Electron Spin Resonance Studies of Coals and Coal-Derived

10 Electron Spin Resonance Studies of Coals and Coal

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 27, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0071.ch010

-Derived Asphaltenes H. L. RETCOFSKY, G. P. THOMPSON, M. HOUGH, and R. A. FRIEDEL U.S. Department of Energy, Pittsburgh Energy Research Center, 4800 Forbes Avenue, Pittsburgh,PA15213

The discovery of e l e c t r o n s p i n resonance (esr) absorption i n n a t u r a l carbons by Uebersfeld (1) and Ingram (2) prompted a number of i n v e s t i g a t o r s to apply the technique to c o a l and m a t e r i a l s derived from c o a l . At l e a s t three e x c e l l e n t review articles d e s c r i b i n g the e a r l y esr s t u d i e s o f c o a l s have been published (3-5). Although the exact nature of the species r e s p o n s i b l e f o r the esr absorption has not been e s t a b l i s h e d unambiguously, it is g e n e r a l l y thought that the unpaired e l e c trons, at l e a s t i n n o n - a n t h r a c i t i c and p o s s i b l y young anthracitic c o a l s , are a s s o c i a t e d with organic f r e e r a d i c a l s t r u c t u r e s . One group of i n v e s t i g a t o r s , however, has proposed that charge-transfer complexes r a t h e r than s t a b l e f r e e r a d i c a l s may be r e s p o n s i b l e f o r the absorption (6). During the present i n v e s t i g a t i o n , esr spectra were obt a i n e d f o r v i t r a i n s and f u s a i n s from a l a r g e number of c o a l s . For most ranks of c o a l , samples of both l i t h o t y p e s were studi e d . The o b j e c t i v e s of the i n v e s t i g a t i o n were: 1) to b e t t e r c h a r a c t e r i z e the immediate chemical environment o f the unpaired e l e c t r o n s ; 2) to deduce information about the metamorphic changes that occur during vitrinization and f u s i n i z a t i o n ; and 3) to provide needed background information f o r future s t u d i e s of the r o l e of f r e e r a d i c a l s i n c o a l l i q u e f a c t i o n . To f u r t h e r pursue the l a t t e r o b j e c t i v e , esr s p e c t r a of asphaltenes, which are considered by many c o a l researchers to be intermediates i n the conversion of c o a l to liquid f u e l s , were a l s o obtained. A secondary purpose f o r examining the asphaltenes was to explore the recent hypothesis (7) that charge-transfer i n t e r a c t i o n s may be important binding f o r c e s between the a c i d / n e u t r a l and base components i n these m a t e r i a l s . EXPERIMENTAL Samples.

Most of the v i t r a i n s and

f u s a i n s studied were o f

0-8412-0427-6/78/47-071-142$05.00/0 This chapter not subject to U.S. copyright. Published 1978 American Chemical Society Larsen; Organic Chemistry of Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


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high pétrographie p u r i t y , although s e v e r a l of the v i t r a i n s are more a p p r o p r i a t e l y described as v i t r a i n - r i c h samples. Sixtythree v i t r a i n s , i n c l u d i n g samples from A n t a r c t i c a , A u s t r i a , Canada, Germany, Japan, P a k i s t a n , Peru, the United States, and Yugoslavia, and 30 f u s a i n s , most of which were separated from U. S. c o a l s , were i n v e s t i g a t e d . Pétrographie c h a r a c t e r i z a t i o n and other p r o p e r t i e s of many of these samples were published p r e v i o u s l y (8). The asphaltene samples were d e r i v e d from products from the P i t t s b u r g h Energy Research Center's SYNTHOIL (9) c o a l l i q u e f a c t i o n Process Development Unit using a r e c e n t l y described s o l vent s e p a r a t i o n method (10). The a c i d / n e u t r a l and base components of the asphaltenes were prepared according to published procedures (11). S p e c t r a l Measurements. The esr measurements were made over a p e r i o d of approximately ten years. The experimental techniques were e s s e n t i a l l y those published e a r l i e r (12) except for minor m o d i f i c a t i o n s that were made over the years to f a c i l i t a t e the measurements.^ A l l esr measurements were made on evacuated samples (*°10 t o r r ) to prevent l i n e broadening by oxygen i n the a i r . The e l e c t r i c a l p r o p e r t i e s of many of the higher rank samples n e c e s s i t a t e d that the samples be dispersed i n a non-conducting medium to avoid microwave s k i n e f f e c t s . RESULTS AND

DISCUSSION

V i t r a i n s and F u s a i n s . ESR data obtained f o r v i t r a i n s and f u s a i n s during the present i n v e s t i g a t i o n are f a r too numerous to t a b u l a t e here; complete l i s t i n g s of the data are a v a i l a b l e from the authors upon request. The p l o t s of Figures 1-6 show some of the more s i g n i f i c a n t c o r r e l a t i o n s of the data with c o a l composition or c o a l rank. The concentrations of unpaired e l e c t r o n s i n the v i t r a i n s , as estimated by comparing the esr i n t e n s i t y of each sample with that of a standard sample of d i p h e n y l p i c r y l h y d r a z y l , are shown as a f u n c t i o n of the carbon contents of the samples i n F i g u r e 1 The r e l a t i o n s h i p of F i g u r e 1 i s s i m i l a r to those from e a r l i e r s t u d i e s (3-5, 12), although the s c a t t e r of the data p o i n t s i s more pronounced i n the present work. These data show t h a t , i n general, the concentrations of unpaired e l e c t r o n s i n c r e a s e with i n c r e a s i n g c o a l rank up to a carbon content of approximately 94% a f t e r which the s p i n concentrations decrease r a p i d l y . The i n i t i a l , crudely exponential i n c r e a s e i n s p i n c o n c e n t r a t i o n i s g e n e r a l l y a t t r i b u t e d to the formation of organic f r e e r a d i c a l s during v i t r i n i z a t i o n . The f r e e r a d i c a l e l e c t r o n s are thought to be d e l o c a l i z e d over aromatic r i n g s and thus s t a b i l i z e d by resonance. Resonance s t a b i l i z a t i o n of the r a d i c a l s i s g r e a t e r for the v i t r a i n s from higher rank c o a l s s i n c e these presumably c o n t a i n the l a r g e r polynuclear condensed aromatic r i n g systems.

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


ORGANIC CHEMISTRY OF COAL

Figure 1. Concentrations of unpaired electrons as a function of carbon content for vitrains from selected coals



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The p r e c i p i t o u s decrease i n s p i n c o n c e n t r a t i o n above 94% C r e s u l t s from the increased c o n d u c t i v i t y of the samples. In c o n t r a s t to the r e s u l t s f o r v i t r a i n s , the s p i n concent r a t i o n s of the f u s a i n s (Figure 2) e x h i b i t no r e a d i l y d i s c e r n able dependence on carbon content. The wide v a r i a t i o n i n thermal h i s t o r y experienced by f u s a i n s during t h e i r formation i s the most l i k e l y explanation f o r t h i s behavior (13). The changes that occur i n esr l i n e w i d t h s and g values during v i t r i n i z a t i o n and f u s i n i z a t i o n are shown i n F i g u r e s 3-6. The a b s c i s s a used i n these f i g u r e s i s based on the c o a l i f i c a t i o n p l o t s of Schopf (14) with s l i g h t m o d i f i c a t i o n s by Parks (15) and the present authors. The ranks shown on the p l o t s are those of the parent c o a l s as determined by standard procedures (16) . For the v i t r a i n s , an i n c r e a s e i n esr l i n e w i d t h with i n c r e a s i n g rank i s f i r s t observed (Figure 3); t h i s trend i s reversed at the low rank bituminous stage. The r a t e of decrease becomes l a r g e r as c o a l i f i c a t i o n progresses through the higher rank bituminous stages to the e a r l y a n t h r a c i t i c stages. Some of the a n t h r a c i t e s and most of the meta-anthracites exh i b i t very broad l i n e s (not shown i n the f i g u r e ) ; a l i n e w i d t h i n excess of 60 gauss was observed f o r one such c o a l . The l i n e w i d t h r e s u l t s can be i n t e r p r e t e d as f o l l o w s : Nuclear broadening, i . e . , unresolved p r o t o n - e l e c t r o n h y p e r f i n e i n t e r a c t i o n s , plays an important r o l e i n the observed l i n e w i d t h s of peats, l i g n i t e s , and bituminous c o a l s . The r e l a t i v e l y narrow l i n e s observed i n the s p e c t r a of some of the a n t h r a c i t e s probab l y r e s u l t from the smaller number of protons i n the samples, although exchange narrowing of the esr resonances may a l s o be o c c u r r i n g . The proton l i n e broadening hypothesis i s supported by a recent esr study of c o a l s before and a f t e r c a t a l y t i c dehydrogenation (17). The very l a r g e l i n e w i d t h s of the highest rank m a t e r i a l s are undoubtedly due to the presence of g r a p h i t e l i k e s t r u c t u r e s which form during the latt,er stages of c o a l i f i cation. The d i f f i c u l t y i n d i f f e r e n t i a t i n g between a n t h r a c i t e s and meta-anthracites (18) may be r e s p o n s i b l e f o r the apparent l a c k of p r e d i c t a b i l i t y of l i n e w i d t h s i n v i t r a i n s from c o a l s of these ranks. The esr l i n e w i d t h s f o r the f u s a i n s (Figure 4) are very small, f r e q u e n t l y l e s s than one gauss, except f o r samples from the lowest rank c o a l s . U n l i k e the r e s u l t s f o r the v i t r a i n s , no evidence was found f o r the formation of g r a p h i t i c s t r u c t u r e s during the l a t t e r stages of f u s i n i z a t i o n . In a d d i t i o n , the gradual decrease i n l i n e w i d t h s of v i t r a i n s as c o a l i f i c a t i o n proceeds from the low rank bituminous stages to the e a r l y a n t h r a c i t i c stages (Figure 3) appears as a very r a p i d change i n the f u s i n i z a t i o n p l o t (Figure 4 ) . This i s i n accord with S c h o p f s (14) r e p r e s e n t a t i o n of f u s i n i z a t i o n as a process which has an e a r l y i n c e p t i o n and progresses r a p i d l y i n the peat and l i g n i t i c stages, a f t e r which the metamorphic changes become 1


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Figure 2. Concentrations of unpaired electrons as a function of carbon content for fusains from selected coals



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n e a r l y imperceptible. V i t r i n i z a t i o n , on the other hand, i n v o l v e s a p r o g r e s s i v e change throughout a l l stages of rank development. The r e l a t i o n s h i p between the g values of the l i t h o t y p e s and c o a l rank (Figures 5 and 6) a l s o supports S c h o p f s t h e o r i e s of v i t r i n i z a t i o n and f u s i n i z a t i o n . The l a r g e g values found f o r the v i t r a i n s from meta-anthracites i s i n accord with the f i n a l step i n v i t r i n i z a t i o n being the f u s i n g of aromatic r i n g s i n t o g r a p h i t e - l i k e s t r u c t u r e s . The g value of each of the v i t r a i n s and f u s a i n s i s higher than that of the f r e e e l e c t r o n and l i e s i n the s p e c t r a l r e g i o n expected f o r simple organic free radicals. The only exceptions are v i t r a i n s from the more h i g h l y metamorphized c o a l s , one of which e x h i b i t e d a g value of 2.011. The f a c t that esr g values of organic f r e e r a d i c a l s are g r e a t e s t f o r r a d i c a l s i n which the unpaired e l e c t r o n i s l o c a l i z e d or p a r t i a l l y l o c a l i z e d on atoms having high s p i n - o r b i t coupling constants can be used to e x p l a i n the g value r e s u l t s for vitrains. Since the heteroatom contents of c o a l s decrease w i t h i n c r e a s i n g rank, the high g values f o r peats and l i g n i t e s can be i n t e r p r e t e d i n terms of aromatic r a d i c a l s with some p a r t i a l l o c a l i z a t i o n of the unpaired e l e c t r o n s on heteroatoms, p a r t i c u l a r l y but not e x c l u s i v e l y oxygen. As c o a l i f i c a t i o n progresses the g values decrease, suggesting that the r a d i c a l s become more "hydrocarbon-like. The g values of many of the v i t r a i n s from bituminous and young a n t h r a c i t i c c o a l s compare f a v o r a b l y with those e x h i b i t e d by aromatic hydrocarbon r a d i c a l s . During the f i n a l stages of c o a l i f i c a t i o n , the g values become q u i t e l a r g e as one would expect i f continued condensat i o n of the aromatic r i n g s i n t o g r a p h i t e s t r u c t u r e s occurs. The o b s e r v a t i o n of a s m a l l , but r e p r o d u c i b l e , anisotropy i n the g value of c e r t a i n a n t h r a c i t e s (Figure 7) suggests that some o r d e r i n g of the polynuclear condensed aromatic r i n g s i s o c c u r r i n g . 11

Coal-Derived Asphaltenes. To b e t t e r understand the chemist r y of c o a l l i q u e f a c t i o n , an esr i n v e s t i g a t i o n of c o a l - d e r i v e d asphaltenes was i n i t i a t e d . P r e l i m i n a r y r e s u l t s are presented here. Of p a r t i c u l a r concern was the temperature v a r i a t i o n of the esr i n t e n s i t i e s of asphaltenes and t h e i r a c i d / n e u t r a l and base components (Figure 8). The most s i g n i f i c a n t f i n d i n g to date i s that the weighted average of the temperature dependencies of the two components reproduces the temperature dependence of the t o t a l asphaltene (before separation) e x c e p t i o n a l l y w e l l . T h i s suggests that charge t r a n s f e r i n t e r a c t i o n s , at l e a s t i n the M u l l i k a n sense, are r e l a t i v e l y unimportant b i n d i n g f o r c e s between the a c i d / n e u t r a l and base components of the asphaltenes.




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Electron

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Electron Spin Resonance Studies of Coals and Coal-Derived

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