Catalytic Oxidation Of Aromatic Compounds Over Modified Molybdovanadophosphoric Heteropolyacid Catalysts

Oxidation is an important method to functionalize aromatic compounds to produce phenols, aldehydes, ketones, quinones, and carboxylic acids that are important organic intermediates and fine chemicals. The selective oxidation of aromatic compounds has been one of the most hot but difficult topics in the catalytic research field. Hydrogen peroxide has attracted great attention as an environmentally begin oxidant, because it is quite cheap, readily available, easy to handle, and gives water as the only by-product. Therefore, liquid-phase oxidations with H2O2as the oxidant are considered to be environmentally friendly processes, which has been received extensive attention. Vanadium containing heteropolyacids (HPAs) were proven to be effective catalysts for the selective oxidation of aromatic compounds. However, pure HPAs utilized in bulk form, which can be dissolved in a polar reaction medium, have drawbacks such as difficulty for catalyst separation and recovery. Therefore, the immobilization of HPAs onto porous supports becomes the commonly used method to heterogenize HPA catalysts. However, the catalyst preparation by the immobilization of HPAs onto porous supports is generally complicated, and the prepared catalysts usually show low activities and the leaching of active species inevitably takes place during the oxidation reaction. Apparently, the heterogeneous hydroxylation of benzene with hydrogen peroxide catalyzed by HPAs-based catalysts still remains a challenge.The main results in this thesis are summarized as follows:1) Inorganic-organic dual modified HPA catalysts were prepared by partially Cs+ion exchanging with the protons in the V-containing HPA followed by the immobilization of different organic compounds, and characterized by FT-IR,13C NMR, XRD, SEM, N2adsorption and ESR techniques as well as by the elemental analysis, and used in the hydroxylation of benzene to phenol with H2O2as oxidant. Among the catalysts investigated, Cs2.5(MIMPS)1.5PMo11VO40[MIMPS is the abbreviation of3-(1-methylimidazolium-3-yl) propane-1-sulfonate], shows the best catalytic performance, over which a benzene conversion of15.5%with absolute selectivity for phenol can be obtained. By the introduction of Cs+, the soluble HPA can be transformed into insoluble HPA salts via inorganic modification. The acidic SO3H group from the MIMPS immobilized onto Cs2.5H1.5PMo11VO40might effectively compensate for the loss of the proton acid sites due to the protons in Cs2.5H1.5PMo11VO40substituted by MIMPS. In addition, the hydrophobic organic segments in this hybrid catalyst are also beneficial to the hydroxylation of benzene via the enhanced encapsulability of relatively non-polar benzene and the promoted releasability of polar phenol. Furthermore, as a heterogeneous catalyst after4-times repeated use, a comparable high catalytic activity is still obtained. However, Cs2.5H1.5PMo11VO40reveals a low reusability. It seems likely that Cs2.5H1.5PMo11VO40is a solid mixture of H4PMo11VO40and Cs2.5H1.5PMo11VO40, in which the H4PMo11VO40is inevitably dissolved in the reaction medium, leading to the catalyst deactivation. Differently, there is no free H4PMo11VO40existing in Cs2.5(MIMPS)1.5PMo11VO40and consequently, the leaching of the H4PMo11VO40hardly takes places during the reaction, leading to a comparable high reusability.2) HPA-based amphiphilic catalysts are prepared by the functionalization of the Ⅴ-containing HPA with surfactants with different carbon-chain lengths, and characterized by FT-IR, TG,1H NMR, XRD, SEM and TEM techniques as well as by the elemental analysis, and used in the selective oxidation of benzyl alcohol to benzaldehyde with H2O2as oxidant under the organic solvent-free condition. Both higher activity and selectivity are observed over the amphiphilic catalyst, compared to those over the HPA as the catalyst. During the reaction, the emulsion system between HPA amphiphilic catalyst and reaction medium under the presence of H2O2is formed and the catalyst can be easily separated by demulsification after H2O2is completely consumed. After the four-run reaction, the activity of the HPA amphiphilic catalyst is reserved. A comparison of the characterization results of the fresh catalyst with those of the recovered catalyst after the four-run reaction indicates that the structure of the catalyst does not change.