Role of Fe Complexes as Initiators in the Oxidative Degradation of Amine Resins for CO2 Capture: Molecular Modeling and Experimental Results Compared

Abstract

CO₂ capture is an emerging technology to reduce the effects of CO₂ emissions on the atmosphere. Amine resins could play an important role to realize this goal not as a storage material but as an option to produce highly concentrated CO₂ streams which can be used further in the chain. Air oxidation is a major point of concern with respect to the operational lifetime of the resins and its economic viability. The oxidation of the resins follows the so-called Basic Autoxidation Scheme or Free Radical Chain Autoxidation scheme which consists of three steps: (1) Initiation, (2) Propagation, and (3) Termination. From both bioinorganic chemistry and oxidation catalysis, it is known that Initiation of Free Radical Chain Autoxidation is the step with the highest activation energy. In the limiting case, Initiation occurs at high temperature via H-abstraction by O₂ itself. Experimentally obtained activation barriers on oxidative degradation for Branched Polyethylene Imine and Lewatit R VP OC 1065 are 135.0 and 122.7 kJ/mol, respectively. The computational values for Branched Polyethylene Imine and Lewatit R VP OC 1065 are 133.2 and 117.5 kJ/mol, respectively. Transition metal ions like Fe(II)/Fe(III) play an important role in Initiation, leading to much lower activation barriers. Two plausible types of Initiation with Fe(II)/Fe(III) were investigated by comparing previously published experimental findings with newly obtained computational results. The two mechanisms are (1) Outer Sphere Electron Transfer by Fe(III) and (2) Dioxygen Activation by Fe(II). It was found that the Outer Sphere Electron Transfer mechanism is very unlikely as no applicable exothermic reaction between Fe(III) complexes and an amine resin model could be determined. Dioxygen Activation by Fe(II) complexes of primary amines in Branched PolyEthylene Imine, most likely, is responsible for the Initiation of oxidative degradation of amine resins under Direct Air Capture CO₂ process conditions. The computational activation barrier for Dioxygen Activation of a Branched Polyethylene Imine model is 68.6 kJ/mol. The latter is much lower than the experimentally obtained activation barriers for Branched Polyethylene Imine and Lewatit R VP OC 1065 in their limiting cases. Molecular Modeling was able to make a clear distinction between the various initiation processes. This provides an improved understanding of oxidative degradation of Branched Polyethylene Imine and Lewatit R VP OC 1065 in general. It also provides an outlook to the application of Polyethylene Imine resins in Direct Air Capture CO₂ processes. The upfront removal of all possible initiators should lead to drastically increased lifetimes. From the activation barrier of Branched Polyethylene Imine as determined experimentally and computationally, a lifetime of approximately 5 years between 30 and 50 °C seems possible under ideal process conditions.