Carbocatalysts: Graphene Oxide and Its Derivatives

Abstract

Graphene oxide (GO) sheets are emerging as a new class of carbocatalysts. Conventionally, researchers exfoliate graphite oxide into submicrometer-sized, water-dispersible flakes to produce these sheets. The presence of oxygen functional groups on the aromatic scaffold of GO allows these sheets to mediate ionic and nonionic interactions with a wide range of molecules. GO shows remarkable catalytic properties on its own and when hybridized with a second material. It is a perfect platform for molecular engineering. This Account examines the different classes of synthetic transformations catalyzed by GO and correlates its reactivity with chemical properties. First, we raise the question of whether GO behaves as a reactant or catalyst during oxidation. Due to its myriad oxygen atoms, GO can function as an oxidant during anaerobic oxidation and become reduced at the end of the first catalytic cycle. However, partially reduced GO can continue to activate molecular oxygen during aerobic oxidation. Most importantly, we can enhance the conversion and selectivity by engineering the morphology and functionalities on the G/GO scaffold. GO can also be hybridized with organic dyes or organocatalysts. The photosensitization by dyes and facile charge transfer across the graphene interface produce synergistic effects that enhance catalytic conversion. Using GO as a building block in supramolecular chemistry, we can extend the scope of functionalities in GO hybrids. The presence of epoxy and hydroxyl functional groups on either side of the GO sheet imparts bifunctional properties that allow it to act as a structural node within metal-organic frameworks (MOFs). For example, known homogeneous molecular catalysts can be anchored on the GO surface by employing them as scaffolds linking organometallic nodes. We have demonstrated that porphyrin building blocks with GO can lead to facile four-electron oxygen transfer reactions. We have also evaluated the advantages and disadvantages of GO as a catalytic material relative to other types of catalysts, both metallic and nonmetallic. Researchers would like to increase the potency of GO catalysts because many catalytic reactions currently require high loading of GO. Further research is also needed to identify a low-cost and environmentally friendly method for the synthesis of GO.