CO 2 Capture via Crystalline Hydrogen-Bonded Bicarbonate DimersA crystallization-based CO 2 -separation method involving a simple aqueous guanidine sorbent offers the prospect of energy-efficient and cost-effective carbon-capture technologies that could help mitigate climate change.

We report a bench-scale direct air capture (DAC) process comprising CO2 absorption with aqueous amino acid salts (i.e., potassium glycinate, potassium sarcosinate), followed by room-temperature regeneration of the amino acids by reaction with solid meta-benzene-bis(iminoguanidine) (m-BBIG), resulting in crystallization of the hydrated m-BBIG carbonate salt, (m-BBIGH2)(CO3)(H2O)n (n = 3-4). The CO2 is subsequently released by mild heating (60-120 °C) of the carbonate crystals, which regenerates the m-BBIG solid quantitatively. This low-temperature crystallization-based DAC process circumvents the need to heat the aqueous amino acid sorbents, thereby minimizing their loss through thermal and oxidative degradation. The CO2 cyclic capacity for the sarcosine/m-BBIG system, measured over three consecutive absorption/regeneration cycles, is in the range of 0.12-0.20 mol/mol. The regeneration energy of m-BBIG, comprising the enthalpy of CO2 and water release, and the sensible heat, is 360 kJ/mol (8.2 GJ/ton CO2). Alternatively, the aqueous amino acids can be regenerated by boiling under reflux, with measured cyclic capacities of up to 0.64 mol/mol.

Direct air capture (DAC) technologies that extract carbon dioxide directly from the atmosphere using chemical processes have the potential to achieve zero or negative emissions and restore the atmospheric composition to an optimal level with respect to the CO2 concentration. However, before such technologies can be developed and applied energy-efficiently, we need to gain fundamental understanding of DAC chemistry. Here we report a structure-properties relationship study of DAC by crystallization of bis-iminoguanidine (BIG) carbonate salts. The study focuses on a series of basic BIG structures including the glyoxal-bis(iminoguanidine) prototype (GBIG) and its simple analogs methylglyoxal-bis(iminoguanidine) (MGBIG) and diacetyl-bis(iminoguanidine) (DABIG). The crystal structures of the BIGs and their carbonate salts have been analyzed by single-crystal X-ray and neutron diffraction to accurately measure key structural parameters including molecular conformations, hydrogen bonding, and p-stacking. Experimental measurements of key BIG properties, such as aqueous solubilities, and regeneration energies and temperatures, are complemented by first-principles calculations of lattice and hydration free energies, as well as free energies of reactions with CO2, and BIG regenerations. We find that minor structural modifications in the molecular structure of GBIG, such as substituting one or two hydrogen atoms with methyl groups, result in major changes in the crystal structures, induced by the increased conformational flexibility and steric hindrance. As a result, the corresponding aqueous solubilities within the series increase significantly, leading in turn to enhanced DAC performances.

A hybrid solvent/solid-state approach to CO2 separation from flue gas is demonstrated based on absorption with aqueous amino acids (i.e., glycine, sarcosine), followed by crystallization of the bicarbonate salt of glyoxal-bis(iminoguanidine) (GBIG), and subsequent solid-state CO2 release from the bicarbonate crystals. In this process, the GBIG bicarbonate crystallization regenerates the amino acid sorbent, and the CO2 is subsequently released by mild heating of the GBIG bicarbonate crystals, which results in quantitative, energy-efficient regeneration of GBIG. The cyclic capacities measured from multiple absorption-regeneration cycles are in the range of 0.2-0.3 mol CO2/mol amino acid. This hybrid CO2-separation approach reduces the sorbent regeneration energy by 24% and 40% compared to the regeneration energy needed for benchmark industrial sorbents monoethanolamine and sodium glycinate, respectively, while minimizing the amount of the amino acid sorbent loss through evaporation or degradation.

We report a novel di(imino)guanidinium anion extractant with unparalleled selectivity for sulfate in a liquid-liquid separation system. In addition to a 4.4 order-of-magnitude enhancement in affinity compared to a standard benchmark, our alkylated di(imino)guanidinium receptor is economically synthesized and features good compatibility with application-relevant aliphatic solvents. Small-angle X-ray scattering results reveal the formation of reverse-micelles, which together with the significant organic-phase water content challenge traditional notions of selectivity in extraction of superhydrophilic anions.