Hybrid materials from covalent organic frameworks and ionic liquids

Abstract

Covalent organic frameworks (COFs) are new type material, whereby their wide range of properties derived from the chemistry of frameworks and their regular porosity. Here in, we reported the successful syntheses of the novel hybrid materials between porous COF material and ionic liquids (ILs). The crystalline porous powder of boroxine‑based COF, so called COF‑5, is reproducible via a condensation reaction between hexahydroxyltriphenylene and phenyldiboronic acid under a mild condition using a conventional heating method. The as‑synthesized COF-5 was characterized with PXRD, FT-IR, TGA and BET techniques, showing that the material contained 1D regular channels caused by π-π stacking of the gauze sheets with a surface area of 1700 m2 g‑1. Ionic liquid, namely 1-vinyl-3-ethylimidazolium bromide (VEIM[Br]) and 1-vinyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)imide (VEIM[NTf2]), were infiltrated into the pores of COF-5 comparing with a silica porous material SBA‑15 and imine‑based COF, named imine‑1, using a vacuum infiltration method. The BET analysis of VEIM[Br]@COF‑5 shows a decrease in surface area of 170 m2 g‑1 along with the PXRD and TGA profile shows that 20 % of infiltrated IL are occupied in pore cavities without a disruption on the host structure. The in‑situ polymerization of VEIM[Br] in porous material was carried out using thermal radical initiator in solvent‑free condition under an inert atmosphere of nitrogen yielding a polyVEIM[Br]@COF-5. The BET surface area of 518 m2 g‑1 implies the higher ordered‑alignment of polymer of VEIM[Br] along the 1D-channel of COF‑5. Moreover, the new hybrid material, VEIM[Br]@COF‑5 was attempted to use as a heterogeneous catalyst in a cycloaddition of CO2 with propylene oxide. The result shows a turnover number (TON) of 64 within 20 hours and turnover frequency (TOF) of 3.2, which calculated from the measured conversions by 1H‑NMR technique. Even through, the hybrid material is not yet be a high effective catalyst, but it has shed some light on the further development of new hybrid materials.