Preparation of a solid superacid of sulfated tin oxide with acidity higherthan that of sulfated zirconia and its applications to aldol condensation and benzoylation

A highly active solid superacid of sulfated tin oxide was prepared from tin oxide gel, which was precipitated by the hydrolysis of SnCl4 and washed wi th aqueous ammonium acetate solution, followed by exposure to aqueous sulfu ric acid and calcining. Differencial thermal analysis suggested that the ac etate ion remaining on the surface was replaced with sulfate ion. The acid strength of the sulfated tin oxide estimated by temperature-programmed deso rption using argon was higher than that of sulfated zirconia, a well-known solid superacid, the activation energy of Ar desorption by the former being 10.6 kJ mol(-1) in comparison with 9.3 kJ mol(-1) by the latter. The sulfa ted tin oxide showed activities much higher than those of the sulfated zirc onia for the skeletal isomerization of n-pentane, the Mukaiyama aldol conde nsation of 1-trimethylsilyloxy-1-cyclohexene with benzaldehyde, and the ben zoylation of toluene with benzoic anhydride.