Zeolites transition metal ions

Catalysts include oxides, mixed oxides (perovskites) and zeolites [3]. The latter, transition metal ion-exchanged systems, have been shown to exhibit high activities for the decomposition reaction [4-9]. Most studies deal with Fe-zeolites [5-8,10,11], but also Co- and Cu-systems exhibit high activities [4,5]. Especially ZSM-5 catalysts are quite active [3]. Detailed kinetic studies, and those accounting for the influence of other components that may be present, like O2, H2O, NO and SO2, have hardly been reported. For Fe-zeolites mainly a first order in N2O and a zero order in O2 is reported [7,8], although also a positive influence of O2 has been found [11]. Mechanistic studies mainly concern Fe-systems, too [5,7,8,10]. Generally, the reaction can be described by an oxidation of active sites, followed by a removal of the deposited oxygen, either by N2O itself or by recombination, eqs. (2)-(4). 

Figure 2.17. Diagram showing nitroside and nitrosonium ways of NO activation upon coordination to the transition-metal ions in zeolites along with basic characteristics of the NOs+ and NO5-species.

One of the most promising techniques for studying transition metal ions involves the use of zeolite single crystals. Such crystals offer a unique opportunity to carry out single crystal measurements on a large surface area material. Suitable crystals of the natural large pore zeolites are available, and fairly small crystals of the synthetic zeolites can be obtained. The spectra in the faujasite-type crystals will not be simple because of the magnetically inequivalent sites however, the lines should be sharp and symmetric. Work on Mn2+ in hydrated chabazite has indicated that there is only one symmetry axis in that material 173), and a current study in the author s laboratory on Cu2+ in partially dehydrated chabazite tends to confirm this observation. 

Camel through the eye of a needle" syntheses, in zeolites, 231, 232/ Carboxylate esters, alcoholysis of, with transition metal ion and Ln3 + catalysts, 288-294... 

Extremely high ion exchange affinities are however sometimes observed for alkali metals (e.g. Cs) and transition metal ion complexes in clay minerals and zeolites. The objective of this paper is to give an account of the factors which are involved in these high selectivity phenomena. The discussion will be focussed mostly on montmorillonites and faujasites as representatives of the phyllosilicate and tectosilicate groups. 

Exchange in zeolites of alkali, alkaline earth, transition metal ions and small organic ammonium ions, has been reviewed (111), and in general, the exchange is characterized by small AG values comparable to those found in clay minerals. Althoufft identical selectivity orders for alkali and alkaline earth metal ions are obtained, as in montmorillonite, the opposite variation of AG with charge density is found. 

From the previous paragraphes it follows that a substantial amount of experimental data exist that illustrate the oxygenation properties of zeolite catalysts. In very general terms zeolites are used to heterogenize transition metal ions in ion exchange or lattice positions, to stabilize transition metal oxide dispersions and to prepare ship-in-bottle complexes. 

The behavior of transition metal ions exchanged in zeolites is very similar to that in a homogeneous medium CuPdY zeolites are efficient substitutes for Wacker chemistry in absence of chloride ions. 

Fig. 1. TPD chromatograms of oxygen from several transition metal ion-exchanged Y zeolites. A Na Y, B Ni2+Y, C Mn2+Y, D Co2+Y,andE Cu2+Y.

K. The results indicate that Cu-ZSM5 is the most active catalyst at 773 K for the decomposition of dilute NO gas. The order of activity is Cu-ZSM5 > Ag-Co304 > La-Sr-Co(Cu)-0 > Pt/Al203 > Y-Ba-Cu-O/MgO. Regarding transition metal ion-exchanged zeolites, catalytic activities in the reduction of NO with or and the adsorption state of NO were extensively... 

The reduction of Cu to Cu in the zeolite lattice is more difficult than reduction of platinum and palladium ions but easier than that of other transition metal ions.25 The resulting Cu" " ion in the zeolite is fairly stable both in a reductive atmosphere and imder degassing treatment at elevated temperatures, wh eas the precious metal ions are easily reduced to the respective metals and collect to yield metal particles. Die easy reducibility of Cu and the stability of Cu" " lead to a reversible redox behaivor betweoi Cu and Cu and result in the iqipearance of the specific catalytic activity. 

The replacement of Si4+ by Al3+ ions in the tetrahedra generates a deficit of one positive charge per aluminum ion, which must be compensated by the incorporation of extrinsic cations in the zeolite structure. The sodium or calcium ions which are most commonly found in natural or synthetic zeolites can be exchanged with other alkali, alkaline-earth, rare-earth, or transition metal ions. The zeolite open structure can accommodate not only the extraframework cations, but also various molecules provided that their size is smaller than the zeolite apertures. A key feature of cation-exchanged zeolites is the local electrostatic field associated with the cations. This has led to the view of zeolites as solid solvents (258 and references therein). 

There are several examples where irradiation is not necessary to produce Oj ions. In such cases, a thermal activation is sufficient because of the presence of transition metal ions which can easily transfer one electron to oxygen. Iron is the most common impurity found in zeolites and the formation of OJ depends very much on the iron content (239, 240). Transition metal ions can also be exchanged in zeolites and this will be discussed later. There is also some indication that the types of OJ can be influenced by the level of exchange. When cations of different valence states are involved in the exchange, an incomplete exchange will leave two types of cations present, creating the possibility of at least two types of adsorption sites. This has been observed for both Mg and CaY zeolites (263). 

Hi. Zeolites exchanged with transition metal ions. In the first row, scandium-, titanium-, cobalt-, and nickel-exchanged zeolites have been the most studied. Cobalt-exchanged zeolites are discussed in Section IV,E since they lead to oxygen adducts on adsorption of oxygen. There are several cases where copper and particularly iron ions are found as impurity cations which affect the oxygen adsorption properties of the zeolite. 

The gzz values were assigned to a given oxidation state of the adsorption site on the basis of spectroscopic and chemical evidence. For transition metal ions, the oxidation state was deduced from reactions of the type M1"- u+ + 02 - Mn+02, which were ascertained by a decrease in the EPR signal of Moxidation state has been taken. For the +1 oxidation state observed only in alkali zeolites there is a large range of gzz values 2.054-2.166 (Table X), which has been used in Fig. 3. It appears,... 

Zeolites also provide convenient framework sites for activating transition metal ions for redox catalysis. Iwamolo30 has described a CutlI)/Cu(I) exchanged zeolite that holds promise for the high-temperature conversion of NO, tin diesel and auto exhaust) to N and O, ... 

This paper presents some data relating to these aspects which have been obtained in the course of an extensive experimental study of the ion-exchange behavior of transition-metal ions in X and Y zeolites. 

The temperature-dependent irreversibility demonstrates that the ion-exchange behavior of NaX towards bivalent cations depends strongly upon the thermal history of the sample. The rather pronounced differences in behavior of transition-metal ions, also observed in synthetic zeolite 4 A (9) is in very sharp contrast with the nearly identical, either hydrated or crystallographic, dimensions of these ions (10). Obviously, this observation raises important questions as to the value of the current interpretation (nearly) exclusively in terms of physical dimensions of ions and pore width. In contrast, the similarity of behavior in mont-morillonite is remarkably close the AG0 value for the replacement of Na by either Ni, Co, Cu, or Zn is —175 cal ( ll)/equivalent, irrespective of the nature of the cation (11). Therefore, the understanding of their difference in behavior in zeolites must take other effects into consideration. 

Transition metal ion-exchanged zeolites possess interesting properties. 

Another characteristic feature of the hydrogen-reduced transition metal zeolites is their acidic properties, as demonstrated by their catalytic behavior (7). Naccache and Ben Taarit (8) gave IR evidence of the subsequent formation of protons on hydrogen-reduced Cu(II)-Y zeolite. Furthermore, transition metal ions have various oxidation states. Owing to the shielding effect caused by the zeolite network and the electric fields, the transition metal ions may be stabilized in unusual oxidation states—i.e. Ni(I) (9). 

Correlations between structure and catalytic activity have been described for carbonium-ion type reactions (1). Much effort was also spent to establish a correlation between structural and compositional factors and the activity for redox type reactions (1, 9-12). Transition metal ions in zeolites were shown to be active in the oxidation and hydrogenation of hydrocarbons. In this connection various techniques were used to locate the cations in the framework of the faujasite-type zeolites (13-20). These ions migrate upon thermal treatment or by the adsorption of various substances. Thus, methods are needed to determine the location of the cations under reaction conditions. 

As it was shown in the case of pentasil zeolites (MOR, FER, MFI), UV-VIS-NIR DR spectroscopy and UV-VIS emission spectroscopy appear to be extremely powerfiill tools for the characterization of transition metal ions in molecular sieve matrices [1,5]. The aim of this study is to employ this promissing and powerfull technique for the characterization of siting of metal cations exchanged into the (A1)MCM-41 matrix. 

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. 

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