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 10 wt.%, 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 11.500 h-1, slightly better than the related acetoxy-functionalized NHC iridium homogeneous catalyst. A good catalyst recyclability and stability were achieved.