Oxidation reactions aqueous catalysis

A fascinating area is micellar autocatalysis reactions in which surfactant micelles catalyse the reaction by which the surfactant itself is synthesized. Thus synthesis of dimethyldoceylamino oxide (reaction between dimethyl dodecyl amine and H2O2) benefits from this strategy. Here an aqueous phase can be used and an organic solvent can be avoided. Synthesis of mesoporous molecular sieves benefit through micellar catalysis and silicate polymerization rates have been increased by a factor 2000 in the presence of cetyltrimethyl ammonium chloride (Rathman, 1996). 

The Wacker process (Eq. 1) was developed nearly 50 years ago [1-3] and represents one of the most successful examples of homogeneous catalysis in industry [4-9]. This palladium-catalyzed method for the oxidation of ethylene to acetaldehyde in aqueous solution employs a copper cocatalyst to facilitate aerobic oxidation of Pd° (Scheme 1). Despite the success of this process, certain features of the reaction have Umited the development of related aerobic oxidation reactions. Many organic molecules are only sparingly sol-... 

Building on earlier work in these laboratories (8) we have overcome the typical mass transfer limitations of phase transfer catalysis for propylene oxidation by the use of 3-component liquid phases based on CO2 expanded liquids (CXLs). For the application to oxidations by aqueous H2O2, the organic component of the CXL is chosen because it is miscible with both dense CO2 and water. In this way, homogeneous systems are produced which decrease mass-transfer limitations and intensify chemical reactions. Previous reports using CXL systems have shown that they enhance the oxidation of the substrate and improve the selectivity at moderate reaction temperatures and pressures (3, 8, 9). 

FTIR, Laser Raman, SAXS and nitrogen adsorption. The experimental validation of these materials has been mainly catalysis, especially dedicated to oxidation reactions. Filtration, separation and extraction are other possibilities. It can be done from organic solution, but also from aqueous solutions as recently some progresses have been done on the hydrostabilistion of the materials. Here are presented some of the results obtained in all these areas, from the synthesis side to the applications. The specificity of each partner involved in the project is not fully detailed here but may be found on the internet site whose address is provided at the top of the reference section. 

The area where aqueous-organic biphasic catalysis has had the greatest impact is in oxidation reactions arguably the most important industrial catalysed reaction. Many oxidation catalysts lend themselves well to the biphasic technique, as they do not require any modification to induce water solubility.15 Other complexes that are active oxidation catalysts are insoluble in water and in order to induce water solubility hydrophilic groups are attached to the periphery of the ligand. Some examples of such modified ligands are shown in Figure 2. 

Even by 1989, the importance of rhenium compounds in oxidation catalysis was still minimal [1], This picture has changed dramatically since organorhenium(VII) oxides, especially the water-soluble methyltrioxorhenium (MTO, 1), have proven to be excellent catalyst precursors for a surprisingly broad variety of processes, most notably for a variety of oxidation reactions [2, 3]. This article summarizes the behavior of MTO and its peroxo derivatives in the presence of water and under catalytic condition in aqueous systems, particular attention being given to the most recent findings. 

This prediction has been realized. Various aromatic nitro compounds have been reduced to the corresponding amines by treatment at 25 °C in aqueous solution of glyme containing catalytic quantities of Fe(CO)5 and large amounts of triethylamine under a pressure of 1700 psi of CO (18). To observe catalysis, however, it was necessary to maintain the concentration of the oxidant, i.e., the nitrobenzene, low at all times. If this were not done, then rapid loss of the CO ligands occurred (Reaction 9) and irreversible formation of iron oxides (Reaction 10) resulted. The concentration of the oxidant was maintained low inside the reactor vessel simply by pumping in the nitrobenzene over a... 

The importance of the solid support and the way in which certain supports can influence the outcome of the catalysis also needs deeper understanding and rationalization. The interplay between media, conditions and support must also be delineated. This issue is certainly related to the operating reaction mechanism and the fact that certain pathways compete or one may prevail, depending on the conditions. The apparent contradiction about the lack of influence of the support on oxidation in aqueous media compared to solventless conditions deserves specific studies. 

Transition metal cations solvated by weakly coordinating solvents are useful precursors for a variety of nonaqueous synthetic applications. The dissociation of acetonitrile ligands opens up vacant coordination sites for catalysis, and allows transition metals to be introduced into extended arrays formed by condensation reactions with polydentate ligandsAlthough salts of homoleptic acetonitrile metal cations with tetrafluoroborate anions have been known since the 1960s, there has been no detailed description of the syntheses and characterization of these compounds. Likewise, there have been general descriptions of the syntheses of trifluoromethanesulfonate salts, but no specific outline for individual metals has been published. In this report, the syntheses of [M (CH3CN)x][BF4]2 (M = Cr,Mn,Fe,Co,Ni,Cu) and [M CHaCNlJlSOsCFsla (M = Mn,Fe,Co,Ni) are described. Two different methods are used to prepare tetrafluoroborate salts (1) the oxidation of metals with nitrosonium tetrafluoroborate, first described by Hathaway et al. and (2) the dehydration of aqueous tetrafluoroborate salts in a Soxhlet extractor with molecular sieves. The oxidation reaction is rapid, anhydrous, and recommended for smaller-scale preparations because of the relatively... 

Water is so extensively used in catalytic oxidation reactions that usually this fact is regarded as a natural feature and remains unnoticed. Wacker oxidation of olefins by palladium complexes involves water as a nucleophilic reagent, and thus the whole Wacker-type chemistry, which has developed into a powerful and versatile method of organic synthesis, is derived from aqueous catalysis [178]. The role of the nature of the co-oxidant and the mechanism of deactivation of the palladium catalyst due to aggregation and growth of inactive metal particles were recently investigated, and such study may have relevance for other processes catalyzed by phosphine-less palladium catalysts [179]. 

The subject of micellar catalysis and inhibition of reactions can be divided into the types of reaction occurring, e.g. base-catalysed and acid-catalysed hydrolyses, oxidation, etc., or in terms of mechanisms, e.g. juxtaposition of reactive groups in micelles, attraction of counterions to an oppositely charged micellar surface, protection by solubilization within non-ionic micelles, etc. It is not possible to adhere rigidly to either scheme but we will attempt here to consider, in turn, hydrolysis, oxidation in aqueous micelles, reactions in inverse micelles, reactions involving drugs and miscellaneous reactions of interest. Bunton s summary of the topic in his recent review of the subject is worth repeating here [12] ... 

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