Chemical Communications Blog

A group of scientists from Washington University at St. Louis, USA have disclosed experimentally how CO₂ is captured by polymers with quaternary nitrogen cations. Using solid-state nuclear magnetic resonance (NMR), the authors established that CO₂ molecules were absorbed as bicarbonate anions (HCO₃^–).

The increasing amount of CO₂ has posed a number of concerning environmental issues such as climate change, rising sea level and ocean acidification. Capturing CO₂ from the atmosphere is an effective way to lower the CO₂ concentration. Recently, a family of polymer absorbents containing quaternary nitrogen functional groups, termed humidity-swing polymers, have been identified as promising absorbents to absorb CO₂ directly from air. However, the limited understanding of the chemical mechanism related to their CO₂-capturing capability hindered the development of these promising absorbents.

In ChemComm, Yang et al. used solid-state ¹³C NMR to explore how CO₂ molecules were captured and released. Figure 1a presents the NMR spectra of a humidity-swing polymer absorbent itself (top), upon contacting with CO₂ (middle) and after releasing CO₂ (bottom). The most striking feature is the appearance of an additional sharp peak at a chemical shift of 161 ppm in the middle spectrum, which did not show up in the other two spectra. The authors further studied the shape evolution of the additional peak, with respect to temperature, and concluded that the peak was due to HCO₃^– anions. Additionally, the authors also identified the presence of hydroxide anions in the absorbent after CO₂ was released.

Figure 1. (a) The solid-state ¹³C NMR spectra of the humidity-swing polymeric absorbent (structure shown in the inset of the middle spectrum) itself (top), upon contacting with CO₂ (middle) and after releasing CO₂ (bottom). (b) The proposed pathways of how CO₂ molecules interact with the quaternary-N anions of the absorbent.

The researchers then proposed the CO₂ adsorption-desorption mechanism (illustrated in Figure 1b) based on the experimental results. The storage and release of CO₂ depend on the humidity level of the surroundings: When the humidity is low, the polymer absorbs CO₂ and forms HCO₃^– anions; the negative charge of HCO₃^– is counter-balanced by the neighboring quaternary N cations. When the humidity is increased, HCO₃^– anions combine with water and decompose to CO₂ and hydroxide anions. This proposed pathway does not involve CO₃^2- anions, which differs from the previously-reported mechanisms derived from theoretical simulations.

The published results represent the first set of experimental evidence elucidating how CO₂ molecules interact with humidity-swing polymeric absorbents. The acquired mechanistic insight could provide valuable guidelines for the design of CO₂ absorbents with ultrahigh absorption capacity.

To find out more please read:

Humidity-Swing Mechanism for CO₂ Capture from Ambient Air
Hao Yang, Manmilan Singh and Jacob Schaefer
Chem. Commun., 2018, DOI: 10.1039/c8cc02109k

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.