molecules
Review Crystallization of Covalent Organic Frameworks for Gas Storage Applications
Lijuan Zhu and Yue-Biao Zhang * School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China; [email protected] * Correspondence: [email protected]; Tel.: +86-21-2068-5334
Received: 13 June 2017; Accepted: 4 July 2017; Published: 10 July 2017
Abstract: Covalent organic frameworks (COFs) have emerged as a new class of crystalline porous materials prepared by integrating organic molecular building blocks into predetermined network structures entirely through strong covalent bonds. The consequently encountered “crystallization problem” has been conquered by dynamic covalent chemistry in syntheses and reticular chemistry in materials design. In this contribution, we have reviewed the progress in the crystallization of COF materials and their hydrogen, methane and carbon dioxide gas storage properties for clean energy applications.
Keywords: covalent organic framework; crystallization; dynamic covalent chemistry; hydrogen storage; methane storage; CO2 capture
1. Introduction In recent decades interest in organic materials with permanent nanometer scale pores has grown very quickly due to their specific properties and broad applications in gas storage, gas separation, drug delivery, energy conversion, catalysis, and optoelectronics. Scientists have designed and constructed a large number of porous materials, such as zeolites, metal-organic frameworks (MOFs), mesoporous silica, organosilica, microporous polymers, porous carbons, covalent organic frameworks (COFs) and organic molecular cages. These porous materials can be designed with variation of components, functionalities, pore metrics and skeletons. During the past decades, construction of persistent organic porous materials via covalent bonds is attracting increasing interest due to their unique stability and advanced applications. However, the wide pore-size-distribution and disordered extended structures of these amorphous networks restrict their further applications. Therefore, COFs as a class of covalent porous crystalline polymer in which the organic building blocks are covalently integrated to form extended ordered crystal structures, makes them different from the amorphous porous organic polymers [1–6]. The first synthetic challenge to make COFs, which was even thought impossible due to the difficulty of linking organic building blocks into crystalline extended structures entirely through strong covalent bonds, is the so-called “crystallization problem”, the lack of solubility and reversibility for correcting errors during crystallization. Such difficulties were first conquered by the judicious choice of reversible condensation reactions where water is the byproduct, whose retention can be further used to control the direction of reactions for good reversibility. Meanwhile, the bond energy is also a key parameter for the ease of bond breaking and reformation during crystallization. It is worth mentioning that it is easy to used weaker covalent bonds to achieve higher crystalline products, but the framework’s robustness necessary to achieve practical functions, such as permanent porosity and structure integrity in applications, would then be compromised. Obviously, there is a trade-off between crystallinity and robustness in making COF in the current state of the art. We do believe, however, that this situation will be resolved in the near future by knowing how to control the reversibility and the precision of molecular design.
Molecules 2017, 22, 1149; doi:10.3390/molecules22071149 www.mdpi.com/journal/molecules Molecules 2017, 22, 1149 2 of 29
2.Molecules Dynamic 2017 Covalent, 22, 1149 Chemistry of COFs 2 of 29 2. DynamicIn order Covalent to achieve Chemistry higher of reversibility COFs for efficient error correction during crystallization, dynamicIn order covalent to achieve chemistry higher (DCC) reversibility hasbeen for efficien wellt applied error correction in the constructionduring crystallization, of COFs. dynamic To stitch discretecovalent molecular chemistry building(DCC) has blocks been intowell periodicapplied in networks, the construction DCC enables of COFs. strong To stitch covalent discrete bond formationmolecular accompanied building blocks by into the assemblyperiodic networks, of molecular DCC building enables blocksstrong covalent into predetermined bond formation spatial arrangementsaccompanied by [7, 8the]. Consideringassembly of molecular the possible building spatial blocks arrangements into predetermined of the moiety spatial formed arrangements in the reaction,[7,8]. thereConsidering are multiple the possible products spatial in the arrangements equilibrium of with the moiety various formed degrees in the of crystallinity.reaction, there Such are multiple reversible assemblyproducts of in molecular the equilibrium components with canvarious proceed degrees along of kinetic crystallinity. or thermodynamic Such reversible pathways assembly (Scheme of 1). Duemolecular to the lower components barrier can of activation proceed along (∆G‡ ),kinetic kinetic or intermediates thermodynamic dominate pathways initially (Scheme due 1). to Due a fast to rate ofthe formation lower barrier the kinetic of activation products, (ΔG which‡), kinetic can beintermediates trapped during dominate quenching initially of due the to reaction a fast rate and of form amorphousformation the products kinetic withproducts, the gain which of entropycan be tra frompped their during randomness quenching of of arrangement, the reaction and meanwhile form theamorphous thermodynamic products products with the gain have of the entropy lowest from overall their randomness Gibb’s free of energy arrangement, (∆G◦), someanwhile that given the the opportunitythermodynamic the reaction products will have re-equilibrate the lowest towards overall theGibb’s global free minimum energy ( toΔG form°), so the that most given stable the and probablyopportunity crystalline the reaction products. will re-equilibrate It is also possible towards that the theglobal kinetic minimum products to form can the also most be stable transformed and intoprobably thermodynamic crystalline productsproducts. byIt is using also possible certain that thermo-treatments the kinetic products for recrystallization.can also be transformed It is common into inthermodynamic COF chemistry products that amorphous by using productscertain thermo-treatments are initially obtained for recrystallization. until the synthetic It is conditionscommon in are COF chemistry that amorphous products are initially obtained until the synthetic conditions are optimized by varying the crystallization times, reaction temperatures, reaction solvents, and catalysts optimized by varying the crystallization times, reaction temperatures, reaction solvents, and catalysts (such as acids, bases, and metal salts acting as Lewis acids). The discovery process may change the (such as acids, bases, and metal salts acting as Lewis acids). The discovery process may change the activation energy of thermodynamic products and their reaction rates to accelerate the crystallization activation energy of thermodynamic products and their reaction rates to accelerate the crystallization of COFs. On the other hand, rational design of building blocks may also increase the correctness of of COFs. On the other hand, rational design of building blocks may also increase the correctness of thethe assembly assembly of of molecular molecular components,components, thus thus enhance enhance the the crystallinity crystallinity of of the the obtained obtained products. products. It is It is generallygenerally believed believed thatthat 2D2D COFsCOFs are easier to to form form having having less less probability probability of ofresulting resulting in networks in networks thanthan the the 3D 3D ones ones with with more more structuralstructural diversity.
SchemeScheme 1.1. ThermodynamicThermodynamic control control in in the the dynamic dynamic covalent covalent chemistry chemistry of ofCOFs. COFs.
From the chemistry viewpoint, the difficulty of constructing COF materials can be classified by From the chemistry viewpoint, the difficulty of constructing COF materials can be classified by the difference of bond energy of the linkages between the molecular building blocks (Figure 1). It is theeasier difference for weak of covalent bond energy bonds of to the break linkages and re-form between to correct the molecular assembly building errors, such blocks as the (Figure reversible1). It is easiercovalent for weakbond covalenthave enabled bonds to to make break products and re-form with tolarge correct single assembly crystals errors,[9]. However, such as the lack reversible of covalentrobustness bond prevents have enabledtheir permanent to make porosity products for with further large applications. single crystals For stronger [9]. However, covalent the bonds, lack of robustnesstheir reversibility prevents during their permanentthe reactions porosity are highly for dependent further applications. on the reaction For strongerconditions, covalent which can bonds, theirperform reversibility both crystallinity during the and reactions robustness are after highly isolation dependent from the on system. the reaction conditions, which can perform both crystallinity and robustness after isolation from the system.
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FigureFigure 1. 1.Several Several reversible reversiblereactions reactions usedused forfor constructionconstruction ofof COFCOF materials.materials. Dynamic Dynamic reversible reversible reactions are essential for the formation of crystal covalent organic frameworks (COFs). These reactions are essential for the formation of crystal covalent organic frameworks (COFs). These reversible reversible reactions for the formation of COFs are boron-containing linkages (for example, boroxine, reactions for the formation of COFs are boron-containing linkages (for example, boroxine, boronate boronate ester, borazine and spirobrate), nitrogen-containing linkages (for example, imine, ester, borazine and spirobrate), nitrogen-containing linkages (for example, imine, hydrazone, azine, hydrazone, azine, squaraine, phenazine, triazine, imide and azodioxy), and a double-stage linkage squaraine, phenazine, triazine, imide and azodioxy), and a double-stage linkage containing the boronate containing the boronate ester linkage and the imine linkage in one COF via a one-pot reaction. ester linkage and the imine linkage in one COF via a one-pot reaction. 2.1. B-O Linkages 2.1. B-O Linkages Featuring light-weight compositions and high crystallinity, boron-based COFs have been intensivelyFeaturing studied light-weight including compositions understanding and their high crystallization crystallinity, mechanism, boron-based to the designing COFs have of new been intensivelytopologies, studied achieving including high surface understanding area, and the their util crystallizationization in gas mechanism,storage applications. to the designing The successful of new
Molecules 2017, 22, 1149 4 of 29 construction of such crystalline materials benefited from the decent reversibility of the B-O linkages, including boroxine (-B3O3), dioxaborole (-BO2C2), and spiroborate (-O4B¯).
2.1.1. Boroxine-Linked COFs The emergence of COF chemistry was triggered by Yaghi and co-workers’ seminal work on the self-condensation of aromatic polyboronic acids (Figure 2) [10]. The successful crystallization of these materials was attributed to the implementation of a closed reaction system to sustain the availability of water for maintaining reversible conditions conducive to crystal growth. The solvents used and their mixture are a handle to control the diffusion of building blocks into the crystallization mother liquor. The crystallization was intentionally processed slowly and terminated after sufficient time had elapsed.
MoleculesThe2017 choice, 22, 1149 of solvent and temperature to maximize the error correction associated with4 ofthe 29 boroxine ring-forming reactions can optimize the crystallinity of the boroxine-based COFs. COF-1 with two-dimensional (2D) architectures, was prepared by the self-condensation of 1,4-phenylenediboronic acidtopologies, (BDBA) achieving under solvothermal high surface area,conditions, and the to utilization produce inan gasextended storage staggered applications. layered The successfulstructure withconstruction hexagonal of suchpores crystalline of 15 Å diameter materials and benefited a BET surface from the area decent of 711 reversibility m2·g−1. Yaghi of the and B-O co-workers linkages, alsoincluding synthesized boroxine three-dimensional (-B3O3), dioxaborole (3D) (-BO COFs,2C2), andCOF-102 spiroborate and COF-103 (-O4B¯). by self-condensation of tetrahedral (3D-Td) nodes, tetra(4-dihydroxyborylphenyl)methane (TBPM) and its silane analog 2.1.1. Boroxine-Linked COFs (TBPS), respectively (Figure 2) [11]. These two COFs are the crystal porous organic frameworks with high Thesurface emergence areas (3472 of COF m2 g chemistry-1 for COF-102 was triggeredand 4210 m by2 Yaghig-1 for andCOF-103) co-workers’ and pore seminal size distributions work on the ofself-condensation 11.5 Å (COF-102) of and aromatic 12.5 Å polyboronic (COF-103). acids (Figure2)[ 10]. The successful crystallization of these materialsMicrowave was attributed heating can to the accelerate implementation the reaction of a closedtimes and reaction obtain system much to cleaner sustain COFs the availability with high yields.of water Cooper for maintaining and co-workers reversible demonstrated conditions the conducive synthesis to and crystal purification growth. of The COF-5 solvents and usedCOF-102 and bytheir microwave mixture are heating. a handle The to microwave control the synthetic diffusion me ofthod building reported blocks by into Cooper the crystallization and co-workers mother was 200liquor. times The faster crystallization than solvothermal was intentionally methods reported processed by slowly Yaghi et and al. terminated without changing after sufficient the physical time propertieshad elapsed. of COFs [12].
Figure 2. ChemicalChemical structures structures of of classic boron-cont boron-containingaining COFs (COF-1, COF-102, COF-103) with boroxine connections.
2.1.2. Dioxaborole-Linked COFs The choice of solvent and temperature to maximize the error correction associated with the boroxineThe cross-condensation ring-forming reactions of polybornic can optimize acids the with crystallinity catechols opened of the boroxine-based the way to stich COFs. different COF-1 building with blockstwo-dimensional into one framework (2D) architectures, (Figure 3). was The prepared popular byutilization the self-condensation of 2,3,6,7,10,11- ofhexahydroxyt 1,4-phenylenediboronicriphenylene (HHTP)acid (BDBA) as a undertriangular solvothermal building block conditions, with ditopic, to produce tirtopic an extended and tetratopic staggered boronic layered acids structure leads to with 2D andhexagonal 3D networks pores of(Figure 15 Å diameter3) [10,11,13]. and With a BET a surfacesealed reaction area of 711system, m2· gCOF-5,−1. Yaghi -6, -8 and and co-workers -10 were successfullyalso synthesized prepared three-dimensional in the cocktail solvent (3D) COFs, system COF-102 by mixing and mesistylene COF-103 byand self-condensation 1,4-dioxane for 3–5 of daystetrahedral at 85–100 (3D- °CTd )[10,13]. nodes, tetra(4-dihydroxyborylphenyl)methaneA similar COF with the same HHTP as (TBPM) co-linker and can its be silane also analog prepared (TBPS), by stirringrespectively under (Figure an inert2)[ atmosphere11]. These [14]. two COFs are the crystal porous organic frameworks with high surface areas (3472 m2 g−1 for COF-102 and 4210 m2 g−1 for COF-103) and pore size distributions of 11.5 Å (COF-102) and 12.5 Å (COF-103). Microwave heating can accelerate the reaction times and obtain much cleaner COFs with high yields. Cooper and co-workers demonstrated the synthesis and purification of COF-5 and COF-102 by microwave heating. The microwave synthetic method reported by Cooper and co-workers was 200 times faster than solvothermal methods reported by Yaghi et al. without changing the physical properties of COFs [12].
2.1.2. Dioxaborole-Linked COFs The cross-condensation of polybornic acids with catechols opened the way to stich different building blocks into one framework (Figure3). The popular utilization of 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) as a triangular building block with ditopic, tirtopic and tetratopic boronic acids leads to 2D and 3D networks (Figure3)[ 10,11,13]. With a sealed reaction system, COF-5, -6, -8 and -10 were Molecules 2017, 22, 1149 5 of 29 successfully prepared in the cocktail solvent system by mixing mesistylene and 1,4-dioxane for 3–5 days at 85–100 ◦C[10,13]. A similar COF with the same HHTP as co-linker can be also prepared by stirring underMolecules an 2017 inert, 22,atmosphere 1149 [14]. 5 of 29
FigureFigure 3.3. ConstructionConstruction ofof dioxaborole-linkeddioxaborole-linked COFsCOFs viavia thethe cross-condensationcross-condensation ofof triangulartriangular HHTPHHTP withwith ditopicditopic oror tritopictritopic boronicboronic acidsacids toto formform 2D2D honeycombhoneycomb layer layer structures. structures.
By using the tetrahydroxybenzene and its derivatives, 2D COFs can be prepared by stitching By using the tetrahydroxybenzene and its derivatives, 2D COFs can be prepared by stitching tritopic boronic acids together. The reactions were conducted in the mixture of tetrahydrofuran (THF) tritopic boronic acids together. The reactions were conducted in the mixture of tetrahydrofuran (THF) and methanol (MeOH) by stirring under an inert atmosphere [15,16]. and methanol (MeOH) by stirring under an inert atmosphere [15,16]. Boronate ester linked COFs have high crystallinity, but low hydrolytic stability and chemical Boronate ester linked COFs have high crystallinity, but low hydrolytic stability and chemical stability because of the reversibility of the reactions, which leads to decomposition upon exposure to stability because of the reversibility of the reactions, which leads to decomposition upon exposure water or acid. This disadvantage greatly limits their applications. Alkylation in the pores of COFs to water or acid. This disadvantage greatly limits their applications. Alkylation in the pores of improves the stability by decreasing the hydrolysis rate of the connected linkage. Lavigne and COFs improves the stability by decreasing the hydrolysis rate of the connected linkage. Lavigne and co-workers were the first to incorporate alkyl groups into the channel walls during the synthesis of co-workers were the first to incorporate alkyl groups into the channel walls during the synthesis of the the boronate ester linked COFs to increase the stability of COFs, as well as to fine tune the pore sizes [16]. boronate ester linked COFs to increase the stability of COFs, as well as to fine tune the pore sizes [16]. The presence of alkyl chains in the channels control the pore size from 1.8 nm to 1.1 nm (Figure 4), The presence of alkyl chains in the channels control the pore size from 1.8 nm to 1.1 nm (Figure4), which were observed for the series of COF-18 Å (no alkyl group), COF-16 Å (methyl), COF-14 Å which were observed for the series of COF-18 Å (no alkyl group), COF-16 Å (methyl), COF-14 Å (ethyl), (ethyl), and COF-11 Å (propyl). Meanwhile, the surface area decreased from 1263 to 105 m2·g−1 and and COF-11 Å (propyl). Meanwhile, the surface area decreased from 1263 to 105 m2·g−1 and the pore the pore volume decreased from 0.69 to 0.052 cm3·g−1, as the chain length increased. Interestingly, 3· −1 volumemodification decreased of the from pore 0.69 interior to 0.052 with cm increasinglyg , as the chainlarger length alkyl increased.groups causes Interestingly, a decline modification in nitrogen ofuptake the pore but interioran increase with in increasingly the molar largeramount alkyl of hydrogen groups causes adsorbed. a decline Once in submersion nitrogen uptake in aqueous but an increasemedia, the in theporosity molar of amount alkylated of hydrogen COFs decreased adsorbed. by Once about submersion 25%, while in the aqueous non-alkylated media, the COFs porosity were ofalmost alkylated completely COFs decreasedhydrolyzed, by losing about 25%,virtually while all the porosity. non-alkylated In addition, COFs the were degree almost of crystallinity completely hydrolyzed,decreased by losing about virtually40% for alkylated all porosity. COFs In addition,and 95% thefor degreenon-alkylated of crystallinity COFs. By decreased using microwave by about 40%synthesis, for alkylated Bein et COFsal. extended and 95% the for framework non-alkylated into materials COFs. By having using microwavelarge 4 nm synthesis,openings (Figure Bein et 4), al. extendedfeaturing theone frameworkof the largest into pores materials in crystalline having materials large 4 nm at openingsthis time [17]. (Figure 4), featuring one of the largest pores in crystalline materials at this time [17].
Molecules 2017, 22, 1149 6 of 29 Molecules 2017, 22, 1149 6 of 29 Molecules 2017, 22, 1149 6 of 29
Figure 4. Construction of dioxaborole-linked COFs via the cross-condensation of ditopic catechol- Figure 4. Construction of dioxaborole-linked COFs via the cross-condensation of ditopic catechol-based based linkers with tritopic boronic acids to form the 2D honeycomb layer structures. The pore sizes linkersFigure with 4. Construction tritopic boronic of dioxaborole-linked acids to form the 2DCOFs honeycomb via the cross-condensation layer structures. Theof ditopic pore sizes catechol- can be can be modulated by the decorated functionalities of the ditopic linkers, and expanded by elongating modulatedbased linkers by thewith decorated tritopic boronic functionalities acids to of form the ditopicthe 2D linkers,honeycomb and expandedlayer structures. by elongating The pore both sizes the both the boronic acid and catechol linkers. boroniccan be modulated acid and catechol by the decorated linkers. functionalities of the ditopic linkers, and expanded by elongating both the boronic acid and catechol linkers. By using octahydroxyphthalocyanine and its metalated version, the Dichtel group was able to By using octahydroxyphthalocyanine and its metalated version, the Dichtel group was able to expandBy usingthe pore octahydroxyphthalocyanine size of the ZnPc lattice from and 2.7its metalatedto 4.4 nm (Figureversion, 5) the [18]. Dichtel The Dichtelgroup was group able also to expandexpandreported the the the porepore selective sizesize of ofand thethe patterned ZnPcZnPc latticelattice growth from of 2.7 a 2Dto 4.4 phthalocyanine nm (Figure (Figure 5 5)COF)[ [18].18 on]. The Thesingle-layer Dichtel Dichtel group graphene. group also also reportedreportedThis provides the the selective selective a promising andand patterned patternedpathway to growth direct of ofthe a a growth 2D phthalocyanine phthalocyanine of COF films COF COF[19]. on on single-layer single-layer graphene. graphene. ThisThis provides provides a a promising promising pathwaypathway toto directdirect thethe growth of COF films films [19]. [19].
Figure 5. Construction of dioxaborole-linked COFs via the cross-condensation of ditopic catechol- based linkers with tritopic boronic acids to form 2D honeycomb layer structures. The pore sizes can FigureFigure 5. 5.Construction Construction of of dioxaborole-linked dioxaborole-linked COFs COFs via via the the cross-condensation cross-condensation ofditopic of ditopic catechol-based catechol- be modulated by the decorated functionalities of the ditopic linkers, and expanded by elongating both linkersbased linkers with tritopic with tritopic boronic boronic acids acids to form to form 2D honeycomb 2D honeycomb layer layer structures. structures. The The pore pore sizes sizes can can be the boronic acid and catechol linkers. modulatedbe modulated by the by decoratedthe decorated functionalities functionalities of theof th ditopice ditopic linkers, linkers, and and expanded expanded by by elongating elongating both both the the boronic acid and catechol linkers. boronicTo avoid acid the and oxidation catechol linkers. and insolubility of polycatechols, protected catechols were proposed as startingTo avoidmaterials the oxidationfor making and COFs insolubility which can of bepolyca deprotectedtechols, protectedin-situ under catechols catalysis were by proposeda Lewis acid as starting materials for making COFs which can be deprotected in-situ under catalysis by a Lewis acid
Molecules 2017, 22, 1149 7 of 29
To avoid the oxidation and insolubility of polycatechols, protected catechols were proposed as startingMolecules 2017 materials, 22, 1149for making COFs which can be deprotected in-situ under catalysis by a Lewis7 of 29 acid such as BF3·OEt2 [20]. With this strategy, triangular HHTP and square phthalocyanine as wellsuch asas itsBF3 metalated·OEt2 [20]. With derivatives this strategy, have been triangular successfully HHTP incorporatedand square phthalocyanine into boronate as ester-linked well as its COFsmetalated (Figure derivatives6). Mechanistic have been studies successfully show incorporated that before adding into borona the Lewiste ester-linked acid, the COFs boronic (Figure acids 6). self-condensateMechanistic studies reversibly show tothat form before boroxine-linked adding the Lewis products acid, and the H 2boronicO. The additionacids self-condensate of BF3·OEt2 catalyzesreversibly acetonide to form boroxine-linked hydrolysis of protected products catechols, and H2O. and The the addition resulting of catechol BF3·OEt will2 catalyzes rapidly acetonide condense withhydrolysis the boronic of protected acids catechols, to form the and boronate-linked the resulting catechol COFs [will21]. rapidly Once thecondense free boronic with the acids boronic are consumed,acids to form the the formed boronate-linked products will COFs hydrolyze [21]. Once and the release free boronic more boronic acids are acids, consumed, which will the produce formed moreproducts boronate-linked will hydrolyze COFs. and release more boronic acids, which will produce more boronate-linked COFs.
FigureFigure 6.6. SynthesisSynthesis ofof dioxaborole-linkeddioxaborole-linked COFsCOFs startingstarting fromfrom protectedprotected catechol-basedcatechol-based linkerslinkers withwith boronicboronic acidsacids forfor improved solubilitysolubility andand anti-oxidation inin comparisoncomparison withwith pristinepristine catechol linkers.linkers.
To accelerate the synthesis of highly crystalline COF material, a two-step microwave synthesis To accelerate the synthesis of highly crystalline COF material, a two-step microwave synthesis has been proposed for protected boronic acids as starting materials with the addition of HHTP has been proposed for protected boronic acids as starting materials with the addition of HHTP (Figure 7). The resulting product proved to be highly crystalline and possess large pore openings. (Figure7). The resulting product proved to be highly crystalline and possess large pore openings. Control experiments didn’t yield COF via the one-pot reaction. This work also provided the first Control experiments didn’t yield COF via the one-pot reaction. This work also provided the first high-resolution TEM image of a COF showing the pores clearly [22]. high-resolution TEM image of a COF showing the pores clearly [22].
Molecules 2017, 22, 1149 8 of 29 Molecules 2017, 22, 1149 8 of 29 Molecules 2017, 22, 1149 8 of 29
Figure 7. Synthesis of the dioxaborole-linked COF by in-situ deprotection of boronic pinacol ester Figure 7. Synthesis of the dioxaborole-linked COF by in-situ deprotection of boronic pinacol ester followed with the addition of HHTP via microwave heating method. followedFigure 7. with Synthesis the addition of the ofdioxaborole-linked HHTP via microwave COF heatingby in-situ method. deprotection of boronic pinacol ester followed with the addition of HHTP via microwave heating method. A modulation approach has been developed to control the structure and crystallinity of COF materials,AA modulation modulation introducing approachapproach a modulator hashas beenbeen in the developeddeveloped synthesis tothat control can compete the the structure structure with one and and of crystallinitythe crystallinity building ofblocks of COF COF materials,materials,during the introducing introducing solvothermal a a modulator modulator COF growth inin thetheto form synthesissynthesis highly that crystalline can compete compete frameworks with with one one with of of the large the building building domain blocks blockssize duringduringand very the the solvothermal solvothermalhigh porosity. COF COF Bein growth growth and to co-workersto form form highly highly used crystalline crystalline monoboronic frameworks frameworks acids with aswith largemodulators large domain domain in size thesize and veryandsolvothermal highvery porosity. high synthesis porosity. Bein andof Beinthe co-workers archetypicaland co-workers used COF-5 monoboronic used to optimize monoboronic acids the as crystallinity, modulators acids as modulators indomain the solvothermal size, in and the synthesissolvothermalporosity of of the 2D synthesis archetypical COFs (Figure of the COF-5 8). archetypical Moreover, to optimize theCOF-5 the ad crystallinity,dition to optimize of the domain monoboronicthe crystallinity, size, and acids porosity domain also provides ofsize, 2D COFsand a (Figureporositypotential8). Moreover,of construction 2D COFs the (Figure of addition function 8). ofMoreover,al thecrystalline monoboronic the COF addition materials acids of alsothe [23]. monoboronic provides a potential acids also construction provides a of functional crystalline COF materials [23]. potential construction of functionalOH crystalline COF materials [23]. HO OH OH B FG HO OH HO OH OH B FG FG = SH, HO HO B B COOH HO OH HO OH OH FG = SH, HO OH COOH B ModulatorB HHTP BDBAHO OH HO OH OH OH HHTP BDBA Modulator
mesitylene/dioxane 100 °C GF mesitylene/dioxane 100 °C GF B O O B O O
O O B O B B O B O O O O
O O O O B O B B O B O O
O O O B B O O B B O O O
O O GF B O B B O B FG O O O O
O O O O GF B O B B O B FG O O
O O O B B O O B B O O O
O O B O B B O B O O O O
O O O O B O COF-5-B Bx O B O O O O B O COF-5-x O B Figure 8. The modulation approach to synthesize COF-5-X via the cross-condensation of HHTP and
1,4-diboronic acid combining with terminal boronic acids with various functional groups to achieve FigureFigurehigher 8. 8.crystallinityThe The modulation modulation and ex approachapproachternal functionalities. toto synthesizesynthesize COF-5-XX viavia the the cross-condensation cross-condensation of of HHTP HHTP and and 1,4-diboronic1,4-diboronic acid acid combining combining withwith terminalterminal boronic acids with various various functional functional groups groups to to achieve achieve higher crystallinity and external functionalities. higher crystallinity and external functionalities.
Molecules 2017, 22, 1149 9 of 29
Molecules 2017, 22, 1149 9 of 29 2.1.3. Spiroborate-Linked COFs 2.1.3. Spiroborate-Linked COFs Researchers have shown great interest in exploring new methods to enhance the chemical stabilityResearchers of boron-containing have shown COFs. great Recently,interest in the exploring condensation new methods of diols to with enhance trialkyl the borate chemical in the presencestability of of basic boron-containing catalysts has COFs been. exploredRecently, forthe thecondensation synthesis of spiroborate-linkeddiols with trialkyl borate ionic COFsin the in whichpresence the negatively of basic catalysts charged has borons been explored are located for onthe the synthesis edges withof spiroborate-linked different cations ionic as counter COFs in ions. Zhangwhich and the co-workers negatively charged [24] constructed borons are a located novel on type the of edges spiroborate-linked with different cations ionic as covalent counter organicions. frameworkZhang and (ICOF, co-workers Figure [24]9), constructed which contains a novel sp ty3 pehybridized of spiroborate-linked boron anionic ionic centers covalent and organic tunable framework (ICOF, Figure 9), which contains sp3 hybridized boron anionic centers and tunable countercations (lithium or dimethylammonium). The synthesized ICOFs (ICOF-1 and ICOF-2) countercations (lithium or dimethylammonium). The synthesized ICOFs (ICOF-1 and ICOF-2) show show good thermal stabilities and excellent resistance to hydrolysis, remaining nearly intact when good thermal stabilities and excellent resistance to hydrolysis, remaining nearly intact when immersed in water or basic solution for up to two days. The SEM images support the single-crystalline immersed in water or basic solution for up to two days. The SEM images support the single- morphology of ICOF-1 and ICOF-2. The PXRD results also show multiple sharp peaks, which indicate crystalline morphology of ICOF-1 and ICOF-2. The PXRD results also show multiple sharp peaks, thewhich orderliness indicate structures the orderliness in the structures framework. in the fr However,amework. theHowever, crystal the structures crystal structures of ICOFs of haveICOFs not 2· −1 beenhave characterized. not been characterized. These ICOFs These alsoICOFs have also highhave high BET BET surface surface areas areas up up to to 1259 1259 m m2·gg−1 andand adsorb adsorb a significanta significant amount amount ofof H2 (up(up to to 3.11 3.11 wt wt %, %, 77 77K, K,1 bar) 1 bar) and and CH4 CH (up4 to(up 4.62 to wt 4.62 %, wt273 %, K, 2731 bar). K, The 1 bar). Theexistence existence of ofpermanently permanently immobilized immobilized ion centers ion centers in ICOFs in ICOFs enables enables the transportation the transportation of lithium of lithium ions −5 −1 + ionswith with room-temperature room-temperature lithium-ion lithium-ion conductivity conductivity of 3.05 × of 10 3.05−5 S·Lcm× 10−1 andS an·Lcm averageand Li+ an transference average Li transferencenumber value number of 0.80 value ± 0.02. of 0.80 ± 0.02.
Figure 9. Construction of spiroborate-linked COFs featuring anionic skeletons with various counter Figure 9. Construction of spiroborate-linked COFs featuring anionic skeletons with various counter ions for high ionic conductivity. ions for high ionic conductivity. 2.2. C-N Linkages 2.2. C-N Linkages 2.2.1. Imine-Linked COFs 2.2.1. Imine-Linked COFs To avoid the structural fragility of boron-containing COFs, imine-based COFs were discovered byTo a avoidreversible the structuralcondensation fragility of polytopic of boron-containing anilines with COFs, polybenzaldehydes imine-based COFs or werepolyketone discovered with by a reversibleelimination condensation of water by a of catalyst polytopic of aqueous anilines acetic with polybenzaldehydesacid (Figure 10). Generally, or polyketone the crystallinity with elimination of the ofimine-based water by a catalyst COFs is of lower aqueous compared acetic with acid that (Figure of boronate 10). Generally, ester-linked the crystallinity COFs, whereas of the the imine-based chemical COFsstability is lower in the compared presencewith of water, that ofacids boronate and bases ester-linked is significantly COFs, enhanced. whereas the chemical stability in the presenceThe of first water, imine-linked acids and COF bases was is developed significantly by enhanced.Yaghi and co-workers in 2009 [25]. They constructed an Theimine-based first imine-linked COF (COF-300), COF was which developed possesses by Yaghi a 3D and five-fold co-workers interpenetrating in 2009 [25]. They diamond-like constructed anstructure. imine-based COF-300 COF has (COF-300), a BET surface which area possesses of 1360 m a2·g 3D−1 and five-fold a pore interpenetratingsize of 7.8 Å, which diamond-like provides structure.better hydrolytic COF-300 stability has a BET compared surface to area the of boro 1360n-containing m2·g−1 and COFs. a pore Moreover, size of 7.8 the Å, broad which range provides of bettermultifunctional hydrolytic amines stability and compared aldehydes to provides the boron-containing a large number of COFs. structural Moreover, possibilities the broadfor imine- range based COFs.
Molecules 2017, 22, 1149 10 of 29
Imine formed COF-320 was prepared through solvothermal condensation of tetra-(4-anilyl)- methane and 4,4’-biphenyldialdehyde in 1,4-dioxane at 120 °C. A 9-fold interlaced diamond network was formed and the final crystalline material exhibits high porosity with a Langmuir surface area of 2400 m2·g−1. The crystal structure of the COF-320 solid was elucidated by single-crystal 3D electron diffraction [26]. Recently, a pyridyl functionalized version of COF-320 was prepared, namely LZU- 301, in which the reversible dynamic response upon guest accommodation and release was uncovered. Thus, the design principle of 3D dynamic COFs was claimed [27]. Wang and co-workers synthesized a novel 3D pyrene-based COF (3D-Py-COF) by condensation of tetra(p-aminophenyl)methane and 1,3,6,8-tetrakis(4-formylphenyl)-pyrene. This 3D-Py-COF has a two-fold interpenetrated pts topology, which was first reported in this work. Due to the existence of pyrene units in the 3D frameworks, 3D-Py-COF exhibits fluorescent properties and can be used in explosives detection [28]. Yan and co-workers designed and synthesized two new 3D base-functionalized COFs with micropores (BF-COF-1 and BF-COF-2), by the reaction of a tetrahedral alkyl amine, 1,3,5,7- tetraaminoadamantane (TAA), and 1,3,5-triformylbenzene (TFB) or triformylphloroglucinol (TFP) [29]. Molecules 2017, 22, 1149 10 of 29 These BF-COFs were used for the Knoevenagel condensation reaction with high conversion (BF-COF-1: 96% and BF-COF-2: 98%), highly efficient size selectivity, and good recyclability. The structural rigidityof multifunctional of the admantane amines building and aldehydes unit is providesthe key for a largeforming number targeted of structuralproducts. possibilitiesIn contrast, the for combinationimine-based COFs.of tetra(p-aminophenyl) with TFB could not produce crystalline product.
Figure 10. 10. ConstructionConstruction of imine-linked of imine-linked COFs COFsby co-condensation by co-condensation of tetratopic of tetratopic anilines (tetra-(4- anilines (tetra-(4-anilyl)methane,anilyl)methane, and 1,3,5,7-tetraaminoadamantan and 1,3,5,7-tetraaminoadamantane)e) with various with various ditopic, ditopic, tetraaldehydes tetraaldehydes and triangularand triangular aldehydes aldehydes to form to form 3D networks. 3D networks.
To enhance both the chemical stability and crystallinity in 2D porphyrin COFs, Banerjee and co- Imine formed COF-320 was prepared through solvothermal condensation of tetra-(4-anilyl)-methane workers proposed a new strategy to protect the COF interior by introducing -OH to the Schiff base [-C=N] and 4,40-biphenyldialdehyde in 1,4-dioxane at 120 ◦C. A 9-fold interlaced diamond network was formed centers in COFs and creating an intramolecular [O-H···N=C] hydrogen bond (Figure 11) [30]. This and the final crystalline material exhibits high porosity with a Langmuir surface area of 2400 m2·g−1. hydrogen-bonding interaction enhances the stability of imine bonds in the presence of water and acid The crystal structure of the COF-320 solid was elucidated by single-crystal 3D electron diffraction [26]. (3 N HCl). Otherwise, this hydrogen bond in DhaTph also could enhance the crystallinity and Recently, a pyridyl functionalized version of COF-320 was prepared, namely LZU-301, in which the porosity, compared to the methoxy-substituted COF DmaTph in which this intramolecular hydrogen reversible dynamic response upon guest accommodation and release was uncovered. Thus, the design bond does not exist. This work demonstrates that the introduction of intramolecular hydrogen bonds principle of 3D dynamic COFs was claimed [27]. can not only improve the chemical stability, but also the crystallinity and porosity of 2D COFs, which Wang and co-workers synthesized a novel 3D pyrene-based COF (3D-Py-COF) by condensation would broaden the potential applications of COF materials. of tetra(p-aminophenyl)methane and 1,3,6,8-tetrakis(4-formylphenyl)-pyrene. This 3D-Py-COF has a two-fold interpenetrated pts topology, which was first reported in this work. Due to the existence of pyrene units in the 3D frameworks, 3D-Py-COF exhibits fluorescent properties and can be used in explosives detection [28]. Yan and co-workers designed and synthesized two new 3D base-functionalized COFs with micropores (BF-COF-1 and BF-COF-2), by the reaction of a tetrahedral alkyl amine, 1,3,5,7-tetraaminoadamantane (TAA), and 1,3,5-triformylbenzene (TFB) or triformylphloroglucinol (TFP) [29]. These BF-COFs were used for the Knoevenagel condensation reaction with high conversion (BF-COF-1: 96% and BF-COF-2: 98%), highly efficient size selectivity, and good recyclability. The structural rigidity of the admantane building unit is the key for forming targeted products. In contrast, the combination of tetra(p-aminophenyl) with TFB could not produce crystalline product. To enhance both the chemical stability and crystallinity in 2D porphyrin COFs, Banerjee and co-workers proposed a new strategy to protect the COF interior by introducing -OH to the Schiff base [-C=N] centers in COFs and creating an intramolecular [O-H···N=C] hydrogen bond (Figure 11)[30]. This hydrogen-bonding interaction enhances the stability of imine bonds in the presence of water and acid (3 N HCl). Otherwise, this hydrogen bond in DhaTph also could enhance the crystallinity and porosity, compared to the methoxy-substituted COF DmaTph in which this intramolecular hydrogen Molecules 2017, 22, 1149 11 of 29 bond does not exist. This work demonstrates that the introduction of intramolecular hydrogen bonds can not only improve the chemical stability, but also the crystallinity and porosity of 2D COFs, which would broaden the potential applications of COF materials. Molecules 2017, 22, 1149 11 of 29
Molecules 2017, 22, 1149 11 of 29
Figure 11. Syntheses of DmaTph and DhaTph by the condensation of square planar Tph building unit Figure 11. (blue)Syntheses and linear of Dma/Dha DmaTph building and DhaTph unit (red). by Th thee introducing condensation of -OH of to squarethe Schiff planar base [-C=N] Tph building unit (blue)centers and linearin COFs Dma/Dha can create buildingan intramolecular unit (red). [O-H···N=C] The introducing hydrogen ofbond -OH to toenhance the Schiff both basethe [-C=N] centers inchemical COFs can stability create and an crystallinity intramolecular of COFs. [O-H ···N=C] hydrogen bond to enhance both the chemical stability and crystallinity of COFs. FigureThe formation 11. Syntheses of ofcrystalline DmaTph andstructures DhaTph viaby therelati condvelyensation weak of interactions square planar between Tph building planar unit layers has been(blue) well and definedlinear Dma/Dha and designed building in unitcrystal (red). engine The ering.introducing Jiang ofand -OH co-workers to the Schiff synthesized base [-C=N] a series Theof formation 2Dcenters COFs in locked ofCOFs crystalline with can createintra-layer structuresan intramolecular hydrogen-bonding via [O-H···N=C] relatively (H-bonding) hydrogen weak interactions bond to enhance (Figure between both 12) [31].the planar The layers has beenH-bonding wellchemical defined interaction stability and and sites designedcrystallinity were located of in COFs. crystal on the edge engineering. units of the imine-linked Jiang and co-workerstetragonal porphyrin synthesized a series of 2DCOFs, COFs and lockedthe contents with of intra-layer the H-bonding hydrogen-bonding sites in the COFs were (H-bonding) synthetically interactions regulated by (Figureusing a 12)[31]. three-componentThe formation condensation of crystalline system. structures The viaintra-la relatiyervely H-bonding weak interactions interactions between suppress planar the torsionlayers The H-bondinghasof the been edge interactionwell units defined and siteslockand designedthe were tetragonal located in crystal sheets on engine thein a edgeplanarering. units conformation.Jiang ofand the co-workers imine-linked This planarization synthesized tetragonal enhances a series porphyrin COFs, andofthe 2D the interlayer COFs contents locked interactions with of the intra-layer and H-bonding triggers hydrogen-bonding extended sites in π-cloud the (H-bonding) COFs delocalization were interactions synthetically over the (Figure 2D sheets. regulated 12) Upon[31]. The AA by using a three-componentH-bondingstacking, the condensationinteraction resulting sites COFs system.were with located layered The on intra-layer the2D edgesheet sunits amplify H-bonding of the these imine-linked interactionseffects an tetragonald strongly suppress porphyrin affect the the torsion of the edgeCOFs,physical units and and properties the lock contents the of the tetragonalof material,the H-bonding including sheets sites in improving in a the planar COFs their conformation. were crystallinity, synthetica enhancinglly This regulated planarization their by porosity, using a enhances three-componentincreasing their condensationlight-harvesting system. capability, The intra-la reducingyer H-bonding their band interactions gap, and suppress enhancing the torsion their the interlayer interactions and triggers extended π-cloud delocalization over the 2D sheets. Upon AA ofphotocatalytic the edge units activity and lock toward the tetragonal the generation sheets ofin singleta planar oxygen. conformation. This planarization enhances stacking,the the interlayer resulting interactions COFs and with triggers layered extended 2D sheetsπ-cloud amplifydelocalization these over effects the 2D sheets. and strongly Upon AA affect the physicalstacking, properties the resulting of the material, COFs with including layered 2D improving sheets amplify their these crystallinity, effects and strongly enhancing affect their the porosity, increasingphysical their light-harvestingproperties of the material, capability, including reducing improving their their band crystallinity, gap, and enhancing enhancing their their porosity, photocatalytic activity towardincreasing the their generation light-harvesting of singlet capability, oxygen. reducing their band gap, and enhancing their photocatalytic activity toward the generation of singlet oxygen.
Figure 12. Schematic of the synthesis of 2D porphyrin COFs with designable content of hydrogen- bonding structures.
Figure 12. Schematic of the synthesis of 2D porphyrin COFs with designable content of hydrogen- Figure 12.bondingSchematic structures. of the synthesis of 2D porphyrin COFs with designable content of hydrogen-bonding structures.
Molecules 2017, 22, 1149 12 of 29 Molecules 2017, 22, 1149 12 of 29
In addition to H-bonding interactions, self-complementary π-electronic forces also provide new In addition to H-bonding interactions, self-complementary π-electronic forces also provide new opportunities for enhancing the crystallinity of COFs. Jiang and co-workers reported a synthetic opportunities for enhancing the crystallinity of COFs. Jiang and co-workers reported a synthetic method to control the crystallinity and porosity of COFs by managing interlayer interactions based on method to control the crystallinity and porosity of COFs by managing interlayer interactions based self-complementary π-electronic forces (Figure 13). on self-complementary π-electronic forces (Figure 13).
Figure 13. Schematic of the synthesis of 2D COFs integrated with self-complementary π-electronic forces (CuP−TFPhX, X = 25, 50, and 75 mol %) and the CuP−Ph and CuP−TFPh controls. forces (CuP−TFPhX, X = 25, 50, and 75 mol %) and the CuP−Ph and CuP−TFPh controls.
They controlled the crystal structure of CuP-Ph COF by introducing 2,3,5,6- They controlled the crystal structure of CuP-Ph COF by introducing 2,3,5,6-tetrafluoroterephthalaldehyde tetrafluoroterephthalaldehyde into the COF structure. Fluoro-substituted aromatic units in different into the COF structure. Fluoro-substituted aromatic units in different ratios were integrated into the edge ratios were integrated into the edge units, which will induce the self-complementary π-electronic units, which will induce the self-complementary π-electronic interactions in the COFs. This interaction interactions in the COFs. This interaction increases the crystallinity and the porosity of COFs by increases the crystallinity and the porosity of COFs by maximizing the total crystal stacking energy maximizing the total crystal stacking energy and minimizing the unit cell size. This work provides a and minimizing the unit cell size. This work provides a new pathway to improve the crystals of COFs new pathway to improve the crystals of COFs by controlling the interlayer interactions [32]. by controlling the interlayer interactions [32]. The stability and crystallinity of 2D COFs are highly related to the factors of the interlayer The stability and crystallinity of 2D COFs are highly related to the factors of the interlayer interactions between the 2D layers. In imine-based COFs, the C=N bond is polarized to yield partially interactions between the 2D layers. In imine-based COFs, the C=N bond is polarized to yield positively charged carbon and negatively charged nitrogen, which will cause electrostatic repulsion partially positively charged carbon and negatively charged nitrogen, which will cause electrostatic between the layers and could destabilize the layered structure staking. Jiang and co-workers introduced repulsion between the layers and could destabilize the layered structure staking. Jiang and co-workers methoxy groups (-OMe) into the pore walls to reinforce the interlayer interactions (Figure 14). The oxygen introduced methoxy groups (-OMe) into the pore walls to reinforce the interlayer interactions lone pairs on the methoxy-substituted C2-phenyl edges soften the polarization through resonance (Figure 14). The oxygen lone pairs on the methoxy-substituted C -phenyl edges soften the polarization and increase the stability and high crystallinity of the COF. Compared2 to the TPB-TP-COF and TPB- through resonance and increase the stability and high crystallinity of the COF. Compared to the DHTP-COF, the methoxy-functionalized TPB-DMTP-COF has much higher crystal stacking energy, TPB-TP-COF and TPB-DHTP-COF, the methoxy-functionalized TPB-DMTP-COF has much higher showing high crystallinity and high BET surface area. Through the introduction of chiral centers, crystal stacking energy, showing high crystallinity and high BET surface area. Through the introduction (S)-Py moieties, onto the walls of the open channels, they obtained chiral COFs which can be used of chiral centers, (S)-Py moieties, onto the walls of the open channels, they obtained chiral COFs for heterogeneous asymmetric organocatalysis of asymmetric Michael reactions. The Jiang group also which can be used for heterogeneous asymmetric organocatalysis of asymmetric Michael reactions. investigated the proton conduction properties of this highly crystalline and stable COF and explored The Jiang group also investigated the proton conduction properties of this highly crystalline and the potential applications in many areas, including fuel cells, electrochemical sensors, electrochemical stable COF and explored the potential applications in many areas, including fuel cells, electrochemical reactors and electrochromic devices [33].From Figure 15, we can observe that TPB-DMTP-COF also sensors, electrochemical reactors and electrochromic devices [33].From Figure 15, we can observe that exhibits much higher BET and Langmuir surface area compared to the COFs without enhanced TPB-DMTP-COF also exhibits much higher BET and Langmuir surface area compared to the COFs interlayer interactions and hydrogen bond enhanced COFs. As shown in Figure 15, the BET surface without enhanced interlayer interactions and hydrogen bond enhanced COFs. As shown in Figure 15, area decreases with the decrement of the methoxy group ratio, which demonstrates that the the BET surface area decreases with the decrement of the methoxy group ratio, which demonstrates introduced methoxy groups greatly improve the crystallinity of the COFs. This kind of COF also shows enhanced chemical stability. After immersing in 12-M HCl, boiling water and 14-M NaOH for 1 week, the surface area of TPB-DMTP-COFs still retains the original values. However, the non-methoxy
Molecules 2017, 22, 1149 13 of 29 that the introduced methoxy groups greatly improve the crystallinity of the COFs. This kind of
MoleculesCOF also 2017 shows, 22, 1149 enhanced chemical stability. After immersing in 12-M HCl, boiling water and13 14-M of 29 NaOHMolecules for 2017 1, week,22, 1149the surface area of TPB-DMTP-COFs still retains the original values. However,13 of the 29 group-basednon-methoxy COFs group-based (TPB-TP-COF COFs (TPB-TP-COFand TPB-DHTP-COF) and TPB-DHTP-COF) show very week show acid, very base week or boing acid, base water or group-based COFs (TPB-TP-COF and TPB-DHTP-COF) show very week acid, base or boing water tolerance.boing water tolerance. tolerance.
Figure 14. Synthesis and structure of stable crystalline porous COFs through softened interlayer Figure 14.14. SynthesisSynthesis and and structure structure of stableof stable crystalline crystalline porous porous COFs COFs through through softened softened interlayer interlayer charge charge repulsion. (a) TPB-TP COF control; (b) TPB-DHTP COF control; (c) stable crystalline porous repulsion.charge repulsion. (a) TPB-TP (a) TPB-TP COF control; COF control; (b) TPB-DHTP (b) TPB-DHTP COF control;COF control; (c) stable (c) stable crystalline crystalline porous porous COF COF of TPB-DMTP COF; (d) functionalization of TPB-DMTP COF ([(S)-Py]x -TPB-DMTP-COFs, ofCOF TPB-DMTP of TPB-DMTP COF; (COF;d) functionalization (d) functionalization of TPB-DMTP of TPB-DMTP COF ([( SCOF)-Py] x([(-TPB-DMTP-COFs,S)-Py]x -TPB-DMTP-COFs,x = 0.17, x = 0.17, 0.34, 0.50). 0.34,x = 0.17, 0.50). 0.34, 0.50).
Figure 15. The surface area vs the methoxy group ratio in TPB-DMTP-COF (left). The BET surface Figure 15. The surface area vs the methoxy group ratio in TPB-DMTP-COF (left). The BET surface areasFigure of 15.TPB-DMTPThe surface and area (S)-Py-TPB-DMTP vs the methoxy COFs group us ratioe TPB-TP in TPB-DMTP-COF COF and TPB-DHTP (left). COF The BETas control. surface areas of TPB-DMTP andand (S)-Py-TPB-DMTP(S)-Py-TPB-DMTP COFsCOFs useuse TPB-TPTPB-TP COFCOF andand TPB-DHTPTPB-DHTP COFCOF asas control.control. Bein and co-workers found that a successive COF layer may nucleate at more than one location Bein and co-workers found that a successive COF layer may nucleate at more than one location duringBein the and crystallizaation co-workers found procedure, that a and successive the latera COFl offset layer of may the nucleategrowing at pattern more thanis more one likely location to during the crystallizaation procedure, and the lateral offset of the growing pattern is more likely to beduring in various the crystallizaation directions, which procedure, can induce and lattice the lateral strain offset and defect of the into growing the crystals pattern of is COF more materials likely to be in various directions, which can induce lattice strain and defect into the crystals of COF materials (Figurebe in various 16). To directions, prevent these which defects can induce from latticehappeni strainng, they and defectdeveloped into thea synthetic crystals ofconcept COF materialsto allow (Figure 16). To prevent these defects from happening, they developed a synthetic concept to allow consecutive(Figure 16). COF To prevent sheets to these lock defects into position from happening,during crystal they growth, developed and thus a synthetic minimize concept the occurrence to allow consecutive COF sheets to lock into position during crystal growth, and thus minimize the occurrence ofconsecutive stacking COFfaults sheets and dislocations. to lock into position To prove during thei crystalr concept, growth, they and constructed thus minimize a series the occurrenceof highly of stacking faults and dislocations. To prove their concept, they constructed a series of highly crystalline star-shaped, dual-pore 4PE-based COFs by co-condensation of 1,1,2,2-tetrakis(4- crystalline star-shaped, dual-pore 4PE-based COFs by co-condensation of 1,1,2,2-tetrakis(4- aminophenyl)ethane with the linear dialdehyde bridging units 1P, 2P, 3P and TT. They also aminophenyl)ethane with the linear dialdehyde bridging units 1P, 2P, 3P and TT. They also synthesized a set of hexagonal 3PA- and 3PB-based COFs by combination of the linear dialdehyde synthesized a set of hexagonal 3PA- and 3PB-based COFs by combination of the linear dialdehyde bridging units 2P and TT as control COFs [34]. This approach enabled the achievement of a very high bridging units 2P and TT as control COFs [34]. This approach enabled the achievement of a very high crystallinity for a series of COFs that comprise tri- and tetradentate central building blocks. Bein and crystallinity for a series of COFs that comprise tri- and tetradentate central building blocks. Bein and
Molecules 2017, 22, 1149 14 of 29 of stacking faults and dislocations. To prove their concept, they constructed a series of highly crystalline star-shaped, dual-pore 4PE-based COFs by co-condensation of 1,1,2,2-tetrakis(4-aminophenyl)ethane with the linear dialdehyde bridging units 1P, 2P, 3P and TT. They also synthesized a set of hexagonal Molecules3PA- and 2017 3PB-based, 22, 1149 COFs by combination of the linear dialdehyde bridging units 2P and TT as control14 of 29 COFs [34]. This approach enabled the achievement of a very high crystallinity for a series of COFs that co-workerscomprise tri- also and proposed tetradentate that the central molecular building geometry blocks. of Bein the central and co-workers building block alsoproposed in COFs provides that the amolecular uniquely geometry defined docking of the central site that building can lead block the in COFsattachment provides of asuccessive uniquely definedCOF layers, docking which site will that furthercan lead enhance the attachment the crystallinity of successive of the COF COFs layers, [35]. They which chose will further1,3,6,8-tetrakis(4-aminophenyl)pyrene enhance the crystallinity of the asCOFs a central [35]. Theybuilding chose block 1,3,6,8-tetrakis(4-aminophenyl)pyrene to synthesize a series of highly crysta as alline central pyrene-based building block COFs. to synthesizeAs shown ina series Figure of 16, highly the single-crystal crystalline pyrene-based structures of COFs. their Asmolecular shown inmodel Figure compounds 16, the single-crystal also revealed structures different of packingtheir molecular preferences model for compounds different molecules. also revealed different packing preferences for different molecules.
Figure 16. DifferentDifferent packing packing fashions fashions between between buildin buildingg units units observed observed in in the discrete model compounds of ( a) 4PE-1P COF with 1,1,2,2-tetraphenylethene as as the central unit and ( b) Py-1P COF with 1,3,6,8-tetrakisphenylpyrene as the centralcentral unit,unit, respectively.respectively.
To achieve remarkable chemical stability, BanerjeeBanerjee and co-workers explored a new method to enhance thethe chemicalchemical stability stability of of imine imine COFs COFs by by the th transformatione transformation of combined of combined reversible reversible Schiff Schiff base basebonds bonds to irreversible to irreversibleβ-ketoenamine β-ketoenamine bonds bonds (Figure (Figure 17)[ 3617)]. [36] They. They synthesized synthesized the COFthe COF TpPa-1 TpPa-1 and TpPa-2and TpPa-2 by the by Schiffthe Schiff base base reactions reactions of 1,3,5-triformylphloroglucinolof 1,3,5-triformylphloroglucinol (Tp) (Tp) with withp p-phenylenediamine-phenylenediamine (Pa-1) and 2,5-dimethyl-p-phenylenediamine-phenylenediamine (Pa-2),(Pa-2), respectively.respectively. The The expected expected enol-imine enol-imine (OH)(OH) form underwent irreversible proton tautomerization into irreversible keto-enamine form, which confersconfers outstanding stability stability to to boiling boiling water, water, aqueous aqueous acid acid (9 N (9HCl) N HCl)and base and (9 base N NaOH). (9 N NaOH). This linkage This has linkage also beenhas also synthesized been synthesized by a simple, by solvent-free, a simple, solvent-free, room-temperature room-temperature mechanochemical mechanochemical synthetic route synthetic [37].The amide-linkedroute [37].The amide-linkedCOFs show improved COFs show chemical improved stabilit chemicaly relative stability to relativetheir imine to their progenitors, imine progenitors, but low degreebut low of degree crystallinity. of crystallinity. Recently, Recently,Yaghi and Yaghi co-worke andrs co-workers reported a reportedchemical aconversion chemical conversionof imine bond of toimine amide bond bond to amide inside bond COFs inside to improve COFs toboth improve the chem bothical the stability chemical (including stability (including base and acid base stability) and acid andstability) crystallinity and crystallinity (Figure 18). (Figure They 18 constructed). They constructed two layered two layered imine COFs, imine COFs,TPB-TP-COF TPB-TP-COF (1) and (1 )4PE- and 1P-COF4PE-1P-COF (2) as(2 starting) as starting materials materials and then and converted then converted the imine-linked the imine-linked COFs to COFs the amide-linked to the amide-linked COFs, [CCOFs,6H3(C [C6H6H4NH)3(C63]H2[C4NH)6H4(CO)3]2[C2]63H, 14’(CO) and2 [(C]3, 61H’ and4NH) [(C4][C6H6H4NH)4(CO)4][C2]2, 62H’ without4(CO)2]2 losing, 2’ without their crystallinity losing their orcrystallinity topology. orFT-IR topology. and 13C FT-IR CP-MAS and 13NMRC CP-MAS spectra NMRindicated spectra the indicatedsuccessful theconversion successful of conversionCOF 1 and COFof COF 2 to1 andCOF COF 1’ and2 to COF COF 21’.’ PXRD and COF results2’. PXRD demonstrated results demonstrated the high crystal the structure high crystal of COF structure 1’ and of COF 12’’. andThe COFreduction2’. The of reduction BET surface of BETarea surface of COF area 1’ and of COFCOF 12’’ andcompared COF 2 ’to compared COF 1 and to COF 21 dueand toCOF the2 increasedue to the of increaseframework of frameworkmass and decrease mass and of decrease pore volume. of pore This volume. method This offers method a new offers pathway a new to pathwayovercome tothe overcome usual crystallization the usual crystallization problem in COF problem chemistry in COF [38]. chemistry [38].
2.2.2. Hydrazone-Linked COFs Like the reactions of imine-based COFs, the reactionreaction of benzaldehydes and hydrazide groups could yield hydrazone-linkedhydrazone-linked COFsCOFs (Figure(Figure 1919).). Hydrazone-linked Hydrazone-linked COFs COFs show show good chemical stability relative to imine-based ones due to the hydrogen-bonding interactions between the oxygen atoms in the alkoxyl chains and the hydrogens in -CONH- units. However, the functional monomers available for construction of hydrazone-based COFs are limited. Yaghi and co-workers first condensed a di-functional acylhydrazide (2,5-diethoxyterephthalohydrazide) and trifunctional aldehydes (1,3,5-triformylbenzene or 1,3,5-tris(4-formylphenyl)benzene) to provide two crystalline mesoporous COFs, COF-42 and COF-43 [39].
Molecules 2017, 22, 1149 15 of 29 stability relative to imine-based ones due to the hydrogen-bonding interactions between the oxygen atoms in the alkoxyl chains and the hydrogens in -CONH- units. However, the functional monomers available for construction of hydrazone-based COFs are limited. Yaghi and co-workers first condensed a di-functional acylhydrazide (2,5-diethoxyterephthalohydrazide) and trifunctional aldehydes (1,3,5-triformylbenzene or 1,3,5-tris(4-formylphenyl)benzene) to provide two crystalline mesoporous COFs, COF-42 and COF-43 [39]. Molecules 2017, 22, 1149 15 of 29
CHO HO OH 3 OHC CHO OH
NH2 NH2 reversible reversible H2N NH2 R R NH 2 2 reversible N reversible R Shiff base reaction 2 Shiff base reaction 2 2 NN NH2 Shiff base reaction Shiff base reaction NH R 2 NH2 NH2
HO OH HO OH R HO OH R HO OH 1 N N N OH OH OH N R N OH N R OH OH N N 1 N N OH N N OH R2 HO OH N OH HO OH R2 N OH R NN N HO OH N N N R NN R N HO OH HO OH 1 HO OH R N Enol form R N N N 2 R N Enol form N Enol form R2 Enol form HO OH N HO OH HO OH R1 HO OH N R N N OH N N R OH N HO OH OH N R2 HO OH R2 HO OH N N OH N N R N OH N OH NN 1 HO OH OH OH N N OH N N R HO OH HO OH 1 R R OH R OH R N N irreversible irreversible HO OH irreversible tautomerism tautomerism tautomerism irreversible tautomerism O O O O R1 H H N O O R H N O O O O O HN R N O HN N 1 N H H O HN R N R2 O H O O R2 NH O O O NH O N O HN N O HN HN NN N R H R 1 O O O O N O O R H NN HN O O R2 Keto form HN HN R Keto form NH R2 HN NH Keto form NH O O R O O R1 O O NH NH Keto form NH O O O HN R2 O O R O HN O O H R2 H H NH O HN N N R N R1 NH O NH O N O O O O NH O N N NN H R O O H O O 1 H O O O HN N R R O R O R N N N H2N NH2 TpPa-1 (R1=H, R2=H) H H N O O TpPa-2 (R1=Me, R2=Me) N NH TpPa-3 (R1=H, R2=NO2) (R=H) 2 H2N NH2 H2N N (R=Me) O (R=OMe) NH2 (R=NO2) H2N O H2N NH2 Figure 17. Construction of secondary amine-linked COFs via tautomerism by linking Tp with ditopic Figure 17. Construction of secondary amine-linked COFs via tautomerism by linking Tp with ditopic or tripic aniline-based linkers. or tripic aniline-based linkers. Dichtel et al. reported the exfoliation of the COF-43 in various solvents to yield few-layer 2D polymers.Dichtel The et al.obtained reported exfoliated the exfoliation COF-43 of polymers the COF-43 showed in various an apparent solvents loss to yieldof crystallinity few-layer 2Dby polymers.PXRD, but The no apparent obtained exfoliatedchanges in COF-43 its covalent polymers linkages showed and anhigh apparent aspect lossratios of [40]. crystallinity by PXRD, but noStegbauer apparent et changes al. reported in its covalenta hydrazone-based linkages and CO highF (TFTP-COF) aspect ratios capable [40]. of visible-light driven hydrogenStegbauer generation et al. reportedwith Pt as a hydrazone-baseda proton reductio COFn catalyst (TFTP-COF) (PRC). The capable COF, of constructed visible-light by driven 1,3,5- hydrogentris-(4-formyl-phenyl)triazine generation with Pt (TFPT) as a proton and 2,5- reductiondiethoxy-terephthalohydrazide catalyst (PRC). The COF, (DETH) constructed building by 1,3,5-tris-(4-formyl-phenyl)triazineblocks, shows a layered structure with (TFPT) a honeycomb- and 2,5-diethoxy-terephthalohydrazidetype lattice featuring mesopores (DETH) of 3.8 building nm and blocks,gives a showsBET surface a layered area structure of 1603 with m2·g a−1 honeycomb-type. When illuminated lattice with featuring visible mesoporeslight, the Pt-doped of 3.8 nm COF and 2 −1 givescontinuously a BET surface produces area hydrogen of 1603 mfrom·g water. When withou illuminatedt signs of withdegradation. visible light, This thenew Pt-doped application COF of continuouslyCOFs in photocatalysis produces hydrogento open new from pathwa waterys without for heterogeneous signs of degradation. photocatalysts This [41]. new application of COFsDing in photocatalysis et al. rationally to opendesigned new pathwaysa thioether- forbased heterogeneous hydrazone-linked photocatalysts COF-LZU8, [41]. and use it for highlyDing sensitive et al. rationallydetection and designed effective a thioether-based removal of Hg hydrazone-linked2+. Due to the high COF-LZU8,stability of the and hydrazone use it for 2+ highlylinkages, sensitive together detection with the and dense effective distribution removal of of the Hg thioether. Due to groups the high and stability the straight of the channels hydrazone in linkages,COF-LZU8, together the recycling with the of COF-LZ dense distributionU8 achieved of the the simultaneous thioether groups detection and theand straightremoval channels of the toxic in Hg2+ [42].
Molecules 2017, 22, 1149 16 of 29
COF-LZU8, the recycling of COF-LZU8 achieved the simultaneous detection and removal of the toxic 2+ HgMolecules[42 2017]. , 22, 1149 16 of 29
Figure 18. Construction of imide-linked COFs via post-oxidization of imine-linked COFs which Figure 18. Construction of imide-linked COFs via post-oxidization of imine-linked COFs which bypass bypass the difficulty of crystallization problem for irreversible imide bonds. the difficulty of crystallization problem for irreversible imide bonds. 2.2.3. Azine-Linked COFs 2.2.3. Azine-Linked COFs Azines (−C=N−N=C−) are formed by condensation of hydrazine with aldehydes (Figure 20). Azines (−C=N−N=C−) are formed by condensation of hydrazine with aldehydes (Figure 20). Azine-linked COFs usually have small micropores and are stable in water, acid and base. Dalapati et Azine-linked COFs usually have small micropores and are stable in water, acid and base. Dalapati et al. al. constructed a highly crystalline two-dimensional the Py-Azine COF by condensation of hydrazine constructed a highly crystalline two-dimensional the Py-Azine COF by condensation of hydrazine with 1,3,6,8-tetrakis(4-formylphenyl)pyrene (TFPPy) under solvothermal conditions [43]. The azines with 1,3,6,8-tetrakis(4-formylphenyl)pyrene (TFPPy) under solvothermal conditions [43]. The azines inside of Py-Azine COF serve as Lewis basic sites to bind guest and offers a selective detection of inside of Py-Azine COF serve as Lewis basic sites to bind guest and offers a selective detection of 2,4,6-trinitrophenol (TNP), which exhibits a potential application for chemosensing systems. Li et al. 2,4,6-trinitrophenol (TNP), which exhibits a potential application for chemosensing systems. Li et al. synthesized an azine-linked covalent organic framework, ACOF-1, by condensation of hydrazine synthesized an azine-linked covalent organic framework, ACOF-1, by condensation of hydrazine hydrate and 1,3,5-triformylbenzene. The high surface areas (1318 m2·g−1) and small pore size make it hydrate and 1,3,5-triformylbenzene. The high surface areas (1318 m2·g−1) and small pore size make 2 −1 2 −1 useful as a gas storage medium for CO (177 mg·g at 273−1 K and 1 bar), H (9.9mg·g at 273 K− and1 1 bar), it useful as a gas storage medium for CO2 (177 mg·g at 273 K and 1 bar), H2 (9.9mg·g at 273 K CH4 (11.5 mg·g−1 at 273 K and 1 bar) [44,45]. Smaldone and co-workers developed a novel azine-linked and 1 bar), CH (11.5 mg·g−1 at 273 K and 1 bar) [44,45]. Smaldone and co-workers developed a hexaphenylbenzene-based4 covalent organic framework, HEX-COF 1, which shows excellent sorption novel azine-linked hexaphenylbenzene-based covalent organic framework, HEX-COF 1, which shows capability for carbon dioxide (20 wt %) and methane (2.3 wt %) at 273 K and 1 atm [46]. Lotsch et al. excellent sorption capability for carbon dioxide (20 wt %) and methane (2.3 wt %) at 273 K and reported a tunable water- and photostable azine COF by hydrazine and triphenylarene aldehydes for 1 atm [46]. Lotsch et al. reported a tunable water- and photostable azine COF by hydrazine and visible light induced hydrogen generation [47]. Zhang et al. reported an azo-containing COF by introducing a novel azobenzene monomer through the borate ester formation reaction of azobenzene- 4,4′-diboronic acid (ABDA) and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP). The Azo-COF has a hexagonal skeleton and permanent porosity. The azo units in azo-COFs endow the COF material with photoisomerization properties [48].
Molecules 2017, 22, 1149 17 of 29 triphenylarene aldehydes for visible light induced hydrogen generation [47]. Zhang et al. reported an azo-containing COF by introducing a novel azobenzene monomer through the borate ester formation reaction of azobenzene-4,40-diboronic acid (ABDA) and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP). The Azo-COF has a hexagonal skeleton and permanent porosity. The azo units in azo-COFs endow the
COFMolecules material 2017, with22, 1149 photoisomerization properties [48]. 17 of 29 Molecules 2017, 22, 1149 17 of 29
Figure 19. Synthetic reaction of hydrazone-linked COFs (COF-42, COF-43, and TFPT-COF). Figure 19. Figure 19.Synthetic Syntheticreaction reaction ofof hydrazone-linked COFs COFs (COF-42, (COF-42, COF-43, COF-43, and and TFPT-COF). TFPT-COF).
Figure 20. Synthetic reaction of azine-linked COFs (Py-Azine COF, HEX-COF 1, and Nx-COF). Figure 20. Synthetic reaction of azine-linked COFs (Py-Azine COF, HEX-COF 1, and Nx-COF). Figure 20. Synthetic reaction of azine-linked COFs (Py-Azine COF, HEX-COF 1, and Nx-COF). 2.2.4. Squaraine-Linked COF 2.2.4. Squaraine-Linked COF The use of squaric acids and amines or hydrazines as building blocks has been developed for the synthesisThe use of of squaric squaraine-linked acids and aminesCOFs that or hydrazinesallow the integration as building of blocks zwitterion has been structures developed into forthe theskeletons. synthesis Nagai of squaraine-linked et al. reported a newCOFs reaction that allow based the on integration squaraine forof zwitterionthe synthesis structures of a new into type the of skeletons.squaraine-based Nagai 2Det al.COF. reported They used a new copper(II) reaction 5, based10,15,20-tetrakis(4-aminophen on squaraine for the synthesisyl)porphyrin of a new (TAP-CuP) type of squaraine-based 2D COF. They used copper(II) 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TAP-CuP)
Molecules 2017, 22, 1149 18 of 29
2.2.4. Squaraine-Linked COF The use of squaric acids and amines or hydrazines as building blocks has been developed for the synthesis of squaraine-linked COFs that allow the integration of zwitterion structures into the Molecules 2017, 22, 1149 18 of 29 skeletons. Nagai et al. reported a new reaction based on squaraine for the synthesis of a new type of as a squaraine-basedbuilding block 2D to COF. construct They used a crystalline copper(II) 5,10,15,20-tetrakis(4-aminophenyl)porphyrin 2D conjugated COF (CuP-SQ COF) with (TAP-CuP) a tetragonal as a building block to construct a crystalline 2D conjugated COF (CuP-SQ COF) with a tetragonal mesoporous mesoporous skeleton (Figure 21). The CuP-SQ COF has a BET surface area of 2289 m2·g−1 with a pore skeleton (Figure 21). The CuP-SQ COF has a BET surface area of 2289 m2·g−1 with a pore size of 2.1 nm. size Thisof 2.1 research nm. This expands research the types expands of COFs. the Zhangtypes andof COFs. their co-workersZhang and constructed their co-workers a novel functionalconstructed a novelcovalent functional organic covalent framework organic by the porphyrinframework building by the block porphyrin (meso-tetra-4-hydrazidocarbonylphenyl) building block (meso-tetra-4- hydrazidocarbonylphenyl)porphyrin, THCPP) and squaricporphyrin, acid viaTHCPP) a bottom-up and squaric approach. acid The via reported a bottom-up covalent approach. porphyrin The reportedframework covalent with porphyrin coordinated framework manganese with (III) ionscoordi (Mn–CPF-2)nated manganese presents promising (III) ions catalytic(Mn–CPF-2) properties presents promisingfor the catalytic selective oxidationproperties of for olefins the [selective49]. oxidation of olefins [49].
FigureFigure 21. 21. SyntheticSynthetic reaction reaction of of squaraine-linked squaraine-linked COFs COFs (CuP-SQ (CuP-SQ COF). COF).
2.2.5.2.2.5. Imide-Linked Imide-Linked COFs COFs PolyimidesPolyimides are areformed formed by bycondensing condensing primary primary am aminesines and and anhydrides anhydrides (Figure 22 22).). YanYan and and co- workersco-workers synthesized synthesized three-dimensional three-dimensional porous porous crystalline crystalline polyimide covalent covalent organic organic frameworks frameworks (PI-COFs)(PI-COFs) by choosing by choosing tetrahedral tetrahedral building building units units ofof differentdifferent sizes sizes [50 [50].]. These PI-COFs show high thermal stability (>450 ◦C) and surface area (up to 2403 m2·g−1), as well as narrow pore size distribution (13 Å for PI-COF-4 and 10 Å for PI-COF-5). Due to the high surface area and large pore size, they were the first to be employed in controlled drug delivery, where they show high loadings and good release control for ibuprofen (IBU). These results provide a pathway of COFs for pharmaceutical applications. The Yan group also synthesized a series of polyimide-based COFs with different pore size by changing the length of tri-amines, using the imidization reaction [51]. Among them, the PI-COF-3, synthesized by condensation of PMDA and the extended triamine 1,3,5-tris[4-amino(1,1-biphenyl-4-yl)]benzene (TABPB) has 5.3 nm wide pores and were loaded with Rhodamine B, a dye probe for biological applications. The dye-doped PI-COF-3 exhibits clear temperature-dependent photoluminescence, which facilitate the development of dye-doped porous materials for applications in temperature-sensing devices.
Figure 22. Construction of imide-linked COFs by stitching tritopic or tetratopic polyaniline with benzene-1,2,4,5-tetracarboxylic dianhydride into 2D and 3D networks.
Molecules 2017, 22, 1149 18 of 29
as a building block to construct a crystalline 2D conjugated COF (CuP-SQ COF) with a tetragonal mesoporous skeleton (Figure 21). The CuP-SQ COF has a BET surface area of 2289 m2·g−1 with a pore size of 2.1 nm. This research expands the types of COFs. Zhang and their co-workers constructed a novel functional covalent organic framework by the porphyrin building block (meso-tetra-4- hydrazidocarbonylphenyl) porphyrin, THCPP) and squaric acid via a bottom-up approach. The reported covalent porphyrin framework with coordinated manganese (III) ions (Mn–CPF-2) presents promising catalytic properties for the selective oxidation of olefins [49].
Figure 21. Synthetic reaction of squaraine-linked COFs (CuP-SQ COF).
2.2.5. Imide-Linked COFs Polyimides are formed by condensing primary amines and anhydrides (Figure 22). Yan and co- Moleculesworkers2017, 22 synthesized, 1149 three-dimensional porous crystalline polyimide covalent organic frameworks19 of 29 (PI-COFs) by choosing tetrahedral building units of different sizes [50].
Molecules 2017, 22, 1149 19 of 29
These PI-COFs show high thermal stability (>450 °C) and surface area (up to 2403 m2·g−1), as well as narrow pore size distribution (13 Å for PI-COF-4 and 10 Å for PI-COF-5). Due to the high surface area and large pore size, they were the first to be employed in controlled drug delivery, where they show high loadings and good release control for ibuprofen (IBU). These results provide a pathway of COFs for pharmaceutical applications. The Yan group also synthesized a series of polyimide-based COFs with different pore size by changing the length of tri-amines, using the imidization reaction [51]. AmongFigure them, 22. the Construction PI-COF-3, of synthesized imide-linked by COFs condensa by stitchtioning of PMDAtritopic andor tetratopic the extended polyaniline triamine with 1,3,5- Figure 22. Construction of imide-linked COFs by stitching tritopic or tetratopic polyaniline with tris[4-amino(1,1-biphenyl-4-yl)]benzenebenzene-1,2,4,5-tetracarboxylic dianhydride (TABPB) into 2D has and 5.3 3D networks.nm wide pores and were loaded with benzene-1,2,4,5-tetracarboxylicRhodamine B, a dye probe for dianhydride biological intoapplic 2Dations. and 3D The networks. dye-doped PI-COF-3 exhibits clear temperature-dependent photoluminescence, which facilitate the development of dye-doped porous
2.2.6.materials Phenazine-Linked for applications COFs in temperature-sensing devices.
Aza-fused2.2.6. Phenazine-Linked linkages were COFs pursued in making conductive organic materials that have many potential applications in electrochemical devices [52]. However, the great challenge is how to make Aza-fused linkages were pursued in making conductive organic materials that have many them highly crystalline. Jiang and co-workers prepared a phenazine-linked COF (CS-COF, Figure 23) potential applications in electrochemical devices [52]. However, the great challenge is how to make through ring-fusing reactions between quinone and amine derivatives catalyzed by acidic forms. them highly crystalline. Jiang and co-workers prepared a phenazine-linked COF (CS-COF, Figure 23) Theythrough used C 3ring-fusing-symmetric reactions triphenylene between hexamine quinone (TPHA)and amine and derivatives C2-symmetric catalyzed tertbutylpyrene by acidic forms. tetraone (PT) as building blocks to prepare the crystalline phenazine-linked CS-COF under solvothermal They used C3-symmetric triphenylene hexamine (TPHA) and C2-symmetric tertbutylpyrene tetraone (PT) conditions.as building The blocks polygon to prepare skeletons the ofcrystalline CS-COF phenazine-li are highlynkedπ conjugated CS-COF under and chemicallysolvothermal stable conditions. in various solventsThe polygon [53]. skeletons of CS-COF are highly π conjugated and chemically stable in various solvents [53].
N N
H2N NH2 N N N N
N N
H2N NH2
H2N NH2 TPHA N N NN
O O NN NN
N N O O N N PT N N
N N
Cs-COF
FigureFigure 23. 23.Synthetic Synthetic reactionreaction of the the phenazine-linked phenazine-linked COF. COF.
2.2.7. Triazine-Linked COFs The self-condensation of aromatic nitriles at high temperature in molten salts or at room temperature in the presence of a strong acid catalyst, such as trifluoromethylsulfonic acid, can form crystalline, porous, covalent triazine frameworks (CTFs, Figure 24). The first CTF was reported by Thomas and co-workers in 2008, and is synthesized through the dynamic trimerization of aromatic nitriles (1,3,5-triazines) using a ZnCl2 ionothermal method [54]. CTFs provide high surface area (up to 3000 m2·g−1), low densities, and outstanding thermal/chemical stability. However, CTFs exhibit short-range crystalline order and limited pore size distributions in some cases [55].
Molecules 2017, 22, 1149 20 of 29
2.2.7. Triazine-Linked COFs The self-condensation of aromatic nitriles at high temperature in molten salts or at room temperature in the presence of a strong acid catalyst, such as trifluoromethylsulfonic acid, can form crystalline, porous, covalent triazine frameworks (CTFs, Figure 24). The first CTF was reported by Thomas and co-workers in 2008, and is synthesized through the dynamic trimerization of aromatic nitriles (1,3,5-triazines) using a ZnCl2 ionothermal method [54]. CTFs provide high surface area (up to 3000 m2·g−1), low densities, and outstanding thermal/chemical stability. However, CTFs exhibit
Moleculesshort-range 2017, crystalline22, 1149 order and limited pore size distributions in some cases [55]. 20 of 29
Figure 24. Construction of crystalline covalent triazine frameworks by self-condensation of aromatic nitriles. Figure 24. Construction of crystalline covalent triazine frameworks by self-condensation of aromatic nitriles. Some CTFs were prepared in molten ZnCl2 at high reaction temperatures (400 °C~700 °C). Yet these conditions for CTF preparation obtained less crystalline CTFs of limited functionality, and only Some CTFs were prepared in molten ZnCl at high reaction temperatures (400 ◦C~700 ◦C). from 1,4-dicyanobenzene and 2,6-dicyanonaphthalene2 [56]. Then, Cooper et al. [57] developed a Yet these conditions for CTF preparation obtained less crystalline CTFs of limited functionality, microwave-assisted, acid-catalyzed method to synthesize triazine networks with fewer discolored and only from 1,4-dicyanobenzene and 2,6-dicyanonaphthalene [56]. Then, Cooper et al. [57] impurities (P1M-P6M). Their structural and synthesized tunability developed their specific developed a microwave-assisted, acid-catalyzed method to synthesize triazine networks with fewer applications, such as gas storage, gas separation, catalyst support, and organic dye separation. discolored impurities (P1M-P6M). Their structural and synthesized tunability developed their specific Thomas et al. synthesized a covalent triazine framework (CTF-0) by trimerization of 1,3,5- applications, such as gas storage, gas separation, catalyst support, and organic dye separation. tricyanobenzene in molten ZnCl2 [58]. This material showed high CO2 uptakes and good catalytic Thomas et al. synthesized a covalent triazine framework (CTF-0) by trimerization of 1,3,5-tricyanobenzene activity for CO2 cycloaddition. The high surface area and robust porous structures of the organic in molten ZnCl [58]. This material showed high CO uptakes and good catalytic activity for CO framework were2 explored for highly selective gas capture.2 2 cycloaddition. The high surface area and robust porous structures of the organic framework were Ghosh et al. explored a strategy to obtain a porous covalent triazine framework (CTF-IP-10) with explored for highly selective gas capture. bimodal functionality by acid-catalysed room temperature reaction of a tricyanomonomer (PCN-M1) Ghosh et al. explored a strategy to obtain a porous covalent triazine framework (CTF-IP-10) with in CHCl3 [59]. The obtained CTF-IP 10 consists both an electron-deficient central triazine core and bimodal functionality by acid-catalysed room temperature reaction of a tricyanomonomer (PCN-M1) electron-rich aromatic building blocks. in CHCl3 [59]. The obtained CTF-IP 10 consists both an electron-deficient central triazine core and 2.3.electron-rich Hetero Linkages aromatic in One building COF Structure blocks. 2.3. HeteroTo improve Linkages the in One functionality COF Structure and stability of imine-based COF structures, design and construction of COFs connected by two or more types of covalent bonds based on orthogonal To improve the functionality and stability of imine-based COF structures, design and construction reactions were developed. of COFs connected by two or more types of covalent bonds based on orthogonal reactions First the Zhao group constructed a 2D COF (NTU-COF-1, Figure 25) by the formation of imine were developed. groups and B3O3 boroxine rings, involving the utilization of two building blocks: 4-formyl- First the Zhao group constructed a 2D COF (NTU-COF-1, Figure 25) by the formation of imine groups phenylboronic acid (FPBA) and 1,3,5-tris(4-aminophenyl)-benzene (TAPB). They also constructed a and B3O3 boroxine rings, involving the utilization of two building blocks: 4-formyl-phenylboronic acid ternary COF (NTU-COF-2, Figure 25) through the formation of imine groups and C2O2B boronate rings from FPBA, TAPB and HHTP, which involved the formation of two types of covalent bonds. The NTU-COF-2 possesses high BET surface areas of 1619 m2·g−1 and mesopores with a diameter of 2.6 nm. The design and construction of COFs by this orthogonal reaction strategy is a promising approach in rational COF development [60].
Molecules 2017, 22, 1149 21 of 29
(FPBA) and 1,3,5-tris(4-aminophenyl)-benzene (TAPB). They also constructed a ternary COF (NTU-COF-2, Figure 25) through the formation of imine groups and C2O2B boronate rings from FPBA, TAPB and HHTP, which involved the formation of two types of covalent bonds. The NTU-COF-2 possesses high BET surface areas of 1619 m2·g−1 and mesopores with a diameter of 2.6 nm. The design and construction of
COFsMolecules by this 2017 orthogonal, 22, 1149 reaction strategy is a promising approach in rational COF development21 of 29 [60].
Molecules 2017, 22, 1149 21 of 29
Figure 25. Syntheses of (i) NTU-COF-1 and (ii) NTU-COF-2 by two types of covalent bonds from Figure 25. Syntheses of (i) NTU-COF-1 and (ii) NTU-COF-2 by two types of covalent bonds from orthogonal reactions. orthogonalFigure reactions.25. Syntheses of (i) NTU-COF-1 and (ii) NTU-COF-2 by two types of covalent bonds from Theorthogonal Qiu group reactions. synthesized two extended 3D COFs (DL-COF-1, DL-COF-2) with dual linkages, Theimine Qiu groups group and synthesized B3O3 boroxine two extended rings (Figure 3D COFs 26), (DL-COF-1, involving DL-COF-2)the utilization with of dual two linkages, 1,3,5,7- imine tetraaminoadamantaneThe Qiu group synthesized (TAA) twoand extended 4-formylph 3D COFsenylboronic (DL-COF-1, acid DL-COF-2) (FPBA) with or dual 2-fluoro-4- linkages, groups and B3O3 boroxine rings (Figure 26), involving the utilization of two 1,3,5,7-tetraaminoadamantane imine groups and B3O3 boroxine rings (Figure 26), involving the utilization of two 1,3,5,7- (TAA)formylphenylboronic and 4-formylphenylboronic acid (FFPBA) acid building (FPBA) blocks or 2-fluoro-4-formylphenylboronic [61]. The obtained DL-COF-1 and DL-COF-2 acid (FFPBA) showed building tetraaminoadamantane (TAA) 2 and−1 4-formylph2 −1enylboronic acid (FPBA) or 2-fluoro-4- high BET surface areas of 2259 m ·g and 2071 m ·g , respectively. 2· −1 blocksformylphenylboronic [61]. The obtained acid DL-COF-1 (FFPBA) andbuilding DL-COF-2 blocks [61]. showed The obtained high BET DL-COF-1 surface and areas DL-COF-2 of 2259 showed m g and 2 −1 2071high m ·g BET, respectively.surface areas of 2259 m2·g−1 and 2071 m2·g−1, respectively.
Figure 26. Syntheses 3D COFs of (i) DL-COF-1 and (ii) DL-COF-2 by two types of covalent bonds from orthogonal reactions. Figure 26. Syntheses 3D COFs of (i) DL-COF-1 and (ii) DL-COF-2 by two types of covalent bonds from Figure 26. Syntheses 3D COFs of (i) DL-COF-1 and (ii) DL-COF-2 by two types of covalent bonds from 3. Gasorthogonal Storage forreactions. Clean Energy Applications orthogonal reactions. 3. GasCOFs Storage have for been Clean touted Energy as a new Applications class of porous crystalline materials for gas storage applications due to their high inherent surface area, very low densities (i.e., being made of the elements H, C, N, B, O, COFsSi), high have stability been (formationtouted as a of new covalent class of bonds), porous and crystalline tunable pore materials dimensions for gas (from storage ultramicropores applications due to their high inherent surface area, very low densities (i.e., being made of the elements H, C, N, B, O, Si), high stability (formation of covalent bonds), and tunable pore dimensions (from ultramicropores
Molecules 2017, 22, 1149 22 of 29
3. Gas Storage for Clean Energy Applications COFs have been touted as a new class of porous crystalline materials for gas storage applications due to their high inherent surface area, very low densities (i.e., being made of the elements H, C, N, B, O, Si), high stability (formation of covalent bonds), and tunable pore dimensions (from ultramicropores to mesopores). The gas storage capacity of COFs for hydrogen, methane, and carbon dioxide, has attracted increasing interest in the past decades [62].
3.1. Hydrogen Storage The demand for the reduction of air pollution has led to the search of new and clean energy sources. Hydrogen gas storage is one of the bottleneck for the realization of an energy economy based on hydrogen’s high chemical abundance, being a clean energy carrier, and high chemical energy density. The demand of the US Department of Energy (DOE) for hydrogen storage is 5.5 wt % in gravimetric capacity, and 40 kg·m−3 of volumetric capacity at operating temperature of 233–333 K with a pressure of 100 atm by the year 2017. Recently, a large number of porous materials have been explored for the application of hydrogen storage. However, none of the candidate materials developed so far has satisfied the DOE target. COF materials, as new crystal porous materials, have also attracted increasing interest in hydrogen storage applications. Figure 27 and Table1 show the capabilities of hydrogen storage reported for different COF materials. The plot of hydrogen uptake against surface area and pore volume for the selected COFs at 2 −1 3 −1 77 K are shown in Table1. COF-18Å (S BET: 1263 m ·g , pore volume: 0.65 cm ·g , pore size: 1.8 nm) showed the highest hydrogen uptake at low pressure (1 bar and 77 K) among the similar 2D COFs with different alkyl chain lengths (compared to COF-11Å, COF-14Å, and COF-16Å). For context, 3D COFs (COF-102 and COF-103) show much higher hydrogen storage capacities due to the higher surface areas and lower densities, compared to 2D COFs. Among the different COF materials for H2 uptake, the one 2 −1 3 −1 with the largest storage capacity is 3D COF-102 (Table1,S BET: 3620 m ·g , pore volume: 1.55 cm ·g , pore size: 1.2 nm), which uptakes 72 mg per gram at 1 bar and 77 K. This capacity is comparable −1 2 −1 −1 2 −1 to those of MOF-177 (75 mg·g ,SBET: 4500 m ·g ), MOF-5 (76 mg·g ,SBET: 3800 m ·g ), and −1 2 −1 the porous aromatic framework PAF-1 (75 mg·g ,SBET: 5600 m ·g ). These capacities significantly demonstrate the potential of COFs for use as hydrogen storage materials.
Table 1. BET surface area, porosity parameters and H2 capture of selected COFs.
2 −1 3 −1 COF Materials BET Surface Area (m ·g ) Pore Size (Å) Pore Volume (Vp, cm ·g ) H2 Uptake (wt %) Ref. COF-1 750 9 0.3 1.48 [62] COF-5 1670 27 1.07 3.58 COF-6 750 6.4 0.32 2.26 COF-8 1350 18.7 0.69 3.5 COF-10 1760 34.1 1.44 3.92 COF-102 3620 11.5 1.55 7.24 COF-103 3530 12.5 1.54 7.05 COF-1 628 9 0.36 1.28 (1 bar) COF-11Å 105 11 0.05 1.22 (1 bar) COF-14Å 805 14 0.41 1.23(1 bar) COF-16Å 753 16 0.39 1.4 (1 bar) COF-18Å 1263 18 0.69 1.55 (1 bar) CTC-COF 1710 22.6 1.03 1.12 (1.05 bar) dualpore-COF 1771 7.3, 25.2 3.189 (P/P◦ 0.99) 1.37 (1 bar)
From the summarized H2 storage results in Figure 27, we find that the high-pressure H2 adsorption of COFs with higher surface areas and larger pore possess higher hydrogen uptake capacities when measured under the same conditions. Meanwhile, the COFs with higher pore volume show higher hydrogen uptake. These results demonstrate that the high-pressure H2 uptake capacities are dominantly determined by pore volumes and SBET of COFs. The H2 uptake capacity at 77 K and 1 bar increased in the order of COF-11Å < COF-14Å < COF-16Å < COF-18Å, revealing that the low-pressure H2 uptake also depends on the pore size of COFs. Molecules 2017, 22, 1149 23 of 29 Molecules 2017, 22, 1149 23 of 29
2 Figure 27.27. Plot ofof HH2 uptake against BET surface area (left) and pore volumevolume ((right)) forfor thethe selectedselected COFs at 77 K.
However, the hydrogen storage of COFs at ambient pressure and temperature is still low and However, the hydrogen storage of COFs at ambient pressure and temperature is still low and far from meeting the DOE requirements. Adsorption simulations of the COFs show that doping with far from meeting the DOE requirements. Adsorption simulations of the COFs show that doping charged species can affect their capacity. Theoretical studies [63,64] show that metal-doped COFs can with charged species can affect their capacity. Theoretical studies [63,64] show that metal-doped enhance the hydrogen storage at a temperature range of 273 to 298 K. Recently, scientists have COFs can enhance the hydrogen storage at a temperature range of 273 to 298 K. Recently, scientists theoretically predicted that lithium doped COFs would exhibit improved hydrogen storage capacity have theoretically predicted that lithium doped COFs would exhibit improved hydrogen storage due to the increased binding energy between the H2 and the Li atom. These simulation studies predict capacity due to the increased binding energy between the H2 and the Li atom. These simulation that the hydrogen uptakes for Li-doped COF-105 and COF-108 (6.84 and 6.73 wt %, respectively, at studies predict that the hydrogen uptakes for Li-doped COF-105 and COF-108 (6.84 and 6.73 wt %, 298 K and 100 bar) excelled beyond the non-doped COFs and the previous reported MOF materials. respectively, at 298 K and 100 bar) excelled beyond the non-doped COFs and the previous reported Although the experimental and theoretical results indicate that COFs are promising candidates for MOF materials. Although the experimental and theoretical results indicate that COFs are promising hydrogen storage, the practical and industrial use of COF materials toward the DOE target for candidates for hydrogen storage, the practical and industrial use of COF materials toward the DOE hydrogen uptakes is still far away. target for hydrogen uptakes is still far away.
3.2. Methane Storage Methane, as an abundant and inexpensive gas, standsstands out as a potential vehicular fuel to replace conventional fossilfossil fuels. fuels. According According to theto the US DOEUS DOE requirement, requirement, the current the current target fortarget methane for methane storage storage is3 350 cm−3STP3 cm−3adsorbent (v/v) and 0.5 gCH4 gadsorbent−1−1 (699 cm33STP g−1)1 at 35 bar and 298 K (STP: is 350 cm STP cm adsorbent (v/v) and 0.5 gCH4 gadsorbent (699 cm STP g ) at 35 bar and 298 K (STP: standard temperature and pressure). From Table 2, the 2D COF, COF-1 shows CH4 uptake −of1 standard temperature and pressure). From Table2, the 2D COF, COF-1 shows CH 4 uptake of 40 mg·g −1 at40 298mg·g K, 35 at bar. 298 However,K, 35 bar. upon However, increasing upon the increasing pressure the further, pressure no increase further, of no methane increase adsorption of methane is adsorption is observed due to the small pores in COF-1. The pores in COF-1 are quickly occupied by observed due to the small pores in COF-1. The pores in COF-1 are quickly occupied by CH4 molecules atCH low4 molecules pressures at and low higher pressures pressures and higher maynot pressure compacts may the not gas compact any more. the The gas largestany more. pore, The 3D largest COFs, pore, 3D COFs, COF-102 shows the highest observed CH4 storage capacity−1 (187 mg·g−1). COF-103 also COF-102 shows the highest observed CH4 storage capacity (187 mg·g ). COF-103 also possesses a −1 highpossesses methane a high capacity methane of 175capacity mg·g −of1 .175 These mg·g values. These are comparablevalues are comparable to the highest to the methane highest uptake methane for −1 MOFsuptake (MOF-210; for MOFs 220(MOF-210; mg·g−1 ).Theoretical220 mg·g ).Theoretical simulation studiessimulation indicate studie thats indicate lithium that doped lithium COFs doped could + significantlyCOFs could significantly strengthen thestrengthen binding the between binding methane between and methane Li+ cations, and Li which cations, will which further will improve further theimprove methane the capacitymethane ofcapacity COFs. Theof COFs. results The show results that show the methane that the storagemethane of storage lithium of doped lithium COFs doped are doubleCOFs are at 298double K and at 298 low K pressures and low ( Table 2. BET surface area, porosity parameters and CH4 capture of selected COFs. COF BET Surface Pore Size Pore Volume CH4 Uptake Ref. Materials Area (m2·g−1) (Å) (Vp, cm3·g−1) (mg·g−1) a COF-1 750 9 0.3 40 [62] COF-5 1670 27 1.07 89 COF-6 750 6.4 0.32 65 Molecules 2017, 22, 1149 24 of 29 Table 2. BET surface area, porosity parameters and CH4 capture of selected COFs. 2 −1 3 −1 −1 a COF Materials BET Surface Area (m ·g ) Pore Size (Å) Pore Volume (Vp, cm ·g ) CH4 Uptake (mg·g ) Ref. COF-1 750 9 0.3 40 [62] COF-5 1670 27 1.07 89 COF-6 750 6.4 0.32 65 COF-8 1350 18.7 0.69 87 COF-10 1760 34.1 1.44 80 COF-102 3620 11.5 1.55 187 COF-103 3530 12.5 1.54 175 ILCOF-1 2723 23 1.21 129 (L·L−1) a CH4 uptake at 35 bar and 298 K. 3.3. CO2 Capture The rise of carbon dioxide is one of the global issues, which is causing global warming, rising sea levels, and the increasing acidity of the oceans. The development of effective strategies for capturing CO2 has attracted broad interest. Among the current strategies, capture of CO2 by crystal porous materials is energetically efficient and technically feasible. Yaghi and co-workers first studied a series of 2D COFs with 1D micropores (COF-1 and COF-6), 2D COFs with 1D mesopores (COF-5, COF-8 and COF-10), and 3D COFs with 3D medium sized pores (COF-102 and COF-103) on application of CO2 capture in 2009. From Table3, we can observe that the storage of CO 2 at 298 K and 55 bar, follow the order of COF-102 > COF-103 > COF-10 > COF-5 > COF-8 > COF-6 > COF-1, which indicates that the high-pressure CO2 capture are highly related to the pore volumes and SABET of COFs. Table 3. BET surface area, porosity parameters and CO2 capture of selected COFs. 3 −1 CO2 Uptake (cm ·g ) 2 −1 3 −1 COF Materials BET Surface Area (m ·g ) Pore Size (nm) Pore Volume (Vp, cm ·g ) Ref. Low a High b COF-1 750 0.9 0.3 51 117 [39] COF-5 1670 2.7 1.07 31 443 COF-6 750 0.64 0.32 85 158 COF-8 1350 1.87 0.69 33 321 COF-10 1760 3.41 1.44 27 514 COF-102 3620 1.15 1.55 34 611 COF-103 3530 1.25 1.54 38 606 FCTF-1 662 0.46,0.54 - 105 [65] FCTF-1-600 1535 0.46,0.59 - 124 CTF-1 746 0.54 55 CTF-1-600 1553 - 80 [HO]25%-H2P-COF 1054 2.5 27 [66] [HO]50%-H2P-COF 1089 2.5 23 [HO]75%-H2P-COF 1153 2.5 26 [HO]100%-H2P-COF 1284 2.5 32 [HO2C]25%-H2P-COF 786 2.2 49 [HO2C]50%-H2P-COF 673 1.9 68 [HO2C]75%-H2P-COF 482 1.7 80 [HO2C]100%-H2P-COF 364 1.4 89 [CH≡C]0-H2P-COF 1474 2.5 0.75 37 [67] [CH≡C]25-H2P-COF 1413 2.3 0.71 27 [CH≡C]50-H2P-COF 962 2.1 0.57 24 [CH≡C]75-H2P-COF 683 1.9 0.42 22 [CH≡C]100-H2P-COF 462 1.6 0.28 20 [Et]25-H2P-COF 1326 2.2 0.55 28 [Et]50-H2P-COF 821 1.9 0.48 23 [Et]75-H2P-COF 485 1.6 0.34 21 [Et]100-H2P-COF 187 1.5 0.18 19 [MeOAc]25-H2P-COF 1238 2.1 0.51 43 [MeOAc]50-H2P-COF 754 1.8 0.42 45 [MeOAc]75-H2P-COF 472 1.5 0.31 42 [MeOAc]100-H2P-COF 156 1.1 0.14 33 [AcOH]25-H2P-COF 1252 2.2 0.52 48 [AcOH]50-H2P-COF 866 1.8 0.45 60 [AcOH]75-H2P-COF 402 1.5 0.32 55 [AcOH]100-H2P-COF 186 1.3 0.18 49 [EtOH]25-H2P-COF 1248 2.2 0.56 47 [EtOH]50-H2P-COF 784 1.9 0.43 63 [EtOH]75-H2P-COF 486 1.6 0.36 60 [EtOH]100-H2P-COF 214 1.4 0.19 43 [EtNH2]25-H2P-COF 1402 2.2 0.58 59 [EtNH2]50-H2P-COF 1044 1.9 0.5 80 [EtNH2]75-H2P-COF 568 1.6 0.36 68 [EtNH2]100-H2P-COF 382 1.3 0.21 49 a b CO2 uptake was measured at 1 bar and 273 K; CO2 uptake was measured at 55 bar and 298 K. Molecules 2017, 22, 1149 25 of 29 [AcOH]100-H2P-COF 186 1.3 0.18 49 [EtOH]25-H2P-COF 1248 2.2 0.56 47 [EtOH]50-H2P-COF 784 1.9 0.43 63 [EtOH]75-H2P-COF 486 1.6 0.36 60 [EtOH]100-H2P-COF 214 1.4 0.19 43 [EtNH2]25-H2P-COF 1402 2.2 0.58 59 [EtNH2]50-H2P-COF 1044 1.9 0.5 80 Molecules 2017, 22, 1149 25 of 29 [EtNH2]75-H2P-COF 568 1.6 0.36 68 [EtNH2]100-H2P-COF 382 1.3 0.21 49 a CO2 uptake was measured at 1 bar and 273 K; b CO2 uptake was measured at 55 bar and 298 K Functional groups inside the channel walls of COF materials can enhance the CO2 adsorption and separation. Fluorinated alkanes exhibit an extraordinary affinity to CO2 molecules. Han et al. Functional groups inside the channel walls of COF materials can enhance the CO2 adsorption reported a fluorinated triazine-based COF (FCFT-1 and FCTF-1-600) for CO2 capture by using and separation. Fluorinated alkanes exhibit an extraordinary affinity to CO2 molecules. Han et al. tetrafluoroterephthalonitrile as a monomer [65]. The triazine-based COFs haveN-rich frameworks, reported a fluorinated triazine-based COF (FCFT-1 and FCTF-1-600) for CO2 capture by using which are favored for CO adsorption, meanwhile, the high electronegativity of F enhances CO tetrafluoroterephthalonitrile2 as a monomer [65]. The triazine-based COFs haveN-rich frameworks,2 electrostatic interaction. The CO2 adsorption of these triazine-based COF materials at 273 K and which are favored for CO2 adsorption, meanwhile, the high electronegativity of F enhances CO2 1 bar were as follows, FCTF-1-600 (124 cm3·g−1) > FCTF-1 (105 cm3·g−1) > CTF-1-600 (86 cm3·g−1) electrostatic interaction. The CO2 adsorption of these triazine-based COF materials at 273 K and 1 bar > CTF-1 (55 cm3·g−1). These results demonstrate that the fluorizated COF materials exhibit much were as follows, FCTF-1-600 (124 cm3·g−1) > FCTF-1 (105 cm3·g−1) > CTF-1-600 (86 cm3·g−1) > CTF-1 better CO2 uptake under low pressure. Jiang et al. converted a conventional hydroxyl group modified (55 cm3·g−1). These results demonstrate that the fluorizated COF materials exhibit much better CO2 2D COF ([HO] -H P-COFs) into an outstanding carbondioxide-capture material, carboxyl group uptake under lowx% pressure.2 Jiang et al. converted a conventional hydroxyl group modified 2D COF modified 2D COF ([HO2C]x%-H2P-COFs) [64]. The [HO2C]x%-H2P-COFs show micropores between ([HO]x%-H2P-COFs) into an outstanding carbondioxide-capture material, carboxyl group modified 1.4-2.2 nm instead of the mesopores of [HO]x%-H2P-COFs (2.5 nm). From the results of Table3, the 2D COF ([HO2C]x%-H2P-COFs) [64]. The [HO2C]x%-H2P-COFs show3 − 1micropores between 1.4-2.2 nm [HO]x%-H2P-COFs have low CO2 capacity between 23-32 cm ·g at 273 K and 1 bar. However, instead of the mesopores of [HO]x%-H2P-COFs (2.5 nm). From the results of Table 3, the [HO]3 −1x%-H2P- [HO2C]x%-H2P-COFs exhibit a dramatically enhanced CO2 adsorption between 49–89 m ·g under COFs have low CO2 capacity between 23-32 cm3·g−1 at 273 K and 1 bar. However, [HO2C]x%-H2P-COFs the same conditions. exhibit a dramatically enhanced CO2 adsorption between 49–89 m3·g−1 under the same conditions. Jiang et al. also carried out a similar strategy to modify the pore surface with different Jiang et al. also carried out a similar strategy to modify the pore surface with different functional functional groups through click reactions [66,67]. From Table3, we can observe that different groups through click reactions [66,67]. From Table 3, we can observe that different function of pore function of pore walls of COF materials can induce the decrease of surface area, pore sizes and pore walls of COF materials can induce the decrease of surface area, pore sizes and pore volumes. volumes. However, the capacity for CO2 adsorption was highly dependent on the structures of the However, the capacity for CO2 adsorption was highly dependent on the structures of the functional functional groups. Both from Table3 and Figure 28, ethyl functionalized COFs, [Et] x-H2P-COFs, groups. Both from Table 3 and Figure 28, ethyl functionalized COFs, [Et]x-H2P-COFs, exhibit similar exhibit similar CO2 adsorption capacity to those of [CH≡C]x-H2P-COFs. The CO2 adsorption CO2 adsorption capacity to those of [CH≡C]x-H2P-COFs. The CO2 adsorption capacities of [CH≡C]x- capacities of [CH≡C]x-H2P-COFs and [EtNH2]x-H2P-COFs enhanced with the increase of BET surface H2P-COFs and [EtNH2]x-H2P-COFs enhanced with the increase of BET surface area and pore size. area and pore size. However, [MeOAc]x-H2P-COFs, [AcOH]x-H2P-COFs, [EtOH]x-H2P-COFs and However, [MeOAc]x-H2P-COFs, [AcOH]x-H2P-COFs, [EtOH]x-H2P-COFs and [EtNH2]x-H2P-COFs [EtNH2]x-H2P-COFs show much higher CO2 adsorption capacity than those of [CH≡C]x-H2P-COFs. show much higher CO2 adsorption capacity than those of [CH≡C]x-H2P-COFs. The CO2 adsorption The CO2 adsorption capacities are in order of [CH≡C]x-H2P-COFs ≈ [Et]x-H2P-COFs capacities are in order of [CH≡C]x-H2P-COFs ≈ [Et]x-H2P-COFs ≪ [MeOAc]x-H2P-COFs < [AcOH]x- [MeOAc]x-H2P-COFs < [AcOH]x-H2P-COFs ≈ [EtOH]x-H2P-COFs < [EtNH2]x-H2P-COFs, which H2P-COFs ≈ [EtOH]x-H2P-COFs < [EtNH2]x-H2P-COFs, which is due to the interaction between the is due to the interaction between the functional group and CO2 molecule. These results indicate that functional group and CO2 molecule. These results indicate that the CO2 adsorption performances are the CO adsorption performances are highly related to the pore surface of the COF materials. highly related2 to the pore surface of the COF materials. Figure 28. Plot of low-pressure CO2 uptake against BET surface area (left) and pore size (right) for Figure 28. Plot of low-pressure CO2 uptake against BET surface area (left) and pore size (right) for the theselected selected COFs COFs at 273 at 273 K and K and 1 bar. 1 bar. 4. Perspectives and Challenges The past decade has been witnessed the blooming COF chemistry and their applications. Their unique features, such as flexible molecular structure design, permanent porosity, low density, modest to high crystallinity and controllable pore size from ultraporous to mesoporous, and the diversity of available building blocks, easily make them targets for structural and functional design, Molecules 2017, 22, 1149 26 of 29 and promise their great potential as materials for application in various areas. However, there are still a lot of challenges in the design and synthesis of COFs. First, compared to the high ordered structure of MOF materials, further investigation on the control of crystal growth, the skeletons, and pores is still of great importance. Currently, thermodynamically controlled solvothermal reactions are widely used to synthesize COF materials. The investigations reveal that the formation and growth of crystal nuclei to improve the crystallinity of COFs is quite important. Such studies provide a general pathway for the preparation of high crystallinity COFs. Moreover, control over the defect sites, the morphology and the stacking mode of COF’s structures are also important and effective methods to construct precise crystal COF structures are necessary. At present, no direct analytical tools can be used to visualize and analyze the stacking and topological structures of COFs. Thus, the creation of single-crystal COFs an unattainable target. Meanwhile, through the existing approaches, the methods to increase the surface area and porosities of COF materials are a big bottleneck. The surface area and porosities are the most important parameters for COF materials for gas adsorption and gas separation applications. How to design COFs structures which can improve both surface area and these properties is the key to realize the potential application of COFs in these fields. For the 2D COFs, spatially enlarging the interlayered spaces would increase the surface areas. 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