Graphene–NHC–iridium hybrid catalysts built through –OH covalent linkage

Abstract

Graphene oxide (GO) and thermally reduced graphene oxide (TRGO) were covalently modified with imidazolium salts through their hydroxyl surface groups. The selective reaction of the –OH groups with p-nitrophenylchloroformate produced labile intermediate organic carbonate functions which were used for the covalent anchoring of a hydroxy-functionalized imidazolium salt. Nanohybrid materials containing iridium N-heterocyclic carbene (NHC)-type organometallic complexes were prepared by causing the imidazolium-functionalized materials to react with [Ir(μ-OMe)(cod)]2. The iridium content of the graphene-based hybrid catalysts, as determined by XPS and ICP-MS was the order of ∼5 and 10wt.%, for the TRGO and GO-based materials, respectively. The graphene-supported iridium hybrid materials were active in the heterogeneous hydrogen-transfer reduction of cyclohexanone to cyclohexanol with 2-propanol/KOH as the hydrogen source. The thermally reduced graphene–NHC–iridium hybrid catalyst showed the best catalytic performance with an initial TOF of 11.500h−1, slightly better than the related acetoxy-functionalized NHC iridium homogeneous catalyst. A good catalyst recyclability and stability were achieved.