Article pubs.acs.org/IC
Paddle Wheel Based Triazolyl Isophthalate MOFs: Impact of Linker Modification on Crystal Structure and Gas Sorption Properties Merten Kobalz,† Jörg Lincke,† Karolin Kobalz,† Oliver Erhart,† Jens Bergmann,† Daniel Las̈ sig,† Marcus Lange,‡ Jens Möllmer,‡ Roger Glas̈ er,†,‡ Reiner Staudt,§ and Harald Krautscheid*,† †
Fakultät für Chemie und Mineralogie, Universität Leipzig, Johannisallee 29, D-04103 Leipzig, Germany Institut für Nichtklassische Chemie e.V., Permoserstraße 15, D-04318 Leipzig, Germany § Fakultät Maschinenbau und Verfahrenstechnik, Hochschule Offenburg, Badstraße 24, D-77652 Offenburg, Germany ‡
S Supporting Information *
ABSTRACT: Syntheses and comprehensive characterization of two closely related series of isomorphous metal−organic frameworks (MOFs) based on triazolyl isophthalate linkers with the general formula ∞3[M2(R1−R2−trz−ia)2] (M = Cu, Zn) are presented. Using solvothermal synthesis and synthesis of microcrystalline materials on the gram scale by refluxing a solution of the starting materials, 11 MOFs are readily available for a systematic investigation of structure−property relationships. The networks of the two series are assigned to rutile (rtl) (1−4) and α-PbO2 (apo) (5−9) topology, respectively. Due to the orientation of the triazole substituents toward the cavities, both the pore volume and the pore diameter can be adjusted by choice of the alkyl substituents. Compounds 1−9 exhibit pronounced microporosity with calculated porosities of 31−53% and show thermal stability up to 390 °C as confirmed by simultaneous thermal analysis. Systematic investigation of adsorption properties by CO2 (298 K) and N2 (77 K) adsorption studies reveal remarkable network flexibility induced by alkyl substituents on the linker. Fine-tuning of the gate opening pressure and of the hysteresis shape is possible by adjusting the substitution pattern and by choice of the metal ion.
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INTRODUCTION Due to their structural diversity and tunable properties, porous coordination polymers (PCPs) or metal−organic frameworks (MOFs) have been intensively studied in the past 15 years.1,2 This unique class of highly porous solids contains promising adsorbents for gas separation3 and storage4,5 as well as for liquid phase separation.6 Further applications in sensor design,7 heterogeneous catalysis,8 and as proton conductors9 illustrate the variety of these materials with targeted functionalities. To ensure accessibility of the pore system for guest molecules removal of incorporated solvent molecules is essential. Regarding their behavior during this activation procedure, MOFs are classified in three generations according to Kitagawa et al.2,10,11 In recent years, more and more research has focused on the investigation and possible applications of MOFs belonging to the third generation.12−17 This particular class of porous materials, also called soft porous crystals, consists of structurally flexible, porous coordination polymers possessing the ability to reversibly respond to external stimuli like pressure,18−21 temperature,22,23 or light.24 Their structural flexibility causes stepwise adsorption and a characteristic adsorption/desorption hysteresis, which is described as a gate opening process or breathing, respectively. In order to investigate the origin of structural transformations occurring in such gate opening processes, isostructural series of flexible © XXXX American Chemical Society
coordination polymers are most promising to understand structure−property relationships. Thereby, variation of a given MOF system is possible either by linker functionalization or metal substitution.5,14,15,17,19,21,25−27 With knowledge of gate opening mechanisms on the atomic level, the targeted adjustment of adsorption properties, e.g., gate opening pressure and hysteresis shape, will be possible. Herein, we present the syntheses, crystal structures, and thermal and gas adsorption properties of two closely related series of MOFs based on triazolyl isophthalate linkers28,29 and the well-known paddle wheel motif. Seven 5-(1,2,4-triazol-4yl)isophthalate ligands ((R1−R2−trz−ia)2−) were used to synthesize 11 MOFs with the general formula ∞3[M2(R1− R2−trz−ia)2] (M = Cu, Zn). The ligands differ in the sterical demand of the triazole substituents R1 and R2, i.e., hydrogen, methyl, ethyl, and n-propyl. This allows a comprehensive investigation of the impact of gradual changes in the linker, combined with the choice of metal ion, on gas adsorption properties. Received: December 18, 2015
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DOI: 10.1021/acs.inorgchem.5b02921 Inorg. Chem. XXXX, XXX, XXX−XXX
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SYNTHESIS AND X-RAY CRYSTALLOGRAPHY The crystal structures of 1, 2, and 4−9 were determined by single crystal X-ray diffraction. Suitable crystals were obtained starting from the appropriate metal salt and the protonated ligands via solvothermal synthesis and via diffusion methods using H2O/MeCN or DMF/EtOH (1:1, v/v) as solvents. Furthermore, compounds 1, 2, 3, 4, and 8Cu are easily accessible in gram scale by refluxing a solution of the starting materials. Detailed MOF synthesis protocols and crystal structure data are reported in the Supporting Information. Compounds 1−4 and 5−9 are members of two closely related series of isomorphous MOFs (Figure 1). While 1, 2, and 4
focus on the systematic investigation of both paddle wheel based triazolyl isophthalate MOF series, including crystal structures and thermal and adsorption properties. Considering the unit cell parameters of 2 (mP) and 5 (oP), the close structural relationship of both series becomes obvious. Whereas the axes a and b of 5 correspond to b and c of 2, the c axis of 5 is related to the doubled a axis of 2 (Table 2). This structural similarity is illustrated by comparison of the unit cell packings of 2 and 5, respectively (Figure 2 and Figure S1). Both MOF series contain the well-known dinuclear paddle wheel motif as a secondary building unit (SBU).27,29,33−36 The bridging coordination of the carboxylate groups results in a square planar MO4 (M = Cu, Zn) environment for the metal ion. Through coordination of N1 (triazole group) in the apical position, the NO4 coordination sphere is completed (Figure 1). This structural motif is known for M = Cu, Zn, Co, and Ni.27,35 Within the two series, the M···M distance varies for M = Cu from 267.9(1) pm to 274.49(4) pm (Table 3), which is in good agreement with other paddle-wheel-based MOF structures.30−33,36 Interestingly, disubstituted triazole groups cause a small extension of the M1···M1a distance compared to linkers with only one substituent on the triazole ring, which is probably due to the enhanced electron density of N1. In the cases of 7Zn and 8Zn, the M1···M1a distance is significantly larger, i.e., 309.38(3) pm and 307.83(5) pm, respectively. In addition, the O−M1 and N1d−M1 bond lengths differ between the Zn2+ and Cu2+ paddle wheel units. For the latter, O−M1 distances in the narrow range of 196.6(3)−200.1(2) pm are observed. Due to Jahn−Teller distortion of the Cu2+ coordination sphere, the N1d−Cu1 bond length is elongated (214.7(6)−217.3(3) pm). In contrast, no stretching of the Zn1−N1d bond is observed for 7Zn and 8Zn. As expected for a d10 metal ion, the coordinative bonds to Zn1 are almost uniform in length (203.1(2)− 206.7(2) pm). As described earlier for similar paddle-wheelbased MOFs, the elongated Zn1···Zn1a distance is partially compensated by the shortened Zn1−N1d bond.37 Note that in all crystal structures of 1, 2, and 4−9, the triazole ring coordinates exclusively via N1, which is the sterically less hindered donor atom. In the cases of 2 and 5, the methyl group is found to be disordered on both possible sides of the triazole ring with minor probability near N1. While the paddle wheel units represent six-connected nodes, the linker molecules act as three-connected nodes, due to their coordination via both carboxylate groups and the triazole group to different Cu2+ ions. Through bridging of paddle wheel units by triazolyl isophthalate ligands, a three-dimensional network is built up. Topological analysis of the networks resulted in two
Figure 1. Fragments of the crystal structures of 4 (rtl-topology, left) and 8Cu (apo-topology, right): coordination sphere of the paddle wheel motif (50% ellipsoids). Symmetry codes, 4: (a) −x, 1 − y, 1 − z; (b) x, 0.5 − y, 0.5 + z; (c) −x, 0.5 + y, 0.5 − z; (d) 1 − x, 1 − y, 1 − z. 8Cu: (a) 1 − x, y, 1.5 − z; (b) 0.5 − x, 0.5 + y, z; (c) 0.5 + x, 0.5 + y, 1.5 − z; (d) 0.5 + x, 1.5 − y, 1 − z.
crystallize in the monoclinic space group P21/c (no. 14, abbr. as mP) with two formula units per unit cell, 5−9 crystallize in the orthorhombic space group Pbcn (no. 60, abbr. as oP) with four formula units per unit cell (Tables 1 and 2). Interestingly, the use of the monosubstituted ligands (H−Me−trz−ia)2− and (H−Et−trz−ia)2− leads to both, monoclinic and orthorhombic paddle wheel based MOFs (2 and 5; 3 and 6, respectively). In case of the disubstituted ligands, only the orthorhombic structure type is observed. In context of isoreticular platforms, the crystal structure of the monoclinic 3∞[Cu2(trz−ia)2] (1) was described earlier.30 In addition, the crystal structures and gas adsorption properties of 3 orthorhombic ∞ [Cu 2 (trz−ia) 2 ] 3 1 and monoclinic 32 3 were reported previously, which ∞ [Cu2(tetrazole−ia)2] implied the potential of this class of MOFs. In this work, we
Table 1. Overview 1−9: Metal Ion and Linker Substitution Pattern
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DOI: 10.1021/acs.inorgchem.5b02921 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 2. Unit Cell Parameters of Paddle Wheel Based 3∞[M2(R1−R2−trz−ia)2] MOFs 1, 2, and 4−9 space group a/pm b/pm c/pm β/deg V/106 pm3 Z
1
2
4
5
6
7Cu
7Zn
8Cu
8Zn
9
P21/c 1096.8(2) 1220.3(1) 1436.6(2) 110.30(1) 1803.3(5) 2
P21/c 1085.9(1) 1278.6(2) 1402.6(2) 110.508(8) 1823.9(4) 2
P21/c 1087.8(2) 1257.9(2) 1411.4(2) 110.02(2) 1814.5(5) 2
Pbcn 1298.7(1) 1406.2(1) 2040.4(2) 90 3726.3(5) 4
Pbcn 1345.9(1) 1320.5(1) 2018.4(2) 90 3587.3(5) 4
Pbcn 1340.1(1) 1337.94(8) 2028.1(1) 90 3636.3(4) 4
Pbcn 1349.52(7) 1324.45(6) 2044.0(1) 90 3653.4(4) 4
Pbcn 1343.44(9) 1334.9(1) 2018.1(2) 90 3619.0(5) 4
Pbcn 1356.3(1) 1333.7(1) 2033.7(2) 90 3678.9(6) 4
Pbcn 1333.4(2) 1365.5(1) 2004.2(2) 90 3649.1(6) 4
Figure 2. Comparison of the unit cells of 2 (rtl topology, left, view in b direction) and 5 (apo topology, right, view in a direction). H atoms are omitted for clarity.
Table 3. Selected Atom Distances and Angles of 1, 2, and 4−9 1
2
4
M1···M1a M1−O1 M1−O2a M1−O3b M1−O4c M1−N1d
269.58(9) 198.2(4) 197.9(4) 197.8(3) 197.0(3) 216.9(4)
268.98(8) 196.6(3) 197.1(3) 196.9(3) 197.7(3) 215.0(3)
267.9(1) 198.3(6) 198.5(6) 197.0(5) 197.6(5) 214.7(6)
O1−M1−N1d O3b−M1−N1d O2a−M1−O3b O3b−M1−O4c
92.5(1) 94.3(1) 90.2(1) 166.9(1)
94.1(1) 96.4(1) 90.6(1) 166.7(1)
92.8(2) 94.4(2) 90.9(2) 167.4(2)
5
6
atom distance/pm 270.84(4) 270.07(4) 198.6(2) 196.9(2) 199.0(2) 197.2(2) 198.4(2) 197.7(2) 199.1(2) 198.1(2) 216.6(2) 214.9(2) bond angle/deg 96.64(8) 99.94(9) 94.81(8) 93.19(9) 166.89(7) 167.72(9) 90.36(7) 90.39(9)
7Cu
7Zn
8Cu
8Zn
9
274.49(4) 199.3(2) 197.2(2) 197.5(2) 200.1(2) 217.2(2)
309.38(3) 205.3(2) 203.1(2) 204.5(2) 205.1(2) 203.4(2)
273.51(3) 197.6(2) 196.8(2) 197.1(2) 198.7(2) 215.9(2)
307.83(5) 205.5(2) 204.6(2) 206.7(2) 205.3(2) 203.7(3)
272.03(5) 197.5(2) 197.9(2) 197.6(2) 198.3(2) 217.3(3)
103.00(8) 91.89(8) 169.13(8) 90.05(8)
107.13(6) 94.68(6) 161.08(6) 88.83(6)
103.01(6) 92.45(6) 169.44(6) 90.11(6)
108.04(9) 94.13(9) 163.30(9) 88.70(9)
103.6(1) 91.4(1) 167.5(1) 90.8(1)
interconnected three-dimensional pore system for both isomorphous MOF series (Figure 4). Even in the structure of 9 (R1 = R2 = Et), very narrow nonlinear pore channels are visible. For 1−4 and 5−9, the void fractions are calculated to be in the range of 53−35% and 50−31%, respectively (Table 4).38 For a detailed analysis of the pore system, pore size distributions (PSDs) were calculated on the basis of the crystal structures (Figure 5).40 In case of 3, the PSD was calculated on the basis of the crystal structure of 4 (replacing n-propyl with ethyl group) in combination with lattice constants as determined from the X-ray powder diffraction pattern of 3. The pore diameters in 1−9 vary in the range of 0.25 nm up to 0.60 nm, which illustrates their pronounced microporosity. Interestingly, compound 1 does not show the largest pore
closely related network types. Whereas 1, 2, and 4 possess rtl (rutile) topology with point symbol {4.62}2{42.610.83}, the networks of 5−9 are of apo (α-PbO2) topology with point symbol {4.62}2{42.69.84}. Thus, the networks of the two isomorphic series differ only slightly in the number of sixand eight-membered circuits, which illustrates their close relationship. For both framework types, narrow pore channels are visible along the a (1, 2, 4) or c direction (5−9), accordingly. The pore surface is built up by aromatic rings of the linkers, whereas the R2 substituents are arranged inside the pore windows (Figure 3). In this fashion, the choice of the substituent influences the solvent accessible volume of the framework. A detailed view inside the crystal structures reveals an C
DOI: 10.1021/acs.inorgchem.5b02921 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 3. Representation of the 3D networks of 1 (top left), 4 (top right), 5 (bottom left), and 9 (bottom right). The alignment of the substituents is emphasized (green, R2; yellow, R1).
Table 4. Calculated Pore Volumes for CO2 Adsorption at 298 K According to Gurvich Equation45 (p0= 6.4121 MPa)
1 2 3 4 6 7Cu 8Cu 8Zn 9
Figure 4. Visualization of the 3D interconnected pore system (gray, 2 × 2 × 2 super cell) of 1 (left) and 9 (right).39
diameter of the monoclinic MOF series although the network is built up by the ligand with the smallest substituent (R1 = R2 = H). Instead, the network of 2 containing the methyl substituted linker shows the largest pore size of 0.49 nm. In addition, in the cases of 1 and 2, a smaller aperture is present at 0.33 and 0.32 nm, respectively. Replacement of the methyl by an ethyl group leads to a unimodal PSD of 3 with a maximum frequency at 0.34 nm. Hence, the bulkier ethyl group induces a blocking of the larger pore. By further elongation of the alkyl chain (npropyl, 4) the pore diameter is reduced to 0.32 nm, which is in good agreement with the orientation of the substituent toward the cavities (Figure 3). In contrast to 1−4, PSDs of 5−9 are
a
calculated from X-ray structure data
after first pore filling
porosity/%
ρ/g cm−3
VPore/ cm3 g−1
p/p0
VPore/ cm3 g−1
52.6 47.1/50.4
1.086 1.124
0.48 0.43
39.9a 35.0 38.5 40.7 35.9 36.1 30.8
1.197a 1.233 1.195 1.179 1.236 1.223 1.277
0.33a 0.28 0.32 0.35 0.29 0.30 0.24
0.796 0.015 0.309 0.118 0.045 0.377 0.341 0.241 0.054 0.115
0.49 0.01 0.29 0.05 0.03 0.26 0.22 0.22 0.15 0.10
after second pore filling
p/p0
VPore/ cm3 g−1
0.664
0.40
0.867 0.869 0.882 0.798 0.844 0.931 0.793
0.34 0.29 0.34 0.28 0.28 0.32 0.26
Calculated based on unit cell dimensions determined by PXRD.
multimodal, and their shape is influenced by the choice of the linker and, to a smaller extent, by the metal ions used. The networks of 7Cu and 7Zn, containing the disubstituted ligand (Me2−trz−ia)2−, form the largest pores of the orthorhombic MOF series with maximum pore diameters of 0.56 and 0.57 nm, respectively. It can be concluded that an increase of the sterical demand of one triazole substituent results in reduction D
DOI: 10.1021/acs.inorgchem.5b02921 Inorg. Chem. XXXX, XXX, XXX−XXX
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indexing the powder diffraction pattern (Table S3). Indexing of the as-synthesized PXRD of 4, which shows minor deviations in comparison to the simulated one, including broadening of individual reflections, revealed a monoclinic unit cell similar to the single crystal data (Figure S4 and Table S4). The broadening of the 1̅11 reflection at 2θ = 11.5° (1 and 4) was observed earlier for 130 and for an isostructural MOF based on a tetrazolyl isophthalate ligand.32 Interestingly, the PXRD pattern of as-synthesized 6 shows deviations from the simulated one. An explanation for this deviation could be a slightly smaller orthorhombic unit cell identified by indexing of the experimental pattern (Figure S5 and Table S5). As discussed below, the thermal behavior of 6 fits perfectly to the orthorhombic MOF series (Figure S10). The compilation of the as-synthesized powder patterns of 1−4 and 6−9 is shown in Figure S7 and illustrates the structural similarity within the two isomorphous series. In order to achieve a complete solvent exchange prior to thermal stability and gas adsorption studies, Soxhlet extraction with methanol was carried out over 7 days. After this postsynthetic treatment, the PXRD patterns remain unchanged except in case of 7Cu, indicating a response of this framework to solvent exchange. However, after adsorption experiments with N2 and CO2 including activation procedures and multiple adsorption/desorption cycles, the powder diffraction patterns change as a response to the sorption stress. After resolvation with methanol, the pristine PXRD patterns and, thus, the crystal structures are restored in the cases of 2, 3, 7Cu, 8Zn, and 9, indicating unchanged network connectivities during activation and adsorption/desorption processes. For 1, 4, 6, and 8Cu, the PXRD patterns (Figures S2−S6) after adsorption experiments and resolvation do not fit to the pattern of the Soxhlet extracted sample. However, as discussed below, the adsorption/desorption isotherms are fully reproducible. Prior to adsorption experiments the thermal stability of Soxhlet extracted samples was investigated by simultaneous thermal analysis (TG-DTA-MS) and temperature dependent X-ray diffraction (TD-PXRD). In contrast to all other analyzed samples, the TD-PXRD pattern of 1 shows no phase transition, although solvent molecules are evaporated from the pore system (Figures S8 and S11). Interestingly, even after Soxhlet extraction over 7 days with methanol, a great portion of water remains in the pores. After desolvation, the framework collapses at 250 °C associated with the release of CO2 from the organic linker. This observation is in good agreement with the TDPXRD pattern showing no reflections above this temperature. For all other compounds, a phase transition is observed due to
Figure 5. Pore size distribution of 1−9 calculated from single-crystal structure data (3: structure model based on 4; solid and dashed lines correspond to the two limiting cases due to disorder of the substituent (2, 5)).
of the maximum pore diameter by 50 pm (8Cu and 8Zn). This observation is consistent with the concept of isoreticular MOFs (IR-MOFs), indicating that pore size, shape, and functionality can be tuned systematically.41 All in all, the PSDs show cavities with varying diameters that can be fine-tuned via linker substitution. Thus, the two series are promising candidates for the systematic investigation of linker influence on gas adsorption properties.
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X-RAY POWDER DIFFRACTION AND THERMAL STABILITY The comparison of X-ray powder diffraction (PXRD) patterns of 1−4 and 6−9 with simulated powder patterns based on single crystal data confirm phase purity of all samples used for adsorption studies (Figures S2−S6). Exemplarily for the monoclinic and orthorhombic MOF series, the PXRD patterns of 2 and 9 are shown in Figure 6. In general, the experimental powder patterns of the as-synthesized samples are in good agreement with the simulated ones. As mentioned earlier, 3 is not accessible as single crystals but could be identified by
Figure 6. X-ray powder diffraction patterns (λ(Cu−Kα1) = 154.060 pm) of 2 (left) and 9 (right) obtained by several synthetic methods and the powder patterns after postsynthetic treatments. E
DOI: 10.1021/acs.inorgchem.5b02921 Inorg. Chem. XXXX, XXX, XXX−XXX
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temperature phases show reflections up to 250 °C, in the case of 8Zn even up to 390 °C. The thermal decomposition accompanied by release of CO2 starts at 250 °C (9) to 280 °C (4, 6, 7Cu) and in the case of 8Zn at 390 °C.
solvent evaporation. The peculiarity of 1 showing no phase transition indicates that the absence of triazole substituents is related to the observed rigidity, and, on the other hand, introduction of alkyl substituents in the linker allows the framework to respond to solvent evaporation with structural changes. This key impact of linker side chains on the thermal behavior was also discussed for a series of MOFs based on alkoxy functionalized bdc-linkers.22 It also appears to be related to the structural flexibility observed during CO2 adsorption experiments (see below). Exemplarily for the monoclinic and orthorhombic MOF series, the TD-PXRD patterns of 2 and 9 are shown in Figure 7.
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ADSORPTION STUDIES In order to evaluate their adsorption characteristics, high pressure CO2 (298 K) and low pressure N2 (77 K) adsorption measurements with 1, 2, 3, 4, 6, 7Cu, 8Cu, 8Zn, and 9 were carried out. Although these frameworks exhibit calculated pore volumes of up to 0.48 cm3 g−1 (PLATON),38 N2 is not adsorbed by 2, 8Cu, 8Zn, and 9 at 77 K. For 1, 3, 4, 6, and 7Cu, only a small uptake of N2 is observed (Figures S16−S18). In contrast, CO2 (298 K) is adsorbed in all investigated MOF materials. Whereas 1 shows a type I isotherm with a saturation loading of 9 mmol g−1 (Figure S16), the CO2 isotherms of 2−9 do not correspond to any type of the recommended IUPAC classification of physisorption isotherms.42 Instead, S-shaped isotherms originating from a stepwise pore filling are observed in combination with broad adsorption/desorption hystereses (Figures S16−S18). This type of isotherm is characteristic of flexible MOFs showing a steep increase in the adsorption branch at the so-called gate opening pressure.11,20,21,26 This flexibility is based on flexible linkers and/or deformation of secondary building units (SBUs).12 In particular, paddle-wheelbased MOFs are well-known for their high degree of structural flexibility.5,14−16,27,37,43 As discussed before, the PXRD patterns of several members of the two series after adsorption experiments and resolvation with methanol do not fit to the patterns of the Soxhlet extracted samples. However, the shape of all isotherms is reproducible, demonstrating the reversibility of structural changes occurring during the gate opening process and the robustness of the adsorbents against sorption stress. Again, the pore diameters can be fine-tuned by choice of the ligand. Hence, the pore volume is dependent on the sterical demand of the substituent (Table 4). For the monoclinic MOF series, this relationship is proven experimentally, as the CO2 saturation loading decreases with elongated alkyl chain (Figure 8). The experimentally determined pore volumes of 1−4 are in good agreement with the calculated ones based on single crystal data. Thus, CO2 is able to access the entire pore system (Table 4). It becomes obvious that the substituent not only influences the accessible pore volume but also affects the gate opening pressure. For 2, a steep increase is observed at 0.3 MPa followed by a plateau
Figure 7. Temperature dependent PXRD patterns (Guinier-Simon diagrams) of 2 (left) and 9 (right) (λ(Cu−Kα1) = 154.060 pm).
Whereas 9 shows a distinct change in reflection positions at 110 °C, for 2 a gradual structural change is observed up to 130 °C followed by two phase transitions. With the exception of 4 and 9, the simultaneous thermal analysis of all samples reveals the endothermic loss of methanol and small amounts of water up to 180 °C, which indicates that in the cases of 4 and 9 all solvent is already removed during sample preparation and premeasurement evacuation in the instrument (Figures S11−S15). As shown in Figure S10, the high temperature phases of 2 and 3 are very similar, whereas the pattern of 4 is different. During the phase transition, the monoclinic symmetry is preserved associated with a cell volume contraction by 36% (Table S6). Contraction of the unit cell volume during activation is probably the reason for the decay of single crystals as observed in SEM images of 2 (Figure S23). The high temperature phases of the orthorhombic MOF series are in accordance except for 8Zn. Nevertheless, the volumes of the orthorhombic unit cells determined at 200 °C are reduced by 30%. These high
Figure 8. CO2 adsorption isotherms (298 K; closed symbols, adsorption; open symbols, desorption; lines are to guide the eyes) for 1−4 (left) and 6−9 (right) in semilogarithmic plot. F
DOI: 10.1021/acs.inorgchem.5b02921 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 9. CO2 adsorption isotherms (298 K; closed symbols, adsorption; open symbols, desorption; lines are to guide the eyes) for 3 and 6 (left) and 8Cu and 8Zn (right) in semilogarithmic plot.
indicating a complete filling of the accessible pore system (p/p0 = 0.309; 0.29 cm3 g−1). At 2.0 MPa another gate opening occurs, generating new sorption sites. The saturation loading of 7 mmol g−1 corresponds to a pore volume of 0.40 cm3 g−1, which is in quite good agreement with the theoretical value of 0.43 cm3 g−1. Up to 0.2 MPa, almost no CO2 is adsorbed (
