Methods of separating a bio-oil are disclosed. The method comprises contacting a bio-oil with a supercritical alkane to dissolve non-polar compounds of the bio-oil in the supercritical alkane. The bio-oil is produced from a biomass by a thermochemical process. The non-polar compounds are removed from the bio-oil to produce a fractionated bio-oil comprising polar compounds.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
 This invention was made with government support under Contract Number DE-AC07-051D14517 awarded by the United States Department of Energy. The government has certain rights in the invention.