用分子动力学模拟天然气水合物的抑制效应

用分子动力学模拟方法确定了结构H型(SH)天然气水合物的稳定晶体生长面为(001), 系统研究了277 K时三种动力学抑制剂对此晶面的影响. 模拟显示抑制剂中的氧与表面水分子形成氢键, 从而破坏原有的稳定结构, 造成水合物笼型结构坍塌, 达到抑制水合物形成的效果. 比较三种不同动力学抑制剂对SH的抑制效果得出: PVCap＞PEO＞PVP. 在此基础上研究了PVCap对天然气水合物结构I型(SI), 结构II型(SII)和SH三种不同晶型的抑制效应. 模拟发现抑制效果的次序为: SH＞SI＞SII.     

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.     

碳纳米管和碳纳米管-四氢呋喃水合物的储氢特性

研究了单壁碳纳米管(SWNTs)干法储氢和碳纳米管(SWNTs)-四氢呋喃(THF)水合物法储氢的过程. 结果表明, 实验所用的SWNTs在16.5 MPa压力下, 温度为0.5 ℃时, 氢气的吸附存储量为0.75%(质量分数), 经浓酸处理后, 氢气的存储量可以达到1.15%, SWNTs-THF水合物法储氢量为0.37%, 与碳纳米管干法储氢相比, 储氢量有所降低.     

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.     

B12Sc4和B12Ti4团簇的储氢性质

提出了两个稳定的团簇B12Sc4和B12Ti4,基于理论计算,研究了它们的结构与储氢性质.结果发现,在这两个稳定的团簇中,过渡金属原子不会聚合在一起而影响它们对氢气的吸附. B12Sc4最多可以吸附12个氢分子,达到7.25%(质量分数)的储氢量,它的平均每氢分子吸附能量为-10.5 kJ·mol-1. B12Ti4最多只能吸附8个氢分子,储氢量为4.78%,但其平均每氢分子吸附能量可达-50.2 kJ·mol-1.进一步计算表明,即使在77 K,也需要很高的氢气压力才能使12个氢分子都吸附到B12Sc4上.电子结构分析表明, B12Ti4-nH2吸附结构中的Kubas作用要大于相应B12Sc4-nH2结构中的Kubas作用     

氢气在Na-MAZ和Li-MAZ沸石原子簇上的吸附

采用基于密度泛函理论(DFT)的广义梯度近似(GGA)/PBE(Perdew-Burke-Ernzerhof)交换相关泛函和双数值基加p极化(DNP)基组对氢气分子在Na-MAZ和Li-MAZ沸石原子簇上的吸附进行了研究, 计算得到吸附复合物的平衡几何结构参数、振动频率以及吸附能等数据. 结果表明: MAZ沸石中存在四个稳定的吸附位点, 分别为SI′、SI″、SII′和SII″位点; 氢气分子在Na-MAZ沸石的SII″位点吸附时最稳定, 而在Li-MAZ沸石中, 氢气分子处于SI″和SII″位点时最稳定. 吸附能越大, 氢气分子键长越长, 振动频率减少也越多. Li-MAZ沸石对氢气的吸附能力要明显强于Na-MAZ沸石的吸附能力, 理论上Li-MAZ沸石具有更高的氢气储量, 可能是一种潜在的储氢材料.     

甲硫醇在Cu(111)表面的解离吸附: 密度泛函理论研究

采用基于密度泛函理论的第一性原理方法和平板模型研究了CH₃SH分子在Cu(111)表面的吸附反应.系统地计算了S原子在不同位置以不同方式吸附的一系列构型, 第一次得到未解离的CH₃SH分子在Cu(111)表面顶位上的稳定吸附构型,该构型吸附属于弱的化学吸附, 吸附能为0.39 eV. 计算同时发现在热力学上解离结构比未解离结构更加稳定. 解离的CH₃S吸附在桥位和中空位之间, 吸附能为0.75-0.77 eV. 计算分析了未解离吸附到解离吸附的两条反应路径, 最小能量路径的能垒为0.57 eV. 计算结果还表明S―H键断裂后的H原子并不是以H₂分子的形式从表面解吸附而是以与表面成键的形式存在. 通过比较S原子在独立的CH₃SH分子和吸附状态下的局域态密度, 发现S―H键断裂后S原子和表面的键合强于未断裂时S原子和表面的键合.     

二氧化钛纳米管储氢能力的分子动力学研究

二氧化钛纳米管是一种有前景的储氢材料,因此,在本文中通过卷曲锐钛矿单分子层,获得锯齿型(Zig-zag)和手性型(Chiral)二氧化钛纳米管结构。并采用分子动力学方法(Molecular dynamics)研究了氢分子分别在锯齿型和手性型二氧化钛纳米管和碳纳米管中的分布情况,并计算其储氢能力。结果表明,与碳纳米管一样,锯齿型和手性型二氧化钛纳米管存在管间储氢和管内储氢情况,并且氢分子在管间和管内的分布与二氧化钛纳米管内、外两侧的氧原子相关。Lennard-Jones势能模型表明:氢分子向纳米管内部和管间隙处的低能处聚集,形成氢分子环结构。储氢量计算结果表明,虽然锯齿型和手性型二氧化钛纳米管储存的氢分子数目较多,但由于系统重量较大,储氢量较低,低于美国能源部6%的商业标准,不能满足实际需要,而碳纳米管储氢量接近这一标准。     

外电场对锂修饰氧化石墨烯结构储氢性能的影响

基于密度泛函理论(DFT)的第一性原理方法,研究了外加电场对锂修饰氧化石墨烯结构(Li@GO)储氢性能的影响.考察Li@GO结构的稳定性及其对外加电场的响应,研究H_2-Li@GO结构的H_2分子吸附能、几何构型与外加电场的关系.研究结果表明,当外加电场方向垂直Li@GO平面向下(负电场)时,随电场强度增加,H_2分子的吸附能逐渐降低,H_2分子逐渐接近Li原子;当外加电场方向向上(正电场)时,随电场强度增加,H_2分子的吸附能逐渐升高,H_2分子逐渐远离Li原子,分波态密度(PDOS)分析表明:与无外加电场体系的PDOS相比,当对体系施加负电场时,H_2-Li的杂化峰向低能量方向位移,H_2分子与Li@GO结合更加紧密,提升了储氢稳定性;施加正电场时,H_2-Li的杂化峰向高能量方向位移,H_2分子与Li@GO作用减弱提升了氢气释放动力学性能.进一步计算表明,在无外加电场情况下,Li@GO结构最大储氢量在3.1%以上.     

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.     

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.     

Multiple H2 occupancy of cages of clathrate hydrate under mild conditions

Experiments were carried out by reacting H(2) gas with N(2) hydrate at a temperature of 243 K and a pressure of 15 MPa. The characterizations of the reaction products indicated that multiple H(2) molecules can be loaded into both large and small cages of structure II clathrate hydrates. The realization of multiple H(2) occupancy of hydrate cages under moderate conditions not only brings new insights into hydrogen clathrates but also refreshes the perspective of clathrate hydrates as hydrogen storage media.     

Structure, dynamics and ordering in structure i ether clathrate hydrates from single-crystal X-ray diffraction and 2H NMR spectroscopy

The structure and dynamics of trimethylene oxide (TMO) and ethylene oxide (EO) structure I (sI) hydrates are reported from single-crystal X-ray diffraction and 2H NMR spectroscopic measurements. The guest molecule positions in the large cage were determined with considerable improvement over previous diffraction work so that a dynamic model that was consistent with these orientations could be developed to explain the 2H NMR data. Reorientations are shown to take place among both symmetry-related and symmetry-independent sites, 16 positions in all. Because of the prochiral nature of the molecules, both guests show 2H NMR line shapes with large asymmetry parameters, rather unusual for guest molecules in the sI hydrate large cage. The results also show that the dipolar axis of the TMO molecule lies close to the 4 bar axis of the cage on average, whereas for EO, this is not the case. For TMO, progressive alignment of the polar axis with decrease of temperature then allows the dipoles to interact more strongly until dipole reversal is quenched at the ordering transition. The lack of ordering of EO is consistent with the much weaker alignment of the molecular dipoles along the 4 bar axis. With the new complementary information on the structure and dynamics from crystallography and NMR, it is possible to understand why the large cage guests order in the large cage of sI hydrate for TMO hydrate but not for EO hydrate.     

Effect of Guest–Host Hydrogen Bonding on the Structures and Properties of Clathrate Hydrates

To provide improved understanding of guest–host interactions in clathrate hydrates, we present some correlations between guest chemical structures and observations on the corresponding hydrate properties. From these correlations it is clear that directional interactions such as hydrogen bonding between guest and host are likely, although these have been ignored to greater or lesser degrees because there has been no direct structural evidence for such interactions. For the first time, single‐crystal X‐ray crystallography has been used to detect guest–host hydrogen bonding in structure II (sII) and structure H (sH) clathrate hydrates. The clathrates studied are the tert‐butylamine (tBA) sII clathrate with H₂S/Xe help gases and the pinacolone + H₂S binary sH clathrate. X‐ray structural analysis shows that the tBA nitrogen atom lies at a distance of 2.64 Å from the closest clathrate hydrate water oxygen atom, whereas the pinacolone oxygen atom is determined to lie at a distance of 2.96 Å from the closest water oxygen atom. These distances are compatible with guest–water hydrogen bonding. Results of molecular dynamics simulations on these systems are consistent with the X‐ray crystallographic observations. The tBA guest shows long‐lived guest–host hydrogen bonding with the nitrogen atom tethered to a water HO group that rotates towards the cage center to face the guest nitrogen atom. Pinacolone forms thermally activated guest–host hydrogen bonds with the lattice water molecules; these have been studied for temperatures in the range of 100–250 K. Guest–host hydrogen bonding leads to the formation of Bjerrum L‐defects in the clathrate water lattice between two adjacent water molecules, and these are implicated in the stabilities of the hydrate lattices, the water dynamics, and the dielectric properties. The reported stable hydrogen‐bonded guest–host structures also tend to blur the longstanding distinction between true clathrates and semiclathrates.     

Raman spectra of gas hydrates--differences and analogies to ice 1h and (gas saturated) water

It is generally accepted that Raman spectroscopic investigations of gas hydrates provide vital information regarding the structure of the hydrate, hydrate composition and cage occupancies, but most research is focused on the vibrational spectra of the guest molecules. We show that the shape and position of the Raman signals of the host molecules (H(2)O) also contain useful additional information. In this study, Raman spectra (200-4000 cm(-1)) of (mixed) gas hydrates with variable compositions and different structures are presented. The bands in the OH stretching region (3000-3800 cm(-1)), the O-H bending region (1600-1700 cm(-1)) and the O-O hydrogen bonded stretching region (100-400 cm(-1)) are compared with the corresponding bands in Raman spectra of ice Ih and liquid water. The interpretation of the differences and similarities with respect to the crystal structure and possible interactions between guest and host molecules are presented.     

Hydrogen storage in sH hydrates: a Monte Carlo study

Grand canonical Monte Carlo simulations are performed to evaluate the hydrogen-storage capacity of the recently discovered hydrogen hydrates of the sH type, at 274 K and up to 500 MPa. First, the pure H2 hydrate is investigated in order to determine the upper limit of H 2 content in sH hydrates. It is found that the storage capacity of the hypothetical pure H2 hydrate could reach 3.6 wt % at 500 MPa. Depending on pressure, the large cavity of this hydrate can accommodate up to eight H2 molecules, while the small and medium ones are singly occupied even at pressures as high as 500 MPa. Next, the binary H2-methylcyclohexane sH hydrate is examined. In this case, the small and medium cavities are again singly occupied, resulting in a maximum H2 uptake of 1.4 wt %. Finally, the results from simulations on pure H2 and binary hydrates are utilized to investigate the potential of H2 storage in sH hydrates where the promoter molecules occupy the medium instead of the large cavities.     

Spectroscopic methods in gas hydrate research

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) CO₂ 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.     

Effect of small cage guests on hydrogen bonding of tetrahydrofuran in binary structure II clathrate hydrates

Molecular dynamics simulations of the pure structure II tetrahydrofuran clathrate hydrate and binary structure II tetrahydrofuran clathrate hydrate with CO(2), CH(4), H(2)S, and Xe small cage guests are performed to study the effect of the shape, size, and intermolecular forces of the small cages guests on the structure and dynamics of the hydrate. The simulations show that the number and nature of the guest in the small cage affects the probability of hydrogen bonding of the tetrahydrofuran guest with the large cage water molecules. The effect on hydrogen bonding of tetrahydrofuran occurs despite the fact that the guests in the small cage do not themselves form hydrogen bonds with water. These results indicate that nearest neighbour guest-guest interactions (mediated through the water lattice framework) can affect the clathrate structure and stability. The implications of these subtle small guest effects on clathrate hydrate stability are discussed.     

High-pressure gas hydrates

It has long been known that crystalline hydrates are formed by many simple gases that do not interact strongly with water, and in most cases the gas molecules or atoms occupy 'cages' formed by a framework of water molecules. The majority of these gas hydrates adopt one of two cubic cage structures and are called clathrate hydrates. Notable exceptions are hydrogen and helium which form 'exotic' hydrates with structures based on ice structures, rather than clathrate hydrates, even at low pressures. Clathrate hydrates have been extensively studied because they occur widely in nature, have important industrial applications, and provide insight into water-guest hydrophobic interactions. Until recently, the expectation-based on calculations-had been that all clathrate hydrates were dissociated into ice and gas by the application of pressures of 1 GPa or so. However, over the past five years, studies have shown that this view is incorrect. Instead, all the systems so far studied undergo structural rearrangement to other, new types of hydrate structure that remain stable to much higher pressures than had been thought possible. In this paper we review work on gas hydrates at pressures above 0.5 GPa, identify common trends in transformations and structures, and note areas of uncertainty where further work is needed.