Jiajia Wang,Lizhi Wang,You Wang,Du Zhang,Qin Xiao,Jianhan Huang*,You-Nian Liu

College of Chemistry and Chemical Engineering,Hunan Provincial Key Laboratory of Micro and Nano Material Interface Science,Central South University,Changsha 410083,China

Keywords:Porous organic polymers Selectivity Adsorbent CO2 capture

ABSTRACT Carbon capture,storage,and utilization(CCSU)is recognized as an effective method to reduce the excessive emission of CO2.Absorption by amine aqueous solutions is considered highly efficient for CO2 capture from the flue gas because of the large CO2 capture capacity and high selectivity.However,it is often limited by the equipment corrosion and the high desorption energy consumption,and adsorption of CO2 using solid adsorbents has been receiving more attention in recent years due to its simplicity and high efficiency.More recently,a great number of porous organic polymers(POPs)have been designed and constructed for CO2 capture,and they are proven promising solid adsorbents for CO2 capture due to their high Brunauer-Emmett-Teller (BET) surface area (SBET),adjustable pore size and easy functionalization.In particular,they usually have rigid skeleton,permanent porosity,and good physiochemical stability.In this review,we have a detailed review for the different POPs developed in recent years,not only the design strategy,but also the special structure for CO2 capture.The outlook of the opportunities and challenges of the POPs is also proposed.

1.Introduction

The excessive emission of CO2has caused serious global warming,which is a great menace for the living conditions of human beings [1,2].How to reduce the emission of CO2had been caught increasing attentions.Although the development and utilization of the clean energy has greatly improved,the fossil fuels still keep the main position[3].Hence,the excessive emission of CO2would constantly occur for the foreseeable future.Mentionable,CO2is considered as a kind of cheap,innoxious,abundant,and green C1 resources [4].It is meaningful to capture the excessive CO2from the environment and convert CO2into high value-added chemical products such as CO,methane,ethanol,formic acid,etc[5,6].For this purpose,the enrichment of CO2is of great importance.

The common adsorptions of CO2were physical and chemical adsorption [7].As a kind of chemical adsorbent,aqueous amine solutions have been widely used for CO2capture from the flue gas.However,liquid ammonia could cause equipment corrosion,and the high desorption energy consumption would cause large energy penalty [8].Hence,the solid adsorbents have caught more attentions for their low heat capacities,tunable pore structure,and high Brunauer-Emmett-Teller (BET) surface area (SBET).For instance,metal-organic frameworks (MOFs) [9,10],activated carbons[11],and porous organic polymers(POPs) [12,13]are usually used for CO2capture.However,in the complex conditions such as flue gas(N2,O2,CO2,H2O,SOxand NOx),MOFs are suffered from the chemical stability for their unsteady structure,and the release of organic ligand and metal ions are also harmful for the environment.Activated carbons also have some disadvantages such as poor selectivity and difficult functionalization.Among them,POPs have attracted more and more attentions due to their easy preparation and post-functionalization,high physiochemical stability and selectivity [14].

The POPs are constructed by the cross-linking of lighter elements (C,H,B,O,N,S,P,etc.) through covalent bonds [15].As a kind of good carbon capture materials,POPs have several advantages:(1) POPs are constructed by rigid monomers,resulting permanent porosity in the polymers;(2) There are many bond formation methods and crosslinking reactions,which will take the polymers with various topological structures and tunable pore structures;(3)POPs are constructed by the cross-linking of lighter elements,which will make the polymers with high mass capacity of CO2;(4)POPs are constructed with covalent bonds,which make them good physiochemical stability.In this way,special POPs with specific performance and pore structures could be designed and synthesized to fit the need of CO2capture,energy storage,catalysis,etc.[16,17]Till now,the POPs could be divided to six kinds for their different structures and construction methods:hyper-cross-linked polymers (HCPs),conjugated microporous polymers (CMPs),polymers of intrinsic microporosity (PIMs),porous aromatic frameworks (PAFs),covalent organic frameworks (COFs),and covalent triazine frameworks (CTFs).The HCPs,PIMs,CMPs,and PAFs are amorphous structure,which are synthesized through irreversible reactions under kinetic control.While the COFs and CTFs are crystalline structure and are synthesized by reversible reactions under thermodynamics control [18].

As reported in the literatures,there were several factors played key roles in CO2capture,such asSBET,heteroatom,and microporous content [19,20].As forSBET,the higherSBETwill take much more adsorption sites.While under low pressure,SBEThave little influence on CO2capture due to the low interaction between the sites and CO2moleculars.However,under high pressure,CO2moleculars were pressed to the adsorption sites,which caused higher CO2uptake.Heteroatoms also have grent influence on CO2capture.For instance,N atoms could interact with CO2moleculars by dipole-quadrupole interactions or hydrogen bonding,which will obviously increase the CO2capture of the polymers.As reported in the literatures,O,S,P heteroatoms were also played the positive roles in CO2capture [21-23].In addition,the microporous structure of polymers also a key factor for CO2capture.It was reported that ultramicropores (d＜1.0 nm) would greatly increase the adsorption capacity and selectivity of CO2due to the size selection effect [24].Hence,the design of POPs for CO2capture should considered three points:highSBET,heteroatom content,micropores content.

To the best of our knowledge,there were few reviews about POPs for CO2capture in recent years.In 2017,Yuan’s group [25]mainly reviewed the amorphous POPs for CO2capture in ‘‘Journal of Materials Chemistry A” and summarized the factors for influence on CO2capture capacity.However,the crystalline POPs were not mentioned.In the same year,Zhou’s group [26] also reported POPs for CO2capture.In this review,they mainly summarized the strategies for POPs to enhance CO2uptake and selectivity.Tan’s group [16] described the synthetic strategies and principles of HCPs,and the major applications in environmental remediation and energy storage,such as gases capture and storage,pollutants removal,sensing,etc.In 2020,Sun’s group [27] reported a review on crystalline POPs (COFs) for gases capture,organics pollutants and heavy metals adsorption.Zhang’s group [28] reviewed the COFs for gases separation,and briefly introduced the application of COFs in CO2separation before 2018.Nevertheless,the comprehensive and detailed reviews about design and preparation of POPs for CO2capture were not reported.Thus,we mainly concluded and discussed the design and preparation of the POPs for CO2capture and separation in the last few years.The outlook of the opportunities and challenges of POPs were also considered.

2.Amorphous POPs for CO2 Adsorption

Different from the harsh synthesis conditions of crystalline structure,amorphous POPs were more commonly seen.For instance,the HCPs were commonly constructed by Friedel-Crafts alkylation reaction between the aromatic rings and the crosslinkers.Because of the multiple substitution of alkyl occurs on the aromatic rings,the indeterminacy makes HCPs amorphous structure.Although the amorphous structure makes the polymers difficult to clear insight into the inner structure,the C-C bonds make them excellent thermal and chemical stability,and they can still maintain their original properties in very harsh chemical environment.For POPs materials,the HCPs,CMPs,PIMs and PAFs were belongs to amorphous polymers.The applications of these amorphous POPs in CO2adsorption and separation are briefly introduce in the following.

2.1.HCPs for CO2 adsorption

The HCPs usually formed a large number of rigid cross-linking bridges between polymer chains or aromatic monomers through Friedel-Crafts reaction [29].There are some permanent channels among the molecular chains in HCPs because of the high degree of cross-linking,which prevents the polymers chains from packing tightly[30].The rigidity structure of HCPs also brings the polymers with high chemical stability,highSBETand micropore volume(Vmicro).In addition,HCPs also have great advantages in synthesis,such as the simple reaction procedures,cheap catalysts,and various methods.In this way,HCPs had greatly developed in the recent years.At present,HCPs can be produced by three ways:(1) Postcrosslinking of precursor polymers.(2) Direct one-step condensation of multifunctional monomers (or mixtures).(3) Knitting method with external crosslinking agents.Abundant building monomers and synthesis strategies have produced various of HCPs,which have become one of the most potential and valuable materials in CO2adsorption [31].

2.1.1.Post-crosslinking of precursor polymers

Davankov’s group[32]firstly reported that a series of polymers with permanent micropores were post-cross-linked by Friedel-Crafts reaction.Linear polystyrene or low crosslinked polystyrene polymers chains were swelled in suitable solvents.With the help of Lewis acid catalysts,different external crosslinking agents were post-crosslinked with the adjacent benzene rings of the precursor polymers.This strategy provides a new method for synthesis the porous polymers.Until 2002,Davankovet al.[33] firstly proposed the concept of ultra-high crosslinked polymers,and the hypercross-linked resin based on polystyrene framework was named‘‘Davankov resin”.The formation process was mainly divided into three stages.Firstly,the polymers chains were fully dissolved or swelled in the solvent;then they were quickly cross-linked by external crosslinking agents;and finally the solvents were removed by drying to obtain HCPs with permanent micropores.

Post-crosslinking of precursor polymers for HCPs construction had greatly developed and caught a lot of attentions.For instance,Li and co-workers [34] synthesized the F-HCP-x by using postcrosslinking method.Phenylmethylsilicone was chosen for the precursor polymer.Then formaldehyde dimethyl ether (FDA) was added to crosslink the benzene rings in the polymers chains,resulting the polymers with moderateSBET.What’s more,the silicon-oxygen bonds of phenylmethylsilicone could be etched by hydrofluoric acid.The resulting polymers showed great increasing ofSBET(from 440 to 1201 m2·g-1) and ultramicropore volume(Vultra) (from 0.18 to 0.49 cm3·g-1),resulting the polymers with high CO2capture of 153.00 mg·g-1(273 K and 0.1 MPa),low isosteric enthalpy of adsorption (Qst) of 30.7 kJ·mol-1,and good adsorption selectivity for CO2/N2of 36.6.

The precursor polymers for the construction of post-crosslinked HCPs were various sources.Our group [35] synthesized a kind of precursor polymers by Friedel-Crafts acylation reaction.Although the polymers showed very lowSBETand almost no pores,the resulting polymers which were post-crosslinked by the carbonyl of the precursor polymers and amidogen of melamine through the Schiff-base reaction,showed a great increasing ofSBET(from 4 to 645 m2·g-1) andVultra(up to 0.16 cm3·g-1).The postfunctionalization of melamine also takes abundant N species to the polymers.Hence,the capture capacity of CO2was also greatly improved(from 48.30 to 153.00 mg·g-1at 273 K and 0.1 MPa).This strategy was also expanded to other ketone functionalized polymers,which were constructed by aromatic monomers and acyl chloride (or anhydride) through the Friedel-Crafts acylation reaction.These precursor polymers also showed greatly improvement ofSBET,Vultra,N content,and adsorption capacity of CO2(up to 193.00 mg·g-1) after the post-crosslinked of melamine through the Schiff-base reaction.The results indicated that the postcrosslinked of the precursor polymers with melamine could be widely used for ketone based precursor polymers (Fig.1).This strategy not only elevate theSBETandVultraof the polymers,but also takes abundant N species to the polymers,resulting the final polymers with good performance in CO2capture and selectivity for CO2/N2[36,37].

Although divinyl benzene (DVB) can be used for the construction of HCPs directly by free radical polymerization,the resulting polymers have no functionalized sites.Hence,Wang and coworkers[38]synthesized the crosslinked precursors by free radical polymerization of DVB,4-vinylbenzyl chloride(VBC),and 1-(3′-car boxylpropyl)-3-vinylimidazolium bromide.Then,the functionalized ionic liquid crosslinking polymers of IHCPs was postcrosslinked by the self-crosslinking of the benzene ring and chloromethyl (-CH2Cl) in the precursor polymers through the Friedel-Crafts alkylation reaction.The polymers showed good performance for CO2capture (94.60 mg·g-1at 273 K and 0.1 MPa).

2.1.2.Direct condensation of multifunctional monomers (or mixtures)

Different from the post-crosslinking method,the direct condensation of multifunctional monomers(or mixtures)to construct the HCPs could reduce the complexity of preparation procedures.Special reactions were chosen for this strategy,such as the Friedel-Crafts alkylation reaction,free radical polymerization,hydroxylaldehyde condensation reaction,and Scholl coupling reaction.In 2008,Cooper’s group [39] firstly reported the HCPs were synthesized by self-condensation of multifunctional monomers of dichloroxylene (DCX) and 4,4′-bis(chloromethyl)-1,10-biphenyl(BCMBP).The resulting polymers showed highSBETof 1307 m2·g-1.Subsequently,the other multifunctional monomers were also performed for the construction of HCPs,which had reported in the recent review [16].For this kind of HCPs,with the help of Lewis acid catalysts,the chloromethyl,bromomethyl,and benzyl alcohol groups could crosslinked with the neighboring phenyl rings through multi-substitution,resulting the polymers with highSBETand good performance in CO2capture.For instance,Tan and co-workers[40]constructed two kinds of HCPs by the selfpolycondensation of 1,4-benzenedimethanol (BDM) and benzyl alcohol through FeCl3catalytic Friedel-Crafts alkylation reaction.The polymer of HCP-BDM showed highSBET(847 m2·g-1),Vmicro(0.21 cm3·g-1),and good performance in CO2capture(126.00 mg·g-1at 273 K and 0.1 MPa).

Free radical polymerization of styryl content monomers also caught a lot of attentions.For instance,Tu and co-workers [41]synthesized a series of sponge-like quaternary ammonium-based mesoporous poly(ionic liquid)s (MPILs) using ionic liquids functionalized styryl content monomers by free-radical polymerization.Although the polymers showed lowSBET,total pore volume(Vtotal),and CO2capture (54.00 mg·g-1at 273 K and 0.1 MPa),the polymers performed good for CO2cycloaddition reaction (yield of 99.6% at 90 °C and 0.1 MPa).Subsequently,a series of crosslinked imidazolyl MPILs with highSBETandVtotalwere prepared by the same method.The maximumSBETandVtotalwere 798 m2·g-1and 2.05 cm3·g-1,respectively,and the adsorption capacity of CO2was up to 158.40 mg·g-1[42].Dai and co-worker [43] prepared the nitrogen-functionalized POPs (PDVB-VP) with highSBETandVtotalby the one-step copolymerization of divinylbenzene (DVB)with vinyl-contained monomers (e.g.,4-vinylpyridine [VP] and 1-vinylimidazole [VI]) under solvothermal conditions.It shows good CO2adsorption performance under low pressure conditions(64.68 mg·g-1at 273 K and 0.015 MPa).Subsequently,a series of P(VCB-HMTA)s were prepared by aqueous and template-free route herein.And the adsorption capacity of CO2was up to 69.52 mg·g-1at 273 K and 0.015 MPa [44].

Hydroxyl-aldehyde condensation reaction was also a kind of useful reaction for the HCPs construction.For example,Han’s group [45] prepared two hydrophobic pyrene-based hypercrosslinked microporous resins (ZLYs) by condensation of 1,3,6,8-tetrakis(p-formylphenyl)pyrene (TFPPy) with phloroglucinol(L1)/1,5-dihydroxynaphthalene(L2)(Fig.2).The polymers showed highSBETof 647 m2·g-1,and the structure of ZLYs exist intrinsic hydroxyl groups on the pore-walls.Hydroxyl group could enhance the force between CO2and ZLYs,resulting the polymers with high CO2uptake of 112.60 mg·g-1andQstof 31.6 kJ·mol-1.Scholl coupling reaction had also widely used for the construction of HCPs.In this strategy,phenyl rings of aromatic monomers were linked directly by elimination of hydrogen atoms and forming a new aryl-aryl bond by the catalysis of AlCl3.Unlike the HCPs were constructed by external crosslinkers,Scholl-coupled could clearly increase theSBET.For instance,Tan and coworkers[46] firstly constructed a series of HCPs by Scholl coupling reaction of naphthalene,pyrene,triphenylphosphine (PPh3),1,10-phenanthroline monohydrate,benzyl amine,benzoic acid,pyrrole,thiophene,furan,phenyl cyanide,phenylene diamines,and 2,2′-bipyridine through self-crosslinking.The resulting polymers all showed highSBET(as high as 1421 m2·g-1) and abundantVmicro,which made the polymers showed high CO2capture (202.00 mg·g-1at 273 K and 0.1 MPa).Cui and co-workers [47] also constructed a kind of HCPs by self-crosslinked of 3,5-diphenyl-1H-pyrazole through Scholl coupling reaction.The resulting polymer of MOP-PZ had highSBETof 965 m2·g-1and N content,the CO2capture as high as 202 mg·g-1.The common HCPs have low selectivity for CO2/N2for the lake of heteroatoms.However,the HCPs with highSBETand CO2capacity were hardly got by crosslinked of heteroatomic functionalized monomers directly.For this purpose,Cooper and co-workers [48] reported a kind of Scholl-coupled microporous polymers and post-functionalized by fluoride.The postsynthetic fluorination polymers showed higher CO2capacity(132.00 mg·g-1at 273 K and 0.1 MPa) and selectivity for CO2/N2(IAST:26.0) than the direct crosslinking of fluorobenzene (CO2:106.00 mg·g-1,selectivity for CO2/N2:10.0).

Fig.1.(a) Synthetic route for melamine-based HCPs;(b) CO2 isotherms at 273 K,and (c) selectivity for CO2/N2 based on IAST at 273 K.

Fig.2.(a) Synthetic route for ZLYs;(b) CO2 isotherms at 273 K,and (c) Qst of the ZLYs.

2.1.3.Knitting method with external crosslinking agent

At present,the method of knitting method with external crosslinking agents was most commonly used for preparation of the HCPs.This method include two kinds of external crosslinking agents,one was the common external crosslinking agents which were added directly for crosslinking,the other one was solvent knitting method,for which the solvents itself were used as crosslinking agent.For the first kind of cross-linkers,formaldehyde dimethyl acetate (FDA),DCX,1,4-dimethoxybenzene (DMB) and cyanuric chloride (CC),etc.were widely used as the external cross-linkers.

FDA was the most popular external cross-linker for HCPs construction.As was firstly reported by Tan and co-workers in 2011[49],a great amount of HCPs were prepared in this way.In recent years,Dai and co-worked [50] used heterocyclic carbazole as raw materials,FDA for external cross-linker,and trifluoromethanesulfonic acid (Br?nsted acid) instead of traditional Lewis acid to induce a‘‘knitting”.And a series of HCPs(HC-PCz-(1-8))were successfully fabricated by Friedel-Crafts alkylation reaction (Fig.3).The highestSBETwas 1688 m2·g-1,the maximum CO2uptake of 189.20 mg·g-1at 273 K and 0.1 MPa,and the selectivity for CO2/N2was 17.0.Lau’s group [51] also used the Br?nsted acid as the catalyst for catalyzed the Friedel-Crafts alkylation reaction,for which triptycene and FDA were used for constructing the HCPs.The reaction time could be greatly reduce by replacing Lewis acids with Br?nsted acids as catalyst,and theSBETwas improved for 27%,CO2uptake increased 24% (up to 146.08 mg·g-1at 273 K and0.1 MPa).CC and DMB were also widely used for HCPs construction.Sadak [52] reported three kinds of HCPs by crosslink of 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (YBN-DMB) and three crosslinkers of FDA,CC,and DMB.TheSBETfor the three HCPs were 968,784,and 579 m2·g-1,corresponding to FDA,DMB,and CC,respectively.For FDA,the multi-substitution will greatly increase theSBET,however,the steric effect made CC and DMB hardly multi-substitute in one benzene ring.

The higherSBETof the polymers made them perform well in CO2capture(88.00-127.00 mg·g-1at 273 K and 0.1 MPa).Daset al.[53]constructed three HCPs by triptycene and three crosslinkers(2,6-dichloropyrazine,3,6-dichloropyridazine,and 2,4-dichloropyrimidine) through AlCl3catalytic Friedel-Crafts alkylation reaction.Among them,TNHCP2 showed highSBETof more than 751 m2·g-1and high N contents,resulting high CO2capture of 127.00-160.00 mg·g-1at 273 K and 0.1 MPa.Wanget al.[54]used DCX as external crosslinking agent,a variety of imidazolium salts for functional monomers,a series of imidazolium-functionalized ionic HCPs with abundant ionic sites,large surface area,and rich micro/mesoporosity were straightforwardly synthesized through Friedel-Crafts alkylation.Abundant microporosity and rich ionic moieties gave the polymers remarkably CO2capture performance.The adsorption capacity of CO2was up to 134.20 mg·g-1(at 273 K and 0.1 MPa) and selectivity for CO2/N2was as high as 86.0(Henry)/336.0 (IAST).Our group [55] prepared a series of benzimidazole-based hyper-cross-linked poly(ionic liquid)s(HPILs).In this reaction system,benzimidazole as ionic liquids(ILs) monomers,benzyl chloride or benzyl bromide for external crosslinking agents,the Friedel-Crafts alkylation reaction was occurred between ILs monomers and benzyl halides.With the production of imidazolium salts,rich ionic active sites were loaded in HPILs.The highSBETand abundant nitrogen(N)content was beneficial to CO2adsorption,and the HPILs showed good CO2capture performance of 143.00 mg·g-1at 273 K and 0.1 MPa,and the selectivity for CO2/N2was 44.0.

Fig.3.The synthesis route of nanoporous HCPs.

Solvent knitting method had also caught improved attentions for constructing of HCPs.Recently,Tan and co-workers [56] used polycyclic aromatic hydrocarbons (PAHs) such as fluoranthene,binaphthalene,naphthalene and phenanthrene,etc.as raw materials,dichloromethane(DCM)as reaction solvent and external crosslinker,a series of functionalized HCPs were prepared by Friedel-Crafts alkylation reaction under the catalysis of AlCl3.Comparison of the four PAH-based HCPs,theSBETof the four polymers ranged from 978 to 1788 m2·g-1.And the CO2uptake capacity ranged from 181.50 to 247.90 mg·g-1at 273 K and 0.1 MPa.More importantly,the electron-riched PAHs contain π conjugated systems were considered to be good candidates for the efficient separation of CO2and energy storage materials.The other HCPs reported in recent years for CO2capture were listed in the Table 1.

Table 1CO2 adsorption and separation performance of amorphous POPs

In recent decades,HCPs have developed greatly.Both the synthetic methods and the application fields have been greatly expanded.However,there are still many challenges in the development of HCPs.For example,the biggest disadvantage of the HCPs is the randomness,and the resulting polymers networks usually do not have regular structures,which greatly limits the performance of polymers networks for applications.

2.2.CMPs for CO2 adsorption

As a kind of amorphous POPs with large conjugated skeleton,CMPs were constructed by π-π conjugated rigid structural units.The way of producing micropores structures in the framework was directly connected with aromatic ring structural units or indirectly connected with other aromatic ring monomers through C=C/C≡C bonds.The whole rigid framework could be completely conjugated through the single-double/triple bonds tautomerism[65].A large amount of common reactions could be used for the construction of CMPs,such as Suzuki cross coupling reaction[66],Yamamoto coupling reaction [67],Schiff-base reaction [68],Sonogashira-Hagihara coupling reaction [69],Friede-Crafts reaction [70] and Buchwald Hartwig amination [71],etc.CMPs as typical amorphous polymers,3D network structure could be formed by π-π conjugated polymerized between the monomers,resulting the polymers with highSBETandVtotal.The covalent bonds connected structures made the CMPs stable enough under high temperature,therefore,CMPs were extensive used for CO2adsorption[72].

For example,Hanet al.[73]prepared two sp2carbon CMPs(BOCMP-1 and BO-CMP-2) with analogous structure through palladium-catalyzed Suzuki-Miyaura cross-coupling reaction by 9,10-bis(dibromomethylene)-9,10-dihydroanthracene (BBMA)and 1,4-phenylenediboronic acid.Then the CMPs were further post-oxidated (oBO-CMP-1 and oBO-CMP-2) to enhance the rigidity of CMPs structure.The adsorption performance of CMPs couldbe adjusted by adjusting the porosity of CMPs through postoxidation reaction.BO-CMP-1,BO-CMP-2,oBO-CMP-1 and oBOCMP-2 polymers displayed CO2uptake of 79.00,106.00,51.00 and 75.00 mg·g-1(273 K and 0.1 MPa),respectively,and theQstof the four CMPs were less than 36.5 kJ·mol-1,indicated that CO2has a good affinity with aromatic micropore walls with large porosity.Sunet al.[74] fabricated two novel CMPs (NUT-14 and NUT-15) by Yamamoto coupling reaction and Sonogashira-Hagihara coupling reaction.The results showed that NUT-15 with largerSBET(415 m2·g-1) andVtotal(0.24 cm3·g-1) than NUT-14(SBET:302 m2·g-1;Vtotal:0.18 cm3·g-1).The adsorption and separation performance for CO2of the two kinds of NUT were compared,NUT-15 have the CO2uptake of 127.60 mg·g-1,which was the twice of NUT-14 (74.80 mg·g-1,273 K and 0.1 MPa),and the CO2/CH4selectivity of NUT-14 and NUT-15 were 5.0 and 12.4,respectively.Xu’s group [75] synthesized three CMPs with abundance N and oxygen(O)by imidizing tetra-(4-aminophenyl)ethylene and anhydrides (CMP@1～3).TheSBETandVtotalof the three CMPs were calculated to be 346 m2·g-1and 0.22 cm3·g-1,325 m2·g-1and 0.60 cm3·g-1,and 343 m2·g-1and 0.70 cm3·g-1for CMP@1,CMP@2,and CMP@3,respectively.The abundant N and O contents of the three CMPs provide sufficient adsorption sites for CO2.The maximum adsorption capacity of CO2was 101.80 mg·g-1(273 K and 0.1 MPa) and the selectivity for CO2/CH4was 16.6.

CMPs always prepared by different reactions and monomers,resulting the polymers with different CO2adsorption properties for different porosity and functioned groups.Huet al.[76]prepared three CMPs through cationic cyclization polymerization by multiple enediyne moieties monomers.TheSBETvalue of CMP1,CMP2 and CMP3 were 767,624 and 780 m2·g-1,respectively.TheSmicro(481 m2·g-1) andVmicro(0.19 cm3·g-1) of CMP1 were larger than those of CMP2 (302 m2·g-1and 0.12 cm3·g-1) and CMP3(134 m2·g-1and 0.02 cm3·g-1).The higherSmicroandVmicroendow the good CO2adsorption performance(129.60 mg·g-1at 273 K and 0.1 MPa) for CMP1.The porphyrin-based polymers were widely used for their stable structure and porosity.If loaded with metal ions,the active sites could be enhanced and the polymers could have better performance in gas capture.Su and co-workers [77]fabricated two porphyrin-based CMPs (Porp-TPE-CMP and Porp-Py-CMP) through coupling reaction catalyzed by FeCl3.TheSBETof Porp-TPE-CMP (547 m2·g-1) was one order of magnitude larger than Porp-Py-CMP(31 m2·g-1),indicative higher CO2uptake of Porp-TPE-CMP(88.75 mg·g-1)than Porp-Py-CMP(54.41 mg·g-1)at 273 K and 0.1 MPa.The selectivity for CO2/N2and CO2/CH4of Porp-TPE-CMP and Porp-Py-CMP were calculated to be 55.3 and 40.8,4.1 and 5.1,respectively.The other CMPs reported in recent years for CO2capture were listed in the Table 1.

CMPs have emerged as a broad platform for CO2adsorption due to their highSBET,Vtotaland thermal stability.By changing the structure and synthetic strategy of CMPs,the adsorption capacity and selectivity of CO2can be adjusted.However,the use of expensive metals in some CMPs synthetic processes,which limits their application in large-scale application.

2.3.PIMs for CO2 adsorption

The PIMs are a kind of novel materials with micropores in own structures,they are also called intrinsic microporous polymers.The monomers for the construction of PIMs usually have dense ring and trapezoidal twisted molecular chain.The trapezoidal skeleton and twisted structure limit the rotation of polymer chains,which make the molecular chains cannot be effectively stacked.Therefore,they have high free volume and microporous structure,which makes them have excellent gas capture performance[78].In 2002,McKeownet al.[79,80]firstly used the benzodioxane to connected diphthalonitrile units and the polymer with microporous structure and highSBET(895 m2·g-1) was obtained.Then using a similar method,porphyrin as the monomer,they obtained a porphyrinbased microporous polymer withSBETof up to 910 m2·g-1.In 2005,Buddet al.[81]synthesized a series of linear PIMs.Different from the above-mentioned insoluble 3D PIMs network frameworks,linear PIMs can be dissolved in many organic solvents and spin-coated into a membrane.Compared with other rigid structured polymers,the CO2permeability of PIMs membranes was significantly higher [82].For this reason,large amount of PIMs as separation membranes were used for selectively gases separation.

Filiz’s group [83] prepared a mixed matrix membrane (MMM)with excellent gas transport performance based on PIM-1.The gas transport performance of the membrane can be adjusted by controlling the loading of Potassium dodecahydrodecaborate(K2B12H12) inorganic particles (IPs).When the loading of IPs increased up to 10%,the CO2permeability of PIM-1 was maximum(12954± 23 Barrer,1 Barrer=7.52×10-18m3(STP)·m2·m·s-1·Pa-1),which was increased for 31% by comparing with the pure PIM-1(9896±28 barrer).Fuoco’s group[84]analyzed the details of basic transport parameters of triptycene-based ultrapermeable PIMTMN-Trip membrane and PIM-BTrip membrane (Fig.4).Those two PIMs membranes have already been reported,but without basic parameters of gas transmission.Therefore,they obtained the activation energy of permeation,diffusion and sorption for PIMs membranes through theoretical calculation.They also obtained the selectivity of diffusion,entropy and energy by calculated.The results showed that the permeability of PIMs membranes was increased with the increasing of diffusion coefficient and solubility coefficient.Meanwhile,the size of PIMs membranes and the temperature also have greatly influence on permeability and selectivity.This work provides a strong theoretical basis for the gas adsorption performance of PIMs membranes and lays a foundation for the design and development in the future.

Regenet al.[85] synthesized a series of polyelectrolyte multilayers containing PIMsviathe layer-by-layer deposition method.The thickness of the membranes was changed with the number of deposition layers.The influence of membranes thickness on gas separation had been investigated.The results indicated that the 6 nm multilayers derived from 1 plus poly(diallyldimethylammonium chloride)showed exceptional permeation properties with CO2and N2.The CO2permeability was 130 GPU and the selectivity for CO2/N2was 33.0.McKeownet al.[86] fabricated four ethanoanthracene-(EA) and triptycene-(Trip) based polymer membranes (TB-PIM) with/without bridgehead methyl substituents.They researched the influence ofSBETand gas separation performance of the polymer membranes by methyl substituents.The results showed that theSBETandVtotalwere increased after bridgehead methyl substituents,nevertheless,theSBETandVtotalwere not the decisive factors for the selective permeability of gases.It mainly depends on the size of the porosity and the interconnection among pores.And the types,orientation,final arrangement and expansion of macromolecular groups in polymer backbone were also played important roles in CO2selectively separation performance.

Qin’s group [87] for the first time used the Al2O3atomic layer deposition (ALD) technique to modify the microporosity of PIM(PIM-1)at the atomic level to adjust the performance of CO2separation.With the increasing of ALD cycle numbers,the permeability of CO2decreased,and the selectively significant increased.The maximum selectivity for CO2/CH4(56.2) and CO2/N2(29.3) were obtained at 6 ALD cycles.The results showed that the microporous porosity of PIM-1 could be adjusted systematically by changing the cycle numbers of ALD,the adsorption and separation performance of gas were also adjusted.The method of ALD technology provides a new way to construct PIMs membranes.The other PIMs reported in recent years for CO2capture were listed in the Table 1.

The PIMs have widely used for CO2capture for their highSBETandVmicro.Some linear PIMs could be dissolved in many organic solvents and spin-coated into membranes.The PIMs membrane has already been one of the famous separation membranes for their excellent selectivity and permeability of gases.Nevertheless,PIMs membrane inevitably has the defect of physical aging.Therefore,how to improve this limitation through the design of the structure is of great important.

2.4.PAFs for CO2 adsorption

Fig.4.(a)Molecular structures of PIM-1,PIM-TMN-Trip and PIM-BTrip;Robeson plots for the(b)CO2/CH4 and(c)CO2/N2(1.0 Barrer=7.52×10-18 m3(STP)·m2·m·s-1·Pa-1).

The PAFs are a kind of porous solid materials with rigid skeleton structures and synthesize according to topology theory.PAFs are always constructed by C-C bond coupling between the rigid organic monomers,resulting the structure with higherSBETand stability.In 2009,Zhu’s group [88] firstly reported the PAFs with 2D/3D periodic aromatic frameworks.The PAFs were effectively assembled from organic building blocks through irreversible coupling reactions (usually C-C bond coupling).This method was based on topological orientation,which could realize the unprecedented feasibility of synthesizing and pre-determining the structure of POPs.In that way,porous materials could be designed on the basis of specific uses.And diversified functions could be designed and synthesized according to the inherent chemical properties of the PAFs structural units,or achieved by post-modification of the aromatic skeleton through common organic reactions.Hence,PAFs were widely used in CO2capture and separation [89].

With time going on,researches on PAFs had gradually increased.In 2011,Qiuet al.[90] synthesized two PAFs (PAF-3 and PAF-4) by using four-coordinated silicon and germanium as raw materials by Yamamoto-Ullmann reaction.The resulting polymers showed highSBETand stability.TheSBETandVtotalof PAF-3 and PAF-4 were 2932 m2·g-1and 1.54 cm3·g-1,2246 m2·g-1and 1.45 cm3·g-1,respectively.The largeSBETandVtotalwas conducive for CO2adsorption,with the value of 153.00 mg·g-1and 107.00 mg·g-1for PAF-3 and PAF-4,respectively.Hill and coworkers [91] developed a method for the lithiated of PAN-1(Li@PAF-1).This strategy enhanced the surface activity of PAN-1,the adsorption enthalpy,and the gas adsorption performance.Compared with the original PAN-1 polymer,Li@PAF-1 showed an increasing of the adsorption capacity of CO2for 320%(395.50 mg·g-1at 273 K and 0.1 MPa).

With the development of PAFs,increasing interests were focus on gas adsorption.In recent years,Braccoet al.[92] synthesized a porous 3D fluorinated PAF (F-PAF1) by condensation of fluorinated tetraphenylmethane (TPM) monomer.TheSBETwas 2050 m2·g-1.After fluorination,the C-F dipoles were partially built on the PAFs framework.In this,the C-F dipoles were arranged in the swellable architecture by non-centrosymmetric,giving the entirep-phenyl groups a dipole moment.This noncentrosymmetric way of regularly spaced fluorine substitution made the substitution amount of fluorine atoms limited and ensured that the skeleton as light as possible.In the PAFs structure,the uniformly distributed C-F dipoles were exposed to the diffused gas.Compared with the unfluorinated PAF1,the affinity of CO2was greatly promoted,and the selectivity for CO2/N2was also improved.In order to obtain a better PAFs-based CO2adsorbent,it is necessary to precisely adjust the pore size andVtotalto increase the affinity of PAFs polymers for CO2[93].Jiang’s group[89]made full use of the advantages of ionic liquids and PAFs,impregnated charged PAFs with ionic liquids.They confined cations and anions within the polymers framework through electrostatic interaction.The loading of the ionic liquids made the PAFs structures produced nano-scale space,which could improve the capture of the gases.And the ionic liquids could also promote the transmission of the gas in the porous structure.In that way,the permeability of CO2and the selectivity for CO2/CH4could be adjusted by adjusting the loading of ionic liquids.Liuet al.[94] used biphenyl and diphenylamine as build units to prepared PAFs (PAF-45DPA) with functional groups and high porosity.The rigid structure of biphenyl provides a porous structure for PAFs,diphenylamine provides basic N functional groups,and the porous structure provides channels for gas transmission.According to the acid-base theory,basic N functional groups can effectively adsorb CO2and other acid gases.Hence,PAF-45DPA showed highSBET(679 m2·g-1),CO2uptake(88.98 mg·g-1at 298 K and 0.1 MPa),and selectivity for CO2/N2(63.1).The other PAFs reported in recent years for CO2capture were listed in the Table 1.

In the past decade,the PAFs had also developed rapidly,and PAFs showed excellent performance in CO2adsorption and separation.However,organometallic catalysts were often used in the process of PAFs synthesis.Those catalysts were expensive and difficult to remove from the framework,which will block the pores and decrease the porosity.For this reason,it is meaningful for researchers to develop a method for the construction of PAFs with high porosity and low cost.

3.Crystalline POPs for CO2 Adsorption

Compared with POPs with amorphous structure,crystalline structured POPs have adjustable structure,pore channels,and highly ordered structures.The COFs and CTFs are belongs to crystalline POPs,and they are briefly introduced as followed.

3.1.COFs for CO2 adsorption

The COFs are a kind of porous organic framework polymers with long-range ordered structure and are connected by covalent bonds (B-O,C-C,C=H,C=N,C-N,etc.).COFs always obtained through reversible condensation under thermodynamic control,the resulting structure is more stable than MOFs.As a kind of typical POPs with crystal structure,COFs have attracted a lot of attentions in recent years [27,28,95].Since Yaghi’s group[96] first reported COFs in Science,COFs have received widespread attention.The methods for the construction of COFs include microwave and mechanically assisted synthesis,solvothermal synthesis,and room temperature synthesis,etc.Generally speaking,most of the reported COFs were synthesized in a sealed container under solvothermal conditions.By selecting different organic monomers,adjusting the ratio of monomers,and controlling the reaction conditions,COFs with different structures and morphologies could be pre-designed accurately.Therefore,this article briefly summarizes the adsorption and separation of CO2by COFs with different covalent bonding methods.

3.1.1.Imine linked

The COFs connected by imine are considered as a more advantageous gas adsorbent for the stable structure in some harsh conditions.Therefore imine linked strategy has been one of the most extensive covalent bonding for COFs construction.El-Kaderiet al.[97] prepared the ILCOF-1 with highSBET(5070 m2·g-1) by using tetrakis(p-formylphenyl)pyrene and 1,4-p-phenylenediamine as the raw materials.And the ILCOF-1 showed good CO2uptake (492.80 mg·g-1at 298 K and 4.0 MPa),and lowQst(18.3 kJ·mol-1).Neogiet al.[98] synthetized an imine linked 3D COF with the structure of ninefold interpenetrationviatetra(p-aminophenyl)methane (TAPM)and N-functionalized linear strut 2,2′-bipyridine-5,5′-dicarbalde hyde (BPDCA).The COF had one-dimensional porous channels,highSBET(5070 m2·g-1) andVtotal(1.93 cm3·g-1),resulting the COF with high CO2adsorption capacity of 173.50 mg·g-1.And theQstwas 35.0 kJ·mol-1,CO2adsorption selectivity were as high as 64.2 (selectivity for CO2/N2) and 10.5 (selectivity for CO2/CH4),respectively.Thereafter,they anchored the Ru into COF to catalyze the conversion of CO2into cyclic carbonates.Nagarajaet al.[99] prepared a N-riched COF with highSBET(736 m2·g-1) by Schiff-base condensation reaction between 4,4′-azodianiline (AZO) and benzene-1,3,5-tricarboxaldehyde(BTA).The COF exhibits CO2adsorption capacity of 52.25 mg·g-1at 273 K,134.55 mg·g-1at 195 K,and 31.23 mg·g-1at 298 K,and theQstwas 32.3 kJ·mol-1.

3.1.2.β-ketoenamine linked

The synthesis of β-ketoenamine linked COFs are based on the reversible Schiff base reaction between trialdehyde phloroglucinol and amino monomers to form an eno-limine structure,which is then irreversibly convert to a stable structure of ketoenamine.Compared with eno-limine isomers,the ketoenamine isomers are dominated (Fig.5) [100].Wei’s group [101] synthetized a 2D βketoenamine TpTa-COF under lower temperatures and shorter reaction timeviarapid microwave assisted solvothermal method.The TpTa-COF had high stability andSBET(725 m2·g-1)and showed a good performance of CO2capture (224.40 mg·g-1at 273 K and 0.1 MPa),Qstof TpTa-COF was 34.1 kJ·mol-1and the selectivity for CO2/N2was as high as 32.0.Triformylphloroglucinol and azo substituted 4,4′-biphenyl diamine were also used to prepared azo functionalized β-ketoenamine-linked AzoCOFs (Tg-AzoCOF,C10-AzoCOF and H-AzoCOF) by the Schiff base reaction.Among them,H-AzoCOF showed good adsorption effect on CO2for its higherSBETandVmicro[102].Lai and co-workers [103] synthetized chlorinefunctionalized β-ketoenamine-based CAA-COF-1 and CAA-COF-2 by solvothermal method between triformylphloroglucinol and 2,5-dichloro-1,4-phenylene diamine/3,3′-dichlorobenzidine dihydrochloride.The CO2uptake of CAA-COF-1 and CAA-COF-2 were 251.68 and 119.68 mg·g-1at 273 K and 0.1 MPa,respectively.And theQstof CAA-COF-1 and CAA-COF-2 were 29.9 and 29.5 kJ·mol-1,respectively.The selectivity under dry and humid mixed gas conditions was investigated,CAA-COF-1 showed better selectivity for CO2/N2and CO2/CH4than CAA-COF-2.The selectivity for CO2/N2and CO2/CH4of CAA-COF-1 in dry and humid mixed gas conditions were 95 and 29,87 and 26,respectively.The dipole interaction between the electron-riched chlorine in CAA-COF structure,the electron-deficient carbon in the CO2molecule,and the N-riched environment in the structure significantly all improved the CO2/N2selective separation performance.

3.1.3.Other linked

Thanks to the abundant monomers and multitudinous connection methods,a large amount of COFs with different linked types had constructed.In addition to the above two connection methods,there were boronate ester-linked,hydrazone-linked,azine-linked and boroxine-linked,etc.El-Kaderiet al.[104] used 1,4-benzenediboronic acid and hexahydroxytriptycene as raw materials,the boronate ester-linked TDCOF-5 with highSBET(2497 m2·g-1) was synthetized.The adsorption capacity of CO2was 92 mg·g-1(273 K and 0.1 MPa)and theQstwas 21.8 kJ·mol-1.At present,there are short of new boronate ester-linked COFs for CO2capture due to the boronate ester-linked COFs structure not stable enough under acidic or basic conditions.Mothé Esteveset al.[105] prepared a series of hydroxylated and azine-based microporous COFs by condensation of hydrazine hydrate and hydroxylated-1,3,5-triformylbenzenes.TheSBETof COFs with different hydroxyl contents was quite different,largeSBETalways have more hydroxyl.And theSBETwas ranged from 242 to 1205 m2·g-1.Therefore,the CO2adsorption capacity were obviously different,and the CO2adsorption capacity was ranged from 5.98 to 24.99 mg·g-1at 273 K and 0.1 MPa.

The present 2D hydrazine-linked COFs were demonstrated good chemical stability in boiling water and strong acidic media.Therefore,Karimiet al.[106] used 2,4,6-tris(p-formylphenoxy)-1,3,5-triazine and oxalyl dihydrazide as raw materials,synthesized a stable 2D hydrazine-linked COF (TPT/OH COF) with highSBETand N-riched porous structure.The synthetic COFs showed the CO2uptake of 39.60 mg·g-1under 0.1 MPa and 298 K.The other COFs reported in recent years for CO2capture were listed in the Table 2.Membrane separation technology was already a mature technology for CO2separation.As a novel porous open framework materials,COFs have already widely prepared into membranes for their high porosity,SBET,and regular pore size,which endowed them excellent CO2separation performance [107].For instance,Tan’s group [108] reported a strategy of 3D MOF-mediated COF-based membrane,in which MOF film provides the binding sites,and the 2D COF structural building units were anchored along the vertical direction.The 2D COF-based composite membrane H2P-DHPh COF-UiO-66 was prepared by strong chemical bonding between 2D COF and 3D MOF.The composite membrane showed ultra-high CO2permeability.

Table 2CO2 adsorption and separation performance of crystalline POPs

As a kind of crystalline structure POPs,COFs have adjustable structure and pore channels.Although the pore size can be adjusted through the design of the structure,it is difficult to construct COFs with ultramicropores due to the limit of the size of the construction units.Therefore,building a COFs with abundant ultramicropores to achieve high selective CO2adsorption and separation is an urgent problem.

3.2.CTFs for CO2 adsorption

Fig.5.Illustration of (a) formation of β-ketoenamine linkage in COFs and (b) aldimine via an iminium ion intermediate.

The CTFs are a kind of high-performance organic porous materials which were widely developed in recent years.They are a new type of polymers based on the cyano ring trimerization(formation of triazine ring)of aromatic nitrile compounds,and was firstly proposed by Thomaset al.in 2008 [109].As a sub-category of COFs,the pore size,SBET,pore structure and functional groups of CTFs can be adjusted by selecting different monomers,and CTFs have permanent open and stable pores.CTFs with different structures(2D/3D),pore structures and different N contents can be obtained by selecting different polymerization reactions (ionothermal reaction,Friedel-Crafts reaction and Schiff base reaction,etc.).CTFs polymers usually have largeSBET,uniform porous structure,high thermal/chemical stability,and rich N content,making them widely used in CO2capture.

Fig.6.(a-c) Structural units;(d,e) CO2 adsorption isotherms and (f) Qst of CTFs (1 bar=0.1 MPa).

As an important CO2capture and storage adsorbent,CTFs have received extensive attentions.Dai’s group[110]synthesized a series of fluorine-functionalized CTFs-derived polymers of F12CTF-3-T by using fluorinated trinitrile monomer through ionothermal under different temperatures.TheSBETwas 2085 m2·g-1for F12CTF-3-T and the pore size was about 0.7 nm which matches well with the twice size of CO2.Therefore,F12CTF-3-T had highefficiency CO2capture performance (289.52 mg·g-1at 273 K and 0.1 MPa).Kuo and co-workers [111] synthesized a series of bicarbazole-based CTFs (Car-CTFs) by the same method through the self-condensation of [9,9′-bicarbazole]-3,3′,6,6′-tetracarboni trile (Car-4CN).The Car-CTFs showed highSBETof 1400 m2·g-1,resulting very high CO2adsorption capacity of 334.40 mg·g-1(273 K and 0.1 MPa) and 172.04 mg·g-1(298 K and 0.1 MPa),respectively.The triazine structure has abundant N contents,which were conducive to the adsorption of CO2.TCNQ-derived CTFs (TCNQ-CTF) was prepared by 7,7,8,8-tetracyquinodimethane(TCNQ).TheSBETwas as high as 4000 m2·g-1,CO2uptake was 263.56 mg·g-1at 273 K and 0.1 MPa,and theQstwas 29.2 kJ·mol-1[112].Chen’s group[22]synthesized three CTFs(CTFs-N4,CTFs-O4 CTFs-S4) by using 2,4,6-tris(4-cyanophenylamino)-1,3,5-triazine(TAT),2,4,6-tris(4-cyanophenoxy)-1,3,5-triazine (TOT),and 2,4,6-t ris(4-cyanobenzenesulfenyl)-1,3,5-triazine (TST) as building units through ZnCl2-mediated heating cyclotrimerization method.The maximumSBETwas 758 m2·g-1andVtotalwas 0.29 cm3·g-1.The abundance N heteroatoms were extremely improved the CO2capture.Adsorption capacity for CO2were 149.60 mg·g-1(273 K) and 96.80 mg·g-1(298 K)under 0.1 MPa,theQstwas 44.0 kJ·mol-1and selectivity for CO2/N2was 45.0 (Fig.6).Mukhtar and co-workers[113] prepared high pressure selective CO2/CH4adsorption CTFNH by using cyanuric chloride and 1,4-phenylenediamine.They researched the CO2selectivity adsorption performance under different temperatures (298 to 348 K) and pressures (1.0 to 2.0 MPa).The results showed that the CO2adsorption capacity was decreased with the increasing of temperature,while the selectivity for CO2/CH4obviously improved (2.7 to 5.1).After amine functionalization the adsorption capacity of CO2improved for 94.99% and the maximum CO2uptake was 342.32 mg·g-1.The other CTFs reported in recent years for CO2capture were listed in the Table 2.

As an emerging porous material,CTFs has been widely used in CO2adsorption and separation.However,there were few CTFs with crystalline structure,and amorphous CTFs are limited by CO2adsorption sites,resulting unsatisfactory adsorption performance.The common methods for the construction of CTFs were thermosynthesis under 400 °C,the structure of CTFs could be partly carbonized.The high energy consumption and uncertain structure would limit the application of the CTFs for CO2capture.

4.Conclusion and Prospect

We systematically reviewed the construction of different POPs for CO2capture.As a kind of promising polymers,the POPs were constructed by lighter elements,rigid monomers,many chemical bond formation methods,crosslinking reactions,and rigid covalent bonds.Therefore,a large amount of POPs was constructed.The highSBETandVmicro,abundant heteroatoms,high physiochemical stability,and unique porosity endowed the POPs excellent performance for CO2.By comparing with other porous materials,the pore structure,SBET,and function groups of POPs could be adjusted.In this way,POPs with highSBET,heteroatoms contents,and ultramicropore contents could be designed and constructed.

However,the application of POPs in CO2capture was limited by the adsorption capacity.As was reported by the literature,SBET,Vul-tra,and heteroatoms play the key role in CO2capture.Take HCPs for example,aromatic rings were always multi-substituted and hypercross-linked through Friedel-Crafts reaction,resulting the HCPs with highSBETand abundant ultramicropores.However,the hyper-cross-linking would create quite a few closed pores or even solid structures,which were useless for CO2capture.Hence,high temperature carbonization was a promising way to create much more micropores for the polymers.Although activated carbons showed good CO2capture,they were limited by the low selectivity for CO2/N2for the lack of functionalized groups and heteroatoms,which were considered useful for the selectivity for CO2/N2.The porous carbons derived from the POPs always showed good CO2capture and selectivity for CO2/N2for the abundant heteroatom content of the POPs.

The utilization of the captured CO2was also a promising strategy for the reduction of CO2excessive emissions.As a kind of C1 source,CO2could be used for the preparation of cyclic carbonates through cycloaddition reaction between CO2and the epoxide.With the functionalization of ionic liquids or metal ions,POPs could be used for the catalysts of cycloaddition reaction for CO2fixation.POPs could be constructed with photo-responsive monomers(porphyrin,carbazole,phthalocyanine,etc.),and the polymers exhibit good performance for photocatalysis.In this way,CO2could be converted into high value-added chemical products (CO,methane,ethanol,formic acid,etc.) through the photocatalysis.

The excessive emissions of CO2have caused serious extreme climate.The reduction of CO2has caused increasing attentions.Hence,as a kind of promising materials,more and more POPs with new functionalities would be designed and constructed.We hope this work will provide guidance for the design and synthesis of functional POPs for the development of advanced adsorption and other performance.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.