Atmospheric Hydrodeoxygenation of Guaiacol over Alumina-, Zirconia-, and Silica-Supported Nickel Phosphide Catalysts

This study investigated atmospheric hydrodeoxygenation (HDO) of guaiacol over Ni2P-supported catalysts. Alumina, zirconia, and silica served as the supports of Ni2P catalysts. The physicochemical properties of these catalysts were surveyed by N-2 physisorption, X-ray diffraction (XRD), CO chemisorption, H-2 temperature-programmed reduction (H-2-TPR), H-2 temperature-programmed desorption (H-2-TPD), and NH3 temperature-programmed desorption (NH3-TPD). The catalytic performance of these catalysts was tested in a continuous fixed-bed system. This paper proposes a plausible network of atmospheric guaiacol HDO, containing demethoxylation (DMO), demethylation (DME), direct deoxygenation (DDO), hydrogenation (HYD), transalkylation, and methylation. Pseudo-first-order kinetics analysis shows that the intrinsic activity declined in the following order: Ni2P/ZrO2 > Ni2P/Al2O3 > Ni2P/SiO2. Product selectivity at zero guaiacol conversion indicates that Ni2P/SiO2 promotes DMO and DDO routes, whereas Ni2P/ZrO2 and Ni2P/Al2O3 enhance DME. These differences were attributed to Ni2P morphologies on these supports: SiO2 hosted small Ni2P particles, which were more active in H-transfer than large Ni2P clusters supported on ZrO2 and Al2O3. Within the first hour of time on-stream testing, Ni2P/SiO2 possessed relatively higher activity than Ni2P/ZrO2 and Ni2P/Al2O3. Low coke accumulation and excess phosphorus, which may replenish the Ni2P phase to maintain its fully phosphided state, were likely responsible for the high activity of Ni2P/SiO2 at the outset.