Interaction of platinum and molybdophosphoric heteropoly acid under conditions of catalyst preparation for benzene oxidation to phenol with an O-2-H-2 gas mixture

摘要

The transformations of platinum and a heteropoly acid (HPA) in binary systems prepared from H(2)ptCl(6) or H2PtCl4 and H3PMo12O40 were studied using IR and UV-VIS spectroscopy, elemental analysis, XPS, EXAFS, TPR, and HREM. The calcination of platinum chloride with the HPA to 450 degrees C resulted in the formation of a platinum salt of the HPA along with decomposition products (mixture I). The reduction of calcined samples containing Pt : HPA = 1 : 1 with hydrogen at 300 degrees C (mixture II) followed by exposure to air resulted in the regeneration of the HPA structure. The resulting solid samples of (Pt1-nPtnII)-Pt-0 ClmOxHy) ((H3+pPMo12-pMoPV)-Mo-VI O-40) (III) contained platinum and molybdenum in both oxidized and reduced states. The following association species were isolated from mixtures I and II by dissolving in water: [Pt-n(II) PMo12O40] (I-s) (n = 0.3-0.8) and [(PtnPMo12red)-P-0 O-40] (IIs) (n = 1). Under exposure to air, the solutions of I-s were stable ((PtnClmOxHy)-Cl-II) whereas pt(met) was released from IIs After the drying of I-s, the solid association species (Pt-n(II) ClmOxHy) n (H3PMo12O40), where n = 0.3-0.8, m = 0.2-1, and x = 3-0, (I-solid) were obtained. The I-solid/SiO2 supported samples were prepared by impregnating SiO2 with a solution of I-s and drying at 100 degrees C. Platinum metal particles of size similar to 20 angstrom and a mixed-valence association species of platinum with the HPA were observed after the reduction of I-solid/SiO2 with hydrogen at 100-250 degrees C. These samples were active in the gas-phase oxidation of benzene to phenol at 180 degrees C with the use of an O-2-H-2-N-2 mixture.