Metal Complexes in Fossil Fuels - American Chemical Society

Chapter 23

Analysis of Metal Species in Petroleum and Tar Sands Using the Electron Paramagnetic Resonance and Fourier Transform Infrared Techniques W. R. M. Graham

Downloaded by MONASH UNIV on November 26, 2015 | http://pubs.acs.org Publication Date: July 6, 1987 | doi: 10.1021/bk-1987-0344.ch023

Department of Physics, Texas Christian University, Fort Worth, TX 76129

The results are reviewed from studies carried out to demonstrate the versatility of the electron paramagnetic resonance (EPR) technique supplemented by Fourier transform infrared (FTIR) spectroscopy in characterizing the metals in crude oils and tar sands. Application of the highly sensitive, but nondestructive, EPR technique can provide detailed information on the occurrence of metals in metalloporphyrins, in other organic complexes, or in associated mineral matter. The vanadyl ion has been identified bound to a porphyrin and associated with clay in tar sand samples. Examples of manganese in bitumen and in carbonate minerals, and the origin of ferric iron are also discussed. The identification of metal species in crude oils, source rocks, tar sands, and oil shales is of fundamental interest in understanding the geochemical origin of petroleum, and has important implications for the extraction, processing, and upgrading of petroleum and synfuels. Just as significant as the identification of the metal elements present, which can be done by standard analytical techniques, is the determination of the site of the metal species. Whether they occur in metalloporphyrins, in other organic complexes in the bitumen, or are associated with minerals or clays, has important consequences for their behavior and influence in processing. Electron paramagnetic resonance (EPR) has several unique advantages over more commonly used analytical techniques. It is extremely sensitive, with the capability of detecting concentrations of metal ions as low as a few ppm. At the same time it is nondestructive, which means that EPR analysis can be carried out without altering the original environment, site, or complex containing the metal species. This is an important advantage since the magnetic constants derived from standard EPR measurements, particularly the g-value and hyperfine splitting constants, are extremely sensitive to the chemical bonding to the paramagnetic ion and the structure in which it is located, and thus they provide an extremely useful probe for the characterization of metals in organic complexes and minerals. 0097-6156/87/0344-0358$06.00/0 © 1987 American Chemical Society

In Metal Complexes in Fossil Fuels; Filby, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


23.

GRAHAM

Metal Species in Petroleum and Tar Sands

359

The purpose o f t h i s paper i s t o demonstrate the k i n d o f i n f o r m a t i o n about m e t a l s p e c i e s i n p e t r o l e u m and s y n f u e l s which can be drawn from the a p p l i c a t i o n o f EPR, supplemented t o some e x t e n t by F o u r i e r t r a n s f o r m i n f r a r e d (FTIR) s p e c t r o s c o p y . Examples are d i s c u s s e d from the r e s u l t s o f work c a r r i e d out i n t h i s l a b o r a t o r y ( 1 - 6 ) , p r i n c i p a l l y on t a r sands, but a l s o on c o a l and p e t r o l e u m . The p r i n c i p l e s i n v o l v e d are g e n e r a l l y a p p l i c a b l e t o p e t r o l e u m and s y n f u e l s . An i n t r i n s i c l i m i t a t i o n o f the t e c h n i q u e which s h o u l d be acknowledged, i s t h a t the m e t a l s p e c i e s i n v e s t i g a t e d must be e i t h e r p a r a m a g n e t i c , f e r r o m a g n e t i c , or f e r r i m a g n e t i c i n o r d e r t o be d e t e c t a b l e u s i n g an EPR s p e c t r o m e t e r ; however, a l a r g e p e r c e n t a g e o f the m e t a l s o f i n t e r e s t such as i r o n , vanadium, and manganese f a l l i n t o these c a t e g o r i e s . B e f o r e a d i s c u s s i o n o f some examples o f the a p p l i c a t i o n o f the EPR t e c h n i q u e to m e t a l s i n p e t r o l e u m and t a r sands, a b r i e f o u t l i n e o f the p r i n c i p l e s o f EPR s p e c t r o s c o p y i s p r e s e n t e d i n the next s e c t i o n . D e t a i l e d t r e a t m e n t s o f EPR t h e o r y at both the i n t r o d u c t o r y (7) and advanced (8) l e v e l s are a v a i l a b l e e l s e w h e r e . H a l l (9,10) has r e viewed the a p p l i c a t i o n o f EPR to s t u d i e s o f c l a y m i n e r a l s . P r i n c i p l e s o f EPR

Spectroscopy

The EPR phenomenon depends on the p r o p e r t y t h a t any atomic or molecu l a r system w i t h one or more u n p a i r e d e l e c t r o n s p o s s e s s e s a magnetic moment which i n t e r a c t s w i t h an a p p l i e d magnetic f i e l d , H. Of p r i n c i p a l i n t e r e s t i n the case o f f o s s i l f u e l s are the t r a n s i t i o n m e t a l i o n s , such as V , M n , or F e , and f r e e r a d i c a l s , which are f r a g ments o f o r g a n i c m o l e c u l e s . In the s i m p l e s t case o f one u n p a i r e d e l e c t r o n ( t o t a l s p i n S = 1/2) the magnetic moment i s a l i g n e d e i t h e r p a r a l l e l or a n t i p a r a l l e l to the e x t e r n a l magnetic f i e l d , c o r r e s p o n d ing to the two s p i n s t a t e s M = -1/2 and +1/2. The d i f f e r e n c e i n energy between these two s p i n s t a t e s i s z e r o i n the absence o f the magnetic f i e l d , but e q u a l t o ggH ( t h e Zeeman s p l i t t i n g ) i n i t s p r e s ence, where $ i Bohr magneton and g i s the g f a c t o r . Thus, as shown i n F i g u r e 1, the energy d i f f e r e n c e i n c r e a s e s w i t h H. A transit i o n can be i n d u c e d between the two s t a t e s by r a d i a t i o n o f f r e q u e n c y V , p r o v i d i n g the resonance c o n d i t i o n H +

2 +

s t

n

3 +

e

hv=

g$H

(1)

is f u l f i l l e d . For an x-band EPR s p e c t r o m e t e r the r e q u i r e d microwave energy i s o f f r e q u e n c y v = 9.2 GHz, and i s f i x e d . The EPR spectrum i s then o b t a i n e d by v a r y i n g the magnetic f i e l d u n t i l the resonance c o n d i t i o n g i v e n by E q u a t i o n 1 i s met. For a f r e e e l e c t r o n g = 2.0023 and the t r a n s i t i o n o c c u r s at -330 mT ( c e n t e r f i e l d ) . The t r a n s i t i o n i s r e c o r d e d as the f i r s t d e r i v a t i v e o f the a b s o r p t i o n c u r v e . C o m p l e x i t y i s i n t r o d u c e d t o the spectrum when the paramagnetic ion p o s s e s s e s a nonzero n u c l e a r s p i n I, which i n t e r a c t s w i t h the e l e c t r o n s p i n S. T h i s i n t e r a c t i o n causes h y p e r f i n e s p l i t t i n g o f the s p i n l e v e l s and, t h e r e f o r e , o f the EPR l i n e s i n t o 2 1 + 1 components. Thus f o r an i o n w i t h I = 1/2 the o r i g i n a l EPR l i n e i s s p l i t i n t o a d o u b l e t as shown i n F i g u r e 1. The magnitude o f the h y p e r f i n e s p l i t t i n g , A, r e f l e c t s the c h a r a c t e r o f the m e t a l l i g a n d bonds. In the more g e n e r a l case o f more than one u n p a i r e d e l e c t r o n , the e x t e r n a l magnetic f i e l d produces 2S + 1 s p i n l e v e l s , and t r a n s i t i o n s



360

METAL COMPLEXES IN FOSSIL FUELS

H

F i g u r e 1. Energy l e v e l s and a l l o w e d EPR t r a n s i t i o n s f o r a s y s tem w i t h : ( t o p ) a s i n g l e u n p a i r e d e l e c t r o n , S = 1/2; ( m i d d l e ) n u c l e a r s p i n I = 1/2 and S = 1/2, showing the h y p e r f i n e s p l i t t i n g , A; and (bottom) S = 3/2, showing the z e r o - f i e l d s p l i t t i n g , D.


23.

GRAHAM


361


are induced between a d j a c e n t l e v e l s a c c o r d i n g t o the s e l e c t i o n r u l e AM = +1. When the paramagnetic i o n o c c u r s i n a c r y s t a l ( f o r example, i n a m i n e r a l ) the c r y s t a l l i n e f i e l d may be s u f f i c i e n t t o s p l i t the s p i n l e v e l s ( z e r o - f i e l d s p l i t t i n g ) , even i n t h e absence o f an e x t e r n a l f i e l d , as shown i n F i g u r e 1 f o r S = 3/2. S i n c e the c r y s t a l l i n e f i e l d i s determined by the e l e c t r i c charge on n e i g h b o r i n g atoms, the z e r o - f i e l d s p l i t t i n g parameter, D, measured from the EPR spectrum r e f l e c t s the environment o f the paramagnetic i o n . In g e n e r a l , the r e l e v a n t magnetic parameters a r e o b t a i n e d by s o l v i n g the a p p r o p r i a t e s p i n H a m i l t o n i a n H, f o r the r e q u i r e d sym metry : H = 3H.£-S + S-A-I + SvD.S (2) In E q u a t i o n 2, A i s the h y p e r f i n e s p l i t t i n g c o n s t a n t , and D, the z e r o - f i e l d s p l i t t i n g c o n s t a n t . I t i s the e v a l u a t i o n o f g, A, and D which can l e a d unambiguously t o the i d e n t i f i c a t i o n o f both the m e t a l i o n , and the m i n e r a l c r y s t a l o r o r g a n i c m o l e c u l a r s t r u c t u r e i n which i t occurs. Procedure The EPR measurements i n t h i s work were a l l made u s i n g a V a r i a n V-4500 s p e c t r o m e t e r o p e r a t i n g a t 9.2 GHz w i t h 100 kHz magnetic f i e l d modula tion. Samples were p l a c e d i n 3 mm O.D. q u a r t z t u b e s , and the temper a t u r e was c o n t i n u o u s l y m o n i t o r e d w i t h a c h r o m e l - c o n s t a n t a n thermo c o u p l e . A V a r i a n V-4557 v a r i a b l e temperature a c c e s s o r y was used i n r e c o r d i n g s p e c t r a a t temperatures down t o 80 K. The supplementary i n f r a r e d s p e c t r a were r e c o r d e d on a N i c o l e t 5DXE FTIR s p e c t r o m e t e r . Samples were u s u a l l y made f o l l o w i n g p r o c e dures d i s c u s s e d elsewhere (5) i n the form o f p r e s s e d p e l l e t s o f mix t u r e s o f m i n e r a l i n KBr i n a 1:100 r a t i o by w e i g h t . In the case o f the t a r sand samples d i s c u s s e d h e r e , v a r i o u s f r a c t i o n s were o b t a i n e d u s i n g e x t r a c t i o n t e c h n i q u e s which have been p r e v i o u s l y reported i n d e t a i l (3,11). R e s u l t s and D i s c u s s i o n Vanadium. Vanadium i n d i v a l e n t and t e t r a v a l e n t o x i d a t i o n s t a t e s w i t h c o n f i g u r a t i o n s 3 d and 3d , r e s p e c t i v e l y , i s r e a d i l y o b s e r v a b l e by EPR ( 7 , 8 ) . I n c o n t r a s t t o V + and V**"", V + would o n l y be o b s e r v a b l e under s p e c i a l c i r c u m s t a n c e s (7) a t low t e m p e r a t u r e s , and i s l e s s l i k e l y t o be i m p o r t a n t i n f o s s i l f u e l s . I n p e t r o l e u m o r t a r sands V Ή- i s expected t o o c c u r most commonly i n the form o f the v a n a d y l c a t i o n , V 0 + . I t s EPR spectrum i s c h a r a c t e r i z e d by an 8 - l i n e p a t t e r n a r i s i n g from the s p l i t t i n g i n t o 2 1 + 1 components which i s produced by the h y p e r f i n e i n t e r a c t i o n between the S = 1/2 s p i n o f the u n p a i r e d e l e c t r o n and the I = 7/2 s p i n o f the vanadium n u c l e u s a c c o r d i n g t o the second term i n the s p i n H a m i l t o n i a n g i v e n above ( E q u a t i o n 2 ) . F i g u r e 2 i s the c e n t r a l p o r t i o n o f the EPR spectrum r e c o r d e d f o r the a s p h a l t e n e f r a c t i o n o f C i r c l e C l i f f s t a r sand ( 4 ) . Two s e t s o f f e a t u r e s each show the 8 - l i n e p a t t e r n c h a r a c t e r i s t i c o f vanadium. The EPR spectrum o f a x i a l l y symmetric paramagnetic systems which a r e randomly o r i e n t e d i n a powder e x h i b i t s two c h a r a c t e r i s t i c l i n e com ponents c o r r e s p o n d i n g t o the o r i e n t a t i o n o f the symmetry a x i s p a r a l 3

1

2

1

3

2


362

METAL COMPLEXES IN FOSSIL FUELS

BOSCAN


ASPHALTENE

IN ι ι ι ι I PERPENDICULAR LINES CIRCLE

CLIFFS

ASPHALTENE

PARALLEL LINES I

I

I L n ι 11 l i t PERPENDICULAR LINES

P.R.

SPRING

MINERAL FINES

2 +

Figure 2. EPR spectra of V 0 observed for Boscan asphaltene, C i r c l e C l i f f s tar sand asphaltene, and mineral fines extracted from P.R. Spring bitumen.


23.

TABLE I .

Comparison o f EPR

Sample

gx

Parameters f o r V02+

g//

A//( mT)

mT)

Ref.

1..986

1..962

6. 08

17. 20

4

1..9861

1,.9598

5. 93

17. 30

3

Vanadyl e t i o p o r p h y r i n

1,.9862

1..9629

6. 01

17. 13

12

Vanadyl t e t r a b e n z o porphyrin

1..985

1,.962

5. 0

15. 0

13

1,.9928

1,.9432

7. 26

18. 51

7. 1

19. 4

14

Circle Cliffs Boscan


363


GRAHAM

P.R.

asphaltene

Spring mineral

2 +

Mg -hectorite γ-Α1 0 2

Si0

asphaltene

(2% V 0 ) 2 +

2

2 +

(5% V 0 ) 2 +

3

(2% V 0 )

fines

3

1 .989

1 .916

7. 08

18. 12

15

1 .982

1 .922

7. 72

19. 51

15

l e i and p e r p e n d i c u l a r t o the magnetic f i e l d . The t y p i c a l , p a r a l l e l and p e r p e n d i c u l a r l i n e shapes observed i n F i g u r e 2 i n d i c a t e t h a t V** + o c c u r s at an a x i a l l y symmetric s i t e . A v e r y s i m i l a r spectrum i s ob served f o r Boscan petroleum a s p h a l t e n e s . In Table I , an e x a m i n a t i o n o f the p e r p e n d i c u l a r and p a r a l l e l components o f the magnetic con s t a n t s g and A (lmT = 10 g a u s s ) , f o r the C i r c l e C l i f f s and Boscan a s p h a l t e n e s r e a d i l y c o n f i r m s t h a t the vanadium p r e s e n t i n b o t h samples i s i n the form of v a n a d y l e t i o p o r p h y r i n . Table I a l s o g i v e s the magnetic c o n s t a n t s measured f o r a v a n a d y l spectrum which was observed f o r m i n e r a l f i n e s w h i c h remained i n P.R. S p r i n g t a r sand bitumen a f t e r c o n v e n t i o n a l S o x h l e t e x t r a c t i o n t e c h n i q u e s had been a p p l i e d ( 3 ) . Subsequent e x t r a c t i o n (11) o f these f i n e s from the bitumen was accomplished by c e n t r i f u g i n g the m i x t u r e , which had been r e d i s s o l v e d i n benzene, at 8000 r e v m i n " f o r 35 min at 4° C, d e c a n t i n g the l i q u i d , and then r e p e a t e d l y washing the r e maining c l a y p a r t i c l e s w i t h benzene u n t i l the s o l v e n t was c o l o r l e s s . Even though the c l a y p a r t i c l e s were e x h a u s t i v e l y t r e a t e d to remove a l l o r g a n i c m a t e r i a l , the p o s s i b i l i t y e x i s t s , o f c o u r s e , t h a t the v a n a d y l i n t h i s case o r i g i n a t e s from p o r p h y r i n i n bitumen which had been adsorbed onto the s u r f a c e s o f the m i n e r a l f i n e s . Table I I shows the r e s u l t s o f an FTIR a n a l y s i s o f the b u l k o f the P.R. S p r i n g t a r sand m i n e r a l m a t t e r o b t a i n e d by the s p e c t r a l s u b t r a c t i o n method and d i s c u s s e d i n d e t a i l elsewhere ( 5 ) . As can be seen, the b u l k m i n e r a l m a t t e r , which was o b t a i n e d by the c o n v e n t i o n a l S o x h l e t e x t r a c t i o n method to remove the bitumen, c o n t a i n s

Metal Complexes in Fossil Fuels - American Chemical Society

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