Alumina-promoted sulfated mesoporous zirconia catalysts

Mesoporous zirconia, hydrothermally synthesized from surfactant templating, was directly impregnated with aluminum sulfate to give the acidic Al-promoted sulfated mesoporous zirconia (AS/MP-ZrO₂). A series of AS/MP-ZrO ₂ catalysts were characterized by Brunauer-Emmett-Teller and X-ray diffraction for their texture properties and crystalline phases. The catalytic behavior for n-butane isomerization was found to be strongly promoted at relatively low temperature by the addition of a proper amount of alumina as a promoter. ²⁷Al S.S. magic-angle spinning nuclear magnetic resonance results indicated that Zr atoms were partially substituted by Al, giving a considerable increased concentration of Brønsted acids. X-ray photoelectron spectroscopy and diffuse-reflectance infrared Fourier-transformed spectra (DRIFT) analysis were then employed to identify and relatively quantify properties of acid sites on catalyst surface. A balanced distribution of acid sites strength was proven to prevent a catalyst from deactivating rapidly due to coke formation on the catalyst surface. A small concentration of olefins formed by oxidation of n-butane and proven to be key intermediates during n-butane isomerization on sulfated zirconia was found by the Baeyer test. Electron paramagnetic resonance and in situ DRIFT 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. A modified biomolecular mechanism for the isomerization of butane is examined to explain the catalysis results.