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J. Phys. Chem. C 2007, 111, 6694-6699
Confined Detection of High-Energy-Density Materials Gerardo Majano,† Svetlana Mintova,*,† Thomas Bein,‡ and Thomas M. Klapo1 tke*,‡ Laboratoire de Mate´ riaux a` Porosite´ Controˆ le´ e, UMR-7016 CNRS, 68093 Mulhouse, France, and Department of Chemistry and Biochemistry, Ludwig-Maximilians UniVersity Munich, 81377 Munich, Germany ReceiVed: December 22, 2006; In Final Form: March 2, 2007
Detection of a low-sensitivity, high-energy-density material, 1,1-diamino-2,2-dinitroethylene (FOX-7), stabilized in the pores of MFI-type nanocrystals is demonstrated. Diverse spectroscopic methods, including NMR, Raman, and IR spectroscopies, combined with differential scanning calorimetry (DSC) were applied for the recognition of the FOX-7 compound under thermal stress. A remarkably higher thermal stability of FOX-7 immobilized in the channels of MFI nanocrystals was achieved because of the different arrangement and isolation of the molecules inside the MFI-type zeolite. In addition, the confined detection approach demonstrates the possibility of stabilizing the imine form of FOX-7 in zeolite nanocrystals treated at elevated temperature. A different exothermal response of the imine form of FOX-7 was observed, which provides further information on the decomposition path and nature of temperature-treated FOX-7. The confined recognition approach opens new alternatives for the stabilization and detection of sensitive high-energy materials such as primary explosives and provides a new possibility for the development of safe standards.
1. Introduction The detection of energetic materials commonly used for improvised explosive devices is indisputably a significant focus of research and has attracted considerable attention during the past decade. A wide variety of detection techniques for highly energetic materials are currently being applied. Mostly, detection by explosives-sniffing dogs has been used, but now, recognition through physicochemical methods has attracted considerable attention.1-3 For the detection of high-energy materials based on analytical techniques, extreme care is required in the preparation and handling of each individual sample. The task obviously requires well-trained personnel, and even then, this does not eliminate the possibility of accident. For further minimization of the hazards associated with energetic materials, it is necessary to desensitize them, i.e., to reduce the friction, shock, and heat sensitivity by means of stabilizers or binders. The traditional way of desensitizing highenergy materials is through a plasticizing technique with the help of a binder and a plasticizer.4 However, methods such as this and many others, including mixing the energetic material with inert substances such as salts or oil,5,6 coating the crystals with waxes,7,8 or regulating the oxygen balance of the energetic material with the help of nonexplosive nitro compounds,9 are not aimed toward detection purposes. Therefore, all of the desensitization approaches entail a considerable amount of spectral pollution that highly compromises their precision and selectivity. This problem and others such as compatibility with the environment, cost, and sensitivity during processing make the search for better ways to desensitize energetic materials a topic of great interest. Inclusion in framework-type materials by chemical means is widely regarded as a method for successful tailoring of the * To whom correspondence should be addressed: Tel.: +33 389336739. Fax: +33 389336885. E-mail:
[email protected] (S.M.),
[email protected] (T.M.K.). † UMR-7016 CNRS. ‡ Ludwig-Maximilians University Munich.
physicochemical properties of the guest molecules through orientation and/or conformation control, as mentioned in the literature.10-12 Various properties, such as nonlinear optical responses for second harmonic generation and micro-antenna properties, can be tailored to differ from those of the pure compounds by making use of inclusion chemistry techniques.11,12 Prime candidates as hosting matrixes for inclusion chemistry are molecular sieves, particularly zeolites. Zeolites are crystalline aluminosilicates with an open-framework-type structure based on tetrahedrally coordinated silicon and aluminum.13 Such materials offer unique pore structures with high pore volumes, high surface areas, and variable pore size openings. These characteristics allow for guest molecules, either neutral or charged, to penetrate and leave the hosting matrix through diffusion processes. Work on inclusion chemistry has been carried out using zeolites for stabilization of labile species through isolation of attacking agents.14 However, these investigations have not included studies on the reactivity and sensitivity of high-energy materials incorporated inside the porous matrix. Investigations regarding the interaction of energetic materials with microporous materials such as zeolites are few and are limited to reducing the toxicity of gaseous explosion products15 and using them as filling agents in nitrates mixtures.16 Herein, we report on the detection of FOX-7, a high-energydensity material, within MFI nanosized crystals by means of spectroscopic and thermal methods. Additionally, the confined detection of the high-energy compound permits a study the stabilization of different forms of FOX-7 in the host channels upon high-temperature treatment. 2. Experimental Section Silicalite-1, the all-silica representative of the MFI-type zeolite family, was used as a hosting matrix in this work because of its chemical inertness resulting from a lack of reactive metal centers.17 Silicalite-1 exhibits a porous network composed of
10.1021/jp068863n CCC: $37.00 © 2007 American Chemical Society Published on Web 04/18/2007
Confined Detection of High-Energy-Density Materials straight channels (5.3 × 5.6 Å) interconnected by sinusoidal channels (5.1 × 5.5 Å).13,18 1,1-Diamino-2,2-dinitroethylene (FOX-7) is a high-energydensity material chosen as a model molecule for detection because of its highly stable nature, thus providing a safe candidate for the current study. 2.1. Synthesis of Silicalite-1 Nanocrystals. The silica source tetraethyl orthosilicate (TEOS, Aldrich, 98%) was mixed with the structure-directing agent tetrapropylammonium hydroxide (TPAOH, Aldrich, 20 wt % in water) to prepare precursor solutions with the composition 9 TPAOH:25 SiO2:420 H2O: 100 EtOH. After hydrolysis of the above solutions for 48 h, hydrothermal treatment at 90 °C for 4 days was carried out, resulting in the crystallization of nanosized pure-silica MFI-type zeolites. The purified zeolite suspensions with a solid concentration of 25 wt % were freeze-dried and calcined (500 °C, 6 h, heating ramp of 2 °C/min) to remove the template (TPAOH) and make free space for the incorporation of FOX-7. 2.2. Inclusion of FOX-7 in Silicalite-1 Nanocrystals. FOX-7 was provided by NEXPLO (Bofors, Sweden) in wet form, and prior to use, it was dried at 50 °C for 3 h. The FOX-7 sample (100 mg) was dissolved in a mixture of 12 mL of acetone and 120 µL of dimethyl sulfoxide (DMSO), the calcined silicalite-1 was then added to the solution, and the mixture was stirred at room temperature for 6 h. The zeolite nanocrystals were separated from the solution by centrifugation (1000 rpm, 10 min), washed with pentane, and dried at room temperature for 18 h. (Optionally, some of the samples were dried at 178 °C for 3 h.) The zeolite samples treated with 40 wt % FOX-7 solutions are abbreviated as F-silicalite-1. 2.3. Confined Detection of FOX-7. Before and after immobilization of FOX-7, the silicalite-1 nanocrystals were characterized by X-ray powder diffraction; the patterns were collected on a STOE STADI-P diffractometer in the DebyeScherer geometry using Cu KR radiation. The size and morphology of the silicalite-1 crystals were determined by scanning electron microscopy (SEM) using a JSM-6500F instrument (JEOL). The FOX-7 immobilized in the zeolite nanocrystals was characterized by Raman spectroscopy using a Perkin-Elmer Spectrum 2000 NIR FT-Raman spectrometer equipped with a Nd:YAG laser (400 mW, 1064 nm, 2000 scans). FT-IR spectra of the samples were collected on a Perkin-Elmer Spectrum One instrument with a CsI beam splitter. In addition, the samples were characterized by solid-state MAS NMR spectroscopy. 1H and 13C NMR spectra were collected with a Bruker DSX Avance 500 spectrometer under the following conditions: for 1H spectra, 500.206 MHz, 3.40 µs pulse length, 12 kHz spinning rate, and for 13C spectra, 125.787 MHz, 3.0 µs pulse length, 6 kHz spinning rate. Prior to the NMR studies, some samples were heated at 178 °C for 3 h in air and then filled in zirconia rotors under nitrogen. Differential scanning calorimetry (DSC) and thermogravimetry (TG) were used to probe the thermal behavior of FOX-7 both while free and while immobilized in the zeolite hosting material. The DSC and TG data were collected on a PerkinElmer Pyris 6 DSC instrument (heating ramp, 5°/min; nitrogen flow, 20 mL/min) and on a Setaram TG92-24 DTA instrument (heating ramp, 10°/min; helium flow, 20 mL/min), respectively. 3. Results and Discussion The stability of the crystalline structure of the nanosized silicalite-1 samples before and after incorporation of FOX-7 was
J. Phys. Chem. C, Vol. 111, No. 18, 2007 6695
Figure 1. XRD patterns of (a) pure FOX-7, (b) F-silicalite-1, and (c) pure silicalite-1. Insets show the SEM images of silicalite-1 crystals (a) before and (b) after incorporation of FOX-7. M ) 100 nm.
verified by recording the X-ray patterns from the powders (Figure 1). The Bragg reflections characteristic of MFI-type zeolite did not change in either position or intensity after incorporation of FOX-7 into the hosting nanocrystals (Figure 1b,c). Moreover, the morphology of the zeolite crystals remained the same, and both samples contained very well-defined roundshaped particles with sizes between 90 and 100 nm (see SEM insets in Figure 1). In addition, Bragg reflections originating from pure FOX-7 were not present in the XRD patterns of the F-silicalite-1 sample (Figure 1a,b), which is in a good agreement with the SEM observations. These reveal no excess of FOX-7 either as separated agglomerates or as shells on the zeolite nanocrystals, which could give rise to any considerable reflections in the XRD patterns. Because the detection of high-energy-density materials in a pure form is never risk-free, the immobilization of highsensitivity substances in an inorganic matrix is considered as an appropriate approach for safe spectroscopic characterization. In our case, solid-state NMR spectroscopy was used for recognition of FOX-7 incorporated in the zeolite hosts, because several nuclei including 1H, 29Si, and 13C are present in great abundance in the samples discussed here. It has to be taken into account that 1H NMR spectroscopy is not typically used to study zeolites because of the broad silanol peak around 5 ppm arising from residual water at the surface of MFI zeolites.19,20 Additionally, a second peak below 2 ppm coming from nonacidic silanol groups in zeolite samples can sometimes be observed. However, the goal in our case was to confirm or refute the presence of FOX-7 in the zeolite hosts; for that purpose, a comparison of the 1H NMR spectra of pure silicalite-1, pure FOX-7, and F-silicalite-1 is adequate (Figure 2). It should be noticed that the proton signal from pure FOX-7 [-C(NH2)2] in the liquid-state 1H NMR spectrum appears at about 8.7 ppm, whereas the same signal in the solid-state spectrum is shifted to higher field at 4.6 ppm.21 Therefore, it was anticipated that the above-mentioned silanol signal originating from the zeolite surface and the proton signal arising from pure FOX-7 would be close together and difficult to resolve in the solid-state spectra if they appeared as one broad signal. Nonetheless, it was expected that a disordered state of FOX-7 molecules in the voids of the zeolitic matrix with relatively few H-bonds between them would give rise to a signal similar to that observed in solution. To promote this possibility and eliminate residual solvent from the zeolite surface, some samples were dried prior to the NMR study (178 °C, 3 h) and were packed in the rotor under nitrogen.
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Figure 2. 1H MAS NMR spectra of (a) F-silicalite-1, (b) pure FOX7, and (c) pure silicalite-1.
As can be seen, the NMR spectrum of pure silicalite-1 contains a Si-OH signal at 5.16 ppm, which, after inclusion, undergoes an evident shift combined with a broadening that can be assigned to the presence of FOX-7 in a disordered manner inside the zeolite channels (see Figure 2a). Thus, the 1H signals in confined and pure FOX-7 samples are located at 4.54 and 4.58 ppm, respectively. The 1H NMR data for pure FOX-7 and for FOX-7 stabilized in zeolite matrix complement the recently obtainted data on FOX-7 incorporated at different concentrations in zeolite nanocrystals.22 The results of 13C NMR measurements reveal no change in the spectra of FOX-7 immobilized within the zeolite crystals prior to treatment at higher temperature. However, for samples heated at 178 °C, the signal of dCs (NO2)2 at 155.63 ppm is shifted to lower field, and the signal of dCs(NH2)2 is split into two peaks centered at 134.24 and 132.73 ppm. For comparison, the signals of pure FOX-7 in the liquid-state 13C NMR spectra appear at 158.2 and 128.4 ppm.21 A reaction with the silanol groups is ruled out, as most reaction products of FOX-7 are known to show a major shift of more than 10 ppm toward higher field and show a white color resulting from the loss of conjugation,23 changes that were not observed in our case. Detection of FOX-7 in the zeolite matrix was additionally performed by recording the Raman spectra of solid samples before and after heating. The Raman spectra of zeolites are usually dominated by vibrations characteristic of the organic molecules used as structure-directing agents (templates). In the current case, the TPAOH used as a template for the preparation of highly crystalline silicalite-1 was completely removed through calcination, and then FOX-7 was incorporated. The Raman spectrum of a calcined silicalite-1 sample contains an intense band at 381 cm-1, which is associated with the presence of pairs of five-membered rings, which are the secondary building units of MFI-type zeolites. In addition, the general features of FOX-7 such as the intensity and position of the Raman signals are not affected by the presence of the zeolite matrix (Figure 3a,b). However, a signal at 3332 cm-1 arises in the spectrum of F-silicalite-1 prior to heating that was assigned to asymmetric NH stretching.24 This signal is most likely due to the highly disordered hydrogen bonds of FOX-7 immobilized in the zeolite channels, which are not observed in the pure crystalline FOX-7 sample (Figure 3a). This observation is supported by a temperature-dependent study of the phase transitions of FOX-7, where the NH band at 3332 cm-1 shows an increase in intensity just before the phase transition, followed by a decrease in intensity (Figure 4). This process points toward a reorganization of the H-bonding in the crystalline FOX-7
Figure 3. Raman spectra of (a) pure FOX-7, (b) F-silicalite-1 dried at 25 °C (asterisk marks the vibration at 3332 cm-1), (c) F-silicalite-1 dried at 178 °C, and (d) silicalite-1 exposed at room temperature. The inset shows the spectrum of silicalite-1 (d) in the range of 280-650 cm-1.
Figure 4. Raman spectra of pure FOX-7 representing progressive heating at (a) 50, (b) 75, (c) 100, and (d) 105 °C, revealing the transition from the R phase to the β phase (the asterisk marks the vibration at 3332 cm-1).
material, leading to a stabilization of a different form of FOX7. Figure 4 presents the Raman spectra of pure FOX-7 subjected to progressive heating, which clearly shows a transition from the R phase to β phase. The behavior of the 3332 cm-1 vibration seen in these spectra clearly correlates with that observed in the F-silicalite-1 sample. The most revealing change after drying of the F-silicalite-1 sample at 178 °C occurs in the highwavenumber region (Figure 3c). The vibration at 3332 cm-1 associated with asymmetric NH stretching begins to disappear, and several bands belonging mainly to in- and out-of-layer H and NH wagging also appear with less intensity. Although Raman spectroscopy is a more sensitive method for the investigation of organic materials loaded in inorganic matrixes, IR spectroscopy was also used as a faster technique for FOX-7 detection. The vibrations sensitive to the zeolite structure appear at 569 cm-1 (pairs of 5-rings), 790 cm-1 (symmetric SiO4 stretch), and 1080 cm-1 (asymmetric SiO4 stretch). In addition, marked differences in the IR spectra of the F-silicalite-1 sample dried at room temperature and that heated at 178 °C can be seen (Figure 5). Several vibrations disappear from the spectrum of the F-silicalite-1 sample subjected to additional heating (Figure 5c). The disappearance
Confined Detection of High-Energy-Density Materials
Figure 5. IR spectra of (a) pure FOX-7, (b) F-silicalite-1, (c) F-silicalite-1 dried at 178 °C, and (d) silicalite-1. The ellipse encloses the vibration assigned to sCdNH+sHsOSi at 2235 cm-1.
TABLE 1: IR/Raman Vibrations Disappearing from the Spectra of the F-Silicalite-1 Sample Treated at 178 °C vibration frequency activity (cm-1) (IR/Raman) 623 859 1025 1066 1350 1396 1474 1525 1609 3332
Raman IR Raman Raman IR IR IR Raman Raman Raman
vibrational mode24 out-of-layer symmetric H wagging NO and NH rocking in-layer asymmetric H wagging in-layer symmetric H wagging symm C-NO stretching + NH wagging C(NH) wagging + asymmetric NH wagging CC stretching + in-layer H wagging NH + NO rocking CC + symmetric NH stretching asymmetric N-H stretching
of these bands was assigned to the elimination of solvents coordinating both the FOX-7 and the silanol groups from the surface of the zeolite crystals. The latter conclusion is also based on the 13C NMR data and DSC measurements (Vide infra), as these reveal no degradation products from FOX-7, which is also in a good agreement with the observations reported in refs 25 and 26. In this case, it is expected that some of the few defect silanol groups and oxygen atoms from Si-O-Si bridges inside SCHEME 1: Resonance Hybrids of FOX-7
J. Phys. Chem. C, Vol. 111, No. 18, 2007 6697 the channel structure are forming H-bonds with the amino groups from FOX-7, thus diminishing the intensities of the bands containing NH vibrations. Most of the bands of free FOX-7 are ascribed to NH and H vibrations as previously reported in ref 24. Small shifts of the bands at 1521-1507 and 16361625 cm-1, assigned to NH, NO rocking, C-C stretching, and symmetric NH stretching, respectively, are observed in the samples as well (Figure 5). In conclusion, the disappearance and weakening of the bands in the IR and Raman spectra correspond to in- and out-of-layer NH and H vibrations (see Table 1). This can be inferred as a further disarrangement of FOX-7 molecules upon heating, starting from a semicrystalline arrangement inside the zeolite channels. The latter explanation can also be regarded as a stabilization of another mesomeric form of FOX-7 inside the zeolite. As a validation of this statement, the more significant appearance of two new bands, one at 2235 cm-1 and an overtone band at 2339 cm-1, is evident (Figure 5c). These vibrations can be assigned to an imine moiety coordinated to the zeolitic framework (sCdNH+sHsOSi), because the sCdNH+sH bands from pure FOX-7 appear at 2200-1800 cm-1 and have medium intensity.24 Moreover, the free imine (sCdNH) vibrations, which generally appear at 3400-3300 cm-1 are also missing in the spectrum of the F-silicalite-1 sample.27 FOX-7 is also known to have an atypical reactivity and crystalline ordering that points toward a lower bond order on the C-C axis.23,28,29 Thus, if the resonance hybrids of FOX-7 are taken into consideration, the traditional structure of FOX-7 (see Scheme 1, forms 1 and 3) with a CdC bond should have a lower weight in the resonance hybrid than traditionally expected, whereas the imine structures should make a higher contribution to it. In this case, imine forms 4-7 are in agreement with the above observations and thus can coordinate to the zeolite structure (see Scheme 1). This conclusion is also supported by the 13C NMR spectra reported earlier,22 showing a splitting of the signal at 133.60 ppm corresponding to the -C(NH2)2 moiety. The combination of spectroscopic methods clearly reveals that another form of FOX-7 is stabilized inside the zeolite structure different from the free crystalline material, and this form was found to be an imine mesomeric form.
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Figure 6. DSC curves of F-silicalite-1 dried at (a) 25 and (b) 178 °C and of (c) pure FOX-7. The curve for FOX-7 has been downscaled by a factor of 20 because of its explosive decomposition at 225 °C.
Differential scanning calorimetry is commonly used to characterize the exothermal features of energetic materials. DSC measurements of the starting, calcined zeolite (not shown) did not reveal any thermal response in the temperature range of 50-440 °C. However, the DSC curves of pure FOX-7 and zeolite nanocrystals containing FOX-7 exhibited marked changes in the two exothermic maxima of the material (Figure 6a,b). First, the peak corresponding to the transition from the β phase to the γ phase of FOX-7 is situated at 264 °C, which is about 30 °C higher than in the pure crystalline FOX-7 compound (Figure 6c),30 and second a peak corresponding to the transition from the γ phase to the δ phase of FOX-7 and the decomposition increases up to 377 °C. The most prominent exothermic features of pure FOX-7, namely, the small endotherm due to the R to β phase transition at 115 °C, the first exothermal peak at around 225 °C, and the explosive decomposition beyond 275 °C, can also be seen in the DSC plot (Figure 6c). Because of the high intensity of the explosive decomposition, the curve of pure FOX-7 has been downscaled by a factor of 20. For the preparation of F-silicalite-1, a minimum quantity of DMSO was used as a cosolvent (about 120 µL for 12 mL of acetone). Apparently, the small quantity of DMSO was capable of holding some FOX-7 at the surface of the zeolite crystals, as the calorimetric curves show (Figure 6a). A small endothermic peak with a modest exothermic signal at around 220 °C, which is typical for pure FOX-7, was detected.30 The signal at around 200 °C arose through the pronounced endothermic process coming from DMSO at 178 °C. After heating of the samples (178 °C, 3 h), the removal of the remaining DMSO was observed, which caused an increase of the first exothermic peak at 264 °C (Figure 6b). This sample also exhibited a significant weight loss based on the TG measurements, but had a positive impact on the exothermic behavior, as can be seen from the DSC curves. Because no decomposition products of FOX-7 were detected by 13C MAS NMR spectroscopy, the difference between the energies of the exotherm at 264 °C in the F-silicalite-1 sample treated at room temperature (∆H ) -92.57 J/g) and that treated at 178 °C (∆H ) -118.34 J/g) indicates a decomposition and can be related to the stabilization of the imine form of the FOX-7 energetic compound. In this case, the double bond would not be localized along the C-C axis, but would preferentially be on one of the C-N bonds, thus reducing the order of the central C-C bond and labilizing the neighboring C-NO2 bonds. This is known to play a critical role in the decomposition of high-energy nitro compounds, causing an increase in the exothermic response. This observation further supports the stabilization of the imine form of FOX-7 in the porous hosting material.
FOX-7, a high-energy-density material, was immobilized inside the channels of pure-silica MFI-type zeolite. FOX-7 exhibits high stability inside the zeolite matrix, which nullifies the intrinsic explosive decomposition of the substance and sets the exothermic maximum up to 100 °C higher than for the free substance. However, the spectral features of FOX-7 in the Raman, IR, and NMR data are not affected by inclusion in the zeolite matrix. This opens new possibilities for the creation of safer standards for energetic materials such as primary and secondary explosives and for the detection of improvised devices by inclusion in a matrix based on harmless microporous silicate materials. Inclusion chemistry also made possible the characterization of the imine form of FOX-7 obtained under treatment at high temperature. This imine form shows a slightly different exothermal response and brings further light on the decomposition path and nature of FOX-7 treated at elevated temperatures. Acknowledgment. PROCOPE, LMU, Fund of the Chemical Industry, European Research Office of the U.S. ARL (N6255805-C-0027), and Bundeswehr Research Institute for MaterialsWIWEB (E/E210/4 D 004/X 5143) are gratefully acknowledged for financial support. References and Notes (1) Steinfield, J. I.; Wormhoudt, J. Annu. ReV. Phys. Chem. 1998, 49, 203-232. (2) Hannum, D. W.; Parmeter, J. E. SurVey of Commercially AVailable ExplosiVes Detection Technologies and Equipment; Sandia National Laboratories: Albuquerque, NM, Sep 1998. (3) Rouhi, A. M. Chem. Eng. News 1997, Sep 29. (4) Hoffman, D. M.; Cunnigham, B. J.; Tran, T. D. J. Energy Mater. 2003, 21, 201-222. (5) Holm, B. German Patent DE 3224477, 1983. (6) Matya´sˇ, R. Influence of oil on sensitivity and thermal stability of triacetone triperoxide and hexamethylenetriperoxide diamine. In New Trends Res. Energ. Mater., Proc. Semin., 8th 2005, 2, 687-692. (7) Reichel, A.; Roos, O. German Patent DE 2308430, 1974. (8) Wagstaff, D. C.; Clive, D. British Patent GB 2374867, 2002. (9) Enoksson, B. P. (Nitro Nobel AB). German Patent DE 1808922, 1969. (10) (a) Langley, P. J.; Hulliger, J. Chem. Soc. ReV. 1999, 28, 279291. (b) Ramamurthy, V.; Eaton, D. F. Chem. Mater. 1994, 6, 1128-1136. (11) Kim, H. S.; Lee, S. M.; Ha, K.; Jung, C.; Lee, Y.-J.; Chun, Y. S.; Kim, D.; Rhee, B. K.; Yoon, K. B. J. Am. Chem. Soc. 2004, 126, 673682. (12) Calzaferri, G.; Huber, S.; Maas, H.; Minkowski, C. Angew. Chem., Int. Ed. 2003, 42, 3732-3758. (13) Van Bekkum, H; Flanigen, E. M.; Jansen, J. C.; Jacobs, P. A. Introduction to Zeolite Science and Practice; Elsevier Science Publishers: New York, 1991. (14) (a) Cozens, F. L.; Garcia, H.; Scaiano, J. C. J. Am. Chem. Soc. 1993, 115, 11134-11140. (b) Rohdes, C. J.; Reid, I. D.; Roduner, E. J. Chem. Soc., Chem. Commun. 1993, 6, 512-513. (c) Quin, X. Z.; Trifunac, A. D. J. Phys. Chem. 1990, 94, 4751-4754. (15) Dunne, S. R. U.S. Patent 6,251,200 B1, 2001. (16) Fleming, W. C.; McSpadden, H. J.; Olander, D. E. U.S. Patent 5,583,315, 2002. (17) Holderich, W. F.; van Bekkum, H. Stud. Surf. Sci. Catal. 2001, 137, 821-910. (18) Mintova, S.; Olson, N. H.; Senker, J.; Bein, T. Angew. Chem., Int. Ed. 2002, 41, 2558-2561. (19) Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites; John Wiley & Sons: New York, 1987. (20) Koller, H.; Lobo, R. F.; Burkett, S. L. J. Phys. Chem. 1995, 99, 12588-12596. (21) Latypov, N. V.; Bergman, J. Tetrahedron 1998, 54, 11525-11536. (22) Majano, G.; Mintova, S.; Klapo¨tke, T. M.; Bein, T. AdV. Mater. 2006, 18, 2440-2443. (23) Herve´, G.; Jacob, G.; Latypov, N. Tetrahedron 2005, 61, 67436748. (24) Peiris, S. M.; Wong, C. P.; Zerilli, F. J. J. Phys. Chem. 2004, 120, 8060-8066.
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J. Phys. Chem. C, Vol. 111, No. 18, 2007 6699 (27) Fabian, J.; Legrand, M.; Poirier, P. Bull. Soc. Chim. Fr. 1956, 1499-1509. (28) Evers, J.; Klapo¨tke, T. M.; Mayer, P.; Oehlinger, G.; Welch, J. Inorg. Chem. 2006, 45, 4996-5007. (29) Baum, K.; Nguyen, N. V. J. Org. Chem. 1992, 57, 3026-3030. (30) Burnham, A. K.; Weese, R. K.; Wang, R.; Kwok, Q. S. M.; Jones, D. E. G. Thermal properties of FOX-7, International Annual Conference of ICT, 36th (Energetic Materials), 2005, 150/1-87/12.
