Sustainable hyperbranched polyester production through Brønsted Acid Catalyst Heterogenization

Glycerol, an abundant byproduct of biodiesel production, is an attractive renewable platform molecule for synthesizing value-added bio-based materials. Among these, hyperbranched polyesters have gained attention due to their highly functionalized three-dimensional architecture, tunable viscosity, and applications in coatings, resins, additives, and sustainable polymer formulations. Developing catalytic routes based on renewable feedstocks is a promising strategy for more sustainable polymer production.

This work investigates the heterogenization of Brønsted acid catalysis for the solvent-free polyesterification of glycerol with dicarboxylic acids. The influence of catalyst nature and diacid chain length on glycerol conversion, polymer growth, thermal stability, and polymer structure was evaluated. Methanesulfonic acid (MSA) was used as a homogeneous catalyst, while SO₃H-C-Al₂O₃ was synthesized as a heterogeneous alternative. The catalyst consisted of a carbon-coated Al₂O₃ support functionalized with strongly bonded SO₃H groups, minimizing the contribution of alumina surface sites, reducing hydrophilicity, and facilitating catalyst recovery.

Polyesterification reactions were conducted in a batch reactor equipped with a Dean–Stark collector for continuous water removal and conversion monitoring. Reactions were performed at 150 °C for 7 h using a glycerol/diacid molar ratio of 1:1. Succinic and glutaric acids were selected as model dicarboxylic acids. Reaction progress was monitored by IR spectroscopy, and the resulting hyperbranched polyesters were characterized by CHN, TGA-DTA, NMR, and complementary physicochemical techniques.

Catalyst nature and diacid structure strongly influenced glycerol conversion and polymer growth. Without catalyst, glycerol conversion reached 58.9% and 53.8% for succinic and glutaric acids, respectively. MSA achieved the highest conversion (74.4% with succinic acid), while SO₃H-C-Al₂O₃ showed comparable performance, particularly with glutaric acid (61.2%). These results demonstrate the potential of heterogeneous sulfonic Brønsted acid catalysts for cleaner and more sustainable production of bio-based hyperbranched polyesters.