Fe Terephthalate Metal-Organic

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Polymorphism of Mixed Metal Cr/Fe Terephthalate MetalOrganic Framework Utilizing Microwave Synthetic Method Ladawan Pukdeejorhor, Kanyaporn Adpakpang, Panyapat Ponchai, Suttipong Wannapaiboon, Somlak Ittisanronnachai, Makoto Ogawa, Satoshi Horike, and Sareeya Bureekaew Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00508 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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Crystal Growth & Design

Polymorphism of Mixed Metal Cr/Fe Terephthalate Metal-Organic Framework Utilizing Microwave Synthetic Method Ladawan

Pukdeejorhor,†

Kanyaporn

Adpakpang,*,†

Panyapat

Ponchai,† Suttipong

Wannapaiboon,§ Somlak Ittisanronnachai,‡ Makoto Ogawa,† Satoshi Horike,‖,±, # and Sareeya Bureekaew,*,† †Department

of Chemical and Biomolecular Engineering, School of Energy Science and

Engineering, ‡Frontier Research Center (FRC), and £Research Network of NANOTEC-VISTEC on Nanotechnology for Energy, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand. §Synchrotron

Light Research Institute, University Avenue, Nakhon Ratchasima 30000,

Thailand. ‖Institute

for Integrated Cell-Material Sciences, Institute for Advanced Study, ±Department of

Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, and #AISTKyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan.

ACS Paragon Plus Environment

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KEYWORDS Polymorphism; metal-organic frameworks; mixed-metal terephthalate MOF; phase transformation; microwave synthetic method

ABSTRACT: Polymorphic metal-organic frameworks (MOFs) possess distinct characteristics due to a variation of linking patterns and spatial orientations of similar chemical components. Herein, microwave synthesis is used to prepare polymorphic mixed-metal Cr/Fe-terephthalate MOFs, of which the formation is different from the single-metal Cr-terephthalate. By varying the reaction time at a fixed concentration of Fe at 33 mol%, three polymorphic Cr/Fe-MOFs, i.e. MIL-101, MIL-88B and MIL-53, with remarkably different properties were obtained. Phase-pure Cr/Fe-MIL-101 is obtained in 5 min which shows well-defined octahedral shape with larger particle size, highlighting a kinetically preferable formation of the Cr/Fe-MIL-101. Further prolongation of the reaction time conveys the formation of more-thermodynamicallystable phases namely Cr/Fe-MIL-88B and then Cr/Fe-MIL-53 at the reaction time longer than 2 h. Cr K-edge and Fe K-edge XANES and EXAFS data unveil the presence of both Cr and Fe at the same metal nodes within the mixed-metal MOFs. Moreover, the EXAFS results suggest the distortion of the trinuclear-oxo cluster within the mixed-metal MOFs, which possibly facilitates the facile phase transformation in the Cr/Fe system alternative to the single Cr


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system. This study provides a way to monitor the phase formation and/or transformation of the polymorphic Cr/Fe-terephthalate MOFs ascribable to the effect of the additional second metal. Moreover, it can be an example to understand the polymorphic phenomenon in metalterephthalate MOFs and is believed to broaden the knowledge of MOF synthesis and design. INTRODUCTION Metal-organic frameworks (MOFs) are porous materials which consist of metal ion/metal ion cluster bonding with organic linker to construct 2D or 3D frameworks. MOFs can be rationally synthesized from various types of metal ions and linkers resulting in different chemical and structural properties based on the designated chemical components.1-7 In many cases, combining similar metal ions and organic ligands but in different synthetic conditions possibly form polymorphs or framework isomers.4,8-10 Despite the similar chemical components, they are linked in different spatial patterns to lead polymorphs. Hence, they possess distinct characteristics, e.g., pore architecture, void, surface area, and thermal/chemical stability or activity4,5,11 which draw an attention of many research groups including our group to the polymorphism in MOFs from both experimental and theoretical approaches in order to control the properties of MOF materials more precisely.4,5,8,9 Self-assemblies of trivalent metals (Cr, Fe, Ti, In, Ga, Sc, V, etc.) and terephthalate organic linker results in many types of MOFs so-called polymorphic MOFs, e.g. MIL-101, MIL88B, MIL-53 (or MIL-47) and MIL-68 depending on synthetic conditions (MIL stands for Materials from Institut Lavoisier).4,10,12-22 Specifically, MIL-101 and MIL-88B are constructed


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from trinuclear oxo-centered metal–carboxylate M3O(COO)6 building unit linked with ligands to form 3D frameworks. According to their linking patterns, MIL-101 and MIL-88B are classified in mtn and acs-a network topology, respectively. Also, they provide remarkably different properties. MIL-101 is a rigid framework with the highest specific surface area (5900 m2/g) among aforementioned polymorphs, while MIL-88B shows structural response to external stimuli e.g., accommodation of guest molecule and temperature change.4,13,23,24 Instead of constructing polymorphic MOFs from the trivalent metal-terephthalate, the formation of MOFs by connecting the infinite chains of edge-sharing metal octahedral MO4(OH)2 by terephthalate linkers results in the 3D frameworks with 1D channels known as MIL-53 and MIL-68 (Figure 1). In detail, MIL-53, possessing 1D diamond-shaped channels, is thermally/chemically stable and shows structural flexibility induced by temperature and pressure change, and the adsorption of guest molecules.25,26 MIL-68 provides two types of pore channels with diameter of 6.0−6.4 and 16−17 Å for the triangular and hexagonal rings, respectively.27-29


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Figure 1. (a) Trinuclear oxo-centered metal–carboxylate M3O(COO)6 building unit within (b) MIL-88B and (c) MIL-101. (d) Edge-sharing metal octahedral MO4(OH)2 building unit within (e) MIL-53 and (f) MIL-68.

Ascribing the selective phase formation to the synthetic conditions, e.g., temperature, pH, solvent and reaction time, different MOFs can be obtained.4,8-11,18,30,31 Several studies on the polymorph formation of MOFs have been exerted. For example, Zou et al. reported the phase-selective synthesis of V-terephthalate polymorphs. V-MIL-101 is obtained by solvothermal synthesis of VCl3 and terephthalic acid in ethanol at 120 °C. An addition of HCl to the reaction mixture leads to the formation of V-MIL-88B instead. According to the lower


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pH caused by the addition of HCl, the deprotonation of terephthalic acid forming terephthalate to coordinate with V(III) is slower than in the HCl-free system. The slower deprotonation leads to the formation of more thermodynamically stable V-MIL-88B more than the kinetically favored V-MIL-101. Furthermore, V-MIL-88B and V-MIL-101 transform to the thermodynamically stable phase (MIL-47) by a higher temperature synthesis (i.e. 200 °C). Recently, our group reported phase and morphology change of the Cr-based terephthalate MOF by the introduction of Fe as the second metal into the synthesis system. Using the typical solvothermal synthesis for Cr-MIL-101, an introduction of Fe up to 33 mmol% induces the selective formation of rod-shaped Cr/Fe-MIL-88B.5 Those previous works inspire us to monitor the selective phase formation and/or transformation in the polymorphic Cr- and Cr/Fe-terephthalate frameworks. In this study, we employed a microwave synthetic method to investigate the formation of polymorphs in the mixed metal Cr/Fe terephthalate MOFs. Typical for the microwave synthesis, the system is directly heated at the molecular level by microwave irradiation, facilitating a homogeneous reaction in short reaction time. The problem of heat transfer usually found in conventional solvothermal synthesis can be substantially diminished. Therefore, the kinetically controlled phase transformation could be precisely investigated. The effect of the reaction time on the occurrence of Cr- and Cr/Fe-terephthalate polymorphs was examined. Based on our previous study, the second metal, i.e. Fe of 33 mol% was used for all the mixed-metal systems. It should be noted that, at this concentration, only Cr/Fe-MIL-88B was found under the typical hydrothermal condition.5 Herein, by fixing the concentration of


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Fe at 33 mol% and varying the reaction time, three different polymorphs of mixed-metal Cr/Fe MOFs, i.e. MIL-101, MIL-88B and MIL-53 were obtained. Details of phase formations, morphologies, elemental compositions, and local structure of the secondary building units (or metal nodes) of the resulting MOFs were thoroughly examined.

EXPERIMENTAL SECTION Chemicals. Chromium(III) nitrate nonahydrate (Cr(NO3)3·9H2O, 98.0%) and iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, 98.0%) were procured from Daejung, Korea. Terephthalic acid or 1,4-benzendicarboxylic acid (H2BDC, 98.0%) was purchased from SigmaAldrich. N,N-dimethylformamide (DMF, 99.8%) and ethanol (99.9%) were provided by Qrec Chemical. All chemicals were used as received without further purification. Material preparation. Cr-terephthalate MOFs were synthesized at 200 °C under microwave irradiation at 300 W. 400.1 mg (1 mmol) Cr(NO3)3·9H2O and 4.5 mL of DI water were homogenously mixed in 35 mL microwave vials and then 166.1 mg (1 mmol) of terephthalic acid was added to the reaction mixture. The synthesis of mixed-metal Cr/Fe-terephthalate MOF was carried out following the abovementioned procedure except that two metal salts, 268.1 mg (0.67 mmol) of Cr(NO3)3·9H2O and 133.3 mg (0.33 mmol) of Fe(NO3)3·9H2O were used. The reactant mixtures were sealed and placed in a microwave oven (CEM Discovery). The mixtures were further heated at 200 °C at the designated reaction times of 5, 10, 30, 60, 120 and 240 min in the microwave reactor. Note that, potency and pressure during the synthesis for Cr-MOF(n) and Cr/Fe-MOF(n) are approximately 75 W (power set-point = 300


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W) and 170 psi, respectively. After cooling down to room temperature, the resulting products were collected by centrifugation (KUBOTA 3700; operated at 13,000 rpm at 10 °C for 10 min) and then washed with fresh DMF twice and ethanol twice to remove the remaining unreacted organic linkers. Further, the samples were dried under vacuum overnight at room temperature. The mass of dried products was reported in Table S1. Hereafter, notation of CrMOF(n) and Cr/Fe-MOF(n) indicate the products synthesized by microwave irradiation at 200 °C for n minutes using single-metal Cr and mixed-metal Cr/Fe, respectively. Material Characterization. The crystal phase and the crystallinity of synthesized samples were examined by using powder XRD analysis (Cu Kα radiation (λ = 1.54 Å), Bruker, New D8). Functional groups in the resulting materials were characterized by infrared spectroscopy (IR, Perkin Elmer, Frontier FT-IR, Universal-ATR). The morphology and particle size were analyzed using a field-emission scanning electron microscope (FE-SEM, JEOL, JSM-7610F) and a high-resolution transmission electron microscope (TEM, JEOL, JEMARM200F, operated at 200 kV). For SEM sample preparation, the powders were attached on carbon tape and coated with Pt (20 sec sputtering at 10 mA under vacuum) to increase the electrical conductivity. Nanobeam electron diffraction mode (NBED) was also employed to identify the crystal phases of the specific crystals with different morphologies according to the TEM images. Elemental distribution was derived by a scanning transmission electron microscopic (STEM) mode equipped with an energy dispersive X-ray spectroscopy (EDS). Additionally, elemental analysis


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by synchrotron-based X-ray fluorescence spectroscopy (XRF), X-ray absorption near edge structure (XANES) and X-ray absorption fine structure (EXAFS) analyses of Cr K-edge and Fe K-edge were performed at beamline BL1.1W: Multiple X-ray Techniques Beamline, Synchrotron Light Research Institute, Thailand. The measurement was conducted at ambient temperature and pressure by simultaneously measuring the samples together with the Cr and Fe foils as standard references for an in-line alignment of the energy shift during the synchrotron-operating time. The obtained data were analyzed using ATHENA and ARTEMIS softwares.32

RESULTS AND DISCUSSION Referring to our previous study, the addition of Fe as the second metal into the hydrothermal reaction, which is typically suitable for the formation of Cr-MIL-101, enhanced the formation of polymorphic Cr/Fe-MIL-88B.5 The phase formation of Cr/Fe-MIL-88B increased by increasing the amount of Fe. Eventually, phase-pure Cr/Fe-MIL-88B was obtained when 33 mol% Fe was added to the reaction mixture. Similar to those obtained from the hydrothermal synthesis,5 the Cr-MOFs synthesized from the microwave method also show bright-green color till the reaction time of 60 min. The prolongation of reaction times to 120 and 240 min results in brownish-green powders instead (as shown in Figure S1). The PXRD patterns of all the collected products of the single-metal Cr system with various microwave reaction times, shown in Figure 2(a), were indexed to be Cr-MIL-101 phase. However, the broader XRD peaks of the products obtained from the microwave synthesis in 5 to 240 min indicated less


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crystallinity and/or smaller crystallite size as compared with the ones from the hydrothermal synthesis. In the case of mixed-metal Cr/Fe-MOF(n), the brown solids were obtained for all conditions due to the incorporation of Fe as shown in Figure S2. According to the PXRD patterns in Figure 2(b), Cr/Fe-MIL-101 is the main phase obtained from the microwave synthesis. This is distinct from our previous finding using hydrothermal synthesis, in which the addition of 33 mol% Fe enhances the phase-selective formation of Cr/Fe-MIL-88B.5 This finding supports the hypothesis that MIL-101 is the kinetic phase which quickly forms due to the fast and effective heating by microwave radiation whereas the solvothermal method yields more thermodynamically stable MIL-88B which slowly grows under the conventional heating system. Moreover, as evidenced by the sharp peaks of PXRD patterns (Figure 2(b)), the crystallinity of Cr/Fe-MOF(n) is higher than that of Cr-MOF(n) prepared using the same reaction time. It indicates a kinetically-preferable formation of the Cr/Fe-terephthalate MOFs in the presence of Fe. Also, additional phase was observed in the PXRD pattern of Cr/FeMOF(240) which is closely similar to the as-synthesized form of MIL-53.33 To confirm the presence of MIL-53 phase by its guest-induced structural responsiveness, the solvothermal exchange of excess terephthalic acid with DMF was performed for the as-synthesized Cr/Fe-MOF(240) product. The as-synthesized MIL-53 comprised in the Cr/Fe-MOF(240) should accommodate the excess terephthalic acid as guest molecules. After the guest exchange reaction, PXRD peaks of the Cr/Fe-MOF(240) corresponding to the as-synthesized MIL-53 entirely change while those for MIL-101 remain


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unchanged (Figure 3(a)). This evidence indicates the exchange of the excess terephthalic acid guests by DMF molecules and eventually only DMF is enclosed in the framework. In addition, IR spectroscopy was used to confirm the complete exchange of terephthalic acid by DMF. The as-synthesized Cr/Fe-MOF(240) sample shows the peaks of carbonyl group (C=O) of both terephthalic acid (at 1693 cm-1) in MIL-53 and DMF (at 1657 cm-1) used during the washing step (Figure 3(b)). After solvothermal solvent exchange, only the peak of DMF at 1657 cm-1 is observed without an evidence of the characteristic peaks of terephthalic acid.34 According to the alteration of both PXRD patterns and IR spectra, the emerged phase in Cr/Fe-MOF(240) is confirmed to be the Cr/Fe-MIL-53.


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Figure 2. PXRD patterns of (a) Cr-MOF(n) and (b) Cr/Fe-MOF(n) compared to the patterns of Cr-MOF5, MIL-101, MIL-88B and MIL-53 for phase identification.11,33,35 Characteristic peaks of MIL-53 in mixed phase Cr/Fe-MOF(240) are indexed by *.


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Figure 3. (a) PXRD patterns and (b) IR spectra of Cr/Fr-MOF(240) before and after solvothermal solvent exchange with DMF. The symbols #, *, and * describe the characteristic peaks of MIL-101, MIL-53 (before solvent exchange) and MIL-53 (after solvent exchange), respectively. Considering the sizes and morphologies of the products, SEM images of all Cr-MOF(n) depicted in Figure 4 clearly show the aggregates of spherical particles with the small size of ca.


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40 nm for all reaction times. Table S2 summarizes the particle size of each phase. According to the PXRD data (Figure 2(a)), those spherical particles are Cr-MIL-101. Note that, MIL-101 forms octahedral-shaped particles in a typical solvothermal synthesis.5,36,37 Since microwave radiation accelerates the nucleation of MIL-101 rather than the secondary growth to form the large and well-defined crystals, the obtained crystals are usually smaller (sometimes with illdefined shape) than those obtained using conventional heating method.38 This result agrees very well with the broad PXRD patterns (Figure 2(a)) as discussed above. Interestingly, trace amounts of the secondary phase as rod-shaped particles was observed in Cr-MOF(240), that was not observed by PXRD analysis due to its detection limit. The small spherical particles of Cr-MIL-101 require short reaction time to form, suggesting that this phase is considered as the kinetic product. The rod-shaped particles, which occur only in the long reaction time (240 min), is therefore considered as more thermodynamically stable product. In the case of mixed-metal Cr/Fe-MOF(n), well-defined octahedral particles with the larger size of ca. 150  200 nm are clearly observed as presented in Figure 5. According to the PXRD patterns and the SEM image shown in Figure 5(a), only single-phase Cr/Fe-MIL-101(5) is obtained at the shortest reaction time of 5 min. Compared with Cr-MIL-101(5), the addition of Fe leads to the formation of the larger particles with better-defined octahedral shape of Cr/Fe-MIL-101(5). Unlike solvothermal synthesis with the same Fe concentration,5 fast reaction using microwave method does not produce Cr/Fe-MIL-88B but the kinetic product Cr/Fe-MIL-101 instead. When the reaction time increases to 10 min, the second phase with


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long hexagonal rod particles starts to be observed (Figure 5(b)) in the mixed-metal Cr/Fe system, which is much faster than that in single-metal Cr case (Figure 4(f)). This clearly emphasizes the role of Fe (as the second metal) in accelerating the formation of more thermodynamically stable phase (herein, MIL-88B) which is normally formed at longer reaction time. The amount of hexagonal rod particles increases with the prolongation of reaction time but the size of this phase remains similar as shown in Table S2. Of note, this phase cannot be detected by PXRD analysis because of its trace amounts. In addition, when the reaction time increases to 120 min, large brick-like particles with the size of >3.5 µm occur as the third phase, but still undetectable by PXRD. This phase needs much longer reaction time and it does not appear in the case of Cr-MOF(n). Hence, the addition of Fe as the second metal in the reaction is obligated. The amount of the brick-like particles significantly increases, when the reaction is held for 240 min. It is worth noting that unlike the hydrothermal synthesis,5 no Fe2O3 particle is observed at any reaction time under the microwave synthesis. Based on this circumstance, we discover that the required reaction time for the synthesis of more thermodynamically stable polymorphic phases can be shortened by performing microwave synthesis and adding the second metal Fe into the reaction. However, the phasepure polymorphic Cr/Fe-terephthalate MOFs cannot be achieved via the present synthetic conditions. Based on the PXRD result of Cr/Fe-MOF(240), only two phases, i.e. MIL-101 and MIL53, are well identified. However, according to the SEM images, three different phases were


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observed in Cr/Fe-MOF(240). In order to characterize the polymorphic phases found in the samples, Cr-MOF(240) containing spherical and hexagonal rod particles and Cr/Fe-MOF(240) containing octahedral, hexagonal rod and brick-like particles were chosen to be further examined by TEM analysis. The TEM images and their corresponding electron diffraction (ED) patterns of spherical and hexagonal rod particles in Cr-MOF(240) are shown in Figure 6. The ED patterns are indexed using the unit cell parameters generated from the calculated XRD data of MIL-101, MIL-88B and MIL-53 for octahedral/spherical, hexagonal rod, and brick-like particles, respectively. The ED patterns of the spherical and hexagonal rod particles in CrMOF(240) (Figure 6) can be indexed in cubic crystal system (space group of Fd3m) of MIL-101 and hexagonal crystal system (space group of P-62C) of MIL-88B, respectively.11,35 In the case of mixed-metal Cr/Fe-MOF(240), the ED patterns of the octahedral, hexagonal-rod, and bricklike particles can be indexed in cubic phase (space group of Fd3m) of MIL-101, hexagonal phase (space group of P-62C) of MIL-88B and orthorhombic phase (space group of Pnma) of MIL53, respectively.11,33,35 In all, the ED patterns of all particles match well with the reference data assuring the presence of MIL-101, MIL-53 and also MIL-88B in trace amount which cannot significantly be defined by PXRD patterns.


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Figure 4. FE-SEM images of Cr-MOF(n) synthesized by microwave method at 200 oC for (a) 5 min, (b) 10 min, (c) 30 min, (d) 60 min, (e) 120 min, and (f) 240 min.


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Figure 5. FE-SEM images of Cr/Fe-MOF(n) synthesized by microwave method at 200 oC for (a) 5 min, (b) 10 min, (c) 30 min, (d) 60 min, (e)-(f) 120 min, and (g)-(h) 240 min.


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Figure 6. TEM images and their corresponding ED patterns of (a) spherical particle, and (b) long hexagonal rod particle found in Cr-MOF(240).


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Figure 7. TEM images and their corresponding ED patterns of (a) octahedral, (b) hexagonalrod, and (c) brick-like particles found in Cr/Fe-MOF(240).


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In principle, it should be possible to obtain pure thermodynamically stable MIL-53 by increasing the reaction time. Attempts were made on both Cr-MOF and Cr/Fe-MOF. The reaction time to synthesize Cr-MOF was prolonged to 360 min, yielding Cr-MOF(360). The PXRD of Cr-MOF(360), shown in Figure S3, indicates that at the reaction time of 360 min, mixture of MIL-101 and MIL-88B could be obtained. The characteristic pattern of MIL-88B are discernible, which is consistent with a large number of hexagonal-rod particles observed in SEM images (Figure S4). In the case of mixed-metal Cr/Fe-MOF, we additionally performed the reaction at 180 and 720 min. As depicted in Figure S5, PXRD patterns of Cr/Fe-MOF(180) and Cr/Fe-MOF(720) indicate the co-existence of both MIL-101 and MIL-53. Based on the peak intensity, more fraction of MIL-53 was found in Cr/Fe-MOF(720). Although, the PXRD pattern of MIL-88B cannot be observed due to its trace amount, SEM images of both compounds (Figure S6) exhibit the presence of hexagonal-rod MIL-88B. Similar to Cr-MOF, it is not possible to obtain phase-pure MIL-53 under the present synthetic conditions. The elemental homogeneity of Cr and Fe composed in each polymorphic phase of the resulting Cr/Fe-MOF(240) sample was further determined by STEM/EDS. The STEM/EDS elemental mappings (shown in Figure 8) indicate the uniform dispersions of Fe and Cr elements in MIL-101 and MIL-88B whereas MIL-53 clearly exhibits the core-shell structure. The elemental compositions from STEM/EDS of Cr and Fe in each polymorphic phase were determined. Cr/Fe-MIL-101(240), Cr/Fe-MIL-88B(240) and Cr/Fe-MIL-53(240) possess


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different mol% of both metal elements; namely 23.4, 13.3 and 30.8 mol% of Fe and 76.6, 86.7 and 69.2 mol% of Cr, respectively. For further insightful investigation on the elemental distribution, the STEM/EDS point analysis results for the core and the shell of each polymorph in Cr/Fe-MOF(5) and Cr/Fe-MOF(240) are summarized in Table 1. The present values are average numbers of more than three particles for each phase. The results show that octahedral particles present the homogeneous distribution of Cr and Fe but the hexagonal rod and brick-like particles exhibit inhomogeneous distribution of both metals. Note that the composition results from EDS are non-stoichiometric due to the semi-quantitative nature of the technique. Also, a very small size of the EDS probe projecting to the small area of sample can sensitively affect an altered composition as compared to the bulk powders. So that the analysis of bulk composition was determined by synchrotron-based XRF measurement as described in Figure 9 (Table S3). The XRF result revealed that Fe content was maximum at the reaction time of 5 min, then gradually decreased until the reaction time of 60 min. When the reaction time reached 120 min, the Fe content slightly increased and then decreased again when the reaction time was prolonged to 240 min. These results supported the hypothesis that Fe as the second metal, crucially accelerated the formation of Cr/Fe-MOFs, ascribing to its more active nature as compared to the inert Cr.39 As the reaction occurs rapidly, the Fe-rich phase (MIL-101) occurs first at the short reaction times (5 - 60 min). When the reaction time increased, the Cr ion incorporates gradually, leading to the decrease of Fe content with prolonged reaction time, and hence the formation of Fe-less MIL-88B. At the reaction time of 120 min, the Fe content increased. This might be attributed to the initial formation of the additional MIL-53 phase.


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Note that, MIL-53 is constructed from the infinite chains of edge-sharing metal octahedral (Cr/Fe)O4(OH)2 which is different from the trinuclear-oxo cluster as a building unit of MIL101 and MIL-88B. The formation of MIL-53 phase may cause a shift of system equilibrium. Also, more kinetically favored Fe ions may rapidly be incorporated in the MIL-53 product in comparison with the Cr ions, leading to the abrupt increase of the Fe content. As the reaction extended to 240 min, the increase of MIL-53 content was found. The elemental composition of Cr-rich MIL-53, according to the STEM/EDS results in Figure 8 and Table 1, was again anticipated from the gradual contribution of Cr. This is more thermodynamically favored upon the extended time. The elemental dispersions of Fe and Cr obtained from STEM/EDS elemental mapping and the elemental composition from XRF remain limited to explain the co-existence of Cr and Fe in the discrete M3O(COO)6 building unit. Therefore, local geometry/structure analysis using XANES/EXAFS was further carried out.


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Figure 8. STEM/EDS elemental maps of (a) octahedral particle, (b) long hexagonal rod, and (c) brick-like particles composed in Cr/Fe-MOF(240). Table 1. STEM/EDS point analysis of Cr/Fe-MOF(5) and Cr/Fe-MOF(240). Morphology

Initial content

Resulting content (mol %)


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Sample

Cr/Fe-MOF(5)

(Phase)

(mol %)

Octahedral particle Octahedral particle

Cr/Fe-MOF(240)

Hexagonal rod particle Brick-like particle

Core (mol %)

Shell (mol %)

Fe

Cr

Fe

Cr

Fe

Cr

33.3

66.7

27.6

72.4

19.5

80.5

33.3

66.7

22.9

77.1

13.6

86.4

33.3

66.7

24.6

75.4

3.7

96.3

33.3

66.7

40.4

59.6

19.0

81.0

Figure 9. Schematic representation of the formation of different building units with Fe/Cr ratios depending on reaction time for Cr/Fe-MOF(n). M3O(COO)6 and MO4(OH)2 building units present the framework of MIL-101/MIL-88B and MIL-53, respectively.


25


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To probe the phase formation/transformation as well as the incorporation of the second metal Fe into the discrete M3O(COO)6 building unit of the resulting products, Cr K-edge and Fe K-edge XANES and EXAFS measurements were employed. Cr K-edge XANES spectra of Cr-MOF(n) and Cr/Fe MOF(n) were depicted in Figure 10. Cr K-edge XANES spectra of all Cr-MOF(n) and Cr/Fe-MOF(n) possess spectral feature composed of pre-edge and edge positions at 5991-5993 eV and 6001 eV, respectively. Both pre-edge and edge positions are close to that of Cr2O3, indicating the octahedral-like coordination environment and the trivalent oxidation state of Cr ion. The results suggest that the oxidation state of Cr within CrMOF(n) and Cr/Fe-MOF(n) remains unchanged upon extending the reaction times. In details, an existence of pre-edge peaks is typically attributed to the dipole-forbidden 1s3d transition of non-centrosymmetric coordination, attributing to the slightly distorted octahedral coordination. Despite the position, the pre-edge intensity at ca. 5993 eV are also altered. In case of Cr-MOF(n), the pre-edge intensities remain nearly identical up to the reaction time of 60 min. Further extending the reaction time obviously increases the pre-edge intensity for the Cr-MOF(120) and Cr-MOF(240) as shown in Figure 10(a). These results suggest that the prolonged reaction times of 120 and 240 min may lead to the increased degree of distortion of the CrO6 octahedra in which the electronic state might be modified, resulting in an intense pre-edge intensity. This evidence supports the observation of hexagonal-rod morphology of MIL-88B at the reaction time of 240 min in the SEM image (Figure 2(f)). This


26

Page 27 of 44

structural alteration with modified electronic state at a phase transformation stage from MIL101 to MIL-88B (after prolongation of reaction time more than 120 min) agrees well with the change in the product color from bright green to brownish green. Contrary to Cr-MOF(n), the intense pre-edge peak was observed for Cr/Fe-MOF(n) at the short reaction times up to 120 min (Figure 10(b)). This may be due to the highly distorted octahedra by an incorporation of Fe(III) ion into the trinuclear-oxo cluster. However, when the reaction time increases to 240 min, the intensity of pre-edge peak decreases. This depleted pre-edge intensity may be caused by the increased amount of Cr/Fe-MIL-53, possessing the less distorted octahedral (Cr/Fe)O4(OH)2 composed in the 1D-chain of Cr/Fe-MIL-53.

(a)

(b)

(i)

(i)

Normalized x / a.u.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(ii) (iii) (iv) (v) (vi)

(ii) (iii) (iv) (v) (vi)

(vii)

(vii)

(viii)

(viii)

(ix)

(ix)

5980

6000

6020

Energy / eV

6040 5980

6000

6020

6040

Energy / eV

Figure 10. Cr K-edge XANES spectra of the (a) Cr-MOF(n) and (b) Cr/Fe-MOF(n) products obtained from microwave synthesis using the reaction times (n) of (i) 5, (ii) 10, (iii) 30, (iv) 60,


27


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(v) 120 and (vi) 240 min compared to the standard reference compounds (vii) Cr2O3, (viii) CrO2 and (ix) CrO3. Fe K-edge XANES spectra of Cr/Fe-MOF(n) were depicted in Figure 11. All the Cr/FeMOF(n) samples possess spectral feature nearly identical to that of Fe2O3, indicating the preserved trivalent state of Fe ion without any effect of reaction time. The intensities of preedge peaks are comparable for all the reaction times. A strong depletion of pre-edge peak intensity is obvious for the Cr/Fe-MOF(240) similar to what was observed in Cr K-edge. This is also expected to be ascribable to the formation of MIL-53, a 1D-chain structure of (Cr/Fe)O6 octahedra which are considerably less distorted than the (Cr/Fe)O6 octahedra with (Cr/Fe)3O bridging in MIL-101 and MIL-88B, as depicted in Figure 9. Therefore, the richness of MIL-53 phase in Cr/Fe-MOF(240) leads to the decrease in the pre-edge intensity as compared to those of the samples from the other reaction times.


28

(i)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 29 of 44

(ii) (iii) (iv) (v) (vi) (vii) (viii)

7100

7120

7140

7160

Energy / eV

7110

7115

Energy / eV

7120

Figure 11. Fe K-edge XANES spectra of Cr/Fe-MOF(n) with the reaction times (n) of (i) 5 min, (ii) 10 min, (iii) 30 min, (iv) 60 min, (v) 120 min and (vi) 240 min compared to the standard reference compounds (vii) Fe2O3, and (viii) FeO. To confirm the co-existence of Fe with Cr in the same (Cr/Fe)3O cluster also called secondary building unit (SBU) within the MOF framework, both Cr K-edge and Fe K-edge EXAFS analyses were performed as depicted in Figure 12(a) and Figure 12(b), respectively. To avoid the effect of different SBU, Cr/Fe-MOF(5) was selected since it is composed of single phase of MIL-101(Cr/Fe), according to PXRD and SEM results. The Cr K-edge EXAFS spectra in R-space possess the first shell centered at ca. 1.55 Å and the second shell centered at ca. 2.45 Å while the Fe K-edge EXAFS spectra show R-space of the first shell and the second shell at

ca. 1.50 Å and 2.55 Å, respectively. Considering the first-shell scattering from the (Cr/Fe)O6


29


octahedra, the results suggest that the Fe-O bonds are slightly shorter than the Cr-O bonds. According to the XRF result for the formation of Fe-rich Cr/Fe-MIL-101(5), the Cr K-edge and Fe K-edge spectra were fitted according to the (Cr1Fe2)O(COO)63H2O model obtained from the structure optimization using the DFT calculation. The detailed fitting results of Cr K-edge and Fe K-edge spectra of Cr/Fe-MOF(5) were provided in Table S4 and Table S5, respectively. The fitting data revealed that, for the first-shell scattering, one Cr ion coordinates as CrO(3), CrO(C), and CrO(water) with the bond lengths of 1.75, 1.95, and 2.43 Å, respectively. While, two Fe(III) ions coordination are FeO(3), FeO(C), and FeO(water) as 1.57, 1.97, and 2.25 Å, respectively.

FT |(R)| / Å4

12 8

(a)

4 0 -4 -8

-12 0.5

8 6

FT |(R)| / Å4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 44

1.0

1.5

1.0

1.5

2.0

2.5

3.0

3.5

2.0

2.5

3.0

3.5

R/Å

(b)

4 2 0 -2 -4 -6 -8 0.5

R/Å


30

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Figure 12. (a) Cr K-edge and (b) Fe K-edge EXAFS spectra of Cr/Fe-MOF(5). R-space is illustrated without the phase correction. To further confirm the effect of the second metal Fe toward the bond shortening and distortion of the Cr/Fe trinuclear-oxo cluster, the EXAFS analysis of single metal MIL-101(Cr) and MIL-101(Fe) was carried out. Note that, MIL-101(Cr) was obtained by microwave synthesis (Cr-MOF(5)) while MIL-101(Fe) was obtained by solvothermal synthesis since the single MIL-101(Fe) phase cannot be achieved by microwave synthetic condition applied in the present study. The EXAFS fitting results are shown in Figure S7 with the detailed fitting data in Table S6 and Table S7 for MIL-101(Cr) and MIL-101(Fe), respectively. Their metal-oxygen bond lengths (summarized in Table 2) composed in the trinuclear-oxo clusters (illustrated Figure 13) are compared to those of the microwave-synthesized Cr/Fe-MOF(5) sample. The results demonstrate that the bond lengths of Cr-O and Fe-O presented in the single-metal system are quite similar indicating less distortion of the octahedral coordination within the SBU cluster. However, in the mixed-metal Cr/Fe system, the Fe-O3 shrinks by nearly 19% while the Fe-Owater elongates by 18%. The similar trend is found in EXAFS fitting of Cr K-edge results, in which Cr-O3 shrinks by 6% while the Cr-Owater elongates by 18%. These results support the intense pre-edge peaks observed in both Cr K-edge and Fe K-edge regions of the Cr/Fe-MOF(5). The EXAFS fitting results strongly support that Fe as the second metal acts as an efficient cationic competitor. As the short reaction time of 5 min, Fe resides in the trinuclear-


31


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oxo cluster for ca. 2/3 of the metal despite only 33 mol% (1/3 of the metal) was added into the reaction mixture. Accordingly, the formed Cr/Fe-MIL-101 at 5 min possesses a well-defined octahedral shape with the Fe-rich SBU.

(a) 1.96 Å

(b)

2.06 Å

2.02 Å

1.87 Å

1.88 Å

(c) 1.97 Å

2.24 Å

1.58 Å

1.94 Å

1.57 Å 1.75 Å

2.43 Å 1.95 Å

1.97 Å

Figure 13. Illustration for trinuclear-oxo clusters of (a) MIL-101(Cr), (b) MIL-101(Fe) and (c) MIL-101(Fe/Cr) composed in Cr/Fe-MOF(5) sample. Table 2. EXAFS derived average bond lengths of metal-oxygen in the trinuclear-oxo clusters of mixed-metal Cr/Fe-MOF(5) together with those of the single-metal MIL-101(Cr) and MIL101(Fe). Cr K-edge

CrO(3)

CrO(C)

CrO(water)

Cr/Fe-MOF(5)

1.754 Å

1.954 Å

2.434 Å

MIL-101(Cr)

1.869 Å

1.959 Å

2.055 Å

Fe K-edge

FeO(3)

FeO(C)

FeO(water)

Cr/Fe-MOF(5)

1.574 Å

1.971 Å

2.225 Å

MIL-101(Fe)

1.938 Å

2.025 Å

1.881 Å

2.25 Å


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The overall results demonstrate the crucial role of Fe as a cationic competitor, affecting both phase formation and chemical composition of polymorphic Cr/Fe-terephthalate frameworks by an efficient utilization of microwave synthetic method to monitor the phase formation and/or transformation.

CONCLUSION In this work, the competitive and the enhancing roles of the second metal Fe(III) by employing the microwave synthesis and varying the reaction time on the formation of polymorphic Cr/Fe-terephthalate MOFs have been revealed. Without the addition of Fe as the second metal, microwave synthesis mainly provides fast formation of 40 nm-sized spherical Cr-MIL-101 crystal particles for the reaction time ranging from 5 to 120 min. Addition of Fe as the second metal significantly affects the phase formation and the morphology of the obtained polymorphic Cr/Fe-terephthalate MOF products. Well-defined octahedral crystal particles of Cr/Fe-MIL-101 are achieved at the reaction time of only 5 min, highlighting the kinetic enhancement of the Cr/Fe-terephthalate formation by an introduction of Fe. Unlike the typical solvothermal synthesis, polymorphic Cr/Fe-terephthalate MOFs are observed depending on the microwave reaction time. The more-thermodynamically-stable polymorphic phases are formed at the extended reaction time; namely the hexagonal-rod Cr/Fe-MIL-88B crystals started to form since the reaction time reached 10 min and the brinklike Cr/Fe-MIL-53 crystals formed when the reaction time increases to 120 and 240 min.


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According to the analysis of XAS data, the co-existence of Cr and Fe in the discrete metal trinuclear-oxo cluster within the MOF frameworks is proved. Moreover, EXAFS fitting indicates that the incorporation of Fe(III) into the trinuclear-oxo cluster leads to the distortion of the metal octahedral units. This observed local distortion may be a driving force for the polymorphic-phase transformation to the more-stable phases consisting of lower degree of local distortion when extending the reaction time. Also, Fe is kinetically more favorable than Cr, attributing to the formation of Fe-rich MIL-101(Cr/Fe) in a very short reaction time of 5 min. In all, this work can be contributed to broaden the knowledge of the polymorphic phenomena in MOF synthesis and design.

ASSOCIATED CONTENT Supporting Information Additional data of products’ physical appearance, PXRD, SEM, elemental composition by XRF, EXAFS fitting results, N2 sorption isotherm, products’ yield, summary of particle size and morphology

AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Tel.: (+66)33014256


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ORCID Ladawan Pukdeejorhor: 0000-0001-6866-1567 Kanyaporn Adpakpang: 0000-0002-1388-3389 Suttipong Wannapaiboon: 0000-0002-6765-9809 Makoto Ogawa: 0000-0002-3781-2016 244 Satoshi Horike: 0000-0001-8530-6364 242 Sareeya Bureekaew: 0000-0001-9302-2038

Note The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by student grant and postdoctoral fellowship from Vidyasirimedhi Institute of Science and Technology and by the Thailand Research Fund (Grant RSA6080068). National Science and Technology Development Agency, Ministry of Science and Technology, Thailand, also support through the Research Chair Grant 2017 (Grant FDA-CO-2560-5655) and through the Research Network of NANOTEC-Nanotechnology for EnergyVidyasirimedhi Institute of Science and Technology (RNN-Energy-VISTEC). REFERENCES


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20. Volkringer, C.; Loiseau, T.; Guillou, N.; Férey, G.; Elkaïm, E.; Vimont, A., XRD and IR structural investigations of a particular breathing effect in the MOF-type gallium terephthalate MIL-53(Ga). Dalton Trans. 2009, 0, 2241-2249. 21. Whitfield, T. R.; Wang, X.; Liu, L.; Jacobson, A. J., Metal-organic frameworks based on iron oxide octahedral chains connected by benzenedicarboxylate dianions. Solid State Sci. 2005, 7 , 1096-1103. 22. Volkringer, C.; Meddouri, M.; Loiseau, T.; Guillou, N.; Marrot, J.; Férey, G.; Haouas, M.; Taulelle, F.; Audebrand, N.; Latroche, M., The Kagomé topology of the gallium and indium metal-organic framework types with a mil-68 structure: Synthesis, XRD, solid-state NMR characterizations, and hydrogen adsorption. Inorg. Chem. 2008, 47, 11892-11901. 23. Serre, C.; Mellot-Draznieks, C.; Surblé, S.; Audebrand, N.; Filinchuk, Y.; Férey, G., Role of solvent-host interactions that lead to very large swelling of hybrid frameworks. Science 2007, 315, 1828-1831. 24. Ramsahye, N. A.; Trung, T. K.; Scott, L.; Nouar, F.; Devic, T.; Horcajada, P.; Magnier, E.; David, O.; Serre, C.; Trens, P., Impact of the flexible character of mil-88 iron(III) dicarboxylates on the adsorption of n-alkanes. Chem. Mater. 2013, 25, 479-488. 25. Alhamami, M.; Doan, H.; Cheng, C.-H., A review on breathing behaviors of metalorganic-frameworks (MOFs) for gas adsorption. Materials 2014, 7, 3198-3250.


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39. Yang, L.; Zhao, T.; Boldog, I.; Janiak, C.; Yang, X.-Y.; Li, Q.; Zhou, Y.-J.; Xia, Y.; Lai, D.-W.; Liu, Y.-J., Benzoic acid as a selector–modulator in the synthesis of MIL-88B(Cr) and nano-MIL-101(Cr). Dalton Trans. 2019, 48, 989-996.


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For Table of Contents Use Only

Polymorphism of Mixed Metal Cr/Fe Terephthalate Metal-Organic Framework Utilizing Microwave Synthetic Method

Ladawan Pukdeejorhor,† Kanyaporn Adpakpang,*,† Panyapat Ponchai,† Suttipong Wannapaiboon,§ Somlak Ittisanronnachai,‡ Makoto Ogawa,† Satoshi Horike,‖,± ,# and Sareeya Bureekaew,*,†

Synopsis: The occurrence of polymorphic mixed metal Cr/Fe MOFs, i.e., MIL-101, MIL-88B and MIL53 was investigated under microwave synthetic method.


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44

Fe Terephthalate Metal-Organic

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