Oxidation catalysts destruction

These porphyrins with chiral superstructures are expected to be insufficiently robust against oxidative catalyst destruction when H2O2 is used as the source... 

Porphyrines and phthallocyanines suffer from oxidative degradation and oxidative dimerization [68]. The improved activity of the zeolitic systems is due to the effective site isolation within the pores, which prevents any bimolecular pathways to catalyst destruction [63]. Therefore, deactivation is more severe for the homogeneous catalysts than for the heterogenized TMPc. FePc itself is a poor catalyst for alkane oxidation with a high initial turn-over, but after less than 45 minutes it becomes completely inactive. On the contrary, FePc encaged in zeolite Y is stable for 24 hours [63]. 

Cao et. al. also examined titanium dioxide photocatalysts doped with 0.5% platinum. These doped catalysts displayed complete activity recovery following thermal regeneration at 350°C. Presumably, the addition of platinum, which may act as a thermal oxidation catalyst, allows for the destruction of accumulated intermediates generated during the photocatalytic oxidation at lower temperatures than untreated titanium dioxide. 

Since the porphyrin ring is liable to oxidative self-destruction and catalyst recovery is problematic, studies have focused on the development of porphyrins with... 

As in the ethylene oxide system kinetics are complex and do not lend themselves to exact interpretation (19). The boron fluoride — water catalyst system appears to be most effective at a boron fluoride/water ratio of about three, a surprising and probably fortuitous similarity to the efficiency of this catalyst in the isomerization of some hydrocarbons (20). At low water concentrations the number of polymer molecules formed equals the number of water molecules added and chain transfer may be assumed, though it has not actually been demonstrated. There is some indication of a maximum molecular weight of 15,000—20,000 at — 20° C but the present data are inadequate to establish this point. The order in monomer appears to be first at low water concentrations rising to second at higher water levels, but it seems quite possible that this apparent change in order is due to some factor such as catalyst destruction. 

The most important role of U03 is in the production of UF4 [10049-14-6] and UF6 [7783-81-5], which are used in the isotopic enrichment of uranium for use in nuclear fuels (119—121). The trioxide also plays a part in the production of U02 for fuel pellets (122). In addition to these important synthetic applications, microspheres of U03 can themselves be used as nuclear fuel. Fabrication of U03 microspheres has been accomplished using sol-gel or internal gelation processes (19,123—125). Finally, U03 is also a support for destructive oxidation catalysts of oiganics (126,127). 

The key issue in effective catalytic oxidation of organics is finding a suitable catalyst. Oxidation of aqueous phenol solutions by using different transition metal oxides as heterogeneous catalysts is already known [4-6]. On the other hand, the potential of molecular sieves to catalyze oxidative phenol destruction has not been examined yet. The objective of this contribution is to provide kinetic and mechanistic data on the catalytic liquid-phase oxidation of aqueous phenol solutions obtained in the presence of various transition metal oxides and molecular sieves. The reaction was studied in a semibatch slurry as well as two-and three-phase continuous-flow reactors. Another matter of concern was the chemical stability of catalysts under the reaction conditions. 

Thereafter total oxidation proceeds via gas phase radical chemistry. The conversion-temperature profile for a given substrate depends upon the activity of the catalyst and the inherent destructibility of the substrate. Here we propose that the lower the bond dissociation enthalpy of the weakest bond the more readily the substrate can be activated by the catalyst. At the basis of this hypothesis is the suggestion that active dtes in total oxidation catalysts distinguish between C-H bonds on a bond strength basis only. 

To meet the 1996 EU emission regulations all diesel engined cars are now fitted with oxidation catalysts. Some, but not many, heavy duty vehicles also use oxidation catalysts. The fitment of an oxidation catalyst allows overall reductions in particulate levels of up to 50%, destruction of the organic fraction of the particulate and significant reductions in CO, HC and the characteristic diesel odour. Very low back pressures mean little effect on performance and economy. Operation wifli low sulphur fuel is desirable since sulphur inhibits the performance of the catalyst and catalyst activity must be reduced to avoid oxidation of sulphur to the acidic sulphur trioxide. 

This paper deals with the hydrothermal deactivation, under an air + 10 vol. % H2O mixture between 923 and 1173 K, of Cu-MFI solids, catalysts for the selective reduction of NO by propane. Fresh and aged solids were characterized by various techniques and compared with a parent H-ZSM-5 solid. The catalytic activities were measured in the absence and in the presence of water. The differences between fresh and aged Cu-ZSM-5 catalysts (destruction of the framework, extent of dealumination...) were shown to be small in spite of the strong decreases in activity. Cu-ZSM-5 is more resistant to dealumination than the parent H-ZSM-5 zeolite. The rate of NO reduction into N2 increases with the number of isolated Cu VCu ions. These isolated ions partially migrate to inaccessible sites upon hydrothermal treatments. At very high aging temperatures a part of the copper ions agglomerates into CuO particles accessible to CO, but these bulk oxides are inactive. Under catalytic conditions and in the presence of water, dealumination is observed at a lower temperature (873 K) than under the (air + 10 % H2O) mixture, because of nitric acid formation linked to NO2 which is either formed in the pipes of the apparatus or on the catalyst itself... 

Certain of the rare earth oxides are very active catalysts. The introduction of small amounts of samarium oxide to a copper oxide catalyst enabled lower temperatures to be used in the oxidation of ethanol although it also reduced the yields of aldehyde obtainable probably because of the catalytic effect of the rare earth oxide on the decomposition of aldehyde.80 Thus, with 482° C. as the best temperature under the conditions with a copper oxide catalyst, the introduction of 0.5, 1.0, 2.0, and 5.0 per cent amounts of samarium oxide lowered the effective temperatures to 460°, 465°, 395°, and 370° C., respectively. Considerable excess of oxygen was used in these experiments, 0.40 to 0.43 liters of oxygen per gram of ethanol as compared to 0.2434 liters theoretically necessary, and probably accounts for a part of the alcohol destruction. However, considerable charring was evident with these catalysts unless an excess of air was present. It is apparent that such active catalysts are of no utility in the oxidation of such readily oxidized substances as the alcohols. 

B. Chen, C. Bai, R. Cook, J. Wright, C. Wang. Gold/cobalt oxide catalysts for oxidative destruction of dichloromethane. Catalysis Today, 30 15-20,1996. 

The most widely used metal oxidation catalysts are platinum, palladium, copper, and silver. With all these, except silver, hydrocarbons undergo oxidation with complete destruction of the molecular skeleton into CO 2 and H20. Silver is the sole catalyst for obtaining ethylene... 

The destruction of halogenated VOCs, particularly those of short chain length, are of great industrial and environmental importance, and a considerably number of studies using oxide catalyst have investigated this area. The oxidation of 0.74 vol.% dichloroethylene by a wide range of oxides has been studied by Imamura at 3,600 h space velocity [66]. Catalytic activity was defined in terms of CO2 yield and at 650°C the activity was ranked in the order ... 

Catalysts based on uranium oxide are also particularly active for the destruction of the chlorinated VOCs chlorobenzene and chlorobutane [77]. Both were destroyed by U3O8 at 350°C and 70,000 h space velocity, showing 99.7% and >99.5% conversions respectively. Time-on-line studies for the destruction of 0.12% chlorobenzene at 450°C showed that the catalyst was not deactivated as 99.9% conversion was maintained during 400 hours continuous operation. These catalysts were also active for the oxidative abatement of other VOCs and it has been demonstrated that toluene, butylacetate and cyclohexanone can also be destroyed at relatively low temperatures. Considering the high space velocities employed in these studies, uranium based catalysts are amongst some of the most active oxide catalysts investigated for VOC destruction. 

The addition of metals to oxide catalysts, which are already active for combustion processes, is an interesting approach which attempts to combine the beneficial aspects of both types of catalyst system. The incorporation of the metal component generally increases the activity, however, the destruction of halogenated VOCs have not been investigated and problems associated with deactivation may take place. 

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