Immobilization of transition metal (Fe2+, Co2+, VO2+ or Cu2+) Schiff base complexes onto graphene oxide as efficient and recyclable catalysts for epoxidation of styrene

Abstract

Transition metal (Fe²⁺, Co²⁺, VO²⁺ or Cu²⁺) Schiff base complexes were immobilized onto graphene oxide previously functionalized with 3-aminopropyltriethoxysilane (3-APTES). X-ray diffraction (XRD), IR spectroscopy, thermal gravimetric analyses (TGA) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) confirmed the successful incorporation of the metal Schiff base onto the graphene oxide. N₂ adsorption–desorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed the intact structure of the graphene oxide. Catalytic results showed that the immobilized copper(II) Schiff base complex catalyst was more active than the immobilized iron(II), cobalt(II) and oxovanadium(IV) complexes in the epooxidation of styrene. Above 94% styrene conversion and excellent ∼99% selectivity to the epoxide could be achieved over the copper(II) Schiff base heterogeneous catalyst in the epoxidation of styrene using tert-butyl hydroperoxide (TBHP) as oxidant after 7 h reaction. The recycling experiment results indicated that the catalyst could maintain very high styrene conversion (>93%) and epoxide selectivity (>99%) even after being used for four cycles.