Dec., 1961
Low PRESSURE, FASTFLOWPYROLYSIS OF METHYLAMINES
2139
LOW PRESSURE, FAST FLOW PYROLYSIS OF METHYLAMINES BY G. W. MEADOWS AND J. J. KIRKLAND Industrial and Biochemicals Department, E. I . du Pont de h’emours &. Go., Ine., Wilmington, Delaware Received May 16, 1961
The pyrolysis of monomethylamine under conditions of low pressure, short contact time and rapid quenching produced relatively high yields of hexahydro-1,3,5-trimethyl-s-triazineand N-methylglycinonitrile, and smaller quantities of glycinonitrile. A solid, believed to be a poly-XG’-2-(methylene methy1imino)-acetamidine, was recovered from the quenching surface. Under similar pyrolysis conditions, dimethylamine and trimethylamine gave the same liquid products, plus K,N-dimethylglycinonitrile, the principal product in the case of dimethylamine. It is suggested that methyleneimine, N-methylmethyleneimine, and hydrogen cyanide, which are formed in the pyrolysis zone, undergo addition reactions on condensation to give the observed products.
Introduction Previous studies of the thermal decomposition of methylamines have shown that a wide variety of products may be formed, depending on the pyrolysis conditions. These materials range from gases, such as hydrogen cyanide, ammonia, hydrogen and various hydrocarbons1-* to complex unidentified oils and solid^.^.^ Reaction mechanisms have been proposed in which me thyleneimine and N-methylmethyleneimine are postulated as intermediate products, n-hich may either decompose further or p o l y m e r i ~ e . ~The . ~ transient formation of N-methvlmethyleneimine has been suggested to acoiount lor products obtained from the pyrolysis of ethylamine,’ the photolysis of di- and trimethylamine8 and the decomposition of dimethylamine by high frequency electric discharge.9 Methyleneimine was believed to be produced in the thermal decomposition of dimethylazide.10 On the other hand, a free radical mechanism, which does not involve imine intermediates, has been advanced to explain the occurrence of methylhydrasines in the products obtained by cracking dimethylamine.l1 A study of the pyrolysis of the methylamines was undertaken in order to identify the complex oils and solids previously reported. Pyrolysis conditions giving rise to optimum yields of these materials were investigated. Experimental Materials.-The methylamines used were commercial cylinder gases and samplefi analyzed by gas chromatography were found to contain only small amounts (less than 2Yo) of air and other amines. Removal of these impurities by repeated varuum distillation, in the case of methylamine, did not result in any difference in behavior on pyrolysis compared with gas taketi directly from a cylinder. Apparatus.-The pyrolysis chamber was a 25-mpi. “Vycor” tube, heated by a resistance furnace, the temperature inside the pyrolysis tube being measured with a thermocouple which could be positioned along the axis of the tube by sliding it inside 9 “T’ycor” sleeve. A steady supply of amine was obtained by passing cylinder gas into a five-liter buffer bulb, and regulating the flow from this bulb to the furnace by means of I: needle valve. The prrssure in the (I) RI. A. Muller, B r l l . soc. chim.. 46, 439 (1880). (2) L. J. Jolley, J . C h e n . Soc., 1957 (1934). (3) H.J. Emelem and L. J. Jolley, ibid., 929 (1935). (4) M. W. Travers, Trans. Faraday SOC.,33, 1342 (1937). (5) J. Romney, Bet.. 11, 83.5 (1878). (6) A. G. Carter, P. A. Rosanquet. C. G. Silcocks, M. A ‘ .’ Trarers hnd A. F. Wilshire, J . C‘liem. Soc.. 495 (19391. (7) C. D. Hurd and F. L. Carnaban, J. Am. Chem. Soc., 62, 4151
(1930). (8) C. H. Bamford, J- Chem. Soc., 17 (1939). (9) F. 0.Rice and C. J. Grelecki. J . Am. Chcm. Hoc., 61, 824 (1957). (10) F. 0.Rice and C. J. Grelecki. ibid.. 61, 830 (1957). (11) €1. A. Taylor, J . Phyr. Chem., 36, 1960 (1932).
reaction zone was measured with a mercury manometer, and flow rates were determined by means of a calibrated rotameter. An ice-cooled finger was situated two inches from the end of the pyrolysis chamber, and gases impinged on this finger 1-2 msec. after leaving the hot zone. The resulting quenched gases then passed through Dry Ice and liquid nitroger, traps, and the uncondensed products, mainly hydrogen and methane, were pumped from the system. In order to achieve high flow rates and maintain low pressures in the furnace, there were no valves in the trapping system. Five hundred-ml. traps were used so that large amounts of condensate could be collected without restricting the rate of removal of gaseous products. Procedure.-The apparatus was evacuated to mm. pressure, and the furnace was brought to temperature. The temperature of the pyrolysis tube was checked a t 2” intervals along the tube, and the power input was adjusted to keep the temperature constant to lt4’ over a 9 to 10” hot zone. The refrigerants were applied to the quenching finger and the receiving traps, and the amine then was admitted to the pyrolysis chamber at the desired flow rate. The pressure was established by adjusting a valve in the line, through which noncondensable products were removed. At the end of a run the amine was shut off and the apparatus was filled with nitrogm. The refrigerated traps then were disconnected, stoppered and transferred to a distillation apparatus. After pumping out the nitrogen, the traps were warined to about 25’ and the volatile products were distilled into a liquid nitrogen-cooled trap. The condensate was re-evaporated into a calibrated volume, and the total amount of gaseous condensable products and undecomposed amine was measured. The gaseous fraction was analyzed by gas chromatography and components of the gas were identified by infrared spectrophotometric analysis and by peak retention times. The liquid products, which remained in the traps after distilling off the volatile fraction, also were qualitatively analyzed by gas chromatography. A column of 25% (by weight) “Carbowax” 400, suspended on sodium hydroxide-treated, SO/lOO mesh “Celite” 545, was used a t a temperature of 125’ and a helium carrier gas flow rate of 50 cc. per minute. In addition to the gaseous and liquid products, a solid deposit was collected on the quenching finger. This was removed from the finger as a flexible, transparent film, and was characteiized by chemical and infrared analysis.
Results Monomethylamine.-The volatile fraction consisted of ammonia and monomethylamine. The composition of the gas was calculated using gas chromatography peak area calibrations, and this was checked by measuring the average molecular weight of the mixed gases. No monomethylamine was found in the gaseous fraction recovered from the pyrolysis a t l l O O o , and infrared analysis of this gas showed hydrogen cyanide and ammonia were the major constituents. Gas chromatographic analysis showed only ammonia to be present, since hydrogen cyanide was not eluted from the alkaline column. Calculations from peak area indicated
G. W. MEADOWS ASD J. J. KIRKLSKD
2130
Yoi. 65
TABLE I PYROLYSIS OF METHYLAMINE Run no.
Temp.,a OC.
Press., mm.
Contact time 8ec.
% Smine dec.
Mole ratio, NHa/CHaNHz dec.
Liquid products recovered, 70 wt. of CH3NHz dec. Trap B Trap A (Dry Ice) (liquid Nz)
Total
.. 0.28 54 0.302 52 .. 860 5.0 .. .. 49 900 5.0 .27 83 .228 40 .. 2.5 945 .20 80 .319 .. 3 56 4 .. .. 960 .09 68 ,345 2.0 75 5 20 55 1.5-2.0 ,017 25 ,325 945 62 6 1005 1.o ,011 62 .445 28 34 55 1000 ,011 55 I 1.0 .405 30 25 24 8* 1000 1.o ,011 .. ... 33 57 45 9 2.0 ,022 70 ... 58 13c 995 74d 1015 2.0-2.5 ,027 >80 ... 77 10 3d 11 .OlG 71 ,310 10 1050 1.5 68 58 12 1055 87 ,375 63 5G 7 016 1.5 c 1100 ,049 100 ,039 13 3.0 Temperature variation along pyrolysis tube 1 4 ’ . * Long duration pyrolysis to prepare 30-40 g. of liquid products. Pale brown liquid collected in trap B. Dark brown liquid in both traps. e Darkviscous liquid collectedin trap A amounted to only 5 % of the amine decomposed. Kon-volatile brown liquid accumulated between furnace and trap A . The gaseous fraction was found to be mainly HCN. 1
2
-
6-foot, 31-mm. i.d. columii of “Carbowax” 400 supported on sodium hydroxide-treated “Celite” 545, and operated a t a temperature of 125’ with a helium carrier gas flow of 2.8 liters per minute. The purified materials were identified as hexahydro-1,3,5-trimethyl-s-triazine, N-methylglycinonitrile and glyciiioiiitrile (Table 11). The triazine v a s obtained mainly from the fraction condensed in the liquid nitrogen trap and aminonitriles from the Dry Ice trap. A sample of hexahydro-1,3,5trimethyl-s-triazine was prepared from formaldehyde and methylamine, and the infrared absorption curve was identical m-ith that of the purified component from the pyrolysis of methylamine. Hydrolysis of the N-methylglycinoiiitrile fraction with barium hydroxide, followed by acidification with carbon dioxide,13yielded a crystalline product 0 5 10 15 20 25 30 35 40 45 which was identified as sarcosine by infrared Time, sec. analysis. Fig. 1.-Gas chromatographic separation of liquids from Solid Products.-In addition to the gaseous and monomethylamine pyrolysis: “A”-hexahydro-1,3,5-triliquid products obtained from the pyrolysis of methyl-s-triazine; “B”-N-methylglycinonitrile; “C”monomethylamine, a solid product was formed. glycinonitrile. that the gas contained 14 mole 70 ammonia. The This material was removed from the quenching total amount of hydrogen cyanide recovered corre- finger as a clear flexible film. It mas slightly hygrosponded to 29.4 mole % of the amine pyrolyzed, scopic when first exposed to air, but quickly lost and this high conversion coincided \\-ith a large this characteristic on standing in contact with the atmosphere. Under the conditions used to obtain decrease in the yield of non-volatile products. the liquid products in good yield, the solid material The extent of amine decomposition over a range of conditions studied and the amounts of ammonia produced amounted to less than 1;: by weight of and non-volatile pyrolysis products recovered are the amine decomposed. I t was found that somesummarized in Table I. I t was found that non- what higher pressure and lower temperature favored volatile products were best obtained at furnace the formation of this product, aiid also that the temperatures of 1OO0-105Oo, contact times of 0.01 yield was dependent on the flow rate and quenching temperature. Figure 2 shom the relationship to 0.02 second and pressures below 2 mm. Gas chromatographic analysis of the contents hetween the amount of solid polymer formed aiid of the liquid nitrogen and Dry Ice traps showed contact time aiid furnace temperature. The yieldb peaks corresponding to the volatilc constituenti are expressed ab the percentage by weight of the identified previously, together with other compo- amine dccompoied. Elementary alialyws on ceveral films are giwn nents having longer retention times (Figb. l a and 1b). Highly purified samples of these compoiienti in Table 111. In all cases, the films were exposed to were obtained by low pressure distillation, followed air, a t least oxernight, before anal by preparative-scale gas chromatography, l 2 using a ses are consistent with an empirical formula C4H,(12) J. J. Kirkland, in V. J. Coates, et al , eds “Gas Chromatography,” Academic Piess Kew P o r h N 1- 1988. pp. 203-222
(13) J. H Ford, “Organic Syntheses ” Coll Vol. 111, John F3 iley and Sons, In? , S e i \ Tork Pi Y , 1935, p 34.
Dee., 1961
L O W PRESSURE,
FASTFLOW PYROLYSIS O F METHYLAMINES
2141
TABLE I1 Hexahydro-l,3,5-trimethyl-8-triazine, CeHisNa Theory or reported Found
Analysis or determinatiou
N-methyl: glycinonitnle CiHsNn Theory or reported Found
Carbon, % 55.8 Hydrogen, yo 11.9 Nitrogen, %(Duni:ts) 32.5 Neut. equiv." 43.0 Mol. wt.6 129 B.p., O C. 162.5
56.3,56.1 11.8,11.6 32.5,32.0 43.8 126,131 162
Derivative, m.p.,
96"
101.5-102 5
...
...
Refractive index
O
C.
51.4 8.6 40.0
70 70
51.6 8.7 39.2,39.3d 70,73,72 71,73
Theory or reported
nI9D
Found
42.8 7.2 50.0 56 56
43.5,43.8 7.4,7.3 45.1,45.2 61 58, 69
57.2 57.4,57.4 9.5 9.7,9.8 3 3 . 3 33.4,33.1 84 85 84 82,94 137137.5 138
....
....
.. ..
N,N'-,Dimethylglyoinonitrile CaHsNo Theory or reported Found
Glycinonitrile CzH4Nz
164-166.5" (withdec.)
...
165 (with dec.)
...
n26D
1 4604-1 4632 1.4604 a Titration with 10.1 LYperchloric acid in glacial acetic acid * Cryoscopic method, benzene solvent. Reaction with CSz t o form C ~ H I O N ~11.1. S ~DeIBpine, , Bull. soc. chzm. Francp, [3] 15, 89 (1896). Sealed tube Kjeldahl method. e React.ion with picric acid in alcohol to form picrate salt.
S3-,0,, assuming that oxygen constitutes the balance of material.
I
I
I
I
I
1
TABLE I11 -4NALYTICAL IXFORMATlOK
ON
SOLID POLYAiVJNE Elemental C,H,N formulaa
%, C ( 6 ,H 40, N 76 48.89 8 08 35.39 92 36 46.92 7.95 38 55 93 62 7 90 39 26 97 66 50 50 50 07 7 76 37 14 94.97 49 06 8 42 35 49 92 97 47 87 7 79 41 65 97 31 Assuming material not accounted for in gen. Sample
1 2 3 4 5 6
C4.iHa.iNz.jOo.s
C3.gHa.oN2.iOo.4 C4.2H7.9N2.800.2
C4.2H7.8Nz.jOo.t C4.1H8.4N~.~00.4 CaoH7.sXa.oOo.z analysis is oxy-
Additional information on the composition of the solid pyrolysis product was obtained by infrared studies using a Perkin-Elmer &lode1 21 infrared spectrophotometer with a sodium chloride prism. A spectrum was run first on a freshly prepared film and re-run on the same sample after aging under various conditioiis. The original spectrum showed close similarities with the cwrves for N-methylglycinoiiitrile and glycinonitrile. Absorptions characteristic of bonded K-H (3.05 p ) ; C-H stretching (3.55 p ) ; C-H deformation (6.85 and 7.35 p ) and rather weak C=K) (4.48 and 4.55 p ) were noted. In addition, a band a t 6.12 p suggested N-H deformation absorption. Exposure to the atmosphere produced a decrease in the absorption intensity due to bonded N-H and C r N and a slight decrease in C-H, which showed up in the weakest band >it6.85 y. The increased absorption at 5.95 p war probably due to the formation of C=O, possibly :is part of an amide group. The abhorption inteiitity at 6.12 1.1 remained unchanged, and two new iinjderitificd peaks appeared at 12.26 and 14.90 y. The absorption spectra obtained after a further period of 11 days, during which the film was stored in a desiccator, showed essentially no change. However, aftcr again expohing the film to the atmosphcrr for 32 dayy changrs in allsorptioii inteiiqity vere evident at thc w n e wave lengths previously notcd. The strorigcr C -fI absorptions a t 3.55 and 7.33 p also sholved slight decreases in intensity aftrr prolonged exposure to the atmosphere. On the basis of the above observations, it waq
CONTACT T I M E (SECS.). -
2
1 0
-
2t 800
1
850 900 950 1000 FURNACE TEMP ('C
Fig. 2.--Relationship between polymer yield and (a) coritact time a t 950" and 5 mm. pressure; (b) pyrolysis temperature a t 0.3 and 0.35 sec. contact time and 5 mm. pressure.
concluded that the solid product was a basic polymer containing )NH; )C=NH; )C=NR and -CN groups, the imine and nitrile groups being susceptible to hydrolysis in the presence of moisture, as shown below ,C=SH \
+ HzO +>C=O + KH3
(1)
0
-CSV
I1 + HZO +-C-NH2
(2)
It is suggested that the film is a polymer, consisting of a polyamine chain with methyl and imine substituents and an enipirical formula (GHci?JJr.
(TZ
YH3 )
-C-NH-CH2-X-CHZ-
\
/
G. W. MEADOWS AND J. J. KIRKLAND
2142
Vol. 65
TABLE IV PYROLYSIS OF DIMETHYLAMINE AND TRIMETHYLAMINE Run no.
Temp.,a
PreSS..
1 2 3b
800 855 850 945
5.0 2.0 2.0
OC.
4
Contact time, 8ec.
mm.
0.30 .030 ,030 -46
3.0
”14
Amine dec.
-Mole NH:/ Amine dec.
Dimethylamine 85 0.281 93 .244 90 100 .225
ratiaHk/ Amine dec.
0.024 ,075
...
...
Liquid products recovered, % wt. of Amine decomp. Total Trap Trap B A (Dry (lrq. N1) Ice)
45 63 64 4
16 35 36
..
29 28 28
..
Trimethylamine 1.o 770 .016 ”-N,N-dimethylglycinonitrile.
Dimethylamine and Trimethylamine.-A more limited study of the pyrolysis of dimethylamine and trimethylamine was made using the low pressure, fast flow technique employed in the case of monomethylamine. The volatile and liquid products were separated and identified, using the same procedure. The amine conversion and the amounts of gaseous and liquid products obtained are summarized in Table IV. It was found that in order to avoid the formation of hydrogen cyanide and the production of black viscous tarry products, it was necessary to use progressively milder conditions, with increasing methyl substitution on the amine nitrogen. Infrared and gas chromatographic analyses of the condensed gaseous products showed that slightly
..
..
less ammonia was obtained from dimethylamine and much less from trimethylamine. Traces of ethylene also were obtained from the dimethylamine pyrolysis and in the case of trimethylamine ethylene was a major component of the volatile fraction, with a small amount of acetylene also being evident. Gas chromatographic analysis of the condensed products from the dimethylamine pyrolysis showed the presence of a new and major component in addition to the triazine obtained from monomethylamine (Figs. 3a and 3b). This material was isolated and purified as previously described, and identified as N,N-dimethylglycinonitrile (Table 11). The characterization was confirmed by infrared analysis. All four of the compounds obtained from the cracking of monomethylamine and dimethylamine were found in the pyrolysis products of trimethylamine (Figs. 4a and 4b). The major pyrolysis product found in the liquid nitrogen trap was hexahydro- 1,3,5-trimethyl-s- triazine, with much smaller quantities of h’,N-dimethylglycinonitrile and Nmethylglycinonitrile in evidence (Fig. 4a). The Dry Ice trap contents were composed mainly of Kmethylglycinonitrile, with smaller amounts of N,N-dimethylglycinonitrile and glycinonitrile also present (Fig. 4b). Solid Product.-The basic polymer which collected on the quenching finger as a transparent flexible film in the pyrolysis of monomethylamine also was obtained from dimethylamine; however, this product was not recovered from the pyrolysis of trimethylamine. Infrared analysis of films prepared from dimethylamine showed them to be practically identical with those obtained from monomethylamine, the only difference being a weak band a t 6.00 p , most probably due to olefinic unsaturation. The close similarity with the absorption spectrum of Ihe monomethylamine product implies that the structure is essentially thc same, with an olefinic substituent present a t a few sites along the polymer chain. Discussion Methyleneimine and h’-methylmethyleneimine
Low PRESSURE, FASTFLOW PYROLYSIS OF METHYLAMINES
Dec., 1961
2143 .
have been postulated as intermediates in the exhaustive cracking of monomethylamine
-
----
+
NH=CHz -+ HCN Hz CHaN=CH, -+ HCN CHd ICHsNHz +HCN “=CHI
+
+
+ CHI + NHI
It is suggested that the products obtained in this work are formed when the active species leaving the pyrolysis zone are rapidly quenched and condensed (A) CHsN=CH2 (B) CHaN=CRz
-+ (CHaN-CH2)s
I
DRY ICE TRAP
+ HCN +CHaNHCHzCN (C) NH=C€Iz + HCN +,h;H&HzCN (D) HCN + N.K==CHe + CHsN=CHi +
I’i
’E‘ h
OH8 )
-/‘ E N H - - c H r N - - C H w \
/Z
These reactions indicate that the relative proportions of the varilous products will depend upon the amounts of hydrogen cyanide and imines formed in the pyrolysis zone. Decomposition of the imines, giving hydrogen cyanide, ammonia and gaseous hydrocarbons is minimized a t the lowest temperatures and shortest contact times. These conditions favor the production of cyclic triazine and solid polymer. Only small amounts of Nmethylglycinonitrile and glycinonitrile are formed. As the conditions are made more severe, the solid polymer and the cyclic triazine yields decrease, with a corresponding increase in the amount of Nmethylglycinonitrile. This is due to the formation of more HCN, which results in a greater opportunity for the production of the aminonitrile by reaction B. Since neither glycinonitrile nor the solid polymer are formed as the temperature and contact time are increased, it is concluded that methyleneimine is decomposed more readily to hydrogen cyanide than is N-methylmethyleneimine. Under the most extreme conditions, both imines decompose completely to gaseous products and black tars. The imine intermediates also appear to be produced from dirrtethylamine and trimethylamine, since the same products were obtained as in the monomethylamine pyrolysis. The greater complexity of the secondary and tertiary amines also results in other side reactions. Increasing amounts of unsaturated hydrocarbons and less arnmoni~am r e obtained in the sequence mono-, di- and trimethylamine. Thus the conditions which give rise to the polyamines and amino-
L-
0
5
10
15 20
25
30 35
40
45
Time, min. Fig. 4.-Gas chromatographic se aration of liquids from trimethylamine pyrolysis: ‘LA”-Rex&hydrO-l,a,bt~methylglycinonitrile; “C”-glycinonitrile; “D”-N,N-dimethylglycinonitrile.
nitriles are progressively milder, and the optimum yields of liquid products are less the more complex the amine, as shown in Table V.
TABLEV OPTIMUMCONDITIONS FOR 80-00% CONVERSION OF AMINES WlTHOUT PRODUCING
Amine
MonomethylDimethylTrimethyl-
Temp., OC.
HYDROGEN CYANIDE
Press., mm.
1000-1050 1.5-2 800- 860 2-5 800- 850 3-4
Contact
time, Rec.
0.1-0.02
0.03 0.044.06 CU.
% . Yield liquid products
50-70 45-65 3545
The formation of N,N-dimethylglycinonitrile, from both dimethylamine and trimethylamine, cannot be explained by addition reactions between the various intermediates described above. Since N,N-dimethylglycinonitrile was not obtained from monomethylamine, its formation must be related to the presence of two methyl groups on the original amine nitrogen. The only analogous process producing this compound would be an addition reaction between acetonitrile and N-methylmethyleneimine. CHaN=CHz
+ CHiCN --+(CHa)tN-CH*CN
While acetonitrile was not observed in any of the gas chromatographic analyses, it was detected by mass spectrometric analysis in a sample of the volatile components recovered from one pyrolysis.
