Hydrogen abstraction of alkyl radicals from polycyclic aromatic hydrocarbons and heterocyclic aromatic hydrocarbons

Abstract To understand the condensation during heavy oil thermal processing, the formation of aromatic carbon radicals by H-abstraction of methyl radicals ( CH3) or ethyl radicals ( C2H5) from polycyclic aromatic hydrocarbons (PAHs) or heterocyclic aromatic hydrocarbons (hetero-PAHs) was studied by density functional calculation. The H-abstraction from different PAHs all has a positive standard state Gibbs free energy change, while increasing reaction temperature or radical concentration promotes the reaction to proceed spontaneously. CH3 mainly contributes to the formation of aromatic carbon radicals on PAHs, and the activation entropy of H-abstraction determines the difference in reaction rate constants. The particularity of H-abstraction from hetero-PAHs is reflected at the α-site of heterocycles. The H-abstraction from N-containing hetero-PAHs occurs preferentially at the α-site of heterocycles, and both CH3 and C2H5 could participate in abstracting the α-H. The H-abstraction from S-containing hetero-PAHs occurs preferentially on coupled aromatic rings, showing reaction behavior similar to the H-abstraction on PAHs.