Systematic study of the chemical and hydrothermal stability of selected “stable” Metal Organic Frameworks

Accepted Manuscript Systematic study of the chemical and hydrothermal stability of selected “stable” Metal Organic Frameworks Karen Leus, Thomas Bogaerts, Jeroen De Decker, Hannes Depauw, Kevin Hendrickx, Henk Vrielinck, Veronique Van Speybroeck, Pascal Van Der Voort PII:

S1387-1811(15)00720-9

DOI:

10.1016/j.micromeso.2015.11.055

Reference:

MICMAT 7492

To appear in:

Microporous and Mesoporous Materials

Received Date: 31 August 2015 Revised Date:

17 November 2015

Accepted Date: 18 November 2015

Please cite this article as: K. Leus, T. Bogaerts, J. De Decker, H. Depauw, K. Hendrickx, H. Vrielinck, V. Van Speybroeck, P. Van Der Voort, Systematic study of the chemical and hydrothermal stability of selected “stable” Metal Organic Frameworks, Microporous and Mesoporous Materials (2016), doi: 10.1016/j.micromeso.2015.11.055. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Systematic study of the chemical and hydrothermal stability of selected “stable” Metal Organic Frameworks Karen Leusa*, Thomas Bogaertsa,b, Jeroen De Deckera, Hannes Depauwa, Kevin Hendrickxa,b, Henk Vrielinckc, Veronique Van Speybroeckb, Pascal Van Der Voorta* a

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Department of Inorganic and Physical Chemistry, Center for Ordered Materials, Organometallics and Catalysis, Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium. b Center for Molecular Modeling, Ghent University, Technologiepark 903, 9052 Zwijnaarde, Belgium. c Department of Solid State Sciences, Ghent University, Krijgslaan 281-S1, 9000 Ghent, Belgium. *corresponding authors: [email protected], [email protected]

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Abstract: In this work, the hydrothermal and chemical stability towards acids, bases, air, water, and peroxides of Metal Organic Frameworks, that are commonly considered to be stable, is presented. As a proof of stability both the crystallinity and porosity are measured before and after exposure to the stress test. The major part of the MOFs examined in this study show a good hydrothermal stability except for UiO-67, NH2-MIL-101 (Al) and CuBTC. The chemical stabilities towards acids and bases show a similar tendency and an ordering can be proposed as: MIL-101(Cr)>NH2-UiO-66>UiO-66>UiO-67>NH2-MIL-53>MIL53(Al)>ZIF-8>CuBTC>NH2-MIL-101(Al). In the tests with H2O2 most materials behaved poorly, only the UiO-66 and NH2-UiO-66 frameworks show a good stability.

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Keywords: metal organic framework, hydrothermal, chemical, stability

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Introduction Metal Organic frameworks (MOFs) are a class of inorganic-organic hybrid materials that have received great interest over the past decades. While the initial research on MOFs was focused on the synthesis and structural characterization, an increasing number of MOFs are now being examined for their interesting properties, including optical, magnetic and electronic properties as well as for their various potential applications such as in catalysis, ion exchange, gas storage and separation, sensing, polymerization and drug delivery 1-3. However, one of the major problems that limits the use of MOFs is their relatively poor stability. Besides their low thermal stability (limited to 350-400°C), few MOFs are known to be stable in the presence of water. This is due to the hydrophilic properties of the metal nodes which results in a strong interaction with water molecules and therefore leads to the cleavage of coordination bonds, hence, destroying the framework4. Since the pioneering study of Low et al. many other studies have been carried out on the water sensitivity of MOFs5-7. Very few MOFs showed no structural integrity loss in the presence of water. Within this context, the pyrazolate based frameworks showed a remarkable stability after exposure to boiling water and other solvents which was attributed to the high pKa value of the imidazole ligands8. Besides the pyrazolate based frameworks, the hydrothermally synthesized MIL series constructed from octahedrally coordinated aluminium or chromium metal clusters (MIL-53 or MIL-101 type) and zeolitic imidazolate frameworks (ZIFs) have been reported to be stable in water9,10. In particular, the ZIF-8 material, in which the zinc atoms are coordinated to methylimidazolate ligands via ZnN bonds possess a very high stability, not only under mechanical pressure but also in aqueous solutions11,12. The higher basicity of the imidazolate linker, in comparison to the carboxylate

ACCEPTED MANUSCRIPT linkers, results in stronger metal-ligands bonds and therefore in an enhanced stability towards water8. Interesting work on the water-stability of MIL-101(Cr) was performed in a study on dehumidification over hierarchically porous MOFs and the use as advanced water adsorbents, by Chang et al13. Furthermore, MOFs constructed from Zr6 based nodes also show a remarkable high stability (mechanical, hydrolytical, and chemical stability). The high stability of these Zr-based MOFs, of which UiO-66 is a prototypical example, is due to the Coulombic interaction of the highly oxophilic ZrIV metal sites with the negatively charged termini of the carboxylate linkers14.

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Nevertheless, for the evaluation of MOFs towards processes in industry, the chemical, thermal, and hydrothermal stability are important factors as many processes are performed in the presence of acids or bases (for reactions in the liquid phase) or at elevated temperature (for gas phase reactions). Furthermore, the stability of MOFs towards commonly used oxidants is also very crucial in oxidative processes. Although there have already been studies on the chemical, thermal, and hydrothermal stability of MOFs, to the best of our knowledge no systematic and no long-term stability tests have been carried out 15.

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Here, we present for the first time a systematic comparative study of reportedly “stable” Metal Organic Frameworks which will be denoted in the following as MIL-101(Cr), NH2-MIL101(Al), MIL-53(Al), NH2-MIL-53, UiO-66, NH2-UiO-66, UiO-67, ZIF- 8 and CuBTC. More specifically, their hydrothermal and chemical stability to aqueous acids (pH=0 or pH=4), -bases (pH=12), and oxidative environment (5 wt. % H2O2) is studied on short-term (3 days) and long-term (60 days). Additionally, their short and long-term exposure to water and air has been evaluated. 2 Materials and Methods

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2.1 General procedures All chemicals were purchased from Sigma Aldrich or TCI Europe and used without further purification. Nitrogen adsorption experiments were carried out at -196 °C using a Belsorpmini II gas analyzer. Prior to analysis, the samples were dried under vacuum at 120 °C to remove adsorbed water. X-Ray powder diffraction (XRPD) patterns were collected on an ARL X’TRA X-ray diffractometer with Cu Kα radiation of 0.15418 nm wavelength and a solid state detector. Thermo Gravimetric Analysis (TGA) was performed on an SDT 2960 from TA Instruments.

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2.2 Synthesis of the MOF materials MIL-101 (Cr) MIL-101(Cr) was synthesized according to an adapted recipe from Edler et al.16. In a typical experiment, 0.665 g terephthalic acid (4 mmol) and 1.608 g Cr(NO3)3·9H2O (4 mmol) were added to 20 mL deionised water. The resulting suspension was placed in a Teflon-lined autoclave at 210 °C during 8 hours under autogenous pressure (2 hours warm-up). After cooling down to room temperature, the mixture was filtered and the green solid was collected and washed thoroughly with dimethylformamide (DMF) and water in order to purify the material by removing any unreacted reagents. The material was not subjected to additional activation steps. NH2-MIL-101(Al) The NH2-MIL-101(Al) material was prepared in a few smaller batches as proposed by Fischer et al.17 270 mg (1.49 mmol) of 2-aminoterephthalic acid was dissolved in 60 mL of DMF.

ACCEPTED MANUSCRIPT This solution was heated to 110°C and 730 mg (3.0 mmol) AlCl3.6H2O was added in 6 equal portions, one each 15 minutes. The solid material began to form after approximately 30 minutes of reaction. After adding the last portion, the mixture was stirred for an additional 3 hours. In a final step the mixture was placed under heating without stirring for 16 hours. Afterwards, the solid was filtered off and washed several times with DMF after which a soxhlet extraction with acetone was performed for 6 hours in order to remove any free linkers and AlCl3.

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CuBTC

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For the synthesis of CuBTC, 2g (9.52 mmol) of benzene-1,3,5-tricarboxylic acid was added to 50 mL of a 1:1:1 mixture of DMF/EtOH/H2O. 3.4 g (17.24 mmol) of Cu(OAc)2.H2O was added with 50 mL of the same solvent mixture and both mixtures were combined under stirring. Finally triethylamine (2 mL) was added, after which the resulting mixture was stirred for 23 hours at room temperature. The product was collected by filtration and washed 2 times with 25 mL of DMF18. UiO-66-X (X=H, NH2)

UiO-67

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The UiO-66-X materials were synthesized according to a slightly modified procedure of Van Der Voort et al.19 Typically 0.3g (0.89 mmol) ZrO2Cl2.8H2O and 0.1545g (0.93 mmol) terephthalic acid or 0.168g (0.93 mmol) 2-aminoterephthalic acid were added to 3.6 mL formic acid and 9 mL dimethyl acetamide. After 20 minutes of sonication, the solution was transferred to a Teflon-lined autoclave and placed in an oven at 150°C for 12 hours. When cooled down, the solid was filtered off and subsequently stirred in DMF (12 hours) and methanol (24 hours) to remove respectively free linker and DMF from the pores. The resulting materials were dried under dynamic vacuum at 65° (X=H) and 220° (X=NH2) for 24 hours.

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The synthesis procedure of UiO-67 was based on the recipe of Farha et al.20. 0.27 mmol ZrCl4 and 0.38 mmol 4,4'-diphenyldicarboxylic acid (BPDC) was dissolved in 15 mL DMF and 0.5 mL concentrated HCl. The resulting solution was sonicated for 20 minutes and transferred afterwards in a thermoblock at 80° for 12 hours. After filtration and washing with DMF and ethanol, the samples were dried under a dynamic vacuum at 90°C and activated at 150°C (3 hours). MIL-53 (Al)

MIL-53 (Al) was synthesized according to a slightly modified procedure of Ferey et al. 21. A mixture of AlCl3·6H2O (2.90 g, 12.0 mmol), terephthalic acid (2.00 g, 12.0 mmol), and deionised water (60 mL) is placed in a Teflon-lined autoclave at 210 °C for 24 hours. Afterwards the MIL-53 (Al) as is collected by filtration and washed with acetone, followed by a calcination at 300°C for 72 hours to obtain MIL-53 (Al). NH2-MIL-53 The NH2-MIL-53 was synthesized following the recipe of Stock et al.22. 1,48 g (6.13 mmol) of AlCl3.6H2O was mixed with 1,13 g (6.24 mmol) of 2-aminoterephthalic acid in 15 mL of

ACCEPTED MANUSCRIPT deionised water. The resulting solution was placed in a Teflon-lined autoclave at 150 °C (heating rate: 1 hour) for 5 hours. Afterwards the solid material was filtered off and placed with DMF in an autoclave for an additional 15 hours at 150 °C to remove unreacted linker. The DMF molecules were removed by a thermal treatment in air at 130°C in a muffle furnace.

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For the synthesis of ZIF-8, 0.733 g Zn(NO3)2.6H2O (2.46 mmol) was dissolved in 50 mL deionised water. A second solution of 1.622 g HMe-Im (19.75 mmol) and 2.00 g TEA (19.76 mmol) in 50 mL deionised water was prepared. The Zn solution was added to the second solution under stirring which resulted immediately in an opaque white solution. After stirring for an additional 10 minutes at room temperature, the solid was separated through centrifugation and placed in water for 12 hours. This procedure was repeated 2 times. Hereafter the solid was collected and dried in air at 110 °C. Finally, the sample was dried under vacuum at 150 °C for 1 hour 23.

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2.3 Chemical stability The chemical stability tests were performed by exposing the MOFs for 3 or 60 days to acidic conditions (HCl, pH=0 and pH=4), basic conditions (NaOH, pH=12) or under oxidizing conditions (5 wt. % H2O2). Additionally, the stability of the MOFs was examined by exposing them to air and water for respectively 3 or 60 days. All the tests were carried out at room temperature (RT) without stirring.

3. Results and discussion

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2.4 Hydrothermal stability The hydrothermal stability study was conducted by exposing the MOF samples to saturated steam for 5 hours at 200°C.

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3.1 Hydrothermal stability In Table 1 the Langmuir surface area is presented of all the pristine MOF materials and after exposing them to water, air, acidic-, basic-, oxidative-, and a hydrothermal environment. In Table 2 the results of the hydrothermal treatment as well as those of the short and long term exposure to air and water are illustrated (based on XRPD spectra). As can be seen from Table 2 and Figure S1, MIL-101 (Cr) preserved its crystalline structure after the steaming test. This observation is in agreement with the earlier report of Ferey et al. in which the high moisture stability of MIL-101 (Cr) was already stated 24. Furthermore, the hydrothermal stability of this chromium based MOF was confirmed in the work of Kang et al. in which they found that the MIL-101 (Cr) was structurally stable after exposure to water at 80°C for 5 days, as well as in the work of Chang et al. demonstrating that the MIL-101 (Cr) preserved its crystalline structure after exposure to boiling water for 1 week 25,26. In contrast to the high hydrothermal stability of the MIL-101 (Cr), it is known that the MIL-101 analogues of Fe3+ and Al3+ show a much lower resistance to hydrolysis. This last conclusion was also confirmed by our study demonstrating that the NH2-MIL-101 (Al) is highly sensitive to water which leads to the transformation into the thermodynamically more stable NH2-MIL-53 (Al). The latter observation is in agreement with the recent report of Senker et al. demonstrating that the NH2MIL-101 (Al) already is transformed into NH2-MIL-53 (Al) after only 5 minutes exposure to water 27. Besides the MIL-101 (Cr), the NH2-MIL-53 is also stable under the examined steaming conditions (see Table 2 and Fig S1) as no changes are observed in the XRPD pattern

ACCEPTED MANUSCRIPT in comparison to the pristine NH2-MIL-53. In contrast to the NH2-MIL-53, the MIL-53 (Al) exhibits a partial transformation towards a new crystalline phase, as new diffraction signals arises at 2θ=11° and 21°. Furthermore, as can be seen from Table 1, the Langmuir surface area of the MIL-53 (Al) is reduced from 1531 m2/g to 792 m2/g which suggests a partial hydrolysis of the MOF framework. The latter observation was also noted in the work of Bellat et al. and Jhung et al. who attributed this effect to the production of free organic linker H2BDC and to the formation of γ-AlO(OH) species under reflux conditions in water 15,28.

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The copper paddle-wheeled framework, CuBTC, is known to be unstable under steaming conditions29. As can be seen from Fig S1 and Table 1, not only was the framework converted into another structure, also the porosity was completely lost after the hydrothermal treatment. Based on literature, the structure of the green phase obtained after the hydrothermal treatment could be assigned to [Cu2OH(BTC)(H2O)]n.2nH2O30. While the CuBTC material is completely destroyed, the ZIF-8 framework can withstand the hydrothermal treatment as no changes are observed in the crystal structure and only a minor decrease in the Langmuir surface area is noted (from 772 m2/g to 660 m2/g). This observation is in agreement with previous studies showing that ZIF-8 is resistant to steam for hours and to boiling water for at least a week6,12. Additionally, the UiO-66 and NH2-UiO-66 framework are stable after 5 hours at 200°C under an autogeneous pressure while the UiO-67 material is completely destroyed. The lack of stability of the UiO-67 material to H2O exposure was recently assigned to the linker hydrolysis engineered by clustering of H2O molecules near the Zr6 nodes and to rotational effects of the extended organic linker 31.

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3.2 Stability towards water and air As can be seen from Table 2 and Fig S2, the MIL-101 (Cr) preserved its crystalline structure after 3 days in air or water which is in agreement with earlier literature reports on this Crbased MOF25,32. Moreover, even after exposure for 2 months to air or water no changes are observed in the Langmuir surface area and in the XRPD pattern of the MIL-101 (Cr), confirming the remarkable stability of this framework. Noteworthy is the starting value of the Langmuir surface area of MIL-101(Cr) (~2000 m²/g), which is considerably lower than the value reported by Ferey et al24. This is due to the fact that the material was not subjected to an activation step after synthesis, in order to check if the applied stability conditions could also serve as purification solvents. In contrast to the MIL-101 (Cr), the NH2-MIL-101 (Al) is already completely destroyed after exposure to air for 3 days. Table 1. Effect of the hydrothermal treatment, chemical and oxidative treatment and exposure to water and air (short-term and long-term) on the Langmuir Surface Area (expressed in m2/g) UiO66

NH2-UiO66

UiO67

MIL-53 (Al)

NH2-MIL-53 (Al)

MIL-101 (Cr)

NH2-MIL-101 (Al)

CuBTC

ZIF8

pristine material

1008

885

2395

1531

NP

2001*

2026

846

772

pH=0, RT

549

712

35

D

NP

3180

D

D

D

pH=4, RT

1063

994

2182

1068

NP

3200

182

1103

673

pH=12, RT

1051

590

1395

76

NP

3109

187

153

652

water, RT

1018

632

2436

1380

NP

2227

1060

823

656

steam, 5 h

886

548

33

792

NP

2200

253

4

660

Air

651

492

172

1385

NP

2005

244

701

801

5% H2O2, RT

979

686

19

607

NP

2040

430

402

502

pH=0, RT

534

591

29

D

NP

3787

D

D

D

pH=4, RT

973

729

16

136

NP

3470

173

266

160

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pH=12, RT

866

646

2042

740

NP

3250

191

289

139

water, RT

1145

740

2320

1165

NP

2322

183

469

673

Air

738

562

523

1530

NP

1998

119

284

755

5% H2O2, RT

1142

486

77

241

NP

<20

377

41

107

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NP= narrow pore ,Langmuir surface area is < 50 m /g, D= dissolved. (*) Specific surface area as obtained after synthesis, without additional purification steps.

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Additionally, slight changes are observed in the XRPD pattern of the NH2-MIL-101 (Al) after exposure for 3 days to water, and a decrease in the Langmuir surface area is observed (from 2026 to 1060 m2/g), suggesting a partial transformation of the framework into NH2-MIL-53. In contrast to the report of Senker et al., who observed this transformation after only 5 minutes in water, we observed this framework transformation after 2 months 27. Besides the MIL-101 (Cr), also the MIL-53 (Al) and NH2-MIL-53 material exhibit a good stability to air and water after a contact time of 3 days. Moreover, even after 2 months in air or water no significant changes are observed in the XRPD patterns for both materials. The high stability of MIL-53 (Al) could be due to the strong chemical bond between the Al sites in the secondary building unit (SBU) and the oxygen atoms. Additionally, Huang et al. stated that the hydrophobic aromatic walls can prevent the attack of the AlO4(OH)4 units by water molecules 33. Nevertheless, it should be noted that a small degradation of the framework will not be visible in the XRPD pattern due to the high intensities. As the Langmuir surface area of the MIL-53 (Al) decreases from 1531 to 1165 m2/g, a partial hydrolysis/transformation, similar to the one observed during the hydrothermal treatment (see section 3.1), is suggested. In contrast to the MIL-53 (Al) framework which shows a good stability in water, the CuBTC material is known to be unstable in the presence of water. This observation was also confirmed by our experiments. From the XRPD data in Fig S2 it can be seen that even after a contact time of 3 days to water the characteristic diffraction peaks of the CuBTC material are still present besides the appearance of new diffractions. Many groups have examined the degradation of the latter framework and the assumption was made that water molecules can coordinate along the Cu–Cu axis and therefore can distort the SBU along that axis. However, this distortion is not able to break the Cu–carboxylate bonds, and water clusters need to be formed around the SBU before degradation will take place34,35

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In comparison to the CuBTC material which has free metal sites, a defect-free zirconium based MOF,UiO-66, should have fully saturated sites which makes it impossible for water molecules to coordinate with the framework, hence leading to a higher water stability. No changes are observed in the XRPD pattern and in the Langmuir surface area in comparison to the pristine material even after exposure for 2 months to water. Furthermore, the incorporation of functional groups into this framework resulted in a similar stability to water and even a higher stability after exposure to air. As can be seen from Fig S2, the UiO-66 material starts already to degrade after only 3 days exposure to air while the NH2-UiO-66 shows no structural changes based on the XRPD data. This similar or better stability in comparison to the parent framework was already observed by Lillerud et al.36 Additionally, the extended UiO-66 framework, denoted as UiO-67, shows a good stability towards linker hydrolysis in water which was also noted in the study of Farha et al. and Maurin et al.14,37. Although, no changes are observed in the XRPD pattern of the UiO-67 after exposure for 2 months to air, there is a significant decrease in the Langmuir surface (from 2395 m2/g for the pristine MOF material, to 172 m2/g after 3 days and 523 m2/g after 2 months exposure to air). Concerning the stability of ZIF-8 there is a lot of debate whether the

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material is stable in water or not, as conflicting results have been published. From our hydrothermal stability tests (see section 3.1) it was clear that the ZIF-8 showed no structural degradation. However, after storage at room temperature for 2 months new diffraction peaks appear in the XRPD pattern which were also observed in the work of Kaskel et al. 5(see Fig.S2). In a recent report of Friscic et al. these diffraction peaks have been assigned to complex carbonates, the hydrolytic degradation products of ZIF-8, which are formed due to the chemical reaction of the framework with CO2 in the presence of water or moisture 38.

3 days

2 months

water

Hydrothermal stability

+

+

+

-

T (NH2-MIL-53)

T (NH2-MIL-53)

+

+

PT

+

+

+

+

PD

+

+

+

+

+

+

+

+

+

D

MIL-101 (Cr)

+

+

NH2-MIL-101 (Al)

-

PT

MIL-53 (Al)

+

+

NH2-MIL-53

+

+

UiO-66

PD

NH2-UiO-66

+

UiO-67

+

EP +

PT

+

PT

T([Cu2OH(BTC)(H2O)]n .2n H2O)

+

+

+

PT

+

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ZIF-8

air

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air

CuBTC

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Table 2. Effect of the hydrothermal stability test and short and long time exposure to air and water on the XRPD pattern of the examined MOF materials.

+: crystallinity preserved, -: MOF is destroyed, PT: partial transformation, PD: partial destruction, T: transformation, D: dissolved

3.3 Chemical stability 3.3.1 Stability towards acids In Table 3. the effects of exposing the MOFs for 3 days or 2 months to acidic solutions (pH=0 and pH=4), basic conditions (pH=12) and peroxides are reported. From this table and Fig S3 it can be seen that the MIL-101 (Cr) exhibits an extremely high stability under acidic conditions as no changes can be observed in the XRPD patterns even after exposing the material for 2 months in a pH=0 or pH=4 solution. Moreover, from Table 1, it is clear that the Langmuir surface area increased drastically after treating the MOF with acidic solutions (from

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2001 m2/g towards 3180 m2/g or higher). To determine if this increase in surface area is due to the dissolution of left-over starting reagents (e.g. terephthalic acid), TGA experiments were carried out. Figure 1 depicts the TGA spectra of the pristine MIL-101(Cr) and after a 1M HCl treatment for 3 days. From this figure one can clearly see that the pristine MIL-101(Cr) exhibits a first weight loss at approximately 250-260°C, which can be assigned to free terephthalic acid molecules that are still clogged in the pores as this temperature corresponds to the flash point of terephthalic acid. In a second weight loss at 320°C the MIL-101 (Cr) starts to decompose. The thermostability is in agreement with the earlier reports on MIL-101 (Cr) 39. After acid exposure, the first weight loss step at 250° is absent, indicating that the acid treatment removes the free organic linker from the pores. 100 90

MIL-101(Cr) prior 1M HCl treatment

80 % /70 e ga t 60 n e cr 50 e p40 ss a30 M 20

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MIL-101(Cr) post 1M HCl treatment

0 0

100

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10 200

300 400 Temperature /°C

500

600

700

Figure 1. TGA curve of MIL-101 (Cr) before and after the treatment in 1M HCl (heating rate 2°C min-1)

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In contrast to MIL-101 (Cr), NH2-MIL-101 (Al) shows a very low stability towards acids. The NH2-MIL-101 (Al) structure is completely converted into the thermodynamically more stable structure NH2-MIL-53 after 3 days in a pH=4 solution whereas in a pH=0 solution the MOF was completely dissolved after a few minutes. Besides the NH2-MIL-101 (Al), the ZIF-8 and CuBTC structures were also immediately dissolved after exposure to a solution of pH=0. Nevertheless, ZIF-8 still possesses a good crystallinity after 3 days in a pH=4 solution, whereas the CuBTC material already starts to form a new crystalline phase, which was also observed after exposing it to water (see section 3.2). After remaining for 2 months at pH=4, the ZIF-8 shows a significant degradation. Although part of the MOF diffraction peaks are still present, new diffraction peaks can be observed, which were also noted after storage for 2 months in water and which could be assigned to carbonates (see section 3.2)38.

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Next to the MIL-101 (Cr), UiO-66 as well as the functionalized NH2-UiO-66 exhibit an exceptional stability under acidic conditions, even after exposure for 2 months in pH=0 and pH=4 solutions. The short-term stability of both materials was already seen by the group of Lillerud, who demonstrated that no loss in crystallinity could be observed after 2 hours in HCl (pH=1)36. In the recent report of Zhong et al. it was shown that UiO-66 can be kept intact in solutions of pH=2 to 6 for at least 24 hours, in the absence of fluorine40. Although both materials show no loss in crystallinity, one can clearly see from Table 1 that the Langmuir surface area decreases significantly for both MOFs after exposure for 3 days at pH=0. The Langmuir surface areas of the pristine UiO-66 and NH2-UiO-66 are respectively 1008 m2/g and 885 m2/g, whereas afterwards they dropped to respectively 549 m2/g and 712 m2/g. In contrast to the UiO-66 materials, UiO-67 reveals a much lower stability towards acids. In a pH=4 solution the framework is still well preserved as can be seen from the XRPD pattern in Fig.S3. Also no significant loss in the Langmuir surface area is noted (see Table 1). However, after immersing the material for 2 months in the latter solution, the framework shows a significant decrease in crystallinity and in surface area. Moreover, the framework is already completely destroyed after 3 days at pH=0, as only the diffraction peaks of the free organic

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linker, (4,4'-diphenyldicarboxylic acid), can be observed in the XRPD pattern (Fig.S3). From Fig S3 it can be seen that the MIL-53 (Al) material exhibits a rather good stability at pH=4 for 3 days. No changes in the crystallinity are observed, which is consistent with the report of Huang et al. who showed that the MIL-53 (Al) structure was highly resistent to hydrolysis in acidic solutions (pH=2) after immersing the material for 7 days at room temperature or even at 100°C33. However, although no changes are observed in the XRPD pattern, a significant decrease is noted in the Langmuir surface area in comparison to the pristine material. After exposure for 3 days in a pH=4 solution the Langmuir surface area is decreased to 1068 m2/g whereas the pristine MOF started out at 1531 m2/g. Furthermore, after exposure for 2 months to a solution of pH=4 the Langmuir surface area decreased to 136 m2/g which shows that the MIL-53 (Al) gradually decomposes. After 2 months at pH=4 a new crystalline phase starts to appear which can be assigned to γ-AlO(OH) and terephthalic acid intercalated into the pores28. Additionally, after immersing the material for a period of 3 days in a solution of pH=0, the structure is completely destroyed and only the diffraction peaks of the terephthalic acid linkers can be observed in the XRPD pattern. In comparison to MIL-53 (Al), the functionalized NH2-MIL-53 exhibits an enhanced stability under acidic conditions, as even exposure for 2 months at pH=4 shows no differences in the crystallinity of this MOF. However, in stronger acidic conditions (pH=0) the material experiences a gradual decomposition. After suspending the material for 2 months at pH=0 the structure of NH2MIL-53 is almost completely destroyed and only the diffraction peaks of the free 2aminoterepthalic acid are observed.

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3.3.2 Stability towards bases Besides the very high stability of MIL-101 (Cr) in acidic media, it can be seen from Table 3 and Fig S4 that it also possesses a good stability towards bases. No changes in the crystallinity are observed after 2 months in a pH=12 solution. Additionally the Langmuir surface area increases significantly, an effect which was also observed after the treatment with acid. The Langmuir surface area of the MIL-101 (Cr) after 2 months is increased to 3250 m2/g whereas the parent MOF had a Langmuir surface are of 2001 m2/g. Table 3. Effect of a short and long time exposure to acids (pH=0 and pH=4), bases (pH=12) and peroxides on the XRPD pattern of the examined MOF materials.

pH=0

EP

3 days

2 months

pH=4

pH=12

H 2O2

pH=0

pH=4

pH=12

H 2O2

+

+

+

+

+

+

T (MIL-53 as)

+

NH 2-MIL-101 (Al)

D

T (NH 2-MIL-53)

T (NH2-MIL-53)

T (NH2-MIL-53)

D

T (NH2-MIL-53)

T (NH 2-MIL-53)

T (NH2-MIL-53)

MIL-53 (Al)

D

+

+

PT

D

PT

+

PT

NH 2-MIL-53

PD

+

+

+

D

+

+

PT

UiO-66

+

+

+

+

+

+

+

+

NH 2-UiO-66

+

+

+

+

+

+

+

+

UiO-67

D

+

+

D

D

PD

+

D

CuBTC

D

PT

T several crystalline phases)

+

D

PT

T (several crystalline phases)

D

ZIF-8

D

+

+

PD

D

PD

PT

D

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MIL-101 (Cr)

+: crystallinity preserved, -: MOF is destroyed, PT: partial transformation, PD: partial destruction, T: transformation, D: dissolved

ACCEPTED MANUSCRIPT In contrast to the very high stability of the MIL-101 (Cr), the NH2-MIL-101 (Al) and CuBTC framework show an exceptional low stability towards bases as can be seen from Fig. S4. After contact for only 3 days at pH=12 a complete structure transformation of NH2-MIL-101 (Al) into NH2-MIL-53 is observed, whereas the CuBTC framework is converted into other crystalline phases, which were also seen after contact in water and in acidic media. The formed phases can probably be assigned to free organic linker, benzene-1,3,5-tricarboxylic acid, [Cu2OH(BTC)(H2O)]n.2nH2O and the original CuBTC framework30.

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MIL-53 (Al) and NH2-MIL-53 exhibit a good stability in basic media. No loss of crystallinity is observed from the XRPD patterns (Fig.S4). Nonetheless, there is a significant decrease in the Langmuir surface area of the MIL-53 (Al) in comparison to the pristine MOF. This result is in agreement with the report of Jhung et al. who noticed a decrease in the surface area and XRD intensities as a function of contact time after stirring the MIL-53 (Al) in a 7.0 x 10-2 M NaOH solution at room temperature15. Also the group of Huang observed no significant changes in the degree of crystallinity after uninterrupted exposure up to 7 days at room temperature or 50°C at pH=1433. However, after increasing the temperature to 100°C, the framework shows limited structure stability in a pH=14 solution and structural transformations start to occur after 2 days. Besides the MIL-53 (Al) and NH2-MIL-53, the ZIF-8 shows proper stability in basic solutions as well, as can be seen from Fig. S4. After 3 days in a pH=12 solution no changes are observed in the XRPD pattern and only a minor decrease in the Langmuir surface area is noted. This is in agreement with the report of Yaghi et al. demonstrating that the ZIF-8 stayed unchanged for up to 24 h in 0.1 and 8 M aqueous sodium hydroxide at 100°C12. However, after contact for 2 months in a pH=12 solution extra diffraction peaks are observed, which can be assigned to the formation of carbonates 38. Next to the ZIF-8 material, the UiO-66, NH2-UiO-66 and UiO-67 also reveal a high resistance towards bases. No changes are observed in the XRPD pattern and only a slight decrease is observed in the Langmuir surface area. However at higher pH’s Lillerud et al. have noticed that the UiO-66 framework starts to transform in a less crystalline material within 2 hours, whereas the treatment of NH2-UiO-66 with NaOH led to a total decomposition of the MOF into an amorphous phase36.

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3.3.3 Stability towards peroxides In contrast to the previous examined media in which the MIL-101 (Cr) showed an exceptional stability, it can be seen from Table 3 and Fig. S5 that the MIL-101 (Cr) is completely converted into another crystalline phase, more specifically the MIL-53 as structure, after exposure for 2 months to the oxidative solution. The ZIF-8 material is almost fully amorphized after 2 months in the H2O2 solution, whereas the CuBTC material is transformed into a crystalline phase which we could not directly identify. While the already mentioned MOFs preserved their crystallinity after 3 days contact time (although their Langmuir surface area did decrease), the UiO-67 and NH2-MIL-101 (Al) showed a complete degradation after only 3 days in the oxidative environment. The NH2-MIL-101 (Al) is converted into the thermodynamically more stable NH2-MIL-53 structure whereas from the UiO-67 only the diffraction peaks of the free linker 4,4'-diphenyldicarboxylic acid can be observed.

ACCEPTED MANUSCRIPT Table 4. Concluding overview on the stability (N2 and XRD) of the examined MOFs in the various media ( the XRPD pattern and surface area is largely preserved, the XRPD pattern and surface area is completely destroyed, the XRPD pattern and surface area is partially decreased/degraded or transformed) 3 days pH=0

pH=4

pH=12

XRD N2 XRD N2 XRD N2

2 months

H2O2 XRD

N2

H2O XRD

air

N2

XRD

pH=0 N2

XRD

N2

pH=4 XRD

N2

pH=12 XRD

N2

H2O2 XRD

N2

H2O XRD

N2

air XRD

Steam test N2

XRD

N2

NH2-MIL-101 (Al) MIL-53 (Al) NH2-MIL-53

NP

NP

NP

NP

NP

NP

UiO-66 NH2-UiO-66 UiO-67

NP

NP

NP

NP

NP

NP

SC

CuBTC

NP

RI PT

MIL-101 (Cr)

ZIF-8

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In contrast to the extended UiO-67 framework, UiO-66 and NH2-UiO-66 show a remarkable stability in the peroxide solution. Even after exposure for 2 months, no changes are observed in the XRPD pattern (see Fig. S5). Moreover, the Langmuir surface area of UiO-66 stayed unaltered whereas for the NH2-UiO-66 a significant decrease is observed (from 885 m2/g to 486 m2/g after 2 months). Also for the MIL-53 (Al), a gradual decrease in the Langmuir surface area is noted as a function of time (from 1531 m2/g to 607 m2/g and 241 m2g/ after respectively 3 days and 2 months) in addition to progressive structure transformation into a new crystalline phase. This phase was also observed after the hydrothermal treatment and after exposure to acidic media and could be assigned to free organic linker and γ-AlO(OH)28. A gradual degradation of the framework was also noted for the NH2-MIL-53. After 2 months in the H2O2 solution, new diffraction peaks appear showing that the NH2-MIL-53 structure is modified.

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Conclusions In this work we presented for the first time a systematic comparison of the stability of several metal organic frameworks. In order to compare these materials, not only the crystallinity before and after the tests were compared, but also the porosity. In the reports of many new MOF materials some kind of stability test is already proposed but in this work all the materials were treated under the same conditions to allow a fair comparison. For most of the frameworks under examination (MIL-101(Cr), NH2-MIL-101(Al), MIL-53(Al), NH2-MIL-53, UiO-66, NH2-UiO-66, UiO-67, ZIF- 8, CuBTC) the hydrothermal stability was confirmed (see Table 4). The chemical stability towards acids and bases was overall disappointing, especially at a 2 months exposure time, with a few notable exceptions. One example worth noting was MIL-101(Cr) that reached a higher internal surface area after an acid or basic washing step. The NH2-MIL-101(Al) structure shows a very low stability as it can be easily converted to the more thermodynamically stable NH2-MIL-53 framework. Very few of the examined MOFs showed a good stability in the 5 wt. % H2O2 solution. Only the UiO-66 and NH2-UiO-66 material showed no loss in crystallinity after storage for 2 months. Furthermore, it was shown that it is important to not only consider the crystallinity to assess the stability of a material as many materials which appeared to retain crystallinity based on the XRPD analysis exhibited a significant decrease in internal surface area, indicating that porosity measurements should also be consistently applied to verify the stability of a material.

ACCEPTED MANUSCRIPT Acknowledgements K.L. acknowledges the financial support from the Ghent University BOF postdoctoral grant 01P06813T. K.L. and T.B. thank UGent for the financial support via GOA Grant 01G00710. H.D. thanks the Research Foundation Flanders (FWO-Vlaanderen) Grant No. G.0048.13N. J.D.D. acknowledges the Special Research Fund Grants Nr. 05V00812 (AUGent).

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Very few MOFs show a good chemical stability after 2 months exposure time NH2-MIL-101 (Al) show a very low stability and can be easily converted into NH2-MIL-53 The Langmuir surface area of MIL-101 (Cr) can be increased by treatment with acid of base.

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