Transition Metal Exchanged Zeolites

It was suggested by a number of authors that sintering similar to metal catalysts leads to deactivation of copper-modified zeolites. 

Sulfate formation and coke deposition were often excluded as reasons for deactivation, using EXAFS spectroscopy. The formation of larger clusters in the zeolite, which can also lead to destruction of the host structure, can be observed by an increase in the coordination number of the Cu—Cu scattering contribution to the EXAFS. 

The formation of extended oxidic species was deemed detrimental to the catalytic performance by most authors. XAS has been instrumental in the detection of these species. 

Deactivation of the copper zeolites under de-NO, conditions was one of the major reasons why the catalyst was never used in a commercial application. Recent environmental legislation intensified the hunt for a water- and sulfur-stable active catalyst One of the most successful preparative methods was reported by HaU and Feng [42, 43]. They reported excellent de-NO, performance based on an iron exchanged ZSM-5 zeolite. The activity was reported to remain constant for extended times, even under high water and sulfur content conditions. The initial catalytic study initiated a whole raft of characterization studies by a number of groups. The interest was significantly increased when it became obvious that there are issues with catalyst preparation reprodudbihty [44, 45]. XAS was crucial in the discussion of the structure of active sites for de-NO, and the site responsible for the high 

Discussion of the active site structure remains current. The catalytic process and activity of these materials is so unique that much effort is being put into their structural elucidation. 

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Only a few attempts have been made to relate the catalytic activity to the properties of cations on the transition metal-exchanged zeolites. Cross, Kemball, and Leach (5) studied the isomerization of 1-butenes over a series of the ion-exchanged X zeolites. Their results with CeX zeolite and the majority of other zeolites indicated a carbonium ion mechanism however a radical mechanism was operative with NiX and in some cases with ZnX. 

The catalytic activity for the aniline formation from chlorobenzene and ammonia of the Y zeolites with various cations was studied at 395° C (Table I). It is clear that the transition metal-exchanged zeolites have the catalytic activity for the reaction, while alkali metal and alkaline earth metal zeolites do not. The fact that alkaline earth metal-exchanged zeolites usually have high activity for carbonium ion-type reactions denies the possibility that Bronsted acid sites are responsible for the reaction. Thus, catalytic activity of zeolites for this reaction may be caused by the... 

Scheme 10.3 Formation of MePc complexes in the supercages of FAU-type zeolites via tetramerization of 1,2-dicyanobenzene around transition metal exchanged zeolite.

Despite the enormous importance of zeolites (molecular sieves) as catalysts in the petrochemical industry, few studies have been made of the use of zeolites exchanged with transition metal ions in oxidation reactions.6338- 634a-f van Sickle and Prest635 observed large increases in the rates of oxidation of butenes and cyclopentene in the liquid phase at 70°C catalyzed by cobalt-exchanged zeolites. However, the reactions were rather nonselective and led to substantial amounts of nonvolatile and sieve-bound products. Nevertheless, the use of transition metal-exchanged zeolites in oxidation reactions warrants further investigation. 

The tendency to obtain incomplete degrees of transition metal chelation starting from transition-metal exchanged zeolites, thus yielding ions that can start a radical cycle, may be circumvented, at least for iron, starting from ferrocene adsorbed NaY. 

Alkene oxidation over transition metal exchanged zeolites has been of recent interest. Yu and Kevan have studied the partial oxidation of propene to acrolein over Cu2+ and Cu2+/alkali-alkaline earth exchanged zeolites.33 In both... 

Mann, D. E, Pratt, K. F. E., Paraskeva, T., Parkin, I. P. and Williams, D. E. (2007) Transition Metal Exchanged Zeolite Layers for Selectivity Enhancement of Metal-Oxide Semiconductor Gas Sensors. Sensors Journal, IEEE 7,551-6. 

Although the main applications of zeohtic sohds in catalysis will continue to be as solid acids in the synthesis and transformations of petrochemicals and commodity chemicals they continue to be considered as catalysts and catalyst supports for a range of reactions of synthetic and industrial relevance. The most important of these are of titanium- and tin-containing solids in selective oxidations. Other well-studied reactions over zeohtes include light hydrocar-bons-to-aromatics (Ga-zeolites) selective catalytic reduction of NO (transition metal exchanged zeolites) C C bond formation (Pd zeohtes) selective alkane oxyfunctionalisation with air (MAPOs, M Mn, Fe, Co) and chiral catalysis over encapsulated chiral complexes. 

Fedeyko JM, Chen B, Chen H-Y (2010) Mechanistic study of the low temperature activity of transition metal exchanged zeolite SCR catalysts. Catalysis Today 151 231-236... 

However, if transition metal exchanged zeolites are active materials in NH3-SCR, N2O emission can be also observed. N2O emission constitutes one of the main drawbacks of this system. For instance, Wilken et al. [112] reports a maximum N2O production at 200 °C over Cu-Beta zeolite. Mechanism of N2O emission is proposed to proceed through the decomposition of ammonium nitrates (reaction 19.28) ... 

Fig.4.35 for the case of SnOi M0O3. Formation of ketones from olefins and water probably in a similar manner have been reported for Pt/AfeOj, ", M0O3/AI2O3, transition metal-exchanged zeolite, H —ZSM-5 and heteropoly acid. ° ... 

Alkoxyl species form very readily from the reaction of alkyl halides on alkali, alkaline earth, transition metal, and lanthanide exchanged zeolites (128, 129). The more basic the zeolite, the more readily the reaction proceeds. Alkyl halides have been used to generate methoxyl, ethoxyl, isopro-poxyl, and ferf-butoxyl species on metal-exchanged zeolites. The mechanistic significance of alkoxyl species in zeolite acid catalysis is not in general clear in some reactions they may be true intermediates, and in others mere spectators. 

The abovementioned rate acceleration and selectivity enhancement brought about by catalysts are particularly marked when unactivated dienes and dienophiles are involved. Two molecules of 1,3-butadiene can react in a Diels-Alder reaction, one acting as diene and the other as a dienophile to produce 4-vinylcyclohexene (in 0.1% yield at 250°C in the absence of a catalyst). Cs+, Cu,+ and trivalent transition-metal exchanged montmorillonites534 as well as large-pore sodium zeolites (Na ZSM-20, NaY) and carbon molecular sieves,535 result in 20-35% yields with 95% selectivity. Large rate enhancement was observed when 1,3-cyclohexadiene underwent a similar cycloaddition536 in the presence of K10 montmorillonite doped with Fe3+ ... 

Jones and Landis (12) assumed the formation of the ammine complexes and their participation in the reaction of toluene with ammonia to form benzonitrile over a variety of transition metal-exchanged X zeolites. 

Yashima, T., Ushida, Y., Ehisawa, M. and Hara, N. Polymerization of ethylene over transition-metal exchanged Y-zeolites. J. Catal., 1975, 36, 320-326. 

Scheme 10.7 Co-tetramerization of pyrrole and aldehydes in the presence of transition metal exchanged Y zeolite (MeY), yielding entrapped metalloporphyrin (after Jacobs1131).

Transition metals exchanged into Y-zeolite offer a basis upon which to build an understanding of the important parameters involved in designing zeolitic redox catalysts. Y-zeolite was chosen for this study because it is the most thoroughly characterized catalytic zeolite. Thus, one can address such questions as what non-framework cation sites are occupied, whether the cations move between sites, whether interactions between the cations themselves are important and how these factors relate to the kinetics of catalytic reactions. 

The cluster size of the transition metal in zeolites was determined for a number of different preparations. In the mesoporous MCM-41 materials [48, 49] isolated clusters were observed, whereas for some solid-state exchanged and chemical vapor deposition samples dimeric species similar to methane monooxygenase were suggested [50, 51]. To date the discussion centers on clustered versus isolated species present in the various zeoHtes. 

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-... 

Ce-Y and Pr-Y zeolites were found to exhibit significant activity using propene as reductant and they show temperatures of maximum NO conversion within the range reported for the transition-metal-exchanged ZSM-5 and Y zeolites. In this way, Nishizaka and Misono (1993) have reported that, when methane was used as the reductant, the activity of two palladium loaded catalysts (i.e., Pd-H- and Pd-Ce-H-ZSM-5) was found to be comparable to the activity of transition-metal-exchanged ZSM-5. However, these catalysts were ineffective for the reduction of NO by propylene. On the other hand, the addition of alkaline-earth-metal ions (Mg, Ca, Sr, and Ba) enhanced the activity of the Ce-ZSM-5 catalysts, particularly at temperatures above 350°C (Yokoyama and Misono 1992, 1994a,b). 

Shimizu K,Akahane H,KodamaT and Kitayama Y (2004), Selective photo-oxidation of benzene over transition metal-exchanged BEA zeolite , Appl Catal A Gen, 269,75-80. 

The results based on chemical shifts should be interpreted with caution because the difference between the chemical shifts of the supported sample and those of the zeolite support depends on the xenon pressure, [149] on the type of cations exchanged into zeolite (e.g. divalent vs. monovalent, [150-153]) and on the temperature and size of the zeolite crystals. [154, 155] A recent publication by Ryoo et al. [149] illustrates the application of the technique in characterizing dusters of several different transition metals in zeolite Y. 

Zeolite catalysts have also been proposed for stationary SCR applications, mainly in gas-fired cogeneration plants. Zeolites in the acid form, in which transition metal ions (eg, Fe, Co, Cu, Ni) are introduced in the structure to improve the SCR activity, guarantee high de-NOx activity even at high temperatures to a maximum of 600° C, where metal oxide based catalysts are thermally unstable. The use of metal-exchanged zeolite-based catalysts with distinct structures has been proposed, for example, mordenite, faujasite (both of X and Y types), and ZSM-5 (21,22). Techniques to remove the aluminum oxide from the crystal matrix can be conveniently applied to increase the Si/Al ratio and accordingly the thermal stability of the zeolite and at the same time to limit its tendency to sulfatation. 

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