(401c) CO2 Adsorption Equilibrium of Modified Amine-Based Sorbent for Direct Air Capture

Direct air capture (DAC) continues to draw a lot of attention as it has been identified as a major technology for net-zero emissions control. Since it involves CO₂ separation directly from the atmosphere, there is a wide range of location options for deploying DAC plants under different climatic conditions. Therefore, it is important to understand the adsorption characteristics of a sorbent under different temperature and humidity conditions. However, to the best of our knowledge, there is a limited number of previous studies on adsorption isotherms under very dilute CO₂ concentrations relevant to DAC.

In this study, equilibrium adsorption data were obtained under ambient and sub-ambient CO₂ concentrations for unmodified and modified amine-based sorbents and described using the Toth isotherm model. Modified amine-based sorbents can reduce the shortcomings of thermal and oxidative degradations of unmodified amine-based sorbents. The isotherm data for modified amine sorbents showed a shift in equilibrium resulting from the amine modification, leading to a decrease in the heat of CO₂ adsorption. The modification most likely benefits from lowering the basicity and increasing the steric hindrance of the amine center, leading to lower binding energy between adsorbed CO₂ molecules and modified amine sites. Our differential scanning calorimetry (DSC) data for saturated sorbents also support that modified amine-based sorbents show lower heat of desorption than that of unmodified amine sorbent. These data support that modified amine-based sorbents can provide lower heat of desorption as well as the resistances to thermal and oxidative degradations.

Our DAC system includes a sorbent-washcoated monolith structure. The isotherm model determined from lab-scale testing of the powdered sorbent is a key part which will be employed in the coating layer of the monolith to predict the adsorption performance. This is done under various flow conditions within the system. Adsorption isotherm parameters determined from this effort provide a means to successful scale-up of DAC following key performance metrics like CO₂ capture efficiency, overall volumetric productivity, and pressure drop.