4367
THERMAL DECOMPOSITION OF METHYL-SUBSTITUTED AMINE-TYPE PERCHLORATES
The Study of the Thermal Decomposition of Methyl-Substituted Amine-Type Perchlorates by William A. Guillory and Morgan King Nacal Ordnance Research, Naval Ordnance Station, I n d i a n Head, Maryland
80640
(Received M a y 86, 1969)
The thermal decompositions of the methyl-, dimethyl-, and trimethylammoniumperchlorates have been studied Over the temperature range 130-320" by use of a Bendix time-of-flight mass spectrometer. All three methylsubstituted amine salts decompose principally via (CHs).NH4-.C104 + (CH,), NHa-. HC104, in addition to some heterogeneous decomposition. Perchloric acid, formed upoiz dissociation, appears to undergo limited decomposition to C102 and HCl.
+
Introduction The thermal decomposition of ammonium perchlorate (AP) has been exhaustively studied.' Over the temperature range from 130 to 200", the salt undergoes simple dissociation into NH3 and HC104, presumably by a proton transfer process. However, above 200" other modes of decomposition occur which do not yield HClOd as a final product. It has been suggested by Mack and Wilmot2 that an approximate proportional relationship might exist between the relative vaporization temperatures of amine salts and the pK, values of their corresponding bases in aqueous solution. Therefore, the substitution of methyl groups for hydrogen atoms in the ammonium cation should lead to higher dissociation temperatures because of the stabilizing effect of methyl groups with respect to the proton transfer mechanism. As a consequence, reactions of the evaporated products from simple dissociation could be studied under Knudsen conditions at higher temperatures; of particular interest are reactions involving HC104. This study revealed that such a proportional relationship as suggested above does exist in the case of the methylsubstituted amine-type perchlorates. In addition, HClO4 produced as an evaporated product remains relatively stable up to 320".
Experimental Section Materials. The various methyl-substituted amine salts were prepared by titrating aqueous solutions of the methyl-substituted amines against a slightly diluted solution of 70 wt % HC104 using a methyl red indicator. The water was removed by liquid nitrogen cryo-pumping for 2 days. The methyl-substituted amine salts were recrystallized twice with a hot ethanol solution and a few drops of HzO. The salts were then filtered and placed in a 60" oven for 2 hr. The 70 mt % HClOd was obtained from Allied Chemical. Prior to each HClOd run, the 70 wt % HC104 sample was pumped
until the mass spectrum revealed peaks due solely to HCIOl and no HzOfrom the sample. Mass Spectrometric Method. The mass spectral observations were performed with a Bendix time-offlight mass spectrometer (Model 12-101) with the modified 5-12-101 source in conjunction with a heater inlet system. The heater inlet system consisted of a 2-in. diameter copper tube soldered to a stainless steel flange that adapted directly to the bottom of the source cross. The Pyrex glass sample tubes were mounted axially to the inlet system about 6 cm below the entrance slit. Further details of the experimental setup have been described previ~usly.~The inlet system used for the comparison spectrum of HC104was all glass with two Teflon stopcocks in series for pressure monitoring. The delivery tube was attached to a flange that was mounted to the bottom of the source cross such that the sample came into contact with only glass and Teflon.
Results Methylammonium Perchlorate ( M A P ) . The thermal decomposition of MAP under free-evaporation conditions was studied over the temperature range 150 to 257". New features in the spectrum appeared at approximately 150". The prominent features are indicative of two major products, HC104 and CH3n"2.4 Other products formed in small quantities were NzO, HzO, and, possibly, OZand NZ(Figure 1). However, one striking feature of the spectrum was the apparent change in the cracking pattern of HC104as the temperature was increased. The peaks due to the 67-69 isotopic pair were dominant below approximately 176", Above 176", there was a reversal in the relative (1) A. R. Hall and G. S. Pearson, "A Review of Ammonium Perchlorate Combustion," Ministry of Technology, London, Jan 1967. (2) J. L.Mack and G. B. Wilmot, J. Phys. Chem., 71,2155 (1967). (3) W. A. Guillory, J. L.Mack, and AM.King, ibid., 73,4370 (1969). (4) "American Petroleum Institute Research Project 44,Mass Spectral Data," Serial No. 1123. Volume 73, Number 18 December 1969
4368
WILLIAM A. GUILLORY AND MORGAN KING
lool--
I
1 a5
102 100
a3
69 61
I
53
44
51
37 35 !9 27 38 36 32 30 20 26
14
18
I 7 15
12
Mass Numbers
Figure 1. &lass spectrum of thermally dissociated methylammonium perchlorate a t 230” [free-evaporation experiment].
peak heights of the 67-69 and 83-85 isotopic pairs, such that the 83-85 pair became the dominant peaks in the isotopic pattern (reversal effect). The peaks then remained proportional as the temperature was further increased, The true HC104 cracking pattern was shown to be the one observed at high temperatures as proven by a comparison spectrum of KC104 alone (Figure 2). Dimethylammonium Perchlorate (DMAP). The thermal decomposition of DMAP under free evaporation and Knudsen conditions was studied over the temperature range from 165 to 290”. The resulting spectra showed little difference between the two conditions. The products spect,rum at 190” is shown in Figure 3. The major features observed are those due to HC104, (CH@H,5 a chlorine-oxygen species, and their degradation products. There is also production of some HzO, Oz, Xz,HCl, NOz, and, possibly, 11~0. When comparing the spectrum of HClO4 with the DMAP chlorine isotopic peaks, one notices that the 67-69 and 51-53 pairs are not proportional to the HC104 parent peak (100-102). These two pairs 100
i
I 69
Mass Numbers
Figure 2.
Mass spectrum of perchloric acid.
The Journal of Physical Chemistrg
remained greater than their sole contribution from HC104 over the entire temperature range, Trimethylammonium Perchlorate ( T M A P ) . The thermal decomposition of solid TMAP was studied in both a free-evaporation and a Knudsen cell over the temperature range 250-320’. The products spectrum is shown in Figure 4. The major features observed are and their degradation those due to HC104, (CHB)BN,~ products. In addition, some HC1, NzO, Oz, Nz, and HzO were formed. The Knudsen experiments revealed that there was no tendency toward further gas-phase reactions between the evaporated products. Again, the so-called “reversal effect” was observed.
Discussion All three of the methyl-substituted amine salts appear to decompose principally to the corresponding methylsubstituted amine and HCIOd via (CH,).NH4-,C104
+ (CH*).NH3,,
+ HC104
(1)
where n ranges from 1 to 3. In each case there is an additional mode of decomposition giving rise to small contributions of HC1, NOz, HzO, 02,Nz, and some NzO. The substitution of methyl groups into the cation does appear to stabilize these perchlorate salts. This stability is indicated by the higher dissociation temperatures as the number of methyl groups increases. This observation supports the suggestion that one should observe this trend where decomposition occurs via proton transfer. The Knudsen experiments revealed that HC104 and the various methyl-substituted amines are relatively stable with respect to each other in the gas phase over the temperature ranges investigated. The most striking feature in the spectra of the vaporized salts was the so-called “reversal effect” in the (6) See ref 4,Serial No, 1124. (6) See ref 4,Serial No, 1127.
4369
THERMAL DECOMPOSITION OF METHYL-SUBSTITUTED AMINE-TYPE PERCHLORATES
-
I
I
I Mass Humhers
Figure 3.
Mass epectrum of thermally dissociated dimethylammonium perchlorate a t 190’ [free-evaporation experiment]
cracking pattern of HC104. Upon closer examination of the isotopic peaks, it can be shown that this effect is due to the production of ClOz. This species has also been observed by Pellet and Saundersl upon laser flashing an AP sample and in the thermal decomposition of NH&104.3 Since this effect is observed in all of these perchlorate systems, it would appear to arise from a common component, namely HC104. Apparently, HC1 and CIOzproduction results from the heterogeneous
I
~
decomposition of HC104 on the salt surface as suggested by Pellet and Saunders.l
Conclusion The thermal decompositions of methyl-,dimethyl-, and trimethylammonium perchlorates occur principally by dissociation into perchloric acid and the corresponding methyl-substituted amine. The dissociation temperatures increase with increasing methyl substitution into the ammonium cation. Experiments with a Knudsen cell revealed that practically no reactions occur between the evaporated products. Perchloric acid appears to undergo limited decomposition in all cases leading to HC1 and ClOz production.
Acknowledgments. This work funded under the Foundation Research Program of the Naval Ordnance Systems Command. The authors wish to express their appreciation to Dr. George Wilmot and Dr. Julius Mack, who read the manuscript and made helpful suggestions. 102 100
85 83
69 61
60 58 5351
59
44 42 31 35 30 28 3836 32
I8 1614 12 1115
”0
Mass Numhsrs
Figure 4. Mass spectrum of thermally dissociated trimethylammonium perchlorate a t 300’ [Knudsen experiment].
(7) G. L. Pellet and A. R. Saunders, “Heterogeneous Decomposition of Ammonium Perchlorate-Catapult Mixtures Using Pulsed Laser Mass Spectrometry,” presented at the AIAA Sixth Aerospace Sciences Meeting (Jan 1967),NASA Langely Research Center, Hampton, Va.
Volume 73,Number 18 December 1060
