Microporous organic polymers as CO2 adsorbents: advances and challenges

Materials Today Advances 6 (2020) 100052

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Microporous organic polymers as CO2 adsorbents: advances and challenges C. Xu a, G. Yu b, J. Yuan c, M. Strømme a, N. Hedin c, * €m Laboratory, Uppsala University, 751 21, Uppsala, Nanotechnology and Functional Materials, Department of Materials Science and Engineering, Ångstro Sweden b College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China c Department of Materials and Environmental Chemistry, Stockholm University, SE-10691, Stockholm, Sweden a

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 September 2019 Received in revised form 3 December 2019 Accepted 11 December 2019 Available online xxx

Microporous organic polymers (MOPs) with internal pores less than 2 nm have potential use in gas separation, sensing, and storage, in the form of membranes, monoliths, fibers, or adsorbent granules. These covalently bonded polymers are being formed by reacting with rigid organic monomers, and MOPs have lately been studied for capturing CO2 from gas mixtures in the form of membranes and adsorbents. Especially, the potential of MOPs in the processes of carbon capture and storage has been in the focus and small pore MOPs are preferred for regular separation processes but larger pores could be suitable if cryogenic processes would be used. Recent studies (2014 e mid 2019) on the potential use of MOPs as CO2 adsorbents and, to some degree, CO2-selective membranes are reviewed. © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Keywords: Microporous polymers Ionic microporous polymers CO2 capture Adsorption Heat of sorption

1. Introduction The global emissions of greenhouse gases (GHGs) are increasing and with those the risks for irreversible changes to the environment. Numerous approaches are needed to decrease the emissions of GHGs, and among these carbon capture and storage (CCS) is commonly judged to be important [1]. CCS comprises processes for capture, transport, and storage of CO2 and its principles are straightforward. However, it has not yet been commercially implemented because of the lack of global policies, negative public perception of CO2 storage, association to the fossil-fuel industry, and the costly capture step of CCS. The potential of using microporous (pores < 2 nm) organic polymers (MOPs) in CCS should first be analyzed in relation to processes for capturing CO2 at existing point sources by retrofitting separation processes to large power plants, cement and steel factories, and pulp mills. The industry standard for CO2 capture is the use of costly amine scrubbers that could entail environmental concerns with respect to amine emissions [2e4]. Using MOPs as advanced functional materials in the adsorption- or membrane-

* Corresponding author. E-mail address: [email protected] (N. Hedin).

based capture of CO2 instead of amine scrubbers could lower down the critically high operational costs for the CO2 capture by being more energy efficient [5e7]. More energy-efficient processes make investment decisions less risky and also the global energy balances of the power plants or industrial processes more attractive. Alternative processes for CO2 capture at point sources include those based on adsorption, partitioning over polymer membranes, and combinations thereof. The cost reduction prospects for membrane-based separation processes are in principle advantageous, as they would not necessarily involve a phase change in the membrane. The prospects of using hollow fibers, such as the Matrimid® 5218, to capture CO2 from gas mixtures in combination with cryogenic separation are promising. It seems that the capital expenses associated with the recovery of CO2 can be lowered using cryogenic conditions [8,9]. Irrespective of the temperature used, the properties of the MOPs in the form of hollow fibers, granules, and so on are critical to the cost of the CO2 separation. In general, the MOPs should have high CO2-over-N2 selectivities, low costs, small environmental footprints and allow for high fluxes of flue gas and good mechanical and chemical resistances. In light of our expertise, this review is mainly focused on the prospects of using MOPs as CO2 adsorbents but instances of the potential use of MOPs in CO2 separation membranes are also being highlighted.

https://doi.org/10.1016/j.mtadv.2019.100052 2590-0498/© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

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C. Xu et al. / Materials Today Advances 6 (2020) 100052

2. Advances and challenges for MOPs as adsorbents for CO2 The composition and temperature of emitted gases at large point sources of CO2 vary but they consist of dilute CO2 in mainly N2 at ambient pressure with a temperature of typically 323 K. Upon contact with such gas mixtures, CO2 will preferably adsorb over N2 on the internal surfaces of the MOPs. In general, the adsorption of CO2 occurs via physisorption or chemisorption. Under physisorption, the gas-solid interactions are dominantly weak with contributions from, for example, electric field gradient (efg)eelectric quadrupole moment interactions and van der Waals interactions. Chemisorption of CO2 involves electronic reconfiguration of CO2 during the interaction with the solid surface or porous polymer backbone. The heat of adsorption (Qst) of CO2 and CO2-over-N2 selectivity is higher for the MOPs that operate on chemisorption than on physisorption and such MOPs are typically regenerated thermally in temperature swing adsorption (TSA) processes. In turn, TSA is applicable when the CO2 concentration is low. Relevant gas mixtures with low concentrations of CO2 are found in natural gas, flue gas of natural gas power plants, and indoor air. MOP-based chemisorbents typically contain aliphatic amines and operate in analogy with CO2eamine system used in amine scrubbers. With the assistance of solid-state 13C NMR and in situ infrared spectroscopy, recent studies have revealed that CO2 chemisorbs on aminemodified sorbents as carbamic acid, bicarbonates, and ammonium-carbamate ion pairs depending on the amine density, presence of water, amine-amine distance, and so on. [10e12] Since the seminal study of Lu et al. [13] on amine tethered porous polymer networks, several synthetic strategies (i.e., chemical modification, physical impregnation) have been adopted to introduce alkyl amine species into MOPs aiming to increase their CO2 capture performance (Table 1) [12,14e21]. It should be noted that although the amine-modified MOPs have high CO2 adsorption capacities and high CO2-over-N2 selectivities, the high Qst (up to 80 kJ/mol) would result in a large energy use during the regeneration of the sorbents. In addition, the adsorption kinetics and the cyclic adsorption performance are other concerns. In this context, it would be desirable to balance the effects of chemisorption and physisorption of CO2 by, for instance, tuning the amine densities and introducing amines with appropriate basicity in the MOPs to achieve both high separation and energy efficiencies [18]. MOPs that operate on physisorption of CO2 can be used in vacuum swing adsorption (VSA) and pressure swing adsorption (PSA) processes, and the processes put different requirements on the adsorbents. In VSA processes, the CO2 capacity at a pressure of 0.05e0.15 bar at a temperature of about 323 K is of large importance [26,27]. In PSA processes, it will be the effective CO2 capacity over the pressure swing range (typically several bars) that is of large importance. One argument against the applicability of PSA processes for the CO2 capture at large point sources relates to the compression of large volumes of gas containing mainly N2. The flow rate of the flue gas from a normal sized coal-fired power plant is very large, around 200e500 m3/s [28,29]. Many of the CO2 adsorption studies of MOPs have, hence, been focused on their capacity to capture CO2 at low pressures with a VSA process implicit in mind. It is worth noticing that most studies have been conducted at a standard temperature of 273 K. This choice may be relevant for the sake of comparison; however, the real temperature is higher which drastically reduces the actual CO2 adsorption capacity. In the linear regime of the CO2 adsorption isotherm, the uptake of CO2 will only be about 13% for a MOP at an industrially relevant temperature of 323 K as compared with 273 K when assuming a Qst of 30 kJ/mol, which is typical for physisorption. For MOPs with very small pores [22,30e33], the loss in capacity at 323 K would be smaller as the

corresponding CO2 adsorption isotherm will curve in the pressure regime of 0e0.15 bar of CO2 at a temperature of 273 K. In general, MOPs with very small pores could be suitable for the VSA-driven CO2 capture, and the CO2 capacity may be quite significant even though they do not have ultrahigh surface areas [34e37]. The CO2over-N2 selectivity of MOPs with very small pores can be enhanced kinetically as N2 is expected to diffuse slower in the MOPs than CO2. Kinetically enhanced selectivity of adsorbents is used in the production of N2 from air by using carbon molecular sieves [38], however, its applicability for high flow rates is not yet established. To use the effect of kinetics for the removal of CO2 from CH4 in the low-flow upgrading of raw biogas seems uncomplicated with a suitable sorbent. For high flow applications such as flue gas capture of CO2, it is critical to assure that the intraparticle diffusion rate of CO2 is high for the kinetically active sorbent and that the adsorbed CH4 that might eventually build up in the sorbent can be recovered by intermittent regeneration. Several of the MOPs that contain small micropores have been shown to display relatively high and kinetically enhanced CO2-over-N2 selectivity with values of about 100 [39e47]. Zhang et al. [48] prepared a set of benzoxazole-based MOPs with tunable micropore sizes. The CO2-over-N2 selectivity for the MOPs increased from 69 to 97 by decreasing the average pore size from 1.4 to 1.0 nm. On the other hand, the thermodynamic CO2over-N2 selectivity of a MOP (linear regime) is dependent on the Qst of CO2 and N2 and the shapes of the isotherms, and the Qst (CO2) is roughly 20e40 kJ/mol. The values for Qst (N2) have not been determined for MOPs to our best knowledge. They are expected to be low and maybe about 12 kJ/mol that has been reported for porous carbons [49]. Yassin et al. [50] synthesized a family of cyanovinyl-based MOPs and observed that the values of Qst (CO2) were proportional to the density of the cyanovinyl groups. By increasing the cyanovinyl content from 0 to 50.4%, the Qst increased from 20.9 to 40.3 kJ/mol. As indicated by theoretical calculations, the sharp efgs induced by the cyanovinyl groups in the MOPs served as high-energy sites for the adsorption of CO2. Consequently, the CO2-over-N2 selectivity was significantly increased (from 14 to 96) when the cyanovinyl content was increased as aforementioned. The revealed structure-thermodynamic-property relationships for MOPs provided a consistent description. Also the combination of kinetic and thermodynamic effects can enhance the CO2 capture performance of MOPs, and they are not always easy to separate. For example, The studies by Arab et al. [23], Abdelmoaty et al. [51], Rabbani et al. [52], Sekizkardes et al. [53], and Islamoglu et al. [54] developed a range of MOPs consisting of Nrich building units (i.e., aminal, imidazole, thiazole, oxazole, triazine, aniline, azo). Both small pores and N-containing species in these MOPs seemed to have enhanced the efgs in the MOPs, which in turn promoted a strong interaction with CO2. The MOPs had high CO2 adsorption capacities at 0.05e0.15 bar and high CO2-over-N2 selectivity of up to 95, here, the small pores could have enhanced the selectivity also by kinetics. By introducing N- and O-containing moieties as efg-enhancing (CO2-philic) sites in hexaazatriphenylene-based conjugated triazine frameworks, relatively high CO2 adsorption capacities of up to 2.0 mmol/g at 0.15 bar were observed at a temperature of 297 K [24]. Huang et al. [25,55] also varied the efgs of covalent organic frameworks (COFs; a type of crystalline MOPs with atomically ordered frameworks and porous channels) by elaborated chemical manners. The enhanced Qst with values up to 43.5 kJ/mol resulted in significantly enhanced CO2over-N2 selectivity of up to 323, which was comparable with those of top performing CO2 sorbents (i.e., CuBTC, Mg-MOF-74, NaX zeolite). The studies mentioned previously have demonstrated that pore surface engineering in MOPs through, for instance, predesign of the monomers, selection of appropriate polymerization reactions, postmodification, is a powerful approach to exploit high

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Table 1 Selected microporous organic polymers and their CO2 capture performances. Sorbent

SBET (m2/g)

Pore size (nm)

CO2 uptake at 273 K (mmol/g) 0.05 bar

CO2/N2 selectivity

Qst (kJ/mol)

Ref.

0.15 bar

1 bar

Amine-modified MOPs (chemisorbents) 435 0.56 0.76

1.36

3.0

97a

68

[16]

72

1.7

1.13

1.43

2.30

1040b

80

[12]

100d

e

0.91

1.16

1.80

10140c

75

[15]

555

1

2.60e

3.0e

4.3e

555b

55

[13]

Ultramicroporous/functionalized MOPs (physisorbents) 1108 0.6 0.73 1.79

5.40

40a (35c)

33

[22]

1235

1.0

0.86

1.84

5.36

35a (40c)

29

[23]

1090

0.35e0.90

1.60

3.0

6.30

183c

27

[24]

364

1.40

0.63

1.46

3.95

323c

44

[25]

MOPs, microporous organic polymers. a Initial slope selectivity. b Simplified ideal adsorbed solution theory (IAST) selectivity. c IAST selectivity. d Calculated from CO2 adsorption isotherm at 273 K. e Recorded at 295 K.

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performance CO2 sorbents by increasing the efgs or by inducing sites with a specific tendency to interact with CO2. If the flue gas can be compressed, or integrate with cryogenic cooling, economically in a similar manner as for membrane-based CO2 separation [8,9], then the properties of the optimal MOPs would be quite different as compared with those suitable for VSAbased separation at 323 K. For PSA or cryogenic VSA/PSA processes, it would be the effective CO2 capacity at high pressures or low temperatures that would be the most defining parameters of the MOPs. A high-pressure CO2 adsorption capacity can be assured by a high specific surface area of the MOP. It has been shown that by using rigid and tetrahedrally shaped monomers one can prepare MOPs with ultrahigh surface areas of over 6000 m2/g with associated very high CO2 adsorption capacities of up to 48.2 mmol/g at 295 K and 50 bar [56]. Alternatively, by introducing flexible skeletons into the MOPs one could effectively increase the CO2 adsorption capacity at high pressures by physical swelling [57,58]. In addition, it is important to note that MOPs with a very high volumetric capacity (the volume of the sorbate adsorbed per volume of the sorbent) are ideal in processes as they can contribute to reduce the size of the columns [59]. 3. Classification and synthesis of MOPs as CO2 sorbents There are numerous ways of categorizing the MOPs, and no standard has been agreed on; however, a first natural division is to single out the linear MOPs from network-based ones. The microporosity in linear MOPs emerges from the high free volume stemming from the contorted building blocks leaving the polymers porous. Since Budd et al. [60] developed the first polymer of intrinsic microporosity (PIM); PIM-1, several PIMs linked by amide € ger's base [63], aromatic units [64] and their [61], imide [62], Tro modified versions [65e67] have been reported. Although most of the PIMs do not show outstanding CO2 adsorption capacities, their typically good solubility in organic solvents makes them processable into membranes for the separation of CO2 from mixed gases [68,69]. Recently, several studies of mixed matrix membranes (MMMs) based on PIMs and CO2-philic fillers (i.e., metal organic frameworks (MOFs) [70,71], silica nanoparticles [72], polymers [73], porous carbons [74]) have aimed to break the Robeson upper bounds for CO2/N2 and CO2/CH4 separation, assuring both high permeability and selectivity. In addition, it has been demonstrated that by introducing functional polar groups in either the PIMs or fillers, the intermolecular interactions between the components were enhanced and thus the rigidity of the membranes [70,75,76]. As a result, the rigidified membranes showed a significantly enhanced aging and operation stability as compared with the PIM1 membranes. Enhanced stability is of great importance for practical applications. Another group of MOPs that have been studied since along time are the hyper-cross-linked polymers (HCPs) [77]. They have already been commercialized for applications such as solvent cleanup and so on. Friedel-Crafts alkylation and Scholl coupling reaction are common and powerful approaches for the synthesis of HCPs [78,79]. Owing to the general applicability of the reaction, almost all of the aromatic compounds and heterocycles can be used as monomers to construct the HCPs. The advantages of HCPs over other porous materials are in their synthetic diversity and potential € rnerba €ck and Hedin synthesized a class of low cost. Recently, Bjo HCPs from biobased molecules in sulfolane [80]. The resultant HCPs showed tunable pore structures with surface areas of 610e1440 m2/g and large CO2 uptakes of 2.6e4.0 mmol/g (273 K, 1 bar). The economical and environmentally friendly synthesis of biobased HCPs provides a sustainable approach for the development of MOPs.

Covalent triazine frameworks (CTFs) are a new type of MOPs linked by triazine units [81]. Because of their high stability, rich microporosity, and abundant nitrogen content, CTFs are being considered as promising candidates for CO2 capture [26,82,83]. Since Kuhn et al. [84] synthesized a family of CTFs by trimerization of aromatic nitrile monomers under ionothermal conditions, several groups have dedicated significant efforts in developing new CTF materials for various applications. Apart from ionothermal trimerization reactions taking place under harsh conditions, a range of mild synthesis strategies including superacid catalyzed reactions [85,86], Friedel-Crafts reactions [87], polycondensation reactions [88] have been recently developed for the synthesis of CTFs. For example, Wang et al. [89] synthesized a new family of CTFs (CTF-Huazhong University of Science and Technologies via polycondensation of aldehyde and amidine dihydrochloride monomers at relatively low temperature (120
C) and ambient pressure and atmosphere. The resultant CTFs showed high CO2 adsorption capacities up to 3.16 mmol/g at 273 K and 1 bar. More significantly, the synthesis procedures can be easily scaled up to prepare CTFs in gram quantities, which is beneficial to practical applications. Condensation reactions are most common in the synthesis of MOPs and can be performed with and without the use of catalysis. For example, the condensation reactions between amines and aldehydes have been used to synthesize MOPs being linked by imine [36], aminal [90,91], and benzimidazole [52] units. The N-containing species and the ultramicropores in these MOPs could enable strong interaction with CO2, which is as mentioned of great advantage in the CO2 capture at low pressures. Notably, condensation reactions that are reversible and under kinetic control allow construction of crystalline COFs possible. In this context, COFs with predesigned architectures, tunable porosities, and optimized CO2 separation performances can be synthesized in terms of reticular chemistry and crystal engineering [92]. The catalytic coupling chemistries are well developed within organic chemistry and have also been used extensively for the synthesis of MOPs. A class of conjugated microporous polymers and porous aromatic frameworks has been synthesized by Yamamoto, Sonogashira, and Suzuki coupling reactions, and so on. [93,94] These polymers usually possess high specific surface areas because of their high degree of polymerization and rigid 2D or 3D networks. Apart from these porous polymers with CeC linkages, recent studies have used other coupling reactions (i.e., Buchwald-Hartwig coupling, oxidation coupling, diazocoupling reaction) to enable the synthesis of MOPs with azo [95e97], secondary amine [98,99], and polycarbazole [100,101] linkages (Table 2). An alternative way to enhance the efgs in MOPs is to decorate them with ionic moieties. The corresponding ionic microporous organic polymers (IMOPs) have attracted a growing interest in the field of CO2 capture. The incorporated ion pairs are either covalently bonded to the pore wall or attached to it with either cations or anions as pendant units. As such, their physicochemical properties, functionalities, and active sites for CO2 binding, can be regulated through pore surface engineering, that is, the screening of building units to backbones, pendant groups, and ionic tectonism. Several key design requirements are generally being considered for the IMOPs, which include the pore parameters, host-guest interactions, and charge effects. The high surface area and large pore volume of the IMOPs assure many accessible binding sites for CO2 and sufficient accommodating space, which are considered crucial to a high CO2 uptake capacity [105e107]. Generally, also for IMOPs, the introduction of rigid and contortive building blocks into the polymer backbone is the most frequently used approach to achieve high porosity. However, synthesizing IMOPs with high surface areas and small pores is far more challenging than for neutral MOPs because

C. Xu et al. / Materials Today Advances 6 (2020) 100052

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Table 2 Selected reactions for the synthesis of microporous organic polymers. Chemical equations

Ref. [63]

[102]

[52,103]

[104]

[23]

[98]

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of the strong electrostatic repulsion that emerges in the micropores. Small pores can be introduced by using hard or soft templates, which have been popular to use from a synthetic point of view, however, their uses are time and energy intensive. It is of our opinion that a template-free approach is more favorable, such as the postmodification of presynthesized nonionic MOPs. For this purpose, various synthetic strategies including coupling reactions (i.e., Sonogashira reaction [108,109], Suzuki-Miyaura crosscoupling reaction [110], Yamamoto reaction [111,112], and Eglinton coupling [113]), Friedel-Crafts reaction [114], free-radical polymerization [115], trimerization reaction [116], and Schiff-base reaction [117] can be used. IMOPs with polar pore surfaces have strong efgs, which enhance the efg-quadrupole interactions between the frameworks and guest CO2 molecules [118]. As for the other MOPs, the incorporation of heteroatoms (i.e., N, O, P, S, and B) or CO2-philic ionic moieties into the framework/pore surface through direct polymerization or postsynthesis modification would be beneficial for the CO2 capture. The charges themselves enhance the efgs as well [119]. It seems that tetraalkylammonium cations have a stronger interaction with CO2 than imidazoliums as the latter have a delocalized positive charge [120]. Similarly, the adsorption ability of CO2 also can be affected by the nature of the anion. All in all, the ions in the IMOPs can promote the CO2 adsorption, and a proper design of the IMOPs is especially important [55]. The IMOPs have both covalent and ionic bonds, which allow the pores and interaction potentials with CO2 to be controlled and enhanced. Till now, a series of IMOPs have been studied for achieving a high CO2 uptake and selectivity and their pore-surface functionalities have been moderated by backbone modification, adjustment of the pendant groups, and counter-ion exchange. Both direct polymerization and postsynthetic modification have been used. With direct polymerization, several types of IMOPs with backbone building units including cationic [121], anionic [122], and zwitterionic moieties [123] have been studied. Note that the CO2 adsorption performance would be readily tuned by controlling the ionic moieties of the backbone. For instance, IMOPs synthesized from vinyl-functionalized quaternary phosphonium salts through freeradical polymerization reaction (Table 3) have exhibited high surface areas (up to 758 m2/g) and high CO2 adsorption capacity (2.23 mmol/g at 273 K) [124]. Luo et al. reported on a series of IMOPs being synthesized via a one-pot free-radical copolymerization of ionic vinylimidazolium-based metallosalen moieties with divinylbenzene as a cross-linker under solvothermal polymerization conditions [125]. The obtained polyvinylimidazolium-based metallosalen IMOPs (DVB@ISA) had a surface area of 590 m2/g, pore volume of 0.55 cm3/g, and high CO2/N2 adsorptive selectivity. Liu and Landskron [126] synthesized anionic IMOPs with covalently bonded phosphate groups in the backbone phosphate porous organic frameworks (PA-POFs) through the Yamamoto coupling reaction. These PA-POFs showed a high surface area of up to 2408 m2/g and excellent CO2 adsorption capacity of up to 4.59 mmol/g at 273 K and 1 bar. This excellent CO2 capture ability of these IMOPs could be ascribed to the high surface area and intrinsic high CO2 affinity of the ion pairs in the IMOPs. In contrast to direct polymerization, CO2-philic organic functionalization can be more easily performed postsynthetically, which potentially can take the advantage of an existent neutral MOP having a high surface area. Chen et al. prepared IMOPs via the phenol-formaldehyde condensation chemistry [127], where the neutral framework of the as-synthesized FIP-IM@QA was postsynthetically modified with quaternary ammonium units using the Williamson ether reaction. The ionized IMOP showed a CO2 adsorption capacity up to 0.9 mmol/g, despite of its low surface area (10 m2/g) after the surface modification (Table 3).

The ability to alter the counter ions of IMOPs is a distinctive advantage that allows the properties of CO2 capture and selectivity without changing the polymer backbone to be engineered. Direct polymerization via ionic monomers containing different anions is one strategy to alter the counter ions of IMOPs. Xie et al. [128] prepared a series of sponge-like IMOPs with different anions (BF4, PF6, and Cl), which were synthesized by quaternary ammoniumbased monomers via a free-radical polymerization reaction. The CO2 sorption capacity of the IMOPs was reduced in the order of BF4 > PF6 > Cl. Ion exchange of already prepared IMOPs is another approach to tune the CO2 adsorption performance. Fischer et al. [112] synthesized two IMOPs (CPN-1-Br and CPN-2-Br) with tetrakis (4-bromophenyl) methane using the Yamamoto coupling and Songashira-Hagihara reaction. The initial prepared polymers CPN1-Br and CPN-2-Br exhibited high surface areas of 1455 and 540 m2/ g and good CO2 uptakes of 2.49 and 1.55 mmol/g at 273 K and 1 bar, respectively. The CPN-1-Br was anion exchanged into the CPN-1-Cl, which had an enhanced CO2 uptake (2.85 mmol/g), being consistent with the properties of the smaller anion (Cl). Rukmani et al. [129] studied the CO2 adsorption and selectivity of the ionic and functionalized PIM-1 (IonomIM-1) with counter ions being Liþ, Naþ, Kþ, Rbþ, or Mg2þ. The counter ion type enabled the CO2 adsorption capacity and selectivity for CO2/CH4 and CO2/N2 gas mixtures to be tuned. Overall, it is clear that the type of counter ion influences the CO2 adsorption performance of the IMOPs. In short, IMOPs are promising adsorbents for CCS. Generally, they feature a higher CO2 capture ability than their neutral analogs of similar surface areas owing to the enhanced efgs, and we summarized important strategies to the design and methods to engineer the pore surfaces. Challenges, important to overcome in the future, are that IMOPs typically have low surface areas, broad pore size distributions, and are difficult to obtain directly by using coupling reactions. 4. Processing and shaping of MOPs Processing of powders of network polymerebased MOPs into functional forms is critical to their applications in gas separation processes. Except for a few soluble MOPs such as PIMs with linear structures, they are usually insoluble in common solvents and have no observable glass transition temperatures because of the rigid, conjugated, or highly cross-linked structures. Therefore, solution and melt processing used for conventional polymers are not suitable for the processing of MOPs. The weak processability of MOPs has largely hampered their practical applications. Only recently, this issue has been started to be addressed by researchers. A number of MOP aerogels have been successfully synthesized from the corresponding organogels via freeze-drying, subcritical, or supercritical drying [130e132], which are powerful methods to prepare low-density materials. Certain polymerization reactions that have been performed without stirring or agitation have been shown successful to form directly MOPs as aerogels and monoliths [133,134]. In addition, flexible and freestanding MOP membranes have also been recently developed [135,136]. Noteworthy, 3Dprinting techniques have been shown to hold great potential for the structuring and processing of MOPs [137,138]. Such structured MOP materials can not only be used as sorbents for CO2 capture but also have a great potential for applications in nanofiltration, heterogeneous catalysis, oil cleanup, solar steam generation, artificial muscles, and so on. 5. Summary The fully organic backbone of MOPs and the possibilities of the detailed control achieved by polymer and organic chemistry render

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Table 3 Selected reactions for the synthesis of microporous organic polymers. Chemical equations

Ref. [124]

SBET: 402e758 m2/g; Q (CO2)a: 1.88  2.23 mmol/g [126]

SBET: 478e2408 m2/g; Q (CO2)a: 2.5  4.59 mmol/g [127]

SBET: < 10 m2/g; Q (CO2)a: 1.45 mmol/g [112]

SBET: 540 m2/g; Q (CO2)a: 1.55 mmol/g a

CO2 adsorption capacity at 273 K and 1 bar.

the MOPs with certain advantages over zeolites, activated carbons, and MOF compositions, as adsorbents for CO2. Their relatively low hydrophilicity, high surface area, and low densities are of great advantage when the MOPs are used to support amines for chemisorption of CO2. Decorating the internal pores of the MOPs by efgenhancing groups such as the ionic species of the IMOPs enhances the physisorption abilities of the MOPs. The recent progress made in the processing and shaping of MOPs could potentially boost their practical applications in the CO2 capture and other fields. The high cost of the MOPs is still a big obstacle for their large-scale preparation and the road to industrialization. In addition, most reported MOPs have been synthesized in organic solvents at elevated temperatures with catalysts (e.g., noble metal catalysts, organic catalysts), followed by procedures of filtration, purification,

and drying. Obviously, the current synthesis technologies for MOPs are not economically and environmentally friendly. In this regard, further exploration of inexpensive and more sustainable monomers and facile and green synthesis routes are greatly being desired. In contrast to crystalline porous materials such as MOFs and zeolites, most MOPs have been obtained in amorphous forms. Except for a few examples of COFs and CTFs synthesized under delicate conditions, it is still challenging to create ordered structures in MOPs, which has hampered the design of MOPs with predetermined pore structures. It would be highly advantageous to develop advanced simulation models allowing precise prediction of the pore size and the pore geometry in MOPs at the molecular level, which could guide the design of MOPs with high performances in CO2 capture.

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At the moment, it is still unclear if the MOPs are most promising as gas separation membranes for CO2 separation, adsorbents for VSA-based capture, or adsorbents for cryogenic swing adsorption processes. The ideal properties of the MOPs are very different for these three applications. Hence, it is crucial that the engineering and chemistry communities stay in close contact when further developing the MOPs for CO2 separation purposes. 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 article. Acknowledgments CX thanks the ÅForsk, Sweden (19-493), NH the Swedish Research Council (2016-03568), and JYY the ERC (Starting Grant NAPOLI-639720). References [1] M.E. Boot-Handford, J.C. Abanades, E.J. Anthony, M.J. Blunt, S. Brandani, N. Mac Dowell, J.R. Fern andez, M.-C. Ferrari, R. Gross, J.P. Hallett, R.S. Haszeldine, P. Heptonstall, A. Lyngfelt, Z. Makuch, E. Mangano, R.T.J. Porter, M. Pourkashanian, G.T. Rochelle, N. Shah, J.G. Yao, P.S. Fennell, Energy Environ. Sci. 7 (2014) 130e189. [2] G.T. Rochelle, Science 325 (2009) 1652e1654. [3] L.M. Romeo, I. Bolea, J.M. Escosa, Appl. Therm. Eng. 28 (2008) 1039e1046. [4] B. Dutcher, M. Fan, A.G. Russell, ACS Appl. Mater. Interfaces 7 (2015) 2137e2148. [5] C. Xu, N. Hedin, Mater. Today 17 (2014) 397e403. [6] W. Wang, M. Zhou, D. Yuan, J. Mater. Chem. A 5 (2017) 1334e1347. € rnerba €ck, N. Hedin, Sci. China Chem. 60 [7] T.L. Church, A.B. Jasso-Salcedo, F. Bjo (2017) 1033e1055. [8] L. Liu, E.S. Sanders, S.S. Kulkarni, D.J. Hasse, W.J. Koros, J. Membr. Sci. 465 (2014) 49e55. [9] D. Hasse, J. Ma, S. Kulkarni, P. Terrien, J.-P. Tranier, E. Sanders, T. Chaubey, J. Brumback, Energy Procedia 63 (2014) 186e193. [10] R.W. Flaig, T.M. Osborn Popp, A.M. Fracaroli, E.A. Kapustin, M.J. Kalmutzki, R.M. Altamimi, F. Fathieh, J.A. Reimer, O.M. Yaghi, J. Am. Chem. Soc. 139 (2017) 12125e12128. [11] N. Hedin, Z. Bacsik, Curr. Opin. Green Sustain. Chem. 16 (2019) 13e19. [12] C. Xu, Z. Bacsik, N. Hedin, J. Mater. Chem. A 3 (2015) 16229e16234. [13] W. Lu, J.P. Sculley, D. Yuan, R. Krishna, Z. Wei, H.-C. Zhou, Angew. Chem. Int. Ed. 51 (2012) 7480e7484.  ski, M. Alkordi, M.I.H. Mohideen, Y. Belmabkhout, [14] V. Guillerm, Ł.J. Weselin A.J. Cairns, M. Eddaoudi, Chem. Commun. 50 (2014) 1937e1940. [15] L.-B. Sun, Y.-H. Kang, Y.-Q. Shi, Y. Jiang, X.-Q. Liu, ACS Sustain. Chem. Eng. 3 (2015) 3077e3085. [16] P. Puthiaraj, Y.-R. Lee, W.-S. Ahn, Chem. Eng. J. 319 (2017) 65e74. [17] S. Sung, M.P. Suh, J. Mater. Chem. A 2 (2014) 13245e13249. [18] X. Kong, S. Li, M. Strømme, C. Xu, Nanomaterials 9 (2019) 1020. [19] D. Thirion, V. Rozyyev, J. Park, J. Byun, Y. Jung, M. Atilhan, C.T. Yavuz, Phys. Chem. Chem. Phys. 18 (2016) 14177e14181. [20] J. Tao, Y. Wang, J. Tang, S. Xiong, M.U. Javaid, G. Li, Q. Sun, C. Liu, C. Pan, G. Yu, Chem. Eng. J. 373 (2019) 338e344. [21] Y. Yang, C.Y. Chuah, T.-H. Bae, Chem. Eng. J. 358 (2019) 1227e1234. [22] J. Yan, B. Zhang, Z. Wang, Polym. Chem. 7 (2016) 7295e7303. lu, H.M. El-Kaderi, Chem. [23] P. Arab, M.G. Rabbani, A.K. Sekizkardes, T. slamog Mater. 26 (2014) 1385e1392. [24] X. Zhu, C. Tian, G.M. Veith, C.W. Abney, J. Dehaudt, S. Dai, J. Am. Chem. Soc. 138 (2016) 11497e11500. [25] N. Huang, X. Chen, R. Krishna, D. Jiang, Angew. Chem. Int. Ed. 54 (2015) 2986e2990. [26] S. Hug, L. Stegbauer, H. Oh, M. Hirscher, B.V. Lotsch, Chem. Mater. 27 (2015) 8001e8010. [27] X. Zhang, J. Lu, J. Zhang, Chem. Mater. 26 (2014) 4023e4029. [28] M. Radosz, X. Hu, K. Krutkramelis, Y. Shen, Ind. Eng. Chem. Res. 47 (2008) 3783e3794. [29] Y. Zheng, S. Kiil, J.E. Johnsson, Chem. Eng. Sci. 58 (2003) 4695e4703. [30] C. Klumpen, M. Breunig, T. Homburg, N. Stock, J. Senker, Chem. Mater. 28 (2016) 5461e5470. [31] C. Xu, N. Hedin, Microporous Mesoporous Mater. 222 (2016) 80e86. [32] Sang H. Je, O. Buyukcakir, D. Kim, A. Coskun, Chem 1 (2016) 482e493. [33] B. Lopez-Iglesias, F. Suarez-Garcia, C. Aguilar-Lugo, A.G. Ortega, C. Bartolome, J.M. Martinez-Ilarduya, J.G. de la Campa, A.E. Lozano, C. Alvarez, ACS Appl. Mater. Interfaces 10 (2018) 26195e26205.

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