Adsorption of Acetone Vapor by Cu-BTC: An Experimental and

Article pubs.acs.org/JPCC

Adsorption of Acetone Vapor by Cu-BTC: An Experimental and Computational Study Thibault Terencio, Francesco Di Renzo, Dorothée Berthomieu, and Philippe Trens* Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-UM2-ENSCM-UM1, ENSCM, 8 Rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France S Supporting Information *

ABSTRACT: We report an experimental and theoretical study of acetone adsorption in the metal−organic framework (MOF) compound Cu-BTC. The isosteric heat of adsorption could be derived experimentally and was found to be −60 kJ mol−1. This value matches the theoretical data obtained by DFT-based methods at zero coverage. In situ DRIFT measurements allowed us to precisely describe the adsorption steps from zero coverage to saturation. Two main adsorption sites were determined for the adsorption of acetone. The small cavities were found to interact through van der Waals interaction with acetone, while the Cu(II) site was found to interact with the carbonyl function of acetone. On the basis of the in situ infrared experiments, it was demonstrated that the small cavities were first in interaction with acetone. DFT proved consistent with these findings by giving the energy of interaction in the different sites explored but also by providing calculated infrared spectra of adsorbed acetone in Cu-BTC. Using acetone as a probe allowed showing that dispersive interactions with the pore sites of the Cu-BTC can be dominant among all other interactions. Additionally, the adsorption of acetone in Cu-BTC proved not fully reversible unless exposed to atmospheric moisture.

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INTRODUCTION Metal−organic frameworks (MOFs) are now a well-known class of porous materials which have attracted increasing interest, due to the tunability of the properties of these materials.1,2 Compared to mineral zeolites or porous carbons, MOFs have specific features including the possibility to obtain flexible structures under stress or very high surface areas.3−5 Furthermore, the possibility to vary the inorganic centers as well as the organic ligands6,7 opens the way to a wide range of applications such as gas storage, 8,9 sensors, 10,11 drug delivery,12,13 adsorption,8,14,15 heterogeneous catalysis,16−18 and separation.8,19−21 It is therefore possible to choose the organic ligand for enhancing or disabling the interaction with a host.14,22 On the other hand, the inorganic centers may also be chosen to develop an interaction with a specific host.23 Indeed, if many MOFs have saturated inorganic centers, others have coordinatively unsaturated site metal (CUS). This feature provides additional sites which may interact with host species. It is the case of Cu-BTC (also named as HKUST-1) which possesses unsaturated copper centers.3 This MOF contains copper carboxylate paddlewheel dimers, [Cu2(COO)4−(H2O)2], with one apical water ligand on each copper moiety which allows a coordinative unsaturation on the metal center when evacuated by outgassing. This is one of the reasons why this MOF has attracted much interest in the past decade. In the literature, many studies were focused on its catalytic properties,16,17 for instance, in the case of the © 2013 American Chemical Society

heterogeneous oxidation of hydrocarbons or the cyanosilylation of aldehydes.24,25 Interestingly, in ref 25, Kaskel et al. suggested that Cu-BTC could not be recovered after reaction without damaging the crystalline structure of Cu-BTC. This is a very interesting point when thinking of removing traces of polar hydrocarbons, from airstreams for instance. In fact, the ability of Cu-BTC to adsorb gases or vapors of various polarities has already been studied many times. According to the literature, polar or polarizable molecules such as NO and CO,26,27 CO2,9,28 water,22,29,30 or alkenes31,32 preferentially coordinate to the copper ions of Cu-BTC. During the sorption of polarizable molecules such as CO, Szanyi et al. suggested the occurrence of redox reactions involving Cu2+ species.27 They also suggested that the adsorption phenomenon could lead to a so-called Cu2+/Cu+ interchange. On the other hand, apolar molecules such as alkanes preferentially adsorb on sites where van der Waals interactions with Cu-BTC are maximized, that is, the small cages of Cu-BTC.31,33 Some quantification of these interactions is available in the literature. Most of the data have been obtained by using the isosteric method, either after modeling the adsorption isotherms33,34 or experimentally.35,37 Some others have been provided by microcalorimetric measurements.9 Received: October 13, 2013 Revised: November 25, 2013 Published: November 25, 2013 26156

dx.doi.org/10.1021/jp410152p | J. Phys. Chem. C 2013, 117, 26156−26165

The Journal of Physical Chemistry C

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Figure 1. Cluster model (left) and periodic model (right). Small green balls are copper; red parts are oxygen from carboxylates; gray parts are carbon atoms; and the hydrogen atoms are in white. The big green ball is a type 1 cage; the big parts of the yellow balls are a type 3 cage; and the medium magenta ball is a type 2 cage.

Figure 2. Different views of the seven sites selected for the modeling of adsorption sites of acetone. For simplicity, the extended paddlewheel cluster is used. The notation used is TxN with T for type (window or cage), N for cage number, and x for site index in each cage.

involve a solvent exchange activation. This would prevent any damage of the crystal structure during the solvent removal at the end of the syntheses. In our study, a commercial Cu-BTC is used. This material, provided by Sigma-Aldrich, has been prepared, including this solvent exchange. We mentioned that the adsorption of CO and CO2 on CuBTC was already studied.9,26 Alkanes were also studied; however, very few studies were focused on the adsorption of ketones or aldehydes on Cu-BTC.25 Sorbates having both a nonpolar hydrocarbon side and a polar carbonyl function clearly represent interesting candidates. We decided to focus on one of the most typical of these compounds, which is acetone. The current work focuses on the adsorption of acetone by Cu-BTC. As far as we know, this system has not been investigated yet. Our major objective is to gain insights into the adsorption sites of acetone in the cavities of this MOF. We also aim at clarifying the interaction involved at low pressure. Our study uses a combination of quantum chemical calculations by DFT and experiments by means of gravimetry of adsorption and in situ spectroscopy.

In the case of polarizable systems, the enthalpy of adsorption at zero coverage was calculated to be in the range between −17 and −35 kJ mol−1 (adsorption of CO and ethylene, respectively).34 In the case of apolar systems, the enthalpy of adsorption was located between −16.5 kJ mol−1 for methane and −60 kJ mol−1 for isobutane.33,37 This influence of the alkyl chain length is consistent with former investigations related to the adsorption of alkanes in other MOFs such as modified MIL-53(Cr, Fe)7,38,39 or MIL-47(V).38 The van der Waals interaction was shown to increase proportionally with the alkyl chain length of the sorbates. There is therefore a wide range of adsorption enthalpies. This depends not only on the polarity of the guests but also on the host adsorption sites providing or not dispersive interactions. Furthermore, different enthalpy profiles were obtained, depending on the system of interest. In the case of polar systems, rather constant enthalpies were found with increasing coverage, whereas an apolar system led to u-shape profiles. The flat enthalpy profiles were classically attributed to a rather homogeneous surface with mild interaction,9 whereas the ushape profiles proved difficult to explain.31,36 In many of the experimental or theoretical studies dealing with adsorption by Cu-BTC, type I or pseudo type I adsorption isotherms were obtained.31,33 Further, clearly defined saturation plateaus were also obtained. However, the total pore volume obtained very scarcely approached the theoretical pore volume of Cu-BTC. This discrepancy was recently argued by Yang et al.40 These authors suggested that the synthesis route should

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EXPERIMENTAL SECTION Methods. Theoretical Calculations. Periodic Models. Each Cu-BTC unit cell (u.c.), belonging to the Fm3̅m space group, contains 624 atoms: 48 copper atoms, 192 oxygen atoms, 288 carbon atoms, and 96 hydrogen atoms. To save computational time, a rhombohedric primitive unit cell of Cu12O48C72H24 formula was used.30 The optimized cell parameters were a = b = 26157



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c = 18.708 Å; the angles were α = β = γ = 60°; and the cell volume was 4629.82 Å3. These optimized lattice parameters were fixed in all further periodic structures of the primitive unit cell Cu-BTC containing acetone as well as for isolated acetone. A series of different positions for adsorption of acetone in all the different cages of the unit cell were systematically investigated. Seven positions displaying the lowest energies were selected. These positions involve two large cages, one small cage, copper(II), and the windows connecting the cages (Figures 1 and 2). Sites are labeled as TxN with T for type (window or cage), N for a number indicating the type of cage, and x for site index in each cage (Figure 2). For instance, C11 refers to the large type 1 cage without copper. Cu-BTC presents two types of large cubooctahedral cages communicating through 9 Å square windows. They are indicated in this paper as cage type 1 (without copper cations) and type 2 (with copper cations). Smaller truncated tetrahedral cages (sometimes indicated in the literature as octahedral despite their true symmetry) are opened on the large cages of type 2 through 3.5 Å windows and are indicated here as cages of type 3. Cluster Models. A cluster model with an extended ″paddlewheel″ structure with Cu2O8C28H20 formula was cut in the optimized primitive u.c. structure (see above). Dangling bonds were saturated with H atoms. It contains two copper atoms and four trimesic ligands (Figure 1). This cluster model is much larger than the usual reported “paddlewheel” structure, for instance in ref 30. Another cluster model was considered: the [acetone··· trimesic acid] complex in which one acetone molecule is in interaction with one trimesic acid molecule. Calculation Methods. Periodic structures were optimized using the PBE exchange-correlation functional,41,42 the VASP program, and the projector-augmented wave approximation (PAW) of Blöchl.43 All calculations were done using a cutoff of plane waves of 600 eV, sampling only the gamma-point of the Brillouin zone as reported in recent studies.9 The cell parameters were optimized using the PBE method. Each copper has a d9 electronic structure and thus possesses an unpaired electron, allowing different magnetic states, high spin states (HS), and low spin states (LS). The Cu-BTC structure was optimized constraining the spin state to the highest HS. The electronic structure of the whole Cu-BTC u.c. contains 12 Cu(II), each having a high spin. Thus, a triplet state electronic configuration was considered for each Cu pair. The electronic structure of the whole Cu-BTC u.c. contains six paired Cu(II), each having an antiferromagnetic electronic configuration (singlet low spin state). This structure was calculated to be only 48 kJ mol−1 more stable than the structure containing Cu(II), each having a triplet state electronic configuration. Because this energy difference is relatively small, only the HS structure was considered. This result is supported by recent calculations using a paddlewheel cluster and two water molecules adsorbed on the paddlewheel cluster.30 It was shown that the HS−LS gap remains relatively unchanged. We calculated similar behavior between HS and LS when acetone interacts with Cu-BTC. All the atoms in the u.c. were allowed to relax to minimize both the forces on the atoms and the pressure on the unit cell containing acetone, while the cell parameters were fixed. The dispersion and force-field parameters available in the VASP program named as the DFT-D2 method of Grimme were

considered for both Cu-BTC and [Cu-BTC···acetone] structures.44 Partial frequency calculations were performed using the finite differences method in the harmonic approximation. Only frequency calculations of the acetone atoms were considered. All the calculations of the molecular models were performed using the Gaussian09 program. 45 Along the geometry optimization processes, all the C atoms of the Cu-BTC and [Cu-BTC···acetone] clusters were frozen. Different methods and basis sets were used including MP2. The B3LYP, ωB97, and ωB97X-D functionals were used with the 6-31+G(d) and 6-311++G(d,p) basis set. Dispersion correction of Grimme was included along the B3LYP calculation and was named as B3LYP-D2.44 The energies were not BSSE corrected. Adsorption Gravimetric Experiments. The adsorption experiments have been performed using a purpose-built adsorption apparatus already described.46,47 This setup is based on gravimetric measurements, using a magnetic compensation balance provided by SETARAM, with a resolution better than 0.05 μg. The pressure in the sample cell can be recorded by two capacitive pressure gauges (0−10 Torr and 0−1000 Torr). Before adsorption, the sample cell can undergo a thermal treatment up to 500 K (Cu-BTC is thermostable at 510 K)3 under a vacuum of 10−4 Torr. This adsorption setup allows for the choice of the pressure of the adsorbate to be introduced (instead of the equilibrium pressure). Vapor adsorption was performed at 313 K with a thermal stability of the sample better than 0.1 K. For each point on the adsorption isotherm obtained, the system was considered to have reached a state of thermodynamic equilibrium when the mass did not vary over a period of at least 600 s. Allowing longer equilibration times gave the same sorption isotherms, thus validating the choice of 600 s as a equilibrium criterion. The exact time to reach equilibrium depended on the relative vapor pressure considered. Adsorption−Desorption Followed by Spectroscopy. Diffuse reflectance infrared fourier transform experiments (DRIFT) have been performed using an IFS55 Bruker spectrometer (MCT cryodetector) with a 2 cm−1 resolution in a spectral domain 6000−400 cm−1 with a Thermo SpectraTech DRIFT environmental cell. The obtained spectra were transformed using the Kubelka−Munk transform equation. The Kubelka−Munk theory is generally used for the analysis of diffuse reflectance spectra obtained from weakly absorbing samples. It provides a correlation between reflectance and concentration. The relationship is as follows F (R ) =

(1 − R )2 2R

(1)

where F(R) is the Kubelka−Munk function and R is the reflectance. A vapor saturator was connected to the DRIFT accessory to introduce the acetone vapor onto Cu-BTC. The carrier gas used was nitrogen which does not adsorb on Cu-BTC at the experimental temperature. Along with adjustable nitrogen flow, the saturator placed at different temperatures allowed us to introduce controlled partial pressures of acetone (calculated using the empirical Antoine’s equation). The temperature range used for obtaining these partial pressures was between 180 and 273 K. The lower limit of this temperature range allowed for very low partial pressure of acetone to be introduced. In these particular pressure conditions, the very first adsorption sites of Cu-BTC could be probed. The sample cell was kept at 303 K 26158



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during the adsorption measurements. The desorption was performed by increasing the temperature of the DRIFT environmental cell up to 403 at 2 K min−1. This desorption temperature was chosen for desorbing most of the adsorbed acetone species at a temperature compatible with the thermal stability of Cu-BTC. During the desorption of acetone, the sample was subjected to different vapor compositions: (i) a nitrogen flow of 20 cm3 min−1 and (ii) an acetone-saturated vapor flow of 20 cm3 min−1. It has already been shown that benzaldehyde may be irreversibly adsorbed on Cu-BTC, Cu-BTC starting to decompose with some benzaldehyde molecules adsorbed on the most active sites of Cu-BTC.25 We will show that acetone does not exhibit the same affinity for Cu-BTC. A blank spectroscopic spectrum was obtained by replacing the crucible containing Cu-BTC with a mirror. Since the surface of the mirror is very limited, this is a way to measure the infrared absorption of acetone in the vapor phase contained in the DRIFT environmental cell. This blank experiment showed that in our experimental conditions (in terms of partial pressure of acetone) the acetone in the vapor phase could not be detected. Materials. Acetone used as adsorbate (provided by Aldrich, purity >99.9%) was stored over an activated 3 Å molecular sieve. Cu-BTC was purchased from Sigma-Aldrich (Basolite C300) and used as received. Even though this porous material is widely known, it is worth noting its structure since adsorption may take place in some specific sites. Cu-BTC is made of dimeric clusters of copper coordinated with the three carboxylate groups of benzene tricarboxylic acid. In the three-dimensional porous structure Cu-BTC, three types of cages can be distinguished. The first type of cage is called ″side pockets″. These small cavities are characterized by a window opening of 4.6 Å. Arranging eight side pockets at the corners of a cube generates two different large cages with internal diameters of 10 and 12 Å. The window opening between the large cages is 6.5 Å in diameter. Upon heating CuBTC, the axial ligand on the copper dimer occupied by a water molecule can be removed. This heating treatment therefore allows for the generation of a coordinatively unsaturated site (CUS). Before nitrogen adsorption measurements, Cu-BTC was dehydrated at 423 K under vacuum for eight hours. The stability of the dehydrated form of Cu-BTC was proved stable under vacuum by infrared spectroscopy (not shown here). After this outgassing stage, its specific surface area was determined by nitrogen adsorption at 77 K using a commercial apparatus (Micromeritics 2010). The Langmuir surface area was found to be 1560 m2 g−1. Powder X-ray diffraction revealed a crystalline solid typical for this material (see later).3 Results. Experimental Section. The amount of acetone adsorbed on Cu-BTC was measured by gravimetry at three different temperatures: 303, 313, and 323 K. Before adsorption, dehydration at 423 K under vacuum brought about a change of color of Cu-BTC from turquoise to deep blue, indicating a successful decrease of the coordination of copper ions. The adsorption isotherms at different temperatures, reported in Figure 3, are of pseudo type I, corresponding to the adsorption in a microporous material. A simple Langmuir model does not provide a satisfactory fit of the adsorption isotherms. In fact, the adsorption isotherms can be correctly fitted by a two-site composite Langmuir model.

Figure 3. Gravimetric adsorption isotherms of acetone in Cu-BTC at 303 K (triangles), 313 K (diamonds), and 323 K (squares). The curves have been obtained by a two-site Langmuir model.

The two-site composite Langmuir model can be written as follows Cads,1 Csat,1

=

b1p1 2

1 + ∑ j = 1 bjpj

(2)

where Cads,1 is the surface concentration on one site and Csat,1 the saturation concentration on this site. b1 stands for the rate constant for adsorption to that for desorption on the first site, and pj is the pressure of species j.48 At 303 K, the adsorption at saturation corresponds to 108 molecules per unit cell. This value exactly matches the number of molecules of liquid acetone (density 0.79) which can fill the volume of a cell of Cu-BTC with the largest experimental pore volume reported in the literature: 0.823 cm3 g−1.40 The experimental setup used for the gravimetric experiments did not provide reliable results below p/p0 = 5 × 10−4. Hence, gravimetry did not provide significant information on the first steps of adsorption. However, the two-site Langmuir fit suggests that nearly 60% of the adsorption takes place with a Langmuir constant (relative pressure at half coverage) near 1 × 10−4. The remaining adsorption takes place with a Langmuir constant near 7 × 10−3. From the isotherms at 303, 313, and 323 K, the adsorption enthalpy calculated from the Clausius− Clapeyron equation is −60 ± 1 kJ mol−1 in the domain from 63 to 85% of the saturation coverage. As seen in Figure 4, adsorption of acetone at low pressure leads to the appearance of one CO stretching mode, at 1711 cm−1. At relative pressure of acetone higher than 2 × 10−6, new and intense bands appear at 1708 and 1704 cm−1, accompanied by a less intense band at 1698 cm−1. All these bands are at lower energies (frequencies) by comparison with the 1740 cm−1 band of acetone in the gas phase and result from the formation of new species. The formation of new species as the pressure increases would result from a different kind of adsorbed acetone on the Cu-BTC, in cages and on the copper site. The bands at the lower energy appearing at the lowest pressure indicate that while the sites exhibit the highest energy of adsorption the perturbation of the CO bond of adsorbed acetone is smaller. As it seems logical to correlate the strongest perturbation of the carbonyl bond with the interaction with the Cu(II) sites, the most likely interpretation of the CO stretching at 1711 cm−1 is the attribution to acetone adsorbed in the small truncated tetrahedral cages, viz., the sites in which acetone can be more tightly confined. The vibrations at 1708 26159



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Figure 4. FTIR spectra in the region of CO stretching of acetone. Cu-BTC activated (a), Cu-BTC equilibrated at 303 K with an acetone partial pressure of 1.9 × 10−6 (b), 2.1 × 10−6 (c), 3.3 × 10−6 (d), 9.3 × 10−6 (e), 3.5 × 10−5 (f), 2.5 × 10−4 (g), 3.5 × 10−4 (h), 7 × 10−3 (i), 2.6 × 10−2 (j), and 2.7 × 10−1 (k), after desorption under N2 flow (l), and after equilibration with room atmosphere (m).

and 1704 cm−1 would correspond to the interaction with copper sites, with a stronger perturbation of the carbonyl bond of acetone. Beyond relative pressure 0.01 (j and k curves in Figure 4), new intense bands appear at higher energy (1712 and 1720 cm−1) or lower energy (1693 cm−1) as compared to the previous bands. The bands at the highest wavenumbers probably correspond to acetone filling the large cages without copper sites. The band at 1693 cm−1 can be tentatively attributed to cooperative interactions with the copper sites. Spectra at relative pressure 2.6 × 10−2 and 2.7 × 10−1 are virtually identical, and they are accompanied by the appearance of incipient acetone gas bands near 1740 cm−1, indicating that the microporous adsorbent has been saturated at a relative pressure lower than 0.02. The reversibility of the adsorption has been monitored by desorption overnight in situ at 403 K under N2 flow. Some acetone is still retained in these conditions, as indicated by residual bands at 1708 and 1693 cm−1, suggesting that the desorption is still out of equilibrium. The retained amount roughly corresponds to the amount adsorbed at p/p0 = 3 × 10−6. All acetone is removed upon exposure at room atmosphere, suggesting that residual acetone has been exchanged for water at the room relative humidity. It can be observed that neither the parent sample nor the hydrated sample present the absorption band at 1707 cm−1 observed in other commercial samples and attributed to free hydroxyls.49 A quantitative exploitation of the Kubelka−Munk transform is usually poorly reliable at high coverage in a porous material, when the scattering of the materials is likely to be modified. However, an attempt at a spectroscopic isotherm has been carried out by integration of the Kubelka−Munk transform between 1685 and 1735 cm−1 and normalization of the results on the amount adsorbed at the highest pressure attained. The results, reported in Figure 5, are in good agreement with the adsorption on a microporous solid. However, there is a limited number of available data for determining this spectroscopic adsorption isotherm. The phase stability of the sample upon cycles of activation− adsorption−desorption−rehydration was monitored by powder X-ray diffraction (Figure S1, Supporting Information). The parameter of the cubic face-centered cell remained stable at 2.633 nm, and no bands corresponding to phases different from Cu-BTC were observed. However, some line broadening was observed. The size of the coherent diffraction domain as

Figure 5. Spectroscopic isotherm of the adsorption of acetone on CuBTC at 303 K and based on the integration of the Kubelka−Munk transform between 1685 and 1735 cm−1. The line is only a guide for the eyes.

evaluated by Williamson−Hall plots decreased from more than 100 nm to about 50 nm.50

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RESULTS Theoretical Section. Acetone Adsorption on Copper (C12). The “paddlewheel” cluster was first considered to study the interaction of acetone at the metal site. This cluster allows us to investigate different DFT methods using the paddlewheel cluster with acetone adsorbed on Cu(II): B3LYP, CAMB3LYP, ωB97, ωB97XD (Table 1). The last three functionals are longrange corrected functionals: CAMB3LYP uses the Coulomb Attenuated Method,51 while ωB97XD uses an additional empirical dispersion term.52 As shown in Table 1, the CuBTC geometry parameters depend on the DFT methods used. It can be further observed that the geometry of Cu-BTC changes upon acetone adsorption. Adsorption of acetone on a copper site leads to an orientation of the acetone molecules regardless of the methods used. However, it clearly appears that the geometry parameters of the [Cu-BTC···acetone] depend on the method. Furthermore, the inclusion of a dispersion term also induces structural changes even if the basis sets are the same. The adsorption energies are reported in Table 2. The inclusion of long-range correction and dispersion forces using CAMB3LYP, ωB97, and ωB97XD significantly modifies the 26160



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Table 1. Geometry Parameters of the Cu-BTC Cluster Model before and after Acetone Adsorption on Cu1(II) Using DFT Methods and the 6-31+G(d,p) Basis Seta system

paddlewheel

acetone + paddlewheel

functionalb

UB3LYP

CAMB3LYP

ωB97

ωB97XD

UB3LYP

CAMB3LYP

ωB97

ωB97XD

d(Cu1−Otrimesic) d(Cu2−Otrimesic) d(Cu1−Cu2) A(O−Cu1−O) A(O−Cu2−O) d(Cu1−Oacetone) A(Cu1−Oacetone−Cacetone)

1.969 1.969 2.514 172.4 172.4 / /

1.959 1.959 2.502 172.2 172.2 / /

1.961 1.961 2.532 171.7 171.7 / /

1.964 1.964 2.519 171.8 171.8 / /

1.964 1.991 2.583 166.1 174.7 2.249 135.4

1.955 1.983 2.572 165.8 174.7 2.190 134.3

1.957 1.985 2.600 165.4 174.2 2.195 130.6

1.960 1.987 2.587 165.5 174.4 2.193 131.8

a In the Cu-BTC cluster model, one acetone was adsorbed on Cu1 of the pair of Cu atoms. The second Cu atom is labeled Cu2. bd(X−Y) is for distance in Å, and A(X−Y−Z) is for angles in degrees.

[acetone···trimesic acid] complex was located on the potential energy surface (PES) using B3LYP/6-31+G(d). In contrast, as shown in Figure 6, one minimum corresponding to an [acetone···trimesic acid] complex was located using MP2/6-31+G(d) (Figure 6). Similarly, the use of B3LYP-D2/6-31+G(d) leads to a minimum with a very similar structure (Figure 6). These preliminary investigations highlight the prominent role of dispersion in such complexes. These results clearly indicate that DFT methods taking into account dispersion interactions are required to accurately model the adsorption of molecules in MOF cages and windows. Acetone Adsorption in Cu-BTC. Structure parameters of the seven selected complexes resulting from the acetone adsorption in the Cu-BTC cages, windows, and the metal site are reported in Tables S2 and S3 (Supporting Information). To have structural information about changes occurring upon adsorption, average distances for each type of bond reflect the general changes in structure. The Cu-BTC structure is only slightly modified upon acetone adsorption in Cu-BTC cages and windows. The largest changes, 0.023 and 0.011 Å, are calculated for the Cu−Cu and COacetone, respectively, when acetone is adsorbed on the copper sites. Calculated adsorption energies range between −21 and −62 kJ mol−1. According to the energy values reported in Table 3,

Table 2. Adsorption Energies Calculated for One Acetone Molecule at Cu(II) of the Paddlewheel Complex Using the 6-31+G(d,p) Basis Set and Four Functionalsa adsorption energy/kJ mol−1

UB3LYP

CAMB3LYP

ωB97

ωB97XD

−36

−50

−65

−66

Adsorption energy calculated as the difference E(Cu‑BTC+acetone) − (Eacetone + ECu‑BTC). a

adsorption energy values. Indeed, it is about twice larger using the ωB97XD than using the B3LYP method. These investigations indicate that inclusion of dispersion plays a significant role to model the adsorption of acetone on the copper site. This result is in line with the modeling of water adsorption at the copper site in Cu-BTC.30 Because large energy differences are calculated depending on the method employed, additional investigations would be required for accurately predicting acetone adsorption energy at the copper site in Cu-BTC. Using the PBE-D2 method with PAW plane waves and a periodic model, an adsorption energy of −62 kJ mol−1 was calculated. This energy value is in the range of the calculated values of −66 and −65 kJ mol−1 obtained when using the ωB97 methods and a cluster model. Since these values are not BSSE corrected, they are slightly overestimated. The value of −62 kJ mol−1 is slightly larger than the recent calculated energy value of water adsorption of −46 ± 6 kJ mol−1 on copper at zero coverage.29 Acetone Adsorption on Trimesic Acid. Before any investigation of the formation of complexes corresponding to adsorption of acetone in MOF cages, the adsorption of acetone with one trimesic acid only was investigated. No stable

Table 3. Calculated Adsorption Energies of Acetone in the Seven Sites of Periodic Cu-BTC Using PBE-D2a site EAds/kJ mol

−1

C11

C21

W11−2

W21−2

W12−3

C13

C12

−21

−29

−25

−33

−38

−51

−62

Adsorption energy calculated as the difference E(Cu‑BTC+acetone) − (Eacetone + ECu‑BTC) for the different sites described above. a

Figure 6. [Acetone···trimesic acid] complex optimized using MP2/6-31+G(d) and B3LYP-D2/6-31+G(d) methods. Bond lengths are in Å. Carbon in blue, oxygen in red, hydrogen in white. 26161



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Table 4. νCO Stretching Frequencies Calculated for Acetone Adsorbed in Seven Selected Sites of the Cu-BTC Using the PBE +D2 site CO frequency/cm dCO/Å

−1

isolated acetone

C11

C21

1 W1−2

W21−2

W11−3

C13

C12

1725 1.225

1723 1.226

1716 1.227

1717 1.227

1716 1.226

1719 1.226

1713 1.228

1687 1.236

of Cu-BTC, 0.823 cm3 g−1.40 It is interesting to point out that the earliest measurements of pore volume of Cu-BTC indicated values of 0.34 cm3 g−1,3,35 corresponding to severely degraded samples. Indeed, the full achievement of the theoretical pore volume of Cu-BTC has only been obtained by overcoming the poor stability of the structure in aqueous media22,54 by the use of alternative solvents.40 Only one molecule of acetone can enter each of the eight small type 3 cages of a unit cell, suggesting a maximum capacity of eight molecules per cell in the small truncated tetrahedral cages. This suggests that about 100 acetone molecules share the remaining microporosity, significantly less confined. This larger porosity of Cu-BTC corresponds to the two types of communicating large cubooctahedral cages. The orientation of the Cu-centered paddlewheel carboxylate complexes which form the structure is such that metal sites are only accessible from the cages of type 2. Adsorption of one molecule of acetone per copper site would lead to the adsorption of 48 molecules of acetone per cell, corresponding to about half the amount of acetone expected to be adsorbed in the whole system of the largest cages, very close to the total occupation of the type 2 cages. The DFT-D2 calculations allow attributing the absorption bands observed by in situ DRIFT. The band observed at 1711 cm−1 is the first one to grow while increasing relative pressure, thus adsorption on this site is the most favorable for acetone. DFT-D2 calculation predicts that the C13 site presents one of the strongest energies, and vibrational calculation on this site gives also a frequency of 1712 cm−1 for the CO stretching mode. Additionally, this site is also rapidly saturated when increasing relative pressure, indicating that only a few acetone molecules can adsorb onto this type of site, in good agreement with the limited number of molecules which can enter the truncated tetrahedral cages. Moreover, among sites possessing the highest affinity for acetone (i.e., first absorption bands to appear when increasing relative pressure), it is logical that the CO stretching of acetone in this site of lower polarity is less shifted than acetone on Cu(II) sites of higher polarity. Absorption bands at 1708, 1704, and 1698 cm−1 appear simultaneously at a slightly higher relative pressure, suggesting an energy of adsorption quite close to that corresponding to the adsorption at 1711 cm−1. In DFT-D2 calculations, the only adsorption site at a similar energy to C13 is C12, i.e., acetone coordinated to Cu(II). The spectroscopic isotherm of Figure 5 suggests that the saturation of the bands between 1708 and 1698 cm−1 (Figure 4) corresponds to about 60% of the maximum adsorbed amount. This value is in close agreement with the expected adsorbed amount for complete adsorption on the Cu(II) sites after saturation of the type 3 small cages. Hence both the calculated adsorption energy and the amount adsorbed suggest that the band region 1708−1698 cm−1 can be attributed to the interaction of acetone with Cu(II) sites. The multiplicity of bands in this region can be easily explained by multimolecular interactions, either on a single Cu(II) or on the opposite sites of a copper cluster.55 In our vibrational calculations, single acetone molecules coordinated to Cu(II)

adsorption at metal site C12 leads to the largest adsorption energy of −62 kJ mol−1. Nevertheless, adsorption of acetone in the type 3 truncated tetrahedral cage C13 is only 11 kJ mol−1 less stable than adsorption at the metal site. Minima were also located in windows suggesting that adsorption of acetone may also occur in the window sites as the acetone pressure increases. The νCO stretching frequency values of acetone adsorbed in the seven sites were calculated using the PBE+D2 method (Table 4). As seen in Table 4, the νCO stretching frequency for isolated acetone was calculated to be 1725 cm−1. Upon acetone adsorption in the seven different sites the νCO frequencies are red-shifted, the largest one corresponding to acetone adsorbed at the copper site. The calculated νCO stretching value of acetone in the six cages ranges between 1713 and 1723 cm−1. These values show that in the cages and windows the νCO frequencies of acetone change. The red-shift depends not only on the location of acetone in the cages and windows but also on its orientation in the Cu-BTC.

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DISCUSSION The adsorption on Cu-BTC is a controversial matter, as different evaluations have been proposed not only about the specificity of adsorption on different sites but also about the total amount of adsorbate which can enter the microporosity.3,35,40,53 This topic is strictly related to the stability of the adsorbent through activation−adsorption−desorption cycles.25,54 The comparison of experimental results and modeling of the adsorption of a polar molecule like acetone can shed some light on the interaction existing between the adsorbates and the different sites of the coordination polymer. Acetone is a highly polar molecule and aprotic. It is therefore an especially interesting probe for measuring the adsorption on Cu-BTC, as it can interact with both the metal site and the organic linker. The oxygen of the carbonyl group can donate electrons to the copper cation, while the methyl groups can interact with the aromatic rings of the ligands. Cu-BTC has a quite peculiar structure, as copper cation pairs are coordinated to the organic linkers of the MOF framework with a free position available on each copper to reach a 6-fold coordination. The sphere of coordination of the metal ions is completed by less strongly bonded hydrotropes, whose desorption provides an easily accessible copper site. The accessibility of the metal site is further improved by the geometry of the framework of Cu-BTC, as all copper cations decorate the surface of the large type 2 cages, easily accessible through the tridimensional network of large pores. Cu-BTC also presents small truncated tetrahedral cages which are opened on the large cages through windows of 3.5 Å. Steric and compositional considerations provide preliminary information on the adsorption capacity of the different kinds of cages. The saturation of the porosity of Cu-BTC with acetone corresponds to the adsorption of 108 molecules per unit cell, as indicated by our thermogravimetric data. This result is not trivial, as this adsorbed amount corresponds both to the theoretical pore volume and to the best measured pore volume 26162



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were found to vibrate at 1687 cm−1, somewhat more red-shifted than the observed frequencies. At relative pressure above p/p0 = 0.01, the infrared spectra show significant absorption bands between 1712 and 1720 cm−1 (see Figure 4). DFT-D2 calculations provide different adsorption sites, interacting mainly by van der Waals interaction. These sites correspond to frequencies located between 1715 and 1723 cm−1. Since the adsorption energies involved are weak as compared to C13 or C12 these bands may be attributed to acetone physisorption within the cages and pores of Cu-BTC. A full adsorption process for the system acetone/Cu-BTC can therefore be proposed, going from zero coverage to saturation. This process takes into account the experimental observation of the adsorption but also the results obtained with DFT calculations. However, the different interactions involved cannot simply be added since the most favorable adsorption sites are quite far from one another. In the literature, various energetic profiles have been suggested, some of them being unexpected. More precisely, as the adsorption first takes place on the most energetic sites, it is admitted that the enthalpy profiles should exhibit a monotonous decreasing slope. This decrease depends on the distribution of the surface sites. When adsorption is governed by mild sorbate/sorbent interaction, the enthalpy profile is rather flat, up to high coverages. In this study, we obtained two complementary quantitative pieces of information about acetone/Cu-BTC interaction. At zero coverage, an energy value of −62 kJ mol−1 could be obtained by PBE-D2. Experimentally, a ΔH value of −60 kJ mol−1 could be derived from the adsorption isotherms in the region 0.6 < θ < 0.85 (Figure S4, Supporting Information). On the basis of this observation, it is pretty difficult to distinguish which site should interact at zero coverage. However, the infrared results indicate that adsorption first takes place in the small cavities at very low relative pressure. In these cavities, confinement effects have a prominent influence on the adsorption process. However, the enthalpy of condensation of acetone in the bulk is around −44 kJ mol−1. This is only 15 kJ mol−1 more than the enthalpy of adsorption of acetone by Cu-BTC. It can be concluded that despite confinement effects the adsorption process is not driven by strong interactions. The DFT calculations favor the adsorption of acetone onto the Cu(II) sites. Nevertheless, the energy difference between the copper site and the octahedral site is small, typically in the range of 10 kJ/mol. Indeed long-range interactions most probably contribute differently in the two complexes. Thus the calculated energy for the adsorption of acetone on Cu(II) may exhibit a larger error than for the adsorption of acetone in cages and windows. Experimentally, adsorption first takes place in the octahedral cavities. However, the enthalpy difference with the octahedral cavities is not very important. This discrepancy between experimental observation and calculations can be accounted for by DFT calculations and also by diffusion effects. Indeed, the present DFT calculations do not take diffusion limitations into account. The stability of Cu-BTC upon adsorption of polar sorbates as water or benzaldehyde25 has been shown to be limited. From this point of view, the ease of desorption without structural damage is an important parameter. Our observation of a significant decrease of the size of the coherent diffraction domains of Cu-BTC after complete desorption of acetone indicates that some structural damage takes place also in our

experiments. However, the decrease of the crystallite size with no other significant modifications of the diffraction patterns suggests that the damage is localized at defects of the structure and does not imply the whole adsorbent. The conditions of desorption are probably important for the stability of the adsorbent. We have seen that desorption of most of the acetone under vacuum is quite easy. However, a complete desorption in our conditions is only obtained in the presence of ambient humidity. Water is likely to only interact strongly with the Cu(II) sites but not with the pores of the Cu-BTC. The pressure of water vapor in the atmosphere (25 Torr) perfectly justifies the observed winning competition of water for acetone on Cu(II) sites. At a first glance, it seems more difficult to understand why a fraction of the less polar adsorption sites in the truncated tetrahedral cages retain acetone unless some water is adsorbed. However, this effect can be better understood by considering the accessibility of the truncated tetrahedral cages. The type 3 small cages are opened on the cation-lined type 2 large cages through windows with an opening of nearly the same size as the molecule of acetone. These windows can be obstructed by the retention of acetone molecules on the copper sites of the type 2 cages. The replacement of the smaller water molecule for acetone on the Cu(II) sites liberates the openings of the type 3 small cages and allows complete desorption of acetone, without necessarily implying an easy water adsorption inside the small cages.

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CONCLUSION Study of acetone adsorption inside Cu-BTC illustrates the limiting case between preferential coordination of polar molecules to a metal center and preferential adsorption inside small cages of apolar molecules. DFT-D2 data combined with experimental results allowed us to propose an adsorption process. Furthermore, in the case of acetone within Cu-BTC, one site maximizing van der Waals interactions (center of octahedral cage) and coordination to Cu(II) are almost energetically equivalent. Due to the matching between the size of the octahedral cages and acetone, a rather flat enthalpy profile can be suggested. Indeed, adsorbing in the small cavities of Cu-BTC or onto Cu(II) sites leads to similar energies of adsorption, that is between −50 and −60 kJ mol−1. It can be anticipated that in the case of bulkier sorbates the interaction undergone in the small truncated tetrahedral cavities would be limited. Therefore, the prominent interaction would be related to the metal centers. The present study confirms the effectiveness of a combined experimental and quantum chemical approach in the understanding of adsorption phenomena. The potential competition between adsorption on metal sites and confinement in small cavities can shed some light on the behavior of Cu-BTC, a largely studied and industrially produced MOF. Understanding the mechanisms of interaction of adsorbed molecules with CuBTC seems especially important, as its large-scale application as an adsorbent is challenged by its limited stability toward highly polar sorbates such as water.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org. 26163



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Dr. Hugo Petitjean for his expertise on the practical side of DRIFT experiments. This work was performed using HPC resources from GENCI- (Grant 2013x2013081071). Computations were also carried out on the IBM SP4 computers of CINES in Montpellier (France) and at IDRIS, Orsay (France).

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Adsorption of Acetone Vapor by Cu-BTC: An Experimental and

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