Rogelio Cuevas

Characterization of Al2O3-ZrO2 mixed oxide catalytic supports prepared by the sol-gel method

Abstract A series of Al 2 O 3 -ZrO 2 mixed oxides was prepared by the sol-gel method with variable amounts of ZrO 2 between pure alumina and pure zirconia. Textural, bulk and surface characterization of the samples was carried out by nitrogen physisorption ( S BET , porosity), surface acidity, zero point charge (ZPC), thermal analysis (DSC, TGA), X-ray diffraction (XRD) and FT-Raman spectroscopy. The textural results show that at low zirconia contents, higher surface areas than those of pure alumina are obtained, and that the mixed oxides samples show a bimodal pore size distribution different from that of a mechanical mixture of the pure oxides. Also, in the zirconia-rich samples, higher surface areas than for pure zirconia are stabilized. The ZPC results indicate the formation of a surface composition equivalent to the bulk composition of the two oxides. The acidity measurements show that, as the density of acid sites in the mixed oxides increases steadily with zirconia content, a sharp increase is observed between the zirconia-rich mixed oxides and the pure ZrO 2 . It appears possible then to tune the acidity of the mixed oxide by changing its composition. The XRD and FT-Raman results show that the incorporation of alumina in the support stabilizes the metastable cubic and tetragonal zirconia phases, possibly by a matrix effect which constrains the size of the ZrO 2 particles below the critical size beyond which the crystallization and transformation process to a more stable zirconia phase occurs.

Effect of boron addition on the activity and selectivity of hydrotreating CoMo/Al2O3 catalysts

Abstract The influence of the incorporation of different amounts of boron ( x =0.0, 0.2, 0.8, 1.5 and 2.0 wt.-%B) to CoMo/Al 2 O 3 catalysts on the thiophene hydrodesulfurization catalytic activity and chemical state of the CoMo/Al 2 O 3 -B( x ) catalysts has been investigated by UV-vis diffuse reflectance spectroscopy, X-ray diffraction spectroscopy, scanning electron microscopy, FT-IR of adsorbed nitric oxide, temperature-programmed reduction and acidity measurements. The results indicate that boria is deposited as a monolayer on the surface of the alumina up to a loading of 0.8% B. The incorporation of boron into the catalyst supports leads to a change in the distribution of the cobalt and molybdenum oxidic species which results in a decrease in the proportion of tetrahedrally coordinated Co 2+ and Mo 6+ species in strong interaction with the alumina support. As a result of this change in species in the oxidic precursors, the number of cobalt sites probed by nitric oxide in the sulfided catalysts increases with boron contents, while the molybdenum sites remain practically unchanged. The net result is a beneficial effect on the catalytic activity of the thiophene hydrodesulfurization reaction which reaches a maximum at a boron content of 0.8 wt.-% B.

Effect of the support porosity on the thiophene and dibenzothiophene hydrodesulfurization reactions. Al2O3-TiO2 mixed oxide support

A series of Mo and NiMo catalysts supported on Al2O3-TiO2 mixed oxides, with the same composition (Al2O3:TiO2 molar ratio=1:1) and different pore structures, was prepared and tested in the thiophene and dibenzothiophene hydrodesulfurization reactions. It was shown that for DBT and even for simple small model molecules such as thiophene, changes in the pore structure can significantly alter the catalyst formulation performance in terms of conversion and even induce some small changes in product distribution.