Catalyst‐Free Cleavage of C─O/C─C Bonds in Lignin Linkages by Water Microdroplets

Abstract

AbstractBulk water serves as an inert environment for lignin linkages, resulting in their natural half‐lives that extend over centuries. In this study, we present the striking results of the spontaneous and ultrafast C─O/C─C bond cleavage of various lignin models in water microdroplets, yielding value‐added aromatic chemicals. The β‐O‐4 linkages, the most abundant interunit linkages in natural lignin, were selectively cleaved at Cβ─O bonds, producing phenols in yields exceeding 70%. Mechanistic studies elucidated that the cleavage of β‐O‐4 linkages is derived from the unique alkaline environment at the air–water interface of a negatively charged water microdroplet, even in the absence of extra alkalis. The challenging cleavage of the highly stable Cα─Cβ bonds in β‐O‐4 and β‐1 lignin linkages was also accomplished, yielding valuable benzoic acid product. Mechanistic investigations revealed that the oxidation of the substrates by molecular oxygen is the key step for the Cα─Cβ bond cleavage. Notably, all intermediates, including the fragile peroxide intermediates, were identified using mass spectrometry. Accompanied by evidence from radical scavenging and 18O labeling, the mechanisms for the selective C─O/C─C bond cleavages have been unambiguously characterized, paving a new and green way for the cleavage of lignin linkages.