Preparation And Catalytic Behaviors Of Water-Tolerant Polyoxometalates

Water-tolerant polyoxometalates were prepared and characterized. Based on the characteristic of prepared water-tolerant polyoxometalates, the catalytic activity of these catalysts were studied by three kinds of catalytic reactions.Firstly, nanocomposite photocatalysts, cesium hydrogen salts of 12-tungstophosphoric acid coupled with anatase TiO2 (Cs_xH_3-xPW₁₂O₄₀/TiO2, x = 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0, respectively), were prepared via one-step sol–gel method followed by solvothermal treatment. The phase structure and optical absorption properties of the composites were characterized by several characterization techniques including FT-IR, XRD and Raman scattering spectroscopy. The results indicated that the primary Keggin structure of starting POM in as-prepared composites remain intact after formation of the composites, and the strong chemical interactions between POM molecules and titania network existed. The morpholygy and surface textural properties of the prepared composites were characterized by FESEM and N2 adsorption/desorption analysis, indicating Cs_xH_3-xPW₁₂O₄₀/TiO₂ composites had excellent surface physicochemical properties (mesoporosity, large BET surface area and uniform pore size). The photocatalytic activities of Cs_xH_3-xPW₁₂O₄₀/TiO₂ were evaluated by the degradation of three model organic molecules including 4-nitrophenol (4-NP), methyl orange (MO), and rhodamine B (RB) under UV-light irradiation. For comparison, the photocatalytic activityies of the parent CsxH3?xPW12O40 and Degussa P25 were studied under the same conditions. The results showed that as-prepared nanocomposite photocatalysts were substantially more effective than the starting CsxH3?xPW12O40 and Degussa P25. The high photoactivities of as-prepared nanocomposites could be attributed the higher surface acidity, mesoporosity, and the synergistic effect existed between the CsxH3?xPW12O40 and the TiO2 matrix.Secondly, a series of water-tolerent cesium partly substituted Dawson type heteropolyacids, Cs_xH_6-xP₂W₁₈O₆₂ (x = 1.5, 2.0, 2.5, 3.5, 4.5, and 6.0), and cesium partly substituted Keggin type heteropolyacids, (Cs_xH_3-xPW₁₂O₄₀, x = 1.0, 1.5, 2.0, 2.5, and 3.0), were synthesized and characterized using the techniques including UV-vis/DRS, FT-IR, XRD, XPS, and N2 porosimetry. As the unique and reusable solid acid catalysts, Cs_xH_6-xP₂W₁₈O₆₂ salts were applied to produce diphenolic acid by the condensation reaction of phenol with bio-platform molecule, levulinic acid. For comparison, Cs_xH_3-xPW₁₂O₄₀ (x = 1.0?3.0), HCl, HZSM-5, and MCM-49 were also tested. Adsorption behavior of the catalysts and the influence factors on the catalytic activity and selectivity including solvent, molar ratio of phenol to levulinic acid, amount of catalyst, reaction temperature, stirring speed, and reaction time were considered. The experimental results demonstrated that both Cs_1.5H_4.5P₂W₁₈O₆₂ and Cs_<sub>2.5H_0.5PW₁₂O₄₀ exhibited excellent catalytic performance under solvent-free conditions. Furthermore, both selectivity and activity of Cs_xH_6-xP₂W₁₈O₆₂ were higher than those of Cs_xH_3-xPW₁₂O₄₀. Reasons for the different catalytic behaviors between two types of cesium partly substituted heteropolyacids were investigated.Lastly, the mesoporous composites, [(n-C₄H₉)₄N]₄[γ-SiW₁₀O₃₄(H₂O)2]/SiO₂ (TBA-1^*/SiO₂, where TBA-1^* = [(n-C₄H₉)₄N]₄[γ-SiW₁₀O₃₄(H₂O)₂]), with TBA-1^* loading of 4.3-14.8% were prepared by sol–gel method followed by solvothermal treatment in the presence of triblock copolymer surfactant (P123). The as-prepared composites were characterized by ICP-AES, FT-IR, XRD, Raman scattering spectroscopy, 31Si (MAS) NMR, TEM, and N2 adsorption/desorption analysis. The results indicated that the primary Keggin structure of parent TBA-1^* in as-prepared composites remain intact after formation of the composites, and the strong chemical interactions between TBA-1^* molecules and silica network existed. The materials exhibited excellent surface physicochemical properties including higer BET surface area, larger interconnected pore structure, and uniform pore size distribution. As-prepared TBA-1^*/SiO₂ were used as the heterogeneous oxidation catalyst for the styrene epoxidation reaction to synthesize styrene oxide in the presence of dilute H2O₂ (30%). In addition, influences of solvent, molar ratio of styrene to H2O₂, TBA-1^* loading on the styrene conversion, styrene oxide yield and selectivity were considered.