Effect of Alkyl Chain Length in Anions on the Physicochemical Properties of Cellulose-Dissolving Protic Ionic Liquids
Hideki Hanabusa A , Yuko Takeoka A , Masahiro Rikukawa A and Masahiro Yoshizawa-Fujita A BA Department of Materials and Life Sciences, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan.
B Corresponding author. Email: masahi-f@sophia.ac.jp
Australian Journal of Chemistry 72(2) 55-60 https://doi.org/10.1071/CH18170
Submitted: 16 April 2018 Accepted: 6 July 2018 Published: 20 August 2018
Journal Compilation © CSIRO 2019 Open Access CC BY-NC-ND
Abstract
A protic ionic liquid (PIL) composed of 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) and acetic acid can dissolve cellulose under mild conditions and catalyse its transesterification. To investigate the relationship between physicochemical properties and chemical structures, PILs composed of DBU and carboxylic acids with varying alkyl chain lengths were prepared as cellulose-dissolving solvents. The thermal behaviours of the PILs were analysed by thermogravimetry and differential scanning calorimetry, and their viscosities, ionic conductivities, and cellulose-dissolution abilities were determined. The effect of the alkyl chain length in the carboxylate ion on the physicochemical properties of the PILs was investigated. With increasing chain length, the thermal stability and ionic conductivity increased, whereas the melting point (Tm), glass-transition temperature (Tg), cellulose solubility, and viscosity decreased. The cellulose solubility increased as the difference between the pKa values of the DBU and carboxylic acid (ΔpKa) increased. In addition, the cellulose solubility increased with the increasing density of the PIL. It was revealed that PILs with a high ΔpKa value and a carboxylate ion with a short alkyl chain are suitable for cellulose dissolution.
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