In addition to the use of acids and bases, catalysis is currently dominated by the use of transition metals, as free ions, coordination complexes, clusters, or nanoparticles, acting as active sites [1–6]. Some of these transition metals are rare, and questions of sustainability make the search of alternatives mandatory. This is one of the reasons why there is a growing interest in exploring the potential of carbonaceous materials with large specific surface area as heterogeneous organocatalysts [7–12]. Carbon‐based materials in catalysis as such is not a new topic, and there have been several reactions such as removal of nitroxides, ozonizations, and other oxidations that are known to be catalyzed by active carbons (ACs). However, with the recently massively improved availability of graphenes (G), fullerenes, graphene oxides (GO), and other more sophisticated carbon nanostructures (CNS), the field has experienced “explosive” progress, worth to be discussed [10, 13–15].

One advantage of nanocarbons is that they have a large specific surface area making indeed most of the constituting atoms accessible for catalysis. In general terms, transition metals can promote reactions by acting as Lewis acids, by redox centers transferring electrons, and through the formation of organometallic intermediates. Indeed, nanocarbons can exhibit similar catalytic properties, by introducing acid and electrophilic centers, redox pairs, and by charge transfer interactions. The aim of this chapter is to summarize the types of centers that have been currently proposed as responsible for the catalytic activity of carbon materials in different reactions, presenting the evidence that supports these claims and describing the reaction mechanism proposed for different reaction types. The aspects emphasizing the importance of the collective properties of carbon in catalysis by adsorbing substrates and activating them through charge transfer have also been included. The final section provides our view on some of the perspectives of the field.

10.1 How to Identify Active Sites?

Determining the nature of the active sites in the progress of carbocatalysis is problematic [16]. Knowledge of the structure of the active sites can, thereby, also help in devising the optimal preparation conditions and suitable post‐synthetic modification treatments to increase their population. The long‐term goal is of course manipulation of nanocarbons with atomic precision in such a way that it could be possible to create high populations of isolated single sites on the frameworks.

One of the most impressive tools to propose active sites in carbocatalysis is to establish valid models of the proposed sites and perform quantum chemical calculations, i.e. how these models interact with substrates establishing plausible reaction mechanisms with estimation of energy barriers. However, several issues that limit the validity of this approach are as follows: how close the models really are to the reality of nanocarbons, the size of the models, and the accuracy level of the calculations. In addition, theoretical studies have always shown their highest utility when they are accompanied by experimental data to validate conclusions from calculations.

A more elemental methodology to propose the nature of the active sites is based on the comparison of the activity of series of different well‐characterized samples in which some parameter is gradually changed. As an example, one simple case would be to determine the activity of a dopant element via preparing a series of nanocarbons with different dopant contents. The problem is that most frequently, other parameters are changed at the same time, e.g. oxygen percentage or the electronic work functions, and the resulting variations of the catalytic activity can be derived from the concurrent alteration of effects.

One of the most convincing strategies to provide evidence in support of the nature of the active sites is post‐functionalization of the parent AC material by performing a selective reaction that masks one type of functional group, i.e. a “knock out” experiment. If the group masked is involved in the catalysis, a significant decrease in the catalytic activity should be observed. This strategy was, for instance, used by Su and coworkers to analyze the nature of active sites on carbon nanotubes for the hydrogenation of nitro groups by hydrazine, observing that only derivatization of the carbonyl groups leads to deactivation of carbon nanotubes.

All these techniques about active sites complement each other and could also be used favorably.

10.2 Oxygen Atoms in Carbon‐Driving Catalysis

Several oxygen functional groups have been reported as active sites in various carbocatalysts, particularly in GO and reduced graphene oxide (rGO)‐based systems. Concerning nucleophilicity, density functional theory (DFT) calculations have revealed that the carbonyl oxygen of quinone‐like groups is the most nucleophilic site compared with the other oxygen atoms, such as carboxyl, 1,2‐ and 1,3‐diketones, isolated ketones, or lactones [17]. Ag⁺‐binding energy calculations were employed to theoretically establish the relative oxygen nucleophilicity order based on electron density parameters. The best coordinating center is in fact a diketone in zigzag configuration.

Selective reduction of nitroarenes to the corresponding anilines is known to be catalyzed by natural graphite [18], fullerene [19], and rGO [20]. Recently, the role of different oxygen functional groups on a carbon catalyst in this reduction of nitrobenzene by hydrazine has been studied using a series of model molecules [21]. It was observed that the carbonyl and hydroxyl groups are the most likely centers in the activation of hydrazine as a reducing molecule. In contrast, the ester, ether, and lactone groups seemed to be inactive to promote this reduction, whereas the carboxylic group has a negative effect, probably due to acid–base interaction with hydrazine. It was found that the use of 9,10‐anthraquinone as a model catalyst affords 97.7% conversion with 98.4% selectivity to aniline. The reduction of nitrobenzene by 9,10‐anthraquinone occurs both under helium atmosphere and in air, which suggests that the active hydrogen resulting from hydrazine decomposition is the real reducing species. It is believed that the reduction of nitrobenzene can take place either through direct or through condensation pathway [22]. In the direct pathway, nitrobenzene is reduced to nitrosobenzene, hydroxylamine, and aniline, successively, whereas for the condensation pathway, nitrosobenzene reacts with hydroxylamine to form azoxybenzene, which is further reduced to azobenzene, hydrazobenzene, and aniline. A series of control experiments revealed that the most likely mechanism while using 9,10‐anthaquinone as a catalyst proceeds via the direct route.

In the context of simultaneous removal of SO₂ and NO_x from flue gases at temperatures lower than 150 °C, calculations at DFT level predict that the epoxy groups of GO can oxidize both SO₂ and NO_x close to room temperature [23]. Neighboring hydroxyl groups on the GO surface can enhance the adsorption and oxidation of SO₂ and NO. In the case of SO₂, oxidation can occur through charge transfer, taking place between SO₂ and epoxide and hydroxyl groups. The oxidation is enhanced by the introduction of more hydroxyl groups near the active site, as there are more adsorption sites. In the case of NO, a somewhat different mechanism occurs, and interaction with the hydroxyl group leads to the formation of covalent NC bond between the adsorbed NO molecules and the GO surface.

Another field where oxygen functionalities have disclosed their remarkable role is advanced oxidation processes (AOPs) for wastewater remediation [24, 25]. They are based on the generation of highly reactive oxidant species such as hydroxyl radicals (HO^·), superoxide (O₂^·−), or sulfate radicals (SO₄^·−). Here, the Fenton reaction has attracted considerable attention from academic and industrial point of view [26, 27] and is usually based on the generation of hydroxyl radicals by reduction of H₂O₂ using transition metals such as Fe²⁺. This process can be assisted by irradiation using UV–Vis light [28, 29]. Current drawbacks hampering the wide application of this process are (i) the need of acidic pH values (∼3) to achieve appropriate Fe²⁺ speciation, (ii) the high H₂O₂ consumption due to the parallel disproportionation to H₂O and O₂, (iii) the stoichiometric consumption of transition metal leading to the formation of sludges that have to be removed at the end of the treatment, and (iv) the need of irradiation using artificial UV–Vis lamps. In order to overcome at least some of these limitations, heterogeneous catalytic Fenton reactions based on the use of sub‐stoichiometric amounts of metals [30], metal oxides [30, 31], aluminosilicates [32], but also nanocarbons, have been considered an alternative [16, 33].

10.1

10.2

In this context, Garcia and coworkers have proposed that hydroquinone/quinone‐like functional groups of carbonaceous materials can act as catalytic sites, making these materials efficient carbocatalysts for the (photo) Fenton reaction [34]. Among the different graphenes tested under dark conditions, it was found that the order of activity for phenol degradation and H₂O₂ decomposition is G∼rGO > (B)G > (B,N)G > (N)G > GO, while no activity was found in the absence of catalyst [34]. For the most active “G” catalyst (derived from pyrolysis of alginate, having an oxygen content of 8 wt%) and rGO, similar kinetic profiles for phenol degradation and H₂O₂ decomposition were found. In contrast, the activity of GO is negligible for both phenol degradation and H₂O₂ decomposition. This observation indicates that the total oxygen content in this case does not correlate with the observed catalytic activity. In the case of using (N)G as a catalyst, H₂O₂ decomposes at much higher reaction rates than phenol disappears, i.e. this material catalyzes the disproportionation. In the case of (B)G, an induction period was observed for phenol degradation, but not for H₂O₂ decomposition [34]. This induction period characterized by H₂O₂ consumption was correlated with the observation of boron leaching from the solid material to the solution, as revealed by ICP measurements. Interestingly, using the most active materials G and rGO, the apparent activation energies for phenol degradation and H₂O₂ decomposition were almost similar, 31 and 30 kJ mol⁻¹, respectively. This observation is compatible with the fact that the generation of HO^· radicals from H₂O₂ is the rate‐determining step of the apparent activation energy for phenol decomposition. Once formed, the HO^· radicals would react with phenol in a barrierless process. On the contrary, for the less‐active carbocatalysts tested, Ea for phenol degradation is higher than for H₂O₂ decomposition, indicating that other processes beyond a Fenton‐like reaction take place. Importantly, the possible contribution of metal traces present on rGO as active sites for the observed catalytic activity was ruled out by performing additional catalytic experiments with purposely added Mn²⁺ (Figure 10.1).

DFT calculations and the use of simple molecules as organocatalysts suggest that hydroquinone/quinone‐like moieties present in the rGO are presumably the active sites for the decomposition of H₂O₂ to HO^· radicals. Enhanced catalytic activity was achieved by using hydroquinone substituted with electron‐donating groups such as OCH₃ or CH₃. Quantification by X‐ray photoelectron spectroscopy (XPS) of quinone‐like centers on rGO based on deconvolution of C 1s peak was used to determine the population of active sites, and the maximum amount of decomposed phenol was determined after the reuse experiments, resulting in a turnover number for phenol degradation and H₂O₂ decomposition as high as 4540 and 15 023, respectively.

In a similar manner, the same authors showed that rGO can act as an efficient and reusable carbocatalyst for the photo‐Fenton reaction under natural sunlight irradiation [35]. The reaction rate and the pH of the solution increase by light irradiation. A minimum H₂O₂ to phenol molar ratio of 5.5 is needed to degrade phenol and its more toxic reaction intermediates formed in the decomposition (hydroquinone, catechol, and p‐benzoquinone). In this study, a relationship was established using three G samples between their oxygen content and the catalytic activity. The lower oxygen content of rGO (∼18%) < rGO (∼33%) < GO (∼44%) resulted in the higher catalytic activity for the photo‐Fenton reaction, underlining the importance of a sufficiently conjugated optical absorber to remediate the light energy. This catalytic activity of G to promote the Fenton reaction is a clear example of how nanocarbons with adequate composition can replace metals in reactions that have been assumed to be promoted exclusively by metals. In any case, the need of acidic pH values to promote the (photo)Fenton reaction is still a severe limitation for the general implementation of the process.

Recently, rGO has also been employed to catalyze the decomposition of p‐hydroxybenzoic acid as a model pollutant in water by ozone [36]. It was found that rGO promotes the O₃ decomposition to O₂^·− and ¹O₂ as evidenced by electron paramagnetic resonance (EPR) measurements as well as selective quenching experiments. The catalytic activity of rGO was higher than that of acidic AC or GO. Catalyst deactivation occurs due to the oxidation of the rGO surface, although a thermal treatment of the used rGO catalyst could completely restore the catalytic activity.

Complementing these processes, catalytic oxidation using potassium peroxymonosulfate (PMS; KHSO₅·0.5KHSO₄·0.5K₂SO₄) as an oxidant [37] is also promising for pollutant degradation in water, particularly for neutral pH values. The main advantage of using PMS compared with H₂O₂ is the lower pH dependence of the reaction for the generation of SO₄^·− compared with the generation of HO^· radicals from H₂O₂ [38]. Typically, sulfate radicals are generated from PMS employing Co(II) or Mn(II) as homogeneous catalysts. Heterogeneous catalysts for this process include supported or unsupported cobalt oxides [39, 40] or manganese oxides [41, 42]. Shaobin and coworkers have reported the use of graphitic carbons as metal‐free catalysts for this reaction. In a preliminary screening, it was observed that rGO showed the highest activity for PMS activation when compared with AC, graphite powder, GO, and multiwall carbon nanotube (MWCNT) [43]. Further investigations on rGO showed that its catalytic activity for PMS activation correlates in a series of thermally annealed rGO with the I_D:I_G ratio and the presence of relatively electron‐enriched ketonic groups [44]. Among the three rGO samples prepared, the one with the highest ketonic content exhibited the highest activity even though its I_D:I_G ratio, surface area, and phenol adsorption were lower. The main feature of ketonic group is the presence of lone pair electrons acting as a weak Lewis basic site able to coordinate and participate in electron transfer processes with PMS. It should be mentioned that PMS decomposition occurs easily even at basic conditions, in contrast to H₂O₂.

10.3 Carbon–Carbon and Carbon–Nitrogen Coupling Catalyzed by Carbonaceous Materials

Another exciting reaction where noble metals were considered to be mandatory are aromatic carbon–carbon coupling reactions Recently, H₂‐treated GO in combination with KO^tBu has been employed as an efficient heterogeneous catalyst for the direct CH arylation of benzene by iodobenzene to from biphenyl [45]. This is related to the classical Suzuki–Miyaura cross‐coupling reaction that is catalyzed by Pd. In the present case, it was found that Mn or Fe impurities do not play any role. Reactions with various oxygen‐containing model compounds and DFT calculations support that negatively charged oxygen atoms are likely to be responsible for the overall transformation. The stabilizing and activating effect of K⁺ ions is proposed to facilitate the activation of the CI bond of C₆H₅‐I after adsorption. This hypothesis was further supported by performing a reaction in the presence of 18‐crown‐6 to trap the K⁺ ions, observing that the CH arylation of benzene with 4‐iodoanisole was effectively stopped. In addition, the graphitic π system greatly favors the reaction by adsorbing the aromatic reagents before coupling.

To investigate the nature of oxygen species, four catalysts with different oxygen contents were synthesized by controlling the H₂ annealing temperature of GO at 300, 500, 700, and 900 °C to obtain GO‐300, GO‐500, GO‐700, and GO‐900, respectively. XPS spectra indicated that the peak intensity of the oxygen species decreases from 16.1 to 4.2 wt% as the treatment temperature increases. Catalytic data showed that the lower the oxygen content in GO, the lower the catalytic activity. A series of oxygen‐containing model compounds showed that oxygen functional groups directly attached to the benzene ring gave the desired product in very low yield or not at all, while oxygen functionalities at a benzylic position catalyzed the desired reaction. Ordinary benzyl alcohol turned out to be the most active of the organic molecules to promote the coupling. It is, however, unusual to expect a high density of benzylic alcohols on GOs that ideally should have exclusively sp² carbons.

Recently, a general strategy for alkylation of arenes with styrenes and benzylic alcohols catalyzed by GO was presented, affording valuable diarylalkanes in high yields and excellent regioselectivity [46] (Scheme 10.1).

It was observed that the recovered GO in suspension measured by methylene blue adsorption exhibits a surface area of 367 m² g⁻¹, indicating the occurrence of partial π stacking of the GO sheets as compared with the parent GO material (1371 m² g⁻¹). The increase in the π–π stacking is consistent with a partial reduction of initial GO to rGO during the course of the reaction as it was indicated in XPS by a decrease in CO/CO functional groups on the GO surface from 49.4% to 40.9% with a concomitant increase in the intensity of peaks corresponding to CC bonds (from 37.0% to 52.0%). The partial reduction of GO to rGO during the process points toward the importance of oxygen‐containing functionalities anchored on the GO surface as active sites. Based on these observations, it was proposed that the reaction mechanism of the alkylation involves activation of both coupling partners by adsorption. The key step would be activation of olefin by hydration to an adsorbed alcohol and a transition state in which the arene nucleophile is prepositioned by π‐stacking interactions with the GO sheet for a concerted CC bond forming step to give the alkylated product and regenerate the catalyst after the release of water. It should be noted, however, that classical mechanism for arene alkylation requires the presence of strong acid sites, whose presence in this case has not been discussed. It will be discussed below that indeed electron transfer via charge transfer interactions between the substrate and the catalyst can have a similar promotional effect as an acid, i.e. partial charge transfer from the aromatics to the carbocatalyst would leave a positive partial charge on the substrate, which then drives the reaction. It is clear that further studies are required to provide some support to this proposed mechanism.

An efficient protocol for the formation of 2,3‐dihydroquinazolinones has been reported employing GO as a carbocatalyst [47]. The catalytic activity of GO is higher than that of other catalysts such as PEG‐SO3H, rGO, SnO₂‐QDs (quantum dots), or nano‐CuFe₂O₄. GO retains more than 90% of its catalytic activity of the fresh sample after five uses. In addition, the presence of oxone as an oxidant in the system allows expanding this cyclocondensation for the selective preparation of quinazolin‐4(3H)‐one derivatives (Scheme 10.2).

10.4 Acidic Sites at Nanocarbons for Carbocatalysis

Hydrothermally treated graphene oxide (HGO) was found to be a new metal‐free catalyst for activating NaBH₄, here exemplified by the reduction of 4‐nitrophenol to 4‐aminophenol [28]. Increasing the reactivity of metal hydrides derives from the polarization of the metal–hydrogen bond that increases with the electropositivity of the metal. Combined experimental and theoretical investigations revealed that using NaBH₄ as a reducing agent in acidic hydroxyl groups, as well as holes, is beneficial in promoting the reduction, while in contrast, epoxy and carboxyl groups should not exhibit catalytic activity.

Active acidic sites on G can also be impurities or adventitious groups introduced in the material during its preparation procedure [16]. One of these cases is the catalytic activity of sulfonic or sulfate groups introduced on rGO or GO due to excess H₂SO₄ employed in graphite oxidation. In this context, Garcia and coworkers have reported the activity of GO as a metal‐free carbocatalyst for the room temperature ring opening of epoxides using methanol and other primary alcohols as nucleophile and solvent [47]. Interestingly, GO at 0.19 wt% exhibited, in the ring opening of styrene oxide by methanol, 99% conversion with 97% selectivity toward the desired product (Scheme 10.3). The amount of GO used as a catalyst in this reaction is much lower than that used for benzyl alcohol oxidation or alkyne hydration (200 wt%) [48] or for hydration of propylene oxide (3.4 wt%) [49]. It was confirmed that acidic impurities present in GO are responsible for the catalytic activity of GO. Indeed, the catalytic activity decreased when S was partially removed by thermal treatment at 200 °C. Furthermore, the addition of pyridine in the reaction mixture completely stopped the reaction due to neutralization of acid sites. A comparable activity to GO was observed for the molecular strong acids H₂SO₄ and p‐toluenesulfonic acid, whereas glacial acetic acid showed no conversion of styrene oxide.

Room temperature acetalization of benzaldehyde by methanol could also be performed in a quantitative yield and selectivity using GO as a carbocatalyst (Scheme 10.4) [50]. In contrast, the use of other carbonaceous materials, such as graphite or AC (Norit A), highly porous metal–organic frameworks [Fe(BTC) or Cu₃(BTC)₂ (BTC = 1,3,5‐benzenetricarboxylate)], or acidic resins such as Amberlite XAD4 as a catalyst, resulted in yields below 14% for acetalization of benzaldehyde. In this reaction, hydrogen sulfate groups on GO, whose proportion can be decreased by exhaustive washings with methanol, were proposed as being responsible for the observed catalytic activity.

Acidic GO was also reported to be an efficient carbocatalyst for the dehydration of fructose into 5‐hydroxymethylfurfural (HMF) (Scheme 10.5), yielding 87% at 120 °C after 6 h [51]. A series of control experiments were performed. Glacial acetic acid, p‐toluenesulfonic acid, concentrated sulfuric acid, and Amberlyst‐15 were used as catalysts and, except glacial acetic acid, could facilely promote the dehydration reaction. This suggests that the active sites of GO are probably the sulfonic groups present as impurities rather than the carboxylic groups. The catalytic activity of GO declined rapidly after GO treatment above 200 °C, whereas its activity is retained for GO treated below 150 °C. In addition, the affinity of GO and Amberlyst‐15 for fructose and HMF was evaluated by adsorption measurements, indicating that the amount of fructose adsorbed on GO is much higher than that on Amberlyst‐15. The higher affinity of GO for fructose was attributed to the presence of surface oxygen‐containing groups forming hydrogen bonds with the hydroxy groups of fructose.

Similar to the previous case of fructose dehydration, GO has been reported as an acid catalyst for the synthesis of polyoxymethylene dimethyl ethers (PODE_n) from methanol and trioxymethylene [52]. It was proposed that the active sites are the combination of the sulfonic groups and the hydroxyl and carboxyl groups present on the surface of GO establishing a cooperative synergy. This conclusion was reached by comparing the catalytic activity in the synthesis of PODE_n of various modified GO catalysts differing in the oxygenated functional groups. Three types of modified GOs masking selectively one type of OH group were prepared. The hydroxyl groups were selectively blocked by silylation with tetramethoxysilane. The carbonyl and carboxyl groups on GO were selectively removed by reduction with NaBH₄. Similarly, 84.7% of the SO₃H groups on GO was removed by hydrothermal treatment at 150 °C for 12 h. When the hydroxyl and carboxyl groups are selectively masked or removed, the modified GO exhibited much lower activity and selectivity to PODE_2–8 than the parent GO under identical conditions. All these control experiments clearly support that the hydroxyl and carboxyl groups contribute to the catalytic activity of GO for the synthesis of PODE_n, but the main active sites appear to be strong sulfonic acid groups.

Acidic GO was also efficient to promote in high yields ring opening reactions of chiral aromatic epoxides by indoles in regio/enantioselective manner in solvent‐ and metal‐free conditions [53]. The Friedel–Crafts products were obtained with yields of the range of 25–80% and enantioselectivity up to 99%. It was believed that the presence of different acid functionalities on the carbocatalyst surface activates the epoxide ring opening and forms an incipient carbocation that reacts with the nucleophilic carbon of the indole. Furthermore, the high stereoselectivity achieved in all cases with complete inversion of stereochemistry indicates that no free carbocation is formed, but a surface‐bound one: the nucleophilic attack by indole taking place regioselectively at the benzylic position as in S_N1‐type mechanism.

10.5 Carbocatalysis with Carbon Holes and Edges

Loh and coworkers have reported that simple base and acid washings of as‐prepared GO (ba‐GO) can release and enlarge the defects in GO, thereby enhancing the catalytic activity of treated ba‐GO for the oxidative coupling of amines to imines to 98% yield under solvent‐free and open‐air conditions [54]. In contrast, untreated, as‐prepared GO exhibited 44% yield under identical conditions. The ¹³C NMR spectrum of ba‐GO revealed a significant decrease in the concentration of oxygen functionalities appearing in the spectrum at 71 ppm (COH) and 61.9 ppm (COC), which are attributable to hydroxyl and epoxide groups, respectively. In situ FTIR analysis of ba‐GO indicated that residual oxygen functional groups, mainly as ketones, epoxides (significantly reduced), and carboxylic acids, still remain in ba‐GO. Performing a control experiment with 1‐pyrene carboxylic acid as a molecular analog gave a yield of 95%, comparable with that of ba‐GO. Scanning tunneling microscopy revealed that GO sheets become highly porous after the base treatment with an average pore area of around 5 nm². These holes are likely to be created during the harsh oxidation process and are freed from amorphous carbon debris by base treatment. The available data thereby suggest that the active catalysis sites in ba‐GO are likely to be hole defects in the conjugated domain as well as the edges of the hole defect terminated by the carboxylic acids.

The role of carbon vacancies in the catalytic activity of ba‐GO was supported by observing a broad and a sharp signal in electron spin resonance spectroscopy (ESR) with an intensity ratio of about 5 : 1. Therefore, it was proposed that the observed catalytic ability is related to the localized spins present at the edges of π electron system. Furthermore, EPR spectroscopy showed that the characteristic spectrum with hyperfine couplings of the adduct corresponding to superoxide radical being trapped by 5,5‐dimethyl‐1‐pyrroline‐N‐oxide (DMPO). Also, the formation of H₂O₂ was detected using horseradish peroxidase to promote the enzymatic oxidation of N,N‐diethyl‐1,4‐phenylenediammonium sulfate (DPD). Therefore, the edge sites with unpaired electrons obviously enhance the kinetics for molecular oxygen activation by a sequence of electron transport and reduction to the superoxide radical.

The carboxylic groups at the edges of defects work along with the localized unpaired electrons synergistically to trap molecular oxygen and the amine molecules. The overall mechanism is summarized in Figure 10.2.

As a continuation of the concept of an active site constituted by a carboxylic acid group and free radicals located at edges, the use of GO with TEMPO as a cocatalyst has been reported for the selective oxidation of HMF to 2,5‐diformylfuran (DFF) with 100% HMF conversion with 99.6% selectivity to DFF using 80 wt% GO loading at 1 atm air pressure [55]. A series of model catalysts were tested with various oxygen functional groups such as hydroxyl, carbonyl, anhydride, and carboxyl groups with TEMPO as a cocatalyst. The activity of various carboxylic acids in the tested substrates increased in the order: acetic acid ∼ hexanoic acid < benzoic acid < 1‐pyrene carboxylic acid ≪ GO. The data with model compounds demonstrate that the carboxylic acid−TEMPO system has always intrinsic activity for the aerobic oxidation of HMF. Nevertheless, a large π conjugation system together with the connected carboxylic acid group leads to an enhanced catalytic activity. These findings suggest that the active catalytic sites in GO are likely to be holes surrounded by conjugated domains, with some positions terminated by carboxylic acid groups.

Another possibility of carbon edges with their partly complicated electron‐pairing situation to act as catalytic sites is to activate covalent bonds by interacting with them. A combination of experimental and theoretical calculations has been used to rationalize the catalytic activity of G nanofibers for hydrogen release from sodium alanate (NaAlH₄) [56]. It was found that G nanofibers considerably improve hydrogen release from NaAlH₄. DFT revealed that carbon atoms at sheet edges, regardless of whether zigzag or armchair, can weaken AlH bonds in sodium alanate. This effect is believed to be due to a combination of NaAlH₄ destabilization and dissociation product stabilization.

10.6 Frustrated Lewis Pairs in Nanocarbon Structures

A series of metal‐free carbocatalysts has been reported for hydrogenation of carbon–carbon multiple bonds [57]. The selective hydrogenation of acetylene in the presence of excess ethylene is here an industrial question of key importance. The selective conversion of acetylene can be promoted by a carbocatalyst obtained from pyrolysis of alginate at 900 °C followed by exfoliation of the graphitic carbon residue. The activity of this material increases along the temperature, reaching an optimal value of 99% of acetylene conversion with only 21% conversion of ethylene at 120 °C. Hydrogenations are classical examples of reactions catalyzed by noble or transition metals and, therefore, the finding that carbons can catalyze selective hydrogenation represents an important step toward replacement/complementation of metal catalysts. Under the optimized reaction conditions, GO was able to achieve 50.6% of acetylene conversion to ethylene with no measurable ethylene hydrogenation at 150 °C, while rGO reached complete acetylene conversion with only 5% ethylene hydrogenation at 130 °C. XPS spectra indicated no changes on the used G and rGO catalysts with respect to the fresh samples, while in contrast, GO is first converted into rGO.

To explain the catalytic activity of graphitic carbons for hydrogenation, an analogy with organocatalysis was suggested. It has been well documented in the literature that Lewis acid–base pairs rigidly separated at a short distance without undergoing neutralization (“frustrated Lewis acid–base pairs,” FLPs) that can activate hydrogen and act as metal‐free hydrogenation catalysts [58–61]. It was proposed that the activation of H₂ on graphitic carbons takes place similarly, resulting in the simultaneous formation of H⁺‐like and H⁻‐like sites. This claim was supported by preadsorbing D₂ on G, followed by evacuation and subsequent heating on a H₂ stream, whereby the detection of H–D by mass spectrometry confirmed H₂ activation by observation of isotopic H–D scrambling. This was further supported by control experiments wherein acetylene conversion was found to depend on the presence of CO₂ (increasing conversion by 33%) or NH₃ (decreasing conversion by 9%) in the stream. These gas effects were reversible, and the catalytic activity of G was recovered to the initial state in the absence of CO₂ or NH₃. Ir is an interesting question if such FLPs are located in the same graphitic layer or – sterically more easy to control – on neighboring positions in stacked layers.

The products obtained by pyrolysis of alginate at 900 °C under inert atmosphere have also been reported as metal‐free catalysts for the reduction of nitro to the corresponding amino compounds, using hydrogen as a reducing agent [62]. It is worth mentioning that supported Au NPs exhibited a preferential selectivity higher than 90% at high substrate conversions for the selective hydrogenation of nitro groups with respect to CC double bonds [63, 64]. In contrast to these observations, G was found much less selective, the selectivity depending on the nature of substrate. Reduction of 4‐nitrostyrene using G as a catalyst gives rise to 4‐aminostyrene at low conversions, whereas at higher conversions, 4‐aminoethylbenzene was obtained, corresponding to the hydrogenation of both the nitro and the CC double bond. FLPs as the proposed active sites for hydrogenation were further supported by performing two additional tests in which 4‐nitrostyrene and 1‐nitrocyclohexene were submitted to hydrogenation using G as a catalyst, with the addition of a minute amount of acetic acid or ethyl acetate. Addition of 134 μmol of acetic acid results in no hydrogenation of 4‐nitrostyrene and decreased the conversion of 1‐nitrocyclohexene from 86% to 9%. In contrast, addition of the same amount of ethyl acetate had much less effect on the process, which shows that the quenching of acetic acid has some effect with its acidic character, leading to neutralization of the basic centers of the FLPs.

Very recently, theoretical models support that a G bilayer and G ribbon having boron and nitrogen as dopant elements should be effective frustrated Lewis pair catalysts for activation of molecular hydrogen [65]. Analysis of the structures along the reaction path suggested that the hydrogen molecule should be heterolytically dissociated. The activation is in the models asynchronous as one of the hydrogen atoms interacts initially with the active site followed by the other one. According to this model, it would be very important to assure that the synthesized doped carbon catalysts have separated boron and nitrogen dopants in different G sheets in order to act as frustrated Lewis pair catalysts. This already points to the discussion that graphenes are more than molecules, but packed solid‐state structures, and that a catalytic site can extend over more than one layer and involves the electronic environment of a number of separated units. Especially, simultaneous acid–base interactions, charge or bond frustration (note that graphitic stacking with about 0.34 nm is much longer than any chemical bond, but in the optimal range for FLPs), or bipolar charge transfer activation works much better “over the layers.”

10.7 Beyond Localized Chemical Functionality as the Active Site: Collective Solid‐State Effects in Catalysis

Adsorption of substrate and reagents on the graphitic carbon plane is generally claimed as one of the reasons of the high activity observed for graphitic carbons when compared with their reference systems (“binding precedes reaction”). Particularly, π–π interactions of G with condensed polycyclic aromatic compounds are known to be very strong, and intermolecular complexes have been reported between G and pyrenes, among other aromatic compounds [66]. In many of the already discussed cases, this preassociation of substrates with the graphitic basal plane has been proposed as the cause of a synergy with other active sites resulting in high activity of nanocarbons as metal‐free catalysts.

The above comments lead to the interesting discussion on how much heterogeneous carbocatalyst can really be described by the techniques developed for molecules and how much collective properties of an electronically coupled solid state material play a role. It is clear that substrate binding via charge transfer interactions, electron transfer, or so‐called π–π interactions will depend on the electron density of the carbocatalyst, and the higher the difference in electron density, the stronger the binding and the activation/deactivation toward a specific chemical reaction. A remarkable clear case was reported for related graphitic carbon nitrides in Friedel–Crafts catalysis [67]. Here, electron transfer from the benzene to graphitic carbon nitride does the same role as a solid acid catalyst, and benzene could be successfully activated to be alkylated by methanol or substituted by cyano groups using urea. Similar arguments hold true for the catalyzed trimerization of nitriles [68], which was also shown to be driven by charge transfer interactions of nitriles with the graphitic catalyst. It is clear that such “diffuse” effects due to charge transfer without the existence of a precise center do overlap with many of the sites described above.

Another useful concept from solid‐state physics with impact in catalysis is the “flat band potential” of electrons within an electronically coupled system, which corresponds to the HOMO in molecular orbital theory. The flat band potential is a collective property, i.e. the accessibility of electrons at a specific site depends on the electron acceptor and electron donor properties of the complete conjugated system. This is especially important for the heteroatom doping discussed below, as for instance nitrogen is known to make the HOMO more positive, i.e. more electron poor and thereby more “acidic”/electron accepting. Nitrogen‐doped carbon made by appropriate techniques is thereby “noble,” has a metallic luster, and cannot be oxidized in air even up to 700 °C [69]. It is a quantum‐mechanical effect that electron density cannot be discontinuous but “bends” into any other medium. This means that even acidic groups bound to such carbocatalysts will sense the electron density, and these acid sites on doped G will behave in a way as molecular acids having electron‐pulling units: a more noble support will increase the acid strength. In that sense, controlling the electron density within a carbon framework can be regarded as the heterogenous version of controlling electron density by ligand substitution or by modifying an organic environment [70]: it is now in the hands of the chemist to fine‐tune catalytic activity and selectivity.

In the case of graphitic carbons, it has also been reported that preadsorption can drive the selectivity of a reaction toward a given stereoisomer that is different to the one observed in homogeneous catalysis. Thus, recently, the Mukaiyama–Michael addition of 2‐(trimethylsiloxy)furan (TMSOF) and β‐nitrostyrene (Scheme 10.6) has been reported using graphite oxide/GO‐based catalyst [71]. Lowering the graphite oxide loading from 12 to 0.2 wt% results in good yields in all cases, whereas the selectivity in favor of the anti‐diastereoisomer increases continuously. For instance, 0.2 wt% graphite oxide exhibited 85% yield after 6 h at room temperature with syn:anti ratio of 25/75. Furthermore, the use of 0.2 wt% exfoliated graphite oxide further improved the yield to 90% after 6 h at room temperature and also the unprecedented anti‐selectivity up to a diastereoisomeric ratio of 23:77 (syn:anti).

DFT calculations for a graphene model was constructed by considering 54 C atoms consisting of 19 fused six‐membered rings. The geometry of the transition states shown in Figure 10.3 indicates that the β‐nitrostyrene molecule establishes a π‐stacking interaction with the G layer. Due to this interaction, TMSOF can attack the activated β‐nitrostyrene only from the top. The main difference between the two transition states is in the relative orientation of TMSOF, which according to the calculations establishes a series of dispersive interactions with the C atoms of the G in the anti‐transition state (see the short distances in Figure 10.3a) because the SiMe₃ group protrudes away from the reactants. In contrast, in the syn transition state, the predicted geometry shows that the SiMe₃ group is basically oriented above the NO₂ group of β‐nitrostyrene, and only weak interactions can occur with the graphitic basal planer, as indicated by a long distance of ≥5 Å (see Figure 10.3b). On the other hand, in the case of the uncatalyzed reaction, a favorable electrostatic interaction between the positively charged SiMe₃ group and the negatively charged nitro group is the main force that drives the formation of the syn product. In contrast to the results attained with graphitic materials in the reaction catalyzed by BINOL‐phosphoric acid, the geometrical constraint imposed by the phosphoric group activating the addition better matches the geometry of the syn compared with the anti‐transition state.

10.8 The Heterojunction and Dyad Concepts in Catalysis

A related concept coming from solid‐state chemistry is the concept of heterojunction. This happens when two carbocatalysts of different character are in tight contact with each other, e.g. GO@G or N‐doped G@G. As the electron density cannot behave discontinuously, surface charges build up, which, in case of atomic or thin layers, can extend over the whole sample. This means that by the right synthesis or processing, one and the same graphitic system can be brought to possess different electronegativities, due to this non‐covalent neighborhood effect. This concept lays the unusual selective oxidation of saturated hydrocarbons by dioxygen, using a C@C₃N₄ heterojunction [72]. The classical molecular orbital description of this interaction is then, of course, like a charge transfer complex, where electron density is sucked into the π* orbital of the electron poor partner, thus leaving a stronger oxidizing carbocatalyst as the support phase. If molecular entities are involved, this activation was termed as “dyadic.”

10.9 Nitrogen, Sulfur, and Boron Doping to Construct Active Sites

As a follow‐up of the previous discussion on the influence of heteroatoms on the collective properties of G such as electron density and flat band potential, the next sections will review some publications in which heteroatoms have been proposed as active sites in reactions promoted by carbocatalysts.

Selective aerobic oxidation of benzyl alcohol has been achieved by engineering of active sites on graphitic carbons via N‐doping through a high temperature (800–1000 °C) nitridation [73]. Among the three types of nitrogen atoms present within the graphitic framework, pyridinic‐N, pyrrolic‐N, and graphitic‐N, the graphitic sp² N atoms were declared to be the catalytically active centers for this aerobic oxidation. The intensity of the N 1s signal can be used to determine the N content that decreases as the nitridation temperature increases in the order NG‐800 (4.16 atm%) > NG‐900 (3.48 atm%) > NG‐1000 (1.71 atm%). The presence of N heteroatoms on carbocatalysts promotes benzyl alcohol oxidation with 100% selectivity to the benzaldehyde by NG‐900 that exhibited an approximately fourfold enhancement in activity upon increasing the reaction temperature from 313 to 343 K for benzyl, p‐nitrobenzyl, p‐fluorobenzyl, p‐methylbenzyl, and p‐methoxybenzyl alcohols, thus indicating the wide substrate scope of N‐doped carbons as catalysts for the aerobic oxidation of alcohols. However, NG‐900 catalyst was catalytically inactive for 1‐phenylethanol. The spatial constrains on the reactive sites is apparently one of the important factors determining the value of the pre‐exponential constant of the kinetic equation for aerobic alcohol oxidation over multilayer N‐doped graphitic carbons as catalysts.

NG‐T and G samples prepared by high‐temperature exfoliation are believed to react with methanol to produce methoxyl radicals, which were trapped by DMPO allowing to record the sextet peaks corresponding to DMPO−CH₃O^· adducts [74]. It is proposed that the formation of CH₃O^· is closely correlated with the presence of electron‐deficient defects on the NG‐T surface. It is interesting to note that NG‐900 with the fewest density of defects (I_D:I_G 1.51) generates a larger amount of DMPO–CH₃O^· compared with NG‐800 (I_D:I_G 1.58) and G (I_D:I_G 1.56), suggesting that any N atom is able to activate methanol to form the methoxyl radical. Addition of benzyl alcohol decreased the EPR line intensity of DMPO–CH₃O^·, indicating that benzyl alcohol may be activated by the electron‐deficient sites to form a benzyl radical, which competes with the generation of CH₃O^·. Apart from N atoms as catalytically active sites, the contribution of other sites like defects, including edges and carbon holes, cannot be excluded [75]. Adsorption of molecular oxygen over the graphitic N atoms to form a sp² N–O₂ adduct transition state seems to be the elementary step leading to O₂ activation. The activated oxygen exhibits high chemical reactivity toward primary alcohols as it can abstract α‐H atoms of alcohol to finally form water in several steps. No H₂O₂ was detected during the catalytic reaction.

The importance of the preparation method of N‐doped G and the proposal of graphitic‐N as active sites were again proposed in another work [76]. The most active N‐doped G sample (NG‐700), obtained by reaction of GO and melamine and subsequent pyrolysis at 700 °C, showed an enhancement of catalytic activity with respect to rGO‐700 of 80 times (Figure 10.4). In addition, NG‐700 exhibits much higher activity than that of other sample prepared from GO and ammonium nitrate at 350 °C. It is important to note the high N content achieved using melamine as a precursor (NG‐700, 9.68 atm%; 54.41% pyridinic, 23.09% pyrrolic, and 22.49% graphitic) compared with the use of ammonium nitrate (5.61 atm%) [77], annealing of GO with NH₃ (3–5 atm%) [78], and CVD (5.0 atm%) [79], among others. Interestingly, the NG‐700 sample is even more active than the benchmark heterogeneous Co₃O₄ catalyst and than that of single‐walled carbon nanotubes (SWCNTs), N‐doped CNT, and G nanosheets (GNs) (Figure 10.4).

In this study, the proposed mechanism for PMS activation by N‐G involved transfer of delocalized electrons from the zigzag edges of G to PMS [76]. Importantly, theoretical calculations at DFT level revealed that the presence of graphitic‐N enhances the adsorption of PMS and the electron transfer from G to PMS compared with pristine G. These calculations also predict that the activity of pyridinic‐N or pyrrolic‐N should not be significant compared with that of graphitic‐N. Furthermore, EPR measurements using DMPO as spin trap as well as radical quenching experiments using ethanol and t‐butanol provide direct and indirect evidence in support of the generation of HO^· and SO₄^·− radicals. Unfortunately, as in all the previous G‐based catalysts commented, the NG‐700 catalyst deactivates upon reuse and thermal annealing of the used catalyst to diminish the presence of oxygenated functional groups can hardly recover part of the catalytic activity. XPS showed that the used catalyst has a higher oxygen (13.74 versus 3.11 atm%) content and lower N‐doping (1.56 versus 9.68 wt%) with respect to the fresh material giving hints that deactivation should originate from the introduction of oxygenated functional groups and N loss.

In line with the above commented results, N‐doping of rGO (∼9 wt% N) improves the catalytic activity with respect to rGO also for the catalytic wet air oxidation (∼160 °C at 7 bar O₂) and catalytic ozonation (room temperature and atmospheric pressure) [80]. The N content was assigned to pyridinic‐, pyrrolic‐, and quaternary‐N atoms based on XPS. Further studies, however, about the nature of the N atoms that are acting as catalytic sites in these oxidation processes are still needed.

N‐doped G has also been reported as a metal‐free catalyst for the reduction of 4‐nitrophenol to 4‐aminophenol by NaBH₄ [81]. N‐G exhibited a pseudo zero‐order kinetics, which was found different from pseudo first‐order reactions catalyzed by metal NPs [82]. From characterization by in situ FT‐IR spectroscopy and theoretical studies, it was suggested that 4‐nitrophenol tend to interact with N–G via the O atom of the hydroxyl group. Also, evidence was obtained in favor to consider adsorption of 4‐nitrophenol as the decisive elementary step responsible for the pseudo zero‐order kinetics. Theoretical studies, on the other hand, suggested that only the carbon atoms bonded to N atoms are activated and exhibit more favorable charge density than the rest of C atoms on the G sheet. All the four kinds of N atoms (pyridinic, pyrrolic, graphitic, and amine) present on G have good adsorption capability for 4‐nitrophenol and the nitro groups of the adsorbed 4‐nitrophenol become always activated by adsorption.

The presence of two or more dopant elements enlarges the potential to introduce active sites on G and the possibility to tune the catalytic activity of this material. For activation of PMS, it has been observed that co‐doping with S (0.69%) and N (8.15%) causes a synergism for this process [83]. The amount of residual O surely present on rGO, its variation in the different samples, and its possible influence on the catalytic activity were although not considered. The low S content with respect to N was attributed to the larger difference in atomic diameter 1.03 and 0.71 Å, respectively, with respect to the carbon atoms (0.75 Å) that should make S atom more difficult to accommodate in the G lattice. Based on XPS, the N content mainly distributed in pyridine‐like (39.8%), pyrrole‐like (51.6), and quaternary or graphitic (8.6%). The sulfur content was assigned to C‐S‐C and C‐SO_X‐C. DFT calculations of models for undoped G, S‐G, N‐G, S‐N‐G, and S‐S‐N‐G were carried out to get some insights into the effect of dopants on the electronic density of bonded carbon atoms on a G model (Figure 10.5C). In the case of S‐doped G, insignificant charge transfer was found, whereas N doping induces a positive charge density on the adjacent carbons (C1, C2, and C3). Simultaneous co‐doping with N and S of G sheet further increases the charge density of the atom carbon C2 (Figure 10.5) from 0.31 to 0.48. Thus, a good correlation between polarization of electron density by the dopant elements and the experimental catalytic activity for PMS decomposition was observed. Interestingly, an experimental optimal S loading was observed for the catalytic activity of SN‐rGO. EPR measurements showed the formation of SO₄^·− and HO^· radicals by NS‐rGO decomposition of PMS. Unfortunately, this special SN‐rGO deactivates upon use, and its catalytic activity could not be recovered. Further synthetic refinements are, therefore, still needed to increase the stability of co‐doped NS‐rGO catalyst to allow reusability of the material.

The activity of nitrogen‐ and boron‐co‐doped graphene (B,N‐G) was tested for acetylene hydrochlorination [84]. XPS and Raman spectroscopy strongly supported that N and B atoms are covalently doped into the G sheets. Under the same reaction conditions, the initial acetylene conversion for GO, B‐G, N‐G, and B,N‐G catalysts follows the order B,N‐G > N‐G > B‐G > GO. Thus, the estimated TOF values for N‐G and B,N‐G catalysts were 8.33 × 10⁻³ and 3.32 × 10⁻² min⁻¹, respectively, indicating the benefits of co‐doping on G sheet. These TOF values are higher than those of the SiC@N–C composite reported in an earlier precedent (∼5.63 × 10⁻³ min⁻¹) [85], and also higher than those of the C₃N₄ material (1.42 × 10⁻² min⁻¹) [86]. To be clear, the TOF values for the two most active catalyst, namely Au/AC and Hg/AC, are 5.38 and 0.22 min⁻¹, respectively [87]. Hence, the catalytic performance of B,N‐G is significant, but still noncompetitive.

Both experimental and theoretical studies suggest that the carbon atoms bonded to N species are the active sites. The enhanced activity of B,N‐G (10.96% N; 15.47% B) may be due to its higher N content compared with N‐G (5.47% N). Also, the presence of B atoms changes the electronic density of the N atom and, therefore, influences HCl adsorption on the N active sites. Adsorption of HCl on the G catalyst is the rate‐determining step of the acetylene hydrochlorination. In the matrix of B,N‐G, carbon atoms act as the adsorbing site for C₂H₂, and the nitrogen atom provides the catalytic active site for HCl adsorption. Therefore, the active site is an ensemble of carbon atoms bonded to the N atoms. In contrast to N atoms, the presence of oxygen atoms on N‐G and B,N‐G catalysts decreases the adsorption of HCl. DFT calculations anticipate that a higher HCl adsorption value should occur for B,N‐G due to the combined interaction between pyridinic‐N and ‐B atoms. In agreement with this theoretical prediction, it was measured that the presence of B dramatically increases the adsorption energy. TPD analysis showed that the binding strength of HCl with various G materials increases in the order GO < N‐G < B,N‐G. One of the major drawbacks of B,N‐G catalyst is that acetylene conversion decreases from 94.89% to 61.88% within 4 h on stream, indicating its poor catalytic stability of the current model versions. This decay in activity was ascribed to the formation of coke around the active sites.

N,P atoms in the corresponding graphitic carbons can also act as catalytic centers in aerobic oxidations. In this context, a simple and efficient approach was reported to synthesize P‐doped graphitic porous carbon (PGc) materials by heating phytic acid in a domestic microwave oven (1100 W) for 40 s [88]. Using this preparation methodology, it is possible to control the coordination of P atoms. XPS measurements indicated that the porous P‐doped carbon contains 4.9 atm% P in agreement with the presence of P in the carbon material. This porous P‐doped carbon was used for the oxidation of benzyl alcohol and resulted in benzyl alcohol conversion of 33.4% at 60 °C with >99% selectivity to benzaldehyde, which is approximately three times higher than the conversion achieved with a sample N‐doped G under similar reaction conditions [73]. Interestingly, the P‐doped carbon was active in the aerobic oxidation of 1‐phenylethanol reaching 46.5% conversion with complete selectivity to acetophenone at 80 °C, whereas in contrast, the N‐doped carbon catalysts were inactive [73]. It was shown that the formation of a large amount of H₂O₂ by‐product seems to be unavoidable when using noble‐metal‐based catalysts for selective oxidation of alcohols to aldehydes by molecular oxygen [89]. However, for P‐doped G as a catalyst, no detectable H₂O₂ is generated, which is another specific feature of this P‐doped material compared with transition metal catalysts [90, 91]. As P has the same number of valence electrons as that of N, it is believed that the mechanism of O₂ activation could be similar to that calculated for N‐doped G, involving the formation of peroxo‐like species on the heteroatom [92]. XPS analysis indicated that P‐doped porous carbon contains mainly PO, instead of CO, whereas COC groups are below the detection limit, and the majority of OH present on it are directly bonded to P with abundant POH functionalities. Further, the atomic O:P ratio of ∼4 measured by XPS for P‐doped carbon suggests that some P atoms possibly connect with two or more oxygen‐containing functionalities, such as OH groups. A series of control experiments using PGc materials with similar oxygen content but different POH populations suggest that the POH functional groups are likely to play an important role in the oxidation promoted by P‐doped carbon. On the other hand, benzyl alcohol oxidation reaction as such did not work in the presence of molecules containing just PO and POH functional groups, such as phytic acid and phosphorous acid. These data suggest the importance of the graphitic regions on the P‐doped material establishing π–π interactions with the aromatic substrates.

10.10 Summary of the Current State of the Art of Carbocatalysis and Future Developments

It was shown that nanostructured carbonaceous materials are indeed metal‐free catalysts for a large diversity of different organic reactions, well beyond radical chain aerobic oxidations, and include even some reaction types in which transition metals were considered necessary, such as the Fenton oxidation, hydrogenations, and CC couplings. Considering the research interest that is currently attracted by carbon catalysis, we are sure that many other reaction types will be described in the near future as being catalyzed by carbon materials in the absence of metals. Current approaches tend to attribute the catalytic activity to discrete substructures present on the carbonaceous frameworks such as certain functional groups and vacancies, while the importance of the collective properties of nanocarbon materials such as electrophilicity or work function is usually underestimated. This is well understandable, taking into account the difficulty to address the issue of the impact of the electron density in the catalysis using organic models or even calculations. The unique property of conjugated nanocarbons to offer a joint, delocalized electron pool, which can be modified even by distance atoms in any case, certainly add to the possibilities that just have started to be explored.

In addition to expanding carbon catalysis to more and more reactions, another aspect of interest is to prepare more active materials, best even exceeding benchmark metal catalysts. In this area, the knowledge of the active sites should serve as a powerful tool to direct carbon synthesis. The long‐term goal should be to make nanocarbons the catalyst of choice for certain important reactions. Carbocatalysis indeed offer promising potential in novel areas where metal catalysts are not already well optimized or metals are notoriously weak, say for biomass transformation and environmental remediation. In these areas, carbocatalysts offer, as advantages, their compatibility with biomass feedstocks or polluted wastes and the fact that G can be considered a renewable catalyst that do not need to be recovered after the reaction and can be processed with the final residue, e.g. simply burned. As a whole set of novel reactions are seeking for suitable catalysts to be commercially implemented, there is in these cases a distinct possibility to compete with transition and precious metals to be used as catalysts at an industrial scale.

The potentials of carbocatalysis, however, can go significantly further. With the advent of zeolite‐like heteroatom‐doped carbons [93, 94], “carbolithes” can be envisaged where the catalytic activity of carbons is combined with substrate enrichment and size/steric selectivity within the controlled pore system. As such materials, contrary to inorganic zeolites, are also electronically conducting, electrocatalysis or electrically biased chemocatalysis is an exciting new option. Integrating metal particles within the nanometer and subnanometer pores of nanocarbons will result in new hybrid or “fusion” catalysts, as all nanocarbons are catalysts themselves, i.e. they represent “non‐innocent” supports. In spite of the fact that carbons in addition have delocalized electrons, heterojunction effects will set‐in, and the carbons with the small metal entities might merge into an interface‐driven, joint electronic system with previously unknown properties.

In spite of all those potentialities, we believe in the fact that the era of carbocatalysis is just at the beginning of an exciting phase, still full of surprises.

Acknowledgements

MA wants to thank the German Excellence Cluster Unicat for continued support. Financial support by the Spanish Ministry of Economy and Competitiveness (CTQ 2015‐69153‐CO2‐1, CTQ2014‐53292‐R, Severo Ochoa) and Generalitat Valenciana (Prometeo 2013‐014) is gratefully acknowledged.

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