Transition-metal ion-exchanged

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). 

Soma et al. (12) have generalized the trends for aromatic compound polymerization as follows (1) aromatic compounds with ionization potentials lower than approximately 9.7 eV formg radical cations upon adsorption in the interlayer of transition-metal ion-exchanged montmorillonites, (2) parasubstituted benzenes and biphenyls are sorbed as the radical cations and prevented from coupling reactions due to blockage of the para position, (3) monosubstituted benzenes react to 4,4 -substituted biphenyls which are stably sorbed, (4) benzene, biphenyl, and p-terphenyl polymerized, and (5) biphenyl methane, naphthalene, and anthracene are nonreactive due to hindered access to reaction sites. However, they observed a number of exceptions that did not fit this scheme and these were not explained. 

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... 

Transition metal ion-exchanged zeolites possess interesting properties. 

Maes, A. and Cremers, A., "Thermodynamics of Transition Metal Ion Exchange in Mbntmorillonite" Proc. Int. Clay Conf., Mexico City Bailey, S. W., Ed. Applied Publishing Wilmette, Illinois, 1975. 

The oxidation of CO is widely used as a test reaction for oxidation catalysts because of its simplicity. Thus, there is quite an extensive literature on CO oxidation using various zeolite catalysts. The parent (sodium forms) of zeolites show very little oxidation activity as might be expected and therefore the majority of the studies have concentrated on transition metal ion-exchanged forms. 

It has also been shown (20) that for transition metal ion-exchanged zeolites X and Y, the activities for CO oxidation increase exponentially with in-... 

More recently it has been shown (21-24) that the equilibrated pH of the transition metal ion-exchange solution is also critical in determining the specific activity of zeolite catalysts. The results obtained for the CuY system (21) are shown in Fig. 8. 

The oxidative dehydrogenation of cyclohexane to benzene has been studied more extensively. Transition metal ion-exchanged forms of zeolite Y have been shown (34-39) to be particularly active catalysts for this reaction. Although the platinum metal ions exhibit the highest activity, CuY was found to be the most selective for benzene formation (38, 39). 

Transition metal ion-exchanged zeolites are active catalysts for alkene oxidation but generally result in deep oxidation to carbon dioxide and water (43-45). In common with CO and alkane oxidation, the platinum metal ions are more active than the first-row transition metal ions. Mochida et al. (43) have been able to correlate the catalytic activity of ion-exchanged Y zeolites for propylene oxidation with a so-called Y parameter as shown in Fig. 9. This parameter was considered to express the tendency of the metal ion toward the formation of a dative re-bond with propylene. Further, it was shown that with increasing Y factor there was a decrease in reaction order, which was considered evidence of increased propylene adsorption. In a more recent study of CuX zeolites, Gentry et al. (45) found some evidence... 

The oxidation of NO and N02 is of industrial importance for cleaning combustion-flue gases. Transition metal ion-exchanged zeolites have been shown (51) to be highly active catalysts for this reaction. The relative activities are shown in Fig. 10, from which it can be seen that equilibrium conversions of NO to N02 can be achieved with Cu2 + X at reaction temperatures as low as 350"C. Kinetic studies showed that the reaction rates were fractional order in both NO and 02. The following reaction mechanism was therefore proposed, for example, with Cu2 + X,... 

A number of transition metal ion-exchange zeolites are active for acetylene trimerization (159, 160), and the criterion for activity appears to be an even, partially filled d-orbital, i.e., d8 (Ni2 +, Co+), d( (Fe2+), d4 (Cr2 + ). This has led to the suggestion that the mechanism must involve a complex in which there is simultaneous coordination of two acetylene molecules to the transition metal ion. The active oxidation state for CuNaY butadiene cyclodimerization catalysts has been unambiguously defined as monovalent copper (172-180). The d10 electronic configuration of Cu+ is consistent with the fact that isoelectronic complexes of Ni° and Pd° are active homogeneous catalysts for this reaction. The almost quantitative cyclodimerization selec-... 

Maes, A., P. Peigneur, and A. Creners. 1976. Thermodynamics of transition metal ion exchange in montmorillonite. Proc. Int. Clay Conf. Mexico City 1975 319-329. 

The electron spin echo modulation technique detects directly the coordination environment around a paramagnetic ion by observing the dipolar coupling to nuclei of weakly coordinated ligands. This technique has been used extensively by Kevan, for example, to examine transition metal ion exchanged and substituted zeolite materials [19]. 

Recently there have been numerous studies on gas-phase catalytic reactions induced by transition metal ion-exchanged zeolites such as reduction, oxidation, and carbonylation. The transition metal ions in zeolites are held to the zeolite framework by coordinating lattice oxide ions hence, the zeolite framework is considered to be a mono- to polydentate macro ligand for the... 

It has been reported that metal-zeolite catalysts have high possibility as new hydrodesulfurization catalysts for petroleum [1-9]. The catalytic desulfurization of organic sulfur compounds over zeolites has been investigated systematically [10-12]. On the basis of fundamental catalytic studies, active zeolite-based hydrodesulfurization catalysts such as transition metal ion-exchanged Y zeolites(MeY)[13-16], MoS2/NiHY[17] and Rh/USY[18,19] were developed. 

Transition metal ion-exchanged faujasites CuY, PdY and HPdY (metal content in weight% indicated as suffix), activated at 623 K (CuY) or 723 K (PdY, HPdY), respectively, in an oxygen flow of ca. 20 ml/min were used as catalysts. Details about preparation of the zeolite samples as well as experimental procedures are reported elsewhere [4]. The home-made flow apparatus was equipped with an analytical tube for chlorine detection from Draeger, Liibeck,... 

Several catalyst candidates, including transition metal ion-exchanged zeolites have been suggested for application on an industrial scale. Among zeolite catalysts group the Cu, Fe, Co and NF -exchanged ZSM-5 materials have proved to be of considerable promise [6]. 

Transition metal ion-exchanged ZSM-5 zeolites were prepared by using conventional and solid-state ion-exchange procedures. The samples proved to be crystalline and no structural destruction was found as a consequence of the modifications. 

Kumarraja, M., PItchumanI, K. Divalent transition metal Ion-exchanged faujasites as mild, efficient, heterogeneous Frledel-Crafts benzylatlon catalysts. Synth. Commun. 2003, 33, 105-111. 

Selective catalytic reduction (SCR) of NO, by hydrocarbons is under investigation as an alternative NO, removal technology. NO reduction by NHj is presently the commercial state-of-the-art technology available for reducing NO, from stationary sources and from the exhausts of lean-bum gasoline and diesel engines. A number of catalysts for the selective reduction of NO by hydrocarbons have been examined in previous studies [1-18], Transition metal ion-exchang zeolite catalysts such as mordenite and ZSM-5 are the most effective SCR catalysts. 

Most flue gases from the combustion sources contain, in addition to NO, HjO vapor and SO2, which may cause SCR catalysts to lose activity. Therefore, sulfur and water tolerances of catalysts are needed for the successful commercial application of the catalytic process. Since a dramatic loss of NO removal activity of transition-metal ion-exchanged zeolite catalysts was observed for NO reduction by hydrocarbons at wet conditions [1,4,9, 11-13], severd studies have focused on the cause of activity loss by HjO [14-18]. However, effects of SOj on SCR catalysts employing hydrocarbons as a reductant have received considerably less attention. 

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