Conceptual model, based on experiment and simulation, of the changes in cellulose structure upon aqueous-based SEP pretreatment. Before pretreatment, cellulose fibrils (green) are separated by layers of hydration H2O (blue background) which are ordered relative to the bulk but disorder the cellulose surface (see inset with hydrogen bonds shown in blue, H in white and O in red). At higher temperatures H2O is released and the cellulose fibrils coalesce.
Co-solvents. Use of THF-water co-solvents can significantly enhance biomass deconstruction by increased lignin removal. At the molecular level, this is achieved by the lignin adopting random coil conformations, as the co-solvent is a “theta” solvent because THF, unlike water, interacts favorably with lignin. Further, the THF:water co-solvent segregates locally when interaction with cellulose. THF preferentially solvates the hydrophobic surfaces of cellulose, and water the hydrophilic surfaces [Mostofian, B. et al.]. Unlike cellulose and lignin, the solvation of hemicellulose depends strongly on temperature. Hemicellulose is solvated by both THF and water at pretreatment temperatures, which erduces the hemicellulose hydrolysis rate compared to a water-only environment [PAPER]. The co-solvent exhibits an interesting behavior: water and THF are mixed and high and low temperatures, but phase separate at intermediate temperatures (~60-145 oC). We paired molecular simulation and experimental evidence, and revealed how the solvation of xylan in an water–THF pretreatment can lead to single-pot conversion of biomass to furfural and 5-hydroxymethylfurfural. In aqueous solution, xylan is depolymerized faster than cellulose is, making it difficult to convert both biopolymers to fuel precursors at the same time. We showed that solvation by THF:water (CELF) slows the rate of xylan hydrolysis, allowing an economically desirable “single-pot” conversion of both xylan and cellulose to fuels and products [Smith, M. D. et al.].
(a) Lignin in Water (b) Lignin in THF-Water (c) Cellulose in THF-Water
(a) Lignin adopts collapsed conformations in water. (b) Lignin adopts random-coil conformations in THF:water co-solvent. (c) THF (orange) and water (blue) locally segregate on the hydrophobic and hydfrophilic surfaces of cellulose (green), respectively.
Depending on the solvent environment and the temperature, lignin adopts different conformaitonal states. Lignin can be modeled as ~15 monolignol “blobs”, coloured differently in the image below. A | In H2O and at room temperature lignin collapses to compact, spherical ‘equilibrium’ globules. B In H2O and at pretreatment temperatures the solvent remains poor, but lignin adopts more extended, aspherical ‘crumpled’ globules with separated blobs. C | THF:H2O is a “theta” solvent, leading to adoption of random coil lignin configurations, in which the “blobs” are not formed and the lignin chain is exposed to the solvent along its entire contour.
Molecular dynamics models of the three states of lignin. The ~15 monolignol “blobs” are coloured differently.
Ionic liquids. Our studies show that both the anions and cations of ionic liquids (IL) enhance the dissolution of biomass. Anions predominantly interact with cellulose surface hydroxyl groups, disrupting intrachain hydrogen bonds on the origin chains but not those on the center chains. In contrast, cations stack preferentially on the hydrophobic cellulose surface, governed by non-polar interactions with cellulose. Complementary to the polar interactions between Cl- and cellulose, the stacking interaction between solvent cation rings and cellulose pyranose rings can compensate the interaction between stacked cellulose layers, thus stabilizing detached cellulose chains [Mostofian et al.; Mostofian et al.]. The interactions of ILs with the other major biomass polymers, hemicellulose and lignin, are also of interest [Moyer, P. et al.]. The calculated second virial coefficients between ILs and hemicellullose match the trend of the experimental solubility measurements. The simulations also reveal that the interactions between the ILs and hemicellulose are an important factor in determining the overall biomass solubility, whereas lignin-IL interactions were found to not vary significantly.
Sphere-and-stick model of 1-butyl-3-methylimidazolium chloride (BmimCl). The ion pair consists of an organic cation with an imidazolium ring and a chloride anion.