A theoretical study on bond dissociation enthalpies of coal based model compounds

The homolytic bond dissociation enthalpies (BDE) of various bonds (CH, CC, CO, and OH) for coal based model compounds that are representative of the functionalities present in coal were computed by using a double-hybrid method mPW2PLYP. The BDE for CH, CC, CO, OH cover a range from 111.4 to 81.2 kcal/mol, 114.1 to 62.8 kcal/mol, 107.6 to 52.6 kcal/mol, and 111.2 to 86.6 kcal/mol, respectively. The so-called “weak bond” in coal does not mean a specific bond type, but it is associated with the BDE and highly depends on the stabilization of the fragmented radicals. Our work suggests that the initiation steps of the coal pyrolysis are most probably the cleavage of CO and CC bond to form the phenoxy radical or benzyl radicals. With the increase of temperature, the loss of alkyl and hydrogen groups becomes feasible, whereas the phenyl-like radicals are the most difficult to form.