Metal Complexes Of Schiff Base Supported On Graphene Oxide: Synthesis, Characterization And Catalytic Properties In Olefin Epoxidation

The catalytic epoxidation of olefins is a significant theme in the production offine chemicals, pharmaceuticals and agrochemicals. Epoxidation of olefin tocorresponding epoxides, is one of the most important chemical intermediates with thehighest economic value in organic synthesis. Schiff base is a bidentate ligand derivedfrom an aldehyde and an amine and it can coordinate with stable transition metalcomplexes. Schiff base complexes have received much attention, mainly because oftheir extensive application in the fields of synthesis and catalysis. Homogeneoustransition metal Schiff base complexes have attracted much interest in the field ofcatalysis because of their reversible bonding oxygen ability and high activities in theoxidation reactions.In recent decades, the heterogenization of homogeneous catalysis has becomeone of the important research areas because it allows the design of eco-friendlycatalysts. Many materials have been tested for heterogenization, including carbonmaterials, mesoporous silica, clays, organic polymers and metal organic frameworksetc. The immobilization of transition metal complexes onto supports is a significantresearch to make costly homogeneous catalysts high efficiency, recyclability,low-price and environmental desire.Graphene has been raised great expectation in the field of optical, sensor, drugsdeliver, composite materials, catalysis and cells and many other fields due to itsunique nanostructure and remarkable properties—high specific surface areas, specialelectronic properties, high thermal conductivity. Graphene oxide (GO), as a precursorto graphene, is synthesized by oxidization and exfoliation of graphite. For example,graphene oxide is heavily oxygenated and has hydroxyl and epoxide groups on itsbasal planes as well as carbonyl and carboxyl groups located at the sheet edges. Theplenty oxygenic functional groups in graphene oxide could immobilize varioushomogeneous materials as active sites. (1) Transition metal (Fe2+, Co2+, VO2+or Cu2+) Schiff base complexes were directlyanchored onto amino-modified grephene oxide. Catalytic results showed that theimmobilized copper(Ⅱ) Schiff base complex catalyst was more active than theimmobilized iron(Ⅱ), cobalt(Ⅱ) and oxovanadium(Ⅳ) complexes in theepooxidation of styrene. Above94%styrene conversion and excellent99%selectivity to the epoxide could be achieved over the copper(Ⅱ) Schiff baseheterogeneous catalyst in the epoxidation of styrene using tert-butylhydroperoxide (TBHP) as oxidant after7h reaction. The recycling experimentresults indicated that the catalyst could maintain very high styrene conversion(>93%) and epoxide selectivity (>99%) even after being used for four cycles.(2) Cobalt(Ⅱ), iron(Ⅲ) or oxovanadium(Ⅱ) salicylaldimine Schiff base metalcomplexes have been covalently grafted onto graphene oxide (GO). Co-GO andFe-GO showed high styrene conversion (90.8%vs.86.7%) and epoxideselectivity (63.7%vs.51.4%). Nevertheless, VO-GO showed lower catalyticperformance compared with Co-GO and Fe-GO. The recycling results of theseheterogeneous catalysts showed good recoverability without significant loss ofactivity and selectivity within successive four runs.