(420c) Pyrolysis Kinetics of Anisole a Simple Lignin Model Compound

Pyrolysis Kinetics of
Anisole a Simple Lignin Model Compound

Abstract

    The thermal decomposition of lignin, one of the
major components of biomass, is not well characterized and leads to formation
of undesirable products that require substantial upgrading before using it as a
drop-in fuel. An improved understanding of the underlying decomposition reactions
of lignin could provide insight into process control conditions that could
mitigate the formation of these undesirable products during the fast pyrolysis process
to produce biofuels. Because the structure of lignin is so complex, model
compounds are often utilized to gain insight into this underlying chemistry.
The simplest model that can be employed to investigate the weak phenolic-carbon
linkage in lignin is anisole (C₆H₅OCH₃). Although
several studies have been carried out to understand the decomposition of
anisole, there are limited data available under fast pyrolysis process
conditions. Furthermore, the existing models have not been able to predict the
observed product formation. In this work, we present anisole thermal
decomposition data that was collected under conditions that closely resemble
fast pyrolysis (T=525-650°C, t
~0.9s, and P~0.8
atm) using a tubular flow reactor. These data were modeled using a mechanism
that was developed by Pecullan et al.¹ , but it was modified to
include updated rate constants for the reactions involving the  o-methylcyclohexadienone
potential energy surface that were derived from electronic structure calculations.

The stationary points and transition
states located on this PES were calculated using the CBS-QB3 level of theory
using Gaussian 03.² To improve the accuracy of the D_fH₂₉₈,
S₂₉₈, and C_p values, low frequency vibrational modes that
resemble torsions around single bonds are treated as hindered internal rotors;
the hindrance potentials were calculated at the B3LYP/6-31G(d) level of theory
via relaxed surface scans. The electronic energy of each species was converted
to its heat of formation using the atomization method. High-pressure rate
coefficients were calculated using canonical transition state theory. These
high pressure rate constants were used to obtain apparent pressure and
temperature dependent rate constants using Quantum-Rice-Ramsperger-Kassel
theory with a modified strong collision approximation.³ This
mechanism was used to model the data collected here as well as previously
published data;  the simulations were performed using the ChemKin-Pro software
package.⁴ 

Figure
1. Potential Energy Surface for o-methyl
cyclohexadienone (units:  kcal mol ^-1)

A simplified version of o-MecyHDOE
potential energy surface calculated at the CBS-QB3 level of theory is shown in
Figure 1.  The barrier for isomerization to form o-cresol (3) is predicted to
be ~53 kcal mol^-1, while those to form the two
other methyl cyclohexadienone isomers (7 and 10) are much lower (~41 and ~43 kcal mol^-1). Note that these latter
isomerization reactions proceeds through a diradical intermediate, which can
eliminate CO to form 1- and 2-methyl cyclopentadiene (P3 and P4). The overall
barriers for the formation of these two species are ~57 and ~63 kcal mol^-1, respectively. The barrier to
form 1-methyl cyclopentadiene is comparable to that for ortho-cresol formation,
suggesting that these channels might be of comparable
importance. The surface also contains several
reaction pathways to methyl phenoxy plus H products (P1). Although these
reactions are ~6 kcal mol^-1 higher in energy
than the entrance channel (P2), these dissociation reactions are entropically
favored when compared to isomerization. These channels are potentially
important since these H atoms can add to methyl phenoxy radical to directly
form o-cresol or they can add to phenoxy radical to form phenol. 

The potential
energy surface determined in this work has some important differences compared
to the one originally estimated by Pecullan et al.¹ (Note that at
the time that the Pecullan et al. mechanism was assembled, high level
electronic structure calculations were generally not feasible for systems of
this size. Thus, this PES and the corresponding high pressure rate constants
were estimated in analogy to other similar reactions.) The
CBS-QB3 barrier for ortho-cresol formation is ~14 kcal mol^-1
higher in energy, while those for
methylcyclopentadiene formation are at least 25
kcal mol^-1 higher in energy. We find that o-methyl
cyclohexadienone reacts to form the 1- and 2-methyl cyclopentadiene isomers,
while Pecullan's analysis predicts the formation of the 5-methyl
cyclopentadiene isomer.

 Incorporation of these rate constants leads to improved
predictions for many of the observed products. Selected comparisons are shown in
Fig. 2. Although this updated mechanism is an improvement, it is clear that
further progress is needed. Current efforts are focused on extending the type
of analysis described here to other reactions that occur during anisole
pyrolysis.  

Figure 2. Comparisons of anisole pyrolysis products
distribution data with model predictions where solid lines indicate the updated
model and the dashed lines indicate the Pecullan model.

References

1. Pecullan, M,; Brezinsky, K,;  and Glassman, I, J. Phys Chem A., 1997, 101, 3305 ? 3316.
2. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A. et al. Gaussian 03, Revision A. 1, Gaussian, Inc: Pittsburgh, PA, 2003.

3.       Chang,
A. Y.; Bozzelli, J. W.; Dean, A. M., Z. Phys. Chem. 2000, 214,
1533-68

4.       ChemKin-Pro,
Reaction Design, San Diego, 2008