Catalytic depolymerization of lignin over cesium exchanged and transition-metal substituted heterogeneous polyoxometalates

It is of great importance to develop new pathways for low-cost, high-selectivity conversion of lignin into aromatics. In this work, a series of cesium exchanged and transition-metal substituted heterogeneous polyoxometalates were prepared and applied as efficient catalysts for lignin depolymerization. The catalytic oxidation of lignin has been monitored by GPC, 2D NMR HSQC, and GC/MS technologies, and the reaction pathway was also confirmed by the examination with a dimeric β-O-4 lignin model compound. Under optimal conditions, these catalysts showed high activity toward oxidative cleavage of β-O-4 linkages, as well as β-5 and β-β CC linkages. In particular, the Co substituted polyoxometalates gave high yield of ca. 9.58% monomeric products at 150 °C within 3 h under an oxygen atmosphere. Results demonstrated the catalyst was easily separated from products and could be repeatedly used at least five cycles without significant loss of activity. Further, the possible reaction mechanism was proposed by a two-step oxygen-based electron transfer and oxygen transfer reaction mechanism. The design and application of the multifunctional POMs based heterogeneous catalytic system make this strategy of great interest in the production of aromatic products from lignin and inspire a new insight to utilize the entire lignocellulosic biomass.