Zeolites contracted lattice parameters

The absence of initial contraction in zeolite 8 Li-4 NaA is probably caused by the fact that Li+ cations may be situated in other positions inside the voids because of their small sizes. This is corroborated by the fact that Li exchange increases the lattice parameter a of zeolite LiA by 2.6% compared with zeolite NaA. Then, the same processes occur as in NaA, leading to crystal expansion. Perhaps at high fillings Li cations extend from six-membered oxygen windows into the large voids this would decrease the contraction of their positive charges and expand the crystal lattice. 

Most EXAFS data show that the lattice parameter of a small particle is contracted as compared with the bulk metal. For example, Renouprez and co-workers125,135 observe for Pt encased in Y-zeolite a Pt-Pt distance contracted from 0.277 to 0.270 nm. Similarly, Hamilton et al.123 observe Cu-Cu distances varying from 0.233 for very small clusters, to 0.250 for... 

Sr2+ ion exchange for Na+ in zeolite X is an example of the occurrence of a phase reaction as a result of the limited solubility of the end members of an ion exchange series. Olson and Sherry (45) have reported an x-ray powder diffraction study that shows that as Sr2+ ions are exchanged into NaX, the cubic unit cell contracts. At 71% Sr loading, the unit cell suddenly expands and a new phase forms that is richer in Sr2+ and has a much more expanded unit cell. Evidence for the presence of 2 solid phases is the double x-ray diffraction pattern that they observed. The new phase has the faujasite structure, an Sr exchange level of 87%, and is in equilibrium with the old phase. As the miscibility gap of 71 to 87% is traversed, the Sr-poorer phase disappears and the amount of the Sr-richer phase increases until at 87% Sr loading, only the expanded phase exists. The lattice parameter data are shown in Figure 8. 

Demontis et al. (94) reported an early MD study of the sorption and mobility of benzene in zeolite NaY. The zeolite was modeled with a Si/Al ratio of 3.0, as in previous calculations for Xe and methane. The zeolite and benzene molecules were treated as rigid. The authors supported the assumption of a rigid zeolite lattice by quoting structural studies (95), in which the cell parameter of NaY zeolite was found to contract little upon uptake of benzene. It is, however, more than possible that the lattice undergoes substantial deformation without an overall change in volume quantum chemical calculations (96) have shown that the Si-O-Si bending potential is very soft. When these calculations were performed, the assumption of a rigid lattice was more a matter of computational necessity than it is today. 

Hydrothermal treatment can however be used constructively to modify the properties of an adsorbent. Perhaps the best example is the formation of ultrastable Y by hydrothermal treatment of sodium ammonium Y zeolite. The change is accompanied by a contraction in the unit cell parameter and an increase in the Si/Al ratio due to elimination of aluminum from the lattice. The resulting product shows greater thermal stability than Y zeolites of similar composition which have not been subjected to the hydrothermal treatment. However, with X zeolite the usual result is a loss of crystallinity with attendant deterioration of the adsorptive properties while with A zeolite a more subtle effect referred to as pore closure occurs. Hydrothermally treated 4A zeolite behaves as if the window aperture is somewhat smaller than in normal 4A sieve. This effect can be useful since a pore-closed 4A does not admit chlorinated hydrocarbons and is therefore useful for drying freon refrigerants. If a wider pore sieve is used for this purpose premature breakdown and loss of capacity may occur due to formation of HF and/or HCl by hydrolysis. The precise mechanism of pore closure has not yet been established and it remains uncertain whether it involves a true modification of the ciystal structure or merely a rearrangement of the surface layers. 

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