Microporous Mixed Oxide Catalysts

The most efficient catalysts in liquid-phase oxidation of organic compoimds were crystalline mked oxides [1]. They are ionic mixed oxides or mixed oxides containing oxides supported on oxides. In the latter case, the catalytic activity of the oxide support is increased by adding one or more metal components or is obtained by immobilization of metal oxides on inactive oxide support. Metal ions were isomorphously substituted in framework positions of molecular sieves, for example, zeolites, silicalites, silica, aluminosilicate, aluminophosphates, silico-aluminophosphates, and so on, via hydrothermal synthesis or postsynthesis modification. Among these many mixed oxides with crystalline microporous or mesoporous structure, perovskites were also used as catalysts in liquid-phase oxidation. 

Microporous crystalline solids in which transition metals are tetrahedrally substituted via template-mediated hydrothermal synthesis have remarkable properties in selective oxidation reactions [66]. Unfortunately, the microporous structure and the rigidity of the crystalline frameworks limit the substitution degree, variety of substituted metal, and their general applicability. For this reason, the amorphous microporous mixed oxides (AMMs) with uniform microporosity and wide compositional variability are devoid of Bronsted acidity, but are not associated with the redox-active elements. However, metals such as Ti, V, and Mo incorporated in amorphous silica were good catalysts in allylic oxidation or epoxidation of olefins [67]. 

The metal cations can also be immobilized onto supports by cation exchange. Already classic examples are those of metal-zeolites, metal-acidic clays, or metal-aluminophosphates (APO) [5,6,38,70-72]. Fe-substituted molecular sieves can be considered as very good examples for this variety of catalysts and the oxidation of pinacol to pinacolone evidenced the effect of the crystalline support structures on the catalytic activity. Thus, for a series of APOs and silicalites, the activity decreased in the following order APO-5 APO-11 APO-8 VPI-5 silicalite-1. Since the catalytic activity was independent of the pore diameter of these supports, the liquid-phase oxidation was considered to proceed mainly on the outer surfaces of the catalysts. The hydrophilicity of the aluminophos-phate surface was in the fevor of catalyzing the pinacol reaction, which in fact corresponds to the case of polar reactants and less polar products. Moreover, a high pinacol conversion was achieved by using solvents of low polarity [70]. 

More complex microporous support compositions such as SAPO (a mixture of Si, Al, and P oxides) can also be substituted by metals such as titanium. Using hydrogen peroxide as the oxidant, such Ti-substituted SAPO molecular sieves marked catalytic activity in phenol hydroxylation reaction. However, using the same catalysts, only little catalytic activity was observed in the oxidation of organic substrates of low polarity, such as alkenes [71]. Correlations between the structure and catalytic activity of titanium sites and oxo-titanium intermediates are also described [72]. 

Many 3d transition ions were supported on mesoporous silicas by substitution of Si from silica network. The studies have shown a high activity and selectivity of such catalysts in the oxidation of cyclohexene, aromatic hydrocarbons, phenols, and alcohols. Thus, V, Ti, Cr, Mn, Fe, Ni, and Co incorporated into MCM-41 materials showed activity in liquid-phase oxidation of styrene and benzene with hydrogen peroxide [15,29,79]. The best activity in the oxidation of benzene was obtained for Ti-MCM-41, while for the oxidation of styrene the most active were Cr-MCM-41 and CrNi-MCM-41. The activity of these catalysts decreased with an increase of the number of 3d electrons of the metal ions. Ti, 

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The sol-gel method has also been used to put a NHC(S)NHC(0)Ph on to silica.72 A rhodium catalyst made from this could be used at least five times in the hydro-formylation of styrene. By including some methyltri-ethoxysilane in the cohydrolysis of tetraethoxysilane and titanium(IV) isopropoxide, it was possible to vary the surface polarity of the amorphous microporous mixed oxide catalysts used in oxidations with hydrogen peroxide.73 The methyl groups slowed the deactivation of the catalyst and made it possible to regenerate them thermally. 

Holzwarth et al. (51) reported the synthesis and IR thermographic-imaging screening of a 37-member, focused discrete heterogeneous catalyst library L6 for oxidations and reductions. The library was prepared using sol-gel solution synthetic protocols (47, 51) to produce the library individuals as amorphous microporous mixed oxides (AMMs), which have previously shown heterogeneous catalytic properties (54, 55). The scaffolding metal oxides contained either Ti (subset 1, Fig. 11.7) or Si (subset 2), and many active metal components were used. The complete structure of L6 is reported... 

The activity exhibited by the pretreated G-66A catalyst could be attributed to the presence of a CuVCu redox couple in a spinel matrix. This is also in agreement with previous studies where catalytic activity of mixed oxide catalysts with a spinel structure in the phenol oxidation was found to be higher than that of the pure oxides [11, 12]. On the other hand, the specific catalytic activity of molecular sieves is limited by the concentration of isolated redox centres located in the microporous voids or in the fiamework. Although molecular sieves have a much lower concentration of metals (Table 1), they show fairly good activity for liquid-... 

Maier W.F. (1998) Amorphous microporous mixed oxides, new selective catalysts with chemo- and shape-selective properties. Prep. Symp. Amer. Chem. Soc., Div. Fuel Chem. 43,534-537. 

W.F. (1996) Amorphous microporous mixed oxides as selective redox catalysts. Catal. Lett, 38 (3-4), 209-214. 

Klein S., Thorimbert S., Maier W.F. Amorphous microporous titania-silica mixed oxides preparation, characterization, and catalytic redox properties. J. Catal. 1996 163 476 88 Liu Z., Tabora J., Davis R.J. Relationship between microstructure and surface acidity ofTi-Si mixed oxide catalysts. J. Catal. 1994 149 117-126... 

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

Abstract. Nanocarbon materials and method of their production, developed by TMSpetsmash Ltd. (Kyiv, Ukraine), are reviewed. Multiwall carbon nanotubes with surface area 200-500 m2/g are produced in industrial scale with use of CVD method. Ethylene is used as a source of carbon and Fe-Mo-Al- mixed oxides as catalysts. Fumed silica is used as a pseudo-liquid diluent in order to decrease aggregation of nanotubes and bulk density of the products. Porous carbon nanofibers with surface area near 300-500 m2/g are produced from acetylene with use of (Fe, Co, Sn)/C/Al203-Si02 catalysts prepared mechanochemically. High surface area microporous nanocarbon materials were prepared by activation of carbon nanofibers. Effective surface area of these nanomaterials reaches 4000-6000 m2/g (by argon desorption method). Such materials are prospective for electrochemical applications. Methods of catalysts synthesis for CVD of nanocarbon materials and mechanisms of catalytic CVD are discussed. 

Amorphous Sn-, Si-, and Al-containing mixed oxides with homogeneous elemental distribution, elemental domains, and well-characterized pore architecture, including micropores and mesopores, can be prepared under controlled conditions by use of two different sol-gel processes. Sn-Si mixed oxides with low Sn content are very active and selective mild acid catalysts which are useful for esterification and etherification reactions [121]. These materials have large surface areas, and their catalytic activity and selectivity are excellent. In the esterification reaction of pentaerythritol and stearic acid catalytic activity can be correlated with surface area and decreasing tin content. The trend of decreasing tin content points to the potential importance of isolated Sn centers as active sites. 

Metal oxides are widely used as catalyst supports but can also be catalytically active and useful in their own right. Alumina, for example, is used to manufacture ethene from ethanol by dehydration. Very many mixed metal oxide catalysts are now used in commercial processes. The best understood and most interesting of these are zeolites that offer the particular advantage of shape selectivity resulting from their narrow microporous pore structure. Zeolites are now used in a number of large-scale catalytic processes. Their use in fine chemical synthesis is discussed in Chapter 2. 

Both microporous [23] and mesoporous [24] hydrophobic Ti-Si mixed oxides have been synthesized but their activities as epoxidation catalysts with aqueous hydrogen peroxide are, as yet, disappointingly low compared with the corresponding reactions with TBHP in organic media or with TS-1 (Section 9.1.6). 

Sol-gel chemistry (Chapter 5) is a preparation method, which can easily be adapted to synthesis robots. The application of this method to high-throughput catalysis was first described by the group of Maier, who prepared amorphous microporous mixed-metal oxides in small cavities of a carrier slate plate [95, 96]. Libraries of doped Ti02, Sn02, and WO3 have been prepared in larger amounts in sets of HPLC flasks [97]. The robot-assisted sol-gel preparation has been applied to mixed-metal oxide catalysts of various composition and the catalysts have been tested for several reactions in gas phases as well as in liquid phase (see Table 11.3). 

Palazzi C., Oliva L., Signoretto M., Strukul G., Catal J. Microporous zirconia-silica mixed oxides made by sol-gel as catalysts for the liquid-phase oxidation of olefins with hydrogen peroxide. J. Catal. 2000 194 286-293... 

Mixed-valence Ru"-Ru" paddlewheel carboxylate complexes also have potential for oxidation reactions after incorporation in a microporous lattice with porphyrinic ligands. This MOF can be used for oxidation of alcohols and for hydrogenation of ethylene. Both the porosity of the lattice and the abihty of the diruthenium centers to chemisorb dioxygen are essential for the performance of the catalyst [62, 64]. 

This review concerns the synthesis, characterization, and catalytic activity of microporous ferrierite zeolites and octahedral molecular sieves (QMS) and octahedral layer (OL) complexes of mixed valent manganese oxides. The ferrierite zeolite materials along with borosilicate materials have been studied as catalysts for the isomerization of n-butenes to isobutylene, which is an important intermediate in the production of methyltertiarybutylether (MTBE). The CMS materials have tunnels on the order of 4.6 to 6.9 A. These materials have been used in the total oxidation of CO to C02, decomposition of H2O2. dehydrogenation of CeHi4, C0H14 oxidation, 1-C4H3 isomerization, and CH4 oxidation. The manuscript will be divided into two major areas that describes zeolites and OMS/OL materials. Each of these two sections will include a discussion of synthesis, characterization, and catalytic activity. 

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