A more sustainable synthesis approach for cellulose acetate using the DBU/CO2 switchable solvent system

Abstract

Cellulose acetate is one of the most important cellulose derivatives and commercially mainly produced using the Acetic Acid Process, in which overstoichiometric amounts of acetic anhydride and concentrated acetic acid are used to obtain cellulose triacetate. A subsequent partial hydrolysis is necessary to achieve evenly substituted cellulose acetates with lower degrees of substitution. Homogeneous acetylations in ionic liquids or other cellulose dissolving solvent systems often offer milder conditions and the possibility of a one-step synthesis of cellulose acetates with lower degrees of substitution by simply adjusting the equivalents of the acetylation agent. Here, we show an efficient homogeneous cellulose acetylation process without the need of any additional catalyst or activation step using the DBU/CO₂ switchable solvent system. Vinyl acetate was used as a more benign acetylation agent under mild conditions and straightforward recyclability of all employed components was demonstrated with high recycling ratios (87.0–98.9%). Less cellulose backbone degradation compared to a cellulose acetate sample synthesized by the Acetic Acid Process from the same cellulose source was shown by size exclusion chromatography (M_n = 35 kDa vs. 12 kDa), which resulted in improved mechanical properties of solvent casted foils. Other homogeneous procedures reported so far (e.g. in ionic liquids) reached lower degrees of substitution, needed additional catalysts, proved to be less advantageous in terms of recycling, or required more reactive acetylation agents. Our results thus demonstrate a cellulose acetylation method with full focus on sustainability, efficiency, and applicability, resulting in an E-factor of 1.92 for the overall process.