Efficient Recovery of CO2 from Flue Gas by Clathrate Hydrate Formation in Porous Silica Gels

Thermodynamic measurements and NMR spectroscopic analysis were used to show that it is possible to recover CO2 from flue gas by forming a mixed hydrate that removes CO2 preferentially from CO2/N2 gas mixtures using water dispersed in the pores of silica gel. Kinetic studies with 1H NMR microimaging showed that the dispersed water in the silica gel pore system reacts readily with the gas, thus obviating the need for a stirred reactor and excess water. Hydrate phase equilibria for the ternary CO2-N2-water system in silica gel pores were measured, which show that the three-phase hydrate-water-rich liquid-vapor equilibrium curves were shifted to higher pressures at a specific temperature when the concentration of CO2 in the vapor phase decreased. 13C cross-polarization NMR spectral analysis and direct measurement of the CO2 content in the hydrate phase suggested that the mixed hydrate is structure I at gas compositions of more than 10 mol % CO2, and that the CO2 molecules occupy mainly the more abundant 51262 cages. This makes it possible to achieve concentrations of more than 96 mol % CO2 gas in the product after three cycles of hydrate formation and dissociation. 1H NMR microimaging showed that hydrate yields of better than 85%, based on the amount of water, could be obtained in 1 h when a steady state was reached, although ~90% of this yield was achieved after ~20 min of reaction time.NRC publication: Ye

39K NMR of solid potassium salts at 21 T : effect of quadrupolar and chemical shift tensors

39K Solid State NMR spectra (static and magic angle spinning (MAS)) on a set of potassium salts measured at 21.14 T show that the chemical shift range for K+ ions in diamagnetic salts is well in excess of 100 ppm contrary to previous assumptions that it was quite small. Inequivalent potassium sites in crystals can be resolved through differences in chemical shifts, with chemically similar sites showing differences of over 10 ppm. The quadrupolar coupling constants obtained from MAS and solid echo experiments on powders cover the range from zero for potassium in cubic environments in halides to over 3 MHz for the highly asymmetric sites in K2CO3. Although the quadrupolar effects generally dominate the 39K spectra, in several instances, we have observed subtle but significant contributions of chemical shift anisotropy with values up to 45 ppm, a first such observation. Careful analysis of static and MAS spectra allows the observation of the various chemical shift and quadrupole coupling tensor components as well as their relative orientations, thereby demonstrating that high-field 39K NMR spectroscopy in the solid state has a substantial sensitivity to the local environment with parameters that will be of considerable value in materials characterization and electronic structure studies.Peer reviewed: YesNRC publication: Ye

IN SITU NMR STUDIES OF HYDROGEN STORAGE KINETICS AND MOLECULAR DIFFUSION IN CLATHRATE HYDRATE AT ELEVATED HYDROGEN PRESSURES

Clathrate hydrates can be reasonable choices for high-density hydrogen storage into compact host media, which is an essential task for hydrogen-based future society. However, conventional storage scheme where aqueous solution is frozen with hydrogen gas was impractically slow for practical use. Here we propose a much faster scheme where hydrogen gas was directly charged into hydrogen-free, crystalline hydrate powders. The storage kinetics was observed in situ by nuclear magnetic resonance (NMR) spectroscopy in a pressurized tube cell. At pressures up to 20 MPa the storage was complete within 80 minutes, as observed by growth of stored-hydrogen peak into the hydrate. Since the rate-determining step of current storage scheme is body diffusion of hydrogen within the crystalline hydrate media, we have measured the diffusion coefficient of hydrogen molecules using the pulsed field gradient NMR method. The results show that at temperatures down to 250 K the stored hydrogen is highly mobile, so that the powdered hydrate media should work well even in cold environments. Compared with more prevailing hydrogen storage media such as metal hydrides, the clathrate hydrate could offer even more advantages: It is free from hydrogen embrittlement, more chemically durable, more environmentally benign, as well as economically quite affordable.Non UBCUnreviewe

Mg-25 ultra-high field solid state NMR spectroscopy and first principles calculations of magnesium compounds

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.En raison de probl\ue8mes de sensibilit\ue9, le noyau du (25)Mg n\u2019a pas \ue9t\ue9 suffisamment explor\ue9 par spectroscopie de r\ue9sonance magn\ue9tique nucl\ue9aire (nuclear magnetic resonance ou NMR) en phase solide (SS). Nous avons \ue9tudi\ue9, en pr\ue9sence d\u2019un tr\ue8s fort champ magn\ue9tique de 21,1 T, les spectres NMR SS du (25)Mg en abondance naturelle de certains compos\ue9s du magn\ue9sium non rapport\ue9s jusqu'\ue0 pr\ue9sent, mais dont la structure cristalline est connue. Nous avons r\ue9examin\ue9 certains des compos\ue9s connus de fa\ue7on \ue0 clarifier leurs spectres, pr\ue9alablement enregistr\ue9s dans des champs plus faibles et r\ue9solus de fa\ue7on partielle ou mal interpr\ue9t\ue9s. Pour effectuer les calculs de premiers principes des param\ue8tres NMR SS du (25)Mg, nous avons employ\ue9 des jeux de bases d'ondes planes et des conditions p\ue9riodiques aux limites (CASTEP). Les r\ue9sultats obtenus ont ensuite \ue9t\ue9 compar\ue9s aux donn\ue9es exp\ue9rimentales. Les calculs fournissent une \ue9chelle absolue d\u2019\ue9cran du (25)Mg et des indications sur la relation entre les spectres NMR et les propri\ue9t\ue9s structurales. \uc0 une fr\ue9quence de 21,1 T, les effets des interactions quadripolaires d\ue9croissent de mani\ue8re significative et la sensibilit\ue9 et la pr\ue9cision dans la d\ue9termination des d\ue9placements chimiques et des param\ue8tres de couplage quadripolaire s\u2019am\ue9liorent consid\ue9rablement. Bien que n\u2019ayant pas effectu\ue9 explicitement des mesures de T (1), celles-ci se sont av\ue9r\ue9es plus longues que ne l\u2019indiquent la plupart des travaux ant\ue9rieurs. Nous d\ue9montrons que l\u2019\ue9tendue des d\ue9placements chimiques du magn\ue9sium dans des compos\ue9s diamagn\ue9tiques peut avoisiner les 200 ppm. Toutefois, les d\ue9placements observ\ue9s sont g\ue9n\ue9ralement compris entre - 15 et + 25 ppm. Les effets quadripolaires dominent les spectres (25)Mg des cations du magn\ue9sium dans des formes non cubiques. L\u2019anisotropie du d\ue9placement chimique semble relativement faible et sa contribution ne peut \ueatre d\ue9tect\ue9e de fa\ue7on fiable que dans un faible nombre de cas. Nous avons observ\ue9 une bonne corr\ue9lation entre les valeurs calcul\ue9es des constantes d\u2019\ue9cran et les valeurs exp\ue9rimentales des d\ue9placements chimiques, ce qui d\ue9montre le potentiel satisfaisant des m\ue9thodes de calcul des attributions spectrales par spectroscopie NMR.Peer reviewed: YesNRC publication: Ye