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Jul 5, 2016 - ABSTRACT: A postsynthetic functionalization approach was used to tailor the hydrophobicity of DUT-67, a metal−organic framework (MOF) ...
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Postsynthetic Inner-Surface Functionalization of the Highly Stable Zirconium-Based Metal−Organic Framework DUT-67 Franziska Drache,† Volodymyr Bon,† Irena Senkovska,*,† Claudia Marschelke,‡ Alla Synytska,‡ and Stefan Kaskel† †

Department of Inorganic Chemistry, Technische Universität Dresden, Bergstraße 66, 01062 Dresden, Germany Leibniz-Institut für Polymerforschung Dresden eV, Hohe Straße 6, 01069 Dresden, Germany

‡

S Supporting Information *

ABSTRACT: A postsynthetic functionalization approach was used to tailor the hydrophobicity of DUT-67, a metal−organic framework (MOF) consisting of 8-connected Zr6O6(OH)2 clusters and 2,5-thiophenedicarboxylate as the ligand, using postsynthetic exchange of the modulator by fluorinated monocarboxylates. Water adsorption isotherms demonstrated that, by the incorporation of such hydrophobic molecules, the hydrophobicity of the inner surface of the network can be tuned. Furthermore, tolerance of the material toward the removal of adsorbed water can be significantly enhanced compared to the parent DUT-67 MOF.

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INTRODUCTION Metal−organic frameworks (MOFs) are crystalline porous materials with exceptionally high porosity and a strictly defined pore system. Additionally, the rational building block concept allows tuning of the pore size and chemical functionality of the surface over a wide range.1 Attractive properties of MOFs are responsible for versatile potential applications in the fields of gas storage, separation processes, sensing, catalysis, etc.2−4 Furthermore, because of the high water adsorption capacity, MOFs are extremely interesting for heat transformation applications, like heat pumps or chillers.5−9 The main limitations, however, in this case arise from the low hydrolytic stability of many MOFs, while water is the preferred working fluid in solid sorption heat transformation systems.10 Since the discovery of UiO-66 in 2008,11 zirconium-based MOFs have attracted considerable interest because of their thermal and, most notably, chemical stability.12 The majority of Zr-based frameworks are stable in water and acidic media over long periods of time. However, repeated water adsorption/ desorption cycling on some UiOs and related materials, nevertheless, leads to a pronounced decrease in water uptake after first adsorption/desorption cycle, making the application of MOFs in heat transformation processes inefficient because multiple adsorption cycling is a crucial prerequisite.13,14 Moreover, not only the water stability but also the hydrophilic/hydrophobic balance and polarity of the inner surface are extremely important for these types of applications. Recently, Farha et al. pointed out that such cyclic instability arises to a lesser extent from the hydrolytic instability through water clustering and more from the strong capillary forces arising during water removal. Therefore, an increased stability against © XXXX American Chemical Society

cyclic water adsorption/desorption by tuning the hydrophilic/ hydrophobic properties of the inner surface of Zr-based MOFs is still a key issue.15−21 One approach to improving the hydrolytic stability is based on ligand functionalization;22 however, this approach is time-consuming and cost-intensive. Another promising way to introduce the water-repelling molecules on the inner surface of the MOF could be functionalization of the metal cluster. However, the latter is only feasible if the metal cluster contains additional coordination capacity. In the case of Zr-based MOFs, only frameworks with reduced cluster connectivity are suitable. The proof of principle was successfully performed on NU-1000, where terminal −OH groups were functionalized with benzoic acid, perfluoropentanoic acid, and perfluorodecanoic acid.23 Recently, we discovered a Zr-based MOF with reduced cluster connectivity, Zr6O6(OH)2(tdc)4(CH3COO)2 (tdc = 2,5thiophenedicarboxylate), also known as DUT-67 (DUT Dresden University of Technology).24 In contrast to UiO-type frameworks, forming a 12-connected fcu net, the connectivity of the cluster in DUT-67 is reduced to 8 (reo topology). For charge balancing and to complete the coordination sphere of Zr atoms, two additional monocarboxylate anions (modulator used in the synthesis) and solvent molecules (N,Ndimethylformamide, DMF) were coordinated to the cluster (Figure 1). This cluster configuration is ideal for the postsynthetic functionalization approach because the monoSpecial Issue: Metal-Organic Frameworks for Energy Applications Received: April 4, 2016

A

DOI: 10.1021/acs.inorgchem.6b00829 Inorg. Chem. XXXX, XXX, XXX−XXX

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modulator in the synthesis (Figure S22). The change of the modulator does not affect the experimental porosity significantly (Table 1). The discrepancies between the theoretical and experimental pore volumes can originate from some defects, partial collapse of some pores after solvent removal, or additional clusters,24 which could be localized crystallographically in the structure of DUT-67. For functionalization, the larger modulator molecules, namely, trifluoroacetic acid (HTfa), 4-(trifluoromethyl)benzoic acid (HTfmba), pentafluorobenzoic acid (HPfba), and perfluorooctanoic acid (HPfoa) containing hydrophobic groups, were integrated into DUT-67 in order to tune the polarity of the inner surface (Table 2). Because the larger

Figure 1. Left: Zr6O6(OH)214+ cluster in DUT-67. The blue spheres represent the coordination sites of the modulator or DMF molecules. Right: Crystal structure of DUT-67. Clusters are shown in green, carbon in gray, oxygen in red, and sulfur in yellow. Hydrogen atoms are omitted for clarity.

Table 2. List of Carboxylic Acids Used for Postsynthetic Modulator Exchange in DUT-67

Thus, using carboxylic acids with water-repellent functional groups (fluorinated monocarboxylates) should critically tune the stability and water adsorption behavior of DUT-67. At the same time, the postsynthetic modulator exchange approach is less complex and less expensive in comparison to the proposed earlier linker functionalization route,26 because no special synthetic protocol of organic components is involved, and commercially available monocarboxylic acids can be utilized. It should also be pointed out that the direct synthesis of frameworks using linkers with additional functionality in some cases causes the undesired formation of nonporous byproducts. In this contribution, we used several fluorinated monocarboxylic acids, such as trifluoroacetic acid, pentafluorobenzoic acid, 4-(trifluoromethyl)benzoic acid, and perfluorooctanoic acid, in the postsynthetic functionalization of DUT-67(Zr) and studied their influence on the hydrophobicity of the inner surface and on the cyclability of water adsorption.

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RESULTS AND DISCUSSION Surface Functionalization and Textural Properties of the Investigated Compounds. As a starting material for the study, a MOF containing formate as the monocarboxylic ligand (DUT-67-Fa) was used. After activation, this compound has a specific surface area of 1143 m2 g−1 and a total pore volume of 0.47 cm3 g−1. Instead of formic acid (HFa), it is also possible to use acetic acid (HAc)24 or propionic acid (HPa) as the

molecules could not be utilized directly during the synthesis, subsequent experiments were performed using a postsynthetic pathway. Functionalization of DUT-67 was performed by

Table 1. Textural Properties of DUT-67 and the Functionalized Analogues DUT67-Fa

DUT67-Ac

DUT67-Pa

DUT-67Tfa

DUT-67Tfmba

DUT-67Pfba

DUT-67Pfoa

0.62 2152 0.47 1143 0.40 85 0.31

0.60 2080 0.47 1171 0.41 87 0.38

0.58 2128 0.46 1063 0.40 87 0.37

0.55 2007 0.41 1012 0.35 85 0.38

0.47 1608 0.45 1111 0.36 80 0.39

0.46 1721 0.42 1021 0.35 83 0.40

0.33 1053 0.32 773 0.23 72 032

34

20

20

7

13

5

0.3

theoretical pore volume, cm3 g−1 theoretical surface area, m2 g−1 pore volume derived from nitrogen adsorption, cm3 g−1 surface area (BET) derived from nitrogen adsorption, m2 g−1 pore volume derived from water adsorption, cm3 g−1 water pore-filling degreea, % pore volume derived from nitrogen adsorption for materials after water adsorption/desorption, cm3 g−1 loss of porosity after water desorption, % a

Calculated as the pore volume derived from water adsorption isotherm divided by the pore volume derived from nitrogen adsorption isotherm. Both pore volumes were calculated at p/p0 = 0.97. B

DOI: 10.1021/acs.inorgchem.6b00829 Inorg. Chem. XXXX, XXX, XXX−XXX

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connectivity opens the possibility of performing postsynthetic modification of the cluster. Careful analysis of the crystal structure shows that only the inner surface of the cages with the center in the Wyckoff positions 8c (octahedral pore, A) and 4b (cuboctahedral pore, B) could be modified using postsynthetic modulator exchange. The remaining cuboctahedral pore (C) should not be modified because all terminal oxygen atoms, which could participate in the exchange, are oriented outward from this cage. In order to gain deeper insight into the chemistry of the pore surfaces, crystal structures containing modulators used in this study were simulated and subjected to detailed analysis of the pore-size distribution using Zeo++ software (Figure 3). Three DMF molecules were placed in the smallest octahedral pore. The presence of DMF molecules leads to a formal splitting of the pore space into two voids with much smaller pore size. Zeo ++ identifies this pore as two different pores with 4.4 and 5.7 Å in diameter (marked yellow in Figure 3). The size of the cuboctahedral pore B in DUT-67 after insertion of modulator molecules varies from 17.1 Å (hypothetical structure without the modulator) to 3.6 Å (for DUT-67-Pfoa). Certainly, the positions of DMF and modulator molecules on the cluster are statistically disordered within the above-mentioned cages and the model for calculation is only an approximation. Nonetheless, the structural models presented here, in general, describe well the changes on the inner surface of the material and are helpful for understanding the functionalization process in detail. To compare the pore sizes calculated on the basis of the simulated structures with the experimentally observed pore-size distributions, nonlocal density functional theory methods (see the Supporting Information, section 1.2) were applied to nitrogen adsorption isotherms (at 77 K) of the corresponding MOFs. The best fit could be achieved using a silica kernel based on a spherical and cylindrical pore model. At relative pressures higher than 10−5, the calculated and measured isotherms fit quite well. At lower pressures, some deviations are observed that can be attributed to inappropriate equilibration or quadrupole interactions, a common problem in high-resolution nitrogen adsorption isotherms.28 Thus, pore sizes estimated for the smallest pore cannot be considered to reflect realistic values and therefore are not further discussed. In the pore-size distribution of DUT-67-Fa (Figure S8), three distinct maxima are presented corresponding to different pore types. The cuboctahedral pore C (purple sphere in Figure 3a), which does not have any modulator molecules inside, has a diameter of 14.3 Å, according to the pore-size distributions in DUT-67-Fa, DUT-67-Ac, DUT-67-Tfa, and DUT-67-Tfmba. This is in good agreement with the calculated pore size from simulated structures. For DUT-67-Pa, DUT-67-Pfba, and DUT-67-Pfoa, the pore diameter could not be determined exactly because of overlapping of the peaks (Figures S10, S13, and S14). The size of cuboctahedral pore B (red sphere in Figure 3a), where modulator exchange takes place, decreases with the size of the modulator. However, the peaks are broadened, indicating the rotational mobility of the modulator inside the pore. For DUT-67-Fa, the pore size was estimated to be 16.3 Å, for DUT-67-Ac 12.4 Å and for DUT-67-Pa 13.2 Å. The values for DUT-67-Tfa, DUT-67-Pfba, and DUT-67Tfmba are similar (approximately 12.7 Å), which can be explained with a different degree of modulator exchange. Thermal Stability. Thermogravimetric analysis (TGA; Figure S26−S32) and in situ thermo XRD measurements were carried out (Figure S25) to study the thermal stability of

exposing parent DUT-67-Fa crystals to a concentrated solution of the selected monocarboxylic acid in a suitable solvent. Initially, HTfmba was used for ligand exchange, and the exchange rate was monitored by 1H NMR (Figure S47). The kinetics follow pseudo-second-order, illustrating a successful exchange, which is almost complete after 24 h. Finally, a conversion of 87% after 3 days could be achieved. The DMF molecules coordinated to the cluster are retained if exchange is performed in DMF as the solvent. If ethanol is used as the solvent during the exchange, the amount of DMF on the cluster decreases drastically to almost zero (Figures S44−S46). The successful exchange of the other fluorinated monocarboxylates was proven by 1H and 19F NMR spectra (Figures 2 and S33−S43). The 1H NMR spectroscopy show that the amount of the initial modulator (Fa) is significantly decreased in all cases.

Figure 2. 1H NMR (top) and 19F NMR (bottom) spectra of desolved DUT-67-Tfmba. 1H-NMR: (DMSO/DCl): δ/ppm = 7.87 (d, 2H, tfmba), δ/ppm= 8.13 (d, 2H, tfmba), δ/ppm= 7.71 (s, 2H, tdc), δ/ ppm = 7.93 (s, 1H, DMF), δ/ppm = 8.14 (s, 1H, fa); 19F-NMR (DMSO/DCl): δ/ppm = −62.17.

Each of the postsynthetically modified DUT-67 analogues retains a high degree of crystallinity (Figure S22), as well as porosity (Table 1). In comparison to DUT-67-Fa, the fluorinated analogues have slightly lower pore volumes calculated from nitrogen adsorption isotherms, which correspond well to the theoretical total pore volumes trend because they decrease with increased size of the modulator molecule. Pore-Size Distribution. DUT-67(Zr) has three types of pores with different pore sizes. If no modulator is considered and the original crystal structure is taken for calculation (the vacant positions on the cluster is occupied by oxygen atoms only), pore-size distribution analysis, performed using Zeo++,26 results in three maxima at 9.1, 14.6, and 17.1 Å (Figure 3). As was mentioned before, the reduced secondary building unit C

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Figure 3. (a) Pore system of DUT-67 without modulator and solvent molecules. (b) Pore system of DUT-67-Pfba: the cuboctahedral pore B (red sphere in part a) is affected by the modulator exchange; the octaheadral pore A (yellow sphere in part a) splits by DMF molecules. Zr polyhedra are illustrated in green, carbon atoms in dark gray, oxygen atoms in red, sulfur atoms in yellow, nitrogen atoms in dark blue, fluorine atoms in pale blue, and hydrogen atoms in light gray. (c) Pore-size distribution, calculated for the structural models of DUT-67(Zr) materials containing different modulator molecules, using Zeo++.

Water Adsorption/Desorption Isotherms. To gain a deeper understanding of the influence of postsynthetic functionalization on the surface properties, the water physisorption isotherms were studied for all investigated compounds. The water adsorption isotherm of DUT-67-Fa at 298 K shows three distinct steps characteristic for the hierarchical three-modal pore system discussed above26 (Figure 4). The isotherm reaches saturation at 499 cm3 g−1, corresponding to a pore volume of 0.40 cm3 g−1. Interestingly, the nitrogen adsorption isotherm measured on the sample used for the water adsorption experiment and reactivated at 100 °C in vacuum shows a drastic drop of the porosity, which is reflected in a pore volume decrease of 34% (Figure S1 and Table 1). Significant differences are observed in the water adsorption isotherms after functionalization (Figure 5). Up to a relative pressure of p/p0 = 0.2, the isotherms of all materials show the

the DUT-67 series. In air, decomposition of DUT-67-Fa takes place at temperatures between 275 and 300 °C. In this temperature range, DMF and modulator molecules are eliminated. DUT-67 materials containing bulkier modulator molecules are even less thermally stable because of the weaker modulator bonding (Figure S24). The strength of the Zr− O(modulator) bond depends on the basicity of the monocarboxylic acid used for exchange. The tendency to release the monocarboxylic ligand decreases with an increase of their basicity (Table S1), confirming the observations reported in the literature.29 The crystallinity is preserved after activation in vacuum at 100 °C for all compounds (Figure S22). At 150 °C, DUT-67Tfa and DUT-67-Tfmba become less crystalline according to XRD analysis, and DUT-67-Pfba even becomes amorphous. DUT-67-Pfoa is the exception; it seems to be stable at these conditions (Figure S24). D

DOI: 10.1021/acs.inorgchem.6b00829 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Water adsorption (solid symbols) and desorption (blank symbols) isotherms at 298 K of DUT-67-Fa (left) and DUT-67-Tfmba (right). The red lines represent the slope of the isotherm for pore filling of the cuboctaheral pore B (red sphere in Figure 3), and the purple lines represent the slope of the isotherm for pore filling of the cuboctahedral pore C (purple sphere in Figure 3).

Figure 5. Water adsorption isotherms at 298 K. Left: DUT-67-Fa (black circles), DUT-67-Ac (dark-gray rhombuses), DUT-67-Pa (gray triangles), and DUT-67-Tfa (light-gray stripes). Right: DUT-67-Tfmba (dark-gray twisted crosses), DUT-67-Pfba (gray crosses), and DUT-67-Pfoa (light-gray stars). The background color represents filling of the corresponding pore (yellow, small octahedral pore; purple, cuboctahedral pore without the modulator; red, cuboctahedral pore with the modulator).

C (purple sphere in Figure 3: d = 14.6 Å). Since the pore walls of C are decorated only by tdc linker molecules and no terminal monocarboxylates, a similar relative pressure for filling this pore is expected. As a consequence, no modulator molecules protrude into the pore and the surface properties cannot be influenced by the choice of modulator. Summarizing, the use of different modulators in the synthesis of DUT-67 influences only the pore filling of the cuboctahedral pore B (red sphere in Figure 3), where the exchange of modulator takes place. A similar pore filling mechanism can be assumed for DUT-67-Tfa that could be obtained only utilizing postsynthetic modulator exchange. If sterically demanding fluorinated monocarboxylic acids are used for the postsynthetic exchange, the pore-filling mechanism for DUT-67-Tfmba, DUT-67-Pfba, and DUT-67-Pfoa changes drastically. The large fluorinated carboxylic acids shield the polar Zr cluster and repell water molecules from the cuboctahedral pore B (red sphere in Figure 3), leading to a switching of the pore-filling order of the two different cuboctahedral pores: the filling of the cuboctahedral pore C (purple sphere in Figure 3), which has no free coordination sites, corresponds now to the second step in the water adsorption isotherm. This assumption is supported by a comparison of the corresponding slopes in the water adsorption

same slope, indicating that the inner surface of the smallest pore (represented as a yellow sphere in Figure 3) was not affected by modulator exchange. Obviously, this pore is too tiny for modulator molecules larger than HFa and, therefore, all free coordination sites, which protrude in this pore, are only occupied by DMF molecules. The second plateau in the isotherm of DUT-67-Fa (starting at p/p0 ≈ 0.30) can be attributed to the filling of the cuboctahedral pore B (red sphere in Figure 3), exposing a surface that is most prone to be affected by modulator chemistry. Therefore, the major differences in water adsorption isotherms including different modulator molecules are visible in the isotherm step, which is responsible for filling of pore B (red sphere in Figure 3). Comparing the second step of the isotherm for DUT-67-Fa, DUT-Ac, and DUT-67-Pa, a clear trend is detected: longer alkyl chain of the modulator causes a decreased slope of the isotherms as well as shift of the saturation point to higher humidity. This behavior indicates that the hydrophobicity of this cage is changing by the functionalization of the inner surface. The plateau between the second and third steps in the isotherm of DUT-67-Fa is wider in comparison to the other materials. The third step starts at p/ p0 = 0.4 and has a smaller slope and is quite similar for all three isotherms and therefore can be referred to cuboctahedral pore E

DOI: 10.1021/acs.inorgchem.6b00829 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Advancing water contact angle for investigated compounds.

67-Tfmba, to 7% for DUT-67-Tfa, to 5% for DUT-67-Pfba and even to 0.3% for DUT-67-Pfoa (Table 1 and Figures S4−S7). Water physisorption isotherms of the latter also show a hysteretic behavior, but the hysteresis loops are almost closing during desorption (Figures S18−S21), indicating reversibility of the adsorption/desorption process. In addition, not only the porosity but also the crystallinity is preserved after water desorption in vacuum at 100 °C. The powder X-ray diffraction (XRD) patterns of DUT-67-Pfoa, DUT-67-Tfa, DUT-67Tfmba, and DUT-67-Pfba (Figure S23), and even of DUT67-Fa, match the calculated one well, confirming phase purity and a high degree of crystallinity. Contact Angle. Contact-angle measurements are commonly used to examine the hydrophobic/hydrophilic properties of the materials. For water contact-angle measurement, the materials were compacted into a pellet. DUT-67-Fa was found to be very hydrophilic, absorbing the water droplet completely. Thus, the contact angle could not be estimated in this case. In contrast, no significant water adsorption was observed for all other investigated samples. DUT-67-Ac, DUT-67-Pa, and DUT-67-Tfa have similar hydrophilicity, displaying an advancing contact angle close to 60° (Figure 6). However, the DUT67 samples modified with Tfmba and Pfoa show advancing contact angles of ≥103° and can be regarded as more hydrophobic materials. The increasing contact angle also develops well along the line as the stability increases during water removal.

isotherms (Figures 4 and S15−S21). Thus, the slope of the second step in the DUT-67-Pfba and DUT-67-Tfmba water adsorption isotherms corresponds exactly to the third step of the parent DUT-67-Fa. At the same time, the slope of the third step in the adsorption isotherms of DUT-67-Pfba and DUT-67Tfmba is extremely small. The isotherms reach saturation at p/ p0 = 0.68 (Pfba), 0.78 (Pfmba), and ≈0.6 (Pfoa), which are significantly higher in comparison with the water adsorption isotherm of DUT-67-Fa, showing saturation at p/p0 = 0.5. Furthermore, the pore volume of the cuboctahedral pore C (Figure 3, purple sphere), calculated from the third step of the DUT-67-Fa water adsorption isotherm (0.13 cm3 g−1), is similar to the pore volume calculated for DUT-67-Tfa and from the second step of the DUT-67-Pfba and DUT-67-Tfmba isotherms (Vp = 0.14 cm3 g−1). Similar calculations for the cuboctahedral pore B with modulator molecules inside (Figure 3, red sphere) result in 0.11 cm3 g−1 for DUT-67-Tfa (p/p0 = 0.2−0.57) and 0.12 cm3 g−1 for DUT-67-Pfba (p/p0 > 0.47) and DUT-67-Tfmba (p/p0 > 0.55). These values are significantly smaller than 0.18 cm3 g−1 calculated from the second step of the DUT-67-Fa isotherm (p/p0 = 0.22−0.4). Thus, on the basis of the isotherm slopes and pore volumes, calculated for the different pore types, it is concluded that fluorinated species introduced into the pores not only hinder water adsorption in the pores but also affect the pore-filling mechanism (Figure 4 and 5). In the case of DUT-67, this may even cause a reversal of the pore-filling sequence. In the case of DUT-67-Pfoa, the flexibility of the alkyl chains and fluorination degree of octanoic acid affects all steps in the water adsorption isotherm (Figure 5). Thus, Pfoa is the only tested modulator molecule that is able to influence the hydrophobicity of both cuboctahedral pore types. This could be due to the nearly complete blocking of the cuboctahedral pore B (red sphere in Figure 3), hindering water molecules from diffusing into the cuboctahedral pore C with no modulator inside (purple sphere in Figure 3). In summary, selective functionalization of the inner surface of DUT-67 by modulator exchange not only affects the slope of the isotherms but also changes the sequential pore-filling mechanism, allowing one to switch the pore-filling sequence of the two different cuboctahedral pores B and C. With the introduction of fluorinated species into the pore system of DUT-67, the water stability in the sense of thermal reactivation is substantially improved. The use of HPa or HAc as the modulator decreases the loss of porosity from 34% (DUT-67-Fa) to 20% (Table 1 and Figures S1−S3) after water desorption. It is also worth mentioning that the water isotherms of both compounds do not close (Figures S16 and S17), indicating that the adsorbed water cannot be completely removed during desorption, possibly caused by the partial chemical transformation of the framework. The use of fluorinated carboxylic acids significantly improves the situation. Thus, the loss of porosity could be minimized to 13% for DUT-

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CONCLUSIONS Rational functionalization of the inner surface of DUT-67(Zr) is possible by integrating monocarboxylic ligands (modulator) coordinated to the Zr cluster. In DUT-67, only one out of three available pore types can be functionalized because of the peculiarity of the framework topology. Functionalization of the inner pore surface by fluorinated monocarboxylic acids not only leads to the change of the hydrophobicity reflected by the changes in the water contact angle but also causes a reversal in the pore-filling sequence. These changes in the water adsorption mechanism are detected using water adsorption isotherms analysis, and in parallel pore-size distribution analysis, performed on the model crystal structures. In the case of DUT-67, the stability of the framework during water removal could be significantly improved by integrating fluorinated molecules, rendering this material a potential candidate for adsorption-based heat-exchange applications and solar cooling. We believe our approach can be widely applied to other Zr-based MOFs with reduced cluster connectivity.

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EXPERIMENTAL SECTION

General Remarks. ZrCl4 (98.0% purity) was purchased from Sigma-Aldrich and 2,5-thiophenedicarboxylic acid (H2tdc; 99% purity) from TCI. The solvents N,N-dimethylformamide (DMF; p.a. purity), N-methyl-2-pyrrolidione (NMP; 99% purity), and ethanol (99.0% F

DOI: 10.1021/acs.inorgchem.6b00829 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Synthesis of DUT-67-Pa. DUT-67-Pa was synthesized using the same procedure, but instead of HFa, 120 equiv of HPa was used as the modulating agent [Zr6O6(OH)2(tdc)4(CH3CH2COO)2 (DMF)· (H2O)5]. Yield: 85%. Elem anal. Calcd: C, 23.77; H, 2.24; N, 0.84; S, 7.69. Found: C, 24.5; H, 2.08; N, 2.09; S, 7.62. Found: C, 25.86; H, 2.11; N, 0.75; S, 8.22. 1H NMR (500 MHz, DMSO/DCl): δ 0.94 (t, 3H, HPa), 2.17 (q, 2H, HPa), 7.68 (s, 2H, tdc), 2.86, 2.69 (s, 3H, DMF), 7.92 (s, 1H, DMF). Postsynthetic Exchange of Modulator Molecules. A total of 150 mg of DUT-67-Fa was suspended in 5 mL of a 0.125 mol L−1 DMF solution of the respective carboxylic acid (Table 2) at room temperature for 5 days. Afterward, the resulting product was washed several times with DMF and ethanol.

purity) were purchased from ABCR GmbH. Formic acid (HFa; 99.0%) was purchased from Grüssing, and propionic acid (HPa; 99.5%), trifluoromethylbenzoic acid (HTfmba; 98.0%), pentafluorobenzoic acid (HPfba), and trifluoroacetic acid (HTfa; 99.0%) were purchased from Alfar Aesar. All chemicals were used without further purification. Nitrogen physisorption measurements were performed on a BELSORP-max (MicrotracBEL, Japan) apparatus at 77 K up to 1 bar. For measurements, high-purity nitrogen gas was used (99.999%). Water physisorption measurements were performed on a Hydrosorb 1000 apparatus (Quantachrome Co.) at 298 K with an equilibration time of 300 s for adsorption and desorption processes. Before all adsorption measurements, the samples were outgassed in vacuum at 373 K for at least 12 h. TGA and differential thermal analysis measurements were performed on a STA 409 PC Luxx (Netzsch) thermal analyzer in air with a heating rate of 2 K min−1 (range: 25−1000 °C). The powder XRD data were collected in transmission geometry on a STOE STADI P diffractometer with Cu Kα1 radiation (λ = 1.5405 Å) at room temperature. Contact angles were measured by the sessile-drop method using a conventional contact angle and drop contour analysis system OCA 35 XL (DataPhysics Instruments GmbH). All measurements were recorded at 24 °C and 40% relative humidity. Deionized water was used for the measurements. Because of pinning effects, water receding angles could not be measured exactly. 1 H NMR spectra were recorded on a Bruker DRX 500 P at 500.13 MHz. The spectra were referenced against the deuterated solvent dimethyl sulfoxide (DMSO). 19F NMR spectra were carried out at 282.38 MHz on a Bruker AC 300 P. MOF digestion for NMR studies was achieved by mixing a small amount (ca. 15 mg) of CsF with 3 drops of deuterated hydrochloric acid (DCl) and adding this mixture to a same amount of the sample under investigation. After 6 h, deuterated DMSO (0.9 mL) was added and then K2CO3 for neutralization of the hydrochloric acid. The crystal structures of functionalized DUT-67(Zr) materials were simulated using the visualization module of Materials Studio 5.0 software.30 In order to obtain the substituted structure, consistent with the material composition, the symmetry of the crystal structure was reduced from Fm3̅m (original structure) to Pa3̅. Subsequently, two modulator and one DMF molecules were modeled in such a way that two modulator molecules are positioned in the cuboctahedral pores and DMF in the smallest octahedral cage. The geometry optimization tool with a universal force field was used for the final structure optimization. The residual five positions of the terminal ligands are occupied by water molecules. The number and positions of the terminal ligands coordinated to the Zr cluster were derived from 1H NMR and water adsorption isotherms. These structural models were used for calculation of the pore-size distribution using the Zeo++ program.27,31,32 A probe radius of 1.2 Å and a bin size of 0.1 Å were used for calculations. Synthesis of DUT-67 Analogues. Synthesis of DUT67-Fa. ZrCl4 (1.38 g, 6 mmol) was dissolved in 150 mL of a mixture of DMF and NMP (a 1:1 volume ratio) by ultrasonication for 10 min. H2tdc (0.66 g, 4 mmol) was added, and the mixture was sonicated for 5 min more. HFa (26.8 mL, 120 equiv) was added. The resulting mixture was placed in an oven for 48 h at 120 °C. The product (further denoted as DUT-67-Fa) was filtered, washed several times with DMF and ethanol, and dried in vacuum at 120 °C [Zr6O6(OH)2(tdc)4(HCOO)2(DMF)· (H2O)5]. Yield: 89%. Elem anal. Calcd: C, 24.5; H, 2.08; N, 0.87; S, 7.62. Found: C, 24.5; H, 2.08; N, 2.09; S, 7.62. 1H NMR (500 MHz, DMSO/DCl): δ 8.12 (s, 1H, HFa), 7.70 (s, 2H, tdc), 2.87, 2.70 (s, 3H, DMF), 7.92 (s, 1H, DMF). Synthesis of DUT-67-Ac. DUT-67-Ac was synthesized with the same procedure, but instead of HFa, 150 equiv of HAc was used as the modulating agent [Zr6O6(OH)2(tdc)4(CH3COO)2 (DMF)·(H2O)5]. Yield: 76%. Elem anal. Calcd: C, 22.7; H, 2.03; N, 0.85; S, 7.82. Found C, 24.35; H, 1.90; N, 0.74; S, 8.57. 1H NMR (500 MHz, DMSO/ DCl): δ 1.89 (s, 3H, HAc), 7.68 (s, 2H, tdc), 2.85, 2.68 (s, 3H, DMF), 7.9 (s, 1H, DMF).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00829. Physisorption data (nitrogen and water physisorption isotherms), powder XRD and TGA data, and NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel. +49 351 463-32564. Fax: +49 351 463-37287. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Martin Heise and Simon Krause (DUT) for their support during in situ thermo−XRD measurements. The Deutsche Forschungsgemeinschaft is acknowledged for financial support through Grant KA 1698/19-1.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b00829 Inorg. Chem. XXXX, XXX, XXX−XXX