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.     

过氧化氢水溶液作用下甲烷水合物分解特性的分子动力学模拟研究

采用分子动力学模拟方法研究过氧化氢水(HP)溶液作用下结构I型(SI)甲烷水合物晶体分解特性. 系统分析甲烷水合物在过氧化氢水溶液作用下由晶态向液态转变过程的机理, 对比相同摩尔浓度乙二醇(EG)溶液作用下甲烷水合物分解变化规律, 得出HP与水合物热力学抑制剂EG一样对甲烷水合物分解具有促进作用, 为HP溶液促进甲烷水合物分解实验研究提供参考.     

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.     

微波场中乙烷及丙烷气体水合物的分解特性

研究了2.45GHz微波场中I型乙烷水合物及II型丙烷水合物的热激分解过程,基于晶体表面两步分解机制的动力学模型,结合传热传质分析了其分解特性.结果表明:水合物在微波场中的加热分解是一个与实际微波电磁场相互耦合的过程,微波体积加热的特点强化了水合物颗粒表层的传热传质过程,时间累积的热效应增大了水合物晶体破解速率;在120至540W入射功率下,乙烷、丙烷水合物气化速率分别达到0.109-0.400mol·min-·1L-1及0.090-0.222mol·min-1·L-1.在一定范围内增大微波功率可显著提高水合物分解速率,其中乙烷水合物一直处于功率主控区,丙烷水合物更早进入功率和分解动力机制共同控制区.     

配合物的稳定性与配位体的酸碱强度之间的直线自由能关系VIII:N,N'-双(对位取代苯基)乙二胺与铜(II),镍(II)生成配合物的稳定性研究

在25℃, I=0.1mol.dm^-^3NaClO4的条件下,在混合溶剂[50%(υ/υ)二氧六环和70%(υ/υ)乙醇]中用pH法测定了N,N'-双(对位取代苯基)乙二胺的质子化常数及其与铜(II),镍(II)生成二元配合物的生成常数.结果表明:在配位体的质子化常数与配合物的生成常数之间,在配位体的σ'值与Hammett方程中的取代基常数σ之间,以及稳定化因子S~f与σ值之间,均存在着良好的线性关系.从实验进一步验证了在配位化学中直线自由能关系存在的普遍性.     

基于Ag3PO4的全固态Z型光催化体系研究进展

随着现代工业的迅猛发展,人类面临的能源危机和环境污染问题日益严重.光催化剂技术有望利用太阳能同时解决这两大问题,其关键在于设计高效的光催化体系.传统光催化材料TiO2具有价廉、活性高及稳定性好等优点,然而其带隙宽(Eg=3.2 eV),仅能利用占太阳光谱约4%的紫外光,从而限制其利用太阳能.可见光占太阳光谱的40%以上,因此开发可见光响应的光催化材料成为光催化领域研究焦点.2010年,叶金花课题组报道了Ag3PO4在可见光照射下可高效分解水产氧及降解水体中有机污染物,从而使其迅速成为研究热点.Ag3PO4是目前为止报道的光量子效率最高的可见光响应的催化材料,带隙能在2.3~2.5 eV范围内,其高效的光催化活性归结于其独特的电子结构利于光生电荷的分离及转移.然而,由于Ag3PO4本身易光蚀,稳定性差,必然限制其实际应用.近年来,为在进一步提升Ag3PO4活性的基础上增强稳定性,研究者通过多种方法对其进行修饰,包括贵金属沉积、碳材料修饰、负载及半导体异质复合等.相对于前面几种修饰方法,半导体复合相对高效且成本低.半导体复合主要构成II型异质结构和Z型光催化体系.II型异质结构由于内建电场的存在可以促进光生电荷的定向转移,从而提高光生电荷的分离效率,进而提高光催化活性.然而,这种电荷的定向迁移会降低光生电荷的氧化还原能力.模拟绿色植物的光合作用过程,一种全固态Z型光催化体系应运而生,其是将两种导带和价带位置匹配的可见光驱动的催化剂分别作为光催化系统I(PS I)和光催化系统II(PS II),同时选用导电性能优良的材料(Ag,Au和RGO等)作为电子介体.可见光照条件下,PS I和PS II均被激发产生电子和空穴,PS II导带上的电子通过电子介质与PS I价带空穴复合,一方面抑制了PS I和PS II本身电子和空穴的复合,另一方面保留了PS I导带电子的强还原性和PS II价带空穴的强氧化性.另外,PS I和PS II紧密结合形成具有准连续能级的固-固接触界面,PS II导带上的电子直接与PS I价带空穴复合,形成无电子介体的直接Z型光催化体系.Ag3PO4价带顶相对靠下,氧化能力强,往往作为PS II组分,其与导带顶相对靠上的催化剂(PS I)构成Z型体系,这样Ag3PO4导带电子可与PS I的价带空穴复合,减弱电子对Ag3PO4本身的还原,提高其稳定性;另一方面,Ag3PO4价带空穴可参与氧化反应.基于Ag3PO4的Z型体系主要以Ag作为电子介体,归因于在制备及光催化过程中原位产生的少量Ag可直接作为电子介体.此外,还原氧化石墨烯(RGO)也可作为电子介体,并且其存在可进一步提高Ag3PO4的稳定性.需要指出的是,基于Ag的等离子体共振效应,Ag3PO4基等离子体Z型光催化体系也受到关注.目前,Z型光催化体系处在发展阶段,必然存在一些问题,比如,II型异质光催化体系与直接Z型光催化体系如何区分,有待进一步研究.另外,报道的基于Ag3PO4的Z型体系主要用来光催化降解水体中的有机污染物,催化剂的回收再利用受到限制,今后可开发磁性Ag3PO4基Z型体系,解决回收再利用的问题;另外,通过能带调控,可将基于Ag3PO4的Z型体系多用于光催化产氢、还原CO2及处理有害气体.     

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.     

二十四元六氮大环双核铜(I)配合物的氧化去甲基作用: 单加氧酶的模拟

以5-溴-2-甲氧基-1, 3-苯二甲醛与二乙烯三胺通过[2+2]缩合,合成了一个新的六氮杂二十四元大环配体, 并在[Cu(CH3CN)4]ClO4存在下生成Cu(I)大环配合物, 然后在空气(或氧气)中氧化, 得到了新的大环双核Cu(II)配合物, 用多种方法对其进行了表征, 用1H NMR谱等方法鉴定了氧化产物。实验结果表明: 在Cu(I)配合物氧化过程中,能使配体环上的一个甲氧基发生断裂, 形成苯氧桥和水桥联的Cu(II)配合物。在木质素酶等单加氧酶的氧化过程中也伴随着氧化去甲基作用。本文首次用大环配合物对这一过程进行了模拟, 并测定了氧化反应中的吸氧量和吸氧速率常数。     

二十四元六氮大环双核铜(I)配合物的氧化去甲基作用: 单加氧酶的模拟

以5-溴-2-甲氧基-1, 3-苯二甲醛与二乙烯三胺通过[2+2]缩合,合成了一个新的六氮杂二十四元大环配体, 并在[Cu(CH3CN)4]ClO4存在下生成Cu(I)大环配合物, 然后在空气(或氧气)中氧化, 得到了新的大环双核Cu(II)配合物, 用多种方法对其进行了表征, 用1H NMR谱等方法鉴定了氧化产物。实验结果表明: 在Cu(I)配合物氧化过程中,能使配体环上的一个甲氧基发生断裂, 形成苯氧桥和水桥联的Cu(II)配合物。在木质素酶等单加氧酶的氧化过程中也伴随着氧化去甲基作用。本文首次用大环配合物对这一过程进行了模拟, 并测定了氧化反应中的吸氧量和吸氧速率常数。     

两种晶型酞菁氧钒纳米颗粒的制备及形成机理

在水溶液中利用激光消融制备了酞菁氧钒(VOPc)相I型纳米颗粒，在加入一种非离子型表面活性剂的情况下通过激光消融制备得到了其相II型纳米颗粒.X射线衍射(XRD)、紫外可见吸收光谱(UV-Vis)和傅立叶变换红外光谱(FT-IR)表征了其纳米颗粒中的晶体结构.扫描电子显微镜(SEM)观察显示相I和相II型酞菁氧钒纳米颗粒的直径分别约为100和60 nm.对相II型酞菁氧钒纳米颗粒的形成机理进行了讨论.     

Unusual crystalline and polycrystalline structures in methane hydrates

We present previously unreported crystalline and polycrystalline structures for methane hydrates obtained at relatively low pressures. These structures, which contain unusual cages with 12 pentagonal faces and 3 hexagonal faces, were observed during the atomistic simulations of the crystal growth of sI and sII methane hydrates. These 51263 cages have a significant impact on the structure of the resulting crystal and could explain several experimental observations regarding in-situ transformations between sI and sII hydrates. We document a previously unidentified structure of methane hydrates which we designate structure sK. Additionally, we predict a polycrystalline structure consisting of this new hydrate and sI and suggest a mechanism for the formation of a polycrystalline structure consisting of sequences of sI and sII hydrates.     

Structures of hydrocarbon hydrates during formation with and without inhibitors

The formation of hydrates from a methane-ethane-propane mixture is more complex than with single gases. Using nuclear magnetic resonance (NMR) and high-pressure powder X-ray diffraction (PXRD), we have investigated the structural properties of natural gas hydrates crystallized in the presence of kinetic hydrate inhibitors (KHIs), two commercial inhibitors and two biological ice inhibitors, or antifreeze proteins (AFPs). NMR analyses indicated that hydrate cage occupancy was at near saturation for controls and most inhibitor types. Some exceptions were found in systems containing a new commercial KHI (HIW85281) and a recombinant plant AFP, suggesting that these two inhibitors could impact the kinetics of cavity formation. NMR analysis confirmed that the hydrate composition varies during crystal growth by kinetic effects. Strikingly, the coexistence of both structures I (sI) and II (sII) were observed in NMR spectra and PXRD profiles. It is suggested that sI phases may form more readily from liquid water. Real time PXRD monitoring showed that sI hydrates were less stable than sII crystals, and there was a conversion to the stable phase over time. Both commercial KHIs and AFPs had an impact on hydrate metastability, but transient sI PXRD intensity profiles indicated significantly different modes of interaction with the various inhibitors and the natural gas hydrate system.     

Structure transition and swapping pattern of clathrate hydrates driven by external guest molecules

We first report here that under strong surrounding gas of external CH4 guest molecules the sII and sH methane hydrates are structurally transformed to the crystalline framework of sI, leading to a favorable change of the lattice dimension of the host-guest networks. The high power decoupling 13C NMR and Raman spectroscopies were used to identify structure transitions of the mixed CH4 + C2H6 hydrates (sII) and hydrocarbons (methylcyclohexane, isopentane) + CH4 hydrates (sH). The present findings might be expected to provide rational evidences regarding the preponderant occurrence of naturally occurring sI methane hydrates in marine sediments. More importantly, we note that the unique and cage-specific swapping pattern of multiguests is expected to provide a new insight for better understanding the inclusion phenomena of clathrate materials.     

Temperature- and Pressure-induced Structural Transition of Binary Clathrate Hydrates

We discover new structure II (sII) hydrate forming agents of two C₄H₈O molecules (2-methyl-2-propen-1-ol and 2-butanone) and report the abnormal structural transition of binary C₄H₈O+CH₄ hydrates between structure I (sI) and sII with varying temperature and pressure conditions. In both (2-methyl-2-propen-1-ol+CH₄) and (2-butanone+CH₄) systems, the phase boundary of the two different hydrate phases (sI and sII) exists at the slope change of the phase-equilibrium curve in the semi-logarithmic plots. We confirm the crystal structures of two hydrates synthesized at low (278 K and 6 MPa) and high (286 K and 15 MPa) temperature and pressure conditions by using high-resolution powder diffraction and Raman spectroscopy. 2-Methyl-2-propen-1-ol and 2-butanone can occupy the large cages of sII hydrate at low temperature and pressure conditions; however, they are excluded from the hydrate phase at high temperature and pressure conditions, resulting in the formation of pure sI CH₄ hydrate.     

Time-resolved in situ neutron diffraction studies of gas hydrate: transformation of structure II (sII) to structure I (sI).

We report the in situ observation from diffraction data of the conversion of a gas hydrate with the structure II (sII) lattice to one with the structure I (sI) lattice. Initially, the in situ formation, dissociation, and reactivity of argon gas clathrate hydrate was investigated by time-of-flight neutron powder diffraction at temperatures ranging from 230 to 263 K and pressures up to 5000 psi (34.5 MPa). These samples were prepared from deuterated ice crystals and transformed to hydrate by pressurizing the system with argon gas. Complete transformation from D(2)O ice to sII Ar hydrate was observed as the sample temperature was slowly increased through the D(2)O ice melting point. The transformation of sII argon hydrate to sI hydrate was achieved by removing excess Ar gas and exposing the hydrate to liquid CO(2) by pressurizing the Ar hydrate with CO(2). Results suggest the sI hydrate formed from CO(2) exchange in argon sII hydrate is a mixed Ar/CO(2) hydrate. The proposed exchange mechanism is consistent with clathrate hydrate being an equilibrium system in which guest molecules are exchanging between encapsulated molecules in the solid hydrate and free molecules in the surrounding gas or liquid phase.     

Can amorphous nuclei grow crystalline clathrates? The size and crystallinity of critical clathrate nuclei

Recent studies reveal that amorphous intermediates are involved in the formation of clathrate hydrates under conditions of high driving force, raising two questions: first, how could amorphous nuclei grow into crystalline clathrates and, second, whether amorphous nuclei are intermediates in the formation of clathrate crystals for temperatures close to equilibrium. In this work, we address these two questions through large-scale molecular simulations. We investigate the stability and growth of amorphous and crystalline clathrate nuclei and assess the thermodynamics and kinetic factors that affect the crystallization pathway of clathrates. Our calculations show that the dissociation temperature of amorphous clathrates is just 10% lower than for the crystals, facilitating the formation of metastable amorphous intermediates. We find that, at any temperatures, the critical crystalline nuclei are smaller than critical amorphous nuclei. The temperature dependence of the critical nucleus size is well described by the Gibbs-Thomson relation, from which we extract a liquid-crystal surface tension in excellent agreement with experiments. Our analysis suggests that at high driving force the amorphous nuclei may be kinetically favored over crystalline nuclei because of lower free energy barriers of formation. We investigated the role of the initial structure and size of the nucleus on the subsequent growth of the clathrates, and found that both amorphous and sI crystalline nuclei yield crystalline clathrates. Interestingly, growth of the metastable sII crystal polymorph is always favored over the most stable sI crystal, revealing kinetic control of the growth and indicating that a further step of ripening from sII to sI is needed to reach the stable crystal phase. The latter results are in agreement with the observed metastable formation of sII CO(2) and CH(4) clathrate hydrates and their slow conversion to sI under experimental conditions.     

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.     

Molecular simulation of nanoclusters of gas hydrates in a water shell. The mechanical state of the system

The molecular dynamics method is employed to study hydrates of methane (sI), and krypton hydrate (sII), as well as an ice nanocluster in a supercooled water shell. The main attention is focused on the local structure and the mechanical state of two-phase nanosized systems, which is described using the local pressure tensor. Analysis of the temperature dependence of the local pressure allows one to compare two possible mechanisms responsible for the anomalous stability of gas hydrates at ambient pressure. According to the first mechanism, the water shell plays the role of a barrier that prevents the gas from escaping from the hydrate core. The second mechanism implies that the water shell generates additional pressure, which transfers the hydrate to a thermodynamically stable state. Results of molecular dynamics simulation indicate that both mechanisms are simultaneously involved in the stabilization of the hydrate nanocluster.     

Dissociation enthalpies of synthesized multicomponent gas hydrates with respect to the guest composition and cage occupancy

This study presents the influences of additional guest molecules such as C2H6, C3H8, and CO2 on methane hydrates regarding their thermal behavior. For this purpose, the onset temperatures of decomposition as well as the enthalpies of dissociation were determined for synthesized multicomponent gas hydrates in the range of 173-290 K at atmospheric pressure using a Calvet heat-flow calorimeter. Furthermore, the structures and the compositions of the hydrates were obtained using X-ray diffraction and Raman spectroscopy as well as hydrate prediction program calculations. It is shown that the onset temperature of decomposition of both sI and sII hydrates tends to increase with an increasing number of larger guest molecules than methane occupying the large cavities. The results of the calorimetric measurements also indicate that the molar dissociation enthalpy depends on the guest-to-cavity size ratio and the actual concentration of the guest occupying the large cavities of the hydrate. To our knowledge, this is the first study that observes this behavior using calorimetrical measurements on mixed gas hydrates at these temperature and pressure conditions.     

Thermal expansivity for sI and sII clathrate hydrates

Knowledge of thermal expansivity can aid in the understanding of both microscopic and macroscopic behavior of clathrate hydrates. Diffraction studies have shown that hydrate volume changes significantly (as much as 1.5% over 50 K) as a function of temperature. It has been demonstrated previously via statistical mechanics that a minor change in hydrate volume (e.g., a 1.5% change in volume or 0.5% change in lattice parameter) can lead to a major change in the predicted hydrate formation pressure (e.g., >15% at >100 MPa for methane). Because of this sensitivity, hydrate thermal expansivity measurements, for both Structures I and II with various guests, are needed help quantify volume distortions in hydrate lattices to ensure accurate hydrate phase equilibria predictions. In addition to macroscopic phase equilibria, the thermal expansion of different hydrates can give information about the interactions between the guest molecules and the host lattice. In this work, the hydrate lattice parameters for four Structure I (C2H6, CO2, 47% C2H6 + 53% CO2, and 85% CH4 + 15% CO2) and seven Structure II (C3H8, 60% CH4 + 40% C3H8, 30% C2H6 + 70% C3H8, 18% CO2 + 82% C3H8, 87.6% CH4 + 12.4% i-C4H10, 95% CH4 + 5% C5H10O, and a natural gas mixture) systems were measured as a function of temperature. The lattice parameter measurements were combined with existing literature values. Both sI and sII hydrates, with a few exceptions, had a common thermal expansivity, independent of hydrate guest. Many guest-dependent correlations for linear thermal expansivity have been proposed. However, we present two guest-independent, structure-dependent correlations for sI and sII lattices, which have been developed to express the normalized hydrate lattice parameters (and therefore volume) as a function of temperature.