Faujasite lattice

Recent work by Rabo et al. (57) opens new possibilities for controlling the activity and selectivity of zeolite catalysts. Occlusion of various guest molecules into the sodalite cavities of Y zeolites can significantly change the catalytic properties of the zeolites for carbonium-type reactions. Anions of occluded salts are located close to the center of the sodalite cavity and strongly influence the arrangement of cations in the faujasite lattice and hence the catalytic activity. 

The relative crystalline stability of these materials was evaluated by DTA/TG (Fig.2). It is seen that the exothermic peak (indicating the collapse of the faujasite lattice) around 1133 K in NaY( Sample A) progressively shifts to lower temperataures with increasing iron content in the zeolite reaching 1033 K for Sample E indicating that the presence of iron reduces the thermal stability of the lattice... 

It was also noted that the stretching vibration frequencies of these groups somewhat increased (by 20-30 cm 1) with increasing content of aluminum in a faujasite lattice (83). From the cluster quantum-chemical viewpoint, this should be attributed to the effects of the aliminum atoms of the second and next coordination spheres of a cluster. 

Another example is the extensive study by Haase and Sauer, whose calculations were carried out for a methanol molecule with a number of cluster models, the most sophisticated of which was sufficiently large to represent a portion of the faujasite lattice formed by three 4-R rings and one 6-R. All structures were fully optimized at the SCF and/or MP2 level. The authors found that, for all cases studied, the IP complex was a transition state, whereas the HB complex corresponded to a minimum on the PES of the system. However, the adsorption energies for both structures were very close to one other, with a barrier for proton transfer of only a few kilojoules per mole. A strong H-bond between the Bronsted proton and the O atoms of methanol, and a long and weak H-bond between the H atom of the molecule and the O atom of the zeolite... 

Fe with the template ion. DTA studies indicate that Fe-faujasites have lower thermal stability than their A1-analogs. The vibration frequency shifts from 3540 and 3630 to 3570 and 3643 cm-1 respectively on isomorphous substitution of A1 by Fe. Relative changes in the intensity of the ESR peak at g = 4.3 at low temperatures also support the conclusion that iron can be inserted in the faujasite lattice positions. 

Values of (SiO /Al O ) were taken from solid state NMR data (presented later). The difference between lines A and B (indicating unit cell expansion) is due to the incorporation of Fe in the lattice framework. However if all the Fe in the solids were to occupy lattice positions the unit cell parameters would have been larger than those observed (line A) and would have increased with Fe content. The observed intermediate position of line A leads to the conclusion that only a fraction of Fe is in the faujasite lattice the balance occupying nonframework positions. Thus while the observed a... 

Figure 1. The molecular graphics picture of 12-membered window in faujasite lattice.

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