**1. Introduction**

Carbon nanomaterials, including carbon nanotubes [both single-walled (SWCNTs) and multiwalled (MWCNTs)], graphene (G) or graphene oxide (GO), and carbon nanoparticles (CNPs), have attracted increasing attention owing to their unique structural regularity, high surface area, electrical conductivity, chemical inertness, biocompatibility, mechanical, and thermal stability [1, 2]. Graphene is a 2D single-atom-thick sheet of sp2 -hybridized carbon, and it can be stacked to form 3D graphite and rolled to form 1D carbon nanotubes (CNTs). The longrange π-conjugation in graphene possesses astonishing thermal, mechanical, and electrical properties [3, 4]. Because of their outstanding physicochemical properties, researchers turned

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

straight away into the exploration and modification of graphene and CNTs. To date, the potential applications of graphene and CNTs are diverse, which include catalyst carrier, energy storage, absorbents, biomedical, textiles, and sensors and support in many areas. As a catalyst carrier, the role of graphene and CNTs is just outstanding [5]. Particularly, in heterogeneous catalysts, the carbon materials often employed as a support to disperse the metal nanoparticles [3, 6]. In fact, the metal nanoparticles can easily agglomerate to form big nanoparticles due to their high surface energy, and it can be avoided by using support materials. Generally, the activity of the heterogeneous catalyst is mainly due to the structure of the catalyst, size of the metal nanoparticles, nature of the support, metal-support interaction, and fine dispersion of catalyst in reaction medium [7, 8]. To date, there are several metal nanoparticles supported graphene or CNT catalysts developed and reported for various organic transformations. The catalytic products are highly valuable in various fields including pharmaceutical, biomedical, agricultural, and material sciences [9]. In recent days, the interest on carbon nanocomposites in organic reaction has been increased significantly due to their unexpected positive outcomes. In this chapter, we discuss the main advances in the field over the last few years and explore the novel preparation methods of carbon nanocomposites (metal nanostructures/ carbon materials) and their applications in various catalytic organic transformations.

Noble metals such as silver (Ag), gold (Au), ruthenium (Ru), and palladium (Pd) nanoparticles have been widely employed as promoters and catalysts in various organic transformations. The carbon-based supports such as CNTs and graphene/graphene oxide (G/GO) are often used as support for the immobilization of Ag, Ru, Pd, and Au nanoparticles. It was found that the preparation method has huge influences on the structure and surface morphology of the carbon nanocomposites. Recently, Salam et al. [13] obtained a highly efficient silver-graphene nanocomposite (Ag-G) through a simple wet chemical route. They used silica-coated Ag nanoparticle solution as Ag sources. In a typical wet synthesis, the silica-coated Ag nanoparticle solution was added with aqueous GO solution under stirring for 15 min followed by the addition of hydrazine solution. The solution was heated at 80°C for 15–20 min, and the resultant precipitate (Ag-G) was filtered and dried. The Ag-G has been characterized by XRD, TEM, and Raman spectroscopy. The results confirmed the uniform dispersion of Ag nanoparticles with good

Carbon Nanocomposites: Preparation and Its Application in Catalytic Organic Transformations

the preparation of carbon nanocomposites. The Ag/graphene nanocomposites were prepared

at 100°C. Finally, the Ag/G nanocomposites were obtained by centrifugation, washing, and freeze-drying. The SEM and TEM results confirmed that the Ag nanoparticles (ranging from 5

Bozkurt [15] obtained Ag/graphene nanocomposite by the sonochemical method in situ reducing reaction of silver ions and GO with sodium citrate as a green reducing agent. At first, GO was

above suspension under vigorous stirring condition. Finally, sodium citrate was added to the above mixture and sonicated for 1 h. The resultant black solid product (Ag/graphene nanocomposite) was centrifuged and dried in a vacuum. The authors have proposed mechanism for the formation of Ag nanoparticles on GO. Briefly, at first, silver nitrate precursor deposits on the surface of the GO nanosheets. Subsequently, the applied ultrasonic irradiation assists the deposited silver nitrate precursor to homogeneously disperse on the GO surface. The functional groups such as epoxy groups, hydroxyl groups (–OH), carbonyl groups (C = O), and carboxylic acid (–COOH) groups on the surface of GO would also act as the active sites for the metal cations. In general, the oxygen functional groups interact with the metal cations through electrostatic interactions. In the final step, the addition of sodium citrate reduces the GOAg+ to Ag nanoparticles on the GO surface. In comparison with other methods, this ultrasonic irradiation method has advantages such as simplicity and high efficiency. The characterization results confirmed the merit of the ultrasonic irradiation method. TEM results showed the most of Ag nanoparticles deposited on the GO, which are spherical in shape with good attachment over GO surface.


with water. The carbonic acid can act as a catalyst for TEOS hydrolysis. Certainly,

can also promote the deposition of nanoparticles on a solid support. The


solved in TX-100 aqueous solution dispersed with GO, followed by the addition of compressed

). Here the aim of utilizing compressed CO2

solution mixture was stirred at room temperature for 7 hours. Finally, the CO2

to 25 nm) were orderly decorated and closely attached on the graphene nanosheets.

solution and stirred at 100°C, followed by the addition of aqueous NaBH<sup>4</sup>

well dispersed in distilled water, and an aqueous solution of AgNO<sup>3</sup>

A one-pot strategy was designed for forming the Au-SiO2

[16]. To prepare Au-SiO2

carbon dioxide (CO2

the compressed CO2

the product Au-SiO2

reacting CO2

as a reducing agent [14]. In a typical procedure, GO was mixed with CH3

is a strong reducing agent and often used for

http://dx.doi.org/10.5772/intechopen.81109

COOAg

19

solution and stirred

was gradually added to the


is to form carbonic acid by

were dis-

was released, and

attachment with GO. Well known that the NaBH<sup>4</sup>

using NaBH4
