Flame spray synthesis of CoMo/Al2O3 hydrotreating catalysts

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 32wt.% Mo with atomic ratio Co/Mo=1/3 and 16wt.% 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-Al2O3 with some CoAl2O4, as evidenced by X-ray diffraction (XRD) and UV–vis spectroscopy. Bulk MoO3 was not detected by XRD, except at the highest molybdenum content (32wt.%) 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 90m2/g when going from the lowest loading supported catalyst (8wt.%) to the unsupported reference. Transmission electron microscopy (TEM) images showed that at low molybdenum loadings nanoparticle agglomerates with 5–10nm primary particles were produced.

As the molybdenum loading on the alumina was increased from 8 to 32wt.% and for the unsupported reference the primary particle size increased to up to 20nm 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 16wt.% Mo (atomic ratio Co/Mo=1/3), which did not contain crystalline MoO3 and only small amounts of CoAl2O4. 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 32wt.% Mo the average length of supported MoS2 entities increased from 3 to 4nm (for the unsupported catalyst it was 8.5nm), while the average number of MoS2 layers per particle increased from 1.1 to 2.5.

The increase in MoS2 particle size resulted in lower activity.