Temperature dependence of the levitation effect Implications for separation of multicomponent mixtures

Abstract

Sufficiently long molecular dynamics simulations have been carried out on spherical monatomic sorbates in NaY zeolite, interacting via simple Lennard-Jones potentials, to investigate the dependence of the levitation effect on the temperature. Simulations carried out in the range 100–300 K suggest that the anomalous peak in the diffusion coefficient (observed when the levitation parameter, γ, is near unity) decreases in intensity with increase in temperature. The rate of cage-to-cage migrations also exhibits a similar trend. The activation energy obtained from Arrhenius plots is found to exhibit a minimum when the diffusion coefficient is a maximum, corresponding to the γ≈1 sorbate diameter. In the linear or normal regime, the activation energy increases with increase in sorbate diameter until it shows a sharp decrease in the anomalous regime. Locations and energies of the adsorption sites and their dependence on the sorbate size gives interesting insight into the nature of the underlying potential-energy surface and further explain the observed trend in the activation energy with sorbate size. Cage residence times, τ_c, show little or no change with temperature for the sorbate with diameter corresponding to γ≈1, whereas there is a significant decrease in τ_c with increase in temperature for sorbates in the linear regime. The implications of the present study for the separation of mixtures of sorbates are discussed.

References

1. J. Chem. Soc., Faraday Trans., 1991, 87(13).
2. R. M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves, Academic Press, New York, 1978.
3. J. Karger and D. M. Ruthven, Diffusion in Zeolites and Other Microporous Solids, Wiley, New York, 1992.
4. D. M. Ruthven, Principles of Adsorption and Adsorption Processes, Wiley, New York, 1984.
5. R. L. June, T. A. Bell and D. N. Theodorou, J. Phys. Chem., 1990, 94, 1508.
6. J. O. Titiloye, S. C. Parker, F. S. Stone and C. R. A. Catlow, J. Phys. Chem., 1991, 95, 4038.
7. N. J. Henson, A. K. Cheetham, B. K. Peterson, S. D. Pickett and J. M. Thomas, J. Comput. Aided Mater. Des., 1993, 1, 41.
8. D. W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974.
9. S. Yashonath and P. Santikary, Mol. Phys., 1993, 78, 1.
10. S. Yashonath and P. Santikary, J. Phys. Chem., 1994, 98, 6368.
11. S. Bandyopadhyay and S. Yashonath, J. Phys. Chem., 1995, 99, 4286.
12. R. Chitra and S. Yashonath, unpublished results.
13. A. N. Fitch, H. Jobic and A. Renouprez, J. Phys. Chem., 1986, 90, 1311.
14. A. V. Kiselev and P. Q. Du, J. Chem. Soc., Faraday Trans 2, 1981, 77, 1.
15. M. P. Allen and D. J. Tildesley, Computer Simulations of Liquids, Clarendon Press, Oxford, 1987.
16. S. Yashonath and P. Santikary, J. Phys. Chem., 1993, 97, 3849.
17. R. Chitra and S. Yashonath, J. Phys. Chem., 1997, in press.
18. S. Yashonath and P. Santikary, J. Phys. Chem., 1993, 97, 13778.
19. P. R. van Tassel, S. A. Somers, H. T. Davis and A. V. McCormick, Chem. Eng. Sci., 1994, 49, 2979.
20. D. Chandler, J. Chem. Phys., 1978, 68, 2959.
21. J. A. Montgomery, D. Chandler and B. J. Berne, J. Chem. Phys., 1979, 70, 4056.
22. T. Mosell, G. Schrimpf, C. Hahn and J. Brickmann, J. Phys. Chem., 1996, 100, 4571.