Multinuclear solid-state NMR investigation of nanoporous silica prepared by sol-gel polymerization using sodium silicate

Abstract

Multinuclear solid-state nuclear magnetic resonance (NMR) experiments were performed to investigate the local structure changes of nanoporous silica during hydrothermal treatment and surface modification with 3- aminopropyltriethoxysilane (3-APTES). The nanoporous silica was prepared by sol-gel polymerization using inexpensive sodium silicate as a silica precursor. Using 1H magic angle spinning (MAS) NMR spectra, the hydroxyl groups, which play an important role in surface reactions, were probed. Various silicon sites such as Q2, Q3, Q4, T2, and T3 were identified with 29Si cross polarization (CP) MAS NMR spectra and quantified with 29Si MAS NMR spectra. The results indicated that about 25% of the silica surface was modified. 1H and 29Si NMR data proved that the hydrothermal treatment induced dehydration and dehyroxylation. The 13C CP MAS and 1H MAS NMR spectra of 3-APTES attached on the surface of nanoporous silica revealed that the amines of the 3-aminopropyl groups were in the chemical state of NH3+ rather than NH2.