity in the production of 9-oxabicyclo[3.3.1]nonane-2,6-dioles while internally immobilized catalyst HPA（in）-MCM shows better activity in the production of 2-hydroxy-9-oxabicyclo[3.3.1]nonane-6-one.3.Relationship between moiety distribution pattern of aminosilyated MCM-41 and catalytic performance of immobilized phosphotungstic acidWe report that solvents used for grafting organosilanes on mesoporous silicas strongly affect the distribution patterns of grafted organic moieties.Internally aminosilylated material NH2（in）-MCM-E was synthesized in polar solvent ethanol while internally aminosilylated material NH2（in）-MCM-T in nonpolar solvent toluene.The intrinsic differences between NH2（in）-MCM-E and NH2（in）-MCM-T were thoroughly studied through various characterization methods.The results suggest that grafted organoamine groups were spatially isolated（site-isolation） on the surface of NH2（in）-MCM-E while organoamine groups should be in much closer proximity to each other on NH2（in）-MCM-T.Catalyst HPA（in）-E has much lower activity in o-heterocyclization of cycloocta-1,5-diene than its counterpart HPA（in）-T.However,HPA（in）-E presents excellent catalytic performance in the epoxidation of cycloocta-1,5-diene,in which the TON（turnover number） value could reach 1000.Different catalytic performance of HPA（in）-E and HPA（in）-T should be assigned to different dispersion condition led by site-isolation and site-aggregation of organoamine groups.4.Study of the selective oxidation of styrene to benzaldehyde by tungsten-contained catalystsSelective oxidation of styrene to benzaldhyde over different kinds of tungsten-contained catalysts has been carried out with aqueous hydrogen peroxide. With the increase of reaction temperature,the reaction rate accelerates while the reaction selectivity remains unchanged.To restrict the amount of oxidant H2O2 would not influence the selectivity of benzaldhyde.Heterogeneous catalysts HPA（in）-E and PW（in）-E both show superior activity to homogeneous catalyst HPA.While n（styrene）/n（H2O2） is 2/1 and reaction temperature is 70℃,the TOF（turnover frequency） value of HPA（in）-E catalyzed reaction is 547 and that of PW（in）-E is 262 while that of HPA is only 67.The pretreatment of catalysts with aqueous hydrogen peroxide before reaction could enhance their catalytic performance.Under optimal reaction condition,HPA（in）-E could be reused for 8 times without deactivation and the total TON value is up to 4500.

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Chemical kinetics,catalysis > Catalytic > Catalyst
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