Abstract
We study the thermal decomposition of cellulose using molecular simulations based on the ReaxFF reactive force field. Our analysis focuses on the mechanism and kinetics of chain scission, and their sensitivity on the condensed phase environment. For this purpose, we simulate the thermal decomposition of amorphous and partially crystalline cellulose at various heating rates. We find that thermal degradation begins with depolymerization via glycosidic bond cleavage, and that the order of events corresponds to a randomly initiated chain reaction. Depolymerization is followed by ring fragmentation reactions that lead to the formation of a number of light oxygenates. Water is formed mainly in intermolecular dehydration reactions at a later stage. The reaction rate of glycosidic bond cleavage follows a sigmoidal reaction model, with an apparent activation energy of 166 ± 4 kJ/mol. Neither the condensed phase environment nor the heating programme have appreciable effects on the reactions. We make several observations that are compatible with mechanisms proposed for cellulose fast pyrolysis. However, due to the absence of anhydrosugar forming reactions, the simulations offer limited insight for conditions of industrial interest. It remains unclear whether this is a natural consequence of the reaction conditions, or a shortcoming of the force field or its parameter set.Graphic abstract
Highlights
We study the thermal decomposition of cellulose using molecular simulations based on the ReaxFF reactive force field
We carried out stochastic ReaxFF-Molecular dynamics (MD) simulations on the thermal decomposition of isolated cellulose chains. (Paajanen and Vaari 2017) We found that the decomposition begins with depolymerization via glycosidic bond cleavage, and that the apparent activation energy is comparable to values reported in global mass loss kinetics studies
We find that crystallinity has no appreciable effects on the mechanism or kinetics of chain scission, the evolution of the molecular weight distribution, or the low molecular weight products (LMWPs)
Summary
We study the thermal decomposition of cellulose using molecular simulations based on the ReaxFF reactive force field. Our analysis focuses on the mechanism and kinetics of chain scission, and their sensitivity on the condensed phase environment For this purpose, we simulate the thermal decomposition of amorphous and partially crystalline cellulose at various heating rates. Molecular dynamics (MD) simulations based on reactive force fields are of particular interest, as they can predict both reaction mechanisms and rates without pre-determined chemistry. One factor that differentiates the earlier pyrolysis-related studies is their description of the condensed phase environment This includes periodic crystals (Qiao et al 2020), amorphous melts (Zheng et al 2016; Chen et al 2017; Rismiller et al 2018) and individual chains (Wang et al 2017). Another question that has received limited attention is that of reaction kinetics
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