Metallosilicate zeolites

Survey of ZSM-5 Type Metallosilicate Zeolites Synthesis and Catalysis. Luo, Jian Tan, Changyu Dong, Qingnian Shiyou Huagong, 18(3), 202-8, 201, 1990. 

The past nearly six decades have seen a chronological progression in molecular sieve materials from the aluminosilicate zeolites to microporous silica polymorphs, microporous aluminophosphate-based polymorphs, metallosilicate and metaHo-phosphate compositions, octahedral-tetrahedral frameworks, mesoporous molecular sieves and most recently hybrid metal organic frameworks (MOFs). A brief discussion of the historical progression is reviewed here. For a more detailed description prior to 2001 the reader is referred to [1]. The robustness of the field is evident from the fact that publications and patents are steadily increasing each year. 

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],... 

In this paper, we present the "atom-planting" method for the preparation of metallosilicates. We discuss evidence of the introduction of metal elements into the zeolite framework as well as the acidity of the metallosilicates prepared by this new method. 

Bulk amounts of elements were determined by atomic absorption spectrophotometry. The amount of framework A1 was determined by Al MAS NMR. The acidic properties of the metallosilicates were determined by IR and NH3-TPD measurements. Before the IR measurements, the sample wafer was evacuated at 773 K for 1.5 h. In the observation of pyridine adsorbed on metallosilicates, the sample wafer was exposed to pyridine vapor (1.3 kPa) at 423 K for 1 h, then was evacuated at the same temperature for 1 h. All IR spectra were recorded at room temperature. NH3-TPD experiments were performed using a quadrupole mass spectrometer as a detector for ammonia desorbed. The sample zeolite dehydrated at 773 K for 1 h was brought into contact with a 21 kPa of NH3 gas at 423 K for 0.5 h, then evacuated at the same temperature for 1 h. The samples were cooled to room temperature, and the spectra obtained at a heating rate of 10 K min from 314 to 848 K. 

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. 

Kim and Inui[54] have reported the synthesis of MCM-41 with incorporation of various metal components such as Al, Ga or Fe with different Si/metal ratio. These catalysts were used for the oligomerization of propene, and it was found that the order of activity was Al-MCM-41 > Fe-MCM-41 > Ga-MCM-41 with the optimum Si/metal ratio being equal to 200. The propene conversion increases with the temperature from 423 up to 523 K. The catalytic activity of mesoporous silicates was lower than zeolitic catalysts, such as MFI metallosilicates. However,... 

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. 

Very striking results on the interactions of molecules with a catalyst have been recently reported in zeolite catalysis because of the well ordered structure of these materials it is worth mentioning the subjects of zeolite design [10] and of acidic properties of metallosilicates [11]. In other areas where polycrystallinic or even amorphous materials arc applied, highly interesting results are now numerously emerging (such as hydrocarbon oxidation on vanadium-based catalysts [12] location of transition metal cations on Si(100) [13] CO molecules on MgO surfaces [14] CH4 and O2 interaction with sodium- and zinc-doped CaO surfaces [15] CO and NO on heavy metal surfaces [16]). An illustration of the computerized visualization of molecular dynamics of Pd clusters on MgO(lOO) and on a three-dimensional trajectory of Ar in Na mordenitc, is the recent publication of Miura et al. [17]. 

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. 

Ga can be incorporated into the zeolite skeleton instead of Al. These metallosilicates molecular sieves are denoted as B-ZSM-5 and Ga-ZSM-5 in this report. Moreover, a part of Al atoms in ZSM-5 zeolite lattice can be also replaced by B atoms. This metallosilicate molecular sieve is denoted as [B-AlJ-ZSM-5. 

The metallosilicates molecular sieves, [Ga]-ZSM-5, [B]-ZSM-5, and [B,A1]-ZSM-5 are also used as solid-acid catalysts as well as zeolites. There is a possibility that the mobility of protons is influenced by incorporating Ga and B atoms into the ZSM-5 zeolite skeleton instead of A1 atom, because the extent of incorporation of the isomorphous replacement affects the catalytic properties, e.g. the activities of the zeohtes. Quantification can be attained by appropriate test reactions. ... 

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. 

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... 

The addition of small amounts of certain oxyanions such as phosphate, perchlorate, arsenate, chlorate, bromate ect. in the synthesis mixture of zeolites and their metallosilicate / silicate analogues significantly enhances the nucleation and crystallization rates, thereby reducing the overall crystallization time by as much as five times. Sensitivity enhanced high resolution liquid state Si and P NMR studies, using a specially designed probe, on low temperature (358 K) synthesis of Silicalite-1 in the presence of NaH2P04 as promoter indicate a catalytic role of the promoter. 

The data obtained show that in phosphate-based molecular sieves as well as in metallosilicates or modified silica gels with molecular sieve properties, unusual Broensted and/or Lewis acid sites may exist which erfiibit different properties toward weak or strong bases. This should be taken into account when the behaviour of such zeolite-like materials in catalytic reactions and adsorption phenconena is discussed. 

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