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Abstract: Active, inactive, coked and resulfated SZ and Pt/SZ samples were investigated by XRD, TG, TPE, TPD and IR measurements. As test reaction conversion of n-heptane at 200°C and atmospheric pressure was chosen. The active catalysts exhibit the pure tetragonal phase, have a sulfate content corresponding to a surface coverage of more than half a monolayer and show Brønsted acidity. It is assumed that the active species consist of pyrosulfate groups, which can oxidize hydrogen (alkanes) to water (and alkenes) by decomposing into sulfate groups and adsorbed SO2. The atmosphere of the first activation step of Pt/SZ does not seem to have any influence on the performance of Pt/SZ. For regeneration an oxidative atmosphere, a temperature of 500°C and reduction in hydrogen at 200°C is necessary to restore the catalytic activity of Pt/SZ completely. If, however, regeneration of Pt/SZ is done in He or N2, a remarkable loss of SO2 was observed and the catalyst became irreversibly inactive. The sulfur species, which are more weakly bonded to the surface after a reductive step are evolved at lower temperatures (between 300 and 600°C) in an inert gas atmosphere. This species are essential for catalytic activity, whereas the sulfate groups, which are removed at temperatures higher than 600°C, are inactive for n-alkane conversion. The inactive SZ and Pt/SZ samples exhibit the tetragonal and monoclinic structure, have lost approximately 40% of their sulfate groups and possessed only Lewis acid sites. If an inactive Pt/SZ sample was resulfated Brønsted acidity was regenerated and the sample was active, however to a lesser extent than a fresh catalyst. The changes occurring during coking did not affect the textural properties; no changes of the crystal structure were detected by XRD. Almost all coking was observed in the first minutes of reaction. Regeneration in air reestablished the activity whereas regeneration in an inert atmosphere led to a loss of sulfate groups and therefore to inactivity. [Copyright &y& Elsevier]

Abstract: Sulfated zirconias (SZ) with different sulfur content were prepared by controlled impregnation of zirconium hydroxide with corresponding amounts of 1N H2SO4. After drying the samples were impregnated with 2.5 wt% Pt. All samples exhibited pure tetragonal structure except one with the lowest sulfur content where small amounts of the monoclinic ZrO2 phase were detected. Temperature-programmed desorption of ammonia and pyridine adsorption followed by IR measurements were carried out in order to determine the nature of acid sites. It was found that the concentration of Brønsted acid sites depends on the concentration of surface sulfates. On samples with more than half of a monolayer sulfates Brønsted acid sites were detected. On SZ with 46% of a monolayer sulfates and on a sample, which was inactive after regeneration in an inert atmosphere only Lewis acid sites were observed. The n-heptane conversion was studied at 473K and atmospheric pressure. SZ catalysts were only active when Brønsted acid sites were present on the surface. The activity increased with increasing sulfate content whereas the isomerization selectivity decreased. [Copyright &y& Elsevier]

In situ infrared spectroscopy (IR), thermogravimetric measurements (TG) and temperature-programmed evolution of gases (TPE) have been used to study activation and regeneration of sulfated zirconia with and without Pt (SZ, Pt/SZ). Isomerization and cracking of n-heptane was used as test reaction. The first activation of Pt/SZ is independent of the atmosphere, air or He and N2, in each case the catalyst was active. But it was necessary to regenerate the catalyst in air to obtain the initial activity and selectivity. When regeneration was done in helium or nitrogen the catalyst lost its acid properties and only hydrogenolysis of n-heptane to propane and n-butane was observed. This can be explained by a loss of a part of sulfates between 400 and 500 °C measured by TG and TPE. In the IR spectrum the intensity of the SO band at 1400 cm-1 decreased and shifted to lower wavenumbers whereas the band of terminal Zr–OH groups at 3740 cm-1, which was absent in the first spectrum after activation increased. The activation conditions of SZ are independent of the atmosphere, no loss of sulfate groups was observed. The reason for deactivation of SZ is coke formation during alkane conversion. [ABSTRACT FROM AUTHOR]