A Comparison of Asphaltenes from Naturally ... - ACS Publications

9

Downloaded by UCSF LIB CKM RSCS MGMT on November 30, 2014 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0163.ch009

A Comparison of Asphaltenes from Naturally Occurring Shale Bitumen and Retorted Shale Oils: The Influence of Temperature on Asphaltene Structure FENG FANG SHUE and TEH FU YEN School of Engineering, University of Southern California, Los Angeles, CA 90007 The majority of the organic material in oil shale is known as kerogen (organic solvent-insoluble fraction). Bitumen (organic solvent-soluble fraction) generally comprises only a small part of the total organic matter in oil shale. During retorting of the oil shale, kerogen and bitumen undergo thermal decomposition to oil, gas and carbon residue. According to a number of investigators (1,2,3), the mechanism for thermal cracking of oil shale is by decomposition of kerogen to bitumen and subsequently decomposition of bitumen to oil, gas and coke. Since bitumen is the intermediate formed from kerogen during thermal cracking, it may be accepted as representative of the natural-occuring organic matter in oil shale. According to the solvent fractionation scheme for the fossil fuel products, asphaltene is defined as the pentane-insoluble and benzene-soluble fraction. The role of asphaltene is significant (4). Asphaltene is suggested to be the "transitional stage" in the conversion of fossil to oil (5). Therefore the structures of asphaltene fractions produced at various temperatures may be useful indicators of the severity of temperature effects during processing. In this research, the asphaltene fractions produced from natural-occuring shale bitumen and shale oils retorted at 425 and 500°C were compared to investigate structural changes during thermal cracking. Due to the complexity of the asphaltene structure, an approach using the so called "average structural parameters" (6,7) was used to characterize the gross structural features. The average structural parameters were calculated principally from proton and carbon-13 NMR data (8-14). Comparisons of the relative values of these structural parameters clearly indicate the effects of thermal treatment on asphaltene structures. Experimental Solvent E x t r a c t i o n . A sample o f the Green R i v e r o i l shale from A n v i l P o i n t s , Colorado was crushed to 8-20 mesh s i z e p r i o r 0097-6156/81/0163-0129$05.00/0 © 1981 American Chemical Society

In Oil Shale, Tar Sands, and Related Materials; Stauffer, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

130

OIL SHALE, TAR SANDS, AND RELATED

MATERIALS


to s o l v e n t e x t r a c t i o n or r e t o r t i n g . Bitumen was e x t r a c t e d from the shale by exhaustive Soxhlet e x t r a c t i o n with 10% methanol i n benzene f o r 72 h r s . T o t a l bitumen y i e l d was 2.1% by weight. Shale R e t o r t i n g . The r e t o r t chamber was a c y l i n d r i c a l quartz column, 47 mm i n diameter and 300 mm i n length with a screen o f 20 mesh s i z e welded to the bottom. Heat was t r a n s f e r e d to the shale through the quartz w a l l wrapped with two h e a t i n g wires connected t o a transformer. A s t a i n l e s s s t e e l sheathed chromel-alumel (type k) thermocouple was i n s e r t e d i n the center o f the r e t o r t chamber. The temperature was r a i s e d r a p i d l y to 425°C i n one experiment and to 500°C i n another, and maintained there f o r three hours. A i r entered through the top o f the chamber at a flow r a t e o f 1 cc/sec and moved downward together with the product o i l through the c o o l i n g column. The product o i l was c o l l e c t e d i n t o the r e c e i v e r and then seaprated from the water phase. The y i e l d s o f shale o i l s were 10.0% at 425°C and 12.5% at 500°C. Gas and coke were not analyzed. I s o l a t i o n o f Asphaltene. Asphaltenes were i s o l a t e d by prec i p t a t i n g with a 2 0 - f o l d volume o f n-pentane. The o i l / r e s i n f r a c t i o n was separated from the p r e c i p t a t e by f i l t r a t i o n through the thimble f o l l o w e d by Soxhlet e x t r a c t i o n with n-pentane. The asphaltene f r a c t i o n was obtained by Soxhlet e x t r a c t i o n o f the residue with benzene. In the f o l l o w i n g t e x t , the three asphaltenes w i l l be abbreviated as B i t u , R and R ^ Q Q r e p r e s e n t i n g bitumen asphaltene and asphaltenes d e r i v e d from shale o i l s r e t o r t e d at 425 and 500°C r e s p e c t i v e l y . 4 2 5

P h y s i c a l and Chemical A n a l y s i s . Elemental analyses were done by Elek M i c r o a n a l y t i c a l L a b o r a t o r i e s , Torrance, C a l i f o r n i a . Mol e c u l a r weights were determined on a Mechrolab Model 301A Vapor Pressure Osmometer using THF as the s o l v e n t at 40°C. IR s p e c t r a were recorded at a c o n c e n t r a t i o n o f 25 mg/ml i n CH C1 u s i n g 0.5 mm NaCl c e l l s on a Beckman Acculab 6 instrument. Proton and carbon-13 NMR s p e c t r a were obtained on a V a r i a n XL-100 spectrometer o p e r a t i n g at 100.1 MHz f o r proton o r 25.2 MHz f o r carbon. Proton NMR was measured i n C D C 1 ( c e n t r a l residue peak at 5.3 ppm). Carbon-13 NMR was measured i n CDCl^ ( c e n t r a l peak at 77.1 ppm). A sample o f asphaltene (0.5 g) was d i s s o l v e d i n 2.5 ml o f CDC1_ with 35 mg o f C r ( a c a c ) added to i t . To o b t a i n r e l i a b l e q u a n t i t a t i v e r e s u l t s , a delay time o f 4 sec a f t e r each 35° p u l s e and 0.68 sec a c q u i s i t i o n time was used i n the gated decoupling sequence. A l l chemical s h i f t s were reported i n ppm downfield from TMS. 2

2

2

3

Results and D i s c u s s i o n F r a c t i o n a t i o n o f the three samples a f f o r d e d the asphaltene f r a c t i o n s c o n s t i t u t i n g 7.3%, 0.39% and 0.74% by weight f o r the shale bitumen, shale o i l s r e t o r t e d at 425 and 500°C r e s p e c t i v e l y .


9.

SHUE A N D Y E N

A Comparison

of

131

Asphaltenes

Elemental Composition. Elemental a n a l y s i s data f o r the three asphaltene samples are presented i n Table I.


Table I .

Elemental Compositions

of Green R i v e r Asphaltenes

Bitu

R

Wt. % C

74.53

76.50

78.22

wt. % H

8.86

7.97

7.02

wt. % N

2.71

4.49

5.03

wt.

% S

1.80

1.37

1.08

wt.

% o

8.01

7.71

wt.

% Ash

1.78

1.59

0.94

H/C

1.42

1.25

1.08

N/C

0.031

0.050

0.055

s/c

0.009

0.0067

0.0052

0/C

0.104

0.079

0.074

1100

660

620

a

Molecular

10.32

425

R

500

3

a

by d i f f e r e n c e

b

by VPO i n THF

The r e s u l t s o f the elemental a n a l y s i s show that asphaltenes d e r i v e d from r e t o r t e d shale o i l s have s m a l l e r values o f H/C r a t i o and s m a l l e r oxygen and s u l f u r contents, but g r e a t e r n i t r o g e n content than that d e r i v e d from shale bitumen. I n f r a r e d Data. I n f r a r e d s p e c t r a o f the three asphaltene samp l e s are presented i n F i g u r e 1. A number o f w e l l d e f i n e d ban^s were used f o r comparison: 0-H s t r e c h i n g o f phenols (3600 cm" ) , N-H s t r e c h i n g o f p y r r o l e s (3460 cm" ) , C-H s t r e c h i n g o f a l i p h a t i c a l k y l groups (2925 and 2860 cm" ) , C=0 s t r e c h i n g o f carbonyl com^ pounds (1800 - 1650 cm" ) , C-C s t r e c h i n g o f aromatics («1600 cm" ) and C-0 s t r e c h i n g o f aromatic ethers and phenols (1320 - 1200 1



132


4000

4000

5000

3000

2000 1800 Wavenumber, c m

2000

MATERIALS

1600

1400

1200

1000

1600

1400

1200

1000

- 1

1800

Wavenumber, cm"**

4000

2000

1800

1600

1400

1200

1000

Wavenumber, cnr*

Figure 1.

IR spectra of shale oil

asphaltene


9.

A Comparison

SHUE A N D Y E N

R

a n c

R

a

v

of

133

Asphaltenes

es t r

cm" ) . A2S * 500 ^ ° n g e r absorption i n t e n s i t i e s f o r p h e n o l i c 0-H and p y r r o l i c N-H groups and C-0 s t r e c h i n g o f aromatic ethers and phenols. As the p r o c e s s i n g temperature i n c r e a s e s , the a l i p h a t i c C-H absorptions decrease and the aromatic C-C a b s o r p t i o n i n c r e a s e s . Band shapes f o r the carbonyl s t r e c h i n g r e g i o n o f the three samples a l s o show remarkable d i f f e r e n c e s . B i t u has very strong carbanyl absorption at 1700 cm" . F o r d 5QQ> " bonyl absorptions were s h i f t e d toward lower f r e q u e n c i e s . Since i t i s w e l l known that e i t h e r hydrogen bonding o r conjugation with an o l e f i n i c o r phenyl group causes a s h i f t o f the carbonyl abs o r p t i o n a t lower f r e q u e n c i e s , the r e s u l t seems t o i n d i c a t e a r e l a t i v e l y g r e a t e r p r o p o r t i o n o f hydorgen-bonded and/or conjugated carbonyl groups f o r R and R Q Q than f o r B i t u .


a

4 2 5

n

R

C

A

a

r

R

5

e

*H NMR Data. Proton NMR s p e c t r a o f B i t u , R425 and R500 shown i n Figures 2-4. I f c o n t r i b u t i o n s from a c i d i c protons ( i . e . 0-H and N-H, such protons show very broad chemical s h i f t ranges (15)) are neglected, the s p e c t r a can be d i v i d e d i n t o d i f f e r e n t proton groups based upon chemical s h i f t ranges. Proton type d i s t r i b u t i o n s are d e f i n e d as f o l l o w s : H^(9.0-6.0 ppm), protons on aromatic r i n g s ; H (4.5-1.9 ppm), alpha a l k y l protons; Hg(1.9-1.0 ppm), beta a l k y l protons p l u s methine and methylene protons i n the gamma p o s i t i o n s o r f u r t h e r from aromatic r i n g s ; Hy(1.0-0.4 ppm), methyl protons i n the gamma p o s i t i o n s o r f u r t h e r from aromatic r i n g s . The i n t e g r a t e d proton i n t e n s i t i e s f o r each s p e c i f i c group were obtained d i r e c t l y from i n t e g r a t i o n curves. Normalized proton type d i s t r i b u t i o n s are given i n Table I I . A s p e c i f i c group o f protons with chemical s h i f t s i n the range o f 4.5-3.3 ppm, assigned as protons on carbons alpha t o two aromatic r i n g s , were observed i n the NMR s p e c t r a o f B i t u and R425- The r e s u l t seems t o i n d i c a t e that diphenyl methane type o f s t r u c t u r e remains s t a b l e at 425°C but i s cleaved a t 500°C. a

Carbon-13 NMR Data. Carbon-13 NMR data were f r a c t i o n a t e d i n t o groups based upon chemical s h i f t ranges: carbonyl carbons (220168 ppm); aromatic carbons j o i n e d t o oxygens, i . e . , aromatic ethers and phenols (168-148 ppm); aromatic carbons not j o i n e d t o oxygens (148-100 ppm) and a l i p h a t i c carbons (60-9 ppm). Aromatic i t y was c a l c u l a t e d as f r a c t i o n o f aromatic carbons (168-100 ppm) over t o t a l carbons. For s i m p l i c i t y , carbons a s s o c i a t e d with nitrogens were discounted i n the above scheme due t o the r e l a t i v e l y small percentage o f n i t r o g e n present i n the samples. Carbon group d i s t r i b u t i o n s and a r o m a t i c i t i e s o f three asphaltene samples are compared i n Table I I I . We have observed very poor agreement between the oxygen contents determined by u l t i m a t e a n a l y s i s (see Table I) and by C NMR (see Table III) f o r a l l three samples. For B i t u , 0/C = 0.104 by elemental a n a l y s i s but 0/C =0.024-0.037 by 13Q NMR. We think q u a n t i t a t i v e a n a l y s i s o f carbonyl carbons by !3c ^MR ^ d i f f i c u l t f o r two reasons: 1) carbonyl carbons gener a l l y have very long r e l a x a t i o n times (16) and are q u a n t i t a t i v e l y underestimated even under current c o n d i t i o n s ; 2) the disappearance of a large number o f carbonyl absorptions i n the n o i s e i s accen1 3

s



134


9.0

8.0

7.0

Figure 2.

9.0

8.0

Figure 3.

9.0

8.0

Figure 4.

6.0

5.0 4.0 3.0 2.0 Chemical Shift,ppm

1.0

H-l NMR spectrum of asphaltene from shale

7.0

6.0

5.0 4.0 3.0 2.0 Chemical Shift,ppm

1.0

MATERIALS

0.0

bitumen

0.0

H-l NMR spectrum of asphaltene from shale oil retorted at

6.0

5.0 4.0 3.0 2.0 Chemical Shift,pp:s

425°C

0.0

H-l NMR spectrum of asphaltene from shale oil retorted at

500°C

( % and 13c NMR are being used only to c h a r a c t e r i z e the hydrocarbon p o r t i o n and, thus, the c o n t r i b u t i o n from heteroatom p o r t i o n s of the molecule have been minimized.)


SHUE AND Y E N

A Comparison

of

Asphaltenes


Table I I . F r a c t i o n a l Proton D i s t r i b u t i o n s o f Green R i v e r Asphaltenes by *H NMR

Proton Type

Bitu

R

H

A

(9.0-6.0 ppm)

0.046

0.145

0.179

H

a

(4.5-1.9 ppm)

0.204

0.337

0.402

Hg (1.9-1.0 ppm)

0.525

0.389

0.330

Hy (1.0-0.4 ppm)

0.225

0.129

0.089

Table I I I .

R

425

500

Percentage Carbon Group D i s t r i b u t i o n s o f Green R i v e r Asphaltenes by l^C NMR

Carbon Type

Bitu

R

4 2 5

R

5 Q 0

carbonyl carbons(220-168 ppm) 1.3 aromatic carbons j o i n e d t o oxygens(168-148 ppm)

1.1

2.4

2.8

to oxygens(148-100 ppm)

23.0

48.7

56.8

a l i p h a t i c carbons(60-9 ppm)

74.6

49.9

40.4

aromatic carbons not j o i n e d

Aromaticity

0.24

0.51

0.60


OIL SHALE, TAR SANDS, AND RELATED MATERIALS

136

tuated due to the i n e f f i c i e n t number o f scans and the broader range o f carbonyl resonance f o r a sample c o n t a i n i n g many components. A conversion o f C=0 f u n c t i o n s i n t o CHOH o r CH groups by r e d u c t i o n would b e n e f i t t h e i r d e t e c t i o n by l^C NMR. Indeed we have observed new absorption i n the a l i p h a t i c e t h e r and a l c o h o l region (75-60 ppm) and i n c r e a s e d a b s o r p t i o n i n the a l i p h a t i c amine region (60-50 ppm) i n the C NMR spectrum o f Green R i v e r Bitumen asphaltene reduced by LiAlH4(17). Reductions o f carbox y l i c acids and ketones to a l c o h o l s , e s t e r s to ethers and amides to amines by L i A l H are w e l l known r e a c t i o n s (18). Further e v i dence f o r the presence o f carbonyl f u n c t i o n s i"n~"these samples was discussed e a r l i e r from i n f r a r e d data. 2

1 3


4

S t r u c t u r a l Parameters. Average s t r u c t u r a l parameters i n c l u d e : a r o m a t i c i t y ( f ) , degree o f s u b s t i t u t i o n o f the aromatic s h e e t ( a ) , number o f carbon atoms per a l k y l s u b s t i t u e n t (n), r a t i o o f p e r i pheral carbons p e r aromatic sheet to t o t a l aromatic carbons (Haru/Car) and a l i p h a t i c H/C r a t i o can be c a l c u l a t e d according to Eq. ( D - ( 5 ) a

C/H-QWX + Ho/Y

a

= —

n-

+

H /3) y

Hyx a

( 2 )

H. + H /X A ct Hq/X + H /Y 6

+ Hy/3

m

H /X a

H

+H /X A a C/H - (H /X + H /Y H H aliphatic — g — ratio = -g- x A

Haru C

a

^

r

a

3

+

H /3) A y

1 _ H

^

(5)

F

a where X and Y are the assumed atomic H/C r a t i o s f o r H and Hg f r a c t i o n s . Assuming X=Y, these values were chosen to give f (aromaticity) values c o n s i s t e n t with those measured d i r e c t l y by C NMR. The c a l c u l a t e d values o f X o r Y were 1 . 6 0 f o r B i t u , 2.08 f o r R 4 2 5 and 2 . 1 5 f o r R Q Q r e s p e c t i v e l y . C a l c u l a t e d average s t r u c t u r a l parameters are presented i n Table IV. These s t r u c t u r a l parameters h i g h l i g h t the d i f f e r e n c e s among the three asphaltene samples. As the temperature o f treatment i n c r e a s e s , the aromatic i t y a l s o increases but average a l k y l chain length, degree o f subs t i t u t i o n and degree o f condensation o f the aromatic system decrease. The o v e r a l l a l i p h a t i c H/C r a t i o f o r B i t u was 1 . 8 , implyi n g a s i g n i f i c a n t amount of condensed naphthenic r i n g s t r u c t u r e . The a l i p h a t i c H/C r a t i o f o r R 4 2 5 o r R Q Q was above 2 , implying the growing number o f methyl groups upon h e a t i n g . Although assumptions were made i n c a l c u l a t i o n s o f these s t r u c t u r a l paramet e r s , the trend o f changes i n s t r u c t u r e with temperature was clearly indicated. a

a

l 3

5

5


9.

SHUE A N D Y E N

Table IV.

A Comparison

137

Asphaltenes

Average S t r u c t u r a l Parameters o f Green R i v e r Asphaltenes

S t r u t u r a l Parameter


of

Bitu

R

425

R

500

f

0,.24

0,.51

0.60

a o

0..73

0,.53

0.51

n

4..16

2,.42

1.98

0,.99

0 .75

0.66

1,.80

2 .18

2.22

Haru/Car a l i p h a t i c H/C r a t i o

Summary A s t r u c t u r a l comparison o f asphaltenes d e r i v e d from shale bitumen and r e t o r t e d shale o i l has been undertaken i n order t o i n v e s t i g a t e s t r u c t u r a l changes during thermal c r a c k i n g . The r e s u l t s o f elemental a n a l y s i s i n d i c a t e that asphaltene d e r i v e d from r e t o r t e d s h a l e o i l has lower H/C r a t i o and l e s s oxygen and s u l f u r contents, but more n i t r o g e n content than that d e r i v e d from shale bitumen. Heteroatom f u n c t i o n a l groups have a l s o been i n v e s t i g a t e d f o r these asphaltenes. The r e t o r t e d shale o i l asphaltene has more 0-H and N-H f u n c t i o n a l groups as i n d i c a t e d i n the IR s p e c t r a . R e t o r t i n g processes a l s o i n c r e a s e the aromatic i t y , decrease the degrees o f s u b s t i t u t i o n and condensation o f the aromatic sheet and shorten the chain length o f the a l k y l substituent. Acknowledgement This work has supported by U.S. Department o f Energy, O f f i c e o f Environment under Contract No. 79EV10017,000.

Literature Cited 1. Allred, V.D., Quart. Colorado School of Mines, 1967, 62 (3), 91. 2. Hubbard, A.B. and Robinson, W.E., "A Thermal Decomposition Study of Colorado Oil Shale," U.S. Bureau of Mines, 1950, R.I. 4744. 3. Wen, C.S. and Yen, T.F., Chem. Eng. Sci., 1977, 32, 346.


138

OIL SHALE, TAR SANDS, AND RELATED MATERIALS

4. Yen, T.F., "The Role of Asphaltene in Heavy Crudes and Tar Sands," Proceeding of the First International Conference on the Future of Heavy Crude and Tar Sands, paper 54, UNITAR, 1979.


5. Weller, S., Pelipetz, M.G. and Friedman, S., Ind. Eng. Chem. 1951, 43, 1572. 6. Williams, R.B., Sym. on Composition of Petroleum Oils, Determination and evaluation, ASTM Spec. Tech. Publ., 1958, 224, 168. 7. Brown, J.K. and Ladner, W.R., Fuel, 1960, 39, 87. 8. Bartle, K. D., Martin, T.G. and Williams, D. F., Fuel 1975, 54, 226. 9. Cantor, D. M., Anal. Chem., 1978, 50, 1185. 10. Wooton, D. L., Coleman, W. M., Taylor, L. T. and Dom, H. C., Fuel, 1978, 57, 17. 11. Dereppe, J. M., Moreaux, C. and Castex, H. Fuel, 1978, 57, 435. 12. Bartle, K. D., Ladner, W. R., Martin, T. G., Snape, C. E. and Williams, D. F., Fuel, 1979, 58, 413. 13. Ladner, W. R., Martin, T. F. and Snape, C. E., ACS Div. Fuel Chem. Preprints, 1980, 25(4), 67. 14. Dickinson, E. M., Fuel, 1980, 59, 290. 15. Jackman, L. M. and Sternhell, S., "Applications of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry," Pergamon Press, 1969, p.215. 16. Levy, G. C. and Nelson, G. L., "Carbon-13 Nuclear Magnetic Resonance for Organic Chemists", Wiley-Interscience, 1972. 17. Shue, F. F. and Yen, T. F., unpublished work. 18. House, H. O., "Mordern Synthetic Reactions", W. A. Benjamin, Inc., 1972. RECEIVED March 19, 1981.


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