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

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.