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

MARINA GORSD; GUSTAVO ROMANELLI; GABRIEL SATHICQ; LUIS PIZZIO; MIRTA BLANCO

JOURNAL OF MOLECULAR CATALYSIS A-CHEMICAL

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2016 p. 294 - 302

1381-1169

Materials based on tungstophosphoric acid supported on core-shell polystyrene-silica microspheres orhollow silica spheres were prepared, characterized and used as catalysts in the synthesis of trisubstitutedimidazoles by a multicomponent reaction under solvent-free conditions.For the preparation of the two different silica-based structures, silica was synthesized using tetraethy-lorthosilicate (TEOS) as precursor in an ethanol-ammonium solution and polystyrene spheres as templateor core. The formation and growth of the silica layer to obtain the core-shell microspheres were followedby 24 h. Then, the hollow spheres were obtained by calcination of the core-shell material at 500◦C for3 h. Both supports were impregnated with tungstophosphoric acid (TPA) solutions.A smooth appearance of the spheres was observed by scanning electron microscopy (SEM). Mesoporousmaterials were obtained, without important microporosity, as determined from N2adsorption-desorption isotherms. The Fourier transform infrared (FT-IR) spectra confirmed that complete removal ofthe polystyrene core can directly be achieved by calcination at 500◦C, and also that Keggin undegradedTPA species are present in the impregnated solids. The acidic characteristics of the solids were evaluatedby potentiometric titration, showing that they exhibit very strong acid sites. Moreover, peaks assignedto 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 eighttrisubstituted imidazoles were high, without formation of by-products resulting from competitive reac-tions or decomposition products, so the prepared materials are highly selective catalysts.