The surprising stability of Cu3(btc)2 Metal-Organic Framework under

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The surprising stability of Cu3(btc)2 Metal-Organic Framework under steam flow at high temperature raynald giovine, Frédérique Pourpoint, Sylvain Duval, Olivier Lafon, Jean-Paul Amoureux, Thierry Loiseau, and christophe volkringer Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00931 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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Crystal Growth & Design

The surprising stability of Cu3(btc)2 Metal-Organic Framework under steam flow at high temperature Raynald Giovine,1 Frédérique Pourpoint,1 Sylvain Duval,1 Olivier Lafon,1,2 Jean-Paul Amoureux,1,3 Thierry Loiseau,1 Christophe Volkringer*1,2

AUTHOR ADDRESS. 1

Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité de Catalyse et Chimie du Solide, F-59000 Lille, France. 2

3

Institut Universitaire de France, 1 rue Descartes, 75231 Paris Cedex 05, France.

Bruker, Biospin, Wissembourg, France

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT.

It is widely admitted that Metal-Organic Frameworks (MOFs) are water sensitive and that high temperatures accelerate the hydrolysis phenomena and the collapse of the porous network. We have investigated here the resistance of the prototypical HKUST-1 MOF under steam flow at temperatures ranging from RT to 200 °C. Surprisingly, the porous framework is not degraded at 200 °C, whereas temperatures close to 100 °C lead to the total destruction of the MOF. This unexpected behavior is assigned to the extended interaction of water with the porous framework at temperature about 100 °C, whereas above 150 °C the transient interactions between gaseous water and the HKUST-1 framework limit the decomposition process.


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I.

INTRODUCTION

Since their discovery in the late 1980s,1 Metal-Organic Frameworks (MOFs), also known as porous coordination polymers (PCPs), have had impact on numerous fields of research (gas sorption, catalysis, medicine, nuclear power…).2 These materials are promising owing to their very high porosity and their chemical versatility, which allows adjusting their framework to the desired application.3 Despite a great potential of MOFs in several domains, their industrial utilization remains limited and other porous materials, such as aluminosilicates or charcoals, are usually preferred. These porous inorganic materials or activated carbons benefit not only from a high efficiency in several chemical processes but also from a good stability in a large variety of conditions. Conversely, a major limitation of MOFs, compared to these porous solids, remains their weak stability at high temperature, as well as in the presence of water.4 It is well established that the thermal stability of MOFs mainly depends on that of the organic ligand itself, which typically decomposes between 300 and 400 °C for most MOFs. Only a handful of MOFs stable at higher temperature have been reported. For instance, UiO-66(Zr)5 or MIL-53(Al)6 start to decompose around 500 °C under air. The stability of MOFs in the presence of water remains something difficult to predict and highly depends on the structure.7 In the presence of water (liquid or vapor), the metal dimer turns out to be the fragile part of the hybrid compound. The hydrolysis reactions weaken the bonds within the dimeric unit or between the metal and its ligands. Several structural factors limiting the effect of hydrolysis and improving the stability of MOFs in the presence of water have been identified:



the Lewis acidity of the metal, the lability of the metal node, the hydrophobicity of the framework and the steric hindrance around the inorganic unit.8 In the almost infinite family of MOF compounds, copper trimesate HKUST-1 (a.k.a. [Cu3(btc)2] with btc = benzene-1,3,5-tricarboxylate, also called trimesate) represents a prototypical structure for studying the effect of water.9 The HKUST-1 structure (see Figure 1a) is composed of copper(II) paddlewheel dimer linked to each other via btc ligands. The axial position of the square pyramidal copper is occupied by a water molecule, which can be removed upon thermal activation (see Figure 1b). This structural transformation gives rise to a coordinatively unsatured site, which is the cause of the hydrophilic behavior of this porous framework.10 The pore structure can be viewed as a stacking of three types of cavities (see Figures 1a and c). Two of them have a diameter of 13.2 and 11.1 Å, with an aperture of 6.9 Å. In addition, there are small secondary cavities that are around 5 Å in diameter, accessible through windows with 4.1 Å diameter.


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Figure 1. (a) Illustration of the HKUST-1 structure. The three cavities are displayed as three colored spheres. (b) Scheme illustrating the loss of aqua ligand occupying the axial position of the copper paddle wheel unit during the heating of the HKUST-1. (c) Structure of the three types of cavities in HKUST-1. Hydrogens are not represented for sake of clarity.

Based on the TGA analysis, HKUST-1 is usually reported to be stable up to 350 °C.11 However, this value has to be taken with precaution, since studies indicate the appearance of structural damages from 70 °C.12, 13 Schlesinger et al. revealed that the stability of HKUST-1 is dependent on its hydration.12 Indeed, whereas the anhydrous material is not deteriorated until 320 °C, its hydrated version is irreversibly decomposed from 110 °C and converted to another porous copper trimesate [Cu2(btc)(OH)(H2O)]·2H2O.14 At room temperature (RT) and when exposed to ambient air for short periods (up to 10 hours), the hydrated solid is not altered.15 However, after an exposition of six months to ambient air at RT, the partial decomposition of the solid is 5 ACS Paragon Plus Environment


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irreversible and leads to a 60 % loss of specific surface area. A similar decomposition is observed after the immersion of HKUST-1 in pure liquid water during 24 h at RT16 or 50 °C.17 Furthermore, it has been showed that HKUST-1 is stable at RT over 60 days when 0.5 mol equivalent of water with respect to copper amount was adsorbed.18 Conversely, when 0.75 mol equivalent of water is adsorbed, the HKUST-1 structure is altered after 6 days at room temperature. Due to the hydrophilic nature of HKUST-1, this material proved a great potential for the separation of water vapor from other gaseous molecules.19, 20 This interest has motivated studies dedicated to the effect of controlled steaming conditions and relative humidity on the copper hybrid framework.15, 18, 19, 21-27 Low et al. reported first the behavior of HKUST-1 under steaming conditions (50 % mol% steam) during 2 hours, at different temperatures (85, 200 and 300 °C), using powder X-ray diffraction (PXRD) as the unique technique of characterization.24 Based on these conditions, the authors did not observe structural transformations up to 200 °C, but noted the complete decomposition of the framework at 300 °C. More recently, experiments performed at RT with high relative humidity (RH = 90 %),23, 25, 27 induced a complete degradation of the bulk of the material in one week. The decomposition process occurs only in 3 days when the temperature was increased up to 40 °C.27 Whereas PXRD, gas sorption and infrared (IR) analysis remain the most common techniques used to characterize the degradation of MOFs, other methods were also employed to determine the effect of water on the HKUST-1 structure, such as small-angle PXRD scattering,25 electron paramagnetic resonance,22 or solid state NMR (1H and

13

C).18 To the best of our knowledge,

there have been so far very few solid-state NMR studies relative to the stability of MOFs in the presence of water,18, 28 and these investigations were not carried out on MOFs subject to a steam 6 ACS Paragon Plus Environment

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flow. The solid-state NMR characterization of HKUST-1 is challenging since Cu2+ ions are paramagnetic. However, this technique has already been employed for the structural analysis of HKUST-1, loaded or not with small molecules18, 29-31, as well as other Cu(II)-32 and or Cr(III)based MOFs.33 Based on the different studies dedicated to the stability of MOFs in the presence of water, it is widely accepted that the most severe conditions, in terms of temperature and water concentration, favor the decomposition of the framework. Nevertheless, so far and to the best of our knowledge, no in-depth analysis of the stability of MOFs subject to steam flow at different temperatures has been reported to support such statement. In order to fill this gap, we exposed HKUST-1 to steam with a controlled amount of water (29 mol% steam) and temperature (80, 100, 150 and 200 °C). The MOF samples were fully characterized by means of PXRD, gas sorption, and IR and NMR spectroscopies. Surprisingly, we observed the decomposition of the framework for the softest conditions (80, 100 and 150 °C), whereas its structure remains primarily unaltered at higher temperatures (200 °C).

II.

EXPERIMENTAL METHODS

II.1. Synthesis. The synthesis of HKUST-1 was inspired and scaled-up from the published procedure,9 using a mixture of water and ethanol. II.2. Stability experiments



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A controlled amount of steam was generated by a Vapor Delivery Module (VDM-Series Bronkhorst), including a controlled evaporation and a mixing system connected to a pressurized water reservoir of one liter. The module is equipped with a single carrier gas (non diluted argon) supply (Figure S1). The flows of liquid water and argon were adjusted to 5 g.h-1 and 15 Ln.h-1, respectively. Furthermore, this flow value of liquid water corresponds to the maximal output capacity of the Vapor Delivery Module use in this study. These process conditions correspond to a volume fraction of steam in argon of 29 % at 1.5 bar (absolute) with a RH of 99 % at a dew point of 69 °C. This equipment is connected to a glass reactor including a sintered disc filter used as sample holder. The glass reactor is surrounded by a furnace and all the connections between the vapor generator and the reactor are insured by insulating glass tubes. The relative humidity (RH) in the glass reactor is given by equation (1):

× (1) ∗ ∗ the partial pressure of water vapor in the flow (considered as a perfect gas) and =

with

the equilibrium vapor pressure of water for the corresponding reactor temperature. In our study, is given by equation (2):

= × = 29.296 × 150 = 44.526

(2)

where = 29.296 % is the molar fraction of steam in the flow and = 1.5 is the pressure inside the glass reactor.


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∗ at the corresponding glass reactor temperature was obtained from the literature34 and RH is

calculated accordingly (Table 1). Table 1: Equilibrium vapor pressure and relative humidity calculated in the reactor for the temperature used in this study. Glass reactor temperature (°C) 80

100

150

200

∗ (kPa) (ref handbook)

47.373 101.32 475.72 1553.6

Calculated RH (%)

92.8

43.4

9.2

2.8

For the analysis of MOF stability, the glass reactor is filled with 500 mg of HKUST-1. Prior to experiments, the powder bed is flushed with argon (15 Ln.h-1) in order to evacuate physisorbed H2O molecules during 10 minutes. This step is followed by heating the material at a rate of 10 °C per min. The temperature inside the reactor is followed by means of a thermocouple. After the heating phase, the targeted temperature is hold and the vapor steam flow is started. Immediately after the end of the experiment, the powdered sample is collected, kept in a closed glass vial and transferred into a glove box under argon, in order to prevent the effect of moisture from ambient air. No specific cooling phase was used during the collet ion of the solid and its transfer in the glove box. For this study, the effect of temperature was analyzed for an exposure time of either 24 h or ranging from 0.5 to 24 h in the case of the kinetic study performed at 100 °C. For the 24 h duration analyses, the experiments were repeated without the addition of water in order to distinguish the effect of water and temperature, respectively.



II.3. Solid state NMR NMR experiments were performed at 9.4 T (1H Larmor frequency of 400 MHz) using a wide-bore Bruker BioSpin spectrometer equipped with an AVANCE-II console. Spectra were recorded with a 2.5 mm triple resonance HXY Magic-Angle Spinning (MAS) probe used in double resonance mode (1H-13C). The samples were packed in a glove box under argon in 2.5 mm zirconia rotors which were closed with two Vespel caps and spun at νR = 30 kHz. 1H spectra were recorded using a DEPTH sequence in order to remove the background signal.35 For these experiments, the nutation frequency during the radiofrequency pulses applied to the 1H channel was 83 kHz. The 1H DEPTH spectra result from averaging 64 scans with a recovery delay of 1 s, resulting in an experimental time of 1 min. The 1H isotropic chemical shifts (δiso) were externally referenced to tetramethylsilane (TMS) using the resonance of adamantane (1.8 ppm) as a secondary reference. 13

C MAS NMR spectra were acquired using spin echo sequence and acquiring the half-echo in

order to avoid large first-order phase correction. The nutation frequency during the radiofrequency pulses applied to the

13

C channel was 86 kHz. The π-pulse in the spin-echo

sequence was bracketed by two 66.6 µs delays, corresponding to 2 rotor periods. The 13C spectra result from averaging 102400 transients, with a recovery delay of 100 ms, resulting in an experimental time of 4 h. TPPM-15 1H decoupling with an rf nutation frequency of 78 kHz was applied during the acquisition. The 13C isotropic chemical shifts (δiso) were externally referenced to TMS (δiso = 0 ppm) using the deshielded resonance (38.7 ppm) of adamantane. In order to confirm the assignment of 1H spectra of as-synthesized HKUST-1, we recorded twodimensional (2D) dipolar-mediated refocused Insensitive Nuclei Enhanced by Polarization 10 ACS Paragon Plus Environment

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Transfer (D-R-INEPT) spectrum of that sample with 1H excitation and 13C detection (1H→13C). The 1H-13C dipolar couplings were reintroduced under MAS conditions by the application of Rotational Echo, Double-Resonance (REDOR) recoupling.36 Such D-R-INEPT sequence is also called Transferred Echo DOuble Resonance (TEDOR,37 see Figure S7). The nutation frequencies during rf pulses applied to 1H and

13

C channels were equal to 78 and 89 kHz, respectively. The

defocusing and refocusing delays were equal to 66.7 µs. The 2D spectrum results from averaging 81920 transients for each of 30 t1 increment with ∆t1 = 33.33 µs and a recovery delay of 50 ms, resulting in an experimental of time of 64 h. II.4. Powder X-ray Diffraction The PXRD patterns were collected at room temperature with a D8 advance A25 Bruker apparatus with a Bragg-Brentano geometry. This diffractometer is equipped with a LynxEye detector with CuKα1/α2 radiation. The 2θ range was 3-50° with a step of 0.02° and a counting time of 0.5 s per step. II.5. IR spectroscopy IR analysis was carried out by using a Perkin–Elmer Spectrum 2 instrument equipped with a single reflection diamond module (ATR). IR spectra were recorded in the 400-4000 cm-1 range with 4 cm-1 resolution. II.6. N2 adsorption N2 adsorption isotherms were measured on a Micromeritics ASAP 2020 analyzer at 77 K. Before gas sorption, the sample was evacuated at 150 °C under vacuum overnight. The porosity of the



sample was estimated by gas sorption isotherm experiments in liquid nitrogen (at 77 K). For surface area calculations, BET model was applied in the 0.01-0.2 p/p0 range.

III.

RESULTS AND DISCUSSION

III.1. Experiments under dry conditions PXRD. A powdered sample of HKUST-1 was firstly analyzed under dry conditions for temperatures ranging from 80 to 200 °C. After heating for 24 h, the ex-situ PXRD patterns collected at RT remains very similar to that of the initial compound (Figure S3) and do not reveal any significant alteration of the long-range structure. We just notice a slight broadening of the base of the peak centered at 2θ = 12.5° at all temperatures. N2 adsorption. The porosity of each sample indicates very different specific surface area (see Figure 6 and Table S4). We observed that the solids heated at 150 and 200 °C exhibit very high specific surface areas (BET surface ≈ 1898 and 1870 m2/g, respectively) comparable to that of the reference material (BET surface ≈ 1877 m2/g). The samples heated at 80 and 100 °C present a significantly lower porosity (1634 and 1481 m2/g, respectively). IR. The IR spectra of the samples subject to a flow of argon do not exhibit peaks at 3300 and 1043 cm-1, which are detected in the spectrum of the reference HKUST-1 (see Figure S4). The disappearance of these peaks indicates the evacuation of water and ethanol molecules from the pores of HKUST-1, when it is subject to dry argon flow. No other significant change is observed for the samples subjected to dry argon flow. Hence, the IR spectra do not allow detecting the creation of defects or other modifications of the framework due to heating. 12 ACS Paragon Plus Environment

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Solid-state NMR. 1H NMR spectrum of the as-synthesized material shows four signals at −2.7, 2.9, 7.0 and 8.7 ppm (see Figure 2). The 1H signal at 7.0 ppm correlates to that of aromatic 13CH one resonating at 228 ppm in the 2D 1H→13C TEDOR spectrum shown in Figure S8, and hence, was assigned to that of aromatic protons (Har). The 1H signal at 8.7 ppm is assigned to water adsorbed within the pores. In a previous study,38 such signal has been denoted H2O-III and it has been shown that its isotropic chemical shift depends on the amount of water within the pores. Assuming a fast chemical exchange between two sites, water bonded to copper (H2O-II) resonating at 12 ppm and bulk water in the pores at 3.8 ppm, we can estimate that the assynthesized HKUST-1 contains approximately 1.7 water molecules per Cu ions.38 Such water amount is smaller than that measured for HKUST-1 hydrated during more than 67 h at ambient air. Such difference probably stems from the presence of ethanol molecules in the pores. The signals at −2.7 and 2.9 ppm are assigned to ethanol protons. Furthermore, the signal at -2.7 ppm can be assigned to CH3 group while signal at 2.9 ppm is assigned to CH2 as stated by the relative intensity of their integrals. In previous studies,18,

38

the ethanol was evacuated under vacuum

before rehydration and its NMR signal was not observed in the 1H spectrum of the hydrated HKUST-1 material. After treating the material under dry argon flow, we only observed two 1H resonances at 8.1 and 13.8 ppm. The first signal corresponds to Har of the organic part of the structure, and the second to the proton of a single water molecule bonded to the copper from the paddle-wheel unit of the MOF compound (noted H2O-I).18, 30 The lack of ethanol signals and the strong reduction of the water signal intensity relative to that of the aromatic one indicates the evacuation of water and ethanol from the pores of HKUST-1 treated by dry warm argon flow.



When increasing the temperature, the signal intensity at 13.8 ppm decreased, which indicates a further dehydration of the MOF. The Har resonance retains the same δiso value and linewidth for all investigated temperatures, indicating that the treatment with dry Ar flow does not alter its framework.

Figure 2. 1H DEPTH NMR spectra of HKUST-1 at 9.4 T with MAS frequency of 30 kHz for HKUST-1 as-synthesized and subjected to dry Ar flow with temperature ranging from 80 to 200 °C. The assignment of the peaks is indicated on the figure. The δiso value of H2O-II site (12 ppm), corresponding to two ligands bonded to paddlewheel Cu unit is also indicated, although such signal was not detected in those sample. In the samples treated at 80, 100 and 150 °C, the remaining fractions of H2O-I signals with respect to the Har is indicated in the figure.


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The 13C NMR spectrum of the as-synthesized material shows five signals at 853, 228, 56.8, 13.0 and -46.5 ppm (Figure S8 and Figure 3). The resonances at 56.8 and 13 ppm arise from the ethanol, while the resonances at 853, 228 and -46.5 ppm are assigned to the aromatic Cα, aromatic CH and carboxylate signal, respectively.18, 29, 39 As already reported, the Cα signal is broad and is difficult to detect. It is not displayed in Figure 3. When the HKUST-1 compound is treated under dry Ar flow, only the three resonances from the linker remain, confirming the evacuation of ethanol. The evacuation of ethanol and the dehydration of HKUST-1 shift the signals of the COO− sites toward lower δiso value in agreement with a previous study.29 Conversely the δiso value of CH site is not modified by the treatment with dry Ar flow. Furthermore, the linewidth of the 13C signals are comparable for all investigated samples. These observations indicate that the HKUST-1 framework is preserved under treatment by dry Ar flow.



Figure 3.

13

C half-echo spectra at 9.4 T with MAS frequency of 30 kHz of as-synthesized

HKUST-1 and subjected to dry Ar flow with temperature ranging from 80 to 200 °C. The assignment of the peaks is indicated on the figure. * denotes the spinning sidebands.

Discussion. As expected, IR and NMR spectra indicate that the treatment of as-synthesized HKUST-1 by dry Ar flow at temperatures higher than 80 °C evacuates the ethanol and water molecules within its pores. Furthermore, those spectra as well as PXRD data do not indicate significant alteration of the HKUST-1 framework subject to that treatment. Conversely a significant reduction of the specific surface area of HKUST-1 was measured for Ar flow at about 100 °C. Based on the results obtained for HKUST-1 subjected to steam flow (see section III-2), we can assume that water molecules present in the pores of as-synthesized HKUST-1 can hydrolyze at about 100 °C a weak amount of the bonds between the copper cations and the 16 ACS Paragon Plus Environment

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trimesate ligands and hence, create point defects in the framework. Such defects can block the access to some of the pores, thus resulting in a smaller specific surface area. Nevertheless, the concentration of these point defects is too small to be detected by PXRD, IR and NMR spectroscopy. We did not observe the formation of any other crystalized phases. However, it has been reported that slightly higher temperatures ranging from 110 to 120 °C allow for the decomposition

of

hydrated

HKUST-1

and

the

irreversible

crystallization

into

[Cu2(btc)(OH)(H2O)].14 In this study, we do not detect a decomposition of the framework when HKUST-1 is subjected to dry Ar flow at temperature of 150 °C and 200 °C. At these temperature, the removal of water molecules must be rapid and complete, which prevents the significant hydrolysis of the bonds between Cu2+ ions and ligands, hindering the formation of defects blocking the access to the pores.

III.2. Experiments under steam flow Optical images. The HKUST-1 compound has been treated under steam flow at the temperatures of 80, 100, 150 and 200 °C during 24h. As seen in Figure 4, the as-synthesized sample has a typical light blue color, representative of hydrated HKUST-1, whereas it appears either lighter after treatment at 100°C or darker after treatment at 150 and 200 °C, respectively. The dark-blue color is characteristic of the dehydrated form of HKUST-1. Hence, the color of HKUST-1 samples indicates that the treatment at 150 and 200 °C dehydrates the HKUST-1 sample, whereas that at 100 °C hydrates it. SEM images. SEM images reveal that typical octahedrally-shaped crystals of HKUST-1 are still present after a treatment at temperatures 150 and 200 °C. Conversely the powdered samples 17 ACS Paragon Plus Environment


heated at 100 °C or less exhibit degraded octahedral-shaped crystals, showing surface cracks and the appearance of new flat crystallites. Such observation indicates the structural decomposition of HKUST-1 with the formation of new phases at the surface of the crystallites.27

Figure 4. Optical and SEM pictures of HKUST-1 as-synthesized and after thermal treatments at 100, 150 and 200 °C under steam.

PXRD. PXRD signatures of solids heated at 150 and 200 °C (Figure 5) are comparable to the pattern of as-synthesized HKUST-1 with characteristic peaks at 2θ = 6.7, 9.5 and 11.6°. On the contrary, for solids treated at 80 and 100 °C, PXRD patterns exhibit new Bragg peaks at 2θ = 9.5°, 10.3°, 11.6°, 29.1° and 19.4°, 26.0°, indicating the crystallization of new phases along with the structural collapse of HKUST-1. At 80 °C, traces of HKUST-1 are still visible, and they are mixed with two other distinct hydrated copper trimesates, [Cu2(btc)(OH)(H2O)]14 and [Cu(Hbtc)(H2O)3],40 easily identifiable by the Bragg peaks at 2θ = 10.3°, 29.1° and 19.4°, 26.0°, respectively. At 100 °C, only the Bragg peaks of these two hydrated copper trimesates are observed, proving the full structural transformation of the HKUST-1 structure. The expansion


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shown on the right, displays additional small reflections at 2θ = 9.5 and 11.6° corresponding to an unidentified solid.

Figure 5. Ex-situ PXRD patterns of HKUST-1 as-synthesized and subjected to steam flow at 80, 100, 150 and 200 °C. The main Bragg peaks of [Cu2(btc)(OH)(H2O)] and [Cu(Hbtc)(H2O)3] are marked with circles and squares, respectively. An expansion of the PXRD pattern over the 2θ = 8.5-12.0° range is displayed on the right of the figure.

N2 adsorption. The preservation of the HKUST-1 structure after an exposition to steam at 200 °C is confirmed by a relative high BET surface of 1744 m2.g-1 (Figure 6), close to the value measured for a sample at the same temperature under a dry flow of argon (1870 m2.g-1, ∆BET = 7 %).

At 150 °C, the presence of crystallized HKUST-1 leads to a significant porosity

(1117 m2.g-1), which is, however, much lower than the value obtained from dry conditions (∆BET = -41 %). At lower temperatures (80 and 100 °C), the BET surfaces are very low (175 and 19 ACS Paragon Plus Environment


68 m2.g-1, respectively) and stems from traces of preserved HKUST-1 as well as the presence of the open framework [Cu2(btc)(OH)(H2O)].14

Figure 6. BET surface areas measured for the compounds submitted to dry Ar (black square) and steam (red circle) flows at temperatures ranging from 80 to 200°C for 24h. The dashed line represents the BET surface of the as-synthesized material.

IR. The IR spectrum of the solid exposed to steam at 200 °C is quite comparable to that of the as-synthesized one (Figure S5), except the loss of peaks at 3300 and 1043 cm-1. As explained in the section III-1, the disappearance of these peaks indicates the evacuation of water and ethanol from pores. Hence, paradoxically, the treatment with steam dehydrates the sample. After a treatment at 150 °C (Figure 7), we note the growth of new small vibrations at 1705 and 1619 cm-1, as well as an intense band at around 1550 cm-1. The first vibration peak (1705 cm-1) is assigned to C=O bond of the uncoordinated carboxylic function, and indicates the breaking of the bonds connecting the trimesate ligand to the copper center. The very low intensity of this 20 ACS Paragon Plus Environment

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vibration suggests that only a small amount of bonds is broken. The two other vibrations at 1619 and 1550 cm-1 are ascribed to the bending mode of water molecules coordinated to copper.15 The detection of these bands indicates that the HKUST-1 subjected to steam flow at 150 °C contains aqua ligand. These bands are hardly visible in the IR spectrum of the as-synthesized HKUST-1 because they are broadened by the hydrogen bonds between aqua ligands and water molecules within the pores. The steam treatment at 80 and 100 °C induces major modifications in the 1800-1100 cm-1 spectral range (Figure 7), with notably an increased intensities of the bands associated to the C=O from the uncoordinated carboxylic function and aqua ligand. The shoulder at 3592 cm-1 (Figure S5) could be assigned to the stretching band of hydroxyl groups in copper hydroxide or [Cu2(btc)(OH)(H2O)].

Figure 7. 1800-1000 cm-1 spectral region of FTIR spectra of HKUST-1 as-synthesized and subjected to steam flow at temperature 80, 100, 150 and 200 °C. The FTIR spectra of those compounds over the range 4000-400 cm-1 are displayed in Figure S5. 21 ACS Paragon Plus Environment


Solid-state NMR. The 1H NMR spectra of the materials treated by steam at 150 and 200 °C, (see Figure 8) exhibit a resonance at 7.7-8 ppm ascribed to Har. In addition, the 1H NMR spectrum of the sample treated at 200 °C under steam flow displays an additional resonance at 13.3 ppm assigned to H2O-I site. The comparison of Figures 2 and 8 shows that the H2O-I intensity is higher for HKUST-1 treated by steam flow than by dry Ar gas. The sample treated at 150 °C exhibits a resonance at 12 ppm assigned to H2O-II, showing that the amount of aqua ligands is higher for decreasing steam temperature. This spectrum differs from the one obtained under dry condition at the same temperature (see Fig. 2) where we observed only H2O-I site. As expected, the steam treatment leads to a higher amount of aqua ligands than the dry Ar flow. The 1H NMR spectra of the samples treated at 80 and 100 °C display very broad signals ranging from -5 to 20 ppm. Such broad resonances indicate a distribution of 1H local environments, which is consistent with the collapse of the HKUST-1 framework and the formation of new phases.


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Figure 8. 1H DEPTH spectra at 9.4 T with MAS frequency of 30 kHz of HKUST-1 as synthesized and treated by steam flow at 80, 100, 150 and 200 °C.



Figure 9.

13

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C half-echo spectra at 9.4 T with MAS frequency of 30 kHz of HKUST-1

as-synthesized and treated by steam flow at 80, 100, 150 and 200 °C. For the samples treated at 150 and 200 °C under steam flow, the

13

C NMR spectra exhibit two

signals at 228 and -87.4 ppm assigned to aromatic carbon CH and carboxylate, respectively (Figure 9). Those spectra are similar to those recorded for HKUST-1 treated by dry Ar gas at the same temperature (see Figure 3). It is noted that the

13

C spectrum was also recorded with an

offset frequency set on the resonance at 850 ppm in order to detect the Cα site (not displayed). Unfortunately, this signal was difficult to detect as stated above. This observation indicates that steam flow at 150 and 200 °C does not alter the structure of the framework. On the contrary, the 13

C NMR spectra of the samples treated by steam flow at 80 and 100 °C exhibit numerous

resonances, indicating the formation of new phases. However, the assignment of these signals is challenging owing to the large number of signals and the paramagnetic shifts.


13

C

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Discussion. So far, it was commonly established that the stability of MOF materials in the presence of water, decreased when the temperature increased. The various data obtained here (colors, SEM, surface area, PXRD, FTIR, solid-state NMR) all indicated that the destructive effect of the water steam is maximum around 100 °C. At such temperature, water molecules can condense in the pore and hence, strongly interact with the framework. Furthermore, the temperature is high enough to allow the hydrolysis of the bond between carboxylate ligands and copper centers. Above 150 °C, the interaction between water and HKUST-1 framework is transient, which prevents the decomposition process. III.3. Kinetic study of the behavior of HKUST-1 under steam flow at 100 °C In order to follow the kinetics of the framework decomposition, HKUST-1 samples were subjected to water vapor at 100 °C for exposure times, ranging from 0.5 to 24 h. Optical and SEM. After 0.5 h, the crystals exposed to water vapor are dark blue, a color typical of dehydrated HKUST-1 (see Figure 10). After 2 h, they become light blue, which indicates the binding of water molecules to the metal center and the formation of penta-coordinated copper. The octahedral geometry of the crystals is conserved up to 16 h. After 24 h, plate-like crystallites appear which indicate the formation of a new phase.



Figure 10. Optical and SEM pictures of HKUST-1 as-synthesized and treated by steam (29 %) flow at 100 °C for time exposure ranging from 30 min to 24 h.

PXRD. After 1 h, the collected ex-situ PXRD diagram is similar to that of the as-synthesized material in terms of intensity and width (Figure 11). At 2 h, the quality of the Bragg peak profiles is not modified, but we note the appearance of a new peak at 2θ = 5.8°. This new reflection is related to the level of hydration of the compound and (i) it steps up when the amount of water increases, but (ii) it disappears after reaching complete hydration.9, 15, 21 From 4 to 8 h, this peak characterizing the hydration of the solid is still present but its width increases. From 8 h, two 26 ACS Paragon Plus Environment

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new Bragg peaks are detected: one at 2θ = 10.3° ascribed to the phase [Cu2(btc)(OH)(H2O)],14 and the second at 2θ = 9.5° assigned to the undefined solid mentioned above. After 16 h, the diffractogram is that of a mixture of [Cu2(btc)(OH)(H2O)], the unknown phase and traces of HKUST-1. After 24 h, the HKUST-1 structure is not detectable anymore by PXRD and [Cu2(btc)(OH)(H2O)] represents the main compound and appears in the form of platelet crystals (see Figure 10). Furthermore, at 24 h, the amount of the unknown phase yielding a peak at 2θ = 9.5° is decreased, whereas [Cu(Hbtc)(H2O)3],40 phase yielding reflections at 2θ = 9.3 and 24.6° is detected.

Figure 11. Evolution of PXRD of HKUST-1 as function of the exposure time (up to 24 h) to steam (29 %) at 100 °C. The main peaks [Cu2(btc)(OH)(H2O)] and [Cu(Hbtc)(H2O)3] are marked with circles and squares, respectively. The expansion over the 2θ interval 8.5-12.0° shows the evolution of the two peaks of HKUST-1 at 2θ = 9.4 and 11.6°.



N2 adsorption. The disappearance of HKUST-1 is also confirmed by the decrease of the porosity of the collected sample after hot steam treatment (Figure 12). Only after 30 min of exposure to steam, the compound lost approximately 35 % of specific surface (1213 m2.g-1). The porous capacity remained constant some hours and decreased to a minimum reached after 1 day (68 m2.g-1).

Figure 12. Evolution of BET surface areas of HKUST-1 as function of the exposure time (up to 24 h) to steam (29 %) at 100 °C.

IR. As seen in Figure 13, the band at 1043 cm-1, assigned to the stretching of the C-O bond of ethanol, disappears after 1 h, showing the rapid removal of ethanol from the pores. After 1h, the intensification of the band centered at around 1549 cm-1 assigned to the water bending mode shows the hydrophilicity of HKUST-1. The stretching band of free carboxylic functions at 1707 cm-1 is also detected from 30 min, which indicates the breaking of the bond connecting the Cu2+ ions and btc ligands. For increasing exposure time, we observed a shift of the stretching wavelength of carboxylate bonded to copper as well as the additional peaks in the range 1700 28 ACS Paragon Plus Environment

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and 1600 cm-1. Such observation is consistent with the formation of new phases in which btc ligands coordinate Cu2+ ions but with geometry distinct from that in HKUST-1.

Figure 13. Evolution of the 1800-1000 cm-1 region of the FTIR spectrum of HKUST-1 as function of the exposure time (up to 24 h) to steam (2 9%) at 100 °C. The FTIR spectra of those compounds over the range 4000-400 cm-1 are displayed in Figure S6.

Solid-state NMR. The 1H NMR spectrum (Figure 14) recorded on a sample treated during 30 minutes at 100 °C under 29 % steam flow presents the same resonances as the spectrum recorded on the initial HKUST-1: two signals at -2.7 and 2.2 ppm ascribed to the protons of ethanol molecules (-CH3 and -CH2 respectively) trapped in the porosity and two other ones at 7 and 9.5 ppm corresponding to the Har protons and water molecules within the pores (H2O-III), respectively. The main difference comes from a slight decrease of intensity of the ethanol resonances due to an evacuation of these molecules by the steam flow. After 30 min, the H2O-III signal is shifted to higher isotropic chemical shifts and exhibits lower intensity with respect to 29 ACS Paragon Plus Environment


that of Har one than in the as-syntheszied material. These modifications suggest an evacuation of water from the pores. However, the signal at 7 ppm is unaffected, which indicate the preservation of the HKUST-1 framework.

Figure 14. Evolution of 1H DEPTH spectra of HKUST-1 at 9.4 T and MAS frequency of 30 kHz as function of the exposure time (up to 24 h) to steam (29 %) at 100 °C. The HKUST-1 compound treated by a flow of dry Ar gas at 100 °C during 24 h is also shown on top.

After 1 h exposure time, the intensity of the H2O-III peak is strongly reduced, indicating that steam at 100 °C flows first evacuate the water from pores. Furthermore, a significant broadening 30 ACS Paragon Plus Environment

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of the Har peak is observed and indicates significant structural disorders around aromatic protons and the formation of defects. After 2 h, the 1H spectrum consists in a broad resonance extending from 20 to −5 ppm, which must result from the overlap of several resonances. In particular, the intensities around the isotropic chemical shifts of H2O-I and H2O-II sites are increased with respect to the spectrum acquired after 1 h of exposure time. These higher intensities indicate that water molecules from steam coordinate to the copper ions after exposure time of 2 h or longer. Hence, steam flow at 100 °C dehydrates the sample for exposure time of 1 h, whereas it rehydrates it after 2 h. This broad line displays shoulders at 20, 14.5, 1.7 and −1.8 ppm. These shoulders can be 1H signals of trimesic acid formed by hydrolysis of the bonds between carboxylate groups and Cu2+ ions as well as those of new phases, including [Cu2(btc)(OH)(H2O)] and the unidentified phase giving a reflection at 2θ = 9.5° (see Figure 11). For longer exposure time to steam at 100 °C, those signals become more intense with respect to the Har one. Such results are consistent with the decomposition of HKUST-1 framework exposed to steam at 100 °C and the formation of new phases. The relative intensity of the signals resonating at 20 and −1.5 ppm decreases after 24 h exposure, whereas the signals at 10 and 1 ppm become more intense. In view of the PXRD data (see Figure 11), the signals at 20 and −1.5 ppm may be those of the unidentified phase, whereas those at 10 and 1 ppm could be produced by the [Cu2(btc)(OH)(H2O)] phase.



Figure 15. Evolution of 13C half-echo NMR spectra of HKUST-1 at 9.4 T and MAS frequency of 30 kHz as function of the exposure time (up to 24 h) to steam (29%) at 100 °C.

Figure 15 shows the

13

C NMR spectra of HKUST-1 treated by steam flow at 100 °C. It is also

noted that even though the spectrum was also recorded with the offset frequency set on 850 ppm, the signal of the Cα is not detected (as explained above). After 30 min, the 13C spectrum remains similar to that of the as-synthesized HKUST-1. We only observe a reduction of the signal intensities of ethanol, which is evacuated from the pores by the steam treatment. Such result is consistent with the 1H NMR spectra of Figure 14. After 1 h exposure to steam, 13C peaks at 174 and 138 ppm can be detected. They are ascribed to trimesic acid molecules, which are not in the proximity of Cu2+ ions. Hence, the steam hydrolyzes the bonds between carboxylate groups and copper ions and evacuates trimesic acid from the HKUST-1 pores. The relative intensities of 32 ACS Paragon Plus Environment

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those signals are maximal for thermal treatment of 2 h. After an exposure time of 16 h, new 13C peaks are detected in the range 460 to -40 ppm. The full list of observed 13C sites is reported in the Table S1. For an exposure time of 24 h, the relative intensities of the peaks at 282, 258, 243, 232 and 110 ppm decrease, whereas those of the peaks at 456, 225, 184, 173, 167, 157, 134, 77, -32 and -84 ppm increase. Therefore, in view of the PXRD data, the former group of peaks can be assigned to those of the unidentified phase, whereas the latter ones can originate from those of [Cu2(btc)(OH)(H2O)] phase. In parallel, 2D 1H→13C TEDOR spectrum of HKUST-1 after an exposure time of 16 h and 24 h display numerous correlations from 280 to 130 ppm and from 471 to 77 ppm respectively (Figure S10 and S11). The

13

C to 1H correlations observed in

these 2D 1H→13C TEDOR spectra are listed in the Table S2 and S3. Nevertheless, a precise attribution of these signals is quite challenging and beyond the scope of this article. Finally, these 13

C NMR data confirms the formation of new phases for steam treatment longer than 16 h.

Discussion. Optical images, FTIR and NMR data show that during the first 30 min of steam treatment, water and ethanol molecules are eliminated from the pores of HKUST-1. The N2 adsorption indicates a 35 % reduction in the surface area. Such reduction may stem from the creation of point defects by hydrolysis of the bonds between carboxylate groups and Cu2+ ions, which block the access to the pores. However, the concentration of these defects may be too small to be detected by the employed characterization techniques. After 1 h, IR and

13

C NMR

spectra indicate the formation of trimesic acid molecules by hydrolysis of the bonds between the ligands and the Cu2+ ions of HKUST-1. After 8 h, PXRD indicates the formation of an unidentified crystalline phase and [Cu2(btc)(OH)(H2O)]. After 16 h, the decomposition of the HKUST-1 leads to an increase of the amounts of the unidentified phase and the [Cu2(btc)(OH)(H2O)], which goes with a further decrease of the surface area. After 24 h, the 33 ACS Paragon Plus Environment


amount of the unidentified phase decreases at the expense of the formation of [Cu2(btc)(OH)(H2O)] and [Cu(Hbtc)(H2O)3] and the surface area is reduced. This kinetic study indicates that a temperature of 100°C induces the hydrolysis of the copper carboxylate node and the formation of uncoordinated carboxylic acid functions. At this temperature and in the presence of water steam, the residues of trimesic acid and copper react and crystallize as new copper trimesates. In HKUST-1 network, the trimesate ligand is hexacoordinated to copper centers (CuO4(H2O) engaged in paddle wheel units) via bidendate connection mode. During the steam treatment, a first crystallization is observed with the appearance of the [Cu2(btc)(OH)(H2O)] compound, showing a 3D network, based on the connection of infinite chains of square bipyramids CuO3(OH)2 and octahedra CuO4(OH)(H2O) units linked through the trimesate ligands. In this structure, the trimesate is only coordinated to five copper atoms, since one carboxylate arm adopts a chelating mode, the second one bidentate connection mode and the third one, monodentate connection mode toward copper centers. During this structural rearrangement, only one Cu-O-C bond is broken and the trimesate ligand is still attached to five copper atoms. When the water reaction proceeds further, a second crystallized phase has been identified, resulting in one-dimensional coordination polymer [Cu(Hbtc)(H2O)3]. The copper is coordinated to a larger number of aqua ligands in a square bipyramidal unit CuO2(H2O)3, preventing any further bonding with trimesate molecule. Indeed, the latter organic species is only coordinated twice to the copper center, in a monodentate fashion. These different structural transformations show the progressive substitution of the carboxylate groups around the copper center by hydroxid/aqua species, when the amount of water increases.


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Figure 16. Schematic representation of the decomposition pathway of HKUST-1 in the presence of steam at 100°C, deduced from the observation of identified crystallized phases.

IV.

CONCLUSION

One of the main limits of Metal-Organic Framework materials remains their weak stability in the presence of H2O. In the case of aqueous solution, it was proved that the elevation of temperature accelerates the destructive effect of water, mainly based on the hydrolysis of the bond connecting the organic ligand to the inorganic unit of the hybrid framework. This conclusion was usually transposed to water steam, assuming that high temperatures favor the destruction of the MOF. In this study, we proved that the behavior of the prototypical compound HKUST-1 does not follow this assumed trend. Indeed, this solid remains unchanged by the combination of water steam and high temperature (200 °C), whereas softer conditions engender the irreversible collapse of the porous solid. The destruction is maximal for temperature below or equal to the 35 ACS Paragon Plus Environment


boiling point of water, and it leads to the precipitation of new copper trimesates. This result indicates that high temperatures, significantly above the boiling point of water, inhibit the adsorption of water and limits the hydrolysis process. This means that MOF compounds, which are denigrated due to their apparent low stability in presence of water, would find a new set of applications at high temperatures. This result is very important for the industrial utilization of MOF and open new perspectives for processes involving high temperatures and gaseous water, like catalytic reactions or nuclear accident management.

ASSOCIATED CONTENT Supporting Information. AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Any funds used to support the research of the manuscript should be placed here (per journal style).


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ACKNOWLEDGMENT. Chevreul Institute (FR 2638), Ministere de l’Enseignement Supérieur et de la Recherche, Région Hauts-de-France and FEDER are acknowledged for supporting and funding partially this work. Authors are also grateful for funding supported by contract ANR-14-CE07-0009-01. REFERENCES 1. Hoskins, B. F.; Robson, R., INFINITE POLYMERIC FRAMEWORKS CONSISTING OF 3 DIMENSIONALLY LINKED ROD-LIKE SEGMENTS. J. Am. Chem. Soc. 1989, 111 (15), 5962-5964; Robson, R., Design and its limitations in the construction of bi- and polynuclear coordination complexes and coordination polymers (aka MOFs): a personal view. Dalton Trans. 2008, (38), 5113-5131. 2. Silva, P.; Vilela, S. M. F.; Tome, J. P. C.; Paz, F. A. A., Multifunctional metal-organic frameworks: from academia to industrial applications. Chem. Soc. Rev. 2015, 44 (19), 67746803; Nandanwar, S. U.; Coldsnow, K.; Utgikar, V.; Sabharwall, P.; Aston, D. E., Capture of harmful radioactive contaminants from off-gas stream using porous solid sorbents for clean environment - A review. Chem. Eng. J. 2016, 306, 369-381. 3. Cohen, S. M., The Postsynthetic Renaissance in Porous Solids. J. Am. Chem. Soc. 2017, 139 (8), 2855-2863; Lu, W. G.; Wei, Z. W.; Gu, Z. Y.; Liu, T. F.; Park, J.; Tian, J.; Zhang, M. W.; Zhang, Q.; Gentle, T.; Bosch, M.; Zhou, H. C., Tuning the structure and function of metalorganic frameworks via linker design. Chem. Soc. Rev. 2014, 43 (16), 5561-5593. 4. Canivet, J.; Fateeva, A.; Guo, Y. M.; Coasne, B.; Farrusseng, D., Water adsorption in MOFs: fundamentals and applications. Chemical Society Reviews 2014, 43 (16), 5594-5617. 5. Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.; Bordiga, S.; Lillerud, K. P., A new zirconium inorganic building brick forming metal organic frameworks with exceptional stability. J. Am. Chem. Soc. 2008, 130 (42), 13850-13851. 6. Loiseau, T.; Serre, C.; Huguenard, C.; Fink, G.; Taulelle, F.; Henry, M.; Bataille, T.; Ferey, G., A rationale for the large breathing of the porous aluminum terephthalate (MIL-53) upon hydration. Chem. Eur. J. 2004, 10 (6), 1373-1382. 7. Gelfand, B. S.; Shimizu, G. K. H., Parameterizing and grading hydrolytic stability in metal-organic frameworks. Dalton Trans. 2016, 45 (9), 3668-3678. 8. Burtch, N. C.; Jasuja, H.; Walton, K. S., Water Stability and Adsorption in Metal-Organic Frameworks. Chem. Rev. 2014, 114 (20), 10575-10612. 9. Chui, S. S. Y.; Lo, S. M. F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D., A chemically functionalizable nanoporous material Cu-3(TMA)(2)(H2O)(3) (n). Science 1999, 283 (5405), 1148-1150. 10. Grajciar, L.; Bludsky, O.; Nachtigall, P., Water Adsorption on Coordinatively Unsaturated Sites in CuBTC MOF. J. Phys. Chem. Lett. 2010, 1 (23), 3354-3359. 11. Raoof, J. B.; Hosseini, S. R.; Ojani, R.; Mandegarzad, S., MOF-derived Cu/nanoporous carbon composite and its application for electro-catalysis of hydrogen evolution reaction. Energy 2015, 90, 1075-1081; Lee, Y. R.; Kim, J.; Ahn, W. S., Synthesis of metal-organic frameworks: A mini review. Korean J. Chem. Eng. 2013, 30 (9), 1667-1680. 37 ACS Paragon Plus Environment


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The surprising stability of Cu3(btc)2 Metal-Organic Framework under

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