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Exceptional CO2 Adsorbing Materials under Different Conditions Mahasweta Nandi*[a] and Hiroshi Uyama*[b] Department of Integrated Science Education and Research, Siksha Bhavana, Visva-Bharati, Santiniketan 731 235 (India) E-mail: [email protected] [b] Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita 565-0871 (Japan) E-mail: [email protected]

[a]

Received: July 10, 2014

ABSTRACT: In this article we discuss those materials that have recorded the highest adsorption capacities for the greenhouse gas CO2 under ambient conditions as well as at different temperatures and pressures. For convenience, the materials have been categorized under four categories, viz., porous carbon, metal–organic, zeolite and mesoporous silica, and porous organic frameworks. It has been found that the gas adsorption property significantly relies on several factors such as high surface area and pore volume and the presence of N-, O- and S-containing moieties. The presence of a microporous structure and strong interaction between the CO2 molecules with the framework through H-bonding or dipole–quadrupole interactions facilitates adsorption of the gas. DOI 10.1002/tcr.201402062 Keywords: carbon storage, metal–organic frameworks, polymers, silicates, zeolites

1. Introduction Carbon dioxide (CO2) present in the atmosphere is one of the chief contributors towards global warming as it absorbs and emits radiation within the thermal infrared region causing the greenhouse effect. It is present in the atmosphere naturally as part of the earth’s carbon cycle but its concentration has been increased due to various human activities. On the one hand, extensive industrialization and modernization has increased CO2 production over the years, while deforestation has destroyed the natural sinks for the gas, posing great threat towards mankind and environment. Thus, capture and storage of this gas has become a big challenge to scientists across the world. Storage of gas in a material instead of inside a container is preferred, since a given volume of solid under ambient pressure can store a relatively greater amount of gas than a storage tank can even under higher pressures.[1] Adsorption of CO2 on

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a material can occur due to various types of interacting forces between the host and the gas molecules; these can be broadly classified as chemisorption and physisorption. Chemisorption is a process where acidic CO2 molecules are adsorbed onto basic amine-containing sites through formation of N–C bonded carbamate species.[2] The amine moieties are subsequently regenerated via rupture of the covalent carbamate bond by heating with the release of CO2. This process consumes a huge amount of energy and sometimes also leads to corrosion, instability and volatility of the amines. Physisorption, on the other hand, is a reversible process involving adsorption and desorption of the gas only under the influence of temperature and pressure.[3] Since the interaction here is mainly van der Waals in nature and there is no chemical interaction between the gas molecules and the adsorbent, there is insignificant

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adverse effect of the process of adsorption on the nature of the material. The materials that adsorb CO2 by either of the above types of interactions may be either a bulk solid where the gas is adsorbed onto the surface of the solid[4] or a porous material where the gas is stored inside the pores. Herein, we concentrate on those solids that have a porous structure and make good CO2 storage materials, either by physical or chemical means. As mentioned previously, chemisorption involves actual chemical interaction via bond formation between the gas molecule and the substrate. Thus, the presence of basic sites alongside a highly porous structure is important for this process. Chemisorption may be reversible or irreversible, depending on the strength of interaction. On the other hand, adsorbents with high surface area and fewer basic sites are desirable for physisorption where there is almost no possibility for chemisorption. Physisorption is generally reversible in nature as the interaction between the gas and the substrate is relatively weak. A desorption isotherm that follows exactly the same path as that of the adsorption isotherm signifies a reversible gas uptake and in such cases almost all the gas stored is accessible for use. If the adsorption and desorption branches do not follow the same path or there is large hysteresis between the two it indicates an irreversible gas uptake. Materials based on porous carbon[5] and graphenes,[6] metal–organic frameworks,[7] zeolites[8] and mesoporous silica,[9] and several types of covalent and polymeric organic frameworks[10] have been reported in the literature to be efficient for CO2 adsorption and storage. Porous materials with exceptional ability to adsorb and store CO2 will be discussed in the present article. We shall be confined here to those materials that have been documented as showing very high adsorption capacities of gaseous carbon dioxide in their respective genre Mahasweta Nandi was born in Kolkata, India, in 1980. She obtained her Masters in Chemistry in 2003 from the University of Calcutta, India, with a specialization in Inorganic Chemistry. She then joined the laboratory of Prof. Asim Bhaumik to pursue a Ph.D. degree at the Indian Association for the Cultivation of Science, Kolkata. In 2008, she received her Ph.D. degree and then joined Visva-Bharati University, Santiniketan, India, as an Assistant Professor in 2009, where she is presently working. In 2010, she joined the group of Prof. Hiroshi Uyama as a postdoctoral researcher and continued as a fellow of the Japan Society for the Promotion of Science (JSPS) until 2012. Her research interests include the synthesis of nanoporous materials by various techniques and their potential applications in adsorption, optoelectronics, catalysis, etc.

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from time to time. When we discuss materials with respect to a particular type of property, the description becomes more lucid if we can organize the materials under different classes. In the following sections we have classified the porous CO2 adsorbents under four major categories.

2. Porous Carbon Frameworks Materials based on porous carbon are capable of adsorbing considerable amounts of CO2. They are cheap, widely available and chemically inert with exceptionally high surface area and large pore volume.[11] Due to the hydrophobic nature of their surface, their highly stable structure, and good thermal and mechanical stability, their regeneration is also quite facile. Porous carbons are generally synthesized by two different techniques (Figure 1). In one approach, carbons with controlled pore structure can be obtained by pyrolyzing a suitable porous polymeric material like mesophase pitch or resin (Figure 1a).[12] In another procedure, porous carbons are produced by using a “hard template” such as porous ordered silica material.[13] Here, a mesoporous silica material (MCM-41, MCM-48 or SBA-15) is filled with a precursor solution (carbon source), then this system is calcined at high temperatures and finally the inorganic silica matrix is washed away using a HF or NaOH solution (Figure 1b). Very recently, another technique for the preparation of nanoporous carbon has emerged where metal–organic frameworks (MOFs)/porous coordination polymers (PCPs) are directly carbonized after impregnation with carbon sources like furfuryl alcohol. The MOFs/PCPs here function as a hard template as well as a secondary carbon precursor, as they too have a high carbon content from the organic moieties.[14] In the Hiroshi Uyama was born in Kobe, Japan, in 1962. He obtained his Master Degree of Engineering from Kyoto University, Japan, in 1987, and his Doctor Degree of Engineering from Tohoku University in 1991 under the direction of Prof. Shiro Kobayashi. In 1987 he joined Tohoku University as an Assistant Professor, thereafter moved to Kyoto University, and is presently a Professor in the Department of Applied Chemistry, Osaka University. He is the recipient of several awards and recognitions, including the Young Scientist Award (Chemical Society of Japan) and the Highest Award of Japan Bio-technology Business Competition. His research interests include biomass plastics, nanoprocessing of polymers and production of biomaterials from biopolymers.

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E x c e p t i o n a l C O 2 A d s o r b i n g M a t e r i a l s u n d e r D i ff e r e n t C o n d i t i o n s

Fig. 1. Two different techniques to prepare porous carbon: (a) pyrolyzing mesophase pitch and (b) “hard template” route.

case of hard templating, the template, which is generally silica, is washed away completely after the synthesis by treatment with NaOH or HF, whereas in soft templating the organic surfactants are removed by combustion during carbonization. While the basic approach towards the synthesis of the nanoporous carbon structures has remained the same, several modifications have been carried out over the years to generate materials with enhanced adsorption ability. Carbon molecular sieves in the form of powder or monolith have been produced without the use of a binder from petroleum pitch by using potassium hydroxide as the activating agent.[15] These molecular sieves are microporous with a very high surface area of ca. 3100 m2/g, thus making them an interesting material for gas adsorption. In fact, this material exhibited the highest adsorption capacity for CO2 at 1 bar and 273 K when the work was reported in 2010. The value 8.64 mmol/g is an exceptionally high value as long as carbon-based materials are considered and therefore we chose this work to begin our discussion with. In another work, sustainable porous carbons[16] were prepared by Sevilla et al. by hydrothermal treatment of polysaccharides (starch and cellulose) or biomass (sawdust) by chemical activation using KOH. These materials registered a very high CO2 uptake of 4.8 mmol/g at 298 K and 1 bar, which at the time of report was recognized as the highest recorded CO2 uptake at room temperature for any activated carbon. In this study it was further demonstrated how severe (KOH four times the precursor) and mild activation conditions (KOH two times the precursor) at various temperatures affect the texture of the porous carbons and their CO2 adsorption ability. Whereas the carbons obtained under mild conditions showed small surface area (1300 m2/g) and narrow micropores (2000 m2/g) and larger pore sizes (1–3 nm). For CO2 storage, however, the mildly activated carbons exhibited a better performance, which was attributed to the presence of a large number of narrow micropores. Fabrication of activated carbons can be even more interesting if they can be derived from biomass, which can make the process much more economic. One such example worth mentioning here is the use of bamboo[17] as an appropriate precursor for obtaining granular activated carbon using the KOH activation method. Though not claimed to be any highest quantity, the amount adsorbed, ca. 7.0 and 4.5 mmol/g at 1 bar and 273 and 298 K, respectively, is higher than most of the carbon materials reported. The adsorbent showed high selectivity for CO2 and highly reversible adsorption characteristics. Similar to the previous example, the KOH amount and activation temperature here also had ample effect on the gas uptake capacity, whereas the bamboo mesh size had very little effect. Porous carbons prepared under severe activation conditions are not favorable for gas adsorption though they have higher surface area; it is the microporosity of the adsorbent that contributes. The hard-templating approach towards generation of microporous carbon frameworks can also result in adsorbents with high CO2 uptake capacity. According to a recent report,[18] nanoporous carbon prepared by zeolite Y templating could adsorb ca. 22.4 mmol/g of CO2 at 40 bar and 298 K, which exceeds the high-pressure adsorption capacities of all previously reported carbon materials. These nanoporous carbons with high microporosity were synthesized by chemical vapor deposition of furfuryl alcohol–butylene as the carbon source over zeolite Y as the hard template followed by pyrolysis. An increase

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in pyrolysis temperature from 1073 to 1273 K resulted in increases in the specific surface area from 2563 to 3010 m2/g and the total pore volume from 1.53 to 1.84 cm3/g. The amount of physisorbed gas is proportional to the surface area of the materials obtained but the uptake capacity relied to a large extent on the pore size effect as well as the surface chemistry of the adsorbent. As mentioned previously, in an attempt to increase the adsorption capacity of the materials various alterations have being carried out over the skeleton of the materials. In this respect, introduction of basic nitrogen sites has been proven to produce excellent results by improving the selectivity for CO2 adsorption. Although incorporating N-containing groups into the carbon framework can sometimes increase the preparation cost and make regeneration difficult, a suitable choice of precursor can help overcome both of these problems. It was in 2011 that Sevilla et al. reported highly porous N-doped carbons[5] by using polypyrrole (PPy) as the carbon precursor. The activating agent in this case was again KOH at temperatures of 873 to 1073 K under severe and mild conditions with 4:1 and 2:1 ratios of KOH and polypyrrole, respectively. An investigation of CO2 adsorption capacity revealed that the carbon prepared under mild conditions was superior to the severely treated counterpart. The values for the material prepared at 873 K with a 2:1 ratio of KOH and polypyrrole were 6.2 mmol/g and 3.9 mmol/g at ambient pressure and 273 and 298 K respectively, which are very high. A closer look into the material showed two important features: the presence of a large number of N-functional groups (ca. 10 wt %), mostly as pyridonic N with a small contribution from pyridinic N, and a narrower microporous structure. The specific surface area of the material was 1700 m2/g and the pore size ca. 1 nm; the nitrogen content was one of the highest reported values. In the same year a high-surface-area N-containing microporous carbon[19] was prepared by Xia et al. and at the time of its report it recorded the highest CO2 uptake capacity for any carbon material as well as for any inorganic or organic porous material. The synthetic approach in this case was different, however, with a zeolite EMC-2 as a hard template and acetonitrile as the carbon source. This method was very easy with excellent recyclability and regeneration stability and the materials exhibited high selectivity for CO2 adsorption. The uptake capacity was found to be 6.9 mmol/g and 4.4 mmol/g at ambient pressure and 273 and 298 K respectively, which is much higher than the CO2 adsorption capacity required for practical applications.[20] The BET surface area of the material was 3360 m2/g with 4.7 wt % of N content. The completely reversible adsorption isotherms suggested that the uptake of CO2 is primarily via physisorption or very weak chemisorption. Before discussing more about carbon-based adsorbents where activation is carried out chemically, e.g., in the pres-

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ence of KOH, we will briefly discuss a recent work reported by our group. Instead of employing harsh chemicals, the N-containing activated carbon materials were obtained by physical activation of polyacrylonitrile monolith[21] in an oxidizing CO2 environment. The adsorbents, which are obtained in the form of monoliths, are easy to handle and were prepared by a very simple thermally induced phase-separation technique.[22] The materials showed reversible physisorption with exceptionally high CO2 uptakes of 11.51 and 5.14 mmol/g at ambient pressure and 273 and 298 K, respectively, which are among the highest reported values in the literature. Here, a 3:1 Ar/CO2 atmosphere with a heating temperature of 1273 K was used to produce a microporous sorbent with a BET surface area of 2501 m2/g and a pore size of ca. 8.5 nm. The N content of this material was ca. 1.8 wt %, which supports the reversible physisorption characteristic. Our discussion on carbon-based adsorbents would not be complete without mentioning a very recent work reported by Ma et al.[23] This work described a unique activation approach to the fabrication of N-doped carbon monoliths with exceptional CO2 adsorption capacity. A natural biopolymer, namely sodium alginate, was used as the carbon precursor in this case where a mixture of acids, H3PO4 and HNO3, served as co-activating agent. This is a novel methodology to generate N-doped carbon monoliths and the adsorbents showed very high CO2 adsorption capacities of 2.98, 4.57 and 8.99 mmol/g at 318, 298 and 273 K, respectively, and 1 atm pressure. The surface area of this material was 1740 m2/g, with narrow micropore size distribution and a high pyrrolic N content (3.38 wt %). Additionally, the material recorded a CO2 adsorption capacity of 1.51 mmol/g at 298 K under the low pressure of 0.15 atm, which is the highest reported value so far under these conditions. Thus, from the above discussion it is clear that the CO2 adsorption ability of porous carbons is significantly dependent on the existence of narrow micropores[5] (less than 1 nm) with high adsorption potential that increases their gas uptake capacity.[16] In addition to that, the adsorption capacity for acidic CO2 molecules is enhanced in the presence of basic nitrogen functional groups in a material[24] through strong polar interaction between the large quadrupole moment of the CO2 molecule and the polar site linked to N groups (13.4×10−40 C·m2 for CO2 against 4.7×10−40 C·m2 for N2).[3a] The hard-templating method for the synthesis of porous carbons is quite useful as it gives rise to ordered structures, particularly when a highly ordered silica is used as the mold. However, the primary drawback of this process is the sacrificial use of the silica backbone as well as harsh chemicals like NaOH or HF. In this context, soft-templating or nontemplating routes are more desirable and by suitable choice of precursors it is possible to generate highly ordered carbon frameworks by these techniques.

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Fig. 2. Bridging organic linkers in the synthesis of MOFs.

3. Metal–Organic Frameworks Metal–organic frameworks, popularly abbreviated as MOFs, constitute an important class of porous polymeric ordered crystalline structures, comprising of metal ions linked to each other by bridging groups, particularly organic moieties (Figure 2). They establish a connecting link between molecular coordination chemistry on one side and material science on the other. Their facile synthesis, reproducibility, high porosity, ease of

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chemical modification, high thermal stability and outstanding sorption performances with excellent reversibility make them interesting for various applications. The greatest advantage of these materials is their highly crystalline structures, which makes their characterization extremely precise by using singlecrystal X-ray diffraction techniques. By an appropriate choice of metal ion and organic linker it is possible to design MOFs with the desired properties. The accurate three-dimensional representations of the crystal structures obtained from diffraction studies give an estimate of the amount of porosity in these materials. Any discussion regarding the CO2 uptake capacity of MOFs cannot proceed without mentioning MOF-177,[25] Zn4O(BTB)2 (BTB = 1,3,5-benzenetribenzoate), an extra-high porous framework reported by Yaghi’s group as early as in 1999. The surface area of the compound is ca. 4500 m2/g and the pore volume 1.59 cm3/g, with an ordered extra-large pore structure. The CO2 adsorption property of this Zn-containing MOF with organic carboxylate as the linker[26] has been studied along with other MOFs at room temperature. The measurements were performed at high pressures of up to 35 bar, which is a common practice in the adsorption studies of MOFs. The adsorption of CO2 was recorded as 33.5 mmol/g, which at that time exceeded the capacity of standard materials under similar conditions by 150%. This report actually gave a new direction to the study of porous MOFs for their application in gas adsorption. Since then a number of studies have been carried out on CO2 uptake,[27] many of them describing the selective adsorption of the gas.[28] However, as mentioned earlier, in this article we are highlighting those materials that showed the highest values when reported. The 2,5-dihydroxyterephalic acid linker is known to form microporous coordination polymers based on the tetraanionic form 2,5-dioxido-1,4benzenedicarboxylate (DOBDC). Zn/DOBDC[29] was reported first followed by Co/DOBDC[30] and Ni/DOBDC,[31] all of which are isostructural. In a bid to expand the choice of metal, a lighter, harder metal, namely Mg, was used to obtain Mg/DOBDC,[32] similar to the other compounds in this series. The yellow microcrystalline product, after activation in methanol to remove high-boiling solvents from its pores and subsequent treatment under vacuum, showed a BET surface area of 1495 m2/g. The CO2 adsorption experiments on this Mg analogue at 296 K showed a reversible uptake of 5.36 and 8.0 mmol/g at 0.1 and 1 atm, respectively. These values are considerably higher than any other material in this series and are ascribed to the higher ionic character of the Mg–O bond. For comparison, the Ni analogue[32,33] showed adsorption capacities of 2.63 and 5.82 mmol/g at 0.1 and 1 atm, respectively, at 296 K. Chromium terephthalate based MOF MIL-101,[34] with very large surface area (4230 m2/g) and pore volume (2.15 cm3/g), has been shown to adsorb a very high quantity of CO2,

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viz., 40 mmol/g at ca. 50 bar and 303 K.[35] Trimers of Cr octahedra with terminal (H2O, F) ligands are linked to the rigid carboxylate ligands to generate microporous supertetrahedral units where the cages are accessible through microporous windows. The activation conditions are vital, and the carbon dioxide isotherms strongly depend on them. Along with the as-synthesized MIL-101, two more samples have been studied. The first one was a hot-ethanol-treated sample, which results in the removal of unreacted ligands from the structure, and the second was obtained by treatment of this hot-ethanoltreated sample with a mixture of ethanol+NH4F. The gas uptake capacity at 50 bar and 303 K was found to increase gradually from 28 mmol/g for as-synthesized MIL-101 to 34 mmol/g for the hot-ethanol-treated sample and finally up to a record value of 40 mmol/g for the ethanol+NH4F-treated material. A number of isoreticular frameworks derived from dendritic hexacarboxylate ligands having C3 symmetry resulted in (3,24)-paddlewheel connected[36,37] networks on reaction with copper salts. The most important of these was NU-100 (also known as PCN-610),[36] with the formula (H3PTEI = 1,3,5-tris[(1,3-carboxylic [Cu3(PTEI)(H2O)3]n acid-5-(4-(ethynyl)phenyl))ethynyl]-benzene). Activation of the compound in supercritical carbon dioxide removed the guest solvent molecules (high-boiling dimethylformamide being previously replaced by ethanol) without affecting the structure. The BET surface area of this activated NU-100 was as high as 6143 m2/g, which is one of the highest reported values for any porous material, and the pore volume was 2.82 cm3/g. At 40 bar and 298 K, the total CO2 uptake capacity was 52.6 mmol/g, which is also the highest value reported so far for any MOF material. Furthermore, another MOF, PCN-68[37] [(Cu3(H2O)3-(PTEI)·13H2O·33dmf )] with a BET surface area and pore volume of 5109 m2/g and 2.13 cm3/g, respectively, showed a CO2 uptake capacity of 30.4 mmol/g at 298 K and 35 bar. Such high adsorption has been attributed to the formation of mesocavities as a result of metal–ligand interaction, which are interconnected by microwindows stabilizing the framework. The size of the microwindows is fixed due to formation of cuboctahedra sustained by the isophthalate ligand and extended throughout the structure. Highly porous, thermally stable DUT-9, [Ni5O2(BTB)2·21H2O·21def ] (BTB = benzene-1,3,5-tribenzoate, def = N,N-diethylformamide) is another interesting material for CO2 uptake.[38] It can be obtained by the solvothermal reaction of Ni(NO3)2·6H2O with H3BTB in N,N-dimethylformamide or N,Ndiethylformamide. The framework has a high concentration of open metal sites per cluster and a few of the solvent molecules (dmf/def and water) can be removed by heating to create accessible metal sites in the structure. An additional activation in vacuum at 393 K leading to a partial removal of coordinated solvent molecules imparts interesting adsorption characteristics

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in the material. While the material shows a CO2 uptake of 25.22 mmol/g at 47 bar and 298 K, after additional activation the value increases to 37.27 mmol/g under similar conditions. A microporous copper-based MOF with the 5-(3-methyl5-(pyridin-4-yl)-4H-1,2,4-triazol-4-yl)isophthalate (MPTP) ligand,[39] containing a combination of carboxylate, triazole and pyridine functions, showed extraordinarily high CO2 uptake at ambient pressure. Pores are partially blocked in the as-synthesized sample, as a result of which it shows poor adsorption capacity. However, the behavior of the material drastically changes leading to phase transition by reversible intercrystalline structural transformations[40] when certain post-synthetic treatments are carried out on it. These include treatment of the sample in vacuum at room/higher temperature or Soxhlet extraction in MeOH followed by evacuation at 298 K. In the latter case, the stability of the MOF increases up to 523 K and it shows its maximum adsorption capacity, with a BET surface area of 1473 m2/g. The CO2 uptake of this post-treated MOF followed a typical type I isotherm with adsorption capacities of 9.2 and 6.1 mmol/g at 273 and 298 K, which are among the highest values bearing in mind that the pressure was ambient, i.e., 1 bar. The cuboctahedral metal–organic polyhedron DUT49,[41] based on copper paddlewheels and carbazole-3,6dicarboxylate, was developed using the tetratopic ligand H4BBCDC (9,9′-([1,1′-biphenyl]-4,4′-diyl)bis(9H-carbazole3,6-dicarboxylic acid)). Cube-shaped blue crystals of DUT-49 [Cu2(BBCDC)](H2O)x(NMP)y, were obtained solvothermally by the reaction of H4BBCDC with Cu(NO3)2·3H2O in N-methyl-2-pyrrolidone (NMP). The polyhedral framework shows twelve accessible metal sites on removal of coordinated solvent molecules. The specific surface area and pore volume of 5476 m2/g and 2.91 cm3/g, respectively, are among the highest reported values to date for any porous material. High-pressure CO2 adsorption studies showed an uptake capacity of 55.68 mmol/g at 50 bar and 298 K, which exceeds most MOFs. Though it is not among the highest reported CO2 uptake capacities, yet another interesting category of MOF that should not go without mentioning is MAF-66.[42] This has a zeolite-type structure, [Zn(atz)2] (Hatz = 3-amino-1,2,4triazole) consisting of a metal azolate framework. The atz behaves as an imidazolate-type linker that connects tetrahedral Zn(II) to form a non-interpenetrating framework. The pore surface of the narrow, three-dimensional intersecting channel system thus formed is decorated with amino groups and uncoordinated triazolate N atoms. The CO2 uptake values at 273 and 298 K were 6.26 and 4.41 mmol/g, respectively, at 1 atm. The compound shows enhanced adsorption ability due to its narrow channel size and high density of uncoordinated nitrogen atoms on the pore surface, which behave as basic sites for the acidic CO2 molecules.

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Tricarboxylate organic linkers, viz., 5-(2carboxyvinyl)isophthalic acid (CVPA) and 3,3′-(5-carboxy1,3-phenylene)diacrylic acid (CPAA), have been used for the synthesis of ZJU-35 and ZJU-36.[43] Blue crystals of these MOFs were obtained by reacting the organic linkers with Cu(NO3)2·3H2O in acidified DMF/H2O at 338 K for two days. They showed higher porosities because of their enlarged pore spaces, with BET surface areas of 2899 and 4014 m2/g, respectively. The absolute gravimetric CO2 capture capacities of the MOF materials are more or less linearly dependent on their specific surface areas. High gas uptake values of 14.64 and 13.88 mmol/g were recorded for the activated samples of ZJU-35 and ZJU-36 at 300 K and 30 bar, which are among the highest reported values. The second-generation MOF NU-111,[44] formed from Cu ions and a hexacarboxylic acid linker (named as HCA) with three pairs of triple-bond spacers, has been found to exhibit exceptionally high CO2 uptake capacity. The internal surface area of the activated material was 4930 m2/g with a pore volume of 2.09 cm3/g. CO2 adsorption at 0.15 bar and 298 K was ca. 0.41 mmol/g, whereas at 30 bar and 298 K it exhibited an uptake capacity of 38 mmol/g, which is one of the largest values reported for MOFs.[45] Incorporation of pyridyl moieties into the framework of MOFs results in interesting structural characteristics and excellent stability. They can improve the gas uptake capacity due to the presence of Lewis base sites on the surface of the pore without compromising on the porosity and robustness of the structure.[46] One such example is the MOF UiO(BPDC),[47] Zr6(μ3-O)4(OH)4(BPDC)12, prepared using the N-heterocyclic carboxylate ligand 2,2′-bipyridine-5,5′dicarboxylate (BPDC), where free Lewis base sites have been incorporated into the MOF in the form of bipyridyl moieties. Octahedral crystals of this compound were obtained solvothermally by reaction of the BPDC ligand with ZrCl4. The compound can be fully activated by Soxhlet extraction with ethanol to replace high-boiling-point solvents and other compounds, facilitating the removal of guest molecules. The BET surface area was 2646 m2/g and at 293 K and 20 bar it adsorbed 18.1 mmol/g of CO2, which is among the top uptakes on MOF materials. The enhancement of CO2 uptake here can be ascribed to the high surface area and presence of Lewis basic bipyridyl sites. The gas uptakes at 0.15 and 1 bar have been studied and found to be 1.5 and 8.0 wt %, respectively. Thus, we can see that the CO2 uptake capacities of MOFs depend on several factors, including high surface area and pore volume and also the concentration of open metal sites[33,38,41,48] present in the framework and their accessibility. Large pore volumes as well as high internal surface areas enhance the storage capacity, particularly in the moderate to high pressure region. On the other hand, MOFs with large pore sizes are not suitable for CO2 capture under low pressures, such as from flue

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gas where the partial pressure of carbon dioxide is very low, typically ca. 0.1 bar.[49]

4. Zeolites and Mesoporous Silica Frameworks Zeolites are hydrated crystalline aluminosilicates, later extended to metallosilicates, with well-defined microporous structures. The crystalline nature of zeolites creates regularity in their structure and imposes an inherent limit to the pore diameter of 1.3 nm.[50] Mesoporous silica-based frameworks, on the other hand, have higher surface areas, exceptionally high pore volume and a wide and controllable range of pore diameters (>2 nm). Use of zeolites and mesoporous materials is energetically more efficient for large-scale CO2 capture and recycling and has appeared as a low-cost alternative to amine-based solutions, which require continuous regeneration and hence consume a lot of energy. Progress has been made in the structural, compositional and morphological control and in the stability of these silica-based micro- and mesoporous materials so that they can efficiently adsorb and store carbon dioxide. However, the adsorption capacities of these porous inorganic zeolites and silicates are much lower compared to the MOFs, which show exceptionally high values of CO2 uptake. We shall here discuss briefly those zeolites and mesoporous silica-based frameworks that have shown exceptionally good performance in their genre in the uptake of CO2. Any discussion in this category cannot start without mentioning zeolite 13X, one of the oldest known large-pore aluminosilicates, which has the chemical formula Na86[(AlO2)86(SiO2)106]·H2O and is among the very best sorbents for CO2 adsorption. The average pore diameter of this zeolite is ca. 0.8–1.0 nm and the BET surface area is 506 m2/ g.[51] High-pressure adsorption of carbon dioxide on zeolite 13X has been measured under different conditions.[52] The values obtained were 7.372 mmol/g at 298 K and 32 bar, 6.920 mmol/g at 308 K and 33.65 bar, and 5.762 mmol/g at 323 K and 33.95 bar, which are very high for zeolite- or silicatebased frameworks. Acidic and copper-exchanged forms of SSZ13, an aluminosilicate zeolite belonging to the ABC-6 family, have shown significantly high carbon dioxide adsorption capacity at low pressure.[8] Zeolite SSZ-13, with a SiO2 to Al2O3 ratio of 1:12, is composed of corner-sharing AlO4/SiO4 tetrahedra that form double six-membered-ring cages that are further linked to form a cavity with eight-membered windows. The primary binding site for CO2 is situated at the center of this eight-membered-ring pore window. The Langmuir surface areas of SSZ-13, which is the acidic form, and its copper-exchanged form were found to be 764 and 710 m2/g, respectively, whereas the micropore volumes were 0.27 and 0.25 cm3/g, respectively. The CO2 uptakes under ambient conditions were 3.98 and 3.75 mmol/g for the acidic and Cu-exchanged forms,

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Fig. 3. N-containing functional groups have a strong interaction with CO2.

respectively, which are among the best uptake capacities at standard temperature and pressure. The high adsorption capacity of the acidic form is attributed to the ideal diameter of the cavity for CO2 adsorption, which remains unoccupied by cations, and the electrostatic interactions with the gas, which differ between the acidic and Cu-exchanged frameworks. Mesoporous silica supports with disordered pores are not appropriate as solid supports for amine grafting in order to enhance their gas adsorption properties, as their entire pore volume cannot be accessed by the amine molecules. In this respect, ordered mesoporous silica supports with high surface areas and well-defined pore arrangements are much better. The total pore volume and pore size[53] of the supports influence their performance for CO2 adsorption, as a larger pore volume can accommodate a higher quantity of amine and thus lead to higher CO2 adsorption capacity. Pure mesoporous silica is a poor adsorbent for CO2 due to its weak interaction with the gas. However, amine-functionalized porous silicates show very good CO2 uptake capacities due to the presence of the basic amine sites (Figure 3), which strengthened the interaction between CO2 and MCM-41.[54–57] These can be synthesized by two techniques, viz., grafting/chemical conjugation of aminecontaining alkoxysilanes on the surface of the support and deposition/physical adsorption of amine compounds[56,58] onto it. Chemical conjugation improves the chemical stability and prevents leaching of the amines but only a low concentration of amine can be loaded. The amine-deposited solids, on the other hand, have poor thermal stability due to the volatility of the amines, and as a result these materials can only be used in a narrow temperature range. The gas uptake capacity is also low, as limited numbers of amines are retained on the solid support. Polyethylenimine (PEI) is the most commonly used macromolecule for CO2 adsorbents due to its high amine content (ca. 33 wt % N). The OH groups on the silica surface interact strongly

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with the amino groups of PEI, enabling the uniform distribution of the polymer throughout the porous framework. Branched PEI can be uniformly dispersed into the channels of MCM-41 to obtain a “molecular basket-type” nanoporous solid adsorbent (MCM-41-PEI),[54,59] which can synergistically enhance the uptake of CO2 in condensed form. The adsorbents have channels of large pore size and volume consisting of plenty of basic sites acting as the basket for capturing CO2. PEI is thus a suitable choice, with branched chains and numerous CO2-capturing amino groups that can enhance the adsorption/ desorption of CO2.[60] The PEI-modified MCM-41, MBS-1, is obtained by impregnation of the amine over the mesoporous silica. The surface area and pore volume of MBS-1 decreased to 4.2 m2/g and 0.011 cm3/g, respectively, from 1480 m2/g and 1.0 cm3/g, respectively, in MCM-41 due to filling of the mesopores with PEI. The CO2 adsorption experiments showed the highest adsorption capacity of 3.02 mmol/g when the PEI loading was 75 wt %, at a higher temperature of 348 K and 1 atm pressure, which is 15.5 times higher than that of MCM41. Impregnation of linear PEI over mesoporous silica SBA-15 gives MBS-2,[61] which has a BET surface area of 80 m2/g. MBS-2 with 50 wt % loading of amine gave a high CO2 adsorption capacity of 3.18 mmol/g at 348 K and 0.15 bar, which is attributed to its high amine-group density of ca. 12.3 mmol/g. Use of periodic mesoporous silica with grafted amine groups and whose pores have been further expanded through post-synthetic treatment[62] is a better choice in this regard. One such example is pore-expanded MCM-41 silica functionalized with diethanolamine (DEA, named as PE-MCM-41),[56] which has been used as a water-tolerant high-capacity recyclable adsorbent for the greenhouse gas. The materials are obtained by hydrothermal treatment of MCM-41 in an aqueous emulsion of N,N-dimethylalkylamine of various concentrations. The pore sizes typically vary from 3.5 to 25 nm and accordingly the pore volumes from 0.8 to 3.6 cm3/g, whereas the surface areas remain almost unaffected (ca. 917 m2/g).[63] PE-MCM-41 is able to accommodate a huge quantity of amine moieties because of its very large pore volume, which results in its high CO2 adsorption capacity. With increasing amine content the uptake capacity initially increases but it then declines gradually due to the deposition of excess amine on the external surface of the particles and within interparticle spaces. Unlike zeolite 13X, PE-MCM-41 is insensitive to humidity and below partial pressures of 0.15 atm it has better performance than the former. For the sample with 7.26 mmol of amine loading per gram of adsorbent the CO2 uptake at 0.15 atm and 298 K was as high as 2.65 mmol/g; higher amounts of amine resulted in lower uptake capacities. Organically modified amine-tethered silica with high amine content (>6 mmol/g) is capable of binding CO2 reversibly.[64,65] Covalently tethered hyperbranched aminosilica

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(SBA-HA) containing 7.0 mmol/g of N can be synthesized by a one-step reaction between aziridine and the silica support SBA-15. The BET surface area of this organic–inorganic hybrid was ca. 170 m2/g and the pore volume was 0.246 cm3/g. SBA-HA has been analyzed for CO2 capture at 298 and 348 K and the adsorption capacities were found to be very high, viz., 3.1 and 2.1 mmol/g under simulated flue gas conditions. Mesoporous silica hollow capsules, MCx/y, where x and y represent the approximate diameter and shell thickness of the capsules in nanometers, can be impregnated with the oligomeric amine tetraethylenepentamine, to obtain functionalized nanocomposite sorbent. This nanocomposite at 348 K exhibited CO2 capture capacities[66] of 6.6 and 7.9 mmol/g at 1 atm under dried CO2 gas and simulated flue gas conditions (0.13 to 0.16 bar) with pre-humidified 10% CO2, respectively. The sorbent is recyclable with high stability and the high uptake capacity is due to the increased amount of reactive amine sites. Comparison of the particle size of the supports with similar shell thickness showed that the maximum adsorption capacity as well as the optimal amine content was higher for the larger particles. On the other hand, for particles with similar size, a thinner shell is preferred for optimal amine loading and higher CO2 uptake. With the increase in shell thickness the surface area and the pore volume decrease, resulting in blockage of pores. Mesostructured cellular foams (MCFs) of silica impregnated with PEI of various molecular weights also showed a very high ability for CO2 uptake.[67] The MCF solid support used here has a well-defined three-dimensional interconnected mesoporous structure with a large cell diameter of 30.3 nm and large window diameter of 11.3 nm. The PEI-impregnated MCFs have cell diameters in the range of 29.4 to 31.2 nm, while the window diameters vary in the range of 10.8 to 11.3 nm depending on the amine loading. The BET surface area and pore volume of this MCF were 628 m2/g and 3.14 cm3/g, respectively. When dispersed with 50 wt % branched amine (mol wt = 600), the surface area and pore volume were reduced to 199 m2/g and 1.3 cm3/g, respectively, as expected. On increasing it to 70 wt % branched amine (mol wt = 600, ca. 22.3 wt % N), significant decreases in BET surface area and pore volumes to ca. 20 m2/g and 0.13 cm3/g were observed, which is due to almost complete filling of the pores. The highest uptake capacity of 4.1 mmol/g was obtained for MCP loaded with 50 wt % branched PEI (mol wt = 600) at 348 K and 1 atm. The adsorbents have good recyclability due to the presence of large pore diameter and three-dimensionally interconnected porous channel networks, which resist blocking of the pores and maintain the porosity even after amine impregnation. Here, it may be mentioned that the amine with a higher molecular weight of 25000 is an inferior performer. The CO2 uptake capacities after 50 wt % amine loading for the branched and linear amines were 3.66 and 2.55 mmol/g, respectively, under similar conditions.

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Thus, from the above discussion, we learn that in order to develop a high-capacity CO2 adsorbent, microporous and mesoporous inorganic solid supports should have high pore density, large pore size and sufficiently large surface area. Most of these adsorbents rely on the presence of basic amine groups that have high affinity for CO2 adsorption. A well-defined support structure plays an important role for developing more efficient amine-functionalized sorbents. In addition to the adsorption of CO2 alone, there are several works that highlight the selective adsorption of the gas in comparison to H2, N2, CH4, etc.[8,68] Although the silica-based frameworks offer better selectivities than carbon-based materials, their stability as well as adsorption capacity degrade[69] significantly in the presence of moisture.

5. Porous Organic Frameworks Nanoporous organic polymers are an important class of materials known for their low skeletal density, high chemical stability and controllable pore size distribution. They include hypercross-linked polymers (HCPs),[70] conjugated microporous polymers (CMPs),[71] covalent organic frameworks (COFs)[72] and polymers with intrinsic microporosity (PIMs).[73] Compared to metal–organic frameworks, porous organic polymers display better stability by replacing chemically susceptible coordination bonds with robust covalent bonds.[74] A lot of attention has been devoted in the past few years to developing various nanoporous organic frameworks with chemical and physical tunability that find applications in gas storage and separation.[75] In this regard, materials with polar surfaces[76] have been extensively studied for efficient and selective capture of CO2. The building blocks constituting the polymeric frameworks that shall be discussed in this section are shown in Figure 4. Functional groups can be introduced into COFs either by appropriate design of the organic building blocks or by postsynthetic functionalization. Based on gas adsorption behavior and capacity, COFs can be classified as group 1, 2 or 3 depending on their structural dimensions and pore sizes.[77] Group 1 includes two-dimensional structures having small onedimensional pores (ca. 9 Å), group 2 consists of twodimensional structures with large one-dimensional pores (ca. 27–32 Å), and group 3 COFs have three-dimensional structures with medium-sized three-dimensional pores (ca. 12 Å for each). The group 3 COFs, COF-102 and COF-103 (having compositions C25H24B4O8 and C24H24B4O8Si, respectively), are found to be better than group 1 and group 2 COFs in terms of their gas uptake capacities. The BET surface area of COF102 is 3620 m2/g with a pore volume of 1.55 cm3/g. At 35 bar and 298 K, the evacuated form of COF-102 showed an uptake capacity of 27.2 mmol/g, which is similar to the performance of COF-103 (ca. 27 mmol/g).

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Fig. 4. Building blocks of the porous organic polymers.

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Cucurbit[7]uril (CB[7]) is an amorphous molecular cage compound that is easy to synthesize with low manufacturing cost. It has a “hollowed-out” structure with a large inner cavity of ca. 279 Å3 and preserves significant porosity in its solid state with a BET surface area of 293 m2/g. It can be prepared in high yield from inexpensive starting materials, viz., glycoluril and formaldehyde by a one-pot synthesis.[78] At ambient temperature (298 K) the CO2 sorption capacities of CB[7] were 1.1 and 2.3 mmol/g at 0.1 and 1 bar, respectively,[79] which at the time of report were the highest among known organic porous materials. The high uptake of CO2 can be ascribed to the high charge density at the oxygen sites of the inner cavity and the lone pair of electrons on nitrogen that can interact strongly with CO2 molecules. Porous organic polymer PAF-1, obtained through Yamamoto homocoupling of the tetrahedral monomer tetrakis(4-bromophenyl)methane,[80] has a high BET surface area of 5600 m2/g and exhibits excellent CO2 uptake capacity. The tetrahedral monomers on polymerization result in a diamondoid framework with wide openings and interconnected pores, which gives rise to a high surface area and reduces the amount of “dead space”.[76,81] PPN-4, the PAF-1 analogue obtained by replacing the central carbon of the quadricovalent building block by silicon, can give rise to even higher surface areas as a result of more expanded tetrahedral monomers.[82] The BET surface area of PPN-4 is 6461 m2/g, which is extraordinarily high, with a pore volume of 3.04 cm3/g, and the size of the pores lies in the microporous or microporous/mesoporous range. Thus it is expected to have a high gas uptake capacity and indeed it was found that PPN-4 can adsorb 48.2 mmol/g CO2 at 295 K and 50 bar, which is among the highest for porous materials reported to date. Low-cost, robust covalent organic polymer (COP-1) can be obtained by the reaction of cyanuric chloride (2,4,6trichloro-1,3,5-triazine) with the linker piperazine and N,Ndiisopropylethylamine (DIPEA)[83] as the base in dioxane. No post-processing or cross-linking is necessary to attain their permanent porosity, which distinguishes them from regular chain macromolecules, and their porosity is retained even on boiling in water for over a week. The BET surface area of COP-1 is 168 m2/g and it is capable of adsorbing a large amount of CO2 under industrially relevant temperatures. The CO2 uptake capacities at 338 K were 1.36 and 127.6 mmol/g at 1 bar and 200 bar, respectively, and the isotherms displayed 100% recyclability. Benzimidazole-linked polymers (BILPs), derived from nitrogen-rich building units, are synthesized in a template-free approach by the condensation reaction between aryl-o-diamine and arylaldehyde building blocks. It has been found that BILPs derived from 3D building blocks generally have higher surface areas than those obtained from 2D building blocks. The highest storage capacity for CO2 was recorded for BILP-4,[84]

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which has the remarkable value of 5.34 mmol/g at 273 K and 1 bar. The yellowish-brown solid product can be synthesized by the reaction of 1,2,4,5-benzenetetraamine tetrahydrochloride with tetrakis(4-formylphenyl)methane. BILP-4 has outstanding chemical and physical stability and a BET surface area of 708 m2/g with pore size and pore volume of 6.8 Å and 0.65 cm3/g, respectively. Another highly porous analogue, BILP-1, can be obtained by template-free synthesis via the condensation reaction between 2,3,6,7,10,11-hexaaminotriphenylene and tetrakis(4-formylphenyl)methane.[85] The yellow powder polymer has a BET surface area of 1172 m2/g, a pore size of ca. 6.8 Å and a total pore volume of 0.70 cm3/g. The CO2 isotherms up to 1 bar are completely reversible and the uptake capacities were 4.27 and 2.97 mmol/g at 273 and 298 K, respectively. Such high gas uptake arises from favorable interactions of the polarizable CO2 molecules through hydrogen bonding accompanied by dipole–quadrupole interactions between the protonated and proton-free N-sites of the imidazole rings. The reversibility of the sorption isotherms further indicates that the interaction of CO2 with the pore walls of the polymer is weak, facilitating regeneration of the material without applying heat. The porous sulfur-bridged covalent organic polymer COP-3 can be synthesized through nucleophilic aromatic substitution by 1,3,5-benzenetrithiol on the highly reactive triazine ring of cyanuric chloride that is destabilized by three chlorides.[86] This is a one-pot, low-cost, high-yield and catalyst-free process, giving a highly networked insoluble polymer structure primarily consisting of thioether (R–S–R) bonding. The robust material that is produced has a BET surface area of 413 m2/g, an average pore size of 2.7 nm and a pore volume of 0.31 cm3/g. Due to its microporosity and high content of nitrogen and sulfur within the networks, this material can adsorb a large amount of CO2 and the adsorption capacity of the polymers is retained even on boiling in water for at least one week. The uptakes at atmospheric pressure and 273 and 298 K were ca. 1.6 and 1.1 mmol/g, respectively, and show reversible adsorption characteristics. The high-pressure adsorption isotherms at warm conditions, e.g., 318, 328, and 338 K, show uptake capacities of 74.8, 70.1 and 64.4 mmol/g, respectively, up to 200 bar. The porous polymer framework PPF-1 shows an excellent performance in CO2 adsorption.[87] This polymer is obtained through imine condensation between the tetratopic building block tetra-4-anilylmethane and 1,3,5-triformylbenzene in a ratio of 3:4 in a solvent mixture of 1,4-dioxane/mesitylene. The specific surface area of PPF-1 is 1740 m2/g, the pore volume is 1.18 cm3/g, and it is dominated by micropores with a pore width of less than 1.5 nm. The material adsorbs 6.07 mmol/g CO2 at 273 K and 1 bar and on further lowering the temperature to 195 K adsorption capacity reaches 19.8 mmol/g at 1 bar; these are among the highest reported for all organic porous materials.

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The extremely stable crystalline H-bonded organic framework HOF-8[88] can be prepared from organic building block N1,N3,N5-tris(pyridin-4-yl)benzene-1,3,5tricarboxamide (TPBTC), containing pyridine N atoms and amide H atoms as H-bond acceptors and donors. HOF-8 is highly stable in water and organic solvents and its desolvated form exhibits high CO2 uptake under ambient conditions, ascribed to the pyridyl and amide N atoms having polar interactions with CO2. The sorption isotherms are typically of type I and adsorption of CO2 at 298 K and 1 bar was 2.56 mmol/g, reported to be the highest among porous organic crystalline materials. Porous azo-linked polymers (ALPs) with high porosity and remarkable CO2 adsorption ability can be obtained by homocoupling of aniline-like building units in the presence of CuBr and pyridine via azo bond formation. The bestperforming polymer, ALP-1,[89] stores 5.36 and 3.25 mmol/g of CO2 at 1 bar and 273 and 298 K, respectively, whereas its uptake at 298 K and the higher pressure of 40 bar was 15 mmol/g. ALP-1, synthesized from the reaction of 2,6,12triaminotriptycene and pyridine in the presence of CuBr, displayed the highest surface area of 1235 m2/g with pore size and volume of ca. 10 Å and 0.66 cm3/g, respectively. The CO2 sorption isotherms are completely reversible, indicating that the interactions between CO2 and ALPs are weak enough to allow material regeneration without heating. The high uptake of CO2 by ALP-1 can be attributed to high values of internal molecular free volume, surface area and N content. Thus, we see that the adsorption capacity of organic polymeric frameworks for the greenhouse gas is attributed to several factors. Interconnected pores offer high surface area by reducing the amount of “dead space”, while high pore volume[83,89] and microporous structure enhance the gas uptake capacity. On the other hand, high contents of nitrogen, oxygen and sulfur in the structures[80,86] can lead to strong interactions with CO2 molecules and enhance the adsorption behavior. High adsorption selectivity[87–90] for CO2 over other small molecules like hydrogen, methane, nitrogen, etc. can be attributed to the interaction between the CO2 molecules and the framework through H-bonding or dipole–quadrupole interactions.[78,88,91] In the above discussion we have explored different categories of porous materials, viz., carbons, metal–organic frameworks, zeolites and mesoporous silica, and polymers, for their carbon dioxide uptake capacity under various conditions. While in general we see that with the increase of surface area and pore volume more physical adsorption takes place with high CO2 adsorption uptake, the presence of basic sites in the framework also increases the affinity for the acidic carbon dioxide molecules. If the adsorption capacities are compared we find that MOFs and porous carbons are the best adsorbents for this greenhouse gas, followed by porous organic polymers (POPs) and zeolites and mesoporous silica. However, the MOFs and POPs that show very high adsorption capacities

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mostly involve high-pressure conditions, which is a drawback for their practical utility. Compared to this, porous carbons under ambient conditions are better adsorbents and hence more useful. All these types of materials are related more to gas storage and require high surface area and pore volume. When comparing the adsorbents on the basis of gas storage capacity their volumetric uptake should be considered, i.e., cm3 of the gas adsorbed per cm3 of the material. Thus, for example, zeolite 13X, which has a higher density than MOF-177, will show a higher volumetric uptake in comparison to MOF-177. However, presenting these data in terms of cm3/cm3 is outside the scope of this article, which concisely summarizes various reported data. The results presented so far are based on the study of the adsorption of the gas from a sole source of CO2 or a mixture of one or two gases with a higher concentration of CO2 than that present in the atmosphere. But when we go to a realistic situation, where CO2 constitutes only ca. 0.035 mole percent of the total gas mixture content, its capture from air becomes challenging. To meet this challenge a lot of research has been done and very recently Shekhah and Belmabkhout[92] have reported a breakthrough work that will be the emerging trend of future research. They prepared a metal–organic framework, SIFSIX-3-Cu, based on pyrazine/copper(II) twodimensional periodic 44 square grids pillared by silicon hexafluoride. The BET surface area of this material is 300 m2/g, with a reduced pore size of 0.35 nm. The smaller pores enhance the energetics of adsorption[92,93] and CO2 uptake and selectivity increases at low values of partial pressure, which is applicable for capture of the trace quantities of CO2 present in highly diluted gas streams, e.g., air. These materials functionalized with basic sites like amines or having high charge density not only increase the gas uptake at low pressure but also the CO2 selectivity. These materials, which do not show exceptionally high adsorption capacities as such, are dedicated more for the separation of CO2 and do not necessarily require high surface area and pore volume. Thus, we see that the adsorption capacity for greenhouse gases is definitely important but until and unless it can be conveniently utilized for practical applications its utility remains incomplete.

6. Conclusions

Acknowledgements M.N. wishes to thank Visva-Bharati, Santiniketan, for providing research funding. H.U. thanks the Japan Society for the Promotion of Science for a Grant-in-Aid for Scientific Research (No. 21350124) and a Project for Creating Start-ups from Advanced Research and Technology, MEXT.

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