Mechanochemical nitration of toluene with metal oxide catalysts

Results of an experimental study of the mechanochemical nitration of toluene are presented. The focus is on the effect of acidity of metal oxide catalysts on the yield of mononitrotoluene. No solvents were used during the reaction. Sodium nitrate served as a source of nitronium. Gas chromatography-mass spectrometry of the products showed that the nitration rate scaled with the catalyst's acidity and specific surface area. Homogenizing NaNO₃ with the catalyst by additional milling prior to reaction with toluene led to a rapid, nearly complete nitration. Distribution of nitrate over the surface of catalyst is likely rate limiting when toluene is mechanochemically nitrated without the preliminary milling step. The observed for varied reactant ratios trends in yield and isomer ratios suggest that nitronium participates in the nitration while localized on the active sites of the catalyst. Excess of toluene blocks acid sites, inhibiting the formation of nitronium and impeding the nitration.