Structural Differences Between Asphaltenes Isolated from Petroleum

4 Structural Differences Between Asphaltenes Isolated from Petroleum and Downloaded by UNIV MASSACHUSETTS AMHERST on April 18, 2013 | http://pubs.acs.org Publication Date: January 1, 1982 | doi: 10.1021/ba-1981-0195.ch004

from Coal Liquid TEH FU YEN School of Engineering, University of Southern California, Los Angeles, CA 90007 The structure of asphaltenes derived from petroleum is different from those of coal. In general, coal-derived asphaltenes have higher aromaticity than petroleum-based asphaltenes. The aromatic system within the petroleum asphaltene is largely peri-condensed, while that of the coal asphaltene is kata-condensed. The substitution of the peripheral carbons in the aromatic system of coal-derived asphaltenes is less than and shorter than that in asphaltenes of petroleum origin. The unit molecular weight of coal asphaltenes is 400-600, whereas that of petroleum asphaltenes is 800-2500, which is reflected in stacking height as well as layer diameter. Another major characteristic difference is that coal asphaltenes contain more hydroxyl and pyrrolic groups in addition to ether-oxygen or basic nitrogen functions than do petroleum asphaltenes.

A

sphaltene is an essential component of any dark-colored, heavy, viscous a n d nonvolatile o i l , regardless of the o i l source. Asphaltene can be obtained f r o m the o i l extracted f r o m a naturally o c c u r r i n g o r g a n i c - r i c h fossil m a t e r i a l b y a simple solvent fractionation. Asphaltene also can be obtained f r o m the c h e m i c a l conversion product of a solid fuel, such as pyrolysis or catalytic hydrogénation of coal or shale. T h e f o r m e r is an example of the asphaltene isolated f r o m native petroleum o i l . A n example of the latter is the asphaltene obtained f r o m a synthetic crude, such as shale o i l or coal l i q u i d . In this chapter, I emphasize the o r i g i n or source f r o m w h i c h the asphaltenes came, rather than the environment or conditions to w h i c h they w e r e exposed. F o r example, I w i l l group asphaltenes f r o m refinery bottoms w i t h petroleum-derived asphaltenes i n the discussion. S i m i l a r l y , asphaltenes f r o m the solvent extracts of raw coal are also classified as coal-derived asphaltenes. 0065-2393/81/0195-0039$05.00/0 © 1981 American Chemical Society In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

40

CHEMISTRY O F ASPHALTENES

Concept of Average Structure

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F o r well-defined, simple organic molecules, the c h e m i c a l structure c a n b e elucidated b y composition analysis and some spectroscopic methods. As the m o l e c u l e becomes more complex i n nature, the traditional c h e m i c a l structures d o not a p p l y . N e w methodology must be developed to characterize them. T h e f o l l o w i n g is a list of some general classes of substances r a n g i n g f r o m simple to complex: 1. simple compounds—organic molecules described i n most general textbooks or handbooks, along w i t h alkaloids, vitamins, and drugs. 2. regular polymers—substances containing fixed repeating sequences, for example, polystyrene. 3. random polymers—substances similar to those i n 2, w i t h the exception that their b u i l d i n g blocks occur i n r a n d o m fashion, such as l i g n i n a n d m e l a n i n . 4. intrinsic mixtures—generally, mixtures of similar molecules, perhaps isomers or homologs, of a narrow carbon-number distribution range associated b y intermolecular force; petroporphyrins and coal tar p i t c h w o u l d be good examples. 5. multipolymers—the most complex substances k n o w n , w h i c h are referred to as polycondensates w i t h different repeating blocks, w i t h copolymers containing 2 blocks, terpolymers 3, a n d m u l t i p o l y m e r s expected to contain η blocks. Because of the inter- a n d intramolecular interactions w i t h i n the structure, this class also has the nature of class 4, w i t h association between molecules. Asphaltenes generally belong to this class. Because of the variance i n m u l t i p o l y m e r s , a n exact c h e m i c a l structure is not possible. T o differentiate between different asphaltenes, the methodology l e a d i n g to a n average structure is necessary. Microstructure

Versus Macrostructure

F o r a discrete molecule w i t h a simple structure, a microstructure is sufficient to characterize the g i v e n molecule. F o r a complex system such as that of asphaltene, the i n f o r m a t i o n r e q u i r e d for characterization has to i n c l u d e association as w e l l as m i c e l l e formation. T h e microstructure has been chosen a r b i t r a r i l y to refer to short-range b o n d i n g , that is, distances between 0.5 Â - 2 . 0 Â ; whereas the macrostructure (bulk structure) pertains to m o l e c u l a r interactions or orders at larger distances (20 Â - 2 0 0 0 Â ) . Structural

Parameters

T o characterize the most complex system, m a n y selected physical a n d c h e m i c a l methods should be e m p l o y e d . S i m p l y , the reason is that one g i v e n m e t h o d m a y p r o v i d e only partial structural i n f o r m a t i o n about the system.

In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

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41

Structural Differences Between Asphaltenes

W h a t is important is that, if different methods (with or without certain assumptions) y i e l d identical results, m u l t i p l e methods w i l l increase the p r e c i sion of structural i n f o r m a t i o n . Structural i n f o r m a t i o n obtained f r o m various methods c a n usually be represented b y a set of structural parameters, S) η (C /C J H /C A

s

N

N

c /c m

p

C„/C(/) C /C(/„) N

c„./c C/R F/C A

A

M

Ν Empirical formula

Methods VPO elemental analysis elemental analysis direct determination elemental analysis elemental analysis elemental analysis NMR NMR NMR NMR NMR NMR x-ray x-ray NMR NMR NMR NMR NMR IR IR IR IR densimetric densimetric MS MW and x-ray

Reference Experimental Calculated (2) (3) (3) (3) (3) (3) (iJ) (4) (4) (4) (4) (4) (4) (5) (5) (4) (4) (4) (4) (4) (6) (6) (6) (β) (3) (7) (2) (8) (8)

4750 84.2 7.9 1.6 2.0 4.5 0.055 0.055 0.27 0.95 0.31 0.19 0.18 0.41 9.7 Â 0.43 0.71 3.2 1.25 0.47 0.35 0.24 0.15 1.7 6.2 0.32 576 4 Laquinillas asphaltene

4276 83.0 8.1 1.5 1.3 6.0 0.054 0.046 0.25 0.95 0.31 0.21 0.18 0.42 9.85 Â 0.44 0.71 4.3 1.06 0.50 0.36 0.22 0.16 1.1 5.0 0.60 524 4 (C H NS 0 74


87

2

44

CHEMISTRY OF

ASPHALTENES


o r d e r e d and consist of a n u m b e r of planar aromatic molecules stacked i n layers v i a the π-π association. In general, the stacking occurs w i t h five or six layers, as discovered b y x-ray diffraction. Some c o m m o n l y used structural parameters for the macrostructure of p e t r o l e u m - d e r i v e d asphaltene are shown i n T a b l e II. T h e conventional m e a n i n g of molecular weight does not a p p l y to the case of petroleum asphaltene; however, d e p e n d i n g on the methods used, a unit sheet weight (1000-4000), a cluster or particle weight (4000-10,000), a n d a m i c e l l e weight (40,000-40,000,000) are possible. Most intramolecular asso ciated particles can be taken as the basic molecular weights, such as i n the case o f the uniparticle evaporation (13). Between the sheets of the stacks, small p l a n a r aromatic molecules can be inserted. Also, interchelation complexes can b e m a d e (14) i n such a w a y that m a n y catalytic effects m a y be explained (15). T h e association-disassociation energy of the sheets is 14-20 k c a l / m o l (16). These interactions involve charge-transfer mechanisms, as f o u n d b y studies using both IR spectroscopy (17) a n d E S R (18). M a n y of the aromatic portions of the sheet contain defective centers (gaps a n d holes) (19), a n d usually these are the coordination centers for metals (20). Schematically, the arrangement of sheets w i t h the zig-zag chains can be best illustrated as i n F i g u r e 3. T h e π-π association w i l l result i n the p r o d u c t i o n of a crystallite ( F i g u r e 3 A). T h e zig-zag c h a i n can orient i n b u n d l e - l i k e fashion (B) d e p e n d i n g on the torsional angle w h e n the propagation of s p - b o n d e d orbital takes place. These island-like stacks are t e r m e d particles (C). Usually, particles c a n be further associated to micelles (D); the p e r i p h e r a l groups are polar. T h e r e are a n u m b e r of weak links (E) along the c h a i n configuration; some of these are ether, thioether, or even h y d r o g e n bonds, w h i c h can undergo cleavage. T h e r e are imperfections i n the aromatic systems w i t h d i - or trivalent heteroatoms, such as nitrogen, sulfur, or oxygen. Clusters m a y also be l i n k e d 3

Table II. Some Commonly Used Structural Parameters for the Macrostructure of a Petroleum-Derived Asphaltene Structural Parameters L M D c

e

k

De R D

L

ι r D M[ M

e

Methods x-ray x-ray EM EM SAS SAS SAS SAS ultracentrifuge l o w angle scattering f i l m balance

Reference

Value

(5) (5) (13) (13) (21) (21) (21) (21) (2) (21) (22)

16-20 Â 5-6 19-37 Â 7-97 Â 33 86 300-400 Â 500-600 Â 22 Â 50 Â 80,000-140,000



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YEN

45


Macrostructure of Asphaltics A. Crystallite C. Particle E. Weak Link G. Intracluster I. Resin K. Petroporphyrin

B. Chain Bundle D. Micelle F. Gap and Hole H. Intercluster J . Single Layer L. Metal (M)

Figure 3. The general features of the macrostructure of asphaltenes and related substances

petroleum-derived

b y f o l d i n g of the chains (intracluster G ) or b y b r i d g i n g through weak links (intercluster H). T h e resin (I) is a small h o m o l o g of asphaltene that is larger, a n d the molecular weight is higher. A simple layer (J) of the stacked clusters c a n be f o r m e d b y disassociation, p r o v i d e d that conditions do not favor the reassociation. Petroporphyrins (K) are a single layer containing extended conjugation of pyrroles. T h e metal ( L ) c a n be a coordination center for several clusters.

Coal-Derived

Asphaltene

M i c r o s t r u c t u r e . T h e characterization of coal-derived asphaltene is q u i t e similar to that of petroleum-derived asphaltene. Since it is anticipated that coal-derived asphaltene w i l l have acid/neutral a n d base characteristics (26, 38), the average structure of both must be considered. I n T a b l e III, Structure I is amphoteric (or slightly basic), a n d Structure II is a n acid/neutral representation. A m i x t u r e of both m a y be t y p i c a l of the average structure of a c o a l - d e r i v e d asphaltene. A t present, w e w i l l illustrate this b y a n asphaltene o b t a i n e d f r o m coal l i q u i d of the Synthoil process. ( T h e coal is h v A b , West K e n t u c k y , Homestead Seam; the coal l i q u i d is obtained b y catalytic hydrogéna t i o n at 4 5 0 ° C a n d 4000 psig h a v i n g % C , 86.7; % H , 8.38; % N , 0.93; %S, 0.09; % 0 , 3.2; a n d % A s h , 0.7.)


46


Especially important is the fact that the coal-derived asphaltenes defi n i t e l y have a lower molecular weight, that is, usually o n the order of 500-600. T h e y also contain a m u c h smaller aromatic system that appears to be kata-condensed as judged b y H / C value (24), the r i n g condensation index (30), a n d r i n g compactness (30). Investigations b y C N M R (24) a n d I R spectroscopy (36) reveal no l o n g paraffinic substituents. I n general, coald e r i v e d asphaltenes have 3 0 % - 5 0 % substitution at the p e r i p h e r a l position of the aromatic system. Consequently, the asphaltene is expected to b e more reactive. Both the phenolic a n d p y r r o l i c groups are abundant i n coal-derived asphaltene (32). Structures I a n d II, utilized for the calculation of the last t w o c o l u m n s of T a b l e III, are shown i n F i g u r e 4. These structures are consistent w i t h those reported i n recent literature (46). a r u

a r


1 3

M a c r o s t r u c t u r e . A very distinctive feature of coal-derived asphaltenes is that they appear to be associated both i n concentrated a n d d i l u t e d solutions, a n d , particularly, i n nonpolar solvents. T h i s fact is supported b y the viscosity measurement (40). T h e fitting of a m o d e l of m o n o m e r - d i m e r - t r i m e r for the c o a l - d e r i v e d asphaltenes is almost perfect (34). T h e h y d r o g e n - b o n d i n g nature of the interaction w i t h i n the coal-derived asphaltenes is important (35, 41, 42). T h e other interaction is still the π-π association. X - r a y data give a l o w value of

Table III. Examples of the Microstructure of a Coal-Derived Asphaltene Structural Parameters M %C %H %N %S

%o x/c

Η ; ( Η / Η , K) Γ

Α

Η (Η /Η) H α

/.

β

α

0

Methods VPO elemental analysis elemental analysis elemental analysis elemental analysis elemental analysis elemental analysis NMR NMR NMR C NMR NMR NMR NMR C NMR C NMR C NMR densimetric densimetric GPC x-ray IR IR refractive index 13

iwcuivcj

σίΟ,/Η,) n(C /CJ a — Me β - Me Ν C/R F/C M s

13

13

13

A

%Oo_H

%N .„ N

Empirical formula

Calculated Calculated / Reference Experimental (25) (23) (23) (23) (23) (23) (11) (24) (24) (24) (24) (24) (24) (24) (30) (30) (30) (30) (30) (31) (28) (32) (32) (39) (29)

561 87.27 6.51 1.63 0.66 3.93 0.044 0.31 0.42 0.25 0.69 0.67 0.45 1.6 1.91 1.3 1.9 6.57 -0.08 548 10.3 A 2.67 0.93 14 Synthoil asphaltene

549 85.26 6.38 2.40 0 5.83 0.077 0.29 0.40 0.29 0.69 0.71 0.47 1.5 2 1 2 6.5 0 549 7.0 A 2.91 0 14 1

559 88.80 6.62 2.50 0 2.86 0.048 0.30 0.43 0.27 0.68 0.66 0.47 1.6 2 1 2 5.9 0.036 559 7.4 À 2.86 2.50 13

1

I

C^NO,

CuH^ON


4.

YEN

ACID / NEUTRAL


47

Structural Differences Between Asphaltenes H

Figure 4. Hypothetical structures for the Synthoil asphaltenes of both acid/ neutral and amphoteric fractions

three to four layers, probably because of the small groups of aromatic rings (29, 34). T h e support for the π-π association is best exemplified b y the charge-transfer properties of I a n d T C N E (33), along w i t h their E S R properties. Structural parameters for the characterization of the macrostruc ture of coal-derived asphaltenes are shown i n T a b l e I V . T h e weak linkage i n coal a n d coal-derived products m a y be attributable to excessive inter- a n d intramolecular b o n d i n g (41). T h e fact that the solubili zation a n d swelling curves are similar is important (43). T h i s type of b o n d i n g m a y increase its extent of crosslinking so m u c h that carboid (or preasphaltene) is no longer benzene-soluble. So far, there is no exciton interaction for an exponential temperature dependence of the spins for E S R work (37, 44). T h e r e has been l i m i t e d work on the long-range order of p e t r o l e u m - d e r i v e d asphaltene. B y using the shadowing technique of T E M , the height of the synthoil carboid was f o u n d to be 10-30 Â. This order of discontinuity is not 2

T a b l e IV. P a r a m e t e r s for the C h a r a c t e r i z a t i o n of the M a c r o s t r u c t u r e of C o a l - D e r i v e d Asphaltenes and Related Derivatives Structural Parameter L

c

h W

Methods x-ray x-ray charge-transfer association TEM TEM

Reference

Value

(29) (29) (33) (34) (39) (39)

10.5-14 Â 4 5/2 3 10-30 Â 100-200 Â

American Chemical Society Library In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

48


unreasonable,

i n v i e w of the 2 0 - Â b a n d i n coal a n d i n the pyrine-soluble

f r a c t i o n of vitrinite as an intercrystallite distance (45). Summary and Conclusion T h e particular example discussed above typifies the asphaltenes so far studied. T h e f o l l o w i n g general statements c a n be deduced f r o m the average structure so far obtained (47). 1. T h e aromaticity of petroleum-derived asphaltene (/ = 0 . 2 0.5) is lower than that of coal-derived asphaltene (/ = 0.6-0.7). 2. T h e aromatic r i n g systems w i t h i n petroleum-derived asphaltene are m u c h more condensed ( H / C = 0.3-0.5, w h i c h is peri) than those of coal-derived asphaltene ( H / C = 0.5-0.7, w h i c h is kata). 3. T h e substituents of the petroleum-derived asphaltenes are longer (n = 4-6) than those of coal-derived asphaltenes (n = 1-2). 4. T h e aromatic system of petroleum-derived asphaltene is extensively substituted (50%-70%) whereas the coal-derived asphaltene is sparingly substituted (30%-50%). 5. T h e molecular weight of petroleum-derived asphaltene is about ten times higher than that of the coal-derived asphaltene. U n i t molecular weight of coal asphaltene is 4 0 0 - 6 0 0 whereas that of petroleum asphaltene is 800-2500. 6. P e t r o l e u m - d e r i v e d asphaltene is less reactive to physical or c h e m i c a l agents than is coal-derived asphaltene. 7. Petroleum-derived asphaltene is more h i g h l y associated ( M e = 5-7) than is coal-derived asphaltene ( M e = 2-4); this w i l l be reflected i n the ease of processing. 8. T h e aromatic system i n coal-derived asphaltene is small ( L = 7-14 Â ) , c o m p a r e d w i t h that of petroleum ( L = 10-15 Â ) . 9. Petroleum-derived asphaltene is less polar than coal-derived asphaltene ( X / C for coal-derived asphaltene is about 0.08, a n d for petroleum-derived asphaltene is 0.05). 10. C o a l - d e r i v e d asphaltene contains more h y d r o x y l a n d p y r r o l i c groups i n a d d i t i o n to ether-oxygen or basic nitrogen functions. 11. T h e h i g h polarity a n d l o w association of coal-derived asphaltenes c a n be used to explain the nature (hydrogen-bonding) a n d reactivity of coal conversion. 12. T h e charge-transfer nature of donor-acceptor properties of petroleum asphaltenes c a n influence the processing of petroleum. a


a

a r u

a r

a r u

a r

a

a

Acknowledgment Support f r o m the U . S . D e p a r t m e n t of E n e r g y , under contracts E X 76-01-2031, E F - 7 7 - G - 0 1 - 2 7 3 8 , a n d E T - 7 8 - G - 0 1 - 3 3 7 9 , a n d f r o m Gas Research Institute, under contract 5010-362-0036, is acknowledged.


4.

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49


Glossary A r o m a t i c Sheet W e i g h t f r o m Mass Spectrometry

A

m

C

N u m b e r of Carbons

C/R C

N u m b e r of A r o m a t i c Carbons

A

Car

C

C

A r o m a t i c Carbons P e r Structural U n i t

C Downloaded by UNIV MASSACHUSETTS AMHERST on April 18, 2013 | http://pubs.acs.org Publication Date: January 1, 1982 | doi: 10.1021/ba-1981-0195.ch004

R i n g Condensation Index

a u

C

N u m b e r of M e t h y l e n i c Carbons

s u

Chain Length

N

N u m b e r of N a p h t h e n i c Carbons

C

N u m b e r of Paraffinic Carbons

P

C

N u m b e r of Saturated Carbons ( C = C + C

s

s

A

A r i t h m e t i c M e a n D i a m e t e r of Particles L o w A n g l e Scattering Distance

e

G

G e o m e t r i c M e a n D i a m e t e r of Particles

F

N u m b e r Interval Quaternary C a r b o n i n A r o m a t i c System

D F/C

= C — C )

M i c e l l e or Particle D i a m e t e r

A

D

N

N u m b e r of Substitutable Carbons i n A r o m a t i c System

s u

D D

p

N u m b e r of Saturated M e t h y l Carbons

Qme C

A

M e t h y l Content

m p

m

C

= C , another converter

N u m b e r of M e t h y l Carbons

m e

Cme/C C p/C

a r

Degree of F u s i o n

A

/

a

/

n

/

p

/

A r o m a t i c i t y (/ = C / C ) a

A

Naphthenicity Parafhnity F r a c t i o n of Saturated C a r b o n to T o t a l C a r b o n ( / = / + / = 1 -

s

s

n

p

fa)

Cluster H e i g h t

h H

N u m b e r of H y d r o g e n

h

a

H y d r o g e n A r o m a t i c i t y (/i = H / H )

H

A

N u m b e r of A r o m a t i c H y d r o g e n s

H

a

N u m b e r of α-Substituted H y d r o g e n s Next to a n A r o m a t i c System

a

A

H °

H ° = H /H

H ° ar

H ° = H / H= /i

H

a r u

N u m b e r of F u l l y Substitutable H y d r o g e n s i n the A r o m a t i c System

a

a

Hi H,/C

H, = H

A

a

a r u

Extent of Condensation

A

H

a

ar

N u m b e r of N a p h t h e n i c H y d r o g e n s

N

Ho°

H ° = 1 — ( H ° -ι- Η 0

Hp H

α

β

)

N u m b e r of Paraffinic H y d r o g e n s N u m b e r of M e t h y l e n i c H y d r o g e n s

R

s

H y d r o g e n N o n - A r o m a t i c i t y (h = 1 — /i )

s

N u m b e r of Saturated H y d r o g e n s ( H = H

/

Inhomogeneity L e n g t h

h H

ar

a

s

s

N

+ H ) P


50

CHEMISTRY OF

ASPHALTENES

L a y e r D i a m e t e r of the A r o m a t i c System κ κ M M M

a

e

a-Me β-Me M Downloaded by UNIV MASSACHUSETTS AMHERST on April 18, 2013 | http://pubs.acs.org Publication Date: January 1, 1982 | doi: 10.1021/ba-1981-0195.ch004

f

Afi

n N N %N _„ O N

%Oo_ R R H

' M

Coherence L e n g t h Cluster D i a m e t e r V P O Molecular Weight Association L a y e r Effective L a y e r N u m b e r of α-Substituted M e t h y l s N u m b e r of ^-Substituted M e t h y l s M o l e c u l a r O b t a i n e d F i l m Balance Charge-Transfer Layer Average C h a i n L e n g t h (n = C / C ) s

s u

N u m b e r of Nitrogens Oligomer Number I m i n o Content N u m b e r of Oxygens Phenolic H y d r o x y l Content N u m b e r of Rings Radius of G y r a t i o n Radius of Particle i n Sedimentation

X σ

N u m b e r of Sulfurs W i d t h of Clusters N u m b e r of Heteroatoms (X = S + Ν + Ο) Degree of Substitution (σ = C /Hi)

%

W e i g h t Percent

S W

m

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Lee, W. C.; Schwager,I.;Yen, T. F. Anal. Chem. 1977, 49, 2363-2365. Dickie, J. P.; Yen, T. F. Anal. Chem. 1967, 39, 1847-1852. Erdman, J. G.; Hanson, W. E.; Yen, T. F. J. Chem. Eng. Data 1961, 6, 443-448. Erdman, J. G.; Yen, T. F. Am. Chem. Soc., Div. Pet. Chem., Prepr. (Wash. D.C., Mar., 1962) 7(3), 99-111. Erdman, J. G.; Pollack, S. S.; Yen, T. F. Anal. Chem. 1961, 33, 1587-1594. Erdman, J. G.; Yen, T. F. Am. Chem. Soc., Div. Pet. Chem., Prepr. (Wash. D.C., Mar., 1962) 7(1), 5-18. Dickie, J. P.; Yen, T. F. J. Inst. Pet. 1968, 54, 50-53. Yen, T. F. Energy Sources 1974, 1(4), 447-463. Erdman, J. G.; Ramsey, V. G. Geochim. Cosmochim. Acta 1961, 25, 175. Yen, T. F. Nature Phys. Sci. 1971, 233(9), 36. Sprang, S. R.; Yen, T. F. Geochim. Cosmochim. Acta 1977, 41, 1007-1018. Yen, T. F., presented at the 13th Biennial Conf. Carbon, Irvine, CA 1977, 322. Dickie, J. P.; Haller, N. M.; Yen, T. F. J. Coll. Interface Sci. 1969, 29, 475. Sill, G. Α.; Yen, T. F. Fuel 1969, 43, 161. Yen, T. F. Energy Sources 1978, 4(3), 339. Tynan, E. C.; Yen, T. F. Fuel 1969, 43, 191. Yen, T. F. Fuel 1973, 52, 93. Yen, T. F.; Young, D. K. Carbon 1973, 11, 33. Erdman, J. G.; Saraceno, A. J.; Yen, T. F. Anal. Chem. 1962, 34, 694. Boucher, L. J.; Dickie, J. P.; Tynan, E. C.; Vaughan, G. B.; Yen, T. F. J. Inst. Pet. 1969, 55, 87.



4. YEN


51

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Structural Differences Between Asphaltenes Isolated from Petroleum

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