Explosive Oxidation of Boranes - Advances in Chemistry (ACS

Explosive

Oxidation

of

Boranes

Downloaded by COLUMBIA UNIV on April 22, 2013 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch013

WALTER H. BAUER and STEPHEN E. WIBERLEY Department of Chemistry, Rensselaer Polytechnic Institute, Troy, N. Y.

A t certain conditions of temperature and pressure, borane-oxygen mixtures are spontaneously explosive, with combustion features similar in many respects to those exhibited in the chain isothermal explosions of hydrogen-oxygen and hydrocarbon-oxygen mixtures. Pressure-temperature explosion limits for diborane and oxygen were found by Price (4) and Whatley and Pease (10) with characteristics indicating a branched chain mechanism. When oxygen was rapidly added to pentaborane, Price (5) found a first explosion limit region at low pressures at room temperature, as well as a glow reaction at mixture pressures below the explosion limit pressures. Stock (9) reported the spontaneous explosion of decaborane and oxygen at a temperature of 100°C. Further studies of the oxidation of the boranes have been carried out in this laboratory. Initially diborane was studied because it has the simplest molecular structure of the borane series. Later, pentaborane and decaborane were investigated. Oxidation of Diborane Detailed studies of the combustion of diborane-oxygen mixtures at the second explosion limit were carried out by Roth and Bauer (6, 7) in an attempt to discover the cause of disagreement between the results of the investigations of Price and of Whatley and Pease. In general agreement with Price (4), well defined pressure-temperature explosion limits were found by Roth and Bauer (6, 7) for premixed samples of diborane-oxygen in borosilicate glass bulbs rapidly heated to the explosion temperature. Anomalous crossing over of the first limit reported by Price was found not to occur when the reaction vessels were outgassed at increased temperatures. Similar results were obtained in clean glass vessels and in vessels coated with reaction products if the coating was outgassed at temperatures above 170°C. First, second, and third explosion limits were defined, Figures 1 and 2. The second limit was yirtually independent of the vessel diameter, but the position of the third limit was strongly dependent on the vessel size. The second explosion limit was studied at various mixture ratios, and analysis of explosion temperatures at 23 mm. of mercury pressure indicated that oxygen was at least 1.5 times as efficient as diborane as a third body in the chain-breaking reaction. Addition of helium and nitrogen had no significant effect on the explosion limit at 23 mm. of mercury, but argon lowered the explosion temperature. Hydrogen additions, at concentrations up to 0.10 mole fraction, lowered the explosion temperature slightly at 23 mm. of pressure. At higher hydrogen concentrations, the explosion temperature increased. Further studies of the effect of additives on the second explosion limit were made by Snyder (8). Nitrogen dioxide lowered the explosion temperature from 153° to 74°C. at concentrations of 1.20% of nitrogen dioxide in a stoichiometric diborane-oxygen mixture. At higher concentrations of nitrogen dioxide, up to 4.5%, the explosion temperature increased again, to 148°C; thus all concentrations of nitrogen dioxide used sensitized the reaction. When an attempt was made to prepare a diborane-oxygen mixture with nitric oxide, NO, as an additive, immediate explosion occurred at room temperature when the nitric oxide was added. When iron carbonyl, Fe(CO) , was added, the explosion temperature of a diborane-oxygen mixture at 23 mm. was raised, and at 0.57% of iron carbonyl the 5

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

ADVANCES IN CHEMISTRY SERIES


116

200

175

150

TEMPERATURE, °C. C O U R T E S Y R E I N H OLD P U B L I S H I N G

Figure 1.

CORP.

Explosion limits for the stoichiometric mixture of diborane a n d oxygen, 3 to 1 mole ratio

• . Price (averaged results) X. This work, 4.6-cm. spherical, borosilicate glass vessel, evacuated at bath temperature, 150°-200°C. 1. Vessel evacuated at room temperature 2. Diborane decomposed in vessel prior to experiment

40

T E E

«

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uJ

or 3

cn 2 0 UJ

or Q.

1 «

10

i

130

160

1 190

TEMPERATURE, Figure 2 . Second explosion limits for various mixtures of diborane a n d oxygen Curve 5. ( B H « ) / ( 0 ) 1:4 2

2

Curve 4. 1:3

Curve 3. 1:2.5 Curve 2. 1:2

Curve 1. 1:1

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

BAUER AND WIBERLEY

Explosive Oxidation of Boranes

117

explosion temperature was 190° C , at the limit of temperature of operation for the silicon oil bath.

Oxidation of Decaborane


When the oxidation of decaborane was studied, explosive mixtures were prepared by means of slow vaporization of a weighed quantity of decaborane into a n oxygen atmosphere of known pressure. These mixtures were then allowed to come to equilibrium at a temperature at which the decaborane was completely vaporized and at which no slow oxidation occurred.

10 20 OXYGEN PRESSURE,mm. Hg COURTESY

JOURNAL

OP P H Y S I C A L C H E M I S T R Y

Figure 3. Composition explosion limits for pentaborane-oxygen mixtures in clean borosilicate glass vessels a n d in borosilicate glass vessels coated with reaction products, at 1 5 ° C . exceptas otherwise noted Explosion limits were determined for mixtures of 22 oxygens to one decaborane i n borosilicate glass vessels, 6.7 cm. i n diameter, coated with explosion products, b y a rapid heating method. A first explosion limit for mixtures of 22 oxygens to one decaborane was found at a total pressure of 85 m m . of mercury. A second explosion limit extended from 100 to 114 m m . of mercury over a temperature range of 78° t o 112°C. The peninsula t i p , between the first and second limits, could not be studied since i t In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

118


was situated i n a region of incomplete vaporization. The upper portion of the second explosion limit appeared to be sloping toward a third or thermal explosion limit juncture. A t total pressures greater than 114 m m . explosions occurred below 80°C. When a lower mixture ratio of oxygen to decaborane was used, with the stoichiometric ratio of 11 to 1, explosions occurred before the solid decaborane could be vaporized. Addition of nitrogen to the decaborane-oxygen mixture decreased the temperature


60

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/

I 40| UJ 01

X

20

*

25

45 65 TEMPERATURE °C. COURTESY

JOURNAL

85 OF P H Y S I C A L C H E M I S T R Y

Figure 4. Temperature-pressure explosion limits for pentaborane-oxygen, mixture ratio 1 to 3 , with 0 . 1 % iron carbonyl, in clean borosilicate glass bulbs, 4.4 cm. in diameter X. •. G. E. N.

Explosion on heating Explosion on withdrawal Glow on mixing Explosion on mixing No reaction on mixing

necessary for explosion at a given total pressure. Prolonged heating of the test decaborane-oxygen mixtures d i d not affect the explosion temperature, indicating that very little prereaction occurred i n the mixtures before the critical explosion temperature was reached. F o r oxygen-decaborane mixture ratios of 16.5 to 1, a n erratically defined explosion limit region of from 80° to 115°C. was found at mixture pressures above 90 m m . A t total pressures less than 90 mm., the 16.5 to 1 oxygen-decaborane mixtures were nonexplosive at temperatures tested, reaching to 150°C. When nitrogen was added to the 22 to 1 mixtures, the second explosion limit pressures were i n Table I. Second Explosion Limit Pressures for Mixtures of 22 0 to O n e B i H i Containing Nitrogen at 1 0 0 ° C . 2

0

4

N , % by volume Total pressure i n mm. H g 2

0 105.4

10 113.4

20 115.4


BAUER AND WIBERLEY


119

creased, over a range of explosion temperatures of from 80° to 120°C. are shown i n Table I .

T y p i c a l data

Oxidation of Pentaborane on Rapid Mixing Price (5) studied the luminous reaction of pentaborane with oxygen below the first explosion limit and the first explosion limit at room temperature b y means of a A.

60

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H

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EXPLOSION

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Figure 10. Spectra showing formation of intermediate by heating diborane with oxygen Spectra 1-7. Room temperature to 1 2 2 ° C . for 2 hours Spectra 8, 9. Temperature, 1 2 2 ° to 1 5 0 ° C . for 0.5 hour Spectra 10-12. Temperature, 1 5 0 ° C . for 0.5 hour



124

subsequently titrated corresponded to a formula with four atoms of boron per molecule. The mass spectrum of the intermediate is shown i n Figure 8. T h e highest mass peak occurs at mass 72. When the oxidation was carried out b y slow addition of oxygen enriched 1.5% i n oxygen-18 the resultant intermediate had a mass spectrum with the highest peak at 74; hence i t was concluded that oxygen was present i n the molecule. The infrared spectrum of the intermediate is shown i n Figure 9 and i n this same figure is the spectrum of the intermediate formed when B D was slowly oxidized. These spectra are most informative. N o bands characteristic of bridge hydrogens are observed. Also, because no O H bands or O D bands were observed, the oxygen atom must be connected to boron atoms. The band at 1390 c m . corresponds to the B — 0 linkage. T h e relative simplicity of the spectrum for a molecule of so many atoms indicates that the molecule must be highly symmetrical. A plausible formula for the intermediate is B H 0 , i f the results of hydrolysis are taken into account. Recently this same intermediate has been detected i n diborane-oxygen mixtures heated at temperatures near the third thermal limit of explosion. Infrared spectra taken during this reaction are shown i n Figure 10. T h e characteristic bands of the intermediate at 882 and 893 c m . - are evident. I n a l l cases the oxide formed had a characteristic absorption spectrum attributed 5

9

- 1


4

1 2

1

6

8

10

WAVE LENGTH, MICRONS Figure 11. Infrared spectra of boron oxide explosion product formed in oxidation of diborane, and of the explosion product after hydrolysis In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

BAUER AND WIBERLEY


125

to boron oxide. When water was allowed to react with this compound, even in the absence of oxygen the product was boric acid. In Figure 11 the absorption spectrum of the oxide is shown together with the spectrum of H B 0 resulting from its hydrolysis. 3

3

Discussion


For the three borane hydrides studied, critical pressures and temperatures were found defining an explosive region, outside of which the mixtures of boranes and oxygen were stable or reacted nonexplosively. In the case of diborane oxidation, effects of addition of argon, nitrogen, helium, and hydrogen support a reaction involving bimolecular chain branching and trimolecular chain breaking. Sensitivity of the reaction to addition of nitrogen dioxide, nitric oxide, and iron carbonyl is in agreement with the participation of oxygen atoms in the reaction chains. A mechanism analogous to that for hydrogen oxidation at the second limit was proposed (6), as follows: BH + 0 -*BH OH-F 0 0 + B H « - B H O H + BHS \ , . B H O H + B H -> BHS + BTH.OH/

• «

(1) (2) (3)

BHS + 0 + M -* H B 0 + H + M chain breaking

(4)

8

2

2

2

2

C

2

2

E

2

2

H

A

M

B R A N C H I N

2

B H + B H« BSHT + H BSH -» higher hydrides B H O H — further oxidation 3

2

2

7

2

6

(5) (6) (7)

The mechanism proposed does not explain the fact that when explosions were conducted in infrared absorption cells, the reaction products always contained water and boron trioxide, but no boric acid. This indicates that the explosive oxidation of the boranes may proceed with the direct formation of boron trioxide and that the hydrogen present in the hydrides is oxidized directly to water. In the case of the partial oxidation of pentaborane at 25° to 30°C, hydrogen and diborane are formed and are obtained as reaction products. It may be that hydrogen is first formed from the hydrides in the explosive reaction also, but that in this case the temperature soon rises above the explosive limit for hydrogen-oxygen mixtures, thus igniting the hydrogen. In the slow oxidation of diborane above the second limit and in the partial oxidation of pentaborane, the concentration of the intermediate increases to a maximum and finally falls off to zero during the reaction. From this it appears that an alternative path to the explosive oxidation is the formation of the intermediate from active fragments formed in the initial oxidation steps. The effect on the propagation of chains of the formation of nuclei of solid boron trioxide must be considered. Even though the mechanism of the oxidation of the boranes is far from being understood, further study of the oxidation should be valuable because of the characteristics intermediate between those of the oxidations of hydrogen and carbon monoxide and those of the hydrocarbons. A notable feature is that the boranes and the intermediate thus far detected, as well as the boron trioxide found, all have distinctive infrared absorption spectra, permitting study of the progress of the oxidation in cells equipped with rock salt windows. The sensitivity of the explosive oxidations to iron carbonyl and other additives indicates the importance of further studies of the mechanism of inhibition, with the possibility of effective control of the region of pressures and temperatures of spontaneous explosion.

References (1) Baden, H . C., Wiberley, S. E., Bauer, W. H., J. Phys. Chem. 59, 287 (1955).

(2) Ibid., 62, 331 (1958). (3) Hammond, J. A., Ph. D. dissertation, Rensselaer Polytechnic Institute, Troy,N.Y., 1958. (4) Price, F. P., J. Am. Chem. Soc. 72, 5361 (1950). (5) Ibid., 73, 2141 (1951). (6) Roth, Walter, Bauer, W. H., "Fifth Symposium on Combustion," p. 710, Reinhold, New York, 1955. In BORAX TO BORANES; Advances in Chemistry; American Chemical Society: Washington, DC, 1961.

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(7) Roth, Walter, Bauer, W. H., J. Phys. Chem. 60, 639 (1956). (8) Snyder, A. D., Ph. D. dissertation, Rensselaer Polytechnic Institute, Troy, N. Y., 1957. (9) Stock, A., "Hydrides of Boron and Silicon," Cornell Univ. Press, Ithaca, N. Y., 1933. (10) Whatley, A. T., Pease, R. N., J. Am. Chem. Soc. 76, 1997 (1954).


Explosive Oxidation of Boranes - Advances in Chemistry (ACS

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