Flame spray synthesis of CoMo/Al 2O 3 hydrotreating catalysts

Abstract

The first alumina supported and unsupported cobalt molybdenum hydrotreating catalysts have been prepared by one-step flame spray pyrolysis (FSP) by spraying and combusting tris(acetylacetonato)aluminum, cobalt 2-ethylhexanoate and molybdenum 2-ethylhexaoate dissolved in toluene. The oxide particles produced contained varying amounts of transition metals (8, 16, 24 and 32 wt.% Mo with atomic ratio Co/Mo = 1/3 and 16 wt.% Mo with atomic ratios Co/Mo = 2/3 and 1/1) with alumina constituting the balance. In addition, an unsupported reference catalyst (atomic ratio Co/Mo = 1/3) was produced. The particles obtained consisted mostly of γ-Al 2O 3 with some CoAl 2O 4, as evidenced by X-ray diffraction (XRD) and UV–vis spectroscopy. Bulk MoO 3 was not detected by XRD, except at the highest molybdenum content (32 wt.%) and in the unsupported sample, indicating that molybdenum is well dispersed on the surface of the support. The specific surface area as measured by nitrogen adsorption (BET) decreased from 221 to 90 m 2/g when going from the lowest loading supported catalyst (8 wt.%) to the unsupported reference. Transmission electron microscopy (TEM) images showed that at low molybdenum loadings nanoparticle agglomerates with 5–10 nm primary particles were produced. As the molybdenum loading on the alumina was increased from 8 to 32 wt.% and for the unsupported reference the primary particle size increased to up to 20 nm and the morphology became more irregular due to primary particle sintering and aggregation. After activation by sulfidation the activity of the catalysts were measured for the three hydrotreating reactions hydrodesulfurization, hydrodenitrogenation and hydrogenation using a model oil containing dibenzothiophene, indole and naphthalene in n-heptane solution. The best catalyst was the FSP-produced material containing 16 wt.% Mo (atomic ratio Co/Mo = 1/3), which did not contain crystalline MoO 3 and only small amounts of CoAl 2O 4. The hydrotreating activity was approximately 75% of that of commercial cobalt molybdenum catalysts prepared by wet impregnation of pre-shaped alumina extrudates. Since the commercial catalyst is the product of years of development, this shows the potential of the flame spray pyrolysis technique. The Co–Mo–S phase, active for hydrotreating, is formed upon sulfidation of the flame made oxide precursor. TEM images of the spent catalysts showed that as the metal loading was increased from 8 to 32 wt.% Mo the average length of supported MoS 2 entities increased from 3 to 4 nm (for the unsupported catalyst it was 8.5 nm), while the average number of MoS 2 layers per particle increased from 1.1 to 2.5. The increase in MoS 2 particle size resulted in lower activity.