Tailoring alumina surface chemistry for efficient use of supported MoS2

Reported activity trends for hydrodesulfurization (HDS) over MoS2/gamma-Al2O3 catalysts show a maximum in activity with Mo loading when activity is normalized to Mo content. In contrast, simple monotonic decreases in normalized activity are observed over TiO2 and ZrO2 supports. While earlier work ascribes these different activity trends to differences in MoS2 morphology, activity measurements and transmission electron microscope images presented here conclusively demonstrate that the two different trends can occur on support materials that give rise to virtually identical MoS2 morphologies. Since differences in morphology cannot explain this result, we instead propose the following chemical explanation involving the formation of inactive molybdate species on gamma-Al2O3 at low Mo coverages. Reaction of aqueous molybdates with the highest frequency, or type I-a, OH groups on gamma-Al2O3 is known to form stable MoO42- species at low Mo coverages, which are difficult to convert into the active MoS2 form. As a result, normalized HDS activity is very low As Mo coverage increases the type I-a OH groups are consumed and formation of more easily sulfided molybdate species begins to predominate, and normalized activity increases. Ultimately, normalized activity goes through a maximum with Mo coverage as the average size of the MoS2 platelets begins to grow, resulting in a decrease in the fraction of Mo atoms located at active edge sites. Since the type-I-a hydroxyls on gamma-Al2O3 are associated with tetrahedrally coordinated Al cations, it should be possible to prevent the formation of inactive molybdates, and thereby eliminate the maximum in activity with coverage, by removing all tetrahedrally coordinated Al cations from the surface. This removal has been accomplished through the use of alpha-Al2O3, which contains only octahedrally coordinated Al atoms, and through titration of the type I-a hydroxyls on gamma-Al2O3 with titanium isopropoxide prior to Mo loading. In both cases, no maximum in activity is observed and activity at all Mo loadings is higher than on gamma-Al2O3 Fourier transform infrared measurements of OH group consumption coupled with X-ray photoelectron spectroscopy measurements of molybdate reducibility support the chemical explanation by demonstrating that reaction of gamma-Al2O3 with titanium isopropoxide preferentially consumes type I-a hydroxyls and that molybdates are more easily reduced on alpha-Al2O3 and titania coated gamma-Al2O3 than on pure gamma-Al2O3. Thus, titration of type I-a OH groups on gamma-alumina by a suitable modifier, such as titania, offers a simple method for increasing the overall activity of supported MoS2 catalysts, while retaining the advantageous properties of gamma-Al2O3 supports, such as high surface area and thermal stability. (C) 1998 Academic Press.