Mesoporous Silica Supported Polyoxometalates: Synthesis, Characterization And Application In Heterogeneous Catalysis

Polyoxometalates (POMs) are constituted primarily by early transition metalcations (Mo, W, V, Nb, Ta, etc.) and oxide anions. POMs show specific aciditiy, redoxand tunable catalytic ability, which make POM chemistry a rapidly growing area.Some POM catalyzed reactions have been applied in industry. Nowadays, effectiveand environmental friendly catalytic oxidation is being received increasing attention.The application of POM catalysts, however, suffers from its low surface area and highsolubility. Thus, heterogenizing of POM homogeneous catalysts is very desirable.Mesoporous materials with large surface area and stable structure could beemployed as supporter of POM catalysts. When POM is supported onto mesoporousmaterials through the conventional incipient wetness method, the loading is normallylow. The catalysts show poor reusability in liquid-solid reaction, as the supportedmetal ions tend to leach and cause environmental problem. When POM is introducedinto mesoporous materials through co-synthesis, much POMs are buried insideamorphous wall, which in generally lower the catalytic activity of POM. The mainpurpose of this dissertation is to develop some novel methods to prepare supportedPOM catalysts, which could be applied in oxidation of organic compounds under mildconditions with high performance. Some important issues are considered, e.g. toimprove the dispersion of the active species and the interaction between the activespecies and the supporter, to minimize the leaching of active species and to boost thereusability of catalyst. In addition, a number of analytical and spectroscopictechniques were employed to investigate the relationship between the structure ofactive species and their catalytic behavior.In this dissertation, POMs are immobilized onto aminosilylated mesoporous silica,and the resulting catalysts exhibit high activity and stability and low leaching. Aminosilane compounds are used to modify the surface of mesoporous silica. Afteraminosilylation, the surface of silica turns from weak acidity to basicity, which is ableto interact with early transition metals and hold POMs firmly inside the channels. Theinteractions help immobilizing highly dispersive active species and keeping themfrom leaching. 1. Preparation and application of aminosilylated SBA-15 supportedmolybdovanadophosphoric acids.Keggin-type molybdovanadophosphoric acids H₄PMo₁₁VO₄₀, H₅PMo₁₀V₂O₄₀ andH₆PMo₉V₃O₄₀, are anchored ontoγ-aminopropyl-triethoxysilane (APTS)aminosilylated SBA-15 through acid-base neutralization and the resultingPOM/APTS/SBA-15 were characterized by BET, TEM, XRD, TCP, FT-IR and ³¹pMAS NMR. The characterization results indicate that the POMs enter into themesoporous channels and the Keggin-structure of these POMs is preserved within themesoporous silica host. The catalysts were tested for catalytic aerobic oxidation ofacetaldehyde heterogeneously in liquid phase under ambient condition. Theelectrostatic force between POM and amino groups grafted on the silica channelsurface leads to high POM load and strong immobilization of POM inside SBA-15which is against the leaching during the reaction. The catalysts give higher catalyticperformance (acetaldehyde conversion at about 73%) than the supported POMcontaining no vanadium, and better reusability than the bulk POMs.Nanowires of mixed oxides MoO₃-V₂O₅ inside the channels of mesoporous silicaSBA-15 could be obtained by calcining POM/APTS/SBA-15. The immobilization ofthe heteropolyacid containing mixed addenda makes the molar ratio of the loadedcomponents controllable. The formation of the MoO₃-V₂O₅ nanowires inside thechannels occurs at 250-400℃, monitored by variable temperature in situ XRD. Thematerials obtained by heat treatment at 400℃for 5 h were characterized by TEM,N₂-sorption measurements, laser Raman spectra and UV-Vis diffuse reflectancespectra. Further heat treatment of the MoO₃-V₂O₅ nanowires inside the SBA-15channels at higher temperature (700℃) destroys the framework integrity of SBA-15by complete sublimation of MoO₃ through the SBA-15 channel walls.2. Rare earth metal sandwiched polyoxometalates immobilized on SBA-15:Synthesis, characterization and catalytic application in cyclohexeneepoxidationRare earth metal sandwiched Keggin-type heteropolyoxometalates,K₁₁[RE(PW₁₁O₃₉)₂] (RE-PW₁₁, RE=La, Ce, Pr, Nd, Sm, Eu, Dy and Y), are anchoredonto APTS aminosilylated mesoporous silica SBA-15 and the resulting RE-PW₁₁/APTS/SBA-15 materials are characterized. ICP, FT-IR and ³¹P MAS NMRresults indicate that the RE-PW₁₁ clusters preserve their structure within thesurface-modified mesoporous host. The similar RE-PW₁₁ loading indicates that thereare similar electrostatic interaction between different POM clusters and amine groups.Low angel XRD and BET test suggest that the POMs enter into the host channels,while the primary structure of SBA-15 is maintained after aminosilylation andimmobilization of RE-PW₁₁. The catalytic activity of RE-PW₁₁ clusters was tested inheterogeneous oxidation of cyclohexene by H₂O₂. The main product is cyclohexeneepoxide. Y-PW₁₁/APTS/SBA-15 shows the best activity with epoxide yield at 64%.RE-PW₁₁/APTS/SBA-15 show higher catalytic efficiency per POM unit than the bulkPOMs. There is strong interaction between RE-PW₁₁ and amino groups grafted on thesilica channel surface, leading to negligible leaching of RE-PW₁₁ species and ease inseparation in solid-liquid phase after catalytic reaction.3. Immobilization of heteropolyphosphotungstate on HMS spheres and itscatalytic application in eyelooetene epoxidationH₃PW₁₂O₄₀ are immobilized on 3-amino-propyltriethoxysilane (APS)aminosilylated HMS silica spheres with various loading. No changing of POMcomposition and structure was found. The catalytic activity of PW₁₂/APS/sphere istested in the heterogeneous oxidation of cyclooctene by 30% H₂O₂. Catalyst withPW₁₂ loading of 30 wt.% gives epoxide yield of ca. 100% after 24 h reaction. Afterrecycled 6 times, the epoxide yield is still more than 80%. Catalysts with the samePOM content but supported on un-aminosilylated HMS spheres give much loweractivity and reusability than the aminosilylated ones, which indicate thataminosilylation helps enhancing the immobilizing of POMs onto silica surface.4. Immobilization of heteropolyphosphotungstate on co-condensationaminosilylated SBA-15 and its catalytic applications in cyclohexene epoxidationIn order to simplify the preparation of aminosilylated mesoporous silica, a simpleone-pot approach was developed. A series of aminosilylated SBA-15 with variedAPTS to triblock copolymer Pluronic P123 ratio have been synthesized through asimple one-pot method. H₃PW₁₂O₄₀ are immobilized within the modified SBA-15 channels and calcined at different temperature. The obtained materialsW/APTS-SBA-15 were characterized by ICP, FTIR, N₂ adsorption, TEM, XRD, ²⁹Siand ³¹P MAS NMR. The results indicate that the PW₁₂ clusters were stronglyimmobilized within channels of the surface modified SBA-15 host and exhibitanti-leaching effect during the reaction. The effect of different loading content andtreatment temperature were also discussed. More importantly, the catalytic activity ofthe immobilized catalyst was tested in heterogeneous oxidation of cyclohexene usingH₂O₂ as the oxidant. Among the samples with the same PW₁₂ content, the catalystscalcined at 400℃show the best activity and recycling ability.5. Photoeatalytie degradation of aqueous 4-chlorophenol by silica-immobilizedpolyoxometalatesThe degradation of 4-chlorophenol with near-UV light by silica-immobilizedpolyoxometalate (POM-in-SiO₂) catalysts has been studied. The silica-immobilizedNa₆W₇O₂₄ (W₇), H₄W₁₀O₃₂ (W₁₀), H₃PW₁₂O₄₀ (PW₁₂) and H₆P₂W₁₈O₆₂ (P₂W₁₈) wereprepared by means of the sol-gel hydrotherrnal technique through the hydrolysis oftetraethoxysilane in aqueous solution of the corresponding polyoxometalate, followedby hydrothermal treatment. During irradiation, 4-chlorophenol first dechlorinated toform hydroquinone and p-benzoquinone, and then these intermediates furthermineralized to form CO₂ and H₂O. The degree to which 4-chlorophenol wasmineralized by photocatalytic oxidation was investigated. Results indicate that W₁₀has the highest activity, whereas W₇ has the lowest. A nearly complete mineralizationwas found over W₁₀ after 60 min of photoirradiation. The present studies suggest thatPOM-in-SiO₂ catalysts have potential in photocatalytic treatment of waste water.