Comparison of the promotion effects on sulfated mesoporous zirconia catalysts achieved by alumina and gallium

Mesoporous zirconia, hydrothermally synthesized from surfactant templating, was directly impregnated with aluminum sulfate or gallium sulfate to give the solid acids, Al- and Ga-promoted sulfated mesoporous zirconia catalysts (AS/MP-ZrO₂ and GS/MP-ZrO₂). The promotional effects achieved by these two catalysts for the n-butane isomerization were compared to clarify the differences in mechanism in catalytic activities in butane isomerization. The catalysis was found to be strongly promoted by the addition of a proper amount of alumina or gallium. The activities of Ga-promoted catalysts were higher than those of Al-promoted catalysts. Diffuse-reflectance infrared Fourier-transformed (DRIFT) spectra and temperature programmed desorption mass (TPD-MS) analyses were employed to identify and quantify properties of the acid sites on catalyst surfaces. The 3AS/MP-ZrO₂ catalyst shows a slightly higher acidity in strength and amount than that of 3GS/MP-ZrO₂, but much higher than that of the unpromoted 3S/MP-ZrO₂ catalyst. All three catalysts were further compared in their redox properties, with the H₂-temperature programmed reduction (TPR) analysis showing totally different tendencies (redox ability: 3GS > 3S > 3AS/MP-ZrO₂). A small concentration of olefins accompanied with water formation were observed by in situ DRIFT spectra and quantified by the Baeyer test. The results show that this occurs via oxidative dehydrogenation of butane by the sulfate groups to form butene, which leads to butyl carbenium species for skeleton isomerization. The catalytic promotion effect of alumina on sulfated zirconia is mainly due to a balanced increase of acid sites; on the other hand, gallium promotes sulfated zirconia through enhancing both the surface acidity and its redox properties on the surface.