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43 Thermal Decomposition of Hexamethylenetetramine
Thermal Decomposition of Hexamethylenetetramine I. N. STRANSKI IN COLLABORATION WITH G. KLIPPING, A. F. BOGENSCHUETZ, H. J. HEINRICH, AND H. MAENNIG Fritz Haber Institute of the Max Planck Gesellschaft, Berlin-Dahlem, Germany The vapor pressure ( p ) of hexamethylenetetramine (“hexa”) was measured i n the temperature range of 20 t o 280” and found t o correspond t o the equation log p = -(3940/T) 10.0. The thermal decomposition of “hexa” is a catalytic reaction of good reproducibility. It was studied in the vapor phase in the presence (18& 250”) and i n the absence of the solid phase (250-500”). I n both systems t h e decomposition has similar characteristics. It involves a n autocatalytic reaction leading t o incomplete conversion. The ratio of the final pressure t o the initial pressure increases with t h e temperature. The molecular decomposition occurs only at phase boundaries. There is a distinct difference between the induction of the reaction depending on one catalyst (I) and the reaction proper occurring under t h e influence of a second catalyst (11)formed during the induction period. Various phase boundaries (glasses, quartz, solid “hexa”) act as catalyst I of varying activity which affect only the induction period. Quartz and solid “hexa” give the shortest induction periods. Carbon formed during t h e reaction acts probably as catalyst 11.
+
In connection with the present study and with other work relating to crystal growth processes, it was thought necessary to measure the vapor pressure of hexamethylenetetramine ((‘hexa’’) ( 1 ) . The vapor-pressure measurements were carried out with “hexa” sublimed in high vacuum, Pressures of lop3to 10-’ mm. Hg, corresponding to 20 to 85”, were measured with a quartz filament manometer ( 2 , s ) At . temperatures of 120 to 210” a simple mercury manometer was used, and the vapor pressures were obtained by extrapolating the pressure-time curves plotted in Fig. 1 to time zero. The pressure increase occurring during the time of observation indicated that thermal decomposition of the ‘(hexa” occurred and that this decomposition was accelerated by the catalytic action of the solid phase itself. By quick heating, however, it was possible to vaporize entirely a certain amount 406
43.
THERMAL DECOMPOSITION O F HEXAMETHYLENETETRAMINE
0
-time
[h]
407
200
FIG.1. Vapor-pressure measurements with Hg manometer.
of substance without noticeable decomposition. Observing these conditions, the vapor pressure measurements could be extended up to 280". The measurement principle used is illustrated in Fig. 2 . Curve 1 represents the vapor pressure based on the preceding measurements. The other curves were obtained as follows: While the reaction vessel mas being heated, the pressure was lower than the saturation vapor pressure. The heating was continued till the solid phase was completely vaporized. On cooling the system to the range of supersaturation, the pressure assumed a value corresponding to the gas law. As soon as nucleation of the solid phase set in, the pressure dropped rapidly while the temperature remained constant, and finally, the real vapor pressure was reached. These measurements were made with a glass spring gage (4)which permitted pressure measurements between 1 and 2000 mm. Hg. The results of the vapor-pressure measurements for temperatures between 20 and 280" are plotted in Fig. 3 and can be represented by the equation logp
=
- 3940 T
+ 10.0
Accordingly, the melting point of hexa is above 280". The pressure-time curves for the thermal decomposition of 0.5 g. hexa in a reaction volume of 400 cc., at temperatures between 195 and 405" are plotted in Fig. 4. The dotted curves refer to the reaction system vapor/ crystal. With increasing temperature, the sold phase decreased in size and was completely vaporized a t 248". Above this temperature only vapor was
FIG.
2. Vapor-pressure measurements by rapid condensation.
FIG.3. Vapor-premure of hexamethylenetetramine log p vs. 1/T. 408
43.
THERMAL DECOMPOSITION OF HEXAMETHYLENETETRAMINE
409
1200
9 E
L 9 0 0 W L
a
In
E
1
600
300
300
600
-time [h] FIG.4. Thermal decomposition of 0.5 g. “hexa.!!
present in the decomposition reaction. As can be seen from the discontinuity of the decomposition times, the solid bulk phase itself catalyzed the decomposition of the molecule. In spite of the temperature increase from 240 to 255”, a tenfold time was needed for the decomposition of a given amount of substance. The decomposition process was, therefore, separately studied in the presence and in the absence of the solid phase. It was observed that in the presence of the solid phase only the solid became discolored during the decomposition process, while in the absence of the solid, the walls of the reaction vessel were blackened uniformly by carbon deposition. From these results it appears that “hexa” is a suitable model substance for studying the decomposition of solid organic compounds. The sudden rise of the rate of decomposition in the presence of the solid phase is caused by the higher catalytic activity of the crystal surfaces compared to that of the glass walls. It was also observed that the rounded sections of the crystal surface were particularly active, as was indicated by their deeper coloring during decomposition. The sublimation pressure of “hexa” reaches considerable values even at moderate temperatures. Owing to the catalysis of the decomposition reaction at the surface of the crystal, sublimation occurred rather fast and hence the “hexa” crystals showed typical forms of dissolution.
410
I. N. STRANSKI
-
time [hl
FIG.5. Thermal decomposition of “hexa” a t 220” in the presence of the solid phase. Variation. of the amount of “hexa.”
During the experiments conducted up to this point, it was, therefore, not possible to give the solid bulk phase definite forms. Still, it could be shown by the experiments illustrated in Fig. 5 that the rate of reaction depended on the size of the solid phase which was here varied between 0.1 and 2.0 g. In the absence of the condensed phase, the conditions of reaction are considerably simplified. The reactions presented in Fig. 4 show the following peculiarities. The ratio of the final pressure to the initial pressure p,/po, increases linearly with the temperature up to 500” (Fig. 6). The ratio p,/po can be accurately reproduced. If the temperature is increased after stopping the reaction, a p , / p , value belonging to the higher temperature is reached. A lowering of the temperature, however, does not bring forth the corresponding lower value of p,/po. The loop produced on stopping the reaction below 315” can be ascribed to the condensation of a reaction product. At lower temperatures the decomposition of ‘‘hexa” vapor showed marked autocatalytic character which appeared less pronounced with increasing temperature and disappeared nearly entirely at 405”. This is demonstrated by “normalizing” the respective curves (Fig. 7). Each run is plotted against a different unit of time chosen in such a way that the max-
43.
THERMAL DECOMPOSITION OF HEXAMETHYLENETETRAMINE
41 1
FIG.6. Temperature dependence of reaction depth.
FIQ.7.
phase.
imum rate ( d p l d t ) is reached at time 1 = 1. The stoppage of the reaction, however, is plotted on the other side of the dotted vertical line at normal scale. This indicates the portion of the reaction not taken into account when normalizing. In the slowest reaction the individual steps stand out particularly clearly. The observed phenomena can be interpreted as follows.
412
I. N. STRANSKI
-
time[h]
FIG.8. Thermal decomposition of “hexa.” Variation of the specific surface area f = area/volume. (1)f = 1 cm-1, (2) f = 2 cm-1, (3) f = 4 cm-1.
The reaction is started by catalyst I, which may be the wall of the vessel, whereupon a highly effective catalyst I1 is released. After a certain amount of this catalyst I1 has been formed, further reaction is promoted solely by it. The performance of catalysts of the type I was studied in two series of measurements, using various amounts and grades of glass. The amount was varied by increasing the surface area at constant volume of the vapor phase (Fig. 8). Increasing the specific surface area caused a shortening of the total reaction time by reducing the induction period without affecting appreciablythe rate of the fast reaction. Avariation of the grade of catalyst I was accomplished by using different types of glass (Fig 9). The lowest catalytic activity was exhibited by AR-glass. The three hightemperature glasses showed similar activity and quartz was the most active catalyst. It was evident once more that by varying catalyst I, the duration of the induction period could be appreciably reduced. Another striking observation was the catalytic acceleration of the reaction in quartz vessels in the presence of the solid phase. This indicates that the support material influences the decomposition of the crystal. In order to clarify the autocatalysis by carbon, a series of runs was carried out with carbon samples of different origin (Fig. 10). Primary interest was on “hexa” carbon. Curve 1 represents the normal reaction in AR-glass. The reactions 2 and 3 were conducted in an apparatus in which a reaction had previously taken place. The volatile reaction products were removed by pumping, without letting the carbon film come into contact with air. For this, the apparatus was baked at 400” for 4 hrs. (curve 2) or 20 days (curve 3). It is apparent that the “hexa” carbon shortens the induction period, and that an increase in the baking time increases its activity. Brief exposure to air prior to baking increases the activity of the carbon film (curve 4). Particularly revealing was the effect of carbon of foreign origin. One g. of
43.
THERMAL DECOMPOSITION
9 u
OF HEXAMETHYLENETETRAMINE
413
600
rw
w I 3
u) VI
E
200
300
600 -time
[h]
FIG.9. Activity of various glasses.
-600 9 E
25
$2
1
CL
300
300
600
900
-time [h] FIG.10. Decomposition of “hexa” in the presence of various kinds of carbon.
well-degassed activated carbon “Degussa Eponit” catalyzed the reaction (curve 5) to such an extent that the induction period was entirely eliminated, and the decomposition reaction itself was also considerably accelerated. In reaction 6 the gaseous reaction products were removed without baking the apparatus. Thus, the carbon film was left covered by reaction products of low volatility in order to study poisoning effects. These products, because of their own vapor pressure, prevented the free evaporation of the solid phase during the warming up of the apparatus. Thus, the reaction vapor/ crystal was caused to occur. This is mentioned in order to demonstrate the catalytic activity of the solid bulk phase at this point. A summary of the experiments performed at 255” is given in Fig. 11,
414
I. N. STRANSKI
Induction period durotion AP
Nr.
I
2 3 4
5
6 7 8 9 10
II
12
13
14
'
50
50
'
50
50 time [h]
[h]
[mmHg]
980 488
98 89
0
0 98 74
306 91 92 81
980 162 160 144 40 980
643
76 82 90 98 96
310 203
84
75
5b
FIG.11. Summary of rate studies.
showing the pressure increase during the final 50 hrs. before the maximum pressure was reached. Also shown is the duration of the induction period and the pressure increase A p , for each run. This figure illustrates once more the striking effect of activated carbon and shows how extensively the induction period can be changed by variations of the reaction conditions (catalyst I) without affecting the reaction itself. All of these results lead to the conclusion that carbon may act as catalyst 11, or at least as its carrier. Received: February 27, 1956
REFERENCES 1. Stranski, I. N., and Honingmann, B. 2.physik. Chem. 194, 180 (1950). 2. Haber, F., and Kerschbaum, F., 2. Elektrochem. 20, 296 (1914). 3. Wetterer, G., Wiss. Verii$entl. Siemenswerken 19, 68 (1940). 4. Bodenstein, M., 2. physik. Chem. A69, 26 (1909).
+
In connection with the present study and with other work relating to crystal growth processes, it was thought necessary to measure the vapor pressure of hexamethylenetetramine ((‘hexa’’) ( 1 ) . The vapor-pressure measurements were carried out with “hexa” sublimed in high vacuum, Pressures of lop3to 10-’ mm. Hg, corresponding to 20 to 85”, were measured with a quartz filament manometer ( 2 , s ) At . temperatures of 120 to 210” a simple mercury manometer was used, and the vapor pressures were obtained by extrapolating the pressure-time curves plotted in Fig. 1 to time zero. The pressure increase occurring during the time of observation indicated that thermal decomposition of the ‘(hexa” occurred and that this decomposition was accelerated by the catalytic action of the solid phase itself. By quick heating, however, it was possible to vaporize entirely a certain amount 406
43.
THERMAL DECOMPOSITION O F HEXAMETHYLENETETRAMINE
0
-time
[h]
407
200
FIG.1. Vapor-pressure measurements with Hg manometer.
of substance without noticeable decomposition. Observing these conditions, the vapor pressure measurements could be extended up to 280". The measurement principle used is illustrated in Fig. 2 . Curve 1 represents the vapor pressure based on the preceding measurements. The other curves were obtained as follows: While the reaction vessel mas being heated, the pressure was lower than the saturation vapor pressure. The heating was continued till the solid phase was completely vaporized. On cooling the system to the range of supersaturation, the pressure assumed a value corresponding to the gas law. As soon as nucleation of the solid phase set in, the pressure dropped rapidly while the temperature remained constant, and finally, the real vapor pressure was reached. These measurements were made with a glass spring gage (4)which permitted pressure measurements between 1 and 2000 mm. Hg. The results of the vapor-pressure measurements for temperatures between 20 and 280" are plotted in Fig. 3 and can be represented by the equation logp
=
- 3940 T
+ 10.0
Accordingly, the melting point of hexa is above 280". The pressure-time curves for the thermal decomposition of 0.5 g. hexa in a reaction volume of 400 cc., at temperatures between 195 and 405" are plotted in Fig. 4. The dotted curves refer to the reaction system vapor/ crystal. With increasing temperature, the sold phase decreased in size and was completely vaporized a t 248". Above this temperature only vapor was
FIG.
2. Vapor-pressure measurements by rapid condensation.
FIG.3. Vapor-premure of hexamethylenetetramine log p vs. 1/T. 408
43.
THERMAL DECOMPOSITION OF HEXAMETHYLENETETRAMINE
409
1200
9 E
L 9 0 0 W L
a
In
E
1
600
300
300
600
-time [h] FIG.4. Thermal decomposition of 0.5 g. “hexa.!!
present in the decomposition reaction. As can be seen from the discontinuity of the decomposition times, the solid bulk phase itself catalyzed the decomposition of the molecule. In spite of the temperature increase from 240 to 255”, a tenfold time was needed for the decomposition of a given amount of substance. The decomposition process was, therefore, separately studied in the presence and in the absence of the solid phase. It was observed that in the presence of the solid phase only the solid became discolored during the decomposition process, while in the absence of the solid, the walls of the reaction vessel were blackened uniformly by carbon deposition. From these results it appears that “hexa” is a suitable model substance for studying the decomposition of solid organic compounds. The sudden rise of the rate of decomposition in the presence of the solid phase is caused by the higher catalytic activity of the crystal surfaces compared to that of the glass walls. It was also observed that the rounded sections of the crystal surface were particularly active, as was indicated by their deeper coloring during decomposition. The sublimation pressure of “hexa” reaches considerable values even at moderate temperatures. Owing to the catalysis of the decomposition reaction at the surface of the crystal, sublimation occurred rather fast and hence the “hexa” crystals showed typical forms of dissolution.
410
I. N. STRANSKI
-
time [hl
FIG.5. Thermal decomposition of “hexa” a t 220” in the presence of the solid phase. Variation. of the amount of “hexa.”
During the experiments conducted up to this point, it was, therefore, not possible to give the solid bulk phase definite forms. Still, it could be shown by the experiments illustrated in Fig. 5 that the rate of reaction depended on the size of the solid phase which was here varied between 0.1 and 2.0 g. In the absence of the condensed phase, the conditions of reaction are considerably simplified. The reactions presented in Fig. 4 show the following peculiarities. The ratio of the final pressure to the initial pressure p,/po, increases linearly with the temperature up to 500” (Fig. 6). The ratio p,/po can be accurately reproduced. If the temperature is increased after stopping the reaction, a p , / p , value belonging to the higher temperature is reached. A lowering of the temperature, however, does not bring forth the corresponding lower value of p,/po. The loop produced on stopping the reaction below 315” can be ascribed to the condensation of a reaction product. At lower temperatures the decomposition of ‘‘hexa” vapor showed marked autocatalytic character which appeared less pronounced with increasing temperature and disappeared nearly entirely at 405”. This is demonstrated by “normalizing” the respective curves (Fig. 7). Each run is plotted against a different unit of time chosen in such a way that the max-
43.
THERMAL DECOMPOSITION OF HEXAMETHYLENETETRAMINE
41 1
FIG.6. Temperature dependence of reaction depth.
FIQ.7.
phase.
imum rate ( d p l d t ) is reached at time 1 = 1. The stoppage of the reaction, however, is plotted on the other side of the dotted vertical line at normal scale. This indicates the portion of the reaction not taken into account when normalizing. In the slowest reaction the individual steps stand out particularly clearly. The observed phenomena can be interpreted as follows.
412
I. N. STRANSKI
-
time[h]
FIG.8. Thermal decomposition of “hexa.” Variation of the specific surface area f = area/volume. (1)f = 1 cm-1, (2) f = 2 cm-1, (3) f = 4 cm-1.
The reaction is started by catalyst I, which may be the wall of the vessel, whereupon a highly effective catalyst I1 is released. After a certain amount of this catalyst I1 has been formed, further reaction is promoted solely by it. The performance of catalysts of the type I was studied in two series of measurements, using various amounts and grades of glass. The amount was varied by increasing the surface area at constant volume of the vapor phase (Fig. 8). Increasing the specific surface area caused a shortening of the total reaction time by reducing the induction period without affecting appreciablythe rate of the fast reaction. Avariation of the grade of catalyst I was accomplished by using different types of glass (Fig 9). The lowest catalytic activity was exhibited by AR-glass. The three hightemperature glasses showed similar activity and quartz was the most active catalyst. It was evident once more that by varying catalyst I, the duration of the induction period could be appreciably reduced. Another striking observation was the catalytic acceleration of the reaction in quartz vessels in the presence of the solid phase. This indicates that the support material influences the decomposition of the crystal. In order to clarify the autocatalysis by carbon, a series of runs was carried out with carbon samples of different origin (Fig. 10). Primary interest was on “hexa” carbon. Curve 1 represents the normal reaction in AR-glass. The reactions 2 and 3 were conducted in an apparatus in which a reaction had previously taken place. The volatile reaction products were removed by pumping, without letting the carbon film come into contact with air. For this, the apparatus was baked at 400” for 4 hrs. (curve 2) or 20 days (curve 3). It is apparent that the “hexa” carbon shortens the induction period, and that an increase in the baking time increases its activity. Brief exposure to air prior to baking increases the activity of the carbon film (curve 4). Particularly revealing was the effect of carbon of foreign origin. One g. of
43.
THERMAL DECOMPOSITION
9 u
OF HEXAMETHYLENETETRAMINE
413
600
rw
w I 3
u) VI
E
200
300
600 -time
[h]
FIG.9. Activity of various glasses.
-600 9 E
25
$2
1
CL
300
300
600
900
-time [h] FIG.10. Decomposition of “hexa” in the presence of various kinds of carbon.
well-degassed activated carbon “Degussa Eponit” catalyzed the reaction (curve 5) to such an extent that the induction period was entirely eliminated, and the decomposition reaction itself was also considerably accelerated. In reaction 6 the gaseous reaction products were removed without baking the apparatus. Thus, the carbon film was left covered by reaction products of low volatility in order to study poisoning effects. These products, because of their own vapor pressure, prevented the free evaporation of the solid phase during the warming up of the apparatus. Thus, the reaction vapor/ crystal was caused to occur. This is mentioned in order to demonstrate the catalytic activity of the solid bulk phase at this point. A summary of the experiments performed at 255” is given in Fig. 11,
414
I. N. STRANSKI
Induction period durotion AP
Nr.
I
2 3 4
5
6 7 8 9 10
II
12
13
14
'
50
50
'
50
50 time [h]
[h]
[mmHg]
980 488
98 89
0
0 98 74
306 91 92 81
980 162 160 144 40 980
643
76 82 90 98 96
310 203
84
75
5b
FIG.11. Summary of rate studies.
showing the pressure increase during the final 50 hrs. before the maximum pressure was reached. Also shown is the duration of the induction period and the pressure increase A p , for each run. This figure illustrates once more the striking effect of activated carbon and shows how extensively the induction period can be changed by variations of the reaction conditions (catalyst I) without affecting the reaction itself. All of these results lead to the conclusion that carbon may act as catalyst 11, or at least as its carrier. Received: February 27, 1956
REFERENCES 1. Stranski, I. N., and Honingmann, B. 2.physik. Chem. 194, 180 (1950). 2. Haber, F., and Kerschbaum, F., 2. Elektrochem. 20, 296 (1914). 3. Wetterer, G., Wiss. Verii$entl. Siemenswerken 19, 68 (1940). 4. Bodenstein, M., 2. physik. Chem. A69, 26 (1909).