(34c) Roles of Rotamerism in Concerted Mechanism of Cellulose Pyrolysis: A Molecular Modeling Study.

Cellulose is the most abundant lignocellulosic biomass as a renewable energy source. Fast pyrolysis is one of the most common processes to convert biomass into value-added products. Significant efforts have been conducted to understand the reaction pathways of cellulose pyrolysis, and several mechanisms were proposed to explain the formation of levoglucosan, the main product of cellulose pyrolysis. However, the kinetics of the glycosidic bond cleavage reaction, the main cellulose decomposition pathway, are still debated.

This presentation aims to elucidate how rotamerism of cellulose affect the lowest-energy path of the concerted glycosidic bond cleavage reaction during its pyrolysis. The low-energy conformers of reactants, transition structures, and products were discovered by the density functional theory. Our results show that ring conformation plays an important role in the cleavage of the glycosidic bond. Not all isomers are in a conformation to follow the glycosidic bond cleavage. The ¹C₄-inverted chair conformer was identified as the most favorable conformation that can follow the path for glycosidic bond cleavage (Fig.1). Our calculation shows that the lowest-energy path has a free energy difference of 49.1 kcal/mol. The activation energy barrier is calculated as 47.5 kcal/mol using the transition state theory, and the pre-exponential factor is estimated to be 5.4 x 10¹² s^-1. The hypothesized concerted mechanism via ¹C₄-inverted chair is energetically more favorable than that of ²S_O-twisted-boat by 6 kcal/mol.

Our dimer model cannot capture hydrogen bonding between linear chains of cellulose. We hypothesize that the activation energy barrier would increase due to the additional energy requirement of hydrogen bond breakage and that our model would be in very good agreement with the experiment value of 53.7 kcal/mol as a result. In summary, our hypothesized pyrolysis mechanism at the molecular level, incorporating the conformational analysis of cellulose, provides novel insights into the formation of levoglucosan.