Polyoxometalate-Intercalated Layered Double Hydroxides as Efficient and Recyclable Bifunctional Catalysts for Cascade Reactions

The polyoxometalate (POM) intercalated layered double hydroxides (LDHs) have been widely used as heterogeneous catalysts. However, the application of POM-LDHs as bifunctional catalysts for cascade reaction has seldom been studied compared to the noble-metal-based catalysts. Herein, a series of POM-LDHs catalysts of Tris-LDH-X-4(PW9)(2) (X=Mn, Fe, Co, Ni, Cu, and Zn; Tris=Tris(hydroxymethtyl)aminomethane) have been prepared. The efficacy of Tris-LDH-Zn-4(PW9)(2) as efficient bifunctional catalyst has been demonstrated for cascade reactions involving oxidation of benzyl alcohol to benzaldehyde followed by Knoevenagel condensation with ethyl cyanoacetate to produce benzylidene ethyl cyanoacetate. The combination of POM's redox/acidic sites and LDHs's basic sites led to a composite catalyst with excellent activity (99%) and selectivity (99%) under mild and soluble-base-free conditions. This work offers a new design strategy for the fabrication of efficient bifunctional catalysts for the promotion of one-pot cascade reactions.