Zeolite exchanged

Ion exchange is a chemical reaction. For uni-univalent ion exchange, it can be represented by  

For type X and Y zeolites, below a level of 34 cations/unit cell (or, 40% ion exchange of a typical X zeolite with 86 cations per unit cell), the order of selectivity for univalent ions follow (Sherry, 1966 Breck, 1974)  

This series corresponds to occupancy of the most accessible eation sites (sites IB and IV) within the supercage. At 50% exchange of X zeolite, which includes site 11 in the 6-ring adjacent to the supercage, the selectivity series is (Breck, 1974) 

These sites (II, III, and IV) are exposed to the supercage and hence important for adsorption. 

In tailoring sorbents for rr-complexation, both the cation-sorbate bond strength and the total number of cations are important. The density of cations depends on the cation exchange capacity of the zeolite. Table 8.3 provides useful information on the total cation capacities for a number of zeolites. 

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Zeolites with lower UCS are initially less active than the conventional rare earth exchanged zeolites (Figure 3-5). However, the lower UCS zeolites tend to retain a greater fraction of their activity under severe thermal and hydrothermal treatments, hence the name ultrastable Y. 

A fully rare-earth-exchanged zeolite equilibrates at a high UCS. whereas a non-rare-earth zeolite equilibrates at a very low UCS in the range of 24.25 [3]. All intermediate levels of rare-earth-exchanged zeolite can be produced. The rare earth increases zeolite activity and... 

Figure 3-5. Comparison of activity retention between rare-earth-exchanged zeolites versus USY zeolites. (Source Grace Davison Octane Handbook.)...

A rare-earth-exchanged zeolite increases hydrogen transfer reactions. In simple terms, rare earth forms bridges between two to three acid sites in the catalyst framework. In doing so, the rare earth protects... 

Our own earlier work on the chlorination of toluene had been subject to similar constraints. In this case, chlorination with ferf-butyl hypochlorite had proved to be advantageous. In the presence of silica gel as catalyst the yield of chlorotoluenes was quantitative but the regioselectivity was more or less statistical (ref. 8). However, the use of proton-exchanged zeolite X allowed the production of chlorotoluenes with a para-selectivity of more than 90 % (Fig. 4) (ref. 9). No HCl is generated in this process since the by-product is tert-butanol, and there is no inhibition of the catalyst. Indeed, the catalyst can be reused if necessary. 

Cu-exchanged zeolites have been examined in the nucleophilic substitution of halobenzenes towards aminated and oxygenated systems. Selectivities are dependent on the zeolite s pore sizes. 

Carbohydrate-coupling or glycosylation, is a major synthesis method in carbohydrate preparation. Silver silicates and Ag(I)-exchanged zeolite A - so-called insoluble Ag(I) - have been advocated as promoting agents, applied in more than stoichiometric amount (Fig. 9). All hydroxyl groups except the attacking one are suitably protected. 

Low concentrations of Cd(l) species can be obtained by allowing Cd vapor to react with Cd and trapping the resultant Cd(I) species in a zeolite . When Cd -exchanged zeolite A is exposed to Cd metal vapor at 350°C for 2.5 d, both Cd and Cd2 " are formed, the latter having a Cd-Cd internuclear distance of 235 pm. 

By minimizing the Fe concentration (i.e., avoiding extensive Fe-exchange), zeolites, or mesoporous compounds can be detemplated at low temperatures without the need for high-temperature calcination. This third concept refers to the low-temperature Fenton detemplation. Strictly speaking, Fenton requires thermal activation but always below 100 °C. We refer here to quasi room temperature as compared to the high temperatures usually applied for calcination. 

Ion exchange zeolites are builders in washing powder, where they have gradually replaced phosphates to bind calcium. Calcium and, to a lesser extent, magnesium in water are exchanged for sodium in zeolite A. This is the largest application of zeolites today. Zeolites are essentially nontoxic, and pose no... 

For infrared spectroscopy, 20-50 mg of the cobalt-exchanged zeolite was pressed into a self-supporting wafer and placed into an infrared cell similar to that described by Joly et al. [21], Spectra were recorded on a Digilab FTS-50 Fourier-transform infrared spectrometer at a resolution of 4 cm-i. Typically, 64 or 256 scans were coadded to obtain a good signal-to-noise ratio. A reference spectrum of Co-ZSM-5 in He taken at the same temperature was subtracted from each spectrum. 

It is important for oxygen to be absent in the feed to avoid oxidation of cuprous to cupric ions rendering adsorption with reaction unworkable. This strategy can be extended to the removal/recovery of olefinic compounds, for which cuprous-exchanged zeolites may also be useful. 

Large-pore zeolites such as Y zeolites are efficient for the hydroamination of several olefins. For example, propene reacts with NH3 over SK-500 (a pelleted lanthanum-exchanged zeolite) or La-Y or H-Y zeolites with 6-15% conversion to give i-PrNHj with high selectivity (95-100%) (Eq. 4.5) [50]. 

Schneider, W.F., Hass, K.C., Ramprasad, R. et al. (1998) Density functional theory study of transformations of nitrogen oxides catalyzed by Cu-exchanged zeolites, J. Phys. Chem. B, 102, 3692. 

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. 

Pietrzyk, P., Sojka, Z. (2007) Co2+/Co° redox couple revealed by EPR spectroscopy triggers preferential coordination of reactants during SCR of NOx with propene over cobalt-exchanged zeolites, Chem. Commun., 1930. 

Iwamoto, M., Mizuno, N. and Yahiro, H. (1991) Removal of nitrogen monoxide over copper ion-exchanged zeolite catalysts, Sekiyu Gakkaishi, 34, 375. 

ASPECTS OF CATALYST DEVELOPMENT FOR MOBILE UREA-SCR SYSTEMS - FROM VANADIA-TITANIA CATALYSTS TO METAL-EXCHANGED ZEOLITES... 

Comparison of metal-exchanged zeolites and vanadia catalysts under standard-SCR conditions... 

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