Enhanced performance on CO2 adsorption and release induced by structural transition that occurred in TBAB·26H2O hydrates

Abstract

The applications of ionic clathrate hydrates have greatly improved the efficiency and the conditions required for hydrate-based CO2 capture, but high energy input for hydrate growth and complicated treatment of hydrate slurry still hinder their commercial use. Here we chose TBAB·26H2O hydrate particles to adsorb CO2 molecules instead of TBAB solutions below 2 MPa and release them at ambient pressure. Results showed that the TBAB·26H2O hydrate could adsorb CO2 without induction time and enhance the gas storage capacity by structural transition, especially under high pressure. By using in situ Raman, CO2 molecules were found to fill the empty cages in TBAB·26H2O hydrate first, the formed nCO2·TBAB·26H2O hydrate then converted to nCO2·TBAB·38H2O and TBAB·21/3H2O hydrates at 2 MPa. Macroscopic measurements revealed that around 20 volume of CO2 in standard state could be adsorbed by 1 volume of TBAB·26H2O hydrate sample at 1 MPa, but this volume ratio could reach 67 v/v at 2 MPa where structural change was thought to take place. The pressurized CO2 trapped in hydrate phase was assumed to destroy the structure of TBAB·26H2O hydrate easily, and force the water molecules to form a structure that more compatible with CO2 molecules. This may explain why nCO2·TBAB·26H2O hydrates barely grow from TBAB solutions when pressurized CO2 is injected. In the CO2 release process, the nCO2·TBAB·38H2O and TBAB·21/3H2O hydrates quickly transformed back to nCO2·TBAB·26H2O hydrates and 60–80% of the captured CO2 could be released. Combing with their excellent gas selectivity, TBAB·26H2O hydrate particles would be an ideal material for hydrate-based CO2 capture.