Synthesis and characterization of bronze-type mixed oxides for the selective activation of hydrocarbons

Resumen

Herein an exhaustive study on the synthesis, characterization and reactivity of mixed metal oxides that present the orthorhombic M1 phase has been conducted, aiming to unravel the influence of the composition on the catalytic performance of said materials in the oxidative dehydrogenation (ODH) of ethane to produce ethylene, an interesting alternative for the obtention of light olefins, which could suppose a drastic decrease in the greenhouse gas emissions. In a first place, the hydrothermal synthesis of a bimetallic form of the M1 phase, Mo-V-O, has been explored, taking into account all the possible parameters such as temperature, atmosphere, pH of the synthesis gel, activation temperature and post-synthesis treatments. Results indicate that the combination of these parameters is capital not only for the appropriate formation of the M1 phase, but also to the catalytic performance. This is, a set of differently synthesized Mo-V-O catalysts, all of them presenting the M1 phase, may differ in the activity and selectivity to ethylene by up to 25 % depending on the chemical composition on the surface species of the catalyst. Moreover, it was investigated the synthesis and characterization of trimetallic oxides Mo-V-Te-O presenting the M1 phase with different Te-loadings (0 < Te/Mo < 0.17), trying to modulate the acid properties, as well as the thermal stability and catalytic behavior in the ethane ODH. Accordingly, we observed that the introduction of tellurium into the structure of the M1 phase leads to an enhanced thermal stability, in addition to an increase on the selectivity to ethylene. Subsequently, it was performed a comparative study between bi-, tri- and tetrametallic (Mo-V-Te-Nb-O) catalysts about the influence of composition and/or thermal activation on the catalytic behavior. Obtained results suggest that there is a correlation between surface composition, as well as the thermal activation temperature, with the selectivity of these catalysts to ethylene. Then, it was found a direct relationship between surface V4+ species (results from X-ray photoelectron spectroscopy, XPS) and the selectivity to ethylene. Furthermore, since all these mixed oxides can be considered as semiconducting materials, an electrochemical parallel study was also conducted. Then, these results suggest that the oxides that showed the best selectivity to ethylene (i.e., catalyst Mo-V-O activated at 400 ºC; and Mo-V-Te-O and Mo-V-Te-Nb-O catalysts treated at 600 ºC) are also the ones that present the highest amount of V4+ species on the surface of the solid, in addition to the highest values of electric resistance. Finally, a comparative study was performed between the three catalytic systems that have offered the best properties in the oxidative dehydrogenation of ethane in the literature: i) alumina supported vanadium oxide (VOx/Al2O3), ii) tin promoted nickel oxide (Sn-NiO) and iii) multicomponent Mo-V-Te-Nb-O (M1) catalyst optimized in the present thesis. Thus, both the catalytic and the characterization (by means of conventional and in situ techniques) results indicate important differences in the selectivity to ethylene of these catalysts at high ethane conversion values, as a consequence of a greater or lesser degradation of the ethylene over each catalyst. In this sense, the catalyst based on mixed metal oxides, Mo-V-Te-Nb-O, shows a very low reactivity for ethylene deep oxidation (which favors a high selectivity during ethane ODH), whereas the rest of the catalytic systems display higher reactivity for deep oxidation of ethylene (which drastically reduces the selectivity to ethene during ethane ODH, specially at high ethane conversion). These results will be discussed in terms of the different adsorption properties of both ethane/ethene over the three catalytic systems.