Metallosilicates with MFI structure

Preparation of Metallosilicates with MFI Structure by Atom-Planting Method... 

The atom-planting method for the preparation of several metallosilicates with MFI structure was studied. By the treatment of silicalite or ZSM-5 type zeolite with metal chloride vapor at elevated temperatures, metal atom could be introduced into the zeolite framework. From the results of alumination of silicalite it is estimated that the metal atoms are inserted into defect sites, such as hydroxyl nests in zeolite framework. The metallosilicate prepared had both Bronsted and Lewis acid sites with specific acid strength corresponding to the kind of metal element. 

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],... 

It has been reported that aluminium can be introduced into the framework of silicalite with MFI structure by the treatment with AICI3 vapor at elevated temperatures [4-8]. By such treatment, not only Bronsted acid sites but Lewis acid sites are also generated, because aluminium atoms are introduced not only into the framework sites but alkso into the non-framework sites [6-8]. It is expected that this method can be applied to prepare some metallosilicates with MFI structure. Namely, by treating silicalite with metal chloride vapor at... 

T. Yashima, K. Yamagishi, and S. Namba, Preparation of Metallosilicates with MFI Structure by Atom-Planting Method. Stud. Surf. Sci. Catal., 1991, 60, 171-178. 

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. 

The results in Tables 5 probably represent the level of selectivity in 2-MN methylation that has been achieved and reported in open literature so far using metallosilicates with MFI structure. When a catalytically inert compound is introduced to the external surface or a bulky basic nitrogen compound is used to poison the external acid sites, the (5-selective or shape-selective methylation... 

Titanium (TS-1), niobium (NbS-1), and tantalum silicalite-1 (TaS-1) with MFI structure were synthesized by microwave heating of the TPAOH impregnated xerogels which were prepared by sol-gel process. Highly crystalline products were obtained in 30 min to 2 h with yields over 90%. The metallosilicates showed high catalytic performances in Beckmann recirrangement of cyclohexanone oxime to caprolactam. 

By using halides of other metals, metallosilicates with the MFI structure (Ga, In, Sb,... 

It should be reminded at this juncture (see Chapter 1) that only a fraction of the zeolite structures are used commercially and that with few exceptions (SAPO-11 for instance), almost no metallosilicates have found a large scale use. As far as the catalytic and adsorption properties are concerned, zeolite molecular sieves are further classified according to their pore openings, defined by the number of 02 anions delineating their pore mouths. The most useful zeolites are those containing 12, 10 or 8 of these Cr and are commonly referred to as 12-, 10-, and 8- ring structures. The zeolites FAU and MFI are the major components of the FCC catalysts. 

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