笼形水合物的科学与技术研究以及在能源和环境领域中的应用

能源与环境是人们越来越关注的问题,而笼形水合物同时在这两方面展示出了巨大的应用前景:(1) 在广阔的海底大陆架中蕴藏着大量的以甲烷为主体的笼型水合物——天然气可燃冰;(2) 将大气中日益增多的二氧化碳装入笼型水分子中并沉于海底以降减温室效应;(3) 氢气可燃冰作为一种清洁的能源载体具有很高的能量密度、可重复再生、可快速充氢;(4) 利用笼形水合物的形成过程,可以有效地分离气体、降低能耗、减轻污染.虽然笼形水合物在工程应用以及科学研究中有着重要的价值, 但其晶体结构, 成键机制, 温压相图, 热化学与力学稳定性, 合成与分解的反应动力学, 声学弹性, 与海底沉积的反应, 以及扩散和输运性质等都有待深入的研究.目前,科学家已经研发或者正在探讨将高压和低温装置与中子衍射技术以及激光光谱、热学测量、超声技术等有机的结合在一起, 从而能够进行一系列的实验研究来解决诸多的基本科学问题. 高压及低温环境下的中子衍射装置在氢气、甲烷以及二氧化碳气体水合物体系研究中显示了巨大的优势, 并且在确定水合物晶体结构, 气体分子在水合物笼格中的占据情况, 以及气体分子在笼格中的分布状态等方面取得了巨大的进展.     

笼形水合物的科学与技术研究以及在能源和环境领域中的应用

能源与环境是人们越来越关注的问题,而笼形水合物同时在这两方面展示出了巨大的应用前景:(1)在广阔的海底大陆架中蕴藏着大量的以甲烷为主体的笼型水合物--天然气可燃冰;(2)将大气中日益增多的二氧化碳装入笼型水分子中并沉于海底以降减温室效应;(3)氢气可燃冰作为一种清洁的能源载体具有很高的能量密度、可重复再生、可快速允氢;(4)利用笼形水合物的形成过程,可以有效地分离气体、降低能耗、减轻污染,虽然笼形水合物在工程应用以及科学研究中有着重要的价值,仉其晶体结构,成键机制,温压相图,热化学与力学稳定性,合成与分解的反应动力学,声学弹性,与海底沉积的反应,以及扩散和输运性质等郜有待深入的研究.目前,科学家已经研发或者正在探讨将高压和低温装置与中子衍射技术以及激光光谱、热学测量、超声技术等有机的结合在一起,从而能够进行一系列的实验研究米解决诸多的基本科学问题.高压及低温环境下的中子衍射装置在氢气、甲烷以及二氧化碳气体水合物体系研究中显示了巨大的优势,并且在确定水合物晶体结构,气体分子在水合物笼格中的占据情况,以及气体分子在笼格中的分布状态等方面取得了巨大的进展.     

应用中子衍射研究笼型水合物的结构及反应动力学

随着国民经济的快速发展,能源成为推进我国现代化建设的关键物质因素和原动力,开发新能源特别是具有洁净环保能源理念的新能源成为建设可持续发展社会的迫切需要。笼型水合物在新型清洁能源开发、储存、和温室气体减排上具有巨大的应用前景。本文介绍了我们利用原位高压、低温中子衍射实验手段,并辅以分子动力学模拟,研究多种气体笼型水合物的结构、反应动力学模型,揭示了客体气体分子极性及尺寸影响气体笼型水合物性质的物理机制,以及今后气体笼型水合物的应用前景。     

显微激光拉曼光谱测定甲烷水合物的水合指数

甲烷水合物是由甲烷气体分子与水分子在低温高压下形成的一种笼型结构化合物,广泛存在于海底陆架区和陆地冻土区,被认为是一种潜在的能源资源。在水合物的晶格中,水分子在氢键的作用下形成大小不同的笼子,甲烷分子可分别进入大笼(51262)和小笼(512)中。在自行研制的实验装置上,分别合成了一系列不同体系下的甲烷水合物,包括十二烷基硫酸钠(SDS)水溶液-甲烷体系、冰粉-甲烷体系以及冰粉-不同粒度砂-甲烷体系。对这些甲烷水合物样品进行了激光拉曼光谱分析,测定了其水合指数,笼占有率等结构参数。结果表明,这些甲烷水合物都为Ⅰ型结构,其水合指数和笼占有率基本不受沉积物粒径大小的影响。在3种体系中生成的水合物,大笼中甲烷分子基本占满,占有率大于97%;小笼中甲烷分子占有率为80%～86%,测得的水合指数为6.05～6.15。     

CO2在气体水合物结构特性的固体核磁共振波谱与拉曼光谱实验研究

天然气水合物是蕴含着巨大能源潜力的非常规能源,2017年和2020年两次我国南海探索性试采的成功,加快了天然气水合物项目的进展。二氧化碳置换开采法,既能开发CH₄,又能封存CO₂。同时水合物法分离烟气中CO₂具有很好的应用前景,而CO₂在气体水合物的微观结构和特性尚不明确,实际应用存在一定的未知影响。为了考察其特性,利用13C固体核磁技术（NMR）和拉曼光谱（Raman）进行CO₂置换CH₄水合物、合成13CO₂-H₂-CP混合水合物实验表征,讨论CO₂在水合物中的定量问题,研究CO₂分子在笼型结构中的分布,探讨CO₂分子在气体水合物中的结构类型和特性。结果表明：（1）利用Raman费米低频共振1 277.5 cm-1峰积分得到CO₂在I型大笼（51262笼）的占有率为0.978 2,CH₄在Ⅰ型小笼（512笼）和大笼（51262笼）的占有率为0.059 3和0.009 5,水合数7.61,Raman费米高频共振1 381.3 m-1峰积分得到CO₂在51262笼的占有率为0.984 3,CH₄在512笼和51262笼的占有率为0.023 7和0.003 3,水合数7.70,CO₂几乎占满了大笼,CO₂气体的加入会导致水合物中,CH₄的大、小笼占有率均大幅度降低,置换后水合数略低于纯甲烷水合物,未标记的CO₂水合物在核磁中较难测出信号,CO₂气体置换后CH₄在小笼的占有率仅0.097 5,大笼占有率为0.317 2,两种方法差异主要原因为核磁的CO₂未出峰。（2）利用拉曼费米低频共振1 273.4 cm-1峰积分得到H₂、CO₂在512笼、CP在51262的占有率分别为0.124 8,0.304 2和0.997 8,水合数9.16;Raman费米高频共振1 380.6 cm-1峰积分得到H₂、CO₂在512笼、CP在51262的占有率分别为0.123 6,0.577 1和0.985 1,水合数7.12。13C标记CO₂分子在水合物中达到较好的固体核磁分辨率,首次确认CO₂在Ⅱ型小笼中的化学位移为124.8 ppm,计算得到CO₂的小笼占有率为0.783 1,CP的大笼占有率为0.971 8,水合数6.70,Raman高频频费米共振峰（1 380.6 cm-1）定量计算与13C NMR结果更接近。（3）对CO₂的13C NMR化学位移进行了归属,并结合Raman与13C NMR的对比分析,为CO₂水合物的13C NMR研究和拉曼定量提供参考。     

CH_4-CO_2混合气体水合物在NaCl溶液中生成过程

本实验着重研究了纯水、质量分数为3.4%和5.0%的NaCl溶液中,甲烷-二氧化碳混合气体水合物的生长速率以及气体在水合物相中分布规律。研究结果表明水合物生成过程中对甲烷和二氧化碳的吸收存在一定的选择性。二氧化碳在水合物晶核生长初期更容易被捕获,表明二氧化碳更易于占据水合物中大笼,同时有利于稳定水合物的晶体结构。NaCl的存在提高了水合物三相平衡压力,降低了水合物的气体消耗总量。随着溶液中初始盐浓度的提高,水合物生长初期对二氧化碳的选择性吸收得到了强化。表明溶液中NaCl浓度的上升提高了水合物晶核的不稳定性同时抑制了晶核生成。因此,NaCl对水合物的生长过程具有一定的抑制作用。     

醇类抑制剂对甲烷水合物形成的影响

在本工作中,甲烷水合物的生长动力学是通过甲醇、乙醇、乙二醇三种不同醇类抑制剂存在下的分子动力学模拟研究的.模拟结果发现,三种醇类都可作为甲烷水合物的抑制剂,醇类分子中的亲水性羟基极大地破坏了水合物笼的结构,并且羟基可以与局部的液态水分子形成氢键,从而增加了形成水合物笼型结构的难度,导致甲烷水合物的生长速率降低.对于甲醇分子,甲醇分子的亲水性羟基与水分子形成氢键从而破坏了水分子结构,而亲油性甲基对周围的水分子具有簇效应,两者都会降低水合物生长速率;对于乙二醇和乙醇分子,它们只含有羟基,特别是乙二醇分子含有两个羟基,其对H₂O分子有很强的吸附作用,导致水合物生长速率降低.在抑制效果方面,甲醇分子最优,乙二醇稍微优于乙醇.     

激光拉曼光谱在气体水合物研究中的应用

天然气水合物是一种重要的潜在能源。用激光拉曼光谱法表征气体水合物能够为研究水合物形成机理和开采方法提供重要信息。系统介绍了激光拉曼光谱法的基本原理,综述了激光拉曼光谱仪在气体水合物微观表征上的各种实际应用。通过激光拉曼测试可分析水合物气体组成、推测结构类型,再利用经验公式或者相对定量法可计算出其大/小笼的气体占有率和水合数;利用原位拉曼技术可以观测水合物形成和分解的微观过程,解析气体分子进入和离开笼子的进程、进行水合物形成和分解过程中气体浓度变化及水合物形成过程中气体溶解度的测定,辨识水合物系统中的相变过程,进而研究水合物形成和分解动力学;激光拉曼光谱法还可用于研究超高压条件下气体水合物的结构及其变化过程。原位拉曼光谱能够对深海天然气水合物及其环境在原位进行表征;利用拉曼成像技术可以对水合物晶体表面进行系统测定,探求气体组分在晶体表面的分布。随着激光拉曼技术的发展及与其他设备联用水平的提高,激光拉曼光谱仪向便携,高灵敏度发展,能够更广泛深入地进行气体水合物微观研究。     

普适的基于能量的分块方法计算甲烷水合物簇的精确结合能和拉曼光谱

甲烷水合物在化学、能源和环境科学等领域中都具有重要作用. 本文采用普适的基于能量的分块(GEBF)方法计算了多种甲烷水合簇的结合能和拉曼光谱. 首先使用这些甲烷水合簇的在显相关耦合簇[CCSD(T)(F12^*)]水平下得到的GEBF结合能,评估了一系列密度泛函计算的结果. 计算结果表明B3PW91-D3和B97D泛函表现最佳,与GEBF-CCSD(T)(F12^*)基准相比的平均绝对误差分别仅为0.27和0.47 kcal/mol. 然后用GEBF-B3PW91-D3方法计算得到了单、双笼甲烷水合簇的结构和拉曼光谱,得到的甲烷C-H键伸缩拉曼振动峰与实验值的偏差小于3 cm^-1,说明B3PW91-D3泛函可以很好地重现实验结果. 随着水笼尺寸的增加. 甲烷C-H键伸缩拉曼峰发生红移,该现象与实验中提出的“松笼-紧笼”模型吻合. 此外,甲烷分子邻近环境(水笼)的改变对拉曼光谱的影响很小,环境从单笼变为双笼导致C-H键伸缩拉曼振动峰的蓝移不超过3 cm^-1. 甲烷水合簇的理论拉曼光谱与实验拉曼光谱结合可以用来研究海底或星际冰体内部的甲烷水合物的结构. 结合B3PW91-D3或B97D泛函和机器学习模型,可以进一步应用分子动力学模拟研究甲烷水合物的成核/生长机制和相变过程.     

甲烷水合物的固体核磁共振碳谱与激光拉曼光谱研究

甲烷水合物（CH₄·nH₂O）是主要由甲烷和水分子构成的冰状笼型化合物,在自然界储量巨大．固体核磁共振（NMR）波谱和激光拉曼光谱是在分子水平分析甲烷水合物的重要手段．该文利用低温固体核磁共振碳谱（¹³C NMR）对合成的甲烷水合物结构进行了研究,分别使用¹³C交叉极化（¹³C CP）和高功率质子去偶（¹H HPDEC）2种脉冲程序采集甲烷水合物的¹³C NMR谱图,结合实验结果分析及理论推导可知,使用¹H HPDEC方法得到的¹³C NMR谱图信号更强,更利于定量分析;甲烷气体与冰粉合成的甲烷水合物为I型,其大笼和小笼占有率分别为0.988和0.824,水合数为6.07;甲烷气体与SH2站位沉积物和冰粉合成的甲烷水合物也为I型,其大笼和小笼占有率分别为0.987和0.887,水合数为5.98;SH2站位沉积物使合成的甲烷水合物的小笼占有率提高、水合数降低、水合物饱和度提高．激光拉曼光谱结果证实了上述结果的准确性．该文为甲烷水合物测试提供了重要的方法参考．     

Tensile properties of structural I clathrate hydrates: Role of guest–host hydrogen bonding ability

Clathrate hydrates (CHs) are one of the most promising molecular structures in applications of gas capture and storage, and gas separations. Fundamental knowledge of mechanical characteristics of CHs is of crucial importance for assessing gas storage and separations at cold conditions, as well as understanding their stability and formation mechanisms. Here, the tensile mechanical properties of structural I CHs encapsulating a variety of guest species (CH₄, NH₃, H₂S, CH₂O, CH₃OH, and CH₃SH) that have different abilities to form hydrogen (H-) bonds with water molecule are explored by classical molecular dynamics (MD) simulations. All investigated CHs are structurally stable clathrate structures. Basic mechanical properties of CHs including tensile limit and Young’s modulus are dominated by the H-bonding ability of host–guest molecules and the guest molecular polarity. CHs containing small CH₄, CH₂O and H₂S guest molecules that possess weak H-bonding ability are mechanically robust clathrate structures and mechanically destabilized via brittle failure on the (1 0 1) plane. However, those entrapping CH₃SH, CH₃OH, and NH₃ that have strong H-bonding ability are mechanically weak molecular structures and mechanically destabilized through ductile failure as a result of gradual global dissociation of clathrate cages.     

Total Cross Sections for Electron Scattering on Polyatomic Molecules (CH4, CO2,NO2, and N2O) at 10 ～ 3000 eV

A new modified formulation of the Additivity Rule (AR) was proposed to calculate the total electron scattering cross sections for CH₄, CO₂, NO₂, and N₂O, considering the overlapping between atoms in molecules and the not fully transparency of the molecules. The present calculation covers the range of impact energy from 10 to 3000 eV. The results are compared with experimental data and other theories where available. The atoms are presented by spherical complex optical potential, which is composed of static, exchange, polarization, and absorption terms.     

Theoretical investigation of structures and compositions of double neon-methane clathrate hydrates,depending on gas phase composition and pressure

The region of existence of neon clathrate hydrates is an actual problem of hydrate chemistry. The current work presents theoretical study of the equilibrium formation conditions of pure neon clathrate hydrates and double clathrate hydrates of neon-methane mixture. The structures and properties of double clathrate hydrates were described within the scope of the previously developed molecular clathrate hydrate model that takes into account the influence of guest molecules on the host lattice, interaction of guest molecules between themselves, and the possibility of multiple filling of host lattice cages by guest molecules. The model makes it possible to find an equilibrium state and thermodynamic properties of clathrate hydrates at given values of p and T. In the present work, we considered the properties of double clathrate hydrates in the range of pressures from 0 to 4 kbar at 250 K. The results of modeling have shown that the mass fraction of neon in double clathrate hydrate of Ne and CH₄ mixture of cubic structure I (sI) can reach 26%, and 22.5% in double hydrate of cubic structure II (sII) even at a low methane concentration (1%) in gas phase, at high pressure. It is shown that in double clathrate hydrates of the Ne and CH₄ mixture at high pressures, phase transition sII-sI can occur.     

Guest-host coupling and anharmonicity in clathrate hydrates

We present a review of our work on the dynamics of clathrate hydrates (gas hydrates). The experimental results obtained with inelastic neutron scattering are compared with molecular-dynamics calculations. The vibrations of the guest molecules and their coupling to the cages is found to depend critically on the size, shape and electrostatic properties of the encaged guest. Atoms like xenon, that are large enough to fill the cages, show close-to-harmonic behaviour and couple strongly to the cage vibrations. Small atoms and molecules fully explore the anharmonicities of the potential within the cage, in particular at low frequencies and low temperatures. Their dynamic response is broad in energy and they couple weakly to the cage vibrations. The relevance of the microscopic dynamics for cage stability and the glass-like thermal conductivity is discussed. We equally place the observed dynamic peculiarities into the broader context of vibrations in disordered systems. Raman spectroscopic results on internal guest vibrations at high frequencies reflect also the influence of guest-host interactions and are discussed in the framework of the loose-cage tight-cage model.Received: 1 January 2003, Published online: 30 October 2003PACS: 82.75.-z Molecular sieves, zeolites, clathrates, and other complex solids - 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 63.20.-e Phonons in crystal lattices     

Using clathrate hydrates for gas storage and gas-mixture separations: experimental and computational studies at multiple length scales

ABSTRACT

Clathrate hydrates have characteristic properties that render them attractive for a number of industrial applications. Of particular interest are the following two cases: (i) the incorporation of large amounts of gas molecules into the solid structure has resulted in considering hydrates as possible material for the storage/transportation of energy or environmental gases, and (ii) the selective incorporation of guest molecules into the solid structure has resulted in considering hydrates for gas-mixture separations. For the proper design of such industrial applications, it is essential to know accurately a number of thermodynamic, structural and transport properties. Such properties can either be measured experimentally or calculated at different scales that span the molecular scale-up to the continuum scale. By using clathrate hydrates as a particular case study, we demonstrate that performing studies at multiple length scales can be utilised in order to obtain properties that are essential to process design.     

Li修饰的C$lt;sub$gt;6$lt;/sub$gt;分子对H$lt;sub$gt;2$lt;/sub$gt;O的吸附研究

采用密度泛函理论中的广义梯度近似研究C₆Li吸附H₂O分子并将之进行分解的催化过程. 几何优化发现：Li原子最稳定的吸附位置是位于C 原子顶位上方. 研究表明,第一个H₂O 分子吸附在C₆Li上需要克服1.77 eV的能量势垒,然后分解为H和OH且与Li原子成键. 当吸附第二个H₂O分子时,第二个H₂O分子需要克服1.2 eV的能量势垒分解为H和OH,其中H与Li原子上的H原子结合成H₂,OH则替代Li 原子上的H结合在Li原子上. 因此C₆Li 可以作为催化剂将H₂O分子进行分解得到H₂. 分析可知：C₆Li主要是通过Li原子与H₂O之间形成的偶极矩作用来吸附H₂O 分子,与C₆₀Li₁₂ 的储氢机制类似. 研究结果可为储氢材料的制备提供一个新的思路. 关键词： 6')" href="#">C₆ Li 2O')" href="#">H₂O 密度泛函理论     

A Correlation Potential Method for Electron Scattering Total Cross Section Calculations on Several Diatomic and Polyatomic Molecules over Energy Range 10 ～ 5000 eV

A complex optical model potential correlated by the concept of bonded atom, which considers the overlapping effect of electron clouds between two atoms in a molecule, is firstly employed to calculate the total cross sections for electron scattering on several molecules (NH₃, H₂O, CH₄, CO, N₂, O₂, and C₂H₄) over the energy range 10～5000 eV using the additivity rule model at Hartree-Fock level. The difference between the bonded atom and the free one in states is that the overlapping effect of electron clouds of bonded atoms in a molecule is considered. The quantitative total cross sections are compared with the experimental data and with the other calculations wherever available and good agreement is obtained over the energy range 10～5000 eV. It is shown that the correlated calculations are much closer to the available experimental data than the uncorrelated ones at lower energies, especially below 500 eV. Therefore, considering the overlapping effect of electron clouds in the complex optical model potential could be helpful for the better accuracy of the total cross section calculations of electron scattering from molecules.     

Effect of metal catalyst on the mechanism of hydrogen spillover in three-dimensional covalent-organic frameworks

Hydrogen spillover mechanism of metal-supported covalent-organic frameworks COF-105 is investigated by means of the density functional theory, and the effects of metal catalysts M_4(Pt_4, Pd_4, and Ni_4) on the whole spillover process are systematically analyzed. These three metal catalysts exhibit several similar phenomena:(i) they prefer to deposit on the tetra(_4-dihydroxyborylphenyl) silane(TBPS) cluster with surface-contacted configuration;(ii) only the H atoms at the bridge site can migrate to 2,3,6,7,10,11-hexahydroxy triphenylene(HHTP) and TBPS surfaces, and the migration process is an endothermic reaction and not stable;(iii) the introduction of M_4 catalyst can greatly reduce the diffusion energy barrier of H atoms, which makes it easier for the H atoms to diffuse on the substrate surface. Differently, all of the H2 molecules spontaneously dissociate into H atoms onto Pt_4 and Pd_4clusters. However, the adsorbed H2 molecules on Ni_4 cluster show two types of adsorption states: one activated state with stretched H–H bond length of 0.88 ?A via the Kubas interaction and five dissociated states with separated hydrogen atoms. Among all the M_4 catalysts, the orders of the binding energy of M_4 deposited on the substrate and average chemisorption energy per H2 molecule are Pt_4Ni_4Pd_4. On the contrary, the orders of the migration and diffusion barriers of H atoms are Pt_4Ni_4Pd_4, which indicates that Pt_4 is the most promising catalyst for the hydrogen spillover with the lowest migration and diffusion energy barriers. However, the migration of H atoms from Pt_4 toward the substrate is still endothermic. Thus direct migration of H atom from metal catalyst toward the substrate is thermodynamically unfavorable.