Tungstophosphoric acid supported on core-shell polystyrene-silica microspheres or hollow silica spheres catalyzed trisubstituted imidazole synthesis by multicomponent reaction

Abstract

Materials based on tungstophosphoric acid supported on core-shell polystyrene-silica microspheres or hollow silica spheres were prepared, characterized and used as catalysts in the synthesis of trisubstituted imidazoles by a multicomponent reaction under solvent-free conditions. For the preparation of the two different silica-based structures, silica was synthesized using tetraethylorthosilicate (TEOS) as precursor in an ethanol-ammonium solution and polystyrene spheres as template or core. The formation and growth of the silica layer to obtain the core-shell microspheres were followed by 24 h. Then, the hollow spheres were obtained by calcination of the core-shell material at 500 °C for 3 h. Both supports were impregnated with tungstophosphoric acid (TPA) solutions. A smooth appearance of the spheres was observed by scanning electron microscopy (SEM). Mesoporous materials were obtained, without important microporosity, as determined from N2 adsorption-desorption isotherms. The Fourier transform infrared (FT-IR) spectra confirmed that complete removal of the polystyrene core can directly be achieved by calcination at 500 °C, and also that Keggin undegraded TPA species are present in the impregnated solids. The acidic characteristics of the solids were evaluated by potentiometric titration, showing that they exhibit very strong acid sites. Moreover, peaks assigned to crystalline forms of TPA were observed by XRD. The yields obtained in the solvent-free synthesis of 2,4,5-triphenyl-1H-imidazole and other eight trisubstituted imidazoles were high, without formation of by-products resulting from competitive reactions or decomposition products, so the prepared materials are highly selective catalysts.