Synthesis and hydrodesulfurization properties of silica-supported nickel-ruthenium phosphide catalysts

Abstract

A series of Ni2-xRuxP/SiO2 catalysts spanning the full composition space (0 ≤ x ≤ 2) were prepared from hypophosphite-based precursors (P/M = 0.72, M = Ni + Ru). The Ni2-xRuxP/SiO2 catalysts having Ni-rich compositions (x < 1.00) formed solid solutions as determined by X-ray diffraction. Temperature-programmed reduction (TPR-MS) measurements revealed that a primary role of the incorporated Ru in the catalyst synthesis was to lower the temperature of precursor reduction, while increasing the extent of P retained in the supported Ni2-xRuxP phase. Bulk and surface composition measurements showed a trend of increasing P/M molar ratios for Ni- and Ru-rich compositions, with only the Ni2P/SiO2 catalyst having a P deficient surface. The 4,6-DMDBT hydrodesulfurization (HDS) activity and selectivity showed a strong dependence on Ru content, with a Ni1.85Ru0.15P/SiO2 catalyst 15 and 6 times more active (on a mass basis) than Ni2P/SiO2 and Ru2P/SiO2 catalysts, respectively. A similar trend was observed for the turnover frequencies, but an important finding was that although the Ni1.85Ru0.15P/SiO2 catalyst was 1.3 times more active than an optimized Ni2P/SiO2 catalyst (P/Ni = 1.5) on a mass basis, it had a two-fold lower TOF for 4,6-DMDBT HDS. Incorporation of low Ru contents resulted in significant increases in selectivity for products of the hydrogenation desulfurization pathway (86.2% for Ni1.85Ru0.15P/SiO2) compared to Ni2P/SiO2 (64.2%). The primary role of incorporated Ru in enhancing the HDS properties of the Ni2-xRuxP/SiO2 catalysts was surface P enrichment, but the possibility of Ru modification of the active Ni sites in the phosphide catalysts could not be excluded.