Fe–rich saponite as a catalyst for the solvent free synthesis of tetrasubstituted imidazoles

Fe–rich saponite clay mineral solids have been tested as heterogeneous catalysts for organic synthesis. The major aim was the synthesis of substituted imidazoles under solvent free conditions. Saponite was used in the natural rock form, purified by dispersion–decantation (<2 μm fraction), and calcined at 750 °C. The catalysts were characterized by X–ray diffraction, FT–IR spectroscopy, thermal analyses, N₂ adsorption–desorption and electron microscopy. The calcined solid showed the best catalytic performance (86 %), and the purified uncalcined solid also gave good yield (78 %). The temperature and time of the reaction, and the catalyst doses were optimized, 120 °C, 3 h and 30 mg being the optimal conditions. The reaction was also carried out using different substrates, finding that electron–withdrawing groups from aldehydes tended to increase electrophilicity, while slightly electron–withdrawing groups from amines improved reactivity. The best catalyst was reusable over four cycles without appreciable loss in activity. The prepared solids are environment friendly catalysts for this reaction under solvent–free conditions.