Rehydrogenation

The conversion of glucose proceeds via its splitting into pyruvic acid and hydrogen, which is bound as NADPH. Pyruvic acid is subsequently decarboxylated to C02 and acetaldehyde (bound to the coenzyme-A), which is subsequently rehydrogenated to ethanol. The overall reaction delivers therefore two molecules of ethanol and two C02 for every glucose unit. Notice that such a simplified metabolic pathway does not display the energy fluxes, e.g., in the form of ATP/ADP interconversion. 

The metabolic pathways can be diverted to other products, however. For instance, the pyruvic acid can be rehydrogenated to lactic acid. Accordingly, glucose is converted into two molecules of lactic acid, which is the building block for Cargill s polylacate polymer [65],... 

In addition to the physical interactions described above, the tip may also be used to alter the local chemical conditions within the tunnel junction. For example, catalytic rehydrogenation of carbonaceous fragments on Pt(lll) by tip-directed production of atomized hydrogen in vacuum at the Pt-Ir tip has been described [524]. Similar modification schemes may also be envisioned based on limiting the transport of reactants and products into or away from the partly occluded tunnel junction. As noted earlier, such effects may be important in the study of electrodeposition and etching process [126-131]. Nonetheless, much remains to be understood about the detailed physics and chemistry of the immersed tunnel junction. 

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 ... 

In coal liquefaction, highly dispersed, disposable, catalysts are needed because maximal contact between coal and catalysts is essential. It is assumed that one of the roles of the coal liquefaction catalyst is to assist in the rehydrogenation of the donor solvent (e.g. tetralin) by facilitating the hydrogen transfer from the gas phase.36,37... 

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 extent of this rehydrogenation depends on the hydrogen pressure. Rehydrogenation is detectable at a pressure of about 2 mm. and is practically complete at % atm. There is some indication, but no positive ... 

A detailed band assignment is not presented in this work, but a summary is given in Table II. Throughout all the deuter-ation and rehydrogenation procedures there are three main features to note in the spectra. 

Gel-film (Fig. 8). Deuteration ( B) shows that ROD-D2O (cryst) is present with a slight reduction in H2O (cryst). Therefore only a small proportion of the water of crystallization is exchangeable. Differences in the deuterated spectrum lead to the assumption that some of the OH groups undergo an exchange reaction to form 0D. Rehydrogenation proves that this is a completely reversible reaction and that the D2O (cryst) is exchangeable, contrary to the "buried" H2O (cryst). 

Figure 8. IR spectra obtained from curdlan sample gelled in water (a) original film (b) film after deuteration (c) rehydrogenated film...

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