One-pot synthesis of naturanol from α-pinene oxide on bifunctional Pt-Sn/SiO2 heterogeneous catalysts Part I:: The catalytic system

Bifunctional Pt-Sn/SiO2 catalysts with different Sn/Pt ratios have been prepared by co-impregnation, reduced in H-2 and tested by means of the one-pot synthesis of alpha-campholenic alcohol (naturanol) from a-pinene oxide. All results are reported in two sections. In Part 1, here presented, characterization of catalysts in terms of microstructure chemical and surface properties by X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and acid properties investigated by FT-IR of adsorbed pyridine are included. In the reduced Pt-Sn/SiO2 catalysts, oxidized and/or metallic species, having different microstructural and acid characteristics, strongly dependent on the Sn/Pt ratio have been identified. For Sn/Pt ratios <1, elemental Pt and the Pt-rich platinum-tin alloy Pt3Sn were found as main species. No ionic tin was detected by FT-IR analysis of adsorbed pyridine, further indicating that tin alloyed with platinum does not show acid properties. At Sn/Pt ratios higher than 1, the PtSn alloy was detected as the main species, whereas the excess of Sn was found in an oxidized state as suggested by the presence of strong IR bands at 1453 and 1608 cm(-1) attributed to pyridine bonded to Lewis acid sites associated with ionic tin species. A clear relationship between the intensity of FT-IR bands and the activity in a-pinene oxide isomerization was observed indicating that Lewis acid sites, associated with ionic tin species, present on the surface of bifunctional Pt-Sn/SiO2 catalysts, are active centres favouring the isomerization reaction. Sn addition also increases the selectivity to naturanol, in the subsequent consecutive hydrogenation of campholenic aldehyde intermediate. However, at high loading, the bifunctional effect is less significant because the excess of oxidized tin strongly decreases the catalytic activity. Our results indicate that the Sn/Pt ratio is the key factor in Pt-Sn/SiO2 bifunctional catalysts and influences both activity and selectivity in the one-pot synthesis of naturanol from alpha-pinene oxide. (C) 2007 Elsevier B.V. All rights reserved..