Aerobic oxidation of α-pinene catalyzed by homogeneous and MOF-based Mn catalysts

Abstract

Manganese catalysts were investigated for the liquid-phase aerobic oxidation of α‐pinene. First, the influence of reaction parameters such as time, solvent, temperature, oxidant flow rate and catalyst concentration on conversion, yield and selectivity were evaluated using Mn(III) acetate as homogeneous catalyst. Mn(III) acetate afforded pinene oxide as the main product along with small amounts of verbenol and verbenone. The optimized reaction conditions were likewise applied to a novel mixed-linker metal-organic framework (MOF) based on MIL-53-NH2(Al), which was successfully synthesized and modified in a two-step post-synthetic reaction using maleic anhydride and Mn(III) acetate. The performance of the Mn-containing MOF catalyst was directly compared to its homogeneous counterpart Mn(III) acetate, showing very similar activity in a mixture of diethyl carbonate/dimethylformamide (DEC/DMF) as solvent. In both cases, a conversion of 31% and the formation of 17% pinene oxide (55% selectivity) were observed after 6 h. The heterogeneous MOF catalyst was easily removed from the reaction mixture by filtration and reused for at least five catalytic cycles without significant loss of activity. A hot filtration test showed that the catalysis mainly proceeded heterogeneously, although minor contributions of homogeneous species could not be completely excluded.