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roduct and Process Development section devoted to information on the development of products and the processes for making them on any scale with industrial implications, and including economics and market development
Aluminum Borohydride Preparation JAMES B. HINKAMP
AND
VINCENT HNIZDA
Ethyl Corporation Research Laboratories, Detroit 20, Mich.
A
LUMINUM borohydride, Al(BHb)s, is a material of consider-
able scientific, and perhaps practical, interest as a n igniter and high energy fuel. Its handling and preparation are complicated, however, by high volatility, ease of hydrolysis, and spontaneous flammability in air. The original method of synthesizing aluminum borohydride was to treat trimethyl aluminum with excess diborane until all the methyl groups had been removed (6). Alz( CI&)s
+ 4B&,
--c
2B(CHs)3
+ 2A1( BH4)a
This method is not practical for large scale application, however. A process which could be adapted to larger scale operations was later developed by Schlesinger and Brown (5, 5 ) . This involved the reaction of an alkali metal borohydride with an aluminum halide. This paper discusses the mechanical development of a suitable system for reacting sodium borohydride and aluminum chloride to give pound quantities of aluminum borohydride.
3NaBH4
+ AIClj Fr? 3NaCl + Al(BH4)s
Side reactions which are encountered include 2AI(BH4)3 Al(BH4)a
+ 6HC1
+ 12H2O
+ 3BzHe + 6H2 3H3BOa + Al(0H)s + 12Hz
--F
+
2AlCla
These reactions account for the production of diborane and hydrogen, the principal by-products. The sodium borohydride used in this study was prepared in substantial quantities in the Ethyl Corporation Research Laboratories from sodium hydride and trimethyl borate (2, 4). Recrystallization from liquid ammonia or isopropyl amine was employed t o ensure adequate purity (95 to 96%). Aluminum chloride was purchased in the form of sublimed powder or was prepared by resublimation of commercial material. TheJeaction vessel was a horizontal, 2-liter, stirred autoclave, 1560
heated electrically and cooled by a cold water coil wound around the autoclave body. The wall temperature was determined by a thermocouple placed in the discharge plug and connected to a recorder. The autoclave was fitted with a charging port, a discharge port, a safety blowoff disk, and a vacuum gage, and was connected to the glass product collecting system by a length of &/%-inchcopper tubing. The product collecting system consisted of a series of three graduated glass receivers, the first cooled to -80" C. by dry ice and the second and third cooled to - 196' C. by liquid nitrogen. Blanketing nitrogen or vacuum could be supplied separately to any part of the system. The entire product collecting system was located behind a safety barrier of steel and wire mesh glass. Metal to glass connections were made with specially constructed brass to borosilicate glass spherical joints. The stopcocks and spherical joints were lubricated with Apiezon N grease. Hoke blunt point needle valves were used exclusively throughout the metal portion of the apparatus with the exception of the large bellows-gland valve between the autoclave and the product collecting system.
All
o p e r a t i o n s are c o n d u c t e d in a b s e n c e of air or m o i s t u r e
Since both the reagents and the volatile products were extremely sensitive to oxidation or hydrolysis, it was necessary to carry out all operations in the absence of air or moisture. The reagents were weighed to h O . 1 gram in a nitrogen box and were added to the nitrogen-filled autoclave while the stirrer wm in operation. The charging port was sealed, and the autoclave was evacuated t o about 1 mm. , the maximum vacuum obtainable with a Cenco Megavac pump through the train of receivers. The autoclave was progressively heated, and the volatile products distilled t o the product collecting system. Most of the
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Vol. 47, No. 8
PRODUCT AND PROCESS DEVELOPMENT
~
aluminum borohydride condensed in the first receiver a t -80" C., and the remainder and all the by-product diborane were condensed in the second and third receivers a t - 196' C., while the by-product hydrogen passed through the system. Although reaction took place a t room temperature, the product did not begin to distill from the autoclave until the temperature reached 45' to 55" C. The distillation rate increased with temperature, reaching a maximum a t 90' to 110" C. Above this temperature the rate decreased and the reaction was essentially completed when the mass had reached 140" to 150' C. A minimum of 6 hours was necessary to reach 140" t o 150" C. without exceeding an operating pressure of 2 mm. Higher operating pressurea increased the residence time and product decomposition in the reaction zone with an accompanying decrease in yield. After the reaction was complete, the bellows-gland valve connecting the autoclave to the product collecting system was closed. The autoclave was cooled and filled with nitrogen, and the residue was discharged. Residual activity in the discharge masa was destroyed by treatment q-ith ethanol. The volatile products, aluminum borohvdride and diborane, were separated by fractional condensation from the collector flasks. As the diborane was vaporized, it was measured for a material balance by passage to a large U-tube filled with powdered sodium trimethoxyborohydride which absorbed the diborane rapidly in accordance with the equation (6)
+ 2NaBH(OCHa)3
ILH6
-+
2NaBH4 f 2B(OCHs)3
The volume of trimethylborate subsequently distilled from the U-tube was a measure of the quantity of diborane formed in the reaction. The yield of diborane varied from 4 to 7 % in the various runs. The aluminum borohydride was allowed to warm to room temperature; the volume was noted, and the material was siphoned t o an evacuated steel storage cylinder. Finally, the product collecting system was filled with nitrogen to avoid air leakage and possible explosive mixtures with any residues remaining in the receivers. The importance of this final operation cannot be overemphasized. The violently explosive qualities of mixtures of aluminum borohydride with air or oxygen ( 1 ) make it imperative to take every precaution possible to prevent the formation of such mixtures. Although aluminum borohydride may be stored in a glass or steel container a t room temperature for a period of years without decomposition, the small amount of diborane usually present as an impurity will slowly decompose during the first few months with the evolution of hydrogen. Therefore, aluminum boro-
Table
I.
Effect of Aluminum Chloride Quantity Variations
Reaction conditions: Temperature raised to 145' C. in 7 hours 160 grams NaBH4 40 to 50y0 niinernl oil 5 to 7% graphite AICls, Mole % Excess .41(BHa)8, % Yielda 5 61 25 64 50 63 75 63 e Obtained from crude sodium borohydride.
Table
II.
Effect of Charge Size Variations
Reaction oonditions: Temperature raised to 145' C. in 6 to 7 hours 40 t o 50% mineral oil 2.5 to 5Y0 graphite Number of Aluminum Borohydride Yield, '%a NaBHd Charged, Wt. Grams Reactions Variations Average 120 3 66-72 69.3 160 7 63.4 61-66 2 200 57-65 59.9 ' Obtained from crude sodium borohydride.
August
1955
hydride should be purified before storage, and the container should be vented periodically during the first few months of storage. Major problems encountered in autoclaving were mechanical rather than chemical
Stirring the dry reaction mixture gave rise to a number of difficulties. The dry mass had a tendency to form lumps, to cake on the autoclave walls beneath the stirrer shoes, and to wear the stirrer shaft bearings excessively. The lumps formed were very difficult t o discharge in contrast to that portion (about 50%) of the solid product which was produced as a freeflowing powder. Moreover, the stirring produced a quantity of very finely divided solid material which was entrained by the product vapors and carried to the manifold and the first product receiver, with consequent plugging of the lines.
I
A WORKABLE PROCESS
. . . for production of pound quantities
of a possible igniter and high energy fuel
. . . susceptible
to extrapolation for large scale manufacture
The addition of flake graphite t o the reaction mass partially alleviated the caking, lump formation, and abrasive nature of the mass. However, difficulties still were experienced, and it was thought that the presence of a nonvolatile inert liquid lubricant would be beneficial. A vacuum pump oil was first selected but it contaminated the product. Stanolind heavy, white mineral oil was completely inert, eliminated the abrasive action of the mass and the entrainment of solid particles from the autoclave. The use of graphite with the mineral oil effectively prevented lumping and caking of the reaction mass and allowed the residue to discharge freely and completely. It was no longer necessary t o shut down for cleaning or repairs. Best results were obtained with a weight of mineral oil approximately equal to the weight of the dry reagents, and a weight of graphite equal to 5 to 10% of the weight of the reagents. The order in which the reagents and lubricants were charged to the autoclave had no effect on the reaction. However, the charging operation was facilitated by starting the agitator, adding the graphite first, the sodium borohydride and aluminum chloride next, and the mineral oil last. Operating variables studied were ratio of reagents and size of reactor charge
Effect of Excess Aluminum Chloride. Earlier small scale laboratory work had indicated that a large excess of aluminum chloride increased the yield of aluminum borohydride, but this was not so in the present work, as seen in Table I. The difference is probably due to the lack of agitation in the laboratory runs. Effect of Size of Reactor Charge. Decreasing yields were obtained with increasing size of autoclave charge as summarized in Table 11. This adverse effect of larger reactor charges on yield was attributed to the autoclave outlet design. The llrinch outlet was too small to permit as low an operating pressure with the larger batches. The smaller batches permitted operation a t 1-mm. pressure whereas the larger batches caused operation a t close to 2.0 mm. Operation a t the higher pressure increased the holding
INDUSTRIAL AND ENGINEERING CHEMISTRY
1561
PRODUCT AND PROCESS DEVELOPMENT time of the product in the heated reaction zone with a consequent loss in yield due to product decomposition. I n order to verify this explanation, a reaction mixture containing 200 grams of sodium borohydride was heated to 145" C. over a period of 10 hours giving a total operating time of 12 hours. By heating the reaction mixture more slowly (the usual time of reaction had been 6 to 7 hours) the average pressure was maintained a t 1 mm., and a 65% yield was obtained. This was a yield increase of 7 to 8% over the 57 and 58% yields previously obtained for runs of this size. This demonstrates that larger runs can be made in the normal operating time (6 to 7 hr.) without a sacrifice of yield in equipment of proper design.
venting it to the atmosphere. With a backing pressure of less than 1 mm., the temperature in a mercury diffusion pump would be no higher than that of the autoclave. 3. An agitator that would give a grinding action might, give more complete reaction. Reaction residues were of the same particle size as the reactants, indicating that unreacted material may remain in the center of these particles. 4. The entire product collecting system should be constructed of metal for safety reasons. I n conclusion, a workable process for the production of pound quantities of aluminum borohydride has been developed that has scale-up possibilities. literature cited
Discussion
The equipment employed in this work was not ideally suited to the conduct of the aluminum borohydride synthesis. I n future production the changes recommended are
1. A larger vapor outlet should be provided on the autoclave to permit minimum autoclave pressures for minimum product residence times and maximum yields. 2. A further improvement in operating pressure might be obtained by placing a diffusion pump between the autoclave and the product collecting system to maintain a very low autoclave pressure; the Cenco Megavac pump would then act as a roughing pump, removing the by-product hydrogen from the receivers and
(1) Badin, E. J., Hunter, P. C., and Pease, R. N., J . Am. Chem. SOC., 71, 2950 (1949). (2) Brown, H. C., Schlesinger, H. I., Sheft, I., and Ritter, D. M., Ibid., 75, 192 (1953). (3) Schlesinger, H. I., and Brown, H. C. (to U. S. Atomic Energy Commission), U. 8. Patent 2,599,203 (June 3, 1962). (4) Schlesinger, H. I., Brown, H. C., Hoekstra, H. R., and Rapp. L. R., J. Am. Chem. SOC.,75, 199 (1953). (5) Schlesinger, H. I., Brown, H. C., and Hyde, E. K., Ibid., 75, 209 (1953). (6) Schlesinger, H. I., Sanderson, R. T.,and Burg, A. B., Ibid.. 62, 3421-5 (1940). RECEIVED for review November 19, 1954.
.~CCEPTED February 28, 1955.
Bw tyl-Type Polymers Conta ning Bromine R. T. MORRISSEY The 6. F. Goodrich C o m p a n y Research Cenfer, 6recksvil/e, Ohio
B
UTYL rubber has become well established as a useful synthetic rubber in industry. But despite its many unique properties, i t has never developed beyond the status of a specialty rubber. Since its introduction in the United States, the major use has been in inner tubes. Other useful applications are believed possible through certain modifications of the butyl rubber. The main properties in which improvement might be obtained are:
1. Rate of cure comparable to that of other vulcanizable rubbers 2. Adhesion to other elastomers and metals 3. Compatibility of cure with natural and synthetic rubbers The need for keeping butyl stocks separated from other rubber compounds to prevent contamination has become a most im-
BUTYL RUBBER'S SPECIAL PROPERTIES c a n now be imparted fo natural rubber and GR-S by covulcanization with a new brominated butyl polymer. Easy adherence to other elastomers and metals is a property of this polymer with outstanding commercial significance 1562
.
portant problem. This difficulty, along with its poor adhesion to metal, has greatly limited the uses of butyl and eliminated i t from consideration for many products in which its outstanding properties would be useful. The bromine-modified butyl polymers were discovered as a result of an intensive effort to improve these properties of butyl rubber (7'). It was found during this study that the introduction of 1.0 to 3.5% (by weight) of bromine along the butyl polymer chain causes an appreciable increase in the vulcanization rate without affecting certain base polymer properties such as air diffusion and ozone resistance. The reaction has been considered as mainly one of addition t o the double bonds of the isoprene units contained in butyl rubber so as to produce units of the structure
H H CH, H
Although addition predominates i t is possible that some substitution of the bromine does occur. It is postulated that the bromine present in the polymer is sufficient to saturate only part of the isoprene units, leaving the remaining units available for the curing of the polymer by sulfur. If the modified butyl polymer is considered as a whole, the polymer chains may be visualized as having the saturated and unsaturated isoprene units distributed randomly throughout the length of the chain, Therefore, there are available two principal types of curing mechanisms-namely, the normal crom linking, by means of sulfur and dehalogenation, and bonding through bivalent oxide exemplified by zinc oxide. As is well
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 8
