Recovery of Acid-Gas-Degraded Zeolitic Imidazolate Frameworks by

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Recovery of Acid Gas-Degraded Zeolitic Imidazolate Frameworks by Solvent-Assisted Crystal Redemption (SACRed) Krishna C. Jayachandrababu, Souryadeep Bhattacharyya, Yadong Chiang, David S. Sholl, and Sankar Nair ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11686 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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ACS Applied Materials & Interfaces

Recovery of Acid Gas-Degraded Zeolitic Imidazolate Frameworks by Solvent-Assisted Crystal Redemption (SACRed) Krishna C. Jayachandrababu, Souryadeep Bhattacharyya, Yadong Chiang, David S. Sholl, and Sankar Nair* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA 30332-0100 AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The acid stability of zeolitic imidazolate frameworks (ZIFs) is an important issue hindering their application. Acid gas damage of ZIFs has been considered irreversible. However, we demonstrate a methodology called ‘Solvent Assisted Crystal Redemption’ (SACRed) to reverse acid gas damage to ZIFs with a high degree of structural and functional recovery. For example, post-SACRed ZIF-8 is shown to be structurally and chemically near-identical to the original pristine ZIF-8 that suffered a large loss of surface area, porosity, and crystallinity during acid gas exposure. We also provide mechanistic insight into the recovery process using deuterium-labeled linkers and 2H NMR spectroscopy. SACRed treatments could allow large extensions in lifetime of ZIF-based membranes and adsorbents that degrade over time.

KEYWORDS MOFs, ZIFs, Acid gas, Degradation, Recovery


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Zeolitic Imidazolate Frameworks (ZIFs) are a widely studied class of nanoporous Metal Organic Frameworks (MOFs). They possess attractive characteristics such as facile synthesis, good thermal and chemical stability, and pore sizes suited for molecular recognition. For example, ZIF-8 is investigated for gas and liquid mixture separations, catalysis, chemical sensors, and drug delivery.1-7 Unlike many other classes of MOFs, ZIFs are stable under a range of conditions.8-9 Their acid stability, however, is generally poor owing to the use of imidazole bases as linkers.10 In many practical separation applications (e.g., CO2 removal from flue gases, natural gas purification, petrochemical separations), ZIFs will come into contact with varying concentrations of acid gases such as CO2, SOx, NOx, or H2S, usually in combination with water vapor. Recent studies have shown that even ppm-levels of humid SO2 cause significant damage to ZIF-8 crystals over time.11-12 The damage is caused by cleavage of the Zn-imidazole bond and protonation of the N atom by the humid acid gas, and can be quantified by loss of surface area, porosity and crystallinity.11 Visible changes in appearance and texture of the crystals also occur. Acid gas damage to ZIFs is generally regarded as permanent and irreversible.

Until now, efforts for improving MOF/ZIF stability have involved structural modifications such as linker functionalization or metal substitution.13-16 These modifications often lead to loss in surface area/pore volume (due to bulkier linkers), altered properties, or more cumbersome synthesis. An alternative strategy is to periodically regenerate MOFs after damage caused by environmental exposure. Here we find that a solution-based linker delivery route (which we call ‘Solvent Assisted Crystal Redemption’ – SACRed) can reverse acid gas damage to ZIFs, with examples of ZIF-8 (SOD topology, 2-methylimidazole linkers) and ZIF-14 (ANA topology, 2-ethylimidazole linkers) presented. We demonstrate that the crystallinity, surface


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area, and pore volume can be completely recovered for ZIF-8 (and substantially for ZIF-14) using SACRed, even for severely degraded materials. We show that recovered ZIF-8 is structurally and chemically near-identical to its pristine counterpart, and investigate the mechanism for this process using isotopically labelled linkers. ZIF-8 and ZIF-14 synthesis, characterization (by XRD, SEM, nitrogen physisorption, and FTIR spectroscopy), controlled exposure to humid SO2, and activation were carried out according to previously reported procedures3,11,17-18 (Supporting Information). Pristine ZIF crystals were divided into batches of 150-200 mg and exposed to humid SO2 to obtain varying levels of structural degradation. After exposure, the SACRed treatments were performed by treating the degraded ZIF materials with 0.25 M methanolic linker solutions at 90 °C inside a sealed reactor for 48 hours. Table 1 shows the loss of surface area and pore volume after SO2 exposure, and the remarkable regeneration of these properties after SACRed procedures. It is seen that even after losing ~100% of surface area and pore volume to acid gas attack, ZIF-8 (ZIF-14) crystals are amenable to complete (substantial) recovery using this method.

Figure 1 shows XRD patterns of ZIF-8 and ZIF-14 in pristine, post-degradation, and post-SACRed form. After humid SO2 exposure, the increase in background and loss of peak intensities are attributed to degradation and gradual amorphization.11 In contrast, post-SACRed ZIF-8 regained most of the peak intensities and the background signal is considerably reduced. A similar trend is observed for ZIF-14, although the recovery of crystallinity is less than for ZIF-8. In both materials, SACRed led to reduction in the amorphous phase and regeneration of lost peak intensity induced by acid-gas exposure.


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Table 1. Surface area (SA) and pore volume (PV) of ZIF-8 and ZIF-14 before SO2 exposure, after SO2 exposure, and after SACRed treatment. Experiments were done in duplicate with independent samples to obtain error values, which are based upon the distance from the mean value.

Humid SO2 Exposure

Batch

SA (m /g)

PV (cm /g)

-

-

1341±44

0.64±0.01

-

-

545±45

0.20±0.01

ZIF-8 exposed to humid SO2 (activated)

100 ppm-days

1 2 3

510±30 484±24 245±20

0.25±0.01 0.24±0.01 0.13±0.01

200 ppm-days

4

~0

~0

ZIF-14 exposed to humid SO2 (activated)

30 ppm-days 70 ppm-days 125 ppm-days 300 ppm-days

ZIF-8 post-SACRed (activated)

100 ppm-days

1 2 3 4 1 2 3

360±70 210±30 150±46 ~0 1355±51 1329±28 1270±32

0.13±0.03 0.08±0.01 0.08±0.01 0.01 0.63±0.02 0.63±0.01 0.62±0.02

200 ppm-days

4

1214±20

0.58±0.01

30 ppm-days 70 ppm-days 125 ppm-days 300 ppm-days

1 2 3 4

421 442 470 89

0.15 0.16 0.18 0.04

Material Pristine ZIF-8 (activated) Pristine ZIF-14 (activated)

ZIF-14 post-SACRed (activated)

2

3


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Figure 1. XRD patterns of (a) ZIF-8: pristine, exposed to 100 ppm-days humid SO2, and postSACRed; and (b) ZIF-14: pristine, exposed to 125 ppm-days humid SO2, and post-SACRed. The ordinate axis is logarithmic without any offsets. Patterns are normalized to the highest peak at 2θ = 7.33° (ZIF-8) and 8.23° (ZIF-14).


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Figure 2. (a-c) Optical photographs and (d-f) corresponding SEM images of ZIF-8 powders: pristine, after exposure to 100 ppm-days of humid SO2, and post-SACRed respectively. Photographs in (a-c) depict a 2.5×2.5 cm2 area.


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Figure 2 shows photographs and SEM images of ZIF-8 at various stages. The pristine ZIF-8 crystal samples are colorless with a fine powdery texture, and individual crystals have a rhombic dodecahedral morphology. After SO2 exposure the crystals turn off-white to yellowish with more agglomeration. SEM images show some morphological changes, with a veneer of apparently amorphous material covering the crystals. After SACRed, the crystal texture is restored but a pale pink tint (indicative of trace amounts of sulfur-containing species) remains, and which fades with repeated washing of the crystals in fresh methanol. SEM images show small unidentified particles adhering to the crystals.

Figure 3. Butanol, and water (inset), adsorption at 30 °C in pristine (black) and post-SACRed ZIF-8 (blue). The rapid rise in water uptake at >80% RH is due to interparticle capillary condensation.

We tested the functionality of the regenerated ZIF-8 crystals by measuring 1-butanol and water adsorption at 30°C (Figure 3).

Both pristine and redeemed crystals showed similar


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adsorption profiles, with post-SACRed crystals exhibiting a slightly higher uptake than pristine ZIF-8. This is consistent with the slightly higher measured surface area of the post-SACRed material (1399 m2/g) versus the pristine material (1297 m2/g). It is clear that SACRed restores ZIF-8 to the full functionality of the pristine material. The hydrophobicity of post-SACRed ZIF is as good (or slightly better) than pristine ZIF-8, indicating that there are no hydrophilic defect sites retained that may have been created by acid-gas exposure.


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Figure 4. FTIR spectra in the (a) 500-2000 cm-1 and (b) 2000-4000 cm-1 wavenumber regions for ZIF-8: pristine (black), exposed to 100-ppm days of humid SO2 and before activation (red), after activation (green), and post-SACRed treatment (blue). We used FTIR spectroscopy (Figure 4) to understand the chemical transformations occurring during SACRed in ZIF-8. The spectrum of pristine ZIF-8 agrees with previous reports.11, 19-21 After 100 ppm-days of humid SO2 exposure (and before reactivation), several new peaks are observed as characterized in our recent work.11 These peaks have been assigned to (bi)sulfite or (bi)sulfate groups along with adsorbed SO2. A more detailed description of the peak assignments in these two materials is given in the Supporting Information. After reactivation, the humid SO2 exposed sample shows substantial decreases (but not complete disappearance) in peak intensities in the broad (bi)sulfite stretch region centered around 870 cm-1 as well as the (bi)sulfate and adsorbed SO2 regions. The water bending peak at 1630 cm-1 disappears, and the intensities of the N-H stretch and O-H stretch decrease. The FTIR spectrum of this sample is still significantly different from the pristine ZIF-8 due to Zn-linker bond cleavage and incomplete removal of sulfur-containing species. However, the FTIR spectrum of post-SACRed ZIF-8 reveals a close match with pristine ZIF-8. The (bi)sulfite and (bi)sulfate peaks disappear completely along with the broad stretch region from 2300-3600 cm-1. A small decrease in the intensity of the ring vibrations (1350-1500 cm-1) is observed, while other small differences are a result of manual baseline corrections. The FTIR spectra thus indicate nearcomplete removal of any residual (bi)sulfite or (bi)sulfate groups by SACRed and the nearcomplete reformation of Zn-linker bonds (the N-H and O-H stretch regions are no longer observed). EDX analysis (Figure S1, Supporting Information) strongly corroborates the nearcomplete removal of sulfur-containing species from the recovered sample. The sulfur peaks (Kα:


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2.309 keV, Kβ: 2.465 keV) in the EDX spectrum disappear almost completely after SACRed. Thus, we can consider the post-SACRed ZIF-8 material as chemically near-identical to pristine ZIF-8. The absence of S species in the post-SACRed material suggested that their removal might be critical to the recovery process. We recently showed that the ZIF-8 degradation mechanism involves an acid proton cleaving the Zn-N coordination bond, leading to loss of surface area and porosity due to the dangling linker-acid complex.11 Therefore, it is possible that restoration of crystallinity and porosity is due to a simple removal of acid species by reaction with a base such as 2-methylimidazole (2-MeIm) used in SACRed, allowing the dangling linker to re-establish the coordination bond. To test this possibility, we attempted ZIF-8 regeneration with three other basic solutions: aqueous 2-MeIm, aqueous NaOH, and methanolic NaOH at the same temperature as the SACRed treatment. ZIF-8 crystals did not recover any crystallinity or porosity. While the failure of aqueous treatments could be attributed to residual hydrophobicity of ZIF-8 even after degradation, the failure of NaOH/methanol treatment showed that ZIF-8 regeneration is not due to simple removal of acid species by a base. To further test the possibility that the recovery of textural characteristics is simply due to removal of defective material at the crystal surfaces (e.g., Figure 2e), we performed a control experiment in which the ZIF-8 crystals were treated in methanolic conditions identical to that of SACRed, but without any linker present. No recovery in surface area or porosity was obtained.

The question of whether SACRed is truly a crystal repair process (versus the possible dissolution of degraded ZIF crystals and recrystallization as pristine crystals) was also


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considered. We recently examined a similar question in the mechanism of Solvent-Assisted Linker Exchange (SALE) in ZIFs), wherein it was found that dissolution and recrystallization was not a significant factor.24 Harsh conditions were required to induce partial dissolution of ZIF-8, which also led to large morphological defects such as pits and holes in the crystals. In the present case, it is important to note that there is no sodium formate in any of the SACRed solutions. The formation of ZIF-8 crystals in the micrometer-size range (Supporting Information) requires a coordination modulator such as sodium formate, without which one would only obtain nanocrystals of ZIF-8 in methanolic or aqueous conditions. In the present SACRed conditions, significant quantities of ZIF-8 microcrystals cannot be formed even if sufficient quantities of Zn2+ were available from the initial acid-damaged crystals. Even if new nanocrystals of ZIF-8 could form, they cannot account for the large recovery of pore volume and surface area unless they greatly outnumber the initial ZIF-8 microcrystals in mass and number. As seen in Figure 2f, the initial ZIF-8 crystals remain morphologically intact save for the small unidentified particles adhering to the crystals. We also performed a control experiment in which we collected samples of the linker solution at different durations (6 h, 12 h, 24 h, and 48 h) during SACRed processing of degraded ZIF-8. We analyzed these solutions using ICP-MS, and found the Zn concentrations to be very small (57±8 ppm). A ZIF-8 synthesis was then conducted with an initial 80 ppm Zn concentration, and linker concentration identical to the SACRed treatments (10 mmol of 2-MeIM in 40 mL methanol). No microscale or nanoscale crystals of ZIF-8 were detected by SEM or XRD. ) Similarly, degraded ZIF-14 was dispersed in methanol and heated at 90°C for 48 hours. No formation of ZIF-14 nanocrystals or mass loss from the original ZIF-14 sample was seen. Degraded ZIF-14 and 1-methylimidazole (1-MeIM) were subjected to conditions identical to the SACRed treatment. 1-MeIM is an isomer of the ZIF-8 linker but it cannot form a ZIF due to the


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methyl group being bonded to one of the N atoms. No measurable mass loss from the degraded ZIF-14 sample was observed. suggesting that even the basicity of the linker does not leach out Zn from the crystals.

Figure 5. Solution-state 2H NMR spectra of pristine ZIF-8 before and after treatment with deuterated 2-MeIm, and SO2-degraded ZIF-8 after SACRed treatment with deuterated 2-MeIm.

An alternative (and more interesting) mechanism is then hypothesized to involve replacement of the entire dangling linker-acid complex with a fresh linker during SACRed. It is already known that linkers can be substituted even in pristine ZIF materials by Solvent Assisted Linker Exchange (SALE).22 The experimental conditions (temperature, duration, linker concentration) under which we carried out SACRed are similar to the conditions under which


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SALE is usually performed in ZIFs. We hypothesized that such a mechanism could be greatly accelerated in degraded materials containing partially detached (dangling) linkers, relative to pristine materials containing fully coordinated linkers. To test this hypothesis, we performed SACRed with fully deuterium-substituted 2-MeIm (C4D6N2) and measured the incorporation of these linkers by solution state 2H NMR after dissolving the material in acetic acid (Figure 5). Known amounts of deuterated acetic acid (C2D4O2) were added as a standard for quantification. First, we performed a control experiment by treating pristine ZIF-8 with deuterated linkers. This measures the ‘background’ linker exchange on pristine sites due to SALE. Pristine ZIF-8 has no measurable 2H peaks from the methyl functional group or the imidazole ring, due to the low natural abundance (0.01%) of deuterium. Pristine ZIF-8 treated with deuterated 2-MeIm shows weak but noticeable peaks at both positions. Quantification via the C2D4O2 standard revealed that no more than 6% of linkers in pristine ZIF-8 were exchanged with isotopically labeled SALE linkers. However, a similar analysis of SO2-degraded ZIF-8 showed that 40% of linkers were replaced by C4D6N2. This much larger exchange is clear evidence that the SACRed mechanism involves preferential replacement of the partially detached linker-acid complex by a fresh linker from the SACRed solution. For every 100 mg of degraded (sulfated) ZIF-8 that was subjected to SACRed treatment, we were able to recover 75-80 mg of the material after the treatment. If 40% of 2-MeIm linkers were complexed with HSO3-/HSO4- species, then replacing them with fresh 2MeIm linkers is calculated to lead to roughly 20% loss in mass. This is consistent with the observed reduction in mass of the recovered post-SACRed material.

As discussed earlier, we also performed SACRed on ZIF-14 crystals that were exposed to humid SO2. ZIF-14 is chemically similar to ZIF-8 with a methyl functional group replaced by an


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ethyl group, but it has a different (ANA) crystal topology.23 As a result, ZIF-14 has very small pore apertures (nominal size 2.2 Å) as compared to ZIF-8 (3.4 Å), and its relatively dense structure has lower surface area and porosity. ZIF-14 showed surface area recoveries in the 1980% range depending on the level of SO2-induced degradation. The absence of complete recovery in ZIF-14 is attributable (inconclusively) to its very small pore size, which may strongly impede diffusion of fresh linkers into the framework and removal of existing linker-acid complexes. We have recently shown that SALE is diffusion-controlled in the ZIF-8 ZIF-90 system24. However, diffusion limitations did not prevent near-complete SALE from occurring in ZIF-8 when the processing time and ZIF-90 linker concentration in solution were high enough. However, the very small pore size of ZIF-14 will likely impose stronger limitations on the rate of SALE and SACRed processes.

Figure 6. Schematic of acid-gas degradation and SACRed recovery of ZIFs. In conclusion, we have demonstrated a solution-based treatment for reversal of damage caused to ZIF crystals by acid gas exposure. SACRed has potential as a relatively inexpensive method to recover lost surface area, porosity, and crystallinity of degraded ZIF materials. These


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findings also challenge a prevailing belief that acid gas-induced damage to ZIF/MOF materials is permanent. ZIF-8 crystals that lost nearly 100% of their surface area and porosity showed almost complete recovery of structure and adsorption functions. A schematic of the degradation process and proposed mechanism of SACRed is shown in Figure 6. Isotopically labeled linkers were used to show that fresh linkers from the solution can preferentially replace the dangling/partially detached linker-acid complexes. The apparent simplicity of the method presents an opportunity to extend it to other ZIF/MOF systems that may be affected by slow degradation during operational conditions, thereby potentially extending membrane/adsorbent lifetime and reducing process costs. It has been shown that treatment of freshly synthesized ZIF-8 membranes with linker solutions at elevated temperatures can improve their separation factor and stability.25-27 Similarly, water-degraded Co-MOF-74 crystals were partially restored by exposure to methanol vapors.28 These membranes and crystals were not degraded by exposure to acid gases. The improved performance was attributed to healing of grain boundary defects remaining from the membrane growth or the annealing of crystal surfaces by methanol. Our results suggest that these membrane treatments may also have healed defects within the polycrystalline grains. Finally, SACRed is shown to proceed at conditions under which other linker exchange processes such as SALE are not in operation, suggesting that controlled degradation followed by SACRed processing with different linkers could allow new hybrid ZIFs to be synthesized.

ASSOCIATED CONTENT Supporting Information.


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The following files are available free of charge. Procedures for ZIF-8 and ZIF-14 synthesis, details of X-ray diffraction, scanning electron microscopy, nitrogen physisorption, NMR spectroscopy, and adsorption experiments, details of SACRed treatment, FTIR band assignments, and supporting figures. (PDF) AUTHOR INFORMATION Corresponding Author *Sankar Nair: Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported partially by the National Science Foundation (NSF-CBET #1264874); and also as part of the Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy (UNCAGE-ME), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award #DESC0012577.

REFERENCES 1.

Bux, H.; Chmelik, C.; Krishna, R.; Caro, J., Ethene/Ethane Separation by the MOF

Membrane ZIF-8: Molecular Correlation of Permeation, Adsorption, Diffusion. J. Membr. Sci. 2011, 369, 284-289.


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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

2.

Page 18 of 21

Eum, K.; Rownaghi, A.; Choi, D.; Bhave, R. R.; Jones, C. W.; Nair, S., Fluidic

Processing of High-Performance ZIF-8 Membranes on Polymeric Hollow Fibers: Mechanistic Insights and Microstructure Control. Adv. Funct. Mater. 2016, 26, 5011-5018. 3.

Zhang, C.; Lively, R. P.; Zhang, K.; Johnson, J. R.; Karvan, O.; Koros, W. J.,

Unexpected Molecular Sieving Properties of Zeolitic Imidazolate Framework-8. J. Phys. Chem. Lett. 2012, 3, 2130-2134. 4.

Tran, U. P. N.; Le, K. K. A.; Phan, N. T. S., Expanding Applications of Metal−Organic

Frameworks: Zeolite Imidazolate Framework ZIF-8 as an Efficient Heterogeneous Catalyst for the Knoevenagel Reaction. ACS Catal. 2011, 1, 120-127. 5.

Lu, G.; Hupp, J. T., Metal−Organic Frameworks as Sensors: A ZIF-8 Based Fabry−Pérot

Device as a Selective Sensor for Chemical Vapors and Gases. J. Am. Chem. Soc. 2010, 132, 7832-7833. 6.

Sun, C.-Y.; Qin, C.; Wang, X.-L.; Yang, G.-S.; Shao, K.-Z.; Lan, Y.-Q.; Su, Z.-M.;

Huang, P.; Wang, C.-G.; Wang, E.-B., Zeolitic Imidazolate Framework-8 as Efficient PhSensitive Drug Delivery Vehicle. Dalton Trans. 2012, 41, 6906-6909. 7.

Jiang, J.-Q.; Yang, C.-X.; Yan, X.-P., Zeolitic Imidazolate Framework-8 for Fast

Adsorption and Removal of Benzotriazoles from Aqueous Solution. ACS Appl. Mater. Interfaces 2013, 5, 9837-9842. 8.

Park, K. S.; Ni, Z.; Cote, A. P.; Choi, J. Y.; Huang, R.; Uribe-Romo, F. J.; Chae, H. K.;

O'Keeffe, M.; Yaghi, O. M., Exceptional Chemical and Thermal Stability of Zeolitic Imidazolate Frameworks. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 10186-10191. 9.

Burtch, N. C.; Jasuja, H.; Walton, K. S., Water Stability and Adsorption in Metal-Organic

Frameworks. Chem. Rev. 2014, 114, 10575-10612. 10. Thompson, J. A.; Blad, C. R.; Brunelli, N. A.; Lydon, M. E.; Lively, R. P.; Jones, C. W.; Nair, S., Hybrid Zeolitic Imidazolate Frameworks: Controlling Framework Porosity and Functionality by Mixed-Linker Synthesis. Chem. Mater. 2012, 24, 1930-1936.


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11. Bhattacharyya, S.; Pang, S. H.; Dutzer, M. R.; Lively, R. P.; Walton, K. S.; Sholl, D. S.; Nair, S., Interactions of SO2-Containing Acid Gases with Zif-8: Structural Changes and Mechanistic Investigations. J. Phys. Chem. C 2016, 120, 27221-27229. 12. Pang, S. H.; Han, C.; Sholl, D. S.; Jones, C. W.; Lively, R. P., Facet-Specific Stability of ZIF-8 in the Presence of Acid Gases Dissolved in Aqueous Solutions. Chem. Mater. 2016, 28, 6960-6967. 13. Tan, Y.-X.; He, Y.-P.; Zhang, J., Tuning Mof Stability and Porosity Via Adding Rigid Pillars. Inorg. Chem. 2012, 51, 9649-9654. 14. Wu, T.; Shen, L.; Luebbers, M.; Hu, C.; Chen, Q.; Ni, Z.; Masel, R. I., Enhancing the Stability of Metal-Organic Frameworks in Humid Air by Incorporating Water Repellent Functional Groups. Chem. Commun. 2010, 46, 6120-6122. 15. Ma, D.; Li, Y.; Li, Z., Tuning the Moisture Stability of Metal-Organic Frameworks by Incorporating Hydrophobic Functional Groups at Different Positions of Ligands. Chem. Commun. 2011, 47, 7377-7379. 16. Jiao, Y.; Morelock, C. R.; Burtch, N. C.; Mounfield, W. P.; Hungerford, J. T.; Walton, K. S., Tuning the Kinetic Water Stability and Adsorption Interactions of Mg-MOF-74 by Partial Substitution with Co or Ni. Ind. Eng. Chem. Res. 2015, 54, 12408-12414. 17. Han, S.; Huang, Y.; Watanabe, T.; Nair, S.; Walton, K. S.; Sholl, D. S.; Meredith, J. C., MOF Stability and Gas Adsorption as a Function of Exposure to Water, Humid Air, SO2, and NO2. Microporous Mesoporous Mater. 2013, 173, 86-91. 18. Mounfield, W. P.; Han, C.; Pang, S. H.; Tumuluri, U.; Jiao, Y.; Bhattacharyya, S.; Dutzer, M. R.; Nair, S.; Wu, Z.; Lively, R. P.; Sholl, D. S.; Walton, K. S., Synergistic Effects of Water and SO2 on Degradation of MIL-125 in the Presence of Acid Gases. J. Phys. Chem. C 2016, 120, 27230-27240. 19. Barbosa, P.; Rosero-Navarro, N. C.; Shi, F.-N.; Figueiredo, F. M. L., Protonic Conductivity of Nanocrystalline Zeolitic Imidazolate Framework 8. Electrochim. Acta 2015, 153, 19-27.


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Page 20 of 21

20. Cheng, P.; Hu, Y. H., H2O-Functionalized Zeolitic Zn(2-Methylimidazole)2 Framework (ZIF-8) for H2 Storage. J. Phys. Chem. C 2014, 118, 21866-21872. 21. Hu, Y.; Kazemian, H.; Rohani, S.; Huang, Y.; Song, Y., In Situ High Pressure Study of ZIF-8 by FTIR Spectroscopy. Chem. Commun. 2011, 47, 12694-12696. 22. Wang, Z.; Cohen, S. M., Postsynthetic Modification of Metal-Organic Frameworks. Chem. Soc. Rev. 2009, 38, 1315-1329. 23. Banerjee, R.; Phan, A.; Wang, B.; Knobler, C. B.; Furukawa, H.; O'Keeffe, M.; Yaghi, O. M., High-Throughput Synthesis of Zeolitic Imidazolate Frameworks and Application to CO2 Capture. Science 2008, 319, 939-943. 24. Jayachandrababu, K. C.; Sholl, D. S.; Nair, S., Structural and Mechanistic Differences in Mixed-Linker Zeolitic Imidazolate Framework Synthesis by Solvent Assisted Linker Exchange and De Novo Routes. J. Am. Chem. Soc. 2017, 139, 5906-5915. 25. Zhang, H.; Liu, D.; Yao, Y.; Zhang, B.; Lin, Y. S., Stability of ZIF-8 Membranes and Crystalline Powders in Water at Room Temperature. J. Membr. Sci. 2015, 485, 103-111. 26. Kwon, H. T.; Jeong, H.-K.; Lee, A. S.; An, H. S.; Lee, J. S., Heteroepitaxially Grown Zeolitic Imidazolate Framework Membranes with Unprecedented Propylene/Propane Separation Performances. J. Am. Chem. Soc. 2015, 137, 12304-12311. 27. Lee, M. J.; Kwon, H. T.; Jeong, H.-K., Defect-Dependent Stability of Highly PropyleneSelective Zeolitic-Imidazolate Framework ZIF-8 Membranes. J. Membr. Sci. 2017, 529, 105113. 28. Chmelik, C.; Mundstock, A.; Dietzel, P. D. C.; Caro, J., Idiosyncrasies of Co2(dhtp): In situ-annealing by methanol. Microporous Mesoporous Mater. 2014, 183, 117-123.


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Recovery of Acid-Gas-Degraded Zeolitic Imidazolate Frameworks by

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