Bismuth promotion

Wenkin, M., Touillaux, R., Ruiz, P., Delmon, B., and Devillers, M. (1996) Influence of metallic precursors on the properties of carbon-supported bismuth-promoted palladium catalysts for the selective oxidation of glucose to gluconic acid. Appl. Catal., A, 148, 181-199. 

The Effect of Bismuth Promotion on the Selective Oxidation of Alcohols Using... 

Bonnemann, H. etal., Selective oxidation of glucose on bismuth-promoted Pd-Pt/C catalysts prepared from N(Oct)4Cl-stabilized Pd-Pt colloids, lnorg. Chim. Acta., 270, 95, 1998. 

Promotion and deactivation of unsupported and alumina-supported platinum catalysts were studied in the selective oxidation of 1-phenyl-ethanol to acetophenone, as a model reaction. The oxidation was performed with atmospheric air in an aqueous alkaline solution. The oxidation state of the catalyst was followed by measuring the open circuit potential of the slurry during reaction. It is proposed that the primary reason for deactivation is the destructive adsorption of alcohol substrate on the platinum surface at the very beginning of the reaction, leading to irreversibly adsorbed species. Over-oxidation of Pt active sites occurs after a substantial reduction in the number of free sites. Deactivation could be efficiently suppressed by partial blocking of surface platinum atoms with a submonolayer of bismuth promoter. At optimum Bi/Ptj ratio the yield increased from 18 to 99 %. 

In this paper we report the application of bimetallic catalysts which were prepared by consecutive reduction of a submonolayer of bismuth promoter onto the surface of platinum. The technique of modifying metal surfaces at controlled electrode potential with a monolayer or sub-monolayer of foreign metal ("underpotential" deposition) is widely used in electrocatalysis (77,72). Here we apply the theory of underpotential metal deposition without the use of a potentiostat. The catalyst potential during promotion was controlled by proper selection of the reducing agent (hydrogen), pH and metal ion concentration. 

Before bismuth-promotion the Pt-on-alumina catalyst was pre-reduced in water with hydrogen. The pH was decreased to 3 with acetic acid and the appropriate amount of bismuth nitrate dissolved in water (10 - lO " M) was added into the mixed slurry in 15-20 min, in a hydrogen atmosphere. Promotion of unsupported Pt was carried out similarly. The metal composition of the bimetallic catalysts was determined by atomic absorption spectroscopy. 

Bismuth promotion suppresses the hydrogen sorption on platinum (curves b-d). The peak at -0.05 V indicates the oxidation of adsorbed bismuth, which overlaps the OH adsorption on uncovered platinum surface sites 18). Bismuth adatoms are discharged in the low potential region (Bi°) and occupy three platinum sites at low surface coverages. The structure of the oxygen-containing species are (BiOH), (BiO), and [Bi(OH)J,d 19). 

In Figure 5 the conversion of 1-phenylethanol and the open circuit potential of alumina-supported catalysts are plotted as a function of reaction time. There is a striking difference between the curves of unpromoted (a, a ) and bismuth-promoted (c, c ) catalysts. When air is introduced to the reactor, the potential of the platinum-on-alumina catalyst quickly increases to the anodic direction and after one minute the catalyst potential is above -300 mV. One may conclude that there is practically no hydrogen on the platinum surface and after a short period an increasing fraction of platinum is covered by OH. The influence of bismuth promotion is a higher reaction rate (final conversion) and lower catalyst potential during reaction. 

M. Besson, F. Lahmer, P. Gallezot, P. Fuertes, and G. Fleche, Catalytic oxidation of glucose on bismuth-promoted palladium catalyst, J. Catal., 152 (1995) 116-121. 

Additional examples of the bismuth-promoted oxidation include the bismuth phthalocyanine catalyzed hydroxylation of phenol to hydroquinone and catechol with hydrogen peroxide [91SSSC(66)455], and bismuth acetate-mediated oxidation (modified Prevost reaction) of cyclohexenes to 1,2-cyclo-hexandiol derivatives (Scheme 5.14) [89CC407]. 

Data for the analysis of the reaction medium after reaction is also presented in Table 1. After 22 hours of reaction, levels of leached platinum were below the liimt of detection of the analytical method. However, significant quantities of the bismuth promoter were leached (26 %-50%, across the pH range 2-6). 

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. 

By using close-to-neutral conditions for the catalytic oxidation of glyceric and tartronic acids on platinum-bismuth catalysts to their respective keto derivatives, deactivation of the catalyst by adsorbed acids may be reduced, leading to higher degrees of conversion and improved yields. In addition, higher loadings of bismuth promoter may also serve to reduce deactivation by adsorbed acids. 

In general, the platinum component of the catalyst appears to be highly stable under the conditions employed, with very little or no leaching observed. The bismuth promoter, however, is clearly prone to lixiviation. Unfortunately, it would app)ear that the conditions required to optimise selectivity also lead to dissolution of the promoter. 

In summary, this work demonstrates that high selectivities for oxygenated keto-acids derived from glycerol may be obtained by catalytic oxidation on bismuth-promoted platinum under acidic conditions. However, problems of catalyst deactivation by adsorbed acids, overoxidation of targeted products and leaching of the promoter need to be overcome to attain the ultimate goal of theoretical yield. 

Heinen et al. [35] investigated the oxidation of D-fructose on Pt/C catalyst at 30 °C and pH 7.3. Oxidation of both the C, primary alcohol and the C5 secondary alcohol occurred. The reaction stopped at ca 80 % conversion giving mainly 2-keto-D-gluconic acid (45% selectivity) and D-t/n-eo-hexo-2,5-diulose or 5-ketofructose (27 % selectivity). In the presence of bismuth-promoted catalysts selectivity to the former was slightly increased. This was attributed to the complexation by bismuth of the y9-o-fructofuranose stmcture via the c/s-diol functions. 

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