Structural effects of amine polymers on stability and energy efficiency of adsorbents in post-combustion CO2 capture

Linear/branched polyethyleneimines (PEI) and their modified structures have been widely used to prepare CO2 adsorbents due to their low material cost and high amine content. However, few studies have been carried out to comprehensively understand the effects of polymer structures on the properties of adsorbents, especially other than CO2 capacity. In this study, we rigorously investigated the effects of polymer structures on the CO2 adsorption capacity, kinetics, adsorbent stability, and regeneration heat of adsorbents using four amine polymers with different molecular weights, amine distributions, and ppm-level metal impurities. Linear tetraethylenepentamine (TEPA) exhibited the highest CO2 adsorption capacity due to its low tertiary amine content. However, unlike common intuition, the high CO2 capacity of TEPA did not lead to high energy efficiency of the CO2 capture process because of excessively strong CO2 adsorption and substantial H2O co-adsorption under typical flue gas adsorption conditions. Furthermore, the use of TEPA led to the slowest CO2 adsorption kinetics and the fastest thermochemical degradation, limiting its practical applicability. In contrast, epoxide-functionalized branched PEI exhibited the lowest CO2 capacity, but enabled the most energy-efficient CO2 capture due to its moderate CO2 adsorption strength and suppressed H2O co-adsorption. It also exhibited the fastest adsorption kinetics and the highest stability. Conventional branched PEIs showed intermediate behaviors. The present results indicated that the CO2 adsorption capacity of adsorbents, which was the major focus of previous studies, should not be overemphasized for CO2 adsorbent development and other important engineering aspects (e.g., adsorption kinetics, thermochemical stability, and regeneration heat) need to be considered collectively.