Composition of Asphaltenes from Coal Liquids - Advances in

14 Composition of Asphaltenes from

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Coal Liquids THOMAS ACZEL, R. B. WILLIAMS, N. F. CHAMBERLAIN, and H. E. LUMPKIN Exxon Research and Engineering Company, Baytown, TX 77520 Asphaltenes from heavy coal distillates were characterized with a combination of high resolution mass spectrometry, nuclear magnetic resonance, infrared, gel permeation chromatography, and elemental analyses. More than 1000 aromatic and heterocyclic carbon number components were identified and determined in one sample alone. Structural assumptions were cross-checked by comparing average values on carbon atom types calculated from the MS composition with averages obtained directly from NMR. Coal asphaltenes consist predominantly of units of three to ten ring heteroaromatic components, with one to two or three of these units per molecule. Most abundant heteroaromatic ring systems are those with two oxygen atoms per molecule, followed by monooxygen, trioxygen, nitrogen, nitrogen oxygen, sulfur oxygen, and hydrocarbon type units. Sidechains and naphthenic rings are also present, with one or two naphthenic rings and one or two short sidechains associated with each heteroaromatic or aromatic nucleus.

A

sphaltenes are generally defined as those components i n petroleum a n d coal liquids that under certain conditions are soluble i n benzene but insoluble i n aliphatic solvents, such as n-pentane, n-heptane, or cyclohexane. This definition obviously includes a broad variety of components, as insolubility i n the above solvents can be caused b y h i g h molecular weight, h i g h polarity, hydrogen b o n d i n g , acid-base c o m p l e x i n g , or combinations of these parameters. Asphaltenes are thought to be a major intermediate i n the t h e r m a l processes leading to coal liquefaction. Increased interest i n coal liquefaction techniques has, therefore, resulted i n renewed attempts to unravel the structural characteristics of these materials. Fortunately, this effort is facilitated b y 0065-2393/81/0195-0237$05.00/0 © 1981 American Chemical Society In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

238

CHEMISTRY OF

ASPHALTENES

the v e r y nature of c o a l - d e r i v e d asphaltenes, w h i c h are generally m o r e polar but of lower molecular weights than p e t r o l e u m asphaltenes, a n d , thus, are m o r e amenable to detailed compositional analyses. W e have recently d e t e r m i n e d the structural parameters a n d composition of some asphaltene samples obtained f r o m the S y n t h o i l a n d E x x o n D o n o r Solvent ( E D S ) l i q u e f a c t i o n processes. T h e particular E D S sample used was sufficiently volatile for analysis b y u l t r a h i g h resolution mass spectrometry, so we c o u l d obtain very detailed data on its composition i n terms of

the


distribution of i n d i v i d u a l c a r b o n - n u m b e r homologs. I n f o r m a t i o n f r o m this approach, integrated w i t h data f r o m N M R , IR, m o l e c u l a r weight d e t e r m i n a tions, elemental analyses, a n d separations f u r n i s h e d us w i t h a novel a n d detailed insight into the nature of these asphaltenes. T h e excellent agreement observed between composites calculated f r o m the detailed M S data, where available, a n d the averages d e t e r m i n e d b y N M R , IR, a n d elemental analyses reinforces the c r e d i b i l i t y of the approaches used a n d allows extrapolations to heavier samples that are not amenable to detailed M S characterization. T h i s chapter describes briefly the a n a l y t i c a l methodology used i n this work, summarizes the m o r e than 1200

i n d i v i d u a l asphaltene

components

d e t e r m i n e d , a n d discusses the overall structural i m p l i c a t i o n s of the integrated analytical

findings.

Discussion Analytical

Methodology.

Analytical

techniques

used i n this w o r k

i n c l u d e d separations, h i g h a n d l o w resolution M S , N M R , IR, U V , m o l e c u l a r weight determinations, a n d elemental analyses. These are discussed i n detail i n our w o r k on the c h e m i c a l characterization of S y n t h o i l feeds a n d products (J, 2). C o m p o s i t i o n of the E D S asphaltenes was d e t e r m i n e d w i t h the a i d of a K r a t o s - A E I m o d e l M S 5 0 u l t r a h i g h resolution mass spectrometer that became available o n l y after we h a d already c o m p l e t e d the w o r k on the S y n t h o i l samples. T h e roles of the various a n a l y t i c a l approaches selected are listed i n T a b l e I. T h e most important techniques are also s u m m a r i z e d i n the f o l l o w i n g paragraphs. SEPARATIONS.

T h e asphaltene fractions were obtained b y solvent extrac

tion w i t h benzene a n d subsequent p r e c i p i t a t i o n w i t h cyclohexane. T h e c y c l o hexane-soluble fractions were separated into saturate, aromatic, a n d polar aromatic fractions b y the c l a y - g e l technique, A S T M D - 2 0 0 7 (modified). T h i s separation is also applicable to asphaltenes. HIGH RESOLUTION MASS SPECTROMETRY.

H i g h resolution mass spectrom

etry was used to i d e n t i f y the m a x i m u m possible n u m b e r of components, or rather, c a r b o n - n u m b e r homologs, i n the samples. T h i s can be achieved o n l y on compounds that are volatile at the conditions used ( 3 0 0 ° C a n d 1 χ 1 0 "

2

torr)

a n d y i e l d stable molecular ions. These conditions are satisfied b y esentially a l l aromatic

and

heteroaromatic

components

boiling

below

approximately

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

14.

239

Asphaltenes from Coal Liquids

A C Z E L ET AL.

Table I. Roles of Major Analytical Techniques in Characterization of Asphaltene Fractions Examples

Technique Solvent extraction

0

C h r o m a t o g r a p h i c separation L o w resolution mass spectrometry


0

U l t r a h i g h resolution mass spectrome try

N u c l e a r magnetic resonance Infrared spectrometry

of Structural

Information

Concentration of asphaltenes i n sam ple Saturates/aromatics/polar aromatics Residual solvents, volatiles, initial survey of sample nature, M W dis tribution, determination of satu rate types D e t e r m i n a t i o n of i n d i v i d u a l aro m a t i c a n d heteroaromatic carbonn u m b e r homologs. C a l c u l a t i o n of composite structural features f r o m M S composition D e t e r m i n a t i o n a n d calculation of av erage structural parameters F u n c t i o n a l groups ( O H , C O O H , CONH ) A r o m a t i c r i n g structures C , Η, Ο, N , basic N , S 2

U l t r a violet/visible spectrometry Miscellaneous conventional tech niques a n d neutron activation analysis G e l permeation chromatography a n d vapor pressure osmometry

Average molecular weights, m o l e c u lar weight distribution

"Used also to prepare fractions for subsequent additional separations and/or instrumental analysis.

6 0 0 ° C a n d likely to be present i n coal liquids. In the case of the E D S asphaltenes, these amounted to 77% of the sample; i n case of the Synthoil asphaltenes, to 10%-30%. T h e mass spectrometer was operated i n the low voltage m o d e at a resolving power of about 50,000. T h i s mode of operation restricts the mass spectra to aromatic a n d heteroaromatic molecular ions a n d allows the separate determination of essentially a l l volatile hydrocarbons, aromatic oxygen, a n d aromatic nitrogen compounds that are present i n coal liquefaction products. T h e h i g h accuracy of the c o m p u t e r i z e d mass measurements (better than 0.0005 a m a units on the average) allows the identification of some of the sulfur-containing components, even if the instrument is operated at a resolving power below that r e q u i r e d (approximately 1:90,000 at mass 300) to separate these components f r o m interfering hydrocarbons ( C — S H doublet; Δ Μ = 0.0034 ama). T h e power of u l t r a h i g h resolution mass spectrometry is illus trated b y the determination of approximately 1200 components/carbonn u m b e r homologs i n the spectrum of the E D S asphaltenes. 3

4


240

CHEMISTRY OF

ASPHALTENES

T h e M S data are quantitated w i t h the a i d of p r o p r i e t a r y c o m p u t e r programs. These i n c l u d e programs that calculate composite values f r o m the M S composition for average parameters t y p i c a l l y d e t e r m i n e d b y N M R , such as fraction of carbons i n aromatic structures a n d angular H atoms. T h e scope of these programs is still increasing. P r o t o n nuclear

PROTON NUCLEAR MAGNETIC RESONANCE. resonance ( N M R ) a n d i n one case

1 3

magnetic

C N M R were the major techniques used to

determine the average structural features of the asphaltene fractions. Interpre-


tations were based on correlations published i n the literature (3) a n d on calculations based on the c o m b i n a t i o n of data f r o m N M R , G P C , a n d elemental analyses (4, 5). These latter are based on equations relating total rings (R), aromatic rings (R ), naphthenic rings (R ), total hydrogens ( H ) , total carbons a

(C), aromatic

n

r i n g carbons

( C ) , p e r i p h e r a l (substitutable) a

aromatic

ring

carbons ( C J a n d n u m b e r of a l k y l links between aromatic r i n g groupings ( L ) (6)· 2(R 2(R -

a

-

1)/C - 2 -

1)/C = ( C . -

H/C - C /C

C

a

x

+ 2(R

n

+ L))/C

A detailed d e r i v a t i o n of these equations is g i v e n i n References 1 a n d 2. T h e i r m a i n f u n c t i o n is to relate N M R data to atomic carbon types a n d to convert the relative ratios a m o n g these to n u m b e r values. T h e calculations y i e l d only average values a n d i n c l u d e uncertainties, as for example that between a l k y l linkages a n d naphthenic rings (only the s u m of these can be

calculated).

Nevertheless, they can be used to define reasonable structural limits. P a r t i c u larly interesting is the c o m p a r i s o n of these N M R parameters w i t h M S data, because this approach, as w i l l be shown later, allows the d e d u c t i o n of structural parameters that c o u l d not be possibly obtained f r o m M S or N M R . Infrared Spectrometry.

Infrared spectrometry was used for the deter-

m i n a t i o n of f u n c t i o n a l groups a n d for survey purposes. Samples were r u n both i n solution a n d as K B r pellets. Assignments were based on the work of M c K a y et al. (7), Snyder (8), a n d Petersen et al. (9). A very detailed description of the overall a n a l y t i c a l methodology is reported i n Reference 1.

Results T h e structural i n f o r m a t i o n obtained w i l l be illustrated w i t h data on asphaltenes f r o m a C l e a r f i e l d , P e n n s y l v a n i a coal extract, asphaltenes f r o m the Synthoil coal l i q u e f a c t i o n process of the D e p a r t m e n t of E n e r g y , a n d a lighter distillate asphaltene f r o m the E x x o n D o n o r L i q u e f a c t i o n Process ( E D S ) . N o attempt is m a d e to correlate composition w i t h sample o r i g i n or treatment as the samples d e r i v e f r o m different coals a n d were obtained at conditions.


different

14.

ACZEL ET AL.

241


T h e data y i e l d a considerable a n d novel insight into the structure of asphaltenes f r o m coal liquids. T h i s is particularly true for the lighter distillate asphaltenes f r o m the E D S process, for w h i c h detailed M S data are available. T h e good agreement observed between data f r o m M S , N M R , a n d other techniques increases the c r e d i b i l i t y of the latter even i n the cases where the samples were not sufficiently volatile for detailed M S characterization.


O u r major findings are s u m m a r i z e d below: 1. T h e asphaltenes studied are extremely complex mixtures of hydrocarbons and heteroatomic compounds. 2. V o l a t i l i z e d asphaltene molecules contain prevalently only one structural unit per molecule. H e a v i e r molecules contain one to three of these units per molecule. 3. Basic structural units are two- to e i g h t - r i n g condensed aromatic hydrocarbons, associated w i t h one to three functional groups and/or n a p h t h e n i c rings a n d short sidechains. A v e r a g e aromatic r i n g condensation ranges f r o m eight to ten i n the heavier asphaltenes, three to five i n the lighter ( E D S ) asphal tenes. There is no evidence of a l k y l links i n the latter. 4. Average molecular weight is 500-800. T h e above conclusions a p p l y to a l l asphaltenes studied, although detailed M S characterization was l i m i t e d to the lighter E D S asphaltene samples. H i g h resolution M S spectra obtained for the 10%-30% volatiles f r o m the other asphaltene samples show the presence of the same type of components as the ones d e t e r m i n e d i n the light asphaltenes. T h e high resolution M S analysis of the light asphaltene sample is s u m m a r i z e d i n Tables II—VI. T h e overall distribution of the components d e t e r m i n e d is shown i n T a b l e II. Items reported for each class of components i n c l u d e total amount, the n u m b e r of homologous series (in Ζ number), a n d i n d i v i d u a l carbon-number homologs d e t e r m i n e d i n that class and their ranges (in carbon number). T h e Ζ n u m b e r indicates the hydrogen deficiency i n a homologous series w i t h general f o r m u l a C H _ . Most abundant components are those w i t h two oxygens atoms per molecule, f o l l o w e d b y those w i t h one oxygen atom per molecule, three oxygen atoms per molecule, a n d hydrocarbons. Some of the miscellaneous components, although present only i n very m i n o r amounts, contain as m a n y as five heteroatoms per molecule. In a l l , w e have d e t e r m i n e d a total of 1269 components, r a n g i n g f r o m 30 p p m to 20,000 p p m concentra tion i n this asphaltene fraction a n d belonging to 234 homologous series. R i n g condensation ranges f r o m one to eight (from Ζ = 6 to Ζ = 44), a n d the carbon-number homologs w i t h i n the series range f r o m C to C . T h e presence of hydrocarbons a n d of l o w molecular weight heterocompounds identical to those f o u n d i n the cyclohexane- or pentane-soluble fractions of coal liquids was rather unexpected. H o w e v e r , neutral hydrocar bons have been identified i n other coal asphaltenes (10), a n d more recently, n

2 n

z

6

3 0


242

CHEMISTRY O F ASPHALTENES

Table I I . C o m p o u n d Types D e t e r m i n e d i n L i g h t Asphaltenes F r o m E D S Process Homologous Series


Class Dioxygen compounds Monooxygen com pounds Hydrocarbons Trioxygen com pounds Nitrogen compounds Nitrogen-oxygen compounds Thiophenes Miscellaneous N, S, Ο compounds Sulfur-oxygen com pounds Sulfur-dioxygen compounds Miscellaneous S, Ο compounds Tetraoxygen com pounds Nonvolatile residue Totals

Homologs Range in Carbon Number

Weight Percent

Number

Range in Ζ Number

Number

27.26 16.81

20 19

6-44 6-42

130 139

6-29 10-30

6.29 6.26

18 14

12-46 12-38

101 104

10-31 12-28

5.84 3.57

20 15

5-43 7-39

131 118

9-30 8-29

2.40 2.19

11 69

10-36 8-35

64 231

8-28 9-27

2.18

12

10-32

41

10-28

1.90

13

10-34

37

10-23

1.54

10

10-38

119

13-23

0.96

13

10-34

54

14-24

22.80 100.00

234

—

—

—

—

1269

5-46

6-31

we have also separated saturate, aromatic, a n d polar aromatic fractions f r o m a coal asphaltene sample using the clay g e l percolation procedure ( A S T M D 2007). W e speculate that these neutral molecules coprecipitate w i t h the surrounding polar materials, possibly entrapped w i t h i n a h y d r o g e n - b o n d network. T h e aromatic r i n g distribution d e t e r m i n e d f o r the major classes of compounds is shown i n T a b l e III. F o u r a n d five r i n g types are the most abundant. H y d r o c a r b o n s show higher average condensation than the heterocompounds; however, most of the latter contain a d d i t i o n a l f u n c t i o n a l groups such as h y d r o x y groups a n d f u r a n i c rings. T h e distribution of homologous series w i t h i n the major classes is shown i n T a b l e I V . It is essentially a Gaussian type of distribution; the most abundant series are those w i t h general formulas r a n g i n g f r o m C H _ 6 to C H _ , C „ H _ O to C H _ 3 o O , a n d C H _ 0 to C H _ 0 . T h e sudden increase i n concentration f o r the C H _ 0 series c o u l d be a n i n d i c a t i o n of the presence of hydroxydibenzofurans. n

2 n

2 0

n

2 n

n

n

2 n

2 n

1 6

1 6

2

n

2 n

2 6

2 n

2

n

2 n

3 0

2

2

T a b l e V illustrates the carbon-number distribution i n selected series. S i m i l a r trends were observed i n most others. W i t h one exception, the h o m o l o g present i n the largest concentration w i t h i n a series has three to four carbon atoms more than the first m e m b e r of the series. T h i s c o u l d indicate short sidechains or, possibly, the appearance of isomeric naphthenoaromatic series


In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982. 4.06

4.12

3.36

—

27.26

16.81

—

—

0.40 4.92 9.76 9.22 2.37 0.59

0.34 0.78 2.05 9.42 2.86 1.36

0.93 0.71 2.09 1.85 0.67 0.04 6.29

Dioxygen Compounds

Monooxygen Compounds

Hydrocarbons

—

3.45

6.26

3.91

5.84

—

0.22 0.49 0.71 2.99 1.03 0.40

—_

1.28 1.70 2.59 0.59 0.10

Nitrogen Compounds

Trioxygen Compounds

3.88

2.40

—

—

0.02 0.92 0.86 0.54 0.06

Thiophenes

3.69

0.96 8.42 15.85 27.19 9.24 3.18 0.04 64.86

Total

0

"Ring distribution in minor unreported classes, amounting to 12.34%, is similar to that shown above; degree of condensation should be higher in 22.80% nonvolatiles.

Totals Average number of aromatic rings/molecule

1 2 3 4 5 6 7-8

Number of Aromatic Rings/Molecule

Weight Percent

T a b l e III. A r o m a t i c R i n g D i s t r i b u t i o n i n M a j o r C l a s s e s F o u n d i n E D S A s p h a l t e n e s


244

CHEMISTRY OF ASPHALTENES

T a b l e I V . D i s t r i b u t i o n of H o m o l o g o u s Series i n Selected Classes F o u n d i n E D S Asphaltenes


Ζ Number Formula,

in General C H _ X n

2 n

6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 Total Average Ζ n u m b e r overall average Ζ number, 22.49

z

Hydro carbons

— — —

0.24 0.30 0.38 0.41 0.30 0.68 0.54 0.87 1.29 0.56 0.51 0.07 0.09 0.02 0.01 0.01 0.006° 0.004° 6.29 24.47

Monooxygen Compounds x = o

Dioxygen Compounds x = o

0.15 0.13 0.06 0.13 0.13 0.52 0.81 1.24 3.57 3.93 1.92 1.82 1.03 0.73 0.42 0.22 0.16° 0.10° 0.04°

— —

17.11 24.30

2

0.34 0.04 0.02 0.06 0.68 4.19 4.72 5.02 4.87 2.66 1.71 1.56 0.81 0.43 0 15 0.01 0.003° 0.001° 0.001° traces 0

—

27.27 20.82

"These types were included in the miscellaneous category in Tables II and III.

starting at higher carbon numbers. B o t h structural types are probably present. T h i s type of carbon-number distribution is similar to that observed i n cyclohexane-soluble coal liquids, a n d i n organics d i r e c t l y volatilized f r o m coal (1), a n d it is an i n d i c a t i o n of intermediate coal rank a n d relatively m i l d temperature conditions i n processing. O n e of the most salient features of the detailed M S data s u m m a r i z e d i n Tables I I - V is the s i m i l a r i t y of the components d e t e r m i n e d i n the light asphaltenes to those generally f o u n d i n the cyclohexane-soluble fractions. Asphaltenes are s i m p l y more polar than the cyclohexane solubles, but less polar than the polar fraction separated f r o m the cyclohexane-soluble fraction. T h i s is attributable to the fact that asphaltenes contain significant amounts of neutral components. T h e overall amount of polar components is m u c h higher i n asphaltenes than i n the cyclohexane solubles, a n d this is the factor that is



Number

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Totals

Carbon

—

0.68

—

0.02

—

0.02

—

0.02

0.15 0.08 0.07 0.07 0.10 0.08 0.07

1.29

0.04 0.18 0.29 0.24 0.16 0.12 0.05 0.04 0.13 0.04 3.57

— — —

1.82

4.87

— —

0.18 0.66 0.86 0.96 0.66 0.55 0.38 0.28 0.18 0.09 0.04 0.03 0.27 0.37 0.32 0.26 0.21 0.18 0.09 0.06 0.02 0.04

C"n^2n-22^2

0.04 0.32 0.69 0.82 0.63 0.48 0.28 0.13 0.08 0.06 0.04

n

Percent

C £*2n-22^

Weight

T a b l e V . C a r b o n - N u m b e r D i s t r i b u t i o n i n Selected H o m o l o g o u s Series i n E D S A s p h a l t e n e s


n

1.56

0.35 0.30 0.18 0.20 0.19 0.13 0.09 0.06 0.04 0.02

C #2n-25

246

CHEMISTRY O F ASPHALTENES


probably responsible for their insolubility at the conditions used. Asphaltenes also seem to have, o n the average, more heteroatoms p e r molecule than the polar components of the cyclohexane-soluble fractions. Components w i t h t w o oxygen atoms per molecule, for example, are m u c h more abundant than those w i t h one oxygen atom per molecule i n asphaltenes, w h i l e monooxygen compounds are more abundant than dioxygen compounds i n the polar fraction separated f r o m the corresponding cyclohexane solubles (27% a n d 17%, respectively, i n asphaltenes vs. 14% a n d 26% i n the polars). Some of these trends are illustrated i n T a b l e V I . A l t h o u g h M S data c a n be used to obtain structural i n f o r m a t i o n , these data usually need to be extended a n d corroborated b y other analytical techniques such as elemental analyses, molecular weight measurements, N M R , IR, a n d U V . T h e remainder of this chapter deals w i t h i n f o r m a t i o n gathered f r o m these techniques a n d w i t h the integration of the data w i t h the above-discussed M S characterization. As volatility is not a l i m i t i n g factor for these d e t e r m i n a tions, the data reported i n c l u d e those obtained on the heavier asphaltenes. E l e m e n t a l analyses of three selected asphaltenes are reported i n T a b l e V I I . C o m p a r i s o n of the data f r o m conventional methods, i n particular the H / C ratio, confirms the " t r u e " asphaltenic nature of the asphaltene fraction f r o m the E D S distillate. T h e higher oxygen content of the latter indicates higher polarity, a n d it m i g h t explain its lower molecular weight a n d higher volatility i n the M S inlet (Table VIII). C o m p a r i s o n of conventional analyses on the total sample a n d its volatile fraction indicates a lower H / C a n d higher oxygen content for the 23% nonvolatiles. T h e molecular weight data given i n T a b l e V I I I were obtained b y g e l permeation chromatography using both tetrahydrofuran a n d N - m e t h y l p y r r o l i d o n e solvents and were c o n f i r m e d b y V P O .

Table V I . Relative Concentrations of Heterocyclics i n A r o m a t i c , Polar, and Asphaltene Fractions from E D S Asphaltenes Weight Percent Compound Class

Aromatics

Asphaltenes

Polar Aromatics

2.94 Hydrocarbons 6.29 94.58 Monooxygen compounds 4.97 16.81 26.39 Dioxygen compounds 0 27.26 13.94 Other oxygen compounds" 0 7.22 5.29 Nitrogen compounds traces 11.60 25.49 Sulfur compounds 0.45 2.40 0.20 Miscellaneous heterocyclics traces 5.62 traces Nonvolatile residue 22.80 25.80 0.00 100.00 100.00 Totals 100.00 Percent of atomic oxygen in hydroxy groups 0 30.3 80.2 "Includes tri- and tetraoxygen compounds as well as compounds containing both oxygen and sulfur atoms.


14. ACZEL ET AL.

247


T a b l e V I I . E l e m e n t a l A n a l y s i s of A s p h a l t e n e s S t u d i e d Weight Source of Asphaltenes

Coal

Extract

Synthoil

EDS

Product

Distillate

B A A A 83.52 Carbon 82.13 85.85 86.82 6.29 Hydrogen 5.61 6.01 6.29 8.20 Oxygen 5.74 9.66 4.39 1.28 Sulfur 1.78 1.81 0.36 0.71 Nitrogen 0.79 0.97 2.11 — 0.34 basic 0.40 1.17 — 0.45 nonbasic 0.57 0.94 100.00 99.97 Total 100.38 99.97 0.904 0.820 Atomic H / C 0.864 0.835 A : Conventional methods, including oxygen determination by neutron activation analysis. *Β: Calculated from MS composition of 77.20% volatiles. a


Percent

b

a

a

e

F u n c t i o n a l groups d e t e r m i n e d b y IR techniques are shown i n T a b l e I X . T h e relatively small fraction of the total organic oxygen present i n c a r b o x y l i c a n d h y d r o x y groups indicates significant concentrations of etheric bridges a n d f u r a n i c functions. As one w o u l d predict f r o m their t h e r m a l lability, acids a n d amides were f o u n d o n l y i n the asphaltenes f r o m coal extracts. Structural

parameters

calculated

from

N M R , G P C , and

elemental

analyses are s u m m a r i z e d i n T a b l e X . T h e prevalently aromatic character of these asphaltenes is c o n f i r m e d b y the 0.7 to 0.8 f r a c t i o n of C atoms i n aromatic rings. T h e ratio of aromatic to substitutable p e r i p h e r a l aromatic C atoms is about 1.50:1.60. T h i s indicates a n average condensation of four to five rings per unit structure, as i n pyrene, 0 , / Q j = 1 6 0 , benzopyrene, C /C a

= 1.67, cholanthrene, C /C

e

a

e

= 1.50.

T h e ratio of C atoms a l p h a to aromatic rings to p e r i p h e r a l aromatic carbon atoms is an i n d i c a t i o n of the degree of substitution. T h e a p p r o x i m a t e l y 30% value f o u n d for these asphaltenes corresponds to an average of three to four substituted aromatic carbons, if one assumes four to five r i n g aromatic structures

that

generally

contain

ten or twelve substitutable

peripheral

aromatic carbons (e.g., 10 i n pyrene, 12 i n benzopyrene). Proton N M R cannot determine i n d e p e n d e n t l y the n u m b e r of naphthenic rings (R ) a n d a l k y l linkages ( L ) , o n l y their sum. Therefore, T a b l e X contains n

three alternate possibilities for R a n d the related parameters, considering the n

changes i n structural parameters corresponding to L = 0, 1, or 2. T h i s is the most reasonable set of values for L , as it is u n l i k e l y that it exceeds the value of R a n d m i g h t be considerably less. n

L

values of 0, 1, a n d 2 correspond, respectively, to 1, 2, a n d 3 unit

structures per molecule. T h i s is consistent w i t h l i m i t e d U V data o n the heavier asphaltenes that indicate one or two unit structures per molecule (I ). U V data

American Chemical Society Library 1155 16th St. N. W. In Chemistry of Asphaltenes; Bunger, J., et al.; Washington, D. C. 2003(1 Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

248

CHEMISTRY OF ASPHALTENES

Table V I I I . M o l e c u l a r Weight Distribution i n Asphaltenes


Source

Coal Extract

Property Number-average M W Weight-average M W W e i g h t percent material in M W ranges

Composition of Asphaltenes from Coal Liquids - Advances in

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