Polyoxoniobate as a Recyclable Proton Shuttle Enabling Efficient Lignin β- <i>O</i> -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 (O2–), facilitates a proton-shuttling mechanism that drives both Cβ–O and Cα–Cβ 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.