1. Department of Chemical Engineering and Resource Recycling, Wuzhou University, Wuzhou 543000, China
2. Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences，Guangzhou 510640, China
3. Key Laboratory of Gas Hydrate，Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences，Guangzhou 510640, China
4. Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China
5. Key Laboratory of Natural Gas Hydrate, Ministry of Land and Resources, Qingdao Institute of Marine Geology, China Geological Survey, Qingdao 266071, China
Abstract：TBAB semi-clathrate hydrate has a huge potential for effective application of carbon dioxide (CO2) capture. Because of the complexity of the crystal structure, the kinetics of TBAB hydrate remains poorly understood. In this work, the spectral characteristics of nCO2·TBAB·26H2O and nCO2·TBAB·38H2O were analyzed by Raman and powder X-ray diffraction (PXRD). To understand the gas storage characteristics of TBAB hydrate, the processes of CO2 molecules entering 2 kinds of crystal structures were measured using in situ Raman spectroscopy. Results showed that the Raman spectra of 2 crystal structures had high similarity. The Raman peaks at 1 309.5 and 1 326.9 cm-1 were assigned to be the C—C deformation vibration mode of TBA+ cations in nCO2·TBAB·26H2O hydrate. They did not shift in nCO2·TBAB·38H2O hydrate, but became detached and narrow in half-peak width. Meanwhile, the peaks at 1 446.6 and 1 458 cm-1 were assigned to be the C—H shear vibration mode of TBA+ cations in nCO2·TBAB·26H2O hydrate. They shifted away from each other and had lower less overlap region in nCO2·TBAB·38H2O hydrate. Those features in Raman spectra were helpful to distinguish the 2 kinds of structures. The PXRD patterns of the 2 TBAB hydrates showed large difference from each other. nCO2·TBAB·26H2O hydrate was tetragonal which had the space group of (P4/m), while nCO2·TBAB·38H2O hydrate was orthorhombic which had the space group of (Pmma). In the PXRD patterns, the peaks at 2θ=8.406° and 10.941° were (200) and (220) planes of nCO2·TBAB·38H2O hydrate respectively. The structure of nCO2·TBAB·26H2O hydrate was characterized by the (012) and (003) planes at 2θ=5.976° and 6.969° respectively. During the in situ Raman measurements, nCO2·TBAB·26H2O and nCO2·TBAB·38H2O hydrates grew directly from the prepared TBAB·26H2O and TBAB·38H2O hydrates at 276 K, 2 MPa. The CO2 molecules were captured by the 512 hydrate cages in the 2 kinds of hydrates, formed the characteristic peaks of CO2 at 1 275.4 and 1 379.3 cm-1 and increased continuously. The Raman peaks at 1 110.3 cm-1 were chosen as reference peak to compare the CO2 concentration growth in the 2 kinds of hydrates. In the initial 75 minutes of in situ Raman measurements, the content of CO2 in hydrate phase grew linearly with generally the same growth rates in 2 kinds of crystals. As the measuring spots were on the hydrate surface where the gas diffusion resistance in hydrate phase could be neglected and the cage structures used for gas storage were all 512 cage, the similar gas storage rates were obtained. The microcosmic experimental study provides a theoretical basis for CO2 capture technology by forming TBAB semi-clathrate hydrate.
Key words：Natural gas hydrate; Kinetics; Carbon dioxide; Raman; X-ray diffraction
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