Industrial and Laboratory Pyrolyses

6 Kinetics and Mechanism of Hydrogenolysis of Propylene in the Presence of Deuterium YOSHINOBU YOKOMORI, HIROMICHI ARAI, and HIROO TOMINAGA

Downloaded by GEORGETOWN UNIV on February 6, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0032.ch006

Faculty of Engineering, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, Japan

Introduction In our previous study, (1) the introduction of large amounts of hydrogen into a hydrocarbon p y r o l y s i s system was found to enhance the rate of p y r o l y s i s and to r e s u l t i n increased y i e l d s of ethylene. The role of hydrogen was discussed i n terms of the reaction kinetics and mechanism where hydrogenolysis of higher olefins into ethylene has an important r o l e . In t h i s connection, hydrogenolysis of 1-butene (2) and isobutene (3) had been investigated to demonstrate the k i n e t i c s of consecutive demethylation of the o l e f i n s into ethylene. On the other hand, a k i n e t i c i n v e s t i g a t i o n had been reported on thermal hydrogenolytic demethylation of propylene by Amano, A. and Uchiyama, M. (4) The reaction can be expressed by the stoichiometry, C C CH 3 H 6 + H2 = 2H4 + 4, and i s s u f f i c i e n t l y clean for rate measurements. observed rate was described by

The

d[C2H4]/dt = k[C3H6][H2]1/2, mol/1, sec k = 1012.3 exp(-55,900/RT). The values of o v e r a l l three-halves order and Arrhenius k i n e t i c parameters were interpreted i n terms of a free r a d i c a l chain mechanism, which was subsequently supported by Benson, S.W. and Shaw, R. (5) The free r a d i c a l chain mechanism, which also could be successfully applied to the hydrogenolytic demethylation of toluene, (6,7,8) i s , to our understanding, e s s e n t i a l l y as follows. C

3H6

C H

2=CHCH2'

+

H·

84

Albright and Crynes; Industrial and Laboratory Pyrolyses ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

6.

YOKOMORi

Hydrogenolysis of Propylene

ET AL.

85

c H- + C3 H. . , H"2 + CH 6 ^ n=CHCH " 2-' N

J

Tll

0

u o u o 0

0

T

2

Η

·

+

C

H

3 6 - ^

•CH + H 3

2

H

+

4

C H

- 3

> CH + H4

g ^ •CH + C H ν - CH + CH =CHCH Reaction (e) was p o s t u l a t e d by Amano (7) as a process i n v o l v i n g a hot η-propyl r a d i c a l . Benson and Shaw (5) suggested that r e a c t i o n (e) was a stepwise one c o n s i s t ing o f a d d i t i o n of hydrogen atom t o propylene followed by e l i m i n a t i o n o f methyl r a d i c a l , where the former step being rate c o n t r o l l i n g . Most i n t e r e s t i n g i s the behav i o r o f the hot η-propyl r a d i c a l , namely i t s decomposi t i o n i n t o ethylene and a methyl r a d i c a l or i n t o propy lene and a hydrogen atom, or s t a b i l i z a t i o n t o give propane a f t e r hydrogen a b s t r a c t i o n . Furthermore, the terminal a d d i t i o n of a hydrogen atom to propylene to give a hot sec-propyl r a d i c a l and i t s behavior are a l s o noteworthy. I t was attempted i n t h i s study, t h e r e f o r e , to have a more d e t a i l e d i n v e s t i g a t i o n o f propylene p y r o l y s i s i n the presence of hydrogen. Furthermore propylene was subjected t o p y r o l y s i s i n the presence of deuterium. K i n e t i c isotope e f f e c t was measured and deuterium d i s t r i b u t i o n s i n the p y r o l y s i s products were analyzed. The experimental r e s u l t s were i n t e r p r e t e d i n l i n e w i t h the r e a c t i o n mechanism above mentioned, and discussed i n the l i g h t o f RRKM theory which q u a n t i t a t i v e l y deals w i t h the behavior of chemical a c t i v a t i o n system such as the decomposition rates of hot a l k y l r a d i c a l s . 3


f 2

C

5

6

h

4

2

2

Experimental M a t e r i a l s . C y l i n d e r propylene, 99.7 m o l l p u r i t y w i t h p r i n c i p a l i m p u r i t i e s being methane and ethane, was used without f u r t h e r p u r i f i c a t i o n . C y l i n d e r hydrogen, 99.99 moll was used a f t e r p u r i f i c a t i o n over Deoxo cata l y s t and z e o l i t e 13Y. C y l i n d e r deuterium, 99.5 m o l l p u r i t y w i t h p r i n c i p a l i m p u r i t i e s being H2 and HD and n i t r o g e n , was used a f t e r p u r i f i c a t i o n over Pd/Al2Û3 and 13Y. Cylinder n i t r o g e n , 99.99 m o l l was used without further p u r i f i c a t i o n . Apparatus and Procedure. A conventional flow system was used. Hydrogen (or deuterium) and propylene were separately supplied at c o n t r o l l e d and constant



Temp. Methane (mol%) (°C) 57.6 800 27.5 800 36.1 775 14.9 775 22.5 750 11.6 750 19.2 725 8.1 725 7.5 700 4.4 700 1.9 700 3.7 675 1.6 675

1.0 0.2 0.2 0.1 0.1 0.1 0.03

—

—

Ethane (moll) 4.8 1.0 2.0 0.4

Ethylene (moll) 55.6 27.8 33.6 15.0 19.8 9.6 18.9 8.1 7.9 4.6 1.9 3.6 1.7

Reaction time (sec) 0.92 0.45 1.00 0.46 1.02 0.53 2.20 1.00 1.98 1.13 0.51 2.14 1.08 2

6

H C3H 9.5 10.2 10.0 10.2 10.0 13.0 11.1 10.7 10.0 9.8 10.5 10.6 10.1 1

2

ôbs (ll/2/mol / .sec) 8.36 7.17 4.05 3.42 2.19 1.84 0.904 0.802 0.391 0.393 0.355 0.158 0.147

Table I. P y r o l y s i s of Propylene i n the Presence of Hydrogen



6. YOKOMORi ET AL.

87


flow r a t e s , mixed at d e s i r e d mole r a t i o s , and then introduced i n t o a r e a c t o r . The r e a c t o r was c o n s i s t e d of two quartz tubes (6 mm outer diameter of the smaller tube and 11 mm inner diameter o f the bigger tube); t h e i r length was 100 mm. The reactants flowed through the annular space of 2.5 mm width. The r e a c t o r was f i t t e d i n an s t a i n l e s s - s t e e l block heated by an elect r i c furnace. Temperature p r o f i l e s of the r e a c t o r were measured during each run by a movable chromelalumel thermocouple along the c e n t r a l a x i s of the v e s s e l . The average e f f e c t i v e temperature and r e a c t o r volume were determined by usual method. (9) The product gas was passed through a trap (immersed i n an ice-methanol bath) i n which condensation products c o l l e c t e d . The r e a c t i o n was s t u d i e d under c o n d i t i o n s v a r i e d over the f o l l o w i n g range; temperature, 675 ^ 800°C; contact time, 0.5^3.8 sec; hydrogen or deuterium/propylene mole r a t i o , up t o approximately 10. A n a l y s i s . Gaseous products, c o n s i s t i n g of hydrocarbons from methane up to € 3 ' s were analyzed by gas chromatography using a Porapak Q column (2 m long) a t 40°C w i t h n i t r o g e n as c a r r i e r gas. D i s t r i b u t i o n of deuterated products were determined by gas chromatographic-mass spectrography using a Porapak Q column (1 m long) at 40 * 90°C w i t h helium as c a r r i e r gas. P a t t e r n c o e f f i c i e n t s o f ethylene-do, - d i and propylene-do were obtained from mass s p e c t r a l data of API Research P r o j e c t 44. The fragmentation mode o f propylene-di was assumed to be the same w i t h propylenedo. I t was assumed that both do and d i compounds had the same s e n s i t i v i t y . Results The experimental r e s u l t s w i t h hydrogen and deuterium are summarized i n Tables I and I I . The o v e r a l l rates o f ethylene formation were described by 1.5 order rate equation as f o l l o w s : d [ C H ] / d t = k [ C H ] [ H or D ]1/2 2

4

o b s

3

6

2

2

Figure 1 shows Arrhenius p l o t s of the 1.5 order r a t e constants, which are r e s p e c t i v e l y given by, 1/2 /mol 1/2 10 14.0 exp(-64,400/RT), 1 sec 1/2 10 15.1 exp(-70,600/RT), 1 1/2 /mol sec



Temp. Methane (°C) (do+ dx) (moll) 59.1 800 42.7 800 775 34.1 28.0 775 9.4 750 9.3 725 3.7 725 5.8 700 3.6 700 3.2 675 2.5 675

3.0 1.6 1.4 0.7 0.1 0.2 0.1 0.2 0.1 0.1 0.1

Ethane (mol%) (d^di+d^)

Ethylene Reaction time (sec) (mol*) 1.47 53.6 1.12 42.3 1.28 26.9 1.02 23.6 0.83 9.4 2.11 9.1 1.02 3.9 3.00 5.9 2.12 3.6 3.76 3.1 3.00 2.5 °2 6

9.7 9.7 9.8 12.3 10.8 9.3 9.7 10.5 9.7 11.2 10.3

Λ

C3H 5.24 5.40 2.21 2.56 1.14 0.433 0.373 0.189 0.163 0.077 0.078

1

2

kobs (ll/2/mol / -sec)

Table I I . P y r o l y s i s o f Propylene i n the Presence of Deuterium


89


YOKOMORI ET AL.

ο

Η ΑΜΑΝΟ 2

Ηο • Q )ΤΗ IS WORK δ

î.o μ

2\

2

Ο Αν

\\

0.5


°Ο

\2

0,0

Α

Χ -0.5 h

\

\ -1.0 0.90

0.95

1.00

1.05

1/Τ Χ 10

Figure 1. Arrhenius plots of propyl ene hydrogenolysis rate constants

50 ο METHANE-d

0

10

S

•

METHANE- i

δ

METHANE- 2

d

d

30

20

0 Figure 2.

10 20 30 i*0 50 PROPYLENE HYDRODEMETHYLATED (MOLES) Deuterium distribution in methane (675° 800°C). D,/C,H, — ca. 10.

~


90

INDUSTRIAL AND LABORATORY PYROLYSES

The r a t e constant obtained i n the presence of hydrogen i s i n f a i r l y good agreement w i t h that obtained by Amano.

(1) The o v e r a l l k i n e t i c isotope e f f e c t i s defined as the r a t i o of k values f o r hydrogen and deuterium, and the values were found to vary as (kobs)H2/Côbs)D7 1.45^2.13 i n the temperature range studied. Table I I I shows d i s t r i b u t i o n s of deuterated products. The expres s i o n " f r a c t i o n " i n d i c a t e s a mole composition of the re spective d ^ d i compounds. The expression " y i e l d " i n d i c a t e s the moles of products obtained from 100 mol of propylene. Figures 2, 3, and 4 show the changes i n deuterium d i s t r i b u t i o n s as a f u n c t i o n of propylene hydrodemethylation. Discussion =


0

K i n e t i c Isotope E f f e c t , The steady-state assumpt i o n was a p p l i e d to the f r e e r a d i c a l chain mechanism, (a) *v> (h), g i v i n g r i s e to the o v e r a l l r a t e equation as follows. d C H ]/dt = k [H-][C H ] = ^ ^ ^ ) t

2

4

e

3

6

1

/

2

[α Η ][Η ] 3

6

1

/

2

2

The o v e r a l l r a t e constant can be separated i n t o two terms; kç and ( k k^/kb k c ) / . The isotope e f f e c t s are examined f o r both terms below: [1] kg term Equation (e) i s separated i n t o two steps, that i s , a d d i t i o n and decomposition steps. 1

2

a

H. + C H

e

1

(.CH CHCH )* -2l£> CH =CH + ·CH H (a) A d d i t i o n step (e-1) The C-D bond formed i s stronger than the C-H bond by about 1^2 kcal/mol, so that ( k - i ) n / ( k - l ) D term i s expected to be smaller than u n i t y , where the symbol (ê-l)p represents the r a t e constant f o r a d d i t i o n of deuterium atom, e t c . Based on the k i n e t i c isotope eff e c t at 25°C reported by Daby et a l . , (10) ( k _ i ) H / ( e-l)D 1000°K i s estimated at 0.91. (b) Decomposition step (e-2) This i s an unimolecular decomposition process of a hot η-propyl r a d i c a l . This rate constant k -2 can be estimated by RRKM theory. D e t a i l s of the c a l c u l a t i o n are given i n the appendix. 3

6

">

2

3

2

e

2

3

e

e

k

a t

e

(k _ ) e

2

9

H

= 3.39 χ 1 0 ,

sec"

1


YOKOMORi ET AL.


25

20

ο

ETHYLENE-d

•

ETHYLENE-di

δ

ETHYLENE-d

0

2

CO LU

/


^15

10

0 Figure 3.

10 20 30 i»0 50 PROPYLENE HYDRODEMETHYLATED (MOLES) Deuterium distribution in ethylene (675° ~ 80O°C). DJC H = ca. 10. S

S

o PR0PYLENE-d

c

0 Figure 4.

ο

PR0PYLENE-ëi

δ

PROPYLENE-^

10 20 30 HO 50 PROPYLENE HYDRODEMETHYLATED (MOLES) Deuterium distribution in propylene (675° ~ D /C H = ca. 10. t

3

800°C).

6



nj-l Π I I..« «I - I "

il

di d.2 Q3 800°C 1.47sec Fraction 0.23 0.57 0.18 0.02 conv. 53.6% Yield(l) 13.6 33.7 10.6 1.2 800°C 1.12sec Fraction 0.21 0.60 0.17 0.02 conv. 42.31 Yield(l) 9.0 25.6 7.3 0.9 775°C 1.28sec Fraction 0.25 0.62 0.13 conv. 26.9% Yield(l) 8.5 21.1 4.4 775°C 1.02sec Fraction 0.17 0.68 0.15 conv. 23.6% Yield(%) 4.8 19.0 4.2 750°C 0.84sec Fraction 0.22 0.70 0.08 conv. 9.4% Yield(%) 2.1 6.6 0.8 725°C 2.01sec Fraction 0.17 0.72 0.11 conv. 8.3% Yield(%) 1.4 6.0 0.9 725°C 0.99sec Fraction 0.24 0.67 0.09 conv. 4.1% Yield(%) 1.2 3.2 0.4 700°C 2.98sec Fraction 0.18 0.72 0.10 conv. 5.6% Yield(%) 1.0 4.0 0.6 700°C 2.12sec Fraction 0.19 0.74 0.07 conv. 3.59% Yield(%) 0.7 2.6 0.2 675°C 3.76sec Fraction 0.20 0.72 0.08 conv. 3.12% Yield(%) 0.6 2.3 0.3 675°C 3.00sec Fraction 0.16 0.75 0.09 conv. 2.54% Yield(%) 0.4 1.9 0.2 -

1 .11 .1 . MMii ι,

Methane I

' ~Î

H

f

"

dp di dz 0.10 d$ 0.16 0.43 0.30 8.6 23.1 16.1 5.4 0.21 0.49 0.22 0.06 8.9 20.7 9.3 2.5 0.32 0.51 0.17 8.6 13.7 4.6 0.32 0.53 0.15 7.5 12.5 3.5 0.39 0.55 0.06 3.7 5.2 0.6 0.45 0.47 0.08 3.7 3.9 0.7 0.49 0.46 0.05 2.4 2.2 0.2 0.47 0.47 0.06 2.6 2.6 0.3 0.50 0.46 0.04 1.8 1.7 0.1 0.54 0.43 0.03 1.7 1.3 0.1 0.57 0.40 0.03 1.4 1.0 0.1 -

II

Ethylene ~

di» 0.01 0.5 0.02 0.8 -

IW Ij«

» Jill • W• - •

dp di 0.2 0.31 0.36 0.24 0.06 0.03 13.4 15.6 10.4 2.6 1.3 0.39 0.39 0.16 0.06 22.5 22.5 9.2 3.5 0.58 0.29 0.13 42.4 21.2 9.5 0.66 0.26 0.08 50.5 19.9 6.1 0.85 0.15 77.0 13.6 0.80 0.18 0.02 73.3 16.5 1.8 0.91 0.09 86.6 8.6 0.87 0.13 82.2 12.3 0.93 0.07 89.7 6.7 0.91 0.08 0.01 88.2 7.8 1.0 0.94 0.06 - ·91.6 5.8 -

«..!

Propylene

Table I I I . Deuterium D i s t r i b u t i o n i n the P y r o l y s i s Products of Propylene


6.

YOKOMORi


ET AL.

93

-1

This computation shows that there i s no appreciable isotope e f f e c t i n the decomposition step. These considerations i n d i c a t e that k term, ( k ) r l / ( k ) D °· > account f o r our e x p e r i mental r e s u l t s , that i s (k bs)H2/ Côbs)D2 1.45^2.13. The i m p l i c a t i o n i s that the second term should be much l a r g e r than u n i t y . [2] ( k kd/kb k ) l / 2 term The ( k kd/kb k ) / term i s c o r r e l a t e d w i t h hy drogen atom concentration as f o l l o w s . 1/2 e

=

e

9

i s

u n a b l e

t 0

e

=


0

a

c

i

a

z

c

In t h i s equation, k and kb should remain nearly con stant i n the presence of hydrogen or deuterium, and thus these terms do not c o n t r i b u t e to k i n e t i c isotope e f f e c t . Small d i f f e r e n c e s i n k can be estimated from the Evans-Polanyi-Semenov r u l e . (11) E log A kcal/mol 1/mol sec 5.0 10.1 10.1 a

c

a

1/2 On the other hand f o r r e a c t i o n ( d ) , the a l l y l r a d i c a l i s much l e s s r e a c t i v e , so that zero point energy d i f ference between H-H and D-D bonds p o s s i b l y contributes to the isotope e f f e c t t o a large extent. I f i t i s as sumed that the d i f f e r e n c e , 2 kcal/mol, i s f u l l y r e f l e c t e d , the isotope e f f e c t i s c a l c u l a t e d as f o l l o w s . 1/2 3

exp(2.0 χ 10 /RT)} In summary,

exp(0.9 χ

10VRT).


94


The k i n e t i c isotope e f f e c t can be c a l c u l a t e d from t h i s equation. The c a l c u l a t e d and observed values are shown as f o l l o w s . Temperature (°C) 700 800 750 1.45 1.68 1.96 1.53 1.56 1.60

O v e r a l l K i n e t i c Isotope E f f e c t


Observed Calculated

The r e l a t i v e l y good agreement between the c a l c u l a t e d and observed values seemingly supports the v a l i d i t y of the proposed r e a c t i o n mechanism. Deuterium D i s t r i b u t i o n of Products. Methane. Figure 2 i n d i c a t e s that methane-di i s produced i n l a r g e s t concentrations of a l l methanes produced when deuterium i s mixed w i t h the propylene, and methane-do i s the second l a r g e s t . The methyl r a d i c a l , produced by hydrogenolysis of propylene or by r e a c t i o n (e), i s converted e i t h e r to methane-di by r e a c t i o n ( f ) or to methane-do by r e a c t i o n (g) f

•CH + D 3

>

2

CH D + D3

Experimental values of [ C H 3 D ] / [ C H 4 ] = 3 suggests that kf'/kg equals to 0.3. This seems reasonable from a k i n e t i c standpoint. Formation of methane-d2 i n the advanced stage o f hydrogenolysis i s remarkable. This probably comes from CH2D- which i s produced by hydrogenolysis of deuterated propylene given by the reactions (d ) and ( i - l § - 2 ) , f

f

D- + CH =CH-CH 2

3

lf

1

" >

(CH -CH-CH )* 2

3

D CH D-CH=CH + H2

2

CHD=CH-CH + H3

This explanation i s apparently supported by the observ a t i o n that [CH D ]/[CH D] = [C H D] / [ C ^ ] 2

*

2

3

3

5

A n a l y s i s by Professor K. Kuratani and T. Yano of mono-deuterated propylenes by use o f microwave spectroscopy has revealed that they are mainly CH D-CH=CH . ( P r i v a t e communication) ?

?


6.

YOKOMORi ET AL.


95

Ethylene. I t should be noted i n Figure 3 that both ethylene-di and -do are the primary products i n the i n i t i a l stage o f hydrogenolysis. Formation of ethylenedi i s n a t u r a l l y expected by the presence o f r e a c t i o n (e'), CH =CH-CH + D> CH =CHD + -CH e >

2

3

2

3

The formation o f ethylene-d i s r a t h e r unexpectedly l a r g e , but the p r e v i o u s l y mentioned r e a c t i o n ( i ) can e x p l a i n the production o f Η· i n the r e a c t i o n system where the hydrogen molecule i s i n i t i a l l y absent, and the attack of Η· on propylene may produce e t h y l e n e - d . The decrease i n ethylene-do y i e l d i n the advanced stage i s probably due to the H-D exchange of propylene and ethy lene. The gradual increase i n e t h y l e n e - d y i e l d i s p a r t l y accounted by the hydrogenolysis, w i t h D«, o f propylene-di produced by r e a c t i o n ( i - 3 ) . Propylene. Figure 3 shows the successive deuterat i o n of propylene along w i t h demethylation. The rate of d e u t e r a t i o n o f propylene i s roughly two times l a r g e r than that of hydrogenolytic demethylation. Furthermore, a n a l y s i s of propylene-di and of ethylene-di i n the i n i t i a l stage, or below ca. 101 conversion o f propylene, gives the rate r a t i o of d e u t e r a t i o n (mostly v i a termi nal a d d i t i o n of D« to propylene) to demethylation ( v i a i n t e r n a l a d d i t i o n ) as approximately 4 . 5 . In view of the high temperature employed i n t h i s study, t h i s r a t i o i s not incompatible w i t h the observation at room tem perature that the r a t i o o f t e r m i n a l a d d i t i o n t o i n t e r nal a d d i t i o n i s more than 1 0 . (12) 0


f

0

2

f

Conclusions The k i n e t i c isotope e f f e c t observed on the o v e r a l l rates o f hydrogenolytic demethylation o f propylene i n the presence o f deuterium was s u c c e s s f u l l y i n t e r p r e t e d i n terms of a free r a d i c a l chain mechanism. L i t t l e d i f f e r e n c e s were i n f e r r e d to e x i s t between the rates o f a d d i t i o n of Η· or D- to propylene and also between those o f unimolecular decomposition o f the produced hot η-propyl r a d i c a l s , and thus the k i n e t i c isotope e f f e c t was a s c r i b e d mainly t o the d i f f e r e n c e between the steady state concentrations of [Η·] i n the presence o f hydrogen and the concentrations o f [D«] + [Η·] i n the presence o f deuterium. In more d e t a i l , conversion o f rather i n a c t i v e a l l y l r a d i c a l by metathesis w i t h deu terium i n t o an a c t i v e D- i s r e l a t i v e l y slow. This was concluded to be the main cause o f the observed k i n e t i c isotope e f f e c t , which agrees w e l l w i t h the c a l c u l a t e d


96



values based on the above free r a d i c a l chain mechanism. Deuterium d i s t r i b u t i o n i n the r e a c t i o n products a l s o supports the mechanism proposed. In summary, the most important r o l e of hydrogen molecule i n p y r o l y s i s of propylene i s to convert the a l l y l r a d i c a l , which other wise e a s i l y polymerizes, i n t o an a c t i v e hydrogen atom that promotes the key r e a c t i o n , namely hydrogenolytic demethylation of propylene. Acknowledgement. We wish to thank professors Akira Amanο and Osamu Horie f o r t h e i r h e l p f u l d i s c u s s i o n s on chemical a c t i v a t i o n system. We are a l s o g r a t e f u l to professor Tadao Yoshida f o r h i s suggestion on RRKM pro gram. We are favoured to have the a s s i s t a n c e of Mr. Shozo Yamamiya who c o n t r i b u t e d h i s experimental s k i l l . Summary The k i n e t i c s and mechanism of hydrogenolysis of propylene to give ethylene and methane were investigated i n a flow r e a c t o r made of quartz, at temperatures be tween 675 and 800°C. The molar r a t i o of deuterium or hydrogen to propylene employed was up to approx. 10. The main o b j e c t i v e of t h i s i n v e s t i g a t i o n i s to elucidate the r o l e played by hydrogen i n the p y r o l y s i s of hydro carbons when hydrogen i s employed to enhance the rates of hydrocarbon decomposition and to promote ethylene formation. The o v e r a l l rates of ethylene formation were described by 1.5 order r a t e equations as f o l l o w s : d[C H ]/dt 2

= k

4

o b s

[C H ][H 3

6

2

or D ] 2

1 / 2

, mol/1

sec,

where the r a t e constants are r e s p e c t i v e l y given by, (k ( k

Q b s

)

1 4

= 10 ·

H 2

)

obs D

= 2

1 θ 1 5 β 1

0

exp(-64,400/RT) exp(-70,600/RT).

The k i n e t i c isotope e f f e c t observed on the o v e r a l l r a t e s , ranging from 2.0 at 700°C to 1.5 at 800°C, was i n t e r p r e t e d i n terms of a free r a d i c a l chain mechanism where a unimolecular decomposition of a hot n-propyl r a d i c a l , produced by a d d i t i o n of Η· or D- to propylene, plays a key r o l e . The d i f f e r e n c e i n the observed over a l l rates was mainly a s c r i b e d to that i n steady state concentrations of Η· and D« which are produced, i n part, through the r e a c t i o n between a l l y l r a d i c a l and H o r D 2 . No appreciable d i f f e r e n c e between the decomposition rates of C3H7 and C3H6D was i n f e r e d by RRKM theory. 2


6. YOKOMORI ET AL.

Literature (1) (2) (3) Downloaded by GEORGETOWN UNIV on February 6, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0032.ch006

(4) (5) (6) (7) (8) (9) (10) (11) (12) (13)


97

Cited

Kunugi, T., Tominaga, Η . , and Abiko, S., Proceed ings of the 7th World Petroleum Congress (Mexico City) (1967). Kunugi, T., Tominaga, Η . , Abiko, S., Uehara, Κ . , and Ohno, T . , Kogyo Kagaku Zasshi, (1967) 70, 1477. Tominaga, H . , A r a i , H . , Moghul, K . A . , Takahashi, Ν . , and Kunugi, T., ibid., (1971) 74, 371 Amano, A. and Uchiyama, M . , J . Phys. Chem., (1963) 67, 1242. Benson, S.W. and Shaw, R., J . Chem. Phys., (1967) 47, 4052. Matsui, Η . , Amano, Α . , and Tokuhisa, Η . , B u l l . Japan P e t r o l . I n s t . , (1959) 1, 67. Amano, Α . , Tominaga, H . , and Tokuhisa, Η., ibid., (1965) 7, 59. Tominaga, Η . , A r a i , Η . , Kunugi, T., Amano, Α . , Uchiyama, Μ . , and Sato, Y., B u l l . Chem. Soc. Japan, (1970) 43, 3658. Hougen, O.A. and Watson, K . M . , "Chemical Process P r i n c i p l e s " . p.884, John Wiely & Sons, I n c . , New York, (1950). Daby, E.E., N i k i , Η . , and Weinstock, B . , J . Phys. Chem., (1971) 75, 1601. Semenov, N . N . , "Some Problems i n Chemical Kinetics and R e a c t i v i t y " , Pergamon Press and Princeton U n i v e r s i t y , (1958). Falconer, W . E . , Rabinovitch, B . S . , and Cvetanovic, R . J . , J . Chem. Phys., (1963) 39, 40. Rabinovitch, B.S. and Setser, D.W., Adv. Photochem., (1964) 3, 1.

Appendix RRKM Calculation on the Unimolecular Decomposition of Hot n- and sec-Propyl Radicals. The basic concept of the computation i s given elsewhere. (12) The thermochemical data were obtained from the l i t e r a t u r e . (13) V i b r a t i o n models and the other parameters were mostly adopted from the work by Falconer, W.E. et a l . , (12) but with some modifications. The calculated rate con stants shown below are at 998°K and atmospheric pres sure .



2

0

T

Η·

H

CH =CHCH ^ 2 3 7*

3

CH =CHCH

D >

k l 0

Z

2

+ H-

2

^ CH =CHD + -CH

3

3

2

2

3

3

>

2

2

(ΟΗ €Η ΟΗ )*-^>ΟΗ =αΗ

3

2

+ -CH

3

3

(CH CHCH )*-^Z CH =CHCH + H-

V S k

2

CH =CDCH + H-

k

(CH CHDCH )* S

k

3

^3^CHD=CHCH + H-

>

CH,-CHCH_ + D2 3

5

3

4

2

2

2

CH =CHCH + D-

(CH DCHCH )* 2i CH DCH=CH

Elementary Reactions

9

9

e

e

2

2

(k _ )

3.39 χ ΙΟ = ( k _ )

1.95 χ 1 0

9

9

9

9

9

9

3.48 χ 1 0 =

1.89 χ 1 0

1.18 χ 1 0

0. 757 χ 1 0

1.88 χ 1 0

0.400 χ 1 0

Rate Constant, sec ^


H

D

5

ο

*

| g

|

CO 00

Industrial and Laboratory Pyrolyses

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