Zeolites containing transition metals

Zeolites containing transition metal ions (such as Cr3+, Ag+, and Cu2 + ) are active as oxidation catalysts. Comprehensive reviews dealing with various aspects of the structure (34-37) sorption (35), catalysis (38-42), and other chemical properties of zeolites are available in the literature. 

The Cr A and several other zeolites containing transition metal ions, which may exist in two or more valence states, were also found to be oxidation catalysts. One such system of note is the copper containing Type Y zeolite, the redox chemistry of which was studied in several recent investigations (2, 3.4, 5). These studies established the range of conditions at which copper exists in divalent, monovalent, or zerovalent state and in particular determined the reduction conditions in hydrogen and carbon monoxide atmospheres for a complete conversion of Cu Y to Cu Y but no further to Cu°. The Cu ions in type Y zeolite were reported to be specific adsorption centers for carbon monoxide ( 6), ethylene ( 7), and to catalyze the oxidation of CO (8). In the present work the Cu ions were also found to be specific adsorption centers for oxygen. 

Catalytic activity of cation-exchanged zeolites containing transition metal ions was reported by Rouchaud et al. (82), who found that the oxidation of n-hexane to acetic acid is catalyzed by Mn -exchanged Y at 160° and 25 atm. As another example of the effect of transition metal ions, Kruerke (52) found that acetylene trimerized to benzene on transition metal-exchanged Y at near room temperature. The activity changes with the transition metal Ca and Na = O < Mn " < < Co " = Ni " > > Cu " = = O. This activity pattern is consistent with a model accord-... 

Jones and Landis (155) have shown that aromatic nitriles can be produced in good yield by reacting methylaromatic hydrocarbons with NHs in the presence of certain heavy metal-containing crystalline aluminosilicates. Generally, large pore zeolites containing transition metal cations capable of forming coordination complexes were required for... 

Selective Aerobic Epoxidation of Olefins over NaY and NaZSM-5 Zeolites Containing Transition Metal Ions... 

Numerous studies have been devoted to zeolites containing transition metals under cationic form, or as reduced metal particles, or as coordination complexes (1, 2, 3). Among these, very few deal with encaged transition metal sulfides. We recently examined the transformation of nickel ions into sulfide in various zeolite structures, mainly from the stoichiometric point of view (4, 5). It... 

Unlike hydrogen adsorbed on alkali or alkaline earth cations in zeolites, detailed IR studies of H2 adsorbed on zeolites containing transition metal cations have been reported only relatively recently. Kazansky et al. (1999a) observed five bands in the DRIFT spectrum of ZnNaY, two of which were attributed to hydrogen adsorbed on Zn cations. Serykh et al. (2000) found... 

Tsitovskaya, I. L., Catalytic Properties of Zeolites Containing Transition Metal Ions in Oxidation Reduction Reactions, Dokl. Akad. Nauk SSSR (1971) 196,872. 

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. 

Catalytic oxidation processes are usually connected with transfer of electrons and changes of structure and valence state of active catalyst components. This chapter presents methods that are especially suitable for monitoring these kinds of changes (UV-vis-DRS, EPR, X-ray scattering, XPS, XAS, TPO, TPR, TPRS, TAP and SSITKA). After a short section on basic principles and experimental details, the potential of each technique is illustrated by selected application examples that include a wide variety of oxidation catalysts such as mixed metal oxides and oxynitrides, zeolites containing transition metal ions, heteropoly acids and supported noble metals. 

Zeolite-like Transition Metal Containing Porous Compounds... 

The number of reactions catalyzed by zeolites is continually increasing. Synthetic faujasites containing transition metal cations are active for the complete oxidation of H2, CO, C2H4, NH3 (9). Upon interaction of NH3 with 02 over CrY and AgY, N20 and N2 are formed (9). Mahida et al. (21) investigated the oxidation of propylene over Cu2+Y. Depending on the temperature and on the water vapor content in the reaction mixture,... 

Minachev et al. (41, 42) have recently examined alkali metal ion forms of various zeolites (A, X, Y, L, chabazite, erionite, and mordenite) for cyclohexane oxidative dehydrogenation. Not surprisingly these alkali metal ion forms are considerably less active than those containing transition metal ions (reaction temperatures of approximately 300° and 450°C, respectively). Further, cyclohexene rather than benzene is the predominant product (selectivity to cyclohexane 67-84%), particularly with small-pore zeolites. In fact, NaA was the most active zeolite tested (42), which strongly suggests that the reaction is simply occurring on the outer surface of the zeolite crystallites. 

Use of Catalysts Containing Transition Metal Cations. Ethyl -ene being alkylated over certain zeolite catalysts reacts specifically. Ethylene can not, however, be alkylated with Isobutane In the presence of H2SO., because of the formation of stable ethylsulphates. We examined the Isobutane - ethylene alkylation over crystalline aluminosilicates and found that those catalysts containing RE and/or Ca In combination with transition metal cations were most active. The alkylation has resulted In not hexanes as would be expected, but an alkylate containing octane Isomers as the major product (about 80%). Moreover, the product composition was similar to that obtained from n-butene over CaREY. The TMP-to-DMH ratios were 7.8 and 7.1 respectively. 

Zeolites containing transition metai offer interesting possibilities to combine acid properties and a hydrogenation function in bifunctional catalysts. For hydrotreating, e.g. for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) reactions, both functions are essential the acid properties are provided by the zeolite, the hydrogenation activity by the sulfided transition metal cations [1,2]. It can be expected that these catalysts will be attractive alternatives to alumina supported Co-Mo and Ni-Mo hydrotreating catalysts because of their superior catalytic properties. 

Since Hamada et al. (1) reported that solids not containing transition metals catalyze the selective reduction of NO with hydrocarbons in the presence of O2, many research groups have studied this matter. Yogo et al. (2) reported that H-zeolites are able to reduce NO with CH4. They found that the selectivity for this reaction decreases in the order H-ferrierite > H-mordenite > H-ZSM5 H-Y. 

Epoxidation of alkenes can be performed efficiently by molecular oxygen (air) in the presence of isobutyraldehyde and NaY or NaZSM-5 zeolite catalysts containing transition metal ions. High selectivity of epoxidation is achieved (up to 99%) in spite of the chain radical nature of the reaction. 

The great potential of electron spin resonance in zeolite science, in particular in the characterization of zeolitic systems containing transition metal cations, paramagnetic clusters, or molecules or metal particles, is demonstrated by B.M. Weckhuysen, R. Heidler and R. Schoonheydt, who co-authored Chapter 3. 

KY zeolite induces mono-Y-alkylation of amines and amides by alkyl halides at 80°C with good yields and selectivity [49]. Sabu et al. investigated the acid properties and catalytic activities of natural kaolinitic clays containing transition metals for Friedel-Crafts alkylation [50]. High catalytic activity and selectivity for the reaction of benzyl chloride and benzene to diphenylmethane was reported. 

The main classes of catalysts used for heterogeneous WHPCO reaction are clays and anionic clays (hydrotalcites), metal-ion exchanged zeolites and mesoporous silica containing transition metals, and doped metal oxides. Although some other transition metals have been also used (Mn, V), most catalysts contain iron and/or copper as the active elements. Leaching of the active metal is also a significant problem in this case. While different types of catalysts have been reported, only a few of them have been effectively proven to have a stable activity in long-term continuous experiments or at least in several repeated batch tests. Between the stable catalysts, Fe- and Cu-PILC (pillared clays) materials " have the best combination of activity and stability. However, the limited quantity of active elements (around 2% wt. of iron or copper) necessary to achieve stable performances, limits the overall activity. 

Copper hydroxyphosphate also allows the direct hydroxylation of 2,3,6-TMP, leading to selectivities for TMHQ (1) of >80%. The main by-product is TMQ (5). Heterogeneous catalysts which have been used for the oxidation of 3 include zeolites, mesoporous materials and molecular sieves contaiiung transition metals. Molecular sieves containing transition metals such as vanadium or copper in the framework can simply be mixed with 2,3,6-TMP and H2O2 in acetoiutrile for the oxidation. ... 

Therefore, the subsequent examples are not categorized with respect to the methods used for monitoring solid-state reactions of compounds containing transition metal cations with zeolites. Rather, a classification of the examples is adopted according to the type of metal cations to be introduced into the zeolite matrix. 

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