Due to the dramatically increased atmospheric CO 2 concentration and consequential climate change, significant effort has been made to develop sorbents to directly capture CO 2 from ambient air (direct air capture, DAC) to achieve negative CO 2 emissions in the immediate future. However, most developed sorbents have been studied under a limited array of temperature (>20 °C) and humidity conditions. In particular, the dearth of experimental data on DAC at sub-ambient conditions (e.g., −30 to 20 °C) and under humid conditions will severely hinder the large-scale implementation of DAC because the world has annual average temperatures ranging from −30 to 30 °C depending on the location and essentially no place has a zero absolute humidity. To this end, we suggest that understanding CO 2 adsorption from ambient air at sub-ambient temperatures, below 20 °C, is crucial because colder temperatures represent important practical operating conditions and because such temperatures may provide conditions where new sorbent materials or enhanced process performance might be achieved. Here we demonstrate that MIL-101(Cr) materials impregnated with amines (TEPA, tetraethylenepentamine, or PEI, poly(ethylenimine)) offer promising adsorption and desorption behavior under DAC conditions in both the presence and absence of humidity under a wide range of temperatures (−20 to 25 °C). Depending on the amine loading and adsorption temperature, the sorbents show different CO 2 capture behavior. With 30 and 50 wt % amine loadings, the sorbents show weak and strong chemisorption-dominant CO 2 capture behavior, respectively. Interestingly, at −20 °C, the CO 2 adsorption capacity of 30 wt % TEPA-impregnated MIL-101(Cr) significantly increased up to 1.12 mmol/g from 0.39 mmol/g at ambient conditions (25 °C) due to the enhanced weak chemisorption. More importantly, the sorbents also show promising working capacities (0.72 mmol/g) over 15 small temperature swing cycles with an ultralow regeneration temperature (−20 °C sorption to 25 °C desorption). The sub-ambient DAC performance of the sorbents is further enhanced under humid conditions, showing promising and stable CO 2 working capacities over multiple humid small temperature swing cycles. These results demonstrate that appropriately designed DAC sorbents can operate in a weak chemisorption modality at low temperatures even in the presence of humidity. Significant energy savings may be realized via the utilization of small temperature swings enabled by this weak chemisorption behavior. This work suggests that significant work on DAC materials that operate at low, sub-ambient temperatures is warranted for possible deployment in temperate and polar climates.

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The CO 2 sorption behavior of commercially available zeolites such as 3A, 4A, 5A, and 13X is considered at low temperatures for CO 2 removal from ambient air or direct air capture (DAC). Low silica zeolites are typically not effective CO 2 sorbents in the presence of water, as they preferentially competitively adsorb water from humid gas streams, resulting in high sorbent regeneration costs. We hypothesize that low silica zeolites may function as efficient physisorbents for DAC if deployed at cold temperatures where the absolute humidity of air is low. Two modes of deployment of low silica zeolites for DAC at cold temperatures are explored here. Based on the CO 2 isotherms of the zeolites at −20 °C with different H 2 O surface loadings, zeolite 5A was selected for evaluation in a competitive H 2 O and CO 2 coadsorption process as the first mode of deployment. Despite the low absolute humidity at −20 °C compared to that at 25 °C, H 2 O adsorption and accumulation result in a 39% decrease in the CO 2 adsorption capacity of 5A, rendering the process energetically expensive. In the second mode of deployment, focusing on estimates of the thermal energy requirements, zeolite 13X with silica gel as a desiccant in a two-stage, two-bed process is found to provide a potentially energetically feasible process (4359 MJ/tCO 2 ) for cold-temperature DAC. Cyclic adsorption and desorption cycles swinging between −20 and 200 °C with 0.04% and 99.9% CO 2 , respectively, are conducted to experimentally support the thermal energy calculations using a temperature swing adsorption (TSA) process. Water production using available cooling energy from cold ambient air offers the potential to reduce the cost of DAC, as do additional process design modes such as vacuum swing adsorption and advanced heat management systems.

A variety of amine-impregnated porous solid sorbents for direct air capture (DAC) of CO 2 have been developed, yet the effect of amine-solid support interactions on the CO 2 adsorption behavior is still poorly understood. When tetraethylenepentamine (TEPA) is impregnated on two different supports, commercial γ-Al 2 O 3 and MIL-101(Cr), they show different trends in CO 2 sorption when the temperature (−20 to 25 °C) and humidity (0−70% RH) of the simulated air stream are varied. In situ IR spectroscopy is used to probe the mechanism of CO 2 sorption on the two supported amine materials, with weak chemisorption (formation of carbamic acid) being the dominant pathway over MIL-101(Cr)-supported TEPA and strong chemisorption (formation of carbamate) occurring over γ-Al 2 O 3supported TEPA. Formation of both carbamic acid and carbamate species is enhanced over the supported TEPA materials under humid conditions, with the most significant enhancement observed at −20 °C. However, while equilibrium H 2 O sorption is high at cold temperatures (e.g., −20 °C), the effect of humidity on a practical cyclic DAC process is expected to be minimal due to slow H 2 O uptake kinetics. This work suggests that the CO 2 capture mechanisms of impregnated amines can be controlled by adjusting the degree of amine-solid support interaction and that H 2 O adsorption behavior is strongly affected by the properties of the support materials. Thus, proper selection of solid support materials for amine impregnation will be important for achieving optimized DAC performance under varied deployment conditions, such as cold (e.g., −20 °C) or ambient temperature (e.g., 25 °C) operations.

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.