Converting CO2 into heterocyclic compounds under accelerated performance through Fe3O4-grafted ionic liquid catalysts

Abstract

Solid supported catalysts such as amines are in high demand for the chemical fixation of CO₂ into commodity chemicals. Here, we demonstrate an accelerated platform for 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-ionic liquid (IL) catalyzed CO₂ fixation via the grafting of DBU-ILs over magnetically separable Fe₃O₄ nanoparticles (MNPs). The DBU-ILs were covalently immobilized over Fe₃O₄ MNPs utilizing the thio–ene reaction of allyl-DBU with –SH modified Fe₃O₄ MNPs. The DBU-IL-grafted Fe₃O₄ (Fe₃O₄@DBU-ILs) materials were characterized by Fourier-transform infrared (FT-IR) spectroscopy, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and vibrating-sample magnetometry (VSM). TEM revealed that the MNPs have a spherical structure with a particle size of 12–20 nm. VSM showed the gradual decrease in magnetization after stepwise grafting from Fe₃O₄ to Fe₃O₄@DBU-ILs. The efficacy of the two different Fe₃O₄@DBU-ILs, Fe₃O₄@[HDBU⁺][TFE⁻] and Fe₃O₄@[HDBU⁺][AcO⁻] prepared by neutralization of  Fe₃O₄@DBU with 2,2,2-trifluoroethanol (TFE) and acetic acid (AcOH), was investigated by simultaneous fixation of CO₂ into the important heterocyclic compounds quinazoline-2,4(1H,3H)-dione and benzimidazolone. This approach shows excellent recyclability with a nominal decrease (2–3%) in product yields after each cycle. In particular, the energy-dispersive X-ray spectroscopy (EDX) mapping of Fe₃O₄@[HDBU⁺][TFE⁻] used for five cycles demonstrated significant leaching of TFE. Interestingly, after retreatment with TFE, Fe₃O₄@[HDBU⁺][TFE⁻] showed a similar yield to that of a fresh catalyst.