The Fumed Silica Surface:  A Study by NMR

Abstract

High-resolution solid-state NMR techniques were used to investigate the surface structure of Cab-O-Sil fumed silica. 1H NMR results obtained from CRAMPS, MAS-only and relaxation studies reveal the existence of both hydrogen-bonded silanols and isolated silanols on the Cab-O-Sil surface. A systematic dehydration study of fumed silica was carried out, with results on the quantity of each type of silanol on the surface at various dehydration stages. 29Si CP-MAS experiments, including CP spin dynamics studies and various other relaxation studies, were employed to probe hydrogen bonding and the local structural environments of various hydroxyl groups of silica surfaces. 29Si CP-MAS experiments on water-treated and deuterium-exchanged Cab-O-Sil indicate the existence of interparticle silanols and internal silanols in fumed silica. 1H and 29Si NMR show that for fumed silica both isolated and hydrogen-bonded silanols are present on the surface of an untreated sample, in contrast to the case of silica gel, where all silanols of an untreated sample are hydrogen bonded.

Surface silylation of mesoporous/macroporous diatomite (diatomaceous earth) and its function in Cu(II) adsorption: The effects of heating pretreatment

“…Its intensity evidently decreases in the spectrum of Dt-1000. This band is attributed to the isolated hydroxyl groups, which has been confirmed by various studies [32,39,40]. The band is not resolved in the spectra of diatomite samples heated at temperatures lower than 600°C because the isolated hydroxyl groups are H-bonded with the physically adsorbed H 2 O at these temperatures, such that their vibration signals are covered by the water signals.…”

“…It is attributed to the Hbonded silanols [39,41]. Silanols of this type are condensed more easily than the isolated silanols [32], so their vibration signals disappear in the spectrum of Dt-800.…”

Controlled deposition of titanium oxide overcoats by non-hydrolytic sol gel for improved catalyst selectivity and stability

“…H magic angle spinning (MAS) solid-state nuclear magnetic resonance (ssNMR) was investigated, as chemical shifts are very sensitive to the nature of surface hydroxyl groups [54]. The spectra of pure SBA-15 displayed a single peak at 2 ppm, which is characteristic of surface silanols.…”

Silane Coupling Agents

“…Solid-state silicon NMR has also been a significant aid in the understanding of the reactions of the coupling agent with the surface of the filler. These investigations have been aided by detailed studies of the fillers alone (24). A number of studies have focused on the reactions with surface species and yield a detailed view of the species present, similar to that done for solutions, though with less resolution (25,26).…”