Molecular Modeling Study to the Relation between Structure of LPEI, Including Water-Induced Phase Transitions and CO2 Capturing Reactions

A Molecular Modeling Study on the Propagation in Free Radical Chain Oxidation of (B)PEI

Air oxidation of PEI is a Free Radical Chain Autoxidation process, described as a process following the Basic Autoxidation Scheme with Initiation, Propagation and Termination as discriminating steps. Molecular Modeling was able to identify the most important propagation reactions. HO2(d) is the most likely candidate as the main oxidation chain carrying radical. α-H-abstraction from PEI α-amino hydroperoxides by HO2(d), leading to amide PEI repeat units and eventually to HO2(d) again, is the first step in Propagation. Apart from well-know propagation reactions, the reaction of PEI α-amino CH(d) radicals with H2O2 is of major importance, too, with an estimated contribution of ~50% to Propagation. Furthermore, it provides an explanation for the formation of NH3 and various imine PEI repeat units. PEI α-amino alkoxy radicals might contribute to some extent to Propagation and can lead to chain breaks in PEI and the formation of CO2. Amide and imine PEI repeat units contribute to ~90% of the fully oxidized PEI.

Recent advances in direct air capture by adsorption

Significant progress has been made in direct air capture (DAC) in recent years. Evidence suggests that the large-scale deployment of DAC by adsorption would be technically feasible for gigatons of CO₂ capture annually. However, great efforts in adsorption-based DAC technologies are still required. This review provides an exhaustive description of materials development, adsorbent shaping, in situ characterization, adsorption mechanism simulation, process design, system integration, and techno-economic analysis of adsorption-based DAC over the past five years; and in terms of adsorbent development, affordable DAC adsorbents such as amine-containing porous materials with large CO₂ adsorption capacities, fast kinetics, high selectivity, and long-term stability under ultra-low CO₂ concentration and humid conditions. It is also critically important to develop efficient DAC adsorptive processes. Research and development in structured adsorbents that operate at low-temperature with excellent CO₂ adsorption capacities and kinetics, novel gas-solid contactors with low heat and mass transfer resistances, and energy-efficient regeneration methods using heat, vacuum, and steam purge is needed to commercialize adsorption-based DAC. The synergy between DAC and carbon capture technologies for point sources can help in mitigating climate change effects in the long-term. Further investigations into DAC applications in the aviation, agriculture, energy, and chemical industries are required as well. This work benefits researchers concerned about global energy and environmental issues, and delivers perspective views for further deployment of negative-emission technologies.

Branched versus Linear Structure: Lowering the CO2 Desorption Temperature of Polyethylenimine-Functionalized Silica Adsorbents

Lowering the regeneration temperature for solid CO2-capture materials is one of the critical tasks for economizing CO2-capturing processes. Based on reported pKa values and nucleophilicity, we compared two different polyethylenimines (PEIs): branched PEI (BPEI) and linear PEI (LPEI). LPEI outperformed BPEI in terms of adsorption and desorption properties. Because LPEI is a solid below 73–75 °C, even a high loading amount of LPEI can effectively adsorb CO2 without diffusive barriers. Temperature-programmed desorption (TPD) demonstrated that the desorption peak top dropped to 50.8 °C for LPEI, compared to 78.0 °C for BPEI. We also revisited the classical adsorption model of CO2 on secondary amines by using in situ modulation excitation IR spectroscopy, and proposed a new adsorption configuration, R1(R2)-NCOOH. Even though LPEI is more expensive than BPEI, considering the long-term operation of a CO2-capturing system, the low regeneration temperature makes LPEI attractive for industrial applications.