**3. Carbon nanocomposites catalyzed organic reactions**

Recently, carbon nanocomposites have been widely used as heterogeneous catalysts in various organic transformations. Less than 10% of the chemical processes in chemical industries are still conducted without the addition of catalyst [34]. The catalytic products such as organic building blocks, pharmaceuticals, natural products, and agricultural derivatives are very valuable in chemical industries [35]. Numerous metal catalysts (supported and unsupported) are reported for the industrially important organic transformations. Carbon nanocomposites, particularly structural carbon (graphene and CNTs) based materials, are recently being used as heterogeneous catalysts in organic transformations. In fact, the high surface area, fine dispersion, stability, reusability, and easy recovery are the key factors. Moreover, the immobilization of metal nanoparticles onto the carbon support has revealed more versatility in carrying out the highly selective catalytic processes [36]. In comparison with CNTs, graphene or GO has been preferred due to its low cost, large-scale preparation, and less health risk.

#### **3.1. Noble metals supported carbon catalysts**

Pd nanoparticles supported carbon materials have been widely used as heterogeneous or semi-heterogeneous catalysts for C▬C coupling reactions, such as Mizoroki-Heck, Suzuki-Miyaura, and Sonogashira reactions [37]. These cross-coupling reactions are the most efficient methods for the construction of C▬C bonds. The Pd as a catalyst can assemble C▬C bonds between various functionalized substrates allowed researcher to achieve the reactions that were previously impossible (or possible with multiple steps) [38]. Hence, these methodologies have found extensive use in organic synthesis and material science. Moreover, these cross-coupling reactions found to play an important role in pharmaceutical, fine chemical, and agrochemical industries.

Li et al. [39] demonstrated Pd-graphene nanocomposites as an efficient nanocatalyst for Suzuki reaction. The Pd-graphene nanocomposites showed an efficient catalytic activity toward Suzuki reaction in water under aerobic condition for a short time. **Scheme 1** shows the Suzuki reaction of iodobenzene with phenylboronic acid catalyzed by Pd-graphene nanocomposites. The catalyst is not only efficient but also easily recovered and reused several times for the low-cost and environmentally friendly synthesis of biaryls. Using 1.1 mol% of Pd-graphene nanocomposite with sodium dodecyl sulfate (SDS) at 100°C, the catalytic system affords 100% of yield with 95.5% selectivity. Interestingly, the catalyst can be reused at least for 10 times (at 10th cycle, the yield was 78.6%).

The complete recovery and excellent reusability are the major advantages of using heterogeneous catalysts [40]. However, in most of the heterogeneous catalytic systems, the isolation of catalysts from the reaction mixture by conventional filtration methods is inefficient and time consuming. Therefore, magnetically recoverable carbon nanocomposites have gained much attention due to it easily and complete recovery of the catalyst from reaction mixture. Fe3 O4 and Pd nanoparticles were decorated on sulfonated graphene (s-G) by a facile chemical approach [41]. The prepared carbon nanocomposite Pd/Fe3 O4 /s-G was used as an excellent semi-heterogeneous catalyst for the Suzuki-Miyaura cross-coupling reaction in an environmentally friendly solvent (water/ethanol (1:1)) under ligand-free ambient conditions (**Scheme 2**). It was found that even a low amount of catalyst Pd/Fe3 O4 /s-G (0.15 mol% Pd) is also enough to achieve 97% of the product after 30 min of the reaction time. The small size and homogeneous distribution of Pd nanoparticles on the Fe3 O4 /s-G matrix are the main reason for the excellent catalytic activity. The activity of Pd/Fe3 O4 /s-G did not deteriorate even after 10th cycle, which may be due to the easy and efficient magnetic separation of the catalyst and the high dispersion and stability of the catalyst in an aqueous solution. At 10th cycle, the Pd/Fe3 O4 /s-G catalyst gave 84% of the product. Similarly, magnetically recoverable Pd/Fe3 O4 nanoparticles supported graphene nanosheets (Pd/Fe3 O4 /G) were prepared for Suzuki and Heck coupling reactions (**Figure 4**) [42]. The Pd/Fe3 O4 /G system gave excellent yields over a broad range of highly functionalized substrates in both Suzuki and Heck coupling reactions. With 7.6 wt% of Pd, the Pd/Fe3 O4 /G worked well in Suzuki cross-coupling reaction with a high turnover number (TON) of 9250 and turnover frequency (TOF) of 111,000 h−<sup>1</sup> . Due to the good magnetic property of the Pd/Fe3 O4 /G, it was easily recovered using a simple magnet and reused for 10 times (**Figure 4**).

XRD patterns, and Raman of GNPs-RuO<sup>2</sup>

strongly attached on the surface of SWCNTs.

**3.1. Noble metals supported carbon catalysts**

and agrochemical industries.

10th cycle, the yield was 78.6%).

[31], and GNPs-RuO2

24 Nanocomposites - Recent Evolutions

be 416 m2 g−<sup>1</sup>

NPs. Later, CuO/MWCNTs [30], RuO2

NPs [32] were synthesized by the dry synthesis method. It was dem-

onstrated that the SWCNTs were also utilized to successfully decorate the RuO2

. Moreover, Raman and XPS results confirmed that the RuO<sup>2</sup>

Recently, carbon nanocomposites have been widely used as heterogeneous catalysts in various organic transformations. Less than 10% of the chemical processes in chemical industries are still conducted without the addition of catalyst [34]. The catalytic products such as organic building blocks, pharmaceuticals, natural products, and agricultural derivatives are very valuable in chemical industries [35]. Numerous metal catalysts (supported and unsupported) are reported for the industrially important organic transformations. Carbon nanocomposites, particularly structural carbon (graphene and CNTs) based materials, are recently being used as heterogeneous catalysts in organic transformations. In fact, the high surface area, fine dispersion, stability, reusability, and easy recovery are the key factors. Moreover, the immobilization of metal nanoparticles onto the carbon support has revealed more versatility in carrying out the highly selective catalytic processes [36]. In comparison with CNTs, graphene or GO has been preferred due to its low cost, large-scale preparation, and less health risk.

Pd nanoparticles supported carbon materials have been widely used as heterogeneous or semi-heterogeneous catalysts for C▬C coupling reactions, such as Mizoroki-Heck, Suzuki-Miyaura, and Sonogashira reactions [37]. These cross-coupling reactions are the most efficient methods for the construction of C▬C bonds. The Pd as a catalyst can assemble C▬C bonds between various functionalized substrates allowed researcher to achieve the reactions that were previously impossible (or possible with multiple steps) [38]. Hence, these methodologies have found extensive use in organic synthesis and material science. Moreover, these cross-coupling reactions found to play an important role in pharmaceutical, fine chemical,

Li et al. [39] demonstrated Pd-graphene nanocomposites as an efficient nanocatalyst for Suzuki reaction. The Pd-graphene nanocomposites showed an efficient catalytic activity toward Suzuki reaction in water under aerobic condition for a short time. **Scheme 1** shows the Suzuki reaction of iodobenzene with phenylboronic acid catalyzed by Pd-graphene nanocomposites. The catalyst is not only efficient but also easily recovered and reused several times for the low-cost and environmentally friendly synthesis of biaryls. Using 1.1 mol% of Pd-graphene nanocomposite with sodium dodecyl sulfate (SDS) at 100°C, the catalytic system affords 100% of yield with 95.5% selectivity. Interestingly, the catalyst can be reused at least for 10 times (at

thesis method [33]. Astonishingly, the mean diameter of the RuO2

SWCNTs was found to be about 0.9 nm. The BET surface area of RuO<sup>2</sup>

**3. Carbon nanocomposites catalyzed organic reactions**

/MWCNTs

via dry syn-

nanoparticles attached to

/SWCNT was found to

nanoparticles were

Similarly, various Pd nanoparticles supported graphene nanocomposites were prepared and used as an excellent nanocatalyst for the cross-coupling reaction. Pd nanoparticles supported graphitic carbon nitride (Pd/g-C3 N4 ) was prepared through a one-step photodeposition strategy, and it was used for Suzuki-Miyaura coupling reactions by Sun and coworkers [43]. They found that the Pd/g-C3 N4 was worked well at room temperature without any phase transfer agents, toxic solvents, and inert atmosphere. Under the optimized conditions, the Pd/g-C3 N4 achieved a complete conversion (100%) of the reactant and a high yield of 97% for biphenyl. Unlike other supports, the g-C3 N4 with plenty of nitrogen-containing anchor sites was a suitable platform for Pd atoms, which could favor fine dispersion and stabilization of the ultrafine Pd nanoparticles on g-C3 N4 . Siamak et al. [44] used single- or multi-walled carbon nanotubes (SWCNTs and MWCNTs) as a support for the decoration of Pd nanoparticles. Both the supported catalysts (Pd/MWCNT)M and (Pd/SWCNT)M) were successfully employed in Suzuki cross-coupling reactions with a wide variety of functionalized substrates. Interestingly, they noticed that the MWCNTs supported Pd nanoparticles catalyst (Pd/MWCNT)M) showed slightly better yield

**Scheme 2.** Suzuki-Miyaura cross-coupling reaction catalyzed by Pd/Fe3 O4 /s-G catalyst (from Elazab et al. [41]).

the synthesis of triazoles from anilines by one-pot two-step click reaction in water medium at room temperature (**Scheme 3(b)**). The excellent catalytic activity is due to the synergistic effect of GO. In fact, GO has high adsorption nature toward reactants through p-p stacking interactions. Hence, the GO could help the reactant to go closer to the Ag nanoparticles on GO, leading to good contact between the reactant and Ag on GO. In addition, electron transfer from the GO to Ag nanoparticles increases the local electron concentration, facilitating the

Carbon Nanocomposites: Preparation and Its Application in Catalytic Organic Transformations

The catalytic conversion of nitrophenols to valuable aminophenols in water by using NaBH4 is one of the important organic conversions [47]. In general, the nitrophenols are the major organic pollutants, which can be found in industrial and agricultural wastewaters. They are highly water soluble and stable in the soil and thus cause harmful effects to human beings, animals, and agricultural plants [48]. Very recently, a simple and efficient method for the reduction of nitrophenols to aminophenols was developed by using carbon nanocomposites as a catalyst. The catalytic products (aminophenols) can be used as anticorrosion-lubricant, corrosion inhibitor, photographic developer, and analgesic and antipyretic drugs [49]. Ag nanoparticles supported carbon nanofiber composites (CNFs/AgNPs) were fabricated for

very fine Ag nanoparticles were homogenously dispersed on the CNFs (**Figure 5**). The results showed an excellent catalytic activity of CNFs/AgNPs in the reduction of 4-nitrophenol. The reason for the superior catalytic activity of CNFs/AgNPs is mainly due to the high surface areas and synergistic effect on delivery of electrons between CNFs and Ag nanoparticles. Notably, the CNFs catalyst could be easily recycled at least for three times without loss in its activity. Possible catalytic mechanism is elucidated schematically in **Figure 5(e)**. Similarly, Wang et al. [51] found that Au nanoparticles supported functionalized CNTs [with cyclotriphosphazene-containing polyphosphazenes (PZS)] (Au@PZS@CNTs nanohybrids) are highly

Among noble metals, Ru has shown the ability to catalyze a remarkable range of organic transformations because of its wide range of oxidation states (−2 to +8) and tunable properties [52]. The Ru metal is well known for oxidation-reduction and cross-coupling reactions. The catalytic products are high-functional components in the perfume industry and pharmaceuticals.

**Figure 5.** (a and b) TEM images of CNFs/AgNPs, (c) UV-vis absorption spectra during the catalytic reduction of 4-NP over CNFs/AgNPs, (d) reusability test, and (e) postulate mechanism of the catalytic reduction of 4-NP with the CNFs/

in water [50]. The TEM images confirmed that

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27

uptake of electrons by reactant molecules [13].

the reduction of 4-nitrophenol with NaBH4

suitable catalyst for the reduction of 4-nitrophenol.

AgNPs (from Zhang et al. [50]).

**Figure 4.** Pd/Fe3 O4 /G catalyzed (a) Suzuki cross coupling, (b) Heck coupling reactions, (c) the reaction mixture with Pd/ Fe3 O4 /G, and (d) separation of spent catalyst from reaction mixture using a simple magnet (from Hu et al. [42]).

when compared with SWCNTs supported Pd catalyst (Pd/SWCNT)M). They concluded that the superior catalytic activity and excellent reusability of (Pd/MWCNT)M mainly due to the larger diameter of the MWCNTs (20–150 nm) offer stronger surface interactions and provide large number of anchoring sites for the Pd nanoparticles, thus facilitating the deposition of the greater number of Pd nanoparticles on the surface of MWCNTs with strong attachment.

In organic synthesis, multi-component reactions (MCRs) are very important and essential for the synthesis of diverse complex molecules through a combination of three or more starting materials in a one-pot reaction [45]. For instance, synthesis of propargylamine through coupling reaction of aldehydes, amines, and alkynes (A3 coupling) is one of the important MCRs. The propargylamines are highly valuable in the synthesis of various biologically active compounds and natural products [46]. To synthesis the propargylamines, graphene-based composite with silver nanoparticles (Ag-G) was prepared *through* a simple chemical route by Salam and coworkers [13]. After being optimized the reaction conditions, the scope of the catalytic was extended. The catalytic system worked well for a wide range of substrates including aromatic and aliphatic aldehydes, including those bearing functional groups such as ▬OH, ▬Cl, and ▬Br additions. The Ag-G is air-stable, heterogeneous, cost-effective, easily recoverable, and reusable without loss in activity and selectivity. **Scheme 3(a)** shows three-component (A3 ) coupling reaction catalyzed by the Ag-G. Moreover, the Ag-G catalyst is also suitable for

**Scheme 3.** Ag-G catalyzed (a) three-component (A3 ) coupling reaction and (b) synthesis of triazoles from anilines (from Salam et al. [13]).

the synthesis of triazoles from anilines by one-pot two-step click reaction in water medium at room temperature (**Scheme 3(b)**). The excellent catalytic activity is due to the synergistic effect of GO. In fact, GO has high adsorption nature toward reactants through p-p stacking interactions. Hence, the GO could help the reactant to go closer to the Ag nanoparticles on GO, leading to good contact between the reactant and Ag on GO. In addition, electron transfer from the GO to Ag nanoparticles increases the local electron concentration, facilitating the uptake of electrons by reactant molecules [13].

The catalytic conversion of nitrophenols to valuable aminophenols in water by using NaBH4 is one of the important organic conversions [47]. In general, the nitrophenols are the major organic pollutants, which can be found in industrial and agricultural wastewaters. They are highly water soluble and stable in the soil and thus cause harmful effects to human beings, animals, and agricultural plants [48]. Very recently, a simple and efficient method for the reduction of nitrophenols to aminophenols was developed by using carbon nanocomposites as a catalyst. The catalytic products (aminophenols) can be used as anticorrosion-lubricant, corrosion inhibitor, photographic developer, and analgesic and antipyretic drugs [49]. Ag nanoparticles supported carbon nanofiber composites (CNFs/AgNPs) were fabricated for the reduction of 4-nitrophenol with NaBH4 in water [50]. The TEM images confirmed that very fine Ag nanoparticles were homogenously dispersed on the CNFs (**Figure 5**). The results showed an excellent catalytic activity of CNFs/AgNPs in the reduction of 4-nitrophenol. The reason for the superior catalytic activity of CNFs/AgNPs is mainly due to the high surface areas and synergistic effect on delivery of electrons between CNFs and Ag nanoparticles. Notably, the CNFs catalyst could be easily recycled at least for three times without loss in its activity. Possible catalytic mechanism is elucidated schematically in **Figure 5(e)**. Similarly, Wang et al. [51] found that Au nanoparticles supported functionalized CNTs [with cyclotriphosphazene-containing polyphosphazenes (PZS)] (Au@PZS@CNTs nanohybrids) are highly suitable catalyst for the reduction of 4-nitrophenol.

when compared with SWCNTs supported Pd catalyst (Pd/SWCNT)M). They concluded that the superior catalytic activity and excellent reusability of (Pd/MWCNT)M mainly due to the larger diameter of the MWCNTs (20–150 nm) offer stronger surface interactions and provide large number of anchoring sites for the Pd nanoparticles, thus facilitating the deposition of the greater number of Pd nanoparticles on the surface of MWCNTs with strong attachment.

/G, and (d) separation of spent catalyst from reaction mixture using a simple magnet (from Hu et al. [42]).

/G catalyzed (a) Suzuki cross coupling, (b) Heck coupling reactions, (c) the reaction mixture with Pd/

In organic synthesis, multi-component reactions (MCRs) are very important and essential for the synthesis of diverse complex molecules through a combination of three or more starting materials in a one-pot reaction [45]. For instance, synthesis of propargylamine through

MCRs. The propargylamines are highly valuable in the synthesis of various biologically active compounds and natural products [46]. To synthesis the propargylamines, graphene-based composite with silver nanoparticles (Ag-G) was prepared *through* a simple chemical route by Salam and coworkers [13]. After being optimized the reaction conditions, the scope of the catalytic was extended. The catalytic system worked well for a wide range of substrates including aromatic and aliphatic aldehydes, including those bearing functional groups such as ▬OH, ▬Cl, and ▬Br additions. The Ag-G is air-stable, heterogeneous, cost-effective, easily recoverable, and reusable without loss in activity and selectivity. **Scheme 3(a)** shows three-component

) coupling reaction catalyzed by the Ag-G. Moreover, the Ag-G catalyst is also suitable for

coupling) is one of the important

) coupling reaction and (b) synthesis of triazoles from anilines (from

coupling reaction of aldehydes, amines, and alkynes (A3

**Scheme 3.** Ag-G catalyzed (a) three-component (A3

Salam et al. [13]).

(A3

**Figure 4.** Pd/Fe3

Fe3 O4 O4

26 Nanocomposites - Recent Evolutions

Among noble metals, Ru has shown the ability to catalyze a remarkable range of organic transformations because of its wide range of oxidation states (−2 to +8) and tunable properties [52]. The Ru metal is well known for oxidation-reduction and cross-coupling reactions. The catalytic products are high-functional components in the perfume industry and pharmaceuticals.

**Figure 5.** (a and b) TEM images of CNFs/AgNPs, (c) UV-vis absorption spectra during the catalytic reduction of 4-NP over CNFs/AgNPs, (d) reusability test, and (e) postulate mechanism of the catalytic reduction of 4-NP with the CNFs/ AgNPs (from Zhang et al. [50]).

So far, several Ru nanoparticles supported CNTs or GO catalyst are developed for the organic transformations [53, 54]. Kim's group prepared various Ru or RuO<sup>2</sup> nanoparticles supported carbon nanocomposites and used as heterogeneous catalysts in organic transformation [29, 31–33]. For example, 0.5–3 nm size of metallic Ru nanoparticles decorated graphene nanosheets (GNSs) was used for the oxidation of alcohols [29]. Results revealed that various alcohols (aliphatic, aromatic, alicyclic, benzylic, allylic, amino, and heterocyclic alcohols) can be oxidized into their corresponding carbonyl compounds in good to excellent yields with high selectivity (**Scheme 4**). Very interestingly, a 0.036 mol% Ru (5 mg) of catalyst (GNS-RuNPs) was more than enough for complete oxidation of alcohols (the lowest amount of catalyst so far reported), which shows the merit of the GNS support. The formation of active Ru-oxo species during the reaction was confirmed. The GNS-RuNPs was found to be highly efficient, chemoselective, heterogeneous, stable, and reusable. The GNS-RuNPs catalyst was reused for four times without significant loss in its catalytic activity. After 4th cycle, the used GNS-RuNPs were calcinated at high temperature and used for transfer hydrogenation of carbonyl compounds. It was concluded that the excellent catalytic activity of GNS-RuNPs is due to the smaller size of the Ru nanoparticles, higher surface area, strong interaction between Ru nanoparticles and GNSs, and an effective dispersion of the catalyst in the reaction medium. Similarly, RuO2 NPs/MWCNTs [31] and RuO2 NRs/GNPs [55] were prepared and used for both aerial oxidation of alcohols and transfer hydrogenation of carbonyl compounds. Aliphatic and aromatic tert-amine oxides (amine N-oxides) are essential and key components in the formulation of several cosmetic products as well as in biomedical applications. The GNPs-RuO2 NPs demonstrated excellent catalytic activity toward oxidation of tertiary amines to their corresponding N-oxides in good to excellent yields [32]. The results showed that the scope of the reaction can be extended to various aliphatic, alicyclic, and aromatic tertiary amines.

amines including less reactive aliphatic amines can be transformed by the RuO2

and (c) oxidative coupling of benzyl alcohol and substituted primary amines (from Yuan et al. [21]).

nanoparticles (RuO2

heterogeneity of RuO2

Similarly, ultrafine RuO<sup>2</sup>

and carbon vacancies.

**Scheme 6.** RuO2

efficient RuO<sup>2</sup>

**Scheme 5.** RuO2

RuO2

the corresponding imines in good yields (98–58%) with an excellent selectivity (100%). In addition, an indirect two-step protocol was adopted for the coupling of alcohols and amines to obtain imines, and the results were found to be excellent. The reusability, stability, and

SWCNTs by a straightforward "dry synthesis" method and used it for Heck olefination of aryl halides (**Scheme 6**) [33]. Although Ru has showed good catalytic activity toward Heck reaction, the bromo- and chloroarenes are less reactive. Interestingly, the SWCNTs supported

/SWCNT catalyzed Heck type olefination of aryl halides (from Gopiraman et al. [33]).

 catalyst worked well for the olefination of less reactive chloro- and bromoarenes. In case of supported heterogeneous catalysts, the activity is dependent on the nature of the support, metal-support interaction, and the particle size. It was believed that the inert SWCNTs might be transformed to a very active catalyst through the strong interactions between RuO2

/GO were also investigated. The authors claimed that this is the most

NPs) with 0.9 nm in size were immobilized on


/GO catalyzed (a) self-coupling of amines, (b) cross coupling of aniline with substituted primary amines,

Carbon Nanocomposites: Preparation and Its Application in Catalytic Organic Transformations

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29

/GO to obtain

Imines are very important moieties for the formation of fine chemicals, biologically active compounds, and their intermediates [56]. Interconnected ruthenium dioxide nanoparticles (RuO2 NPs) anchored graphite oxide nanocatalyst (RuO2 /GO) with good BET surface area (285 m2 /g) were prepared and used as a catalyst for the synthesis of imines (**Scheme 5**) [21]. Generally, the graphene-based nanocomposites are often suffered from the lower BET surface area due to the face-to-face aggregation of graphene sheets. However, in case of RuO2 / GO, the interconnected RuO2 network strongly prevented the further aggregation of GO, leading to the high-specific surface area of RuO<sup>2</sup> /GO. It was noticed that a broad range of

**Scheme 4.** Ru-graphene catalyzed (a) oxidation of alcohols, (b) transfer hydrogenation of carbonyl compounds, and (c) chemoselectivity oxidation of alcohols (from Gopiraman et al. [29]).

Carbon Nanocomposites: Preparation and Its Application in Catalytic Organic Transformations http://dx.doi.org/10.5772/intechopen.81109 29

So far, several Ru nanoparticles supported CNTs or GO catalyst are developed for the organic

carbon nanocomposites and used as heterogeneous catalysts in organic transformation [29, 31–33]. For example, 0.5–3 nm size of metallic Ru nanoparticles decorated graphene nanosheets (GNSs) was used for the oxidation of alcohols [29]. Results revealed that various alcohols (aliphatic, aromatic, alicyclic, benzylic, allylic, amino, and heterocyclic alcohols) can be oxidized into their corresponding carbonyl compounds in good to excellent yields with high selectivity (**Scheme 4**). Very interestingly, a 0.036 mol% Ru (5 mg) of catalyst (GNS-RuNPs) was more than enough for complete oxidation of alcohols (the lowest amount of catalyst so far reported), which shows the merit of the GNS support. The formation of active Ru-oxo species during the reaction was confirmed. The GNS-RuNPs was found to be highly efficient, chemoselective, heterogeneous, stable, and reusable. The GNS-RuNPs catalyst was reused for four times without significant loss in its catalytic activity. After 4th cycle, the used GNS-RuNPs were calcinated at high temperature and used for transfer hydrogenation of carbonyl compounds. It was concluded that the excellent catalytic activity of GNS-RuNPs is due to the smaller size of the Ru nanoparticles, higher surface area, strong interaction between Ru nanoparticles and GNSs, and an effective dispersion of the catalyst in the reaction medium.

aerial oxidation of alcohols and transfer hydrogenation of carbonyl compounds. Aliphatic and aromatic tert-amine oxides (amine N-oxides) are essential and key components in the formulation of several cosmetic products as well as in biomedical applications. The GNPs-RuO2

demonstrated excellent catalytic activity toward oxidation of tertiary amines to their corresponding N-oxides in good to excellent yields [32]. The results showed that the scope of the

Imines are very important moieties for the formation of fine chemicals, biologically active compounds, and their intermediates [56]. Interconnected ruthenium dioxide nanoparticles

Generally, the graphene-based nanocomposites are often suffered from the lower BET surface area due to the face-to-face aggregation of graphene sheets. However, in case of RuO2

**Scheme 4.** Ru-graphene catalyzed (a) oxidation of alcohols, (b) transfer hydrogenation of carbonyl compounds, and (c)

/g) were prepared and used as a catalyst for the synthesis of imines (**Scheme 5**) [21].

network strongly prevented the further aggregation of GO,

reaction can be extended to various aliphatic, alicyclic, and aromatic tertiary amines.

nanoparticles supported

NPs

/

NRs/GNPs [55] were prepared and used for both

/GO) with good BET surface area

/GO. It was noticed that a broad range of

transformations [53, 54]. Kim's group prepared various Ru or RuO<sup>2</sup>

NPs/MWCNTs [31] and RuO2

NPs) anchored graphite oxide nanocatalyst (RuO2

leading to the high-specific surface area of RuO<sup>2</sup>

chemoselectivity oxidation of alcohols (from Gopiraman et al. [29]).

Similarly, RuO2

28 Nanocomposites - Recent Evolutions

(RuO2

(285 m2

GO, the interconnected RuO2

**Scheme 5.** RuO2 /GO catalyzed (a) self-coupling of amines, (b) cross coupling of aniline with substituted primary amines, and (c) oxidative coupling of benzyl alcohol and substituted primary amines (from Yuan et al. [21]).

amines including less reactive aliphatic amines can be transformed by the RuO2 /GO to obtain the corresponding imines in good yields (98–58%) with an excellent selectivity (100%). In addition, an indirect two-step protocol was adopted for the coupling of alcohols and amines to obtain imines, and the results were found to be excellent. The reusability, stability, and heterogeneity of RuO2 /GO were also investigated. The authors claimed that this is the most efficient RuO<sup>2</sup> -based nanocatalyst for the synthesis of imines among those reported to date. Similarly, ultrafine RuO<sup>2</sup> nanoparticles (RuO2 NPs) with 0.9 nm in size were immobilized on SWCNTs by a straightforward "dry synthesis" method and used it for Heck olefination of aryl halides (**Scheme 6**) [33]. Although Ru has showed good catalytic activity toward Heck reaction, the bromo- and chloroarenes are less reactive. Interestingly, the SWCNTs supported RuO2 catalyst worked well for the olefination of less reactive chloro- and bromoarenes. In case of supported heterogeneous catalysts, the activity is dependent on the nature of the support, metal-support interaction, and the particle size. It was believed that the inert SWCNTs might be transformed to a very active catalyst through the strong interactions between RuO2 and carbon vacancies.

**Scheme 6.** RuO2 /SWCNT catalyzed Heck type olefination of aryl halides (from Gopiraman et al. [33]).

#### **3.2. Non-noble metal supported carbon nanocomposites**

Due to less cost, high activity, and less toxic nature, non-noble (Ni, Cu, Fe, Al, V, Ce, and Mn) nanoparticles are extensively employed studied as efficient catalysts for the organic transformations [57]. Particularly, Ni, Cu, and Fe nanoparticles have been widely studied for the organic conversion. Formamides are valuable intermediates in the synthesis of pharmaceutically important compounds [58]. Fakhri et al. [59] prepared Cu nanoparticles supported GO catalyst (rGO/CuNPs), and it was used for the synthesis of formamides and primary amines (**Scheme 7**). It was demonstrated that the rGO/CuNPs are highly efficient and reusable. Similarly, highly sustainable and versatile carbon nanocomposite CuO/GNS was prepared and used as catalysts for base-free coupling reactions (**Scheme 8**) [24]. Under very mild reaction conditions (CuO/GNS 0.7 mol%, acetonitrile 5 mL, air atmosphere, 3.5–12 h, 82°C), the CuO/GNS demonstrated outstanding catalytic activity in terms of yield (52–98%) and TON/TOF under base-free reaction conditions. A wide range of aromatic aldehydes, amines, and alkynes were employed to extend the scope of the catalytic system. In addition to the heterogeneous, stable, and reusable nature, the versatility of CuO/GNS was realized from the higher yield in aza-Michael reaction (**Scheme 7(b)**). After use, the GNS and CuO NPs (as CuCl2 ) were successfully recovered from the u-CuO/GNS (**Figure 6**). The recovered GNS and CuCl2 can be used for other applications. Recently, a highly efficient and cost-effective CuO/carbon-nanoparticle catalyst (CuO/CNP) was prepared by a simple "mix-and-heat" method and used for the self-coupling of amines [24]. The CuO/CNP demonstrated excellent catalytic activity toward the synthesis of imines under optimal reaction conditions involving 12 h of reaction time, 25 mg of catalyst, air atmosphere, and 110°C. A wide range of amines (aromatic, aliphatic, alicyclic, and heterocyclic amines) were efficiently catalyzed by CuO/ GNS. Heterogeneity, stability, and reusability of CuO/CNP were found to be excellent.

showed that this is the smallest amount of catalyst used for N-arylation of imidazole reported to date. Chemical and physical stability, heterogeneity, and reusability of CuO/MWCNT were found to be excellent. After 4th cycle, MWCNTs were successfully separated from the used CuO/MWCNT, and it was confirmed. Based on the results obtained, it was concluded that the good catalytic activity of CuO/MWCNT is due to high surface area and effective dispersion of

**Scheme 8.** CuO/GNS catalyzed (a) three-component coupling of aldehyde, amine, and alkynea and (b) aza-Michael

Carbon Nanocomposites: Preparation and Its Application in Catalytic Organic Transformations

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31

Formic acid is often produced from biomass cellulose as well as from fats and oils. This simple acid can be used for storage of hydrogen for different applications [61]. Several metal catalysts including Pt and Cu were employed to decompose formic acid [62]. However, the stability and reusability of the catalysts are limited because the sintering of Cu leads to deactivation in catalytic reactions. Bulushev et al. [63] Cu nanoparticles supported N-doped expanded graphite oxide for the decomposition of formic acid. They showed that the problem of sintering of Cu leaching could be resolved by N-doping of the carbon support. The N-doping leads to a strong interaction of the Cu species with the support by pyridinic nitrogen atoms present in the carbon support. The results showed that the N-doped Cu catalyst has good stability in the formic acid decomposition even at 478 K for at least 7 h on-stream and a significantly

**Figure 6.** (a) Reusability and heterogeneity tests of CuO/GNS, (b) TEM images of used CuO/GNS, (c) photographic

from used CuO/GNS, and (d) TEM images of recovered GNS (from

the CuO/MWCNT in the reaction medium.

reaction of amines with acrylonitrile (from Gopiraman et al. [24]).

higher catalytic activity.

image showing the recovery of GNS and CuCl2

Gopiraman et al. [24]).

Nitrogen-containing heterocycles including imidazole and its derivatives are prevalent structural motifs in various fields such as biological, pharmaceutical, and material sciences [60]. They are highly efficient antibacterial, antimalarial, antiviral, antimycobacterial, and antifungal compounds. Gopiraman and coworkers [30] have prepared highly efficient and reusable CuO/MWCNT catalyst for N-arylation of imidazole (**Scheme 9**). It was found that a 0.98 mol% (5 mg) of the CuO/MWCNT was sufficient for the efficient N-arylation of imidazole. The results

**Scheme 7.** rGO/Cu NPs catalyzed (a) formylation of different arylboronic acids and (b) amination of different arylboronic acids (from Fakhri et al. [59]).

Carbon Nanocomposites: Preparation and Its Application in Catalytic Organic Transformations http://dx.doi.org/10.5772/intechopen.81109 31

**3.2. Non-noble metal supported carbon nanocomposites**

30 Nanocomposites - Recent Evolutions

(as CuCl2

and CuCl2

acids (from Fakhri et al. [59]).

Due to less cost, high activity, and less toxic nature, non-noble (Ni, Cu, Fe, Al, V, Ce, and Mn) nanoparticles are extensively employed studied as efficient catalysts for the organic transformations [57]. Particularly, Ni, Cu, and Fe nanoparticles have been widely studied for the organic conversion. Formamides are valuable intermediates in the synthesis of pharmaceutically important compounds [58]. Fakhri et al. [59] prepared Cu nanoparticles supported GO catalyst (rGO/CuNPs), and it was used for the synthesis of formamides and primary amines (**Scheme 7**). It was demonstrated that the rGO/CuNPs are highly efficient and reusable. Similarly, highly sustainable and versatile carbon nanocomposite CuO/GNS was prepared and used as catalysts for base-free coupling reactions (**Scheme 8**) [24]. Under very mild reaction conditions (CuO/GNS 0.7 mol%, acetonitrile 5 mL, air atmosphere, 3.5–12 h, 82°C), the CuO/GNS demonstrated outstanding catalytic activity in terms of yield (52–98%) and TON/TOF under base-free reaction conditions. A wide range of aromatic aldehydes, amines, and alkynes were employed to extend the scope of the catalytic system. In addition to the heterogeneous, stable, and reusable nature, the versatility of CuO/GNS was realized from the higher yield in aza-Michael reaction (**Scheme 7(b)**). After use, the GNS and CuO NPs

) were successfully recovered from the u-CuO/GNS (**Figure 6**). The recovered GNS

can be used for other applications. Recently, a highly efficient and cost-effective

CuO/carbon-nanoparticle catalyst (CuO/CNP) was prepared by a simple "mix-and-heat" method and used for the self-coupling of amines [24]. The CuO/CNP demonstrated excellent catalytic activity toward the synthesis of imines under optimal reaction conditions involving 12 h of reaction time, 25 mg of catalyst, air atmosphere, and 110°C. A wide range of amines (aromatic, aliphatic, alicyclic, and heterocyclic amines) were efficiently catalyzed by CuO/ GNS. Heterogeneity, stability, and reusability of CuO/CNP were found to be excellent.

Nitrogen-containing heterocycles including imidazole and its derivatives are prevalent structural motifs in various fields such as biological, pharmaceutical, and material sciences [60]. They are highly efficient antibacterial, antimalarial, antiviral, antimycobacterial, and antifungal compounds. Gopiraman and coworkers [30] have prepared highly efficient and reusable CuO/MWCNT catalyst for N-arylation of imidazole (**Scheme 9**). It was found that a 0.98 mol% (5 mg) of the CuO/MWCNT was sufficient for the efficient N-arylation of imidazole. The results

**Scheme 7.** rGO/Cu NPs catalyzed (a) formylation of different arylboronic acids and (b) amination of different arylboronic

**Scheme 8.** CuO/GNS catalyzed (a) three-component coupling of aldehyde, amine, and alkynea and (b) aza-Michael reaction of amines with acrylonitrile (from Gopiraman et al. [24]).

showed that this is the smallest amount of catalyst used for N-arylation of imidazole reported to date. Chemical and physical stability, heterogeneity, and reusability of CuO/MWCNT were found to be excellent. After 4th cycle, MWCNTs were successfully separated from the used CuO/MWCNT, and it was confirmed. Based on the results obtained, it was concluded that the good catalytic activity of CuO/MWCNT is due to high surface area and effective dispersion of the CuO/MWCNT in the reaction medium.

Formic acid is often produced from biomass cellulose as well as from fats and oils. This simple acid can be used for storage of hydrogen for different applications [61]. Several metal catalysts including Pt and Cu were employed to decompose formic acid [62]. However, the stability and reusability of the catalysts are limited because the sintering of Cu leads to deactivation in catalytic reactions. Bulushev et al. [63] Cu nanoparticles supported N-doped expanded graphite oxide for the decomposition of formic acid. They showed that the problem of sintering of Cu leaching could be resolved by N-doping of the carbon support. The N-doping leads to a strong interaction of the Cu species with the support by pyridinic nitrogen atoms present in the carbon support. The results showed that the N-doped Cu catalyst has good stability in the formic acid decomposition even at 478 K for at least 7 h on-stream and a significantly higher catalytic activity.

**Figure 6.** (a) Reusability and heterogeneity tests of CuO/GNS, (b) TEM images of used CuO/GNS, (c) photographic image showing the recovery of GNS and CuCl2 from used CuO/GNS, and (d) TEM images of recovered GNS (from Gopiraman et al. [24]).

**Scheme 9.** CuO/MWCNT catalyzed N-arylation of imidazole with various aryl halides (from Gopiraman et al. [30]).

Kamal et al. [64] prepared GO-based nanocomposite (CuO@GO), and it was utilized for ligandfree and solvent-free C▬N and C▬S cross-coupling reactions with weak bases such as triethylamine (**Scheme 10**). They found that the CuO@GO is a simple and efficient catalyst for solvent- and ligand-free C▬S cross-coupling reactions in the presence of weak bases and relatively mild reaction conditions by using the CuO@GO catalytic system. In addition, the CuO@ GO was readily separated by centrifugation and could be reused six times under the solvent-free conditions with only a marginal loss of catalytic activity. Catalytic conversion of biomass-derived acids to valuable products is an important process in various chemical industries. Similarly, Ni nanoparticles supported reduced graphene oxide (Ni/RGO) was prepared and used as a heterogeneous catalyst for the C▬S cross-coupling reaction between aryl halides and thiols (**Scheme 11**) [65]. They found that the catalytic performance is mainly dependent on the sizes of the Ni nanoparticles. Moreover, the electron-rich planar surface of RGO helps in stabilizing the nanoparticles and prevents agglomeration.

Very recently, carbon black (CB) supported Ni catalyst (Ni/CB) has been prepared by a facile

**Scheme 11.** Ni/RGO-40 catalyzed C▬S cross coupling between aryl halide and thiol (from Sengupta et al. [65]).

The Ni/CB catalyst showed excellent activity toward hydrogenation of nitrophenols in water at room temperature. Results showed that the synergistic effect of nano-Ni and carbon black, the presence of oxygen functional groups on carbon black for anchoring Ni atoms, strong adsorption ability for organic molecules, and good conductivity for electron transfer from the carbon black to Ni nanoparticles are the main reason of the superior catalytic activity of the Ni/CB. Moreover, the Ni/CB catalyst is not only cheap but also magnetically separable, and therefore, this approach facilitates achieving the cost-effective reduction of nitrophenols to aminophenols. Similarly, Saravanamoorthy et al. [67] prepared highly efficient and costeffective NiO-based carbon nanocomposite (NiO/CNP) by a simple "mix-and-heat" method.

were calculated for the reduction of 4- and 2-nitrophenols. Interestingly, the catalyst worked well for the transfer hydrogenation of carbonyl compounds under mild reaction conditions (5 mg of NiO/CNP, 9 h of reaction time, 2 mmol of NaOH, air atmosphere, and room temperature). It was found that the NiO/CNP composite is chemoselectivity and heterogeneous

Nitrogen-doped carbon materials are found to be highly efficient support for metal nanoparticles [68]. In fact, the N-dopants in the carbon matrix act as efficient anchoring sites or defects for enhancing the nanoparticle nucleation and reducing the nanoparticle size [69]. Interestingly, the N-dopants can modify the electronic structure of the carbon matrix and tune the activity of

 carbon and metal nanoparticles, thus promoting the higher catalytic activity. In addition, the hydrophilicity and basicity of carbon supports can be improved by N-doping; therefore, the N-doped carbon materials could be effectively used to prepare catalysts in the aqueous phase. However, the recent studies on the N-doped carbon supports are mainly focused on noble metals. Very recently, Nie et al. [70] prepared porous N-doped carbon black supported Ni catalyst (Ni/NCB) by a simple chemical method. The prepared Ni/NCB catalyst showed high performance in the hydrogenation of vanillin (4-hydroxy-3-methoxybenzaldehyde) to 2-methoxy-4-methylphenol under mild conditions at low hydrogen pressure (0.5 MPa) and mild temperature (<150°C), which is significantly superior to other frequently used Ni catalysts. The nanostructure of Ni/NCB, intimate interaction between the Ni nanoparticles and the N species, and lower oxidation state are the main reason for higher catalytic activity of Ni/

nanoparticles have played a crucial role as a heterogeneous catalyst due to its

environmentally benign, high catalytic activity, good magnetic separation performance, and

The NiO/CNP exhibited that high-rate constant (kapp) values of 4.2 × 10−<sup>2</sup> s−<sup>1</sup>

NCB. Moreover, the Ni/NCB catalyst is cost-effective and easily separable.

as the nickel source and hydrazine hydrate as the reducing agent [66].

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33

and 3.06 × 10−<sup>2</sup> s−<sup>1</sup>

method using NiCl2

in nature, stable, and reusable.

the sp2

The Fe3

O4

**Scheme 10.** CuO@GO catalyzed (a) S-arylation of various thiols with different aryl halides, (b) S-arylation of various thiols with different aryl chlorides, (c) reactions of various iodobenzenes with thiourea, and (d) cascade C▬S and C▬N cross coupling of aryl ortho-dihalides and ortho-aminobenzenethiols (from Kamal et al. [64]).

Carbon Nanocomposites: Preparation and Its Application in Catalytic Organic Transformations http://dx.doi.org/10.5772/intechopen.81109 33

**Scheme 11.** Ni/RGO-40 catalyzed C▬S cross coupling between aryl halide and thiol (from Sengupta et al. [65]).

Kamal et al. [64] prepared GO-based nanocomposite (CuO@GO), and it was utilized for ligandfree and solvent-free C▬N and C▬S cross-coupling reactions with weak bases such as triethylamine (**Scheme 10**). They found that the CuO@GO is a simple and efficient catalyst for solvent- and ligand-free C▬S cross-coupling reactions in the presence of weak bases and relatively mild reaction conditions by using the CuO@GO catalytic system. In addition, the CuO@ GO was readily separated by centrifugation and could be reused six times under the solvent-free conditions with only a marginal loss of catalytic activity. Catalytic conversion of biomass-derived acids to valuable products is an important process in various chemical industries. Similarly, Ni nanoparticles supported reduced graphene oxide (Ni/RGO) was prepared and used as a heterogeneous catalyst for the C▬S cross-coupling reaction between aryl halides and thiols (**Scheme 11**) [65]. They found that the catalytic performance is mainly dependent on the sizes of the Ni nanoparticles. Moreover, the electron-rich planar surface of RGO helps in stabilizing the

**Scheme 9.** CuO/MWCNT catalyzed N-arylation of imidazole with various aryl halides (from Gopiraman et al. [30]).

**Scheme 10.** CuO@GO catalyzed (a) S-arylation of various thiols with different aryl halides, (b) S-arylation of various thiols with different aryl chlorides, (c) reactions of various iodobenzenes with thiourea, and (d) cascade C▬S and C▬N

cross coupling of aryl ortho-dihalides and ortho-aminobenzenethiols (from Kamal et al. [64]).

nanoparticles and prevents agglomeration.

32 Nanocomposites - Recent Evolutions

Very recently, carbon black (CB) supported Ni catalyst (Ni/CB) has been prepared by a facile method using NiCl2 as the nickel source and hydrazine hydrate as the reducing agent [66]. The Ni/CB catalyst showed excellent activity toward hydrogenation of nitrophenols in water at room temperature. Results showed that the synergistic effect of nano-Ni and carbon black, the presence of oxygen functional groups on carbon black for anchoring Ni atoms, strong adsorption ability for organic molecules, and good conductivity for electron transfer from the carbon black to Ni nanoparticles are the main reason of the superior catalytic activity of the Ni/CB. Moreover, the Ni/CB catalyst is not only cheap but also magnetically separable, and therefore, this approach facilitates achieving the cost-effective reduction of nitrophenols to aminophenols. Similarly, Saravanamoorthy et al. [67] prepared highly efficient and costeffective NiO-based carbon nanocomposite (NiO/CNP) by a simple "mix-and-heat" method. The NiO/CNP exhibited that high-rate constant (kapp) values of 4.2 × 10−<sup>2</sup> s−<sup>1</sup> and 3.06 × 10−<sup>2</sup> s−<sup>1</sup> were calculated for the reduction of 4- and 2-nitrophenols. Interestingly, the catalyst worked well for the transfer hydrogenation of carbonyl compounds under mild reaction conditions (5 mg of NiO/CNP, 9 h of reaction time, 2 mmol of NaOH, air atmosphere, and room temperature). It was found that the NiO/CNP composite is chemoselectivity and heterogeneous in nature, stable, and reusable.

Nitrogen-doped carbon materials are found to be highly efficient support for metal nanoparticles [68]. In fact, the N-dopants in the carbon matrix act as efficient anchoring sites or defects for enhancing the nanoparticle nucleation and reducing the nanoparticle size [69]. Interestingly, the N-dopants can modify the electronic structure of the carbon matrix and tune the activity of the sp2 carbon and metal nanoparticles, thus promoting the higher catalytic activity. In addition, the hydrophilicity and basicity of carbon supports can be improved by N-doping; therefore, the N-doped carbon materials could be effectively used to prepare catalysts in the aqueous phase. However, the recent studies on the N-doped carbon supports are mainly focused on noble metals. Very recently, Nie et al. [70] prepared porous N-doped carbon black supported Ni catalyst (Ni/NCB) by a simple chemical method. The prepared Ni/NCB catalyst showed high performance in the hydrogenation of vanillin (4-hydroxy-3-methoxybenzaldehyde) to 2-methoxy-4-methylphenol under mild conditions at low hydrogen pressure (0.5 MPa) and mild temperature (<150°C), which is significantly superior to other frequently used Ni catalysts. The nanostructure of Ni/NCB, intimate interaction between the Ni nanoparticles and the N species, and lower oxidation state are the main reason for higher catalytic activity of Ni/ NCB. Moreover, the Ni/NCB catalyst is cost-effective and easily separable.

The Fe3 O4 nanoparticles have played a crucial role as a heterogeneous catalyst due to its environmentally benign, high catalytic activity, good magnetic separation performance, and high chemical stability [71]. Huo et al. [72] prepared graphene-Fe3 O4 nanocomposite for the A3 coupling of aldehydes, alkynes, and amines (**Scheme 12**). The catalytic system produced a diverse range of propargylamines in a moderate to high yield under mild conditions. Interestingly, this catalyst could be reused up to eight times with essentially no loss of activity. Moreover, the separation and reuse of graphene-Fe3 O4 were very simple, effective, and economical. Similarly, Stein and coworkers [58] prepared Fe nanoparticles supported GO for the preparation of hydrogenation of different olefins and alkynes with H<sup>2</sup> .

affording the targeted biaryl products in high yields. They concluded that the rGO-CuPd catalytic system has obvious advantages such as recyclable, easy to operate, and environmentally friendly over the conventional Sonogashira couplings. Goksu et al. [77] developed bimetallic Ni-Pd nanoparticles supported GO catalytic system for the tandem dehydrogenation of ammonia borane and hydrogenation of nitro/nitrile compounds (**Scheme 15**). They found that the G-NiPd catalyst is highly active and reusable. Moreover, the reaction can be performed in an environmentally friendly process with short-reaction times and high yields.

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**Scheme 13.** Au-Ag/SLG catalyzed (a) hydroarylation of alkynes with arenes, (b) direct arylation of 1,1-diphenylethylenes

**Scheme 14.** rGO-CuPd catalyzed Sonogashira couplings of various aryl halides and phenyl acetylene (from Diyarbakir

compounds and (b) nitrile and/or nitro compounds

with iodobenzene, and (c) hydrophenoxylation of alkynes with substituted phenols (from Babu et al. [17]).

**Scheme 15.** G-NiPd catalyzed tandem reaction of (a) various R-NO2

et al. [76]).

(from Goksu et al. [77]).

#### **3.3. Bimetallic carbon nanocomposites**

Bimetallic alloy nanoparticles show an enhancement in the catalytic properties owing to the synergistic effects between the two or more distinct metals [73]. In particular, carbon materials supported bi- or multi-metallic nanocomposites are often show dramatic change in the catalytic activity when compared with the mono metallic carbon supported catalysts. Babu et al. [17] prepared bimetallic Au-Ag nanoparticles supported single-layer graphene (SLG) nanocomposites (Au-Ag/SLG) for the hydroarylation, C-arylation, and hydrophenoxylation reactions under mild and ligand-free conditions (**Scheme 13**). They found that the catalytic activity of the Au-Ag/SLG found to be better than the mixture of monometallic nanocatalysts (Au/SLG and Ag/SLG). Interestingly, more than twofold synergy was obtained by this bimetallic nanocatalyst (Au-Ag/SLG). Usage of meager amount of precious metals (0.09 mol% of Au and 0.22 mol% of Ag) and very good reusability made this catalytic system economically feasible. Similarly, Lv and coworkers [74] prepared porous Pt-Au nanodendrites supported on reduced graphene oxide nanosheets (Pt-Au pNDs/RGOs) for the reduction of 4-nitrophenol. They found that the Pt-Au pNDs/RGOs exhibited significantly enhanced catalytic performance toward the reduction of 4-nitrophenol, as compared to commercial Pt black and home-made Au nanocrystals. The reason for the enhancement in the catalytic activity of the Pt-Au pNDs/RGOs is due to its unique interconnected nanostructures of Pt-Au pNDs, which provide more available active sites and the improved mass transport by using RGOs as a support, along with the synergistic effects between Pt and Au.

Aryl-substituted alkynes are versatile intermediates in the formation of various agrochemicals, medicines, and functional organic molecules [75]. Sonogashira cross-coupling reaction of terminal alkynes with aryl halides is one of the most efficient routes for the construction of substituted aryl alkynes. Supported Pd-Cu catalyst has been found to be highly efficient for the Sonogashira cross-coupling reaction in good yield. Diyarbakir et al. [76] prepared Cu-Pd alloy nanoparticles immobilized GO catalyst (rGO-CuPd) for the Sonogashira cross-coupling reactions of various aryl halides with phenylacetylene (**Scheme 14**). The rGO-CuPd catalyst worked well for both electron-rich and electron deficient aryl iodides and aryl bromides,

**Scheme 12.** Graphene-Fe3 O4 nanocomposite catalyzed A3 coupling of aldehydes, alkynes, and amines (from Huo et al. [72]).

affording the targeted biaryl products in high yields. They concluded that the rGO-CuPd catalytic system has obvious advantages such as recyclable, easy to operate, and environmentally friendly over the conventional Sonogashira couplings. Goksu et al. [77] developed bimetallic Ni-Pd nanoparticles supported GO catalytic system for the tandem dehydrogenation of ammonia borane and hydrogenation of nitro/nitrile compounds (**Scheme 15**). They found that the G-NiPd catalyst is highly active and reusable. Moreover, the reaction can be performed in an environmentally friendly process with short-reaction times and high yields.

high chemical stability [71]. Huo et al. [72] prepared graphene-Fe3

the preparation of hydrogenation of different olefins and alkynes with H<sup>2</sup>

ity. Moreover, the separation and reuse of graphene-Fe3

support, along with the synergistic effects between Pt and Au.

**Scheme 12.** Graphene-Fe3

[72]).

O4

nanocomposite catalyzed A3

**3.3. Bimetallic carbon nanocomposites**

 coupling of aldehydes, alkynes, and amines (**Scheme 12**). The catalytic system produced a diverse range of propargylamines in a moderate to high yield under mild conditions. Interestingly, this catalyst could be reused up to eight times with essentially no loss of activ-

economical. Similarly, Stein and coworkers [58] prepared Fe nanoparticles supported GO for

Bimetallic alloy nanoparticles show an enhancement in the catalytic properties owing to the synergistic effects between the two or more distinct metals [73]. In particular, carbon materials supported bi- or multi-metallic nanocomposites are often show dramatic change in the catalytic activity when compared with the mono metallic carbon supported catalysts. Babu et al. [17] prepared bimetallic Au-Ag nanoparticles supported single-layer graphene (SLG) nanocomposites (Au-Ag/SLG) for the hydroarylation, C-arylation, and hydrophenoxylation reactions under mild and ligand-free conditions (**Scheme 13**). They found that the catalytic activity of the Au-Ag/SLG found to be better than the mixture of monometallic nanocatalysts (Au/SLG and Ag/SLG). Interestingly, more than twofold synergy was obtained by this bimetallic nanocatalyst (Au-Ag/SLG). Usage of meager amount of precious metals (0.09 mol% of Au and 0.22 mol% of Ag) and very good reusability made this catalytic system economically feasible. Similarly, Lv and coworkers [74] prepared porous Pt-Au nanodendrites supported on reduced graphene oxide nanosheets (Pt-Au pNDs/RGOs) for the reduction of 4-nitrophenol. They found that the Pt-Au pNDs/RGOs exhibited significantly enhanced catalytic performance toward the reduction of 4-nitrophenol, as compared to commercial Pt black and home-made Au nanocrystals. The reason for the enhancement in the catalytic activity of the Pt-Au pNDs/RGOs is due to its unique interconnected nanostructures of Pt-Au pNDs, which provide more available active sites and the improved mass transport by using RGOs as a

Aryl-substituted alkynes are versatile intermediates in the formation of various agrochemicals, medicines, and functional organic molecules [75]. Sonogashira cross-coupling reaction of terminal alkynes with aryl halides is one of the most efficient routes for the construction of substituted aryl alkynes. Supported Pd-Cu catalyst has been found to be highly efficient for the Sonogashira cross-coupling reaction in good yield. Diyarbakir et al. [76] prepared Cu-Pd alloy nanoparticles immobilized GO catalyst (rGO-CuPd) for the Sonogashira cross-coupling reactions of various aryl halides with phenylacetylene (**Scheme 14**). The rGO-CuPd catalyst worked well for both electron-rich and electron deficient aryl iodides and aryl bromides,

O4

A3

34 Nanocomposites - Recent Evolutions

O4

coupling of aldehydes, alkynes, and amines (from Huo et al.

nanocomposite for the

were very simple, effective, and

.

**Scheme 13.** Au-Ag/SLG catalyzed (a) hydroarylation of alkynes with arenes, (b) direct arylation of 1,1-diphenylethylenes with iodobenzene, and (c) hydrophenoxylation of alkynes with substituted phenols (from Babu et al. [17]).

**Scheme 14.** rGO-CuPd catalyzed Sonogashira couplings of various aryl halides and phenyl acetylene (from Diyarbakir et al. [76]).

**Scheme 15.** G-NiPd catalyzed tandem reaction of (a) various R-NO2 compounds and (b) nitrile and/or nitro compounds (from Goksu et al. [77]).
