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11 Lewis Acids Assisted Degradation of Athabasca Asphaltene 1
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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
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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
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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
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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.
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
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CHEMISTRY OF ASPHALTENES
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
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3
3
2
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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
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
11.
IGNASIAK ET AL.
187
Degradation of Athabasca Asphaltene
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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
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
1.1
CHEMISTRY OF ASPHALTENES
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
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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
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
11.
IGNASIAK ET AL.
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Degradation of Athabasca Asphaltene
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.
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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.
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
fl
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CHEMISTRY OF ASPHALTENES
Table V . Hydrocarbon Type Class Separation of the Pentane-Soluble Fraction
Fraction
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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
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
11.
IGNASIAK ET AL.
191
Degradation of Athabasca Asphaltene
Product distribution
Residue 1 3
Benzene fraction Pentane fraction
H/C ratio of fractions Original Asphaltene
1.2 h
1.0
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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 .
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
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
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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
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
11.
IGNASIAK ET AL.
193
Degradation of Athabasca Asphaltene
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
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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
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
194
CHEMISTRY OF ASPHALTENES
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).
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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
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
11.
IGNASIAK ET AL.
195
Degradation of Athabasca Asphaltene
Nitrogen Sulfur
80
60 Oxygen 40
20
150
200
250
300
350
400
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°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
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
2
196
CHEMISTRY OF ASPHALTENES
Polars
Polyaromatics Diaromatics
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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
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
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
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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
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
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
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11.
IGNASIAK ET AL.
199
Degradation of Athabasca Asphaltene
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
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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
In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
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.
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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.
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