Capture of Iodine Species in MIL-53(Al), MIL-120(Al), and HKUST-1

Article Cite This: J. Phys. Chem. C 2017, 121, 25283-25291

pubs.acs.org/JPCC

Capture of Iodine Species in MIL-53(Al), MIL-120(Al), and HKUST1(Cu) Periodic DFT and Ab-Initio Molecular Dynamics Studies Siwar Chibani,*,† Fatah Chiter,† Laurent Cantrel,‡ and Jean-François Paul*,† †

Univ. Lille, CNRS, ENSCL, Centrale Lille, Univ. Artois, UMR 8181 - UCCS - Unité de Catalyse et de Chimie du Solide, F-59000 Lille, France ‡ Institut de Radioprotection et de Sûreté Nucléaire, CE Cadarache, F-13115 Saint Paul lez Durance, France S Supporting Information *

ABSTRACT: The potential use of three metal−organic frameworks (MIL-53(Al), MIL-120(Al) and HKUST-1(Cu)) to adsorb iodine species (I2 and ICH3), which can be released during a severe nuclear accident, is investigated using periodic dispersion density functional theory for the first time. Competitive adsorption of iodine in the presence of water molecules is also characterized for the hydrophilic HKUST1(Cu). In the first step, we have found that the absolute values of interaction energies of I2 and ICH3 are higher in the hydrated form of HKUST-1(Cu) than in the dehydrated one, which is of very high interest for iodine trapping. In a second stage, iodine species are strongly adsorbed in MIL-53(Al) than in MIL-120(Al) and HKUST-1(Cu) MOFs and therefore this material could potentially trap iodine compounds. Moreover, we study the influence of the functionalization of the MIL-53(Al) organic linkers on the adsorption behavior of iodine and it turns out that the substitutions does not present a significant effect for this purpose. The factors governing the interaction energies between iodine (I2 and ICH3) and MOF structures are analyzed and the important role of van der Waals interactions in these materials is highlighted. diversity.4,7,8,11,12 This class of porous materials is composed of organic molecules connected by metal salts (Al, Cu, Zn, etc.).19,20 The MOF materials have been the subject of the largest number of investigations for different applications: catalysis,21 gas storage/separation,22,23 drug delivery as well as nuclear facilities.24 Indeed, there have been several work reported on the adsorption of I2 compound into the MOFs materials, but to the very best of our knowledge no previous study has been made on the radioactive ICH3 molecule, except the experimental PhD work of Chebbi.25 Recently, Nenoff et al. have investigated the potential use of the MOFs to capture I2 molecules in nuclear facilities.4,7,8,12,17 Using the zeolite imidazole framework (ZIF-8), the authors have determined the pair distribution function (PDF) for I−I and I−framework interactions through X-ray diffraction experiment and they found that the local structure of the captive I2 is unchanged upon amorphization of the framework.4 Remaining with I2 capture within ZIF-8, the authors show in another experimental and theoretical work that the adsorption of I2 forms stable charge-transfer (CT) complexes with aromatic carbon molecules. Also, the Nenoff group has studied the competitive sorption of I2 from humid gas stream into Cu-BTC, also known as HKUST-1.12 The results demonstrated that I2 adsorbs in preference to water vapor and it forms an effective hydrophobic

1. INTRODUCTION Considerable attention has recently been focused on the management of fission products and on their release into the environment in the case of a severe nuclear accident. Among the most dangerous radioactive compounds, iodine species (I2 and ICH3), presented relatively in small amounts, have a severe impact on the nature and on the human metabolic processes due to their radiotoxicity and volatility.1 For this reason, iodine capture and separation are the subjects of a large number of studies.2−6 In this vein, various technologies based on solid sorbents have been used and discussed in the literature, such as activated carbon, silver-exchanged mordenite (Ag-MOR) and metal−organic frameworks (MOFs).2−4,7−17 However, in the case of a severe accident, the activated carbon is unsuitable for this purpose because of its low ignition temperature and the poisoning effect of nitrogen. In another way, Ag-MOR is one of the most effective sorbents for iodine compounds trapping.2,3,13,14 Indeed, Ag-exchanged zeolite presents good stability toward temperature, oxidizing, and irradiation conditions.3,18 Unfortunately, Ag-MOR suffers from several disadvantages, such as generally low sorption capacity, high cost of silver, and silver’s severe environmental effects. Besides, the formation of solid silver iodide in zeolites is very stable for a long period of time, involving a difficult regeneration of adsorbents. All these drawbacks encourage scientists to develop alternative materials. In this context, MOF structures appear to be promising candidates for iodine uptake process due to their high stability, low-cost, high surface areas, and a wide structural © 2017 American Chemical Society

Received: September 7, 2017 Revised: October 20, 2017 Published: October 27, 2017 25283

DOI: 10.1021/acs.jpcc.7b08903 J. Phys. Chem. C 2017, 121, 25283−25291

Article

The Journal of Physical Chemistry C

Figure 1. Representation of the metastable NP (left) and LP (right) structures of the MIL-53(Al) during the dehydration/hydration cycles.

that the PBE-D3 approach, which we have used in the present work, predicts more accurate structures (unit cell volume, bonded parameters, and pore description) than PBE compared to experiment for diverse MOFs. The plane wave cutoff energy was set to 550 eV and partial orbital occupancies were smeared using the Gaussian smearing scheme34 with a smearing parameter σ = 0.2 eV. The relaxation of the structures was performed until all forces acting on the atoms were smaller than 0.03 eV/Å. Because of the large unit cell used in calculations, the Brillouin-zone sampling was restricted to the Γ-point. The interaction energies of the MOF with N molecules of adsorbate, ΔEint, are calculated as follows,

barrier to minimize water sorption. The obtained results are relevant for the scientists in industrial nuclear energy and in the case of a severe nuclear accident due to the presence of a large amount of water in the nuclear containment, as the highly desired goal is to capture the iodine species. In another work, Falaise et al.9 investigated the sorption of I2 in cyclohexane solvent in a series of aluminum-based MOFs. The authors found that the functional groups attached to the organic ligand of the MOFs improve the capture of I2, tentatively explained by the formation of charge-transfer between electron donor and iodine. Assfour et al.11 using Grand canonical Monte Carlo (GCMC) classical simulations have determined the adsorption isotherms at room temperature for iodine in several MOFs. The simulation results exhibit that MOFs with high pore volume and surface are preferred for iodine storage at ambient conditions of pressure and temperature, while at low pressure, MOFs with smaller pore volume are more qualified for iodine uptake. On the basis of all these results, in the present work, we have selected a set of three diverse MOFs: MIL-53(Al), MIL120(Al), and HKUST-1(Cu) for their different structural features, surface areas, and metallic clusters. MIL-53(Al) exhibits a wide variety of functionalization. MIL-120(Al) material presents a high concentration of aluminum placed in 1D chains. To extend our study, HKUST-1(Cu) was chosen due to its metal cluster, high pore size, and competitive iodine sorption in the presence of water, as well as its resistance under gamma irradiation.16 The difference between the selected materials allows us to distinct which parameters affect mostly on the adsorption of iodine molecules. Combining density functional theory (DFT) with molecular dynamics simulations, we have elucidated the interaction processes of I2 with the MOF framework and determined the infrared spectra. It is important to highlight that the methodology followed had never been used in the iodine uptake process elucidation. The paper is organized as follows: in section 2, the computational details are presented. In section 3, the interaction energies and infrared spectra of I2 in MOFs are analyzed. Finally, the main conclusions are summarized in section 4.

ΔE int = EMOF − N * X − EMOF − NE X

(1)

where EMOF is the total energy of clean MOF, EX [X = I2 or ICH3] corresponds to the isolated iodine molecules, and EMOF−N*X is the interacting system of MOF with N adsorbate molecules. The contribution of dispersion energies (ΔEdisp) to the interaction energies is simply: ΔEdisp = Edisp ‐ MOF − N * X − Edisp ‐ MOF − NEdisp ‐ X

(2)

2.2. Molecular Dynamics Simulations. All our Born− Oppenheimer ab initio molecular dynamics (AI-MD) simulations have been performed in the NVT ensemble. The simulation temperature was controlled using a Nosé thermostat,35,36 and its value was set to T = 373 K, which corresponds to typical experimental conditions. A time step is fixed to 0.5 fs for the integration of the equations of motion. The length of trajectories in standard MD runs presented in this work was 50 ps whereby the initial period of 6 ps is considered as equilibration and the corresponding data are not included in the calculations. 2.3. Structural Model. As stated in the Introduction three MOF structures (MIL-53(Al), MIL-120(Al), and HKUST1(Cu)) have been used in the present work. As the synthesis of the MOFs is most often carried out hydrothermally (the used solvent is water), the activation of MOF by the removal of water molecules is crucial to adsorb the molecules of interest (I2 and ICH3). After dehydration, MIL-120(Al) and HKUST1(Cu) MOFs change relatively their cell volumes (3%). On the contrary, the MIL-53(Al) structure shows a remarkable variation of the cell volume (up to 40%) after water molecules located in the center of the pores are removed. This phenomenon, named breathing, induces a reversible transition from the narrow-pore phase (NP) to the large-pore phase (LP) (Figure 1). Indeed, the breathing transitions of MIL-53(Al) have been widely characterized by molecular simulations in different studies.37−39 This characteristic is important for a large range of potential practical applications especially on the specific gas separations.37

2. COMPUTATIONAL DETAILS 2.1. Electronic Structure Calculations. All periodic density-functional calculations were carried out with the Vienna Ab initio Simulation Package (VASP).26−29 The Kohn−Sham equations were solved self-consistently until the energy was converged within 10−6 eV and the electron−ion interactions were described using the projector-augmented wave (PAW) method developed by Blöchl.30 The semilocal PBE exchange− correlation functional31 was employed. This level of theory is known to lack the correct description of long-range dispersion interactions.32 To correct this problem, Nazarian et al.,33 found 25284


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Figure 2. Representation of the LP-MIL-53(Al) (left), MIL-120(Al) (middle), and HKUST-1(Cu) (right) structures. Dashed lines indicate the orthorhombic, monoclinic, and rhombohedral unit cells for MIL-53(Al), MIL-120(Al), and HKUST-1(Cu), respectively. Color code: O atoms are in red, C atoms in gray, Al atoms in pink, and H atoms in white.

Table 1. Properties (Surface Area and Pore Volume/Size) of the Different Tested MOFs, MIL-53(Al), MIL-120(Al), and HKUST-1(Cu), with the Structural Parameters of the Supercell Used in This Work MOF

cation

MIL-53(Al) MIL-120(Al) HKUST-1(Cu)

3+

Al Al3+ Cu2+

surface area (m2/g)

pore volume (cm3/g)

pore size (Å)

ref

supercell

a, b, c (Å)

α, β, γ (deg)

1254 145 1164

0.5 0.04 0.43

8 5.4 × 4.7 6.9 × 4.1

11 9 12

orthorhombic monoclinic rhombohedral

a = 17.1, b= 6.6, c = 12.1 a = 9.7, b = 20.1, c = 7.4 a = b = c = 18.6

α = β = γ = 90 α = γ = 90, β = 134.4 α = β = γ = 60

computed interaction energies of I2 are quite similar to that of ICH3 for the three MOF structures. This can be explained technically by the large pore size of the selected MOFs compared to the longest molecular axes, which are approximately 2.7 and 2.4 Å for I2 and ICH3, respectively. However, Table 2 clearly indicates that marked differences between the computed interaction energies of the iodine species occur for each MOF structure. Let us now start by analyzing the ΔEint results in HKUST-1(Cu) MOF; relaxed structures are available in the Supporting Information (Figures S1 and S2). As pointed out above, HKUST-1(Cu) preferentially adsorbs I2 over water vapor.12 It is noteworthy that the water molecules in HKUST1(Cu) are located in the axial positions in the Cu cluster, which can be removed after dehydration. To allow meaningful comparison with experiments, we have also determined the interaction energies of iodine species in HKUST-1(Cu) with water and the results are listed in the end of Table 2. The HKUST-1(Cu) structure contains two types of cavities: a small cage with a free diameter of 5 Å, where molecules interact with the diatomic Cu cation and a large cavity: cage center/cage window of 13.5 Å, respectively. According to Nenoff group work,12 I2 initially adsorbs in the small cage close to copper with a tiny amount, and then it adsorbs within the large pore close to the organic linker. Upon comparison of our findings with the obtained results of Nenoff group12 in HKUST-1(Cu), the arrangement of the iodine species within the cages yields an excellent agreement. In the small cage, the absolute values of the interaction energies of iodine species are relatively higher compared to the energies computed in the large cage, due to the interaction with the copper. The computed differences ΔEint (small cage) − ΔEint (large cage) are −28.7 and −20 kJ/ mol in the dehydrated form of HKUST-1(Cu) for I2 and ICH3, respectively. Similar results were also found for the hydrated form of HKUST-1(Cu), but the difference in ΔEint (small cage) − ΔEint (large cage) is even larger (−40.5 and −29.9 kJ/mol for I2 and ICH3, respectively). Furthermore, the interaction of iodine species with organic linkers is dominated by dispersion interactions, especially in the larger cavity (more than 50% of the total interaction energy), in line with the work of Rubes et al.40 and Nazarian et al.33 showed that the long-range

In the present work, we performed simulation for the LPMIL-53(Al), MIL-120(Al), and HKUST-1(Cu) configurations experimentally obtained and given in refs 11, 19, and 12. For the selected structures, all water molecules have been removed and our calculations were carried out in the orthorhombic, monoclinic, and rhombohedral supercell for MIL-53(Al), MIL120(Al), and HKUST-1(Cu) MOFs, respectively (Figure 2). The lattice parameters of the conventional cell with the surface area, pore volume, and pore size are given in Table 1. As one can see from Table 1, MIL-53(Al) and MIL-120(Al) materials present very different properties; for example, the surface area of MIL-53(Al) is 8 times greater than that of MIL120(Al), despite these two materials being based on the same Al metal salts linked together by 1,4-benzenedicarboxylate (BDC). HKUST-1(Cu) composed of trimesate organic molecules and connected by Cu metal presents the largest pore size (6.9 × 4.1 Å), indicating that they can be considered as a promising material to accommodate a large amount iodine compounds.

3. RESULTS AND DISCUSSION 3.1. Adsorption of Iodine Compounds in MOFs. With eqs 1 and 2, the calculated interaction energies for both I2 and ICH3 molecules with the corresponding contributions of the dispersion are presented in Table 2. It is seen that the Table 2. Calculated Interaction Energies (ΔEint) for I2 and ICH3 Molecules (kJ/mol) for the Three Tested MOFsa MOFs MIL-53(Al) MIL-120(Al) HKUST-1(Cu): small cage HKUST-1(Cu): large cage HKUST-1(Cu) + H2O: small cage HKUST-1(Cu) + H2O: large cage

ΔEint(I2) (kJ/mol) −111.2 −64.1 −52.9 −24.2 −67.7 −27.2

(−47.1) (−81.8) (−35.9) (−13.7) (−45.6) (−14.2)

ΔEint(ICH3) (kJ/mol) −94.3 −50.4 −35.7 −15.4 −55.4 −20.3

(−34.6) (−66.5) (−23.9) (−10.6) (−40.7) (−10.8)

a

The calculated interaction energy corresponds to only one molecule. The contribution of dispersion interactions (ΔEdisp) (kJ/mol) to interaction energies is given in parentheses. 25285


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that iodine species are more strongly adsorbed in MIL-53(Al) than in MIL-120(Al) and HKUST-1(Cu) MOFs and therefore this material could potentially trap iodine compounds, in agreement with the experimental work of Chebbi et al.25 In the rest of the paper we will only discuss about the MIL-53(Al) MOF. 3.2. Effect of the Functionalization with MIL-53(Al) on the Adsorption of Iodine Compounds. As mentioned before, Falaise et al.9 have investigated the sorption of I2 dissolved in the cyclohexane solvent in the MIL-53(Al) series studying the effect of the functionalization. The authors have found that the nonfunctionalized MIL-53(Al) presents the worst structure for iodine uptake, a very small amount of I2 molecules have been removed from the solution after 48 h. This value progressively increases with the functionalization in diluted cyclohexane in MIL-53(Al) improving the I2 capture. Indeed, the authors have shown that the best removal efficiency is obtained with MIL-53(Al)−NH2, tentatively explained by the formation of strong charge-transfer complexes with iodine. To determine the effect of the functionalization with MIL-53(Al) on the adsorption, nine different groups (−H, −Br, −Cl, −OH, −NH2, −CH3, −NO2, −COOH) that have been tested experimentally9 have been used in the present work. One can expect that the highest removal efficiency determined experimentally can be correlated to an interesting interaction energy. Computed values of interaction energy and the contribution of dispersion energy to this quantity of gaseous iodine species are collected in Table 3. As can be seen, the

nonbonding interactions of molecules with the MOFs are far from being negligible. For instance, I2 and ICH3 located in the small cage have larger interaction energies in HKUST-1(Cu) with water than in the dehydrated one, in agreement with the work of Nenoff et al.12 In that case, the ΔEint of I2 and ICH3 are, respectively, −67.7 and −55.4 kJ/mol in the hydrated HKUST-1(Cu), whereas those in the dehydrated material are relatively lower in absolute value (−52.9 kJ/mol (I2) and −35.7 kJ/mol (ICH3)). At this stage, it is worth mentioning that the interaction energy of water with the copper cation is −36.6 kJ/mol and thus the water molecules do not prevent the iodine adsorption. In this small cage, the difference of the ΔEint of iodine species in the two environments is about 20 kJ/mol. This can be explained not only by the adsorption of molecules closer to the copper in the small cage but also by the interaction energies of I2 and ICH3 with the water molecules (located in the axial positions of the Cu cluster) being affected by local interactions such as hydrogen bonding (Figure S2 in the Supporting Information). Nevertheless, in the case of a large cage, the computed interaction energies of iodine compounds are quite similar for both hydrated and dehydrated HKUST-1(Cu) structures. For I2, the ΔEint are −24.2 and −27.2 kJ/mol in the large cage of the dehydrated and hydrated forms of HKUST-1(Cu), respectively. This is because of the absence of water molecules inside the large cage. Clearly, iodine species are more strongly adsorbed in the small cage of the hydrated HKUST-1(Cu) material than in the dehydrated one. The present finding is particularly interesting for iodine trapping, revealing that water is not a competitive component versus iodine, but the absence of strong adsorption sites promotes rather physisorption interactions. Let us now turn to the results obtained with the dehydrated MIL-53(Al) and MIL-120(Al). In these two MOFs, we have only a single type of cavity where one iodine molecule is adsorbed. At this stage, it is important to highlight that for the hydrated forms of MIL-53(Al) and MIL-120(Al), water molecules located in the center of the pores block the adsorption of iodine compounds. Thus, we only focus on the dehydrated MIL-53(Al) and MIL-120(Al) MOF structures. Analyzing geometric parameters, we have found that iodine species placed in the center of the pores are distant by around 3.5 Å for MIL-53(Al) and MIL-120(Al) with respect to the closest neighboring hydrogen (Figures S3 and S4 in the Supporting Information). It is clear from Table 2 that the lowest absolute values of interaction energies of iodine species are obtained with HKUST-1(Cu) in the dehydrated form followed by MIL-120(Al). This trend nicely agrees with the experimental work of Whitehead et al.,41 MOFs bearing aluminum appear as an efficient adsorbent for iodine uptake. The computed ΔEint of I2 and ICH3 in MIL-120(Al) are −64.1 and −50.4 kJ/mol, respectively, and the dispersion energy contributions to this quantity are significantly stronger −81.8 kJ/mol (I2) and −66.5 kJ/mol (ICH3). This effect is caused by an increased short-range repulsion between the molecular adsorbates and the MOF structure, showing the complexity of the adsorbate−substrate interactions in MIL-120(Al). For instance, I2 and ICH3 have larger interaction energies in MIL-53(Al) than in MIL-120(Al) and HKUST-1(Cu): the ΔEint are −111.2 kJ/mol (I2) and −94.3 (ICH3) and the dispersion energy contribution to this quantity is relatively small in absolute value for I2 (≈47 kJ/mol) and ICH3 (≈34 kJ/ mol). From the computed interaction energies, we conclude

Table 3. Calculated Interaction Energies (ΔEint) for I2 and ICH3 Molecules (kJ/mol) in the MIL-53 Seriesa MIL-53(Al) −H −Br −Cl −OH2 −NH2 −CH3 −NO2 −COOH

ΔEint(I2) (kJ/mol) −111.2 −99.8 −102.5 −103.1 −109.7 −103.4 −104.5 −96.2

(−47.1) (−54.3) (−50.4) (−48.4) (−51.2) (−53.1) (−54.5) (−61.6)

ΔEint(ICH3) (kJ/mol) −94.3 −56.5 −57.3 −61.3 −66.2 −61.3 −59.7 −59.7

(−34.6) (−44.4) (−39.2) (−37.3) (−41.1) (−42.5) (−42.8) (−49.9)

a

The contribution of dispersion interactions (ΔEdisp) (kJ/mol) to interaction energies is given in parentheses.

absolute value of the interaction energy for I2 is higher than that for ICH3 in the MIL-53(Al) series. The ΔEint for I2 (ICH3) ranges between −96 kJ/mol (−56 kJ/mol) and −111 kJ/mol (−94 kJ/mol). The dispersion energy contribution to this quantity is relatively large in absolute value for I2 (≈50 kJ/mol) and ICH 3 (≈40 kJ/mol). Interestingly, the calculated interaction energy of iodine species is almost unaffected by the nature of the functionalization with MIL-53(Al) discussed here. For I2, the difference between the interactions energies for the different groups in MIL-53(Al) is not very large, as it does not exceed 15 kJ/mol. This difference is slightly higher for ICH3 and it can reach 30 kJ/mol. Whatever the difference, the highest interaction energies for both I2 and ICH3 in the gas phase are obtained with the nonfunctionalized MIL-53(Al). Due the modest size of the MIL-53 pore, the substitution of the hydrogen by the other groups increase the steric repulsion, which cannot be overcome by the increase of the van der Waals interaction. This result is not in the line with the experimental work of Falaise et al.9 This outcome can be explained by the 25286


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Figure 3. Right: representation of the complexes (I2 and the terephthalate unit) extracted from the relaxed configurations with the respect of the periodic calculations. Left: representation of the different cluster models (the terephthalate unit) decorated by the different functionalization investigated here.

fact that the sorption of iodine was realized experimentally9 in solution (cyclohexane), which may prevent the transition between LP and NP and inhibit the absorption. Within DFT+D3 calculations, from various initial orientations of iodine species in the MIL-53(Al) series (for example, the I atom is in direct contact with the electron-donor group attached to the organic ligand), the corresponding molecules go toward the center of the pores during the structural relaxations, such that the molecule tends to maximize the long-range interactions and that the electron-donor groups do not play an important role on the adsorption behavior of iodine species. To provide further information about the interaction between iodine and MIL-53(Al) series for a better performance in the uptake of I2, high-quality correlated quantum mechanical calculations are needed. In this respect, the coupled-cluster method, CCSD(T),42 used most often as a benchmark, is one of the most accurate method. In this way, Rezac et al.43 have estimated the interaction energies of BrCH3, ICH3, BrCF3, and ICF3 with benzene at the basis set limit using CCSD(T) calculations. Indeed, recent theoretical work indicates that we must perform CCSD(T) calculations for studying the interactions of aromatic molecules.44−47 In the present work, the interaction of I2 with smaller cluster models decorated by the different functionalization as a good representative of the MIL-53(Al) structural building blocks was investigated using the CCSD(T) method (Figure 3). In this part, our calculations have been performed with the Gaussian 09 program.48 The counterpoise method has been used to determine the interaction energies and to avoid the basis set superposition error (BSSE). The calculations have been determined with 631G(d,p) atomic basis sets. Pseudopotential is used for iodine with the basis sets LanL2DZ, to include relativistic effects and to reduce the size of the calculations. In the present part, our complexes are composed of I2 and the terephthalate unit. The latter is the organic ligand of MIL-53(Al) decorated with the different substitutions. The corresponding complexes are extracted from the relaxed configurations with the respect to the periodic calculations (Figure 3). Interaction energies of I2 with terephthalate−X (X = −H, −Br, −Cl, −OH2, −NH2, −CH3, −NO2, and −COOH) are displayed in Figure 4 and the corresponding values can be found in Table S1 in the Supporting Information. One can see that the interaction energy potentials indicate that the

Figure 4. Interaction energies calculated with CCSD(T) for the I2− terephthalate(X) complex (X = −H, −Br, −Cl, −OH2, −NH2, −CH3, −NO2, and −COOH).

attraction decreases slowly with the different substitutions. The calculated interaction energies are rather small (do not exceed −4 kJ/mol) and they are practically zero from 5 Å for all the structures (Figure 4). At this stage, we can find that the functionalization does not present a significant effect on the adsorption behavior of the iodine species in MOFs. The obtained results agree well with the CCSD(T)/CBS calculations of Rezac et al.,43 as the interaction energy of the iodomethanebenzene complex is about −8 kJ/mol. All these results reflect a well-known fact that the long-range interactions (dispersion) are the primary source of the attraction. At this stage, one can expect that the short-range interactions in the MOF structures (charge transfer and exchange−repulsion), which appear at the distance when the molecular wave functions overlap significantly, do not play a prominent role on the interaction between iodine and terephthalate unit. This is in agreement with the previous results performed with periodic DFT+D3 calculations in MIL-53(Al): whatever the substitutions, (i) iodine species are adsorbed in the center of the pores and they do not have any preference over any terephthalate unit, (ii) more than 30% of the total interaction energies of iodine compounds is of long-range interaction 25287


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(ν CC) modes of the benzene unit, respectively, which are in good agreement with experimental values (see ref 10). The 1410 and 1620 cm−1 wavenumbers correspond to the symmetric and asymmetric stretching modes of the dicarboxylique entities linked to the benzene cycle and to aluminum. Finally, at higher wavenumbers, we can clearly distinguish two intense bands located at 3120 and 3710 cm−1. These bands are assigned to the CH and the hydroxyl (OH) groups stretching modes, respectively. For infrared spectra, theory provides an overall accurate reproduction of the experimental topologies, as the relative positions of all the peaks are well restored. Simulated infrared spectrum of I2 in the gas phase are presented in Figure 6. The I−I stretching mode is located at 201 cm−1. The infrared spectrum of ICH3 is composed of three characteristic bands located at 550, 1200, and 3120 cm−1 (Figure S5 in the Supporting Information). These bands represent the stretching and the bending vibrations of the I−C and the CH3 groups. Upon the adsorption of the iodine species, the I−I (I2) stretching mode is shifted from 201 cm−1 in the gas phase to 187 cm−1 (inset of Figure 6), which is in line with the experimental observations of Chebbi.25 Moreover, a significant perturbation in the hydroxyl region has been observed. In fact, the intensity of the OH band is reduced and shifted: it goes from 3710 cm−1 in the MIL-53(Al) to 3695 cm−1 in I2/MIL53(Al). In addition, it is worthwhile to mention the appearance of a new band located at 3630 cm−1 (Figure 6). The latter corresponds to an effective hydrogen bonding formed between I2 molecule and the hydroxyl group of MIL-53(Al). The same behavior is also observed in the case of ICH3 (Figure S5). To better understand this behavior, distances between iodine and OH group (r1 and r2) have been investigated (Figure 7 and Figure S6 in the Supporting Information). During the whole simulation time, the iodine species oscillate inside the pores of MIL-53(Al) and it forms an H-bonded complex for most of the time. As can be seen in Figure 7, for the variation of the I−OH distance (r1 and r2) during the simulation time, the formation of a hydrogen bond is involved at each minimum of the two curves. The lengths of the H-bonds are about 2.5 Å in

(section 3.1). Therefore, the van der Waals (dispersion) interactions can be considered as the decisive factor for the nature of the interaction between iodine and MOF structures. 3.3. Infrared Spectra of the MIL-53(Al). So far, the nonfunctionalized MIL-53(Al) exhibits the most negative interaction energies for both I2 (−111.2 kJ/mol) and ICH3 (−94.3 kJ/mol). To gain insight into the nature of interactions between the nonfunctionalized MIL-53(Al) and iodine species, infrared spectra have been simulated through molecular dynamics (MD) calculations at 373 K. Although our MD simulations were started from the relaxed configurations. The simulated infrared spectra of the LP form of MIL-53(Al) is displayed in Figure 5. The agreement between experimental

Figure 5. Simulated infrared spectrum of MIL-53(Al).

and theoretical spectrum is satisfactory.10,25 In particular, we have been interested in assigning the vibrational modes associated with the dominant spectral lines (Figure 5). Let us start from the low wavenumbers to the high wavenumbers. The stretching and the bending modes of the Al−O−Al group are observed in the range 325−490 cm−1. The 660 and 1350 cm−1 bands correspond to the twisting (γ CCC) and the stretching

Figure 6. Simulated infrared spectrum of I2 alone (left) and I2 in MIL-53(Al) (right). The I−I elongation extracted from the simulated infrared spectrum of I2 in MIL-53(Al) is given in the inset. 25288


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Figure 7. Time evolution of the parameters r1 and r2 with I2/MIL-53(Al) LP.

accurate CCSD(T) method. In that case, the calculated interaction energies of iodine are rather small and do not exceed −4 kJ/mol for all the substitutions. This shows that the long-range interaction (dispersion) is the decisive factor of the nature of the interaction between iodine and MOF structures and one can expect that the short-range interactions (chargetransfer and exchange−repulsion) do not play a prominent role on the interaction between iodine and MOF. In the second time, we have analyzed I2 and ICH3 in MIL-53(Al) structures by determining their infrared spectrum, which has been simulated through molecular dynamics calculations at 373 K. It follows from our results that an effective hydrogen bonding is formed between iodine molecules and the hydroxyl group of MIL-53(Al) and is located at 3630 cm−1. This work represents a further step in our systematic effort to study the interaction between iodine species and MOF structures. In the future work, we plan to extend this study using different MOF structures, which have been proposed and tested on the iodine trapping in recent experimental work.

I2/MIL-53(Al) and 2 Å in ICH3/MIL-53(Al). The present result is in line with the CCSD(T) calculations (section 3.2), as the charge-transfer and exchange−repulsion interactions do not play a prominent role on the interaction between iodine and MOF structures.

4. CONCLUSIONS For the first time, adsorption properties of I2 and ICH3 in MIL53(Al), MIL-120(Al), and HKUST-1(Cu) have been investigated by using periodic DFT simulations. The activation of MIL-53(Al) and MIL-120(Al) materials by the removal of water molecules is crucial to have enough space to adsorb the molecules of interest. However, in the case of HKUST-1(Cu), water molecules are located in the axial positions of the Cu cluster and they do not obstruct the adsorption of iodine species. We have first focused on the hydrated and dehydrated HKUST-1(Cu) structures. It turns out that the interaction energies of I2 and ICH3 are higher in the hydrated form of HKUST-1(Cu) than in the dehydrated one, especially in the smaller cage. I2 and ICH3 located in the small cage strongly interact with water molecules where stronger hydrogen bonds are formed. In the larger cage of HKUST-1(Cu), due to a longer distance between iodine species and the framework (metal cluster and organic linkers), lowest interaction energies of iodine species are noted. The obtained results with the hydrated HKUST-1(Cu) material are promising for safety nuclear applications due to the presence of a large amount of water in the nuclear containment. It follows from our results that iodine species are strongly adsorbed in MIL-53(Al) than in MIL-120(Al) and HKUST-1(Cu) MOFs and therefore this material could potentially trap iodine compounds. The calculated interaction energies are −111.2 kJ/mol (I2) and −94.3 kJ/mol (ICH3) in MIL-53(Al). Moreover, more than 30% of the total interaction energies of iodine compounds are of long-range (dispersion) interaction. We studied the effect of the functionalization (−Br, −Cl, −OH, −NH2, −CH3, −NO2, − COOH) of the MIL-53(Al) organic linkers on the adsorption behavior of iodine, and it has been seen that the substitutions do not present a significant effect for this purpose. To provide further information about the interaction between iodine and MIL-53(Al) series for a better understanding, we have performed for the first time, calculations with the more

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08903. Relaxed structures of iodine species over HKUST-1 (Cu), MIL-53(Al), and MIL-120(Al) structures; interaction energies calculated with CCSD(T) for the complexe I2−terephthalate(X); simulated infrared spectrum of ICH3 in MIL-53(Al); time evolution of r1 and r2; POSCAR files (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*S. Chibani. E-mail: [email protected]. *J.-F. Paul. E-mail: [email protected]. ORCID

Jean-François Paul: 0000-0003-1935-1428 Notes

The authors declare no competing financial interest. 25289


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ACKNOWLEDGMENTS S.C. thanks the National Research Agency (ANR, ANR-11RSNR-0013-01) for her postdoctoral grant. This work has been supported by the French State under the program “Investissements d’Avenir” MiRE managed by the ANR under grant agreement. This research used resources of the Centre de Ressources Informatiques (CRI) supported by the University of Lille1, CPER Nord-Pas-de-Calais/FEDER, France Grille, and CNRS. This work was performed in the frame of the international collaboration agreement between IRSN and University of Lille 1. The authors thank also Dr. B. Azambre (University of Lorraine), Dr. T. Loiseau (University of Lille1), and Pr. C. Volkringer (University of Lille1) for fruitful discussions.

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Capture of Iodine Species in MIL-53(Al), MIL-120(Al), and HKUST-1

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