Predicting cellulose solvating capabilities of acid-base conjugate ionic liquids

Area of Science:

• Materials Science
• Chemistry
• Polymer Science

Background:

• Cellulose dissolution is crucial for biomass processing and material applications.
• Ionic liquids offer tunable properties for dissolving challenging polymers like cellulose.
• Understanding the fundamental interactions governing cellulose dissolution is key to designing efficient solvents.

Purpose of the Study:

• To investigate the efficacy of various acid-base conjugates in dissolving cellulose.
• To identify the key chemical properties influencing cellulose dissolution.
• To explore the role of cation acidity and hydrogen bonding in the dissolution process.

Main Methods:

• Synthesis of acid-base conjugates using acetic and propionic acids with bases and superbases.
• Calculation of proton affinities for the studied bases using ab initio methods (MP2/6-311+G(d,p)).
• X-ray diffraction (XRD) analysis to study crystal structures and hydrogen bonding.
• Viscosity measurements of the conjugates over a temperature range.

Main Results:

• Only conjugates containing superbases dissolved cellulose.
• A proton affinity threshold of approximately 240 kcal/mol was identified for cellulose dissolution.
• Cation acidity was found to be a significant factor, alongside anion basicity.
• Hydrogen bonding between anion and cation dominated interactions in the crystalline state.
• Low viscosity conjugates showed high dissolution rates, but not necessarily high solubility.

Conclusions:

• Cellulose dissolution in these systems is primarily driven by superbases and requires a specific proton affinity.
• Both cation acidity and anion basicity contribute to cellulose dissolution, with hydrogen bonding playing a key role in the solid state.
• Viscosity and dissolution rate are correlated, but quantitative solubility is influenced by other factors.
• Recyclability of effective ionic liquids, like 1,5-Diazabicyclo[4.3.0]non-5-enium propionate, was demonstrated.