Spectroscopic Identification of the Mixed Hydrogen and Carbon Dioxide Clathrate Hydrate

In this contribution, we first found the novel clathrate hydrate containing two gaseous guests of hydrogen and carbon dioxide by spectroscopic analysis. X-ray powder diffraction and NMR spectroscopy were used to identify structure and guest distribution of the mixed H2 + CO2 hydrate. X-ray diffraction result confirmed that the unit cell parameter was 11.8602 ± 0.0010 Å, and the formed hydrate was identified as structure I hydrate. 1H magic angle spinning (MAS) NMR and 13C cross-polarization (CP) NMR spectroscopy were used to examine the distribution of hydrogen and carbon dioxide molecules in the cages of structure I, respectively. These NMR spectra showed that carbon dioxide molecules occupied both small 512 cages and large 51262 cages, and hydrogen molecules only were occluded in small 512 cages of structure I. The new finding of the mixed hydrogen hydrate is expected to contribute toward the development of hydrogen production technology and, particularly, inclusion chemistry

Spectroscopic Identification of Amyl Alcohol Hydrates through Free OH Observation

In this study, we identify the crystal structures of amyl alcohol + CH4 hydrates and demonstrate that the free OH observation of alcohol hydrates provides evidence of OH incorporation into the host framework occurring in some amyl alcohols. While two amyl alcohols, 3-methyl-2-butanol and 2-methyl-2-butanol, were identified as encaged in the 51268 large cage of structure-H hydrate, as expected from their molecular sizes above 7.5 Å, two other amyl alcohols, 3-methyl-1-butanol and 2,2-dimethyl-1-propanol, were identified to be abnormally included in the 51264 large cage of structure-II hydrate in spite of their too large sizes of 9.04 and 7.76 Å, respectively. The Raman spectra of two “normal” amyl alcohol hydrates evolved free OH peaks around 3600 cm−1, implying that there is no strong hydrogen bonding interaction between alcohol guest and water host; however, for two “abnormal” amyl alcohol hydrates, the corresponding peaks were not detected, which indicates that the OH is incorporated into the host lattice in order to make the large alcohol guest fit into the relatively small 51264 cage of structure-II. The present findings are expected to provide useful information for a better understanding of alcohol guest dynamic behavior that might be significantly affected by structural dimensions and host−guest interactions

Structure Transition and Swapping Pattern of Clathrate Hydrates Driven by External Guest Molecules

We first report here that under strong surrounding gas of external CH4 guest molecules the sII and sH methane hydrates are structurally transformed to the crystalline framework of sI, leading to a favorable change of the lattice dimension of the host−guest networks. The high power decoupling 13C NMR and Raman spectroscopies were used to identify structure transitions of the mixed CH4 + C2H6 hydrates (sII) and hydrocarbons (methylcyclohexane, isopentane) + CH4 hydrates (sH). The present findings might be expected to provide rational evidences regarding the preponderant occurrence of naturally occurring sI methane hydrates in marine sediments. More importantly, we note that the unique and cage-specific swapping pattern of multiguests is expected to provide a new insight for better understanding the inclusion phenomena of clathrate materials

Thermodynamic and Spectroscopic Identification of Methane Inclusion in the Binary Heterocyclic Compound Hydrates

Two kinds of heterocyclic organic compounds, a five-membered ring with the chemical formula of C4H4NH (pyrrole, PRL) and a six-membered ring with the chemical formula of C5H5N (pyridine, PRD), are introduced into clathrate hydrate structures as a coguest with methane (CH4) gas. The results from powder X-ray diffraction of the binary (pyrrole + CH4) and (pyridine + CH4) clathrate hydrates showed that the lattice size of the former is smaller than that of the latter but the crystal structure of both binary hydrates is identified as a cubic Fd3m structure II hydrate. Raman spectroscopy also provided the clear evidence of CH4 and each aromatic ring compound occupying in the small and large cavities of structure II hydrates. The thermodynamic behaviors of the two binary systems were compared with those of the pure methane system to identify the role of the two aromatic compounds in the clathrate hydrate system by using high-pressure micro-differential scanning microcalorimetry. It is striking that the two heterocyclic compounds containing a part of hydrate inhibiting functional groups have a promoting effect on the hydrate formation of the methane–water system

Size-Selective Synthesis of Gold and Platinum Nanoparticles Using Novel Thiol-Functionalized Ionic Liquids

One-phase synthesis of gold and platinum nanoparticles using new thiol-functionalized ionic liquids (TFILs) is described for the first time. TFILs as stabilizing agents for gold and platinum nanoparticles were designed to have thiol groups on either the cation or anion and symmetrical or unsymmetrical positions only in the cation. Transmission electron microscopy, electron diffraction, and NMR were used for the characterization of nanoparticles. The metal nanoparticles formed using TFILs are crystalline structures with face-centered cubic packing arrangements and have small sizes (the average diameters are 3.5, 3.1, and 2.0 nm for Au and 3.2, 2.2, and 2.0 nm for Pt, respectively) and uniform distributions (the standard deviations are 0.7, 0.5, and 0.1 nm for Au and 1.1, 0.2, and 0.1 for Pt, respectively). It is believed that the nanoparticle size and distribution depend on the number and position of thiol groups in the IL

Temperature-Dependent Structural Transitions in Methane–Ethane Mixed Gas Hydrates

A thermodynamic interpretation of the interconversion between structures I and II occurring in methane (CH₄) + ethane (C₂H₆) mixed gas hydrates is of great importance from both fundamental and applied perspectives. The present study experimentally confirms the predicted temperature dependence of structural changes in the lower transition region (72–74 mol % of CH₄ balanced with C₂H₆) of the CH₄ + C₂H₆ + H₂O system. The measurements of phase equilibria and Raman spectra, at the macroscopic and microscopic levels, respectively, reveal the phase transition point at which the structural rearrangements occur. The isothermal data reported here clearly demonstrate significant changes of transition behavior from sII inhibition to sII promotion in accordance with increased equilibrium temperatures. This solid–solid transition trend may be dictated by the peculiar structural feature of the CH₄ + C₂H₆ mixed gas hydrates on the basis of the comprehensive experimental and theoretical data published previously. The predominance of CH₄ over C₂H₆ in cage occupancy may lead to a change in guest molecules playing a dominant role in determining the preferential hydrate structure

Experimental Verification of Methane–Carbon Dioxide Replacement in Natural Gas Hydrates Using a Differential Scanning Calorimeter

The methane (CH4) – carbon dioxide (CO2) swapping phenomenon in naturally occurring gas hydrates is regarded as an attractive method of CO2 sequestration and CH4 recovery. In this study, a high pressure microdifferential scanning calorimeter (HP μ-DSC) was used to monitor and quantify the CH4 – CO2 replacement in the gas hydrate structure. The HP μ-DSC provided reliable measurements of the hydrate dissociation equilibrium and hydrate heat of dissociation for the pure and mixed gas hydrates. The hydrate dissociation equilibrium data obtained from the endothermic thermograms of the replaced gas hydrates indicate that at least 60% of CH4 is recoverable after reaction with CO2, which is consistent with the result obtained via direct dissociation of the replaced gas hydrates. The heat of dissociation values of the CH4 + CO2 hydrates were between that of the pure CH4 hydrate and that of the pure CO2 hydrate, and the values increased as the CO2 compositions in the hydrate phase increased. By monitoring the heat flows from the HP μ-DSC, it was found that the noticeable dissociation or formation of a gas hydrate was not detected during the CH4 – CO2 replacement process, which indicates that a substantial portion of CH4 hydrate does not dissociate into liquid water or ice and then forms the CH4 + CO2 hydrate. This study provides the first experimental evidence using a DSC to reveal that the conversion of the CH4 hydrate to the CH4 + CO2 hydrate occurs without significant hydrate dissociation

Phase Equilibria of CO₂ and CH₄ Hydrates in Intergranular Meso/Macro Pores of MIL-53 Metal Organic Framework

The formation of gas hydrates in porous media is expected to bring out beneficial properties for gas storage and separation. Appropriate combined use of both gas hydrate and highly porous metal organic frameworks (MOFs) can be useful for achieving advances in the field of gas storage and separation. This makes understanding the behavior of gas hydrates in the confining pores of MOF crucial. The formation and phase equilibria of CO₂ and CH₄ hydrates in MOF were investigated using MIL-53 MOF through low-temperature synchrotron high-resolution powder diffraction (HRPD) and <i>P</i>–<i>T</i> traces. MIL-53 has both intrinsic micropores and intergranular meso/macropores. Gas hydrate forms in meso/macropores, and its thermodynamic behavior is relatively inhibited compared to its behavior in bulk phase due to reduced water activity. However, a strong CO₂ dissolution appeared instead of gas hydrates in the intrinsic micropores of MIL-53. This led to a notable phenomenon in which the cooling and heating lines in the <i>P</i>–<i>T</i> trace curves of CO₂ hydrate did not intersect near the dissociation point of CO₂ hydrate

Thermal Expansivity of Tetrahydrofuran Clathrate Hydrate with Diatomic Guest Molecules

The guest dynamics and thermal behavior occurring in the cages of clathrate hydrates appear to be too complex to be clearly understood through various structural and spectroscopic approaches, even for the well-known structures of sI, sII, and sH. Neutron diffraction studies have recently been carried out to clarify the special role of guests in expanding the host water lattices and have contributed to revealing the influence factors on thermal expansivity. Through this letter we attempt to address three noteworthy features occurring in guest inclusion: (1) the effect of guest dimension on host water lattice expansion; (2) the effect of thermal history on host water lattice expansion; and (3) the effect of coherent/incoherent scattering cross sections on guest thermal patterns. The diatomic guests of H2, D2, N2, and O2 have been selected for study, and their size and mass dependence on the degree of lattice expansion have been examined, and four sII clathrate hydrates with tetrahydrofuran (THF) have been synthesized in order to determine their neutron powder diffraction patterns. After thermal cycling, the THF + H2 clathrate hydrate is observed to exhibit an irreversible plastic deformation-like pattern, implying that the expanded lattices fail to recover the original state by contraction. The host-water cage dimension after degassing the guest molecules remains as it was expanded, and thus host−guest as well as guest−guest interactions will be altered if guest uptake reoccurs

Structural Transformation due to Co-Host Inclusion in Ionic Clathrate Hydrates

Structural Transformation due to Co-Host Inclusion in Ionic Clathrate Hydrate