Influence of silica gel structure on hydrocarbon separation by gas chromatography

Abstract

IN GAS-ADSORPTION chromatography, silical gel has been widely used for the separation of light hydrocarbons as a sufficiently homogeneous adsorbent [1-3]. Static studies showed that adsorption of hydrocarbons increases with the diminution in size of silica gel pores [4, 5]. I t was found of interest to study the effect of structure and surface chemistry on the gas-chromatographic separation of hydrocarbon mixtures. The effect of the surface chemistry of silica gels on the gas-chromatographic separation of hydrocarbons was studied in papers [6-11]. Similar investigations of porous glasses [12-15] are closely related to these papers. The effect of silica gel structure on the gas-chromatographic separation of light hydrocarbons formed the subject of paper [16]. With an increase of specific surface and reduction of silica gel pore size, more complete separation was achieved. By t rea tment of silica gels with HC1 and KOH solutions, adsorption was reduced as a result of increased pore sizes and reduced specific surface. With the removal of imp.urities polymerization of propylene and butenes on untreated silica gel was eliminated. I t was also shown in paper [17] that during t reatment of silica gels with acids and alkalies, the fine pores are reduced and the large ones increased. Paper [18] studies the effect on the separation of hydrocarbon gases of calcining silica gels of varying structure at 110-800°C. On increasing the temperature of treatment, silica gel activity increased to a maximum and then rapidly decreased. In the case of ethylene, activity increase was observed only before the elimination of adsorbed water, on eliminating the chemically combined water, activity markedly decreased. Ttmse results can be explained by the reduction of silica gel surface during sintering and by the reduction of adsorption energy of unsaturated (and aromatic) hydrocarbons during dehydroxylation of the surface as a consequence of the reduced specific interaction of ~-electron bonds with the hydroxyl groups on the silica surface [4, 12, 19-21]. In paper [18] a study was made of methods of obtaining symmetrical peaks on active silica gels by increasing working temperatures and deactivation of