Synthesis of mesoporous carbon materials from renewable plant polyphenols for environmental and energy applications

FENG You-you, CHEN Yi-qing, WANG Zheng, WEI Jing,

（1.Institute of Analytical Chemistry and Instrument for Life Science, the Key Laboratory of Biomedical Information Engineering of Ministry of Education, School of Life Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China;2.State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China）

Abstract： Mesoporous carbon materials have a high specific surface area, tunable surface chemistry and pore structure, and good chemical stability and conductivity.They have attracted great attention for use in environmental remediation, industrial catalysis, energy conversion and storage.The carbon precursor is important for the synthesis of mesoporous carbons with different properties.Plant polyphenols are a kind of universal biomass with low cost, nontoxicity and sustainability that can be used as a carbon source.Most importantly, their good adhesion and metal chelating ability make them suitable for the synthesis of mesoporous carbon composites.Methods for the synthesis of different forms of mesoporous carbon from plant polyphenols are provided, including porous carbon foams, ordered mesoporous carbons, mesoporous carbon spheres, heteroatom-doped mesoporous carbons, and composites of mesoporous carbon with metals.Their uses in environmental and energy studies are summarized.

Key words: Plant polyphenol；Mesoporous carbon；Adsorption；Catalysis；Energy storage

1 Introduction

Mesoporous carbon materials have attracted extensive attentions due to the advantages of large specific surface area, tunable porous structure, good conductivity, excellent stability and multifarious applications in catalysis, adsorption and separation, energy conversion and storage, environmental remediation and biomedicine[1-10].Carbonization is a main method to synthesize porous carbon materials[11].However,carbon materials prepared by direct carbonization of different carbon sources generally do not have mesoporous framework[12].Therefore, it is necessary to introduce pore-forming agent to create mesopores during the carbonization process[13].According to the different types of pore forming agent, it can be generally divided into hard-template method and soft-template method[14].In the hard-template synthesis, mesoporous silica is usually employed as the template[15-16].The carbon source is firstly filled into the mesopores silica.After carbonization process, the carbon materials are formed in the mesoporous silica.Then, mesoporous silica is selectively removed by hot alkaline solution or highly toxic hydrofluoric acid.In the softtemplate method, amphiphilic block copolymers are usually served as the templates[17-21].Due to the organic-organic self-assembly process, a mesophase is formed by the block copolymers and organic source.After carbonization process, block copolymers are decomposed, resulting in mesoporous framework.Except for the typical soft/hard-template method, selftemplate method has also been widely investigated for synthesis of mesoporous carbon[22-25].In this process,an organic/inorganic hybrid is served as a carbon precursor.During the carbonization process, the organic species are turned into carbon materials.At the same time, inorganic species tends to form crystalline nanoparticles.After removal of inorganic nanoparticles,mesoporous carbon can be formed.Comparing to the typical soft/hard template methods, the mesoporous frameworks, especially ordered mesopores are difficult to form by self-template method.However, the organic/inorganic hybrids can be easily obtained withvarious morphologies, compositions and nanostructures.Therefore, the self-template method has also been widely investigated for mesoporous carbon nanostructures.Metal-organic frameworks, as a typical organic/inorganic hybrid, have been widely used for fabrication of nanoporous carbon due to the well-reserved morphology, tunable compositions, and ease of preparation and modification[26-27].

There are different kinds of methods for the synthesis of mesoporous carbon materials.However,some issues should still be concerned.For example,most of carbon sources for mesoporous carbon materials are petrochemical products.These carbon sources are not sustainable in the industrial application, such as phenolic resin.Therefore, it is particularly important to explore the nontoxic and renewable carbon materials precursors.Recently, biomass, including lignin,cellulose, glucose, fructose and sucrose, has been widely surved as the carbon source for carbon materials synthesis[7,11,28-30].However, the obtained carbon materials from biomass usually exhibit microporous structure, which inhibits the loading of large size of guests such as biomolecule and nanoparticles.

Plant polyphenols are one kind of water-soluble biomass with the molecular weight ranging from 500 to 3 000-4 000Da.Plant polyphenols possess 12-16 of phenolic hydroxy groups on 5-7 of aromatic rings per 1 000Da[31-32].Plant polyphenols are generally divided into three classes: proanthocyanidins, gallo- and ellagitannins, and phlorotannins.They are widely existed in the fruits, vegetables, seeds, tea leaves, and many other related foodstuffs.They are low-cost, nontoxic, renewable carbon sources (Fig.1).Specially,they have three unique properties.Firstly, plant polyphenols have large number of catechol or pyrogallol groups.Such groups show strong hydrogen bonding interactions with polyethylene oxide (PEO)-based block copolymers.Therefore, an organic-organic selfassembly strategy can be used for the synthesis of mesoporous polymer precursors and carbons with desirable pore architectures.Moreover, the catechol groups from plant polyphenols show strong chelate ability with metal species.It is reasonable to prepare metal/metal oxide nanoparticles decorated mesoporous carbon materials, which could further extend the functionalities and applications of mesoporous carbons.Secondly, plant polyphenols are ubiquitous,showing similar property with phenol or resorcinol.They can be polymerized by formaldehyde to form polyphenol-formaldehyde resin.The phenol-formaldehyde resin has been widely used as a carbon source for mesoporous carbon[33-35].Compared with the phen-ol or resorcinol, plant polyphenols are renewable and nontoxic.They are one of ideal candidates to replace the commonly-used phenol.Thirdly, mussel-inspired adhesive materials have attracted intensive attentions for interfacial modification and functional composites[36].Plant polyphenols have been widely used as the molecular glues for interfacial assembly or modification[31].Based on this adhesive property,polyphenols or their hybrids can be easily deposited on various substrates.After carbonization, different kinds of functional carbon composites can be obtained.

Based on the above properties, plant polyphenols have recently received enormous intentions for the synthesis of nanoporous carbon materials[37-42].For example, Zhang and co-workers reported the solid-phase synthesis of ordered mesoporous carbon using one kind of plant polyphenols, tannic acid (TA), as a carbon source, Pluronic block copolymers as a template,and metal ions (Zn2+, Ni2+) as a crosslinker[40].After carbonization process, metal nanoparticles can be well loaded in the ordered mesoporous carbon.Such materials are excellent catalysts for catalytic hydrogenation.Wei and co-workers reported the sol-gel synthesis of metal-polyphenol-formaldehyde resin and their derived porous metal/carbon colloidal spheres[39], showing high catalytic activity for oxygen reduction reaction.So far, there are many excellent reviews on mesoporous carbon materials[14,17,43-46].However, there are very few summaries of mesoporous carbon materials derived from plant polyphenols.Based on recent progresses, it is desirable to summarize the mesoporous carbon constructed from plant polyphenols.

Herein, the synthesis methods and various applications of mesoporous carbon materials derived from plant polyphenols have been systematically reviewed (Fig.2), listed in Table 1[37-42,47-80].Firstly,mesoporous carbon materials with tunable pore structures are summarized.By changing the synthesis methods and introducing of soft template molecules,the porous structure can be easily adjusted.Specially,ordered mesostructure decorated with metal nanoparticles can be obtained.Secondly, the morphology control of the mesoporous carbon is discussed.Inspired from the synthesis of the resorcinol-formaldehyde resin colloidal sphere, metal-polyphenol-formaldehyde colloidal sphere was prepared and used as the carbon source.Mesoporous carbon/metal composites can be easily prepared.Thirdly, based on the excellent adhesive property of polyphenols, mesoporous carbon can be deposited on various substrates, enabling the fabrication of functional composites.Fourthly, applications of mesoporous carbon materials in adsorption, catalysis, supercapacitor are summarized based on the mesoporous carbon with tunable pore structure, composition and morphology.Finally, the prospects and challenges of the synthesis and application of plant polyphenol-derived mesoporous carbon materials are summarized.

Table 1 Mesoporous carbon materials derived from plant polyphenols.(Continued).

Table 1 Mesoporous carbon materials derived from plant polyphenols.

2 Mesoporous carbon with tunable pore structure

Pore structure such as pore size, pore shape and pore connection is one important parameter for nanoporous materials.It could affect the mass transport,the loading of other guest molecules or nanoparticles,the accessibility of the surface and the number of exposed active sites[3,81-82].Therefore, the performances of nanoporous materials in various applications are mainly determined by the pore structure.Soft-template synthesis of mesoporous materials is one efficient strategy to adjust the pore structure due to the abundant mesophase structure of block copolymers, as well as tunable micelle size[3].For the soft-template synthesis of mesoporous carbon, the non-covalent interactions between carbon precursor and soft template are very important.However, most of biomass carbon sources have weak interaction with template, resulting into an uncontrollable pore structure.Different from the common carbon source such as cellulose,polyphenols have plenty of phenol groups.They could form hydrogen bond with the commonly used PEO-based block copolymers.It is possible to adjust the pore structure of mesoporous carbon based on the assembly between block copolymer and polyphenols.In this section, polyphenol-derived porous carbon with different pore structure is summarized.

Plant polyphenols have been widely used as the carbon source for preparation of porous carbon.For example, Tondi and co-workers used natural plant tannin extract as the carbon precursor[83].After crosslinking of tannin with formaldehyde, the carbon foam material based on tannin-formaldehyde system was synthesized with the assistance of foaming agent (Fig.3).The density of the obtained carbon foams is 1.98 g cm−3.The average pore size in the z-direction is 250 μm.The average pore size in the xy-direction is 135 μm.The specific surface area is 0.89 m2g−1.The thermal conductivity of tannin-based carbon foam is between that of thermal insulators and thermal conductors.It possesses high absorbance, reaching 93%-96%.At the same time, the material exhibits certain electrical conductivity.Its thermal expansion coefficient is 1.8×10−6-2.2×10−6K−1, which was muchlower than that of most solid materials.The thermal expansion coefficients for copper and aluminum were 17×10−6and 23×10−6K−1, respectively.After baking at 900 °C, the material exhibited good flame-retardant performance.

Subsequently, Celzard and co-workers studied the radiation properties of tannin-based carbon foam[84].The parameters of porous carbon foam before and after calcination were measured.The emissivity of carbon foam was temperature dependent.As the porosity of carbon foam increasing, the thermal conductivity decreased[65].Further, the tanninbased carbon foam was updated.A carbon foam with seasonal thermal storage properties was obtained[85].Based on tannin-based carbon foam, the exfoliated graphite was suspended to improve the thermal conductivity and mechanical properties of the material.The thermal conductivity increased from 0.11 to 3.65 W m−1K−1.The compressive strength increased from 3.6 to 6.9 MPa.The elastic modulus increased from 30.1 to 61 MPa.The material had good thermal conductivity and enough mechanical strength for thermal management applications.Subsequently, the effects of different types and quantities of graphite fillers on the thermal properties of carbon foam were further explored.The results showed that the size of material particles affected various physical properties.Small particles leaded to better conductivity and better mechanical properties[86-87].

Other templates were also used to synthesize porous carbon blocks.Szczurek and co-workers synthesized porous carbon monolith by a emulsion-templated method[76].They used polymerized high internal phase emulsion (polyHIPE), a kind of oil in water emulsion,as a template agent.Oil droplets contact each other at high concentrations.The precursors polymerize in the aqueous phase and form interconnected pore structures after removing the oil phase.Carbon materials with macroporous structure were obtained by carbonization of precursors.The porosity and pore size can be controlled by adjusting oil droplets.The material with the initial oil phase volume fraction of about 70% has the smallest pore size of 1.5 μm, the highest specific surface area of 400 m2g−1and the highest mechanical properties.

Carbon gels, like carbon foams, are usually prepared by carbonization of resorcinol-formaldehyde resin (RF resin) gels in inert gas[88-89].Choosing natural plant polyphenols to instead resorcinol is an attracted method to lower the cost and toxicity effects[64,68,90-91].Tannin can be used as a substitute for resorcinol or phenol and formaldehyde can be used as a crosslinking agent[92-93].The two components can be cross-linked to form tannin-formaldehyde gel[73,94].The pore structure of the carbon gel can be adjusted by pH value.Szczurek and co-workers synthesized a tannin-formaldehyde gel in a wide pH range of 3-8 and the carbon aerogels can be obtained by the following carbonization process.Under certain pH condition, the porosity and specific surface area of the carbon aerogel reached to 95% and 715 m2g−1respectively.Subsequently, Szczurek and co-workers also studied the preparation of organogels and their derived carbon gels from tannin-resorcinol-formaldehyde system.

Doping N into nanoporous carbon materials has agreat impact on the properties of materials[95].Braghiroli and co-workers synthesized N-doped carbon material from plant tannin by one-pot synthesis method[56].The tannin-ammonia solution was hydrothermally treated at 180 °C for 24 h.After the calcination at 900 °C under nitrogen atmosphere, a carbon gel with a specific surface area of about 60 m2g−1was obtained.The nitrogen content is as high as 6.3 wt%.The results show that the hydrothermal treatment of tannin in ammonia solution increases the amination efficiency of the material to obtain the highly N-doped carbon gel material.

Based on the synthesis of carbon foams and carbon gels, mesoporous carbon materials have also attracted intensive attentions.As for the synthesis of mesoporous carbon, a proper pore forming agent is required.Surfactant and block polymer are the best candidates.They firstly form the micelle in the carbon precursor.After polymerization and carbonization process, mesoporous carbon materials can be obtained.Zhao and co-workers selected several natural nitrogen sources such as amino acid, urea, guanine and melamine, to introduce into the solid-phase synthesis system[52].The N species can be effectively introduced into the carbon network.However, the synthesized mesoporous carbon materials show disordered structure.These natural nitrogen-containing molecules are prone to gasification and pyrolysis during the calcination progress, which eventually leads to the disorder of the pore structure.

To synthesize ordered mesoporous carbon materials, soft-template strategy was usually employd by applying block copolymer as the template[3,58].The carbon sources and block copolymers can crosslink into an ordered and mesostructured precursor via the process of organic-organic self-assembly.After thermal pyrolysis process, the ordered mesoporous carbon with open framework and periodic pore arrays will be obtained[3].Liang and co-workers firstly synthesized ordered mesoporous carbon films using RF resin as a carbon source and PS-b-P4VP as a pore forming agent[18].Later, Meng and co-workers prepared highly ordered mesoporous polymers and carbon materials with different mesostructures.Pluronic block copolymers were used as templates, and a soluble resol (a low-molecular-weight phenol-formaldehyde resin) was used as a carbon source via a evaporation induced self-assembly process (EISA)[19].During the synthesis process, resol was firstly synthesized.Then, resol and block copolymers were mixed in the organic solvent.As the solvent evaporated, the block copolymers formed micelles together with resol.Then, the mesostructure was fixed via further thermal polymerization process.Above 600 °C, the phenolformaldehyde resin was converted into rigid carbon framework.The block copolymers were decomposed due to high oxygen content.The ordered mesoporous structure was adjustable by changing the ratio of pore forming agent to precursor or using different surfactants.2D hexagonal (p6m) structure, 3D cubic (Imm)structure and lamellar mesostructure were obtained.Ordered mesoporous carbon spheres using RF resin as a precursor were also prepared[33].In accordance with the organic-organic self-assembly process, plant polyphenols were also used for the synthesis of ordered mesoporous carbon.The synthesis methods can be mainly divided into solution-phase method and solidphase method.

Schlieger and co-workers reported the synthesis of ordered mesoporous carbon materials using plant tannin as a carbon source, and F127 (PEO106-PPO70-PEO106) as a soft template[74].Firstly, multiple adjacent phenolic hydroxyl groups from wattle tannin can interact with the hydrophilic PEO segments from block copolymer by hydrogen bond.The precursors of tannin and the template are formed in solution.Through the self-assembly method induced by the solvent evaporation, the hydrophobic PPO region gradually separates from the hydrophilic tannin and PEO region in the precursor.Then the ordered mesophase is formed.Formaldehyde was not used in this synthesis process, because it is more toxic and environment-unfriendly.Ordered mesoporous carbon was formed after further high temperature carbonization.The ordered domains are around 200-500 nm.The pore size of the material is changed from 9 to 7 nmwith the temperature raising from 400 to 900 °C.However, the specific surface area of the material do not change monotonically according to the change of temperature due to both the decomposition of template and the shrinkage of carbon network during carbonization.In the temperature range of 400-900 °C,the BET specific surface area of the material varies in the range of 483-545 m2g−1.

In addition to the above wattle tannins, other tannin molecules can also be used for the synthesis of ordered mesoporous carbon, such as tannic acid,Mimosa tannin[60], ellagic acid, etc.Mimosa tanninbased ordered mesoporous carbon materials were prepared by Braghiroli and co-workers[72]with F127 as the soft template.The whole reaction system do not need formaldehyde crosslinking or the assistance of other organic solvents.At the beginning, phase separation occurs rapidly in solution at room temperature.The solid phase in the reaction system is the mesophase state from tannin-F127 composite.The synthesis process of the material can be realized under acidic pH conditions.The pH value (0-4.2) had no significant effect during the preparation process.With the increasement of the calcination temperature, the BET specific surface area increased gradually.When pH was 4.2, the BET specific surface area of the material increased from 563 to 722 m2g−1with the increasement of temperature from 400 to 900 °C.The ordered mesoporous carbon derived from Mimosa tannin is amorphous.With the calcination temperature increasing, the order of crystalline structure increases gradually (Fig.4).

The synthesis of ordered mesoporous carbon generally adopts the solution-phase synthesis route.However, this process still requires solvents.Zhang and co-workers reported a method for solid-phase synthesis of ordered mesoporous carbon[40].They used plant tannin as a carbon source and Pluronic triblock copolymers PEO-PPO-PEO as a pore forming agent.Firstly, tannin and amphiphilic block copolymer were ground in a vibrating ball miller.The catechol and pyrogallol groups of tannin interacted with the hydrophilic PEO region to form a network structure by hydrogen bond.Zn(OAc)2·H2O or Ni(OAc)2·H2O were added and grounded together.The coordination between metal ions and phenolic hydroxyl groups from tannins further cross-linked the tannins around block copolymer micelles.Finally, gel-like products were obtained.The ordered mesoporous carbon materials could be obtained by calcination of the gel-like composites in nitrogen atmosphere.During calcination process, Pluronic F127 was completely decom-posed.Tannins were converted to carbon network.Zn2+was firstly transformed into zinc oxide during calcination.Then zinc oxide was reduced to Zn nanoparticles during calcination at higher temperature and finally evaporated.When Ni2+was added, it would eventually be reduced to Ni nanoparticles and evenly distributed in ordered mesoporous carbon materials.Pluronic F127 and metal ions were indispensable in the synthesis process, otherwise disordered carbon structure will be formed.The adjustment of mesoporous size was also related to the mass ratio of tannin and F127, the type of block copolymer, carbonization temperature and so on.The pore size was 7.7-8.6 nm.Ni nanoparticles loaded ordered mesoporous carbon was also synthesized by this method.The tannin networks limited the aggregation of Ni nanoparticles and makes them evenly dispersed in the material.Even if the temperature was as high as 600 °C, the size of Ni nanoparticles was as small as 5.4 nm.The size increased to 10 nm when the temperature rose to above 800 °C.The Ni nanoparticles loaded ordered mesoporous carbon had similar Ni content compared to the commercial activated carbon by wet synthesis, which is of great significance in catalysis.The Ni loading content reached to 16.1 wt% (Fig.5).

Other kinds of metal nanoparticles loaded ordered mesoporous carbon materials can also be synthesized by solid-phase synthesis.Zhang and coworkers subsequently synthesized cobalt@ordered mesoporous carbon material (Co@OMCs)[48].Gel-like precursors were synthesized by adding Co2+into tannic acid and Pluronic F127 during mechanical attrition treatment.OMCs were obtained by calcination in nitrogen atmosphere.The calcination temperature had a great influence on the reduction of Co2+.There was no Co nanoparticles found in the samples treated at 400 °C.The reduction of Co2+occurred at 600 °C or higher.The Co content was as high as 9.0 wt%-21.5 wt%.In terms of pore structure, the pore size distribution was 4.3-9.4 nm.The specific surface area was 432-700 m2g−1for Co@OMCs.

Wang and co-workers synthesized a kind of Ru nanoparticles loaded ordered mesoporous carbon materials[96].The coordination ability between Ru and tannin was weaker compared with the above ordered mesoporous carbon materials synthesized by solid phase.Although tannic acid could provide the coordination sites for Ru3+, it was difficult for Ru3+to further crosslink the tannic acid/F127 complex to metal polyphenol precursors.At the same time, its weak coordination ability could not maintain the morphology of the precursor in the calcination process.If it was needed to reach the amount of Ru required for crosslinking tannin, the final product would containexcessive Ru nanoparticles catalyst.Too many Ru nanoparticles were not needed.At the same time, Ru nanoparticles would further aggregate, reducing its atom utilization while increasing the cost.To solve the above problems, other types of metal ions were added to the reaction system as auxiliary crosslinking agent.It played the role of diluting Ru ion concentration and strengthening the stability of precursor network.At the same time, the newly introduced metal ions needed to be removed in the calcination process.Herein, Zn ions were chosen as auxiliary.Zn2+could effectively crosslink plant tannins, reduce the required concentration of Ru3+and avoid the sintering of Ru nanoparticles.After calcination and reduction at 600 °C, Zn2+was converted to ZnO.When the temperature further increased to 800 °C, ZnO was reduced to Zn and removed by evaporation at higher temperature.Ru3+in the carbon network was reduced to Ru nanoclusters with a size distribution of 1.4-1.7 nm.Ru@OMC catalyst had excellent catalytic performance for the preparation of benzaldehyde from benzyl alcohol.

3 Mesoporous carbon with tunable morphology

The morphology is also an important parameter for mesoporous carbon.For example, mesoporous carbon spheres, mesoporous carbon nanosheets have been widely investigated using other carbon source[97-101].Based on the assembly of plant polyphenol and other functional units, plant polyphenolderived mesoporous carbon materials with different morphologies have also been prepared.

The preparation of carbon nanospheres using spherical polymer precursors is a very effective method.In 2011, Liu and co-workers developed the synthesis of resorcinol/formaldehyde resin colloid spheres using the extended Stöber method[34].Due to the similarity for the sol-gel process between silica and RF resin, the RF resin colloid spheres were synthesized by the polymerization between resorcinol and formaldehyde with lysine and other basic amino acids as catalysts.Firstly, the hydrogen bonds were formed by the molecular of water, alcohol, resorcinol and formaldehyde, which form the emulsion droplets.Next, the colloid spheres were formed inside the droplets with the catalysis of alkaline condition.The size of the spheres was at the mean of 500 nm, and it could be easily controlled by the temperature, ratio of the reactant and pH value.For example, with increasing the concentration of ammonia from 0.052 9 to 0.158 7 mol L−1, the sphere size increased from 520 to 740 nm.When the ethanol/water ratio increased from 0 to 1.75, the size increased from 349 to 850 nm.And the size increased from 260 to 1 200 nm while resorcinol concentration increased from 0.032 4 to 0.099 6 mol L−1.Then the product was calcined in inert atmosphere to obtain carbon spheres.The resin polymer spheres showed tunable morphology and particle size (Fig.6).

Subsequently, more RF resin derived mesoporous carbon nanospheres were prepared[102].By using cationic surfactant or block copolymer as a template,mesoporous carbon spheres can be synthesized.The diameter and pore size of mesoporous carbon spheres derived from resorcinol/formaldehyde resin can be easily adjusted.For example, Du and co-workers found that the pore size of mesoporous carbon spheres obtained by carbonizing resorcinol/formaldehyde resin spheres was 3.1 nm[102].However, if dense silica shell was coated on polymer spheres before calcination process, the pore size of mesoporous carbon spheres increased to 10.0 nm, with the specific surface area increasing from 696 to 1 186 m2g−1.The dense silica shell could retain the polymer framework and decrease the sharp shrinkage of polymer framework, resulting into a much larger pore size.They further found that different morphologies of yolk-shell mesoporous carbon nanospheres could be obtained by using silica shell with different density[103].After that,the method of preparing mesoporous carbon spheres with dopamine as carbon source was gradually realized[104].

Natural plant polyphenols have similar properties to resorcinol and are more environmentallyfriendly.Therefore, it is an excellent candidate for the synthesis of mesoporous carbon spheres.Plant polyphenols can also be crosslinked with formaldehyde.These plant polyphenols are rich in catechol or galloyl groups, which can be strongly chelated with metal ions through coordination bonds[105].For example,tannic acid with abundant catechol and galloyl groups,was usually used as an organic ligand[105].Based on this, it is even possible to directly use tea extract for chemical synthesis[106].In 2013, Ejima and co-workers synthesized metal-polyphenol coordination capsules by using the chelation between plant polyphenols and metal ions[107].They used Fe(III) ions as crosslinking agent and tannic acid as an organic ligand to form metal polyphenol coordination, which could be coated on variety of substrates.Compared with resorcinol, catechol group could coordinate with metal ions strongly.Guo and co-workers verified that the natural plant polyphenols could chelate with various metal ions[108].They used 18 different metal ions (including cobalt (Co), nickel (Ni), copper (Cu), manganese(Mn), zinc (Zn), iron (Fe), molybdenum (Mo), aluminium (Al), vanadium (V), chromium (Cr), ruthenium (Ru), gadolinium (Gd), terbium (Tb), rhodium(Rh), cadmium (Cd), cerium (Ce), europium (Eu) and zirconium (Zr)) to achieve the coordination with tannic acid.Because tannic acid had strong adhesion,these metal ions could easily coordinate with tannic acid, and be coated on the spherical substrate.After removing the substrate, the metal-polyphenol coordination capsule with hollow structure was obtained.

Based on the chelation between the above metal ions and plant polyphenols, Wei and co-workers synthesized cobalt-polyphenol coordination crystal[38].The pH value of the reaction system increased to 8 by adding ammonium hydroxide.Tannic acid was used as an organic ligand.Under alkaline conditions, cobalt ion could chelate with catechol groups to crosslink tannic acid molecules and realize polymerization.At this time, the synthesized Co-TA particles showed amorphous structure.The amorphous coordination polymers were gradually crystallized during hydro-thermal process.Finally, Co-TA crystalline particles(～5 μm) were obtained.To incorporate N heteroatom, dicyandiamide was used as N source to grind with Co-TA crystal.Subsequently, Co-TA crystals were carbonized under nitrogen atmosphere.Finally,the Co/N-doped carbon composites were obtained,which had high electrocatalytic properties for both ORR and OER reactions (Fig.7).Like the coordination assembly of the above Co-TA nanocrystals, other plant polyphenols and metal ions can also assemble and realize the synthesis of metal-phenolic coordination materials[41].Ellagic acid (EA) can also be used as a polyphenol ligand.Zn ions were coordinated with phenolic hydroxyl groups from EA to synthesize EAZn2 complex crystal materials.Hierarchically carbon microparticles were synthesized by direct carbonization under 900 °C.During this process, EA acted as carbon source and directly transformed into carbon frameworks.Zn2+was first converted to ZnO, then further reduced to Zn and evaporated at high temperature.Hierarchically carbon microparticles could be used for adsorption (Fig.7).

According to the above coordination between metal ions and plant polyphenol, Wei and co-workers have developed a universal method to synthesize the metal-phenolic colloidal spheres[39].The highlight of this synthesis was the usage of formaldehyde as a cross-linking agent to control the coordination assembly between metal ions and natural plant polyphenols.This method was similar with the Stöber method.Firstly, tannic acid was pre-crosslinked by formaldehyde to form oligomers in alkaline ethanol/water solution.F127 was added to mediate the assembly process.After obtaining oligomers, metal ions were added to crosslink these oligomers through coordination bonds.Because tannic acid has the strong coordination ability, many metal ions could coordinate with oligomers.According to the different valence states of metals, different coordination forms could also be synthesized, such as divalent metal complexes(Co2+, Fe2+, Ni2+, Cu2+, Zn2+) and trivalent metal complexes (Fe3+, Al3+, Ce3+).Furthermore, they could not only synthesize metal polyphenol coordination particles with mono-metal, but also could obtain the bi-metal (Fe-Co and Zn-Co) and multi-metal (Fe-Co-Ni-Cu-Zn) with different metal components.The metal polyphenol coordination nanospheres prepared by this method had a uniform particle size distribution of about 300 nm.By changing the ratio of reactants in the synthesis process, the composition of the materialcould be easily adjusted.Not only the content of metal ions could be adjusted, but also the proportion of different metals in bi-metal and multi-metal components could be adjusted.Formaldehyde played an important role in the synthesis of materials.If formaldehyde was not added in the synthesis process, tannic acid would directly form rod-shaped crystals through the self-polymerization process under dissolved oxygen, which cannot obtain spherical materials.After adding formaldehyde, it could cross-link with tannic acid, forming small tannic acid-formaldehyde oligomer.Then the metal ions could introduce the formation of stable metal-polyphenol colloidal spheres(Fig.8).

Based on the synthesis of metal-polyphenol coordination spheres, the carbon materials could be synthesized by a simple carbonization process[39].With calcining in nitrogen atmosphere, the polyphenol framework was converted to carbon.At the same time, the metal ions were reduced to metal particles.The specific surface area of the carbon material obtained from the calcination of the metal polyphenol complex was about 400 m2g−1.The spherical morphology was well maintained.This one-step conversion method could be used to synthesize carbon materials loaded with metal nanoparticles.This method could also be used to obtain carbon materials without loading metal nanoparticles after acid treatment.The pure mesoporous carbon material was obtained by using zinc salt as crosslinking agent.Due to the low boiling point of zinc, the reduced zinc nanoparticles would be removed by evaporation when calcined in nitrogen atmosphere at high temperature.In this way, Wei and co-workers synthesized a kind of Zn-TA coordination sphere and transformed it into mesoporous carbon sphere[62].The mesoporous carbon sphere had uniform size of about 120 nm and ultra-high specific surface area of 2 221 m2g−1(Fig.9).

Mesoporous carbon nanospheres loaded with magnetic nanoparticle could also be synthesized using the similar method.Wang and co-workers synthesized a magnetic mesoporous carbon nanosphere for the adsorption of hexavalent chromium in water[53].Firstly, Fe-TA nanospheres were synthesized by co-ordination between tannic acid and Fe2+.Then magnetic nanoparticles were obtained by one-step carbonization.Mesoporous carbon nanospheres withα-Fe/Fe3C particles were obtained.The organic ligands inside the nanospheres were gradually decomposed with the increasement of calcination temperature from 400 to 850 °C.Thermogravimetric analysis (TGA)showed that the mass of carbon decreased from 68.8 wt% to 47.1 wt%.The diameter of nanospheres was about 60 nm.Magnetic nanoparticles were about 10 nm, which evenly distributed in the material.The main components wereα-Fe and Fe3C.The pore diameter was about 6.7 nm, and the BET specific surface area was 512.3 m2g−1.Theα-Fe/Fe3C loaded magnetic mesoporous carbon spheres showed excellent adsorption properties of hexavalent chromium in water.At the same time, the magnetism of the material allowed to realize easy separation by magnetic field(Fig.10).Another magnetic mesoporous carbon material was also successfully prepared.Plant polyphenols were the carbon source.Fe3O4nanoparticles were loaded in the carbon framework.This kind of materials were used for the extraction of chlorophenols[109].

The above mesoporous carbon nanospheres loaded with metal elements is generally used in catalytic applications.Single atom doping can be selected to improve the atomic utilization of supported metals and investigate the catalysis mechanism.Wei and co-workers synthesized a kind of covalently crosslinked poly(HCCP-TA-BPS) (BPS = 4,4‘-sulfonyldiphenol)nanospheres (PSTA)[42].Tannic acid was used as an organic ligand.The co-monomers were first formed with hexachlorochlorophosphazene.Hexachlorocellulose phosphate introduced N and P elements into the material.Tannic acid could coordinate with metal ions.The diameter of the synthesized PSTA sphere was about 50 nm.With further carbonization at 1 000 °C, hollow carbon nanospheres with mesoporous structure were obtained.The concentration of TA could adjust the hollow structure of the material.With the gradual increasement of TA concentration, the material gradually changes from the solid sphere to the hollow sphere.The thickness of the spherical shell gradually became thinner and finally became a bowl.To realize the application of electrocatalysis, the materials were calcined in nitrogen atmosphere to obtain mesoporous carbon spheres.SEM results showed that the material could maintain its hollow spherical morphology after calcination.Heteroatoms such as P and N in the carbon spheres improved the catalytic activity of the materials.Co element existed in the form of Co single atom inside the carbon network.This kind of material could be used as an efficient catalyst for ORR reaction (Fig.11).

4 Mesoporous carbon on different substrates

Tannic acid has excellent adhesion properties and can be used in many applications such as adhesives.Based on its adhesion, metal-tannic acid coordination polymer can adhere on the various of substrates[105].Then the mesoporous carbon materials on the surface of different materials can be prepared by carbonization.Wei and co-workers synthesized Fe3C/Fe-N-C catalyst on commercial filter paper, tissue and cotton[37].In this synthesis, cellulose could be acted as the carbon source and substrate, and Fe-tannic acid nano-ink was the iron source and carbon source.Dicyandiamide was introduced as the nitrogen source.Firstly, black Fe-tannic acid complexes were coated on cellulose.The obtained products were mixed and ground with dicyandiamide.N element was introduced into the material.Finally, mesoporous carbon materials were prepared by carbonization and acid etching.The main components of the material obtained by carbonization were Fe3C and graphite carbon.Fe3C nanoparticles were well protected by graphite carbon.Therefore, Fe3C could be retained after acid etching.The N element from dicyandiamide existed in the form of pyridinic nitrogen, pyrrolic nitrogen, graphical nitrogen and pyridinic N+-O−.The con-tents were 28.13%, 21.10%, 32.61% and 18.17%, respectively.The material had excellent ORR catalytic properties (Fig.12).

Metal-tannic acid complex can not only be coated on cellulose, but also be loaded in many carriers.Li and co-workers synthesized high density Fe nanoclusters loaded N, S co-doped carbon[110].Tannic acid was used as an organic ligand, and iron ions was used as a crosslinking agent to crosslink TA molecules.The polyaniline (PANI) gel containing Fe-TA was formed.The Fe-N/C catalyst was then obtained by freeze-drying for 48 h and calcination at 900 °C.Fe element mainly existed in the form of Fe nanoclusters in the material.The amino or imino groups of PANI could interact with Fe3+.After carbonization,Fe-Nxactive sites were formed in the material.N and S heteroatoms were also doped in the material, with a content of 4.3 wt% and 7.5 wt%, respectively.The number of Fe monatomic active sites would gradually decrease when the calcination temperature was higher than 900 °C.After carbonization, the ORR properties of the material were improved, because of the removal of the internal Fe particles by acid treatment.The N, S co-doped carbon had excellent ORR catalytic performance (Fig.13).

5 Mesoporous carbon for different applications

5.1 Adsorption

Porous carbon material is a kind of excellent adsorbent[111].It plays a very important role in both water and air purification.The appearance of mesoporous carbon materials further improves the performance due to their high specific surface area and large pore size.Yang and co-workers synthesized the hierarchical carbon microparticles for the adsorption of macromolecular dyes (Fig.14)[41].The porous carbon material could adsorb 70-110 mg g−1rhodamine B(RhB) dye in 10 min.The hierarchical structure of the material could shorten the time for dye molecules to enter the material and improve the adsorption efficiency.In addition to dyes, heavy metal ions in water have adverse effects on human health.Mesoporous carbon materials could realize the adsorption and removal of heavy metal ions in natural water.The magnetic mesoporous carbon composites synthesized by Wang and co-workers could effectively adsorb Cr6+ions in water[53].The ultra-high specific surface area of the material provided excellent adsorption properties.The removal of Cr6+was realized by electrostaticinteraction and reduction.Its maximum adsorption capacity for Cr6+ions was 336.7 mg g−1.The adsorption rate was 1.60×10−3mg g−1min−1.Mesoporous carbon materials can not only adsorb pollutants in water, but also realize gas adsorption[50].The N-doped mesoporous carbons prepared by Zhao and co-workers had CO2and light hydrocarbon sorption properties[52].The material had ultra-high specific surface area and large pore diameter.N doping improved the acid gas capture performance of the material.The adsorption capacity of N-doped mesoporous carbon materials for CO2was higher than that without N atoms.The adsorption capacity was as high as 3.45 mmol g−1.It also realized the adsorption for other alkane molecules,such as acetylene of 3.35 mmol g−1, ethylene of 2.23 mmol g−1, ethane of 2.62 mmol g−1, acetone of 3.35 mmol g−1, propylene of 2.62 mmol g−1and propane of 2.69 mmol g−1.

5.2 Catalysis

Mesoporous carbon materials derived from plant tannin also possess excellent electrocatalytic performance[112].For fuel cell or metal-air cell, oxygen reduction reaction (ORR) and oxygen evaluation reaction(OER) are the two most important processes.Noble metal or non-noble metal catalysts are generally required to achieve the efficient catalysis[113].Mesoporous carbon loaded with metal nanoparticles is a potential candidate material.

Wei and co-workers deposited Fe-TA on cellulose, and then prepared Fe3C/Fe―N―C material forORR[37].When Fe3C/Fe―N―C catalysts and Pt/C(20 wt%, Sigma-Aldrich) possessed the same loading(i.e.0.1 mg cm−2), they exhibited similar catalytic performance in the alkaline conditions.Furthermore,when the loadings were two- or three-times than that of Pt/C in alkaline conditions, Fe3C/Fe―N―C catalysts showed a much higher performance.The ORR performance of Fe3C/Fe―N―C catalysts was one of the best of the reported non-precious metal catalysts.Fe3C/Fe―N―C catalysts were even comparable with some non-precious metal catalysts on nano carbon substrates.This coating method could obtain a variety of catalyst materials.Subsequently, the metal/N-doped carbon composites synthesized by Wei and coworkers had the characteristics of Co nanoparticle loading[38].The cyclic voltammetry curve showed that the material had oxygen reducing activity.The catalytic activity and long-term stability of Co-TA-800(mesoporous carbon material calcination by Co-TA polymer at 800 °C) were higher than those of commercial Pt/C electrode.The onset potential was 0.95 V.The catalytic process was dominated by a four-electron process.The OER catalytic performance of the material revealed a low onset potential (1.69 V) when the current density reached 10 mA cm−2.In general,Co-TA-800 was an excellent ORR and OER electrocatalyst (Fig.15).

Nonprecious metal catalysts derived from tannic acid as carbon source can not only support metal nanoparticles, but also use metal single atom as catalytic site to realize oxygen reduction.Wei and co-workers designed and synthesized nanoporous carbon material with hollow structure, in which cobalt single atoms were uniformly dispersed[42].Under alkaline conditions, the half wave potential was 0.878 V, and limit current density was 6.4 mA cm−2.The catalytic performance of the material was better than that of carbon material loading Co nanoparticles, showing theimportance of cobalt single atom.The material also had good catalytic stability.

The ordered mesoporous carbon material prepared by Zhang and co-workers through solid-state synthesis was a good catalyst carrier[40].Ni nanoparticles were uniformly dispersed in ordered mesoporous carbon.Ordered mesoporous carbon with Ni nanoparticles was an efficient macromolecular selective hydrogenation catalyst.The key point to the preparation of alkanes by olefin catalytic hydrogenation was the mass transfer in the catalyst.Ordered mesoporous carbon would solve this problem well.The catalytic hydrogenation of cyclohexene, 1-octadecene and cholesteryl acetate was carried out.It was found that Ni-OMC@F1270.8-450, Ni-AC@450 and Ni-STOMC@450 showed different catalytic properties for macromolecules, such as 1-octadecene and cholesteryl acetate.The pore structure in Ni-AC@450 and Ni-ST-OMC@450 was blocked by large Ni particles, which affected the transport of macromolecular substrate.Therefore, their catalytic performance was much lower than that of Ni-OMC@F1270.8-450 materials.For small molecule, cyclohexene, the above three catalysts have high catalytic performance(Fig.16).For the activated carbon or post-impregnated ordered mesoporous carbon, their micropores or even mesopores could be easily blocked by large Ni nanoparticles.The bulky cholesteryl acetate had a rigid backbone, which likely prevented to enter the pores.It was hard to be transported to the active sites,resulting in lower catalytic performance.

The palladium loaded mesoporous carbon material was used as a catalyst for ligand free Suzuki-Miyaura couplings of aryl bromides[47].The worm-like mesoporous carbon networks were used as carrier.Pd nanoparticles were encapsulated in it.Mesoporous carbon network effectively limited the precipitation of palladium.In ethanol or aqueous solution, Pd loaded mesoporous carbon material realized Suzuki-Miyaura coupling catalytic effect.Its performance was betterthan the commercial Pt/C electrode, and had a good repeatability.Then Peter and co-workers improved the material.They loaded magnetic nanoparticles in the ordered mesoporous carbon.A kind of magnetically recoverable Pd catalyst was obtained for Suzuki reaction[114].Zhang and co-workers synthesized ordered mesoporous carbon materials with Co nanoparticles in the way of solid-state synthesis.The Co@OMCs could catalyze the deoxidation of alcohols, aldehydes or ketones to form alkanes.The high loading rate and dispersion of Co nanoparticles in ordered mesoporous carbon greatly improved the catalytic performance.Wang and co-workers synthesized a mesoporous carbon material loaded with CuO and CuFe2O4nanoparticles using tannic acid as a carbon source[115].The material was used for the adsorption and catalytic degradation of bisphenol A (BPA).The carbon skeleton of the material would adsorb BPA, support and disperse metal oxide nanoparticles, reduce their aggregation and improve the catalytic efficiency.Specifically,removal rate was 43% via adsorption within 5 min.The overall removal rate was 98%.Finally, it realized the effective removal of BPA (Fig.17).

5.3 Energy storage

As an excellent electrode material, porous carbon materials have been widely used in energy storage[43].Mesoporous carbon materials have porous structure, high specific surface area, good electron transport efficiency, and high electrochemical performance[28,116-117].The usage of plant polyphenols isan efficient strategy for the sustainable synthesis of mesoporous carbon electrode materials[10,117].

The N, O-doped carbons derived from pine tannins were used as an excellent supercapacitor[57].According to the cyclic voltammetry (CV) curves, the curve had a wide redox peak at a low scan rate.While at high scan rate, the CV curve was rectangular, indicating that the capacitance property of the material was excellent.It has the advantages of high capacitance retention, high power density, high energy density and long-term stability.The maximum energy density was about 1 500 mA g−1(Fig.18).Castro-Gutiérrez andco-workers synthesized a kind of ordered mesoporous carbon based on Mimosa tannin[61].The OMCs had 2D hexagonal ordered mesoporous structure through CO2activation.After the activation, the BET specific surface area of the material was greatly improved to 2 000 m2g−1.The capacitance values of the material were 37 and 27 F g−1in aqueous and organic electrolytes respectively.The capacitance retention was 70%(80 A g−1) and 44% (40 A g−1) in aqueous and organic electrolytes respectively.The high capacitance retention is owing to the abundant mesopores and efficient mass transfer.

6 Conclusion and perspectives

The synthesis of mesoporous carbon materials with different structures and compositions using renewable plant polyphenols as a carbon source has been comprehensively reviewed.Plant polyphenols are nontoxic, renewable and low-cost biomass.They have plenty of catechol or pyrogallol groups to form hydrogen bonding with PEO-based block copolymers.Soft-template strategy can be used for the synthesis of ordered mesoporous carbon.Due to the strong metal chelate ability, different metal species can be incorporated into the polymer network.The metal species can also be used as a template to enhance the porosity of mesoporous carbon.Therefore, metal-polyphenol hybrids are an important precursor for mesoporous carbon composites without using any external template.Like the phenolic resin colloids, metal-polyphenol-formaldehyde colloidal spheres can also be prepared and used for mesoporous carbon sphere.More importantly, polyphenols show universal adhesive property on different interfaces based on musselinspired chemistry.This made the mesoporous carbon composites can be deposited on various substrates.Due to the high specific surface area (up to 2 254 m2g−1), tunable compositions such as heteroatom doping (N―, P―), metal doping(Fe―N―C), nanoparticles loading including metal(i.e., Co, Ni), metal oxide (i.e., Fe2O3, ZnO), metal carbide (i.e., Fe3C), as well as diverse morphologies and pore structures, these mesoporous carbon shows promising applications in adsorption, catalysis, energy conversion and storage.Despite the great progresses, there are still tremendous room to explore.

Firstly, there are over 8 000 kinds of plant polyphenols in the nature.They usually show different chemical properties.Using these different polyphenols as a carbon source, the synthesis strategy for creating mesopores in a controllable manner should be considered.Different kinds of synthesis strategy should be developed to tailor the pore structure and compositions.Secondly, mesoporous carbon composites, especially mesoporous metal/carbon composites have been prepared using metal-polyphenol hybrids.It should be noted that these metal species have obvious effects on their physicochemical properties and performance.The distributions, particle size, coordination state, location for metal species, as well as the pore structure of mesoporous carbon are also important parameters.It is still a big challenge to control these parameters.Thirdly, metal-polyphenol hybrids can be deposited on various substrates.It’s possible to develop functional composites or modify the surface.Metal-polyphenol film on various substrates have been intensively investigated.However, the conversion of metal-polyphenol film to porous carbon has been rarely investigated.It may be an important research direction.Fourth, phenol-formaldehyde resin has been widely used as a mesoporous carbon source.The replace of toxic phenol with renewable plant polyphenols is an efficient strategy for large-scale and sustainable synthesis of mesoporous carbon.However, due to the different chemical properties, the polymerization of polyphenol-formaldehyde is still needed to investigate.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China(21701130), Key Research and Development Program of Shaanxi (2021GY-225) and the Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (2020-KF-42).