Study on the characteristic spectral bands of water molecule and hydrogen bond of methane hydrate

https://doi.org/10.1016/j.ces.2021.117117Get rights and content

Highlights

  • The evolution of water molecular characteristics during hydrate formation is studied.

  • Quantitative analysis of spectra can reveal the disorder-order character of hydrate.

  • The peak of the hydrogen bond changes abruptly before and after the phase transition.

Abstract

In this study, in situ observation on the formation of methane hydrate with a laser confocal Raman imaging microscope is conducted, to reveal the interaction between the guest molecule and the lattice water molecule. The evolutions of the broad band of the stretching vibration of water molecules and intermolecular hydrogen bonds in the Raman spectrum are observed in the course of methane hydrate formation, revealing that quantitative analysis of Raman spectrum can be considered as a criterion of the disorder–order character of the water–hydrate phase transition. Moreover, based on the recognition that the characteristic peak of the hydrogen bond will change abruptly before and after the phase transition, a promising technique is proposed for field hydrate exploration and laboratory identification of hydrate growth.

Introduction

Nature gas hydrate is a crystalline ice-like solid formed by water and guest molecules at relatively low temperature and high pressure conditions(Sloan Jr. and Koh, 2007). The structure of hydrate includes a host lattice of hydrogen-bonded water molecules that form polyhedral cages. Guest molecules are enclosed within the cages, which interact with the host lattice via Van der Waals interactions. More than 80% of the molecules in the hydrate are water molecules, thus most of the unique properties of hydrate are related to water molecules, which is dominated by the tetrahedral hydrogen bonded structure.

Raman spectroscopy is a molecular structure characterization technique based on the Raman effect, and its signals come from the vibration and rotation of molecules(Ye and Liu, 2012). Laser Raman microscopy is currently widely used for hydrate structure analysis(Chazallon et al., 2007, Subramanian and Sloan, 1999, Sum et al., 1997) and the mechanism of hydrate formation and decomposition(Kawamura et al., 2003, Komai et al., 2004, Taylor et al., 2007). In the Raman spectrum of hydrate, the number, intensity, and shift of spectral bands generated by each molecule will be inevitably affected by the interaction between the guest molecule and the water molecule lattice, which is the basis for qualitative or quantitative analysis of gas hydrate. However, previous studies mainly analyzed the Raman characteristic peaks of guest molecules enclosed in cages, to determine key parameters such as cage occupancy and hydration number. However, few investigations have been conducted about the host lattice of the water molecules, which leads to poor understanding of how the host-guest interaction affects water molecules.

The Raman spectrum is sensitive to the molecular environments, making it appropriate for the analysis of water and its liquid–solid transition(Auer and Skinner, 2008, Li et al., 2004). Particularly, the broad Raman spectrum band of OH-stretching, corresponding to 2800–3800 cm−1 wavenumber region, is the most informative region about water structure(Walrafen, 1964). This broad region originates from symmetric and asymmetric OH– stretching vibrations, which is closed related to water molecule structure and can intuitively reflect the modifications induced by the change of phase or temperature.(Bunkin et al., 2004, Kargovsky, 2006, Xue et al., 2013)

At present, the study on the characteristic peaks of water molecular vibration in the Raman spectra of gas hydrate is still insufficient. Schicks et al.(Schicks et al., 2005) experimentally compared the difference in the stretching vibration peaks of water molecules in the Raman spectra of gas hydrate, ice and liquid water. Meng et al. (Meng et al., 2010) analyzed the evolution of the Raman characteristic peaks of water molecules during the hydrate decomposition process. In this work, in situ observation is conducted with a state-of-the-art Raman imaging microscope, and the vibration of both hydrogen-oxygen bonds and intermolecular hydrogen bonds were analyzed, providing a more in-depth insight into the phase transition behavior of gas hydrate.

Section snippets

Material and methods

In the experiment, a stainless-steel high-pressure reactor equipped with a sapphire lens was used, as shown in Fig. 1. The reactor was also equipped with a thermocouple and a pressure transducer, which can monitor and adjust the internal temperature and pressure flexibly during the experiment. For better understanding the phase transition behavior of gas hydrate, the formation of methane hydrate was observed in situ by WITec’s alpha300 confocal Raman imaging microscope, with an ultimate lateral

Results and discussion

The water molecule is nonlinear, with three normal vibrations, namely symmetric stretching vibration, antisymmetric stretching vibration and bending vibration. For liquid water, due to the coupling between water molecules, the stretching vibration of O-H bonds is a steamed-bread-like band between 2800 and 3800 cm−1 in the Raman spectrum(Li et al., 2004, Ye and Liu, 2012). This broad band is actually a convolution of four superposed peaks, which are, respectively, the symmetric stretching

Conclusion

In situ observation on the formation of methane hydrate is conducted with a confocal Raman imaging microscope, which helps to reveal the evolution of water molecular characteristics during the hydrate formation process. The steady evolution of the broad band of the stretching vibration of water molecules in the Raman spectrum along with the formation of methane hydrate is observed. It is found that the Raman characteristic peak intensity of the symmetric stretching vibration of water molecules

CRediT authorship contribution statement

Xin Huang: Conceptualization, Methodology, Investigation, Resources, Formal analysis, Visualization, Writing – original draft. Le Zhang: Validation, Writing – review & editing. Wenjiu Cai: Validation, Formal analysis, Visualization. Jiayuan He: Writing – review & editing, Funding acquisition. Hailong Lu: Writing – review & editing, Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Sinopec Ministry of Science and Technology for financial support (Grant No. P20040-4).

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