Sustainable Transesterification of Cellulose with High Oleic Sunflower Oil in a DBU-CO₂ Switchable Solvent

The direct transesterification of cellulose with high oleic sunflower oil, without any activating steps, was achieved in a DBU-CO₂ solvent system to obtain fatty acid cellulose esters (FACEs). Optimization of the reaction parameters (i.e., concentration, temperature, plant oil equivalents, as well as reaction time) was performed using microcrystalline cellulose (MCC) and followed by Fourier-transform infrared spectroscopy (FT-IR). Further confirmation of the FACEs structures was achieved via ¹H and ¹³C NMR, and ³¹P NMR revealed DS (degree of substitution) values of up to 1.59. The optimized conditions were successfully applied to filter paper (FP) and cellulose pulp (CP). Characterization of the FACEs showed improved thermal stability after transesterification reactions (up to 30 °C by TGA) and a single broad 2θ peak around 19.8° by XRD, which is characteristic of a more amorphous material. In addition, films were prepared via solvent casting and their mechanical properties obtained from tensile strength measurements, revealing an elastic modulus (E) of up to 478 MPa with elongation of about 35% and a maximum stress of 22 MPa. The film morphology was studied by scanning electron microscopy (SEM) and showed homogeneous surfaces. In this report, we thus demonstrated a more sustainable approach toward FACEs that combines cellulose and plant oil (two renewable resources) directly, resulting in fully renewable polymeric materials with appealing properties.