Catalysis by Heteropoly Compounds. 20. An NMR Study of Ethanol Dehydration in the Pseudoliquid Phase of 12-Tungstophosphoric Acid

Dehydration of ethanol in the pseudoliquid phase of 12-tungstophosphoric acid, H₃PW₁₂O₄₀, was studied by means of solid-state NMR combined with IR, thermal desorption, and transient-response methods. It was confirmed by the transient-response method using isotopically labeled ethanol that a large amount of ethanol was absorbed in the lattice of the catalyst bulk (pseudoliquid phase) under the reaction conditions, and the reaction proceeded in the bulk phase. Probable reaction intermediates of the ethanol dehydration such as protonated ethanol dimer ((C₂H₅OH)₂H⁺) and monomer (C₂H₅OH₂⁺) were directly identified in this phase by solid-state NMR spectroscopy. The assignments were supported by IR spectroscopy and stoichiometry of ethanol absorption. By comparison of the NMR data with the results of thermal desorption of ethanol, the dimer and monomer species are very likely the intermediates for diethyl ether and ethylene formation, respectively. The unusual pressure dependence observed for the dehydration of ethanol was reasonably explained by the changes in the concentration of these intermediates in the pseudoliquid phase.