Infra-red spectroscopic studies of adsorption and catalysis. Part 2.—Aliphatic aldehydes and ketones adsorbed on silica

Abstract

Infra-red spectra between 4000 and 1300 cm^–1 have been obtained for propionaldehyde, isobutyraldehyde, pivalaldehyde, methyl ethyl ketone, methyl isopropyl ketone and methyl tert. butyl ketone (pinacolone) adsorbed on discs pressed from silica powder. In all cases the spectra show a strong band at about 3400 cm^–1, characteristic of a hydrogen-bonded interaction between the hydroxyl groups of the silica surface and the carbonyl oxygen atoms, plus bands due to adsorbed molecules. The intensity of all the absorption bands of the ketones falls smoothly towards zero as the vapour phase pressure is reduced from 10 to 10^–5 torr, indicating reversible physical adsorption of a single type. Weak absorption in the νCC region which occurs for adsorbed ketones may be caused by a small degree of enolization in the adsorbed state.

The adsorption of aldehydes is also mainly reversible at room temperature. However, particularly with isobutyraldehyde, residual νCH absorption bands of adsorbed species are found on evacuation to low pressures. These are consistent with the occurrence of either hydration or polymerization on the surface. Raising the temperature to 120°C normally leads to desorption of the aldehyde. However, with propionaldehyde, new carbonyl bands are found at 120°C which may be interpreted in terms of the presence of both 2-methyl-2-pentene-1-al and the aldol intermediate, propionaldol, on the surface. Acetaldehyde has been shown previously to give the analogous product crotonaldehyde on adsorption on silica at similar temperatures.