Oxidative cleavage of β-O-4 bonds in lignin model compounds with polymer-supported Ni-Salen catalysts

Transition metal-catalyzed lignin oxidative cleavage reactions have attracted considerable attention. In this work, polymerized ionic liquid-tagged Salen ligands have been initially synthesized, followed by anion exchange, and then coordination with Ni(ii) via a -N2O2- tetradentate structure. Finally, the as-obtained Ni-Salen complexes were polymerized to give a Ni-Salen polymer catalyst (poly-Ni-[Salen-Vim][OAc]2). The resulting catalyst showed 99% conversion and 88% selectivity to oxidative cleavage products for the oxidative cleavage of a lignin model compound (2-phenoxy-1-phenylethanone) without any base additive at 110 degrees C. The polymeric ionic liquid-tagged Salen(Ni) catalysts can be separated easily by centrifugation after the reaction and recycled for five runs with a slight loss of activity. Additionally, studies on birch lignin depolymerization indicated that polymer-supported Ni Salen catalysts were able to cleave beta-O-4 linkages to produce dimeric products. Further investigation suggests that the oxidative cleavage reaction was proceeded via a radical pathway. The polymerized Salen-Ni(ii) complex can catalyze oxidative cleavage of the lignin model compounds into high-value aromatic chemicals without any base additive.