Characterization of adsorbed water in MIL-53(Al) by FTIR spectroscopy and ab-initio calculations J. M. Salazar, G. Weber, J. M. Simon, I. Bezverkhyy, and J. P. Bellat Citation: The Journal of Chemical Physics 142, 124702 (2015); doi: 10.1063/1.4914903 View online: http://dx.doi.org/10.1063/1.4914903 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/142/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Structural, vibrational, and quasiparticle band structure of 1,1-diamino-2,2-dinitroethelene from ab initio calculations J. Chem. Phys. 140, 014105 (2014); 10.1063/1.4855056 A prototypical ionic liquid explored by ab initio molecular dynamics and Raman spectroscopy J. Chem. Phys. 139, 144309 (2013); 10.1063/1.4823824 Ab initio and classical molecular dynamics studies of the structural and dynamical behavior of water near a hydrophobic graphene sheet J. Chem. Phys. 138, 204702 (2013); 10.1063/1.4804300 The effects of electronic polarization on water adsorption in metal-organic frameworks: H2O in MIL-53(Cr) J. Chem. Phys. 137, 054704 (2012); 10.1063/1.4739254 Adsorption and vibrational spectroscopy of ammonia at mordenite: Ab initio study J. Chem. Phys. 120, 10263 (2004); 10.1063/1.1737302

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THE JOURNAL OF CHEMICAL PHYSICS 142, 124702 (2015)

Characterization of adsorbed water in MIL-53(Al) by FTIR spectroscopy and ab-initio calculations J. M. Salazar,a) G. Weber, J. M. Simon, I. Bezverkhyy, and J. P. Bellat Laboratoire Interdisciplinaire Carnot de Bourgogne, Adsorption Sur Solides Poreux (ASP), UMR-6303 CNRS-Université de Bourgogne, 9, Av. Alain Savary B.P. 47870 F-21078 Dijon Cedex, France

(Received 23 December 2014; accepted 3 March 2015; published online 23 March 2015) Here, we report ab-initio calculations developed with a twofold purpose: understand how adsorbed water molecules alter the infrared spectrum of the metal-organic framework MIL-53(Al) and to investigate which are the associated physico-chemical processes. The analyzed structures are the two anhydrous narrow (np⊘) and large (lp⊘) pore forms and the hydrated narrow pore form (np-H2O) of the MIL-53(Al). For these structures, we determined their corresponding infrared spectra (FTIR) and we identified the vibrational modes associated to the dominant spectral lines. We show that wagging and scissoring modes of CO2 give flexibility to the structure for facilitating the lp⊘- np⊘ transition. In our studies, this transition is identified by eight vibrational modes including the δCH(18a) vibrational mode currently used to identify the mentioned transition. We report an exhaustive band identification of the infrared spectra associated to the analyzed structures. Moreover, the FTIR for the np-H2O structure allowed us to identify four types of water molecules linked to the host structure by one to three hydrogen bonds. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4914903]

I. INTRODUCTION

Metal organic frameworks (MOFs) are the subject of a considerable number of studies since the last decade.1–21 Some appealing features of these materials are their high specific surface area and a uniform pore size distribution ranging from 0.9 nm for MIL-5322 up to about 10 nm for IRMOF-74XI.23 Undoubtedly, these properties place MOFs as promising materials for industrial applications (i.e., gas storage,4,6,23–25 separation and purification,2,3,10,14,23,26–28 catalysis,21,29 gas detection,2,14,30 and drug delivery5,31). Another interesting property of some MOFs concerns the framework flexibility. This is revealed when an external stimulus is applied (e.g., mechanical pressure,32–34 temperature change,23,35,36 gas or liquid exposure37,38) and reflected by a reversible opening and closing of the framework pores. This phenomenon, called breathing effect, was first reported by Zhou et al.,21 for the highly stable MIL-53(Cr) structure. In a relative recent work by Liu et al.,22 was reported the passage from the large pore (lp⊘) to the narrow pore (np⊘) without guest molecules and showed the presence of a bistable behavior of MIL- 53(Al). Moreover, Walker et al.35 suggested that the driving force for this transition is provided by dispersion interactions, whose action may result in a np⊘ phase more stable than the lp⊘ phase at low temperatures. During the last decade, the MIL53 (M = Fe, Al, Cr) materials have been studied in the presence of water39–43 by molecular dynamics simulations, X-ray diffraction, electron spin resonance, solid state NMR spectroscopy, microcalorimetry, and neutron scattering.38,44–49 Among these studies, those using FTIR spectroscopy are less numerous.49–52 Assuredly, spectral band identification is a laborious and tedious task, nonetheless, useful to identify the a)Electronic mail: [email protected]

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role of each component of the structure. Knowing which are the major vibrational modes of a given porous structure is a worth information for guiding the formulation of a theoretical model concerning the flexibility or elasticity of MOFs. If we consider the use of MOFs for industrial applications, an important question concerns their stability in the presence of water. In this context, knowing how water molecules interact with porous materials becomes a relevant issue. The results given by publications devoted to this point show that water adsorption mechanisms in MIL-53 have not been yet completely elucidated.49,51,53–57 From a general point of view, MIL-53 materials may be considered as hydrophobic materials because the enthalpy of water adsorption at zero loading is lower than the liquefaction enthalpy.49 Water molecules are only physisorbed at the surface by hydrogen bonding interactions. Furthermore, the adsorption affinity depends on the nature of both the MOF metallic center and its organic linker.58,59 In addition, water adsorption induces a shrinkage of MIL-53(Al) framework57 and even when water is physisorbed the adsorption and desorption process is not always reversible. A large hysteresis loop is observed on the adsorption-desorption isotherm for MIL53(Al),57 whereas water adsorption-desorption is reversible for gallium or chromium based MIL-53 materials. NMR, X-ray diffraction, and molecular simulations give some information about the location of water molecules inside the pores. Basically, three types of water have been identified during the hydration of MIL-53: (i) the first implies a hydrogen bond between the oxygen atom of the guest molecule and the hydrogen atom of the µ2-OH groups of the [AlO4(OH)2] octahedra interconnected by terephthalate linkers, (ii) the second implies two hydrogen bonds with hydrogen atoms of the guest molecules and an oxygen atom of the terephthalate ligand, (iii) the third type corresponds to water molecules with

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strong guest-guest interactions at the center of the channel.60 In the large pore form adsorbate-adsorbate interactions prevail on adsorbate-adsorbent interactions whereas the reverse situation holds in the narrow pore form. These three types of water-guest structures have similar adsorption energies and they cannot be differentiated by calorimetry. FTIR spectroscopy is a very sensitive and powerful technique, which can be used to distinguish different types of adsorbed water molecules. However, the assignment of the absorption bands is a subtle task which prevented the identification of relevant vibrational modes in previous studies.36,39,49,51 Here, are reviewed the adsorption process of water by MIL-53(Al). For this, we combined in situ FTIR spectroscopic measurements and ab-initio calculations performed with the VASP package.61 The structures of MIL53(Al) studied are the hydrated narrow pore form (np-H2O) and the two dehydrated narrow (np⊘) and large (lp⊘) forms. The spectroscopic measurements were performed in the range 400–4000 cm−1 and the dominant absorption bands were assigned by comparing experimental and numerical FTIR spectra. In particular, our analysis allowed us to identify the most relevant vibrational modes of the lp⊘ and np⊘ phases and permitted us to establish a detailed IR signature of the MIL-53(Al) structural (np-H2O)-lp⊘ phase transition.

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the np⊘, lp⊘, and np-H2O of MIL-53(Al) under periodic boundary conditions. We used the np⊘ and lp⊘ forms configurations experimentally obtained and given in Refs. 48 and 50 (Fig. 1). Here, the Perdew-Burke-Ernzerhof (PBE)63 functional was adopted. This functional is less accurate than the hybrid PBE0 functional nonetheless, gives a satisfactory description of hydrogen bonded systems.64 When studying the structuring of confined water molecules, the hybrid PBE0 functional seems to be more suitable. This is because it gives better positions of the stretching and bending bands.65 The computational cost of PBE0 calculations is heavier than those

II. EXPERIMENTAL A. Sample preparation

The hydrated narrow pore (np-H2O) of MIL-53(Al) (i.e., Al(OH)BDC.H2O, BDC = 1,4-benzene dicarboxylate) was synthesized according to the procedure described elsewhere.9 The dehydrated np⊘ and lp⊘ pore forms were obtained, after dehydration of the synthesized narrow pore (np-H2O), under a vacuum of approximately 10−4 hPa at room temperature and at 433 K, respectively. B. In situ FTIR measurements

In situ FTIR absorption measurements were performed in an optical cell specially built to study the interaction of a gas with a nanoporous material.36,62 The FTIR spectrometer was a Bruker Equinox 55 instrument equipped with a globar source, a DTGS detector, and a KBr beamsplitter. The spectra were recorded in transmission mode at room temperature, in the range of 400-4000 cm−1 with a resolution of 2 cm−1 and 40 accumulated number of scans. Measurements were performed on powdered samples deposited between two potassium bromide wafers. The spectrum of the starting synthesized material in the form np-H2O is first collected under ambient atmosphere. Then, the initial material is dehydrated during 12 h under vacuum at room temperature or then temperature is raised to 433 K to prepare the forms np⊘ or lp⊘ of MIL-53(Al). III. SIMULATIONS A. Ab-initio calculations

As mentioned above, the results presented in this work were obtained by using VASP. We performed simulations for

FIG. 1. In this figure are given the Mil-53(Al) a) lp⊘, b) np⊘, and c) np-H20 optimized structures by ab-initio calculation at 0 K. For the optimization, we used the experimental structures (see Sec. III A for more details).

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using PBE, indeed, and a compromise needs to be found between accuracy and technical feasibility. For describing the van der Waals interactions, we used the semi-empirical method proposed by S. Grimme where the interactions are described via a simple pair-wise force field.66 The optimization of the structure was performed by allowing shape and volume relaxation. The ionic relaxation to their instantaneous ground state was obtained by using the conjugate-gradient algorithm. For ensuring a convergence of both total energy and forces, we used a plane wave cutoff of 500 eV. The Brillouin-zone was sampled by using an equally spaced mesh provided by the Monkhorst and Pack method67 with 2 × 5 × 3 K-points. B. IR spectra identification by using density-functional perturbation theory (DFPT)

Today band position assignment is a routine task in density functional theory (DFT) calculations. However, determine the intensities requires a careful choice of the method to be used. For instance, for each vibrational direction, the intensity can be calculated based on the change of dipole moment. This approach has the limitation to give the intensities for each atom separately. Nonetheless, this limitation is overcome by using density functional perturbation theory. This advanced method, available in VASP package, uses the formula given in Refs. 68–70 for calculating the change of atoms polarizabilities (i.e., the matrix of the Born effective charges). We used this method for calculating the vibrational modes intensities of MIL-53(Al). In our calculations, we systematically determined the Hessian matrix (∂ 2 E/∂r 2, where E is the free energy and r the ion position), the stress tensor and we allowed the ions to relax.

IV. RESULTS AND DISCUSSION A. Stabilized structures of the anhydrous np⊘ and lp⊘ MIL-53(Al) phases

During the last five decades, ab-initio calculations have been used for studying a large variety of biological, physical, and chemical processes.71 The results obtained have proved the method can give access to information difficult, if not impossible, to access by experiments72 and reveals useful to investigate the mechanisms of adsorption. The results reported below were obtained by optimizing the experimental MIL-53(Al) np⊘ and lp⊘ structures. As was mentioned above, this was performed by allowing a relaxation of shape and volume. The ionic relaxation into their instantaneous ground state was achieved by using the

conjugate-gradient algorithm. In all the numerical results reported here, otherwise stated, the van der Waals interactions resulting from dynamical correlations between fluctuating charge distributions are calculated. lp⊘ The initial lp⊘ structure39 with a volume Vexp = 1437.71 Å3 (orthorhombic structure with a = 6.84 Å, b = 17.40 Å, c = 12.08 Å) after being quenched at 0 K presents a volume lp⊘ contraction of ≈3 % with Vopt = 1392.24 Å3 (a = 6.69 Å, b = 17.49 Å, c = 11.89 Å). A similar calculation performed with the CRYSTAL software gives 1455.4 Å3.72 np⊘ The initial np⊘ phase Vexp = 976.93 Å3 (a = 19.51 Å, b = 7.61 Å, c = 6.58 Å) followed, after quenching, a np⊘ volume contraction of ≈19% (Vopt = 797.0 Å3, a = 19.48 Å, b = 6.19 Å, c = 6.61 Å). Our calculations in comparison to those reported with the CRYSTAL simulation package72 show a volume difference for the optimized lp⊘ and np⊘ phases of approximately 6% and 4%, respectively. Our calculations give a volume difference between both phases of approximately of 57%. While those reported with CRYSTAL give a volume difference of 72%. We suppose the differences between our results and those reported by Walker et al.73 are due to the finite temperature used by the latter authors. This last is supposed, because we use the same exchange-correlation functional with a similar cutoff. The minimum energies obtained, by ignoring the van der Waals interactions, for the np⊘ and lp⊘ structures (Fig. 1) are −534.99 eV and −536.52 eV, respectively. This result indicates that the lp⊘ structure is the most stable, which is in contradiction with experimental results. Nonetheless, if dispersive interactions are considered, we obtain for the np⊘ phase −540.91 eV and −540.26 eV for the lp⊘ phase (Table I). This indicates that at low temperature, the np⊘ structure is the most stable and corroborates previous numerical and experimental results.22,41,51,73,74 We obtained a rather small energy difference between the lp⊘ and np⊘ phases (∼0.647 eV) when compared to the total energy (around 0.1%). This small energy difference may be the reason why the metastable lp⊘ phase can also be observed experimentally. B. Stabilized structure of the hydrated MIL-53(Al) np-H2O phase

We studied the MIL-53(Al) np⊘ with four water molecules per unit cell. The introduced water molecules can be differentiated by their positions and the number of hydrogen bonds they make with the host structure. In Fig. 2 are illustrated the most probable positions obtained in our simulations and denoted A, B, C, and D. These positions were obtained by a trial and error insertion of more than 80 H2O molecules in

TABLE I. Volumes and energies of the optimized MIL-53(Al) np⊘, lp⊘, and np-H2O structures. MIL-53(Al) form

a (Å)

b (Å)

c (Å)

α

β

γ

Volume (Å3)

Energy (eV)

lp⊘ lp⊘ without VdW np⊘ np⊘ without VdW np-H2O

6.69 6.69 19.48 19.48 19.81

17.49 17.46 6.19 6.19 7.43

11.89 12.03 6.61 6.61 6.69

90.00 90.00 86.8 86.8 86.8

90.0 90.0 101.9 101.9 105.5

90.0 90.0 93.5 93.5 92.6

1392.24 1406.14 797.04 792.04 947.47

−540.26 −536.52 −540.91 −534.99 −598.84

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FIG. 2. Water molecule positions in the MIL-53(Al) np-H2O structure. See Subsections IV A and IV B.

the vicinity of the hydroxyl µ2-OH group of an octahedron and in the neighborhood of the different carboxylate groups of the organic linker. For water molecules of type A, the oxygen atom of water is linked to one hydrogen atom of the paradisubstituted benzene ring with a bond length of 2.15 Å. For adsorption sites of type B, water molecules interact with the structure through three hydrogen bondings: one between the oxygen atom of water and one hydrogen atom of a hydroxyl µ2-OH group of an octahedron, and two bonds between the two hydrogen atoms of water and the oxygen atoms of two distinct carboxylate groups of the organic linker, the bond lengths are 2.14, 1.77, and 1.93 Å , respectively. For adsorption sites of type C, the water molecule forms the bond between the oxygen atom of water and a hydrogen atom of a hydroxyl µ2-OH group of an octahedron (1.98 Å) and a second bond between a hydrogen atom of water and an oxygen atom of the carboxylate group of the organic linker (1.86 Å). Finally, for adsorption sites of type D, the water molecule is bonded to the structure through two hydrogen bondings between the two hydrogen atoms of water and the oxygen atoms of two different carboxylate groups with a respective length of 1.73 and 1.94 Å. For comparison, we recall the hydrogen bond length H2O-H2O in liquid water is around 1.88 Å. The optimized energy obtained for the host structure including four water molecules is −598.84 eV with a n p−H O volume Vopt 2 = 947.47 Å3 (a = 19.81 Å, b = 7.43 Å, c = 6.69 Å). The volume difference between the np-H2O and np⊘ phases is approximately of 20% and is originated by an anisotropic dilation of the anhydrous structure with the largest deformation along the b-axis (approximately 19%). For calculating the adsorption energy, we computed the energy of a free water molecule in the gaseous state, for which we obtained −14.21 eV. When four H2O molecules are introduced in the np⊘ phase its energy is reduced by 57.93 eV. The average adsorption energy per H2O is given by the expression (−57.93 + 4 ∗ 14.21)/4 = −0.27 eV (−26.3 kJ/mol). This result is lower than the liquefaction energy (−40.66 kJ/mol). We also performed simulations with one H2O molecule and we found an adsorption energy of

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−0.41 ev (−39.4 kJ/mol) which is in excellent agreement with the energy found for H2O adsorption ion MIL-53(Cr) at low loading (approximately −39 kJ/mol).49 Based on experimental studies, Férey60 suggested that MOF flexibility is due to the presence of weak points in the structure allowing rotation/translation. He suggested that these points play the role of a kneecap during the topological modification of the structure. It is worth noticing that in our simulations, the adsorption of water molecules induces a rotation of the benzene rings (Fig. 3). The relative angles formed between the normal vectors of the benzene rings (Fig. 3) (Ri for i = 1, 4) in the np⊘ and np-H2O phases are 54◦, 45◦, 56◦, and 56◦, respectively. For example, consider ring R3 or R4 (Fig. 1(b)), the introduction of water molecules will produce a ring rotation along the line joining the two opposite CO2. The measurement of the rotation angle is not completely accurate due to the fact that opposite CO2 are not perfectly aligned but slightly bended. Interestingly, this last implies a channel asymmetry (Fig. 1(c)). This result goes along the existence of kneecap points (weak points) in the hydrated structure and, as we will see below, these points have well-defined vibrational modes. The studies by Salles et al.,75 for the N2 breathing of the Co(1,4-ben-zenedipyrazolate) MOF by in situ pressurecontrolled X-ray diffraction and molecular dynamics, showed that a necessary condition for observing multiple step breathing is the rotation of the pyrazolate rings (associated to CN stretching vibrations). Intuitively, one may expect that for materials of the MIL-53 family, the passage from a large pore to a narrow pore (and vice-versa) has to be assisted by stretching and/or bending of the CO2 making the link to the octahedron. However, less intuitive is the wagging mode observed in our simulations (see Sec. IV C) and which, partially, allows the benzene ring rotation. Undoubtedly, the rotation observed for two different kinds of MOFs appears as a dynamical effect giving flexibility to the structure. C. Infrared spectra of the MIL-53(Al) lp⊘, np⊘ phases

In this subsection, we detail the infrared spectra obtained experimentally and by simulations for the anhydrous phases of

FIG. 3. In this figure is illustrated the angle rotation of the benzene rings after water adsorption in the MIL-53(Al) np⊘ phase. It is important to remark that although the ring rotation is produced along the initial line joining two opposite CO2, these last entities slightly bend after water molecules have been introduced.

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MIL-53(Al). Although the experimental spectra were obtained at room temperature and those by simulations at 0 K, this was not a hindrance to establish a frequency correspondence between both spectra. The experimental and simulated infrared spectra for the np⊘ and lp⊘ forms of MIL-53(Al) are shown in Figures 4(a) and 4(b), respectively. In particular for the np⊘ phase, the general tendency is a red shift of the simulated bands with respect to the experimental ones, characterized by a more pronounced red shift at low wavenumbers (≈100 cm−1) than for high wavenumbers (≈60 cm−1). At 450 cm−1, we observe similar wavenumbers in the spectrum but with a pronounced difference in the intensities. From 550 to 800 cm−1, some of the experimental maxima seem to be associated to several maxima of the simulated spectrum. Between 1300 and 1600 cm−1, we can clearly distinguish the pronounced experimental maxima in the simulated spectrum. The simulated spectrum of the lp⊘ phase (Fig. 4(b)) present, as for the np⊘ phase, a general red shift with respect to the experimental spectrum. Roughly speaking, for low

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wavenumbers, the red shift varies between 70 to 100 cm−1 and for high wavenumbers the shift diminishes. The comparison between numerical and experimental spectra suggests that in the range of 400-700 cm−1, similar vibrational modes are present. From 1000 to 1300 cm−1, despite the intensity difference in the spectra, similar vibrational modes can be distinguished. Clearly, from 1400 and 1600 cm−1, it is difficult to ignore the close resemblance between both spectra. As we will see below, this correspondence of band positions in simulated and experimental spectra allowed to propose plausible identification of vibrational modes. D. Infrared spectrum of the MIL-53(Al) np-H2O phase

The spectrum of the MIL-53(Al) np-H2O phase is quite complex and molecules of water may vibrate involving symmetric stretching, νs , asymmetric stretching, νas , and bending, δ, of the covalent bonds. The vibrational intensities vary according to the states of free water (Table II). We can say without loss of generality, that free water molecules have their

FIG. 4. In figures (a) and (b) are given the experimental and the obtained spectra by DFTP in the range of 400-2000 cm−1 for the np⊘ and lp⊘, respectively. We can see in both spectra the correspondence of wavenumbers although the intensities are different and which maybe associated, partially, to the temperature difference.

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TABLE II. Main vibrational modes for different states of water. STATE

ν s OH (H2O) (cm−1)

ν a s OH (H2O) (cm−1)

δ OH (H2O) (cm−1)

∆ = (ν a s −ν s ) (cm−1)

Gas Liquid Solid

3657.1 3280 3085

3799.9 3490 3220

1594.7 1644 1650

98.8 210 135

symmetric and asymmetric stretching values above 3000 cm−1 and bending modes around 1600 cm−1. In the MIL-53(Al)-np-H2O phase, we can distinguish OH groups belonging either to water molecules or to MIL-53(Al). Here, this distinction is expressed by writing in parentheses the location of the water molecule linked to the structure by H2OA,B,C,D. If the link is made with the MOF we write (µ). For “OH(µ),” the vibrational response strongly depends if the interaction is with a water molecule or not. If the OH group interacts with water, we write “OH(µ) ⊂ (H2O)type of water.” On the contrary, we simply write “OH(µ) free.” As mentioned above, the free water vibrational modes are bending, symmetric, and asymmetric stretching. When water molecules interact with MIL-53(Al), we observe two additional types of vibrations: rocking ( β) in the plane and wagging (ω) out-of-plane. These additional modes appear when OH(µ) interacts with H2O. Our studies revealed that the interaction between MIL-53(Al) and H2O gives a total of eight vibrational modes, namely, νas OH(H2O), νs OH(H2O), νas OH(µ) ⊂ (H2O), δOH(H2O), δOH(µ) ⊂ (H2O), β(H2O), δ(H2O), and ω(H2O) (Table III). Interestingly, the adsorbed water molecules can be differentiated according to the number of links they make with the host structure (Fig. 2) and this allows to identify the vibrational signature of each water molecule. This result was obtained by a comparative analysis of experimental and numerical spectra. For the symmetric stretching vibrational mode, νs OH(H2O), the wavenumber are 3687, 3697, 3606, and 3489 cm−1 for water molecules A, B, C, and D, respectively. For scissoring (δ), the bands are located in the range of 16061653 cm−1. This vibrational mode was observed for water molecules linked to both the hydroxyl, (OH) µ , group of the octahedron and the carboxylate group of the organic linker. For (H2O)C , we observed a rocking mode, β, at 770 cm−1 and for (H2O) D a band at 671 cm−1 corresponding to an out-of-plane

mode (wagging). The role of the observed scissoring, rocking, and wagging vibrational modes is to give some flexibility to the structure. In Fig. 5, all the discussed modes have their corresponding bands in the experimental spectrum which undoubtedly is an asset for validating our numerical results. In Table IV is given a detailed spectral band identification for the three studied MIL-53(Al) phases. E. Vibrational similarities between MIL-53(Al) and MIL-53(Cr)

Relatively recent studies have focused to determine the OH-stretch vibrational frequency. This vibrational mode is crucial when studying the confinement of water molecules in porous materials since it determines how molecules selforganize. Recently, Cirera et al.76 used a fully polarizable Thole-type model (TTM), and showed that the inclusion of polarized effects can have an impact on the distribution of H2O molecules inside the nanopores. Moreover, Haigis et al.56 studied as well the MIL-53(Cr) structure by means of DFT-based Car-Parrinello molecular dynamics simulations (CPMD package). The simulation cell used by the last authors represents a unit cell of the empty (non-hydrated) and the hydrated narrow- and large-pore phases. The last two structures were modeled by adding 4 and 24 water molecules, respectively. In the case of the narrow pore fully hydrated, our ab-initio calculations for the MIL-53(Al) give a distance of 1.86 Å for the distance H µ2−OH − Owater. This value is similar to the value reported by Haigis et al.,56 of 1.8 Å for MIL53(Cr) and slightly smaller than the 2.0 Å reported by Cirera et al.76 with the polarizable TTM force field. For the distance Owater − Owater, we obtained 3.2 Å which is rather similar to the value reported by Medders and Paesani77 of 3.1 Å, while

TABLE III. Characteristic vibrational modes of water molecules depicted in Fig. 2. In parentheses are given the experimental values. Vibrational mode (cm−1) ν s OH(H2O) exp ν ssi m −ν s ν a s OH(H2O) exp ν asism −ν a s δOH(H2O) δ si m − δ exp ωOH(H2O) ω si m − ω exp βOH(H2O) β si m − β exp

(H2O) A

(H2O) B

(H2O)C

(H2O) D

3687 (3547) 140 3775 (3606) 169 ... ... ... ... ... ...

3697 (3400-3500) 297/197 3695 (3547) 148 1653 (1630) 23 ... ... ... ...

3606 (3400-3500) 206/106 3744 (3400-3500) 344/244 1606 (1630) −24 ... ... 770 (658) 112

3489 (3400-3500) 89/−11 3602 (3400-3500) 202/102 1618 (1630) −12 671 (627) 44 ... ...

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FIG. 5. In this figure are given the obtained spectra for experimental and simulated lp⊘ and np-H2O structures. Figs. 5(a)–5(c) give the spectra for the experimental and simulated np-H2O from 400-3800, cm−1. The last figure shows four pictures associated to the adsorbed water molecules (see Sec. IV D) which allowed us to identify the signature of each absorbed water molecule. In Table IV are detailed the dominate vibrations modes when water molecules are adsorbed in the MIL-53(Al) np⊘.

Haigis et al.56 reported 2.8 Å. This last suggests that the three approaches lead to similar distances for H µ2−OH − Owater and Owater − Owater. However, these similarities need to be taken with precaution, since our studies were developed for MIL53(Al) and not for MIL-53(Cr). Experimentally for the large pore configuration of MIL53(Al), the OH-stretch frequency is located around 3700 cm−1 and our ab-initio calculations give 3808 cm−1. This corresponds to a blue shift of 108 cm−1. For the narrow pore of the empty and the fully hydrated structures of MIL-53(Al), we obtained numerically 3545 cm−1 and 3592 cm−1, respectively. The latter frequency exhibits a red shift of 88 cm−1 with

respect to the value reported by Medders and Paesani for the MIL-53(Cr), namely, 3680 cm−1.77 Certainly, the use of a hybrid exchange-correlation function (e.g., PBE0), may lead to reduce slightly the difference between the experimental and our ab-initio results. However, Guidon et al.78 indicated that negligible changes in the number and character of hydrogen bonds were found with PBE and PBE0 functionals. It is worthwhile to mention that quantitative discrepancies between theory and experiments remain, e.g., the position of the IR stretching band, and the origin of these discrepancies is not well understood. Based on this last, we can assume that ab-initio calculations with the PBE functional for studying

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TABLE IV. In this table, we give a detailed spectral band identification of the MIL-53(Al) in the forms: np⊘, lp⊘, and hydrated (np-H2O). It is worthwhile to mention, that for the latter structure, we obtained vibrational modes associated to the CO2’s linked to the benzene cycle and to the octahedrons which can be used to identify the breathing transition (Sec. IV F). Vibration mode (cm−1)

np⊘

lp⊘

np-H2O

νOH (µ) free Model Experiment ∆

3393/3396 3701 +360/+160

3541/3545 3707 −102/−112

3809/3812 3701 −106

3819/3823

3807

ν a s OH (H2O)C, A Model Experiment ∆

3744/3775 3606 −138/−169

ν s OH (H2O) A +ν a s OH (H2O) B Model Experiment ∆

3687/3695 3547 −140/−148

νOH (µ)C(H2O)C +ν s OH (H2O)C, D +ν a s OH (H2O) D Model Experiment ∆

3489-3602 3400-3500

νOH (µ)C(H2O) B Model Experiment ∆ νCC(8a)

3348 3276 −72

Model Experiment

1668 1735

1730

1737

∆ δOH (H2O)C, D, B Model

1606/1618/1653

Experiment

1630 +24/+12/−23

∆ ν a sCO(CO2)[νCC(8a ′)] Model

1531-1605

1528-1629

1509-1583

Experiment

1504/1578

1511/1596

1504/1573

Model

1342-1422

1395-1431

1379/1415

Experiment

1408*/1442

1415*/1444

1397*/1408/1445

∆ ν sCO(CO2)[νCC(19b)]∗

∆ νCC(14) Model Experiment

1363 1317



1346/1360

1319

1318

−44

−28/−42

1293

1293

δC H (3) Model Experiment

1291



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TABLE IV. (Continued.) Vibration mode (cm−1)

np⊘

lp⊘

np-H2O

δC H (18a)[δC H (9a)] Model

1150

1158

1154

Experiment

1159

1164

1159

+9

+8

+5

∆ δOH (µ)C(H2O)C, B Model

1130/1197

Experiment

1116 −14/−81

∆ δC H (18b) Model Experiment

1093

1097

1086/1103

1090/1101 −3/+8

∆ δC H (18a ′) Model

1002

1009

1008

Experiment

1017

1026

1017



+15

+17

+9

1070/1121

927

954/981

977

986

980

−93/−144

+59

+26/−1

874 887/891 +13/+17

867 887 +20

889/892

870/825 836/848 +29/+23

807 838/853 +31/+46

813 837/848 +24/+35

733 757/765 +24/+32

723 752/756 +29/+33

733 760/767 +27/+34

δOH (µ) f r ee Model Experiment ∆ γC H (17b) Model Experiment ∆ δ/βCO(CO2) Model Experiment ∆ ωC H (11) Model Experiment ∆

δOH (µ)C(H2O)C, B + β(H2O)C + δ(H2O) B Model Experiment ∆

750/770 698 −52/−72

γCCC(6a) −γCCC(4) Model Experiment ∆

703 671

660/683

676

δOH (µ)C(H2O)C, B + ω((H2O) D ) Model Experiment ∆

648/671 627 −21/−44

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TABLE IV. (Continued.) Vibration mode (cm−1)

np⊘

lp⊘

np-H2O

δ AlO Al(O AlO) +ν AlO Al(O AlO) Model Experiment ∆

500-600 510-640

500-600 510-640

500-600 510-640

425-500 430-510

425-500 430-510

425-500 430-510

443

425-500 448 23

ν/ωOH (µ) f r ee + δ AlO Al +γCCC(16b) Model Experiment ∆ νOH (µ) f r ee + δ AlO Al +γCCC(16b) Model Experiment ∆

TABLE V. Vibrational modes associated to the transition lp⊘ to np-H2O of MIL53-(Al). The values in bold correspond to the experimental values. Vibrational mode (cm−1)

NP-H2O

ω CH(11) δ CO (CO2) δOH (µ) free δCH (18a) δCH (18a) [δCH (9a)] ν s CO (CO2) ν a s CO (CO2) ν a s CO (CO2)

764 733 848 980 954-981 1017 1008 1159 1097 1402 1397 1504 1509 1573 1562

the adsorption of water molecules in MIL-53(Al), even when they can be improved by hybrid functionals, are reliable enough for giving a detailed description of the vibrational modes characterizing the adsorption of H2O by MIL-53(Al). F. Phase transition in MIL-53(Al)

The experimental signature of the breathing effect in MIL53(Al) is usually associated to a shift of a vibrational mode in lp⊘ at 1026 cm−1 to 1017 cm−1 in the np-H2O.36,51 This wavenumber has been observed in our experimental results (Fig. 5) and our analysis suggests that it may be associated to a CH in-plane deformation of the aromatic ring δCH(18a). For this molecular group, we also obtained a wagging mode, ωCH, shifting from 764 cm−1 in the np-H2O phase to 754 cm−1 in the lp⊘ phase which seems to be equally important as δCH during the phase transition. Our analysis shows that other vibrational modes can be used to identify the mentioned transition (Table V). These modes are associated to CO2 which are linked to the benzene cycle and to the octahedrons. The role of these CO2 vibrations is to give flexibility to the structure. From the eight identified modes, the shift varies in the range (−7, 35) cm−1 when the transition takes place. In particular, the presence of the vibrational mode νasCO(CO2) implies the

np⊘ 761 733 848 825 977 1070-1121 1017 1002 1159 1090 1408 1410 1504 1531 1578 1563

lp⊘ 754 723 852 807 986 927 1026 1009 1164 1093 1415 1405 1511 1528 1596 1529

≈ ∆ [lp-(np-H2O)] −7 −10 +4 −18 +6 −27/−54 +9 −1 +9 −4 +7 +8 +7 +8 +23 +36

unlocking of the in and out-of plane movements (δ CO(CO2) and ωCH) of the benzene cycles.

V. CONCLUSIONS

We show that an agreement between experimental results and ab-initio simulations requires to include dispersive interactions. This result corroborates the proposition developed in Ref. 72. We show that DFPT spectral calculations and experimental infrared spectra are powerful tools for analyzing the effect of water molecules when the transition from lp⊘ to np-H2O of MIL-53(Al) takes place. Our analysis permitted us to identify four types of adsorbed water molecules (Fig. 2) characterized by their vibrational signature instead of the three H2O suggested by Férey.60 In fact what is important is to know where the adsorption takes place inside the host structure. The reason is that different vibrational modes will be present depending on the nature of the site. The [AlO4(OH)2] octahedra is a rigid structure and, intuitively, one may expect that the structure flexibility may arise from a stretching of the CO2 (e.g., νCO(CO2)) for increasing or reducing the surface of the pore. However, we observed less intuitive modes, namely, in and out-of-plane-

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vibrational modes of the benzene rings. These modes imply a rotation of the benzene rings not reported before for MIL53(Al). This rotation was observed for another type of MOF, (Co(BDP) (BDP2-) 1,4-benzene-dipyrozolate), by adsorption of N2.51 This rotation mode seems to be a dominant vibrational mode for the MOFs breathing behavior. In Secs. IV C and IV D, we gave a detailed spectra for the analyzed structures. The laborious and meticulous identification of the main vibrational modes allowed us to give a detailed spectrum for each structure studied, for the first time. These spectra can be seen as a band identification tool of the adsorption processes of MIL-53(Al) and probably for other MOFs of the MIL-53 family. Usually, the breathing transition is identified by the vibrational mode δCH(18a). Our work shows that other important vibrational modes, seven more in fact, can be used to fully characterize the transition from lp⊘ to np-H2O. Finally, our studies suggest that the role of the wagging and scissoring modes of the CO2, linking the benzene cycle to the octahedron, is to give flexibility to the structure for accessing the breathing transition. ACKNOWLEDGMENTS

The authors thank the Agence National de la Recherche (ANR) for fundings under the project “SOFTCRYSTAB” (No. ANR-2010-BLAN-082). J. P. Bellat and J. M. Salazar thank B. Diawara for enlightening discussion and suggestions about VASP. 1S.

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Characterization of adsorbed water in MIL-53(Al) by FTIR spectroscopy and ab-initio calculations.

Here, we report ab-initio calculations developed with a twofold purpose: understand how adsorbed water molecules alter the infrared spectrum of the me...
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