TAILORING ALUMINA SURFACE-CHEMISTRY FOR EFFICIENT USE OF SUPPORTED MOS2

Reported activity trends for hydrodesulfurization (HDS) over MoS2/gamm a-Al2O3 catalysts show a maximum in activity with Mo loading when acti vity is normalized to Mo content. In contrast, simple monotonic decrea ses in normalized activity are observed over TiO2 and ZrO2 supports. W hile earlier work ascribes these different activity trends to differen ces in MoS2 morphology, activity measurements and transmission electro n microscope images presented here conclusively demonstrate that the t wo different trends can occur on support materials that give rise to v irtually 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 g amma-Al2O3 at low Mo coverages. Reaction of aqueous molybdates with th e 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 t o convert into the active MoS2 form. As a result, normalized HDS activ ity is very low As Mo coverage increases the type I-a OH groups are co nsumed and formation of more easily sulfided molybdate species begins to predominate, and normalized activity increases. Ultimately, normali zed activity goes through a maximum with Mo coverage as the average si ze of the MoS2 platelets begins to grow, resulting in a decrease in th e fraction of Mo atoms located at active edge sites. Since the type-I- a hydroxyls on gamma-Al2O3 are associated with tetrahedrally coordinat ed Al cations, it should be possible to prevent the formation of inact ive molybdates, and thereby eliminate the maximum in activity with cov erage, by removing all tetrahedrally coordinated Al cations from the s urface. This removal has been accomplished through the use of alpha-Al 2O3, which contains only octahedrally coordinated Al atoms, and throug h titration of the type I-a hydroxyls on gamma-Al2O3 with titanium iso propoxide prior to Mo loading. In both cases, no maximum in activity i s observed and activity at all Mo loadings is higher than on gamma-Al2 O3 Fourier transform infrared measurements of OH group consumption cou pled with X-ray photoelectron spectroscopy measurements of molybdate r educibility support the chemical explanation by demonstrating that rea ction of gamma-Al2O3 with titanium isopropoxide preferentially consume s type I-a hydroxyls and that molybdates are more easily reduced on al pha-Al2O3 and titania coated gamma-Al2O3 than on pure gamma-Al2O3. Thu s, titration of type I-a OH groups on gamma-alumina by a suitable modi fier, such as titania, offers a simple method for increasing the overa ll activity of supported MoS2 catalysts, while retaining the advantage ous properties of gamma-Al2O3 supports, such as high surface area and thermal stability. (C) 1998 Academic Press.