Carbocatalysis by Graphene-Based Materials - Chemical Reviews

Review pubs.acs.org/CR

Carbocatalysis by Graphene-Based Materials Sergio Navalon,† Amarajothi Dhakshinamoorthy,‡ Mercedes Alvaro,† and Hermenegildo Garcia*,†,§ †

Instituto Universitario de Tecnología Química CSIC-UPV and Departamento de Química, Universidad Politécnica de Valencia, Avenida de los Naranjos s/n, 46022 Valencia, Spain ‡ Centre for Green Chemistry Processes, School of Chemistry, Madurai Kamaraj University, Madurai 625 021, Tamil Nadu, India § Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia Biographies Acknowledgments References

AD AE AE

1. INTRODUCTION AND SCOPE OF THE REVIEW 1.1. Interest of Carbocatalysis

Many homogeneous and heterogeneous catalysts are based on transition metals, both noble1−3 and base metals.3−5 For the sake of sustainability, it would be convenient to develop alternative catalysts based on renewable resources. Carbon materials derived from biomass are considered paradigmatic examples of sustainability.6,7 Other factors besides sustainability that may also favor carbon-based versus inorganic materials as catalysts are economic considerations, since the price of some precious metals is considerably higher than the feedstock carbon materials. For these reasons, there is much interest in developing metal-free catalysts; “carbocatalysis” is one of the most important examples of this tendency to reduce the dependency on metals.8−10 Carbocatalysts can be defined as those catalysts whose elemental composition contains predominantly carbon that is performing as active component. In this sense, carbocatalysts are different from organocatalysts, which are constituted by organic molecules rather than carbonaceous materials. Carbons with large surface area such as active carbons and carbon black powders have since ever been considered as supports to deposit transition metals that were the relevant catalytic centers.7,11 In this type of materials the role of carbon is to disperse the metal on a very large surface area and maybe in the best case also adsorb reactants near the metal particles. In the case of carbocatalysts the key point is to incorporate the active sites into the carbon by suitable modifications, making metals unnecessary. In contrast to organocatalysis, in which molecules are the active centers, carbocatalysis is based on materials lacking molecular entities. For this reason, carbocatalysis is in principle better suited for heterogeneous rather than homogeneous catalysts. However, by reducing the particle size of the carbon material to a few nanometers (carbon dots) or using single or few-layers bidimensional materials of subnanometric thickness, carbocatalysis provides a bridge toward homogeneous catalysis where reactants and catalysts are in a single phase. In some cases, even though the carbocatalyst is in a single phase with reactants it can be recovered after the reaction by means of membranes or centrifugation. In these cases the process is based on the use of indefinitely persistent colloidal dispersions (“pseudo-homoge-

CONTENTS 1. Introduction and Scope of the Review 1.1. Interest of Carbocatalysis 2. Definition of Graphene-Based Materials and General Remarks 3. Preparation Methods of Graphene-Based Materials with Relevance as Carbocatalysts 4. Physical Properties of Graphene-Based Materials with Relevance in Catalysis 5. Characterization Techniques for Graphene-Based Materials as Catalysts 6. Chemical Methods of Functionalization 7. Overview of the Catalytic Reactions Performed Using Graphene-Based Catalysts 8. Graphene as Carbocatalyst 8.1. Oxidation Reactions 8.1.1. Oxidation Reactions Promoted by Molecular Oxygen 8.1.2. Oxidation Reactions Promoted by Nonoxygen Oxidants 8.2. Reduction Reactions 8.3. Acid or Base Reactions 8.3.1. Acid Reactions 8.3.2. Base Reactions 8.4. Thermal Decomposition Reactions 9. Graphene Oxide as Carbocatalyst 9.1. Oxidation Reactions 9.1.1. Oxidation Reactions Promoted by Molecular Oxygen 9.1.2. Oxidation Reactions Promoted by Nonoxygen Oxidants 9.2. Acid or Base Reactions 9.3. Thermal Decomposition Reactions 10. Concluding Remarks and Future Prospects Author Information Corresponding Author Notes © XXXX American Chemical Society

A A B C F H J K K M M P S U U W X X X X AA AA AC AC AD AD AD

Received: December 28, 2013

A

dx.doi.org/10.1021/cr4007347 | Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Scheme 1. Possible Catalytic Active Sites Present in G-mat, Including Impurities, Holes and Defects, Armchair and Zigzag Edges, Dopant Elements, and Acidic and Basic Sites

characterization techniques widely applied in heterogeneous catalysis can be adapted to the study of G-mat. The main body of this review deals with reports on the use of G as catalysts. This section has been grouped according to the nature of the active sites and the type of G material responsible for catalysis. It is worth commenting that although the term G implies a singlesheet morphology, there are some articles in which even though this term has been used, there is reasonable doubt about the single or few-layer sheet morphology of the catalysts used. In those cases in which no convincing evidence of the single or fewlayer morphology of the material used as catalyst is provided, we opted to omit them to avoid introducing misconceptions in the field under review. Scheme 1 illustrates the types of sites that can be present on Gmat, according to which the present review has been organized. It should be commented that compared to other areas, the use of G in catalysis has started very recently after the pioneering work of Bielaswki50 using graphene oxide (GO) as catalyst for oxidation of benzyl alcohol; although now the number of papers focused on G in catalysis is large, this area is still nascent and the number of studies dealing with G-based catalysis is still relatively low compared to other types of carbons. Considering the added value that these materials could have as catalysts, their affordability, and the sustainability of their use compared to metal-based catalysts, it can be easily anticipated that this area will grow considerably in years to come. This forecast and our view on future developments in the field will be summarized in the concluding section.

neous”) in which the catalyst is apparently in the same phase as reactants and products during the reaction but can be separated from the reaction mixture after the reaction. In the present review, we will focus on the use of graphene (G) and G-based materials (G-mat) as carbocatalysts. While there are many different types of carbon materials including amorphous and diamond NPs, herein we will deal with applications of 2D Gmat as carbocatalysts. The reader is directed to a recent review on advanced catalysts based on carbon nanoforms for in-depth coverage of the catalytic activity of carbon nanotubes (CNTs) and other various nanostructured carbons.12 The main purpose of the present review is to show the advantages and possibilities that G-mat offer in catalysis due to their 2D morphology and to the possibility of introducing heteroatoms and functional groups on the G sheet in such a way that they can act as catalytic centers. In addition, the large surface area of G (up to 2600 m2 g−1)13 is also of interest for its use as support to deposit inorganic NPs for development of catalysts, but this topic is a field in its own and will not be included in the present review, which is focused on carbocatalysis.14 Our review covers the chemical literature on the use of G as catalyst or support up to end of 2013. Considering the vast number of papers reporting preparation methods of various Gmat,12,13,15−33 characterization techniques,17,18,23,34 as well as their properties as electronics18,23,34−40 or as conductive materials,18,26,30,35,38−41 these aspects will also not be covered and the reader should refer to the existing literature in these areas. Specifically, the area of electro- and photocatalysis using Gmat exclusively or in combination with metals or metal oxides will not be reviewed in the present manuscript. The reader should be aware that electrocatalysis20,34,42−47 and photocatalysis48,49 have been extensively studied using G-mat as catalysts; however, there is growing interest in the properties of G-mat as semiconductors. Only those characterization techniques with relevance in catalysis to determine the presence and density of catalytic sites will be discussed here. Our review starts with general considerations of G-mat that are important in catalysis having emphasis in the morphology and structure of the sheets, adsorption capacity, and electron density, followed by a general classification of the nature and position of active sites on the G sheets. Then, we will address how general

2. DEFINITION OF GRAPHENE-BASED MATERIALS AND GENERAL REMARKS G is a bidimensional layer of one atom thick sp2 carbons in a hexagonal arrangement.51−55 The perfect stacking of planar G sheets perpendicular to the layer forms graphite. The term G-mat includes not only G but also G-mat doped with heteroatoms as well as some other one atom thick 2D layered materials containing a substantial population of other elements besides carbon. In doped G-mat, a certain percentage of carbon atoms, typically below 10 wt %, are replaced by other elements such as nitrogen,56−58 boron,59 phosphorus,60 or sulfur.61 G materials also include those G-mat that have been submitted to a chemical B


Chemical Reviews

Review

Scheme 2. G and Related Materials Covered in This Review

Scheme 3. Overview of Most-Used Preparation Procedures for G-mats

reaction to effect covalent bonding with pendant units.62−72 This strategy of covalent functionalization can be used to introduce active sites or ligands that can bind to metals and metal ions. Another G-mat is G oxide (GO), which is obtained by deep oxidation of graphite and subsequent exfoliation of the resulting graphite oxide. GO may contain a high proportion of oxygen, in some cases higher than that of carbon.15 Due to the ease of preparation in large quantities from available graphite, GO has been one of the preferred G-mat as catalysts. The stability of G makes partial reduction of GO going toward G relatively easy, decreasing the O content. However, GO reduction is generally not complete, leaving in the material various extents of residual oxygenated functional groups. For this reason, it is very common to distinguish G obtained from GO that is generally referred as “reduced GO” (rGO) from those obtained by chemical vapor deposition (CVD) that, prepared by pyrolysis of hydrocarbons, should not contain any oxygen. It should be mentioned, however, that in contrast to rGO, CVD G-mats have been rarely used as catalysts due to their difficult preparation in sufficient quantities, although new developments in this technique are increasing continuously the amount of G that can be prepared, and it should be now possible to have large enough quantities for performing catalytic tests.73 Nevertheless, CVD techniques require specialized equipment and are still not routine in most of catalytic laboratories. In addition, it could be that the activity of highquality G lacking defects and oxygen functionalities from CVD synthesis could not be so appropriate for carbocatalysis in view of the current understanding of the possible active sites present on G-mat. Scheme 2 summarizes some possible G-based materials

that will be covered in the present review.74−76 We excluded in this definition other materials with the same morphology and structure as G but not containing any carbon such as silicene or hexagonal boron nitrides.

3. PREPARATION METHODS OF GRAPHENE-BASED MATERIALS WITH RELEVANCE AS CARBOCATALYSTS The subject of the present review is the application of G-mat in catalysis. Toward this goal it is convenient to briefly summarize some of the preferred preparation procedures of these materials that have been subsequently employed as catalysts. Scheme 3 summarizes these methods. The reader is referred to extensive reviews and articles in this area to obtain in-depth coverage of the various preparation procedures.13,17,18,20,21,23,77−82 It is generally assumed that the highest quality G, i.e., the one without defects in the hexagonal structure and with very low, or even absent, oxygen content, is prepared only by CVD of precursors on hot metallic surfaces.17,30,79,81−84 Copper and nickel are among the preferred metals in CVD preparation.30,80,82,85 The precursors can be hydrocarbons such as methane that at high temperature decompose, forming a carbon layer on the metal surface. It is proposed that the mechanism of G formation relies on the templating effect of metal atoms whose spherical shape and size fits in a six carbon ring and during growth of the G sheet act as dehydrogenating centers giving rise to hydrogen and carbon. Scheme 4 illustrates the proposed mechanism of G formation due to the template action of metals. Incidentally, carbon residues generally amorphous and not C


Chemical Reviews

Review

The most general preparation procedure for G being used as catalyst is by far the one that starts from graphite and leads to GO that should contain a considerable degree of defects.15 Preparation of G-mat through this route requires a series of consecutive steps involving deep oxidation of graphite to graphite oxide, subsequent exfoliation by ultrasound irradiation, and partial reduction by chemical or thermal methods (Scheme 5).15,22,86 The quality of resulting rGO is much lower than G obtained by CVD because the density of holes and defects and the percentage of residual oxygen in the material can be high. In addition, graphite oxide and GO can also be used directly as carbocatalyst since their structure contains in principle the same type of sites as imperfect G.15 Oxidation of graphite is very frequently performed following the recipe reported by Hummers and Offeman that consists of a two-step oxidation that is started at low temperature with permanganate in a mixture of concentrated nitric and sulfuric acids and then is followed by addition of H2O2 at ambient temperature.86 Besides direct oxidation of graphite by permanganate and nitric acid, it is very possible that H2O2 in combination with Mn2+ produces, in addition to reduction of the excess of permanganate, a Fenton chemistry that generates hydroxyl radicals (•OH) attacking graphite G layers.11,87,88 There are some variations of Hummers oxidation using different oxidants and acids.89,90 Phosphoric acid can be a good alternative to sulfuric acid, since it increases the viscosity of the reaction mixture (Tour method).90 There are, however, other chemical oxidation methods53,89 to prepare graphite oxide that have been described but considerably less used to obtain materials for catalysis, such as those developed by Brodie,91 Staudenmaier,92 and Hofmann.93 Oxidation of the G layers in graphite decreases significantly the strong interaction between them, expanding the interlayer distance and introducing negative Coulombic charges that can later facilitate exfoliation of the layers. One point that is highly relevant in catalysis is that as consequence of being used in high concentrations manganese, sulfuric acid, or other oxidizing agents and acids can later impurify the carbocatalyst, introducing some catalytic activity.9 This possibility of the presence of impurities on G, even in low percentages, is particularly high considering the large surface area of the material and the

Scheme 4. Pictorial Illustration of G Formation by CVD and Templating Effect of Metal Atomsa

a

Copper, carbon, and hydrogen atoms are in red, blue, and white, respectively.

forming a monolayer, termed as “coke”, have been very wellknown poisons since a long time ago, and in catalysis they were considered as undesirable residues deactivating the solid catalysts containing transition or noble metals. In the present case, this ability of copper and nickel as dehydrogenation catalysts originating carbon residues is used advantageously, the key points being carbon atom arrangement in a crystalline hexagonal structure and monolayer coverage of the metal surface.84 However, few-layer G can also evolve by prolonging the CVD process.9,15,52 Since CVD is carried out in the absence of oxygen, the most common defects of G-based catalysts due to the presence of oxygenated functional groups are avoided. CVD, however, is not the best-suited process for the large scale production of affordable carbocatalyst, and in addition, use of a film is not common in catalysis, which typically prefers either colloidal suspensions or pelletized particles in suspension or immobilized beds. In addition, defects on the G sheet are increasingly recognized as active sites in catalysis (Scheme 1), and accordingly, it may very well be that ideal G sheets are less catalytically active than other types of G. It should be commented, however, that considering the potential role of impurities and defects in catalysis it would be interesting to check systematically the catalytic activity of this high-quality G as a reference material devoid of catalytic activity.

Scheme 5. Protocol for Chemically Assisted Exfoliation of Graphite Based on the Hummers Oxidation and Subsequent Sonication

D


Chemical Reviews

Review

presence of reactive functional groups on GO (Scheme 1 for ideal location of metal ions). For this reason, it is always advisable to provide complete analytical data of any G-mat used as catalyst or even to study the influence on the catalytic activity of impurities added on purpose in known amounts to the reaction mixture. We will discuss this point later in sections 8 and 9 when discussing specifically the role of impurities in carbocatalysis. Due to the high percentage of oxygen and the reactivity of oxygenated functional groups, GO can surely undergo degradation and aging under catalytic conditions. Since the oxygen content becomes reduced upon GO manipulation and also because of the remarkable changes in infrared (IR) spectroscopy showing the disappearance of oxygenated functional groups, GO tends to evolve spontaneously, rendering materials with larger graphenic domains that are termed rGO but are different from higher quality G not containing oxygen. Moreover, reduction of GO to rGO can be performed on purpose.13,15,18,22 Chemical agents such as hydrogen, metal ion borohydride, and hydrazine are among the preferred reducing agents to prepare rGO, but even heating above 100 °C in aqueous solution can be sufficient to obtain rGO (Scheme 5).25,74−76 The ease of GO reduction raises the issue of how the initial Gmat can evolve and become modified during its use as catalyst. In general, a good practice in this area is to characterize the G-mat after being employed as catalyst searching for possible structural alterations. Recovery and reuse of the carbocatalyst is also a good practice to determine the stability of the material under the reaction conditions. As mentioned earlier, Hummers oxidation of graphite contributes remarkably to make possible exfoliation of the resulting graphite oxide. It has been, however, reported that direct exfoliation of graphite can be performed on a detectable level in certain solvents without the need of an intermediate oxidation step.18,21,32 For this direct exfoliation process, solvents such as ionic liquids,94,95 N-methylpyrrolidone (NMP), dimethylformamide (DMF), or other amides are among the best-suited media (Scheme 6). The main problem of direct

A variation, however, that appears to be very useful consists in direct exfoliation of graphitic carbonaceous residues obtained by pyrolysis of turbostratic carbon precursors.97,98 Apparently, exfoliation of graphite is very difficult due to the high crystallinity of the material. When graphitic carbons are poorly crystallized, the exfoliation process becomes easier (Scheme 7). Typical Scheme 7. Preparation of G Suspensions by Biomass Pyrolysis and Subsequent Exfoliation of Low-Crystallinity Turbostratic Carbons

precursors for these types of carbonaceous residues are natural biopolymers, particularly polysaccharides such as alginate or chitosan.97−99 Carbohydrates are well known as carbon precursors upon pyrolysis under moderate conditions resulting in poorly crystalline graphite. Some of these natural biopolymers, considered biomass wastes, are inexpensive feedstocks. The crystallinity of these graphitic residues can be easily assessed by X-ray diffraction (XRD) of pyrolyzed powders in which the sharp (002) peak of graphite appearing at around 25.0° is observed as a much broader band of low intensity as a consequence of the imperfect G layer.98 Transmission electron microscopy (TEM) of these graphitic residues shows the presence of thin G platelets constituted by few-layers G that are almost independent or weakly stacked with the rest of the material.100 With respect to preparation of doped G, the most widely used procedure consists in introducing the dopant element during synthesis of the carbon material (Scheme 8).101 Arc discharge in which graphitic electrodes are separated by a few millimeters while a high-intensity current is flowing between the anode and the cathode causes ablation and erosion of the graphite rods rendering some carbon debris containing G-mat.102 When this arc discharge consuming the graphite electrodes is carried out in an atmosphere that contains P, N, B, or another element, incorporation of this element into G can occur due to the high energy of the process.101,103 Other procedures of G doping are based on the high reactivity of GO that can undergo substitution of oxygen by other elements104 and may lead to incorporation of these elements by addition of suitable reagents.101,104−106 When later GO is reduced, a percentage of this heteroatom can remain on the carbon layer. One alternative doping procedure that we reported recently is based on the use of a natural biopolymer, modified107 or not,97,107 that may already contain the doping element. During pyrolysis, the heteroatom becomes easily incorporated into the G layer that is formed in the process. It has been found that the percentage of doping decreases as the pyrolysis temperature increases in the range from 600 to 1200 °C. Apparently, the increase in the pyrolysis temperature improves the quality of the resulting G sheet by healing defects. In this regard, it can be considered that doping is like a defect on the G layer since the heteroatoms introduce some stress in the layers and a percentage of the dopant atoms is expelled out of the G sheet during thermal heating.97 Besides natural polymers such as chitosan that act as a simultaneous source of carbon and nitrogen or carragenate that is a source of carbon and sulfur, another possibility is to modify the polymer adding another element not present in the composition

Scheme 6. Direct Graphite Exfoliation by Sonication and Other Physical Means in Highly Viscous Solvents

exfoliation is, however, the low G concentration achieved, typically below 0.01 mg mL−1 and the difficulty to concentrate or remove these high boiling point solvents. Also, these very polar and viscous solvents are often not the best option for some catalytic reactions, making this procedure of G preparation of limited use for catalytic applications. Another alternative to produce larger amounts of high-quality single or few-layers G includes thermal expansion of graphite at high temperatures (1000 °C, 60 s) under H2(5%)/Ar(95%) atmosphere to weaken or break the van der Waals forces between graphitic layers that should make easier subsequent liquid exfoliation coupled with sonication and final centrifugation.96 E


Chemical Reviews

Review

Scheme 8. Procedure for Synthesis of G-mat Doped with Heteroatoms

approach is, however, bond connectivity forming the extended 2D sheet and overall reaction efficiency. In addition G-mat could be heavily contaminated with reagents and catalyst used in these organic syntheses, particularly when homogeneous catalysts are employed. In order to form the G layer, the cyclization− coupling−aromatization reactions must proceed with very high selectivity that will be difficult to achieve, particularly considering that as the molecular weight grows the material should become increasingly insoluble, precipitating from the solution. Also, multigram-scale G production by organic synthesis seems to be unrealistic at present, and this bottom-up approach appears appealing only to check concepts and principles rather than for massive production. Another recent alternative has been a catalyst-free gas-phase hydrocarbon detonation.110 G-mat can form strong supramolecular conjugates with molecules through Coulombic attraction, π−π interactions, and van der Waals forces. For instance, it has been estimated that the association constant of methylene blue with GO is higher than 106 mol−1 L−1 (Scheme 12).111 Also, it is well known for CNTs that pyrene and other condensed polycyclic aromatics form strong association complexes on the curved G walls of CNTs.112,113 It would be of interest to exploit also the potential of G conjugates in catalysis, modifying the electron density of the G layer by association with small organic molecules.

of the biopolymer. For instance, it is possible to easily modify carbohydrates with phosphates108 and borates,107 and the resulting modified polymer can serve to introduce phosphorus and boron in the final G-mat material. Scheme 9 illustrates the doping process based on pyrolysis of biopolymers. Scheme 9. Pictorial Illustration of the Preparation Procedure of N-Doped G Films by (i) Spin Coating of Chitosan or Other Polymer Forming Films Followed by (ii) Annealing at 200 °C for 2 h and (iii) Calcination at Temperatures Between 600 and 1200 °Ca

a

Reprinted with permission from ref 97. Copyright 2012 Royal Society of Chemistry.

As an example of codoping, chitosan, already containing N and leading to N-doped G, can be modified with boric acid. The resulting G after pyrolysis of boronated chitosan and exfoliation contains simultaneously B and N (Scheme 10).107 This codoping illustrates the large range of possibilities that G offers as a platform to develop carbocatalysts of various compositions and properties. It is believed that G doping is a powerful strategy to introduce catalytically relevant active sites; therefore, doped G materials are expected to be increasingly applied as carbocatalysts in the near future. Concerning G preparation, there is continuous interest in applying a bottom-up approach and organic synthesis routes such as Diels−Alder reaction to build well-defined G-mat (Scheme 11).21,109 In this methodology, multiple cyclization and aromatization from adequate unsaturated precursors such as conjugated alkenes and alkynes will end up in G-mat with small dimensions in the nanometric range. The problem of this

4. PHYSICAL PROPERTIES OF GRAPHENE-BASED MATERIALS WITH RELEVANCE IN CATALYSIS There have been a considerable number of reports focused on the physical properties on G and related materials, and the reader is referred to the existing literature for a comprehensive coverage of them.13,18,23,35,52 In the present review, we will limit to those physical properties that are relevant for the use of G-mat in catalysis. As discussed earlier, the key structural feature of G is its 2D morphology structure as an individual, one atom thick layer. The dimensions of these layers can vary considerably from micrometers to a few nanometers, but for many applications in

Scheme 10. Codoped (B,N)-G Formed by Pyrolysis of Boronated Chitosan

F


Chemical Reviews

Review

Scheme 11. Bottom-Up Approach Based on Organic Synthesis for Preparation of G-mat; Synthesis of Monodisperse Ribbon-Type Polyphenylenes and Subsequent Cyclodehydrogenated Nanoribbonsa

a

Adapted with permission from ref 21. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA.

charged in aqueous solution at neutral pH values exhibiting negative zeta potentials in this pH region and having high stability. For these samples the point of zero charge in water, corresponding to the pH value of minimum stability, is between 3 and 5; therefore, working in this pH range should be avoided to minimize aggregation and precipitation of the samples. Besides water, which is the solvent of choice for GO derivatives but probably not for G, viscous organic solvents are also suited as medium to disperse G materials.117 For catalytic applications, organic solvents have the obvious advantage with respect to water of exhibiting generally better solubility for substrates and products. The fact that G-based catalysts are dispersed in a liquid phase containing the substrates would be similar to perform homogeneous catalysis, although eventually G could be recovered by filtration or centrifugation; for this reason these systems are better described as colloidal dispersions. G-mat have notable mechanical properties at the microscopic level that can be summarized as having high elasticity, a large Young modulus, and notable resistance to undergo rupture of the layer.35 Although G cannot be expected to break, agglomeration and stacking of the dispersed sheets can lead to a decrease on the surface area exposed to substrates. G has a high thermal stability, and it can be heated up to temperatures of 300 °C without having oxidation according to thermogravimetric measurements.20 In the absence of oxygen or other oxidizing reagents, this temperature range of stability increases considerably. Other doped G, such as those having N atoms, are even more stable against chemical oxidation.118 In contrast, GO is very labile, and heating even at mild temperatures in the presence of oxygen can lead to decomposition; if reducing reagents are present, the material can experience significant chemical changes in the nature of the functional groups.119,120 For this reason, GO should only be considered as a suitable catalyst for low-temperature reactions or as oxidation catalyst under low temperatures. Even under these cases the material should be surveyed for stability and catalytic studies should contain a discussion addressing possible changes in the material under operation conditions and recyclability studies. G-mat have many other properties that can affect to their performance as catalyst. These properties, such as electrical13,121,122 and thermal conductivity,35 can be of particular relevance in some processes since polarization and charge

Scheme 12. Methylene Blue Forms Strong Conjugates with GO and G Materials Due to Coulombic Attractive Forces and π−π Stacking111

catalysis, suspension of these layers in water or organic solvents is used. In this case, dispersibility is very important leading to permanent suspensions in the form of inks. Stability of these dispersions requires a maximum size of the layers in the range of a few micrometers or smaller. Also, the point of zero charge is important since being far from this point disfavors aggregation and results in an increase in the stability of the inks. Laser scattering techniques can be applied to have an estimation of the dynamic dimension of the G sheets present in the liquid phase,114 although most of the models have been developed for spherical particles and may not apply for G-based sheets. However, it has to be considered that due to the flexibility of G sheets they are not flat and could even be present as balls or spherical particles due to the wrinkling of the sheet. Presumably, in the course of the catalytic reaction G sheets can undergo some bent having a distribution of particles with various geometries that cannot be considered flat. Besides the size and point of zero charge a quantitative estimation of the persistence of a suspension can be obtained from the zeta potential value.115 The magnitude of this value is higher for more stable dispersions and can be positive or negative depending on the net charge of the G sheets. Values of zeta potential outside the −30/+30 mV region are considered that ensure sufficient stability of the suspension; although for short reactions performed under sonication or suitable stirring, lower zeta potential values can be sufficient to ensure the lack of aggregation and sedimentation.116 Due to the presence of carboxylic acid groups in GO, most of rGO are negatively G


Chemical Reviews

Review

In this regard, the morphology of the G-based materials is typically established by imaging techniques such as electron microscopy and atomic force microscopy (AFM).17,18 These imaging techniques should address one point of major concern, that is, the single-layer constitution of the catalyst. It should be mentioned that not infrequently materials denoted by the authors as G are really graphitic platelets since they are not singlelayer, 2D particles but multilayer. Aggregation can even increase during the use of the material. However, since imaging techniques cannot be performed in suspension but only for dried samples it could be that the species present in the suspension are different from those characterized after drying, particularly if the suspension is extensively sonicated. Actually, this requirement of dry samples is a general problem also common in the characterization based on many other techniques. TEM images at high resolution (HR) should show the hexagonal arrangement of the atoms in G.17,18 Selected area electron diffraction gives information about the crystallinity of G at the microscopic level in certain areas and the presence of large or small crystalline domains. Also, high-resolution TEM can provide elemental analysis of selected areas and comparison of the carbon distribution with that of other elements, report on the existence of microphases or domains, or support homogeneous distribution of heteroatoms in the sample.127 TEM is particularly informative when using G as support of MNPs since in this case it is crucial to know the particle size distribution and the morphology of the MNPs. One case in which the information obtained for dried powders is accepted not to be relevant for dispersions is surface area and porosity measurements based on gas adsorption. Thus, typically the Brunauer−Emmett−Teller (BET) surface area of G-mat determined by isothermal N2 measurements gives low values below 100 m2 g−1 and for some G materials sometimes even below 10 m2 g−1 that are very far from the maximum theoretical values for fully exfoliated G-mat that can be considered around 2600 m2 g−1.13,111,128 This contrasting discrepancy is considered to arise from the restacking and strong interaction of G layers when they are dried, without a close relationship with the area of the microsheets in the colloidal suspension present in the liquid phase. It can be said that unfortunately surface area measurements of dried powders are irrelevant with regard to understanding the properties of G suspensions. Similarly, XRD patterns can be useful to follow the transformation of graphite into graphite oxide by the change in the diffraction peak from 26.6° to 11.2° and even to support that ultrasound irradiation affects the solid, increasing the disorder of the stacking as expected for exfoliated powders.17 However, the fact that XRD is performed for dried powders in which some extent of restacking of the layers can occur as well as the fact that amorphous and delaminated materials should not give any peak in XRD makes this technique always biased toward providing information on the fraction of this material presenting some crystalline domains that maybe does not reflect the largest proportion of the solid. IR spectroscopy has been widely used for GO characterization, but it is relatively useless for rGO and other G materials that do not exhibit any intense absorption vibrations in IR. In contrast, Raman spectroscopy is a very powerful technique to characterize G materials by monitoring the 2D, G (“graphenic”) and D (“defects”) bands present on all these materials.17,18,35,101,129 Carbon nanoforms generally exhibit intense Raman scattering to the point that it is possible to record the Raman spectrum of a single, one-atom thick G sheet, and hybrid inorganic-G-mat

transfer at local points can take place between substrates and G in some of the steps of a reaction mechanism. In fact, transfer of electronic density from G to supported metal nanoparticles (MNPs) deposited on G is frequently invoked to understand the catalytic performance of these MNP-G materials. Also, activation of substrates can take place through electron donation from G microsheets. Considering that in an ideal G system π orbitals extend their electron density on the upper and lower sides of G sheets it could happen, as in electronics, that the interaction of a G film with a support affects the performance of the exposed G face in catalysis. At the current stage of G catalysis there are not convincing examples showing how this effect related to electron density and faster or slower electron mobility influence affects to the outcome of a catalytic process. Surely it would be of interest to show the operation of charge transfer and support influence on the outcome of a catalytic process, since this feature would be unique for G-mat. Also, the thermal conductivity of G-mat could be important in catalysis, particularly for those reactions exhibiting a strong endoor exothermicity for which the active sites will benefit from a fast heat management. Heat transfer is an important issue in reactor design and engineering, but it may also have an impact for reactions carried out at smaller scale. Many other G properties can have an influence on catalysis such as, for instance, the high Seebeck factor that reflects the interconnection between heat and electron mobility and could be perhaps useful to promote redox reactions by heating.123 Transparency is other feature of G films. It can be anticipated that as the use of G-mat as catalysts develops, examples in which these properties not found in conventional homogeneous or heterogeneous catalysis play a key role will be reported, showing the uniqueness of G-mat among other alternative catalysts.

5. CHARACTERIZATION TECHNIQUES FOR GRAPHENE-BASED MATERIALS AS CATALYSTS As discussed above, there are a considerable number of articles describing characterization of the properties of G-mat by a multitude of different techniques; the reader is referred to these existing references for comprehensive coverage of this field.13,16−18,23,34,124−126 Herein, we briefly summarize those types of characterization techniques that can provide useful information about active sites relevant to catalysis. A summary of the characterization techniques adequate to study G samples and the relevant information that they provide is given in Scheme 13. Scheme 13. Summary of Techniques Used To Characterize Gmat as Catalysts

H


Chemical Reviews

Review

5%, 13C NMR spectroscopy reports on the major families of carbons in G-mat. Combustion elemental analysis provides important information in G-mat. The percentage of C and the presence of heteroatoms doping the G layer can be revealed by combustion analysis. In some cases, the percentage of oxygen is not directly measured, being assumed to be the difference of the sum in the percentage of the other elements with respect to 100%. However, since oxygen content can be measured directly with appropriate elemental analyzers it is highly recommended to determine experimentally the oxygen content. Also, considering the high adsorption capacity of G, coadsorbed water, and G hydration can disguise carbon analysis, giving lower values than really present in the dry material. When metals are present, it is necessary to perform also other types of elemental analysis generally based on atomic emission (AES) or absorption spectroscopy (AAS) after calcining the material and dissolving the residue with an appropriate mixture of acids. Combination of imaging and analytical data can serve to address the homogeneity of the sample by comparing bulk data referring to the average of all parts of the samples with that of electron microscopy that corresponds to the microscopic domain probed by the electron beam. X-ray photoelectron spectroscopy (XPS) is one of the most suited techniques to gain information about the nature of the active sites in G-mat.101 XPS can serve to determine the presence or absence of elements on the sample as well as, after applying appropriate correction factors, quantifying their relative percentage. These analytical data should be in agreement with those obtained by chemical analyses if the sample is homogeneous. In addition, XPS gives information about the types of C and other elements present in the sample. The experimental XPS band can be deconvoluted to determine the relative contribution of different types of atoms to the overall XPS peak. In the case of G sp2 carbon atoms the binding energy is about 284.6 eV and can be accompanied by other components corresponding to carbon atoms associated with defects, particularly those bonded to oxygen appearing at a binding energy of 286 eV and sp3 carbons.133 For doped G the presence of another component in the XPS C 1s peak deconvolution corresponding to those carbon atoms bonded to the doping element can also be seen in a proportion that should match with the doping content. Besides carbon, the presence of a peak corresponding to oxygen is also observed and can be attributed to the presence of oxygenated functional groups such as carboxylates, phenol, epoxides, and ketones. One should be aware that XPS can detect also oxygen coming from adsorbed water or the support on which the G sample is deposited as a monolayer or very thin film to perform the measurement. Accordingly, oxygen should be detectable even if the element should not be present on the sample. XPS can also serve to detect the dopant element, if any, determining its percentage and distribution among different families according to their coordination sphere. One particular case in which XPS is also useful is when there are metals supported on G.14,134 Thermogravimetric and thermodesorption methods are also very useful for studying G-based materials having application in catalysis.131 In principle, thermogravimetry should show the absence of any residue and a total loss of weight when the sample is heated under air at increasing temperatures up to 800 °C or above. Typically, combustion starts at about 300 °C with the highest slope in the weight loss around 500 °C.25,135,136 Before the weight loss associated with combustion, water desorption can

exhibit almost exclusively or predominantly the signals corresponding to G. The technique can even be applied to count the number of layers based on the intensity of the Raman signals and also the position and relative intensity ratio of the 2D band. In general, the position of these three bands appears around 2600, 1600, and 1350 cm−1 for 2D, G, and D, respectively. High-quality G, lacking defects, oxygen functionalities, and dopant elements, should have absent the D band; therefore, the intensity ratio of the G vs D bands can be taken as a quantitative indicator of the quality of G. The presence of dopant elements also increases the intensity of the D band. With regard to catalysis, where the active sites are generally due to the defects or dopants, the presence of a relatively intense D band is frequently observed.107 With respect to characterization of the active sites, general techniques in catalysis include adsorption of probes from the gas phase onto dried powders and subsequent monitoring of the adsorbed probe IR spectrum.130 However, it is very common that G-based materials cannot be pelletized as transparent wafers, and they can only be prepared as thin films. Therefore, measurements requiring the use of thick G wafers such as for pyridine adsorption to determine the density and strength of the active sites or CO adsorption for assessing metal dispersion or the presence of Lewis acids are difficult to be applied in the present case. Nonspectroscopic techniques such as thermoprogrammed desorption in which acid sites can be titrated by ammonia adsorption/desorption on dry powder are more suitable.131,132 Solid-state 13C NMR spectroscopy is also a powerful tool to distinguish between GO and rGO as well as to monitor the presence of various functional groups.74 Figure 1 indicates the

Figure 1. 13C NMR chemical shifts (δ) of the major types of carbons that could be present in GO and rGO. The intensity of each peak depends on the recording technique (1H−13C cross-polarization or direct 13C pulse) and on the population of each type of carbon.74

chemical shift of various possible types of carbons, graphitic or bonded to oxygen, that have been reported for 13C NMR spectra of GO. Spectra can be recorded by direct 13C pulse or 1H−13C cross-polarization. The latter technique enhances the intensity of the carbons at the vicinity of hydrogen atoms, generally carbon bonded to −OH groups. GO has two overlapping peaks of high intensity about 70 and 61 ppm corresponding to sp3 carbons bonded to −OH and the ether linkage, respectively. CO carbons of ketones and carboxylates appear at about 190 and 165 ppm, respectively. When GO is reduced to rGO the intensity of these peaks decreases, increasing concomitantly the peak corresponding to graphitic sp2 carbons at about 130 ppm. Depending on the degree of the reduction, this peak of graphitic sp2 carbons could be the only one recorded for rGO. In spite of the low sensitivity of 13C NMR spectroscopic techniques that generally do not detect those populations of carbon atoms below I


Chemical Reviews

Review

be observed as a decrease in the weight of the sample at temperatures about 100 °C. Although G can be considered a hydrophobic material the presence of π orbitals and the large surface area determines the high water adsorption capacity even without considering the presence of oxygenated functional groups and structural defects. In the case of GO, a weight loss in the range between 100 and 300 °C is generally observed that is associated with the reduction of the G layer and decomposition of some labile oxygenated functional groups, particularly carboxylic acids. Besides thermogravimetry, other methods that can provide useful information is thermodesorption measurements.133 In these techniques, the small molecules evolved from the solid upon heating can be characterized by adequate detectors. When using a mass spectrometer as a detector, the identity of the desorbed species can be determined. In this way, profiles of water, CO, and CO2 desorption as a function of temperature can be determined. In the case of G-mat for application in catalysis, thermodesorption can be combined with prior treatment of the sample with known amounts of a probe. These probes can serve to assess the presence of acidic (ammonia) and basic (CO2) sites or even probe the dispersion of metals (CO adsorption) on G. Thus, for instance, exposure of G-based catalysts to ammonia will produce an acid/base reaction leading to ammonia chemisorption by interaction with the acid sites. When this G-based sample previously exposed to ammonia is heated, a gradual loss of ammonia should occur and detection of the amount of ammonia desorbed as a function of temperature provides a quantitative estimation of the acidity of the sample, weaker acid sites desorbing ammonia at lower temperatures than stronger acid sites. This provides a rough estimation of the acid strength distribution of the solid. However, since G-based materials may not be totally thermostable or they may undergo significant changes upon heating, these thermal techniques have to be interpreted carefully and require controls to avoid misinterpretation of the data. Similarly to characterization of sites in other catalysts, it is very common to use a probe to assess the properties and population of the catalytic centers using spectroscopic techniques to monitor the probe. For instance, in the case of G samples containing noble metals, adsorption of CO followed by desorption under controlled conditions can be followed by IR monitoring the CO vibration band around 2000 cm−1.137 Also, monitoring adsorption/desorption of pyridine, methanol, and other probes can be used to gain information on the catalytic sites on G-mat.130,138 However, the problem in this case is to make wafers with G that are self-supported and transparent.

Scheme 14. Some General Reactions That Have Been Widely Used for Covalent CNT Functionalizationa

a

Adapted with permission from ref 147. Copyright 2005.

Since G chemistry is more recent, the number of examples of covalent G modification is considerably smaller, but a research line aimed at preparing G derivatives is currently under intense development.12,13,15,17,20,23,151−153 There is no doubt that these modified G-mat will have application in catalysis, mainly using the G sheet as a platform to anchor metal complexes or other units that exhibit intrinsic catalytic activity but when anchored on G can become recoverable and reusable.9,12,154 Besides esterification of carboxylic acid groups or amide formation, dipolar cycloadditions, radical attacks, and electrophilic additions, doped G could also be modified by specific reactions on the dopant heteroatom present on the G layer. Scheme 15 illustrates some of the possibilities of chemical functionalization of doped G-mat depending on the nucleophilic or electrophilic nature of the dopant element. Scheme 15. Covalent Functionalization of Doped G through the Dopant Elementa

6. CHEMICAL METHODS OF FUNCTIONALIZATION In heterogeneous catalysis one active field has been heterogenization of highly active homogeneous catalysts by covalent attachment on insoluble high surface area solid supports.139−141 The most clear examples of this methodology have been the use of high surface area silicas that have been employed as insoluble supports to anchor active sites and ligands by means of surface chemistry.139,140 More related to the present case has been the modification and functionalization of active carbons142,143 and CNTs.12,144−148 CNTs have a single or multiple G walls with a long aspect ratio. Essentially, the chemistry for CNT functionalization can be applied straightforward to functionalization of G.12,13,15,17,20,29,149 Scheme 14 summarizes some of the reactions that have been employed for CNTs modification.150

a

X = N, P, or B; Y = Cl, Br, or I; Nu = HO−, halide, and CN.

In spite of the above comments it is clear that the simplest strategy for covalent functionalization of G is based on the use of GO as a key synthetic intermediate.15,17,20 As discussed earlier, starting from graphite preparation of GO dispersions in water is very convenient. The high degree of functionalization of GO obtained by Hummers oxidation with a multitude of epoxide and hydroxyl groups makes this material suitable for undergoing further derivatization through nucleophilic−epoxide ring openJ


Chemical Reviews

Review

ing or reactions based on −OH substitution. The main limitation of the use of GO as a starting material for functionalization is that water is usually the medium in which GO is dispersed and water can be incompatible with many common organic reagents. Generally, the conditions for GO derivatization should lead also to partial GO reduction, but this side effect can even be favorable from the point of view of gaining stability on the G sheet for its subsequent use as catalyst. An example of this strategy based on GO is, for instance, attachment of aza crown ethers on GO (Scheme 16)72 or anchoring of carbene to CC.155

and CNTs, also in future years many additional examples illustrating the potential of chemical modification for implementing novel catalytic activities on G will grow. The reader is referred to recent comprehensive reviews discussing the chemical reactivity of G,12,13,20,23,27,33 GO,12,13,15,17,20,23,27,33 and related materials24 for complete coverage of this area.

7. OVERVIEW OF THE CATALYTIC REACTIONS PERFORMED USING GRAPHENE-BASED CATALYSTS In this section, we compile reports on the use of G and rGO (Table 1) as well as GO (Table 2) catalysts. A description and comments on these articles make up the main body of the review and will be the focus of the next section. The overview tables include the type of catalyst, general reaction conditions, and some relevant remarks.

Scheme 16. Aza-Crown-Substituted G Obtained Starting from GO as Intermediatea

8. GRAPHENE AS CARBOCATALYST As discussed in section 3 there are different procedures to obtain G, each of them leading to a different density of defects. Regardless the high-quality G obtained by CVD, only a few articles have reported this material as carbocatalyst, most probably due to the lack of catalytic activity derived from the absence of active sites to promote catalytic reactions. In contrast, low-quality rGO, obtained by chemical or thermal reduction of GO, has been amply used as carbocatayst for a variety of reactions. In this context, the level of reduction should be determined and this information provided in order to ensure reproducibility. Since combustion chemical analyses do not measure directly the oxygen content, XPS analysis could be more informative than just the C content. Moderate temperatures

a

Adapted with permission from ref 72. Copyright 2013 Royal Society of Chemistry.

The number of examples of G-mat should still grow, since the opportunities that this strategy offers are very large (Scheme 17). It can be easily predicted that similarly to functionalized silicas Scheme 17. Possibilities of Covalent Functionalization of GO

K


Chemical Reviews

Review

Table 1. Catalytic Reactions Using G and rGO as Catalysts

L


Chemical Reviews

Review

Table 1. continued

a Legend: graphene (G), X-doped G [(X)G, where X represents B, N, or Si), (B,N)-doped holey G [(BN)HolG], carbon nitride (g-C3N4), reduced graphene oxide (rGO), hydrogenated G (HG), X-doped rGO [(X)rGO, where X represents N or codoped (B,N)], triethylamine-modified rGO (rGO-NEt3), rGO functionalized with −SO3H or aryl functionalized with −SO3H functional groups (rGO-SO3H and rGO-Ar-SO3H), poly(amidoamine)-modified rGO (rGO-PAMAM). bTON: turnover number

above 100 °C can produce the reduction of GO in various degrees depending on the severity of the treatment. Chemical reduction commonly uses hydrazine as a reducing agent, although its toxicity in water can make it advisable to find other alternatives such as NaBH4,202 hydroquinone,203−205 vitamin C,206 reducing sugars,207 and even natural antioxidants208 like those found in green tea. Reduction of GO makes the resulting rGO become hydrophobic with low solubility in water and polar solvents and also with enhanced tendency to undergo stacking.22 In order to stabilize rGO in water some

hydrophilic polymers such as poly(styrene sulfonic acid),183 polyethylenimine,209 chitosan,162 poly(vinylpyrrolidone),210 or dendrimers186 have been used. Other authors have employed G obtained by mechanical exfoliation of graphite assisted by solvents as an active catalyst. In this section we review those papers describing the use of Gmat as catalysts (Table 1). 8.1. Oxidation Reactions

8.1.1. Oxidation Reactions Promoted by Molecular Oxygen. An efficient and cost-effective approach has been M


Chemical Reviews

Review

Table 2. Catalytic Reactions Using GO as Catalyst

a Legend: graphene oxide (GO), reduced GO (rGO), GO or rGO functionalized with aryl −SO3H groups (GO-Ar-SO3H and GO-Ar-SO3H), GO functionalized with −SO3H groups (GO-SO3H), amino-functionalized GO with 3-[2-(2-aminoethylamino)ethylamino] propyl-trimethoxysilane (AEPTMS) and aryl −SO3H groups (GO-AEPTMS/SO3H); amino borane-supported GO (GO-AB). bTON: turnover number

N


Chemical Reviews

Review

Scheme 18. Schematic Representation for the Exfoliation of Graphite in DMF or NMP by KOH To Obtain a Solution of the Mono/ Multilayer G-Supported KOH Compositea

a

Reprinted with permission from ref 156. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.

reactor (100 L) with the final yield of 97%, demonstrating the practicality of this oxidation for large-scale synthesis. In addition, the catalyst was reused 5 times with no decay in its activity, clearly demonstrating its stability and durability. An interesting application of G-mat related to aerobic oxidation could be their use as additives to accelerate combustion of future high-speed propellants.157,211 Thus, it has been reported that G dispersed in liquid nitromethane can increase the burning rate of this compound up to 175% over neat nitromethane, outperforming other conventional additives such as silica or alumina NPs. Theoretical studies suggest that the active sites in this process would be defects in the G sheet such as carbon vacancies and particularly oxygen functionalities such as hydroxyls, ethers, and carbonyls around sp2 dangling bonds at the vacancy edges.157,211 Defectless G has been frequently found to be inactive to promote aerobic oxidations.107,158,160 However, the presence of heteroatoms in the layer can introduce catalytic activity toward this type of reactions. In particular, N-doped G [(N)rGO] has been reported to produce materials with activity toward oxidation of hydrocarbons. A simple way to effect N doping is to start with reactive GO and submit it to nitridation with NH3 gas. N heteroatoms may be introduced as substitutional N atoms during nitridation. In this way, three (N)rGO-T (T represents the nitridation temperature between 800 and 1000 °C) were prepared and their catalytic activity investigated for aerobic oxidation of benzyl alcohol.158 The disappearance of GO and formation of G sheets was supported by powder XRD monitoring the intensity decrease of the broad peak at 2θ 15.5° corresponding to GO and observing growth of the rGO peak at 22.6°. HRTEM and AFM revealed that the (N)rGO-T samples consist of 1−10 layer G-mat with an interlayer distance of 0.38 nm, slightly larger than the interlayer distance of graphite (0.34 nm). Although elemental analysis confirmed the presence of nitrogen atoms, a key parameter such as N loading was not determined, making reproducibility problematic. XP spectra showed three components for the N peak, two of them at binding energies of 398.1 and 399.4 eV attributed to pyridinic and pyrrolic N, respectively. The third peak at 401.7 eV was assigned to graphitic N. According to XPS the N atomic content in (N)rGO-T decreases monotonically from 4.16% to 1.71% with the increase in the annealing temperature from 800 to 1000 °C. (N)rGO-T catalyzes aerobic oxidation of benzyl alcohols in water (Scheme 20).158 Undoped G resulted in negligible conversion of benzyl alcohol. In contrast, introduction of N

developed for synthesis of 9-fluorenones by aerobic oxidation of 9H-fluorenes using mono/multilayer G-supported KOH as catalyst in DMF as solvent (Scheme 18).156 It was proposed that the effect of KOH can be due to its dissolution in a small quantity in DMF or NMP, ionizing into −OH and K+ that can become intercalated into graphite interlayers, decreasing attractive forces between the layers that would be exfoliated from the graphite sheets to form G accompanied by multilayer graphitic platelets. Powder XRD supports formation of multilayer graphitic platelets incorporating KOH. HRTEM images are compatible with the suspension comprising a distribution of multilayer G and graphitic platelets supported KOH composite.156 Statistical AFM measurements showed that the thickness of the sample of G and multilayer graphitic flakes-supported KOH composite is in the range of 0.8−3.0 nm. Further, it also showed that the G surface is not smooth, indicating that presumably there are some KOH crystallites on the G surface. The optical absorbance of the G/multilayer graphitic platelets-supported KOH composite is 10 times stronger than graphite in DMF at the same concentration, indicating a certain dispersion of G and KOH-containing platelets in DMF. It would have been convenient to address the degree of exfoliation in a more convincing way. The synergy between KOH and graphite is also evidenced by the fact that the pH value of 50 mg of KOH in 3 mL of DMF was 9, while for KOH supported on G and multilayer graphitic flakes (50 mg of KOH and 2.5 mg of graphite) in 3 mL of DMF increases to 13, suggesting more KOH is dispersed in DMF when graphite is present. KOH supported on G and multilayer graphitic platelets in DMF was used as catalyst for aerobic oxidation of 9Hfluorenes to 9-fluorenones (Scheme 19), achieving 98% yield at room temperature. Graphite and rGO did not show any activity. Using the same protocol, a series of 9H-fluorenes was smoothly oxidized under the same conditions to the corresponding 9fluorenones in high yields. Remarkably, the present experimental conditions allowed oxidation of 5 kg of 9H-fluorene in a glass Scheme 19. Aerobic Oxidation of 9H-Fluorene to 9Fluorenone Using KOH Supported on G and Multilayer Graphitic Flakes (KOH-G) as Catalyst

O


Chemical Reviews

Review

presence of dicyandiamide within the interlayers of lamellar carbon nitrides by a thermal pyrolytic process (Scheme 21). Although the system is certainly of interest and can serve to develop doped G materials, no individual (BN)HolG layers have been obtained.

Scheme 20. Aerobic Oxidation of Benzyl Alcohol and 1Phenylethanol Catalyzed by (N)G-900 as Catalyst

Scheme 21. Proposed Protocol for Synthesis of (BN)HolGa

into the flat lattice of G microsheets enhanced the benzyl alcohol conversion to 4% at 40 °C and 12.8% at 70 °C for (N)G-900 without compromising complete selectivity to aldehyde. On the other hand, (N)rGO-800 and (N)rGO-1000 resulted in 6.5% and 4.0% benzyl alcohol conversion at 70 °C, respectively, indicating that there is an optimum for N content. The scope of this reaction includes primary benzyl alcohols such as pnitrobenzyl alcohol, p-fluorobenzyl alcohol, p-methyl benzyl alcohol, and p-methoxybenzyl alcohol, while 1-phenylethanol gave only 1.8% conversion, indicating that oxidation of secondary alcohols is sluggish. Additional investigation is necessary to identify the reasons for this chemoselectivity since, in principle, radical chain autoxidations should take place for secondary alcohols at higher rate than the primary ones. Concerning the nature of the active sites a good linear relationship between the graphitic N/C atomic ratio and the initial reaction rate of benzyl alcohol oxidation was found, while a similar correlation could not be established for pyridinic and pyrrolic N atoms. On the basis of this linearity it was proposed that activation of molecular oxygen takes place on graphitic N atoms to form a sp2 N−O2 adduct. The activated oxygen complex should show high chemical reactivity for primary alcohols, possibly due to steric effects. Importantly, no evidence was found for formation of H2O2 during the catalytic reaction, although this compound is usually generated from reactive oxygen species. In addition, the study does not provide data supporting catalyst stability in multiple uses. One interesting point of this study was comparison of the catalytic activity of (N)rGO-T with N-doped multiwall CNTs [(N)MWCNTs]. (N)MWCNTs showed 6.1% conversion of benzyl alcohol at 70 °C. This common catalytic activity of (N)G-Ts and (N)MWCNTs suggests the existence of similar active sites in both materials. Garcia and co-workers reported the use of nitrogen (N)-, boron (B)-, and boron, nitrogen (B,N)-doped G as carbocatalysts to promote aerobic oxidation of the benzylic positions of aromatic hydrocarbons and cyclooctane to the corresponding alcohol/ketone mixture with more than 90% selectivity.107 Aerobic oxidation of styrene was also included in the study. Use of doped G as catalyst appears to offer broad scope for aerobic oxidation of benzylic compounds and styrene, for which low catalyst loading (substrate/doped G ratio of 200), mild reaction temperatures, and no additional solvents are required. The main advantages of the procedure developed in this study are the absence of any transition metal, high catalyst activities, and the absence of additional solvent. The catalyst was found to be reusable. In other work, Li and Antonietti reported a simple bottom-up method for synthesis of 2D B- and N-codoped holey G assembly [(BN)HolG] and further use as efficient carbocatalyst for aerobic oxidative coupling of several amines to imines.159 (BN)HolG was prepared by copolymerization of glucose and boric acid in the

a

Reprinted with permission from ref 159. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.

Other approach to promote catalytic reactions has been exemplified by Li, Antonietti, and co-workers synthesizing a G sheet/polymeric carbon nitride (g-C3N4) nanocomposite and used as a metal-free catalyst to activate molecular oxygen for selective oxidation of primary or secondary C−H bonds.160 The G/g-C3N4 material was obtained by irradiation using a xenon lamp of a mixture of GO, obtained by Hummers method, and cyanamide that is the precursor of g-C3N4 upon calcination under N2 at 600 °C. Available data suggest that G sheets, rather than graphitic stacks, are well dispersed in the layered g-C3N4 host. It is believed that intimate dispersion of G in g-C3N4 is relevant for electron exchange between the components that is the origin of O2 activation. Controls show that while bare g-C3N4 or G or the physical mixture of them (g-C3N4+G) resulted in no conversion of cyclohexane, the G/g-C3N4 composite at different weight ratios between the two components gave some conversion (up to 12%) for aerobic oxidation of cyclohexane.160 8.1.2. Oxidation Reactions Promoted by Nonoxygen Oxidants. Other authors have focused on the use of G materials to activate peroxides such as H2O2, tert-butylhydroperoxide (TBHP), and peroxymonosulfate (PMS). Although molecular oxygen as a terminal oxidant should be the preferred oxidizing reagent due to the availability of other tolerable oxidants (H2O2, TBHP, or PMS) it can be advantageous to achieve high conversions and high selectivity (both in terms of substrate and oxidant), justifying their use among other applications in the synthesis of high added value chemicals or for environmental remediation. For this reason it is also of interest to evaluate the activity G-mat with respect to the spurious decomposition of the oxidizing reagent and to determine which percentage of the oxidant is consumed in the wanted oxidation and which percentage corresponds to undesirable decomposition. In this sense, Ma and co-workers reported the direct, one-step hydroxylation of benzene to phenol using H2O2 as oxidant and rGO as catalyst.161 Oxidation proceeds with high selectivity, and the catalyst can be reused seven times without significant loss of P


Chemical Reviews

Review

MWCNTs, whereby benzene conversion was low (around 6%) with a very high selectivity (99%). Thus, the catalytic activity of MWCNTs was much lower than that of rGO.212 On the other hand, Pt/SiO2 exhibits a barrier-free Ea for H2O2 decomposition toward O2, hampering the benzene oxidation process.161 For comparison, titanium silicalite (TS-1), a catalyst commercially used for benzene oxidation, was also tested under the same conditions observing poor phenol selectivity. Other substrates including toluene, naphthalene, p-xylene, and ethylbenzene were also studied, observing yields and selectivities ranging between 0.5% and 9.6% and 3% and 100%, respectively.161 These positive results were interpreted as reflecting different π−π interaction between rGO and the aromatics with the steric hindrance being the major factor to be considered to rationalize the reactivity. These promising results leave still room for improvement in order to reduce as much as possible the H2O2 to substrate molar ratio, increasing the economic attractiveness of the process. Furthermore, deeper insight into the active sites responsible for H2O2 decomposition toward reactive oxygen species is necessary to prepare optimized generations of rGO catalysts and provide evidence supporting the proposed peroxidase-like mechanism. In other work, Weng and co-workers found that the peroxidase catalytic activity of few-layer G/chitosan composite is 45 and 4 times higher than that of GO and rGO, respectively.162 For this purpose, the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by H2O2 was studied (Scheme 23). This composite was obtained

catalytic activity and maintaining unaltered the catalyst structure (Table 3). The possibility that the observed catalysis is due to Table 3. Direct Oxidation of Benzene to Phenol under Various Conditions161 entry 1 2 3 4 5 6 7 8 9 10d 11e

catalyst blank graphite rGOb rGO rGO Pt/SiO2c TS-1 (0.1−0.2 μm) TS-1 (1 μm) GO rGO-H Mn/rGO

t (h)

conversion (%)

4 4 4 8 16 16 16

Carbocatalysis by Graphene-Based Materials - Chemical Reviews

Recommend Documents