Institute of Analytical and Bioanalytical Chemistry, University of Ulm, Ulm, Germany.
Anal Bioanal Chem. 2012 Jan;402(1):163-73. doi: 10.1007/s00216-011-5522-1. Epub 2011 Nov 18.
Gas hydrates are crystalline structures comprising a guest molecule surrounded by a water cage, and are particularly relevant due to their natural occurrence in the deep sea and in permafrost areas. Low molecular weight molecules such as methane and carbon dioxide can be sequestered into that cage at suitable temperatures and pressures, facilitating the transition to the solid phase. While the composition and structure of gas hydrates appear to be well understood, their formation and dissociation mechanisms, along with the dynamics and kinetics associated with those processes, remain ambiguous. In order to take advantage of gas hydrates as an energy resource (e.g., methane hydrate), as a sequestration matrix in (for example) CO(2) storage, or for chemical energy conservation/storage, a more detailed molecular level understanding of their formation and dissociation processes, as well as the chemical, physical, and biological parameters that affect these processes, is required. Spectroscopic techniques appear to be most suitable for analyzing the structures of gas hydrates (sometimes in situ), thus providing access to such information across the electromagnetic spectrum. A variety of spectroscopic methods are currently used in gas hydrate research to determine the composition, structure, cage occupancy, guest molecule position, and binding/formation/dissociation mechanisms of the hydrate. To date, the most commonly applied techniques are Raman spectroscopy and solid-state nuclear magnetic resonance (NMR) spectroscopy. Diffraction methods such as neutron and X-ray diffraction are used to determine gas hydrate structures, and to study lattice expansions. Furthermore, UV-vis spectroscopic techniques and scanning electron microscopy (SEM) have assisted in structural studies of gas hydrates. Most recently, waveguide-coupled mid-infrared spectroscopy in the 3-20 μm spectral range has demonstrated its value for in situ studies on the formation and dissociation of gas hydrates. This comprehensive review summarizes the importance of spectroscopic analytical techniques to our understanding of the structure and dynamics of gas hydrate systems, and highlights selected examples that illustrate the utility of these individual methods.
天然气水合物是由客体分子被水分子笼包围而成的结晶结构，由于其在深海和永久冻土层中的自然存在，因此具有重要意义。在适当的温度和压力下，低分子量分子（如甲烷和二氧化碳）可以被隔离在该笼中，从而促进向固相的转变。尽管天然气水合物的组成和结构似乎已经得到很好的理解，但它们的形成和分解机制，以及与这些过程相关的动力学和热力学，仍然存在模糊性。为了利用天然气水合物作为能源（例如甲烷水合物）、作为（例如）二氧化碳储存的隔离基质，或者用于化学能量的节约/储存，需要更详细地了解其形成和分解过程的分子水平，以及影响这些过程的化学、物理和生物参数。光谱技术似乎最适合分析天然气水合物的结构（有时是原位分析），从而可以在整个电磁光谱范围内获得这些信息。目前，在天然气水合物研究中使用了多种光谱方法来确定水合物的组成、结构、笼占有率、客体分子位置以及结合/形成/分解机制。迄今为止，最常用的技术是拉曼光谱和固态核磁共振（NMR）光谱。衍射方法，如中子和 X 射线衍射，用于确定天然气水合物的结构，并研究晶格膨胀。此外，紫外可见光谱技术和扫描电子显微镜（SEM）也有助于天然气水合物的结构研究。最近，在 3-20μm 光谱范围内的波导耦合中红外光谱技术已证明其在天然气水合物形成和分解的原位研究中的价值。本综述总结了光谱分析技术对我们理解天然气水合物体系结构和动力学的重要性，并突出了说明这些方法的实用性的一些实例。
