Vapor-phase isomerization of 3-pentenal over amorphous SiO2 catalyst

Vapor-phase isomerization of 3-pentenal to 2-pentenal was performed over various oxide catalysts. An amorphous SiO2 catalyst with a weak acidity and medium mesopore size showed a conversion over 82% with 2-pentenal selectivity higher than 95% at 250 °C. Several characterizations, such as thermogravimetric analysis, diffuse reflectance infrared Fourier-transform, temperature-programmed desorption of adsorbed NH3, and isotope experiment using deuterated SiO2, were performed to investigate the active sites on SiO2. A SiO2 with a specific pore size showed a stable catalytic activity after a rapid decrease at the initial stage of the reaction. The isomerization of 3-pentenal to 2-pentenal was found to be an equilibrium reaction: the pressure equilibrium constant was estimated to be ca. 5 at 250 °C. The weak acidic silanol groups on SiO2 were proved to be the active sites for the isomerization of 3-pentenal to 2-pentenal. A possible reaction mechanism was proposed: hydrogen of the silanol groups on SiO2 was coordinated to 2-pentenal with a form of six-membered ring, which can explain the primary product of cis isomer.