Modeling of asphaltenes: Assessment of sensitivity of 13C SSNMR to molecular structure

Asphaltenes are an important constituent of many oils and one of the major components in heavy oils sources for liquid hydrocarbon fuels. With light oil sources rapidly depleting, it is necessary to use alternative sources from heavy oil reservoirs. Refining these heavy oil resources presents new challenges and the study of the detailed molecular composition and structure of asphaltenes is becoming of great importance for optimizing the entire refinery process. At present, the structure of asphaltenes is not fully understood and only a few validated 3D (three dimensional) representative structures of asphaltenes have been published in the literature. Three dimensional models representative of asphaltene structures are a necessary requirement to use advance computational modeling techniques to address asphaltene chemistry and aggregation during extraction, transportation and refinement. Atomistic modeling is routinely used in many industries (pharmaceutical, polymers, coatings, explosives, membrane proteins, etc.) to gain insight into the properties of materials. Using 3D molecular models it becomes possible to calculate molecular properties that can be correlated with experimental data obtained from solid and liquid state 13C nuclear magnetic resonance (NMR) spectroscopy, atomic pair-wise distribution functions, thermo gravimetric analysis (TGA) data on pyrolysis kinetics, small angle X-ray scattering (SAXS) and ion cyclotron resonance mass spectroscopy (ICR-MS), etc. Calculated properties that are sensitive to the 3D models and the underlying 2D chemical models used in their construction can be used as guideposts in the validation of the proposed models. The application of these techniques to the elucidation of asphaltene structures has been limited to a few applications reported in Ref. 2. Among them it is noteworthy to mention the recent work by Ruiz-Morales and Mullins to understand UV spectra and fluorescence of asphaltenes. Solid state NMR (SSNMR) has been used to characterize complex materials such as coals, but there are very few applications of SSNMR to asphaltenes. The effective use of 13C SSNMR in the characterization of asphaltenes must be predicated on the intrinsic sensitivity of 13C SSNMR spectra to the structure of these materials, a fact that has not yet been established in the literature. In this paper we calculate 13C SSNMR spectra of model asphaltene structures with the overall goal of assessing how valuable 13C SSNMR studies of asphaltenes can be in guiding the development of representative 3D models of asphaltenes. We accomplish this by analyzing the sensitivity of the 13C SSNMR spectra to: i) the 2D chemical model used to develop the 3D model, ii) the different thermally accessible 3D conformations for the same 2D model and iii) the aggregation of asphaltenes.