Cellulose Extraction From Rice Hulls Using Ionic Liquids

Over one fifth of calories consumed by humanity are from rice.  Consequently,  vast quantities of rice hulls are disposed of as waste, since the hull constitutes 20% of rice mass. Standard practice for disposal of rice hulls is burning, generating pollution and CO₂, or thrown away in vast quantities. An alternative would be to extract cellulose from rice hulls them using a low temperature, environmentally safe process.   Removing cellulose for other uses, such as conversion to glucose and then fermentation to bio-ethanol or use in polymers, would be a better solution to this problem. In addition, this pretreatment process could render the rice hull residue into a biomass readily accessible for enzymatic hydrolysis, so even more cellulose could be converted to glucose for biofuels use.  Hemicellulose or pretreated biomass high in hemicellulose can also be used for enzymatic hydrolysis and fermentation to biofuels.

Ionic liquids are basically molten salts with a melting point at least below 100 °C and generally below room temperature.  Ionic liquids have been found which dissolve isolated lignin and cellulose, and which dissolve these components in wood powder. However, the dissolution capability of these solvents has not been determined for rice hulls. Also, when ionic liquids are applied to biomass, the behavior of hemicellulose has not been well characterized.

In this study, the ionic liquids 1-butyl-2,3-dimethylimidazolium chloride, 1-hexyl-3- methylimidazolium chloride, 1-ethyl-3- methylimidazolium acetate, and 1-allyl-3- methylimidazolium chloride were used to dissolve rice hulls.  A 10% (w/w) loading of rice hulls was used. The room temperature ionic liquid 1-ethyl-3- methylimidazolium acetate  (EMIM Ac) was found to completely dissolve the rice hulls when a temperature of 110°C was used with a holding time of 24 hours.  When EMIM Ac was used on rice hulls for 4 hours at  90°C, no hemicellulose was removed. For EMIM Ac, the conditions of 4 hours at 110°C removed one third of the hemicellulose and 8 hours at 90°C removed one half of the hemicellulose.  The reaction conditions of 110°C for 8 hours removed 86% of the hemicellulose when EMIM Ac was used.  Cellulose and lignin were also extracted from the rice hull biomass by EMIM Ac treatment and precipitation by water addition.