Catalytic action of sulfated tin oxide for etherification and esterification in comparison with sulfated zirconia

Two highly active solid superacids of sulfated tin oxide were prepared, and their catalytic action was compared with that of sulfated zirconia. One was prepared from tin oxide gel, which was precipitated by the hydrolysis of SnCl4 and washed with aqueous ammonium acetate solution, followed by exposure to aqueous sulfuric acid and calcination. The other was prepared from meta-stannic acid following the above procedures. The latter material was higher than the former in catalytic activities for the etherification of methanol and esterification of n-octanoic acid with methanol, and both showed activities higher than those of sulfated zirconia for the reactions. TPD measurements using ammonia and pyridine as probes showed that most of the acid sites on the surface of sulfated tin oxide have high strength of acidity with a narrow distribution, indicating the steady adsorption of both probes up to 673 K. On the other hand, the acidity on sulfated zirconia was shown to be heterogeneous, covering a wide range of strength, the temperature range of desorption of the probes being 473-823 K. The acid strength was estimated by the heat of Ar adsorption to be 29.7-31.0 and 24.3 U mol(-1) for the sulfated tin and zirconium oxides, respectively. Differences in activities between the two Sn materials were explained on the basis of their IR spectra. (C) 2004 Elsevier B.V. All rights reserved.