Metal well defined alumina surface

Metal Carbonyls on a Well Defined Alumina Surface. 119... 

Supported vanadium catalysts, whereby vanadium oxide is dispersed on a support such as alumina or titania are of particular importance in, for instance, the oxidative dehydrogenation of alkanes [58-64]. Such materials have attracted considerable interest in the direct dehydrogenation of butane, where a key driver is to identify the relationship between catalytic activity and structural properties [5, 6, 65-68]. In the pure (solid) metal oxides the coordination of vanadium is well defined. However, this is not necessarily true in the case of supported catalysts. Vanadium may be present on the support surface as isolated vanadium ions dimeric or polymeric species one- and two-dimensional chains of vanadium ions ... 

The higher the active surface area of the catalyst, the greater the number of product molecules produced per unit time. Therefore, much of the art and science of catalyst preparation deals with high-surface-area materials. Usually materials with 100- to 400-m /g surface area are prepared from alumina, silica, or carbon and more recently other oxides (Mg, Zr, Ti, V oxides), phosphates, sulfides, or carbonates have been used. These are prepared in such a way that they are often crystalline with well-defined microstructures and behave as active components of the catalyst system in spite of their accepted name supports. Transition-metal ions or atoms are then deposited in the micropores, which are then heated and reduced to produce small metal particles 10-10" A in size with virtually all the atoms located on the surface... 

The use of a silica-supported, tantalum alkyhdene as a precursor for alkane metathesis was found to result in a stoichiometric, alkane cross metathesis in which an initial pendant alkyl-alkylidene group was transformed to produce the desired, active species [76, 90]. This reaction was later observed to work with additional, well-defined systems supported on alumina [58] and sihca-alumma [53]. As mentioned previously, this transformation does not occur when the surface organometallic precursor bears no alkyl group. Exposing these supported, metal neopentyl, neopentylidene, and neopentyhdyne complexes to alkane at 150 °C produced alkanes containing a neopentyl fragment (CH jiBu) via cross metathesis. Propane metathesis with these alkyl-alkyhdene surface complexes typically generates stoichiometric amounts of dimethylpropane, 2,2-dimethylbutane, and 2,2-dimethylpentane (Scheme 2.11). 

Finally, catalysis is not confined to well-defined. Miller-indexed metal surfaces. One area of recent interest is in the use of certain minerals to catalyze reactions. Some aluminosilicates—minerals with mixed alumina and silica structures—have pores in which molecules can enter and react catalytically [Figure 22.25(a)]. One type of aluminosilicate is called zeolite, shown in Figure 22.25(b). Thanks to the pores in zeolites (which can vary in size and geometry depending on the exact type of zeolite), the properly sized reactant molecules can enter the structure and a particular reaction can be promoted. In fact, it is thought that such minerals are the future of designed catalysts that can be used to promote a preferred chemical reaction—if the pore size is just right. 

Catalysts with better ability to control polymer morphology can be prepared in many different ways. One of these uses the well defined morphology of silica, silica-alumina, or alumina which is loaded with TiCU or other active transition metal compound. The best silicas used as supports have good mechanical strength, high porosity, and surface area. Thus, they do not break during the early steps of polymerization. 

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