Bifunctional nature of a SiO2-supported Ni2P catalyst for hydrotreating: EXAFS and FTIR studies

A Ni2P catalyst supported on a high-surface area SiO2 (350 m2 g−1) was prepared by temperature-programmed reduction, and its structural and surface properties were studied. X-ray diffraction and extended X-ray absorption fine structure measurements were used to obtain structural parameters for the supported Ni2P phase, and Fourier transform infrared (FTIR) analysis with the probe molecules CO and pyridine was carried out to characterize the surface properties. The catalytic activity was measured at 573 K and 3.1 MPa in a three-phase fixed-bed reactor for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) using a model liquid feed. At standard conditions using 500 ppm S as 4,6-dimethyldibenzothiophene (4,6-DMDBT), 3000 ppm S as dimethyldisulfide, 200 ppm N as quinoline, and 1% tetralin in a tridecane solvent, the Ni2P/SiO2 gave an HDS conversion of 85%, an HDN conversion of 100%, and a tetralin conversion of 37%, which were much higher than those of a commercial Ni–Mo–S/Al2O3 catalyst, which gave an HDS conversion of 41%, an HDN conversion of 98%, and a tetralin conversion of 20% based on equal numbers of sites (240 μmol) loaded in the reactor. The sites were counted by CO chemisorption for the phosphide and by low-temperature O2 chemisorption for the sulfide. The Ni2P/SiO2 catalyst favored the hydrogenation (HYD) pathway for 4,6-DMDBT HDS to generate methylcyclohexyltoluene and dimethylbicyclohexane with a relative HYD selectivity of 95%. It also favored hydrogenation for tetralin to give decalin with a relative HYD selectivity of 89%. The Ni2P/SiO2 catalyst also showed better resistance to N-compounds than the Ni–Mo–S/Al2O3 catalyst. The FTIR spectra of adsorbed CO showed that the Ni site in the Ni2P phase gave rise to considerable π-back bonding, which was related to the high activity of the Ni2P/SiO2 catalyst in the hydrogenation of aromatics. The FTIR spectra of adsorbed pyridine showed that the Ni2P phase had a POH group associated with Brønsted acidity that was active for the protonation of N compounds. These results thus suggest that the supported Ni2P catalyst has bifunctional properties that are beneficial for catalytic activity in hydroprocessing.