Systematic Investigations of the Transition between Framework

Sep 26, 2018 - The decisive factors for the product formation and hence the transition between the framework topologies are the HCOOH/metal ratio and ...
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Cite This: Inorg. Chem. 2018, 57, 12820−12826

Systematic Investigations of the Transition between Framework Topologies in Ce/Zr-MOFs Jannick Jacobsen, Helge Reinsch, and Norbert Stock* Institute of Inorganic Chemistry, Christian-Albrechts-Universität, Max-Eyth Straße 2, D-24118 Kiel, Germany

Inorg. Chem. 2018.57:12820-12826. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/20/18. For personal use only.

S Supporting Information *

ABSTRACT: 1-H-Pyrazole-3,5-dicarboxylic acid (H2PZDC), a small, strongly bent linker molecule with an angle of 147.4° between the carboxylate groups, was used in the synthesis of metal organic frameworks (MOFs) with fcu, bcu and reo topology. In systematic studies of the chemical system Ce4+/Zr4+/H2PZDC/ HCOOH, their fields of formations were established. The decisive factors for the product formation and hence the transition between the framework topologies are the HCOOH/metal ratio and the molar ratio of Ce4+/Zr4+ employed in the synthesis. All title compounds crystallize with the well-known hexanuclear cluster {M6(μ3-O)4(μ3-OH)4(−CO2)n}, with n = 8 or 12 and M = Ce4+ and Zr4+, as the inorganic building unit (IBU). Connection through 12 or eight linker molecules leads to three framework topologies: fcu, bcu, and reo, respectively. The dominant phase observed in this system crystallizes with reo topology and is known as DUT-67. The pure Zr-MOF of composition [Zr6(μ3-O)4(μ3-OH)4(PZDC)4(OH)2(H2O)2] (Zr-DUT-67-PZDC) as well as the mixed-metal compounds Ce/Zr-DUT-67-PZDC are accessible and the molar ratio Ce4+/Zr4+ can be adjusted between 0 and 1. At low HCOOH/metal ratios, surprisingly, the UiO-66 type structure with fcu topology is formed despite the nonlinear geometry of the linker. Thus, using exclusively Zr4+ ions in the starting mixture the pure Zr-MOF with ideal composition [Zr6(μ3-O)4(μ3-OH)4(PZDC)6] (Zr-UiO-66-PZDC) was obtained. Variation of the Ce/Zr molar ratio leads to a continuous increase in linker defects with increasing Ce content in the MOF. At a Ce/Zr value of ∼ 1:1 a transition from the fcu to the reo framework topology takes place. Using high HCOOH/metal ratios, a transition from the reo to the bcu topology is observed when a molar ratio of Ce/Zr ≥ 1:5 is employed. Irrespective of the molar ratio used in the reaction mixture, the mixed-metal MOF of composition [CeZr5(μ3-O)4(μ3-OH)4(PZDC)4(OH)2(H2O)2] (Ce/Zr-CAU-38-PZDC) is always formed as confirmed by comprehensive EDX analyses. Rietveld refinement strongly indicates the presence of exclusively hexanuclear {CeZr5(μ3-O)4(μ3-OH)4} clusters and thus CAU-38 is the first Ce/Zr-MOF which solely occurs at a specific metal stoichiometry. In addition to the detailed synthetic study, the compounds were thoroughly characterized regarding their composition, lattice parameters, and porosity, as well as thermal and chemical stability.

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(IV),14 and neptunium(IV).15 In addition, some isoreticular compounds with UiO-66 structure type have also been reported employing a large variety of linear linker molecules.9,16−18 Furthermore, various functionalized terephthalic acids have also been used, of which the UiO-66 structure type can be formed as well.6,19−22 Remarkably, up to now only two bent linker molecules are know which form a three-dimensional network by 12-fold connected hexanuclear clusters, 9-fluorenone-2,7dicarboxylic acid (H2FDCA, 157°, BUT-10) and dibenzo[b,d]thiophene-3,7-dicarboxylic acid 5,5-dioxide (H2DTDAO, 160°, BUT-11).23 The interconnection of the hexanuclear clusters {Zr6(μ3-O)4(μ3-OH)4(−CO2)n}, through n = 10, 8, or 6 carboxylate groups is also known.18,24 The remaining coordination sites of the IBU are occupied by hydroxide ions and water molecules25 or formate or acetate ions.18,24 Recently, a few new

INTRODUCTION Metal−organic frameworks (MOFs) are porous coordination polymers formed by linking of organic (linker) and inorganic building units (IBUs). They often exhibit high specific surface areas, different chemical functionalities, and tunable pore sizes.1−3 These properties make them suitable for potential applications like gas separation and storage,4,5 catalysis,6 heat transformation,7 and drug delivery.8 Since the first Zr-MOF of composition [Zr6(μ3-O)4(μ3OH)4(BDC)6], Zr-UiO-66-BDC, H2BDC = terephthalic acid, was published in 2008 by Cavka et al.,9 the popularity of zirconium based porous materials has increased continuously.10 The three-dimensional network of Zr-UiO-66-BDC is formed by hexanuclear clusters [Zr6(μ3-O)4(μ3-OH)4]12+ which are 12fold connected by terephthalate linker molecules to form a framework with fcu topology.9,11 In the past few years other tetravalent metal ions were used to build UiO-66 type MOFs such as cerium(IV),6 hafnium(IV),12 uranium(IV),13 thorium© 2018 American Chemical Society

Received: July 18, 2018 Published: September 26, 2018 12820

DOI: 10.1021/acs.inorgchem.8b02019 Inorg. Chem. 2018, 57, 12820−12826

Article

Inorganic Chemistry

Table 1. Overview of the Optimized Synthesis Conditions for the Two Zr-MOFs Zr-UiO-66-PZDC and Zr-DUT-67-PZDC and the New Mixed-Metal MOF Ce/Zr-CAU-38-PZDCa compound

Ce

Zr

Zr-DUT-67-PZDC (P01) Zr-UiO-66-PZDC (P11) Ce/Zr-CAU-38-PZDC (P20)

0 0 0.4

2.0 2.0 1.6

linker 2.0 3.0 3.0

mod.

Ce [μL]

Zr [μL]

H2PZDC [mg]

DMF [μL]

HCOOH [μL]

175 75 350

0 0 80

400 400 320

37.1 55.7 55.7

800 800 800

704 302 1409

The synthesis conditions for all other mixed-metal compounds are given in Tables S1−S3. The molar ratios are given in columnds two to five.

a

CAU-38 structure were synthesized using Pyrex glass vials (Vmax = 7 mL). First the linker 1-H-pyrazole-3,5-dicarboxylic acid (H2PZDC, 98%) was dissolved in DMF (800 μL) in the glass reactor. Thereafter, the formic acid (HCOOH, 100%) was added. Finally, the aqueous solutions of (NH4)2Ce(NO3)6 (0.533 mol/L) and ZrO(NO3)2·H2O (0.533 mol/L) were introduced into the glass reactor to achieve the desired molar ratio of Ce/Zr. The accumulated volume of the metal salt solutions was always 400 μL. The Pyrex glass vials were heated under stirring using an aluminum heating block. While a reaction time and temperature of 15 min and 100 °C were used for the synthesis of the UiO-66 and DUT-67 based compounds, the reaction time was increased to 25 min to get highly crystalline CAU-38. After the synthesis the glass reactors were cooled down to room temperature. Thereafter, the mother liquor was removed by centrifugation, and the product was redispersed and centrifuged two times in DMF (2 mL) and two times in acetone (2 mL). Finally, the product was dried in air at 70 °C. The optimized synthesis conditions for Zr-UiO-66-PZDC, ZrDUT-67-PZDC, and the new mixed-metal MOF Ce/Zr-CAU-38PZDC are given in Table 1. Details for the individual reactions that were carried out are given in Tables S1−S3. Various products with different molar ratios of Ce/Zr have been obtained. The products with DUT-67 (P01−P10), UiO-66 (P11−P17), and CAU-38 (P20−P25) are discussed in more detail in the following sections. Sample Treatment Prior to 1H NMR Measurements. 1H NMR spectroscopy was utilized to detect possible linker modification and the amounts of incorporated formic acid used as modulator or DMF used as solvent in the reaction. Therefore, the MOFs were dissolved in a mixture of deuterated dimethyl sulfoxide (DMSO-d6) and 10% deuterochloric acid (DCl) in D2O (molar ratio 7:1), and the 1H NMR spectra were measured. The spectra are shown and discussed in Figures S66−S74 and Table S18.

Zr-MOFs with new IBUs composed of condensed hexanuclear clusters have been reported.26−29 We are interested in the investigation of Ce-MOF and succeeded in the synthesis of many compounds isoreticular to Zr-MOFs.6,25,30−32 Since the latter exhibit superior thermal and chemical stabilities,9,33 mixed-metal Ce/Zr-MOFs were investigated. Tuning of the Ce/Zr ratio between 0 and 100% Ce was demonstrated by varying the molar amounts of the metal(IV) solutions in the synthesis approach. The resulting materials have improved stabilities at low Ce contents.31 Detailed systematic studies on the influence of modulator concentration, i.e., acetic acid, on the product formation have been carried out by Bon et al.24 Thus, variation of the acetic acid concentration results in Zr-MOFs with 8 or 10 connected hexanuclear cluster, as observed in DUT-67, DUT-68, and DUT-69, respectively.24 We report the results of our systematic study of the system Ce4+/Zr4+/H2PZDC/HCOOH that allowed us to establish important synthesis-structure relationships. Variation of the molar ratios of HCOOH/metal and Ce/Zr results in three distinct phases with fcu, bcu, or reo topology. Uniquely, we demonstrate the transition of framework structures in Ce/ZrMOFs as a result of the Ce/Zr ratio and the formation of a MOF with a fixed Ce/Zr molar ratio = 1−5.

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EXPERIMENTAL SECTION

Materials. Cerium ammonium nitrate (98%, (NH4)2Ce(NO3)6, Alfa Aesar), zirconyl nitrate monohydrate (99%, ZrO(NO3)2·H2O, ABCR), 1-H-pyrazole-3,5-carboxylic acid monohydrate (98%, C5H4N2O4·H2O, ABCR), N,N-dimethylformamide (99%, DMF, Grüssing GmbH), and formic acid (99−100%, HCOOH, BASF) were used as obtained. Methods. Powder X-ray diffraction (PXRD) for product identification and variable temperature (VT) PXRD were carried out on a Stadi P Combi diffractometer with Cu Kα1 radiation. PXRD patterns for Le Bail and Rietveld refinements were recorded on a Stadi MP diffractometer (Cu Kα1 radiation) with a MYTHEN detector. The variable temperature (VT) PXRD measurement was carried out in a STOE capillary oven on a Stadi P Combi diffractometer with Cu Kα1 radiation. The energy-dispersive X-ray (EDX) spectroscopy data were recorded on a Philips XL30 FEG microscope. Each sample was measured a few times at different sample spots. The average values in atom % of these measurements of each metal (Ce/Zr) and the standard deviation were calculated. Sorption experiments were performed using a BEL Japan Inc. Belsorpmax. Before sorption measurements all samples were first treated in acetone for 24 h under stirring and subsequently heated at 50 °C under reduced pressure (10−2 kPa). The specific surface areas were determined using the Rouquerol method.34 The theoretical surface areas were calculated with Material Studio,35 using a probe with the diameter of N2 (3.64 Å) as guest molecule. 1H NMR spectra were measured on a Bruker DRX 500 spectrometer. MIR spectra were recorded on a Bruker ALPHA-FT-IR A220/D-01 spectrometer with ATR-unit. Thermogravimetric measurements were performed using Netzsch, STA-409CD under air flow (75 mL/min) with a heating rate of 4 K/min between 20 and 800 °C. General Synthesis Procedure. The pure zirconium and the different mixed-metal Ce/Zr-compounds with DUT-67, UiO-66 and

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RESULTS AND DISCUSSION Synthesis. The systematic investigation of the chemical system Ce4+/Zr4+/H2PZDC/HCOOH led to the formation of Ce/Zr-MOFs crystallizing in three different topologies, fcu, reo, or bcu. As known from the synthesis of Ce- and mixed-metal Ce/Zr-MOFs, all compounds were obtained after very short reaction times (15−25 min) at mild reaction temperatures (100 °C) starting from a mixture of the dissolved linker (H2PZDC) in DMF and aqueous metal salt solutions.6,31 By varying the molar ratios of HCOOH/metal and Ce/Zr, the different well-defined fields of formation of the title compounds Ce/Zr-DUT-67PZDC, Ce/Zr-UiO-66-PZDC, and Ce/Zr-CAU-38-PZDC were identified (Figure 1, and a more extended version in Figure S1). The figure also contains results from EDX and TG measurements. According to Figure 1, a reaction mixture containing molar ratios of HCOOH/metal of 75:2 and Ce/Zr < 0.8:1.2 (P11 to P17) results in the formation of MOFs crystallizing in the UiO66 structure (fcu topology). Increasing the amount of Ce further leads first to phase mixtures of UiO-66 and DUT-67 and finally to a phase pure Ce-DUT-67-PZDC as previously reported in the literature.25 Increasing the molar ratio of HCOOH/metal to 175:2 crystalline products with DUT-67 structure are almost exclusively obtained; see P01−P10. Only in reaction mixtures 12821

DOI: 10.1021/acs.inorgchem.8b02019 Inorg. Chem. 2018, 57, 12820−12826

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Inorganic Chemistry

∼ 1:1 and the ratio employed in the reaction mixture corresponds to the one in the reaction product (Figure 1, Table S6). Due to the larger ionic radius of Ce4+ compared to Zr4+ (CN = 8 pm, Ce = 97 pm, Zr = 84 pm),36 a shift of reflection positions to smaller 2θ values is observed in the PXRD patterns (Figure S3). At Ce/Zr > 1.4:0.6, the formation of the DUT-67 structure is proven by PXRD (Figure S4). Evaluation of the TG analyses demonstrate a continuous increase in the number of linker defects with increasing Ce content (Figures S11−S14 and Table S8). This transition of framework topology from fcu to reo is in accordance with the decrease of cluster connectivities from 10.7-c to 8.7-c within the series of the UiO-66 type compounds. For the dominating phase in this chemical system, Ce/Zr-DUT-67-PZDC (P01−P10), similar trends as established for Ce/Zr-UiO-66-PZDC are observed (Figures S06− S10, Table S8). With increasing molar ratio of Ce/Zr, a decrease in the linker connectivity (8.2-c to 6.2-c) and a continuous increase of Ce in the product is observed, which also leads to a shift of reflection positions in the PXRD patterns (Figures 2 and S3). EDX analysis shown Zr is preferably incorporated in the mixed-metal MOF. Thus, in P10 a molar ratio of Ce/Zr of 1.8− 0.2 was used in the synthesis, but the final product contains a molar ratio of Ce/Zr of 1.5−0.5 (Table S5). In contrast to these results, Ce/Zr-CAU-38-PZDC is only observed with one, welldefined composition [CeZr5(μ3-O)4(μ3OH)4(PZDC)4(OH)4(H2O)4] (P20−P25). This is reflected in the identical reflection positions in the PXRD patterns of the different products (Figure 2, right), and the results are confirmed by the EDX (Table S7) and TG measurements (Figures S15−S17 and Table S8). By PXRD measurements all compounds were investigated after the TG analyses, the observed patterns are given in Figure S18. Crystal Structures. Although many crystalline MOFs have been synthesized in this study, only one member of each of the three phases was structurally characterized via Rietveld refinement. For the UiO-66 and DUT-67 type compounds, the pure Zr-MOFs were chosen (P11 and P01, respectively) since they were obtained as highly crystalline products with the smallest number of linker defects. For CAU-38, sample P20 was used for the crystal structure elucidation. The structures of the Zr-UiO66-PZDC, Zr-DUT-67-PZDC and Ce/Zr-CAU-38-PZDC

Figure 1. Crystallization diagram of the chemical system Ce4+/Zr4+/ H2PZDC/HCOOH with fields of formations of the different types of crystalline products. A value of 1 on the x and y axis corresponds to 0.1067 mmol of a reactant employed in the synthesis. Product assignments are based on the results of the PXRD measurements (Figures 2 and S2−S4). Sample numbering corresponds to the reaction conditions given in the Tables S1−S3. Molar ratios of Ce/Zr in the reaction products obtained from EDX measurements are given as red numbers below the phase symbol. Numbers in black above the phase symbol correspond to the cluster connectivity and were determined from TG analyses.

containing large amounts of cerium (1.6−2 equiv) is the formation of cerium formate, Ce(HCOO)3, observed (P26− P28). At a molar ratio of HCOOH/metal of 350:2 a new crystalline phase, Ce/Zr-CAU-38-PZDC is detected when molar ratios 0.4:1.6 ≤ Ce/Zr ≤ 1.4:0.6 are employed in the reaction mixture (P20−P25). Results of the reactions carried out at molar ratios of HCOOH/metal of 225:2 and 275:2 are given in Figure S1. At 225:2, phase mixtures of Ce/Zr-DUT-67PZDC and Ce/Zr-CAU-38-PZDC and the pure DUT-67 structure type are observed. In contrast, at 275:2 mostly pure structure types of both are formed. The combination of the results of the PXRD measurements and the ones of the EDX and TG analyses allow for a more detailed discussion of the observed synthesis-structure-trends. For the mixed-metal compounds with UiO-66 structure (P11 to 17) the molar ratio of Ce/Zr can be tuned up to a value of Ce/Zr

Figure 2. Left: PXRD patterns (λ = 1.5401 Å) of Ce/Zr-DUT-67-PZDC with increasing amounts of Ce in the reaction mixture (P01 to P10, Table S1). Right: PXRD patterns of the reaction products (P18 to P28, Table S3) obtained by increasing the molar ratio of Ce/Zr in the presence of 350 equiv of HCOOH (P18: Zr-DUT-67-PZDC; P20−P25: Ce/Zr-CAU-38-PZDC, P26−P28: Ce(HCOO)3). The 222 peak of Zr-DUT-67-PZDC (P01) is marked with a red line to illustrate the peak shifting with increasing cerium ratio. 12822

DOI: 10.1021/acs.inorgchem.8b02019 Inorg. Chem. 2018, 57, 12820−12826

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Scheme 1. Bent Linker Molecule H2PZDC (Left Column) Is Connected to the IBUs with Different Types of Connectivity {M6(μ3O)4(μ3-OH)4(−CO2)n} (Middle Column) to Build up the Three Types of MOFs with Different Topology, Connectivity, and Composition (Right Column)a

a

Only the ideal connectivities and compositions are presented.

contain the hexanuclear clusters {M6(μ3-O)4(μ3-OH)4} as IBU which form frameworks with fcu, reo, and bcu topology (Scheme 1). The structures of these compounds were determined by Rietveld refinement (Figures 1, S21, and S23) from PXRD data of the as-synthesized (as) compounds, employing TOPAS37 for indexing and Rietveld refinements and Materials Studio35 for structural manipulations and force-field optimization using the universal force-field. Details on the derivation of the structural models are given in the Supporting Information. For the UiO-66 and DUT-67 type compounds, structural models from the literature were used without changing the space group symmetry.23,24 For Ce/Ze-CAU-38-PZDC, no suitable structural model could be identified from the literature; hence, it had to be set up from the available analytical data and assume the presence of hexanuclear clusters as the IBU. The results of the Rietveld refinements are shown in Figures 3, S19, and S21, and an overview of some crystallographic parameters is given in Table 2. In the following sections, the structures of Zr-DUT-67-PZDC and Zr-UiO-66-PZDC are only briefly described. Our focus will be on the structure of Ce/Zr-CAU-38-PZDC. Zr-DUT-67PZDC is isostructural to Zr-DUT-67-TDC,24 where TDC2− = thiophenedicarboxylate, and crystallizes in the cubic space group Fm3̅m with a lattice parameter of a = 38.2803(9) Å. This parameter is quite similar to the published lattice parameter of Zr-DUT-67-TDC (a = 39.120(5) Å)24 due to the fact of the similar bonding angles between the carboxylate groups of the linkers PZDC2− (147.4°) and TDC2− (147.9°).25 The hexanuclear clusters [Zr6(μ3-O)4(μ3-OH)4]12+ are connected by eight linker molecules (PZDC2−) to build up a threedimensional network (Figure 4). The cluster connectivity leads to the reo topology; thus, two different pores are formed with 16.6 Å (cuboctahedral) and 8.8 Å (octahedral) diameters, taking the van der Waals radii into account.18 Zr-UiO-66-PZDC crystallizes in the cubic space group Pa3̅ with a lattice parameter of a = 19.5811(10) Å. The hexanuclear clusters [Zr6(μ3-O)4(μ3-OH)4]12+ are connected by 12 linker molecules (PZDC2−) to build up a three-dimensional network with fcu topology. The crystal structure is isostructural to the

Figure 3. Rietveld plot of Ce/Zr-CAU-38-PZDC (P20). The observed PXRD pattern (λ = 1.5401 Å) (black), the calculated curve (red), and the difference plot (blue) are shown. The allowed peak positions are marked as black ticks.

published structures of BUT-1023 and BUT-1123 and compared with the UiO-66 structure with a linear linker molecule (BDC2−) in Figure 5. To compensate for the bond angle between the carboxylate groups (147.4°), the IBUs units are tilted compared to the terephthalate-based framework. The new mixed-metal MOF Ce/Zr-CAU-38-PZDC crystallizes in the orthorhombic space group Pnnm. The IBU is a hexanuclear cluster with a constant stoichiometry of cerium to zirconium of 1:5. Thus, clusters of composition [CeZr5(μ3O)4(μ3-OH)4]12+ are present which are connected by eight PZDC2− ions (Figure 6, middle) to form a three-dimensional network with bcu topology (Figure 6) and distorted octahedral pores of 10.4 × 6.0 Å2 in diameter. On the basis of the results of the Rietveld refinement, the cerium ions are exclusively located with an occupancy of 0.25 in the equatorial plane of the hexanuclear clusters. Within this plane, the coordination spheres of the Ce/Zr ions are completed by four hydroxyl groups and four water molecules to give the sum formula [CeZr5(μ3O)4(μ3-OH)4(PZDC)4(OH)4(H2O)4]. 12823

DOI: 10.1021/acs.inorgchem.8b02019 Inorg. Chem. 2018, 57, 12820−12826

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Inorganic Chemistry

Table 2. Crystallographic Parameters of the As-Synthesized Compounds Zr-DUT-67-PZDC (P01), Zr-UiO-66-PZDC (P11), and Ce/Zr-CAU-38-PZDC (P20) compound space group crystal system

Zr-DUT-67-PZDC (P01) Fm3̅m cubic

Zr-UiO-66-PZDC (P11) Pa3̅ cubic

lattice parameters

a = 38.28031(98) Å

a = 19.5811(10) Å

rwp (%) GoF rBragg (%)

8.918 2.776 2.976

4.849 2.194 1.214

Ce/Zr-CAU-38-PZDC (P20) Pnnm orthorhombic a = 13.3898(11) Å b = 19.4667(19) Å c = 14.1355(12) Å 4.551 2.320 1.409

PXRD. The lattice parameters were determined by Le Bail fitting (Figures S24−S32 and S34−S39; Table S12 and S14) and compared to the ones using Vergard’s law (Figures S33 and S40; Table S13 and S15). As previously described for the other known Ce/Zr-MOFs,31 the lattice parameters follow Vergard’s law; thus, solid solutions are formed. The Ce content in mixedmetal Ce/Zr-MOFs has also a strong influence on their thermal stability.31 Thermal and Chemical Stability. To assess the thermal stability and the framework flexibility, VT-PXRD measurements of the three compounds that were structurally characterized by Rietveld refinement and three mixed-metal compounds of the DUT-67 and UiO-66 structure types were each carried out. ZrDUT-67-PZDC (Figure S41) and Zr-UiO-66-PZDC (Figure S45) show very similar thermal stabilities up to 265 and 270 °C, respectively. For the mixed-metal compounds of these structure types, the thermal stability decrease with higher cerium content as known from the literature31 (Figures S42−S44 and S46− S48). For Ce/Zr-CAU-38-PZDC, strong shifts of reflection positions above 110 °C and decomposition of the compound at 250 °C are clearly seen (Figure S49). This structural flexibility has also been observed in chemical stability studies and is in agreement with the reported flexibility of framework with bcu topology.38 Ce/Zr-CAU-38-PZDC is stable in some aprotic, polar solvents (e.g., DMF and DMSO), nonpolar solvents (e.g., n-hexane and toluene), and in protic, polar solvents a shift in reflection positions is observed (Figure S52). Zr-DUT-67PZDC and Zr-UiO-66-PZDC are stable in several aprotic, polar solvents (e.g., acetone, DMF, DMSO, and acetonitrile) and protic polar solvents (e.g., water and ethanol) (Figures S50 and S51). Sorption Measurements. N2 sorption measurements of the activated compounds were performed to determine the

Figure 4. Crystal structure of Zr-DUT-67-PZDC. Atom color scheme: C (gray), N (blue), O (red), Zr and Zr−O−polyhedra (light blue).

Figure 5. Comparison of the crystal structures of the UiO-66 structure type with a linear linker molecule (BDC2−, left) and the bent linker molecule (PZDC2−, 147.4°, right). Atom color scheme: C (gray), N (blue), O (red), Zr and Zr−O−polyhedra (light blue).

The mixed-metal Ce/Zr-MOFs with UiO-66 (P12−P17) and DUT-67 (P02−P10) structure were further characterized by

Figure 6. Crystal structure of Ce/Zr-CAU-38-PZDC as seen along the a axis (left) and c axis (right) and the 8-fold connected hexanuclear cluster (middle). Atom color scheme: C (gray), N (blue), O (red), Zr and Zr−O−polyhedra (light blue), Ce and Ce−O−polyhedra (orange). The cerium only resides in the orange marked plane of the cluster. The cerium(IV) occupation level in the plane of the octrahedron is 0.25 for each position. This results in a stoichiometry of cerium to zirconium of 1:5 in the hexanuclear clusters. 12824

DOI: 10.1021/acs.inorgchem.8b02019 Inorg. Chem. 2018, 57, 12820−12826

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Inorganic Chemistry

established as the decisive factors that control the framework structures. In all MOFs hexanuclear clusters are observed as IBUs that are connected to frameworks with fcu, bcu or reo topology. The Ce/Zr molar ratios can be gradually varied in the UiO-66 and DUT-67 type compounds and effect the number of linker defects and hence the thermal stability. The transition from the fcu to the reo topology occurs at high cerium contents. Another transition from the reo to the bcu topology takes place upon increase of the HCOOH/metal ratio. The new MOF Ce/ Zr-CAU-38-PZDC exhibits always a fixed Ce/Zr ratio of 1:5 independent of the molar ratio used in the reaction mixture. Our results support the presence of exclusively {CeZr5(μ3-O)4(μ3OH)4} clusters as the IBU. Our investigations show the importance of the systematic variation of multiple chemical parameters in the discovery of unprecedented MOFs.

specific surface area and porosity (Figure 7). The activation process has a strong influence on the sorption properties and

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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.8b02019. Figure 7. N2 sorption measurements of Zr-DUT-67-PZDC, Zr-UiO66-PZDC, and Ce/Zr-CAU-38-PZDC at 77 K. The filled symbols mark the adsorption isotherm, the empty symbols mark the desorption isotherm.

Results and additional data of PXRD patterns, EDX analysis, Rietveld refinements, Le Bail plots, VT-PXRD measurements, N2 and H2O sorption measurements, chemical stability, 1H NMR and IR spectra (PDF) Accession Codes

solvent exchange using acetone followed by a mild activation step at 50 °C under reduced pressure (10−2 Pa) turned out to be optimal (Figures S53−S56). The solvent exchange step was necessary since DMF was observed with IR spectroscopy in all as synthesized products (Figures S75−S77; Table S19). All compounds with UiO-66 and DUT-67 structure type exhibit a Type I(a) adsorption isotherm34 with a high N2 uptake at small p/p0 values (Figures 7, S57, and S59). The small hysteresis observed for Zr-UiO-66-PZDC and some mixedmetal MOFs is probably due to interparticle porosity. PXRD measurements of the MOFs after the N2 sorption measurements confirm the crystallinity and stability of the pure Zr-MOFs, and a decrease in long-range order of the compounds with an increasing cerium content (Figures S58 and S60). The theoretical specific surface area of Ce/Zr-CAU-38-PZDC is 1225 m2 g−1 but it is not porous toward N2 even when very mild activation conditions are employed, and a decrease in long-range order is observed by PXRD measurements (Figure S61). This observation is in line with the results of the VT-PXRD and chemical stability studies. The specific BET surface areas of the two Zr-MOFs with DUT-67 and UiO-66 structure are 1057 and 545 m2 g−1, respectively, and are considerably smaller than the theoretical values (1841 and 729 m2 g−1). This is probably due to the limited stability of the MOFs as demonstrated by the susceptibility of the porosity to the activation conditions. The specific surface areas and pore volume of the mixed-metal Ce/ Zr-MOFs are given in Table S17. Zr-DUT-67-PZDC and ZrUiO-66-PZDC also show high water uptake in vapor sorption experiments at 25 °C of 34.0 and 28.9 wt %, respectively, without loss of crystallinity (Figures S62−S65).

CCDC 1853454−1853456 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 Author

*E-mail: [email protected]. ORCID

Helge Reinsch: 0000-0001-5288-1135 Norbert Stock: 0000-0002-0339-7352 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate support from the group of Prof Dr. Sönnichsen (University of Kiel) for NMR measurements and the group of Prof. Dr. Bensch for carrying out TG measurements. Furthermore, we thank the province of Schleswig-Holstein for the financial support.

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REFERENCES

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CONCLUSION In our contribution, we have demonstrated the dependence of product formation by systematically investigating the chemical system Ce4+/Zr4+/H2PZDC/HCOOH. Thus, the HCOOH/ metal concentration as well as the Ce/Zr ratio have been 12825

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Article

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