Si-29 and C-13 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 silyla tion with 3-chloropropyltriethoxysilane in anhydrous toluene. The modified sample (Cl-MTS) was then end-capped by vapor phase treatment with hexamethy ldisilazane (HMDZ), which led to the AZA-Cl-MTS sample. The textural proper ties of the support were preserved during the different modifications. A co mparison of Si-29 MAS-NMR spectra of Cl-MTS with those of the parent suppor t (MTS) showed that silylation occurred through nucleophilic attack at sili con atoms located in the hydrophobic part of the surface. This mechanism di ffered from the sol-gel mechanism, which involves H-bonded silanol groups d uring silylation in polar or aqueous solvent. Residual silanol groups of th e Cl-MTS surface were not totally end-capped during HMDZ treatment, This re sult 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.