Selective Oxygenation of 3,4‐Dihydro‐2 <i>H</i> ‐Pyran to 5,6‐Dihydro‐2 <i>H</i> ‐Pyran‐2‐One With Pd/C Catalyst and Molecular Oxygen

The oxidation of 3,4-dihydro-2H-pyran (DHP), which can be produced from lignocellulose-derived furfural, was investigated with various catalysts and molecular oxygen as an oxidant in a H₂O solvent. 5,6-Dihydro-2H-pyran-2-one (DHPO) was obtained in moderate yield with a carbon black-supported Pd catalyst (Pd/C). The addition of base (NaHCO₃) suppressed the side reaction of DHP (i.e., hydration of DHP) and increased DHPO yield up to 52%. The activity of the catalyst was slightly decreased after three-time use but recovered by the heat-treatment under H₂. Through the investigation of various Pd/support catalysts, the catalysts with low valent and stable Pd⁰ phase during the reaction, such as Pd/C and Pd/BN, showed high activity in DHPO formation. Positive and almost zero reaction orders were observed with respect to O₂ pressure and DHP concentration, respectively, indicating the involvement of oxygen species in the rate-determining step and the strong adsorption of DHP with catalyst. The addition of radical scavenger lowered the formation amount of DHPO, suggesting that a radical intermediate is involved in the reaction.