Carbon dioxide capture by an amine functionalized ionic liquid: fundamental differences of surface and bulk behavior

Area of Science:

• Chemical Engineering
• Materials Science
• Physical Chemistry

Background:

• Ionic liquids (ILs) are promising for carbon capture.
• Amine-functionalized ILs offer enhanced CO2 absorption capabilities.
• Understanding CO2 speciation in ILs is crucial for optimizing capture processes.

Purpose of the Study:

• To investigate the absorption mechanism of carbon dioxide (CO2) by dihydroxyethyldimethylammonium taurinate, an amine-functionalized ionic liquid.
• To differentiate CO2 binding states (carbamate vs. carbamic acid) in both the near-surface and bulk regions of the IL.
• To elucidate the influence of pressure and solvation on CO2 speciation within the ionic liquid.

Main Methods:

• Surface-sensitive X-ray photoelectron spectroscopy (XPS) was employed to analyze the near-surface region.
• Bulk-sensitive isothermal gas uptake experiments were conducted.
• Fourier-transform infrared (FTIR) spectroscopy was used in conjunction with uptake studies to identify CO2 species.

Main Results:

• At 0.9 mbar CO2, the near-surface region showed approximately 0.58 mol CO2/mol IL, predominantly as carbamic acid (~0.43 mol) with some carbamate (~0.15 mol).
• In the bulk, carbamate was the dominant species (~0.5 mol CO2/mol IL) up to 2.5 bar CO2.
• At higher pressures (>2.5 bar), carbamic acid became the dominant species in the bulk (~1 mol CO2/mol IL).

Conclusions:

• CO2 speciation in the ionic liquid is pressure-dependent, with distinct behaviors observed in the near-surface and bulk regions.
• Differences in solvation between the outermost IL layers and the bulk environment influence the formation of carbamic acid and carbamate species.
• The findings provide insights into the mechanism of CO2 absorption by amine-functionalized ionic liquids, relevant for designing efficient carbon capture technologies.