Selective catalytic oxidation over highly

The selective catalytic oxidation over highly dispersed noble metal catalysts in aqueous media is gaining interest (1-5). One of the main problems consists of the deactivation of the catalyst. The role of oxygen is recognized as being crucial in this matter (6,7). 

All these catalytic results, however, were usually achieved at very low (2-3%) conversions. The only exception is a paper reporting up to 80% selectivity at 20% conversion over a M0CI5—R4Sn-on-silica olefin metathesis catalyst (700°C, 1 atm, CH4 air = l).42 In general, higher temperature and lower—about ambient— pressure compared to homogeneous oxidation, and high excess of methane are required for the selective formation of formaldehyde in catalytic oxidations.43 The selectivity, however, decreases dramatically at conversions above 1%, which is attributed to the decomposition and secondary oxidation of formaldehyde.43,44 It is a common observation that about 30% selectivity can be achieved at about 1% conversion. 

Although it was not suggested that defects are required for selective oxidation over other catalysts, the results indicated that defects and bismuth must be present for high activity and selectivity over scheelite-type catalysts. The authors concluded that the defects which were introduced into the bulk of these phases must manifest themselves in some manner at the surface. The question of how the introduction of defects into these phases affected their catalytic properties was not resolved. However, the active site for catalysis was suggested as a cation vacancy which could abstract a proton from an olefin to form the well-established allyl intermediate and should offer considerable stabilization to a surface hydroxyl group. 

Oxidation over zeolites The results of Iwamoto et al. on the oxidation of benzene to phenol [58] stimulated further efforts in the search for new and more efficient catalytic systems. As a result, in 1988, ZSM-5 zeolites were shown to be the best catalysts for this reaction [59-61]. Over zeolites, the reaction proceeded at much lower temperature and, which was even more important, with a very high selectivity, approaching 100%. Further studies involving many other groups [62-80] contributed much to the improvement of ZSM-5 catalysts. Some other type zeolites and FeP04 were shown to be also active [69, 70, 73, 80, 81], although their efficiency was inferior to that of ZSM-5. 

Liquid phase catalytic oxidation of ethylbenzene with hydrogen peroxide over TS-1 molecular sieves is most appropriate for the production of 1-phenylethanol with high selectivity (up to 93 % of all the oxidation products in methanol) under the reaction conditions studied here. An additional increase of the 1-phenylethanol selectivity could be achieved with smaller amounts of the catalyst. The highest conversion to acetophenone is found over TS-2 zeolites but further oxidation easily takes place in this case. 

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