Abstract A series of tungsten oxide-silica (WO3–SiO2) composite nanomaterials were synthesized through a novel, template-free sol-gel method, in which supercritical-CO2 (scCO2) was utilized as synthesis medium. The efficacy of the synthesis method stems from a tailored reactor design that allows the contact of the reactants only in the presence of scCO2. Selected synthetic parameters were screened with the purpose of enhancing the performance of the resulting materials as heterogeneous catalysts in epoxidation reactions with H2O2 as environmentally friendly oxidant. A cyclooctene conversion of 73% with epoxide selectivity of > 99% was achieved over the best WO3–SiO2 catalyst under mild reaction conditions (80 °C), equimolar H2O2 amount (1:1) and low WO3 loading (~2.5 wt%). The turnover number achieved with this catalyst (TON = 328), is significantly higher than that of a WO3–SiO2 prepared via a similar sol-gel route but without supercritical CO2, and that of commercial WO3. A thorough characterization with a combination of techniques (ICP-OES, N2-physisorption, XRD, TEM, STEM-EDX, SEM-EDX, FT-IR and Raman spectroscopy, XPS, TGA and FT-IR analysis of adsorbed pyridine) allowed correlating the physicochemical properties of the WO3–SiO2 nanomaterials with their catalytic performance. The high catalytic activity was attributed to: (i) the very high surface area (892 m2/g) and (ii) good dispersion of the W species acting as Lewis acid sites, which were both brought about by the synthesis in supercritical CO2, and (iii) the relatively low hydrophilicity, which was tuned by optimizing the tetramethyl orthosilicate concentration and the amount of basic solution used in the synthesis of the materials. Our optimum catalyst was also tested in the reaction of cyclohexene with H2O2, resulting in cyclohexane diol as main product due to the presence of strong Bronsted acid sites in the catalyst, whereas the reaction with limonene yielded the internal epoxide as the major product and the corresponding diol as side product. Importantly, the catalyst did not show leaching and could be reused in five consecutive runs without any decrease in activity.

中文翻译：

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在超临界 CO2 中使用新型无模板法合成的 WO3-SiO2 纳米材料作为 H2O2 环氧化的多相催化剂

摘要 以超临界CO2 (scCO2)为合成介质，采用新型的无模板溶胶-凝胶法合成了一系列氧化钨-二氧化硅(WO3-SiO2)复合纳米材料。合成方法的功效源于定制的反应器设计，该设计允许反应物仅在存在 scCO2 的情况下接触。筛选了选定的合成参数，目的是提高所得材料作为非均相催化剂在 H2O2 作为环境友好氧化剂的环氧化反应中的性能。在温和的反应条件 (80 °C)、等摩尔的 H2O2 量 (1:1) 和低 WO3 负载量 (~2.5 wt%) 下，在最佳 WO3-SiO2 催化剂上实现了 73% 的环辛烯转化率和 > 99% 的环氧化物选择性. 使用该催化剂实现的营业额数 (TON = 328)，显着高于通过类似溶胶-凝胶路线但没有超临界 CO2 制备的 WO3-SiO2 和商业 WO3。结合技术（ICP-OES、N2 物理吸附、XRD、TEM、STEM-EDX、SEM-EDX、FT-IR 和拉曼光谱、XPS、TGA 和吸附吡啶的 FT-IR 分析）进行彻底表征，允许关联WO3-SiO2 纳米材料的理化性质及其催化性能。高催化活性归因于：(i) 非常高的表面积 (892 m2/g) 和 (ii) 作为路易斯酸位点的 W 物质的良好分散，这两者都是由超临界 CO2 合成带来的， (iii) 相对较低的亲水性，通过优化原硅酸四甲酯浓度和材料合成中使用的碱性溶液的量来调整。我们的最佳催化剂也在环己烯与 H2O2 的反应中进行了测试，由于催化剂中存在强布朗斯台德酸位点，导致环己二醇作为主要产物，而与柠檬烯的反应产生内环氧化物作为主要产物和相应的产物。二醇作为副产品。重要的是，该催化剂没有出现浸出现象，可以在连续五次运行中重复使用，而不会降低活性。而与柠檬烯的反应产生内环氧化物作为主要产物和相应的二醇作为副产物。重要的是，该催化剂没有出现浸出现象，可以在连续五次运行中重复使用，而不会降低活性。而与柠檬烯的反应产生内环氧化物作为主要产物和相应的二醇作为副产物。重要的是，该催化剂没有出现浸出现象，可以在连续五次运行中重复使用，而不会降低活性。