Estudo do método de mistura das suspensões precipitadas (MPS) para a síntese de catalisadores de tungstênio suportado em zircônia (WOx/ZrO2)

Abstract

Zirconia-supported tungsten oxide (WOx/ZrO2) is one of the main acid catalysts. The development of new methods for its preparation has been widely studied lately. This thesis presents a study of a new synthesis method, named Mixed Precipited Suspension (MPS), to prepare WOx/ZrO2 catalysts. This method consists in the mix of the precipitated suspensions containing the elements of interest, appropriate homogeneity during the mixing step and activation by heat treatment. The dependence of the catalyst properties were investigated as function of synthesis conditions (time and temperature during the mixing step), tungsten content and calcination temperature. The catalysts were characterized and their activity evaluated for oxidative desulfurization (ODS) of model fuel containing dibenzothiophene (DBT). Results show that high mixing time of the suspensions increase the tetragonal ZrO2 phase concentration and the surface area of the catalysts. Furthermore, an improvement of the activity is observed in the catalysts obtained after larger mixing time, which was attributed to the high dispersed distorted Zr-WOx clusters, polytungstate species and no formation of crystalline WO3. The presence of tungstite and the increase of the temperature during the mixing step are prejudicial to achieve highly active catalysts. In addition, the increase in the WOx surface density to values above the monolayer surface coverage (4.5 W nm-2) aid the formation of active species and, consequently, catalysts more effective. Moreover, MPS method manteined the nominal composition and prepared catalysts with higher activity than those obtained by impregnation, for all calcination temperature studied. These results demonstrate the potentiality of MPS method for preparing WOx/ZrO2 catalysts.