Tailored Distribution of MoO3 in the TiO2 and ZrO2 Supported Catalysts by Water-Assisted Spreading
Advancing the chemical engineering fundamentals
Catalysis - I (T2-13a)
Keywords: MoO3/TiO2, MoO3/ZrO2, eggshell catalysts, hydrodesulfurization, solvent-assisted spreading
Luděk Kaluža*, Daniela Gulková, Zdeněk Vít, and Miroslav Zdražil
Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Rozvojová 135, 165 02 Prague 6 – Suchdol, Czech Republic; *Email: kaluza@icpf.cas.cz
Catalysts MoO3 supported on TiO2 and ZrO2 are studied because of their activity in various industrially important reactions, such as hydrodesulfurization, partial oxidation of methanol, vapor-phase ammoxidation of toluene, the water-gas shift reaction, epoxidation of allyl acetate with tert.-butyl hydroperoxide, oxidation of 1-butene and butadiene, and selective catalytic reduction of NOx by NH3. A controlled distribution of MoO3, eggshell/uniform, in the TiO2 and ZrO2 is both of theoretical and practical interest. The eggshell catalysts find application when a rate-determining step is internal diffusion. A desirable reaction product is transported from the eggshell structure more easily than from the interior of the catalyst particles preventing it from further and unwanted consecutive reactions.
A new method proposed here for preparation both eggshell and uniformly distributed MoO3 titania and zirconia supported catalysts is based on reaction (spreading) of MoO3 (Fluka, activated in a planetary mill for 27 h) with the support (TiO2 of S.A. 140 m2g-1, or ZrO2 of S.A. 108 m2g-1; both 1/8” pellets, AlphaAesar, Germany) in water. Points of zero charge 5.5 and 6.0 of the TiO2 and ZrO2, respectively, were found to be well above the pH 2.1 of the aqueous impregnation slurry of MoO3. An adsorption of dissolved Mo anionic species was therefore enhanced. The dissolved species that penetrated the pellets of support adsorbed there to from gradually saturated adsorption monolayer of MoO3. After filling that monolayer, the impregnation continued deeper into the carrier up to the point where uniform distribution throughout the supports was achieved. A thickness of a desired eggshell is easy to control either by amount of MoO3 or by reaction time. The process of water?assisted spreading was followed by electron probe microanalysis and visually after samples sulfidation (white MoO3 deposited was transformed to black MoS2, supports remained white). The MoO3 concentration in the eggshell of TiO2 pellets, for example, was high, about 9-10 wt.%. The content of MoO3 in the saturated pellets with the uniform MoO3 distribution was additionally determined by chemical analysis AAS and was about 10 and 7.6 wt% in MoO3/TiO2 and MoO3/ZrO2 catalyst, respectively. These values corresponded to the density of a saturated monolayer of 3.3 and 3.2 atoms Mo per nm2 of TiO2 and ZrO2, respectively, which was closed to the value 3.4 atoms Mo nm-2 obtained previously with MoO3/Al2O3 system. It was also found that high temperature speed up the water-assisted spreading. For example, uniform saturation of whole TiO2 pellets was achieved after 45 h at 95 °C, whereas the eggshell of about 0.6 mm thick was obtained at 25 °C after 52 h. The spreading over ZrO2 pellets was considerably slower probably due to its lower S.A. in comparison with TiO2. Uniform Mo distribution was received after 20-day reaction including 75-hour heating at 95 °C.
Activity of the prepared and reference catalysts were tested in hydrodeslufurization (HDS) of thiophene after presulfiding in H2S/H2 mixture. The uniformly saturated pellets were crushed to particle size 0.16-0.32 mm. Particles 0.16-0.32 mm of both carriers were also impregnated with (NH4)6Mo7O24 for comparison. It was determined that the samples obtained by solvent-assisted spreading were at least as active as the sample impregnated conventionally. This confirmed good dispersion of molybdena species achieved by water-assisted spreading. MoO3/TiO2 and MoO3/ZrO2 catalysts exhibited relative HDS activity, normalized per gram of reference MoO3/Al2O3 catalyst (15wt.% MoO3, S.A. 210 m2g-1, BASF M8-30, Germany), of about 1.5 and 1.0, respectively.
Financial support of the Grant Agency of the Czech Republic is gratefully acknowledged. L.K. acknowledges Grant No. 104/06/P034 and D.G., Z.V., and M.Z. acknowledge Grant No. 104/06/0705.
See the full pdf manuscript of the abstract.
Presented Thursday 20, 10:05 to 10:25, in session Catalysis - I (T2-13a).
