MIL-53: Modulating Breathing Behavior through Intr - American

Article pubs.acs.org/Langmuir

Interplay of Metal Node and Amine Functionality in NH2‑MIL-53: Modulating Breathing Behavior through Intra-framework Interactions Pablo Serra-Crespo,† Elena Gobechiya,‡ Enrique V. Ramos-Fernandez,† Jana Juan-Alcañiz,† Alberto Martinez-Joaristi,† Eli Stavitski,§ Christine E. A. Kirschhock,‡ Johan A. Martens,‡ Freek Kapteijn,† and Jorge Gascon*,† †

Catalysis Engineering, Chemical Engineering Department, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands ‡ Centre for Surface Chemistry and Catalysis, Faculty of Bioscience Engineering, University of Leuven, 3001 Leuven, Belgium § National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *

ABSTRACT: A series of amino-functionalized MIL-53 with different metals as nodes has been synthesized. By determining adsorption properties and spectroscopic characterization, we unequivocally show that the interaction between the amines of the organic linker and bridging μ2-OH of the inorganic scaffold modulates metal organic framework (MOF) flexibility. The strength of the interaction has been found to correlate with the electropositivity of the metal.

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INTRODUCTION A special class of metal organic frameworks (MOFs) is those whose pore dimensions change without breaking chemical bonds within the framework. This results in special properties, such as the breathing effect1−3 and the gate phenomenon,4−6 where pores contract or open during molecule adsorption. An example of a breathing-type material is the MIL-53 series, first reported and rigorously characterized by Ferey, Serre, and coworkers (MIL stands for Material Institute Lavoisier).3 MIL-53 is built from MO6 octahedra (where M can be Fe3+, Cr3+, Al3+, Ga3+, In3+, or Sc3+) formed from trans-bridging OH ions and the oxygens of the coordinate, bridging 1,4-benzenedicarboxylate linkers. In this way, a crystalline material with one-dimensional (1D) diamond-shaped pores is formed. Upon adsorption of guest molecules, e.g., CO2 or H2O, or by changing operating conditions, the framework structure reversibly changes.2,7 For the Cr- or Al-containing forms of MIL-53, MIL-53(Cr) or MIL-53(Al), the structure in which the pores are in the “open” form is the most stable form after thermal activation. These two forms of MIL-53 show a transition from a large pore form (lp form, the initial dehydrated form) to a narrow pore form (np form) during adsorption of certain molecules. When the driving force (the partial pressure of the adsorbing molecule) is large enough, the pores reopen to their original lp form. Thermodynamical, stress-based, and molecular models have been developed to describe and analyze these breathing transitions in MIL-53.8−11 The iron and gallium forms of MIL-53, MIL-53(Fe) and MIL-53(Ga), display a more complex and different behavior. In © XXXX American Chemical Society

the case of gallium, the stability domain of the narrow pore structure MIL-53(Ga) np is larger (up to 160 °C, instead of 20−30 °C for Al, for instance), while the iron form of MIL-53 is in a “very narrow pore” form (vnp form) when it is initially dehydrated. The latter can hardly accommodate guest molecules. With increasing pressure, this structure passes via intermediate forms to the lp form.12−14 Amino-MIL-53, hereafter NH2-MIL-53 (see Figure 1), is a material with the topology of MIL-53. During the synthesis of NH2-MIL-53, 2-amino-terephthalic acid is used as the linker molecule, instead of terephthalic acid. The isoreticular material obtained has the formula X(OH)[O2C−C6H3NH2−CO2], with X denoting a trivalent metal at the nodes.15−17 During the past few years, we have extensively investigated the Al version of the amine-functionalized MIL-53 framework.18−21 NH2-MIL53(Al) appears to be an excellent candidate for CO2 capture from multicomponent mixtures, displaying an almost infinite selectivity for CO2 over other gases, e.g., CH4, H2O, N2, and CO, based on shape selectivity.22,23 Although we initially assumed amino-MIL-53(Al) to behave similar to MIL-53(Al or Cr), i.e., starting in the open pore configuration after thermal activation, followed by a transition to the narrow pore form after adsorption of CO2 molecules at relatively low pressures, more detailed combined spectroscopic (IR), in situ diffraction (XRD), and theoretical (DFT) studies Received: July 12, 2012 Revised: August 12, 2012

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in a series of block 13 NH2-MIL-53, viz., NH2-MIL-53(Al), NH2-MIL-53(Ga), and NH2-MIL-53(In).

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EXPERIMENTAL SECTION

Synthesis. For the preparation of NH2-MIL-53(Al, Ga, and In), 5.3 mmol of the metal precursor (AlCl3·6H2O, 99.9% Ga(NO3)3·xH2O, or 99.9% In(NO3)3·xH2O from Sigma Aldrich) and 8.3 mmol of 2amino-terephthalic acid (HO2C−C6H3NH2−CO2H, Sigma Aldrich, 99%) are dissolved in deionized water (20 mL). The reactants are then placed in a Teflon-lined autoclave and heated for 5 h at 423 K in an oven under static conditions. The resulting yellow powders were filtered under vacuum and washed with acetone. To remove organic species trapped within the pores, the samples were activated. The activation process consists of exchanging the trapped linker with N,Ndimethylformamide (DMF) at 423 K overnight. In the preparation of the deuterated version of NH2-MIL-53(Al), deionized water was replaced by deuterium oxide. Gas Adsorption Isotherms. Low-pressure adsorption isotherms of CO2 (purity of 99.995%) were measured at 273 K in a device built by Bruker based on the volumetric technique. High-pressure adsorption isotherms of CO2 (purity of 99.995%) and CH4 (purity of 99.95%) were determined using the volumetric technique with an apparatus from BEL Japan (Belsorp HP). Around 0.5 g of NH2-MIL53(X) samples were placed in the sample container. Before every measurement, the adsorbent was pretreated by increasing the temperature to 473 K at a rate of 10 K/min under vacuum and maintaining the temperature for 2 h. The measurements for carbon dioxide and methane adsorption were carried out at 273 K for the complete series of NH2-MIL-53. Fourier Transform Infrared (FTIR) Transmission Spectroscopy. Self-supported pellets made of different NH2-MIL-53 were prepared with a mass around 8 mg. The pellets were placed in an IR quartz cell equipped with CaF2 windows. A movable sample holder allows for the sample to be placed in the IR beam for the measurements or into the furnace for thermal treatments. The cell is connected to a vacuum line for pretreatment. The specimen is activated in vacuum at 180 °C for 2 h to remove adsorbed species. After this step, the samples were cooled to room temperature. Transmission spectra were recorded in the 400−4000 cm−1 range at 4 cm−1 resolution on a Nicolet Nexus spectrometer equipped with an extended KBr beam splitter and a mercury cadmium telluride (MCT) cryodetector. Methanol Desorption Followed by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). DRIFT spectra were recorded in a Bruker model IFS66 spectrometer, equipped with a high-temperature cell with CaF2 windows and a 633 nm laser. The spectra were collected after accumulation of 128 scans with a resolution of 4 cm−1. A flow of helium at 10 mL/min was maintained during the measurements. Before collecting the spectra, the sample was pretreated in the equipment under helium flow at 473 K for 1 h. Previous to the experiment, the sample was saturated with deuterated methanol (CH3OD, Sigma Aldrich, 99.5% D). After a spectrum acquired at room temperature, the sample was heated in intervals of 10 K up to 473 K, and after every increase, a new spectrum was recorded. KBr was used as a background. Carbon Dioxide Followed by DRIFTS. DRIFT spectra were recorded in a Bruker model IFS66 spectrometer, equipped with a highpressure cell with ZnSe windows and a 633 nm laser. The spectra were collected after accumulation of 256 scans with a resolution of 4 cm−1. Before the spectra were collected, the sample was pretreated in the equipment under helium flow at 473 K for 1 h. Carbon dioxide pressure was increased first in steps of 0.2 bar up to 1 bar and then in steps of 1 bar up to 25 bar, and all of the spectra were recorded at 298 K. KBr was used as a background. Powder XRD. Powder XRD patterns were collected at the Swiss− Norwegian Beamline (SNBL) BM01A at the European Synchrotron Radiation Facility (ESRF, Grenoble, France), using a MAR345 image plate detector and at 250 mm from the sample. The data were collected on 0.7 mm glass capillaries (Hilgenberg) filled with the

Figure 1. NH2-MIL-53(Al) in its vnp and lp forms. Color legend: yellow, aluminum; red, oxygen; gray, carbon; and blue, nitrogen.

recently indicated that NH2-MIL-53 behaves more like MIL53(Fe), where a narrow pore form prevails after thermal activation.22,24 The amine moieties of the framework do not interact chemically with CO2 upon its adsorption but modulate the framework flexibility. In the absence of guest molecules, NH2-MIL-53(Al) is in a very narrow pore form because contracted framework results in a more efficient hydrogen bonding between −NH2 groups and the framework hydroxyls (μ2-hydroxo groups), keeping the framework contracted. In addition, the NH2-MIL-53(Al) in its vnp and np forms possesses a non-centrosymmetric structure, while expansion of the framework breaks this non-centrosymmetric arrangement. As a result, the NH2-MIL-53(Al) behaves as a highcontrast reversible solid-state nonlinear optical switch.25 The effect of framework functionalization on breathing in the MIL-53 family has been addressed by several research groups: Serre and co-workers have reported the most extensive study on a large series of MIL-53(Fe) functionalized solids [−Cl, −Br, −CH3, −(CF3)2, −NH2, −(OH)2, and −(CO2H)2] and have evaluated by X-ray diffraction (XRD), in situ infrared (IR) spectroscopy, thermogravimetric (TG) analyses, and 57Fe Mössbauer spectrometry the effect of such organic modifications on both the surface properties and the breathing phenomenon.26 A related study on functionalization of another flexible framework, MIL-88(Fe), has led to the conclusion that the presence of additional functional groups, with a complex combination of steric hindrance and intra-framework interactions, leads to a slightly different flexible behavior.27,28 Along the same lines, Stock et al. have reported the synthesis and breathing behavior of a whole series of functionalized MIL53(Al).29 In the specific case of amine functionalization, Farruseng et al. have studied the effect of partial amine functionalization (using mixtures of amino-terephthalic acid and terephthalic acid) on (NH2)x-MIL-53(Al), with x < 1.8,30 Despite the remarkable breadth and high quality of the previous work, rationalization of breathing modulation has not yet been achieved. In this report, we make a crucial step in rationalizing the mechanism that controls breathing in the specific case of amine functionalization. By means of a combined diffraction, adsorption, and IR characterization, we are able to rationalize how the breathing behavior is controlled B

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Table 1. Unit Cell Parameters for the Different NH2-MIL-53 Samples vnp form NH2-MIL-53(Al) NH2-MIL-53(Ga) NH2-MIL-53(In) a

space group Cc Cc C2/c

a (Å) 19.757 (2) 19.9033 (21) 21.8641 (22)

b (Å) 7.481 (1) 7.1956 (7) 6.6247 (8)

c (Å) a

6.589 (1) 6.6897 (7)a 7.1565 (9)a

β (deg)

V (Å3)

105.731 (8) 104.667 (10) 115.007 (9)

937.53 (19) 926.85 (19) 939.41 (20)

Parameter along the channel direction.

sample and attached to a gas system able to operate from vacuum conditions up to 20 bar. The capillaries with the sample were rotated during the measurements. In this way, the full powder rings are collected: integrating over the rings and rotation around the axis sums up all of the scattering in three dimension (3D), resulting in a true average with no preferred orientation. The data were integrated using the Fit2D program (Dr. A. Hammersley, ESRF) using National Institute of Standards and Technology (NIST) LaB6 as a reference (λ = 0.709 659 Å). Prior to the experiments, the samples were outgassed under vacuum at 473 K for 2 h. The temperature was then adjusted to 253 K, and the data were collected in vacuum and under CO2 pressure. The temperature was controlled using an Oxford Cryosystem device. The powder XRD patterns of the activated NH2-MIL-53(Al), NH2MIL-53(Ga), and NH2-MIL-53(In) were indexed using DICVOL04,31 followed by the Le Bail fit performed with FOX.32 The crystal structure models were obtained directly from powder diffraction using direct-space techniques implemented in FOX and refined with the GSAS/EXPGUI software package.33,34 Refined unit cell parameters of the activated samples (vnp form) are listed in Table 1.

than that of the aluminum framework. The purity of the synthesized samples is evidenced by the TG analysis. The amounts of metal oxide residue are in agreement with the empirical formula XOH−(COO−C6H3NH2−COO), where X is Al, Ga, or In. IR spectra (DRIFT) collected on the series of NH2-MIL-53, after pretreatment in a He flow at 423 K, are shown in Figure 2

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Figure 2. FTIR spectra of the NH2-MIL-53 series. Color legend: red, aluminum; blue, gallium; and black, indium. (Left) Detail of the hydroxyl region. (Right) Detail of the NH2 stretching region.

RESULTS AND DISCUSSION The structure model for the vnp form of NH2-MIL-53(Ga) has been obtained directly from powder diffraction in Cc (noncentrosymmetric) and in C2/c (centrosymmetric) space groups with very good fit to the data for both models (Rwp = 4.00 and 3.82%, respectively). In contrast, the structure model for the vnp form of NH2-MIL-53(In) has been obtained in C2/c (centrosymmetric) space groups only (Rwp = 5.77%), while no solution for the Cc (non-centrosymmetric) space group directly from powder diffraction has been found. Very recently, we reported the structure solution for NH2-MIL-53(Al), also in the Cc (non-centrosymmetric) space group.25 The obtained models were imported into GSAS for the Rietveld refinement.33 The linker molecule was modeled as a rigid body, where orientation and position were freely refined. At the initial stage of the refinement, a high mobility of Ga atoms and linker molecules along the c axis (channel direction) in the non-centrosymmetric structure (Cc) where Ga and origin of the rigid body of the linker occupy general positions was observed. Thereafter, the refinement of (x, y, and z) parameters for Ga atom and origin of the rigid body was performed one by one to avoid strong correlation. Isotropic temperature factor parameters (Uiso) for the atoms of the linker molecule were refined as constraints with the same value for all atoms in both cases, NH2-MIL-53(Ga) and NH2-MIL-53(In). A much better fit for the vnp form of NH2-MIL-53(Ga) was obtained with the non-centrosymmetric (Cc) structure with an ordered position of the amino group of the linker molecule (see Figure S1 of the Supporting Information). R values for non-centrosymmetric and centrosymmetric structure refinements are listed in the Table S3 of the Supporting Information. More detailed refinement information is given in the Supporting Information. The difference in thermal stability among the different MIL53 is observed in Figure S5 of the Supporting Information. The Al, Ga, and In samples are thermally stable in air up to temperatures close to 673, 623, and 598 K, respectively. The thermal stability of the Ga and In materials is slightly lower

for the two most interesting spectral regions, namely, the OH and NH2 stretching regions. In the case of the Al sample, the broad band centered around 3680 cm−1 along with a shoulder at 3693 cm−1 are assigned to the bridging hydroxyl groups of the MIL-53, i.e., Al−OH−Al (μ2-hydroxo groups). The shoulder position is very close to that reported for unfunctionalized MIL-53(Al),35 whereas the main band is slightly red-shifted. We attribute this main band to hydroxyl groups, which are directly interacting with the neighboring amines, while the shoulder at 3693 cm−1 corresponds to OH groups not engaged in such hydrogen bonding.23,36 The two sharp bands at 3393 and 3505 cm−1 in the spectrum of the hydrothermally synthesized NH2-MIL-53(Al) represent the symmetric and asymmetric N−H vibrations, respectively.24,37,38 When different MIL-53 are compared, the OH stretching bands are red-shifted in the order Al−Ga−In. The amine region also shows clear differences between the samples. Although the maxima of the symmetric and asymmetric bands are centered at the same position for all samples, the materials based on aluminum and gallium present sharp absorbances, whereas for indium, both amine stretchings are much broader and asymmetric, tailing to lower wavenumbers. To study the influence of the metal node on the breathing properties of the activated NH2-MIL-53, CO2 and CH4 adsorption isotherms were determined. Figure 3 shows the results obtained for CO2 at 273 K. Even at low pressures, clear differences are observed. While the Al sample presents a typical Langmuir isotherm, Ga and In samples display a stepped adsorption profile. Further steps are observed at increasing pressures (bottom panel of Figure 3), in good agreement with the well-known breathing behavior of this type of material. Experiments performed with NH2-MIL-53(Ga) samples of different particle size demonstrate that this does not influence the breathing behavior and that differences in Figure 3 are truly C

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Figure 4. In situ XRD patterns obtained during CO2 adsorption at different pressures and 253 K using synchrotron radiation (λ = 0.709 659 Å). A change in color indicates a phase transition.

the Le Bail fits for the np and lp forms can be found in the Supporting Information. With regard to the solvent-free structural features, it is clear that metal nodes do play a major role in the pore opening of the vnp form stable at room temperature, with pore sizes decreasing in the order Al > Ga > In. This change in pore opening has very interesting consequences for adsorption selectivity, as observed in Figure 3. The IR spectra of the NH2-MIL-53 series strongly suggest the different strengths of the intra-framework interactions. A clear correlation with the electropositivity of the metal node on the position of the hydroxyl stretching and the flexibility of the framework is observed. The higher the electropositivity of the metal, the lower the wavenumber of the hydroxyl stretchings. From the structure refinements, it can also be observed that the distance N−Oμ2 decreases with increasing electropositivity of the metal (see Figure 5). The length of the N−Oμ2 distance

Figure 3. Carbon dioxide adsorption isotherms at up to 120 kPa (top) and 3.5 MPa (bottom) at 273 K. Color legend: red, aluminum; blue, gallium; and black, indium.

due to the metal nodes. We attribute the difference in the pressure needed for the vnp → np and np → lp transitions to the electropositivity of the metal, as explained below. When CO2 uptake is represented in moles of CO2 per moles of metal, in all cases, the maximum uptake of both the np and lp configurations is similar (CO2 uptake at saturation, on a mass basis, is 47, 40, and 35 wt % for the Al, Ga, and In samples, respectively). In contrast to CO2, CH4 does not adsorb at all on the Ga and In NH2-MIL-53, even at very high pressures (see Figure S6 of the Supporting Information). XRD using synchrotron radiation was performed during adsorption of carbon dioxide and methane to unravel transitional changes in the structure of the NH2-MIL-53(X) series and to correlate such changes with the isotherms discussed above. The diffraction patterns under different CO2 pressures at 253 K for the whole series of materials are shown in Figure 4, while those for CH4 are shown in the Supporting Information. The color of the lines is changed when a conformational change occurs. Patterns collected under vacuum conditions are plotted in black (vnp form). Right after the introduction of CO2 (at sub-atmospheric pressures), a small change in the diffraction pattern is observed for all of the materials (vnp → np). In very good agreement with Figure 3, the stability of the vnp form is higher for the Ga and In materials [vnp → np takes place at PCO2 ≈ 0.2 bar for NH2MIL-53(Ga) and NH2-MIL-53(In), while this transition already occurs at much lower PCO2 for the Al MOF]. As pressure increases, all NH2-MIL-53 samples expand to their corresponding lp forms. At 273 K, the np to lp transition occurs at pressures of around 7, 13, and 25 bar for the aluminum, gallium, and indium samples, respectively. In contrast, in the presence of methane, only the vnp → np transition occurs (see the Supporting Information). Refined unit cell parameters and

Figure 5. N−Oμ2 distances for the different NH2-MIL-53 as calculated from the structure refinement in the vnp form.

correlates with the position of the linker molecule in the NH2MIL-53 framework, in good agreement with previous reports on NH2-MIL-53(Fe).26 The location of the geometrical center of the linker molecule in the center of inversion (4c) in the C2/ c space group gives a shorter N−Oμ2 distance and partial disorder of the amino groups like in the case of the NH2-MIL53(In) framework. A shift of the geometrical center of the linker molecule to a general position leads to the noncentrosymmetric arrangement of the structure with the ordered D

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amino groups and increasing N−Oμ2 distances in the case of NH2-MIL-53(Ga) and NH2-MIL-53(Al) material. At the same time, the observed distances inversely correlate with the breathing pressure. It is easy to envisage that a higher metal electropositivity results in a higher charge density at the μ2 bridging O and, therefore, a stronger acidic character of μ2-OH. IR spectrum-wise, this translates in a red shift of the OH stretching. The different acidity of bridging μ2-OH together with the presence of slightly basic amine moieties at the linker clearly modulate the breathing behavior of the resulting adsorbents. From the amine perspective, intra-framework NH2−Oμ2 interactions are also evident in the IR spectra, as demonstrated from the broadening observed in both the O−H and N−H vibration regions going from Al to In. All of the above spectroscopic and structure refinement results demonstrate that intra-framework interactions modulate the breathing of the adsorbents, as macroscopically observed in the adsorption isotherms and by in situ XRD experiments, where the framework opening pressure increases with an increasing interaction strength and a decreasing N−Oμ2 distance. These results are in very good agreement with recent reports.39 The strength of the interaction has very interesting consequences for adsorption, because only certain adsorbates are able to induce the expansion of the framework [e.g., CO2, toluene, or MeOH (see the Supporting Information) but not CH4]. We speculate that only adsorbates able to break N−Oμ2 via similar hydrogen-bonding interactions (i.e., MeOH) or with strong adsorbate−adsorbent affinity (i.e., CO2 or toluene) are able to expand the framework. Because the amino groups do not interact with the adsorbates, it seems reasonable to state that the primary adsorption sites in these materials are bridging μ2-OH. To undoubtedly demonstrate this hypothesis, we performed two different series of experiments. In the first series, we followed with in situ DRIFTS the adsorption and desorption of methanol on the three NH2-MIL-53. In the second series of experiments, adsorption of CO2 up to 24 bar was followed in situ with DRIFTS on a partially deuterated NH2-MIL-53(Al). The use of deuterated methanol as a probe molecule for IR studies is well-known, and its use is convenient in the present case because all MeOD vibrations occur at wavenumbers below 3000 cm−1. When the materials are saturated (red lines in Figure 6), a broad band, because of hydrogen bonding between the microporous materials and MeOD, from ∼3600 to 3125 cm−1 is visible and covers the region of interest. Once desorption of MeOD starts (blue lines), the amine stretchings reappear at the same wavenumbers. Moreover, the new doublet centered between 2715 and 2720 cm−1 indicates a certain degree of deuterium exchange between MeOD and μ2-OH, resulting in μ2-OD vibrations (2715 and 2730 cm−1). The adsorption strength can also be predicted on the basis of Figure 6, Recovery of the μ2-OH vibration is much faster for Al and Ga NH2-MIL-53, in contrast to the stronger interaction observed in the case of the In sample. These results are also in line with CO2 and MeOH adsorption isotherms. Finally, in situ DRIFTS experiments were performed using CO2 as a probe molecule on a NH2-MIL-53(Al) sample deuterated at the bridging hydroxyls (hereafter μ2-OD). Figure 7 shows the IR spectra of the O−D region at increasing CO2 pressures and after evacuation of the cell. After deuteration, similar stretching vibrations as those shown in Figure 2 for μ2OH appear at ∼1000 cm−1 lower wavenumbers, demonstrating the successful deuteration of the sample at the bridging

Figure 6. In situ DRIFTS study of deuterated methanol (MeOD) desorption. Color legend: black, material before adsorption; red, material saturated with MeOD at 298 K; dark blue, material heated at 311 K; and light blue, material heated at 343 K.

Figure 7. DRIFTS spectra of deuterated NH2-MIL-53(Al) in the 2680−2740 cm−1 region upon CO2 adsorption. Color legend: solid black, 0 bar; red, 0.2 bar; orange, 0.8 bar; light green, 3.5 bar; green, 12 bar; blue, 24 bar; and dashed black, 0 bar after desorption.

hydroxyls (see the Supporting Information for the full IR spectrum). In this case, the band associated with μ2-OD interacting with the amines of the framework is centered at 2715 cm−1, while the shoulder at 2730 cm−1 corresponds to OD groups not engaged in hydrogen bonding. As soon as CO2 is dosed into the in situ cell, the bands associated with the free OD groups diminish and a new band appears at ∼2705 cm−1, while the peak at 2730 cm−1 is also slightly red-shifted. The interaction of adsorbate−μ2 -OH results in the appearance of a new band (centered at 2705 cm−1 in the deuterated sample). This new band is attributed to the noninteracting OH of the structure that upon adsorption of CO2 shifts by 30 cm−1. Because these OHs are not involved in hydrogen bonding with the amine moieties of the structure, they are the first to interact with CO2, in very good agreement with experiments performed on non-functionalized MIL-53.40 Although a much larger OH shift would be expected in the case of experiments performed in Figure 6,41 ca. 300 cm−1, according for non-functionalized MIL-53 and MeOD,42 most likely this shift is not observed because it is hidden under the broad and intense absorption because of the amines in that range for all samples. On the other hand, the minor red shift in the interacting OH in Figure 7 (2715 cm−1 band for the deuterated sample) demonstrates that, once the N−Oμ2 E

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interaction is broken, these sites are occupied by the adsorbate. The change in the relative intensity of both bands as shown in Figure 7 is attributed to the variation in the extinction coefficients of the O−D bands and, therefore, cannot be used as a quantification tool.43,44 In contrast, as earlier reported, amine bands are hardly affected by the presence of adsorbates, confirming bridging μ2-OH as the preferred adsorption sites.

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CONCLUSION A series of amino-functionalized MIL-53 with different metals as nodes has been synthesized. Using a combined adsorption and spectroscopic analysis, we have undoubtedly shown that both metal nodes and linker functionality play a major role in modulating the MOF flexibility. The strong interaction between the amines of the organic linker and bridging μ2-OH of the inorganic scaffold is responsible for the initial vnp configuration. This interaction is determined on the electropositivity of the metal used and, therefore, the final acidity of μ2-OH and clearly influences flexibility of the framework. Last, the primary adsorption sites in the whole NH2-MIL-53 series are μ2-OH, similar to their unfunctionalized counterparts, and not the amines in the framework.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental details and crystallographic information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Elena Gobechiya, Christine E. A. Kirschhock, and Johan A. Martens acknowledge financial support from the Belgian Prodex Office and long-term structural funding from the Flemish Government (Methusalem). Jorge Gascon gratefully thanks the Dutch National Science Foundation (NWO-CWVENI) for its financial support. We thank the ESRF for the provision of the beamtime at the BM01A beamline and Dr. Yaroslav Filinchuk for his assistance during the use of the beamline (SNBL at ESRF).

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REFERENCES

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