Influence of the Oxidation State of the Metal Center on the Flexibility

ARTICLE pubs.acs.org/JPCC

Influence of the Oxidation State of the Metal Center on the Flexibility and Adsorption Properties of a Porous Metal Organic Framework: MIL-47(V) Herve Leclerc,† Thomas Devic,‡ Sabine Devautour-Vinot,§ Philippe Bazin,† Nathalie Audebrand,|| Gerard Ferey,‡ Marco Daturi,† Alexandre Vimont,† and Guillaume Clet*,† †

Laboratoire Catalyse et Spectrochimie, ENSICAEN, Universite de Caen Basse-Normandie, CNRS, 6 Bd. du Marechal Juin, 14050 Caen, France Institut Lavoisier, UMR CNRS 8180, Universite de Versailles Saint-Quentin-en-Yvelines, 45 avenue des Etats-Unis, 78035 Versailles cedex, France § Institut Charles Gerhardt Montpellier, UMR CNRS 5253, UM2, ENSCM, Place E. Bataillon, 34095 Montpellier cedex 05, France Sciences Chimiques de Rennes (UMR CNRS 6226), Universite de Rennes 1, 263 Av. du General Leclerc, 35042 Rennes cedex, France

)

‡

bS Supporting Information ABSTRACT: An alternative activation method was developed to stabilize the Metal Organic Framework (MOF) MIL-47(V) material in the VIII oxidation state. This solid and the oxidized forms were investigated by in situ infrared and Raman spectroscopies, X-ray diffraction (XRD), and Complex Impedance Spectroscopy (CIS). Unlike MIL-47(VIV), MIL-47(V III) is a flexible structure which presents μ2-hydroxyl groups acting as preferential adsorption sites for H2O or CO2 . The modulation of the oxidation state of the metal center of this porous material leads to new intermediate porous solids with mixed oxidation states V III/VIV. In these materials, the VIII and VIV centers seem to occur in close vicinity. However, the presence of VIV centers inhibits the flexibility to a large extent.

’ INTRODUCTION Metal Organic Frameworks (MOFs) are promising new materials for a large number of applications.13 Many porous structures present valuable adsorption properties4 and high sorption capacities for hydrogen,5,6 CO2, CH4,713 polar vapors,1416 linear hydrocarbons,17,18 substituted aromatics,1921 or drugs for biological applications.22,23 In some cases, the adsorption properties were tailored by an unusual property of the structure: its flexibility (“breathing”) between different porous structures, which corresponds to an adaptation of the pore size and shape to the guest molecules without any loss of crystallinity.24,25 Metal terephthalates (1,4-benzenedicarboxylate) such as the MIL-53 (MIII(OH)(L) 3 (guest)x; L = terephthalate, MIII = Cr, Al, Ga, Fe, ...)2630 may give rise to this phenomenon. These solids, built up from chains or μ2-OH corner sharing MO6 octahedra connected through the terephthalate ligands to define a 1-D pore system, can indeed expand or contract reversibly upon thermal31 or mechanical32 stimuli or upon adsorption of gases (pure and in mixture)10,12,18 or liquids.33 In such processes, the cell volume can vary by 40% between the narrow pore form (NP) and the large pore form (LP). Organic modifications brought to the ligand itself can also help tune the flexibility.34 Alternative properties are found on the related MIL-47(V) material,35 which is also made of infinite chains of corner-sharing metal octahedra interconnected by the carboxylate groups but shows a structure which is essentially rigid and remains in an LP form. r 2011 American Chemical Society

The as-synthesized solid exhibits a slight structural modification upon guest exchange, but its calcination under air induces simultaneously a permanent modification of the solid, which was attributed to a change of the oxidation state of the vanadium centers from VIII to VIV by single-crystal XRD analysis.35 This activation step was shown to be quite sensitive to experimental parameters such as temperature, time, and amount of solid.36 This leads to the final formulation VIV(O)L and a complete loss of flexibility.35 In the case of the deposition of liquid organics on crystals of MIL-47(VIV), some slight deformations of the structure can still be observed but, except with acetone, the cell volume is not modified sufficiently to represent a real “breathing”.37,38 However, even in the case of the shrinking observed with acetone, the final pore dimensions were far larger than those that are obtained when MIL-53(Cr) is in its NP form. The rigidity of this MIL-47 structure influences considerably its adsorption properties compared to MIL-53, and several studies have reported differences between MIL-47 and MIL-53 despite their similar structure and porosity in the LP form. During CO2 adsorption, the structure change induces a twostep adsorption on MIL-53 (Cr, Al) which is not observed on MIL-47(V).9 This was attributed to specific interactions between Received: July 13, 2011 Revised: August 29, 2011 Published: August 30, 2011 19828

dx.doi.org/10.1021/jp206655y | J. Phys. Chem. C 2011, 115, 19828–19840

The Journal of Physical Chemistry C the hydroxyl groups and CO2 in MIL-53.10,39,40 OH groups in this latter solid were also found to influence the adsorption of H2O16,41 or H2S.42,43 Similarly, adsorption of substituted alkyl aromatics gave different results on MIL-47(V) and MIL-53(Al).1921 For gas-phase adsorptions of xylenes, the breathing of the structure of MIL-53 strongly modified the adsorption profile compared to MIL-47.21 Differences in selectivities were also observed in liquid-phase separations, but they were suggested to be due to different polarizations of the carboxylate groups following the presence of different metals in the structure.19 Both the metal center and its environment can thus play a role in the adsorption properties. Therefore, the permanent change induced on the structure of MIL-47 after calcination under air raises questions on the role of the oxidation state of the metal in the flexibility of the material. This will be the focus of this paper. For this purpose, an alternative pretreatment was developed to obtain a well-activated material without oxidation of the metal, i.e., with vanadium in the VIII state. This results in a flexible solid, as highlighted by the sorption of CO2 and water. This material was characterized and compared with MIL-47 in the VIV state, the differences in flexibility being highlighted by several techniques. Alternatively, a solid in a mixed oxidation state VIII/VIV was developed after partial oxidation of the MIL-47(VIII). All these MIL-47(V) solids were extensively characterized by XRD with or without coupling with Raman spectroscopy, Complex Impedance Spectroscopy (CIS), and vibrational spectroscopies (FTIR and Raman).

’ EXPERIMENTAL SECTION Synthesis. The preparation of the solid by hydrothermal synthesis was carried out as previously reported.35 This leads first to the as-synthesized form of MIL-47, H2L@MIL-47, formulated following Barthelet et al.35 as VIII(OH)L 3 (H2L)x (x ∼ 0.7). The free terephthalic acid was then exchanged by dimethylformamide (DMF) at 423 K10 to produce DMF@MIL47 with DMF/L ∼1.2. The pores in this form were then emptied by activation under secondary vacuum (P < 105 mbar), following the transformation by in situ infrared spectroscopy. In a first step, the solid was very slowly heated to remove most of the impurities before reaching 423 K, as their presence at higher temperatures induces the oxidation of the solid. In a second step, the solid was treated at 523 K to ascertain the complete removal of free acid. This gave rise to the activated MIL-47 (impurityfree), later labeled MIL-47(VIII). The time requested for each step was dependent on the amount of solid used (from 1 h for each step for 2 mg of sample to 8 h for 400 mg batches). Partial oxidation was achieved by keeping the activated solid in air flow (for Raman experiments) or under O2 (133 mbar) for various times from 15 min to 2 h at 453 or 473 K. These solids will be later labeled MIL-47(VIII/VIV). Different VIII/VIV ratios can be obtained by tuning the temperature of oxidation and duration of the oxidative treatment. The ratio is controlled more easily when carrying out the oxidative treatment at a lower temperature with a reduced amount of O2. Total oxidation of the MIL-47 was achieved by calcination of the as-synthesized solid as previously reported35 or from the activated MIL-47 (MIL-47(VIII)) by successive treatments with O2 at 423 K (2 h) and 473 K (1 h). This solid will be finally labeled MIL-47(VIV). The activation procedure is summarized in Scheme 1.

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Scheme 1. Summary of the Different Activation Procedures of MIL-47(V)

X-ray Powder Diffraction. X-ray powder diffraction (XRPD) patterns were collected at the BM01A station at the Swiss Norwegian Beamline of the European Synchrotron Radiation Facility (Grenoble, France), using a MAR345 imaging plate with a sample-to-detector distance of 400 mm and a time of exposure of 60 s (λ = 0.701277 or 0.709622 Å depending on the experiments). The powdered sample VIII(OH)L 3 (H2O)x was introduced in 1 mm quartz capillaries. The structural behavior of the solid was tested in air, by simply heating the capillary with a gas blower from 293 to 500 K, with a heating ramp of about 200 K h1 and a data collection every 150 s. In a second range of experiments, the behavior of MIL-47(VIII) was explored upon adsorption of CO2. To this purpose, the capillary was connected to a homemade gas dosing system.44 Prior to the experiments, the sample, initially in its hydrated state, was outgassed under vacuum (pressure of about 103 mbar) at room temperature for a few hours. The temperature was then adjusted to 303 K, and aliquots of gas were introduced. X-ray powder diffractograms were collected one minute after the gas introduction. New X-ray powder diffraction patterns were recorded at the same pressure every five minutes, and equilibrium (at a given pressure) was assumed when no change was observed between the successive patterns. The data were integrated using the Fit2D program (Dr. A. Hammersley, ESRF) and a calibration measurement of a NIST LaB6 standard sample. The patterns were indexed using the Dicvol software.45 Le Bail fits were then performed with the Fullprof2k software package using the Winplotr interface.46,47 The resulting unit-cell parameters are summarized in Table 1 and Table 2. Raman Spectroscopy. Raman experiments were conducted on a Jobin Yvon Labram 300 confocal microscope. The solid was placed in an environmental chamber (Linkam CCR-1000) connected to a gas-flow system. The sample was heated in the cell. The confocal spectrometer was equipped with a laser at 532 nm and a 1800 lines/mm grating. Laser power on the sample was ca. 0.4 mW, and acquisition time varied from 10 s to 1 min. The MIL-47(VIII) samples were used after activation as reported above. They were quickly transferred to the cell and flushed with inert gas. They were then reactivated at 453 K under gas flow. In some cases, water vapor was introduced after the gas flow passed through a saturator at 295 K. For coupled XRPD/Raman measurements, Raman spectra were collected in situ in the capillary at the position of the X-ray beam. The Renishaw confocal spectrometer was equipped with a laser at 532 nm and a 1800 lines/mm grating. The laser and Raman signal were transferred between the spectrometer and the 19829

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The Journal of Physical Chemistry C

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Table 1. Cell Parameters of MIL-47 in Various Statesa T (K)

solid MIL-47(VIII) NP (H2O)

303

a (Å)

S.G. C2/c

MIL-47(VIII) INT

∼400

MIL-47(VIII) CP

∼450

MIL-47(VIII) LPb

500

MIL-47(VIV) empty

303

Pmcn

19.808(1)

b (Å)

c (Å)

7.5931(4)

6.8159(3)

P-1

6.9

C2/c

21.2

Pmcn or Imcm

16.515(2)

13.647(2)

16.070(1)

13.960(1)

6.818(1)

10.7 6.9

α (deg)

β (deg)

90

104.29(1)

γ (deg) 90

V (Å3)

form

993.5(1)

NP

914

INT

908

CP

13.7

111

89

104

6.8

90

115

90

6.860(1)

90

90

90

1546.1(3)

LP

90

90

90

1529.5(1)

LP

a

In the case of the INT and CP forms, the a and b cell parameters were evaluated from the position of the 110 and 200 Bragg peaks, starting from the values determined for MIL-53(Fe) (see ref 30). b For MIL-47(VIII) LP, the accurate space group could not be determined.

Table 2. Cell Parameters of MIL-47(VIII) and MIL-47(VIV) under CO2 Pressure at 303 K solid

P (bar)

S.G.

a (Å)

b (Å)

c (Å)

β (deg)

V (Å3)

form

MIL-47(VIII) MIL-47(VIII)

0 3.6

C2/c C2/c

19.868(2) 19.858(2)

7.6102(6) 7.6785(8)

6.8370(5) 6.8404(6)

104.33(1) 104.40(1)

1001.6(1) 1010.3(2)

NP (H2O) NP

MIL-47(VIII)

7.9

C2/c

19.854(2)

7.6762(7)

6.8413(6)

104.35(1)

1010.1(2)

NP

MIL-47(VIII)

30.9

Imcm

16.130(1)

14.269(1)

6.9087(4)

90

1590.1(2)

LP

MIL-47(VIV)

0

Pmcn

16.070(1)

13.960(1)

6.818(1)

90

1529.5(1)

LP

MIL-47(VIV)

29

Pmcn

16.120(1)

13.928(1)

6.827(1)

90

1532.9(1)

LP

experimental hutch via optical fibers. Laser power on the sample was 25 mW, and acquisition time varied from 1 to 8 min. Fractions of narrow pore and large pore were calculated from the characteristic νsym(COO) vibrations at ca. 14201430 and 1440 cm1, respectively, as previously reported.12 Spectra in this region were curve-fitted with three bands at 14201432, 1437 1442, and 14531456 cm1 using 85% Lorentzian + 15% Gaussian bands which showed the best fitting. fwhm's were kept between 8 and 12 cm1. Spectra of samples with only narrow pores or only large pores were used to estimate the relative Raman cross sections of these two bands. The band at 1444 cm1 obtained for a 100% LP was found to be on average 3.1 times larger than the band at 14201430 cm1 for a 100% NP. The area of the band at 1432 cm1 was thus multiplied by this factor for the calculation of the NP fraction. In the presence of water, this band was found at lower wavenumbers (ca. 1422 cm1) than in the presence of CO2 (ca. 1430 cm1). This has also been observed in the case of MIL-53(Cr). CO2 was quantified with the ν1 band at ∼1383 cm1. Its area was related to the area of the structure band at ∼1140 cm1. Infrared Spectroscopy. The sample was dispersed in absolute ethanol (12 mg of powder) and deposited on a silicium plate. Ethanol was then removed by evaporation at room temperature (rt). Samples were placed in a quartz cell equipped with KBr windows. A movable quartz sample holder allowed both activation in situ under vacuum and the acquisition of IR spectra at rt. Transmission IR spectra were recorded in the 5005500 cm1 range, with 4 cm1 resolution on a Nicolet Nexus spectrometer equipped with an extended KBr beam splitter device and, depending on the case, either a MCT or a DTGS detector. Activation was carried out in situ under a secondary vacuum (∼105 mbar). The sample was heated at 523 K (ramp 3.75 K/ min) for 1 h. H/D exchange experiments were done by repeated introductions of 13.3 mbar of D2O at room temperature and further evacuation at 373 K. CO2 adsorption was carried out at 220 K, after activation of the solid, by successive introductions of known amounts of CO2. The optical isotherms were monitored by the side band at 2272 cm1 due to 13CO2 as the 12CO2 signal was often too intense to be quantified.

Complex Impedance Spectroscopy. Isothermal conductivity measurements were performed in the 102104 Hz frequency range, with a Novocontrol dielectric alpha analyzer. The temperature of the sample was controlled by the Quatro Novocontrol system, via nitrogen flux. To avoid the use of high mechanical pressure, Complex Impedance Spectroscopy (CIS) experiments were not carried out on pellets prepared from compressed powder. We rather employed a specific cell similar to that generally used for liquids. About 100 mg of the activated powder sample was poured on the lower electrode of the cell and then very gently pressed by the upper metallic electrode, in a glovebox to prevent any contamination from the air. The cell was then quickly transferred into the cryostat of the impedancemeter. The solids were treated in situ at 473 K under dried nitrogen, for 2 h. They were then cooled to 100 K, and CIS measurements were carried out from T = 100 to 473 K, every 10 K.

’ RESULTS AND DISCUSSION 1. Characterization of MIL-47(V) in Different Oxidation States. Depending on the activation conditions, different mate-

rials can be obtained with the vanadium centers in various oxidation states. These materials and the oxidation process will be first discussed in this section after their characterization by vibrational spectroscopies and CIS. 1.1. Impurities and OH Groups of the Nonoxidized Solid. The IR spectrum of the DMF-exchanged MIL-47 (Figure 1) recorded at 293 K displays characteristic ν(CdO) bands of free terephthalic acid and DMF at 1692 and 1675 cm1, respectively, indicating that the exchange was not complete. While heating under vacuum at increasing temperatures for 30 min at each step, DMF remained up to 373423 K. Free terephthalic acid vanished around 473523 K under vacuum. A broad band around 3200 cm1 (not shown) and small bands at 3605 and 3580 cm1 were also observed on the solid initially. These bands are probably due to the presence of the free acid, and they also disappeared upon heating at 523 K. 19830



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Figure 1. FTIR spectra during heat treatment under vacuum of the DMF-exchanged MIL-47.

Figure 2. FTIR spectra of MIL-47 in various oxidation states: (a) activated MIL-47; (b) after 45 min oxidation at 473 K; (c) after 90 min oxidation at 473 K; (d) after 14 h oxidation at 483 K; (e) same as spectrum (a) after exchange with D2O and evacuation; (f) same as spectrum (d) after exchange with D2O and evacuation.

Several bands are present on the activated MIL-47 in the 6001600 cm1 region, which can be ascribed to the vibrations of the ligand,39,48,49 notably bands around 14001600 cm1 due to carboxylate vibrations which overwhelm the spectra. These IR bands, very broad and intense, will not be discussed further here, but the sample will be rather studied by Raman spectroscopy. The most characteristic feature on the IR spectra is a band at 3642 cm1 assigned to the ν(OH) band due to unperturbed (free) OH groups. This band is accompanied by a band at 901 cm1, which may be attributed to δ(OH). Both bands indeed grow simultaneously upon the removal of the impurities and the disappearance of the H-bonded species. In addition, a small band is also observed at 4511 cm1 (see Supporting Information, Figure S1), which increases with the removal of impurities. It is thus consistent with a ν(OH) + δ(OH) combination. An additional band observed at 512 cm1 (see Supporting Information, Figure S1) is likely due to the γ(VOH) deformation mode. The presence of these bands confirms that hydroxyl groups are present on this solid. Considering the structure of this solid deduced from the singlecrystal XRD study published by Barthelet et al.,35 these OH bands are assigned to hydroxyls bridged over two octahedral vanadium cations in the +III oxidation state, leading to the formulation VIII(OH)(L) (or MIL-47(VIII)). To ascertain the assignments, this sample was deuterated by addition of D2O (Figure 2e). After sample deuteration, the ν(OH) band at 3642 cm1 shifted to 2685 cm1, and the band at 901 cm1 was displaced to 712 cm1. These shifts are consistent with the positions which can be calculated for an isotopic substitution50 and confirm thus the formation of OD hydroxyls. The band at 4511 cm1 also disappeared upon sample

deuteration, while a weak band arose at 3397 cm1 (see Supporting Information, Figure S1), as expected for the ν(OH) + δ(OH) combination band with these new positions. This confirms thus the δ(OH) character of the 901 cm1 band in the activated MIL-47(VIII) form. 1.2. In Situ IR and Raman Analysis of the Oxidative Treatment. Oxidation of MIL-47 was originally obtained after direct calcination of the as-synthesized material in air,35 but it can also be expected from the activated impurity-free form. During the treatment of the activated solid under O2 (Figure 2), the intensity of the main IR bands due to the organic ligand was not appreciably modified, whereas the bands related to the free hydroxyl groups at 3642, 4511, and 512 cm1 (Figure 2 and Supporting Information, Figure S1) vanished progressively. On the partially oxidized form (Figure 2b and Figure 2c), new OH groups were also observed at 3633 and 3649 cm1 in addition to the initial OH group at 3642 cm1. These bands disappeared in the spectrum of the solid treated in O2 in the most severe conditions (Figure 2d). Conversely, the band at 901 cm1 was slightly modified during oxidation as a shoulder appeared on the mildly oxidized samples, but it remained nearly unchanged after the complete oxidative treatment. It is noteworthy that a similar position could be expected for δ(OH) and ν(VdO) bands. In the case of the oxidized solid, the absence of hydroxyl groups evidenced by the elimination of the ν(OH), the γ(OH), and the (ν(OH) + δ(OH)) bands would discard the assignment of the 901 cm1 band to the OH bending. This was confirmed by H/D exchange experiments. The solid initially deuterated was treated at 483 K in O2 and again treated with D2O (Figure 2f). In this case, an intense band was observed at 901 cm1, while the bands at 712 and 2685 cm1 19831



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Figure 3. Oxidation of the activated MIL-47 (impurity-free) followed by Raman spectroscopy. Spectra of (a) the initial solid after heating at 453 K under inert gas; and after oxidation in a 10% O2 gas flow: (b) mildly oxidized at 453 K and (c) subsequently oxidized at 473 K.

were absent and the band at 3642 cm1 had not reappeared. Therefore, although the position and intensity remained in the same range compared to the activated solid, the band at 901 cm1 cannot be attributed to the δ(OH) band on the O2treated solid but is assigned instead to the ν(VdO) mode. To fully characterize the modifications of the solid during the O2 treatment, Raman experiments (Figure 3) were performed under a gas flow (vs under vacuum for FTIR), which is more representative of common operating conditions. Compared to IR, Raman spectra show a limited number of bands due mainly to the terephthalate moieties. Notably, carboxylate bands at ca. 14001600 cm1 appear as sharper bands with a relatively lower intensity. Their shift is indicative of the structural modifications due to the flexibility of the structure12 and will be developed specifically in Section 2. In Raman also, a weak band was observed at 3645 cm1 on the activated, nonoxidized MIL-47. This band is attributed to the hydroxyl groups and confirms their presence on this solid. The δ(OH) mode could not be observed by Raman. As in the case of the FTIR measurements, the OH band decreased upon oxidation and disappeared when the solid was treated in O2 at 473 K. Meanwhile, new bands developed at lower wavenumbers in the presence of oxygen at 453 K. Besides the terephthalate vibration at 867 cm1, which is likely due to a (ring +COO) bending mode,51 a band absent on the nonoxidized solid appeared at 900 cm1 upon treatment under oxygen and increased with the time under flow at this temperature. This latter feature can thus be attributed to ν(VdO). From the study of many vanadium compounds, Hardcastle and Wachs52 found an empirical relation between the position of the VO bond and its length or covalent strength. From their relation, a bond length of 1.65 Å can be deduced from the band at 900 cm1. From single-crystal X-ray data, Barthelet et al.35 found that MIL-47 calcined in air showed alternations of short (1.672 Å) and long (2.102 Å) bonds. The Raman band at 900 cm1 is thus consistent with the short bond. According to the latter relation, the new bands developing at 362 and 378 cm1 could correlate with VO bonds of 2.10 and 2.13 Å, respectively, which is in agreement with the XRD data for the long bond. However, several other bands grew simultaneously at low Raman shifts. These are more difficult to assign due to the many modes of deformations and combinations certainly present, as calculated on MOF-5 material.51

Within a limited time, and after subsequent oxidation at 473 K, the ν(VdO) band at 900 cm1 did not increase anymore, probably indicating the complete conversion of the activated form to the fully oxidized form under O2. The presence of a shoulder at 894 cm1 on the solid mildly oxidized (Figure 3b) is consistent with that observed by FTIR and might indicate the existence of a different VO environment compared to the fully oxidized solid. Once oxidized, it was, however, impossible to return to the initial hydroxylated form even after treatments with reducing agents such as H2, NH3, or hydrazine, which implies that the oxidized solid is the most stable form of the MIL-47(V). Our results show that calcination (even in the presence of minor amounts of O2) induces a change in the oxidation state of the metal center. The disappearance of the μ2-OH bands present on the metal chains upon oxidation is correlated to the development of new VdO bonds. This is consistent with a change from the VIII to the VIV oxidation state, as initially proposed by Barthelet et al.,35 but it is in contrast with recent results of Centrone et al.,53 who prepared MIL-47 from microwave synthesis. From their study by XPS, magnetic susceptibility, and XAFS, the initial form after synthesis is in the VIII state, as expected. For the calcined solid, they found that the metal centers are in a VIII state in asymmetrically distorted octahedral sites, where a short VO bond is present. However, solely distorted sites would not account for the disappearance of the hydroxyl groups. Besides, other results confirm the VIV state of the oxidized MIL-47(V). UVvis diffuse reflectance spectroscopy shows that calcination of the as-synthesized MIL-47 in air induces the development of broad bands around 10 500 and 18 000 cm1, characteristic of VIV dd transitions.36 In another work on the MIL-47(V), Meilikhov et al.54 have shown by ESR, magnetic susceptibility, and DFT that the calcined solid showed metal centers in axially compressed distorted octahedral sites, but in the VIV state. Another study proposed by Djerdj et al.55 investigated closely related hybrid materials based on corner-sharing vanadium chains, in which the ligand was derived from benzoic acid instead of terephthalic acid. These materials were characterized by XPS, ESR, magnetic susceptibility, and DFT. In this case, the authors found that the vanadium centers were in the VIV state. Therefore, the vanadium centers are certainly in the VIV state in the oxidized MIL-47(V), while a VIII state was maintained in the hydroxylated form. 19832



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Figure 4. Real part of the conductivity as a function of the frequency of the electrical field, recorded at 473 K for the (a) MIL-47(VIV), (b) MIL47(VIII), and (c) MIL-47(VIII/VIV).

1.3. Complex Impedance Spectroscopy. The change of the oxidation state between the hydroxylated and the fully dehydroxylated solids raises questions about the mildly oxidized samples. Complementary experiments were thus performed by CIS to give more insight into the different oxidation states of the MIL-47(V) samples and notably into the solid mildly oxidized, presumably in a mixed oxidation state MIL-47(VIII/VIV). Applied to MOFs, this method has proved recently to be efficient either to address the molecular mobility of the organic ligands in the IRMOF-2 or MOF-5 solids at low temperature56,57 or to capture the structural flexibility of the MIL-53(Cr, Fe) materials upon desorption of guest molecules.58 For conducting solids, CIS gives access to the real part of the global conductivity, σac(f,T), which results in many cases from the combination of MaxwellWagnerSillars polarization, σMWS0 (f,T), dc conductivity, σdc(T), and polarization conductivity, σpol(f,T), contributions 0

σac ðf , TÞ ¼ σ MWS ðf , TÞ þ σ dc ðTÞ þ σpol ðf , TÞ

ð1Þ

Basically, dc conductivity corresponds to long-range redistribution of charges, i.e., ionic or electron transport, while the polarization contribution arises from local rearrangement of charges or dipoles causing dipolar reorientation and thus resulting in the intrinsic bulk polarization. MaxwellWagnerSillars polarization appearing at low frequency is mainly due to the accumulation of ionic charge carriers at the sample/electrode interface and is consequently observable if ionic species are responsible for the dc conductivity contribution. Noteworthy, in insulators, σdc(T) ≈ 0, so that only the polarization conductivity is detectable. Figure 4 illustrates the global conductivity profiles measured at 473 K as a function of the frequency of the electrical field, in the case of the three investigated anhydrous solids MIL-47(VIII), MIL-47(VIV), and MIL-47(VIII/VIV). The conductivity properties of the investigated materials sharply differ from one sample to the other. The MIL-47(VIV) response is characterized by the absence of the dc conductivity plateau as shown by the continuous increase of the σac(f) signal with the frequency in the whole explored domain. Noteworthy, this behavior is characteristic of an extremely poor conductor, i.e., an insulator, in agreement with the low values of the polarization conductivity. The absence of dc conductivity signal also suggests that this solid does not contain any conducting charges, whatever their nature, i.e., electronic or

Figure 5. Thermodiffractogram of the hydrated form of MIL-47(VIII), VIII(OH)L 3 H2O (λ = 0.709622 Å). Blue, hydrated NP form; red, hydrated triclinic intermediate INT form; green, closed pore CP form; black, LP form. The indexation of the two main peaks corresponds to the monoclinic (NP and CP) and orthorhombic (LP) unit cells reported in Table 1(a ∼ long axis of the pore, b ∼ short axis, c ∼chain axis). On the right, the four structures are plotted schematically along the chain axis, based on the structures reported for MIL-53(Fe).30

ionic. This confirms that MIL-47(VIV) presents one single oxidation state and is exempt of any conducting ionic species, in agreement with the absence of μ2-OH groups and their replacement by μ2-oxo moieties linked to the VIV metal. At 473 K, the MIL-47(VIII) solid exhibits a conductivity response, whose values are nearly in the same order of magnitude as those of the MIL-47(VIV). This suggests that MIL-47(VIII) is also a poorly conducting solid. However, in that case and contrarily to what was observed for MIL-47(VIV), a dc plateau is likely to appear in the low-frequency domain. It would probably be more pronounced at lower frequency as well as at higher temperature. As reported for the MIL-53(Cr) solid,58,59 the dc plateau is probably due to a protonic conduction, arising from the μ2-OH groups linked to the VIII metal. In that case, we note that the conductivity value of this plateau is rather low, in relation with a poorly favored proton hopping process from one site to another. This suggests that (i) the density of the charge carriers is low and (ii) the μ2-OH groups do not highly interact with each other via hydrogen bonds, in relation with the large pore form of the MIL-47(VIII) structure after activation at 473 K, as highlighted by vibrational spectroscopy and XRD measurements (see Section 2). Finally, the MIL-47(VIII/VIV) exhibits a conductivity signal drastically different from both other solids. In that case, the dc conductivity plateau is predominant in almost the whole frequency domain, with values drastically higher than those recorded for the MIL-47(VIII) or MIL-47(VIV) solids. This behavior indicates that the conductivity properties are sharply enhanced in MIL-47(VIII/VIV). In addition, no decrease of the conductivity signal at low frequency is observed, suggesting that the Maxwell WagnerSillars polarization σMWS(f,T) does not take place. The absence of such σMWS(f,T) contribution indicates that the charge carriers responsible for the dc plateau are mainly electronic. This is thus consistent with the hypothesis of a mixed oxidation state for the MIL-47(VIII/VIV) compound with charge carriers in close contact through VOV bonds, i.e., in mixed valence VIII/VIV chains rather than in a mixture of segregated VIII and VIV chains. Several mixed valence states MOFs have already been reported, prepared either by direct synthesis60 or by postsynthetic redox 19833



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Figure 6. Flexibility of MIL-47(VIII) upon hydration/dehydration evidenced by (A) FTIR (under vacuum) and (B) Raman (under gas flow): (a) after activation; (b) after exposure to water vapor; and (c) after dehydration at 453 K.

reactions, notably in the course of the activation process.61 For MIL-53(Fe), a mixed valence state FeII/FeIII has been obtained by insertion within the pores of extra cations, either by direct synthesis (dimethyl ammonium)62 or by postsynthetic treatment (Li+).63 Regarding MIL-47(V), a mixed oxidation state was found after incorporation of cobaltocene in the pores and subsequent partial reduction of the VIV metal centers.54 The existence of such mixed valence states in porous MOFs is known to strongly modify their electronic conductivity,6466 as also observed in the case of the MIL-47(VIII/VIV) compound. The above results have thus confirmed the existence of the MIL-47(V) with the metal centers in the VIII state, the VIV state, and a mixed oxidation state VIII/VIV. The flexibility of these different solids and their abilities for CO2 adsorption will be developed in the next sections. 2. Factors Controlling the Flexibility. Unlike what was previously reported for MIL-47 in the VIV state,35 the structure of the MIL-47(VIII) developed here is flexible and “breathes” like MIL-53. We will show below how the flexibility varies on the solids with different oxidation states of the vanadium. 2.1. Flexibility of MIL-47(VIII). After activation, the MIL-47(VIII) adopts an NP hydrated form in ambient conditions (a minor amount of LP form was also found), similar to the one observed for MIL-53(Cr, Fe, Al, Ga) 3 H2O (see the blue diagrams in Figure 5 and Table 1 for the unit-cell parameters). This is a first striking difference from the MIL-47(VIV) which is known to remain in the LP form in the same conditions9,35,67 and already points out the major role of the oxidation state of the cation on the structural features of the MIL-47/53 materials. The thermal behavior of MIL-47(VIII) 3 H2O was followed by thermodiffraction in air. Three structural transformations occur upon heating. When possible, the unit-cell parameters were determined, but the occurrence of a mixture of phases and traces of impurities (especially for the intermediates states) complicated the process. It was thus sometimes impossible to get accurate crystallographic parameters, but the comparison with the wellknown MIL-53(Cr, Fe, Ga) solids allowed elucidating the flexible character of MIL-47(VIII) (see Table 1). The two main Bragg peaks (110 and 200 for the monoclinic and orthorhombic unit cells, see Figure 5), especially, give a clear picture of the pore opening and allow us to evaluate the a and b cell parameters. Upon water departure, a first change is observed at 380 K, characterized by a shift of the 110 peak to high angle (associated with the contraction of the pore) and the appearance of a new

peak at low angle (2θ = 3.1°, red diagrams in Figure 5), which is characteristic of the intermediate triclinic form (INT) already observed with MIL-53(Fe).30,44 In this form, half of the pores are completely closed and half slightly open. At higher temperature (about 420 K), the first peak vanishes, leading to the monoclinic closed pore (CP) form (green diagrams in Figure 5), similar to the one observed for activated MIL-53(Fe) and MIL-53(Al, Ga) at low temperatures.28 At high temperature (T > 480 K), the Bragg peaks associated with the LP form start to grow, to lead to an almost pure phase at 500 K (black diagrams in Figure 5). This pore opening may be associated either with a thermal transformation (similar to MIL-53(Al, Ga)) or to the oxidation of VIII to VIV. When comparing MIL-47(VIII) in the LP form and MIL-47(VIV), the shortest parameter of the cell appears to decrease. This was also the case in the work of Barthelet et al.35 between the as-synthesized solid (H2L@MIL-47: 6.875 Å) and the activated form (MIL-47(VIV): 6.818 Å). This is consistent with a contraction along the chain axis due to the presence of a shorter bond on MIL-47(VIV).35 However, the exact cell parameters and the space group of the MIL-47(VIII) in the empty LP form could not be accurately determined with the present data. This will be more clearly demonstrated after adsorption of CO2 (see Section 3.1). The low stability of the mixed-valence solid MIL-47(VIII/ VIV) also precluded its deep structural study. The flexibility was also monitored by infrared spectroscopy and Raman spectroscopy (Figure 6). For FTIR, the shift of the band at 10161022 cm1 was used (δ(CH), ν18a mode of the terephthalate species). On the hydrated MIL-47(VIII), the ν18a band is found at 1016 cm1 which is characteristic of the NP form on MIL-53(Cr).10 In these conditions (Figure 6A), the spectrum displays a large band at ca. 3320 cm1 and a decrease of the free OH band indicating a perturbation of the hydroxyl groups with H-bonding species. Upon calcination under secondary vacuum, the ν18a band shifts to 1022 cm1, indicating the progressive opening of the structure. Meanwhile, the unperturbed OH groups are recovered, showing the departure of adsorbed water. This phenomenon is reversible for several hydration/calcination cycles. In conjunction with the aforementioned XRD studies, this proves that the MIL-47(VIII) behaves similarly to MIL-53(Ga, Al), i.e., with a thermal opening of the pore (in the activated form) and a CP to LP transition around 453 K and a large hysteresis (transition from LP to CP not seen above 293 K). By Raman spectroscopy, information on the flexibility is obtained with the shift of the carboxylate bands (Figure 6B). In this case, 19834



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Figure 7. Effect of the adsorption of water on the flexibility of (a) MIL-47(VIII), (b) MIL-47(VIII/VIV), and (c) MIL-47(VIV). (A) Raman spectra and quantifications after adsorption of H2O. (B) FTIR spectra and quantifications after adsorption of D2O. The quantifications show the VIV fraction (red blocks) and the NP fraction (green blocks).

the symmetric νs(COO) band at 1440 cm1 in the LP form shifts down to 1422 cm1 in the NP form, as shown for MIL-53(Cr).12 The extent of the LP f NP transformation can then be calculated from the relative abundance of these bands (the NP/CP/INT forms were not clearly differentiated from each other). On MIL-47(VIII), this confirmed reversible closing and opening of the structure. Reversible flexibility was thus observed on this material either under vacuum (FTIR) or under gas flow (Raman). 2.2. VIII/VIV Mixed Form. Water vapor was also introduced after the VIII solid was partly oxidized. The same experiment was done by IR spectroscopy in vacuum and by Raman spectroscopy in gas flow. The extent of the possible flexibility was determined by the fraction of NP present after water adsorption and compared with the fraction of VIV phase present. After this treatment, the solids did not degrade as shown by the absence of modifications in the structure bands (see Supporting Information, Figure S2). In FTIR, the fraction of VIV was calculated from the intensity of the hydroxyl, which decreased upon oxidation or from the increase of the 901 cm1 band. In this case, D2O was adsorbed instead of H2O to eliminate the contribution of the δ(OH) in the 901 cm1 band. For the Raman spectra, the development of the band at 900 cm1 was used to quantify the fraction of VIV. Although performed in different conditions, both Raman and FTIR experiments yielded similar results (Figure 7). The increase in VIV fraction leads to the decrease of the fraction of NP in the presence of water. On the MIL-47(VIII), the fraction of NP was maximal. It quickly decreased on oxidized samples. On the mixed oxidation state form MIL-47(VIII/VIV), the amount of NP which could form was limited, while it was not observed at all on the MIL-47(VIV). Therefore, the oxidation of the solid inhibits the flexibility. However, the oxidation state and the amount of NP form were not directly correlated. Indeed, the remaining NP form was quickly very low, even for a mild oxidation (see for example the case of the 50% VIII/50% VIV in Figure 7). This is a further indication that this mixed-valence sample is not a mixture of pure MIL-47(VIII) and MIL-47(VIV) but rather a new phase. 3. Influence of the Oxidation State of Vanadium on the Adsorption of CO2. 3.1. High-Pressure Adsorption. Adsorption

Figure 8. Influence of the pressure of CO2 on the flexibility of MIL47(VIII). (A) XRPD and (B) Raman spectra at increasing pressures. (Pressures underlined in A refer to Table 2.)

of CO2 at room temperature (i.e., at high pressure) was followed in situ by coupled XRDRaman techniques on the VIII form and compared with the VIV form. This latter form was previously shown by in situ XRD to remain rigid upon adsorption of CO2, leading to a type I adsorption isotherm.9,68 A sample of hydrated MIL-47(VIII) 3 H2O was exposed to increasing amounts of CO2 at 303 K. Up to 9 bar, only slight shifts of the Bragg peaks were observed, in agreement with the 19835



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Figure 9. Adsorption of CO2 for (A) MIL-47(VIII) and (B) MIL-47(VIV). Quantification of the CO2 adsorbed (9) and fraction of NP (]) determined from the Raman spectra.

occurrence of a NP form evolving upon replacement of H2O by CO2 (see Table 2 for the unit cells and blue patterns in Figure 8A). The cell volume of this form is indeed known to be dependent on the pore content, with only a very slight difference between the H2O and CO2 NP forms, as already exemplified with MIL-53(Cr).10 Above 9 bar, the structure evolves to a LP form (black diagrams in Figure 8A). MIL-47(VIII) and MIL-47(VIV) thus present a drastically different behavior upon adsorption of CO2, behaving either as flexible or rigid solids. Table 2 gives the cell parameters of MIL-47(VIII) and MIL-47(VIV) under CO2 pressure at 303 K. The change of oxidation state seems to be associated with a change of symmetry in the LP form (Pmcn vs Imcm), although the Bragg peaks associated with this change are very weak, and this point should be taken with caution. Nevertheless, this is consistent with the loss of symmetry along the chain axis from one set of MIIIμ2-OH distance vs two sets of VIVμ2O distances. MIL-53(Fe or Cr), with their regular MIIIμ2-OH distances, were indeed indexed in Imcm in the LP form,18,26,44 while MIL-47(VIV) belongs to Pmcn (equivalent to Pnma with a different setting) space group,35,67,68 both with empty and gasfilled pores. Furthermore, as already observed in Section 2.1, the comparison of MIL-47(VIII) and MIL-47(VIV) shows that the c parameter of the LP form decreases upon oxidation, in agreement with a shortening of the averaged Vμ2-O(H) distances. When CO2 is adsorbed on MIL-47(VIV), the adsorption curve, obtained by the development of the Raman band at 1383 cm1, shows a type I isotherm (Figure 9B), in agreement with previous studies.9,68 Meanwhile, as expected, the spectra only show the presence of the LP form. A large difference is found during the adsorption on MIL47(VIII) (Figure 9A). The development of the LP form was determined from the Raman spectra, along with the relative amount of CO2 on the solid. Although some LP form could be observed on the Raman spectra initially, the hydrated solid, just evacuated at rt, was predominantly in the NP form as shown above. Upon adsorption of CO2, the NP form was found to reach a maximum intensity at around 3 bar. Above this pressure, the LP form started to develop. The presence of CO2 in the structure is shown by the development of a band at 1383 cm1 (Figure 8B). Due to its very low intensity, this band was difficult to quantify at low pressure. It seems, however, to increase first, probably due to

the complete closing of the structure of the remaining LP form, as previously observed in the case of MIL-53.9,10,12 A second adsorption step is then clearly observed and coincides with the development of the LP form and the filling of these pores. A difference is observed between the results obtained by XRD (∼7 bar) and Raman (∼3 bar) concerning the pressure at which the maximum of NP form is observed. Several reasons might account for this discrepancy. First, the intensities of the XRD peaks are dependent on the pore content. They cannot be thus directly related to the exact amount of NP/LP forms, and problems will notably arise when mixtures of NP/LP are found. Besides, XRD will highlight a long-range order, while Raman could evidence smaller ordering. Nevertheless, it is important to note that even if the CO2 pressure inducing the structure change was not strictly consistent between both techniques the opening of the structure under high CO2 pressures is assessed in both cases, confirming the flexibility of this material. Finally, at high pressure, the analysis of the solid, both by Raman and XRD, showed that it was fully converted to its LP form. 3.2. Influence of the Hydroxyl Groups: Low-T, -P Adsorption. To gain more insight into the adsorption of CO2 and notably into the role of the hydroxyl groups present on MIL-47(VIII) in this process, the interaction of the adsorbed species with the hydroxyls was followed by FTIR spectroscopy of the solids after activation and introduction of known amounts of CO2 at 220 K (low pressures). The spectra are presented in Figure 10 for all the solids. The presence of CO2 adsorbed was evidenced by the development of the ν3 stretching mode at ca. 2335 cm1 and by the bands around 660650 cm1 (ν2 CO2 bending mode). The amount of CO2 adsorbed plotted in Figure 11A was followed by the integrated intensity of the associated ν3 13CO2 band at 2272 cm1, whose signal does not saturate like the broad band at ca. 2335 cm1 (see Supporting Information, Figure S3). Figure 11B shows the variations of the structure opening, followed by the relative intensities of the 1017 and 1022 cm1 bands. Adsorption of CO2 on MIL-47(VIV) only yielded one band at 660 cm1 similar to that in MIL-53(Cr) in the LP form,10 indicating weak interactions. The band at 1022 cm1 remained at this position in the whole pressure range, confirming that the solid remained in the LP form. A small perturbation of the ν(VdO) 19836



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Figure 10. FTIR spectra after adsorption of CO2 at 220 K on (a) MIL-47(VIII), (b) MIL-47(VIII/VIV), and (c) MIL-47(VIV). Spectrum in black: After activation; red, 13 mbar; blue, 67 mbar; green, 160 mbar (107 mbar for (c)). * denotes the (ν1 + ν3) combination band of CO2.

Figure 11. Adsorption of CO2 at 220 K on (a) MIL-47(VIII), (b) MIL-47(VIII/VIV), and (c) MIL-47(VIV). Quantifications by FTIR of (A) the amount of CO2 (band at 2272 cm1) and (B) the fraction of LP. The inset in A corresponds to the enlargement of the amount of CO2 adsorbed in the 013 mbar pressure range.

band was observed from 905 to 891 cm1, indicating a weak interaction between CO2 and VO. A similar perturbation has been observed on this solid after interaction with H2S.43 Ramsahye et al.40 have shown by DFT that in the case of MIL-47(VIV) the adsorption enthalpy was slightly higher when CO2 adsorbed on μ2-O groups instead of on other possible sites like the carboxylate groups. However, the differences observed between the different possible sites were not sufficient to account for specific interaction with CO2. The absence of any specific interaction will influence the type of isotherm observed. On MIL-47(VIII), two bands are observed at low CO2 pressures at 660 and 649 cm1. This latter band tends to disappear at higher pressure (above 133 mbar), while the band at 660 cm1 becomes predominant. The development of these bands is

accompanied by a perturbation of the hydroxyls: the ν(OH) is indeed shifted from 3645 to 3617 cm1, while the δ(OH) is shifted at the same time from 902 to 933 cm1. Meanwhile, the ν3 band of CO2 is observed at 2335 cm1, with an additional shoulder at ca. 2325 cm1, clearly lower than the gas phase (2349 cm1). The features of the ν2 and ν3 bands of CO2, as well as the perturbation of the hydroxyls, are consistent with the specific interactions observed on MIL-53(Cr) in the NP form.39 They have been attributed to the formation of electron donoracceptor complexes in which CO2 interacts with the hydroxyl groups via the C-atom. These changes are accompanied by changes of the structure bands. The band at ca. 1020 cm1, initially at 1022 cm1 in the solid after activation and characteristic of a LP form, is shifted to 19837


The Journal of Physical Chemistry C 1017 cm1 as soon as low amounts of CO2 are introduced, indicating a structural modification to the NP form. This band returns to its initial position (i.e., LP form) above 133 mbar. On the mildly oxidized MIL-47(VIII/VIV), the main ν2 CO2 band is observed around 660 cm1. The weak band at 649 cm1 may only be observed at low pressures as a weak shoulder. This band is consistent with the shift observed on the remaining OH, which are perturbed in the same way as on MIL-47(VIII). In addition, although the band at 1022 cm1 remains predominant, a shoulder can be seen at 1017 cm1. These three features thus imply that the specific interactions in the NP form between CO2 and the μ2-OH are present also on this solid. The interactions described above explain the various adsorption profiles observed for the three solids (Figure 11A). Due to the lack of specific interaction, the VIV form only shows a type I adsorption isotherm, CO2 quickly reaching saturation. Conversely, both VIII and mixed VIII/VIV forms show an intermediate step around 50110 mbar. However, despite this step, the amount of CO2 at saturation was the same on all the solids. This is consistent with the fact that at the highest pressures all the solids are in the LP form. This had been previously described in the comparison of the LP forms of MIL-53(Cr or Al) and MIL47(VIV) for the adsorption of CO29 or for the adsorption of xylenes.21 Differences are larger at lower pressures. The two-step adsorption observed on MIL-47(VIII) and MIL-47(VIII/VIV) corresponds to the closing of the structure, as previously observed on MIL-53(Cr),9 and it is more pronounced in the case of MIL-47(VIII) for which the closing is more complete. The inset in Figure 11A shows the behavior of these two solids at very low pressure. Initially, the adsorption of CO2 is important on MIL-47(VIII) until a first plateau is reached around 50 mbar. This corresponds to the immediate closing of the structure. Figure 11B indeed shows that the fraction of LP decreases sharply, while the amount of adsorbed CO2 first increases. Meanwhile, the amount of free OH groups largely decreases, while the band at 3617 cm1 grows in parallel. Closing of the structure is thus accompanied by the interaction of CO2 with the hydroxyls. The initial adsorption on the MIL-47(VIII/VIV) differs. In this case, no adsorption was observed until 2 mbar. This might be due to a lower affinity of the solid with CO2 due to the decreased number of hydroxyls present. Above 2 mbar, CO2 was steadily adsorbed. The structure finally started to close, although only partially, above 5 mbar. Therefore, the presence and the abundance of μ2-OH groups have a large influence on the behavior of MIL-47. The specificity of the MIL-47(VIII) compared to the oxidized states influences its adsorption properties and confirms the importance of the presence of these OH groups for the adsorption and the flexibility.

’ CONCLUSIONS MIL-47(V) is a versatile MOF. By carefully controlling the activation, solids free of impurities can be obtained. We have shown here that this material could be stabilized in different oxidation states: the expected VIV form but also a new form with a lower oxidation state (VIII) and intermediate forms with mixed valence states VIII/VIV. The new MIL-47(VIII) solid presents hydroxyl groups as the analogous structure MIL-53. This brings a new property to MIL-47, the flexibility in the presence of adsorbates (due to

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these similarities, this form VIII might as well be named MIL53(V)). The flexibility of this solid was studied in the presence of water and CO2. Both adsorbates are able to induce flexibility, and as in the case of MIL-53, a two-step adsorption of CO2 is observed induced by the closing and reopening of the structure. As in the case of MIL-53, hydroxyl groups are responsible for flexibility as they are the adsorption sites. However, flexibility in MIL-47 is particularly inhibited by the presence of VIV in the structure. The presence of such tetravalent centers, even in low amounts, shows a dramatic effect on the flexibility. Solids with an intermediate oxidation VIII/VIV state can also be produced by the careful oxidation of the MIL-47(VIII) solid. They differ from both MIL-47(VIII) and MIL-47(VIV), showing for instance the presence of new hydroxyl groups and VO environments. Conductivity measurements also show that they behave in a radically different manner than the pure oxidation state solids, with a large enhancement of the electronic conductivity, suggesting that regions of different valences might be in contact with each other. This suggests that a mixed-valence state was formed. The careful control of the activation conditions of MIL-47(V) and of the oxidation state of the metal centers brings thus additional properties to this class of materials, with the possibility of fine-tuning the flexibility and the adsorption features.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional FTIR data concerning Figures 2, 7, and 10. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors acknowledge the financial support of the French ANR (“NOMAC” ANR-06-CO2-008) and the “Basse-Normandie” region for funding and ESRF for providing beamtime. We thank Dr Y. Filinchuk and Dr. W. van Beek for their help on the beamline (BM01A); Dr. P.L. Llewellyn for the development of the gasintroducing system on this line; Dr. S. Bourrelly, Dr. P. Horcajada, Dr. P. Yot, and Dr. D. Phanon for their assistance during some of the experiments at ESRF; F. Ragon for the synthesis of a batch of solid; and Dr. C. Serre for fruitful discussions. ’ REFERENCES (1) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334. (2) Ferey, G. Chem. Soc. Rev. 2008, 37, 191. (3) See the special issue devoted to MOFs: Chem. Soc. Rev. 2009, 38(5). (4) Li, J.-R.; Kuppler, R. J.; Zhou, H.-C. Chem. Soc. Rev. 2009, 38, 1477. (5) Rowsell, J. L. C.; Millward, A. R.; Park, K. S.; Yaghi, O. M. J. Am. Chem. Soc. 2004, 126, 5666. (6) Murray, L. J.; Dinc, M.; Long, J. R. Chem. Soc. Rev. 2009, 38, 1294. (7) Millward, A. R.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 17998. (8) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469. 19838


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