Thermodynamic Modeling of Hydrogen Storage Capacity in Mg-Na Alloys
Thermodynamic modeling of the H-Mg-Na system is performed for the first time in this work in order to understand the phase relationships in this system. A new thermodynamic description of the stable NaMgH3 hydride is performed and the thermodynamic models for the H-Mg, Mg-Na, and H-Na systems are reassessed using the modified quasichemical model for the liquid phase. The thermodynamic properties of the ternary system are estimated from the models of the binary systems and the ternary compound using CALPHAD technique. The constructed database is successfully used to reproduce the pressure-composition isotherms for MgH2 + 10 wt.% NaH mixtures. Also, the pressure-temperature equilibrium diagram and reaction paths for the same composition are predicted at different temperatures and pressures. Even though it is proved that H-Mg-Na does not meet the DOE hydrogen storage requirements for onboard applications, the best working temperatures and pressures to benefit from its full catalytic role are given. Also, the present database can be used for thermodynamic assessments of higher order systems.
Thermodynamic analysis of dehydrogenation path of Mg–Al–Li–Na alloys
Abstract A thermodynamic study on the Mg–Al–Li–Na–H system is conducted in this work in order to investigate its hydrogen storage properties. For that purpose, a thermodynamic database is constructed using the CALPHAD technique. In this work, Li is added to the previously studied Mg–Al–Na–H system and the related binaries (Li–Mg, Li–Na) and ternaries (Al–Li–Mg, Al–Li–H, Li–Mg–H) are reassessed using the Compound Energy Formalism (CEF) for the terminal solid solutions and the Modified Quasichemical Model (MQM) for the liquid phase. The quaternary hydride Na 2 LiAlH 6 is assessed for the first time. The constructed database is used to calculate de/re-hydrogenation reaction pathways of MgH 2 –LiAlH 4 /Li 3 AlH 6 composites in a wide temperature and pressure ranges. The results are analyzed and compared with the experimental data to better understand and predict the reaction mechanisms. It is shown that Al resulted from the decomposition of LiAlH 4 /Li 3 AlH 6 hydrides destabilizes MgH 2 by the formation of Mg–Al phases: β and γ, and that LiH does not contribute to these reactions. Only negligible amounts of hydrogen are released from LiH because of the limited solubility of Li (from LiH) in Mg at high temperatures. It is concluded that MgH 2 –Li 3 AlH 6 system has better reversibility than MgH 2 –LiAlH 4 system and promising compositions (e.g. 37.09 mol% and 53.65 mol% MgH 2 contents) are predicted. Also, a new destabilization reaction is predicted in the system. The MgH 2 and Na 2 LiAlH 6 hydrides destabilize mutually at 308.4 K (35.25 °C) and 1 bar because of the formation of MgNaH 3 . However, the hydrogen capacity is reduced. It is concluded that Na considerably reduces the hydrogen storage potential of the system.
Synthesis of 2,4-dideoxy-.beta.-D-erythro-hexopyranosyl nucleosides
The synthesis of 1-(2,4-dideoxy-β-D-erythro-hexopyranosyl)thymine (12) 1 was accomplished using two different synthetic routes. It was obtained starting either from 1,2:5,6-di-O-isopropylidene-α-D-glucofuranose or from tri-O-acetyl-D-glucal. The other modified nucleosides, with either a cytosine, guanine, or adenine moiety, were synthesized using the second reaction scheme. Deoxygenation reactions were accomplished with the (2,4-dichlorophenoxy)thiocarbonyl derivatives generated in situ