Aromatic total oxidation

The development of noble metal catalysts and transition metal oxides for catalytic oxidation of VOCs has been widely reported in the literature. " The review paper published in 1987 by Spivey presents a good overview of catalytic combustion of VOCs. More recent reviews, focusing on the catalytic combustion of a wide range of VOCs by a wide variety of catalysts and on chlorinated VOCs, were published in 2004. In the last two years, two more reviews have been published. These reviews focused on the development of non-noble metal oxide catalysts for catalytic combustion of VOCs and on catalytic combustion catalysts for the removal of polycyclic aromatic hydrocarbons. This review is not intended to be an exhaustive account, but should provide an overview of the current state of research for catalysts used for alkane and aromatic total oxidation. The aim is also to identify the types of catalysts that are likely to be of use in the future, and the obstacles that must be overcome to produce viable catalysts. The development of a catalyst that may be used for the combustion of all classes of compounds under the general term VOC presents a major challenge for future research, as this has not yet been achieved. 

As part of the total synthesis of the triterpene (+)-a-onocerin, one of the first total syntheses in which RuO played a key role, a diphenylethyleneacetoxyketone was oxidised to the corresponding acetoxyketoacid by RuO /aq. Na(10yacetone. Aromatic ring oxidation was also involved (cf. 3.3.1 below) [219]. An oxidative cyclisation of a 1,5-diene to a diol by RuCl3/Na(10 )/wet SiO /THF formed part of the synthesis of the antitumour agent cw-solamin [220]. 

In addition, this study confirms that, in neat-liquid phase, the aromatic ring is not affected as previously observed in the case of alkyltoluenes (refs 4, 5). Moreover, the presence of an aromatic ring seems to stabilize the molecule with respect to total oxidation since a very small amount of CO2 is produced (0.4 %). 

The oxidation of butane (or butylene or mixtures thereof) to maleic anhydride is a successful example of the replacement of a feedstock (in this case benzene) by a more economical one (Table 1, entry 5). Process conditions are similar to the conventional process starting from aromatics or butylene. Catalysts are based on vanadium and phosphorus oxides [11]. The reaction can be performed in multitubular fixed bed or in fluidized bed reactors. To achieve high selectivity the conversion is limited to <20 % in the fixed bed reactor and the concentration of C4 is limited to values below the explosion limit of approx. 2 mol% in the feed of fixed bed reactors. The fluidized-bed reactor can be operated above the explosion limits but the selectivity is lower than for a fixed bed process. The synthesis of maleic anhydride is also an example of the intensive process development that has occurred in recent decades. In the 1990s DuPont developed and introduced a so called cataloreactant concept on a technical scale. In this process hydrocarbons are oxidized by a catalyst in a high oxidation state and the catalyst is reduced in this first reaction step. In a second reaction step the catalyst is reoxidized separately. DuPont s circulating reactor-regenerator principle thus limits total oxidation of feed and products by the absence of gas phase oxygen in the reaction step of hydrocarbon oxidation [12]. 

As we were not only interested in the development of a racemization method but also wanted to evaluate an asymmetric synthesis articulated around the imine intermediate 7 vide infra), we initially investigated its controlled preparation by oxidation of the unwanted (7 )-TH[3C 4 obtained from the mother liquors. Among the various methods initially tested, good results (approximately 75 to 80% in situ yield of imine 7) were obtained with NaOCl in methanol/THF at 0 to 5°C for 3 h. The major by-product is the overoxidized (3-carboline derivative (5 to 10%), although in some experiments, low levels of the unstable N-chloroamine intermediate were also detected. Later on, approximately 68% in situ yield was obtained with tetra-n-propylammonium perruthenate (0.05 equiv) as catalyst with iV-methylmorpholine oxide (1.5 equiv) as cooxidant in acetonitrile at room temperature. However, in this latter method, up to 16% of totally oxidized (3-carboline was also formed. The imine 7 was then directly reduced with sodium borohydride to produce the racemic material in approximately 50% isolated overall yield. Although the aromatic (3-carboline by-product was easily removed upon salt formation, the above approach suffered from several major drawbacks difficulty to control the overoxidation of the desired dihydro-(3-carboline to the (3-carboline on... 

Mixed oxide systems of well-defined ABO3 structure (perovskites), although not exclusively, are more stable in the presence of such compoimds and appeared as a reasonable alternative. Based on this, the aim of this chapter is to highlight and compare selected examples of perovskites and related oxides in total oxidation of heavy hydrocarbons and aromatics and their halogenated derivatives. The perovskite structure encompasses a wide array of materials with differing physical, chemical, and electronic properties. 

Toluene is a representative example of aromatic compounds investigated due to its extensive use in industrial processes as solvent or as feedstock for the production of polymers or adhesives, but also due to its existence in petroleum-derived fuels. In a close relation, catalytic total oxidation takes into consideration the serious environmental concern generated by its perilous health effects but also by its contribution to global warming smog processes as a result of the emission of toluene from both stationary and mobile sources. 

The role of support on the performance of noble metals-based catalysts for the total oxidation of aromatic hydrocarbons is essential [38]. Although with a smaller surface area than the typical supports, perovskites also demonstrate good properties as carriers for noble metals. Thus, perovskites of type LaBOs (B = Co, Mn, Fe, Ni) synthesized using the citrate route were used as support for noble metals in total oxidation of toluene [39]. The performances of these catalysts varied in the order Fe>Mn>Co>Ni, and the superior behavior of iron was attributed to the low temperature of calcination and the high stability of the perovskite lattice irrespective of the nature of the stream it was exposed to. The dispersion of palladium at the different stages of the process remained unchanged. 

Literature reports provided arguments demonstrating that the performance of catalysts for the total combustion of alkanes and monoaromatic compounds cannot be directly extrapolated to predict their efficacy for the total oxidation of polycyclic aromatic hydrocarbons [43]. Therefore, the total oxidation of these molecules should consider specific catalytic systems. 

Studies reported to date confirmed the successful replacement of noble metals by perovskites in total oxidation of heavy hydrocarbons and aromatics. Besides activity, these materials allow a good stability, especially in the presence of halogens. Another relevant advantage is the very large composition of these... 

---