Humidity-Enhanced Direct Air Capture of Carbon Dioxide Using Amine-Grafted Covalent Organic Frameworks Under Ambient and Sub-ambient Temperatures

TL;DR

A new material captures more carbon dioxide from air when it's cold and humid, which could help reduce atmospheric CO2 in most real-world conditions.

Contribution

A novel amine-grafted COF shows enhanced CO2 capture under cold and humid conditions, which is critical for DAC in diverse climates.

Findings

ImCOF-TAEA achieves a 137% increase in CO2 adsorption capacity under 70% humidity at 25°C.

CO2 uptake further increases by 205% at 15°C under 70% humidity.

The material shows excellent recyclability and low regeneration temperature requirements.

Abstract

The significant rise
in atmospheric CO2 and its impact
on accelerating climate change have triggered intense efforts to develop
porous sorbents for direct air capture (DAC), a route toward carbon-neutrality.
Amine-functionalized covalent organic frameworks (COFs), an emerging
class of crystalline porous materials, have recently shown promising
potential for DAC at ambient, indoor temperatures (25 °C). However,
most of Earth’s land area has annual mean temperatures below
25 °C, accompanied by nonzero and variable relative humidity
(RH). The performance of amine-grafted COFs under cold, humid conditions
remains largely unexplored, even though such climates represent the
majority of potential DAC deployment sites. Herein, we report a systematic
investigation of a tetrahydroquinoline-linked COF covalently functionalized
with diverse amines, evaluating its CO2 adsorption behavior
across a broad range of ambient to sub-ambient temperatures (25 °C
to −20 °C) and relative humidities (0%–70%). A
unique tris­(2-aminoethyl)­amine-functionalized COF (ImCOF-TAEA) achieved
a pseudoequilibrium capacity of 0.46 ± 0.02 mmol g–1 under dry conditions, rising ∼137% to 1.09 ± 0.09 mmol
g–1 under 70% RH using 400 ppm of CO2 at 25 °C. Upon cooling to 15 °C under 70% RH, the uptake
further increased to 1.25 ± 0.02 mmol g–1,
showing a 205% enhancement relative to dry conditions. In
situ spectroscopic analysis supports the mechanism behind
the unusually high enhancement in CO2 adsorption under
humid conditions. ImCOF-TAEA also demonstrates excellent recyclability
under ambient/sub-ambient conditions and has modest (45 °C-65
°C) requirements for regeneration.

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