(165a) Nascent Decomposition Pathways of Cellulose From First Principles

Fast pyrolysis is a burgeoning
technology that converts lignocellulosic biomass to processable
bio-oil.^[1] Commercializing pyrolysis would require efficient process design,
especially reactors as they are one of the most energy intensive
units in the whole process. This would in turn require detailed
understanding of pyrolysis chemistries. Biomass is mainly composed of
the biopolymer cellulose; therefore, understanding complex cellulose
pyrolysis chemistries is important for efficiently modeling and
optimizing fast pyrolysis reactors. We have modeled nascent
decomposition pathways of cellulose at 600 ^OC
using Car-Parrinello molecular dynamics (CPMD)^[2]
simulations. We used a simulation cell of 4 cellobiose residues
periodically repeated in all directions to model decomposition of
cellulose matrix. We generated initial configurations by performing
classical NPT
molecular dynamics
simulations on large periodic cells of cellulose^[3]
and then clipping out the required simulation cell of 4 cellobiose
residues. By applying the metadynamics method^[4]
using multiple sets of collective variables, we have found various
possible nascent processes that may occur during pyrolysis such as
depolymerization, fragmentation, ring opening, and ring contraction.
The nascent processes observed can explain the formation of
precursors to major products observed during cellulose pyrolysis such
as levoglucosan (LGA), hydroxy-methylfurfurral (HMF) and
fragmentation products such as formic acid. We found that a low
barrier process to a precursor of LGA is a concertered process with a
likely intermediate/transition-state stabilized by resonance and
nearby hydrogen bonding. We computed a barrier of 48.6 kcal/mol for
this process, which is in excellent agreement with experimentally
estimated activation energies, suggesting that depolymerization of
cellulose to this precursor of LGA (pre-LGA) is an important,
rate-limiting step. We found that free-energy barriers are in the
order of pre-LGA < pre-HMF < formic acid. We also found LGA to
be thermodynamically more stable than HMF. Kinetic and thermodynamic
favorability taken together explain why LGA is the major product
observed during pyrolysis.

[1] Y.-C. Lin, J. Cho, G. a. Tompsett, P. R.
Westmoreland, G. W. Huber, The Journal of
Physical Chemistry C 2009,
113, 20097.

[2] R. Car, M. Parrinello, Physical
Review Letters 1985,
55, 2471.

[3] V. Agarwal, G. W. Huber, W. C. Conner, S. M.
Auerbach, The Journal of chemical physics
2011, 135,
134506.

[4] A. Laio, M. Parrinello, Proceedings
of the National Academy of Sciences of the United States of America
2002, 99,
12562.