Platinum oxidation, alcohols

For more selective hydrogenations, supported 5—10 wt % palladium on activated carbon is preferred for reductions in which ring hydrogenation is not wanted. Mild conditions, a neutral solvent, and a stoichiometric amount of hydrogen are used to avoid ring hydrogenation. There are also appHcations for 35—40 wt % cobalt on kieselguhr, copper chromite (nonpromoted or promoted with barium), 5—10 wt % platinum on activated carbon, platinum (IV) oxide (Adams catalyst), and rhenium heptasulfide. Alcohol yields can sometimes be increased by the use of nonpolar (nonacidic) solvents and small amounts of bases, such as tertiary amines, which act as catalyst inhibitors. 

Before showing how direct peroxidation of active group works, it should be mentioned that platinum catalyses oxidation of alcohols at ambient temperature in aldehydes (provided they are primary). The exothermicity of the reaction is sufficient to cause the alcohol to combust by heating a thread of platinum at a high temperature. Note that 2-butanol (but not isopropanol) is classified in list B, producing peroxides that become expiosive at a certain concentration in the lists of peroxidable compounds set up by Du Pont de Nemours company (see p.261). 

Rightmire RA, Rowland RL, Boos DL, Beals DL. 1964. Ethyl alcohol oxidation at platinum electrodes. J Electrochem Soc 111 242-247. 

Alcohol oxidation is inliibited at platinum surfaces with high surface step densities due to the formation of an organic film [12]. More recent attempts to improve the direct electiochemical oxidation of alcohols involve the use of a platinum... 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

It has recently been recognized that crystal structure and particle size can also influence photoelectrochemical activity. For example, titanium dioxide crystals exist in the anatase phase in samples which have been calcined at temperatures below 500 °C, as rutile at calcination temperatures above 600 °C, and as a mixture of the two phases at intermediate temperature ranges. When a range of such samples were examined for photocatalytic oxidation of 2-propanol and reduction of silver sulfate, anatase samples were found to be active for both systems, with increased efficiency observed with crystal growth. The activity for alcohol oxidation, but not silver ion reduction, was observed when the catalyst was partially covered with platinum black. On rutile, comparable activity was observed for Ag, but the activity towards alcohol oxidation was negligibly small . Photoinduced activity could also be correlated with particle size. 

The chlorine atoms in these compounds are unusually labile, especially in the case of the cis isomer, which in aqueous and alcoholic solution yields an immediate precipitate with silver nitrate, while the trans isomer reacts more slowly. This lability also causes interconversion to occur readily, as during recrystallization from water or often in metatheses. The two isomers can readily be distinguished by treatment with silver(I) oxide.2 The cis compound reacts immediately to give the soluble strong base [Pt (C2-H5)2S 2(OH)2], while the trans compound slowly decomposes to black platinum(II) oxide and diethyl sulfide, the solution remaining neutral. 

Platinum-catalyzed oxidation of alcohols in aqueous solutions. The role of Bi-promotion in suppression of catalyst deactivation... 

A two-step hydrolysis of crude 2 with trifluoroacetic acid and lithium hydroxide gives the optically pure a-hydrazino acids 3 (ee >98 %). Reduction with hydrogen on platinum(II) oxide reduces 3 to the a-amino acids 4 in high yields. Alternatively, 2 can also be reduced with metal hydrides (LiAIH4/Et20) to, V-inethylcphedrinc and /i-hydrazino alcohols 5. After transformation into the Mosher ester [with (- )-(S)-methoxy-4-(tri fluoromcthyljphenylacctyl chloride], 5 (R = CH3) shows a diastereomeric ratio d.r. [(2,S,2, S )/(2,S, 2 7f)] >95 5. 

The reaction mechanism of alcohol oxidation on smooth platinum electrode was also investigated by combined electrochemical, analytical and spectroscopic techniques. On line chromatographic techniques, particularly High Performance Liquid Chromatography, were developed to analyze quantitatively the reaction prod-ucts." °... 

Among the aliphatic alcohols, oxidation of methanol has been studied most extensively [122-125]. At a platinum anode in acidic aqueous solutions, methanol oxidizes completely to CO2. Higher primary alcohols oxidize to aldehydes and acids under these conditions, though detailed mechanistic studies are lacking [126,127]. Anodic oxidation of secondary alcohols in aqueous acid leads to the corresponding ketones in high yield, but the reaction has received little attention over the years [126,128]. Indirect oxidation methods employing mediators are of considerable interest in this area and are treated elsewhere. 

The platinum-catalyzed oxidation with oxygen can also be applied for selective oxidation of secondary alcohols if no primary alcohol is present [73]. Like the tin-bromine method, axial secondary hydroxy groups will undergo preferential oxidation over equatorial hydroxy groups. However, as described above large amounts of platinum metal are required for these oxidations. Some improvement in catalyst activity has been achieved by promotion of platinum with bismuth or lead [76]. This also causes a change in selectivity and makes it possible in... 

The platinum catalyzed oxidation of alcohols with molecular oxygen has been known for a long time.52,53 Palladium and iridium are also effective for this reaction but not rhodium or ruthenium.54 The reaction proceeds by the initial dehydrogenation of the alcohol to produce the aldehyde or ketone with the adsorbed hydrogen then reacting with the oxygen to give water.53>55-58 These... 

This selectivity for axial alcohol oxidation is not, apparently, universal since both the 3a- (18) and the 3P-cholestanol (19) are oxidized to cholestanone (20) with a better yield obtained on oxidation of the equatorial, 3p-alcohol (Eqn. 21.31). Selectivity in the platinum catalyzed oxidation of steroid alcohols. 

Oxidation of hydrocarbons and alcohols If reasonably effective oxidation catalysts can be identified for aqueous electrolytes, hydrocarbon and alcohol oxidation processes would make possible promising fuel cells operating directly on quite practical fuels at moderate temperatures. The currently used platinum and platinum-family metals and alloys have substantial activity, but it is not sufficient for practical fuel cells with aqueous electrolytes. With the many electrons involved in the complete oxidation, the detailed mechanisms for the oxidation are likely to be quite complex. To avoid incomplete oxidation it is probably necessary to have the reactants remain adsorbed on the electrode surface through the complete oxidation to C02 and H20. Here again, new promising catalysts and new experimental approaches are needed. 

The currently most widely accepted reaction mechanism for alcohol oxidation catalyzed by supported gold nanoparticles has been proposed by analogy with the assumed reaction mechanism for alcohol oxidation in palladium and platinum metals taking into consideration the kinetic data and mechanistic information obtained from gold-catalyzed CO oxidation [110]. Scheme 12.11 illustrates a reasonable mechanistic proposal. 

Platinum and palladium are effective catalysts for alcohol oxidation when used alone however, significant stability and selectivity improvements have been observed on incorporation of a second (usually less active) metal promoter such as Bi, Pb, and Sn [63-65]. These observations are common to aerobic selox of allyhc and benzylic alcohols, as well as polyols such as propylene glycol [64] and glycerol [66]. In the case of Bi, in situ X-ray absorption spectroscopy (XAS) and attenuated total reflection infrared spectroscopy (ATR-IR) indicate that the promoter protects Pt against deactivation by overoxidation and prevents site blocking by, for example, aromatic solvents [67]. 

With a different approach, the influence of bismuth on the electrocatalysis of glycerol was also investigated by Koper and co-workers [67]. They observed that a carbon supported platinum electrode in a bismuth-saturated solution is highly selective to the electro-oxidation of the secondary alcohol of the glycerol, leading to 100 % of dihydroxyacetone at a carefully chosen potential. Using a combinatiOTi of online HPLC and in situ FTIR, the authors have shown that bismuth not only blocks the pathway towards the primary alcohol oxidation but also provides a... 

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