南海神狐海域及祁连山冻土区天然气水合物的拉曼光谱特征

采用显微激光拉曼光谱对我国在南海神狐海域及祁连山冻土区首次钻获的天然气水合物实物样品进行了详细的研究, 探讨了其笼型结构特征及其气体组成. 结果表明, 南海神狐海域天然气水合物样品是典型的I型结构(sI)水合物, 气体组分主要是甲烷, 占99%以上; 水合物大笼的甲烷占有率大于99%, 小笼为86%, 水合指数为5.99. 祁连山冻土区天然气水合物气体组分相对复杂, 主要成分除甲烷外(70%左右), 还有相当数量的乙烷、丙烷及丁烷等烃类气体, 从拉曼谱图上可初步判断其为II型结构(sII)水合物; 水合物的小、大笼的甲烷占有率的比值(θ_S/θ_L)为26.38, 远远大于南海神弧海域水合物的0.87, 这主要是由于祁连山水合物气体组分中的大分子(乙烷、丙烷及丁烷等)优先占据水合物的大笼, 大大减少了大笼中甲烷分子的数量.     

客体分子所处晶穴结构不同对甲烷水合物稳定性的影响

采用分子动力学方法模拟了SⅠ型甲烷水合物受热分解微观过程,并对水合物分解过程中不同晶穴结构内客体分子对甲烷水合物稳定性的作用进行了研究.通过最终构象、均方位移和势能等性质的变化规律对分别缺失大晶穴和小晶穴中客体分子的2种水合物体系随模拟温度升高稳定性的变化进行了分析.模拟结果显示,随温度的上升,水合物稳定性逐渐下降直至彻底分解;而水合物分解速度与2种晶穴各自部分晶穴占有率相关,不能简单的通过整体晶穴占有率表示.对比相同注热过程中2种水合物体系分解状况,发现位于大晶穴内的客体分子对水合物稳定性影响更大,缺失大晶穴内客体分子的水合物更容易随温度升高而分解.     

多孔甲烷水合物样品导热系数的测定和模拟

天然气水合物是由水分子和气体分子在一定温压条件下形成的一种类冰状笼形化合物．天然气水合物主要存在于海底和大陆的永久冻土区和青藏高原等多年冻土区,是一种潜在的替代能源．在考虑对天然气水合物资源进行开采和考察地球温压变化对含水合物层的影响时,有必要掌握水合物的热物性和含水合物层的有效导热系数,但目前报道的水合物导热系数数据不一、差异很大．水合物是一种非化学计量的化合物,因此很难获得一个不含自由气、自由水的零孔隙率完美样品．利用多孔介质的理论模型对多孔水合物的导热系数进行预测是获得水合物本征导热系数的有效途径之一．我们在一个自行设计的实验台上使用HotDisk系统独特的单面测试技术,并利用瞬态平面热源法测定了含甲烷气的多孔甲烷水合物的有效导热系数,获得该样品的导热系数和温度以及所加压力的关系．为了研究含甲烷气的多孔甲烷水合物的有效导热系数与孔隙率的关系,我们利用自相似的Sierpinski地毯分形模型,先假设多孔介质体系由多孔介质和流体两部分组成,而多孔介质颗粒则由随机分布不相接触的颗粒和带有接触热阻的自相似分布颗粒组成,再通过一维热流假设和采用等价电阻网络（即通过电一热阻模拟分析得到系统的热导率）分别模拟了干砂（含空气）和多孔甲烷水合物样品（含自由甲烷气）导热系数与样品孔隙度的关系,推测了无孔隙水合物样品的导热系数．实验和模拟结果均显示样品的有效导热系数随着孔隙度的增大而降低,样品的有效导热系数在30％的孔隙度时降低了25％．通过分析实验结果和模拟结果发现,无孔隙甲烷水合物样品的导热系数约为0．7Wm^-1K^-1．     

不同浓度聚乙烯醇对甲烷水合物分解作用的分子动力学模拟

采用分子动力学方法,模拟了273.15K下聚乙烯醇(PVA)对甲烷水合物的分解作用.研究发现,PVA浓度为2.7wt%时,水分子始终在其平衡位置附近波动,扩散系数仅为1.04×10-11m2/s;PVA浓度为5.2wt%时,水合物笼型结构坍塌,水分子以液态水的形式存在,甲烷分子从孔穴中逸出,聚乙烯醇的羟基在分子内部形成氢键,形成团簇,产生空间位阻,阻止了水分子再生成水合物,水分子的扩散系数1.61×10-9m2/s;PVA浓度为7.6wt%时,甲烷水合物周围有部分笼型结构被破坏,部分甲烷分子从孔穴中逸出,水分子扩散系数为3.55×10-10m2/s.得出聚乙烯醇对甲烷水合物的分解作用大小为:5.2wt%7.6wt%2.7wt%,为PVA溶液促进甲烷水合物分解实验研究提供参考.     

石英砂中甲烷水合物稳定条件研究

采用不同粒径(160～200目、200～300目、300～400目、400～600目)细颗粒石英砂模拟自然界中沉积物实验合成甲烷水合物, 采用多步升温分解法测定了甲烷水合物的稳定温度-压力(p～T)条件. 结果发现细颗粒石英砂对甲烷水合物的稳定条件有明显影响, 温度最大下降1.5 K. 其中160～200目的石英砂是实验测试条件下石英砂对甲烷水合物稳定条件有无影响的分界目数. 每一种石英砂在不同的温度压力下甲烷水合物p-T数据点较为离散, 并且不同石英砂中p-T数据曲线出现交叉, 这与文献中采用规则的硅胶、玻璃球等测得的结果有所不同. 文献中测定的甲烷水合物p-T数据点温度相对于纯水中甲烷水合物p-T数据点温度向左偏移量相同.     

聚乙烯唑啉作用下甲烷水合物分解的分子动力学模拟

利用分子动力学模拟系统研究了不同质量浓度下(1.25%、2.50%、6.06%)聚乙烯唑啉(PEtO)对甲烷水合物的分解作用. 模拟体系为甲烷水合物2′2′2的超胞和聚合物对接体系. 模拟发现水分子间氢键构架的水合物笼型结构在PEtO的作用下出现扭曲, 最终导致水合物笼型结构完全坍塌. 通过氧原子径向分布函数、均方位移以及扩散系数比较不同浓度PEtO的作用, 证实在一定浓度范围内, PEtO的浓度越高, 其水合物分解作用越强. 此外, PEtO 具有一定的可生物降解性. PEtO 对水合物的作用为: PEtO 吸附在水合物表面, 其中的酰胺基(N―C＝O)与成笼的水分子形成氢键, 破坏邻近的笼形结构, 令水合物分解; PEtO不断分解表面的水合物, 直到水合物笼完全分解.     

二元包合水合物储氢及促进剂作用的理论研究

尽管包合水合物因其高储能密度和环境友好等优点,可以作为大规模工业化应用的潜在储氢材料,但在理解包合水合物中氢气的结构和扩散方面尚存在较大争议.本文应用密度泛函理论(DFT)探讨了sI型包合水合物的储氢结构和性质.结果表明,纯氢气水合物中5¹²笼中可容纳2个H₂分子, CH₄和C₂H₆促进剂对其结构和性质没有明显影响;而在N₂和CO₂促进剂作用下5¹²笼可容纳的H₂分子增加到3个,氢气水合物稳定性得到提高,主客体间的静电和氢键相互作用增强且H—H伸缩振动峰值蓝移.当N₂和CO₂包合物5¹²笼中氢气的占据达3个H₂分子时, 1个H₂分子通过五边形面的扩散能垒接近于纯氢气水合物中的2倍,有效阻碍了氢气在笼间的扩散.     

金属-有机骨架材料中吸附气体的扩散速率

采用分子动力学方法,以甲烷为探针分子研究了不同压力条件下气体在具有不同孔道结构的金属-有机骨架材料(MOFs)中的扩散速率.通过计算气体在八种材料中的自扩散系数,并结合气体分子在材料中的质心分布图等,讨论了气体扩散速率与孔道结构之间的关系.研究结果表明:对于同时含有孔笼(pocket)和三维正交孔道(channel)结构的MOF材料(P-C材料),低压时甲烷气体吸附在孔笼结构中,随着压力的升高,气体分子开始进入正交孔道,同时其自扩散系数增加;而对于只含有三维立方孔道结构的IRMOF(isoreticular MOF)系列材料,在中低压范围内,气体分子在其中的自扩散系数随压力变化较小.当压力进一步升高时,气体分子在材料孔道中的吸附逐渐接近饱和,其自扩散系数均降低.因此,在不同MOF材料中气体分子扩散速率的差异主要取决于孔道结构的不同.对P-C材料,中低压下通过控制压力可以控制气体在其中的扩散速率,从而为MOF材料在气体存储、分离等方面的实际应用提供参考信息.     

甲烷水合物膜生长动力学研究

采用水中悬浮气泡法测定了温度为273.4～279.4 K、压力为3.60～11.90 MPa范围内甲烷微小气泡表面水合物膜生长动力学数据. 应用无因次Gibbs自由能差(－ΔG^exp/RT)作为推动力, 提出了具物理意义的水合物膜生长动力学模型, 并回归得到甲烷水合物膜生长动力学反应级数为1.60, 表观活化能为55.95 kJ•mol^－1, 指前因子为1.65×10¹¹ mm²•s^－1. 同时考察了温度和压力对甲烷水合物膜生长速率的影响.     

水合物CH4-CO2置换的双重机理:空位辅助与竞争

过去的CO2置换甲烷水合物的微观机理研究,主要集中在客体分子(CH4、CO2)之间的交换、占据状态,孤立地研究分解过程或生成过程,忽视主体-客体之间的作用、主体分子(H2O)的空位辅助和客体分子的多重竞争通道。本文基于水合物分解的过冷水及其水空位辅助,以及水合物生成的串滴链及其客体分子竞争的研究,进一步评论水合物CH4-CO2置换的双重机理。然后,对微观机理的动态性和未来研究的相关问题进行讨论。通过综述和评论,文章得出以下初步结果:过冷水通过水空位推动客体分子的跳跃、扩散,实现置换过程的自组装;CO2分子在分解前沿形成一个有序结构的CO2串滴链,其动态性伴随水的组织到获取包合物笼的结构,以及非晶形包合物转变成晶形包合物的生长过程;CO2和CH4在中晶穴中必然产生竞争,并且存在多种竞争类型;成核过程中,不稳定簇导致竞争结构,且有多重竞争通道。最后,结果表明水合物的CH4-CO2置换机理具有双重性,即主体分子的空位辅助和客体分子的竞争,是分解过程和生成过程的自然统一。     

Molecular dynamics simulations of the mechanisms of thermal conduction in methane hydrates

The thermal conductivity of methane hydrate is an important physical parameter affecting the processes of methane hydrate exploration,mining,gas hydrate storage and transportation as well as other applications.Equilibrium molecular dynamics simulations and the Green-Kubo method have been employed for systems from fully occupied to vacant occupied sI methane hydrate in order to estimate their thermal conductivity.The estimations were carried out at temperatures from 203.15 to 263.15 K and at pressures from 3 to 100 MPa.Potential models selected for water were TIP4P,TIP4P-Ew,TIP4P/2005,TIP4P-FQ and TIP4P/Ice.The effects of varying the ratio of the host and guest molecules and the external thermobaric conditions on the thermal conductivity of methane hydrate were studied.The results indicated that the thermal conductivity of methane hydrate is essentially determined by the cage framework which constitutes the hydrate lattice and the cage framework has only slightly higher thermal conductivity in the presence of the guest molecules.Inclusion of more guest molecules in the cage improves the thermal conductivity of methane hydrate.It is also revealed that the thermal conductivity of the sI hydrate shows a similar variation with temperature.Pressure also has an effect on the thermal conductivity,particularly at higher pressures.As the pressure increases,slightly higher thermal conductivities result.Changes in density have little impact on the thermal conductivity of methane hydrate.     

A potential model for methane in water describing correctly the solubility of the gas and the properties of the methane hydrate

We have obtained the excess chemical potential of methane in water, over a broad range of temperatures, from computer simulation. The methane molecules are described as simple Lennard-Jones interaction sites, while water is modeled by the recently proposed TIP4P/2005 model. We have observed that the experimental values of the chemical potential are not reproduced when using the Lorentz-Berthelot combining rules. However, we also noticed that the deviation is systematic, suggesting that this may be corrected. In fact, by introducing positive deviations from the energetic Lorentz-Berthelot rule to account indirectly for the polarization methane-water energy, we are able to describe accurately the excess chemical potential of methane in water. Thus, by using a model capable of describing accurately the density of pure water in a wide range of temperatures and by deviating from the Lorentz-Berthelot combining rules, it is possible to reproduce the properties of methane in water at infinite dilution. In addition, we have applied this methane-water potential to the study of the solid methane hydrate structure, commonly denoted as sI, and find that the model describes the experimental value of the unit cell of the hydrate with an error of about 0.2%. Moreover, we have considered the effect of the amount of methane contained in the hydrate. In doing so, we determine that the presence of methane increases slightly the value of the unit cell and decreases slightly the compressibility of the structure. We also note that the presence of methane increases greatly the range of pressures where the sI hydrate is mechanically stable.     

Molecular dynamics study of structure H clathrate hydrates of methane and large guest molecules

Methane storage in structure H (sH) clathrate hydrates is attractive due to the relatively higher stability of sH as compared to structure I methane hydrate. The additional stability is gained without losing a significant amount of gas storage density as happens in the case of structure II (sII) methane clathrate. Our previous work has showed that the selection of a specific large molecule guest substance (LMGS) as the sH hydrate former is critical in obtaining the optimum conditions for crystallization kinetics, hydrate stability, and methane content. In this work, molecular dynamics simulations are employed to provide further insight regarding the dependence of methane occupancy on the type of the LMGS and pressure. Moreover, the preference of methane molecules to occupy the small (5(12)) or medium (4(3)5(6)6(3)) cages and the minimum cage occupancy required to maintain sH clathrate mechanical stability are examined. We found that thermodynamically, methane occupancy depends on pressure but not on the nature of the LMGS. The experimentally observed differences in methane occupancy for different LMGS may be attributed to the differences in crystallization kinetics and/or the nonequilibrium conditions during the formation. It is also predicted that full methane occupancies in both small and medium clathrate cages are preferred at higher pressures but these cages are not fully occupied at lower pressures. It was found that both small and medium cages are equally favored for occupancy by methane guests and at the same methane content, the system suffers a free energy penalty if only one type of cage is occupied. The simulations confirm the instability of the hydrate when the small and medium cages are empty. Hydrate decomposition was observed when less than 40% of the small and medium cages are occupied.     

Mechanisms for thermal conduction in hydrogen hydrate

Extensive equilibrium molecular dynamics simulations have been performed to investigate thermal conduction mechanisms via the Green-Kubo approach for (type II) hydrogen hydrate, at 0.05 kbar and between 30 and 250 K, for both lightly filled H(2) hydrates (1s4l) and for more densely filled H(2) systems (2s4l), in which four H(2) molecules are present in the large cavities, with respective single- and double-occupation of the small cages. The TIP4P water model was used in conjunction with a fully atomistic hydrogen potential along with long-range Ewald electrostatics. It was found that substantially less damping in guest-host energy transfer is present in hydrogen hydrate as is observed in common type I clathrates (e.g., methane hydrate), but more akin in to previous results for type II and H methane hydrate polymorphs. This gives rise to larger thermal conductivities relative to common type I hydrates, and also larger than type II and H methane hydrate polymorphs, and a more crystal-like temperature dependence of the thermal conductivity.     

Thermal conductivity of methane hydrate from experiment and molecular simulation

A single-sided transient plane source technique has been used to determine the thermal conductivity and thermal diffusivity of a compacted methane hydrate sample over the temperature range of 261.5-277.4 K and at gas-phase pressures ranging from 3.8 to 14.2 MPa. The average thermal conductivity, 0.68 +/- 0.01 W/(m K), and thermal diffusivity, 2.04 x 10(-7) +/- 0.04 x 10(-7) m2/s, values are, respectively, higher and lower than previously reported values. Equilibrium molecular dynamics (MD) simulations of methane hydrate have also been performed in the NPT ensemble to estimate the thermal conductivity for methane compositions ranging from 80 to 100% of the maximum theoretical occupation, at 276 K and at pressures ranging from 0.1 to 100 MPa. Calculations were performed with three rigid potential models for water, namely, SPC/E, TIP4P-Ew, and TIP4P-FQ, the last of which includes the effects of polarizability. The thermal conductivities predicted from MD simulations were in reasonable agreement with experimental results, ranging from about 0.52 to 0.77 W/(m K) for the different potential models with the polarizable water model giving the best agreement with experiments. The MD simulation method was validated by comparing calculated and experimental thermal conductivity values for ice and liquid water. The simulations were in reasonable agreement with experimental data. The simulations predict a slight increase in the thermal conductivity with decreasing methane occupation of the hydrate cages. The thermal conductivity was found to be essentially independent of pressure in both simulations and experiments. Our experimental and simulation thermal conductivity results provide data to help predict gas hydrate stability in sediments for the purposes of production or estimating methane release into the environment due to gradual warming.     

Optical microscopy and in situ Raman scattering of single crystalline ethylene hydrate and binary methane-ethylene hydrate at high pressures

Visual observations through a microscope and in situ Raman measurements have been made for single crystalline ethylene hydrate (EH) and binary methane-ethylene hydrate (MEH) at pressures up to 3.7 GPa and room temperature. Both hydrates showed pressure-induced phase transitions at 1.6, 2.0, and 3.0 GPa for EH and at 1.7, 2.1, and 3.3 GPa for MEH. The cubic sI phase of EH and MEH remains stable up to 1.6 and 1.7 GPa, respectively, which are more widely ranging values than the values for the methane hydrate sI phase. In this sI phase of binary MEH, the cage occupancies by methane and ethylene molecules are investigated from Raman spectra. Above P = 3.0 GPa for EH and 3.3 GPa for MEH, they decomposed by associating with the formation of the polyethylene.     

Micro-Raman investigations of mixed gas hydrates

We report laser Raman spectroscopic measurements on mixed hydrates (clathrates), with guest molecules tetrahydrofuran (THF) and methane (CH₄), at ambient pressure and at temperatures from 175 to 280 K. Gas hydrates were synthesized with different concentrations of THF ranging from 5.88 to 1.46 mol%. In all cases THF molecules occupied the large cages of sII hydrate. The present studies demonstrate formation of sII clathrates with CH₄ molecules occupying unfilled cages for concentrations of THF ranging from 5.88 to 2.95 mol%. The Raman spectral signature of hydrates with 1.46 mol% THF are distinctly different; hydrate growth was non-uniform and structural transformation occurred from sII to sI prior to clathrate melting.     

All-Atom Molecular Dynamics of Pure Water–Methane Gas Hydrate Systems under Pre-Nucleation Conditions: A Direct Comparison between Experiments and Simulations of Transport Properties for the Tip4p/Ice Water Model

(1) Background: New technologies involving gas hydrates under pre-nucleation conditions such as gas separations and storage have become more prominent. This has necessitated the characterization and modeling of the transport properties of such systems. (2) Methodology: This work explored methane hydrate systems under pre-nucleation conditions. All-atom molecular dynamics simulations were used to quantify the performance of the TIP4P/2005 and TIP4P/Ice water models to predict the viscosity, diffusivity, and thermal conductivity using various formulations. (3) Results: Molecular simulation equilibrium was robustly demonstrated using various measures. The Green–Kubo estimation of viscosity outperformed other formulations when combined with TIP4P/Ice, and the same combination outperformed all TIP4P/2005 formulations. The Green–Kubo TIP4P/Ice estimation of viscosity overestimates (by 84% on average) the viscosity of methane hydrate systems under pre-nucleation conditions across all pressures considered (0–5 MPag). The presence of methane was found to increase the average number of hydrogen bonds over time (6.7–7.8%). TIP4P/Ice methane systems were also found to have 16–19% longer hydrogen bond lifetimes over pure water systems. (4) Conclusion: An inherent limitation in the current water force field for its application in the context of transport properties estimations for methane gas hydrate systems. A re-parametrization of the current force field is suggested as a starting point. Until then, this work may serve as a characterization of the deviance in viscosity prediction.     

Melting and superheating of sI methane hydrate: molecular dynamics study

Melting and decay of the superheated sI methane structure are studied using molecular dynamics simulation. The melting curve is calculated by the direct coexistence simulations in a wide range of pressures up to 5000 bar for the SPC/E, TIP4P/2005 and TIP4P/Ice water models and the united-atom model for methane. We locate the kinetic stability boundary of the superheated metastable sI structure that is found to be surprisingly high comparing with the predictions based on the classical nucleation theory.     

A molecular dynamics study of ethanol-water hydrogen bonding in binary structure I clathrate hydrate with CO2

Guest-host hydrogen bonding in clathrate hydrates occurs when in addition to the hydrophilic moiety which causes the molecule to form hydrates under high pressure-low temperature conditions, the guests contain a hydrophilic, hydrogen bonding functional group. In the presence of carbon dioxide, ethanol clathrate hydrate has been synthesized with 10% of large structure I (sI) cages occupied by ethanol. In this work, we use molecular dynamics simulations to study hydrogen bonding structure and dynamics in this binary sI clathrate hydrate in the temperature range of 100-250 K. We observe that ethanol forms long-lived (>500 ps) proton-donating and accepting hydrogen bonds with cage water molecules from both hexagonal and pentagonal faces of the large cages while maintaining the general cage integrity of the sI clathrate hydrate. The presence of the nondipolar CO(2) molecules stabilizes the hydrate phase, despite the strong and prevalent alcohol-water hydrogen bonding. The distortions of the large cages from the ideal form, the radial distribution functions of the guest-host interactions, and the ethanol guest dynamics are characterized in this study. In previous work through dielectric and NMR relaxation time studies, single crystal x-ray diffraction, and molecular dynamics simulations we have observed guest-water hydrogen bonding in structure II and structure H clathrate hydrates. The present work extends the observation of hydrogen bonding to structure I hydrates.