Cellulose Aggregation under Hydrothermal Pretreatment Conditions

Abstract

Cellulose, the most abundant biopolymer on Earth, represents a resource for sustainable production of biofuels. Thermochemical treatments make lignocellulosic biomaterials more amenable to depolymerization by exposing cellulose microfibrils to enzymatic or chemical attacks. In such treatments, the solvent plays fundamental roles in biomass modification, but the molecular events underlying these changes are still poorly understood. Here, the 3D-RISM-KH molecular theory of solvation has been employed to analyze the role of water in cellulose aggregation under different thermodynamic conditions. The results show that, under ambient conditions, highly structured hydration shells around cellulose create repulsive forces that protect cellulose microfibrils from aggregating. Under hydrothermal pretreatment conditions, however, the hydration shells lose structure, and cellulose aggregation is favored. These effects are largely due to a decrease in cellulose-water interactions relative to those at ambient conditions, so that cellulose-cellulose attractive interactions become prevalent. Our results provide an explanation to the observed increase in the lateral size of cellulose crystallites when biomass is subject to pretreatments and deepen the current understanding of the mechanisms of biomass modification.

Water in cellulose: evidence and identification of immobile and mobile adsorbed phases by 2H MAS NMR

“…Across this surface, often termed the internal surface, 67 numerous hydrogen bonds join the microfibrils that belong to the same aggregate. 68 Despite earlier suggestions that cellulose surfaces within aggregates are inaccessible to solvents, 40 it has been shown that moisture is able to penetrate between microfibrils to some extent. 45 Hence, we propose here that the mobile WAC consists of inter-aggregate water molecules adsorbed at the external surfaces, while water molecules responsible for the observed immobile WAC phase reside at intra-aggregate microfibril interfaces.…”

“…Assumedly, the microfibril-microfibril contact within the aggregates is more regular and stronger than the contact between different aggregates. 68,69 If so, swelling of the aggregates and thereby the resulting microfibril-microfibril separation is limited since it proceeds against strong forces between the microfibrils within the aggregate. That limitation must be weaker at the external surfaces.…”

Cellulose Elementary Fibrils Assemble into Helical Bundles in S1 Layer of Spruce Tracheid Wall

“…10,16 Mathematical modelling has also become a powerful, complementary analytical technique to study the structure and behavior of cellulose in various environments. 16,17 In a recent study, applying transmission electron microscopy on ultrathin wood sections, we observed that EFs form an out-of-plane angle with respect to the longitudinal cell axis. 18 Here, following our previous work, transmission electron tomography and mathematical modelling have been applied to observe nanoscale geometries and assembly of EFs in native cell walls.…”

Hydrothermal pretreatment for deconstruction of plant cell wall: Part II. Effect on cellulose structure and bioconversion

“…It is also found from C4 region in Figure a that the accessibility to cellulose decreases and the inaccessibility increases with the increase of pretreatment severity, which means the average fibril aggregate dimension is increaseing. The enlargement of the fibril and fibril aggregate can be explained by thermally induced recrystallization and aggregate growth, which is probably driven by prevalent cellulose‐cellulose attractive interactions under hydrothermal pretreatment . With the increase of pH in Figure b, the signal of accessible fibril becomes increasing, which indicates high pH does not favor the fibril aggregate growth and will leave more void space (or pores) for facilitating the penetration of cellulase.…”

“…The efficiency of enzymatic hydrolysis of cellulose could be greatly increased after thermal and/or chemical pretreatment. A lot of work has been done on the effect of the pretreatment on the alterations of ultrastructural cellulose and the impact of these ultrastructural modifications of cellulose on the efficiency of enzymatic hydrolysis …”