BORAX TO BORANES

Conversion of Diborane to Higher Molecular Weight Boranes in the Presence of Certain Heterogeneous Catalysts

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E. DOWWHITNEY, J.PHILIPFAUST, DONALD G.POWELL, and EDWARD J. LONGOSZ

Downloaded by CORNELL UNIV on October 21, 2016 | http://pubs.acs.org Publication Date: June 1, 1961 | doi: 10.1021/ba-1961-0032.ch018

High Energy Fuels Organization, Research and Development Division, Olin Mathieson ChemicalCorp.,Niagara Falls, Ν. Y.

This paper reviews some approaches taken by the research staff of Olin Mathieson Chemical Corp. on the behavior of diborane when in contact with certain catalytic materials. In such studies it is desirable to employ relatively low temperatures, be cause diborane is known to react with other boranes to form nonvolatile solids of variable composition at higher temperatures. Some of these solids are colorless, but with increasing temperature they lose hydrogen and progressively change in color through yellow and brown toward black. The structural and chemical characteristics of these materials, referred to as "yellow solids," are still virtually unknown. Relatively little work has been done on the catalytic decomposition of diborane. Schlesinger (6), at the University of Chicago, has considered the possibility that some catalyst might be found which would promote the formation of pentaborane(9) from diborane. Schlesinger found that the conversion temperature was lowered in the presence of dehydrogenation catalysts. Relatively pure pentaborane(9) was formed in 50% yield with a 50% diborane conversion when diborane was passed over a Universal Oil Products dehydrogenation catalyst at 115° to 135°C. Pentaborane(11) however, was obtained in good yields when hydrogenation catalysts were employed at higher temperatures (about 195°C.). Several investigators have been interested in the effects of various gases on the conversion of diborane to higher molecular weight hydrides—in particular, Schlesinger and Burg (7), Stock and Mathing (9), and McCarty and DiGiorgio (4). None of the gaseous catalysts employed produced any significant change in the conversion of diborane or in the distribution of the products. During the course of early work in these laboratories it was observed that many materials resulted in very large conversions of diborane. Unfortunately, however, this apparent "catalytic activity" in the majority of cases was of an undesirable nature, resulting in the formation of large amounts of yellow solids. In many instances a reaction occurred between diborane and the catalytic surface to form undetermined products. On the other hand, many of the materials screened were found to act only as chemically inert heat transfer agents. Experimental The reactor shown i n Figure 1 was used i n these experiments. The catalyst and/or supporting material to be tested rested on the coarse sintered-glass plate. The Present address, Research and Development Division, Carborundum Co., P.O. Box 337, Niagara Falls, Ν. Y . Present address, Energy Division, Olin Mathieson Chemical Corp., New Haven, Conn. Present address, Chemicals Division, Olin Mathieson Chemical Corp., New Haven, Conn. Present address, Products Research Division, Esso Research and Engineering Co., Linden, N . J . 168 1

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

WHITNEY ET AL.

169

Conversion of Diborane

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L^Ls

10

PRODUCT TRAPS AND HIGH VACUUM FRACTIONATING SYSTEM

J * " ^ 24/40 I

JOINT

THERMOCOUPLE WELL (6mm. GLASS TUBING)

22-mm. GLASS

TUBING

CATALYST CHAMBER SINTEREO

GLASS PLATE (COARSE)


5-mm. GLASS TUBING

12/30

$ JOINT

DIBORANE

Figure 1.

INPUT

Catalytic reactor

thermocouple well was designed to allow the temperature of the interior of the catalyst bed to be measured. This particular arrangement for temperature measurement was important, especially where an exothermic reaction occurred between the catalyst and diborane. During the experiment, both the 22-mm. tube enclosing the catalyst bed and the entire 5-mm. spiral glass tubing immediately below the sinteredglass plate were enclosed by a furnace and heated to the same temperature. T h e purpose of the spiral tubing was to preheat the diborane and diluent gas to the reaction temperature before they came i n contact with the catalyst bed. Care had to be taken i n all experiments with this reactor that extensive pyrolysis of diborane did not take place i n the spiral preheater as evidenced b y the appearance of yellow solids. I n Figure 2 is shown an outline of the catalyst evaluation apparatus used i n this .ICE WATER COOLER CONDENSER

GLASS WOOL PLUG TO FRACTIONATING SYSTEM AND VACUUM PUMP

MINERAL OIL PRESSURE RELEASE VENT |"MULTIPLE U N I T * ^ FURNACE (FISHER SCIENTIFIC CO.)

TO VACUUM PUMP MERCURY PRESSURE RELEASE VENT

Figure 2.

Catalyst evaluation apparatus


ADVANCES IN CHEMISTRY SERIES

170

work. The ice water-cooled condenser was effective in removing decaborane from the gas stream leaving the catalyst bed. Unreacted diborane, along with other volatile boranes, was collected in three traps cooled to —196°C. After the experiment, the catalytic reactor was isolated from the system and the contents of the —196°C. traps were analyzed by fractional condensation. Infrared analysis of the fractions served to confirm the results. Before testing, each potential catalyst was baked out for at least 12 hours at about 400° under the vacuum obtainable with a glass three-stage mercury diffusion pump. The final vacuum at room temperature was usually about 1 Χ 1 0 m m . of mercury. - 4


I t was soon realized i n the early stages of this activated alumina-supported catalysts so commonly unsuitable for use with the boron hydrides. This sideration of Table I, i n which is shown the effect Table I.

work that the common silica and used i n the petroleum field were may be best illustrated by con of some common hydrogenation-

Catalytic Conversion of Diborane to Higher Hydrides A

Catalyst Activated alumina 0.5% P t on alumina M o O t on alumina Silica gel N i c k e l o n charcoal

Temp., °C.

Temp. During Run, °C.

156 156 25 154 148

+22 + 12 +94 +30 +6

Corrected Mole % C o n v e r s i o n of Diborane, %

-

B4H10 0.77 0 1.3 Trace 0

54.5 100 37.5 51.0 71.6

Yields BwHu 0 0 0 0 0

B«H»(n) 6.0 0 1.7 Trace 0

dehydrogenation catalysts on diborane i n the single-pass reactor system. The second column refers to the temperature of the catalyst bed at the beginning of the experi ment, and the third column to the maximum temperature rise occurring within the catalyst bed after exposure to diborane. Although the experimental runs lasted 15 to 20 minutes each, the maximum temperature rise was always noticed within 1 or 2 minutes after exposure of the catalyst to diborane. The last three columns of the table refer to the corrected mole per cent yields of the four boron hydrides most com monly found as a result of the decomposition of diborane. F o r the sake of brevity, the yields of pentaborane(9) and pentaborane(ll) have been combined. Infrared anal ysis has repeatedly shown that the amount of hexaborane formed i n these experiments is very small; thus it may be assumed that the corrected yields for the four hydrides shown i n the table should equal 100% unless diborane was being lost through chemical interaction with the catalyst itself. The high temperature increase observed with the molybdena on alumina catalyst could not have been due simply to oxidation of the diborane, because the catalyst was treated with hydrogen at 400° for a p proximately 6 hours prior to the r u n . I n an attempt to determine the nature of the interaction of diborane with catalytic surfaces, a series of experiments with activated alumina was undertaken (Table I I ) . The first two columns refer to the way i n which the activated alumina Table II.

Effect of Heat Treatment of Alumina on Conversion of Diborane (Catalyst.

P r e t r e a t m e n t of

Time, hours

Temp., ° C . 750 920 920 1000 Anhydrous alumina (Alundum)

Activated

15 15 38 15

Surface Area, Sq. M . / G r a m 80.5 132 80.4 69.0 70.2 10.1

alumina)

•

Catalyst Temp., °C. 157 150 154 148 148 150

Temp, during Run, °C. +36 +22 +20 + 12 +20 +2

Diborane Conversion, % 86.0 82.6 76.1 64.9 73.0 16.9

Corrected M o l e %

Yields

B4H10

BioHw 0 0 0 0 0 0

0 Trace Trace Trace Trace 6.4

B»H (n) 9

1 4.9 6.6 6.2 7.3 39.4

e

β

Commercial material.

was pretreated prior to a run. The heating of the alumina samples was performed i n Vycor tubes under a pressure of approximately 1 0 ~ m m . of mercury. Surface 4


WHITNEY ET AL.

Conversion of Diborane

171


area measurements of the treated aluminas were obtained employing the procedure of Brunauer, Emmett, and Teller, using nitrogen as the adsorbing gas. I n most cases when the diborane flow was begun, the temperature of the alumina bed began to rise, reaching a maximum value i n several minutes. This temperature rise is, no doubt, due to a reaction between diborane and the chemically bound water on the alumina surface. T h e observed temperature rise was greatest i n those experiments which showed the greatest conversions of diborane. After reaction, the alumina i n most cases had a strawlike color. T h e catalyst beds were analyzed for total boron, and the results were applied to the boron recovery i n the material balance calcula tion. O n the basis of only the volatile boron hydrides formed over the alumina surface, the boron recovery ranged from 15.0 to 39.4%. However, considering the boron i n the form of yellow solids and/or other by-products on the catalyst bed, the total boron recovery ranged from 75.6 to 99.5%. Results The results of these experiments are shown graphically on Figure 3. 150

oc M .

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BORAX TO BORANES

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