**1. Introduction**

In recent years, there has been tremendous focus towards developing greener synthetic methods for the industrial production of fine and commodity chemicals. Towards the development of economical and sustainable routes for large scale synthesis, the ideal protocols are characterized by four parameters: catalytic activity, selectivity, atom-economy and step-selectivity. Among them, development of catalytic systems with high activity and selectivity plays the

© 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.

most important role. A catalyst provides an alternative pathway with a lower activation energy barrier for an organic reaction without being transformed itself [1]. If there are several products and several reaction barriers, a catalyst can drive the reaction selectivity with a particular product. The efficiency of a catalyst lies on the formation of the desired product with high yield and selectivity at relatively milder reaction conditions. Hence, the search for suitable catalytic systems to achieve green and sustainable production of chemicals is growing continuously, that promotes reduction of toxic and hazardous chemicals, stoichiometric amount of promoter, less consumption of energy, fewer side products and less number of steps involved.

easy recovery and recyclability, the active nanocatalysts are often immobilized on a solid inert matrix, which is in a different phase to the reactants. The solid catalysts can in most cases simply be filtered out and used for the next cycle of reactions, making the process economical and ecological. Due to the complex physicochemical properties at the nanometer level and possibility of multiple surface initiated reactions at the active site, synthesis-structure-catalysis performance relationships are poorly understood in many catalytic systems. A variety of heterogeneous catalytic systems based on metal nanoparticles, semiconductors, oxides/sulfides, mesoporous materials, metal-organic framework etc. and their combinations have been developed for photochemical and electrochemical catalysis, environmental remediation as well as catalysis for important chemical conversions. Other metal-free nanomaterials such as fullerene, graphite, graphene oxide (GO), carbon nanotube (CNT) and carbon nanodots (CNDs) have also been studied either as effective carbocatalyst or as support for metal/oxide

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In heterogeneous catalysis, the modus operandi is through surface mediated reactions. Therefore, availability of large surface area is a prerequisite for high catalytic conversions [6, 7]. Although, unsupported nanoparticles with well-defined surface structure and clean exposed facets are predicted to be highly active for catalysis as shown by several theoretical studies, their applicability in real catalysis is a challenge. Due to high surface energy resulting from the large fraction of atoms present on the surface of NPs, they tend to reduce the surface energy and stabilize themselves through agglomeration and coagulation, resulting in decreased active surface area [7]. This leads to an exponential decrease in the surface area for effective catalysis and the prime objective of using nanoparticles for repeated cycles with high activity is practically lost. For example, in case of Au nanoparticles, size plays a critical role during oxidation reactions, as nanoparticles with the diameter 1–5 nm are highly active, whereas larger nanoparticles becomes inactive. The protection of nanoparticle surface from agglomeration can be achieved by using a surface stabilizing agent or immobilizing the NPs into a solid matrix with high surface areas [8, 9]. Soft organic materials such as polymers, surfactants, dendrimers, and ionic liquids have been used as effective capping agents. These systems often show high catalytic performances [10] e.g. reduction of nitrobenzene, olefin hydrogenation and CO oxidation at low temperatures by Au nanoparticles, C─C coupling or hydrosilylation reactions of olefins by Pd nanoparticles. However, these polymeric supports suffer certain disadvantages such as providing low surface area which limits the interface between the catalyst and the reaction substrate. Moreover, soft nature of the materials and low mechanical stability often leads to separation of the support from the NPs surface after repeated reaction cycles which eventually results in agglomeration of the particles and decrease in catalytic activity. Alternatively, nanoparticles have been immobilized or grafted onto solid supports to improve their stabilization and recycling ability [8]. "Inert" porous solid materials such as zeolites, charcoals, metal-organic frameworks, layered-double hydroxides etc. have been used for immobilization of active metal catalysts. Being inert and hard, these materials are highly stable even under harsh reaction conditions and can be easily recovered from the reaction mixture for further applications. Porous materials also allow control over the nanoparticle growth in the porous matrix, prevents agglomeration and enhances active site exposure [11]. The host support materials stabilizes the nanoparticles, however they have

for various catalytic applications.

In the past, the main objective of catalysis was to enhance the activity and selectivity of a catalyst, the recovery and reusability of a catalyst was rarely a major concern. Traditional homogeneous organocatalysts, transition metal catalysts as well as bio-catalytic systems are highly efficient because the catalytic activity can be defined on a molecular level. These single site catalysts are highly accessible to reactant molecules, readily soluble in reaction medium and often give rise to high catalytic activity and selectivity even under mild conditions [2]. However, often use of expensive metal salts, expensive ligands, expensive and tedious purification steps as well as recovery of the catalyst is the major issue. A variety of homogeneous and heterogeneous catalysts have been developed, however there is still a vast scope for development of suitable catalysts that not only offers high activity and selectivity, but also provide a greener route. Fabricating single step synthetic methods using heterogeneous catalysts is a challenging goal as it aims at decreasing the energy and time consumption by operation of single step synthesis. This consequently eliminates the need of separation of products from the reaction mixture again and again.

#### **1.1. Nanocatalysis**

In the area of heterogeneous catalysis, the primary focus is towards development of materials with high surface area as the reactions involve surface initiated pathway. With exceptional nanodimensional properties (high surface to volume ratio), nanoparticles have produced tremendous interest in wide range of research activities including pharmaceuticals, fine chemicals, renewable energy and biotransformation. The key objective of nanocatalysis research is to produce catalysts with 100% selectivity with extremely high activity, low energy consumption, and long lifetime. This can be achieved only by precisely controlling the size, shape, spatial distribution, surface composition and electronic structure, and thermal and chemical stability of the individual nanocomponents. In addition, surface atoms which are at the edges or in the corners are more active than those in planes, and their number also increases with decreasing particle size. Thus nanoparticles have shown tremendous applicability at the interface between homogeneous and heterogeneous catalysis [3–5]. Homogeneous nanocatalysts are used in the same medium as the reactants. The main concern with homogeneous nanocatalysts is their recovery from the solution for repeated cycles as nanoparticles are extremely difficult to be removed from a solution and the extra steps needed to do so could completely negate the process simplification. If the nanoparticles cannot be recovered, they pose an environmental risk, as well as threatening the profitability of the process. For easy recovery and recyclability, the active nanocatalysts are often immobilized on a solid inert matrix, which is in a different phase to the reactants. The solid catalysts can in most cases simply be filtered out and used for the next cycle of reactions, making the process economical and ecological. Due to the complex physicochemical properties at the nanometer level and possibility of multiple surface initiated reactions at the active site, synthesis-structure-catalysis performance relationships are poorly understood in many catalytic systems. A variety of heterogeneous catalytic systems based on metal nanoparticles, semiconductors, oxides/sulfides, mesoporous materials, metal-organic framework etc. and their combinations have been developed for photochemical and electrochemical catalysis, environmental remediation as well as catalysis for important chemical conversions. Other metal-free nanomaterials such as fullerene, graphite, graphene oxide (GO), carbon nanotube (CNT) and carbon nanodots (CNDs) have also been studied either as effective carbocatalyst or as support for metal/oxide for various catalytic applications.

most important role. A catalyst provides an alternative pathway with a lower activation energy barrier for an organic reaction without being transformed itself [1]. If there are several products and several reaction barriers, a catalyst can drive the reaction selectivity with a particular product. The efficiency of a catalyst lies on the formation of the desired product with high yield and selectivity at relatively milder reaction conditions. Hence, the search for suitable catalytic systems to achieve green and sustainable production of chemicals is growing continuously, that promotes reduction of toxic and hazardous chemicals, stoichiometric amount of promoter, less consumption of energy, fewer side products and less number of

In the past, the main objective of catalysis was to enhance the activity and selectivity of a catalyst, the recovery and reusability of a catalyst was rarely a major concern. Traditional homogeneous organocatalysts, transition metal catalysts as well as bio-catalytic systems are highly efficient because the catalytic activity can be defined on a molecular level. These single site catalysts are highly accessible to reactant molecules, readily soluble in reaction medium and often give rise to high catalytic activity and selectivity even under mild conditions [2]. However, often use of expensive metal salts, expensive ligands, expensive and tedious purification steps as well as recovery of the catalyst is the major issue. A variety of homogeneous and heterogeneous catalysts have been developed, however there is still a vast scope for development of suitable catalysts that not only offers high activity and selectivity, but also provide a greener route. Fabricating single step synthetic methods using heterogeneous catalysts is a challenging goal as it aims at decreasing the energy and time consumption by operation of single step synthesis. This consequently eliminates the need of separation of products

In the area of heterogeneous catalysis, the primary focus is towards development of materials with high surface area as the reactions involve surface initiated pathway. With exceptional nanodimensional properties (high surface to volume ratio), nanoparticles have produced tremendous interest in wide range of research activities including pharmaceuticals, fine chemicals, renewable energy and biotransformation. The key objective of nanocatalysis research is to produce catalysts with 100% selectivity with extremely high activity, low energy consumption, and long lifetime. This can be achieved only by precisely controlling the size, shape, spatial distribution, surface composition and electronic structure, and thermal and chemical stability of the individual nanocomponents. In addition, surface atoms which are at the edges or in the corners are more active than those in planes, and their number also increases with decreasing particle size. Thus nanoparticles have shown tremendous applicability at the interface between homogeneous and heterogeneous catalysis [3–5]. Homogeneous nanocatalysts are used in the same medium as the reactants. The main concern with homogeneous nanocatalysts is their recovery from the solution for repeated cycles as nanoparticles are extremely difficult to be removed from a solution and the extra steps needed to do so could completely negate the process simplification. If the nanoparticles cannot be recovered, they pose an environmental risk, as well as threatening the profitability of the process. For

steps involved.

26 Graphene Oxide - Applications and Opportunities

**1.1. Nanocatalysis**

from the reaction mixture again and again.

In heterogeneous catalysis, the modus operandi is through surface mediated reactions. Therefore, availability of large surface area is a prerequisite for high catalytic conversions [6, 7]. Although, unsupported nanoparticles with well-defined surface structure and clean exposed facets are predicted to be highly active for catalysis as shown by several theoretical studies, their applicability in real catalysis is a challenge. Due to high surface energy resulting from the large fraction of atoms present on the surface of NPs, they tend to reduce the surface energy and stabilize themselves through agglomeration and coagulation, resulting in decreased active surface area [7]. This leads to an exponential decrease in the surface area for effective catalysis and the prime objective of using nanoparticles for repeated cycles with high activity is practically lost. For example, in case of Au nanoparticles, size plays a critical role during oxidation reactions, as nanoparticles with the diameter 1–5 nm are highly active, whereas larger nanoparticles becomes inactive. The protection of nanoparticle surface from agglomeration can be achieved by using a surface stabilizing agent or immobilizing the NPs into a solid matrix with high surface areas [8, 9]. Soft organic materials such as polymers, surfactants, dendrimers, and ionic liquids have been used as effective capping agents. These systems often show high catalytic performances [10] e.g. reduction of nitrobenzene, olefin hydrogenation and CO oxidation at low temperatures by Au nanoparticles, C─C coupling or hydrosilylation reactions of olefins by Pd nanoparticles. However, these polymeric supports suffer certain disadvantages such as providing low surface area which limits the interface between the catalyst and the reaction substrate. Moreover, soft nature of the materials and low mechanical stability often leads to separation of the support from the NPs surface after repeated reaction cycles which eventually results in agglomeration of the particles and decrease in catalytic activity. Alternatively, nanoparticles have been immobilized or grafted onto solid supports to improve their stabilization and recycling ability [8]. "Inert" porous solid materials such as zeolites, charcoals, metal-organic frameworks, layered-double hydroxides etc. have been used for immobilization of active metal catalysts. Being inert and hard, these materials are highly stable even under harsh reaction conditions and can be easily recovered from the reaction mixture for further applications. Porous materials also allow control over the nanoparticle growth in the porous matrix, prevents agglomeration and enhances active site exposure [11]. The host support materials stabilizes the nanoparticles, however they have no role in manipulating the activity of the nanoparticles for catalysis. In the last few years, there has been tremendous focus on the development of "active" supports, which along with stabilizing the nanoparticles also contribute towards overall catalytic activity in synergy with the nanoparticles [12, 13]. For example, the charge state of the Au nanoparticles is known to influence their reactivity, in the case of the negatively charged Au nanoparticles, an extra electron from the gold readily transfers to the anti-bonding 2π ∗ orbital of the adsorbed O2 , which weakens the O─O bond and activates oxygen molecule for further catalytic reaction. On the other hand, the positive charge accumulated on the gold can promote adsorption of some reactants, such as CO and hydrocarbons. An active support can transfer charges to/ from the active catalytic surface, hence influencing the activity of the reaction. For example, Au nanoparticles anchored on rutile TiO2 (110) surface shows high activity for the oxidation of CO to CO2 . Along with providing significant exposed catalytic active sites for the reaction, TiO2 also involves in charge transfer process with the Au NPs making the NPs surface highly negative for dioxygen activation leading to oxidation of CO to CO2 [14]. Recently, research works involving GO as a support for immobilizing active metal nanoparticles have gained attention. GO, not only provides a large surface area with high exposure of active catalysts, but also can influence the catalytic activity [15]. Possible surface to metal electron transfer from GO to nanoparticles activating dioxygen molecule over NPs surface for several oxidation reactions has been reported [15]. Hence, choice of a suitable support for NPs stabilization with possible cooperativity might play an important role in controlling the reaction yield and selectivity of products.

and easy removal etc. Moreover, metal salts can be reduced to active metallic forms in these

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

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

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

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

large impacts in the area of catalysis, mainly because of their unique electronic properties


mesoporous materials, making them highly competent as metal supports.

genated hydrocarbon [16].

nanotubes [21] (**Figure 1**).

**2.1. Graphene oxide as a carbocatalyst**

Graphene and other two-dimensional sp2

activity for several organic transformations.

surface.
