Efficient solid acid catalysts based on sulfated tin oxides for liquid phase esterification of levulinic acid with ethanol

Tin oxide nanomaterials prepared by hydrothermal synthesis at 100 degrees C or 140 degrees C with or without template and further calcination step were modified with sulfate groups by post synthesis treatment. The catalysts were characterized by X-ray powder diffraction (XRD), N-2 physisorption, IN Vis spectroscopy, TG analysis, XPS and solid state NMR spectroscopy. The acidity of the materials was characterized by temperature programmed desorption (TPD) of ammonia. The catalytic performance of nanosized SnO2 catalysts and their sulfated analogues was studied in levulinic acid (LA) esterification with ethanol. Sulfated materials show significantly higher activity compared to non-sulfated ones. It was found that the synthesis parameters (temperature, template) for preparation of the parent SnO2 nanoparticles influence significantly their textural properties and have a pronounced effect on the structural characteristics of the obtained sulfated tin oxide based materials and their catalytic performance in levulinic acid esterification. Skipping the calcination step during the preparation of parent SnO2 samples synthesized without template resulted in the formation of new, highly crystalline phase based on hydrated tin(IV) sulfate [Sn(SO4)(2)center dot xH(2)O], tin(IV) bisulfate [Sn(HSO4)(4)center dot xH(2)O] and/or tin(IV) pyrosulfate [Sn(S2O7)center dot xH(2)O] species in the sulfated nanomaterials with superior catalytic performance. The formation of this new and catalytically very active phase not reported so far in the literature for sulfated tin oxide-based materials is discussed. The catalytically active sites for esterification of levulinic acid with ethanol is suggested to result from the formation of strong Bronsted and Lewis acid sites with high density in the newly registered phase. The results indicate that the chemical structure and catalytic performance of the obtained sulfated tin oxide based materials strongly depend on the treatment of the SnO2 nanoparticles before the sulfation procedure.