Synthesis and characterization of nanoparticulate silica with organized multimodal porous structure impregnated with 12-phosphotungstic acid for its use in heterogeneous catalysis

Abstract

Mesoporous silica (MESSI) nanoparticles with ordered mesoporous frameworks were successfully obtained using the triblock copolymer P123 as the mesoporous template and agar as micropore former. The agar/TEOS ratio influences the specific surface area (SBET) and the micropore area (SMIC). The SBET values increase with the increment of the agar/TEOS ratio used in the synthesis. SAXS (small angle X-ray scattering) patterns and TEM (transmission electron microscopy) images of MESSI samples reveal the presence of a 2D hexagonal mesopore array. The study of all the materials impregnated with PTA by 31P NMR and FT-IR confirmed the existence of undegraded [H3-XPW12O40](3−X)− and [PW12O40]3− anions interacting with the ≡Si-OH2 + groups. The potentiometric titration results show that the MESSI-2PTA solids present very strong acid sites and that both the acid strength and the number of sites increase with the increment of the PTA amount in the samples. The performance of MESSI-2PTA materials as catalysts in the solvent-free synthesis of a series of substituted 3H-1,5-benzodiazepines was evaluated. The yields achieved were high, without formation of by-products resulting from competitive reactions or decomposition products, so the prepared materials are highly selective and reusable catalysts.