Iron-substituted zeolites

Figure 1. Comparison of the Framework Region Infrared Spectra of Some Titanium or Iron Substituted Zeolites with the Untreated Starting Materials.

Preferred conditions for the reaction of the zeolite with the metal ammonium fluoride are as follows. A zeolite-water slurry containing about 10-25 gm of zeolite per 100 cm3 of water is preheated to 75-95°C. When substituting Ti, the titanium salt is added to the zeolite as a water slurry containing finely divided crystals, 10 gm of (NH4>2T1F6 per 100 cm3 of water. With iron substitution, the iron salt, when added as (NH4>3FeF6, is added from a 10 wt.% solution of the salt in water. Alternatively, FeF3 can be added to a solution of NH4HF2 or NH F such that the stoichiometry of fluorine (F2) to Fe3+ is 3.0 and the total amount of salt in solution is about 10 wt.%. The addition rate of the metal ammonium fluoride salt to the zeolite slurry is about 0.005 moles of the metal ion per minute per mole of aluminum in the zeolite. 

The hydronium exchanged synthetic mordenite does have a band, as shown in spectrum 10 in Figure 1. The iron substituted mordenite samples (spectra 8 and 9) do not show the presence of the band it was "removed" as a result of the substitution reaction. An absorption band at 950 cm- is normally attributed to an Si-OH stretch vibration (14, 15), and is typically observed in some acid or hydrothermally treated zeolites. 

A. TITANIUM SUBSTITUTED ZEOLITES B. IRON SUSTITUTED ZEOLITES... 

The n-butane cracking values obtained with the titanium substituted zeolites all show an increase in kA value over that obtained with the starting zeolite. This is notable in that the Ti is tetra-valent, and does not require a cation. With reduced cation content, the acidity should be reduced. However, the result obtained was the reverse. With iron-substituted products, the resulting kA values varied with the zeolite. Although not discussed in detail in this paper, all Fe-containing products did show indications of metals activity there was a dramatic increase in the amount of olefins produced. 

Experiments of the type described above and illustrated in Figure 4 have been carried out for a series of zeolites which included H-ZSM-5, H-offretite, and their iron substituted analogs, using light alcohols, dimethyl ether, and ethylene as reactants. 

Due to their particular catalytic properties, iron- and iron-aluminium-substituted zeolites are especially interesting for application in oxidation processes. In this paper the preparation procedure and the adsorption properties of Fe-Al-BEA and Fe-Al-MOR zeolites are described and the results are compared with the data obtained for the corresponding aluminium forms of both zeolites. 

Aluminosilicate zeolite ([Al]ZSM-5), borosilicate zeolite ([B]ZSM-5), and iron silicate zeolite ([Fe]ZSM-5), doped with transition metal or rare-earth or noble metal were tested as the catalysts. Different modifications of the zeolite do not improve the olefin conversion. Neither isomorphous substitution of Al atom in the zeolite structure by B or Fe, nor doping the zeolite with metals influences notably the conversion of olefin into the acid compared with [Al]ZSM-5 (Table 23). 

The 1980s saw major developments in secondary synthesis and modification chemistry of zeolites. SUicon-enriched frameworks of over a dozen zeolites were described using methods of (i) thermochemical modification (prolonged steaming) with or without subsequent acid extraction, (ii) mild aqueous ammonium fluorosilicate chemistry, (iii) high-temperature treatment with silicon tetrachloride and (iv) low-temperature treatment with fluorine gas. Similarly, framework metal substitution using mild aqueous ammonium fluorometaUate chemistry was reported to incorporate iron, titanium, chromium and tin into zeolite frameworks by secondary synthesis techniques. 

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. 

Fe-substituted ZSM-5 type zeolites, allowing the hydroxylation of benzene with N20 with the help of the so-called redox properties of lattice substituted a-iron [34]. 

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