Chemistry of Asphaltenes - American Chemical Society

10 Molecular Interactions Involving Coal-Derived Asphaltenes KRISHNA C. TEWARI and NORMAN C. LI 1

Downloaded by IOWA STATE UNIV on March 9, 2017 | http://pubs.acs.org Publication Date: January 1, 1982 | doi: 10.1021/ba-1981-0195.ch010

Department of Chemistry, Duquesne University, Pittsburgh, PA 15219

Interaction of quinoline (Qu) with coal-derived asphaltenes (A), acid/neutral (AA) and base (BA) components of A, silylated asphaltenes A(TMS), and pentane-soluble heavy oil (HO) fractions, obtained from same feed coal, in solvent benzene has been studied by calorimetric method. The linear variation of molar enthalpy (for a 1:1 complexation) with the phenolic oxygen content of the fractions has been attributed to the dominance of hydrogen bonding effects involving phenolic OH over other types of molecular interactions. In Qu-A(TMS) systems, the degree of complexation largely depends on some entropy effects. For interaction between HO and asphaltenes in benzene, both viscosity and molar enthalpy change in the order BA > A > AA. These correlate with the NMR downfield shift of o-phenylphenol-OH signal as a function of asphaltene (A, AA, BA) concentration and suggest hydrogen bonding involving largely phenolic OH as a mechanism by which asphaltene-HO interactions are achieved. Other measurements with amine mixtures indicate that hydrogen bonding involving phenolic-OH and nitrogen bases gives rise to proton-transfer complexes, which partially accounts for the high viscosity and non-Newtonianflowof the coal liquids.

T

he h i g h viscosity at ambient temperature of coal liquids d e r i v e d f r o m hydrogénation processes has been related to the asphaltene (toluenesoluble, pentane-insoluble) a n d preasphaltene (toluene-insoluble, p y r i d i n e soluble) fractions (1-5). A l t h o u g h the effect of preasphaltene concentration on the viscosity of coal liquids is d r a m a t i c , the increase caused b y asphaltene materials has been attributed to h y d r o g e n - b o n d i n g (6) a n d acid-base salt Current address: Air Products and Chemicals, Inc., Box 538, Allentown, PA 18105.

1

0065-2393/81/0915-0173$05.00/0 © 1981 American Chemical Society Bunger and Li; Chemistry of Asphaltenes Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

174

CHEMISTRY OF ASPHALTENES

f o r m a t i o n (2, 4) interactions. W e report here c a l o r i m e t r i c a n d N M R results on the h y d r o g e n - b o n d i n g interaction i n v o l v e d i n coal-derived subfractions a n d its effect on the viscosity of coal liquids.


Experimental Centrifuged liquid product (CLP) samples, FB53 batch 1 and 59, and FB57 batch 42 were made in the Process Development Unit at Pittsburgh Energy Technology Center, under different process conditions (7) from the same feed coal, Kentucky hvAb, from Homestead Mine. SRC II process solvent (referred to as SRC II) was prepared by Pittsburgh and Midway Coal Company, from Kentucky #9 and #14 blend bituminous coal. The isolation of toluene-insoluble (TI), asphaltene (A), and heavy oil (HO) from C L P was accomplished by solvent fractionation based upon solubility in toluene and pentane (7). The A fraction was further separated into acid/neutral (AA) and base (BA) components by bubbling hydrogen chloride gas through a stirred toluene solution (8). The same method was used to separate SRC II into acid/neutral (SRC II-acid) and base fractions. A 2:1 (v/v mixture of hexamethyldisilazane and N-trimethylsilyldiethylamine was used for the hydroxyl silylation of asphaltene (A, A A , BA) and H O samples (9). N M R integration and elemental analysis data were used to calculate the phenolic oxygen content of these fractions. The Bolles and Drago (10) calorimetric method was used to determine simultaneously the molar enthalpy, ΔΗ°, and equilibrium constant, K , for a 1:1 complexation in a donoracceptor type of reaction A + Β C. The applicability of the calorimetric method and reliability of the assumed 1:1 complex in systems involving coal-liquid fractions have been described previously (7, 8, 9).

Results and Discussion T h e viscosities (11) at 355 Κ of the three C L P samples, F B 5 3 - 1 , F B 5 7 - 4 2 , a n d F B 5 3 - 5 9 d e r i v e d f r o m the same feed coal, were 25.1, a p p r o x i m a t e l y 128, a n d greater than 700 saybolt seconds, respectively. T h e results of solvent fractionation, ultimate analyses, a n d molecular weight determinations ( V P O ) of the fractions are g i v e n i n T a b l e I (7, 8). T h e N M R proton distribution i n carbon disulfide solutions a n d calculated structural parameters of the frac tions, using B r o w n a n d L a d n e r (12) equations, are shown i n T a b l e II (7). T h e results indicate that the lower viscosity l i q u i d product contains a lower weight percent of T I , A a n d a higher weight percent of B A i n A . T h e A , A A , B A a n d H O fractions isolated f r o m l o w viscosity l i q u i d contain lower oxygen contents than do fractions f r o m h i g h viscosity l i q u i d product. F u r t h e r m o r e , i n agree ment w i t h the trend of C / H ratio (see T a b l e I), the structural parameters such as aromaticity, / , a n d aromatic/benzylic h y d r o g e n ratio are the same for the fl

three H O fractions a n d decrease i n the order of F B 5 3 - 1 > F B 5 7 - 4 2 > F B 5 3 - 5 9 for the A fractions.

Quinoline Interaction With A, AA, ΒA and HO Fractions T h e phenolic oxygen contents i n A , A A , B A a n d H O fractions a n d the thermodynamic

parameters

of their interactions

w i t h q u i n o l i n e (Qu) i n

solvent benzene are s u m m a r i z e d i n T a b l e III (IS). F o r a g i v e n system, Q u + A

Bunger and Li; Chemistry of Asphaltenes Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

10. TEWARI AND Li

175

Asphaltenes Molecular Interactions

T a b l e I. W e i g h t D i s t r i b u t i o n a n d U l t i m a t e A n a l y s i s of C o a l - L i q u i d Fractions' 1

Source

Fraction

FB53-1

TI A HO A (TMS) TI A AA BA (HCl-free) HO A (TMS) TI A AA BA (HCl-free) HO A (TMS)

FB57-42


Weight Percent Ash Free

FB53-59

Molecular Weight —

5.6 19.0 75.4 —

680 240 770

9.3 28.4 13.3 15.1

530 430 680

—

C/H

O/C

1.45 1.25 0.84

0.055 0.023 0.010 0.026 0.019 0.003 0.014 0.008 0.002

1.33 1.12 1.06 1.18

0.082 0.036 0.041 0.030

s/c

N/C

0.028 0.020 0.012 0.030

0.033 0.004 0.003 0.005

260 660

0.85

0.025 0.011 0.003

10.4 33.3 17.7 15.6

—

740 620 950

1.36 1.10 0.97 1.08

0.120 0.038 0.049 0.037

0.84

0.028 0.010 0.003

0.86 0.86

0.027 0.013 0.002 0.027 0.008 0.001

290 830 190 210

56.3 —

100 90.7

SRC II-Acid

Ratio

—

62.3

SRCII

Atomic

0.024 0.020 0.011 0.028

0.051 0.004 0.004 0.003

Ref. (7, 8).

e

T a b l e II. P r o t o n D i s t r i b u t i o n a n d S t r u c t u r a l P a r a m e t e r s of C o a l - L i q u i d F r a c t i o n s (7) A r e a Percent N M R Spectra Aromatic Source

Fraction

FB53-1 FB57-42 FB53-59

A HO A HO A AA BA HO

(HJ 38.8 23.7 36.5 25.8 32.6 25.6 32.9 23.8

Benzylic"

(H ) a

30.3 31.1 33.5 32.9 34.5 22.9 32.4 31.9

Aliphatic* (Ho)

30.9 45.2 30.0 41.3 32.9 51.6 34.7 44.3

fa

σ

0.75 0.55 0.72 0.56 0.70 0.62 0.69 0.54

0.32 0.41 0.36 0.42 0.40 0.39 0.38 0.43

"Separation point between H and H„ chosen at 1.94 ppm from TMS. a


H

a u

/C

a

0.61 0.87 0.72 0.93 0.70 0.70 0.71 0.93 Fuel

176


a n d Q u + H O , the values of the e q u i l i b r i u m constant, K, are the same w i t h i n experimental error w h i l e the m o l a r enthalpies of interaction, Δ Η ° , increase m a r k e d l y w i t h the increase i n oxygen content of the c o a l - l i q u i d fraction (in the order F B 5 3 - 1 < F B 5 7 - 4 2 < F B 5 3 - 5 9 ) . C o a l - l i q u i d fractions are complex mixtures of substituted heterocyclic aromatics; therefore, the observed values of Κ a n d AH° correspond to the total interaction i n v o l v i n g h y d r o g e n - b o n d i n g a n d other types of r a p i d l y reversible intermolecular interactions. Since C / H ratio, aromaticity ( / J , a n d other structural parameters are the same for the three H O fractions (see T a b l e II), the π-bonding contributions to the observed ΔΗ° values of Q u + H O systems, to a large extent, w o u l d be the same. Since the C / H ratio a n d f

a

decrease i n the order of F B 5 3 - 1 > F B 5 7 - 4 2 > F B 5 3 - 5 9


for the A fraction, the c o m p u t e d Κ values for the interaction of Q u w i t h A , A A , a n d Β A fractions are almost the same, a n d the C / H ratio decreases i n the order B A > A > A A , the observed rectilinear variation of Δ Η ° w i t h the phenolic oxygen content of c o a l - l i q u i d fractions, F i g u r e 1 (IS), c o u l d be attributed to the d o m i n a n c e of h y d r o g e n - b o n d i n g effects, i n v o l v i n g phenolic O H , over other types of m o l e c u l a r interactions i n solution. In the absence of phenolic O H , the calculated Κ values for the interaction of Q u w i t h silylated asphaltenes [ A ( T M S ) , T a b l e III] are not the same a n d are quite small c o m p a r e d w i t h those observed for the Q u + A system. In the Qu

+

A ( T M S ) systems the results indicate that although the strength of

interaction (AH°) is appreciable, the degree of c o m p l e x a t i o n largely depends u p o n some unusual entropy effect. It is difficult to speculate o n the factors influencing entropy; however, the observed Δ Η ° values increase w i t h decreasTable III. Hydroxyl Distribution in Coal-Derived L i q u i d Fractions and Thermodynamic Parameters of Their Interaction with Quinoline in Solvent Benzene at 298 ± 0.5 K e

Source

Fraction

FB53-1

FB57-42

FB53-59

e

A HO A (TMS) A AA BA HO A (TMS) A HO A (TMS)

Phenolic Oxygen (g/mol) 12.1 3.0 —

14.3 12.5 9.4 6.1 —

22.3 7.6 —

Constants 1

dm~ )

0.0515 0.0323 0.6309 0.0532 0.0549 0.0543 0.0328 0.2354 0.0585 0.0352 0.4553

3

(kj 14.98 4.23 13.31 16.92 14.74 11.77 7.49 14.14 26.02 8.28 10.84

mol' ) 1

± ± ± ± ± ± ± ± ± ± ±

Interaction

-AS°

—AH°

κ(mol

Thermodynamic of Quinoline

0.13 0.04 0.25 0.21 0.04 0.04 0.12 0.22 0.13 0.08 0.35

(jmol^K 25.5 -14.2 40.8 32.2 25.3 15.2 -3.3 35.5 63.6 0.0 29.8

Ref. (13).



10. TEWARI AND L i

Asphaltenes Molecular

5 10 15 20 25 Phenolic Oxygen Content (g) per mol Fuel

111

Interactions

Figure 1. Dependence of ΔΗ° on the phenolic oxygen content of the coal fraction: (A) A , (O) AA (U) B A , (·) HO (IS) y

i n g molecular weight (see T a b l e I) a n d , therefore,

decreasing size of the

A ( T M S ) " m o l e c u l e . " T h e decrease i n entropy of the Q u - A ( T M S ) complex w i t h the increase i n aromaticity,

of the A fraction (Table II) indicate the possible

restriction associated w i t h the Q u molecule j o i n i n g the larger polynuclear condensed aromatic f r a m e w o r k of the A ( T M S ) " m o l e c u l e . "

Interaction Between Asphaltene and Heavy Oil Fractions T h e relative viscosity (η ) changes for benzene solutions of H O (0.305M) Ί

at 293 K , w i t h a d d e d A A a n d B A isolated f r o m F B 5 7 - 4 2 , are shown i n F i g u r e 2 (Curves 2 a n d 5). T h e results indicate that at a g i v e n concentration (above 0 . 0 3 5 M ) a n d temperature, Β A has a larger effect o n viscosity than does A A .

0.04

0.08 0.12 [Asphaltene], M

0.16

Analytical Chemistry

Figure 2. Relative viscosity change with asphaltene concentration in C H and in 0.305M solution of HO in C H at 293 K. Sample FB57-42: (·) AA in C H , (O) AA in HO + C H , (A) BA in C H , (A) Β A in HO + C H ; Sample FB53-59: (*) AA in C H (9). 6

6

6

6

6

6

6

6

6

6


6

6

6

e

178


T a b l e IV. S u m m a r y of T h e r m o d y n a m i c C o n s t a n t s " at 298 ± 0.5 Κ (9)

-ΔΗ°

Κ Source

System

(dm

FB53-59

H O + A in C H H O + A A in C H H O + B A in C H

3

6

6

6

6

6

6

mol' )

(kjmol' )

1

1

9.3 9.1 9.4

19.12 ± 0.57 15.02 ± 0.48 25.90 ± 0.78

"Uncertainties in ΔΗ° values are standard deviations. Error in Κ is about 10%. Analytical Chemistry

Since coal-derived asphaltenes are k n o w n to associate even i n d i l u t e solution


and, more significantly, i n nonpolar solvents (14, 15), the effect of m o l e c u l a r size on the observed viscosity can be seen q u a l i t a t i v e l y f r o m the variation of η

Ί

w i t h concentration of a d d e d asphaltene fraction i n pure benzene ( F i g u r e 2, C u r v e s 1, 3 a n d 4). A t a g i v e n temperature

a n d concentration, η

Ί

varies

linearly w i t h the molecular weight of the a d d e d fraction, a n d for a g i v e n fraction ( A A or B A ) the value of η i n benzene is smaller than that for the same Ί

fraction i n benzene c o n t a i n i n g H O . F u r t h e r m o r e , i n the presence of H O a n d at a g i v e n concentration of asphaltene, the increase i n η for B A is larger than Ί

that for A A . Since solvent benzene is less polar than benzene c o n t a i n i n g H O , it is assumed that i n a d d i t i o n to the molecular weight of the a d d e d asphaltene fraction, part of the effect on viscosity is attributable to f u n c t i o n a l groups such as phenolic or alcoholic h y d r o x y l as w e l l as a c i d i c N H groups, w h i c h serve as h y d r o g e n donors i n intermolecular association. T h e contribution i n v o l v i n g p y r r o l type of i m i n o groups as h y d r o g e n donors is negligible since the p X

a

of

phenol a n d p y r r o l i n aqueous solution at 293 Κ are 9.89 a n d 15, respectively. T h e t h e r m o d y n a m i c constants for the interaction of A , A A , a n d B A w i t h H O i n benzene are s u m m a r i z e d i n T a b l e I V . T h e c o m p u t e d Κ values, w i t h i n experimental error, are the same w h i l e Δ Η ° increases m a r k e d l y w i t h the increase i n molecular weight a n d nitrogen content a n d the decrease i n oxygen content of the asphaltene fraction (in the order A A < A < B A ) . T h e c o m p u t e d Κ values also show a direct correlation w i t h the viscosity results shown i n F i g u r e 2. Since the C / H ratio a n d N M R structural parameters of both A a n d Β A (Table II) are the same, the 7r-bonding contribution to the observed

AH°

values of H O interaction w i t h A a n d w i t h B A w o u l d be largely the same. T h e observed increase of Δ Η ° value i n the order A A < A < B A , therefore,

is

attributable to the degree of h y d r o g e n - b o n d i n g basicity of these fractions. In a d d i t i o n , the h y d r o g e n - b o n d i n g interactions

i n v o l v i n g phenolic

hydroxyl

protons can be seen f r o m the N M R d o w n f i e l d c h e m i c a l shift of the phenolic O H signal of o - p h e n y l p h e n o l ( O P P ) . O P P is a m o d e l used to study phenolic O H i n H O as a f u n c t i o n of a d d e d aslphaltene (A, A A , B A ) i n solvent C S

2

( F i g u r e 3). T h e observed shift of the O P P - O H signal at a g i v e n asphaltene (A, A A , B A ) concentration is f a i r l y large, d o w n f i e l d i n the order B A > A > A A , and correlates w e l l w i t h the viscosity a n d c a l o r i m e t r i c results reported above. F u r t h e r m o r e , a d d i t i o n of silylated asphaltene, A ( T M S ) , into 0 . 2 M O P P i n C S


2


10. TEWARI AND Li


179

Interactions

Figure 3. Chemical shift changes at 60 MHz of the proton-OH resonance of o-phenylphenol (0.2M) as a function of asphaltene concentration in CS solu tion; added asphaltene: (O) A , ( · ) A A , (β) B A , ( A ) A(TMS), (w) AA(TMS) (9). 2

( F i g u r e 3) moves the O P P - O H signal considerably d o w n f i e l d c o m p a r e d w i t h that observed for A a d d i t i o n . T h i s is expected because the r e m o v a l of phenolic hydrogens i n silylation w o u l d leave a n appreciable amount of basic r i n g nitrogens available f o r association w i t h O P P - O H protons. T h e above qualitative correlation of viscosity, calorimetric, a n d N M R results suggests that, i n coal-liquids, asphaltene

a n d pentane-soluble H O

fractions are associated i n t e r m o l e c u l a r l y through h y d r o g e n b o n d i n g i n v o l v i n g largely phenolic hydrogens as proton donors. T h e h y d r o g e n b o n d i n g is, i n part, responsible f o r the increase of viscosity of the product o i l .

Influence of Hydrogen Bonding on the Viscosity of Coal Liquids T o further investigate the effect of h y d r o g e n b o n d i n g i n v o l v i n g phenolic O H o n the viscosity of coal-liquids, w e have c o m p a r e d viscosity data f o r c o a l - l i q u i d - a m i n e systems w i t h those for a m o d e l system,

o-cresol-amine

mixtures. T h e results are shown i n F i g u r e 4. I n systems i n v o l v i n g acid-base interaction ( — O H — N ) , the viscosity of the mixtures as a f u n c t i o n of o-cresol or S R C - I I - A c i d mole fraction shows a single m a x i m u m . O u r results are s i m i l a r to those reported b y F e l i x a n d Huyskens (16) f o r mixtures of phenol a n d aliphatic amines, where viscosity, electric c o n d u c t i v i t y , a n d v o l u m e contrac tion have been correlated w i t h the size a n d concentration of ionic protontransfer complexes ( — O " — H N R ) . N o m a x i m u m i n the viscosity vs. mole 3

fraction curve was observed f o r the S R C II + E t N system because b o n d e d 3



180


0.2 0.4 0.6 Q8 Mole Fraction of Phenol

0.2 0.4 0.6 0.8 Mole Fraction of Coal-liquid

1.0

Fuel Processing Technology

Figure 4. (a) Viscosity, in cp, as a function of mole fraction of the phenol at 293 K: (A) anisole and Qu, (Π) o-cresol and CC1 , (·) o-cresol and Qu, (O) o-cresol and Et N. (b) Viscosity, in cp, as a function of the mole fraction of the coal liquid at 298 Κ: (M) SRC II and Et N, (O) SRC-II-Acid and Et N; (·) SRC-II-Acid and Qu. (Insert) Effect of rate of shear (τ) on the viscosity (η) of SRC-II-Acid and Et N mixtures. Mole fraction of SRC-II-Acid: (a) 0.491, (b) 0.795, (c) 0.689 (17). 4

3

3

3

3

phenolic O H groups already exist i n the unfractionated l i q u i d ( S R C II). T h e l i q u i d mixtures show N e w t o n i a n behavior over the entire mole fraction region. H o w e v e r , i n S R C - I I - A c i d + E t N a n d S R C - I I - A c i d + Q u systems, 3

h y d r o g e n b o n d i n g i n v o l v i n g phenolic O H as proton donor not only defines the viscosity b u t also the plastic nature of these systems as shown i n F i g u r e 4b (insert).

F e l i x a n d H u y s k e n s (16) have shown that the size of the ionic

complexes i n the mole fraction region below the m a x i m u m is larger than that

T a b l e V . Number-Average M o l e c u l a r W e i g h t of S R C - I I - A c i d + E t N M i x t u r e s (17) 3

Mole Fraction

of SRC II-Acid

0.677 0.763 0.805 1.000

M

M

obs

250 230 220 210

a

cak

175 184 189 210

"Calculated from mole fractions and molecular weights of SRC-II-Acid (M = 210) and Et N (M = 101) assuming no complex formation. 3




10. TEWARI AND L i


181

Interactions

of the ionic complexes i n the mole fraction region above the m a x i m u m . T h e results of our molecular

weight d e t e r m i n a t i o n

of S R C - I I - A c i d +

Et N 3

mixtures at various mole fractions [Table V (17)] q u a l i t a t i v e l y indicate that the relative size of the molecules is larger at mole fractions below the observed m a x i m u m (at mole fraction 0.75) than above the m a x i m u m . T h e transition f r o m a h y d r o g e n - b o n d i n g complex ( Ο — Η

· · · Ν) to a

proton-transfer complex ( Ο " · · · H — N ) can be f o l l o w e d b y IR spectrosco +

py. T h e IR spectra of S R C - I I - A c i d a n d S R C - I I - A c i d + E t N m i x t u r e at 0.69 3

mole fraction of S R C - I I - A c i d are shown i n F i g u r e 5. A b s o r p t i o n bands at 2680 cm"

1

a n d 2500 c m " , ascribed to N — Η 1

+

absorption b a n d at 3 2 0 0 - 3 6 0 0 c m "

1

. . .

Ο (18,

19)

and a broad

clearly indicates the presence of both

O H · · · Ν a n d O " · · · H — Ν species i n the S R C - I I - A c i d + E t N system. +

3

T h e self-association of S R C - I I - A c i d is obvious f r o m the IR spectrum of neat SRC-II-Acid. F i g u r e 6 shows the IR spectra of C C 1 solutions of the S R C II + E t N a n d 4

SRC-II-Acid +

3

E t N mixtures. A d d i t i o n of E t N to S R C - I I - A c i d i n C C 1 3

3

results i n the appearance of strong N — Η . . . +

Ο bands (18,

4

19) at 2630,

2610, a n d 2500 c m " . O n the other h a n d , the a d d i t i o n of E t N to S R C II shows 1

3

no significant absorption bands i n the region 2 5 0 0 - 2 8 0 0 c m " . These results 1

indicate that h y d r o g e n b o n d i n g i n v o l v i n g largely phenolic O H a n d nitrogenc o n t a i n i n g bases yields proton-transfer complexes that are, i n part, responsible for the viscosity a n d n o n - N e w t o n i a n flow of the coal liquids.

Acknowledgments T h e authors thank the D e p a r t m e n t of E n e r g y for support of this w o r k under Contract N o . E Y - 7 6 - S - 0 2 - 0 0 6 3 . A 0 0 3 a n d D E - A C 2 2 - 8 0 P C 30252. W e thank B r a d l e y C . B o c k r a t h for h e l p f u l discussions.




182


Figure 6. Infrared spectra in CCl solutions, 5-mm cell (KBr): (a) SRC II (0.02M), Et N (0.01M); (b) SRC-II-Acid (0.02M), Et N (0.01M) (17). 4

3

3

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Chemistry of Asphaltenes - American Chemical Society

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