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Rehydrogenation of Titanium Doped Lithium and Sodium Aluminum Hydride Monitored by High Pressure NMR

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Item Summary

Title: Rehydrogenation of Titanium Doped Lithium and Sodium Aluminum Hydride Monitored by High Pressure NMR
Authors: Birkmire, Derek
Keywords: metal hydride
titanium doped
solvent mediation
Issue Date: Aug 2015
Publisher: [Honolulu] : [University of Hawaii at Manoa], [August 2015]
Abstract: The direct hydrogenation of MH/AlTi-doped to MAlH4 (M = Na and Li) was studied by high pressure NMR. The dehydrogenation pathways have been well documented as the systems have been studied for their potential as lightweight hydrogen storage materials. The high pressure required for the hydrogenation had previously hindered the investigation. The rehydrogenation of NaH/AlTi-doped to Ti-doped NaAlH4 under 140 bar H2 at 120°C was monitored by 27Al and 23Na wideline NMR Spectroscopy. The rehydrogenation was observed to proceed via the Na3AlH6 intermediate. Further conversion to NaAlH4 was observed rapidly after the initial appearance of Na3AlH6. This demonstrates that the rehydrogenation does not proceed via the micro-reverse of the dehydrogenation where complete conversion to Na3AlH6 occurs before final conversion to NaH/AlTi-doped. Of note, the 27Al NMR spectra revealed 2 overlapping peaks (one broad and one narrow) assigned to NaAlH4. The broad peak is characteristic of solid NaAlH4, whereas the narrow peak indicates motional averaging. The discrete nature of the peak indicates 2 distinct populations of static versus mobile NaAlH4. The mobile NaAlH4 species was also generated by air-exposure of pure NaAlH4, followed by exposure to 200 °C and 180 bar H2 pressure. This material was unable to cycle hydrogen under the conditions required to cycle Tidoped NaAlH4. Therefore, the mobile species does not play a role in the hydrogen cycling of Tidoped NaAlH4, under modest conditions.
The solvent mediated rehydrogenation of LiH/AlTi-doped to Ti-doped LiAlH4 was monitored by 27Al and 7Li wideline NMR spectroscopy. The mediation by the solvents THF and DME were utilized for the rehydrogenation. The solvent mediation, by both solvents, was confirmed to bypass the Li3AlH6 intermediate observed in the solid state dehydrogenation. Most notably, the 7Li NMR spectra revealed a downfield shift for the DME adduct compared to the THF adduct. This indicates DME has a higher coordination number to LiAlH4. These NMR studies not only furthered the understanding of the alanate systems, but also demonstrated novel high pressure NMR techniques.
Description: Ph.D. University of Hawaii at Manoa 2015.
Includes bibliographical references.
Appears in Collections:Ph.D. - Chemistry

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