Ab Initio Dynamics of Cellulose Pyrolysis: Nascent Decomposition Pathways at 327 and 600 °C

We modeled nascent decomposition processes in cellulose
pyrolysis
at 327 and 600 °C using Car–Parrinello molecular dynamics
(CPMD) simulations with rare events accelerated with the metadynamics
method. We used a simulation cell comprised of two unit cells of cellulose
Iβ periodically repeated in three dimensions to mimic the solid
cellulose. To obtain initial conditions at reasonable densities, we
extracted coordinates from larger classical NPT simulations at the
target temperatures. CPMD-metadynamics implemented with various sets
of collective variables, such as coordination numbers of the glycosidic
oxygen, yielded a variety of chemical reactions such as depolymerization,
fragmentation, ring opening, and ring contraction. These reactions
yielded precursors to levoglucosan (LGA)the major product
of pyrolysisand also to minor products such as 5-hydroxy-methylfurfural
(HMF) and formic acid. At 327 °C, we found that depolymerization
via ring contraction of the glucopyranose ring to the glucofuranose
ring occurs with the lowest free-energy barrier (20 kcal/mol). We
suggest that this process is key for formation of liquid intermediate
cellulose, observed experimentally above 260 °C. At 600 °C,
we found that a precursor to LGA (pre-LGA) forms with a free-energy
barrier of 36 kcal/mol via an intermediate/transition state stabilized
by anchimeric assistance and hydrogen bonding. Conformational freedom
provided by expansion of the cellulose matrix at 600 °C was
found to be crucial for formation of pre-LGA. We performed several
comparison calculations to gauge the accuracy of CPMD-metadynamics
barriers with respect to basis set and level of theory. We found that
free-energy barriers at 600 °C are in the order pre-LGA <
pre-HMF < formic acid, explaining why LGA is the kinetically favored
product of fast cellulose pyrolysis.