Petroleum molecular structure

Because of the existence of numerous isomers, hydrocarbon mixtures having a large number of carbon atoms can not be easily analyzed in detail. It is common practice either to group the constituents around key components that have large concentrations and whose properties are representative, or to use the concept of petroleum fractions. It is obvious that the grouping around a component or in a fraction can only be done if their chemical natures are similar. It should be kept in mind that the accuracy will be diminished when estimating certain properties particularly sensitive to molecular structure such as octane number or crystallization point. 

Petroleum chemistry is concerned with the origin, composition, and properties of naturally occurring petroleum deposits, whether in liquid (crude oil or petroleum), gaseous (natural gas), or solid (tars and asphalts) form. All of them are essentially mixtures of hydrocarbons. Whereas natural gas contains a few lighter hydrocarbons, both crude oil and tar deposits may consist of a large number of different hydrocarbons that cannot be easily identified for molecular structure or analyzed for composition. 

NMR spectroscopy is one of the most widely used analytical tools for the study of molecular structure and dynamics. Spin relaxation and diffusion have been used to characterize protein dynamics [1, 2], polymer systems[3, 4], porous media [5-8], and heterogeneous fluids such as crude oils [9-12]. There has been a growing body of work to extend NMR to other areas of applications, such as material science [13] and the petroleum industry [11, 14—16]. NMR and MRI have been used extensively for research in food science and in production quality control [17-20]. For example, NMR is used to determine moisture content and solid fat fraction [20]. Multi-component analysis techniques, such as chemometrics as used by Brown et al. [21], are often employed to distinguish the components, e.g., oil and water. 

I thank the Research Corporation, the Petroleum Research Fund administered by the American Chemical Society and the U.S. Department of Energy (DE-FG02-80ER10125) for their financial support. I also acknowledge my fruitful collaborations with Dr. John C. Huffman, Director of the Molecular Structure Center, Indiana University, as well as my own group Whose contributions are cited in the references. 

The use of molecular wires and devices for electronics applications is destined to occur. The ability to control molecular structures at the subnanometer scale is obvious throughout chemical synthesis. These are the same techniques that have been optimized over the last 50 years for the synthesis and modification of compounds for pharmaceutical, dye, petroleum, and fine chemical indus-... 

Naphthenes or cycloparaffins are formed by joining the carbon atoms in ring-type structures, the most common molecular structures in petroleum. These hydrocarbons are also referred to as saturated hydrocarbons since all the available carbon atoms are saturated with hydrogen. Typical naphthenes and their respective physical properties are listed in Table 4.2 and shown in Figure 4.3. 

In terms of the elemental composition of petroleum, the carbon content is relatively constant it is the hydrogen and heteroatom contents that are responsible for the major differences. Nitrogen, oxygen, and sulfur are present in only trace amounts in some petroleum, which thus consists primarily of hydrocarbons. On the other hand, a crude oil containing 9.5% heteroatoms may contain essentially no true hydrocarbon constituents insofar as the constituents contain at least one or more nitrogen, oxygen, and/or sulfur atoms within the molecular structures. 

All organisms synthesize carbohydrates, lipids, proteins, and polynucleotides, although the details of their molecular structures can be somewhat species specific. These basic classes of macromolecules have changed little over geologic time. The secondary metabolites are more species specific and have also changed little over geologic time. Many are resistant to degradation, and those provide excellent biomarkers that have been preserved in ancient marine sediments and petroleum deposits. 

Relation of Properties to Molecular Structure for Petroleum Hydrocarbons... 

The foregoing pages have shown a number of ways in which physical constants vary with molecular structure. Petroleum chemists and automotive engineers have come to recognize that the performance of a motor fuel in an internal combustion engine is also dependent upon the structure of the hydrocarbon molecules which the fuel contains. This does not mean that engine performance is a function of the physical constants, but rather that the features of molecular structure which determine the one also determine the other. 

Typical molecular structures of coal, petroleum, and natural gas. 

Chemical classifications of petroleums relate to the molecular structures of the molecules in the oil. Of course the smaller molecules, six carbon atoms and less, are predominately paraffins. So chemical classification is usually based on analysis of the petroleum after most of the light molecules are removed. 

This section will review the nature of nickel and vanadium compounds present in petroleum oil, beginning with a detailed discussion of the metal compound categories and their molecular structure. The final section of this discussion will consider the manner in which these two classes of metal-bearing compounds are distributed or associated in the oil. 

Unlike metalloporphyrins, the nonporphyrinic metal compounds are poorly characterized with respect to molecular structure and properties. Examining the nature of the nickel and vanadium in these compounds is important from the standpoint that often most of the Ni and V in a petroleum is nonporphyrinic, as shown in Table II. Sugihara et al. (1970) suggested that the nonporphyrin metal compounds comprise a wide variety of coordinated complexes resulting from the reaction of inorganic forms of the metals with polar organic molecules. Larson and Beuther (1966) speculated that the nonporphyrinic metal complexes are simply... 

The composition of the various feedstocks may, at first sight, seem to be of minor importance when the problem of the hydrodesulfurization of heavy oils and residua comes under consideration. However, consideration of the variation in process conditions that were outlined in the previous section (Table 6-6) for different feedstocks (where the feedstocks are relatively well-defined boiling fractions of petroleum) presents some indication of the problems that may be encountered where the feedstocks are less well defined. Molecular composition is as important as molecular weight (or boiling range). Such is the nature of the problem when dealing with various residua and heavy oils which are (to say the least) unknown in terms of their chemical composition. In fact, the complexity of these particular materials (Chapter 3) has allowed little more than speculation as to the molecular structure of the constituents. 

Polar aromatics resins the constituents of petroleum that are predominantly aromatic in character and contain polar (nitrogen, oxygen, and sulfur) functions in their molecular structure(s). 

Advances in petroleum characterization at the molecular structure level by GC-MS methods renewed interest in OSC. Within the past few years, at least one-thousand new and novel OSC that previously were not known to be present in petroleum and bitumens have been reported. Tentative molecular structures inferred from GC-MS and other techniques have been confirmed in many cases by synthesis of authentic reference-compounds. The difficult and time-consuming synthetic work has been crucial in validating many of the novel structures. Another key finding has been that immature bitumens and crude oils (samples that have not received significant thermal stress) differ markedly from the previously known OSC in that they have carbon-skeletons resembling ubiquitous biomarker hydrocarbons (e.g., n-alkanes, isoprenoid alkanes, steranes, and hopanes). This similarity, of course, suggests that the hydrocarbons and OSC have common biogenic precursors. 

The pyrolysis properties of four model compounds were examined. Their molecular structures are shown in Table II. The three and four-ring molecules, containing a benzo [b] thiophenic unit were synthesized by Dr. Cagniant (L.S.C.O., Metz University). A polycyclohexanesulfide is an aliphatic sulfide synthesized by Dr. N. Spassky (Laboratory of Macromolecular Chemistry, Paris VI University). Polymeric aromatic sulfide was represented by a polybenzosulfide provided by Philips Petroleum. 

This advantage is due to molecular structure and the lack of crystalline wax particles, present in some refined petroleum oils. Fully synthetic engine oil lubricants offer excellent low temperature flow and viscosity properties.(Demmin etal., 1992 Lakes, 1999). 

We thank the National Science Foundation, the donors of the Petroleum Research Fund administered by the American Chemical Society, and the Wrubel Computing Center for financial support. We are also indebted to Drs. John Huffman, Kirsten Folting, and William Streib at the Molecular Structure Center for single-crystal X-ray studies, to Mr. David L. Clark for assistance with the Fenske-Hall calculations, and to Drs. Dennis Lichtenberger and Edward Kober for obtaining photoelectron spectra. 

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. 

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