This thesis is concerned with the thermal treatment of lignocellulosic
biomass using ionic liquids for the purpose of comminution via
dissolution, for fractionating the biological composite and for obtaining
aqueous solutions of carbohydrate monomers from the pulp via
enzymatic hydrolysis. A major focus was the relationship between the
choice of the anion and the effectiveness of the treatment.
The synthesis of a range of 1-butyl-3-methylimidazolium ionic liquids
with strongly hydrogen-bond basic anions was accomplished. Selected,
process-relevant physicochemical properties were measured, such as
the Kamlet-Taft solvent polarity, hygroscopicity and thermal stability. It
was shown that 1-butyl-3-methylimidazolium acetate is not stable at
120°C, while other ionic liquids e.g. 1-butyl-3-methylimidazolium
hydrogen sulfate exhibit very good long-term thermal stability. It was
shown that hydrogen-bond basic 1-butyl-3-methylimidazolium ionic
liquids attract more than stoichiometric quantities of water when exposed
to air, suggesting that ionic liquid pretreatment under anhydrous
conditions is difficult to achieve.
Dissolution of air-dried wood chips in 1-butyl-3-methylimidazolium ionic
liquids was attempted. It was shown that the large particle size and the
moisture contained in the biomass hamper complete dissolution. The
hydrogen-bond basicity of the ionic liquid, described by the Kamlet-Taft
parameter ß, was correlated with the ability to expand as well as partially
and anisotropically dissolve wood chips.
Pretreatment of lignocellulosic biomass with 1-butyl-3-
methylimidazolium methyl sulfate, 1-butyl-3-methylimidazolium hydrogen
sulfate and 1-butyl-3-methylimidazolium methanesulfonate was explored
and high saccharification yields were reported. It was found that
successful application of methyl sulfate and hydrogen sulfate ionic liquids
requires addition of water and that comparatively high water contents are
tolerated. Fractionation of lignocellulose into an insoluble cellulose
fraction, a solubilised hemicellulose fraction and a lignin containing
precipitate was achieved. The influence of water content, pretreatment time and biomass type on the enzymatic saccharification yield and the
extent of hemicellulose solubilisation, hydrolysis and dehydration were
examined.