The influence of sulfur-containing compounds on the donor solvent dissolution of Illinois coals

Fuel Processing Technology, 12 (1986) 287--298

287

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE INFLUENCE OF SULFUR-CONTAININGCOMPOUNDSON THE DONORSOLVENT DISSOLUTION OF ILLINOIS COALS

L.M. STOCK, J.E. DURAN, M.G. NOEL AND V.R. SRINIVAS Department of Chemistry, The University of Chicago, Chicago, IL 60637 (USA)

ABSTRACT The influence of benzenethiol, benzyl phenyl t h i o e t h e r , diphenyl d i s u l f i d e and other s u l f u r - c o n t a i n i n g compounds on the conversion of coals from the I l l i nois Basin into soluble products in short duration, donor solvent d i s s o l u t i o n reactions has been investigated from 300 to 400°C using t e t r a l i n . Benzenethiol is an e s p e c i a l l y e f f e c t i v e reagent. The reactions carried out in the presence of t h i s compound provide 60 to 100% more soluble products at 350°C. Hydrogen s u l f i d e is also a reasonably e f f e c t i v e reagent, but p y r i t e has a lesser i n f l u ence on the conversion. The information now a v a i l a b l e is most compatible with the view that t h i o l s , which are extremely e f f e c t i v e hydrogen atom transfer agents, exert t h e i r primary e f f e c t through the reduction of the reactive radicals formed during the fast thermal decomposition reactions of the coals. The pr incipal products of these reactions are r e l a t i v e l y stable, soluble preasphaltenes. INTRODUCTION Sulfur compounds in coal influence the extent of i t s conversion to products that are soluble in conventional organic solvents such as py r id in e , toluene, and hexane.

Given and his coworkers called special a t t e n t i o n to this fact in t h e i r

discussion of the r e l a t i o n s h i p between the chemical composition of coals and t h e i r r e a c t i v i t y in long duration, 60 minute, donor solvent d i s s o l u t i o n reactions (refs.

I and 2).

S t a t i s t i c a l analyses indicated that the s u l f u r content

was one of the important variables.

Generally, the more s u l f u r - r i c h coals pro-

vided somewhat more soluble products, but there was no d e f i n i t e r e l a t i o n s h i p between the r e l a t i v e amounts of the p y r i t i c and organic s u l f u r derivatives in the coal and the f a c i l i t y

with which the coal underwent d i s s o l u t i o n .

Winans

and his colleagues found that s u l f u r content was also an important v a r i a b l e in a s t a t i s t i c a l analysis of the data f o r short duration d i s s o l u t i o n reactions of 50 other coals ( r e f . 3).

Hoover examined the q u a n t i t a t i v e r e l a t i o n s h i p between

coal composition and d i s s o l u t i o n y i e l d s f or a v a r i e t y of Kentucky coals ( r e f . 4).

His s t a t i s t i c a l work, in contrast to the results of the other investiga-

tors, pointed to the importance of dispersed p y r i t e on the reaction.

Davidson

has reviewed the information concerning the c a t a l y t i c r o l e of mineral matter, e s p e c i a l l y p y r i t e , on d i s s o l u t i o n reactions ( r e f . 5) and Stenberg and his 0378-3820/86/$03.50

© 1986 Elsevier Science Publishers B.V.

288

associates have discussed the beneficial influence of hydrogen sulfide on the reaction (ref. 6).

In contrast, the role of organosulfur compounds in such

chemistry has received much less attention.

Several lines of reasoning led us

to suspect that such substances would be highly reactive agents in liquefaction and we, therefore, undertook a study of their effect on the hydrogen atom transfer reaction between t e t r a l i n and diphenylmethane, on the decomposition of 1,3diphenylpropane, and on the donor solvent dissolution reactions of representative bituminous coals from the I l l i n o i s Basin (refs. 7 and 8).

The results

stimulated more work on the coals to obtain a better perspective of t h e i r react i v i t y patterns in the presence of organic sulfur reagents.

These observations

are discussed in this report. METHODS Materials The organic s u l f u r compounds and t e t r a l i n were obtained from commercial sources and were p u r i f i e d as necessary by f r a c t i o n a l d i s t i l l a t i o n l i z a t i o n p r i o r to use.

or r e c r y s t a l -

The physical and spectroscopic properties of these mate-

r i a l s were in accord with chemical l i t e r a t u r e values. pared in the reaction vessel via the reaction

Hydrogen s u l f i d e was pre-

of sulfur with t e t r a l i n .

This

reaction (eqn. ( I ) ) which produces only minor quantities of organosulfur com(1)

S + T e t r a l i n ~ H2S + Naphthalene

pounds, is completed during the time, 2 minutes, allotted for the achievement of the desired reaction temperature.

The analytical data for the coals have

been presented previously (ref. 8). Procedure The coal conversion reactions were carried out and the products were isolated as described previously (refs. 7 and 8).

The reactions discussed in

this report were conducted with the coal (0.75-0.80g), purified t e t r a l i n (1.61.7g, 12.5 mmol), and the organosulfur compound (0.3 to 1.4 mmol) in an argon atmosphere. Two minutes were allowed for the vessel to achieve the desired temperature. RESULTS AND DISCUSSION Preliminary studies with an I l l i n o i s No. 6 coal from the Peabody No. 10 mine in Pawnee, I l l i n o i s revealed that benzenethiol and benzyl phenyl thioether s i g n i f i c a n t l y increased the degree of conversion of the coal to products that were soluble in organic solvents (ref. 7).

Another sample from an en-

289

t i r e l y d i f f e r e n t location in the same mine provided equivalent r e s u l t s .

These

observations and the reports of Markuszewski and A t t a r and t h e i r coworkers (refs. 9 and 10) that l l l i n o i s

No. 6 coal contains a spectrum of organic s u l -

f u r compounds including t h i o l s and substances capable of forming t h i o l s during thermal reactions prompted more work on the influences of benzenethiol and other organic s u l f u r compounds on the d i s s o l u t i o n reactions of three other coals from the l l l i n o i s

Basin.

These samples, which are thought to be repre-

sentative of the coals in t h i s large deposit were prepared and provided by the Illinois

Coal Board.

The results obtained during the course of t h i s new study and our previous study ( r e f . 8) are summarized in Table I f o r convenient comparison. TABLE I The influences of s u l f u r compounds on the donor solvent d i s s o l u t i o n reactions of three representative I l l i n o i s

coals.

Reaction conditions Reagent(mmole)

Temp.(°C) Sample IA

Conversion (%soluble) Time(min)

Pyridine

Herrin I l l i n o i s

Unreacted Coal

Toluene

Hexane

No. 6 Coal 25

None

300

5

24

23

23

C6H5SH, 1.33

300

5

32

27

24

None

300

30

26

23

19

C6H5SH, 1.33

300

30

41

24

22

(C6H5S) 2, 1.33 None

300 350

30 5

43 39

33 28

30 24

C6H5SH, 1.33

350a

5

68

33

26

C6H5SH, 0.67

350 a

5

69

38

29

C6H5SH, 0.33

350a

5

56

32

24

C6H5SSC6H5, 1.33

350a

5

67

31

24

C6H5CH2SC6H5, 1.33

350a

5

60

31

25

H2S, 1.33

350a

5

44

35

23

FeS2, 1.33

350a

5

42

24

19

Na2S, 1.33

350

5

34

20

14

None

400

5

82

46

25

400 5 86 54 Sample 2B Colchester 111inois No. 2 Coal

31

C6H5SH Unreacted coal

18

None

300

30

21

19

17

C6H5SH, 1.33

300

30

21

17

5

C6H5SSC6H5, 1.33

300

30

23

20

16

290

None

350

5

34

28

25

C6H5SH, 1.33

350a

5

54

30

24 31

C6H5SSC6H5, 1.33

350

5

48

33

C6H5SCH2C6H5, 1.33

350

5

38

20

17

H2S, 1.33

350

5

48

42

38

None

400

5

87

44

30

C6H5SH, 1.33

400a

5

82

47

32

Sample 3C A Blend of 80% Springfield I l l i n o i s No. 5 and 20% Herrin I l l i n o i s No. 6 Coals Unreacted coal

17

None

300

30

10

9

9

C6H5SH, 1.33

300

30

28

26

23

C6H5SSC6H5, 1.33

300

30

33

31

29

None

350

5

27

15

11

C6H5SH, 1.33

350

5

44

13

11

C6H5SSC6H5, 1.33

350

5

38

14

10

C6H5CH2SC6H5, 1.33

350

5

37

35

32

H2S, 1.33

350

5

41

25

22

None

400

5

69

36

20

C6H5SH, 1.33

400a

5

82

36

23

C6H5SSC6H5, 1.33

400

5

74

29

18

C6H5SCH2C6H5, 1.33

400

5

94

39

30

aRef. 8. The general pattern of r e a c t i v i t y of the three coals in the presence and absence of benzenethiol is i l l u s t r a t e d in Figures I-3.

As observed previous-

ly ( r e f . 8), coals IA and 2B are so r e a d i l y converted to soluble products in 5 minutes in t e t r a l i n at 400°C that the influence of the t h i o l is completely obscured at that temperature.

However, benzenethiol does notably enhance the con-

version of coal 3C, which is much less r e a d i l y converted to soluble products. L i t t l e conversion is realized in 5 minutes reactions at 300°C, but there are large differences in the degree of conversion of a l l the coals at the intermediate temperature of 350°C. The t h i o l enhances the conversion by factors of 2, 1.6, and 1.6 f o r coals IA, 2B, and 3C, respectively in these short duration experiments at 350°C. I t was necessary to carry out somewhat longer reactions at 300°C to est a b l i s h the e f f i c a c y of the t h i o l at the lower temperature, the results are presented in Figure 4.

In this case, the t h i o l influences the r e a c t i v i t y of

coals IA and 3C, but not coal 2B. We next turned our a t t e n t i o n to the r e l a t i v e effectiveness of a v a r i e t y of d i f f e r e n t s u l f u r - c o n t a i n i n g materials.

The reactions were carried out with

291

I00 Cool IA, Reaction for 5min. ~T •=- 80

.:_ 80

~ 60

~o

¢/

I00

=40 o

Cool 2B, Reaction for 5 min.

60

-

40

o

= 20 o

~

20

o

I

300

I

I

I

350 400 Temperature,°C

I

300

I

350 400 Temperature,°C

Fig. I . The r e l a t i o n s h i p between the conversion of Coal IA and temperature f o r reactions carried out in the presence, closed c i r c l e s , and absence, open c i r c l e s , of benzenethiol. Fig. 2. The r e l a t i o n s h i p between the conversion of Coal 2B and temperature f o r reactions carried out in the presence, closed c i r c l e s , and absence, open c i r c l e s , of benzenethiol.

I

I00

.s

80

,]

C6H5SH

~ 6o "

~ g

I

I

C6H~sSH

40

I

I

Reaction at 300°C for 30min.

Cool 3C, Reoction for 5min.

1A

2B

20

3C 0

I 300

I

I

350 400 Temperature, °C

I

0

I

I

20 40 60 80 I00 Conversion,% Soluble in Py

Fig. 3. The r e l a t i o n s h i p between the conversion of Coal 3C and temperature f o r reactions carried out in the presence, closed c i r c l e s , and absence, open c i r c l e s of benzenethiol. Fig. 4. The conversions of coals IA,2B, and 3C at 300°C in the presence and absence of benzenethio1.

equivalent molar amounts of the reagents f o r convenient comparison of the data. The observations presented in Figures 5,6, and 7 indicate t h a t t h i o l s , t h i o ethers, and d i s u l f i d e s as well as hydrogen s u l f i d e a l l promote the conversion to s i g n i f i c a n t degree but t h a t the t h i o l appears to be the most e f f e c t i v e reagent with a l l three coals.

292

Cool |A at 350°C for 5min ]

None

N°zS

E

Coal 2B at 350°C for 5min.

[

E

None

FeS2 I H2S I C6H5CHzSC6H5 ] C6HsSSC6H5 ] I C`HSsH h 20 40 6JO 80 lO0 Conversion, % Soluble in Py

c~

I

;6H5SCHzC6H5I HzS C6H5SSC6H5

iC6HSSH, I I I 20 40 60 80 lO0 Conversion,°/° Soluble in Py

Fig. 5. The influences of s u l f u r - c o n t a i n i n g compounds on the conversion of Coal IA. Fig. 6. The influences of s u l f u r - c o n t a i n i n g compounds on the conversion of Coal 2B.

Cool 3C of 550°C for 5min.

CI3

0

C6HsSH 20 40

60

80

I00

Conversion,% Soluble in Py

Fig. 7. The influences of s u l f u r - c o n t a i n i n g compounds on the conversion of coal 3C. In our previous report, we pointed out that the added reagents can enhance the d i s s o l u t i o n reaction of the coal in two p r i n c i p a l ways.

F i r s t , substances

such as diphenyl d i s u l f i d e and benzyl phenyl t h i o e t h e r , which have r e l a t i v e l y weak s u l f u r - s u l f u r

and carbon-sulfur bonds (eqns. (2) and (3)) can i n i t i t a t e

C6H5SSC6H5 ~ 2C6H5S.

AH = 230kJ mole -I

C6H5CH2SC6H5 - C6H5CH2. + C6H5S.

AH = 220 kJ mole -I

(2) (3)

293

free radical decomposition reactions of the coal molecules as i l l u s t r a t e d f o r the B-scission reaction (eqns. (4) and ( 5 ) ) .

Second, the t h i y l radicals formed

CoaICH2CH2CH2Ar + C6H5S. ~ CoalCH2CH2CHAr + C6H5SH

(4)

CoaICH2CH2CHAr ~ CoalCH2. + ArCH=CH2

(5)

in the reactions of the d i s u l f i d e and the t h l o e t h e r and from the t h i o l s provide alternate low energy pathways f o r the t r a n s f e r of hydrogen atoms from the donor solvent to the coal molecules (eqn. ( 6 ) ) . C6H5SH + CoalR. - C6H5S. + CoalRH

(6)

The observation that benzenethiol is the most e f f e c t i v e reagent with a l l three coals provides quite secure evidence f o r the viewpoint that the promotion of hydrogen t r a n s f e r reactions is a key f a c t o r in t h i s chemistry. Several exchange reactions between t e t r a l i n and diphenylmethane (eqn. (7)) were carried out in stainless steel vessels in the presence of about 12 atmoT e t r a l i n + (C6H5)2CH2

D2 7FO-O-~-C~- T e t r a l i n - d + (C6H5)2CHD

spheres of dihydrogen or dideuterium at 400°C.

(7)

Spectroscopic measurements

of the deuterium content of the recovered hydrocarbons revealed that the gases did not p a r t i c i p a t e in the reaction.

The reaction was also carried out in the

presence of benzenethiol in dihydrogen and dideuterium atmospheres.

Again, the

spectroscopic studies of the products revealed that the gases did not p a r t i c i pate in the reaction. mole - I ,

Apparently, the strength of the hydrogen bond, 430 kJ

is so great that diphenylmethyl and 1 - t e t r a l y l radicals s e l e c t i v e l y

abstract hydrogen atoms from the hydrocarbons rather than from the gas.

I t is

p e r t i n e n t , however, that the exchange reactions between dideuterium and t e t r a f i n do occur in the presence of coal molecules in longer duration, 60 minute, donor solvent reactions.

These exchange reactions presumably occur via reac-

t i v e aromatic compounds, which undergo reduction possibly by the d i r e c t addit i o n of dihydrogen (dideuterium), to form labeled hydroaromatic compounds capable of exchange with the donor solvent ( r e f .

11).

We also investigated the d i s s o l u t i o n reaction of O-methylated d e r i v a t i v e . The same proportions of coal and t e t r a l i n were used in these experiments (Table 2).

The O-methy! l l l i n o i s

No. 6 coal y i e l d s considerably more p y r i d i n e -

soluble products than the unmodified coal.

More importantly, the addition of

benzenethiol to the reactions of the O-methylated coal leads to a f u r t h e r i n crease in the y i e l d of soluble products.

These increases are realized even

294

though the presumably a c t i v a t i n g influences of the hydroxyl groups have been eliminated by the e t h e r i f i c a t i o n . TABLE 2 The donor solvent d i s s o l u t i o n of an I l l i n o i s coal and i t s O-methylation product at 350°C f o r 5 min. Substrate

Conversion (%Soluble in Pyridine)

Coal

51

O-Methylated Coal

60

O-Methylated Coal, C6H5SH, 1.33 mmo]e

75

though the presumably a c t i v a t i n g influences of the hydroxyl groups have been eliminated by the e t h e r i f i c a t i o n reaction. The data are consistent with the idea that the added methyl groups part1cipate in the reaction via i n t r a p a r t i c l e hydrogen atom t r a n s f e r reactions, (eqn. (8)).

(8)

CoaIArOCH3 + CoalRadical. ~ CoalArOCH2. + CoalRadicalH This reaction is important, even though the bond d i s s o c i a t i o n energy of the carbon-hydrogen bond in anisole, about 90 kcal mole - I ,

is somewhat greater

than the energy of the carbon-hydrogen bond in t e t r a l i n , about 85 kcal mole - I . The anisole donors are w i t h i n the p a r t i c l e and are, therefore, very near to the sites of the thermal decomposition reactions of the coal molecules.

The fact

that the y i e l d is improved when benzenethiol is also present is in accord with the greater importance of this substance as a hydrogen t r a n s f e r reagent. S t a t i s t i c a l analyses of the data f o r long duration, 60 minute, donor solvent d i s s o l u t i o n reactions as mentioned in the INTRODUCTION imply that there is no d i r e c t c o r r e l a t i o n between the success of the reaction and the quantities of the mineral or organic s u l f u r compounds in the coal. made in our two studies are in accord with this conclusion.

The observations For example,

coals IA and 3C contain e s s e n t i a l l y equal amounts of p y r i t e , but undergo the d i s s o l u t i o n reaction to a comparable extent, even though IA contains more organic s u l f u r . less reactive.

Coal 3C, which contains more organic s u l f u r than 2B, is also Thus, there is no simple r e l a t i o n s h i p between the degree of

conversion of the coal to soluble products in the short duration reaction and the amount of p y r i t i c or organic s u l f u r in the coal. The influences of benzenethiol and iron p y r i t e are contrasted in Figure 8.

295

I00 % -= 8C

~- 60

m 40

I

I

I

I

[

ssH

g 2O

00

I

I

I

I

0.3 0.6 0.9 1.2 1.5 Added Sulfur Compound,mmole

Fig. 8. The conversion of coal IA into p y r i d i n e - s o l u b l e products in the presence of added benzenethiol or p y r i t e in t e t r a l i n at 350°C f o r 5 minutes. Clearly, benzenethio] is a more e f f e c t i v e reagent than p y r i t e .

This ob-

servation is hardly surprising inasmuch as p y r i t e must be decomposed to pyrrh o t l t e and s u l f u r f o r e f f e c t i v e c a t a l y s i s .

Montano and his coworkers estab-

lished t h a t pure p y r i t e does not decompose at 350°C in the presence of I l l i nois No. 6 coal, t e t r a l i n , and dihydrogen and that the decomposition reaction proceeds slowly at t h i s temperature in the presence of t h i s coal, SRCII solvent, and dihydrogen ( r e f .

12).

To examine t h i s point more completely, we

measured the conversion of t e t r a l i n to naphthalene using p y r i t e as the o x i d i zing agent at 350°C (eqn. (9)).

Only 16% of the t e t r a l i n is converted to

naphthalene a f t e r 30 minutes even in the presence of benzenethiol. T e t r a l i n + FeS2

350°C~

Naphthalene + 2H2S + FeS

(9)

CONCLUSION Organic s u l f u r compounds t h a t are known to enhance the rates of hydrogen atom t r a n s f e r reactions of t e t r a l i n and carbon-carbon bond cleavage reactions of 1,3-diphenylpropane also enhance the degree of conversion of f i v e l l l i n o i s coals to soluble products.

The r e l a t i v e effectiveness of the s u l f u r compounds

f o r the promotion of coal l i q u e f a c t i o n depends upon the nature of the coal. But, generally speaking, aromatic t h i o l s are more e f f e c t i v e than other compounds such as thioethers or hydrogen s u l f i d e .

Heterocyclic s u l f u r compounds

and iron p y r i t e are unreactive under the mild conditions used in t h i s study. The observed order of r e a c t i v i t y also implies that coals with a preponderance of t h i o l s or thioethers should be much more r e a d i l y converted to soluble products than other coals with a preponderance of heterocyclic s u l f u r compounds under low severity reaction conditions.

In addit ion, our observations are in

296

accord with the view that coals containing s u l f u r compounds which can be converted to aromatic t h i y l radicals under the reaction conditions should be noticeably more r e a d i l y converted to soluble products.

For example, the con-

version of the reactive thioethers present in coals to aromatic t h i o l s by reductive carbon-sulfur bond cleavage would accelerate preasphaltene formation in donor solvent d i s s o l u t i o n reactions.

S i m i l a r l y , hydrodesulfurization reac-

tions that provide substances such as 2-phenylbenzenethiol or hydrogen sulf i d e from dibenzothiophene would hasten the d i s s o l u t i o n of the coal. We conclude that only c e r ta i n types of organic s u l f u r compounds are eff e c t i v e catalysts f o r short duration thermal coal conversion reactions.

The

fact t h a t there is a d i s t i n c t r e l a t i o n s h i p between the structure and r e a c t i v i ty of the s u l f u r compounds nicely accounts f o r the previous f i n d i n g (refs. I and 2) t h a t, although the reaction y i e l d depends upon the t o t a l quantity of s u l f u r containing compounds in the coal, there is no d i r e c t c o r r e l a t i o n bebetween e i t h e r the p y r i t i c s u l f u r or organic s u l f u r content of a coal and it s ease of conversion.

The addition of p y r i t e to coals that already are rich in

p y r i t e does not lead to any increase in the q u a n t i t i e s of the liq u id s and gases produced in the reaction ( r e f .

13).

However, a l l the coals examined in

this study responded favorably to the addition of small amounts of organic s u l f u r compounds. The increases in the yields of the preasphaltenes are particularly striking.

Apparently the t h i y l radicals intervene in the early

stages of the donor solvent | i q u e f a c t i o n reactions p r i n c i p a l l y by accelerating hydrogen atom t r a n s f e r reactions and by preventing undesirable recombination reactions as well as by promoting the essential molecular weight reducing fragmentation reactions. While i t is evident that no simple series of equations can describe the chemistry of donor solvent d i s s o l u t i o n reactions, i t is per t inent that there are at least two important radical reaction sequences.

The thermal decompo-

s i t i o n reactions of coal molecules by homolytic bond scission reactions (eqn. (I0)).

RiRj -R i. + Rj.

(10)

Aromatic t h i o l s intervene in this chemistry by the rapid donation of hydrogen atoms to the i n i t i a l

products (eqn. ( I ~ ) ) .

These rapid reactions scavange the

Ri . + ArSH - RiH + ArS.

(11)

coal radicals before undesirable recombination or radical addition reactions can occur.

The high yields of preasphaltenes and the pattern of r e a c t i v i t y

shown in the figures are best explained on this basis.

A d d i t i o n a l l y , the coal

297

radicals and the t h i y l radicals i n i t i a t e bond fragmentation reactions as i l l u strated in (eqns. (12) and (13)). Ri. + Coal(CH2)3Coal - RiH + CoalCH(CH2)2Coal

(12a)

ArS. + Coal(CH2)3Coal - ArSH + CoalCH(CH2)2Coal

(12b)

CoalCHCH2CH2Coal ~ CoalCH=CH2 + CoalCH2,

(13)

The results obtained in t h i s study are best rationalized on the basis of the occurrence of the e n t i r e set of reactions (eqns. (10)-(13)) and present f u r ther evidence for the essential need for rapid hydrogen transfer reactions in the i n i t i a l

stages of the dissolution reaction.

ACKNOWLEDGEMENT This research was supported by a grant from the I l l i n o i s Coal Board administered through the I l l i n o i s Department of Energy and Natural Resources.

REFERENCES I 2 3 4 5 6

7 8 9 10 11

M . B . Abdel-Baset, R.F. Yarzab and P.H. Given, Fuel, 57 (1978) 89-94. R.F. Yarzab, P.H. Given, W. Spackman and A. Davis, Fuel, 59 (1980) 81-92. R.E. Winans, H.H. King, R.L. McBeth and R.B. Botto, Am. Chem. Soc., Div. Fuel Chem. Preprints, 28(5) (1983) 8-16. D.S. Hoover, Am. Chem. Soc., Div. Fuel Chem. Preprints, 28(5) (1983) 4857. R.M. Davidson, Mineral effects in coal conversion, IEA Coal Research, London, 1983. Hydrogen s u l f i d e promotes the conversion of coals to soluble products. Patents concerning i t s use in liquefaction have been issued: J.G. Gatsis, U.S. Patent 3,503,863 (1970); R. Bearden, Jr. and C.L. Aldridge, U.S. Patents 4,094,765 (1978) and 4,149,959 (1979) and several groups have presented evidence concerning i t s effectiveness: M.B. Abdel-Baset and C.T. R a t c l i f f e , Am. Chem. Soc., Div. Fuel Chem. Preprints, 25(I) (1980) I-7, V.I. Stenberg, R.J. Baltisberger, T. Ogawa, K. Raman and N.F. Woolsey, Am. Chem. Soc., Div. Fuel Chem. Preprints, 27(3/4) (1982) 22-27 subsequent presentations from t h e i r laboratory, and J. Lambert, J r . , Fuel, 61 (1982) 777-778. C.B. Huang and L.M. Stock, Am. Chem. Soc., Div. Fuel Chem. Preprints, 27(3/4) (1982) 28-34. L.M. Stock, J.E. Duran, C.B. Huang, V.R. Srinivas and R.S. W i l l i s , Fuel, 64 (1985) 754-760. A. Attar and F. Dupuis, "Coal Structure," Am. Chem. Soc., Adv. Chem. Ser., 192 (1981) 239-256. R. Markuszewski, L.J. M i l l e r , W.E. Straszheim, C.W. Fan, T.D. Wheelock and R.T. Greer, New Approaches in Coal Chemistry, Am. Chem. Soc., Symp. Ser., 169 (1981) 401-414. (a) D.C. Cronauer, R.I. McNeil, D.C. Young and R.G. Rubrto, Fuel, 61 (1982) 610-619. (b) F.J. Derbyshire, P. Varghese and D.D. Whitehurst, Fuel, 61 (1982) 859-864.

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P.A. Montano, A.S. Bommannavar and V. Shah, Fuel, 60 (1981) 703-711 and related a r t i c l e s in t h i s series. R . C . Neavel, Phil. Trans. Roy. Soc., A300 (1981) 141-156. (a) B.C. Bockrath, Coal Science, 2 (1984) 65-124. (b) L.M. Stock, Hydrogen Transfer Reaction, in The Chemistry of Coal Conversion, R. Schlosberg, ed., Plenum Publishing Co. (1985).