Acta Physico-Chimica Sinica ›› 2020, Vol. 36 ›› Issue (11): 1910010.doi: 10.3866/PKU.WHXB201910010

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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)

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