Use as oxidation catalysts

The vanadium(IV) complex of salen in zeolite was found to be an effective catalyst for the room temperature epoxidation of cyclohexene using t-butyl hydroperoxide as oxidant.88 Well-characterized vanadyl bis-bipyridine complexes encapsulated in Y zeolite were used as oxidation catalysts.101 Ligation of manganese ions in zeolites with 1,4,7-triazacyclononanes gives rise to a binu-clear complex stabilized by the zeolites but allows oxidation with excellent selectivity (Scheme 7.4). 

This chapter is essentially a review of those ruthenium complexes which have been used as oxidation catalysts for organic substrates, emphasis being placed on such species which have been chemically well-defined and are effective catalysts. Of all the ruthenium oxidants dealt with here those which have the greatest diversity of use are RuO, [RuO ] , [RuO ], the tetramesityl porphyrinato (TMP) complex fran.y-Ru(0)2(TMP), RuCl3(PPh3)3, and cw-RuCl3(dmso). Many other catalysts are covered, and the uses of two principal starting materials, RuO. nH O and RuClj. nH O as precursors for a number of catalysts, discussed. 

Since the first synthesis of TS-1 in 1983 [1], considerable efforts have been devoted to the synthesis of titanium-containing zeolites [2, 3]. Recently, Ti-beta, a large-pore molecular sieve, has been extensively studied [4, 5]. Owing to its unique large-pore channel system, Ti-beta seems to be more active than the medium-pore TS-1 catalyst for the oxidation of cyclic and branched alkenes with aqueous hydrogen peroxide. Under the usual synthesis conditions, however, Ti-beta crystallizes with some Al as a framework constituent [4], This leads to the presence of acid centers, which may have a detrimental effect on the activity or selectivity of this type of catalyst. Since 1992, the discovery of a new family of mesoporous molecular sieves has received much attention [6,7], Because of their mesoporous nature (20-100A), the Ti-MCM-41 zeolites may be useful as oxidation catalysts for larger molecules [8], In this... 

The use of certain vanadium compounds as catalysts has been increasing. Vanadium oxy trichloride is a catalyst in making ediylene-propylene rubber. Ammonium metavanadate and vanadium pentoxide aie used as oxidation catalysts, particularly in the production of polyamides, such as nylon, in the manufacture of H>S04 by the contact process, in the production of phdialic and maleic anhydrides, and in numerous other oxidation reactions, such as alcohol to acetaldehyde, anthracene to anthraquinone, sugar to oxalic acid, and diphenylamine to carbazole. Vanadium compounds have been used for many years 111 die ceramics field for enamels and glazes. Colors are produced by various combinations of vanadium oxide and silica, zirconia, zinc, lead, tin, selenium, and cadmium. Vanadium intermediate compounds also are used in the making of aniline Mack used by the dye industry... 

Bulk materials used as oxidation catalysts not only allow for oxygen transport but also accommodate a wide and homogeneous modification of their electronic... 

Figure 9. Structures of Fe(TPP), Fe(TMP), and Fe(TDCPP) used as oxidation catalysts.

Ruthenium(V), (VI), (VII), (VIII) (d3, d2, d, d°) These complexes are restricted to halo, oxo and nitrido species such as [RuNClJ-, [RuNC15]2 and [Ru04]2-, . Oxo complexes of ruthenium have been used as oxidation catalysts. 

The role of transition metals as counter-cations in polyoxometaUates used as oxidation catalysts has been reviewed [75]. Transition metals have important and complex effects on textural, acid-base and redox properties of the heteropolyanions, as described in a number of studies. The interaction of the molybdophos-phoric Keggin heteropolyanion with the iron counter-ion has been studied and... 

One of the most smdied examples is the mimic of the enzyme cytochrome P-450 in the pores of a faujasite zeolite [196,204,225], The iron-phthalocyanine complex was encapsulated in the FAU supercage and is used as oxidation catalyst for the conversion of cyclohexane and cyclohexanone to adipic acid, an important intermediate in the nylon production. In this case the two step process using homogeneous catalysts could be replaced by a one step process using a heterogeneous catalyst [196]. This allowed better control of the selectivity and inhibited the auto oxidation of the active compound. In order to simulate a catalyst and the reaction conditions which are close to the enzymatic process, the so obtained catalyst was embedded in a polydimethylsiloxane membrane (mimics the phospholipid membrane in the living body) and the membrane was used to limit oxygen availability. With this catalyst alkanes were oxidized at room temperature with rates comparable to those of the enzyme [205]. 

Some of the Group VIII metals have uses as oxidation catalysts (5). All except platinum do however tend to oxidize under vigorous conditions, such as are used in ammonia oxidation and the Andrussow process, for which only platinum and its alloys are acceptable catalysts. Under milder conditions both platinum and palladium have somewhat limited applications in liquid-phase oxidation processes, as for example in the carbohydrate field. 

Manganese and cobalt compounds are particularly effective in decomposing hydroperoxides (eqs. (17) and (19)) and hence are frequently used as oxidation catalysts [47]. In spite of their similar abilities in this regard, the results of their use in a given instance can be quite different. Manganous ions are reported to have a pronounced ability to reduce peroxy radicals directly to peroxy anions (eq. (20)) [50] ... 

Mo hexacarbonyl was used as oxidation catalyst in SCCO2 with BuOOH as oxidant by several groups [15, 26, 27]. Depending on the water content and the reaction temperature, epoxides or vicinal irans-dioh are obtained in good yields. The epoxidation (cf. also Section 2.4.3) of allylic and homoallylic alcohols with BuOOH also occurs smoothly in liquid CO2 with [V0(0 Pr)3] or [Ti(0 Pr)4] as a catalyst [27]. Using di(isopropyl) tartrate (DIPT) as a chiral ligand for the Ti catalyst yields up to 87 % ee at 0 °C (eq. (2)). The isopropyl groups in the... 

C.L. Gibson and co-workers developed an efficient synthesis for chiral ring annulet 2,6-disubstituted 1,4,7-trimethyl-1,4,7-triazamacrocycles. This class of molecules is capable of stabilizing transition metals in their high oxidation states and therefore can be used as oxidation catalysts. The A/-methylation of the three nitrogens in the last step was conducted using the original Eschweiler-Clarke methylation conditions. 

A variety of cyclic tetraamides, e.g., (61), which stabilize metal ions in high oxidation states and are resistant to oxidation, have been synthesized for use as oxidation catalysts for peroxide bleaching.61... 

Electrolytic oxidation of anthracene in 20 per cent sulfuric acid solution with 1 per cent of vanadium pentoxide present is carried out at 80° C. with lead electrodes and a current density of 300 amperes per square meter at 1.6 volts. Good yields have been claimed 10 for this process. Air under pressure has been used for the oxidation of anthracene in the form of dispersions in aqueous ferric sulfate solutions,20 or as a solution iu pyridine or dispersion in aqueous alkaline solutions preferably in the presence of catalysts 21 of copper, cobalt, nickel or lead compounds. Vanadium compounds have been found more active than chromium compounds for use as oxidation catalysts in the form of suspensions in the liquid phase, as in the preparation of aniline black.22 Anthracene suspended in water or dilute sulfuric arid or dissolved in a solvent as acetone is oxidized with ozone, or ozonized oxygen at ordinary temperatures.28... 

Finally, a few words should be said about results that fell beyond the scope of this report. The reduced copper forms of zeolite Y possess a high catalytic activity in relation to the reaction 0216 + 0218 = 2 O10O18 at temperatures as low as 100°-300°C. Thus, zeolites could be used as oxidizing catalysts. 

The industrial uses of tellurium are limited. In metallurgy, tellurium is used as an additive to improve alloys. The addition of tellurium improves the creep strength of tin and the mechanical properties of lead. Powdered tellurium is used as a secondary vulcanizing agent in various types of rubbers (natural rubber and styrene-butadiene rubbers) as it reduces the time of curing and endows the rubbers with increased resistance to heat and abrasion. In addition, tellurium and its compounds have been used as oxidation catalysts in organic syntheses. Due to its photoelectric properties, tellurium and its compounds are also employed in the semiconductor and electronics industry. In much smaller quantities, tellurium is used in pottery glazes. For further details, see Fishbein (1991). 

Noble metals are defined herein as Pt, Pd, Ag, and Au. These noble metals are frequently alloyed with the closely related metals Ru, Rh, Os, and Ir, and they are usually supported on an oxide support such as 7-AI2O3 or SiO. Although in principle any of these metals may be used as oxidation catalysts, in most practical systems only Pt, Pd, and a few alloys are used because the generally high temperatures employed for most oxidation catalyst applications (e.g., catalytic incineration, automotive exhaust catalysts) can cause sintering, volatility loss, and irreversible oxidation of the other metals. Limited supply and the resulting high cost of these other metals also minimizes their use. As a result, most reported research deals with supported Pt and Pd (and alloys of these with other metals). 

The electron-deficient nature of some metal-connecting points and clusters makes the resulting MOFs efficient oxidation catalysts. It is well established that binuc-lear copper complexes can be used as oxidation catalysts with oxygen or H2O2 as the oxidant [24]. Mori and coworkers reported MOFs with [Cu2(OOCR)4]... 

Several other MOFs have also been used as oxidation catalysts. Kim and coworkers used [Zn2(BDC)(L-Lac)(DMF)] (DMF) as a heterogeneous catalyst for the oxidation of thioethers to sulfoxides by urea hydroperoxide (UHP) or hydrogen peroxide (H2O2) [26]. Its size selectivity was illustrated by the higher conversion of smaller sulfides over larger ones. Snejko and coworkers reported the use of ln2(OH)3(BDC)i,5 [27] and [Sc2(BDC)3] [28] as active catalysts for the oxidation of alkylphenylsulfides. 

Figure 2. Formula of some third generation iron porphyrins used as oxidation catalysts.

Murugavel, R., and H.W. Roesky, Titanosihcates Recent Developments in Synthesis and Use as Oxidation Catalysts, Angew. Chem. Int. Ed. Engl 36 477-419 (1997). 

Gonsalves, A.M., R.A.W. Johnstone, M.M. Pereira, J. Shaw, and A.J.F. do N. Sobral (1991). Metal-assisted reactions. Part 22. Synthesis of perhalogenated porphyrins and their use as oxidation catalysts. Tetrahedron Lett. 32, 1355-1358. 

Owing to their extensive use as oxidation catalysts, metalloporphyrins will be discussed in a separate section. 

Pillared LDHs are special in that there is freely accessible interlayer space resulting from the intercalation of large polyvalent anions, such as polyoxometalates (POMs) and phthalocyanines (Pc) (see Sec. II.D and HLA.S.b). POMs are acidic, introducing acidic properties to the basic LDH or LDO and making LDH-POM an acidic catalyst. The pillared LDHs can be also used as oxidation catalysts with intercalation of redox-active anions, such as Co-phthalocyaninetetrasulfonate (CoPcTs). The usage of pillared LDH is described according to the reaction types in the following. 

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