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.