Large-molecule Guest Substance Research Articles

Enlargement of Structure-H Clathrate Hydrates by New Large Molecular Guest Substances and Lattice Size Limitation

The size of structure-H clathrate hydrates (sH hydrates) depends on the size of large molecule guest substances (LMGSs) that are en-captured in icosahedral (51268, E) cages. Especially, the c-axis in an sH hydrate structure is sensitive toward the longest molecular length of the LMGSs. In this study, we report four new LMGSs for structure-H hydrates containing CH4, namely, iodocyclopentane (ICP), chlorocycloheptane (ClCHP), bromocycloheptane (BrCHP), and trans-1,4-dichlorocyclohexane (t14DClCH). The longest molecular length of t14DClCH is 0.975 nm, considering its van der Waals radius. Moreover, t14DClCH is assumed to be the longest possible LMGS for sH hydrates containing CH4 because of the relationship between its lattice parameter and its longest molecular length. The phase equilibrium pressure–temperature conditions in the four CH4–LMGS–water systems were also measured by an isochoric method. The temperature shifts of the four LMGS systems from the pure CH4–water system increased in the following order: ICP ∼ BrCHP (3.7 K) < ClCHP (4.7 K) < t14DClCH (5.1 K).

Kinetic studies of structure-H hydrate using a Langmuir adsorption model: Effects of methyl cyclohexane and methyl cyclopentane as large-molecule guest substances on methane hydrate

The present study investigates the kinetics of structure-H hydrate formation in the presence of large molecule guest substances (LMGS), Methyl cyclohexane (MCH) and Methyl cyclopentane (MCP), as well as methane as a help gas. The experiments were done at the temperatures range of 274.1–277.1 K. For studying the effect of LMGS on the kinetics of the hydrate formation, the initial pressure of each experiment was considered to be higher than the equilibrium pressure of structure-I methane hydrate formation. The molar consumptions of the help gas and guest substances during hydrate formation were calculated by using an algorithm based on the Langmuir adsorption theory. It was found that first step of a two-step process of hydrate formation in the presence of MCH and MCP is more related to the structure-I methane hydrate formation, so that the transition of sI to sH occurs after that. The results showed that the molar consumption of methane in the presence of MCH is more than that of MCP, and more structure-H hydrate is formed at all operational conditions. MCH could lower the gas/water interfacial tension dramatically and also affect the rate of sI hydrate formation. However, the MCP is more effective LMGS on the rate of sH hydrate formation as having the higher equilibrium pressures at all conditions. Finally, the consumption of help gas depends on the consumption of LMGS that it causes a constant participation ratio.

Complex Phase Behaviors and Structural Coexistence of Natural Gas Hydrates Containing Large-Molecule Guest Substances

The complex phase behaviors and structural coexistence of natural gas hydrates (NGHs) that contain large-molecule guest substances (LMGSs) were examined for their significance in the exploration and exploitation of NGHs as well as natural gas storage and transportation. Methylcyclopentane (MCP) and 2,2-dimethylbutane [neohexane (NH)] were chosen as LMGSs, and the simplest composition of NGHs and natural gas was simulated by a gas mixture of CH4 (90%) + C2H6 (10%). The coexistence of structure II (sII) and structure H (sH) hydrates in the CH4 + C2H6 + LMGS + water mixtures was revealed by 13C nuclear magnetic resonance (NMR) and powder X-ray diffraction (PXRD). CH4 and LMGSs were captured in sH, whereas CH4 and C2H6 were enclathrated in sII. Endothermic heat flow curves, which were obtained by a differential scanning calorimeter (DSC), and pressure (P)–temperature (T) traces exhibited two-step dissociation of formed gas hydrates (sH dissociation followed by sII dissociation). The experimental results for the PXRD patterns, 13C NMR spectra, phase equilibria, DSC heating curves, and P–T traces clearly demonstrated that the CH4 (90%) + C2H6 (10%) + LMGS + water mixtures formed both sII hydrates and sH hydrates and that the final hydrate structure at equilibrium dissociation points was sII.

Experimental investigation of the exact role of large-molecule guest substances (LMGSs) in determining phase equilibria and structures of natural gas hydrates

This study examined the complex phase behavior and structures of natural gas hydrates containing large-molecule guest substances (LMGSs). Methylcyclopentane (MCP) and 2,2-dimethylbutane (neohexane, NH) were selected as LMGSs for CH4 (90%) + C3H8 (10%) hydrates. To verify the exact role of LMGSs, the four-phase (hydrate (H) – liquid water (LW) – liquid LMGS (LLMGS) – vapor (V)) equilibria of the CH4 + C3H8 + LMGS + water mixtures were measured. The hydrate phase equilibria demonstrated that the presence of LMGSs did not contribute to shifts in the equilibrium curves of the CH4 + C3H8 hydrates. The structures and guest distributions of the gas hydrates containing LMGSs were analyzed using nuclear magnetic resonance (NMR) and powder X-ray diffraction (PXRD). 13C NMR spectra and PXRD patterns revealed the coexistence of structure II (sII) and structure H (sH) hydrates in the CH4 + C3H8 + LMGS + water mixtures. CH4 and LMGSs were enclathrated in sH, while CH4 and C3H8 were encapsulated in sII. The experimental results covering complex phase equilibria and structural characteristics of natural gas hydrates containing LMGSs will contribute to a better understanding of the exact role of LMGSs in natural gas hydrates.

Structure H Clathrate Hydrates in Methane−Halogenic Large Molecule Substance–Water Systems

We investigated crystallographic variations in structure H (sH) hydrates hosting CH4 and halogenic large-molecule guest substances [halogenic large-molecule guest substance (LMGS)]. The three halogenic LMGSs, namely, chlorocyclohexane (ClCH), bromocyclohexane (BrCH), and iodocyclohexane (ICH), share a common molecular structure (X-cyclohexane). The lattice constants along a and c axes of sH hydrates hosting X-cyclohexane increased with increasing molecular size: ClCH < BrCH < ICH. The c lattice constant was especially dependent on the molecular size, possibly because X-cyclohexanes align along the longitudinal direction of the 51268 cage, which coincides with the c-axis. Raman spectroscopy revealed that LMGSs altered the surroundings of CH4 molecules in the 512 and 435663 cages. As the crystal size increases, CH4 molecules encounter more attractive (less repulsive) guest–host interactions. The wavenumber shifts of the C–H vibrations of CH4 in the 512 and 435663 cages increased with temperature and were slightly greater in the 435663 cages than in the 512 cages. Different thermal responses between the 512 and 435663 cages may be caused by anisotropic lattice expansion of the sH hydrates. Finally, the phase stabilities of the sH (CH4 and X-cyclohexane) hydrates were evaluated by an isochoric method. The pressure region of equilibrium pressure–temperature conditions was lower in the sH (CH4 and X-cyclohexane) hydrates than in the pure CH4 hydrate system. Moreover, the temperature region of the equilibrium pressure–temperature conditions increased in the order ICH < ClCH < BrCH. The dissociation enthalpies of the sH (CH4 + ClCH) and sH (CH4 + ICH) hydrates were estimated as 380 kJ/mol–1.

Effects of non-ionic surfactants on formation kinetics of structure H hydrate regarding transportation and storage of natural gas

Clathrate hydrates are one of the proposed mediums for transportation and storage of natural gas. Among the known hydrate structures, structure H seems the most fitting crystal due to its moderate formation pressure and high storage capacity. In the present work, the kinetics of methane hydrate formation is investigated in a high pressure batch stirred reactor with and without additives. Methylcyclohexane (MCH), methylcyclopentane (MCP) and tertbutylmethylether (TBME) were used as large molecule guest substance (LMGS). The effects of one anionic and five non-ionic surfactants on hydrate promotion at 2 °C and 55 bar pressure are experimentally investigated. These surfactants belong to the family of Nonyl Phenol Ethoxylates (NPE), Lauryl Alcohol Ethoxylates (LAE), Ethylene Oxide/Propylene Oxide copolymer (EO/PO), polyoxyethylene sorbitan mono palmitate (Tween®), Butyl Phenol ethoxylates (Triton™X100) and Sodium alkyl sulfate. The results show that while the system with MCH and MCP exhibit high induction times and low formation rates, addition of surfactants can intensely decrease the induction times and increase the formation rates. Among the studied surfactants, Triton™X100 and NPE6EO are the best kinetic promoters of structure H methane hydrate, resulting more than 500% increase in the hydrate formation rate. Also effects of non-ionic surfactants on MCP and TBME follow the same trend. Finally, systems containing two LMGSs (e.g. MCH+TBME) were tested and proved to be better candidates for storage and transportation of natural gas in sH hydrated form.

Experimental investigation on structure H hydrates formation kinetics: Effects of surfactants on interfacial tension

Among the known gas hydrate structures, structure H (sH) is the most fitting crystal for storage and transportation of natural gas due to its moderate formation pressure and high storage capacity. In the present work, the kinetics of methane (as help gas) hydrate formation is investigated in a high pressure batch stirred reactor with and without additives. Methylcyclohexane (MCH), methylcyclopentane (MCP) and tertbutylmethylether (TBME) were used as large molecule guest substance (LMGS). The effects of five surfactants on sH hydrate promotion are experimentally investigated. These surfactants belong to the family of Nonyl Phenol Ethoxylates (NPE), Lauryl Alcohol Ethoxylates (LAE), Ethylene Oxide/Propylene Oxide copolymer (EO/PO), polyoxyethylene sorbitan mono palmitate (Tween), Butyl Phenol ethoxylates (TritonX100) and Sodium alkyl sulfate. The results show that while the system with MCH and MCP exhibit high induction times and low formation rates, addition of surfactants can highly decrease the induction times and increase the formation rates. Investigation of the Liquid/Liquid (L/L) interfacial tension (IFT) of these systems in their pure state and in present of surfactants indicates that IFT plays an important role in the kinetic promotion of these systems. Studied surfactants can reduce L/L IFTs dramatically. Furthermore our results indicate that, in general, the systems with lower interfacial tension tend to have higher hydrate formation rates and lower induction times. Among the studied surfactants, TritonX100 and NPE6EO are the best kinetic promoters of structure H methane hydrate. The rate of hydrate formation in MCH system is increased more than 400% and the induction time is lowered to 0.05 its original value. Also effects of non-ionic surfactants on MCP and TBME follow the same trend.

Crystal Phase Boundaries of Structure-H (sH) Clathrate Hydrates with Rare Gas (Krypton and Xenon) and Bromide Large Molecule Guest Substances

Phase equilibrium pressure–temperature (pT) boundaries of structure-H clathrate hydrates (sH hydrates) with rare gas (Kr and Xe)-bromide large molecule guest substances (LMGSs: bromocyclohexane, BrCH and bromocyclopentane, BrCP) were measured. The phase boundaries for the sH hydrates in the Kr–LMGS–water systems shifted to lower pressures than those for the pure Kr hydrate in the temperature range of (273.2 to 279.3) K. In this study, sH hydrate formation was not confirmed in the Xe–BrCP–water system, but sH hydrates were found in the Xe–BrCH–water system. At temperatures below 277 K, equilibrium conditions were observed at lower pressures for the Xe–BrCH–water system than for the pure Xe hydrate. However, the equilibrium pT curve for the Xe–BrCH–water system crossed over the equilibrium pT curve for the Xe hydrate at around 277 K. Intersections between the equilibrium pT curves for the Xe hydrates and the sH hydrates (Xe + LMGS) have also been found in Xe–methylcyclohexane–water systems. Using the Kr–and Xe–bromide LMGS–water systems showed that the sH hydrate phase stabilities are strongly related to the encaptured LMGS.

Structure H (sH) Clathrate Hydrate with New Large Molecule Guest Substances

This study characterized new structure H (sH) clathrate hydrates with bromide large-molecule guest substances (LMGSs) bromocyclopentane (BrCP) and bromocyclohexane (BrCH), using powder X-ray diffraction (PXRD) and Raman spectroscopy. The lattice parameters of sH hydrates with (CH4 + BrCP) and (CH4 + BrCH) were determined from their PXRD profiles. On the basis of their Raman spectra, the M-cage to S-cage occupancy ratio (435663 and 512 cages, respectively), θM/θS, was estimated to be approximately 1.3, and the Raman shift of the symmetric C–H vibrational modes of CH4 in S- and M-cages was 2911.1 and 2909.1 cm–1, respectively. The phase-equilibrium conditions of sH hydrates with (CH4 + BrCP) and (CH4 + BrCH) were determined by an isochoric method. A comparison between the equilibria of sH hydrates with BrCP and BrCH and those with other typical nonpolar and polar LMGSs (methylcyclopentane, MCP; methylcyclohexane, MCH; neohexane, NH; and tert-butyl methyl ether, TBME) at the same temperature revealed that th...

Effect of Guest Size and Conformation on Crystal Structure and Stability of Structure H Clathrate Hydrates: Experimental and Molecular Dynamics Simulation Studies

To better understand the effect of size and flexibility of large molecule guest substances (LMGSs) on crystal lattice structure and thermodynamic stability of structure H (sH) clathrate hydrates, we performed powder X-ray diffraction (PXRD) measurements and Parrinello–Rahman molecular dynamics (MD) simulations on six alkane and cycloalkane LMGS in the presence of methane help gas. We first quantitatively analyze the dependence of the experimental lattice constants and formation pressure on the average maximum length of the LMGS guests as determined by MD simulation. The PXRD results show that within a family of LMGSs with similar molecular structures molecules of optimum size give better stability of sH hydrate phase. The most stable hydrate in each class has larger lattice constants along the a-axis and smaller lattice constant along the c-axis. The lattice expansion/shrinkage is well reproduced in the MD simulations. From the MD simulations, we determine the changes in the conformation as a result of en...

Thermodynamic Stability of Structure H Hydrates Based on the Molecular Properties of Large Guest Molecules

This paper report analyses of thermodynamic stability of structure-H clathrate hydrates formed with methane and large guest molecules in terms of their gas phase molecular sizes and molar masses for the selection of a large guest molecule providing better hydrate stability. We investigated the correlation among the gas phase molecular sizes, the molar masses of large molecule guest substances, and the equilibrium pressures. The results suggest that there exists a molecular-size value for the best stability. Also, at a given molecule size, better stability may be available when the large molecule guest substance has a larger molar mass.

Molecular dynamics simulation of structure H clathrate-hydrates of binary guest molecules

Molecular dynamics (MD) simulations are performed to study the stability of structure H clathrate-hydrates of methane+large-molecule guest substance (LMGS) at temperatures of 270, 273, 278 and 280 K under canonical (NVT-) ensemble condition in a 3×3×3 structure H unit cell replica with 918 TIP4P water molecules. The studied LMGS are 2-methylbutane (2-MB), 2,3-dimethylbutane (2,3-DMB), neohexane (NH), methylcyclohexane (MCH), adamantane and tert-butyl methyl ether (TBME). In the process of MD simulation, achieving equilibrium of the studied system is recognized by stability in calculated pressure for NVT-ensemble. So, for the accuracy of MD simulations, the obtained pressures are compared with the experimental phase diagrams. Therefore, the obtained equilibrium pressures by MD simulations are presented for studying the structure H clathrate-hydrates. The results show that the calculated temperature and pressure conditions by MD simulations are consistent with the experimental phase diagrams. Also, the radial distribution functions (RDFs) of host-host, host-guest and guest-guest molecules are used to analysis the characteristic configurations of the structure H clathrate-hydrate.

Inhibition of Natural Gas Hydrates in the Presence of Liquid Hydrocarbons Forming Structure H

The effects of LMGS (large molecule guest substance) amount on the thermodynamics of natural gas hydrates, as well as structural characteristics of mixed hydrates of LMGS and natural gas, have been studied. The addition of 1.7 wt % neohexane (NH) to water induced inhibition of natural gas hydrates, and this inhibition effect increased with increased addition of NH up to 7.8 wt %. However, the hydrate equilibrium condition changed slightly when the concentration of NH further increased from 7.8 to 14.5 wt %. Investigations on structural characteristics were carried out by analyzing (13)C NMR spectra of mixed hydrates formed from the mixture of natural gas and NH. They indicate that two hydrate structures of II and H coexist simultaneously, and the ratio of structure H to II decreased from 0.97 to 0.43 when the NH concentration decreased from 14.5 to 7.8 wt %. In addition, it was confirmed that ethane, propane, and iso-butane gas molecules do not participate in the formation of structure H and only enclathrated in large cages of structure II. These results indicate the existence of multiple hydrate structures, which must be considered in many industrial applications when mixed hydrates are formed from multicomponent gas mixtures and liquid hydrocarbons.

Interactions between Structure H Hydrate Formers and Water Molecules

Studies on methane storage in hexagonal structure H (sH) hydrates reveal that hydrate stability conditions, formation rates, and cage occupancies differ for the three large molecule guest substances (LMGSs) studied: tert-butyl methyl ether (TBME), neo-hexane (NH), and methyl-cyclohexane (MCH). In this work, the structures of aqueous solutions with the LMGS are examined with molecular dynamics to gain insights into the interactions between water and LMGS. Water molecules form solvation cavities that follow the contours of the LMGS molecules and have diameters between 4 and 7 Å. The cavity size increases from TBME < NH < MCH, and the nearest water molecules to the LMGS have a local density that is higher by a factor of 2 than that of the bulk water phase. Water molecules interact strongly with the ether oxygen of TBME but not with other atoms in the LMGS molecules. This strong attraction gives TBME a greater solubility in water and better wetting on ice than NH or MCH. Moreover, TBME also has a larger diffusivity than NH in water. The greater contact between TBME and the host water molecules may cause the faster rate of initial hydrate growth from solid ice particles (fixed bed) or liquid water with mixing for this guest species. However, the strong attraction of the TBME molecule with water may also slow down the subsequent crystal growth and may perhaps limit the occupancy of the other guests (methane) in the sH clathrate. The volume change due to solvation is negative for all three LMGSs. This suggests a more compact hydrogen bond network in the hydration shell that may facilitate the formation of clathrates when a hydrate seed is available and when mass and heat transfer resistances are negligible. For example, this may occur during hydrate formation from a fixed ice bed with temperature ramping. Hence, the clathrate growth with hydrophobic guests (NH and MCH) proceeds rapidly toward completion as the temperature is increased above the ice melting point. On the other hand, the formation of hydrate cages for the TBME system is disrupted because of the strong water−solute interactions. Consequently, hydrate formation from ice with thermal ramping with TBME is slower.

Molecular dynamics study of structure H clathrate hydrates of methane and large guest molecules

Methane storage in structure H (sH) clathrate hydrates is attractive due to the relatively higher stability of sH as compared to structure I methane hydrate. The additional stability is gained without losing a significant amount of gas storage density as happens in the case of structure II (sII) methane clathrate. Our previous work has showed that the selection of a specific large molecule guest substance (LMGS) as the sH hydrate former is critical in obtaining the optimum conditions for crystallization kinetics, hydrate stability, and methane content. In this work, molecular dynamics simulations are employed to provide further insight regarding the dependence of methane occupancy on the type of the LMGS and pressure. Moreover, the preference of methane molecules to occupy the small (5(12)) or medium (4(3)5(6)6(3)) cages and the minimum cage occupancy required to maintain sH clathrate mechanical stability are examined. We found that thermodynamically, methane occupancy depends on pressure but not on the nature of the LMGS. The experimentally observed differences in methane occupancy for different LMGS may be attributed to the differences in crystallization kinetics and/or the nonequilibrium conditions during the formation. It is also predicted that full methane occupancies in both small and medium clathrate cages are preferred at higher pressures but these cages are not fully occupied at lower pressures. It was found that both small and medium cages are equally favored for occupancy by methane guests and at the same methane content, the system suffers a free energy penalty if only one type of cage is occupied. The simulations confirm the instability of the hydrate when the small and medium cages are empty. Hydrate decomposition was observed when less than 40% of the small and medium cages are occupied.

Methane conversion rate into structure H hydrate crystals from ice

AbstractThe methane uptake and conversion rate to structure H (sH) hydrates was measured and compared to crystallization kinetics models. Three large molecule guest substances (LMGS) were used as sH hydrate formers: neohexane (NH), methylcyclohexane (MCH), and tert‐butyl methyl ether ( TBME). The initial crystallization occurred quickly at the LMGS liquid‐ice interface until ∼20–30% of ice was converted into hydrate (hydrate growth stage I). Slower hydrate crystal growth was observed after a hydrate film covered the ice surface at a rate of 3–400 nm2/h (hydrate growth stage II). The TBME system showed the fastest kinetics at the beginning of the reaction followed by NH and MCH system. However the trend changed when the temperature was increased (“reaction” stage III). Surprisingly, the conversion rate achieved with the TBME system upon melting the ice was the smallest. This was attributed to the strong interaction of TBME with water molecules that increased the energy barrier for water molecules to form hydrate cages. The conversion rates were well correlated with the Avrami equation and the shrinking core model. Finally, NH was found to be the best LMGS in this study to obtain full conversion within a short reaction time and achieving high methane gas storage in the hydrate. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Characterization of gas hydrates with PXRD, DSC, NMR, and Raman spectroscopy

Structure I (sI) and H (sH) hydrates containing methane were synthesized and characterized with PXRD, DSC, NMR, and Raman spectroscopy. Three well-known large molecule guest substances (LMGSs) were selected as sH hydrate formers: 2,2-dimetylbutane (NH), methylcyclohexane (MCH), and tert-butyl methyl ether (TBME). The solid phase analysis confirmed the presence of sH hydrate whenever a LMGS was present. The presence of a non-hydrate former (n-heptane) did not affect the methane hydrate structure or cage occupancies. Ice to hydrate conversion was limited when the LMGS amount was less than stoichiometric and synthesized at low methane pressure, but nearly complete conversion was achieved with temperature ramping and excess LMGS. The methane occupancies were found to depend on the type of LMGS and increased with pressure. The hydrate with TBME was found to have the smallest methane content followed by the hydrates with NH and MCH. Both NMR and Raman identified methane and LMGS signals from the hydrate phase, however, the cage occupancy values of sH hydrate can only be obtained from NMR spectroscopy. The hydrate structures, ice to hydrate conversion, gas content in hydrate and cage occupancy from the various measurements are consistent with each other.

Predicting Thermodynamic Stability of Clathrate Hydrates Based on Molecular-Dynamics Simulations and Its Confirmation by Phase-Equilibrium Measurements

In order to establish a method to predict the formation of new clathrate hydrates under milder temperature-pressure conditions, we performed a molecular-dynamics (MD)-based free energy calculation, and then hydrate phase equilibrium measurements were done to confirm the prediction. The free energy differences are calculated for the structure-H hydrates each formed with methane and each of the following large-molecule guest substances: 2-methylbutane, 2,3-dimethylbutane, 2,2-dimethylbutane, 2,2,3-trimethybutane, and 2,2,3,3-tetramethylbutane. MD simulations were performed under a constant pressure and temperature with 6120 TIP4P water molecules, 900 OPLS-UA methane molecules, and a multisite modeled 180 OPLS-UA LMGS molecule. The results of the free energy calculation indicated the structure-H hydrate formed with 2,2,3,3-tetramethylbutane is the most stable hydrate although the relevant equilibrium pressure data were not previously reported in the literature. The experimental phase equilibrium measurement...

Hydrate Kinetics Study in the Presence of Nonaqueous Liquid by Nuclear Magnetic Resonance Spectroscopy and Imaging

The dynamics of methane hydrate growth and decomposition were studied by nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI). Three well-known large molecule guest substances (LMGS) were used as structure H hydrate formers: 2,2-dimethylbutane (NH), methylcyclohexane (MCH), tert-butyl methyl ether (TBME). In addition, the impact of a non-hydrate former (n-heptane/nC7) was studied. The methane diffusion and hydrate growth were monitored by recording the 2H NMR spectra at 253 K and approximately 4.5 MPa for 20 h. The results revealed that methane diffuses faster in TBME and NH, slower in nC7, and slowest in MCH. The TBME system gives the fastest hydrate formation kinetics followed by NH, MCH, and nC7. The conversion of water into hydrate was also observed. The imaging study showed that TBME has a strong affinity toward ice, which is not the case for the NH and MCH systems. The degree of ice packing was also found to affect the LMGS distribution between ice particles. Highly packed ice increases the mass transfer resistance and hence limits the contact between LMGS and ice. It was also found that "temperature ramping" above the ice point improves the conversion significantly. Finally, hydrates were found to dissociate quickly within the first hour at atmospheric pressure and subsequently at a much slower rate. Methane dissolved in LMGS was also seen. The residual methane in hydrate phase and dissolved in LMGS phase explain the faster kinetics during hydrate re-formation.

Crystal Lattice Size and Stability of Type H Clathrate Hydrates with Various Large-Molecule Guest Substances

To gain a better understanding of the effects of guest molecules on the lattice and stability of type H hydrates, we performed powder X-ray diffraction (PXRD) measurements and semiempirical molecular orbital calculations. The unit cell parameters and cohesive energies of various type H hydrates that contain methane (CH4) were analyzed. PXRD measurements indicated that an increase in the large-molecule guest volume caused the unit cell volume to increase. It was also indicated that a large-molecule guest substance caused the a-axis-direction of the unit cell to increase with little decrease in the c-axis direction. Calculations of cohesive energy by means of a semiempirical molecular orbital method indicated that the functional group and configuration of large-molecule guest substances affects the stability of type H hydrates. It was concluded that the icosahedron (5(12)6(8)) cages do not easily increase in length along the c-axis direction when larger guest molecules are used to form the hydrate, but the 5(12)6(8) cage and the layer of dodecahedron (5(12)) cages can easily increase in length along the a-axis direction due to interactions of the guest-host molecule.