Cu-faujasite

Delahay G, Coq B, Kieger S et al (1999) The origin of N2O formation in the selective catalytic reduction of NOx by NH3 in O2 rich atmosphere on Cu-faujasite catalysts. Catalysis Today 54 431 38... 

Delahay G, Kieger S, Tanchoux N et al (2004) Kinetics of the selective catalytic reduction of NO by NH3 on a Cu-faujasite catalyst. Applied Catalysis B Environmental 52 251-257... 

An important group of catalysts for NOx reduction through anunonia SCR is represented by copper exchanged zeolites [1-9]. Many different zeolites have been investigated, for example, Cu-ZSM-5 [10-13], Cu-faujasite [14], Cu-Beta [15,16], and Cu-Y [17]. Recently, copper zeolites with chabazite (CHA) structure have received great attention due to their high thermal stability and hydro carbon resistance [18]. Both Cu-SAPO-34 [19] and Cu-SSZ-13 [18] have a CHA structure... 

Kinetic models for ammonia SCR have been developed for vanadia on titania [20-24], Cu-ZSM-5 [10-13], Cu-faujasite [14], HZSM-5 [25], and Fe zeolites [26-28]. In this chapter, the focus is on kinetic models for ammonia SCR over copper zeolites. Both global and detailed kinetic models will be described and many subreactions in the mechanism, such as ammonia storage, ammonia oxidation, NO oxidation etc., will be discussed in detail. 

Zeolites can be ion-exchanged with cations or impregnated with various metals to modify their performance for use in applications such as separations, adsorption and catalysis. For example, faujasite zeolites exchanged with Na, Li, K, Ca, Rb, Cs, Mg, Sr, Ba, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Pd, Ag, Cd, In, Pt, H, Pb, La, Ce, Nd, Gd, Dy and Yb have been made and studied due to their use in separation and catalysis [135]. The ability to determine the distributions of these cations in the zeolitic structure is one of the key parameters needed in understanding adsorption mechanisms and molecular selectivities. Little has compiled an excellent reference... 

The active component for olefin oxidation is Pd2+, while Cu2+ acts as a promoter for the reoxidation of Pd. The sequence of ion exchange of Pd + and Cu2+ on the faujasite zeolite influences the catalytic performance. Best results seem to be obtained when Pd + is introduced in the second step of the ion exchange as it will then be located mainly at the more easily accessible cation sites II and/or III [23], The amount of exchanged Pd + determines the catalytic activity of Pd +Cu +Y, provided that Cu2+ is present in sufficient amounts to assure fast regeneration of Pd2+. A Pd/Cu atomic ratio of four is required here. Increasing acidity in Pd +Cu +NaY results in a decrease of both the activity and selectivity in the olefin oxidation [26]. 

L. A. Luke and J. V. Brunnock, Separation of naphthenic and paraffinic hydrocarbons up to Cu from hydrocarbon mixtures by gas chromatography on faujasite molecular sieves , Ger. Offen. 1 908 418 1968). 

It is clear that the Wacker cycle in a CuPdY zeolite incorporates the traditional features of the homogeneous catalysis combined with typical effects of a zeolite (303, 310). It also follows that whereas other cation exchangers in principle will show Wacker activity after cation exchange with Cu/Pd ions, the cage and pore architecture will probably be less suitable for Wacker chemistry than those of the faujasite structure. This is the case for fluoro-tetrasilicic mica, a synthetic layer silicate that swells under reaction conditions and allows access to the interlayer space (311). 

Fig. 17. Perspective view showing the siting of Cu(III) cations at the pore entrance to the supercage (Cu2 +-exchanged faujasite, dehydrated at I50°C, butadiene adsorbed). The occupancy factors are such that there is approximately one Cu(III) cation per two pore entrances.

Table 1 Yield of arabinose and erythrose in the oxidative degradation of calcium gluconate by hydrogen peroxide catalysed by Cu(II)-exchanged faujasite. Comparison with homogeneous catalysis,...

With increasing temperatures the NO conversion over most catalysts of this group passes throu a maximum (33). Cu/ZSM-5 has the highest maximum at the lowest temperature, namely, 500°C. The activity of Cu in other zeolites, e.g., mordenite, ferrierite, faujasite, /3 and L, is distinctly lower (33,35). A rough correlation seems to exist between activity and Si/Al ratio of these zeolites (33) Li and Hall state, however, that the Si/Al cannot be the sole controlling factor, because Cu/Y and dealuminated Cu/Y have very similar activities (i5). 

Metastable species derived from (L)Cu(02), where L- = 2,4-di-te/V-butyl-phenolate linked to l,4-di-z, so-propyl-l,4,7-triazacyclononane, show multiple vCuO bands in the wavenumber range 500-550 cm-1.389 The resonance Raman spectrum of [Cu2(p-0)2(flf4-Me2-etpy)2]2+ has a vCu-O-Cu band at 579 cm-1 (551 cm-1 for lsO).390 Selective catalytic reduction of NO by NH3/02 on copper-faujasite catalysts gave rise to IR bands showing changes in copper oxidation states (using the vas[Cu-0-Cu]+ band near 900 cm-1).391... 

Faujasite-X zeolite (NaX) (Si/Al = 1.23, ca. 2 pm particle size, from Aldrich Chemical Company), ammonium hydroxide (assay 29+%, from Fisher Chemical Company), cobalt(II) chloride (99+ % assay) and copper nitrate hydrate [Cu(N03). H20, 101.7 % by EDTA complexation, from J.T. Baker Chemical Company] were used. 

This work shows the spectroscopic characteristics of ammonia in the presence of Cu and Co " " cations exchanged in the Faujasite-X zeolite and its ligand strength. This data may be used to identify and semi-quantify the presence of ammonia in the presence of pyridine, acetone and water. Specifically, both DRS and IR need to be used together in any attempt to formalize the sensoring of ammonia in the environment. Further research is, however, needed to quantify ammonia in the environment. 

In the first method the metal complex is assembled in the zeolite cavities by allowing the metal-exchanged zeolite to react with ligands that are small enough to access the micropores. The metal complex, once formed, is too large to diffuse out. For example, bis- or tris-bipyridyl complexes of Fe", Ru", Mn", Co" and Cu" have been encapsulated in zeolite Y (FAU) [12-15, 36], Metal-Salen and related SchifFs base complexes have been similarly encapsulated in faujasites [12-15, 37, 38]. However, in this case there is virtually no difference in kinetic diameter between the complex and the free ligand and metal-Salen complexes are readily leached by protic solvents, such as ethanol [12]. 

Computational modeling was successfully used to identify the locations of cations in zeolites and to clarify their interaction with zeolite hosts Cu" and Cu in ZSM-5 (MFI), ferrierite (PER), and faujasite (FAU) zeolites [134-136] alkali and alkaline-earth cations in MFI [133] Zn " in FAU and MFI [137,138] also some divalent cations in MFI [139]. Yet, these investigations represent only first steps in the exploration of the large variety of both metal cations (especially of transition metals) and zeolite structures. Moreover, the interaction of cations in zeolites with probe molecules or reagents [137,140,141] is much less investigated computationally, despite the fact that these interactions are crucial for interpreting results of spectroscopic methods used to characterize the cations as well as for rationalizing specific catalytic, adsorption, sensor, or other properties of the materials. 

A combination of DRIFT spectroscopy and TPD of CO adsorbed on faujasite-type zeolites, which have been exchanged with transition metal cations (Cu +, Fe +, Co +, Ni +),was employed by Rakic et al. [775]. Except on Cu, Na-Y, disproportionation of CO and carbon deposition occurred. The Lewis acid, charge-compensating sites were assumed to be the sites of adsorption. 

In specific cases, the exact composition of the first coordination sphere around Cu + can be derived from the superhyperfine splitting pattern in ESR. This is illustrated in Fig. 11 for a faujasite-type encagedbisCu(histidine) complex [47]. The axially symmetric ESR spectriun has a hyperfine splitting with values for g, gx and (j of around 2.27,2.06 and 17 mT, respectively and a seven-line superhyperfine structme with an value of 1.23 mT. The additional splitting is due to the presence of three nitrogen atoms (2 nN-tl=7 with and n equal to 1... 

The Si/Al ratio in zeolites controls acidobasic properties, and influences electronic distribution, and thus metal ions in the stracture. Cu in faujasite-type zeolites is present as monomers and dimers at Si/Al = 5.6, but when Si/Al reaches high values (13.9 and 390), only monomers are observed. Photocatalytic activity is therefore higher on the sample with Si/Al = 13.9, although its Cu content is much lower [29]. 

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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