BORAX TO BORANES

Impact of Recent Developments in Boron Chemistry on Some Scientific and Engineering Problems

PAUL F. WINTERNITZ Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 22, 2015 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch019

Department of Chemical Engineering, New York University, New York, Ν. Y.

In the last few years the study of the chemistry of boron compounds has been very much intensified, resulting in profound effects on both science and technology. The reasons for this increased interest include: Boron is an excellent absorbent for thermal neutrons; therefore, boron itself and some of its compounds are important in nuclear engineering. The incomplete electron shell of the boron atom gives rise to interesting structural and valence problems. Furthermore, boron is, because of its electronic structure, an extremely versatile element and forms a great variety of compounds which have numerous and unique applications in chemistry. Some boron compounds are, because of their high heat of combustion, excellent fuels for jet propulsion, known under such names as high-energy or exotic fuels. Effect of Recent Developments on Industry and Science Boron Compounds as Fuels for Jet Propulsion. H E A T I N G V A L U E . Boron com pounds are excellent propellants because of the high energy released i n the combustion of boron. Evidently the first question to be answered is, " W h y is heat of combustion so i m p o r t a n t ? " T h e answer is given by the following range formulas for various jetpropelled devices: Jet-propelled aircraft #max = η X AH Χ ^ X In M where -Rmax = η = AH = L/D = M =

(1)

range propulsion system efficiency energy released per unit mass lift to drag ratio mass ratio (ratio of structure plus fuel weights to structure weight)

Rocket on surface of earth #max = const. X AH X In M (horizontal range) 2

(2)

Rocket ascending vertically (maximum height) -Rmax ~ const. AH In M (vertical range after cutoff) (3) These formulas are, of course, greatly simplified. B u t even from these rough approximations the importance of the energy released during combustion is apparent. Range or height i n a l l cases is proportional to this energy. The energy released (AH i n the equations) is, however, not identical with the heat of combustion measured i n a calorimeter. Rather, i t is the energy released by the combustion gases during the working cycle. The exact determination of this quantity is rather cumbersome. Basically, i t is the fraction of the heat being developed i n the combustion chamber which can generate the propulsive force b y being converted into directed motion of the exhaust jet. 2

174 In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

WINTERNITZ

Recent Developments in Boron Chemistry

175

For a preliminary rough estimate of the heat released by compounds containing various atomic species two sweeping assumptions are made.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 22, 2015 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch019

First, it is assumed that the heat released is about proportional to the heat of combustion measured calorimetrically. This assumption is justified b y experience. Second, it is assumed that the heats of combustion of the elements contained i n a molecule add up to give the heat of combustion of the compound. T h e justification for this assumption is that the heat of formation of most fuels is only a small fraction of their heats of combustion. For more accurate computations i t would be necessary to use the most recent thermochemical measurements, and lacking such values, the heats of formation must be estimated from bond energies. Evidently, fuel combinations with small bond energies to be broken are desirable and endothermic compounds are even better (*4,80). Comparison of Boron with Other Elements Used in Fuels. W i t h the limitations involved i n the assumptions made, the heating values of the elements will give some idea of the performance of propellants made u p from different species. Table I shows the heat of combustion for various light weight elements at stoichio metric mixture ratios. Table I.

Heat of Combustion of Elements in the First and Second Row of the Periodic Table

Atomic No.

Element

Atomic (Molecular) Weight

1 3 4 5 6 11 12 13 14 15 16

H Li Be Β C Na Mg Al Si Ρ S

2 6.9 9.0 10.8 12.0 23.0 24.3 27.0 28.1 31.0 32.1

2

H e a t of Combustion, K g . C a l . / G r a m Kg. Cal./Mole Fuel 68.4 141.7 154.8 280.0 94.4 99.1 145.8 399.1 201.3 365.8 91.5

34.2 10.3 17.2 13.0 7.9 2.2 6.0 7.4 7.2 5.7 2.85

Prod.

M.W. Prod.

H 0(g) Li 0(8) BeO(s) B 0»(s) C0 (g) Na 0(s) MgO(s) Al Oi(s) Si0 (g) P*0»(s) SO,(g)

18 29.8 25.0 69.6 44.0 62.0 40.3 102.0 60.1 142.0 80.1

2

2

2

2

2

2

2

Kg.

Cal./Gram Prod. 3.7 4.75 6.2 4.0 2.14 1.60 3.6 3.9 3.54 2.5 1.14

Electronegative elements and noble gases have been omitted for obvious reasons. The table has been discontinued with the element sulfur because elements with higher atomic weight will give more and more decreasing values of heat of combustion per unit weight. Evidently only light weight elements of the first and possibly of the second row can form compounds suitable as propellants for jet propulsion. Two values are shown for each element: the heat released per unit weight of fuel alone and the heat released per unit weight of fuel plus oxidizer. T h e first value applies to the air-breathing engines, the second one to rockets. The difference i n the application of fuels to these two types of engines is obvious. This difference becomes even more clear i n Table I I , where the figures are arranged i n decreasing order for both cases. Table II. Comparison of Heats of Combustion per Unit Weight of Fuel and Unit W e i g h t of Fuel plus O x i d i z e r (Systems a r r a n g e d i n decreasing order) Air-Breathing Engines No. 1 2 3 4 5 6 7 8 9 10 11

Element H Be Β Li C Al Si Mg Ρ S Na 2

Rocket Engines

Kg.-cal./gram

Element

Kg.-cal./gram

34.2 17.2 13.0 10.3 7.9 7.4 7.2 6.0 5.75 2.85 2.2

Be Li Β Al H« Mg Si Ρ C Na S

6.2 4.75 4.0 3.9 3.7 3.6 3.55 2.5 2.2 1.6 1.15

In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

ADVANCES IN CHEMISTRY SERIES


176

This table makes it very clear that the rank of the elements as fuel constituents changes with the application. Boron ranks in the third place i n both cases. Hydrogen is i n first place for air-breathing engines by a great margin, but falls behind for rocket engines. Actually it gives the highest specific impulse i n this case because the heat release per unit weight is much higher at off mixture ratios and because the actual performance of metal oxides is greatly reduced by their evaporation and dissociation. Unfortunately, hydrogen has very unsatisfactory physical properties. It is, therefore, advantageous to use hydrogen in combination with other elements which act as hydrogen carriers. L i t h i u m and beryllium have to be excluded for large scale applications. N o liquid or suitable solid compound of these elements is known, and they carry much less hydrogen than does boron per unit weight. F o r both applications carbon ranges below boron and is also undesirable because of the high energy loss resulting from the ease of dissociation of carbon dioxide. Therefore, from these admittedly very crude considerations, i t can be inferred that compounds consisting primarily of hydrogen attached to boron should give the most desirable propellants and should be superior to conventional hydrocarbon fuels. This conclusion is borne out by the figures i n Table I I I , which compare the heat of Table III.

Heating Value of Propellants and Boron Hydrides (26)

Hydrogen Diborane Pentaborane Decaborane Ethyldiborane JP-4 E t h y l alcohol Hydrazine

Fuel Only, Kg.-Cal./Gram

F u e l P l u s Oxidizer, Kg.-Cal./Gram

28.8 17.6 16.6 15.5 14.9 9.5 6.4 4.1

3.21 3.95 4.12 4.00 3.74 2.10 2.08 2.00

combustion of some boron hydrides with the energy released by hydrocarbon fuels and hydrazine. Remembering that the range of jet-propelled devices is approximately proportional to the heating value of the fuel, it is obvious that the use of boron compounds as fuels will increase the range by about 25 to 5 0 % and it becomes immediately evident that the successful development of chemical boron fuels will have a tremendous impact on aviation and the technology of rockets. On the other hand the application of these new fuels has raised many practical problems, the solution of which has required intensive fundamental research. One of these problems is discussed here briefly. F o r the first approximation, calorimetric heating values based i n the case of boron on solid boron trioxide, B 0 , have been tabulated. B u t at elevated temperatures or i n the presence of other combustion products, such as steam, boron trioxide is neither the only oxygen compound of boron formed nor is i t always present i n the solid state. The loss by evaporation alone is nearly 2 5 % of the tabulated heat of combustion. F o r a correct evaluation of boron compounds as fuels one must, therefore, know the type of boron oxides present i n the combustion chamber and in the exhaust, as well as their vapor pressures and thermodynamic functions. Also, the equilibrium constants for the interaction of the various oxides between themselves and with other combustion products must be known. The study of these fundamental properties has received a great impetus by the use of newly developed boron compounds in jet propulsion. M a n y of the pertinent problems have been solved by Brewer, Margrave, Inghram, and Bauer, a few of the many contributors (1, 9, H, 15, 21). One important result seems to be that the evaporation of boron oxide is promoted by the presence of steam. 2

3



WINTERNITZ

Λ77


These results of basic research i n boron chemistry are especially important for an evaluation of rocket power plants with their very high chamber and exhaust tempera tures; they affect only to a lesser degree air-breathing engines. Superiority of Boron Compounds for Reasons Other Than Heating Value. U p to now the superiority of boron compounds as jet fuels has been related to their thermochemical properties. T h e contribution of the chemical characteristics of boron is probably equally great. Boron is an excellent hydrogen carrier. Possibly just as important is its extreme versatility, which is the result of its position i n the periodic system. I t approaches i n this respect its neighbor, carbon, although the chemistry of boron has, of course, been developed to a far lesser extent than organic chemistry. As a result of this versatility the physical properties of boron compounds can be tailored to meet most requirements for boron fuels, and vice versa, the necessity to meet these requirements has led to the discovery of entirely new classes of boron com pounds and of numerous unique and unexpected reactions. New Boron Compounds in Industry and in Basic Chemistry. B O R O N H Y D R I D E S . Compounds containing only boron and hydrogen would be the best jet fuels. They have also been the first unusual compounds to be studied and have, be cause of their reactions and structure, aroused the interest of chemists ever since A . Stock i n Germany discovered them early i n this century. B u t the real starting point for the study of boron hydrides i n this country was the work b y H . I . Schlesinger and his coworkers at the University of Chicago, which began i n the thirties of this century. Table I V gives the formulas and the physical properties of the more common representatives among them. Table IV. Diborane(6) Tetraborane(lO) Pentaborane(9) Pentaborane(ll) Hexaborane(lO) Decaborane(14)

Physical Properties of Some Boron Hydrides

Formula

M.W.

B:He B4H10 B H, BsHu BeHio

27.7 53.4 63.2 65.2 75.0 122.3

6

B10H14

M.P.,

°C.

-165.5 -120 -45.6 -128.6 -65.1 99.7

B.P., ° C .

d (Liquid) 0.447 ( - 1 1 2 ° ) 0.59 (-70°) 0.61 ( 0 ° )

-92.5 + 18 +60

0.70 0.78

+213

(0°) (100°)

Evidence for the existence of many other boron hydrides such as octaborane, B H , and nonaborane, B H , has been obtained (5, 6, 17). B H is known i n its coordination compounds with Lewis bases (8)> and also as a product of the cleavage of B H . A t least the transitory existence of dodecaborane has been reported (22). Diborane(6), the lowest and the simplest member of the series, is most easily pre pared. A l l other boron hydrides are obtained from this compound b y controlled pyrolysis. Methods for large scale manufacture of diborane(6) have been developed b y a number of organizations. B u t these methods have found until now only very limited practical application. The simplest and smoothest reaction for the laboratory preparation of diborane is still the hydrogénation of boron trifluoride etherate b y lithium hydride. This reaction was, to the author's knowledge, discovered independently i n the year 1944 b y H . I . Schlesinger (19) i n Chicago and Sherman Lesesne (12) of the since defunct Lithaloys Corp. I t made diborane available for the first time i n substantial amounts. A t the end of the war the equivalent of one pound of diborane per day was produced at Lithaloys. T h e over-all reaction is shown i n the first line of Table V . 8

1 2

4

9

1 5

3

7

1 0

Table V .

Laboratory Preparation of Diborane (1)

6LiH + 2BF

(2)

3L1BH4 + B F i

3

-> B H e + 2

6LiF

2B*H« + 3 L i F



178

Unfortunately lithium hydride and boron trifluoride are rather expensive materials and, therefore, not suitable for the production of diborane in tonnage quantities as required by the mushrooming demand for the manufacture of high energy fuels. B u t attempts to replace them in the direct hydrogénation reaction by less expensive starting materials, such as B C 1 or B ( O C H ) and N a H met, as far as is known, only moderate success or no success at all. A n indirect way proved to be more successful. M E T A L O B O R O H Y D R I D E S . This indirect method is based on the use of metaloborohydrides as intermediates. Metaloborohydrides can be considered as double hydrides of a metal hydride and the radical borine, B H , which is the unstable monomer dimerizing to diborane. They were discovered in 1939, again by H . I . Schlesinger and coworkers. Since their discovery they have turned out to be essential for the manufacture of boron hydrides, and consequently, also of high energy fuels and they have become indispensable for hydrogénation reactions i n organic chemistry. A l u m i n u m borohydride, A 1 ( B H ) , the first of these compounds to be synthesized, was prepared i n 1939 by H . I. Schlesinger, R . T . Sanderson, and A . B . B u r g (20) by reacting aluminum trimethyl with diborane. Since 1940 a large number of metaloborohydrides have been prepared, mostly by an interchange reaction of N a B H and the pertinent salt i n an appropriate solvent. Sodium borohydride itself is now manufactured i n tonnage quantities by the hydrogénation of the easily accessible trimethoxyborane with sodium hydride. Table V I shows, as an example, the sequence of steps i n the manufacture of potassium borohydride as it is carried out by M e t a l Hydrides Inc. 3

3

3


3

4

3

4

Table VI. 1.

Na +

Manufacture of Potassium Borohydride (7) in H2 —* N a H d i s p e r s i o n oil

2. 4 N a H +

B(OCHI)I liq.

in • NaBH* + mixture

3Na(OCH!)»

3. E x t r a c t i o n of N a B H 4 w i t h water 4.

P r e c i p i t a t i o n of K B H * w i t h K O H

The decisive step in Table V I is the second one leading to sodium borohydride. It proceeds, according to B r o w n and coworkers (3), exceedingly smoothly i n suitable solvents such as diethylene glycol (diglyme) or tetrahydrofuran. B u t most important, sodium borohydride reacts in the same solvents with boron trichloride to give excellent yields of diborane (2). This sequence of reactions represents, therefore, an excellent route for the large scale production of diborane. M a n y derivatives of metaloborohydrides, where the hydrogen is replaced b y organic groups, have also been reported. Attempts to prepare ammonium borohydrides have, however, remained unsuccessful to date; but the author obtained quaternary ammonium borohydrides by the reaction shown i n Table V I I . Table VII. A n a l o g y Leading to First Preparation of a Quaternary Ammonium Borohydride (1)

K B F 4 from B F i and K O H 4BFi + 3 K O H 3KBF4 +

(2)

K B H < from BiHe and

(3)

( C H ) 4 N B H 4 from B H e and K O H (28) 2 Β Η + 3 K O H - » 3(CH»)4BH4 + B ( O H ) s

2Β Η 2

β

+

B(OH)i

KOH

3 K O H -> 3 K B H 4 +

B(OH),

(29)

2

8

2

β

The analogy between B F and the borine radical, B H , is evident when comparing the three reactions shown, including the unpublished preparation of K B H . Inciden3

3

4


WINTERNITZ

179


tally, the author hit on this preparation of K B H by accident when he tried to obtain potassium hypoborate according to A . Stock's description by the reaction of potassium hydroxide with diborane and was unable to verify Stock's findings. I t appears, however, possible that the hypoborates are intermediates i n this reaction. The hydrogénation effect i n general is at least as important as the use of metaloborohydrides i n the production of borohydrides. I n the words of Wiberg (27), the foremost German expert, "one can today not imagine preparative inorganic and organic chemistry without the metaloborohydrides (which he calls borànates) and the analogous aluminum compounds." M a n y hundreds of papers have been published on this subject and i n this brief survey not even the surface of this vast field can be touched. Table V I I I is taken from a paper given by E . Wiberg.


4

Table V l l l . ^ Hydrogénation of Organic Groups b y Metaloborohydrides NaBH

4

L1BH4

Ca(BH4)

2

Al(BH4)s

NaBH

4

+

A1C1»

Ο - C ^

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

CI

ο —

+

R Ο

-c OR Ο - C OH Ο

^NRj Ο

I n the first column a series of carbonyl compounds is arranged according to de creasing ease of hydrogénation and i n the first row a number of metaloborohydrides are listed i n increasing order of hydrogénation ability. The table shows the high selectivity of the hydrogenating effect. This selectivity extends also to groups other than those shown in the table. As an example, conjugated double bonds are not attacked by metaloborohydrides, a fact which is widely being applied i n the synthesis of pharmaceuticals and fine chemicals—for instance, i n the preparation of vitamin A . Thus, the recent developments i n boron chemistry have affected also seemingly remote fields. M a n y organic derivatives of boron hydrides, metaloborohydrides, boric acid, and other organic boron compounds have been prepared and extensively studied i n recent years. A n excellent review of their complicated chemistry by Lappert (11) i n London, England, has been published recently i n Chemical Reviews. M a n y of them exhibit properties and reactions which are highly interesting for the theoretical chemist. Boron-Nitrogen Compounds. The simplest nitrogen-boron compounds result from the addition of an amine to borine, B H , the monomelic radical mentioned before. U p to now nitrogen-boron compounds have had very little effect on industry, and 3



180

their influence on preparative chemistry can i n no way be compared with that of the two classes of compounds which have been discussed before. B u t some of them appear to be stepping stones in the preparation of inorganic or semiorganic polymers which may become industrially important some day. Their principal interest at present is, however, their contribution to the theoretical concepts of chemical bonds. There are three types of these borine-amine addition products, a l l containing coordination bonds between boron and nitrogen. They are analogous to saturated, olefinic, and acetylenic hydrocarbons (Table I X ) .


Table IX. Systematic Arrangement of Hydrocarbon and Borine-Amine Addition Compounds Alkanes

C—C

Boranes

B

BORAX TO BORANES

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