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

Kyuchul Shin, Konstantin A. Udachin, Igor L. Moudrakovski, Donald M. Leek, Saman Alavi, Christopher I. Ratcliffe, and John A. Ripmeester
PNAS May 21, 2013 110 (21) 8437-8442; https://doi.org/10.1073/pnas.1302812110

  1. Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved April 15, 2013 (received for review February 14, 2013)

Abstract

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.

Methanol is the quintessential antifreeze. It works by altering thermodynamic conditions of aqueous solution to suppress or delay the formation of icy phases when the temperature is reduced to below the normal freezing point of water. The action in aqueous solution is readily understood by considering strong hydrogen-bonding interactions between water and methanol that lower the chemical potential of the aqueous phase, leading to strongly nonideal solution behavior (1). The low-temperature phase diagram of water-methanol is well known (2), with a single solid 1:1 methanol hydrate compound identified (3) in addition to the pure solid phases. The eutectic in the water-methanol system occurs at rather low temperatures (between 155 and 160 K), which has led to speculations that liquid aqueous oceans may exist on icy planetary bodies, such as Titan and Enceladus (2⇓⇓–5). For the formation of clathrate hydrates, which are ice-like host lattices composed of water molecules so as to form cages that encapsulate guest molecules, the behavior of methanol as an antifreeze has always been assumed to parallel that in water-methanol solutions. Even after some 70 y of applied research of hydrate inhibition by methanol as part of gas-pipeline flow assurance programs in the oil and gas industry (6) and, more recently, applied research of methanol as a material of interest in the extraterrestrial sciences, there has been little or no direct evidence of methanol participation in clathrate formation. This includes a number of deliberate attempts to include methanol in a clathrate, in which the evidence was either indefinite (7⇓⇓⇓–11) or the interpretation of results proved to be in error as shown by subsequent work (12). However, methanol is a molecule of the correct size and shape such that it could occupy one or more cavity types in the hydrate lattices. Our recent work on the participation of ammonia in clathrate formation (13) suggests that methanol could well be a coguest in a clathrate hydrate lattice, and that it could also modify hydrate formation processes at low temperatures.

Methanol has been detected in star-forming regions (14, 15), on comets (16), and in the cryovolcanic gas plumes of Enceladus (5). Models predict that about 5% (vol/vol) methanol in Titan's primordial oceans (and/or similar amounts of ammonia) could maintain a subsurface aqueous ocean (2). The fact that in the presence of ice phases, methanol, as well as ammonia, may be incorporated in the hydrate phases of methane and other hydrocarbons can seriously affect the equilibrium models used to describe the icy planets’ compositions and dynamics.

To address the behavior and role of methanol in these systems, we performed experiments and molecular simulations to determine the nature of the participation of methanol in clathrate hydrate structures and processes.

Results

To test whether tetrahydrofuran (THF) hydrate, a material often used as a model hydrate system, might host methanol, a THF/H2O/CH3OH solution mole ratio of 1:17:2.4 was cooled to −40 °C/233 K. Crystals formed and were grown from the solutions for a few days, and those suitable for diffraction were mounted on a low-temperature X-ray diffractometer. Single-crystal structural data were recorded at 100 K (additional details are provided in Materials and Methods). The structure found is the usual cubic structure II (sII) clathrate hydrate, space group Fd-3m, with a unit cell edge of 1.71237(3) nm. This lattice constant is smaller than the range of values between 1.7166 and 1.7197 nm measured for the pure THF sII clathrate hydrate prepared under similar conditions (17, 18). The THF molecules occupy three symmetry-inequivalent positions in the large cages, and methanol was found in the sII small cages (Fig. 1), with a total methanol cage occupancy of 4.4%. This gives a THF/H2O/CH3OH solution mole ratio of 1:17:0.09 in the clathrate hydrate phase. The position of the methanol O atom overlaps the position of a water O atom of the small cage framework (nominally, the two sites are 0.183 nm apart). Thus, when methanol is present, this water O atom is absent and the methanol O atom hydrogen-bonds to the three adjacent framework O atoms. In effect, the methanol OH is incorporated into the hydrogen-bonded water framework with the methyl group sitting toward the center of the small cage. Thus, hydrogen bonding allows a close approach of the methanol molecule to the cage wall and the incorporation of the relatively large methanol molecule into the small cages of the clathrate hydrate structure.

Fig. 1.
Fig. 1.

(A) Single-crystal X-ray structure of the sII THF + methanol clathrate hydrate. Methanol occupies 4.4% of the small cages. (B) Alternative view of methanol inside the small cage from the X-ray structure. When methanol is present in the cage, the framework oxygen site 1.83 Å away from the methanol oxygen is vacant. (C) Snapshot of methanol in a small cage from the molecular dynamics simulation in which the oxygen atom of methanol hydrogen bonds with two water molecules of the clathrate hydrate lattice framework.

Powder X-ray diffraction (PXRD) of the solid formed by quenching a THF/H2O/CH3OH solution mole ratio of 1:17:2 to 183 K is shown in Fig. S1A and Table S1. Analysis of the PXRD pattern (Materials and Methods) gave a much higher methanol occupancy of 49% in the small sII cages compared with the single-crystal sample. The measured lattice constant of 1.72482(5) nm from the PXRD sample was larger than the 1.72231(7)-nm value for pure THF hydrate formed under similar conditions in the presence of air (shown in Fig. S1B and Table S1). The high percentage of methanol incorporation in the small cages reflects the method of preparation, yielding samples with a highly nonequilibrium composition compared with the single-crystal result in this case. Below, we also present NMR evidence for the migration of methanol from an initially formed nonequilibrium clathrate hydrate phase.

To study the THF and methanol hydrate formation from the solution phase in more detail, solutions of THF/D2O/CH3OH with mole ratios of 1:17:1 and 1:17:0.1 were placed in NMR tubes, degassed, sealed, and quickly cooled to form hydrate phases (temperature range of 243–223 K). Spectra were taken during cooling to temperatures of 223 K (blue lines in Fig. S2 and spectra at 10 °C and −50 °C in Fig. S3). The ice/hydrate sample was allowed to equilibrate for 30 min and then heated back up to the liquid phase at 283 K. To follow the hydrate decomposition process, 1H spectra of the sample were taken at different temperatures (red lines in Fig. S2 and spectra at −30 °C and 10 °C in Fig. S3). By comparing the integrated 1H peaks for the up-field THF and methanol methyl group in the sample cell at different temperatures in the hydrate stability region and after hydrate decomposition, the percentages of incorporation of THF and methanol from the solution in the hydrate phase were determined. Under these conditions, the methanol occupancies of the small cages of the hydrate formed from the liquid mixtures with 1- and 0.1-mol fractions of methanol were measured to be between 3.8% and 6.5%, respectively. A higher percentage of the available methanol entered the clathrate hydrate cages in the 1:17:0.1 mixture than in the more concentrated 1:17:1 mixture. The results indicate that rapid freezing of a subcooled solution traps a relatively large quantity of methanol in the hydrate, some of which is again released on annealing during warming from −50 to −30 °C, a process accompanied by additional incorporation of THF into the hydrate and decrease of the THF peak in the liquid phase.

We next characterized methanol incorporation into clathrate hydrate phases with simple hydrocarbon guests at relatively high temperatures because such processes may occur during hydrate formation in gas and oil pipelines. Hydrates were prepared by quenching an aqueous solution containing 2.5% (vol/vol) methanol (enriched to 11% in 13C) at 77 K, grinding the solid to a fine powder at 77 K, and exposing the powder to 2.0 MPa of methane or 0.5 MPa of propane gas for about a week at 253 K. The 13C high-power decoupling (HPDEC), magic-angle spinning (MAS), and cross-polarization (CP) NMR spectra of the resulting solid phase for methane and propane as the hydrocarbon guests are shown in Fig. 2, and chemical shifts are given in Table 1 (further details are provided in Figs. S4–S6). The peaks in the ∼50-ppm region of the spectrum are assigned to methanol. Because the spectra were obtained both with and without CP, spectral lines have vastly different intensities in the spectra obtained using the different methods. This reflects the fact that species with widely varying dipolar couplings to protons are present. There are two peaks (in the range of 49.2–49.5 ppm) that cross-polarize rather slowly in all samples, and we associate these with highly dynamic methanol adsorbed on the ice or hydrate surface in two distinguishable sites. The peaks cross-polarizing rather rapidly are then assigned to more confined methanol in the hydrate lattice.

Fig. 2.
Fig. 2.

13C HPDEC MAS (A) and 13C CP MAS (B) 3-kHz NMR of system of ice + methanol exposed to methane at 20-bar pressure at 193 K. The two peaks in the ∼50-ppm region in the MAS spectrum indicate the incorporation of methanol in the sI clathrate hydrate solid phase. 13C HPDEC MAS (C) and 13C CP MAS (D) 3-kHz NMR spectrum of the binary C3H8 − 13CH3OH sII clathrate hydrate at T = 193 K with number of scans (NS) = 128 and contact time (tcont) = 4 ms. The splitting of the propane peaks can be related to sII large cages with and without methanol in their neighboring small cages.

View this table:
Table 1.

13C NMR shifts of frozen methanol/water, methane/methanol hydrate, and propane/methanol hydrate

The 13C methane resonances in the mixed hydrate in Fig. 2 A and B occur at their usual positions for structure I (sI) hydrate. An interesting feature of the propane hydrate 13C NMR spectrum is the weak splitting of both propane peaks of the sII clathrate hydrate (Fig. 2 C and D). This means that in the binary hydrate, there are two slightly different 51264 cage environments for propane, likely due to the fact that the large cages are distorted because the cage wall is shared with a small cage in which methanol is hydrogen-bonded into the water lattice, as shown, for example, in Fig. 1. This is also consistent with the presence of the low-field methanol signal observed at 48.76 ppm; because of rather restricted dynamics, this methanol signal cross-polarizes easily, and the low-field shift is consistent with the presence of a hydrogen-bonded species. The high field signals (49.93 and 50.61 ppm) for both hydrates therefore can be assigned to methanol trapped in cages with little or no hydrogen bonding or with rapidly forming and breaking hydrogen bonding; however, the NMR peaks cross-polarize quite well because the methanol molecules are still trapped in their cages and have significant nuclear dipolar couplings to nearby protons. Note that the low-field signal observed for methanol in propane sII hydrate is absent for methane hydrate, and that the methane signals are not split. This suggests that methanol does not hydrogen-bond into the sI lattice as it does into sII at these rather high temperatures of study. There is a marked difference in the symmetry of the sI and sII small cages (m3 and −3m, respectively), and this may lead to the difference in behavior of methanol in these cages. Methanol hydrogen bonding in the sI hydrates is discussed further in the section on molecular simulation.

Aqueous methanol solutions cannot be frozen to yield a clathrate hydrate, and unlike the case of ammonia (13), a pure methanol hydrate has not been obtained by vapor codepositing and annealing methanol-water mixtures at low temperatures. Of course, these facts are the basis for the use of methanol as an ice antifreeze and clathrate hydrate formation inhibitor. Therefore, we investigated the formation of methanol-including clathrates at low temperatures by using methane as a coguest.

An amorphous solid material was synthesized by the codeposition of water, methanol, and methane at 16 K. The details of the synthesis and characterization methods are presented in Materials and Methods. At 170 K, the PXRD patterns of the resulting solid phases prepared with a 5–20% (vol/vol) initial mass percentage of methanol show the signature diffraction lines of the cubic sI hydrate, hexagonal ice, methanol monohydrate, and a number of reflections that were not so easily assigned. A number of hydrate structure patterns, including those of sII, hexagonal structure H (sH), and hexagonal structure I (HS-1), were considered in obtaining satisfactory fits to the data. Tetragonal structure IV for argon and tetragonal structure III bromine hydrate did not fit the XPRD data. On considering the NMR evidence (see below), we present a likely possibility of the structure. Fig. 3A and Fig. S7 give a description of the temperature-dependent phase transformations.

Fig. 3.
Fig. 3.

(A) PXRD patterns of (methanol solution + methane) codeposited samples recorded at 170 K (160 K for 15 wt% sample). Vertical thin lines indicate the peaks from the P-6m2 mixed-layer structure. The red dots indicate the peaks from the Pm-3n sI hydrate. Lattice parameters are given in Table S3. The top two patterns were magnified (2×) to show the weak reflections better. (B) HPDEC 13C NMR spectra of (methanol solution + methane) codeposited samples recorded at 170 K and 3 kHz spinning.

The solid-state 13C NMR spectra of vapor-deposited samples, given in Fig. 3B and Fig. S8, show peaks for the encapsulated methane guests in three regions at −4, −6, and −8 ppm. As verified by NMR measurements of 13C spectra of methane in hydrate phases, extrapolation from 129Xe NMR shifts (Table S2), these peaks can be assigned to methane in the 512, 51262, 435663, and 51264 cages, which occur in a number of actual sI and hypothetical methane hydrate structures. HS-1, considered in the PXRD fits of Fig. 3A, was eliminated because there was no resonance that corresponds to the expected −7.5-ppm shift of the 51263 cage in the 13C NMR spectrum. As noted above and seen in Fig. 3A, clear evidence of sII and sH clathrates is absent from the PXRD patterns. However, a number of actual hydrate structures may be present, including cubic sII, hexagonal sII, and another known structure that can be derived by the stacking of layers of pentagonal dodecahedra with various stacking sequences (19) (Fig. S9). Hexagonal sII is a hypothetical example of such a polytype. We now propose a unique polytype that can account for the methane 13C NMR resonances, a hexagonal P6-m2 structure that has 512, 435663, 51264, and 51268 cages in this case (Fig. S9 and Table S4). In addition to explaining the NMR results, the inclusion of the P6-m2 diffraction pattern into the PXRD fitting procedure gave satisfactory results (Fig. 3A). We cannot specify the guest for the large 51268 cage, which is present both in sH and in the newly described P6-m2 layered hydrate structure. As shown above, methanol can hydrogen-bond into the hydrate lattice as well as participate in hydrate formation as a guest, yet our experiments are not able to locate the methanol molecules explicitly because the 13C methanol peak at ∼50 ppm is rather broad for the annealed codeposits. It is obvious that methanol is essential for the formation of the hydrate phases formed in the temperature region below 150 K. Pure methane-water codeposits do not form any distinguishable clathrate hydrate phases (12) under these conditions. Of course, other possibilities exist, including less ordered structures involving layer stacking.

Further evidence for the incorporation of methanol in hydrate formation comes from 129Xe NMR of two samples prepared at low temperatures from 15 wt% methanol solution (Fig. 4). In Fig. 4A, a sample codeposited from the vapors at ∼15 K and then warmed to 77 K gave a broad 129Xe spectrum similar to that obtained previously for a pure Xe/H2O codeposit (20⇓⇓–23). This broad spectrum is indicative of an intimate amorphous mixture. However, the material also shows a significant side peak in the shift range characteristic of Xe in a small cage-sized cavity, which only appears in the spectra of the pure Xe/H2O codeposit after annealing at considerably higher temperatures (147–157 K) (20). After annealing the new sample at 157 K (Fig. 4B), the spectrum has separated into two peaks corresponding to Xe in the sI small (S) and large (L) cages in the ratio S/L = 1:2.5, but with spectral widths broader than for crystalline Xe hydrate. In Fig. 4C, a sample produced at 77 K by freezing the 15 wt% methanol solution, adding Xe, and then storing for several days shows lines for the sI S and L cages, which are already much sharper and in the ratio of 1:2. It is significant that in both samples, the S/L ratio is greater than the ∼1:4 ratio usually seen for pure Xe hydrate, indicating reduced Xe occupancy of the large cage. These results show that the methanol can enhance hydrate-forming rearrangement processes at significantly lower temperatures than in pure Xe/ice mixtures, and that the methanol is replacing Xe in some of the large cages of sI.

Fig. 4.
Fig. 4.

The 129Xe CP/static spectra at 77 K of the amorphous material formed from Xe/15 wt% methanol solution vapor codeposited at 15 K (A), the same material after annealing at 157 K for 15 min (B), and the material formed at 77 K from Xe and a frozen solution of 15 wt% methanol (C).

Molecular dynamics simulations of (a) the binary sII clathrate hydrate of THF (in large cages) + CH3OH (in 49% of small cages), (b) the pure CH3OH sI clathrate hydrate, and (c) binary CH3OH (large cages) + CH4 (small cages) sI clathrate hydrates were carried out for temperatures in the range of 100–240 K. The simulation methodology is described in detail in SI Materials and Methods. In the binary sII THF(L) + CH3OH(S) and binary sI CH3OH(L) + CH4(S) clathrate hydrates, methanol shows strong hydrogen bonding with the small and large cage water molecules. Sample snapshots of methanol in the small and large cages are shown in Figs. 1C and 5D. The radial distribution function (RDF) plots that characterize hydrogen bonding for the CH3OH(L) + CH4(S) clathrate hydrate are shown in Fig. 5. Hydrogen bonding from both the oxygen (Fig. 5B) and the hydrogen (Fig. 5C) of the alcohol OH group are seen as peaks in the O···H RDF plot at distances less than 0.2 nm. The snapshot shows that different methanol-water hydrogen-bonding configurations are observed with methanol molecules, primarily hydrogen bonding with the waters of the hexagonal face of large sI cages. The hydrogen bonding introduces many Bjerrum L-defects in the clathrate hydrate lattice. Methanol also forms hydrogen bonds in the small cages of the binary sII clathrate hydrate with THF. Interestingly, methanol also affects the hydrogen-bonding tendencies of the THF guest with water molecules in the large cages (24). In the simulations at 240 K, the structure of the binary sI clathrate hydrate of methanol and methane shows sign of decomposition.

Fig. 5.
Fig. 5.

RDF for the methanol oxygen atom with the water oxygen atom (A), the methanol oxygen atom with the water hydrogen atom (B), the methanol hydroxyl hydrogen atom with the water oxygen atom for an sI clathrate hydrate with methanol in large cages and methane in small cages (C), and snapshots of methanol in adjacent large sI clathrate hydrate cages (D). The distortions caused by methanol in the cages will affect the environment of the methane guests in the neighboring cages. The panel on the right shows a snapshot of the three sI methanol clathrate hydrate large cages from the simulation showing different hydrogen bonded configurations of methanol with the cage water molecules. g, radial distribution function; HO, methanol hydroxyl group hydrogen; HW, water hydrogen; OH, methanol oxygen; OW, water oxygen; r, distance.

To study the relative thermodynamic stability of methanol and methane in the large cages of the sI clathrate hydrate, we calculate the free energy of the hypothetical reaction,Embedded Image

The free energy per CH4 guest substitution in the large cages at 200 K with a CH3OH guest is calculated to be −3.7 kJ⋅mol−1. This somewhat surprising result shows the CH3OH guests are actually more stable in the sI clathrate hydrate large cages than the CH4 guests. The free energy penalty associated with removal of the polar CH3OH molecules from the aqueous solution phase opposes this substitution and makes the reaction unfavorable from the liquid phase.

Discussion

Our experiments and computations highlight the dual role of methanol in clathrate hydrate formation. In addition to its traditional role as a thermodynamic inhibitor of hydrate formation from aqueous phases, we observe that methanol can be included in the hydrate in amounts that depend on the details of preparative methods. Single-crystal X-ray diffraction (XRD) of THF/methanol hydrate shows that methanol is incorporated in 4.4% of the small cages in a slowly grown single crystal and that methanol is located in the small sII cage with its OH group hydrogen-bonded into the hydrate framework.

It has been argued (8, 9), based on the van der Waals radii, that methanol will not fit into the small 512 cages of the sI or sII phase. However, this argument did not consider the possibility of hydrogen bonding of methanol with the cage water molecules. Indeed, as we have shown here, methanol is strongly incorporated into the hydrogen-bonding network of the hydrate; however, up to temperatures near 240 K, the hydrate lattice tolerates the defects caused by incorporation of small amounts of methanol.

We also showed that methanol can be incorporated into hydrates that are formed from solid ice and hydrocarbon guests. At the low temperatures in question, methanol must be largely present as an aqueous surface film, but this does not necessarily inhibit hydrate formation from ice. This observation can have repercussions on the use of methanol as a clathrate hydrate inhibitor at low temperatures. If pipelines pass through conditions where moisture may freeze on the walls of the pipes, the presence of methanol in the gas stream may actually catalyze the formation of solid clathrate hydrate phases. This can occur in gas pipelines in polar climates and in extraction of methane gas from clathrate hydrate deposits in permafrost layers.

The kind of molecular-scale information we present is essential for the development of knowledge-based models that can be used to provide more realistic predictions of hydrate formation in the presence of methanol inhibitors. Models routinely used to predict the effectiveness of hydrocarbon hydrate inhibition all assume that methanol is not incorporated in the hydrate lattice. Of course, the fact that methanol can be incorporated in the hydrate phases will affect the interpretation of experimental results, because the solid phases are no longer pure hydrocarbon hydrates as they are in the absence of methanol inhibitor.

Although it is known that pure methanol clathrate hydrates have not yet been prepared, methanol can accelerate hydrate formation in solid phases formed by codepositing water, methanol, and methane at 20 K and annealing this mixture above ∼120 K. Clearly, methanol plays an important role, because methane-water codeposits only form sI hydrate at much higher temperatures (>160 K) on the same time scale. Much more complex phase formation occurs in the methanol-water-methane codeposits. From the PXRD and NMR experiments on annealed codeposits, sI hydrate can easily be identified. To account for the other NMR signals and PXRD reflections, we have postulated a layer-based hydrate structure that agrees reasonably well with the data. The exact location of methanol in these low-temperature phases could not be determined, nor could the presence of less ordered layered hydrate phases be excluded.

The dual promoter/inhibitor roles of methanol may be relevant in assigning functions to methanol in planetary environments, such as Saturn’s icy moons Titan and Enceladus. Methanol can act as antifreeze, which contributes to inhibiting the freezing of Titan’s primordial subsurface aqueous ocean. On the other hand, in the presence of ice phases, methanol can accelerate the capture of methane (and other small hydrocarbons) into clathrate hydrate structures, potentially modifying Titan’s atmosphere.

Methane gas is released through vents and cryovolcanoes to the surface of Enceladus. Methanol may be formed photochemically from methane from the moon’s interior or be brought to the surface from interior sources (5). The present work shows that despite the relatively low local methane pressure at the plumes, the surface methanol may catalyze the formation of local methane hydrate deposits at the surface of Enceladus. This could account for the relatively high concentrations of surface methane and other hydrocarbons on the moon surface near the plume vents, despite predictions that most hydrocarbons would have been lost long ago from the atmosphere through escape. These observations are also relevant to the formation of hydrates under vapor deposition conditions on icy planets, comets, and planetary and other extraterrestrial bodies, where methanol can catalyze the formation of mixtures of nonstoichiometric gas hydrates, which are then trapped in icy grains and accreted into planetary objects.

Polar water-miscible molecules, such as methanol and ammonia, are traditionally considered to be hydrate formation inhibitors. The present work shows that the inhibition effects of the polar molecule are not due to its destabilizing the solid clathrate hydrate framework structure but rather to the stabilizing effect of these guests on the aqueous solution.

Materials and Methods

Sample Preparation.

Vapor codepositions of water-methanol solution (5, 10, 15, and 20 mass % mixtures) and methane were performed in an evacuated chamber at ∼0.01 mbar. The solution was vaporized from a pipette into the evacuation chamber by low pressure and deposited on a copper plate, which was cooled down to ∼16 K by a DISPLEX cryocompressor (Air Products). In each experiment, the methane gas, initially at a pressure of 700 mmHg in a 1-L bulb, was slowly injected into the deposition chamber. The solution and guest gas vapor flows were controlled by means of a leak valve, and after 24 h, ∼2 g of solution and ∼0.6 g of guest vapors were deposited on the copper plate. The copper plate covered with the product deposit was removed from the evacuation chamber and quickly immersed in liquid nitrogen with minimal exposure to air. Significant moisture condensation on the sample during the short transfer period is not expected. For the various characterization experiments, the materials were transferred to the PXRD and NMR instruments.

In the single-crystal experiments, a THF + CH3OH binary sII hydrate was prepared from a solution of THF/H2O/CH3OH at a mole ratio of 1:17:2.4. On cooling the solution to 233 K, masses of crystals appeared. Crystals suitable for diffraction were identified by examination under a microscope mounted in a cold box. The structure obtained from the single-crystal diffraction experiment was used to determine that 4.4% of the small cages were occupied by methanol, which gives an approximate clathrate hydrate composition of 1:17:0.09 for THF/H2O/CH3OH. Different methanol small cage occupancies are likely to be found starting from different initial compositions. Different methanol small cage occupancies will be possible from different starting conditions.

X-Ray Data Collection and Structure Solution.

Single-crystal XRD data were measured on a Bruker Apex 2 Kappa diffractometer at 100 K, using graphite monochromatized Mo Kα radiation (λ = 0.71073 Å). The unit cell was determined from randomly selected reflections obtained using the Apex 2 automatic search, center, index, and least-squares routines. Integration was carried out using the program SAINT (Bruker), and an absorption correction was performed using SADABS (25). The crystal structures were solved by direct methods, and the structure was refined by full-matrix least-squares routines using the SHELXTL program suite (26). All atoms were refined anisotropically. Hydrogen atoms on guest molecules were placed in calculated positions and allowed to ride on the parent atoms. This structure is deposited at the Cambridge Crystal Database under accession no. 922778.

PXRD.

The PXRD pattern was recorded on a Bruker AXS model D8 Advance diffractometer using Cu Kα radiation [λ1 = 1.5406 Å, λ2 = 1.5444 Å, and intensity ratio (I2/I1) = 0.5]. The samples stored in liquid nitrogen were quickly transferred to the X-ray stage cooled down to ∼100 K in air, and diffraction patterns were measured with increasing temperature. The experiments were carried out in the step mode with a fixed time of 1 s and a step size of 0.0196551° for 2θ = 8–42° at each temperature for vapor codeposition samples and a fixed time of 2 s and a step size of 0.0196551° for 2θ = 8–100° at 183 K for the THF hydrates, which had been quenched in liquid nitrogen. The powder patterns for the codeposited samples were refined by the Le Bail method using the profile matching method within FULLPROF (27). The structural analysis of the THF hydrates followed method used in the previously reported work by Takeya et al. (28). The direct space method using the FOX (free objects for crystallography) program was used to determine initial THF and methanol positions in the cages (29, 30). With these guest coordinates, the PXRD patterns were refined by the Rietveld method using FULLPROF.

NMR Spectroscopy.

Methane-methanol and propane-methanol hydrate 13C NMR.

The 13C NMR spectra were collected on a Bruker AVANCE-III 400-MHz instrument (magnetic field = 9.4 T, Larmor frequency of 13C = 100.67 MHz). The experiments were performed with a Bruker BL7 MAS double-resonance probe. The spinning speed and the temperature inside the probe were controlled using standard Bruker equipment. Hydrate samples were packed in 7-mm zirconia rotors in liquid nitrogen, and the measurements were performed at 170 K and 3-kHz spinning speed unless indicated otherwise. Both Bloch decay (single-pulse excitation) and CP experiments were performed. Bloch decay experiments used a π/2 pulse of 5 μs and composite pulse proton high power decoupling (HPDEC). The pulse repetition delay in the HPDEC experiments was set at 40 s, which was found to be sufficient for complete relaxation of all carbon resonances for the hydrates studied. The CP experiments were performed with a ramped spin-lock and accompanied composite pulse proton decoupling. The high frequency signal of adamantine was used as a secondary chemical shift reference set to 38.56 ppm at 298 K.

Methanol-THF hydrate 1H NMR.

For the 1H NMR of a liquid sample, a solution of 17:1:1 D2O/methanol/THF was prepared and then degassed and sealed in the NMR tube. Spectra were recorded using a Bruker AV-III 400 NMR spectrometer. A 1H NMR spectrum was recorded at +10 °C and then at −30, −40, and −50 °C. The sample was allowed to sit at −50 °C for an additional 30 min, and spectra were then recorded as the sample was warmed. The methanol methyl signal and the up-field methylene signal of THF were integrated. The methanol integrals were multiplied by 4/3 so that integral ratios on the graph (Fig. S2) would represent molar ratios.

Methanol-methane vapor codeposits.

The codeposited samples were cold-loaded into a zirconia rotor with a 7-mm outer diameter and then loaded into a variable temperature probe in a Bruker AVANCE 400 MHz instrument. The HPDEC and CP 13C NMR spectra were recorded with MAS at ∼3 kHz, and the signal of adamantine, assigned a chemical shift of 38.56 ppm relative to tetramethylsilane, was used as an external chemical shift reference. The pulse repetition delay time for HPDEC spectra was 40 s, and the contact time for CP spectra was 2 ms. The experiments were performed at 170 K.

Methanol/Xe codeposits.

The 129Xe solid-state CP/static NMR spectra were obtained at 77 K on a Bruker AMX 300 instrument (82.98 MHz) using a 5-mm probe from Morris Instruments. Acquisitions used CP times of 5 ms. Chemical shifts were referenced using the shift of solid Xe at 317 ppm (77 K) relative to Xe gas at zero pressure.

Acknowledgments

We thank G. L. McLaurin and S. Lang for technical support. We also thank the National Research Council of Canada for financial support.

Footnotes

  • ↵1Present address: Division of Ocean Systems Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea.

  • ↵2To whom correspondence should be addressed. E-mail: john.ripmeester{at}nrc-cnrc.gc.ca.

  • Author contributions: K.S. and J.A.R. designed research; K.S., K.A.U., I.L.M., D.M.L., S.A., and J.A.R. performed research; K.S., K.A.U., I.L.M., D.M.L., S.A., C.I.R., and J.A.R. analyzed data; and S.A., C.I.R., and J.A.R. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • Data deposition: Crystallography, atomic coordinates, and structure factors have been deposited in the Cambridge Crystal Database (accession no. CCDC 922778).

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1302812110/-/DCSupplemental.

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Methanol incorporation in clathrate hydrates
Kyuchul Shin, Konstantin A. Udachin, Igor L. Moudrakovski, Donald M. Leek, Saman Alavi, Christopher I. Ratcliffe, John A. Ripmeester
Proceedings of the National Academy of Sciences May 2013, 110 (21) 8437-8442; DOI: 10.1073/pnas.1302812110
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