Methane is considered to be a cubic structure I (CS-I) clathrate hydrate former, although in a number of instances, small amounts of structure II (CS-II) clathrate hydrate have been transiently observed as well. In this work, solid-state magic angle spinning 13C NMR spectra of methane hydrate formed at low temperatures inside silica-based nanoporous materials with pores in the range of 3.8–20.0 nm (CPG-20, Vycor, and MCM-41) show methane in several different environments. In addition to methane encapsulated in the dodecahedral 512 (D) and tetrakaidecahedral 51262 (T) cages typical of the CS-I clathrate hydrate phase, methane guests in pentakaidecahedral 51263 (P) and hexakaidecahedral 51264 (H) cages are also identified, and these appear to be stabilized for extended periods of time. The ratio of methane guests among the D and T cages determined from the line intensities is significantly different from that of bulk CS-I samples and indicates that both CS-I and CS-II are present as the dominant species. This is the first observation of methane in P cages, and the possible structures in which they could be present are discussed. Broad and relatively strong methane peaks, which are also observed in the spectra, can be related to methane dissolved in an amorphous component of water adjacent to the pore walls. Nanoconfinement and interaction with the pore walls clearly have a strong influence on the hydrate formed and may reflect species present in the early stages of hydrate growth.

A dendrimer with three-fold symmetry and piperazinoamido moieties was prepared in similar to 35% yield. This molecule was observed to exhibit columnar phases on thermal treatment and have free void space in the mesogenic and solid states based on a Xe-129-NMR study. The piperazinoamido moiety was indicated in a theoretical calculation to cause the dendritic framework to become twisted between the central core and periphery-thus resulting in the dendrimer forming a free void space inside the framework after having self-assembled into columnar stacks.

Thermodynamic and statistical mechanics of clathrate hydrate substances are discussed in this chapter. The occupancy of guests in clathrate hydrate cages is an important factor in determining hydrate properties and performance in different applications. Experimental methods of determining the hydration numbers that allow the determination of cage occupancies are described. The van der Waals–Platteeuw statistical mechanical theory and some of its extensions are introduced, and its use in predicting cage occupancy numbers and the phase stability boundary are described. Other thermodynamic-related topics, such as the use of thermodynamic clathrate hydrate inhibitors, the guest encagement energies, and hydrate–hydrate phase transitions, are discussed.

A brief overview of key highlights of clathrate hydrate research is given, with a separate section outlining research performed at the National Research Council of Canada in Ottawa and the researchers contributing to this work. For reference, lists of books, review articles, and conferences on clathrate hydrates are given at the end of this chapter.

Methanol is a widely known thermodynamic hydrate inhibitor, but recent studies revealed that an ammonium fluoride ion-pair doped into the host clathrate hydrate lattice can stabilize methanol in the hydrate cages. In this work, we examined THF + methanol clathrate formed from 27 mol % NH4F aqueous solution with powder X-ray diffraction (PXRD) and molecular dynamics (MD) simulations to investigate the effect of methanol guest molecules on the thermal properties of ammonium fluoride doped THF clathrate hydrates. The PXRD pattern analyses revealed that the dissociation of THF + methanol clathrate of NH4F/H2O occurs at higher temperature than 260 K (the dissociation temperature of the nondoped THF + methanol clathrate) and that the thermal expansivity of this doped clathrate is ~67% that of pure THF hydrate in the range of 100 K to 260 K. The MD simulation results demonstrated that the smaller thermal expansivity is due to the strong interactions between methanol guests and host NH4F pairs and revealed that methanol and NH4F complementarily stabilize each other in the doped clathrate system. The findings in this work provide a better understanding of the nature of ion-pair doped clathrate systems and the crystal engineering approach of clathrate hydrates for their applications in gas storage.

The clathrate hydrates can be seen as members of a number of super-classes, including the guest–host materials, the inclusion compounds, and supramolecular materials. The clathrate hydrate family can be divided into sub-groups that depend largely on the size and chemistry of the guest materials and a descriptive phase diagram. Since neutral molecules make up by far the largest group of guests and have been studied most extensively, details for each are presented in tabular form.

The van der Waals diameters of N-2, O-2, and CH4 are almost identical; however, they have different electrostatic charge distributions, and their preferred hydrate structures are different. The O-2 and N2 molecules form structure II (sII) clathrate hydrates under moderate pressure conditions up to 1 kbar, while the CH4 molecule forms a structure I (sI) clathrate under these pressure conditions. In this work, we investigated the effect of NH4F doping on N-2, O-2, and CH(4 )hydrates with powder X-ray diffraction (PXRD) measurement. From the PXRD pattern analyses, the lattice parameter decreased for all three hydrates as the concentration of NH4F doping in the framework increased. The sizes and electrostatic charge distributions within hydrate cages were "tuned " by the NH4F doping, and the transition of the preferred clathrate structure of N2 hydrate from sII to sI occurred at doping concentrations greater than 5 mol %. This transition was not observed in O2 and CH4 hydrates. The findings in this work reveal that the guest-host van der Waals and electrostatic interactions can be adjusted by the NH4F doping to the host framework and suggest that the crystal engineering of the hydrate lattice can be an alternative to improve hydrate-based gas separation technologies.

Clathrate (gas) hydrates as materials have received great interest due to their high-density gas storage potential and separation applications for their ability to preferentially separate a targeted component such as CO2 from waste streams. Among the three clathrate hydrate structures, only sH (structure H) hydrates require a large molecule such as neohexane (NH) or tert-butyl methyl ether (TBME) as well as a "help-gas" molecule such as methane (CH4) to form a stable structure. However, attempts to use CO2 as a help-gas came up with mixed results where it appeared that the sH hydrate formed was stable only at temperatures below the ice point, whereas the compound formed with CH4 was considerably more stable. sH hydrates have considerable potential for gas separation, for instance, of CH4-CO2 mixtures. Thus, in this work, several compositions were tested for their hydrate forming ability. The large cage guests tested were NH and TBME, the help-gas mixtures were a CO2-rich mixture (76% CO2 and 24% CH4), and a CO2 lean mixture (24% CO2 and 76% CH4). The phase behavior of hydrates formed from the various combinations was tested by measuring the endo- and exotherms associated with hydrate formation and decomposition in a high-pressure differential calorimeter. The different phases indicated from the DSC results were identified by employing an in situ high-pressure cell on a powder X-ray diffractometer. Powder patterns were recorded to identify the crystal phase arising from nucleation and possible re-crystallization events after annealing. It was confirmed that the CO 2 lean mixture (24% CO2 and 76% CH4) forms structure H (sH) hydrate, while the CO2-rich mixture (76% CO2 and 24% CO4) forms structure I (sI) hydrate presenting a structure transition pattern across the gas mixtures investigated. Further, it was observed that the CO2 lean mixture, which forms sH hydrate, also starts as sI hydrate and gradually converts to the thermodynamically stable sH hydrate. This study, in essence, helps to understand the preference of CH4 and CO2 for three different types of cages in sH hydrates and presents a design framework for a suitable gas separation mechanism for this gas mixture of interest.

Nuclear magnetic resonance (NMR), discussed in this and the following two chapters (11, 12), has played a very important role in studying numerous aspects of gas hydrate properties, and has led to numerous discoveries. This chapter will introduce the basics of solid-state NMR necessary to understand and interpret the spectra of clathrate hydrates, and show how this has been applied. Initially only the abundant nuclei 1 H and 19 F were accessible for study: the fine structure of broadline dipolar lineshapes at liquid helium temperatures gave information on internuclear distances and conformations of guest molecules in hydrate cages, and second moment and relaxation studies were used extensively to define guest and host dynamics (Chapter 12). With the advent of higher magnetic fields and high-resolution solid-state techniques, major contributions became possible, such as 13 C and 129 Xe NMR studies, that led to the relationship between the NMR chemical shielding shifts and anisotropies and the size, shape, and symmetry of the hydrate cages. In turn, this, together with quadrupolar 2 H NMR line shapes, led to the first experimental measurements of relative cage occupancies and hydration numbers based on instrumental techniques. 129 Xe NMR, especially, became very useful for differentiating clathrate hydrate structures. NMR lineshapes of non-integer quadrupolar nuclei 131 Xe and 83 Kr showed influences due to vacancies in neighboring hydrate cages.

The kinetics of hydrate formation has followed different paths depending on the goals of a particular community of researchers. Much of the initial work on hydrate kinetics came from the need to understand processes important for industry such as hydrate plug control in pipelines and seawater desalination with gas hydrates. Engineering researchers first developed models of hydrate processes based on activated chemical kinetics. Following a phase transition model, we present the classical theory of homogeneous and heterogeneous nucleation. Models of hydrate processes based on such considerations are still emerging, and their description takes account of theory, experimental data, and computer modeling studies. Other aspects related to kinetics that are presented include modifiers of kinetics, metastability, morphology, and molecular simulations of kinetic processes.

This chapter presents detailed structural features of the various non-canonical clathrate hydrates, including the canonical amine clathrate hydrates, semi-clathrates, and ionic clathrates. The similarities and structural differences with the canonical clathrate hydrates of Chapter 5 are discussed.

Alternative interpretations of the experimental results given in the Communication of Petuya et al.1 are presented. There is evidence that under certain conditions, ammonia can be incorporated into clathrate hydrate cages.

After introducing the importance of motional timescales built into the various techniques used to study dynamics, this chapter introduces dielectric relaxation as one of the first techniques to offer insight into guest and host dynamics. Although limited in the extent to which molecular motions can be described by this method, many aspects relevant to dielectric relaxation were eventually transferred to NMR spectroscopy. After noting early advances, the molecular dynamics (MD) of hydrate guest and host molecules and its importance in developing a molecular picture of clathrate hydrates are discussed in some detail. Both hydrate cages and hydrate guests can be differentiated on the basis of size and symmetry, thus there are many possible combinations, some of which are analyzed in detail. Since the cages are held together by hydrogen bonds, guest chemistry can be expected to play a role, for instance, competing guest–host and host–host hydrogen bonding interactions will affect hydrate properties.

Methods of clathrate hydrate synthesis are classified according to the solubility of guest molecules in water and their physical state. Examples of hydrate synthesis techniques and reactors are given for ambient and high-pressure conditions and for near ice freezing and colder temperatures. Currently a variety of equipment has been designed and used for the preparation of hydrates and the measurement of their properties. For guests with low water solubility, hydrate synthesis technique requires both high pressure (particularly for gas-phase guests) and rigorous mixing of phases through stirring or bubbling of guests through the aqueous phase. For the important case of methane hydrate synthesis, semi-batch, flow loop, and continuous reactors are described. Techniques for obtaining single crystals of clathrate hydrates are briefly described. It is also recognized that the presence of air during hydrate synthesis resulted in a hydrate with changed properties because of air entrainment.

Principles of molecular dynamics and Monte Carlo simulations, as applies to molecular simulations of clathrate hydrates, are outlined. Specialized topics, including simulation force fields, use of order parameters to characterize clathrate hydrate phases in a simulation, assignment of disordered water hydrogen atom positions in the clathrate hydrate unit cell, and simulations of hydrate dissociation, are discussed. Characterization methods, such as the use of the radial distribution function and autocorrelation functions, are outlined. Quantum mechanical calculations of translational–rotational states of the small guests such as H 2 and CH 4 in clathrate hydrate cages are discussed.

CO2 capture preference of semiclathrate hydrate analyzed by single crystal XRD. Asymmetrically distorted cages preferentially capture CO2.

Antifreezes are widely used in preventing hydrate formation in oil and gas flowlines. Recent studies have revealed that methanol and ammonia can be incorporated into clathrate hydrate phases along with a more hydrophobic guest such as THF or propane and that these antifreezes act as catalysts for methane hydrate formation from ice. In this work, we demonstrated that these antifreezes can enhance the methane storage content of binary clathrate hydrates, namely those of THF and TBAB. THF + methane and TBAB + methane binary hydrates with/without methanol or ammonia were synthesized and analyzed with 13C NMR spectroscopy and Xray diffraction (XRD) methods. In the THF hydrate system, 84% and 81% of the 512 small cages were occupied by methane in the presence of methanol and ammonia respectively, while only 44% of the small cages were occupied in the absence of these antifreezes. In the TBAB-H2O system, the powder XRD (PXRD) patterns of 1TBAB:38H2O samples without antifreeze both before and after methane introduction showed mostly tetragonal structures. On the other hand, it was confirmed that methanol can easily induce TBAB hydrates to form the orthorhombic structure which is more suitable for methane storage than the tetragonal structures of TBAB hydrates. The single crystal XRD analysis of a crystal grown from the 1TBAB:1CH3OH:38H2O solution at 277 K showed that methanol was present in the 512 cage of the orthorhombic TBAB hydrate phase. The 13C NMR spectra of TBAB + methane hydrates also showed an enhanced methane content in the presence of methanol. The present findings on the enhancement of methane storage induced by antifreezes suggest that methanol can be a key material for hydrate-based methane storage systems.

Methanol is known as one of few small molecules that cannot stabilize a solid clathrate hydrate host lattice as a guest molecule in a simple hydrate phase. Recently, it was discovered that water-NH4F solutions can form clathrates consisting of solid solutions of water and NH4F, which have the same structure as the canonical clathrate hydrates. These doped phases were found to be able to incorporate strongly hydrophilic guests such as methanol. As the next step in testing the utility of these novel hydrates, we prepared NH4F-doped clathrates with simple CO2 and binary CO2 + methanol guest molecules and characterized these by powder X-ray diffraction (PXRD), Raman spectroscopy, and molecular dynamics (MD) simulations. From the PXRD analysis, it was confirmed that CO2 interacts more strongly with the NH4F-doped 5(12) cages than the 5(12) cages without dopants. The MD simulations supported the PXRD results by demonstrating a strong interaction between the O atom of CO2 and the dopant NH4+ in the small cages. The incorporation of methanol into the CO2 + methanol clathrates was confirmed by PXRD analysis. With low concentrations of methanol, this guest shows a preference for the 5(12) cages and may serve as a site blocker for the 5(12) cage that normally would be occupied by small molecules such as CH4 and N-2 in hydrate-based gas separation (HGBS) processes. Phase boundary conditions for hydrate stability in CO2-NH4F-CH3OH- water were obtained, and it was determined that a solution of 5 mol % NH4F and 2.2 mol % CH3OH is a reasonable choice for operating an HBGS process. The present findings provide insight into the potential of the NH4F-doped hydrate lattice, aided by quantities of catalytic methanol, for use in HBGS processes.

Clathrate hydrates are host–guest crystalline compounds which can capture both small gases such as H2, CH4, and CO2 and larger guest molecules which are in the liquid state. Large guests (LGs) such...