Dehydrogenation-rehydrogenation

Comparison of the Al peaks observed by synchrotron X-ray diffraction for NaAIH4 doped with 2mol% TiCIa before and after seven cycles of dehydrogenation/rehydrogenation. 

Therefore, highly acidic components have been sought to be associated with the platinum usually thought of as the best dehydrogenation-rehydrogenation function. 

In the patent literature, processes are described for the epimerization of the benzyhc chiral center at the 4-posihon using an alkoxide base [28] and for reagent-based dehydrogenation, then rehydrogenation, of the amine [29]. We envisaged racemization of the chiral amine center using the SCRAM catalyst and the tertiary carbon center using an alkoxide base (Scheme 13.10). 

A unique mechanism was suggested to interpret the difference observed in the isomerization and hydrogenation of 1-butene and ds-2-butene over a stepped Pt(775) surface.360 It was observed that the hydrogenation rates were insensitive to surface structure for both 1-butene and ds-2-butene. The isomerization rates of cis-2-butene to give only trans-2-butene on the stepped Pt(775) surface, however, was double that of 1-butene to yield both cis- and trans-2-butenes. The Horiuti-Polanyi associative mechanism, that is, the involvement of the 2-butyl intermediate (see Section 4.3.2), cannot explain this difference. However, a facile dehydrogenation of ds-2-butene to 2-butyne followed by a rehydrogenation is consistent with the experimental observations ... 

When ethylene is adsorbed on bare nickel at 35° C. or on either bare or hydrogen-covered nickel at 150° C., the intensity of the C—H bands, shown as A of Fig. 3, is small compared with those of the associated chemisorbed ethylene shown in Fig. 2. When the species represented by A is treated with H2 at 35° C., the band intensities increased as is shown in B of Fig. 3. This behavior shows that A is due to a dissociatively chemisorbed ethylene in which the number of hydrogens per carbon is low (7). The species obtained by dissociative chemisorption will be referred to as a surface complex. It is doubtful whether the surface complex has a specific stoichiometric composition. Rather it appears that the carbon-hydrogen ratio will depend on the severity of the dehydrogenation conditions. In some cases it appears that a surface carbide, which has no hydrogens, is obtained. Even in this case the carbons appear to be easily rehydrogenated to adsorbed alkyl groups. 

The first step was confirmed to be exothermic (AH —7.5 kj moH ) which makes a reversible rehydrogenation under reasonable physical conditions almost impossible. The reaction enthalpy for the second step was determined to be about 32 kJ mol H2. The experimental data have been confirmed by first-principles DFT calculations of the dehydrogenation enthalpies, showing that the first step of dehydrogenation is indeed exothermic [187, 188]. 

Boron hydrides do have high hydrogen contents. However, the thermodynamics is in most cases unfavorable, the kinetics or the dehydrogenation and rehydrogenation are slow and the possible evolution of diborane is an additional problem. Therefore boron hydrides do not at present not allow any use as reversible hydrogen storage material. 

Fundamental research is necessary to understand all the mechanisms involved in the dehydrogenation and rehydrogenation in complex hydrides. Additionally to the understanding of the kinetics it is exceedingly important to develop new systems with much more advantageous thermodynamic properties. 

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