**2. Carbocatalysis**

Carbon is one of the most abundant elements on earth and is central to life. Hence, catalytic application of carbon is very attractive and both organic and inorganic carbons play a key role in catalysis. A huge amount of organic compounds act as highly efficient homogeneous catalysts, forming a dedicated branch of chemistry "organocatalysis." Carbon is often the main constituent of the organic ligands surrounding the metallic center in organometallics. In enzymatic catalysis it constitutes the backbone of the active species. In heterogeneous catalysis, carbon materials act as unique catalyst supports by anchoring different active species through its active site and can also act as catalysts by themselves. The physical and chemical properties of carbon materials, such as their tunable porosity and surface chemistry, make them suitable for application in many catalytic processes.

Among the carbon catalysts developed, activated carbon (AC) and carbon black (CB) are the most commonly used carbon supports. The typically large surface area and high porosity of activated carbon catalysts favor the dispersion of the active phase over the support and increase its resistance to sintering at high metal loadings. The pore size distribution can be tuned to suit the requirements of active supports and substrates. The activated carbon shows several advantages owing to their several outstanding properties, such as low cost, resistance to acids and bases, high stability even at elevated temperature, high surface area (>1000 m2 /g) and easy removal etc. Moreover, metal salts can be reduced to active metallic forms in these mesoporous materials, making them highly competent as metal supports.

The study of chemical reactions using carbon materials are termed as carbocatalysis. The catalysts are prepared and used in the powder form and hence they are heterogeneous. Carbocatalysis has been known for decades since the first discovery of catalytic activities of carbon materials [16] when Rideal and Wright [17] showed the charcoal catalyzed oxidation of oxalic acid, which was one of the ground breaking discovery of carbocatalysis. Moreover, 45 years earlier also, carbon materials were shown to be effective for the conversion of halogenated hydrocarbon [16].

With the development of fullerenes, the research activities for the growth of nanocarbon materials have gained momentum. Several polytypes of carbon which include fullerenes, nanotubes, graphene, nanodiamonds and amorphous porous carbon and their derivatives represent a rich class of solid carbonaceous materials with environmental acceptability and reusability and all are found to be catalytically active in certain reactions. However, most of these carbon materials are highly hydrophobic without any functional groups on their surface.

Fullerene black is an efficient catalyst for dehydrogenation, cracking, methylation, and demethylation reactions. C60 and C70 were found to be suitable catalysts for the reduction of nitrobenzene, using hydrogen gas under UV light [18]. Further, several organometallic compounds involving fullerene as a ligand have been developed that showed efficient catalytic activity for several organic transformations.

The 1D and 2D carbon materials such as carbon nanotubes and graphene offered high surface area and continuous efforts are focused on surface functionalization of these materials, both through covalent and non-covalent approach. Oxidation in presence of strong acids and oxidants could introduce oxyfunctionalized groups on the surfaces of these carbon materials, making them hydrophilic and suitable for anchoring several active catalytic groups on their surfaces. The work on the oxidative dehydrogenation reaction by Mestl et al. [19] and Zhang et al. [20] opened a new window in carbocatalysis. Carbon nanotubes, in its oxygenated forms, showed efficient catalysis for oxidative dehydrogenations e.g. conversion of n-butane to 1-butene [20]. In the catalytic hydrogenation of ethylbenzene to styrene, a process of high industrial relevance, CNTs perform better than activated carbon and graphite as catalysts. It was reasoned that the reactant molecules were first adsorbed on the CNT surface via π-interactions next to basic oxygen moieties, which facilitated dehydrogenation with concomitant formation of surface hydroxyl groups [19]. Taking advantage of surface modification techniques, various nanoparticle as well as molecular catalysts could be anchored on carbon nanotubes [21] (**Figure 1**).

### **2.1. Graphene oxide as a carbocatalyst**

no role in manipulating the activity of the nanoparticles for catalysis. In the last few years, there has been tremendous focus on the development of "active" supports, which along with stabilizing the nanoparticles also contribute towards overall catalytic activity in synergy with the nanoparticles [12, 13]. For example, the charge state of the Au nanoparticles is known to influence their reactivity, in the case of the negatively charged Au nanoparticles, an extra electron from the gold readily transfers to the anti-bonding 2π ∗ orbital of the adsorbed O2

which weakens the O─O bond and activates oxygen molecule for further catalytic reaction. On the other hand, the positive charge accumulated on the gold can promote adsorption of some reactants, such as CO and hydrocarbons. An active support can transfer charges to/ from the active catalytic surface, hence influencing the activity of the reaction. For example,

. Along with providing significant exposed catalytic active sites for the reaction,

also involves in charge transfer process with the Au NPs making the NPs surface highly

works involving GO as a support for immobilizing active metal nanoparticles have gained attention. GO, not only provides a large surface area with high exposure of active catalysts, but also can influence the catalytic activity [15]. Possible surface to metal electron transfer from GO to nanoparticles activating dioxygen molecule over NPs surface for several oxidation reactions has been reported [15]. Hence, choice of a suitable support for NPs stabilization with possible cooperativity might play an important role in controlling the reaction yield and

Carbon is one of the most abundant elements on earth and is central to life. Hence, catalytic application of carbon is very attractive and both organic and inorganic carbons play a key role in catalysis. A huge amount of organic compounds act as highly efficient homogeneous catalysts, forming a dedicated branch of chemistry "organocatalysis." Carbon is often the main constituent of the organic ligands surrounding the metallic center in organometallics. In enzymatic catalysis it constitutes the backbone of the active species. In heterogeneous catalysis, carbon materials act as unique catalyst supports by anchoring different active species through its active site and can also act as catalysts by themselves. The physical and chemical properties of carbon materials, such as their tunable porosity and surface chemistry, make them suitable

Among the carbon catalysts developed, activated carbon (AC) and carbon black (CB) are the most commonly used carbon supports. The typically large surface area and high porosity of activated carbon catalysts favor the dispersion of the active phase over the support and increase its resistance to sintering at high metal loadings. The pore size distribution can be tuned to suit the requirements of active supports and substrates. The activated carbon shows several advantages owing to their several outstanding properties, such as low cost, resistance to acids and bases, high stability even at elevated temperature, high surface area (>1000 m2

negative for dioxygen activation leading to oxidation of CO to CO2

(110) surface shows high activity for the oxidation

[14]. Recently, research

Au nanoparticles anchored on rutile TiO2

28 Graphene Oxide - Applications and Opportunities

for application in many catalytic processes.

of CO to CO2

selectivity of products.

**2. Carbocatalysis**

TiO2

,

/g)

Graphene and other two-dimensional sp2 -hybridized carbon scaffolds are expected to have large impacts in the area of catalysis, mainly because of their unique electronic properties

**Figure 1.** Various forms of carbon nanomaterials.

and high surface area in comparison to other carbon materials [22]. Although graphene was known to exist within graphite materials, it was assumed to be thermodynamically unstable in distinct 2D structures at finite temperatures. Geim et al. (2004) [23] mechanically exfoliated single sheets from the π-stack layers in graphite for the first time. The unique electron transfer properties of graphene, such as a half-integer quantum Hall effect, the massless Dirac fermion behavior of its charge carriers, and quantum capacitance, have been extensively studied making them one of the most important materials in optoelectronics utility. The use of graphenebased nanomaterials as catalyst support was hampered by the high price associated with the laborious synthesis and processing (e.g., sublimation of silicon from silicon carbide wafers, chemical vapor deposition, oxidation/reduction protocols etc. However, the process for liquid phase exfoliation through oxidation of graphite in presence of strong oxidizing agents generating the graphene analogue with oxygenated functionalities on their surface (popularly known as Hummer's method) has brought tremendous excitement in the nanocatalyst research community. These materials termed as "graphene oxide" can be obtained in sufficient quantities from commercially available graphite through reliable, now well-established preparation procedures. Further potential chemical modifications of the graphene surface introduces different newer catalytically active site important for specific catalytic reactions (**Figure 2**) [23].

Oxidation of graphite leads to the decoration of the graphene surface with oxygen functionalities that increases the inter-layer separation, thus helping in exfoliation into single or a few layer two-dimensional surfaces. During this process, several sites are induced those are important from catalytic or surface modification point of view. The extensive π-conjugated domains provide interactions between aromatic compounds with the graphene surfaces and greatly facilitate the adsorption/activation of aromatic compounds on graphene based carbon [23]. There are at least five different oxygen functional groups decorated over the graphene surface. These include carboxyl (─COOH), hydroxyl (─OH), carbonyl (─C═O), epoxy (─C─O─C─ and ketone (─C═O) groups. These oxygenated groups provide four different categories of catalytic activity to the carbon material: (1) their acidic properties promote acid-catalyzed reactions; (2) their intermediate form reacts with oxidants to catalyze oxidation reaction; (3) their nucleophilic nature promote coupling reactions; and (4) their perfect π-conjugated structure with significant defects/holes can also promote several catalytic reactions. Further, reduction of graphene oxides can be performed using common reducing agents such as hydrogen, metal ion borohydride and hydrazine. Moreover, the graphene

**Figure 2.** Possibilities of covalent functionalization of GO (reprinted with permission from Ref. [23]. American Chemical

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Society).

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**Figure 2.** Possibilities of covalent functionalization of GO (reprinted with permission from Ref. [23]. American Chemical Society).

and high surface area in comparison to other carbon materials [22]. Although graphene was known to exist within graphite materials, it was assumed to be thermodynamically unstable in distinct 2D structures at finite temperatures. Geim et al. (2004) [23] mechanically exfoliated single sheets from the π-stack layers in graphite for the first time. The unique electron transfer properties of graphene, such as a half-integer quantum Hall effect, the massless Dirac fermion behavior of its charge carriers, and quantum capacitance, have been extensively studied making them one of the most important materials in optoelectronics utility. The use of graphenebased nanomaterials as catalyst support was hampered by the high price associated with the laborious synthesis and processing (e.g., sublimation of silicon from silicon carbide wafers, chemical vapor deposition, oxidation/reduction protocols etc. However, the process for liquid phase exfoliation through oxidation of graphite in presence of strong oxidizing agents generating the graphene analogue with oxygenated functionalities on their surface (popularly known as Hummer's method) has brought tremendous excitement in the nanocatalyst research community. These materials termed as "graphene oxide" can be obtained in sufficient quantities from commercially available graphite through reliable, now well-established preparation procedures. Further potential chemical modifications of the graphene surface introduces different newer catalytically active site important for specific catalytic reactions

(**Figure 2**) [23].

**Figure 1.** Various forms of carbon nanomaterials.

30 Graphene Oxide - Applications and Opportunities

Oxidation of graphite leads to the decoration of the graphene surface with oxygen functionalities that increases the inter-layer separation, thus helping in exfoliation into single or a few layer two-dimensional surfaces. During this process, several sites are induced those are important from catalytic or surface modification point of view. The extensive π-conjugated domains provide interactions between aromatic compounds with the graphene surfaces and greatly facilitate the adsorption/activation of aromatic compounds on graphene based carbon [23]. There are at least five different oxygen functional groups decorated over the graphene surface. These include carboxyl (─COOH), hydroxyl (─OH), carbonyl (─C═O), epoxy (─C─O─C─ and ketone (─C═O) groups. These oxygenated groups provide four different categories of catalytic activity to the carbon material: (1) their acidic properties promote acid-catalyzed reactions; (2) their intermediate form reacts with oxidants to catalyze oxidation reaction; (3) their nucleophilic nature promote coupling reactions; and (4) their perfect π-conjugated structure with significant defects/holes can also promote several catalytic reactions. Further, reduction of graphene oxides can be performed using common reducing agents such as hydrogen, metal ion borohydride and hydrazine. Moreover, the graphene oxide surfaces can be reduced by heating at elevated temperature. Various heteroatoms such as N, B, P, Se, S, F, and Cl [24] can be incorporated into the lattice of graphene sheets. Several organic reactions can also incorporate acidic functional groups such as ─SO3 H groups onto graphene sheets [25].

surface enhances several folds compared to the unsupported enzymes towards the oxidation reaction of pyrogallol [27]. The importance of oxygen functional groups on GO surface has been exploited towards several C─H activation and C─C coupling reactions. The carbocatalytic activity of graphene oxide has successfully been exploited by Ma et al. for the C─H bond arylation of benzene enabling biaryl construction. The oxygen functional groups in these gra-

catalytic activity. Several model reactions and DFT calculations confirmed that the negatively charged oxygen atoms promote the overall transformation by stabilizing and activating K+ ions, which in turns facilitates the activation of the C─I bond. The π basal plane also greatly facilitates the overall reaction as the aromatic coupling partners are easily adsorbed on the 2d surface [36]. Transition-metal-catalyzed alkylation reactions of arenes have turn out to be a central transformation in organic synthesis. Szostak et al. developed the first general strategy for alkylation of arenes with styrenes and alcohols catalyzed by carbon-based materials, exploiting the unique surface property of graphene oxide to produce valuable diarylalkanes with excellent yields and regioselectivity. Remarkably, this protocol represents the first general application of graphene oxide to promote direct C─C bond formation utilizing oxygenated functional groups present on GO surface [37]. Recently, our research group have demonstrated the dual carbocatalytic activity of graphene oxide for the C─N coupling reaction towards the formation of α-ketoamides through a cross-dehydrogenative coupling pathway [38]. The presence of polar functional groups (e.g., carboxyl, hydroxyl, ketonic, and epoxides) on graphene oxide surface induce acidic as well as oxidizing properties to the material. This dual catalytic property of the material is explored towards the generation of α-ketoamides where surface acidity favors the initial formation of hemiaminal intermediate followed by oxidation leading to the desired final product. Several control experiments as well as thermal treatment showed that it is the oxygen functional groups, especially carboxylic acid group that is only responsible for the observed catalytic activity. The protocol could also be extended towards the synthesis of biologically important α-ketoamides. On the other hand graphene surface can also be used as support for immobilization of several metal/metal oxide nanoparticles and used for several electrocatalysis, photocatalysis and organic transformations [39, 40]. For example, Pd nanoparticle immobilized on graphene oxide gave remarkable turnover frequencies (TOF > 39,000 h−1) in Suzuki-Miyaura cross-coupling reactions. Microwave assisted reduction of well-dispersed GO and palladium salt to form Pd/rGO [41] demonstrated outstanding catalytic activity for the Suzuki-Miyaura coupling reaction (TOF up to 108,000 h−1) under ligand-free conditions, which

was attributed to the high concentration of well dispersed Pd-NPs.

The carbon based nanomaterials have already demonstrated their enormous potential either as catalysts or heterogeneous catalyst supports. Graphene oxide with oxygenated functional groups on their surface could act as active sites for various acid catalyzed and oxidative catalytic reactions. Recent advancement of these graphene based materials shows that the modification of graphene surface by different methods leads to generation of holes which acts as traps

**3. Current and future prospect**

Bu were demonstrated to be essential for the observed

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phene oxide sheets and presence of KOt

GO and their chemically converted forms have shown broad spectrum of catalytic activity ranging from oxidation reactions and thermal decomposition reactions. Bielawski et al., first demonstrated catalytic activity of graphene oxide for liquid phase organic transformations [26]. Since then, a variety of organic transformations have been explored taking advantage of the functional groups present on the graphitic surface. **Table 1** summarizes a variety of reported reactions catalyzed by GO and chemically converted GO.

Further, the two-dimensional surface of graphene based materials can be used to anchor other active catalysts as well as biocatalysts. For example, the catalytic activity of several enzymes including cytochromes, peroxidases, myoglobins, and hemoglobins supported on graphene


**Table 1.** Catalytic reactions by GO and chemically converted GO. (B, N)-doped holy G [(BN)HolG], reduced graphene oxide (rGO), triethylamine modified rGO (rGO-NEt<sup>3</sup> ), rGO functionalized with ─SO3 H (rGO-SO3 H) and poly(amidoamine)-modified rGO (rGO-PAMAM).

surface enhances several folds compared to the unsupported enzymes towards the oxidation reaction of pyrogallol [27]. The importance of oxygen functional groups on GO surface has been exploited towards several C─H activation and C─C coupling reactions. The carbocatalytic activity of graphene oxide has successfully been exploited by Ma et al. for the C─H bond arylation of benzene enabling biaryl construction. The oxygen functional groups in these graphene oxide sheets and presence of KOt Bu were demonstrated to be essential for the observed catalytic activity. Several model reactions and DFT calculations confirmed that the negatively charged oxygen atoms promote the overall transformation by stabilizing and activating K+ ions, which in turns facilitates the activation of the C─I bond. The π basal plane also greatly facilitates the overall reaction as the aromatic coupling partners are easily adsorbed on the 2d surface [36]. Transition-metal-catalyzed alkylation reactions of arenes have turn out to be a central transformation in organic synthesis. Szostak et al. developed the first general strategy for alkylation of arenes with styrenes and alcohols catalyzed by carbon-based materials, exploiting the unique surface property of graphene oxide to produce valuable diarylalkanes with excellent yields and regioselectivity. Remarkably, this protocol represents the first general application of graphene oxide to promote direct C─C bond formation utilizing oxygenated functional groups present on GO surface [37]. Recently, our research group have demonstrated the dual carbocatalytic activity of graphene oxide for the C─N coupling reaction towards the formation of α-ketoamides through a cross-dehydrogenative coupling pathway [38]. The presence of polar functional groups (e.g., carboxyl, hydroxyl, ketonic, and epoxides) on graphene oxide surface induce acidic as well as oxidizing properties to the material. This dual catalytic property of the material is explored towards the generation of α-ketoamides where surface acidity favors the initial formation of hemiaminal intermediate followed by oxidation leading to the desired final product. Several control experiments as well as thermal treatment showed that it is the oxygen functional groups, especially carboxylic acid group that is only responsible for the observed catalytic activity. The protocol could also be extended towards the synthesis of biologically important α-ketoamides. On the other hand graphene surface can also be used as support for immobilization of several metal/metal oxide nanoparticles and used for several electrocatalysis, photocatalysis and organic transformations [39, 40]. For example, Pd nanoparticle immobilized on graphene oxide gave remarkable turnover frequencies (TOF > 39,000 h−1) in Suzuki-Miyaura cross-coupling reactions. Microwave assisted reduction of well-dispersed GO and palladium salt to form Pd/rGO [41] demonstrated outstanding catalytic activity for the Suzuki-Miyaura coupling reaction (TOF up to 108,000 h−1) under ligand-free conditions, which was attributed to the high concentration of well dispersed Pd-NPs.
