物理化学学报 >> 2020, Vol. 36 >> Issue (11): 1910010.doi: 10.3866/PKU.WHXB201910010

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常见客体分子对笼型水合物晶格常数的影响

孟庆国1,2,3, 刘昌岭1,2,*(), 李承峰1,2, 郝锡荦1,2, 胡高伟1,2, 孙建业1,2, 吴能友1,2   

  1. 1 青岛海洋地质研究所,自然资源部天然气水合物重点实验室,山东 青岛 266071
    2 青岛海洋科学与技术试点国家实验室海洋矿产资源评价与探测技术功能实验室,山东 青岛 266237
    3 中国地质科学院,北京 100037
  • 收稿日期:2019-10-08 录用日期:2019-12-09 发布日期:2019-12-11
  • 通讯作者: 刘昌岭 E-mail:qdliuchangling@163.com
  • 基金资助:
    国家海洋地质调查专项项目(DD20190221);国家自然科学基金项目(41876051);国家自然科学基金项目(41976074);国家自然科学基金项目(41302034);国家重点研发计划项目(2017YFC0307600)

Effect of Common Guest Molecules on the Lattice Constants of Clathrate Hydrates

Qingguo Meng1,2,3, Changling Liu1,2,*(), Chengfeng Li1,2, Xiluo Hao1,2, Gaowei Hu1,2, Jianye Sun1,2, Nengyou Wu1,2   

  1. 1 The Key Laboratory of Gas Hydrate, Ministry of Natural Resources, Qingdao Institute of Marine Geology, Qingdao 266071, Shandong Province, P. R. China
    2 Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology, Qingdao 266237, Shandong Province, P. R. China
    3 Chinese Academy of Geological Sciences, Beijing 100037, P. R. China
  • Received:2019-10-08 Accepted:2019-12-09 Published:2019-12-11
  • Contact: Changling Liu E-mail:qdliuchangling@163.com
  • Supported by:
    the National Marine Geological Survey projects(DD20190221);National Natural Science Foundation of China(41876051);National Natural Science Foundation of China(41976074);National Natural Science Foundation of China(41302034);National Key R & D Program of China(2017YFC0307600)

摘要:

天然气水合物被认为是未来理想替代能源之一,相关基础与应用研究已成为国内外研究热点。客体分子对水合物晶胞参数的影响规律研究是水合物结构特征研究的重要内容,对理解水合物主客体分子间相互作用、形成分解微观机理及其稳定性变化具有重要意义。为考察不同客体分子对水合物微观结构特征的影响,本文合成了系列水合物样品(甲烷、乙烷、丙烷、异丁烷、二氧化碳、四氢呋喃、甲烷+ 2, 2-二甲基丁烷和甲烷+甲基环己烷),并基于低温X射线衍射技术对样品结构特征进行了分析,探讨了水合物样品晶格常数与客体分子尺寸间的关系。结果表明,(1)低温X射线衍射技术能够有效获取天然气水合物样品的晶格常数及结构类型等特征。(2)甲烷、乙烷、丙烷及异丁烷等烷烃类同系物形成的单一组分水合物样品晶格常数与客体分子的尺寸(范德华直径)呈现正相关关系。同等条件下,分子尺寸越大,相应水合物的晶格常数越大。尺寸较小的甲烷、乙烷生成Ⅰ型结构水合物,而丙烷、异丁烷等相对较大的分子则形成Ⅱ型结构水合物。(3)含氧原子客体分子(如CO2、THF)水合物样品的结构类型依然受范德华直径的控制,但其晶格常数与客体分子尺寸间的相关性不遵从烷烃类水合物的正相关关系,分析认为该现象是氧原子特殊的电负性致使客体分子与“水笼”间产生异常相互作用所致。(5) H型水合物样品中,大分子与辅助小分子对水合物不同方向晶轴长度影响分别呈现反向趋势特征。与甲烷+ 2, 2-二甲基丁烷水合物样品相比,甲烷+甲基环己烷水合物样品a轴方向长度较短,而c轴方向长度略长。与相同温度下甲烷+ 2, 2-二甲基丁烷水合物样品相比,由小分子氮气辅助形成的H型水合物样品晶胞a轴方向略长,而c轴方向长度较短。

关键词: 天然气水合物, 粉末X射线衍射, 结构特征, 晶格常数, 客体分子尺寸

Abstract:

Natural gas hydrates are considered as ideal alternative energy resources for the future, and the relevant basic and applied research has become more attractive in recent years. The influence of guest molecules on the hydrate crystal lattice parameters is of great significances to the understanding of hydrate structural characteristics, hydrate formation/decomposition mechanisms, and phase stability behaviors. In this study, we test a series of artificial hydrate samples containing different guest molecules (e.g. methane, ethane, propane, iso-butane, carbon dioxide, tetrahydrofuran, methane + 2, 2-dimethylbutane, and methane + methyl cyclohexane) by a low-temperature powder X-ray diffraction (PXRD). Results show that PXRD effectively elucidates structural characteristics of the natural gas hydrate samples, including crystal lattice parameters and structure types. The relationships between guest molecule sizes and crystal lattice parameters reveal that different guest molecules have different controlling behaviors on the hydrate types and crystal lattice constants. First, a positive correlation between the lattice constants and the van der Waals diameters of homologous hydrocarbon gases was observed in the single-guest-component hydrates. Small hydrocarbon homologous gases, such as methane and ethane, tended to form sI hydrates, whereas relatively larger molecules, such as propane and iso-butane, generated sⅡ hydrates. The hydrate crystal lattice constants increased with increasing guest molecule size. The types of hydrates composed of oxygen-containing guest molecules (such as CO2 and THF) were also controlled by the van der Waals diameters. However, no positive correlation between the lattice constants and the van der Waals diameters of guest molecules in hydrocarbon hydrates was observed for CO2 hydrate and THF hydrate, probably due to the special interactions between the guest oxygen atoms and hydrate "cages". Furthermore, the influences of the macromolecules and auxiliary small molecules on the lengths of the different crystal axes of the sH hydrates showed inverse trends. Compared to the methane + 2, 2-dimethylbutane hydrate sample, the length of the a-axis direction of the methane + methyl cyclohexane hydrate sample was slightly smaller, whereas the length of the c-axis direction was slightly longer. The crystal a-axis length of the sH hydrate sample formed with nitrogen molecules was slightly longer, whereas the c-axis was shorter than that of the methane + 2, 2-dimethylbutane hydrate sample at the same temperature.

Key words: Natural gas hydrate, PXRD, Structure characteristic, Lattice constant, Guest molecule size

MSC2000: 

  • O641