Bimetallic Nb−Mo Carbide Hydroprocessing Catalysts: Synthesis, Characterization, and Activity Studies

Abstract

A series of Nb1.0MoxOC (x = 0.67−2.0) catalysts were prepared by a temperature-programmed reaction technique. The catalysts were synthesized from oxide precursors in a flow of 20% CH4/H2 reactant gas mixture, while the temperature was increased linearly at 5 K/min (8.3 × 10-2 K s-1). The samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), elemental analysis, CO chemisorption, surface area measurements, and temperature-programmed reduction. XRD patterns of the fresh catalysts indicated that Nb1.0Mo1.5OC and Nb1.0Mo1.75OC consisted of pure bimetallic carbide phases, while the other compositions showed impurity phases of NbO2 or Mo2C at high concentrations of Nb and Mo, respectively, in the starting oxide. The hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) activity of these materials was studied in a high-pressure reactor system. The reactions were carried out at 3.1 MPa and 643 K using model liquid compounds containing moderate concentrations of sulfur, nitrogen, oxygen, and aromatics. All the catalysts were found to be active for quinoline HDN, and the activity did not show much variation with changes in the ratio of the two metals (Mo/Nb). However, the HDS activity was found to be more sensitive to the composition (Mo/Nb) and Nb1.0Mo1.75OC showed the highest HDS activity among the catalysts tested. The bimetallic compounds showed enhancement in the activity and stability compared to the corresponding monometallic carbides. X-ray diffraction patterns of the spent catalysts did not show any sulfide, oxide, or metal peaks, indicating that the catalysts were stable and tolerant of sulfur.