Heterogeneous selective oxidation reactions

The EM studies show that the novel glide shear mechanism in the solid state heterogeneous catalytic process preserves active acid sites, accommodates non-stoichiometry without collapsing the catalyst bulk structure and allows oxide catalysts to continue to operate in selective oxidation reactions (Gai 1997, Gai et al 1995). This understanding of which defects make catalysts function may lead to the development of novel catalysts. Thus electron microscopy of VPO catalysts has provided new insights into the reaction mechanism of the butane oxidation catalysis, catalyst aging and regeneration. 

A key objective in heterogeneous catalysis is the development of catalysts that are 100% selective for the desired reaction. One path toward this goal is to synthesize catalysts with uniform isolated active sites. The engineering of these catalysts, especially for selective oxidation reactions,... 

Various heterogeneous asymmetric oxidation reactions using inorganic supports are listed in Table 2.2. A few selected examples will be described herein on the basis of reachon type. 

Very recently, ultrafme metal oxides have attracted much research interests in terms of materials science and heterogeneous catalysis[10-12]. These new catalytic materials are expected to have unique catalytic properties because of their nano-scale particle sizes. In this work, a novel catalyst for selective oxidation of toluene to benzaldehyde, i.e. ultrafme complex molybdenum based oxide particles, has been developed. It has been found that the reactivity of lattice oxygen ions can be improved by decreasing the oxide particle size to nano-scale and that the ultrafme oxide particles exhibit unique catalytic properties for selective oxidation. Our results have revealed that the ultrafme complex oxide particles are potentially new catalytic materials for selective oxidation reactions. 

Phthalocyanine (Pc) complexes of transition metals have received much attention in the scientific literature of the last two decades, not only as mild catalysts for selective oxidation reactions but also as functional models for enzymes. Unfortunately, their use is hampered by their reduced solubility in solvents and their tendency to form adducts even when used in solution. Provided such complexes can be immobilized individually on a catalyst carrier, it is expected that an enhanced dispersion of the complex will be achieved. The use of heterogenized Pc complexes will also no longer be restricted by the nature of the reaction medium or solvent. The issue has been reviewed as early as 1986 [3]. 

Another good example of greener chemistry through the use of heterogeneous catalysis is the use of TSl, a titanium siHcate catalyst for selective oxidation reactions [21] such as the 4-hydroxylation of phenol to the commercially important hydroquinone (Scheme 1.1-2). 

Table I Examples of Selective Heterogeneous Catalytic Oxidation Reactions. ...

Shell catalysts consist of an compact inert support, usually in sphere or ring form, and a thin active shell that encloses it [4]. Since the active shell has a thickness of only 0.1-0.3 mm, the diffusion paths for the reactants are short. There are many heterogeneously catalyzed reactions in which it would be advantageous to eliminate the role of pore difiusion. This is particularly important in selective oxidation reactions, in which further reactions of intermediate products can drastically lower the selectivity. An example is acrolein synthesis two catalysts with the same active mass but different shell thicknesses differed greatly in selectivity at the high conversions desired in industry (Fig. 6-5). Therefore, if acrolein synthesis is to be operated economically, the shell thickness must be optimized. 

Novel and innovative approaches for the selective catalytic homogeneous oxidation of alkanes and alkenes to produce industrially important alcohols, aldehydes, ketones, and epoxides aie still urgently needed. Homogeneous catalytic oxidation is a process that is more selective to products and, as well, the reactions aie carried out at lower temperatures in comparison to heterogeneous catalytic oxidation reactions. However, the separation of the homogeneous catalyst from the oxidation products is energy intensive for example, distillation, as well as possible thermal decomposition of the catalyst during distillation. We will discuss... 

Despite this limitation, the development of TS-1 assisted in the creation of a new type of solid, recyclable catalyst called a redox molecular sieve and subsequently enabled organic selective oxidation reactions to encompass many principles of green chemistry via heterogeneous catalysis. To alleviate the diffusion limitations of TS-1, incorporation of Ti into larger mesoporous silicas has been widely reported [118,131-133]. 

All these molecules are synthesized by selective oxidation reactions, which can be gas-phase heterogeneously catalyzed (maleic and phthalic anhydride) or... 

A wide variety of examples have been pubhshed in which these two techniques were used to explore the role of certain TMIs in heterogeneous catalytic oxidation reactions, some of which have been discussed in review papers. An exhaustive presentation of the application potential of these methods is impossible within this chapter. Instead, an instructive example highlighting the benefits of combining EPR and UV-vis spectroscopy for elucidating active sites in vanadium-containing oxidation catalysts will be presented in more detail, followed by a summary of selected more recent applications on other oxidation catalysts, for which the reader is referred to the respective literature for a more detailed description. 

Microenvironment Heterogeneous catalysis Oxidation reactions Organic nitrations Para-selectivity in nitration Synthesis of Macrocycles Calix[4] pyrroles MCM-41... 

Supported noble metal catalysts ate most widely used among heterogeneous catalysts because they are active for hydrogenation and dehydrogenation reactions as well as for complete and selective oxidation reactions (1). Although they ate usually prepared via impregnation, the influence of preparation method on their fine stmcture and catalytic properties has not been fully understood yet. 

Other important uses of stannic oxide are as a putty powder for polishing marble, granite, glass, and plastic lenses and as a catalyst. The most widely used heterogeneous tin catalysts are those based on binary oxide systems with stannic oxide for use in organic oxidation reactions. The tin—antimony oxide system is particularly selective in the oxidation and ammoxidation of propylene to acrolein, acryHc acid, and acrylonitrile. Research has been conducted for many years on the catalytic properties of stannic oxide and its effectiveness in catalyzing the oxidation of carbon monoxide at below 150°C has been described (25). 

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