Biomass Solvent Pretreatment

Production of ethanol by bioconversion of lignocellulosic biomass is an very interesting topic. The pretreatment process for increasing the enzymatic digestibility of cellulose has become a key step in production of bioethanol at the commercial scale. However, for the efficient production of bioethanol, lignin serves as a barrier. Therefore, it is an essential to remove lignin from the lignocellulosic biomass. For the pretreatment of lignocellulosic biomass, there are a number of ways: alcohol, organic acid, acetone pretreatment methods. One of the novel ways is co-solvent based pretreatment method so-called co-solvent enhanced lignocellulosic fractionation which employs tetrahydrofuran (THF) in a single phase mixture with water to increase the deconstruction of lignocellulosic biomass. We focus on how co-solvent affects the structure of lignin employing molecular dynamics simulations.

Aqueous. We have revealed two fundamental process that are responsible for the morphological changes in biomass during dilute acid & steam explosion pretreatments. The first is cellulose dehydration at pretreatment temperatures (160 0C) that leads to cellulose fibril coalescence [Langan, P. et al; Pingali, S. et al.]. The second is lignin-hemicellulose phase separation that leads to lignin aggregation on the surface of cellulose [Lindner, B. et al.], which in turn blocks enzyme from deconstructing cellulose [Vermaas, J. V. et al.]. Lignin aggregation is driven by lignin hydrophobicity, making aqueous pretreatments poor solvent environments for lignin [Petridis, L. et al; Petridis, L. & Smith, J. C.].

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 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. 

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.