(704f) Investigation of CO2 Adsorbents for Direct Air Capture: Equilibrium, Kinetic, and Stability Data

The 2015 Paris Agreement saw 190 countries plus the European Union commit to limiting global temperature rise to below 2 °C, and preferably 1.5 °C. According to the latest reports by the Intergovernmental Panel on Climate Change, this can only be achieved with negative emission technologies (NETs). The use of solid adsorbents to remove CO₂ directly from the atmosphere, i.e. direct air capture (DAC), is a NET actively being investigated. However, due to the low concentration of CO₂ in the atmosphere, it remains a challenging process.

A first generation of pilot scale adsorption-based DAC units have already been commercialized. However, there is still a need for accurate performance, energy, and cost estimates for these processes. This requires the collection of critical data such as an adsorbent’s working capacity in the relevant low-pressure range, mass transfer kinetics, selectivity towards other gases, and long-term stability. Today, much of this data is scarcely available.

Our work aims to address this knowledge gap by collecting such data, which is necessary to evaluate DAC adsorbents using process modelling, and eventually to assess their potential at scale. This will allow for improvements in the adsorbent design, which could then further reduce the cost and energy requirements and improve their competitiveness in comparison to other negative emission approaches or technologies.

In this contribution, we have investigated as potential DAC adsorbents: a commercial resin (adsorbent A), and TIFSIX-3-Ni, an ultramicroporous MOF first developed by Kumar et al. [1]. We have compared their performance to that of a benchmark DAC adsorbent, i.e. Lewatit VP OC 1065. To study the equilibrium sorption properties of these adsorbents, we have collected volumetric isotherms of CO₂ and N₂ between 15 °C and 120 °C, of H₂Oat 15 °C, 25 °C, and 35 °C, and of Ar and O₂ at 25 °C. We are using this data to perform isotherm fittings and to calculate the heats of adsorption for CO₂, N_2, and H₂O. To investigate the dynamic sorption properties of these adsorbents, we are collecting kinetic data using both an in-house dynamic adsorption setup and a thermogravimetric analyzer (TGA). To assess the stability of these adsorbents, we are performing cyclic adsorption-desorption tests at 400 ppm CO₂via TGA, and are investigating the CO₂ uptake performance and structural stability of TIFSIX-3-Ni after exposure to 90% relative humidity.

Our results demonstrate that adsorbent A has the highest CO₂ uptake at 400 ppm levels, but still maintains comparable N₂, Ar, O₂, and H₂O uptakes to the benchmark. TIFSIX-3-Ni is shown to have the lowest CO₂ uptake at 400 ppm levels and the highest N₂, Ar, O₂, and H₂O uptakes. Yet, preliminary investigations point to faster adsorption kinetics compared to the other two adsorbents. Based on these results thus far, both adsorbent A and TIFSIX-3-Ni could prove to be strong candidate adsorbents for DAC.

References:

1. A. Kumar, C. Hua, D. G. Madden, D. O’Nolan, K.-J. Chen, L.-A. J. Keane, J. J. Perry, and M. J. Zaworotko, “Hybrid ultramicroporous materials (HUMs) with enhanced stability and trace carbon capture performance,” Chemical Communications, vol. 53, no. 44, pp. 5946–5949, Apr. 2017.