Infra-red spectra and energy of adsorption of aromatic compounds on silica

Abstract

The change in spectrum of surface hydroxyl groups and in the spectra of benzene, toluene, p-xylene and mesityfene molecules during adsorption on aerosils of various degrees of dehydroxylation has been studied. It was observed that mainly the free hydroxyl groups of the silica surface participate in the adsorption interaction. The shift of the absorption band of the free hydroxyl groups depends on the structure of the adsorbed molecuIe and on the surface coverage. There exists a relation between the magnitude of this shift and the heat of adsorption. It is established that the degree of hydroxylation and the surface coverage influence the location and intensity of the bands of the out-of-plane CH deformation of the benzene molecule. The changes in the spectrum during adsorption are due to the interaction between the n-electron system of the aromatic molecules and the hydroxyl groups of the silica surface.

Nature and Estimation of Functional Groups on Solid Surfaces

“…Steric restrictions towards H-bonding of silanol groups with adsorbates having bulky substituent groups occur occasionally [170,171]. Linear correlations were also reported between L\ VOH and the specific H-bond interaction energy L\HOH [172][173][174], were L\HOH is defined as the difference…”

Surface photochemistry: the effect of surface modification on the photophysics of naphthalene and pyrene adsorbed on silica gel

“…Water not only binds to the isolated silanol groups through hydrogen bonding but also hydrolyses strained siloxane bonds formed by heating, thereby increasing the number of vicinal hydroxyl groups. Probably the first effect precedes the second, since rehydration is slow with drastically dehydrated silica gel and since the water has little to which it can anchor because the siloxane bonds are known to be hydrophobic (35). Iler (39) and others (40) have suggested that the rate-limiting step may instead involve the association of molecular water with uncondensed silanol functions.…”

“…Furthermore, of the various environments in which the silanol function may find itself, it is what is isolated, freely vibrating, and not hydrogen bonded to another silanol or to water, that is the most effective in binding (35)(36)(37). In the present instance, because of the dimension of the adsorbed naphthalene and pyrene, even when the {ehydration of the surface is taken to about -2 silanols/100 A' (21), there must be more than one hydrogen bond per molecule.…”

Partial Gas Phase Hydrogenation of Benzene on Sol-Gel Ruthenium Catalysts

“…Thus, a rather dense water adsorption layer on the ruthenium surface can be formed by the addition of water vapor. Since it is wellknown that benzene molecules adsorb on hydroxyl groups of the catalyst surface under formation of p-complexes [16,17], the presence of water may favor the adsorption of benzene in comparison to cyclohexene adsorption.…”