A molecular mechanics-density functional study of the adsorption of fragments of asphaltenes and resins on the (001) surface of Fe2O3

A combination of Molecular Mechanics and Density Functional Theory allowed the calculation of the charge distribution and the adsorption energy on the (001) surface of hematite (alpha -Fe2O3) of a set of twelve molecules that represent fragments of resins and asphaltenes. The results showed that no covalent bond was formed with the inorganic surface upon adsorption, which indicates that physisorption instead of chemisorption takes place. In aroma tic hydrocarbons, the adsorption energy was found to be proportional to the molecular surface, which indicates the main role, played for the van der W aals forces in the adsorption of these molecules. For 2-substituted naphtha lenes, the energy decreased with the increase in the length of the alkyl ch ain. This decrease was related to the gain in deformation energy of the alk yl chain that was required by the adsorption process. The aromaticity and t he WC ratios were found to be good indicators of the ability of the molecul es to be adsorbed on the hematite surface. In particular, molecules with hi gh aromaticity and low H/C ratio show high adsorption energy. This result c an explain the experimental finding of a high content of high aromatic and low hydrogen content molecules found in asphaltenic deposits extracted from wells. Substitution of the central -CH2- group by N-H, O or S at the five- member ring in fluorene produced complex changes in the adsorption energies . In some these molecules, the atoms protruding from the molecular plane pr oduced reduced adsorption energies.