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Cite this:DOI:10.1039/c2cs35301f Direct synthesis of ordered mesoporous carbons w

Tian-Yi Ma,Lei Liu and Zhong-Yong Yuan*

Received 31st July 2012DOI:10.1039/c2cs35301f

Ordered mesoporous carbon materials have recently aroused great research interest because of their

widespread applications in many areas such as adsorbents,catalysts and supports,gas storage hosts,and electrode materials.The direct synthesis strategy from organic–organic self-assembly involving the

combination of polymerizable precursors and block copolymer templates is expected to be more ?exible in preparing mesoporous carbons,compared with the traditional nanocasting strategy of complicated and high-cost procedures using mesoporous silica materials as the hard template.In this review,we present the fundamentals and recent advances related to the ?eld of ordered mesoporous carbon materials from the direct synthesis strategy of block copolymer soft-templating,with a focus on their controllable

preparation,modi?cation and potential applications.Under the guidance of their formation mechanism,the preparation of ordered mesoporous carbons are discussed in detail by consulting di?erent

experimental conditions,including synthetic pathways,precursors,catalysts and templates.Both the mesopore size and morphology control are introduced.The potential applications of pure mesoporous carbons,nonmetallic-and metallic-modi?ed mesoporous carbons,and some interpenetrating carbon-based composites are demonstrated.Furthermore,remarks on the challenges and perspectives of research directions are proposed for further development of the ordered mesoporous carbons (232references).

1.Introduction

Porous materials are of great importance for fundamental research and practical applications ranging from catalysis,

adsorption,separation,sensing to biotechnology.1–3Notice-ably,ever since the exciting discovery of the novel family of M41S was reported by the researchers at Mobil Research and Development Corporation in 1992,4,5mesoporous materials have attracted more and more attention and shown great potentials in many ?elds,due to outstanding properties,including high surface areas,periodically arranged monodis-persed mesopore space,tunable pore sizes,alternative pore shapes,uniform nanosized frameworks,large particle sizes,and abundant compositions.6,7Besides typical silica-based

Institute of New Catalytic Materials Science,Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),College of Chemistry,Nankai University,Tianjin 300071,China.E-mail:zyyuan@556dfbc94028915f804dc266;Fax:+86-22-23502604;Tel:+86-22-23509610

w Part of the mesoporous materials themed

issue.

Tian-Yi Ma

Tian-Yi Ma received his BSc degree in Chemistry at Nankai University in 2008.He is cur-rently a PhD candidate in Phy-sical Chemistry under the supervision of Prof.Zhong-Yong Yuan.He won the Top-Ten Youth of Nankai Univer-sity and the YANG Shixian Scholarship in 2010,and the Academic Award for Doctoral Students by the Ministry of Education in 2011.His current research is focused on the rational synthesis and applica-tion exploration of mesoporous

materials with complex compositions from carbon,organic poly-mer to metal–organic

frameworks.

Lei Liu

Lei Liu received her BSc degree in Chemistry in 2006at Liao-cheng University,and obtained her MSc and PhD degrees in Physical Chemistry at Nankai University in 2009and 2012,respectively,under the supervi-sion of Prof.Zhong-Yong Yuan.Her work was focused on nano-porous carbon-based materials for applications in heterogeneous catalysis,gas storage and elec-trode materials.She is currently working in the Shandong University of Science and Tech-nology as a Lecturer.

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mesoporous materials,such as the M41S family,4,5SBA series 8,9and their related mesostructures,10–12ordered mesoporous materials with di?erent compositions from pure inorganic or pure organic frameworks to organic–inorganic hybrid frame-works have been widely reported in the past two decades.For example,mesoporous metal oxides and mixed oxides with semicrystalline frameworks,such as TiO 2,Ta 2O 5and ZrTiO 4,13,14mesoporous metal phosphates,borates and chalcogenides,15and other mesoporous non-oxide materials,such as SiC 16and BCN,17were successfully prepared by a direct synthesis strategy using amphiphilic copolymer templates.For organic–inorganic hybrid mesoporous materials with more complex compositions,periodic mesoporous organosilicas (PMOs)containing various bridged organic groups inside the hybrid network have been the most widely investigated.18,19Mesoporous metal phosphonates with organically bridged polyphosphonates were reported,consisting of di?erent functional groups inside the hybrid framework,with tunable mesoporosity from the hexagonal 20to cubic phase.21Periodic hexagonal mesoporous metal sulfonates were synthesized by the coupling of naphthalenedisulfonic acid or ethanedisulfonic acid with metal centers like Cd,La,Cu or Sr in the presence of surfactant templates and crown ether.22These two classes of non-silica-based hybrid mesoporous materials were proved to be useful in heavy metal ion and protein adsorption,CO 2capture,photocatalysis,acid catalysis and catalytic oxidation reactions.23

Porous carbon materials,as another huge family of porous materials,have also been widely studied due to their applications in water and air puri?cation,shape-selective catalysts,gas hosts,templates and components of electrodes for electrochemical double-layer capacitors.24,25The widespread applications of porous carbons are attributed to their remarkable physico-chemical properties,including the hydrophobicity of their surfaces,high corrosion resistance,good thermal stability,high surface area,large pore volume,good mechanical stability,easy handling and low cost of manufacture.26,27Thus the designed synthesis of porous carbon materials with controlled surface properties and structural ordering is important from a fundamental and application point of view.Porous carbons

with di?erent nanoscaled pores were investigated.The most representative method for the synthesis of disordered microporous carbons is the pyrolysis of appropriate carbon precursors.28,29Ordered microporous carbon materials were mainly synthesized using zeolites as templates,30,31and the resultant materials with a high surface area could retain the structural regularity of zeolite templates.For the synthesis of ordered macroporous carbons,various hard templates were utilized,such as a silica colloidal crystal,polymer microsphere,anodic aluminum oxide (AAO)and some mixed hard templates.32,33However,as to a number of other potential uses,such as large molecule separation and recognition,dye adsorption and catalyst supports for biomolecules,the presence of mesopores would be more preferable than micropores and macropores.Mesoporous carbons with disordered pores were obtained through various methods,including catalytic activation using a metal species,34carbonization of polymer/polymer blends 35and carbonization of organic aerogels.36Ordered mesoporous carbons can be synthesized by the nanocasting strategy using mesoporous silicas as the hard template.Ryoo and co-workers 37and Hyeon and colleagues 38were the ?rst to independently report the synthesis of ordered mesoporous carbons using cubic mesoporous silica MCM-48as the hard template in 1999.Subsequently,many other mesoporous silicas,such as the SBA series,39,40MSU-H 41and HMS,42were employed as the hard template.Based on this nanocasting strategy,some more cost-e?ective methods were also invented by making use of as-synthesized mesoporous silica/surfactant meso-phases as starting materials,followed by the introduction of extra carbon precursors.43,44By carbonization of phenyl-bridged meso-porous organosilica/surfactant mesophases without extra carbon precursors,mesoporous carbon with an average pore size of 2.5nm was reported by Lu et al.45To synthesize mesoporous carbons with larger pore sizes,colloid silica particles and silica gels have been explored as hard templates.46Mesocellular carbon foam,with uniform large mesopores,has been produced by partially impregnating mesocellular aluminosilicate foam with phenol/formaldehyde.47

However,some limitations have already emerged for the nanocasting strategy to prepare ordered mesoporous carbons.As shown in Fig.1,many experimental steps were

usually

Fig.1Two typical methods for the preparation of ordered mesoporous carbon materials:the nanocasting strategy from mesoporous silica hard templates and the direct synthesis from block copolymer soft

templates.

Zhong-Yong Yuan

Zhong-Yong Yuan received his BSc degree from Zhejiang Normal University in 1990,and obtained his MSc and PhD degrees from Nankai University in 1996and 1999,respectively.After his post doctoral research at the Insti-tute of Physics,Chinese Acad-emy of Sciences,he joined the Laboratory of Inorganic Materials Chemistry at the University of Namur,Belgium in 2001.In 2005,he was engaged as a Professor at Nankai University.In 2006,

he was awarded the ‘‘Program for New Century Excellent Talents in University’’by the Ministry of Education.He is coauthor of 170ISI publications,3book chapters and 9patents.

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involved:(a)the preparation of ordered mesophase silica/surfactant composites;(b)the removal of surfactant molecules by calcination,extraction or other techniques (for the cost-e?ective method,surfactant molecules were preserved);(c)?lling of suitable carbon precursors into the template pores by either wet impregnation,chemical vapour deposition,or a combination of these two;(d)the carbonization of the carbon precursors inside the pore system of mesoporous silicas;(e)the removal of silica templates by etching.The nanocasting mechanism could lead to a relatively precise negative replica of the template and the syntheses are easy to control due to the ?xed template structure.However,the pore size distribution of the synthesized mesoporous carbons is relatively wider than the mother silica template;and the use of mesoporous silica as a sca?old makes the process expensive,complicated,and time-consuming,and consequently unsuitable for large-scale production and industrial applications.Meanwhile,much progress has been achieved on the direct synthesis of ordered mesoporous carbon materials by self-assembly of copolymer molecular arrays and carbon precursors,which is pioneered by the work of Dai et al.,48,49Nishiyama et al.,50and Zhao et al.51,52This opens a new way for the preparation of ordered mesoporous carbon materials with many advantages com-pared with the previous methods.First,fewer steps are needed in the preparation process (Fig.1):(a)the self-assembly of phenolic resin and block copolymer surfactants into three-dimensional ordered mesostructures according to corres-ponding symmetries (up to now only certain block copolymers could be utilized as the templates);(b)removal of the surfac-tants leaving mesoporous polymers and opened pores with the dimension,shape and topology depending on the size and structure of the supramolecular aggregates;(c)carbonization of the mesoporous polymers to give the ?nal mesoporous carbon materials.The directly synthesized carbon architec-tures via soft templates are more varied and mechanically stable due to the continuous framework.Their formation is dependent on the temperature,type of solvent and ionic strength,which makes the pore structure and surface properties easy to adjust.53–55The entire procedure is low-cost,simple,convenient,and suitable for large-scale industrial production.A comparison between the direct synthesis and nanocasting strategies is summarized in Fig.2.

Due to many advantages of this speci?c strategy and the resultant materials,the present review focuses mainly on the preparation of ordered mesoporous carbon materials by the direct strategy.The synthesis mechanism and the experimental condi-tions that a?ect the mesoporous structure will be discussed,including di?erent carbon precursors and block copolymers,the acidity of the reaction process,the use of various catalysts,and the di?erent preparation pathways [including evaporation-induced self-assembly (EISA)],aqueous route,macroscopic phase separa-tion process and hydrothermal synthesis through autoclaving treatment.Then the pore size and morphology control,the further functionalization and potential applications of the directly synthe-sized mesoporous carbons are presented.Finally we o?er some concluding remarks on the development and challenge of meso-porous carbon materials by direct synthesis.

There are some review articles on porous carbon materials.Lee et al.25and Xia et al.26independently reviewed carbon materials with micropores,mesopores and macropores synthe-sized from hard or soft templates.Liang et al.reviewed the mesoporous carbons obtained from both nanocasting and direct synthesis strategies,as well as some applications of carbons synthesized by hard templates.24Wan et al.presented a short review on the ordered mesoporous carbons and phenolic resin polymers from soft templates,mainly based on their contribution to this area;the history and development of the mechanism with supramolecular aggregates as the directing agent was clari?ed.54The present review highlights the direct-synthesized mesoporous carbons by combining the excellent work from worldwide researchers.The key elements in the preparation are discussed in detail,which aims at guiding the designed synthesis of mesoporous carbons and enlightening the future work in this speci?c ?eld.Moreover,the modi?cation and application of direct-synthesized carbons are elaborated,in an attempt to attract more attention and determine the real value of these materials.

2.Synthesis

2.1Synthesis mechanism

There were ?rst some attempts for mesoporous carbon preparation,consulted by the synthesis of ordered mesoporous silicates,in which the electrostatic interaction between the silicate precursor and the positively charged surfactant micelle leads to a supramolecular assembly.6,56However,by using phenol-fomaldehyde,57resorcinol-formaldehyde 58,59or oxidized polycondensed aromatic hydrocarbons 60as the precursors and cationic quaternary ammonium (e.g.,hexadecyl trimethyl ammonium bromide)as the surfactant,the majority products were disordered mesostructures with wide pore size distribu-tions,or the mesophases collapsed after aging at low tempera-tures.This could be attributed to the uncontrollable polymerization of organic precursors,the di?culty in removal of cationic micellar templates without destruction or obstruc-tion of pore structures,and the weakened interaction between organic polymer frameworks and quaternary ammonium surfactants.57,60,61

Then inspired by the successful preparation of ordered mesoporous silicates from amphiphilic block copolymers,

9,10

Fig.2A comparison between the direct synthesis and nanocasting strategies to prepare ordered mesoporous carbons.

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these templates were used in the synthesis of mesoporous carbons.However,mesoporous carbons could not be fabricated by the direct pyrolysis of self-assembled block copolymers,62because of their very poor carbon yields in carbonization reactions.The melting of the linearly structured copolymers before carboniza-tion reactions occur and the massive loss of carbon in the form of volatile carbon-containing species during pyrolysis,both result in the inability to preserve the pre-organized ordered nanostructures.Therefore,adding suitable carbon precursors to the supra-molecular aggregates of block copolymers is necessary.By blending an amphiphilic diblock copolymer,poly(ethylene oxide)–poly(ethylene-alt -propylene)(PEO–PEP),with a poly-merizable epoxy resin,Bates et al.obtained ordered meso-structured thermosetting materials.63The epoxy resin selec-tively mixed with the PEO block and the separation of the block copolymer and epoxy was avoided due to the strong dipole polarization between them.Di?erent morphologies containing lamellar,cubic bicontinuous,hexagonally packed cylinders and body-centered cubic packed spheres were obtained depending on epoxy concentration.However,the template was not removed,possibly because of the small di?erence in the chemical and thermal stability between the block copolymers and epoxy resins.

The direct synthesis of ordered mesoporous carbons is now resorting to the hydrogen-bonding interaction between templates and precursors,which has been proved to be an e?cient route to prepare mesoporous materials.9,64The hydrogen-bonding interactions between the pyridine-group-containing or PEO-containing block-copolymers and the hydroxyl-group-containing organic precursors are demonstrated in Fig.3.Ikkala and colleagues prepared ordered mesostructured or mesoporous polymeric composites by utilizing the strong hydrogen-bonding interaction between phenolic hydroxyl groups and poly(2-vinyl-pyridine)–poly(isoprene)(P2VP–PI)65or poly(4-vinylpyridine)–polystyrene (P4VP–PS).66–68The self-assembled mesostructure could be tuned from lamellar to cylindrical and spherical phases,by tailoring the block lengths of PI or PS.Unfortunately,the pore network of the synthesized materials were often thermally unstable,perhaps because of the lack of the 3D interconnecting sites of the linear novolac phenolic resin.54Thus,only porous

polymers and polymeric composites were obtained instead of the pure carbon framework.

The real breakthrough of the preparation of ordered mesoporous carbons was achieved by Dai and co-workers,who developed a stepwise assembly approach to fabricate highly ordered nano-porous carbon ?lms using polystyrene–blockpoly(4-vinylpyridine)(PS–P4VP)as the soft template.48Using N ,N 0-dimethylformamide (DMF)as the solvent,the resorcinol monomers were ?rst pre-organized into a well-ordered PS–P4VP nanostructured ?lm through spin-coating followed by solvent annealing in DMF/benzene vapor.The self-assembly of the PS–P4VP/resorcinol mixture was driven by the hydrogen-bonding inter-action between resorcinol and the P4VP block.The su?cient repulsion between the PS domains and the P4VP/resorcinol domains favors the reserve of the highly ordered alignment together with the enrichment of resorcinol in the matrix upon solvent evaporation.The microphase-separated ?lm was then exposed to formaldehyde vapor to form a highly cross-linked phenolic resin located in the P4VP domain,followed by the decomposition of the block copolymer template to generate ordered nanopores,and the carbonization of the nanostruc-tured phenolic resin to form the carbon pore walls at 8001C in nitrogen.Highly ordered and crack-free mesoporous carbon thin ?lms were obtained by this method.The mesopores were oriented perpendicular to the ?lm surface with a pore diameter of 33.7?2.5nm and a wall thickness of 9.0?1.1nm.The amphiphilic copolymers direct the formation of the phenolic resin nanostructure and serve as templates for the nanopores.Nishiyama and co-workers 50reported the synthesis of mesoporous carbons with an ordered channel structure (COU-1),which were fabricated via the direct carbonization of an organic–organic nanocomposite.The choice of an appropriate set of thermosetting polymers and thermally decomposable surfactants is the most important factor in controlling the structure of these ordered mesoporous carbons.Resorcinol/formaldehyde and triethyl orthoacetate were used as the carbon co-precursors;commercially available and inexpensive pluronic PEO–PPO–PEO [poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide)]block copolymer F127(EO 106PO 70EO 106)was used as the surfactant.The reaction mixture containing the surfactant and carbon precursors was spin-casted on a silicon substrate,followed by polymerization and carbonization at di?erent temperatures.The resultant COU-1materials showed a periodic mesostructure consisting of hexagonally arranged pores,which were parallel to the ?lm surface.The whole preparation procedure of their method could be accomplished in the liquid phase instead of the formaldehyde gas that was used in the work of Dai et al.48A carbon co-precursor triethyl orthoacetate has to be used,but its role is uncertain.The addition of triethyl orthoacetate apparently has a favorable e?ect on the stabilization of the periodic mesostructure,since a poor periodic structure was obtained in the absence of triethyl orthoacetate.

Zhao’s group has made major contributions to the direct synthesis of ordered mesoporous carbon materials through an organic–organic self-assembly approach.Highly ordered mesoporous carbons,denoted as FDU-15and FDU-16,were synthesized through a solvent evaporation induced self-assembly method (EISA)by using amphiphilic triblock

copolymers

Fig.3Hydrogen-bonding interaction between (a)pyridine-group-containing and (b)PEO-containing block-copolymers and the hydroxyl-group-containing organic precursors.Reprinted with permission from ref.54.

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(PEO–PPO–PEO)as templates and a soluble low-molecular weight resol of phenol and formaldehyde as the precursor.51Fig.4illustrates the preparation procedure of the ordered mesoporous carbons.Low-molecular-weight resol was ?rst mixed with F127in an ethanolic solvent,followed by the evaporation of the solvent.The choice of resol as a precursor is essential for the successful organization of organic–organic mesostructures because resol has a large number of hydroxy groups,which can interact strongly with F127surfactant through the formation of hydrogen bonds.Driven by the hydrogen-bonding interaction between the PEO block and resol,an ordered mesostructure of the resol/F127composite formed.Curing the resol at 1001C solidi?ed the polymeric framework,yielding a rigid zeolite-like hydrocarbon network with three-connected benzene rings through the formation of covalent bonds.Because of the di?erence in chemical and thermal stability between the resin and the triblock copolymer,the template could be removed by calcination at 3501C in nitrogen.The carbonization was carried at 700–14001C under an inert atmosphere to transform the polymeric framework into the carbon mesostructures.The resultant solids had well-structured mesoporosity with uniform pore size distribution.After carbonization at 9001C,FDU-15had a large BET surface area of 968m 2g à1and a pore volume of 0.56cm 3g à1;FDU-16had a surface area of 778m 2g à1and a pore volume of 0.44cm 3g à1.

Although many self-assembly mechanisms have been utilized for the synthesis of mesoporous oxide materials under di?erent pH conditions,9,10,14,15such as I àS +,I +X àS +,and I 0S 0(I:inorganic precursor,S:surfactant,X à:Cl à,Br à,I à,NO 3à,H y SO 4à2+y ,H y PO 4à3+y ,etc.),only self-assembly induced by hydrogen-bonding (I 0S 0)has been mostly utilized for making ordered mesoporous carbon materials.Noticeably,by using strongly acidic conditions,Dai and co-workers expanded the self-assembly process to coulombic interaction (I +X àS +)for the successful preparation of ordered meso-porous carbons,denoted as C-ORNL-1.69C-ORNL-1could be synthesized in a wide range of acid concentrations (e.g.,0.5–2.0mol L à1)in the mixed solution of ethanol and water,

with resorcinol/formaldehyde and pluronic surfactant F127as precursors and template,respectively.Under highly acidic conditions,phenols and cross-linked phenols are protonated,and the EO blocks of F127are also protonated.The presence of Cl àwas con?rmed as a mediator.Thus the I +X àS +mechanism driven by coulombic interactions was suggested.10The self-assembly of surfactant–polymer nanocomposites could be attributed to the combination of coulombic inter-actions and hydrogen-bonding interaction.This method was suitable for large-scale synthesis of ordered mesoporous carbons with high thermal stability,but mant halogenide ions were introduced to the reaction system,which would be unfavorable for some applications and green production.As we can see through the evolution of the direct synthesis strategy of ordered mesoporous carbons,the addition of carbon precursors besides the surfactant template is necessary.The development trend,considering economical and clean production,is to utilize fewer types of carbon sources without extra carbon co-precursors and to use precursors in liquid or even solid phases,avoiding toxic gas emission.Although the PEO–PPO–PEO type copolymers are the most utilized templates,and the driving force in the organic–organic assembly is mainly based on the hydrogen-bonding interaction,other interactions like coulombic and van der Waals interactions are also very promising to be involved in the assembly.If so,more kinds of templates,like cationic quaternary ammonium and nonionic alkyl PEO oligomeric surfactants,can be used independently or together with block copolymers in the preparation,which can break the threshold of the pore size in the present carbons by the direct strategy to obtain smaller or even multi-scaled mesoporosity.2.2Synthesis pathway

2.2.1EISA method.Ordered mesoporous carbon materials can be successfully synthesized through di?erent pathways,and some typical examples are summarized in Table 1,which contains the main experimental parameters.The EISA of block copolymer and resols (phenol,resorcinol,or phloroglucinol/formaldehyde)has been demonstrated as one of the e?cient paths.48,50,51The EISA method avoids the cooperative assembling process between the precursor and the surfactant template,which can separate the cross-linking and thermopolymerization processes of the resols from the assembly.70It is a facile method under wide synthetic conditions,such as acidity of the reaction solution,surfactant and phenol/template ratio.Evaporation of the solvent can be simply performed in an open-mouthed dish,51and a spin-coating or dip-coating technique is sometimes applied.48,50Since the mesostructures are formed on the surface by the EISA method,it meets the demands of preparing mesostructured ?lms.

By utilizing the EISA method,a series of ordered meso-porous carbons with various structures including 2D hexago-nal (p 6mm ),3D bicontinuous (Ia %3

d ),body-centered cubic (Im %3

m )and lamellar mesostructures could be prepared from an organic–organic assembly of PEO–PPO–PEO triblock copolymers with low-molecular-weight phenolic resin precursors.71The step-wise synthesis procedure is shown in Fig.4.The precursor derived from phenol and formaldehyde was formed in a basic

solution

Fig.4Scheme for the preparation of ordered mesoporous carbon frameworks by the EISA method.Reprinted with permission from ref.51.

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catalyzed by NaOH,while the EISA process was carried out in a nearly neutral system.Ethanol and other organic solvents,such as methanol,acetone,and tetrahydrofuran (THF),can be used as a solvent,but water is not adopted because the evaporation is too slow.The evaporation of ethanol progres-sively enriches the concentration of the copolymer;and the organic–organic assembly between the block copolymer and the resol precursor is mainly driven by the hydrogen-bonding interaction.71The obtained carbons exhibited well-structured porosity with high surface area,large pore volume and uniform pore size.The high thermal stability of mesoporous carbons is attributed to the covalently bonded constructions and the very thick pore walls.They also have a higher mechanical stability in comparison with CMK-3that was prepared by the nanocasting strategy,39which may be attributed to the continuous frame-work of the former.

Complicated self-assembly behaviors may occur during the solvent evaporation and thermopolymerization stages of the EISA synthesis method,and thus many e?orts have been made to study the surfactant–resol system using di?erent techniques.72,73Very recently,Goldfarb and co-workers studied the transformation of F127-phenolic resol composites

in the preparation of FDU-16(Im %3

m )by the EISA method,74utilizing the spin probe electron paramagnetic resonance (EPR)method,which is widely used in mechanistic studies of ordered mesoporous silica materials.75,76In the EPR measurement,two pluronic-derived spin probes with di?erent lengths of PPO and PEO blocks were used to sense the PEO

region (by the probe with long PEO chains)and the PPO–PEO interface (by the probe with relatively shorter PEO chains),respectively.77,78A better understanding of the EISA pathway was accessed,and it was found that the F127-resol composite was formed through hydrogen-bond interactions during the solvent evaporation process.The resol was distributed throughout the PEO blocks,all the way to the PPO–PEO interface.Weaker hydrogen bonds between the PPO block and resol may also be formed.At the end of the evaporation there is no polarity gradient along the PEO blocks,and the meso-phase is not well-structured without a de?nite PPO–PEO interface.During the following thermopolymerization stage,the hydrogen bonds of resol with the PPO block and close to the PEO–PPO interface are disrupted,and the PPO and the PEO regions are better segregated.The polymerizing resol is driven out to the outer region of the PEO corona,where it continues to polymerize,trapping part of the PEO chains to form the pore wall.A corona of resin–pluronic composite and a resol-free PPO core are formed,and the ?nal periodic meso-structure is ?xed during this thermopolymerization step.742.2.2Dilute aqueous route.Despite the success of the EISA method,it can barely meet the demands of large-scaled industrial production of powder materials due to the engineering di?culties,including sample collection,reactor design (large surface vessel),etc.Moreover,uniaxial structural distortion is always observed for the mesoporous materials prepared by the EISA method,and pore size tailoring is still a great challenge.79

Table 1Synthetic conditions for ordered mesoporous carbon materials by the direct strategy Reaction pH Precursor Pathway Solvent Catalyst Template Mesophase Annotation a Ref.Neutral Resorcinol EISA DMF Not

available PS–P4VP p 6mm Formaldehyde gas 48Neutral Phenol EISA THF Not

available PEO–PS Fm %3

m Resol precursor 105Neutral Phenol EISA THF NaOH PEO–PMMA Fm %3

m Resol precursor

106

Neutral Phenol EISA Ethanol NaOH P123,F127

p 6mm,Ia %3d,Im %3m Resol precursor 51,71,74,149

Neutral Phenol EISA Ethanol NaOH PPO–PEO–PPO Fd %3

m Resol precursor 107,108Basic Phenol Dilute aqueous

Water

NaOH P123,F127p 6mm,Ia %3d,Im %3m

Hexadecane or decane

52,80,115,116Basic Phenol Hydrothermal Water NaOH P123,F127p 6mm,Im %3

m Autoclaving 87,124Basic Resorcinol EISA

Ethanol NaOH F108Im %3m

—

90Basic Resorcinol Hydrothermal

Water/ethanol

Lysine F127p 6mm Autoclaving 101Acidic Resorcinol Hydrothermal Water/

ethanol

HCl F127p 6mm Autoclaving 86,134Acidic Resorcinol Hydrothermal Water/

ethanol

Citric acid F127p 6mm Autoclaving 102,103,104Acidic Resorcinol EISA Ethanol HCl F127p 6mm,Im %3

m Furfural 95Acidic Resorcinol EISA Water/

ethanol

HCl F127p 6mm Triethyl orthoacetate 50Acidic Resorcinol Phase separation Water/ethanol HCl F127p 6mm Strong acid condition 69,84,85,146Acidic Resorcinol Phase separation Water Glutamic acid F127p 6mm —

100

Acidic Resorcinol/phloroglucinol EISA Water/

ethanol

HCl F127Fmmm,p 6mm Spin or dip coating 91,92,93Acidic Phloroglucinol Phase separation Water/ethanol HCl F127p 6mm Low HCl concentration 49,146Acidic

Phloroglucinol

Phase separation Water/ethanol

HCl

F127

p 6mm

Glyoxal

Special experimental conditions or reagents needed.

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Chem.Soc.Rev.

The surfactant induced self-assembly method in aqueous solution,which has been widely used in the synthesis of many ordered mesoporous siliceous materials,is considered as an alternative path to prepare mesoporous carbons.The ordered mesostructures derived from this method have the advantages of few defects,easily tailored pore sizes,and well-controlled 556dfbc94028915f804dc266pared with the EISA method,the aqueous route exhibits better reproducibility and an unlimited fabrica-tion batch size,which is suitable for industrial production.A dilute aqueous pathway was used to fabricate the ordered mesoporous carbon (FDU-14)for the ?rst time by Zhao’s group.52The synthesis was performed through the organic–organic self-assembly of pluronic surfactant P123(EO 20PO 70EO 20)with resols (phenol/formaldehyde)in a dilute aqueous solution.FDU-14possessed a typical 3D bicontinuous cubic

space group (Ia %3

d ).Th

e synthesized materials,after carboni-zation at 7001C,had a high surface area o

f 1150m 2

g à1and uniform pore size distribution at 3.8nm.Many micropores were derived on the pore wall during the carbonization process.Subsequently,the aqueous pathway was used to the synthesize a series of ordered mesoporous carbon materials wit

h hexagonal and cubic structures through the self-assembly of phenol/formaldehyde resols and triblock copolymer templates.With the assistance of hydrocarbon (hexadecane or decane)as a swelling agent,highly ordered mesoporous carbons FDU-15with 2D hexagonal (p 6mm )structure and enlarged pore sizes could be synthesized by using P123as the template.80The hydrocarbon molecules can interact with the hydrophobic PPO segment of the triblock copolymers leading to the lattice enlargement or the phase transition,which have been used as pore size swelling agents for the synthesis of mesoporous silicate materials in aqueous conditions with triblock copolymers as the template.81–83Ordered mesoporous carbon FDU-16

with body-centered cubic structure (Im %3

m )was also synthesized under basic aqueous conditions by using template F127with a relatively wide range of concentrations.80

The synthesis in the aqueous system was supposed to follow a cooperative self-assembly route.The resol interacts with block copolymers through one-layer hydrogen bonds between the phenolic groups of the resol and the ether groups of the PEO.The resulting complexes further assemble into a mesostructure,and the resol polymerizes around the block copolymer aggregates to form large phenolic resin–pluronic composite particles.After removal of the triblock copolymers and carbonization,the ?nal mesoporous carbon materials are obtained.Basic conditions,with a pH value of around 9.0,of the reaction solution is necessary to obtain a rigid and stable mesoporous framework,while the hydrogen bonds are weakened in the relatively strong basic solution and the polymerization is too slow in neutral or weak acidic media.52,80Thus,the narrow acidity range is a limitation of the aqueous pathway.

2.2.3Macroscopic phase separation.When resorcinol or phloroglucinol is used as the precursor to polymerize with formaldehyde in a acidic system,it undergoes a macroscopic phase separation process during the generation of ordered mesoporous carbons.49,69,84,85For example,Zhang and co-workers synthesized ordered mesoporous carbons through the self-assembly

of resorcinol/formaldehyde and surfactant F127in the mixed solution of ethanol and water.84,85The synthesis was performed in a strong acidic system with hydrochloric acid as the catalyst.After adding the precursors,F127and hydro-chloric acid,into the ethanol/water solution,the mixture turned cloudy and began to separate into two phases.The upper phase (consisting mainly of water/ethanol)was discarded,whereas the composite of resorcinol/formaldehyde oligomers and F127was formed in the lower polymer-rich phase.After aging,curing and carbonization,mesoporous carbons could be obtained.The phase separation of the polymer-rich phase from the ethanol/water solvent is the key step in their synthesis.Resorcinol and formaldehyde ?rst polymerized slowly to ethanol-soluble resol oligomers with a low polymerization degree,which would interact with the PEO segments of the self-assembled triblock copolymers through hydrogen bonds.When the linear resols further polymerized with each other to achieve a relatively large molecular weight,a glue-like polymer product began to separate from the solvent,resulting in macroscopic phase separation.The larger amount of resorcinol would lead to stronger hydrogen-bonding interactions and the longer aging time would lead to less defects,thus it was found that both the excess amount of resorcinol (resorcinol/formaldehyde Z 1/2)and the long aging time (96h)were favorable for the formation of highly ordered mesoporosity.84,85The resultant mesoporous carbons possessed high surface areas of 674–781m 2g à1and large pore volumes of 0.52–0.72cm 3g à1.84The same macroscopic phase separation phenomenon was also observed when Dai and co-workers performed the synthesis of ordered mesoporous carbons in the acidic solution by using phloroglucinol 49or resorcinol 69as the monomer.

2.2.4Hydrothermal autoclaving process.Hydrothermal preparation through an autoclaving treatment is a more e?cient approach under controlled temperature and pressure that has been extensively used in the preparation of many nanoporous materials.This powerful method is faster and more energy e?cient than conventional aqueous chemical processing pathways.Yuan and co-workers demonstrated a low-temperature autoclaving method to prepare ordered meso-porous carbon monoliths with F127template and resorcinol/formaldehyde precursor in a mixed ethanol/water acidic solution.86The precursors,surfactant F127and catalyst HCl were mixed at room temperature into the ethanol/water solution,followed by autoclaving treatment at 501C.Ordered mesoporous carbons could be collected after ?ltration,washing,drying and carbonization.A 2D hexagonal pore system with uniform pore size of B 5nm and high surface area of B 675m 2g à1was obtained.The optimal conditions to access ordered mesoporosity were ?xed at the formaldehyde/resorcinol molar ratio r 2and the autoclaving treatment time Z 48hours.In comparison,they also prepared mesoporous carbons under the identical conditions using a mixed ethanol/water solvent,but through the EISA method.It was interesting to ?nd that only wormhole-like mesopores were obtained when they tried to mold the meso-porous carbons from the EISA method into di?erent morphologies.It could be attributed to the problem that the shapes of the molds made the reaction mixture possess only a small area to come into

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contact with the atmosphere during the EISA process,and the existence of water caused the solvent evaporation rate to not be easily controlled.However,highly ordered structures can be obtained if a directional force is applied to the polymer.In the low-temperature autoclaving method,a certain temperature (501C)and autogenous pressure enhanced the interaction between the molecules and induced the faster polymerization rate,which led to the periodic assembly between precursor and template molecules.86Zhao and co-workers synthesized meso-porous carbons by using phenolic resols (phenol/formaldehyde)as precursors and mixed triblock copolymers of P123and F127as templates via a hydrothermal autoclaving approach in a basic system.The autoclaving treatment was carried out at 1001C for 10hours,leading to ordered 2D hexagonal mesoporous carbon materials with a uniform pore size around 3nm.87

Each synthetic pathway has its own advantages and short-comings.The dilute aqueous route is limited by the narrow reaction acidity,while the EISA method can operate under wide preparation conditions,like acidity and precursor ratios.On the other hand,the aqueous route is much more facile for large-scaled production compared with the EISA method,which is restricted by the reactor size.Although the hydro-thermal autoclaving process is a quick and e?cient route,energy consumption is required for the high temperature and pressure.The macroscopic phase separation method is easily operated and has high yields,but strong acidic conditions are often needed,which may result in the corrosion of equipment and halogenide ion pollution.Thus,one should select the most suitable pathway depending on the practical situation.The development of a low-cost,environmentally friendly and reproducible method is still urgent.2.3

Precursor

Phenol,resorcinol,and phloroglucinol are the three most common phenolic resin monomers used for the preparation of mesostructured resin polymers and the corresponding mesoporous carbons.These three monomers can form single,double,and triple hydrogen bonds to the polyethylene oxide (PEO)chains of the PEO–PPO–PEO block copolymers,which is the driving force of the organic–organic self-assembly.The reactivity of di?erent monomers with formaldehyde follows a trend of phloroglucinol c resorcinol c phenol.Under acidic conditions,the reaction and polymerization of phloroglucinol,resorcinol or phenol with formaldehyde involves the proto-nation of hydrated formaldehyde and electrophilic aromatic substitution reactions of phenols.88Therefore,the concen-tration of acid and the reactivity of di?erent phenols are two key factors in determining the polymerization rate of phenolic resins.

The utilization of di?erent monomers to prepare porous carbons was compared at low acid concentration (10à2mol L à1)and room temperature in a mixed solution of ethanol and water.49The polymerization of phenol was very slow in the presence of F127,and the resulting polymer yielded porous carbons possessing very low surface areas (o 5m 2g à1).Resorcinol polymerized slightly faster than phenol at room temperature,but the speci?c surface area was mainly contributed to micropore surfaces.Phloroglucinol,which was found to be an excellent

precursor for the synthesis at low acid concentration,poly-merized much faster than either resorcinol or phenol.The macroscopic phase separation of the reaction mixture occurred within 30min,and the phloroglucinol/formaldehyde oligomers existed in the lower layer of a polymer-rich phase.The super-iority of using phloroglucinol was attributed to the high hydroxy density in the oligomers formed from phloroglucinol,providing the greater driving force for the self-assembly inter-action with the PEO blocks than those provided by the oligomers from phenol and resorcinol.49The ordered meso-porous carbons could be obtained under mild reaction condi-tions and in a wide composition range (weight ratios of phloroglucinol:F127:formaldehyde :ethanol :water :HCl E 1:1:0.26–0.48:150–1:90–10:0.03–0.003).However,the fast polymerization rate of phloroglucinol with formaldehyde is some-times not favorable for the formation of ordered mesostructures.For example,only irregular mesoporous carbons were obtained when the synthesis was performed under higher acid concentra-tions (HCl/phloroglucinol weight ratio of 0.036–0.144).89

On the other hand,resorcinol monomers were used in the original reports for ordered mesoporous carbon preparation through the spin-coating technique,which had to be polymerized with formaldehyde vapor 48or by the assistance of co-precursor triethyl orthoacetate.50Although it seems not to be a suitable monomer at low acid concentrations and room temperature (mainly microporosity)compared with phloroglucinol,49resorcinol/formaldehyde precursors have been used to success-fully prepare ordered mesoporous carbons under strong acidic conditions as we mentioned above (Table 1).69,84–86In addi-tion,Song and co-workers prepared mesoporous carbons with Im %3

m symmetry by direct carbonization of self-assembled F108(EO 132PO 50EO 132)and resorcinol/formaldehyde compo-sites.90Their synthesis was accomplished in a basic ethanol solution catalyzed by NaOH through the EISA method.Moreover,by using mixed phenolic resin monomers of resor-cinol and phloroglucinol,Tanaka and co-workers synthesized ordered mesoporous carbon membranes with F127as the template catalyzed by HCl.91–93The obtained materials possessed a face-centered orthorhombic Fmmm symmetry,and high speci?c surface area of B 670m 2g à1after carboniza-tion at 6001C.93It was demonstrated that using a combination of phloroglucinol and resorcinol reduced the weight loss of the resin compared with using only resorcinol/formaldehyde and thus improved the thermal stability.

Phenol,as a cheaper monomer than resorcinol and phloro-glucinol,has been widely used to prepare ordered mesoporous carbons through the EISA,51,71dilute aqueous,52,80and hydrothermal pathways (Table 1).87Low-molecular-weight and soluble resols derived from the polymerization of phenol and formaldehyde in a basic system were chosen as the organic precursors.There are many advantages for this speci?c precursor.First,phenol has fewer reactive sites than those in resorcinol and phloroglucinol,and thus the polymerization rate of resols is easily controlled,which favors the ordered assembly between phenolic resins and copolymer templates.Many benzyl hydroxyl groups,besides phenolic hydroxyl groups,formed during the polymerization of phenol and formaldehyde in basic system,which enhanced the hydrogen-bonding interactions with PEO blocks of the copolymer

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PEO–PPO–PEO templates.Furthermore,a rigid zeolite-like hydrocarbon network with three-connected benzene rings through the formation of covalent bonds was obtained for phenolic resin after the thermopolymerization process,94and this stable mesostructure could be preserved upon template removal by calcination.For the EISA method,resol precursors are di?cult to evaporate,which results in not only the constant composition with high yield but also the avoidance of formal-dehyde emission.71

In fact,phenol,resorcinol,and phloroglucinol/formaldehyde pairs are not the only reported precursors to prepare meso-porous polymers and corresponding mesoporous carbons.There are also some illustrations of replacing formaldehyde that is a severe air pollutant.For example,highly ordered mesoporous carbon spheres with hexagonal and body-centered cubic phases were designed inside an ethanol-in-oil emulsion system by utilizing the EISA method combined with the suspension polymerization method.95The organic–organic assembly was accomplished between the surfactant F127and the carbon precursor of resorcinol–furfural oligomers catalyzed by HCl in the presence of hexamethylenetetramine.The typical hexagonally arranged structures with large domains are shown in the TEM images (Fig.5a and b).With decreasing the added amount of F127,the cubic mesophase Imm was then obtained,shown in Fig.5c–e.Another example is the use of glyoxal instead of formaldehyde.As mentioned above,phloroglucinol reacts faster than resorcinol or phenol with formaldehyde.96To obtain an ordered porous carbon,the fast reactivity of phloroglucinol must be countered by a low acid concentration 49or a slower reacting aldehyde than formaldehyde.Glyoxal is a slow reacting aldehyde,potentially due to the possible enol-tautomerization that can stabilize a reaction intermediate structure and hinder further cross-linking.Dai and co-workers synthesized highly ordered mesoporous carbons with a surface area of 410m 2g à1and a uniform pore size at 7.5nm from the phloroglucinol/glyoxal precursor in the presence of F127.97Moreover,Yuan and co-workers developed a novel method to synthesize porous polymer and carbon by using hexamethyl-enetetramine as one of the carbon precursors under the

hydrothermal conditions.98First,the partial hydrolysis of hexamethylenetetramine produced formaldehyde and ammo-nia.Ammonia could catalyze the polymerization of resorcinol and formaldehyde and initiate their condensation process.As the reaction proceeded,more hexamethylenetetramine slowly decomposed to provide a gradual and controlled supply of formaldehyde and ammonia,which quickly reacted with resorcinol to form numerous hydroxymethyl-substituted and/or amino-methyl-substituted species.These hydroxymethyl-substituted and/or aminomethyl-substituted species were positioned at the surface of the polymer owing to the electrostatic interaction with the ammonia molecules,and further cross-linking of these species occurred during the hydrothermal treatment.This strategy is user-friendly,avoiding the use of toxic formaldehyde,which is promising to construct many mesostructured carbon materials.Although most of the organic–organic self-assemblies have been applied for phenolic resins up to now,there are still some attempts of other polymer compositions with a comparable mesostructure.Melamine resins belong to the class of amino resins or amino plastics,which are closely related to their phenolic analogs both in synthesis and application.Hong and co-workers successfully synthesized mesoporous melamine resins using hexamethoxymethyl melamine as the monomer and F127as the template.99The template could be removed by solvent extraction,yielding 2D hexagonal mesoporous polymers with a surface area of up to 258m 2g à1and pore diameter of 7.8nm.Though mesoporous carbons were not yet obtained,which is probably due to the ?exible pore walls of melamine resins,this method extends the composition of mesoporous polymeric frameworks.2.4Catalyst

The polymerization of phenolic resins can be performed in either acidic or basic systems.For the preparation of ordered mesoporous carbons,the polymerization between phloro-glucinol or resorcinol precursors with formaldehyde is often catalyzed by hydrochloric acid,49,50,69,84–86or sodium hydro-xide,90while low-molecular-weight resols derived from the polymerization of phenol and formaldehyde in basic systems are also chosen as the precursors (Table 1).51,52,71,80However,it is still necessary to ?nd alternative catalysts for the commonly used NaOH and HCl,which are not convenient and environ-mentally friendly due to the corrosion of the equipment and the pollution of halogenide ions.

An amino acid was used as the catalyst in the direct synthesis of ordered mesoporous carbons.Lu et al.synthesized mesoporous carbons through the self-assembly of resorcinol/formaldehyde polymer and surfactant F127in aqueous phase with glutamic acid as the catalyst.100The highly ordered hexagonal mesophase was obtained with a p 6mm space group.The obtained materials possessed high surface areas of around 720m 2g à1,which could be easily tuned by varying the pyrolysis temperature.The glutamic acid acted as a catalyst to polymerize resorcinol and formaldehyde,and simultaneously it initiated the interaction between resorcinol/formaldehyde polymer and F127through hydrogen-bonding,resulting in self-assembly into a mesostructure.Thus,the amount of glutamic acid was important in determining the mesopore ordering of

carbons.

Fig.5TEM images of ordered hexagonal mesoporous carbon spheres recorded from the (a)[110]and (b)[001]directions,and cubic mesoporous carbon spheres recorded from the (c)[110],(d)[111]and (e)[100]directions.The insets are the corresponding fast Fourier transform di?ractographs.Reprinted with permission from ref.95.

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The particular feature of their synthesis is that no inorganic compounds were involved,and therefore carbon materials with high purity can be achieved.Lu and co-workers further investigated the function of amino acids in the direct synthesis of mesoporous carbons by using organic base lysine as both the polymerization catalyst and mesostructure assembly promoter.101As shown in Fig.6,lysine molecules could form intra-molecule salts,so the deprotonated carboxyl group and the protonated NH 3+group could form hydrogen bonds with the –OH group of resorcinol and the hydrophilic EO (N–H áááO)segment of F127.Thus the lysine molecule would enhance the assembly of the mesostructure between F127and phenolic resins.Moreover,the basic lysine could provide a basic environment for the polymerization of resorcinol and formaldehyde.101

Independently,Yuan and co-workers demonstrated the synthesis of ordered mesoporous carbon materials with the use of citric acid to catalyze the polymerization of resorcinol/formaldehyde resin.102–104The obtained carbon materials,with high thermal stability,have a 2D hexagonal mesopore system with a uniform pore size of B 5.2nm and a high surface area of 612–851m 2g à1.102A reaction temperature of 50–801C and a molar ratio of formaldehyde to citric acid of Z 3favored the formation of the highly ordered p 6mm mesostruc-tures.Citric acid played an important role in the formation of a periodic mesostructure,being attributed to the abundant –COOH groups in citric acid,which may enhance the inter-action between the resorcinol/formaldehyde precursor and F127.The enhancement e?ect further caused the polyethylene oxide (PEO)segments of triblock copolymer to embed into the resins during the polymerization process.Thus,more micro-porosity was introduced than when catalyzed by HCl,because the removal of the PEO block would cause some dis?gurement in the pore wall of the ?nal products.10The synthesized mesoporous carbons also had a high thermal stability,due to the highly cross-linked resorcinol/formaldehyde polymers and the resulting rigid carbon framework.A good mesoscopic order was still re?ected after carbonization at 600–10001C,and the pore size was almost unchanged,which is superior to

the previously reported ordered mesoporous carbons su?ering from serious structure shrinkage and collapse.51,52,71,80,902.5Template

A proper template should interact strongly with both precursors and be easily removed.The PEO–PPO–PEO type amphiphilic triblock copolymers,such as F127,F108and P123,are most widely utilized (Table 1).The organic–organic self-assembly is driven by the hydrogen-bonding interaction,and the PEO–PPO–PEO templates can be easily removed at low temperature without destroying the resin framework,because they contain many oxygen atoms and have a low glass transition temperature (lower than room temperature).71Although cationic quaternary ammonium and nonionic alkyl PEO oligomeric surfactants were also used to prepare mesostructured resin-surfactant composites,these templates were di?cult to remove.

Following this concept,other PEO-containing block copolymers can be chosen as templates for the mesoporous carbon preparation.By using a laboratory-made poly(ethylene oxide)–polystyrene diblock copolymer PEO 125–PS 230as the template,ordered mesoporous carbons with ultralarge pore sizes and face-centered cubic closed-packing mesostructures (Fm %3

m )were obtained through the EISA method.105

The long PS segment was the key to obtaining the large mesopores with a cubic closed-packing structure.During the solvent evaporation process,resol precursors could interact with the PEO segment of the diblock copolymer by hydrogen-bonding,and discrete cubic-packed spheres of PS blocks were formed in a matrix of the PEO block and resin.Upon thermosetting,the framework polymerized,causing the mesostructure of discrete micelles to

form the Fm %3

m mesophase.The pore size depended on the length of the hydrophobic PS blocks,while the micropores were generated to connect the primary mesopores due to the pyrolysis of the PEO blocks.Therefore,mesoporous carbon frameworks possessing a uniform large pore size of B 23nm were obtained with a large amount of microporosity.105

A similar poly(ethylene oxide)–poly(methyl methacrylate)diblock copolymer PEO 125–PMMA 144was also used as the template to synthesize mesoporous carbon materials.106The preparation was carried out through the EISA method by using soluble resol and THF as the carbon source and the solvent,respectively.The obtained carbons had a high surface area up to 1050m 2g à1and a uniform large mesopore

B 10.5nm after carbonized at 8001C,and a face-centered cubic close-packed mesostructure (Fm %3

m ),which was the same as that prepared from PEO 125–PS 230.105Despite the similar experi-mental procedures and the identical hydrophilic PEO chain length,the mesoporous carbons prepared by PEO 125–PMMA 144had an ultra thick pore wall of 11.5–12.4nm,com-pared with that prepared by PEO 125–PS 230(B 9.9nm)and the previously reported mesoporous carbons.This could be explained by the weaker hydrophobic PMMA block containing ester groups,which could partially associate with the resol molecules,while the PS segment formed an extremely hydrophobic core and could not interact with the hydrophilic resol molecules.Thus,more resol molecules were present around the hydro-phobic phase in the PEO–PMMA/resol composites,leading to the ultra thick pore wall.106Enhanced

mechanical/chemical

Fig.6Schematic of the rapid synthesis of mesoporous carbons with lysine as the catalyst.Reprinted with permission from ref.101.

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stability and related applications were expected for these thick wall carbon materials.

Although reverse-type PPO–PEO–PPO copolymers are thermodynamically di?cult to form mesostructures under general hydrophilic synthesis conditions,which makes them seldom used to synthesize mesoporous carbon materials,the PPO–PEO–PPO copolymer possessing a long PEO chain is capable of lowering the curving energy and forming the mesophase with high curvature.Huang et al.chose phenolic resol and PO 53EO 136PO 53as the precursor and the template respectively,to prepare ordered mesoporous carbons with a

3D face-centered cubic (Fd %3

m )structure through the EISA method.107,108

The ordered mesoporous materials had high thermal stability,and the BET surface area was around 870m 2g à1with a pore volume of 0.54cm 3g à1after carbonization

at 10001C.107The formation of the cubic Fd %3

m mesophase was attributed to the characteristic phase behaviors of the reverse PPO–PEO–PPO copolymers.The two outer PPO blocks in a chain participate in two di?erent micelles or aggregates,109,110forming interconnected micelles.111,112The assembly of the reversed PPO–PEO–PPO with a large PEO weight fraction of 45%and the resol precursor rendered the face-centered cubic packed spheres of PPO blocks in the PEO/

phenolic resin matrix,and the resultant Fd %3

m mesophase with the high interface curvature was not observed in the PEO–PPO–PEO system.

Various mesostructured carbon materials with lamellar,

bicontinuous cubic (Ia %3

d ),hexagonal (p 6mm )and body-centered cubic (Im %3

m )mesophases were obtained by using di?erent triblock copolymer templates.It is found that the mesophase of the ?nal mesoporous carbons are determined by the hydrophilic/hydrophobic volume ratio in the entire system,which is referred to the hydrophilic to hydrophobic PEO/PPO ratio in copolymer templates and the phenol/surfactant ratio.On the one hand,with the increase of PEO/PPO ratio in the triblock copolymer template,the mesophase with a higher interface curvature is obtained,and the mesophase transforms from lamellar to the bicontinuous cubic,hexagonal,and body-centered cubic structure.On the other hand,hydrophilic resols can interact with PEO segments,leading to the hydro-philic volume swelling,while the hydrophobic volume per block remains essentially constant.Thus,with the increase of the phenol/surfactant ratio,the hydrophilic/hydrophobic volume ratio in this system may also increase,resulting in the mesophase with a higher interface curvature.51,52,71,80For example,when using the aqueous synthesis pathway,F127(EO 106PO 70EO 106)with a larger PEO/PPO ratio induced the

formation of the body-centered cubic (Im %3

m )mesostructure with high curvature,while P123(EO 20PO 70EO 20)with a smaller PEO/PPO ratio induced a bicontinuous cubic meso-phase (Ia %3

d )with low interfac

e curvature.In addition,a phase transition from Ia %3

d to p 6mm was observed with increasing of th

e phenol/P123molar ratio.80When using the EISA method,

the mesostructure transformed from lamellae to Ia %3

d and then to p 6mm with an increasing phenol/P123ratio,but th

e Im %3

m structure was not obtained,even with a high phenol/P123ratio,due to the small PEO/PPO ratio in P123.In the case of

the F127template,the mesophase shifted from p 6mm to Im %3

m with an increasing phenol/F127ratio.71For the F108template

(EO 132PO 50EO 132)with a much higher PEO/PPO ratio than

F127and P123,only the Im %3

m mesostructure with high inter-face curvature was obtained in the literature.71,90

As mentioned above,carbon sources,templates and cata-lysts are the three main constituents added into the solvent to prepare ordered mesoporous carbons.For carbon sources,phenol,resorcinol,and phloroglucinol/formaldehyde pairs are most commonly used so far;and with the reactivity increasing from phenol to phloroglucinol,lower acid concentrations or slower reacting aldehydes have to be adopted to control the reaction rate.The replacing of conventional precursors and catalysts always aims at decreasing formaldehyde and halogenide pollution,protecting reaction equipment from corrosion,as well as obtaining enhanced organic–organic assembly and various compositions in the mesopore walls.Thus,amino acids and citric acid were used as alternative catalysts;hexamethylenetetramine,furfural and glyoxal were chosen instead of 556dfbc94028915f804dc266anic acids/bases and even solid acids/bases should be used as catalysts in the future.New mesoporous polymeric frameworks,besides phenolic resins,are still needed,in which the melamine resin is a promising candidate.Among many of the diblock copolymer and tri-block copolymer templates,the PEO–PPO–PEO type copolymer remains dominant because of its commercial availability and easy-handling,though the utilization of special surfactants may some-times generate novel pore structures.The rational adjustment of the mesophase is now mainly based on changing the hydrophilic/hydrophobic volume ratio of the system.2.6Pore and morphology control

2.6.1Pore size control.The pore sizes of mesoporous carbon materials mainly depend on the hydrophobic groups in surfactants.As for the conventional PEO–PPO–PEO tri-block copolymers,the pore sizes can be enlarged with the increase of the molecular weight of the hydrophobic blocks rather than those of the copolymers.In the organic–organic self-assembly to mesoporous carbon materials,triblock copolymers,i.e.,F127and P123,are used as the template to produce mesopores.However,the pore size of the obtained carbons is usually smaller than 4nm when phenol/formaldehyde is used as the carbon precursor as a result of the low molecular weights of the hydrophobic blocks of F127or P12

3.52

Adding organic swelling agents is a signi?cant way to expand the pore sizes.The hydrophobic organic species can be solubilized inside the hydrophobic regions of surfactant micelles,which leads to the micelle swelling.The pore sizes are expanded by the additives of large organic hydrocarbons,such as decane and hexadecane.For example,Zhao and co-workers reported that with the aid of hexadecane molecules,the pore size could be enlarged from 3.1to 3.8nm for the 8001C-carbonized sample;when utilizing decane as the swelling agent,the pore size could be further enlarged to 4.1nm.80These results suggest that hydrocarbon molecules with relatively shorter chains have a stronger interaction with the copolymer templates and conse-quently lead to larger pores and a larger lattice parameter.83However,when hydrocarbon molecules smaller than decane,such as 1,3,5-trimethylbenzene,heptane,hexane,etc.,were used as swelling agents in this system,the resulting mesoporous

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carbon materials were rather disordered,though the reason for this phenomenon is not yet clear.Considering that the pore size of carbons usually decreased signi?cantly with the increase of the carbonization temperature due to the serious structure shrinkage of the polymeric framework,silicate was combined through a triconstituent co-assembly process into the material network.113The presence of rigid silicates in the nanocompo-sites can greatly reduce structural shrinkage during the carbo-nization.After etching of the silica,carbon with enlarged mesopores of 5.8nm could be obtained.

Since the concentration of the acid and the reactivity of phenols are two key factors that determine the polymerization rate of the phenolic resins,49when phloroglucinol,which is of high reactivity,was employed as the carbon precursor,the pore size of the obtained carbon was enlarged to 19.2nm (Fig.7a and b)using F127or P123as template under high acid concentration (phloroglucinol/HCl molar ratio of 2).89The high acid concentration facilitated the formation of high-molecular-weight phloroglucinol/formaldehyde polymer particles,which randomly distribute around the F127or P123template micelles through hydrogen-bonding (Fig.7),instead of assembling with the PEO segments.In addition,the spherical particle packing preferably reduces the structure shrinkage during the carbonization process,causing pore size enlargement of the ?nal carbons.This indicates that the high acid concentration could e?ciently bene?t the improvement of the porosity and textural properties of the resultant carbon materials,but decrease the pore regularity.In addition,carbons with ultra-large pores of 420nm could be accessible with the use of unusual copolymers with large molecular weight as template,e.g.,PS–P4VP could fabricate large pores with a dimension of 33nm,48PEO–PS revealed large mesopores of B 23nm,105and PEO–PMMA fabricated uniform large meso-pores of B 10.5nm.106

Hierarchical pore structures are bene?cial for special applica-tions.Multimicellar systems can give bimodal pore architec-tures.Carbons with bimodal mesoporous structures consisting of small (3.2–4.0nm)and large (5.4–6.9nm)mesopores could

be produced with the use of the reverse amphiphilic triblock copolymer PO 53EO 136PO 53.107,108Bridging con?guration was energetically favorable for the reverse PPO–PEO–PPO copolymer with a long PEO chain,in which the two outer PPO blocks participated in two sets of intercrossing micelles of di?erent sizes.This resulted in the formation of a unique bimodal pore mesostructure.When monodispersed silica colloidal crystals were introduced to the amphiphilic triblock copolymer/resols system,hierarchically ordered macro-/meso-porous carbons with tunable pore sizes of 230–430nm and interconnected windows with a size of 30–65nm were obtained from a dual-templating approach.114The rigid silica spheres could prevent the shrinkage of the mesostructure during the thermosetting and carbonization procedure,resulting in large cell parameters (B 18nm)and pore sizes (B 11nm).Mean-while,the bimodal porous carbons had large BET surface areas up to 760m 2g à1and large pore volumes of B 1.25cm 3g à1,with partially graphitized frameworks.

2.6.2Morphology control.The morphology of mesoporous carbons is important for industrial applications,for example,released mesoporous carbon microwires and nanowires as lightweight functional ?ller materials,?lms in catalysis and separation,monoliths in optics,and uniformly sized spheres in chromatography.Controllable synthesis on both the meso-scale (mesostructure)and macroscale (morphology)is there-fore necessary.

Since ordered mesoporous carbon is always based on the self-assembly of block copolymers and surfactants.The mesopore arrays thus formed may show a well-developed local order.Morphologies of the mesoporous carbon materials synthesized from the EISA method are usually ?lms and monolithic,whereas the aqueous route usually yields powder carbon materials with particle sizes in the micrometer or millimeter scale.Otherwise,the control over shape and arrangement of the mesoporous entities on a macroscopic scale has also made great progress.The aqueous cooperative assembly route from phenol/formaldehyde and triblock copolymer (F127and P123)under weakly basic conditions produced pellet-like mesoporous carbons in the size range of 1–5mm,rod-like particles ranging from 5to 200m m,and 3D cubic mesoporous carbon FDU-16rhombdodecahedron-like single-crystals with a uniform size of 5m m.115,116The match of the rate between the polymerization of resols and the growth of mesostructured crystals might be responsible for these phenomena.Both the stirring rate and synthesis temperature are key factors for the formation of high-quality single-crystals.The medium temperature (661C)could balance the assembly and condensation;the medium stirring rate (B 300rpm)favored the mass transport for the precipitation of large single crystals.Thus,large single crystals (B 5m m)of body-centered cubic

(Im %3

m )mesoporous carbon with a perfect rhombdodecahe-dral morphology could be obtained (Fig.8).A higher stirring rate (B 500rpm)could result in irregular morphology,while small crystals (1–2m m)were obtained at a lower rate (B 150rpm).No precipitation was observed at a temperature of B 701C,and only irregular crystals were obtained at B 601C.It is highlighted that the body-centered cubic single crystals were formed by a layer-by-layer growth mode from the centers of twelve

{110}

Fig.7(a)Nitrogen sorption analysis,(b)the corresponding pore size distribution,and (c)the formation mechanism of carbon materials with ultra-large mesopores.Reprinted with permission from ref.89.

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planes.2D hexagonal mesoporous carbon FDU-15crystals with discus-like morphology have also been fabricated by mixing F127with P123as templates.

Mesostructured carbon ?bers can be obtained by growing within a con?ned space,like in an anodic aluminum oxide (AAO)membrane.117,118To obtain ?bers with well-aligned mesopores,a shear-aligned block copolymer/polymeric matrix is usually needed,i.e.,a phenolic resin/F127composite formed by phloroglucinol/F127complex reacting with formaldehyde.Macroscopic alignment by shearing force,such as spin-coating and ?ber-extrusion produced mesostructured ?lms and ?bers (Fig.9a and b).48,49Martin et al.119reported mesoporous carbon microwires and nanowires by in?ltration of a solvent-free carbon precursor into porous alumina.The removal of the solvents prior to the in?ltration step circumvented the occurrence of macro-scopic phase separation and hydrodynamic instabilities during solvent evaporation from the pores.The mesoporous carbon nanowires released by etching alumina had a pore diameter of B 60nm with lengths of a few tens of micrometers (Fig.9c).The wormy mesostructures could be assembled to more ordered hexagonal mesostructures through controlled solvent evapora-tion or shear force.Both spin-coating and ?ber-extrusion were employed to provide the shear force for synthesizing the meso-porous carbon ?lms and ?bers with more ordered mesostruc-tures.By varying the P123and/or the amount of phenol/formaldehyde resol introduced into the phenolic resin/surfactants system,1D carbon ?bers with a core–shell structure and carbon ribbons with circular mesopores running parallel to the long-itudinal axis of the ribbon were obtained within the AAO membranes (Fig.9d).120Otherwise,biological templates are generally abundant,renewable,inexpensive and environmentally benign compared with arti?cial templates.Considering its unique morphology and structure,natural crab shell was also used to fabricate carbon nano?ber arrays with ordered mesopores (11nm)and macroporous voids (interspacing void of 70nm between nano?bers).121

Aerosol-assisted co-assembly has been regarded as an e?cient and productive route for simultaneously controlling

the morphology and mesostructures.122Mesoporous carbo-naceous polydisperse spheres with diameters from 100nm to 5mm were prepared via an aerosol-assisted organic–organic assembly (Fig.9e).123The pore size and mesostructures can be varied by changing the amphiphilic surfactant and polymeric oligomer assembly process.In order to get the spherical diameters limited to the nanoscale with uniform size,Fang et al.124proposed a low-concentration hydrothermal autoclaving

route and fabricated highly ordered body-centered cubic (Im %3

m )mesoporous carbon nanoparticles with spherical morphology (Fig.9f).The ordered mesostructures were retained while the spherical diameters were tuned from 20to 140nm by simply varying the reagent concentration.A spherical monomicelle assembly process promoted by high-temperature hydrothermal treatment at 1301C was proposed to explain the formation of the ordered mesoporous carbon nanospheres.The low-concentration (ca.10à7mol L à1surfactant)controlled hydrothermal treat-ment was crucial to obtain the nanostructure and con?ne the particle size.

Phenol/formaldehyde oligomers are always elastic but non-sticky.Thus,it is easily molded.When the oligomers and templates are assembled under hydrothermal autoclaving condi-tions,carbonaceous monoliths with certain shapes were usually formed.Huang et al.87hydrothermally synthesized carbonaceous monoliths with a hierarchical macro-mesomicroporous

structure

Fig.8(A,B)SEM images and (C)structural model of mesoporous carbon FDU-16.Reprinted with permission from ref.

115.

Fig.9Mesoporous carbons with di?erent morphologies:(a)the electron microscopy image from the Z -axle of the carbon ?lm in a 4m m ?3m m area (scale bar 1m m),(b)the photo of the carbon sheet fabric,(c)the SEM image of released microwires (scale bar 20m m),(d)the TEM image of the mesoporous carbon nano?ber and nano-ribbon (scale bar 100nm),(e)the SEM image of mesoporous carbon spheres (inset:a TEM image),and (f)the high-resolution SEM image of ordered mesoporous carbon nanospheres.Reprinted with permis-sion from ref.48,49,119,120,123and 124.

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by employing F127-P123mixed triblock copolymers as templates and phenolic resols as carbon precursors.Their monoliths showed an ordered 2D hexagonal mesostructure with a uniform pore size of B 3nm,but the synthesis temperature was relatively high (1001C)and the volume shrinkage was as high as 75%during the carbonization process.A low-temperature (501C)autoclaving method was applied to produce crack-free monolithic carbon materials with an ordered mesostructure.86The polymer monolith obtained from this process had a columned shape with a diameter of approx.16mm and length of approx.30mm (Fig.10a and b).Carbonization caused the color of the polymer monolith to change to black,but the columned shape was well retained,and the monolith was crack-free,even when an obvious shrinkage (31%)in size occurred (Fig.10c,12mm in diameter and 21mm in length).A mooncake-like polymer monolith (diameter of approx.21mm and height of approx.7mm)and its corresponding carbon monolith (diameter of approx.17mm and height of approx.5mm)could also be obtained (Fig.10d and e)by changing the size of the Te?on-lined autoclave.The formation mechanism of the monolithic mesoporous carbons is shown in Fig.10f.In comparison,monolithic mesoporous carbon materials prepared through an EISA method were partly cracked with a disordered wormhole-like mesostructure.

The controlled synthesis of mesoporous carbons includes the adjustment of mesopore size,mesophase symmetry,micro-architecture as well as macroscopic morphology of the obtained materials.Despite much progress on changing meso-porous size by adding organic swelling agents,incorporating

rigid silicates and tuning reaction acidity,one can still not obtain very small mesopores (o 3nm)by the direct strategy due to the copolymer templates used.Thus,mesoporous carbons with a much wider pore-size range still needs to be prepared.The micro-architectures,like nanowires,nano-spheres and nanocrystals,can be obtained by a spontaneous process or with external assistance,in which dual templates and an aerosol-assisted method are typical examples.Notice-ably,crack-free monoliths with various macroscopic shapes and high mechanical strength are easy to obtain for meso-porous carbons,which is expected to ?nd broad applications as devices in industrial ?elds.

3.Modi?cation and applications

The applications of nanocasting-synthesized ordered meso-porous carbons have been widely reported in the past few decades.125–130In this section,we discuss only the potentials of direct-synthesized mesoporous carbons after functionalization or not,summarized in Fig.11.Some recent progress is presented as follows.

3.1Pure mesoporous carbons

Dehydrogenation of propane is the main approach to meet the growing demand for propylene.The industrial catalytic processes generally use chromia-or platinum-based catalysts supported on alumina and are promoted with alkali metals.131Although nanocarbons have been found to be e?cient in the oxidative dehydrogenation of ethylbenzene and butane,132,133little e?ort to date has been focused on the direct dehydro-genation reaction over mesoporous carbon materials.Yuan and co-workers reported ordered mesoporous carbons (OMC-1and OMC-2with a similar mesoporous structure)being applied as a new catalyst for the direct dehydrogenation of propane to propylene under steam-free conditions,exhibiting high activity and selectivity,as well as good stability.134The catalytic activity of OMC-1and OMC-2was compared with that of CMK-3,carbon nanotubes and graphite carbon.

Fig.10(a–e)Photographs of the polymeric monoliths (orange)and the corresponding carbon monoliths (black)obtained through the low temperature autoclaving approach after carbonization at 6001C.(f)Formation mechanism of the monolithic mesoporous carbons through di?erent approaches.Reprinted with permission from ref.

86.

Fig.11A summary of various morphologies,modi?cations and potential applications of ordered mesoporous carbons prepared from the direct strategy.

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shown in Fig.12,the initial activities (15min at 873K)measured by the conversion for OMC-1and OMC-2were 69.3%and 65.7%with propylene yields of 43.1%and 46.4%,respectively,much greater than that of the CMK-3carbon catalyst (propane conversion of 54.3%with propylene yield of 15%).Poor dehydrogenation activity (only 6.5–9%of propane conversion)was observed for carbon nanotubes and graphite carbon due to the lack of reactant O 2.132,133The propylene selectivity of OMC materials remained stable over a 100h reaction,which is related to the large void volume of the composite available for the gas stream,avoiding local trans-port limitations and hot spots.135The ?nal conversions after 100h were 44.5%,39.3%and 49.8%,with ?nal yields of 37.9%,34.8%and 32.9%and ?nal selectivities of 85.1%,88.6%and 68.0%for OMC-1,OMC-2and CMK-3,respec-tively.The stable propylene yield of higher than 38%over OMC-1was comparable with that of the previously reported Pt/Sn–Ce based catalysts,136and much higher than that of the Pt/Sn based catalysts.137,138

For ordered mesoporous carbon catalysts,the presence of carbonyl/quinone groups is essential for the high catalytic activity and reaction rate.Unsaturated ketonic C Q O groups and quinone groups have substantial electron density at the oxygen atom,which can serve as a Lewis base to activate saturated hydrocarbon propane.132,133,139Propylene is pro-duced and hydroxyl groups (C–OH)remain as intermediates.The reaction cycle is closed by the thermal decomposition of C–OH to C Q O and molecular hydrogen.Meanwhile,deacti-vation of the C Q O active site can be further induced by hydrogen passivation during the prolonged reaction period.140On the other hand,the high catalytic activity of mesoporous carbons can also be related to their unique structure of large porosity that is advantageous for mass transport and good thermal stability,141but the abundance of micropores in CMK-3would hinder the produced propylene coming out of the pores and bring about the subsequent deep cracking,decreasing the propylene selectivity.Monolithic OMC materials were also used for direct dehydrogenation and oxidative dehydro-genation applications,exhibiting high catalytic activity and stability.103,104The basic oxygen groups on the catalyst surface were the active sites for selective dehydrogenation.The regen-eration of the active sites was achieved by oxidation of C–OH

in the oxidative dehydrogenation process and thermal decom-position of C–OH in the direct dehydrogenation process.Due to the highly ordered mesopores of the directly synthe-sized carbons,they exhibit much superiority over conventional carbon materials,such as carbon aerogels,activated carbons and carbon blacks,the most surface area of which is from micropores.Capacitive deionization is being developed as a potential method for removing salts from aqueous solution.142,143Capacitive deionization has many advantages compared with conventional desalination methods,including reversibility,operation at low voltages,the potential for low energy require-ments,and reduction of secondary wastes.144,145Electrodes of low electrical resistivity and high surface area are favorable for a higher ion removal capacity.Tsouris and co-workers synthe-sized mesoporous carbons using both resorcinol/formaldehyde and phloroglucinol/formaldehyde as the precursors,and then assembled the single-cell capacitive deionization reactor for water deionization (Fig.13).146The performance of these mesoporous carbons was compared with that of a carbon aerogel in salt concentrations between 1000and 35000ppm.It was shown that the mesoporous carbons removed up to 15.2–21mg salt per gram of carbon,which was much higher than the 5.8mg salt per gram of carbon removed by the carbon aerogel.The mesoporous carbon-coated electrode exhibited not only a larger ion removal capacity than the carbon aerogel,but also more e?cient removal at a higher temperature.It can be explained that a large majority of the carbon aerogel surface area comes from micropores that cannot be accessed by ions.A higher surface area associated with microporosity does not contribute to a higher ion capa-city because of the electrical double-layer overlapping,144and the microporosity limits mass transport rates and is more susceptible to collapse at higher temperatures.147Therefore,although the carbon aerogel has a relatively high surface area and was previously used in capacitive deionization,142,148its removal capacity is not as high as that of mesoporous materials.Moreover,by incorporation of the macroporosity into the mesoporous carbons,the in?uence of a pore hierarchy on the capacitive deionization process was also investigated.97,146The hierarchical porosity could enable higher active surface areas and increased transport rates of ions through the mesoporous

structure.

Fig.12(a)Conversion and (b)selectivity of the dehydrogenation of propane to propylene over di?erent mesoporous carbons.Reprinted with permission from ref.

134.

Fig.13Single-cell capacitive deionization reactor with two half cells,each of which containing (1)a Plexiglas cover,(2)a current collector,(3)a carbon electrode,and (4)a middle hollow plate.Right:STEM images of the mesoporous carbon-coated graphite electrode materials.Reprinted with permission from ref.146.

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Capacitive deionization tests indicated that the material with a meso-/macroporous hierarchical structure was a better electrode than that of mesoporous carbons for the removal of salts from brackish solution due to the faster ion uptake kinetics.97

The ordered mesoporous carbons also exhibited excellent performances in adsorption.Wan and co-workers systemically investigated ordered mesoporous carbons with large pore sizes in the e?cient removal of bulky dye molecules from wastewater,including methylthionine chloride,fuchsin basic,rhodamine B,brilliant yellow,victoria blue B,methyl orange and Sudan G.149The mesoporous carbons were directly added into wastewater without any treatment and adjustment of the pH value.The ordered mesoporous carbons exhibited large adsorption capacities,almost one time higher than those of the activated carbons for basic,acidic and azo dyes,except for large victoria blue B molecules,the adsorption capacity of which was similar on these two types of adsorbents.The adsorption of dyes with a low concentration was also investigated,because this value is a decisive factor for the residue concentration of dyes in water.The adsorption rates of all mesoporous carbons for low-concentration dyes were higher than 99.9%,suggesting complete removal.The adsorption properties for dyes were sensitively dependent on the pore properties and dye molecular sizes,which were proved by using adsorbents with various pore sizes.The desorption of partial adsorbed dyes could be realized by acetic acid elution,and mesoporous adsorbents were stable with well preserved ordered mesostructures,indicative of reusability.149It is found that in the case of adsorption of dyes with large molecular sizes,the mesoporous carbon from the direct strategy with a large pore size,large pore volume,and high surface area is a good candidate.

The adsorption property of mesoporous carbons can also be extended to biological ?elds,such as drug delivery and cell imaging.For example,ordered mesoporous carbon spheres were used for the drug delivery of ibuprofen.124The adsorp-tion amount of ibuprofen in organic solvent was found to increase as the concentration of the mesoporous carbon nano-spheres increased,and the largest quantity reached 30mg g à1in a short time,demonstrating a good adsorption capacity.The cell permeability and the cell viability were investigated to ensure the practicability of the mesoporous carbon spheres as a drug carrier.124The carbon materials were functionalized by ?uorescein isothiocyanate as a tracer before penetrating into living cells.The remarkable luminescence from the cytoplasm of the cells was observed,indicating that the mesoporous carbon nanospheres could enter into the cells.Measurements of the cell viability by the methyl thiazolyl tetrazolium assay revealed that the nanospheres seemed to not be highly cyto-toxic.The cell viability remained above 90%and above 65%after culturing in high concentrations of mesoporous carbon spheres of 50and 100m g mL à1,respectively.Therefore,the large delivery capacity with good cell permeability and high cell viability made the mesoporous carbon spheres promising materials for biological and clinical applications.

The applications of pure mesoporous carbons are relatively limited because of the lack of functional groups or active sites on the pore wall surface.Although some attempts were presented,such as catalytic propane dehydrogenation,capacitive desalination,

dye adsorption and drug delivery,they mainly utilized the uniform pore structure with the high surface area of meso-porous carbons.For more speci?c and e?cient applications,post-treatment of carbons are needed.3.2Functionalized mesoporous carbons

Unlike mesoporous silicates that have plenty of reactive hydroxyls on the surface,the functionalization of carbon is encounters the problems of poor controllability due to the inert surface,and the high-temperature carbonization can eliminate the functional groups and weak C–X bonds.Although functionalization of ordered mesoporous carbons from the direct strategy is still lacking so far compared with that synthesized from the nanocasting strategy,there are some successful illustrations of metallic and non-metallic modi?cation through in situ polymerization or a post-treatment method.

3.2.1Nonmetallic modi?cation.N-doped ordered mesoporous carbons could be utilized as an e?cient oxygen reduction reaction catalyst,potentially useful in polymer electrolyte membrane fuel cells.150N-doped mesoporous carbons were prepared by heat treatment of pre-made mesoporous carbons 69under ?owing NH 3at high temperatures.The reactions between carbon and NH 3involves the replacement of oxygen-bearing species by N-containing groups and the etching of carbon fragments by the radicals generated by the decomposition of NH 3at elevated temperatures.151Con?rmed by the polarization curves of oxygen reduction on N-doped ordered mesoporous carbons,the oxygen reduction reaction activity increased with the NH 3treatment temperature,reaching an onset positional at about 720mV (vs.NHE),which was higher than other reported N-doped irregular mesoporous or microporous carbon catalysts.152,153Pyridinic and graphitic types of nitrogen were believed to be responsible for the oxygen reduction reaction activity.Moreover,N-doped ordered mesoporous carbons were much more stable than the platinum-based electrocatalysts and exhibited a much higher resistance to CO poisoning.Platinum nanoparticles supported on high surface area carbons often su?er from instability due to the corrosion of the carbon support under the operation conditions and the agglomeration and detachment of platinum particles,leading to a decrease in catalytic surface areas.154The higher stability of the N-doped mesoporous carbons was attributed to the nature of its active sites,which were induced by N doping and were less amenable to change by carbon corrosion and other degradation mechanisms that Pt supported catalysts su?er.

N-doped mesoporous carbons can also be used for CO 2capture.CO 2capture has been at the center of interests in the scienti?c community in recent years,this is because of the implications for global warming.155–158The development of e?cient methods for capturing CO 2from industrial ?ue gas has become an important issue.159–163Ordered mesoporous carbons were treated in a NH 3?ow at the temperatures of 650–10001C,102which was demonstrated to be e?ective in introducing basic functionalities that enhanced the speci?c interaction of CO 2and adsorbent.The nitrogen content increased with the temperature from 650to 8001C,reaching a maximum of 4.52wt%,and then decreased with the further

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increase of the nitridation temperature.The N-doped carbons contained a large amount of a pyrrolic type of nitrogen,which is useful for CO 2capture.164,165Compared with pristine carbons,the N-doped carbons exhibited enhanced CO 2uptake with the highest CO 2capture capacity of 3.46mmol g à1at 251C and 760mmHg for the 10001C-nitrided sample.Both the textural and surface chemistry in?uenced the CO 2capture performance of the resultant mesoporous carbon adsorbents.102The CO 2capture capacities of the mesoporous carbons nitrided below 8001C were strongly in?uenced by the surface chemistry properties,and the covalently tethered nitrogen functional groups served as Lewis-base active sites for bonding the acidic CO 2;while the capacities of samples nitrided at 900and 10001C were mainly controlled by the adsorption on the narrow micropores,because the useful pyrrole N for CO 2adsorption slowly converted to pyridinic or quaternary N when treated at higher than 8001C.166Therefore,the CO 2capture capacity is considerably enhanced if an adequate and well developed porosity is joined to a favorable chemistry.Besides by ?owing NH 3treatment at high temperatures,Lu and co-workers accomplished the N-doping by employing organic amines during the organic–organic assembly of the resorcinol/formal-dehyde precursor and the F127template.167They prepared ordered mesoporous carbon monoliths with a hierarchical structure in the presence of 1,6-diaminohexane,which led to a N-containing framework.The obtained materials exhibited out-standing CO 2capture and separation capacities,high selectivity,and facile regeneration at room temperature.

Fluorinated mesoporous carbons were synthesized by organic–organic assembly of F127and organic precursors of phenol,formaldehyde,and a functional monomer of p -?uorophenol through the EISA process.168The co-condensation of p -?uoro-phenol,phenol,and formaldehyde ?rst formed F-resols,which assembled with F127during the evaporation of ethanol.Then the thermopolymerization of the phenolic resins was proceeded to solidify the F-phenolic resin/F127nanocomposites,followed by removal of the template and carbonization to give F-containing ordered mesoporous carbons.The obtained ?uorinated carbons possessed highly ordered mesostructures,high surface areas of 693–998m 2g à1and large pore sizes of 3.0–4.4nm.Di?erent mesophases of 2D hexagonal and 3D body-centered cubic struc-tures could be synthesized by tuning the ratios of p -?uorophenol/phenol or phenol/F127.The molar ratio p -?uorophenol/total phenol in the organic precursors could be adjusted from 0to 83%,but the content of p -?uorophenol showed a minor e?ect on the structural regularity.The ?uorinated mesoporous carbons have carbon frameworks with C–F covalent bonds,which could be retained even after high temperature carbonization owing to the high energy.169The ?uorinated carbon-modi?ed glassy carbon electrode exhibited a higher electron transfer rate than both the pure mesoporous carbon modi?ed and bare glassy carbon electrodes.168The high redox ability may be attributed to the integrity of the carbon framework,170large pore sizes and F incorporation,making the F-modi?ed ordered mesoporous carbon materials potentially useful in electrocatalytic reactions.Dai and co-workers developed the method for the post-synthesis phosphorylation of mesoporous carbon materials without a?ecting the adsorption and structural properties of the starting mesoporous carbon support.171The mesoporous

carbons were synthesized through the direct strategy 48and the phosphorylation was accomplished by stirring the obtained carbon supports in phosphoric acid at room temperature.1wt%phosphorus was doped on the mesoporous carbon pore wall.This method did not induce the activation of the original carbons as the ?nal surface areas and micropore volumes were similar to the starting material.The phosphorylation was proposed to be a simple dehydration involving surface hydroxyl species and phosphoric acid.172NH 3-TPD (temperature-programmed desorption)results con?rmed that a total amount of 132m mol NH 3g à1desorbed from the acid sites,arising from the acidic phosphate groups on the mesoporous carbons.Isopropanol dehydration was used as the probe reaction to characterize the catalytic site.The point of 50%conversion of isopropanol was observed at 2451C.Despite the relatively low phosphate contents,the sites were highly active,resulting in similar activity to carbons with higher phosphate contents (3–4wt%)as observed in other work.173Such high activities and selectivities were attributed to the large and uniform mesopores of the carbon materials.

B-incorporated,P-incorporated,and B,P-coincorporated ordered mesoporous carbons were prepared by the direct strategy.85The incorporation was achieved by simply adding boric acid or phosphoric acid or mixed boric acid and phosphoric acid into the reaction solution.Heteroatom-incorporated carbons possessed highly ordered hexagonal mesoporous structures,uniform pore size distributions,and large surface areas ranging from 500to 700m 2g à1.The incorporation of these nonmetal elements could e?ectively limit the framework shrinkage during the carbonization process,which brought about the same e?ect to the incorporation of metal or metal oxide into the carbon framework.84The simultaneous incorporation of B and P not only increased the actual concentration of B and P in the materials compared with that in the solo-incorporated counter-parts because of the interaction between B and P species,174but also facilitated the formation of the mesoporous structure.The formation mechanism of the heteroatom-incorporated meso-porous carbons is shown in Fig.14.Since the reaction was performed in a strong acidic system,the self-assembly of surfactant–polymer nanocomposites could be attributed to the combination of coulombic interactions and hydrogen-bonding interaction.69The nonmetal element-incorporated mesoporous carbons were evaluated as the electrodes of supercapacitors.The results showed that the B-incorporated,P-incorporated,and B,P-coincorporated carbons exhibited superior electro-chemical performances compared to the nonincorporated coun-terpart.It could be explained that the incorporation of P or/and B into the carbon framework not only produced the heteroatom functional groups on the carbon surface,but also yielded additional surface oxygen groups.The high speci?c surface area with ordered mesoporosity and abundant functional groups can contribute to the enhanced capacitance.175–177

SO 3H-functionalized porous carbons are usually used as ?ne powders in catalysis.Xiao and co-workers reported SO 3H-functionalized ordered mesoporous resin and carbon materials.178Sulfonation of the samples was performed by chlorosulfonic acid treatment in CH 2Cl 2.The acid concentrations of the SO 3H-functionalized resin and carbon were con?rmed to be 1.56and 0.91mmol g à1,respectively.The solid acid catalysts

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showed high activity in the esteri?cation of acetic acid with butanol,esteri?cation of acetic acid with cyclohexanol,and condensation of benzaldehyde with ethylene glycol.They also exhibited excellent recyclability compared with conventional acid catalysts.Wu and co-workers reported the SO 3H-func-tionalized ordered mesoporous polymers from direct synthesis strategy.179Careful sulfonation of the samples was carried out through a gas–solid reaction,in which the sample powders were contacted with the vapor from 50%SO 3/H 2SO 4.This process was very important for inducing an e?ective but structurally nondestructive sulfonation modi?cation.The concentration of acid sites quanti?ed by an acid–base titration was 2.1–2.5mmol g à1,while the element analyses indicated 1.8–2.2mmol g à1of sulfonic acid groups were attached.The catalytic properties of the solid acids were tested for the liquid-phase Beckmann rearrangement of cyclohexanone oxime as well as the condensation of bulky aldehydes with alcohol.Owing to their large mesopore dimensions (42nm)and considerable amounts of Bro nsted acid sites,they showed superior activity to the conventional acid Dowex resin and many microporous zeolite catalysts.

Surface oxidation is one of the most convenient and simplest methods for modifying the carbon surface,with not only the attachment of oxygen-containing groups,but also alteration of the surface hydrophobic/hydrophilic balance.180–182As a result,the inert and hydrophobic nature with poor wettability and dispersibility in polar solvents of pristine mesoporous carbon materials,which is unfavorable for many applications,can be changed.Zhao and co-workers systematically investigated the pore evolution,mesostructural oxidation resistance,and simultaneous surface functionalization of ordered mesoporous carbon FDU-15under various oxidation conditions.183When wet oxidation was performed by an acidic (NH 4)2S 2O 8solution

as a gentle oxidant,the mesostructural regularity of the products was retained.However,the micropores/small meso-pores were blocked by the attached surface functional groups.Abundant surface oxygen-containing groups,especially carboxylic groups could be generated during oxidation,and the sample became much more hydrophilic and showed a higher adsorption a?nity toward water.The high and strong adsorbed NH 3capacity was also observed,attributed to the surface acidic carboxylic and phenolic groups.When the HNO 3solution was used as a strong oxidant,the blocked micropores could be reopened.(NH 4)2S 2O 8treatment could introduce more carboxylic groups than the HNO 3solution under mild conditions.The mesostructural analogue CMK-3showed much poorer oxidation stability compared with the FDU-15,due to the relatively unstable structure of CMK-3.The surface functionalized mesoporous carbon materials could be used as e?cient adsorbents for heavy metal ions such as Fe 3+,Cu 2+,Cd 2+,Pb 2+ions and organic dyes,and could also be used for drug delivery.

In nonmetallic modi?cation,both the post-treatment and one-pot synthesis method were used.For N-doping,NH 3treatment on pre-made mesoporous carbons and the addition of organic amines during the organic–organic assembly were both e?cient.P-doping could be realized by either dehydra-tion condensation between phosphoric acid and mesoporous carbons or the addition of phosphoric acid during the meso-porous carbon preparation.B-or B,P-coincorporated carbons could be obtained by the one-pot synthesis.Functional groups like SO 3H–and COOH–were usually grafted by post-sulfona-tion and post-oxidation,556dfbc94028915f804dc266pared with the unfunctionalized pure carbons,these materials are expected to have more speci?c functions.The carbons with a basic surface (N-doping)may favor acidic gas (CO 2)adsorption;while carbons with an acidic surface (phosphorylated carbons)may have a large capacity for basic gas (NH 3)adsorption.The sulfonated carbons can act as a strong solid acid catalyst,and the oxygen-containing groups are useful claws for adsorbates like metal ions and dyes.

3.2.2Metallic modi?cation.A series of highly ordered iridium-containing ordered mesoporous carbons (Ir-OMC)through a one-pot direct strategy were synthesized by adding H 2IrCl 6into the reaction system of resorcinol/formaldehyde polymer and surfactant F127.84H 2IrCl 6was then reduced to metallic Ir by the emitted reducing gases,such as CO and H 2,or were even reduced by carbon itself under a N 2atmosphere.Ir particles,with loading amounts of 2.6–7.7wt%and uniform sizes of B 2nm,were observed on the carbon supports,while the ordered mesostructure of the carbons remained.The high dispersity and the high thermal stability of the Ir particles should result from its imbedding into the carbon matrix during the one-pot synthesis process.Ir-doped mesoporous carbons obtained by post-impregnating the carbon supports with H 2IrCl 6were also prepared for comparison.However,the post-impregnation method led to an uncontrollable and thus polydispersed particle size distribution.Hydrazine (N 2H 4)decomposition was chosen as a probe reaction,which has been widely used in the altitude control of spacecrafts.184–187The N 2H 4conversions on Ir-OMC catalysts attained

100%

Fig.14Proposed mechanism for the multicomponent self-assembly of resorcinol,formaldehyde,F127,and B or/and P precursors.Reprinted with permission from ref.85.

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even at 301C,and no decay was observed over a 300min run,demonstrating their high activities and stabilities;while the N 2H 4conversion on the Ir-doped mesoporous carbon catalysts prepared by the post-impregnation method under the same reaction conditions dropped rapidly with the reaction time.The enhanced mass transfer caused by the ordered mesoporosity and the highly dispersed uniform iridium particles both contrib-uted to their outstanding catalytic performance.

Ordered mesoporous carbon-supported calcium oxide materials were synthesized by the three-constitute co-assembly of F127,resol precursor and Ca(NO 3)2through the EISA method.188The obtained materials possessed a 2D hexagonal mesophase with high surface areas of 615–1058m 2g à1and CaO weight percentages of 3.0–20.4%.The CaO particles were anchored within and/or between the mesopore walls of the carbon support with good dispersion when their contents were lower than 13.5wt%;while further increasing the CaO content would lead to larger nanoparticles with obvious aggregation and a less ordered mesostructure.The CaO-loaded carbon materials were used for low-temperature physisorption of CO 2,showing a high adsorption capacity of up to B 5.0mmol g à1at 251C and B 7.0mmol g à1at 01C,which is comparable with the previously reported adsorbents for CO 2capture at low temperatures.189–191Moreover,the CaO-loaded carbon materials delivered good adsorption selectivity for CO 2over N 2.Due to their small particle sizes,the nanocrystalline calcium oxides in the mesoporous carbons were also active for high-temperature CO 2chemisorption,with a high adsorption capacity at 200–5001C (up to B 3.25mmol g à1),high initial conversions (nearly 100%)and fast reaction-based kinetics at 4501C.The cyclic stability was improved due to the con?nement of nanoparticles within the mesopores.188

The loading of magnesium oxides into ordered mesoporous carbons can also lead to the excellent solid base property as well as the enhanced adsorption capacity for CO 2.These materials were prepared by the simple one-pot assembly of triblock copolymer F127,resol,and magnesium nitrate,followed by thermal curing and carbonization.192The periclase MgO nanocrystals with a uniform size of 4.5–13nm were well-dispersed in the carbon framework.The highest incorporation amount of MgO was around 37wt%,and a phase transforma-tion from the hexagonal (p 6mm )to body-centred cubic (Im %3

m )mesostructure occurred as the Mg content increased,which could be explained by a higher MgO content leading to a larger volume ratio of hydrophilic to hydrophobic PEO/PPO.71,80When the MgO particles had sizes larger than the wall thick-ness,they could extend from the carbon walls into the mesopore channels,resulting in a rougher pore surface and a lower degree of mesostructure regularity.Moreover,con?rmed by the CO 2-TPD analysis,the basicity was greatly enhanced due to the incorporation of the Mg species,and the number and strength of the basic sites were quite related to the content of MgO in the resultant solids.The well-structured mesoporosity and the outstanding basic property make these materials promising candidates for selective adsorption and catalysis.

The doping of Fe and Ni species can make the resultant carbon materials magnetically separable.For example,maghemite-loaded ordered mesoporous carbons with superparamagnetism were synthesized via the one-pot co-assembly of F127,resol,

and ferric citrate,followed by thermal curing and carboniza-tion.193The mesoporous materials with a low g -Fe 2O 3content of 9.0wt%,possessed an ordered 2D hexagonal structure with a uniform mesopore of 4.0nm and a high surface area of 590m 2g à1.Maghemite nanocrystals with a small particle size of 9.3nm were con?ned in the matrix of the amorphous carbon frameworks,and their high dispersion was due to the interaction between the iron citrate complex and the phenolic resin matrix during the cooperative assembly process,which was important to avoid the aggregation of iron ions.194The g -Fe 2O 3loaded carbons exhibited excellent superparamagnetic behaviors,and high stability during the H 2O 2oxidation process.They showed a high adsorption capacity of up to 220mg g à1for fuchsin base dye molecules.The large mesopores,with a high pore volume,hydrophilic property after H 2O 2treatment,and magnetic framework for easy separation,make these g -Fe 2O 3-loaded ordered mesoporous carbons promising adsorbents to treat wastewater for industrial use.

Ni-doped ordered mesoporous carbons were synthesized via the co-assembly of resol,tetraethyl orthosilicate,Ni(NO 3)2á6H 2O and F127,followed by carbonization and silicate removal with NaOH etching.195The Ni 2+was reduced to Ni by carbon and/or by the emitted reducing gas CO during the pyrolysis.The magnetic Ni nanocrystals with particle sizes of 16.0nm were con?ned in the matrix of the carbon frame-works.As the Ni,Co and Fe species could act as catalysts for the formation of the graphitic structure during the carboniza-tion step,196–200in some regions of the high resolution TEM image of the Ni-doped mesoporous carbons,the Ni nano-particles were fully crystallized and the carbons nearby were partially graphitized.The incorporation of rigid silicates in the pore walls could reduce the framework shrinkage signi?cantly,creating large mesopores.The obtained material possessed an ordered 2D hexagonal structure with a large mesopore of 6.8nm and a high surface area of 1580m 2g à1.Due to the enlarged mesopores with high surface area for free mass transportation,149these materials showed higher adsorption capacity for fuchsin base (up to 420mg g à1)than pure mesoporous carbon FDU-15(118mg g à1).Moreover,the adsorbents could be easily separated from solution by a magnetic ?eld because of the existence of the magnetic Ni nanoparticles in the framework.Doong and co-workers reported magnetically-separable hierarchically ordered porous carbons with graphitic structures,which were directly synthesized by one-pot dual-templating EISA method.201Polystyrene latex spheres and F127were used as macroporous and mesoporous structure-directing agents,while phenol/formaldehyde resins and Ni species were chosen as the carbon source and graphitization catalyst,respectively.Addition of a nickel species catalyzed the graphitization at relatively low carbonization temperatures under di?erent atmospheres (N 2or H 2/N 2).The graphitic structures in these hierarchically ordered porous carbon materials could be obtained at 800–10001C,which was much lower than the high graphitization temperature of B 20001C without catalysts.202,203The obtained materials contained amorphous carbons in the pore walls and graphitized carbons around the surface of Ni nanoparticles.These carbon materials exhibited improved electrocatalytic activity towards I 3àand I 2reductions in dye-sensitized solar cells with a conversion e?ciency up to 5.2%,

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which could be due to the high conductivity and low charge transfer resistance caused by the high graphitization degree.The magnetically-separable materials also showed acid resistance and a high adsorption capacity for methylene blue.201Thus,the doping of a Ni species into mesoporous carbons can not only introduce a magnet property to the composites but also increases the graphi-tization degree at lower temperatures,which is important for electrochemical applications due to the enhanced conductivity.However,the in?uence of the graphitization degree on the adsorp-tion behaviors is not obvious.

Inspired by using ferric citrates as precursors to prepare maghemite-loaded carbons,193other metal citrates were also tested.For example,ordered mesoporous TiC/carbon materials were prepared by the EISA method from resol,titanium citrate,and F127,combined with in situ carbothermal reduction.204The cubic crystalline TiC were con?ned in the matrix of the amorphous carbon pore walls with controllable titanium contents of 12–32wt%,and small particle sizes of 4–6nm.The utilization of titanium citrate as a precursor was crucial to obtain highly dispersed TiC.The polycarboxylate chelated titanium ions,related to titanium citrate and resol involved in the Pechini-like process,61could restrict the aggregation and condensation of the TiO species and the esteri?cation between titanium and the phenol hydroxyl group,205,206while other titanium precursors,such as titanium isopropoxide,would result in serious precipitation and macrophase separation.This method facilitated the absence of the crystalline titanium oxide phase before the formation of TiC nanocrystals by carbo-thermal reduction,and thus the structural ordering was well preserved.The loading of TiC nanocrystals could not only enhance the oxidation resistance ability of the amorphous carbons but also improve their electrical properties,making the resultant solids potentially useful as electrode materials and alternative options for noble metal-supported catalysts.

Unlike nonmetallic doping,the incorporation of a metal species into direct-synthesized mesoporous carbons is often achieved by the one-pot co-assembly of copolymer templates,carbon sources and metal precursors.Metal oxides,like CaO and MgO,can introduce an excellent solid base property and an enhanced adsorption capacity for CO 2;magnetic Fe 2O 3and Ni can make the mesoporous carbon adsorbents or catalysts easily separable.It is still promising for mesoporous carbons to load more kinds of metal species with distinctive functions,including Ir-doped carbons for hydrazine decom-position,CuO/carbons for CO oxidation,noble metal-doped carbons as highly-e?cient catalysts for many reactions and TiC/carbons for electrode materials,etc.Importantly,the incorporation of both nonmetallic and metallic species should avoid damage to the integrity of the chemical and porous structures of the mesoporous carbons.207–210

3.2.3Interpenetrating carbon-based composites.The major component of the above-mentioned mesoporous carbonac-eous materials,functionalized by either metallic or nonmetallic elements,is still carbon,and the heterogeneous species are modi?ed on the carbon surface or/and enchased inside the mesopore wall.However,in some cases,especially carbon–silica nanocomposites,interpenetrating networks of carbon and silica can be accessed with homogenous and continuous

composition with the ratios ranging from zero to in?nity for the two constituents.For example,mesoporous polymer–silica and carbon–silica nanocomposites could be prepared by the triconstituent co-assembly of soluble resols,prehydrolyzed silica oligomers from tetraethyl orthosilicate,and F127.113,211–213Silicate species in the nanocomposite with a complex chemical environ-ment had a relatively high crosslinking degree and some interac-tions with the carbon species,but no Si–C bonding modes were formed.214The ratios of polymer/silica or carbon/silica could be tuned from zero to in?nity by varying the mass ratios of resols to tetraethyl orthosilicate.The synthesized ordered mesoporous materials were con?rmed to have ‘‘reinforced concrete’’-structured frameworks (Fig.15).During the EISA process,resols exhibited low condensation rates under acidic conditions at room temperature,forming nanosized ‘‘concrete’’structures in the nanocomposites.Silicate oligomers not only assembled with the surfactants,but also condensed and cross-linked together around the nanosized resols to generate 3D ‘‘reinforcing-steel-bar’’frameworks.Then the interpenetrating large domain sized polymer resins and silicates shaped during the succedent thermopolymerization with the homogenous composition.The ordered mesoporous carbon–silica nanocomposites can be used as catalyst supports for noble metals,such as palladium,by isochoric impregnation using PdCl 2as the precursor.215The palladium content was about 5wt%.Accessible Pd nano-particles were well dispersed inside the pores with a particle size of about 3nm,which could be explained by the hybrid nature of the carrier.The silica and carbon components uniformly dispersed inside the pore walls to construct a continuous framework.Metallic ions were selectively adsorbed on the surface of silica,while the inert and hydrophobic component carbon possibly played a role in separating 556dfbc94028915f804dc266pared with the pure polymer,silica or carbon supports,the mesoporous carbon–silica nanocomposites were superior.They exhibited a high yield of B 60%for trans -stilbene at 1001C in the Heck coupling reaction of chlorobenzene and styrene,as well as a high yield of 46%for biphenyl at 301C in the Ullmann coupling of chlorobenzene with water as the solvent.The high catalytic activity was also observed when using substituted aryl chlorides in the system.The Pd catalysts supported on mesoporous carbon–silica nanocomposites were stable,which showed negligible metal leaching and could be reused more than 20

times.

Fig.15Formation mechanism of ordered mesoporous polymer–silica and carbon–silica nanocomposites,and the corresponding ordered mesoporous silica and carbon frameworks.Reprinted with permission from ref.113.

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When 1,3,5-trimethylbenzene was introduced as an organic cosolvent into the triconstituent co-assembly system,carbon–silica nanocomposites with multilayer vesicle structures could be obtained.216Mesoporous carbon–silica composites after carbonization consisted of small irregularly spherical particles,and each of the particles exhibited a hollow lamellar structure.The layer number varied in the range of 3–9.Many small pillars were located between the layers to stabilize the lamellar structures.The key issue for the successful synthesis was the aqueous co-assembly process on the emulsion oil/water inter-face under a hydrothermal condition.The silicate oligomers and resols could interact with F127through hydrogen bonds to generate the uniform lamellar mesostructure on the oil/water emulsion interface;1,3,5-trimethylbenzene served as a core in the emulsion solution and hollow vesicular particles were formed.217Mesoporous carbon multilayer vesicle struc-tures could also be prepared by etching the silica from the carbon–silica nanocomposites.

Similar to the carbon–silica nanocomposites,an interpene-trating hybrid network consisting of titania and carbon could be synthesized.218The mesoporous carbon–titania nano-composites could be obtained in a wide range,from 20to 80wt%TiO 2,by adjusting the initial mass ratios.They exhibited ordered hexagonal mesophases with surface areas of 191–465m 2g à1,and pore sizes of 3.2–4.1nm.The carbon–titania nanocomposites possessed ‘‘bricked-mortar’’frame-works.The amorphous carbon acted as a glue,linking with the TiO 2nanocrystals through covalent Ti–O–C bonds on the surface and/or the coordination between Ti and HO–C groups,preserving the mesostructure and stabilizing the average regu-larity.The mesoporous carbon–titania materials showed good photocatalytic activity for the photodegradation of rhodamine B in an aqueous suspension,attributed to the highly crystallized titania crystals in the hybrid frameworks and the large adsorp-tive capacity related to the high surface areas.

Pure ordered mesoporous carbon materials can be obtained from the above-mentioned carbon-based 556dfbc94028915f804dc266-monly,etching with hydro?uoric acid is the e?ective way to remove the silica species.However,highly toxic hydro?uoric acid is not user-friendly.Kaskel and co-workers reported free-standing ?lms of ordered mesoporous silicon carbide–carbon and titanium carbide–carbon composites and the derived mesoporous carbons using a novel soft templating approach without employing hydro?uoric acid etching.219A high tem-perature chlorine treatment was used instead.220–222The resin precursor,F127and titanium citrate or tetraethyl orthosilicate underwent the EISA process,followed by pyrolysis in Ar at 800–13001C to generate the mesoporous TiC–carbon or SiC–carbon materials.Then the Cl 2extraction for Ti and Si components was performed at 10001C,according to the following reactions,respectively:

TiC +2Cl 2-TiCl 4+C (ref.219)(1)SiC +2Cl 2-SiCl 4+C (ref.223)

(2)

The ?nal carbide-derived carbons consisted of a hierarchi-cally ordered meso-/microporous structure with a high surface area of 1538m 2g à1and a large pore volume up to 2.53cm 3g à1.

They could be used for catalyst supports,high-pressure gas storage,electrochemical capacitors,etc.

Although many applications have been mentioned above for direct-synthesized mesoporous carbons,the carbon mate-rials prepared from the nanocasting strategy exhibited similar functions,and are also being widely reported.125–130Because the mesoporous carbons from these two distinct strategies possess nearly the same chemical compositions and surface functional groups,the di?erences in their performance could be related to the pore structure,stability and graphitization degree.First,the carbons (e.g.,CMK-3)from the nanocasting method may have plenty of micropores due to their meso-porous silica templates.39The micropores indeed increase the surface area,but sometimes they are not conducive to cata-lysis.For example,in the direct dehydrogenation of propane to propylene,the large number of micropores in CMK-3may lead to a block of the products and subsequent deep cracking,decreasing the selectivity.134The pore size distribution of the direct-synthesized carbons is more uniform compared with those from hard templates,and the mesophase symmetry of the former can be rationally adjusted,which is suitable for particular applications.25,26,80Secondly,the stability,under thermal and oxidation treatments,and mechanical pressure,of the direct-synthesized carbons is much higher due to their thick pore walls and continuous framework.71,183Thus they are useful in some high-temperature reactions;and many oxygen-containing groups can be grafted through oxidation onto their surface as binding sites for various adsorbates.Thirdly,much progress has been made towards the prepara-tion of graphitic mesoporous carbons through the nanocasting method,224–228while the graphitization of direct-synthesized carbons is relatively di?cult.The good conductivity caused by the high graphitization degree favors the electrical and photo-electrical applications of the mesoporous carbons from hard templates.

4.Summary and outlook

Ordered mesoporous carbon,surface-functionalized carbon,and related carbon-based nanocomposite materials have attracted great research interest in the past decade due to their outstanding physicochemical characteristics.To date,the nanocasting strategy (from pre-formed hard templates)and the direct synthesis strategy (induced by supramolecular assembly of the block copolymer soft templates)are the two main routes to prepare these ordered mesoporous carbons.Considering the obvious superiority of the latter method,this review only focused on the controlled preparation,modi?ca-tion and application of ordered mesoporous carbons by the direct strategy.The direct strategy undergoes di?erent pathways,including the EISA route,dilute aqueous route,macroscopic phase separation and hydrothermal autoclaving 556dfbc94028915f804dc266mon precursors,pairing formaldehyde with phenol,resorcinol or phloroglucinol,are used as well as some new attempts,such as furfural,glyoxal,hexamethylenetetramine and hexamethoxymethyl melamine.Due to their enhancement e?ect on the assembly of the phenolic resin and templates,amino acids and citric acid are chosen as alternative catalysts besides NaOH and HCl.Various types of block copolymers

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