Ultra-low Mn(salen) supported on lignin and selective epoxidation of olefins

Precise design of highly efficient, low metal load and eco-friendly catalysts is the key to sustainable catalysis and green chemistry. In this work, we develop an ultra-low loading of Mn(salen) complexes covalent grafted with lignin and applied in the selective epoxidation of olefins using O-2 as an oxidant. Lignin not only provides the Mn(salen) graft sites but also promotes substrate enrichment via the pi-stacking interaction between substrate and supports. In particular, high turnover frequencies (TOF) of 1833 h(-1) can be obtained for styrene epoxidation under mild conditions, which can be superior to most reported Mn(salen) supported catalysts. Compared with cellulose, the lignin-based catalysts exhibit a higher selectivity toward styrene oxide during the epoxidation of styrene under mild conditions. Controlled experiments indicate that the synergistic effect between phenolic hydroxyl groups and the Mn(salen) complex can improve the conversion rate and selectivity during the selective epoxidation of styrene. This work designs a series of lignin-based catalysts with ultra-low Mn(salen) loading, which broadens the utilization of lignin and gives enlightenment for the rational design of lignin-based catalysts.