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‐2 H ‐pyran (DHP), which can be produced from lignocellulose‐derived furfural, was investigated with various catalysts and molecular oxygen as an oxidant in a H 2 O solvent. 5,6‐Dihydro‐2 H ‐pyran‐2‐one (DHPO) was obtained in moderate yield with a carbon black‐supported Pd catalyst (Pd/C). The addition of base (NaHCO 3 ) 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 H 2 . Through the investigation of various Pd/support catalysts, the catalysts with low valent and stable Pd 0 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 O 2 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.