3820
Ind. Eng. Chem. Res. 2008, 47, 3820–3826
Decontamination Reactions of Chemical Warfare Agent Simulants with Alcohols in the Basic Ionic Liquid Tetramethylammonium Hydroxide/ 1,2-Dimethyl-3-propylimidazolium Bis(trifluoromethylsulfonyl)amide John S. Wilkes, Patrick J. Castle,* Joseph A. Levisky, Cynthia A. Corley, Adrian Hermosillo, Matthew F. Ditson, Paul J. Coˆte´, Donald M. Bird, Ralph R. Hutchinson, Keith A. Sanders, and R. Larry Vaughn USAFA, 2355 Fairchild DriVe, Suite 2N225, USAF Academy, Colorado 80840
Increased acts of international terrorism called for the development of rapid and reliable chemical agent decontamination reactions to minimize human exposure and material loss. Even though many chemical reaction processes have been reported, most remain unattractive for various reasons. In order to eliminate many of the adversities, these processes were studied in an ionic liquid solvent. In this report, we describe the reaction between chemical warfare simulants with alcohols in the ionic liquid 1,2-dimethyl-3-propylimmidazolium bis(trifluoromethylsulfonyl)amide made basic with tetramethylammonium hydroxide pentahydrate. The chemical agent simulants used in this study were diisopropylfluorophosphate, bis(2-ethylhexyl) phosphite, and chloroethylphenyl sulfide, which simulate agents GD, VX, and HD, respectively. The reactions were rapid, and products were identified by LC/MS-TOF and GC/MS. All of the reaction products of the VX and HD simulants remained in the ionic liquid, whereas the products from GD simulant partitioned between the ionic liquid and water extract. Introduction Despite the greatly reduced threat of Cold War superpower confrontation, the threat of chemical, biological, radiological, and nuclear attack remains serious and credible.1 There is now renewed interest to develop biological and chemical reaction systems to neutralize a variety of chemical warfare agents (CWAs). These interests center on establishing rapid and reliable decontamination processes for the nerve agents Sarin (GD) and VX and for the blister agent sulfur mustard (HD). Established criteria asserted that, for a decontamination/neutralization process to be effective, it must be rapid, generate nontoxic reaction products, and be contained in a medium that is environmentally safe.2 In investigating decontamination and neutralization reactions, we use simulants instead of the actual chemical warfare agent. Simulants are chemical compounds that are similar in chemical composition and physical properties to the chemical agent. In this study, diisopropylfluorophosphate (DFP) was used to simulate the nerve agent Sarin, bis(2-ethylhexyl) phosphate (BEHP) was used to simulate the nerve agent VX, and 2-chloroethylphenyl sulfide (CEPS) was used to simulate the blister agent sulfur mustard (HD) (see Figure 1). Isopropylmethyl phosphonofluoridate (Sarin), O-ethyl-S-(2diisopropylaminoethyl)methylphosphonothioate (VX), and other closely related organophosphites and phosphates undergo aqueous alkaline hydrolysis and substitution reactions but remain unattractive because many of these large-scale reactions can result in toxic substances entering the drinking water supply.3 Containment of these reactions remains a prerequisite to the development of a rapid and safe process. To resolve some of these environmental and containment issues, decontamination reactions in ionic liquids were proposed. Ionic liquids are liquids that contain only ions, remain liquid at low temperatures, and have low vapor pressures.4 They are * To whom correspondence should be addressed. E-mail:
[email protected].
considered an excellent substitute for volatile organic solvents in decontamination reactions because of their extraordinarily wide liquid range, low melting points, chemical and thermal stability, high conductivity, and nonvolatility.5–9 Ionic liquids are used as solvents for a wide range of chemical reactions.10 In fact, these materials are proven to be more effective as solvents for chemical warfare agents than traditional solvents.11 Also, because ionic liquids are compatible with alcohol/alkaline decontamination agents, they are ideal for use as chemical warfare decontamination solvents. The objective of studying decontamination reactions in ionic liquids was 3-fold: (1) to identify those chemical compounds that react with simulants in an ionic liquid, (2) to determine the composition of the reaction products, and (3) to develop a reaction matrix that allows for containment of the reactants and products.
Figure 1. Chemical warfare agents and simulants.
10.1021/ie800237z CCC: $40.75 2008 American Chemical Society Published on Web 04/15/2008
Ind. Eng. Chem. Res., Vol. 47, No. 11, 2008 3821
Preliminary investigations identified two potential ionic liquids for this investigation: 1,2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)amide (DMPITf2N) and trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide. We selected DMPITf2N as the ionic liquid for this study because it has excellent hydrophobic/hydrophilic properties, offers a reaction matrix that is immiscible with water, and possesses excellent containment potential. In this report, we describe the decontamination reactions of each of the simulants as well as a mixture containing the three chemical warfare simulants with a mixture of methanol (the reagent), tetramethyl ammonium hydroxide (TMAOH) (ionic base), and DMPITf2N (the ionic liquid medium). Materials and Methods A. Ionic Liquids. 1,2-Dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)amide (DMPITf2N) was prepared according to the procedure described by Sutto et al.12 The chloride salt of DMPI was prepared by refluxing a mixture of 51.3 g of 1,2-dimethylimidazole (Aldrich, 136131) and 70.5 mL of 1-chloropropane (Aldrich, C68555) in a pressurized glass apparatus at 88 °C for 7 days. The contents were removed, placed in an ice bath at 0 °C, and allowed to crystallize. The solid material was rinsed with freshly distilled ethyl acetate and dried under vacuum. Sufficient acetonitrile was added to dissolve the solid. Excess ethyl acetate then was added to this solution to cause precipitation. The precipitate was filtered and dried under vacuum overnight at 88 °C. The ionic liquid (DMPITf2N) was then prepared by adding a 1.5 mol excess of lithiotrifluoromethane sulfonimide (Aldrich, 544094) in 100 mL of H2O to the dried precipitate (DMPI+Cl-) and shaken vigorously. Two layers were formed, the aqueous layer was discarded, and the ionic liquid layer was washed twice with water and dried overnight (under vacuum at 88 °C). The purity of DMPITf2N was determined by proton NMR and liquid chromatography/mass spectrometry-time-of-flight (LC/MSTOF) with electrospray ionization source (ESI). B. 1,2-Dimethyl-3-ethylimidazolium Bis(trifluoromethylsulfonyl)amide (DMEITf2N). 1,2-Dimethyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)amide (DMEITf2N) was prepared by substituting ethyl chloride (Aldrich, 29,531-0) for 1-chloropropane in the procedure described above. The purity was established by proton NMR and LC/MS-TOF. C. Tetraalkylammonium Hydroxides (R4N+OH-). Tetramethylammonium hydroxide pentahydrate (Sigma, T7505), tetraethylammonium hydroxide 20% solution (ACROS Organics, 176072500), and tetrabutylammonium hydroxide 30-hydrate (Fluka, GA11796) were used as purchased. D. Chemical Warfare Agent Simulants and Reaction Products. Diisopropylfluorophosphate (Sigma, D0879), bis(2ethylhexyl) phosphate (Aldrich, 248959), 2-chloroethylphenyl sulfide (Aldrich, 417602), and chloroethylphenyl sulfone (Aldrich, 417645) were used as purchased. E. Reaction Mixture Containing a Single Simulant. A mole ratio of 10:2:1 (DMPITf2N/TMAOH/simulant) was used throughout this study. In addition, a 1:1 ratio of internal standard 1,2-dimethylethyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)amide (DMEITf 2 N) to simulant was used to monitor all reactions. In separate experiments, 18.4 mg of diisopropylfluorophosphate (0.100 mmol), or 30.6 mg of bis(2-ethylhexyl) phosphite (0.100 mmol) or 18.1 mg (0.100 mmol) of 2-chloroethylphenyl sulfide, was added to 419 mg of DMPITf2N (1.00 mmol) and 40.5 mg of DMEITf 2 N (0.100 mmol). Each mixture was vortexed for 1 min and allowed to equilibrate for 1 h. To
each of the mixtures, 0.3 mL (7.4 mmol) of methanol containing 36.2 mg of TMAOH (0.200 mmol) was added. A homogeneous mixture was obtained after vortexing for approximately 15 s. A 0.010 mL aliquot was removed and placed in 1.0 mL of 1% acidified methanol (0.1 mL of concentrated HCl in 10 mL of methanol). The quenched mixture was vortexed and a 10 µL aliquot of this mixture was diluted in 1 mL of methanol. (When aliquots were quenched in methanol without acid, there was no change in reaction products.) This solution was analyzed by LC/MS-TOF. F. Reaction Mixture Containing Three Simulants. To 1.00 mmol (419 mg) of DMPITf2N and 0.1 mmol (40.5 mg) of DMEITf 2N were added 0.100 mmol (18.4 mg) of DFP, 0.100 mmol (30.6 mg) of BEHP, and 0.100 mmol (17.2 mg) of CEPS. The mixture was vortexed for 1 min and allowed to equilibrate for 1 h. To this homogeneous mixture, a solution of 0.6 mmol (108.6 mg) of TMAOH in 0.3 mL of methanol was added. A 0.010 mL aliquot was removed, and the reaction was quenched by placing it in 1.0 mL of acidified methanol. A 0.010 mL aliquot of this mixture was diluted with 1 mL of methanol and then analyzed by LC/MS-TOF. G. Instrument Conditions. A polar and aromatic reversedphase selectivity ether-linked phenyl with polar endcapping (Synergi Polar-RP) liquid chromatography column was employed. A mobile phase consisting of methanol and 5 mM ammonium formate employing gradient elution from 30% methanol to 90% methanol with a run time of 12 min and a flow rate of 0.3 mL/min provided good retention and resolution. H. MS/TOF. Drying gas temperature and pressure were 350 °C and 9.5 L/min, respectively. A capillary voltage of 3000 V, corona voltage of 26 V, and fragmentor voltages from 125-175 V were used. Autotune values for pusher, puller, puller offset, and other parameters were accepted as default values. Electrospray ionization (ESI) in both positive and negative modes was used. Results and Discussion The reactions between TMAOH in methanol and the three simulants, with and without an ionic liquid, as part of the reaction matrix were investigated. Reactions under similar aqueous alkaline conditions leading to alcoholysis have been reported.13,14 Although ionic liquids are excellent solvents for many chemical reactions, they frequently present obstacles with routine instrumental analysis, especially those equipped with front-end gas chromatographs. Many ionic liquids adhere to the GC injector lining and affect vaporization, alter the nature of the stationary phases of many capillary columns, and interfere with the partitioning of analytes in the column. In the GC analysis of solutions containing ionic liquids, it is often necessary to clean the entire inlet system and remove large sections of the column after only a few injections. By running a decontamination reaction in a matrix without an ionic liquid, the course of the reaction and the identification of the reaction products were readily determined. After optimizing the reaction conditions and identifying the products without the ionic liquid, the same reactions were carried out in a matrix where the ionic liquid was incorporated as the solvent. Liquid chromatography/mass spectrometry-time-of-flight (LC/MSTOF) equipped with an electrospray ionization (ESI) chamber was used in the analysis, and many of the problems associated with gas chromatography were eliminated. Methanol and ammonium formate (5 mM) made up the mobile phase. Gradient elution from 30% to 90% methanol effectively eluted and separated the cation/anion components of the ionic liquid and the reactants and the products without contaminating the system.
3822 Ind. Eng. Chem. Res., Vol. 47, No. 11, 2008
Figure 2. Decontamination reaction of diisopropyl phosphate.
A. DFP Reaction in Basic Methanol/Water Solution. We examined one known decontamination method in methanol/ water, where the methanol and water served as solvent and either or both could be reactants.2,15 The reaction between 0.1 mmol (18.4 mg) DFP (I) and a solution containing 36.2 mg of TMAOH•5H2O (0.2 mmol TMAOH; 1 mmol H2O) in 0.3 mL (7.4 mmol) of methanol was monitored by following the disappearance of DFP and the appearance of reaction products. All components of the reaction mixture were in excess over DFP: hydroxide (2-fold as TMAOH), water (10-fold), and methanol (70-fold). The reaction was extremely rapid with complete removal of the starting material in
