Nanostructured mesoporous carbons: Tuning texture and surface chemistry

Accepted Manuscript Nanostructured mesoporous carbons: Tuning texture and surface chemistry M. Enterría, J.L. Figueiredo PII:

S0008-6223(16)30563-2

DOI:

10.1016/j.carbon.2016.06.108

Reference:

CARBON 11127

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Carbon

Received Date: 18 May 2016 Revised Date:

28 June 2016

Accepted Date: 29 June 2016

Please cite this article as: M. Enterría, J.L. Figueiredo, Nanostructured mesoporous carbons: Tuning texture and surface chemistry, Carbon (2016), doi: 10.1016/j.carbon.2016.06.108. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Nanostructured mesoporous carbons: tuning texture and surface chemistry M. Enterría*, J. L. Figueiredo

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Laboratory of Separation and Reaction Engineering – Laboratory of Catalysis and Materials (LSRE-LCM), Faculty of Engineering, University of Porto Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal

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TABLE OF CONTENTS Abstract 1. Introduction

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2. Control of the mesoporosity and the nanostructure 2.1. Carbon gels: control of the mesopore size.

2.1.1. Hydroxybenzene/aldehyde derived gels: conventional carbon gels. 2.1.2. New forms of carbon gels: graphene and carbon nanotube aerogels.

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2.2. Templating: control of both the pore size and the tridimensional structure. 2.2.1. Exotemplating of ordered mesoporous carbons. 2.2.2. Endotemplating of ordered mesoporous carbons.

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2.3. Biomass-derived mesoporous materials: sustainable carbon gels/ordered mesoporous carbons. 3. Control of the surface chemistry.

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3.1. Post-synthesis functionalization. 3.2. Functionalization during-synthesis. 3.3. Phosphorous: the next heteroatom to be explored. 3.4. Biomass derived nanostructured carbons containing heteroatoms. 3.5. Bifunctional mesoporous materials. 4. Conclusion. *

Corresponding Author. E-mail: [email protected] (Marina Enterría) Telephone number: +351.22 041 4919

ACCEPTED MANUSCRIPT Acknowledgements. References.

Abstract

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Nanostructured mesoporous carbons are widely used in catalysis, adsorption and energy storage. Unlike conventional carbons, the textural properties of nanostructured carbons can be easily adjusted during synthesis. This design of the mesostructure allows to

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process large molecules by addressing the diffusional limitations/pore blockage which might occur in purely microporous carbon materials. Many synthetic approaches have

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been reported for the synthesis of mesoporous materials, either with a disordered mesoporosity (carbon gels), or with ordered porous systems (templated carbons). Carbon gels are synthesized by sol-gel processing through the polymerization of hydroxybenzenes and aldehydes. On the other hand, carbons with a periodically

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arranged mesoporous network can be obtained from the same precursors in the presence of templates, which may be rigid inorganic solids (hard-templating) or supramolecular assemblies (soft-templating). Controlling the surface chemistry is also a critical step in

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the development of high-performance materials; this can be achieved by doping with heteroatoms and functionalization with surface groups. The design of both texture and

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surface chemistry allows tuning the properties, thus leading to materials that meet the requirements of the targeted applications. The fundamentals, recent advances and the main challenges related to the preparation of nanostructured mesoporous carbon materials are presented in this review.

1. Introduction

ACCEPTED MANUSCRIPT Pores are inherently present in solid matter and play an important role on materials behaviour. Initially proposed by Dubinin in 1960 [1] and subsequently accepted by the International Union of Pure and Applied Chemistry (IUPAC) in 1985 [2], pores can be classified according to their size. Micropores have a width smaller than 2 nm,

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mesopores present sizes ranging from 2 to 50 nm, and macropores are those larger than 50 nm. Pores can also be classified according to their shape since they can present a large variety of geometries such as slit-shaped, cylindrical, spherical, conical, ink-bottle

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or interstitial. These classifications are quite relevant because adsorption processes largely depend on the size/geometry of both the pores and the targeted molecules to be

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adsorbed. Micropores are mainly responsible for the high adsorption capacity of carbon materials, and it can be generally established that micropores can accommodate a wide range of adsorptives from the gas phase. Adsorption of organic and bulky molecules can occur on mesopores, which can act also as diffusion channels towards the micropores.

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Finally, macropores have negligible impact on the adsorption capacity but can also favour the access of molecules to the internal surface of the solids. The emerging interest for mesoporous carbons arose during the past three decades due to the large

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mass transfer limitations in microporous carbon materials. The advent of carbon gels [3], presenting tuneable nanosized pores, boosted the interest for mesopore control and

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nanoarchitecture design. Carbon gels were prepared for the very first time by sol-gel processing through the polymerization of hydroxybenzenes and aldehydes. Such versatile design of a well-defined mesoporous system enabled the processing of large molecules and eliminated the diffusional limitations of conventional activated carbons. Furthermore, the discovery of ordered mesoporous carbons in the late 1990s [4] introduced a new concept in the field of porous materials: the tridimensional structure of the mesopores can also be designed. Thus, carbon materials with a periodically arranged

ACCEPTED MANUSCRIPT mesoporous network were obtained in similar conditions as carbon gels but in the presence of “nanotemplates”. These particular porous structures were found to enhance molecular diffusion. Molecules move more easily through ordered porous channels rather than through the tortuous pores of conventional carbon materials. Nevertheless,

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the increasing public concern with environmental protection triggered the interest for cleaner approaches [5] and sustainable carbon sources [6] which could replace the tedious synthesis procedures and/or toxic precursors used in the synthesis of carbon gels

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and ordered mesoporous carbons. Only three years ago, researchers of the Zhejiang University went a step further and presented the self-assembled graphene/nanotube

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carbon gels [7]. Unlike conventional carbon gels (disordered mesopores/amorphous carbon walls) or templated carbons (ordered mesopores/amorphous carbon walls), these newly developed materials yield a disordered but tuneable mesoporosity confined within highly structured carbon walls. The unusual properties of both carbon gels and

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templated carbons, together with the requirement of new living standards and societal demands, provoked an incredible interest for nanostructured mesoporous carbons. Consequently, carbons evolved from the conventional wide spectrum adsorbents to

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advanced materials for novel applications such as catalysis, microelectronics, medical diagnosis, sustainable energy production and storage, or environmental protection. On

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the other hand, most applications of carbon materials involve interfacial interactions between the carbon support and the adsorptive in gas or liquid phase. Keeping this in mind, the functionalization of carbon materials also became an important issue for materials science in recent years. The presence of heteroatoms (such as oxygen, nitrogen, boron, sulphur or phosphorus) in carbon materials enables controlling their electronic properties. In this context, the performance of carbon materials can be tuned by chemically attaching heteroatoms on their surface or by incorporating such atoms

ACCEPTED MANUSCRIPT into the carbon backbone. The role of surface chemistry is of paramount importance for carbon materials applications, since they usually involve the transfer of electrons between the carbon material and the surrounding fluid phase. Classically, surface modification is performed by post-treatments of the original carbon material either in

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oxidizing media or in the presence of the target heteroatom containing source. Such treatments can lead to considerable changes in the textural properties, and they also involve multi-step procedures. Consequently, the development of novel and efficient

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“functionalization during synthesis” strategies is highly attractive. However, as compared with conventional methods, one-pot procedures generally entail low

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functionalization degrees, limited control of the surface speciation and can even hinder the pore control. While doping carbons with oxygen or nitrogen has experienced an enormous progress, other heteroatoms have gained increasing interest during the last years. Thus, boron, sulphur or phosphorous have been found to induce interesting

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modifications on the surface properties of carbon materials. Moreover, bifunctionally or multifunctionally doped carbons, bearing more than one heteroatom in their surface/bulk, have recently emerged as very promising materials. In summary, whatever

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specific molecular interactions are needed for a given application, they usually occur as a result of specific geometric and electronic properties of the adsorbent and adsorptive.

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The design of both the texture and the surface chemistry would therefore enable the fine tuning of the carbon materials properties and, consequently, their performance for a given application. The development of novel synthetic procedures enabling the controlled modification of the amount, distribution and structure of the pores, as well as the nature and concentration of functional groups on the surface, would allow the preparation of advanced carbon materials for novel applications.

ACCEPTED MANUSCRIPT 2. Control of the mesoporosity and the nanostructure. 2.1. Carbon gels: control of the mesopore size. 2.1.1. Hydroxybenzene/aldehyde derived gels: conventional carbon gels. Mesoporous carbon gels are carbon materials yielding very narrow pore size

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distributions but randomly oriented mesopores. They are prepared by sol-gel synthesis through the polymerization of hydroxybenzenes (phenol [8, 9], resorcinol [10, 11], phloroglucinol [12] or cresol [13, 14]) with aldehydes (formaldehyde [11] or furfural

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[15]) and different catalysts (NaOH [11], Na2CO3 [16], HCl, acetic acid or hexamethylenetetramine [15]). Nonetheless, the most common gels are those prepared

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with resorcinol and formaldehyde as monomers and Na2CO3 as catalyst [3]. The general steps involved in the synthesis of carbon gels are: (a) Polymerization of resorcinol and formaldehyde in aqueous solution in the presence of a base catalyst, (b) gelation and ageing to promote aggregation and crosslinking, (c) drying and d) carbonization in inert

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atmosphere. As represented in Figure 1, the polymerization of hydroxybenzenes is carried out through two main stages [17, 18]. The first one is the addition reaction and consists in the formation of hydroxymethyl derivatives by hydrogen abstraction from

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phenolic groups. Subsequent nucleophilic attack on formaldehyde occurs through the 2, 4 and 6 positions in the aromatic ring (activated by –OH groups through resonance) of

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the as-generated phenolates. In the second stage, the formed hydroxymethyl derivatives condensate to form linear polymeric chains with different molecular weights (n monomers, Figure 1) by formation of methylene or ether bridges. As the polymerization proceeds, the linear resin becomes a robust polymeric network (Figure 1) by crosslinking of smaller polymer clusters. In the case of acid catalysts, the gelling process is slightly different from that represented in Figure 1. Hence, it is the aldehyde which forms first a carbocation and performs an electrophilic attack on hydroxybenzene

ACCEPTED MANUSCRIPT [15]. The thus formed reactive species condensate by methylene or methylene ether bridges. In both basic and acid media, the sol-gel transformation occurs from a few minutes to many hours depending on the preparation conditions. This large influence of the operation parameters on both the polymerization and the ageing stages enables a

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controllable design of the organic gels porosity. The customized texture of the hydrogel

can be transferred to the final carbon material by suitable drying and carbonization. Figure 1.

Polymerization stages of the organic gels synthesis and subsequent

crosslinking process.

ACCEPTED MANUSCRIPT There are many parameters affecting the properties of carbon gels, but it has been firmly established that the amount of catalyst (pH) is the main variable determining both the structure and texture [17, 19, 20]. Job et al. [21] demonstrated that, for a given set of synthesis variables, it is possible to tailor the mesopore size by fixing the initial pH in a

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narrow range. Hence, a pH below 5.45 leads to macroporous carbon gels, whereas a pH above 7.35 generates purely microporous solids. Between those pH limits, micro/mesoporous materials with a well-defined mesopore size are obtained. Within

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this range of optimum pH, the average mesopore size steadily decreases with the increase of the initial pH of the reaction mixture. Figure 2a [22] presents the nitrogen

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adsorption isotherms for two carbon gels with quite different textural properties which were synthesized under similar conditions, except for the pH (adjusted with NaOH solution) which was 5.6 for sample 37CXUA or 6.0 for sample 39CXUA. The specific surface areas of these samples resulted to be very similar (653 m2/g for 37CXUA and

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645 m2/g for 39CXUA), but the average mesopore diameters are quite different: 16 nm

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for sample 37CXUA and 4 nm for sample 39CXUA (Fig. 2b).

Figure 2. Textural properties of two carbon xerogels synthesized at different pH: a) nitrogen adsorption isotherms at 77 K; b) mesopore size distributions. Reprinted from:

ACCEPTED MANUSCRIPT J. L. Figueiredo, Boletín del Grupo Español del Carbón (ISSN 2172−6094), no 26, Dec. 2012, 12−17. http://www.gecarbon.org/boletin.asp

The observed evolution of the average mesopore size is easily explained bearing in

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mind the gelation mechanism. For strong basic media, the addition step is favoured due to the rapid generation of hydroxymethyl phenolates. These highly reactive species form very tiny and unstable clusters which tend to form a highly branched polymer network.

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A suitable balance between clusters growing (polymerization step) and crosslinking degree (ageing step) leads to micro/mesoporous gels (Figure 1b) consisting of a network

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of interconnected nanosized primary particles where the micropores are formed in the primary particles while the larger pores have their origin in the interparticle voids. An excessively high pH could derive in very tiny but highly crosslinked primary clusters leading to purely microporous gels (Figure 1c). In the case of acid medium, the addition

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step is slowed down resulting in barely interconnected clusters which tend to grow instead of crosslink each other. The thus formed colloidal suspensions (tiny particles) would produce meso/macroporous gels (Figure 1a). The dilution of the initial synthesis

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mixture (D) can also affect the aqueous gel characteristics since the primary clusters diffuse through the solution and tend to aggregate by colliding. Rey-Rapp et al. [23]

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performed an exhaustive study on the effect of both pH and dilution of the initial resorcinol/formaldehyde reaction mixture, and demonstrated that an increase in the dilution ratio (i.e. solvent/reactants ratio) increases the distance between clusters and prevents the organic gel from ageing. On the other hand, from a chemical point of view, an increase of the dilution entails decrease of the initial pH of the solution when a basic catalyst is used. Hence, the formation of a small number of large clusters (high dilution ratio and lower pH) leads to low density gels with large pores, whereas high

ACCEPTED MANUSCRIPT concentration of smaller clusters (low dilution ratio and higher pH) generates high density materials with smaller pores [24]. The in-depth study of the gelation mechanism performed by these authors enables the preparation of customized carbon gels with an average pore size ranging from 2 nm to 1000 nm by simply adjusting the range of pH-

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dilution ratio of the synthesis mixture. The type of catalyst used in the synthesis also influences the polymerization mechanism. Al-Muhtaseb et al. [25] revised the existing literature dealing with the effect of the catalyst nature on the textural characteristics of

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carbon gels. They concluded that the hydroxybenzene/aldehyde reaction depends mainly on the acidic or basic effect of the catalyst itself. Therefore, textural properties

taken

in

consideration

are

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would depend on the initial pH provided by the catalyst. Other factors that must be the

hydroxybenzene/catalyst

(H/C)

and

the

hydroxybenzene/aldehyde (H/A) ratios. By changing the H/C molar ratio, the mesoporosity of the polymerized gels (and therefore of the final carbon materials) can

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be modified; a low H/C ratio results in polymeric gels, whereas high H/C ratios give rise to colloidal gels (Figure 1). The polymerization rate greatly increases by decreasing the H/A ratio because formaldehyde promotes polymer crosslinking. On the other hand,

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an increase of the reaction time increases the probability of particles to aggregate [26] leading to more compact gels. Job et al. [27] observed that, for high catalyst

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concentration, the aging time has almost no influence on either the pore size or pore volume because the polymerization degree reaches an equilibrium. In this situation, the extent of polymerization depends on pH (i.e precursors/catalyst ratio), synthesis temperature or nature of the used monomers. Particularly, for low dilution ratios or high pH, the pore size decreases with increasing synthesis temperature since the reactivity of the primary clusters is increased. The same effect is observed by changing the nature of the monomers. The more reactive they are (higher electronic density in the aromatic

ACCEPTED MANUSCRIPT ring) the faster they polymerize/crosslink, leading to a decrease of the pore size. In this sense, –OH groups are known to activate the –ortho and –para positions of the aromatic ring by means of resonance. In that sense, an increasing number of hydroxyl groups in β positions results in enhanced reactivity of the aromatic ring. Similarly, the methyl

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groups also increase the basic character of the hydroxybenzene molecules by inductive effect. Bearing this in mind, the reactivity of the different hydroxybenzenes follows the sequence: phenol (hydroxybenzene) ˂ m-cresol (1-hydroxy-3-metylbenzene) ˂

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resorcinol (1,3-dihydroxybenzene) ˂ phloroglucinol (1,3,5-trihydroxibenzene). On the other hand, α-substituted hydroxybenzenes (1,2-dihydroxybenzene, 1-hydroxy-2-

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methylbenzene…) are more likely to originate linear polymers, resulting in lower density gels with larger pore size [28]. In order to transfer the textural characteristics of the designed wet gel to the stabilized carbon material, suitable drying/carbonization of the gel must be performed. When water is removed from the gel, the space that it used

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to occupy becomes accessible and porosity is generated. However, the strong capillary forces of water molecules going out of the solid can provoke the collapse of the tridimensional network. If the external surface shrinks much faster than the interior, the

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differential strain can cause cracking or even fracture of the material. Several strategies were proposed in order to minimize the damage of the designed structures during the

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drying step. Calvo et al. [18] thoroughly reviewed the different drying strategies that can be found in the literature comparing their advantages and disadvantages. In summary, ambient drying consists simply in evaporating the solvent at room temperature. To avoid damages in structure, water can be exchanged by another solvent with lower surface tension (i.e. acetone). To avoid drastic shrinkages, Pekala et al. [3] proposed the supercritical drying of the hydrated organic gel. It consists in heating the gel under pressure beyond the critical point of the solvent and, at this point, the solvent

ACCEPTED MANUSCRIPT is exchanged by inert gas. The last method of drying organic gels is freeze drying [29]. Thus, the solvent is frozen and the gel is dried by sublimation at low pressure. Both supercritical drying and freeze drying produce highly porous dry gels which maintain the initial nanostructure with very low shrinkage. On the other hand, it was commonly

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accepted that ambient drying would destroy the pore texture of phenolic gels. However, Job et al. [21, 30] demonstrated that porous carbon xerogels with a good preservation of the porous system can be also obtained by evaporative drying of water by correctly

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selecting the operating variables (controlled temperature, drying rate and humidity). To finally obtain a carbon material, the prepared organic resins must be pyrolized at high

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temperature in order to remove oxygen and hydrogen atoms to increase the material aromaticity. Regarding the drying procedure (supercritical drying, freeze-drying or ambient drying) carbon gels are generally referred as "aerogels", "cryogels", or "xerogels", respectively. Nevetheless, the term “aerogel” is not always used in

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connection to supercritical drying. Indeed, it has also been used for low-density gels regardless of the drying method, which in fact, is the case of the graphene/nanotube aerogels (Section 2.1.2). Generally, organic aerogels are carbonized under inert

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atmosphere between 600-900 ºC with heating rates of 5-15 ºC/min in order to prevent collapse of the structure. Lin et al. [31] studied the impact of carbonization variables on

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the carbon xerogels structure and observed that increasing the carbonization temperature leads to a decrease of the surface area, but does not affect the mesopore volume. All the mentioned drying methods present advantages and disadvantages, and should be selected according to the desired textural properties. In terms of industrial scalability, evaporative drying without any preliminary treatment is obviously the most attractive drying procedure due to its simplicity and low-cost. It was recently discovered that the use of cationic surfactants during synthesis stabilizes the pore structure during solvent

ACCEPTED MANUSCRIPT removal [32-34]. Hence, surfactants adsorb on the hydrogel reducing the interfacial tension and preventing collapse of the pores during water removal. Concerning the carbonization of the dry organic gels, higher temperatures [35] or heating rates [36] result in a notable decrease of the adsorption capacity due to the decrease of the

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micropore volume. On the other hand, the carbonization temperature does not present significant effect on the mesoporosity, but a high heating rate could provoke the collapse of mesopores [35]. Generally, carbonization ramps around 5-10 ºC/min

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generate carbon gels with specific surface areas in the 500-700 m2/g range. Depending on the desired application, the carbonization extent must be adjusted, since the

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electrochemical conductivity of the final material greatly depends on this stage. Alternatively to conventional carbonization, Calvo et al. [37] described a microwaveassisted synthesis of tailored mesoporous carbon gels. The preparation of mesoporous carbon gels with similar characteristics to those prepared through conventional methods

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was made feasible in a few hours. The synthesis, drying and carbonization steps are carried out simultaneously, entailing considerable savings of time and energy. As an additional value, the authors observed that this newly developed technique extends the

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range of pH in which a micro/mesopore gel is obtained.

2.1.2. New forms of carbon gels: graphene and carbon nanotube aerogels. Beyond conventional carbon aerogels, the preparation of tridimensional structures from carbon nanotubes or graphene sheets has been reported. The self-assembly of this kind of nanostructures in aqueous solution to form a gel phase leads to unusual materials with extremely low density and controlled mesoporosity. These highly attractive materials are obtained by i) suitable dispersion of the graphene sheets and/or carbon nanotubes, ii) gelling the dispersion by physical or chemical assembly of the

ACCEPTED MANUSCRIPT nanostructures and iii) drying the obtained gel. Carbon nanotubes are inherently hydrophobic so a previous oxidation or the use of surfactants will facilitate their dispersion. Thus, highly dispersed carbon nanotube suspensions can be prepared by using poly(p-phenyleneethynylene)s [38] or sodium dodecylbenzene sulfonate [39] as

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surfactants. The dispersed nanotubes can self-assemble to form a tridimensional gel structure either by physical or chemical procedures. Hence, the crosslinking of the carbon nanotube units is promoted by decreasing the solvent concentration [40] or by

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the use of chemical binders [41]. The fabrication of self-assembled graphene structures is also possible following the same overall procedure. The controllable reduction of

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graphene oxide dispersions generates tridimensional porous structures by crosslinking of the graphenic sheets. Cheng et al. [42] studied the effect of different reduction agents such as NaHSO2, Na2S, Vitamin C or hydrazine, and observed that the reaction time required for gel formation varies as function of the selected reductant. Thus, fast

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gelation times are obtained with Na2S and Vitamin C, while no tridimensional structures are obtained with hydrazine. On the other hand, Zhang et al. [43] reported an interesting fabrication method for the preparation of carbon gels using hydrothermal reduction of a

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combined graphene oxide/carbon nanotube suspension without using any reducing agent. A hydrothermal treatment causes the crosslinking of the sheets by condensation

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of the hydroxyl and carboxylic groups present either at the edges or on the defects of the graphene lattice. By adjustment of the hydrothermal treatment time, the extent of gelation can vary and different textures are obtained in the final carbon materials. The final texture of graphene aerogels strongly depends on the assembly process and the binding conditions. Thus, the textural properties of the wet gels depend on the initial concentration of building units, the evaporation conditions (physical gelation), the binder nature/proportion (chemical gelation of nanotubes) or the reducing agent

ACCEPTED MANUSCRIPT nature/concentration (chemical gelation of graphene oxide). Therefore, the density and the mesoporosity of the final gels can be adjusted by properly selecting the crosslinking parameters. The drying step is critical for the preservation of the porosity generated during the preparation of the tridimensional gel. The inert nature of the used “building

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blocks” (either carbon nanotubes or graphene) reduces the interconnection of the structures as compared with conventional aerogels which, therefore, are very likely to collapse. Consequently, both supercritical drying [44] and freeze drying [45] are

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generally used to stabilize graphene or carbon nanotube gels. A suitable selection of the gelling/binding conditions, together with careful drying, allows the synthesis of

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extremely low density carbon gels with tuneable porosity and highly ordered carbon walls.

2.2. Templating: control of both the pore size and the tridimensional structure.

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Templating is one of the most powerful techniques for preparing materials with ordered and uniform pores [46-48]. Highly structured porous networks are obtained by using ordered porous solids as moulds (exotemplating) or molecules (endotemplating) to

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obtain inverse carbon replicas [49].

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Figure 3.

Schematic representation of exotemplating (template represented by a

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square) and endotemplating (template represented by triangles).

In the case of exotemplating, a rigid inorganic solid is used as template to create another solid confined in its porous network. By contrast, endotemplating consists in occluding

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atoms, molecules or supramolecular structures in the prepared solid framework. In both cases, when the template is removed the space becomes accessible and an ordered pore

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system is developed. The thus obtained carbon materials inherit the tridimensional structure of the parent template, with highly ordered porous networks and amorphous carbon walls. Since the used templates are crystalline solids or liquid crystals, the pore size distributions of the carbons replicas are very narrow, this being an excellent

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approach for the control of the pore size.

2.2.1. Exotemplating of ordered mesoporous carbons

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The exotemplating of carbon materials is generally referred as “nanocasting” or “hard

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templating”, since rigid inorganic solids are used as templates. The earliest synthesis of a hard-templated carbon was reported in 1982 by Gilbert et al. [50]. They obtained a carbon with a well-defined macroporosity by polymerizing a resin around the spherical particles of a commercial silica (Porasil ®). Henceforth, several types of templates for the mesoporous carbons production were reported [51]. For instance, Kyotani et al. [52] used 2D montmorillonite as template and demonstrated that some carbon precursors, such as polyfurfuryl alcohol, can be graphitized in the confined 2D spaces between inorganic lamellas of this layered clay. The same group demonstrated that, by

ACCEPTED MANUSCRIPT using an anodic aluminium oxide (AAO) film as a hard template, it is possible to produce a periodic macropore network in carbon materials [53] or carbon nanotubes with uniform diameter (20-200 nm). Despite the wide variety of materials that can be used as hard-templates, ordered mesoporous silicas readily become the most used due to

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their particular structural characteristics, their stability and their reproducible preparation [4, 47, 48, 54]. Ordered mesoporous silicas can be prepared by the sol-gel method with the assistance of surfactants as structure directing agents [55]. As indicated

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in Figure 4, this approach involves the i) synthesis of the silica template (Fig. 4a), ii) infiltration of the template with a suitable carbon precursor and subsequent

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polymerization inside the porous network (Fig. 4b), iii) carbonization of the silica/ carbon composite (Fig. 4c) and iv) removal of the template by treatment with HF or

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NaOH (Fig. 4d).

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Figure 4. General procedure of nanocasting method. Adapted from reference [46].

Bearing this in mind, the mesoporous structure of the final carbon can be easily tuned by changing the parent silica template. The versatility of the technique derives from the wide range of silica structures which can be obtained in a reproducible way. After the synthesis of the first mesoporous silica by Beck et al. [56] in 1992, many others were reported: MCM [56], KIT [57], SBA [58], HMS [59], MSU [60], FSM [61], AMS [62], FDU [63] or HOM [64]. The infiltration of the silica template is the most critical stage

ACCEPTED MANUSCRIPT for the replication success. The carbon precursor must have suitable dimensions and favourable interactions with the silica walls [65]. A correct penetration of the precursor into the porous channels can be achieved either by liquid or gas-phase infiltration. Liquid infiltration consists in impregnating the template with a solution of the carbon

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precursor. Wet impregnation is carried out in excess volume of solution, while a volume of solution equal to the pore volume is used for incipient wetness. Some of the most used precursors are sucrose, furfuryl alcohol, phenolic resins, acrylonitrile or pitch.

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Depending on the carbon precursor, the polymerization step may require the presence of a catalyst [4]. On the other hand, gas-phase infiltration can be carried out by chemical

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vapour deposition (CVD). Hydrocarbons such as benzene, methane, propylene or acetylene are thermally decomposed at high temperature inside the porous network of the template [66]. Finally, the composites obtained by the different infiltration methods are carbonized at 700-900 ºC and subsequently treated with HF or NaOH in order to

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remove the silica template. The first synthesis of an ordered mesoporous carbon using an ordered mesoporous silica as template was reported by Ryoo et al. [4]. Sucrose was converted to carbon inside the mesopores of MCM-48 using sulfuric acid as

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polymerization catalyst. Nevertheless, the obtained carbon material was not a precise replica of the parent template [67]. The first faithful carbon replica of a silica template

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was achieved by Zhao et al. using SBA-15 as template [68]. The structure of SBA-15 was thought to be similar to MCM-41 but the latter generated disordered microporous carbons [69]. Some years ago, Imperor-Clerc et al. [70] discovered that in order to transfer the structure from the silica template to the carbon replica the former must present interconnected mesoporous channels. Afterwards, Ryoo et al. reported the preparation of high-quality cubic ordered mesoporous carbons using KIT-6 [57] as template, instead MCM-48. The main difference between both structures is that KIT-6

ACCEPTED MANUSCRIPT presents interconnections between the mesoporous channels. The importance of this factor can be easily appreciated in Figure 5. The XRD pattern of a carbon derived from SBA-15 (CMK-3) reveals the preservation of the hexagonal p6mm structure of the template (Fig. 5a). However, the carbon derived from MCM-48 displays patterns

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different from those of template (Fig. 5b). The enantiomerically paired channels of MCM-48 silica are not connected [4] and, after template removal, the two chiral carbon

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bars change their spatial arrangement (Ia3d) to a new ordered cubic structure (I41/a,).

Figure 5. Importance of the interconnection of the mesoporous channels in the silica

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templates. Low-angle XRD patterns of a) SBA-15 and its carbon replica (CMK-3) and

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b) MCM-48 and its carbon replica (CMK-1).

Hard-templating permits a strict control of the porosity in a wide range of pore sizes. Figure 6a displays the nitrogen adsorption isotherms of SBA-15, the silica-carbon composite after infiltration and the resulting carbon after template elimination (CMK3). Figure 6b shows their corresponding pore size distributions. When SBA-15 is infiltrated with propylene at 750 ºC during 6 h, a substantial decrease of pore volume is observed (compare SBA-15 and silica-carbon composite isotherm). Homogeneous

ACCEPTED MANUSCRIPT infiltration and high carbon loading (50 wt. %) take place with no carbon deposit on the external surface of the particles (Fig 7b) Both specific surface area and mesopore volume increase after template removal (see CMK-3 isotherm) and a very narrow pore

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size distribution is obtained (Fig. 6b).

Figure 6. a) Nitrogen adsorption isotherms of SBA-15 (squares), the silica-carbon composite after propylene infiltration (line) and the carbon replica (circles) and b) their

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corresponding pore size distributions calculated by QSDFT.

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Parmentier et al. [71] studied the influence of the carbon precursor nature, the infiltration degree and the infiltration method on the final properties of the prepared

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carbons. Infiltrations above 50 % lead to the deposition of the carbon precursor at the external surface of the template and, consequently, amorphous carbons are obtained. They fixed the infiltration degree at 36% and prepared one carbon with glucose by wet impregnation, a second carbon with propylene by CVD method and a third carbon with pitch by incipient wetness. Both liquid impregnation of glucose and gas phase carbon deposition from propylene provide ordered mesoporous carbons. Nevertheless, liquid impregnation produced carbons with higher surface areas due to the release of volatile species during the carbonization of the sucrose. In contrast, the use of pitch as carbon

ACCEPTED MANUSCRIPT precursor results in lower surface areas and mesopore volumes. The large-size pitch molecules deposit preferentially on the surface of the template instead of inside the mesopores. In spite of the largest surface areas, liquid infiltration is not much homogeneous and can cause pore-blocking. CVD achieves highly homogeneous

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infiltrations leading to dense carbons with lower microporosity but high mesopore volumes. Figure 7 shows the Scanning Electron Microscopy (SEM) and the Transmission Electron Microscopy (TEM) images for a lab-prepared ordered

with propylene (CMK-3).

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mesoporous silica (SBA-15) and its corresponding carbon replica prepared by CVD The carbon replicas obtained by nanocasting not only

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preserve the porosity of the parent template but also faithfully reproduce their morphology and microstructure. As can be observed by SEM imaging, the morphology of the final ordered mesoporous carbon (Fig. 7b) is indistinguishable from the template (Fig. 7a). The observation of the carbon structure in TEM reveals also the replication of

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7d).

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the honeycomb-like porous structure from SBA-15 (Fig. 7c) to the carbon material (Fig.

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ACCEPTED MANUSCRIPT

Figure 7. SEM images of a) SBA-15 and b) CMK-3 and TEM images of c) SBA-15

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and d) CMK-3.

Nanocasting is a very versatile and easily-controlled technique providing high density

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carbons bearing a precise replication of the nanomould. Nevertheless, as deduced from the described procedure, it presents some limitations like multi-step synthesis (prior

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synthesis and subsequent elimination of the template) and the use of highly corrosive HF. These aspects hinder the industrial scaling-up and green manufacture of hard templated ordered mesoporous carbons. As a consequence, the direct preparation of ordered mesoporous carbons using flexible molecular aggregates as templates has been the focus of intense investigation during the last decade.

2.2.2 Endotemplating of ordered mesoporous carbons.

ACCEPTED MANUSCRIPT The synthesis of ordered mesoporous materials using surfactants as endotemplates is commonly known as “soft-templating” or “self-assembly”. Similarly to carbon gels, this technique uses the sol-gel technology to obtain organic resins as parent materials of shaped carbons. In this context, the aspects influencing the preparation of carbon gels

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ordered mesoporous carbons by soft-templating.

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(polymerization and ageing) can be also taken into consideration for the preparation of

Figure 8. Schematic representation of the soft- templating method with surfactant

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micelles as template.

The soft-templating method involves three main steps: i) polymerization of the carbon

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precursor in the presence of a structure directing agent (Fig. 8a), ii) elimination of the

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template by heat treatment (Fig. 8b) and ii) carbonization of the obtained polymeric resin (Fig. 8c). Considering that the elimination of the template and the carbonization stages can be carried out consequently, the number of steps is considerably reduced as compared with hard templating. Nonetheless, soft-templating is not as predictable as nanocasting since the templates are not fixed structures, and largely depends on synthesis conditions. Surfactants have been extensively used as “structure directing agents” in the direct preparation of ordered mesoporous carbons due to their amphiphilic nature. They form micelles in the bulk aqueous phase; the hydrophilic

ACCEPTED MANUSCRIPT portions are oriented outwardly (attracted to the water molecules) and the hydrophobic portions meet inwardly forming the core of the aggregate. The peculiarity that makes surfactants versatile directing agents is the formation of highly ordered mesophases by micelles under certain conditions. These phases bear intermediate properties between

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glass and liquid and their structure/geometry can be easily changed by changing the

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reaction medium (Fig. 9).

Figure 9. Surfactant geometric parameters and resulting structures.

Israelachvili et al. [72] presented the first model that rationalized the dependency of the micelles structure on different synthesis conditions (Fig. 9). They defined a micelle packing parameter (g) which predicts the phase transitions in micelles. The effective volume of the surfactant chain (v), the aggregate surface area associated with the

ACCEPTED MANUSCRIPT hydrophilic head (ao) and the effective length of the hydrophobic tail (l) determine the liquid crystal spatial conformation. These parameters are not constant and depend on several conditions (nature and concentration of the reactants, the pH, the temperature or the reaction time), enabling the design of flexible templates in solution [73] and,

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consequently, the design of different porous materials by “liquid crystal templating” [74]. The success of this templating approach requires that two circumstances occur simultaneously; i) the template has to acquire a stable liquid-crystal structure and ii) the

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carbon precursor must form a highly cross-linked polymeric network around the organic template. On this basis, a suitable interaction between the surfactant and the carbon

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precursor is of major importance. Appropriate interactions will provoke the migration of the carbon monomers towards the micelles, which subsequently polymerize occupying the voids between them. All the above mentioned circumstances are likely to happen in a very narrow operation range, evidencing the limitations of this technique. For

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instance, the carbon precursor should not be able to interact with the soft-template in the same conditions in which it is capable to polymerize. Strong acidic media are usually needed for hydrogen bonding (main interaction in soft-templating) while the formation

EP

of crosslinked polymers is more likely to occur in basic media, as discussed in section

AC C

2.1.1. Surfactants can be classified according to the state in which they are in solution [75]. Thus, there are cationic, anionic and non-ionic surfactants depending on whether the polar head has positive, negative or neutral charge. Although non-ionic surfactants have non ionizable groups such as alcohols, ethers or esters, they are polarisable. In this context, ionic surfactants interact with carbon precursors by electrostatic interactions [81] while non-ionic surfactants establish interactions based on hydrogen bonds [59, 77]. Generally, ionic surfactants are not used as templates for the synthesis of ordered mesoporous carbons because they have very weak interactions with organic polymers

ACCEPTED MANUSCRIPT and they are difficult to remove after synthesis (high thermal stability) [78, 79]. The first successful synthesis of ordered mesoporous carbons by soft templating was reported in 2004 by Dai et al. [80] using a lab-prepared polystyrene-block-poly(4vinylpyridine) (PS4V). The vinylpyridine section interacts with resols by hydrogen

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bonding and induces the organization of the monomers around the template. Contrary to PS4V, amphiphilic triblock copolymers consisting of polyethylene-polypropylene oxides (PEO-PPO-PEO, Pluronics®) are commercially available and cheap. They are

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able to interact with organic carbon precursors in strong acidic media by hydrogen bonds and decompose without significant carbon residue by thermal treatment. Carbon

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precursor monomers prefer to interact with the more hydrophilic PEO segments of the Pluronics® rather than with the hydrophobic PPO segments. Pluronic® F127 presents a high PEO/PPO ratio [81] and consequently is broadly used as structure directing agent. Similarly to carbon gels, hydroxybenzenes and aldehydes (phenol/formaldehyde,

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resorcinol/formaldehyde and phloroglucinol/formaldehyde) are the most common carbon precursors in organic-organic self-assembly. They have great ability for thermosetting and greatly interact with the structure directing agents that are used in

EP

templating. Due to the high hydroxyl group density, phenol, resorcinol and phloroglucinol can form single, double and triple hydrogen bonds with polyethylene

AC C

oxides, respectively. As mentioned, the organic-organic self-assembly requires hydrogen bonds to provide interaction between carbon precursor and surfactants. (Fig. 10a). Nevertheless, a strong acidic medium is not always mandatory; directional forces can be applied to the carbon precursor monomers in order to orient them towards surfactant micelles (Fig. 10b). Hence, by concentrating (concentration gradient) or autoclaving (pressure gradient) the reaction mixtures, close H-bonding can be promoted in neutral or basic medium (Fig. 10b and Table 1). The polymerization of

ACCEPTED MANUSCRIPT hydroxybenze-derived resins can be performed either in basic or acidic conditions but it is in the former medium where it occurs faster. The control of the polymerization rate is a key point in direct templating of ordered mesoporous carbons. Fast polymerization will make the carbon precursor to polymerize independently from the template,

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producing amorphous materials. On the other hand, a very slow polymerization will produce weak structures, very likely to collapse during template removal. As happened in mesoporous carbon gels (Section 2.1.1), the polymerization/ageing of the resins can

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be controlled by modification of the synthesis parameters such as pH, temperature,

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reaction time, type of monomers, H/C ratio or H/A ratio.

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Figure 10. Existing driving forces in organic-organic self-assembly: a) hydrogen bonds

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and b) concentration gradient.

The formation of ordered mesostructures involves two competitive processes; i) addition/condensation reactions to form the polymeric resin and ii) organic–organic self-assembly between the resin oligomers and the surfactant. Keeping this in mind, Table 1 summarizes the different approaches to prepare ordered mesoporous carbons by soft templating [82].

ACCEPTED MANUSCRIPT

Driving force

Precursor/conditions

EISA (Evaporation Induced Self-Assembly)

Concentration gradient

Phenol/ basic or neutral Resorcinol/neutral or acid

Hydrothermal autoclaving

Autogenous pressure

Phenol/ basic Resorcinol/acid

Diluted route

Hydrogen bonds

Phase separation

Hydrogen bonds

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Method

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Phenol/ basic

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Resorcinol/acid Phloroglucinol/acid

Table 1. Existing approaches to prepare ordered mesoporous carbons by soft

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templating.

The evaporation induced self-assembly method (EISA) consists in casting thin films of the synthesis solution by coating a planar surface [80]. The solvent evaporates and the concentration gradient induces the ordering of precursors around templates. Generally, it is performed either in basic or neutral medium using phenol and formaldehyde as carbon precursors. Prior to casting, a low-molecular-weight copolymer of phenol and formaldehyde is prepared with sodium hydroxide as a catalyst. The as-made resol is neutralized and poured into an ethanolic solution of Pluronic®. At this stage, the resol

ACCEPTED MANUSCRIPT oligomers interact with the surfactant in order to assemble into a polymeric mesostructure. Further progressive evaporation of the solvent (increase of the surfactant concentration) would favour the formation of H-bonds between the carbon precursor and the template [83-85]. The polymerization rate is easily controlled when phenol is

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used as carbon precursor in EISA. Resorcinol can also be used as carbon precursor but polymerization must be performed in acid medium because of its higher reactivity [86] (Table 1). Although EISA is a powerful tool for the preparation of ordered mesoporous

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carbons, it involves the evaporation of a large amount of organic solvents and it is very sensitive to the casted film thickness. Disordered non-porous carbons are obtained by

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simply thickening the solution film some few nanometers. To avoid this inconvenient, the planar surface can be replaced by a polyurethane or silicon substrate [86]. Their tridimensional macroporous structure provides large voids and interfaces for the selfassembly of the ordered mesostructures. The problem could be also solved by exposing

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the block copolymer-carbon precursor solution to formaldehyde gas for an “in situ” polymerization and cross-linking [80]. The interaction between carbon oligomers and the soft template can be also enhanced by applying an autogenous pressure (Table 1).

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Hydrothermal autoclaving favours the self-assembly of carbon precursors and template in neutral or basic media as well as the polymerization of the carbon precursors. Huang

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et al. [87] succeeded in preparing a highly-ordered mesoporous carbon with the assistance of hydrothermal polymerization using phenol as carbon precursor. A basic mixture of phenol and formaldehyde was heated up to 60-80 ºC to prepare a resol precursor. Subsequently, a neutral solution of Pluronic® was added to the mixture and transferred to an autoclave. A dense polymeric resin was obtained after overnight treatment at 100 ºC. Similarly to EISA, phenol is the most commonly used precursor for autoclaving since the polymerization of either resorcinol or phloroglucinol is barely

ACCEPTED MANUSCRIPT controllable under high pressures. Nevertheless, Lui et al. [88] demonstrated that the use of resorcinol for autoclaving methods was possible. They obtained a resorcinol-derived ordered mesoporous carbon by a “Low Temperature Autoclaving Method” (LTA). In this approach, an acidic aqueous mixture of resorcinol and formaldehyde is polymerized The mild

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without any pre-polymerization stage inside an autoclave at 50 ºC.

hydrothermal treatment permits the control over the resorcinol/formaldehyde polymerization rate. Despite the low polymerization degree of phenol in diluted media,

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Zhang et al. [89] achieved the preparation of ordered mesoporous carbons with phenol without solvent evaporation or directing gradient. This strategy, named as “diluted

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route” (Table 1), follows the same fundamentals as the autoclaving method, but the pH under which the synthesis is performed is highly limited. A pre-polymerization of phenol and formaldehyde is performed in basic medium previous to the addition of an aqueous neutral solution of Pluronic®. At that stage, the pH must be adjusted; values

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below 8 lead to very slow polymerization of phenol; values between 8 and 9 provide adequate polymerization rates as well as enabling the formation of weak hydrogen bonds between Pluronic® and the carbon precursor; finally, values above 9 hinder the

EP

formation of any hydrogen bonding. Liang et al. [90] extended the applicability of Zhang´s approach by eliminating the strict pH limitation. The alcoholic end-groups of

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the block copolymer F127 can be oxidized “in situ” to aldehydes enhancing the interactions with the surfactant. In this way, the template can form stable covalent bonds with the phenol/formaldehyde resols during the self-assembly process in a wide range of pH. Subsequently, other authors extended the principle developed by Zhao and tried to prepare ordered mesoporous carbons using resorcinol [91] and phloroglucinol [92] as carbon precursors using the “one-pot” strategy. They discovered that both resorcinol and phloroglucinol undergo macroscopic phase separation without the need for a pre-

ACCEPTED MANUSCRIPT polymerization stage. Hence, these highly reactive hydroxybenzenes are able to assemble “one-pot” with the block copolymer simultaneously forming a dense polymer around the template. An acidic medium would provide resorcinol or phloroglucinol with protons to interact with the template as well as controlling the polymerization rate.

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Phloroglucinol is the most reactive hydroxybenzene and, consequently, the phase separation occurs at room temperature from few minutes to hours depending on the conditions. In the case of resorcinol, two different phases are only observed after some

SC

days [91]. Nevertheless, mild heating (50 ºC) of the initial reaction mixture leads to resorcinol polymerization in shorter times (hours) [93]. Because of its simplicity, the

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phase separation method has been widely used for the preparation of ordered mesoporous carbons since their discovery to the present day. Regardless of the selected synthesis approach (Table 1), the average mesopore size and the tridimensional arrangement of the pores can be tuned by simply varying the synthesis parameters. The

TE D

modification of pH, type of carbon precursor, reagent proportions, catalyst concentration, temperature or solvent can change both the micelles spatial organization (structure) and area (pore size). In addition, the modification of the operation conditions

EP

changes the polymerization behaviour of the carbon source leading to differences in the materials density. In this regard, the textural properties of the final carbon materials can

AC C

be adjusted by optimization of all the above mentioned variables. Hence, the pore size can be varied from 4 to 19 nm and the structure can be designed from hexagonal (p6mm), cubic bicontinuous (Ia3d), body centred cubic (Im3m), lamellar to wormlike. For a given method, differences in the pore size are generally small (no more than 2 nm) limiting the design of ordered mesoporous carbons over the wide nanoscale. Some advances have been achieved by using mixtures of Pluronics® [85, 87, 94, 95] since the size of the micelles depends on the length of the PPO hydrophobic segment of the

ACCEPTED MANUSCRIPT surfactant. The use of copolymers with a different PEO/PPO ratio (F127, F108 or P123) or even their mixtures, enables to obtain different pore sizes. On the other hand, organic molecules with long hydrocarbon chains such as decane or hexadecane can solubilise inside the micelles and expand their size [96]. Figure 11 displays the nitrogen

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adsorption isotherms and the corresponding pore size distributions of a series of carbons prepared in our group by phase separation and using different resorcinol/Pluronic® F127 ratios (R/P). The dependency of the pore size distribution on the concentration of

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carbon precursor is clearly observed. The average mesopore size increases from 4.6 to 6.5 nm by simply increasing the R/P ratio. The proportion of resorcinol changes the

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strength of the hydrogen bonds which govern self-assembly producing the expansion of the micelles. Regarding the tridimensional structure of the mesopores, Figure 12 shows the TEM images of a carbon series prepared through the EISA method using phenol/formaldehyde resins. The structure can be varied from three-dimensional

dimensional

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bicontinuous (Ia3d, Fig. 12a) to body centered cubic (Im3m, Fig. 12b) or twohexagonal

(p6mm,

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EP

phenol/Pluronic® 123 ratio.

Fig.

12c)

only

by

simply

adjusting

the

ACCEPTED MANUSCRIPT Figure 11. a) Nitrogen adsorption isotherms at 77 K of a series of ordered mesoporous carbons prepared by the diluted route with different resorcinol/ F127 proportions and b)

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their corresponding pore size distributions calculated by QSDFT.

Figure 12. TEM images of a family of carbons prepared by the EISA method with a)

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Ia3d b) Im3m and c) p6mm structures. Adapted from [85], with permission from ACS. Regardless of the used strategy, suitable curing, drying and carbonization of the organic

EP

resins are necessary to obtain high quality ordered mesoporous carbons. The organic resins obtained by soft-templating in acidic media are generally low density polymeric

AC C

structures. In that sense, a curing step is necessary to promote crosslinking, enhancing their density. After the appearance of the insoluble organic phase in the aqueous reaction medium, the remaining solvent is decanted off and the polymeric phase is heated up to 80-100º C either at atmospheric pressure or in vacuum during one day. The drying step after curing the resins is not as critical as in carbon gels, since the template prevents the collapse of the designed structure. The solid resin is dried in an oven around 100 ºC during additional 24h. The dry and dense phenolic resin is finally transformed into a carbon material by heating in a furnace in inert atmosphere. The

ACCEPTED MANUSCRIPT template removal and carbonization steps can be performed in the same treatment. In this regard, the heat treatment is normally performed in two stages; i) increase of temperature with a very slow ramp (1 or 2 ºC/min) up to 550 ºC in order to decompose the organic template during 5 h and ii) further increment of the temperature (700-1000

carbonization of the ordered mesoporous organic resin.

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ºC) at higher heating rates (5-10 ºC/min) during some hours to perform the

2.3. Biomass-derived mesoporous materials: sustainable carbon gels/ordered

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mesoporous carbons.

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Despite the versatility of soft-templating and sol-gel preparation of carbon gels, the available literature is still dominated by the use of hydroxybenzenes and aldehydes as carbon precursors and Pluronics® as structure directing agents. The use of novel structure directing agents, carbon precursors or self-assembly mechanisms constitutes a great challenge in the synthesis of nanostructured mesoporous carbon materials. The

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exclusivity of hydroxybenzenes in the preparation of mesoporous carbons derives from their ability to polymerize and thermoset as well as interacting with the block

EP

copolymers used in templating approaches. Nonetheless, the toxic (or even carcinogenic) nature of this type of compounds prompted the scientific community to

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search for more environmental friendly carbon precursors. Biomass-derived products (pure carbohydrates and lignocellulosic materials) are considered as potential carbon sources to produce valuable porous carbon materials. They are a renewable resource available in high quantities, they are inexpensive and in many cases they are waste products from industry. The reader is referred to the reviews presented by White and Titirici [97, 98] reporting on the production of porous carbon materials from biomass resources. The utilization of biomass to produce porous materials has met with limited success, since the low degradation ability of complex polysaccharides produces non-

ACCEPTED MANUSCRIPT porous carbon materials, and the poor reactivity of simpler carbohydrates (i.e., glucose, sucrose, fructose, starch, etc) originates carbons suffering from poor mechanical resistance. Alternative to conventional polymerization/condensation approaches, Budarin et al. [6] presented in 2006 a novel strategy for the preparation of mesoporous

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carbon gels. They observed that polysaccharides undergo retrodegradation by inducing them to an expanded hydrogel state for further self-association in controlled conditions. These newly developed materials were designated as “Starbons®” since the first carbon

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was made from starch. The steps involved in the preparation of Starbons® can be summarized as; i) polymer expansion by gelatinization in aqueous solution, ii) slow

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recrystallization of the polymeric chains, iii) solvent exchanging (water with ethanol or acetone) iv) drying and v) thermal carbonization/dehydration. In the gelatinization step, the dense polymeric network of the polysaccharides becomes hydrated and swollen leading to an expanded gel structure. The structure disaggregates into small crystalline and breaking of hydrogen bonds

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domains by collapse of the structures

(retrodegradation). If the solution is cooled down or dried and heated, the disaggregated clusters aggregate again into partially ordered domains different from the initial ones

EP

(recrystallization). The extent of this disaggregation and further recrystallization depends on the temperature or duration of treatment. The first carbon gel derived from

AC C

starch was prepared by heating amylose corn starch in water and cooling the solution to 5 ºC during two days [6]. The water was then exchanged with ethanol, acetone, and oven-dried. The carbonization of the gel must be performed in the presence of an acid catalyst, otherwise the polymer will melt before dehydration. The starch gel was therefore impregnated with a catalytic amount of p-toluene sulfonic acid prior to heating under vacuum to various temperatures up to 700º C. In 2011, the Starbon® concept was extended to the preparation of carbon gels derived from pectin [99] and alginic acid

ACCEPTED MANUSCRIPT [100]. The porous pectin gel was prepared by gelling the polysaccharide in water at 90º C during 2 h and further recrystallization at 5 ºC during 24h. Since pectin gels can either be formed through thermal gelation or by lowering the system pH, the authors also performed a chemical gelation. Pectin was dissolved by stirring a small quantity of the

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polysaccharide in an acid aqueous solution (HCl, 0.5 M). The as-obtained gel was cured at room temperature during 48 h to yield a strong solid gel structure. On the other hand, the alginic acid carbon gel was prepared by mixing the polysaccharide in water and

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heating the solution at 90 ºC during 2 hours inside an autoclave. The resulting gel was physically recrystallized at 5 ºC during 24 h. As an advantage, the impregnation with an

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organic acid previous to the carbonization could be avoided both for pectin and alginic acid because these two new polysaccharides present inherent acidic nature (low pKa). The textural properties of retrodegradated polymers are largely related to the extent of gelatinization and subsequent duration and temperature of storage, enabling the design

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of mesoporous carbon gels derived from green carbon precursors. Dodson et al. [101] succeed in preparing organic gels directly from fresh algae. The method is very simple and involves the swelling of the milled raw material in water. The expansion of the

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polymer was performed in water during 24 h at room temperature. The solvent was exchanged by ethanol several times and subsequently with CO2. The gas was carefully

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evacuated in vacuum overnight to yield highly porous organic gels. In addition to the Starbon® approach, other strategies have been reported for the preparation of carbon gels from simple carbohydrates. It has been found that robust polymeric structures can be obtained from carbohydrates in solution by using certain polymerization/ condensation promoters. Hydrothermal carbonization (HTC) is a mild process (180-250 ºC) that has been largely demonstrated to favour the condensation, polymerization, and aromatization of saccharides in solution. Hydrothermal carbonization of carbohydrates

ACCEPTED MANUSCRIPT generates monodisperse colloidal carbonaceous spheres with non-porous character. Nevertheless, Brun et al. [102] successfully prepared porous carbon gels with controllable mesoporosity using monosaccharides such as glucose, fructose or xylose as carbon source and using phloroglucinol as crosslinking agent and hydrothermal

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carbonization as polymerization promoter. Martin-Jimeno et al. [103] showed that the acidic sites on graphene oxide can also promote the dehydration/condensation reactions of carbohydrates whilst providing a scaffold that supports the growth of glucose-derived

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carbon xerogels. These authors successfully prepared monolithic xerogels with a particular mesoporous nanomorphology through hydrothermal carbonization of glucose

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in the presence of graphene oxide sheets as morphology directing agents. On the other hand, Rey-Raap et al. [104] or Braghiroli et al. [105] prepared tuneable mesoporous carbon gels replacing hydroxybenzenes by eco-friendly and low-cost tannins as carbon precursors. Moreover, some works reporting on the preparation of ordered mesoporous

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carbons using simple carbohydrates as carbon source can also be found in the literature. For instance, Enterría et al. [106] obtained ordered mesoporous carbons using carbohydrates as precursors by combination of hard and soft templating approaches.

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This strategy (known as “tri-constituent co-assembly”) has been extensively used to introduce microporosity in ordered mesoporous carbons to avoid post-activation

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procedures [107]. Hence, the hydrolysis of a silica precursor (typically tetraethyl orthosilicate) originates micropores on soft-templated carbons by occlusion of the formed silica particles into the carbon bulk. Nevertheless, to that date, there was not any work using this approach to prepare sugar-derived carbon materials yielding highly defined mesopores. The silica precursor provides an endo-scaffold that forces the carbohydrate molecules to aggregate around the Pluronic® micelles. Therefore, hierarchical materials combining high surface areas and well-defined mesoporosity are

ACCEPTED MANUSCRIPT obtained using starch, glucose or arabic gum as carbon precursors. Even though the combination of templates enables the synthesis of ordered mesostructures with high carbon yields, it still involves the use of hydrofluoric acid to eliminate the hard template. In that regard, Kubo et al. [108] reported the first preparation of a fructose-

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derived ordered mesoporous carbon by direct templating. They succeed in obtaining ordered mesoporous networks by simply treating an aqueous solution of Fructose and Pluronic® on an autoclave at 130 ºC during 3 or 5 days. It is worth pointing out that

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both Starbons® and hydrothermal carbons present much higher stability than conventional organic gels. This fact enables an unprecedented control of the surface

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chemistry (oxygen groups) by changing the carbonization temperature from 200 to 1000 º C. This strategy is unfeasible for carbon gels or ordered mesoporous carbons because they require higher carbonization temperatures (above 500 ºC) to be stabilized and to avoid the collapse of pores or micelles.

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A compilation of representative values of the textural characteristics of the materials discussed in Sections 2.1.1, 2.1.2, 2.2.1, 2.2.2 and 2.3 is displayed in Table 2 as well as some general remarks. As will be reviewed in the following section, the control of the

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surface functionalities also determines the properties of the materials.

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ACCEPTED MANUSCRIPT

reviewed in Section 2.

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Table 2. Representative values of textural parameters and general remarks for materials

Surface area (m2/g)

Vmeso (cm3/g)

cryogels

800-1300

1.30-2.5

aerogels

500-1000

Materials

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Hydroxybenzene/ aldehyde- derived gels

dmeso (nm)

0.7-2.0

6-50

250-700

0.5-1.1

EP

xerogels

100-600

0.15-1.30

3.5-32

Exotemplated ordered mesoporous carbon

300-500

0.78-1.20

3.0-7.0

Endotemplated ordered mesoporous carbons

500-1500

0.10-0.78

3-10

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Graphene and carbon nanotube aerogels

Remarks

• High porosity, moderate density and good conductivity. • High reproducibility, low production costs and toxic reactants. • Adsorption, catalysis and electrochemical applications. • No porosity, low density and excellent conductivity. • High reproducibility and high production costs. • Catalysis and electronic applications. • High porosity, high density and good conductivity. • High reproducibility and high production costs. • Adsorption, catalysis and electrochemical applications. • High porosity, high density and good conductivity. • Moderate reproducibility, low production costs and use of surfactants. • Adsorption, catalysis and electrochemical

Ref.

[18], [29] [18], [25-26] [28] [8-17] [21-24] [27], [31-37]

[41-45]

[46], [54] [65-71]

[82-93]

ACCEPTED MANUSCRIPT applications.

0.9-1.60

14-22

450-740

0.25-0.50

6-20

[102] [104], [105]

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3. Control of the surface chemistry.

[97-101] [103],[106] [108]

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derived from tannines or phloroglucinol

200-550

• Moderate porosity, lowmoderate density, moderate conductivity. • High reproducibility, low production costs and environmental friendly production. • Adsorption, catalysis and electrochemical applications.

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Biomass-derived mesoporous materials derived from starch, pectin, chitosan, glucose or alginic acid

The surface of carbon materials can be modified by doping with heteroatoms such as hydrogen, oxygen, nitrogen, sulfur, boron or phosphorus. These heteroatoms are generally present as surface functional groups at the edges of the graphenic layers.

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Boron and nitrogen can even be embedded in the material´s backbone since they have similar sizes as the carbon atom. Although there are numerous techniques for the functionalization of carbon materials, the present chapter will focus on those dealing

EP

with porous carbons with designed nanostructure. The introduction of functional groups on carbon materials can be achieved in-situ during synthesis, or ex-situ using

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appropriate post-treatments. In the case of in-situ doping, the materials are subjected to carbonization by heating under inert atmosphere at high temperatures. On the other hand, ex-situ approaches generally involve milder thermal treatments. The selected temperature for these thermal treatments will largely define the surface chemistry of the final carbon material regardless of the synthesis strategy. The reductive conditions of carbonization will promote the evolution of heteroatom-containing functional groups to lower oxidation states. On the other hand, the progressive aromatization of the carbon structure will involve a gradual decrease of the doping degree. A lack of control over

ACCEPTED MANUSCRIPT the type of functionalities introduced is still evident in the available literature which, in turn, is much focused in achieving high doping ratios. In this context, doping strategies which enable the control of the surface speciation would be most welcome. Additionally, the development of advanced “multifunctional” materials has been

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increasing in recent years. The simultaneous presence of two or more heteroatoms on

preparation of materials with uncommon properties.

3.1. Post-synthesis functionalization.

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the carbon materials surface gives rise to interesting synergistic effects, enabling the

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The oxygen atom is able to combine with carbon to form a wide spectrum of functionalities such as carboxyls, carbonyls, quinones, phenols, pyrones, chromenes, lactones, lactols, anhydrides or ethers (Figure 13) [109]. This huge variety of oxygencontaining groups can be introduced onto the carbon material surface by treatment with

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oxidizing agents, either in the liquid phase (e.g., nitric acid, hydrogen peroxide, ammonium persulphate) or in the gas phase (e.g., with oxygen, ozone, nitrogen oxides) [110-116]. For liquid phase oxidation, a diluted or concentrated solution of the

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oxidizing agent is boiled in the presence of the carbon material. After washing and drying, carbons with different oxygen contents are obtained. In the case of gas phase

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oxidation, the sample is heated up to a suitable temperature (usually in the range 350– 450 º C) under a flow of the gaseous oxidizing agent. Concentrated nitric acid and diluted oxygen (typically, 5% O2 in N2) are the most frequently used oxidants for wet and dry methods, respectively. In both cases, the extent of oxidation is dictated by the heating temperature, the oxidation agent concentration and the duration of the treatment. The amount and type of oxygen groups can be also adjusted to the desired degree by subsequent thermal treatments in inert atmosphere. In this regard, Figueiredo et al. [117]

ACCEPTED MANUSCRIPT presented a comprehensive study covering different oxidation strategies, changing the oxidation conditions and performing various thermal treatments after oxidation. By combination of Elemental Analysis (EA), X-Ray Photoelectron Spectroscopy (XPS) and Temperature Programmed Desorption (TPD), the authors established solid

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correlations between the oxidation parameters and the type of functional groups that are introduced in carbon materials. Gas-phase treatments are more effective for oxygen introduction but could be more aggressive in terms of texture preservation. In fact, the

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oxidative gasification increases both the micropore and mesopore volumes as well as the micropore average size. By increasing the duration of the treatment (5 % O2 in N2)

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from 3 to 20 h, the oxygen content was found to increase from 6.56 to 12.82 wt. % (measured by EA). The extent of oxidation also varies as function of the selected strategy. Hence, the gas-phase oxidation with N2O (50% in N2) provided lower oxygen content (8.9 wt. %) than O2 gasification. On the other hand, the liquid-phase treatment

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in presence of HNO3 was much more effective than H2O2 (12.28 vs 4.86 wt. %). The surface chemistry of the O2-oxidized materials was tuned by subsequent thermal treatment in inert atmosphere at 1100 ºC. As a result, the oxygen content decreased

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from 9.37 to 2.93 wt. %. The evolution of the surface speciation was also studied by XPS and TPD. The nitric acid treatment was found to increase the amount of carboxylic

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acid groups whereas gas-phase oxidation promoted the introduction of anhydrides, lactones, phenols and carbonyl /quinones. It was also observed (by comparison of EA and XPS values) that the liquid-phase treatments are more likely to oxidize the external surface of the carbon materials rather than the bulk. Further thermal treatments in inert atmosphere decrease the quantity of all the oxygen functionalities on the carbon surface. Nevertheless, the following evolution of the surface species with the temperature was proposed: i) temperatures above 600 ºC remove the carboxylic anhydrides, most of the

ACCEPTED MANUSCRIPT lactones and some phenols, ii) temperatures above 750 ºC remove the remaining lactones and phenols and, finally, iii) temperatures above 1100 ºC remove carbonyls and ethers. For a given oxidation method, low oxidation degrees promote the appearance of carbonyls and ethers, in contrast to phenol and anhydrides. On the other hand, high

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oxidation degrees benefit the incorporation of the latter groups. The oxidation of the carbon materials surface is also feasible by treatment with oxygen plasma. Mahata et al. [11] exposed a series of carbon xerogels to a 70 W oxygen plasma for 10 min and

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observed an increase of the amount of carboxylic acid groups. The plasma treatment showed a selective oxidation of the external surface and larger pores, providing

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materials with enhanced diffusion. Some examples of oxygen enriched ordered mesoporous carbons can be also found in the literature. As carbon gels, ordered mesoporous carbons are susceptible to oxidation by treatment in HNO3, H2O2 or O2 in gas phase [114, 118-120]. Post-synthesis oxidation treatments are generally aggressive

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and do not provide control in the surface speciation. In that regard, Silva et al. [121] developed an innovative hydrothermal method for the fine control of the surface oxidation in carbon xerogels. By using highly diluted HNO3 and changing the acid

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concentration/temperature of the hydrothermal treatment, the extent and the type of oxygen groups can be adjusted. The newly developed strategy increases the

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concentration of all oxygen functional groups as compared to conventional approaches. The proportion of strong carboxylic acids, phenols and carbonyls/quinones increases throughout the hydrothermal treatment while the texture of the carbon material is not significantly affected. Morales-Torres et al. [122] adapted the hydrothermal oxidation by using sulfuric acid containing ammonium persulfate as an oxidizing agent and observed a notable incorporation of sulfonic acid groups. The amount of both oxygen and sulfur containing groups was correlated with the concentration of each oxidizing

ACCEPTED MANUSCRIPT agent. The decoration of nanostructured mesoporous carbons with other heteroatoms different from oxygen can be performed by impregnating the material with a compound containing the target heteroatom. Nitrogen can be present in the carbon materials surface as nitrogen oxides, amines, amides, pyridinic or pyrrolic groups. Nitrogen can

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also substitute the carbon atom into the lattice as quaternary nitrogen (Figure 13, [109]). Nitrogen-doped mesoporous carbons can be prepared by physical mixture or by impregnation with melamine or urea and further heat treatment under inert atmosphere.

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Depending on the temperature of the treatment, the quantity and the type of nitrogen groups can be controlled. A mild thermal treatment (lower than 500 ºC) leads to the

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formation of lactams, imides, and amines, whereas stronger thermal treatments (above 600 ºC) result in an increase of quaternary nitrogen, pyridinic, and pyrrole-type structures [123]. Sousa et al. [115, 124] prepared functionalized carbon xerogels by dipping them in a 1M solution of urea during 24h, drying and thermal treatment at 600

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ºC in N2 flow. Carbons bearing nitrogen contents up to 1% (measured by EA) and showing pyrrolic and pyridinic groups (studied by XPS) were obtained by this impregnation approach. Sousa et al. [115] also studied the effects of the hydrothermal

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functionalization with nitrogen by treating the xerogels with a 0.1 M urea solution on an autoclave at 200 ºC during 2 h. The hydrothermally functionalized gels showed very

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similar nitrogen contents as those prepared by conventional impregnation, but using much less aggressive conditions. XPS analysis suggested that the hydrothermal treatment tends to introduce the nitrogen functionalities on the carbon surface rather than in the bulk, as well as promoting the formation of nitrogen oxides. Post-synthesis strategies, using carbonization temperatures above 600 ºC, barely provide nitrogen contents above 4 wt. %. Soares et al. [125] developed a new functionalization method that not only avoids the use of solvents and production of wastes, but also provides

ACCEPTED MANUSCRIPT unusually high nitrogen contents. The ball-milling of the carbon material/nitrogen precursor mixture prior to thermal annealing rises the nitrogen content up to 7.6 wt. %. Although this new strategy looks very promising, it has not been yet applied to porous carbon materials. The functionalization of ordered mesoporous carbons using nitrogen

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precursors led to materials with both designed texture and controlled surface chemistry. Li et al [126] decorated a FDU-15 carbon with nitrogen by either gas-phase or liquidphase functionalization. For the gas-phase treatment, they exposed the material to a pure

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ammonia flow at 700 ºC for 10 h. In the liquid-phase, the material was heated at 80 ºC in an ethanol/melamine solution until solvent evaporation. The liquid-phase

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functionalization notably reduces the surface area while providing nitrogen contents up to 7.8 wt. % (measured by EA). The treatment with ammonia produces a two-fold increase of the porosity and provides nitrogen contents of 8.6 wt. %. Activation of the carbon materials was ascribed to the formation of radicals during the heating treatment.

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Thus, species such as NH2· , NH· or H· can remove carbon atoms from the material structure, originating pores. The ordered mesoporous network remains invariable throughout the doping treatment. As expected from the carbonization temperature (700

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ºC), both pyridinic and quaternary nitrogen groups were detected by XPS analysis. Giraudet et al. [127] prepared a CMK-3 ordered mesoporous carbon and further

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enriched it with nitrogen by heat treatment in ammonia. The materials were subjected to thermal treatment up to different temperatures (500, 550 and 700 ºC) during 5 h. Contrasting with Li´s results, the treatment with ammonia does not cause an increase of the surface area. The carbon used in the Giraudet´s study was obtained by liquid infiltration with sucrose, while Liu´s was a commercial carbon prepared by chemical vapour deposition. The latter technique is known to produce high density carbon materials with underdeveloped microporosity. In this context, the carbons prepared by

ACCEPTED MANUSCRIPT Giraudet would be more susceptible to increase their specific surface area through the activation process. Regarding the surface chemistry, the nitrogen content increases from 1.2 to 3.9 wt. % (measured by XPS) by increasing the temperature from 400 to 700 ºC. The samples treated at 400 and 500 º C presented pyridinic and pyrrolic moieties while

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those prepared at 700 ºC showed pyridine, pyrrole, quaternary and nitrogen oxide functionalities. Wang et al. [128] prepared an ordered mesoporous carbon by softtemplating and proposed the direct functionalization with ammonia during the

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carbonization process. The obtained resorcinol/formaldehyde organic resin was milled after curing. The powder was heated under NH3 flow up to different temperatures (700-

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850 ºC) and for different times (1 to 4 h). The specific surface area increased by increasing the carbonization temperature due to the loss of volatile species such as CO or CO2. The nitrogen content increased from 7.1 to 9.3 % by extending the 800 ºC treatment from 1 to 4 hours (XPS analysis). An increment of the treatment duration also

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favoured the presence of pyridinic-type functionalities rather than pyrrolic. On the other hand, the increase of temperature from 700 to 850 ºC did not lead to any difference on

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groups.

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the nitrogen content after a 2 h treatment, origination similar distribution of nitrogen

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Figure 13. Nitrogen and oxygen surface groups on carbon. Reprinted from reference

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[129] with permission from Elsevier.

Boron can be either present on the surface of the carbon material as polynuclear oxides

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(B2O3), or covalently linked at the edges of the graphenic layers as -BC2O and -BCO2 functionalities. Similarly to nitrogen, it can also be inserted into the carbon material

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backbone as –BC3 species (Figure 14). Functionalization with boron, particularly if substitutional, enables the design of the electronic band gap of graphitic carbon materials [130, 131]. Consequently, the literature provides a large number of works dealing with ex-situ functionalization of graphene materials (highly conductive) and very few reporting on boron-doped porous carbons (less conductive). For instance, Fang et al. [132] functionalized graphene by annealing a physical mixture of graphene and boron oxides at 1200 ºC for 4h. The content of boron was easily tuned from 0.09 to 2.36 at. % (measured by XPS) by varying the B2O3 / graphene oxide mass ratio. Even

ACCEPTED MANUSCRIPT though XPS analysis detected both substitutional and surface functionalization, -BC2Otype species prevailed over other types of functionalities. Kim et al. [123] mixed boric acid (H3BO3) with graphene and heated it up to 2450 ºC. The obtained carbon materials presented 2.35 wt. % of pure substitutional boron measured by XPS. Another strategy

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consists in performing a chemical vapour deposition using thermally labile boron compounds. Romanos et al. [134] deposited decaborane (B10H14) on the surface of activated carbons. The nature of the surface functionalities was measured by

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microscopic Fourier Transform Infrared Spectroscopy (FTIR). The activated carbon contained 6.8 wt. % of substitutional boron and 2-3 wt. % of superficial boron oxides.

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Wu et al. [135] obtained boron doped graphene by exposing the pristine material for 2 hours to a BCl3 flow (1/4 v/v% in Ar) at 800 ºC. The prepared materials showed 1 at. %

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of substitutional boron measured by XPS.

Figure 14. Boron surface groups on carbon. Reprinted from reference [89] with permission from Elsevier. Concerning the preparation of sulfur-doped carbon materials, the reader is referred to the review published by Kicinski et al. [136]. Similarly to oxygen, sulfur presents multiple chemical states when present on the surface of carbon materials (Figure 15). It can be adsorbed on the carbon surface as elemental sulfur (S8 rings) or covalently

ACCEPTED MANUSCRIPT bonded to the edges of the graphene layers as oxides (sulfate, sulfone or sulfoxide), sulfides (mono and bi), thiol species or organic sulfur (thiophene). Similarly to other heteroatoms, the quantity and the proportion of each sulfur functional group depend on the doping strategy and the post-synthesis thermal treatments. Kelemen et al. [137]

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performed a thorough study about the evolution of the sulfur species with the carbonization temperature. The different sulfur forms were studied by combination of XPS and X-ray absorption near edge structure (XANES). The authors extracted the

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following conclusions: i) low carbonization temperatures (200 ºC) lead to sulfonic groups, ii) moderate pyrolysis temperatures (400 ºC) promote the formation of aliphatic

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sulfur mainly as thiol and disulfide, iii) the former species prevail over the latter since disulfide moieties normally decompose to form thiols and iv) severe carbonization conditions (large times or high temperatures) promote the formation of thiophene as a result of the transformation of sulfides, thiols and sulfones. The introduction of sulfur

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groups into carbon materials is carried out in an atmosphere of a sulfurizing agent. Asasian et al. [138] produced sulfurized mesoporous carbons by treating a commercial granular carbon either with dimethyl disulfide ((CH3)2S2) or carbon disulfide (CS2). The

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materials were immersed in a solution containing the sulfur precursor and stirred during 48 h at 30 ºC. Samples were dried at 60 ºC for 24 h but further carbonization was not

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performed. The XPS surface characterization showed 9.6 wt. % of sulfur when (CH3)2S2 was used as doping agent. The sulfur amount decreased to 3.2 wt. % when using CS2 as sulfurizing agent. For both functionalization methods, sulfide, disulfide and elemental sulfur dominated the speciation of the carbon materials surface. Kishnan et al. [139] doped activated carbons with sulfur by exposing them to a SO2/H2S stream. The material was treated at 400 ºC with a flow of SO2 and, subsequently, with gaseous H2S. The amount of sulfur retained on the carbon surface was adjusted from 0.8 to 8.9 wt. %

ACCEPTED MANUSCRIPT (EA measurement) by changing the flow time of either SO2 or H2S. The observation of C=S, S=O or S-S bands, by FTIR, revealed the presence of thioketones, sulfoxides and sulfides, respectively. The gas-phase sulfurization is associated with the decomposition of existing oxygen surface functionalities, which create active sites for sulfur bonding.

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Thus, sulfur can either react directly with carbon to form C-S bonds, releasing hydrogen, or substitute the existing oxygen functionalities, releasing H2O [140]. Resorcinol/formaldehyde-derived carbon gels were functionalized with sulfur by Baker

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et al. [141] by including an additional step in the drying process. An ethanolic solution of 3-thiophenecarboxaldehyde was used to wash the gel prior to carbonization at 1000

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ºC during 1 h. The obtained materials yielded sulfur amounts of 0.49 at. %, mainly in thiophenic form (XPS analysis). Among the sulfur enriched porous carbons, materials containing sulfonic groups are of particular interest for applications in catalysis. A liquid-phase treatment of carbon materials with sulfuric acid at high temperatures

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results in a range of sulfur containing surface groups, where thiol (–SH) and sulfonic acid (–SO3H) are the predominant ones. For instance, Rocha et al. [111, 116] treated resorcinol derived carbon xerogels with concentrated sulfuric acid at 150 ºC during 6 h.

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The content of sulfur (measured by XPS) was increased from 1.89 to 3.18 wt. % by increasing the concentration of sulfuric acid. Subsequent heating at 250 ºC for 1h

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reduced the sulfur content to 0.60 wt. %. The nature of the created sulfur groups was also studied by XPS. The authors concluded that no organic forms of sulfur were introduced by sulfuric acid treatment. Instead, large amounts of terminal sulfonic groups (-SO3H) were observed on the carbon surface.

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Figure 15. Sulfur surface groups on carbon. Adapted from reference [136]

3.2. Functionalization during synthesis.

The “in-situ” doping of mesoporous carbon materials is possible through two different

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strategies; i) nanocasting with a heteroatom-containing compound using either liquid impregnation or chemical vapour deposition or ii) by using heteroatom-containing monomers in the sol-gel synthesis of either soft-templated carbons or carbon gels. As

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commented in Section 2.3, the use of complex polysaccharides as carbon precursors for Starbons® production provides for much greater stability than conventional gels. The

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low temperatures required for Starbons® stabilization generate materials with high oxygen content on their surface. White et al. [97] studied the evolution of a Starbon® surface chemistry with the temperature of carbonization. By using diffuse reflectance infrared spectroscopy (DRIFT) the authors extracted some interesting conclusions: i) temperatures above 250 ºC promote the dehydration and loss of hydroxyl groups alongside carbonyl groups conjugated with olefinic and vinyl ethers, ii) heating up to 200–600 ºC leads to the formation of aromatic structures and iii) temperatures above 700 º C resulted in the formation of very condensed extended aromatic structures. The

ACCEPTED MANUSCRIPT surface chemistry of starch-derived carbons can be therefore controlled by adjusting the carbonization parameters. It should be noted that this temperature-assisted strategy is only suitable for materials targeted for certain applications (such as catalysis or adsorption) since the use of carbon materials in electronic/electrochemical devices

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would require graphitic conductive structures. The design of the oxygen speciation by “in-situ” control of the carbonization process is also possible for carbon materials derived from hydrothermal sol-gel procedures. Hence, similarly to the retrodegradation

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of polymers, hydrothermal carbonization produces materials containing large amounts of oxygen [110, 142]. Concerning other heteroatoms, the sol-gel polymerization of

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heteroatom-containing precursors provides a simple alternative to the tedious “ex-situ” approaches. As a disadvantage, the low participation of this kind of precursors in the polymerization processes generally leads to amorphous materials (with no defined nanostructure) or to low doping levels. Nitrogen-doped carbon xerogels were prepared

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by adding either melamine or urea into the resorcinol/formaldehyde synthesis mixture by Gorgulho [143]. To that end, different amounts of resorcinol and melamine were mixed in distilled water and heated to 90 ºC until melamine dissolution. The solution

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was then cooled to room temperature, and formaldehyde was added. Gelation was performed at 85 ºC for 72 h and, the wet gels were dried in an oven at 100 ºC for 3 days.

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The same synthesis procedure was followed to prepare an additional set of nitrogendoped materials using urea as nitrogen precursor instead of melamine. The dried gels were carbonized at 800 ºC during 2 h. The nitrogen content, measured by EA, was larger for the xerogels prepared from melamine due to the higher initial concentration of nitrogen of this precursor. As compared with urea, larger amounts of nitrogen are preserved upon pyrolysis when melamine is used as doping agent. However, the surface nitrogen concentration determined by XPS was slightly higher in the urea-derived

ACCEPTED MANUSCRIPT xerogels. It was therefore concluded that melamine promoted the introduction of nitrogen on the carbon matrix while urea favoured the nitrogen bonding at the edges of the graphenic layers. Additional information about the nature of the nitrogen bonding was extracted by XPS analysis: 28 wt. % of the nitrogen existed as pyridinic, 30 wt. %

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as pyrrolic and 35 wt. % as quaternary nitrogen for the urea-derived materials and, 34 wt. % as pyridinic, 27 wt. % as pyrrolic and 31 wt. % as quaternary nitrogen for those prepared with melamine. The surface of different xerogels, obtained by carbonization at

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600-800 ºC, was studied in order to investigate the evolution of the nitrogen type of bonding. Pyridinic-type nitrogen was the main species observed at 600 ºC. In the case of

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800 ºC, the contribution of the pyrrolic nitrogen decreased while the amount of quaternary nitrogen increased. Temperatures above 800 ºC showed a large decrease of the pyridinic-like functionalities as well as some nitrogen oxides. In a similar way, Sousa et al. [144] prepared nitrogen-doped carbon xerogels using either melamine or

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urea as nitrogen precursors. Further sol-gel gelation at different pH (5.3, 6.0 and 6.9) and carbonization temperatures (500, 700 and 900 ºC) originated a set of different carbon gels. Similarly to [143], the authors observed that doping with melamine

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promotes the presence of quaternary nitrogen, while urea is more likely to generate surface functionalities. On the other hand, the nitrogen groups introduced with urea

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presented higher thermal stability than those introduced with melamine. Thus, nitrogen contents between 3.8 and 5.3 wt. % were provided by using melamine as carbon precursor, while nitrogen contents of 2.7-2.9 wt. % were obtained by using urea (EA measurements). The preparation of nitrogen-doped ordered mesoporous carbons by “hard-templating” has also been reported. Sevilla et al. [145] prepared functionalized nanocasted carbons by using either SBA-15 or amorphous silica xerogel as templates. Both structures were replicated by chemical polymerization of polypyrrole using FeCl3

ACCEPTED MANUSCRIPT as catalyst and further carbonization at 900 ºC for 1h. The obtained carbons showed highly ordered mesoporosity and yielded nitrogen contents up to 5.45 wt. % (SBA-15) or 4.91 wt. % (xerogel). Even though pyrrolic and pyridinic species were observed by XPS analysis, the main contribution corresponded to quaternary nitrogen. Sanchez et al.

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[146] achieved, for the first time, the replication of the SBA-15 structure using polyamides as nitrogen precursor. This newly developed strategy was accomplished through two different synthetic pathways: i) “in-situ” infiltration by chemical

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polymerization of 3-aminobenzoic acid and ii) thermal polymerization of the same precursor. The composites obtained from both approaches were carbonized at 900 ºC

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for 30 min and treated with HF to remove the silica template. The reported materials showed much better defined mesoporosity together with large nitrogen contents, 6 wt. % (measured by EA). By conducting an XPS study, the authors observed that 50% of the nitrogen existed as quaternary nitrogen, 40% as pyridinic or pyrrolic groups and

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only 7% belonged to oxidized forms of nitrogen. Sanchez et al. [147] also proposed the chemical vapour deposition from acetonitrile as suitable technique to prepare functionalized carbon materials. An SBA-15 was infiltrated at 850 ºC with an

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acetonitrile/argon flow with different concentrations (3 to 8.8 % v/v). The carbon deposition time was varied from 2 to 7 hours. By increasing the concentration of

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acetonitrile or extending the duration of the treatment, the mesopore size decreased from 10 to 3 nm. Simultaneously, the total content of nitrogen increased from 6.37 to 9.42 wt. % (measured by EA). All samples presented very similar surface speciation: 55 % of quaternary nitrogen, 13 % of pyridine and 16 % of pyrrole-type functionalities. Wei et al. [148] achieved high nitrogen loadings in soft-templated ordered mesoporous carbons by adding dicyanamide to a pre-synthesized resol. Thus, an ethanolic solution of dicyanamide and Pluronic ® F127 was added to a resorcinol/formaldehyde-derived

ACCEPTED MANUSCRIPT resol. The synthesis mixture was poured into Petri dishes to evaporate the solvent at 50 ° C for 6 h. Subsequently, the obtained films were aged at 100 ° C for 24 h, ground to powder and carbonized at 600 ° C for 3 h. The mesostructure of the prepared carbon materials was studied by Small Angle X-Ray Scattering (SAXS) and several

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conclusions were extracted. On one hand, the variation of the Pluronic®/resol mass ratio originated a phase transition: low concentrations of the surfactant produced p6m hexagonal structures while higher proportions led to cubic Im3m mesophases. On the

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other hand, the variation of the dicyanamide/resol mass ratio led to changes in both the mesoporous system and the nitrogen content. The unit cell parameter could be adjusted

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by increasing the dycianamide/resol ratio. It was observed that ratios above 2.5 hampered the co-assembly process, originating disordered mesoporous systems. Interestingly, an increase of the dycianamide/resol ratio from 0 to 2.5 caused a huge increase of the nitrogen content from 0.08 to 13.1 wt. % (measured by EA). Similar

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nitrogen speciation was observed for all the prepared carbons when investigated by XPS: 44.2 wt. % pyridinic, 16 wt. % pyrrolic, 11.8 wt. % quaternary and 27.9 wt. % of pyridinic N-oxide. Shen et al. [149] prepared nitrogen-rich ordered mesoporous carbons

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via organic-organic self-assembly using hexamethylenetetramine (HMTA) as formaldehyde source and Pluronic® F127 as structure directing agent. The slow

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delivery of formaldehyde in the reaction medium via progressive hydrolysis of HMTA provided a tight control of the 3-aminophenol and formaldehyde polymerization. The obtained resin was aged at 350 ºC for 3 hours and subsequently carbonized at 600 or 800 ºC for 5 hours. Porous carbon materials yielding highly ordered cubic Im3m mesoporous structure were observed by SAXS analysis. The nitrogen content was measured by XPS and decreased from 5.40 to 3.42 at. % with increasing carbonization temperature. The chemical state of the surface nitrogen groups changed along the

ACCEPTED MANUSCRIPT carbonization temperature. Both pyridinic and pyrrolic groups were the prevailing functionalities for materials carbonized at 600 ºC. When the carbonization temperature was increased to 800 ºC, a large fraction of the pyridinic functionalities was converted to quaternary nitrogen. Regarding “in-situ” strategies for direct production of boron-

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doped nanostructured mesoporous carbons, Zapata-Benabihe et al. [150] prepared carbon xerogels containing boron. The authors conducted the polymerization of resorcinol and formaldehyde by using either boric or phenyl boronic acid as

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catalysts/dopants. After pH adjustment, the hydrogels were cured at 80 ºC during several days. Different drying methods were performed: i) supercritical drying with CO2

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after exchanging water with acetone, ii) freeze drying at −15 °C and vacuum drying at around 10−3 mbar, iii) microwave drying at 384 W in a N2 atmosphere and iv) convective drying at 60 °C and 10 mbar. The dried organic gels were carbonized at 900 ºC during 5 h. The boron content of the gels varied according to the drying method

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used. Supercritical drying led to the lowest boron contents due to the loss of boron precursor during the solvent exchanging step. The utilization of phenyl boronic acid led to the highest doping ratios (~ 2.4 wt. %, measured by EA). Boron oxycarbides, boron

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carbides and substitutional boron were observed by XPS on the surface of all the prepared carbons. Moreno-Castilla et al. [151] prepared resorcinol and pyrocatechol-

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derived carbon gels using boric acid as catalyst. The boron-containing hydrogels were cured during 5 days at 80 ºC and dried under supercritical conditions. A considerable loss of boron during the exchange of water by acetone was also observed in this work. XPS characterization showed the presence of boric oxide on the surface of the doped organic gels. Carbonization at temperatures above 500 ºC caused the appearance of boron oxycarbides, while temperatures above 900 ºC promoted the insertion of boron into the carbon backbone. On the other hand, the total boron content remained constant

ACCEPTED MANUSCRIPT between 0.1 and 0.3 wt. % irrespective of the carbonization temperature. Nsabimana et al. [152] prepared ordered mesoporous carbons by hard-templating using sucrose as carbon precursor and 4-hydroxyphenylboronic acid as polymerization catalyst. It was observed that the boron content could be controlled by adjusting the 4-

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hydroxyphenylboronic acid/sucrose ratio. The materials, carbonized at 800 ºC for 2h, were studied by XPS and displayed boron contents around 1.3 at. %. The chemical state of the surface boron species was also investigated. The presence of boron carbides,

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boron oxycarbides and substitutional boron was observed on the surface of the prepared carbon gels. Wang et al. [153] impregnated the pores of SBA-15 with glucose. Further

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thermal polymerization at 160 ºC for 6h using boric acid as catalyst provided borondoped ordered mesoporous carbon. The materials were carbonized at 900 ºC during 4h and the surface of the carbon replicas was studied by XPS. The amount of introduced boron increased from 0.2 to 0.6 at. % by increasing the initial mass of boric acid. Boron

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oxycarbide and carbide species were observed on the surface of the ordered mesoporous carbons. Zhai et al. [154] reported the preparation of boron-doped carbon materials yielding

high

specific

surface

area

via

organic-inorganic

co-assembly.

A

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phenol/formaldehyde mixture was polymerized in the presence of boric acid (catalyst and boron source), TEOS (micropore generator) and Pluronic® P125 (mesopore

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template). The resins were carbonized at 900 ºC for 3h and immersed in NaOH in order to remove the silica template. Materials with ordered mesoporosity and specific surface areas between 1367 and 1534 m2/g were obtained. The boron content varied from 0.22 to 0.33 wt. % (measured by XPS) by increasing the proportion of boric acid. Both boron oxycarbide and boron oxide functionalities were observed by XPS compositional analysis. Boron-doped ordered mesoporous carbons were prepared via solvent evaporation induced self-assembly (EISA) by Song et al. [155]. Pluronic® F127 was

ACCEPTED MANUSCRIPT used as a soft template, a low-molecular weight phenol/formaldehyde resin as carbon source and boric acid as boron doping agent. The effect of formaldehyde/phenol molar ratio on both the carbon porosity and the surface composition was investigated. The results showed that the specific surface area increases with the increase of the

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formaldehyde proportion. Carbons prepared using formaldehyde/phenol ratios between 1.5 and 1 presented the highest degree of order, the largest specific surface area, and the highest boron contents measured by Inductively Coupled Plasma Mass Spectroscopy

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(ICP-MS). The XPS compositional study also detected mixed B–C and B–O bonding on the carbon surface. The autogenous pressure generated during hydrothermal

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treatments has demonstrated advantages for the functionalization of porous carbons. Bearing this on mind, Enterría et al. [93] developed a new synthesis strategy to obtain boron-doped ordered mesoporous carbons with highly controlled porosity and surface speciation. The “soft-templating” approach used not only enabled adequate control of

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the total boron content but also the adjustment of the boron chemical state. By increasing the hydrothermal treatment temperature or duration, the oxidation state of boron decreased since the self-generated pressures favour B-C bond formation. The

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total content of oxygen increased simultaneously with the boron content, and its speciation was also influenced by the boron chemical state. Hence, the presence of BC3

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and BC2O species induces the oxidation of phenols to carbonyls. The solubility of boric acid in the organic phase increases as a consequence of the generated pressure. Hence, the boron precursor could actively participate in the organic phase polymerization instead of hydrolysing in the aqueous phase. Performing a hydrothermal treatment (100 ºC for 28 h) prior to carbonization increased the boron content from 0.42 to 2.37 wt. % (measured by XPS). The direct preparation of nanostructured mesoporous carbons containing sulfur was also reported by several authors. Sevilla et al. [156, 157]

ACCEPTED MANUSCRIPT performed an “in-situ” activation of a lab-prepared polythiophene polymer during the carbonization step. Carbon materials bearing defined mesoporosity together with high surface area were prepared. The sulfur content, measured by XPS, varied from 11.8 to 3.5 wt. % as the carbonization temperature was changed from 600 to 850 ºC. The sulfur

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present in these carbons formed either sulfide or sulfone groups which act as bridges between adjacent aromatic rings. It was observed that an increase of the carbonization temperature produced a gradual enlargement of the mesopores as well as a decrease of

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the sulfone bridge fraction. Lee et al. [158] replicated a MSU-H mesoporous silica with 2-D hexagonal structure by subsequent infiltrations with p-toluene sulfonic acid and

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heating up to 160 ºC for 6 hours. Carbonization was conducted at 900 ºC for 3 h and the silica template was removed with HF. The thus obtained ordered mesoporous carbons showed sulfur contents up to 2.3 wt. % when studied by EA. On the other hand, thiol and sulfate groups were the main surface functionalities detected by XPS. Kicinski et al.

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[159] reported the preparation of sulfur doped carbon xerogels by replacing formaldehyde by 2-thiophenecarboxaldehyde as monomer in the preparation of resorcinol-derived hydrogels. The carbon gels obtained by carbonization at 800 ºC for

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1h yielded high sulfur loadings. The variation of the carboxyaldehyde proportion during the synthesis enabled the adjustment of the sulfur content. The change in the sulfur

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content throughout the carbonization process was monitored by EA in the range of 600– 1000 ºC. The sulfur amount dropped from 19.6 wt. % to 14 wt. % after pyrolysis at 600 ºC. After 2h of carbonization at 1000 ºC only 4.83 wt. % of the sulfur was retained. The nature of the chemical bonding between carbon and sulfur was investigated by XPS. The XPS results confirmed that the sulfur atoms were structurally inserted into the carbon xerogel network as thiophenic sulfur. Minor contributions of oxidized sulfur species such as sulfate, sulfonate or thioketone were also observed.

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3.3. Phosphorus: the next heteroatom to be explored.

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The scarce reports available on phosphorus-doped porous carbon materials call for future research on this heteroatom. Developing novel synthetic approaches to prepare phosphorus decorated carbons or getting further insights on the chemical state of

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phosphorus in this kind of materials would be of great interest. The lack of knowledge is the reason for some controversies about the chemical state of phosphorus when

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present in the surface of carbon materials. Nevertheless, phosphate (C-O-P) and phosphonate (C-P-O) species have been generally proposed as possible phosphorus configurations (Figure 16). Puziy et al. [160] demonstrated that the most abundant phosphorus species in carbons obtained at 400–1000ºC by phosphoric acid activation is

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the phosphate-like structure. Nevertheless, the authors observed a small contribution of phosphonates in the 500–700 ºC temperature range. The presence of phosphonates at higher temperatures was not observed, showing higher thermal stability of phosphates

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over the phosphonates.

Figure 16. Phosphorus surface groups on carbon. Adapted from reference [161]

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A few works reporting on the preparation and characterization of phosphorus doped porous carbon materials can be found in the literature. For instance, Wu et al. [162] prepared phosphorus doped carbon xerogels by ex-situ functionalization. Resorcinol

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and formaldehyde were polymerized in the presence of cobalt nitrate and the obtained organic gel was carbonized at 800 ºC for 1 h. The as-prepared carbon was impregnated with different proportions of phosphoric acid (H3PO4) at 85 ºC for 3 h and treated up to

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800 ºC for 1 h in inert atmosphere. The carbon samples were finally treated with 1 M HCl in order to remove the cobalt salts. The phosphorus content was measured by

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Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) analysis and varied from 0.78 at. % to 3.56 at. % as function of the initial proportion of the doping agent. The presence of phosphonates on the surface of the carbon xerogels was confirmed by XPS analysis. Nevertheless, phosphate-like groups were the majority. The

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authors proposed the existence of phosphate functions mainly as tetrahedral forms such as C3PO, C2PO2 and CPO3 since the huge size of the phosphorus atom favours the sp3orbital configuration in molecules. Yang et al. [163] prepared a phosphorus

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functionalized ordered mesoporous carbon by nanocasting using SBA-15 as template. The silica was infiltrated at room temperature with a triphenylphosphine/phenol mixture

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and pyrolyzed at 900 °C for 2h. The investigation of the surface composition by XPS revealed the presence of both P−O and P−C bonding as well as phosphorus contents of 1.36 at. %. Zhu et al. [164] recently reported on the direct synthesis of phosphorusdoped ordered mesoporous carbons through soft-templating approaches. Resorcinol and formaldehyde were polymerized in the presence of 1-hydroxyethylidene-1,1diphosphonic acid (HEDP) as phosphorus source and Pluronic® F127 as porogenic agent. The resulting mesoporous materials presented high phosphorus content, large

ACCEPTED MANUSCRIPT surface area and an interconnected mesoporous system. It was confirmed by XPS characterization that the prepared materials yielded phosphorus amounts up to 1.62 at. % and that phosphonates were the prevailing chemical state of phosphorus. Even though phosphorus remained somewhat less popular, it has been widely used as co-dopant in

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order to obtain multifunctional carbon materials (see Section 3.5).

3.4. Biomass derived nanostructured carbons containing heteroatoms.

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Complementarily to the oxygen-containing hydrothermal carbons and Starbons® (Section 2.3), the direct preparation of heteroatom-enriched carbon materials from

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biomass is also possible. Natural polysaccharides deriving from seaweeds are made from polymers with several functional groups that can be used to prepare decorated carbon gels. For instance, chitin is a nitrogen-rich polymer which is mainly obtained from the exoskeletons of crustaceans. The insoluble nature of this polymer hinders its

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manipulation but, partial deacetylation of chitin under alkaline conditions leads to the extraction of the soluble chitosan. Chitosan can be retrodegradated by dissolution on weak acids and further recrystallized by chemical gelation in basic media. In this basis,

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Wu et al. [165] produced porous organic carbon gels by decomposition and dehydration of chitosan. To that end, chitosan powder was dissolved in acetic acid and sealed in an

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autoclave at 180 ºC during 12 h. The materials, carbonized at 900 ºC during 4 h, yielded high nitrogen contents between 7.0 and 11.0 wt. % (measured by EA). Both quaternary and pyrindinic-type groups were observed by XPS analysis. White et al. [166] prepared nitrogen doped carbon gels using crude prawn shells as carbon source. The shells were mixed with water and treated in a stainless steel autoclave at 180-200 ºC for 24 h. The resulting material was dried, carbonized at 750 ºC during 4h and treated with acetic acid in order to remove the inorganic content. The obtained porous carbon gel showed a

ACCEPTED MANUSCRIPT nitrogen content of 5.8 wt. % (measured by EA) and quaternary, pyridinic and pyrrolic type groups (observed by XPS). Zhao et al. [167] prepared nanostructured carbon materials by hydrothermal carbonization of either chitosan or glucosamine. Both biomass-derivatives were mixed together with water and treated in an autoclave at 180

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ºC overnight. The organic gels were pyrolyzed at 750°C during 1 h. The chitosanderived carbon presented nitrogen contents around 7.7 wt. % while that deriving from glucosamine showed 6.3 wt. % of nitrogen. In both cases, quaternary and pyridinic-like

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groups were observed by XPS. White et al. [168] also reported on the sustainable synthesis of nitrogen doped carbon aerogels by using the hydrothermal approach. The

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authors heated an aqueous solution of ovalbumin and glucose during 5.5 h at 180 ºC until formation of an organic aerogel. Further carbonization on inert atmosphere allows to adjust the nitrogen content from 7.5 to 5.9 wt. % (measured by EA) by changing the temperature in the 350–900 ºC range. An interesting dual role was observed for

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ovalbumin: it acts both as nitrogen source and structure directing agent to achieve a porous glucose hydrogel. This approach was extended to the synthesis of bifunctionalized carbon gels. Thus, Wohlgemuth et al. [169] prepared hydrothermal

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organic carbon aerogels doped with nitrogen and sulfur by using S-(2- thienyl)-Lcysteine and 2-thienyl carboxaldehyde as carbon precursors. The gels obtained after

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hydrothermal carbonization were carbonized at 900 ºC during 4 hours. The final carbon materials showed nitrogen contents up to 5 wt. % and sulfur loadings of 4 wt. %. The chemical state of both heteroatoms was studied by XPS. Quaternary and pyiridinic nitrogen groups as well as thiophenic sulfur were confirmed as the main functionalities on the materials surface. Interestingly, sulphur containing organic gels can also be obtained by using carrageenans as carbon precursors. K-carrageenan is a natural polymer containing sulfonic groups that can be extracted from red algae. It is able to

ACCEPTED MANUSCRIPT form gels induced by some monovalent cations such as potassium, rubidium and cesium. In fact, Quignard et al. [170] prepared a carrageean-derived hydrogel by dissolving the crude polymer in water at 80 ºC for 30 min and further addition to a cold KCl solution. The supercritical drying of the as-obtained hydrogel led to sulphur doped

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mesoporous materials. Bearing in mind the above mentioned achievements, the development of new technological strategies in order to maximize the use of the non-

3.5. Bifunctional mesoporous materials.

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edible part of the biomass is a key challenge for a sustainable future.

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The simultaneous presence of more than one heteroatom with different electronic and/or redox properties on the carbon materials surface takes the design of the surface chemistry one step further. As previously mentioned, heteroatoms can be present as functional groups on the carbon surface or embedded into the material backbone as

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dopants. In the case of surface functionalization, electron withdrawing groups originate acid active centres whereas electron donating groups act as basic sites. The combination of different kinds of surface groups would therefore lead to materials having novel

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redox or acid/basic characteristics. Concerning bulk doping, heteroatoms bearing fewer valence electrons than the carbon atom will act as p-dopants by decreasing the Fermi

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level of graphitic structures (thermodynamic work required to add one electron to the carbon body). In contrast, atoms with more electrons than the carbon atom will behave as n-dopants increasing the electronic density of the carbon materials. The combination of more than one of the above mentioned situations will enable extensive modulation of both the electronic and surface properties of carbon materials. Nitrogen and boron can either accommodate on the carbon lattice or exist as functional groups on the surface of carbon materials. On one hand, the boron atom has lower electronegativity than carbon

ACCEPTED MANUSCRIPT and one valence electron less in the valence band. On the other hand, nitrogen presents higher electronegativity and one electron more in the valence band compared to the carbon atom. Consequently, the simultaneous doping with nitrogen and boron produces basic and acid sites in the carbon surface as well as a wide range of electronic states on

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the graphitic basal plane. Guo et al. [171] prepared bifunctional carbon gels using citric acid as carbon precursor, boric acid as polymerization catalyst and ammonia as pH regulator. The carbonization of the dry gel at 900 ºC for 4h in the presence of nickel

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chloride lead to multifunctionally doped carbons with defined mesoporosity. The composition of the surface and the chemical states of both heteroatoms were studied by

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XPS. Boron and nitrogen contents up to 9.6 and 9.2 at. % were observed, respectively. The surface speciation was studied by analysis of the XPS core level spectrum of C1s. Even though both C-B-O and C-B-N types of bonding were observed, XRD analysis did not show any evidence of boron nitride phases. The authors suggested that two adjacent

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carbon atoms at the periphery of graphene sheets could be substituted by boron and nitrogen atoms simultaneously. The orbitals of one of these carbon atoms could be first polarized by nitrogen, subsequently giving extra electrons to the surrounding boron

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atoms. It was also observed that both the composition and the texture of the carbon gel depended on the nickel chloride mass ratio: i) increasing the nickel concentration

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produced larger mesoporous volumes and specific surface areas and ii) higher proportions of nickel originated lower doping ratios, since not only Ni+2 ions promote graphitization, but nickel chloride also reacts with boron during carbonization. Sepehri et al. [172] prepared boron/nitrogen co-doped cryogels by using a mixture of tertbutanol and ammonia borane during the solvent exchanging step. The organic cryogels were carbonized at 1050 ºC for 4h. A XPS study of the prepared materials revealed nitrogen and boron contents of 0.1 and 2.2 wt. %, respectively. The low nitrogen/boron

ACCEPTED MANUSCRIPT ratio together with high oxygen contents suggested that nitrogen leaves the pyrolized samples whereas boron is retained. The chemical state of boron changes from boron tert-butoxide in the organic gels to B-OH type functionalities after pyrolysis. Ryu et al. [173] made use of the nitrogen contained in orange peels, and prepared mesoporous

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carbon materials doped with boron and nitrogen. Dried orange peel wastes were immersed in an aqueous solution of boric acid and heated up to 60 ºC for 3 h. Samples were dried at 60ºC for 24 and carbonized at 900 ºC for 3 h. The nitrogen and boron

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contents, estimated by XPS, were 2.40 and 1.57 at. %, respectively. It was also observed that the surfaces of the obtained materials were dominated by pyridinic-type

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functionalities, presenting also important contributions of quaternary nitrogen. In the case of boron, both B-C and B-O types of bonding were detected, but substitutional boron prevailed over other chemical states. Interesting synergistic effects are also observed by combining sulfur and nitrogen as dopants. Despite having similar

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electronegativity as the carbon atom, sulfur induces the formation of defects in the carbon structure as a consequence of its large atomic size. While nitrogen and boron enable the design of the electronic properties of the carbon materials, sulfur or

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phosphorus have been used to increase the chemical reactivity of the carbon surface. The large size of both sulfur and phosphorus provides polarizable electrons and

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promotes the creation of defects on the carbon structure. Liu et al. [174] replicated SBA-15 by using sucrose as carbon precursor and thiourea as nitrogen/sulfur source. The surface chemistry of the carbons obtained after carbonization at 1000 ºC for 2h was studied by XPS. The surface of the ordered mesoporous carbons presented 6.53 wt. % of sulfur mainly as thiophenic forms. On the other hand, 2.88 wt. % of nitrogen was detected on the carbon surface in the form of pyridinic, pyrrolic and quaternary nitrogen. Zhang et al. [175] prepared nanocasted carbons by using polypyrrole as

ACCEPTED MANUSCRIPT nitrogen/carbon precursor and phosphoric acid as catalyst. The wet infiltrated silica was dried at 80 ºC for 6 h and calcined at 160 ºC for 6 h to obtain a pre-carbonized sample. The thus obtained composites were carbonized at different temperatures from 650 to 950 ºC and the changes in their surface chemistry were monitored by means of XPS.

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The concentration of nitrogen decreased from 5.18 to 2.06 at. % by increasing the carbonization temperature, while the phosphorus content dropped from 3.42 to 0.88 at. %. Pyridinic, pyrrolic and quaternary nitrogen were detected on the carbon surface. In

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the case of phosphorus, phosphonate (C-P-O) and phosphate (C-O-P- or P-O-P) like groups were observed in the ordered mesoporous carbons. Sanchez et al. [176] prepared

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nanocasted carbon materials using 3-aminobenzoic acid as carbon and nitrogen precursor and phosphoric acid to achieve the phosphorus doping. The aminobenzoic acid was polymerized at 160 ºC for 1 h inside the pores of SBA-15. Aqueous solutions with different concentrations of phosphoric acid were added to the carbon/silica

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composite prior to carbonization at different temperatures (700, 800 or 900 ºC). The evolution of the surface chemistry with the phosphoric acid concentration or the carbonization temperature was thoroughly studied by XPS. It was observed that the

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increase of phosphoric acid concentration has the same effect on the nitrogen functionalities as the increase in carbonization temperature. Hence, the amount of

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quaternary nitrogen increases at the expense of pyridinic and pyrrolic functionalities when the proportion of H3PO4 increased. This was ascribed to enhanced dehydration and condensation of the carbon structure produced by the catalytic effect of phosphoric acid. The dependency of the carbonization temperature on both the oxygen and the phosphorus evolution was also investigated. The authors observed that temperatures above 800 ºC led to higher oxygen and phosphorus contents as a consequence of the following reactions: i) phosphates and polyphosphates are formed by phosphoric acid

ACCEPTED MANUSCRIPT dehydration ii) elemental phosphorus is generated by reduction of phosphates with carbon, iii) the reduction of phosphates to elemental phosphorus causes the oxidation of the carbon material. In the case of temperatures above 900 ºC, the reduction of phosphates becomes more significant, increasing the retention of either oxygen or

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phosphorus. On this basis, the utilization of high H3PO4 proportions (150 wt. %) or high carbonization temperatures (900 ºC) produced materials with high oxygen (10.20 wt. %) and phosphorus (5.43 wt. %) contents but less nitrogen (3.49 wt. %). The boron-

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phosphorus combination could also be attractive because boron would modify the electronic structure of the basal plane while phosphorus would increase the reactivity of

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the carbon bulk. Zhao et al. [177] prepared boron/phosphorus doped ordered mesoporous carbons through hydrothermal soft-templating. Resorcinol was used as the carbon source while boric acid and phosphoric acid were selected as the heteroatom precursors. A mixture of the mentioned precursors was poured into an ethanolic acid

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solution of Pluronic ® F127 for further addition of formaldehyde. The synthesis mixture was then transferred to an autoclave and heated to different temperatures (50, 100, and 150 ºC) and reacted for 10 h. The organic phase obtained after the phase separation was

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dried overnight at 80 ºC and carbonized at 600 ºC for 2 h. The XPS compositional analysis revealed that the increase of the hydrothermal temperature increases the

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concentration of boron from 0.8 to 1.6 wt. % and the amount of phosphorus from 2.3 to 3.6 wt. %. Hydrothermal temperatures below 150 ºC led to materials with prevailing phosphorus species while higher hydrothermal temperatures caused the appearance of phosphonate-like groups. Concerning the chemical state of boron, B–C and B–O species were detected on the carbon surface. Interestingly, the formation of boron phosphide (B–P) was observed at hydrothermal temperatures above 100 ºC. Panja et al. [178] prepared ordered mesoporous carbons decorated with boron and phosphorus by

ACCEPTED MANUSCRIPT nanocasting, using KIT-6 silica as template. In this work the authors selected urea and phytic acid as nitrogen and phosphorus precursors, respectively. The silica/carbon composites were carbonized at 900 ºC for 5h. The functionalization degree and the surface speciation were studied by XPS. The prepared carbon materials presented 3.2

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wt. % of nitrogen belonging to pyridinic and pyrrolic groups. Moreover, 1.7 wt. % of phosphorus was detected on the surface of the ordered mesoporous carbons. The latter heteroatom was observed to exist in the form of tetragonal phosphorus and phosphate–

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like structures. The works reviewed in the present section highlight the need for future research on the co-doping of porous carbon materials. In fact, despite of the larger

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interest on multifunctional graphene materials, synergistic effects arising from different heteroatoms have also been observed with nanostructured mesoporous carbon materials.

4. Conclusion.

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The existing methods for the synthesis of nanostructured mesoporous carbons have been reviewed, emphasizing the control of their texture and surface chemistry. A representative compilation of the most relevant works was made to provide the reader

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with a solid basis for fine manipulation of the mesoporous carbons properties. As explained in Section 2, the diffusional and adsorptive properties of porous carbons

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materials can be adjusted by varying the synthesis parameters of different sol-gel procedures such as polymerization, templating, hydrothermal carbonization or retrodegradation. On the other hand, the electronic, acid/base or redox properties of nanostructured mesoporous carbons can be designed by controllable functionalization with different heteroatoms (Section 3). Understanding the fundamentals of the available preparation methods is a key factor towards developing customized carbon materials to target applications. Although the present review shows the recent progress in synthesis

ACCEPTED MANUSCRIPT and functionalization of nanostructured mesoporous carbons, some challenges to be tackled have also been highlighted, such as: i) maximizing the use of biomass to produce sustainable carbon materials from environmental friendly procedures (Section 2.3 and 3.4), ii) strengthening the control on the type of functionalities introduced in

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contrast to the total amount of dopant (Section 3) and, iii) exploring the functionalization of porous carbon materials with boron, sulfur and phosphorous (Section 3.3), and co-doping with different heteroatoms (Section 3.5). Although the last

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item has been extensively explored for non-porous graphene and carbon nanotubes, wide commercial application is strongly hindered by unacceptably high production

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costs. In this context, porous carbon materials obtained from sustainable precursors and cheap synthetic procedures, and presenting controllable porosity or designed surface chemistry, seem to emerge as the carbon materials for the future.

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Acknowledgements

This work was financially supported by: Project POCI-01-0145-FEDER-006984 – Associated Laboratory LSRE-LCM funded by FEDER through COMPETE2020 -

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Programa Operacional Competitividade e Internacionalização (POCI) – and by national

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funds through FCT - Fundação para a Ciência e a Tecnologia.

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