Influence of the synthesis method on the nanostructure and reactivity of mesoporous Pt/Mn-WO x-ZrO 2 catalysts

Abstract

The influence of the synthesis method on acid site generation, nanostructure and reactivity of Mn-promoted WO x -ZrO 2 catalysts was investigated. The materials were synthesized by impregnation (IMP), coprecipitation (COP) and reflux (COP-R and IMP-R), using cetyl-trimethylammonium bromide (CTAB) as surfactant in order to promote the textural properties of the catalysts. The Mn-WO x -ZrO 2 samples were calcined at 800 °C and characterized by X-ray diffraction (XRD), nitrogen adsorption–desorption, laser Raman spectroscopy, ultraviolet–visible (UV–vis) spectroscopy, high-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS). The Mn-WO x -ZrO 2 materials were impregnated with 0.3 wt% of platinum (Pt/Mn-WO x -ZrO 2) in order to enhance H 2 spillover and diminish coke formation and they were evaluated for n-hexane hydroisomerization. The XRD results show that the incorporation of 0.5 wt% Mn cation stabilizes the metastable tetragonal zirconia phase and the Rietveld refinement of the crystalline phases shows that only a fraction of tungsten (about 5 wt%) leads to the formation of the WO 3 segregated phase. Laser Raman and UV–vis spectroscopies verify the presence of different tungsten oxide domains, WO 3 crystallites and a WO x phase not detected by XRD formed by oligomeric and polymeric WO x clusters. The surfactant assisted methods produce mesoporous materials with narrow pore size distributions, allowing access to the WO x clusters of suitable domain size for the hydroisomerization reaction. The simultaneous precipitation of tungsten with the formation of hydrous zirconia and manganese generates a larger number of active sites by the formation of polytungstate species on the zirconia surface with sizes and structures suitable for the isomerization of alkenes. Additionally, the coprecipitation-reflux method (COP-R) results in the formation of WO x species with W O bonds which generates acid sites suitable and accessible to the n-hexane molecule, leading to an increase in the conversion (70%) and selectivity to the high octane biramified products 2,3-dimethyl-butane (2,3-DMB) and 2,2-dimethyl-butane (2,2-DMB). It was found that besides the role of Mn in the stabilization of the tetragonal phase, this cation promotes the catalytic activity of these materials.