Mechanism of propene hydration over heteropolyacid catalysts

The catalytic properties of bulk and supported HPW catalysts in the gaz phase hydration of propene were studied and compared to the reactivity of H-ZSM-5 zeolite. The nature of the intermediate products was investigated by FrIR spectroscopy. HPW-based catalysts showed higher performance in alcohol formation than H-ZSM-5. Different mechanisms of hydration were proposed on the basis of kinetic and spectral investigations. The classical acidic mechanism proceeding via propyl carbenium ion intermediate formation prevailed for the acidic zeolite. Hydroxonium ion H3O+ is proposed to be the active site of bulk HPW and on high-loaded HPW/SiO2. The hydration reaction proceeds as the direct reaction of gaseous propene molecules with adsorbed hydroxonium ions. The intermediate isopropyl alcohol ions C3H7OH2+ could be stabilized within bulk HPW or within the two or three-dimensional structure of HPW clusters supported on high-loaded silica (70 wt.%). Both mechanisms exist over silica loaded with low HPW amount (10 wt.%) accounting for the modifications of the intrinsic acidity of 12-phosphotungstic acid combined with modifications of the collective properties of Keggin units. (C) 2003 Elsevier B.V. All rights reserved.