New Approaches in Coal Chemistry - American Chemical Society

10 Interaction of Naphthalene with Bituminous Coal: A n E P R and N M R Study JOSEPH J. R A T T O and IRA B. GOLDBERG

Downloaded by EMORY UNIV on February 16, 2016 | http://pubs.acs.org Publication Date: October 26, 1981 | doi: 10.1021/bk-1981-0169.ch010

Rockwell International Science Center, Thousand Oaks, C A 91360

Coal, mixtures of coal with naphthalene-d , or mixtures of coal with naphthalene-h were heated at 400°C for periods up to 10 h. After cooling to ambient temperature, the heated coal was investigated by EPR spectrometry, and the reacted naphthalene was investigated by NMR spectrometry. The radical concentrations in the heated coal increased s l i g h t l y with time, while the radical concentrations in the coal heated with naphthalene increased more rapidly suggesting that the solvent may act as a s t a b i l i z i n g agent for the thermally generated radicals. The g-factors of the sam ples of treated coal decreased rapidly from the room temperature value of 2.00282 to a constant value in the range 2.00265 ± 0.00003 during the time period from 1 h to 10 h, suggesting that the radicals rapidly become more hydrocarbon in nature. The peak -to-peak linewidths from the coal-naphthalene-d samples were s l i g h t l y narrower than the linewidths from the coal-naphthalene-h8 samples, indicating that deuterium was not preferentially incorpo rated close to the stable radical centers. S i g n i f i c a n t h y d r o g e n 8

8

8

e x c h a n g e was o b s e r v e d between t h e i n c o r p o r a t i o n o f protium i n t o the o c c u r r e d more r a p i d l y t h a n i n t h e developed to account f o r the r a t e

c o a l and n a p h t h a l e n e - d e . The α p o s i t i o n o f naphthalene-de 3 p o s i t i o n , and a model was o f t h i s exchange.

One o f t h e methods used i n t h e p r o d u c t i o n o f l i q u i d f u e l s from c o a l i s t o heat c o a l i n the presence o f s o l v e n t s i n o r d e r t o d i s s o l v e and s t a b i l i z e l o w m o l e c u l a r w e i g h t f r a g m e n t s . Many s t u d i e s ( 1 - 6 ) have been d e v o t e d t o e l u c i d a t i n g t h e c h e m i c a l mechanism o f p r o d u c t f o r m a t i o n i n h y d r o g e n d o n o r and n o n - d o n o r s o l v e n t s . In most o f t h e s e s t u d i e s , t h e t i m e dependence o f t h e p r o d u c t y i e l d was used as a measure o f t h e r a t e s o f r e a c t i o n , o r t h e p r o d u c t y i e l d was c o r r e l a t e d w i t h t h e s o l v e n t , t h e r a n k o f t h e c o a l o r other properties o f the c o a l . In o r d e r t o g a i n a b e t t e r u n d e r s t a n d i n g o f t h e n a t u r e o f t h e c o a l - s o l v e n t i n t e r a c t i o n s , we

0097-6156/81/0169-0173$05.00/0

© 1981 American Chemical Society

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

NEW


174

APPROACHES IN

COAL CHEMISTRY

m o n i t o r e d t h e t i m e dependence o f t h e r a d i c a l p r o d u c t i o n and t h e d e g r e e o f h y d r o g e n e x c h a n g e between t h e c o a l and s o l v e n t . A number o f w o r k e r s ( 7 - 1 5 ) have used i s o t o p e l a b e l i n g o f d o n o r and nondonor s o l v e n t s t o i n v e s t i g a t e h y d r o g e n t r a n s f e r , h y d r o g e n e x c h a n g e r e a c t i o n s between c o a l and model compounds, and s t r u c t u r a l rearrangements o f t h e s o l v e n t under l i q u e f a c t i o n c o n d i t i o n s . In t h i s s t u d y , I l l i n o i s b i t u m i n o u s c o a l , and m i x t u r e s o f c o a l and e i t h e r n a p h t h a l e n e - d e o r n a p h t h a l e n e - h e were h e a t e d a t 400°C t o examine t h e t i m e dependence o f t h e r e a c t i o n between c o a l and n a p h thalene. A r o m a t i c h y d r o c a r b o n s s u c h as n a p h t h a l e n e c a n p a r t i a l l y d i s s o l v e c o a l at a t e m p e r a t u r e of 350°C. The mechanism o f t h i s s o l v a t i o n ( 6 , 1 6 ) has been a t t r i b u t e d t o p h y s i c a l b r e a k - d o w n o f t h e c o a l p a r t i c l e s due t o normal s o l v e n t a c t i v i t y and t o c h e m i c a l i n t e r a c t i o n of the solvent with the c o a l . This chemical a c t i v i t y may be t h e r e s u l t o f t r a n s f e r o f h y d r o g e n v i a s o l v e n t f r o m one part of the coal to t h e r m a l l y generated r a d i c a l s i n another part of the c o a l . R e s u l t i n g r a d i c a l s may t h e n be s t a b i l i z e d by t h e s o l v e n t , may r e a r r a n g e t o f o r m l o w m o l e c u l a r w e i g h t p r o d u c t s , o r may combine w i t h l a r g e m o l e c u l a r g r o u p s t o f o r m c h a r s . To d e t e r m i n e t h e c o n c e n t r a t i o n and n a t u r e o f t h e r a d i c a l s i n t h e s e r e a c t i o n s , t h e c o a l and c o a l p r o d u c t s were e x a m i n e d by e l e c t r o n p a r a m a g n e t i c r e s o n a n c e (EPR) s p e c t r o m e t r y . Deuterated naphthalene was used t o d i s t i n g u i s h between h y d r o g e n ( H ) o r i g i n a t i n g f r o m t h e s o l v e n t and h y d r o g e n ( H ) o r i g i n a t i n g f r o m t h e c o a l . D e u t e r i u m incorporated i n t o p o s i t i o n s i n the coal close to r a d i c a l centers w i l l n a r r o w t h e l i n e w i d t h o f an EPR a b s o r p t i o n . Isotope l a b e l l i n g was a l s o u s e d t o measure t h e e x c h a n g e o f h y d r o g e n between t h e s o l v e n t and t h e c o a l . The s p e n t s o l v e n t was a n a l y z e d by H and H n u c l e a r m a g n e t i c r e s o n a n c e (NMR) s p e c t r o m e t r y t o f o l l o w t h e exchange process i n d i f f e r e n t s t r u c t u r a l p o s i t i o n s . 2

X

X

2

Experimental Materials I l l i n o i s No.6 c o a l ( 7 4 . 5 % C, 5.1% H, 1.3% N, 1 9 . 1 % S and 0 , by w e i g h t , d a f b a s i s ) ; 4 . 0 % w a t e r and 1 3 . 1 % ash by w e i g h t , -200 mesh was d r i e d a t 110°C i n vacuum f o r 4 h p r i o r t o use. Naphthalene-de, c o n t a i n i n g 98.5% H e n r i c h m e n t , n a p h t h a l e n e - h e ( A l d r i c h C h e m i c a l C o . ) and a n h y d r o u s e t h y l e t h e r ( J . T . B a k e r C h e m i c a l Co.) were u s e d . NMR a n a l y s i s showed t h a t t h e s t a r t i n g n a p h t h a l e n e - d e c o n t a i n e d 1.5% H ; 0.9% was i n t h e α p o s i t i o n and 0.6% was i n t h e 3 p o s i t i o n . 2

X

Sample P r e p a r a t i o n M u l t i p l e s a m p l e s f r o m S e t s 1-4 were p r e pared. Set 1: A 30 mg s a m p l e o f c o a l was e v a c u a t e d f o r 18 h a t < 0.2 Pa ( 1 0 " t o r r ) and s e a l e d i n a 4 . 4 mm o . d . , 12 cm l o n g quartz tube. S e t s 2 and 3: Coal and n a p h t h a l e n e - d e ( S e t 2) o r c o a l and n a p h t h a l e n e - h e ( S e t 3) were m i x e d i n a 1:1 w e i g h t r a t i o . A 150 mg sample was p l a c e d i n a 6.5 mm o . d . , 15 cm l o n g t h i c k w a l l e d pyrex tube. The t u b e was c o o l e d i n l i q u i d n i t r o g e n , e v a c u a t e d a t < 0 . 2 Pa f o r 15 m i n , c l o s e d t o vacuum, t h e n a l l o w e d t o H


Naphthalene

RATTO AND GOLDBERG

10.

and Bituminous

Coal

175


warm t o room t e m p e r a t u r e . These s t e p s were r e p e a t e d t h r e e t i m e s b e f o r e s e a l i n g t h e sample u n d e r vacuum. Set 4 : A 60 mg sample o f c o a l and n a p h t h a l e n e - d e was d e g a s s e d as d e s c r i b e d f o r S e t s 2 and 3 and was s e a l e d i n a 4 . 4 mm o . d . , 12 cm l o n g q u a r t z t u b e . E x p e r i m e n t a l P r o c e d u r e The f o u r s e t s o f s a m p l e s were h e a t e d f o r d i f f e r e n t l e n g t h s o f t i m e , r a n g i n g f r o m 10 t o 600 min i n a M a r s h a l l c y l i n d r i c a l f u r n a c e at 400°C. The g l a s s t u b e s were attached lengthwise t o a rod extending through the furnace which was t i l t e d a t an a n g l e o f 20° f r o m t h e h o r i z o n t a l p o s i t i o n . The t u b e s were r o t a t e d a t 200 rpm t o p r o v i d e a g i t a t i o n . The t i m e s r e q u i r e d f o r sample h e a t - u p and c o o l - d o w n between 25°C and 400°C were r e s p e c t i v e l y 200 s and 80 s. A f t e r c o o l - d o w n , S e t s 1 and 4 were examined unopened by EPR s p e c t r o m e t r y . Samples f r o m S e t 1 were r e p e a t e d l y h e a t e d and r e - e x a m i n e d by EPR s p e c t r o m e t r y a t room temperature. S e t s 2 and 3 were o p e n e d , and t h e r e a c t e d n a p h t h a l e n e was e x t r a c t e d f r o m t h e sample w i t h e t h y l e t h e r a t a m b i e n t temperature. Based on t h e i n i t i a l and f i n a l w e i g h t s o f t h e c o a l , l e s s t h a n 4 mg ( - 5 wt%) o f c o a l was e x t r a c t e d f r o m t h e s p e n t c o a l using t h i s procedure. The r e m a i n i n g s p e n t c o a l was e v a c u a t e d a t < 0.2 Pa f o r 18 h and s e a l e d u n d e r vacuum i n q u a r t z t u b e s f o r a n a l y s i s by EPR s p e c t r o m e t r y . Based on t h e i n i t i a l w e i g h t , t h e amount o f s p e n t n a p h t h a l e n e e x t r a c t e d f r o m t h e c o a l r a n g e d f r o m 7 0 - 9 5 w t % . The s p e n t n a p h t h a l e n e was s u b l i m e d , and t h e n a p h t h a l e n e was examined by NMR s p e c t r o m e t r y . NMR and EPR H and H NMR s p e c t r a o f n a p h t h a l e n e samples were o b t a i n e d w i t h a JE0L F X - 6 0 - Q s p e c t r o m e t e r a t 9.18 MHz and 5 9 . 7 9 MHz, r e s p e c t i v e l y . S p e c t r a were a c c u m u l a t e d i n t h e f r e e i n d u c t i o n d e c a y mode and were r e c o n s t r u c t e d by F o u r i e r t r a n s f o r m . A 45° p u l s e i n t e r v a l was used c o r r e s p o n d i n g t o 75 us f o r H and 14 us f o r H. C h l o r o f o r m and a c e t o n e - d o r c h l o r o f o r m - d w e r e used as H o r H NMR s o l v e n t s w i t h TMS as r e f e r e n c e f o r H NMR s p e c t r a . Sam p l e s were c o n t a i n e d i n 10 mm o . d . q u a r t z t u b e s , and t h e p r o b e t e m p e r a t u r e was 3 0 ° C . Q u a n t i t a t i v e measurements were made u s i n g pd i o x a n e - d e ( H ) and p - d i o x a n e ( H ) as i n t e r n a l s t a n d a r d s . Trans f o r m s and n u m e r i c a l i n t e g r a t i o n s were o b t a i n e d u s i n g s o f t w a r e s u p p l i e d by J E 0 L , I n c . EPR measurements were made a t 9 . 5 3 3 GHz u s i n g a TEio»+ mode d u a l sample c a v i t y . The c o m p u t e r - c o n t r o l l e d s p e c t r o m e t e r and t h e i n t e g r a t i o n t e c h n i q u e s have been d e s c r i b e d elsewhere (17). Data a n a l y s i s was c a r r i e d out w i t h o f f - l i n e BASIC programs t o d e t e r m i n e t h e d o u b l e i n t e g r a l , g - f a c t o r and l i n e w i d t h ( Δ Η ρ ρ ) . L e s s t h a n 250 pg o f d i p h e n y l p i c r y l h y d r a z y l (95%) were used to c a l i b r a t e the spectrometer. The C u r i e t e m p e r a t u r e o f t h e s t a n d a r d was assumed t o be -26 K. Sample s i z e s were s e l e c t e d t o e n s u r e t h a t t h e c a v i t y was n o t o v e r l o a d e d , w h i c h would c a u s e n o n l i n e a r r e s p o n s e o f t h e EPR s i g n a l ( 1 8 ) . g - F a c t o r s were measured a g a i n s t Mn* i n CaO ( 1 9 ) . The m a g n e t i c f i e l d a t w h i c h t h e d e r i v a t i v e of the a b s o r p t i o n crossed the b a s e l i n e of the spectrum was s e l e c t e d f o r t h e d e t e r m i n a t i o n o f t h e g - f a c t o r . 2

l

2

l

a

6

2

l

2

X


176


Results

NEW

APPROACHES IN COAL CHEMISTRY

and D i s c u s s i o n

EPR Measurements o f H e a t e d Samples The EPR s p e c t r u m o f t h e u n h e a t e d c o a l ( F i g u r e l a ) i s composed o f a s h a r p and a b r o a d s p e c t r a l component. The n a r r o w component has a l a r g e r s i g n a l a m p l i t u d e t h a n t h e b r o a d e r one and a l m o s t o b s c u r e s t h e h i g h f i e l d l o b e o f t h e b r o a d e r component. T h i s s h a r p component has been a t t r i b u t e d t o f u s a i n s i n t h e c o a l ( 2 0 ) , w h i l e t h e b r o a d component has been a s s o c i a t e d w i t h e x i n i t e and v i t r a i n s ( 2 1 ) . When t h e c o a l o r m i x t u r e s o f c o a l and n a p h t h a l e n e a r e h e a t e d , t h e i n t e n s i t i e s o f t h e s h a r p component d e c r e a s e s r e l a t i v e t o t h a t o f t h e b r o a d component. T h i s a p p e a r s t o o c c u r : (a) b e c a u s e t h e l i n e w i d t h o f t h e b r o a d e r peak d e c r e a s e s so t h a t t h e a m p l i t u d e o f t h e d e r i v a t i v e i n c r e a s e s t h e r e b y m a s k i n g t h e s h a r p c o m p o n e n t , and (b) b e c a u s e t h e i n t e n s i t y o f t h e s h a r p component d e c r e a s e s . B o t h s h a r p and b r o a d components were o b s e r v e d o n l y f o r t h e c o a l i n t h e a b s e n c e o f n o n - d o n o r s o l vent. The p r e s e n c e o f oxygen c a n c a u s e c o n s i d e r a b l e b r o a d e n i n g o f t h e l i n e w i d t h s ; t h e r e f o r e , t h e measurements must be made a f t e r r e m o v i n g oxygen i n o r d e r t o o b s e r v e d e t a i l s o f t h e s p e c t r a . Radical Concentrations The number o f r a d i c a l s r e l a t i v e t o t h e c o n c e n t r a t i o n i n u n h e a t e d c o a l was d e t e r m i n e d f r o m t h e s u s c e p t i b i l i t y a c c o r d i n g t o t h e C u r i e - W e i s s l a w ( 2 2 ) . The r a d i c a l c o n c e n t r a t i o n s f r o m S e t 1 a r e shown i n F i g u r e 2. The r e p e a t e d h e a t i n g o f a s i n g l e c o a l sample i s shown by t h e l i n e s c o n n e c t i n g t h e data points. In g e n e r a l , as t h e c o a l sample i s h e a t e d f o r l o n g e r p e r i o d s , more r a d i c a l s a r e p r o d u c e d as shown by t h e upward t r e n d o f t h e d a t a p o i n t s between t h e p a i r s o f d o t t e d and b r o k e n l i n e s . Samples o f c o a l h e a t e d f o r a s h o r t p e r i o d o f t i m e (10-15 m i n ) show a s l i g h t r e d u c t i o n i n t h e number o f r a d i c a l s even a f t e r c o o l i n g t o room t e m p e r a t u r e . A maximum o f 40% more r a d i c a l s were p r e s e n t a f t e r h e a t i n g f o r 120 m i n . Grandy and P e t r a k i s ( 2 3 ) r e p o r t e d a s e v e n - f o l d i n c r e a s e o f t h e r a d i c a l c o n c e n t r a t i o n o f a Powhatan No. 5 HVb c o a l on h e a t i n g t o 450°C w i t h 1 0 . 3 5 Pa o f h y d r o g e n meas u r e d a t t h e e l e v a t e d t e m p e r a t u r e i n a r a p i d h e a t - u p EPR c a v i t y . R e h e a t i n g o u r samples o f c o a l r e s u l t e d i n a s l i g h t i n c r e a s e o r v i r t u a l l y no change i n t h e r a d i c a l c o n c e n t r a t i o n . On t h e o t h e r h a n d , f r e s h s a m p l e s h e a t e d f o r 25 t o 120 min show a s i g n i f i c a n t i n c r e a s e i n t h e nunber o f r a d i c a l s p r o d u c e d . It i s d i f f i c u l t to a s c e r t a i n t h e experimental s c a t t e r o f these samples. Double i n t e g r a l s were r e p r o d u c i b l e t o c a . t 2 % ; h o w e v e r , h e a t i n g and r o t a t i o n o f t h e samples can cause a r e d i s t r i b u t i o n o f t h e p a r a m a g n e t i c m a t e r i a l w i t h i n t h e sample t u b e d u r i n g r e a c t i o n . This c a n c a u s e c o n s i d e r a b l e s c a t t e r i n t h e d a t a , and we have no way t o evaluate t h i s source o f e r r o r . The number o f r a d i c a l s p r o d u c e d a f t e r h e a t i n g t h e c o a l samples f r o m S e t s 2 and 3 r e l a t i v e t o t h e r a d i c a l s p r e s e n t i n u n h e a t e d c o a l a r e shown i n F i g u r e 3. These c o a l samples were s e p a r a t e d f r o m t h e s o l v e n t and s e a l e d u n d e r vacuum b e f o r e t h e EPR m e a s u r e ments were made. The b r o k e n and d o t t e d l i n e s i n d i c a t e t h e g e n e r a l



10.

RATTO AND

GOLDBERG

Naphthalene

and Bituminous

Coal

177

Figure 1. EPR spectra of coal, coal-naphthalene-h and coal-naphthalene-d heated at 400°C: a. unheated coal; b. coal heated for 30 min; c. coal heated for 10 h; d. coal heated with naphthalene-d for 10 h; e. coal heated with naphthalenehsforlOh. 8

8


8


Figure 2.

Radical concentration in samples of heated coal (Set 1) relative to unheated coal measured for periods up to 600 min


H

CO

ο s w g

ι

3

C/5

m

ο se

> •τ

m

00

-J


Figure 3.

8

Radical concentration in samples of coal heated with naphthalene-d (Set 2, V relative to unheated coal measured for periods up to 600 min

TIME OF HEATING (min) o r


8

naphthalene-h

(Set 3, vo

S*

S

î

ο

w

s

ο ο r

α

>

2


180

NEW

APPROACHES IN

COAL CHEMISTRY

trend of the data. The g r e a t e s t change o f t h e r a d i c a l c o n c e n t r a t i o n occurs w i t h i n the f i r s t hour. The r e l a t i v e r a d i c a l c o n c e n t r a t i o n i n c r e a s e s t o a maximum o f a p p r o x i m a t e l y 1.3 t o 1.4 a f t e r 25 m i n . The c o n c e n t r a t i o n t h e n s l o w l y d e c r e a s e s t o c a . 1.2 a f t e r 60 min and t o c a . 0.9 a f t e r 600 m i n . This steady decrease i n the c o n c e n t r a t i o n o f t h e r a d i c a l s a f t e r 25 min i s c o n s i s t e n t w i t h t h e h y p o t h e s i s ( 6 , 1 6 ) t h a t n a p h t h a l e n e a c t s as a h y d r o g e n t r a n s f e r a g e n t , t h a t i s , h y d r o g e n i s t r a n s f e r r e d f r o m one p a r t o f t h e c o a l to t h e r m a l l y generated r a d i c a l s i n another p a r t . Resulting n a p h t h y l r a d i c a l s c a n e i t h e r c o n t i n u e t h e c h a i n o r end i t by f o r m ing other products. A d i s c u s s i o n of the s t a b i l i t y of the naphthyl r a d i c a l s i s given elsewhere (14). Some o f t h e r e d u c t i o n i n r a d i c a l c o n c e n t r a t i o n may a l s o o c c u r by c o m b i n a t i o n o f d i s s o l v e d r a d i c a l s d e r i v e d f r o m t h e c o a l o r by c o m b i n a t i o n o f t h e s e r a d i c a l s w i t h e i t h e r heterogeneous coal or o t h e r d i s s o l v e d r a d i c a l s p e c i e s . g-Factors The g - f a c t o r s o f s a m p l e s f r o m S e t s 1-4 a r e shown i n F i g u r e 4. In g e n e r a l , t h e s e t s o f d a t a p a r a l l e l each o t h e r w i t h t h e g - f a c t o r s o f Set 4 * Set 2 > Set 3 > Set 1. To show t h e g e n e r a l t r e n d s , l i n e s a r e drawn t h r o u g h t h e d a t a f r o m t h e h e a t e d c o a l - n a p h t h a l e n e m i x t u r e s ( • , • , and Δ ) and t h r o u g h t h e d a t a f r o m t h e heated coal ( · ) . A l t h o u g h no measurements were made on t h e c o a l h e a t e d between 0 and 10 min b e c a u s e o f t h e f i n i t e h e a t - u p t i m e , t h e r e i s a l a r g e d i f f e r e n c e between t h e room t e m p e r a t u r e v a l u e (2.00282 + 0 . 0 0 0 0 2 ) and t h e v a l u e s o b t a i n e d a f t e r 10 min o f h e a t i n g each o f t h e f o u r s e t s ( 2 . 0 0 2 6 3 - 2 . 0 0 2 6 8 ) . This suggests t h a t the c o n c e n t r a t i o n of heteroatom c o n t a i n i n g r a d i c a l s decreases r a p i d l y d u r i n g t h e f i r s t few m i n u t e s o f h e a t i n g . Other workers ( 2 2 , 2 4 , 2 5 ) have d e s c r i b e d s i m i l a r b e h a v i o r f o r h e a t e d c o a l s . The a v e r a g e v a l u e o f t h e g - f a c t o r s o f s a m p l e s o f c o a l h e a t e d between 1 h and 10 h i s 2.00263 w i t h a s t a n d a r d d e v i a t i o n o f 0.00003. This i n d i c a t e s t h a t only small s t r u c t u r a l changes, i f any, occur. Studies of u n s u b s t i t u t e d aromatic hydrocarbon r a d i c a l s ( 2 6 , 2 7 ) have shown t h a t e i t h e r n e u t r a l r a d i c a l s o r r a d i c a l i o n s o f l a r g e s i z e s h o u l d e x h i b i t g - f a c t o r s o f 2.00262 ± 0.00001. σ-Radicals are expected to e x h i b i t g - f a c t o r s smaller than 2.00262. Thus, i t i s p o s s i b l e t h a t t h e r a d i c a l s i n heated c o a l a r e π - a r o m a t i c , o f n e u t r a l c h a r g e , and c o n t a i n v e r y l i t t l e heteroatom c h a r a c t e r , g-Factors o f c o a l s heated w i t h naphthalene ( S e t s 2 and 3) a r e l a r g e r ( 2 . 0 0 2 6 6 t o 2 . 0 0 2 7 0 ) but p a r a l l e l t h e gf a c t o r s o f t h e heated c o a l (Set 1 ) , s u g g e s t i n g t h a t these r a d i c a l s may have s i g n i f i c a n t l y l a r g e r h e t e r o a t o m c o n t e n t ; t h e y may be s t a b i l i z e d by t h e s o l v e n t . P a r t o f t h e d i f f e r e n c e between g - f a c t o r s o f S e t s 2 and 3 (ca_. 0 . 0 0 0 0 3 u n i t s ) may be an a r t i f a c t due t o t h e d i f f e r e n t l i n e s h a p e s ( F i g u r e 1 ) ; t h e b r o a d e r l i n e o f Set 3 a l l o w s more w e i g h t t o t h e s h a r p e r s p e c t r a l component a t t h e p o i n t a t which t h e d e r i v a t i v e c r o s s e s the b a s e l i n e . g - F a c t o r s o f t h e sam p l e s f r o m S e t s 2 and 3 i n c r e a s e a f t e r h e a t i n g between 8 t o 10 h as shown by t h e d o t t e d l i n e i n F i g u r e 4. A possible explanation for t h i s i n c r e a s e i s t h a t r a d i c a l s are formed i n t h e c o a l a f t e r long


In New Approaches in Coal Chemistry; Blaustein, Bernard D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981. 8

8

8

Figure 4 The factors of Illinois No. 6 coal heated for the same periods of time under different conditions: Set 1, · , coal heated in sealed tubes; Set 2, U> coal heated with naphthalene-d ; Set 3, • , coal heated with naphthalene-h ; Set 4, Δ , coal heated with naphthalene-d and unopened.



182

NEW

APPROACHES IN

COAL CHEMISTRY

h e a t i n g t i m e s w h i c h r e a c t i r r e v e r s i b l y w i t h oxygen o r t h e s o l v e n t on e x p o s u r e a t a m b i e n t t e m p e r a t u r e t h u s i n c r e a s i n g t h e g - f a c t o r s . L e w i s and S i n g e r (28) have r e p o r t e d t h e measurement o f t h e gf a c t o r s o f 25 a r y l o x y r a d i c a l s . The g - f a c t o r s v a r i e d between 2.0032 and 2 . 0 0 4 0 . The s a m p l e s f r o m S e t s 2 and 4 can be compared t o examine t h e e f f e c t o f e x p o s u r e o f t h e spent c o a l t o a i r b e f o r e r e s e a l i n g under vacuum f o l l o w e d by measurement o f t h e g - f a c t o r s and l i n e w i d t h s . The s a m p l e s f r o m Set 2 were o p e n e d , and t h e s a m p l e s f r o m Set 4 w e r e unopened. As shown i n F i g u r e 4 , t h e g - f a c t o r s f r o m S e t 4 a r e e q u a l t o t h e g - f a c t o r s f r o m Set 2 w i t h i n e x p e r i m e n t a l u n c e r tainty. T h i s i n d i c a t e s t h a t e x p o s u r e t o a i r and s o l v e n t a t a m b i e n t t e m p e r a t u r e does n o t s i g n i f i c a n t l y a f f e c t t h e g - f a c t o r of the product. I f oxygen does have an a f f e c t on t h e r a d i c a l s i n t h e s a m p l e , t h o s e r a d i c a l s w h i c h a r e changed a r e n o t d e t e c t e d . T h i s r e s u l t i n d i c a t e s t h a t p e r o x y r a d i c a l s , i f f o r m e d , do not i n f l u e n c e t h e s e measurements. Linewidths The EPR s p e c t r a o f t h e s p e n t c o a l s a m p l e s f r o m S e t s 1-4 c o n t a i n e d b o t h s h a r p and b r o a d c o m p o n e n t s . Several ex a m p l e s a r e shown i n F i g u r e 1. The measured v a l u e s a r e shown i n F i g u r e 5. The s h a r p component was o n l y o b s e r v a b l e f o r Set 1 o v e r t h e e n t i r e range o f heated samples. It decreases s l i g h t l y from 0.13 mT (1 mT = 10 Oe) a f t e r h e a t i n g f o r 20 min t o 0.09 mT a f t e r h e a t i n g f o r 600 m i n . The s h a r p components o f t h e s p e c t r a o f h e a t e d c o a l - n a p h t h a l e n e m i x t u r e s ( S e t s 2 and 3) c o u l d be measured o n l y f o r s a m p l e s h e a t e d f o r up t o 40 min b e f o r e b e i n g o b s c u r e d by t h e h i g h f i e l d l o b e o f t h e b r o a d component o f t h e d e r i v a t i v e s p e c trum. These v a l u e s were v e r y c l o s e t o t h e v a l u e s o f t h e h e a t e d c o a l s a m p l e s , Set 1. The l i n e w i d t h s (ΔΗρρ) o f t h e b r o a d component f r o m Set 1 d e c r e a s e r a p i d l y d u r i n g i n i t i a l h e a t i n g and a r e c o n s t a n t a t 0.45 ± 0 . 0 2 mT f o r s a m p l e s h e a t e d f r o m 10 t o 600 m i n . ΔΗρρ o f t h e b r o a d component o f S e t s 2 and 3 a r e much b r o a d e r i n i t i a l l y , s t a r t i n g a t 0 . 6 8 mT (10 min) and d e c r e a s i n g t o 0.49 mT (180 m i n ) . ΔΗρρ o f S e t 2 d e c r e a s e d f u r t h e r t o 0.45 mT a f t e r 600 m i n o f h e a t i n g w h i l e t h a t o f S e t 3 r e m a i n e d c o n s t a n t a t 0.49 mT. In F i g u r e 4 , t h e v a l u e s o f ΔΗρρ f r o m Set 4 , s e a l e d s a m p l e s o f h e a t e d c o a l - n a p h t h a l e n e - d e w h i c h have n o t been e x p o s e d t o a i r , p a r a l l e l t h e v a l u e s o f ΔΗρρ f r o m Set 2 , t h e c o r r e s p o n d i n g s e p a r a t e d s a m p l e s e x p o s e d t o a i r , but t h o s e o f S e t 2 a r e much b r o a d e r . T h i s b r o a d e n i n g may be due t o t h e s a m p l e s i r r e v e r s i b l y a b s o r b i n g oxygen from t h e a i r d u r i n g t h e e x t r a c t i o n o f n a p h t h a l e n e . T h i s r e s u l t s u g g e s t s t h a t even p r o l o n g e d e v a c u a t i o n does n o t remove a l l o f t h e oxygen f r o m t h e c o a l p r o d u c t . It i s a l s o p o s s i b l e t h a t e x p o s u r e t o t h e s o l v e n t c a u s e s some change i n t h e e n v i r o n m e n t o f the r a d i c a l , although the g - f a c t o r s suggest t h a t the r a d i c a l s are chemically equivalent. L i n e w i d t h s o f r a d i c a l s can r e f l e c t t h e degree o f d e r e a l i z a t i o n o f an u n p a i r e d e l e c t r o n i n an a r o m a t i c r a d i c a l and t h e e x t e n t



120

8

240

8

TIME (min)

360

8

480

600

Figure 5. Peak-to-peak linewidths of coals heated for different periods of time: Set 1, · , coal heated in sealed tubes; Set 2, •> coal heated with naphthalene-d ; Set 3, • , coal heated with naphthalene-h ; Set 4, Δ , coal heated with naphthalened and unopened.

°·°0



184

NEW

APPROACHES IN COAL CHEMISTRY

o f h y d r o g e n i n c o r p o r a t i o n . A h y d r o g e n atom bonded t o a c a r b o n atom i n a π - e l e c t r o n system e x h i b i t s a h y p e r f i n e s p l i t t i n g which i s p r o p o r t i o n a l t o t h e f r a c t i o n o f e l e c t r o n s p i n d e n s i t y on t h a t t r i gonal c a r b o n . As t h e e l e c t r o n becomes more d e l o c a l i z e d , t h e l i n e w i d t h t e n d s t o n a r r o w f o r two r e a s o n s : ( a ) The s p i n d e n s i t y on e a c h c a r b o n d e c r e a s e s s o t h a t e a c h h y p e r f i n e s p l i t t i n g due t o h y d r o g e n i s s m a l l e r , and t h e number o f h y d r o g e n atoms i n t e r a c t i n g w i t h t h e e l e c t r o n i n c r e a s e s , so t h a t t h e s p e c t r a l d e n s i t y i n c r e a s e s i n t h e r e g i o n i n which t h e t o t a l n u c l e a r s p i n i s near zero, (b) A consequence o f i n c r e a s e d d e r e a l i z a t i o n i s t h a t t h e r a d i c a l contains a higher proportion o f t e r t i a r y (bridgehead) c a r b o n atoms so t h a t s p i n d e n s i t y i s consumed w i t h o u t a c o r r e sponding i n c r e a s e o f h y p e r f i n e i n t e r a c t i o n s . A r e d u c t i o n i n t h e n i m b e r o f 3 h y d r o g e n atoms (bonded t o c a r b o n atoms α t o t h e a r o m a t i c system) w i l l a l s o d e c r e a s e t h e l i n e w i d t h . T h i s o c c u r s b e cause o f fewer h y p e r f i n e i n t e r a c t i o n s which r e s u l t from t h e i n t e r a c t i o n o f t h e s p i n on t h e t r i g o n a l c a r b o n w i t h two o r t h r e e h y d r o gens. A p p l y i n g t h i s r e a s o n i n g t o c o a l s a m p l e s o f S e t s 1 and 3 i n d i c a t e s t h a t c o a l h e a t e d w i t h o u t s o l v e n t f o r m s more h i g h l y conjugated systems than t h e c o a l s heated w i t h s o l v e n t . This i s a l s o s u p p o r t e d by t h e l o w e r g - f a c t o r . DeuteritiTi l a b e l l i n g p e r m i t s t h e e x t e n t o f hydrogen i n c o r p o r a t i o n n e a r t h e r a d i c a l c e n t e r s t o be e s t i m a t e d . Sample S e t s 2 and 3 were t r e a t e d i d e n t i c a l l y , e x c e p t t h a t i n S e t 2, n a p h t h a l e n e - d e was used as a s o l v e n t , w h i l e i n S e t 3, n a p h t h a l e n e - h e was u s e d . I f t h e o n l y s o u r c e o f l i n e w i d t h o f t h e c o a l i s due t o inhomogeneous h y p e r f i n e b r o a d e n i n g , a s d e s c r i b e d a b o v e , and i f a l l o f t h e H i n t h e c o a l i s r e p l a c e d b y H , t h e l i n e w i d t h s h o u l d d e c r e a s e by a f a c t o r o f 3.25 (22J. A f t e r h e a t i n g f o r 10 h , 21.1% o f t h e H i n t h e n a p h t h a l e n e o f S e t 2 i s r e p l a c e d b y H ( s e e F i g u r e 6). Based on t h i s amount o f H b e i n g i n c o r p o r a t e d i n t h e c o a l , t h e l i n e w i d t h o f S e t 2 s h o u l d be a b o u t 0.03 mT [(1/3.25) (.211)(.49 mT)] s m a l l e r t h a n t h a t o f S e t 3 a f t e r h e a t i n g f o r 10 h. The e x p e r i m e n t a l d i f f e r e n c e i s 0.04 mT, w h i c h i s c l o s e t o t h e p r e d i c t e d amount. This r e s u l t s u g g e s t s t h a t exchange i s n o t s i g n i f i c a n t l y p r e f e r e n t i a l near t h e s i t e s o f t h e s t a b l e r a d i c a l p r o d u c t s , i n c o n t r a s t w i t h r e s u l t s o b t a i n e d f o r a u t o c l a v e e x p e r i m e n t s (22) i n w h i c h c o a l was h e a t e d w i t h D2 and T e t r a l i n - d i 2 . A

2

2

X

2

Exchange o f Hydrogen Between C o a l and N a p h t h a l e n e Incorporat i o n o f H i n t o t n e α a n d 3 p o s i t i o n s o f n a p n t h a l e n e - d e a s mea s u r e d by H NMR s p e c t r o m e t r y i s shown i n F i g u r e 6. Progressively more p r o t i u n was f o u n d t o be i n c o r p o r a t e d i n t o t h e n a p h t h a l e n e - d e s o l v e n t as t h e r e a c t i o n t i m e was i n c r e a s e d . During t h e f i r s t few m i n u t e s , t h e i n c o r p o r a t i o n i n b o t h p o s i t i o n s o c c u r r e d more r a p i d l y t h a n l a t e r i n t h e r e a c t i o n . T h r o u g h o u t t h e measured d u r a t i o n o f the r e a c t i o n s , t h e rate o f incorporation o f protium i n t o t h eα p o s i t i o n was more r a p i d t h a n i n t h e 3 p o s i t i o n . I f we d e f i n e t h e t o t a l h y d r o g e n ( d e u t e r i u m and p r o t i u m ) i n e i t h e r t h e α and 3 p o s i t i o n s a s u n i t y , so t h a t f o r f ^ r e p r e s e n t A

X

a



Figure 6.

a

β

2

β

8

β

2

β

Fraction of protium incorporated in the a and β positions of naphthalene-d as a function of reaction time. The fraction of 'H and *Η is defined such that Ή « + Η — 1.0 and *Η + Η — 1.0.

TIME (min)


00

ρ

NEW APPROACHES IN COAL CHEMISTRY

186

r e s p e c t i v e l y t h e f r a c t i o n o f protium i n t h e α o r β p o s i t i o n s , then t h e r e s p e c t i v e f r a c t i o n s o f d e u t e r i u m a r e g i v e n by 1 - f and 1 - f g . A f t e r h e a t i n g f o r 600 m i n , f i n c r e a s e d f r o m Ο.Ο18 t o 0.33, w h i l e f i n c r e a s e d f r o m 0.012 t o 0.092. T h i s r e p r e s e n t s an i n c r e a s e f r o m 1.5% [100%·(0.018 + 0.012)/2] p r o t i u m t o 21.1% p r o tium. Our measurement o f f i s e q u a l t o 0.138 a f t e r 60 m i n and i s c l o s e t o t h e v a l u e o f 0.108 r e p o r t e d (14) f o r n a p h t h a l e n e - d e and HVB L o v e r i d g e M i n e , P i t t s b u r g h Seam c o a f h e a t e d i n an a u t o c l a v e r e a c t o r a t c o n d i t i o n s o f 1 h and 400°C. A s i m p l e c h e m i c a l model t o i n t e r p r e t t h e d a t a i n F i g u r e 6 i s b a s e d on t h e f o l l o w i n g c h e m i c a l mechanism f o r t h e exchange reactions: D H


6

The α - h y d r o g e n e x c h a n g e , R e a c t i o n 1, and t h e s - h y d r o g e n e x c h a n g e , R e a c t i o n 2, a r e i n t e r r e l a t e d by a 1,2-hydrogen exchange as shown i n R e a c t i o n 3. Each o f t h e s e r e a c t i o n s may be more c o m p l i c a t e d t h a n t h a t shown a b o v e , s i n c e t h e y p r o b a b l y i n v o l v e s o l v e n t o r c o a l radical intermediates. In r e a c t i o n s 1 and 2, f o r e x a m p l e , t h e c o a l probably a b s t r a c t s H from t h e naphthalene-de t o form a n a p h t h y l r a d i c a l , and s u b s e q u e n t l y t h e n a p h t h y l r a d i c a l a b s t r a c t s Η· f r o m a d i f f e r e n t p a r t o f t h e c o a l as shown i n R e a c t i o n s 4 and 5: 2

#

ι

+ coal*

+ coal—D


(4)

Naphthalene

RATTO AND GOLDBERG

10.

and Bituminous

187

Coal

H

By u s i n g t h e a d d i t i o n a l s i m p l i f y i n g a s s u m p t i o n s t h a t t h e number o f a c t i v e c o a l s i t e s r e m a i n s c o n s t a n t d u r i n g t h e r e a c t i o n and t h a t t h e f o r w a r d and r e v e r s e r a t e c o e f f i c i e n t s a r e e q u a l , k = k_ , 3 3 * k-i» equations f o r the incorjura t i o n o f H i n t o t h e " a and 3 p o s i t i o n s o f n a p h t h a l e n e r e a c t i o n s can be w r i t t e n k

a

k

=

n

d

k

=

t

h

e

n

t

h

e

r

a

t

e

a


i

df -HT

at

=

k

Ν f

a

- k N f

C C

-

a C a

k*H {f 1

a

a

3

-

f j

(6)

3

and d f

3

=

k

e c c N

f

k

B C 3 N

1

f

α

β

1

(7)

a'

where t h e f i r s t t e r m o f R e a c t i o n s 6 and 7 i s t h e f o r w a r d r a t e , t h e second t e r m i s t h e r e v e r s e r a t e , and t h e t h i r d t e r m i s t h e 1 , 2 h y d r o g e n exchange r a t e . In t h e s e e q u a t i o n s , f i s the f r a c t i o n of a c t i v e s i t e s i n t h e c o a l s u b s t i t u t e d w i t h p r o t i u m , and Ν , Ν and N a r e t h e n i m b e r o f a c t i v e s i t e s i n t h e n a p h t h a l e n e a n d c o a l respectively. I f a l l o f t h e p r o t i u m and d e u t e r i u m r e m a i n s i n t h e r e a c t i v e c o m p o n e n t s , t h e c o n s e r v a t i o n o f p r o t i u n c a n be w r i t t e n ç

β

a

p

c

f Ν

α α

Since N

f

Q

+ f Ν = (1 - f ) Ν 3 3 C C β

= N^, s o l v i n g f o r f

c

(8)

we o b t a i n

= - ( « V iç · 1

c

.

0

f

+

(9)

T h i s c a n be s u b s t i t u t e d i n t o E q u a t i o n s

Ν

df

—rr = k Ν dt

α c

l - f

'

( « + l) - f

orN

Ν «

3 Ν

4 and 5 t o - k,N 1

α

(f

χ

α

give

- f j

3

and


(10)

NEW

188 df

Ν

e

"Λ

TÉ"

APPROACHES IN

COAL CHEMISTRY

Ν

V T T

+

1

" W W

) - a N : f

(ID The e x a c t s o l u t i o n s o f E q u a t i o n s 10 and 11 c a n be o b t a i n e d by Laplace transforms or the s o l u t i o n of simultaneous d i f f e r e n t i a l equations. The r e s u l t i n g c o e f f i c i e n t s o f f and f ^ a r e v e r y complex f u n c t i o n s o f k k , k , Ν and Ν . However, as seen i n F i g u r e 6 , t h e r e a c t i v i t y of t h e 3 p o s i t i o n i s s m a l l r e l a t i v e t o the α p o s i t i o n . T h u s , i n t h e l i m i t i n g c o n d i t i o n where f ^ i s c l o s e t o t h e i n i t i a l v a l u e and n e g l e c t i n g e x c h a n g e between t h e α and 3 positions, a


l f

TF ' Integrating *n

k

« c * N

~ a f

New Approaches in Coal Chemistry - American Chemical Society

Recommend Documents