Direct Determination and Quantification of Sulfur Forms in Heavy

Chapter 10

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Direct Determination and Quantification of Sulfur Forms in Heavy Petroleum and Coal Sulfur Κ Edge X-ray Absorption and X-ray Spectroscopic Approaches

Photoelectron

Martin L. Gorbaty, Graham N. George, and Simon R. Kelemen Exxon Research and Engineering Company, Annandale, NJ 08801

A Sulfur Κ Edge X-ray Absorption Near Edge Structure (XANES) Spectroscopy method has been developed for

the direct determination and quantification of the forms of organically bound sulfur in nonvolatile petroleum and coal samples. XANES spectra were taken of a number of model compounds, mixtures of model

compounds, heavy petroleum and coal samples. Analysis of the third derivatives of these spectra allowed approximate quantification of the sulfidic and thiophenic components of the model mixtures and of

heavy petroleum and coal samples. These results are compared with those obtained by X-ray Photoelectron Spectroscopy (XPS).

Although many attempts have been made t o determine t h e forms o f o r g a n i c a l l y bound s u l f u r i n n o n v o l a t i l e and s o l i d hydrocarbonaceous m a t e r i a l s , v i r t u a l l y a l l have i n v o l v e d i n one way o r another some chemical change t o t h e s t r u c t u r e s , and l e a v e open q u e s t i o n s as t o what i s being measured (1). While l e s s o f an i s s u e f o r petroleum based samples (2), chemical d e r i v a t i z a t i o n s o f coal r e q u i r e good mass t r a n s p o r t t o be sure t h a t t h e r e a c t i o n i s complete and a l l products a r e accounted f o r . P y r o l y s i s type experiments (3a) l e a v e open t h e q u e s t i o n as t o whether s u l f u r forms a r e i n t e r c o n v e r t i n g (3b). To remove these a m b i g u i t i e s , a d i r e c t measurement i s r e q u i r e d , which must be element s p e c i f i c , environment s e n s i t i v e , and must be a b l e t o observe t h e e n t i r e sample. X-ray A b s o r p t i o n Spectroscopy (XAS) i s one such method. In e a r l i e r work Hussain e t a l . (4), S p i r o e t a l . (5), and l a t e r Huffman e t a l . (6,7) demonstrated t h e p o t e n t i a l o f s u l f u r X-ray a b s o r p t i o n s p e c t r o s c o p y f o r t h e q u a l i t a t i v e d e t e r m i n a t i o n o f s u l f u r forms i n c o a l s , however these workers made no attempt a t q u a n t i f i c a t i o n . T h i s r e p o r t i n v e s t i g a t e s t h e a p p l i c a t i o n s o f X-ray a b s o r p t i o n near edge s t r u c t u r e (XANES) s p e c t r o s c o p y f o r t h e purpose o f s p e c i a t i n g and q u a n t i f y i n g t h e forms o f o r g a n i c s u l f u r i n s o l i d s and n o n v o l a t i l e liquids. 0O97-6156/91/0461-O127$06.00/0 © 1991 American Chemical Society

In Coal Science II; Schobert, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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The d e t a i l s o f the XANES experimental setup and data a n a l y s e s have been d e s c r i b e d p r e v i o u s l y (3b,8). A l l model compounds used i n t h i s study were o b t a i n e d from A l d r i c h Chemical Company and were used without f u r t h e r p u r i f i c a t i o n . The asphaltene samples were prepared from t h e i r r e s p e c t i v e petroleum r e s i d u a by p r e c i p i t a t i o n from η-heptane f o l l o w i n g the procedure o f C o r b e t t ( 9 ) . A sample o f Rasa coal was g e n e r o u s l y p r o v i d e d by Dr. Curt White o f the P i t t s b u r g h Energy Technology Center. R e s u l t s and D i s c s s i o n XANES o f Model Compounds. T a b l e I l i s t s the f i r s t i n f l e c t i o n p o i n t s o f the s u l f u r Κ edge s p e c t r a f o r a s e r i e s o f model compounds whose s t r u c t u r e s are b e l i e v e d t o be r e p r e s e n t a t i v e o f the types o f o r g a n i c a l l y bound s u l f u r found i n heavy petroleum and c o a l s . Although the a b s o l u t e v a l u e f o r the energy c a l i b r a t i o n c o n t a i n s some u n c e r t a i n t y , the r e l a t i v e p r e c i s i o n o f the energy s c a l e proved to be r e p r o d u c i b l e t o w i t h i n l e s s than 0.1 eV. The t o t a l span i n energy i s q u i t e l a r g e , being some 12.4 eV between t h i o h e m i a n t h r a quinone and potassium s u l f a t e . As might be a n t i c i p a t e d (10,11), the f i r s t i n f l e c t i o n s o f compounds with more o x i d i z e d s u l f u r are n o t a b l y h i g h e r i n energy than f o r those with reduced s u l f u r . The s u l f u r XANES s p e c t r a o f compounds with s i m i l a r s u l f u r e l e c t r o n i c environments were found t o be s i m i l a r . For example, dibenzothiophene and benzothiophene were found t o g i v e s i m i l a r s u l f u r XANES s p e c t r a , whereas dibenzothiophene and t h i a n t h r e n e , whose s u l f u r atoms are i n s i g n i f i c a n t l y d i f f e r e n t environments, e x h i b i t d i s s i m i l a r s u l f u r XANES. The XANES s p e c t r a o f the compounds l i s t e d i n T a b l e I (3b,8) can r e a d i l y be used as a f i n g e r p r i n t f o r the e l e c t r o n i c nature o f o r g a n i c a l l y bound s u l f u r , and f o r d i s t i n g u i s h i n g between the forms of s u l f u r i n the pure compounds. Examination o f Table I r e v e a l s t h a t the edge o f dibenzothiophene i s d i s p l a c e d from t h a t o f d i b e n z y l s u l f i d e , the f i r s t i n f l e c t i o n energy being some 0.6 eV h i g h e r f o r the former compound. From p r e v i o u s XANES data on dibenzothiophene and d i b e n z y l s u l f i d e and p h y s i c a l mixtures o f the two, i t proved p o s s i b l e t o i d e n t i f y each compound i n the presence o f the o t h e r (3b,8). A d d i t i o n a l l y by simply measuring the h e i g h t s o f the t h i r d d e r i v a t i v e f e a t u r e s at 2469.8 eV and 2470.4 eV r e l a t i v e t o the base l i n e i n the model compound mixtures, a c a l i b r a t i o n was e s t a b l i s h e d which allowed an approximate estimate o f the amounts o f each component i n hydrocarbon samples t o be o b t a i n e d . XANES o f Petroleum Residua. On the l e f t s i d e o f F i g u r e 1 the s u l f u r Κ edge s p e c t r a f o r t h r e e d i f f e r e n t petroleum r e s i d u a and the a s p h a l t e n e samples prepared from them are shown. While the a b s o r p t i o n s p e c t r a a l l appear t o be s i m i l a r , d i f f e r e n c e s are r e v e a l e d by examining the t h i r d d e r i v a t i v e s o f the s p e c t r a , which are shown on the r i g h t s i d e o f the f i g u r e . A l l the r e s i d u a samples appear t o c o n t a i n s u l f u r bound i n s u l f i d i c and t h i o p h e n i c forms, the amount o f s u l f i d i c s u l f u r i n c r e a s i n g from sample 1 t o sample 3. The asphaltene samples prepared from r e s i d u a 2 and 3 a l s o appear t o c o n t a i n both forms. Assuming t h a t the composition o f the s u l f u r


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Table I. S u l f u r Κ Edge F i r s t I n f l e c t i o n E n e r g i e s (a)

Compound K2SO4

anthraquinone 6-sulfonate(Na) tetramethylenesulfone di phenylsulfone di benzothi ophenesulfone dimethyl s u l f o x i d e t e t r a m e t h y l e n e s u l f o x i de 2-methylth i ophene benzoth i ophene d i benzoth i ophene methionine thianthrene dioctylsulphide cystine t e t r a m e t h y l t h i ophene benzyl p h e n y l s u l p h i d e dibenzylsulphide 2-naphthalenethiol cysteine di phenyldi s u l p h i de di b e n z y l d i s u l p h i d e sulfur iron pyrite th i ohemi anthraqui none

F i r s t I n f l e c t i o n (b) (e\H 2478.5 2477.6 2475.6 2474.8 2474.7 2472.8 2472.4 2470.6 2470.4 2470.4 2470.3 2470.2 2470.1 2470.1 2470.0 2469.9 2469.8 2469.8 2469.2 2469.2 2469.1 2469.1 2468.4 2466.1

a) M o d i f i e d from Reference 8. b) F i r s t i n f l e c t i o n p o i n t s were o b t a i n e d from the p o s i t i o n o f t h e lowest energy maximum o f t h e f i r s t d e r i a t i v e , and a r e c o n s i d e r e d a c c u r a t e t o b e t t e r than 0.1 eV.



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forms i n these samples i s approximated by the simple two component mixture o f dibenzothiophene and d i b e n z y l s u l f i d e models and t h a t any t h i c k n e s s e f f e c t s are minimal, an e s t i m a t e o f the r e l a t i v e molar q u a n t i t i e s o f s u l f i d i c and t h i o p h e n i c forms can be o b t a i n e d as d e s c r i b e d above. These approximate values are l i s t e d i n T a b l e I I .


Table I I . Approximate Q u a n t i f i c a t i o n o f O r g a n i c a l l y Bound S u l f u r Forms i n Heavy Hydrocarbons Sample

% S u l f i d i c f± 10)

% T h i o p h e n i c (± 10)

Petroleum Residuum 1 Asphaltene 1

29 0

Residuum 2 Asphaltene 2

42(58) 43(54)

30(42) 37(46)


65 50

35 50

Coal Rasa

30

70

71 100

In sample 1, i t i s c l e a r t h a t the predominant form o f s u l f u r i n the residuum i s t h i o p h e n i c , w h i l e the s u l f i d e forms seem t o predominate i n r e s i d u a 2 and 3. In asphaltene samples 1 and 3, the t h i o p h e n i c s u l f u r content i n c r e a s e s r e l a t i v e t o the r e s p e c t i v e r e s i d u a , i n d i c a t i n g t h a t the molecules c o n t a i n i n g s u l f i d e s u l f u r may be more s o l u b l e i n heptane than those c o n t a i n i n g thiophenes. For sample 2, t h e r e appears t o be no such d i s c r i m i n a t i o n w i t h i n the 10% accuracy l i m i t s c u r r e n t l y e s t a b l i s h e d f o r the XANES a n a l y s i s . In the l a t t e r case, the t o t a l s do not add t o 100%. T h i c k n e s s e f f e c t s may be r e s p o n s i b l e f o r t h i s . Another p o s s i b l e e x p l a n a t i o n i s t h a t a range o f s l i g h t l y d i f f e r e n t s u l f u r t y p e s , o f both s u l f i d i c and t h i o p h e n i c forms e x i s t i n t h i s m a t e r i a l , which causes a broadening o f the f e a t u r e s o f the XANES spectrum and making q u a n t i f i c a t i o n based on mixtures o f two model compounds l e s s a c c u r a t e . In agreement w i t h t h i s p o s s i b i l i t y , the s t r u c t u r e o f the t h i r d d e r i v a t i v e s p e c t r a o f both the residuum sample 2 and the asphaltene d e r i v e d from i t do appear t o be broadened r e l a t i v e t o t h a t o f the s p e c t r a o f sample 3 i n F i g u r e 1. S i n c e the broadening c o u l d be due to s t r u c t u r a l v a r i a t i o n s o f both forms, the data s e t s were n o r m a l i z e d t o 100%, g i v i n g a rough approximation o f the amounts o f s u l f i d i c and t h i o p h e n i c s u l f u r , which are shown i n parentheses i n the t a b l e . I f i t i s determined i n ongoing s t u d i e s t h a t the broadening i s due t o t h i c k n e s s e f f e c t s , then v a l u e s r e p o r t e d here f o r s u l f i d e s are o v e r e s t i m a t e s . I f n o n - t h i o p h e n i c s u l f u r forms such as d i s u l f i d e s o r elemental s u l f u r are found, the s u l f i d e v a l u e s f o r sample 2 w i l l be r e v i s e d upward. XPS o f Petroleum Residua. XPS s u l f u r 2p s i g n a l s from a v a r i e t y o f model compounds have been p r e v i o u s l y measured t o o b t a i n the



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instrumental response f o r s i n g l e component species as well as t o determine the b i n d i n g energies r e p r e s e n t a t i v e o f d i f f e r e n t s u l f u r environments (14,15). These model compound r e s u l t s enable the deconvolution o f XPS s u l f u r 2p s p e c t r a o f complex m a t e r i a l s i n t o c o n t r i b u t i o n s from s u l f i d e (163.3 eV) and thiophene (164.1 eV) types. XPS s p e c t r a were c o l l e c t e d f o r the same petroleum r e s i d u a and asphaltene samples used i n t h e XANES s t u d i e s d e s c r i b e d above. F o r a l l samples the t o t a l amount o f s u l f u r r e l a t i v e t o carbon as measured by XPS was comparable t o t h a t determined by bulk elemental a n a l y s i s . The s p e c t r a were deconvoluted by curve f i t t i n g , and t h e approximate q u a n t i f i c a t i o n s thus d e r i v e d a r e shown i n Table I I I . In agreement with t h e XANES a n a l y s i s , residuum sample 1 c o n t a i n s the h i g h e s t l e v e l o f t h i o p h e n i c - l i k e forms and residuum sample 3 the l e a s t . Table I I I . Mole Percent S u l f i d e and Thiophenic S u l f u r Forms Determined From A n a l y s i s o f t h e XPS S u l f u r 2p Signal SamDle

% S u l f i d i c f± 10)

%ThioDhenic (±10) % O x i d i z e d

Petroleum Residuum 1 Asphaltene 1

24 12

74 85

2 3


33 26

64 70

3 4


42 28

55 67

3 5

Coal Rasa

26

70

4

XANES o f C o a l . In Figure 2 t h e XANES spectrum and i t s t h i r d d e r i v a t i v e o f Rasa coal a r e d i s p l a y e d and Table II c o n t a i n s t h e approximate q u a n t i f i c a t i o n o f s u l f u r types. Rasa coal was chosen f o r t h i s i n i t i a l study because i t has an unusually high amount o f o r g a n i c a l l y bound s u l f u r , and an unusually low l e v e l o f p y r i t i c s u l f u r (12). XANES a n a l y s i s i n d i c a t e s t h a t about 30% o f the s u l f u r in Rasa coal i s s u l f i d i c and 70% i s t h i o p h e n i c . These numbers a r e in agreement with those found by XPS (14), shown i n Table I I I . P a r t i a l c o n f i r m a t i o n o f these values a l s o comes from t h e work o f Kavcic (13), who showed t h a t about 75% o f t h e s u l f u r i n t h i s coal was not r e a c t i v e toward methyl i o d i d e ; t h i s l a c k o f r e a c t i v i t y was a t t r i b u t e d t o t h e s u l f u r being bound i n r i n g s t r u c t u r e s . Even r e c o g n i z i n g t h e p o t e n t i a l o r inherent e r r o r s o f t h e methyl i o d i d e method such as degree o f r e a c t i o n , p o s s i b l e s i d e r e a c t i o n , e t c . , the extent o f agreement o f t h e d i r e c t and i n d i r e c t techniques i s good.


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Figure 2. XANES of Rasa Coal; Top: XANES spectrum; Bottom: 3rd Derivative of XANES spectrum.


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Comparison o f XANES and XPS Data f o r t h e t h i o p h e n i c s u l f u r content determined by XANES and XPS f o r a l l samples s t u d i e d a r e p l o t t e d i n F i g u r e 3. The s o l i d l i n e i n the f i g u r e r e p r e s e n t s the p a r i t y s i t u a t i o n . Most o f t h e data l i e o f f t h e p a r i t y l i n e i n t h e f i g u r e , but i n a non-random f a s h i o n . The apparent s y s t e m a t i c nature o f t h e d i s p a r i t i e s i m p l i e s t h a t the d i f f e r e n c e s i n t h e values d e r i v e d from XANES and XPS a r i s e from t h e u n d e r l y i n g assumptions used i n each technique s i n c e t h e experimental p r e c i s i o n f o r both methods i s b e t t e r than d i f f e r e n c e s between t h e data from XPS and XANES. Both XANES and XPS r e s u l t s g i v e t h e same r e l a t i v e r a n k i n g f o r the t h i o p h e n i c content o f any sample, r e c o g n i z i n g t h e c u r r e n t l y e s t a b l i s h e d accuracy o f ± 10%. F o r example, e i t h e r method confirms t h a t petroleum residuum sample 1 c o n t a i n s t h e most t h i o p h e n i c s u l f u r o f those s t u d i e d , and sample 3 t h e l e a s t . Both methods show t h a t t h i o p h e n i c s u l f u r c o n c e n t r a t e s i n t h e asphaltenes prepared from r e s i d u a 1 and 3, but not i n t h e asphaltenes from residuum 2. However, t h e q u a n t i t a t i v e values a r e d i f f e r e n t . There a r e s e v e r a l p o t e n t i a l sources o f e r r o r . Both methods o f a n a l y s i s use a b i n a r y model mixture, composed o f s u l f i d i c and t h i o p h e n i c components. Thickness e f f e c t s i n t h e XANES o f these model systems would a l t e r t h e c a l i b r a t i o n s . There may be c o n t r i b u t i o n s from s p e c i e s not adequately r e p r e s e n t e d by a simple d i b e n z o t h i o p h e n e - d i b e n z y l s u l f i d e model. While the XPS d a t a a r e r e p r e s e n t e d by 163.3 eV and 164.1 eV components, t h e model compound data base i s as y e t l i m i t e d and not s u f f i c i e n t f o r a d e f i n i t i v e i n t e r p r e t a t i o n i n terms o f a l k y l s u l f i d e and t h i o p h e n i c forms. Examination by both XPS and XANES o f a wider v a r i e t y o f model compounds and m u l t i p l e component model compound mixtures w i l l b e t t e r d e f i n e the s u l f u r s p e c i e s r e p r e s e n t e d by these q u a n t i f i c a t i o n methods. Conclusions T h i s work has demonstrated t h a t o r g a n i c a l l y bound s u l f u r forms can be d i s t i n g u i s h e d and i n some manner q u a n t i f i e d d i r e c t l y i n model compound mixtures, and i n petroleum and c o a l . The use o f t h i r d d e r i v a t i v e s o f t h e XANES s p e c t r a was t h e c r i t i c a l f a c t o r i n a l l o w i n g t h i s a n a l y s i s . The t e n t a t i v e q u a n t i t a t i v e i d e n t i f i c a t i o n s o f s u l f u r forms appear t o be c o n s i s t e n t with the chemical behavior o f t h e petroleum and coal samples. XANES and XPS a n a l y s e s o f t h e same samples show t h e same trends i n r e l a t i v e l e v e l s o f s u l f i d e and t h i o p h e n i c forms, but with s i g n i f i c a n t numerical d i f f e r e n c e s . T h i s r e f l e c t s t h e f a c t t h a t use o f both XPS and XANES methods f o r q u a n t i t a t i v e d e t e r m i n a t i o n s o f s u l f u r forms a r e i n an e a r l y development stage. Work i s c u r r e n t l y i n progress t o r e s o l v e i s s u e s o f t h i c k n e s s e f f e c t s f o r XANES s p e c t r a and t o d e f i n e t h e p o s s i b l e i n t e r f e r e n c e s from p y r i t i c s u l f u r i n both approaches. In a d d i t i o n these techniques a r e being extended t o o t h e r n o n v o l a t i l e and s o l i d hydrocarbon m a t e r i a l s .



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% THIOPHENIC BY XPS

F i g u r e 3. Comparison o f XPS and XANES Values f o r T h i o p h e n i c S u l f u r Content o f Samples S t u d i e d .


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136 Acknowledgements


X-ray a b s o r p t i o n s p e c t r a were r e c o r d e d a t t h e S t a n f o r d S y n c h r o t r o n R a d i a t i o n L a b o r a t o r y , which i s funded by the Department o f Energy, under c o n t r a c t DE-AC03-82ER-13000, O f f i c e o f B a s i c Energy S c i e n c e s , D i v i s i o n o f Chemical S c i e n c e s and the N a t i o n a l I n s t i t u t e s o f H e a l t h , B i o t e c h n o l o g y Resource Program, D i v i s i o n o f Research Resources. The w r i t e r s wish t o thank Dr. C u r t White (PETC) f o r p r o v i d i n g a sample o f Rasa c o a l . References Cited 1. Van Krevelen, D.W. Coal, Elsevier, Amsterdam, 1961, 171. 2. Rose, K.D.; Francisco, M.A. J. Am. Chem. Soc., 1988, 110, 637. 3. (a) See for example Calkins, W. H. Energy Fuels, 1987, 1, 59. (b) Gorbaty, M. L.; George, G. N.; Kelemen, S. R. in "Geochemistry of Sulfur in Fossil Fuels", ACS Symposium Series, in press. 4. Hussain, Z.; Umbach, E.; Shirley, D.A.; Stohr, J.; Feldhaus J . Nucl. Instrum. Methods, 1982, 195, 115. 5.

Spiro, C.L.; Wong, J.; Lytle, F.W.; Greegor, R.B.; Maylotte,

6.

D.H.; Lamson, S.H. Science, 1984, 226, 48. Huffman, G.P.; Huggins, F.E.; Shah, N.; Bhattacharyya, D.; Pugmire, R.J.; Davis, B.; Lytle, F.W.; Greegor, R.B. in

7.

Y. P. and Caudle, R. D., eds. Elsevier, Amsterdam, 1987, 3. Huffman, G.P.; Huggins, F.E.; Shah, N.; Bhattacharyya, D.;

"Processing and Utilization of High Sulfur Coals II", Chugh, Pugmire, R . J . ; Davis, B.; Lytle, F.W.; Greegor, R.B.; ACS Divison of Fuel Chemistry Preprints, 1988, 33, 200.; Huffman,

G.P.;

Huggins, F.E.;

Mitra, S.; Shah, N.; Pugmire, R.J.;

Davis, B.; Lytle, F.W.; Greegor, R.B. Energy Fuels, 1989,

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Received November5,1990


Direct Determination and Quantification of Sulfur Forms in Heavy

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