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Engineering Copper Carboxylate Functionalities on Water Stable MetalOrganic Frameworks for Enhancement of Ammonia Removal Capacities Jayraj N. Joshi, Erika Y. Garcia-Gutierrez, Colton M. Moran, Jacob I. Deneff, and Krista S. Walton J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08610 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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The Journal of Physical Chemistry

Engineering Copper Carboxylate Functionalities on Water Stable Metal-Organic Frameworks for Enhancement of Ammonia Removal Capacities

Jayraj N. Joshi, Erika Y. Garcia-Gutierrez, Colton M. Moran, Jacob I. Deneff, Krista S. Walton* School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332, United States

-All authors affiliated with the Georgia Institute of Technology -All work was done at the Georgia Institute of Technology

Corresponding Author (*) Krista Walton: [email protected]

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Abstract Functionalization of copper carboxylate groups on a series of UiO-66 metal organic framework (MOF) analogues and their corresponding impact on humid and dry ammonia adsorption behavior was studied. Relative locations of possible carboxylic acid binding sites for copper on the MOF analogues were varied on ligand and missing linker defect sites. Materials after copper incorporation exhibited increased water vapor and ammonia affinity during isothermal adsorption and breakthrough experiments, respectively. The introduction of copper markedly increased ammonia adsorption capacities for all adsorbents possessing carboxyl binding sites. In particular, the new MOF UiO-66(COOCu)2 displayed the highest ammonia breakthrough capacities of 6.38 and 6.84 mmol g-1 under dry and humid conditions, respectively, while retaining crystallinity and porosity. Relative carboxylic acid site locations were also found to impact sorbent stability, as missing linker defect functionalized materials degraded under humid conditions after copper incorporation. Post-synthetic metal insertion provides a method for adding sites that are analogous to open metal sites while maintaining good structural stability.


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Introduction The development of novel materials for the sequestration and/or destruction of toxic industrial chemicals (TICs) and chemical warfare agents (CWAs) is of importance for both civilian and military air purification applications.1,2 Modern adsorbents are needed for chemical, biological, radiological, and nuclear (CBRN) personal breathing devices, as the currently used carbon-based filtration media (ASZM-TEDA)3 offers protection against many toxic gases, but fails to provide sufficient protection against the full range of possible TICs/CWAs.4 Specifically, variable volatilities and molecular weights amongst TICs and CWAs dictate the efficiency in which they are removed by CBRN devices.5 Ammonia (NH3) is of particular interest, where its abundant worldwide production6, high toxicity, and precedence of use as a CWA2, makes capture or conversion of ammonia in CBRN devices of great importance. Metal-organic frameworks (MOFs) are a growing class of crystalline, nanoporous materials, whose characteristics qualify them for potential use in ammonia filtration equipment. Vapor adsorption on MOFs occurs not only through their large accessible surface areas7, but also via linker-based functionalities4,8,9 and open metal sites,8,10–12 which provide tunable pathways to facilitate adsorption. Post-synthetic modification strategies for MOFs have been pursued, through binding additional functionalities into the frameworks13–15 cross-linking16, and metal coordination.17 Adding new chemical functionalities via this methodology has been shown to improve adsorbate loadings in MOFs at lower relative pressures, increasing their applicability for air purification applications.18–20 MOFs possessing open metal sites (OMS) are particularly well known to exhibit strong adsorption interactions with molecules such as ammonia,21 carbon monoxide,22 and carbon dioxide.23–28 These sites possess Lewis acidity and are also potentially useful as single-site catalysts. In fact, MOFs with OMS provide the strongest adsorption interactions of any feature that can be built into MOFs. At the same time, these sites also strongly adsorb water, which often leads to hydrolysis and degradation of the MOF structure.24,29 Given the application advantages of MOFs with OMS, it is desirable to develop the techniques for creating open metal sites in materials that also maintain 3 ACS Paragon Plus Environment


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structural stability in the presence of water and other challenges. A promising strategy to accomplish this goal is to use post-synthetic synthesis methods to insert metal sites into the MOF framework. Thus, the open metal site will be accessible to incoming molecules but will not be part of the MOF building unit. Such a material will allow the desirable strong interactions between adsorbate molecules and the open metal site while minimizing the potential for degradation. A computational screening of MOF functional groups by Kim et al.30 showed metal coordination to linker functionalities to be potentially advantageous for ammonia binding, as R-COOCu groups were calculated to have the most favorable binding energies with ammonia out of all the functionalities considered, as well as strong energetic binding affinity of ammonia over water. In contrast to impregnation, the functionalization of guest metals onto active sites in MOFs is under studied, however, metal alkoxide formation on organic linkers has been reported through conversion of hydroxide groups by Hupp and coworkers,31 and a few investigations of iridium and copper coordination to MOF ligands were carried out.32–35 Results from these studies suggest copper carboxylate groups may be formed through post-synthetic modification strategies on MOFs containing carboxylic acid moieties. Previous work by Walton and Sholl36 showed that the presence of bare metal clusters in MOFs do not impact properties in this way, indicating that metal clusters of this kind will not provide the same adsorption advantage as anchored metal sites. The goal of this work is to evaluate UiO-66 as a platform material for metal insertion to provide improved adsorption interactions with ammonia from air. This zirconium-based MOF UiO-66 does not possess open metal sites but features high chemical and thermal stabilities37,38 and has shown considerable promise as a platform material for water-stable, ammonia removal from air. A myriad of linker substitutions and functional groups have been tested on the UiO-66 structure, where –COOH, –NH2, –OH, and other functionalities have been observed to enhance ammonia capture capabilities through acidbase and hydrogen bonding interactions.39–41 Similarly, metal impregnation into UiO-66 has been observed to create interesting catalytic and selective adsorption properties.42,43 Functionalized UiO-66 candidates for copper carboxylation have been previously 4 ACS Paragon Plus Environment

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synthesized: UiO-66-COOH44 and UiO-66-(COOH)2,45 where characterization of these MOFs suggest uncoordinated carboxylic acid groups reside on the organic linker of the respective materials.16 Acid modulation experiments to create missing linker sites in the UiO-66 framework46,47 have subsequently lead to the incorporation of carboxylic acid groups in defect sites, where DeCoste et al.48 reported a coordination of –COOH groups onto missing linker moieties through post synthetic treatment with oxalic acid, named UiO-66-ox. Importantly, the incorporation of carboxylic acid functionalities through missing linker defect sites was shown to largely mitigate losses in BET surface area that were observed with UiO-66 containing linker functionalized carboxylates,39,49 affording greater accessibility for adsorbate species in the pore space.50 Following on these promising studies, this present work aims to develop a post synthetic modification strategy for novel copper carboxylate functionalization on water stable UiO-66 analogues. Ammonia adsorption properties for filtration applications are typically only relevant when low partial pressures and mass transfer limitations exist,5 so dynamic ammonia breakthrough experiments under dry and humid conditions will be employed to evaluate the ammonia capture performance of the new MOFs. Experimental Methods All chemicals used for MOF synthesis in this study were commercially available and utilized without further purification. MOF syntheses performed were based on previously published work. Detailed synthesis procedures and materials characterization techniques mentioned here are available in the Supporting Information (SI). Water vapor adsorption measurements were conducted at 298 K and 1 bar. An Intelligent Gravimetric Analyzer (IGA-3) was used for data collection. Samples were activated insitu under their respective activation conditions until sample weight loss was no longer observed. Tested materials were exposed to water vapor up to 90% RH to avoid condensation. A total gas flow rate of 200 mL min-1 was used, with balance of dry air. Ammonia breakthrough measurements were taken in dry and humid conditions at room temperature. Ammonia was introduced to the system at a concentration of 7155 ppm and a flow rate of 4 mL min-1, and mixed with air at a flow rate of 16 mL min-1 before 5 ACS Paragon Plus Environment


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entering the packed bed. Setup for wet breakthrough was observed to result in ~80% RH for wet breakthrough runs, and a calculated NH3 concentration of 1431 ppm. In dry conditions, the air stream circumvents the bubbler. N2 was flowed in at 20 mL min-1 following breakthrough for desorption measurements. Further detail on the dynamic ammonia breakthrough experiments is available in the Supporting Information. Results and Discussion Materials Characterization Parent UiO-66 was synthesized along with the previously described carboxylic acid functionalized versions of UiO-66: UiO-66-COOH, UiO-66-(COOH)2, and UiO-66-ox. Synthesis procedures are detailed in the Supporting Information. Powder X-ray diffraction (PXRD) measurements were employed to assess the crystallinity of the MOF samples in this study. Patterns for the materials before metal insertion are shown in Figure S1, and suggest that all materials are of the same isostructural family. N2 physisorption isotherms taken at 77 K for the materials in Figure S2 are in agreement with previous observations of microporosity in all UiO-66 type MOFs.40 MOFs with linker based functionalities show diminished accessible surface areas, in agreement with established porosity trends for functionalized UiO-66.51 This decrease in surface area has been previously attributed to pore blocking from carboxylic acid functionalities.16,49 Conversely, the N2 physisorption curves for UiO-66-vac and UiO-66-ox show significantly higher vapor loadings. This behavior is the result of missing linker defects created through modulation with HCl, increasing the accessible surface area of the framework.52,53 The observed decrease in N2 physisorption loading from UiO-66-vac to UiO-66-ox is due to the inclusion of carboxylic acid moieties in the framework.48 Predicted coordination behaviors for samples after copper incorporation are delineated in Figure S3 in the Supporting Information. Experimental copper mass fractions for materials after metal loading were measured through ICP-OES analysis, and are summarized in Table 1 with their respective experimental BET surface areas. Normalized fractional metal loadings for the materials were approximated by taking the ratio of the experimental mass fractions over theoretically calculated ones. Theoretical loadings were 6 ACS Paragon Plus Environment

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calculated based on the number of uncoordinated carboxylic acid sites in each MOF, assuming one Cu2+ binds to two COO- groups to ensure charge balancing, and based on previous observations of copper chelation to MOF carboxyl groups.54 For UiO-66-COOH and UiO-66-(COOH)2, the number of uncoordinated carboxylic acid groups were estimated from the number of carboxyl groups on each ligand in the ideal unit cells of the respective materials. Uncoordinated carboxylic acid sites in UiO-66-ox were approximated based on previously published elemental analysis47 and 13C NMR results,48 where the number of oxalic acid species quantified in the framework through these analyses were assumed to be equal to the number of free carboxylic acid sites in the MOF. The ideal structure of UiO-66 contains no free carboxylic acid species, so the theoretical copper loading for Cu@UiO-66 was assumed to be zero (0 wt%). It should be noted however, that inherent linker vacancies in UiO-66 that may lead to open carboxylic acid moieties through structural defects have been reported.55 Table 1. BET Surface Areas, Pore Volumes, and Copper Loadings for Activated Materials. BET

Total

Fractional

Surface

Pore

Loading

Area

Volume

Copper Mass Fraction (wt %)

2

MOF

-1

-1

Experimental

Theoretical

(-)

(m g )

(cc g )

UiO-66

----

----

----

1111

0.73

Cu@UiO-66

4.51

N/Aa

----

781

0.46

UiO-66-vac

----

----

----

1534

0.80

UiO-66-ox

----

----

----

1158

0.59

UiO-66-ox-Cu

4.84

5.50

0.88

1116

0.56

UiO-66-COOH

----

----

----

658

0.28

UiO-66-COOCu

7.88

9.62

0.82

564

0.24

----

----

----

364

0.22

3.15

14.52

0.22

357

0.18

UiO-66(COOH)

2

UiO-66(COOCu) a.

2

Cu@UiO-66 structure assumed to be free of open –COOH bonding sites for copper.



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The highest fractional copper loadings were observed for UiO-66-ox-Cu and UiO-66COOCu, despite UiO-66-(COOCu)2 possessing the highest theoretical loading, due to the high density of linker –COOH groups. Relatively low fractional loading on UiO-66(COOCu)2 is attributed to steric hinderance of copper species for binding sites on the bulky dicarboxylate organic linker groups. This may explain the significantly higher loading for monocarboxylate ligands used in UiO-66-COOCu, where half as many carboxyl groups populate the pore space. Nearly theoretical metal loading in UiO-66-oxCu was achieved, suggesting a greater accessibility of –COO- sites for copper species during metal insertion, although this may also be a result of impregnation, which is likely more prevalent in this case than with UiO-66-COOCu and UiO-66-(COOCu)2. This behavior can be explained by the greater pore accessibility of UiO-66-ox-Cu through missing linker defects in the framework as opposed to linker functional groups, as suggested by the significantly higher experimental BET surface area of the MOF as compared to UiO-66-COOCu and UiO-66-(COOCu)2. Interestingly, the BET surface areas of all materials featuring open carboxylic acid binding sites are largely unchanged, in contrast to that of Cu@UiO-66. This observation supports the initial prediction of metal insertion into parent UiO-66 being more representative of an impregnation, as supported by previously observed cases in which BET surface areas decrease after MOF impregnation;42,56 this result is in sharp contrast to the envisioned metal-carboxylic acid coordination in the frameworks of the other three materials studied. Thermogravimetric analysis curves for the new MOFs are provided in Figure S4 and show that the materials exhibit minimal losses in thermal stability following metal insertion. This behavior has been previously observed in post synthesis metal incorporation into MOFs, such as with the grafting of lithium tert-butoxide on UiO-66, where metal guests appear to not significantly reduce the thermal stability of the framework.57 Residual mass percentages are higher than calculated for parent MOFs due to presence of copper in the materials. Metal properties on the surface of MOF materials were investigated using high resolution XPS scans of the Cu 2p3/2 core. Cu+ and Cu2+ chemical states at respective binding energies of 933 and 935 eV were identified in the samples.58–60 Spectra for all four copper 8 ACS Paragon Plus Environment

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containing materials in Figure 1 suggest the majority of copper present in the top 5-10 nm of the samples exists in the Cu2+ state. Complete scans provided in Figure S5 contain strong shake-up satellite peaks at binding energies of 944.3 and 963 eV for all samples, characteristic of Cu2+.61 Broad Cu 2p1/2 peaks around 955 eV also support a major presence of Cu2+ in the samples, and possibly mask sharper Cu+ peaks in that region.62 Minor presence of Cu+ could be attributed to a mixed bonding environment for copper with other residual guest molecules (solvent, water, etc.) in the frameworks. Copper Kedge X-ray absorption (XAS) measurements are presented for the MOFs and impregnate salt (copper nitrate trihydate) in Figure S6. Pre-edge features around 8977 eV and shoulders at 8986-8988 eV for the samples reflect the 1s→3d and 1s→4s electron transitions, respectively, and corroborate XPS data.63 The sensitivity of XANES to the stereochemistry of the probe species64,65 suggests apparent spectral differences between copper nitrate trihydrate and the MOFs indicates a unique coordination geometry of copper in the frameworks, suggestive of the metal coordination to binding sites within the samples. For Cu@UiO-66, this result is puzzling, since the structure ideally possesses no open carboxylic binding sites. However, the previously discussed inherent point defect sites in the structure may create similar binding sites in the structure for copper coordination.



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Figure 1. XPS data for Cu 2p3/2 region for (a) Cu@UiO-66 (b) UiO-66-ox-Cu (c) UiO66-COOCu (d) UiO-66-(COOCu)2 Water Vapor Adsorption Experiments A recent screening by Moghadam et al.66 showed TIC adsorption in humid streams to be affected through both competitive and cooperative adsorbate-adsorbate interactions through a wide range of MOFs. Competitive binding effects are typical for hydrophilic materials and/or sorbent active sites.5,29 However, water has also conversely enhanced low pressure CO2 uptake in MIL-100(Fe)67. In the case of ammonia, cooperative binding in the copper based MOF HKUST-1 was observed to increase ammonia capture capabilities of the material for humid ammonia environments68, and spectroscopic studies by Nijem et al.69 suggest water strengthens metal-ammonia affinity through hydrogen bonding between the adsorbates. UiO-66 possesses slight hydrophobicity29, although functionalization with copper carboxylate groups and missing linker defects70 likely affect water affinity and stability of the material in humidity, so water vapor adsorption experiments were conducted to evaluate changes in interactions with water for the MOFs studied. 10 ACS Paragon Plus Environment

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Water vapor adsorption isotherms were collected for materials prior to metal insertion at 25°C, and are shown in Figure 2a with previous measurements for UiO-6671. All materials exhibit hydrophobicity at relative humidities less than 20%, and low pressure hysteresis (H1).72 The presence of missing linker defects generates more hydrophilic behavior in UiO-66-vac, as predicted through GCMC water adsorption simulations for the UiO-66 structure by Ghosh and coworkers70. Interestingly, MOFs containing carboxylic acid groups, which can chemically bind with water species through acid-base interactions, appear to exhibit increased hydrophobic interactions here. The sigmoidal isotherm shape for UiO-66-(COOH)2 is in disagreement with a previous report by Hu et al.49 of Type I isothermal water vapor uptake, which is indicative of favorable adsorbateadsorbent interactions in microporous materials.29,72 However, BET surface area measurements for UiO-66-(COOH)2 in this study are significantly lower than those reported by Hu et al., suggesting the materials may possess disparate textural properties that may affect water vapor affinity. The water adsorption isotherms for UiO-66-ox, UiO-66-COOH, and UiO-66-(COOH)2 in Figure 2a also indicate that these materials possess a hydrophobic character, which may be introduced through pore blocking by the carboxylic acid groups. All three materials belong to the same isostructural family, and contain carboxylic acid functional groups, so their water vapor adsorption behavior is predicted to differ primarily through surface area and pore accessibility. The highest observed water vapor loading in UiO-66-ox amongst the other two sorbents is subsequently attributed to this. The relatively high water vapor retention at 0% RH for UiO-66-ox may also suggest the defect-coordinated hydrophilic carboxyl species are more accessible to water guest molecules than linker functionalized groups on UiO-66-COOH and UiO-66-(COOH)2.



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Figure 2. Water vapor adsorption isotherms at 25°C for (a) UiO-66, UiO-66-vac, UiO66-ox, UiO-66-COOH, and UiO-66-(COOH)2 (b) UiO-66-ox-Cu, UiO-66-COOCu, and UiO-66-(COOCu)2. Data for UiO-66 taken from Schoenecker et al.71 Open symbols— desorption, closed symbols—adsorption. Figure 2b shows water vapor adsorption isotherms for MOFs predicted to possess copper carboxylate groups after metal insertion. Water vapor loadings increased for all materials following metal insertion, in agreement with high calculated binding energy of water for –COOCu groups.30 Similar water vapor loadings and Type I adsorption curves are observed for UiO-66-COOCu and UiO-66-(COOCu)2, unlike their respective parent materials. This result may suggest water is primarily adsorbed in both materials on the active copper sites, because similar metal coordination exists in the two materials. Low water affinity is still observed in the low pressure region for UiO-66-ox-Cu, although it exhibits the highest loading at 90% RH. A comparison of Figure 2b with ICP-OES results in Table 1 shows that water vapor loadings do not correlate with metal content. BET surface areas for MOFs before and after water exposure are summarized in Table 2, along with their respective water vapor loadings at 90% RH. The data indicates that all materials retain the characteristic water stability of UiO-6673 with exception of UiO-66ox-Cu, whose BET surface area decreased by about 50% after water vapor exposure. A significant BET surface area loss of around 30% was also observed for the respective parent MOF, UiO-66-ox. Framework collapse via hydrolysis has been previously observed in many MOF systems29,74. Still this result is somewhat surprising, as other functionalized UiO-66 analogues have been observed to retain porosity in humidity and 12 ACS Paragon Plus Environment

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even liquid water.29,49,73,75,76 Losses in porosity are not apparent in UiO-66-vac, as shown in Table 2. Changes in pore structure for UiO-66-ox and UiO-66-ox-Cu are therefore consequent of introduced species following post-synthetic modification. PXRD patterns for investigated MOFs containing missing linker defects in Figure S7 indicate overall structural retention, but large losses in accessible surface area for UiO-66-ox and UiO66-ox-Cu are strongly suggestive of a partial collapse of the pore space. Instability of these MOFs in humid air can explain their sigmoidal isotherm shapes in Figure 2. Gravimetric measurements (Figure S8) for UiO-66-ox do not reveal an experimentally significant mass loss between activated samples before and after exposure to humidity, indicating the lower observed porosities are likely not due to the strong retention of water on hydrophilic functionalities, as observed for other frameworks.73 This behavior is not observed with UiO-66-COOCu or UiO-66-(COOCu)2, which are thought to possess copper carboxylate sites on linker functionalities, as opposed to defect-based moieties that would be in closer proximity to zirconium metal centers. The high connectivity of UiO-66 and many other Zr, Hf, and La based MOFs has been argued to sterically hinder water molecules from clustering near metal centers and hydrolyzing metal-ligand bonds.29 It is therefore expected that reducing framework connectivity while concurrently introducing hydrophilic species near zirconium-ligand bonds facilitates water degradation of the framework, as with UiO-66-ox and UiO-66-ox-Cu. Despite probable partial pore collapse, relatively high vapor loadings may be caused by released active sites during the degradation process, as previously suggested for HKUST-1 composites by Bandosz and coworkers.77



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Table 2. Water Vapor Loadings and BET Surface Area for Materials before and after Water Vapor Exposure. Loading at

BET Surface Area (m2 g-1)

90% RH (mL g-1)

Before

After

UiO-66-vac

0.59

1534

1566

UiO-66-ox

0.28

1158

835

UiO-66-ox-Cu

0.37

1116

538

UiO-66-COOH

0.12

658

632

UiO-66-COOCu

0.28

564

634

UiO-66-(COOH)2

0.14

364

317

0.31

357

394

MOF

UiO-66(COOCu)2

Ammonia Breakthrough Experiments Ammonia breakthrough experiments under dry and humid (80% RH) conditions were conducted for the materials after copper insertion, and breakthrough capacities were ascertained in accordance with previously discussed methods.21,78 Breakthrough curves for dry and humid experiments are provided in Figure 3 and Figure 4, respectively. Instead of conducting a total breakthrough experiment, which continues until the detected outlet gas concentration equals the inlet concentration, ammonia flow was terminated when effluent concentrations exceeded 500 ppm NH3 to preserve the lifetime of the chemical sensor utilized in the experiments. This results in the sharp peaks evidenced in Figure 3 and Figure 4. Additionally, the breakthrough time was evaluated as the time required for the outlet ammonia concentration to reach the OSHA PEL of 50 ppm NH3. UiO-66-(COOCu)2 exhibits the longest breakthrough times, and consequently highest ammonia capacities, under both conditions for all materials studied, whereas Cu@UiO66 possesses the lowest. Dynamic breakthrough capacities for the four materials are contrasted with their respective parent MOF capacities in Table 3. UiO-66-(COOCu)2 14 ACS Paragon Plus Environment

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exhibited the largest improvement in capacity at a 125% increase of the materials studied. Significant increases in uptake for UiO-66-ox-Cu and UiO-66-COOCu are also observed. These results were expected, as functional groups with high affinity for ammonia have been predicted to enhance low pressure ammonia loadings.79 Increases in ammonia capacity for MOFs thought to contain copper carboxylate groups are attributed to the improved binding affinity of –COOCu groups with ammonia, as these binding energies have been calculated to be four times stronger than with carboxylic acid functional groups alone.30 Ammonia loadings do not correlate with the copper loadings reported earlier in Table 1, suggesting the respective coordination nature and accessibility of copper carboxylate sites in the materials are more impactful on ammonia uptake than copper content. Disparate behaviors in ammonia capture performance after metal insertion between Cu@UiO-66 and the other materials studied suggests copper is incorporated differently into the framework that is mostly absent of free carboxylic acid coordination sites. This is in agreement with the original hypothesis of metal impregnation in UiO-66, as opposed to coordination to carboxylic acid sites in the other investigated materials. The slight ammonia capacity increase in Cu@UiO-66 under dry conditions is still attributed to the formation of copper carboxylate groups, which may form through interaction with open carboxyl groups existing from the small number of missing linker defects inherent to the structure of UiO-66.38 Interestingly, Cu@UiO-66 is the only MOF in this study to also have a lower dynamic ammonia capacity than its parent material after metal insertion. Water chemisorption in HKUST-1 was found to impact MOF adsorption characteristics by Watanabe et al.80, and other reports suggest synergistic binding for mixed water and ammonia environments is typically indicative of cooperative hydrogen bonding between the adsorbate species and secondary binding on active sites in the material69. The presence of these weak interactions are evidenced by more shallow desorption curves in humid breakthrough experiments, as depicted for the materials in Figure 4. However, these cooperative physisorption effects are probably limited in Cu@UiO-66 due to blocking of active sites in the pore space by impregnated metal species. Decreased ammonia capture in humid conditions may indicate competitive binding between water and ammonia on active sites for the adsorbent material, as reported previously by Jasuja 15 ACS Paragon Plus Environment


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et al. for functionalized analogues of UiO-66.25 Hindering access to binding sites could also lead to competitive binding between water and ammonia molecules, which has been attributed to a decrease in material acidity from water bonding68, creating less favorable adsorption conditions for basic ammonia guests.

Figure 3. Dry (0% RH) dynamic ammonia breakthrough curves. Breakthrough times are normalized by sample weight.

Figure 4. Humid (80% RH) dynamic ammonia breakthrough curves. Breakthrough times are normalized by sample weight.


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Table 3. Dynamic Ammonia Breakthrough Capacities, Measured at Ambient Conditions under Balance Air Flow. MOF

NH3 Dynamic Capacity (mmol g-1)a Dry (0% RH)

Wet (80% RH)

1.79

2.75

Cu@UiO-66

2.25

1.15

UiO-66-ox

2.50

2.30

UiO-66-ox-Cu

4.31

4.02

UiO-66-COOH

2.24

2.21

UiO-66-COOCu

3.04

3.10

UiO-66-(COOH)2

2.83

1.83

UiO-66-(COOCu)2

6.38

6.84

UiO-66

b

a.

Capacities are normalized by sample mass

b.

Data reported from Jasuja et al.39

PXRD and N2 physisorption measurements were collected after dry and humid ammonia exposure to assess material stability. PXRD patterns in Figure 5 for activated materials after copper insertion, with exception to Cu@UiO-66, contain an extra peak around 2θ = 12°, alluding to structural changes created only in MOF samples with uncoordinated carboxylic acid groups during metal incorporation. Inspection of the PXRD patterns shows no apparent change in crystallinity after dry ammonia breakthrough experiments. However, structural degradation of UiO-66-ox-Cu is suggested by the diffraction measurements for the material following humid ammonia exposure. BET surface area analysis results for the samples support this, where UiO-66-ox-Cu was found to have a loss of 45% and 71% accessible surface area in dry and humid ammonia breakthrough experiments, respectively. PXRD patterns for UiO-66-ox were also collected, and reveal the degradation of UiO-66-ox-Cu is also dependent on metal insertion, as diffraction data for the parent MOF in Figure 5 does not exhibit similar changes before and after breakthrough experiments. Surface area losses for UiO-66-ox-Cu reflect previously discussed behavior for the MOF after water vapor adsorption experiments. Analogous experimental observations by Ebrahim et al.81 for surface area loss after dry and humid 17 ACS Paragon Plus Environment


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NO2 exposure indicated a combinatorial degradation effect between water and the adsorbate species on weakening the accessible Zr-O bonds in UiO-67, but not for the more confined framework of UiO-66. However, the defect-laden structure of UiO-66-oxCu corroborates this proposed stability behavior by fostering the opposite effect: previously restricted water-adsorbate interactions within the UiO-66 framework in UiO66-ox-Cu can occur with less spatial hindrance than in the other materials investigated in this study. Recently published electronic structure calculations additionally support the thermodynamic favorability of degradation reactions with water for MOFs found to contain point defects, as well as in systems with other adsorbate species in the presence of water, as in the case here.82,83 Similar atomistic modeling studies for UiO-66-ox-Cu in this case would be markedly more difficult due to the random distribution of point defects, oxalic acid, and copper functional sites throughout the material. Although –COOCu groups are expected to interact with water and ammonia more strongly than zirconium centers, the previously observed clustering of ammonia and water in MOFs through secondary binding may lead to sufficient steric interference to destabilize zirconium metal center bonds with terephthalate linkers, enhancing susceptibility of organic linker bonds with MOF nodes to replacement reactions with ammonia and water.


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Figure 5. PXRD patterns of copper incorporated MOFs and UiO-66-ox: as-synthesized and following ammonia breakthrough experiments. Conclusions A post-synthetic modification procedure for incorporating copper into a series of UiO-66 analogues with carboxylic acid functional groups was developed in effort to create copper carboxylate functionalities to enhance ammonia uptake capacity. Carboxylic acid groups in the crystal lattices of the MOFs were varied by studying both ligand and missing linker defect site functionalized UiO-66 analogues. Novel materials developed in this study exhibited large increases in dynamic ammonia uptake capacity, attributed to favorable binding energies of ammonia with copper carboxylate functionalities in both dry and humid streams.30 Furthermore, incorporating copper into UiO-66 samples absent of intentionally synthesized carboxylic acid coordination sites did not produce similar improvements in ammonia capture, suggesting interactions between copper species and carboxylic acid moieties facilitate the enhancement of ammonia uptake observed for the materials studied. Through the work discussed here, it was found that the new material 19 ACS Paragon Plus Environment


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UiO-66-(COOCu)2 possesses dynamic ammonia capacities in excess of 6 mmol g-1 under dry and humid environments, and retains stability in water vapor as well as dry and humid ammonia streams. To the best of the author’s knowledge, this is the highest reported ammonia uptake of any previously studied metal-organic framework that retains framework stability in the evaluated conditions.5,10,21,39,40 Results from this study provide perspective onto the efficacy of employing copper carboxylate groups for ammonia filtration applications, as well as the effects and considerations that should be made on engineering locations for active binding sites in regard to adsorption efficiency and material stability. For instance, utilization of linker functionalities in MOFs to target adsorbates has been relatively well studied, however, studies on functionalizing active sites on deliberately created MOF defects as reported here is in nascent. Consequently, exacting novel manipulation strategies of MOF characteristics through defects and postsynthetic modification routes may prove beneficial towards the development of new and effective sorbent materials for a wide range of adsorption applications.

Supporting Information for Publication Detailed experimental methods, synthesis procedures, and material characterization information are detailed online in the Supporting Information (SI). Acknowledgements Funding for the work presented herein was provided by the Army Research Office (ARO) contract W911NF-15-1-0640 (EG; JD). Work by JNJ and CMM was supported by the Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy (UNCAGE-ME), an Energy Frontier Research Center funded by U.S. Department of Energy (US DoE), Office of Science, Basic Energy Sciences (BES) under Award # DE-SC0012577. The authors would also like to acknowledge the contribution from the M.S. Thesis work of Erika Garcia-Gutierrez:


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Garcia-Gutierrez, E. Copper Insertion in a Series of Metal-Organic Frameworks with Uncoordinated Carboxylic Acid Groups for Ammonia Removal. M.S. Thesis, Georgia Institute of Technology, May 2015.



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Engineering Copper Carboxylate Functionalities ... - ACS Publications

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