Langmuir 2006, 22, 11077-11084
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Adsorption and Thermal Reaction of DMMP in Nanocrystalline NaY Kevin Knagge, Matthew Johnson, Vicki H. Grassian,* and Sarah C. Larsen* Department of Chemistry, UniVersity of Iowa, Iowa City, Iowa 52242 ReceiVed May 11, 2006. In Final Form: August 4, 2006 In this study, FTIR spectroscopy and solid-state magic angle spinning (MAS) NMR were used to investigate the adsorption and thermal reaction of the nerve gas simulant dimethyl methylphosphonate (DMMP) in nanocrystalline NaY with a crystal size of ∼30 nm. DMMP adsorbs molecularly in nanocrystalline NaY at 25 °C. Gas-phase products of the reaction of DMMP and oxygen in nanocrystalline NaY at 200 °C were monitored by FTIR spectroscopy and determined to be carbon dioxide (major product), formaldehyde, and dimethyl ether. In the presence of water, the thermal reaction of DMMP in nanocrystalline NaY at 200 °C yielded methanol (major product), carbon dioxide, and dimethyl ether. When the thermal reaction of DMMP in nanocrystalline NaY at 200 °C was conducted in the presence of water and oxygen, the predominant products were methanol and carbon dioxide. Hydroxyl sites located on the external zeolite surface were consumed during the DMMP thermal reactions as monitored by FTIR spectroscopy and were therefore determined to be the active sites in this reaction. 31P solid-state MAS NMR experiments were used to identify the surface-bound phosphorus complexes. The reactivity per gram of zeolite was comparable to other recently studied metal oxides such as MgO, Al2O3, and TiO2, and was found to have comparable, if not higher reactivity. Future improvements in reactivity may be achieved by incorporating a reactive transition metal ion or metal oxide nanocluster into the nanocrystalline NaY to enhance reaction rates and to achieve complete reaction of DMMP.
Introduction Decontamination of chemical warfare agents (CWAs), such as the nerve gas VX [O-ethyl S-(2-diisopropylamino) ethyl methylphosphonothioate] (I), is required in a variety of situations
including battlefields, laboratories, storage facilities, and destruction sites. Most research has dealt with battlefield decontamination due to the speed and ease of use of a decontaminant required in this situation. Battlefield decontamination has been defined as the rapid removal of chemical agents from military vehicles, equipment, personnel, and facilities by both chemical and physical methods.1 Because of the requirements of battlefield decontamination, reactive inorganic powders have been widely explored as possible catalysts for CWA decontamination. Studies have been performed examining the neutralization of VX on nanosized MgO,2 nanosized CaO,3 AgY,4 and nanosized Al2O35 and other similar compounds on nanosized MgO.6 Due to safety requirements, government regulations, and difficulties in obtaining actual CWAs, most work is performed using simulant compounds.7 The most common simulant of phosphorus-containing CWAs, such as VX, is dimethyl methyl (1) Yang, Y. C.; Baker, J. A.; Ward, J. R. Chem. ReV. 1992, 92, 1729. (2) Wagner, G. W.; Bartram, P. W.; Koper, O.; Klabunde, K. J. J. Phys. Chem. B 1999, 103, 3225. (3) Wagner, G. W.; Koper, O. B.; Lucas, E.; Decker, S.; Klabunde, K. J. J. Phys. Chem. B 2000, 104, 5118. (4) Wagner, G. W.; Bartram, P. W. Langmuir 1999, 15, 8113. (5) Wagner, G. W.; Procell, L. R.; O’Connor, R. J.; Munavalli, S.; Carnes, C. L.; Kapoor, P. N.; Klabunde, K. J. J. Am. Chem. Soc. 2001, 123, 1636. (6) Rajagopalan, S.; Koper, O.; Decker, S.; Klabunde, K. J. Chem.sEur. J. 2002, 8, 2602. (7) Yang, Y. C. Acc. Chem. Res. 1999, 32, 109.
phosphonate (DMMP) (II). There have been a number of studies that have investigated the reaction and decomposition of DMMP on metal oxides and supported metal oxides. Theoretical8 and experimental9 studies of the decomposition of DMMP on MgO, on metal-supported TiO210 and Al2O3,11,12 and on cerium oxide supported on Al2O313 have been reported. Oxidation studies involving platinum-coated oxides14,15 as a potential lowtemperature oxidation catalyst have been performed. The adsorption and photocatalytic reaction of DMMP on TiO2 have been intensively investigated.16-19 The adsorptive and reactive properties of VX and the simulant DMMP on aluminosilicates, such as zeolites, have also been examined. Zeolites are aluminosilicate molecular sieves with pores of molecular dimensions that are synthesized with a variety of different pore sizes and crystal morphologies, leading to a wide range of applications in catalysis, separations, adsorption, and ion exchange. Different chemical composition, pore size, pore dimension, and framework topologies, as well as the introduction of transition metals, lead to differences in reactivity, selectivity, and adsorptivity of molecules interacting with zeolites. For use as decontamination materials, it has been shown by solid-state NMR that VX hydrolyzes on NaY and AgY at room temperature through cleavage of the P-S bond to yield ethyl methylphosphonate (EMPA).4 The reaction proceeds more (8) Zhanpeisov, N. U.; Zhidomirov, G. M.; Yudanov, I. V.; Klabunde, K. J. J. Phys. Chem. 1994, 98, 10032. (9) Li, Y. X.; Koper, O.; Atteya, M.; Klabunde, K. J. Chem. Mater. 1992, 4, 323. (10) Ma, S.; Zhou, J.; Kang, Y. C.; Reddic, J. E.; Chen, D. A. Langmuir 2004, 20, 9686. (11) Cao, L. X.; Segal, S. R.; Suib, S. L.; Tang, X.; Satyapal, S. J. Catal. 2000, 194, 61. (12) Sheinker, V. N.; Mitchell, M. B. Chem. Mater. 2002, 14, 1257. (13) Mitchell, M. B.; Sheinker, V. N.; Cox, W. W.; Gatimu, E. N.; Tesfamichael, A. B. J. Phys. Chem. B 2004, 108, 1634. (14) Hsu, C. C.; Dulcey, C. S.; Horwitz, J. S.; Lin, M. C. J. Mol. Catal. 1990, 60, 389. (15) Tzou, T. Z.; Weller, S. W. J. Catal. 1994, 146, 370. (16) Trubitsyn, D. A.; Vorontsov, A. V. J. Phys. Chem. B 2005, 109, 21884. (17) Vorontsov, A. V.; Kozlov, D. V.; Smirniotis, P. G.; Parmon, V. N. Kinet. Catal. 2005, 46, 189. (18) Kozlova, E. A.; Vorontsov, A. V. Appl. Catal., B 2006, 63, 114. (19) Oshea, K. E.; Garcia, I.; Aguilar, M. Res. Chem. Intermed. 1997, 23, 325.
10.1021/la061341e CCC: $33.50 © 2006 American Chemical Society Published on Web 11/17/2006
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quickly on AgY relative to NaY, and the reaction continues on AgY as EMPA reacts further to form the desulfurized analogue of VX, 2-(diisopropylamino) ethyl methylphosphonate, also called QB. This is important since QB has an LD50 that is 3 orders of magnitude less than the LD50 of VX. Recently, another zeolite application of CWA detection was reported in which zeolite films immobilized on a quartz crystal microbalance were used as gas sensors for DMMP.20 Though zeolite pore dimensions are on the scale of nanometers or angstroms, only recently have methods for synthesizing zeolite crystals on a nanometer scale been thoroughly studied.21-23 Nanometer-scale or nanocrystalline zeolites offer greater surface areas and greater rates of diffusion because of shorter diffusive path lengths.24-26 The increased surface areas of nanocrystalline zeolites have been shown to lead to greater adsorption of reactant molecules27-29 and enhanced reactivities relative to larger micrometer-sized zeolite crystals.30-32 The objective of this study is to evaluate the use of nanocrystalline NaY for the thermal reaction of DMMP, thus taking advantage of the inherent reactivity of Y zeolites and the enhanced properties of the nanocrystalline form of Y zeolites. In this study, nanocrystalline NaY with a crystal size of ∼30 nm was evaluated for the adsorption and thermal reaction of the VX simulant DMMP. FTIR and solidstate NMR spectroscopy were used to identify the gas-phase and adsorbed species formed during the adsorption and reaction of DMMP in nanocrystalline NaY. Experimental Section Synthesis and Characterization of Nanocrystalline NaY. Uniform, monodisperse nanocrystalline NaY was synthesized using a method previously reported by Li et al.21 and modified by Larsen and co-workers.27,33 Aluminum isopropoxide (Alfa Aesar), distilled water, and tetramethylammonium hydroxide (TMAOH, 25 wt % from Aldrich) were mixed until a clear solution was formed. Tetraethyl orthosilicate (TEOS from Alfa Aesar), sodium hydroxide, and additional water and TMAOH were then added, and the resulting solution was stirred overnight to ensure complete hydrolysis of the aluminum and silicon precursors. The original synthesis gel composition for zeolite Y was 0.07Na:2.4TMAOH:1.0Al:2.0Si:132H2O:3.0i-PrOH:8.0EtOH where i-PrOH and EtOH are the hydrolysis products of aluminum isopropoxide and TEOS, respectively. The solution was then placed into a flask with an air-cooled condenser and placed in an oil bath at 95 °C for approximately 5 days. Products were removed by centrifugation, dried at 80 °C, and calcined under oxygen flow at 550 °C. Subsequent batches were made by recycling the synthesis solution as previously described.33 (20) Xie, H. F.; Yang, Q. D.; Sun, X. X.; Yu, T.; Zhou, J.; Huang, Y. P. Sens. Mater. 2005, 17, 21. (21) Li, Q. H.; Creaser, D.; Sterte, J. Chem. Mater. 2002, 14, 1319. (22) Mintova, S.; Olson, N. H.; Bein, T. Angew. Chem., Int. Ed. 1999, 38, 3201. (23) Tosheva, L.; Valtchev, V. P. Chem. Mater. 2005, 17, 2494. (24) Bein, T. Chem. Mater. 1996, 8, 1636. (25) Petrik, L. F.; Oconnor, C. T.; Schwarz, S. In Catalysis by Microporous Materials; Beyer, H. K., Karge, H. G., Kiricsi, I., Nagy, J. B., Eds.; Elsevier Science: Amsterdam, 1995; Vol. 94, p 517. (26) Vogel, B.; Schneider, C.; Klemm, E. Catal. Lett. 2002, 79, 107. (27) Song, W. G.; Li, G. H.; Grassian, V. H.; Larsen, S. C. EnViron. Sci. Technol. 2005, 39, 1214. (28) Song, W.; Justice, R. E.; Jones, C. A.; Grassian, V. H.; Larsen, S. C. Langmuir 2004, 20, 8301. (29) Song, W.; Justice, R. E.; Jones, C. A.; Grassian, V. H.; Larsen, S. C. Langmuir 2004, 20, 4696. (30) Li, G. H.; Jones, C. A.; Grassian, V. H.; Larsen, S. C. J. Catal. 2005, 234, 401. (31) Li, G. H.; Larsen, S. C.; Grassian, V. H. Catal. Lett. 2005, 103, 23. (32) Li, G. H.; Larsen, S. C.; Grassian, V. H. J. Mol. Catal. A: Chem. 2005, 227, 25. (33) Song, W.; Grassian, V. H.; Larsen, S. C. Chem. Commun. 2005, 2951.
Knagge et al. The resulting nanocrystalline NaY was characterized using powder X-ray diffraction (XRD) to determine crystallinity, BET nitrogen adsorption isotherms to measure surface area, electron microscopy (SEM (Hitachi S-4000) and TEM (JEOL JEM-1230)) to determine crystal size and morphology, and inductively coupled plasma atomic emission spectroscopy (ICP/AES) to measure elemental composition. A Siemens D5000 X-ray diffractometer with a Cu KR target and nickel filter was used to collect XRD powder patterns between 2θ angles of 5° and 35°. The XRD pattern confirmed the presence of NaY zeolite. The Si/Al was determined to be 1.6 from ICP/AES (Perkin-Elmer Plasma 400). Nitrogen adsorption isotherms were obtained on a Quantachrome Nova 1200 multipoint BET apparatus using approximately 0.2 g of sample for each measurement. Immediately prior to the N2 adsorption, each sample was degassed at 140 °C under vacuum for 12 h. The specific surface area was measured by the BET method, which was performed automatically by the instrument. BET adsorption isotherms were collected before and after calcinations of nanocrystalline NaY. The surface area before calcination provides the external surface area of the zeolite since access to the internal surface is blocked by template molecules. The surface area obtained after calcination reflects the total surface area (internal and external) of the NaY. Transmission FTIR Spectroscopy. The transmission infrared cell and gas handling system have been described previously.31 For FTIR experiments, zeolite samples were prepared by mixing approximately 30 mg of nanocrystalline NaY with a few drops of methanol. The resulting hydrosol was sonicated for 5-10 min and applied to one side of a 3 cm × 2 cm tungsten grid (mesh 100). The grid was placed inside two nickel jaws which were attached to copper leads for resistive heating. A thermocouple was also attached to the tungsten grid for temperature measurement. The zeolite-containing grid was placed into a stainless steel infrared cell with BaF2 windows and attached to a Matteson Galaxy 6000 infrared spectrometer. The IR cell was placed on a linear translator to allow different regions of the grid (gas phase or zeolite) to be scanned at different times. The FTIR spectra were obtained by averaging 64 scans with an instrument resolution of 4 cm-1. The system is connected to a vacuum/gas handling system. For all experiments, nanocrystalline NaY was first heated under vacuum for 12 h at 350 °C. Reaction gases were introduced into a premix chamber with a known volume and then expanded into the infrared cell containing the zeolite. The extinction coefficient of individual gases was calibrated using the characteristic IR absorption band and measuring the pressure using an absolute pressure transducer in the pressure range from 30 mtorr to 3 torr. The IR cell was sealed off from the premix chamber before any heating was started. For thermal reactions, the zeolite sample was resistively heated to 200 °C and maintained there for a given amount of time (up to 5 h). In the adsorption of DMMP in nanocrystalline NaY, 22 µmol of DMMP was exposed to approximately 10-15 mg of NaY in a volume of 1.155 L (volume of the premix chamber and the IR cell combined) for 30 min before FTIR spectra were acquired. The nanocrystalline NaY adsorbs approximately 1.43 mmol/g of DMMP. NMR Experiments. NMR experiments were performed using sealed zeolite samples. Approximately 50 mg of nanocrystalline NaY was placed in a small Pyrex tube and attached to a gas manifold. The sample was heated under vacuum at 140 °C for 12 h to dehydrate the zeolite. The sample was then placed in a liquid nitrogen bath and exposed to reactants (DMMP, H2O, and O2). Each sample was loaded with approximately 50 µmol of DMMP with equal parts of 50 µmol of H2O and O2 adsorbed on the sample for various experiments. The sample was then torch-sealed and placed in an NMR 7.5 mm rotor for NMR experiments. Samples were later heated by placing the sealed sample tube into a 200 °C oven for 5 h. All NMR spectra were obtained at room temperature. 31P (121.5 MHz) solid-state magic angle spinning (MAS) NMR spectra were obtained using a 300 MHz wide bore magnet with a TecMag Discovery console and a Chemagnetics double-channel 7.5 mm pencil MAS probe. For 31P MAS NMR spectra, 128 scans were averaged with a 9.5 µs pulse and 10 s delay. 31P NMR spectra were
DMMP in Nanocrystalline NaY
Figure 1. TEM image of nanocrystalline NaY. referenced to 85% H3PO4 (0 ppm). Samples were spun at the magic angle at a spinning rate of approximately 1 kHz. Due to technical difficulties associated with spinning of the samples sealed in Pyrex tubes and inserted into rotors, higher spinning rates could not be achieved with this sample configuration. The use of sealed samples was preferred so that both gas-phase and adsorbed species could be detected in the NMR experiments.
Results Characterization of Nanocrystalline NaY Zeolites. Nanocrystalline NaY was synthesized with a crystal size of approximately 30 nm. The size and morphology of the synthesized NaY are obtained from the TEM image shown in Figure 1. The cubic shape of the nanocrystalline NaY crystals is evident in the TEM image, and the size was estimated to be 31 ( 14 nm based on averages of several samples. The lattice spacing of 14 Å, also observed in the TEM image, is attributed to the [111] planes of the faujasite (FAU)-type structure.34 Powder XRD confirmed the NaY crystal structure, and nitrogen adsorption isotherms using the BET method were used to determine the total and external surface areas of the nanocrystalline NaY. The BET surface area of uncalcined nanocrystalline NaY (with template present) was 136 m2/g, which represents the external surface area and can be used to estimate the particle size using the equation from Song et al.27
particle size (nm) ) 4061/external surface area (m2/g) This equation gives an approximate crystal size of 32 ( 9 nm. This is in very good agreement with the crystal size of 31 ( 14 nm determined from electron microscopy. The total surface area of the calcined nanocrystalline NaY sample was 538 ( 47 m2/g. The large standard deviations of the size and surface area measurements are due to the fact that several batches were combined from the recycling synthesis procedure so that the size and surface areas represent an average of several batches of zeolite. FTIR Spectroscopy: Adsorption of DMMP in Nanocrystalline NaY. The gas-phase and adsorbed phase infrared spectra of DMMP in nanocrystalline NaY are shown in Figure 2. The spectra were divided into two spectral regions. The higher frequency spectral region (2800-3100 cm-1) contains the methyl group C-H stretches, and the lower frequency spectral region (900-1700 cm-1) includes the C-O and PdO stretches and methyl group deformation vibrations. A summary of the gasphase and adsorbed phase infrared spectra peak assignments is listed in Table 1.35 (34) Valtchev, V. P.; Bozhilov, K. N. J. Phys. Chem. B 2004, 108, 15587.
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In the C-H stretching region (Figure 2a), absorption bands at 3002, 2959, 2928, and 2854 cm-1 are observed for DMMP adsorbed on nanocrystalline NaY and are assigned to the CH3P and CH3O asymmetric stretches and the CH3P and CH3O symmetric stretches, respectively. In the lower frequency spectral region, absorption bands for adsorbed DMMP on nanocrystalline NaY are observed at 1256, 1315, 1414, and 1455 cm-1 and are assigned to the PdO stretching mode, the CH3P symmetric bend, the CH3P asymmetric bend, and the CH3O symmetric bend, respectively. It should be noted that two peaks (1046 and 915 cm-1) are observed in the DMMP gas-phase spectrum that have no corresponding adsorbed phase peaks. The zeolite background signal in this region obscures features in the infrared spectrum below 1150 cm-1. The peak occurring at 1651 cm-1 in the infrared spectrum of the adsorbed DMMP spectrum is assigned to residual water in the zeolite. When water was introduced into the system containing DMMP adsorbed in nanocrystalline NaY this band was observed to increase and was also found to adsorb on the surface and compete with DMMP for surface sites. More gasphase DMMP was observed after water was introduced suggesting that water displaces the DMMP from the zeolite surface. In previous work, dissociative adsorption of DMMP on metal oxides, such as TiO2 and Al2O3, was observed such that adsorbed methanol (methoxy) was also on the surface.35-37 The peaks for methanol adsorbed on nanocrystalline NaY occur at 2837 cm-1 for the symmetric stretch νs(CH3O) and 2945 cm-1 for the asymmetric stretch νs(CH3O). A clearly resolved methoxy symmetric stretch νs(CH3O) at 2837 cm-1 is not observed in the spectrum in Figure 2a, although a shoulder in this range may be present. FTIR Spectroscopy: Reaction of DMMP and Oxygen in Nanocrystalline NaY at 200 °C. The thermal reaction of DMMP and O2 in nanocrystalline NaY was investigated using infrared spectroscopy. Gas-phase products of the thermal reaction of DMMP and oxygen in nanocrystalline NaY were monitored by infrared spectroscopy as a function of time at 200 °C. Figure 3 shows representative infrared spectra monitoring the evolution of gas-phase products every 10 min for 2 h for DMMP reaction in nanocrystalline NaY at 200 °C. The gas-phase products detected from the reaction of DMMP at 200 °C are CH3OCH3 (1179, 1457, 2891, and 2926 cm-1), CO2 (2345), CO (2145 cm-1), and CH2O (1745 cm-1). After 4 h of reaction time, a small amount of DMMP remained on the surface indicating that the reaction did not reach completion. It is possible that other products, such as methanol, are formed in the reaction of DMMP, but their characteristic peaks are not resolved in the infrared spectra obtained here. The gas-phase products identified and the vibrational frequencies used to monitor these products are listed in Table 2.38 FTIR Spectroscopy: Reaction of DMMP with Water in Nanocrystalline NaY at 200 °C. The thermal reaction of DMMP and H2O in nanocrystalline NaY was also investigated using infrared spectroscopy. The infrared spectra of the gas-phase species formed when DMMP and H2O in nanocrystalline NaY were heated to 200 °C are shown in Figure 4. The major gasphase products observed in the infrared spectra are CH3OH (1035, 2845, 2960 cm-1), CH3OCH3 (1179, 1457, 2891, and 2926 cm-1), CO2 (2345 cm-1), and a possible phosphoric acid species (1370 (35) Rusu, C. N.; Yates, J. T. J. Phys. Chem. B 2000, 104, 12292. (36) Templeton, M. K.; Wieinberg, W. H. J. Am. Chem. Soc. 1985, 107, 97. (37) Mitchell, M. B.; Sheinker, V. N.; Mintz, E. A. J. Phys. Chem. B 1997, 101, 11192. (38) NIST Chemistry WebBook; http://webbook.nist.gov/chemistry (accessed May 2006).
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Figure 2. FTIR spectra of (a) DMMP adsorbed on nanocrystalline NaY at room temperature and (b) gas-phase DMMP. The spectrum shown in (a) was recorded in the presence of gas-phase DMMP and was referenced to clean nanocrystalline NaY prior to adsorption. Table 1. Vibrational Mode Frequencies of DMMP in the Gas Phase and Adsorbed on Nanocrystalline NaY adsorbed frequency (cm-1)
Table 2. Vibrational Frequencies and Assignments of Gas-Phase Products Formed from the Thermal Reactions of DMMP on Nanocrystalline NaY
vibrational modea
gas-phase frequency (cm-1) literaturea
NaY (30 nm)
TiO2b
νa(CH3P) νa(CH3O) νs(CH3P) νs(CH3O) δs(CH3O) δa(CH3P) δs(CH3P) ν(PdO) F(CH3P) νa(C-O) νa(C-O) CH3 rock
3014 2962 2924 2860 1467 1421 1314 1276 1188 1073 1049 915
3002 2959 2928 2855 1455 1414 1315 1256 c c c c
2992 2960 2924 2856 1463 1424 1315 1242
formaldehyde (CH2O)
ν(CdO)
1745
methanol (CH3OH)
1052 1052
ν(C-O) νs(CH3) νa(CH3)
1035 2845 2960
HP(O2)O
ν(PdO)
1370
gas-phase product
vibrational modea
frequency (cm-1)
dimethyl ether (CH3OCH3)
νa(CH3) νs(CH3) νdeform(CH3) CH3 rock
2926 2891 1457 1179
carbon dioxide (CO2)
νs(CdO)
2345
carbon monoxide (CO)
ν(CtO)
2145
a a Gas-phase vibrational mode assignments taken from ref 35. Vibrational mode assignments taken from ref 35. c The vibrational modes below 1000 cm-1 for DMMP adsorbed on NaY are not observed due to a strong zeolite background in this region.
Vibrational mode assignments taken from ref 38.
b
Figure 3. FTIR spectra of gas-phase products of the thermal reaction of DMMP and oxygen on nanocrystalline NaY at 200 °C. Representative spectra obtained every 10 min for 2 h are shown here.
cm-1). The spectral assignments for the observed products are listed in Table 2. It should also be noted that peaks corresponding to DMMP are absent from the gas-phase spectra. One possible explanation
Figure 4. FTIR spectra of gas-phase products of the thermal reaction of DMMP in the presence of water on nanocrystalline NaY at 200 °C. Gas-phase water absorption bands between 1300 and 1750 cm-1 are clearly seen in the spectra. Representative spectra obtained every 20 min for 4 h are shown here.
for this is that water facilitates a complete reaction of DMMP. It should also be noted that, in the thermal reaction with oxygen, CO2 is the dominant product, while in the hydrolysis reaction, CH3OH is the dominant product.
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Figure 5. FTIR spectra of gas-phase products of the thermal reaction of DMMP in the presence of water and oxygen on nanocrystalline NaY at 200 °C. Representative spectra obtained every 20 min for 4 h are shown here.
FTIR Spectroscopy: Reaction of DMMP in the Presence of Oxygen and Water in Nanocrystalline NaY at 200 °C. The thermal reaction of DMMP, O2 and H2O in nanocrystalline NaY was investigated using infrared spectroscopy. Gas-phase products of the thermal reaction of DMMP, O2, and H2O in nanocrystalline NaY were monitored by infrared spectroscopy as a function of time at 200 °C. Gas-phase FTIR spectra were acquired every minute for 5 h. Representative infrared spectra of the reaction of DMMP, O2, and H2O in nanocrystalline NaY at 200 °C are shown in Figure 5. The major gas-phase products observed are CH3OH and CO2; CH3OCH3 is another possible product, but a large degree of spectral overlap with methanol vibrational peaks makes quantification difficult. In this reaction, major DMMP peaks are observed at 1050, 1272, and 1312 cm-1. This indicates that DMMP does not completely react on the surface of nanocrystalline NaY and remains in the gas phase before reacting to form products, such as CO2 and CH3OH. FTIR Spectroscopy: Surface OH Groups Before and After Reaction. As reported previously, several bands due to hydroxyl groups are observed in the FTIR spectrum (Figure 6, solid line) of nanocrystalline NaY.27,30-32 An intense band at 3745 cm-1 is assigned to terminal silanol groups that are located on the external surface of the nanocrystalline NaY.27 A weaker absorption band assigned to hydroxyl groups at defect sites is observed at 3723 cm-1, and a band at 3694 cm-1 is assigned to surface hydroxyls near Na+ sites.39 The band at 3654 is attributed to hydroxyl groups attached to extraframework alumina (EFAL) species, which have been shown to be located on the external zeolite surface.40 The hydroxyl group region of the FTIR spectrum of nanocrystalline NaY before (solid line) and after thermal treatment (dashed line) with (a) DMMP and O2, (b) DMMP and H2O, and (c) DMMP, O2, and H2O at 200 °C for 5 h are shown in Figure 6a-c. After thermal treatment of DMMP on nanocrystalline NaY with O2, the absorption band associated with the hydroxyl group near the EFAL sites at 3654 cm-1 disappears from the spectrum. The absorption band associated with the external surface silanol groups at 3745 cm-1 is seen to decrease slightly in intensity. (39) Fritz, P. O.; Lunsford, J. H. J. Catal. 1989, 118, 85. (40) Cairon, O.; Khabtou, S.; Balanzat, E.; Janin, A.; Marzin, M.; Chambellan, A.; Lavalley, J. C.; Chevreau, T. In Zeolites and Related Microporous Materials: State of the Art 1994; Weitkamp, J., Karge, H. G., Pfeifer, H., Ho¨lderich, W., Eds.; Elsevier Science: Amsterdam, 1994; Vol. 84, p 997.
Figure 6. FTIR spectra of the hydroxyl region of nanocrystalline NaY before (solid line) and after thermal treatment (dashed line) with (a) DMMP and O2, (b) DMMP and H2O, and (c) DMMP, O2, and H2O at 200 °C for 5 h.
Similarly, after thermal reaction of DMMP and water on nanocrystalline NaY, the hydroxyl bands decrease in intensity as shown in Figure 6b. The hydroxyl group region shows a complete loss of spectral intensity for all three absorption bands after reaction of DMMP on nanocrystalline NaY with O2 and H2O. These results indicate that hydroxyl groups located on the external zeolite surface participate in these decomposition reactions of DMMP and thus represent active sites for the decontamination of DMMP. Solid-State 31P MAS NMR Spectroscopy: Adsorption of DMMP in Nanocrystalline NaY. 31P MAS NMR experiments were conducted to investigate the phosphorus surface species formed during the DMMP thermal reactions in nanocrystalline NaY since this information was difficult to obtain from the FTIR experiments due to spectral overlap and broadening. The 31P MAS NMR spectra of (a) neat DMMP, (b) DMMP adsorbed in nanocrystalline NaY, and (c) DMMP and water adsorbed in nanocrystalline NaY are shown in Figure 7a-c. Spectral assignments for 31P chemical shifts are listed in Table 3.39,41-46 The 31P chemical shift for neat DMMP (Figure 7a) is 33 ppm. When DMMP is adsorbed in nanocrystalline NaY, the peak shifts slightly downfield to 34 ppm and spinning sidebands (marked with asterisks) appear in the NMR spectrum (Figure 7b). The spinning sidebands indicate appreciable chemical shift anisotropy and a decrease in mobility suggesting that DMMP strongly adsorbs on the nanocrystalline NaY surface. When water is coadsorbed with DMMP, the peak shifts further downfield to 37 ppm and is narrower although spinning sidebands are still observed in the NMR spectrum (Figure 7c). (41) Pouchert, C. J. Aldrich Library of NMR Spectra; Aldrich Chemical Co.: Milwaukee, WI, 1983. (42) Moedritzer, K.; Maier, L.; Groenweghe, L. C. D. J. Chem. Eng. Data 1962, 7, 307. (43) Mitchell, M. C.; Taylor, R. J.; Kee, T. P. Polyhedron 1998, 17, 433. (44) Salehirad, F.; Anderson, M. W. J. Catal. 1996, 164, 301. (45) Beaudry, W. T.; Wagner, G. W.; Ward, J. R. J. Mol. Catal. 1992, 73, 77. (46) Dahn, H.; Pechy, P. Magn. Reson. Chem. 1996, 34, 723.
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Figure 7. 31P MAS NMR spectra of (a) DMMP neat, (b) DMMP adsorbed on nanocrystalline NaY, and (c) DMMP adsorbed with water on nanocrystalline NaY. 31P MAS NMR spectra of DMMP reacted on nanocrystalline NaY at 200 °C for 5 h with (d) O2, (e) H2O, and (f) O2 and H2O. Table 3.
31P
NMR Chemical Shifts and Assignments for DMMP and Thermal Reaction Products
Figure 8. Structures for possible adsorption complexes of DMMP on nanocrystalline NaY: (A) dissociative adsorption of DMMP on the surface and (B) molecular adsorption of DMMP on the surface.
spectrum is shown in Figure 7f, and several different peaks are observed in the range of 10-50 ppm. The peak at 37 ppm corresponds to unreacted DMMP. The peak at 25 ppm is close to the chemical shift of HMPA as observed in Figure 7e and discussed above. Another peak appears at 12 ppm, and this peak is close to the chemical shift for an authentic sample of dimethyl phosphite (CH3O)2P(O)H or DMP. There is also a broad peak present in the 31P NMR spectrum due to a strongly adsorbed immobile surface species analogous to Figure 7d. This peak is difficult to assign due to the width of the peak.
Discussion
Solid-State MAS NMR Spectroscopy: Reactions of DMMP in Nanocrystalline NaY at 200 °C. The 31P MAS NMR spectra obtained after thermal reaction of DMMP and O2, DMMP and H2O, and DMMP, O2, and H2O all in nanocrystalline NaY at 200 °C are shown in Figure 7d-f. The NMR samples were contained in sealed sample tubes that were heated to 200 °C ex situ. All NMR spectra were recorded at room temperature under conditions of thermal equilibrium. The 31P MAS NMR spectrum of DMMP on nanocrystalline NaY after reaction with O2 at 200 °C is shown in Figure 7d. The spectrum is broad and featureless. The large line width indicates significant chemical shift anisotropy and an immobile and strongly bound phosphorus-containing surface species. 31P chemical shifts and relevant literature references are provided in Table 3. The 31P NMR spectrum of the products formed when DMMP is reacted with H2O at 200 °C is shown in Figure 7e. A peak at 25 ppm is observed in the 31P MAS NMR spectrum, which is close to the chemical shift of hydroxy methyl phosphonic acid (OHCH2PO(OH)2) or HMPA. DMMP was then reacted with H2O and O2 at 200 °C for 5 h. The resulting 31P MAS NMR
Adsorption of DMMP in Nanocrystalline NaY. Previous work reported in the literature on DMMP adsorption on metal oxide surfaces shows that DMMP adsorbs dissociatively at room temperature and above to form adsorbed methoxy species and a methyl methylphosphonate complex on the surface as shown in Figure 8A.37 It has also been demonstrated that under certain conditions DMMP adsorbs molecularly on the surface as illustrated schematically in Figure 8B.35 Studies of DMMP adsorption on TiO2 have shown dissociative adsorption at temperatures as low as -65 °C.35 Studies of DMMP adsorption on MgO have shown dissociative adsorption and the formation of an immobilized methyl methylphosphonate species with a bridging O-P-O group at 200 °C47 and methoxy species from dissociative adsorption at 100 °C.37 Studies of DMMP adsorption on MgO at room temperature indicate that molecular adsorption is occurring.47 Theoretical studies of DMMP adsorption on MgO suggest that dissociative adsorption is thermodynamically favorable compared to molecular adsorption.8 Studies on alumina indicate molecular adsorption at -73 °C and room temperature, but they have also shown dissociative adsorption above 22 °C.36,37 The formation of methoxy species from dissociative adsorption occurring at the ceria phase regions of Ce/Al2O3 catalysts has also been observed.13 (47) Li, Y. X.; Klabunde, K. J. Langmuir 1991, 7, 1388.
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Scheme 1. Proposed Reaction Scheme for the Thermal Reactions of DMMP, H2O, and O2 on Nanocrystalline NaY at 200 °C
Although the FTIR and NMR spectra of DMMP adsorption on nanocrystalline NaY reported here show no indication of dissociative adsorption, it should be noted that the FTIR results are inconclusive with respect to differentiating between the two modes of adsorption for DMMP on nanocrystalline NaY. The characteristic methoxy band at 2830 cm-1 is not apparent but may be present as a shoulder in the FTIR spectrum. Similarly, solid-state NMR does not provide much insight into the mode of DMMP adsorption. What is apparent from the solid-state NMR data is that the adsorbed DMMP species is strongly bound as evidenced by increased line broadening in the 31P NMR spectra after adsorption and the presence of spinning sidebands. Proposed Reaction Pathway for the Thermal Reaction of DMMP in Nanocrystalline NaY. The thermal reaction of DMMP in nanocrystalline NaY at 200 °C was monitored by in situ FTIR spectroscopy and by ex situ solid-state MAS NMR spectroscopy. Due to the limitations on the sensitivity of NMR, all NMR spectra were obtained at room temperature under conditions of thermal equilibrium using sealed sample tubes. The differences in experimental conditions between the FTIR and NMR experiments may account for some of the differences in the observed products. For the thermal reaction of DMMP and O2 at 200 °C, the gasphase products observed were CH3OCH3, CO2, CO, and CH2O by FTIR. The predominant gas-phase products were CH3OCH3 and CO2. Presumably, a surface species such as methyl phosphonic acid, PO2, P2O5, or some other phosphate species also formed although this was not resolved in FTIR or 31P NMR due to spectral broadening.11,14,48,49 A possible reaction path resulting in the formation of these products is shown in Scheme 1. The formation of dimethyl ether has been observed previously on other surfaces and has been attributed to an acid-catalyzed reaction between two methanol product molecules or two surface-adsorbed methoxy groups formed from the decomposition of DMMP.13,35,47 The FTIR spectra of the hydroxyl region before and after thermal reaction with O2 indicate that hydroxyl sites near EFAL play a role in the thermal reaction of DMMP and are consumed in the reaction. A slight decrease in intensity in the silanol region is also observed. Notably, both the silanol sites and the hydroxyl groups near EFAL are located on the external zeolite surface. (48) Moss, J. A.; Szczepankiewicz, S. H.; Park, E.; Hoffmann, M. R. J. Phys. Chem. B 2005, 109, 19779. (49) Segal, S. R.; Cao, L. X.; Suib, S. L.; Tang, X.; Satyapal, S. J. Catal. 2001, 198, 66.
Since there is greater external surface area and an increase in the OH groups for nanocrystalline zeolites compared to micrometersized zeolites, these data suggest that the nanocrystalline zeolites are superior zeolites for DMMP decomposition and perhaps VX decontamination. Influence of Water on the Thermal Reaction of DMMP in Nanocrystalline NaY. The product distribution changes dramatically in the presence of water. When DMMP and water are coadsorbed on nanocrystalline NaY and then reacted thermally at 200 °C, a different product distribution results. In FTIR, CH3OH is the dominant gas-phase product with the formation of CO2 and CH3OCH3 also observed. The 31P MAS NMR spectrum indicates the presence of a new phosphorus species at 25 ppm that is assigned to be HMPA. When both water and O2 were present for the thermal reaction of DMMP on nanocrystalline NaY at 200 °C, CH3OH and CO2 were the main products observed by FTIR. The 31P MAS NMR spectrum indicated the presence of several compounds, unreacted DMMP, HMPA, and DMP. All of the phosphorus species appear to be strongly bound to the zeolite surface as concluded based on the presence of spinning sidebands. A proposed reaction scheme indicating possible reaction paths to form these products on nanocrystalline NaY is shown in Scheme 1. The hydroxyl group region of the FTIR spectrum (Figure 6c) showed a complete loss of spectral intensity after thermal reaction of DMMP with water and O2 suggesting that all of the hydroxyl sites were involved in the thermal reaction of DMMP with water. Comparison of Reactivity of DMMP in Nanocrystalline Zeolites with Metal Oxides. Many studies have previously been conducted to examine the decomposition of DMMP on metal oxide surfaces including TiO2,10,16-18,35,48,50-52 MgO,2,9,47 and Al2O35 or alumina-supported catalysts.13 A discussion of these results provides the context for interpreting the current work on DMMP adsorption and thermal reactions on nanocrystalline NaY. Studies of reactions of DMMP on TiO2 indicate some phosphorus species form on the surface such as MPA, phosphate, methyl methyl phosphonate, and possibly P2O5.14,48 Studies of DMMP on MgO are not as specific, indicating the formation of (50) Rusu, C. N.; Yates, J. T. J. Phys. Chem. B 2000, 104, 12299. (51) Cao, L. X.; Gao, Z.; Suib, S. L.; Obee, T. N.; Hay, S. O.; Freihaut, J. D. J. Catal. 2000, 196, 253. (52) Oshea, K. E.; Beightol, S.; Garcia, I.; Aguilar, M.; Kalen, D. V.; Cooper, W. J. J. Photochem. Photobiol., A 1997, 107, 221.
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some phosphate species and methyl methylphosphonate at 200 °C.47,49 In one study of DMMP reaction on Al2O3, the destruction of P-CH3 bonds at 400 °C was reported.37 Some studies indicate the formation of many products such as dimethyl phosphate, monomethyl phosphate, phosphoric acid, methyl methyl phosphonate, and methyl phosphonic acid.15 Other studies indicate the presence of methyl phosphonic acid as high as 400 °C and the presence of P2O5.11 In the study reported here of DMMP thermal reactions on nanocrystalline NaY, phosphorus species such as DMP and HMPA were observed by 31P MAS NMR. However, after complete thermal reaction of DMMP, the disappearance of all these products with the exception of some strongly adsorbed phosphate species (possibly P2O5) was noted. Thermal hydrolysis reactions of DMMP on nanocrystalline NaY similarly indicated the destruction of all phosphorus species but HMPA. The partial destruction of the P-CH3 bond was observed as low as 200 °C in this study. It was suggested by Cao et al.11 that phosphorus is actually incorporating into the catalysts to form bonds such as AlPO4. This is definitely a possibility considering the very broad 31P NMR peak that is observed with the reaction of DMMP on nanocrystalline NaY. Many studies performed on DMMP decomposition or reaction on metal oxides have involved flow-through systems in contrast to the batch reactor studies reported here. Previous results have shown 1.1 mmol/g DMMP destruction on TiO2,16 1.28 mmol/g destruction on MgO,47 and 1.1 mmol/g destruction on Ce/Al2O310 systems. This compares well with the 1.43 mmol/g adsorption and corresponding destruction of DMMP observed for reaction of DMMP on nanocrystalline NaY. The reported result that is significantly improved relative to other results in the literature is for a mixed metal oxide system containing a supported transition metal, such that 20.18 mmol/g DMMP was decomposed on V/Al2O3.11 Future improvements in reactivity for our zeolite-based system may be achieved by incorporating a reactive transition metal ion or oxide cluster, such as iron oxide or cerium oxide, into the nanocrystalline NaY. Supported vanadium oxide has shown
Knagge et al.
unique reactivity for DMMP decomposition and would be a good candidate for incorporation into nanocrystalline NaY.11 Alternatively, aluminum-supported cerium oxide materials have shown ambient temperature DMMP decomposition activity13 suggesting that cerium oxide could improve the decomposition activity of nanocrystalline NaY by providing additional reactive sites.
Conclusions Adsorption and thermal reaction of DMMP on nanocrystalline NaY with a crystal size of ∼30 nm was investigated using FTIR and solid-state NMR spectroscopy. Molecular adsorption of DMMP on nanocrystalline NaY was observed at room temperature. Thermal reaction of DMMP and O2 on nanocrystalline NaY at 200 °C yielded several gas-phase products including dimethyl ether, carbon dioxide, carbon monoxide, and formaldehyde. Strongly bound phosphorus surface species were observed by 31P solid-state MAS NMR spectrum. Thermal reaction of DMMP in the presence of water in nanocrystalline NaY at 200 °C resulted in the formation of methanol, carbon dioxide, and phosphorus decomposition products such as hydroxy methylphosphonic acid and dimethyl phosphite. The external surface hydroxyl sites (silanol and EFAL) which are uniquely present in nanocrystalline NaY are important in the reaction and decomposition of DMMP. The reactivity per gram of zeolite was comparable to other recently studied metal oxides such as MgO, Al2O3, and TiO2. Future improvements in reactivity may be achieved by incorporating a reactive transition metal ion or oxide into the nanocrystalline NaY to provide additional reactive sites. Acknowledgment. The authors acknowledge Dr. Weiguo Song for his contributions to the synthesis and characterization of the nanocrystalline zeolites used in this study and Karna Barquist for performing ICP/AES experiments. This material is based on work supported by the U.S. Army Research Laboratory and the U.S. Army Research Office under Grant No. W911NF04-1-0160. LA061341E
