Non-specific, specific molecular and chemical adsorption of tetrahydrofuran on progressively dehydroxylated aerosilogel

Abstract

The effect on the heats of adsorption of tetrahydrofuran of the progressive dehydroxylation of a wide-pore aerosilogel has been studied. Three distinct mechanisms of adsorption were observed: non-specific adsorption, characterized by differential heats of adsorption Q∼9 kcal mole^–1, occurred on siloxane (Si—O—Si) groups (formed as the major product of dehydroxlyation) and in 2nd layer adsorption; specific molecular adsorption (Q∼15 kcal mole^–1) occurred on surface hydroxyl groups; on samples outgassed at high temperatures (800–1100°C), adsorption centres showing a high affinity for tetrahydrofuran (Q∼40 kcal mole^–1) were found. These are associated with loss of isolated hydroxyl groups from the surface without sintering, the maximum growth in their number occurring on outgassing between 900 and 1000°.

Adsorbed water molecules, although having differential heats of adsorption on these centres, considerably higher than those on hydroxylated silica surfaces, apparently did not poison these centres to any great extent.