Recent Development of Hypercrosslinked Microporous Organic Polymers
Shujun Xu , Yali Luo , Bien Tan *
Hypercrosslinked polymers (HCPs) are currently receiving great interest due to their easy preparation, high chemical and thermal stability, and low cost. Combined with the light-weight properties and high surface areas HCPs can be considered as promising materials for gas storage and separation, catalysis, and heavy metal ions removal in wastewater treatment. This Feature Article summa-rizes strategies for the preparation of HCPs, comprising the post-crosslinking of “Davankov-type” resins, direct polycondensation of aromatic chloromethyl (or hydroxymethyl) monomers, and knitting aromatic compound polymers (KAPs). The HCPs applica-tions, such as H 2 storage, CO 2 capture, and heterogeneous catal-ysis, are also discussed throughout in the article. Finally, the outlook of this research area is given.
1. Introduction
Microporous materials, with pores smaller than 2 nm, are nowadays at the forefront of materials research due to their potential applications in gas storage and separa-tion, catalysis, and conductivity. [ 1 ] During the last few dec-ades, the surge to develop such useful materials has led to produce a number of microporous solids such as carbon materials, [ 2 ] zeolites, [ 3 ] silica, [ 4 ] metal-organic frameworks (MOFs), [ 5 ] zeolitic imidazolate frameworks, [ 6 ] polysisesqui-oxanes, [ 7 ] and porous organic cages. [ 8, , 9 ] Among them, microporous organic polymers (MOPs), which are com-posed of light, non-metallic elements such as C, H, O, N, and B, [ 10,11 ] have attracted considerable attention and are becoming an important class of microporous materials. Compared with other microporous solids, MOPs possess
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S. Xu, Y. Luo, Prof. B. TanSchool of Chemistry and Chemical Engineering,Huazhong University of Science and Technology,Wuhan 430074, China E-mail: [email protected]
a number of potential advantages. For example, the great choice of monomers available makes it easy to introduce various functional groups in the pore walls. [ 12 ] Micro-porous polymer networks can be constructed using a plethora of organic reactions and building blocks, which provides fl exibility for the material design to achieve desirable pore properties. [ 13 ] Furthermore, polymers are a scalable technology and there are already examples of sys-tems that are produced commercially on a large scale. [ 14,15 ] In the past few years, several types of chemical reactions have been used to develop various MOPs, including soluble and crosslinked polymers of intrinsic microporosity (PIMs) obtained through dioxane-forming polymerization, [ 16 ] cov-alent organic frameworks (COFs) through reversible borate chemistry, [ 17 ] palladium-catalyzed Sonogashira-Hagihara cross-coupling for conjugated microporous polymers (CMPs), [ 18,19 ] Friedel-Crafts reaction for hypercrosslinked polymers (HCPs), [ 20 ] covalent triazine-based frameworks (CTFs) obtained from the trimerization of aromatic nitrile compounds, [ 21,22 ] porous aromatic frameworks (PAFs) by homocoupling of aromatic bromides, [ 23–25 ] element-organic frameworks (EOFs) via an organometallic polymer synthesis route, [ 26 ] oxidative coupling of thiophenes, [ 27,28 ]
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Bien Tan received his B.Sc. (1993) and M.Sc. degree (1996) from Hubei University. He obtained his Ph.D. in 1999 from South China University of Technology. He then joined in Beijing Institute of Aeronautical Materials for postdoctoral research (1999-2001). He worked as a PDRA at The University of Liverpool (2001–2007). He then returned to China and joined Huazhong University of Science and Technology (HUST) in September 2007 as a professor. In December 2009, Tan was awarded New Century Excellent Talents in University by the Ministry of Education, and in the same year he was awarded Chutian Scholar Distinguished Professor by Hubei province. Currently, he is a Professor of HUST, and the director of Hubei key laboratory of materials chemistry and service failure. His main research interests are polymeric materials, supercritical fl uids, micro-porous materials, hydrogen storage, metal nan-oparticles, emulsion-templated materials, and high-throughput materials methodology.
Shujun Xu obtained his M.S. (2006) degree in organic chemistry from Huazhong University of Science and Technology. After 4 years stay in a pharmaceutical company as a research, he started his Ph.D. program course in 2010 under the supervision of Professor Bien Tan. His research focuses on design and synthesis of microporous organic polymers for catalysis applications.
Yali Luo received her M.Sc. degree in 2009 from Henan university. Then, she joined the research group of Prof. Bien Tan at the Huazhong University of Science and Technology. Currently, she is completing her Ph.D. in the design, syn-thesis, and application of hypercrosslinked microporous organic polymers (HCPs).
trimerization of ethynyl groups, [ 27 , 29 ] amide or imide for-mation, [ 12 , 30–32 ] and reversible imine formation. [ 13 , 33 ] How-ever, the transition-metal catalysts or noble metal catalysts used for the synthesis of CMPs, PAFs, and other porous organic polymers are rare and expensive thus limiting the practical application of these synthetic strategies.
HCPs [ 34 ] represent a novel class of porous materials, which are low cost and mainly prepared by the Friedel-Crafts alkylation reaction. The permanent porosity in HCPs is a result of extensive crosslinking reactions, which prevents the polymer chains from collapsing into a dense, nonporous state. Such highly crosslinked nature of the materials confers them high thermal stability that is not commonly expected for organic polymers. Combined with their lightweight properties and high surface areas, HCPs can be considered as promising materials for the gas storage applications, especially in the fi eld of clean energy and environmental issue, such as hydrogen storage and carbon dioxide capture. [ 35,36 ] According to the synthetic methods, HCPs can be produced by three ways: 1) inter-molecular and intramolecular crosslinking of preformed polymer chains (either linear chains or lightly crosslinked gels), 2) direct step growth polycondensation of suitable monomers, and 3) knitting aromatic compound polymers (KAPs) using the external crosslinking. In this perspective article, our goal is to introduce the recent development of HCP along with their current and potential applications to the readers.
2. HCPs by Post-Crosslinking of Polymers
Pioneering work on the HCPs was reported by Webster and co-workers. [ 37,38 ] Some hypercrosslinked polyarylcar-binols with high surface areas (400–1000 m 2 g − 1 ) were pre-pared by reacting of 4,4 ′ -dilithiobiphenyl as well as other multi-lithiated aromatic compounds with dimethyl car-bonate. “Davankov-type” resins are another type of hyper-crosslinked materials, which have been well-studied and are prepared by post-crosslinking of linear polystyrene
Figure 1 . Schematic of the hypercrosslinking process. [ 39 ]
(PS), poly(vinylbenzyl chloride), or their pre-crosslinked copolymers with a divi-nylbenzene (DVB) moiety. [ 34 ] As shown in Figure 1 , fi rst, the linear or lightly crosslinked polymer precursors are dis-solved or swollen in a thermodynami-cally good solvent; this introduces space between the polymer chains. Then the precursors are quickly cross-linked to leave the polymer chains locked in an expanded form. When the solvent is removed from the system, the space that
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it had previously occupied becomes pore volume and a net-work of intercommunicating pores is left behind. [ 11 , 39 ]
Hypercrosslinked PS was fi rst introduced in the early 1970s. [ 40 ] Since then a signifi cant research effort has been directed at synthesizing such materials. Dav-ankov et al. [ 41,42 ] have prepared ‘‘nano-sponges’’ via intramolecularly hyper-crosslinking reaction of linear PS. In this case, monochlorodimethyl ether (MCDE) was used as the crosslinking agent and multiple Friedel-Crafts-alkylation catalyzed by Lewis acids has been established as a synthetic protocol for the build-up of the nanoporous polymer networks. The resulting nanosponges have high surface area of ca. 680–1000 m 2 g − 1 . These materials can be used in chromatography, [ 43–45 ] the separation of contami-nants from liquid solutions, [ 34 ] and the sorption of organic vapors. [ 43 , 46 ] Compared with monochlorodimethyl ether, tetrachloromethane is a less carcinogenic crosslinker. Hradil and Králová [ 47 ] reported the permanently porous functionalized styrene copolymers by post-crosslinking gel or macroporous styrene-DVB copolymers with tetra-chloromethane in the presence of a Friedel-Crafts catalyst (aluminium or ferric chloride). In crosslinking, reactive groups are also introduced into the polymer, which can be used for subsequent transformations. In addition, very high surface area resins (up to 1000 m 2 g − 1 ), equivalent to Davankov’s hypercrosslinked resins, have been obtained by the modifi cation of the macroporous copolymers. Using the same crosslinker, cross-linked PS hollow nano-spheres also have been prepared. Fu et al. [ 48 ] fi rst synthe-sized PS- b -PMMA-grafted silica nanoparticles and then crosslinked by ultraviolet (UV) treatment, followed by HF etching of the silica core. The UV treatment is utilized to
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Figure 2 . Preparation process of novel NPN polymer and carbon matwith permission. [ 49 ] Copyright 2011, American Chemical Society.
concurrently crosslink the PS interior shell and decompose the PMMA exterior shell. Moreover, the shell thickness of nanospheres can be readily tuned by controlling PS chain lengths. By the similar method, Wu et al. [ 49 ] developed a new class of nanostructured porous network (NPN) mate-rials, in which the network units are microporous PS nanoparticles crosslinked via formation carbonyl (–CO–) bridges between PS chains. The fi nal NPNs contain three types of pores: (i) micropores induced by crosslinking of the hairy PS shell, (ii) mesopores obtained by removal of sacrifi cial silica nanoparticle core, and (iii) meso-/macro-porous network formed through interparticle cross-linking of PS (Figure 2 ). For the textural characterization, the specifi c and micropore surface areas were determined from the N 2 adsorption isotherms at 77 K. BET calculation shows that the BET surface area ( S BET ) is up to 642 m 2 g − 1 ; and micropore surface area ( S mic ), and external (i.e., mes-opore and macropore) surface area ( S ext ) are calculated to be 237 and 405 m 2 g − 1 , respectively, according to a t-plot method. [ 50 ] It should be noted, however, that although nitrogen sorption has generally been proven to be an extremely versatile tool in the analysis of porous mate-rials, it has several drawbacks when employed for micro-porous organic materials. [ 51 ] For example, the swelling effects of the “soft” polymeric materials may lead to a signifi cant hysteresis of the isotherms in the low pressure regime. In addition, some ultramicropores are inaccessible to nitrogen molecule. As a result, the nitrogen sorption data does not refl ect the real pore structure information of the microporous polymers but just can give estimates. In spite of these disadvantages, an analysis of micropo-rous polymers by nitrogen sorption still is useful and con-
erials. Reproduced
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venient to compare the pore properties of different polymer materials.
Poly(divinylbenzene- co -vinylbenzyl chloride) (DVB-VBC) is proved as another excellent precursor for the synthesis of the HCPs with higher surface areas where the –CH 2 Cl substituent actually yields an internal electrophile to form the basis of the crosslinks. [ 52 ] Over the past years, almost all HCPs were synthe-sized by using such polymer as the pre-cursors. Sherrington and co-workers [ 53 ] have studied the effect of synthesis conditions (such as the ratios of dif-ferent vinylbenzyl chloride monomers, catalyst, reaction solvent) on the pore properties of the resulting networks. It is found that manipulation of the Friedel-Crafts reaction variables and the structure of the precursor resin can allow fi nal resin products to be pre-pared with surface areas in the range
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Figure 3 . Reaction scheme for the synthesis of a hypercrosslinked polymer prepared from gel poly(divinylbenzene-co-vinylbenzyl chloride). Reproduced with permission. [ 53 ] Copyright 2006, American Chemical Society.
of 300–2000 m 2 g − 1 . Of the Lewis acids examined (FeCl 3 , AlCl 3 , SnCl 4 ), FeCl 3 was by far the most effective catalyst. Rather remarkably in the case of a gel-type 2 mol% DVB-VBC precursors, extensive microporosity was generated within only 15 min of initiating the cross-linking reac-tion, yielding a surface area of ≈ 1200 m 2 g − 1 , which rose steadily to a maximum approaching 2090 m 2 g − 1 after 18 h (Figure 3 ). [ 53 ] Typically, most pores in such a material are about 1.8 nm in size. The high surface areas and small pore sizes make these materials excellent gas adsorbent candidates for clean energy applications. Hydrogen is considered as an ideal energy carrier in future trans-portation application. Currently, numerous hydrogen storage materials are being developed, including metal/chemical hydrides and sorption-based materials. Distinct from hydride-based adsorbents, sorption-based materials adsorb hydrogen through physisorption without breaking the H-H bond and therefore consume minimum parasitic energy during H 2 extraction. Simultaneously, Germain
et al. [ 20 ] and Lee et al. have [ 54 ] reported the use of hypercrosslinked PSs for hydrogen storage. Germain’s material with the surface area of 1930 m 2 g − 1 can reversibly adsorb 1.5 wt% H 2 at 0.12 MPa and 77 K; Lee’s polymer with the surface area of 1470 m 2 g − 1 can adsorb 1.3 wt% under the same conditions. At the high pressure, the hydrogen storage capaci-ties are 3.0 wt% at 77 K/3 MPa and 5.4 wt% at 77 K/8.0 MPa for Germain’s and Lee’s materials, respectively. This con-fi rms that nanoporous polymers can
approach the capacities required by the DOE for hydrogen storage. Recent experimental studies have shown that a further enhancement of the hydrogen storage capacity of porous materials can be achieved by “hydrogen spillover” technique. [ 55–57 ] This process involves the use of a metallic catalyst to dissociate the hydrogen molecule into atoms, which are then adsorbed into the material. To achieve this remarkable result, our group [ 58 ] synthesized microporous HCPs with highly dispersed Pt nanoparticles (Figure 4 ). Hydrogen adsorption isotherms are measured at 77.3 K and up to 1.13 bar, and 298.15 K and up to 19 bar. By containing 2 wt% Pt nanoparticles, the hydrogen storage capacity of HCPs is enhanced to 0.21 wt% at 298.15 K and 19 bar. Compared to the similar materials without Pt nano particles, the H 2 adsorption amount has been enhanced by a factor of 1.75.
Following these studies, our group [ 59 ] investigated the infl uence of the precursor structure on the observable porosity in greater detail. They have demonstrated that
the pore size of the poly(divinylbenzene- co -vinylbenzyl chloride) (HCP-DVB-VBC) can be adjusted from macropore to micropore scale by changing the DVB contents. With the DVB content varying from 0–10%, the pore size of HCP-DVB-VBC decreases, the pore size distribution becomes narrower and the micropore volume content increases from 6.82 to 61.90% (Table 1 ). When the DVB con-tent is higher than 7%, pure micropo-rous organic polymer will be obtained. The mechanism of effect of DVB con-tent in controlling the pore structure also is proposed. Usually, the hyper-crosslinking reaction occurs in a com-pact polymer chains region. For the DVB-VBC precursor with 0% DVB, the loose and disordered twisted macromolecular chains dissolve well in solvent, the dis-tance between two neighboring chains (or two space neighbors but distant
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Table 1. Composition, porosity, and gas adsorption properties of samples.
No. DVB [mol%]
S BET [m 2 g − 1 ] a)
PV [cm 3 g − 1 ] b)
MPV [cm 3 g − 1 ] c)
H 2 uptake [mmol g − 1 ] d)
CO 2 uptake [mmol g − 1 ] e)
1 0 1420 1.29 0.088 6.00 2.26
2 0.5 1790 1.86 0.0045 6.40 2.27
3 1 1860 1.26 0.078 6.95 2.27
4 1.5 1890 1.20 0.098 7.00 2.28
5 2 2060 1.23 0.086 7.20 2.29
6 3 1920 1.10 0.19 7.35 2.52
7 4 1840 1.04 0.26 7.40 2.57
8 5 1760 0.99 0.27 7.50 2.57
9 6 1520 0.82 0.39 7.25 2.61
10 7 1430 0.77 0.37 7.15 2.70
11 8 1370 0.70 0.39 7.15 2.76
12 10 1260 0.63 0.39 7.10 2.82
a) Surface area calculated from nitrogen adsorption isotherms at 77.3 K using BET equation; b) Pore volume calculated from nitrogen iso-therm at P/P 0 = 0.995; c) t-Plot micropore volume; d) H 2 uptake determined volumetrically using a Micromeritics ASAP 2020 M analyzer at 1.13 bar and 77.3 K; e) CO 2 uptake determined volumetrically using a Micromeritics ASAP 2020 M analyzer at 1.00 bar and 273.15 K.
segments of one chain) is random. So not every –CH 2 Cl group possesses neighboring benzene ring, which results in random hypercrosslinking density and the appear-ance of macropores in HCP-VBC. However, the nonlinear crosslinked precursor with higher DVB content presents a more uniform texture (Figure 5 ), which favored a uniform pore structure after the hypercrosslinking reaction. On the other hand, the precursor with a higher crosslinking degree possesses a more rigid network, and the distance between each chain tended to be more identical at every location, which means the hypercrosslinking degree will be more uniform throughout the polymer. The fracture section of HCP-DVB-VBC with 2% to 10% DVB thus rep-resents smoother, narrower, and more uniform pore size distribution. Gas adsorption properties of these materials indicate that the smaller micropore size and higher micro-porous volume favors the H 2 and CO 2 uptakes (Table 1 ). Compared to other costly porous organic materials, these cost-effi cient polymers (HCP-DVB-VBC) with a tunable pore structure would have broad feasible applications as molecular sieves, chromatographic column packing mate-rials, and selective catalysis.
However, the particle size of traditional “Davankov Resins” prepared via suspension polymerization usually is polydisperse (diameter of ca. 10–500 μ m), [ 60 ] which may limit their practical applications in some fi elds. Therefore, many methods have been used to prepare the HCPs with uniform nanoparticle morphology. Sieving and sedimentation of suspended particles in water or acetone has been used to obtain relatively uniform fractions and many commercial resins with fairly uniform microscale
dimensions, such as Purolite MacroNet resins, have been used as packing materials for high-performance liquid chromatography for many years. [ 61,62 ] Sherrington and co-workers [ 63 ] reported a kind of uniform porous polymer nanospheres, which were prepared via soap-free emul-sion polymerization, with particle size around 500 nm and surface area up to 1200 m 2 g − 1 . This work demonstrated the emulsion polymerization method can effi ciently con-trol the spherical morphology and uniform particle size of “Davankov Resins.” More recently, Liu [ 64 ] synthesized ultrahigh surface area uniform nanospheres with particle size around 650 nm and surface area up to 1223 m 2 g − 1 via emulsifi er-free miniemulsion polymerization. Our group further optimized the reaction conditions of the emulsion polymerization and synthesized the uniform microporous polymer nanoparicles (MPNs) with nano-size. By adjusting the emulsifi er dose, the particle size can be fi ne tuned from 36 to 131 nm. The BET surface area of the MPNs is up to 1500 m 2 g − 1 and hydrogen adsorp-tion capacity is about 1.59 wt% at 77.3 K/1.13 bar. Due to the effi cient gas hydrogen diffusion through micropores inside the nanoparticles, the materials present faster adsorption rate compared with polydisperse micro-size “Davankov Resins” previously reported. Sulfonic acid-modifi ed microporous hypercrosslinked polymers (SAM-HCPs) were also synthesized by direct sulfonation reac-tion of HCPs. [ 65 ] Due to the good hydrophilic nature of SAM-HCPs, synergic effect of microporous structure, and active sites, they can be used as high-capacity adsorbents for toxic metal ions (e.g., Cu 2 + 51.45 mg g − 1 at 303 K, 54.82 mg g − 1 at 313 K, and 57.68 mg g − 1 at 323 K). Salek et al. [ 66 ]
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Figure 5 . A FE-SEM image of fracture section of samples with 0%, 0.5%, 1%, 2%, 5% and 10% DVB before (a, scale 200 nm) and after (b, scale 100 nm) hypercrosslinking reaction. Reproduced with permission. [ 59 ] Copyright 2011, Royal Society of Chemistry.
obtained the superparamagnetic iron oxide nanoparticles by precipitation of precipitation of Fe(II)/Fe(III) salts in the sulfonated hypercrosslinked styrene-DVB copolymers microspheres. This is the fi rst example of preparation of strongly magnetic supports by taking advantage of highly microporous structure of hypercrosslinked microspheres. The microspheres with immobilized anti-OVA antibody
integrated in a sandwich-type electrochemical immuno-sensor and ovalbumin can be served for construction of an electrochemical immunosensor for the detection of ovalbumin protein, which can be easily assayed using linear sweep voltammetry with a three-electrode screen-printed sensor with platinum working electrode.
Besides the hypercrosslinked PS, microporous poly-aniline [ 67 ] have also been prepared by following the same design concept. Polyaniline and diaminobenzene were coupled with diiodobenzene and tribromobenzene using Ullman and Buchwald synthetic routes. The resulting porous polymer networks consisted of aromatic rings linked through a trivalent nitrogen atom. The Buchwald reaction appears to be more effective than the Ullman synthesis for the production of such materials. The use of solvents with higher Hildebrand solubility coeffi cients during synthesis affords polymers with higher surface areas. However, that hypercrosslinked networks of aro-matic rings with pores that are too small to allow pene-tration of nitrogen but large enough for hydrogen adsorp-tion can be generated. Germain et al. [ 39 ] synthesized the hypercrosslinked polyamines with nanoporous structure and high surface area using both conventional and micro-wave-assisted processes. Short crosslinks such as those formed using paraformaldehyde and diiodomethane lead to materials with the highest surface areas exceeding 630 m 2 g − 1 . Surface area also increased with the concen-tration of polyaniline in solution used during preparation. The hydrogen storage capacities of these materials were also tested and a capacity of 2.2 wt% at 77 K and 3.0 MPa was found for the best adsorbent. Using the same chem-istry, they also synthesized the hypercrosslink undoped polypyrrole (Figure 6 ). [ 68 ] The diiodomethane-hyper-crosslinked polypyrrole, which exhibits very small pores and remarkably high BET and Langmuir surface areas of 732 and 543 m 2 g − 1 , reversibly adsorbs 1.6 wt% hydrogen at 77 K and 0.4 MPa. Under optimal conditions, nanopo-rous polypyrroles can be formed using a variety of dif-ferent crosslinking units. The size of the crosslinking unit then determines the prevailing pore size. The use of boron as a single-atom crosslinker is completely new chemistry in the fi eld of hypercrosslinking with a potential to be extended to other polymers.
3. HCPs by Direct Polycondensation
HCPs can also be produced by the direct polycondensation of small molecule monomers without the need to make the precursor crosslinked polymer. This direct approach to microporous organic networks uses bis(chloromethyl) aromatic monomers such as dichloroxylene (DCX), bis(chloromethyl)biphenyl (BCMBP), and bis(chloromethyl) anthracene (Figure 7 ). [ 18 ] The resulting networks can be
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Figure 6 . Reactions used for the preparation of nanoporous hypercross-linked polypyr-roles, and idealized schematic structures thereof. [ 68 ]
considered the analogous of the Friedel-Crafts linked PS materials. Cooper and co-workers [ 14 ] have synthesized a series of similar networks using different amounts of Lewis acid and copolymers of the different isomers of DCX and BCMBP, utilizing high-throughput synthesis and charac-terization techniques. These materials are predominantly microporous and exhibit apparent BET surface areas of up to 1904 m 2 g − 1 as measured by nitrogen adsorption at 77.3 K. Networks based on BCMBP exhibit a gravimetric storage capacity of 3.68 wt% at 15 bar and 77.3 K. The isosteric heat of sorption for hydrogen on these materials is found to be in the range 6–7.5 kJ mol − 1 . A molecular model is presented for a p -DCX network that simulates well certain key physical properties such as pore volume, pore width,
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Figure 7 . Monomers used for the synthesis of the HCP networks. Reproduced with per-mission. [ 14 ] Copyright 2007, American Chemical Society.
absolute density, and bulk density. This model also predicts the isotherm shape and isosteric heat for H 2 sorption at 77.3 and 87.2 K but overestimates the abso-lute degree of H 2 uptake, most likely because of a degree of occluded, inac-cessible porosity in the real physical samples (Figure 8 ). The methane uptake capacities were also investigated. [ 15 ] At 20 bar/298 K, the copolymers of DCX and BCMBP were shown to adsorb up to 5.2 mmol g − 1 (116 cm 3 g − 1 ) of methane, which is comparable with many other microporous systems but falls short of materials with higher micropore vol-umes. [ 69–71 ] However, it is worth noting that these materials demonstrate some of the highest peak isosteric heats of sorption for methane (20.8 kJ mol − 1 ) reported thus far. Martin et al. [ 72 ] studied the CO 2 adsorption capacity of these poly-mers using a thermogravimetric ana-lyzer (atmospheric pressure tests) and a high-pressure magnetic suspension bal-ance (high pressure tests). It was found that the CO 2 capture capacities were related to the textural properties of the HCPs. The performance of these materials to adsorb CO 2 at atmospheric
pressure was characterized by maximum CO 2 uptakes of 1.7 mmol g − 1 (7.4 wt%) at 298 K. At higher pressures (30 bar), the polymers show CO 2 uptake of up to 13.4 mmol g − 1 (59 wt%), superior to zeolite-based materials (zeolite 13X, [ 73 ] zeolite NaX [ 74 ] ) and commercial activated carbons (BPL, [ 75 ] Norit R [ 74,75 ] ). In addition, these polymers showed low isosteric heats of CO 2 adsorption and good selectivity towards CO 2 . These results confi rmed that the HCPs have potential to be applied as CO 2 adsorbents in pre-combustion capture processes where high CO 2 partial pres-sures are involved.
Recently, copolymerizations of BCMBP with a set of non-functionalized fl uorene-based comonomers such as fl uorine (FLUO), 9,9 ′ -spirobi(fl uorene) (sFLUO), diben-
zofuran (DBF), and dibenzothiophene (DBT) have also been reported (Figure 9 ). [ 76 ] By introducing these strongly twisted geometry or heterocyclic central rings, the copolymer networks exhibit high BET surface areas of up to 1800 m 2 g − 1 and adsorb 9.9 wt% (6.20 mmol g − 1 ) methane at 35 bar and reach a max-imum uptake of 13.5 wt% (8.43 mmol g − 1 ) at 100 bar. Subsequently, hydrogen sorption was volumetrically determined
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Figure 8 . (a-c) Molecular simulation of p -DCX network and (d) H 2 sorption properties. (a) Simulated poly( p -DCX) network, Connolly surface area = 2519 m 2 g − 1 , simulated micro-pore volume) 0.551 cm 3 g − 1 . Dimension of simulation box (the “amorphous cell”) = 3.3175 nm. (b) Three-dimensional array of eight amorphous cells with periodic boundary conditions. A Connolly surface is shown in gray/white. (c) Two-dimensional “slice” through an array of amorphous cells in the simulated pore structure. The occupied and unoccupied volumes are shown in gray and black, respectively. (d) Snapshot of H 2 sorption in the simulated pore structure at 1.1 bar/77 K. The density fi eld of the adsorbed H 2 molecules is shown as white spots. Reproduced with permission. [ 14 ] Copyright 2007, American Chemical Society.
at 77 K and revealed high gas uptake for the materials. A fully reversible sorption behavior was observed for all of the samples. FLUO-10 (16.3 g kg − 1 ) and FLUO-25 (14.9 g kg − 1 ) exhibited the highest adsorption among the copolymer network series at 1 bar, respectively. All of these uptake values are in the same range as those of previously
Figure 9 . Molecular structure of the building blocks 4,4 ′ -bis(chloromethyl)biphenyl (BCMBP), fl uorene (FLUO), 9,9 ′ -spirobi(fl uorene) (sFLUO), dibenzofuran (DBF), and diben-zothiophene (DBT). [ 76 ]
reported HCPs obtained by the Friedel-Craft crosslinking method. [ 14 , 20 , 54 ]
Based on the similar synthetic strategy, Chaikittisilp et al. [ 77 ] synthe-sized a hierarchically porous, hyper-crosslinked siloxane-organic hybrid (PSN-5) by Friedel-Crafts self-conden-sation of benzyl chloride-terminated double-four-ring cubic siloxane cages as a singular molecular precursor (Figure 10 ). [ 77 ] During the synthesis, polymerization of the organic functional groups is simultaneous with destruc-tion of the siloxane cages. Although the siloxane cages collapse during syn-thesis, this benzyl chloride-terminated unit can serve as a good and singular precursor for the construction of hyper-crosslinked porous networks with extremely high surface areas ( ≈ 2500 m 2 g − 1 ) and large pore volumes ( ≈ 3.3 cm 3 g − 1 ). Such high surface area is attribut-able to (i) a highly connectable or hyper-branched feature of the D4R cages that should intensify the hyper-crosslinking of the network and (ii) substantial cleavage of the D4R cages to form less-dense siloxane units during the reaction, possibly as a result of cage distortion in the hyper-cross-linked networks. Due to the high surface area and narrow pore size, the H 2 uptake for PSN-5 at 77 K/60
Torr was 1.25 wt% and the isosteric heats of adsorption at low H 2 coverage was calculated to be 8.3 kJ mol − 1 . These values are similar to those of promising candidates for H 2 storage applications, such as MOFs and porous organic frameworks. [ 78–81 ] Yuan et al. [ 82 ] reported porous organic polymers containing carborane through the Friedel-Crafts alkylation reactions. In this study, they designed and syn-
thesized the carborane monomer mCB-3 by linking two chloromethyl functional groups to an m -carborane (1,7-dicar-badodecaborane). The monomer thus prepared was subsequently used for the synthesis of PmCB-3 and PmCB-4 via self-condensation and copolymerization with BCMBP, respectively (Figure 11 ). The maximum hydrogen uptake for polymers PmCB-3 (BET, 864 m 2 g − 1 ) and PmCB-4 (BET, 1037 m 2 g − 1 ) at 77 K was 21.0 mg g − 1 and 28.4 mg g − 1 , respec-tively. The uptake capacity is generally proportional to their respective BET surface areas. The extrapolated heats of
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Figure 10 . Synthesis of PSN-5 by Friedel Crafts Self-Condensation of 1. Reproduced with permission. [ 77 ] Copyright 2011, American Chemical Society.
Figure 11 . Synthesis of polymers PmCB-3 and PmCB-4. Reproduced with permission [ 82 ] Copyright 2012, John Wiley & Sons, Inc.
adsorption, Δ H ads , for hydrogen at near zero coverage for PmCB-3 and PmCB-4 are approximately 9.0 and 8.2 kJ mol − 1 , respectively. These Δ H ads values correlate positively with the concentration of car-borane in the porous polymer network: the higher the carborane content, the higher the initial isosteric heat of adsorp-tion at near zero hydrogen coverage. This could presumably be attributed to the strong electrophilicity of the highly elec-tronically defi cient carborane units.
Recently, our group has demon-strated the synthesis of highly porous polymers by F-C-catalyzed self-conden-sation of aromatic monohydroxyme-thyl compound, benzyl alcohol (BA) (Figure 12 ). [ 83 ] This is different from the previous reports where multifunctional
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Figure 12 . Synthesis of HCP–BA by the Friedel-Crafts catalyzed self-condensation. Reproduced with permission. [ 83 ] Copyright 2013, Royal Society of Chemistry.
monomers or cross-linkers are crucial for constructing the porous polymer networks. Network HCP-BA shows the BET surface areas of 742 m 2 g − 1 . Based on these results, a pos-sible mechanism is proposed to explain the formation of the pores in the as-synthesized polymer. The primary step in the polycondensation reaction seems to be the genera-tion of benzyl carbocation, which attacks another mono mer molecule to form a dimer. It should be noted that, in the dimer, one phenyl ring contains two predominantly ortho-para orienting groups, which signifi cantly enhance the susceptibility of the ring to be attacked by cations. There-fore, the dimer can grow by the attacking other mole cules with its hydroxymethyl group (benzyl carbonium ion) and by being attacked by benzyl alcohol. Such growth by the addition of benzyl groups to the dimer will result in highly substituted rings and many short branches. The porosity arises from the ineffi cient packing of the highly branched polymer chains. The gas uptake capacity show
Figure 13 . Showing the Synthetic Pathway to the Network Structurepermission. [ 84 ] Copyright 2011, American Chemical Society.
that BA can store a signifi cant amount of CO 2 (8.46 wt%) and H 2 (0.97 wt%) at 1.0 bar, and at 273 K and 77 K, respec-tively. This study opens up the possibility of synthesizing porous materials using monofunctional monomers. Given their high thermal stability and the fact that only stoichio-metric amounts of water are generated as a side product, the networks are promising gas adsorbent candidates for clean energy and environmental applications.
4. HCPs by External-Crosslinker
Although the signifi cant progress has been made in the fi eld, the polymeric precursors or monomers used for constructing HCPs are limited to swollen PS and poly(chloromethystyrene), as well as polyfunctional benzyl chlorides, which may limit the synthetic diversifi cation of the resulting materials. In addition, the environmental issues should also be taken into account because of large quantities of harmful byproducts such as hydrochloric acid generated during the Friedel-Crafts alkylation. With these considerations in mind, our group recently developed a versatile strategy for preparing microporous polymers by using the simple one-step Friedel-Crafts reaction. In this approach, formaldehyde dimethyl acetal (FDA) was used as an external cross-linker and various aromatic mono-mers can be directly crosslinked to form the highly porous networks (Figure 13 ). [ 84 ] This avoids the need for mono-mers with specifi c polymerisable groups and also avoids the use of precious metal coupling catalysts. Via adjusting the external cross-linker ratio, the surface area and pore volume also can be fi nally controlled. When the molar ratio of FeCl 3 with respect to benzene is 3, the highest BET of 1391 m 2 g − 1 was obtained. Various functional groups can
. Reproduced with
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also be easily introduced into the porous frameworks just by choosing the proper monomers. For example, networks with –Cl and –OH can be synthesized by using the chlorobenzene and phenol as the corresponding monomers, respectively. The resulting network based on phenol exhibits the BET surface area about 400 m 2 g − 1 and can adsorb 1.07 wt% hydrogen at 1.13 bar/77.3 K and 9.4 wt% carbon dioxide at 1.00 bar/273.15 K (Table 2 ). This high gas uptake capacity may result from enhanced interactions with the phenolic hydroxyl group with the gas molecular, or alternatively it may be an effect of pore size.
Cooper group subsequently demon-strated that this “knitting” approach can produce network with surface area of up to 1470 m 2 g − 1 when using the
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Table 2. Porosity and CO 2 gas adsorption of the samples.
a) (Surface area calculated from nitrogen adsorption isotherms at 77.3 K using BET equation ( S BET )); b) (Pore volume (PV) calculated from nitrogen isotherm at P/P 0 = 0.995, 77.3 K); c) (Micropore pore volume (MPV) calculated from the nitrogen isotherm at P/P 0 = 0.050); d) (CO 2 uptake determined at 1.00 bar); f) (Calculated over the pressure range P/P 0 = 0.05–0.15); g) (Apparent BET surface areas calculated over the relative pressure range P/P 0 = 0.015 − 0.1);
tetrahedral monomer, tetraphenylmethane. [ 85 ] At present, only a little chiral microporous networks are reported, and these have either given low levels of porous or require complicated route in order to induce chiral monomers into frameworks. For example, Ritter et al. [ 86 ] reported for the fi rst time on a chiral poly(imide), which were pre-pared via polycondensation from R( + )-2, 2 ′ -diamino-1, 1 ′ -binaphthalene (( + )BINAM), and pyromellitic dianhy-dride. The apparent BET surface area was calculated to be about 500 m 2 g − 1 as derived from the nitrogen sorption isotherm. Subsequently, Bleschke et al. [ 87 ] applied thienyl-functionalized binaphthol (BINOL) derivatives as novel tectons to introduce microporosity, chirality, and catalytic active centers in a polymer network at the same time. For the BINOL polymer network, a BET surface area of 560 m 2 g − 1 was calculated from the nitrogen sorption while its corresponding phosphoric acid (BNPPA) polymer showed 88 m 2 g − 1 . By using the simple “knitting” method, Dawson et al. [ 36 ] synthesized both racemic and chiral microporous BINOL networks with high levels of porosity (BET sur-face areas up to 1000 m 2 g − 1 ). The BINOL networks show
higher CO 2 capture capacities than their naphthol coun-terparts under idealized, dry conditions. Many literatures have shown that the present of amines on the surfaces of porous materials may enhance the CO 2 uptake capacity of the materials. [ 88,89 ] However, only few MOPs containing –NH 2 groups have been synthesized. [ 90 ] Dawson et al. [ 36 ] describe the introduction of the simplest amine-function-alized aromatic monomer aniline into MOPs networks by co-polymerization with benzene (Figure 14 ). [ 91 ] In compar-ison with other methods, the co-polymerization strategy allows the fi ne tuning of properties. Networks synthe-sized solely from aniline were non-porous to nitrogen. However, co-polymerization with benzene resulted in net-works with apparent BET surface areas up to 1100 m 2 g − 1 . In addition, increased aniline content in the hyper-crosslinked networks led to improved CO 2 /N 2 selectivity. For example, the 100% benzene network showed a CO 2 /N 2 selectivity of 15.9:1, while the 100% aniline network has a selectivity of 49.2:1.
Following the previous study, we applied the sim-ilar chemistry to crosslink the aromatic heterocycle
Figure 16 . Synthetic route of KAPs(Ph-PPh 3 )-Pd. Reproduced with permission. [ 93 ] Copyright 2012, John Wiley & Sons, Inc.
molecules to get the microporous heterocyclic polymers (Figure 15 ). [ 92 ] The BET surface area of these polymers is up to 726 m 2 g − 1 . Owing to the narrow pore system and the heteroatoms-riched pore surface, the microporous aromatic heterocyclic polymers exhibit high adsorption capacity for H 2 (1.11 wt%, 77 K, and 1.13 bar) and CO 2 (12.7 wt%, 273 K, and 1 bar) (Table 2 ). These CO 2 adsorp-tion capacities are comparable or even higher than those of amine- or carboxylic acid-functionalized materials. Specifi cally, Py-1 shows an extraordinarily high selec-tive adsorption of CO 2 over N 2 (about 117 at 273 K). To our knowledge, this selective CO 2 sorption is the highest among all the microporous materials reported to date. Collectively, these results emphasize the importance of utilizing heterocyclic molecules as building blocks to produce MOPs for CO 2 capture application.
This “knitting” strategy can also be used to pre-pare Pd(II) organometallic catalysts immobilized on the triphenylphosphine-functionalized KAPs(Ph-PPh 3 ) (Figure 16 ). [ 93 ] The microporous structure ensured the high dispersion of Pd active sites and heterogeneous porous structure also improved the diffusion of organic reactant molecules. As a result, the KAPs(Ph-PPh 3 )-Pd exhibited excellent activity and selectivity combined with the mild conditions and aqueous reaction media for the Suzuki-Miyaura cross-coupling reaction of aryl chlorides. This work also highlights that the microporous polymers can not only play the role of support materials, but also protect the catalyst and positively affect the cat-alytic activity.
Figure 15 . Schematic representation of the synthesis of aromatic heterocyclic microporous polymers. Reproduced with permis-sion. [ 92 ] Copyright 2012, John Wiley & Sons, Inc.
First discovered by Davankov and co-workers, the HCPs rep-resent a fl ourishing area of interdisciplinary research. As with other MOPs such as PIMs and CMPs, the HCPs mate-rials possess the advantages of robustness, good thermal stability, and excellent chemical robustness (e.g., to strong acids and bases). In addition, they can be produced in a molded monolithic form. Another distinct advantage of the HCPs is their relative low heat of adsorption in com-parison with other materials. Regarding the generation of microporosity in an HCP, two general methods can be dis-tinguished. The fi rst relies on the extensive crosslinking of polymer chains in an expanded state whereas the second approach is based on the bottom-up assembly of rigid, mostly aromatic building units. Following these approaches, polymeric precursors like swollen poly(styrene) and poly(chloromethylstyrene) as well as functionalized aromatic benzyl chlorides were developed. Landmark in this fi eld is the knitting aromatic polymers (KAPs) syn-thesized by a simple one-step Friedel-Crafts alkylation of aromatic monomers using an FDA crosslinker. There are several outstanding characteristics of this strategy: (I) the range of possible structural building block is extensive (only a few possible examples are demonstrated here), and hence this method is versatile and fl exible; (II) the mild synthesis conditions and the cheap reagents should allow economical and larger-scale production of such materials; (III) it has been demonstrated that the method can lead to materials with high surface area, abundant micropore structure and/or various functional groups; (IV) different structural building blocks and adjustable cross-linker ratio can give rise to diverse pore structures and, potentially, unique properties for certain applications. Based on these characteristics, we have reason to believe that KAPs mate-rials effectively bridge the gap between the laboratory method and the industrial production.
In spite of these advantages, HCPs present some chal-lenges. For example, the large amount of heats generated during the hypercrosslinking has to be eliminated when large-scale production of HCPs was carried out. In addi-tion, the construction of HCPs materials with highly
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periodic frameworks also should be taken into account. Seeking for appropriate external crosslinkers or reac-tion conditions may be helpful to obtain the HCPs with higher periodicity. Of course, new synthetic strategies for constructing novel HCPs networks are, as always, highly desired. We are now trying to develop new materials, which present some ordered structures and unusual pho-toelectric properties. It can be assumed that once a poten-tially useful material is recognized its performance can be tuned incrementally by synthesis. Further developments may be aimed to prepare the HCPs with long range order and well-defi ned outer shapes.
Acknowledgements: The authors would like to thank fi nancially supported by the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1014), and the National Science Foundation of China (Grant no. 50973037, 51173058). Y. Luo thanks the Program of Scholarship Award for Excellent Doctoral Student granted by Ministry of Education.
Received: December 7, 2012 ; Revised: December 30, 2012; Pub-lished online: January 30, 2013; DOI: 10.1002/marc.201200788
Keywords: gas storage; hypercrosslinking; microporous poly-mers; networks; specifi c surface area
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