Covalent Methods for Functional Carbons' Synthesis

J.-P. TESSONNIER, R. G. RAO, G. GIAMBASTIANI AND G. TUCI

1.1 Towards a Molecular Understanding of the Reactivity of Carbon Surfaces

1.1.1 Role of Curvature and Topological Defects on Surface Reactivity

The unique chemico–physical properties of carbon-based materials in their various allotropic forms have made them valuable candidates for application in many catalytic processes. Traditionally, carbon materials have been employed as supports for metal-based multi-phase catalysts in heterogeneous processes although their role (innocent or non-innocent) in the catalytic performance remains largely questionable. The last few years have witnessed a technological renaissance that has boosted the exploitation of carbon-based nanomaterials for running a number of key industrial transformations with good to excellent catalytic outcomes. This renaissance is in line with seminal papers published in the 1960s by Donnet, Boehm, and Coughlin, who showed that the catalytic performance (activity and selectivity) of carbon catalysts is related to two largely interconnected features: the chemistry and electronic properties of their outer surfaces.

Many efforts in the scientific community have been devoted to the understanding of molecular-level phenomena that govern carbon material reactivity. The discovery of well-defined carbon nanomaterials in the 1980s and 1990s, namely fullerene and carbon nanotubes, offered unprecedented opportunities to advance our understanding of carbon materials' rich chemistry. Works on defect-free fullerenes and single-walled carbon nanotubes (SWCNTs) revealed notable connections between the curvature and the reactivity of carbon surfaces. For instance, for fullerenes, the reactivity of sp2 hybridized carbons was demonstrated to be a result of the strain induced by the curvature. Indeed, any C-sp2 carbon atom within a high curvature environment (radius of curvature from 5 to 10 A) will undergo a strain-induced pyramidalization of its orbitals. This pyramidalization strongly facilitates addition reactions, thus breaking C=C bonds and inducing atoms' relaxation due to rehybridization to C-sp3. Chirality ordinarily present in SWCNTs creates additional strain on C=C bonds due to the "twisting" of the rolled graphene sheet that leads to a misalignment of the pz orbitals. The high reactivity of strained carbon surfaces is not limited to well-defined nanocarbons like fullerene and SWCNTs, but it actually occurs on any site with a high local curvature. In the case of Stone–Wales defects (Figure 1.1), the presence of C5 and C7 rings would induce curvature in the carbon materials.

1.1.2 Surface Modification through Heteroatom Insertion

Typically, topological defects are reactive sites and their interaction with oxygen leads to modification of surface properties. Their oxidation generates a variety of oxygen-containing functional groups (carboxylic acid, anhydride, hydroxyl, lactone, ether), which alter both the polarity and acid–base properties of the carbon surface. Deep oxidation eventually results in the formation of CO2 and the creation of a vacancy. This reactivity explains why carbons present a quilted surface consisting of defect-free graphitic domains interconnected by regions with amorphous carbon (sp3), point defects, and vacancies. The interaction of defects with oxygen also introduces oxygen-doping functionalities on the surface of carbon materials, for example around vacancies as well as at the edge/prismatic plane carbon atoms as these sites are far more reactive towards oxidation than that of basal plane carbons. For instance, carbon atoms on the edges of graphene can lead to activation of O2 to form epoxides that migrate or hop on the surface of the basal plane. Therefore, the presence of edges or defects is essential for introducing heteroatoms in the basal plane of carbons.

Defects are generally introduced in carbon materials during synthesis as a result of fast reaction kinetics compared to the thermodynamically-favored formation of a defect-free honeycomb structure. Alternatively, the post-synthetic introduction of new defects typically relies on the use of strong oxidants able to disrupt carbon–carbon double bonds, such as the mixture of nitric acid and potassium chlorate as proposed by B. C. Brodie in 1859 for the oxidation of graphite. Similarly, the works of Staudenmaier, Hummers, and Offeman utilized potassium permanganate and sulfuric acid for further improving the process of graphene oxide (GO) synthesis. The introduction of defects and heteroatoms disrupts the delocalized electron cloud and alters the electronic properties of the surfaces, allowing the grafting of functional groups on the surface of nanocarbons.

In recent years, the heteroatom-doping (N, B, P, S) of carbon nanomaterials has emerged as an effective way to manipulate the electronic properties of nanocarbons. The inclusion of hetero-elements in the honeycomb carbon structure induces a deep redistribution of the electronic properties. An ever-growing number of light hetero-doped carbons (preferentially containing nitrogen) are nowadays employed with success as metal-free catalysts. Selected systems from this series can offer higher catalytic activity and long-term operational stability than those measured with benchmark state-of-the-art metal-based systems. Post-synthetic chemical and thermal treatments (typically with strong and oxidizing acids or bases) have also been used to tune the materials' surface chemistry. The control of porosity and defect-sites' surface density along with the introduction of dangling (acidic or basic) functional groups is the new contemporary approach to prepare a family of new powerful catalysts. However, both heteroatom-doping and post-synthetic treatments generally suffer from moderate control on the nature of the chemical surface...