Room-Temperature Superbase-Derived Ionic Liquids with Facile Synthesis and Low Viscosity: Powerful Solvents for Cellulose Dissolution by Destroying the Cellulose Aggregate Structure

Twenty-two superbase-derived ionic liquids (SILs) (16 novel) including 16 1,8-diazabicyclo[5.4.0]undec-7-enium carboxylate (DBU-SILs) and 6 1,5-diazabicyclo[4.3.0]non-5-enium carboxylate (DBN-SILs) were facilely synthesized by coupling superbase cations with different carboxylic anions for cellulose dissolution. Systematic investigations revealed that the combination of the electron-donating groups, small steric hindrance groups, and short-chain groups in carboxylate anions with a larger ring in superbase cations facilitated cellulose dissolution. The regenerated cellulose films produced from seven SILs ([DBUH][CH3CH2OCH2COO], [DBUH][CH3OCH2COO], [DBUH][CH2═CHCOO], [DBUH][CH3COO], [DBUH][CH3CH2COO], [DBNH][CH3CH2OCH2COO], and [DBNH][CH3OCH2COO]) with excellent cellulose solubility exhibited similar chemical structures, a high degree of polymerization, sufficient thermostability, smooth morphology, and high mechanical strength. Moreover, room-temperature SILs with low viscosity displayed a promising opportunity for large-scale production of renewable packaging. Particularly, in addition to hydrogen bond destruction by the joint action of anions and cations, the interactions on (200) and (110) crystal planes of cellulose such as intermolecular hydrogen bonds (O6–H···O3, O6–H···O2, and O2–H···O6) and van der Waals forces were destroyed preferentially and violently by the SILs. This work presented an available protocol in designing novel ILs for commercial processing of cellulose.