Oxidative Cleavage of β-O-4 bonds in Lignin Model Compounds with Polymer-Supported Ni-Salen Catalysts

Abstract

The transition metal-catalyzed lignin oxidative cleavage reactions have attracted considerable attention. In this work, the polymerized ionic liquid-tagged Salen ligands have been initially synthesized, followed by anion exchange, and then coordination with Ni(Ⅱ) via the -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 the oxidative cleavage products for the oxidative cleavage of the lignin model compound (2-phenoxy-1-phenylethanone) without any base additive at 110°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, the studies on birch lignin depolymerization indicated that the polymer-supported Ni Salen catalyst was able to cleave the β-O-4 linkages to produce the dimeric products. The further investigation suggests that the oxidative cleavage reaction was proceeded via a radical pathway.