TBAB‐CO2水合物形成过程的微观实验

四丁基溴化铵（TBAB）半笼型水合物在二氧化碳（CO₂）捕集和封存技术中具有巨大的发展与应用潜力。由于晶体结构的复杂性，TBAB半笼型水合物的动力学过程尚未得到充分的研究。为了解TBAB半笼型水合物在储气方面的动力学特性，实验采用原位激光拉曼技术和多晶粉末X射线衍射仪（PXRD）对nCO₂·TBAB·26H₂O和nCO₂·TBAB·38H₂O水合物的光谱特征进行了鉴别与分析，利用原位激光拉曼技术考察了CO₂分子分别进入2种晶体结构的动力学过程。研究结果表明，2种晶体结构的拉曼光谱具有较高的相似性，值得注意的是nCO₂·TBAB·26H₂O中位于1 309.5和1 326.9 cm-1的拉曼峰为TBA+阳离子中C-C键的变形振动峰，在nCO₂·TBAB·38H₂O水合物中峰基本不发生改变，但半峰宽降低，峰形也变得相对清晰；同时，nCO₂·TBAB·26H₂O中位于1 446.6和1 458 cm-1的拉曼峰为TBA+阳离子中C-H键的剪切振动峰，在nCO₂·TBAB·38H₂O水合物中分别向左、右两边发生了位移，峰形的重叠度也随之下降。依据上述2处拉曼光谱特征可以对2种晶体结构进行辨别。通过PXRD图谱可以发现2种晶体结构的衍射图谱存在着比较明显的差距。nCO₂·TBAB·26H₂O晶体属于四方晶系，空间群(P4/m)，而nCO₂·TBAB·38H₂O属于正交晶系，空间群(Pmma)。图谱中2θ=8.406°和10.941°分别为nCO₂·TBAB·38H₂O的(200)和(220)晶面的特征峰，而2θ=5.976°和6.969°分别为nCO₂·TBAB·26H₂O的(012)和(003)晶面特征峰，可以用来判别样品中水合物的晶体结构。在原位拉曼测量过程中，nCO₂·TBAB·26H₂O和nCO₂·TBAB·38H₂O分别在已经合成好的TBAB·26H₂O和TBAB·38H₂O水合物表面形成。在276 K，2 MPa条件下，气相中的CO₂分子分别进入2种晶体结构中用于储气的512笼形结构，在1 275.4和1 379.3 cm-1处形成特征峰并逐渐增长。实验以2种TBAB水合物位于1 110.3 cm-1的拉曼峰作为参考，比较了CO₂在水合物中的增长速率。研究发现在反应初期的75 min内CO₂在2种水合物中的含量基本保持线性增长且上升速率的差别不大。由于测量点位于水合物表面，受气体在水合物中扩散的阻力较小同时2种TBAB水合物均采用512笼形结构储气导致了储气速率相近。以上的微观晶体结构研究结果对TBAB水合物法捕集和封存CO₂技术应用具有重要的意义。

Microscopic Experimental Study on the Crystallization of TBAB-CO2 Hydrate

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.