CO₂ Capture from Ambient Air by Crystallization with a Guanidine Sorbent

Abstract

Carbon capture and storage is an important strategy for stabilizing the increasing concentration of atmospheric CO and the global temperature. A possible approach toward reversing this trend and decreasing the atmospheric CO concentration is to remove the CO directly from air (direct air capture). Herein we report a simple aqueous guanidine sorbent that captures CO from ambient air and binds it as a crystalline carbonate salt by guanidinium hydrogen bonding. The resulting solid has very low aqueous solubility (K =1.0(4)×10 ), which facilitates its separation from solution by filtration. The bound CO can be released by relatively mild heating of the crystals at 80-120 °C, which regenerates the guanidine sorbent quantitatively. Thus, this crystallization-based approach to CO separation from air requires minimal energy and chemical input, and offers the prospect for low-cost direct air capture technologies.

Direct Air Capture of CO2 with Aqueous Amino Acids and Solid Bis-iminoguanidines (BIGs)

“…The reaction mixture was heated at 65 °C with mechanical stirring (350 rpm) for 4 h. The m-BBIG chloride salt (m-BBIGH2Cl2) formed as a solid suspension and the white crystalline product was collected by vacuum filtration and washed with ethanol followed by diethyl ether, then allowed to dry for a few hours. The yield of m-BBIGH2Cl2 was 228.92 g (96% obtained by this method was lower than the corresponding yield of the PyBIG analog [24] due to the significantly higher solubility of m-BBIG (50 mM) compared to PyBIG (10 mM). The solid cake of m-BBIG carbonate was transferred to a large crystallization dish, breaking it up and spreading it into a thin layer.…”

“…Following the proof of principle with PyBIG [23][24][25], we now seek to establish the generality of the amino acid/BIG approach to DAC. One advantage of the BIG family of compounds is that their syntheses are straightforward and modular, consisting of imine condensation of aminoguanidinium salts with different dialdehydes [28,29].…”

Direct air capture of CO2– topological analysis of the experimental electron density (QTAIM) of the highly insoluble carbonate salt of a 2,6-pyridine-bis(iminoguanidine), (PyBIGH2)(CO3)(H2O)4

“…The crystal structure of PyBIG carbonate was reported previously (Seipp et al, 2017;Brethomé et al, 2018) and we have preserved the setting of the unit cell and the atom numbering used in that work. Based on our experimental neutron data, the crystal structure was re-refined within the SHELXTL program suite (Sheldrick, 2015) using the previously reported structure as the starting model.…”

“…The previously reported X-ray crystal structure of (PyBIGH 2 )(CO 3 )(H 2 O) 4 showed that the hydrated carbonate salt comprises an elaborate hydrogen-bonded network involving the carbonate anion, guanidinium cations, water molecules and the pyridine N atom (Seipp et al, 2017;Brethomé et al, 2018). However, the precise geometrical parameters and energetics of the hydrogen bonds and other intermolecular interactions present in this structure, which presumably play important roles in the unusually low aqueous solubility of (PyBIGH 2 )(CO 3 )(H 2 O) 4 , have yet to be determined.…”

“…In a recent study, an aqueous solution of a 2,6-pyridinebis(iminoguanidine), PyBIG [the displayed resonance form is based on the reported crystal structure (Seipp et al, 2017;Brethomé et al, 2018)], was found to efficiently absorb CO 2 from the atmosphere and convert it into the crystalline carbonate salt (PyBIGH 2 )(CO 3 )(H 2 O) 4 (Seipp et al, 2017;Brethomé et al, 2018). The main driver for this reaction is the extremely low aqueous solubility of the carbonate salt (K sp = 1.0 Â 10 À9 ), comparable to CaCO 3 (3.7-8.7 Â 10 À9 ), which pushes the overall equilibrium towards the carbonate formation despite the very low concentration of CO 2 in the air.…”