Reactions of VX, GD, and HD with Zr(OH)4: Near Instantaneous

Article pubs.acs.org/JPCC

Reactions of VX, GD, and HD with Zr(OH)4: Near Instantaneous Decontamination of VX Teresa J. Bandosz,† Matt Laskoski,‡ John Mahle,§ Gregory Mogilevsky,§ Gregory W. Peterson,*,§ Joseph A. Rossin,∥ and George W. Wagner§ †

Department of Chemistry, The City College of New York, 160 Convent Avenue, New York, New York 10038, United States Code 6127, Chemistry Division, Naval Research Laboratory, Washington, DC 20375, United States § Edgewood Chemical Biological Center, 5183 Blackhawk Road, APG, Maryland 21010, United States ∥ Guild Associates, Inc., 5750 Shier Rings Road, Dublin, Ohio 43016, United States ‡

ABSTRACT: Zirconium hydroxide was evaluated for the ability to detoxify chemical warfare agents GD, HD, and VX. Observed half-lives were 8.7 min, 2.3 h, and 1 min, respectively. Owing to its extremely fast reaction rate, the mechanism for VX was further characterized. Zirconium hydroxide samples were calcined at temperatures ranging from 150 to 900 °C to investigate the effect of surface speciation on VX hydrolysis rates. NMR, TGA/DSC, TEM, and potentiometric tritration reveal the importance of the acidic, bridging OH groups of Zr(OH)4 which are proposed to protonate and catalytically hydrolyze VX in a manner similar to autocatalysis by EMPA in solution.

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INTRODUCTION The efficient detoxification of highly toxic chemical warfare agents such as VX, soman (GD), and distilled mustard (HD) has been a major concern for over 50 years. In particular, VX (O-ethyl S-(2-diisopropylamino)ethyl methylphosphonothioate], a nerve agent, is one of the most toxic chemicals known to man. Developed as a highly persistent terrain-denial weapon, VX persists in the environment and is difficult to efficiently neutralize in short periods of time. VX is primarily a percutaneous threat; however, inhalation is also lethal. When considering methods for protecting against VX and other similar compounds, reactive removal is preferential to physical adsorption, as the latter will primarily retain, and not quickly detoxify, the compound. For example, VX has been shown to persist for weeks on activated carbon.1 Therefore, materials with fast kinetics that favor nontoxic byproducts are sought. Metal oxides (MO) generally show reactivity toward VX, GD, and HD, resulting in their hydrolysis.2−4 For HD, elimination also occurs on basic metal oxides. Reaction paths for these agents with MOs are shown in Scheme 1. VX is by far the slowest-reacting agent. Of particular note are a variety of nanometal oxides, including magnesium oxide (MgO),2 calcium oxide (CaO),3 and alumina (Al2O3).4 These materials, due to enhanced surface areas from their nanodimensions, as well as the increased number of defect sites, show enhanced reactivity (hydrolysis) toward many chemical warfare agents. However, these materials also have shortcomings; specifically, some are known to be unstable in air and moisture.5 In addition, metal oxides such as MgO and CaO are so basic that they irreversibly sorb the autocatalytic byproduct EMPA6 (Scheme 1) onto the © 2012 American Chemical Society

surfaceeffectively self-poisoning the EMPA-catalyzed hydrolysis reaction5resulting in quite slow VX-reaction rates. Previous work by Wagner and co-workers5,7 demonstrated that titania-based materials overcame these shortcomings, as the autocatalytic EMPA product6 did not bind to the surface, resulting in half-lives of less than 30 minrepresenting an order-of-magnitude improvement compared to hydrolysis rates recorded using typical metal oxides.2−4 The EMPA-catalyzed “water reaction” for VX is shown in Scheme 2.6 Owing to their high surface areas and abundance of surface hydroxyls, nanotubular titania (NTT) and nanocrystalline titania (nTiO2) were found to be particularly effective, with the latter exhibiting extremely short half-lives for adsorbed VX of less than 2 min.7 In the past two years, zirconium hydroxide, or polymorphic zirconia, has been investigated as a means of removing various toxic chemicals.8−11 Peterson and co-workers found it to be especially reactive toward acidic gases due to its basic terminal hydroxyl sites, yet they also found it possessed a combination of terminal and bridging groups, lending itself to both acidic and basic properties. Zirconium hydroxide has demonstrated surface areas exceeding 400 m2/gquite high for metal oxides. It is amorphous to X-ray diffraction but exhibits localized crystallinity, leading to a highly porous material. Because zirconium falls directly below titanium on the periodic table, it was thought that similar structures and chemistries may be present on the surface of the material. We Received: March 26, 2012 Revised: April 27, 2012 Published: May 8, 2012 11606

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Scheme 1. Reactions for VX, GD, and HD on Metal Oxides

temperatures of 150, 300, 500, and 900 °C as described below. VX, GD, and HD (13C-labeled)13 were obtained locally at ECBC. NMR Analysis. Caution! Experiments should be run by trained personnel using appropriate safety procedures. 1H, 13C, and 31P MAS NMR spectra were obtained using Varian INOVA 600 and Unityplus 300 NMR spectrometers, equipped with Doty Scientific XC5 5-mm and Supersonic 7-mm MAS NMR probes, respectively. Reactions of VX, GD, and HD with Zr(OH)4 were monitored using the Varian Unityplus 300 NMR spectrometer as previously described.13 A typical 5 μL drop size was used for GD and HD reactions; a larger (more slowly reacting) 20 μL drop size was employed for the extremely fast VX reaction so that an accurate half-life could be obtained. Spectra were referenced to external TMS (1H and 13C, 0 ppm) and 85% H3PO4 (31P, 0 ppm). Half-lives were determined from the slopes (m) of plots of ln(concentration) vs time: t1/2 = ln(2)/ m. Further 1H NMR spectra were collected from five zirconium hydroxide samples to explore their intrinsic hydroxylation as it relates to surface chemistry. The samples were treated at room temperature (as-received sample) and 150, 300, 500, and 900 °C. The initial dehydration temperature of 150 °C was chosen to not induce structural or chemical changes but drive off water from the samples, leaving behind intrinsic hydroxyl groups. Temperatures above 150 °C were chosen corresponding to phase transitions in zirconium hydroxide. A 150 mg portion of Zr(OH)4 was packed into a 5 mm Doty Scientific Si3N4 NMR rotor. The thermal treatments of all Zr(OH)4 samples at different temperatures took place by calcining the uncapped NMR rotor filled with the material for 4 h in a tube furnace. The tube furnace was purged with dry N2 to drive off water from zirconium hydroxide. Immediately after outgassing, the cooled rotor was capped with Kel-F end-caps which do not

Scheme 2. VX Auto-Catalysis by EMPA

report here, by zirconium hydroxide, the fastest known solidbased reactive decontamination of VX. We examine the mechanism of removal and correlate the physical characteristics of the material to its efficiency. To accomplish this, we investigated by changing surface properties most notably the ratio and total amount of hydroxyl groups, and by calcining and eventually crystallizing the amorphous material. Nitrogen porosimetry and potentiometric titration were used to quantify changes in surface structure and acid/base behavior, respectively, and MAS NMR was used to determine the resulting reaction rates of VX on zirconium hydroxide. For a complete view on persistent chemical agent removal, GD and HD removal characteristics by zirconium hydroxide were also investigated.

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EXPERIMENTAL SECTION Materials. Zirconium hydroxide powder was purchased from Magnesium Electron, Inc. (MEL) and had a mean particle diameter of approximately 1 μm.12 Materials were made into granular form using a Carver press, followed by crushing and sieving to a 20 × 40 mesh. Zirconium hydroxide was calcined at 11607

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allow any humidity to enter the sample. The sample was weighed before and after the calcination step. Magic angle spinning (MAS) 1H NMR was performed on each calcined sample in a Doty Scientific XC5 VT-MAS probe using a narrow bore 600 MHz Varian INOVA NMR spectrometer. The spinning was done at 10 kHz, and all chemical shifts were calculated relative to TMS. The proton signal was collected with a 3 μs pulse width over 128 scans to maximize the signalto-noise ratio. Deconvolution of individual NMR spectra was performed using the software package NMR Nuts from Acorn. Thermogravimetric Analysis and Differential Scanning Calorimetry. DSC and TGA measurements were obtained using a TA Instruments SDT Q600 with aluminum pans. The scan rate was 10 °C/min. The carrier gas was nitrogen at 20 cc/min after an initial 10 min hold at ambient temperature. The sample mass was 10.4 mg. Potentiometric Titration. Potentiometric titrations were performed with a DMS Titrino 716 automatic titrator (Metrohm). The instrument was set at the mode where the equilibrium pH is collected. Subsamples of the initial and exhausted materials (∼100 mg) were added to NaNO3 (0.01 M, 50 mL) and placed in a container maintained at 25 °C overnight for equilibrium. During the titration, to eliminate the influence of atmospheric CO2, the suspension was continuously saturated with N2. The suspension was stirred throughout the measurements. Volumetric standard NaOH (0.1 M) was used as the titrant. The experiments were done in the pH range 3− 10. Each sample was titrated with base after acidifying the sample suspension. The experimental data were transformed into a proton binding isotherm representing the total amount of protonated sites (Q), which is related to the pKa distribution by the following integral equation:

Figure 1. Selected 31P MAS NMR spectra obtained for 5 μL GD added to as-received Zr(OH)4 at the indicated times. Spinning sidebands are marked by asterisks (see text).

∞

Q (pH) =

∫−∞ q(pH, pKa)f (pKa) dpKa

(1)

The solution of this equation was obtained using the SAIEUS numerical procedure,14 which applies regularization combined with non-negativity constraints. On the basis of the spectrum of acidity constants and the history of the samples, the detailed surface chemistry was evaluated.15,16

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RESULTS AND DISCUSSION GD Reaction. 31P MAS NMR spectra obtained for 5 μL of GD added to 0.265 g of as-received Zr(OH)4 (pre-equilibrated at 50% RH for 16 h) are shown in Figure 1. Over time, the twin resonances for GD, owing to P−F coupling, are replaced by a broad, single peak for pinacolyl methylphosphonate (PMPA) and its attendant spinning sidebandswhich emerges under the upfield GD peak near 25 ppm. The half-life for the reaction is 8.7 min. Although GD undergoes both base- and acidcatalyzed hydrolysis,17 it is the basic OH groups of Zr(OH)4 which are probably more efficacious than the acidic OH groups (see below) as half-lives of 28 min and a few hours are observed on (basic) MgO2 and (less-basic) titania materials,7 respectively. HD Reaction. 13C NMR spectra obtained for 5 μL of HD* added to 0.2636 g of as-received Zr(OH)4 (50% RH) are shown in Figure 2. The two HD peaks were steadily replaced by several product peaks, with an observed half-life of 2.3 h. Obviously, oily, water-insoluble HD would not be expected to quickly spread and react like the other water-soluble agents on

Figure 2. Selected 13C NMR spectra obtained, nonspinning, for 5 μL of HD* added to as-received Zr(OH)4 at the indicated times.

humidified, hydrophilic Zr(OH)4, and this behavior at least partially contributes to its slow hydrolysis. Regarding the observed products (Figure 2), that vinyl species form is in agreement with the presence of rather basic surface hydroxyls (see below) which are required to enable the elimination of HCl from HD.17 It has previously been noted that alumina also performs minor elimination of HD,4 whereas less basic titania materials do not.5 In addition to the vinyls, the 11608

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analogous to that of the EMPA-catalyzed water reaction;6 the nonobservance of EA 2192 is in agreement with such a mechanism.6 These intermediates are depicted in Scheme 3. Note that Zr(OH)4 possesses both terminal and bridging OH groups,20,21 with the latter being more acidic and, thus, more susceptible to protonate VX.

typical HD hydrolysis products thiodiglycol (TG) and CH-TG sulfonium ion18 are also observed. VX Reaction. 31P NMR spectra (nonspinning) obtained for 20 μL of VX added to 0.234 g of as-received Zr(OH)4 (preequilibrated at 50% RH for 16 h) are shown in Figure 3. This

Scheme 3. Proposed Intermediate for the Zr(OH)4Catalyzed Hydrolysis of VX Based on That of EMPA5

To investigate the effect of dehydroxylation on the extraordinary high VX reactivity, samples of the as-received Zr(OH)4 were calcined at 150, 300, 500, and 900 °C;9 these temperatures correspond to important events associated with changes to the structure as determined by thermogravimetric analysis and differential scanning calorimetry (Figure 4). A water isotherm plot for Zr(OH)4 is also presented in Figure 4. A gravimetric method was used where the sample was predried in air at 100 °C for 30 min. A type I isotherm is observed with a maximum loading of 0.24 g/g. Data correspond well to earlier work by Sato et al.22 In the current work, most of the weight loss occurs at approximately 50 °C, where the derivative peaks, but continues until 150 °C, as seen in Figure 4b. This weight loss is due to bound water, and is endothermic; beyond this point, weight loss is attributed to restructuring of the substrate via dehydroxylation and crystallization, both exothermic phenomena. The maximum in the derivative weight plot occurs at 70 °C, indicating that water is only weakly adsorbed. The initial water loading obtained using the TGA/DSC, of 0.22 g/g, can be calculated using the dry weight at 100 °C. This loading is comparable to that obtained in the water isotherm (Figure 4a), where a maximum of 0.24 g/g loads near saturation. The heat flow measurement (Figure 4c) exhibits an endotherm at 77 °C corresponding to transition from water loss to dehydroxylation. The rise in the heat flow that occurs from 77 to 400 °C corresponds to the inflection in the gravimetric curve and is due to dehydroxylation. Also, an exotherm is centered at 440 °C, resulting from the transition from an amorphous to crystalline ZrO2. Beyond 460 °C, the heat flow is unchanged. The calcined materials were equilibrated at 50% RH for 16 h and examined for their reactivity toward VX. Prior to the addition of VX, modest-speed (ca. 2000 Hz) 1H MAS NMR spectra (Figure 5) were obtained to assess the amount of adsorbed water on the samples. As seen in Figure 5, even heating at 150 °C noticeably decreases the ability of Zr(OH)4 to adsorb water (50% RH), which is crucial to the VX reactivity, and calcinations at higher temperatures continue the trend. The calcination at various temperatures also caused gradual dehydroxylation of Zr(OH)4, and this process is analyzed in detail below.

Figure 3. 31P NMR spectra obtained for 20 μL of VX added to asreceived Zr(OH)4 (equilibrated at 50% RH) at the indicated times. The sample was nonspinning (MAS was not used).

was a larger aliquot than used in similar studies because a 5 μL challenge completely reacted prior to the initial spectrum. Note that by the 10 min spectrum in Figure 3 no detectable VX remains, having been largely converted to ethyl methylphosphonic acid (EMPA). The apparent half-life is 1 min; the standard 5 μL challenge is assumed to react even faster. Thus, Zr(OH)4a solid, reactive sorbentis able to decontaminate rather large droplets of VX in nearly instantaneous fashion, rivaling, if not out-performing, current liquid decontaminants.19 That the observed half-life of VX is so short on Zr(OH)4 shorter still than the brief half-lives previously noted for nanotubular titania (t1/2 < 30 min)5 and nanocrystalline titania (t1/2 < 2 min)7implies that not only do VX molecules quickly react with the surface but that bulk, liquid VX drops also undergo fast spreading and diffusion. Yang et al.6 have noted phase separation between water and VX mixtures. However, protonating VX would render it more water-soluble. The measured isoelectric point of Zr(OH)4 using potentiometric titration is 6.5, and it is considerably lower than the pKa of VX (8.6);6 thus, it is reasonable to conclude that VX would become protonated in contact with Zr(OH)4, greatly facilitating its solubility and diffusion in the multilayer of adsorbed water present within the pores of the 50% RH pre-equilibrated sample. Furthermore, reaction of VX on the surface could be catalyzed by the acidic hydroxyl groups of Zr(OH)4 as previously proposed for titania materials:7 The surface OH groups would protonate and could hydrolyze VX in a manner 11609

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Figure 4. (a) Water adsorption isotherm measured gravimetrically at 25 °C. The sample was first dried at 100 °C for 30 min. (b) Weight loss and derivative weight loss versus temperature for Zr(OH)4. Most of the weight loss occurs below 100 °C due to bulk water desorption. (c) Heat flow versus temperature for Zr(OH)4. From ∼77 to 300 °C, structural hydroxyl loss occurs, followed by crystallization to ZrO2 at 440 °C.

Figure 5. 1H MAS NMR spectra obtained for Zr(OH)4 as-received and calcined samples equilibrated at 50% RH prior to reaction with VX.

P NMR spectra obtained for 20 μL VX challenges added to the calcined samples are shown in Figure 6. Figure 6 reflects a gradual change in the chemical shift of the VX peak from ca. 60 ppm on the moisture- and acidic OH-rich, low-temperature calcined samples to ca. 52 ppm for the moisture- and acidic 31

OH-deficient high-temperature calcined material, in agreement with previous observations on the chemical shift of VX in protonating/nonprotonating environments.23 Twin peaks in some of the spectra, which are better resolved than those in the spectra of Figure 3, are attributed to protonated/non11610

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Figure 6. 31P NMR spectra (nonspinning) obtained for 20 μL of VX added to calcined Zr(OH)4 samples equilibrated at 50% RH.

protonated VX (both ca. 60 ppm) and EMPA/VX-pyro (ca. 35 and 25 ppm; Scheme 2). Consistent with the changes observed in the 1H MAS NMR spectra of Figure 5, the observed VX halflives increase with increasing calcination temperature, as summarized in Table 1. Thus, loss of adsorbed water and/or Table 1. VX Half-Lives for Zirconium Hydroxide Materialsa material

temperature

VX half life

Zr(OH)4 Z150 Z300 Z500 Z900 nanotubular TiO2 nanocrystalline TiO2 alumina magnesium oxide

as-received 150 °C 300 °C 500 °C 900 °C as-received as-received as-received as-received