Polyoxoniobate as a Recyclable Proton Shuttle Enabling Efficient Lignin β-O-4 Linkage Cleavage: Experimental and Theoretical Insights

Proton shuttles play a vital role in diverse chemical transformations; however, most known examples, such as water and small organic molecules, operate in homogeneous systems with colocated proton donor and acceptor sites. In contrast, recyclable proton shuttle catalysts with spatially separated donor and acceptor sites are highly desirable due to their enhanced flexibility in proton transfer but remain largely unexplored. Herein, we identify the polyoxoniobate K₇HNb₆O₁₉·13H₂O (KNb₆) as a special type of recyclable proton shuttle catalyst that enables rapid cleavage of lignin β-O-4 ketone into value-added phenol (yield: 93%) and benzonitrile (yield: 73%) within 10 min, using NH₂OH·H₂O as the N-source. Combined computational and experimental studies reveal that the unique surface of KNb₆, featuring a proton-donating hydroxyl group (−OH) surrounded by multiple proton-accepting oxo ligands (O^2–), facilitates a proton-shuttling mechanism that drives both C_β–O and C_α–C_β bond cleavage of a ketoxime intermediate formed in situ. Moreover, KNb₆ maintains a stable catalytic performance over multiple cycles and efficiently promotes the depolymerization of oxidized natural lignin. This work not only provides a robust and recyclable catalyst for biomass valorization but also offers insights into the design of recyclable proton transfer systems.