Sorption on porous solids

Abstract

Theories of adsorption and their application to porous sorbents have been outlined. Two explanations of sorption hysteresis based on the capillary-condensation theory are considered, and work has been carried out to provide further information on the conditions for the occurrence of hysteresis. Ferric oxide gel and two silica gels have been prepared. Ethyl alcohol and water isotherms have been determined at 25° to compare the gels with those used by other workers.The adsorption of hydrogen cyanide at 0°, of acetonitrile and propionitrile at 25°, and by butyronitrile, valeronitrile and capronitrile at 35° on silica and ferric oxide gels has been studied. The nitriles from acetonitrile to valeronitrile showed hysteresis on silica gel 26; the systems capronitrile-gel 26 and hydrogen cyanide-gel 26 did not, and these isotherms were concave to the pressure axis throughout. The isotherms for the ferric oxide gel-nitrile systems were all of Type IV, with reproducible hysteresis loops. Hydrogen cyanide was found to polymerise at the surface of the gel. z/m, the monolayer capacity of the ferric oxide gel in moles grm.-1 was calculated. A constant value for the series of nitriles, as for the aliphatic alcohols, would indicate vertical orientation of the molecules at the surface. Isotherms were obtained for the adsorption of nitromethane on ferric oxide and silica gels at 25°. Nitromethane appeared to decompose at the surface of ferric oxide gel. A normal Type IV isotherm was obtained for silica gel with this sorbate, but the plotdid not give an accurate measure of the monolayer capacity of the gel. An attempt to obtain measurements for the system acetic acid-ferric oxide gel showed that reaction began within a few hours of admission of the vapour to the sorbent. Methyl and ethyl alcohol isotherms determined at 25° on silica gel 26 were "linear isothermals" of the type already described in the literature.