lable at ScienceDirect

Materials Today Advances 6 (2020) 100052

Contents lists avai

Materials Today Advances

journal homepage: www.journals .e lsevier .com/mater ia ls- today-advances/

Microporous organic polymers as CO2 adsorbents: advances andchallenges

C. Xu a, G. Yu b, J. Yuan c, M. Strømme a, N. Hedin c, *

a Nanotechnology and Functional Materials, Department of Materials Science and Engineering, Ångstr€om Laboratory, Uppsala University, 751 21, Uppsala,Swedenb College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, Chinac Department of Materials and Environmental Chemistry, Stockholm University, SE-10691, Stockholm, Sweden

a r t i c l e i n f o

Article history:Received 20 September 2019Received in revised form3 December 2019Accepted 11 December 2019Available online xxx

Keywords:Microporous polymersIonic microporous polymersCO2 captureAdsorptionHeat of sorption

* Corresponding author.E-mail address: niklas.hedin@mmk.su.se (N. Hedin

https://doi.org/10.1016/j.mtadv.2019.1000522590-0498/© 2020 The Authors. Published by Elsevie

a b s t r a c t

Microporous organic polymers (MOPs) with internal pores less than 2 nm have potential use in gasseparation, 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 MOPshave 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 andsmall pore MOPs are preferred for regular separation processes but larger pores could be suitable ifcryogenic processes would be used. Recent studies (2014 e mid 2019) on the potential use of MOPs asCO2 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/).

1. Introduction

The global emissions of greenhouse gases (GHGs) are increasingand with those the risks for irreversible changes to the environ-ment. Numerous approaches are needed to decrease the emissionsof GHGs, and among these carbon capture and storage (CCS) iscommonly judged to be important [1]. CCS comprises processes forcapture, transport, and storage of CO2 and its principles arestraightforward. However, it has not yet been commerciallyimplemented because of the lack of global policies, negative publicperception of CO2 storage, association to the fossil-fuel industry,and the costly capture step of CCS.

The potential of using microporous (pores < 2 nm) organicpolymers (MOPs) in CCS should first be analyzed in relation to pro-cesses for capturing CO2 at existing point sources by retrofittingseparation processes to large power plants, cement and steel fac-tories, and pulp mills. The industry standard for CO2 capture is theuse of costly amine scrubbers that could entail environmental con-cerns with respect to amine emissions [2e4]. Using MOPs asadvanced functional materials in the adsorption- or membrane-

).

r Ltd. This is an open access articl

based capture of CO2 instead of amine scrubbers could lower downthe critically high operational costs for the CO2 capture by beingmore energy efficient [5e7]. More energy-efficient processes makeinvestment decisions less risky and also the global energy balances ofthe power plants or industrial processes more attractive.

Alternative processes for CO2 capture at point sources includethose based on adsorption, partitioning over polymer membranes,and combinations thereof. The cost reduction prospects formembrane-based separation processes are in principle advanta-geous, as they would not necessarily involve a phase change in themembrane. The prospects of using hollow fibers, such as theMatrimid® 5218, to capture CO2 from gas mixtures in combinationwith cryogenic separation are promising. It seems that the capitalexpenses associated with the recovery of CO2 can be lowered usingcryogenic 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 fluegas and good mechanical and chemical resistances. In light of ourexpertise, this review is mainly focused on the prospects of usingMOPs as CO2 adsorbents but instances of the potential use of MOPsin CO2 separation membranes are also being highlighted.

e under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

C. Xu et al. / Materials Today Advances 6 (2020) 1000522

2. Advances and challenges for MOPs as adsorbents for CO2

The composition and temperature of emitted gases at largepoint sources of CO2 vary but they consist of dilute CO2 inmainly N2at ambient pressure with a temperature of typically 323 K. Uponcontact with such gas mixtures, CO2 will preferably adsorb over N2on the internal surfaces of the MOPs. In general, the adsorption ofCO2 occurs via physisorption or chemisorption. Under phys-isorption, the gas-solid interactions are dominantly weak withcontributions from, for example, electric field gradient (efg)eelec-tric quadrupole moment interactions and van der Waals in-teractions. Chemisorption of CO2 involves electronicreconfiguration of CO2 during the interaction with the solid surfaceor porous polymer backbone.

The heat of adsorption (Qst) of CO2 and CO2-over-N2 selectivity ishigher for the MOPs that operate on chemisorption than on phys-isorption and such MOPs are typically regenerated thermally intemperature swing adsorption (TSA) processes. In turn, TSA isapplicable when the CO2 concentration is low. Relevant gas mix-tures with low concentrations of CO2 are found in natural gas, fluegas of natural gas power plants, and indoor air. MOP-based chem-isorbents typically contain aliphatic amines and operate in analogywith CO2eamine system used in amine scrubbers. With the assis-tance of solid-state 13C NMR and in situ infrared spectroscopy,recent studies have revealed that CO2 chemisorbs on amine-modified sorbents as carbamic acid, bicarbonates, andammonium-carbamate ion pairs depending on the amine density,presence of water, amine-amine distance, and so on. [10e12] Sincethe seminal study of Lu et al. [13] on amine tethered porous poly-mer networks, several synthetic strategies (i.e., chemical modifi-cation, physical impregnation) have been adopted to introducealkyl amine species into MOPs aiming to increase their CO2 captureperformance (Table 1) [12,14e21]. It should be noted that althoughthe amine-modified MOPs have high CO2 adsorption capacities andhigh CO2-over-N2 selectivities, the high Qst (up to 80 kJ/mol) wouldresult in a large energy use during the regeneration of the sorbents.In addition, the adsorption kinetics and the cyclic adsorption per-formance are other concerns. In this context, it would be desirableto balance the effects of chemisorption and physisorption of CO2 by,for instance, tuning the amine densities and introducing amineswith appropriate basicity in the MOPs to achieve both high sepa-ration and energy efficiencies [18].

MOPs that operate on physisorption of CO2 can be used in vac-uum swing adsorption (VSA) and pressure swing adsorption (PSA)processes, and the processes put different requirements on theadsorbents. In VSA processes, the CO2 capacity at a pressure of0.05e0.15 bar at a temperature of about 323 K is of large impor-tance [26,27]. In PSA processes, it will be the effective CO2 capacityover the pressure swing range (typically several bars) that is oflarge importance. One argument against the applicability of PSAprocesses for the CO2 capture at large point sources relates to thecompression of large volumes of gas containingmainly N2. The flowrate of the flue gas from a normal sized coal-fired power plant isvery large, around 200e500 m3/s [28,29]. Many of the CO2adsorption studies of MOPs have, hence, been focused on theircapacity to capture CO2 at lowpressureswith a VSA process implicitin mind. It is worth noticing that most studies have been conductedat a standard temperature of 273 K. This choice may be relevant forthe sake of comparison; however, the real temperature is higherwhich drastically reduces the actual CO2 adsorption capacity. In thelinear regime of the CO2 adsorption isotherm, the uptake of CO2willonly be about 13% for a MOP at an industrially relevant temperatureof 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 pressureregime 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-drivenCO2 capture, and the CO2 capacity may be quite significant eventhough they do not have ultrahigh surface areas [34e37]. The CO2-over-N2 selectivity of MOPs with very small pores can be enhancedkinetically as N2 is expected to diffuse slower in theMOPs than CO2.Kinetically enhanced selectivity of adsorbents is used in the pro-duction 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 thelow-flow upgrading of raw biogas seems uncomplicated with asuitable sorbent. For high flow applications such as flue gas captureof CO2, it is critical to assure that the intraparticle diffusion rate ofCO2 is high for the kinetically active sorbent and that the adsorbedCH4 that might eventually build up in the sorbent can be recoveredby intermittent regeneration. Several of the MOPs that containsmall micropores have been shown to display relatively high andkinetically enhanced CO2-over-N2 selectivity with values of about100 [39e47]. Zhang et al. [48] prepared a set of benzoxazole-basedMOPs with tunable micropore sizes. The CO2-over-N2 selectivity forthe MOPs increased from 69 to 97 by decreasing the average poresize from 1.4 to 1.0 nm. On the other hand, the thermodynamic CO2-over-N2 selectivity of a MOP (linear regime) is dependent on the Qstof CO2 and N2 and the shapes of the isotherms, and the Qst (CO2) isroughly 20e40 kJ/mol. The values for Qst (N2) have not beendetermined for MOPs to our best knowledge. They are expected tobe low and maybe about 12 kJ/mol that has been reported forporous carbons [49]. Yassin et al. [50] synthesized a family ofcyanovinyl-based MOPs and observed that the values of Qst (CO2)were proportional to the density of the cyanovinyl groups. Byincreasing the cyanovinyl content from 0 to 50.4%, the Qst increasedfrom 20.9 to 40.3 kJ/mol. As indicated by theoretical calculations,the sharp efgs induced by the cyanovinyl groups in the MOPsserved as high-energy sites for the adsorption of CO2. Consequently,the CO2-over-N2 selectivity was significantly increased (from 14 to96) when the cyanovinyl content was increased as aforementioned.The revealed structure-thermodynamic-property relationships forMOPs provided a consistent description.

Also the combination of kinetic and thermodynamic effects canenhance the CO2 capture performance of MOPs, and they are notalways 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 ofMOPs consisting of N-rich building units (i.e., aminal, imidazole, thiazole, oxazole,triazine, aniline, azo). Both small pores and N-containing species inthese MOPs seemed to have enhanced the efgs in the MOPs, whichin turn promoted a strong interactionwith CO2. The MOPs had highCO2 adsorption capacities at 0.05e0.15 bar and high CO2-over-N2selectivity of up to 95, here, the small pores could have enhancedthe selectivity also by kinetics. By introducing N- and O-containingmoieties as efg-enhancing (CO2-philic) sites inhexaazatriphenylene-based conjugated triazine frameworks, rela-tively high CO2 adsorption capacities of up to 2.0 mmol/g at 0.15 barwere observed at a temperature of 297 K [24]. Huang et al. [25,55]also varied the efgs of covalent organic frameworks (COFs; a type ofcrystalline MOPs with atomically ordered frameworks and porouschannels) by elaborated chemical manners. The enhanced Qst withvalues up to 43.5 kJ/mol resulted in significantly enhanced CO2-over-N2 selectivity of up to 323, which was comparable with thoseof top performing CO2 sorbents (i.e., CuBTC, Mg-MOF-74, NaXzeolite). The studies mentioned previously have demonstrated thatpore surface engineering in MOPs through, for instance, predesignof the monomers, selection of appropriate polymerization re-actions, postmodification, is a powerful approach to exploit high

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

Sorbent SBET (m2/g) Pore size (nm) CO2 uptake at 273 K (mmol/g) CO2/N2 selectivity Qst (kJ/mol) Ref.

0.05 bar 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.

C. Xu et al. / Materials Today Advances 6 (2020) 100052 3

C. Xu et al. / Materials Today Advances 6 (2020) 1000524

performance CO2 sorbents by increasing the efgs or by inducingsites with a specific tendency to interact with CO2.

If the flue gas can be compressed, or integrate with cryogeniccooling, economically in a similar manner as for membrane-basedCO2 separation [8,9], then the properties of the optimal MOPswould be quite different as compared with those suitable for VSA-based separation at 323 K. For PSA or cryogenic VSA/PSA processes,it would be the effective CO2 capacity at high pressures or lowtemperatures that would be the most defining parameters of theMOPs. A high-pressure CO2 adsorption capacity can be assured by ahigh specific surface area of the MOP. It has been shown that byusing rigid and tetrahedrally shaped monomers one can prepareMOPs with ultrahigh surface areas of over 6000 m2/g with associ-ated very high CO2 adsorption capacities of up to 48.2 mmol/g at295 K and 50 bar [56]. Alternatively, by introducing flexible skel-etons into the MOPs one could effectively increase the CO2adsorption capacity at high pressures by physical swelling [57,58].In addition, it is important to note that MOPs with a very highvolumetric capacity (the volume of the sorbate adsorbed per vol-ume of the sorbent) are ideal in processes as they can contribute toreduce the size of the columns [59].

3. Classification and synthesis of MOPs as CO2 sorbents

There are numerous ways of categorizing the MOPs, and nostandard has been agreed on; however, a first natural division is tosingle out the linear MOPs from network-based ones. The micro-porosity in linear MOPs emerges from the high free volume stem-ming from the contorted building blocks leaving the polymersporous. Since Budd et al. [60] developed the first polymer ofintrinsic microporosity (PIM); PIM-1, several PIMs linked by amide[61], imide [62], Tr€oger's base [63], aromatic units [64] and theirmodified versions [65e67] have been reported. Although most ofthe PIMs do not show outstanding CO2 adsorption capacities, theirtypically good solubility in organic solvents makes them process-able 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 organicframeworks (MOFs) [70,71], silica nanoparticles [72], polymers[73], porous carbons [74]) have aimed to break the Robeson upperbounds for CO2/N2 and CO2/CH4 separation, assuring both highpermeability and selectivity. In addition, it has been demonstratedthat by introducing functional polar groups in either the PIMs orfillers, the intermolecular interactions between the componentswere enhanced and thus the rigidity of the membranes [70,75,76].As a result, the rigidified membranes showed a significantlyenhanced aging and operation stability as compared with the PIM-1 membranes. Enhanced stability is of great importance for prac-tical applications.

Another group of MOPs that have been studied since along timeare the hyper-cross-linked polymers (HCPs) [77]. They have alreadybeen commercialized for applications such as solvent cleanup andso on. Friedel-Crafts alkylation and Scholl coupling reaction arecommon and powerful approaches for the synthesis of HCPs[78,79]. Owing to the general applicability of the reaction, almostall of the aromatic compounds and heterocycles can be used asmonomers to construct the HCPs. The advantages of HCPs overother porous materials are in their synthetic diversity and potentiallow cost. Recently, Bj€ornerb€ack and Hedin synthesized a class ofHCPs from biobasedmolecules in sulfolane [80]. The resultant HCPsshowed tunable pore structures with surface areas of610e1440 m2/g and large CO2 uptakes of 2.6e4.0 mmol/g (273 K,1 bar). The economical and environmentally friendly synthesis ofbiobased HCPs provides a sustainable approach for the develop-ment of MOPs.

Covalent triazine frameworks (CTFs) are a new type of MOPslinked by triazine units [81]. Because of their high stability, richmicroporosity, and abundant nitrogen content, CTFs are beingconsidered as promising candidates for CO2 capture [26,82,83].Since Kuhn et al. [84] synthesized a family of CTFs by trimerizationof aromatic nitrile monomers under ionothermal conditions,several groups have dedicated significant efforts in developing newCTF materials for various applications. Apart from ionothermaltrimerization reactions taking place under harsh conditions, arange of mild synthesis strategies including superacid catalyzedreactions [85,86], Friedel-Crafts reactions [87], polycondensationreactions [88] have been recently developed for the synthesis ofCTFs. For example, Wang et al. [89] synthesized a new family ofCTFs (CTF-Huazhong University of Science and Technologies viapolycondensation of aldehyde and amidine dihydrochloridemonomers at relatively low temperature (120 �C) and ambientpressure and atmosphere. The resultant CTFs showed high CO2adsorption capacities up to 3.16 mmol/g at 273 K and 1 bar. Moresignificantly, the synthesis procedures can be easily scaled up toprepare CTFs in gram quantities, which is beneficial to practicalapplications.

Condensation reactions are most common in the synthesis ofMOPs and can be performed with and without the use of catalysis.For example, the condensation reactions between amines and al-dehydes have been used to synthesize MOPs being linked by imine[36], aminal [90,91], and benzimidazole [52] units. The N-con-taining species and the ultramicropores in theseMOPs could enablestrong interaction with CO2, which is as mentioned of greatadvantage in the CO2 capture at low pressures. Notably, conden-sation reactions that are reversible and under kinetic control allowconstruction of crystalline COFs possible. In this context, COFs withpredesigned architectures, tunable porosities, and optimized CO2separation performances can be synthesized in terms of reticularchemistry and crystal engineering [92].

The catalytic coupling chemistries are well developed withinorganic chemistry and have also been used extensively for thesynthesis of MOPs. A class of conjugatedmicroporous polymers andporous 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 becauseof their high degree of polymerization and rigid 2D or 3D networks.Apart from these porous polymers with CeC linkages, recentstudies have used other coupling reactions (i.e., Buchwald-Hartwigcoupling, oxidation coupling, diazocoupling reaction) to enable thesynthesis of MOPs with azo [95e97], secondary amine [98,99], andpolycarbazole [100,101] linkages (Table 2).

An alternative way to enhance the efgs in MOPs is to decoratethem with ionic moieties. The corresponding ionic microporousorganic polymers (IMOPs) have attracted a growing interest in thefield of CO2 capture. The incorporated ion pairs are either covalentlybonded to the pore wall or attached to it with either cations oranions as pendant units. As such, their physicochemical properties,functionalities, and active sites for CO2 binding, can be regulatedthrough pore surface engineering, that is, the screening of buildingunits to backbones, pendant groups, and ionic tectonism. Severalkey design requirements are generally being considered for theIMOPs, which include the pore parameters, host-guest interactions,and charge effects. The high surface area and large pore volume ofthe IMOPs assure many accessible binding sites for CO2 and suffi-cient accommodating space, which are considered crucial to a highCO2 uptake capacity [105e107]. Generally, also for IMOPs, theintroduction of rigid and contortive building blocks into the poly-mer backbone is the most frequently used approach to achieve highporosity. However, synthesizing IMOPs with high surface areas andsmall pores is far more challenging than for neutral MOPs because

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

Chemical equations Ref.

[63]

[102]

[52,103]

[104]

[23]

[98]

C. Xu et al. / Materials Today Advances 6 (2020) 100052 5

C. Xu et al. / Materials Today Advances 6 (2020) 1000526

of the strong electrostatic repulsion that emerges in the micro-pores. Small pores can be introduced by using hard or soft tem-plates, which have been popular to use from a synthetic point ofview, however, their uses are time and energy intensive. It is of ouropinion that a template-free approach is more favorable, such asthe postmodification of presynthesized nonionic MOPs. For thispurpose, various synthetic strategies including coupling reactions(i.e., Sonogashira reaction [108,109], Suzuki-Miyaura cross-coupling reaction [110], Yamamoto reaction [111,112], and Eglintoncoupling [113]), Friedel-Crafts reaction [114], free-radical poly-merization [115], trimerization reaction [116], and Schiff-base re-action [117] can be used.

IMOPs with polar pore surfaces have strong efgs, whichenhance the efg-quadrupole interactions between the frameworksand guest CO2 molecules [118]. As for the other MOPs, the incor-poration of heteroatoms (i.e., N, O, P, S, and B) or CO2-philic ionicmoieties into the framework/pore surface through direct poly-merization or postsynthesis modification would be beneficial forthe CO2 capture. The charges themselves enhance the efgs as well[119]. It seems that tetraalkylammonium cations have a strongerinteraction with CO2 than imidazoliums as the latter have a delo-calized positive charge [120]. Similarly, the adsorption ability ofCO2 also can be affected by the nature of the anion. All in all, theions in the IMOPs can promote the CO2 adsorption, and a properdesign of the IMOPs is especially important [55]. The IMOPs haveboth covalent and ionic bonds, which allow the pores and inter-action potentials with CO2 to be controlled and enhanced. Till now,a series of IMOPs have been studied for achieving a high CO2 up-take and selectivity and their pore-surface functionalities havebeen moderated by backbone modification, adjustment of thependant groups, and counter-ion exchange. Both direct polymeri-zation and postsynthetic modification have been used. With directpolymerization, several types of IMOPs with backbone buildingunits including cationic [121], anionic [122], and zwitterionicmoieties [123] have been studied. Note that the CO2 adsorptionperformance would be readily tuned by controlling the ionicmoieties of the backbone. For instance, IMOPs synthesized fromvinyl-functionalized quaternary phosphonium salts through free-radical polymerization reaction (Table 3) have exhibited high sur-face 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 ofIMOPs being synthesized via a one-pot free-radical copolymeri-zation of ionic vinylimidazolium-based metallosalen moieties withdivinylbenzene as a cross-linker under solvothermal polymeriza-tion conditions [125]. The obtained polyvinylimidazolium-basedmetallosalen 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 cova-lently bonded phosphate groups in the backbone phosphateporous organic frameworks (PA-POFs) through the Yamamotocoupling reaction. These PA-POFs showed a high surface area of upto 2408 m2/g and excellent CO2 adsorption capacity of up to4.59 mmol/g at 273 K and 1 bar. This excellent CO2 capture abilityof these IMOPs could be ascribed to the high surface area andintrinsic high CO2 affinity of the ion pairs in the IMOPs. In contrastto direct polymerization, CO2-philic organic functionalization canbe more easily performed postsynthetically, which potentially cantake the advantage of an existent neutral MOP having a high sur-face area. Chen et al. prepared IMOPs via the phenol-formaldehydecondensation chemistry [127], where the neutral framework of theas-synthesized FIP-IM@QA was postsynthetically modified withquaternary ammonium units using the Williamson ether reaction.The ionized IMOP showed a CO2 adsorption capacity up to0.9 mmol/g, despite of its low surface area (10 m2/g) after thesurface modification (Table 3).

The ability to alter the counter ions of IMOPs is a distinctiveadvantage that allows the properties of CO2 capture and selectivitywithout changing the polymer backbone to be engineered. Directpolymerization via ionic monomers containing different anions isone 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 ammonium-based monomers via a free-radical polymerization reaction. TheCO2 sorption capacity of the IMOPs was reduced in the order ofBF4 > PF6 > Cl. Ion exchange of already prepared IMOPs is anotherapproach to tune the CO2 adsorption performance. Fischer et al.[112] synthesized two IMOPs (CPN-1-Br and CPN-2-Br) with tet-rakis (4-bromophenyl) methane using the Yamamoto coupling andSongashira-Hagihara reaction. The initial prepared polymers CPN-1-Br and CPN-2-Br exhibited high surface areas of 1455 and 540m2/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 consis-tent with the properties of the smaller anion (Cl). Rukmani et al.[129] studied the CO2 adsorption and selectivity of the ionic andfunctionalized PIM-1 (IonomIM-1) with counter ions being Liþ,Naþ, Kþ, Rbþ, or Mg2þ. The counter ion type enabled the CO2adsorption capacity and selectivity for CO2/CH4 and CO2/N2 gasmixtures to be tuned. Overall, it is clear that the type of counter ioninfluences 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 analogsof similar surface areas owing to the enhanced efgs, and we sum-marized important strategies to the design and methods to engi-neer the pore surfaces. Challenges, important to overcome in thefuture, are that IMOPs typically have low surface areas, broad poresize distributions, and are difficult to obtain directly by usingcoupling reactions.

4. Processing and shaping of MOPs

Processing of powders of network polymerebased MOPs intofunctional forms is critical to their applications in gas separationprocesses. Except for a few soluble MOPs such as PIMs with linearstructures, they are usually insoluble in common solvents and haveno observable glass transition temperatures because of the rigid,conjugated, or highly cross-linked structures. Therefore, solutionand melt processing used for conventional polymers are not suit-able for the processing of MOPs. The weak processability of MOPshas largely hampered their practical applications. Only recently,this issue has been started to be addressed by researchers. Anumber of MOP aerogels have been successfully synthesized fromthe corresponding organogels via freeze-drying, subcritical, or su-percritical drying [130e132], which are powerful methods to pre-pare low-density materials. Certain polymerization reactions thathave been performed without stirring or agitation have beenshown successful to form directly MOPs as aerogels and monoliths[133,134]. In addition, flexible and freestanding MOP membraneshave also been recently developed [135,136]. Noteworthy, 3D-printing techniques have been shown to hold great potential for thestructuring and processing of MOPs [137,138]. Such structured MOPmaterials can not only be used as sorbents for CO2 capture but alsohave a great potential for applications in nanofiltration, heteroge-neous catalysis, oil cleanup, solar steam generation, artificial mus-cles, and so on.

5. Summary

The fully organic backbone of MOPs and the possibilities of thedetailed control achieved by polymer and organic chemistry render

Table 3Selected 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.

C. Xu et al. / Materials Today Advances 6 (2020) 100052 7

the MOPs with certain advantages over zeolites, activated carbons,and MOF compositions, as adsorbents for CO2. Their relatively lowhydrophilicity, high surface area, and low densities are of greatadvantage when the MOPs are used to support amines for chemi-sorption of CO2. Decorating the internal pores of the MOPs by efg-enhancing groups such as the ionic species of the IMOPs enhancesthe physisorption abilities of the MOPs. The recent progress madein the processing and shaping of MOPs could potentially boost theirpractical applications in the CO2 capture and other fields. The highcost of the MOPs is still a big obstacle for their large-scalepreparation and the road to industrialization. In addition, mostreported MOPs have been synthesized in organic solvents atelevated temperatures with catalysts (e.g., noble metal catalysts,organic catalysts), followed by procedures of filtration, purification,

and drying. Obviously, the current synthesis technologies for MOPsare not economically and environmentally friendly. In this regard,further exploration of inexpensive andmore sustainablemonomersand facile and green synthesis routes are greatly being desired.

In contrast to crystalline porous materials such as MOFs andzeolites, most MOPs have been obtained in amorphous forms.Except for a few examples of COFs and CTFs synthesized underdelicate conditions, it is still challenging to create ordered struc-tures in MOPs, which has hampered the design of MOPs withpredetermined pore structures. It would be highly advantageous todevelop advanced simulationmodels allowing precise prediction ofthe pore size and the pore geometry inMOPs at the molecular level,which could guide the design of MOPs with high performances inCO2 capture.

C. Xu et al. / Materials Today Advances 6 (2020) 1000528

At the moment, it is still unclear if the MOPs are most promisingas gas separation membranes for CO2 separation, adsorbents forVSA-based capture, or adsorbents for cryogenic swing adsorptionprocesses. The ideal properties of the MOPs are very different forthese three applications. Hence, it is crucial that the engineeringand chemistry communities stay in close contact when furtherdeveloping the MOPs for CO2 separation purposes.

Declaration of competing interest

The authors declare that they have no known competingfinancial interests or personal relationships that could haveappeared to influence the work reported in this article.

Acknowledgments

CX thanks the ÅForsk, Sweden (19-493), NH the SwedishResearch Council (2016-03568), and JYY the ERC (Starting GrantNAPOLI-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.[7] T.L. Church, A.B. Jasso-Salcedo, F. Bj€ornerb€ack, N. Hedin, Sci. China Chem. 60

(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.[14] V. Guillerm, Ł.J. Weseli�nski, M. Alkordi, M.I.H. Mohideen, Y. Belmabkhout,

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.[23] P. Arab, M.G. Rabbani, A.K. Sekizkardes, T. slamo�glu, H.M. El-Kaderi, Chem.

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.

[34] Y. Liao, J. Weber, C.F.J. Faul, Chem. Commun. 50 (2014) 8002e8005.[35] C. Xu, E. Dinka, N. Hedin, ChemPlusChem 81 (2016) 58e63.[36] N. Popp, T. Homburg, N. Stock, J. Senker, J. Mater. Chem. A 3 (2015)

18492e18504.[37] V.M. Suresh, S. Bonakala, H.S. Atreya, S. Balasubramanian, T.K. Maji, ACS Appl.

Mater. Interfaces 6 (2014) 4630e4637.[38] D.M. Ruthven, S.C. Reyes, Microporous Mesoporous Mater. 104 (2007)

59e66.[39] K. Yuan, C. Liu, C. Liu, S. Zhang, G. Yu, L. Yang, F. Yang, X. Jian, Polymer 151

(2018) 65e74.[40] K. Yuan, C. Liu, L. Zong, G. Yu, S. Cheng, J. Wang, Z. Weng, X. Jian, ACS Appl.

Mater. Interfaces 9 (2017) 13201e13212.[41] N. Song, H. Yao, T. Ma, T. Wang, K. Shi, Y. Tian, Y. Zou, S. Zhu, Y. Zhang,

S. Guan, Chem. Eng. J. 368 (2019) 618e626.[42] S. Qiao, W. Huang, H. Wei, T. Wang, R. Yang, Polymer 70 (2015) 52e58.[43] K. Wang, H. Huang, D. Liu, C. Wang, J. Li, C. Zhong, Environ. Sci. Technol. 50

(2016) 4869e4876.[44] V.S.P.K. Neti, J. Wang, S. Deng, L. Echegoyen, J. Mater. Chem. A 3 (2015)

10284e10288.[45] M. Saleh, S.B. Baek, H.M. Lee, K.S. Kim, J. Phys. Chem. C 119 (2015)

5395e5402.[46] S. Yao, X. Yang, M. Yu, Y. Zhang, J.-X. Jiang, J. Mater. Chem. A 2 (2014)

8054e8059.[47] M. Saleh, H.M. Lee, K.C. Kemp, K.S. Kim, ACS Appl. Mater. Interfaces 6 (2014)

7325e7333.[48] B. Zhang, J. Yan, Z. Wang, J. Phys. Chem. C 122 (2018) 12831e12838.[49] X. Wang, B. Yuan, X. Zhou, Q. Xia, Y. Li, D. An, Z. Li, Chem. Eng. J. 327 (2017)

51e59.[50] A. Yassin, M. Trunk, F. Czerny, P. Fayon, A. Trewin, J. Schmidt, A. Thomas, Adv.

Funct. Mater. 27 (2017).[51] Y.H. Abdelmoaty, T.-D. Tessema, F.A. Choudhury, O.M. El-Kadri, H.M. El-

Kaderi, ACS Appl. Mater. Interfaces 10 (2018) 16049e16058.[52] M.G. Rabbani, T. Islamoglu, H.M. El-Kaderi, J. Mater. Chem. A 5 (2017)

258e265.[53] A.K. Sekizkardes, S. Altarawneh, Z. Kahveci, T. Islamoglu, H.M. El-Kaderi,

Macromolecules 47 (2014) 8328e8334.[54] T. Islamoglu, T. Kim, Z. Kahveci, O.M. El-Kadri, H.M. El-Kaderi, J. Phys. Chem.

C 120 (2016) 2592e2599.[55] N. Huang, R. Krishna, D. Jiang, J. Am. Chem. Soc. 137 (2015) 7079e7082.[56] D. Yuan, W. Lu, D. Zhao, H.-C. Zhou, Adv. Mater. 23 (2011) 3723e3725.[57] R.T. Woodward, L.A. Stevens, R. Dawson, M. Vijayaraghavan, T. Hasell,

I.P. Silverwood, A.V. Ewing, T. Ratvijitvech, J.D. Exley, S.Y. Chong, F. Blanc,D.J. Adams, S.G. Kazarian, C.E. Snape, T.C. Drage, A.I. Cooper, J. Am. Chem. Soc.136 (2014) 9028e9035.

[58] V. Rozyyev, D. Thirion, R. Ullah, J. Lee, M. Jung, H. Oh, M. Atilhan, C.T. Yavuz,Nat. Energy 4 (2019) 604e611.

[59] Y. Yang, W. Pu, X. Xu, B. Wei, Y. Yang, C.D. Wood, Macromol. Mater. Eng. 304(2019) 1800780.

[60] P.M. Budd, E.S. Elabas, B.S. Ghanem, S. Makhseed, N.B. McKeown, K.J. Msayib,C.E. Tattershall, D. Wang, Adv. Mater. 16 (2004) 456e459.

[61] J. Weber, Q. Su, M. Antonietti, A. Thomas, Macromol. Rapid Commun. 28(2007) 1871e1876.

[62] B. Ghanem, F. Alghunaimi, X. Ma, N. Alaslai, I. Pinnau, Polymer 101 (2016)225e232.

[63] I. Rose, M. Carta, R. Malpass-Evans, M.-C. Ferrari, P. Bernardo, G. Clarizia,J.C. Jansen, N.B. McKeown, ACS Macro Lett. 4 (2015) 912e915.

[64] G. Cheng, B. Bonillo, R.S. Sprick, D.J. Adams, T. Hasell, A.I. Cooper, Adv. Funct.Mater. 24 (2014) 5219e5224.

[65] X. Wang, Y. Liu, X. Ma, S.K. Das, M. Ostwal, I. Gadwal, K. Yao, X. Dong, Y. Han,I. Pinnau, K.-W. Huang, Z. Lai, Adv. Mater. 29 (2017) 1605826.

[66] C.R. Mason, L. Maynard-Atem, K.W.J. Heard, B. Satilmis, P.M. Budd, K. Friess,M. Lanc, P. Bernardo, G. Clarizia, J.C. Jansen, Macromolecules 47 (2014)1021e1029.

[67] J.W. Jeon, D.-G. Kim, E.-h. Sohn, Y. Yoo, Y.S. Kim, B.G. Kim, J.-C. Lee, Macro-molecules 50 (2017) 8019e8027.

[68] S.H. Pang, M.L. Jue, J. Leisen, C.W. Jones, R.P. Lively, ACS Macro Lett. 4 (2015)1415e1419.

[69] B.S. Ghanem, R. Swaidan, E. Litwiller, I. Pinnau, Adv. Mater. 26 (2014)3688e3692.

[70] B. Ghalei, K. Sakurai, Y. Kinoshita, K. Wakimoto, Ali P. Isfahani, Q. Song,K. Doitomi, S. Furukawa, H. Hirao, H. Kusuda, S. Kitagawa, E. Sivaniah, Nat.Energy 2 (2017) 17086.

[71] X. Wu, W. Liu, H. Wu, X. Zong, L. Yang, Y. Wu, Y. Ren, C. Shi, S. Wang, Z. Jiang,J. Membr. Sci. 548 (2018) 309e318.

[72] S. Hasebe, S. Aoyama, M. Tanaka, H. Kawakami, J. Membr. Sci. 536 (2017)148e155.

[73] X. Mei Wu, Q. Gen Zhang, P. Ju Lin, Y. Qu, A. Mei Zhu, Q. Lin Liu, J. Membr. Sci.493 (2015) 147e155.

[74] H. Zhao, L. Feng, X. Ding, Y. Zhao, X. Tan, Y. Zhang, J. Membr. Sci. 564 (2018)800e805.

[75] R. Swaidan, B.S. Ghanem, E. Litwiller, I. Pinnau, J. Membr. Sci. 457 (2014)95e102.

[76] S. Yi, B. Ghanem, Y. Liu, I. Pinnau, W.J. Koros, Sci. Adv. 5 (2019), eaaw5459.[77] L. Tan, B. Tan, Chem. Soc. Rev. 46 (2017) 3322e3356.

C. Xu et al. / Materials Today Advances 6 (2020) 100052 9

[78] C.H. Lau, T.-d. Lu, S.-P. Sun, X. Chen, M. Carta, D.M. Dawson, Chem. Commun.55 (2019) 8571e8574.

[79] A.H. Alahmed, M.E. Briggs, A.I. Cooper, D.J. Adams, J. Mater. Chem. A 7 (2019)549e557.

[80] F. Bj€ornerb€ack, N. Hedin, ChemSusChem 12 (2019) 839e847.[81] M. Liu, L. Guo, S. Jin, B. Tan, J. Mater. Chem. A 7 (2019) 5153e5172.[82] H. Wang, D. Jiang, D. Huang, G. Zeng, P. Xu, C. Lai, M. Chen, M. Cheng,

C. Zhang, Z. Wang, J. Mater. Chem. A 7 (2019) 22848e22870.[83] S. Mukherjee, M. Das, A. Manna, R. Krishna, S. Das, Chem. Mater. 31 (2019)

3929e3940.[84] P. Kuhn, M. Antonietti, A. Thomas, Angew. Chem. Int. Ed. 47 (2008)

3450e3453.[85] S. Ren, M.J. Bojdys, R. Dawson, A. Laybourn, Y.Z. Khimyak, D.J. Adams,

A.I. Cooper, Adv. Mater. 24 (2012) 2357e2361.[86] S. Kuecken, J. Schmidt, L. Zhi, A. Thomas, J. Mater. Chem. A 3 (2015)

24422e24427.[87] P. Puthiaraj, S.-M. Cho, Y.-R. Lee, W.-S. Ahn, J. Mater. Chem. A 3 (2015)

6792e6797.[88] M. Liu, Q. Huang, S. Wang, Z. Li, B. Li, S. Jin, B. Tan, Angew. Chem. Int. Ed. 57

(2018) 11968e11972.[89] K. Wang, L.-M. Yang, X. Wang, L. Guo, G. Cheng, C. Zhang, S. Jin, B. Tan,

A. Cooper, Angew. Chem. Int. Ed. 56 (2017) 14149e14153.[90] G. Li, B. Zhang, J. Yan, Z. Wang, Chem. Commun. 52 (2016) 1143e1146.[91] B. Zhang, J. Yan, G. Li, Z. Wang, Polym. Chem. 10 (2019) 3371e3379.[92] Y. Zeng, R. Zou, Y. Zhao, Adv. Mater. 28 (2016) 2855e2873.[93] Y. Xu, S. Jin, H. Xu, A. Nagai, D. Jiang, Chem. Soc. Rev. 42 (2013) 8012e8031.[94] Y. Yuan, G. Zhu, ACS Cent. Sci. 5 (2019) 409e418.[95] R. Bera, M. Ansari, A. Alam, N. Das, J. CO2 Util. 28 (2018) 385e392.[96] G. Ji, Z. Yang, H. Zhang, Y. Zhao, B. Yu, Z. Ma, Z. Liu, Angew. Chem. Int. Ed. 55

(2016) 9685e9689.[97] R. Bera, M. Ansari, A. Alam, N. Das, ACS Appl. Poly. Mater 1 (2019) 959e968.[98] Y. Liao, H. Wang, M. Zhu, A. Thomas, Adv. Mater. 30 (2018) 1705710.[99] J. Chen, W. Yan, E.J. Townsend, J. Feng, L. Pan, V. Del Angel Hernandez,

C.F.J. Faul, Angew. Chem. Int. Ed. 58 (2019) 11715e11719.[100] L. Pan, Q. Chen, J.-H. Zhu, J.-G. Yu, Y.-J. He, B.-H. Han, Polym. Chem. 6 (2015)

2478e2487.[101] Q. Chen, B.-H. Han, Macromol. Rapid Commun. 39 (2018) 1800040.[102] S. Wang, C. Zhang, Y. Shu, S. Jiang, Q. Xia, L. Chen, S. Jin, I. Hussain,

A.I. Cooper, B. Tan, Sci. Adv. 3 (2017), e1602610.[103] M.G. Rabbani, H.M. El-Kaderi, Chem. Mater. 24 (2012) 1511e1517.[104] E. Jin, M. Asada, Q. Xu, S. Dalapati, M.A. Addicoat, M.A. Brady, H. Xu,

T. Nakamura, T. Heine, Q. Chen, D. Jiang, Science 357 (2017) 673.[105] J.-K. Sun, M. Antonietti, J. Yuan, Chem. Soc. Rev. 45 (2016) 6627e6656.[106] D. Xu, J. Guo, F. Yan, Prog. Polym. Sci. 79 (2018) 121e143.[107] T. Ben, H. Ren, S. Ma, D. Cao, J. Lan, X. Jing, W. Wang, J. Xu, F. Deng,

J.M. Simmons, S. Qiu, G. Zhu, Angew. Chem. Int. Ed. 48 (2009) 9457e9460.[108] Z. Yan, Y. Yuan, Y. Tian, D. Zhang, G. Zhu, Angew. Chem. Int. Ed. 54 (2015)

12733e12737.[109] S. Fischer, J. Schmidt, P. Strauch, A. Thomas, Angew. Chem. Int. Ed. 52 (2013)

12174e12178.[110] H. Zhao, Y. Wang, R. Wang, Chem. Commun. 50 (2014) 10871e10874.[111] Q. Zhang, S. Zhang, S. Li, Macromolecules 45 (2012) 2981e2988.

[112] S. Fischer, A. Schimanowitz, R. Dawson, I. Senkovska, S. Kaskel, A. Thomas,J. Mater. Chem. A 2 (2014) 11825e11829.

[113] J.-L. Wang, C. Wang, K.E. deKrafft, W. Lin, ACS Catal. 2 (2012) 417e424.[114] J. Wang, J.G.W. Yang, G. Yi, Y. Zhang, Chem. Commun. 51 (2015)

15708e15711.[115] Q. Sun, S. Ma, Z. Dai, X. Meng, F.-S. Xiao, J. Mater. Chem. A 3 (2015)

23871e23875.[116] O. Buyukcakir, S.H. Je, S.N. Talapaneni, D. Kim, A. Coskun, ACS Appl. Mater.

Interfaces 9 (2017) 7209e7216.[117] Z.-W. Liu, C.-X. Cao, B.-H. Han, J. Hazard Mater. 367 (2019) 348e355.[118] D.M. D'Alessandro, B. Smit, J.R. Long, Angew. Chem. Int. Ed. 49 (2010)

6058e6082.[119] W. Lu, D. Yuan, J. Sculley, D. Zhao, R. Krishna, H.-C. Zhou, J. Am. Chem. Soc.

133 (2011) 18126e18129.[120] J. Tang, H. Tang, W. Sun, H. Plancher, M. Radosz, Y. Shen, Chem. Commun.

(2005) 3325e3327.[121] J. Cao, W. Shan, Q. Wang, X. Ling, G. Li, Y. Lyu, Y. Zhou, J. Wang, ACS Appl.

Mater. Interfaces 11 (2019) 6031e6041.[122] R. Shen, X. Yan, Y.-J. Guan, W. Zhu, T. Li, X.-G. Liu, Y. Li, Z.-G. Gu, Polym.

Chem. 9 (2018) 4724e4732.[123] A. Nagai, X. Chen, X. Feng, X. Ding, Z. Guo, D. Jiang, Angew. Chem. Int. Ed. 52

(2013) 3770e3774.[124] Q. Sun, Y. Jin, B. Aguila, X. Meng, S. Ma, F.-S. Xiao, ChemSusChem 10 (2017)

1160e1165.[125] R. Luo, Y. Chen, Q. He, X. Lin, Q. Xu, X. He, W. Zhang, X. Zhou, H. Ji, Chem-

SusChem 10 (2017) 1526e1533.[126] Y. Liu, K. Landskron, J. Mater. Chem. A 5 (2017) 23523e23529.[127] Y. Chen, R. Luo, J. Bao, Q. Xu, J. Jiang, X. Zhou, H. Ji, J. Mater. Chem. A 6 (2018)

9172e9182.[128] Y. Xie, Q. Sun, Y. Fu, L. Song, J. Liang, X. Xu, H. Wang, J. Li, S. Tu, X. Lu, J. Li,

J. Mater. Chem. A 5 (2017) 25594e25600.[129] S.J. Rukmani, T.P. Liyana-Arachchi, K.E. Hart, C.M. Colina, Langmuir 34 (2018)

3949e3960.[130] R. Du, N. Zhang, H. Xu, N. Mao, W. Duan, J. Wang, Q. Zhao, Z. Liu, J. Zhang,

Adv. Mater. 26 (2014) 8053e8058.[131] R. Du, Z. Zheng, N. Mao, N. Zhang, W. Hu, J. Zhang, Adv. Sci. 2 (2015)

1400006.[132] J. Zhang, L. Liu, H. Liu, M. Lin, S. Li, G. Ouyang, L. Chen, C.-Y. Su, J. Mater.

Chem. A 3 (2015) 10990e10998.[133] J. Liu, J.M. Tobin, Z. Xu, F. Vilela, Poly.Chem. 6 (2015) 7251e7255.[134] P. Mu, W. Bai, Z. Zhang, J. He, H. Sun, Z. Zhu, W. Liang, A. Li, J. Mater. Chem. A

6 (2018) 18183e18190.[135] S. Kandambeth, P. Biswal Bishnu, D. Chaudhari Harshal, C. Rout Kanhu,

H.S. Kunjattu, S. Mitra, S. Karak, A. Das, R. Mukherjee, K. Kharul Ulhas,R. Banerjee, Adv. Mater. 29 (2016) 1603945.

[136] Z. Wang, Q. Yu, Y. Huang, H. An, Y. Zhao, Y. Feng, X. Li, X. Shi, J. Liang, F. Pan,P. Cheng, Y. Chen, S. Ma, Z. Zhang, ACS Cent. Sci. 5 (2019) 1352e1359.

[137] M. Zhang, L. Li, Q. Lin, M. Tang, Y. Wu, C. Ke, J. Am. Chem. Soc. 141 (2019)5154e5158.

[138] D. Rodríguez-San-Miguel, A. Abrishamkar, J.A.R. Navarro, R. Rodriguez-Tru-jillo, D.B. Amabilino, R. Mas-Ballest�e, F. Zamora, J. Puigmartí-Luis, Chem.Commun. 52 (2016) 9212e9215.