Oxidation of Glycerol

The glycerol metabolism is closely related to the glycolysis involving glycerol metabolites according to the following scheme  

Hutchings and coworkers demonstrated, that in the selective oxidation of glycerol the selectivity to glyceric acid was 100% at a conversion of 50% but gradually decreased to 86% at 72% conversion under 0.3 MPa of 02 at 333 K over Au/AC in the presence of an equimolar amount of NaOH [169, 179-181]. More than an equimolar amount of NaOH was necessary to promote the reaction the reaction did not occur without base. 

The Au-catalyzed glycerol oxidation was influenced by the kind of support, the size of Au particles and the reaction conditions such as concentration of glycerol, p02 and molar ratio of NaOH to glycerol. As metal oxide supports showed inferior selectivity to glyceric acid compared to carbons, due to successive oxidation and C—C bond cleavage to form di-adds such as tartronic acid and glycolic acid, research has focused on Au NPs supported on carbon, as in the case of ethylene glycol oxidation [182]. Indeed, the catalytic activity was influenced by the kind of carbon support in terms of porous texture [183]. 

Interestingly, Pd was glyceric acid seledive whereas Pt showed seledivity for dihydroxyacetone, which was formed by oxidation at the secondary alcoholic group. A promoting effed in catalytic adivity was observed by the introdudion of Pd metal to Au/AC [184, 187, 188]. Dihydroxyacetone was preferentially formed over Au-Pt/C however, the seledivity was lower than that of Bi-modified Pt catalyst [189]. 

Most recently, Taarning et al. reported that the fully oxidized ester dimethyl mesoxalate, which can be used as a monomer to yield poly(ketomalonate) [190], was produced by glycerol oxidation over Au/Ti02 and Au/Fe203 with 10% NaOCH3 in methanol with a seledivity of 89% at a full conversion [177]. Alcoholic media 

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HC CH(0H) CH20H. optically active. D-glyceraldehyde is a colourless syrup. May be prepared by mild oxidation of glycerol or by hydrolysis of glyceraldehyde acetal (prepared by oxidation of acrolein acetol). DL-glyceraldehyde forms colourless dimers, m.p. IBS-S C. Converted to methylglyoxal by warm dilute sulphuric acid. The enantiomers... 

The reduced catalyst deactivation compared to the analogous oxidations of glycerol and tartronic acid was attributed to the use of the calcium salt rather than the free acid. A recent publication describes a similar observation for the oxidation of sodium gluconate [15]. Sodium ions were assumed to counter catalyst deactivation by neutralizing the acid species responsible. 

The liquid-phase oxidation of glycerol was carried out by using carbon-supported gold particles of different sizes (2.7 2 nm) which were prepared by a colloidal route [120]. Indeed, a particle-size effect was observed because the selectivity to glyceric acid was increased to 75% with smaller particle sizes (4)ptmimn = 3.7 nm). 

Table 3. Selective oxidation of glycerol using l%Au/C catalyst prepared via sol immobilisation.

The importance of size control has been depicted for the selective oxidation of glycerol it was shown that by increasing particle size a high selectivity to glycerate has been reached at the expense of the consecutive oxidation of glycerate to tartronate. 

The aqueous phase air oxidation of glycerol with supported noble metal catalysts occurs under mild conditions (60 °C), but is very dependant on the pH of the reaction medium. Relevant data are shown in Fig. 11.3 [48], For Pd, Pt and Bi-promoted Pt the glycerol oxidation rate increases significantly with the pH of the medium, with Pd showing the lower activity. 

The Ru(m)-catalyzed oxidation of glycerol by an acidified solution of bromate (BrCfi ) at 45 °C consumes the required amount of 2 moles of bromate to obtain pure glyceric acid. Traces of Hg(OAc)2 were used as scavenger for potentially formed bromide, thereby eliminating the formation of bromine (formed by reaction of bromide and bromate) as an alternative oxidant [96]. The reaction is first order in Ru(m) (0.58 ms-1 at 45 °C) and zero order in substrate and protons. The addition of RuC163 to protonated bromate is assumed to be rate limiting. Similar catalytic chemistry is obtained with Rh(m)Cl3 [97]. 

S. Carrettin, P. McMom, P. Johnston, K. Griffin, and G. J. Hutchings, Selective oxidation of glycerol to glyceric acid using a gold catalyst in aqueous sodium hydroxide, Chem. Commun. 7, 696-697 (2002). 

Scheme 7 Oxidation of glycerol in acid media leads to dihydroxyacetone, using Pt-Bi/C as catalyst.

Under basic conditions, oxidation of glycerol mainly leads to the formation of glycerate (Scheme 8). By employing a 5 wt% Pd/C catalyst, the selectivity to glycerate can be as high as 70% at 100% conversion at pH = n/4i,i42... 

Scheme 8 Oxidation of glycerol in basic media leads to glyceric acid, using Pd/C or Au/C as catalyst.

Laurie, V. F. and Waterhouse, A. L. (2006b). Oxidation of glycerol in the presence of hydrogen peroxide and iron in model solutions and wine. Potential effects on wine color. J. Agric. Food Ghent. 54, 4668-4673. 

Similar mechanisms were postulated for the oxidation of glycols by periodate (32) and Ce(IV) (33, 34), and for the oxidation of glycerol by Ce(IV) (44). In these cases the existence of intermediate complexes was demonstrated. The oxidation of formaldehyde by Ce(IV) was also claimed to involve a pre-equilibrium of a Ce(IV)-formaldehyde complex (51). A similar complex was postulated in the formalde-hyde-Mn04 reaction (49, 87). The oxidation of isopropyl alcohol by chromate ions follows a similar mechanism, and a chromate ester was formed as intermediate (94). 

Also note that the oxidation of glycerol completely to C02 requires 3.5 moles of 02 for every mole of glycerol consumed. Thus, the degradation of 74 jUmol glycerol L-1 can just be accomplished with the 02 present at saturation (ca. 280 /tM). 

Structurally, these acids are cunsidered to be the oxidation products of polyhydric alcohols. However, a number of them can be formed from the oxidation of sugars. The careful oxidation of glycerol will yield a syrupy liquid, glyceric acid, an example of a dihydroxymonobasic carboxylic acid. 

Scheme 13- Enzymatic oxidation of glycerol phosphate for the in situ preparation of DHAP, and formation of an isosteric phosphonate analog...

Gas-phase oxidation of glycerol has been less investigated than liquid-phase oxidation it occurs via a two-step catalyzed reaction involving first the dehydration of glycerol into acrolein, catalyzed by an acid, and then its oxidation. The same reactions can be conducted in two distinct reactors, in which the first step can be carried out with an acid catalyst such as phosphoric acid over alumina [107]. Then acrolein is oxidized to acrylic acid with a conventional alumina-supported Mo/V/Cu/O catalyst. 

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