Direct Air Capture of CO₂ Using Amine/Alumina Sorbents at Cold Temperature

Rising CO₂ emissions are responsible for increasing global temperatures causing climate change. Significant efforts are underway to develop amine-based sorbents to directly capture CO₂ from air (called direct air capture (DAC)) to combat the effects of climate change. However, the sorbents' performances have usually been evaluated at ambient temperatures (25 °C) or higher, most often under dry conditions. A significant portion of the natural environment where DAC plants can be deployed experiences temperatures below 25 °C, and ambient air always contains some humidity. In this study, we assess the CO₂ adsorption behavior of amine (poly(ethyleneimine) (PEI) and tetraethylenepentamine (TEPA)) impregnated into porous alumina at ambient (25 °C) and cold temperatures (-20 °C) under dry and humid conditions. CO₂ adsorption capacities at 25 °C and 400 ppm CO₂ are highest for 40 wt% TEPA-incorporated γ-Al₂O₃ samples (1.8 mmol CO₂/g sorbent), while 40 wt % PEI-impregnated γ-Al₂O₃ samples exhibit moderate uptakes (0.9 mmol g^-1). CO₂ capacities for both PEI- and TEPA-incorporated γ-Al₂O₃ samples decrease with decreasing amine content and temperatures. The 40 and 20 wt % TEPA sorbents show the best performance at -20 °C under dry conditions (1.6 and 1.1 mmol g^-1, respectively). Both the TEPA samples also exhibit stable and high working capacities (0.9 and 1.2 mmol g^-1) across 10 cycles of adsorption-desorption (adsorption at -20 °C and desorption conducted at 60 °C). Introducing moisture (70% RH at -20 and 25 °C) improves the CO₂ capacity of the amine-impregnated sorbents at both temperatures. The 40 wt% PEI, 40 wt % TEPA, and 20 wt% TEPA samples show good CO₂ uptakes at both temperatures. The results presented here indicate that γ-Al₂O₃ impregnated with PEI and TEPA are potential materials for DAC at ambient and cold conditions, with further opportunities to optimize these materials for the scalable deployment of DAC plants at different environmental conditions.