Dialkyl Phosphate-Related
Ionic Liquids as Selective
Solvents for Xylan
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Abstract
Herein we describe a possibility of selective dissolution
of xylan,
the most important type of hemicellulose, from <i>Eucalyptus
globulus</i> kraft pulp using ionic liquids (ILs). On the basis
of the IL 1-butyl-3-methylimidazolium dimethyl phosphate, which is
well-known to dissolve pulp, the phosphate anion was modified by substituting
one oxygen atom for sulfur and selenium, respectively. This alteration
reduces the hydrogen bond basicity of the IL and therefore prevents
dissolution of cellulose fibers, whereas the less ordered xylan is
still dissolved. <sup>1</sup>H NMR spectra of model solutions and
Kamlet–Taft parameters were used to quantify the solvent polarity
and hydrogen bond acceptor properties of the ILs. These parameters
have been correlated to their ability to dissolve xylan and cellulose,
which was monitored by <sup>13</sup>C NMR spectroscopy. It was found
that the selectivity for xylan dissolution increases to a certain
extent with decreasing hydrogen-bond-accepting ability of anions of
the ILs