Synergy effect in the HDO of phenol over Ni–W catalysts supported on active carbon: Effect of tungsten precursors

Abstract

The aim of this study was to detect any synergistic effect between nickel and tungsten oxide species supported on activated carbon as well as to study the effect of tungsten precursor (silicotungstic (HSiW), phosphotungstic (HPW), and tungstic (HW) acids) on the activity of partially reduced Ni–W/AC catalysts in phenol hydrodeoxygenation (HDO). The catalytic tests were performed in a flow fixed-bed reactor set-up at a hydrogen pressure of 1.5MPa, temperatures ranging from 423 to 573K and weight hourly space velocities of 0.5gphenol/(gcatalysth). Before reaction, the catalysts were heat treated in H2 flow at soft conditions (T=673K for 4h). The XPS analysis of spent catalysts revealed that this pre-treatment led to W(VI) species and a mixture of metallic and NiO species. Moreover, XPS and HRTEM analyses of the spent catalysts indicated that the nickel incorporation into W/AC catalysts led to a substantial improvement in surface exposure of the metal oxide species. All catalysts were active in the HDO of phenol and the most active catalysts were those prepared from heteropolyacids (HPAs). Regardless of the tungsten precursor, after Ni incorporation to the base W/AC system, a large enhancement in catalyst performance was observed. Moreover, supporting Ni and W phases on AC induced some beneficial effect because of the lower coke formation on the surface of AC with respect to classical alumina support. Regardless of the reaction temperature, analysis of the products suggests that reaction occurs via two direction pathways: (I) a hydrogenation (HYD) of phenol's aromatics ring (main reaction route) leading to formation of cylohexane, cyclohexene, cyclohexanol and methylcyclopentane and (II) a direct cleavage of the C–O σ bond leading to benzene formation. Total elimination of the O-containing compounds was archived in the HDO reaction at 573K over oxide Ni–W(P)/AC and Ni–W(Si)/AC catalysts.