Heterogeneous acid catalysts prepared by immobilization of H3PW12O40 on silica through impregnation and inclusion, applied to the synthesis of 3H-1,5-benzodiazepines

Abstract

Solid acid catalysts were successfully prepared using mesoporous silica (MESOSI) as support and tungstophosphoric acid (KTPA) as active phase, which was incorporated by impregnation (MESOSI#KTPA) and inclusion (MESOSI@KTPA). TEM (transmission electron microscopy) images of MESOSI samples revealed the presence of mesopores (worm-like structured) and well-defined pore channels. XRD patterns of MESOSI#KTPA samples showed small crystals of H3PW12O40·6H2O that were not present in the MESOSI@KTPA materials. The specific surface area of the MESOSI modified with KTPA decreases with the increment of the heteropolyacid content. 31P NMR and FT-IR characterizations confirmed the existence of undegraded [H3-XPW12O40] X− and [PW12O40] 3− anions interacting with the ^SieOH2 + groups of MESOSI. Potentiometric titration results showed that both acid strength and number of acid sites increased with the KTPA loading in MESOSI@KTPA and MESOSI@KTPA samples. Moreover, the acidic properties of MESOSI#KTPA materials were remarkably higher than those of MESOSI@KTPA. The catalysts prepared were highly active, selective and reusable in the solvent-free synthesis of 3H-1,5-benzodiazepine from 1,2-phenylenediamine and 1,3-diphenyl-1,3-propanedione.