SUPERACID AND CATALYTIC PROPERTIES OF SULFATED ZIRCONIA

Zirconium hydroxide was sulfated by aqueous H2SO4 solutions of different normalities. The presence of sulfur was observed to slow down the decrease of the surface area and the tetragonalto-monoclinic phase transformation of zirconia with increasing calcination temperature. Acidic properties were studied by the Hammett indicator technique. Charge transfer complex formation was studied by ESR using benzene as a probe to study strong ionizing properties of the samples, since benzene has a high ionizing potential value (9.24 eV). The observed ESR signal was assigned to biphenyl cation with hyperfine splittings equal to aH = 6.74, 3.37, and 0.52 G for 2,4, and 4 protons, respectively. Its intensity was observed to be maximum for a calcination temperature equal to 600°C and a sulfur content equal to 1.5 to 3 wt%. This corresponds very probably to the maximum of strong Lewis acidity. Catalytic properties were studied for the isomerization of n-butane to i-butane in the 150 to 300°C range. The highest catalytic activity in flow reactor was observed for the samples calcined at 600°C as for benzene ionization properties, which indicates a close correlation. Hydrogen was observed to sharply decrease the deactivation rate suggesting that the active sites do not correspond to a sulfur-reduced species and that Brønsted acid sites are probably necessary for the reaction. It is proposed that both strong Lewis and Brønsted sites are involved for n-butane isomerization. Zr3+ cations were also observed by ESR for sulfated zirconia samples outgassed above 600°C with maximum intensity between 650 and 750°C. It is proposed that they arise from the reduction of Zr4+ cations by SO2-4 decomposition into SO2 and O2. The presence of such Zr3+ species was not related to catalytic properties. © 1993 Academic Press, Inc.