Evaluation of sulfated tin oxides in the esterification reaction of free fatty acids

Sulfated tin oxides (SO42−/SnO2) with different SO42− contents (0.15, 0.3 and 0.45 wt%) were synthesized from hydroxylated tin oxide obtained by the precipitation method, followed by ion exchange of the OH groups with SO42− species using a sulfuric acid solution. In samples annealed at 673, 773 and 873 K the characterization of the solids was made by XRD, FT-IR and nitrogen adsorption. The strength and number of acid sites were determined by titration with n-butylamine using Hammett indicators an acid strength of Ho < −8.2 was found. Sulfated tin oxides were tested in the esterification reaction of free fatty acids with ethanol (molar ratio 1:10), at 353 K. XRD profiles showed that the sulfating inhibits the SnO2 crystal growth. The SO4 species remained strongly bonded at the SnO2 surface stabilizing its crystallite size against sintering and it acts as a structure porogen director mediating nanoparticle growth and assembly yielding a mesostructured form of SnO2 with wormhole morphology and high thermal stability. The conversion of oleic acid increases with the increase of surface acidity showing a maximum when the sulfate content and calcination temperature were 0.3 wt% and 773 K, respectively. Regeneration through heating the SO42−/SnO2 with the highest activity for 4 h allows it for reuse without losing of its catalytic activity.