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Featuring research from the group of Professor Hong-Cai Zhou at Department of Chemistry, Texas A&M University, College Station, Texas, USA. Methane storage in advanced porous materials Methane continues to gain attention as a fuel for vehicular applications, but current storage technologies remain a barrier to its large-scale adoption. Advanced porous materials with pore sizes and functionalities tuned to enhance methane uptake serve to significantly increase the density of methane through adsorption.

See Zhou et al., Chem. Soc. Rev., 2012, 41, 7761.

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Methane storage in advanced porous materials


Trevor A. Makal,a Jian-Rong Li,b Weigang Lua and Hong-Cai Zhou*a Received 10th July 2012 DOI: 10.1039/c2cs35251f The need for alternative fuels is greater now than ever before. With considerable sources available and low pollution factor, methane is a natural choice as petroleum replacement in cars and other mobile applications. However, efficient storage methods are still lacking to implement the application of methane in the automotive industry. Advanced porous materials, metal–organic frameworks and porous organic polymers, have received considerable attention in sorptive storage applications owing to their exceptionally high surface areas and chemically-tunable structures. In this critical review we provide an overview of the current status of the application of these two types of advanced porous materials in the storage of methane. Examples of materials exhibiting high methane storage capacities are analyzed and methods for increasing the applicability of these advanced porous materials in methane storage technologies described.

1

Introduction

The continued growth in worldwide consumption of gasoline and diesel has led to increasing concerns over the sustainability of oil reserves. Furthermore, the rising levels of atmospheric carbon dioxide (CO2) produced from burning of fossil fuels have raised awareness to the overall impact on global ecosystems.1 A number of potential solutions for conservation and remediation of the environment due to the impacts of CO2 release are current on-going research topics. These include work in the capture and storage of CO2,2–5 as well as the utilization of cleaner fuels, a

Department of Chemistry, Texas A&M University, College Station, TX 77842-3012, USA. E-mail: [email protected]; Fax: +1 979 845 1595; Tel: +1 979 845 4034 b College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, 100124, P. R. China

Trevor A. Makal obtained his BSc in Chemistry from Texas A&M University in 2008. He then joined the laboratory of Prof. Hong-Cai Zhou at Texas A&M the same year, where he studies the design and synthesis of metal–organic frameworks with a focus on structure–property relationships as related to gas sorption phenomena.

Trevor A. Makal

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such as natural gas (NG) or hydrogen (H2). While natural gas does still produce CO2, it is much cleaner burning than petroleum-based fuels. The preeminent factor preventing the commercialization of these fuels in the automobile industry is the discovery of effective methods to separate, capture, and reversibly store these energy related gases. Hydrogen is one of the most attractive fuel options because of its natural abundance, high energy density, and non-polluting nature, water being the only chemical by-product. A great amount of attention has already been devoted to the development of new materials and methods for storing hydrogen, and is outside the scope of this review.6–12 In 2007 it was reported that 28% of greenhouse gas emissions were generated from transportation sources, which have been the fastest-growing source of U.S. greenhouse gas emissions.13 Natural gas vehicles (NGVs) have the potential to substantially

Jian-Rong ‘‘Jeff’’ Li obtained his PhD in 2005 from Nankai University under the supervision of Prof. Xian-He Bu. In 2008, he joined Prof. HongCai Zhou’s group as a postdoctoral research associate, first at Miami University and later at Texas A&M University; from 2010 he has been an assistant research scientist at the same university. Since 2011, he has been a full professor at Beijing University of Technology. His recent Jian-Rong Li research interest focuses on new porous materials for energy and environmental science. Chem. Soc. Rev., 2012, 41, 7761–7779

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lower polluting emissions compared to current gasoline- and diesel-fuelled vehicles. This is especially important for air quality concerns in urban areas where ozone levels are particularly high and pollutants pose a major public health concern. In addition to reducing the emissions of nitrogen and sulfur oxides, known to cause ‘‘acid rain,’’ it is generally agreed that NGVs have a lower potential for causing global climate change than liquid hydrocarbon-fuelled vehicles.14 Natural gas is composed primarily of methane (>95%), with the remaining fraction a mixture of ethane and heavier hydrocarbons, nitrogen, and carbon dioxide.15 Methane must be pressurized and cooled in order to convert it to a liquid, owing to a critical temperature of 191 K (82 1C). Methane has a gravimetric heat of combustion (55.7 MJ kg1) comparable to that of gasoline (46.4 MJ kg1), boasts the smallest amount of CO2 per unit of heat produced among fossil fuels, and is naturally abundant; but the lack of efficient storage methods has prevented the widespread use of NG in motor vehicles. The two common methods of NG storage currently used are (1) liquefaction at low temperature and (2) compression to 200–300 bar at room temperature. The volumetric energy density (VED) of liquefied natural gas (LNG; 22.2 MJ L1, 161.5 1C) achieves 64% that of gasoline (34.2 MJ L1) but requires storage in expensive cryogenic vessels and suffers from boil-off losses; whereas compressed natural gas (CNG) necessitates the use of heavy, thick-walled cylindrical storage tanks and multi-stage compressors to achieve a reasonable VED (9.2 MJ L1), yet achieves only 27% of the VED of gasoline. Despite efforts to improve cylinders and compressors, the amount of NG stored in a CNG tank allows for only a short driving range on light-duty passenger vehicles, and high pressure storage on vehicles has associated safety concerns. In order to realize the benefits that NG use in vehicles offers, attractive alternatives to CNG and LNG are needed. It has been suggested that porous adsorbents represent a safer, simpler, and potentially more cost-effective method for storing NG at ambient temperature and reasonable pressures (around 35 bar) in the form of adsorbed NG (ANG).16–20

In adsorption technologies a guest species adheres to the surface of materials, forming a layer of adsorbed molecules. Since the adsorbate is not fully surrounded by an adsorbent, the adsorbed species may attract more adsorbate to form multiple layers. In the case of porous materials, most sorbents interact with guest species through weak van der Waals forces, referred to as physical adsorption or physisorption. Alternatively, chemical adsorption, chemisorption, occurs in the case of significant covalent interaction between adsorbents and adsorbates (e.g. chemical hydride formation in hydrogen spillover processes). From an application standpoint, the primary difference between physisorption and chemisorption is the significant disparity in binding energies. For reversible gas storage and delivery, moderate binding energies (heats of adsorption) are required to maximize energy efficiency of the system. Therefore, physisorptive materials are best suited for this application, as chemisorptive materials would require a substantial amount of external heat to deliver the adsorbed gas. The potential for at-home refuelling from domestic pipelines, using formable lightweight fuel tanks, and reduced safety concerns relative to that of LNG and CNG are major benefits to the use of ANG technologies for on-board vehicular fuel storage.15,17,21 Identifying this potential, the United States Department of Energy (DOE) has issued a call for the development of sorbents capable of achieving volumetric and gravimetric capacities >12.5 MJ L1 (314.2 vCH4 (STP)/vsorbent) and >0.5 gCH4/gsorbent (700 cm3CH4 (STP)/gsorbent), respectively, at reasonable pressure and temperature ranges (40 1C to 85 1C and r35 bar) in order to achieve targets of >9.2 MJ L1 and >0.4 gCH4/gsorbent in an ANG fuel storage system for passenger vehicle usage. The ideal sorbent should also show resistance to impurities typically encountered in natural gas sources with a lifetime of at least 100 fill–release cycles, and approach $10/kgsorbent, in addition to other system level targets (desorption rates, tank abuse tests, etc.). Recent advancements in NG-adsorbing, high surface area materials have shown promise for increasing the density of NG under moderate conditions in the form of ANG, and

Weigang Lu received his PhD in Organic Chemistry (2002) from Sun Yat-Sen University under the supervision of Prof. Longmei Zeng. He was an assistant professor at Sun Yat-Sen University (2002–2005), and then worked for Biotechnology Research Institute at Hong Kong University of Science & Technology (2005–2008). In December 2008, he joined Prof. Hong-Cai Zhou’s group as a postdoctoral research Weigang Lu associate at Texas A&M University. His research interests include the rational design and synthesis of porous materials (metal–organic frameworks and porous polymer networks) and their application in gas storage and separation.

Hong-Cai ‘‘Joe’’ Zhou obtained his PhD in 2000 from Texas A&M University under the supervision of F. A. Cotton. After a postdoctoral stint at Harvard University with R. H. Holm, he joined the faculty of Miami University, Oxford, in 2002. Since the fall of 2008, he has been a professor of chemistry at Texas A&M University. His research interest focuses on gas storage and separations that are relevant to clean energy technologies.

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provide a significant opportunity to develop small, light-weight, and high capacity tanks for vehicular application. These materials include activated carbons,5,6 metal–organic frameworks,7,8 and porous organic polymers.9 ANG technology increases NG density by condensation in the sorbent’s micropore structure at ambient temperatures and pressures of 30 to 40 bar (5- to 8-times lower than CNG for comparable VED).10 Historically, activated carbons and zeolites have been the most studied microporous materials (pore diameter o 2 nm) for the storage of gases, including methane. However, the difficulty in tuning pore shapes and sizes in activated carbons, as well as the limited number of structures, low surface areas, and hydrophilicity of zeolites has limited the utility of these materials in gas storage, including NG. Metal–organic frameworks (MOFs) have garnered a significant amount of attention as advanced porous materials in the past two decades.22,23 Despite the relatively short time since MOF research began, the field has grown explosively not only in number of structures and publications but also in the scope of research topics this new class of porous inorganic–organic hybrid materials has broken into. Metal containing units play the role of nodes in the framework system as individual ions/ atoms, discrete polynuclear clusters, or infinite chains/sheets, and are connected to one another through coordinatively bound organic struts, Fig. 1. The connectivity of metal and organic linkers results in crystalline materials with regular porosity that can be structurally characterized using X-ray diffraction methods.24 For some materials, the system is stable enough to realize permanent porosity upon included guest removal. Utilizing knowledge of the geometry of the building units observed in MOFs, several examples of frameworks exhibiting the same net topology throughout a series (termed isoreticular MOFs) have now been synthesized using similar geometry and number of binding sites of metal and organic components.25–28 Using reticular synthesis (synthesis aimed toward a particular net topology) the functionalities of MOFs may be tuned while maintaining desired porous properties, such as a specific pore shape or size. Systematically approaching materials design at the molecular level,29–32 with a clear focus

on the resulting physical properties, has been one of the keys to success in the versatile MOFs field, which has demonstrated potential in catalysis,33–54 ion exchange,55–60 gas storage,16,61–73 separation,74–83 sensing,84–94 polymerization,95–98 and drug delivery,99–106 in addition to fundamentally interesting properties including optic,107–113 magnetic,114–121 and electronic nature.122–130 Along with MOFs, porous organic polymers (POPs, Fig. 1) have been identified to exhibit exceptional porosity and tantalizing potential as materials for gas storage and separation applications.131–135 POPs are porous materials composed predominantly of light, non-metallic elements such as carbon, hydrogen, boron, oxygen, nitrogen, silicon, and phosphorus that are connected through strong covalent bonds.136,137 POPs may be divided into two sub-classes: crystalline and amorphous. Crystalline POPs have much in common with MOFs; they are also formed through reversible bond-forming chemistry and have regular, ordered porosity. A representative example of crystalline POPs is covalent organic frameworks (COFs), which were formed either by the self-condensation of boronic acids or by the condensation reaction of boronic acid with diols to form boroxines and others in limited cases. COFs were first systematically studied by Yaghi and coworkers.138,139 On the other hand, amorphous POPs were usually formed through irreversible condensation reactions, which typically result in disordered structures and wide pore size distributions. Consequently, the structures of amorphous POPs are difficult to determine; instead, a model of a single net may be formulated based upon the geometric combination of building units, and the degree of interpenetration determined from pore size distribution calculated from gas sorption analyses. Due to the rigidity of covalent bonds, amorphous POPs show extraordinary stability and tolerance toward water and other chemicals, which often displaces coordinated organic linkers from metals in MOFs.140,141 Different reactions have been used for synthesizing amorphous POPs, and they were named separately by individual research groups, such as hypercrosslinked polymers (HCPs),142 polymers of intrinsic microporosity (PIMs),143 conjugated microporous polymers (CMPs),144 element– organic frameworks (EOFs),145 porous aromatic frameworks (PAFs),133 and porous polymer networks (PPNs).136 The low density, high porosity, and high stability of POPs have led to increasing interest in application of these materials. Here, we review the current state of methane storage in MOFs and POPs, focusing on methods/strategies for increasing the methane storage capacity of these materials.

2 Methane adsorption in traditional porous materials

Fig. 1 Schematic showing the self-assembly processes in advanced porous materials, metal–organic frameworks and porous organic polymers.

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Many porous materials, such as aluminosilicate zeolites, carbon and metal-oxide molecular-sieves, aluminophosphates, activated carbon, activated alumina, carbon nanotubes, silica gel, pillared clays, inorganic and polymeric resins, MOFs, and porous metal– organic composites, have been explored as adsorbents, some of which are now used in industry. Relevant reviews and monographs have summarized the syntheses, structures, characterizations, adsorption properties, and applications of these materials.146–152 Methane storage in porous materials has been an active area of research for some time, with the adsorptive Chem. Soc. Rev., 2012, 41, 7761–7779

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potential of charcoals toward methane being dated back to at least 1930.152 During the 1990s research into ANG technologies increased in popularity and the prospects and results of several different types of traditional porous materials were reviewed by Menon and Komarneni.15 Based on the considerable amount of data that have been collected on various types of porous materials, it can be concluded that adsorption and storage of methane in porous materials follow similar trends to that of hydrogen. In particular, a direct correlation between surface area with adsorption capacity is observed, irrespective of the chemical character of the adsorbent.64 However, enhancing the VED remains difficult as an increase in the surface area of materials generally leads to a decrease in density of the material, and inefficient packing of bulk samples reduces the enhancement factor achieved by using ANG. A number of experimental and modelling studies on the application of traditional porous materials in ANG technologies have been reported, but only a brief overview is included herein. 2.1

Zeolites for methane storage

In the beginning stages of research into ANG storage systems, synthesis procedures for developing high surface area carbons were not well documented.15 Therefore, zeolites were the first materials looked to as adsorbents for ANG technologies.153 Studies involving methane adsorption in zeolites continue to assist in the understanding and design of adsorbent materials;154–162 however, their application as NG sorbents is limited. Issues that demonstrate a major constraint with the application of zeolites in ANG storage systems are the structural limitations which prevent the realization of accessible surface areas greater than 1000 m2 g1 and the ionic nature of the material. As methane has no permanent dipole or quadrupole moments, the ionic nature of zeolites is not beneficial to the adsorption of methane and leads to preferred adsorption of other guest species. In particular, the pore surfaces of zeolites are exceptionally hydrophilic, leading to preferred adsorption of water in gas mixtures, significant retention of water per charge–discharge cycle, and, therefore, diminished overall storage capacity. Additionally, macroporosity, an unavoidable product during bulk packing of crystals, is not beneficial in the storage of methane in ANG systems.163 This is due to little interaction between the methane guest molecules and the pore surfaces of the sorbent as the pore size increases. A large number of macropores in the form of interparticle voids are formed due to imperfect packing. The packing density of zeolites may be increased, but at the expense of having very low micropore surface areas. A methane storage system utilizing adsorbents for methane-powered vehicles has been patented by Stockmeyer.164 The data from this patent indicate that compacting CaX zeolite to 0.8 g mL1 can improve methane storage capacity to 150 v(STP)/v (volume of methane at standard temperature and pressure, 273.15 K and 1 bar, per unit volume of sorbent; denoted v/v henceforth), at ambient temperature and about 9.1 bar.15 However, the methane storage capacity of CaX zeolite compacted to 0.8 g mL1 leads to a capacity of 98 v/v, according to data from Zhang and coworkers.15,162 7764

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2.2 Carbonaceous materials for methane storage Being one of the most studied types of porous materials, naturally, carbonaceous materials have been investigated for their application in methane storage.21,146,147,150,151,165–199 Activated carbons and carbon nanotubes with very narrow pore size distributions are the primary carbonaceous materials that have been studied for gas adsorption and storage applications. Through various computational and experimental studies on activated carbons, those with slit-shaped pores were predicted to provide the greatest volumetric methane capacity,200,201 and pore widths of 0.8–1.5 nm were proposed to be most effective for methane storage.202–205 Furthermore, a study by Celzard and Fierro determined that, in addition to the micropores, the meso- and macropores play a role in the deliverable capacity of methane in carbonaceous materials.206 They determined that larger pores assist in the diffusion of methane through the pores of the system, thereby enhancing loading/unloading rates, and also contribute to overall uptake capacity, based upon comparison of experimental and computational results. Upon processing the carbons, they were able to obtain a methane storage capacity of B195 v/v at 295 K and 35 bar. Microporosity in carbons is generally created by removal of carbon atoms through activation, often by treatment with acid or base; however, ‘‘over-activation’’ may lead to the generation of macropores and a decrease in packing density.207 Because of this ‘‘over-activation’’ the development of very high surface area carbons often produces low density materials that exhibit low VED. Therefore, the development of porous carbon sorbents has shifted to enhancing the bulk density of samples, rather than only increasing surface areas. Different compaction methods have been explored to decrease the presence of macropores and enhance the bulk densities of materials, including grinding,204 compression,208,209 and formation of monoliths,21,194 among others. Prototypes of ANG-fuelled vehicles utilizing activated carbons have been previously demonstrated with exciting potential.210,211 The opportunity to use agricultural by-products as starting materials for production of high surface area activated carbons is an attractive option for reducing the cost of production of porous materials.212,213 However, limitations in pore size distribution, accessible surface area, pore volume, and surface functionalization lend focus to advanced porous materials to further increase the practicality of ANG technologies.

3 Advanced porous materials for high methane storage 3.1 Metal–organic frameworks (MOFs) MOFs present a unique blend of the benefits of both zeolites and porous carbons.215 The crystalline nature and regular, ordered porosity of the materials make absolute characterization a simple task, and permit in-depth structural and host–guest studies to be conducted. Additionally, exceptionally high surface areas may be obtained and the character of the framework is easily adjusted by incorporation of functional groups or post-synthetic modification of the system.216–222 This journal is

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The first reported measurement of methane uptake by a porous MOF can be dated to 1997 by Kitagawa and co-workers.223 While volumetric storage capacity has been emphasized, to date, the reported volumetric storage capacity of a MOF is usually calculated from the gravimetric capacity and the crystallographic density of the material. This leads to an idealized maximum volumetric capacity for the framework, as it would be impractical to grow a single crystal large enough to accommodate enough methane for any practical use, and there is a limit to the degree of densification that can occur before causing structural damage or severely limiting diffusion of gas. While the application of MOFs in methane storage has not received nearly as much attention as that for hydrogen storage or carbon dioxide capture, a number of researchers have investigated methane uptake in MOFs. A compilation of reported methane uptake in high capacity MOFs, in addition to other thoroughly studied MOFs, is provided in Table 1. Discussion of investigations on methane binding sites in MOFs and then analysis of several examples of high capacity MOFs follows.

3.1.1 Identifying methane adsorption sites. The Raman spectroscopic investigation of methane adsorption in isoreticular MOFs (IRMOFs) indicates that the gas molecules adsorb to the IRMOF linkers inside the framework cavities under conditions of temperature and pressure that are most relevant to a storage system. These results point to the critical role that the linkers play in the adsorption behavior of methane in MOFs, thus revealing that selection of appropriate linkers with the highest affinity for methane will provide an optimal storage material.224 This conclusion is in contrast to that found for H2 storage in MOFs.225–229 In the case of the adsorption of H2 in MOFs the linker has been identified to play a relatively minor role in the adsorption process. Instead, coordinatively unsaturated metal centers (UMCs) have been routinely identified as exhibiting the highest affinity toward dihydrogen, with heats of adsorption generally lower than what has been observed for methane sorption in MOFs, the highest H2 heat of adsorption reported being 13.5 kJ mol1.230 The UMC binding sites are made available upon removal of coordinated solvent or guest molecules. This is typically

Surface area, pore volume, methane storage properties under specific conditions for metal–organic frameworks

Table 1

Surface area m2 g1 a

BET

Compound

Lang.

Condition Methane uptake capacity Pore volume/ DHads/kj mol1 3 1 P/bar T/K wt%e v(STP)/vf (zero coverage) Ref. cm g

0

Co2(4,4 -bipy)3(NO3)4 Cu2(pzdc)2(pyz) Cu2(pzdc)2(4,4 0 -bipy) Cu2(pzdc)2(pia) CuSiF6(4,4 0 -bipy)2 Zn4O(bdc)3 [MOF-5, IRMOF-1] Zn4O(pdc)3 [IRMOF-14] Al(OH)(bdc) [MIL-53(Al)] Cr(OH)(bdc) [MIL-53(Cr)] Cr3F(H2O)3O(btc)2 [MIL-100] Cr3FO(bdc)3 [MIL-101] Cr3O(H2O)2F(ntc)1.5 [MIL-102(Cr)] Al4(OH)8(btec) [MIL-120] Cu2(sbtc) [PCN-11] Cu2(adip) [PCN-14] Cu2(tdm) [PCN-26] Cu2(btei) [PCN-61] Cu2(ntei) [PCN-66] Cu2(ptei) [PCN-68] Cu3(btc)2 [HKUST-1] Zn2(bdc)2dabco Ni2(dhtp) [NiMOF-74, CPO-27-Ni] Mg2(dhtp) [MgMOF-74, CPO-27-Mg] Mn2(dhtp) [MnMOF-74, CPO-27-Mn] Co2(dhtp) [CoMOF-74, CPO-27-Co] Zn2(dhtp) [ZnMOF-74, CPO-27-Zn] Cu3(bhb) [UTSA-20] Cu(1,4-ndc) Cu2(ebtc) Cd2(azpy)3(NO3)4 Cu2(Hbtb)2 Zn4O(btb) [MOF-177] Zn4O(bbc)2 [MOF-200] Zn4O(btb)4/3(ndc) [MOF-205] Zn4O(bte)4/3(bpdc) [MOF-210] [Fe3O(bdc)3][FeCl4] [MOF-235] Zn(bdc)(4,4 0 -bipy)0.5 [MOF-508b] Zn4O(bdc)(btb)4/3 [UMCM-1] Zn8(bhfp)33 [FMOF-2] Zn2(BPnDC)2(4,4 0 -bipy) [SNU-9] Mg(tcpbda) [SNU-25] Zn(mIm)2 [ZIF-8] Zn(Pur)2 [ZIF-20]

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1870 1100 1100 2693 308 1931 1753 1854 3000 4000 5109 1502 1448 1027 1332 1102 1056 885 1156 168 1852 600 4833 4530 4460 6240 974

824 795 (DR) 1264

4800 1590 1500 1900 4492 42 432 2442 2545 3500 4600 6033 2368 2104

2.3 0.59 0.56 1.1 2.15 0.12 0.11 0.91 0.87 0.84 1.36 1.63 2.13 0.82 0.75 0.54

0.63 322.5 2844 5403

946 4100 378 1030 800


1.96 3.59 2.16 3.6 0.93 2.141 0.366 0.368 0.51 0.27

30.4 30.4 30.4 30.4 36.5 36

298 298 298 298 298 300

3.6c 1.3c 3.9c 4.4c 9.4d 13.5b

71c 32c

35 35 50 60 30 10 25 35 1.07 35 35 35 35 75 49.7 (35) 58.3 (35) 35 35 35 35 20 1 36 25 100 80 (35) 80 (35) 80 (35) 10 4.5 25 30 65 1.01 30 1.01

298 298 303 303 303 303 298 290 298 298 298 298 304 298 298 298 298 298 298 298 298 273 298 298 298 298 298 298 298 303 298 298 298 298 298 273

8.8d 8.8d 10.7c 17.9c 2.2c 2.8d 14.0b 15.3b 1.7d 15.7b 15.1b 15.7b 10.2c 14.3c 11.9b 13.7b

155d 165d

2.2d 2.2d 2.8c 5.3d 22.0d 19.0b 20.5b 20.9b 3.2d 11.4d B2.2d 3.4b 0.7d B6.5d 1.2d

124d 110b

56c

12.2 10 17 17 19 18

171b 220b

27 14.6 30

145b 10b 99b 165c 202c 195b (190)b 169b (149)b 158b 174b 171b 178b

18.7 21.5–22

17.7

60c 311d (41)b (93)b (53)b 69.9d 52.6

b

6.5

223 268 268 268 269 270 17, 271 272 272 273 273, 274 275 276 277 19 79 27 27 27 274 274 214, 278 214, 278 214 214 214 238 279 280 281 282 283, 284 251 251 251 285 286, 287 288 289 290 291 292 293

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(continued ) Surface area m2 g1 a

BET


Compound

Zn(bIm)(nIm) [ZIF-68] Zn(cbIm)(nIm) [ZIF-69] Zn(Im)1.13(nIm)0.87 [ZIF-70] Zn(nbIm)(nIm) [ZIF-78] Zn(mbIm)(nIm) [ZIF-79] Zn(bbIm)(nIm) [ZIF-81] Zn(cnIm)(nIm) [ZIF-82] Zn2(bttb) Zn4O(fma)3 [H3O][Zn7(m3-OH)3(bbs)6] Zn2(2,6-ndc)2(dpni), solution synthesis Zn2(2,6-ndc)2(dpni), microwave synthesis Co3(2,4-pdc)2(m3-OH)2 [CUK-1] Zn3(OH)(p-cdc)2.5 Cu3(btc)2, extrudates Mn(2,6-ndc) Cu(tip) Cu2(tmbdi) [NOTT-107] Cu(Hoxonic)(4,4 0 -bpy)0.5 Cu(bdc-OH) Zn5(bta)6(tda)2 Zn4(OH)2(1,2,4-btc) Co3(ndc)(HCOO)3(m3-OH) Zr(bdc) [UiO-66] Cu2(bbcpm)

Lang.

Condition Methane uptake capacity Pore volume/ DHads/kj mol1 3 1 P/bar T/K wt%e v(STP)/vf (zero coverage) Ref. cm g

1090 950 1730 620 810 760 1300 1370 1120

1618 649

802 167 630 152 >2000 810 1822

191 1063

0.24 0.34 0.064 0.26 0.068 0.34 0.767

414 408 1386

584 607 607 1434

0.214 0.24 0.205 0.58

2010

2665

0.72

1.01 (35) 298 0.7d 1.01 298 0.9d 1.01 (35) 298 0.7d 1.01 298 1.0d 1.01 298 0.8d 1.10 298 0.8d 1.01 298 0.8d 17.5 298 5.2d 28 300 8.6d 1.01 273 0.9d 17.5 298 3.1b 17.5 298 3.9b 1.01 298 0.6d 0.5 298 0.1d 50 303 11.2d 1.01 273 1.3d 1.01 298 1.6d 35 298 25 273 0.8d 1.01 296 0.9d 1.01 295 0.7d 1.01 295 0.7d 1 298 1.1d 9.8 273 5.5d 8.03 295 6.1d

(150)d d

(150)

12

16.6

185b 18.5 19.4 15.7

294 294 294 294 294 294 294 295 296 297 298 298 299 300, 301 302 303 304 235 305 306 307 308 309 310 311

4,4 -bipy = 4,4 -bipyridine; pzdc = pyrazine-2,3-dicarboxylate; pyz = pyrazine; pia = N-(pyridin-4-yl)isonicotinamide; bdc2 = benzene1,4-dicarboxylate; pdc2 = pyrene-2,7-dicarboxylate; btc3 = benzene-1,3,5-tricarboxylate; ntc4 = naphthalene-1,4,5,8-tetracarboxylate; btec4 = benzene-1,2,4,5-tetracarboxylate; sbtc4 = trans-stilbene-3,3 0 ,5,5 0 -tetracarboxylate; adip4 = 5,5 0 -(9,10-anthracenyl)di-isophthalate; tdm8 = tetrakis[(3,5-dicarboxyphenyl)oxamethyl]methane; btei6 = 5,5 0 ,500 -benzene-1,3,5-triyltris(1-ethynyl-2-isophthalate); ntei6 = 5,5 0 ,500 -(4,4 0 ,400 nitrilotris(benzene-4,1-diyl)tris(ethyne-2,1-diyl))triisophthalate; ptei6 = 5,5 0 -((5 0 -(4-((3,5-dicarboxyphenyl)ethynyl)phenyl)-[1,1 0 :3 0 ,100 -terphenyl]4,400 -diyl)-bis(ethyne-2,1-diyl))diisophthalate; H2dhtp = 2,5-dihydroxyterphthalic acid; bhb6 = 3,3 0 ,300 ,5,5 0 ,500 -benzene-1,3,5-triyl-hexabenzoate; 1,4-ndc2 = naphthalene-1,4-dicarboxylate; ebtc4 = 1,1 0 -ethynebenzene-3,3 0 ,5,5 0 -tetracarboxylate; bbc3 = 4,4 0 ,4-[benzene-1,3,5-triyltris(benzene-4,1-diyl)]tribenzoate; btb3 = 4,4 0 ,4-benzene-1,3,5-triyl-tribenzoate; bpdc2 = biphenyl-4,4 0 -dicarboxylate; bhfp2 = 2,2-bis(4-carboxyphenyl)hexafluoropropane; BPnDC2 = benzophenone 4,4 0 -dicarboxylate; tcpbda2 = N,N,N 0 ,N 0 -tetrakis(4-carboxyphenyl)biphenyl-4,4 0 -diamine; mIm = 2-methylimidazolate; Pur = purinate; bIm = benzimidazolate; nIm = 2-nitroimidazolate; cbIm = 5-chlorobenzimidazolate; Im = imidazolate; nbIm = 5-nitrobenzimidazolate; mbIm = 5-methylbenzimidazolate; bbIm = 5-bromobenzimidazolate; cnIm = 4-cyanoimidazolate; bttb4 = 4,4 0 ,4,4-benzene-1,2,4,5-tetrayltetrabenzoate; 2,6-ndc2 = 2,6-naphthalenedicarboxylate; dpni = N,N 0 -di-(4-pyridyl)-1,4,5,8-naphthalene tetracarboxydiimide; 2,4-pdc2 = pyridine-2,4-dicarboxylate; p-cdc2 = 1,12-dicarba-closododecaborane-1,12-dicarboxylate; tip2 = 5-(1H-tetrazol-1-yl)isophthalate; tmbdi4 = 5,5 0 -(2,3,5,6-tetramethylbenzene-1,4-diyl)di-isophthalate; H3oxonic = 4,6-dihydroxy-1,3,5-triazine-2-carboxylic acid; bdc-OH2 = 2-hydroxyterephthalate; 1,2,4-btc2 = benzene-1,2,4-tricarboxylate; bta = 1,2,3-benzenetriazolate; tda2 = thiophene-2,5-dicarboxylate; bbcpm4 = 1,1-bis-[3,5-bis(carboxy) phenoxy]methane; azpy = 4,4 0 -azopyridine. b Excess uptake. c Total (absolute) uptake. d Not reported whether excess or total (absolute) uptake. e wt% = massCH4 adsorbed f masssorbent þmassCH adsorbed 100%. Reported volumetric uptake capacities assume crystallographic densities. a

0

0

2

4

accomplished through a solvent exchange process in which relatively high boiling point solvent is decanted from a freshly prepared sample and replaced by a solvent with lower boiling point, such as methanol or dichloromethane. Repeating this process permits the complete replacement of higher boiling point solvent coordinated to the metal center by the lower boiling solvent which may be removed more easily, generally under dynamic vacuum and slight heating. Alternative methods for removing these coordinated molecules include use of supercritical carbon dioxide, freeze-drying, and simple heating under vacuum. However, not all MOFs can withstand the stress placed on the framework due to removal of guest species from the pores and metal centers, particularly under elevated temperatures, and undergo structural collapse upon guest removal. However, for the MMOF-74 (M2(dhtp), M = Mg, Mn, Co, Ni, Zn, dhtp4 = 2,5-dihydroxy terephthalate) series of 7766

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MOFs, also referred to as the CPO-27-M series, neutron powder diffraction measurements led to the identification of UMCs as the primary methane binding sites, Fig. 2.116,214,231 Extension of this type of study to MOFs with a lower density of UMCs (HKUST-1, PCN-11, and PCN-14), and through a combination of neutron diffraction, grand canonical MonteCarlo (GCMC) simulations, and density functional theory (DFT) calculations, the methane sorption sites in these three MOFs were unambiguously identified.232 In this study it was identified that while the UMCs were the primary adsorption sites with the highest heats of adsorption, the amount of methane adsorbed onto these sites was not substantial. Instead, the ligand played the primary role in overall storage capacity of methane in the system. Furthermore, it was identified that accessible small cages and channels that exhibit enhanced van der Waals interactions due to potential overlap are favorable methane binding sites, Fig. 3. This study identified that surface area and ligand This journal is

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Fig. 3 Experimentally determined partial structure of the HKUST-1 crystal with CH4 molecules adsorbed at (a) the open Cu sites and (b) the small cage window sites (top and side views). (c) van der Waals (vdW) surface of the small octahedral cage in HKUST-1 (derived by using N2 as probe molecules), showing the size and geometry of the pore window in an excellent match with a methane molecule. (d) CH4 molecule adsorbed at the center of the small octahedral cage, a secondary adsorption site. (e) CH4 molecule located at the large cage corner site, also a weak adsorption site. Reprinted with permission from ref. 232. Copyright 2010 John Wiley and Sons.

Fig. 2 (a) Crystal unit cell of Mg2(dhtp) with methane adsorbed on site I (the open metal site), as determined from neutron diffraction. Note that each metal ion directly binds to one methane molecule. (b) Experimental Mg2(dhtp) structure with methane adsorbed on both sites I and II. (c) A close view of methane location and orientation with respect to the metal-oxide pyramids and the organic linkers. Reprinted with permission from ref. 214. Copyright 2009 American Chemical Society.

functionality were less important parameters to consider than the tuning of pore shapes and sizes. This, in combination with experimental results,19 signifies the role of larger organic components enhancing the methane capacity of porous MOFs. Some MOFs have shown high methane storage capacities, as exemplified in the following examples. These materials have set the trend for methane storage in MOFs and may be looked at for inspiration in designing high performing adsorbents. 3.1.2 MOF example 1: PCN-14. A theoretical MOF (IRMOF-993) based on 9,10-anthracene dicarboxylate and Zn4O SBUs was predicted to increase the methane isosteric heat of This journal is

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adsorption and achieve a storage capacity of 181 v/v.17 However, attempts to experimentally reproduce the theoretical measurements of IRMOF-993 led to a MOF (PCN-13) exhibiting selective adsorption of hydrogen and oxygen over nitrogen and carbon monoxide, but very limited methane uptake due to the confined pore space (B3.5 A˚).233 The variance between gas uptake properties and pore size from expected (6.3 6.3 A˚2, IRMOF-993) and that experimentally determined in PCN-13 was explained from observation of distortion of the Zn4O(COO)6 metal building units in the crystal structure. Building upon the design of IRMOF-993, the microporous material PCN-14 was synthesized from the self-assembly of 5,50 -(9,10-anthracenediyl)diisophthalate (adip4) and dicopper paddlewheel SBUs.19 The combination of fourconnected square planar metal SBUs and four-connected rectangular planar organic linkers led to the formation of a framework with nbo net topology. This topology has been identified in a number of other frameworks constructed from dimetal paddlewheel and tetratopic organic carboxylate SBUs.26,234 PCN-14 is composed of squashed cuboctahedral cages constructed from the connection of twelve adip4 ligands to six dicopper paddlewheels, Fig. 4, bringing the anthracenyl rings within close proximity to one another (2.6 A˚ between H atoms and center of phenyl rings on the adjacent anthracenyl group). This orientation of organic linkers within the framework system leads to enhanced interactions between guest methane molecules and the pore surfaces. Chem. Soc. Rev., 2012, 41, 7761–7779

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Fig. 4 (a) Squashed cuboctahedral cage; (b) nanoscopic cage with 18 vertices, 30 edges, and 20 faces. Color scheme: C, gray; Cu, turquoise; and O, red. Reprinted with permission from ref. 19. Copyright 2008 American Chemical Society.

The removal of coordinated solvent from the terminal sites of the dicopper paddlewheel leads to the formation of UMCs that are appropriately oriented toward the center of the cuboctahedral cages so as to maximize interactions with guest species. The combination of small pore diameter and available UMCs permits a high total uptake of methane (230 v/v) at 290 K and 35 bar. PCN-14 continues to be the record holder for volumetric methane uptake in a solid porous material, to the best of our knowledge. Since publication of PCN-14, computational studies have analyzed the methane uptake capacity of PCN-14 with a very recent report, based on GCMC simulations, predicting a lower storage capacity of 205 v/v under the same conditions as the experimental measurement.235 Since the field of MOF research is still very young, computational models must rely on fitting to empirical data to evaluate the efficacy of a model. It has been suggested that one force field may not be applicable across the entire range of framework materials; as such, another recent report proposed a new force field for simulating the adsorption of methane in PCN-14 that better fits the experimental data, as well as describes the adsorption behavior and observed adsorption site specificity in PCN-14.236 Based on the study conducted utilizing this new force field, it was found that no energy barrier is observed between strong and weak adsorption sites in PCN-14. Instead, at room temperature, a cooperative binding effect is observed in which UMCs attract incoming methane guests and direct them to nearby binding sites, such as locations with small pore diameter that maximize potential overlap between the pore surfaces and guest species, Fig. 5. Alternatively, at lower temperature, 150 K, the UMCs act as the primary binding site, matching well with experimental results from neutron diffraction studies.232,236 The results defining the UMCs in PCN-14 as weak methane adsorption sites corroborated those previously reported in a study of HKUST-1 (Cu3btc2).237 In the HKUST-1 study methane molecules were found to be uniformly dispersed throughout the unit cell with no specific adsorption sites. This is in stark contrast to the adsorption behavior of carbon monoxide in the same material, in which CO molecules were found to adsorb near the UMCs. This difference in adsorption sites arises from the fact that methane is an essentially spherical molecule (tetrahedral with no dipole moment, freely rotating at room temperature) that is very difficult to polarize, whereas CO is a polar molecule with lone pairs that can easily interact with UMCs. 7768

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Fig. 5 Probability distributions of the centers of mass of methane in PCN-14 at (a) 3500 kPa and 290 K and (b) 5 kPa and 150 K, viewed along the [2 1 1] crystallographic direction. The circles show the probability distributions of the centers of mass of methane molecules in the region where methane molecules are expected to be positioned to populate the open Cu site. Reprinted with permission from ref. 236. Copyright 2011 American Chemical Society.

3.1.3 MOF example 2: NOTT-107. Through the use of a high-throughput computational screening of potential MOFs for efficient methane sorbents, a MOF similar to PCN-14 (NOTT-107) was identified which had been previously synthesized, but the methane uptake capacity remained unstudied until recently.26,235 NOTT-107 is based upon 5,5 0 -(2,3,5,6-tetramethylbenzene-1,4-diyl)di-isophthalate and dicopper paddlewheel SBUs. The methyl substituents on the central phenyl ring of NOTT-107 play a role similar to that of the phenyl rings of the anthracenyl moiety that extend into the pores of the framework in PCN-14. In both systems these groups effectively reduce the pore diameter, in the case of NOTT-107, to 7.0 A˚.26 The methane capacity of NOTT-107 was calculated from GCMC simulations using the Universal Force Field.235 These calculations determined a potential storage capacity of 213 v/v at 298 K and 35 bar. The enhancement in methane uptake is believed to be similar to the reasoning behind that for PCN-14, in which the close contacts created from extension of the ligands into pores of the framework form ‘‘pockets’’ where methane can interact strongly with the framework. To compare their simulated results, a sample of NOTT-107 was prepared, activated, and tested for gas uptake properties. The experimental methane uptake was found to be lower than that of the simulated results (B196 v/v). This was attributed to a likely incomplete activation procedure and was corroborated when the experimental BET surface area (1770 m2 g1) was determined to be lower than what was calculated from simulations (2207 m2 g1). The experimental BET surface area determined from the first account of NOTT-107 was reported to be 1822 m2 g1.26 Fine tuning of synthesis, activation, and handling conditions to realize the full potential of MOFs is a recurring difficulty in the field. 3.1.4 MOF example 3: MMOF-74/CPO-27-M (M = Mg, Mn, Co, Ni, Zn). The MMOF-74 series of MOFs, with high density of UMCs, were evaluated as methane storage materials.214 It was calculated that the adsorption of one methane molecule per UMC could generate methane storage capacities in the range of 160–174 v/v. The isostructural series, with one-dimensional This journal is

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channels of approximately 13.6 A˚ diameters, were synthesized and specifically studied to elucidate the role of UMCs in methane uptake capabilities. Of the materials in the series, NiMOF-74 performed best as a methane adsorbent with an absolute uptake capacity of B200 v/v, Fig. 6. However, the high heats of adsorption on the UMCs, as determined from neutron diffraction studies, lead to high loading at low pressures and, therefore, retention of a large amount of methane at low pressures (B105 v/v at 5 bar).214 The dependence of the identity of the UMC on methane binding energy was discovered to be much weaker than that of hydrogen binding energies.239 This was attributed to the larger size and geometrical constraint of CH4 molecules, increasing the distance between a metal center and a methane molecule and, thereby, decreasing interaction potential. A potential setback involving the use of MOFs with high densities of UMCs is that, typically, only one methane molecule may interact with one UMC. Additionally, increasing the density of UMCs induces a significant increase in framework mass. This leads to realization of low gravimetric loading of methane within the framework. Since the ligand currently plays little role in the adsorption of methane within the MMOF-74 series, one method for enhancing uptake capacities

Fig. 6 (a) Excess CH4 adsorption isotherms of M2(dhtp) at 298 K. (b) The experimental Qst of Ni2(dhtp) and Zn2(dhtp) (the error bar is 5%). The Qst’s of Mg2(dhtp), Mn2(dhtp), and Co2(dhtp) fall between the two curves and, thus, are not shown for clarity. The Qst’s of MOF-5 (from ref. 14) are also plotted for comparison. Reprinted with permission from ref. 214. Copyright 2009 American Chemical Society.

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is to incorporate into the framework ligands with functional groups which may interact more strongly with methane, such as pendant lipophilic alkyl chains. 3.1.5 MOF example 4: UTSA-20. With the design principles in mind to (1) immobilize a high-density of UMCs and (2) construct suitable pore spaces within a MOF for methane to adsorb, the MOF UTSA-20 was formed from the combination of 3,3 0 ,300 ,5,5 0 ,500 -benzene-1,3,5-triyl-hexabenzoate (bhb6) and dicopper paddlewheel SBUs.238 Two different types of onedimensional channels are formed within the framework with UMCs exposed to the pores: rectangular pores with dimension 3.4 4.8 A˚2 and cylindrical pores with a diameter of 8.5 A˚. The BET surface area calculated from the N2 sorption isotherm at 77 K (1156 m2 g1) falls within the ‘‘moderate’’ surface area range that appears optimal for methane storage applications. Additionally, the presence of a high density of UMCs and small pores make UTSA-20 a very promising potential methane storage material. The methane storage density at 300 K and 35 bar in UTSA-20 was determined to be 0.222 g cm3 (178 v/v), nearly reaching the density of compressed methane at 300 K and 340 bar.238 The storage capacity at 150 K and 5 bar is equivalent to 89% of that of liquid methane and exhibits a high isosteric heat of adsorption of 17.7 kJ mol1 at zero coverage. Since it was discovered that full occupation of the UMCs by methane would only attribute to approximately half of the observed methane stored, GCMC calculations were utilized to identify the other binding sites within the framework. The pore spaces between bhb6 linkers within the channels were discovered to have short distances between adjacent linkers that would allow methane molecules to be ‘‘sandwiched’’ between two bhb6 potential surfaces, Fig. 7. The combination of UMCs and this second adsorption site was calculated to contribute to B90% of the experimental uptake at 298 K and 35 bar. The remaining B10% may be easily attributed to secondary binding sites which exhibit lower heats of adsorption. 3.1.6 MOF example 5: PCN-6 series. Generally, to synthesize high surface area MOFs it is necessary to increase the length of organic linkers.240–242 However, doing so tends to decrease the stability of the framework and many materials with extended linkers collapse upon guest removal. As a means to increase surface area and pore volume while maintaining structural integrity it was proposed that MOFs built upon in situ formed large coordination polyhedra (cages) accessible through small pore apertures could stabilize larger pore voids.243 Cuboctahedral coordination cages, constructed with 12 dimetal paddlewheel clusters and 24 isophthalate moieties, have been identified as very common structural units in MOF structures,244–250 and they serve as stabilizing units in the PCN-6 series due to the small pore space and generation of small pore apertures to larger cages.27 The design of extended hexatopic linkers with C3 symmetry combined with dicopper paddlewheel clusters led to the realization of four isoreticular MOFs with framework formula Cu3(L) (L = btei, PCN-61; ntei, PCN-66; ptei, PCN-68; ttei, PCN-610), Fig. 8. The PCN-6 MOFs exhibit impressive pore volumes and surface areas, with PCN-68 having a Langmuir surface Chem. Soc. Rev., 2012, 41, 7761–7779

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Fig. 7 (a) Probability distribution of the CH4 center of mass in UTSA-20 ([0 0 1] view), obtained from GCMC simulation at 298 K and 10 bar. The red regions represent the places where methane molecules are heavily populated in the MOF structure. Note that the open Cu site is preoccupied with CH4 molecules to focus our effort on the search for other strong methane adsorption sites. (b) The pore surface of the interconnected channel pores in UTSA-20 (derived using N2 as probe molecules, based on vdW interactions), with adsorbed methane at the linker channel site (derived from DFT-D calculations). The channel width along the c axis matches well with the size of the adsorbed methane molecules, leading to enhanced vdW interaction (methane molecules are shown in space-filling representation for clarity). Reprinted with permission from ref. 238. Copyright 2011 John Wiley and Sons.

area >6000 m2 g1, among the highest reported for MOFs.251 While PCN-610 is expected to have an even higher surface area than PCN-68, the organic linker is too long for the framework to be stabilized by the cuboctahedral cages and collapses upon guest solvent removal.

Fig. 8 (a) Nanoscopic ligands btei (PCN-61), ntei (PCN-66), ptei (PCN-68), and ttei (PCN-610); (b) (3,24)-connected network in PCN-68; (c) 3D polyhedra packing in PCN-68. Reprinted with permission from ref. 27. Copyright 2010 John Wiley and Sons.

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The methane uptake capacities of PCN-61, -66, and -68 were measured at 298 K and up to 100 bar, Fig. 9a.27 As expected, the gravimetric uptake at high pressures (>60 bar) is dominated by effects of surface area and pore volume, with PCN-68 showing the greatest uptake. However, in the low to medium pressure regime (o30 bar), PCN-61 exhibits greater uptake capacity than the two isoreticular MOFs with greater surface areas, Fig. 9b. This phenomenon was attributed to stronger methane affinity of the framework likely caused by small pore spaces that exhibit better CH4–framework potential overlap. Using the crystal density of each framework, the volumetric capacities of PCN-61, -66, and -68 were estimated to be 145 v/v, 110 v/v, and 99 v/v, respectively. This trend follows the variance in crystal density, as expected (0.56 g cm3 for PCN-61, 0.45 g cm3 for PCN-66, and 0.38 g cm3 for PCN-68). This previous study emphasized the necessity to concentrate not solely on increasing surface area of porous materials to achieve high volumetric methane uptake but to maintain a balance between porosity, density, pore size, surface area, and other factors. 3.2 Porous organic polymers (POPs) Concerns over stability and cost associated with the application of MOFs in methane adsorption have led to the evaluation of POPs as methane sorbents. Their tolerance of water and metal-free design make POPs very attractive options in applications. Furthermore, many POPs exhibit exceptionally high surface areas and low framework density, which make

Fig. 9 (a) Gravimetric and (b) volumetric capacities of CH4 adsorption in the PCN-6 series at 298 K. The inset in (a) shows the medium-pressure region enlarged. Reprinted with permission from ref. 27. Copyright 2010 John Wiley and Sons.

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them ideal in the gravimetric storage of gases.252–254 However, these same features pose a significant hurdle to overcome in terms of volumetric storage capacity. Due to the formation of strong covalent bonds, amorphous POPs often require optimization of synthesis conditions to maximize the surface area and porosity of the material. In this aspect, the resemblance to traditional organic polymers is very apparent. Unlike amorphous POPs, COFs are typically formed from dynamic covalent bonds (B–O bonds, for instance) to facilitate the formation of crystalline phases;255 however, the dynamic nature of the bonds leaves COFs significantly more susceptible to hydrolysis.256,257 Several POPs that exhibit high gravimetric methane storage capacities are presented below and methane uptake data collected in Table 2. Although experimental results still remain short of volumetric storage goals, promising approaches including surface modification and compression are discussed.

5–35 bar, with a total uptake of 206 v/v.259 This deliverable amount shows that COF-103-Eth-trans can store an amount of methane equivalent to 5.6 times that of methane in the pure gas phase at the same pressure. COF-102-Ant performs similarly, 215 v/v total uptake and 180 v/v deliverable, from 5–35 bar. However, at 300 bar COF-102-Ant has a deliverable volume of methane (258 v/v) less than that of pure methane (263 v/v), whereas MOF-177 is calculated to have a deliverable capacity of 336 v/v at the same pressure. To further enhance the methane capacity at readily applicable pressure, theoretical study indicates that Li+ cation doping of COFs can significantly strengthen the binding of methane to the materials because of London dispersion and induced dipole interactions between Li+ cation and methane molecules.260 At 298 K and relatively low pressures (o50 bar), the methane uptakes of Li+ doped COFs nearly doubled, compared to the corresponding non-doped frameworks. The total volumetric uptakes of methane in Li+ doped COF-102 and COF-103 reach 327 and 315 v/v, respectively, at 298 K and 35 bar. In addition, the Li+ doped COFs also exhibit ultrahigh excess methane uptakes, which is evidently originating from the strong affinity as a result of Li+ doping. The excess volumetric methane uptakes of Li+ doped COF-102 and COF-103 reach 303 and 290 v/v, respectively, at 298 K and 35 bar. It is concluded that functionalizing the building blocks of COFs with a Li atom or a Li+ cation could result in much stronger affinity to methane, therefore effectively improving the storage capabilities. The above-mentioned theoretical study sheds light on approaches to enhance methane storage at ambient temperatures and 35 bar. These results provide useful information on modification of not only COFs but also other porous materials for further improving experimental methane storage capacity.

3.2.1 Covalent organic frameworks (COFs). A combined computational–experimental study has revealed that covalent organic frameworks (COFs) show great promise as methane sorbents.258 COF-1 was identified as a material that can adsorb up to 195 v/v at 295 K and 30 bar, excess adsorption, based on computational measurements. Interestingly, upon increasing the pressure further, no increase in methane adsorption is observed. Because of the small pores in COF-1, the pores are quickly occupied by incoming guest molecules at low pressures. Higher pressures may not compact the gas any more than already observed in the framework, leading to lower efficiency than what is observed at 30 bar. The larger pore, three-dimensional COFs, COF-102 and COF-103, on the other hand, do not reach a plateau in methane adsorption until above 100 bar and exhibit methane working capacities of 230 and 234 v/v, respectively (Fig. 10; working capacity is defined as the volume adsorbed from 5 to 100 bar; volumetric capacities calculated from single crystal density). Furthermore, the functionalized COF-103-Eth-trans has been calculated to deliver up to 192 v/v over the range of Table 2

Surface area, pore volume, methane storage properties under specific conditions for porous organic polymers Surface area m2 g1

Compound

wt% =

Conditions

BET

Lang.

Pore volume cm3 g1

P/bar

T/K

Methane uptake capacity/wt%a

DHads/kj mol1 (zero coverage)

750 1670 750 1350 1760 3620 3530

970 1990 980 1400 2080 4650 4630

0.3 1.07 0.32 0.69 1.44 1.55 1.54

35 35 35 35 35 35 35

(85) (85) (85) (85) (85) (85) (85)

298 298 298 298 298 298 298

3.9 (4.3)b 8.2 (11.2)b 6.2 (6.5)b 8.0 (10.2)b 7.4 (11.1)b 15.8 (19.6)b 14.9 (18.7)b

17 8.5 19 12 8.5 8.6 9.5

1904 1307 963 1366

2992 2001 1210 2096

0.54 0.36 0.32 0.55

15 15 15 15

(20) (20) (20) (20)

298 298 298 298

6.6 5.9 4.9 6.5

1249 1764 2840 6461

827 2790 5323 10 063

0.45 1.26 1.7 3.04

35 35 35 35 (55)

295 295 295 295

7.6b 9.8b 12.2b 21.5 (28.0)b

Ref. 253

COFs COF-1 COF-5 COF-6 COF-8 COF-10 COF-102 COF-103 HCPs HCP-1 HCP-2 HCP-3 HCP-4 PPNs PPN-1 PPN-2 PPN-3 PPN-4 a

3.2.2 Hypercrosslinked polymer networks (HCPs). Methane sorption has been much less widely studied in noncrystalline organic polymers. However, the isosteric heats of adsorption for methane in most of these materials are around 15–20 kJ mol1 (Table 2), which is deemed as the appropriate range for methane

142 (7.7)c (6.7)c (5.3)c (6.9)c

20.8 25

massCH4 adsorbed masssorbent þmassCH4 adsorbed

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100%.

b

18.1 16.4 15.2

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Fig. 11 Methane uptake isotherms for porous organic polymers at 298 K and 35 bar. Data extracted from Table 2 (PPNs at 295 K, HCPs at 36 bar). Fig. 10 Predicted deliverable volumetric methane isotherms (the difference between the total amount at pressure p and that at 5 bar) at 298 K for COFs. Here the black dashed line indicates the uptake for free CH4 gas. MOF-177 uptake is added for comparison. Modified and reproduced with permission from ref. 258. Copyright 2010 American Chemical Society.

storage at close to ambient temperatures, as the optimal value of the heat of adsorption was calculated to be 18.8 kJ mol1 by Bhatia and Myers.261 Considering the relatively easy scale-up, organic polymers could have much to offer here. Prepared by Friedel–Crafts alkylation of bischloromethyl monomers such as dichloroxylene (DCX), and 4,40 -bis(chloromethyl)1,10-biphenyl (BCMBP), a series of HCPs were obtained either as precipitated powders or as monolithic blocks.142 These HCPs were shown to adsorb up to 5.2 mmol g1 (116 cm3 g1) of methane at 298 K and 20 bar, which is comparable with many crystalline porous systems. It is noteworthy to point out that, compared to MOFs, these polymers not only have high surface area (SBET: 1900 m2 g1 and SLangmuir: 3000 m2 g1), but also can be produced by simple and scalable steps. Most importantly, these materials usually have much higher physicochemical stability due to covalent bonding in the construction of the network. Isosteric heat of adsorption for methane on microporous polydichloroxylene (HCP–3(DCX(100)) was measured to be 20.8 kJ mol1 at low-loading, which is in good agreement with atomistic simulation results, the maximum value in the simulated distribution of methane–polymer interaction energy was found to be around 22 kJ mol1. 3.2.3 Porous polymer networks (PPNs). In addition to high affinity to methane, large surface area and high micropore volume are the other two most important factors that should be taken into consideration in terms of material design to improve methane uptake capacity. Fig. 11 shows the methane gravimetric uptake at 298 K and 35 bar for various POPs as a function of apparent BET surface area. In general, the amount of methane uptake in the materials increases with increasing surface area. To maximize surface area of organic polymers, 7772

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the two most important criteria are: (1) highly efficient polymerization reaction, such as Yamamoto or Eglinton homocoupling, which can help to form highly connected frameworks with sufficient molecular weight; and (2) rigid monomeric units with reaction sites oriented in different directions, such as tetrahedral monomers, which can help to form default diamondoid framework topology, thereby creating widely open and interconnected pores without the formation of ‘‘dead space’’. Based on these two criteria, the Zhou group successfully synthesized a series of PPNs with exceptionally high surface areas and microporous nature.136 Among them, PPN-4, synthesized from tetrakis(4-bromophenyl)silane through Yamamoto homo-coupling, exhibits a record high BET surface area of 6461 m2 g1, which is close to the predicted value based on the non-interpenetrated molecular model.25 PPN-4 can retain its structural integrity after being exposed to air for one month, which is indicated by virtually no drop of N2 uptake capacity at 77 K after simple reactivation by heating under vacuum. In terms of methane uptake capacity, PPN-4 can adsorb up to 17.1 mmol g1 at 295 K and 35 bar, which transcends all reported organic porous materials, to date. This gravimetric value is much higher than that of PCN-14 (total: 12 mmol g1 at 290 K and 35 bar), which is the current record holder in terms of volumetric methane uptake capacity. For a material with a density of 1.0 g cm3, a methane uptake of 7.9 mmol g1 would be required to reach a volumetric value of 180 v/v.131 As many organic polymers have very low densities, the amount adsorbed would need to be higher; hence, for the PPN-4 network with an approximate density of 0.2 g cm3, a molar uptake of 39.5 mmol g1 is required to reach 180 v/v. Based on this approximate density, the uptake of 17.1 mmol g1 corresponds to only 77 v/v. However, it is noteworthy to point out that, due to its exceptionally high thermal and chemical stability, the preliminary data indicate that PPN-4 could be compressed to half its size without any obvious loss of its porosity. Thus, the volumetric uptake capacity of methane could be increased several times by appropriate compression. This journal is

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In summary, organic polymers, similarly to MOFs, appear to have promise for methane storage. It is currently unclear, though, if these materials can compete on cost grounds with sorbents such as activated carbon. Notwithstanding cost concerns, we think it is necessary to point out that the cost cannot be calculated easily from lab-scale experiments, it would ultimately be linked to recyclability and lifetime, etc. when scaled up for applications.


4

Design principles for optimal methane sorbents

The majority of MOFs and POPs that have been reported in the literature are composed of micropores, with a limited number of materials containing mesopores (2 nm o pore diameter o 50 nm). Mesopores, however, are inefficient at adsorbing methane due to a decrease in the interactions between methane guest molecules and the pore surfaces. This leads to a characteristic more similar to an empty tank and does not significantly enhance the methane packing density. It should be noted, however, that the functionalization of mesoporous materials can serve to effectively reduce the pore size and increase methane affinity of the frameworks, whereas similar functionalization of microporous materials can potentially result in blockage of pores by the additional functional group. The adsorption of methane in porous sorbents occurs through weak dispersive forces (physisorption). The surface area of the sorbent tends to correlate with the quantity of observed physisorption; higher surface area materials typically exhibit greater gas uptake. However, at least in the case of methane sorption, the accessible surface area appears to play less of a decisive role in volumetric and gravimetric capacity.69 This trend is further emphasized in Table 1. Additionally, a number of studies have shown that a cooperative interplay between the accessible surface area, pore volume, isosteric heat of adsorption, and pore topology exists to determine the CH4 storage or deliverable capacity in porous sorbents.17,262,263 Unlike hydrogen, the interaction energy of methane with porous materials is already at a reasonable level. The problem currently facing the implementation of MOFs in ANG storage devices is the volumetric capacity of methane in such a system. Further developments are necessary to enhance the packing density of methane molecules in the porous sorbent. Additionally, it is not enough to only increase the excess or total uptake of methane in adsorbents; the working, or deliverable, quantity of methane must be enhanced. This is the volume of methane that will be released from the system during use. Obviously, in a fuel system the pressure will not reduce to a level below atmospheric pressure, and typical working pressures for internal combustion engines are around 5 bar. Therefore, for vehicular ANG technologies, the deliverable amount of methane may be referred to as the amount of methane adsorbed between 5 bar and the upper working limit of the system, preferably r35 bar, so as to not require heavy, thickwalled cylinders. To maximize the deliverable capacity of the sorbent, it is necessary to minimize the amount of methane stored at unusable pressures, i.e. below B5 bar. Therefore, a trade-off between total storage capacity and deliverable capacity is observed, whereby increased isosteric heat of adsorption may enhance total storage capacity but decrease deliverable capacity by increasing the amount of methane retained at low pressures. This journal is

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In MOFs, methane has been identified to preferentially adsorb in the tight fitting spaces created from close proximity of large ligands to one another, after occupying the UMCs. The enhancement in the density of UMCs and formation of pore spaces tailored to a size similar to that of the kinetic diameter of methane, or twice the kinetic diameter, through incorporation of large aromatic or methyl-substituted ligands should significantly increase the uptake capacity of future MOFs. From the previous section, it can be concluded that a good ligand candidate for methane storage purposes should have a large aromatic ring system or pendant lipophilic groups, so that ligands in the MOFs generated have higher affinity towards methane. Additionally, large surface area and high micropore volume should be considered. However, it has been observed that increasing the gravimetric surface area of MOFs is beneficial until a certain point is reached (B2500–3000 m2 g1), above this point leads only to loss of volumetric methane storage capabilities.235 Furthermore, increasing the pore volume too far beyond the size of two methane molecules is also detrimental to the storage capacity of MOFs. These larger pores are not effective at binding methane due to the large distance between pore surfaces, leading to a decrease in potential overlap between host and guest and fewer methane molecules interacting with pore surfaces. At higher pressure, such as 100 bar, the accessible volume within the pores of the framework becomes the most significant factor to methane uptake. This is within the region in which CH4–CH4 interactions become prevalent and CH4–pore wall interactions are less significant. The methane uptake enhancement through use of adsorbents in this region becomes less substantial and approaches equivalency with that of pure methane at the same pressure.258 The use of dendritic ligands which increase the density of UMCs and create small pores and cages may serve as a means for increasing the storage capacity of MOFs by enhancing framework–guest interactions. The alignment of UMCs through incorporation of isophthalate moieties paired with dicopper paddlewheel SBUs in a MOF has been shown to significantly enhance the binding affinity and storage of hydrogen by focusing the highly directional UMC–guest interactions to an optimal void space where guests can adsorb.264 However, the data investigating the role of UMCs in methane sorption are somewhat contradictory. On the one hand, UMCs have been shown to be the primary binding sites for methane in the MMOF-74 series, and contribute to a significant quantity of stored methane;214 whereas other studies have shown that UMCs are the primary binding site only in the low pressure/low temperature regime.232 Even if UMCs do not significantly enhance methane storage capacities at ambient temperatures, the isophthalate–paddlewheel combination typically forms microporous cages that create small spaces in which guest molecules may interact strongly with pore surfaces. Both of these enhancements together may produce materials with high methane adsorption capacity by maximizing UMC–methane interactions, as well as potential overlap between framework ligands and methane guest molecules. The crystalline property of MOFs makes them good models for molecular simulation studies of their methane uptake capacities. A recent computational study undertook a screening Chem. Soc. Rev., 2012, 41, 7761–7779

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process for identifying the potential of hypothetical MOFs to adsorb methane, with over 300 candidate MOFs discovered to be able to surpass 180 v/v.235 The expanded study simplified the process of predicting possible MOF structures and, therefore, allowed the analysis of more complicated systems than previous studies. Many of the high capacity materials identified through this computational screening process contained free methyl groups thought to be able to interact with incoming guest methane molecules, increasing the overall uptake capacity. Inspection of the nine top performing MOFs (>230 v/v capacity) from the Hypothetical Metal–Organic Frameworks Database generated by the Snurr lab235 reveals that MOFs with channels defined by large, planar aromatic surfaces and pore sizes centered around B4 or B7.75 A˚ should exhibit exceptional methane storage capacity at 298 K and 35 bar, both in terms of gravimetric and volumetric capacities. Considering the kinetic diameter of methane, 3.8 A˚, this pore size distribution intuitively makes sense. Targeting the synthesis of porous frameworks with high surface area and pores close to the size of one or two methane molecules is the most straightforward method for sustaining CH4–framework interactions upon increased loading. Additionally, by targeting pore sizes very near integer multiples of the kinetic diameter of a methane molecule minimizes the amount of unusable ‘‘dead’’ space within the pores. For instance, a pore size of 9.5 A˚ may accommodate 2.5 methane molecules, but since only whole molecules may be included, a large amount of free volume is left unoccupied. The increased distance between pore surfaces also leads to a reduction in CH4–CH4 interactions between adsorbed molecules. Furthermore, as has been observed in the studies of carbonaceous materials for methane storage, low packing densities significantly adversely affect the volumetric uptake capacity of porous materials. This remains a relatively unexplored topic in the study of advanced porous materials, but initial densification studies have shown decreased gravimetric uptake capacity due to reduction in micropore volume resulting from framework collapse. As such, additional investigations into engineering methods for enhancing the packing density of advanced porous materials while maintaining micropore volume are necessary.

5

Challenges, outlook, and conclusions

The greatest challenges facing the efficient storage of methane in porous materials involve designing materials that maintain effective interaction sites at higher pressures and increasing the packing density of materials so as to achieve high volumetric capacities. Currently, porous sorbents tend to strongly adsorb methane at low pressures (o1 bar), occupying a large portion of effective surface area and binding sites. As the pressure is further increased, CH4–CH4 interactions tend to dominate the storage process and, therefore, exhibit low isosteric heats of adsorption upon greater loading. To increase the magnitude of CH4–CH4 interactions, new strategies must be developed that can effectively pack methane within the pores of a sorbent. It can be imagined that two CH4 molecules strongly adsorbed on pore surfaces opposite of one another could have greater adsorption influence on a third CH4 molecule positioned between the two than randomly distributed gas molecules. 7774

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This influence should be enhanced even further by the ‘‘free’’ CH4 molecule interacting with a greater number of surfacebound CH4 molecules, emphasizing the potential of small cages to increase methane capacities. To this end, it is of exceptional importance to finely tune the pore shapes and sizes of porous sorbents. In particular, the application of ANG as a vehicular fuel storage system must address several additional factors. These include impurities in NG lines, which vary based upon regional location, and heat management during charging and discharging. Impurities can substantially deteriorate the adsorption capacity for methane in ANG systems upon multiple adsorption and desorption cycles due to retention of other gases.265 Water and carbon dioxide present the biggest challenges in terms of impurities in NG lines, due to high heats of adsorption for both species in porous materials, as well as longer chain alkanes, which can all block pores within the sorbents. AGLARG has previously addressed the issue of impurities on a prototype ANG storage tank through the use of a preadsorption system or ‘‘guard bed’’.266,267 Guard bed technologies are designed to serve as a filter to purify NG entering the fuel system by removing those impurities that exhibit stronger binding energies than methane toward the sorbent in the primary fuel tank. Guard beds could be composed of mixed-matrix systems so as to adsorb a maximum amount of impurities from the NG that can be released to the engine during discharge. To be effective, a guard bed must have a strong affinity toward the impurities, as they will be present in trace amounts with relatively low partial pressures, as well as high adsorption capacity. However, interaction energies that are too high will adversely affect desorption rates and require frequent regeneration or replacement of the guard bed. Therefore, diligent selection of porous materials for such a system would be required to meet all the necessary demands. Kinetics and heat exchange during charging and discharging are also of importance in the design of porous sorbents for vehicular ANG fuel storage. Adsorption is an exothermic process and, naturally, desorption is endothermic. Maintaining isothermic conditions or cooling during charging while heating when discharging is likely necessary to maximize deliverable capacity of the system. Additionally, insufficient thermal management during discharge may reduce the kinetics of gas diffusion through the porous sorbent, reducing the amount of fuel provided to the engine. Adsorbents should possess high heat capacity and thermal conductivity values. However, since sorbents with high capacities toward methane possess large inherent pore volumes, they will likely exhibit poor heat transfer characteristics.265 Currently, little data evaluating heat transfer properties of advanced porous materials have been reported, but is expected to become a more active research area. The suggested use of advanced porous materials for methane storage has received wide attention due to growing concerns over the energy environment and economy in the world. While use of porous materials in hydrogen storage devices is yet in the distant future to be fully realized, the storage of methane at relatively low pressure and ambient temperature in porous sorbents, specifically MOFs and POPs, is presently an achievable goal. Several advanced porous This journal is

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materials that exhibit exceptional storage potential have been synthesized and evaluated. Ongoing research involving computational screening of both hypothetical and previously reported materials should play an important role in the determination of potential systems to be experimentally validated and studied. Experimental evaluation of the impacts of pendant lipophilic groups in MOFs on the binding of methane is necessary and presents a hopeful step forward in the design of porous materials for the adsorption of methane. The implementation of materials that can efficiently store methane for personal use in vehicular or in-home fuel systems is near the cusp of being realized. At this stage in the development of methane storage technologies it is necessary for computational and experimental chemists, chemical engineers, and materials scientists to work together to harness the full potential of porous adsorbents.

Acknowledgements This work was supported by the U.S. Department of Energy (DOE DE-SC0001015, DE-FC36-07GO17033, and DE-AR0000073), the National Science Foundation (NSF CBET-0930079), and the Welch Foundation (A-1725).

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