Catalytic conversion of a lignin model compound to value-added products using Cu/TEMPO-catalyzed aerobic oxidation

Catalytic oxidation of vanillyl alcohol (VAL), a lignin model compound, to vanillin (VN), a value-added product, is typically implemented using transition metals with non-recyclable H₂O₂ as an oxidant. In this study, an alternative oxidation process is proposed for VAL oxidation to VN by using Cu^I as a catalyst, 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) as a co-catalyst and ambient air as an oxygen source. This Cu/TEMPO has been validated as a highly effective and chemoselective catalytic process for oxidation of alcohols to corresponding aldehydes. Cu^I coordinated with 2,2′-bipyridyl (BIPY)/1-methylimidazole (MIM) could mediate the cyclic oxidative transformation between TEMPOH and TEMPO, which then oxidizes the OH group of VAL to an aldehyde group, leading to the formation of VN. In addition, the effect of temperature seems as the most influencing factor for VAL oxidation. In particular, the conversion of VAL can reach 99% with a high selectivity of 93% at 90 °C, and the corresponding production is the record-high value of 89%, tremendously higher than the reported values in the literature. These features indicate that Cu/TEMPO-catalyzed aerobic oxidation is indeed a promising, highly selective, and efficient means for valorization of VAL, the lignin model compound, to the target product, VN.