Ind. Eng. Chem. Process Servlces, Department of Commerce, Washlngton, DC, 1957. Predei. B.; Schaliner, U. Mater. Sci. Eng. 1976. 5 . 210. Rao, Y. K.; Belton, 0. R. Met. Trans. 1971, 2 , 2215. Robiette, A. 0. E. "Electric Melting Practice”; McGraw-Hili: New York. 1972. Rock, P. A. “Chemical Thermodynamics”; MacMillan: London, 1969; pp 296-307. Rosenqulst, T. J. “Prlnciples of Extractive Metallurgy”; McGraw-Hili: New York. 1974. Rudd, E. J.; Darlington, W. B. J . Electrochem. SOC.1980, 128, 2586. Shunk, F. A. “Constitution of Binary Alloys”, 2nd Suppi., McGraw-HIII: New York, 1969. Smlley, W. 0. “Preparation of Uranium Metal by Carbon Reductlon”, NAASR-8507. Metals, Ceramics, and Materials, Canoga Park, CA, 1963.
Des. Dev. 1984, 2 3 , 217-220
217
Spendlove, M. J. Vac. Met. Symp. of the Electrothermlcs 8 Met. Div. of the Eictrochemicai Society, 192, Boston, MA. 1954. Stoicos. T.; Eckert, C. A. Ind. €ng. Chem. Fondem. submitted (1982). Wllheim, H. A. “The Carbon Reduction of Uranlum Oxide”, USAEC R 8 D Report IS-1023, Metals, Ceramics. and Materlais, UC-25, Ames, IA, 1964. Zapponi, R. P.; Spendlove, M. J. “Use of Molten Lead as a Quenching Medlum in Carbothermic Production of Magnesium”, U. S. Bureau of Mines, Report of Investigtlons 4082, Washington, DC, 1947.
Received f o r review October 7, 1982 Accepted May 31, 1983
An Improved Process for Preparing Tris( N-methy1amino)methylsilane Monomer for Use in Producing Silicon Carbide-Silicon Nitride Fibers BenJamln0. Penn,‘ Frank E. Ledbetter, 111, and Johnny M. Clemons George C. Marshall Space Flight Center, Natbnal Aeronautical and Space Administration, Marshall Space Flight Center, Alabama 35812
A technique is described for the preparation of tris(N-methylamino)methyisilane by a process which may be used for largascale production. The steps include synthesis, filtration to remove the salt byproduct, and distillation to remove the product from the reaction mixture under anhydrous conditions. Trls(N-methyiamin0)methylsilane is a precursor used to prepare silicon carbide-silicon nitride fibers that have been shown to have high tensile modulus (29 X 10’ psi for 0.4 mil diameter), high tensile strength (10.5 X IO‘ psi), and high electrical reslstlvity (7 X IO8 ohm-cm).
Introduction
Tris(N-methy1amino)methylsilane(TMMS) has been used to prepare a polycarbosilazane precursor that has been pyrolyzed to silicon carbide-silicon nitride (SiJVyC,) fibers which possess excellent mechanical properties and high thermal-oxidative stability. For example, Verbeek (1973) reported a modulus of 29 million psi, a tensile strength of 18.8 X lo4 psi, and oxidation resistance up to 1200 “C, for Si,NyC, fibers derived from tris(N-methylamino)methylsilane. In a recent paper, the authors (Penn et al., 1982) described a method for preparing Si,NyC, fibers derived from TMMS using a technique based on that reported by Verbeek (1973). A tensile rupture modulus of 29 million psi and a tensile strength of 10.5 X lo4 psi were obtained. In addition, the electrical resistivity (7 X 108ohm-cm) was found to be 1OI2 times greater than that reported for graphite. TMMS is prepared by the reaction of methyltrichlorosilane and methylamine as shown below
+
-
CH3SiC13 6CH3NH2 CH3Si(NHCH3)3+ 3[CH3NH3+]C1Verbeek (1973) prepared this silazane by reacting methyltrichlorosilane dissolved in petroleum ether with methylamine at 40 “C. The technique used by Tansjo (1960) was that of adding methyltrichlorosilane to an ice-cooled solution of methylamine dissolved in ether. The mixture was then refluxed for 1 h. 0196-4305/84/ 1123-02 17$0 1.50/0
The object of this paper is to present a process for preparing large quantities of tris(N-methylamino)methylsilane, separating it from salt byproduct, and purifying it by distillation using a method that rigorously excludes moisture. Experimental Section A. Chemicals. Methyltrichlorosilane was purchased
from PCR Research Chemicals, Inc., monomethylamine was purchased from Union Carbide Corp., and analytical grade petroleum ether (boiling range of 35-60 “C) from Sargent-Welch Co. Calcium chloride was obtained from J. T. Baker Chemical Co. B. Description of Equipment. The following paragraphs describe the equipment used to prepare tris(Nmethylamino)methylsilane, separate it from salt produced by the reaction, and then purify it by distillation, all under anhydrous conditions. The basic equipment needed for these operations is shown in Figures 1-4. The reaction to produce tris(N-methy1amino)methylsilane was carried out in the system shown in Figure 1. At the start of the reaction operation, a 22.5-L flask (which served as the reaction flask) was equipped as depicted in Figure 1. However, during the course of the reaction it was necessary to install and remove various apparatus as described in the following sections. Filtration operations to separate the ether-product solution from the salt byproduct were carried out in the equipment depicted in Figure 2. A 22.5-L three-neck round bottom flask, whose center neck was plugged, served 0 1984 Amerlcan Chemical Society
218
Ind. Eng. Chem. Process Des. Dev., Vol. 23,No. 2, 1984
PER
/THERMOMETER
PACKED DRYING TUBE
L.l REACTION FLASK
I
OUTLET
CWLING
1
/
TO DRYINGTUBE
('
I)
u
RECOVERY FLASK
Figure 4. Purification operation system. Figure 1. Reaction operation system.
u FILTRATION FLASK
Figure 2. Filtration operation system.
k
THERMOMETER
ADAPTER
\_i
\ K 2
DISTILLATION FLASK
A RECOVERY FLASK
Figure 3. Distillation operation system.
as a filtration flask. During the filtration operation, a filtration device remained in the left neck and a connection to a water asphator in the right neck. However, at the end of the fdtration operation, accesoriea in both outside necks were removed and replaced with calcium chloride drying tubes. The distillation operation to separate the liquid product from the reaction mixture was conducted in a system depicted in Figure 3. During this operation, the equipment shown in Figure 3 remained in place with the exception
of the second recovery flask which was routinely removed and emptied of its contents. To further purify the product by removing impurities, a second distillation was conducted. In this writing, this distillation is termed the purification step, and it was carried out in a system shown in Figure 4. Some of the equipment with the exception of the recovery flask was the same as that used in the distillation operation. However, a clean flamedried l-L three-neck flask served as the recovery flask since its function was to collect highly pure tris(Nmethy1amino)methylsilane. Miscellaneous equipment used in the process included a temperature bath, heating mantle, stopcocks, glass tubing, and tygon tubing. Use of this equipment is illustrated in Figures 1-4. C. Tris(N-methy1amino)methylsilaneSynthesis. Into a clean reaction flask (Figure 1) equipped with stirrer, stopper, and drying tube, there was placed 13 L of petroleum ether. A bath was placed under the flask, filled with 15 gal of ethanol, and cooled to -30 OC in one 1 h by passing liquid nitrogen through cooling coils made of copper tubing. Shortly after the bath temperature reached -30 O C , the outside necks of the reaction flasks were unplugged and into one of these was placed an N2 inlet line. Liquid monomethylamine (4620 g, 148.74 mol) was then added to the reaction flask. Following the addition of the amine, a dropping funnel containing 900 mL (1147 g, 7.7 mol) of methyltrichlorosilane was installed into one neck of the flask. The silane was added slowly to the reaction mixture with stirring while a dry nitrogen atmosphere was maintained in the flask. After completion of the silane addition, the dropping funnel was removed and replaced with a Friedrich condenser equipped with a calcium chloride drying tube. The N2 line was removed from the other outside neck and replaced with a glass stopper. Liquid nitrogen flow through the coils was stopped and followed by flow of ambient air through the cooling coils. After the ambient air flow warmed the coils to a higher temperature, hot water flow through the coils was started and continued until the bath temperature was about 40 "C. This temperature was maintained for 1 h by alternately heating or cooling the coils with hot or cold water. After the reaction was conducted for 1 h at 40 O C , the condenser was removed from the flask and replaced with a calcium chloride drying tube. D. Filtration Operation. The product was separated
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 2, 1984 219
from the salt produced in the reaction by the filtration operation which was carried out in the system depicted by Figure 2. Equipment was arranged in this system to permit product and solvent containing as little salt as possible to flow from the reaction flask through the filtration device and into the filtration flask. In the flow path, the salts were separated from the product and retained in the filtration device. The flow of materials in the filtration operation system was started by establishing a pressure differential between the reaction flask (higher pressure) and the filtration flask (low pressure). In this experimental setup (Figure 2), a water aspirator was used to establish a pressure differential between flasks. Since the product is extremely moisture sensitive, particular care was taken to ascertain that all the joints in the system were leak-proof at a system-atmosphere pressure differential of about 1 atm. To start the operation, the water aspirator was turned on, stopcocks 2 and 3 were opened, and stopcock 1was left closed. After the filtration operation was started, the flow rate of the reaction mixture was controlled by manipulating stopcocks 2 and 3. The filtration operation was stopped once the petroleum ether level in the reaction flask was barely above surface of the salt. Stopcocks 2 and 3 were then closed and stopcock 1was opened to permit use of a handpump to pressurize the reaction flask with dry air. When atmospheric pressure was reached in the reaction flask, the apparatus (which consisted of a line containing stopcock, handpump, and calcium chloride drying tube) was removed from one of the necks and a dry inlet line installed. A funnel was placed in the opening with the nitrogen inlet line and dry petroleum ether (6 L) was poured into the flask. The nitrogen inlet line was then removed, the opening was plugged with a glass stopper, and the stirrer was activated. After about 5 min, the stirrer was stopped and salt immediately settled to the bottom of the flask. The filtration operation was then restarted using the filtration procedure described earlier. When the liquid level in the reaction flask again neared the surface of the salt, the filtration operation was considered complete. The filtration flask now contained about 14 L of ether-product solution which was completely salt free. At this point, stopcock 3 was closed and the water aspirator was turned off. The filtration device was then removed from the system and the opening was quickly plugged under dry nitrogen conditions with a glass stopper. E. Distillation Operation. The distillation operation was carried out in a system depicted in Figure 3 to separate the ether solvent from liquid product. To complete the operation under anhydrous conditions, the filtration flask containing the ether-product solution was connected from one of its outside necks to an outside neck on the distillation flask through the use of stopcocks, rubber stoppers, and glass tubing. After the proper connections and installations were made, stopcocks 1,2,3, and 4 were opened and the water aspirator was turned on. About 500 mL of ether-liquid product solution was allowed to flow into the distillation flask. Stopcock 1was closed and the heating mantle was placed under the distillation flask and turned on. The water aspirator remained on as the distillation progressed, and stopcock 1 was adjusted to permit solution from the filtration flask to trickle into the distillation flask at about the rate at which condensate (ether) was collected in the recovery flask. The distillation operation proceeded continuously until less than 100 mL of ether-liquid product remained in the filtration flask. At this time, the heating mantle was
turned off and removed from the distillation flask. The water aspirator remained on until the temperature of the vapors reached about 40 OC. Stopcocks 1 and 2 were then closed, and devices in both of the outside necks of the distillation flask were replaced under dry nitrogen conditions with drying tubes packed with anhydrous calcium chloride. The solution in the distillation flask, which now contained product in high concentration, was protected from moisture at all times. F. Purification Operation. After the distillation operation was completed, the material in the distillation flask was mostly pure tris(N-methy1amino)methylsilane. However, small amounta of impurities such as side reaction products and dissolved ether were present and further purification of the product was necessary. To begin the purification operation (Figure 4), the drying tubes in the outside necks of the distillation flask were removed and replaced with glass stoppers and stopcock 1was opened. The previously removed heating mantle was placed under the distillation flask and turned on. Shortly after the vapor temperature reached 140 O C , the heating mantle was turned off and removed from the distillation flask. When the distillation flask temperature reached 50 "C,stopcock 1was closed and the recovery flask was removed from the system. The glass tubing and rubber stopper used formerly to connect the take-off and recovery flask were now installed in an outside neck of a clean, flame-dried three-neck distillation flask. The other two necks of this new recovery flask were plugged with drying tubes. All operations to remove the first recovery pot and install the new one were done under dry nitrogen conditions. Stopcock 1 was opened and the heating mantle was again placed under the distillation flask and turned on. The temperature of the vapors quickly reached 146 O C and the product, tris(N-methylamino)methylsilane,was collected until the temperature began to increase. Immediately, when the temperature was greater than 146 O C , the heating mantle was turned off and removed from the distillation flask. In this particular experiment, the yield was 673 mL (601.8 g, 4.51 mol), which is a percent yield of 59% based on the amount of methyltrichlorosilane used in the reaction. Material remaining after the 146 "C purification operation was discarded after the product flask was removed under dry nitrogen conditions from the system. G . Chemical and Elemental Analyses. Infrared spectra of tris(N-methy1amino)methylsilanewere taken with a Perkin-Elmer 137 sodium chloride spectrophotometer. Liquid monomer sample was placed, under anhydrous conditions, into a sodium chloride cell for investigation. NMR spectra were prepared with a Varian EM-300 system with deuterated chloroform as the solvent and methylene chloride as an external reference. C and H contents were determined by a combustion technique (Skoog and West, 1982). The silicon content was determined by a combustion-gravimetric method (Kolthoff and Sandell, 1959) and the nitrogen content by the Kjeldahl technique (Skoog and West, 1982).
Results and Discussion A. Synthetic Procedure. The feasibility of preparing tris(N-methy1amino)methylsilanein good yield (59%) by a large-scale process in which synthesis, filtration to remove salt byproduct, and purification by distillation are carried out in a manner that rigorously excludes moisture was demonstrated. The exclusion of moisture during this type reaction is essential since water is capable of decreasing
220
Ind. Eng. Chem. Process Des. Dev., Vol. 23,No. 2, 1984 WAVELENGTH IMlCRONSi
3
5
4
6
7
8
9
10
11
12
13
14
15
1
0.0
1.5 I
4ooo
,
2wO
3OW
low
15W
9w
8W
7w I
WAVE NUMBER ICM-11
60
Figure 5. Infrared spectrum of tris(N-methy1amino)methylsilane.
Table I. Elemental Analysis of Tris(N-methy1amino)methylsilane element
C H
N Si
theoret wt % 36.1 11.3 31.6 21.0
exptl wt 7% 37.0 11.4 29.3 20.4
the product yield by cleavage of Si-N bonds. The synthesis of TMMS was carried out by the dropwise addition of methyltrichlorosilane to methylamine in petroleum ether a t -30 “C followed by refluxing of the mixture for 1h. A twofold exceas of methylamine was used during the reaction to ensure that the amine was present in sufficient amounts to react with HCl generated in situ to form a salt. As a result, free acid is not present to decrease the product yield by reacting with the silazane. The purpose for maintaining a low temperature (-30 “C) during the addition of liquid methylamine to the reaction vessel and the subsequent addition of metyltrichlorosilane was to minimize the vaporization of methylamine. As stated previously, tris(Nmethy1amino)methylsilane has been prepared by Verbeek (1973) and Tansjo (1960). Tansjo prepared TMMS by the addition of methyltrichlorosilane dropwise to an ice-cooled mixture of methylamine in ether followed by refluxing of the mixture for 1 h. An excess of 66% methylamine was used in the reaction and a yield of 71% was obtained. Only laboratory quantities were prepared by this investigator. Verbeek claims to have prepared the polymerizable monomer by the reaction of methylamine with methyltrichlorosilane dissolved in petroleum ether at 40 “C. The investigator did not present a detailed description of his synthetic procedure nor were characterization and yield data presented. B. Analysis of Tris(N-methy1amino)methylsilane. Evidence for the formation of tris(N-methylamino) methylsilane was provided by infrared (Figure 5), nuclear
50
40
20
30 CHEMICAL
sniw
10
0
7 10
i IPPMI
Figure 6. NMR spectrum of tris(Nmethy1amino)methylsilane.
magnetic resonance spectroscopy (Figure 6), and elemental analysis. The most convincing data is provided by elemental analysis (Table I). Experimental weight percent values of 37.0,11.4,29.3, and 20.4 were obtained for C, H, N, and Si, respectively. As can be seen from Table I, the experimentally determined weight percent values are very close to their theoretical values. Additional evidence that the reaction product is TMMS is provided by IR (Figure 5). Infrared spectral assignments (Smith, 1960; Silverstein et al., 1974) were made for the following functional groups: N-H (3500 cm-’), N-CH, (950-1150 cm-’), C-H (2900 cm-l), and Si-CH, (750-950 cm-l), 1250 cm-’). In the NMR spectrum there appear to be two peaks in the Si-CH3 region (6 = -0.36 ppm and -0.50 ppm). The peak at 6 2.00 ppm is most likely due to N-CH,.
Acknowledgment The authors wish to thank Deborah Malone for help in organizing the manuscript and W. T. White for assistance in doing the experimental work. Registry No. Silicon carbide, 409-21-2;silicon nitride, 12033-89-5;methyltris(methylamino)silane, 18209-75-1;trimethylamine, 74-89-5. chloromethylsilane, 75-79-6;
Literature Cited Kolthoff, I. M.; Sandell, E. E. “textbook of Quantitative Analysis”, 3rd ed.: Macmlllan: New York, 1959; p 888. Penn, B. 0.; Ledbetter, F. E., 111; Clemons, J. M.; Daniels, J. G. J. Appl. Polym. Sei. 1982, 27, 3751. Sliverstein. R.; Basseler 0.; Morrill, T. “Spectroscopic Identification of Oranic Compounds”; Wlley: New York, 1974; Chapter 3. Skoog,D. A.; West, D. M. “Fundamentals of Analytical Chemistry”, 4th ed.; Saunders Cdlege Publishing: New York, 1982: pp 631, 249-51. Smlth. A. Spectrmhim. Acte 1860, 76, 07. Tans@, L. Acta Chem. S a n d . 1960. 74, 2097. Verbeek, W. Ger. Offen. 2218960, 1973.
Received f o r review June 15, 1982 Accepted May 9,1983
