Modeling Free-Radical Reactions, Produced by Hydrocarbon Cracking, with Asphaltenes

摘要

Radical adsorptions and reactions produced in oil hydrocarbon cracking with an asphaltene model molecule were carried out with CATIVIC and MOPAC programs. Density functional theory (DFT) calculations were also performed for comparison. Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) reaction mechanisms, of H·, CH_3·, and CH_3CH_2· radicals with adsorbed H·, were studied on asphaltene unsaturated fragments. Qualitative results indicated that it is possible that radical adsorptions on the asphaltene surface lead to bond activations (surface deformation). The following adsorption energy trend was found for the three different programs: H· > CH_3· > CH_3CH_2·. Approximate reaction energy barriers were evaluated. The ER mechanism has smaller reaction barriers and higher exothermic reactions than the LH mechanism, which means that ER is energetically more feasible than LH. These results indicate that, at low temperatures and high pressure, the asphaltene surface may play an important role in the recombination of free radicals.