RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem.
Full Paper
Binary Ionic Liquid System for Direct Cellulose
Etherification*
A
,
B
A
A
Takeshi Kakibe,
Hajime Kishi
Satoshi Nakamura, Kiyokazu Amakuni, and
A
A
Department of Chemical Engineering, University of Hyogo, 2167, Shosha, Himeji,
Hyogo 671-2201, Japan.
B
Corresponding author. Email: kakibet@eng.u-hyogo.ac.jp
Etherification of cellulose was performed using a mixture of ionic liquids (ILs) playing roles in both cellulose dissolution
and catalysis. We investigated the effects of the reaction time and the ratio of these ILs in the mixture. Cellulose
etherification was performed in these IL mixtures. The proportion of propoxy cellulose exceeded 2.5 after 24 h.
Manuscript received: 31 July 2018.
Manuscript accepted: 30 October 2018.
Published online: 23 November 2018.
Introduction
Ionic liquids (ILs), organic salts with a melting point below
Although Brønsted acidic ILs are employed as versatile
[
19,20]
solvents and catalysts in various reactions,
none of these
1
rials.
008C, have attracted much interest as a unique class of mate-
could dissolve cellulose. In addition, each functionality may be
inhibited by introducing multiple substituents with different
functionalities onto a single ion. Another approach to tuning IL
properties is to use a eutectic mixture of two or more compo-
[
1,2]
In the liquid state at ambient temperature, they contain
no solvents and consist entirely of ions; this endows them with a
wide range of properties. Various ILs have been researched as
[21–23]
‘green’ and ‘sustainable’ solvents due to their low vapour
nents that have attractive properties individually.
Numer-
pressure, thermal stability, and recyclability. Their physical,
chemical, and electrochemical properties can be tuned by
exploiting their diverse component organic cations and anions.
Since they can be designed to exhibit a wide variety of solvent
properties by changing their ions, ILs are expected to function as
alternatives to traditional reaction media. As specific functional
groups can also be incorporated, various functionalised ILs are
ous studies have been conducted on binary and ternary IL
systems, such as molecular dynamics simulations and detailed
studies concerning mixing and miscibility. We previously
reported the homogeneous epoxidation of cellulose in a binary
[
24]
IL mixture as solvent and catalyst in the presence of glycidol.
Since then, the degree of substitution (DS) was controlled by the
composition ratio of acidic ILs, but the relationship between the
structure of the acidic ILs and the DS value was not discussed. It
is necessary to discuss the correlation between the structure of
acidic ILs and the catalytic activity. In this study, a series of IL
mixtures with suitable compositions were synthesised that
played different roles in cellulose dissolution and catalysis.
We investigated the cellulose solubility and catalyst activity
towards etherification in these binary IL mixtures to produce a
cellulose derivative.
[3]
categorised as ‘task-specific’ ILs (TSILs). TSILs have been
designed and synthesised for specific purposes including
[
4]
[5]
[6]
catalysis, organic synthesis, separation, and ion conduc-
[7,8]
tive materials.
Cellulose, the most common biological polymer on earth, has
been widely used as a raw material in textiles, chemicals,
pharmaceuticals, and energy industries. Chemically modified
[
9,10]
[11]
cellulose derivatives
or prepared composites
can be
employed as new functional materials. Over the years, the
functionalisation of cellulose has attracted considerable
research interest. However, well developed intra- and intermo-
lecular hydrogen-bonding networks hinder cellulose dissolution
in conventional solvents, rendering functionalisation challeng-
ing. Thus, the search for homogeneous functionalisation media
has stimulated considerable research interest in developing
suitable solvents for cellulose. Rogers et al. initially reported
Results and Discussion
It is known that some mixtures of hydrophilic and hydrophobic
ILs form two distinct phases; this leads to significant differences
in the structures of not only the cations but also the anions.
Miscibility is the most important factor in preparing the binary
IL system, especially when these are employed as homogeneous
reaction media for preparing cellulose derivatives. We demon-
strated the miscibility and cellulose solubility in binary ILs by
tuning the concentrations of the component ions. The miscibility
[
25]
[
12]
the use of ILs as a new class of solvents for cellulose,
prompting several studies focussed on the regeneration
[13,14]
[
15–18]
and chemical modification
of cellulose in ILs.
*
This manuscript is dedicated to Professor Kenneth Seddon, who passed away early in 2018, to honour his pioneering research on catalytic reactions in ionic
liquids.
Journal compilation � CSIRO 2018
www.publish.csiro.au/journals/ajc