Solid-Phase Detoxification of Chemical Warfare Agents using

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Functional Nanostructured Materials (including low-D carbon)

Solid-Phase Detoxification of Chemical Warfare Agents using Zirconium-based Metal Organic Frameworks and the Moisture Effects – Analyze via Digestion Hui Wang, John J. Mahle, Trenton M. Tovar, Gregory W. Peterson, Morgan G. Hall, Jared B. DeCoste, James H Buchanan, and Christopher J. Karwacki ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 28, 2019

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ACS Applied Materials & Interfaces

Solid-Phase Detoxification of Chemical Warfare Agents using Zirconium-based Metal Organic Frameworks and the Moisture Effects – Analyze via Digestion Hui Wang,* John J. Mahle, Trenton M. Tovar, Gregory W. Peterson, Morgan G. Hall, Jared B. DeCoste, James H. Buchanan, and Christopher J. Karwacki* U.S. Army Combat Capabilities Development Command Chemical Biological Center, 8198 Blackhawk Road, Aberdeen Proving Ground, MD 21010, United States KEYWORDS. chemical warfare agent, hydrolysis, metal-organic framework, catalysis, digestion, solid-phase decontamination, water effect on hydrolysis

ABSTRACT Zirconium-based metal organic frameworks (Zr-MOFs) are highly chemically and thermally stable and have been of particular interest as reactive sorbents for chemical warfare agent (CWA) removal due to their fast and selective reactivity toward CWAs reported in buffer solutions. However, we find that decontamination of neat CWAs directly on Zr-MOFs, UiO-66, UiO-66-NH2 and NU-1000, are rather slow and the reactivity trend and products generated are very different from those in solution. Furthermore, we show that their decontamination rates are affected by the amount of moisture present in the MOFs. While the effects are minor for UiO-66-NH2 and NU-1000, the hydrolytic activity of UiO-66 toward CWAs dramatically improves as the amount of water present increases. Specifically, the half-life of UiO-66 decreases from 19 d with 0 wt % water loading to less than 1 h with 400 wt % water loading. The results reported here suggest that decontamination of CWAs by Zr-MOFs in solid phase behaves very differently than solution decontamination. Additionally, we present for the first time a digestion method for analyzing and quantifying solid-phase decontamination, which is a daunting challenge itself due to lack of a

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convenient analytical method. 1. INTRODUCTION The need for protection against chemical warfare agents (CWAs) has risen in recent years due to potential terrorist attacks and international conflicts. The current protection technologies use activated carbons in their pure or impregnated form to remove CWAs.1 Generally, activated carbons only capture and retain these toxic chemicals but do not destroy them. Therefore, there is an urgent need to develop new materials that are capable of both adsorption and degradation of CWAs under ambient conditions. Metal–organic frameworks (MOFs) are self-assembled crystalline materials with open three-dimensional frameworks constructed from metal-containing nodes connected by organic linkers.2,3 Owing to their high porosity as well as the amenability to modular design,4,5 MOFs are promising materials as adsorbents and catalysts for removal and detoxification of toxic chemicals.6-9 To this end, Katz et al. recently reported that the Zr-based MOF UiO-66, with 1,4-benzene-dicarboxylate (BDC) as the linker and a 12-connected Zr6(µ3O)4(µ3-OH)4 as the node, was highly active for catalytically hydrolyzing a nerve agent simulant, dimethyl 4-nitrophenyl phosphate (DMNP).10 The catalytic activity originated from the strongly acidic ZrIV ions which are excellent in activating coordinated organophosphates. Additionally, the multiple Zr-OH-Zr moieties in each Zr6(µ3-O)4(µ3-OH)4 cluster are mimics of the Lewis-acidic Zn–OH–Zn active site found in phosphotriesterase, an enzyme capable of hydrolyzing organophosphates. Many computational studies also confirmed the catalytic nature of Zr6 clusters.11,12 Since its first report, many researchers have been focusing on improving the catalytic rate of Zr6-based family of MOFs through both metal node and/or linker engineering.13-19 Two notable examples are UiO-66-NH2 and NU-1000. Katz et al. showed that UiO-66NH2, composed of the same Zr6(µ3-O)4(µ3-OH)4 node and 2-aminoterephthlate (2-ATA) linker, had a 20fold increase in hydrolysis rate compared to the parent UiO-66.13 The authors proposed that the pendant amino moiety on the 2-ATA linker acts as a Bronsted base to aid the proton transfer process during the catalytic cycle. However, a more recent finding by Islamoglu et al. ruled out the role of the amino moiety as a Bronsted base and suggested that it rather causes subtle changes in the microsolvation environment 2 ACS Paragon Plus Environment

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around the nodes.14 With a different approach, Mondloch et al. demonstrated that NU-1000 was also a more potent catalyst than UiO-66 in the destruction of DMNP as well as the nerve agent GD (O-pinacolyl methylphosphonofluoridate, also known as Soman).16 The structure of NU-1000, composed of 8-connected Zr6(µ3-O)4(µ3-OH)4(OH)4(H2O)4 nodes and tetratopic1,3,6,8-tetrakis(p-benzoate)pyrene (TBAPy) linkers, presents far more terminal-zirconium-ligated H2O and OH groups for hydrolysis than the defect-free UiO66. Furthermore, the mesopores of NU-1000 (~ 3 nm) allow bulky CWAs to permeate the entire framework, thus enabling the nodes in the interior of MOF to act as catalysts. Overall, the excellent catalytic property and exceptional porosity along with their outstanding hydrothermal, chemical and mechanical stabilities have made Zr-MOFs one of the most promising classes of materials for personal protection against CWAs.20-23 Nevertheless, the decontamination kinetics of Zr-MOFs reported thus far have mostly been performed in solution. To move Zr-MOFs forward toward the application of solid-phase reactive sorbents that are capable of fast adsorption and decontamination of CWAs under ambient conditions, systemic studies on decontamination kinetics of neat CWAs applied directly onto MOFs are highly demanded. UiO-66

UiO-66-NH2

NU-1000

Zr6(µ3-O)4(µ3-OH)4

Zr6(µ3-O)4(µ3-OH)4

Zr6(µ3-O)4(µ3-OH)4 (OH)4(H2O)4

Nodes

O

OH

O

O

OH

O

HO

OH

NH2

Linkers HO

O

HO

O

HO

OH O

O

H2BDC

H2ATA

H4TBAPy

Figure 1: Nodes and linkers of UiO-66, UiO-66-NH2 and NU-1000. Atom colors: Zr (blue), O (red), C (gray). H is omitted for clarity.



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Water is a crucial component of hydrolytic decontamination of CWAs. While it is always abundant for decontamination in an aqueous solution as it is both a reactant and the solvent, it can be limited for solidphase decontamination in which all available water molecules come from moisture in the atmosphere. Under dry conditions, hydrolysis might slow down or not occur at all due to lack of water. Fortunately, MOFs, similar to zeolites, are very capable of adsorbing water from the atmosphere and exhibit various uptake profiles.24 Consequently, for solid-phase decontamination, it is critical to understand the effect of water content adsorbed on the MOF on the decontamination activity. Herein, we report our findings on solid-phase decontamination of CWAs by three Zr-MOFs, UiO-66, UiO-66-NH2 and NU-1000 (Figure 1), and the effect the amount of moisture present has on the decontamination kinetics. In addition, we introduce, for the first time, a new and versatile digestion method for quantifying solid-phase decontamination of CWAs on Zr-MOFs. The goal of this work is to take a step closer toward understanding solid-phase decontamination and its correlation to solution-phase decontamination. 2. EXPERIMENTAL SECTION 2.1 Materials & Instrumentation All chemicals and reagents were purchased from commercial sources and used as received unless stated otherwise. UiO-66 and NU-1000 were purchased from NuMat Technologies. UiO-66-NH2 was purchased from TDA research, Inc. Hydrofluoric acid (HF, 48–51% solution in water) and dimethyl sulfoxide-d6 (DMSO-d6) were purchased from Acros Organic. All other chemicals and reagents were purchased from Sigma-Aldrich. 1H and 31P NMR spectra were recorded on a 300 MHz Varian NMR spectrometer using the residual proton resonance of the solvent as the internal reference and H3PO4 (0 ppm) as the external reference, respectively. PXRD measurements were taken using a Rigaku Miniflex 600 X-ray powder diffractometer with a D/Tex detector. Samples were scanned at 40 kV and 15 mA, using Cu Ka radiation (λ = 1.54Å), and a scan rate of 2° min-1 over a 2θ range of 3 to 50°. Nitrogen adsorption isotherms were measured using a Micromeritics TriStar 3000 analyzer at 77 K. Water isotherms were measured using a Quantachrome Autosorb-1 instrument with vapor option. Scanning electron microscopy (SEM) images 4 ACS Paragon Plus Environment

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were taken using a Phenom ProX desktop SEM. Thermogravimetric analysis (TGA) measurements were collected on a Netzsch TG 209 F1 Libra analyzer. Relative humidity was controlled using a Thunder Scientific Corporation Series 2500 Humidity Chamber at 25°C. 2.2 Activation of Zr-MOFs As-received UiO-66 and UiO-66-NH2 contained a significant amount of residual dimethylformamide (DMF) which was later removed using a solvent exchange method. Specifically, MOF was transferred to a centrifuge tube and methanol was added. The sample was allowed to stay for 48 h to exchange and remove DMF. Methanol was decanted and replaced with fresh methanol three times during this time period. At the end of this process, methanol was decanted and the sample was activated by drying under vacuum for 6 h, then was dried again in a vacuum oven for 24 h at 120 ºC. As-received NU-1000 was properly activated and used without further activation. 2.3 Stability of DMNP, GD and VX in Digestion Media Caution! Experiments involving GD and VX should be run by trained personnel only using appropriate safety procedures. To a NMR tube containing 5 mg of MOF was added 750 µL of digestion medium. The MOF was allowed to fully digest before 1 µL DMNP was added. Conversion of DMNP was then immediately monitored over time using 1H NMR. For H2SO4/DMSO-d6 and H3PO4/DMSO-d6 digestion media, 50 µL of acid and 700 µL DMSO-d6 were used. Note that it took a rather long time and often sonication was needed to fully digest the Zr-MOFs with H2SO4 or H3PO4. In contrast, the Zr-MOFs can be readily digested with HF without sonication (typically fully digested in less than 5 min). For the real CWAs, 10 mg of MOF and 2 µL of GD or VX were used instead. Conversions of GD and VX were monitored over time using 31P NMR. 2.4 Solid-Phase Decontamination of DMNP, GD and VX by Zr-MOFs Vials containing 5 mg of Zr-MOF powder were each dosed with 1 µL DMNP and incubated for various periods of time. After the desired incubation periods, each sample was mixed with 750 µL of 1.8 M HF in 5 ACS Paragon Plus Environment


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DMSO-d6 (50 µL hydrofluoric acid and 700 µL DMSO-d6). The solutions were transferred to NMR tubes after full dissolution of the MOFs and then analyzed by 1H or 31P NMR. For GD and VX decontamination, 10 mg of MOF and 2 µL of GD or VX were used instead. For all our experiments, regular glass vials and NMR tubes were used in handling HF solutions. When digesting the MOFs using HF/DMSO-d6, DMSOd6 was always added first to the MOF before HF was added. This ensured that the concentration of HF was dilute in the glasswares. 2.5 Solid-Phase Decontamination of DMNP by Zr-MOFs under Different RHs Vials containing 5 mg of MOF powder was incubated at the desired RH (20%, 40%, 60% or 80%) in a RH chamber for at least 16 h. 1 µL DMNP was added to each vial and mixed well (to maximize external contact between the MOF and DMNP) with the MOF powder by vortex. The vials were taken back to the RH chamber to equilibrate the headspace volume with the respective RH for additional 30 min. The vials were sealed tightly while they are in the RH chamber and then sit in the fume hood under ambient conditions for different periods of time. After the desired incubation times, 750 µL of 1.8 M HF in DMSO-d6 were added to each vial and the mixture was analyzed by 1H NMR after the complete dissolution of MOFs. 2.6 Solid-Phase Decontamination of DMNP by Zr-MOFs with Different Water Loadings 5 mg of MOF was placed in 4-mL vials. A defined amount (0 µl, 2 µl, 5 µL or 20 µL) of distilled water was added to each vial and mixed well with the MOF powder by vortex. 1 µL DMNP was then added to each vial and mixed well with MOF by vortex again. The vials were sealed tightly and incubated in the fume hood under ambient conditions for different periods of time. After the desired incubation time, 750 µL of 1.8 M HF in DMSO-d6 were added to each vial and the mixture was analyzed by 1H NMR after complete dissolution of MOFs. 2.7 Solution Decontamination of DMNP by Zr-MOFs To a NMR tube containing 2.6 mg of a Zr-MOF was added 1 mL of N-ethylmorpholine aqueous solution (0.45 M) or distilled water. 4 µL DMNP was then added to the mixture and the tube was shaken vigorously 6 ACS Paragon Plus Environment

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before transferring to a NMR spectrometer. The conversion of DMNP was monitored using in situ

31P

NMR. 3. RESULTS AND DISCUSSIONS Analyzing and quantifying the solid-phase decontamination of nerve agents on MOFs has been a daunting challenge due to both physical and chemical adsorption of reactants and products to the pores of MOFs.11 Several techniques have been previously employed for analyzing the decontamination products, including extraction and solid-state magic angle spinning nuclear magnetic resonance (MAS NMR). The extraction method uses various solvents to extract unreacted CWAs and decontamination products after a certain incubation period. However, 100% extraction efficiency is hardly achieved to account for the mass balance, which is primarily caused by the high binding energy of phosphate products chemically bound to the metal nodes of Zr-MOFs.11 On the other hand, while MAS solid-state NMR is a powerful technique, chemicallybound product peaks tend to broaden out compared to the physically adsorbed reactant or product peaks,25,26 rendering the analysis of decontamination semi-quantitative. In addition, the high spinning speed required by MAS NMR can speed up the reaction, causing a faster decontamination rate than the one obtained under a non-spinning condition.25 Therefore, a new method of analyzing solid-phase decontamination of CWAs on MOFs is required. Toward this end, we present a digestion method. Specifically, MOFs after incubation with CWAs are digested to break down their 3D structures, thus unreacted CWAs and tightly bound products are released to the digestion medium which can be easily analyzed using common analytical instruments, such as liquid NMR spectroscopy and gas chromatography-mass spectrometry (GC-MS). The obvious concern with using digestion method to analyze and quantify decontamination products is the stability of CWAs in the digestion medium, i.e. whether the digestion medium itself would hydrolyze CWAs during the course of digestion and spectroscopic analysis. To minimize hydrolysis, two mild digestion media that are known to digest Zr-MOFs were selected, namely, NaHCO3 (in D2O) and HF (in DMSO-d6). Figure 2a shows that DMNP was slowly hydrolyzed in both 0.5 M and 1.0 M NaHCO3 in D2O over time, approximately 13% and 25% hydrolyzed after 10 h, respectively. In comparison, it was quite 7 ACS Paragon Plus Environment


stable in 1.8 M HF in DMSO-d6, approximately 2% hydrolyzed after 10 h and negligible amounts of any hydrolyzed in 0.5 h which is the typical time required to digest and analyze a sample. Digestion media containing different amounts of HF were also investigated. Higher concentrations of HF barely accelerated the hydrolysis of DMNP (Figure S7), suggesting the high stability of DMNP in the HF/DMSO digestion medium. The stability of DMNP in other acidic digestion systems was also tested. It exhibited remarkable stability in both H2SO4/DMSO and H3PO4/DMSO digestion media as well (Figure S7). To extend this method to the actual nerve agents, the stability of GD and VX (O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate) in the digestion media were tested. Since GD and VX are generally more reactive than DMNP, we expected slightly faster hydrolysis rates for the real agents in both digestion media. As shown in Figure 2b, GD was hydrolyzed nearly instantly in 0.5 M NaHCO3 aqueous solution, completely hydrolyzed within 10 min. On the other hand, it was still stable in 1.8 M HF in DMSO-d6, barely any hydrolyzed products detected after 5 hours. Similarly, VX was also highly stable in the HF/DMSO digestion medium (Figure S8). The high stability of both the simulant and real agents in the HF/DMSO mixture suggests that it is a suitable digestion medium for analyzing and quantifying decontamination of CWAs on Zr-MOFs. This is presumably due to the high tendency of fluoride ions toward Zr(IV) to form strong Zr-F bonds thus destroying the secondary building units. On the other hand, the high electronegativity of fluorine renders nucleophilic attack at phosphorus of nerve agents a slow process.

(b)

100 90 80 70 60 50 40 30 20 10 0

0.5 M NaHCO3 in D2O 1.0 M NaHCO3 in D2O 1.8 M HF in DMSO-d6

0

1

2

3

4

5

6

7

8

9 10

Conversion (%)

(a) Conversion (%)

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

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100 90 80 70 60 50 40 30 20 10 0

0.5 M NaHCO3 in D2O 1.8 M HF in DMSO-d6

0

Time (h)

1

2

3

4

5

Time (h)

Figure 2. Conversion of (a) DMNP and (a) GD over time in digestion media. Note that all the samples contained 5 mg of UiO-66-NH2. This ensured that the possible catalytic effect of digested zirconium species was taken into account. 8 ACS Paragon Plus Environment

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To investigate the solid-phase decontamination of CWAs by Zr-MOFs, the conversion profiles of neat DMNP on UiO-66, UiO-66-NH2 and NU-1000 were measured using the digestion method. Briefly, vials containing 5 mg of MOF powder were each dosed with 1 µL DMNP and incubated for various time periods. After the desired incubation periods, MOFs were digested with 750 µL of 1.8 M HF in DMSO-d6 and analyzed by 1H NMR. As shown in Figure 3a, solid-phase hydrolysis rates of DMNP by three Zr-MOFs were rather slow, with half-lives in the range of days to a month. UiO-66-NH2 had a faster conversion profile than UiO-66 under ambient conditions. Surprisingly, NU-1000 exhibited a negligible reactivity toward DMNP which did not reach 50% conversion even after a month. This was in contrary to the reactivity trend observed both in NEM aqueous buffers where UiO-66-NH2 and NU-1000 had faster rates than UiO-66 and in distilled water where UiO-66 and NU-1000 had faster rates than UiO-66-NH2 (Figure S15). It is important to note that all solid-phase and in-house solution decontamination experiments were done using the same batches of Zr-MOFs. This ensures that the rate differences were not caused by batchto-batch variations of Zr-MOFs due to different defects, porosities, and/or particle sizes.27-31 (a) 100 Conversion (%)

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


UiO-66

NU-1000

UiO-66-NH2

DMNP

0

(c)

(b)

UiO-66 UiO-66-NH2 NU-1000

90 80 70 60 50 40 30 20 10 0

5

10

15 20 Time (d)

25

NO2

O P O O O

30

H 2O UiO-66/NU-1000

NO2

, H 2O 6-NH 2 UiO-6 age v a cle P-O UiO

-66-

DMNP

O

3.9

3.8

O P OH O

+

3.7 3.6 ppm

3.5

3.4

NO2 HO

DMP

DMNP

O P O O O

4.0

DMP M4NP

C-O

NH

2, H 2O vage

clea

O

O P OH O

NO2 + HO

DMP

O P O HO O

NO2 +

OH

M4NP



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Figure 3. Solid-phase decontamination of DMNP on Zr-MOFs. (a) Conversion profiles of DMNP by ZrMOFs under ambient conditions. (b) Selected 1H NMR spectra of hydrolysis products of DMNP on ZrMOFs, showing only the methyl proton region of phosphate products. (c) Reaction schemes of hydrolysis of DMNP on Zr-MOFs.

Besides the different hydrolysis profiles, additional hydrolysis products were also observed in solid-phase decontamination. As previously reported, hydrolysis of DMNP by Zr-MOFs in solution typically occurs through the cleavage of P-O bond to exclusively produce dimethyl phosphate (DMP) as the sole product.10,13,16 While the same was observed for UiO-66 and NU-1000 in solid-phase decontamination, decontamination of DMNP by UiO-66-NH2 produced multiple hydrolysis products (Figure 3c), including two major products, dimethyl phosphate and methyl 4-nitrophenyl phosphate (M4NP), and one minor product, methyl phosphate (MP). Even though the formation of M4NP as a DMNP hydrolysis product is not uncommon in other systems,32, 33 this was the first time it is observed for hydrolysis of DMNP by a ZrMOF. Mechanistically, formation of M4NP can occur through nucleophilic attack of DMNP at two reaction centers, namely the phosphorus atom, which is a hard electrophilic site, and the methoxy carbon atom, which is a soft electrophilic site. Indeed, methyl esters of phosphorus acids, similar to dimethyl sulfate, are known to be good alkylating agents.34 Hence, it’s possible to cleave the C–O bond of DMNP to yield M4NP. Since p-nitrophenolate anion is a better leaving group than methoxy anion, nucleophilic attack on the phosphorus center should preferentially generate the dimethyl phosphate product. Therefore, we postulated that generation of M4NP occurs mainly through nucleophilic attack at the softer electrophilic carbon center. Indeed, this was proven to be the case. Nevertheless, besides methanol, another side product, 2(methylamino)terephthalic acid, was detected in the digestion medium, accounting for ~70% of M4NP formed. In other words, generation of M4NP occurs mainly through nucleophilic attack at the soft carbon center by the amino group of 2-ATA linker (Scheme 1), which is a softer nucleophile than hydroxide ion. Furthermore, it was found that generation of M4NP was dependent on the amount of water present in UiO10 ACS Paragon Plus Environment

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66-NH2 (Figure S11). Higher water loadings reduced the amount of M4NP produced, presumably due to an increase in the amount of the hard competitive nucleophile, hydroxide ion, which preferentially attacked the hard phosphorus center to generate dimethyl phosphate as the major product.

Scheme 1. The proposed reaction mechanism of DMNP with the 2-ATA Linker of UiO-66-NH2. O

OH NH2

HO

O

O O P O O O

NO2

OH H N +

HO

O P O HO O

NO2

O

To evaluate the solid-phase decontamination efficiency of three Zr-MOFs against the real nerve agents, hydrolysis of GD and VX by UiO-66, UiO-66-NH2 and NU-1000 were measured. Overall, hydrolysis of GD and VX by three Zr-MOFs were much faster than that of the simulant. These were expected as the P-F and P-S bond of GD and VX are more labile than the P-O bond of DMNP, respectively. As shown in Figure 4b, hydrolysis profiles of GD by UiO-66 and UiO-66-NH2 were very similar, with UiO-66 having a slightly faster initial rate than UiO-66-NH2. Once again, NU-1000 showed a much slower hydrolysis activity toward GD than both UiO-66 and UiO-66-NH2. In terms of decontamination product, only the nontoxic pinacolyl methylphosphonic acid (PMPA) product was observed for all three MOFs (Figure S9). This is probably due to nucleophilic attack of the amino group of 2-ATA at the carbon center of the bulky pinacolyl is not a feasible process in comparison to nucleophilic attack of hydroxide ion at the phosphorus center. Hydrolysis profiles of VX by the three Zr-MOFs (Figure 4c) were comparable to those of GD, with UiO-66 having a faster conversion profile than both UiO-66-NH2 and NU-1000. Similarly, only the nontoxic ethyl methylphosphonic acid (EMPA) product was observed for all three MOFs (Figure S10).



(a)

O P O F

O

O P

UiO-66/UiO-66-NH2/ NU-1000 Solid Phase

+

O

UiO-66/UiO-66-NH2/ NU-1000 Solid Phase

VX

O P

+

OH

HS

N

EMPA

(c)

100 90 80 70 60 50 40 30 20 10 0

100 90 80 70 60 50 40 30 20 10 0

Conversion (%)

(b)

HF

PMPA

H 2O

N

S

O P O OH

H 2O

GD

Conversion (%)

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

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0

1

2 3 Time (d)

4

5


0

1

2 3 Time (d)

4

5

Figure 4. Solid-phase decontamination of GD and VX on Zr-MOFs. (a) Reaction schemes of hydrolysis of GD and VX by Zr-MOFs. Conversion profiles of (b) GD and (c) VX by Zr-MOFs under ambient conditions.

Considering the slow solid-phase decontamination rates of CWAs by NU-1000, we conceived that it might be caused by the low water uptake of NU-1000 at ambient conditions due to its large pore size. To investigate the effects of water on the hydrolytic activities, it’s important to first understand the water uptake behavior of the Zr-MOFs. As shown in Figure 5a, both UiO-66 and UiO-66-NH2 started to adsorb water at relatively low P/P0 (< 0.2). Water uptake of UiO-66-NH2 then gradually increased with increasing pressure up to P/P0 = 0.35 and levels off afterward. UiO-66 exhibited a steep water uptake around P/P0 = 0.3, followed by a saturation capacity that is slightly higher than UiO-66-NH2. In contrast, NU-1000 had a water adsorption isotherm that shows little uptake up to P/P0 = 0.4, followed by a steep rise and attainment of a saturation capacity exceeding 1.5 g/g at P/P0 = 0.7 with a hysteresis loop, similar to the water uptake behavior of activated carbons. Since water is critical in hydrolytic decontamination, we expected that the decontamination of CWAs by Zr-MOFs would be greatly influenced by the amount of water adsorbed in MOFs. Due to the high risk of working with real nerve agents, only decontamination rates of DMNP by ZrMOFs under different RHs were measured. As shown in Figure 5b, all three Zr-MOFs showed enhanced 12 ACS Paragon Plus Environment

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hydrolysis rates toward DMNP as RH increased. At 20% RH, UiO-66-NH2 exhibited a faster kinetics than UiO-66 at 20% RH. This is corresponding to the higher water content of UiO-66-NH2 at 20% P/P0 (12 wt% for UiO-66-NH2 versus 6 wt% for UiO-66). However, UiO-66 outperformed UiO-66-NH2 at 40%, 60% and 80% RHs, respectively, consistent with the higher water uptake of UiO-66 than UiO-66-NH2 at these RHs. In comparison, NU-1000 exhibited slower hydrolysis rates than both UiO-66 and UiO-66-NH2 under all the RHs investigated. Since our experiments were designed in a way such that the weight ratio of MOF to CWA was kept constant, the slower hydrolysis rates of NU-1000 could be caused by the lower number of active sites present on NU-1000 (4 defects per node) than the defected UiO-66 (~ 4.9 defects per node) (Table S1). However, increasing the weight ratio of NU-1000 to DMNP to account for the number of moles of active sites hardly improved the hydrolytic activity (Figure S12). While the slower hydrolysis rates from 20% to 60% RH can be explained by scarce amount of water present for hydrolysis, it is unclear why NU1000 still exhibited a slower hydrolysis rate at 80% RH at which NU-1000 contained a significant amount of water. (a)

(b)

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0

0.2

0.4

3.0 UiO-66 UiO-66-NH2 NU-1000

2.5


Initial rate (µmol/d)

Water uptake (g/g)

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


0.6

0.8

1.0

2.0 1.5 1.0 0.5 0.0

20

P/P0

40

60

80

RH (%)

Figure 5. (a) Water isotherms of UiO-66, UiO-66-NH2 and NU-1000. (b) Solid-phase initial hydrolysis rates of DMNP by UiO-66, UiO-66-NH2 and NU-1000 under different humidity conditions.

To further explore the effect of water on MOF toward decontamination, a slightly different approach was utilized. Briefly, MOFs containing different amounts of water were prepared by manually adding various amounts of water to the MOFs under ambient conditions. This approach provided a more convenient way 13 ACS Paragon Plus Environment


to control the water loading amount over a wider range. To this end, all three Zr-MOFs, UiO-66, UiO-66NH2 and NU-1000, were wetted with 0, 40, 100, and 400 wt % water, respectively, followed by dosing with DMNP. The samples were then incubated at ambient conditions for various periods of time before they were digested and analyzed. As shown in Figure 6, plain UiO-66 exhibited a rather slow hydrolysis rate with an initial rate of 6 µmol/d. However, its activity significantly improved as the water loading was increased. 40 wt % water loading increased its initial hydrolysis rate to 10 µmol/d. 100 wt% water loading further increased its initial rate to 27 µmol/d. The highest activity was observed with 400 wt % water loading, which achieved 95% conversion in 1 d and had an initial rate of 140 µmol/d. In comparison, plain UiO-66-NH2 showed a slightly faster conversion profile. However, wetted UiO-66-NH2 did not show significant enhancements in hydrolytic activity. Interestingly, 20 wt% water loading slightly slowed down the rate. Presumably, this was caused by poor mass transport of DMNP to the catalytic sites as clumping of MOF occurred with 20 wt % water loading. Similar to UiO-66-NH2, increasing water loadings only slightly improved the decontamination rate of DMNP by NU-1000.

0 wt % 40 wt % 100 wt % 400 wt %

0

1

2

3

4

Time (d)

5

6

7

8

(c)

100 90 80 70 60 50 40 30 20 10 0

0 wt % 40 wt % 100 wt % 400 wt %

0

5

10

15

20

25

Time (d)

30

Conversion (%)

(b)

100 90 80 70 60 50 40 30 20 10 0

Conversion (%)

(a) Conversion (%)

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

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100 90 80 70 60 50 40 30 20 10 0

0 wt % 40 wt % 100 wt % 400 wt %

0

5

10

15

20

25

30

Time (d)

Figure 6. Solid-phase conversion profiles of DMNP by (a) UiO-66, (b) UiO-66-NH2 and (c) NU-1000 with various water loadings.

Overall, solid-phase hydrolysis rates are slower than their rates in basic buffer solutions. This is understandable providing severe catalyst poison in solid-phase decontamination and the higher pH of buffer decontamination which helps catalyze the hydrolysis and neutralize the generated acidic products. Another plausible explanation can be, as pointed out by a recent study, that the high activities of Zr-MOFs in 14 ACS Paragon Plus Environment

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solutions with moderate basic pHs are caused by partial breakdown of MOF structures, which creates a lot of defects for hydrolysis.35 Interestingly, the reactivity trend observed in solid phase for the three Zr-MOFs is very different from their trend observed in buffer or non-buffer solution. Particularly, NU-1000 has a much slower hydrolysis rate than the UiO MOFs in solid state, whereas its rate is faster or comparable to those of UiO MOFs in aqueous solutions (both buffer and non-buffer). One possible explanation can be the difference in particle size of UiO MOFs and NU-1000. Smaller MOF particles have been shown to be more efficient in CWA hydrolysis than large MOF particles.31 The particle size of NU-1000 is ~ 10 micron as seen in the SEM images (Figure S2). This is considerably larger than the nanosized UiO MOFs used in this study (Figure S2). While the effect of particle size can be alleviated to some extend by the presence of the solvent to assist in transporting of CWAs throughout the large pores of NU-1000 in solution, large MOF particles could be very disadvantageous in solid-phase hydrolysis due to lack of a solvent to assist the transport process to fully utilize the reactive sites. It’s important to note that for all the solid-phase decontamination experiments investigated, both the moles of catalyst active sites and moles of adsorbed water were in excess of the moles of CWA used (Table S1-3), indicating achieving less than one turnover with rather slow rates. In addition, all the conversion profiles, especially those measured under different RHs, did not truly follow a pseuso first-order kinetics. The rates were much faster at first and slowed down considerably afterward. This is presumably caused by catalyst poison due to binding of products to the active sites. While hydroxide ions may assist in the displacement of phosphate products chemically bound to Zr(IV) ion to regenerate the active site in basic solutions, product inhibition in solid-phase decontamination can be a severe problem and thus renders the MOFs stoichiometric reagents rather than catalysts. Experimentally, we have also observed this phenomenon for solid-phase decontamination. For example, when a CWA contaminated MOF sample was extracted with an organic solvent, only the unreacted CWA was extracted and no phosphate acid products can be detected, suggesting the strong binding of phosphate acid products to the Zr nodes. On the other hand, both the unreacted CWA and phosphate acid products were readily detectable when the same sample was analyzed by digestion method, in which HF attacks the Zr nodes to release the bound phosphate acids as well as the organic linkers. In 15 ACS Paragon Plus Environment


Page 16 of 22

addition, theoretical studies on the mechanism of CWA hydrolysis by Zr-MOFs also suggest the binding of organophosphate acid products to the zirconium nodes and high energy is required to remove these bound species.11 4. CONCLUSIONS In conclusion, solid-phase decontamination of DMNP by three Zr-MOFs, UiO-66, UiO-66-NH2 and NU1000, showed overall slower hydrolysis rates and a very different reactivity trend than solution decontamination. Content of water present in each Zr-MOF affects their hydrolysis rates to different extents. While increasing water content significantly enhances the hydrolysis rate of DMNP by UiO-66, its effect on those of UiO-66-NH2 and NU-1000 is only moderate. Furthermore, we found that UiO-66-NH2 has an additional mechanism of hydrolyzing DMNP offered by the pendant amino moiety, which generates the toxic product, M4NP. However, no toxic products were observed for hydrolysis of GD and VX by UiO66-NH2. It is still unclear on the reasons behind the slow hydrolysis rates of NU-1000 under all the RHs and water loadings tested. All these findings point out the importance of measuring solid-phase decontamination rates which are very different from the rates observed in solution. These differences also indicate that the design rules for enhancing the hydrolysis rates based on solution results don’t necessarily apply to solid-phase decontamination and encourage further research on the solid-phase decontamination of CWAs by Zr-MOFs. With the high amenability of MOFs toward modular designs, we believe that designing highly reactive Zr-MOFs for CWA decontamination in solid phase will be a foreseeable goal in the near future. Finally, we present for the first time a convenient and powerful digestion method for analyzing and quantifying solid-phase decontamination. Extension of this method to other MOFs and metal oxides are currently under investigation in our lab.

ASSOCIATED CONTENT


Page 17 of 22 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


Supporting Information MOF Characterizations (N2 isotherms, PXRD, SEM and TGA), stability of CWAs in digestion media, solution decontamination of DMNP by Zr-MOFs, 1H NMR spectra

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

*E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge Defense Threat Reduction Agency for funding this research under CB3934. This research was performed while Trenton M. Tovar held a National Research Council Research Associate Award.



Page 18 of 22

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Table of Contents

Zr6(µ3-O)4(µ3-OH)4

NO2

H 2O O

UiO-66 Solid-phase

Conversion (%)

O P O O O

100 90 80 70 60 50 40 30 20 10 0

O P OH O

NO2 + HO

Water Loading

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

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Solid-Phase CWA Detoxification 0

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Time (d)


Solid-Phase Detoxification of Chemical Warfare Agents using

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