Cation-Exchanged Faujasite-Type Zeolites

Another method to produce more stable cracking catalysts is the introduction of polyvalent cations into, e.g., Na-Y, in particular La or rare earth (RE) cations [435,436]. Hirschler [437] andPlank [438] independently suggested that formation of OH groups via introduction of polyvalent cations is due to the following chemistry (which is nowadays generally referred to as the Hirschler-Plank mechanism), illustrated here with two selected examples using chlorides of two- or three-valent cations, M and M , respectively  

Similar reactions proceed with salt solutions of other cations (Cu +,Fe +,etc.) as well (cf., e.g.y [445,446]) and produce unavoidably OH groups which, in certain apphcations, may be undesired and have to be removed, for instance, by subsequent sohd-state reaction (cf. Volume 3, Chapter 2 of this series) or via reaction with NaNj [447,448]. 

Dzwigaj et al. [449] re-interpreted the HF OH-band generally appearing in the mid IR spectra of Y-type zeohtes. They offered evidence for their suggestion that the HF-OH band of, e.g., H,Na-Y or H,Mg-Y is composed of two components with wavenumbers of 3648 and 3660 cm originating from two different kinds of OH groups. This result was supported by pyridine adsorption experiments, which showed the appearance of two pairs of pyridinium ion bands, viz. at 1543/1627 and 1552/1636 cm (cf. Sect. 5.5.2.6.2). 

Similar results were obtained upon interaction with 2,6-lutidine, which was claimed to be more selective with respect to Bronsted acid sites. 

The Bronsted and Lewis acidity of La, Na-Y zeolites with various degrees of exchange was studied via IR spectroscopy of the OH stretching region, after adsorption of pyridine as a probe, after dehydroxylation and rehydration by Bal-livet et al. [251]. Rehydration after dehydroxylation restored to a limited extent the OH groups (cf. also Sect. 5.6.3, especially [888]). 

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The influence of the alkali cation exchange in faujasite type zeolites on the zeolite acidity and electronegativity is presented. Correlations between the changes of these parameters and the activity and selectivity of zeolites in the hydrosul-phurization of alcohols are discussed. It was stated that for these processes in which the dissociatively adsorbed H2S takes part, the increase of the zeolite electronegativity causes the decrease in the activity. 

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. 

Rh dispersion appears to be a key factor in the activity of zeolite-supported systems. In studies of the effect of Si/Al ratio in Rh/NaX (faujasite-type zeolite), prepared by cation exchange of NaX with [Rh(NH3)5Cl](OH)2, maximum carbonylation activity (ca. 8 mol/(gRh-h) at 200°C) correlates with maximum Rh dispersion in the prepared catalyst (48). Similarly, cation exchange of NaX with [Rh(NH3)5Cl]Cl2 has been reported to give carbonylation rates of ca. 1 mol/(gRh h) at 150°C (49), and NaY exchanged with rhodium salts (50) gives carbonylation activity of 0.4 mol/(gRh-h) at 170°C. These rates are claimed to be higher than those for Rh impregnated alumina, silica-alumina, silica, or titania (49,50). Optimum Rh dispersion corresponded to approximately two Rh atoms per vmit cell (50). 

It has been conclusively shown that catalytic activity in ion-exchanged faujasites is influenced by cation type (including size and charge) (8,9,40,43,46,47), cation location in the lattice (40,48), zeolite Si/Al ratio (40,48), and the presence of proton donors (49-52). 

Low-silica zeolites such as sodium type A, having a molar ratio of Si Al near unity, contain the maximum number of cation exchange sites that balance the aluminum in the structure and thus have the highest possible cation exchange capacities [104,105,120]. Intermediate-silica zeolites, for example, of the faujasite type, have ratios of 2-5 and high silica zeolites, for example, ZSM types, have ratios of 10-50, respectively [104]. 

Bennett and Smith (1969) determined the crystal structure of La-exchanged faujasite. In the structure (fig. 62), lanthanum atoms and water molecules are located in the large spaces between framework structures of zeolitic type, with partial occupancies. However, they mentioned that the electron density was insufficient to account for all the cations and water molecules, implying that they might move frequently from site to site. The R factor of their analysis was high, i.e. 0.15 for all reflections, and further analysis of the structure is necessary. 

The dealumination process is associated with a change in the porosity within the crystals and may sometimes cause a drastic loss of crystallinity. The microporous adsorbents of the faujasite type are so arranged that the Si/Al ratio increases as the munber of cations and the average electrostatic field within the framework decrease. To assess the effect of the Si/Al ratio on the activity and acidity of Y zeolites, it is desirable to compare samples with similar extents of exchange, since the degree of exchange has a significant influence on the catalytic and acidic properties of faujasites. 

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