Synthesis of less acidic VO-salen complex grafted onto graphene oxide via functionalization of surface carboxyl groups for the selective oxidation of norbornene

Oxidation of norbornene was carried out over oxovanadium Schiff base complex grafted on –OH and –COOH modified graphene oxide (GO) as heterogeneous catalysts using 30% H2O2 as an oxidant and ethylene glycol as a solvent. Amongst them, the carboxylic acid group modified GO was found to be better for this transformation. The lower conversion with –OH modified catalyst was ascribed to the higher acidic nature of the catalyst. NH3-TPD study confirmed the less acidic sites on the –COOH modified catalyst and this less acidic nature and controlled addition of 30% H2O2 became the centre of action for this catalytic transformation. Absolute conversion of norbornene (100%) with 98.8% 2,3-epoxy norbornane was achieved at 60 °C in 1 h. This catalyst was recycled four times without significant loss of activity. This durability of the catalyst was believed to be due to strong π–π stacking interaction of GO sheet and an unsaturated ring of ligand and cyclic structure of ligand which protect the metal to leach out.