1 On the Molecular Nature of Petroleum Asphaltenes JAMES G. SPEIGHT1
Downloaded by ABERDEEN UNIV on August 9, 2013 | http://pubs.acs.org Publication Date: January 1, 1982 | doi: 10.1021/ba-1981-0195.ch001
Corporate Research-Science Laboratories, Exxon Research and Engineering Company, P.O. Box 45, Linden, NJ 07036 SPEROS E. MOSCHOPEDIS Coal Research Department, Alberta Research Council, 11315-87th Avenue, Edmonton, Alberta T6G 2C2, Canada A survey of the methods used to determine asphaltene "structure" indicates that there are serious shortcomings in all of the methods because of the assumptions required to derive the molecular formulae. The continued insistence that a complex fraction such as asphaltenes, derived in a one-step process from petroleum as a consequence of its insolubility in nonpolar solvents, has a definitive molecular structure is of questionable value to petroleum technology, and it is certainly beyond the scope of the available methods to derive such formulae. Asphaltenes would best be described in terms of several structural types rather than definite molecular structures.
T
he molecular nature of the asphaltene fractions of p e t r o l e u m a n d b i t u mens has been the subject of numerous investigations (J, 2) b u t determ i n i n g the actual structures of the constituents of the asphaltene fraction has p r o v e d to be difficult. It is, no doubt, the great c o m p l e x i t y of the asphaltene fraction that has h i n d e r e d the f o r m u l a t i o n of the i n d i v i d u a l molecular structures. Nevertheless, the various investigations have brought to light some significant facts about asphaltene structure; there are indications that asphaltenes consist of condensed aromatic n u c l e i that c a r r y a l k y l a n d a l i c y c l i c systems w i t h heteroelements (i.e., nitrogen, oxygen, a n d sulfur) scattered throughout i n various, i n c l u d i n g heterocyclic, locations. O t h e r basic generalizations also have been noted; w i t h increasing m o l e c u lar weight of the asphaltene fraction, both a r o m a t i c i t y a n d the proportion of heteroelements increase (S, 4, 5). I n addition, the proportion of asphaltenes i n Author to whom correspondence should be sent.
1
0065-2393/81/0195-0001$05.00/0 © 1981 American Chemical Society In Chemistry of Asphaltenes; Bunger, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
2
CHEMISTRY OF
ASPHALTENES
petroleum varies w i t h source, depth of b u r i a l , the specific (or A P I ) gravity of the crude o i l , and the sulfur content of the crude o i l as w e l l as a nonasphaltene sulfur (5). H o w e v e r , m a n y facets of asphaltene structure still r e m a i n u n k n o w n , and it is the purpose of this chapter to b r i n g together the pertinent i n f o r m a t i o n on asphaltene structure as w e l l as the part p l a y e d b y asphaltenes i n the physical structure of petroleum and bitumens.
Downloaded by ABERDEEN UNIV on August 9, 2013 | http://pubs.acs.org Publication Date: January 1, 1982 | doi: 10.1021/ba-1981-0195.ch001
Asphaltene Composition Asphaltenes are dark b r o w n to black friable solids that have no definite m e l t i n g point, a n d w h e n heated, usually intumesce, then decompose l e a v i n g a carbonaceous residue. T h e y are obtained f r o m petroleums and bitumens by addition of a nonpolar solvent (such as a hydrocarbon) w i t h a surface tension lower than 25 dynes c m " at 2 5 ° C (such as liquefied petroleum gases, the l o w - b o i l i n g petroleum naphthas, petroleum ether, pentane, isopentane, and hexane) but are soluble i n l i q u i d s h a v i n g a surface tension above 25 dynes cm (such as p y r i d i n e , carbon disulfide, carbon tetrachloride, and benzene) (6, 7). 1
- 1
Asphaltenes are, by definition, a solubility class (
