One-pot synthesis of naturanol from α-pinene oxide on bifunctional heterogeneous catalysts: Part II. The reaction Original Research Article

Bifunctional Pt–Sn/SiO2 catalysts described in the Part I, prepared by co-impregnation varying the Pt and Sn loading and the Sn/Pt ratio, have been tested in the one-pot synthesis of α-campholenic alcohol (naturanol) from α-pinene oxide. The process involves the sequential isomerization of the substrate into α-campholenic aldehyde and the subsequent hydrogenation of this intermediate to naturanol. In this Part II, a kinetic investigation of the catalytic process is reported. First, the isomerization and hydrogenation steps were studied separately. Results obtained indicate that the role of tin in the Pt–Sn/SiO2 catalysts formulation is crucial, affecting strongly the activity and/or selectivity in both steps. In the isomerization step, the catalytic activity increases linearly with tin loading whereas the selectivity to campholenic aldehyde remains essentially unchanged. In the second step, high tin loadings increase the selectivity to naturanol but a careful control on the Sn/Pt ratio is necessary to obtain a satisfactory hydrogenation rate, because for Sn/Pt > 1 the catalytic activity falls out dramatically. The overall transformation of α-pinene oxide into naturanol was then investigated. It was found that the isomerization of α-pinene oxide to campholenic aldehyde is the step limiting the naturanol yield. Moreover, kinetic tests have shown that the ratio between the isomerization and hydrogenation rate is the other critical factor in order to synthesize naturanol in high yield. Optimizing the formulation of the Pt–Sn/SiO2 catalysts, a maximum naturanol yield of about 60% was obtained.