Sorption of methanol on tungstosilicic acid

Abstract

The sorption of methanol by anhydrous H₄SiW₁₂O₄₀ (HPA) was investigated using classical sorption investigations, thermometric titration, FTIR spectroscopy and DTA and TG analyses. Methanol is sorbed by the whole volume of HPA crystallites. Sorption is governed by diffusion laws and is partially reversible. The existence of a series of methanolates, H₄SiW₁₂O₄₀·4CH₃OH, H₄SiW₁₂O₄₀·12CH₃OH and H₄SiW₁₂O₄₀·16CH₃OH has been stated. The FTIR investigation showed that in anhydrous HPA, anions [Keggin units (KU)] are bonded by hydrogen bonds between neighbouring KUs and in methanolates the bonds joining KUs are due to the interactions between protonated methanol clusters and KUs. The relatively high enthalpy of formation of methoxylate ion, CH₃OH₂⁺ (58.9 kJ mol^-1), shows that protons are more loosely bonded in anhydrous HPA than in methanolate. The latter fact is in good accordance with the model of catalytic synthesis of methyl tert-butyl ether formation assuming different activities of protons depending on the amount of methanol in the quasi-liquid phase.