Substitutional metallosilicates

For framework metallosilicates other than aluminosilicates we should make the distinction between those materials in which the heteroatom substitutes for silicon in a known zeolite-type framework and those in which it takes up a distinctive framework site in stoichiometric quantities within structures that 

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Zeolites are crystalline aluminosilicates with framework structures made up of 3-dimensional networks of AIO4 and Si04 tetrahedra linked to one another by corner sharing of the oxygens. When the Al ions in the structure are replaced by other metal ions (isomorphous substitution) metallosilicates result. During the past two decades, isomorphous substitution by ions such as, Fe. Ti", Zr , V and Cr have been reported. A... 

Substituted aluminophosphates are of interest as acid and oxidation catalysts. Typical substitutions include M for Al (M = Mg, Mn, Fe, Co, Zn), Si for and 2Si" for Al " -i- (described in Chapters 2 and 3). In cases where the degree of substitution is at a few percent, similar methods to those for substitutional metallosilicates can be used for confirmation (measurement of unit cell dimensions, P MAS NMR for measuring the substitutions of cations for A1 and Si NMR for measuring the mode of silicon substitution). For some of the metals, much higher metal substitutions are possible than for silicates, and complete replacement of aluminium by cations such as cobalt in tetrahe-drally connected frameworks has been reported (see Chapter 2). A number of other metals have been claimed to have been substituted into the aluminophosphate framework, including Ti" and C -3+ i03,i04... 

Recently, the preparation of metallosilicates with MFI structure, which are composed of silicone oxide and metal oxide substituted isomorphously to aluminium oxide, has been studied actively [1,2]. It is expected that acid sites of different strength from those of aluminosilicate are generated when some tri-valent elements other than aluminium are introduced into the framework of silicalite. The Bronsted acid sites of metallosilicates must be Si(0H)Me, so the facility of heterogeneous rupture of the OH bond should be due to the properties of the metal element. Therefore, the acidity of metallosilicate could be controlled by choosing the metal element. Moreover, the transition-metal elements introduced into the zeolite framework play specific catalytic roles. For example, Ti-silicate with MFI structure has the high activity and selectivity for the hydroxylation of phenol to produce catechol and hydroquinon [3],... 

Synthesis of transition metal containing molecular sieves (microporous as well as mesoporous) is one of the fastest developing areas in molecular sieve science, as evidenced by recent published reviews [1,2] Several transition metals have been substituted into crystalline silica or aluminophosphate frameworks to yield the corresponding metallosilicate or metalloaluminophosphate molecular sieves, However, the location of the metal species and their state always remain uncertain, despite the employment of numerous different characterization methods comprising IR, NMR and ESR spectroscopy. 

Difluorobenzenes are isomerized under gas-phase conditions in the presence of metallosilicates, containing the structure of pentasil zeolites with isomorphic substitution of some silicon atoms by aluminum, gallium, or iron.4 A German patent describes the isomerization of l-bromo-2,4-difluorobenzene to l-bromo-3,5-difluorobenzene in pentasil-type zeolites in an autoclave at 320 C and 25 x 105 Pa for 1 h, giving 29% conversion and 73% selectivity.5... 

A different approach to the substitution of metal atoms into the framework is the secondary synthesis or post-synthesis method. This is particularly effective in synthesizing metallosilicates that are difficult to crystallize from the gels containing other metal atoms or hardly incorporate metal atoms by the direct synthesis method. Substitution of Ti for A1 goes back to the 1980s. The reaction of zeolites with an aqueous solution of ammonium fluoride salts ofTi or Fe under relatively mild conditions yields materials that are dealuminated and contain substantial amounts of either iron or titanium and are essentially free of defects [58]. However, no sufficient evidence for the Ti incorporation has been provided. 

Transition-state selectivity is sometimes difficult to distinguish from product shape selectivity. A recent study by Kim et al. (8) shows that the high para-selectivity for the alkylation of ethylbenzene with ethanol in metallosilicates (MeZSM-5) is not due to product selectivity alone. They conclude that the primary product of the alkylation on ZSM-5 type metallosilicates is p-diethylbenzene which isomerizes further inside the cavity of ZSM-5 to other isomers. As the acid sites of zeolites becomes weaker (achieved by substituting different metals into the framework of the zeolite), the isomerization of the primarily produced p-isomer is suppressed. Although Kim et al. attribute this suppression of the isomerization activity to restricted transition-state selectivity, it is more likely that this suppression is due to the decrease in acid strength. 

Metallosilicate molecular sieves result when the ions in zeolitic materials (aluminosilicate) are replaced (isomorphous substitution) by other ions. A large number of metal ions have reportedly been incorporated in zeolite lattices. However, doubts arise regarding the location of these metal ions in the framework in many cases. Detailed characterization of the metallosilicates is necessary to identity the nature and location of the metal ions. As an example, the types of V-ions present in vanadosilicate molecular sieves of MEL, MFI and BEA structure types are discussed based on detailed physicochemical characterization of these materials. Also, the influence of preparation methods on the type and location of the V-ions are reported. 

Yashima and coworkers recently reported on the selective formation of 2,6-dimethylnaphthalene (2,6-DMN) from methylation of 2-methylnaphthalene with methanol on ZSM-5 and metallosilicates with MFI structure [Komatsu et al 1994]. They demonstrated that isomorphous substitution of Al by other elements (Table 5) and deactivation of external surface (Table 6, by using basic nitrogen compound) can increase the selectivity to 2,6-DMN. They concluded that in order to obtain 2,6-DMN in high selectivity, it is effective to weaken the acid strength while keeping the pore dimension of MFI structure constant (or, wider, if possible), which can be achieved by using Fe-MFI as a catalyst. 

Mixed Coordination Silicates. As described in the second chapter, there is a small group of microporous metallosilicates in which the metals possess non-tetrahedral coordination. ETS-10 is the best known of these, in which tetrahedral silicon has two types of chemical environments, Si(OSi)3 (OTi) and Si(OSi)4. The titanosilicate end-member is able to take up a range of trivalent cations into the framework upon synthesis, including boron, aluminium and gallium. Solid state NMR reveals the nature of this substitution ... 

The strongest acid sites are present in crystalline aluminosilicates acid sites of other metallosilicates (B, Ga, Fe), substituted aluminophos-phates or amorphous silica-aluminas are weaker. 

The complexity of characterization and the difficulty in interpreting catalytic data substantially grows with metallosilicate samples that have been steamed or treated at high temperatures in moist air. As indicated earlier, the structural, chemical, and catalytic effect of framework cation hydrolysis must be considered in these cases. In the case of B and Ga substitutions for Al, both the fraction of the substituent element remaining in the crystal framework and the one forming a secondary phase usually as an oxide, which is mainly occluded in the crystal, must be determined. 

The use of organic templates has rendered possible the substitution of many elements, including other trivalent (Cr +), bivalent (Be +), and tetravalent ions (Ge" " ", Ti" +). Most of these metallosilicate compositions have been synthesized with ZSM-5 crystal structure. Adsorption microcalorimetry enabled the studying of acidic properties of thus obtained materials, as in the case of parent zeolites. 

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