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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.
Proceedings of the National Academy of Sciences 05/2013; · 9.68 Impact Factor
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ABSTRACT: The formation of hydrates from a methane-ethane-propane mixture is more complex than with single gases. Using nuclear magnetic resonance (NMR) and high-pressure powder X-ray diffraction (PXRD), we have investigated the structural properties of natural gas hydrates crystallized in the presence of kinetic hydrate inhibitors (KHIs), two commercial inhibitors and two biological ice inhibitors, or antifreeze proteins (AFPs). NMR analyses indicated that hydrate cage occupancy was at near saturation for controls and most inhibitor types. Some exceptions were found in systems containing a new commercial KHI (HIW85281) and a recombinant plant AFP, suggesting that these two inhibitors could impact the kinetics of cavity formation. NMR analysis confirmed that the hydrate composition varies during crystal growth by kinetic effects. Strikingly, the coexistence of both structures I (sI) and II (sII) were observed in NMR spectra and PXRD profiles. It is suggested that sI phases may form more readily from liquid water. Real time PXRD monitoring showed that sI hydrates were less stable than sII crystals, and there was a conversion to the stable phase over time. Both commercial KHIs and AFPs had an impact on hydrate metastability, but transient sI PXRD intensity profiles indicated significantly different modes of interaction with the various inhibitors and the natural gas hydrate system.
The Journal of Physical Chemistry A 12/2011; 116(5):1337-43. · 2.95 Impact Factor
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ABSTRACT: The formation of methane hydrate in an unconsolidated bed of silica sand was investigated and spatially resolved by employing the magnetic resonance imaging technique. Different sand particle size ranges (210–297, 125–210, 88–177, and <75 μm) and different initial water saturations (100, 75, 50, and 25%) were used. It was observed that hydrate formation in such porous media is not uniform, and nucleation of hydrate crystals occurs at different times and different positions inside the bed. Also, hydrate formation was found to be faster in a bed with lower water content and smaller particle size. Decomposition of hydrate by thermal stimulation at constant volume showed that the dissociation front moves radially inward starting from the external surface of the hydrate formation vessel.
06/2011;
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ABSTRACT: We have prepared molecularly imprinted mesoporous organosilica (MIMO) using a semicovalent imprinting technique. A thermally reversible covalent bond was used to link a bisphenol A (BPA) imprint molecule to a functional alkoxysilane monomer at two points to generate a covalently bound imprint precursor. This precursor was incorporated into a cross-linked periodic mesoporous silica matrix via a typical acid-catalyzed, triblock copolymer-templated, sol-gel synthesis. Evidence of imprint sites buried in the pore walls was found through careful characterization of the imprinted material and its comparison to similarly prepared non-imprinted mesoporous organosilica (NIMO) and pure periodic mesoporous silica (PMS). After thermal treatment, the imprinted material (MIMO-ir) removed more than 90% of appropriately sized bisphenol species from water, yet showed significantly lower binding for both smaller and larger molecules containing phenol moieties. Identically treated NIMO-ir showed much poorer retention behavior than MIMO-ir for the same bisphenol species and behaved only slightly better than PMS-ir.
ACS Nano 02/2011; 5(3):2277-87. · 10.77 Impact Factor
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ABSTRACT: (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.
The Journal of Physical Chemistry A 02/2011; 115(9):1650-7. · 2.95 Impact Factor
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ABSTRACT: Preliminary DFT investigations into the feasibility of using (33)S solid-state NMR to study organic and biological molecules suggest that very large (33)S quadrupolar coupling constants (>40MHz) are not uncommon. We have therefore investigated the possibility of using recently developed ultra-wideline techniques to record such (33)S powder patterns at a high magnetic field (21.1T). A WURST-echo sequence was used to record the spectrum from a>99.9% enriched sample of elemental sulfur, resulting in the largest (33)S quadrupolar coupling constant yet measured by solid-state NMR (C(Q)=43.3 MHz). Implications of this experiment are briefly discussed.
Journal of Magnetic Resonance 12/2010; 207(2):345-7. · 2.14 Impact Factor
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ABSTRACT: A new phosphonate metal-organic framework (MOF) with a layered motif but not that of the classical hybrid inorganic-organic solid is presented. Zn(3)(L)(H(2)O)(2)·2H(2)O (L = [1,3,5-benzenetriphosphonate](6-)), henceforth denoted as PCMOF-3, contains a polar interlayer lined with Zn-ligated water molecules and phosphonate oxygen atoms. These groups serve to anchor free water molecules into ordered chains, as observed by X-ray crystallography. The potential for proton conduction via the well-defined interlayer was studied by (2)H solid-state NMR spectroscopy and AC impedance spectroscopy. The proton conductivity in H(2) was measured as 3.5 × 10(-5) S cm(-1) at 25 °C and 98% relative humidity. More interestingly, an Arrhenius plot gave a low activation energy of 0.17 eV for proton transfer, corroborating the solid-state NMR data that showed exchange between all deuterium sites in the D(2)O analogue of PCMOF-3, even at -20 °C.
Journal of the American Chemical Society 10/2010; 132(40):14055-7. · 9.91 Impact Factor
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ABSTRACT: Among a variety of cyclic ether, cyclic ester, and cyclic ketone compounds, six new formers were found to form binary sII or sH hydrates with CH(4) gas. Hydrate-phase equilibria for all the hydrate formers were measured. The results obtained showed distinct relationships between the hydrate-phase equilibrium curve and the molecular size of the guests. In addition, 2-methyltetrahydrofuran and 3-methyltetrahydrofuran, or 4-methyl-1,3-dioxane and 4-methyl-1,3-dioxolane, showed different hydrate structures even though they have similar chemical structures. Such structural differences can provide useful information on the critical guest size, which determines hydrate crystal structures according to the size of the captured guest.
The Journal of Physical Chemistry B 10/2010; 114(42):13393-8. · 3.70 Impact Factor
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ABSTRACT: Synchrotron powder X-ray diffraction, ab initio molecular dynamics calculations and solid state (1)H and (2)H NMR are used to refine the structure of crystalline NH(4)BH(4) including H atoms. Rapid reorientations of both ions mean that on average half-hydrogens occupy the corners of a cube around B or N.
Chemical Communications 10/2010; 46(48):9164-6. · 6.17 Impact Factor
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ABSTRACT: This work is a systematic attempt to determine the possibilities and the limitations of the (43)Ca high field solid state NMR in the study of cement-based materials. The low natural abundance (0.135%) and small gyromagnetic ratio of (43)Ca present a serious challenge even in a high magnetic field. The NMR spectra of a number of cement compounds of known structure and composition are examined. The spectra of several phases important in cement science, e.g., anhydrous beta di-calcium silicate (beta-C(2)S) and tri-calcium (C(3)S) silicate were obtained for the first time and the relation of spectroscopic and structural parameters is discussed. The method was also applied to the hydrated C(3)S and synthetic calcium silicate hydrates (C-S-H) of different composition in order to understand the state of calcium and transformations in the structure during hydrolysis. The spectra of hydrated C(3)S reveals a calcium environment similar to that of the C-S-H samples and 11 A Tobermorite. These observations support the validity of using layered crystalline C-S-H systems as structural models for the C-S-H that forms in the hydration of Portland cement.
Physical Chemistry Chemical Physics 07/2010; 12(26):6961-9. · 3.57 Impact Factor
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ABSTRACT: Due to sensitivity problems, (25)Mg remains a largely under-explored nucleus in solid state NMR spectroscopy. In this work at an ultrahigh magnetic field of 21.1 T, we have studied at natural abundance the (25)Mg solid state (SS) NMR spectra for a number of previously unreported magnesium compounds with known crystal structures. Some previously reported compounds have been revisited to clarify the spectra that were obtained at lower fields and were either not sufficiently resolved, or misinterpreted. First principles calculations of the (25)Mg SS NMR parameters have been carried out using plane wave basis sets and periodic boundary conditions (CASTEP) and the results are compared with experimental data. The calculations produce the (25)Mg absolute shielding scale and give us insight into the relationship between the NMR and structural parameters. At 21.1 T the effects of the quadrupolar interactions are reduced significantly and the sensitivity and accuracy in determining chemicals shifts and quadrupole coupling parameters improve dramatically. Although T(1) measurements were not performed explicitly, these proved to be longer than assumed in much of the previously reported work. We demonstrate that the chemical shift range of magnesium in diamagnetic compounds may approach 200 ppm. Most commonly, however, the observed shifts are between -15 and +25 ppm. Quadrupolar effects dominate the (25)Mg spectra of magnesium cations in non-cubic environments. The chemical shift anisotropy appears to be rather small and only in a few cases could the contribution of the CSA be detected reliably. A good correspondence between the calculated shielding constants and experimental chemical shifts was obtained, demonstrating the good potential of computational methods in spectroscopic assignments of solid state (25)Mg NMR spectroscopy.
Physical Chemistry Chemical Physics 12/2009; 11(48):11487-500. · 3.57 Impact Factor
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ABSTRACT: Metal organic frameworks (MOFs) are particularly exciting materials that couple porosity, diversity and crystallinity. But although they have been investigated for a wide range of applications, MOF chemistry focuses almost exclusively on properties intrinsic to the empty frameworks; the use of guest molecules to control functions has been essentially unexamined. Here we report Na(3)(2,4,6-trihydroxy-1,3,5-benzenetrisulfonate) (named β-PCMOF2), a MOF that conducts protons in regular one-dimensional pores lined with sulfonate groups. Proton conduction in β-PCMOF2 was modulated by the controlled loading of 1H-1,2,4-triazole (Tz) guests within the pores and reached 5 × 10(-4) S cm(-1) at 150 °C in anhydrous H(2), as confirmed by electrical measurements in H(2) and D(2), and by solid-state NMR spectroscopy. To confirm its potential as a gas separator membrane, the partially loaded MOF (β-PCMOF2(Tz)(0.45)) was also incorporated into a H(2)/air membrane electrode assembly. The resulting membrane proved to be gas tight, and gave an open circuit voltage of 1.18 V at 100 °C.
Nature Chemistry 12/2009; 1(9):705-10. · 20.52 Impact Factor
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ABSTRACT: Structural determination of crystalline powders, especially those of complex materials, is not a trivial task. For non-stoichiometric guest-host materials, the difficulty lies in how to determine dynamical disorder and partial cage occupancies of the guest molecules without other supporting information or constraints. Here, we show how direct space methods combined with Rietveld analysis can be applied to a class of host-guest materials, in this case the clathrate hydrates. We report crystal structures in the three important hydrate crystal classes, sI, sII, and sH, for the guests CO(2), C(2)H(6), C(3)H(8), and methylcyclohexane + CH(4). The results obtained for powder samples are found to be in good agreement with the experimental data from single crystal X-ray diffraction and (13)C solid-state NMR spectroscopy. This method is also used to determine the guest disorder and cage occupancies of neohexane and tert-butyl methyl ether binary hydrates with CH(4) in the structure H clathrate hydrates. The results are found to be in good agreement with the results from the (13)C solid-state NMR and molecular dynamics simulations. It is demonstrated that the ab initio crystal structure determination methodology reported here is able to determine absolute cage occupancies and the dynamical disorder of guest molecules in clathrate hydrates from powdered crystalline samples.
Journal of the American Chemical Society 12/2009; 132(2):524-31. · 9.91 Impact Factor
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ABSTRACT: Gas hydrates represent an attractive way of storing large quantities of gas such as methane and carbon dioxide, although to date there has been little effort to optimize the storage capacity and to understand the trade-offs between storage conditions and storage capacity. In this work, we present estimates for gas storage based on the ideal structures, and show how these must be modified given the little data available on hydrate composition. We then examine the hypothesis based on solid-solution theory for clathrate hydrates as to how storage capacity may be improved for structure II hydrates, and test the hypothesis for a structure II hydrate of THF and methane, paying special attention to the synthetic approach used. Phase equilibrium data are used to map the region of stability of the double hydrate in P-T space as a function of the concentration of THF. In situ high-pressure NMR experiments were used to measure the kinetics of reaction between frozen THF solutions and methane gas, and (13)C MAS NMR experiments were used to measure the distribution of the guests over the cage sites. As known from previous work, at high concentrations of THF, methane only occupies the small cages in structure II hydrate, and in accordance with the hypothesis posed, we confirm that methane can be introduced into the large cage of structure II hydrate by lowering the concentration of THF to below 1.0 mol %. We note that in some preparations the cage occupancies appear to fluctuate with time and are not necessarily homogeneous over the sample. Although the tuning mechanism is generally valid, the composition and homogeneity of the product vary with the details of the synthetic procedure. The best results, those obtained from the gas-liquid reaction, are in good agreement with thermodynamic predictions; those obtained for the gas-solid reaction do not agree nearly as well.
Chemistry - An Asian Journal 09/2009; 4(8):1266-74. · 4.50 Impact Factor
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ABSTRACT: The formation of guest-host hydrogen bonds in structure H (sH) clathrate hydrates is studied herein. We contrast the structure and guest dynamics of the tert-butylmethylether (TBME) and neohexane (NH) sH clathrates by performing molecular dynamics simulations on these two clathrates and measuring (1)H and (13)C NMR relaxation times of the guests. These two guests are isoelectronic and differ with respect to the presence of the ether oxygen atom in TBME and a CH(2) group in NH. The TBME guest forms long-lived hydrogen bonds with water molecules in the equatorial region of the large sH clathrate cage. These hydrogen bonds effectively tether the TBME guest to the side of the cage and restrict its rattling and rotational motions in the cage compared to NH, which does not become hydrogen bonded to the cage's water molecules.
ChemPhysChem 03/2009; 10(5):824-9. · 3.41 Impact Factor
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Advanced Materials 10/2008; 20(23):4517 - 4520. · 13.88 Impact Factor
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ABSTRACT: The adsorption of CO(2) into the low density form of p-tert-butylcalix[4]arene (tBC) has been studied by (13)C solid state NMR, single crystal X-ray diffraction and volumetric adsorption measurements. The experimental results indicate that tBC and carbon dioxide can form two distinct inclusion compounds. At low loadings the structure of the empty low-density form of the tBC framework (space group P2(1)/n) is preserved with the included CO(2) molecules located within the conical cavities of the tBC molecules. The ideal composition of this form is therefore 1 : 1 (CO(2) : tBC). With higher applied CO(2) pressures the guest loading increases and the structure of the tBC framework transforms to a well studied tetragonal (space group P4/n) form. In this form an additional CO(2) molecule is located on an interstitial site resulting in an ideal composition 2 : 1 (CO(2) : tBC). In agreement with SCXRD and the gas adsorption measurements, (13)C NMR measurements show the change in structure that takes place as a function of sample loading. Inclusion of CO(2) is a rather slow activated process that can be accelerated by increasing the temperature and the transition between crystal forms is inhomogeneous over a bulk sample. After gas release, the empty (or near empty) P4/n structure survives, thus providing another low density phase of tBC. The magnitude and temperature variation of the (13)C chemical shift anisotropy of CO(2) in both low and high occupancy complexes with tBC indicates restricted motion of the CO(2) molecules. The location and dynamics of CO(2) molecules inside the tBC structure are discussed and a motional model for CO(2) is proposed. The CO(2) molecules in the highly loaded compound are shown to exchange rapidly as a single resonance is observed for the two distinct CO(2) molecules.
Physical Chemistry Chemical Physics 09/2008; 10(31):4636-43. · 3.57 Impact Factor
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ABSTRACT: We have followed the loading of xenon into the low density form of a van der Waals solid host, p- tert-butylcalix[4]arene (tBC), with solid-state NMR and X-ray diffraction (XRD), techniques sensitive to local and long-range order, respectively. Even though there was little change in the unit cell parameters, 13C and 129Xe solid-state NMR spectra indicate that significant structural changes occur in local order even at low levels of loading. In particular, 129Xe double quantum experiments, which probe distance-dependent 129Xe− 129Xe dipolar interactions, reveal that the closest Xe−Xe distances increase with Xe loading into the tBC host, suggesting that the tBC undergoes structural rearrangements as it absorbs Xe. Indeed, in light of the solid-state NMR results, a re-examination of partially loaded single crystals by XRD showed that up to a loading level (Xe/tBC ratio) of 0.25, the structure was closely related to that of the empty form with the typical calixarene bilayer structure; however, at higher loading (0.5), the structure is substantially different with a constricted zigzag channel for Xe. In the latter structure, alternate bilayers are distinct because of different molecular orientations and much enhanced thermal parameters. What is remarkable is that the changes described take place with the different structural motifs apparently coexisting in the same single crystal. These different structures have almost identical unit cell parameters; however, the structures are quite different, and the phase transitions are more easily followed with NMR spectroscopy than with diffraction. The fact that ordered domains are always present suggests that cooperative dynamics play an important role, with the experimental results giving snapshots of the loading process at different stages.
05/2008;
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ABSTRACT: Microporous dipeptides, also known as organic zeolites or biozeolites, as examples of small-pore peptide nanotubes provide a convenient set of materials for developing a systematic approach based on 129Xe NMR spectroscopy for the derivation of thermodynamic and molecular scale information on temperature dependent pore filling. The sorption of xenon in the isolated 1D chiral nanochannels of eight microporous dipeptides Ala-Val (AV), Val-Ala (VA), Leu-Ser (LS), Ala-Ile (AI), Val-Val (VV), Ile-Ala (IA), Ile-Val (IV), Val-Ile (VI) (all LL isomers) was monitored in situ with continuous-flow 129Xe NMR spectroscopy over a temperature range of 173−343 K. The materials all showed strongly anisotropic signals, with isotropic chemical shift changing from 95 to 281 ppm depending on the dipeptide used and/or temperature. The isosteric heats of sorption (qst) and entropy factors were determined from two independent models. The sorption process was complicated by reversible phase transformations of some dipeptides and irreversible changes due to aging of samples, both of which may be of considerable importance in applications of soft materials. The interpretation of the line shapes and chemical shift anisotropy as a function of temperature provided information on the structure of the xenon-cavity complex and made it possible taking into account the helicity and flexibility of the nanochannels and the dynamics of xenon. The approach illustrates a powerful way of analyzing pore space in soft microporous materials, yielding a quantitative thermodynamic description of sorption and the characteristics of the pore space and sorption events that occur on molecular-scale level during pore filling.
04/2008;
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Angewandte Chemie International Edition 02/2008; 47(30):5616-8. · 13.45 Impact Factor
