Ionic heterogeneous catalytic oxidation

Heterogeneous catalytic oxidation of sulfides in ionic liquids by anhydrous H2O2 or urea hydroperoxide with MCM-41 and related mesoporous catalysts containing Ti or Ti and Ge was studied by Hardacre and coworkers [51]. The Ti-based catalyst gave a quite selective sulfoxidation. Addition of Ge to Ti increased the rate of the oxidation but reduced the selectivity toward sulfoxide [51a]. 

A heterogeneous catalytic system was prepared upon grafting a cationic dihydroimidazolium-tagged silane on solid Si02. The resulting supported ionic liquid phase (SILP) has been used to immobilize [y-l,2-H2SiV2Wio04o] and was employed in a mixture of acetonitrile/t-butyl alcohol at 20°C for the oxidation of different substrates terminal olefins (66-82% yield), non-hindered internal olefins (> 70% yield) in 24 h, and sulfides (81-95% yield) in 4—lOh. ... 

A heterogeneous catalytic method based on copper(I) chloride absorbed on Kieselghur provides an efficient and convenient oxidation of alkyl halides and alkyl tosylates to the corresponding aldehydes. Ionic liquids, such as l-butyl-3-methylimidazolium... 

A controlled modification of the rate and selectivity of surface reactions on heterogeneous metal or metal oxide catalysts is a well-studied topic. Dopants and metal-support interactions have frequently been applied to improve catalytic performance. Studies on the electric control of catalytic activity, in which reactants were fed over a catalyst interfaced with O2--, Na+-, or H+-conducting solid electrolytes like yttrium-stabilized zirconia (or electronic-ionic conducting supports like Ti02 and Ce02), have led to the discovery of non-Faradaic electrochemical modification of catalytic activity (NEMCA, Stoukides and Vayenas, 1981), in which catalytic activity and selectivity were both found to depend strongly on the electric potential of the catalyst potential, with an increase in catalytic rate exceeding the rate expected on the basis of Faradaic ion flux by up to five orders of... 

Most of the heterogeneous catalyst which are in practical use consist of one or more catalytically active compounds which are impregnated on supporting carrier materials. This method can be chosen to immobilise acids and bases as well as salts, oxides or complexes. The major drawback is leaching of one or more component which leads to irreversible deactivation of the catalyst. Physisorption can be enhanced by choosing the appropriate porous, chemical and electronical properties. This leads to catalysts with sufficient long term stability due to e.g. ionic linkages. 

Molecular sieve 3 A (MS3A) was also used by Lin et al. for the aerobic oxidation of alcohols in ionic liquid (Scheme 14.32) [30]. It seems that the molecular sieve 3 A serves as a heterogeneous Brpnsted base to enhance the reaction rate remarkably. With TEMPO-IL and the use of MS3A, the catalytic system could easily be recycled and reused three times without loss in its activity. 

Intermediates which are involved in heterogeneous catalysis could have ionic character, which require an extention of the general treatment of complex reactions. As an example we can consider the catalytic hydrogenation over oxides and sulphides, where intermediates of cationic character were proposed. Ionic intermediates are also possible in catalysis over metals, for instance in the case of neopentane transformations over electron deficient palladium, which occur via formation of carbocations. If we consider olefin hydrogenation over oxides or sulphide with a heterolytic dissociation of hydrogen, the mechanismn of this reaction can be presented in the following form... 

The type of bond between the surface of the solid and adsorbate molecules determines the kind of surface processes that can take place crystal growth, growth inhibition, nucleation, corrosion, catalytic activity, and chemical passivation. Sometimes there are two types of surfaces involved in the reaction metallic and ionic (many heterogeneous catalysts consist of very small metal particles on oxidic carriers). 

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