CO2 capture via Crystalline Hydrogen-Bonded Bicarbonate Dimers

Research paper Summary

Average increase or decrease in global temperature.

Since we know that the human activity from the industrial revolution has unsettled the natural carbon cycle, mostly from fossil fuels, factories, and even from our homes, which are responsible for carbon dioxide emissions that lead to an increase in global temperature. To establish this unsettled cycle several methods and efforts are made through Carbon Capture and Storage. The aim of the CCS was to reduce carbon emissions by capturing the carbon dioxide and permanently storing it deep underground.

In the paper, the researcher reports a simple Carbon dioxide separation cycle using an aqueous bis(iminoguanidine) sorbent which reacts with carbon dioxide and crystallizes into insoluble bicarbonate salt [GBIGH2(HCO3)2(H2O)2]. Synthesis of GBIG (glyoxal-bis(iminoguanidine)) bicarbonate is done by imine condensation of glyoxal with aminoguanidinium chloride, followed by neutralization of sodium hydroxide (NaOH). Researchers found that glyoxal-bis(iminoguanidine) can form extremely insoluble crystalline salts with oxyanions that allow for selective separation of anions from aqueous solution by crystallization. Also, they studied the bicarbonate crystal and talked about all types of bond lengths. The mean interplanar distance between the GBIGH2 2+ cations stack along the same direction was found 3.25 A° and the cationic stacks flank the anionic clusters in the close-packed arrangement. They also reported anti-electrostatic hydrogen-bonded (HCO3-)2 dimers are stabilized by guanidinium cations and water.

X-Ray Crystal Structure of GBIGH2(HCO3)2(H2O)2 and ORTEP representation with 50% ellipsoids (C, gray; H, white; N, blue; O, red)

They also performed the thermodynamic analysis of carbon dioxide absorption and release. Carbon dioxide absorption was driven mostly from the formation of HCO3- and [GBIGH2(HCO3)2(H2O)2] crystallization which is highly favorable with respect to glyoxal-bis(iminoguanidine) protonation and dissolution of glyoxal-bis(iminoguanidine) and carbon dioxide.

Further carbon dioxide released from crystalline glyoxal-bis(iminoguanidine) bicarbonate salt was investigated through thermogravimetric analysis coupled with mass spectrometry (TGA-MS) and differential scanning calorimetry (DSC). The TGA-MS shows a 48.3% mass loss around 112°C with the evolution of carbon dioxide and water in a 1:2 ratio. From Density functional theory (DFT) it was confirmed that the release of H2CO3 was most thermodynamically favorable with the increase in temperature. DSC analysis shows an endotherm in the same temperature range for carbon dioxide and H2O loss in TGA, with enthalpy measured for reaction (229–257) KJ/mol which corresponds to (114.5–128.5) kJ/mol CO2. They estimated 151.5 KJ/mol CO2 total regeneration energy for GBIG which corresponds to a 24% reduction compared with monoethanolamine (MEA) which was earlier a benchmark sorbent employed in industrial CO2 scrubbing.

Kinetics of CO2 Release from Crystalline GBIGH2(HCO3)2(H2O)2 Obtained from the Isothermal TGA Data (80C, Red; 90C, Blue; 100C, Green; 110C, Magenta)

Researchers also showed several kinetic and mechanistic analysis of CO2 release. They plotted the fractional conversion ( ɑ ) as a function of time. According to them, the best fit was found for the contracting volume model, corresponding to the initiation of reaction on crystal’s surface results in ɑ — t curve plotted with reference to this equation [ 1- (1 — ɑ)⅓ = kt ]. For mechanistic insights, researchers did ab initio molecular dynamics [MD] simulations and observed a direct proton transfer from GBIGH22+ to HCO3- and after 3.5ps several proton transfers back and forth between bicarbonate oxygen and imine nitrogen of ligand. From these analyses, researchers reported that combined molecular dynamics and static DFT calculations strongly support a reaction mechanism that involves low barrier proton transfer from GBIGH22+ to HCO3- with the formation of a carbonic acid dimer as an intermediate, which was later followed by carbon dioxide and water release in the rate-limiting step.

CO2 Separation from a Flue Gas Simulant (EPA Protocol Standard, 12.8% CO2 + 87.2% N2)

In the later part of the paper, researchers tried to demonstrate the practical utility of glyoxal-bis(iminoguanidine) in capturing carbon dioxide. They ran a full carbon dioxide separation cycle using a flue gas simulant containing 12.8% of CO2. They observed that when the flue gas mixture was bubbled through an aqueous solution of glyoxal-bis(iminoguanidine), and the white precipitate was formed within a minute. Continuous heating of isolated [GBIGH2(HCO3)2(H2O)2] solid at 120°C for two hours led to the quantitative release of carbon dioxide and water which was determined by elemental and gravimetric analyses.

CO2 Separation Performance Showing the Measured Cyclic Capacity (mol CO2/mol GBIG) over Ten Consecutive Absorption-Release Cycles

Researchers concluded the report by showing ten consecutive carbon dioxide capture release cycles that were conducted, and it was observed that only a 3% decrease in CO2 absorption capacity, which was remarkable. The minimum energy required for regeneration of glyoxal-bis(iminoguanidine) is found 151.5 KJ/mol CO2, which is 24% lower than the regeneration energy of monoethanolamine (MEA), a benchmark industrial sorbent. Also, this was the energy-efficient and cost-effective carbon capture technology that can help to re-establish the carbon cycle.

Reference: https://doi.org/10.1016/j.chempr.2018.12.025

Authors: Neil J. Williams, Charles A. Seipp, Flavien M. Brethome´, Ying-Zhong Ma, Alexander S. Ivanov, Vyacheslav S. Bryantsev, Michelle K. Kidder, Halie J. Martin, Erick Holguin, Kathleen A. Garrabrant, and Radu Custelcean.