Structure, Composition, and Thermal Expansion of CO₂ Hydrate from Single Crystal X-ray Diffraction Measurements

Abstract

The structure and composition of CO2 hydrate were determined from single-crystal X-ray diffraction data at 173 K for a crystal grown over heavy water and liquid CO2. Superior diffraction data allowed the derivation of a structural model of unprecedented quality for the hydrate, giving the location of the disordered CO2 molecules in the two cages. In the large cage, the guests are shown to be off-center, with a bimodal distribution of out-of-plane orientations for the long axis of the molecule (173 K). Also, the absolute cage occupancies were determined from the structural model, thus allowing a reliable and direct evaluation of the hydrate composition for this crystal, CO2·6.20(15) D2O. The temperature dependence of the lattice parameters for the single crystal was measured between 123 and 223 K and found to be in good agreement with recent neutron powder diffraction results, and data from all sources were fit to a single polynomial function. The hydrate composition and density are discussed in terms of the information needed for the deep-sea sequestration of CO2. The guest disorder and dynamics are discussed in terms of the model derived from earlier NMR data.

Molecular Cage Occupancy of Clathrate Hydrates at Infinite Dilution: Experimental Determination and Thermodynamic Significance

“…At both extremes of pure and infinitely dilute states, the cage occupancy ratio for CH 4 and CO 2 changed from 1.27 to 0.33 and from 3.30 to 1.05, respectively. Considering the large cage occupancy of 0.98, the ratio around 1.05 indicates that small cage occupancy of CO 2 molecules is about 0.93, while smaller cage occupancy in the small cages was reported by Udachin et al 12 with single crystal samples. Such a difference can be mainly explained by different sample preparation methods as well as some unknown factors because the cage occupancy is strongly dependent on formation conditions.…”

Capture of carbon dioxide from flue or fuel gas mixtures by clathrate crystallization in a silica gel column

“…The hydration number is the number of water molecules per guest molecule. It can be determined experimentally by simultaneously using powder X-ray diffraction and Raman spectroscopy on solid hydrate phase Kumar et al, 2008;Sum et al, 1997;Susilo et al, 2007a;Uchida et al, 1999;Udachin et al, 2007Udachin et al, , 2001 or by solid state NMR spectroscopy (Davidson et al, 1983;Ripmeester and Ratcliffe, 1988). In the presence of THF, the conversion of water to hydrate is determined by the next equation…”

Design of Ecological CO2 Enrichment System for Greenhouse Production using TBAB + CO2 Semi-Clathrate Hydrate

“…Three main structural families of gas hydrate are known, and they form depending mainly on the size of the guest molecules. These three are cubic structure I (sI) [33]. Other structural gas hydrates of sII and sH also include CO2 in the presence of other guest molecules, such as tetrahydrofuran (THF) [34] or 3,3-dimethyl-2-butanone (pinacolone) [35].…”

“…Pure CO 2 gas forms sI hydrate, in which CO 2 is encaged in both 5 12 and 5 12 6 2 cages. The CO 2 hydrate contains about 150 volumes of CO 2 gas per volume of hydrate crystal at standard temperature and pressure (150 V/V(STP)) [33]. Other structural gas hydrates of sII and sH also include CO 2 in the presence of other guest molecules, such as tetrahydrofuran (THF) [34] or 3,3-dimethyl-2-butanone (pinacolone) [35].…”

“…Then, the overall apparent cage occupancy of 5 12 cage (~0.15) was estimated. Other results based on cage occupancies determined by X-ray structure analysis [33,34,38] are plotted for comparison. Figure 4 suggests the possibility of forming greater gas occupation in the TBAB + CO 2 + N 2 SC sample, which could be achieved by improving the formation process of the SC sample.…”