Glycerol oxidation catalysts

Au/C was established to be a good candidate for selective oxidation carried out in liquid phase showing a higher resistance to poisoning with respect to classical Pd-or Pt-based catalysts [40]. The reaction pathway for glycerol oxidation (Scheme 1) is complicated as consecutive or parallel reactions could take place. Moreover, in the presence of a base interconversion between different products through keto-enolic equilibria could be possible. 

A possible method for producing glycerol derivatives can be the reactive distillation in the presence of various oxide and mixed oxide catalysts, such as copper-chromite [3], In this reaction acetol, 1,2- and 1,3-propanediols may be obtained. 

Alternatively, acrylic acid can be obtained in a two-step reactor in which glycerol is catalytically dehydrated with an acid catalyst like H3PO4 on a-alumina [67]. The obtained acrolein is then oxidized with a commercially available oxidation catalyst, viz. Mo/V/W/Cu-oxide on a-alumina, yielding up 55% polymerization grade acrylic acid (Scheme 11.8) [68]. 

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 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. 

In a second wave of activity in the area of glycerol oxidation with Pt or Pd-on-carbon catalysts, the high yields for glyceric add at 60 °C and atmospheric pressure described earlier, were initially no longer obtained by other authors. It seems that there is appreciable formation of compounds other than C3 and C2 oxidized... 

Scheme 11.11 Reaction scheme for glycerol oxidation with dioxygen on Au/C catalysts. (After [84]).

In another patent, very high propane-1,2-diol yields were obtained with a multi-component oxide catalyst consisting of the metals Co, Cu, Mn and Mo, and P. At 523 K under 25 MPa, glycerol is almost quantitatively converted into propane-1,2-diol. In contrast to most studies, the reactions were performed with pure (99.5%) glycerol instead of aqueous dilutions [123],... 

Bienholz A, Blume R, Knop-Gericke A, Giergsdies F, Behrens M, Claus P. Prevention of catalyst deactivation in the hydrogenolysis of glycerol by Ga203-modified copper/zinc oxide catalysts. J Phys Chem C. 2011 115 999-1005. 

Demirel S, Lehnert K, Lucas M, Claus P. Use of renewables for the production of chemicals glycerol oxidation over carbon supported gold catalysts. Appl Catal B Environ. 2007 70 637-43. 

Dihydroxyacetone (DHA), the oxidation product of the secondary hydroxy group of glycerol, is an artificial tanning agent in cosmetics and a pharmaceutical intermediate. Glycerol oxidation in acidic medium on a platinum-bismuth catalyst (Bi/ Pt atomic ratio = 3) prepared by coprecipitation of Pt and Bi salts, yielded 20 % DHA at 30 % conversion [70]. The deposition of bismuth on platinum particles by oxido-reduction (Bi/Pt = 0.13) yielded 37 % DHA at 70% conversion... 

A subsequent detailed investigation of glycerol oxidation has been carried out by Prati et al. in Milano. In a first study, the relationship between catalyst morphology and selectivity was explored at full conversion it was found that larger gold particles (20 nm), supported on suitable carbons, show low TOFs but favour glycerate formation under mild conditions (30 °C, 3 bar) allowing yields up to 92% [39]. 

Based on our fundamental studies [13, 53, 68], we believe that the most convenient cell for employing glycerol as fuel is the AFC because the kinetics of glycerol oxidation and oxygen reduction reactions are favored in alkaline media with respect to those in acidic media and, in addition, the variety of non-noble metal-based catalysts which may be used in alkaline media is large. On the other hand, AFCs should work at relatively higher temperatures in order to compensate kinetic problems resulting from the use of non-noble metal-based catalysts. 

P. Paalanen, B. M. Weckhuysen and M. Sankar, Progress in controlling the size, composition and nanostructure of supported gold-paUadium nanoparticles for catafytic applications, Catal Scl Technol, 2013, 3, 2869-2880. A. Villa et al, Glycerol Oxidation Using Gold-Containing Catalysts, Acc. Chem. Res., 2015, 48(5), 1403-1412. 

Metal-functionalized CTFs have other important aspects. The Pt-loaded CTF shows high catalytic activity with recyclability towards methane oxidation at an elevated temperature in acidic media. A triazine network of 1,4-dicyanobenzene loaded with Pd nanoparticles has been shown to exhibit a better catalytic activity and lifetime than Pd-loaded activated carbon in the glycerol oxidation reaction. N-heterocyclic moieties of CTFs provide the required stability to Pd nanoparticles, while the rigid framework facilitates the selectivity of the catalyst. ... 

Figure 10.8 (Top) Supported Pd-based catalysts for glycerol oxidation (AC, activated carbon). (Bottom) Conversion on req cling of 1% Pd/CTF for glycerol oxidation.

Figure 19.7 Glycerol oxidation with H2O2 catalyzed by heterogenized complex 19.2b. Conditions glycerol, 0.21 M H2O2 (50% aqueous), 0.3 M catalyst 19.2b, 5 mg (which is equivalent to 4.4 x 10 M Mn ions) oxalic acid 0.002 M (a) and 0 M (b). Total volume of the reaction solution was 5 mL 22°C. Adapted from Reference 9.

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