Phosphotungstates as catalysts for monoterpenes oxidation: Homo- and heterogeneous performance

Lacunary (PW11), mono-substituted (PW11M) and sandwich type M4(PW9)2 phosphotungstates (M = CoII, FeIII) exhibits high catalytic activity for geraniol and R-(+)-limonene oxidation under mild conditions, using hydrogen peroxide as oxidant and acetonitrile as solvent. The best homogeneous performance was found for the PW11 and for the Co4(PW9)2. Complete conversion of geraniol and R-(+)-limonene was achieved after 1.5 h and 3 h of reaction, respectively, when catalyzed by PW11. The yield of 2,3-epoxygeraniol was 100% in the presence of lacunary PW11 and mono-substituted compounds. PW11 and Co4(PW9)2 were immobilized on an amine-functionalized SBA-15 (aptesSBA-15) and the composites materials were characterized by FT-IR, FT-Raman, UV–vis/DRS, 31P MAS NMR, elemental analysis and SEM-EDS. Their heterogeneous catalytic activity was investigated for the same substrates and compared with that in homogeneous conditions. The PW11@aptesSBA-15 and Co4(PW9)2@aptesSBA-15 showed to be active heterogeneous catalysts for geraniol epoxidation with 100% of selectivity for 2,3-epoxide. R-(+)-limonene was also oxidized by PW11@aptesSBA-15 achieving 90% of conversion after 24 h of reaction. Both composites could be separated after the reaction and reused for at least three cycles without loss of activity. The stability of the heterogeneous catalysts after catalytic reactions was investigated by different techniques of characterization.