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Thermal stability of long-chain aliphatic amine salts
J. inorg,nucl.Chem.1969.Vol.3I. pp. 205 to 21 I. PersamonPress. Printedin Great Britain
THERMAL
STABILITY
OF
LONG-CHAIN
ALIPHATIC
A MIN E SALTS H. G U T M A N N and A. S. KERTES Department of I norganic and Analytical C hemi stry, The Hebrew University, Jerusalem, Israel
(Received 18June 1968) Abstraet-Thermogravimetry and differential thermal analysis have been applied to the determination of the stability of chlorides, nitrates and bisulfates of some long-chain (at least eight carbon atoms per chain) primary, secondary and tertiary normal aliphatic amines. All compounds investigated are thermally stable at least up to their melting point. The decomposition is exothermal in all cases, and the weight-loss corresponds to a total vaporization of the salts. Some of the salts are suMciently stable for use as metal extractants from molten salt media. INTRODUCTION
LONG-CHAIN alkylammonium salts have been of considerable interest because of their usefulness as extractants for metal salts from aqueous solutions. Solvent extraction from molten salt media has recently been extended[l] to include such amine salt-extractants, but previous studies[2-4] were concerned primarily with the extractive properties of the salts and the distribution of anionic metal complexes between molten alkali nitrates and a high-boiling point organic solvent, as the diluent for the amine salt. The present study has been undertaken to determine the conditions under which long-chain alkylammonium salts can be used as extractants at higher temperatures. By obtaining thermal stability data one can ascertain what permissible temperature range can be employed and thereby avoid thermal decomposition of the amine salt and the resulting affects on its extractive properties. EXPERIMENTAL Procedures used for the preparation and purification of solid primary, secondary and tertiary amine salts have been described elsewhere[5, 6] The anhydrous, white salts were crystalline, non-hygroscopic and stable, at least up to their melting points. They were checked for purity by elemental analysis, argentometric or acidimetric titrations of their alcoholic solutions, and were identified by their infrared spectra[6]. None of the salts contained either free acid or base. Differential thermal analysis (DTA) was carried out in air at atmospheric pressure with a Stone DTA Model K A - 2 H equipment. Salt samples were diluted to 20 per cent using ignited c~-alumina. The temperature readings were reliable to within +_ 5 per cent, and a heating rate of 3°C/rain was employed. 1. Y. Marcus, In Solvent Extraction Chemistry (Edited by D. Dyrssen, J. O. Liljenzin and J. Rydberg), p. 556. North-Holland, Amsterdam (1967). 2. Z. Borkowska, M. Mielcarsky and M. Taube,J. inorg, nucl. Chem. 26,359 (1964). 3. I.J. Gal, J. Mendez and J. W. lrvine, Jr., lnorg. Chem. 7,985 (1968). 4. Y. David, M. Zangen and A. S. Kertes, Proc. 37th Ann. Meeting, Israel Chem. Sot., October 1967. 5. A. S. Kertes, J. inorg, nucl. Chem. 27, 209 (1965). 6. A. S. Kertes, H. Gutmann, O. Levy and G. Markovits, lsraeIJ. Chem. 6, 421 (1968). 205
206
H. G U T M A N N and A. S. KERTES
The thermogravimetric data reported were also obtained in air, at atmospheric pressure, using an automatic recording A.D.A.M.E.L., Paris, thermobaiance. The sample size was maintained nearly constant at about 50 mg. A programmed heating rate of 6°C/min was used. Porcelain glazed crucibles were used. Preliminary experiments showed the solvent, used during recrystallization of the salts to be present in an air-dried salt unless it was dried in vacuo over P~Os. All samples were mortar-ground. The samples completely decomposed to gaseous products at about 550°C and no carbonaceous material remained in the crucible at the end of the runs. RESULTS AND DISCUSSION
Typical TG curves obtained for amine chlorides, nitrates and bisulfates are plotted in Fig. 1. Irrespective of the amine class and the chain-length, the curves are quite similar in appearance for any given anion, as shown, for example, in Figs. 2 and 3 for the ammonium chlorides. While all curves, regardless of the anion, approach a 100 per cent weight loss, none of the curves is smooth. The decomposition of chlorides and bisulfates occurs in two distinct steps, that of nitrates in three, again irrespective of the class and chain-length of the amine from which the salts are derived. Decomposition temperatures, observed weight losses and the molecular weights of the decomposition products for the salts investigated are compiled in Table 1. The different melting points obtained by the capillary method and by deduction from D T A curves are probably due to different modifications of the salts. Similar quaternary alkyl ammonium salts have been observed to form polymorphs [7]. The higher melting point-modification is usually obtained when crystallizing the salt from solutions, whereas the lower melting point-modification 500 T °C
B
400 m
300
m
300
2 O0 ~2101K200
100
0 ~
I0
20
30
40
50
mg Fig. 1. Thermogravimetric curves of di-n-decylammonium salts: 1 -Chloride. 2 - Nitrate and 3 - Bisulfate. 7. J. E. Gordon, J.Am. chem. Soc. 89, 4347 (1965).
Thermal stabilityof long-chainaliphaticaminesalts
207
500
T't;
. 4 6 0 "C
450
-
400
--
350
--
300
-
• 4 5 0 *C
-420
3
2 8 0 *C
/t . ~ . .
250
*C
.
.
.
.
~J
2eo
"c
-
200
2C -
zos "c 180 "C
150
-
I00
--
50-
I
I
I0
ZO
I..... 30
I 40
mg Fig. 2. Thermogravimetriccurves of various alkylammoniumchlorides: 1-n-dodecyl-,
50
2-Di-n-dodecyl-. 3 - Tri-n-dodecyl-. is formed, often spontaneously, on slow-heating the solid of higher melting point. Decomposition of amine chlorides and bisulfates. The decomposition occurs in two steps, as shown on the thermogram given in Fig. 4. In agreement with the TG results, the first is endothermic and should be attributed to the melting of the salt, while the second, a doublet, is exothermic and corresponds to the thermal decomposition of the salt. Examination of Table l leads to the conclusion that the first exothermic step is the most important; about 90 per cent of the sample leaves the system. It is a mixture of olefins, nitrous oxide and the inorganic acid which became volatile at the melting point temperature or slightly above it. The "average molecular weight" of the mixture of gases leaving the system increases by about 56 units in going from di-n-octyl to di-n-decyl to di-n-dodecylamine salt, corresponding to the formula weight of C4H8. The substance which disappears during the second volatilization step (> 280°C) is carbon. The formula weight of the substance is roughly a multiple of 12. Decomposition of amine nitrates. Thermal decomposition of nitrates involves three stages. Whereas in the thermal decomposition of amine chlorides and
208
H. G U T M A N N and A. S. KERTES
500
4001--
300
2 --
~
i 200
"
3
Z8
Yii
50
~'off . , e o m
moo --
0
I
I
I
I
I0
20
30
40
mg
50
Fig. 3. Thermogravimetric curves of various secondary alkyl-ammonium chlorides: 1 - Di-n-octyl-. 2 - Di-n-decyl-. 3 - Di-n-dodecyl-.
/ 2
Exothermal~
/
\
Endothermal I 150
I
I 250
I
I 350
I
I 450
TeC
Fig. 4. Differential thermogram of di-n-dodecylammoniumchloride (1) and bisulfate (2).
bisulfates the volatilization of the acid (HCI or H2SO4) occurs simultaneously with the decomposition of the aliphatic chain, in the case of amine nitrates, the decomposition and subsequent volatilization of the acid radical appears to take place at lower temperatures than that of the alkyl chain.
Thermal stability o f long-chain aliphatic amine salts
209
e~ '¢q
I,M
E .r. E
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m_ e~
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c o
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!iiiii!i!
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~ r,-¢¢~ ¢ q
¢q
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~ ¢q
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,--
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.-2
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210
H. G U T M A N N and A. S. KERTES
It has been noted[8] that the thermal decomposition of ammonium nitrate is a complex reaction. That of alkylammonium nitrates is apparently no less complex. Analysing the numerical data in Table 1, it becomes apparent that NO~ is evolved during the first decomposition step. The residue formed is either the corresponding dialkyl hydroxylamine or the amine oxide, the difference in the weight loss between the two ( - 0.3 per cent) being within the experimental error of our measurements. D T A curves (Fig. 5) show two exothermic peaks in this respect. These, we believe, could be attributed to the subsequent volatilization of NO2
Exothermol
i
./
Endothermal 170*C Fig. 5. Differential thermogram of di-n-dodecylammonium nitrate.
and H2. It is still impossible to decide whether the residue is a N,N-dialkylhydroxyl-amine R2NOH or an amine oxide, R~HNO, since it is known that several types of rearrangements and cleavage reactions may take place on heating such and similar compounds[9-11]. Hydroxyl-amines of the type considered here can either be oxidized by air to nitroxide radicals, R2N---O, or may undergo auto-oxidation, yielding a variety of nitrogenous products, depending on the conditions. The first decomposition product formed here is surprisingly stable. The second and third steps of decomposition of amine nitrates parallel the two steps occurring in the thermograms of alkylamine chlorides and bisulfates. The D T A curves are similar, exhibiting two exothermic peaks, in addition to the two mentioned above, corresponding to the second and third steps of weight 8. S. Gordon and C. Campbell, Analyt. Chem. 27, 1102 (1955); A. G. Keenan, J. Am. chem. Soc. 77, 1379 (1955); J. Jaffray, J. Res. CNRS 153 (1947); H. G. McAdie, Analyt. Chem. 35, 1840 (1963). 9. A. C. Cope, T. T. Foster and P. H. Towle,J.Am. chem. Soc. 71, 3929 (1949). 10. P. A. S. Smith, Open-Chain Nitrogen Compounds, Vol. 2, p. 2 I. Benjamin, New York (! 966). 11. H. Goldwhite, In Chemistry of Carbon Compounds (Edited by S. Coffey), Vol. I B, p. 93. Elsevier, Amsterdam (I 964).
Thermal stability of long-chain aliphatic amine salts
211
loss in the TG curve. Again the formula weight of the substance in the third step is a multiple of 12. Effect of time. Because considerable time is frequently required to establish equilibrium during the extraction of metals from molten salt media[12] it was desirable to examine the stability of alkylammonium salts as a function of time at temperatures below those at which complete decomposition occurs. Samples were studied at constant temperature using DTA. At 200°C di-n-dodecylammonium chloride shows a vertical line, indicating that the compound did not decompose in any manner for at least 3 hr (the maximum time of our experiments*). There was still no evidence of any decomposition at 230°C. The same is true for di-n-dodecylammonium nitrate at 180 and 200°C, and for the bisulfate at 180,200 and 230°C. CONCLUSIONS
Long-chain (> 8 carbon atoms per chain) normal alkylammonium chlorides, nitrates and bisulfates are thermally stable at temperatures not exceeding those of their melting point. As a rule the thermal stability of long-chain alkylammonium salts parallels their melting points. There is a decrease in thermal stability with increase in the length of the aliphatic chain. Secondary amine chlorides are less volatile than primary, and much less volatile than the tertiary amine chlorides, with equal numbers of carbon atoms per chain. The same may be suspected to be true for the tertiary amine nitrates and bisulfates.~ The stability of chlorides is generally higher than that of either bisulfates or nitrates with an identical cationic part of the molecule. Secondary alkylammonium salts possess thermal properties useful in metal extraction from molten salt media. *Preliminary experiments[4] on extraction of lanthanides at 150°C from molten potassiumlithium nitrate eutectic mixture using di-decylammonium nitrate dissolved in tetraline, indicate that equilibrium is established in 1 hr. tThe melting point of tri-n-dodecylamine nitrate is 51-52 °, and that of the bisulfate is 64-65°C [6]. 12. J. David, Unpublished results.
THERMAL
STABILITY
OF
LONG-CHAIN
ALIPHATIC
A MIN E SALTS H. G U T M A N N and A. S. KERTES Department of I norganic and Analytical C hemi stry, The Hebrew University, Jerusalem, Israel
(Received 18June 1968) Abstraet-Thermogravimetry and differential thermal analysis have been applied to the determination of the stability of chlorides, nitrates and bisulfates of some long-chain (at least eight carbon atoms per chain) primary, secondary and tertiary normal aliphatic amines. All compounds investigated are thermally stable at least up to their melting point. The decomposition is exothermal in all cases, and the weight-loss corresponds to a total vaporization of the salts. Some of the salts are suMciently stable for use as metal extractants from molten salt media. INTRODUCTION
LONG-CHAIN alkylammonium salts have been of considerable interest because of their usefulness as extractants for metal salts from aqueous solutions. Solvent extraction from molten salt media has recently been extended[l] to include such amine salt-extractants, but previous studies[2-4] were concerned primarily with the extractive properties of the salts and the distribution of anionic metal complexes between molten alkali nitrates and a high-boiling point organic solvent, as the diluent for the amine salt. The present study has been undertaken to determine the conditions under which long-chain alkylammonium salts can be used as extractants at higher temperatures. By obtaining thermal stability data one can ascertain what permissible temperature range can be employed and thereby avoid thermal decomposition of the amine salt and the resulting affects on its extractive properties. EXPERIMENTAL Procedures used for the preparation and purification of solid primary, secondary and tertiary amine salts have been described elsewhere[5, 6] The anhydrous, white salts were crystalline, non-hygroscopic and stable, at least up to their melting points. They were checked for purity by elemental analysis, argentometric or acidimetric titrations of their alcoholic solutions, and were identified by their infrared spectra[6]. None of the salts contained either free acid or base. Differential thermal analysis (DTA) was carried out in air at atmospheric pressure with a Stone DTA Model K A - 2 H equipment. Salt samples were diluted to 20 per cent using ignited c~-alumina. The temperature readings were reliable to within +_ 5 per cent, and a heating rate of 3°C/rain was employed. 1. Y. Marcus, In Solvent Extraction Chemistry (Edited by D. Dyrssen, J. O. Liljenzin and J. Rydberg), p. 556. North-Holland, Amsterdam (1967). 2. Z. Borkowska, M. Mielcarsky and M. Taube,J. inorg, nucl. Chem. 26,359 (1964). 3. I.J. Gal, J. Mendez and J. W. lrvine, Jr., lnorg. Chem. 7,985 (1968). 4. Y. David, M. Zangen and A. S. Kertes, Proc. 37th Ann. Meeting, Israel Chem. Sot., October 1967. 5. A. S. Kertes, J. inorg, nucl. Chem. 27, 209 (1965). 6. A. S. Kertes, H. Gutmann, O. Levy and G. Markovits, lsraeIJ. Chem. 6, 421 (1968). 205
206
H. G U T M A N N and A. S. KERTES
The thermogravimetric data reported were also obtained in air, at atmospheric pressure, using an automatic recording A.D.A.M.E.L., Paris, thermobaiance. The sample size was maintained nearly constant at about 50 mg. A programmed heating rate of 6°C/min was used. Porcelain glazed crucibles were used. Preliminary experiments showed the solvent, used during recrystallization of the salts to be present in an air-dried salt unless it was dried in vacuo over P~Os. All samples were mortar-ground. The samples completely decomposed to gaseous products at about 550°C and no carbonaceous material remained in the crucible at the end of the runs. RESULTS AND DISCUSSION
Typical TG curves obtained for amine chlorides, nitrates and bisulfates are plotted in Fig. 1. Irrespective of the amine class and the chain-length, the curves are quite similar in appearance for any given anion, as shown, for example, in Figs. 2 and 3 for the ammonium chlorides. While all curves, regardless of the anion, approach a 100 per cent weight loss, none of the curves is smooth. The decomposition of chlorides and bisulfates occurs in two distinct steps, that of nitrates in three, again irrespective of the class and chain-length of the amine from which the salts are derived. Decomposition temperatures, observed weight losses and the molecular weights of the decomposition products for the salts investigated are compiled in Table 1. The different melting points obtained by the capillary method and by deduction from D T A curves are probably due to different modifications of the salts. Similar quaternary alkyl ammonium salts have been observed to form polymorphs [7]. The higher melting point-modification is usually obtained when crystallizing the salt from solutions, whereas the lower melting point-modification 500 T °C
B
400 m
300
m
300
2 O0 ~2101K200
100
0 ~
I0
20
30
40
50
mg Fig. 1. Thermogravimetric curves of di-n-decylammonium salts: 1 -Chloride. 2 - Nitrate and 3 - Bisulfate. 7. J. E. Gordon, J.Am. chem. Soc. 89, 4347 (1965).
Thermal stabilityof long-chainaliphaticaminesalts
207
500
T't;
. 4 6 0 "C
450
-
400
--
350
--
300
-
• 4 5 0 *C
-420
3
2 8 0 *C
/t . ~ . .
250
*C
.
.
.
.
~J
2eo
"c
-
200
2C -
zos "c 180 "C
150
-
I00
--
50-
I
I
I0
ZO
I..... 30
I 40
mg Fig. 2. Thermogravimetriccurves of various alkylammoniumchlorides: 1-n-dodecyl-,
50
2-Di-n-dodecyl-. 3 - Tri-n-dodecyl-. is formed, often spontaneously, on slow-heating the solid of higher melting point. Decomposition of amine chlorides and bisulfates. The decomposition occurs in two steps, as shown on the thermogram given in Fig. 4. In agreement with the TG results, the first is endothermic and should be attributed to the melting of the salt, while the second, a doublet, is exothermic and corresponds to the thermal decomposition of the salt. Examination of Table l leads to the conclusion that the first exothermic step is the most important; about 90 per cent of the sample leaves the system. It is a mixture of olefins, nitrous oxide and the inorganic acid which became volatile at the melting point temperature or slightly above it. The "average molecular weight" of the mixture of gases leaving the system increases by about 56 units in going from di-n-octyl to di-n-decyl to di-n-dodecylamine salt, corresponding to the formula weight of C4H8. The substance which disappears during the second volatilization step (> 280°C) is carbon. The formula weight of the substance is roughly a multiple of 12. Decomposition of amine nitrates. Thermal decomposition of nitrates involves three stages. Whereas in the thermal decomposition of amine chlorides and
208
H. G U T M A N N and A. S. KERTES
500
4001--
300
2 --
~
i 200
"
3
Z8
Yii
50
~'off . , e o m
moo --
0
I
I
I
I
I0
20
30
40
mg
50
Fig. 3. Thermogravimetric curves of various secondary alkyl-ammonium chlorides: 1 - Di-n-octyl-. 2 - Di-n-decyl-. 3 - Di-n-dodecyl-.
/ 2
Exothermal~
/
\
Endothermal I 150
I
I 250
I
I 350
I
I 450
TeC
Fig. 4. Differential thermogram of di-n-dodecylammoniumchloride (1) and bisulfate (2).
bisulfates the volatilization of the acid (HCI or H2SO4) occurs simultaneously with the decomposition of the aliphatic chain, in the case of amine nitrates, the decomposition and subsequent volatilization of the acid radical appears to take place at lower temperatures than that of the alkyl chain.
Thermal stability o f long-chain aliphatic amine salts
209
e~ '¢q
I,M
E .r. E
e,,
m_ e~
e~ e~
c o
e~
!iiiii!i!
r'¢q
~ r,-¢¢~ ¢ q
¢q
¢~
~ ¢q
¢¢) ¢'4 ¢q ¢q
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,--
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.-2
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z~
rr :I::
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210
H. G U T M A N N and A. S. KERTES
It has been noted[8] that the thermal decomposition of ammonium nitrate is a complex reaction. That of alkylammonium nitrates is apparently no less complex. Analysing the numerical data in Table 1, it becomes apparent that NO~ is evolved during the first decomposition step. The residue formed is either the corresponding dialkyl hydroxylamine or the amine oxide, the difference in the weight loss between the two ( - 0.3 per cent) being within the experimental error of our measurements. D T A curves (Fig. 5) show two exothermic peaks in this respect. These, we believe, could be attributed to the subsequent volatilization of NO2
Exothermol
i
./
Endothermal 170*C Fig. 5. Differential thermogram of di-n-dodecylammonium nitrate.
and H2. It is still impossible to decide whether the residue is a N,N-dialkylhydroxyl-amine R2NOH or an amine oxide, R~HNO, since it is known that several types of rearrangements and cleavage reactions may take place on heating such and similar compounds[9-11]. Hydroxyl-amines of the type considered here can either be oxidized by air to nitroxide radicals, R2N---O, or may undergo auto-oxidation, yielding a variety of nitrogenous products, depending on the conditions. The first decomposition product formed here is surprisingly stable. The second and third steps of decomposition of amine nitrates parallel the two steps occurring in the thermograms of alkylamine chlorides and bisulfates. The D T A curves are similar, exhibiting two exothermic peaks, in addition to the two mentioned above, corresponding to the second and third steps of weight 8. S. Gordon and C. Campbell, Analyt. Chem. 27, 1102 (1955); A. G. Keenan, J. Am. chem. Soc. 77, 1379 (1955); J. Jaffray, J. Res. CNRS 153 (1947); H. G. McAdie, Analyt. Chem. 35, 1840 (1963). 9. A. C. Cope, T. T. Foster and P. H. Towle,J.Am. chem. Soc. 71, 3929 (1949). 10. P. A. S. Smith, Open-Chain Nitrogen Compounds, Vol. 2, p. 2 I. Benjamin, New York (! 966). 11. H. Goldwhite, In Chemistry of Carbon Compounds (Edited by S. Coffey), Vol. I B, p. 93. Elsevier, Amsterdam (I 964).
Thermal stability of long-chain aliphatic amine salts
211
loss in the TG curve. Again the formula weight of the substance in the third step is a multiple of 12. Effect of time. Because considerable time is frequently required to establish equilibrium during the extraction of metals from molten salt media[12] it was desirable to examine the stability of alkylammonium salts as a function of time at temperatures below those at which complete decomposition occurs. Samples were studied at constant temperature using DTA. At 200°C di-n-dodecylammonium chloride shows a vertical line, indicating that the compound did not decompose in any manner for at least 3 hr (the maximum time of our experiments*). There was still no evidence of any decomposition at 230°C. The same is true for di-n-dodecylammonium nitrate at 180 and 200°C, and for the bisulfate at 180,200 and 230°C. CONCLUSIONS
Long-chain (> 8 carbon atoms per chain) normal alkylammonium chlorides, nitrates and bisulfates are thermally stable at temperatures not exceeding those of their melting point. As a rule the thermal stability of long-chain alkylammonium salts parallels their melting points. There is a decrease in thermal stability with increase in the length of the aliphatic chain. Secondary amine chlorides are less volatile than primary, and much less volatile than the tertiary amine chlorides, with equal numbers of carbon atoms per chain. The same may be suspected to be true for the tertiary amine nitrates and bisulfates.~ The stability of chlorides is generally higher than that of either bisulfates or nitrates with an identical cationic part of the molecule. Secondary alkylammonium salts possess thermal properties useful in metal extraction from molten salt media. *Preliminary experiments[4] on extraction of lanthanides at 150°C from molten potassiumlithium nitrate eutectic mixture using di-decylammonium nitrate dissolved in tetraline, indicate that equilibrium is established in 1 hr. tThe melting point of tri-n-dodecylamine nitrate is 51-52 °, and that of the bisulfate is 64-65°C [6]. 12. J. David, Unpublished results.