Deep Eutectic Solvent–Assisted Green Synthesis of a Covalent Organic Framework Via a Novel Sustainable Route

Abstract

Covalent organic frameworks (COFs) are crystalline porous polymers that allow precise assembly of organic units into predetermined skeletons and nanopores, combining structural tunability, extended conjugation, light-harvesting ability and semiconducting behaviour [1]. In this study, a β-Ketoenamine COF was synthesized via a novel green synthesis using a PTSA-based deep eutectic solvent (DES) as the reaction medium. XRD under varied reaction conditions identified 1:1.5 PTSA DES:H₂O, 24 h, and 120°C as the optimal parameters for high crystallinity. FTIR confirmed formation of the characteristic β-Ketoenamine linkages, while Raman and solid-state NMR spectra showed all relevant COF peaks. The optimised sample achieved a high isolated yield of 83.49%. N₂ sorption revealed a BET surface area of 698.69 m² g⁻¹ and a BJH pore diameter of 2.8 nm, confirming a mesoporous framework. XPS survey spectra verified the presence of carbon, nitrogen, and oxygen in the expected stoichiometry. High-resolution XPS C1s, O1s and N1s spectra exhibited peaks at 284.1, 285.5, and 287.2 eV, at 530.4, 531.8 eV, 399.7 and 402.4 eV attributable to the C–C/C═C, C–N, C═O, C=O and C-O-H, C-N bond and π-π* transition of COF. FESEM images of COF-120 displayed a cross-linked fibrous network connected by wire-like domains, consistent with strong π–π stacking between neighbouring rods; analogous cross-linking was observed by HRTEM. TGA indicated excellent thermal stability up to 400°C. Optical analysis showed that COF-120 was active under visible light, indicating that the DES-synthesised β-Ketoenamine COF is a promising candidate for photocatalytic applications.