Metal-ion catalyzed, liquid-phase oxidation

The metal-ion catalyzed, liquid-phase oxidation of SO2 has received considerable attention as a mechanism for SO2 conversion in plmes and contaminated droplets. In general, the mechanisms proposed are lengthy, and the derived rate expressions are largely empirical. Table VI STjmmarizes a variety of studies on this process. Observed rates vary substantially depending on the particular catalyst, relative humidity, and other conditions. 

Table VI Metal-Ion Catalyzed, Liquid-Phase Oxidation of SO ...

The four remaining papers all deal with the catalysis of liquid-phase oxidation processes by transition metal ions (6). A. T. Betts and N. Uri show in particular how metal complexes can either catalyze or inhibit oxidation according to their concentration. In this investigation, various hydrocarbons (especially 2,6,10,14-tetramethylpentadecane) were used as substrates, and metal ions were present either as salicylaldimine or di-isopropylsalicylate chelates. These compounds are considerably soluble in non-polar media, and this makes it possible to examine their effect over a much wider range of concentration than is usually accessible in this type of work. These studies show that catalyst-inhibitor conversion is always... 

Many other metal ions have been reported as catalysts for oxidations of paraffins or intermediates. Some of the more frequently mentioned ones include cerium, vanadium, molybdenum, nickel, titanium, and ruthenium [21, 77, 105, 106]. These are employed singly or in various combinations, including combinations with cobalt and/or manganese. Activators such as aldehydes or ketones are frequently used. The oxo forms of vanadium and molybdenum may very well have the heterolytic oxidation capability to catalyze the conversion of alcohols or hydroperoxides to carbonyl compounds (see the discussion of chromium, above). There is reported evidence that Ce can oxidize carbonyl compounds via an enol mechanism [107] (see discussion of manganese, above). Although little is reported about the effectiveness of these other catalysts for oxidation of paraffins to acetic acid, tests conducted by Hoechst Celanese have indicated that cerium salts are usable catalysts in liquid-phase oxidation of butane [108]. 

It has been already emphasized that substitution of heteroelements into the framework of molecular sieves creates acidic sites. Incorporation of transition elements such as Ti, V, Mn, Fe, or Co, which have redox properties, provides molecular sieves with redox active sites that are involved in oxidation reactions (323-332). As mentioned in the beginning of the article, the transition metal-substituted molecular sieves, the so-called redox molecular sieves, exhibit several advantages compared with other types of heterogeneous redox catalysts (1) redox sites are isolated in a well-defined internal structure therefore, oligomerization of the active oxometal species is prevented (this is a major reason for the deactivation of homogeneous catalysts) (2) the site isolation (the so-called microenvironment) of redox centers prevents the leaching of the metal ions, which frequently happens in liquid-phase oxidations catalyzed by conventional transition metal-supported catalysts (3) well-defined cavities and channels of molecular dimensions endow the catalysts with unique performances such as the shape selectivity (and traffic control) toward reactants, intermediates, and/or products. 

Metal-catalyzed liquid-phase auto-oxidahon plays a critical role in the manufacture of monomers widely used in polymers such as nylon and polyester. As menhoned earlier, soluble salts of cobalt and manganese catalyze oxidation of cyclohexane by dioxygen to cyclohexanol and cyclohexanone. Cyclohexanol and cyclohexanone are oxidized by nitric acid to give adipic acid, one of the monomers of nylon 6,6. The oxidation by nitric acid is carried out in the presence of and ions to improve... 

Despite the enormous importance of zeolites (molecular sieves) as catalysts in the petrochemical industry, few studies have been made of the use of zeolites exchanged with transition metal ions in oxidation reactions.6338- 634a-f van Sickle and Prest635 observed large increases in the rates of oxidation of butenes and cyclopentene in the liquid phase at 70°C catalyzed by cobalt-exchanged zeolites. However, the reactions were rather nonselective and led to substantial amounts of nonvolatile and sieve-bound products. Nevertheless, the use of transition metal-exchanged zeolites in oxidation reactions warrants further investigation. 

There are several typical oxidation products from alkenes, which can be reached via catalytic routes using molecular oxygen as terminal oxidant. We are only considering liquid phase processes catalyzed by transition metal ions or complexes typically below 100-150 C. Many of these homogeneous catalytic reactions occur at or around room temperature. In addition to a single solvent containing the dissolved catalyst complex, phase-transfer conditions involving liquid-liquid or solid-liquid systems will in some cases be described. Likewise,... 

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