(435f) 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.