Nanoporous organic polymer networks

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Progress in Polymer Science 37 (2012) 530– 563

Contents lists available at SciVerse ScienceDirect

Progress in Polymer Science

j ourna l ho me p ag e: www.elsev ier .com/ locate /ppolysc i

anoporous organic polymer networks

obert Dawson, Andrew I. Cooper, Dave J. Adams ∗

epartment of Chemistry and Centre for Materials Discovery, University of Liverpool, Liverpool, L69 7ZD, United Kingdom

r t i c l e i n f o

rticle history:eceived 16 May 2011eceived in revised form 17 August 2011ccepted 1 September 2011vailable online 22 September 2011

eywords:

a b s t r a c t

Nanoporous organic polymer networks are a class of materials consisting solely of thelighter elements in the periodic table. These materials have potential uses in areas suchas storage, separation, and catalysis. Here, we review the different classes of nanoporouspolymer networks including covalent organic frameworks, hypercrosslinked polymers,conjugated microporous polymers, and polymers of intrinsic microporosity. The growingvariety in synthetic routes to these materials allows a range of different polymer networksto be formed, including crystalline and amorphous structures. It is also possible to incorpo-
icroporous
onjugated microporous polymerolymers of intrinsic microporosityypercrosslinked polymersydrogen storagearbon dioxide capture

rate many different kinds of functional groups in a modular fashion. So far, most networkshave been examined from the perspective of gas sorption, and this area is discussed criti-cally and in depth in this review. The use of nanoporous organic polymers for applicationssuch as catalysis and separations is an important developing area, and we discuss recentdevelopments as well as highlighting potential future opportunities.

© 2011 Elsevier Ltd. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5312. Synthesis of nanoporous organic polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536

2.1. Covalent organic frameworks (COFs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5362.2. Covalent triazine frameworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5372.3. Polymers of intrinsic microporosity (PIMs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5382.4. Hypercrosslinked polymers (HCPs) and other crosslinked polymer networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5392.5. Conjugated microporous polymers (CMPs) and analogues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 542

3. Porous organic polymers as sorbents for gas capture and storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5503.1. Hydrogen storage in nanoporous organic polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 551
3.2. Methane storage in nanoporous organic polymers. . . . . . . . .3.3. Carbon dioxide sorption in nanoporous organic polymers .
4. Catalysis using nanoporous organic polymers . . . . . . . . . . . . . . . . . . . . .

Abbreviations: BCMA, bis(chloromethyl)anthracene; BCMBP, bis(chloromethyl)bugated microporous polymers; COF, covalent organic framework; DCX, dichlorounctional theory; gra, graphite; HHTP, 2,3,6,7,10,11-hexahydroxytriphenylene;

olymer; NCMP, nitrogen-containing conjugated microporous polymer; PIM, pomall angle X-ray diffraction; ssNMR, solid state nuclear magnetic resonance.∗ Corresponding author.

E-mail address: [email protected] (D.J. Adams).

079-6700/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.progpolymsci.2011.09.002

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 554

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 554

iphenyl; BET, Brunauer–Emmett–Teller; bnn, �-boron nitride; CMP, con-xylene; EOF, element organic framework; NL-DFT, non-localised densityIR, infrared; MOF, metal organic framework; MOP, microporous organiclymer of intrinsic microporosity; PXRD, powder X-ray diffraction; SAXS,

dx.doi.org/10.1016/j.progpolymsci.2011.09.002

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R. Dawson et al. / Progress in Polymer Science 37 (2012) 530– 563 531

5. Separations using MOPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5586. Light harvesting in CMPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5587. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559. . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction

The development of nanoporous polymer networks[1,2] in areas such as gas storage [3–5], separations [2,6],and catalysis [7,8] has seen significant developmentsin the last few years, catalyzed by a broader interestin nanoporous materials and the growing sustainabilityagenda; for example, in the search for materials to performless energy-intensive separations. There have been a num-ber of reviews recently in this field [1,2,6,9,10], but the areais developing rapidly and our aim here is to provide an up-to-date overview of the synthesis of novel porous polymernetworks as well as their potential uses.

Porous materials, as classified by IUPAC, are divided intothe following groups according to pore size [11]: macro-porous materials have pore widths greater than 50 nm;mesoporous materials have pore widths between 2 and50 nm; and microporous materials have pore widths lessthan 2 nm. In this review, we use the term ‘nanoporous’ toencompass both microporous and mesoporous materials.The focus is on materials where the porosity is a functionof the molecular structure rather than, for example, poly-mer materials formed by templating methods [12]. It isthe presence of micropores (d < 2 nm) which can lead tothe very high apparent surface areas (>1000 m2/g) that areobserved in some nanoporous polymers. Micropores alsocontribute most to the adsorption of sorbate molecules atlower pressures [5]. Surface areas in polymer networks,as measured by the adsorption of gases (usually nitro-gen at 77 K), can be calculated using either Langmuir orthe Brunauer–Emmett–Teller (BET) theories. Both meth-ods involve assumptions that will frequently be imperfect,such as energetic homogeneity of the sorbent, but they dooffer a common framework to compare different materialsin this class. Apparent BET surface areas are most com-monly reported because the BET theory accommodatesmultilayer adsorption. However, it is possible that somemechanisms – such as pore-filling versus layer formation– would invalidate the theory [11]. Swelling of polymernetworks in response to N2 adsorption may also inval-idate such analysis, which assumes that the sorbent isrigid. There is however support for the applicability of theBET theory to nanoporous frameworks, at least within cer-tain classes of materials. For example, theoretical studiesinto the validity of the BET theory for high surface areametal–organic frameworks (MOFs) have been reported.[13] Nitrogen isotherms derived from the crystal structureof MOFs were used to calculate the surface area using theBET equation, which was then compared to accessible sur-
face areas derived from single crystal structures (makingthe assumption of perfect crystallinity). The surface areascalculated from crystal structures showed good agreementwith those derived experimentally using the BET equation,
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559

providing that an appropriate relative pressure range wasused for the calculation, leading to a positive ‘C value’. Assuch, apparent BET surface areas are one means for com-paring different porous materials, so long as one bears inmind the limitations of the theory and the potential sensi-tivity to measurement conditions and the relative pressurerange used in the calculation. It is also important to remem-ber that BET surface areas are not equivalent to geometricsurface areas, as applied to macroscopic objects, especiallywhen ultrahigh surface area materials (>4000 m2/g) areconsidered. As such, analogies with the surface area offootball fields and the like are illustrative of the sorptioncapacity of materials, but potentially misleading if taken ina literal sense.

This review focuses on organic polymer networks thatcan best be described as microporous, although many ofthese materials do in fact possess some degree of meso-and macroporosity within their structures. The review alsofocuses on synthetic materials which consist of the lighterelements in the periodic table, mainly the first row. Itshould be remembered that there are also a number of nat-urally occurring microporous materials, most importantlyzeolites [14], and also materials such as activated carbon[15] that are derived from natural and (sometimes) renew-able resources – these are not discussed here. We also donot discuss hybrid materials, such as MOFs, which havegiven rise to some of the highest reported surface areasfor any materials to date [16,17].

To create a pore structure which can be accessible toguest molecules, such as gases, an interconnected networkof channels must be created. For these pores to be stable,it is typically necessary to use rigid building blocks thatprevent the networks from collapsing and hence fillingspace in a more efficient manner. Rigid polymer networkshave been known and described for many years [18]. Theawareness of the porosity of these networks has howeveronly recently become the focus of significant attention. Thisrequirement for rigidity has tended to define the chemistryused so far in nanoporous polymer synthesis [18]. Rigidityis often derived from aromatic monomers, either directlylinked together or linked by other rigid groups, such asalkynes [19] or alkenes [20]. For three-dimensional net-work formation, at least one of the monomers (the node)must have a connectivity of >2. When these nodes arelinked together by monomers containing a functionality ofat least 2 (the struts), network formation is possible. Thenetwork will only be permanently porous after the removalof the solvent if the structure cannot collapse and closethe pores. Porous polymer networks may be divided into
two sub-classes: crystalline networks and amorphous net-works. Crystalline networks have well-ordered structuresand hence uniform pore sizes that are related to the dimen-sions of the monomer struts. Amorphous networks have a

532 R. Dawson et al. / Progress in Polymer Science 37 (2012) 530– 563

F mer netr on. Coma

ssic

ig. 1. Examples of condensation reactions used to form nanoporous polying formation, (D) imine reaction, (E) amide reaction and (F) imide reactind the formation of a linking moiety with high chemical rigidity.

tatistical, disordered structure and may have a wider poreize range, although some amorphous polymer networksn fact have quite narrow pore size distributions [21]. Likerystalline networks, pore size in amorphous networks

works. (A) BO2C2 ring formation, (B) boroxine ring formation, (C) dioxanemon features for A–F are the elimination of a small molecule by-product

may also be related to the monomer strut length but in amore statistical manner [22]. Crystalline porous polymersare usually formed using reversible bond-forming chem-istry: for example, a series of materials known as covalent


r netwupling a

Fig. 2. Metal-catalyzed reactions used to form nanoporous polymeSonogashira–Hagihara coupling, (K) Yamamoto coupling, (L) oxidative co

organic frameworks (COFs) were formed either by theself-condensation of boronic acids or by the condensationreaction of boronic acids with diols to form boroxine rings
(Fig. 1A and B). These reversible condensation reactionsallow the formation of the most thermodynamically stableproduct. By definition, crystalline microporous organicmaterials have well-defined pores. However, crystallinity
orks. (G and H) Cyclotrimerisation, (I) Friedel–Crafts acylation, (J)nd, (M) Suzuki coupling.

is not a prerequisite for high levels of porosity. Indeed,some of the highest BET surface areas reported for anymaterials (e.g., PAF-1 = 5640 m2/g; PPN-4 = 6461 m2/g)
have been measured for amorphous polymer frameworks[23,24]. At the time of writing, PPN-4 has, formally,the highest surface area reported for a material of anykind.


Fig. 3. Network E1 (a CMP network) shows a Type I isotherm (triangles) [21], while the BCMBP network shows a Type IV isotherm with hysteresis (circles)[ rption.

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31]. Filled symbols are for the adsorption step, empty symbols for desoeader is referred to the web version of the article.)

Amorphous networks require the use of high-yieldingeactions to achieve as close to full condensation as pos-ible. Some examples of condensation reactions, such ashose used to form COFs (A, B and D) and polymers ofntrinsic microporosity (PIMs) (C), are shown in Fig. 1.ikewise, examples of metal-catalyzed reactions such asyclotrimerisation (G and H), Friedel–Crafts alkylation (I),onogashira–Hagihara coupling (J), Yamamoto couplingK) and Suzuki coupling (M), are shown in Fig. 2.

The inherent insolubility of porous polymer networksakes characterization relatively difficult in comparisonith small molecules and linear, soluble polymers. Conven-

ional polymer analysis such as solution nuclear magneticesonance (NMR), gel permeation chromatography (GPC),r MALDI-TOF mass spectroscopy cannot be used. InfraredIR) spectroscopy can provide qualitative information onhe structure of the network materials. For example, IRas been used to demonstrate monomer condensation andhe depletion of the starting functionalities in the synthe-is of COFs (Fig. 1A) where the reduction in intensity ofhe O–H band indicates condensation [25]. Likewise, foronjugated microporous polymers (CMPs, Fig. 2J), the pres-nce of internal alkynes and a reduction in the intensity oferminal alkynes indicates effective Sonogashira–Hagiharaoupling [19] and for triazine networks, the reduction inhe carbonyl bond at 2228 cm−1 and an increase in the tri-zine ring at 1507 and 352 cm−1 can be observed over theourse of the reaction [26]. Microanalysis of polymer net-orks is often more difficult to interpret. It is common tond that the total percentage mass does not equate to 100%,

ndicating either incomplete combustion or the presence ofndetected impurities, such as the entrapment of solvent orhe presence of physisorbed water from the atmosphere inhe micropores [27]. It should be noted here that elemental

icroanalysis is more commonly carried out for nonporousaterials, where adsorption on the surface of the sample

an reasonably be ignored. This can be a substantial source
f error in microanalysis, since even relatively hydrophobicorous materials might readily physisorb more than 5 wt.%ater vapor under ambient humidity conditions. An indis-ensable analytical tool for insoluble polymer networks
(For interpretation of the references to colour in this figure legend, the

is solid state NMR (ssNMR), which can provide confirma-tion of the structure of the final material and can givequantitative or semi-quantitative structural informationvia the assignment of the different carbon environments[19,23,25].

Likewise, gas sorption is invaluable for the analysisof both crystalline and amorphous porous polymers [11].Here, the pores in the materials are evacuated under highvacuum, usually with heating, to expel any guests. Theanalysis gas is then dosed into the sample and adsorbedonto the surface; typically, sorption isotherms are calcu-lated using volumetric methods, although some groupsalso employ gravimetric techniques, especially for sorp-tion measurements at elevated pressures. Until now, mostmeasurements have been performed using nitrogen gas asthe sorbate at 77 K, although it is also possible to use othergases such as argon, hydrogen, and carbon dioxide [28].The advantage of nitrogen is its low cost and wide avail-ability, and hence it is the most widely studied analysisgas and, hence, the largest array of pore models is avail-able. Hydrogen is the smallest gas molecule and is thereforeuseful in probing the smallest ultramicropores which maybe inaccessible to larger gases, such as nitrogen or argon.For example, differences between hydrogen and nitrogenadsorption capacities have been used to estimate the porevolume arising from pores with sizes between approxi-mately 0.29 and 0.36 nm [29]. It is also possible to measurecarbon dioxide isotherms quite rapidly (a few hours) andsuch measurements are performed much closer to ambi-ent temperatures. The measurement pressure range withCO2 is higher than for nitrogen gas, and it is possible toanalyze smaller pores without having to achieve such lowpressures. Indeed, carbon dioxide sorption at 273 K can beused to selectively probe pores with radii lower than 0.7 nm[30]. Much information can be obtained from the shape ofnitrogen sorption isotherms. At low pressures, the first gasmolecules adsorb onto the surface of the sorbent and form a
monolayer. If the material is completely microporous, thenit will not be possible to adsorb further gas molecules aftermonolayer coverage and the isotherm should be horizon-tal with respect to the pressure axis, so long as there are


synthes
Fig. 4. Summary of COFsReprinted with permission from [47].
no sample morphology effects (e.g., adsorption in interpar-ticulate spaces) at higher relative pressures. This isothermshape is classified by IUPAC as Type I (Fig. 3, black) [11].However, if larger pores are present, for example becausethe material has a broad pore size distribution, then it ispossible to form further layers of adsorbate which becomesapparent in the isotherm as a further increase in the quan-tity of gas adsorbed up to pressures reaching saturation(P/P0 = 1). These isotherms are classified as Type IV (Fig.3, red). In some cases, very large amounts of gas may beadsorbed at high relative pressures. This is attributed to thecondensation of nitrogen between particles of the material,or in larger pores. In the case of polymer networks, swellingmay also contribute to this adsorption at higher pressures,although it is difficult to assign this behavior (i.e., unre-stricted filling in large pores versus swelling effects) fromvolumetric sorption measurements alone. Such isothermsare often accompanied by a hysteresis in the desorptionisotherm. In H1 type hysteresis, the adsorption and desorp-tion branches are parallel. In another common hysteresistype, H2, the desorption branch of the hysteresis loop issteeper than the adsorption branch, and this is often seenwhen a range of larger mesopores are present [11].

Nitrogen adsorption/desorption isotherms can providemuch detail regarding the pore size in polymer networks,
for example by the application of non-local density func-tional theory (NL-DFT). A number of models exist for avariety of porous materials such as carbons, zeolites, andoxides. However, no common or universal model has yet
ized by the Yaghi group.

been developed for nanoporous polymer networks. Onestudy on pore size distributions evaluated a number ofdifferent models [28] and it was shown that the use ofdifferent models resulted in different calculated pore sizedistributions. We recommend that calculated pore sizedistributions are therefore treated with caution and that,ideally, the results of a range of different models arepresented and compared. A further complication is thepossibility of swelling in polymer networks upon guestsorption [28], which may confound models that were orig-inally developed for more rigid materials such as activatedcarbon and zeolites.

A less commonly used but highly complementarymethod for measuring the porosity in polymer networks issmall angle X-ray scattering (SAXS) [32]. For a given mate-rial, higher surface areas may be determined by SAXS incomparison with gas sorption methods. This is due to theability of SAXS to measure occluded volume – that is, iso-lated pores – which cannot be detected by sorption. Othermethods have been applied to estimate the free volume inmicroporous materials include positron annihilation life-time spectroscopy (PALS) [33,34] and 129Xe-NMR. PALS hasbeen used to determine the free volume in PIMs [34,35]. Forexample, nitrogen adsorption measurements showed thata PIM-1 powder had a higher surface area than the solu-
tion processed film [36]; PALS was used to show that thenitrogen adsorption results reflected a greater externallyaccessible surface area of the powder, rather than internalpore structure [37]. 129Xe-NMR has been used to quantify

5 Polymer

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36 R. Dawson et al. / Progress in

he pore size based on the chemical shift of confined Xetoms [28].

. Synthesis of nanoporous organic polymers

Nanoporous materials consisting entirely of the lighterlements of the periodic table such B, C, N and O can haveery low densities. They have a number of potential advan-ages over inorganic-based microporous materials [3–5].or example, when the targeted application of the mate-ial is the storage of gases, the specific surface area for aiven pore volume will be higher if the constituent atoms inhe network are lighter. Moreover, in some MOF structures,hich are often based around metal clusters, these heavieretals can result in the material being less stable to oxygen

r moisture [38]. Perhaps the major advantage of organicaterials, though, is the wide range of synthetic methods

hat are available. It has therefore been possible to pro-uce a number of different structures for porous polymeretworks including COFs [25], PIMs [39], hypercrosslinkedolymers (HCPs) [40], and CMPs [19]. These various sub-lasses of porous organic polymers are discussed in theollowing sections.

.1. Covalent organic frameworks (COFs)

The series of materials referred to as COFs were pio-eered by Yaghi and co-workers [25]. COFs are crystallineaterials synthesized from organic monomers linked

ogether by strong covalent bonds [41]. Many of theseaterials have been prepared by the reversible forma-

ion of strong B–O bonds [42] and these boroxine ringsan be seen as analogues of the metal centers in MOFs.s noted above, the crystallinity in these materials isscribed to the reversible nature of the boroxine ring for-ation [25]. The first COFs (named COF-1 and COF-5) were

ynthesized under solvothermal conditions using a diox-ne/mesitylene mixture [25]. It was later shown that suchaterials can be synthesized using a number of different

eaction conditions, including rapid microwave synthe-is, while still preserving the crystallinity of the resultingetworks [43–45]. COF-1 is the simplest of the materials

n this class and is formed via the self-condensation ofenzene-1,4-diboronic acid, via the elimination of water.he structure of COF-1 is therefore a hexagonal array ofenzene rings connected by B3O3 rings (Fig. 4). COF-5ses a similar condensation reaction between benzene-1,4-iboronic acid with hexahydroxytriphenylene (HHTP) toorm a BC2O2-linking ring. Powder X-ray diffraction (PXRD)f the materials showed a series of crystalline peaks corre-ponding to staggered 2D sheets, as in graphite (gra), forOF-1. COF-5 by contrast exists as eclipsed sheets, as inoron nitride (bnn). The porosity of the networks was mea-ured by nitrogen gas adsorption at 77 K and by applyinghe BET equation over pressure range of 0.04–0.1. COF-

was found to have a Type I isotherm and an apparent
urface area of 711 m2/g. For COF-5, the surface area waseasured to be 1590 m2/g and the isotherm showed two
teps. This is due to the width of the hexagonal channels27 A) and the mesoporous nature of COF-5.

Science 37 (2012) 530– 563

Further 2D COFs were synthesized containing the HHTPlinker (COF-6, COF-8 and COF-10) [46]. Again, these mate-rials were found to be crystalline, all with an eclipsed bnnstructure. The porosity of COF-6, -8 and -10 were probed byargon adsorption at 87 K and the materials were found tohave Langmuir surface areas of 980, 1400 and 2080 m2/g,respectively. COF-6 exhibited a Type I isotherm, while COF-8 and COF-10 exhibited high uptakes of gas at higherrelative pressures, characteristic of Type IV isotherms.

A significant advance in 2007 was the use ofthree-dimensional (3D) monomers, specifically tetra(4-dihydroxyborylphenyl)methane and the silicon-centeredanalogue [48]. These monomers were either self-condensed to produce COF-102 and -103, respectively, orwere reacted with HHTP to form COF-105 and -108. Allof these materials were found to be crystalline and werematched to modeled PXRD patterns. The surface areasof these materials were significantly higher than thosemeasured for 2D COF networks, and BET surface areas of3472 and 4210 m2/g were found for COF-102 and -103,respectively.

Other research groups have also produced crystallineCOFs – for example a material with a pore size of 18 Avia condensation of benzene-1,3,5-triboronic acid with1,2,4,5-tetrahydroxybenzene to form a five membered ring,as seen in COFs-1, -102 and -103 [45]. In contrast withthe materials prepared by the Yaghi group, these materi-als were not synthesized under solvothermal conditions;rather, simple reflux in a mixture of THF and methanol wasemployed. Further work showed that the pore sizes of thenetworks could be tuned by the addition of alkyl chains inone of the monomers [49]. The largest pore size materiallacked any alkyl chains, COF-18Å. When a methyl groupwas added, the pore size was reduced to 16 A. An increasein the chain length, using a propyl side chain, decreased thepore size further to 11 A. As the pore sizes of the networkswere reduced, the ability to adsorb nitrogen gas decreased,resulting in a reduction in BET surface area from 1263 m2/g(COF-18Å) to 105 m2/g (COF-11Å). However, adsorptionof the smaller gas, H2, increased from 4.84 mol H2/mol forCOF-18Å to 5.33 mol H2/mol for COF-11Å. This chemicaltuning of the pore size could be useful for selective gasseparations.

Further COFs with larger pores have also been reported,such as BTP-COF synthesized via a solvothermal routeusing 1,3,5-benzenetris(4-phenylboronic acid) and 2,3,6,7-tetrahydroxy-9,10-dimethylanthracene to yield a poresize of 40 A. [50] Also, a Zn centered porphyrin withphenylboronic acid functionalities was reacted with1,2,4,5-tetrahydroxybenzene to form a crystalline network(ZnP-COF) with pores of 25 A [51].

Other reactions, such as imine formation, can lead to theformation of crystalline networks such as COF-300 whichwas reported in 2009 [52]. This material is linked togetherby imine bonds via the solvothermal reaction betweentetra-(4-anilyl)methane and terephthaldehyde. The result-ing 3D COF has a BET surface area of 1360 m2/g with a pore
size of 7.2 A. A potential advantage of this material overboron-linked COFs is enhanced physicochemical stability.In the same year, we reported the first of a series of iminelinked porous organic cages [53,54] which are composed


isation
Fig. 5. Ionothermal cyclotrimer
of discrete molecules, rather than extended networks, andwhich can hence exhibit unique ‘on/off’ porosity switchingbehavior [55]. The fact that these systems form discretemolecular cages, rather than extended networks such asCOF-300, again illustrates the need for highly rigid tectonsin porous network formation, in this case to prevent thesystem from ‘folding up’ into a more thermodynamicallystable molecular cage.

2.2. Covalent triazine frameworks

The synthesis of triazine-based networks by the self-condensation of nitriles in the presence of ZnCl2 hasproduced a number of highly porous materials. Amongthe first triazine networks reported, a material synthesizedfrom 1,4-dicyanobenzene produced a network which alsoshowed a degree of crystallinity (Fig. 5; apparent BET sur-face area = 791 m2/g)). [26] The X-ray data, while exhibitingbroad peaks, suggested a bnn structure as observed in otherCOFs [25]. Larger quantities of ZnCl2 led to an increasein the surface area to 1123 m2/g but the crystallinity inthe material was lost. Other monomers were investigatedincluding thiophenes, pyridines, and extended monomers,but these too produced amorphous products. The biphenyl
analogue (4,4′-dicyanobiphenyl) was found to produce anamorphous network with some Type IV isotherm char-acteristics and the highest BET surface area in the series(2475 m2/g).
route to triazine networks [26].

Further investigation into the reaction conditions for thesynthesis of triazine networks from 1,4-dicyanobenzenewas undertaken [56]. The surface area and mesoporositywere found to increase with increasing reaction temper-ature, from 920 m2/g at 400 ◦C to 2530 m2/g at 700 ◦C,with the average pore diameters increasing from 2.0 nm to3.6 nm. The highest surface area (3270 m2/g) was obtainedby the use of a two-step heating method, whereby the reac-tion was initially carried out at 400 ◦C for 20 h, after whichthe temperature was increased to 600 ◦C for another fourdays. Further networks have been produced using a vari-ety of cyano monomers including an adamantane-centeredtetrahedral network [57]. Again, increasing temperaturewas shown to increase the surface area and mesoporos-ity of the networks, while the inclusion of coordinatinggroups was thought to have potential applications in thefield of catalysis (see Section 4) [57]. It should also benoted that significant carbonization was observed, as evi-denced by a reduction in the nitrogen to carbon ratio, underthese more robust synthesis conditions. The use of 2,6-dicyanonaphthalene has also been reported, showing thatincreasing the amount of ZnCl2 to 5 eq. improves the sur-face area of the formed network to 2255 m2/g from only90 m2/g when one equivalent of ZnCl2 is used [58].

The use of a carbon-centered tetrahedral nitrilemonomer was reported by Ren et al. [59]. The networkwas reported to have a Type I isotherm with a BET surfacearea of 891 m2/g, rather lower than other reported triazine


ndane (

nfNas

htBwp

2

ifTobrtis[pa3uatrosmh

pipp2aPlp[8a

Fig. 6. 5,5′ ,6,6′-Tetrahydroxy-3,3,3′ ,3′-tetramethyl-1,1′-spirobisi

etworks [26,56] and contrary to the large increase in sur-ace area observed in tetrahedral crystalline COFs [48]. TheL-DFT pore size distribution was found to be centeredround 1.1 nm and the network was shown to have a highelectivity for benzene over cyclohexane.

A related series of materials containing triazine ringsas been based on Schiff base chemistry via the condensa-ion of melamine and di- and trialdehydes [60]. The highestET surface area obtained was 1377 m2/g. These materialsere made from very cheap monomers and contain highercentages of nitrogen [61].

.3. Polymers of intrinsic microporosity (PIMs)

Another method of synthesizing nanoporous polymerss the use of non-reversible condensation reactions toorm polymers which pack ineffectively in the solid state.his is exemplified by PIMs, first developed by McKe-wn and Budd. Porosity in PIMs results from the rigid,ent monomers which contain a tetrahedral carbon atomeferred to as a ‘site of contortion’. This site of contor-ion causes the polymer to fill space ineffectively, resultingn free volume between the polymer chains. When theolvent is removed, this leaves an open pore structure62,63]. PIMs have been shown to have wide-rangingotential applications in gas storage [3,4], separations [64],nd catalysis [64]. The monomer 5,5′,6,6′-tetrahydroxy-,3,3′,3′-tetramethyl-1,1′-spirobisindane (Fig. 6) has beensed in a number of PIMs; it is a cheap, commerciallyvailable monomer that can introduce a site of contor-ion into a polymer chain. PIMs use a dioxane-formingeaction between an ortho-dihydroxy monomer and anrtho-dihalide monomer (usually a difluoride, but someyntheses utilize dichlorides) to form either a linear poly-er or a network in the case where one of the monomers

as more than two pairs of halides [2,65].The first PIMs were based on phthalocyanines and por-

hyrins (B3 and B1, Fig. 7) and contained metal ions or 2H+

n the cavities [62,63]. These networks exhibited micro-orosity, with BET surface areas of 450–950 m2/g for thehthalocyanine networks containing Zn2+, Cu2+, Co2+ andH+ centers. The porphyrin networks had BET surfacereas of 980 m2/g (FeCl) and 910 m2/g (2H+). A subsequentIM network was synthesized from a hexaazatrinaphthy-ene monomer (B2, Fig. 7), which was reacted with the
reviously mentioned contorted spirobisindane monomer66]. The reported BET surface area was between 750 and50 m2/g. This network PIM (referred to as HATN-PIM) waslso shown to be able to bind to a metal (Pd), although this
left) reacts with tetrafluoroterephthalonitrile to form PIM-1[65].

reduced the surface area to 347 m2/g, most likely due to theeffect of the increased mass from the metal center.

Following these metal coordinating network PIMs, aseries of linear PIMs was reported [39]. Unlike networks,linear PIMs were found to be soluble in common organicsolvents and could be processed to form films, constitut-ing a key advance in porous polymer materials research[36]. The most studied material in this series, later namedPIM-1 (Fig. 6), had the highest BET surface area for a lin-ear PIM at 850 m2/g, while other soluble PIMs were foundto have surface areas of between 440 and 600 m2/g [39].Due to their solubility, the molecular weights can also becalculated. PIM-1 had the highest molecular weight of thisseries (270,000 g/mol). PIM-1 was the subject of perme-ability studies and showed good selectivity for a numberdifferent gas combinations (e.g., O2/N2) as well as good per-meability, lying above Robeson’s upper limit of a range ofdifferent polymers used in separations [64].

PIM monomers based on hexahydroxytriptycene (A1,Fig. 7) [67] and cyclotricatechylene (A2, Fig. 7) [68] usingthe same dioxane-forming reaction gave rise to one of thehighest surface area PIMs, Trip-PIM, which was found tohave a BET surface area of 1064 m2/g [67]. By tuning thealkyl chain length of the triptycene monomer (R, A1, Fig. 7),it was found that the surface area of the network could betuned, with the shortest chain (CH3) giving rise to the high-est BET surface area of 1760 m2/g while the longest chain((CH2)7CH3), possessed the lowest surface area (618 m2/g)due to the chains filling the pores [69].

Extension into bis(phenazyl)s, fused fluorenes, andimides has produced further series of PIM materials withBET surface areas in the range 470–900 m2/g. This hasallowed further tuning of the permeability and separationproperties of PIMs [70–72]. Other research groups havealso produced PIMs, including the highest surface area PIMto date (OFP-3; 1159 m2/g) [73]. Functionalization of themonomers used to make PIM-1 has also been achieved.Using these new monomers, it was possible to introducesulfone and carboxylate side groups into the backboneof the polymers [74,75]. Although the porosity was notmeasured in these networks, the gas permeability and sep-aration properties of the films were studied in depth, againexceeding the Robeson upper bound.

The synthesis of PIMs using rigid twisted spirob-ifluorenes has also been reported using imide- and
amide-forming reactions with the more rigid polyimideshowing the highest surface area of 551 m2/g [76]. Theimide forming reaction was also shown to form PIMswith binaphthalene was used as the site of contortion


s used in
Fig. 7. Some monomer
[77,78], however the surface areas were initially rela-tively low [78]. The use of the binaphthalene monomerhas also been used to make the first chiral PIM [79]. Fur-ther polyimide forming reactions involving binaphthaleneand 4,4′-(9-fluorenylidene)dianiline were carried out andshowed selective gas uptakes (CO2 over H2) when the poly-mers were cast as films [80].

Related work has shown that insoluble, infusiblepolymers can be prepared from highly soluble precur-sors by molecular rearrangement at about 350–450 ◦C.Benzoxazole–phenylene or benzithiazole–phenylene
based polymers were thus generated to form microp-orous membranes [81]. Changes in chain conformationand topology on rearrangement created microporositywhich can be used for molecular separations (see below).
the synthesis of PIMs.

Recently, post-synthetic modification of PIM-1 by a[2 + 3] cycloaddition was used to form TZPIM, a tetrazole-functionalized PIM [82]. This membrane demonstratedexceptional gas separation performance.

2.4. Hypercrosslinked polymers (HCPs) and othercrosslinked polymer networks

Crosslinked polymers are a broad group of materials.HCPs were some of the first purely microporous organicmaterials, and the materials showed remarkable properties
that predate significantly the more recent studies reportedover the last 10 years. HCPs were developed by takingthe concepts used for making other crosslinked materials(for example, macroporous resins and polyHIPEs [83]) and


methyl)

enia

mHmtrbbhwmp

nTtdcmaccBmt

Fig. 8. Hypercrosslinking of poly(chloro

xtending the crosslinking even further, thus making theetworks highly rigid and unable to collapse. This results

n materials with small pore sizes and high surface areasnd micropore volumes.

Among the first reports of microporous organic poly-ers derived from non-styrenic monomers were carbinolCPs with surface areas up to 1000 m2/g [84,85]. Theseaterials were synthesized, for example, via a lithia-

ion route (similar to element organic frameworks (EOFs)eported more recently [86]) of dibromobiphenyl followedy the addition of dimethyl carbonate, thus linking theiphenyls together with C–OH groups and resulting in aighly microporous polymer network [84,85]. This workas ahead of its time, however the properties of theseaterials were not investigated extensively and the micro-

orous polymer field really only ‘took off’ in the early 2000s.HCPs based on crosslinked polystyrene [40,87] are

ow sometimes referred to as ‘Davankov resins’ [40].hese materials can be synthesized by the polymeriza-ion of vinylbenzyl chloride with a small amount ofivinylbenzene crosslinker to produce a lightly crosslinkedopolymer. When swollen in a suitable solvent, this copoly-er can then be ‘hypercrosslinked’ via a Friedel–Crafts

lkylation reaction using a Lewis acid such as iron(III)hloride (Fig. 8). Due to the slightly different synthetic pro-
edures used to hypercrosslink the precursor polymer, theET surface areas of these materials vary from approxi-ately 600 m2/g up to 1466 m2/g [88] and 2090 m2/g in
he maximum case reported.

Fig. 9. Monomers used in the synthesis of hypercrossl

styrene using iron(III) chloride [88–91].

Monodisperse particles can be prepared from suchhypercrosslinked polystyrene materials with diameters inthe micrometer range. These particles can absorb signifi-cant amounts of solvent and have uses in separations andas the stationary phase for HPLC [90].

Friedel–Crafts alkylation has also been used to pre-pare high surface area materials without the needto make the precursor crosslinked polymer. Thisdirect approach to microporous organic networks usesbis(chloromethyl) aromatic monomers such as dichlorox-ylene (DCX), bis(chloromethyl)biphenyl (BCMBP), andbis(chloromethyl)anthracene (BCMA) [92]. A number ofmaterials were synthesized using different amounts ofLewis acid and copolymers of the different isomers ofDCX and BCMBP, utilizing high throughput synthesis andcharacterization techniques (Fig. 9). These materials hadapparent BET surface areas of up to 1900 m2/g and weregenerally more porous when BCMBP was included. Themaximum apparent BET surface area for DCX networkswas ∼1400 m2/g. The surface areas were found to increasewith increasing amounts of Lewis acid.

Copolymerizations of BCMBP with fluorene, 9,9′-spirobi(fluorene), dibenzofuran, and dibenzothiophenehave also been reported [93]. Up to 25% of the comonomerwas introduced, although the highest surface areas were
produced using smaller quantities of comonomer. 10%dibenzofuran produced a network with a surface area of1800 m2/g. The inclusion of the comonomer was confirmedusing FTIR and ssNMR spectroscopy.
inked networks by direct polymerization [92].


element
Fig. 10. Schematic structures of ‘
A variety of other reactions have been used success-fully for the synthesis of microporous structures whichcould also be considered as ‘hypercrosslinked’. A seriesof polysilanes (Fig. 10) was synthesized by reactingtetraethylorthosilicate with dilithiated aromatics to pro-duce materials described as EOFs [86]. These materialsare thermally stable and hydrophobic, as shown by wateradsorption isotherms. The apparent BET surface areas ofthe materials were 780 m2/g (EOF-1) and 1046 m2/g (EOF-2). Using a similar reaction between 4,4′-dibromobiphenyland metal chlorides it was possible to prepare tin, anti-mony, and bismuth EOFs [94]. The BET surface areas forthese networks were 445 (Sn), 423 (Sb) and 261 m2/g (Bi),which is lower than the Si EOFs but might in part beexplained by the increased mass of the tetrahedral metalnodes.

Hypercrosslinking of linear polyaniline and polypyr-role precursors has also been developed by Germain etal. [29,95,96]. Initially, commercially available polyani-line was crosslinked using either diiodomethane orparaformaldehyde to form CH2-linked networks (Fig. 11)[95]. The synthesis of these materials does not require theuse of lithiating agents or large quantities (stoichiometricamounts) of Lewis acids and therefore does not produce
large volumes of HCl gas, as do the aforementioned HCPs.The BET surface areas for the polyaniline networks reached632 m2/g for the diiodomethane route and 480 m2/g for theformaldehyde method.
organic frameworks’, EOFs [86].

Further work involving polyanilines, this timecrosslinked using Ullmann and Buchwald couplingreactions, was also carried out, resulting in networkslinked by aromatic groups (Fig. 11) [29]. However, thesenetworks possessed lower apparent BET surface areas(up to 316 m2/g) for the Buchwald coupling method.These synthetic methods were also used to form net-works, without the precursor polymer, directly from thediaminobenzene and tribromobenzene, thus producing apolymer with an apparent BET surface area of 249 m2/g.

Similar methodologies have been used to pre-pare hypercrosslinked networks of polypyrrole usingdiiodomethane, triiodomethane, and triiodoborane toproduce porous structures with CH2, CH and B crosslink-ers, respectively [96]. The porosity of the networks wasconfirmed by the BET method which demonstrated thatthe polymers had apparent surface areas of 732, 401 and19 m2/g.

Amines have also been used to form microporousnetworks when reacted with borontrihalides [97]. Thesenetworks can be seen as analogues of COF and CTF sys-tems forming N3B3 rings, however the resulting networkspossessed no crystallinity. The network formed from p-phenylenediamine and BCl3 formed the highest surface
area material (BET SA = 1364 m2/g).
A number of porous networks have been synthe-sized using imide bond forming reactions under relativelymild conditions. [32,78,98] A tetraamino-spirobifluorene


polyan

mticaatwanbpefc

cmphmwwd

sadnagftw

Fig. 11. Structure of hypercrosslinked

onomer was reacted with pyromellitic acid dianhydrideo produce a network (PI1 see Fig. 12), in a similar fash-on to PIMs, which packed space inefficiently due to itsentral spiro carbon. This network exhibited a BET surfacerea of 982 m2/g [32]. The poly(amide) analogue showed

significantly lower surface area of only 50 m2/g, as didhe imide network made from tetraamino-biphenyl [32]. Itas therefore suggested that the imide networks required

monomer which packs inefficiently. The porosity for aumber of hypothetical polyimides and polyamic acidsuilt from a series of 1, 2 and 3D monomers has also beenredicted in recent theoretical studies [99,100]. The high-st predicted solvent accessible surface areas of 6360 m2/gor polyimides and 6751 m2/g were seen for hypotheticalrystalline networks incorporating 3D monomers.

Recently, imide networks synthesized using cheap,ommercially available melamine and anhydrideonomers have been reported [101]. The use of

yromelitic dianhydride with melamine produced theighest surface area material (660 m2/g). Another cheaponomer – cyanuric chloride – has been polymerizedith piperazine to yield a microporous network (PAF-6)ith a BET surface area of 183 m2/g and has been used forrug release [102].

The monomer tetra-(4-anilyl)methane, as used in theynthesis of COF-300 [52], has also been used to form

porous network via reaction with naphthalene dianhy-ride (Fig. 12) [98]. Unlike COF-300, the network showedo crystallinity and the BET surface area of 750 m2/g waspproximately half that of the ordered imine network. The
as separation properties of the network were also testedor the separation of CO2 from CH4, showing good separa-ions at low pressure. Further work reducing the materialith lithium increased the selectivity of CO2 over CH4 while
Fig. 12. Structures of PI1 (left) [32] and

iline networks (R = CH2 or Ar) [29,95].

the surface area decreased due to pore filling with lithium[103].

Imine formation has also been used to form amorphousnanoporous polymers in a similar fashion to crystallineCOF-300 [52]. These networks used the trifunctionalmonomer 1,3,5-triformylbenzene (as also used in porousorganic cages [53]) combined with a variety of diaminoben-zenes to yield networks with BET surface areas of up to1521 m2/g in the case of a 1,3-diaminobenzene monomer[104].

The cyclotrimerisation of ketones to form truxenes andtruxenones produced networks with surface areas up to1650 m2/g [105]. The pore sizes of the materials werenot reported. The possibility of functionalization of theremaining ketone in the truxenone networks was also sug-gested. Cyclotrimerisations of diketones using either SiCl4or 4-toluene sulfonic acid has also been demonstratedand has produced networks with apparent BET surfaceareas of up to 895 m2/g for the cyclotrimerisation of 1,4-diacetylbenzene using SiCl4 [106]. The cyclotrimerisationof ketones using thionyl chloride was demonstrated usinga wide range of monomers including a 3D tetrahedralketone and a spirobifluorene ketone [107]. Apparent BETsurface areas ranged from 259 m2/g for the spiro-networkto 832 m2/g for the tetrahedral network [107].

2.5. Conjugated microporous polymers (CMPs) andanalogues

CMPs can be considered a subclass of hypercrosslinked
polymers. They differ significantly, however, from otherhypercrosslinked polymers in that they consist of multiplecarbon–carbon bonds and/or aromatic rings that form anextended conjugated network. As such, there is potential
tetrahedral imide (right) [98].


tures fo
Fig. 13. Schematic 2D strucReprinted with permission from [22].
to combine the well known properties of conjugatedlinear polymers with the advantages of nanoporosity. Thelinking together of carbon atoms in a conjugated mannerlends itself to the use of metal catalysis which can beconducted under relatively mild conditions in comparisonto, for example, triazines (molten ZnCl2) [26,56,57], andhypercrosslinked networks (Lewis acid catalysis) [88–92].These mild synthesis conditions suggest potential for
incorporating a wide range of chemical functional groups,and the rapid growth of literature in this area bears this out.
The first CMPs were reported in 2007 [108] andused Sonogashira–Hagihara palladium coupling to link

r a series of CMP networks.

aromatic halides to aromatic alkynes, thus formingpoly(aryleneethynylene) (PAE) networks [19,109]. A seriesof four CMPs was produced initially with BET surface areasin the range 522–834 m2/g. The network CMP-1 (Fig. 13)had both the shortest strut length and highest surface area,whereas the polymer with the longest strut length (CMP-3)exhibited the lowest surface area. This behavior was ratio-nalized by the increased flexibility and cumulative degrees
of conformation freedom as the struts increased in length[22]. It is also possible that as the struts increase in thelength, the fragments of the growing network are more ableto interpenetrate and hence to fill space.


olumesR

t0CBssramsfCcsmtc[

stg

(banlh

rTnise

Fig. 14. Pore size distributions (left) and micropore veprinted with permission from [22].

This series of CMPs was expanded by the addition ofwo more networks, one with a shorter strut length (CMP-) and the other with longer struts (CMP-5) (Fig. 13) [22].MP-0, with the shortest struts, gave rise to an even higherET surface area (1018 m2/g). More importantly, this serieshowed the effect that the strut length has on the poreize and micropore volume, with the smallest struts givingise to the smallest pores and highest micropore volumes,nd the largest struts the largest pore sizes and lowesticropore volumes. A number of copolymers were also

ynthesized by reacting 1,3,5-triethynylbenzene with dif-erent ratios of the two halide monomers used to makeMP-1 and CMP-2 (CPN-1 to CPN-6) [22]. These statisti-al copolymerizations lead to a remarkable and continuousynthetic control over surface area, average pore size, andicropore volume (CPN-1 to -6, Fig. 14). Such fine syn-

hetic control over porosity was previously thought to beonfined to crystalline materials such as COFs and MOFs109].

All of these CMP networks were characterized bysNMR, confirming the expected structures and showinghat side reactions such as homocoupling of the ethynylroups had not occurred.

Homocoupled CMP networks of 1,3,5-triethynylbenzneHCMP-1) and 1,4-diethynylbenzene (HCMP-2) have alsoeen prepared deliberately, leading to networks withpparent surface areas of 842 and 827 m2/g [110]. Thisetwork chemistry served subsequently as a platform for

ithium-decorated materials where remarkable H2 uptakesave been reported (Section 3.1, ref. [111]).

CMP materials have also been synthesized with a wideange of different pendant functional groups [112,113].hese groups have included alcohols, amines, fluorine,
itro groups, pyridyl groups, and methyl groups. The
nclusion of these functionalities was again proven bysNMR and the diverse chemistry resulted in some inter-sting effects on the surface properties of the materials.

(right) of statistically copolymerized CMP networks.

The unsubstituted, mainly aromatic polymer (CMP-1) ishydrophobic but can be rendered hydrophilic (by theinclusion of alcohol groups) or even more hydrophobic(by including fluorinated groups) (Fig. 15). This methodenabled the selective uptake of hydrophilic dyes into thehydrophilic polymers while those with hydrophobic func-tionalities did not adsorb the dye.

Clearly the CMP-forming chemistry is diverse, but inclu-sion of certain functionalities led to CMP networks withrather low BET surface areas (<500 m2/g) in comparisonwith their non-functionalized analogues [112]. Furtherinvestigations showed that the reaction solvent choice hada dramatic effect on the porosity in such networks [113].For example, it was shown that the surface area and per-centage micropore volume of certain networks could beincreased by up to 300% by using DMF as a reaction solventinstead of toluene (Fig. 16).

A series of CMP networks containing nitrogen-centeredmonomers was also synthesized [114]. BET surface areasranging from 1108 to 546 m2/g were observed and, in asimilar manner to the previous CMP series [22], microporevolume and surface area decreased with increasing strutlength. The introduction of nitrogen atoms into the struc-ture presents the possibility of further functionalization,such as metal binding. The nitrogen centers could also beable to act as donors: for example in porous photovoltaicdevices [115].

The synthesis of CMP-type material utilizing spirobiflu-orene monomers was reported by Thomas et al. (Fig. 17)[8,116,117]. The Sonogashira–Hagihara reaction was usedto couple 2,2′,7,7′-tetrabromo-9-9′-spirobifluorene with1,4-diethynylbenzene, producing a network with a BET sur-face area of 510 m2/g, while SAXS implied a surface area
of 1030 m2/g [116]. The N2 isotherm for this network alsoshows a large uptake of gas at high pressures, similar to thatseen in the tetrahedral CMP samples E3 and E4 (see below)[21]. The use of this monomer to form networks using the


atic funts the h

Fig. 15. CMP networks with fluorinated groups (left), unsubstituted aromdroplets in contact with these materials show how the functionality affec

Suzuki reaction was also achieved, yielding BET surfaceareas of 450 and 210 m2/g when reacted with benzene-1,4-diboronic acid (Fig. 17) and 4,4′-biphenyldiboronicacid respectively. The use of 2,5-thiophene diboronic acidwith the spiro monomer has also recently been reported[118]. By changing the ratio of the thiophene and benzene
diboronic acids it is possible to tune the adsorption andemission wavelengths of the networks.
The Suzuki and Sonogashira–Hagihara reactionshave also been used to form networks from hexakis

Fig. 16. Effect of reaction solvent on surface area for CMP networks. Graph shocompared to DMF. Most networks other than CMP-1 (shown in blue) are more p3-fold enhancements in BET surface area, respectively. (For interpretation of the rversion of the article.)Reprinted with permission from [113].

ctionality (center), and alcohol groups (right). The photographs of waterydrophobicity.

(4-bromophenyl)benzene with benzene-1,4-diboronicacid and 1,4-diethynylbenzene [119]. The Suzuki reactionformed the network with the higher apparent BET surfacearea (1148 m2/g).

A novel A4 monomer (2,3,5,6-tetrakis(4-bromophenyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione),
has been used to synthesize a series of networks usingYamamoto and Sonogashira–Hagihara Pd-coupling reac-tions [120]. The surface areas of these networks wererelatively low in comparison to other CMPs ranging
ws a comparison of BET surface areas for CMPs synthesized in tolueneorous when prepared in DMF, those shown in green and red have 2- andeferences to colour in this figure legend, the reader is referred to the web


F spirobifl

fw1

bitw1sradalal[t1whw

wtp[

9putts

ahos(wa

Sws

ig. 17. Synthesis of CMPs by Thomas et al. from 2,2′ ,7,7′-tetrabromo-9-9′-

rom 216 m2/g for the Yamamoto homocoupled net-ork to 477 m2/g for the network synthesized with

,4-diethynylbenzene.Yamamoto coupling of bromoaromatics has also

een reported [117], and this chemistry has led tomportant discoveries in this area. When 2,2′,7,7′-etrabromo-9-9′-spirobifluorene was homocoupled, a net-ork was produced which exhibited a BET surface area of

275 m2/g. Copolymerization of 2,2′,7,7′-tetrabromo-9-9′-pirobifluorene with 1,4-dibromobenzene (1:1) was alsoeported (Fig. 17) giving a network with a higher surfacerea (887 m2/g) than the 1,3- (5 m2/g) and 1,5-substitutedibromobenzene (361 m2/g) analogues. Increasing themount of 1,4-dibromobenzene in the networks lead toower surface areas and decreased micropore volumes,s would be expected from the average increase in strutength as discussed for the CMP series of materials above19,22]. When only 1,3,5-tribromobenzene was used ashe monomer, the homopolymer had a surface area of255 m2/g. Copolymerization of 1,3,5-tribromobenzeneith dibromobenzene increased the strut lengths andence reduced surface areas and micropore volumes, in lineith the discussion above.

Other Yamomoto couplings have produced networksithout the need for twisted monomers. These include

he homocoupling of 1,3,5-tris(4-bromophenyl)benzene toroduce a network (PAF-5) with a BET SA of 1503 m2/g121].

Replacing the bromo moieties of 2,2′,7,7′-tetrabromo--9′-spirobifluorene and 1,3,5-tribromobenzene with thio-hene yielded new monomers which were polymerizedsing oxidative coupling, hence forming networks con-aining bithiophene linkers [8]. The BET surface areas ofhe networks were reported as 577 and 1060 m2/g for thepirobifluorene and benzene network, respectively.

A thiophene-containing spirobifluorene network waslso reported by Yuan et al. [122] (PS4TH) and this time aigher surface area of 971 m2/g was reported. A number ofther networks were also prepared using the cyclotrimeri-ation of 2,2′,7,7′-tetraethynyl-9,9′-spirobifluorenePS4AC2) and tetra(4-ethynylphenyl)methane (PT4AC),hich were reported to have BET surface areas of 1043

nd 762 m2/g, respectively.
Chen et al. [123,124] have produced networks by
uzuki cross-coupling reactions. A polyphenylene CMPhich showed light-harvesting properties was synthe-

ized from 1,2,4,5-tetrabromobenzene and 1,4-benzene

uorine using the Sonogashira, Suzuki and Yamamoto reactions [116,117].

diboronic acid. This polymer exhibited a BET surface areaof 1083 m2/g and pore size of 1.56 nm [123]. A porphyrin-linked network was also synthesized and was investigatedas a heterogeneous catalyst [124]. The isotherm wasreported to be Type III in character showing that the net-work had both micro- and mesoporosity [11]. The poresize distribution for this network showed the pores to be2.69 nm and the BET surface area was calculated to be1270 m2/g, slightly higher than for the polyphenylene net-work [124].

Cyclotrimerisation of a number of di- and tri-alkynemonomers using a Co2(CO)8 catalyst has produced aseries of conjugated networks with surface areas of up to1246 m2/g when using 1,3,5-triethynylbenzene [27]. In asimilar way to the first series of CMP networks [22], Yuanet al. were able to tune the pore sizes of the materials bychanging the strut length of the monomers.

The use of the cubic polyhedral oligomeric silsesquiox-ane (POSS) monomer has been used to produce micro-porous materials via the Sonogashira–Hagihara andYamamoto routes (Fig. 18) as well as the copperchloride mediated homocoupling of alkyne functional-ized POSS monomers [125–127], although of coursethe POSS cubes break the extended conjugation inthese materials. BET surface areas of the Sonogashirareactions range from 846 m2/g for the reaction with 1,4-diethynylbenzene (PSN-1) to 1042 m2/g for the reactionwith 4,4′-diethynylbiphenyl (PSN-2) [125]. The Yamamotohomocoupled network had a similar BET surface to PSN-2(1045 m2/g) [126]. Type I isotherms were measured for allnetworks and pore sizes of POSS networks ranged from 0.7to 1.2 nm. Alkyne homocoupled POSS networks producednetworks were also shown to have BET surface areas of upto 1000 m2/g [127]. Similar monomers were used in a ther-mal decoupling reaction of iodophenyl-substituted POSSwith BET surface areas of up to 555 m2/g [128].

A common theme in the area of porous organic net-works is the use of tetrahedral monomers to increasesurface areas. A number of networks based on tetra-hedral monomers have already been mentioned suchas the COFs [48], triazine frameworks [59], and EOFs[86] discussed above. CMP-like materials have also beenprepared from tetrahedral monomers (Fig. 19) such as
tetrakis(4-iodophenyl)methane (TPM-I) and tetrakis(4-bromophenyl)silane (TPS-Br) [21], although again, asfor the POSS materials mentioned above, the presenceof the tetrahedral center in the monomer breaks the


from sil
Fig. 18. Porous networks synthesized
extended conjugation in the network. The maximumBET surface area achieved from the reaction of TPM-I with 1,4-diethynylbenzene (E1) was 1213 m2/g, whilewith 1,3,5-triethynylbenzene (E2) the surface area was488 m2/g. Both networks gave rise to Type I isotherms.Network E1 showed a very sharp pore distribution cen-tered around 1.1 nm in comparison with previous CMPnetworks which showed somewhat broader pore size dis-tributions. The coupling of tetrakis(iodophenyl)methanewith the tetrakis(ethynylphenyl) methane monomer hasalso been reported [129]. While this network showed abroader pore size distribution, the surface area was shownto increase to 1917 m2/g.

The Sonogashira–Hagihara cross-couplingof the adamantane-centered 1,3,5,7-tetrakis(4-iodophenyl)adamantane monomer with1,4-diethynylbenzene produced a network with a BETsurface area (665 m2/g) that was around half of its carboncentered analogue [130]. It was also shown that thenetwork could be functionalized after the polymerizationusing an azide compound which reacts with the remainingalkyne groups of the network in a ‘click’ reaction whilemaintaining nanoporosity.

The highest known surface areas reported in a microp-orous organic polymers so far belong to materials producedvia the homocoupling of tetrahedral monomers, such
as tetrakis(4-bromophenyl)methane, using the Yamamotoreaction [23]. One network, PAF-1, showed an excep-tionally high BET surface area of 5600 m2/g (Langmuir7100 m2/g). The CO2 uptake at 298 K and 40 bar was also
oxane centered monomers [125,126].

measured to be 1300 mg/g. The capacity of this materialto adsorb organic vapors was also tested – PAF-1 adsorbed1306 mg/g of benzene and 1357 mg/g of toluene. PAF-1 wasalso found to be hydrothermally stable as shown by theretention of porosity even after boiling in water for sevendays. PAF-1 was also found to be thermally stable to 520 ◦C.The first report on PAF-1 implied a degree of crystallineorder, although we have suggested that the compositionand the high level of porosity in this material can alsobe rationalized by an amorphous model that is related toamorphous silica [24]. Further tetrahedral based networkssynthesized via Yamamoto coupling have been reportedrecently, exceeding the surface areas of PAF-1 [131]. Thesilicon-centered monomer tetrakis(bromophenyl)silane,as previously reported by Holst et al. [129], was reactedunder optimized conditions to give a network, PPN-4, witha BET surface area of 6461 m2/g [131], which at the time ofwriting, is the highest reported surface area of any materialto date. This result illustrates that reaction conditions suchas catalysis concentration and temperature, like solvent[113], can have a profound effect on the resulting poros-ity in metal-coupled polymer networks, and suggests thatcareful optimization of reaction conditions is desirable formaterials in this class.

Other tetrahedral monomers have been used toproduce high surface area materials via Yamamoto
coupling [129,132]. These include the direct analoguemonomer of PAF-1 prepared from TPM-I (E, Fig. 19)[23]. The reported BET surface area of this networkwas, however, somewhat lower than that of PAF-1


Fig. 19. Summary of nanoporous organic networks prepared using tetrahedral monomers. This has lead to materials with much high surface areas than2D analogues, and the leading materials in terms of surface area are also some of the most physically and chemically robust.


Table 1Hydrogen storage in MOPs.

Material SABET (m2/g) Vtot (cm3/g) P (bar) T (K) wt.% Ref

COFsCOF-1 750 0.30 Sata 77 1.46 [47]COF-1 628 0.36 1 77 1.28 [151]

100 298 0.26COF-5 1670 1.07 Sata 77 3.54 [47]COF-6 750 0.32 Sata 77 2.23 [47]COF-8 1350 0.69 Sata 77 3.46 [47]COF-10 1760 1.44 Sata 77 3.88 [47]COF-102 3620 1.55 Sata 77 7.16 [47]COF-103 3530 1.54 Sata 77 6.98 [47]COF-18Å 1263 0.69 1 77 1.55 [49]COF-16Å 753 0.39 1 77 1.40 [49]COF-14Å 805 0.41 1 77 1.23 [49]COF-11Å 105 0.052 1 77 1.22 [49]

TriazinesDCBP 2475 2.44 1 77 1.55 [26]

HCPsDavankov resins 1466 1 77 1.28 [88]

10 77 2.7515 77 3.04

2090 1.2 77 1.55 [91]pDCX 1370 1.13 77 1.69 [92]BCMBP/DCX 1904 1.13 77 1.61 [92]EOF-1 780 1 77 0.94 [86]EOF-2 1046 1 77 1.21 [86]Precursor

polyanilines632 0.94 1.2 77 0.96 [95]480 0.55 1.2 77 0.82 [95]54 0.13 1.2 77 0.38 [29]316 0.25 1.2 77 0.85 [29]

Aniline 249 0.13 1.2 77 0.97 [29]Polypyrole 732 0.37 4 77 1.60 [96]

401 0.22 4 77 1.30 [96]19 0.03 1.2 77 0.63 [96]

PI1 982 0.62 1 77 1.15 [32]BLP-1(Cl) 1364 0.75 1 77 1.10 [97]BLP-2(Cl) 1174 0.65 1 77 1.30 [97]P1 616 0.42 1 77 0.95 [107]P2 451 0.17 1 77 1.13 [107]P3 597 0.24 1 77 1.11 [107]P4 832 0.48 1 77 1.25 [107]P5 259 0.28 1 77 1.56 [107]

PIMsPhthalocyanine 895 [62]Porph-PIM 980 1 77 1.20 [4,63]

10 77 1.95HATN-PIM 850 1 77 1.12 [4,66]

10 77 1.56PIM-1 850 0.78 1 77 0.95 [4,39]

10 77 1.45CTC-PIM 830 1 77 1.35 [4,68]

10 77 1.70Trip-PIM 1064 1 77 1.63 [4,67]

10 77 2.71OFP-3 1159 1 77 1.56 [73]

10 77 3.94CMPs

CMP-0 1018 0.56 1.13 77 ∼1.4 [22]1.13 293 0.006 [152]

Pd-CMP-0 604 0.36 1.13 293 0.069 [152]CMP-1 834 0.47 1.13 77 1.14 [22]CMP-2 634 0.53 1.13 77 0.92 [22]Li-CMP 795 1.61 1 77 6.1 [111]

1 273 0.1PSN-1 846 0.79 1 77 0.89 [125]HPOP-1 1148 1.28 1.13 77 1.50 [119]HPOP-2 742 0.46 1.13 77 1.08 [119]PSN-2 1042 0.87 1 77 1.00 [125]PSN-3 982 0.86 1 77 1.19 [125]


Table 1 (Continued)

Material SABET (m2/g) Vtot (cm3/g) P (bar) T (K) wt.% Ref

Siloxane-Yam 1045 1.01 1 77 0.82 [126]PS4TH 971 0.757 60 77 3.6 [122]

70 298 0.45PS4AC2 1043 0.477 60 77 3.7 [122]

70 298 0.43PT4AC 762 0.425 60 77 2.2 [122]

70 298 0.50PAF-1 5600 1 77 1.66 [153]

48 77 7.0 [23]PAF-3 2932 1 77 2.07 [153]

60 77 5.5PAF-4 2246 1 77 1.50 [153]

60 77 4.2

atfoBtabT3tgtuwLpro

h1p

TS

PPN-4 6461

a Indicates that the uptake is at saturation.

t 3160 m2/g [129]. The use of the silicon-centeredetrakis(bromophenyl)methane monomer (used later toorm PPN-4, see above) produced a network using theriginal PAF-1 conditions that exhibited an apparentET surface area of 1102 m2/g [129]. The coupling ofhe 3D monomer tetrakis(bromophenyl)adamantane using

Yamamoto reaction was reported independently byoth Holst et al. [129] and Lu et al. (PPN-3) [132].he reported surface areas for these networks were180 and 2840 m2/g, respectively. The homocoupling ofetrakis(ethynylphenyl)methane was also reported by bothroups, although using different catalysts: Holst et al. usedhe previously described method [110] while Lu et al.sed Cu(OAc)2·H2O. The BET surface areas of these net-orks were 1470 and 1249 m2/g (PPN-1), respectively.

u et al. also used the same method to homocou-le tetrakis(ethynylphenyl)adamantane (PPN-2), with theesulting network exhibiting an apparent BET surface areaf 1764 m2/g [132].

The synthesis of 3D networks using the Suzuki reactionas also been reported [133]. BET surface areas range from380 to 540 m2/g, slightly higher than for the networksroduced with spiro monomers [117] and the networks

able 2ummary of methane uptakes in MOPs.

Material SABET (m2/g) P (bar)

COFsCOF-1 750 35

COF-5 1670 35

COF-6 750 35

COF-8 1350 35

COF-10 1760 35COF-102 3620 35

COF-103 3530 35

HCPs1 1904 15

20

2 1307 15

20

3 963 15

20

36

4 1366 15

20

36PPN-4 6461 35

55 77 8.34 [131]

synthesized by Jiang and co-workers [123,124]. The‘click’ reaction of tetra-(4-anilyl)methane with tetrakis-(4-ethynylphenyl)methane was reported by two groupswith surface areas up to 1440 m2/g [129,134], whilethe use of tetrakis(4-ethynylbenzene)methane with 1,4-diazidobenzene produced a similar network [135]. Theadamantane-centered tetrahedral monomer was also usedin the same way, although the apparent BET surface areawas calculated to be only 494 m2/g [135]. As a strategyfor porous polymer synthesis, the use of 3D tetrahedralmonomers tends to produce networks with higher sur-face areas than networks made from 2D planar monomers,but at the expense of extended conjugation. There is muchscope, however, to produce ultrahigh surface area materi-als [16] that exceed the most porous activated carbons interms of accessible surface area and with the added benefitof allowing synthetic diversification.

3. Porous organic polymers as sorbents for gas
capture and storage
The use of nanoporous organic polymers as potentialgas storage or gas separation materials has been much

T (K) CH4 uptake (mmol/g) Ref

298 2.5 [47]298 5.6 [47]298 4.1 [47]298 5.4 [47]298 5.0 [47]298 11.7 [47]298 10.9 [47]

298 4.4 [31]298 5.2298 3.9 [31]298 4.5298 3.2 [31]298 3.5298 4.3298 4.3 [31]298 4.6298 5.5295 17.1 [131]


etwork
Fig. 20. Hydrogen uptakes for porous polymer n
discussed; in principle such materials can compete effec-tively with zeolites, MOFs, and activated carbons. In thissection, we will summarize the range of studies that hasbeen carried out so far.

3.1. Hydrogen storage in nanoporous organic polymers

The use of hydrogen as an energy source has been pro-posed as alternative to traditional fossil fuels due to itssingle, clean combustion by-product (water) and its chem-ical energy density, 142 MJ/kg, which is more than threetimes that of other fuels such as hydrocarbons [136]. How-ever, even assuming that a more efficient and scalable wayof producing hydrogen can be found, its use for examplein transport would require a safe and compact method ofstorage. Around 4 kg of hydrogen (or 24 kg of petroleum) isrequired to drive an average car a distance of 400 km [136].Unfortunately, 4 kg of H2 gas would require a volume of45 m3 at room temperature and 1 bar. Hydrogen gas usu-ally comes in cylinders pressurized to 200 bar: hence, thismass of hydrogen would require a volume of around 225 L,roughly the equivalent volume of 5 tanks of petrol. Liquidhydrogen has a higher density than compressed hydrogen(70.8 kg/m3) but requires cooling to −241 ◦C which poses arange of practical problems [136].

It is therefore clear that a better method of hydrogenstorage is required. Storing hydrogen as a metal hydridehas been widely researched [137], but there are prob-lems due to slow kinetics of hydride formation and energyinput required to reverse this reaction. Another method forhydrogen storage is physisorption within a highly porousmaterial. The presence of small micropores is crucial forenhanced storage under practicable conditions since mul-tiple sorbent–sorbate interactions increase the enthalpyof adsorption [138]. The optimum enthalpy has been cal-culated to be 15.1 kJ/mol for reversible hydrogen storageat room temperature [139]. The US Department of Energy(DOE) set a target for hydrogen storage of 6 wt.% at room
temperature for 2010 and by 2015 a target of 9 wt.%.While these targets have been exceeded at cryogenic tem-peratures [140–142], no physisorptive porous materialapproaches these values at ambient temperatures. Storage
s at 1 bar and 77 K. Data extracted from Table 1.

at sub-ambient temperatures could however be effective,providing that the associated energetic penalties do notrender the storage cycle unfeasible.

There has been a large amount of research in the fieldof hydrogen storage using porous MOFs, and numerousreviews have described recent progress [140,143–149].There have also been a few reviews which discuss thehydrogen storage capabilities of microporous organicmaterials [3–5]. Table 1 details the reported hydrogenuptakes for nanoporous organic polymers, expressed inunits of weight percentage. The majority of uptakes arereported at 77 K, well below the ideal temperature rangerequired for use as a mobile energy source. However, thereare also a number of reports for sorption at temperaturesranging between 273 K and 298 K, particularly for mate-rials incorporating metals for use in hydrogen ‘spillover’[150]. Furthermore, some values are reported as maximumpossible uptakes, while others reported at a specific pres-sure (often atmospheric pressure); it is therefore difficultto compare all of the reported values directly.

Due to their extremely high surface areas and low densi-ties, 3D COF networks were anticipated to have substantialpotential for gas storage and there have been a number ofreports both on their predicted and measures gas uptakes[146,154–156]. COF-102 showed the highest H2 uptake(7.16 wt.% at saturation; ∼35 bar) at 77 K [47], slightlyhigher than PAF-1 (excess uptake at saturation of 7.0 wt.%),even though the BET surface area for PAF-1 was around2000 m2/g higher than COF-102 [23], probably emphasiz-ing the strong effect of pore size. COF-18Å showed thehighest H2 uptake (1.55 wt. % at 1 bar, 77 K) for the series ofCOFs synthesized with varying alkyl chain lengths, decreas-ing with increasing chain length to 1.22 wt.% for COF-11Å[49]. However, the uptake per mol of COF increased withnarrowing pore size from 4.84 to 5.33 mol H2/mol COF.

Fig. 20 shows the hydrogen uptake at 77 K and 1 barfor various COFs, triazines networks, HCPs, PIMs, and CMPsas a function of apparent BET surface area calculated from
N2 sorption. In general, the amount of hydrogen uptake inthe materials increases with increasing surface area up toaround 1400 m2/g, after which there appears to be no fur-ther gain in hydrogen uptake, despite the increasing surface

5 Polymer

aathCAl[ot–Hf

cni1w1

mtB1hpsat

sra1ahdwaTte(gm

rf1cphtc3

s7ta


rea. The correlation between apparent BET surface areand H2 uptake is however rather weak. For example, theriazine material DCBP with a surface area of 2475 m2/gas a hydrogen uptake of 1.55 wt.% [26], similar to that forOF-18Å which has a surface area of only 1263 m2/g [49].

similar trend was found previously for a combinatorialibrary of HCPs produced with a wide range of surface areas92], most likely pointing to the fact that larger microporesr mesopores contribute more to nitrogen sorption thano hydrogen sorption in this pressure/temperature regimeas such, apparent BET surface area is a poor measure of2 sorption capacity, at least under conditions that are far

rom saturation.Hypercrosslinked polymers prepared from the lightly

rosslinked polymeric precursors were among the firstanoporous polymers investigated for H2 sorption, show-

ng maximum uptakes of 1.28 wt.% at 1 bar, 2.75 wt.% at0 bar, and 3.04 wt.% at 15 bar [88]. Other comparable net-orks with higher surface areas [91] showed H2 uptakes of

.55 wt.% at 1.2 bar.Hydrogen uptakes for HCPs synthesized by direct poly-

erization from BCMBP and DCX were also measured, andhe highest surface area copolymer network made fromCMBP and pDCX (3:1) showed a maximum uptake of.61 wt.% at 1.13 bar and 77 K. These are still some of theighest H2 uptakes for nanoporous organic polymers atressures close to 1 bar: very high surface area polymers,uch as PAF-1, do not offer a benefit at these pressures,gain most likely because small micropores dominate inhis regime.

EOF-1 and EOF-2 were also evaluated for their hydrogentorage potential. Uptakes at 1 bar of 0.94 and 1.21 wt.%,espectively, were reported [86]. Aniline based polymerslso showed hydrogen uptakes of 0.96 and 0.82 wt.% at.2 bar and 77 K [95]. Interestingly, the lower surfacerea material formed from paraformaldehyde showed aigher isosteric heat of adsorption of 9.3 kJ/mol than theiiodomethane linked material. HCPs made by the Buch-ald coupling method showed relatively high hydrogen

dsorption (0.85 wt.%) despite relatively low surface areas.he latter network showed an isosteric heat of adsorp-ion for hydrogen at 18 kJ/mol. This anomalously highnthalpy was attributed to the presence of ultramicropores<0.7 nm) which can only be accessed by the hydro-en molecule, and which are too small for the nitrogenolecules used to determine BET surface areas.The uptake of hydrogen in a number of PIMs has been

eported [3,4]. The network Trip-PIM has the highest sur-ace area and its hydrogen uptake at 1 bar, was found to be.63 wt.% [67], close to that of the best-performing hyper-rosslinked network (1.69 wt.% at 1.13 bar) [92]. At higherressures of 10 bar, the hypercrosslinked p-DCX networkad a marginally higher uptake 2.75 wt.% compared tohat of Trip-PIM (2.71 wt.%). Another PIM (OFP-3) showedomparable H2 adsorption at 77 K (1.56 wt.% at 1 bar and.94 wt.% at 10 bar) [73].

The thiophene-containing spiro network (PS4TH)
howed a H2 uptake of 3.6 wt.% at 70 bar/77 K while at0 bar and 298 K, the uptake was 0.45 wt.% [122], illus-rating how far these materials are from the DoE targetst ambient temperatures. Cyclotrimerisation networks
Science 37 (2012) 530– 563

PS4AC2 and PT4AC gave materials with H2 uptakes of 3.7and 2.2 wt.%, respectively [122].

Hydrogen isotherms were also measured for series ofCMP materials and, as might be expected, the highestsurface area material with the largest micropore volume(CMP-0) gave rise to the largest H2 capacity (1.4 wt.%at 1 bar and 77 K). Increased uptake of H2 by spillovermechanisms in CMPs has also been reported [152]. Theincorporation of Pd nanoparticles into CMP-0 via the useof supercritical CO2 led to higher H2 uptakes of 0.069 wt.%at 293.3 K and 1.13 bar with 9.5 wt.% Pd. This is a significantincrease in comparison with 0.006 wt.% under the sameconditions for the as-synthesized, non-metallated CMP-0material [22], but still far too low to be of any practicalrelevance.

Theoretical studies suggest strategies to improve the H2uptakes for COFs [157] by incorporating lithium alkoxidegroups into the HHTP monomer of COF-105. These simula-tions suggest that the hydrogen molecules interact withthe lithium in the networks, and calculated uptakes aresuggested which surpass the DOE targets at room temper-ature of 6 wt.%. These theoretical studies have not yet beensupported by experimental data. By contrast, a differentresearch group has presented experimental measurementswhich suggest that Li doping can strongly enhance H2 sorp-tion in CMPs at cryogenic temperatures. The incorporationof lithium into the pores of a homocoupled CMP [110] wasreported to increase the H2 uptake from 1.6 wt.% for thenon-doped CMP to 6.1 wt.% for the lithium-doped materialat 1 bar and 77 K [111]. This is a dramatic effect, althoughfurther comparable examples would be required to sub-stantiate this as a viable future strategy. Moreover, evenhere the H2 uptakes reported at 273 K were very low.

In summary, while there are a number of reports of∼7 wt.%, hydrogen storage in nanoporous polymers, the useof these materials for physisorptive energy storage wouldstill appear to be far away as a result of the cryogenic tem-peratures that are required. Theoretical studies suggestingmethods to enhance the hydrogen adsorption enthalpy,such as lithium intercalation [157] and the incorporationof ‘naked’ halide functionalities [158], have yet to be borneout in practice. The ability to produce porous organic poly-mers with extremely high surface areas [16] does howeveroffer potential to use such materials in the future, providingthat a strategy can be implemented to improve the H2 sorp-tion enthalpy significantly. As such, this is still a promisingalbeit highly challenging area of research.

3.2. Methane storage in nanoporous organic polymers

Methane has environmental advantages as a transi-tional fuel source with respect to other fossil fuels due toits high ratio of hydrogen to carbon, although it does ofcourse still produce CO2 upon combustion. The US DOEtargets for methane storage have been set as 180 (v/v) at35 bar and 298 K [159]. The optimal heat of adsorption formethane storage at room temperature was estimated to be
18.8 kJ/mol [139], allowing both adsorption and desorptionto occur at a fast enough rate for use as a fuel. Unlike thecase of H2 storage, a number of materials in the literaturenow hit these targets [160,161] and hence issues of cost,


Table 3Summary of carbon dioxide uptakes in nanoporous organic polymers.

Material SABET (m2/g) P (bar) T (K) CO2 uptake (mmol/g) Ref

COFsCOF-1 750 55 298 5.2 [47]COF-5 1670 55 298 19.8 [47]COF-6 750 55 298 7.0 [47]COF-8 1350 55 298 14.3 [47]COF-10 1760 55 298 23.0 [47]COF-102 3620 55 298 27.3 [47]COF-103 3530 55 298 27.0 [47]

HCPs

BCMBP(100) 1646 1 298 1.7 [174]30 13.3

DCX(25)/BCMBP(75) 1684 1 298 1.7 [174]30 12.6

DCX(50)/BCMBP(50) 1531 1 298 1.6 [174]30 11.6

DCX(25)/BCMBP(75) 1642 1 298 1.6 [174]30 10.6

CMPsCMP-1 837 1 298 1.18 [175]

273 2.05

CMP-1-(CH3)2899 1 298 0.94 [175]

273 1.64CMP-1-(OH)2 1043 1 298 1.07 [175]

273 1.80CMP-1-NH2 710 1 298 0.95 [175]

273 1.64

CMP-1-COOH 522 1 298 0.95 [175]273 1.60

1 3160 1.13 298 1.5 [129]2 1102 1.13 298 0.9 [129]3 3180 1.13 298 1.7 [129]4 1917 1 298 1.5 [129]5 1470 1 298 1.3 [129]PPN-1 1249 60 295 11 [132]PPN-2 1764 60 295 19 [132]PPN-3 2840 60 295 25.3 [132]PPN-4 6461 50 295 38.9 [131]PAF-1 5640 40 298 29.6 [23]

1 atm 298 1.09 [153]1 atm 273 2.07 [153]

PAF-3 2932 1 atm 298 1.82 [153]1 atm 273 3.48

PAF-4 2246 1 atm 298 1.16 [153]1 atm 273 2.43

1,3,5-Triphenylbenzene HCP 1059 1 273 3.61 [176]POF1B 917 1 298 2.1 [177]

273 4.2
BILP-1 1172 1
scalability, and processability may be paramount to com-pete with established compressed or liquefied gas storagetechnologies.

Methane sorption in nanoporous organic polymers hasbeen much less widely studied than hydrogen sorption.However, the isosteric heats of sorption for methane in car-bonaceous materials fall within the appropriate range forstorage close to ambient temperatures, and hence organicmaterials could have much to offer here. Table 2 showsthe reported methane uptakes for a range of nanoporousorganic polymers.

The highest reported methane storage capacities fororganic polymers are found in three-dimensional COFs [47]– COF-102 and COF-103 adsorb 11.7 and 10.9 mmol/g (114and 106 (v/v) based on bulk densities), respectively. This is

298 4.0 [178]273 4.3

around half that of the MOF PCN-14 which has an excessuptake of 220 (v/v) at 35 bar and 290 K [160].

HCPs prepared by Friedel–Crafts alkylation ofchloromethylaromatics have also been tested for methanestorage [31]. A potential advantage of these materials isthat they can be produced as continuous, molded porousmonoliths. Again, the highest surface area material showedthe highest methane uptake at elevated pressures. In allof this, it is important to note that for a material witha density of 1 g/cm3 an uptake of 7.9 mmol/g would berequired to meet the DOE target of 180 (v/v). As many
nanoporous organic polymers have lower densities, theamount adsorbed would need to be higher; hence, for theHCP networks with densities of 0.78 g/cm3 the uptakes of5.5 mmol/g correspond to 97 (v/v) [31]. For comparison,

5 Polymer

a(

aug

3p

piTidtCs(sarsfdbdrnCadopbccttait[f

wwybdiarpaw

rvo


molar uptake of 10.2 mmol/g is required to reach 180v/v).

In summary, nanoporous organic polymers, like MOFs,ppear to have promise for methane storage. It is currentlynclear, though, if these materials can compete on costrounds with sorbents such as activated carbon [162].

.3. Carbon dioxide sorption in nanoporous organicolymers

The combustion of fossil fuels, for example in powerlants and cars, produces large amounts of CO2 which

s thought to be a major contributor to global warming.herefore, the capture and storage of carbon dioxide is anmportant challenge. The problem of CO2 capture can beivided into two main areas: (i) CO2 capture from con-aminated natural gas reserves containing methane andO2, or from a mixture of fuel gases (e.g., in water–gashift reactors), which is known as pre-combustion, and;ii) CO2 capture from an exhaust gas (e.g., from powertations) which is known as post-combustion [163]. Pre-nd post-combustion CO2 capture have quite differentequirements: pre-combustion conditions are at high pres-ures with large amounts of CO2, which must be separatedrom either methane or hydrogen; post-combustion con-itions involve atmospheric pressures and temperaturesetween 40 and 60 ◦C, with a low concentration of CO2iluted in nitrogen [163]. Therefore, any potential mate-ial for carbon capture and storage (CCS) will be requiredot only to reach targets regarding the amount of pureO2 adsorbed, but will also be required to have good sep-ration of CO2 from the other gases present. We recentlyemonstrated that for a range of MOPs, a selectivity of CO2ver N2 of around 9–20 was observed [164]. This com-ares well with other materials in the literature [165,166],ut is lower than some recently reported porous organicages [167] and MOFs. Absolute gas uptakes must also beonsidered in addition to selectivity; we showed recentlyhat there is a clear trade-off between capacity and selec-ivity for a range of materials including MOPs, MOFs,nd organic cages [168]. The importance of selectivity,sosteric heats of sorption, and robustness to impuri-ies was recently reported in detail for a MOF system169]. The practical requirements will be broadly the sameor MOPs.

The capture of CO2 has been demonstrated using aide range of materials: for example, amine solventshich chemically bind CO2 have been known for many

ears [170]. However, the recycling of these materialsy heating leads to a large energy penalty, often ren-ering them uneconomical. The use of solid adsorbents

ncluding carbons [171], silicas [172] and MOFs [163] haslso been widely researched [173]. MOF frameworks haveecently been examined in detail for their suitability forost-combustion CO2 capture [169] via temperature swingdsorption. The presence of strong CO2 adsorption sitesas shown to be essential.

A number of nanoporous organic polymers have beeneported for the adsorption of CO2 under conditions rele-ant to either post- or pre-combustion capture. A summaryf CO2 uptakes is shown in Table 3.

Science 37 (2012) 530– 563

The reported CO2 uptakes for COFs show a maximumuptake of 27.3 mmol/g at pressures of 55 bar and a tem-perature of 298 K [47]. A higher uptake of 29.6 mmol/gat a pressure of 40 bar was reported for the highest sur-face area amorphous organic polymer, PAF-1 [23]. Thesematerials or analogues might therefore be of use for pre-combustion CO2 capture, although the physicochemicalstability of COFs is a potential problem. By contrast, PAF-1 is highly stable but its cost could be prohibitive forreally large-scale use, even if the material was reusableand had good lifetimes. A number of studies on polymernetworks have reported CO2 uptakes at atmospheric pres-sure, with the adamantane-centered tetrahedral network(3180 m2/g) synthesized by Yamamoto coupling showing aCO2 uptake of 1.7 mmol/g at 298 K. A similar network madevia the Sonogashira coupling of tetraphenylmethanes hada BET surface area which was 1200 m2/g lower (1917 m2/g)and yet a CO2 uptake of 1.5 mmol/g [129]. Again, this high-lights the fact that physical surface area is not dominantin gas uptakes at lower pressures, far from saturation.As such, for post-combustion capture the development ofmaterials with ever-increasing surface areas is unlikely toachieve proportional increases in CO2 uptake. It is thereforeimportant to consider the binding energy of the mate-rial with CO2. Calculations on organic struts in MOFs haveshown that carboxylic acid groups might lead to high isos-teric heats, and this was also shown to be the case whenacid functionalities were incorporated into a CMP material,CMP-1-COOH [179]. However, due to the lower surface areaof CMP-1-COOH, the actual amount of CO2 adsorbed waslower than for the non-functionalized CMP analogue [175].

In summary, while MOPs are within reach of some ofthe targets for post-combustion capture, a number of chal-lenges still remain. It is likely that a combination of highsurface area and high isosteric heats will be required inorder for MOPs to be applicable. Moreover, productioncosts for most materials in the literature are probablytoo high for this technology to be immediately scalable,although this is of course connected with recyclability andthe prospective lifetime of any potential sorbent.

4. Catalysis using nanoporous organic polymers

The use of porous organic polymers for heterogeneouscatalysis is by no means new: macroporous resins and gel-type, swellable analogues have been used for many years[180,181]. Nonetheless, the new generation of nanoporousorganic polymers described above presents a range of inter-esting, specific opportunities and a number of reports aswell as a recent review [182] have appeared recently inthis area.

A CMP network has been synthesized incorporatingTröger’s base into the backbone of the network [7]. The net-work was reported to have a BET surface area of 750 m2/g.The incorporated base was used to facilitate the hetero-geneous catalytic addition reaction of diethylzinc to anaromatic aldehyde with yields approaching that of the
homogeneous model reaction (Fig. 21).
There have been a number of reports of catalysisusing the triazine networks. These include the oxidationof methane to methanol by attaching platinum to the


(4-chlo
Fig. 21. Catalysis of 4-chlorobenzaldehyde to 1-
nitrogens in the structure (Fig. 22) [183]. This reactionprovides turnover numbers similar to that of the homoge-neous catalyst but with the added advantage of separation
and recyclability. The inclusion of palladium nanoparticlesinto the same networks can catalyze the oxidation of glyc-erol with improved stability over a similar system usingactivated carbon [184]. Networks containing bithiophene
Fig. 22. Methane oxidation

rophenyl)propan-1-ol by Tröger’s base CMP [7].

linkers were also shown to be able to bind to palladiumnanoparticles. Initial studies of the catalytic properties ofthe palladium incorporated networks showed complete
conversion of diphenylacetylene to 1,2-diphenylethaneafter 2 h [8].
Three PIMs containing metal centers have also beeninvestigated for use as heterogeneous catalysts. A study of

using Pt-CTF [183].


ng meta

tipp

sttoa

t[o[t

tbpmwaml

Fig. 23. Hydrogen peroxide degradation usi

he cobalt-containing phthalocyanine PIM was shown toncrease the rate of hydrogen peroxide degradation com-ared to that achieved using a low molecular weight cobalththalocyanine (Fig. 23) [64].

The iron-porphyrin containing CMPs (FeP-CMP) synthe-ized using Suzuki cross-coupling [124] has been used inhe oxidation of thioanisole to the sulfoxide with selectivi-ies of up to 99%, conversions of 98% and turnover numbersf up to 980 (Fig. 24). This same network has been used for

further catalytic reaction, the epoxidation of olefins [185].The metal-centered EOF materials have been used in

he heterogeneous cyanosilation catalysis of benzaldehyde94]. The activity of the EOFs was higher than the previ-usly reported reaction using a MOF material (HKUST-1)186] but lower than MIL-101 [187], with EOF-5 showinghe fastest reaction times (Fig. 25).

The incorporation of pendant coordinated metal cen-ers for catalysis within microporous organic networks haseen recently been reported [188,189]. An iridium com-lex coordinated to bipyridine functionalities within a CMPaterial was shown to be active in reductive amination,
ith yields comparable to those of the homogeneous cat-
lyst (Fig. 26) [188]. These materials can be thought of asetal–organic CMPs (MO-CMPs): they are amorphous ana-

ogues of MOFs which, unlike MOFs, can include extended

l-containing phthalocyanine networks [64].

�-conjugation in the network struts. Iridium and ruthe-nium metals were also used as catalysts in a photocatalyticAza–Henry reaction when bound to a cyclotrimerised MO-CMP showing higher conversions than the homogeneouscatalysts [189]. The ruthenium network was also used inanother light-driven reaction – the �-arylation of bro-momalonate and the oxyamination of 3-phenylpropanal,again with yields comparable to that of the homogeneouscatalyst [189].

A carbene-containing tetrahedral CMP (T-IM) has beenshown to catalyze the formation of cyclic carbonates fromepoxides and CO2. [190] Yields of up to 92% were obtainedat 160 ◦C when the pressure was increased to 3 MPa(Fig. 27).

Recently chiral MOPs based on the cyclotrimerisation ofethynyl functionalized BINOLs have been used to catalyzethe diethylzinc addition into aromatic aldehydes with ee’sof up to 81% [191].

The incorporation of different functionalities into MOPshas been shown to be useful in a wide range of catalyticreactions producing yields comparable to homogeneous
catalysts but with the added advantage of recyclability andstability. For most heterogeneous reactions, the use of ahigh surface area material is likely to be advantageous.However, a large number of heterogeneous catalysts have


ioaniso
Fig. 24. Oxidation of th
already been reported, many based on silica or aluminasupports. These are likely to be significantly cheaper thanthe MOP-based examples given here. Thus, a specific chem-ical advantage will be required in MOP catalysts if they are

Fig. 25. Cyanosilyation of benza

le with FeP-CMP [124].

to supplant more established supports. Such advantagesmay lie in the microporosity (many silica-based exam-ples are mesoporous rather than microporous) or in theexploitation of inherent light-harvesting capabilities [123]

ldehyde by EOF-5 [94].


-CpIr-3,

tapms

5

i[opp

Fig. 26. Reductive amination using CMP

o achieve, for example, photocatalysis. In principle, MOPsre also more modular than standard supports and it isossible to envisage catalysts where a number of comple-entary functionalities are readily ‘programmed’ into the

ame material.

. Separations using MOPs

Linear PIMs are soluble and hence they can be usedn gas separations in the form of thin solution-cast films
6]. Such soluble MOPs perform well in comparison tother polymer membranes [192,193]. As noted above,ost-synthetic modification of PIMs has been used to incor-orate triazole groups leading to more favourable CO2
Fig. 27. Cyclic carbonate forma

a metal–organic CMP (MO-CMP) [188].

sorption and hence greater selectivity for gas separation[82]. These TZPIM membranes surpassed the upper boundfor selectivity/permeability in polymeric membranes [193]for the important gas pair, CO2 and N2. Other microporousmembrane have also demonstrated the high gas selectivitythat is necessary for effective separations [81]. For furtherinformation on the use of MOPs for separations, we referreaders to the review by Budd and McKeown [194].

6. Light harvesting in CMPs

A polyphenylene-based light-harvesting CMP syn-thesized by Suzuki cross-coupling was found to havelight-harvesting properties [123]. While the network emits

tion using T-IM [190].


ng using
Fig. 28. Light-harvestiReprinted with permission from [123].© 2010 American Chemical Society.
blue luminescence at 443 nm, it was possible to addCoumarin 6 into the pores of the polymer. When excitedemitted at 512 nm, evidence was given for energy transferfrom the network to the Coumarin 6, which is then ableto emit. In its doped state, the network itself exhibits noluminescence (Fig. 28).

7. Conclusions

The field of MOPs has shown dramatic expansion overthe last 10–15 years, and the literature has grown especiallyrapidly in the last 5 years or so. MOPs have a promis-ing future: they can combine high surface areas, goodphysicochemical stability, and an ever-increasing degree offunctional modularity. Key recent synthetic breakthroughsare:

• 2004: Solution-processable, film-forming PIMs [39].• 2007: Synthesis of crystalline COFs with exceptionally

high surface areas [48].• 2007: CMPs which combine porosity with extended �-

conjugation [108].• 2008/2009: More robust crystalline COF analogues

[26,52].• 2009: Robust and highly porous aromatic frameworks

[23].• 2011: Metal–organic CMPs (MO-CMPs) comprising cat-

alytic metal centers [188].

This rapid advancement in synthetic capability hasgiven researchers a broad toolkit for constructing newmaterials, and we would anticipate that the next 10 yearsof research will focus more closely on the applications ofMOPs. There are many opportunities here which could takeadvantage of this synthetic modularity, for example:

• Photochemical water splitting – perhaps exploitingextended conjugation in CMPs [108] along with the abil-ity to append redox-active metals [188].

polyphenylene CMP.

• Molecular imprinting – new analogues of imprinting tech-nologies that were developed using more traditionalvinyl crosslinking chemistry [195,196].

• Sensing – high surface area porous sensors based onsemiconducting CMPs [108], perhaps in combinationwith imprinting technologies, above, to sense particularguests.

• CO2 separation – large scale, processable PIM mem-branes [39] which separate specific gases, such asCO2; sorbents for pressure- and/or temperature-swingadsorption [164,175].

• Artificial enzymes – for example, using self-assembledreactive metal sites within porous scaffolds [188].

• Energy applications – for example, CMPs [108] as materialplatforms for new batteries and supercapacitors.

A number of challenges also need to be addressed. Forexample, while polymers such as linear PIMs are soluble inorganic solvents, networks such as COFs, CMPs, and HCPsare not and this raises the question of how these materialscan best be processed into functional devices. Cost is also aconsideration. For example, the Pd cross-coupling catalysisused to produce CMPs [108] is an obvious concern in termsof scale up.

These challenges, along with the development of appli-cations for MOP materials, constitutes a fertile area ofresearch for materials chemists working in collaborationwith researchers in other related disciplines such as energy,health, and sustainability.

Acknowledgements

The authors would like to thank the Engineer-ing and Physical Sciences Research Council and E.ON(EP/G061785/1) for funding through the E.ON-EPSRCstrategic call on CCS. A.I.C. is a Royal Society Wolfson MeritAward holder.

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Nanoporous organic polymer networks