Registered charity number: 207890

www.rsc.org/MaterialsA

As featured in:

See Lin-Bing Sun, Hong-Cai Zhou et al.,

J. Mater. Chem. A, 2015, 3, 3252.

Showcasing research from Lin-Bing Sun’s Research Laboratory,

State Key Laboratory of Materials-Oriented Chemical

Engineering, College of Chemistry and Chemical Engineering,

Nanjing Tech University, China.

Title: Facile fabrication of cost-eff ective porous polymer

networks for highly selective CO2 capture

Mesoporous polymers were fabricated via facile nucleophilic

substitution reactions under the direction of a template. The

obtained materials, which consist of abundant secondary amines,

are highly active in selective CO2 capture and can be readily

regenerated.

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Facile fabrication

aState Key Laboratory of Materials-Orien

Chemistry and Chemical Engineering, Na

China. E-mail: lbsun@njtech.edu.cnbDepartment of Chemistry, Texas A&M Unive

USA. E-mail: zhou@chem.tamu.edu

† Electronic supplementary informa10.1039/c4ta06039c

Cite this: J. Mater. Chem. A, 2015, 3,3252

Received 8th November 2014Accepted 7th January 2015

DOI: 10.1039/c4ta06039c

www.rsc.org/MaterialsA

3252 | J. Mater. Chem. A, 2015, 3, 325

of cost-effective porous polymernetworks for highly selective CO2 capture†

Lin-Bing Sun,*a Ai-Guo Li,a Xiao-Dan Liu,a Xiao-Qin Liu,a Dawei Feng,b Weigang Lu,b

Daqiang Yuanb and Hong-Cai Zhou*b

Due to their synthetic diversification, low skeletal density, and high

physicochemical stability, porous polymer networks (PPNs) are highly

promising in a variety of applications such as carbon capture. Never-

theless, complicated monomers and/or expensive catalysts are nor-

mally utilized for their synthesis, which makes the process tedious,

costly, and hard to scale up. In this study, a facile nucleophilic

substitution reaction was designed to fabricate PPNs from low-cost

monomers, namely chloromethyl benzene and ethylene diamine. A

surfactant template was also used to direct the assembly, leading to

the formation of PPN with enhanced porosity. It is fascinating that the

polymerization reactions can occur at the low temperature of 63 �C in

the absence of any catalyst. The obtained PPNs contain abundant

secondary amines, which offer appropriate adsorbate–adsorbent

interactions from the viewpoints of selective CO2 capture and energy-

efficient regeneration of the adsorbents. Hence, these PPNs are highly

active in selective adsorption of CO2, and unusually high CO2/N2 and

CO2/CH4 selectivity was obtained. Moreover, the PPN adsorbents can

be completely regenerated under mild conditions.

The worsening climatic situation due to global warming hasbecome an urgent environmental concern nowadays, andexcessive CO2 emission from fossil fuel combustion is consid-ered to be a major anthropogenic source of greenhouse gases inthe atmosphere. CO2 is also a detrimental component of naturalgas. The presence of CO2 can cause corrosion of relevant pipe-lines and equipment as well as reduction in both transportcapacity of the pipeline and heat capacity of the gas. As aresult, separation of CO2 from N2 (post-combustion for ue gas)and from CH4 (pre-combustion for natural gas) has attractedincreasing attention.

ted Chemical Engineering, College of

njing Tech University, Nanjing 210009,

rsity, College Station, Texas 77842-3012,

tion (ESI) available. See DOI:

2–3256

The traditional technique to remove CO2 is “wet scrubbing”by using aqueous amine (e.g. monoethanolamine, MEA) solu-tions. Nevertheless, this process shows several inherent short-comings including high regeneration costs and erosion ofequipment. Among currently available CO2 capture technolo-gies, adsorption using porous materials is regarded as the mostpromising alternative.1,2 The porous solids have specic heatcapacities that are substantially less than those of aqueoussolutions. Furthermore, they are easier to handle and free ofcorrosion problems. In the past decades, some metal–organicframeworks (MOFs) have been reported to possess excellentcapacity for CO2 capture because of their large surface areas andhigh pore volumes. However, most MOFs are unstable in hightemperatures, moisture, and other rigorous environments, andthus cannot meet the harsh industrial demands.3–6 Fortunately,porous polymer networks (PPNs, also known as covalentorganic frameworks,7 hyper-crosslinked polymers,8 conjugatedmicroporous polymers,9 polymer of intrinsic microporosity,10

covalent triazine-based frameworks,11 porous aromatic frame-works,12 etc.) constructed from lightweight elements throughstrong covalent bonding have become a focus of attention.13–15

These materials generally possess low skeletal density, can besynthesized in diverse ways, and display high physicochemicalstability, and thus they are highly competitive in CO2 capture.However, complex monomers, expensive catalysts, or highreaction temperatures are commonly required for the synthesisof most reported PPNs, which make the synthetic processcomplicated, costly, and hard to scale up.11,12,16 A case in point isthe synthesis of porous aromatic frameworks reported by Benet al.,17 for which the expensive catalyst bis(1,5-cyclooctadiene)nickel(0), that is Ni(COD)2, has to be employed for the Yama-moto-type Ullmann polymerization reaction of this synthesis. Inthe preparation of triazine-based porous polyimide polymernetworks reported by Senker's group,11 the complex monomer2,4,6-tris(4-aminophenyl)-1,3,5-triazine was used; this mono-mer was synthesized from the quite preliminary precursor4-bromobenzonitrile via a series of tedious organic reactions.Despite great efforts, fabrication of PPNs from low-cost

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monomers via a catalyst-free and facile polymerization reactionhas been a great challenge.

Amine groups are well-known active sites that can createstrong interactions between adsorbents and CO2.18–21 Varioustypes of amines have been introduced to the frameworks of PPNs.Primary amines can capture CO2 efficiently by forming stablecomplexes, but are difficult to regenerate. Tertiary amines can beregenerated easily under mild conditions, but at the expense ofadsorption capacity and selectivity. Secondary amines, however,are ideal building blocks for PPNs since they can strike anappropriate balance between adsorption performance andenergy-efficient regeneration.18,22–25

Herein, we developed a strategy to construct a new porouspolymer network, PPN-80, by a facile nucleophilic substitutionreaction of 2,4,6-tris(chloromethyl)mesitylene (M1) and ethylenediamine (M2) (Scheme 1). Aiming to enhance the porosity of PPN,a surfactant template was also used to direct the assembly,leading to formation of PPN-81 with mesoporosity. Due to thereactivity of benzyl chloride in M1, the polymerization reactionswere shown to take place at a temperature as low as 63 �C inthe absence of any catalyst. Furthermore, both monomers areinexpensive and readily available. The resultant PPNs containabundant secondary amines, and thus provide an appropriateadsorbate–adsorbent interaction, which is benecial to bothselective CO2 capture and energy-saving regeneration of theadsorbents. In our experiments, our materials efficiently adsor-bed CO2, while N2 and CH4 were scarcely adsorbed, revealing anextremely high selectivity of CO2 over N2 and CH4. Moreover, thePPNs could be completely regenerated under mild conditions.

For the synthesis of PPN-80, the monomers M1 and M2 wereinitially dissolved in tetrahydrofuran (THF), leading to theformation of a clear colorless solution. Aer heating at 63 �C forabout 0.5 h, white powders as the target products began to begenerated, and the yield increased with the reaction time. In asimilar process, PPN-81 was synthesized by the addition oftemplate (triblock copolymer P123) to the initial solution con-taining monomers. No catalysts were used for the synthesis ofboth PPN-80 and PPN-81. The synthesis of the polymer was alsotested at other temperatures, varying from 50 to 63 �C. The

Scheme 1 (A) Polymerization of monomers to form PPN-80 in theabsence of template and PPN-81 in the presence of template. (B)Proposed interaction between template molecules and amine groups.

This journal is © The Royal Society of Chemistry 2015

results show that the polymerization reactions can also takeplace at 60 �C, although the yield of the polymer was 90% of thatsynthesized at 63 �C. Similarly, the polymer can be produced at55 �C, with the yield at about 40% of that synthesized at 63 �C.Upon further decreasing the temperature to 50 �C, no polymerwas obtained at all.

The infrared (IR) spectra of PPN-80 and PPN-81 are similar,as shown in Fig. 1A. Both spectra have bands at 3330 and1110 cm�1, which can be attributed to N–H and C–N stretchingvibrations, respectively. The presence of these bands indicatesthe successful introduction of amine groups into the frame-works. There is also a band at 1650 cm�1, which originated fromthe –NH2 bending vibration, indicating some –NH2 groupsremained free. Indeed, both –NH2 groups in most diamines canreact with the monomer M1, leading to the formation of –NH–

linkages that assemble the polymer into a 3D organic network.Only a few pendant –NH2 groups formed in the polymer; thesegroups derived from the reaction of one of the –NH2 groups ofthe diamine with M1. It is thus reasonable to conclude that alower –NH2 band intensity reects a higher degree of PPNpolymerization. The band intensity of –NH2 was also quantiedby comparing it to the C–C vibration band of benzene rings atabout 1570 cm�1, which was selected as the reference band. Therelative intensity of –NH2 to C–C was calculated to be 4.2 and 2.1for PPN-80 and PPN-81, respectively. The amount of –NH2 inPPN-80 is thus about twice as high as that in PPN-81. On theother hand, the more intense the band at 1110 cm�1 (C–N), thegreater should be the degree of polymerization. Based oncombining the information about the –NH2 and C–N bands, it is

Fig. 1 (A) IR spectra and (B) solid-state 13C NMR spectra of PPN-80and PPN-81.

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Fig. 2 (A) CO2, CH4, and N2 adsorption–desorption isotherms at295 K. (B) IAST-predicted adsorption selectivity of CO2 over CH4

and N2.

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safe to say that PPN-81, with a more intense C–N band andweaker –NH2 band, achieved a higher degree of polymerizationthan did PPN-80.26 Our solid-state 13C nuclear magnetic reso-nance (NMR) experiments yielded spectra that display threepeaks for PPN-80 and PPN-81 (Fig. 1B). The two sharp peaks at132 and 14 ppm can be assigned to the sp2 C of the benzene ringsand to the carbon atoms of the methyl groups directly connectedto the benzene rings, respectively. The broad peak at 47 ppmappears to be a combination of the peaks of carbon atoms con-nected to alkyl carbon and nitrogen. The results of elementalanalysis show that the PPNs mainly consist of three elements,namely C, H, and N, with a tiny amount of Cl (Table S1†). Themeasured elemental contents are in line with the theoreticalvalues, and this agreement indicates that the polymerizationreactions proceeded as designed. The lower Cl content andhigher N content of PPN-81 conrms the IR results, implying thehigher polymerization degree of PPN-81. The IR spectra, alongwith NMR and elemental analysis results, demonstrate thesuccessful construction of PPNs via nucleophilic substitutionreactions.

The pore structure of PPNs was evaluated by N2 adsorption at77 K. The N2 uptake of PPN-80 is generally low, as shown inFig. S1,† which is indicative of the sizes of the pores being quitesmall, hence restricting entry of N2. This is benecial to theselective adsorption of CO2 rather than N2 as described later.Due to the use of a template in the synthetic process, PPN-81showed an apparently higher N2 uptake than did PPN-80.Moreover, the uptake increased gradually with the relativepressure, and the isotherm presents a hysteresis loop. Pore sizedistributions were further calculated, and are shown in Fig. S2.†PPN-81 has an obvious pore size at about 4 nm, which is absentin PPN-80. These results provide evidence for the directing roleof the template, which leads to the formation of mesopores inPPN-81. Thermogravimetric (TG) analysis yielded comparablethermal stabilities for PPN-81 and PPN-80 (Fig. S3 and S4†),suggesting that the presence of mesopores does not reducethe stability. On the basis of the aforementioned results, weconclude that PPN-80 and PPN-81 have similar network struc-tures, while the proportion of primary –NH2 to secondary –NH–

groups as well as the porosity in two polymers are different.The adsorption behavior of CO2, N2, and CH4 on the PPNs

were systematically studied. Despite the different adsorptiontemperatures, the isotherms of CO2 on PPN-80 and PPN-81present a similar shape (Fig. 2, S5 and S6†). High uptakes wereobtained at relatively low pressures, and hysteresis could beclearly observed. This provides evidence for the existence ofplentiful amine groups in the PPNs, which promote the inter-action of CO2 with adsorbents. The adsorbate–adsorbentinteraction was also revealed by the isosteric heat of adsorption(Fig. S7†). At zero loading, the heat of adsorption of PPN-81reached 72 kJ mol�1, which is higher than that of PPN-80 (at54 kJ mol�1). The higher heat of adsorption for PPN-81 isascribed to the higher degree of polymerization, which is alsodemonstrated by the results of IR and elemental analysis. Inother words, the total concentration of amine groups in PPN-81is higher than that in PPN-80. With the increase of CO2 uptake,the heat of adsorption declined progressively (Fig. S7†), which

3254 | J. Mater. Chem. A, 2015, 3, 3252–3256

may be caused by the continuous occupation of active sites.Regardless of the temperatures, CO2 uptake on PPN-81 isobviously higher than that on PPN-80. For instance, the uptakeof CO2 was measured to be 84.9 mg g�1 on PPN-81 at 295 K and1 bar while only 71.2 mg g�1 on PPN-80. The adsorption capacityof PPN-81 is comparable to that of some reported adsorbentsunder similar conditions such as porphyrin porous polymerCuPor-BPDC (31.4 mg g�1),27 microporous metal–organicframework {[Ni(L)2]$4H2O}n (33.9 mg g�1),28 porous polymernetwork PPN-6 (53.7 mg g�1),29 and porous electron-richcovalent organonitridic framework PECONF-4 (86.2 mg g�1).30

Unlike CO2, CH4 and N2 are barely adsorbed on the PPNs. At295 K and 1 bar, the uptake of CH4 on PPN-81 was measured tobe only 2.9 mg g�1, and that of N2 was negligible (1.0 mg g�1). Itshould be stated that the uptakes of CH4 and N2 measured onthe present materials are lower than those of normal porousmaterials reported in the literature. These results suggest highselectivities of CO2 over CH4 and N2. The ideal adsorptionsolution theory (IAST) was further employed to estimate theselectivities. In the calculation, a CO2/N2 ratio of 15/85 and aCO2/CH4 ratio of 50/50 were used, which are typical composi-tions of ue gas emitted from coal-red power plants andgeneral feed compositions of landll gas, respectively. BothPPNs exhibited very high selectivities of CO2/CH4 and CO2/N2,but the selectivities on PPN-81 were measured to be generallyhigher than those on PPN-80. The magnitudes of the selectiv-ities are quite marked: the selectivity of CO2/CH4 on PPN-81reached 1428 at 295 K and 1 bar, and the selectivity of CO2/N2

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reached as high as 4716. To our knowledge, PPN-81 exhibits thegreatest selectivities of CO2 over both CH4 and N2 among thereported materials. The CO2/CH4 selectivity (1428) is apparentlyhigher than that of some well-known materials such as metal–organic framework SIFSIX-3-Zn (231),3 Mg-MOF-74 (137),31

zeolite 13X (103),32 and porous aromatic framework PAF-30(63).33 Similarly, the CO2/N2 selectivity (4716) is also higher thanthat of typical materials, including SIFSIX-3-Zn (1818),3

Mg-MOF-74 (352),31 PPN-6-CH2DETA (442),34 and 13X (220).32

Dynamic breakthrough curves are pretty useful to evaluate anadsorbent.35–37 Gas mixtures, specically CO2/N2 (15/85) andCO2/CH4 (50/50), were used for column breakthrough curveexperiments. In the case of both mixtures, the breakthrough ofCO2 is obviously later than those for N2 and CH4 (Fig. S8†). Theseresults thus conrm the static adsorption results, pointing outthe selective adsorption of CO2 on the present materials.

The recyclability of adsorbents was investigated due to itsimportance in practical applications. No loss of activity wasobserved even aer six cycles, which indicates the excellentrecyclability of our materials (Fig. 3). It is worthwhile noting thatthe saturated adsorbents can be regenerated at only 60 �C for 100min. The mild regeneration conditions are due to the use ofsecondary amines as building blocks, which provides a properCO2–adsorbent interaction. The regeneration of our materials ismore energy efficient than are most reported adsorbents such asPPN-6-CH2DETA (100 �C) tethered with primary amines34 andmicroporous carbon doped with nitrogen (150 �C).38

By use of a nucleophilic substitution reaction of chloromethylbenzene and ethylene diamine, two PPNs were successfullyconstructed. The reaction of chloromethyl benzene with aminegroups resulted in the formation of new C–N bonds. As a result, agreat number of secondary amines were generated and func-tioned as bridges to connect benzene rings (Scheme 1A). Acontinuous spatial network was thus fabricated. The network ismade up of not only rigid groups (benzene rings) but also exiblelinkages (C–C and C–N single bonds). In the synthetic systemcontaining the template, the interaction between PPN precursorsand template molecules may be built via hydrogen bonding(Scheme 1B).39,40 The template thus plays a directing role, makingthe assembly and growth of PPN proceed in the continuoussolvent phase between template molecules. Hence, the meso-porous PPN-81 was constructed. In comparison with the aminesof PPN-80, those of PPN-81 are more accessible owing to the highporosity. The better accessibility of active sites, together with the

Fig. 3 Cycling adsorption of CO2 over PPN-81 at 295 K.

This journal is © The Royal Society of Chemistry 2015

higher polymerization degree, is believed to be responsible forthe better adsorption performance of PPN-81 for CO2 capture.

Despite many dedicated efforts, achieving a cost-efficientsynthesis of PPNs has been a challenge with regards to mono-mer, catalyst, and reaction conditions. In this study, wedesigned a new strategy to construct PPNs via a facile nucleo-philic substitution reaction for which both monomers areinexpensive and readily available. More importantly, the poly-merization reaction can occur under mild conditions withoutthe addition of any catalyst. By use of a templating method, theporosity of the material can be obviously improved, whichincreases the access to active sites in frameworks. It is inter-esting to note that the frameworks of PPNs are comprised ofabundant secondary amines, which offer appropriate adsor-bate–adsorbent interactions that are benecial to selectiveadsorption and energy-saving regeneration of the adsorbents.As a result, the present PPNs are highly active for selectiveadsorption of CO2, and unprecedented high CO2/CH4 and CO2/N2 selectivities were achieved. Furthermore, the materials canbe completely regenerated under quite mild conditions. Thecost-efficient synthesis, outstanding adsorption performance,and energy-saving regeneration make our materials highlypromising in adsorptive separation of CO2 from mixtures suchas ue gas and natural gas.

Conclusions

Two porous polymer networks, namely PPN-80 and PPN-81,were fabricated via a facile nucleophilic substitution reaction ofchloromethyl benzene and ethylene diamine. The presence of atemplate in the synthetic system can promote the formation ofpolymer with enhanced porosity and subsequently superioradsorption performance. The plentiful secondary amines in theframeworks endow the obtained PPNs with excellent capacity inselective adsorption and energy-saving regeneration of theadsorbents. By judicious choice of monomers, the presentstrategy should enable secondary amines to be introduced toframeworks with various pore structures, resulting in theconstruction of new porous polymer networks that have highpotential for applications in adsorption and catalysis.

Experimental section

The PPNs were synthesized by a nucleophilic substitutionreaction of 2,4,6-tris(chloromethyl)mesitylene (namely M1) withethylene diamine (namely M2). In a typical process, M1 (0.561 g,2 mmol) was dissolved in THF (50 mL) followed by the additionof M2 (0.180 g, 3 mmol). The obtained solution was then heatedin a nitrogen atmosphere at 63 �C for 24 h. Aer cooling to roomtemperature, the reaction mixture was centrifuged to removesolvent, and the precipitate was treated with an ethanol–water(20 mL/20 mL) solution of KOH (0.504 g) at 50 �C for 12 h. Thematerial was then washed with an ethanol–water solution threetimes and dried at room temperature. The obtained whitepowder was denoted as PPN-80. In a similar process, PPN-81was synthesized by the addition of triblock copolymer P123(0.5 g) to the initial solution containing monomers. Static

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adsorption experiments of CO2, CH4, and N2 were conductedusing an ASAP 2020 system. Adsorption–desorption isothermsof CO2, CH4, and N2 at 273 K were measured in an ice-waterbath, while isotherms at 283 and 295 K were measured in awater bath.

Acknowledgements

This work was supported by the National Basic ResearchProgram of China (973 Program, 2013CB733504), the NationalHigh Technology Research and Development Program of China(863 Program, 2013AA032003), Distinguished Youth Founda-tion of Jiangsu Province (BK20130045), the Fok Ying-TongEducation Foundation (141069), and the Project of PriorityAcademic Program Development of Jiangsu Higher EducationInstitutions.

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