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

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 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.

Theoretical Study on Thermodynamic Properties of Pyrolysis of Cellulose Dimer Model Compound

“…除了纤维素二聚体不同类型的键均裂热力学变化 外, 我们还对比 Huber 等的实验 [16] , 理论动力学计算结 果以及提出的反应路径 [10] , 考察了产生其它重要小分 子, 比如 HMF, LGA 等分子的热力学变化情况, 如图 6 所示. 由二聚体 A 可以经历分子内重排形成四元环结构 B33 和烯醇 B9i, 体系自由能略微升高 2.7 kcal/mol.…”

High-temperature decomposition of the cellulose molecule: a stochastic molecular dynamics study

“…the elementary reaction mechanisms underlying pyrolysis, is regarded as one of the fundamental challenges for a thorough understanding of cellulose (and in general polymer) pyrolysis (Mettler et al 2012b). In the recent years, there have been attempts to complement the experimental data on cellulose pyrolysis through electronic structure calculations and atomistic simulations that are capable, one way or the other, of capturing individual chemical reactions that occur during the decomposition process (Liu et al 2011b;Zhang et al 2011b;Mayes and Broadbelt 2012;Agarwal et al 2012;Hosoya and Sakaki 2013;Zhang et al 2014;Murillo et al 2015;Zhang et al 2015a, b;Zheng et al 2016). This is a formidable challenge, as the pyrolysis chemistry of cellulose is highly complex, involving thousands of chemical reactions and yielding hundreds of volatile species (Zhou et al 2014).…”

“…Concerning cellulose, Agarwal et al utilised metadynamics-accelerated AIMD to study the decomposition pathways of cellulose pyrolysis at 327°C and 600°C (Agarwal et al 2012). Their simulations revealed a variety of possible initial reaction steps.…”

Biomass Pyrolysis Kinetics: A Review of Molecular-Scale Modeling Contributions

“…inter-sheet hydrogen bonds (Mazeau, 2005, Lin et al, 2009, Agarwal et al, 2011, Agarwal et al, 2012. As opposed to intra-and inter-chain hydrogen bonds, inter-sheet hydrogen bonds were found to provide the crystalline structure with structural integrity and to be the origin of cellulose recalcitrance in solution and at high-temperatures as validated by simulations.…”

“…Agarwal et al used Car-Parinello metadynamics simulations (CPMD) to uncover mechanisms that lead to precursors of levoglucosan, the main product of cellulose fast pyrolysis, and to minor compounds such as 5-hydroxy-methylfurfural (5HMF) and formic acid (Agarwal et al, 2012). At high-temperatures (873 K) Agarwal et al observe the formation of pre-LVG compounds through a transition state stabilized by anchimeric effects and hydrogen bonding with a freeenergy barrier of 36 kcal/mol.…”