Influences of some synthesis parameters and activation procedures on the one-step sol-gel synthesis of sulfated-zirconia catalysts, followed by TG-DSC and mass spectrometry

A series of sulfated zirconia (SZ) were prepared by sol-gel methods, from Zr n-propoxide, either with the in situ one-step addition of H2SO4 during the gelification process, or by ex situ sulfation of a Zr(OH)(4) hydroxide. Several parameters were studied for the in situ sol-gel preparation: (i) the H2O/Zr-IV hydrolysis ratio, (ii) the protocol followed for the sulfate introduction, and (iii) the amount of sulfate (6 to 30%) added. The activation of SZ was followed by TG-DSC and mass spectrometry detection. The textural properties of SZ are dependent on both the hydrolysis ratio and the procedure for H2SO4 introduction. SZ prepared when introducing H2SO4 through hydrolysis water exhibited a narrow distribution of mesopores centered around 2-3 nm and intergranular macroporosity which increased with the hydrolysis ratio. SZ prepared by introducing H2SO4 through Zr n-propoxide solution exhibited a wide distribution of large mesopores from 3 to 20 nm. In all cases the introduction of sulfate delayed the crystallization into the tetragonal phase. The endothermic decomposition of sulfate species occurs at the beginning of the tetragonal to monoclinic transition. Their catalytic properties were evaluated in the hydroconversion of n-hexane using mechanical mixtures of SZ and Pt/Al2O3. SZ's prepared by in situ methods are the most active catalysts. The late became maximum for a S content close to a nominal monolayer of sulfate after activation at ca 900 K in air. The SZ thus prepared, by in situ methods, were twentyfold more active, per nominal sulfate site, than SZ prepared by ex situ sulfation. This higher reactivity was attributed to acid sites of the same type as previously proposed by Kustov et al. (J. Catal. 150, 143 (1994)). (C) 1998 Academic Press.