Abstract:

Enthalpies of formation and bond dissociation energies(BDE) of n-alkanes and the corresponding radicals of up to 14 carbons, involved in pyrolysis of FischerTropsch naphtha were calculated using various density functional theory(DFT) methods. It was found that the MPW1B95/6-311G(d,p) method was the best in term of accuracy among the methods evaluated, including B97-1, BB1K, B1B95, MPWB1K and MPW1B95. On the basis of this good performance, the MPW1B95/6-311G(d,p) method was used for the estimation of large n-alkanes and their resulting radicals. C—C BDEs of n-alkanes were also examined. The enthalpies of formation of n-alkanes and the radicals had deviations of 0.8 and 2.7 kJ/mol respectively compared with experimental data. For C—C BDEs of n-alkanes, this method predicted the results with a mean absolute deviation of 3.1 kJ/mol. It is noted that by means of the MPW1B95/6-311G(d,p) method the enthalpies of formation can be calculated satisfactorily and the C—C BDEs and their change trend can be predicted reasonably.

Key words: density functional theory,bond dissociation energy,enthalpy of formation,n-alkane,alkyl radical