Origin of Free Radical Persistence in Asphaltenes: Cage Effect and Steric Protection

Heavier petroleum fractions contain persistent free radicals, and the highest concentration of persistent free radicals is usually found in asphaltenes. The origin of free radical persistence was studied. The potential influence of exposure to air, free radical caging, and steric protection was evaluated. The free radical content of asphaltenes diluted in toluene remained constant to within 5% over a period of 465 h under both nitrogen and air atmospheres. The persistent free radicals were accessible for reaction in dilute solution. Caging of free radicals due to high viscosity could not explain the observations. Caging of free radicals due to molecular-level aggregation was possible, but explaining all of the observations in terms of the cage effect became tenuous. The asphaltenes appeared unencumbered by steric constraints compared to 9,10-dihydroanthracene during hydrogen transfer reactions, which made it unlikely that steric protection was a major contributor to the persistence of free radicals. The asphaltenes were reactive at 250 °C, both for self-reaction and for conversion of various model molecules (1,2-dihydronaphthalene, α-methylstyrene, 2,4,6-trimethylstyrene, 1,1-diphenylethylene, trans-1,2-diphenylethylene, triphenylethylene, tetraphenylethylene, and 9,10-dihydroanthracene). An alternative explanation for the origin of free radical persistence was forwarded. It was considered unlikely that individual free radical species could remain as persistent free radicals over geologic time but also be reactive. The observed persistence of free radicals could be explained in terms of a dynamic "equilibrium" of free radical pairs, which is a self-stabilizing process that gives the impression of free radical persistence. Dynamic formation and destruction of free radicals through addition and decomposition reactions of radical pairs average out on the macroscopic scale to create the impression of persistent free radicals. Any individual species does not remain persistent beyond what one would expect from a reactive persistent free radical.