多孔炭材料的設(shè)計(jì)合成及CO2吸附分離研究進(jìn)展

張向倩，李文翠，陸安慧

（大連理工大學(xué)化工學(xué)院，精細(xì)化工國(guó)家重點(diǎn)實(shí)驗(yàn)室，遼寧大連116024）

多孔炭材料的設(shè)計(jì)合成及CO2吸附分離研究進(jìn)展

張向倩，李文翠，陸安慧

（大連理工大學(xué)化工學(xué)院，精細(xì)化工國(guó)家重點(diǎn)實(shí)驗(yàn)室，遼寧大連116024）

溫室效應(yīng)日漸顯著，CO2的捕集與利用已成為一個(gè)全球性的科學(xué)研究熱點(diǎn)。碳質(zhì)吸附材料因其結(jié)構(gòu)的可設(shè)計(jì)性、

多孔炭；設(shè)計(jì)合成；CO2吸附分離

1　Introduction

The emission of CO2from industry and power plants has become a worldwide problem with a strong link to global warming.Reduction of the CO2concentration in the atmosphere is currently a hot topic.In addition，a high concentration of CO2is harmful to humans，especially in space-limited chambers like submarines and space ships.Therefore，development of efficient CO2capture and storage（CCS）techniques is urgent and significant to alleviate this issue［1，2］.Fig.1 shows the quantity of academic publications with a keyword of carbon capture and storage in recent years since 2000.We can see that there is a sharp increase of the publications worldwide in recent years and this change may indicate that a significant interest in CO2capture research and development has been taking place since 2006.It is clearly shown that CCS has attracted worldwide attention including developed and developing countries，e.g.，USA，China，England，Germany，et al.

The development of novel materials and new technologies for CO2adsorption and separation is a greatly concerned step for practical CCS applications［3，4］.Currently，for high concentration CO2-containing gases，chemical/physical sorption and cryodistillation processes are used in industry.While for the dilute sources such as natural gas，syngas，biogas，physical adsorption is more effective because of its multiple advantages such as fast kinetics，easy regeneration，low in corrosion to equipment and superior cycling capability［5］.For this issue，adsorbents with a high CO2adsorption capacity and excellent selectivity of CO2over other gases are essential for CCS.Ideal adsorbents should have a high CO2adsorption capacity，excellent adsorption selectivity over other gases，and a good chemical and mechanical stability.Recently，many groups focused on the study of synthetic methods and performance estimations on the specific class of the new materials，such as metal organicframeworks（MOFs），activatedcarbons（ACs），porous organic polymers（POPs），zeolites，covalent organic frameworks（COFs）.

Fig.1 Publications on“carbon capture and storage”（a）in recently years，（b）in countries/regions（Top 16）.Data adapted from ISI Web of Thomson Reuters.

Among these materials，porous carbons have been proven to be competitive candidates by virtue of their high specific surface areas，moderate heat of adsorption，low-cost preparation，relatively easy regeneration，and less sensitivity to the humidity than the other CO2-philic materials.ACs derived from coal and petroleum and coconut shells are the most commonly used form of porous carbons for a long time.However，the surface chemistry and pore size have been uncontrollable due to uncertain structures of various precursors.Thus，their CO2capture efficiency is badly affected due to the low adsorption selectivity.Nowadays，along with the improvement of science and technology，it has been significant to synthesize carbon materials with a defined nanostructure and morphology，tunable surface area and pore size.This，in turn，leads to a high selectivity of CO2over balance gas，in most case，nitrogen gas.

In this review，the most recent developments and emerging concepts in CO2capture by porous carbons are discussed.After that，some widely used strategies to enhance the CO2capture capability are presented and highlighted.The aim is to describe the key advances，predominantly focusing on high-performance CO2capture materials with tailored pore structures and targeted surface properties.Both an in-depth study of porous carbon materials and a high-level evaluation of their performance in CO2capture are focused.Finally，we briefly summarize and discuss the future perspectives of porous carbons for CO2capture.

2　Synthesis of carbonaceous materials for CO2capture

A variety of carbonaceous adsorbents have been considered for CO2capture，which are prepared from diverse precursors.Here in，porous carbons are，for clarity，mainly classified into the following categories：biomass derived porous carbons，synthetic polymer-based porous carbons，newly developed porous carbons from ionic liquids（ILs）and porous organic frameworks（POFs），etc.In each category，synthesis principles，pore structures，macroscopic and microscopic morphologies，as well as their CO2capture behaviour are discussed.

2.1Biomass-based porous carbons

With the demand of a huge amount of solid CO2sorbents，the development of low-cost materials that can sorb CO2efficiently will undoubtedly enhance the competitiveness of adsorptive separation for CO2capture in flue gas applications.According to the United Nation Environment Programme（UNEP），globally 140 billion metric tons of biomass is produced per year from agriculture［6］.The production of carbonmaterials from biomass is a relatively new but rapidly expanding research area.To emphasize the importance，here we separately set a section to update the carbonaceous CO2sorbents from biomass precursors［7，8］.

Up to now，many kinds of ACs from biomass have been well commercialized in gas adsorption/separation including CO2capture.Polysaccharide-derived “Starbons?”carbon is a successful example，which exhibits outstanding mesoporous textural properties.An extensive comparison of chemical compositions of various biomass sources（shells，stones，woods）and adsorption properties of the resulting carbons are presented in reviews［9，10，11］.A wide range of biomass precursors can be used for fabrication of porous carbons，which include nut shells（almond shell，coconut shell，and palm shell），food residues（coffee grounds，bagasse，fungi，bean dreg，and celtuce leaves），wood residues（sawdust，chips and barks，poplar anthers，etc.），marine macroalgae，and so on［12，13，14］.Here，we briefly summarize the recently reported carbonaceous adsorbents from biomass materials.As shown in Table 1，a series of porous carbons were prepared from biomass materials.The specific surface areas and pore volumes were tuned with controlled carbonization and/or activation process.The maximum adsorption of CO2on these carbons at 1 bar and 25℃reached a high value of 4.8 mmol·g-1.

Researchers have been particularly active in the developing useful carbonaceous materials from sugar based biomass via a hydrothermal carbonization approach（HTC）［15，16］.For example，Sevilla et al.reported a series of porous carbon capture materials，which were produced from the chemical activation of hydrothermally treated precursors（polysaccharides and biomass）using KOH as an activating agent［20］.Compared to the raw HTC materials，the chemical activated counterparts show a significant increase of micropores，delivering a high surface area between 1 260 and 2 850 m2·g-1depending on the activation conditions.Thus，the activated HTC-based porous carbons show a high capacity up to 4.8 mmol·g-1at 25℃and 1 atm.They found that the remarkable CO2capture capacity is ascribed to the presence of rich and narrow micropores（＜1 nm），and the surface area plays a less important role.Subsequently，Sevilla and co-workers prepared the highly porous N-doped carbons through chemical activation of hydrothermal carbons derived from mixtures of algae and glucose［17］.They demonstrate that the control of the activation conditions（temperature and amount of KOH）allows the synthesis of exclusively microporous biomassbased materials.These materials possess surface areas in the 1 300-2 400 m2·g-1range and pore volumes up to 1.2 cm3·g-1.They additionally exhibit the N contents in the 1.1-4.7 wt％range，these heteroatoms being mainly present as pyridone-type structures.When tested as CO2sorbents at sub-atmospheric conditions，they show a large CO2capture capacity up to 7.4 mmol·g-1at 0℃and 1 bar.However，the results indicate that the large CO2capture capacity is exclusively due to their high volume of narrow micropores and not to the high surface areas or pore volumes，neither to the presence of heteroatoms.

Table1　A comparison of structure textures and CO2adsorption performance of porous carbon adsorbents derived from biomass.

Besides the above mentioned HTC routes，direct pyrolysis combined with activation process［27］is considered to be a promising approach for the fabrication of porous carbons from biomass，such as waste cul-ture leaves and bamboo.For example，as shown in Fig.2，waste celtuce leaves were used to prepare porous carbons by air-drying，pyrolysis at 600℃in argon，followed by KOH activation［13］.The as-prepared porous carbons show a very high specific surface area of 3 404 m2·g-1and a large pore volume of 1.88 cm3·g-1.They show an excellent CO2adsorption capacity at 1 bar，which is up to 6.04 and 4.36 mmol·g-1at 0 and 25℃，respectively.Wang et al.reported a series of porous carbons with adjustable surface areas and narrow micropore size distributionbyKOHactivationoffungi-basedcarbon sources［28］.The high CO2uptake of 5.5 mmol·g-1and CO2/N2selectivity of 27.3 at 1 bar，0℃of such fungi-based carbons made them promising for CO2capture and separation.Similarly，Shen and co-workers found that yeast is a promising carbon precursor for the synthesis of hierarchical microporous carbons，whichshowahighCO2adsorptioncapacity（4.77 mmol·g-1）and a fast adsorption rate（equilibrium within 10 min）at 25℃［29］.This may stem from their large surface area and hierarchical pore systems as well as the surface-rich basic sites.

Fig.2（a）An illustration of synthesis of carbonaceous CO2adsorbent from biomass materials.（b）CO2uptake capacity at 0 and 25℃.（c）CO2cycling performance at 0℃.Reprinted with permission of Ref.［13］.

Fan et al.employed chitosan，which is the second nitrogen-richest natural organic matter only after protein，as a carbon precursor and K2CO3as a mild activator to prepare CO2sorbents by a simple chemical activation method［30］.The textural and chemical properties of the porous carbons could be easily tuned by changing the mass ratio of K2CO3/chitosan and activation temperature.Due to their large pore volumes，well-defined micropores and relatively high nitrogen contents，these porous carbons were applied as adsorbents for CO2capture and demonstrated excellent CO2uptake performances.In particular，the sample prepared at 635℃with a K2CO3/chitosan ratio of 2 shows a CO2uptake as high as 3.86 mmol·g-1at 25℃and 1 atm.Furthermore，the CO2uptake remains almost constant in a five consecutive adsorption-desorption cycle，indicating this material has a great stability and recyclability as a CO2sorbent.In addition，anextraordinaryseparationselectivity against N2（CO2/N2selectivity of ca.21）was also observed.

In short，carbon materials synthesized from carbohydrates，biomass，and other sustainable sources have been discussed.Such natural source-derived carbons will play an increasingly important role in the fabrication of nanostructured carbon materials in the future.Heteroatoms can be successfully incorporated within the framework of these materials and their chemical environment can also be controlled with the post-annealing temperature.Such dopants confer very special electronic structure as well as CO2adsorption capability to the porous carbons.

2.2Synthetic polymer-based porous carbons

Generally，the CO2capture performances are strongly dependent on the microstructures of adsorbents.Meanwhile，the understanding of the hostguest interactions between the adsorbent and CO2is prerequisite for a great advance of the CO2capture.Conventional porous carbon materials，such as ACs and carbon molecular sieves，are synthesized by pyrolysis and physical or chemical activation of organic precursors，such as coal，wood and fruit shells，at elevated temperatures.These carbon materials normally have relatively broad pore size distributions in both micropore and mesopore ranges.Thus，a precisely controlled synthesis of carbon structures is greatly needed，and also a promising opportunity to authentically understand their physical and chemical properties of carbon materials from molecular level will in turn efficiently guide practical applications.

Compared with conventional ACs and biomass derived carbons，using synthetic polymers as porous carbons precursors enables a better chemical composition control to achieve precise morphology，tunable pore system and targeted surface chemistry easily.This can be achieved by a designed synthesis methodology.Therefore，in this part，the synthesis and CO2capture performance of porous carbon materials derived from rationally designed synthetic polymers are introduced，which have been classified into three groups according to synthesis methods，the hard-template method，softtemplate approach and template-free synthesis.

2.2.1Hard-template method

In the past decades，the hard-template method（also called nanocasting）has been demonstrated as a controllable method in preparing carbon monoliths withatailorablepore-sizeoverseverallength scales［31，32］.Nanocasting is a process，in which a mould（may called as hard template，scaffold）over nanometer scale is filled with a precursor，and after processing，the initial mould is afterwards removed.The keys rely on preparing templates with accessible pores（e.g.，silica，zeolites，MgO，etc.）and thermally stable carbon precursors such as phenolic resin，sucrose，furfuryl alcohol，acrylonitrile，acetonitrile and mesophase pitch.In the following，we discuss the synthesis principle and recent developments of nanocasting approach for porous carbons based on several commonly used templates.

（1）Porous silica as templates

Using this strategy，numerous porous carbons with well-defined pores have been obtained.The successful synthesis of carbon with an ordered pore structure was first achieved by Ryoo's group［33］in 1999，whereby the worldwide used mesoporous silica MCM-48 was used as a template to create a carbon material（CMK-1）.Then，many studies were carried out to synthesize mesoporous carbons with ordered structures.Hu et al.synthesized hierarchically porous carbons with a relatively higher content of graphite-like ordered carbon structure，using meso-/macroporous silica as a template and mesophase pitch as a precursor［34］.Lindén and co-workers prepared a hierarchical porous monolithic carbon with wormhole-like mesopores and macropores［35］.

Considering the advantages of nanocasted porous carbons，Yin'group synthesized a mesoporous nitrogen-doped carbon（N-MC）with highly ordered two dimensionalhexagonalstructuresusingdiaminobenzene（DAB）as carbon and nitrogen sources，ammonium peroxydisulfate（APDS）as an oxidant，and SBA-15 as a hard template［36］.By adjusting the synthesis temperatures in a range of 70-100℃，the pore diameter of the materials can be tuned from 3.4 to 4.2 nm，while the specific surface area of the N-MC with a nitrogen content of 26.5 wt％，can be tuned from 281.8 to 535.2 m2·g-1.The C/N molar ratio of the samples can be tuned in a range of 3.25-3.65 by adjusting the mole ratio of DAB/APDS at a synthesis temperature of 80℃，while the pore diameter of the N-MC can be tuned in a range of 3.7-4.1 nm.Similarly，Suárez-García et al.have recently synthesized ordered mesoporous carbons（OMCs）with high surface areas and pore volumes through polymerization of a polyamide precursor（3-aminobenzoic acid，MABA）inside the SBA-15 template［37］.Vinu et al.prepared two-dimensional mesoporous carbon nitrides（MCN）with tunable pore diameters using SBA-15 as templates through a simple polymerization of ethylenediamine（EDA）andcarbontetrachloride（CTC）［38］.Zhao'group reported a preparation of porous carbon nitride（CN）spheres with partially crystalline frameworks and a high nitrogen content（17.8 wt％）by using spherical mesoporous cellular silica foams（MCFs）as a hard template，and ethylenediamine and carbon tetrachloride as precursors［39］.The hierarchical porous CN spheres have excellent CO2capture properties with a capacity of 2.90 mmol· g-1at 25℃and 0.97 mmol·g-1at 75℃，indicating that the presence of N-containing sites and a large number of micropores and small mesopores greatly enhance the CO2capture.

（2）Crystalline microporous materials as templates

The efforts to construct a molecular-sieve type porous carbon often require well-crystalline zeolite or MOFs-related materials（ZIFs，MOCPs）as sacrificial templates.The nanoporous carbons produced in thisway show the remarkably high surface areas（up to 4 000 m2·g-1）and precisely controlled mciroporous structures（0.5-1.5 nm）that are well suitable for CO2capture［40］.Banerjee et al.reported a synthesis and gas adsorption properties of porous carbons by usingisoreticularzeoliticimidazolateframeworks（IRZIFs）as a template and furfuryl alcohol（FA）as a carbon source［41］.Similarly，Liang and co-workers synthesized a series of porous carbons from nonporous metal-organiccoordinationpolymers（MOCPs），using in-situ polymerized phenol resin as a carbon precursor［42］.The influence of topological structures and functional ligands of different MOFs on final carbon products was reported using ZIF-8，ZIF-68，and ZIF-69asprecursors［42］.Theresultsshowthat CZIF69a derived from ZIF-69 has the highest CO2uptake of 4.76 mmol·g-1at 1 atm and 273 K，owing to its local structure and pore chemical environment.（3）Colloidal crystals as templates

Colloidal crystals are the self-assembled periodic structures consisting of close packed uniform particles.Replication of a colloidal crystal（colloidal silica/polymer sphere）in most cases leads to a high degree of periodicity in three dimensions.Removal of the crystal template leads to a replica with 3D ordered macroporous（3DOM）structures.For example，Adelhelm et al.synthesized a hierarchical meso-and macroporous carbon using mesophase pitch as precursors and polystyrene（PS）or poly（methyl methacrylate）（PMMA）as templates through spinodal decomposition［44］.By carbonization of a thin layer of phenolic resin on the suitable templates，Gierszal et al.synthesized one kind of uniform carbon film with large pore volumes（6 cm3·g-1for 24 nm silica colloids），uniform pore sizes，and controlled thickness［45］.Yang et al［46］have developed a nanoporous carbon with a hierarchical pore structure prepared by a combination of hard-templating of a thermosetting phenolic resin containing silica nanoparticles，pyrolysis and KOH activation.The carbons exhibited an adsorption capacity of 1.9 mmol·g-1at 25℃，a good reproducibility and stability.

（4）Other templates

MgO is also a good template owing to its easy removal by a diluted noncorrosive acid and facile recycled use［47］.Park et al.reported the preparation of nanoporous（styreneedivinylbenzene）-based ion exchange resin-based carbons by MgO-templating synthesis and activated by KOH，which exhibited a high CO2adsorption value of 1 385 mg·g-1at 30 bar［48］.The abovementioned nanocasting method was very successful in preparing porous carbons with nicely controlled structures，but the multi-steps and long synthesis period involved are time consuming.To simplify the tedious procedures，researchers made massive efforts.For instance，a one-step nanocasting technique was used to synthesize porous carbon monolithswithveryhighBETsurfacearea（ca.1 970 m2·g-1）and ca.2 nm mesopores by the cocondensation of β-cyclodextrin with tetramethylorthosilicate［49］.Park et al.reported a series of porous carbonsby the carbonization of a mixture of a weak acid cation exchange resin（CER）with Mg acetate in different ratios［50］.By dissolving the MgO template，the porous carbons exhibited high specific surface areas（326-1 276 m2·g-1）and high pore volumes（0.258-0.687 cm3·g-1）.The CO2adsorption capacities of the porous carbons were enhanced to 164.4 mg·g-1at 1 bar and 1 045 mg·g-1at 30 bar by increasing the Mg acetate to CER ratio.Recently，Zhang et al.presented a one-pot method to synthesize hierarchically bimodal ordered porous carbons with interconnected macropores and mespores，via in situ self-assembly of colloidal polymer（280，370，and 475 nm）and silica spheres（50 nm）using sucrose as the carbon source.Compared with the classical nanocasting procedure，this approach is inevitably simple；neither pre-synthesis of crystal templates nor additional infiltration is needed，and the self-assembly and the infiltration of polymer spheres into the crystal template are finished simultaneously in the same system［51］.

2.2.2Soft-template method

Soft-templating procedure is garnering a great deal of momentum for synthesizing porous carbons using self-assembly of soft templates（e.g.，amphiphilic block copolymers）and carbon precursors（e.g.，phenolic resins）［52］.Using this strategy，much efforts have been devoted to create new types of carbon monoliths with enhanced functions，which include developing new polymerization systems（solvents and/or precursors），precise pore engineering towards multimodal porosities and targeted surface/bulk functionalization for a high-performance CO2capture［53，54，55］.

Dai's group first synthesized highly ordered mesoporous carbons through a solvent annealing accelerated self-assembly method and reported a versatile synthesis of porous carbons（monolith，film，fiber，and particle）using such soft template approach［56］.They found that phloroglucinol with three hydroxyl groups is an excellent precursor for the synthesis of mesoporous carbons with well-organized mesostructure owing to its enhanced hydrogen bonding interaction with triblock copolymers［57］.This type of mesoporous carbon monoliths shows good performance ingas capture［58］.At 800 Torr and 298 K，the equilibrium adsorption capacity of the ordered mesoporous carbon for CO2is 1.49 mmol·g-1.Significantly higher adsorption uptake observed for CO2is 3.26 mmol·g-1at 100 bar and 298 K.More interestingly，the diffusion time constant of CO2decreased with pressures due to the obvious mesoscale pore character.

Fig.3 Schematic of the rapid synthesis of mesoporous carbons with lysine as the catalyst.Reprinted with permission from Ref 62.

Alternatively，Zhao's group synthesized a N-doped ordered mesoporous carbon with a high N content（up to 13.1 wt％）by using dicyandiamide as a nitrogen source via an evaporation-induced self-assembly process［59］.Such carbons showed high CO2captureof 2.8-3.2 mmol·g-1at 25℃and 1.0 bar.Similarly，Xiao's group also reported a hydrothermal synthesis to prepare carbon monoliths with well-ordered hexagonal or cubic mesopore structures［60］.B-/ P-doped ordered mesoporous carbons were also prepared by using boric acid and/or phosphoric acid as B-or P-heteroatom source，RF resin as the carbon precursor and F127 as the mesoporous structure-directing template［61］.Lu's group established a rapid and scalable synthesis of a crack-free and nitrogen-doped carbon monolith with fully interconnected macropores and an ordered mesostructure by using organic base lysine as a polymerization agent and mesostructure assembly promotor，through a rapid sol-gel process at 90℃［62］.Later，the same group reported a novel self-assembly approach based on benzoxazine chemistry for carbon adsorbents（Fig.3）［63］.With designed structure，the carbon monoliths showed outstanding CO2capture and separation capacities，high selectivity and facile regeneration at room temperature.At～1 bar，the equilibrium capacities of the monoliths are in the range of 3.3-4.9 mmol·g-1at 0℃，and of 2.6-3.3 mmol·g-1at 25℃；while the dynamic capacities are in the range of 2.7-4.1 wt％at 25℃using 14％（v/v）CO2in N2.The carbon monoliths exhibit a high selectivity for the capture of CO2over N2from a CO2/N2mixture，with a separation factor ranging from 13 to 28.Meanwhile，they undergo a facile CO2release in argon stream at 25℃，indicating a good regeneration capacity.

Fig.4（a，b）SEM images and（c，d）adsorption performance of the porous carbon nanosheets with different thicknesses.Reprinted with permission from Ref.［64］.

To shorten the diffusion path and enhance the adsorption ability，Lu et al.report a novel type of porous carbon nanosheets（PCNs）whose thicknesses can be precisely controlled over the nanometer length scale（Fig.4）［64］.This feature is distinct from the conventional porous carbons that are composed of micron-sized or larger skeletons，and whose structure is less controlled.The highest CO2adsorption capacities can reach 5.67 and 3.54 CO2molecules per nm3pore volume and per nm2surface area，respectively，at 25℃and～1 bar.The probable reason is that，the interaction of PCNs with CO2molecules is strong owing to（a）the large amount of microporosity with pore size of ca.0.8 nm and（b）the polar surface caused by the residual heteroatom-containing（e.g.，O，N）species.Prof.Zhao and co-workers have done a lot of work about the porous carbons for CO2capture［65，66］.For example，Zhao's group reported the synthesis of nitrogen-doped hollow carbon nanospheres（N-HCSs）via a two-step St?ber method［65］.The resultant N-HCSs possess a uniform size of～220 nm，a high nitrogen content of 14.8 wt％and a high surface area of 767 m2·g-1，as well as exhibit a considerable performance for CO2capture with a capacity of 2.67 mmol·g-1and a high selectivity in a mixture gas（CO2in N2or O2）.

Jaroniec et al.reported a preparation of a series of carbon spheres（CS）by carbonization of phenolic resin spheres via the modified St?ber method.All samples showed spherical morphology with the average diameter from 200 to 570 nm and high surface area from 730 to 2 930 m2·g-1，narrow micropores（＜1 nm）and，importantly，high volume of these micropores from 0.28 to 1.12 cm3·g-1.The remarkably highCO2adsorptioncapacities，4.55and 8.05 mmol·g-1，are measured on these carbon nanospheres at 25 and 0℃and 1 bar，respectively［67］.Wang et al.have established a new synthesis of highly uniform carbon nanospheres with precisely tailored sizes and high monodispersity on the basis of the benzoxazine chemistry［68］.The polybenzoxazine-based spheres can be carbonized with little shrinkage to produce monodisperse carbon spheres with abundant porosity and intrinsic nitrogen-containing groups that make them more useful for CO2adsorption［69］.The CO2adsorption capacity can reach 11.03 mmol·g-1（i.e.，485 mg·g-1）at-50℃and 1 bar，which is highly desirable for the CO2separation from natural gas during the cryogenic process to produce liquefied natural gas.This finding may be beneficial to design sorbents for the separation of dilute CO2-containing gas streams in practical applications.

2.2.3Template-free synthesis

Exploring new template-free preparation methods are urgently needed in the study of porous carbons［70，71，72］.The template-free synthesis of porous carbons generally involves the transformation of molecular precursors into highly cross-linked organic gels based on sol-gel chemistry.Since the pioneering work of Pekala［73］，the polymer based monolithic carbons have scored remarkable achievements in the new polymerization system and further surface/bulk functionalization.Fairén-Jiménez and coworkers synthesized carbon aerogels with monolith density ranging from 0.37 to 0.87 g·cm-3by carbonization of organic aerogels derived from RF polymer prepared in various solvents such as water，methanol，ethanol，tetrahydrofuran，or acetone solution［74］.They found that the samples with a density higher than 0.61 g·cm-3had micropores and mesopores but no macropores.Fu and Wu have investigated the template-free fabrication of porous carbons by constructing carbonyl（-CO-）and-C6H4-crosslinking bridges between polystyrene chains［70］.

Alternatively，the copolymerization and/or cooperative assembly between carbon precursors，and one or more additional modifiers（i.e.，heteroatom-containing components）can be used to directly synthesize functional carbons with enhanced CO2sorption capability［75］.Sepehri et al.synthesized a series of nitrogenboron co-doped carbon cryogels by homogenous dispersion of ammonia borane in RF hydrogel during solvent exchange and followed by freeze-drying and pyrolysis.The nitrogen-boron co-doping results in an improvement of porous structure，and thus accelerating molecule/ions transport properties as compared to the non-modified carbons［76］.Lu's group reported a timesaving synthesis towards a new type nitrogen-doped carbon monolith through a sol-gel co-polymerization of resorcinol，formaldehyde and L-lysine［77］.The monolithic carbon performs very well in CO2capture with a capacity of 3.13 mmol·g-1at 25℃.Gu et al.have developed a template-free synthesis for a new type of porous carbon spheres.In their synthesis，the azide-alkyne 1，3-dipolar Huisgen cycloaddition reaction was employed for the condensation of 1，4-bis（azidomethyl）benzene and 1，3，5-ethynylbenzene［78］.Because the resulting solid product contains periodically arranged aromatic 1，2，3-triazole rings in the polymer backbone，such carbon precursors contain a large percentage of nitrogen atoms for the preparation of N-doped carbon materials.As expected，the N-contents and surface area can be tuned to 4.30 wt％and 423 m2·g-1after pyrolysis under 800℃and the sample can adsorb 126.8 cm3·g-1（5.66 mmol·g-1）of CO2at 196 K.

Similarly，Shen et al.prepared a series of hierarchical porous carbon fibers with a BET surface area of 2 231 m2·g-1and a pore volume of 1.16 cm3·g-1using polyacrylonitrile nanofibers（prepared by dry-wet spinning）as precursors［79］.This type of material contained a large amount of nitrogen-containing groups（N content＞8.1 wt％）and consequently basic sites，resulting in a faster adsorption rate and a higher adsorption capacity for CO2than the commercial zeolite 13X that is conventionally used to capture CO2，in the presence of H2O.As discussed above，the construction of crosslinking bridges has been proven to play a decisive role in obtaining the porous carbons by using the template method.Under optimized conditions，the as-prepared carbons can have a high BET surface and a large pore volume.Moreover，the template-free fabrication method avoids the use of any hard-/soft-templates，thereby endowing the as-prepared porous carbon with a very competitive price-to-performance ratio.Therefore，it is believed that this simple and effective template-free method will open new avenues for nanostructure design and fabrication of various types of carbon materials from low-cost polystyrenes and other polymers.

2.3Carbonaceous adsorbents derived from ionic liquids

The unique features of ionic liquids（ILs），such as negligible vapor pressure and versatile solvation properties，have rendered the wide use of ILs in many emerging areas，for example，as green solvents for synthesis，as media for advanced separation，and as precursors for porous carbons［80，81，82］.Recently，porous carbons made by directly annealing of ILs or using appropriate porous templates has been an emerging field［83，84，85］.By choosing different ILs，materials with various heteroatoms doping and good pore properties can be produced.The attractive features of IL-derived materials，such as facile synthesis，high specific surface area and nitrogen content，make them promising candidates for CO2capture.Exceptional CO2separation performance can be achieved by these facilelymade carbonaceous adsorbents.Thus，in this section，recent research progress on IL-derived carbonaceous materials and their potential CO2separation application are summarized.

The key structural prerequisite of IL precursors is the presence of certain functional groups that can undergo cross-linking reactions under pyrolysis conditions.Given the tunability of task-specific ionic liquid（TSILs），either cations or anions can be functionalizedwithcross-linkinggroups.Todate，nitrile groups，the key factors in determining the high carbon yields of PAN under charring conditions，have been mostly appended onto the structure of ILs because of their cyclotrimerization of triazine rings at high temperatures［86，87］.In addition，TSILs further allow for the preparation of graphitizable carbons with heteroatomdoping（such as nitrogen and boron with their ratios in the carbon materials controlled by their amounts initially present in the cross-linkable ions）.

Fig.5 A schematic illustration of synthesis of ionic liquid-derived carbon materials.Reprinted with permission from Ref.［85］.

As shown in Fig.5，Dai and co-workers reported a strategy for forming functional porous carbon and carbon-oxide composite materials from conventional ILs by confined carbonization［85］.This method does not require the ILs to have cross-linkable groups but instead utilizes the space confinement of ILs inside oxide networks（e.g.，silica and titania）to convert the ILs into efficient carbon precursors with carbonization yields approaching the theoretical limits.Another interesting feature of this approach is that it allows a rational tuning of the pore structure of the corresponding carbon-oxide composites and the derived carbon materials from microporous to mesoporous and macroporous architectures.Furthermore，other elements，such as nitrogen and boron，which are crucial to modify the physical and chemical properties of the carbon materials，can also be easily doped into carbon frameworks by employing heteroatom-substituted ILs.

The formation of eutectic mixtures（described as either deep eutectic solvents，DESs，by Abbott［88］，or lowtransition-temperaturemixtures，LTTMs，by Kroon and coworkers［89］）using some of the most typical synthetic precursors for carbon preparation has also opened interesting perspectives in this yield.The useof DESs is attractive because，when compared to nitrile containing TSILs，they are less expensive and easy to be prepared owing to a wide range of compounds such as regular carbonaceous precursors（e.g.，resorcinol）.For example，the use of DESs based on mixtures of resorcinol（R）and choline chloride（ChCl）has allowed，upon polycondensation with formaldehyde and without the use of further additives，the formation of monolithic carbons consisting of highly cross-linked clusters that aggregated and assembled into a stiff and interconnected hierarchical structure［90］.The application of resorcinol is by no means trivial because it provides high carbonization yields（up to ca.85 wt％）.This feature，besides the capability of recovering the second component of DES that is not involved in carbon formation（e.g.，choline chloride），makes synthetic processes based on DESs especially attractive in terms of efficiency and sustainability［91］.These advantages may allow for efficient synthesis of CO2adsorbents with good CO2separation performance.

For example，del Monte and his co-workers have further explored the versatility of DESs-assisted syntheses for the preparation of hierarchical nitrogen-doped carbon molecular sieves［92］.DESs were composed of resorcinol and 3-hydroxypyridine（as hydrogen donors）and tetraethylammonium bromide（as a hydrogen acceptor）so that 3-hydroxypyridine acted as the nitrogen source of the resulting carbons while the presence of tetraethylammonium bromide determined the formation of a molecular sieve structure.The prepared carbon exhibited CO2adsorption capacities up to 3.7 mmol·g-1and CO2-N2selectivities up to 14.4 from single component gas data，95.9 in the Henry law regime，and 63.1 from IAST simulations.

Further，using deep eutectic salts either as solvents，or as carbonaceous precursors and structure-directing agents，Monte's group prepared carbon monoliths with a high yield（80％）and tailored mesopore diameters［90，93］.Sotiriou-Leventis，Leventis and coworkers，in recent years，have developed several new polymerization systems such as isocyanate-cross-linked RF gels，polyurea gels and polyimide gels，which offer a high degree of flexibility in producing monolithic carbons［94，95］.The carbon products show interconnected hierarchical pore networks and 3D bicontinuous morphology，a high surface area and large pore volume.For example，polyurea（PUA）gels，which eventually convert to highly porous（up to 98.6％v/ v）aerogels over a very wide density range，can be prepared by carefully controlling the relative Desmodur RE（isocyanate）/water/triethylamine（catalyst）ratios in acetone.It is worthy of exploration of their applications as CO2capture materials in the forthgoing research.

Overall，features such as facile and low cost synthesis of carbonaceous materials from ILs open interesting perspectives for the application of the carbons in separation technologies for gases containing low concentration CO2in post-combustion processes and natural gas upgrading.

2.4Porous carbon derived from organic frameworks

Porous organic foamworks（POFs），especially metal-organic frameworks（MOFs）or porous coordination polymers（PCPs）have attracted much attention as a new type of carbon precursors for porous carbons due to their many merits［96，97，98，99，100］.Considering the large carbon content in the organic ligands of MOFs，they could be used as both templates and carbon precursors at the same time without the need for any additives［101］.Moreover，the pore characteristics of POFs play a crucial role in determining the pore texture of the resultant porous carbon.Direct transformation of POFs-related materials（PCPs，MOFs，ZIFs，PAFs，etc）can generate porous carbons with nanopores of a precise and uniform size，which are very important in selective capture of CO［42］.

2

Motivated by the try of a new system of direct conversion of MOFs or PCPs，Yamauchi et al.have pioneered the synthesis of a novel nanoporous carbon with highly developed porosity（5 500 m2·g-1surface area and 4.3 cm3·g-1pore volume）by a direct carbonization ofAl-PCPs（Al（OH）（1，4-NDC）· 2H2O）［101］.The obtained carbon materials had a similar fiber-like morphology.It is noteworthy that such a fiber-like morphology was retained even after HF treatment.In some parts，large cracks/voids were formed after the calcination followed by HF treatment，which was most likely caused by the large weight loss during thermal decomposition of organic components of the Al-PCPs.By applying the appropriate carbonization temperature，both high surface area（5 500 m2·g-1）and large pore volume（4.3 cm3·g-1）are realized for the first time，of which the porosity is much higher than other carbon materials（such as activated carbons and mesoporous carbons）.

By performing the direct carbonization of MOFs，Chaikittisilp［102］and Yang［103］et al.have synthesized nanoporous carbon materials that exhibited promising gas storage performance.Porous carbons with hierarchical pore structures can also be achieved by direct carbonization of the selected MOFs，indicating the tunable pore characteristics of the MOFs-derived nanoporous carbons［103］.Ariga et al.reported a nanoporouscarbons with high surface areas（up to 1 110 m2·g-1），and narrow pore size distributions that are close to its parent ZIF-8［102］.Three types of ZIFs，ZIF-8，ZIF-68 and ZIF-69 with different topological structures and functional imidazolate-derived ligands as precursors without any additional carbon sources，have been directly carbonized to prepare porous carbon materials at 1 000℃［43］.The BET surface areas of the carbon materials activatedwithfusedKOHare2437（CZIF8a），1 861（CZIF68a），and 2 264 m2·g-1（CZIF69a）.CZIF69a has the highest CO2uptake of 4.76 mmol·g-1at 1 atm and 273 K，owing to its local structure and pore chemical environment.Based on all of these reported results，MOFs are really strong candidates as templates and precursors simultaneously for the preparation of porous carbon materials with high surface areas and large pore volumes.

A family of nanoporous carbons have also been prepared bythermaldecompositionofguest-free MOFs（even non-porous MOFs）by Kim and colleagues［104］.They found that the porosity of the carbon materials depend linearly on the Zn/C ratio of MOFs precursors，which allow a precise control of the porosity of the carbon materials in a predictable manner.Using this strategy，Gadipelli Srinivas has reported a new type of hierarchically porous carbon（HPC）structures of simultaneously high surface area and high pore volume，which were synthesised from carefully controlled carbonization of in-house optimized metal-organic frameworks（MOFs）［105］.Changes in synthesis conditions lead to millimetre-sized MOF-5 crystals in a high yield.Subsequent carbonization of the MOFs yield HPCs with simultaneously high surface area，up to 2 734 m2·g-1，and exceptionally high total pore volume，up to 5.53 cm3·g-1.In the HPCs，micropores are mostly retained and meso-and macro-pores are generated from defects in the individual crystals，which is made possible by structural inheritance from the MOF precursor.The resulting HPCs show a significant amount of CO2adsorption，over 27 mmol·g-1（119 wt％）at 30 bar and 27℃，which is one of the highest values reported in the literature for porous carbons.The findings are comparatively analyzed with the literature.Through a simple pyrolysis of crystalline polymer（PAF），Qiu et al.have prepared a series of nanoporous carbons having high surface area and narrow micropore size distributions［106］.The carbonized（at 450℃）sample showed higher adsorption capacities（4.5 mmol·g-1）for CO2than that of original PAF at ambient conditions.These aforementioned works exemplify a striking indication that the facile and one-step pathway replicated from crystalline microporous materials is highly efficient towards highly nanoporous carbons.

Unambiguously，the controlled carbonization of porous organic polymers，a facile approach to replace the high temperature treatment and chemical activation，opens up an avenue to construct hierarchically porous frameworks of carbon from porous MOFs，which shows great potential in gas storage and electrical applications.We believe that this approach provides a facile way of preparing hierarchically porous materials for application in the fields of CO2capture and storage.

3　Strategies to enhance CO2adsorption and separation

Although a large amount of adsorbent materials have been prepared，the enhancement of adsorption efficiency still remains a challenge.Hence，the development of rational strategies to enhance CO2capture and separation is not only necessary，but also practical and urgent.A key concern for solid adsorbents is to balance a strong affinity for removing an undesired component from a gas mixture with the energy consumption required for their regeneration.In addition to the adsorption capacity，the selectivity is a principal property relevant to adsorptive gas separation.While both factors are dependent on the operational temperature and pressure，as well as the nature of the adsorbents and the gas adsorbates，the factors which influence selectivity are more complicated.

Fig.6 Criteria of a high-performance CO2capture material.Reprinted from Ref.［2］.

As shown in Fig.6，an efficient CO2capture material should not only match the attributes of a CO2molecule（kinetic size and polarizability），but also meet the requirements of a CO2capture process（dynamics，operation，etc.）［2］.Only when both of the two criteria are met，the crucial parameters of CO2capture in term of good selectivity，high capacity，facile regeneration at low-energy penalty，and stable cycling can be achieved.Possible mechanisms of ad-sorptive separation include：1）the molecular sieving effect based upon size/shape exclusion of CO2from a gas mixture；2）thethermodynamicequilibrium effect；3）the kinetic effect，due to differences in the diffusion rates of different components of a gas mixture［107］.

Thus，the feasible strategies for enhancing CO2capture performance include：1）control of pore structure by tuning pore size and surface area；2）chemical doping with nonmetal and metal elements；3）functional integration and reinforcement；4）other newlyemerged methods.Here，we pay more attention to the mechanisms of these strategies and their effects on CO2capture and separation by illustrating typical examples instead of the synthesis approaches.

3.1Control of pore structures

Appropriate pore structure is always the first consideration in design synthesis of porous sorbents for selective gas separation［108，109］.First，the adsorbents should have abundant micropores with suitable size matching the CO2molecules and well dispersed surface functional groups which can polarize the CO2molecules.Second，the pore size，pore volume，and mutual matching de gree and mutual interconnected between micro-，meso-，and macropores known as interconnected hierarchical pore structure determine the adsorption diffusion dynamics of CO2.Gogotsi et al.have conducted a systematic experimental investigation to show a linear correlation between the CO2uptake at a certain pressure and the pore volume by using a large number of different carbide derived carbons（CDCs）with and without activation［109］.They found that CO2sorption is not determined by the total pore volume but only by pores smaller than a certain diameter.At 1 bar，pores smaller than 0.8 nm contribute the most to the CO2uptake and at 0.1 bar pores smaller or equal to 0.5 nm are preferred.Smaller pores contribute more to the measured amount of adsorbed CO2under low total pressures.Zhang et al［110］.found that the CO2adsorption capacity of carbon materials is associated with the pores below a temperature-dependent size.This critical pore size increased with decreasing adsorption temperature；the micropores with diameters smaller than 0.54，0.7 and 0.8 nm determine the CO2adsorption capacity at 75，25 and 0℃，respectively.

Most of porous carbons have a large crystallite size in all dimensions and it therefore takes a long time for the CO2molecules to transfer into and out of the inner microporous network.This leads to the starvation of CO2for the inner pores and further a low utilization of the overall surfaces and porosities.To address this issue，approaches including the construction of a hierarchical pore structure［62，63，91］，adding mesoporosity［111，112］and reducing the size of skeletons to the nanoscale［64］have been widely used by researchers.For example，we and other groups have reported various kinds of carbon materials with micron size spherical building blocks and with either micro-macroporous or micro-meso-macroporous pore structures and they showed a high capacity in CO2adsorption.Moreover，Lu and his co-workers indicated that sheetlike carbon exhibited improved kinetics compared with spherical counterparts in CO2capture owing to its shorter diffusion paths and larger exposed geometrical area resulting from the nanosheet structure［113］.

The porosity of porous carbons can be partially controllable by selecting particular precursor chemistry，activation method，and conditions.Activation method is the first choice of researchers which generally includes two categories，i.e.，physical（or thermal）activation and chemical activation.For instance，Lu et al.reported a novel synthesis approach for the fabrication of microporous carbon materials（HCMs）by using discrete chelating-zinc species as dynamic molecular porogens to create extra micropores that enhance their CO2adsorption capacity and selectivity［114］.During carbonization，the evaporation of the in situ formed Zn species would create additional nanopaths that contribute to the additional micropore volume for CO2adsorption.The resulted HCMs show increased amount of micropores with sizes in the range 0.7-1.0 nm，and a high CO2adsorption capacity of 5.4 mmol·g-1（23.8 wt％）at 273 K and 3.8 mmol·g-1（16.7 wt％）at 298 K and 1 atm，which are superior to most carbon-based adsorbents with N-doping or high specific surface area.Yuan et al.reported a new type of spherical nitrogen-containing polymer and its derived microporous carbon materials for CO2adsorption［115］.The microporous carbon spheresexhibithighsurfaceareasof 528-936 m2·g-1with a micropore size of 0.6-1.3 nm.The synthesized microporous carbons show a good CO2capture capacity，which is mainly ascribed to the presence of nitrogen-containing groups and a large amount of narrow micropores（＜1.0 nm）.At 1 atm，the equilibrium CO2capture capacities of the obtained microporouscarbonsareintherangeof3.9-5.6 mmol·g-1at 0℃and 2.7-4.0 mmol·g-1at 25℃.

3.2Chemical doping

Besides pore structures，the nature of the interaction between gas molecules and pore surface is also important for CO2adsorption.Compared with nonpolar or weakly polar N2，CH4，and H2，the CO2ishighly quadrupolar and weakly acidic.Thus，by introduction of basic functional groups into the carbon framework，the surface-modified carbons can polarize CO2moleculesandthusenhancetheir adsorption［116，117，118，119］.

For this issue，the surface properties can be tuned not only by the pre-design of precursors，but also by the post-modification of existing carbons.Due to high affinity of amine groups towards CO2，-NH2functional group has been widely introduced in various adsorbent materials［116，118］.The N-doping mainly includes two approaches，i.e.，using nitrogen-containing precursors and high-temperature reaction and transformation of pre-made carbons with NH3，etc.The rationally selected polymeric carbon precursors mainly involve p-diaminobenzene［120］，polyacrylonitrile［121］，melamine［122］，ammonia［123］and so on.Kowalewski et al.reported a nitrogen-enriched porous carbon nanostructure prepared via the carbonization of polyacrylonitrile containing block copolymer［119］.The typical sample exhibited a good selectivity for CO2manifested by 7 to 10 folds amount of adsorbed CO2over N2.Prof.Zhao also validated the role of N-containing groups（Fig.7）［39］.The mesoporous CN spheres show an excellent CO2capture performance and cycling stability，with a CO2uptake as high as 2.90 mmol·g-1；this can be attributed to the high specific surface area，abundant nitrogen-and oxygencontaining basic sites，hierarchical mesostructure，and stable framework.

Fig.7 CO2capture capacities of（a）the mesoporous CN materials and（b）the pristine carbon spheres at 25 and 75℃.Reprinted with permission of Ref.［39］.

Nandi and coworkers have fabricated a series of N-doped carbons from the mesoporous PAN monolith via thermal treatment in two steps［121］.The typical sampleshowedahighinitialQstvalueupto 65.2 kJ·mol-1.High initial isosteric heats of adsorption（Qst）values indicate a strong adsorbent-adsorbate interaction between the N-containingcarbon framework and CO2molecules.Fan and co-workers have reported an easy，low-cost method for synthesizing a series of nitrogen-enriched porous carbons，in which petroleum coke was used as a carbon precursor first modified by urea and then activated by KOH under varying conditions［124］.Upon urea modification，a significant amount of nitrogen groups were introduced into the carbon matrix.The resulting porous carbons thus featured a high fraction of fine micropores（＜1 nm）and some degree of basic nitrogencontaininggroups.Consequently，thesenitrogendoped porous carbons showed a maximum CO2adsorptioncapacityof4.40 mmol·g-1and 6.75 mmol·g-1at 25℃and 0℃under atmospheric pressure（1 bar），respectively，together with a good stability and a high CO2/N2selectivity.

Similarly，Sevilla et al.reported a chemical activated synthesis（KOH as the activating agent）of highly porous N-doped carbons for CO2capture［125］.In their synthesis，polypyrrole（PPy）was selected as a carbon precursor，which was activated at different temperatures in the 600-800℃range.Mildly activated carbons have two important characteristics：i）they contain a large number of nitrogen functional groups（up to 10.1 wt％N）identified as pyridonic-N with a small proportion of pyridinic-N groups，and ii）they exhibit narrower micropore size.The above two properties ensure the mildly activated carbons large CO2adsorption capacities.Furthermore，the capture of CO2over this type of carbons takes place at high adsorption rates，more than 95％of the CO2being adsorbed in ca.2 min.In contrast，N2adsorption occurs at slow rates；approximately 50 min are necessarytoattainmaximumadsorptionuptake（0.77 mmol·g-1）.

Treating as-made porous carbons with gaseous ammonia under a high temperature（e.g.，900℃）is another popular way used for preparation of N-doped carbons.In principle，the reaction with ammonia isexpected to take place at carboxylic acid sites formed by the oxidation of side groups and the ring system.At a high temperature，ammonia decomposes with the formation of radicals，such as NH2，NH and H，which may react with the carbon surface to form functional groups，such as—NH2—，—CN，pyridinic，pyrrolic，and quaternary nitrogen.High temperature ammonia treatment clearly enhances the CO2adsorption of carbon adsorbents owing to the presence of C—N and C==Ngroups.However，the higher temperature treatment may cause the close of micropores or changes in the size of pores.Pevida and co-workers demonstrated that ammonia treatment at temperatures higher than 600℃incorporated nitrogen mainly into aromatic rings，while at lower temperatures nitrogen was introduced into more labile functionalities，such as amidelike functionalities［127］.They pointed out that it is the specific nitrogen-functionalities rather than the total nitrogen content that are responsible for increasing the CO2-adsorbent affinity.The same group also investigated the ammonia treatment of pristine carbons in the presence of air（ammoxidation）［128］.

Other polar functional groups like—F，—Cl，—Br，—OH，—COOH，—CN，—NO2，and—PO3may have similar effect and offer an increased CO2-framework interaction［129，130，131，132］.For example，Liu and Wilcox［130］theoretically analyzed the role of oxygen-containing groups in CO2capture based on an assumption，in which complex pore structures for natural organic materials（e.g.，coal and gas shale）and carbon-based porous materials were modeled as a collection of independent，non-interconnected，functionalized graphitic slit pores with surface heterogeneities.Wilcox et al［133］.indicated that the introduction of O-containing functional groups on the graphite surface could enhance the adsorption capacity of CO2and the selectivity of CO2over CH4and N2.More interestingly，the embedded positions of functional groups have a significant effect on their gas adsorption behaviors.Very recently，Lu et al.systematically investigated the effect of edge-functionalization on the competitive adsorption of a binary CO2-CH4mixture in nanoporous carbons（NPCs）by combining density functional theory（DFT）and grand canonical Monte Carlo（GCMC）simulation［134］.Sulphur（S）has also been incorporatedinporouscarbonsforCO2adsorption［135］.Xia et al.synthesized a S-doped microporous carbon（SCEMC）by using zeolite EMC-2 as a hard template and 2-thiophenementhanol as both carbon and sulfur sources［136］.SCEMC contains a high S content of 6.5％and gives an impressive isosteric heat of CO2adsorption（Qst=59 kJ·mol-1）at very low CO2coverage，indicating strong interactions between S species and CO2.However，the Qstvalue decreases quickly to＜40 kJ·mol-1with the increase of surface coverage，which suggests a limited number of strong adsorption sites.Consequently，its overall CO2adsorptioncapacityisnotoutstanding（2.46 mmol·g-1at 298 K and 1 bar）.Also other molecular-level simulations of CO2sorption behavior in micropores of porous carbons show that heteroatom doping greatly enhances CO2uptakes and selectivity at low coverage；while the CO2capture performance at high pressure has largely dispensed by the textual parameters［137］.

In addition to non-metal atoms，metal cations such as Mg，Na and Ca.can also be doped into adsorbent materials to enhance CO2capture and separation［138，139，140］.Jang et al has reported a one-pot synthesis of Mg-OMC（OMC：ordered mesoporous carbon）forCO2adsorptionwithacapacityof 92 mg·g-1at 25℃［141］.Przepiórski et al.studied the competitive uptake of SO2and CO2on porous carbon material containing CaO and MgO，prepared by carbonization of poly（ethylene terephthalate）mixed with a natural dolomite［142］.Zhang et al.prepared nitrogen and magnesium co-doped mesoporous carbon materials（NMgCs）［143］.The strategy involves facile mixing，grinding，and thermal treatment of structuredirecting agent and precursors.The obtained composites show an encouraging CO2capture capacity of 2.45 mmol·g-1at 25℃.Interestingly，the CO2adsorptioncapacityat75℃isashighas 1.17 mmol·g-1.The enhanced CO2adsorption in comparison with parent material can be attributed to the doping of highly dispersed MgO sites and nitrogen atoms in the composite.Besides Mg，Lee et al.reported a highly porous sodium-containing and N-doped solid carbon sorbents（SNSs），which together modify the surface chemistry for stronger interactions with CO2at relatively low pressures［144］.The synthesized SNSs not only have a high CO2capacity of 6.84 mmol·g-1at the atmospheric pressure，but also a significant CO2uptake of 3.03 mmol·g-1even at 0.15 bar at temperature of 0℃.

While，the role of heteroatom-involving sites，particularly N-containing groups in CO2capture，remains controversial.At present，there are three viewpoints regarding this issue：acid-base interactions，hydrogen-bonding interaction，and electrostatic interactions.Han et al.have prepared a series of nitrogendoped carbon materials with a very high nitrogen doping concentration（ca.13 wt％）to study the role of N-containing groups in CO2adsorption［145］.Its CO2adsorption uptake in the typical condition of flue gas（e.g.，CO2partial pressure of 0.2 bar，25℃）is as high as 1.75 mmol·g-1，which is far superior to most reportedcarbonmaterials.Thesenewmaterials showedahighCO2adsorptionheats（ca.40 kJ·mol-1at initial adsorption stages），suggesting an enhanced physical adsorption effect by nitrogendoping［120］.More interestingly，they conducted theoretical calculations that correlate the polarizing capabilities of various functional groups（K+，Cl-ions as well as N-containing sites）with their enhancement effects on CO2adsorption，and demonstrated that such effects are essentially based on electrostatic interactions.This represents a new perspective to explain the positive role of heteroatoms in constribution to a high CO2uptake.

In a word，chemical doping has been a promising strategy to enhance adsorption sites，especially for those adsorbent materials without strong electrostatic potential，such as graphene/graphite and CNTs.It is also a good choice to separate gas mixtures with large differences in dipole/quadrupole by introducing electropositive metal cations or electronegative heteroatoms.

3.3Functional integration and reinforcement

Another strategy to enhance CO2capture and separation is to incorporate polar functional groups into adsorbent materials to tune the affinity of framework.To strengthen the performance of porous carbon，new components such as MOFs and metal oxides，which have strong interactions with CO2，have often been introduced to provide efficient and active CO2binding sites.As for MOFs，one of the disadvantages is their strong hydrophilic properties，while the hybrid of block co-polymers produced by MOFs mixed with hydrophobic polymers could prohibit water permeation effectively.As shown in Fig.8，Lu et al.optimized the structural features of hierarchical porous carbon monolith（HCM）by incorporating the advantages of MOFs（Cu3（BTC）2））to maximize the volumetric based CO2capture capability（CO2capacity in cm3per cm3adsorbent）［146］.The octahedral Cu3（BTC）2crystallites are well dispersed within the HCM matrix.The HCM-Cu3（BTC）2composite can achieve a maximum CO2uptake of 22.7 cm3STP per cm3at～1 bar，which is almost twice as much as the uptake of HCM（12.9 cm3STP per cm3）under the same conditions.This result encourages a new principle on a rational design of CO2capture material by maximizing the capacity on a volumetric basis.

Fig.8 Porous carbon monolith containing MOF crystallites inside the macropores and an SEM micrograph of HPC-Cu3（BTC）2.Reproduced with permission of Ref.［146］.

Inspired with silica-based hybrid sorbents（molecular basket）with grafted or impregnated amine groups on porous silica substrates［147］，Zhao et al.reported an aminated carbon adsorbents by using sustainable biomass（glucose）as precursor［148］.The synthesis involves two steps：（1）hydrothermal carbonization of glucose；（2）transformation into porous carbon-amine composite by a post-synthetic modification with a branched tetramine.The substrate carbons show a novel raspberry morphology，which is beneficial for the loading of liquid amines.CO2capture resultsshowaveryhighCO2uptake（upto 4.3 mmol·g-1）at-20℃.However，the introduction of liquid amines through impregnation might result in some other negative effects，such as blockage of pores，the instability of basic sites on the surface during cycle use.To address this issue，Tour's group developed a route to synthesize polymer-carbon composites by the in-situ polymerization of amine species to produce polyethylenimine（PEI）and polyvinylamine（PVA）inside the mesocarbon CMK-3［149］.Thus，this composites exhibit a high stability owing to the formation of interpenetrating composite frameworks between the entrapped polymers and porous carbon.CO2uptake measurements showed that the 37％PVA-CMK-3 composite had a～13 wt％CO2uptake capacity and the 39％PEI-CMK-3 composite had a～12 wt％CO2uptake capacity at 30℃and1 atm.More importantly，the composite showed a good cycle ability（even up to 500 min）and can be easily regenerated at 75℃.

Graphite oxide（GO）is a derivative of graphene and consists of oxygen functional groups on their basal planes and edges.If polyamines covalently attach to its layers，the residual unreacted amine groups can react with CO2and have potential for the removal of CO2.Under this consideration，Zhao et al.prepared GO-amine composites based on the intercalation reaction of GO with amines，including ethylenediamine（EDA），diethylenetriamine（DETA）and triethylene tetramine（TETA）.Dynamic CO2breakthrough test revealed that the aminated GO was an efficient adsorbent for CO2capture.For example，the typical sample of GO/EDA showed an adsorption capacity of 53.62 mg·g-1［150］.Very interestingly，Koenig et al.pioneered one type interesting graphene membranebased molecular sieve［151］.The experimental results reveal the realization of graphene gas separation membranes by molecular sieving，and represent an important step towards the realization of size-selective porous graphene membranes.

External electric field or charges is one of the newly-emerged strategies to enhance CO2capture using polarizable material substrates，such as boron nitride（BN）and CNTs.Electronic structure calculations coupled with van der Waals-inclusive corrections have been performed to investigate the electronic propertiesoffunctionalizedgraphiticsurface［130］.Thus，besides amorphous carbons，graphitic porous carbons are also widely investigated as adsorbents for figuring out CO2capture behavior owing to its ordering at the atomic scale.CNTs are also considered to be effective in selective CO2recognition due to their shortened size，easy functionalization and/or integration with foreign active species.Liu et al.have shown，from molecular dynamics simulations，that the windowed carbon nanotubes are able to separate CO2from the CO2/CH4mixture with a CO2permeance several orders of magnitude higher than the conventional analogues［152］.

The aforementioned examples open the door for the design and preparation of highly effective carbonaceous CO2adsorbents with controlled pore features and tailored surface chemistry.These porous carbons would combine the merits of designed synthesis（controlled pore structure and task-specific surface chemistry）and intrinsic properties（excellent chemical-and thermal-stability，developedporosity）ofcarbon materials，and meet the complex requirements of efficient adsorbents for CO2capture.

4　Summary and outlook

Designed synthesis and application of porous carbons are one of the most exciting areas in current materials sciences，and the development of porous carbons for CO2capture is particularly important for future energy and environmental concerns.This review intended to provide the readers with an overview of the most recent developments in the field of porous carbon materials for capturing carbon dioxide.In this review，the first half part has sought to highlight the various approaches towards the design and production of porous carbons according to the classification of precursors and meanwhile discuss their CO2capture performance.Then，in the second part，the actively studied strategies to enhance CO2capture of carbonaceous adsorbents have been summarized and provided，which may be beneficial for the interested readers for future study.

It is well-recognized that carbonaceous adsorbents are attractive for pre-combustion CO2capture；nevertheless，the selectivity and capacity of carbonaceous adsorbents is too low for post-combustion applications.Materials scientists of the future will need to consider all contributions when it comes to designing an advanced adsorbent to CO2emission control.While，no unique material or solution exists currently to solve the problem of CO2capture.The function integration and manipulation of porous carbons with other components（MOFs，heteroatoms，etc.）can complement each other with advantages between different materials and thus facilitate porous carbons with improved adsorption capacity，selectivity，diffusion rate and mechanical strength for efficient CO2capture.Furthermore，cost remains a dominant factor when it comes to choosing the ultimate material.The availability of some carbon sources（e.g.，biomass and industrial byproducts）makes activated carbons cost-effective at the industrial production stage.Recent discoveries have achieved remarkable advancement in adsorbent materials derived from biomass materials，which may open a new prospect for the future as the global cost of the CO2capture process.In spite of the challenges surrounding CO2capture using carbon materials as adsorbents，we believe that with a better understanding of the carbon chemistry，advanced carbon materials will play a central role for solving CO2capture problem，which can be realized in the near future.

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Designed porous carbon materials for efficient CO2adsorption and separation

ZHANG Xiang-qian，LI Wen-cui，LU An-hui
（School of Chemical Engineering，State Key Laboratory of Fine Chemicals，Dalian University of Technology，Dalian116024，China）

The emission of CO2from industry and power plants has become a worldwide problem with a strong link to global warming.The development of novel materials for efficient CO2capture and utilization is attracting worldwide attention as a hot topic in materials sciences.Among various CO2adsorbents，porous carbons have proven competitive by virtue of their high specific surface area，tunable pore and surface structures，moderate heat of adsorption，and less sensitivity to humidity than other CO2-philic materials.In this review，we summarize the recent significant advances in porous carbon materials for CO2adsorption and separation.Strategies to increasethe CO2capture capability are highlighted.We also briefly discuss the future prospects of porous carbons for CO2capture.

Porous carbon；Designed synthesis；CO2adsorption and separation

date：2015-10-5；Revised date：2015-11-30

National Natural Science Foundation of China（21473021）；National Program on Key Basic Research Project（2013CB934104）；Fundamental Research Funds for the Central Universities（DUT14ZD209）.

LU An-hui，Professor.E-mail：anhuilu@dlut.edu.cn

introduction：ZHANG Xiang-qian，Ph.D.Student.E-mail：xiangqianzhang@dlut.edu.cn

TQ127.1+1

A

國(guó)家自然科學(xué)基金（21473021）；國(guó)家重點(diǎn)基礎(chǔ)研究發(fā)展計(jì)劃（973計(jì)劃）項(xiàng)目（2103CB934104）；中央高校基本科研業(yè)務(wù)費(fèi)專項(xiàng)資金（DUT14ZD209）.

陸安慧，教授.E-mail：anhuilu@dlut.edu.cn

作者介紹：張向倩，博士研究生.E-mail：xiangqianzhang@dlut.edu.cn

1007-8827（2015）06-0481-21

10.1016/S1872-5805（15）60203-7

孔隙結(jié)構(gòu)發(fā)達(dá)和化學(xué)性質(zhì)穩(wěn)定等特點(diǎn)，在氣體分離領(lǐng)域發(fā)揮著重要的作用。本文主要介紹了近年來多孔炭材料在CO2吸附分離領(lǐng)域的研究進(jìn)展情況，著重介紹了提高CO2吸附分離效率的主要方法與策略，并對(duì)碳質(zhì)吸附材料未來的發(fā)展趨勢(shì)進(jìn)行了評(píng)述。

English edition available online ScienceDirect（http：//www.sciencedirect.com/science/journal/18725805）.