Group Additivity Determination for Oxygenates, Oxonium Ions, and Oxygen-Containing Carbenium Ions

Bio-oil produced from biomass fast
pyrolysis often requires catalytic
upgrading to remove oxygen and acidic species over zeolite catalysts.
The elementary reactions in the mechanism for this process involve
carbenium and oxonium ions. In order to develop a detailed kinetic
model for the catalytic upgrading of biomass, rate constants are required
for these elementary reactions. The parameters in the Arrhenius equation
can be related to thermodynamic properties through structure–reactivity
relationships, such as the Evans–Polanyi relationship. For
this relationship, enthalpies of formation of each species are required,
which can be reasonably estimated using group additivity. However,
the literature previously lacked group additivity values for oxygenates,
oxonium ions, and oxygen-containing carbenium ions. In this work,
71 group additivity values for these types of groups were regressed,
65 of which had not been reported previously and six of which were
newly estimated based on regression in the context of the 65 new groups.
Heats of formation based on atomization enthalpy calculations for
a set of reference molecules and isodesmic reactions for a small set
of larger species for which experimental data was available were used
to demonstrate the accuracy of the Gaussian-4 quantum mechanical method
in estimating enthalpies of formation for species involving the moieties
of interest. Isodesmic reactions for a total of 195 species were constructed
from the reference molecules to calculate enthalpies of formation
that were used to regress the group additivity values. The results
showed an average deviation of 1.95 kcal/mol between the values calculated
from Gaussian-4 and isodesmic reactions versus those calculated from
the group additivity values that were newly regressed. Importantly,
the new groups enhance the database for group additivity values, especially
those involving oxonium ions.