Hydroxypyruvic acid, oxidation glycerate

The Pt/C catalyst, compared with Pd/C, showed not only enhanced activity (vide supra) but also reduced selectivity for glyceric acid (only 55% at 90% conversion), favoring dihydroxyacetone formation up to 12%, compared with 8% for the Pd case [48]. The Pt/C catalyst promoted with Bi showed superior yields of dihydroxyacetone (up to 33%), at lower pHs. Glyceric and hydroxypyruvic acids, apparently, are formed as by-product and secondary product, respectively [48], The addition of Bi seems to switch the susceptibility of glycerol oxidation from the primary towards the secondary carbon atoms. 

Furthermore, a base-catalyzed transformation by OH from the reaction medium between glycerate and hydroxypyruvate aldehyde (or hydroxypyruvic acid) could be excluded, while hydroxyacetone and glyceraldehyde interconversion was possible (Scheme 11.11). The existence of two major routes, of which hydroxyacetone and glyceric aldehyde are the primary oxidation products and glycolic and oxalic acid are the end-members, respectively, is now firmly established. Clearly, rapid oxidation of glyceraldehydes favors glyceric acid rather than hydroxyacetone formation. 

As shown in Scheme 3.4, there are several reaction pathways of glycerol oxidation to form dihydroxyacetone, glyceric acid, hydroxypyruvic acid, mesoxalic acid, and so on. Dihydroxyacetone is formed by the oxidation of a secondary hydroxy group under... 

In a critical study of the periodate oxidation of raffinose and related oligosaccharides, Courtois and Wickstrom showed78 that 1 mole of raffinose reduces 5 moles of periodate, with the formation of 2 moles of formic acid plus a hexaldehyde. Conversion of the aldehyde to the hexa-carboxylic acid, followed by hydrolysis, gave the expected amounts of glyoxylic acid, glyceric acid, hydroxypyruvic acid, and glycolaldehyde (by decarboxylation of hydroxypyruvic acid). 

Figure 2.2.12 Reaction network of glycerol oxidation (GLY, glycerol DHA, dihydroxyace-tone GLA, glyceric aldehyde GLS, glyceric acid HBT, hydroxypyruvic acid MOS, mesoxalic acid TS, tartronic acid GOX, glyoxal GOS, glycolic acid GYS, glyoxylic acid OS, oxalic acid).

Oxidation — Apart from C02 and H20, there are three series of products that result from the oxidation of saccharide. They are 2,3-dialdehydes (5.43 and 5.49) formed on the oxidative cleavage of saccharides (5.42 and 5.48) with periodates, the sole oxidants providing such course of oxidation. Such dialdehydes are considered toxic. Further oxidation of dialdehydes leads to glyceric acid (5.45), glyoxalic acid (5.47), hydroxypyruvic acid (5.46), and erythronic acid (5.51), as shown below for the oxidative cleavage of sucrose (5.42) and maltose (5.48). 

Selective oxidation with air of glyceric to hydroxypyruvic acid and tartronic to mesoxalic acid on PtBi/C catalysts... 

Figure 3 Product distribution for the oxidation of glyceric acid on Pt(4.3%)Bi(3.9%)/C at (a) pH 2, (b) pH 4, (c) pH 5 and (d) pH 6, as a function of time (A - glyceric acid, O -hydroxypyruvic acid, o- oxalic acid and - glycolic acid).

Activity and selectivity data for the oxidation of glyceric acid to hydroxypyruvic acid. 

PtBi/C catalysts were reported earlier to enable selective oxidation of the secondary hydroxy function of glyceric and tartronic acid to hydroxypyruvic and mesoxalic acid, respectively [13]. In the work reported here, these two reactions were studied in more detail to determine Ae influence of the following parameters on selectivity and reaction progress pH of the reaction medium, over-oxidation of targeted products, and leaching of catalyst components. 

Under acidic conditions, bismuth-promoted platinum catalysts selectively oxidise the secondary hydroxy function of glyceric and tartronic acids to their respective keto-acids hydroxypyruvic and mesoxalic acids. A complexing mechanism is proposed to increase the rate of oxidation of the secondary hydroxy function. 

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