29Si and 13C MAS-NMR characterization of surface modification of micelle-templated silicas during the grafting of organic moieties and end-capping

The surface of micelle-templated silicas (MTS) was functionalized by silylation with 3-chloropropyltriethoxysilane in anhydrous toluene. The modified sample (Cl-MTS) was then end-capped by vapor phase treatment with hexamethyldisilazane (HMDZ), which led to the AZA-Cl-MTS sample. The textural properties of the support were preserved during the different modifications. A comparison of Si-29 MAS-NMR spectra of Cl-MTS with those of the parent support (MTS) showed that silylation occurred through nucleophilic attack at silicon atoms located in the hydrophobic part of the surface. This mechanism differed from the sol-gel mechanism, which involves H-bonded silanol groups during silylation in polar or aqueous solvent. Residual silanol groups of the Cl-MTS surface were not totally end-capped during HMDZ treatment, This result is explained by the steric hindrance of the trimethylsiloxane groups, which would act as umbrellas that protect the surrounding silanol groups. (C) 1999 Elsevier Science B.V. All rights reserved.