Hydrodeoxygenation of Anisole over Silica-Supported Ni₂P, MoP, and NiMoP Catalysts

Abstract

Ni2P/SiO2, MoP/SiO2, and NiMoP/SiO2 with different Ni/Mo molar ratios were prepared by temperature-programmed reduction (TPR). Their structural properties were characterized by N2 sorption, X-ray diffraction (XRD), CO chemisorption, X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed desorption (H2-TPD), and NH3 temperature-programmed desorption (NH3-TPD). Their performances for the hydrodeoxygenation (HDO) of anisole were tested in a fixed-bed reactor. It was found that there were mainly three reactions that occurred during the HDO, i.e., the demethylation of anisole, the hydrogenolysis of phenol, and the hydrogenation of benzene. The HDO activities decreased in the sequence of Ni2P/SiO2 > NiMoP/SiO2 > MoP/SiO2. The NiMoP/SiO2 catalysts with larger Ni/Mo ratios had higher activities. In the phosphides, the Niδ+ and Moδ+ sites bearing small positive charges acted not only as Lewis acid sites for the demethylation but also as metal sites for the hydrogenolysis and hydrogenation. The Niδ+ site was more active than the Moδ+ site, and there was no synergy between the Niδ+ and Moδ+ sites. The superior activity of Ni2P to that of MoP is attributed to the higher d electron density in Ni2P. PO−H groups, which acted as Brønsted sites and provided active hydrogen species, had less activity for the three reactions compared to the metal sites. In comparison to a conventional NiMo/γ-Al2O3 catalyst, the Ni phosphide-containing catalysts had much higher activities. The catalyst deactivation due to water was preliminarily discussed. The oxidation of phosphide by water might lead to the formation of metal oxide and/or phosphate, leading to the catalyst deactivation. The high stability of Ni2P/SiO2 may be related to the ligand effect of P that lowers the electron density of Ni and inhibits the Ni−O combination.