Metallosilicate

Activities of various metallosilicates for Mukaiyama aldol reaction of benzaldehyde with silyl enol ether... 

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. 

Metallosilicate Molecular Sieves A large number of metaUosihcate molecular sieves have been reported, particularly in the patent literature. Those claimed... 

Microporous, amorphous Mg-Si-O metallosilicates with a very narrow pore size distribution around 6 A diameter and a typical surface area of ca 350 m /g were obtained from the controlled calcination of compound 22. The resulting Mg-Si-O material was found to be very active in 1-butanol conversion even at 200 °C giving both dehydrogenation and dehydration. 

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... 

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. 

Therefore, these metallosilicates may have the Bronsted acid sites, because the Ga, In, Sb, and As introduced into the silicalite framework are trivalent. 

Acidic properties of metallosilicates prepared by atom-planting method... 

The solid acidity of these atom-planted silicalites was examined by ammonia TPD measurement. The ammonia TPD profiles of all these metallosilicates showed one peak at a higher temperature than 453 K. Figure 4 shows the relationship between peak temperatures of ammonia TPD and wavenumber of IR absorption bands due to the acidic SiOH groups of atom-planted silicalites. A straight line was obtained. From these results, the order of strength of acid sites would be as follows ... 

Yashima and his co-workers examined the methylation of 2-MN using MFI-metallosilicates.60 The methylation over HZSM-5 yielded predominantly 2,6-,... 

The results described above suggest that the methylation of naphthalene over MFI- metallosilicates occurs inside crystalline pores by a restricted transition-state mechanism, not with unordered sites at or near external sites. Weaker acid sites preferentially form 2,6-DMN and favor a decease in coke formation. 

The pores of HZSM-5 do not have enough space to accommodate 1-MN and DMN isomers with a-methyl groups. The methylation of 2-MN over HZSM-5 yields predominantly 3, 3-DMN isomers, 2,6-, 2,7- and 2,3-DMN, with the formation of bulky a,P-DMN, 1,2-, 1,3-, and 1,6-, and 1,7-DMN. The selective formation of 2,6- and 2,7-DMN occurs inside pores, because the deactivation of external acid sites of MFI-metallosilicates by 2,4-DMQ reduces the isomerization of 2,6- and 2,7-DMN to a,(3-DMN. However, the pores of HM and HY do not contribute to the shape-selective methylation and related reactions of naphthalene and 2-MN. 

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. 

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

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,... 

Inui, T., Tarumoto, J., Okazumi, F. and Matsuda, H. Highly selective synthesis of high-octane gasoline from light olefins on novel metallosilicates. Chem. Express, 1986, 1, 49-52. 

A novel sizing strategy utilizes a microparticle system that enhances retention.166 A representative example is a combination of cationic potato starch (DS = 0.04), a metallosilicate hydrosol and AKD. In an approach to combine elevated filler retention, effective sizing and high paper strength, calcium carbonate is reacted with a starch-soap complex and combined with AKD.167168... 

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. 

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