Metal incorporation, zeolites

Rapid crystallization would overcome the disadvantages of slow crystallization and, more significantly, hetero elements could be incorporated inside the crystals. Metal-incorporated zeolitic materials serve as bifunctional catalysts, exhibiting properties of both metal catalysts and zeolite catalysts. 

Catalyst System HZSM-5 Zeolite Acid Modified Zeolite Metal Incorporated Zeolite... 

Product distribution of the light naphtha conversion (LNl) over the metal incorporated zeolite catalyst is given in Table 8. As can be seen from data presented, the metal modified catalyst is highly active for aromatization, evidenced by increased conversion to 91.5 wt % and aromatic yield 35.2 wt %. This is more obvious when we compare with the results on the parent catalyst (before metal modification) HZSM-5, which showed only 84% conversion and 22.4% aromatic yield. Increase in aromatic yields obtained over the metal incorporated catalyst can be explained by the active participation of metal in the olefin production by dehydrogenation and aromatization steps of the reaction [39-41]. Since aromatization of paraffins is an endothermic reaction, higher reaction temperatures (500 C) were employed for the maximum production of aromatics. 

Transition metal-incorporated zeolites have been shown to be effident catalysts for direct conversion of methane to benzene and toluene under nonoxidative conditions [45,46]. Bao and co-workers revealed that Mo/ H-MCM-22 catalysts are desirable bifiinctional catalysts for methane dehydroaromatization reaction [47]. In terms of catalytic performances of Mo/H-MCM-22 with varied metal loading, catalyst with a Mo loading of ca. 6 wt% was found to exhibit the optimal benzene selectivity, suppressed naphthalene yield, and prolonged catalyst hfe under a moderate methane conversion. Although both Bronsted and Lewis acid sites are capable of catalysing methane conversion reaction, active sites with higher acidic strengths are anticipated to play the dominant role. 

Photochemical reduction of CO2 is another important strategy for solar energy conversion. Not only does it harvest energy from sunlight to produce valuable organic compounds but also helps to reduce the CO2 level in the atmosphere. In natural photosynthesis, CO2 is reduced by photochemically generated NADPH in photosystem I [30]. Scientists have developed a number of molecular compounds, semiconductors, and metal-incorporated zeolites for artificial photochemical CO2 reduction [31-34]. 

Metal incorporation into the zeolite using metal loaded seed materials. The combination of catalyst metal with zeolite catalyst is one of the most intriguing subjects for bifunctional catalysis. The achievement of prominent effect of the seed crystals on the crystallization of ZSM-34 type catalyst induced an idea that the seed material on which a catalyst metal had been supported previously would also be effective for rapid crystallization. 

Various techniques are available for the introduction of metals into zeolites. Ion exchange and impregnation, e.g., by the incipient wetness or imbiberaent techniques, are often used. The former method introduces preferentially the cation into the zeolite, whereas the latter method also incorporates an equivalent number of anions. In both cases, the introduction of ions has to be followed by calcination and reduction steps. For zeolite-based catalysts the ion-exchange method is often preferred (97,9S). 

The recent EPR study by Kevan et al.[16] of cobalt substimted ALPO-5 illustrates the power of the EPR technique to characterise metal substituted zeolites. CoAPO-5 as synthesised is blue in colour the electronic spectrum is characteristic of tetrahedrally coordinated Co2+, suggesting that Co2+ has been incorporated into the AIPO4 lattice. The corresponding generation of Bronsted acid sites indicates that Co2 + substitutes for Al +.The material does however change colour to yellow-... 

Incorporation of metals in zeolites occurs usually via ion exchange with the respective metal salt and subsequent reduction. A large body of information exists on the preparation and characterization of mono and bimetallic particles in molecular sieves and the reader is referred to that literature for details [197,198,199]. 

From the above observations, it is clear that HZSM-5 zeolite is the suitable catalyst for the LPG production, whereas metal incorporated catalyst is needed for the production of aromatics. 

Transition metal-containing molecular sieves exhibit remarkable properties as catalysts for a variety of oxidation reactions with peroxides as the oxidant [1]. The potential of transition metal containing zeolites is however, limited because of the number and t e of heteroelements that can be incorporated in the framework and also the pore sizes of the resulting molecular... 

The aim of this work is testing of SERS-activity of metal-containing microporous alumosilicates or zeolites. Porous structure of zeolite skeletons caused by coupling of tetrahedral [Si04] and [AIO4] building units is a unique basis for stabilization of a super-lattice of mono-dispersed metal clusters. Zeolite matrices combine the factors of nanoporosity and nanometer-scale chemical reactivity with respect to incorporated foreign ions, clusters, and nanoparticles [1]. 

Different ways have been proposed in the "open" and patent literature for the incorporation of metals into zeolites and for the partial or total substitution of aluminum. Ion exchange methods are very frequently used for the incorporation of mono and bivalent metals, e.g., in the preparation of Cu-MFI type zeolites, extensively studied for the SCR reaction [4, 5]. Solid state reactions are also used for the introduction of copper, iron and other metals [6, 7]. The TVD technique is another interesting method for the preparation of zeolites containing transition metals [8], and finally the direct synthesis in presence of metal salts [9,10] or metal complexes is also used. 

Zeolites, i.e. microporous aluminosilicate materials with pores smaller than 2 nm, play key roles in the fields of sorption and catalysis [134, 135]. The global annual market for zeolites is several million tons. In the past few decades a large variety of zeolites and related zeotype materials have been produced, whereby transition metal incorporation is extensively used to modulate the catalytic characteristics of these materials. Since the catalytic properties depend on the structure and accessibility of the transition metal sites, a lot of effort is put into probing these sites. Nevertheless, the exact nature of the transition metal incorporation is often strongly debated, since most spectroscopic evidence for isomorphous substitution is indirect. 

The secondary synthesis procedure, described earlier, has also been applied to prepare titanium silicates (4,88). This method of incorporation of fame-work metals into zeolites was part of Union Carbide s zeolite research. 

Niobium and tantalum were incoiporated into the faujasite aluminosilicate structure in one-pot synthesis and also by post-synthesis method, i.e. sohd-state ion exchange. One-pot synthesized zeolites exhibit Nb and Ta incorporated into the zeolite skeleton as evidenced by different methods. The efficiency of metal incorporation examined by XPS measurements was found to be higher for tantalum than for niobium. 

Location of transition metals in the framework and extra framework positions can be deduced from their coordination. Tetrahedrally coordinated species should be detected for framework position, whereas the octahedral coordination can point to the extra framework ones. The UV-Vis spectra of the samples prepared in the one-pot syrrlhesis show the bands characteristic of tetrahedrally coordirrated trarrsition metals, especially for the tantalum containing samples in which the efficiency of metal incorporation was higher (see Table 1). Moreover, the spectra of these samples differ from those recorded for materials prepared via post-synthesis modification. Figure 2 presents the UV-Vis spectra of tantalum oxide, and tantalum incorporated into Y zeolite by synthesis and post-synthesis methods. It is clearly seen, that the latter technique leads to the formation of octahedrally coordinated species, which is likely to be bulk tantalum oxide. However, in the one-pot synthesis method all tantalum is characterized by a band at c.a. 221 nm, typical of tetrahedrally coordinated Ta species. Therefore, one can conclude about the location of Ta in the zeolite skeleton. 

For further investigation of metal state in the rtraterials prepared XPS spectroscopy was applied and the results obtained are shown in Table 1. The oxidation state of the metal incorporated was estimated as +5 for both rtiobirrm arrd tantalum. The presence of metal at +5 oxidation state in the zeolite framework shotrld imply generation of Lewis acidity. Irrdeed, the pyridine adsorption followed by FTTR spectroscopy proved the presence of Lewis acid sites in all zeolites (spectra not shown here). As mentioned above, all alurttinum is tetrahedrally coordinated hence the origin of Lewis acidity should be associated with niobium and tantalum incorporated irrto the skeleton of zeolite. Nb and Ta in the zeolite framework forms positive charged tetrahedral species. 

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