ReviewGraphene supported heterogeneous catalysts: An overview
Introduction
The application of catalytic methods in organic chemistry has become one of the intensive research areas. Almost 90% of all commercially produced chemicals utilize the catalyst at some stage in the production process, including food processing, fine chemicals, bulk chemicals, energy processing and environment, which make the manufacture of catalysts alone account for over USD 10 billion in sales revenue [1], [2], [3], [4]. In 2010, one-third of the materials gross national product in the US contained a catalytic process somewhere in the production chain [1], [2], [5]. Nevertheless, the currency generated due to catalyst sales is enormously outweighed by the overall value of the generated products including, pesticides [6], polymers [7], [8], antibiotics [9], [10], cosmetics [11], cleaning products [12] and others. For example, procedures with the longest history are hydrogenations, which are mainly catalyzed by supported metals. Over the course of decades, various metal catalysts have been developed with the purpose of achieving high activity and selectivity. Since the metal catalyst has met a number of special requirements, in addition to ceramics (SiO2, Al2O3, zeolite, etc.), the development of hybrid, oxide, bifunctional catalyst materials have also gained increased importance [18], [19]. Catalytic reactions are sensitive to the catalyst's structure, surface atomic arrangement and coordination, which can be well controlled by tuning the specific composition, morphology and size of catalyst [10], [11], [12], [13], [14]. In supported catalysts, catalytically active components are bound to a(n) (inert) support with large surface area such as alumina, silica, zeolite and other materials. For stabilization of high dispersion of active components such as noble metal nanoparticles against sintering, reduction of costs and utilization of important mechanical and morphological properties of the support.
Industrial catalysis are generally divided into two types, homogeneous and heterogeneous (Fig. 1). Heterogeneous catalysis is where the catalyst and the reactants are in the different physical phases, while homogeneous is where both are in the same phase [18]. In general, homogeneous catalysts exhibit excellent catalytic activities with high selectivities in particular. But, it is difficult for the homogeneous catalyst to be separated from reaction media as compared to heterogeneous catalyst [19], [20]. Thus, most of the industries tend to shift towards the heterogeneous catalyst.
As heterogeneous catalysis have evolved, it has become more apparent that it has a large part to play in green chemistry and thus removing or substantially reducing pollution and undesirable by-products from both the chemical and refining processes [21], [22], [23]. In the other words, the by-products of heterogeneous catalyst reactions would not end up with harmful emissions and nasty waste materials that are dangerous and detrimental to the ecosystems and the environment [22]. However, conventional heterogeneous catalysts are suffering from relatively low activities due to the limitation of exposed active sites [24], [25]. One of the promising routes to overcome is to support these catalytically active centers on porous inorganic solids such as silica, zeolite, mesoporous materials and others [18], [19]. However, with these supports, inorganic materials have high sensitivity towards acid and base conditions. Owing to good stabilities in both alkaline and acid conditions, carbons as supports have attracted much attention [26], [27], [28], [29].
In industrial chemical engineering, it is often desired that a homogeneous catalyst be heterogenized or supported on a porous material [19]. The heterogeneous catalysts are required to be in micro or as mesoporous catalyst, which makes it more easily separated from the fluid (liquid or gaseous) products and reactants, and enable it to be packed inside a reactor vessel or tube, with fluids flowing via the packed and fixed bed. In addition, the agglomeration of heterogeneous catalyst, especially with respect to its nano-sized characteristic, is one of the main reasons to use a support material [22], [30]. However, if the nano-sized heterogeneous catalyst is not strongly anchored on the surface of the support, the agglomeration can still take place [31], However, it is necessary to strongly attach the nano-sized heterogeneous catalyst to the surface, for example, of defect sites on the surface or functional groups bonding to the surface and catalyst itself [24], [32], [33]. With that in mind, researchers have introduced many kinds of support catalyst materials included, zeolite, carbon, silica and others [18].
Another advantage of support catalyst is concerned with the surface area and porosity in relation to the total catalyst loading, in order to achieve high dispersion of the active site phase in the catalyst [34], [35], [36]. It seems clear that a large surface area formed by accessible pores is important for obtaining highly dispersed and active catalyst [35]. Furthermore, surface chemical properties are another carbon characteristic that has to be taken into account to explain the catalytic behavior of a carbon supported catalyst system [31]. The carbon surface contains a given number of heteroatoms (O, N, H) in the form of functional groups, similar to the way that heteroatoms appear in organic compounds, which as a consequence, can confer the carbon surface an acid–base and hydrophilic character [28], [29], [30]. Some effort has been focused on the role of surface oxygen groups in the dispersion and resistance to the sintering of carbon supported metal catalysts [37], [38]. For example, high surface area of carbon black has been heated in hydrogen at the 1000 °C for removing most of Pt oxygen surface functionality before being oxidized with hydrogen peroxide solution, and it was found that more acidic groups have been consequently developed, which could decrease the hydrophobic character of the carbon support and make the Pt/C surface became more accessible to the aqueous solution [39], [40]. Thus, an increment in Pt dispersion with the minimized Pt sintering process is recorded. Similar outcomes have been observed in Ru supported carbon catalyst for ammonium synthesis, even though some of the oxygen in the catalyst system tended to be unstable under heat treatment conditions [41], [42].
Carbon nanomaterials have been considered for use as support materials due to its tensile strength, large surface area, promising thermal stability, ease of recovery and recyclability which in turn is important in adhering to sustainable chemistry protocol [43], [44], [45], [46]. One possible route to improving the support properties of carbon materials is through its surface functionalization [45]. Carbon materials have been used as a catalyst support in fuel cells, sensors and solar cell applications [44], [47], [48], [49].
The combination of nanocarbon materials and heterogeneous catalysis is the key to significant selectivity and cost reduction of catalysts [50], [51]. Catalysts designed in this way generally involve length scales ranging from the atomic to the catalyst particle or pallet scale, but can be a concern on the reactor that the catalyst is to be applied [52], [53], [54]. However, the activity and selectivity of heterogeneous catalysts supported by carbon materials depends on the atomic structure of its active site [53]. The active sites depend on the surface of mesopores that constitute a vast network inside a solid particle, with size varying from nanometers to centimeters [54], [55], [56]. In general, the structure of the active site governs the quantity of any species to be bound and converted on the catalyst surface [55]. The accessibility of the active site in carbon supported materials can be enlarged by distributing the sites in a microscopically non-uniform way [57], [58]. Thus, there is some effort to enhance the accessibility of the catalyst system, where the sites are concentrated within an area close to the external surface of the catalyst and form a new structure of catalyst, called an eggshell catalyst or carbon nanocage (Fig. 2) [59], [60].
Additionally, due to the porous nature of carbon materials, the carbon pores through which both products and reactants move are often very narrow, while the local surface curvature is very large, and could influence the overall reaction rate [45]. Porous carbon materials consist of a very flexible set of supports for the preparation of heterogeneous catalyst [38], [61]. Thus, its physical and chemical surface properties can easily be tailored to develop a large surface area to disperse the active phase [62], [63]. It is believed that the proper pore size distribution brought about by carbon as the support material could facilitate the diffusion of reactants and products to and from the surface, which in consequence acquire an acid–base characteristic required to result in superior catalyst performance [64]. Meanwhile, the carbon particle size might also be determined by mechanical and environmental factors. Too small particles can form a potentially harmful dust. Thus, it is natural to keep the carbon particle size constant and focus on optimization for maximum yield, selectivity towards desired products, stability and/or a combination of these [64], [65], [66].
The above-mentioned characteristics are very significant for carbon as a support material, whereby the calculation of the activation energy required to fit molecules in carbon pore windows could be used to find the right type of heterogeneous catalysis for certain particular industrial applications and reactions [67], [68], [69], [70]. For example, the role of carbon supported catalyst in reactions such as hydrodesulfurization of thiophene and Fischer–Tropsch synthesis with its advantages in industrial-scale application have been widely reported and discussed [71], [72]. More recently, a large variety of carbon as support materials such as granular, powdered, activated, cloth, nanotube, nanofibers and black carbon have been comprehensively studied in different kinds of reactions and applications [69], [73], [74], [75]. It was concluded that there were some advantages over other traditional catalyst supports. This included its resistance towards both acidic and basic media, a stable structure at high temperature, tailored pore structure that resulted in flexible pore size distribution needed for the given applications, preparation of porous carbon with a variety of macroscopic shapes (e.g.: monoliths, granules, powder, fibers, etc.), the possibility of modification of chemical properties of the surface and control polarity and hydrophobicity, and recovery of active phase of the spent catalyst by burning away the carbon support. Most importantly, carbon supports are usually cheaper than other conventional catalyst supports [76], [77], [78].
Furthermore, the presence of carbon support materials could induce some active phase-support interactions [79], [80], [81]. With that in mind, some studies have prepared and compared a Ru/SiO2 with Ru/C catalyst towards hydrogenation of citronellal (3,7-dimethyl-6-octen-1-al) and recorded that the main products on Ru/SiO2 was the unsaturated cyclic alcohols (isopulegols), which are produced through the isomerization of citronellal on the SiO2 surface [82], [83]. However, due to the low activity of the carbon surface towards isomerization, open-chain hydrogenated compounds such as citronellal, 3,5-7 dimethyloctanal and 3,7-dimethyloctanol were obtained from the Ru/C system. The low interaction between the carbon surface and two metals or metal precursors assisted the mutual interaction between catalyst and support materials, which in consequence, resulted in the relatively low inertness of the carbon surface [70], [83], [84]. This is especially interesting as the main focus was the formation of a bi-metallic catalyst system in which the catalytic characteristic of the system could be highlighted by aspects of total conversion and selectivity towards desired products, the oxidation state of the metal/bi-metallic catalyst, and the possibility of formation of the alloy phase [85], [86], [87].
Several methods have been employed for the preparation of carbon supported catalyst including, impregnation [88], [89], chemical vapor deposition [90], [91], precipitation or co-precipitation method [92], [93] and the liquid phase reduction [94], [95]. Conventional producers for the preparation of carbon supported catalyst by either impregnation or precipitation methods normally yield a metal catalyst cluster with a heterogeneous particle size distribution [88], [92]. It was noted that, electrostatic interactions between the carbon surface and the active phase precursors have also been taken into account in the preparation of carbon supported catalysts [96]. The presence of oxygen functionalizes the carbon surface obtained upon the activation process (in the case of activated carbons) and/or by subsequent oxidation treatments rendering it amphoteric [70]. This consequently resulted in a catalyst system with less charge (positive or negative) and was dependent on the pH of the surrounding solution [73], [97]. Synthesis parameters included the pH of the solution, the polarity of solvent, anionic or cationic nature of metal precursor and the isoelectric point of the carbon support, which indicate the extent of precursor-support interactions, total uptake and dispersion of the active phase in the final catalyst [64], [65], [66], [67], [68].
Therefore, many carbon-based materials with tunable properties such as carbon black, carbon gels, carbon nanohorns, nanocoilds, activated carbon and carbon nanotubes have been explored for possible utilization as catalyst supports [98], [99]. Activated carbon, as one of the categorized carbon materials, exhibits surface areas much higher than those of other conventional catalyst supports such as silica, zeolite, alumina and others [80], [100]. Even though most of this surface may be contained within a narrow microcosm, it may not be available to reactants [101], [102], [103]. With that in mind, much effort has been focused on the effect of porosity and surface area on the dispersion of metals as a heterogeneous catalyst and its catalytic activity (Table 1). With reference to metal sulfide with activated carbon as support, the narrow and slit shape of the activated carbon assisted in lowering the vapor pressure of sulfur to such an extent that the support system develops a driving force for sulfur transfer from the active compound to the microcosm which in the end resulted on active vacancies in metal sulfide [106], [107]. The active vacancies are not observed as microporous silica that is used as a support [108], [109], [110]. Compared with carbon nanotubes and fullerence, activated carbon has some unique characteristics, including porous structure, high conductivity and greater surface area, which can attract much attention to their application as catalyst support. Although carbon nanotube has large surface area, undesired bundling and incomplete functionalization commonly afford limited accessible area and defects [80], [102], [103]. Furthermore, metal catalyst supported carbon nanotubes are electrochemically unstable to CO toxicity [38], [41], [43], [45], [83]. Another type of carbon-based support catalyst is carbon nanofibers. Carbon nanofibers have cylindrical nanostructures and are suitable to be used as catalyst supports [81]. The quasi one dimensional tubular carbon nanofibers has been widely studied as stabilizers and supporters for metal nano particles due to its unique structural, electrical and mechanical properties [111], [112], [113].
The main application of carbon support catalyst is in the hydroprocessing of petroleum feedstocks. For example, sulfur containing naphthalene could be converted into tetrahydronaphthalene with a charcoal-supported catalyst containing NiO, MoS, and CaCO3 [114], [115], [116], [117]. Carbon support catalyst systems are also being potentially used for hydrogenation in fine chemical synthesis, as decomposition and environmental catalyst, in ammonia synthesis and electrocatalysis [115].
Although carbon-supported heterogeneous catalysts are considered to be the promising option for a great number of reactions, there are a few large-volume processes that are currently using this system. However, less than 1% of the activated carbon production worldwide is used as catalyst supports [105]. This may be due to the lack of reproducibility that sometimes arises with carbon-supported catalysts as a result of relatively poor knowledge on the properties of carbon supported materials that influence total reaction performance. Another great limitation of carbon as support materials is that it cannot be used in reactions that are carried out under certain extreme conditions (e.g. hydrogenation at temperatures greater than 500 °C; or oxidation above 300 °C) [118].
Furthermore, a large surface area in the carbon supported catalyst system may be detrimental if the narrow micropore structures are not accessible to the reactant molecules, especially in cases involving large molecules [119], [120]. In such cases, reactants or even products may be hindered by the narrow porosity of support material [120]. Some reports have also indicated that the surface area and porosity of carbon do not have any impact on either the active phase dispersion or the catalytic activity of the overall reaction [119], [120], [121]. The type of precursor is documented as the main important aspect that gives the greatest influence on the catalytic activity [118], [119], [120], [121]. In case of Ru supported by a series of carbon materials, included graphitic carbon, carbon nanotubes, carbon black and mesoporous carbon, it was found that the graphitic structure of carbon is critical to the activity of the Ru catalyst, while the surface area and porosity become the secondary factor [17], [83], [85]. Additionally, as carbon support material act as the anchoring center of the catalyst active phase or the precursor, its decomposition could lead to sintering of the metal species and contribute to the reduction of dispersion [85]. This factor also contributed to the weak interaction between carbon and catalyst metal, which as a consequence, decreases the active catalytic area and ultimately results in the loss of performance over a long period of operation [118]. Thus, further re-movement of carbon support materials is required before the reaction occurred, not only to overcome the above-mentioned obstacles, but also to increase the selectivity of the reaction [83].
Section snippets
Graphene in view: characteristics and applications
Graphene is a sp2 hybridized carbon-based material with a hexagonal (benzene ring) monolayer network. In comparison with other carbon allotropes, graphene offers the greatest intrinsic carrier mobility at room temperature, with perfect atomic lattice, promising mechanical strength, chemical and thermal stability, and to heat ballistically similar massless particles [123], [124], [125]. Compared with carbon fibers, the three-dimensional graphene is applied as an interconnected seamless porous
Design and properties of graphene as catalyst support
The main characteristic of a promising support of heterogeneous catalyst is a high surface area with a suitable porosity, but both of these characteristics are not present in graphene [127]. However, it was noted that the porous structure provides a more favorable path for the penetration and transportation of molecules [2] (Fig. 3).
To increase the surface area of graphene, many novel methods such as erosion [137], self-assembly [138], thermolytic cracking [139], and KOH activation [140] have
Cross-coupling reactions
Cross-coupling reactions have typically been performed under homogeneous conditions using a ligand to increase the catalytic activity and selectivity towards desired products in specific reactions [219]. However, the issue related to homogeneous catalysis remains a challenge to pharmaceutical applications of these synthetic tools due to the lack of recyclability and potential contamination from residual metals in the reaction products [474], [475], [476]. To overcome this issue, an effort has
Conclusion
The graphene based carbon nanomaterial as a promising catalyst and catalyst support, has attracted tremendous attention over recent years due to its exceptional properties, including excellent mechanical, electrical, thermal, and optical properties and a very high specific surface area. It is a promising candidate for catalyst support due to its large specific surface area and the large delocalized π-stacking interactions with organic molecules. Furthermore, the influence of chemical and
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