Acidities of sulfate species formed on a superacid of sulfated alumina

Sulfated alumina was prepared by impregnating gamma-Al2O3 with aqueous H2SO4. Various sulfate species formed in these samples depended on the concentration of H2SO4 ([H2SO4]). Three sulfate species, i.e., surface sulfate, multilayer sulfate and crystallized Al-2(SO4)(3) could be clearly identified by the DTG peaks at ca. 950 degrees C, 630 degrees C and 800 degrees C, respectively. Surface sulfates were the main sulfate species at [H2SO4] less than or equal to 0.8 M and induced Lewis superacidic sites (H-0 less than or equal to -13.8). The superacidic sites were also characterized by a NH3-TPD peak around 400 degrees C and showed a catalytic activity towards n-butane isomerization. When [H2SO4] > 0.8 M, multilayer sulfates were additionally formed, which created weaker Bronsted acid sites with a NH3-TPD peak around 250 degrees C and a negligible activity towards n-butane isomerization. Until [H2SO4] > 2.4 M, neutral crystallized Al-2(SO4)(3) was formed concurrently with the surface and the multilayer sulfates.