Polyamine and Schiff base metal complexes incorporated in mesostructured templated porous silicas: tentative application in selective oxidation

Overview & Synopsis

Novel materials were designed from hybrid organic-inorganic silica-based mesoporous materi¬als containing Cu(II) complexes and were applied in selective oxidation reactions. The localization of the metal sites was controlled using three different organosilane-ligands and two different synthetic routes, either the one-pot synthesis or the post-synthesis grafting. The organosilanes were: N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (L1), N-salicylaldimine-propylamine-trimethoxysilane (L2) and N-(salicylaldimine) -(N'-propyltrimethoxyl silane)-diethylenetriamine, (L3). In addition, Ni(II) ion was used as structural probe. The Ni(II)-L1, Cu(II)-L1 and Cu(II)-L2 complexes were co-condensed with sodium silicate using the one-pot synthetic route in the presence of cetyltrimethylammonium tosylate as templating agent to built well-ordered periodic mesoporous organosilicas (PMOs) of MCM-41 type. The as-made materials were submitted to treatments using a mixture of chloro¬trimethylsilane and hexamethyldisilazane or an appropriate amount of HCl washing to extract template and maintain the mesoporous structure. The Ni(II)-L1, Cu(II)-L1 or Cu(II)-L3 com¬plexes have been also grafted in the preformed mesoporous silica and evenly distributed using the a molecular stencil patterning technique. A multiple technique approach has been applied to thoroughly investigate the structure and morphology of the material as well as the coordination of the metal sites, using XRD, TEM, N2 sorption isotherms, elemental analysis, TGA, DRUV, FT-IR and EPR spectroscopies. In addition, the chemical accessibility and the leaching properties of the metal sites were tested using isothiocyanate (SCN-) as a ligand probe, metal displacement of Ni(II) by Cu(II) ions or resistance to acidic leaching. Apart from the known channel species obtained from grafting that are solution-like, two different frame-work species were identified from their structural and chemical properties: the accessible and non-accessible ones, named “embedded” and “showing on” sites, respectively. The catalytic activity in phenol hydroxylation using hydrogen peroxide as oxidant and catechol oxidation reactions using dioxygen as oxidant depends on the metal location. The grafted Cu(II)-L3 com¬plex exhibited the best catalytic activities and was working in water solutions. The con-ver¬sion and selectivity into valuable products, catechol and hydroquinone, were investi¬gated in function of time, temperature, pH and substrate to oxidant ratio. Catalyst recycling has been also investigated.