PYROLYSIS OF DIBORANE

Formation of Decaborane in a Continuous Flow Reactor

R . J . P O L A K A N D C L A Y T O N O B E N L A N D

Oltn .$fathieson Chemical Cor@., .Vew Haven, Conn.

A process for the preparation of decaborane that appears suitable for adaptation to large scale plant pro- duction involves the pyrolysis of diborane at constant temperature and pressure, using a closed recycle system with continuous diborane feed and decaborane withdrawal. Optimum reaction occurred with a reactor residence time of 4 to 6 seconds, and temperatures of 180" to 185" C., using diborane in 30 to 40 mole % concentration at pressures of 10 to 1 1 p.s.i.g. Diborane conversions of 80 to 90% and decaborane cor- rected yields of 50 to 60% were attained, with a material accountability of 90%.

U R I K G a lengthy investigation of organoboron compounds, D t h e preparation of decaborane (BLOHl4) was studied to develop a practical method for large scale production. 'The pyrolysis of either diborane (A2H6) or tetraborane (BtHlo) to produce higher boranes was first achieved by Stock; this method had since been used almost exclusively to produce the higher hydrides. pentaborane-9 (BSHg) and decaborane ( 7 , 70, 73, 75. 76) . All practical decaborane processes now knowm start with diborane, since this is easily prepared by the reduction of boron trichloride (5). Alternative methods of diborane preparation are based on the reduction of boron trifluoride etherate with either lithium hydride ( 7 7 ) or alkali metal borohydrides (72). An electrolytic process has also been reported (77).

When diborane is heated to temperatures in the range of 100" to 200' C., thermal decomposition occurs to produce the higher boron hydrides. Although the kinetics of diborane pyrolysis have not been completely defined. existing data support a series of stepwise reactions. Clark and Pease (3) have postulated an initial dissociation into unstable BH3 groups. followed by combination with another molecule of diborane to form an unstable triborane:

B2H6 + 2BH3 (1)

(2 1 (3)

BzHs + BH3 + B3Hi 4- Hz

B3H; + BzHe 3 BJHio + BH3

This formation of higher hydrides was considered a radical The addition of hydrogen in

Dupont and Schaeffer (.I) have proposed the following two-

type of polymerization process. these reactions depressed reaction rates.

step reaction of tetraborane :

B4Hio * BiHs + Hz B4Hs + B2H6 -+ BjHii + BH3

(4)

(5)

A similar reaction scheme was proposed by Rragg, Mc- Carry. and Korton [ 2 ) > \vho considered that pentaborane-11 could react in t\vo possible ways :

BSHII * B J I g + Hz

BjHll -+ B2H6 + "higher hydrides"

( 6 )

(71

Hillman. Mangold: and Norman (7, 8) performed a tracer study of the pyrol)-sis \vhich indicated that decaborane came primarily from a combination of five boron atoms from di- borane and five atoms from pentaborane-9. They considered

that pentaborane-3 was an intermediate in a sequence of reactions from diborane to decaborane. A possible mecha- nism for the formation of decaborane uas postulated as follows

(8) (9)

B s H ~ + BzH6 * BiHi3 + HL

BjH13 -+ BsHlo + BH3 "stepwisc" ___ -+ B10H14

The over-all pyrolysis reaction may therefore be considered as :

Shapiro and IVilliams (74) have shown that further reaction of decaborane is possible with diborane at pyrolysis tempera- tures :

This reaction product is a light yellow solid and is almost always observed during diborane pyrolysis. As a result, rapid removal of product was essential to prevent loss of decaborane yield.

Since previous work at Oh-for example, that of Hillman, Mangold, and Sorman (7! 8)-had indicated that the inter- mediate hydrides were necessary for t h e formation of deca- borane. it was apparent that these intermediates should be recycled with diborane to obtain higher yields of decaborane.

Two reaction systems, using this recycling procedure for the conversion of diborane to pentaborane, have been described (6, 9). The adaptation and utilization of this procedure were the basis for the design of a system in which the gases could be recycled through the pyrolysis reactor, removing decaborane as it was formed and continually adding diborane to replace that converted to the higher hydrides.

The energy for these reactions can be supplied by electrical discharge, ultrasonic radiation, or heat. Heat can be intro- duced into the reaction by various methods, the most practical being the use of a hot tube reactor.

This paper presents work on the thermal conversion of di- borane to decaborane in a circulating flow system with penta- borane formed as an intermediate product (see Figures 1 and 2) . This type of system, which allo\vs the use of relatively high temperatures (160" to 200" C.) to obtain high conversions, is readily adaptable to large scale industrial operations.

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30 Ib. Rupture Disc

-

1 ' 1

1

1 1

1

Nichrome

1 " S.S. G a t e valve

Primary Condenser

1' W a t e r in

- -1 t Heated '" pipe Decoborane out

Return l ine (%"tubing)

3 q d ' * S.S. G a t e valve

Auxi l io ry Condenser

% " S.S. Re

Hydrogen in

Diborone in

DIMENSIONSI

Flow me tar

Recycle control / valve

t o r . 2" Diom. 4 ' Long Pyrex p ipe

Primary Cond. - 3 " Diam. 1% "S .s .I 3 ' tong Pyrex p ipe

Auxil iary Cond. . 2" Diam.

2 ' Long S.S. Tubing Inner Coil: %"S.S. Tubing 1 " Spocing

Surge tank (1 7- I i te r v o I u m e )

Pump

(1 7- I i te r v o I u m e I

- L A - - k2z-L Figure 1 . Apparatus for pyrolysis of diborane on a prepilot plant scale

I . .. nitial addition of diborane only

Experimental

Apparatus. Originally, the apparatus consisted of four main sections, as shown in the flow diagram in Figure 1. The pyrolysis reactor consisted of a borosilicate glass pipe, 2 inches in diameter and 4 feet long.

A 4 X 6 inch stainless steel shell and tube heat exchanger was installed above the pyrolysis reactor as a preheater for the entering gas stream. This was heated by a stream of circulat- ing hot oil. The borosilicate glass reactor pipe \vas heated with two sections of Nichrome wire, and \vas shielded against convection loss by a glass jacket. T\vo thermocoiiples xvere installed in the reactor, one immediately below the preheater outlet, \vhich placed it above the high temperature zone of the reactor, and the other in a well made of 4-inch stainless-steel tubing. exrending from the bottom of the reactor to a point near the center of the hot zone. This \vel1 \vas radiation- shielded for more accurate temperature measurement.

'The primary water-cooled condenser, connected to the reactor by a 1-inch heated pipe. served for the collection of decaborane ; the auxiliary condenser. cooled IO - 60' C., served for the ' collection of pentaborane-9 if experimental conditions required its removal. The primary condenser \\-a5 constructed of a .?-foot length of 3-inch borosilicate glass pipe. surrounded by a Plexiglas jacket for cooling Lvater. The auxiliary condenser \vas constructed of stainless steel and cooled by alcohol which circulated through a trichloroethylene- dry ice bath. The gas stream from the auxiliary condenser then passed through a surge tank of 17-liter capacit)- and on to the suction side of a circulating pump. 7-hi> \vas a rotary vane air pump (Gast, Type 0446), connected to the surge tank outlet for the circulation of gas mixture back through the reactor system. This pump \vas equipped with good

I

_j+

0446

(about 2 cfm c a p a c ityl

Thermal conductir i ly

Figure 2. Flow diagram of pyrolysis system with constant addition of diborane

Reactor dimensions. Volume of system. 25.3 liters

2 inches by 4 feet

V O L . 3 NO. 3 S E P T E M B E R 1 9 6 4 235

so I

r-;T I I 1 I I I

lo 10 30 40 50 60 10 80 PO TlML MINUTES

Figure 3. time and temperature

Variation of rate of conversion of diborane with

mechanical shaft seals and had a bypass line to allow a closer regulation of flow rate. A 30-pound rupture disk \vas placed on the gas line leading to the reactor preheaier.

For a second series of experiments, thr apparatus was modified to permit continuous addition of diborane, \vith automatic venting of by-product hydrogen. .4 floiv diagram of the revised unit Lvith an improved decaborane condenser is shown in Figure 2 . A feature of the circular decaborane condenser was a series of 1 2 perforated copper disks? connected to a centered rotating brasa pipe, which was cooled by circula- ting alcohol at -10' C. The copper disks \vere in contact with stationary Teflon scrapers attached to t\vo thin b r a s rods placed opposite each other along the inner \Val1 of the condenser.

M'hen decaborane condensed on rhe copper disks: the center brass pipe with dhks attached was rotated, and the scrapers loosened the decaborane, dropping it out 'of the gas stream into the trap below the condenser. In these experiments, no removal of pentaborane was desired and the surge tank was warmed with a heating mantle to prevent pentaborane con- densation a t this point. An air motor was installed in the rotary pump to allow control of the gas flow rate by control of pump speed, rather than using a bypass connection. A low temperature condenser (-125' C.) was placed in the vent line to condense diborane and return it to the system, while by- product hydrogen escaped through the pressure-regulating valve. This condenser was cooled with a mixture of isopentane and liquid nitrogen, sufficient to liquefy the diborane but not solidify it. Traces of diborane escaping with the hydrogen were trapped in a separate water absorber system, not shoivn on the flow sheet.

Procedure with Initial Addition of Diborane Only. Before each experiment, the apparatus \vas purged I\ ith nitro- gen and then evacuated. During evacuation. the reactor \vas heated to pyrolysis temperature. after which hydrogen \vas introduced until the system was at atniosplleric preasure. Diborane was theri added until rhe pressure in the system \vas about 16 p.s.i.g.; then the recycle pump was started. 'The flow \vas adjusted to 9 to 10 liters per minure and the gas mixture \vas sampled. The recycle was continued for about 90 minutes, after nhich another sample of gas mixture was taken. The ,)-stem \vas then vented and purged. After purging. the decaborane condenser. penraborane condenser. and reactor \rere uashed \vith t\vo or three portions of ethyl chloride to remove decaborane and traces of the yelloiv solids. This \\'as follo\ved by a lvash \vith methanol to remove the remaining yellow solids. A sample of the ethl l chloride solu- tion was subjected to infrared analysis ro detcrmine decaborane content (see discussion section). Samples of both the ethyl chloride solution and the methanol wash \vert' evaporated to dr).ness and the residues analyzed for boron content. From rhese data the iveight of decaborane and the total boron con- tent at the start and completion of the experiment rvere deter- mined, and yields and material balances tiere calciilated.

Experimental results, summarized in Table I , show the conversion of diborane to pentaborane-9 and decaborane at

reactor temperatures of 160', 180"; and 200' C. and reactor residence times of 14 and 28 seconds. The variation of con- version rate \vith time and temperature is shown in Figure 3.

Procedure with Constant Addition of Diborane. For the operation of this system, the diborane condenser was cooled to -120' to -125 ' C . with a mixture of liquid nitrogen and isopentane; liquid nitrogen was added as required during the experiment to maintain this temperature range. The system !vas evacuated and a mixture of approximately equal volumes of hydrogen and diborane was added until the pressure in the system was 9 p.s.i.g. The compressor was then started and the flo\i. rate adjusted.

The gab mixture \vas sampled after a few minutes of mixing and the reactor and preheater were heated, ivhile coolant flow \vas started through the decaborane condenser. Approxi- mately one-half hour was required to bring the reactor to the proper temperature. During this period, some diborane condensed in the low temperature condenser, usually colinter- balancing the pressure increase due to reactor heating. If the pressure had not reached 10 p.s i.g., additional diboranp \vas added to reach this pressure. The pressure-control valve \vas usually set to operate a t 0.5 to 1 pound above this point. The diborane feed was then started, with samples of the circulating gas stream being taken a t hourly intervals to follow the rate of reacrion. Condensed decaborane was removed from the condenser b!. rotating the collecting rings from Jime to time. About an hour before shutdoLvn, the liquid nitrogen addition to the low temperature diborane condenser was stopped as well as diborane addition to the system, and the pressure-control valve \vas turned off'. .After an hour, the temperature in the diborane condenser hdd risen suficiently to vaporize all of the condensed diborane: Lvhich then re-entered the system. A final sample of the circulating gas stream was taken arid the system \vas then vented and purged. Decaborane \vas re- moved from the trap and iveighed, and purity \vas determined by infrared analysis. The condenser containing additional decaborane was uashed with ethyl chloride and the solution submitted to infrared for decaborane content. Material balances were made as described for the first series of experi- ments.'

Experimental results for this series, summarized in Table 11, show h e conversion of diborane to peniaborane-9 and deca- borane at a reactor temperature of 185' C. and reactor resi- dence times varying from 22 to 4 seconds.

Discuccion

The toxicity of the boranes, as well as their pyrophoric nature, made it essential that the pyrolysis apparatus be free of leaks. Sample removal and equipment cleanout could be done only in an oxygen-free environment, requiring frequent evacuations and nitrogen purges to prevent fires and decom- position of products. The first series of experiments \\as. therefore. conducted with limited amounts of diborane. In these experiments, proper ranges of operating conditions were determined. proper sampling techniques \vere earablished: methods for removal of decaborane and pentaborane \vcre developed. and. in short, a safe. operable system \vas ertab- lished,

Table I represent, a aeries of expeiinients conducted in the apparatus shown in Figure 1 in kvhich tit.0 variables \\.ere studied. temperature and residence time. '1 he prehcarer temperature \\.as also examined over a range of 100' to 160' C;. but had no effect on conversion or yield data as long as a rnini- mum temperature of 120" C. \vas maintained. These experi- ments, conducted for 1 to 2 hours. indicated. as expecred, that conversion increased with increase in reactor temperature. These conversion rates \vere also higher during the first 30 minutes of the reaction than for the remainder of the experi- ment; this \vas especially evident a t the ZOO0 C. reactor teni- perature (Figure 3 ) . Residence times ( 1 4 and 28 seconds)

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Table 1. Pyrolysis of Diborane, Initial Feed Only Reactor

Reactor Residence B2H6 Concentration, % T e m p . , C. T i m e , Sec. In it ial Final

160 14 1 4 28

2 5 , 9 2 4 . 5 2 6 . 4

1 8 . 8 1 7 . 7 1 7 . 9

28 2 4 . 6 1 6 . 5 180 1 4 1 9 . 1 1 1 . 2

28 2 2 . 2 1 1 . 1 28 2 4 . 5 1 3 . 4 28 2 4 . 3 1 1 . 9

200 1 4 26 3 7 . 5 1 4 2 3 . 6 6 . 0 28 2 5 . 2 6 . 9 28 2 5 . 2 7 . 2

Time of experiments, 90-95 min. Condenser temperature for B~uHl i ( - 3 ' C. ) . Condenser temperature for BiH8 ( - 6 0 " (2.). Reactor pressure range (15.7 to 19.5' p.s.i.g.)

BZH6 Conversion, 7c

2 7 . 5 2 7 . 8 3 2 , 2 3 2 . 1 4 1 . 5 5 0 . 0 4 5 . 3 51 . O 7 1 . 5 7 4 . 5 7 2 . 5 7 1 . 5

Corrected Yield, %

2 8 . 5 3 4 . 6 3 2 . 1 4 1 . 2 4 5 , 4 3 3 . 7 2 9 . 6 3 2 . 1

0 5 0 . 1 40 .0 2 6 . 4 4 3 . 5 4 2 . 2 3 5 . 0 3 3 . 0

6 . 4 3 4 . 9 0 2 5 . 4

3 1 . 5 2 6 . 1 3 2 . 2 ' 2 8 . 2

B E H B BlOHl4

Material Balance, yc

8 3 . 9 9 2 . 0 9 1 . 5 8 9 . 8 9 7 . 0 7 6 . 5 8 5 . 4 8 6 . 5 6 4 . 0 5 3 , o 8 1 . 5 8 6 . 5

Total T i m e , Min.

210

300 285 300 300 240 210 290 300 300 300 300 175 270 285 300

270

Table I I . Pyrolysis of Diborane, Constant Feed Reactor

Residence BzH6 Bh" Corrected Yield, yG Ti me, Sec . Consumed, C. Conurrsion, 70 B S H S BlQHl4

22 22 22 12 12 12 12

6-9 6 6 6 6 6 4 4 4 4

46 7 69 6 74 2 43 2 55 7 62 0 58 0 47 3 74 0 94 6

113 1 106 0 126 8

53 0 98 2

105 2 1 2 1 . 6

62 70 77 68 69 70 69 72 80 75 71 88 88 72 80 81 87

Surge tank warmed in this and all fol lowing experiments to prevent condensation of pentaborane. Reactor temperature, 785' C . ; prtheater temperature, 740' C. ; pressure 70 to 7 1 p.s.i.g. Conccntration of BpXe in g a s stream, 26 to 44 mole %,

26 19 21 20 22 26 22 24 19 22 13O 1 3 11 24

8 11 12

41 44 52 45 46 52 48 57 56 55 58 60 56 50 56 54 54

Material Balance, 7(

81 83 84 82 84 8 5 87 92 89 90 85 97 89 92 78 80 80

had no effect on diborane conversions a t 200' and did not significantly alter decaborane yields. However, the yield of pentaborane seemed to be considerably higher with the higher residence time a t 180' and 200'. Higher decaborane yields were achieved at rc'actor temperature of 180' C. , indicat- ing that a t the fairly high residence times employed, a reactor temperature of 200' C. caused the formation of considerably more higher boranes (yellow solids). A reactor temperature of 160' C. was not sufficient to promote good diborane con- versions. Material accountabilities of 85 to 95% were con- sidered satisfactory, taking into account the type and size of the system used, and the complexity of removing yellow solids from the apparatus.

These data demonstrated that the pyrolysis of diborane to decaborane was feasible in a circulatory, closed system and might be adapted to plant production. Since the conversion of diborane dropped off with time, a continuous feed system was indicated Since only decaborane was desired from this system, the intermediate pentaborane was recycled through the pyrolyzer. An improved decaborane condenser allowed removal of decaborane from the gas stream as it was formed; this prevented further reaction with the hot gases to form undesirable yellow solids. The internal operating pressure was lowered as an improved safety measure.

Table I1 shows the results of the closed recycle system with continuous feed a t constant temperature and pressure. Resi- dence time was the only variable investigated ( 4 to 22 secorids). The experiments were mainly of a 4- to 5-hour duration with a reactor temperature of 180' to 185' C. and an internal pres- sure of 10 to 11 p.s.i.g. No attempt was made to remove pentaborane ; however, in early experiments it condensed in the surge tank and high yields Lvere obtained a t the completion of the experiments when the pentaborane \cas vaporized. When heat \vas applied to the surge tank, this condition was remedied and pentaborane yields fell off to 8 to 137,. The concentration of pentaborane in the gas stream apparently increased to about 5% and then leveled off. Reproducibility of data in these experiments was good (L\ithin an approximate 776 deviation). Diborane conversions \vere high throughout the range of residence times, with the best conversions (71 to 88%) at 4 to 6 seconds. Decaborane yields were also ini- proved, reaching 50 to GOYc at the 4- to 6-second residence times. Material accountability was continually above 805;;-- one experiment showed a diborane conversion of 887,. with a decaborane yield of 6076 and material accountabilit), of 077,.

All decaborane analyses \cere conducted uving infrared techniques developed by the analytical section of the laborator>-. Ethyl chloride \vas used in the reactor apparatus as a deca-

V O L . 3 NO. 3 S E P T E M B E R 1 9 6 4 237

4000 3000 2000 1500 1000 900 800 700

WAVELENGTH (MICRONS)

Infrared spectrum of decaborane Figure 4.

KBr pellet

borane wash to preveiii higher boranes (k ellow solids) from k i n g carried aloiig in the wash solution. L‘sually, the analyst evapomted the ethyl chloride and wdisolved the decaborane in methylcyclohexane for analyses or analyLed the bolid residue using a KBr pellet (Figure 4).

Conclusions

A closed recycle systeni, operating a t constant temperature and pressure with a coiltinuous feed of diborane, ha> been developed for the preparation of decaborane. I t appears suitable for scale up to semibvorks or plant quipmeri t .

Optimum operating conditions a t pressures of 10 to 11 p.s.i.g. were with residence times of 4 to 6 seconds, reactor tempera- tures of 180’ to 185’ C.? and diborane conceritrations of 30 to 40% in the gas stream.

Diborane conversions of 80 to 900/, could be achieved Lvith decaborane yields of 50 to bOOj,. Material accountabilities were in an 80 to 90% range.

literature Cited (1) Bell, R . P., Emeleus, H. J . , Quart. Rer’s. (Lundurz) 2, 132-51

(2) Bragg, J . K., McCarty, L. V., Norton. F. J., J . A m . Chrm. (1948).

SOC. 73, 2134-40 (1951).

(3) Clark, K . P.. Pease, R . N.. Z b ~ d . , 73, 2132-4 (1951). (4) Dupont, J . A , , Schaeft’er. K., J . Inorg. .Yucl. Ch im. 15, 310--15

(5) Finholt, .A. E., Bond, A. C. , Schlesinger, H. I.: J . Am. Chem. (1 960).

.%c. 69. 1199 (1947). ( 6 i Her&k, C. S , ) K:rk, N., Etherington, T. L., Schubert, .A. E.:

(7) Hillman, hf. . Mailgold, D. J. , Norman, J. H.; Ah. Chem. S e r . , Znd. Erig. Chem. 52, 105-12 (1960).

KO 32, 151-6 (1961r 18) Hillinan. .M , Sfangold, D. J.. Norman. J. H., J Inore. .Vucl. \ I

Chem. 24, 1565-70 (1962). (9) hlcCarty, L. \’., DiGiorges, P. A , , J . A m . Chem. Soc. 73,

3138 (1951). (10) Schecter, LV, J. . Jackson, C. B., .\dams. R . M.. ’.Boron

Hydrides and Related Compounds,” 2nd ed., Caller)- Chemical Co., 1954.

(11) Schlesinger, H. I . , Brown, H. C. , U.S. Patent 2,543,511 (Feb. 27. 1951).

(12) Schlesinger, H. I., Brown: H . C.: Hoekstra, €1. R. , Rapp, L. B., J . A m . Chem. Sac. 75, 199 (1953).

(13) Schlesinger: H. I., Burg, A. B., Chem. Recs. 31, 1-31 (1942). (14) Shapiro, I . , ivilliams, K. E., J . A m . Chem. Soc. 81, 4787-90

(15 ) Stock! A . , “Hydrides of Boron and Silicon,’’ Cornel1 Univer-

(16) Stone, F. G. .A,, Quart Revs. (London) 9, 2, 174-201 (1955). (17) Sundermeyer, W., Glemser, O., Angew. Chem. 70, 625 (1958).

(1 959).

sity Press, Ithaca, N. Y., 1933.

RECEIVED for ietiew January 30. 1964 ACCEPTED June 5. 1964

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