GRAHENE AND GRAPHENE-LIKE MATERIALS IN BIOMASS CONVERSION: PAVING THE WAY TO THE FUTURE
"Materials based on graphene and its derivatives have a dual relationship with biomass: (i) they can be synthesized using biomass or its platform chemicals and (ii) they can be employed as efficient catalysts or supports for nanoparticulate catalysts for biomass conversion. In this review article we discuss methods of catalyst synthesis using top-down and bottom-up approaches as well as different synthetic pathways where all or some catalyst components are prefabricated or the whole catalyst is prepared in situ from small molecules. We identify key factors influencing the efficiency of the catalysts formed and suggest our perspective for the development of this field."
Biomass conversion plays a tremendous role in obtaining value-added chemicals and fuels from renewable sources without the use of petrochemicals. It is well known that oil and gas compositions can strongly vary depending on the place and method of their extraction. For biomass, these variations can be even more pronounced depending on the biomass source. The possible biomass sources were well-described in a recent review article, identifying lignocellulosic biomass as the most abundant. It includes three main components: 40–50% cellulose, 20–40% hemicellulose, and 20–30% lignin.1 Pyrolysis or hydrolysis of biomass leads to bio-oils, whose composition, in turn, can vary tremendously. Moreover, raw bio-oil must be upgraded to remove compounds such as phenols and furfurals which are easily polymerized. This creates a whole spectrum of different processes often requiring very different catalysts. An example of such processes is the catalytic oxidation of biomass derived intermediates such as glucose, 5-hydroxymethylfurfural (HMF), furfural, eugenol, isoeugenol, etc. to high value chemicals.2,3 The diversity of the starting materials puts a significant constraint on the development of catalysts for biomass processing. Considering the growing demand for energy and value-added chemicals obtained from renewable sources, the catalysts developed for these numerous processes need to be efficient, selective, robust, and stable. Moreover, their production should be easily scalable to gram or kilogram quantities from the more common milligram quantities used in laboratory settings.
The catalysts used in the biomass conversion include (but are not limited to) metal oxides with and without metal nanoparticles (NPs), zeolites, carbon based catalysts with metal NPs, solid acids, etc.4–9 The recent development of graphene based catalysts opened a new possibility in the exploration of such catalysts in biomass conversion. Moreover, graphene based supports can be synthesized from biomass and its platform chemicals, making it a dual purpose material.
Despite the fact that graphene was first obtained by exfoliation only in 2004,11 its remarkable properties and great utilization potential prompted an explosion of new chemical routes to synthesize graphene and graphene-like materials. However, the synthesis of quality graphene and its derivatives has always remained a challenge. All synthetic methods can be classified into two approaches: (i) top-down and (ii) bottom-up. The top-down approach is realized when graphite is exfoliated into graphene layers via overcoming van der Waals forces between the layers using oxidation or other treatments. The surface defects occurring during sheet separation lead to their re-agglomeration later and low yields, representing a considerable limitation of this approach. On the other hand, this approach is good for manufacturing graphene nanoribbons, nanoplatelets, etc. in large quantities.
A combination of top-down and bottom-up approaches is realized when biomass feedstocks are first subjected to pyrolysis (top-down) and then the pyrolysis product is converted to graphene or its derivatives (bottom-up).14 Several elegant methods to form nonfunctional or functionalized defect-free graphene sheets and ribbons using a bottom-up approach have been developed,15–18 but these methods normally include multiple steps, making their practical application difficult. A more robust bottom-up approach is the conversion of biomass derivatives, for example, glucose, to graphene.19,20 The biomass based approaches for the synthesis of graphene based supports and catalysts pave the way for making these catalysts environmentally friendly and widely available.
The issue of possible graphene toxicity to humans has been raised in the recent review article.1 However, several latest reports indicate minimal toxicity of graphene based materials to lung tissue.21–23 Moreover, graphene based materials are being actively studied as potential cancer treatment agents with minimized toxicity for healthy cells. For example, few-layer graphene (FLG) was reported to kill cancer cells, showing neither toxicity nor activation effects on other immune cells unlike regular chemotherapy.24 These data allow us to propose that graphene based catalysts can be considered safe for catalytic applications, unless contradictory data come to light.
Taking into account two preceding review articles (published in 2014 (ref. 25) and 2015 (ref. 1)), in this review we focus mainly on identifying the key features of graphene, graphene oxide (GO) and reduced graphene oxide (RGO) based catalysts for various processes of biomass conversion in order to determine which catalyst types are the most promising and how intelligent catalyst design can revolutionize this field. Despite numerous review articles describing the structure of graphene and its derivatives, for the sake of clarity of this review, we will discuss the features of different types of graphene based materials pertinent to catalysis and major methods of their synthesis. In the successive sections we will briefly discuss graphene based materials as catalysts in acid or base catalysis, and subsequently, in greater detail, we will review catalysts containing metal and metal compound NPs along with graphene or its derivatives. The question we are raising here is, what does the future hold? In the Concluding remarks we will address this question.
KEY STRUCTURAL FEATURES OF GRAPHENE, GO, AND RGO PERTINENT TO CATALYSIS
Graphene, an allotrope of carbon, is a flat nanolayer (0.35–1.6 nm in thickness) of trigonally bonded sp2 hybridized carbon atoms firmly packed into a two dimensional hexagonal honeycomb lattice (Fig. 2).26–28 It is noteworthy that its aromaticity is different from the aromaticity of standard aromatic compounds such as benzene, coronene, etc. Aromaticity in graphene is local with two π-electrons delocalized over every hexagonal ring.29 It results in a very tightly packed and strongly bonded network of carbon atoms within the layer. Because the carbon–carbon bonds are short and strong (three per carbon atom), it allows for a very high tensile strength (300 times that of steel). At the same time, graphene is a light, low density material. The π electrons are mobile throughout the layer, generating electrical conduction if a potential difference is applied. The ‘large scale’ delocalization bestows chemical stability to graphene.30 Additionally, due to this delocalized system, graphene is unsaturated and atoms can be added to its structure.
Through the oxidation of graphite (a 3D carbon material) using strong oxidizing agents, abundant oxygen containing functionalities such as hydroxyl, carboxyl, and epoxy are introduced on both the basal planes and edges of a graphite structure. These groups expand the layer separation and make the material hydrophilic. This feature allows for exfoliation of graphite oxide (a multilayer system) in water using sonication, eventually creating single or a few layers thick functionalized graphene called graphene oxide (GO). The crucial advantage of GO is that it can be easily dispersed in water. As for electrical conductivity, GO can be considered an electrical insulator due to the disruption of its sp2 bonding networks. In order to restore the honeycomb hexagonal lattice along with the electrical conductivity, the reduction of GO has to be accomplished using thermal, chemical or electrical treatments. RGO is more difficult to disperse because of its propensity to form aggregates, but it still retains some oxygen functionalities which allow for better solubility (although limited to aprotic solvents) than that of native graphene. RGO reveals properties between those of graphene and GO, with the reduction process strongly influencing these properties. In large scale operations, RGO is a more obvious choice compared to graphene, due to the relative ease of obtaining sufficient quantities of the desired quality levels.31
GO, RGO and their doped analogues are often called chemically modified graphenes (CMGs). Due to their numerous functionalities, comparatively low cost, high mechanical strength, and optical and electronic stability, CMGs are appealing alternatives to conventional acidic catalysts and supports for the active catalyst phase.32 Graphene has a theoretical specific surface area of about 2600 m2 g−1, which is nearly twice as high as the value for single-walled carbon nanotubes (CNTs) and considerably higher than those of multi-walled CNTs, most carbon blacks and activated carbons,33 which makes it an ideal candidate for processes involving adsorption or surface reactions. Thus, the typical structural features of graphene make it possible to use it as a two-dimensional catalyst or support in catalysis.34 Furthermore, the locally conjugated structure endows graphene with better capacity to adsorb substrates in the catalytic reactions.35 Also, graphene and its derivatives contain no metallic impurities that are inevitably present in CNTs and which often obstruct the catalytic performance of CNT based materials in chemical reactions.36 Finally, the greater degree of electron mobility of graphene based materials promotes electron transfer during chemical reactions enhancing their catalytic activity.37 Consequently, graphene and its derivatives create a perfect foundation for anchoring nanomaterials and creating a robust interaction between themselves and the active catalytic sites. In turn, the synergistic effect between active sites and graphene stimulates the reactivity. The graphene based template is believed to stabilize active catalytic sites and prevent wandering of nanosized metals from the surface under harsh reaction conditions.
Because of its chemical inertness and zero band gap, pristine graphene (in solid-state form) synthesized by micromechanical exfoliation, epitaxial growth and chemical vapour deposition is not a promising candidate for catalysis.38 The catalytic action of pristine graphene, however, can be enhanced by doping with heteroatoms like nitrogen, boron or sulfur to generate large numbers of active sites.39,40
SYNTHESIS OF CMGs
As has been discussed in the Introduction, two general approaches, top-down and bottom-up, are used for CMG synthesis. In the top-down approach, GO is synthesized by exfoliation of graphite using strong oxidizing agents and then is reduced (using various reducing agents such as hydrazine,41L-ascorbic acid,42 nature based plant extracts,31etc.) to form RGO. When GO is reduced to RGO, the solubility becomes limited to aprotic solvents (DMF, N-methyl-2-pyrrolidone). Reduction of GO combined with nitrogen doping can be realized upon reaction with melamine,43 NH3,44,45etc. and allows for improved solubility due to pyridine moieties.
The first synthesis of GO in modern history was reported by Hummers in 1958 using harsh conditions.46 Considering that this method results in a starting material for RGO, in recent years there has been a significant spike in interest in GO synthesis, leading to the development of several improved procedures where less harsh conditions are explored for graphite exfoliation.47,48 It is noteworthy that GO can be easily prepared in quantities of several grams, while reduction is normally carried out in diluted solutions, so the RGO yields are lower.
Over the past few years, biomass feedstocks have become popular benign precursors for carbon based catalysts or catalyst supports because they are inexpensive and readily obtainable in high quality and quantity. They are also available in abundant morphological and structural varieties, containing multiple elements.49 Lignocellulose has been used as a source of graphene or graphene-like nanostructures, presenting a green chemistry top-down approach.50 The production of high quality graphene from biomass opens new horizons for the conversion of waste into high value products. In light of this, Tour and co-workers grew graphene from food, insects, and waste by chemical vapour deposition (CVD) (Fig. 3).51 To accomplish this, a carbon source was placed on top of a Cu foil, and the foil was introduced into a 1050 °C tube furnace. They developed a cheap approach using six easily obtained, low or negatively valued raw carbon-containing materials (cookies, chocolate, grass, plastics, roaches, and dog feces) used without further purification to grow graphene directly on the reverse side of a Cu foil at 1050 °C under H2/Ar flow.
Zhou et al. prepared nitrogen doped porous graphene by applying KOH activation with eggplant followed by injecting ammonia under an Ar flow at high temperatures.52 Guo et al. pyrolyzed a solid-state mixture of Coprinus comatus biomass and melamine under a nitrogen atmosphere at 600 °C to obtain a nitrogen-doped biocarbon/graphene-like composite, graphitic carbon nitride, which acted as a self-sacrificing template.53 Wang and co-workers produced N-doped mesocellular graphene foam via a one-step pyrolysis of sludge flocs at 900 °C under a N2 atmosphere.54 Gao and co-workers synthesized a three-dimensional porous graphene-like material by using dried honeysuckles as a single precursor heated in a nitrogen atmosphere.55 This resulted in RGO doped with both N and S.
In a mixed bottom-up/top-down approach from a biomass derivative (glucose) and GO, hierarchical nitrogen-doped porous graphene/carbon (NPGC) composites were prepared by hydrothermal carbonization with KOH at 950 °C.56 The authors determined that this allows for a well-ordered 3D network structure, with nitrogen groups homogeneously dispersed in the material (Fig. 4).
Apart from the above biomass sources, well-aligned graphene layers have been synthesized from poultry litter and wastewater biosolids,57Pinus kesiya sawdust,58 seaweed (Sargassum tenerrimum),59 nori biomass,60 bacterial cellulose/lignin,61 biomass residues of the olive oil industry,62 coconut shell,63 pine wood sawdust,64 soybean oil precursor,65 natural chitosan66 and glucose.67
Despite the fact that the synthesis of CMGs from biomass could result in unwanted impurities (along with the desired doping), the ubiquity and low cost of various biomass sources could be a deciding factor for choosing large scale methods to obtain CMGs.
GO/RGO CATALYSED BIOMASS CONVERSION
GO without any additional functionalization or functionalized with acidic groups has been employed as a catalyst for a number of biomass related processes. Several mechanisms could be identified which promote catalytic assistance by GO in addition to the inherent functional group influence. In a number of cases it is accompanied by the conversion of GO to RGO, with the latter most likely playing a crucial role in the process.68,69 The above assistance mechanisms include (i) self-assembly on the GO/RGO surface, (ii) formation of strong hydrogen bonds between the substrate and GO, and (iii) induced defect sites due to the incorporated functional groups. Below we illustrate these mechanisms for different biomass conversion processes.
GRAPHENE, GO, AND RGO AS SUPPORTS FOR METAL AND METAL COMPOUND NANOPARTICLES
Catalysts based on graphene, GO, or RGO and metal NPs can be synthesized using four major approaches. In the first approach, prefabricated NPs and CMGs are combined in a common solvent with an expectation that electrostatic interactions or coordination forces or π–π interactions drive the attachment. This method allows for better control over NP characteristics, but does not necessarily produce intimate contact between CMGs and NPs. In the second approach, the catalytic NPs are formed in situ in the presence of graphene based materials, whose functional groups or the RGO aromatic structure is crucial for the NP formation and stabilization. In this case, some control over the NP structure and morphology may be lost, but better interactions between the NPs and the support can be achieved. In the third approach, both the formation of metal/metal compound NPs and GO-to-RGO transformation occur in situ, allowing for better control over interactions between the components of the catalyst. And finally, in the fourth approach, both NPs and graphene based support are formed in situ from small molecules by a bottom-up, one-step procedure, allowing for a robust pathway for the catalyst synthesis.
MAGNETICALY RECOVERABLE CATALYSTS
In the last decade magnetically recoverable (MR) catalysts have received considerable attention due to more environmentally friendly processes, conservation of energy, and cheaper target products.97–105 Such catalysts are widely used for biomass and bio-oil related processes.106–115 MR graphene based catalysts are also well developed and utilized in many catalytic reactions.116–121 At the same time, to the best of our knowledge, there are only a few recent examples, where MR graphene based catalysts have been used for biomass and bio-oil relevant catalytic reactions. One such example reports a magnetic solid acid, Fe3O4–RGO–SO3H, which was employed to obtain HMF directly from fructose followed by the utilization of another MR catalyst, ZnFe1.65Ru0.35O4, to obtain 2,5-diformylfuran (DFF) and 2,5-furandicarboxylic acid (FDCA) (Fig. 13).122
A catalyst based on MR graphene, bearing –SO3H, –COOH and –OH functional groups, along with iron NPs has been reported for the hydrolysis of cellulose to saccharides (Fig. 14).123 This catalyst allowed for high yields (up to 98%) of monosaccharides and disaccharides at lower reaction temperatures (75 °C) than normally used in this process (125 °C).
A sophisticated catalyst has been developed combining Fe3O4 NPs, N-doped RGO, and bimetallic Pd based NPs through a simple one-pot reaction under mild conditions.124 To accomplish this, GO was mixed in water with Fe(II) and Fe(III) salts as well as Pd and Ag (Co, Au, and Pt) salts. Reduction with hydrazine monohydrate at 90 °C resulted in N-doped RGO, Fe3O4 NPs, and bimetallic PdAg (PdCo, PtAu, and PdPt) NPs (Fig. 15). This is a novel strategy for the preparation of multicomponent and multifunctional transition-metal catalysts. The catalyst developed was employed for the selective upgrading of biomass-derived vanillin, isatin, and two types of benzyl phenyl ethers (lignin biomass-derived model compounds) as well as synthetic feedstocks derived from aromatic compounds. This novel catalyst was effective in the dehydrogenation of formic acid to form H2 and subsequent utilization of the hydrogen generated in situ for upgrading bio-oil and chemical feedstocks. Moreover, the authors discovered that Fe3O4 does not simply facilitate the catalyst separation with a magnetic field; it also improves the TOF of the reaction presumably by facilitating the catalytic cycle of the tandem reactions.
Here, we return to the original question raised in the Introduction: what does the future hold? In other words, what are the most promising avenues for catalyst development for biomass conversion? Numerous reviews including ours have demonstrated that the presence of aromaticity in RGO is an important factor for π–π interactions and for controlling the NP growth along the crystalline graphene sheets. However, if RGO functionalities are limited, poor solubility of such supports can impede further catalyst development. Thus, if RGO is obtained from GO, one should take this aspect into account when choosing the reduction/functionalization conditions. In the case of CMG formation from biomass, again both the graphene quality and solubility are the key factors for successful utilization. These could be controlled by varying the temperature, time, addition of a catalyst, etc.
Another obvious conclusion from the surveyed literature is that a fully in situ catalyst synthesis from small molecules (especially, biomass derived as a carbon source) through a one-pot procedure is the most advantageous from the viewpoint of simplicity and low cost; however, one needs to bear in mind the mechanistic aspects of such a catalyst formation. The formation of graphene moieties needs to either precede or coincide with the formation of metal/metal compound NPs in order to realize the advantages of co-crystallization, well-defined structures and intimate contact between the catalyst components to allow a synergistic effect in catalysis.
Finally, we believe that magnetically recoverable graphene based catalysts are underutilized in biomass conversion. Our recent work demonstrated that even in the case of solid cellulose and heterogeneous MR hydrophilic catalysts, 100% cellulose conversion can be achieved,113 making MR catalysts coupled with CMG components especially appealing. Considering that no data have been reported to suggest that the presence of magnetic NPs in graphene, GO, or RGO based catalysts has a detrimental effect on catalysis, we believe that there is a great niche where catalytic and magnetic NPs can be combined on graphene based supports (instead of traditional porous supports) and used for biomass valorisation reactions.
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