A comparative study of methanol carbonation on unsupported SnO2 and ZrO2

The aim of this work was to explore the catalytic properties of SnO2 in the coupling of methanol with carbon dioxide to afford dimethyl carbonate. SnO2 nanopowders were produced by hydrolysis of tin tetra-tert-butoxide dissolved in n-butanol. The samples were much more active than a commercial one due to their higher surface areas. In addition, they exhibited excellent recyclability. However, comparison with ZrO2, prepared and tested under the same experimental conditions, showed that zirconia-based catalysts were more selective and are, among the heterogeneous catalysts already reported, still the more selective. SnO2 also catalyzed the formation of dimethyl ether likely due to lower rates in the formation of the key intermediate CH3OC(O)O–Sn and its subsequent alkylation by activated methanol. This lack of selectivity contrasts with that of soluble organotin(IV) which are totally selective to DMC formation. Structural characterization of SnO2 was performed by X-ray powder diffraction, laser Raman spectroscopy, transmission electron microscopy, and nitrogen isotherm. As found from X-ray diffraction line broadening, the crystallite size of all powders was in the nanometric range (cassiterite structure) which was confirmed by transmission electron microscopy. Moreover, the low-frequency Raman scattering allowed to determine an average particle size diameter of 4 nm.