- Email: [email protected]
Time-resolved analytical pyrolysis studies of nitramine decomposition with a triple quadrupole mass spectrometer system
83
Journal of Analytical and Applied Pyrolysis, 12 (1987) 83-95 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
TIME-RESOLVED ANALYTICAL PYROLYSIS STUDIES OF NITRAMINE DECOMPOSITION WITH A TRIPLE QUADRUPOLE MASS SPECTROMETER SYSTEM
S.A. LIEBMAN
*
Geo-Centers, Inc., U.S. Army Chemical Research, Aberdeen Proving Ground, MD 21010-5423 (U.S.A.)
A.P. SNYDER,
J.H. KREMER
and Engineering
Center,
and D.J. REU’ITER
U.S. Army Chemical Research, Development MD 21010-5423 (U.S.A.)
M.A. SCHROEDER
Development
and Engineering
Center, Aberdeen Proving Ground,
and R.A. FIFER
Ballistic Research Laboratory, Aberdeen Proving Ground, MD 210055066
(U.S.A.)
ABSTRACT
An energetic nitramine, cyclotrimethylenetrinitramine (RDX), has been studied over selected thermal ranges and environments with a pyrolysis unit interfaced to an atmospheric pressure chemical ionization tandem mass spectrometer (MS-MS) system. Programmed and pulsed thermolytic degradation conditions under oxidative and non-oxidative atmospheres resulted in complex product distributions. Use of selected boron-containing compounds gave evidence of catalytic activity in the RDX degradation. Identification by daughter-ion MS-MS analysis was attempted for key volatiles of m/z 46, 60, 74, 75, 85, 97, 98, and 103. An integrated thermal degradation profile of RDX was thus probed by monitoring the volatile products as they were generated during slow programmed heating (60 o C/mm) from ambient to temperatures near the RDX melting/decomposition region (190-220 o C). Implications of enhanced nitramine performance are suggested.
Cyclotrimethylenetrinitramine;
mass spectrometry;
nitramine;
pyrolysis.
INTRODUCTION
Extensive literature [1,2], and in particular, 01652370/87/$03.50
is available for the study cyclotrimethylenetrinitramine 0 1987 Elsevier
Science Publishers
of nitramine degradation (RDX) [3,4]. However, B.V.
84
the early experimental work was conducted under a multiplicity of conditions that makes it difficult to delineate the fundamental stages in nitramine degradation. The present work was focused on careful thermal processing of small (milligram) amounts of RDX under a wide range of heating rates, final temperatures, dynamic/static atmospheres, and in the presence/absence of potential catalysts. The use of analytical pyrolysis has developed extensively over the past two decades [5] with time-resolved analyses being reported using on-line gas chromatography (GC), Fourier transform infrared (FTIR) and mass spectral systems. Real-time monitoring by the atmospheric pressure chemical ionization (APCI) tandem mass spectrometer (MS-MS) system permitted sensitive detection and, in several cases, distinct identification of the volatile species. A majority of studies have shown that RDX thermally decomposes to give NO,, formaldehyde, CO,, N,O, and several postulated ring-scission fragments, none of which have been satisfactorily identified. Whether the initial step is an N-NO, bond scission (leading to NO,) or C-N bond cleavage (leading to CH,O and N,O) has not been determined. Electron spin resonance (ESR) studies by Morgan and Beyer [6] and recent experiments at the Naval Research Laboratory have produced evidence [7-91 showing that definite free radicals were produced in early thermal decomposition, one of which was consistent with a spectrum of NO,. Other ESR patterns with complex hyperfine structure were also produced both in solution and solid state decomposition treatments.
EXPERIMENTAL
Samples of RDX were obtained by recrystallization from acetone, vacuum dried, and were ground to a fine powder (unsieved) of ca. 100 mesh. Boron-containing compounds were supplied from proprietary sources, Ta *05 from Aldrich Chemical Company, and TaH, from private sources. Mixtures were ca. 75% RDX physically mixed with ca. 25% additive. All runs used ca. l-2 mg powdered sample placed in the quartz tube of the Pyroprobe in the usual manner. Thermal processing of samples was conducted with a PyroprobeR Model 122 (Chemical Data Systems, Oxford, PA, U.S.A.). Many series of pulse pyrolyses were conducted, at the fastest rise-time (ca. 20 OC/ms) to varied final temperatures (from 200” C at several intervals to 1000” C). Programmed pyrolyses were conducted at controlled ramp rates of 60, 120, or 300”C/min to varied final temperatures in the above range. The most informative rate, 60” C/min, was used to compare product distributions under air or nitrogen atmospheres with varied additives. Interfacing the Pyroprobe to the MS-MS system was accomplished by
a-
e Fig. 1. Schematic of the Pyroprobe in the ion source of the atmospheric pressure chemical ionization (APCI) tandem mass spectrometer (MS-MS) TAGA 6000 system. (a) Pyroprobe; (b) sample positioned in a quartz tube in the coil of the Pyroprobe (insert); (c) air/nitrogen inlet flow (80 ml/min); (d) nitrogen gas flow inlet (400 ml/min); (e) static exhaust; (f) interface plate; (g) needle-corona discharge region.
inserting the Pyroprobe platinum coil probe into a glass tube fitted with positio~ng rings to ensure a uniform gas flow over the sample region during thermal treatments. The end of the Pyroprobe coil was positioned within the glass tube such that the end of the coil was about 1.0 cm from the corona discharge needle tip in the ion source. Fig. 1 shows a schematic of the sample probe and its relative position in the ion source. A SCIEX (Toronto, Canada) TAGA 63 6000 APCI triple quadrupole MS-MS system was operated in the MS mode for total and selected ion monitoring. Basic references for the instrumentation and methods are noted elsewhere [lO,ll]. Additionally, important selected parent ions generated in the thermal processing stage were selected for argon collision-induced dissociation (CID) and ex~nation was made of the dau~ter-ion spectra for structural information. A complete description of the TAGA operating conditions will be published elsewhere [12]. Each series of data was preceded by a reference run of RDX to verify full instrumental reproducibility limits.
min ?s-. . .. 82 85-m__--_.
0.1
2.0
I..
4.8
,
.
,
.
e.0
74
id
-.-..
.__
6.8
min
to 25O”C, 1 min hold. m/z 44, 46, 60, 74, 75, 82, 85, 98
.
CATALYST
44&
m--
.y. .le.0, . __._ &-.-.____------________ s.-.
RDX+B
Fig. 2. (a) Time-resolved analytical pyrolysis APCI MS-MS of RDX under air, 60°C/min are presented in the selected ion monitoring (SIM) mode (b) with borohydride additive.
(a)
RDX
==-I (b)
87 RESULTS AND DISCUSSION
RDX thermolysis Based on numerous runs under the experimental conditions noted to be important from an earlier Box-Behnken experimental design [13], the threshold temperature region for programmed thermolysis was chosen as 250°C. The most informative rate was determined to be 60” C/min and both inert (nitrogen) and oxidative (air) atmospheres were used in the study. Holding the sample at the 250°C final temperature for 1 min was also a selected parameter for the series based on comparative runs. Fig. 2 shows the time-resolved analytical pyrolysis results from thermally processing RDX under air at 60” C/mm to 250” C with a 1 min hold. Volatile products detected under these conditions are found at m/z 44, 46, 60, 74, 75, 82, 85, and 98 (Fig. 2). The earliest detected feature, m/z 46, is noted along with m/z 98 (both slightly under 8.0 min). Peaks at m/z 60, 74, 75, and 85 are detected in significant amounts just after 8.2 min, while only small levels of m/z 44 and 82 are seen. Fig. 3 shows the generation and decay profiles of these selected species when the programmed thermal treatment was conducted under nitrogen. A slight delay in appearance of m/z 46 was noted (ca. 8.2 min compared to ca. 7.8 min under air), as well as a steeper initial rate of evolution. Both atmospheres produced a similar rate of decay. Hence, a slight overall delay in volatile product evolution was noted under the inert atmosphere relative to the oxidative one. This is further supported by a comparison of the times for the total ion maxima: 9.2 min for nitrogen and 8.7 under air (Fig. 4). RDX thermolysis with borohydride additives The influence of added borohydride (“B catalyst”) on RDX thermolysis is also shown in Figs. 2-4. Evolution of m/z 46 (NO,) is not kinetically and/or thermodynamically favorable in the presence of B catalyst, relative to the amount detected with RDX alone under the same experimental conditions (Fig. 2). The amount of m/z 44 is somewhat increased, m/z 60 decreased, and m/z 74 is essentially unaffected by the presence of borohydride additive. Furthermore, m/z 75 and 85 are generated in a similar, rapid manner under programmed pyrolysis in both uncatalyzed and borohydride-catalyzed situations. However, a major change in the evolution of m/z 98 is detected when B catalyst is present (Fig. 2). The ring-intact amine oxide, identified by its MS-MS daughter-ion spectrum (Fig. 5) is detected as one of the first and major volatile thermal fragments in uncatalyzed RDX decomposition; however, it is low in abundance when B catalyst is present. Fig. 4 shows the total ion current (TIC) recorded of RDX alone and with borohydride (under
Fig. 3. Time-resolved analytical pyrolysis APCI MS-MS of RDX under nitrogen, 60” C/min to 250” C, 1 min hold. m/z 46, 60, 74, 75, 85, 98 are presented in the selected ion monitoring (SIM) mode. m/z 75, 82, 85, 98 from RDX under air are shown for comparison.
89 TOT&
ICN
&LX 6216891
9.2
TOT%
ION WX
TOT%
ICN Ft% 3299638
7968834
a.2
RDX+ B CATALYST
4.6
min
1
1
RDX+ B CATALYST
RDX+B CATALYST
60*/min
to 360°C, 1 m@ t&i
Fig, 4. Total ion monitoring of RDX alone (in nitrogen) and with borohydride additive (in nitrogen and in air). All at 60 o C/min to 250 o C, 1 min hold, except the lower spectrum, run at 60 o C/min to 360 o C, 1 min hold.
m/z
‘
75
98.0
7e.e
1
(
10.0
Fig. 5. Daughter-ion spectra of the parent m/z 60, 75, 85, and 98 species.
?
Ii!
60
16
a.0
I.
al
3B.e
40.0
de
*r 1
5-l
,
.
de
I
se.8
.
,
-
60.0
,
98
.
a1
Triorine
70.0
69
85
,
a&e
9 c
I
91
TOT%
IOH W
3172228
TOT&
ION IWX ZSS0361
RDX+ TaH2
1
1
1 1
TOT@_ ION t-RX4121158
1
a,
.
@.I
,
.
, 2.0
.
,
,
.
,
,
4.0 "'
Fig. 6. Total ion monitoring 250 o C, 1 min hold.
. min
of RDX
0.a
under
I..
10.0
air with inorganic
.
.
f 1 13.0
additives,
60° C/min
to
92
nitrogen). The latter shows a significant earlier time to maximum ion current (9.2 mm to 8.2 min). Thus, under programmed oxidative pyrolysis (60”C/min to 250” C, 1 min hold), the absence of both NO, and the ring-intact m/z 98, when the borohydride additive is mixed with RDX, are important experimental observations toward understanding its thermal degradation mechanism. To ensure that a true catalytic effect was being monitored rather than an artifact of the experimental processes, identical runs were conducted with non-boron inorganic additives. Both selected and total ion monitoring were recorded, and Fig. 6 shows the total ion current recorded for the TaH, and Ta,O, additives. Likewise, Ta,O, presence showed only minor variations
mln
min
Fig. 7. Time-resolved analytical pyrolysis APCI MS-MS of RDX under air, 60°C/min to 360 o C, 1 min hold. Comparison (without B additive) of m/z 75, 82, 85, and 98 profiles from RDX are shown from programmed run to 250 ’ C.
93
relative to that observed with RDX itself or with the TaH, additive, A duplicate TaH, run is shown for comparison. These data confirm that physical processes such as thermal transport and dilution effects are not dominant in these RDX decompositions. Rather, the borohydride additive shows genuine chemical interactions that change the kinetics and mechanism of RDX thermal decomposition. The nature of these changes is similar to those produced when RDX is subjected to a higher final temperature and faster heating rates. Fig. 7 shows the comparison of selected ion mo~to~ng (SIM) profiles for RDX heated at 60 * C/mm to 360°C with and without the B catalyst, with SIM for m/z 75, 82, 85, 98 shown for reference when programmed to 250 o C. Hence, catalytic activity is evidenced by a higher “effective” temperature produced by RDX when thermally processed with B catalyst. Volatile product identification As noted above, the key set of volatile products from RDX thermolysis was characterized by examination of their daughter-ion spectra. Fig. 5 shows the recorded MS-MS spectra of key RDX volatile products. Final confirmations are in progress of the daughter-ions while the interpretation of the CID fragmentation pattern of the m/z 98 product appears to be the triazine oxide, C,H,N,O (Fig. 8). Additionally, GC-FTIR data obtained in earlier studies support the triazine oxide structure [14]. A more definitive
NO2
thermolysis
HCN m/z 27
H ciN\cH *I O,N’
I
*
-3HN02
N\c/NINO, ‘42
RDX rnf.7
226
m/z
/
81
rn1.z
98
.[~-N=cH-N=CH-N~O~
m/z 97
(3) -HCN CH=N-CH,-N-0 1
H2C=N=0
-
CH+CH=N
v CHpN+
-t!-C=N
-
3.
mp
Fig. 8. Collision-induced spectrum for mass 98.
dissociation
44
HCN
m/z CH
m/z
27
H-C=N=O mfz
43
71
fragmentation
scheme. Interpretation
of the daughter-ion
94
characterization of the full thermal decomposition elsewhere [ 121.
of RDX will be reported
CONCLUSIONS
These experimental studies with an integrated analytical pyrolysis APCI MS-MS system have permitted a unique opportunity to monitor the detailed thermal degradation of RDX. The results may be interpreted towards better understanding the complex chemical interactions in the early stages of RDX thermolysis [13]. An understanding of the role of the physical state(s) (solid, melt, vapor) involved in the thermal region near the RDX melting point of 205” C has not been attempted in these studies. However, use of controlled thermal programming at a relatively slow (60 o C/mm) rate resulted in a time-resolved analysis of some of the major initial volatile thermal fragments evolved under oxidative and non-oxidative conditions. The role of borohydride additives is seen to be significant in the early stages of RDX decomposition and supports earlier work on their effects reported in larger-scale studies [15].
ACKNOWLEDGEMENT
We appreciate the critical comments and suggestions by Dr. R.A. Yost, University of Florida.
REFERENCES 1 R.A. Fifer, Chemistry of nitrate ester and nitramine propellants, in M. Summerfield and K. Kuo (Editors), Fundamentals of Combustion of Solid Propellants, Progress in Astronautics and Aeronautics Series, Vol. 90, American Institute of Aeronautics and Astronautics, New York, 1984, Ch. 4. 2 M.A. Schroeder, 19th Joint Army, Navy, NASA, Air Force (JANNAF) Combustion Mtg., Greenbelt, MD, October 1982, CPIA Publ. 366, Vol. I, CPIA, Laurel, MD, 1982. 3 J. Stals, Rev. Pure Appl. Chem., 20 (1970) 1768. 4 J.N. Bradley, A.K. Butler, W.D. Capey and J.R. Gilbert, J. Chem. Sot. Faraday Trans. 1, 73 (1977) 1789. 5 S.A. Liebman and E.J. Levy (Editors), Pyrolysis and GC in Polymer Analysis, Marcel Dekker, New York, 1985. 6 C.U. Morgan and R.A. Beyer, Combust. Flame, 36 (1979) 99. 7 M.D. Pace, J. Energetic Materials, in press. 8 M.D. Pace and B.S. Holmes, J. Magnetic Resonance, 52 (1983) 143. 9 M.D. Pace, A.D. Britt and W.B. Monitz, J. Energetic Mater., 1 (1983) 141 and 367. 10 H. Kambara, Y. Mitsui and I. Kanomata, Anal. Chem., 51 (1979) 1447.
95 11 D.I. Carroll, I. Dzidic, E.C. Horning and R.N. Stillwell, Appl. Spectrosc. Rev., 17 (1981) 337. 12 A.P. Snyder, J.H. Kremer, M.A. Schroeder, R.A. Fifer and S.A. Liebman, in preparation. 13 S.A. Liebman, P.J. Duff, K.D. Fickie, M.A. Schroeder and R.A. Fifer, J. Hazard. Mater., 13 (1986) 51. 14 P.J. Duff, S.A. Liebman and R.A. Fifer, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, 1985, paper No. 497. 15 R.A. Fifer and J.E. Cole, Catalysts for Nitramine Propellants, U.S. Patent 4,379,007, July 1980.
Journal of Analytical and Applied Pyrolysis, 12 (1987) 83-95 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
TIME-RESOLVED ANALYTICAL PYROLYSIS STUDIES OF NITRAMINE DECOMPOSITION WITH A TRIPLE QUADRUPOLE MASS SPECTROMETER SYSTEM
S.A. LIEBMAN
*
Geo-Centers, Inc., U.S. Army Chemical Research, Aberdeen Proving Ground, MD 21010-5423 (U.S.A.)
A.P. SNYDER,
J.H. KREMER
and Engineering
Center,
and D.J. REU’ITER
U.S. Army Chemical Research, Development MD 21010-5423 (U.S.A.)
M.A. SCHROEDER
Development
and Engineering
Center, Aberdeen Proving Ground,
and R.A. FIFER
Ballistic Research Laboratory, Aberdeen Proving Ground, MD 210055066
(U.S.A.)
ABSTRACT
An energetic nitramine, cyclotrimethylenetrinitramine (RDX), has been studied over selected thermal ranges and environments with a pyrolysis unit interfaced to an atmospheric pressure chemical ionization tandem mass spectrometer (MS-MS) system. Programmed and pulsed thermolytic degradation conditions under oxidative and non-oxidative atmospheres resulted in complex product distributions. Use of selected boron-containing compounds gave evidence of catalytic activity in the RDX degradation. Identification by daughter-ion MS-MS analysis was attempted for key volatiles of m/z 46, 60, 74, 75, 85, 97, 98, and 103. An integrated thermal degradation profile of RDX was thus probed by monitoring the volatile products as they were generated during slow programmed heating (60 o C/mm) from ambient to temperatures near the RDX melting/decomposition region (190-220 o C). Implications of enhanced nitramine performance are suggested.
Cyclotrimethylenetrinitramine;
mass spectrometry;
nitramine;
pyrolysis.
INTRODUCTION
Extensive literature [1,2], and in particular, 01652370/87/$03.50
is available for the study cyclotrimethylenetrinitramine 0 1987 Elsevier
Science Publishers
of nitramine degradation (RDX) [3,4]. However, B.V.
84
the early experimental work was conducted under a multiplicity of conditions that makes it difficult to delineate the fundamental stages in nitramine degradation. The present work was focused on careful thermal processing of small (milligram) amounts of RDX under a wide range of heating rates, final temperatures, dynamic/static atmospheres, and in the presence/absence of potential catalysts. The use of analytical pyrolysis has developed extensively over the past two decades [5] with time-resolved analyses being reported using on-line gas chromatography (GC), Fourier transform infrared (FTIR) and mass spectral systems. Real-time monitoring by the atmospheric pressure chemical ionization (APCI) tandem mass spectrometer (MS-MS) system permitted sensitive detection and, in several cases, distinct identification of the volatile species. A majority of studies have shown that RDX thermally decomposes to give NO,, formaldehyde, CO,, N,O, and several postulated ring-scission fragments, none of which have been satisfactorily identified. Whether the initial step is an N-NO, bond scission (leading to NO,) or C-N bond cleavage (leading to CH,O and N,O) has not been determined. Electron spin resonance (ESR) studies by Morgan and Beyer [6] and recent experiments at the Naval Research Laboratory have produced evidence [7-91 showing that definite free radicals were produced in early thermal decomposition, one of which was consistent with a spectrum of NO,. Other ESR patterns with complex hyperfine structure were also produced both in solution and solid state decomposition treatments.
EXPERIMENTAL
Samples of RDX were obtained by recrystallization from acetone, vacuum dried, and were ground to a fine powder (unsieved) of ca. 100 mesh. Boron-containing compounds were supplied from proprietary sources, Ta *05 from Aldrich Chemical Company, and TaH, from private sources. Mixtures were ca. 75% RDX physically mixed with ca. 25% additive. All runs used ca. l-2 mg powdered sample placed in the quartz tube of the Pyroprobe in the usual manner. Thermal processing of samples was conducted with a PyroprobeR Model 122 (Chemical Data Systems, Oxford, PA, U.S.A.). Many series of pulse pyrolyses were conducted, at the fastest rise-time (ca. 20 OC/ms) to varied final temperatures (from 200” C at several intervals to 1000” C). Programmed pyrolyses were conducted at controlled ramp rates of 60, 120, or 300”C/min to varied final temperatures in the above range. The most informative rate, 60” C/min, was used to compare product distributions under air or nitrogen atmospheres with varied additives. Interfacing the Pyroprobe to the MS-MS system was accomplished by
a-
e Fig. 1. Schematic of the Pyroprobe in the ion source of the atmospheric pressure chemical ionization (APCI) tandem mass spectrometer (MS-MS) TAGA 6000 system. (a) Pyroprobe; (b) sample positioned in a quartz tube in the coil of the Pyroprobe (insert); (c) air/nitrogen inlet flow (80 ml/min); (d) nitrogen gas flow inlet (400 ml/min); (e) static exhaust; (f) interface plate; (g) needle-corona discharge region.
inserting the Pyroprobe platinum coil probe into a glass tube fitted with positio~ng rings to ensure a uniform gas flow over the sample region during thermal treatments. The end of the Pyroprobe coil was positioned within the glass tube such that the end of the coil was about 1.0 cm from the corona discharge needle tip in the ion source. Fig. 1 shows a schematic of the sample probe and its relative position in the ion source. A SCIEX (Toronto, Canada) TAGA 63 6000 APCI triple quadrupole MS-MS system was operated in the MS mode for total and selected ion monitoring. Basic references for the instrumentation and methods are noted elsewhere [lO,ll]. Additionally, important selected parent ions generated in the thermal processing stage were selected for argon collision-induced dissociation (CID) and ex~nation was made of the dau~ter-ion spectra for structural information. A complete description of the TAGA operating conditions will be published elsewhere [12]. Each series of data was preceded by a reference run of RDX to verify full instrumental reproducibility limits.
min ?s-. . .. 82 85-m__--_.
0.1
2.0
I..
4.8
,
.
,
.
e.0
74
id
-.-..
.__
6.8
min
to 25O”C, 1 min hold. m/z 44, 46, 60, 74, 75, 82, 85, 98
.
CATALYST
44&
m--
.y. .le.0, . __._ &-.-.____------________ s.-.
RDX+B
Fig. 2. (a) Time-resolved analytical pyrolysis APCI MS-MS of RDX under air, 60°C/min are presented in the selected ion monitoring (SIM) mode (b) with borohydride additive.
(a)
RDX
==-I (b)
87 RESULTS AND DISCUSSION
RDX thermolysis Based on numerous runs under the experimental conditions noted to be important from an earlier Box-Behnken experimental design [13], the threshold temperature region for programmed thermolysis was chosen as 250°C. The most informative rate was determined to be 60” C/min and both inert (nitrogen) and oxidative (air) atmospheres were used in the study. Holding the sample at the 250°C final temperature for 1 min was also a selected parameter for the series based on comparative runs. Fig. 2 shows the time-resolved analytical pyrolysis results from thermally processing RDX under air at 60” C/mm to 250” C with a 1 min hold. Volatile products detected under these conditions are found at m/z 44, 46, 60, 74, 75, 82, 85, and 98 (Fig. 2). The earliest detected feature, m/z 46, is noted along with m/z 98 (both slightly under 8.0 min). Peaks at m/z 60, 74, 75, and 85 are detected in significant amounts just after 8.2 min, while only small levels of m/z 44 and 82 are seen. Fig. 3 shows the generation and decay profiles of these selected species when the programmed thermal treatment was conducted under nitrogen. A slight delay in appearance of m/z 46 was noted (ca. 8.2 min compared to ca. 7.8 min under air), as well as a steeper initial rate of evolution. Both atmospheres produced a similar rate of decay. Hence, a slight overall delay in volatile product evolution was noted under the inert atmosphere relative to the oxidative one. This is further supported by a comparison of the times for the total ion maxima: 9.2 min for nitrogen and 8.7 under air (Fig. 4). RDX thermolysis with borohydride additives The influence of added borohydride (“B catalyst”) on RDX thermolysis is also shown in Figs. 2-4. Evolution of m/z 46 (NO,) is not kinetically and/or thermodynamically favorable in the presence of B catalyst, relative to the amount detected with RDX alone under the same experimental conditions (Fig. 2). The amount of m/z 44 is somewhat increased, m/z 60 decreased, and m/z 74 is essentially unaffected by the presence of borohydride additive. Furthermore, m/z 75 and 85 are generated in a similar, rapid manner under programmed pyrolysis in both uncatalyzed and borohydride-catalyzed situations. However, a major change in the evolution of m/z 98 is detected when B catalyst is present (Fig. 2). The ring-intact amine oxide, identified by its MS-MS daughter-ion spectrum (Fig. 5) is detected as one of the first and major volatile thermal fragments in uncatalyzed RDX decomposition; however, it is low in abundance when B catalyst is present. Fig. 4 shows the total ion current (TIC) recorded of RDX alone and with borohydride (under
Fig. 3. Time-resolved analytical pyrolysis APCI MS-MS of RDX under nitrogen, 60” C/min to 250” C, 1 min hold. m/z 46, 60, 74, 75, 85, 98 are presented in the selected ion monitoring (SIM) mode. m/z 75, 82, 85, 98 from RDX under air are shown for comparison.
89 TOT&
ICN
&LX 6216891
9.2
TOT%
ION WX
TOT%
ICN Ft% 3299638
7968834
a.2
RDX+ B CATALYST
4.6
min
1
1
RDX+ B CATALYST
RDX+B CATALYST
60*/min
to 360°C, 1 m@ t&i
Fig, 4. Total ion monitoring of RDX alone (in nitrogen) and with borohydride additive (in nitrogen and in air). All at 60 o C/min to 250 o C, 1 min hold, except the lower spectrum, run at 60 o C/min to 360 o C, 1 min hold.
m/z
‘
75
98.0
7e.e
1
(
10.0
Fig. 5. Daughter-ion spectra of the parent m/z 60, 75, 85, and 98 species.
?
Ii!
60
16
a.0
I.
al
3B.e
40.0
de
*r 1
5-l
,
.
de
I
se.8
.
,
-
60.0
,
98
.
a1
Triorine
70.0
69
85
,
a&e
9 c
I
91
TOT%
IOH W
3172228
TOT&
ION IWX ZSS0361
RDX+ TaH2
1
1
1 1
TOT@_ ION t-RX4121158
1
a,
.
@.I
,
.
, 2.0
.
,
,
.
,
,
4.0 "'
Fig. 6. Total ion monitoring 250 o C, 1 min hold.
. min
of RDX
0.a
under
I..
10.0
air with inorganic
.
.
f 1 13.0
additives,
60° C/min
to
92
nitrogen). The latter shows a significant earlier time to maximum ion current (9.2 mm to 8.2 min). Thus, under programmed oxidative pyrolysis (60”C/min to 250” C, 1 min hold), the absence of both NO, and the ring-intact m/z 98, when the borohydride additive is mixed with RDX, are important experimental observations toward understanding its thermal degradation mechanism. To ensure that a true catalytic effect was being monitored rather than an artifact of the experimental processes, identical runs were conducted with non-boron inorganic additives. Both selected and total ion monitoring were recorded, and Fig. 6 shows the total ion current recorded for the TaH, and Ta,O, additives. Likewise, Ta,O, presence showed only minor variations
mln
min
Fig. 7. Time-resolved analytical pyrolysis APCI MS-MS of RDX under air, 60°C/min to 360 o C, 1 min hold. Comparison (without B additive) of m/z 75, 82, 85, and 98 profiles from RDX are shown from programmed run to 250 ’ C.
93
relative to that observed with RDX itself or with the TaH, additive, A duplicate TaH, run is shown for comparison. These data confirm that physical processes such as thermal transport and dilution effects are not dominant in these RDX decompositions. Rather, the borohydride additive shows genuine chemical interactions that change the kinetics and mechanism of RDX thermal decomposition. The nature of these changes is similar to those produced when RDX is subjected to a higher final temperature and faster heating rates. Fig. 7 shows the comparison of selected ion mo~to~ng (SIM) profiles for RDX heated at 60 * C/mm to 360°C with and without the B catalyst, with SIM for m/z 75, 82, 85, 98 shown for reference when programmed to 250 o C. Hence, catalytic activity is evidenced by a higher “effective” temperature produced by RDX when thermally processed with B catalyst. Volatile product identification As noted above, the key set of volatile products from RDX thermolysis was characterized by examination of their daughter-ion spectra. Fig. 5 shows the recorded MS-MS spectra of key RDX volatile products. Final confirmations are in progress of the daughter-ions while the interpretation of the CID fragmentation pattern of the m/z 98 product appears to be the triazine oxide, C,H,N,O (Fig. 8). Additionally, GC-FTIR data obtained in earlier studies support the triazine oxide structure [14]. A more definitive
NO2
thermolysis
HCN m/z 27
H ciN\cH *I O,N’
I
*
-3HN02
N\c/NINO, ‘42
RDX rnf.7
226
m/z
/
81
rn1.z
98
.[~-N=cH-N=CH-N~O~
m/z 97
(3) -HCN CH=N-CH,-N-0 1
H2C=N=0
-
CH+CH=N
v CHpN+
-t!-C=N
-
3.
mp
Fig. 8. Collision-induced spectrum for mass 98.
dissociation
44
HCN
m/z CH
m/z
27
H-C=N=O mfz
43
71
fragmentation
scheme. Interpretation
of the daughter-ion
94
characterization of the full thermal decomposition elsewhere [ 121.
of RDX will be reported
CONCLUSIONS
These experimental studies with an integrated analytical pyrolysis APCI MS-MS system have permitted a unique opportunity to monitor the detailed thermal degradation of RDX. The results may be interpreted towards better understanding the complex chemical interactions in the early stages of RDX thermolysis [13]. An understanding of the role of the physical state(s) (solid, melt, vapor) involved in the thermal region near the RDX melting point of 205” C has not been attempted in these studies. However, use of controlled thermal programming at a relatively slow (60 o C/mm) rate resulted in a time-resolved analysis of some of the major initial volatile thermal fragments evolved under oxidative and non-oxidative conditions. The role of borohydride additives is seen to be significant in the early stages of RDX decomposition and supports earlier work on their effects reported in larger-scale studies [15].
ACKNOWLEDGEMENT
We appreciate the critical comments and suggestions by Dr. R.A. Yost, University of Florida.
REFERENCES 1 R.A. Fifer, Chemistry of nitrate ester and nitramine propellants, in M. Summerfield and K. Kuo (Editors), Fundamentals of Combustion of Solid Propellants, Progress in Astronautics and Aeronautics Series, Vol. 90, American Institute of Aeronautics and Astronautics, New York, 1984, Ch. 4. 2 M.A. Schroeder, 19th Joint Army, Navy, NASA, Air Force (JANNAF) Combustion Mtg., Greenbelt, MD, October 1982, CPIA Publ. 366, Vol. I, CPIA, Laurel, MD, 1982. 3 J. Stals, Rev. Pure Appl. Chem., 20 (1970) 1768. 4 J.N. Bradley, A.K. Butler, W.D. Capey and J.R. Gilbert, J. Chem. Sot. Faraday Trans. 1, 73 (1977) 1789. 5 S.A. Liebman and E.J. Levy (Editors), Pyrolysis and GC in Polymer Analysis, Marcel Dekker, New York, 1985. 6 C.U. Morgan and R.A. Beyer, Combust. Flame, 36 (1979) 99. 7 M.D. Pace, J. Energetic Materials, in press. 8 M.D. Pace and B.S. Holmes, J. Magnetic Resonance, 52 (1983) 143. 9 M.D. Pace, A.D. Britt and W.B. Monitz, J. Energetic Mater., 1 (1983) 141 and 367. 10 H. Kambara, Y. Mitsui and I. Kanomata, Anal. Chem., 51 (1979) 1447.
95 11 D.I. Carroll, I. Dzidic, E.C. Horning and R.N. Stillwell, Appl. Spectrosc. Rev., 17 (1981) 337. 12 A.P. Snyder, J.H. Kremer, M.A. Schroeder, R.A. Fifer and S.A. Liebman, in preparation. 13 S.A. Liebman, P.J. Duff, K.D. Fickie, M.A. Schroeder and R.A. Fifer, J. Hazard. Mater., 13 (1986) 51. 14 P.J. Duff, S.A. Liebman and R.A. Fifer, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, 1985, paper No. 497. 15 R.A. Fifer and J.E. Cole, Catalysts for Nitramine Propellants, U.S. Patent 4,379,007, July 1980.