Development of Detailed Kinetic Models for the Thermal Conversion of Biomass via First Principle Methods and Rate Estimation Rules

ion rate constants for methyl are less uniform by nature, the calculation results will also be less accurate, because the heavier methyl radical produces to two low-frequency bending modes in the transition state. Frequencies depend inversely on the square root of the reduced mass; hence heavier abstracting reactants generate bending modes of lower frequency (if all other parameters are kept the same). Unavoidable small errors in low-frequency vibrational modes lead to large errors in the pre-exponential factor and might create fluctuations in the rate constants of the test set. This makes it difficult to assign deviations of individual rate constants to real reactivity differences. In Figure 6 and Figure 7 we present the results for H abstraction from the hydroxyl, and α-CH and β-CH groups by methyl radicals. The rate constants, especially for the reactions of the OH and β-C-H sites show the expected impact from steric interactions. For example, the rate constants for isoand tertiary butanol in Figure 6a are clearly smaller than those for the remaining alcohols. Deviations are seen over the entire temperature range but they are largest at the lower temperature end. This indicates that both the pre-exponential factors and the activation energies are influenced by steric interactions. Despite these deviations, the corresponding rate rules at 1000 K are still within a factor of two of the TST results as Figure 6b reveals for abstraction reactions of the OH group. The following rate rule expressions for CH3 as abstracting radical have been obtained: Since the OH moiety has its biggest impact on the reactivity of the α-CH bond, we selected this reaction class to compare the Evans-Polanyi relationship for CH3 radicals with that obtained for H atoms (see Figure 8). Both slopes are identical within the error margins and larger than unity. In general one would expect a value between 0 and 1 for the slope, but larger values have been reported for other reactions (46). The large value might be understood in terms of an additional stabilization of the transition states (relative to the products) by hyperconjugation with the alkyl groups. This explanation is equally valid for both, H atoms and CH3 radicals as abstracting species. Further we notice a shift of the straight lines by 4 kcal/mol. This reflects that, similar to H abstraction reactions involving pure hydrocarbons, the barriers for abstractions by methyl groups are higher than those