Other Transition Metal-substituted Molecular Sieves

3 Other Transition Metal-substituted Molecular Sieves 

4-naphthoquinone (16%), and phthalic anhydride (50%) were identified, Sn-silicalite-1 was tested in the hydroxylation of ethylbenzene and phenol. The products of the former were 1-phenylethanol and acetophenone (78 %), with minor amounts (22%) of o- and p-hydroxyethylbenzene [39]. The rate of hydroxylation of phenol was low, but with hydrogen peroxide efficiency up to 80 % ortholpara ratio 1.6). In common with V-silicalites, yields were the highest in water and the 

4- (12 %) and 1,2-dihydroxynaphthalene (5 %), with 85 % selectivity relative to hydrogen peroxide [52], Lower selectivity ( 65 %) was obtained in the hydroxylation of phenol ortho/para 3.3). 

The catalytic properties of V-HMS were investigated in detail by Reddy et al. [44], Naphthalene and 2-methylnaphthalene were oxidized at the 1,4-positions to afford the related 1,4-naphthoquinones. Under analogous conditions, 2,6-DTBP (83 % conv.) led to the corresponding p-quinone (87 %) and p-diphenoqui-none (4 %). The relative role of homogeneous and heterogeneous pathways in catalysis remained uncertain, because different amounts of V ions were released into the liquid phase under different conditions. 

It is quite evident that if there is no major restriction to the application of mesoporous catalysts as a result of pore accessibility, more serious limitations might arise as a result of their relatively low activity with aqueous hydrogen peroxide. It has been shown that the specific activity of Ti sites in the epoxidation of olefins decreased in the order TS-1 [Ti,Al]-/ff Ti-MCM-41 [53]. An analogous, qualitative, trend seems to exist for the hydroxylation of benzene and phenol TS-1 [Ti,Al]-yS, [Ti,Al]-MOR Ti-MCM-41, Ti-HMS. Among several reasons. 

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Another approach to achieve higher conversions is to start from cyclohexene, which is much more reactive than cyclohexane towards autoxidation [6], and can be prepared by hydrogenation of benzene over a ruthenium catalyst [7]. The higher reactivity of cyclohexene also allows for lower reaction temperatures thus further limiting overoxidation. The 2-cyclohexen-l-one product formed by decomposition of cyclohexenyl hydroperoxide can subsequently be hydrogenated to cyclohexanone. The net reaction stoichiometry is the same as the current process. We now report our results on the use of CrAPO-5, CrS-1 and other transition-metal substituted molecular sieves for the decomposition of cyclohexenyl hydroperoxide. 

Other transition metal-substituted molecular sieves have been synthesized and tested as epoxidation catalysts [21]. The stability of many of these materials towards leaching is, however, seriously in doubt [31]. Catalysis by vanadium-substituted molecular sieves has, in all cases studied, been shown to be homogeneous in nature, i.e. because of leached vanadium [31,55]. A priori, one would expect zirconium- [56] and tin- [57] substituted molecular sieves to be more stable but more rigorous proof is needed. 

A promising and cleaner route was opened by the discovery of titanium silica-lite-1 (TS-1) [1,2]. Its successful application in the hydroxylation of phenol started a surge of studies on related catalysts. Since then, and mostly in recent years, the preparation of several other zeolites, with different transition metals in their lattice and of different structure, has been claimed [3]. Few of them have been tested for the hydroxylation of benzene and substituted benzenes with hydrogen peroxide. Ongoing research on suppoi ted metals and metal oxides has continued simultaneously. As a result, knowledge in the field of aromatic hydroxylation has experienced major advances in recent years. For the sake of simplicity, the subject matter will be ordered according to four classes of catalyst medium-pore titanium zeolites, large-pore titanium zeolites, other transition metal-substituted molecular sieves, and supported metals and mixed oxides. 

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. 

On the other hand, tBuOOH forms a chain complex with transition metal ions which severes to produce tBuO radicals. The mechanism proposed, although very incomplete and still under investigation, allows to explain the side chain oxidation and Involves the redox system present in V-substituted molecular sieves. 

The aluminophosphate molecular sieve, AIPO4-5, itself has limited potential as catalyst, since its stnjcture is neutral and has neither catbn exchange capacities nor acidity [1-3]. There are two possibilities for utilizing the molecular sieves one is rrwdification of the framework by substitution of metal atoms such as silicon [3-6] and/or transition metals [5-11], and the other is introducing active site by impregnation. 

The history of mesoporous material synthesis is unintentionally or intentionally duplicating the development of zeolites and microporous molecular sieve. It starts from silicate and aluminosilicate, through heteroatom substitution, to other oxide compounds and sulfides. It is worth mentioning that many unavailable compositions for zeolite (e.g., certain transition metal oxides, even pure metals and carbon) can be made in mesoporous material form. 

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