We report an extremely biocompatible solvent for plant cell walls based on a polar liquid zwitterion that dissolves cellulose, the most recalcitrant component of the plant cell walls. The polar liquid zwitterion does not affect the viability and activity of Escherichia coli, even at high concentrations. We demonstrate conversion of cell walls to ethanol via a starch-like process, namely successive dissolution, hydrolysis and fermentation in the same reaction pot.

Energy-efficient bioethanol production from plant biomass is in high demand, and one of the most promising procedures reported to date is one-pot ethanol production, that is, the production of ethanol from biomass in the same reaction pot, such as industrial first-generation bioethanol. This process requires cellulose solvents whose toxicity toward fermentative microorganisms is extremely low. Herein, we have developed a low-toxic zwitterionic cellulose solvent known as 4-(1-(2-(2-methoxyethoxy)­ethyl)­imidazol-3-io)­butyrate (OE2imC3C). OE2imC3C is the only reported solvent that satisfies the following properties: being liquid at mild temperature and having good cellulose dissolution ability and low toxicity, even when including other types of solvents. We here investigated the relationship between the chemical structures and properties by synthesizing 22 zwitterions. Long alkyl- or oligoether chains attached to the cation (cation tails) were necessary to be a liquid. The zwitterions, except for that with an octyl tail, exhibited biocompatibility. Interestingly, the spacers of the zwitterions, alkyl chains between the cations and anions, were expected to be inert, but affected the toxicity. The molecular mechanisms were investigated using molecular dynamics simulations. The zwitterions exhibiting low toxicity scarcely inserted their cation tails into cell membrane and thus did not rupture the cell membrane. Ionic liquids, which have free cations and anions, induced molecular-level disruption of the cell membrane, suggesting that the zwitterion structure is a critical factor for low toxicity. The spacers, which were expected to be inert, shifted the solvent cluster structures in the bulk phase and induced molecular-level disruption of the cell membrane. The requirements for low-toxic cellulose solvents are zwitterionic structures, carboxylate anions, long polar cation tails, and in some cases, short spacers.

Cellulose was hydrolyzed using a novel biphasic system consisting of water and an acidic and hydrophobic ionic liquid. The biphasic system enabled a simple separation of the resulting glucose aqueous solution and ionic liquid. Additionally, a fermentation inhibitor, 5-(hydroxymethyl)furfural, could be removed from the aqueous phase into the ionic liquid phase. The yield of glucose in cellulose hydrolysis was 12.9% at 190 °C. The distribution ratio of glucose in the aqueous phase was 0.98 with an ionic liquid/water ratio of 0.13 (w/w), indicating that most of the glucose was recovered into the aqueous phase. 5-(Hydroxymethyl)furfural was absorbed into the ionic liquid phase from the aqueous phase. The concentration of 5-(hydroxymethyl)furfural in the aqueous phase decreased from 37 mM to 1.9 mM, which was lower than the concentration at which fermentation is inhibited (24 mM). The acidic and hydrophobic ionic liquids did not decompose during the cellulose hydrolysis and could be recycled four times.

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Abstract:Lignocellulose is known as a renewable resource, and acidic ionic liquids have been highlighted as efficient catalysts for hydrolysis of cellulose. To achieve successive hydrolysis and fermentation, efficient hydrolysis with sufficiently diluted acidic ionic liquids is necessary because acidic ionic liquids are toxic to fermentative microorganisms.