Homogeneous and heterogeneous selective oxidation of ethyl lactate regulated by a novel vanadium complex

Abstract

As a green and sustainable alternative for producing ethyl pyruvate (EP), the selective aerobic oxidation of ethyl lactate (EL) is still facing issues such as low catalytic efficiency, harsh reaction conditions and catalyst instability. Herein, a novel crystalline vanadium-based coordination compound (VTC=VPTPYCl₂(H₂O), PTPY = 4′-phenyl-2,2′:6′,2′’-terpyridine) has been designed and synthesized. A highly efficient homogeneous catalytic system (VTC-400-O₂-CH₃CN, 400 represents the activation temperature) and a recyclable heterogeneous system (VTC-500-O₂-CH₃CN) for the liquid-phase conversion of EL to EP are developed using molecular oxygen as oxidant through activating VTC at different temperature. In VTC-400-O₂-CH₃CN system, EL can be completely converted within 4 h, and the yield of EP is 77.8 %, which outperforms the state-of-art reported catalytic system. Meanwhile, the heterogeneous VTC-500 catalyst can also promote the full conversion of EL within 4 h, and its activity remains basically unchanged after being used for 10 times. Structural characterization reveals that the soluble VTC breaks the weak coordination V-Cl bonds at a lower activation temperature of 400 ° C to expose more vanadium-based catalytic sites, but the framework of ligand remains intact. However, the organic derived carbon skeleton at higher temperature of 500 ° C can effectively confine the V-based active species and prevent the dissolution of the catalyst, which impels the transition of the homogeneous to heterogeneous catalytic system.