纳米孔、径限制条件下笼型水合物性质的原位高压、低温中子衍射研究

Clathrate hydrates are a group of ice-like, crystalline inclusion compounds formed by water and “guest” gas molecules of suitable size, typically under low temperature and high pressure conditions. Because of the storage capacity of CH4 or CO2, clathrate hydrates have important potential applications in energy storage and environmental sciences...Although clathrate hydrates have been known for almost two centuries, it was recognized recently that nature gas clathrate hydrate are in ocean-floor sediments which are in form of nano porous or nano layer structure. This nano structure will exert an influence on the accumulation of clathrate hydrate, however, the fundamental understanding of the nano confinement to clathrate hydrate is still limited. This is because many of the traditional experimental techniques are inapplicable to the clathrate hydrate owing to its requirement for special stable condition (i.e., elevated pressures and low temperature). On the other hand, neutron diffraction, which is light-element sensitive, penetrated to metal, and has strong signal at high diffraction angle, is a great tool for studying the clathrate hydrate...In this project, we propose to synthesis the CH4 and CO2clathrate hydrate in the nano porous silica, silica nano wire and carbon nano wire respectively. Using the in-situ high pressure low temperature neutron diffraction technique on the “typical” clathrate hydrate samples, we will refine the diffraction pattern and obtain the properties of clathrate hydrate synthesized in different nano confinement environment., including space group, atom position and vacancy, atom thermal vibration factors, and the motion of the guest gas molecular. We will then investigate the effect of nano confinement on the clahrate hydrate formation/dissociation reaction, structure, and guest gas condition...In addition, we will examine the physical mechanism of the nano confinement impact on the clahrate hydrate, by performing the NPDF (Neutron Pair Distribution Function) experiments and analysis to get the short range order information, especially between the clathrate hydrate and the silica host, as well as conducting the inelastic neutron diffraction and theoretical simulations. Our proposed research will provide us with a theoretical model of clathrate hydrate formed in nano confined space, which could guide the exploitation of nature gas clathrate hydrate on the sea floor.

笼型水合物是由水和气体分子形成具有笼型结构的化合物，由于可以包含CH4、CO2等气体，笼型水合物在能源和环境领域极具应用前景。目前人们发现除了稳定于高压低温环境外，笼型水合物还生长于纳米孔隙中，这让笼型水合物的研究面临新的挑战。另外，笼型水合物主要由轻元素组成等特性使人们很难利用传统手段对其进行定量化的研究。中子衍射具有对轻元素敏感、高穿透、高衍射角度强度不衰减等特性，对于笼型水合物的研究极具优势。本项目将在多孔二氧化硅、纳米管、亲水碳纳米管等体系中人工合成CH4、CO2笼型水合物；利用原位高压、低温中子衍射等手段，研究不同体系、不同纳米孔径尺寸对笼型水合物形成/分解反应相图、反应动力学、精细结构、客体分子运动状态等性质的影响；进而逐步揭示纳米孔径限制条件作用于笼型水合物的物理机制。提高人们对笼型水合物-负压水相的进一步认识，为天然气笼型水合物的勘探与开发提供必要的实验依据与理论支持。

笼形水合物是一种清洁能源，大量存储在海洋沉积物和多年冻土中。随着化石燃料的日益短缺，人们认识到合理开采天然气水合物储层可以缓解当前的能源危机。但水合物在低温高压下稳定存在且其周围环境复杂多变，任何不合理的开采都会带来巨大灾难，全面掌握水合物性质以及合理的开采手段显得尤为重要。目前来说，相当一部分水合物存在于裂缝、泥沙和孔洞中，其性质与体相水合物有很大不同。此外，除了最常见的sI型水合物，也有相当储量的sII型水合物。但是我们还没有全面深刻的掌握这些水合物的性质。因此，开展这些方面的研究工作尤为重要。.我们的工作分为四个部分。首先，鉴于纳米多孔或纳米级层状结构中存在大量海底水合物，我们研究了水滑石非层间空隙中甲烷水合物的形成及其热力学行为。水与层状双氢氧化物之间的相互作用导致气体水合物的相行为受抑制。甲烷分子更倾向于占据小笼子(512)。其次，我们用含有极低浓度丙烷的甲烷气体合成水合物，运用中子衍射在分子水平上确认了水合物结构为sII，并通过包括氢/氘原子在内的刚性结构分析估算了客体分子的填充率。结合理论计算，探讨了丙烷被优先吸附原因和对sII型水合物结构稳定性作用。从而获得天然sII水合物的相关性质。此外，水合物的稳定性不仅与温度和压力有关，与其周围的气体也有关系。但是目前环境气体对水合物的影响研究较少，因此我们开展非合成气体环境下水合物稳定性研究。当甲烷水合物周围气体（环境气体）变为Ar或者N2时，水合物稳定性变差（形成条件更加苛刻）。这说明水合物稳定性与环境气体有很大的关系。最后，我们还以农业废弃物-玉米芯为运输介质，研究在玉米芯中形成水合物性质，来寻求新的储气法。.通过这些研究，我们对限域中的水合物性质以及自然界中sII的性质有了更深刻的了解，更有利于水合物的探测和开采。此外，我们的研究也为气体运输提供了新的途径。