The group II salts of 12-tungstophophoric acid were studied in the limiting state of hydration using a suite of H-1 solid-state NMR experiments, theoretical analysis, and density functional theory (DFT) energy calculations. Various nonspinning (NS) H-1 NS NMR, magic-angle spinning (MAS) H-1 MAS NMR, and rotational-echo double-resonance (REDOR) H-1{P-31} REDOR NMR experiments were performed. Chemical shift and spinning sideband patterns and comparison of H-1 MAS two-pulse sequence NMR results with predictions of average Hamiltonian theoretical calculations definitively show that the limiting hydrated form of the group II salts of 12-tungstophosphoric acid contain only H+-protons and H2O-protons. Density matrix methods were used to derive expressions for the H-1 NMR signal for a two-pulse NMR experiment as a function of the second pulse length under both NS and MAS conditions for Htprotons and H2O-protons. NMR structural parameters were obtained from the fit of theoretical expressions to the H-1 NS two-pulse sequence NMR data where it was found that all H2O molecules have an interproton distance of 167 pm and the hydrogen atoms have a chemical shift asymmetry parameters of 0.5-1.0. DFT energy calculations consistent with the H-1{P-31} REDOR NMR results were performed to determine the most stable Htproton and H2O-proton structures in Keggin anion dimers (KA(2)(-6)). For the MgHPW, SrHPW, and BaHPW salts, the H+-protons are found to be in static, surface sites with multiple hydrogen-bonding interactions with oxygen atoms of the 1165 pm KA(2)(-6) dimer in the monoclinic unit cell. Each hydrogen atom of the rotating H2O molecules of MgHPW and BaHPW has multiple hydrogen bonding interactions with one KA(-3) of the 1400 pm KA(2)(-6) dimer, whereas each hydrogen atom of the static H2O molecules of SrHPW has multiple hydrogen-bonding interactions with one KA(-3) of the 1165 pm KA(2)(-6) dimer. Similar analysis for CaHPW could not be performed probably because the H+-protons and H2O-protons are very mobile in this salt and no H-1{3113} REDOR dephasing was observed. For the BeHPW salt, the Htproton and H2O-proton resonance lines are not resolved but analysis of the H-1 NS two-pulse sequence NMR data shows that there is one H+-tproton for each H2O molecule in this salt.

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionCorrection to "Clathrate Hydrate Formation: Dependence on Aqueous Hydration Number"Steven F. Dec*Cite this: J. Phys. Chem. C 2012, 116, 10, 6504Publication Date (Web):March 1, 2012Publication History Published online1 March 2012Published inissue 15 March 2012https://pubs.acs.org/doi/10.1021/jp301398fhttps://doi.org/10.1021/jp301398fcorrectionACS PublicationsCopyright © 2012 American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views304Altmetric-Citations3LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (220 KB) Get e-Alertsclose Get e-Alerts

The reaction of methane, ethane, and water to form structure I (sI) and structure II (sII) clathrate hydrates at 268 K was studied using solid-state C-13 NMR The reaction can be described in terms of four stages: stage 1, the simultaneous formation of methane-ethane sI and sII clathrate hydrates; stage 2, cessation Of sI hydrate formation and enhanced sII hydrate formation; stage 3, decomposition of sI hydrate and continued enhanced sII hydrate formation; stage 4, approach to a final steady state of the system. The rates of formation and decomposition of the various methane and ethane sites obtained from analysis of the time-resolved C-13 MAS NMR intensity data provide strong evidence that the sI <-> sII transformation occurs at surfaces and is dominated by two simultaneous reactions; methane-ethane sI hydrate decomposes to gas phase methane, gas phase ethane, and water while forming methane-ethane sII hydrate from gas phase methane, gas phase ethane, and water. The results of a 2D exchange C-13 MAS NMR experiment are consistent with these concurrent methane-ethane sI hydrate decomposition and methane-ethane sII hydrate formation surface reactions. The 2D exchange C-13 MAS NMR spectrum also provides conclusive evidence that methane-ethane sI and sII hydrates are not intimately mixed.

The low-frequency electrical properties of mixtures of silicates and saline H(2)O were measured over broad ranges of temperature and frequency to assess the subfreezing interactions between these materials synoptically, particularly the effects of adsorbed, unfrozen water. Adsorbed water content was determined using nuclear magnetic resonance. Materials were chosen to control effects of grain size and mineralogical complexity, and the initial salt content was also specified. The temperature-dependent DC conductivity of a sand-salt-H(2)O mixture was found to be described well by Archie's law, with either brine or salt hydrate (above and below the eutectic, respectively) as the conductive and partially saturating phase. For materials with pore sizes less than a few micrometers, the brine/hydrate channels become disconnected, and the DC conductivity is controlled by the surrounding ice. High DC conductivity in a montmorillonite-H(2)O mixture is attributed to proton mobility in interlayer adsorbed water. The ice content of the sand mixture was recovered from the static dielectric permittivity using a power-law mixing model. Ice relaxation frequencies were higher than those observed in defect-saturated saline ice, indicating that additional defects are able to form in proximity to silicate surfaces. Five dielectric relaxations related to H(2)O were identified: two orientation polarizations (ice and adsorbed water), two Maxwell-Wagner interfacial polarizations (because of the conductivity differences between hydrate and silicate and adsorbed water and ice, respectively), and a low-frequency dispersion, probably caused by charge hopping. Thicknesses of a few H(2)O monolayers and the preference of hydronium for surface sites, making adsorbed water slightly acidic, favor protons as the mobile charges responsible for these adsorbed-water interfacial polarizations.

We are developing new proton exchange membranes for hotter and drier operating conditions in fuel cells. The materials we are developing are based on the interaction between the heteropolyacids (HPAs) and the proton donating groups in polymers. One set of materials are composite membranes taking advantage of the HPAs and perfluorosulfonic acid (PFSA) ionomers. These composite membranes have superior proton conductivity compared to the native ionomer under hotter and drier conditions and additionally appear to be more durable under the harsh oxidizing environment of the PEM fuel cell. In another set of materials HPAs are functionalized with monomers which are co-polymerized with monomers that donate protons and add structural features to the hybrid films. While we are yet to fabricate a "polyPOM" with superior proton conductivity these materials can be made to conduct protons as well as standard PFSAs without the need for sulfonic acids.

The interconversion of methane-ethane hydrate from metastable to stable structures was studied using Raman spectroscopy. sI and sII hydrates were synthesized from methane-ethane gas mixtures of 65% or 93% methane in ethane and water, both with and without the kinetic hydrate inhibitor, poly(N-vinylcaprolactam). The observed faster structural conversion rate in the higher methane concentration atmosphere can be explained in terms of the differences in driving force (difference in chemical potential of water in sI and sII hydrates) and kinetics (mass transfer of gas and water rearrangement). The kinetic hydrate inhibitor increased the conversion rate at 65% methane in ethane (sI is thermodynamically stable) but retards the rate at 93% methane in ethane (sII is thermodynamically stable), implying there is a complex interaction between the polymer, water, and hydrate guests at crystal surfaces.

Highly conducive to high conductivity: Polyoxometalates were incorporated in the backbone of a hydrocarbon polymer to produce proton-conducting films. These first-generation materials contain large, dispersed clusters of polyoxometalates. Although the morphology of these films is not yet optimal, they already demonstrate practical proton conductivities and proton diffusion within the clusters appears to be very high.

The formation of methane-ethane (C1-C2) clathrate hydrate was studied with high-resolution, solid-state (13)C NMR and density functional theory techniques. The (13)C NMR experiments yield a number of significant findings: (1) the hydration number of C2(aq) is 26, (2) the initial quantity of C2-5(12)6(2) sI hydrate cages outnumber C1-5(12) cages at 274 K, (3) C1-C2 sII hydrate forms at a C1-C2 gas phase composition where only sI hydrate is thermodynamically stable, (4) the initial composition of C1-C2 sII hydrate at 268 K contains less than the original amount of C 1, (5) a quasi-liquid water layer solvating both C I and C2 exists at 268 K, (6) any C1(qll) and C2(qll) present at 253 K is too small to be detected, (7) the initial amounts of C1-C2 sI and sII hydrates formed at 253 K are much smaller than those formed at 268 and 274 K, and (8) Cl(aq), C2(aq) and C1(qll), C2(qll) facilitate the formation of C1-C2 sI and sII clathrate hydrate at 268 and 274 K, respectively. On the basis of these experimental observations, a model is developed that states that the aqueous hydration number of the most water-soluble clathrate hydrate former controls the structure of the clathrate hydrate that forms during the initial stages of the clathrate hydrate formation reaction. For methane-ethane clathrate hydrate, this means that ethane in a water liquid phase or quasi-liquid layer eliminates or adds two water molecules to its hydration shell to form the ethane-filled 5(12)6(2) or 5(12)6(4) cage building blocks of structure I or structure II clathrate hydrate, respectively. Density functional theory computations on methane-filled 5(12), ;5(12)6(2), and 5(12)6(4) and ethane-filled 5(12)6(2), 5(12)6(3), and 5(12)6(4) clathrate hydrate cages yield the stabilization energy of the gas-filled cages and provide theoretical evidence consistent with the experimentally based clathrate hydrate formation model. The proposed model is found to explain the results of other clathrate hydrate formation reactions.

We have fabricated proton conducting films using monomers based on vinyl substituted silicotungstic acid heteropoly acids (HPAs) and acrylate co-monomers. In this work we probe the limits of this system based on increasing the weight loading of the HPA to 85 wt%. Although impressive proton conductivities can be achieved with these films under hotter and drier operating conditions than in conventional proton exchange membranes, the materials have mechanical limitations. We show that very different film morphologies can be prepared based on whether or not the film is polymerized thermally or by UV light. In general the UV cured films have superior proton conductivity, but have a linear morphology resulting in a brittle film. The thermally cured films have a clustered morphology with good mechanical attributes but have poor proton conductivity.

The reformation of methane-ethane hydrate was observed in situ using 13 C MAS NMR spectroscopy. In all reformation experiments, structure I (stable state for the reformation conditions) reformed, and the hydrate cage occupancy ratios were found to be almost the same as those predicted by a statistical thermodynamics program CSMGem, suggesting that there is no preferential formation of large or small cages on the relatively long time scale of this NMR experiment. It was also found that the reformation rate of the sample with PVCap is several times faster compared with the pure system, indicating that the presence of PVCap promotes the hydrate reformation at a high subcooling though this chemical is well-known as a good hydrate inhibitor.

The dissociation behavior of the CH4+CO2 binary gas hydrate has been investigated using Nuclear Magnetic Resonance (NMR) spectroscopy. This technique allows us to distinguish the hydrate structure present, as well as to quantify phase concentrations. Single-pulse excitation was used in combination with magic-angle spinning (MAS). Time-resolved in situ decomposition experiments were carried out at different compositions in sealed, pressurized samples. The decomposition profiles of the CH4+CO2 binary gas hydrate system obtained at various compositions suggest that the decomposition rate is a strong function of the fractional cage occupancy and temperature. An unexpected CH4 hydrate reformation was observed during our decomposition experiments when the temperature reached the ice melting point. A decrease on the CO2 content in the hydrate phase was found during the decomposition experiment, as the pressure and temperature of the system increases.

The heteropoly acids (HPAs) are a class of inorganic oxides that have some of the highest solid state proton conductivities known at room temperature. At elevated temperatures proton diffusion coefficients in these crystalline materials increase, but, not all the protons are mobile resulting in a decrease of the observed proton conductivity. The proton conductivity in these materials is heavily dependant on the hydration state of the HPA. In order to develop new PEMs based on the HPAs for elevated temperature, dry operation, it will be necessary to both immobilize the HPA and ensure that all of the protons are mobile all of the time. If both these objectives can be obtained a new class of PEMs will be developed that could facilitate the use of PEM fuel cells in automotive applications. We have created a new class of PEM by polymerizing HPA monomers with appropriate co-monomers to produce films with unique proton conducting properties.