Synthesis Of WO_x/SBA Heterogeneous Catalysts And Catalytic Performance In Cycloolefins Oxidation

Hydrocarbon derivatives?e.g.,epoxides,aldehydes,acids,and alcohols,etc.?,significant intermediates in organic synthesis,can be synthesized via oxidation of olefins.The emergence of catalytic oxidation technology makes it possible to employ inexpensive oxidants?i.e.,air,O₂,or H₂O₂?for olefins oxidation.The preferred benign oxidant,H₂O₂,which generates the only decomposition product H₂O,may realize green oxidation,and it has aroused considerable attention.In H₂O₂-mediated olefins oxidation systems,transition metal-based complexes are commonly used as catalysts due to their unique acidities and redox properties.Compared with other transition metal complexes,tungsten-based complexes are excellent catalysts with high catalytic activities and selectivity in olefins oxidation reactions.Besides,tungsten is abundant in China?⁵4%of the world?,but it is rarely acted as catalyst?<8%?.Therefore,the research on olefins oxidation using H₂O₂ as oxidant and tungsten species as catalysts can not only meet the increasing domestic demand for hydrocarbon derivatives,but also turn resource advantage into product advantage.Glutaric acid?GAC?,glutaraldehyde?GA?,cyclohexene oxide?CHE-O?,and adipic acid?AA?with much potential value can be synthesized via catalytic oxidation of cyclopentene?CPE?and cyclohexene?CHE?.At present,the common catalysts employed for cycloolefins oxidation are difficult to be recycled.Therefore,the design and synthesis of novel heterogeneous catalysts with excellent catalytic performance and stability in the oxidation of cycloolefins have become an urgent problem to be solved.Tungsten-based mesoporous heterogeneous catalysts are a novel type of catalysts with the properties of recyclability and high catalytic activities,and the catalytic properties of tungsten-based mesoporous heterogeneous catalysts in liquid-liquid two-phase catalytic systems are much better than these of traditional catalysts.Based on the high catalytic activities and easy recycling of tungsten-based heterogeneous catalysts,in this thesis,the synthesis and catalytic properties of a series of supported tungsten-based heterogeneous catalysts were investigated,and mesoporous silica SBA-16 with 3D cage-like mesostructure and SBA-15 with 2D hexagonal mesostructure were acted as supports,respectively.The relationship among the synthesis,structure control,and catalytic performance of tungsten-based heterogeneous catalysts was studied in depth.Moreover,the complex and different catalytic mechanisms of tungsten-based mesoporous heterogeneous catalysts in CPE and CHE oxidation with H₂O₂ as oxidant were also explored.Supported tungsten-based heterogeneous catalysts W-SBA-16?x?,where x was the mole ratio of Si to W,were prepared using SBA-16 as support.The results indicated that tungsten content had a significant effect on the existence state of tungsten active species,acidities and catalytic activities of target catalysts.When tungsten content was?¹3 wt%,isolated tungsten species and octahedrally coordinated tungsten oxide species were detected.When tungsten content was>¹3 wt%,tungsten species agglomerated and WO₃ crystallites appeared.W-SBA-16?x?catalysts had weak acid sites,and when x was 30,medium acid sites appeared.W-SBA-16?30?catalyst exhibited better catalytic performance than that of other W-SBA-16?x?30?catalysts due to its stronger acidity and better dispersibility of tungsten species.W-SBA-16?30?catalyst could be easily separated from reaction mixture via filtration and reused 5times without dramatic deactivation?69%of CPE conversion and 55%of GAC yield?.Nevertheless,mass transfer resistance existed between organic phase?CPE?and aqueous phase?H₂O₂?is an inherent disadvantage of liquid-liquid two-phase reaction system.Therefore,CPE conversion will be further improved via reducing the mass transfer resistance in the oil-water two phases reaction system.In order to reduce the mass transfer resistance in the oil-water two phases reaction system,an efficient hybrid mesoporous heterogeneous catalyst system was synthesized via covalently grafting tungsten chelate complexes W-N,N in organic moieties?diphenyldichlorosilane,DPHS?functionalized mesoporous silica SBA-15?DPHS-SBA-15?or pure SBA-15.Characterization of synthesized materials indicated that W-N,N species were successfully anchored and highly dispersed in the mesochannels of SBA-15 and the ordered mesostructure of SBA-15 was maintained after the introduction of W-N,N and DPHS moieties.W-N,N-DPHS-SBA-15 catalyst exhibited better catalytic activity?99%of CPE conversion,73%of GA selectivity,and72%of GA yield,respectively?than its corresponding counterpart W-N,N-SBA-15.During the 6 cycles of W-N,N-DPHS-SBA-15 catalyst,CPE conversion was always>95%,and this excellent catalytic activity could be ascribed to two aspects:?1?strong covalent binding existed between W-N,N and Si-OH groups made W-N,N species stable in the pores of DPHS-SBA-15;?2?a more hydrophobic surface was obtained via removing part of residual Si-OH groups on the surface of SBA-15 by the introduction of DPHS moieties.The more hydrophobic mesochannels of W-N,N-DPHS-SBA-15offered suitable accommodation for both hydrophilic H₂O₂ and hydrophobic CPE molecules,and this was beneficial to CPE oxidation.In addition,possible catalytic reaction route for CPE oxidation with H₂O₂ over W-N,N-DPHS-SBA-15 catalyst was proposed.Transition metal-doped phosphotungstic heteropoly acids are cheap and effective homogeneous catalysts in olefins oxidation.In order to realize their recycling,a series ofKeggin-typepolyoxometalate-basedheterogeneouscatalysts Co-/Fe-/Cu-POM-octyl-NH₃-SBA-15 were synthesized via immobilizing Co,Fe,or Cumono-substitutedphosphotungsticacidsCo-/Fe-/Cu-POMon octyl-amino-co-functionalized mesoporous silica SBA-15?octyl-NH₂-SBA-15?.Characterization results indicated that Co-/Fe-/Cu-POM units were highly dispersed in mesochannels of SBA-15,and the mesostructure of SBA-15 was maintained after the introduction of-octyl,-NH₂,and(PW₁₁MO₄₀)^n-?M=Co,Fe,or Cu?anions.Both types of Lewis and Br?nsted acid sites existed in Co-/Fe-/Cu-POM-octyl-NH₃-SBA-15,and the order of acidity was Co-POM-octyl-NH₃-SBA-15>Fe-POM-octyl-NH₃-SBA-15>Cu-POM-octyl-NH₃-SBA-15.Co-POM-octyl-NH₃-SBA-15 catalyst showed excellent catalytic performance in H₂O₂-mediated CHE epoxidation with⁸4%of CHE conversion,⁹3%of CHE-O selectivity,and 98/2 of epoxidation/allylic oxidation selectivity.Strong chemical bonding between-NH₃⁺and(PW₁₁CoO₄₀)^7-resulted in excellent stability of Co-POM-octyl-NH₃-SBA-15 catalyst,which could be reused 6times without considerable loss of catalytic activity?⁸3%of CPE conversion and⁹1%of CHE-O selectivity?.In order to investigate the effect of different organic moieties on tungsten loading and catalytic activities of target catalysts,a series of different tungsten-based mesoporous heterogeneous catalysts,W-SBA-15-APTS/-MTES/-DPHS/-C3/-C8/-C12,were synthesized using SBA-15-APTS/-MTES/-DPHS/-C3/-C8/-C12 as supports,respectively.Based on the characterizatic results of Uv-vis DRS,²⁹Si MAS NMR,ICP,and XPS,the mechanism of SBA-15-APTS supported tungsten species was proposed.W-SBA-15-APTS/-MTES/-DPHS/-C3/-C8/-C12 catalysts all showed excellent catalytic activities in GA oxidation with 90⁹9%of GA conversion,97⁹9%of GAC selectivity,and 87⁹8%of GAC yield,respectively.It was worth mentioning that W-SBA-15-APTS catalyst maintained excellent catalytic activity over 20 cycles.In addition,a reaction-controlled phase transfer catalyst{PO₄[WO₃]₄}-DMA16hadbeenfirstsynthesizedandstabilizedinmesochannelsof?3-chloropropyl?trimethoxysilane modified SBA-15?Cl-SBA-15?to build an active nano-environment for triphase CHE oxidation.Tungsten precursor{PO₄[WO₃]₄}^3-and organic amino N,N-dimethylhexadecan-1-amine DMA16?N⁺?self-assembled in mesochannels of Cl-SBA-15 to form a hybrid catalyst{PO₄[WO₃]₄}-DMA16-SBA-15,which exhibited efficient catalytic activity in CHE oxidation with 86%of CHE conversion and 68%of AA yield and could be reused 5 cycles without distinct change in activity.Free remaining-Cl and-DMA16 moieties on the surface of SBA-15allowed{PO₄[WO₃]₄}-DMA16-SBA-15 catalyst to possess more hydrophobic surface property,which further facilitated the compatibility between organic reactant CHE and inorganic H₂O₂ molecules.Insoluble{PO₄[WO₃]₄}-DMA16 species were formed soluble{PO₄[W?O?₂?O₂?]₄}-DMA16 active species with the aid of H₂O₂,and when H₂O₂ was consumed out,{PO₄[WO₃]₄}-DMA16 species were recombined to realize the recycling of the catalyst.Besides,{PO₄[WO₃]₄}-DMA16-SBA-15 catalyst also exhibited excellent catalytic activities in the oxidation of CHE-O and1,2-cyclohexandiol with 100%of CHE-O conversion and 100%of 1,2-cyclohexandiol conversion,respectively.