3168
Synergistic Catalysis of Ammonium Nitrate Decomposition' A. G . Keenan, K. Notz, and N. B. Franco Contribution f r o m the Department of Chemistry, University of Miami, Coral Gables, Florida 33124. Received November 22, 1968 Abstract: Chloride and various transition metal ions, notably chromium and copper, exhibit an unexpectedly large synergistic effect in the catalysis of the thermal decomposition of ammonium nitrate. Mechanisms are proposed for these reactions. In the case of copper and chloride, the proposed mechanism involves the operation of redox cycles in which reactions occur by electron transfer in chloro-bridged activated states. NH4+and NH3are oxidized to NH3+ and NHz, and NHzCl is produced by reaction of NH2. The original cupric catalysts are then regenerated through oxidation by NOZ+and NO+. The proposed mechanism for chromium catalysis depends on the participation of dichromate in a fused-salt acid-base reaction with nitrate in which nitronium and chromate ions are formed. Both mechanisms produce intermediates which also occur in the mechanism proposed in previous publications for the simple chloride-catalyzed decomposition of ammonium nitrate. A survey of the effects of 23 metals indicates that synergistic catalysis with chloride generally requires a metal capable of forming chloro complexes of reasonable stability in two oxidation states differing by one electron unit. Chromium is a special case in which oxo complexes are believed to be involved.
T
he pronounced catalysis of the thermal decomposiappears to be of such complexity, however, that despite tion of ammonium nitrate by chloride i ~ n and ~ - ~ the amount of quantitative data which has been obtained, only qualitative statements have been made by transition metals6-" is well known. A mechanism relating the probable mechanism to redox cycles which has been proposed3 for the chloride catalysis which depend on the facility of chromium and nitrogen for contains two cycles in which chloride ions are oxidized existing in multiple oxidation states. to atoms by NOz+ and NO+ (eq 1 and4) and reduced In the course of recent electrometric measureback to ions by NH,+ and NH3 (eq 2 and 5). Radical ments'?, l 3 using a silver-ion frit electrode in chloriderecombinations in the reaction cages produce nitramide containing ammonium nitrate melts, it was observed (eq 3) and nitrosamine (eq 6), which are the unstable precursors for the products Nz, NzO, and HzO. The that leakage of silver ion caused a synergistic catalysis which was orders of magnitude greater than the sum NOz+ + CI- +NO2 + CI (1) of the catalytic effects to be expected from silver and (2) CI + NHnC +NH3+ + HC1 chloride ions individually. Further study has shown fast this synergistic catalysis to be characteristic of many NOz + NH3+ +[OzNNH3+]+NzO H30+ (3) other metal ions.
+ CI- +NO + C1 NH3 + CI +NH, + HC1
+
(4)
NO+
fast
NH2
+ NO +[HzNNO] --+
Nz
(5)
+ HBO
(6)
NOz+ comes from the ionic self-dehydration of HN03 produced by dissociation of molten ammonium nitrate. NO+ is produced by the disproportionation of NOZ. A third cycle in the mechanism involves chlorine atom recombination and production of Nz from a chloramine precursor (eq 7). This mechanism has received support NH,
fast + Clz +[NHKI] + NB + HCI
(7)
from mass spectrometric4 and other5 studies. Among the transition metals, chromium compounds are outstanding in their catalytic power. The reaction (1) This work was supported b y the Office of Naval Research, Power Program, under Contract Nonr-4008(07), and comprises parts of the Ph.D. Dissertations of K. Notz and N. B. Franco. (2) A . G. Keenan, J . Amer. Chem. SOC.,77, 1379 (1955). (3) A . G. Keenan and B. Dimitriades, J . Chem. Phys., 37, 1583 (1962). ( 4 ) C. I. Colvin, A . G. Keenan, and J. B. Hunt, ibid., 38, 3033 (1963). (5) C. I. Colvin, P. W. Fearnow, and A . G. I III(CH~)~ >> Zn(CH&. The activation energies for these systems are 7.8 0.8, 8.4 i= 0.2, and 17.0 i= 1.0 kcal/mole, respectively. All kinetic parameters have been obtained by complete line-shape analysis of the nmr spectra by use of modifications of the McConneXl equations to simulate theoretical spectra. An attempt is made to correlate the rates of exchange to the Lewis acidity of the organometallic species.
*
N
uclear magnetic resonance techniques have made possible the study of exchange of alkyl groups in organometallic systems. In the past, however, little quantitative work has been done involving organometallic derivatives of groups 11 and 111. Much of the effort of group I11 alkyl exchange has been placed on the study of reactions involving aluminum compounds. It has been reported by Brown and Williams3 that the rate of methyl group exchange between A ~ z ( C Hand ~ ) ~Ga(CH3)3is determined by dissociation of the trimethylaluminum dimer, with an activation energy of 16.5 kcal/mole. The activated monomer of Al(CH3)3 then reacts with a molecule of Ga(CH3)3, thereby effecting exchange. Similar findings were reported for the A12(CH3)6-In(CH 3)3system. This dissociation process is found only in the alkylaluminum series since these compounds are unique among the saturated derivatives of group I11 in that they exist as dimers in s ~ l u t i o n . ~It appears, however, that other compounds of groups I1 and I11 can assume fourcentered structures as intermediates for alkyl exchange and that the rate of formation of such an intermediate would determine the rate of alkyl exchange. It has been found that when two group I11 compounds are involved in the exchange studies, the rate of exchange is very fast. For example, self-exchange of methyl groups between molecules of T1(CH3)3proceeds in a bimolecular fashion with an activation energy of 6.3 k ~ a l / m o l e . ~ The rate of exchange of methyl groups between Ga(1) Presented in part by K. L. Henold, J. Soulati, and J. P. Oliver at the 156th National Meeting of the American Chemical Society, Atlantic City N . J., Sept 1968. (2) Recipient of a NASA Traineeship, 1966-1969. (3) K. C. Williams and T. L. Brown, J. Am. Chem. SOC.,88, 4134 (1966). (4) Unsaturated derivatives of other group I11 metals have been proven dimeric in solution: J. P. Oliver and L. G. Stevens, J. Inorg. Nucl. Chem., 24, 953 (1962); E. A. Jeffery and T,Mole, J . OrganometaI. Chem., 11, 393 (1968). (5) J. P. Maher and D. F. Evans, J . Chem. SOC.,5534 (1963).
(CH3)3and In(CH3)3is so fast on the nmr time scale that only one resonance is observed at - 70°.3v6 Exchange reactions of group I1 alkyls seem to be much slower. Self-exchange does not occur between Cd(CH3)Z molecules in the absence of catalytic impurities,',* and neither Cd(CH3)2 nor ZII(CH~)~ exchanges with Hg(CH3)2 on the nmr time scale.Q Exchange has been observed between Zn(CH3)2and Cd(CH3)2.l0 The results, obtained using simplifying assumptions for the cadmium spectra, indicated a bimolecular reaction very much slower than the group I11 reactions. These studies, along with the earlier investigations of McCoy and Allred, lo show that alkyl exchange between compounds of group I11 and those of group I1 to be intermediate between the extremes mentioned above. In this work, we have studied the reactions of three organometallic compounds, Ga(CH3)3, ~ I ( C H ~ )and ~, Z ~ I ( C H ~ )with ~ , one reference compound, Cd(CH3)2. This has been done in order to study the effect of the various compounds on the formation of the bridged activated species which is reflected in the rates of exchange. Experimental Section Ga(CH&, In(CH&,l 2 Cd(CHa)2,13and Zn(CH3),14 were all
prepared by previously reported methods. All nmr samples in this study were made by standard high vacuum techniques in 5-mm 0.d. sample tubes with either dichloromethane (6) H. D. Visser and J. P. Oliver, unpublished results. (7) N . S. Ham, E. A . Jeffery, T. Mole, J. K. Saunders, and S. N . Stuart, J . Organometal. Chem., 8 . 7 (1967). (8) J. Soulati and J. P. Oliver, unpublished results. (9) R. E. Dessy, F. Kaplan, G. R. Coe, and R. M. Salinger, J . Am. Chem. Soc., 85, 1191 (1963). (10) C. R. McCoy and A. L. Allred, ibid., 84,912 (1962). (11) L. M. Dennis and W. Panode, ibid., 54, 182 (1932). (12) L. M. Dennis, R. W. Work, and E. G . Rochow, ibid., 56, 1047 (1934). (13) H. Gilman and J. F. Nelson, Rec. Trao. Chim., 55, 518 (1936). (14) N . K. Hota and C. J. Willis, J . Organometal. Chem., 9, 171 ( 1967).
Henold, Soulati, Oliver J Alkyl Exchange Reactions with Organometallic Derivatives
