Metal hydride and water

We have analyzed, both theoretically and experimentally, the reaction chemistry of a variety of metal hydrides and water, and the chemical stability of the organic carriers in contact with metal hydrides and spent hydrides. Since detailed hydrolysis reaction kinetics of the metal hydride/organic carrier slurry is not known, we conducted experiments using a high pressure (13.790 MPa or 2000 psi) and high temperature (232°C) vessel with temperature, pressure, and magnetic stirrer control capabilities (500 cm3 internal volume). Some of the selection criteria for the hydride follow. 

Figure 3 The characteristic time tncs for Compton scattering on H and D, as function of scattering angle 0, with energy selection by Au-resonance foils. The widths 3.8 and 5.0 A-1 correspond to proton vibrations in a typical metal hydride and water, respectively. The corresponding widths for deuterons are 5.0 and 6.5 A-1, respectively.

The nucleophilic attack by alkoxides, amines, and water is of great interest to homogeneous catalysis. A dominant reaction in syn-gas systems is the conversion of carbonyls with water to metal hydrides and carbon dioxide ("Shift Reaction"), see Figure 2.27. 

Oxidation reaction also occurs with hydrogen chloride, metal hydrides and a number of metal salts. It dissolves in water reacting to form iodic acid ... 

Another general method is based on oxygen insertion into metal-hydrogen bonds (50,72,79-81) by any of several known mechanisms. Hydrogen abstraction by superoxo complexes followed by oxygenation of the reduced metal, as in the catalytic reaction of Eqs. (3)-(4) (50,72), works well but is limited by the low availability of water-soluble transition metal hydrides and slow hydrogen transfer (equivalent of reaction (3)) for sterically crowded complexes. 

Class D fires involve strong reducing agents such as active metals (magnesium, titanium, zirconium, and alkali metals), metal hydrides, and organome-tallics. Special dry-chemical fire extinguishers are available for these fires (e.g., Ansul Co.). Sand is also useful for small fires of this type. Water should be avoided because it promotes the fire by liberation of hydrogen or hydrocarbons. 

Isomerization of allylic alcohol to ketone has been extensively studied [13], and two different pathways have been established, including tt-allyl metal hydride and the metal hydride addition-elimination mechanisms [5,14]. McGrath and Grubbs [ 15] investigated the ruthenium-catalyzed isomerization of allyl alcohol in water and proposed a modified metal hydride addition-elimination mechanism through an oxygen-functionality-directed Markovnikov addition to the double bond. 

Already familiar is the convenient laboratory preparation of elementary hydrogen by reduction of acids. Generally those metals lying between magnesium and tin in oxidation potential are appropriate. Less convenient but more spectacular is the production of hydrogen from action of the alkali metals on water. For small quantities of hydrogen, reaction of metal hydrides with water has been used such hydrides will be considered later in the chapter. Commercial preparations of H2 by reduction of steam with iron or coke and, finally, by the electrolysis of water should be recalled. 

As mentioned earlier, direct thermal dissociation of water requires temperatures above approximately 2500 K. Since there are not yet technical solutions to the materials problems, the possibility of splitting water instead, by various reaction sequences, has been probed. Historically, the reaction of reactive metals and reactive metal hydrides with water or acid was the standard way of producing pure hydrogen in small quantities. These reactions involved sodium metal with water to form hydrogen or zinc metal with hydrochloric acid or calcium hydride with water. All these... 

Table 2.10 c, x and reactivity of 3d metal ions towards CO, hydride and water... 

Tab. 4 Standard free energies of formation and standard potentials for alkali metal hydrides in water...

Sandia s design thermally actuates an exothermic reaction of metal hydride with water to generate the hydrogen on board the microsystem. The pressure created by this reaction and the low viscosity of the hydrogen enable the high flow rate of the mobile phase, which, in turn, enables the desired 2 -second chromatographic separation. 

In the laboratory, H2 may be prepared by electrolysis of acidified water (H2 is liberated at the cathode), but small quantities of H2 are most conveniently prepared by reactions between dilute acids and suitable metals (e.g. Fe, Zn, equation 9.7), by treating metals that form amphoteric hydroxides (e.g. Zn, Al) with aqueous alkali (equation 9.8) or by reacting metal hydrides with water (equation 9.9). 

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