Unusual species of methane hydrate detected in nanoporous media using solid state 13C NMR.

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.

Chlorine-35 Solid-State Nuclear Magnetic Resonance Spectroscopy as an Indirect Probe of the Oxidation Number of Tin in Tin Chlorides.

Ultrawideline 35Cl solid-state nuclear magnetic resonance (SSNMR) spectra of a series of 12 tin chlorides were recorded. The magnitude of the 35Cl quadrupolar coupling constant (CQ) was shown to consistently indicate the chemical state (oxidation number) of the bound Sn center. The chemical state of the Sn center was independently verified by tin Mössbauer spectroscopy. CQ(35Cl) values of >30 MHz correspond to Sn(IV), while CQ(35Cl) readings of <30 MHz indicate that Sn(II) is present. Tin-119 SSNMR experiments would seem to be the most direct and effective route to interrogating tin in these systems, yet we show that ambiguous results can emerge from this method, which may lead to an incorrect interpretation of the Sn oxidation number. The accumulated 35Cl NMR data are used as a guide to assign the Sn oxidation number in the mixed-valent metal complex Ph3PPdImSnCl2. The synthesis and crystal structure of the related Ph3PPtImSnCl2 are reported, and 195Pt and 35Cl SSNMR experiments were also used to investigate its Pt-Sn bonding. Plane-wave DFT calculations of 35Cl, 119Sn, and 195Pt NMR parameters are used to model and interpret experimental data, supported by computed 119Sn and 195Pt chemical shift tensor orientations. Given the ubiquity of directly bound Cl centers in organometallic and inorganic systems, there is tremendous potential for widespread usage of 35Cl SSNMR parameters to provide a reliable indication of the chemical state in metal chlorides.

The anomalous halogen bonding interactions between chlorine and bromine with water in clathrate hydrates.

Clathrate hydrate phases of Cl2 and Br2 guest molecules have been known for about 200 years. The crystal structure of these phases was recently re-determined with high accuracy by single crystal X-ray diffraction. In these structures, the water oxygen-halogen atom distances are determined to be shorter than the sum of the van der Waals radii, which indicates the action of some type of non-covalent interaction between the dihalogens and water molecules. Given that in the hydrate phases both lone pairs of each water oxygen atom are engaged in hydrogen bonding with other water molecules of the lattice, the nature of the oxygen-halogen interactions may not be the standard halogen bonds characterized recently in the solid state materials and enzyme-substrate compounds. The nature of the halogen-water interactions for the Cl2 and Br2 molecules in two isolated clathrate hydrate cages has recently been studied with ab initio calculations and Natural Bond Order analysis (Ochoa-Resendiz et al. J. Chem. Phys. 2016, 145, 161104). Here we present the results of ab initio calculations and natural localized molecular orbital analysis for Cl2 and Br2 guests in all cage types observed in the cubic structure I and tetragonal structure I clathrate hydrates to characterize the orbital interactions between the dihalogen guests and water. Calculations with isolated cages and cages with one shell of coordinating molecules are considered. The computational analysis is used to understand the nature of the halogen bonding in these materials and to interpret the guest positions in the hydrate cages obtained from the X-ray crystal structures.

Managing Hydrogen Bonding in Clathrate Hydrates by Crystal Engineering.

Methanol is one of the most common inhibitors for clathrate hydrate formation. Crystalline clathrate hydrates containing methanol were synthesized and analyzed by powder X-ray diffraction and 13 C NMR spectroscopy. The data obtained demonstrate that methanol can be a helper guest for forming structure I, structure II, and structure H clathrate hydrates, as long as the lattice framework contains NH4 F. The latter acts as a lattice stabilizer by providing sites for strong hydrogen bonding of the normally disruptive methanol hydroxy group. NH4 F and methanol can be considered key materials for crystal engineering of clathrate hydrates, as the modified lattices allow preparation of hydrates of non-traditional water-soluble guests such as alcohols and diols. Methanol takes on the role of an unconventional helper guest. This extends clathrate chemistry to a realm where neither hydrophobic guests nor high pressures are required. This also suggests that more stable lattices can be engineered for applications such as gas storage.

Disorder of Hydrofluorocarbon Molecules Entrapped in the Water Cages of Structure†I Clathrate Hydrate.

Water versus fluorine: Clathrate hydrates encaging hydrofluorocarbons as guests show both isotropic and anisotropic distributions within host water cages, depending on the number of fluorine atoms in the guest molecule; this is caused by changes in intermolecular interactions to host water molecules in the hydrates.

Methanol incorporation in clathrate hydrates and the implications for oil and gas pipeline flow assurance and icy planetary bodies.

One of the best-known uses of methanol is as antifreeze. Methanol is used in large quantities in industrial applications to prevent methane clathrate hydrate blockages from forming in oil and gas pipelines. Methanol is also assigned a major role as antifreeze in giving icy planetary bodies (e.g., Titan) a liquid subsurface ocean and/or an atmosphere containing significant quantities of methane. In this work, we reveal a previously unverified role for methanol as a guest in clathrate hydrate cages. X-ray diffraction (XRD) and NMR experiments showed that at temperatures near 273 K, methanol is incorporated in the hydrate lattice along with other guest molecules. The amount of included methanol depends on the preparative method used. For instance, single-crystal XRD shows that at low temperatures, the methanol molecules are hydrogen-bonded in 4.4% of the small cages of tetrahydrofuran cubic structure II hydrate. At higher temperatures, NMR spectroscopy reveals a number of methanol species incorporated in hydrocarbon hydrate lattices. At temperatures characteristic of icy planetary bodies, vapor deposits of methanol, water, and methane or xenon show that the presence of methanol accelerates hydrate formation on annealing and that there is unusually complex phase behavior as revealed by powder XRD and NMR spectroscopy. The presence of cubic structure I hydrate was confirmed and a unique hydrate phase was postulated to account for the data. Molecular dynamics calculations confirmed the possibility of methanol incorporation into the hydrate lattice and show that methanol can favorably replace a number of methane guests.

Phase Transition of a Structure†II Cubic Clathrate Hydrate to a Tetragonal Form.

The crystal structure and phase transition of cubic structure II (sII) binary clathrate hydrates of methane (CH4 ) and propanol are reported from powder X-ray diffraction measurements. The deformation of host water cages at the cubic-tetragonal phase transition of 2-propanol+CH4 hydrate, but not 1-propanol+CH4 hydrate, was observed below about 110 K. It is shown that the deformation of the host water cages of 2-propanol+CH4 hydrate can be explained by the restriction of the motion of 2-propanol within the 5(12) 6(4) host water cages. This result provides a low-temperature structure due to a temperature-induced symmetry-lowering transition of clathrate hydrate. This is the first example of a cubic structure of the common clathrate hydrate families at a fixed composition.

Kinetics of methane hydrate replacement with carbon dioxide and nitrogen gas mixture using in situ NMR spectroscopy.

In this study, the kinetics of methane replacement with carbon dioxide and nitrogen gas in methane gas hydrate prepared in porous silica gel matrices has been studied by in situ (1)H and (13)C NMR spectroscopy. The replacement process was monitored by in situ (1)H NMR spectra, where about 42 mol % of the methane in the hydrate cages was replaced in 65 h. Large amounts of free water were not observed during the replacement process, indicating a spontaneous replacement reaction upon exposing methane hydrate to carbon dioxide and nitrogen gas mixture. From in situ (13)C NMR spectra, we confirmed that the replacement ratio was slightly higher in small cages, but due to the composition of structure I hydrate, the amount of methane evolved from the large cages was larger than that of the small cages. Compositional analysis of vapor and hydrate phases was also carried out after the replacement reaction ceased. Notably, the composition changes in hydrate phases after the replacement reaction would be affected by the difference in the chemical potential between the vapor phase and hydrate surface rather than a pore size effect. These results suggest that the replacement technique provides methane recovery as well as stabilization of the resulting carbon dioxide hydrate phase without melting.

Why ice-binding type I antifreeze protein acts as a gas hydrate crystal inhibitor.

Antifreeze proteins (AFPs) prevent ice growth by binding to a specific ice plane. Some AFPs have been found to inhibit the formation of gas hydrates which are a serious safety and operational challenge for the oil and gas industry. Molecular dynamics simulations are used to determine the mechanism of action of the winter flounder AFP (wf-AFP) in inhibiting methane hydrate growth. The wf-AFP adsorbs onto the methane hydrate surface via cooperative binding of a set of hydrophobic methyl pendant groups to the empty half-cages at the hydrate/water interface. Each binding set is composed of the methyl side chain of threonine and two alanine residues, four and seven places further down in the sequence of the protein. Understanding the principle of action of AFPs can lead to the rational design of green hydrate inhibitor molecules with potential superior performance.

Facilitating guest transport in clathrate hydrates by tuning guest-host interactions.

The understanding and eventual control of guest molecule transport in gas hydrates is of central importance for the efficient synthesis and processing of these materials for applications in the storage, separation, and sequestration of gases and natural gas production. Previously, some links have been established between dynamics of the host water molecules and guest-host hydrogen bonding interactions, but direct observation of transport in the form of cage-to-cage guest diffusion is still lacking. Recent calculations have suggested that pairs of different guest molecules in neighboring cages can affect guest-host hydrogen bonding and, therefore, defect injection and water lattice motions. We have chosen two sets of hydrate guest pairs, tetrahydrofuran (THF)-CO2 and isobutane-CO2, that are predicted to enhance or to diminish guest-host hydrogen bonding interactions as compared to those in pure CO2 hydrate and we have studied guest dynamics in each using (13)C nuclear magnetic resonance (NMR) methods. In addition, we have obtained the crystal structure of the THF-CO2 sII hydrate using the combined single crystal X-ray diffraction and (13)C NMR powder pattern data and have performed molecular dynamics-simulation of the CO2 dynamics. The NMR powder line shape studies confirm the enhanced and delayed dynamics for the THF and isobutane containing hydrates, respectively, as compared to those in the CO2 hydrate. In addition, from line shape studies and 2D exchange spectroscopy NMR, we observe cage-to-cage exchange of CO2 molecules in the THF-CO2 hydrate, but not in the other hydrates studied. We conclude that the relatively rapid intercage guest dynamics are the result of synergistic guest A-host water-guest B interactions, thus allowing tuning of the guest transport properties in the hydrates by choice of the appropriate guest molecules. Our experimental value for inter-cage hopping is slower by a factor of 10(6) than a published calculated value.

Ammonia clathrate hydrates as new solid phases for Titan, Enceladus, and other planetary systems.

There is interest in the role of ammonia on Saturn's moons Titan and Enceladus as the presence of water, methane, and ammonia under temperature and pressure conditions of the surface and interior make these moons rich environments for the study of phases formed by these materials. Ammonia is known to form solid hemi-, mono-, and dihydrate crystal phases under conditions consistent with the surface of Titan and Enceladus, but has also been assigned a role as water-ice antifreeze and methane hydrate inhibitor which is thought to contribute to the outgassing of methane clathrate hydrates into these moons' atmospheres. Here we show, through direct synthesis from solution and vapor deposition experiments under conditions consistent with extraterrestrial planetary atmospheres, that ammonia forms clathrate hydrates and participates synergistically in clathrate hydrate formation in the presence of methane gas at low temperatures. The binary structure II tetrahydrofuran + ammonia, structure I ammonia, and binary structure I ammonia + methane clathrate hydrate phases synthesized have been characterized by X-ray diffraction, molecular dynamics simulation, and Raman spectroscopy methods.

Inter-cage dynamics in structure I, II, and H fluoromethane hydrates as studied by NMR and molecular dynamics simulations.

Prospective industrial applications of clathrate hydrates as materials for gas separation require further knowledge of cavity distortion, cavity selectivity, and defects induction by guest-host interactions. The results presented in this contribution show that under certain temperature conditions the guest combination of CH3F and a large polar molecule induces defects on the clathrate hydrate framework that allow intercage guest dynamics. (13)C NMR chemical shifts of a CH3F/CH4/TBME sH hydrate and a temperature analysis of the (2)H NMR powder lineshapes of a CD3F/THF sII and CD3F/TBME sH hydrate, displayed evidence that the populations of CH4 and CH3F in the D and D' cages were in a state of rapid exchange. A hydrogen bonding analysis using molecular dynamics simulations on the TBME/CH3F and TBME/CH4 sH hydrates showed that the presence of CH3F enhances the hydrogen bonding probability of the TBME molecule with the water molecules of the cavity. Similar results were obtained for THF/CH3F and THF/CH4 sII hydrates. The enhanced hydrogen bond formation leads to the formation of defects in the water hydrogen bonding lattice and this can enhance the migration of CH3F molecules between adjacent small cages.

Antifreezes act as catalysts for methane hydrate formation from ice.

Contrary to the thermodynamic inhibiting effect of methanol on methane hydrate formation from aqueous phases, hydrate forms quickly at high yield by exposing frozen water-methanol mixtures with methanol concentrations ranging from 0.6-10 wt% to methane gas at pressures from 125 bars at 253 K. Formation rates are some two orders of magnitude greater than those obtained for samples without methanol and conversion of ice is essentially complete. Ammonia has a similar catalytic effect when used in concentrations of 0.3-2.7 wt%. The structure I methane hydrate formed in this manner was characterized by powder X-ray diffraction and Raman spectroscopy. Steps in the possible mechanism of action of methanol were studied with molecular dynamics simulations of the Ih (0001) basal plane exposed to methanol and methane gas. Simulations show that methanol from a surface aqueous layer slowly migrates into the ice lattice. Methane gas is preferentially adsorbed into the aqueous methanol surface layer. Possible consequences of the catalytic methane hydrate formation on hydrate plug formation in gas pipelines, on large scale energy-efficient gas hydrate formation, and in planetary science are discussed.

Complex gas hydrate from the Cascadia margin.

Natural gas hydrates are a potential source of energy and may play a role in climate change and geological hazards. Most natural gas hydrate appears to be in the form of 'structure I', with methane as the trapped guest molecule, although 'structure II' hydrate has also been identified, with guest molecules such as isobutane and propane, as well as lighter hydrocarbons. A third hydrate structure, 'structure H', which is capable of trapping larger guest molecules, has been produced in the laboratory, but it has not been confirmed that it occurs in the natural environment. Here we characterize the structure, gas content and composition, and distribution of guest molecules in a complex natural hydrate sample recovered from Barkley canyon, on the northern Cascadia margin. We show that the sample contains structure H hydrate, and thus provides direct evidence for the natural occurrence of this hydrate structure. The structure H hydrate is intimately associated with structure II hydrate, and the two structures contain more than 13 different hydrocarbon guest molecules. We also demonstrate that the stability field of the complex gas hydrate lies between those of structure II and structure H hydrates, indicating that this form of hydrate is more stable than structure I and may thus potentially be found in a wider pressure-temperature regime than can methane hydrate deposits.

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.

C-Methyl-calix[4]resorcinarene-1,4-bis-(pyridin-3-yl)-2,3-diaza-1,3-butadiene (1/2).

In the title compound, 2C(12)H(10)N(4)·C(32)H(32)O(8), the calixarene adopts a rctt conformation with dihedral angles of 138.40 (1) and 9.10 (1)° between the opposite rings. The dihedral angles between the rings of the pyridine derivative are 8.80 (1) and 9.20 (1)°. In the crystal, adjacent C-methylcalix[4]resorcinarene molecules are connected into columns parallel to [010] by O-H⋯O hydrogen bonds. O-H⋯N hydrogen bonds between the axial phenoxyl groups and bipyridine molecules link the columns into sheets parallel to (011), which are connected by O-H⋯N hydrogen bonds. Further O-H⋯N hydrogen bonds link the bipyridine and C-methylcalix[4]resorcinarene molecules, giving rise to a three-dimensional network.

Comment on "Cage occupancy of methane clathrate hydrates in the ternary H2O-NH3-CH4 system" by C. Petuya, M. Choukroun, T. H. Vu, A. Desmedt, A. G. Davies, and C. Sotin, Chem. Commun., 2020, 56, 12391.

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.

Superheating of Structure I Gas Hydrates within the Structure II Cyclopentane Hydrate Shell.

The superheated state of methane (CH4) hydrate that exists under the surface ice layer can persist for considerable lengths of time, which showed promise as a method for storing and transporting natural gas. This study extends this further by coating sI CH4 hydrate with one of several sII hydrates, thus eliminating the need for a defect-free continuous interface between the sI and sII hydrates. Gas hydrate crystals were kept intact above their dissociation temperature by immersing them in liquid cyclopentane (CP), as observed with powder X-ray diffraction and X-ray CT methods. It was observed that placing the CH4 hydrate in CP converted the outer layer of CH4 hydrate to a thin layer of CP hydrate at around 270 K under atmospheric pressure, which is ∼80 K higher than the usual dissociation temperature. It was also observed that sI CO2 hydrate and C2H6 hydrate could be preserved by CP hydrate.

13C NMR studies of hydrocarbon guests in synthetic structure H gas hydrates: experiment and computation.

(13)C NMR chemical shifts were measured for pure (neat) liquids and synthetic binary hydrate samples (with methane help gas) for 2-methylbutane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane, methylcyclopentane, and methylcyclohexane and ternary structure H (sH) clathrate hydrates of n-pentane and n-hexane with methane and 2,2-dimethylbutane, all of which form sH hydrates. The (13)C chemical shifts of the guest atoms in the hydrate are different from those in the free form, with some carbon atoms shifting specifically upfield. Such changes can be attributed to conformational changes upon fitting the large guest molecules in hydrate cages and/or interactions between the guests and the water molecules of the hydrate cages. In addition, powder X-ray diffraction measurements revealed that for the hexagonal unit cell, the lattice parameter along the a-axis changes with guest hydrate former molecule size and shape (in the range of 0.1 Å) but a much smaller change in the c-axis (in the range of 0.01 Å) is observed. The (13)C NMR chemical shifts for the pure hydrocarbons and all conformers were calculated using the gauge invariant atomic orbital method at the MP2/6-311+G(2d,p) level of theory to quantify the variation of the chemical shifts with the dihedral angles of the guest molecules. Calculated and measured chemical shifts are compared to determine the relative contribution of changes in the conformation and guest-water interactions to the change in chemical shift of the guest upon clathrate hydrate formation. Understanding factors that affect experimental chemical shifts for the enclathrated hydrocarbons will help in assigning spectra for complex hydrates recovered from natural sites.

Tuning clathrate hydrates for hydrogen storage.

The storage of large quantities of hydrogen at safe pressures is a key factor in establishing a hydrogen-based economy. Previous strategies--where hydrogen has been bound chemically, adsorbed in materials with permanent void space or stored in hybrid materials that combine these elements--have problems arising from either technical considerations or materials cost. A recently reported clathrate hydrate of hydrogen exhibiting two different-sized cages does seem to meet the necessary storage requirements; however, the extreme pressures (approximately 2 kbar) required to produce the material make it impractical. The synthesis pressure can be decreased by filling the larger cavity with tetrahydrofuran (THF) to stabilize the material, but the potential storage capacity of the material is compromised with this approach. Here we report that hydrogen storage capacities in THF-containing binary-clathrate hydrates can be increased to approximately 4 wt% at modest pressures by tuning their composition to allow the hydrogen guests to enter both the larger and the smaller cages, while retaining low-pressure stability. The tuning mechanism is quite general and convenient, using water-soluble hydrate promoters and various small gaseous guests.