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Gas adsorption properties of highly porous metal–organic frameworks containing functionalized naphthalene dicarboxylate linkers† Jaeung Sim,a Haneul Yim,a Nakeun Ko,a Sang Beom Choi,a Youjin Oh,a Hye Jeong Park,*a SangYoun Park*b and Jaheon Kim*a Three functionalized metal–organic frameworks (MOFs), MOF-205-NH2, MOF-205-NO2, and MOF-205OBn, formulated as Zn4O(BTB)4/3(L), where BTB is benzene-1,3,5-tribenzoate and L is 1-aminonaphthalene-3,7-dicarboxylate (NDC-NH2), 1-nitronaphthalene-3,7-dicarboxylate (NDC-NO2) or 1,5-dibenzyloxy-2,6-naphthalenedicarboxylate (NDC-(OBn)2), were synthesized and their gas (H2, CO2, or CH4) adsorption properties were compared to those of the un-functionalized, parent MOF-205. Ordered structural models for MOF-205 and its derivatives were built based on the crystal structures and were subsequently used for predicting porosity properties. Although the Brunauer–Emmett–Teller (BET) surface areas of the three MOF-205 derivatives were reduced (MOF-205, 4460; MOF-205-NH2, 4330; MOF-205NO2, 3980; MOF-205-OBn, 3470 m2 g−1), all three derivatives were shown to have enhanced H2 adsorption capacities at 77 K and CO2 uptakes at 253, 273, and 298 K respectively at 1 bar in comparison with MOF-205. The results indicate the following trend in H2 adsorption: MOF-205 < MOF-205-NO2 < MOF-205-NH2 < MOF-205-OBn. MOF-205-OBn showed good ideal adsorbed solution theory (IAST)
Received 28th July 2014, Accepted 2nd October 2014
selectivity values of 6.5 for CO2/N2 (15/85 in v/v) and 2.7 for CO2/CH4 (50/50 in v/v) at 298 K. Despite the
DOI: 10.1039/c4dt02300e
large reduction (−22%) in the surface area, MOF-205-OBn displayed comparable total volumetric CO2 (at 48 bar) and CH4 (at 35 bar) storage capacities with those of MOF-205 at 298 K: MOF-205-OBn,
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305 (CO2) and 112 (CH4) cm3 cm−3, and for MOF-205, 307 (CO2) and 120 (CH4) cm3 cm−3, respectively.
Introduction Metal–organic frameworks (MOFs) are crystalline porous materials with various pore sizes and environments that have been extensively investigated for the storage and separation of H2,1a,b CO2,1c–f CH4,1g–i and hydrocarbons.1j,k Well-established reports indicate that, in general, gravimetric gas storage capacity at high pressures is linearly related to the Brunauer–Emmett– Teller (BET) surface areas and pore volumes, whereas the gas separation ability of an MOF is dependent on the strength of the intermolecular interactions between the MOF surfaces and gas molecules.1 Thus, control and design of the structure and porosity in MOFs is a crucial consideration for gas storage and separa Department of Chemistry, Soongsil University, 369 Sangdo-Ro, Dongjak-Gu, Seoul 156-743, Republic of Korea. E-mail: [email protected], [email protected] b School of Systems Biomedical Science, Soongsil University, 369 Sangdo-Ro, Dongjak-Gu, Seoul 156-743, Republic of Korea. E-mail: [email protected] † Electronic supplementary information (ESI) available: NMR spectra, TGA data, electrostatic potential maps, PXRD patterns, structure modelling, additional gas sorption data, and single crystal X-ray data. CCDC 1015820, 1015823, 1015821, and 1015822 for H2NDC-NH2, H2NDC-NO2, MOF-205-NH2, and MOF-205-NO2, respectively. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt02300e
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ation applications. It is noted that the surface area and gas adsorption affinity are not independent factors, but rather must be considered together when designing a structure for efficient gas storage and separation. That is, as MOFs exhibiting large surface areas also have sufficient pore spaces, their framework surfaces can be further functionalized to increase interactions with particular gas molecules. This strategy is realized by (i) constructing isoreticular MOFs by linking pre-functionalized organic linkers with metal ions,2 or (ii) post-synthetic modification methods that allow extrinsic functional groups to be introduced to as-prepared MOFs through simple organic reactions.3 Since the emission of CO2 from industrial coal-fired power plants causes an unfavourable greenhouse effect, the efficient capture and short-term storage of CO2 using MOFs has received much attention.1c–f The majority of research focused on reaching this goal has been devoted to increasing Coulombic interactions between framework surfaces and the large quadrupole moment of CO2.4 In this regard, many MOFs were functionalized with –NO2,5 –NH2,6 –CONH–,7 –OH,8 –CN,9 –SO3H,10 or pyridine11 in order to enhance CO2 adsorption. Theoretical calculations alone or in combination with experiments address this interaction, at the molecular level, between CO2 and the various functionalities incorporated on the organic linkers used to make MOFs.12
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In more detail, functional groups such as –CH3, –OH, –NO2, –NH2, and –COOH on the organic linkers can increase interactions with CO2 molecules, and CO2 can also interact with various functional groups on aromatic linkers such as π electrons/H atoms in benzene or the lone pairs/H atoms/partial charges of functionalized linkers. In a certain case, electron-withdrawing groups destabilize π electron⋯quadrupole moment interactions, which is indeed unfavourable for CO2 adsorption.12d,e Although many of the functionalized MOFs mentioned above show enhanced CO2 adsorption behaviours, the increased gas uptake capacities at ambient temperature are not as large when compared to those of amine-modified MOFs, which can form carbamate or carbamic acid through chemisorption processes with CO2.13 It is evident that intermolecular interactions alone may not be sufficient for increasing CO2 adsorption affinity in MOFs. In fact, it is notable that many of the functionalized MOFs showing enhanced gas adsorption behaviours are microporous materials. Within the small pores of these MOFs, enhanced gas adsorption may also be attributed to intermolecular potentials that are a result of reduced pore spaces as observed in catenated MOFs.14 We were interested in the sole contribution of functional groups in MOFs, which motivated us to construct a series of highly porous MOFs that have well-separated functionalized linkers in order to eliminate concerns regarding pore size effects. A suitable model system is MOF-205 (aka DUT-6) formulated as Zn4O(BTB)4/3(NDC) (BTB = benzene-1,3,5-benzoate; NDC = naphthalene-2,6-dicarboxylate) that is a three-dimensional framework with ith-d topology containing 2.3 nm dodecahedron-like cages.15 Herein, we report the gas adsorption properties at 1 bar and higher pressures for the highly porous MOF-205 and its derivatives: MOF-205-NH2, MOF-205-NO2, and MOF-205-OBn. The three derivative MOFs have been synthesized by replacing NDC in MOF-205 with NDC-NH2, NDC-NO2, and NDC-(OBn)2 (-OBn = benzyloxy), respectively (Scheme 1).
Scheme 1 Reactions for MOF-205 and its derivatives are schematically drawn with their building blocks.
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Experimental General procedures All chemicals and solvents used in the syntheses of this series were of reagent grade and used without further purification. Zinc(II) nitrate hexahydrate (Zn(NO3)2·6H2O), ethyl alcohol (EtOH), and N,N-dimethylformamide (DMF) were purchased from Daejung Chemicals & Metals Co., Ltd. H3BTB was prepared according to a reported literature method.16 Powder X-ray diffraction (PXRD) data were collected on a Rigaku MiniFlex diffractometer with CuKα radiation (λ = 1.5418 Å). Thermogravimetric analyses (TGA) were carried out using a Scinco TGA-S1000 thermal analysis system under air with a temperature-increasing rate of 5 °C min−1. Fourier transform infrared spectra (FT-IR) of samples prepared as KBr pellets were measured using a JASCO FT/IR-4000 spectrophotometer. NMR spectra were obtained on a Bruker 400 MHz NMR spectrometer. Elemental analyses were carried out on an EA 1110 (CE instrument) or a Perkin-Elmer 2400 Series II CHN analyser. Synthetic procedures 1-Nitronaphthalene-3,7-dicarboxylic acid (H2NDC-NO2). H2NDC (6.00 g, 27.8 mmol) was dissolved in conc. H2SO4 (600 mL) and 10% HNO3 (23.2 mL, 1.4 equiv.) was slowly added to the reaction mixture. After stirring at room temperature for 30 min, the solution was cooled to 0 °C and ice was added to form white precipitates. The crude product was washed with water, filtered, and dried under vacuum. The recrystallization from acetic acid affords pure white solids (yield, 56%). 1H-NMR (400 MHz, DMSO-d6): δ = 9.09 (1H, s), 9.08 (1H, s), 8.73 (1H, s), 8.50 (1H, d), 8.23 ppm (1H, d); 13 C-NMR (400 MHz, DMSO-d6): δ = 166 (s, COOH), 165 (s, COOH), 146, 136, 135, 132, 133, 130, 129, 127, 125, 124, 123 ppm (Fig. S1†). 1-Aminonaphthalene-3,7-dicarboxylic acid (H2NDC-NH2). To a solution of 1-nitronaphthalene-3,6-dicarboxylic acid (2.00 g, 8.65 mmol) in methanol (537 mL), 10% Pd/C (0.537 g) was slowly added under an N2 atmosphere. The reaction mixture was stirred under an H2 atmosphere (1 atm) for 1 day to form deep green precipitates. After filtration through Celite, the filtrate was evaporated to obtain yellow solids as a product. For a second crop, the filtered deep green precipitates were dissolved in aqueous solution of NaOH (1.0 M, 400 mL). After filtration, the filtrate was acidified with conc. HCl to form yellow solids. The product was again filtered, washed with water and dried (yield, 83%). 1H-NMR (DMSO-d6): δ = 8.77 (1H, s), 7.95 (1H, d), 7.90 (1H, d), 7.79 (1H, s), 7.25 (1H, s), 6.26 ppm (2H, s); 13C-NMR (DMSO-d6): δ = 168 (s, COOH), 167 (s, COOH), 147, 135, 131, 129, 127, 125, 123, 117 ppm (Fig. S2†). 1,5-Dihydroxy-2,6-dibromonaphthalene.17 A mixture of 1,5dihydroxy naphthalene (16 g, 0.10 mmol) and a catalytic amount of iodine was added to acetic acid (440 mL). The mixture was heated to 80 °C, and then bromine (10.5 mL, 0.20 mol) was slowly added. After stirring for 30 min, the reaction mixture was cooled to room temperature. The crude product was filtered and washed with petroleum ether (yield,
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57%). 1H-NMR (CDCl3): δ 7.7 (d, 2H, J = 8.4 Hz), 7.5 (d, 2H, J = 8.8 Hz), 6.0 (s, 2H) ppm. 1,5-Dibenzoxy-2,6-dibromonaphthalene. A solution of 1,5dihydroxy-2,6-dibromonaphthalene (4.65 g, 14.1 mmol) in DMF (75 mL) was cooled to 0 °C under an Ar atmosphere. After stirring for 30 min, sodium hydroxide (1.29 g, 31.0 mmol) was added to the reaction mixture. After 1 h, benzyl bromide (3.83 mL, 31.0 mmol) was added, and the ice bath was removed to allow the mixture temperature to slowly increase to room temperature. The crude product was washed with water and extracted with ethyl acetate. The extracted solution was dried over MgSO4, filtered, and evaporated to obtain brown solids (yield, 93%). 1H-NMR (DMSO-d6): δ 7.83 (d, 2H, J = 8.8 Hz), 7.80 (d, 2H, J = 9.2 Hz), 7.62 (d, 4H, J = 8.0 Hz), 7.44 (m, 6H), and 5.05 (s, 4H) ppm. 1,5-Dibenzoxynaphthalene-2,6-dicarboxylic acid diethyl ester. A solution of 1,5-dibenzoxy-2,6-dibromonaphthalene (1.69 g, 3.39 mmol) in tetrahydrofuran (338 mL) was cooled to −78 °C under an Ar atmosphere. After stirring for 1 h, n-butyllithium (4.46 mL, 7.12 mmol) was added to the reaction mixture. After 1 h, ethyl chloroformate (3.23 mL, 3.92 mmol) was added, and the dry ice bath was removed to allow the mixture temperature to slowly increase to room temperature. The crude product was washed with Na2CO3 aqueous solution and extracted with ethyl acetate. The ethyl acetate solution was dried over MgSO4, filtered and evaporated to give yellow solids (yield, 57%). 1H-NMR (DMSO-d6): δ 8.02 (d, 2H, J = 8.4 Hz), 7.87 (d, 2H, J = 8.8 Hz), 7.56 (d, 4H, J = 6.8 Hz), 7.43 (m, 6H), 5.14 (s, 4H), 4.36 (q, 4H, J = 7.0 Hz), and 1.30 (t, 6H, J = 7.0 Hz) ppm. 1,5-Dibenzyloxy-2,6-naphthalenedicarboxylic acid (H2NDC(OBn)2). 1,5-Dibenzoxynaphthalene-2,6-dicarboxylic acid diethyl ester (1.00 g, 2.06 mmol) was dissolved in a mixture of tetrahydrofuran (40 mL), ethanol (40 mL), and 1.0 M NaOH aqueous solution (10 mL). After refluxing the reaction mixture at 80 °C for 1 d, the solution was evaporated, and then water was added. The aqueous solution was acidified with conc. HCl to form the precipitates, which were filtered, washed with water and dried under vacuum (yield, 96%). 1H-NMR (DMSOd6): δ 12.61 (s, br, 2H), 7.98 (d, 2H, J = 8.0 Hz), 7.85 (d, 2H, J = 8.0 Hz), 7.59 (d, 4H, J = 7.2 Hz), 7.44 (m, 6H), and 5.15 (s, 4H) ppm (Fig. S3†). MOF-205-NH2. Zn(NO3)2·6H2O (0.425 g, 1.42 mmol), H3BTB (0.128 g, 0.292 mmol), and H2NDC-NH2 (0.112 g, 0.484 mmol) were dissolved in a mixture of DMF (30 mL) and EtOH (3 mL). The clear solution was placed in a 30 mL vial and heated at 85 °C for 2 days to form brown polyhedron crystals. The mother liquor was decanted, and the crystals were washed with DMF (5 × 10 mL) and stored in DMF. Yield: 67% based on 1 mole of H3BTB. Elemental analysis for an evacuated sample, calcd/found (%) for Zn4O(BTB)4/3(NDC-NH2): C, 53.00/52.15; H, 2.50/2.64; N, 1.29/1.26. Calcd/found (%) for Zn4O(BTB)4/3(NDC-NH2)·(H2O): C, 52.16/52.15; H, 2.64/2.64; N, 1.27/1.26. MOF-205-NO2. Zn(NO3)2·6H2O (0.336 g, 1.13 mmol), H3BTB (0.0700 g, 0.160 mmol), and H2NDC-NO2 (0.126 g, 0.583 mmol) were dissolved in DMF (30 mL). The clear solu-
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tion was placed in a 30 mL vial and heated at 105 °C for 2 days to form brown polyhedron crystals. The mother liquor was decanted, and the crystals were washed with DMF (5 × 10 mL) and stored in DMF. Yield: 48% based on 1 mole of H3BTB. Elemental analysis for an evacuated sample, calcd/found (%) for Zn4O(BTB)4/3(NDC-NO2): C, 51.57/50.61; H, 2.25/2.42; N, 1.26/1.23. Calcd/found (%) for Zn4O(BTB)4/3(NDC-NO2)·(H2O): C, 50.78/ 50.61; H, 2.40/2.42; N, 1.23/1.23. MOF-205-OBn. Zn(NO3)2·6H2O (42.5 mg, 0.140 mmol), H3BTB (12.8 mg, 0.0290 mmol), and H2NDC-(OBn)2 (20.8 mg, 0.0490 mmol) were dissolved in a mixture of DMF–EtOH (3.0/ 0.3 mL). The solution was placed in a 4 mL vial that was tightly capped and then heated in an isothermal oven at 85 °C for 48 h to give orange-coloured truncated octahedron crystals. The crystals were filtered, washed with DMF (5 × 10 mL), and stored in DMF. Yield: 68% based on 1 mole of H3BTB. Elemental analysis for an evacuated sample, calcd/found (%) for Zn4O(BTB)4/3(NDC-(OBn)2): C, 57.96/56.60; H, 2.98/2.78; N, 0.00/ 0.00. Calcd/found (%) for Zn4O(BTB)4/3(NDC-(OBn)2)·(H2O)4/3: C, 56.91/56.60; H, 3.13/2.78; N, 0.00/0.00. X-ray crystallography The X-ray intensity data sets were collected on a Bruker APEX CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) for H2NDC-NH2, or were collected with a synchrotron light source (λ = 0.70000 Å) on an ADSC Quantum210 detector at 2D SMC with a silicon (111) double crystal monochromator (DCM) at the Pohang Accelerator Laboratory (PAL), Korea for H2NDC-NO2, MOF-205-NH2, and MOF-205NO2. Initial structures were obtained by direct methods using SHELXS-97 and refined by subsequent full-matrix least-squares refinements on F2 using SHELXL-97.18 Non-H atoms were refined anisotropically. H atoms were located with ideal geometry and refined with a riding model. Both MOF-205-NH2 and MOF-205-NO2 have similar unit-cell parameters and belong to the same cubic space group (Pm3ˉ n) as MOF-205. The functionalized NDCs were disordered at the crystallographic special positions with mmm site symmetry while BTBs were at the special positions with 32 site symmetry. The crystal and refinement data for the two linkers and two MOFs are summarized in Table S1 to S4.† The crystal structures are displayed in Fig. S6 to S9.† Structural modelling and porosity calculations As the MOFs in this work suffered from structural disorder, their ordered model structures were built by aid of Materials Studio 6.1.0TM (Accelrys Software Inc.) (section 4 in ESI†).19 The ordered model structures are displayed in Fig. 1. Using these models, their van der Waals, accessible solvent surface areas (SAvdW, SAacc), and pore volumes (Vvdw) were calculated using a probe radius of 1.82 Å by the “Atom Volumes & Surfaces” utility in the Materials Studio (Table 1). For a fair comparison of the porosity with the same volume, the unit cell parameters of all MOFs were changed to those of MOF-205 with a = 30.353 Å. The pore volumes were also calculated using PLATON with a “CALC SOLV probe 1.82” command.20
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Fig. 1 Unit-cell packing diagrams for the ordered model structures of (a) MOF-205, (b) MOF-205-NH2, (c) MOF-205-NO2, and (d) MOF-205-OBn are displayed with Zn atoms as polyhedra, and C, N, and O atoms as balls. Hydrogen atoms are not shown for simplicity.
Table 1
Calculated and measured porosity properties of the ordered MOF-205 and its derivativesa
MOF
f.w.
dcalc. (cm3 g−1)
SAvdW (m2 g−1)
SAacc (m2 g−1)
VvdW (cm3 cm−3)
VPLATON (cm3 cm−3)
SABET (m2 g−1)
SALangmuir (m2 g−1)
VP (cm3 cm−3)
MOF-205 MOF-205-NH2 MOF-205-NO2 MOF-205-OBn
1072.17 1087.19 1117.17 1284.40
0.382 0.387 0.398 0.458
4220 4230 4190 4530
4640 4680 4660 3770
0.850 0.847 0.845 0.808
0.817 0.814 0.811 0.747
4460 4330 3980 3470
6270 6120 5670 4810
0.83 0.83 0.79 0.78
a
The unit cell volumes of all MOFs were adjusted to V = 27 964 Å3, that of MOF-205 with a = 30.353 Å. Acronyms: SAacc, SABET, SALangmuir are the accessible, BET, and Langmuir surface areas; VvdW, VPLATON, VP are the van der Waals, PLATON, and measured pore volumes. Calculated values using Materials Studio 6.1.0TM (Accelrys Software Inc.), except VPLATON. A probe radius of 1.82 Å, appropriate for the N2 molecule. Six formula units in each unit cell (Z = 6).
Results and discussion Synthesis As observed in UMCM family, resulting framework connectivities of ‘mixed-linker’ MOFs are dependent on the length of the linear linker or the arrangement of organic linkers around a Zn4O cluster.21a,b MOF-205 is also a mixed-linker MOF where NDC linkers are located at the trans-positions and BTB linkers are at the four equatorial positions around a Zn4O cluster.15 For the successful preparation of an isoreticular MOF-205 series, two unfavourable possibilities must be avoided through the adjustment of reaction conditions; (i) the formation of byproducts that contain only one type of organic linker such as a derivative of IRMOF-8 (Zn4O(NDC-R)3) or MOF-177 (Zn4O(BTB)2) in our case, and (ii) a topologically different framework formed through a different arrangement of the two linkers around the Zn4O vertices. In order to prevent the formation of different types of frameworks, the structurally more rigid NDC was selected for functionalization, and the reaction conditions for MOF-205 were slightly changed for the preparation of MOF-205 derivatives using the functionalized NDC linkers. Due to the polar nature of H2NDC-NH2 and H2NDC-NO2, which show dipole moment values of 1.70 and 4.54 Debye by DFT calculations, respectively (Fig. S10†), the resulting frameworks would exert favourable electrostatic interactions with CO2. Due to the molecular symmetry, the linker does not have a net electric dipole moment. However, the bulky and flexible benzyloxy groups were anticipated to contribute to the
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enhanced interaction with CO2 via various benzene⋯CO2 interactions.12 H2NDC-NO2 was synthesized from a nitration reaction of H2NDC in an aqueous H2SO4/HNO3 solution, and H2NDC-NH2 was obtained by reduction of H2NDC-NO2 with H2 in the presence of Pd/C. The crystal structures of the linkers showed that the –NH2 or –NO2 functional group was introduced on the corrected position of the naphthalene moiety (Fig. S6 and S7†). The dihedral angle between the –NO2 and NDC planes was 37.6(1)° in the H2NDC-NO2. From 1,5-dihydroxy-2,6-dibromonaphthalene as a starting compound, H2NDC-(OBn)2 was prepared via four synthetic steps. The linker crystallized as thin needles, which were not suitable for single crystal X-ray diffraction analysis. MOF-205-NH2, MOF-205-NO2, and MOF-205-OBn were prepared relatively easily by modifying the solvothermal reaction conditions for MOF-205.15b For example, pure MOF-205-OBn was obtained by employing the same reaction conditions as for MOF-205, but at a slightly lower temperature. The framework formulas, Zn4O(BTB)4/3(NDC-R), for all members of this series were identified by elemental analysis. Calculated porosity Using the ordered structural models, the porosity properties of the MOFs were calculated and are listed in Table 1. The introduction of the –NH2 or –NO2 group did not lead to large changes in the calculated surface areas (SAvdW and SAacc). This is expected from the general formula of the MOF-205 structures, Zn4O(BTB)4/3(NDC-R), which demonstrates that the
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contribution of the functional groups, R = –NH2 or –NO2 to the framework volume is small in comparison with BTB. Therefore, the densities and calculated pore volumes (VvdW and VPLATON) for MOF-205-NH2 and MOF-205-NO2 are very similar to those of MOF-205. In contrast, MOF-205-OBn exhibits significantly increased SAvdW (+7%) and reduced SAacc (−19%) values compared to those of MOF-205. This implies that the –OBn groups contribute more in increasing the framework molecular surface and contribute less to the potential adsorption sites. A more clear effect on the porosity by the functional groups can be shown when the pore volumes in cm3 cm−3 unit are compared. That is, as four MOFs have almost same unit cell volumes, the calculated pore volume is a good indicator for the porosity change. In detail, the void volume fractions (Vvdw and VPLATON) of MOF-205-OBn are smaller by 5 and 9%, respectively, than those of MOF-205; Vvdw is greater than VPLATON by 4–10%. Therefore, the –OBn groups are expected to change the porosity and gas adsorption properties of the unfunctionalized MOF-205 more noticeably than will the MOF-205 derivatives with –NH2 and –NO2 groups. Indeed, the calculations indicate that MOF-205-OBn would have a much smaller surface area than MOF-205 and the other derivatives based on the SAacc values; SAacc is the simulated surface accessible by N2, thus estimating a measured BET surface area (SABET). Measured porosity The N2 gas sorption isotherms for the three functionalized MOFs showed reversible adsorption/desorption traces with the steps at P/P0 = ∼0.05, similar to that of MOF-205 (Fig. 2). This indicates that the functional groups are not bulky enough to diminish the mesoporous character of MOF-205. The BET surface areas of MOF-205, MOF-205-NH2, MOF-205-NO2, and MOF-205-OBn were 4460, 4330, 3980, and 3470 m2 g−1, respectively (Table 1). The reduction in the BET surface areas of the MOF-205 derivatives is in accordance with the prediction observed in the calculated values. In particular, MOF-205OBn exhibits a significantly reduced BET (Langmuir) surface area, which is 78% (77%) of the surface area of MOF-205. This matches well to prediction made by the SAacc calculations
(Table 1). The pore volume was also decreased by 6%, a value that is also in good agreement with the calculations. This observation also supports the fact that the ordered structural model of MOF-205-OBn is a plausible representation of the crystal structure. H2 and CO2 adsorption at 1 bar The H2 gas sorption isotherms at 77 K and 1 bar showed that MOF-205, MOF-205-NH2, MOF-205-NO2 and MOF-205-OBn adsorb 55, 59, 58, and 68 cm3 cm−3 of H2, respectively (Fig. S12† and Table 2). Considering the reduction of surface areas for the MOF-205 derivatives, it is certain that the introduced functional groups contribute to the enhanced H2 uptake. More compelling evidence was provided from the CO2 gas sorption isotherms at 1 bar (Fig. 3 and Table 2). MOF-205 adsorbed 20.0, 11.0, and 6.3 cm3 cm−3 of CO2 at 253, 273, and 298 K, respectively. Under the same temperature and pressure conditions, the volumetric capacities of the MOF-205 derivatives increased to 22.3, 12.8, and 7.2 cm3 cm−3 for MOF-205NH2, 22.0, 12.3, and 7.2 cm3 cm−3 for MOF-205-NO2, and 29.7, 20.7, and 9.5 cm3 cm−3 for MOF-205-OBn, respectively. It is clear that the volumetric capacities of gas adsorption are directly related to the introduction of functional groups, since the unit cell volumes of all the MOFs are very similar to each other. This is evidenced by the following trend in increasing gas uptake in relation to functional groups: MOF-205 < MOF-205-NO2 < MOF-205-NH2 < MOF-205-OBn. It is noted that the electron-donating –NH2 group seems more favorable than the electron-withdrawing –NO2 for enhancing gas adsorption; the electron-withdrawing group may destabilize the contribution from a π⋯quadrupole interaction that arises between the functionalized NDC with CO2 as in the benzene case.12 The larger gas uptake by MOF-205-OBn may be understood when considering related cases, MIL-47(VIV)-OCH3,8a and multivariate (MTV) MOF-5s containing the propoxy (–OC3H5) or benzyloxy (–OC7H7) groups.22 Similar to the BDC derivatives, the benzyloxy-functionalized NDC, NDC-(OBn)2 appears to be responsible for the large enhancement in CO2 adsorption in MOF-205-OBn. In contrast to expectation, the CO2 adsorption enthalpies were almost the same when calculated using a virial-type equation: at zero-coverage 16.1, 15.9, 16.2, and 16.5 kJ mol−1 for MOF-205, MOF-205-NH2, MOF-205-NO2, and MOF-205OBn, respectively (Fig. S13†). This result implies that the increased gas adsorption capacity may be simply attributed to Table 2 Volumetric H2 and CO2 adsorption capacity in cm3 cm−3 of MOF-205 and its derivatives at 1 bar
Fig. 2 N2 adsorption isotherms of MOF-205, MOF-205-NO2, and MOF-205-OBn measured at 77 K.
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MOF-205-NH2,
H2
CO2
MOF
at 77 K
at 253 K
at 273 K
at 298 K
Qst/kJ mol−1
MOF-205 MOF-205-NH2 MOF-205-NO2 MOF-205-OBn
55 59 58 68
20.0 22.3 22.0 30.6
11.0 12.8 12.3 17.5
6.3 7.2 7.2 9.5
16.1 15.9 16.2 16.5
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Fig. 4 Comparison of (a) CO2 and N2, and (b) CO2 and CH4 adsorption isotherms at 298 K of MOF-205 and MOF-205-OBn. The IAST selectivities for (c) CO2/N2 (15/85 in v/v) and (d) CO2/CH4 (50/50 in v/v) mixtures, respectively.
Fig. 3 CO2 adsorption isotherms of MOF-205, MOF-205-NH2, MOF-205-NO2, and MOF-205-OBn measured at (a) 253, (b) 273, and (c) 298 K, respectively.
an increase in van der Waals areas, rather than from the electronic nature of each functional group. In other words, the increased gas uptake cannot be entirely related to the enhancement in the intermolecular interactions of the functionalized framework surfaces with CO2. In fact, the primary adsorption sites in the MOFs having Zn4O(CO2)6 units are the “cup sites” or near the carboxylate groups, which are particularly responsible for the gas adsorption at room temperature and low pressure.1h,23 CO2 selectivity of MOF-205-OBn at 298 K and under 1 bar Among the MOF-205 derivatives, MOF-205-OBn displayed significant enhancement of gas adsorption. In order to verify the role of the –OBn group in more detail, the N2 and CH4 adsorption isotherms at 1 bar were additionally measured. At 298 K and 1 bar, MOF-205-OBn uptakes more CO2 (20.7 cm3 g−1) than CH4 (7.5 cm3 g−1) and N2 (3.3 cm3 g−1), as shown in Fig. 4. The adsorption selectivities of CO2 over N2 and CH4 at 298 K were calculated using the ideal adsorbed solution theory (IAST) to fit the adsorption isotherms of pure components as well as the compositions of the gas mixture that simulate flue
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gas (CO2/N2 = 15/85 in v/v) or landfill gas (CO2/CH4 = 50/50 in v/v). As shown in Fig. 4, the IAST selectivity in a 15 : 85 molar ratio of CO2 and N2 mixtures has its highest value (∼15.2) at a very low pressure and decreases drastically as the pressure increases. It reaches a plateau at about 0.1 bar and remains relatively constant (∼6.5) at higher pressures. The CO2/CH4 selectivity at 1 bar is predicted to be 2.7 for an equimolar gas mixture. The CO2/N2 and CO2/CH4 selectivities at 1 bar are higher than those (4.6 and 2.2) of MOF-205, indicating that MOF-205-OBn becomes more specific in selectively adsorbing CO2 upon introduction of the benzyloxy groups. When compared to the IAST selectivities of other MOFs under the same conditions, the CO2/N2 selectivity of MOF-205-OBn is comparable to that of MIL-47 (∼8),24 but higher than that of MOF-177 (∼5).25 Furthermore, the CO2/CH4 selectivity of MOF-205-OBn is similar to those of ZIF-25 (2.53) and ZIF-71 (2.67),26 but higher than those of MOF-177 (0.89) and UMCM-1 (1.82) at 298 K and 1 bar.27 CO2 and CH4 adsorption of MOF-205-OBn at high pressures The effect of the –OBn groups on the gas adsorption at high pressures was further investigated by measuring CO2 and CH4 adsorption isotherms at 298 K (Fig. 5). Since the density of MOF-205-OBn (0.46 g cm−3) is larger than that of MOF-205 (0.38 g cm−3), it was predicted that the gravimetric capacity of MOF-205-OBn for CO2 and CH4 would be smaller when compared to MOF-205. At 48 bar, where MOF-205-OBn shows the largest gravimetric capacity for CO2 within the measured pressure range, its total gravimetric capacity (668 cm3 g−1) is much smaller than 803 cm3 g−1 of MOF-205. However, the total volumetric capacities for MOF-205 and MOF-205-OBn at this pressure were very similar to each other: 307 and 305 cm3 cm−3, respectively, for MOF-205 and MOF-205-OBn. As the pressure increases, the isotherm of MOF-205 decreases, pointing
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a mixed-linker approach. Despite the reduced BET surface areas, the MOF-205 derivatives showed noticeably increased H2 and CO2 uptake amounts at 1 bar as compared to the parent MOF-205. The electron-donating –NH2 group in the naphthalene moiety seemingly was more effective than the electronwithdrawing –NO2 for enhancing gas adsorption. MOF-205OBn showed significantly enhanced CO2 affinity compared to that of the un-functionalized MOF-205, and good selectivity over N2 and CH4 at 298 K and 1 bar. However, at high pressure MOF-205-OBn adsorbs smaller amounts of CO2 and CH4 than MOF-205. In particular, it is postulated that the attachment of other bulky functional groups within MOF-205 may increase van der Waals interactions with CH4 for methane storage applications.
Fig. 5 High pressure gas adsorption isotherms at 298 K are displayed for the comparison of the total gravimetric and volumetric capacities of MOF-205 and MOF-205-OBn respectively at high pressures: (a) gravimetric and (b) volumetric CO2 storage capacities, and (c) gravimetric and (d) volumetric CH4 storage capacities.
to the fact that MOF-205-OBn would have a greater volumetric capacity above 50 bar. Thus, MOF-205-OBn seems to interact with CO2 molecules more strongly than MOF-205 in the high pressure regime.1f The CH4 adsorption behaviour of MOF-205-OBn was similar to that of MOF-205 except for the smaller storage capacity (Fig. 5). The total volumetric capacities at 35 and 80 bar are 112 and 178 cm3 cm−3, respectively, which are smaller by 7 and 13% when compared to MOF-205 under similar conditions. The capacity difference becomes larger as the pressure increases even higher. This result indicates that the pore volume is an important factor for CH4 storage as the pressure increases, and also shows that the capacity decrease and the pore volume reduction are well correlated with each other. Working capacity is the storage capacity above 5 bar and assesses the practical performance of the storage materials.1i Unfortunately, but expectedly, the working capacities of MOF-205-OBn are much smaller than the total volumetric capacities, at 35 bar, 90 (MOF-205-OBn) and 101 (MOF-205) cm3 cm−3, and at 85 bar, 157 (MOF-205-OBn) and 186 (MOF-205) cm3 cm−3. The performance of MOF-205-OBn is 10 and 16% less when compared with MOF-205 at 35 and 80 bar, respectively. Therefore, the introduced –OBn groups to MOF-205 seem not to be favourable for effectively increasing van der Waals interactions under high-pressure conditions. Furthermore, the reduced surface area and pore volume in MOF-205-OBn resulted in a reduced CH4 storage capacity, indicating that the –OBn group is not effective for increasing intermolecular interactions with CH4 molecules at high pressures.
Conclusions This work presents the successful functionalization of MOF-205 with –NH2, –NO2, or –OBn by direct syntheses using
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Acknowledgements This research was supported by a grant from the Korea CCS R&D Center (KCRC), funded by the Korean government (Ministry of Science, ICT & Future Planning) (NRF-2012-0008900). We thank Accelrys Korea for the Forcite calculations of the model structures, and Pohang Accelerator Laboratory (PAL), Korea for the X-ray data collection at the 2D-SMC beamline. We thank Dr H. Furukawa and Mr K. E. Cordova for their valuable comments.
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Cite this: DOI: 10.1039/c4dt02300e
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Gas adsorption properties of highly porous metal–organic frameworks containing functionalized naphthalene dicarboxylate linkers† Jaeung Sim,a Haneul Yim,a Nakeun Ko,a Sang Beom Choi,a Youjin Oh,a Hye Jeong Park,*a SangYoun Park*b and Jaheon Kim*a Three functionalized metal–organic frameworks (MOFs), MOF-205-NH2, MOF-205-NO2, and MOF-205OBn, formulated as Zn4O(BTB)4/3(L), where BTB is benzene-1,3,5-tribenzoate and L is 1-aminonaphthalene-3,7-dicarboxylate (NDC-NH2), 1-nitronaphthalene-3,7-dicarboxylate (NDC-NO2) or 1,5-dibenzyloxy-2,6-naphthalenedicarboxylate (NDC-(OBn)2), were synthesized and their gas (H2, CO2, or CH4) adsorption properties were compared to those of the un-functionalized, parent MOF-205. Ordered structural models for MOF-205 and its derivatives were built based on the crystal structures and were subsequently used for predicting porosity properties. Although the Brunauer–Emmett–Teller (BET) surface areas of the three MOF-205 derivatives were reduced (MOF-205, 4460; MOF-205-NH2, 4330; MOF-205NO2, 3980; MOF-205-OBn, 3470 m2 g−1), all three derivatives were shown to have enhanced H2 adsorption capacities at 77 K and CO2 uptakes at 253, 273, and 298 K respectively at 1 bar in comparison with MOF-205. The results indicate the following trend in H2 adsorption: MOF-205 < MOF-205-NO2 < MOF-205-NH2 < MOF-205-OBn. MOF-205-OBn showed good ideal adsorbed solution theory (IAST)
Received 28th July 2014, Accepted 2nd October 2014
selectivity values of 6.5 for CO2/N2 (15/85 in v/v) and 2.7 for CO2/CH4 (50/50 in v/v) at 298 K. Despite the
DOI: 10.1039/c4dt02300e
large reduction (−22%) in the surface area, MOF-205-OBn displayed comparable total volumetric CO2 (at 48 bar) and CH4 (at 35 bar) storage capacities with those of MOF-205 at 298 K: MOF-205-OBn,
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305 (CO2) and 112 (CH4) cm3 cm−3, and for MOF-205, 307 (CO2) and 120 (CH4) cm3 cm−3, respectively.
Introduction Metal–organic frameworks (MOFs) are crystalline porous materials with various pore sizes and environments that have been extensively investigated for the storage and separation of H2,1a,b CO2,1c–f CH4,1g–i and hydrocarbons.1j,k Well-established reports indicate that, in general, gravimetric gas storage capacity at high pressures is linearly related to the Brunauer–Emmett– Teller (BET) surface areas and pore volumes, whereas the gas separation ability of an MOF is dependent on the strength of the intermolecular interactions between the MOF surfaces and gas molecules.1 Thus, control and design of the structure and porosity in MOFs is a crucial consideration for gas storage and separa Department of Chemistry, Soongsil University, 369 Sangdo-Ro, Dongjak-Gu, Seoul 156-743, Republic of Korea. E-mail: [email protected], [email protected] b School of Systems Biomedical Science, Soongsil University, 369 Sangdo-Ro, Dongjak-Gu, Seoul 156-743, Republic of Korea. E-mail: [email protected] † Electronic supplementary information (ESI) available: NMR spectra, TGA data, electrostatic potential maps, PXRD patterns, structure modelling, additional gas sorption data, and single crystal X-ray data. CCDC 1015820, 1015823, 1015821, and 1015822 for H2NDC-NH2, H2NDC-NO2, MOF-205-NH2, and MOF-205-NO2, respectively. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt02300e
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ation applications. It is noted that the surface area and gas adsorption affinity are not independent factors, but rather must be considered together when designing a structure for efficient gas storage and separation. That is, as MOFs exhibiting large surface areas also have sufficient pore spaces, their framework surfaces can be further functionalized to increase interactions with particular gas molecules. This strategy is realized by (i) constructing isoreticular MOFs by linking pre-functionalized organic linkers with metal ions,2 or (ii) post-synthetic modification methods that allow extrinsic functional groups to be introduced to as-prepared MOFs through simple organic reactions.3 Since the emission of CO2 from industrial coal-fired power plants causes an unfavourable greenhouse effect, the efficient capture and short-term storage of CO2 using MOFs has received much attention.1c–f The majority of research focused on reaching this goal has been devoted to increasing Coulombic interactions between framework surfaces and the large quadrupole moment of CO2.4 In this regard, many MOFs were functionalized with –NO2,5 –NH2,6 –CONH–,7 –OH,8 –CN,9 –SO3H,10 or pyridine11 in order to enhance CO2 adsorption. Theoretical calculations alone or in combination with experiments address this interaction, at the molecular level, between CO2 and the various functionalities incorporated on the organic linkers used to make MOFs.12
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In more detail, functional groups such as –CH3, –OH, –NO2, –NH2, and –COOH on the organic linkers can increase interactions with CO2 molecules, and CO2 can also interact with various functional groups on aromatic linkers such as π electrons/H atoms in benzene or the lone pairs/H atoms/partial charges of functionalized linkers. In a certain case, electron-withdrawing groups destabilize π electron⋯quadrupole moment interactions, which is indeed unfavourable for CO2 adsorption.12d,e Although many of the functionalized MOFs mentioned above show enhanced CO2 adsorption behaviours, the increased gas uptake capacities at ambient temperature are not as large when compared to those of amine-modified MOFs, which can form carbamate or carbamic acid through chemisorption processes with CO2.13 It is evident that intermolecular interactions alone may not be sufficient for increasing CO2 adsorption affinity in MOFs. In fact, it is notable that many of the functionalized MOFs showing enhanced gas adsorption behaviours are microporous materials. Within the small pores of these MOFs, enhanced gas adsorption may also be attributed to intermolecular potentials that are a result of reduced pore spaces as observed in catenated MOFs.14 We were interested in the sole contribution of functional groups in MOFs, which motivated us to construct a series of highly porous MOFs that have well-separated functionalized linkers in order to eliminate concerns regarding pore size effects. A suitable model system is MOF-205 (aka DUT-6) formulated as Zn4O(BTB)4/3(NDC) (BTB = benzene-1,3,5-benzoate; NDC = naphthalene-2,6-dicarboxylate) that is a three-dimensional framework with ith-d topology containing 2.3 nm dodecahedron-like cages.15 Herein, we report the gas adsorption properties at 1 bar and higher pressures for the highly porous MOF-205 and its derivatives: MOF-205-NH2, MOF-205-NO2, and MOF-205-OBn. The three derivative MOFs have been synthesized by replacing NDC in MOF-205 with NDC-NH2, NDC-NO2, and NDC-(OBn)2 (-OBn = benzyloxy), respectively (Scheme 1).
Scheme 1 Reactions for MOF-205 and its derivatives are schematically drawn with their building blocks.
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Experimental General procedures All chemicals and solvents used in the syntheses of this series were of reagent grade and used without further purification. Zinc(II) nitrate hexahydrate (Zn(NO3)2·6H2O), ethyl alcohol (EtOH), and N,N-dimethylformamide (DMF) were purchased from Daejung Chemicals & Metals Co., Ltd. H3BTB was prepared according to a reported literature method.16 Powder X-ray diffraction (PXRD) data were collected on a Rigaku MiniFlex diffractometer with CuKα radiation (λ = 1.5418 Å). Thermogravimetric analyses (TGA) were carried out using a Scinco TGA-S1000 thermal analysis system under air with a temperature-increasing rate of 5 °C min−1. Fourier transform infrared spectra (FT-IR) of samples prepared as KBr pellets were measured using a JASCO FT/IR-4000 spectrophotometer. NMR spectra were obtained on a Bruker 400 MHz NMR spectrometer. Elemental analyses were carried out on an EA 1110 (CE instrument) or a Perkin-Elmer 2400 Series II CHN analyser. Synthetic procedures 1-Nitronaphthalene-3,7-dicarboxylic acid (H2NDC-NO2). H2NDC (6.00 g, 27.8 mmol) was dissolved in conc. H2SO4 (600 mL) and 10% HNO3 (23.2 mL, 1.4 equiv.) was slowly added to the reaction mixture. After stirring at room temperature for 30 min, the solution was cooled to 0 °C and ice was added to form white precipitates. The crude product was washed with water, filtered, and dried under vacuum. The recrystallization from acetic acid affords pure white solids (yield, 56%). 1H-NMR (400 MHz, DMSO-d6): δ = 9.09 (1H, s), 9.08 (1H, s), 8.73 (1H, s), 8.50 (1H, d), 8.23 ppm (1H, d); 13 C-NMR (400 MHz, DMSO-d6): δ = 166 (s, COOH), 165 (s, COOH), 146, 136, 135, 132, 133, 130, 129, 127, 125, 124, 123 ppm (Fig. S1†). 1-Aminonaphthalene-3,7-dicarboxylic acid (H2NDC-NH2). To a solution of 1-nitronaphthalene-3,6-dicarboxylic acid (2.00 g, 8.65 mmol) in methanol (537 mL), 10% Pd/C (0.537 g) was slowly added under an N2 atmosphere. The reaction mixture was stirred under an H2 atmosphere (1 atm) for 1 day to form deep green precipitates. After filtration through Celite, the filtrate was evaporated to obtain yellow solids as a product. For a second crop, the filtered deep green precipitates were dissolved in aqueous solution of NaOH (1.0 M, 400 mL). After filtration, the filtrate was acidified with conc. HCl to form yellow solids. The product was again filtered, washed with water and dried (yield, 83%). 1H-NMR (DMSO-d6): δ = 8.77 (1H, s), 7.95 (1H, d), 7.90 (1H, d), 7.79 (1H, s), 7.25 (1H, s), 6.26 ppm (2H, s); 13C-NMR (DMSO-d6): δ = 168 (s, COOH), 167 (s, COOH), 147, 135, 131, 129, 127, 125, 123, 117 ppm (Fig. S2†). 1,5-Dihydroxy-2,6-dibromonaphthalene.17 A mixture of 1,5dihydroxy naphthalene (16 g, 0.10 mmol) and a catalytic amount of iodine was added to acetic acid (440 mL). The mixture was heated to 80 °C, and then bromine (10.5 mL, 0.20 mol) was slowly added. After stirring for 30 min, the reaction mixture was cooled to room temperature. The crude product was filtered and washed with petroleum ether (yield,
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57%). 1H-NMR (CDCl3): δ 7.7 (d, 2H, J = 8.4 Hz), 7.5 (d, 2H, J = 8.8 Hz), 6.0 (s, 2H) ppm. 1,5-Dibenzoxy-2,6-dibromonaphthalene. A solution of 1,5dihydroxy-2,6-dibromonaphthalene (4.65 g, 14.1 mmol) in DMF (75 mL) was cooled to 0 °C under an Ar atmosphere. After stirring for 30 min, sodium hydroxide (1.29 g, 31.0 mmol) was added to the reaction mixture. After 1 h, benzyl bromide (3.83 mL, 31.0 mmol) was added, and the ice bath was removed to allow the mixture temperature to slowly increase to room temperature. The crude product was washed with water and extracted with ethyl acetate. The extracted solution was dried over MgSO4, filtered, and evaporated to obtain brown solids (yield, 93%). 1H-NMR (DMSO-d6): δ 7.83 (d, 2H, J = 8.8 Hz), 7.80 (d, 2H, J = 9.2 Hz), 7.62 (d, 4H, J = 8.0 Hz), 7.44 (m, 6H), and 5.05 (s, 4H) ppm. 1,5-Dibenzoxynaphthalene-2,6-dicarboxylic acid diethyl ester. A solution of 1,5-dibenzoxy-2,6-dibromonaphthalene (1.69 g, 3.39 mmol) in tetrahydrofuran (338 mL) was cooled to −78 °C under an Ar atmosphere. After stirring for 1 h, n-butyllithium (4.46 mL, 7.12 mmol) was added to the reaction mixture. After 1 h, ethyl chloroformate (3.23 mL, 3.92 mmol) was added, and the dry ice bath was removed to allow the mixture temperature to slowly increase to room temperature. The crude product was washed with Na2CO3 aqueous solution and extracted with ethyl acetate. The ethyl acetate solution was dried over MgSO4, filtered and evaporated to give yellow solids (yield, 57%). 1H-NMR (DMSO-d6): δ 8.02 (d, 2H, J = 8.4 Hz), 7.87 (d, 2H, J = 8.8 Hz), 7.56 (d, 4H, J = 6.8 Hz), 7.43 (m, 6H), 5.14 (s, 4H), 4.36 (q, 4H, J = 7.0 Hz), and 1.30 (t, 6H, J = 7.0 Hz) ppm. 1,5-Dibenzyloxy-2,6-naphthalenedicarboxylic acid (H2NDC(OBn)2). 1,5-Dibenzoxynaphthalene-2,6-dicarboxylic acid diethyl ester (1.00 g, 2.06 mmol) was dissolved in a mixture of tetrahydrofuran (40 mL), ethanol (40 mL), and 1.0 M NaOH aqueous solution (10 mL). After refluxing the reaction mixture at 80 °C for 1 d, the solution was evaporated, and then water was added. The aqueous solution was acidified with conc. HCl to form the precipitates, which were filtered, washed with water and dried under vacuum (yield, 96%). 1H-NMR (DMSOd6): δ 12.61 (s, br, 2H), 7.98 (d, 2H, J = 8.0 Hz), 7.85 (d, 2H, J = 8.0 Hz), 7.59 (d, 4H, J = 7.2 Hz), 7.44 (m, 6H), and 5.15 (s, 4H) ppm (Fig. S3†). MOF-205-NH2. Zn(NO3)2·6H2O (0.425 g, 1.42 mmol), H3BTB (0.128 g, 0.292 mmol), and H2NDC-NH2 (0.112 g, 0.484 mmol) were dissolved in a mixture of DMF (30 mL) and EtOH (3 mL). The clear solution was placed in a 30 mL vial and heated at 85 °C for 2 days to form brown polyhedron crystals. The mother liquor was decanted, and the crystals were washed with DMF (5 × 10 mL) and stored in DMF. Yield: 67% based on 1 mole of H3BTB. Elemental analysis for an evacuated sample, calcd/found (%) for Zn4O(BTB)4/3(NDC-NH2): C, 53.00/52.15; H, 2.50/2.64; N, 1.29/1.26. Calcd/found (%) for Zn4O(BTB)4/3(NDC-NH2)·(H2O): C, 52.16/52.15; H, 2.64/2.64; N, 1.27/1.26. MOF-205-NO2. Zn(NO3)2·6H2O (0.336 g, 1.13 mmol), H3BTB (0.0700 g, 0.160 mmol), and H2NDC-NO2 (0.126 g, 0.583 mmol) were dissolved in DMF (30 mL). The clear solu-
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tion was placed in a 30 mL vial and heated at 105 °C for 2 days to form brown polyhedron crystals. The mother liquor was decanted, and the crystals were washed with DMF (5 × 10 mL) and stored in DMF. Yield: 48% based on 1 mole of H3BTB. Elemental analysis for an evacuated sample, calcd/found (%) for Zn4O(BTB)4/3(NDC-NO2): C, 51.57/50.61; H, 2.25/2.42; N, 1.26/1.23. Calcd/found (%) for Zn4O(BTB)4/3(NDC-NO2)·(H2O): C, 50.78/ 50.61; H, 2.40/2.42; N, 1.23/1.23. MOF-205-OBn. Zn(NO3)2·6H2O (42.5 mg, 0.140 mmol), H3BTB (12.8 mg, 0.0290 mmol), and H2NDC-(OBn)2 (20.8 mg, 0.0490 mmol) were dissolved in a mixture of DMF–EtOH (3.0/ 0.3 mL). The solution was placed in a 4 mL vial that was tightly capped and then heated in an isothermal oven at 85 °C for 48 h to give orange-coloured truncated octahedron crystals. The crystals were filtered, washed with DMF (5 × 10 mL), and stored in DMF. Yield: 68% based on 1 mole of H3BTB. Elemental analysis for an evacuated sample, calcd/found (%) for Zn4O(BTB)4/3(NDC-(OBn)2): C, 57.96/56.60; H, 2.98/2.78; N, 0.00/ 0.00. Calcd/found (%) for Zn4O(BTB)4/3(NDC-(OBn)2)·(H2O)4/3: C, 56.91/56.60; H, 3.13/2.78; N, 0.00/0.00. X-ray crystallography The X-ray intensity data sets were collected on a Bruker APEX CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) for H2NDC-NH2, or were collected with a synchrotron light source (λ = 0.70000 Å) on an ADSC Quantum210 detector at 2D SMC with a silicon (111) double crystal monochromator (DCM) at the Pohang Accelerator Laboratory (PAL), Korea for H2NDC-NO2, MOF-205-NH2, and MOF-205NO2. Initial structures were obtained by direct methods using SHELXS-97 and refined by subsequent full-matrix least-squares refinements on F2 using SHELXL-97.18 Non-H atoms were refined anisotropically. H atoms were located with ideal geometry and refined with a riding model. Both MOF-205-NH2 and MOF-205-NO2 have similar unit-cell parameters and belong to the same cubic space group (Pm3ˉ n) as MOF-205. The functionalized NDCs were disordered at the crystallographic special positions with mmm site symmetry while BTBs were at the special positions with 32 site symmetry. The crystal and refinement data for the two linkers and two MOFs are summarized in Table S1 to S4.† The crystal structures are displayed in Fig. S6 to S9.† Structural modelling and porosity calculations As the MOFs in this work suffered from structural disorder, their ordered model structures were built by aid of Materials Studio 6.1.0TM (Accelrys Software Inc.) (section 4 in ESI†).19 The ordered model structures are displayed in Fig. 1. Using these models, their van der Waals, accessible solvent surface areas (SAvdW, SAacc), and pore volumes (Vvdw) were calculated using a probe radius of 1.82 Å by the “Atom Volumes & Surfaces” utility in the Materials Studio (Table 1). For a fair comparison of the porosity with the same volume, the unit cell parameters of all MOFs were changed to those of MOF-205 with a = 30.353 Å. The pore volumes were also calculated using PLATON with a “CALC SOLV probe 1.82” command.20
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Fig. 1 Unit-cell packing diagrams for the ordered model structures of (a) MOF-205, (b) MOF-205-NH2, (c) MOF-205-NO2, and (d) MOF-205-OBn are displayed with Zn atoms as polyhedra, and C, N, and O atoms as balls. Hydrogen atoms are not shown for simplicity.
Table 1
Calculated and measured porosity properties of the ordered MOF-205 and its derivativesa
MOF
f.w.
dcalc. (cm3 g−1)
SAvdW (m2 g−1)
SAacc (m2 g−1)
VvdW (cm3 cm−3)
VPLATON (cm3 cm−3)
SABET (m2 g−1)
SALangmuir (m2 g−1)
VP (cm3 cm−3)
MOF-205 MOF-205-NH2 MOF-205-NO2 MOF-205-OBn
1072.17 1087.19 1117.17 1284.40
0.382 0.387 0.398 0.458
4220 4230 4190 4530
4640 4680 4660 3770
0.850 0.847 0.845 0.808
0.817 0.814 0.811 0.747
4460 4330 3980 3470
6270 6120 5670 4810
0.83 0.83 0.79 0.78
a
The unit cell volumes of all MOFs were adjusted to V = 27 964 Å3, that of MOF-205 with a = 30.353 Å. Acronyms: SAacc, SABET, SALangmuir are the accessible, BET, and Langmuir surface areas; VvdW, VPLATON, VP are the van der Waals, PLATON, and measured pore volumes. Calculated values using Materials Studio 6.1.0TM (Accelrys Software Inc.), except VPLATON. A probe radius of 1.82 Å, appropriate for the N2 molecule. Six formula units in each unit cell (Z = 6).
Results and discussion Synthesis As observed in UMCM family, resulting framework connectivities of ‘mixed-linker’ MOFs are dependent on the length of the linear linker or the arrangement of organic linkers around a Zn4O cluster.21a,b MOF-205 is also a mixed-linker MOF where NDC linkers are located at the trans-positions and BTB linkers are at the four equatorial positions around a Zn4O cluster.15 For the successful preparation of an isoreticular MOF-205 series, two unfavourable possibilities must be avoided through the adjustment of reaction conditions; (i) the formation of byproducts that contain only one type of organic linker such as a derivative of IRMOF-8 (Zn4O(NDC-R)3) or MOF-177 (Zn4O(BTB)2) in our case, and (ii) a topologically different framework formed through a different arrangement of the two linkers around the Zn4O vertices. In order to prevent the formation of different types of frameworks, the structurally more rigid NDC was selected for functionalization, and the reaction conditions for MOF-205 were slightly changed for the preparation of MOF-205 derivatives using the functionalized NDC linkers. Due to the polar nature of H2NDC-NH2 and H2NDC-NO2, which show dipole moment values of 1.70 and 4.54 Debye by DFT calculations, respectively (Fig. S10†), the resulting frameworks would exert favourable electrostatic interactions with CO2. Due to the molecular symmetry, the linker does not have a net electric dipole moment. However, the bulky and flexible benzyloxy groups were anticipated to contribute to the
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enhanced interaction with CO2 via various benzene⋯CO2 interactions.12 H2NDC-NO2 was synthesized from a nitration reaction of H2NDC in an aqueous H2SO4/HNO3 solution, and H2NDC-NH2 was obtained by reduction of H2NDC-NO2 with H2 in the presence of Pd/C. The crystal structures of the linkers showed that the –NH2 or –NO2 functional group was introduced on the corrected position of the naphthalene moiety (Fig. S6 and S7†). The dihedral angle between the –NO2 and NDC planes was 37.6(1)° in the H2NDC-NO2. From 1,5-dihydroxy-2,6-dibromonaphthalene as a starting compound, H2NDC-(OBn)2 was prepared via four synthetic steps. The linker crystallized as thin needles, which were not suitable for single crystal X-ray diffraction analysis. MOF-205-NH2, MOF-205-NO2, and MOF-205-OBn were prepared relatively easily by modifying the solvothermal reaction conditions for MOF-205.15b For example, pure MOF-205-OBn was obtained by employing the same reaction conditions as for MOF-205, but at a slightly lower temperature. The framework formulas, Zn4O(BTB)4/3(NDC-R), for all members of this series were identified by elemental analysis. Calculated porosity Using the ordered structural models, the porosity properties of the MOFs were calculated and are listed in Table 1. The introduction of the –NH2 or –NO2 group did not lead to large changes in the calculated surface areas (SAvdW and SAacc). This is expected from the general formula of the MOF-205 structures, Zn4O(BTB)4/3(NDC-R), which demonstrates that the
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contribution of the functional groups, R = –NH2 or –NO2 to the framework volume is small in comparison with BTB. Therefore, the densities and calculated pore volumes (VvdW and VPLATON) for MOF-205-NH2 and MOF-205-NO2 are very similar to those of MOF-205. In contrast, MOF-205-OBn exhibits significantly increased SAvdW (+7%) and reduced SAacc (−19%) values compared to those of MOF-205. This implies that the –OBn groups contribute more in increasing the framework molecular surface and contribute less to the potential adsorption sites. A more clear effect on the porosity by the functional groups can be shown when the pore volumes in cm3 cm−3 unit are compared. That is, as four MOFs have almost same unit cell volumes, the calculated pore volume is a good indicator for the porosity change. In detail, the void volume fractions (Vvdw and VPLATON) of MOF-205-OBn are smaller by 5 and 9%, respectively, than those of MOF-205; Vvdw is greater than VPLATON by 4–10%. Therefore, the –OBn groups are expected to change the porosity and gas adsorption properties of the unfunctionalized MOF-205 more noticeably than will the MOF-205 derivatives with –NH2 and –NO2 groups. Indeed, the calculations indicate that MOF-205-OBn would have a much smaller surface area than MOF-205 and the other derivatives based on the SAacc values; SAacc is the simulated surface accessible by N2, thus estimating a measured BET surface area (SABET). Measured porosity The N2 gas sorption isotherms for the three functionalized MOFs showed reversible adsorption/desorption traces with the steps at P/P0 = ∼0.05, similar to that of MOF-205 (Fig. 2). This indicates that the functional groups are not bulky enough to diminish the mesoporous character of MOF-205. The BET surface areas of MOF-205, MOF-205-NH2, MOF-205-NO2, and MOF-205-OBn were 4460, 4330, 3980, and 3470 m2 g−1, respectively (Table 1). The reduction in the BET surface areas of the MOF-205 derivatives is in accordance with the prediction observed in the calculated values. In particular, MOF-205OBn exhibits a significantly reduced BET (Langmuir) surface area, which is 78% (77%) of the surface area of MOF-205. This matches well to prediction made by the SAacc calculations
(Table 1). The pore volume was also decreased by 6%, a value that is also in good agreement with the calculations. This observation also supports the fact that the ordered structural model of MOF-205-OBn is a plausible representation of the crystal structure. H2 and CO2 adsorption at 1 bar The H2 gas sorption isotherms at 77 K and 1 bar showed that MOF-205, MOF-205-NH2, MOF-205-NO2 and MOF-205-OBn adsorb 55, 59, 58, and 68 cm3 cm−3 of H2, respectively (Fig. S12† and Table 2). Considering the reduction of surface areas for the MOF-205 derivatives, it is certain that the introduced functional groups contribute to the enhanced H2 uptake. More compelling evidence was provided from the CO2 gas sorption isotherms at 1 bar (Fig. 3 and Table 2). MOF-205 adsorbed 20.0, 11.0, and 6.3 cm3 cm−3 of CO2 at 253, 273, and 298 K, respectively. Under the same temperature and pressure conditions, the volumetric capacities of the MOF-205 derivatives increased to 22.3, 12.8, and 7.2 cm3 cm−3 for MOF-205NH2, 22.0, 12.3, and 7.2 cm3 cm−3 for MOF-205-NO2, and 29.7, 20.7, and 9.5 cm3 cm−3 for MOF-205-OBn, respectively. It is clear that the volumetric capacities of gas adsorption are directly related to the introduction of functional groups, since the unit cell volumes of all the MOFs are very similar to each other. This is evidenced by the following trend in increasing gas uptake in relation to functional groups: MOF-205 < MOF-205-NO2 < MOF-205-NH2 < MOF-205-OBn. It is noted that the electron-donating –NH2 group seems more favorable than the electron-withdrawing –NO2 for enhancing gas adsorption; the electron-withdrawing group may destabilize the contribution from a π⋯quadrupole interaction that arises between the functionalized NDC with CO2 as in the benzene case.12 The larger gas uptake by MOF-205-OBn may be understood when considering related cases, MIL-47(VIV)-OCH3,8a and multivariate (MTV) MOF-5s containing the propoxy (–OC3H5) or benzyloxy (–OC7H7) groups.22 Similar to the BDC derivatives, the benzyloxy-functionalized NDC, NDC-(OBn)2 appears to be responsible for the large enhancement in CO2 adsorption in MOF-205-OBn. In contrast to expectation, the CO2 adsorption enthalpies were almost the same when calculated using a virial-type equation: at zero-coverage 16.1, 15.9, 16.2, and 16.5 kJ mol−1 for MOF-205, MOF-205-NH2, MOF-205-NO2, and MOF-205OBn, respectively (Fig. S13†). This result implies that the increased gas adsorption capacity may be simply attributed to Table 2 Volumetric H2 and CO2 adsorption capacity in cm3 cm−3 of MOF-205 and its derivatives at 1 bar
Fig. 2 N2 adsorption isotherms of MOF-205, MOF-205-NO2, and MOF-205-OBn measured at 77 K.
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MOF-205-NH2,
H2
CO2
MOF
at 77 K
at 253 K
at 273 K
at 298 K
Qst/kJ mol−1
MOF-205 MOF-205-NH2 MOF-205-NO2 MOF-205-OBn
55 59 58 68
20.0 22.3 22.0 30.6
11.0 12.8 12.3 17.5
6.3 7.2 7.2 9.5
16.1 15.9 16.2 16.5
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Fig. 4 Comparison of (a) CO2 and N2, and (b) CO2 and CH4 adsorption isotherms at 298 K of MOF-205 and MOF-205-OBn. The IAST selectivities for (c) CO2/N2 (15/85 in v/v) and (d) CO2/CH4 (50/50 in v/v) mixtures, respectively.
Fig. 3 CO2 adsorption isotherms of MOF-205, MOF-205-NH2, MOF-205-NO2, and MOF-205-OBn measured at (a) 253, (b) 273, and (c) 298 K, respectively.
an increase in van der Waals areas, rather than from the electronic nature of each functional group. In other words, the increased gas uptake cannot be entirely related to the enhancement in the intermolecular interactions of the functionalized framework surfaces with CO2. In fact, the primary adsorption sites in the MOFs having Zn4O(CO2)6 units are the “cup sites” or near the carboxylate groups, which are particularly responsible for the gas adsorption at room temperature and low pressure.1h,23 CO2 selectivity of MOF-205-OBn at 298 K and under 1 bar Among the MOF-205 derivatives, MOF-205-OBn displayed significant enhancement of gas adsorption. In order to verify the role of the –OBn group in more detail, the N2 and CH4 adsorption isotherms at 1 bar were additionally measured. At 298 K and 1 bar, MOF-205-OBn uptakes more CO2 (20.7 cm3 g−1) than CH4 (7.5 cm3 g−1) and N2 (3.3 cm3 g−1), as shown in Fig. 4. The adsorption selectivities of CO2 over N2 and CH4 at 298 K were calculated using the ideal adsorbed solution theory (IAST) to fit the adsorption isotherms of pure components as well as the compositions of the gas mixture that simulate flue
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gas (CO2/N2 = 15/85 in v/v) or landfill gas (CO2/CH4 = 50/50 in v/v). As shown in Fig. 4, the IAST selectivity in a 15 : 85 molar ratio of CO2 and N2 mixtures has its highest value (∼15.2) at a very low pressure and decreases drastically as the pressure increases. It reaches a plateau at about 0.1 bar and remains relatively constant (∼6.5) at higher pressures. The CO2/CH4 selectivity at 1 bar is predicted to be 2.7 for an equimolar gas mixture. The CO2/N2 and CO2/CH4 selectivities at 1 bar are higher than those (4.6 and 2.2) of MOF-205, indicating that MOF-205-OBn becomes more specific in selectively adsorbing CO2 upon introduction of the benzyloxy groups. When compared to the IAST selectivities of other MOFs under the same conditions, the CO2/N2 selectivity of MOF-205-OBn is comparable to that of MIL-47 (∼8),24 but higher than that of MOF-177 (∼5).25 Furthermore, the CO2/CH4 selectivity of MOF-205-OBn is similar to those of ZIF-25 (2.53) and ZIF-71 (2.67),26 but higher than those of MOF-177 (0.89) and UMCM-1 (1.82) at 298 K and 1 bar.27 CO2 and CH4 adsorption of MOF-205-OBn at high pressures The effect of the –OBn groups on the gas adsorption at high pressures was further investigated by measuring CO2 and CH4 adsorption isotherms at 298 K (Fig. 5). Since the density of MOF-205-OBn (0.46 g cm−3) is larger than that of MOF-205 (0.38 g cm−3), it was predicted that the gravimetric capacity of MOF-205-OBn for CO2 and CH4 would be smaller when compared to MOF-205. At 48 bar, where MOF-205-OBn shows the largest gravimetric capacity for CO2 within the measured pressure range, its total gravimetric capacity (668 cm3 g−1) is much smaller than 803 cm3 g−1 of MOF-205. However, the total volumetric capacities for MOF-205 and MOF-205-OBn at this pressure were very similar to each other: 307 and 305 cm3 cm−3, respectively, for MOF-205 and MOF-205-OBn. As the pressure increases, the isotherm of MOF-205 decreases, pointing
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a mixed-linker approach. Despite the reduced BET surface areas, the MOF-205 derivatives showed noticeably increased H2 and CO2 uptake amounts at 1 bar as compared to the parent MOF-205. The electron-donating –NH2 group in the naphthalene moiety seemingly was more effective than the electronwithdrawing –NO2 for enhancing gas adsorption. MOF-205OBn showed significantly enhanced CO2 affinity compared to that of the un-functionalized MOF-205, and good selectivity over N2 and CH4 at 298 K and 1 bar. However, at high pressure MOF-205-OBn adsorbs smaller amounts of CO2 and CH4 than MOF-205. In particular, it is postulated that the attachment of other bulky functional groups within MOF-205 may increase van der Waals interactions with CH4 for methane storage applications.
Fig. 5 High pressure gas adsorption isotherms at 298 K are displayed for the comparison of the total gravimetric and volumetric capacities of MOF-205 and MOF-205-OBn respectively at high pressures: (a) gravimetric and (b) volumetric CO2 storage capacities, and (c) gravimetric and (d) volumetric CH4 storage capacities.
to the fact that MOF-205-OBn would have a greater volumetric capacity above 50 bar. Thus, MOF-205-OBn seems to interact with CO2 molecules more strongly than MOF-205 in the high pressure regime.1f The CH4 adsorption behaviour of MOF-205-OBn was similar to that of MOF-205 except for the smaller storage capacity (Fig. 5). The total volumetric capacities at 35 and 80 bar are 112 and 178 cm3 cm−3, respectively, which are smaller by 7 and 13% when compared to MOF-205 under similar conditions. The capacity difference becomes larger as the pressure increases even higher. This result indicates that the pore volume is an important factor for CH4 storage as the pressure increases, and also shows that the capacity decrease and the pore volume reduction are well correlated with each other. Working capacity is the storage capacity above 5 bar and assesses the practical performance of the storage materials.1i Unfortunately, but expectedly, the working capacities of MOF-205-OBn are much smaller than the total volumetric capacities, at 35 bar, 90 (MOF-205-OBn) and 101 (MOF-205) cm3 cm−3, and at 85 bar, 157 (MOF-205-OBn) and 186 (MOF-205) cm3 cm−3. The performance of MOF-205-OBn is 10 and 16% less when compared with MOF-205 at 35 and 80 bar, respectively. Therefore, the introduced –OBn groups to MOF-205 seem not to be favourable for effectively increasing van der Waals interactions under high-pressure conditions. Furthermore, the reduced surface area and pore volume in MOF-205-OBn resulted in a reduced CH4 storage capacity, indicating that the –OBn group is not effective for increasing intermolecular interactions with CH4 molecules at high pressures.
Conclusions This work presents the successful functionalization of MOF-205 with –NH2, –NO2, or –OBn by direct syntheses using
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Acknowledgements This research was supported by a grant from the Korea CCS R&D Center (KCRC), funded by the Korean government (Ministry of Science, ICT & Future Planning) (NRF-2012-0008900). We thank Accelrys Korea for the Forcite calculations of the model structures, and Pohang Accelerator Laboratory (PAL), Korea for the X-ray data collection at the 2D-SMC beamline. We thank Dr H. Furukawa and Mr K. E. Cordova for their valuable comments.
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