Enhanced CO2 capture rate and capacity through alkylation of polyethylenimines forming a microphase separation structure

Among organic amines, polyethylenimines (PEIs) have been widely utilized to functionalize various porous solid sorbents to capture CO2, aiming to mitigate global warming. However, low CO2 capacity (due to limited CO2 diffusion) and CO2-induced amine degradation hinder their commercialization. In this study, we developed a series of alkyl (C4 to C12) grafted PEIs. The optimized specimen, 1:9-C12-25kPEI, in which 1 in 9 amine groups are alkylated with C12 alkyl chains (PEI backbone Mn = 25k Da), demonstrated much higher CO2 capacity (180 mg CO2 per 1 g PEI) compared with the control specimen 25kPEI (10 mg CO2 per 1 g PEI) at ambient temperature and pressure (14 °C and 100 % CO2), with a 0.21-mm thick liquid specimen layer. The corresponding half capacity times (t0.5) of the specimens were 8.8 and 43.7 s, respectively. The improved capture kinetics is due to the formation of a microphase separation structure in the alkylated PEI, where the continuous alkyl-dominated phase facilitates CO2 diffusion into the bulk (about 70 μm in depth) to access amine groups in PEI-dominated nanodots (about 13 nm in diameter). Moreover, the rapid CO2 capture consumes free amine groups quickly, avoiding CO2-induced amine degradation in consecutive absorption-desorption cycles at 65 and 95 °C. The alkylated PEIs possess a strong capability to capture CO2, insensitive to capture temperature (14–65 °C) and the mesoporous structure of solid supports. They are suitable for capturing CO2 across a wide temperature range and at various CO2 partial pressures (from air to post-combustion flue gases).