Immobilization of new macrocyclic Schiff base copper complex on graphene oxide nanosheets and its catalytic activity for olefins epoxidation

A new heterogeneous copper–salophen catalyst was fabricated through the nucleophilic attack and esterification reaction of salophen Schiff base with graphene oxide (GO). At first HO-terminated Schiff base was prepared via a stepwise procedure. Subsequently, the Schiff base was covalently grafted to the activated carboxyl group on GO through esterification reaction and also through the nucleophilic attack to epoxy groups. Then, Cu(NO₃)·3H₂O was deposited on the functionalized graphene oxide. Formation of novel macrocyclic Schiff base and the covalent linkage between Schiff base and GO was confirmed by XRD, FT-IR, TGA, FESEM and EDX techniques. The prepared heterogeneous catalyst was used in the oxidation of olefins with H₂O₂/NaHCO₃ under mild condition. The main product is olefin epoxide monitored by GC. The influence of various parameters such as catalyst dosage, reaction time as well as reaction temperature on the epoxidation of norbornene was evaluated. The catalyst exhibits high activity and selectivity to target product; cyclohexene showed 100% conversion, without any by-product, and norbornene gave 100% conversion and 93% selectivity. The catalyst reused up to four more consecutive times without significant sacrificing activity. The high activity of catalyst can be ascribed to strong π–π stacking interaction between GO sheet and aromatic rings of salophen complex. Furthermore, cyclic ligand guarantees less leaching of metal.