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

Bulk 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 C_β─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 C_α─C_β 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 C_α─C_β 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.