The thermal decomposition of ethylpentaborane at temperatures of 185 degrees to 244 degrees C is approximately a 1.5-order reaction. The products of the decomposition were hydrogen, methane, a nonvolatile boron hydride, and traces of decaborane. Measurements of the rate of decomposition of pentaborane showed that ethylpentaborane has a greater rate of decomposition than pentaborane

Mcdonald, Glen E

English

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RESEARCH MEMORANDUM 
THEREUZAL DECOMPOSITION OF ETHYLPENTABORANE IN GAS PHASE 
B y  Glen E. McDonald 
Lewis Flight Propulsion Laboratory 
Cleveland, Ohio 
CLASS~TATTON CHANGED 
NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS 
WASHINGTON 
J iQ 13, Is56 
https://ntrs.nasa.gov/search.jsp?R=19930089320 2020-06-17T05:39:05+00:00Z
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HACA RM E56D26 
NATIONAL ADVISORY C C N M I T T E E  FOR AERONAUTICS 
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The thermal s t ab i l i t y  of gaseous ethylpentaborane was investigated 
at temperatures from 185O t o  244O C. !Be products of the decomposition 
r;' were hydrogen, methane, a nonvolatile boron hydride, and traces of deca- 
u borane. From experiments at various  concentrations it w a s  determined 0 
that the reaction was approximately 1.5-order. Reaction-rate constants 
were determined for  the in i t ia l   reac t ion  at each temperature investigated. 
A l inear  correlation was found f o r  the ef fec t  of temperature on the in i -  
tial rate of decomposition. Measurements were made of the rate of de- 
composition of pentaborane under conditions similar to those used with 
ethylpentaborane. It was determined that the ethylpentaborane had a 
- 
- greater rate of decomposition than pentaborane. 
Using the following equation yields  the 1.5-order-rate constant f o r  
ethylpentaborane a t  temperatures intermediate between those measured: 
where k i s  the reaction  constant (sec'l cm-Oo5 Hg), and T is the t e m -  
perature (%). 
INTROIXTCTIOB 
A8 a continuation of the study of the thermal- stability of boron hy- 
drides, the previous investigation of the s tab i l f ty  of pentaborane (ref. 
1) has now been extended t o  ethylpentaborane. The alkylboranes are af 
interest  i n  aircraft propulsion because of the possibi l i ty  that their 
chemical properties, such 88 fnflamml)ility and taxici ty ,  and their  phys- 
ical   properties,  such as vo la t i l f t y  and melting point, would be more  de- . sirable  than  those of the boranes. The work w a s  i n i t i a t ed  on ethylpeuta- 
borane, because this was the first alkylborane other than the allryldi- 
boranes that w a ~  available in purified form. 
I 
2 - NACA RM E56D26 
In  the present work the thermal decomposition of ethylpentaborane 
has been measured at temperatures from 185O t o  244O C. The order of the 
reaction has been determined, and the r a t e  of therms1 decomposition of 
the ethylpentaborane has been compared with that of pentaburane. 
The ethylpentaborane after degassing w88 d i s t i l l e d  into an evacuated 
reaction vessel 15 centimeters long and 4.5 centimetere i n  diameter, and 
then sealed of f .  The reaction vessel was open t o  a capillary-bore mercury 
manometer. The manometer was wound with resistance w i r e  and heated t o  
avoid condensation of ethylpentaborane. The reaction vessel was imtwrsed 
i n   a n   o i l  bsth equipped with thermostat, which was electrically  heated. 
Temperatures were measured with a thermocouple, and decomposition was 
measured by noting the change i n  pressure indicated by the mercury manom- 
e t e r .  A diagrammatic sketch of the apparatus is shown in  figure 1. 
RESULTS AND DISCUSSIOES 
The rate of pyrolysis of ethylpentaborane was measured as a function 
of time at the temperatures 185O, 196O, 218O, and W0 C.  For purpose8 
of comparison the rate of pyrolysis of pentaborane was measured at the 
temperatures of 218O and 244O C. The resu l t s  for these decompositions 
are s h m  i n  figures 2 t o  4 where the   t o t a l  pressure is plotted against 
time. A t  several temperatures the reaction wa6 m i e d  u n t i l  near corn- 
pletion. The results indicate & smooth reaction without a pronounced in- 
duction  period  or an autocatalytic  effect .  
.r 
I n  order to determine the characterist ics of the reaction, each ex- 
periment a t  each temperature was plotted as a first-order  reaction aa 
shown i n  figure 5 where the log of t he   r a t io  of the   ini t ia l   pressure t o  
the pressure of the remaining ethylpentaborane log  t is plotted against 
time. The symbols In figure 5 correspond t o  the symbols used i n  figures 
2 t o  4. The pressure of the remaining ethylpentaborane is calculated 
from the following equation: 
POEP 
P = Pm - P& + 2.5 PkP 
where P i s  the total pressure, P& is the  ini t ia l  total  pressure of  
ethylpentaborane, and P& is the pressure at any time during the decem- 
position of ethylpentabor%@. It was determined from experiments, which 
approached complete decomposition, that  the f i n a l  pressure w a s  approximate- 
* 
l y  2.5 times the ini t ia l  gressure.  A t  l eas t  - the in i t ia l   por t ion  of such 
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plots may be approximated by a straight l ine ,  except at EL tersperature of 
244' C where the deviation from a ffrst-order  plot  starts at a law per- 
centage of decomposition. The slope of the l i n e  gives a rate constant 
f o r  the reaction; this pseudo-constant is expressed in f i rs t -order  terms 
and is designated by k' . 
From sepazate calculations, based on the dependence of the rate of  
dPo 
total-pressure increase log x, on the ini t ia l  pressure of ethylpenta- 
borane log  Po, it w a s  determined that the in i t ia l  reac t ion  w a s  approxi- 
mately a 1.5 order. Therefore, the  pseudo-constants k' w e r e  converted 
to 1.5-order constants by dividing them by the square root of the i n i t i a a  
pressure of ethylpentaborane. The values of these derived rate constants, 
designated by.. k, at various temperatures are shown i n  table I. The cor- 
6 +here the reciprocal of the temperature i~ plotted against the average 
of the logs of the rate constants, log k. 
.id re lat ion of these values of k with the tetnperature is shown in figure 
3 
rI 
0 
I 
u In order to cornpare the rate of decomposition of ethylpentaborane 
with the decomposition of pentaborane, the decomposition of the penta- 
of pentaborane against time are shown in  figures 3, 4 (a),  and 4(b). The 
plots of the reactions as first-order reactions are shown in ffgures 5(c) 
- borane -was measured at Z44O and 218' C . The plots of the total pressures 
- and 5 (d) . 
The values  for the 1.5-order reaction-rate constants for pentaborane, 
which are obtained by conversion of the pseudo-first-order constants, are 
shown i n  table I. The variation with temperature is shown in  f igure  6. 
When the results f o r  ethylpentaborane and pentaborane are  compared, 
it appears tihat at any temperature, within the range of this investiga- 
t ion,  the rate of decomposition of ethylpent&mrane is greater than the 
rate of decomposition of pentaborane. Moreover, the r a t e  of' decomposi- 
t ion  of ethylpentaborane increases more'slawly w i t h  an increase in tem- 
perature than the rate of decomposition of pentaborane. 
From the slope of the l i n e   i n  figure 6 an activation energy of 45.5 
kilocalories per mole was calculated  for the ethylpentaborane decomposi- 
tion. From the following expression, the 1.5-order reaction-rate con- 
s t a n t  may be estimated at other temperatures: 
log k = - + 14.9 T 
w h e r e  k, the reaction-rate  constant, is expressed in   sec- l  of - mercury and T is expressed i n  degrees  Eklvin. 
4 1', NACA RM E56D26 
The products of the reaction consisted of hydrogen, methane, high- 
molecular-welght boron hydrides, and traces of decaborane. Infrared 
analyeis of the gaseous products shared only methane. Vapor-pressure 
measurements also indicated the presence of methane. Ethane, w h i c h  m i g h t  
be expected as a decomposition product, was not detected i n  the infrared 
analysis. The high-molecular-weight boron hydrides were light yellow, 
nonvolatile solFds that decomposed before they melted. A small amount of 
decaborane could be isolated by careful   d is t i l la t ion of the reaction 
products. 
CONCLUSIONS 
The results indicate that the ethylpentaborane gas-phase pyrolysis 
proceeds as a 1.5-order reaction that has an mer-all activation energy 
of 45.5 kilocalories per mole. 
The over-all reaction may be formulated aa follows: ethylpenta- 
borane -* high-molecular-weight hydrides +.Hz + CE4 + t races  of BsG14. 
The sum of the hydrogen and methane pressures i8 about 2.5 times greater 
than the initial pressure of ethylpentaborane. The results indicate 
that, Over the ranges of temperature and concentration investigated, 
ethylpentaborane 1s l ess  stable than pentaborane . 
Lewis Flight Propulsion Laboratory 
National Advisory Committee fo r  Aeronautics 
Cleveland, Ohio, April 26, 1956 
m 
(D 
1. McDonald, Glen E.: The R a t e  of Decomposition of Liquid Pentaborane 
from 85' t o  202O C.  NACA RM E55H01, 1955. 
. 
5 
Cemper - 
Lture , 
TY 
OC 
185 
196 
218 
244 
Reciprocal 
rate constant, constants, CDl of tenrperature , 
Log of Reactfon Pseudo- Pressure, 
( 1/T) , 
log k sec-1 cm-0.5 Hg sec-1 oK-l 
constants y k, k' Y 
2. 18xL0"3 
2.13 
2.04 
1.94 
Ethylpentaborane I 
7.9 
59.5 
24.3 
45 .O 
98.0 
18 .O 
46 .O 
65 .O 
80.9 
35.0 
70.0 
-6.92 
-6.92 
-6.35 
-6.41 
-6.49 
-5.72 
-5.66 
-5.66 
-5.64 
-4.52 
-4.54 
Pentaboram I 
218 
244 
2 . 0 ~ 0 - 3  
118.6 
139 .1 
1.94 37.0 
4. 6X10'6 
6. 5XLOm5 
1.3xL0'4 
3. 9 x w 7  
1.1~10-~ 
1. X O - ~  
-6.41 
-4.95 
-4 -92 
6 NACA RM E56D26 
S t i r r e r  
L Heater with 
thermostat 
L Reaction 
vessel 
bnometer 
Figure 1. - Apparatus. 
NACA RM E56D26 
0 20 30 40 50 60 
T k e ,  t, hr 
(a) Temperature, 1850 C. 
Figure 2. - Variation of total pressure of ethylpentaborme 
with time at various temperatures. 
7 
. .  . . . .  . .   . .  . . 690V . .  . . . 
NACA RM E56D26 r 9 
Time, t, hr 
( c )  Temperature, 2189 C. 
Figure 2. - Continued. Var iat ion of t o t a l  g r e e . T e _ . r f  
varioue temperatures. 
NACA RM ES6D26 
2 S 
. 
0 10 20 30 $0 50 60 
T h e ,  t, hr 
(c) Concluded. Temperature 2l8O C. 
Figure 2. - Concluded. Variation of t o t a l  pressure of 
ethylpentabmane with time a t  various temp=raturers. 
12 NACA RM E56D26 
Time, t, hr 
Figure 3. - Variation of total pressure of pentaborme 
with time a t  temperature of 2180 C. 
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14 
T W ,  t, hr - 
(b) Pentabmane. 
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Figure 4. - Concluded. Variatfon of total pressure 
with time a t  a temperature of 244’ C. 
.. .. . 4069 
(a) IPenperatme, 1850 C. 
Figure 5. - Iog of reciprocal of remaining ethylpentaboram fraction 
plotted against t3me at va~icrua temperatures. 
.. . . . . . . . . . . 
16 - NACA RM E56D26 
(b ) Temperature, 196' C . 
Figure 5. - Continued. Log of reciprocal  of remaining ethyl- 
pentaborane traction plotted againet t ime at var'ioue 
temperatures. 
P 
HACA RM E56D26 - 17 
l-i 
a 
(c ) Temperature, 218O C. 
Figure 5. - Continued. Log of reciprocal of remaining  ethylpentaborane 
fraction plotted against time at various temperatures. 
- NACA RM E56D26 
NACA EM E56D26 m 19 
1 
1 
(a) Temperature, 244O C. 
Figure 5. - Concluded. Log of reciprocal of remainlng  ethglpentaborane 
fraction  plotted against time at various temperatures. 
20 - NACA RM L56D26 
0 Ethylpentaborane 
-7 -01 I I I I I I I I I I 
1.92 1.96 2.00 2.04 2.08 2.12 2.16 2-20 
Temperature, 1/Txl@ OK-'. 
Figure 6. - Correlation of rate constant. with temperature. - 
NACA - LansiI 
. 
f 


Thermal decomposition of ethylpentaborane in gas phase

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