Dual-Function Metal–Organic Framework as a Versatile Catalyst for

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Dual-Function Metal
Organic Framework as a Versatile Catalyst for Detoxifying Chemical Warfare Agent Simulants Yangyang Liu,† Su-Young Moon,† Joseph T. Hupp,*,† and Omar K. Farha*,†,‡ †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States and ‡Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 22254, Saudi Arabia

ABSTRACT The nanocrystals of a porphyrin-based zirconium(IV)

metal
organic framework (MOF) are used as a dual-function catalyst for the simultaneous detoxification of two chemical warfare agent simulants at room temperature. Simulants of nerve agent (such as GD, VX) and mustard gas, dimethyl 4-nitrophenyl phosphate and 2-chloroethyl ethyl sulfide, have been hydrolyzed and oxidized, respectively, to nontoxic products via a pair of pathways catalyzed by the same MOF. Phosphotriesterase-like activity of the Zr6-containing node combined with photoactivity of the porphyrin linker gives rise to a versatile MOF catalyst. In addition, bringing the MOF crystals down to the nanoregime leads to acceleration of the catalysis. KEYWORDS: chemical warfare agents . dual function . metal
organic frameworks . heterogeneous catalysis . VX . Soman . GD . Simulant . DMNP . CEES

F

rom World War I (WWI) to the Syrian chemical-weapons attack in 2013, various chemical warfare agents (CWAs) have been developed and used.1
7 Nerve agents and vesicants are two major types of CWAs, and they are among the most toxic and dangerous chemical weapons.8
10 Nerve agents such as sarin, soman (GD), and VX are members of a family of organophosphates (Scheme 1a) that can disrupt signal transmission from the nervous system to muscles, including muscles needed for breathing. As a result, exposure to sarin, GD, or VX rapidly leads to asphyxiation.9,11 Still a threat today, one of the most powerful CWAs used during WWI was sulfur mustard, also known as mustard gas or HD (Scheme 1a).12 Sulfur mustard causes severe blisters on the skin, as well as irritation to the eyes and respiratory tract, and in some cases can lead to death.13,14 Sulfur mustard is also an alkylating agent that can alter the structure of DNA, resulting in programmed cell death and an increased risk of developing cancer.15,16 LIU ET AL.

Developing materials and methods for deactivating CWAs is an area of significant importance. Organophosphate-based nerve agents, for example, can be degraded by hydrolysis of the labile P
X bond (e.g., X = F in the case of sarin, Scheme 1b). The enzyme phosphotriesterase (PTE) is highly active in catalyzing the hydrolysis of phosphate ester bonds;17,18 however, enzyme deactivation and relatively short enzyme shelf lives (at least at ambient temperature) place limits on the circumstances and range of conditions in which PTE may find practical use.19 As a result, bioinspired materials that mimic the active site in PTE have been explored for the destruction of nerve agents.20
31 Zirconium metal
organic frameworks (MOFs) are some of the most promising candidates for catalytic nerve agent hydrolysis, as they have been found to be very effective for rapid hydrolysis of the phosphate ester bond.27
32 As for the degradation of sulfur mustard, hydrolysis can be challenging due to its oily consistency and water immiscibility.33 Therefore, the VOL. XXX

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* Address correspondence to [email protected], [email protected]. Received for review September 9, 2015 and accepted October 19, 2015. Published online 10.1021/acsnano.5b05660 C XXXX American Chemical Society

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hydrolysis of sulfur mustard (Scheme 1c) is usually slow and incomplete, owing to low water solubility and the formation of intermediate products that interfere with subsequent hydrolysis.10,33 A more promising route for sulfur mustard detoxification is partial oxidation, in which sulfur mustard is oxidized to bis-2-chloroethyl sulfoxide (Scheme 1c).34
40 It should be noted that full oxidation of sulfur mustard produces a sulfone that is itself toxic.37,41 Thus, various oxidants and potential catalysts have been evaluated with the goal of achieving partial oxidation of sulfur mustard to the nontoxic sulfoxide product.39,42
45 To this end, we recently demonstrated the use of singlet oxygen (1O2), generated by a porphyrinic MOF, for efficient and selective oxidation of sulfur mustard to the nontoxic sulfoxide product.45,46 To date, most research has been focused on the development of catalytic materials for the detoxification of CWAs where hydrolysis is the primary detoxification pathway.20
32 Since it is difficult to predict which CWAs will be used, and considering the solubility issues of some CWAs in water, multiple detoxification pathways, such as hydrolysis and oxidation (Scheme 1b,c), are needed to occur simultaneously for efficient deactivation of diverse CWAs. This requires different catalytic moieties embedded in one catalyst in order to generate broad-spectrum detoxification materials. MOFs are a family of materials well known for their multifunctional nature, where the organic linkers and inorganic metal nodes that construct the framework are highly tunable.47
50 The key to designing a versatile MOF for an all-in-one application is to incorporate the right combination of structural components (linkers and nodes) in one material so that these components can function synergistically. In addition to the tunability and multifunctionality, the high surface area51
55 and exceptional stability of selected MOFs make them especially attractive as catalysts for the rapid degradation of CWAs.56 The permanent porosity of MOFs can not only enhance the adsorption LIU ET AL.

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Scheme 1. (a) Structural formula of three chemical warfare agents (CWAs): sarin, VX, and sulfur mustard. (b) Hydrolytic degradation pathway of VX. (c) Hydrolytic and oxidative degradation pathways of sulfur mustard. (d) CWA simulants used in our study.

of toxic agents but also facilitate diffusion of the substrate toward catalytically active sites, thus accelerating the catalysis. Taking advantage of this versatility, herein, we report on a dual-function MOF catalyst that can detoxify both nerve agents via hydrolysis and sulfur mustard through oxidation in one system. Given the high toxicity of nerve agents and sulfur mustard, we used the simulants dimethyl 4-nitrophenyl phosphate (DMNP) and 2-chloroethyl ethyl sulfide (CEES) rather than real agents (Scheme 1d). Previous research has shown that these two are effective simulants for organophosphate nerve agents and sulfur mustard, respectively, owing to their structural similarity to real agents. RESULTS AND DISCUSSION Selection and Synthesis of the Catalyst. Recently, we have reported the use of a porphyrin-based MOF, PCN-222/MOF-54557,58 (1), to achieve selective photooxidation of CEES under LED irradiation.45 Combining the free-base porphyrin linker tetrakis(4carboxyphenyl)porphyrin (TCPP4
) and 8-connected Zr6 cluster, the three-dimensional (3-D) free-base PCN-222/MOF-545 (fb-1) is constructed with open channels up to 3.7 nm in diameter (Figure 1). This Zr-MOF is stable in aqueous media over a wide pH range.57 In addition to permanent porosity and excellent chemical stability, the two structural components of the MOF;a porphyrin linker and an 8-connected Zr6 node;endow fb-1 with dual functionality. While singlet oxygen can be generated by the porphyrin moieties under visible (LED) irradiation (Figure 1e), the Zr6 nodes containing Zr
OH
Zr constructs in fb-1 afford the hydrolase function of the Zn
OH
Zn active site in PTE (Figure 1c,d).27,59 We have shown that the particle size of MOF catalysts can also affect the rate of DMNP hydrolysis, where smaller particles increase the rate.31 This is due to the larger relative external surface areas of nanosized MOFs compared to their microcrystalline form and/or faster diffusion of the substrates into the MOF nanocrystals. Compared to nanosized PCN-222/MOF545 (Fe),31 the synthesis of nanosized PCN-222/MOF545 with free-base porphyrin linkers is more challenging due to the higher flexibility of the unmetalated linkers. As a result, samples often contain multiple phases of different Zr-MOF topologies (including csq,57,58,60 ftw,58,61
63 and shp-a64 topologies). By carefully adjusting the modulator, reaction temperature, and time, however, we were able to obtain purephase, nanosized free-base PCN-222/MOF-545 (nfb-1). Briefly, nanocrystals were obtained by lowering the concentration of the modulator (benzoic acid) as well as the total concentration of TCPP and ZrOCl2 in the solution, while also adding to the solvothermal reaction a second modulator, trifluoroacetic acid (TFA) (see Methods section for detailed synthetic procedure). VOL. XXX

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Figure 1. (a) Scanning electron microscopy (SEM) image of nanosized PCN-222/MOF-545 (free base) (nfb-1). (b) 3-D structure of nfb-1, constructed from a [Zr6(μ3-O)8(O)8]8
node and tetrakis(4-carboxyphenyl)porphyrin linker (TCPP4
). (c) View of Zr6 nodes containing Zr
OH
Zr that mimics the (d) Zn
OH
Zn active site in phosphotriesterase. (e) Concept of generating singlet oxygen by the porphyrin moieties in nfb-1 under LED. Hydrogen atoms are not shown in the structures for clarity.

Scanning electron microscopy (SEM, Figure 1a) and powder X-ray diffraction (PXRD, Figure S4) confirmed the phase purity of nfb-1. Nitrogen adsorption experiments (Figures S2, S3) indicate that the gravimetric porosity and the pore size distribution of nfb-1 are similar to those of fb-1. Dual-Function Catalytic Activity. In the dual reaction, 2.4 mg of nfb-1 was dispersed in a mixed solvent of 0.4 M N-ethylmorpholine solution and methanol in a sealed microwave vial equipped with a magnetic stir bar (Figure 2a).65 After purging with O2 for 20 min, DMNP and CEES were added to the microwave vial using microsyringes. While irradiating with a blue LED, both reactions were monitored by GC-FID (gas chromatography with flame ionization detection), and products were identified by GC-MS (gas chromatography with mass spectrometry). Figure 2b illustrates the kinetics of the hydrolysis and oxidation with 4 mol % catalyst (nfb-1) loading. In the hydrolysis of DMNP, cleavage of the phosphate ester bond catalyzed by the Lewis acidic Zr6 cluster in nfb-1 produces dimethyl hydrogen phosphate and 4-nitrophenol (Figure 2a). Concurrently, under LED irradiation, singlet oxygen generated by the porphyrin moieties66 in nfb-1 selectively oxidizes CEES to the nontoxic 2-chloroethyl ethyl sulfoxide (CEESO). A small amount of 1-(ethylsulfinyl)-2-methoxyethane, a nontoxic product, is also produced from methanolysis and oxidation of a small fraction of the CEES (Figures 2a, S10). Notably, interference effects are absent; the rate of oxidation of CEES is not measurably affected by the presence of DMNP and its concurrent hydrolysis, nor is the rate of catalytic hydrolysis measurably affected by the presence of CEES and its concurrent oxidation. We found that if the same catalyst loading is used for both DMNP and CEES, comparable half-lives for both reactions are observed. After 60 min, DMNP and CEES are both converted to nontoxic products, with half-lives of 8 and 12 min, respectively, using 4 mol % LIU ET AL.

Figure 2. (a) MOF-catalyzed dual-reaction setup to transform toxic CWA simulants CEES and DMNP to nontoxic oxidative and hydrolytic products, respectively. The reactions are performed in a 1:1 mixture of 0.4 M N-ethylmorpholine solution and MeOH purged with O2 under blue LED irradiation; 4 mol % catalyst (nfb-1) loading was used. (b) Kinetic profiles for hydrolysis of DMNP (25 μmol) and oxidation of CEES (50 μmol) in one batch, in the presence of 2.4 mg (1 μmol Zr6/2 μmol porphyrin unit) of nfb-1. Halflives for degradation of DMNP and CEES are 8 and 12 min, respectively.

catalyst. To our knowledge, nfb-1 is the best-performing catalyst reported to date for simultaneous degradation of a nerve agent and mustard gas simulant. López-Maya et al. reported half-lives of 25 and 3 min, respectively, for the hydrolysis of DMMP (dimethyl methylphosphonate, similar simulant to DMNP) and CEES using UiO-66@LiOtBu; however, 100 mol % catalyst was used in their study. It is also important to note that the hydrolysis of these two simulants using UiO66@LiOtBu was conducted separately.31 Reusability of the Catalyst. After the simulants were completely converted to nontoxic products, we set out to test the reusability and stability of the MOF catalyst by adding more DMNP and CEES to the system. The mixture was purged with O2 for 20 min to ensure that enough singlet oxygen could be generated for a second cycle. Figure 3a shows the kinetics of DMNP hydrolysis and CEES oxidation in the second cycle. The same amount of each simulant was injected in each cycle, and we did not observe a significant change in half-lives between cycles. The half-life for DMNP hydrolysis in the second cycle remained the same as in the first cycle (8 min); however, the conversion of DMNP plateaued at 80% conversion after 50 min during the second cycle, which is likely a consequence of product inhibition.67 Both phenomena were observed in previous studies and were attributed to the formation of degradation products that inhibit further reaction.30,31 It was found that by simply washing the catalyst with H2O (and presumably removing the putative degradation products), catalytic activity of the MOF can be fully recovered.30 On the other hand, we observed an even shorter half-life for the oxidation of CEES in the second cycle (10 min) than the first cycle (12 min). The same trend was reported in our recent work45 on CEES VOL. XXX

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oxidation using singlet oxygen, in which pure methanol was used as solvent (first injection: t1/2 = 13 min; second injection: t1/2 = 7 min). As shown in Figure 3b, PXRD measurements of nfb-1 confirm that the MOF catalyst maintains its structural integrity after the dual reaction. Inductively coupled plasma-optical emission spectroscopy (ICP-OES, detection limit ca. 5 ppb) was used to probe the filtered solution after the dual reaction, and no Zr was found, which also indicates that the structure of nfb-1 remained intact during the catalysis (Table S1). Additionally, no porphyrin was detected in solution, confirming that nfb-1 remains intact. Control experiment reveals minimal conversion of both DMNP and CEES in the absence of the catalyst (Figure S6). We can therefore conclude that the dual reaction is indeed heterogeneous and catalyzed by solid nfb-1. Effect of Catalyst Particle Size. To study the effect of MOF particle size on the rates of the dual reaction, we performed the same experiment using PCN-222/MOF545 (fb-1) prepared by traditional methods reported previously.57 SEM images show the particle size of fb-1 to be 10
20 μm, whereas the particle size of nfb-1 is within the nanoregime (Figure S7a,b). We found that the rate of CEES oxidation is not significantly affected by MOF particle size (Figure S7c); however, the

METHODS Materials. ZrOCl2 3 8H2O (98%), benzoic acid (>99.5%), 2-chloroetheyl ethyl sulfide (98%), and methanol (anhydrous, 99.8%) were purchased from Sigma-Aldrich. Trifluoroacetic acid (99%) was purchased from VWR. N,N0 -Dimethylformamide (DMF, 99.8%) was obtained from Macron Fine Chemicals. meso-Tetra(4-carboxyphenyl)porphine (>97%) was obtained from Frontier Scientific. All chemicals were used without further purification. Dimethyl 4-nitrophenyl phosphate was synthesized according to reported procedures.24 Synthesis of Nanosized Free-Base PCN-222/MOF-545. Stock solutions A and B were prepared as follows. A: 200 mg of ZrOCl2 3 8H2O, 3.0 g of benzoic acid, and 20 mL of DMF were added into an 8-dram vial; B: 100 mg of TCPP and 20 mL of DMF were added into an 8-dram vial. Both solutions A and B were first sonicated for 30 min and then incubated at 100 °C in an oven for 1 h. The solutions were divided into 20 1.5 dram vials as follows:

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Figure 3. (a) Kinetic profiles for the destruction of a second injection of DMNP (25 μmol) and CEES (50 μmol) into the “MOF-catalyzed dual-reaction” setup with a 4 mol % catalyst loading (1 μmol Zr6/2 μmol porphyrin unit, 2.4 mg of nfb-1), after the first injection of simulants was completely converted to nontoxic products. (b) PXRD of nfb-1, simulated, pre- and postcatalysis.

hydrolysis of DMNP is much faster when the nanosized MOF catalyst is used (reaction half-life of 12 (fb-1) vs 8 min (nfb-1); see Figure S7d). This is attributed to increasing the relative external surface areas and/or a faster diffusion of DMNP when using nfb-1 as the catalyst compared to fb-1. While the mesoporous structure of PCN-222/MOF-545 facilitates substrate diffusion throughout the pores of its 3-D structure (diffusion inside the pores), making the MOF into a nanocrystalline form further enhances the diffusion of the substrate into the MOF particles (diffusion from outside to inside the pores). This ensures a maximum usage of the active sites inside and on the surface of the MOF crystals, which can lead to an accelerated catalytic rate. CONCLUSIONS In summary, a bioinspired (Zr6 cluster) and photoactive (porphyrin linker) MOF nfb-1 (i.e., nanosized, free-base PCN-222/MOF-545) was utilized as a dualfunction catalyst for degradation (detoxification) of simulants of two CWAs: an organo-phosphate-based nerve agent and mustard gas. Under visible (blue LED) irradiation, nfb-1 can simultaneously hydrolyze the nerve agent simulant, DMNP, and oxidize the mustard gas simulant, CEES, to nontoxic products in one system, with half-lives of 8 and 12 min, respectively. The structure of this dual-function MOF remains intact after catalysis, and no significant change in half-lives is observed when adding more of each simulant after the initial simulants are completely degraded. The versatility of the MOF's performance as a catalyst suggests that it, or suitably engineered successors, may eventually prove useful for air-filtration equipment and destruction (detoxification) of stockpiles or spills of chemical warfare agents. In addition, the superior catalytic activity of nfb-1 indicates the advantage of nanosized MOF materials in heterogeneous catalysis applications and the importance of exploring the synthetic strategies for MOF nanocrystals.

1 mL of A solution, 1 mL of B solution, and 0.05 mL of trifluoroacetic acid were added and mixed by swirling for 5 s. The vials were then immersed in a 120 °C oil bath for 1 h. A dark purple precipitate started to form in the vials. After cooling to room temperature, the suspension was transferred into a centrifuge tube and centrifuged for 5 min (7500 rpm) to remove the supernatant. The solid was then washed with fresh DMF (3  30 mL) before soaking in 40 mL of fresh DMF and 1.5 mL of 8 M HCl. It was then heated at 120 °C for 12 h to remove the benzoic acid. The sample was subsequently washed with fresh DMF (3  30 mL) and acetone (3  30 mL). After soaking in acetone overnight, the solid was dried in a vacuum oven at 100 °C for 2 h and activated at 120 °C for 12 h on a SmartVacPrep instrument (Micromeritics Instrument Corporation, Norcross, GA, USA). Nitrogen adsorption isotherm measurements were carried out on a Micromeritics Tristar II 3020 (Micromeritics Instrument Corporation) at 77 K. Around 100 mg of sample was used in the measurement. Pore-size distributions were obtained by DFT

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Conflict of Interest: The authors declare no competing financial interest. Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b05660. Further details on instrumentation and experiments (PDF) Acknowledgment. O.K.F. and J.T.H. gratefully acknowledge DTRA for financial support (grant HDTRA-1-10-0023). This work made use of the J. B. Cohen X-ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center of Northwestern University. This work also made use of the EPIC facility (NUANCE Center-Northwestern University), which has received support from the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); and the State of Illinois, through the IIN.

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calculations using a carbon slit-pore model with a N2 kernel. Particle size and phase purity of nfb-1 were characterized using SEM (SU8030, Hitachi) and PXRD (Smartlab, Rigaku). Dual-Reaction Experiments. For catalytic tests, 2.4 mg (1 μmol Zr6/2 μmol porphyrin unit) of nanosized free-base nfb-1 was dispersed in 0.5 mL of methanol and 0.5 mL of 0.4 M N-ethylmorpholine solution in a sealed glass microwave vial equipped with a stir bar. After purging with O2 for 20 min, 4 μL (25 μmol) of DMNP, 5.8 μL (50 μmol) of CEES, and 5 μL (40 μmol) of internal standard (1-bromo-3,5-difluorobenzene) were added to the microwave vial using microsyringes. The mixture was then exposed to blue LED irradiation. Approximately 2 μL aliquots were drawn from the mixture using a syringe every 5
10 min for up to 60 min. Aliquots were then diluted with MeOH and subjected to GC-FID. The concentrations of both simulants were monitored by GC-FID simultaneously. Half-lives were determined by fitting the initial concentrations of each reactant to a first-order kinetic model (Figure S5). In situ 31P NMR data confirm the hydrolysis product of DMNP (Figure S9). GC-FID was used to monitor the progress of both reactions (DMNP hydrolysis and CEES oxidation) because we cannot apply LED irradiation on the in situ 31P NMR experiment and 31 P NMR measures only the hydrolysis of DMNP. Control experiments were performed under identical conditions without adding catalyst (Figure S6). The heterogeneous nature of this catalysis was confirmed by ICP-OES analysis of the solution after the dual reaction (catalyzed by 2.4 mg of nfb-1). The ICP sample was prepared by diluting 0.20 mL of filtered solution from the reaction to 25 mL using Millipore water.

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65. The slightly basic N-ethylmorpholine solution facilitates the hydrolysis of DMNP, and methanol increases the solubility of the highly hydrophobic CEES simulant. 66. Porphyrins are known to be effective sensitizers for photochemical formation of singlet oxygen. Their effectiveness stems from efficient conversion of singlet photo-excited states to triplet states and from good energy matching of the porphyrin triplet excited state with the higher of the two possible singlet excited states of dioxygen. Thus, even in the absence of heavy atoms, the free-base form of tetraphenyl-porphyrin (TPP) undergoes excited-state singlet-to-triplet intersystem crossing with a yield of greater than 0.7, while ZnTPP achieves intersystem crossing with a yield approaching 0.9. See, for example: Pineiro, M.; Carvalho, A. L.; Pereira, M. M.; Gonsalves, A. M.; Arnaut, L. G.; Formosinho, S. J. Photoacoustic Measurements of Porphyrin Triplet-State Quantum Yields and Singlet-Oxygen Efficiencies. Chem. - Eur. J. 1998, 4, 2299–2307. 67. Product inhibition in the second cycle is caused by the hydrolysis degradation products dimethyl hydrogen phosphate and 4-nitrophenol.

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