Zeolite catalysis decomposition

Cince the catalytic activity of synthetic zeolites was first revealed (1, 2), catalytic properties of zeolites have received increasing attention. The role of zeolites as catalysts, together with their catalytic polyfunctionality, results from specific properties of the individual catalytic reaction and of the individual zeolite. These circumstances as well as the different experimental conditions under which they have been studied make it difficult to generalize on the experimental data from zeolite catalysis. As new data have accumulated, new theories about the nature of the catalytic activity of zeolites have evolved (8-9). The most common theories correlate zeolite catalytic activity with their proton-donating and electron-deficient functions. As proton-donating sites or Bronsted acid sites one considers hydroxyl groups of decationized zeolites these are formed by direct substitution of part of the cations for protons on decomposition of NH4+ cations or as a result of hydrolysis after substitution of alkali cations for rare earth cations. As electron-deficient sites or Lewis acid sites one considers usually three-coordinated aluminum atoms, formed as a result of dehydroxylation of H-zeolites by calcination (8,10-13). 

The N2O Decomposition Reaction Self-Organization in Zeolite Catalysis... 

In order to get the pore system of zeolites available for adsorption and catalysis the template molecules have to be removed. This is generally done by calcination in air at temperatures up to 500 °C. A careful study (ref. 12) of the calcination of as-synthesized TPA-containing MFI-type single crystals by infrared spectroscopy and visible light microscopy showed that quat decomposition sets in around 350 °C. Sometimes special techniques are required, e.g. heating in an ammonia atmosphere (ref. 13) in the case of B-MFI (boron instead of aluminum present) to prevent loss of crystallinity of the zeolite during template quat removal. 

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoint [17], However, the application of zeolites to organic reactions in a liquid-solid system is very limited, because of insufficient acid strength and slow diffusion of reactant molecules in small pores. We reported preliminarily that the microporous Cs salts of H3PW12O40 exhibit shape selectivity in a liquid-solid system [18]. Here we studied in more detail the acidity, micropore structure and catal3rtic activity of the Cs salts and wish to report that the acidic Cs salts exhibit efficient shape selective catalysis toward decomposition of esters, dehydration of alcohol, and alkylation of aromatic compound in liquid-solid system. The results were discussed in relation to the shape selective adsorption and the acidic properties. 

Tajima, N., Hashimoto, M., Toyama, F. et al. (1999) A theoretical study on the catalysis of Cu-exchanged zeolite for the decomposition of nitric oxide, Phys. Chem. Chem. Phys., 1, 3823. 

Zeolite catalysts may also be regarded as mixed oxides, but the crystallographic structures differ from the solids discussed above in that active sites for catalysis occur within the open lattice framework. In consequence, rate data are not directly comparable with similar observations for other heterogeneous reactions since the preexponential factors are calculated and reported on a different basis. For completeness, however, it is appropriate to mention here that instances of compensation behavior on zeolite catalysts are known. Taylor and Walker (282) described such an effect for the decomposition reactions of formic acid and of methyl forma te on cation-exchanged 13X molecular sieves, and comparable trends may be found in data reported for reactions of propene on similar catalysts (283). 

In a relatively few years zeolites were promoted from simple adsorption agents to catalysts of wide spread use in all fields of chemistry. Apart from their acidic properties generated by exchanging their Na+ or K+ starting forms by ammonium ions and subsequent decomposition of the latter, their unique properties as supports for various precious metals and their solution behaviour attracted much of the attention devoted to catalysis. 

Fig. 45. Reaction time profiles of the photocatalytic decomposition of NO into N2 and O2 at 275 K on Cu(I)ZSM-5 (a), Cu(I)Y zeolite (b), and Cu(I)Si02 catalysis (c). No reaction could be observed at 275 K without UV irradiation [reproduced with permission from Anpo et al. (779)].

Perez-Ramirez, J., Kapteijn, F.. Groen, J.C., Domenech, A., Mul, G., and Moulijn, J.A. 2003. Steam-activated FeMFI zeolites. Evolution of iron species and activity in direct NiO decomposition. Journal of Catalysis 214. 33 5. 

Zhang, Y., and M. Flytzani-Stephanopoulos, 1994, Catalytic decomposition of nitric oxide over promoted copper-ion-exchanged ZSM-5 zeolites, in Environmental Catalysis, ed. J.M. Armor, Vol. 552 of ACS Symposium Series (American Chemical Society, Washington, DC) pp. 7-21. 

T. Inui, S. Kojo, M. Shibaka, T. Yoshida and S. Iwamoto "NO Decomposition on Cu-Incorporated A-Zeolites Under the Reaction Conditions of Excess Oxygen with a Small Amount of Hydrocarbons", Zeolite Chemistry and Catalysis, p. 355 (1991)... 

Tliere are two other specific features observed in the catalysis of cerium zeolites. The first is the poor activity of CeNa-Y. A siiuilar low activity of metal-exchanged Y compared to ZSM-5 is reported in the NO decomposition with CuNa-Y [33], and in the NO reduction with methane over Co-Y [34]. Sepulveda-Escribano... 

The most efficient method for NO removal from stationary and mobile sources is catalytic reduction with ammonia, hydrocarbons, CO or H2. Modified zeolites are active catalysts in these processes. For Cu-ZSM-5 especially high activity and stability have been reported. In this work the properties of copper-containing ZSM-5 zeolites prepared by wet or solid state ion-exchange have been investigated. The Bronsted acidity of the Cu -exchanged samples was much lower than that of the parent zeolites, and they had high activity in selective reduction with ammonia, propene or propane. A comparison of Cu-ZSM-5 activity in the decomposition of NO and in the reaction of NO with propene or propane revealed that the hydrocarbons as well as the nitrogen oxides play important role in the performance of NO reduction catalysis. 

Meyer, U lloelderich. WF. Application of basic zeolites in the decomposition reaction of 2-methyl-3-bulyn-2-oi and the isomerization of 3-carenc. Journal of Molecular Catalysis A Chemical. 1999 142,213-222. 

E.P. Reddy, L. Davydov, P. Smirniotis, Ti02-loaded zeolites and mesoporous materials in the sonophotocatalytic decomposition of aqueous organic pollutants the role of the support , Applied Catalysis B Environmental, 42,1-11, (2003). 

Here, we will first present briefly the general chemistry at work in deNOx catalysis. Then we wiU focus on the various specific nanometric issues in the domain. NOx decomposition mainly takes place on a metal center, as do most redox reactions in heterogeneous catalysis. We will therefore focus on how nanometric size influences the role of metal particles in this domain. It is very much linked to interaction with the oxide support, which wiU be the subject of the following part Zeolites are very specific oxide supports, controlling the distance between reactants and imposing chemical reactivity within the nanometer scale. Metal particles formed in zeoHtes have their size controlled between 0.5 and several dozens of nm, sometimes with Angstrom accuracy. We wiU next deal with three-way catalysts, mainly used for automotive deNOx. They involve very complex solids, where particle size is a key issue for activity. Finally, we wiU mention quickly new nanometric sohds like carbon nanotubes, which have appeared recently in deNOx catalysis. 

The kinetic expressions which govern different reaction mechanisms are usually very different. We will illustrate this by comparing expression (2.2) with the kinetic expression of N2O decomposition found for zeolites. The N2O decomposition reaction has a very different reaction sequence when catalyzed by isolated Fe cations in a zeolite, as compared with the reaction sequence foimd for catalysis by transition metals. 

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