Structural CO2 capture preference of semiclathrate hydrate formed with tetra-n-butylammonium chloride

Abstract

The crystal structure of tetra-n-butylammonium chloride (TBAC) + CO₂ semiclathrate hydrate that is known to have high CO₂ selectivity was studied by single crystal X-ray diffraction. The presence of the CO₂ gas leads to occupation of dodecahedral (D) cages by water, CO₂ and chloride. This hydrate structure has three types of D cages, i.e., D_L, D_M and D_N, with the ratio for cage number D_L : D_M : D_N = 2 : 2 : 1. CO₂ occupied D_M and D_N cages which were suitably distorted for the linear CO₂ molecular shape. Chloride anions partly occupied the D_M cages, coordinating to water molecules. The lateral distortion of the D_M and D_N cage leads restricted spatial CO₂ distribution in the cages. The D_L cages were highly distorted by the TBA cation. In D_L cages, partial occupancies with both water and chloride, which coordinated to cage water molecules, were found. Contrary to the water molecules which were placed at the cage center, chloride anions were off-centred at two distinct positions due to slightly longer coordination lengths with the cage water molecules. The present analysis showed a superior CO₂ capture preference of TBAC hydrate in the structural aspect.