Abstract:

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.

Key words: Glycerol oxidation, Heteropolyacid, Encapsulation, Formic acid, Hydrogen peroxide