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Thickness Dependent CO2 Adsorption of Poly(ethyleneimine) Thin Films for Direct Air Capture



John Hoffman, Avery Baumann, Christopher Stafford


Mesoporous silica impregnated with polyethyleneimine (PEI) has been shown to be a suitable material for the direct air capture (DAC) of CO2. Factors such as CO2 concentration, temperature, and amine loading impact overall capture capacity and amine efficiency by altering diffusional resistance and reaction kinetics. When studied in the impregnated 3-dimensional sorbent material, external diffusion impact the evaluation of the reaction kinetics at the air/amine interface. Here, using tandem quartz crystal microbalance with dissipation (QCM-D) and polarization modulation infrared reflective absorption spectroscopy (PM-IRRAS), CO2 adsorption kinetics of the PEI-based amine layer in a 2-dimensional geometry were studied at a variety of film thicknesses (10 nm to 100 nm), temperatures (25 C to 80 C), and CO2 concentrations (5% and 400 ppm). Total CO2 capture capacity increased with film thickness but resulted in a decrease in amine efficiency, as additional diffusional resistance for thicker films limits access to available amine sites. The capture capacity of thick films (>50 nm) is shown to be limited by amine availability, while capture of thin films (<50 nm) is limited by CO2 availability. A 50 nm PEI film was shown to be optimal for capture of 400 ppm CO2. Increasing temperature alters capture capacity by decreasing diffusional resistance, which counteracts worsened kinetic rates of the exothermic adsorption reaction. This trend was not seen for a 10 nm PEI film and capacity generally decreased with increasing temperature. The adsorption profiles for these conditions were fitted to pseudo-first order and Avrami fractional order models. The reaction process switches between a diffusion limited reaction to a kinetic limited reaction at 80 C when using 5% CO2 and 55 C when using 400 ppm CO2. These results offer accurate analysis of adsorption of CO2 at the air/amine interface of PEI films which can be used for the design of future sorbent materials.
Chemical Engineering Journal


Direct air capture, Quartz crystal microbalance, polarization modulation infrared reflection absorption spectroscopy, polyethyleneimine, solid amine sorbent


Hoffman, J. , Baumann, A. and Stafford, C. (2024), Thickness Dependent CO2 Adsorption of Poly(ethyleneimine) Thin Films for Direct Air Capture, Chemical Engineering Journal, [online],, (Accessed April 19, 2024)
Created January 2, 2024, Updated March 11, 2024