二氧化硅包覆的杂多酸在双氧水存在条件下催化氧化甘油

催化剂的酸性和氧化还原性在催化生物质平台分子转化过程中起着非常重要的作用，杂多酸具有较强的酸性以及优良的氧化还原性，因而杂多酸在生物质催化转化领域备受关注。本文利用溶胶-凝胶法和硅烷化方法将杂多酸催化剂封装在二氧化硅载体内部，随后以傅立叶红外光谱、X-射线衍射仪、热重分析仪、透射电子显微镜、扫描电镜等手段对合成的材料进行了表征。红外光谱表明杂多酸在催化剂中保持了其完整结构，X-射线衍射表明杂多酸高度分散在二氧化硅载体上，电镜表征显示催化剂呈球形纳米颗粒形貌。基于以上表征结果，我们将包覆的杂多酸催化剂应用于甘油氧化，在以过氧化氢为氧化剂，温和反应条件下，合成的材料对甘油氧化具有良好的催化活性，其中对甲酸的选择性大约为70%，对乙醇酸的选择性大约为27%。硅烷化过程对于催化剂循环起着重要的作用，单纯二氧化硅的比表面积为287 m²·g^-1，二氧化硅包覆杂多酸经过硅烷化后，其比表面积降为245 m²·g^-1，而且孔径也有所降低。单纯二氧化硅与水的接触角为0°，而二氧化硅包覆的杂多酸在硅烷化之后的催化剂具有很强的疏水性，与水的接触角为137°。根据这些催化剂表征数据说明硅烷化过程不仅可以显著提高催化剂的疏水性，而且同时限制了载体孔径，阻止杂多酸流失到反应体系中，与传统的浸渍法将杂多酸负载在二氧化硅载体上得到的催化剂相比，催化剂的循环利用性显著提高。反应后的催化剂结构与新鲜催化剂相比，并没有发生明显变化。催化剂经过一次循环后，表面暴露了更多的活性中心，活性稍有提高。催化剂在反应体系中加入强质子酸可以显著提高反应的催化性能，揭示了Bronsted酸在甘油氧化过程中对甘油分子的活化起着重要的作用。

Selective Oxidation of Glycerol with Hydrogen Peroxide Using Silica-Encapsulated Heteropolyacid Catalyst

The Keggin type heteropolyacids (HPAs) have attracted increasing attention due to their strong Bronsted acidity and excellent redox properties, which could play an important role in accelerating the conversion of bio-derived molecules. In this work, heteropolyacid (HPA, H₄PMo₁₁VO₄₀) encapsulated by silica was synthesized by a sol-gel method and a sequential silylation technique (HPA@SiO₂-N₂-S). The as-synthesized material was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The FT-IR spectra show that the HPA anions preserved their Keggin structure when incorporated into the catalyst. The XRD patterns show that HPA molecules are uniformly dispersed within the silica network. The SEM and TEM images confirm that the catalyst was composed of spherical nanometer-sized particles. The porous properties of the catalysts measured by the N₂ adsorption-desorption isotherms indicate that the Brunauer, Emmett and Teller (BET) surface area of pure SiO₂ was 287 m²·g^-1, but upon encapsulation of HPA into the silica matrix, a lower surface area (245 m²·g^-1) was measured for the resulting material. In addition, the pore diameter was reduced after silylation. Furthermore, the hydrophobicity of the catalysts was investigated by the measurement of contact angle (CA) with water. The SiO₂ and SiO₂/HPA catalysts were completely hydrophilic and the contact angle was close to 0°. However, the contact angle of the silylated catalyst was determined to be 137°, indicating that the silylation procedure significantly increased the hydrophobicity of the catalyst. The as-prepared catalysts were also used as heterogeneous catalysts for the selective oxidation of glycerol. The prepared material exhibited excellent catalytic activity towards glycerol oxidation, in which the glycerol can be selectively transformed into formic acid (ca. 70% selectivity) and glycolic acid (ca. 27% selectivity) using H₂O₂ as an oxidant under mild reaction conditions. The effect of the silylation procedure on the recyclability of catalyst was also investigated in this work. The characterizations described above indicated that silylation procedure can significantly increase the hydrophobicity and limit the pore sizes, resulting in high leach-resistance towards HPA, thus improving the recyclability of the silica-encapsulated HPA catalyst, as compared to the SiO₂/HPA catalyst prepared with the conventional impregnation method. Furthermore, the conversion in the second catalytic run is even higher than that of the initial run, which is likely because more active sites are exposed after the first run. The catalyst can be reused for at least five cycles without any leaching of HPA. The spent catalyst did not undergo structural changes, as revealed by FT-IR, XRD, and SEM characterization. Moreover, it was found that the strong Bronsted acid additives played a crucial role in the catalytic oxidation of glycerol.