Filling Pore Space in a Microporous Coordination Polymer to Improve

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Filling Pore Space in a Microporous Coordination Polymer to Improve Methane Storage Performance Ly Dieu Tran, Jeremy Ian Feldblyum, Antek G. Wong-Foy, and Adam Jay Matzger Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504607c • Publication Date (Web): 26 Jan 2015 Downloaded from http://pubs.acs.org on February 3, 2015

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Filling Pore Space in a Microporous Coordination Polymer to Improve Methane Storage Performance Ly D. Tran,† Jeremy I. Feldblyum, †Antek G. Wong-Foy, †and Adam J. Matzger*†‡ †

Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, USA. ‡

Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, USA

A strategy that allows the tuning of pore size in microporous coordination polymers (MCPs) through modification of their organic linkers is presented. When large substituents are introduced onto the linker, these pendent groups partially occupy the pores thus reducing pore size while serving as additional adsorption sites for gasses. The approach takes advantage of the fact that, for methane storage materials, small pores (0.4–0.8 nm in diameter) are more desirable than large pores since small pores promote optimal volumetric capacity. This method was demonstrated with IRMOF-8, a MCP constructed from Zn4O metal clusters and 2,6naphthalenedicarboxylate (NDC) linkers. The NDC was functionalized through the addition of substituents including tbutylethynyl or phenylethynyl groups. High pressure methane uptake demonstrates that the IRMOF-8 derivatives have significantly better performance than the unfunctionalized material in terms of both excess volumetric uptake and deliverable capacity. Moreover, IRMOF-8 derivatives also give rise to stronger interactions with methane molecules as shown by higher heat of adsorption values.

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INTRODUCTION Natural gas, predominantly composed of methane, is an affordable, readily available, and relatively clean fuel compared to gasoline. Thus, natural gas is considered to be an attractive transportation fuel. As compared to dihydrogen, natural gas is much easier to store, but still presents some considerable challenges. Due to the low critical temperature of methane (Tc = 191 K), natural gas is typically stored in a cryogenic tank in the form of liquefied natural gas (LNG) at about 113 K or in a pressure vessel as compressed natural gas (CNG) at approximately 250 bar.1 Both of these methods require the use of expensive and heavy equipment to withstand extreme physical conditions, and consequently, prevent natural gas from being widely used in the automotive industry.2 Adsorbed natural gas (ANG), on the other hand, promises to allow storage of high-density methane at ambient temperature and reduced pressures compared to CNG. Therefore, finding suitable sorbents that operate with acceptable capacities over a modest pressure regime are needed to facilitate a more sustainable transportation infrastructure based on natural gas. The utilization of microporous coordination polymers (MCPs) as adsorbents for methane storage has been extensively investigated within the last decade.3-6 Significant accomplishments include the development of PCN-14, a material with one of the highest recorded methane uptakes7-8 that compares favorably to the Department of Energy (DOE) original target of 180 (cm3/cm3) at 35 bar, RT.9 In contrast to hydrogen storage targets, where the DOE has been lowering expectations, the DOE recently revised the methane storage target to 0.5 g/g for gravimetric capacity and 350 (cm3/cm3) for volumetric capacity.6,8,10,11 So far, there are no known materials that can meet both of these targets. It is, therefore, necessary to search for either new MCPs with optimal capacity for methane or a strategy to maximize the capacity of existing


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materials. Studies of methane adsorption on MCPs using single crystal X-ray diffraction,12 neutron powder diffraction,13 as well as computational simulations14-19 have provided a deep understanding of the geometric features of MCPs and their influence on methane uptake capacities. From previous studies, it was concluded that pore size,13,16-20 surface area,16,17 crystal density,18 and functional groups on the linkers17,21 are critical factors in determining capacity. Moreover, coordinatively unsaturated metal sites are known to assist the adsorption of methane into MCPs albeit with the potential to reduce deliverable capacity.8,22 Nonetheless, the effect of adjusting pore size for optimizing methane uptake capacity of MCPs still has not been thoroughly explored. In fact, open space in MCPs is the key to their sorptive properties but can also limit particular aspects of their performance. If pores are too large they can offer only weak attractive potential for gases. Furthermore, large-pore materials can possess residual void space after sorption, leading to poor volumetric sorption capacity.23 Several attempts have been made to efficiently utilize those empty pore spaces such as constructing cage-within-cage networks,24 modifying the inner wall of cage C in rht-type MOFs,25 or using functionalized linkers.26,27 So far these strategies have not delivered significant improvements in methane storage capacities. Here, we demonstrate a strategy to improve the sorption capacity of existing MCP motifs by partially filling pore space to increase the affinity of the framework towards methane. IRMOF-8 is a cubic MCP derived from commercially available components and it has recently been obtained with high surface area (4400 m2/g) using a combination of RT synthesis and flowing supercritical CO2 activation.28-30 The prospect of pore filling of IRMOF-8 during methane adsorption has been thoroughly examined in the pressure range from 0 to 90 bar using positron annihilation lifetime spectroscopy (PALS).23 Even at the highest pressure examined, only 50–65% of the pore was filled by adsorbed methane. Moreover, monolayer coverage of


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methane is estimated to take up 60% of the pore space. These combined data suggest that only a monolayer of methane forms in IRMOF-8 at uptake saturation. We hypothesized that the uptake of IRMOF-8 would be improved if additional sorption sites were introduced to occupy some of the empty pore space unoccupied by adsorbed methane. Ideally, these additional sorption sites would be guest molecules positioned at the center of the pore; however, such a material is unattainable due to the mobility of guest molecules and also their propensity to adsorb to the framework walls, thereby competing with methane during the adsorption process. These problems can be overcome by employing rigid moieties bound to the organic linkers that extend into the pore center. To test this notion, IRMOF-8 was chosen as a prototype material with pores that maintain considerable residual porosity after high pressure methane loading. Unlike terephthalic acid, the ditopic linker used to make MOF-5, naphthalene-2,6-dicarboxylic acid (H2NDC) derivatives are rare. Fortunately the regioselective bromination of the dimethyl ester of H2NDC could be accomplished by reaction with bromine at RT. The aryl bromide is a useful intermediate for the formation of H2NDC derivatives through cross-coupling reactions. Ethynylation was successfully carried out, thereby providing a platform to produce IRMOF-8 analogs

by

partial

or

complete

substitution

of

NDC

with

4-tbutylethynyl-2,6-

naphthalenedicarboxylate (tBuNDC) or 4-phenylethynyl-2,6-napthalenedicarboxylate (PhNDC) (Figure 1a). The tbutylethynyl or phenylethynyl functional groups should occupy part of the pore and serve as additional adsorption sites for methane (Figure 1b). Moreover, because of the rigid nature of these ligands extension into the pores should be favored over folding back onto the naphthalene rings, thus avoiding conformations that would create adsorbate-inaccessible regions. A series of IRMOF-8 derivatives using linker mixtures of NDC and these new linkers were prepared, characterized, and methane uptake capacities of these materials were studied. The


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isosteric heat of adsorption for IRMOF-8 and other functionalized IRMOF-8s were evaluated: 100%tbutylethynyl-IRMOF-8 (1), 50%tbutylethynyl-IRMOF-8 (2), and 39%phenylethynylIRMOF-8 (3) (where the % denotes the molar ratio of NDC derivatives in the final products).

Figure 1 (a) Chemical structures of linkers: H2NDC, H2tBuNDC, and H2PhNDC (b) Models of IRMOF-8, 50%tbutylethynyl-IRMOF-8 (2), composed of a mixture of NDC and tBuNDC linkers in a 1:1 ratio, and 39%phenylethynyl-IRMOF-8 (3), composed of a mixture of 61% NDC and 39% PhNDC linkers. EXPERIMENTAL SECTION Materials All reagents were obtained from commercial vendors and used as-received unless otherwise noted. N,N-Diethylformamide (DEF) was purified by storing over activated carbon for a minimum of 1 month and subsequently passed through a silica gel column before use. To obtain Zn(NO3)2·4H2O, powdered Zn(NO3)2·6H2O was subjected to reduced pressure (∼20 mTorr) for 48 h. The water content was assessed by thermogravimetric analysis (TGA). IRMOF8-RT was prepared and activated as previously described.28


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Synthesis of Zn4O[(NDC)0.61(PhNDC)0.39]3 (MCP 3) A mixture of 4-phenylethynyl-2,6naphthalenedicarboxylic acid (118.9 mg, 0.376 mmol), naphthalene-2,6-dicarboxylic acid (82.3 mg, 0.381 mmol) and Zn(NO3)2·4H2O (0.640 g, 2.44 mmol) was dissolved in purified DEF (30 mL) by sonication (20-30 minutes). The resulting solution was filtered and distributed equally into six 20 mL-scintillation vials. The vials were placed in the oven at 85 C for 24 hours, after which time, light yellow transparent crystals were formed. The reaction mixtures were allowed to cool to room temperature followed by rinsing with fresh DMF (3 times over 1 day) and subsequently with CH2Cl2 (4 times over 3 days). The solvent was removed under high vacuum and the obtained materials were stored a N2 glove box until analysis. The yield of the dry, activated material is 25%, based on H2NDC. Anal. calcd for Zn4O[(C12H6O4)0.61(C20H10O4)0.39]3: C, 52.53; H, 2.20. Found C, 52.49; H, 2.23. Gas sorption measurement N2 sorption experiments were carried out using an Autosorb 1C (Quantachrome Instruments, Boynton Beach, Florida, USA). N2 (99.999% purity) was purchased from Cryogenic Gases and used as received. Activated sample (20 mg) was charged into a glass sample cell in a N2-filled glove box and subsequently transferred to the sorption apparatus for measurement at 77 K. Excess CH4 sorption isotherms were collected on an HPVA-100 highpressure analyzer (VTI Corporation) within the temperature range 266 K – 313 K using 99.99% purity CH4 (Air Products). Activated sample (∼ 300 mg) was charged into a stainless steel sample cell in a N2-filled glovebox and transferred to the sorption apparatus for analysis. The void volume of the cell was determined by He expansion from the dosing manifold into the sample cell. Compression factors of the gases (ZHe, ZCH4) both in the dosing manifold and in the sample cell were determined using the NIST Reference Fluid Thermodynamic and Transport Properties (REFPROP version 7.0) incorporated into the HPVA-100 software. CH4 isotherms


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were then constructed from adsorption data collected from 0 to 60 bar and from desorption data collected from 56 to 5 bar, unless otherwise noted. RESULTS AND DISCUSSION IRMOF-8 was prepared following the roomtemperature procedure28 because this method provides high surface area and non-interpenetrated IRMOF-8 (hereafter denoted as IRMOF-8RT); however this method requires a long incubation time of approximately one week. The solvothermal method yields the product within 24 hours but contains significant amounts of an interpenetrated phase resulting in a low surface area (~1600 m2/g).28 We reasoned that the bulkiness of additional substituents borne by the new linkers would suppress catenation during MCP formation and therefore allow the synthesis of high surface area IRMOF-8 analogues under high temperature solvothermal conditions.31,32 With this in mind, a series of tbutylethynylIRMOF-8 derivatives were prepared by solvothermal reaction at 85 C using mixtures of different ratios of H2NDC and H2tBuNDC linkers. After activation by flowing supercritical CO2,29 BET surface areas of these MCPs were measured and found to be uniformly high (> 3200 m2/g) (Figure S1). Among all of the MCPs evaluated in the series, MCP 2 containing a mixture of tBuNDC and NDC linkers in a 1:1 ratio had the highest surface area (39073958 m2/g). MCP 1, obtained by complete substitution of NDC by tBuNDC, contains 100% functionalized linker and also has a high surface area (31293346 m2/g). Similarly, a series of phenylethynyl-IRMOF8s with different amounts of H2PhNDC in the linker mixtures were also prepared and their surface areas were studied (Figure S2). MCP 3 with 39% incorporated PhNDC is the material with the highest surface area (38474095 m2/g). However, activation of the 100%phenylethynylIRMOF-8 failed to achieve the theoretical surface area (experimental value of 1270 m2/g as compared with 2700 m2/g computed from a hypothetical model) (Table S2).33 In fact, the


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activated framework collapsed after guest removal as suggested by PXRD analysis (see Figure S7). The fact that all modified IRMOF-8s in this study possessed high surface areas suggested that the non-interpenetrated frameworks were achieved. This is confirmed by comparing obtained PXRD data to the simulated PXRD pattern of non-interpenetrated IRMOF-8 (Figure S6). Moreover, high surface areas of MCP 2 and 3 also suggest that they are not physical mixtures of IRMOF-8 and MCP 1 or 100%phenylethynyl-IRMOF-8. To further validate this point, individual single crystals of MCP 2 and 3 were digested and subjected to 1H NMR analysis (See SI). Results show the presence of linker mixtures in the single crystal and in fact confirm the phase purity of these MCPs. Among those MCPs prepared, 1, 2 and 3 were chosen to study methane adsorption due to their relatively high BET surface areas. Table 1 Chemical formula, surface area, crystal density, and pore volume of IRMOF-8-RT, 1, 2, and 3

Material

Formulaa

IRMOF-8-RT Zn4O(NDC)3

Crystal Pore BET SAb densityc volumed 2 (m /g) (g/cm3) (cm3/g) 4326

0.448

1.827

1

Zn4O(tBuNDC)3

3205

0.566

1.315

2

Zn4O(NDC)1.5 (tBuNDC)1.5

3934

0.507

1.547

3

Zn4O(NDC)1.8 (PhNDC)1.2

3991

0.506

1.603

a

Obtained from 1H NMR of digested materials; b evaluated from N2 isotherms, the BET value shown is the average number obtained from activated samples (see SI); c crystallographic densities computed from models of single crystal structure. For MCPs 1, 2, and 3 the models were built based on non-interpenetrated IRMOF-8’s single crystal structure because PXRD patterns of MCPs 1, 2, and 3 are perfectly matched with the pattern of non-interpenetrated IRMOF-8 (Figure S6). d Evaluated from N2 isotherm at P/P0 = 0.91. Other properties of IRMOF-8-RT, 1, 2, and 3 such as crystal density and pore volume are provided in Table 1. When compared to IRMOF-8-RT, the new MCPs 1, 2, and 3 present lower


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BET surface areas, smaller pore volumes, and higher crystal densities. These data are fully consistent with partial occupation of the pores by additional substituents. The high pressure methane adsorption of IRMOF-8-RT, 1, 2, and 3 were studied over a wide range of temperatures and are shown in Figure 2.34 IRMOF-8-RT has a methane uptake of 114 cm3/cm3 at 58 bar and RT, and no saturation is observed within the examined pressure range, even at the lowest studied temperature (284 K). Complete substitution of NDC by tBuNDC results in the significant change of the excess uptake isotherm (Figure 2b). Compared to IRMOF8-RT, 1 can adsorb more methane at low pressure. The methane isotherms of 1 increase rapidly in the range of 0–40 bar and saturation occurs at around 50 bar (for 278 K and 266 K). The difference in the two isotherms suggests that 1 has a higher affinity toward methane than IRMOF-8-RT, consistent with our predictions. 5


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160

160

3

140

140

3

3 3

Excess volumetric uptake (cm /cm )

180


180

120

120

100

100

80 60 40 308 K 298 K 284 K

20 0 0

10

20

30

40

50

80 60 40

313 K 295 K 278 K 266 K

20 0

60

0

Pressure (Bar)

10

20

30

40

50

60

Pressure (Bar)

(a) IRMOF-8-RT

(b) MCP 1 180

160

160

3


180

3


140

140

3

3

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120

120

100

100

80 60 40 313K 295K 278K 266K

20 0 0

10

20

30

40

50

60

80 60 40

313K 295K 278K 266K

20 0 0

10

20

Pressure (Bar)

(c) MCP 2

30

40

50

60

Pressure (Bar)

(d) MCP 3

Figure 2 High pressure volumetric excess uptake of (a) IRMOF-8-RT, (b) MCP 1, (c) MCP 2, and (d) MCP 3. Despite the fact that 1 can adsorb more methane at low pressure than can IRMOF-8-RT, its volumetric methane uptake capacity was not significantly enhanced, especially at high pressure (Figure 3a). At 295 K and 60 bar, 1 has an excess volumetric uptake of 120 cm3/cm3, 5% higher than the uptake of IRMOF-8-RT under similar conditions. The presence of the tbutylethynyl groups increases affinity towards methane but potentially blocks the pores due to the high population and bulkiness of the tbutyl groups. A higher uptake might be achieved by lowering the concentration of the functionalized linker which would tune the free volume in the pores.


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Thus the methane uptake experiments were performed on 2 and 3; materials wherein NDC derivatives were used in conjunction with pure NDC to form mixed-linker materials.32,35,36 MCPs 2 and 3 showed significant improvement in excess volumetric uptake (Figure 2c and 2d) as compared with IRMOF-8-RT. As depicted in Figure 3a, material 2 has an uptake capacity of 108 cm3/cm3 at 35 bar and 295 K, which is 25% higher than the amount adsorbed by IRMOF-8RT under similar conditions. At 60 bar, 2 can achieve an uptake of 132 cm3/cm3, 16% higher than the uptake of IRMOF-8-RT. MCP 3 also has considerably higher methane uptake compared with IRMOF-8-RT. In particular, methane capacities of 107 cm3/cm3 (24% improvement) and 134 cm3/cm3 (17% improvement) were obtained at 35 bar and 60 bar (295 K) respectively. In order to understand whether or not the rigidity of the substituents plays a significant role in enhancing methane uptake capacity, MCP 4 was prepared using 100% of H2neohexylNDC linker (Figure 3b). Direct comparison between uptakes of 1 and 4 is shown in Figure 3b. The linker of 4 bears a more flexible substituent as compared to that of MCP 1; such a flexible alkyl chain can potentially fold back onto the naphthalene ring and lead to the inaccessibility of pore space. However, experimental measurement indicates that 4 has a slightly higher volumetric uptake than MCP 1. This observation can be explained by the fact that the neohexyl group in 4 is a saturated aliphatic group and thus can create favorable interactions with the chemically similar methane molecules. Such behavior of a flexible37 pendant group shows that the rigidity of the substituents is not strictly required and thus opens up the possibility of incorporating various functional groups into the linkers capable of acting as tethered solvents for the adsorbed gas.


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140

3


140

3

120

3

3

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Langmuir

100 80 60 40 IRMOF-8-RT 1 2 3

20 0 0

10

20

30

40

Pressure (Bar)

50

60

1 4

120 100 80 60 40 20

H2neohexylNDC

0 0

10

20

30

40

Pressure (Bar)

(a)

50

60

(b)

Figure 3 (a) Excess volumetric uptake of IRMOF-8-RT, MCP 1, MCP 2, and MCP 3 at RT. (b) Excess volumetric uptake of MCP 1 and MCP 4 at RT; right bottom corner: chemical structure of linker for MCP 4. Surface area and heat of adsorption are two important factors that determine the uptake properties of porous materials. A smaller pore size, which typically corresponds to lower surface area, also results in higher affinity toward adsorbents due to overlapping attractive potentials from the pore walls. In fact, for methane, the optimal pore size for maximum volumetric capacity is estimated to be either 0.4 nm or 0.8 nm, sufficient for the formation of one or two monolayers of adsorbed gas.17,19 Surface area, on the other hand, has a more profound impact on the uptake at high pressures. The fact that 2 and 3 have lower surface areas than IRMOF-8-RT (Table 1) but give almost the same gravimetric uptake (Table S5 and S6) and higher volumetric uptake (Figure 3) suggests that these MCPs interact more efficiently with methane. To better understand this observation, the heats of adsorption were measured at three temperatures for IRMOF-8-RT, 1, 2, 3, and 4 using both the virial method (Figure S13) and the single-site Langmuir model. Results from both methods suggest the same trend, in which the heat of adsorption (Langmuir Qst) of 1 (13 kJ/mol)  4 (12.6 kJ/mol) > 3 (11.6 kJ/mol)  2 (11.2 kJ/mol) > IRMOF-8-RT (9.4 kJ/mol). These values of Qst are in good agreement with the reported value for MOFs constructed form


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Zn4O secondary building units (SBUs).38 In fact, these Qst measurements validate the notion that functionalized IRMOF-8s having substituents occupying part of the pores, possess stronger affinity toward methane molecules than the parent MCP. Moreover, Ar pore size distributions of MCP 1, 2, and 3 were also evaluated and compared with that reported for IRMOF-8-RT.28 These MCPs indeed have a higher population of smaller pores (8-15 Å) than IRMOF-8-RT (Figure S.19-S.21) which correlates to their higher volumetric uptake and higher heat of adsorption values. MCP 1 has the highest proportion of small pores and the strongest interaction with methane; consequently, its volumetric uptake at low pressures is greater than those of the other MCPs. Yet, the surface area of 1 is significantly smaller than those of MCP 2 and 3; this results in an overall lower uptake capacity for MCP 1. For the application of sorbents in ANG technology, total uptake is more relevant than excess uptake to quantify the methane storage capacity. The total uptakes (ntot) of the MCPs were evaluated from measured excess uptake (nex) and pore volume (Vp) (Table 2, see SI). At RT and up to 60 bar, 1, 2, 3, and 4 have higher volumetric capacity than IRMOF-8-RT. At 60 bar, 3 can adsorb up to 183 cm3/cm3, which is 13% higher than the total uptake of IRMOF-8 under similar conditions. Up to 80 bar (extrapolated), 3 can reach the capacity of 207 cm3/cm3; this value is comparable to the uptake of MOF-205 (205 cm3/cm3) and MOF-177 (205 cm3/cm3) under similar conditions.39 Deliverable capacity is calculated with the assumption that operating pressure is in the range of 5 to 35 bar. MCP 3 has the highest deliverable capacity of 112 cm3/cm3, which is 16% higher than that of IRMOF-8 (96 cm3/cm3). When compared to other MCPs constructed from Zn4O SBUs (Table S8), 3 is among those MCPs that have the highest deliverable capacity. Additionally, the density of adsorbed methane at 60 bar (Table 2) describes effectiveness of packing methane inside the pore. All of the modified IRMOF8s have higher


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densities of adsorbed methane than the parent, particularly, the methane inside the pores of 4 is 24% more dense than in IRMOF-8-RT. Taking into consideration that the density of CNG at 250 bar is in the range of 0.14–0.18 g/cm3,1 these studied MCPs can achieve the same value under a fourfold decrease in pressure. These findings, together with other data demonstrated above, confirm that using mixtures of NDC with PhNDC or tBuNDC that the methane uptake capacity of IRMOF-8 can be greatly enhanced. Table 2 Total uptake, deliverable capacity and density of adsorbed methane of materials studied MCPs

IRMOF-8-RT

1

2

3

4

Total uptake at 35 bar RT (cm3/cm3)

114

132

135

136

136


162

166

180

183

172


199a

182a

204a

207a

191

Deliverable capacityb (cm3/cm3)

96

105

111

112

108

Adsorbed methane density (g/cm3)c

0.14

0.16

0.16

0.16

0.17

a

Calculated with a single-site Langmuir model. b Calculated from 5 bar to 35 bar. c At 60 bar, RT. Extremely high gravimetric uptake normally comes at the expense of volumetric uptake for a physisorptive material.40 High volumetric capacity materials usually have low gravimetric capacities and vice versa. For example, HKUST-1, PCN-14 and UTSA-20 are known for their high volumetric capacities; however, their gravimetric uptake values are relatively low compared with other MCPs such as IRMOF-8-RT, MOF-5, and MOF-205 (Figure 4).10,39,41 For ANG vehicular application, volumetric capacity is important because it relates to the driving range of natural gas vehicles for a given tank size. However, gravimetric capacity also plays an essential


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role since it determines the mass of adsorbent required to achieve a certain capacity. In fact, the U.S. DOE recently set up a target for gravimetric uptake of methane storage materials. MOF-200 and MOF-210, the MCPs with the highest gravimetric uptake so far, have surface areas of 4530 and 6240 m2/g respectively, significantly higher than that of MOF-5, yet the density of adsorbed methane on these MCPs are about 30% lower than that of MOF-5 at 35 bar and 298 K.39,42 Furthermore, their volumetric capacities at RT, up to 60 bar, are lower than those of functionalized IRMOF-8s reported in this publication. A strategy that helps increase the volumetric capacity of these MCPs without losing their high gravimetric uptake, therefore, would be highly desirable. It can be seen in Figure 4 that 2 and 3 have similar gravimetric uptakes as IRMOF-8-RT but their volumetric uptakes are significantly higher. Thus, we have demonstrated a strategy for increasing of volumetric capacity without scarifying gravimetric capacity in the pressure range relevant for storage of methane prepared by single stage compression.43 280

MOF‐519

260

3

3

Volumetric uptake (cm /cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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240 220

HKUST‐1

UTSA‐76

PCN‐14 UTSA‐20

MOF‐5

200

NU‐111

180 160

2

3 MOF‐205

1 IRMOF‐8

140

MOF‐210

120

MOF‐200 100 150

200

250

300

350

400

Gravimetric uptake (mg/g)


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Figure 4. Total gravimetric vs. volumetric uptakes of selected MCPs at RT and 60 bar. Data were obtained from the isotherm of HKUST-1,10 PCN-14,10 MOF-5,10 or interpolated from the isotherm of UTSA-20,8 NU-111,8 MOF-200,39 MOF-205,39 MOF-210,39 UTSA-76,3 MOF-519.44 CONCLUSION MCPs possess high surface areas and large pore volumes; however, the pore volume is not always used efficiently for methane adsorption. Partially occupying pore space with pendant groups on linkers to create additional adsorption sites can strengthen adsorbate-adsorbent interaction and improve overall methane sorption characteristics. Using this strategy, the volumetric uptake of IRMOF-8 was significantly improved without compromising its gravimetric capacity. Notably, at 35 bar and RT, 39% phenylethynyl-IRMOF-8 can adsorb 24% more methane than the parent MCP in term of excess volumetric capacity. The implication of this strategy for other high gravimetric uptake materials can potentially produce MCPs that meet both of the U.S. Department of Energy targets for gravimetric and volumetric methane uptake. ASSOCIATED CONTENT Supporting Information. Powder X-ray diffraction, 1H NMR spectra, gas sorption isotherms and disclaimer. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (A.J.M). Notes


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Table of Contents Graphic


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Filling Pore Space in a Microporous Coordination Polymer to Improve

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