Organic Chemistry of Coal - American Chemical Society

1 A Primer on the Chemistry and Constitution of Coal D.D.WHITEHURST

Downloaded by ST JOSEPHS UNIV on September 15, 2013 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0071.ch001

Mobil Research and Development Corp., P.O. Box 125, Princeton, NJ 08540

The purpose of this paper i s to review what i s known about the structure of coal and show how this information relates to the ultimate conversion of coal to conventional liquid fuels. Let us f i r s t consider some common beliefs about coal, as shown below: • • • • • •

Coal is highly aromatic. Its structure contains predominantly condensed polycyclic aromatic rings. The high degree of condensation makes coal d i f f i c u l t to liquify. Extreme pressures and temperatures are required for coal conversion. Organic sulfur is much more d i f f i c u l t to remove than organic oxygen. Liquefaction requires high hydrogen consumption.

By the end of this paper I hope to have shown that all of these statements are wrong. To initiate this discussion, I propose to present three aspects of coal and coal product structure. These include, aromaticity, functionality, and molecular weight. I w i l l then discuss reactivity of coal in non-catalytic hydrogenative processes and f i n a l l y , how structure and reactivity interrelate. Concerning the structure of coal, I would f i r s t like to say a few words about the origin of coal. It is generally agreed that coal originates primarily from plants. Through a series of evolutionary changes the primary products of the original decomposed plant matter becomes transformed through a series of steps in which the f i r s t product is humic acid. The humic acid 0-8412-0427-6/78/47-071-001$10.00/0 ©

1978 American Chemical Society

In Organic Chemistry of Coal; Larsen, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

ORGANIC CHEMISTRY OF COAL


2

i s then transformed s e q u e n t i a l l y i n t o peat, l i g n i t e , subbituminous c o a l , b i t u m i n o u s c o a l , and f i n a l l y t o a n t h r a c i t e as shown i n F i g u r e 1. With t h e s e t r a n s f o r m a t i o n s , the c a r b o n c o n t e n t i n c r e a s e s and the oxygen content decreases. The r e s u l t i s t h a t the c a l o r i f i c v a l u e o f the c o a l i n c r e a s e s w i t h rank. A l s o shown i n F i g u r e 1, i s the f a c t t h a t c o a l as we know i t today, can be i d e n t i f i e d as composed o f a s e r i e s o f macérais, o r f o s s i l i z e d p l a n t fragments. These f o s s i l i z e d p l a n t fragments are r e l a t e d t o the o r i g i n a l p l a n t m a t t e r from which t h e y are d e r i v e d . The c o n s t i t u e n t s o f p l a n t s which c o u l d p o s s i b l y g i v e r i s e t o c o a l and commonly a s s o c i a t e d s t r u c t u r e s are shown i n F i g u r e 2. The s t r u c t u r e s t h a t we f i n d i n c o a l , o r c o a l l i q u i d s , must be t h o s e r e l a t e d t o the most s t a b l e o f the s t r u c t u r e s from the o r i g i n a l p l a n t fragments. There are two s c h o o l s o f thought on the major c o n s t i t u e n t o f c o a l . United States c o a l s c o n s i s t of p r i m a r i l y v i t r i n i t e , u s u a l l y 80% o r more. The c o m p o s i t i o n o f t h i s v i t r i n i t e i s b e l i e v e d t o be the r e s u l t o f the c o a l i f i c a t i o n o f e i t h e r c e l l u l o s e or l i g n i n s t r u c t u r e s , which c o n s t i t u t e the m a j o r i t y o f the p l a n t components (jL) . I t has been shown by G i v e n and o t h e r s , however, t h a t c e l l u l o s e undergoes v e r y r a p i d biodégradation i n p l a n t s which are decomposing t o d a y (2). The same i s t r u e f o r p r o t e i n . P l a n t c o n s t i t u e n t s which are most r e s i s t a n t t o b a c t e r i a l a t t a c k are t h o s e o f waxes, r e s i n s , t a n n i n s , l i g n i n s , f l a v o n o i d s , and p o s s i b l y a l k a l o i d s (_3) . A l t h o u g h the p r e v i o u s l y d i s c u s s e d s t r u c t u r e s are p r e s e n t i n p l a n t s today, and c o u l d be s i m i l a r t o t h o s e o f p l a n t s o f p r e h i s t o r i c t i m e s , i t i s not a n t i c i p a t e d t h a t the s t r u c t u r e s would s u r v i v e i n t a c t o v e r the l o n g p e r i o d s o f time r e q u i r e d f o r t h e i r t r a n s f o r m a t i o n t o coal. Some o f the s t r u c t u r a l f e a t u r e s however may p o s s i b l y be r e c o g n i z a b l e even i n t o d a y ' s c o a l . I t has r e c e n t l y been shown by G i v e n t h a t c e r t a i n components o f c o a l can be r e l a t e d t o s t r u c t u r e s e v o l v e d from l i g n i n s (4). I t s h o u l d a l s o be remembered t h a t the U.S. coals were l a y e d down i n two d i f f e r e n t g e o l o g i c a l ages; about 160 m i l l i o n y e a r s a p a r t , and the s t r u c t u r e s a s s o c i a t e d w i t h two g e o l o g i c a l ages may be s u b s t a n t i a l l y d i f f e r e n t . Aromaticity

of

Coal

T h e r e i s c o n t r o v e r s y on the p r o p o s e d p r i m a r y b a c k bone s t r u c t u r e o f c o a l . Some workers c o n t e n d t h a t c o a l i s p r i m a r i l y g r a p h i t e - l i k e , o t h e r s argue t h a t c o a l i s of a diamond-like s t r u c t u r e .


1.

wmTEHURST

Chemistry and Constitution of Coal

ORIGINAL PLANT

MATERIAL

3

COAL M A C E R A L (VISUAL


RESINOUS

MATERIAL

CUTICLES, SPORES CELLWALLS,RES1NS

PLANT

MICROSCOPY)

EXINiTc RESIN1TE

N

FRAGMENTS

WOOD, CORK SPORES,?OLLEN

V i T R I N l T E j 6 SUBSTANCES CARBONIZATION

FUSINITE M1CRONITE

RANK % C % 0

Ρ E A T — ^ L ! G Ν I T E — * 5 U S B I T U MI N O U S — * B I T U M I N O U S — * A N T H R A C I T E HIGH M E D . L O W CBA 60 70 80 93 35

CALORIFIC 12000 VALUE Bru/^-fnaf)

25 13000

Figure 1.

15 14000

3 16000

15500

Mode of formation of coal


4


C-OH

Cellulose

\

*

OH

C-OH /h

Protein

c-s-s-c-ç-cC^.^o

3

NH

f*o

COJ

2

OH

c,

Waxes

(C, -C )


7

(C -C32>

3I

26

XOOR

Resins

,COOR C-C

Terpenes

COOH

•C-C-C-C-OH HO"

Sterols HO'

Flavonoids

CO"® 0

OH OH

Tannins

^

OH Ο

COO-Sugar

^ O H

OH

OH

OH

OCH

3

Lignins

CHj-OH OH OH COOH

c-c

Alkaloids CH3O.

Figure 2.

Structures of coal precursors



1.

WHITEHURST

5


Graphite

Diamond

B o t h o f t h e s e s t r u c t u r e s a r e low i n H/C r a t i o which i s consistant with the composition o f c o a l s . To g a i n i n s i g h t on t h e s t r u c t u r e o f c o a l , p a s t workers have attempted t o b r e a k c o a l down i n t o r e c o g n i z a b l e u n i t s and t h e n p i e c e them back t o g e t h e r as i s done i n n a t u r a l p r o d u c t c h e m i s t r y today. The most common t e c h n i q u e p r e s e n t l y p u r s u e d i s t h a t o f o x i d a t i v e degradation. W i t h o x i d a n t s such as HNO3, K2Cr207/HN03, KMNO4/OH"-, BuOOH/AIBN, o r p e r a c e t i c a c i d , workers have come t o t h e c o n c l u s i o n t h a t c o a l i s p r e d o m i n a t e l y a r o m a t i c and c o n t a i n s many condensed r i n g s (J5,6) . Other a u t h o r s u s i n g NaOCl/OH" have come t o a d i f f e r e n t conc l u s i o n , i n that they b e l i e v e c o a l contains large amounts o f q u a r t e r n a r y a l i p h a t i c carbon, o r i s diamonl i k e i n s t r u c t u r e and c o n t a i n s 50% a r o m a t i c c a r b o n o r l e s s (_7) . The p r e c e e d i n g methods o f o x i d a t i o n s e l e c t i v e l y oxidize only the a l i p h a t i c p o r t i o n of c o a l . A new method p i o n e e r e d b y Dieno u s e s t r i f l u o r o a c e t i c a c i d s i n c o m b i n a t i o n w i t h hydrogen p e r o x i d e . T h i s method s e l e c t i v e l y o x i d i z e s the aromatic r i n g s . Combination o f t h e s e two t e c h n i q u e s c o u l d be a v e r y p o w e r f u l i n s t r u c t u r a l c h a r a c t e r i z a t i o n o f c o a l (8) . Because o f t h e d i f f i c u l t y o f p i e c i n g back t o g e t h e r t h e fragmented p r o d u c t s o f c o a l , a number o f workers have attempted t o do d i r e c t c h a r a c t e r i z a t i o n o f coal. D i r e c t t e c h n i q u e s s u f f e r from t h e problem t h a t c o a l i s an opaque s o l i d which i s i n s o l u b l e i n i t s n a t u r a l form and r e l a t i v e l y few t o o l s have been a v a i l a b l e up u n t i l t h e p r e s e n t time f o r such d i r e c t measurements. I n t h e p a s t , t e c h n i q u e s such as X - r a y s c a t t e r i n g have



6


been u s e d and c o n f l i c t i n g i n t e r p r e t a t i o n s as t o t h e predominant s t r u c t u r e o f c o a l have been r e p o r t e d . H i r s c h f i r s t r e p o r t e d t h a t c o a l was from 50-80% aromat i c , w i t h p r i m a r i l y 89% o r d e r e d s t r u c t u r e (9). Ergun l a t e r , using X-ray s c a t t e r i n g , concluded that c o a l i s l e s s a r o m a t i c and c o n t a i n s l a r g e q u a n t i t i e s o f amorphous r e g i o n s (10). F r i e d e l using u l t r a v i o l e t techn i q u e s c o n c l u d e d t h a t c o a l c o u l d not be p o l y a r o m a t i c and c o n t a i n e d l a r g e amounts o f a l i p h a t i c s t r u c t u r e (11). Given^ i n c h a r a c t e r i z i n g c o a l e x t r a c t s by polarographic r e d u c t i o n c o n c l u d e d t h a t low rank c o a l s were g r e a t e r than 20% a r o m a t i c and h i g h rank c o a l s were g r e a t e r than 50% a r o m a t i c (12)· P o l y c y c l i c a r o m a t i c r i n g s were bel i e v e d t o be p r e v a l e n t . R e c e n t l y , new t o o l s have e v o l v e d and f o r the f i r s t time c o a l can be c h a r a c t e r i z e d d i r e c t l y i n i t s n a t u r a l form. The most p r o m i s i n g o f t h e s e t o o l s i s a s o l i d s t a t e CP-C^ n m r d e v e l o p ed by P i n e s (JL3) . Working i n c o n j u n c t i o n w i t h P r o f e s s o r P i n e s , we have found t h a t t h e r e i s r e l a t i v e l y l i t t l e c o r r e l a t i o n between the hydrogen c a r b o n mole r a t i o and the p e r c e n t a r o m a t i c c a r b o n found i n c o a l or c o a l l i q u i d s , as shown i n F i g u r e 3. These d a t a g i v e some i n d i c a t i o n as t o why t h e r e has been so much d i f f i c u l t y i n the p a s t c o r r e l a t i n g a r o m a t i c c a r b o n c o n t e n t w i t h the e l e m e n t a l compos i t i o n o f the c o a l . T h e r e is,however, some c o r r e l a t i o n between the rank o f t h e c o a l and i t s a r o m a t i c i t y . This i s shown i n F i g u r e 4. I t can be seen t h a t the a r o m a t i c c a r b o n c o n t e n t i n c r e a s e s from about 40-50% f o r subb i t u m i n o u s c o a l t o over 90% f o r a n t h r a c i t e . I t w i l l be shown l a t e r t h a t t h i s a r o m a t i c i t y changes w i t h c o n v e r s i o n o f the c o a l under l i q u e f a c t i o n c o n d i t i o n s . The CP-C13 t e c h n i q u e i s somewhat new and s t i l l e v o l v i n g . I t does show p r o m i s e , however, i n c h a r a c t e r i z a t i o n o f c o a l i n t o a l i p h a t i c and a r o m a t i c components, but i n a d d i t i o n , h o l d s p r o m i s e f o r f u r t h e r s u b - d i v i s i o n o f the s t r u c t u r a l types. As shown i n F i g u r e 5, i t i s p o s s i b l e t o d i s t i n g u i s h i n model compounds, a r o m a t i c , a l i p h a t i c , a l i p h a t i c e t h e r , and condensed a r o m a t i c c a r b o n . We hope e v e n t u a l l y t o use t h e s e same s u b - d i v i s i o n s i n the c h a r a c t e r i z a t i o n of c o a l . As r e p r e s e n t a t i v e examples o f the s p e c t r a t h a t one can a c h i e v e , F i g u r e 5 shows t y p i c a l model compounds, the p a r e n t c o a l , SRC derived from a c o a l , u n c o n v e r t e d r e s i d u e and the s p h e r i c a l coke formed on extended t h e r m a l r e a c t i o n (14). 3

F u n c t i o n a l i t y of

Coal

F i g u r e 6 shows the major f u n c t i o n a l group t y p e s identified in coal. Oxygen o c c u r s p r e d o m i n a t e l y as



WHiTEHURST


1.1

Figure 3.

Aromatic carbon vs. H/C



8


Figure 4.

Aromaticity increase with rank


1.

wmTEHURST



p-Pt«thoxvb«n««n« (391)*

2.3-Dla«thrln«ohth«l«n« (480)

Mcnf rmγ Coal (7300)

Monf rmv SBC (S000) AC-70

M ^ t f f y R««^u« (8800) AC-70

Soh«ric»l Cok« (9200)

*The numbers i n p a r e n t h e s e s i n d i c a t e t h e number o f s p e c t r a accumulated t o g e n e r a t e t h e spectrum p r e s e n t e d . Note t h a t t h e s c a l e s a r e n o t a l l the same. Figure 5.

CP- C 13

NMR spectra of representative samples


9


10


p h e n o l i c or e t h e r i c groups w i t h l e s s amounts o f c a r b o x y l i c a c i d s or e s t e r s ; some c a r b o n y l s have a l s o been i d e n t i f i e d . S u l f u r has s i m i l a r c h e m i s t r y t o oxygen and a l t h o u g h s u l f o x i d e s have been i d e n t i f i e d i n t a r sands t h e i r p r e s e n c e i n c o a l i s l e s s w e l l d e f i n e d . N i t r o g e n o c c u r s p r e d o m i n a t e l y as p y r i d i n e o r p y r r o l i c t y p e r i n g s . M e t a l s are found as s a l t s or a s s o c i a t e d with porphyrins. Some r e c e n t work c o n d u c t e d by Ruberto i s summarized i n F i g u r e 7 (15). Shown t h e r e are q u a n t i t a t i v e a n a l y s i s o f the major t y p e s o f f u n c t i o n a l i t y , oxygenated s p e c i e found i n c o a l s and one s o l v e n t r e fined coal. I t can be seen t h a t subbituminous c o a l c o n t a i n s c o n s i d e r a b l y more c a r b o x y l i c a c i d t h a n b i t u minous c o a l s and somewhat more c a r b o n y l . After react i o n under l i q u e f a c t i o n c o n d i t i o n s , the c a r b o x y l i c a c i d s and c a r b o n y l s are almost c o m p l e t e l y absent, and the prédominent p r o d u c t s are p h e n o l i c t y p e oxygen. Our r e s u l t s i n d i c a t e t h a t i n a d d i t i o n t o p h e n o l i c type oxygen, e t h e r i c t y p e oxygen i s a major oxygenated s p e c i e (14). As c o a l i s c o n v e r t e d i n p r e s e n t day p r o c e s s e s , the above d e s c r i b e d f u n c t i o n a l i t y , o f c o u r s e , changes w i t h s e v e r i t y , b u t i n a d d i t i o n the s t r u c t u r e o f the c o a l and i t s e l e m e n t a l c a r b o n t o hydrogen r a t i o must a l s o change. F i g u r e 8 shows a comparison o f the hydrogen t o c a r b o n mole r a t i o f o r c o a l and a number o f o t h e r n a t u r a l p r o d u c t s , i n comparison w i t h t h a t o f p e t r o l e u m and the premium p r o d u c t s t h a t are d e s i r e d from the c o a l . It can be seen t h a t t h e r e i s a v e r y l o n g p a t h n e c e s s a r y i n the c o n v e r s i o n o f c o a l t o premium p r o d u c t s such as g a s o l i n e , s i n c e c o a l s c o n t a i n about .8 hydrogen/carbon. The d e s i r e d p r o d u c t s c o n t a i n about 2. This indicates t h a t i n any c o n v e r s i o n p r o c e s s o f c o a l , one o f the p r i mary g o a l s w i l l be e x t r e m e l y e f f i c i e n t use o f hydrogen. Most p r o c e s s e s p r e s e n t l y u s e d today are i n i t i a l l y t h e r m a l i n n a t u r e s i n c e c a t a l y s t s cannot c o n t a c t the b u l k o f the c o a l m a t r i x . But j u s t where does the t h e r m a l c h e m i s t r y o f c o a l become s i g n i f i c a n t ? F i g u r e 9 shows a s u p e r i m p o s i t i o n o f t h r e e t h e r m a l a n a l y s e s o f coal. These c o n s i s t o f t h e r m a l g r a v i m e t r i c , t h e r m a l m e c h a n i c a l , and d i f f e r e n t i a l t h e r m a l a n a l y s i s o f c o a l . T h i s f i g u r e i n d i c a t e s t h a t c o a l undergoes p r i m a r y dec o m p o s i t i o n i n the range o f 400-450°, a s s o c i a t e d w i t h t h i s temperature range i s the b u l k o f the s w e l l i n g o f the c o a l and s i g n i f i c a n t changes i n the t h e r m a l behavior of c o a l . I t i s not s u r p r i s i n g , t h e r e f o r e , t h a t most o f the p r e s e n t p r o c e s s e s b e i n g d e v e l o p e d today, o p e r a t e i n the range o f 400-450°C. But, a t t h i s temperature j u s t how f a s t does c o a l r e a c t ? We have shown t h a t i n the p r e s e n c e o f hydrogen donors



wmTEHURST

11

OXYGEN OH

R-COCS

R-O-R

R-C-R

SULFUR


(§φ R-S-R

(g)

Rearrangement

HO i"

HO

I n c o n j u n c t i o n w i t h t h e l o s s o f m o l e c u l a r weight and i n c r e a s e i n a r o m a t i c i t y , the f u n c t i o n a l i t y o f t h e i n i t i a l l y d i s s o l v e d c o a l undergoes major change. Oxygen i s t h e p r i m a r y element o f c o n c e r n as f a r as f u n c t i o n a l i t y and s o l u b i l i t y c l a s s determination. I f one examines t h e e l e m e n t a l c o m p o s i t i o n o f c o a l and p r o d u c t s o f c o a l a t s h o r t and l o n g t i m e s , a r a t h e r i n t e r e s t i n g r e s u l t c a n be found. As shown below f o r e v e r y 100 c a r b o n atoms i n a c o a l , c o n v e r s i o n a t e i t h e r s h o r t o r l o n g time causes e s s e n t i a l l y no change i n t h e c o n t e n t o f n i t r o g e n . The hydrogen c o n t e n t i s s i m i l a r t o t h e p a r e n t c o a l a t s h o r t time, b u t becomes l e s s a t l o n g e r t i m e s . The oxygen c o n t e n t and s u l f u r c o n t e n t s b o t h a r e reduced s l i g h t l y a t s h o r t time b u t a r e s i g n i f i c a n t l y r e d u c e d a t l o n g e r times.

General Monterey C o a l

(mml)

Formula

C ^ ^ g N ^ g O ^ S j ^

Number Heteroatoms/ 100 C 14.9 12.0 7.5



24


Oxygen i s l o s t p r i m a r i l y as c a r b o n d i o x i d e and water, w i t h s m a l l e r amounts o f c a r b o n monoxide. The r a t e o f oxygen l o s s p a r a l l e l s the r a p i d i n i t i a l d i s s o l u t i o n o f c o a l and r a p i d l o s s o f h i g h m o l e c u l a r weight m a t e r i a l . About 40-50% o f the oxygen i s r e l a t i v e l y easy t o r e move (17). The l o s s o f s u l f u r i s k i n e t i c a l l y p a r a l l e l t o the l o s s o f oxygen as shown i n F i g u r e 17. This might be a n t i c i p a t e d i n view o f the o r i g i n o f the o r g a n i c s u l f u r o f c o a l , which i s b e l i e v e d t o be the r e s u l t o f exchange o f OH o r c a r b o n y l oxygen b y s u l f u r , due t o b i o l o g i c a l a c t i v i t y i n the sediment (20,21). The s i g n i f i c a n c e o f t h i s i s t h a t 40-50% o f the o r g a n i c s u l f u r i s a l s o e a s i l y removed. The r e m a i n i n g s u l f u r i s much more r e s i s t a n t t o a t t a c k and i s p r o b a b l y p r e sent i n h e t e r o c y c l i c r i n g s t r u c t u r e s . Hydrogen Consumption and

Reactive

Moieties

In c o n j u n c t i o n w i t h the l o s s o f oxygen and s u l f u r , as w e l l as m o l e c u l a r weight r e d u c t i o n o f the s o l u b l e c o a l s p e c i e , t h e r e i s hydrogen consumption r e q u i r e d f o r the p r o c e s s . T h i s hydrogen i n t h e c a s e o f s o l v e n t r e f i n i n g i s donated from the s o l v e n t t o the c o a l o r c o a l fragments. I n i t i a l l y , the l o s s o f oxygen r e q u i r e s r e l a t i v e l y l i t t l e hydrogen consumption and i s v e r y c l o s e t o s t o i c h i o m e t r i c r e q u i r e m e n t s (16,17). T h i s i s shown i n F i g u r e 18, where i t can be seen t h a t o n l y a f t e r about 30% o f the oxygen i s l o s t does the hydrogen consumption become g r e a t e r t h a n s t o i c h i o m e t r y . This h y d r o g e n consumption i n e x c e s s o f s t o i c h i o m e t r y i s due p r i m a r i l y t o the f o r m a t i o n o f gaseous p r o d u c t s o r lowe r m o l e c u l a r weight d i s t i l l a t e s such as s o l v e n t and not due t o the i n p u t o f hydrogen i n t o the h i g h e r molecu l a r weight p r o d u c t s o f c o a l . A n o t h e r way t o l o o k a t the c o n v e r s i o n o f oxygen i s t o compare the p r o d u c t c o m p o s i t i o n * o v e r a l l , w i t h the p e r c e n t oxygen removed from the t o t a l p r o d u c t . F i g u r e 19 shows t h a t i n o r d e r t o a c h i e v e maximum s o l u b i l i t y o f the c o a l about 60% o f the oxygen must be lost. A t the same time the SRC y i e l d maximizes. The f o r m a t i o n o f s o l v e n t range m a t e r i a l and l i g h t gases such as methane, t h e n become major p r o d u c t c o n s t i t u e n t s as the oxygen c o n t e n t i s r e d u c e d f u r t h e r . What t h e s e r e s u l t s i n d i c a t e i s t h a t h i g h hydrogen consumpt i o n i s n o t n e c e s s a r y i n o r d e r t o j u s t d i s s o l v e the c o a l o r t o remove a major p o r t i o n o f the oxygen. I n o r d e r t o g a i n an u n d e r s t a n d i n g o f what k i n d s o f r e a c t i o n s can be e n v i s i o n e d t o e x p l a i n the above r e s u l t s , we c o n d u c t e d a s e r i e s o f e x p e r i m e n t s u s i n g model compounds and r e a c t i o n s w i t h t y p i c a l s o l v e n t s


WHTTEHURST


2.5

2.0 \

WEST KENTUCKY 1.3


a*

Κ

0.5

O.Ol

WYODAK

L

J 6

Figure 17.

!

L

8

wr. %o

I 10

1

I ,

12

Fer cent S in SRC vs. percent Ο in SRC

3.0 h-

/ WYODAK

0.0

0.2

0.4

0.6

0.8

MOLES 0 REMOVED

Figure 18.

Hydrogen consumption vs. moles oxygen removed



26


f o r c o a l under c o n d i t i o n s o f c o a l l i q u e f a c t i o n . I n t h e s e s t u d i e s we i d e n t i f i e d a number o f c h e m i c a l c l a s s e s w h i c h c o u l d be c o n v e r t e d a t r a t e s t h a t were comparable t o t h o s e o f r a p i d d i s s o l u t i o n o f c o a l (14). These a r e summarized i n F i g u r e 20, where f a s t r e a c t i o n s a r e t h o s e i n which 80% o r h i g h e r c o n v e r s i o n c a n be a c h i e v e d i n t e n m i n u t e s a t 800°F. I t c a n be seen t h a t b e n z y l i c e t h e r s o r b e n z y l i c t h i o e t h e r s c e r t a i n e s t e r s and q u i n o n e s r e a c t r a p i d l y enough t o account f o r t h e v e r y s h o r t c o n t a c t time c o a l d i s s o l u t i o n . I n a d d i t i o n , r i n g s t r u c t u r e s such as d i h y d r o p h e n a n t h r e n e s w i l l r a p i d l y dehydrogenate under t h e same c o n d i t i o n s . By c o n t r a s t w i t h t h e s e r e l a t i v e l y f a s t r e a c t i o n s we have a l s o i d e n t i f i e d a number o f low r e a c t i v i t y s p e c i e which can be r u l e d o u t as b e i n g r e s p o n s i b l e f o r c o a l d i s s o l u t i o n , t h e s e a r e summarized i n F i g u r e 21. Here i t c a n be seen t h a t s t r u c t u r e s such as a r o m a t i c e t h e r s , o r r i n g s t r u c t u r e d e t h e r s , h e t e r o c y c l i c hydrogen compounds, p o l y c o n d e n s e d r i n g s b o t h a r o m a t i c and a l i p h a t i c , have r e l a t i v e l y low r e a c t i v i t y . I f a r o m a t i c r i n g s a r e subs t i t u t e d o n t o a number o f t h e s e s t r u c t u r e s , t h e r e a c t i v i t y increases dramatically. This increase i n react i v i t y with higher aromatic s u b s t i t u t i o n c o u l d p o s s i b l y account a t l e a s t i n p a r t f o r t h e h i g h e r r e a c t i v i t y o f b i t u m i n o u s c o a l s r e l a t i v e t o subbituminous c o a l s . Speculations

on C o a l

Structure

Up t o t h i s p o i n t , I have d i s c u s s e d p r i m a r i l y t h e c h e m i s t r y o f d i s s o l v e d c o a l and how t h e c h e m i c a l n a t u r e o f t h e c o a l p r o d u c t s change w i t h t h e s e v e r i t y o f c o n version. I t s h o u l d be n o t e d t h a t a t v e r y s h o r t c o n t a c t times the i n i t i a l products o f c o a l d i s s o l u t i o n are very s i m i l a r i n b o t h a r o m a t i c i t y and f u n c t i o n a l i t y t o t h a t of the parent c o a l . I w i l l now d i s c u s s how t h i s i n f o r m a t i o n c a n be used t o h e l p 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 o r i g i n a l s t r u c t u r e o f p a r e n t c o a l . A number o f workers have attempted t o d e r i v e a r e p r e s e n t a t i v e s t r u c t u r e o f c o a l which i s c o n s i s t e n t i n i t s o b s e r v e d chemistry. One o f t h e f i r s t was t h a t o f P r o f e s s o r G i v e n , shown i n F i g u r e 22. T h i s s t r u c t u r e was n o t i n t e n d e d t o be t h e s t r u c t u r e f o r c o a l b u t m e r e l y t o r e p r e s e n t what k i n d s o f s t r u c t u r e s one s h o u l d e n v i s i o n as c o n s t i t u t i n g c o a l ( 2 2 ) . The s t r u c t u r e i s c o n s i s t e n t w i t h h i g h l y s u b s t i t u t e d a r o m a t i c s , which a r e n o t h i g h l y condensed, w i t h f u n c t i o n a l i t i e s which a r e known t o be p r e s e n t i n c o a l and w i t h i t s e l e m e n t a l c o m p o s i t i o n . A more r e c e n t , more s o p h i s t i c a t e d model, was p r e s e n t e d b y P r o f e s s o r W i s e r (23) and i s g i v e n i n F i g u r e 23. The s i g n i f i c a n c e o f t h i s f i g u r e i s t h e l o c a t i o n o f a number o f


1.

27


wmTEHURST

%

SOLUBLE

ο


UJ

SOLVENT^^

0

2 0

4 0 %

Figure 19.

0

6 0

Θ 0

100

CONVERSION

Product yields vs. percent oxygen conversion for West Kentucky coal

OH

CH

SH

CH,

3

[ri]

-CH -S-

Co]

2

+ CH,

Organic Chemistry of Coal - American Chemical Society

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