Calcium chabazite

Figure 33b. Calcic zeolites as a function of their compositions in Al-Si-Cax coordinates assuming Calcium end members. F = feldspar G = gismon-dine La = lawsonite Sc = scolectite Ph = phillipsite Ch = chabazite ...

The location of extra framework cations is a major problem in characterising zeolites. Simulation is becoming an increasingly powerful tool for the exploration and rationalisation of cation positions, since it not only allows atomic level models to be compared to bulk experimental behaviour, but can also make predictions about the behaviour of systems not readily accessible to experimental probing. In the first part of this work we use the Mott-Littleton method in conjunction with empirical potential energy functions to predict and explore the locations of calcium cations in chabazite. Subsequently, we have used periodic non-local density functional calculations to validate these results for some cases. 

The primary objective of this study was to look at the locations of calcium cations in high silica chabazite. In the limiting case, a calcium ion associated with a pair of aluminium atoms in the framework will not be influenced by any other aluminium. In a slightly more realistic case, the influence of the other aluminium atoms can be regarded, to a first approximation, as a uniform isotropic background that will not affect the relative energies of the different calcium sites. A natural approach to this situation, is to model the chabazite initially as being purely siliceous, then introduce just two aluminium atoms and a calcium cation as a defect. 

The Mott-Littleton calculations revealed three sites for calcium in the high-silica chabazite, as show in Figure 3 ... 

In a pioneering study, Stockmeyer (1985) found that the H2 motion in a sample of natural chabazite, predominantly containing calcium ions, was gas-like at room temperature but solid-like below 200 K with a continuous change between the extremes. At low temperature a broad frequency spectrum was derived containing peaks at 48 and 88cm (6 and 11 meV), thought to... 

D. J. C. Yates (Esso Research Co., Linden, N. J. 07036) It is interesting that you find such a large difference in calcium and strontium exchange in chabazite. Would you care to comment on the reason for this ... 

Fig. 13. Historical UV-VIS spectra of iodine adsorbed on chabazite and calcium fluoride compared to solid I2,12 vapor (broken line) and h dissolved in benzene. Reprinted from [17] with kind permission of R. Oldenbourg Verlag GmbH, Miinchen...

Grey TJ, Gale JD, Nicholson DN, Peterson BK (1999) A computational study of calcium cation locations and diffusion in chabazite. Mesoporous and Microporous Solids 31 45-59 Harrison NM, Leslie M (1992) The derivation of shell model potentials for MgCb from ab initio theory. Mol Simul 9 171-174... 

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