Spectroscopic investigation into the oxidation of polyethylenimine for CO2 capture: Mitigation strategies and mechanism

Abstract

One of the most critical challenges facing large-scale application of amine-based CO2 adsorbents is their long-term stability, particularly their oxidative degradation, which shortens their lifetime. Numerous oxidation mitigation strategies were proposed, notably functionalization by epoxybutane (EB) and addition of polyvinyl alcohol (PVA). However, neither the amine oxidation mechanism, nor the antioxidation mechanism of these additives are clear. Here, we investigated the oxidation of polyethylenimine (PEI) using electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), mass spectrometry (MS), and thermogravimetric analysis (TGA). EPR data provided for the first time direct evidence for the occurrence of radical species during PEI oxidation, and their increased content in the presence of transition metal catalysts. Moreover, significant amounts of CO2, water and ammonia were detected by MS. Surprisingly, although EB and PVA enhanced the oxidation resistance of PEI, they did not reduce the amount of radical species. Theoretical calculations by DFT indicated that PEI is more prone to oxidation than EB-functionalized PEI and PVA, because its CH bonds are weaker. Consistent with experimental data, a mechanistic explanation to the slower amine oxidation in the presence of EB and PVA, was put forward, and clues for further improvement were proposed.