Heavy
oils are enriched with polycyclic (or polynuclear) aromatic hydrocarbons (PAH
or PNA), but characterization of their chemical structures has been a great
challenge due to their tremendous diversity. Recently, with the advent of
molecular imaging with noncontact Atomic Force Microscopy (nc-AFM), molecular
structures of petroleum has been imaged and a diverse range of novel PAH
structures was revealed. Understanding these structures will help to understand
their chemical reactivities and the mechanisms of their formation or conversion.
Studies on aromaticity and bonding provide means to recognize their intrinsic
structural patterns which is crucial to reconcile a small number of structures
from AFM and to predict infinite number of diverse molecules in bulk. Four types
of PAH structures can be categorized according to their relative stability and reactivity,
and it was found that the most and least stable types are rarely observed in
AFM, with most molecules as intermediate types in a subtle balance of kinetic
reactivity and thermodynamic stability. Local aromaticity was found maximized when
possible for both alternant and nonalternant PAHs revealed by the aromaticity index
NICS (Nucleus-Independent Chemical Shift) values. The unique role of
five-membered rings in disrupting the electron distribution was recognized. Especially,
the presence of partial double bonds in most petroleum PAHs was identified and
their implications in the structure and reactivity of petroleum are discussed.
Identify Similarities in Diverse Polycyclic Aromatic Hydrocarbons of Asphaltenes and Heavy Oils Revealed by Noncontact Atomic Force Microscopy: Aromaticity, Bonding, and Implications in Reactivity
