Catalyst Enhancement and Recyclability by Immobilization of Metal Complexes onto Graphene Surface by Noncovalent Interactions

Abstract

The immobilization of a homogeneous catalyst onto a solid surface is one of the major challenges in catalysis, because it may facilitate the separation of the catalyst and the reaction products and may also give rise to the reutilization of the catalyst in multiple subsequent cycles. Noncovalent interactions between the catalyst and the support are arising as interesting alternatives to the more widely used covalent interactions, because they avoid the functionalization of both the catalyst and the surface, which may in turn lead to the modification of the inherent properties of the catalyst. However, some other problems may arise, such as leaching. In this work, we have obtained two complexes containing an N-heterocyclic carbene ligand with a pyrene tag, which we immobilized onto the surface of reduced graphene oxide (rGO), by π-stacking. The catalytic properties of the parent molecular complexes and hybrid materials have been studied in the palladium-catalyzed hydrogenation of alkenes and the ruthenium-catalyzed alcohol oxidation. The results show that the catalytic properties are improved in the hybrid materials, compared to the catalytic outcomes provided by the homogeneous analogues. Although the palladium-catalyzed reactions may be due to the formation of Pd nanoparticles, the ruthenium-catalyzed ones are facilitated by the supported molecular catalyst. The catalyst stability was analyzed by means of recyclability studies, hot filtration test, and large-scale experiments. Both hybrid materials have been reused up to 10 times without any decrease in activity, affording quantitative yield of products. The hot filtration experiment reveals that the catalysis is heterogeneous in nature without any detectable leaching or boomerang effect. The work constitutes a clear improvement over other known immobilization methodologies and offers a practical methodology which may inspire future developments of efficient heterogenized catalysts.