Comparative study of hydrodeoxygenation performance over Ni and Ni2P catalysts for upgrading of lignin-derived phenolic compound

To deeply understand the different hydrodeoxygenation (HDO) behavior of the monometallic Ni and Ni2P catalysts in HDO processes, the Ni/SiO2 and Ni2P/SiO2 catalysts were prepared, respectively. The as-prepared catalysts were characterized by X-ray diffraction (XRD), N2 adsorption–desorption, X-ray photoelectron spectra (XPS), H2 temperature-programmed reduction (H2-TPR), transmission electron microscope (TEM), NH3 temperature-programmed desorption (NH3-TPD), and CO uptakes. Taking the lignin-derived phenolic compound as raw material, the HDO performance and reaction route of the monometallic Ni and Ni2P catalysts were compared. The results showed that the phosphorization contributed to highly dispersion of active sites, formation of smaller active sites and significantly enhanced acid sites, showing Ni2P/SiO2 catalyst is a bifunctional metal–acid catalyst. For both of the Ni/SiO2 and Ni2P/SiO2 catalysts, the HDO of m-cresol mainly proceeded by the hydrogenation (HYD) step. However, the acid sites on Ni2P/SiO2 catalyst can significantly promote the conversion of oxygenated 3-methylcyclohexanol (MCHnol) to deoxygenated methylcyclohexane (MCH) owing to its strong hydrogenolysis of C–OH bond originated from the bifunctional metal–acid sites. In addition, the unique electron structure of Ni2P phase involved in two different types of sites (tetrahedral Ni(I) and square pyramidal Ni(II)) and the highly dispersed smaller Ni2P particles also contributed to the outstanding HDO performance. Density functional calculation (DFT) calculations clarified that the Ni2P active phase possessed prior ability to cleavage of C–OH bond in comparison with the metallic Ni species. As a result, Ni2P/SiO2 catalyst showed a significantly higher selectivity to MCH (96.3 %) than that of Ni/SiO2 (14.1 %) catalyst at the conditions of 250 ℃ for 240 min.