Interlamellar adsorption of alcohols 4. Adsorption properties of crystalline silicas

Abstract

Crystalline silicas (crystalline silicic acids) are intracrystalline-reactive layered materials. Two types were studied: H 4Si 20O 42· xH 2O and H 4Si 14O 30· xH 2O. They show a pronounced preference of methanol and ethanol from mixtures with water, n-butanol, and toluene. The surface excess isotherms are U-shaped with long linear sections extending from x 1 = 0.2 or 0.3 to x 1 = 1 ( x 1 molar fraction of methanol or ethanol). Thus, at x 1 ≥ 0.2 or 0.3, only methanol or ethanol is adsorbed between the layers. The preference of H 4Si 20O 42 for methanol and ethanol from mixtures with water is as high as in silicalite and NaZSM-5. With longer alcohols (propanol … n-octanol), the isotherms become S-shaped, and molecules of component 2 (water, n-butanol, toluene) are also adsorbed in the interlayer spaces. Adsorption from mixtures of n-pentanol (or n-hexanol) and n-butanol produces unusual surface excess isotherms. The cause is formation of paraffin-type bilayers of pentanol or hexanol which, at a certain molar fraction of butanol, collapse to monolayers of flat-lying alcohol molecules. When the alcohol molecules lie flat in the interlayer space (basal spacing less than 2.3 nm), the volume of the adsorption phase, V s, comprises the interlamellar volume V int (determined by X-ray powder techniques) and the volume V ex of a few layers of alcohol molecules adsorbed at the external surface. The ratio V s/ V int varies between 1.2 and 1.5. When the layer separation becomes larger (paraffin-type bilayers of alcohols), V int exceeds V s. This behaviour is also observed for hydrophobized clay minerals in the presence of polar/unpolar liquid mixtures.