Chemistry of Asphaltenes - ACS Publications - American Chemical

11 Lewis Acids Assisted Degradation of Athabasca Asphaltene 1

Downloaded by TROY UNIV on October 16, 2014 | http://pubs.acs.org Publication Date: January 1, 1982 | doi: 10.1021/ba-1981-0195.ch011

T. IGNASIAK, J. BIMER, N. SAMMAN, D.S. MONTGOMERY, and O.P. STRAUSZ Hydrocarbon Research Centre, Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 Our objective was to determine the extent to which the Lewis acids AlCl and ZnCl promote degradation of Athabasca asphaltene into lower molecular weight species, and also to identify the chemical changes taking place. In contrast with ZnCl , AlCl degrades asphaltene to some extent at temperatures below 100° C. The solubility of the AlCl -treated asphaltene depends on the reaction medium, and this is interpreted in terms of the mechanism of typical Friedel-Crafts reactions. At higher temperatures (150°-400° C), ZnCl is a more suitable catalyst than AlCl for degradation. Below 250° C, ZnCl promotes insolubilization of asphaltene in benzene, and this is attributed to the formation of polar groups as a result of cleavage reactions. Above 250° C, conversion into low molecular weight pentane solubles improves. ZnCl promotes heteroatom removal, as shown by the decrease in heteroatom content of nonvolatile products with increasing reaction temperature. Hydrogen is essential to compensate for the dehydrogenating effect of ZnCl and to obtain optimal pentane solubility. 3

2

2

3

3

2

3

2

2

2

A

sphaltene is a complex, h i g h molecular weight fraction of petroleum. B e i n g a solubility class, it is an extremely heterogeneous mixture, w h i c h makes structural investigations on this material very difficult. F o r this reason, although a great deal of valuable i n f o r m a t i o n has been reported, m a n y structural details r e m a i n unsettled. T h e most significant aspects of petroleum asphaltenes are the nature of the carbon skeletal f r a m e w o r k , the types of c h e m i c a l functional groups i n w h i c h the heteroatoms appear, a n d the c h e m i cal nature of the bridges connecting the subunits of the molecule. These Current address: Institute of Organic Chemistry of the Polish Academy of Sciences, Warsaw, Poland. 1

0065-2393/81/0195-0183$05.00/0 © 1981 American Chemical Society In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

184

CHEMISTRY OF ASPHALTENES

structural parameters

together w i l l determine the behavior of

asphaltene

d u r i n g a g i v e n process. T o obtain a m o r e clearly defined p i c t u r e of these structural features a n d to establish the relationship between the c h e m i c a l structure of asphaltene a n d its reactivity under a variety of conditions, the potential of c h e m i c a l a n d t h e r m a l degradation reactions

as diagnostic tools has been studied.

The

specific subject of this investigation was the h i g h molecular weight, sulfur r i c h asphaltene f r o m the Athabasca b i t u m e n . D e g r a d a t i o n , as the t e r m implies, converts a h i g h molecular weight molecule into lower molecular weight species. fragmentation

b y the selective

cleavage

It is desirable to

achieve

of b r i d g i n g bonds, l e a v i n g the


backbone intact. In general, the selectivity of any degradation process depends largely on the specific structure a n d reactivity of a g i v e n c o m p o u n d ; however, i n the case of asphaltene the m u l t i p l e structural arrangements present i n the p o l y m e r drastically reduce the effectiveness of a single reagent i n p r o m o t i n g selective degradation. Therefore, the best approach is to c r i t i c a l l y evaluate data that have been obtained b y methods as diverse as possible. W e have shown i n previous w o r k (1) that the m o l e c u l a r weight of Athabasca asphaltene can be significantly decreased d u r i n g reduction b y electron transfer f r o m naphthalene r a d i c a l anions p r o d u c e d i n situ b y treatment

of naphthalene

with

potassium i n tetrahydrofuran solution a n d subsequent octylation. T h e c h e m i c a l changes effected d u r i n g this type of reduction are generally considered to be cleavage of ether a n d thioether bonds, although the cleavage of certain methylene bridges also has been reported (2). Since the oxygen present i n this asphaltene has been shown to be mostly i n the f o r m of h y d r o x y l groups (J, 3), it was c o n c l u d e d that the reaction t a k i n g place i n Athabasca asphaltene is largely cleavage of c a r b o n - s u l f u r bonds. T h e presence of sulfide linkages has been further d o c u m e n t e d b y studies on the t h e r m a l degradation of

the

Athabasca asphaltene i n the presence of tetralin (4). T h e decrease i n molecular weight of the acetylated, m e t h y l a t e d , a n d silylated asphaltene, along w i t h a u x i l i a r y IR

evidence,

suggested

that

the h y d r o x y l groups are

strongly

i n v o l v e d i n intermolecular h y d r o g e n b o n d i n g . F r o m these results it was c o n c l u d e d that sulfide linkages a n d oxygenbased

h y d r o g e n b o n d i n g play i m p o r t a n t roles i n the molecular size of

asphaltene. T h e presence of the relatively weak sulfide bonds m a y e x p l a i n the relative ease w i t h w h i c h asphaltene

can be partly d e p o l y m e r i z e d under

moderate t h e r m a l conditions where extensive c r a c k i n g a n d coke f o r m a t i o n can be excluded (5). T h e reductive systems i n v o l v i n g s o d i u m or l i t h i u m i n l i q u i d a m m o n i a appeared to be less effective i n d e g r a d i n g asphaltene,

as

reflected b y only a moderate decrease i n the molecular weight (6, 7). M o d e l reactions showed that i n these cases certain sulfides, for example, dialiphatics, do not react.

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

11.

185

Degradation of Athabasca Asphaltene

IGNASIAK ET AL.

H o w e v e r , the l i t h i u m - r e d u c e d asphaltene exhibits an unusual t h e r m a l reactivity resulting i n a higher y i e l d of lower molecular weight

pentane-

soluble fraction a n d higher degree of desulfurization, as c o m p a r e d w i t h the o r i g i n a l asphaltene (5). A p p a r e n t l y this is attributable to the cleavage of the c a r b o n - s u l f u r bonds. Moreover, the i n t r o d u c t i o n of h y d r o g e n into the asphaltene molecule d u r i n g reduction leads to saturation produced during enhanced

thermolysis,

thereby

of the free

radicals

preventing repolymerization.

desulfurization o c c u r r i n g on t h e r m a l treatment of the

The

reduced

asphaltene m a y be related to the f o r m a t i o n of t h e r m a l l y unstable thiols d u r i n g reduction. T h e pentane-soluble portion of asphaltene t h e r m o l y z e d at 3 0 0 ° C has


been f o u n d to contain a f u l l c o m p l e m e n t of n-alkanes, a c y c l i c isoprenoids, small r i n g saturated a n d small r i n g aromatic compounds, t y p i c a l of unbiodegraded conventional c r u d e o i l (8). H o w e v e r , i n a l l cases o n l y a s m a l l fraction of the degraded asphaltene can be analyzed i n this manner, a n d therefore the question of the size a n d homogeneity of the r e m a i n i n g h y d r o c a r b o n units remains unresolved. T h e results of the d e p o l y m e r i z a t i o n studies such as metal reductions a n d low temperature solvolysis have shown that the m e c h a n i s m a n d the rate of degradation of asphaltene d e p e n d to a large extent on the c h e m i c a l e n v i r o n ment. It is reasonable to assume that the presence of a catalyst m a y accelerate the scission of g i v e n bonds, a n d thereby reduce the temperature

requirement

for degradation to the point at w h i c h secondary reactions l e a d i n g to extensive c r a c k i n g a n d p o l y m e r i z a t i o n are m i n i m i z e d . T y p i c a l L e w i s acids such as A l C l

3

and Z n C l

2

display excellent

catalytic

activity i n most F r i e d e l - C r a f t s type reactions (9, 10) a n d are also k n o w n to catalyze coal l i q u e f a c t i o n a n d solubilization (11-26).

Because of this latter

property, acid-catalyzed d e p o l y m e r i z a t i o n has become a useful m e t h o d for structural investigation of coal (27).

T h e rationale for solubilization is inter-

preted either i n terms of d e p o l y m e r i z a t i o n v i a r u p t u r e of the bridges or, since the overall reaction i n general is that of F r i e d e l - C r a f t s a l k y l a t i o n or acetylation, i n terms of the effect of the sidechains i n t r o d u c e d on the aromatic r i n g system. In this study the effects of the L e w i s acids A l C l

3

and Z n C l

2

o n the

degradation of Athabasca asphaltene have been investigated under various conditions i n c l u d i n g temperature, r a n g i n g f r o m 4 0 ° to 4 0 0 ° C , a n d pressures u p to 1800 psi of h y d r o g e n or nitrogen. O u r objective was to determine the extent to w h i c h these catalysts promote degradation into lower molecular weight species a n d also to i d e n t i f y the c h e m i c a l changes t a k i n g place, to c o m p l e m e n t our previous structural investigations. In particular it was desirable to establish those conditions that i m p r o v e the selectivity w i t h w h i c h various bonds are cleaved.


186


Experimental M a t e r i a l s . Athabasca asphaltene was prepared from an oil sand sample from the G C O S (now Suncor Inc.) quarry according to a standard procedure used in this laboratory (9). Solvents were refluxed over C a H and redistilled. Nitromethane was distilled and stored over molecular sieves. A l C l was sublimed and ground under nitrogen; Z n C l was dried at 110° C in a vacuum oven. E x p e r i m e n t s U n d e r R e f l u x . To a solution of asphaltene (2 g) dissolved in 100 m L of the required solvent, A l C l (2 g) was added and the reaction mixture was magnetically stirred for 17 h under nitrogen at reflux temperature. When nitromethane was used as the solvent, A l C l was dissolved in the solvent and added to a suspension of asphaltene in C H N 0 , and the reaction temperature was set to 70° C. After cooling the reaction mixture, 100 m L I N H C l was added and stirring was continued for 1 h. When a volatile solvent such as benzene or C H C l was used, it was removed in a Rotavapor. The resulting precipitate was filtered, washed extensively with water followed by a small volume of acetone, and dried first in a vacuum oven overnight at 60° C and then in a desiccator under high vacuum. When benzene was the solvent, the precipitate was very sticky, which made filtration extremely difficult. It was therefore flushed with acetone and dissolved in C H C l . The C H C l solution was dried over anhydrous N a S 0 and centrifuged for 30 min at 2500 rpm, to remove all inorganic matter. The solvent was then removed and the residue was dried overnight in a desiccator under high vacuum. 2

3

2

3

3

3

2


2

2

2

2

2

2

2

4

E x p e r i m e n t s i n a B a t c h S y s t e m . In the experiments with molten catalyst, asphaltene and catalyst were mechanically mixed and placed in a reactor. In the experiments using a solvent, the catalyst was first impregnated on asphaltene and suspended in the solvent. To impregnate asphaltene, the catalyst was dissolved in diethyl ether and the solution added to a solution of asphaltene in benzene. Solvents from the resulting suspension were removed in a Rotavapor and the residue was dried at 110° C in a vacuum oven. Reactions were conducted in a 300-cm , Magne Dash stirred autoclave equipped with a fitted glass liner. After the reactants were introduced, the system was flushed several times with a given gas, which was then adjusted to the required initial pressure. The valves were then closed, thus sealing the autoclave. The temperature sequence was a 30-45 min heating period to the desired reaction temperature, followed by a cooling period of about 3 h. The asphaltene-to-catalyst ratio was 1:1 by weight, and the actual proportions used were as follows: 3

1. 1 g each of asphaltene and A l C l in 50 m L benzene. 2. 2 g each of asphaltene and Z n C l in 70 m L solvent. 3. 3 g each of asphaltene and molten catalyst. 3

2

The reaction mixtures were treated with I N H C l and left overnight to allow the catalyst and metal complexes to decompose. The solvents were then distilled off and the precipitate was filtered, washed exhaustively with I N H C l , followed by 10% N a H C 0 and water. The precipitate was then dried at 60° C in vacuo. The crude products (nonvolatiles) were then Soxhlet extracted to yield the pentane-soluble fraction, the benzene-soluble fraction, and insoluble residue. The yield of gases was calculated from the weight difference between substrate and product. H y d r o c a r b o n C l a s s A n a l y s i s . The hydrocarbon class or type separation of the pentane-soluble fraction was performed on a silica gel-alumina column according 3


11.

IGNASIAK ET AL.

187



to system II of Sawatzky et al. (28) with the following modifications: the Pyrex glass column, 6 m m i.d. χ 50 cm long, fitted with a ground glass joint at the top, was packed in the upper half with a 4 g Merck silica gel 60 (70-230 mesh), activated at 250° C > 12 h, and the lower half packed with 5 g Woelm W200 alumina (activity grade super 1). The column was prewetted with pentane, then a 100-mg sample in 1 m L pentane was admitted onto the column. The eluents were forced downwards through the column as a result of the pressure exerted by weights attached to the barrel of a 50-mL syringe connected to the column via a Luer Lok needle and embedded in a Teflon plug fitting the column joint. The elution rate was kept constant at about 1 m L / m i n by varying the mass of the weights as necessary. The following solvents were then used for consecutive elution of the sample charges: Eluent Pentane 5% Benzene/pentane 15% Benzene/pentane Benzene

Volume 25 30 30 15

(mL)

Elution was then terminated, the column packing removed and Soxhlet extracted with benzene/methanol (60:40) to retrieve the residual portion.

Results and Discussion Reaction with A l u m i n u m Chloride. AlCl

3

Asphaltene reacts r e a d i l y w i t h

even at temperatures w e l l below 1 0 0 ° C . T h e characteristics of the

product, m a i n l y i n terms of solubility, are strongly influenced b y the reaction m e d i u m (Table I). Asphaltene treated w i t h A l C l

i n benzene remains f u l l y

3

soluble, w h i l e w h e n the r e m a i n i n g solvents are used as a reaction m e d i u m the solubility drops to a mere 10%-20%. T h e effect of solvent on the solubility of the product can be e x p l a i n e d on the basis of the general m e c h a n i s m of a t y p i c a l F r i e d e l - C r a f t s reaction. Thus, asphaltene reacts w i t h A l C l to f o r m intermediate c a r b o n i u m ions, w h i c h then 3

undergo electrophilic substitution. If substitution occurs w i t h i n the asphaltene molecule new bonds are f o r m e d a n d , d e p e n d i n g o n the size of

initial

fragments, the molecule m a y g r o w bigger a n d , thus, less soluble.

T a b l e I. C h a r a c t e r i s t i c s of the R e a c t i o n P r o d u c t of A s p h a l t e n e w i t h A l C l 3

Reaction

Medium

Benzene Dichloromethane Nitromethane Nitromethane/benzene Asphaltene

Yield

Solubility in CH Cl

(%)

(%)

106 102 103 102

2

2

H/C

99 23 12 16

1.14 1.09 1.15 1.12

100

1.22

Cl(%)

N(%

1.9 1.1 1.6 1.1

1.5 1.4


1.1


188

H o w e v e r , benzene is a strong nucleophile a n d is very l i k e l y to react w i t h the intermediate carbocations, thus p r e v e n t i n g t h e m f r o m u n d e r g o i n g i n t r a molecular interactions. N i t r o m e t h a n e , on the other h a n d , is a relatively inert solvent a n d the decrease i n solubility indicates that internal p o l y m e r i z a t i o n prevails. D i c h l o r o m e t h a n e itself reacts i n the presence of A l C l , a n d m a y take 3

part i n b i f u n c t i o n a l a l k y l a t i o n of asphaltene fragments w i t h the f o r m a t i o n of new methylene bridges, thus increasing the molecular size a n d l o w e r i n g the solubility. T h e use of a nitromethane-benzene

m i x t u r e was i n t e n d e d to

approximate the f u l l y homogeneous reaction a n d , therefore, the poor solubility of the product i n this case is difficult to explain. In each case, the H / C ratio is always lower than that of the substrate. T h e chlorine content does not exceed


2%. In benzene, small amounts of A l C l

3

(about 10% b y weight of asphaltene)

do not effect degradation a n d for reaction to occur, h i g h concentrations of A l C l (1:1 b y weight) are r e q u i r e d . T h e requirement for excessive amounts of 3

catalyst seems to be a result of poor contact between asphaltene a n d A l C l , 3

attributable to unusual properties of the a s p h a l t e n e - A l C l complex that m a k e 3

it precipitate out of the solution, thus t r a p p i n g most of the unreacted catalyst. T h e characteristics of the product are g i v e n i n T a b l e II. T h e product f r o m the reaction carried out i n benzene shows a decrease i n the average molecular weight, i n d i c a t i n g that degradation has taken place. T h i s was further c o n f i r m e d b y preparative G P C separation o n Bio-Beads SX-1.

T h e results, shown i n T a b l e III, show that the concentrations of the two

highest

molecular weight fractions of asphaltene,

their sum c o m p r i s i n g

a p p r o x i m a t e l y 69% b y weight vs. 45% i n A l C l - t r e a t e d asphaltene, have been 3

r e d u c e d b y one t h i r d . T h e l o w e r H / C ratio for the treated asphaltene is a c c o m p a n i e d b y an increase i n the amount of* aromatic hydrogen, as calculated f r o m the integration c u r v e of the * H N M R spectrum. T h e same reaction c a r r i e d out i n deuterated benzene showed that this increase comes f r o m incorporation of p h e n y l groups f r o m the solvent. T h e n u m b e r of p h e n y l groups introduced, calculated f r o m the carbon content

T a b l e I I . R e a c t i o n of A s p h a l t e n e w i t h A l C l Characteristics M o l e c u l a r weight (in benzene) H/C Sulfur (%) E x t r a c t a b i l i t y (wt %) pentane benzene H (%) Per 100 C atoms p h e n y l groups a d d e d h y d r o g e n atoms (loss) a r

3

before a n d after

in Benzene Treated

Asphaltene 1200 1-14 6.93 22 78 15 1.2 ~4


11.

IGNASIAK ET AL.

189


T a b l e I I I . G P C S e p a r a t i o n of the A l C l - T r e a t e d A s p h a l t e n e 3

Percent Fraction

Molecular

Weight

Distribution

Asphaltene

>6000 4000-6000

Product

43 26 69

26 19 45

reaction a n d also f r o m the aromatic h y d r o g e n increase, amounts to 1.2 groups per 100 carbon atoms or 1 g r o u p per molecule of reaction product. T h e h y d r o g e n balance showed a deficit of 4 h y d r o g e n atoms per 100 carbons. In other words, p h e n y l a t i o n is a c c o m p a n i e d b y dehydrogenation.


As a result of treatment asphaltene

with A l C l

became soluble i n pentane.

3

i n benzene,

m o r e than 20%

T h e differences

i n the

of

chemical

composition of the pentane a n d benzene fractions are shown i n T a b l e I V . T h e pentane-soluble fraction has a low molecular weight, a higher H / C ratio c o m p a r e d w i t h the o r i g i n a l asphaltene, lower sulfur content, a n d very low nitrogen content. Integration of the * H N M R spectrum suggests that about 5 p h e n y l groups are attached per 100 carbon atoms. A l t h o u g h polar materials constitute the major portion of the

pentane

solubles, saturates a n d aromatics are also f o u n d i n appreciable concentrations (Table V ) . T h e relatively low H / C ratio of the saturate f r a c t i o n suggests a h i g h concentration

of saturated

cyclics or olefins. T h e r m o g r a v i m e t r i c

analysis

i n d i c a t e d that o n l y about 40% of the pentane solubles is volatile under t y p i c a l gas chromatographic conditions. Under

the

same

conditions,

experiments

carried

out

with

model

compounds showed that b e n z y l sulfide underwent 100% conversion, the major product b e i n g d i p h e n y l m e t h a n e

(from b e n z y l c a r b o n i u m i o n attack on

benzene), whereas o n l y 23% a n d 26% conversion to mixtures of u n i d e n t i f i e d compounds took place for n - h e p t y l sulfide a n d b i b e n z y l , respectively. T h e effects of temperature, solvent, a n d pressure of a d d e d N or H 2

2

are

listed i n T a b l e V I . T a b l e I V . C h e m i c a l C o m p o s i t i o n of the P e n t a n e a n d B e n z e n e F r a c t i o n s of A l C l - T r e a t e d A s p h a l t e n e 3

Fraction Characteristics M o l e c u l a r weight (in benzene) H/C Sulfur (%) N i t r o g e n (%) H (%) P h e n y l groups per 100 C atoms a r

Pentane

Benzene

380 1.33 4.58 0.10 21 5.1

2400 1.10 7.44 1.03 n.d. n.d.°

"Not determined.


fl

190


Table V . Hydrocarbon Type Class Separation of the Pentane-Soluble Fraction

Fraction


H/C

Ν

S

15

340

1.66

0.0

0.1

15 8 11 51

260

1.17

0.0

2.6

1.20 1.34

0.2 0.3

6.3 6.0

Yield

0

Saturates Aromatics monodipolyPolars

Percent

Molecular Weight*

e

540 630

*% of the pentane solubles, 'in benzene. Tor the combined mono- and diaromatic fractions.

Increasing the temperature appeared to be of no advantage. T h e surpris i n g l y l o w H / C ratio of the product f r o m the reaction c a r r i e d out i n cyclohex ane points to a d v a n c e d dehydrogenation. M o r e o v e r , blank experiments i n d i cated that A l C l

3

promotes the decomposition of these solvents w i t h

the

f o r m a t i o n of a variety of products r a n g i n g f r o m d i - a n d triaromatics to insoluble polymers. T h i s results i n an unreasonably h i g h weight increase of the treated asphaltene. T o a v o i d undesirable side effects resulting f r o m the solvent reactivity, d r y asphaltene was treated w i t h A l C l melt, that is, at a temperature slightly above 3

the m e l t i n g point of A l C l

3

( 2 2 0 ° C ) under h y d r o g e n pressure. T h e results,

shown i n F i g u r e 1, illustrate the general t r e n d towards f o r m a t i o n of gases a n d insoluble products; the y i e l d of the f o r m e r increases r a p i d l y w i t h the t i m e of reaction. T h e reaction, i n terms of product yields a n d heteroatoms r e m o v e d , is

Table VI. Reaction w i t h A l C l

3

i n a Pressurized System Experiment

Number

1 Conditions temperature ( ° C ) solvent gas pressure (psig) Y i e l d (% of substrate) solid product E x t r a c t a b i l i t y (% of product) pentane fraction benzene fraction residue H / C of solid product

2

160 benzene 200 ( N )

300 cyclohexane 850 ( H )

127

158

28 44 28 0.96

13 32 55 0.83

2

2


11.

IGNASIAK ET AL.

191


Product distribution

Residue 1 3

Benzene fraction Pentane fraction

H/C ratio of fractions Original Asphaltene

1.2 h

1.0


H/C 0.2

0.6 4 Hrs

Figure 1. Degradation of asphaltene with molten

AlCl

3

m o r e or less complete i n four hours. T h e H / C ratios of the fractions d o not change w i t h reaction time, b u t they are distinctly lower than that of the o r i g i n a l asphaltene. T h e destructive decomposition of asphaltene that eventually leads to h i g h conversion into gases m a y arise f r o m i m p r o p e r contact between the s o l i d l i q u i d - g a s phases. T h e role of h y d r o g e n is to hydrogenate a n d stabilize the fragments p r o d u c e d u p o n catalyzed b o n d scission. H o w e v e r , since the system discussed above was not stirred, access of h y d r o g e n into the asphaltene molecule m i g h t have been l i m i t e d b y the layer of m e l t e d catalyst s u r r o u n d i n g the substrate, w h i c h w o u l d then be completely vulnerable to the h i g h c r a c k i n g activity of A l C l . 3

It is evident that, d e p e n d i n g on the conditions e m p l o y e d , A l C l — o n e of 3

the strongest L e w i s a c i d s — c a n promote m a n y secondary reactions i n v o l v i n g the p r i m a r y products f o r m e d d u r i n g asphaltene degradation. Thus, i n a d d i tion to i n i t i a l d e p o l y m e r i z a t i o n as a result of sulfide a n d methylene b o n d cleavages, inter- a n d intramolecular a l k y l a t i o n , dehydrogenation of hydroarom a t i c structures, aromatic condensation, a n d i n d i s c r i m i n a t e c r a c k i n g a l l m a y occur. T h e overall result is that the actual extent of degradation into condensible l o w molecular weight species is insignificant. Reaction with Zinc Chloride.

I n contrast to other L e w i s acids, Z n C l is 2

a relatively weak catalyst for hydrogénation a n d h y d r o c r a c k i n g of single r i n g aromatics (17, 29), a n d since it has been reported (SO) to be far more selective i n the cleavage of certain bonds than is A l C l , it w o u l d appear to be a very 3

attractive reagent f r o m the structural point of v i e w .


192

CHEMISTRY O F ASPHALTENES

Since Z n C l does not react w i t h asphaltene under the same conditions as those e m p l o y e d i n the case of A l C l , that is, below 1 0 0 ° C , the reaction was investigated at temperatures r a n g i n g f r o m 1 5 0 ° to 4 0 0 ° C i n a n autoclave. T h e merit of the batch autoclave system is that it is possible to conduct the reaction under various gaseous atmospheres a n d pressures and, i n a d d i t i o n , it permits a variety of solvents to be e m p l o y e d to control c h a i n propagation reactions. T o obtain o p t i m a l conversion of asphaltene into pentane-soluble materials, the presence of a solvent appeared to be essential to i m p r o v e the dispersion of Z n C l a n d facilitate agitation; otherwise, i n the absence of solvent, the interaction of asphaltene w i t h Z n C l melt at 3 0 0 ° C l e d to advanced decomposition to gaseous products similar to the situation observed w h e n the A l C l melt was used. H o w e v e r , Z n C l appeared to be significantly m o r e efficient than A l C l i n converting heteroatoms into gaseous products, the best examples of w h i c h are illustrated i n F i g u r e 2, where it is seen that a n appreciable though slow denitrogenation a n d r a p i d a n d substantial d e s u l f u r i zation take place w i t h time. 2

3

2

2

3

2


3

T h e disadvantage of the closed system is that l o w molecular weight p r i m a r y scission products, attributable to a l o n g residence time, m a y undergo secondary c r a c k i n g a n d p o l y m e r i z a t i o n reactions. I n a d d i t i o n , some of the l o w molecular weight oils m a y be lost u p o n evaporation of the solvent. Therefore, i n this type of experiment, emphasis is placed o n the overall product

Sulfur

Oxygen

••

Nitrogen

(1)

Sulfur Nitrogen Oxygen

20

(2)

0

4

8

Hrs

Figure 2. Degradation of asphaltene with molten Lewis acids (H 850 psig); heteroatom removal: (1 ) AlCl (220° C); (2) ZnCl (300° C) 2

3

2


11.

IGNASIAK ET AL.

193


a, 60

Pentane fraction Volatiles

Insoluble residue Benzene fraction 150

200

250

300

400

350

Figure 3. Effect of temperature on the product distribution with ZnCl catalyst


2

distribution a n d the behavior of h i g h molecular weight components, w h i l e the c h e m i c a l nature of the l o w molecular weight materials is more closely evaluated i n a parallel series of experiments c a r r i e d out i n a flow system (31). T h e latter appears to be more suitable for mechanistic a n d structural studies, since most of the volatile products c a n be c a r r i e d a w a y f r o m the reaction zone i n the carrier gas. A l t h o u g h benzene is a better solvent for asphaltene than cyclohexane, a n d the product distribution obtained f r o m the hydrogénation

of Athabasca

asphaltene i n benzene a n d cyclohexane is almost the same, experiments w i t h m o d e l compounds showed that c a r b o n i u m ions f o r m e d b y ether a n d sulfide b o n d cleavages react w i t h benzene. Thus, cyclohexane, b e i n g less reactive towards Z n C l , is the solvent of choice. It was also noted that the effectiveness 2

of the catalyst i n p r o m o t i n g degradation of asphaltene depends u p o n the m a n n e r i n w h i c h the catalyst is i n t r o d u c e d to the solution. T h e most satisfactory m e t h o d , therefore a p p l i e d i n this study, appeared to be that of i m p r e g n a t i n g asphaltene w i t h a Z n C l solution. T h e effect of temperature o n the product 2

distribution obtained f r o m the reaction of Z n C l w i t h asphaltene i n cyclohex2

ane u n d e r a h y d r o g e n pressure of 850 psig is illustrated i n F i g u r e 3. T h e yields of volatile products a n d of the pentane-soluble f r a c t i o n increase w i t h temperature, u p to 37% a n d 51%, respectively. T h e y i e l d of the benzene-insoluble fraction is substantial at lower temperatures, reaches a m a x i m u m at 2 0 0 ° C , and then g r a d u a l l y decreases to about 10% at 4 0 0 ° C . The

product distribution indicates

between asphaltene a n d Z n C l

2

that

a certain

specific

reaction

takes place at quite l o w temperatures ( 1 5 0 ° -

2 0 0 ° C ) a n d results i n substantial insolubilization of treated asphaltene; f o r example, at 2 0 0 ° C about 5 5 % of asphaltene becomes insoluble i n benzene. T h e f o l l o w i n g possibilities c a n account for this insolubilization: 1. F o r m a t i o n of salts or complexes w i t h Z n C l . 2. Increase i n polarity attributable to the f o r m a t i o n of n e w f u n c tional groups as a result of cleavage of sulfides or ethers. 3. F o r m a t i o n of n e w covalent bonds b y a l k y l a t i o n or h o m o l y t i c recombination. 2


194


A l t h o u g h the benzene-insoluble residue at 2 0 0 ° C contained 1.41% zinc (determined b y atomic absorption), no correlation between the zinc content i n various reaction products a n d the solubility i n benzene was observed. M o r e over, severe and prolonged treatment w i t h I N H C l w o u l d certainly d e c o m pose the complex a n d restore the solubility, but this was not observed. Between 1 5 0 ° a n d 2 0 0 ° C h o m o l y t i c cleavage a n d recombination of the resulting fragments are less likely to occur. H o w e v e r , i n the presence of L e w i s acids, some sulfide a n d ether bridges m a y be i n v o l v e d i n ionic reactions. It has been recently reported (32) that a variety of ethers containing at least one methylene group adjacent to the oxygen a t o m can be cleaved easily b y Z n C l at 3 0 0 ° C . Reactions on m o d e l compounds at 2 0 0 ° C gave similar results for ethers and sulfides a n d i n d i c a t e d that the c a r b o n i u m ions f o r m e d b y ether or sulfide b o n d cleavage m a y react w i t h any aromatic system v i a inter- or intramolecular a l k y l a t i o n (e.g., the major products f r o m the reaction of b e n z y l ether w i t h Z n C l were d i p h e n y l m e t h a n e w h e n benzene was used as solvent, and insoluble p o l y m e r w h e n cyclohexane was the solvent).


2

2

Similar mechanisms resulting i n the f o r m a t i o n of new f u n c t i o n a l groups and simultaneous intramolecular a l k y l a t i o n w i t h or without c h a n g i n g the molecular weight also can be readily envisaged as o c c u r r i n g i n a complex asphaltene molecule:

W h e n the benzene-insoluble residue obtained at 2 0 0 ° C was subjected to nonreductive a l k y l a t i o n (33) the solubility was restored to 80%, a n d this strongly implies that the generation of new functional groups is responsible for the lower solubility at this temperature. H o w e v e r , i n v i e w of inter- or intramolecular alkylations that m a y a c c o m p a n y the cleavage reactions, it is not possible to correlate the extent of degradation w i t h changes i n the molecular weights. W i t h increasing temperature the amount of insolubles decreases, a n d this corresponds to a depletion of the functional groups, as reflected b y increasing losses i n sulfur a n d oxygen contents. Indeed, a significant feature of the Z n C l - c a t a l y z e d degradation of asphaltene is the amount of heteroatoms r e m o v e d f r o m the recovered product as a f u n c t i o n of temperature ( F i g u r e 4). 2


11.

IGNASIAK ET AL.

195


Nitrogen Sulfur

80

60 Oxygen 40

20

150

200

250

300

350

400


°C

Figure 4. Effect of temperature on the removal of heteroatoms from nonvolatile products O x y g e n a n d sulfur r e m o v a l already takes place b e l o w 2 0 0 ° C a n d their rates are almost linear w i t h increasing temperature, r e a c h i n g about 5 3 % a n d 78%, respectively, at 4 0 0 ° C . Most of the nitrogen is r e m o v e d between

350°-

4 0 0 ° C . A b o v e 3 0 0 ° C there is a slight decrease i n the H / C ratio, a substantial decrease i n the proportions of a l i p h a t i c to aromatic hydrogens (Table V I I ) a n d an almost constant concentration of polar compounds i n the pentane-soluble fraction. T h e effect of temperature o n the f o r m a t i o n of hydrocarbons, as revealed b y class separation of the pentane-soluble fraction, is shown i n F i g u r e 5. T o assess the catalytic action of Z n C l o n the degradation ot asphaltene, 2

the results are c o m p a r e d w i t h those obtained i n the absence of Z n C l . A l l the 2

reactions were c a r r i e d out i n cyclohexane a n d i n the presence of either nitrogen or h y d r o g e n at 3 0 0 ° a n d 4 0 0 ° C . T h e results are s u m m a r i z e d i n Tables V I I I a n d I X . T h e reaction temperature of 3 0 0 ° C was chosen to m i n i m i z e c a r b o n - c a r b o n b o n d scission a n d yet b r i n g about the cleavage of bonds i n v o l v i n g sulfur a n d oxygen. Table VII. Effect of Temperature on the Characteristics of the Pentane-Soluble Fraction 0

Temperature

Yield

(°C)

(%)

150 200 250 300 350 400

9.6 10.8 25.1 39.3 46.1 51.2

Molecular Weight 840 770 490 430 390 250

H/C

Haliph/ H

1.36 1.37 1.36 1.37 1.31 1.30

t

u.d 12.0 9.8 8.8 8.4 5.2 b

"Conditions: catalyst, ZnCl ; catalyst asphaltene weight ratio, 1:1; solvent, cyclohexane; H pressure, 850 psig; duration, 1 h. *Not determined. 2


2

196


Polars

Polyaromatics Diaromatics


Monoaromahcs

Saturates

200

300

400

Figure 5. Effect of temperature on the yields of hydrocarbons A t 3 0 0 ° C , thermolysis of asphaltene dissolved i n cyclohexane gives results similar to those previously described (5). U n d e r the same conditions Z n C l accelerates the degradation reactions, the m a i n quantitative difference b e i n g higher yields of gaseous a n d benzene insoluble products as w e l l as a m a r k e d increase i n the concentrations of saturates, mono-, d i - , a n d polyaromatics i n the pentane-soluble fraction. T h e yields of these c o m p o u n d classes were twice as h i g h as those obtained i n the absence of Z n C l . T h e l o w H / C ratio of the recovered solid product points to the dehydrogenating properties of Z n C l . Indeed, bituminous coal has been reported (34) to evolve h y d r o g e n i n the presence of Z n C l under a n inert atmosphere even at 2 0 0 ° C ; this was attributed to dehydrogenation of h y d r o a r o m a t i c structures. 2

2

2

2

R e p l a c i n g nitrogen b y h y d r o g e n substantially improves the pentane solubility a n d simultaneously suppresses the f o r m a t i o n of insolubles. H y d r o gen also helps to compensate for the dehydrogenating action of Z n C l . T h e yields of saturated a n d aromatic hydrocarbons released d u r i n g degradation are not affected b y the nature of the gas. H o w e v e r , the stabilizing role of hydrogen is reflected b y higher yields of lower molecular weight, pentanesoluble, polar materials resulting f r o m the cleavage reactions. Increasing the hydrogen pressure f r o m 200 psi to 1800 psi effects a slight increase i n the yields of pentane solubles (from 33% to 41%), volatiles (from 18% to 24%), a n d hydrocarbons (from 14% to 19%). H o w e v e r , the c h e m i c a l characteristics of the products (e.g., H / C , H / H , m w ) r e m a i n unchanged. T h e d o m i n a n t reaction governing the overall kinetics is the f o r m a t i o n of intermediate ions b y b o n d scission (sulfide, ether, dealkylation), a n d the subsequent reactions w i t h h y d r o g e n are less important. 2

a H p h

a r


IGNASIAK E T A L .

Degradation of Athabasca

LO Ο τρ f-H CM

oo 00 CM t> CD LO CO f-H CO r-H CNl

(Ν ί Ο i> Η ι/3 d

t> τρ CO CO τρ cb ϊ> Ο

Γ-

TP - Η

CO

Ο

CO CO

TP f-H


Asphaltene

CM CO τρ CO

ο

00 |> CO ο CM cxi

CO CO TP CO co co i o cb

s

00 TP Ο Ο - Η CM

t>- CO f-H Ο CO LO oq TP

3 "•Ο

CO

ο CO

14

TP LO CM

Ο Ο CO

^>

S OC

?« ^

H

CM LO τρ CM

Η oo 00 CO 00 d LO cb CM f-H CO

oo CO LO CO TP d oo © LO f-H CO

TP Ο cb © d CO f-H CM

Ο LO d CM

©

LO CO d CO CM CM CO

Tp

ζ ζa

ζ

CO © CM f—5 d TP f-H CO LO CO LO

v.

«a Ο

α; α; ο

>,©

^

(Ν

Μ

χ ^

ο

(Ν

(Ν

ζ κ (Λ

(Λ

υ ν

CO



1.39 1.36 1.24 1.30

410 410 300 250

N H * Ν Η

2

2

2

f

2

2

β

β

Û

β

1.38 1.33 1.37

H/C

650 390 430

Λ

Molecular Weight 0.5 0.2 0.2 0.5 0.8 0.2 0.1

6.4 6.1 3.2 2.0

Ν

7.4 4.5 4.8

S

Percent

Fraction

Ν Ν H *

2

Gas

•15 psig. *850 psig.

300° C no yes yes 400° C no no yes yes

2

ZnCl

Pentane

—

— 970

0.2

0.83

0.98 0.89

1.14 1.05 1.10

H/C

1100 2200

5300 1300 1700

Molecular Weight

— 1.3

—

1.5 1.9

1.3 0.9 1.3

N

2.3

7.3 5.9

7.2 5.8 5.3

S

Percent

Fraction

— —

0.8

0.3 0.1 1.0

Cl

Benzene

0.6

—

1.5 1.3

1.1 0.8 1.9

Cl

0.66 0.80 0.52 0.55

1.8 1.5 1.0 0.6

1.5 1.7

—

N

1.1 1.8 0.7 1.0

4.7 5.0

—

Cl

Residue

Percent

7.2 7.0 1.7 4.3

— 4.3 4.5

—

S

0.77 0.88

H/C

Insoluble

1.01 1.18 0.78 1.17

1.20 0.98 1.16

H/C

6.9 6.2 2.3 2.4

7.3 4.6 4.8

S

1.2 1.2 0.7 0.2

1.1 1.0 0.9

N

Total Solid Product

Table I X . Product Characteristics from the C a t a l y z e d a n d N o n c a t a l y z e d D e g r a d a t i o n of Asphaltene


11.

IGNASIAK ET AL.

199


T h e negligible effect of h y d r o g e n pressure on the degradation of asphaltene is not really too surprising. It has been reported (18, 21, 23) that h y d r o g e n pressure has a strong influence on the Z n C l - c a t a l y z e d h y d r o c r a c k i n g of coal 2

and S R C , but the reactions were c a r r i e d out at higher temperatures, usually above 4 0 0 ° C . Z i e l k e et al. (18) noted that the role of h y d r o g e n pressure is substantially d i m i n i s h e d at lower temperatures. A t 4 0 0 ° C , w i t h a faster rate of t h e r m a l decomposition, the role of h y d r o g e n becomes more important i n stabilizing the intermediate fragments either

asphaltene

a n d , thus, preventing p o l y m e r i z a t i o n a n d dehydrogenation

the catalytic

or the noncatalytic

degradation

m o d e of

in

treatment.

H y d r o g e n also improves the yields of hydrocarbons. A l t h o u g h the proportions


of pentane solubles do not seem to be affected b y the catalyst, i n the presence of Z n C l

the yields of mono- a n d diaromatics are about 70% higher for the

2

catalytic as c o m p a r e d w i t h the noncatalytic degradation. H o w e v e r , at both temperatures

the

presence of

ZnCl

2

significantly

improves the q u a l i t y of the pentane-soluble products i n terms of l o w m o l e c u lar weight and l o w nitrogen a n d sulfur contents. The

gas chromatograms

of the

h y d r o c a r b o n fractions

indicate

that

asphaltene consists of complex macromolecules that decompose to y i e l d a w i d e distribution (from

C

to

1 0

C35) of molecules w i t h i n each of the saturate,

mono-, d i - , a n d p o l y a r o m a t i c a n d polar classes.

Conclusions Asphaltene reacts w i t h A l C l

3

even at temperatures below 1 0 0 ° C . T h e

intermediate c a r b o n i u m ions f o r m e d b y the catalyst action on asphaltene tend to undergo intraelectrophilic substitution, l e a d i n g to p o l y m e r i z a t i o n a n d loss of solubility. A n u c l e o p h i l i c solvent such as benzene suppresses these i n t r a m o lecular interactions, a n d the m w of the phenylated product is lower than that of o r i g i n a l asphaltene, i n d i c a t i n g that degradation has taken place. Reactions w i t h m o d e l compounds showed that, below 1 0 0 ° C , the extent of cleavage of c a r b o n - s u l f u r bonds or methylene bridges is l i m i t e d . O n the other h a n d , the use of

AICI3

at higher temperatures

( 1 5 0 ° - 3 0 0 ° C ) appeared to be of no

advantage as far as the degradation into condensable l o w molecular weight species was concerned; on the contrary, p o l y m e r i z a t i o n , dehydrogenation, a n d advanced c r a c k i n g to gases prevailed. T h e reaction of A l C l w i t h benzene a n d cyclohexane at 1 6 0 ° a n d 3 0 0 ° C , 3

respectively, gave a w i d e variety of products that greatly c o m p l i c a t e d the interpretation of the results w h e n asphaltene dissolved i n these solvents was heated

with A l C l . 3

T h e c o m p l e x i t y of the reactions

catalyzed b y

AlCl

3

suggested that a m i l d e r a c i d catalyst such as Z n C l be e m p l o y e d . 2

As

expected,

ZnCl

2

was a more suitable catalyst

degradation studies carried out at temperatures

above

than A l C l

3

i n the

1 0 0 ° C . A t lower

temperatures ( 1 5 0 ° - 2 0 0 ° C ) certain cleavage reactions occur, l e a d i n g to the f o r m a t i o n of polar f u n c t i o n a l groups that substantially decrease the product


200

CHEMISTRY O F ASPHALTENES

solubility i n benzene. W i t h increasing temperature, as a result of more extensive interaction of Z n C l w i t h heteroatoms, the sulfur, oxygen, a n d nitrogen contents of the products decrease a n d the rate of conversion into lower molecular weight pentane-soluble and volatile products increases. T h i s also results i n a n increase i n the saturate, mono-, d i - , a n d polyaromatic classes, the yields of w h i c h at g i v e n temperature are always higher than those obtained f r o m p u r e l y thermal degradation.


2

As far as structural investigations are concerned, the choice of a 3 0 0 ° C reaction temperature appears to be correct, since at this temperature t h e r m a l c a r b o n - c a r b o n b o n d scission has been shown previously to be m i n i m a l a n d the y i e l d of the l o w molecular weight soluble fraction is relatively high. A t 4 0 0 ° C , thermal degradation of the asphaltene is extensive. H o w e v e r , the catalytically degraded, pentane-soluble portion of asphaltene, as c o m p a r e d w i t h that f r o m the noncatalytic degradation (both at 4 0 0 ° C ) is characterized b y its superior q u a l i t y i n terms of low sulfur a n d nitrogen contents a n d l o w molecular weight. T h e presence of h y d r o g e n is essential to counteract the dehydrogenating effect of Z n C l a n d also to obtain o p t i m a l pentane solubility. T h e results of the degradative methods so far e m p l o y e d a l l point to a great degree of c h e m i c a l c o m p l e x i t y of Athabasca asphaltene a n d clearly show that it is not possible to envision a n asphaltene structure based o n a single b u i l d i n g unit. O n the other h a n d , because of the presence of connecting bridges, of w h i c h sulfides are one example, m i l d degradation c a n produce appreciable yields of l o w molecular weight, pentane-soluble polar materials that, because of the relatively m i l d conditions e m p l o y e d , probably represent the actual structural units of asphaltene. 2

Acknowledgments T h i s work was supported b y the A l b e r t a O i l Sands Technology a n d Research A u t h o r i t y . W e thank D r . E . M . L o w n f o r reading the manuscript. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Ignasiak, T.; Kemp-Jones, Α. V.; Strausz, O. P. J. Org. Chem. 1977, 42, 312. Lazarov, L.; Angelov, S. Fuel 1980, 59, 55. Ignasiak, T.; Strausz, O. P.; Montgomery, D. S. Fuel 1977, 56, 359. Ignasiak, T.; Strausz, O. P. Fuel 1978, 57, 617. Ignasiak, T.; Ruo, T. C. S.; Strausz, O. P. Am. Chem. Soc., Div. Fuel Chem., Prepr. (Honolulu, Apr., 1979) 24(2), 178. Ignasiak, T.; Strausz, O. P. Presented at the Alberta Sulfur Res. Ltd. Sulfur Symp. 27th Canadian Chem. Eng. Conf. Sulfur Week, Calgary, Alberta, Oct. 23-27, 1977. "The Molecular Structure and Chemistry of Alberta Oil Sand Asphaltene," AOSTRA/University Agreement #30, Final Report, Dec., 1979. Rubinstein, I.; Spyckerelle, C.; Strausz, O. P. Geochim. Cosmochim. Acta 1979, 43, 1. Galloway, N. O. Chem. Rev. 1935, 17, 376. Olah, G. A. "Friedel-Crafts Chemistry"; John Wiley & Sons: New York, 1973.



11.

IGNASIAK ET AL.


201

11. Storch, H. H. Ind. Eng. Chem. 1937, 29, 1367. 12. Weisser, O.; Landa, S. "Sulphide Catalysts, their Properties and Applications"; Friedrich Vieweg & Sohn: Braunschweig, West Germany, 1972. 13. Weller, S.; Pelipetz, M. G. Ind. Eng. Chem. 1951, 43, 1243. 14. Storch, H. H. In "Chemistry of Coal Utilization"; Lowry, H. H., Ed.; John Wiley & Sons: New York, 1945; Vol. 2, pp. 1750-1796. 15. Weller, S. "Catalysis"; Emmett, P. H., Ed.,; Reinhold: New York, 1956; Vol. 4, Chap 7, pp. 513-527. 16. Faingold, S. I.; Vallas, K. R. Iz. Akad. Nauk. Est. SSR, Ser. Tekh. Fiz.-Mat. Nauk. 1957, 6, 245. 17. Zielke, C. W.; Struck, R. T.; Evans, N.M.; Costanza, C. P.; Gorin, E.I&EC Process Des. Dev., 1966, 5, 151. 18. Ibid., 158. 19. Kawa, W.; Feldmann, H. F.; Hiteshue, R. W. Am. Chem. Soc., Div. Fuel Chem., Prepr. (Chicago, Sept., 1970). 15, A23. 20. Zielke, C. W.; Rosenhoover, W. Α.; Gorin, E. In "Shale Oil, Tar Sands and Related Fuel Sources," Adv. Chem. Ser. 1976, 151, 153. 21. Wood, R. E.; Wiser, W. Η. I&EC Process Des. Dev. 1976, 15, 144. 22. Low, J. Y.; Ross, D. S. In "Organic Chemistry of Coal," ACS Symp. Ser. 1978, 71, 204. 23. Skinn, J. H.; Vermeulen, T. Am. Chem. Soc., Div. Fuel Chem., Prepr. (Honolulu, Apr., 1979) 24(2), 80. 24. Bugle, R. C.; Wilson, K.; Olsen, G.; Wade, L. G.; Osteryoung, R. A. Nature 1978, 274, 578. 25. Schlosberg, R. H.; Maa, P. S.; Neavel, R. C. U.S. Patent 4 092 235. 26. Schlosberg, R. H.; Neavel, R. C.; Maa, P. S.; Gorbaty, M. L. Fuel 1980, 59, 45. 27. Larsen, J. W.; Kuemmerle, E. W. Fuel 1976, 55, 162. 28. Sawatzky, H.; George, A. E.; Smiley, G.T.; Montgomery, D. S. Fuel 1976, 55, 16. 29. Schmerling, L.; Ipatieff, V. A. U.S. Patent 2 3888 937. 30. Mobley, D. P.; Salin, S.; Tanner, Κ. J.; Taylor, N. D.; Bell, A. T. Am. Chem. Soc., Div. Fuel. Chem., Prepr., (Miami Beach, Sept. 1978) 23(4), 138. 31. Ignasiak, T.; Samman, N.; Bimer, J.; Montgomery, D. S.; Strausz, O. P. submitted for publication in Fuel. 32. Mobley, D. P.; Bell, A. T. Fuel 1979, 58, 661. 33. Ignasiak, B.; Carson, D.; Gawlak, M. Fuel 1979, 58, 833. 34. Bodily, D. M.; Lee, S. H. O.; Wiser, W. H. Am. Chem. Soc., Div. Fuel Chem., Prepr. (Los Angeles, Mar.-Apr., 1974) 19, 163. RECEIVED June 23, 1980.


Chemistry of Asphaltenes - ACS Publications - American Chemical

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