Influence of crystallization parameters on guest selectivity and structures in a CO2-based separation process using TBAB semi-clathrate hydrates

Carbon dioxide (CO2) capture from flue gas mixture analogs based on ionic clathrate hydrates crystallization (using tetra-n-butyl ammonium bromide, TBAB) is investigated by optical microscopy and Raman spectroscopy. The purpose of this research is to investigate the influence of the crystallization protocol (slow versus fast cooling/heating) on the subsequent semi-clathrate structures formed from a 35 wt% TBAB-H2O solution and CO2 separation efficiency from an initial gas mixture of 10%CO2 + 90%N-2. In-situ Raman analyses reveal that structure type B (sometimes associated with several polymorph phases) is readily observed at the onset of the formation process regardless of the crystallization protocol. Structure type B remains stable up to a temperature close to that of dissociation, whereas polymorphs form and evolve continuously until then. Just before dissociation, structure type A is additionally observed. The protocol involving slow cooling and heating steps (similar to the isochoric pressure search method) performs slightly better in terms of selectivity than the one using the multi-cycling temperature approach (fast cooling and heating). However, within the same protocol, the selectivity greatly depends on the specific semi-clathrate structures formed. We find that the best CO2 selectivity is achieved when structure type A is formed compared to all other semi-clathrate phases. The CO2 recovery fraction is found at lower value than that measured in canonical clathrate hydrates.