Polyol dehydrogenation

Adsorbed CO is produced by polyol dehydrogenation in basic medium. The formation of adsorbed hydroxyl anions at lower potential cleans the metal surface from CO, liberating C02 from solution. Sufficient Ce02 has to be present to efficiently release adsorbed CO, while at too high a concentration the current density decreases. A Pt Ce atomic ratio of 1.3 1 shows optimal performance for the system. The decrease in electrode conductivity is assumed to be linked to increasing amounts of the semiconductor Ce02 [54, 61]. 

But GO is also an archetype for more complex polyols. Catalytic hydrogenolysis of polyols leads to C-C and C-0 bond cleavage as well as activation via dehydrogenation of HCOH to C=0 sites (8, 9). With its three vicinal hydroxyl groups, and its two types of carbon atoms (all bearing hydroxyl functionalities) GO can serve as a simple model in which to study the competition between these processes. We aim to gain insight into the individual steps and apply that information to increase selectivity toward desired products. 

Adsorption of polyols over copper surface and dehydrogenation, favoured at the end of the carbon chain, lead to unsaturated species strongly adsorbed which can undergo the nucleophilic attack of adsorbed hydroxyls ("OH"), leading to dehydroxylation (DOH) ... 

Sorbitol is an interesting natural monomer obtained from glucose (sugar waste is used for this purpose) by dehydrogenation. It easily dehydrogenates into isosor-bide which is useful as a polyol for production of polymers/polyurethane [69, 70]. 

In this contribution, we first review the hydrogenation of carbohydrates to polyols. Section 8.4.3 will focus on the reductive amination of carbohydrates and, finally (Section 8.4.4), the facile dehydrogenation of carbohydrates under alkaline conditions will be reported. The hydrogenolysis of carbohydrates is beyond the scope of this chapter. Readers interested in this subject should consult Ref. 3 and references cited therein. 

Glycerol and other polyols can be converted into propane-1,2-diol with high selectivity on sulfur-modified Ru on carbon the reaction is not simple hydrogenolysis but rather dehydrogenation, then dehydroxylation and hydrogenation of the enol produced [16-18],... 

On the other hand, Pt-Sn catalysts have been found to be the most active binary systems for ethanol oxidation, established by electrochemical techniques and fuel cell measurements [62-64]. Polyol method [65, 66] and Bonneman method [27, 67] were employed to synthesize Pt-Sn/C alloy and Pt-SnOx/C catalysts, and Jiang et al. [68] claimed that the greater activity was from Pt-SnOx/C due to the presence of both sufficiently large Pt ensembles for ethanol dehydrogenation and C-C bond splitting and of SnOx for OH generation. Considerable research has been devoted to optimizing the atomic ratio of Pt-Sn electrocatalysts. Lamy et al. [27] found the alloy... 

The main transformations of sorbitol include dehydrogenation of the substrate on the metal sites followed by a decarbonylation step (Path 1) and dehydration on acidic sites (Path 2), followed by hydrogenation. Path 1 leads to formation of polyol species containing one carbon atom fewer than the initial substrate, whereas Path 2 results in formation of hydrocarbons. The first initial step of dehydration can be considered as the rate determining step, while the subsequent fast steps of... 

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