Several Sandia Joint BioEnergy Institute (JBEI) researchers, in collaboration with others in Kentucky and India, have published, “Theoretical insights into the role of water in the dissolution of cellulose using IL/water mixed solvent systems,” which the editors selected for Journal of Physical Chemistry B’s Nov. 12th issue cover.
The continued reliance of the global transportation energy sector on nonrenewable fossil fuels is a major challenge to sustainability, due to concerns related to carbon emissions and dependence on a finite resource. There is growing importance in using biobased feedstocks as advanced renewable resources for the production of liquid transportation fuels.
Transforming polysaccharides present in nonfood biomass feedstocks into fermentable sugars is one of the keys to the biochemical conversion of biomass into renewable fuels and chemicals. The critical challenges in converting biomass into drop-in fuels and chemicals are associated with the compact packing of polysaccharides and their interactions with lignins.
The planet’s most abundant plant polysaccharide, cellulose, exists in nature as microcrystalline cellulose (I) with two distinct crystalline forms (Iα and Iβ) that possess triclinic and monoclinic unit cells, respectively (see box). The cellulose chains are held together strongly by hydrogen bonding (H-bonding) and stacking of glucose units. These must be disrupted, usually through a pretreatment process, into individual chains in order to increase substrate accessibility to hydrolytic enzymes, thus generating high fermentable-sugar yields.
In recent years, biomass pretreatment with certain ionic liquids has received considerable attention due to their superior dissolution capability of lignocellulosic biomass, very low vapor pressure, and relatively low flammability. A fundamental understanding on how these ionic liquids, in aqueous environments, act on cellulose, particularly at lower ionic-liquid concentrations with water as a cosolvent, is essential for optimizing pretreatment efficiency, lowering pretreatment cost, and improving ionic liquid recyclability. The ionic liquid 1-ethyl-3-methylimidazolium acetate is one of the most efficient cellulose solvents known, greatly altering cellulose structure for improved enzymatic saccharification.
Understanding cellulose dissolution and regeneration in aqueous ionic liquid provides knowledge on (1) efficient cellulose dissolution, (2) ionic liquid recycle and recovery, and (3) biomass solute separations—all of which are critical factors to the rational design of a cost-effective ionic liquid pretreatment process. Comparing the cellulose dissolution process under different conditions indicates that temperature has a dominant effect on the cellulose chain dissolution process in the presence of [C2C1Im][OAc] with cellulose bundle remaining intact at 300 K, whereas it is disrupted at 433 K in pure [C2C1Im][OAc].
The paper describes the research team’s investigation of the dissolution mechanism of microcrystalline cellulose in different water ratios at room (300 K) and pretreatment (433 K) temperatures using all atom molecular dynamics (MD) simulations. To understand the role of water as a cosolvent with [C2C1Im][OAc], The team investigated the dissolution mechanism of microcrystalline cellulose, type Iβ, in different [C2C1Im][OAc]:water ratios at room (300 K) and pretreatment (433 K) temperatures using all atom MD simulations. The MD simulations suggest that levels of 50% to 80% [C2C1Im][OAc] can effectively break the H-bonding present in cellulose. On the other hand, the presence of water at certain concentration increases the diffusivity of cellulose in the medium and aids in cellulose dissolution.
![Diffusion of cellulose at different 1-ethyl-3-methylimidazolium acetate ([C2C1Im][OAc]):water ratios.](https://www.sandia.gov/energy/wordpress/../wp-content/uploads/2015/11/Diffusion-of-cellulose-at-diff-H2O-ratios.jpg)
These MD simulations show that 80:20 [C2C1Im][OAc]):water ratios should be considered as “the tipping point” above which [C2C1Im][OAc]:water mixtures are equally effective on decrystallization of cellulose by disrupting the interchain hydrogen bonding interactions. Simulations also reveal that the resulting decrystallized cellulose from 100% [C2C1Im][OAc] begins to repack in the presence of water but into a less crystalline, or more amorphous, form.
The knowledge gained from this study provides a better understanding of the dual role played by the water (as a cosolvent/antisolvent) in dissolving cellulose. Evidence from this study provides possible clues for the targeted design of ionic liquid−water mixtures that are effective for pretreatment of biomass. Furthermore, this work presents a more general computational method for the selective identification of the mixtures of ionic liquid:water solvent systems that are necessary for dissolution of cellulose.