High-temperature decomposition of the cellulose molecule: a stochastic molecular dynamics study
A Paajanen and J Vaari, CELLULOSE, 24, 2713-2725 (2017).
The kinetics and products of cellulose pyrolysis can be studied using large-scale molecular dynamics simulations at high temperatures, where the reaction rates are high enough to make the simulation times practical. We carried out molecular dynamics simulations employing the ReaxFF reactive force field to study the initial step of the thermal decomposition process. We gathered statistics of simulated reactive events at temperatures ranging from 1400 to 2200 K, considering cellulose molecules with different molecular weights and initial conformations. Our simulations suggest that, in gas-phase conditions at these high temperatures, the decomposition occurs primarily through random cleavage of the beta(1 -> 4)-glycosidic bonds, for which we obtained an activation energy of (171 +/- 2) kJ mol(-1) and a frequency factor of s(-1). We did not observe dependency of the kinetic parameters on the molecular weight or initial conformation. Some of the decomposition reactions involved the release of low-molecular-weight products. Excluding radicals, the most commonly observed species were glycolaldehyde, water, formaldehyde and formic acid. Many of our observations are supported by the existing experimental and theoretical knowledge. We did not, however, observe the formation of levoglucosan, which is the dominant product in conventional pyrolysis experiments at much lower temperatures. This is understandable, since the high temperatures can force the dominance of radical reactions over pericyclic reactions. Nevertheless, our results support further use of ReaxFF-based molecular dynamics simulations in the study of cellulose pyrolysis.
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