Sustainable Brønsted-Lewis Acid Deep Eutectic Solvent for High Conversion of Rosin to Polymerized Rosin

The use of renewable rosin as a raw material to prepare rosin-based polymers with good properties had aroused great interest. This work used rosin as raw material and acid deep eutectic solvent [ZnCl2][FA]3 (synthesized by zinc chloride (ZnCl2) and formic acid (FA)) as a catalyst for high conversion of rosin to polymerized rosin. [ZnCl2][FA]3 possessed a suitable acid strength (457 mV), with both a Brønsted acid site and Lewis acid site, in which the Brønsted acid site could effectively promote the isomerization of rosin and the Lewis acid site could effectively promote the polymerization of rosin, so polymerized rosin with a high softening point (148.40 °C) and low acid value (125.23 mg KOH/g) was obtained under the following conditions: rosin = 15 g, mDES:mrosin = 30%, T = 110 °C, and t = 9 h. [ZnCl2][FA]3 maintained good catalytic performance in all 5 cycles; on this basis, the green synthesis technology of polymerized rosin under the DES system was given and not only promoted the further application of polymerized rosin but also realized the high value-added utilization of renewable resource rosin. In addition, the formation mechanism of DES was systematically explained through DFT calculation, and the reaction mechanism of rosin polymerization over [ZnCl2][FA]3 was explored, which could provide a theoretical basis for further research.