TBAB-CO2水合物形成过程的微观实验

Microscopic Experimental Study on the Crystallization of TBAB-CO2 Hydrate

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1309.5和1326.9cm^-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1446.6和1458cm-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276K,2MPa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1275.4和1379.3cm-1处形成特征峰并逐渐增长。实验以2种TBAB水合物位于1110.3cm-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75min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。

TBAB semi-clathrate hydrate has a huge potential for effective application of c arbon dioxide (CO2) capture. Because of the complexity of the crystal structu re, the kinetics of TBAB hydrate remains poorly understood. In this work, the spectral characteristics of n CO2·TBAB·26H 2O and n CO2·TBAB·38H 2O were analyzed by Raman and powder X-ray diffraction (PXRD). To understand the gas storage characteristics of TBAB hydrate, the processes of CO2 molecu les entering 2 kinds of crystal structures were measured using in situ Raman spe ctroscopy. Results showed that the Raman spectra of 2 crystal structures had hi gh similarity. The Raman peaks at 1 309.5 and 1 326.9 cm -1 were assigne d to be the C-C deformation vibration mode of TBA + cations in n CO2·TBA B·26H 2O hydrate. They did not shift in n CO2·TBAB·38H 2O hydrate, but became detached and narrow in half-peak width. Meanwhile, the peaks at 1 4 46.6 and 1 458 cm^-1 were assigned to be the C-H shear vibration mode of TBA + cations in n CO2·TBAB·26H 2O hydrate. They shifted away from eac h other and had lower less overlap region in n CO2·TBAB·38H 2O 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. n CO2·TBAB·26H 2O hydrate was tetragonal which had the space group of ( P4/m ), while n CO2·TBAB·38H 2O hydrate was orthorhombic w hich 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 n CO2·TBAB·38H 2O hydrate respectively. The structure of n CO2·TBAB·26H 2O hydrate w as characterized by the (012) and (003) planes at 2θ=5.976° and 6.969° respectively. During the in situ Raman measurements, n CO2·TBAB·26H 2O and n CO2·TBAB·38H 2O hydrates grew directly from the prepared TBA B·26H 2O and TBAB·38H 2O hydrates at 276 K, 2 MPa. The CO2 molecules wer e captured by the 5 12 hydrate cages in the 2 kinds of hydrates, formed th e characteristic peaks of CO2 at 1 275.4 and 1 379.3 cm -1 and increase d 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 t he initial 75 minutes of in situ Raman measurements, the content of CO2 in hy drate phase grew linearly with generally the same growth rates in 2 kinds of cry stals. As the measuring spots were on the hydrate surface where the gas diffusi on resistance in hydrate phase could be neglected and the cage structures used f or gas storage were all 5 12 cage, the similar gas storage rates were obta ined. The microcosmic experimental study provides a theoretical basis for CO2 capture technology by forming TBAB semi-clathrate hydrate.