Metal–Organic Frameworks from Group 4 Metals and 2,5

Article pubs.acs.org/crystal

Metal−Organic Frameworks from Group 4 Metals and 2,5Dihydroxyterephthalic Acid: Reinvestigation, New Structure, and Challenges Toward Gas Storage and Separation Published as part of a Crystal Growth and Design virtual special issue on Crystal Engineering of Nanoporous Materials for Gas Storage and Separation Hyungphil Chun*,† and Dohyun Moon*,‡ †

Department of Chemical and Molecular Engineering, College of Science and Convergent Technology, Hanyang University, 55 Hanyangdaehak-ro, Ansan 15588, Republic of Korea ‡ Beamline Division, Pohang Accelerator Laboratory, Pohang, Gyeongbuk 37673, Republic of Korea S Supporting Information *

ABSTRACT: The reactions of group 4 metals (Ti, Zr, and Hf) and 2,5-dihydroxyterephthalic acid (H4dobdc) under solvothermal conditions have been systematically explored, and their major crystalline phases have been investigated by single-crystal diffractions. Ti(IV) forms a layered framework [Ti2(Hdobdc)3] where honeycomb-type sheets are interconnected through strong hydrogen bonding. Various gases are reversibly adsorbed within the straight onedimensional channels decorated with polar O atoms, and H2 and CO2 show relatively high isosteric heats of adsorption at 6.6 and 29.4 kJ/mol, respectively. Zr(IV)- and Hf(IV)-based MOFs have also been synthesized using the same ligand and are isostructural with the formula (H3O)x[M(dobdc)(bz)x] (M = Zr or Hf). They have a unique, nonoxo-trinuclear building block that forms a polyhedral network of 6-connected topology. Unlike the two-dimensional net of Ti, the Zr and Hf metal−organic frameworks are hydrothermally stable as unambiguously shown by variable-temperature X-ray diffraction.

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INTRODUCTION The mainstream of research for metal−organic frameworks (MOFs) in the past decade has shifted from the synthesis and discovery of new structures1 toward practical and advanced applications, such as the storage and separation of small molecules,2 heterogeneous catalysis,3 electronics,4 and others.5 Accompanying these changes, a natural selection has occurred from several thousand independent MOFs toward those most fitted for the applications. In general, a facile synthesis from simple building blocks is a prerequisite, and material stability is another important requirement. Some of the prototype MOFs widely utilized across fields these days include ZIF-8,6 MIL101,7 MOF-74,8 and UiO-66.9 We think that the list of such MOFs is rather short considering the possible combinations for metal and multitopic ligands, and this realization has prompted us to explore synthetic strategies for new MOFs from early transition metals that prefer high oxidation states. For example, one of us has reported a new type of heterometallic MOF where Ti4+ and Zn2+ are present at atom-precise positions10 and an unprecedented bistability for a Co/Ti heterometallic MOF where the two metals occupy the same atomic positions.11 In this paper we wish to report some of the results obtained by using 2,5-dihydroxyterephthalic acid (H4dobdc) and group 4 metals (Ti, Zr, Hf). The ligand, well-known as a primary building block for MOF-74-M systems, was chosen partly because the highly polar oxygen atoms could be helpful in the sorption of gases toward storage and separation, and partly © XXXX American Chemical Society

because its high negative charges can readily balance the high oxidation states of group 4 metal ions without oxo/hydroxo groups. Among our M-dobdc MOFs, where M is Ti, Zr or Hf, the Ti phase has been reported twice before by other groups,12 but our work includes previously unknown structural features along with gas sorption data. Zr- and Hf-dobdc phases are isomorphous and to our best knowledge have never been reported in the literature.

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EXPERIMENTAL SECTION

Materials and Methods. All the reagents and solvents were commercially available and used as received. No attempt has been made to protect the reaction from the air or airborne moisture. Fourier-transform infrared spectra were measured using a Varian FTS 1000 instrument with an attenuated total reflection mode on a ZnSe crystal. The thermogravimetric analysis (TGA) was performed using an SDT Q600 (TA Instruments Inc.) with a heating rate of 10 °C/min in air to 800 °C. Ti-dobdc. H4(dobdc) (204 mg, 1.03 mmol) dispersed in 4.0 mL of 2-propanol was added slowly to an acetonitrile solution (4.0 mL) containing 75.0 μL of Ti(OiPr)4 (0.253 mmol). The orange-brown slurry was vigorously stirred for 1 h to homogenize and charged into five glass tubes. After flame-seal, the tubes were heated in an oil bath at 100−120 °C for 24 h during which time the brown mixture turned to dark red crystals. The product was collected, washed thoroughly, and Received: January 20, 2017 Revised: March 10, 2017

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DOI: 10.1021/acs.cgd.7b00092 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Table 1. Summary of Crystal Data and Structure Refinements formula fw crystal system space group λ (Å) temp (K) a (Å) c (Å) V (Å3) Z ρcalc (g cm−3) μ (mm−1) F(000) crystal size (mm3) reflections collected independent (Rint) data/restraints/param Tmax/Tmin GOF on F2 R1, wR2 [I > 2σ(I)] R1, wR2 (all data) largest difference peak/hole (e Å−3)

Ti-dobdc

Zr-dobdc

Hf-dobdc

[Ti2(Hdobdc)3] 681.11 trigonal P3̅1c 0.70000 100(2) 14.637(2) 11.397(2) 2114.7(7) 2 1.070 0.412 682 0.07 × 0.07 × 0.03 21807 2673 (0.0441) 2673/0/74 1.000/0.952 1.143 0.0560, 0.1845 0.0603, 0.1885 0.537/−0.971

(H3O)0.83[Zr(dobdc)(bz)0.83](DMF)0.35 426.17 trigonal R3̅ 0.63000 100(2) 19.677(3) 54.778(11) 18368(6) 36 1.387 2.260 7638 0.20 × 0.10 × 0.09 86841 19098 (0.0472) 19098/7/506 1.000/0.879 1.058 0.0525, 0.1636 0.0570, 0.1670 3.842/−1.476

(H3O)0.75[Hf(dobdc)(bz)0.75] 475.42 trigonal R3̅ 0.63000 223(2) 19.677(3) 55.019(11) 18450(6) 36 1.540 3.709 8037 0.10 × 0.09 × 0.04 84826 18914 (0.0336) 18901/36/455 1.000/0.899 1.037 0.0499, 0.1561 0.0620, 0.1629 3.595/−3.056

surface-dried. Typical yields are 70−90 mg. Elemental analyses were carried out for an evacuated sample which quickly adsorbs moisture. Calcd for [Ti2(Hdobdc)3](H2O)3: C, 39.21; H, 2.06. Found: C, 39.04; H, 2.66%. Zr-dobdc. To a solution containing 171 mg of ZrCl4 (0.734 mmol) in 6 mL of DMF, added were H4(dobdc) (588 mg, 2.97 mmol), benzoic acid (3.59 g, 29.4 mmol) and CH3CN (2.40 mL). The mixture was homogenized for 1 h and charged into eight glass tubes. After flame-seal, the tubes were heated in an oven at 60 °C for 12 h during which time all the solid had dissolved to give a clear brown solution. The temperature was raised to 80, 100, 120, and finally to 140 °C with 12−24 h delay at each step. Total heating time was ∼7 d (Caution! This reaction in a glass tube should NEVER be carried out in an oil bath because the sealed tube may explode after 3−4 days). After being cooled to RT, the glass tubes were immediately put into a bath of liquid nitrogen and were cut open carefully while frozen using a diamond file (Caution! Be aware of the sudden release of pressure during the glass cutting even after freezing in LN2, and all the necessary safety measures should be taken, including a blast shield and protective gear). The product was collected and washed repeatedly with fresh DMF until no fluorescence appeared from the supernatant under an UV light. Typically, 200−250 mg of surface-dried brown crystals are obtained. This reaction may be carried out in a safer vessel such as a Teflon bomb reactor, but in that case the yield is often unacceptably low. Elemental analyses were carried out for an evacuated sample. Calcd for (H3O)0.83[Zr(dobdc)(bz)0.83]: C, 41.30; H, 2.17. Found: C, 41.92; H, 2.63%. Hf-dobdc. This compound can be synthesized by the same procedure for Zr-dobdc using HfCl4 instead of ZrCl4. Elemental analyses were carried out for an as-synthesized sample. Calcd for (H3O)0.75[Hf(dobdc)(bz)0.75](DMF)3: C, 38.34; H, 4.19; N, 6.03. Found: C, 38.80; H, 3.83; N, 5.46%. Gas Sorption Studies. Gas sorption isotherms were measured with a Belsorp Mini-II at 77 (liquid nitrogen) or 195 K (slush baths of 2-propanol and crushed dry ice). The gases used were of the highest quality available (N60 for H2, N50 for CO2 and N2). For Ti-dobdc, the as-synthesized product was thoroughly washed with a mixed solvent of 2-propanol and acetonitrile until no fluorescence appeared from the supernatant under an UV light. Afterward the solvent was exchanged with CHCl3 for 2 days. The exchanged sample was evacuated under a dynamic vacuum first at RT for 15 h and then at 100 °C for 4 h. For Zr-

dobdc, as-synthesized crystals were washed and soaked successively in DMF, CH3CN, and CS2. For each step, the solvent was replenished every 4−6 h for 3 days. Finally, the exchanged sample was evacuated first at 70 °C for 6 h and then at 100 °C for 5 h. Exchange with H2O, alcohols, or other nonpolar organic solvents and/or thermal activation at different temperatures were not effective in removing included guests. The equilibrium criteria were set consistently throughout all the measurements (change in adsorption amounts less than 0.1 cm3/g within 180 s). Powder X-ray Diffraction. Powder X-ray diffraction patterns were recorded at the BL2D SMC beamline of the Pohang Accelerator Laboratory, Korea. Crystalline samples were ground in an agate mortar and packed in a capillary tube (0.4 mm diameter) in the presence of solvents (CH3CN for Ti-dobdc and DMF and H2O for Zr- and Hfdobdc). Debye−Scherrer diffraction data were collected on an ADSC Quantum-210 detector with a fixed wavelength (λ = 1.40000 Å) and an exposure of 60 s. For in situ heating, the sample tubes were attached to a custom-built housing equipped with a vacuum line and temperaturecontrolled stream of nitrogen gas. It was allowed for at least 20−30 min at the designated temperature before recording the diffraction patterns. The PAL BL2D-SMDC program13 was used for data collection, and the Fit2D program14 was used to convert the two-dimensional (2D) to one-dimensional (1D) patterns. X-ray Crystallography. Single crystals were directly picked up from a washed batch of products with a cryoloop attached to a goniohead and transferred to a cold stream of liquid nitrogen. The data collection was carried out using synchrotron X-ray on a ADSC Quantum 210 CCD detector with a silicon(111) double-crystal monochromator at BL2D SMC beamline of the Pohang Accelerator Laboratory, Korea. The PAL BL2D-SMDC program13 was used for data collection, and HKL3000sm (Ver. 703r)15 was used for cell refinement, data integration, and absorption correction. After space group determination, the structures were solved by direct methods and subsequent difference Fourier techniques, and refined by full-matrix least-squares calculations (SHEXLTL).16 For Ti-dobdc, anisotropic refinements of all the non-hydrogen atoms revealed an interlayer H atom between adjacent O atoms. All the non-hydrogen atoms were refined anisotropically. The interlayer H atom was refined isotropically without any restraint or constaint. The diffused electron densities in the void space could not be modeled properly and were removed from the reflection data using the SQUEEZE routine of PLATON.17 For ZrB

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Figure 1. Crystal structure of Ti-dobdc determined in this work. (a) A hexagonal ring of a layer overlaid on the van der Waals surface of the channel. (b) Partially expanded structure viewed along the c axis. (c, d) Side views of the hydrogen bonds holding adjacent layers in a proximity. Color code: green, Ti; red, O; gray, C; dark gray, H.

Figure 2. (Left) Simulated and experimental PXRD patterns for Ti-dobdc. Inset shows optical and scanning electron microscope images of assynthesized crystals. The scale bars correspond to 20 μm. (Right) Changes in the low 2θ region upon in situ heating inside a capillary.

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and Hf-dobdc, the apparent space group was R3̅, but systematic absences indicated potential pseudotranslational symmetry in a merohedral twin with R3̅c. More than 10 sets of data were collected from different batches of crystals, but the results were always the same. The structures can be solved in either space group, but the refinements converge in neither cases. Attempts to refine the data in R3̅ with a twinned component of R3̅c were successful and greatly reduced the R factor from about 20% to 10% or lower. There are two sets of chemically equivalent atoms within the asymmetric unit, and the only notable difference between the two sets is the occupancies of the benzoate ligands. The average occupancy of the benzoate ligand is 0.83 and 0.75 for Zr- and Hf-dobdc, respectively. All the non-hydrogen atoms were refined anisotropically. A disordered DMF molecule in Zrdobdc is partially occupied. The residual electron densities in both Zrand Hf-dobdc could not be modeled properly nor removed using the SQUEEZE probably because of their proximity to O atoms of H3O+. Hydrogen atoms for some of the H3O+ could not be located. The crystal data and results of structure refinements are summarized in Table 1.

RESULTS AND DISCUSSION Ti-dobdc. One of us has noted in 2013 that the salicylatelike chelation by a hydroxycarboxylate moiety is a useful approach to incorporate high-valent metal ions such as Ti4+ into an MOF.10c Later, Liu, Zhang, and co-workers used a similar approach and reported the synthesis and crystal structure of Tidobdc formulated as [Ti2(Hdobdc)2(H2dobdc)].12a It has 2D honeycomb-type layers based on tris-chelated Ti4+ centers (Figure 1a,b). This work, however, has not addressed the charge balance with evidence, and, more importantly, the reported synthesis in neat acetic acid is found irreproducible. In 2016 the Serre group has published a reinvestigation of the Ti-dobdc and its related phases.12b In this work a new synthetic route employing DEF solvent at 200 °C was reported. The product formulated as Ti(Hxdobdc)1.5(DEAH)2−1.5x·nsolv (DEAH = diethylammonium) is a microcrystalline powder, and the powder X-ray diffraction (PXRD) patterns suffer from peak broadening. C

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Figure 3. (Left) Gas sorption isotherms for Ti-dobdc. (Right) Isosteric heats of CO2 and H2 adsorption.

contains diethylammonium cations derived from DEF solvent.12b Under ambient conditions, Ti-dobdc has an over-thebench stability for at least several months; however, the dark red crystals quickly decompose when put into water. As mentioned in the Introduction, one of the motivation for using the dobdc ligand is to know whether the presence of polar oxo moieties would be helpful in the sorption of industrially important gases, such as H2 and CO2. Note that the gas sorption properties have not been investigated in the previous two papers.12 The straight channels observed in Ti-dobdc have the free passage of 7 Å and are indeed decorated with O atoms of the ligand. The BET surface area measured from the N2 sorption at 77 K is 642 m2/g (Figure 3). The total pore volume of 0.29 cm3/g is equivalent to the porosity of ∼33% and is considerably lower than the value expected from the crystal structure which is ∼50%. In fact, we have noticed in a series of repetitive measurements that gas sorption properties of Ti-dobdc are highly dependent on the activation processes. This is probably because some of the solvent molecules cannot be completely removed from the pores without damaging the interlayer packing. The Fourier transform infrared (FT-IR) spectra of as-synthesized and thermally evacuated samples (Figure S1) show broad bands at 3000−3500 cm−1 implying the presence of 2-propanol with extensive hydrogen bonding. Because of the low pore volume, the uptakes for H2 and CO2 at 1 bar are not high at 6.7 mmol/g H2 at 77 K and 2.8 mmol/g CO2 at 298 K. The isosteric heat of adsorption (Qst) was estimated by fitting the sorption isotherms using virial-type equations as reported in the literature,22 and the zero-coverage values for H2 and CO2 are found 6.6 and 29.4 kJ/mol, respectively. The value for H2 is comparable to or rather high for known MOFs without an open metal site (3.8− 9.5 kJ/mol).2b For CO2, the value is much lower than M2(dobdc) (37−47 kJ/mol for M = Co, Ni, Mg) and is similar to hydrated form of HKUST-1 (30 kJ/mol).2c Zr- and Hf-dobdc. The discovery of the Zr-dobdc phase has been achieved only after painstaking control of the synthetic parameters of solvothermal reactions. For example, the ratio between metal, ligand,and monocarboxylate modulator has been systematically varied in combination with different reaction temperatures, times, solvents, and concentrations.23 Under a condition established after hundreds of independent trials on a small scale, Zr-dobdc crystallizes as light brown hexagonal blocks and their aggregates, and is formulated as (H3O)x[Zr(dobdc)(bz)x] (bz = benzoate; x = 0.83 for Zr and 0.75 for Hf) based on X-ray analysis. The structure has a unique Zr3 secondary building unit (SBU) supported locally by six dobdc and three bz ligands (Figure 4a).

Consequently, the cations and hydrogen bonding could not be determined crystallographically. In our current study, a number of synthetic parameters have been adjusted intuitively from our experiences on a series of Ticarboxylate cluster compounds,18 and we found out that the Tidobdc phase can be readily obtained as dark red hexagonal block shape crystals when Ti(OiPr)4 reacts with an excess amount of the ligand in a mixed solvent of CH3CN-iPrOH. The reaction goes to completion within 24 h at 100−120 °C and is highly reproducible. As shown by microscope images and PXRD in Figure 2, the product is highly crystalline and phasepure with a well-defined morphology. Although the atomic connectivities of Ti-dobdc have been established by previous works,12 we redetermined the structure using single crystals on a synchrotron beamline (BL2D SMC) at Pohang Accelerator Laboratory, Korea. In general, 2D layers of coordination polymers tend to stack with a lateral dislocation in the absence of specific interlayer interaction so that nodes of a layer can occupy the void of adjacent layers. The direct, node-to-node packing of honeycomb layers in Ti-dobdc, in this respect, suggests the presence of strong interactions between oxygen atoms that belong to different layers. Indeed, an unusually short contact was noted between noncoordinating O atoms of neighboring layers (dO···O = 2.472(2) Å) in the crystal structure, and refinements of our single-crystal data revealed a hydrogen atom at the midpoint between the two O atoms (Figure 1c,d). The H atom was freely refined with an isotropic temperature factor. The refined O−H distance is 1.24(2) Å and reminiscent of conventional 3-center2-electron bonds in unsubstituted diborane (dB−H = 1.24(4) Å).19 This type of very strong hydrogen bonding has long been known,20 and the O···O and O−H distances found in Ti-dobdc are very similar to those in the Rb+ salt of oxidiacetate determined by a neutron diffraction study (dO···O = 2.449(3), dO−H = 1.226(2) Å).21 Further examinations of the crystal structure did not show any O atom that could potentially be protonated. Because the synthesis does not involve any source of cations other than Ti4+ and H+, the only plausible explanation for the overall charge balance of Ti-dobdc would be having some defects on the ligand or the formula [Ti2(Hdobdc)2.67]. We believe that the presence of interlayer H atoms is responsible for the relatively low thermal stability. As shown by variable-temperature PXRD in Figure 2b, a shrinkage of the framework is observed only along the vertical direction before it loses the crystallinity at around 200 °C. In TGA (Figure S2), Ti-dobdc shows a short plateau only up to about 180 °C after a weight loss of 10% before 100 °C. Note that the Serre group has also reported very similar thermal behavior for Ti-dobdc that D

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Figure 4. Crystal structure of Zr-dobdc. (a) Trinuclear SBU showing hydrogen-bonded H3O+ ions. Color code: green, Zr; red, O; gray, C. (b) Partially expanded view of the 6-ring with one SBU highlighted in the ball-and-stick diagram. (c) Topological representation of the framework showing detailed structures of two independent polyhedra. The 6-nodes in red and the yellow linkers represent the SBUs and dobdc ligands, respectively. (d) Topological analogy between the two polyhedra and corner-sharing cubes in a pcu net.

pocket where strong electron densities corresponding to oxygen atoms are found in difference maps during the early stage of structure refinements. A total of three such sites are found for every Zr3 SBU. The refined distances between O atoms H3O+ and of surrounding carboxylate ligands are 2.704(3)−3.093(3) Å. In FT-IR spectra measured for samples after a complete evacuation, the O−H stretching vibrations are clearly observed at 3200 cm−1, albeit with significant broadening due to hydrogen bonding (Figure S1). Despite the presence of charge-balancing H3O+ in the framework, Zr-dobdc is hydrothermally stable. As-synthesized crystals were thoroughly washed and soaked in water for several days, before measuring in situ PXRD upon heating inside a capillary in the presence of water. As shown in Figure 5a, the solvent exchange from DMF to water and thermal evacuation do not deteriorate the diffraction patterns and only result in gradual shifts of diffraction peaks to higher 2θ, meaning the contraction of the framework. The in situ PXRD patterns could be successfully indexed (Figures S3− S7), and according to the results the decrease in the unit cell volume on going from the as-synthesized form to the one at 250 °C is about 1145 Å3 or 6% (Figure 5b). As expected, the contraction upon desolvation occurs along both lateral and vertical directions. Significant peak broadening and decay in intensities are observed at around 300 °C and above.

Three of the six dobdc ligands extends the connectivity on the crystallographic ab plane to form a puckered layer with a sixmembered ring structure (Figure 4b). The internal diameter of the hexagonal void is about 12 Å when measured from the van der Waals surface. Remaining dobdc ligands are used to interconnect the layers along the vertical direction with −3 symmetry, completing its three-dimensional (3D) network structure. With chemical intuition in mind, the topology of Zr-dobdc can be analyzed by simplifying the Zr3 SBU and dobdc ligand as a 6-connecting node and a linear ditopic linker, respectively. Then it can be readily seen that there are two kinds of polyhedra as shown in Figure 4c. Although the two polyhedra look different from each other, they have a common geometrical characteristic. That is, both are hexahedra having 8 vertices and 12 edges with all the faces being 4-gons. A regular polyhedron of this type is the cube, and therefore the topology underlying the net of Zr-dobdc can be best described as primitive cubic (pcu) with a rhombic distortion along the body diagonal (Figure 4d). The presence of one benzoate for every Zr dictates that the overall framework is anionic, and it is strongly believed that the framework balances the extra negative charges by capturing H3O+ in the vicinity of the Zr3 SBU. As shown in Figure 4a, the arrangement of the three vertical carboxylates creates a small E

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Figure 6. Simulated and experimental PXRD patterns for Hf-dobdc. The evacuated pattern was measured after heating a solvent-exchanged sample to 110 °C for 5 h under a dynamic vacuum.

Figure 5. (Top) Simulated and experimental PXRD patterns under in situ heating for Zr-dobdc. (Bottom) Changes in the unit cell parameters obtained by indexing the PXRD patterns.

Single crystals of Hf-dobdc can be synthesized under a condition identical to that of Zr-dobdc. It is crystal structure and physical properties are also very similar (if not the same) to its Zr analogue. Figure 6 shows PXRD patterns of Hf-dobdc measured for as-synthesized, water-exchanged, and thermally evacuated samples where a number of sharp peaks appear at positions expected from the single-crystal data. Attempts to measure gas sorptions for Zr- and Hf-dobdc have failed many times due to the inability to remove included solvents completely. This is because the access to the polyhedral voids shown in Figure 4c is in fact partially blocked by the benzoate ligand, while some of the solvent molecules are strongly hydrogen-bonded to H3O+. Therefore, the TGA profiles of Zr- and Hf-dobdc do not show a clear step corresponding to the loss of included solvent, and the onset of thermal decomposition cannot be determined directly from thermal analysis (Figure S2). The best results were obtained when initial DMF was exchanged stepwise using low-boiling solvents of small molecular dimensions (see Experimental Section for details), followed by evacuation under mild heating. After the activation, Zr-dobdc shows selective adsorptions for H2 and CO2 at 77 and 195 K, respectively (Figure 7). The fact that even small molecules like H2 and CO2 show stepped adsorption with a large hysteresis between adsorption and desorption suggests the presence of a diffusion barrier inside the framework. Adsorptions for kinetically larger N2 are

Figure 7. Gas sorption isotherms measured for Zr-dobdc.

almost negligible at both 77 and 195 K. This is because Zrdobdc possesses mesh-like narrow channels without a large void, although we do not completely rule out the possibility for residual solvents.

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CONCLUSIONS Zr-based MOFs have been of high interest since Lillerud et al. reported the first of its kind in 2008.9 In a synthesis point of view, the work by Behrens et al. taking advantage of the socalled modulator has played an important role in expanding the family of porous and stable Zr MOFs.24 Most of the known Zr MOFs, however, are based on either a large organic backbone such as extended π systems or multinuclear Zr clusters with oxo/hydroxo groups. We find only three cases of Zr MOFs with nonoxo Zr centers where the ligands are gallate,25 phosphonate,26 and quinonate.27 Zr-dobdc reported here is rather unique in that perspective, and its nonoxo Zr3 SBU maybe a good starting point for a targeted synthesis. We, for example, propose that the benzoate ligands in Zr-dobdc can be replaced F

DOI: 10.1021/acs.cgd.7b00092 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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by other ditopic linkers, and it may result in new Zr MOFs with an interesting 9-connected topology. It is interesting and potentially important that the cation sites of Zr-dobdc are at the corner of a polyhedral cage and open toward a void (Figure 4a−c). Therefore, the water-stable nature of Zr- and Hf-dobdc may be utilized to convert it to new functional materials through cation-exchange with, for example, catalytically active metal ions. A new study is already on the way along this line. Finally, the results we are reporting here are another reminder that simply the presence of polar atoms on the pore surface is not enough to dramatically improve sorption properties for H2 or CO2. The physical environment of pore systems, such as size and shape, are equally or probably more important. The relatively high Qst values for Ti-dobdc is more likely due to the size of 1D channels which is approximately twice the kinetic diameter of adsorbing gases rather than the oxygen atoms. The impending challenge pertinent to this work would be developing experimental approaches for open metal sites in MOFs of high-valent metal ions toward gas storage and separation.

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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.cgd.7b00092. FT-IR spectra and TGA plots for all compounds. Indexed PXRD data for Zr-dobdc (PDF) Accession Codes

CCDC 1528737−1528739 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*(H.C.) Phone: +82-31-400-5506. Fax: +82-31-400-5457. Email: [email protected]. *(D.M.) E-mail: [email protected]. ORCID

Hyungphil Chun: 0000-0003-2181-7469 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea grant funded by the Korean Government [MSIP] (2016R1A2B4006383 and 2016R1A5A1009405). We appreciate the Pohang Accelerator Laboratory for beamline use (2016-2nd-2D-009 and 2016-3rd-2D-011). H.C. thanks Prof. M. S. Lah (UNIST, Korea) for helpful discussions on topological analysis.

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REFERENCES

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DOI: 10.1021/acs.cgd.7b00092 Cryst. Growth Des. XXXX, XXX, XXX−XXX