Amorphous paracrystalline structures from native crystalline cellulose: A molecular dynamics protocol

Abstract

Cellulose amorphization is a suitable process that allows greater accessibility of cellulolytic enzymes to glycosidic bonds of cellulose, thus improving the yield of bioethanol obtained from lignocellulosic biomass. In a theoretical framework, we demonstrated the amorphization process of two cellulose allomorphs (Iα, Iβ) using a new Molecular Dynamics (MD) protocol at high temperatures. At 700 K, in the high-temperature molten phase, these allomorphs showed the formation of structures with quite amorphous-shell and slightly paracrystalline-core (Am-Par structures) when the degree of polymerization is increased. Applying different characterization methods, we confirmed that Am-Par structures were similar at 700 K, as well as over a temperature ranging from 680 K to 380 K. This confirmation was based on low density of intra-chain hydrogen bonds (HBs), the absence of bands in the region of 3330–3470 cm−1 corresponding to such HBs in the simulated IR, low values of the lateral order index calculated from IR, Q4 and Q6 values close to zero, loss of long-range ordering observed in radial distribution functions, total destruction of crystal unit cell corroborated by X-ray diffraction, and similarity between probability density distribution of dihedral angle ψ with that computed for completely amorphous cellulose. Nonetheless, as revealed by X-ray diffraction results, this protocol seems to provide more crystalline structures starting from cellulose microfibrils having a polymerization degree higher than 10.