MOLECULAR REPRESENTATIONS OF RATAWI AND ALASKA NORTH SLOPE ASPHALTENES BASED ON LIQUID-STATE AND SOLID-STATE NMR

Asphaltenes are a class of molecules that are defined by the condition s of their insolubility in a particular solvent, such as heptane. A be tter understanding of the molecular properties of this mixture of mole cules is needed in order to understand the mechanisms of coke and slud ge formation during refinery upgrading operations. In this work we dev elop molecular representations for Ratawi and ANS asphaltenes using C- 13 and H NMR techniques. These representations differ from the ''avera ge structures' given previously in that a molecular weight is not assu med for the ''average molecule''. We show that these NMR techniques ca n be used to generate molecular representations that automatically pro vide estimates of the number average molecular weight and that these e stimates are consistent with the upper bound on the number average mol ecular weight that was given previously for these asphaltenes. Both li quid- and solid-state techniques yield representations that display es sentially equivalent molecular characteristics, although these techniq ues are essentially independent. The representations making use of liq uid-state NMR require a portion of the proton spectrum to be consisten t with a corresponding portion of the C-13 spectrum, while in the soli d-state method, cross polarization and dipolar dephasing techniques ar e required to give self-consistent information. In both cases one make s use of the chemical relationship between the amount of bridgehead ca rbons and the size of certain polynuclear aromatics. The molecular equ ivalence of these representations suggests that the asphaltenic molecu les are smaller than previously assumed and that the R groups attached to the polynuclear aromatic core are not as long as previously believ ed.