Assessment of the metal exchanged phosphomolybdic acid salt-catalyzed nerol oxidation reactions with hydrogen peroxide

Abstract

In this work, phosphomolybdate salts were synthesized by exchanging the protons of the phosphomolybdic acid with metal cations (i.e., Co2+, Cu2+, Zn2+, Mn2+, Ni2+, Al3+, and Fe3+ cations) and evaluated as catalysts in oxidation reactions of terpene alcohols with hydrogen peroxide. All the catalysts were characterized by Raman, UV-Vis, and infrared spectroscopies, powder X-rays diffraction, energy dispersive spectroscopy, scanning electronic microscopy, and thermal analyses. The strength of the acid sites was determined by potentiometric titration curves. Among the catalysts tested in nerol oxidation (i.e., model molecule selected), AlPMo12O40 was the most active and selective catalyst toward nerol epoxide, the main product. Main reaction variables such as the H2O2: substrate molar ratio, time, temperature, type, and catalyst load were assessed. The focus was to correlate the physicochemical properties of the catalysts with their performance. The reaction scope was extended to other terpene alcohols. Allylic alcohols (nerol and geraniol) were mainly converted to epoxides, while the oxidation of secondary (borneol) and tertiary (linalool) alcohols gave carbonylic products (camphor), or cyclization products (furane and pyrane rings), respectively.