Fast and Sustained Degradation of Chemical Warfare Agent Simulants

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Energy, Environmental, and Catalysis Applications

Fast and Sustained Degradation of Chemical Warfare Agent Simulants using Flexible Self-Supported Metal-Organic-Framework Filters Huixin Liang, Aonan Yao, Xiuling Jiao, Cheng Li, and Dairong Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02886 • Publication Date (Web): 28 May 2018 Downloaded from http://pubs.acs.org on May 28, 2018

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ACS Applied Materials & Interfaces

Fast and Sustained Degradation of Chemical Warfare Agent Simulants using Flexible SelfSupported Metal-Organic-Framework Filters Huixin Liang, Aonan Yao, Xiuling Jiao, Cheng Li*, and Dairong Chen* National Engineering Research Center for Colloidal Materials, School of Chemistry and Chemical Engineering, Shandong University, 250100 Jinan, China.

KEYWORDS: metal-organic frameworks, catalysis, chemical warfare agent simulants, nanofibers, self-supported filters

ABSTRACT: Self-detoxification filters against lethal chemical warfare agents (CWAs) are highly desirable for the protection of human beings and the environment. In this report, flexible self-supported filters of a series of Zr(IV)-based MOFs including UiO-66, UiO-67, and UiO-66NH2 were successfully prepared and exhibited fast and sustained degradation of CWA simulants. Half-lives as short as 2.4 min was obtained for the catalytic hydrolysis of DMNP (dimethyl 4nitrophenyl phosphate), and the percent conversion retained above 90% over a long-term exposure of 120 min, well exceeding that of the previous reported composite MOF filters and the corresponding MOF powders. The outstanding detoxification performance of the self-supported fibrous filter comes from the exceptionally high surface area, excellent pore accessibility and

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hierarchical structure from the nano- to macroscale. This work demonstrates for the first time MOF-only filters as efficient self-detoxification media, which will offer new opportunities for the design and fabrication of functional materials for toxic chemical protection.

1. INTRODUCTION Highly toxic chemical warfare agents (CWAs) such as sarin (GB), soman (GD), and VX, emitted accidentally or deliberately, have been grave threats to human beings and the environment all the time.1 Current filters for CWA removal mainly rely on active carbons and their impregnated forms,2-4 which suffer from issues of secondary emitters, low capacity for long-term exposure, and final disposal.5 Thus, novel self-detoxification filters are highly desirable to establish more efficient protections against lethal CWAs. Recently, metal-organic frameworks (MOFs), compounds assembled from the coordination of organic linkers and metal-containing secondary building units,6-8 have been studied and shown interesting properties in CWA removal due to their ultra-high surface area, tunable structures and periodically distributed abundant catalytic sites.9-12 However, the poor processability and handling of MOF powders hamper their wide use in this application. For instance, catalysis using MOF powders can be severely affected by particle aggregation and pipe clog. One solution is to immobilize MOF crystals in polymer matrices or on textiles using various methods including physical spraying,13 coelectrospinning,14-16 direct impregnation,17,

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in situ crystal growth,19-21 and self-

assembly,22 as demonstrated in several recent studies. However, the poor affinity between MOF crystals and support materials requires intricate and tedious modification of the pristine surface.19,

20, 22

In other cases, embedding of MOF crystals in the polymer

matrices leads to diminished pore accessibility.15,

16

More importantly, the MOF


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composites reported so far exhibit reduced catalytic activity for CWA degradation as compared to the powders, probably resulting from the shielding effects of the substrates. Besides, the high contents of inactive materials also add to the weight of filtration devices. In contrast, support-free/self-supported MOF filters can retain high levels of permanent porosity and catalytic active sites, break the constraint from the substrate, minimize the burden for end users, and are likely to outperform MOF powders for CWA decontamination via structuring. Nevertheless, there has been no report on self-supported MOF filters toward efficient detoxification of CWA or simulants to the best of our knowledge. Herein, we report on the preparation of a series of flexible self-supported Zr(IV)-based MOF filters (denoted as Zr-MOFilters) towards CWA simulant degradation. Zr(IV)-based MOFs were chosen because they have been shown as the most robust and promising MOF catalysts for the degradation of organophosphate-based nerve agents owing to their biomimetic phosphotriesterase activity endowed by the Lewis acidic Zr(IV) centers in combination with the basic hydroxide residues.23 Inspired by our previous research,24 here we use electrospun ZrO2 nanofiber mats as the precursor and take advantage of the pseudomorphic oxide-to-MOF transformation to realize fibrous structured Zr-MOFilters that exhibited much faster CWA simulant degradation relative to previously reported composite MOF fabrics. We further demonstrate the sustained catalytic activity of ZrMOFilters under long-term constant CWA simulant exposure, which is superior to MOF powders, revealing their promising application in CWA protection. 2. EXPERIMENTAL SECTION


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2.1. Chemicals. 1,4-benzene-dicarboxylic acid (H2BDC, 99%) was purchased from Sigma-Aldrich. Biphenyl-4,4'-dicarboxylic acid (H2BPC, 99%), 2-aminobenzene-1,4dicarboxylic acid (H2BDC-NH2, 98%), N,N-dimethylformamide (DMF, 99%), dimethyl sulfoxide (DMSO, 99.5%), ZrCl4 (99.9%), N-ethylmorpholine (99%) and dimethyl 4nitrophenyl phosphate (DMNP, 99%) were purchased from Shanghai Macklin Biochemical Co., Ltd. Polyvinylpyrrolidone (PVP, K-90, 99%), Zr(CH3COO)4 (Zr~15.016.0% in ethanol), ethanol (99.8%), and acetic acid (analytic grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. 2.2. Preparation of ZrO2 Precursors. 2.2.1. Nanofiber Mats. First, 0.4 g of PVP was dissolved in 5 mL of ethanol. Then 15 g of Zr(CH3COO)4 were added to this solution under vigorous magnetic stirring at room temperature. Electrospinning was conducted on a programmable floor-stand electrospinning unit. The above prepared solution was loaded into a 20 mL plastic syringe equipped with a metallic needle (inner diameter ~0.8 mm) and pumped at a flow rate of 2.0 mL h-1. The distance between the needle and the collector was adjusted to be 25 cm. The voltage of electric field was set at 20 kV. The environmental humidity was strictly controlled below 10% during the whole process. The as-obtained gel fiber mats were peeled off carefully from the collector, dried in an electric oven at 50 °C for 12 h, and then calcined in a muffle furnace at 500 °C for 2 h at a ramping rate of 1 °C min-1. 2.2.2. Powders. The synthesis was identical to that described in section 2.2.1 except that the eletrospinning step was replaced with a sequential freeze-drying and milling method.


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2.3. Preparation of Zr-MOFilters. 2.3.1. UiO-66. In a typical synthesis, 0.10 g of ZrO2 nanofiber mat was added to a 50 mL Teflon-lined stainless-steel autoclave that contained 10 mL of DMF, 0.474 g of H2BDC, and 1 mL of acetic acid. Then the autoclave was sealed and heated in the oven at 140 °C for 24 h. The product was washed thoroughly with hot ethanol and then water for five times before dried under vacuum at 80 °C. 2.3.2. UiO-67. In a typical synthesis, 0.10 g of ZrO2 nanofiber mat was added to a 20 mL Teflon-lined stainless-steel autoclave that contained 10 mL of DMSO, 0.69 g of H2BPC and 1 mL of acetic acid. Then the autoclave was sealed and heated in the oven at 150 °C for 24 h. The product was washed thoroughly with hot ethanol and then water for five times before dried under vacuum at 80 °C. 2.3.3. UiO-66-NH2. In a typical synthesis, 0.10 g of ZrO2 nanofiber mat was added to a 50 mL Teflon-lined stainless-steel autoclave that contained 10 mL of DMF, 1.03 g of H2BDC-NH2, and 1 mL of acetic acid. Then the autoclave was sealed and heated in the oven at 140 °C for 24 h. The product was washed thoroughly with hot ethanol and then water for five times before dried under vacuum at 80 °C. 2.4. Preparation of 50%-MOF/ZrO2 Filters. The synthetic procedures were identical to those as described in section 2.3 for the case of UiO-66, UiO-67, and UiO-66-NH2, respectively, except for a shortened reaction time of 12 h. 2.5. Preparation of MOF Powders. The synthesis was conducted using the same conditions as described in section 2.3 for the case of UiO-66, UiO-67, and UiO-66-NH2, respectively, except for changing the Zr source to 0.177 g of ZrCl4 (equivalent Zr content to 0.1 g ZrO2), or 0.1 g of ZrO2 powder prepared according to section 2.2.2.


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2.6. Characterizations. Scanning electron microscopic (SEM) images were acquired from a Hitachi-SU8010 field emission scanning electron microscope. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2200PC diffractometer with a graphite-monochromatized CuKα radiation source (λ = 0.15418 nm) with the applied tube voltage and electric current being 40 kV and 20 mA. Thermogravimetric analysis (TGA) was conducted under air atmosphere with a Mettler Toledo TGA/SDTA 851E analyzer at a heating rate of 10 °C min-1 from room temperature to 700 °C. N2 physisorption isotherms were obtained from a Micromeritics ASAP2020HD88 adsorption system at 77 K. UV-visible absorbance spectroscopy was measured on a Hitachi (U-4100) spectrophotometer. 2.7. Degradation of DMNP. 2.7.1. Batch experiment. The evaluation of catalysts for degrading DMNP was implemented in a same way to the method described previously.19 Specifically, a certain quantity of the MOF catalyst (see Table 1) was mixed with 1 mL of N-ethylmorpholine aqueous solution (0.45 M) in a 2 mL Eppendorf tube under vigorous magnetic stirring (1100 rpm) for 30 min. DMNP (4 µL) was then added into the suspension, and this reaction mixture was continuously stirred at room temperature. To monitor the degradation progress, a 20 µL aliquot was drawn from the mixture at each time interval and diluted in 10 mL of N-ethylmorpholine aqueous solution (0.15 M) for UV-visible spectroscopic measurement. 2.7.2. Filter test in solution. To prepare a filter reactor, the Zr-MOFilter was cut into a round shape and embedded in a 10 mm syringe filter holder. The MOF powder sample was loaded on a commercial polymeric membrane fixed in the filter holder by filtration. 4 µL of DMNP was added to 1 mL N-ethylmorpholine aqueous solution (0.45 M). Then


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this solution was passed through the prepared filter reactor at a flow rate of 0.03 mL min-1 under the control of an infusion pump. The filtration was performed successively for 120 min by a continuous support of freshly prepared DMNP solution (4 mL DMNP solution in total). A 20 µL aliquot was drawn from the filtrates at each time interval and diluted in 10 mL of N-ethylmorpholine aqueous solution (0.15 M) for UV-visible spectroscopic measurement. The catalyst was regenerated by washing with 30 ml of N-ethylmorpholine aqueous solution (0.45 M) and soaking in ethanol for 12 h and then dried under vacuum at 80 °C. 2.7.3. Filter test in aerosol. 4 µL of DMNP was added into 1 mL of N-ethylmorpholine aqueous solution (0.45 M) to form a solution with a DMNP concentration of 0.023 mol L-1. Then, the DMNP solution was atomized using a commercial atomizer and passed through Zr-MOFilter that was tightly sandwiched between a glass tube and a collecting bottle under a pressure drop of 8.0 Kpa. The flow rate was adjusted at 0.20 mL min-1 using a flowmeter. After the DMNP solution was completely evacuated from the atomizer, the collecting bottle and the Zr-MOFilter were thoroughly washed using 50 mL of N-ethylmorpholine aqueous solution (0.15 M) to collect the product. 2. RESULTS AND DISCUSSION Scheme 1 illustrate the general procedure to synthesize our Zr-MOFilters. Free-standing ZrO2 nanofiber mats obtained via sol-gel electrospinning and calcination were used directly as the precursor and template to react with the organic linkers and modulators under appropriate solvothermal conditions. We expected that the products can inherit the shape that is identical to that of the parent fibers via the so-called process of ‘coordination replication’,25 thus achieving flexible self-supported MOF filters for highly efficient detoxification of nerve agent simulants.


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The use of three different organic linkers (H2BDC, H2BPC, and H2BDC-NH2) resulted in the construction of Zr-MOFilters presenting three porous properties based on UiO-66, UiO-67 and UiO-66-NH2, 26, 27 which have been demonstrated excellent stability and good catalytic activities for CWA and simulant detoxification.23,

28-32

Acetic acid was used as the modulator for the

solvothermal synthesis and found to play an indispensible role in the conversion of ZrO2 nanofiber. Reaction parameters, including the solvent, concentration, temperature, and reaction time, were also optimized to achieve the desirable pseudomorphic transformation. For instance, DMF were suitable solvents for the cases of UiO-66, but not applied for the case of UiO-67, in which DMSO was selected as the appropriate solvent. The prepared ZrO2 nanofiber mat and Zr-MOFilters exhibited similarly good structural integrity and flexibility and were several inches in size and 0.03–0.06 mm in thickness (Insets of Figure 1, Figure S1). SEM images (Figure 1a, e) show that the precursor ZrO2 mat was composed of interlacing nanofibers with very smooth surfaces and a diameter ranging from 300 to 500 nm. After the reactions, Zr-MOFilters obtained in all three cases inherited well the fiber texture of the oxide parent architecture (Figure 1b-d). In the case of UiO-66 and UiO-67, needleand belt-like nanostructures were grown conformally on the fibers, respectively (Figure 1f, g), which had led to a significant increase of the fiber volume and diminished void space in the mats. For the case of UiO-66-NH2, the whole fiber surface was covered with intergrown nanocrystals of 50-100 nm and the fiber diameter was doubled (Figure 1h). Considering there was no other metal sources added in the synthesis, such volume expansions suggest that the dense oxide phase in the fiber was replaced by a less-dense MOF phase. Meanwhile, the preservation of the fiber structure strongly point to a ‘dissolution-reprecipitation’ mechanism, in which the oxide phase dissolves in the liquid at the solid/liquid interface and immediately


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crystallizes as a stable MOF phase at the same place as the dissolved precursor (Figure S2). The reaction mechanism was confirmed by SEM images (Figure S3) of the mid-term product (denoted as 50%-MOF/ZrO2 filter). The reaction front at the interface between ZrO2 and MOF crystals can be clearly observed in the case of UiO-66 and UiO-67, together with decreased fiber diameters and newly formed nanoflakelets closely anchored on the fibers (Figure S3a, b), providing evidence of ZrO2 consumption to form the MOF crystals. In the case of UiO-66-NH2, the smooth surfaces of the ZrO2 fiber became rougher owing to the coating of small MOF crystals (Figure S3c). Moreover, no loose MOF crystals were found in the products, which also support the growth mechanism. As contrast to most supported MOF fibers, where the particles tend to aggregate and fall off from the substrate, our Zr-MOFilters formed via the ‘dissolutionreprecipitation’ route effectively prevent the aggregation and shedding of particles, which is very important for heterogeneous catalysts. The Zr-MOFilters have reasonable mechanical properties with tensile strength measured to be 0.37, 0.42, and 0.45 MPa for the respective case of UiO-66, UiO-67 and UiO-66-NH2 (Figure S4), providing a good prospect for uses in filtration. The crystalline phases of the products were studied using X-ray diffraction (XRD) (Figure 2). Diffraction peaks in the patterns for Zr-MOFilters agree well with those of the simulated data and conventionally synthesized pure MOF powders (referred to as ‘salt source’, SEM images provided in Figure S5), confirming the formation of targeted phases of UiO-66, UiO-67 and UiO-66-NH2, respectively. Notably, almost complete phase transformations from oxide to MOFs were achieved in all three cases, which is evidenced by the disappearance of diffraction peaks of ZrO2 for the final products. In comparison, MOF and ZrO2 phases coexisted in 50%-MOF/ZrO2 filters as a result of half oxide-to-MOF conversion. For MOF powders prepared using irregular ZrO2 powders as the precursor (referred to as ‘oxide source’), diffraction peaks for MOF phases


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dominated but those for ZrO2 were still obvious after prolonged reaction time of 60 h. This indicates incomplete phase transformation in these cases, which is probably due to significant particle aggregations that may prevent the oxide from further conversion, as shown in Figure S6. These results demonstrate that self-supported Zr-MOFilters can be successfully fabricated by pseudomorphic transformation of ZrO2 nanofiber mats using designed reaction conditions. The pore-texture properties of the filters were demonstrated by N2 physisorption measurement. As shown in Figure 3, Type I isotherms with steep N2 uptake in the low-pressure regions (P/P0 < 0.1) were obtained for all of the samples, indicating the presence of microporous structures due to the MOF contents. In addition, Type H4 hysteresis loops were observed for Zr-MOFilters in the high-pressure range (P/P0 > 0.9), which could be ascribed to slit-like pores arising from the nanostructures on the fibers. Remarkably, Brunauer–Emmett–Teller (BET) surface area as high as 1512, 1894 and 1319 m2 g-1 was obtained for Zr-MOFilter in the case of UiO-66, UiO-67 and UiO-66-NH2, respectively, which was considerably higher than those for the corresponding MOF powder (Table 1). While Zr-MOFilters and MOF powders (salt source) have similar micropore volumes, Zr-MOFilters possess much larger total pore volumes, as given in Table S1. The pore volume distributions (Figure S7) show that mesopores in the range of 4.4 to 11.3 nm exist in Zr-MOFilters, while the MOF powders (salt source) only have micropores. Therefore, the extra large surface area of Zr-MOFilters can be mainly attributed to the hierarchically porous structures. It should be noted that in order to rule out to the most extent the effect of synthesis conditions on modifying the properties of MOFs, namely by presence of defect,34 Zr-MOFilters and MOF powders (both salt and oxide source) in this work were synthesized using the same conditions (i.e. organic linker and acetic acid concentration, temperature, solvent, reaction time, and equivalent Zr). Also importantly, the high porosities of self-supported Zr-MOFilters far


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exceed those of the 50%-MOF/ZrO2 filters (Table 1), as well as previous reported supported MOF membranes/mats (Table S2) that have long suffered from low loading of MOFs, reduced pore accessibility, and consequent diminished activities. Thermogravimetric analysis (TGA) was further performed to investigate the phase transformation progress and determine the MOF mass fraction in the products. As shown in Figure 4, all samples exhibited two-step substantial weight losses. The first one below 300 °C was associated with the solvent escape from the pores and dehydration of Zr6O4(OH)4 nodes, and the second one above 500 (for UiO-66 and UiO-67) or 400 °C (for UiO-66-NH2) was due to the elimination of organic linkers from the framework, during which the inorganic part was transformed into ZrO2. The content of MOF in the products can be hence quantified based on their thermal behaviors normalized to that of similarly prepared MOF powders. MOF mass fraction values of 95%, 96% and 90% were obtained for the final products in the case of UiO-66, UiO-67 and UiO-66-NH2, respectively (Table 1), demonstrating the high effecacy and generalizability of this synthetic protocol to achieve nearly complete oxide-to-MOF conversion. 50%-MOF/ZrO2 filters in all three cases yielded MOF mass fractions of around 50% (Table 1), showing the progressive and controllable transformation of ZrO2 into Zr-based MOFs with varying MOF contents that are in good coordination with the reaction time. Meanwhile, MOF powders (oxide source) contained 82 to 90% MOF phases (Table 1), which is in line with the XRD analyses. With a series of Zr-MOFilter being successfully prepared, we continue to evaluate their catalytic properties toward the degradation of a nerve agent simulant 4-nitrophenyl phosphate (DMNP). Given the high toxicity of CWAs and the risk of handling them, DMNP is used as a simulant for organophosphate-based nerve agents like VX, GB (Sarin), and GD (Soman) in


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normal laboratories, and has proven to be one of essential tools for accessing the destruction of phosphate ester bonds.23, 33 The catalytic hydrolysis of DMNP was carried out using Zr-MOFilter in an aqueous buffer solution of N-ethylmorpholine at pH 10 (Figure 5a). The reaction progress was evaluated by monitoring the absorbance of p-nitrophenoxide (degraded product from DMNP) at 407 nm based on Lambert-Beer Law according to reported procedures.19 The percent conversion of DMNP was obtained from the ratio of the p-nitrophenoxide concentration to the initial DMNP concentration in the reaction mixture, and its natural log was plotted as a function of time to obtain the pseudo-first order rate constant k. The half-lives (t1/2, 50% conversion) were calculated based on pseudo-first-order kinetics. Turn over frequency (TOF) was calculated per Zr6 cluster at t1/2, and the results were given in Table 1. Over the course of 60 min, 90% conversion was obtained for the case of UiO-66 (Figure S8; Fig. 5b, black rounds), yielding an observed half-life of 21.3 min, which is longer than the calculated value of 13.3 min. For the case of UiO-67 and UiO-66-NH2, much faster degradation occurred with 100% conversion obtained after approximately 45 and 20 min, respectively (Figure S9, S10; Fig. 5c, d, black rounds), yielding observed half-lives of 6.6 and 1.9 min, which are slightly shorter than the calculated ones (6.7 and 2.4 min). This activity trend is consistent with previous reports in that compared to UiO-66, the amine moiety of UiO-66-NH2 serves as a Brønsted base to further improve the catalytic activity and the large pore size of UiO-67 favors faster diffusion and more access of reagent to the active sites. 28, 29, 32Importantly, these results represents the fastest known decomposition of a phosphate ester nerve agent simulant by MOF fabrics reported so far (see Table S2). Notably, while the UiO-66-loaded fabric in the literature showed very sluggish hydrolysis of DMNP with a calculated half-life of 113 min, our Zr-MOFilter of UiO-66 significantly shortened the half-life by almost an order of magnitude. It should be noted that the


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performance is compared between MOF fabrics containing the same mass of active materials, which means that the improved performance of Zr-MOFilter does not come from the increase of MOF contents but from the intrinsic advantages of the self-supported fibrous structure that ensures high pore accessibility, exceptionally large surface area, and fast mass transport. More encouragingly, Zr-MOFilters also show faster degradation kinetics than similarly prepared MOF powders (either salt or oxide source) (Figure S11-S16; Fig. 5b-d, red triangles and green hexagon; Table 1), and those reported in the literature (Table S2). Such catalytic performances are difficult to achieve using supported MOF materials because of the unavoidable shielding effect from the inactive substrate on the reactive sites. These results demonstrate the significance of structuring MOF into certain forms, which is not only for the convenience of practical uses but also for the improvement of intrinsic functionality. As further controls, the ZrO2 nanofiber mat and 50%MOF/ZrO2 filters were characterized under identical reaction conditions. The ZrO2 nanofiber mat showed a very low catalytic activity toward DMNP degradation, with only 16% conversion achieved over the course of 60 min (Figure S17, Table 1). This means that the contribution of residual ZrO2 in Zr-MOFilters for the catalytic activity is negligible. 50%-MOF/ZrO2 filters showed improved activity compared to that of the pristine ZrO2 (Figure S18-S20; Fig. 5b-d, blue diamonds), but much lower performance than those of Zr-MOFilters and MOF powders. With the same mass of active component used in the degradation, the reduced performance might be ascribed to their small surface areas and low amounts of active sites developed from the incomplete crystallization of MOFs. Overall, our results demonstrate the advantage of selfsupported fibrous MOF filters for the catalytic hydrolysis of phosphoester bonds. Besides the fast detoxification rate, it is also critical for the catalytic filter or fabric to sustain continuous and prolonged environmental exposure of CWA so that effective protections are


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achieved. As far as we know, there is no report on the long-term filtration test of a MOF fabric toward the degradation of CWAs or simulants. We herein employed Zr-MOFilter made from UiO-66-NH2 to construct a plug-flow reactor (Figure 6a), and evaluated the catalytic degradation of DMNP in a continuous manner. A DMNP solution of the same concentration to the batch experiment was passed through the Zr-MOFilter embedded in a commercial filter holder at a constant flow rate. The real-time percentage conversion of the reaction was obtained from the ratio of the product concentration in the filtrate to the initial DMNP concentration. As shown in Fig. 6b (black cubes), 100% conversion was sustained for 25 min, which was followed by a very small drop within 60 min to reach 98% and a more prominent decrease over the rest time course. Even so, the percentage conversion still maintained above 90% after 120 min, demonstrating the high activity and stability of Zr-MOFilter for CWA simulant removal under conditions of continuous exposure. The decline in catalytic activities is due to decreased microporosity, as evidenced by N2 sorption-desorption isotherms (Figure S21). As a comparison, UiO-66-NH2 power of equivalent weight to Zr-MOFilter was also tested with identical procedures. As shown in Fig. 6b (red rounds), 100% conversion was only sustained for 10 min, after which a very fast decrease occurred with 77% conversion obtained at 60 min, and further decrease in the rest of time eventually led to a diminished percentage conversion of 62% at 120 min. These results signified the much more persistent catalytic activity of Zr-MOFilter than the powder sample, which could be attributed the intrinsic self-supported fibrous structure that effectively prevented the aggregation of catalyst. When compared with supported MOF fabrics, Zr-MOFilter with its unique self-supported nature offer much higher catalytic activity per area and per weight, therefore significantly improving the capability of protection against toxic chemicals and simultaneously minimizing the burden for end users. After 120 min of continuous reaction, the


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filters were regenerated and reused for a successive run, and there was no obvious change in percent conversion after regeneration (Figure S22), showing good recyclability of MOF catalysts. XRD patterns of regenerated Zr-MOFilter show no substantial change in peak intensity or position compared to the original one (Figure S23), confirming the stability of the catalysts. To further demonstrate the utility of Zr-MOFilter as a protective media, they were employed in a homemade device mimicking the deployment of DMNP aerosols (Figure S24). The aerosol was made by atomizing a DMNP buffer solution considering that water is indispensible for the hydrolysis of DMNP and the buffer acts to remove acidic byproducts from the reaction and to deprotonate water to facilitate the reaction.17, 35, 36 After the aerosol was completely evacuated, the total percent conversion was obtained from the ratio of product amount collected in the filtrates and the initial amount of DMNP. Consistent with the catalytic activity of the MOFs, the total percent conversion of DMNP using Zr-MOFilter of UiO-66, UiO-67, and UiO-66-NH2 were 41.2%, 51.5%, and 63.9%, respectively. It should be mentioned that the total percent conversion may be underestimated because of incomplete collection of filtrates in the current device due to vacuum suction. Taken together, our results demonstrated that Zr-MOFilter could serve as a promising protective media for CWA or simulant detoxification. 4. CONCLUSIONS In conclusion, we have developed a series of self-supported Zr-based MOF filters capable of decomposing nerve agent simulant DMNP. Complete phase transformation of oxide nanofiber mat precursor was achieved via a ‘dissolution-reprecipitation’ process under optimized solvothermal conditions, yielding fibrous filters of UiO-66, UiO-67 and UiO-66-NH2 with exceptionally high porosities. Fast and sustained degradation of DMNP was demonstrated owing to the high pore accessibility, large surface area and hierarchical structure of the fibrous MOF


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filters, signifying their advantages over powders and supported filters of identical MOF weight in maximized protection per area and minimized burden for end users. The synthetic strategy and MOF-only self-detoxifying filters represented here will also open new opportunities for the development of advanced devices for air purification of toxic chemicals and beyond.


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Scheme 1. Synthetic procedure for flexible self-supported MOF filters.

Figure 1. Low- and high-magnification SEM images of the ZrO2 nanofiber mat (a, e) and ZrMOFilters: (b, f) UiO-66, (c, g) UiO-67, and (d, h) UiO-66-NH2. Insets in (a-d) are corresponding optical photographs.


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Figure 2. XRD patterns of Zr-MOFilter, 50%-MOF/ZrO2 filter and MOF powder: (a) UiO-66, (b) UiO-67, and (c) UiO-66-NH2, compared to those of the ZrO2 nanofiber mat and simulated data.


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Figure 3. N2 sorption-desorption isotherms of Zr-MOFilter, 50%-MOF/ZrO2 filter and MOF powder: (a) UiO-66, (b) UiO-67, and (c) UiO-66-NH2.


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Figure 4. TGA curves of Zr-MOFilter, 50%-MOF/ZrO2 filter and MOF powder: (a) UiO-66, (b) UiO-67, and (c) UiO-66-NH2.


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Table 1. Material properties and catalytic performances toward the degradation of a nerve agent simulant DMNP. Total MOF Sample

BET surfaces

amount

2

-1

t1/2

wt %

area (m g )

(min)

k (min-1)

TOF (s-1)

(mg) Zr-MOFilter UiO-66

2.6

95

1512

13.3

0.0521

0.0058

UiO-67

2.7

96

1894

6.7

0.104

0.027

UiO-66-NH2

2.8

90

1319

2.4

0.289

0.068

UiO-66

2.5

100

1134

22.7

0.0305

0.0046

UiO-67

2.5

100

1574

9.7

0.0715

0.012

UiO-66-NH2

2.5

100

1052

6.5

0.107

0.021

UiO-66

2.7

90

998

30.7

0.0227

0.0043

UiO-67

2.8

89

1294

13.5

0.0518

0.0098

UiO-66-NH2

3.1

82

897

10.0

0.0692

0.012

UiO-66

5

49

305

117.8

0.00590

0.0010

UiO-67

5

51

376

53.2

0.0130

0.0027

UiO-66-NH2

5

51

299

30.6

0.0226

0.0048

ZrO2 nanofiber mat

2.5

0

45

5417

1.00×10-4

2.0×10-5

MOF powder (salt source)a

MOF powder (oxide source)b

50%-MOF/ZrO2 filter


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a

Page 22 of 29

The MOF powders were prepared using equivalent ZrCl4 as Zr source. bThe MOF powders were prepared

using irregular ZrO2 particles as Zr source.

Figure 5. Reaction scheme (a) and percentage conversion of the catalytic hydrolysis of DMNP versus reaction time using Zr-MOFilter, MOF powder, and 50%-MOF/ZrO2 filter as the catalyst: (b) UiO-66, (c) UiO-67, and (d) UiO-66-NH2.


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Figure 6. Hydrolysis of DMNP in a plug-flow reactor using Zr-MOFilter and MOF powder: (a) schematic illustration of the plug-flow reactor, (b) percentage conversion of DMNP to pnitrophnoxide versus time. The flow rate of solution was 0.03 mL min-1. ASSOCIATED CONTENT Supporting Information. Optical photographs, formation mechanism, SEM images, tensile stress-strain curves, pore volume distributions, UV-vis spectra and kinetic analyses, XRD patterns, N2 sorption isotherms, percent conversion of regenerated filters, scheme depicting the filter testing device, and comparisons of properties between materials. AUTHOR INFORMATION Corresponding Author *C. Li and *D. Chen Email: [email protected], [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENTS The authors acknowledge support from the National Natural Science Foundation of China (NSFC, Grant 21303095, 21771118), and the Taishan Scholars Climbing Program of Shandong Province (Grant tspd20150201). REFERENCES 1. Szinicz, L. History of Chemical and Biological Warfare Agents. Toxicology 2005, 214, 167181. 2. Yang, Y. C.; Baker, J. A.; Ward, J. R. Decontamination of Chemical Warfare Agents. Chem. Rev. 1992, 92, 1729-1743. 3. Council, N. R. Strategies to Protect the Health of Deployed U.S. Forces: Force Protection and Decontamination, The National Academies Press, Washington, DC, 1999. 4. Smart, J. K. History of the Army Protective Mask; U.S. Army Soldier and Biological and Chemical Command: Aberdeen Proving Ground, MD, 1999. 5. Smith, B. M. Catalytic Methods for the Destruction of Chemical Warfare Agents under Ambient Conditions. Chem. Soc. Rev. 2008, 37, 470-478. 6. Kitagawa, S.; Kitaura, R.; Noro, S. Functional Porous Coordination Polymers. Angew. Chem., Int. Ed. 2004, 43, 2334-2375. 7. Zhou, H. C.; Long, J. R.; Yaghi, O. M. Introduction to Metal-Organic Frameworks. Chem. Rev. 2012, 112, 673-674. 8. Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. The Chemistry and Applications of Metal-Organic Frameworks. Science 2013, 1230444. 9. DeCoste, J. B.; Peterson, G. W. Metal-Organic Frameworks for Air Purification of Toxic Chemicals. Chem. Rev. 2014, 114, 5695-5727.


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TABLE OF CONTENT


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Fast and Sustained Degradation of Chemical Warfare Agent Simulants

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