Ab Initio Dynamics of Cellulose Pyrolysis: Nascent Decomposition Pathways at 327 and 600 掳C
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- 作者：Vishal Agarwal ; Paul J. Dauenhauer ; George W. Huber ; Scott M. Auerbach
- 刊名：The Journal of the American Chemical Society
- 出版年：2012
- 出版时间：September 12, 2012
- 年：2012
- 卷：134
- 期：36
- 页码：14958-14972
- 全文大小：876K
- 年卷期：v.134,no.36(September 12, 2012)
- ISSN：1520-5126
文摘
We modeled nascent decomposition processes in cellulose pyrolysis at 327 and 600 掳C using Car鈥揚arrinello 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尾 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)鈥攖he major product of pyrolysis鈥攁nd also to minor products such as 5-hydroxy-methylfurfural (HMF) and formic acid. At 327 掳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 掳C. At 600 掳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 掳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 掳C are in the order pre-LGA < pre-HMF < formic acid, explaining why LGA is the kinetically favored product of fast cellulose pyrolysis.