Cu Zeolites

Figure 10. Reactions in Cu-zeolite catalyzed amination of halobenzenes.

Maximum temperatures and hydrogen consumption during the temperature programmed reduction of Cu-zeolites by hyckogen. 

A. Alix, S. Chassaing, P. Pale, and J. Sommer, Click chemistry in Cu -zeolites A convenient access to glycoconjugates, Tetrahedron, 64 (2008) 8922-8929. 

Pd/Cu zeolite Y associations were found to be selective catalysts for oxidation of olefins in the presence of steam at temperatures ranging from 373 to 433K [22-30]. Acetone and acetaldehyde were obtained by propylene and ethylene oxidation, with selectivities of at least 90%. Neither Pd/Y nor Cu/Y showed good activity in these reactions. The conversion of different olefins under the same experimental conditions decreases in the following order [23] ethylene > propylene > 1-butene > cis-2-butene - trans-2-butene. 

Pd/Cu-zeolites are also catalysts for the oxidative acetoxylation of propylene to allylacetate [32-39]. The best results are obtained on a catalyst which is pretreated with an alkali solution to neutralize the acidic centres and containing Pd and Cu in an atomic ratio of 1.1 [37]. The alkali treatment suppresses the acid catalyzed addition of acetic acid to propylene, resulting in the formation of isopropyl acetate, which is observed over non-neutralized Na- and H-Y, as well as over unreduced and reduced Pd/Cu-NaY. Experiments with... 

The catalytic properties of Cu-ZSM-5 zeolites, first found by Iwamoto and ooworkers, will be outlined in the next section. Here I discuss how it occurred to us that Cu-zeolites are suitable as... 

In IR experiments it was confirmed that NO could adsorb as NO, NO and (NO)2- species on the Cu-zeolite, and the anionic species decreased with adsorption time to yield N2 and N2O in the gas phase whereas NO" " increased. After adsorption of NO for about 1 h, anionic species had almost disappeared and the intensity of NO species became approximately constant. These results indicate that all the Cu ions generated through pretieatment at elevated temperature were oxidized to Cu2 ions by oxygen produced in the NO decomposition at ambient temperature and the resulting CU2+ ions acted as adsorption sites for NO" " (Cu2+ + NO = Cu -NO ). This NO species could not be desorbed by evacuation at room temp ature. The IR spectra indicated the presoice of a large amount of NO and small amounts of NO2 and NO3 after the evacuation, i.e., weakly adsorbed or physisorbed NO molecules were absent from the zeolite under these condititHis. These phenomena were further confirmed by ESR experiments the adsorption-desorption cycles of NO resulted in a decrease-increase in the intensity of Cu2+ ESR signals. 

In the plots of the catalytic activity per Cu2+ ion, to which an NO molecule can be accessible, against the A1 content, Al/(Si+Al), we obtain a good correlation as depicted in Fig. 5. It is worth noting that not only the acid-base catalysis of proton-exchanged zeolites but also other kinds of catalytic reaction are controlled by the A1 content. In the present NO-Cu zeolite system, the zeolite structure would be the factor determining the effectiveness of Cu2+ ions, and the catalytic activity of the effective Cu2+ ion is probably controlled by the A1 content ... 

Finally, it will be discussed why the Cu-Zeolites exhibit such exceptionally high and stable activities. At present, there is no clear answer to this question but it appears that the decomposition activity is based on a combination of the following factors. 

NOx direct decomposition seems the most attractive solution in emission control, because the reaction does not require any reductants added and potentially could lead to the formation of only N2 and 02. Cu zeolite is one of the best catalysts for NOx direct decomposition, but the activity is very poor and needed to be improved several orders of magnitude high (Iwamoto et al., 1981). 

Fig. 39. Evolution pattern of several gases in cycled feed stream at 310°C. (a) Pd/Al203 + NSR catalyst and (b) Pd/Al203 + NSR catalyst+ Cu/Zeolite.

Fig. 40. NOx reduction activity with pre-adsorbed NH3 or C3H6 on Cu/Zeolite. After saturating the adsorbtion of a reductant (a) NH3 and (b) C3H6, on Cu/Zeolite, NOx reduction activity was measured with increasing temperatures at 20°C/min under the oxidizing atmosphere.

The Cu+/zeolite-catalyzed cyclodimerization of 1,3-butadiene at 100°C and 7 atm was found to give 4-vinylcyclohexene [Eq. (13.12)] with high (>99%) selectivity. Subsequent oxidative dehydrogenation over an oxide catalyst in the presence of steam gives styrene. The overall process developed by Dow Chemical113 offers an alternative to usual styrene processes based on ethylation of benzene (see Section 5.5.2). 

Cu+ emission spectra were recorded using a nanosecond laser kinetic spectrometer (Applied Photophysics). Cu+-zeolites were excited by the laser beam of the XeCl excimer laser (Lambda Physik 205, emission wavelength 308 nm, pulse width 28 ns, pulse energy 100 mJ). The 320-nm filter was situated between 2 mm thick silica cell and monochromator. Emission signal was detected with the photomultiplier R 928 (Hamamatsu), recorded with the PM 3325 oscilloscope and processed by a computer. All the luminescence measurements were carried out at room temperature. The Cu+ emission spectra were constructed from the values of luminescence intensity at the individual wavelengths of emission in selected times after excitation (2, 5,10, 20, 50, 100 and 200 ps). For details see Ref [7]. 

Figure 93 Single-crystal structures of Cu zeolite Cu8-A, (Cu2+)J(Cu+)3(0H)Si12Al12048-H20, prepared by ion...

The same authors reported the liquid-phase oxidation of benzene to phenol, with O2 as the oxidant and Cu-zeolite or Cu-MCM as the catalyst (175, 176). However, phenol yields were low, a large amount of supported Cu was required, and ascorbic acid was used as a stoichiometric coreductant. Phenol production was accompanied by the formation of H2O2 in solution ... 

Two methods of preparing Cu+ zeolites have been described (172-174), namely,... 

Pt/zeolite operates at a significantly lower temperature than does Cu/zeolite, and exhibits relatively high resistance to deactivation, but the major drawback is the high selectivity to N20... 

Kinetic and mechanism. Table 16.1 presents the reaction order with respect to NO, NH3 and O2 for the SCR on various Cu-zeolites. On Cu-exchanged zeolites it is... 

The spectra reported in Figs. 5 and 6 and discussed in the previous section have been obtained in a temperature range in which the Cu -zeolite catalysts are not active for the NO decomposition because of too high an activation barrier (A/sino Oj+Nj) as compared to the thermal energy kT. This condition exactly mirrors the scheme reported in Fig. 1 (Section I.A). In this case, the generic molecule A is the NO molecule and the surface site F is an intrazeolitic Cu" ion. 

If the Cu -zeolite/NO systems are allowed to progressively reach room temperature, then kT first starts to be comparable to and then greater than A/sino Oj+Nj, and the activity of the catalyst is switched on. The evolution of the Cu ZSM-5/NO... 

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