Sorption and diffusion in heteropolyoxometalates: 2. Aliphatic saturated hydrocarbons

Abstract

Sorption and diffusion properties of n-hexane, 3-methylpentane, cyclohexane, n-heptane, n-octane, and isooctane have been studied at 293, 308, and 323 K on three heteropoly acids, 12-tungstophosphoric acid, 12-tungstosilicic acid, and 12-molybdophosphoric acid, and their ammonium salts. The microporous salts show considerably larger sorption capacities than the nonporous acids. The sorption capacities of the microporous salts appear to be dependent on the pore size distributions and on the boiling points of the sorbates. The diffusivities of the aliphatic hydrocarbon are of the same order of magnitude for the heteropoly acids and the ammonium salt of 12-tungstophosphoric acid. In contrast, the diffusivities are approximately 20 and 5 times higher for the ammonium salt of 12-tungstosilicic acid and 12-molybdophosphoric acid, respectively, than those for the acids. Heats of adsorption, in general, show no evidence of chemisorption effects. Little or no evidence was found for the penetration of aliphatic, saturated hydrocarbons into the bulk structure of the solid heteropoly acids.