Isomerization of a-pinene oxide to campholenic aldehyde over Lewis acids supported on silica and titania nanoparticles

Lewis acids (FeCl3, ZnCl2, H3BO3) supported on SiO2 and TiO2 nanoparticles have been tested in the selective isomerization of a-pinene oxide to campholenic aldehyde. The spherical nanoparticle supports with a narrow particles size distribution have been synthesized by the sol–gel method. The morphological and microstructural characterization of the supports and catalysts was carried out by XRD, SEM–EDX and BET surface area measurements. Acid properties were investigated by DTA and FT-IR after adsorption of triethylamine and pyridine as probe molecules. SiO2 and TiO2 nanoparticles used as supports have shown negligible acidity and consequently low activity in the isomerization of a-pinene oxide. After Lewis acid promoters addition, the activity was markedly enhanced with a high selectivity to campholenic aldehyde. Lewis acids supported on titania have been found more actives than analogous supported silica catalysts due to the higher surface area of the TiO2 support. The selectivity instead was found to be independent on the support used. Among the investigated samples, Zn-doped catalysts resulted the most selective. On the basis of FT-IR of adsorbed pyridine, the selectivity to campholenic aldehyde on these catalysts has been correlated to the strength of the Lewis acid sites. Optimising the preparation of the catalysts and reaction conditions, a maximum yield to campholenic aldehyde of 74% was obtained.