Homogenous chemical promotion oxidation

Supercritical CO2 is a non-polar, aprotic solvent and promotes radical mechanisms in oxidation reactions, similar to liquid-phase oxidation. Thus, wall effects might occur as known, e.g. from olefin epoxidation with 02 or H202 which may decrease epoxide selectivities. The literature covers the synthesis of fine chemicals by oxidation either without catalysts (alkene epoxidation, cycloalkane oxidation, " Baeyer-Villiger oxidation of aldehydes and ketones to esters ), or with homogeneous metal complex catalysts (epoxidation with porphyrins, salenes or carbonyls ). Also, the homogeneously catalysed oxidation of typical bulk chemicals like cyclohexane (with acetaldehyde as the sacrificial agent ), toluene (with O2, Co +/NaBr ) or the Wacker oxidation of 1-octene or styrene has been demonstrated. 

Catalytic processes frequently require more than a single chemical function, and these bifunctional or polyfunctional materials innst be prepared in away to assure effective communication among the various constitnents. For example, naphtha reforming requires both an acidic function for isomerization and alkylation and a hydrogenation function for aromati-zation and saturation. The acidic function is often a promoted porous metal oxide (e.g., alumina) with a noble metal (e.g., platinum) deposited on its surface to provide the hydrogenation sites. To avoid separation problems, it is not unusual to attach homogeneous catalysts and even enzymes to solid surfaces for use in flow reactors. Although this technique works well in some environmental catalytic systems, such attachment sometimes modifies the catalytic specifici-... 

The electrochemical oxidation of monomers such as pyrrole,2-5 thiophene,6-9 aniline,10-13 etc., or their derivatives, initiates a polymerization process at the electrode/electrolyte interface that promotes the formation of a polymeric film that adheres to the electrode. A similar homogeneous polymerization process can be initiated by chemical oxidation or chemical polymerization.14-21 Some monomers can be polymerized as well by electrochemical or chemical reduction. 

There is evidence in the literature that many of these transition metal derivatives, when chemically bonded or grafted onto an inorganic oxide surface, possess catalytic activity for olefin reactions far greater than that observed when the organometallic compound is used in a homogeneous manner (26). In some examples, catalytic activity was promoted when the original compound had none (3, 27). 

In retrospect, the catalytic reaction following eq. (1) has promoted recognition of organometallic homogeneous catalysis in the bulk-chemicals industry both hydroformylation (Section 2.1.1) and the Wacker ethylene oxidation are key steps in industrial homogeneous catalysis. 

This type of reaction leads to the formation of an inorganic polymer or a three-dimensional network formed of metal oxianions. The above-described process is called metal alkoxide-based sol-gel. The literature contains excellent reports providing in-depth analyses of this method [100,223]. The sol-gel process allows for very good chemical homogeneity and offers the possibility of obtaining metastable phases, including the amorphous phase. This process normally promotes the formation of amorphous metal oxides, which require thermal or hydrothermal treatment to promote crystallization. Several factors affect the sol-gel process, including the kind... 

In the case of oxide solid solutions [28-31] or complex oxide formation [26,32-39], a high chemical homogeneity of the precursor promotes fast and uniform formation of the target product during its thermal processing. However, even in the case of non-reacting components, it was shown that cryogel synthesis is useful for the synthesis of nanocomposites [27, 40-54]. It is also possible to synthesize core-shell oxide particles by this method when precipitatimi of the components is performed not simultaneously, but sequentially [55]. 

The ability to tailor metal oxide systems for physical and chemical applications represents an obvious advantage for improved performance. Interesting examples are cobalt and nickel oxides, which find applications in many oxidation reactions and also as promoters of Mo and W oxide catalysts for the hydrodesulfurization reactions of middle distillates. NiO and CoO can be mixed in all proportions to form homogeneous solid solutions of the type C0jcNii xO, 0 < x < 1, in a well-ordered rock salt crystal structure in the bulk material. When used in redox reactions in alkaline media [38], these solid solutions exhibit electrochemical properties. The reason why CoO and NiO form solid solutions lies in the close match of... 

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