Structure-driven CO₂ selectivity and gas capacity of ionic clathrate hydrates

Area of Science:

• Materials Science
• Chemical Engineering
• Environmental Science

Background:

• Ionic clathrate hydrates are capable of selectively capturing small gas molecules, including carbon dioxide (CO2), nitrogen (N2), methane (CH4), and hydrogen (H2).
• Understanding the gas separation properties of these hydrates is crucial for developing efficient gas capture technologies.

Purpose of the Study:

• To investigate the CO2 + N2 mixed gas separation properties of ionic clathrate hydrates formed with tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium chloride (TBAC), tetra-n-butylphosphonium bromide (TBPB), and tetra-n-butylphosphonium chloride (TBPC).
• To elucidate the relationship between the hydrate structure and its gas separation performance, specifically CO2 selectivity and capacity.

Main Methods:

• Formation and characterization of ionic clathrate hydrates using TBAB, TBAC, TBPB, and TBPC.
• Evaluation of CO2 + N2 mixed gas separation properties, including selectivity and gas capacity.
• Structural analysis using X-ray diffraction and Raman spectroscopy.

Main Results:

• TBAC hydrates exhibited remarkably higher CO2 selectivity compared to other hydrates, despite having lower gas capacity.
• TBAB hydrates demonstrated unusually high CO2 selectivity at low pressures.
• X-ray diffraction and Raman spectroscopy revealed that TBAC forms a stable tetragonal hydrate structure, while TBPB and TBPC form an orthorhombic structure.
• TBAB hydrates displayed polymorphic phases, potentially comprising both orthorhombic and tetragonal structures.
• The tetragonal hydrate structure showed more efficient CO2 capture, whereas the orthorhombic structure possessed the largest gas capacity.

Conclusions:

• The study demonstrates that the tetragonal hydrate structure is more efficient for CO2 capture, while the orthorhombic structure offers higher gas capacity.
• The choice of ionic guest substance significantly influences the hydrate structure and, consequently, its gas separation performance.
• These findings suggest potential for optimizing gas capacity and selectivity in ionic clathrate hydrates by carefully selecting ionic components for specific gas separation applications.