A Fundamental Chemical Kinetics Approach to Coal Conversion

7 A Fundamental Chemical Kinetics Approach to Coal Conversion STEPHEN E . STEIN

Downloaded by UNIV LAVAL on September 27, 2015 | http://pubs.acs.org Publication Date: October 26, 1981 | doi: 10.1021/bk-1981-0169.ch007

Department of Chemistry, West Virginia University, Morgantown, WV 26506

At the present time, few, i f any, details of chemical reaction mechanisms in coal conversion are known with certainty. This situation is particularly distressing in the areas of coal liquefaction and pyrolysis where chemical kinetics may strongly influence process efficiency and product quality. To improve this situation, in recent years a number of research groups have been performing chemical studies of coal and "model" compound reactions. Thermochemical kinetic methods (1) can be of great value for interpreting and generalizing results of these studies. These methods are now indispensable for mechanistic analysis of many practical chemical systems involving highly complex reactions, including oxidation, combustion, atmospheric chemistry and pyrolysis. With recent extensions of thermo chemical kinetics estimation methods to coal-related molecules (2,3,4) and free radicals (5), it is now feasible to apply thermochemical kinetics analysis to a wide range of coal -related chemical systems. Thermochemical and kinetics estimation methods are particularly suited for analysis of coal systems since these methods are applicable not only to reactions of molecules but also to reactions of specific molecular structures. T h i s work presents the f i r s t systematic a p p l i c a t i o n of these methods to c o a l chemistry. This a n a l y s i s i s intended not only to suggest l i k e l y r e a c t i o n mechanisms, but a l s o to demonstrate the unique power of thermochemical k i n e t i c s methods for s e m i - q u a n t i t a t i v e a n a l y s i s of the complex chemistry of c o a l conversion. Conversion of Gas-Phase Data to the L i q u i d Phase Since most p r e d i c t i v e thermochemical and k i n e t i c s methods y i e l d gas-phase v a l u e s , i t i s necessary to consider the a p p l i c a b i l i t y of these v a l u e s to the l i q u i d phase. For e q u i l i b r i a i n v o l v i n g no change i n the number of moles, Δη = 0,

0097-6156/81/0169-0097$07.50/0 © 1981 American Chemical Society

In New Approaches in Coal Chemistry; Blaustein, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

NEW

98

APPROACHES IN C O A L

CHEMISTRY

a good b a s i s e x i s t s f o r assuming t h a t , i n the absence of s i g n i f i c a n t d i f f e r e n c e s i n solvent-molecule i n t e r a c t i o n s between r e a c t a n t s and products, e q u i l i b r i u m constants are n e a r l y the same i n gas and l i q u i d phases (6). W i t h i n the framework of t r a n s i t i o n - s t a t e theory, the same c o n c l u s i o n a p p l i e s to r a t e constants f o r unimolecular r e a c t i o n s not subject to "cage e f f e c t s (7a). In p r a c t i c e , cage e f f e c t s can be s i g n i f i c a n t o n l y f o r bond homolysis r e a c t i o n s ( R i — R Ri» + R »). In c o n t r a s t , when Δη φ 0 one f i n d s c o n s i d e r a b l e disagreement i n the l i t e r a t u r e concerning r e l a t i v e gas and l i q u i d e q u i l i b r i u m (and r a t e ) constants, even i n the absence of s o l v a t i o n e f f e c t s . Depending on the p a r t i c u l a r t h e o r e t i c a l treatment, bimolecular r e a c t i o n r a t e constants have been estimated to be somewhere between 2 and 100 times f a s t e r i n s o l u t i o n than i n the gas phase (6). However, the very l i m i t e d experimental data a v a i l a b l e i n d i c a t e that there i s l i t t l e , i f any, systematic d i f f e r e n c e between bimolecular r a t e constants i n the two phases (6d). 11

2


2

We have examined t h i s problem as f o l l o w s . equilibria

A

+

B

î

For a s s o c i a t i v e

C

(1)

r e l a t i v e l i q u i d and gas-phase e q u i l i b r i u m constants, K^/Kg, have been computed u s i n g v a p o r i z a t i o n data (8) and equation 2 (6d), K

W

l

κ~

g

=

V

~~P7~~

c

(

s

RT

}

κ

(

γ

2

)

where i s the ( i d e a l gas) vapor pressure of neat l i q u i d A, V i s the molar volume of the s o l u t i o n and Κγ i s the r a t i o of γ γ Α Β condensed-phase a c t i v i t y c o e f f i c i e n t s , . An analogous C formula f o r r e l a t i v e b i m o l e c u l a r r a t e constants, k^/kg, holds i f C i s regarded as the t r a n s i t i o n s t a t e . A d e t a i l e d d e s c r i p t i o n of these c a l c u l a t i o n s i s i n p r e p a r a t i o n , but the major c o n c l u s i o n i s that i n the absence of s u b s t a n t i a l s o l v a t i o n d i f f e r e n c e s between r e a c t a n t s and products (or t r a n s i t i o n s t a t e ) , w i t h i n a f a c t o r of ca. 2, ~ Kg and k^ ~ k . For s p e c i f i c e q u i l i b r i a of i n t e r e s t , Κχ ~ Kg may o f t e n be d i r e c t l y evaluated by means of equation 2 using Ρ and Κγ values obtained from the l i t e r a t u r e , from e s t i m a t i o n methods (9) or by analogy to r e l a t e d systems. A v a i l a b l e vapor pressure data (8) may be used to roughly examine the magnitude of s o l v a t i o n e f f e c t s on e q u i l i b r i u m constants. Table I l i s t s (Κχ/Kg) v a l u e s f o r s e l e c t e d e q u i l i b r i a i n v o l v i n g polyaromatic molecules. Clearly, reactions l e a d i n g to increased a r o m a t i c i t y are i n h e r e n t l y favored i n s o l u t i o n r e l a t i v e to the gas phase. Table I I shows changes i n vapor pressures r e s u l t i n g from replacement of (hydrogen-bonding) s

Y

g


Coal

STEIN

7.

99

Conversion

Table I.

K

K

;i/ g

f

o

r

Polyaromatic

Equilibria 2

K-/K 1


2

2

(Ô) * ® Γ ©

@6)

2 @ ©

J 1 - 2 5 ^

:

1.05

T/K

g

1.6

! -

1

350

500

8

0.055

500

H r e q u i r e d to balance r e a c t i o n s may be assumed to be i n the gas phase; K^/Kg values are not a f f e c t e d by H . C a l c u l a t e d with a s l i g h t l y modified form o f equation 2 (6d) where Δη φ - 1 assuming Κγ = 1; solvent i s assumed to be pure reactant except where noted; vapor pressures were taken from r e f e r e n c e 8b and c o r r e c t e d f o r noni d e a l behavior by methods i n r e f e r e n c e 9 . Solvent i s assumed to be an equimolar mixture of toluene and benzene. 2

2


100

NEW

Table I I .

APPROACHES

IN C O A L

CHEMISTRY

E f f e c t s on Vapor Pressures of Replacing Hydroxyl


Groups with Methyl Groups. Ρ

vap

(R-CH )/P (R-OH)* vap 3

T/°C

R—

50

100

n-C H —

48

18

n-C H

21

8.6

4.3

4.4

3.1

2.3

2.4

3

6

7

1 3

—

n-C

1 2

H

2 5

—

n-C

i e

H

3 7

—

(gf

28

14

[8ϊέ)

*

150

200

250

300

2.1

1.9

1.7

8.2

5.4

3.9

2.9

4-0

3.2

2.4

f

f

M

Note that P (R XR )/P (R CH R ) ~ 1 f o r X = -0-, -C-, and vap vap S; Ρ are from r e f e r e n c e 8. vap 2



7.

STEIN

Coal

101

Conversion

hydroxyl groups with methyl groups. These values roughly correspond to c o n t r i b u t i o n s to e q u i l i b r i u m or r a t e constants a r i s i n g from formation or d e s t r u c t i o n of a hydrogen bond: although Κχ/Kg values w i l l depend somewhat on the p a r t i c u l a r s o l v e n t . The negation of such e f f e c t s with i n c r e a s i n g temperature i s s t r i k i n g . At c o a l conversion temperatures changes i n hydrogen bonding i n a r e a c t i o n are not expected to change r a t e or e q u i l i b r i u m constants by more than a f a c t o r of 2-3. Other e f f e c t s due to s e l e c t i v e s o l v a t i o n , i n c l u d i n g p o l a r e f f e c t s and c h a r g e - t r a n s f e r i n t e r a c t i o n s , are a l s o expected to be f a r smaller at c o a l conversion temperatures than at lower temperatures where such e f f e c t s are most o f t e n observed. With the general assumption that e q u i l i b r i u m constants are n e a r l y the same i n the gas and l i q u i d phases, a number of aspects of the thermal chemistry of c o a l - r e l a t e d molecules w i l l now be examined. Bond Breaking Three modes of bond breaking may be d i s t i n g u i s h e d i n homogeneous, non-ionic systems, namely bond homolysis,

f r e e - r a d i c a l β-bond s c i s s i o n RCHCH CH R 2

2

and concerted molecular

«*

RCH—CH

2

+

•CH R 2

decomposition,

Rate constants f o r s p e c i f i c r e a c t i o n s belonging to any of the above c l a s s e s of r e a c t i o n s can o f t e n be estimated to a reasonable degree of accuracy (1). Even f o r cases where estimates are o n l y accurate to an order of magnitude or worse, s p e c i f i c r e a c t i o n s may o f t e n be shown to be e i t h e r f a r too slow to account f o r observed k i n e t i c s or very r a p i d . In a d d i t i o n , r e l a t i v e r a t e constants are o f t e n a l l that i s needed to decide between competing mechanisms, and estimates of r e l a t i v e r a t e constants are o f t e n f a r more accurate than estimates of a b s o l u t e r a t e constants. Each of the above three modes of bond breaking w i l l now be i n d i v i d u a l l y d i s c u s s e d . Bond Homolysis. A s u b s t a n t i a l number of gas-phase bond homolysis r a t e constants and f r e e - r a d i c a l e n t h a l p i e s of formation have been determined (1,11) and a f a r greater number may be r e l i a b l y estimated. However, two f a c t o r s must be considered when a p p l y i n g gas-phase bond homolysis r a t e constants to condensedphase systems. F i r s t , any s e l e c t i v e s o l v a t i o n of product r a d i c a l s w i l l tend to i n c r e a s e (k^/kg). However, s o l v a t i o n e f f e c t s on f r e e r a d i c a l r e a c t i o n r a t e s are g e n e r a l l y the


102

NEW

COAL

CHEMISTRY

exception r a t h e r than the r u l e , even i n f a i r l y p o l a r media a t low temperatures ( i . e . , RH

β-bond scission

+

— ιC — C — R

(3)

(4)

C=C

I I

Because o f i t s r e l a t i v e l y low a c t i v a t i o n energy and i t s involvement i n a c h a i n r e a c t i o n , β-bond s c i s s i o n i s o f t e n the dominant mode of bond breaking i n the p y r o l y s i s of l a r g e a l i p h a t i c hydrocarbons. Moreover, s u b s t i t u t i o n o f an oxygen atom f o r a methylene (CH ) group i n the above s t r u c t u r e w i l l o f t e n lead t o an even more r a p i d c h a i n r e a c t i o n because of the r e s u l t i n g lower endothermicity o f r e a c t i o n 4. The s i g n i f i c a n c e o f β-bond s c i s s i o n i n c o a l chemistry i s l i m i t e d by the c o n c e n t r a t i o n o f s u i t a b l e molecular s t r u c t u r e s . Rapid β-bond s c i s s i o n g e n e r a l l y r e q u i r e s s t r u c t u r e s c o n t a i n i n g three or more consecutive, saturated, p o l y v a l e n t atoms such as carbon, oxygen, n i t r o g e n and s u l f u r . Examples o f c o a l - r e l a t e d s t r u c t u r e s s u s c e p t i b l e to r a p i d d i s s o c i a t i o n by β-bond s c i s s i o n are given i n Figure 1. For each s t r u c t u r e , once the C-X bond i s broken by any means, the r a d i c a l formed w i l l r a p i d l y dissociate. I t should a l s o be noted that β-bond s c i s s i o n i s f o r m a l l y involved i n thermal d e s u b s t i t u t i o n of aromatic r i n g s by, say, Η atoms, as f o l l o w s . Η 2

Η·

+

+

(5)

The r a t e o f t h i s r e a c t i o n i s l i m i t e d by the a v a i l a b i l i t y of Η atoms, although under a p p r o p r i a t e c o n d i t i o n s t h i s r e a c t i o n can be an important means o f bond breaking i n c o a l systems (16). Concerted Decomposition. Both unimolecular and bimolecular r e a c t i o n s i n v o l v i n g only molecular species (no f r e e r a d i c a l s o r ions) may play a r o l e i n c e r t a i n aspects of c o a l conversion. In some types o f concerted r e a c t i o n s , however, such as r e a c t i o n s i n v o l v i n g C-C o r C-0 bond breaking, the s i g n i f i c a n c e of such r e a c t i o n s appears to be d r a s t i c a l l y l i m i t e d by the l a c k of s u i t a b l e molecular s t r u c t u r e s . In the case o f b i b e n z y l , the f o l l o w i n g pathway f o r decomposition has been proposed (17):

(6)


NEW

APPROACHES IN COAL

CHEMISTRY

Bond Homolysis i n T e t r a l i n (Θ = 0.00458 T/K)


log[k

1

tet

(Ri—R )/s- ] 2

References

τ

χ

17.9 - 117.8/Θ

a,b

10

16.9 -

a,b

10

16.0 [16.4 -

94.1/Θ

65.2/Θ 66.4/Θ]

c d

(400°C)

"5

1 2

6

years

years

28 hours 28 hours

17.0 -

76.2/Θ

a,b

1 year

16.0 -

59.3/Θ

e

20 min

16.0 -

53.1/Θ

f

12 sec

16.0 -

62.3/Θ

g

3 hours

16.0 -

62.7/Θ

g

4 hours

16.0 -

57.4/Θ

g

5 min

16.6 -

63.2/Θ

h,i

16.6 -

60.6/Θ

j

14 min

16.6 -

60.6/Θ

j

14 min

1.6 hours


Coal

STEIN

7.

105

Conversion

Table I I I .

(con't)

50 min

16.0 - 60.4/Θ


ortho 16.0 - 66.9/Θ meta 16.0 - 65.2/Θ para 16.0 - 66.3/Θ

16.0 - 62.2/Θ

k k k

100 hours 28 hours 60 hours

3 hours

17.2 - 59.3/Θ

1.3 min

Ph^Ph

14.7 - 50.8/Θ

40 sec

Ph^h

14.8 - 56.7/Θ

50 min

15.8 - 54.6/Θ

1 min

p

15.8 - 60/Θ

1 hour

h

Ph

Ph

Ph

Ph

Ph

16.0 - 72/Θ

200 days

Ph

Ph

16.0 - 62/Θ

3 hours

16.0 - 63.7/Θ

9 hours

15.3 - 70.6/Θ

330 days

Ph

Ph"


106

NEW

Table I I I .

Ph

Ph

a

gas

(Ph

η

14.9 - 67.2/θ

η

(Ri-R )/k 2

7 hours

70 days

/~\ _(Ph Ph) = k

Ph); k ( P t i P h ) determined gas

^k

CHEMISTRY

(con't)

15.7 - 62.4/θ

Estimated assuming k k


0


as I O

1 5

1

- "

(Ri-R )/ 2

6 1

'

6

s"

1

(llj).

(Ri-R ) d e r i v e d from thermodynamic p r o p e r t i e s of R » and R gas ( l , l l f ) and R i - R p r o p e r t i e s given i n r e f e r e n c e _27 or estimated by group a d d i t i v i t y (1 ), assuming Ri«/R » recombination r a t e constant o f 1 0 · M ™ s " . 2

x

2

2

9

5

1

1

T h i s r a t e constant v a l u e i s used to o b t a i n r e l a t i v e r a t e constants f o r bond homolysis of other bonds. ^determined e

i n the authors l a b o r a t o r y (14).

iPh Ph)/k ^ (Ph Ph) = k (X)/k (Γ\) using tet tet gas gas k (/^) d e s c r i b e d i n footnote f . k Ο 0 obtained from gas gas AH° ( ^ ) = 19.0 + 1.5 kcal/mol ( l l b , d , e ) ; S ° (A) =

Assumes k

f

2

9

8

3 ( ) ( )

66.7 cal/mol°K (1); thermodynamic p r o p e r t i e s of 1,2-dimethylbutane(28); i s o p r o p y l r a d i c a l recombination r a t e constant = 1θ9.5 ^ T -i M

f

s

(PtT^PlO/k (Ph~\h) = k () ()/k (ΛΛ) . tet tet gas gas k () () obtained from r e f e r e n c e (11c), and k (/~λ) gas gas estimated u s i n g : Δ Η ° . ( C H » ) = 27.5 + 1 kcal/mol (l,10b,d,

Assumes k

O O Q

e); S °

2 9 8

2

S

( C H ) = 58.0 cal/mol»K (1); chemical thermodynamic 2

5

p r o p e r t i e s o f butane (28); e t h y l r a d i c a l recombination r a t e constant = 1 0 · M" s ~ (1). 1 0

0

1

êstimated r e l a t i v e to k h

F o r each t e t r a l y l assumed that AS

l

t e t

(Ph

Ph) u s i n g equation 9.

(or hydroaromatic) r a d i c a l formed, i t i s i s 3 cal/mol»K greater than AS

homolysis. Note that S° c a l / m o L K (2). i

( (ÔjQ n

t

_ ( (Ôt^)

) - S° i

n

for bibenzyl ) ~ 3

t


2

e

Coal

STEIN

7.

107

Conversion

Table I I I .

(con't)

"Âctivation energy assumed to be 2 kcal/mol lower than f o r Ph~\h C

d i s s o c i a t i o n , s i n c e D(C

, ) ~ 2 kcal/mol (1). secondary —


Êstimated r e l a t i v e to k

k

R

R

( *- a) k (Ph Ph) tet \_y

S

k

S

t e t

u

m

e

S

(Ri-R ) 2

k t e f c

( ®0

k

t e t

a

. -C . ) - D(C . primary primary primary

(

R

l

C

H

3

)

" k (PhCH -CH ) * gas e

=

^ u s i n g equation 9.

a

s

have been determined

2

3

M

1

k

'

S

e

X

C

e

p

t

i n the authors l a b o r a t o r y

( l l i ,i,k,1). 1

e n t h a l p y of formation o f PhSSPh from r e f e r e n c e (28), and of PhS» from ( I l k ) adjusted f o r recent benzyl r a d i c a l enthalpy of formation ( l l f ) . In the authors l a b o r a t o r y t h i s molecule has been found to undergo d i s s o c i a t i o n i n t e t r a l i n much more r a p i d l y due, presumably, to a r a d i c a l displacement c h a i n reaction.

thermochemistry n

from r e f e r e n c e s given i n r e f e r e n c e ( l i e ) .

f r o m gas-phase homolysis r a t e constants reported i n r e f e r e n c e (lia,m).



108

NEW

APPROACHES IN COAL

CHEMISTRY

Figure 1. Coal-related structures susceptible to rapid β-bond scission (radicals formed by breaking C-X bond will decompose by β-bond scission in £10 s at 400°C) 2


7.

STEIN

Coal

109

Conversion

(7)


(8)

While each o f these r e a c t i o n s i s "allowed", the r a t e estimated f o r decomposition of b i b e n z y l by t h i s mechanism i s many orders of magnitude slower than the observed r a t e (18). Even i f step 7 had no a c t i v a t i o n energy (a h i g h l y u n l i k e l y p o s s i b i l i t y ) , the r a t e p r e d i c t e d f o r t h i s pathway i s s t i l l orders of magnitude slower than the observed r a t e . Note that a v a i l a b l e thermochemical evidence s t r o n g l y suggests that the intermediate p o l y o l e f i n i c molecule i n r e a c t i o n 6 does not possess s u b s t a n t i a l resonance energy. I t might be argued that s t r u c t u r e s such as Ph0CH Ph and@)-(""^-^>) s u s c e p t i b l e to such concerted a

r

e

m

o

r

e

2

r e a c t i o n paths than i s b i b e n z y l . However, i t appears to be a general r u l e that s t r u c t u r e s more s u s c e p t i b l e to such concerted r e a c t i o n s are a l s o more s u s c e p t i b l e to bond homolysis. For the above two s t r u c t u r e s , and s e v e r a l others that we have examined, computed r a t e s v i a r e a c t i o n s analogous to r e a c t i o n s 6-8 a r e many orders o f magnitude slower than estimated (or measured) homolysis rates. For the r e a c t i o n o f 1,2-a-dinaphthyl ethane, f o r instance, the above concerted pathway i s estimated to proceed a t a r a t e f the r a d i c a l , R» J Ipf RH J Recent thermochemical determinations suggest that A = 14.21 kcal/mol and Β = 17.21 kcal/mol ( l l g ) . To o b t a i n , f o r i n s t a n c e , b e n z y l i c C-X bond strengths, the s t a b i l i z a t i o n energy c a l c u l a t e d from equation 9 i s subtracted from C-X bond s t r e n g t h s for corresponding saturated s p e c i e s . Recent s t u d i e s of a l k y l benzene p y r o l y s i s (111) imply that w i t h i n current u n c e r t a i n t i e s i n r e l a t i v e bond s t r e n g t h s t h i s formula a p p l i e s to secondary and t e r t i a r y b e n z y l i c r a d i c a l s (e.g., and ). For complex phenoxy-type r a d i c a l s l e s s i s known,although a t e n t a t i v e assumption i s that d i f f e r e n c e s i n resonance s t a b i l i t y of such r a d i c a l s are the same as d i f f e r e n c e s f o r corresponding b e n z y l i c radicals. A second c l a s s o f r a d i c a l s o f r e l e v a n c e to c o a l r e a c t i o n s are those that may form a new aromatic r i n g upon rupture of a s i n g l e bond (or c o n v e r s e l y these r a d i c a l s may be formed by r a d i c a l a d d i t i o n to a polyaromatic molecule). An example of such a r a d i c a l i s fQkjQ) which may decompose to form anthracene by rupture*of a C i ~ H bond. Some modes of formation o f these r a d i c a l s a r e i l l u s t r a t e d below: 0

RH

OTJP


(10)

112

NEW

APPROACHES IN COAL

CHEMISTRY

R

+

(ID

Η Η


(12)

Each o f these r e a c t i o n s and t h e i r reverse r e a c t i o n s may, i n f a c t , be i n v o l v e d i n major r e a c t i o n pathways i n c o a l conversion. Reaction 10, f o r i n s t a n c e , i s expected to be a key step i n Η-transfer and aromatization, r e a c t i o n 11 can lead to c r o s s l i n k i n g and p o l y m e r i z a t i o n while r e a c t i o n -11 breaks up complex molecules, and r e a c t i o n 12 should provide a steady source of f r e e r a d i c a l s i n many p y r o l y t i c systems even i n the absence o f weak covalent bonds (vide i n f r a ) . Furthermore, these f r e e r a d i c a l s c o n t a i n weak C-H bonds that may rupture to y i e l d Η atoms, which can i n t u r n lead to breaking of C-C or C-0 bonds through aromatic displacement r e a c t i o n s ( r e a c t i o n 5 ) . E n t h a l p i e s o f Η-atom a d d i t i o n to v a r i o u s s i t e s i n benzene, naphthalene, phenanthrene, anthracene and pyrene have been derived from a v a i l a b l e thermochemical data (2,27a) and formula 9, and a r e given i n Table IV. D i f f e r e n c e s between e n t h a l p i e s o f a d d i t i o n o f R» and Η· to aromatics a r e expected to be c l o s e to d i f f e r e n c e s between C — R and C — Η bond strengths (C i s a secondary carbon sec sec sec atom). For i n s t a n c e , using a v a i l a b l e thermochemistry ( l , l l f ) a d d i t i o n o f benzyl r a d i c a l s to a given s i t e on an aromatic r i n g i s ca. 22 kcal/mol l e s s exothermic than Η-atom a d d i t i o n to the same s i t e . Use of the thermochemistry discussed above allows rough e s t i m a t i o n of r e a c t i o n enthalpy f o r a wide range of r e a c t i o n s involving resonance-stabilized radicals. Furthermore, r e a c t i o n e n t r o p i e s and heat c a p a c i t i e s may o f t e n be estimated to a good l e v e l o f accuracy (+ 1-2 cal/mol«K) (7b). Hence, e q u i l i b r i u m constants may be estimated to a l e v e l of accuracy determined p r i m a r i l y by the u n c e r t a i n t y i n r e a c t i o n enthalpy. Several other noteworthy f e a t u r e s o f r e s o n a n c e - s t a b i l i z e d r a d i c a l thermochemistry a r e : 1. Aromatic s u b s t i t u e n t s n o t i c e a b l y a f f e c t r e a c t i o n thermochemistry only when such groups e i t h e r d i r e c t l y d e l o c a l i z e the odd-electron or lead to a d i f f e r e n c e i n s t r a i n energy between r e a c t a n t s and products. For example, meta- or p a r a - a l k y l groups, ether l i n k a g e s , hydroxyl groups, e t c . w i l l not n o t i c e a b l y i n f l u e n c e r e a c t i o n thermochemistry (11Z). 2. Replacement of an aromatic C-H group by an Ν ( p y r i d y l ) atom g e n e r a l l y d e s t a b i l i z e s the r a d i c a l s l i g h t l y . The amount of d e s t a b i l i z a t i o n depends on the p a r t i c u l a r p o s i t i o n of the Ν atom w i t h i n the aromatic system ( H i ) . &


7.

Coal

STEIN

Table IV.

Conversion

E n t h a l p i e s of Η-atom A d d i t i o n to (Poly)aromatics -ΔΗ

V k c a l mol"


a

1.

113

b

c

1

f o r Η-atom a d d i t i o n to p o s i t i o n d

e

f

g

I f ΔΗ p e r t a i n s to r e a c t i o n Ar + Η· + ArH, then ΔΗ = Δ Η ( Α τ + H + ArH ) + D ( A r H — Η ) - 2ΔΗ£ (Η·) ; where D(ArH—H) / k c a l mol"" = 94.5 - (resonance energy from formula 9), i s the bond s t r e n g t h f o r breaking A r H — H bond, Δ Η i s a p p r o p r i a t e enthalpy of hydrogénation. When A r H may be 1,2- or 1,4dihydroaromatic, Δ Η was chosen as the average of the two, assuming no r i n g s t r a i n energy f o r e i t h e r (28). Enthalpy of hydrogénation (7c) enthalpy of formation (7a) measurements and estimates based on experimental data (2) support the idea that such s t r a i n energy i s q u i t e s m a l l . Γ

2

2

1

Γ

2

Γ

2.

AH (1,3-cyclohexadienyl f

reference

r a d i c a l ) = 50 kcal/mol

from

(1).


NEW

114

APPROACHES

IN COAL

CHEMISTRY

3. Accurate p r e d i c t i o n of " r i n g s t r a i n " i n hydroaromatic s t r u c t u r e s i s not p o s s i b l e at present (28a), so a v a i l a b l e hydrogénation or other d i r e c t determinations should g e n e r a l l y be used i n preference to estimated v a l u e s . T h i s can be a very troublesome source of u n c e r t a i n t y .


Reactions I n v o l v i n g H i g h l y - S t a b i l i z e d R a d i c a l s Using the thermochemical estimates given above, along with the c o n s i d e r a b l e body of a v a i l a b l e thermochemical and k i n e t i c data, s e v e r a l p l a u s i b l e r e a c t i o n pathways i n c o a l and model compound r e a c t i o n s w i l l now be examined. T h i s a n a l y s i s i s intended to d i s c r i m i n a t e between f e a s i b l e and u n l i k e l y r e a c t i o n mechanisms. I t should be kept i n mind that absolute r a t e constant estimates a r e o f t e n only very approximate, and we a r e t e s t i n g i d e a s , not proving them. Rates o f Molecular D i s p r o p o r t i o n a t i o n . Simple Η-atom t r a n s f e r from a donor molecule t o an acceptor molecule ( r e a c t i o n 1 3 , molecular d i s p r o p o r t i o n a t i o n ) generates two f r e e r a d i c a l s and can lead to the net t r a n s f e r o f two H atoms by the f o l l o w i n g r e a c t i o n sequence. RH

+ /=\

τ~~~ ^ R*

(13)

+ *rC

-13 R'H

+

>"\

F

A

S

T

>

R'*

+

(14)

r\

In t e t r a l i n , f o r example, t h i s r e a c t i o n leads t o s a t u r a t i o n of a double bond (or hydrogénation on an aromatic r i n g ) and formation of two t e t r a l y l r a d i c a l s . Such a r e a c t i o n sequence has, i n f a c t , j u s t been invoked t o e x p l a i n ethylene hydrogénation by cyclohexene i n the gas phase (28b). To examine the p o t e n t i a l importance of molecular d i s p r o p o r t i o n a t i o n , a means f o r e s t i m a t i n g k i must be found. Estimates o f k w i l l be obtained from estimates of both the r a t e constant f o r the r e v e r s e r e a c t i o n ( r a d i c a l d i s p r o p o r t i o n a t i o n , r e a c t i o n -13), and the e q u i l i b r i u m constant, K ; i . e . , k i = Ki3 k _ » The e q u i l i b r i u m constant K w i l l be obtained from estimates of r e a c t i o n enthalpy, A H , and entropy, A S . The f o l l o w i n g formula provides a convenient means of e s t i m a t i n g Δ Η ι 3

13

i 3

3

l3

i 3

i3

i 3

3

ΔΗ

1 3

= D(R-^H) +

+ ΔΗ (/=\ + H Γ

2

+ Γ\ ) - 2ΔΗ (Η·) £

(15)

where D(R-^-H) and T$$-\) a r e bond d i s s o c i a t i o n e n t h a l p i e s of molecules i n equations 13 and 14, Δ Η i s the enthalpy o f Γ

hydrogénation o f /=\ , and AHf(Η·) (= 52.1 kcal/mol a t 298 K) i s


7.

STEIN

Coal

115

Conversion

the enthalpy o f formation of a mole of H atoms. Reaction enthalpy can o f t e n be estimated to w i t h i n + 3 kcal/mol from t h i s formula, although e r r o r s i n some cases may be even l a r g e r and are g e n e r a l l y expected to be the major sources of u n c e r t a i n t y i n estimates o f k i . Reaction entropy may be expressed as 3

Δ5

=

1 3

Rln4

+

R 1«T

A S ^ ^

+

(16)

where the f i r s t term on the r i g h t accounts f o r the e l e c t r o n i c degeneracy of the two product r a d i c a l s (7b), σ i s , i n essence, r e a c t i o n path degeneracy and Δί>ι i i s the net change i n


e

r

n

a

v i b r a t i o n a l and i n t e r n a l r o t a t i o n a l entropy. In a l l cases considered i n t h i s paper, A S > £ i ~ 0, although f o r other n t e r n a

cases i n v o l v i n g s u b s t a n t i a l changes i n number or type of i n t e r n a l r o t a t i o n s t h i s assumption may not be r e l i a b l e , i n which case more accurate estimates should be made (7b). Now an estimate of r a d i c a l d i s p r o p o r t i o n a t i o n , ^ d

i

s

p

must be found. Since r a d i c a l recombination r a t e constants, k and r e l a t i v e r a d i c a l d i s p r o p o r t i o n a t i o n / r e c o m b i n a t i o n r a t e s , ^disp^rec* * ^ d i r e c t l y measured f o r a l a r g e number of i a v e

r e c

,

e e n

r e a c t i o n s (29), k w i l l be estimated u s i n g the formula, — disp 9 5 , = /k ). For k , a value of 10 M" s disp rec d i s p rec rec i s chosen which i s i n the range expected f o r most gas-phase and l i q u i d - p h a s e recombination r a t e s i n v o l v i n g l a r g e f r e e r a d i c a l s and elevated temperatures Q, 7b,c_,ci). The r a t i o (^lap^rec^, JJf

k

on the other hand, depends on the p a r t i c u l a r r a d i c a l p a i r i n v o l v e d and should be c o r r e c t e d f o r r e a c t i o n path degeneracy (k., /k v a l u e s w i l l apply to a s i n g l e r e a c t i o n path). For disp rec two resonance s t a b i l i z e d hydrocarbon r a d i c a l s , ( k ^ ^ / k ^ ^ ) ~ 0.01 i n the range 400°-500°C. T h i s v a l u e i s based on l i t e r a t u r e values f o r t e r m i n a t i o n of b e n z y l i c r a d i c a l s such as PhCHCH CH at 118°C (30) and f o r the 1,3-cyclohexadienyl r a d i c a l (Q*-) at 2

3

100°C (31) assuming t h a t the a c t i v a t i o n energy f o r recombination i s 1.5 kcal/mol g r e a t e r than that f o r d i s p r o p o r t i o n a t i o n (29). Based on the above estimates f o r K and k _ for s t a b i l i z e d hydrocarbon r a d i c a l s , k^/M" s ~ = σ Ι Ο · " ' * . For r e a c t i o n s i n v o l v i n g one s t a b i l i z e d and one n o n - s t a b i l i z e d r a d i c a l , (k /k ) - 0.1 (32). For s e l f r e a c t i o n s of the disp rec — resonance s t a b i l i z e d k e t y l r a d i c a l , Ph C0H, k,. /k - 0.03 9 * 9 disp rec (33); t h i s v a l u e w i l l be used f o r c a r b o n y l d i s p r o p o r t i o n a t i o n with hydroaromatics. For model compound s t u d i e s i n excess t e t r a l i n , r e a c t i o n 13 w i l l occur with a p s e u d o - f i r s t - o r d e r r a t e constant k / s ~ = i 3 1

1 3

l

8

1

Δ Η ι 3

2

β

2

J

l

k

i3Î@0

J ~ ^ k i . However, f o r experiments 3

c a r r i e d out at


3 R T

116

NEW


CHEMISTRY


temperatures above 446°C (the c r i t i c a l temperature of t e t r a l i n ) , c o n c e n t r a t i o n s of t e t r a l i n , hence k, may be somewhat s m a l l e r . Table V presents a l i s t of estimated e n t h a l p i e s and h a l f l i v e s f o r molecular Η-atom t r a n s f e r ( d i s p r o p o r t i o n a t i o n ) from t e t r a l i n to a number of unsaturated molecules. At 450°C, and even at 400°C, many s t r u c t u r e s w i l l undergo r a p i d molecular d i s p r o p o r t i o n a t i o n with t e t r a l i n . Approximate r e l a t i v e r a t e s at 400°C f o r Η-atom donation by molecular d i s p r o p o r t i o n a t i o n for s e l e c t e d hydrogen donors a r e estimated below (sources f o r thermochemistry a r e given i n footnote 1 of Table I V ) :

Some of these values a r e very approximate, e s p e c i a l l y f o r the dihydronaphthalenes due to r i n g s t a i n u n c e r t a i n t i e s i n the r a d i c a l , and a r e meant to serve only as a rough guide u n t i l more d i r e c t evidence i s a v a i l a b l e . Estimated r a t e s o f molecular d i s p r o p o r t i o n a t i o n w i l l now be compared to observed r e a c t i o n r a t e s of unsaturated molecules i n the presence of v a r i o u s hydrogen donors. C o l l i n s et a l . have studied a number of r e a c t i o n s i n excess t e t r a l i n a t 400°C (15). They reported 99% conversion of indene to indane a f t e r 1 hour and c o n v e r s i o n of cyclohexene and 1cyclohexenylbenzene to cyclohexane and cyclohexylbenzene after 18 hours. At 400°C values i n Table V p r e d i c t n e a r l y complete (>90%) hydrogénation of both indene and 1-cyclohexenylbenzene a f t e r 1 hour and a c o n v e r s i o n of cyclohexene to cyclohexane a t a r a t e of 40% per hour. Molecular d i s p r o p o r t i o n a t i o n i s a f e a s i b l e pathway f o r these r e a c t i o n s . Cronauer et a l . (34) reported f i r s t - o r d e r r a t e constants for the hydrogénation of t r a n s - s t i l b e n e of 0.013 min" at 400°C and 0.06 m i n at 450°C; p r e d i c t i o n s from Table V are 0.004 min"" at 400°C and 0.04 min*" a t 450°C. 1

- 1

1

1

Rate constants f o r r e a c t i o n of benzothiophene

©ÇjF

( (^Q? ) >

(gç3

indole ( ) and benzof uran ( ) i n t e t r a l i n at 400°C450°C have r e c e n t l y been reported by M a l l i n s o n et a l . (35a). To match t h e i r observed r a t e s f o r these compounds, assuming that molecular d i s p r o p o r t i o n a t i o n i s the r a t e l i m i t i n g step, an a c t i v a t i o n energy g r e a t e r than the estimated a c t i v a t i o n energy for indene hydrogénation (38.3 kcal/mol) by 9.8, 9.2 and 5.2 kcal/mol, r e s p e c t i v e l y , i s r e q u i r e d . These v a l u e s are c o n s i s t e n t w i t h the idea that formation o f r a d i c a l s from heteroaromatic molecules i n v o l v e s l o s s of resonance energy i n the heteroaromatic r i n g w h i l e r e l a t i v e l y l i t t l e s t a b i l i z a t i o n energy i s l o s t when c o n v e r t i n g indene to the cr-indanyl r a d i c a l


7.

( @ û


Coal

STEIN

)·

117

Conversion

For example, furan

(^/j?) possesses l e s s aromatic

s t a b i l i z a t i o n energy than e i t h e r p y r r o l e or thiophene (^^) which i s c o n s i s t e n t with the observed higher r e a c t i v i t y of benzofuran. P r e d i c t e d r a t e s of molecular d i s p r o p o r t i o n a t i o n f o r the s e r i e s cyclopentadiene, furan and thiophene as given i n Table V are i n the same order as observed f o r the benzo-analogues. However, while p y r r o l e i s p r e d i c t e d to be l e s s r e a c t i v e than thiophene, benzothiophene was found to be l e s s r e a c t i v e than i n d o l e ("benzopyrrole")· V i r k and Garry (24a) have r e c e n t l y i n v e s t i g a t e d hydrogen t r a n s f e r from cyclohexanol to anthracene and phenanthrene and have reported well-behaved second-order k i n e t i c s . These workers suggest that t h i s r e a c t i o n may occur by a concerted molecular H - t r a n s f e r . Simple second-order k i n e t i c behavior, however, i s a l s o c o n s i s t e n t w i t h molecular d i s p r o p o r t i o n a t i o n (and a l s o with hydride, H~, t r a n s f e r ) . However, i f i t i s assumed that ( ^ l e p ^ r e c ^ 0.1, p r e d i c t e d r a t e s are only 1/100 of observed r a t e s at 400°C. We t h e r e f o r e draw the t e n t a t i v e c o n c l u s i o n that such hydrogen t r a n s f e r by a l c o h o l s i s not p r i m a r i l y due to molecular d i s p r o p o r t i o n a t i o n , although hydride t r a n s f e r remains a r e a l i s t i c p o s s i b i l i t y . I t i s c u r i o u s that observed r e l a t i v e r a t e s of hydrogénation of anthracene and phenanthrene by cyclohexanol are v i r t u a l l y i d e n t i c a l to estimated r e l a t i v e r a t e s f o r molecular d i s p r o p o r t i o n a t i o n at 350°C (estimated r a t i o = 30, experimental = 35). 2

=

V i r k and co-workers (24b,c) and King and Stock (35b) have reported r a t e s f o r H - t r a n s f e r to anthracene and phenanthrene i n s o l u t i o n c o n t a i n i n g 1,2- and 1,4-dihydronaphthalene and t e t r a l i n . Comparisons between reported r a t e constants and estimated r a t e constants f o r bimolecular d i s p r o p o r t i o n a t i o n are given i n Table VI. In agreement with Stock, t h i s data does not provide evidence f o r a concerted H - t r a n s f e r mechanism. Our c a l c u l a t i o n s i n d i c a t e that molecular d i s p r o p o r t i o n a t i o n may be a major hydrogénation mechanism i n these r e a c t i o n systems. T r a n s f e r o f hydrogen to carbonyl groups d i f f e r s from analogous t r a n s f e r to unsaturated hydrocarbons p r i m a r i l y due to the greater l i k e l i h o o d f o r involvement of f r e e i o n i c or i o n p a i r intermediates i n the former r e a c t i o n . L i n s t e a d and coworkers (36) have shown that t r a n s f e r from dihydroaromatics to quinones i s best explained by a r a t e l i m i t i n g step i n v o l v i n g hydride i o n t r a n s f e r . The a p p l i c a b i l i t y of t h i s mechanism to other systems i s p r e s e n t l y unclear (40). For example, under a p p r o p r i a t e c o n d i t i o n s quinones can generate f r e e r a d i c a l s and form adducts (37). P s e u d o - f i r s t order r a t e constants f o r 2

2

r e a c t i o n of benzaldehyde ( have been reported

) and

H

acetophenone ( ( ô f ^ 3 )

by Cronauer et a l . (34).

Rate

constants


118

NEW

Table V.

Η-acceptor (A)


@0

(ÔJ

\

Ο

IN

COAL

CHEMISTRY

Molecular Disproportionation with Tetralin as H-Donor ΔΗ°

(§f*

APPROACHES

1

2

τ

2 9 8

kcal mol"

1

3

1 / 2

(Η-acceptor)

400°C

450°C

34.7

3 min

32 sec

38.3

10 min

1.7 min

5:5* 38.8

8 min 43 sec

2.4 min 7.5 sec]

41.2

54 min

6.4 min

42.7

2.8 hrs

18 min

5

13 hrs

26 min

6

26 hrs

3.4 hrs

44.8

45.7

@Ô)

53.5

200 days

12 days

(Ô)

61.6

150 yrs

6 yrs

Or

39.1

11 min

1.5 min

0

45' S

1.5 hr

1.5 hr

14 yr

0.7 yr

600 yr

22 yr

^

57

62

7

υ

0

31.7

8

1 sec

0.2 sec


Coal

STEIN

7.

Table Οχ II

V.

119

Conversion

(con't)

9,10 41"

48

1

10

4.0

min

3.8

min

days

8.1

hrs

Ο II


46

1.

2 hrs

43 hrs

arrow denotes thermodynémically favored s i t e of H-atom a d d i t i o n ; ΔΗ and -^/2 P * * hydrogen t r a n s f e r s i t e . e r t a

τ

2.

10,11

n

t o

t n

s

Derived u s i n g equation 15. When a v a i l a b l e , enthalpy of hydrogénation A + H AH , i s taken from d i r e c t measurements (27c) or d i f f e r e n c e s between measured gas-phase e n t h a l p i e s of formation (27a) otherwise, e s t i m a t i o n methods (1,7b) are employed. Except where noted, bond strengths are d e r i v e d from r e c e n t l y recommended enthalpy of formation at 298 Κ of benzyl r a d i c a l s (47.0 kcal/mol) ( l l f ) . 2

2

3.

Ξ I n 2 / k i 3 [ t e t r a l i n ] ; assumes [ t e t r a l i n ] obtained as d e s c r i b e d i n t e x t .

4.

Estimated value f o r ΔΗ (1,2-dihydronaphthalene) r e f e r e n c e (2).

5.

Uses D(cyclohexyl-H)

6.

Uses AHf(9,10-dihydropenanthrene) = 36.90 + 0.30 obtained by W. D. Good (28).

7.

AHf (AH ) estimated by group a d d i t i v i t y s t r a i n " of 5 kcal/mol;

8.

D ( H 0 — ® — 0-H)

=95.5 kcal/mol

2

=

5 M;

k

i 3

given i n

(lie). kcal/mol

QL) assuming " r i n g

assumed to be equal to D ( @ —

0-H)

(llh).

L a t t e r value i s c o r r e c t e d f o r recent enthalpy of formation of benzyl r a d i c a l s ( l l f ) . 9.

R a d i c a l d e r e a l i z a t i o n energy assumed to be 15 group lowers C-H

kcal/mol.

10.

Assumes β-ΟΗ (llf).

bond s t r e n g t h by 4 kcal/mol

11.

Enthalpy of hydrogénation i s assumed to be same as f o r 3-pentanone (27a).



o

n

0

s

a

r

e

1.1

MD

(1.2)

obs

/k

C

k

6.2

MD

(1.6)°

obs

/k

1,2-Dihydronaphthalene k

1

obs b 1.5

MD

(l/21)

V / k

c

k

ob S

b 2.0

MD

(1/18)°

S / k

1,4-Dihydronaphthalene

*

sa

8

k

ob S

V / k

4 1/3

MD

k

obs

/k

MD

Tetralin

2.1

3

Q

n

these cases i s dubious s i n c e King and Stock (24b) demonstrated t h a t , a t

400°C, dihydronaphthalenes a r e converted to t e t r a l i n and naphthalene f a r more r a p i d l y than they hydrogenate phenanthrene.

Q

(35b).

lower l i m i t f o r molecular d i s p r o p o r t i o n a t i o n r a t e constants s i n c e d u r i n g

r e a c t i o n a s u b s t a n t i a l amount o f the donor i s converted to t e t r a l i n

g

Interpretation of ^ ^ *

the

a

these cases k ^

s

k

0

ν s k k d ^ ^ bimolecular r a t e constants computed from data given by V i r k (24b) and Stock (35b), r e s p e c t i v e l y . Donor c o n c e n t r a t i o n s i n V i r k s experiments are assumed to be 5 M, except for r e a c t i o n s o f 1,4-dihydronaphthalene where the solvent c o n t a i n s 64% o f t h i s donor (3.2 M). I n i t i a l donor concentrations i n experiments o f King and Stock were assumed to be 2.5 M.

În

a

400

°

C

Phenanthrene

/

300

T

2

f o r H - T r a n s f e r from Hydroaromatics, ^ ^ >

Compared to Estimated Molecular D i s p r o p o r t i o n a t i o n Values,

Comparison o f E m p i r i c a l Rate Constants

Anthracene

Table VI.


7.

STEIN

Coal

121

Conversion

estimated f o r molecular d i s p r o p o r t i o n a t i o n , k observed r a t e constants, k

k^^,

a

r

e

:

^

o r

e s t

>

r e l a t i v e to

benzaldehyde,

/k , = 0.005 a t 400°C, 0.01 at 450°C; f o r acetophenone, obs ' /k , =0.7 at 400°C, 1.3 at 450°C. Therefore, while est obs ' ' acetophenone may undergo s i g n i f i c a n t molecular d i s p r o p o r t i o n a t i o n with t e t r a l i n , benzaldehyde decomposes by way o f other more r a p i d pathways. est


k

Formation o f Free R a d i c a l s by Molecular D i s p r o p o r t i o n a t i o n . A s i g n i f i c a n t c o n c l u s i o n that may be drawn from c o n s i d e r a t i o n s of r a t e and e q u i l i b r i u m constants f o r molecular d i s p r o p o r t i o n a t i o n i s that t h i s path can provide a p p r e c i a b l e concentrations of f r e e r a d i c a l s i n many systems long a f t e r most weak chemical bonds have ruptured and bond homolysis has ceased to be a major source of f r e e r a d i c a l s . In "pure" t e t r a l i n , f o r instance, t r a c e concentrations o f 1,2-dihydronaphthalene a r e expected to e q u i l i b r a t e w i t h t e t r a l i n and t e t r a l y l r a d i c a l s ,

+ 2

At a c o n c e n t r a t i o n of, say, 10" mole percent o f 1,2-dihydro naphthalene, the c o n c e n t r a t i o n o f α-tetralyl r a d i c a l s at 400°C and 450°C (assuming [ t e t r a l i n ] = 5 M) i s estimated as 1 0 " · M and 10~6·7 ^, r e s p e c t i v e l y . These concentrations of f r e e r a d i c a l s a r e i n excess of that needed to c a r r y out many c h a i n r e a c t i o n s a t reasonably r a p i d r a t e s . For i n s t a n c e , using Ha b s t r a c t i o n r a t e constants o f b e n z y l i c r a d i c a l s given i n reference (38), the h a l f - l i f e f o r exchange of b e n z y l i c H-atoms i n the presence of the above concentrations o f r a d i c a l s i s ~1 hour a t 400°C and -10 min at 450°C. Close examination of t e t r a l i n p y r o l y s i s i n d i c a t e s that r e a c t i o n s l e a d i n g to i r r e v e r s i b l e termination of t e t r a l y l r a d i c a l s a r e expected to be very slow due to the r e v e r s i b i l i t y of t e t r a l y l r a d i c a l recombination and d i s p r o p o r t i o n a t i o n r e a c t i o n s . This may, i n e f f e c t , lead to s i z a b l e r a d i c a l concentrations even when the net r e a c t i o n r a t e of t e t r a l i n i s very slow. I t i s i n t e r e s t i n g t o note that c a t a l y t i c r e a c t i o n s tending to e q u i l i b r a t e t e t r a l i n , 1,2-dihydronaphthalene and naphthalene (19,39) w i l l serve to generate f r e e r a d i c a l s s i n c e 1,2-dihydro naphthalene r a p i d l y undergoes molecular d i s p r o p o r t i o n a t i o n with tetralin. For i n s t a n c e , a t e q u i l i b r i u m a 0.5% s o l u t i o n of naphthalene i n t e t r a l i n ( t y p i c a l of d i s t i l l e d t e t r a l i n ) w i l l generate ca. 10" mol percent 1,2-dihydronaphthalene a t 400°C, which w i l l , i n t u r n , form ca. 10" M t e t r a l y l r a d i c a l s (see above). 7

1

2

7

Free-Radical Η-Atom Transfer. In competition with molecular Η-atom transfer reactions, radical-induced transfer may occur,


NEW

122

APPROACHES

IN

COAL

CHEMISTRY

e s p e c i a l l y i n environments c o n t a i n i n g high c o n c e n t r a t i o n s of radicals. B e n z y l i c and p h e n o l i c H atoms, and to some degree p a r a f f i n i c H atoms, are expected to undergo exchange by simple Η-atom metathesis, f o r i n s t a n c e ,

è

RH

+

è

(17)

R»

However, r a p i d D/H exchange of very s t r o n g l y bound aromatic H atoms i s known to occur both i n c o a l systems (40) and i n systems c o n t a i n i n g phenanthrene (and anthracene) and PhCD CD Ph although l i t t l e exchange i s observed i n benzene/PhCD CD Ph systems (41). Aromatic C-H bonds are ~25 kcal/mol stronger than b e n z y l i c C-H bonds; hence,rapid D/H atom randomization of aromatic H atoms cannot be explained by simple metathesis. The f o l l o w i n g mechanism may e x p l a i n t h i s randomization: 2


2

\ / —C—C/ * D

\ / —C—C— /

18 -18

H

19 -19

D

2

2

\ / —C—C—

20 -20

D

l y

H—C—C / \

H, c=c:

21

.C—C

\ / -C—C—

-21 N

c - c /

c=c

\

Note that steps 19 and 20 go through t r a n s i t i o n s t a t e s i n v o l v i n g r e l a t i v e l y unstrained 5-membered r i n g s . Related i s o m e r i z a t i o n s are w e l l known i n p a r a f f i n p y r o l y s i s . A very crude k i n e t i c a n a l y s i s of t h i s system i n d i c a t e s that such r e a c t i o n s are p l a u s i b l e f o r r e a c t i o n s i n which step 18 i s exothermic and step 19 i s not a p p r e c i a b l e endothermic. These c o n d i t i o n s hold f o r a d d i t i o n of PhCDCD Ph to a l l p o s i t i o n s i n anthracene and phenanthrene, e s p e c i a l l y to the 9 and 10 p o s i t i o n s (Table IV). For benzene, however, r e a c t i o n 18 i s 5 kcal/mol endothermic; so randomization of Η atoms on benzene i s estimated to be at l e a s t 40 times slower at 400°C than Η-atom randomization at any p o s i t i o n i n anthracene or phenanthrene. The above r a d i c a l a d d i t i o n sequence may a l s o serve as a means of H - t r a n s f e r i f step 22 i s competitive with s t e p s - 1 9 & 20. 2

2

—C—C—

-+

^C=Cf


(22)

7.

STEIN

Coal

123

Conversion

This reaction could cause homogeneous equilibration of certain hydroaromatic structures. Free-Radical Induced Bond Formation and Aromatization. Based on current understanding of free-radical aromatic substitution reactions at low temperatures (7), a major pathway for polymerization and crosslink formation i n coal systems i s expected to be, H

H

R


(23)

+

QH

(24)

where R» and 0· are organic radicals. At coal conversion temperatures, reaction 23 i s expected to be highly reversible and t i t r a t e of crosslink formation may be written, K k [ R » ] [Q«][ô£], with K = k / k _ . Note that the rate of such bond formation i s proportional to the square of radical concentrations; hence,the very high free-radical concentrations in coal conversion would cause the reaction to proceed much faster than in typical model systems. Moreover, Q» may be a very stable radical because of the very weak C-H bond in the adduct radical formed i n reaction 23 (typically 20-40 kcal/mol, see Table IV). High free radical concentrations are also expected to assist aromatization through reactions such as, 22

23

23

23

23

Based on the above considerations, high concentrations of very highly stabilized radicals w i l l f a c i l i t a t e polymerization and aromatization and i n this sense can be deleterious to coal liquefaction processes. Ionic Processes and Water Formation The potential importance of reactions involving ions or ion pairs i n coal and model compound reactions has been emphasized by Ross and co-workers (42) as well as by Brower (43). For many types of reactions there exists considerable debate concerning reactive intermediates and mechanism. However, in the case of water formation, which i s known to be rapid during coal liquefaction under relatively mild conditions and appears to occur i n certain model compound reactions (15), i t i s d i f f i c u l t to construct plausible pathways without postulating ionic intermediates (although these intermediates may reside on solid surfaces). Free-radical schemes for water formation that involve ·0Η radicals are not l i k e l y since the formation of this highly


NEW

124

APPROACHES

IN C O A L

CHEMISTRY

r e a c t i v e f r e e r a d i c a l must i n v o l v e very unfavorable thermo dynamics. Concerted molecular e l i m i n a t i o n o f water has never been unambiguously observed f o r the type of s t r u c t u r e s b e l i e v e d to e x i s t i n c o a l o r to be formed d u r i n g c o a l r e a c t i o n s . Ionic r e a c t i o n s , on the other hand, provide a number of p l a u s i b l e routes f o r H 0 formation from organic oxygen. For instance, p h e n o l i c hydroxyl groups may be removed by hydride transfer OH ϊί ^ + 2

H

("> Downloaded by UNIV LAVAL on September 27, 2015 | http://pubs.acs.org Publication Date: October 26, 1981 | doi: 10.1021/bk-1981-0169.ch007

R

+

)çC

0

H

( R

~*

j^C

}

< ) — *° 0H

+

>H

and a l i p h a t i c hydroxyl groups may be removed by proton t r a n s f e r . OH HOH a n + Α + X + X + HO OH 2

We have found that benzhydrol (φΟφ) r a p i d l y forms d i p h e n y l methane, benzophenone and presumably water i n t e t r a l i n a t 300°C (llj). We cannot c o n s t r u c t a reasonable mechanism to account f o r t h i s r e a c t i o n without p o s t u l a t i n g i o n i c intermediates. Test Case P y r o l y s i s .

Liquid-Phase P y r o l y s i s of B i b e n z y l

To begin the e x p l o r a t i o n of a c t u a l r e a c t i o n pathways i n complex p y r o l y s e s of aromatic substances, we have c a r r i e d out a d e t a i l e d experimental and t h e o r e t i c a l a n a l y s i s of the l i q u i d phase p y r o l y s i s of b i b e n z y l . T h i s p y r o l y s i s system has been studied by others (44,45,46), and the general k i n e t i c f e a t u r e s of t h i s r e a c t i o n system a r e now r a t h e r w e l l agreed on. Complete d e t a i l s o f t h i s work w i l l appear elsewhere (38a) and a few i m p l i c a t i o n s o f t h i s work of p a r t i c u l a r r e l e v a n c e to c o a l r e a c t i o n s w i l l be discussed here. (1) The r e a c t i o n mechanism f o r formation of the major products of t h i s r e a c t i o n i s given i n Table V I I . Rate parameters were obtained from both e s t i m a t i o n procedures and separate experiments and are a l l w i t h i n the r a t h e r narrow range expected on the b a s i s o f thermochemical k i n e t i c s c o n s i d e r a t i o n s ( 1 ) . T h i s mechanism has been found to e x a c t l y reproduce e v o l u t i o n of the major products a t low extents of r e a c t i o n between 350°-425°C. B i b e n z y l p y r o l y s i s proceeds by c o n v e n t i o n a l well-understood, f r e e - r a d i c a l r e a c t i o n steps. (2) A r a t h e r i n d i r e c t mode o f r e a c t i o n accounts f o r 60% o f the product t r a n s - s t i l b e n e ( r e a c t i o n sequence 1, 2, 4, 5, and 6 i n Table V I I ) . The importance o f t h i s path may be traced to both the low d i s p r o p o r t i o n a t i o n / r e c o m b i n a t i o n r a t i o f o r resonance s t a b i l i z e d r a d i c a l s , and the high r a t e constant f o r 3bond s c i s s i o n o f the intermediate r a d i c a l recombination product, 1,2,3,4-tetraphenylbutane. Analogous decomposition routes i n v o l v i n g formation of an adduct, followed by i r r e v e r s i b l e d e s t r u c t i o n o f the adduct, may be important r e a c t i o n paths i n many other c o a l r e l a t e d r e a c t i o n systems.



2

2

2

2

2

2

2

1

o r M"

2

s"

1

9 = 4.576 T/1000

1

2

2

2

3

2

2

2

r a t e s of product

8.62 - 7.9/9

13-23.5/9

14.4-22.3/9°

8.62 - 16.06/9

formation.

PhCHC(Ph)HC(Ph)HCH Ph

PhCH CH Ph + PhC(CH ^HPh t PhCHCH Ph + PhC(CH )HPh

2

PhCHCH Ph Ζ PhC(CH )HPh

2

°rate constant has no i n f l u e n c e on observed

b

V

2

PhCHC(Ph)HC(Ph)HCH Ph t PhCH=CHPh + PhCHCH Ph

2

+

[PhCH C(Ph)H^

2

b

8.62 - 16.06/9°

9.5

2

+ PhCHCH Ph

2PhCHCH Ph t

3

a

15.9 - 65.0/9

8.6

2

2

2PhCHCH Ph t PhCH=CHPh + PhCH CH Ph

2

+ PhCH CH Ph t PhCH

2

PhCHCH Ph + [PhCH C(Ph)H^ t PhCH CH Ph

PhCH

2

PhCH CH Ph t 2PhCH

log k. f

8.62 - 22.0/9

13-9/9

8.0-10/9°

8.62 - 16.06/9

15.9 - 57.5/9

9.8 - 45.9/9°

8.5 - 19.50/9°

9.8

a

k rev

c

Table V I I . Mechanism f o r Formation of Major Products i n the L i q u i d Phase P y r o l y s i s of B i b e n z y l (38).



126

NEW

APPROACHES IN

COAL

CHEMISTRY

(3) Despite the f a c t that i s o m e r i z a t i o n of b i b e n z y l to 1,1-diphenylethane proceeds through a r a t h e r thermodynamically unfavorable intermediate ( r e a c t i o n 7 i n Table V I I ) , high r a d i c a l concentrations, high temperatures and high concentrations i n t h i s p y r o l y s i s allow t h i s i s o m e r i z a t i o n to proceed at a measurable r a t e . T h i s i s o m e r i z a t i o n converts a thermally l a b i l e substance to one that i s l e s s thermally r e a c t i v e (or at l e a s t r e a c t s through d i f f e r e n t pathways). Related r e a c t i o n s may s i g n i f i c a n t l y hinder c o a l d i s s o c i a t i o n s i n c e high f r e e r a d i c a l concentrations i n c o a l systems may cause such i s o m e r i z a t i o n to compete e f f e c t i v e l y w i t h bond breaking r e a c t i o n s . (4) In h i g h l y d i l u t e mixtures of b i b e n z y l i n t e t r a l i n ( l l j ) , (one part b i b e n z y l i n 200-1000 p a r t s t e t r a l i n ) 1,1-diphenylethane i s formed at a r a t e independent of d i l u t i o n and roughly ones i x t h of the r a t e found i n pure b i b e n z y l . T h i s i m p l i e s that f r e e - r a d i c a l concentrations are independent of the c o n c e n t r a t i o n of f r e e - r a d i c a l i n i t i a t o r s i n s u f f i c i e n t l y pure t e t r a l i n . Summary (1) Gas-phase r a t e and e q u i l i b r i u m constants are g e n e r a l l y not very d i f f e r e n t from solution-phase v a l u e s . (2) Bond homolysis r a t e constants are estimated f o r many covalent bonds presumed to be present i n c o a l conversion r e a c t i o n s . Other modes of bond breaking are examined using thermochemical k i n e t i c methods 01). (3) E n t h a l p i e s of formation and e n t r o p i e s of resonance s t a b i l i z e d r a d i c a l s of importance i n aromatic p y r o l y s i s are estimated to a l e v e l of accuracy s u i t a b l e f o r order of magnitude calculations. (4) Molecular d i s p r o p o r t i o n a t i o n may c o n s t i t u t e a major r e a c t i o n pathway i n c o a l - r e l a t e d systems both f o r t r a n s f e r r i n g hydrogen and f o r generating f r e e r a d i c a l s . Estimated r a t e s of t h i s r e a c t i o n are shown to o f t e n be c l o s e to observed r e a c t i o n r a t e s i n model systems. (5) A f r e e - r a d i c a l hydrogen t r a n s f e r mechanism i s proposed that may lead to r a p i d Η-exchange i n polyaromatic systems. (6) Water formation i s b r i e f l y examined and i o n i c species are suggested as the most p l a u s i b l e intermediates. (7) Recent s t u d i e s of b i b e n z y l p y r o l y s i s are discussed i n the context of c o a l - r e l a t e d chemistry. Ac knowled gement Support f o r t h i s r e s e a r c h by DOE, F o s s i l Energy D i v i s i o n under grant EF-77-G-01-2751 i s g r a t e f u l l y acknowledged. This work was a l s o supported, i n p a r t , by a grant from the West V i r g i n i a Energy Research Center. The s c i e n t i f i c o r i g i n s of t h i s work may be traced to P r o f e s s o r S. W. Benson and to DOE supported


7.

STEIN

Coal

Conversion

127

research a t SRI I n t e r n a t i o n a l during the years 1975-76. Discussions with David S. Ross a t SRI were very h e l p f u l i n the development o f t h i s research.

Literature Cited 1. 2.


3. 4. 5. 6.

7.

8.

9. 10.

Benson, S. W. "Thermochemical Kinetics", Second Edition, Wiley and Sons, New York, NY, 1976. Shaw, Robert; Golden, D. M.; Benson, S. W. J. Phys. Chem. 1977, 81, 1716. Stein, S. E.; Golden, D. M.; Benson, S. W. J. Phys. Chem. 1977, 81, 314. Stein, S. E.; Barton, B. D. Thermochim. Acta, 1981, in press. Stein, S. E.; Golden, D. M. J. Org. Chem. 1977, 42, 839. For general discussions of relations between gas and liquid rate constants see, a. Laidler, K. J. "Chemical Kinetics", McGraw-Hill, Inc., New York, NY, Chap. 5. b. Benson, S. W. "Foundations of Chemical Kinetics", McGraw-Hill Book Co., New York, 1980, Chap. 18. c. Hammes, C. G. "Principles of Chemical Kinetics", Academic Press, New York, 1978, Chap. 7. d. Martin, H. Angew. Chem. Int'l. Ed. 1966, 5, 78. e. Stein, S. E. submitted. "Free Radicals, Vol. I and II"; Kochi, J., ed., Wiley and Sons, New York, NY 1973. a. Koenig, T.; Fischer, H. Chap. 4. b. O'Neal, H. E.; Benson, S. W. Chap. 17. c. Ingold, K. U. Chap. 2. d. Kerr, J. Α., Chap. 1. Sources of vapor pressures used in this analysis are: a. Zwolinski, B. J.; Wilhort, R. C. "Handbook of Vapor Pressures and Heats of Vaporization of Hydrocarbons and Related Compounds", API Project 44, Thermodynamics Research Center, College Station, TX, 1971. b. Boublik, T.; Fried, V.; Hala, E. "The Vapor Pressures of Pure Substances", Elsevier, New York, 1973. c. "CRC Handbook of Chemistry and Physics", Chemical Rubber Co., Cleveland, OH, 1980. d. Stein, S. E. J.C.S. Faraday Trans. I, 1981, in press. Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. "The Properties of Gases and Liquids", Third Edition, McGrawHill, Inc., New York, 1977. Hildebrand, J. H.; Prausnitz, J. M.; Scott, R. L. "Regular and Related Solutions", Van Nostrand Reinhold Co., New York, 1970, Chap. 7.


128


11.

i. j.

12. 13.

14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

N E W APPROACHES IN C O A L

CHEMISTRY

Some sources of use in the present work are: a. Benson, S. W.; O'Neal, H. E. "Kinetic Data on Gas Phase Unimolecular Reactions", NSRDS-NBS 21, U. S. Government Printing Office, Washington, D. C., 1970. b. Tsang, W. Int. J. Chem. Kinetics 1978, 10, 821. c. Walker, J. Α.; Tsang, W. ibid. 1979, 11, 867. d. Culshan, Μ. Α.; Baldwin, R. R.; Evans, G. Α.; Walker, R. W. JCS Faraday I, 1977, 366. e. Kerr, J. Α.; Trotman-Dickinson, A. F., "Bond Strengths of Polyatomic Molecules", in reference 8c. f. Rossi, M.; Golden, D. M. J. Am. Chem. Soc. 1979, 101, 1230. g. McMillan, D. F.; Trevor, P. L.; Golden, D. M. J. Am. Chem. Soc. in press. h. Colussi, A. J.; Zabel, F.; Benson, S. W. Int. J. Chem. Kinetics 1977, 9, 161. Barton, B. D.; Stein, S. E. JCS Faraday I, 1981, in press. Miller, R. E.; Robaugh, D. Α.; Stein, S. Ε., work in progress. k. Robaugh, D. Α.; Stein, S. E. Int. J. Chem. Kinetics, in press. l. Barton, B. D.; Stein, S. E. J. Phys. Chem. 1980, 84, 2141. m. Colussi, A. J.; Benson, S. W. Int. J. Chem. Kinetics, 1977, 9, 295. Walling, C.; Bristol, D. J. Org. Chem. 1971, 36, 733. a. Pryor, W. Α.; Smith, K. J. Am. Chem. Soc. 1970, 98, 5403. b. Walling, C.; Waits, H. P. J. Phys. Chem. 1967, 71, 2361. Preliminary results are reported by Miller, R. E.; Stein, S. E. ACS Div. of Fuel Chem. Preprints, 1979, 24 (3), 271. Benjamin, B. M.; Raaen, V. F.; Maupin, P. H.; Brown, L. L.; Collins, C. J. Fuel 1978, 57, 269. Vernon, L. W. Fuel 1980, 59, 102. Virk, P. S. Fuel 1979, 58, 149. Stein, S. E. Fuel 1980, 59, 900. Bergman, M. R.; Comita, P. B.; Moore, C. B.; Bergman, R. G. J. Amer. Chem. Soc. 1980, 102, 5692. Frey, H. M.; Lister, D. H. J. Chem. Soc. (A) 1967, 1800. Frey, H. M.; Krautz, Α.; Stevens, I. D. R. J. Chem. Soc. (A) 1969, 1734. Klein, M. T.; Virk, P. S. ACS Div. Fuel Chem. Preprints 1980, 25 (4), 180. Golden, D. M.; Spokes, G. N.; Benson, S. W. Angew. Chemie, Int. Ed. (English) 1973, 12, 534. a. Garry, M. J.; Virk, P. S. ACS Fuel Div. Preprints, 1980, 25 (4), 132. b. Bass, D. H.; Virk, P. S. ibid, 25 (1), 17.


7.

STEIN

24. 25. 26.


27.

28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

Coal

129

Conversion

c.

Virk, P. S.; Bass, D. H.; Eppig, C. P.; Ekpenyong ibid., 24 (2), 144. The reverse reaction, H elimination from 1,4-cyclohexadiene is well known, see reference 11a. Herndon, W. C.; Ellsey, M. L. J. Amer. Chem. Soc. 1974, 96, 5331; Herndon, W. C., ibid., 1973, 95, 2404. a. Cox, J. D.; Pilcher, G. "Thermochemistry of Organic and Organometallic Compounds", Academic Press, New York, 1970. b. Stull, D. R.; Westrum, E. F., Jr; SInke, G. C. "The Chemical Thermodynamics of Organic Compounds", Wiley and Sons, NY, 1969. c. Williams, R. B. J. Amer. Chem. Soc. 1942, 64, 1395. a. See footnote 25 in reference 2. b. Benson, S. W. Int. J. Chem. Kinetics 1980, 12, 755. Gibian, M. J.; Corley, R. C. Chem. Rev. 1973, 73, 441. Gibian, M. J.; Corley, R. C. J. Amer. Chem. Soc. 1972, 94, 4178. James, D. G. L.; Stuart, R. D. Trans. Faraday Soc. 1968, 64, 2735. Neuman, R. C.; Alhadeff, E. S. J. Org. Chem. 1970, 35, 3401. Weiner, S. A. J. Amer. Chem. Soc. 1971, 93, 6978. Cronauer, D. C.; Jewell, D. M.; Shaw, Y. T.; Modi, R. J. Ind. Eng. Chem. Fundam. 1979, 18, 153. a. Mallinson, R. G.; Chau, K. C.; Greenkorn, R. A. ACS Fuel Div. Preprints, 1980, 25 (4), 120. b. King, H.-H.; Stock, L. Μ., submitted, 1981. Braude, Ε. Α.; Jackman, L. M.; Linstead, R. P. J. Chem. Soc. 1954, 3548; also see later papers in Linstead and co-workers listed in reference 37. Becker, H-D in "The Chemistry of the Quinonoid Compounds", 2

S. Patai, ed., Wiley and Sons, New York, 1974, Chap. 7, p. 335.

38. 39. 40.

41. 42. 43. 44. 45. 46.

a. Miller, R. E.; Stein, S. E. J. Phys. Chem. in press. b. Abstraction rate constant derived from Jackson, R. Α.; O'Neill, D. W. Chem. Commun. 1969, 1210. Gangwer, T.; MacKenzie, D.; Casano, S. J. Phys. Chem. 1979, 83, 2013. a. Cronauer, D. C., poster presentation at "Conference on Chemistry and Physics of Coal Utilization", Morgantown, WV, June 1980. b. Ratto, J. J.; Heredy, L. Α.; Shawronski, ACS Fuel Div. Preprints 1979, 24 (2), 155. Benjamin, Β. Μ., unpublished results described in "Basic Coal Sciences Project Advisory Committee Report", Gas Research Institute, Chicago, IL 1980, p. 6-8. Ross, D. S.; Blessing, J. E. ACS Fuel Div. Preprints, 1979, 24 (2), 125. Brower, K. R. Fuel 1977, 56, 245. Poutsma, M. L. Fuel 1980, 59, 337. Brower, K. R. J. Org. Chem. 1980, 45, 1004. Livingston, R.; Zeldes, H.; Conradi, M. S. J. Amer. Chem. Soc. 1979, 101, 4312.

In New Approaches in Coal Chemistry; Blaustein, B., et al.; RECEIVED May 5, 1981.

ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

A Fundamental Chemical Kinetics Approach to Coal Conversion

Recommend Documents