Noble metal propylene oxidation

With a molar ratio of propylene glycol to H2O2 of 2.5, the selectivity to hydroxyacetone at 32% conversion of the glycol was 94%, and the selectivity based on H202 was 85%. Small amounts of acetic acid and formic acid were detected. The initial oxidation proceeds with high selectivity for the secondary alcohol group. Further oxidation affords oxidative cleavage products rather than pyruvic acid, as is observed when the oxidation of hydroxyacetone is carried out with 02 and noble metal catalysts. 

Fig. 2. Reaction mechanism for propylene oxidation over noble metals (22).

The only direct evidence for the participation of adsorbed oxygen in the selective oxidation of propylene under reaction conditions is the work of Cant and Hall (22, 23). They reported that propylene could be oxidized to acrolein over noble metals (Au, Rh, and Ru) supported on low-surface-area a-alumina or silica. These catalysts do not contain any usable lattice oxygen, therefore, only adsorbed or gas-phase oxygen is available as a... 

Hydrocarbon oxidation on base metal catalysts is also susceptible to lead poisoning, especially if the catalysts are exposed to relatively high temperatures, for at least part of their service time. It was noted above that lead retention, especially on base metal catalysts, also increases with temperature up to a certain point. This behavior is shown by the results of Yao and Kummer (81) in Fig. 18. One should note that the hydrocarbon used for testing catalyst activity, namely propylene, was quite reactive. With a less reactive test hydrocarbon one could expect a still sharper effect. The comparison with a reference production noble metal catalyst, given in Fig. 18, is quite instructive. 

When Pt/Al203 were used and propylene was applied instead of ammonia [29], the effect oxide and noble metal promoters were investigated. Although the oxide promoters exert various effects, the reaction is basically controlled by the platinum sites. N2/N2O ratio is about 1, indicating that both molecules form by a parallel route from NO. 

Other noble metals such as Pd or Rh which have more stable oxides than Pt and therefore tend to remain well-dispersed on 7-AI2O3 even in the absence of additives such as ceria or molybdena are usually poor catalysts for the oxidation of saturated hydrocarbons. In fact, in a mixture of hydrocarbons containing 2/3 propylene and 1/3 propane the propylene will be easily oxidized <300°C while the propane remains untouched even at 600°C [13],... 

Fresh and thermally aged catalysts containing mixtures of platinum and palladium were laboratory tested for the oxidation of carbon monoxide, propane, and propylene. For both monolithic and particulate catalysts, resistance to thermal deactivation was optimum when palladium content was 80%. Full-scale vehicle tests confirmed these findings. Catalysts of this composition were developed which, on the basis of durability tests at Universal Oil Products and General Motors, appeared capable of meeting the 1977 Federal Emissions Standards with as little as 0.56 g noble metal per vehicle. The catalyst support was thermally-stabilized, low density particulate. 

As can be seen in Table 11.1, noble metals Pt, Pd, and Rh are the most usable catalysts in calorimetric gas sensors designed. It was established that of all the catalysts known they have the highest activity with respect to oxidation of combustible gases (see Fig. 11.2) and provide acceptable operation temperatures. Morooka and coworkers (Morooka and Ozaki 1966 Morooka et al. 1967) showed that activity for a model reaction, propylene oxidation, correlates with the strength of the metal-oxygen (M-0) bond. Because an LEL sensor must oxidize all ambient hydrocarbon species, the highest activity catalysts hold the most promise for the application. Therefore, the choice of palladium and platinum and sometimes rhodium for application in combustion gas sensors is natural (Miller 2001). This explains why the automobile exhaust system is treated with platinum or palladium compounds and is called a catalytic converter. 

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