Hydrotreating of Bio-Oils over Metal-Phosphide Catalysts
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A concern over the impact of fossil fuels on the environment has made the researchers to look for more environmental options. One alternative is bio-oils produced from lignocellulosic material. However, the abundance of oxygen decreases the quality of the oil, and it must therefore be removed before it can serve as a fuel. Bio-oil upgrading can be achieved by catalytic hydrodeoxygenation, a process where the oxygen containing compounds are converted to hydrocarbons by removing the oxygen in presence of hydrogen. The aim of this work was to study the hydrodeoxygenation of phenol as model compound for bio-oils on supported molybdenum phosphide (MoP) catalysts. A total of 8 catalysts were prepared by incipient wetness impregnation and temperature programmed reduction; Molybdenum phosphide catalysts deposited on alumina (Al2O3), titania (TiO2), silica (SiO2) and zirconia (ZrO2) with two different loadings (6.8 and 15 wt %). The prepared catalysts were characterized, both prior and after reduction, by BET-surface area measurements, X-ray diffraction, temperature programmed reduction, and CO chemisorption. A bulk MoP catalyst was prepared for comparison purposes. Hydrodeoxygenation activity was tested on six catalysts; 6.8 and 15 wt % MoP/SiO2, 6.8 and 15 wt % /Al2O3, 6.8 wt % MoP/TiO2 and 6.8 wt% MoP/ZrO2. At 623 K and 25 bar, the silica supported catalysts were inactive, while the remaining catalysts followed the order 6.8 wt % MoP/TiO2, 6.8 wt% MoP/ZrO2>6.8 wt % MoP/Al2O3>15 wt % MoP/Al2O3. Through surface area measurements it was found that the preparation and metal loading of the supported catalysts had a significant impact on the pore structure and surface area. Temperature programmed reduction confirmed a complete reduction of the synthesized alumina, titania and zirconia supported catalysts. However, X-ray diffraction patterns of the catalysts did not confirm the formation of MoP. It was concluded that the active phases in the most active catalysts, including the lower loading molybdenum phosphides on zirconia, titania and alumina, were well dispersed. Hydrodeoxygenation of phenol on metal phosphides follows the direct hydrogenolysis pathway. The results demonstrated in this work indicate that the molybdenum phosphides, and in particular deposited on zirconia and titania, are promising for the upgrading of bio-oils. However, further development of the experimental procedure and repetition of the experiments is necessary in order to validate the results from the activity measurements demonstrated in this work.