Kinetic Measurements on CO₂ Hydrate Formation in the Presence of Tetra-<i>n</i>-butyl Ammonium Bromide

Abstract

Ionic clathrate hydrates are promising materials for hydrate-based CO2 capture due to their mild forming conditions. In this work, hydrate crystallizations from different TBAB solutions were measured. The effect of TBAB concentration on hydrate nucleation time, hydrate growth, and gas storage capacity were evaluated. The TBAB concentration ranging from 0.3 to 7.5 mass % was found to be ideal for efficient hydrate formation. The TBAB hydrates were assumed to form first in the liquid bulk which induced the growth of simple CO2 hydrate. In dilute TBAB solutions, guest ions in TBAB tended to inhibit hydrate nucleation and increase induction time, while concentrated TBAB solutions were found to reduce gas storage capacities. PXRD measurements revealed that the orthorhombic TBAB hydrate preferred to form where TBAB concentration was below 35 mass %. When the concentration was above 40 mass %, tetragonal and trigonal TBAB hydrate dominated the solid hydrate phase. In Raman measurements, C–H stretching modes of the TBA+ cation in different hydrate structures were characterized. CO2 was proved to be captured by tetragonal and orthorhombic TBAB hydrates, which was suggested to be beneficial for hydrate stabilization.