Amorphous microporous mixed oxide

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 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. 

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. 

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]. 

J.U., Voss, J Keune, W., and Maier, W.F. (2001) Selective oxidation of benzene to phenol in the liquid phase with amorphous microporous mixed oxides. Appl. Catal. A Gen., 208 (1-2), 343-358. 

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

Figure 11.7 Structure of the amorphous microporous mixed (AMMs) oxides discrete catalytic library L6.

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). 

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. 

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. 

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... 

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