@article{Agarwal2012,
abstract = {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 I$\beta$ 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 {\textless} pre-HMF {\textless} formic acid, explaining why LGA is the kinetically favored product of fast cellulose pyrolysis. {\textcopyright} 2012 American Chemical Society.},
author = {Agarwal, Vishal and Dauenhauer, Paul J. and Huber, George W. and Auerbach, Scott M.},
doi = {10.1021/ja305135u},
journal = {Journal of the American Chemical Society},
number = {36},
pages = {14958--14972},
title = {{Ab initio dynamics of cellulose pyrolysis: Nascent decomposition pathways at 327 and 600 °c}},
volume = {134},
year = {2012}
}