Small Population of One to Three Fused-Aromatic Ring Moieties in Asphaltenes Corie Y. Ralston, †,§ Sudipa Mitra-Kirtley, ‡ and Oliver C. Mullins* ,† Schlumberger-Doll Research, Ridgefield, Connecticut 06877, and Rose-Hulman Institute of Technology, Terre Haute, Indiana 47803 Received December 15, 1995. Revised Manuscript Received February 21, 1996 X The aromatic ring distributions of asphaltenes are examined with emphasis on one- to three- ring moieties by using fluorescence emission spectroscopy: such a nondestructive, bulk analysis of small aromatic moieties in asphaltene has not been achieved by any other method. Asphaltenes are found to lack significant populations of these small rings compared to larger ring systems; increasing relative populations of small ring moieties are found for resins and maltenes, as expected. Additionally, the one asphaltene with higher polarity possesses smaller aromatic ring systems. Potential perturbations of asphaltene spectra by energy transfer effects are ruled out, using analyses based on fluorescence lifetimes. Energy transfer, which yields red-shifted emission, is known to decrease fluorescence lifetimes. Fluorescence lifetimes of dilute asphaltene solutions are shown to be intrinsic by comparison to those of deasphaltened crude oils; thus, fluorescence emissions of dilute asphaltene solutions are unaffected by energy transfer. Introduction Asphaltenes, the heaviest component of crude oil, are of great economic significance. 1-3 The nature and, particularly, the quantity of asphaltenes strongly affect the chemical and physical properties of crude oils and asphalts, in terms of both fundamental properties and applications. 1-3 Asphaltenes increase difficulties as- sociated with crude oil production, transportation, and refining. On the other hand, asphalts (asphaltenes plus resins) are of value for various coating and paving applications. A greater understanding of the chemistry of asphaltenes can help mitigate difficulties related to asphaltenes and expand applications of asphalts. In particular, the tendency of asphaltenes to form molec- ular complexes can be understood only if accurate molecular structures are obtained, especially aromatic moieties. This important topic relates to intra- and intermolecular binding of asphaltenes. Knowledge of the ring size distribution is required for understanding asphaltene complex formation and may prove critical in any asphaltene modification to improve asphalt and crude oil processing. 3 Of course, there is no unique chemical structure of asphaltenes; a very large number of different molecular structures will exist within a given asphaltene sample. Nevertheless, it is reasonable and has been fruitful to pursue probable molecular structures and prevalent moieties for asphaltenes. Asphaltene chemical structure has been the subject of extensive study for many years, producing a general understanding of bulk chemical structural properties. For instance, C 13 NMR provides the aromatic fraction of carbon (∼40%) and infrared (IR) studies show that the hydrogen substitution is mostly on saturated carbon, predominantly in methylene groups and, to a lesser extent, methyl groups. 1,2 X-ray absorption near edge structure (XANES) studies of sulfur show that the small sulfur fraction is predominantly thiophenic with smaller quantities of sulfides. 3 Some asphaltenes have oxidized sulfur, virtually all of it in the sulfoxide form. 3 XANES studies have shown that the small nitrogen fraction of asphaltenes is virtually all aromatic, with pyrrolic nitrogen being more abundant than pyridinic nitrogen. 3 Various fragmentation studies have shown the existence of a large number of chemical moieties. For instance, destructive oxidative studies of asphaltenes have identi- fied relatively large numbers of small alkyl chains, a few long chains, and naphthenic rings, especially five and six membered. 4 Laser desorption mass spectral (LDMS) studies have been employed to estimate mo- lecular weights of asphaltenes, finding values in the range of 1000 amu. 5 This is significantly smaller than molecular weights (∼2000 amu) found by measurements of colligative properties. 1-3 LDMS may suffer from fragmentation, while colligative measurements probably suffer from molecular association of asphaltenes. Despite the wealth of chemical structural information available for asphaltenes, there remain many important structural unknowns. The distribution of aromatic rings moieties in asphaltenes is one of the most signifi- cant unresolved issues regarding asphaltene molecular structure. In particular, it has been very difficult to assess the relative population of small aromatic ring systems (isolated one-ring to three-fused-ring systems) vs larger aromatic ring systems which are ever-present in asphaltenes. This difficulty is because, for most bulk analysis techniques, the small aromatic ring systems are hidden by large rings systems. For instance, C 13 † Schlumberger-Doll Research. ‡ Rose-Hulman Institute of Technology. § Present address: U.C. Davis, Davis, CA 95616. X Abstract published in Advance ACS Abstracts, April 1, 1996. (1) Chilingarian, G. V.; Yen, T. F. Bitumens, Asphalts and Tar Sands; Elsevier Sci. Pub. Co.: New York, 1978. (2) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: New York, 1984. (3) Sheu, E. Y.; Mullins, O. C. Asphaltenes: Fundamentals and Applications; Plenum Pub. Co.: New York, 1995. (4) Strauss, O. P.; Mojelsky, T. W.; Lown, E. M. Fuel 1992, 71, 1355. (5) Hunt, J. E. Private communication. 623 Energy & Fuels 1996, 10, 623-630 0887-0624/96/2510-0623$12.00/0 © 1996 American Chemical Society