Abstract
The present doctoral thesis presents a study on the synthesis and characterization of V- and Mo-containing mixed metal oxide bronzes to be used as catalysts in the partial oxidation of propane to acrylic acid. MoVSb, MoVTe and MoVTeNb mixed oxides have been prepared hydrothermally and, finally, heat-treated (in inert atmosphere) to obtain active and selective catalysts for the selective oxidation of propane. The physical and chemical characteristics of catalysts have been determined by using several spectroscopic techniques (N2 adsorption, XRD, FTIR, Raman, electro microscopy (SEM/HREM), as well as the determination of surface redox and/or acid properties (XPS, NH3-TPD, or the adsorption of probe molecules by infrared spectroscopy).
First, the incorporation of a promoter metal in the synthesis of MoVTeO mixed oxides has been studied. In general, the incorporation of a promoter in the synthesis gel affects both the distribution of the crystalline phases and the catalytic properties of the materials depending on the metal incorporated. Only in the case of Ca-, Ga- or Nb-promoted catalysts, an improvement in the catalytic properties of the materials has been observed. Especially, it should be noted that, in addition to niobium (promoter element already known), the incorporation of gallium improves the selectivity to acrylic acid during the oxidation of propane over these catalysts. Thus, yields of acrylic acid higher than those achieved over ternary metal oxides have been achieved with Ga-promoted catalysts.
Two important parameters on the preparation of these metal mixed oxides have been studied: i) the hydrothermal synthesis temperature; and ii) the catalysts activation temperature. The hydrothermal synthesis temperature mainly affects the M1 phase formation, while the catalysts activation temperature has an influence in the catalysts redox and/or acid surface properties and, thus, on the catalytic behavior of the materials.
Finally, a comparative study on the partial oxidation of propane, propylene and methanol over MoVSbO, MoVTeO and MoVTeNbO catalysts heat-treated at 500 (very active but less selective) and 600ºC (active and very selective) is presented. In addition, the study of the surface characterization of heat-treated (at 500 or 600ºC) catalysts, allowed us to know the nature of the active and selective sites on the catalyst surface. Thus, the comparison of catalytic and characterization results has allowed us to establish the relationship between the modification of the surface properties and the catalytic properties in the partial oxidation of propane to acrylic acid over these materials.