Mechanistic insights into the methyl esterification of 2,6-naphthalenedicarboxylic acid catalyzed by polymeric ferric sulfate: an experimental and DFT study

Polymeric ferric sulfate enables efficient esterification of 2,6-NDA to 2,6-NDC. • Polynuclear hydroxo iron clusters are identified as the catalytically active species. • Methoxy assisted proton transfer is proposed to lower the reaction energy barrier. • 2,7-NDA induced catalyst deactivation is clarified at the molecular level. Poly(ethylene naphthalate) (PEN) is a high-performance polyester renowned for its excellent barrier and mechanical properties. The production of high-molecular-weight PEN with superior mechanical performance is commonly achieved through the transesterification of dimethyl-2,6-naphthalenedicarboxylate (2,6-NDC), which is more readily purified than its acid counterpart. In this study, polymeric ferric sulfate (PFS) was developed as a highly efficient catalyst for the esterification of 2,6-naphthalenedicarboxylic acid (2,6-NDA) to 2,6-NDC. Under optimized conditions of 200 °C for 80 min with a catalyst content of 0.2 wt%, a conversion exceeding 95% was achieved. Deactivation studies and EPR spectroscopy, complemented by DFT calculations of the broken symmetry states, confirmed that the active species are polynuclear hydroxo complexed iron clusters exhibiting antiferromagnetic coupling. Mechanistic insights revealed that proton transfer mediated by methoxy groups significantly lowers the reaction energy barrier, a process validated by topological analysis. UV–Vis spectroscopy combined with TDDFT simulations further demonstrated the formation of catalytically active methoxy coordinated iron species in the methanolic PFS solution. This work provides a molecular level understanding of polynuclear iron catalysis for the synthesis of monomers for high performance polyesters. Download: Download high-res image (120KB) Download: Download full-size image Graphical abstract showing the catalytic mechanism of methyl esterification of 2,6-naphthalenedicarboxylic acid over polymeric ferric sulfate.