Kinetic investigation of phenol hydrodeoxygenation over unsupported nickel phosphides

Three representative nickel phosphides (Ni2P, Ni12P5 and Ni3P) were prepared and characterized comparatively by means of XRD, N2 adsorption-desorption, ICP, TEM, XPS and NH3-TPD. It is shown that the binding energy ascribed to Niδ+ in nickel phosphide decreased, whereas that of Pδ− increased, as the Ni/P molar ratio increased from 2 (Ni2P) to 2.4 (Ni12P5) and to 3 (Ni3P). It is indicated that the slight transfer of electron density from Ni to P atoms decreased from Ni2P, Ni12P5 to Ni3P. NH3-TPD measurements revealed that the total acid amounts of the nickel phosphides decreased with increasing Ni/P molar ratio. Their catalytic performance in hydrodeoxygenation (HDO) were investigated using phenol in decalin (1.0 wt%) as the feed. It is found that phenol was readily hydrogenated to yield cyclohexanol over all the three unsupported nickel phosphides at low temperatures and the subsequent HDO of cyclohexanol was rate-determining. As a result, the overall kinetics of phenol HDO were investigated by measuring the rates of phenol HDO and cyclohexanol HDO. The activation energy for phenol hydrogenation decreased in the order Ni2P > Ni12P5 > Ni3P, whereas that for cyclohexanol dehydration increased.