(240d) Use of UV Irradiation on Photo Isomers to Enhance the Removal of Lignin from Woody Biomass Hydrolysate

Problem:

Conversion of lignocellulose to bioproducts and biofuels is often initiated with a hydrolytic pretreatment process that helps to depolymerize the native polymers in biomass and produce a cellulose-rich pulp stream. Extracting dissolved components from this hydrolysate contributes to deriving value from biomass components other than cellulose.

The mildly hydrophobic components in the hydrolysate, such as dissolved lignin, and dehydration products, such as furfural or levulinic acid, are often recovered using liquid-liquid extraction. Recovery of the desired extracts from the solvent typically involves distillation or back extraction, both of which have high energy demands. In this investigation, we propose a novel method to enable the precipitation of desired solutes using photo-isomers in the extraction solvent. The azobenzene moiety is stable in its linear trans-isomer, but will isomerize when irradiated with UV light into the more polar/bulkier cis-isomer. The cis-isomer will likely interact differently with the extraction solvent, which could change the solubility of the lignin in the solvent. This investigation sought to determine if the presence of a photo-isomer in a solvent could result in a reversible UV light induced precipitation of lignin.

Methods:

Three solvents: hexanol, ethanol, or acetone, were combined with three different photo-isomers: azobenzene, diethylamino azobenzene, or diethoxy-azobenzene, at varying concentrations, to observe the precipitation behavior of lignin dissolved in the solvent-isomer mixture. The lignin used in these experiments was derived from the Organosolve process, which was one motivation for the selection of ethanol and acetone as potential solvents to be studied. Hexanol solvent was chosen as a representative of biphasic liquid liquid extraction of biomass hydrolysates.

Solubility limits of the three photo-isomers in different solvents were determined at different temperatures, as was the solubility of Organosolve lignin in each of the solvents. Experiments to test precipitation were carried out at concentrations at or below the solubility of the photo-isomers and the lignin.

The charging (isomerizing trans to cis) was done by exposure to 365 nm UV light, whereas dis-charging (re-isomerizing cis to trans) occurred through exposure to visible light. Experiments were carried out either in glass beakers that were irradiated with UV light on the upper surface of the solvent-isomer solution, or by using quartz beakers that could be irradiated through the vessel walls. This latter arrangement enabled sealing off the top of the beaker to minimize evaporation of the solvent. Precipitation of lignin under irradiation was observed visually and the amounts of precipitate were quantified by filtering the precipitated solution and measuring the mass of dried lignin.

Results:

Experiments with this system provided insights on solubility concentrations, precipitation, and charging/discharging time durations. Using the simple azobenzene molecule was found to be impractical for developing a precipitation process because the highly rapid rates of isomerization and de-isomerization made the handling too cumbersome. Hexanol has very low solubility of both the photo-isomers and the lignin, and thus was not suitable for extracting or precipitating lignin.

It was found that among the different combinations of solvent and photo-isomer, the acetone and diethoxy azobenzene solution was most efficient at precipitating dissolved lignin in response to UV light exposure. It was found that the UV-induced precipitation was stable for some time, and that the solubility of the system could be restored by shutting off the UV light and allowing the isomerization to reverse and increase the lignin solubility. The concentrations of diethoxy azobenzene in acetone were varied, and the amount of precipitation was recorded. It was observed that precipitation increases with an increased amount of photo-isomer up to a certain point, above which a “shading effect” comes into play, whereby a high concentrations of photo-isomer in one region of the solvent container appears to block the penetration of the UV light, and thus isomerization in the “shaded” regions of the vessel becomes limited.

Implications:

The experimental work validated our hypothesis that a switchable solvent system could be developed that makes use of UV light as the means of activating a switch in solvent activity.

Making use of a UV activated switchable solvent may enable new approaches to separations of different products in dilute solutions. Organosolve lignin in particular may be a suitable material to be separated from biorefinery process streams.